Download Chapter 1: Aqueous Processing Systems

CHAPTER 1. AQUEOUS PROCESSING SYSTEMS
1.1.
Introduction ..................................................................................................
1
1.2 Aqueous Processing Technologies ....................................................................
2
1.2.1
1.2.2
1.2.3
1.2.4
1.2.5
1.2.6
1.2.7
1.2.8
1.2.9
1.2.10
1.2.11
1.2.12
Fields of Application and Academic Homes ....................................
Mineral Processing ...........................................................................
Hydrometallurgical Extraction .........................................................
Ceramic Aqueous Processing ...........................................................
Catalyst Preparation ..........................................................................
Emulsion-based Photographic Technology ......................................
Surface Finishing .............................................................................
Semiconductor Device Microfabrication..........................................
Cement Technology..........................................................................
Battery Technology ..........................................................................
Solar Energy Conversion and Photoelectrochemical Processing .....
Waste Treatment and Control ...........................................................
2
4
12
22
29
31
34
41
48
49
51
52
1.3 Common Denominators in Aqueous Processing .............................................
55
1.3.1
Integrated Processing ......................................................................
55
1.3.2
Reagents, Solutions, and Materials .................................................
56
1.3.3
Unit Processes .................................................................................
56
1.3.4
Physicochemical Processes .............................................................
57
______________________________________________________________________________
1.1 Introduction
Our physical well-being depends, in part, on our ability to take raw materials from the earth and
transform them into technologically useful materials. The raw materials include naturally occurring
minerals (commercially useful mixtures of which are called ores), coal and crude oil (sources of energy
and chemicals), and water (the premier solvent and transport medium). By processing, we refer to the
various physical and chemical steps that connect ores to refined metals and to engineered materials.
And in Aqueous Processing, in particular, our focus is on water-based chemical process technologies.
Water is the most abundant solvent, and it has some unique properties, which facilitate the
dissolution of solids, while at the same time leaving room (under changed conditions) for the reverse
process of solid formation from solution (crystallization and precipitation) to occur. Water is also a
convenient vehicle for the solubilization and transport of ions and molecules, as well as for the
suspension and transport of fine particles. The unique properties of water that make these processes
feasible include: (a) a relatively high dielectric constant, which stabilizes ionic species, (b) the ability to
donate and to receive protons (H+), which enables it to interact with a wide variety of chemical species,
(c) the relatively high chemical and physical stability under ordinary conditions of temperature and
pressure.
1.2 Aqueous Processing Technologies
1.2.1 Fields of Application and Academic Homes
Aqueous processing is practiced in a wide variety of technological fields, as demonstrated in the
non-exhaustive list provided in Figure 1.1. Traditionally, there has been a tendency to organize the
different fields of application into different academic disciplines or majors. For example, aqueous
processing applications in mineral processing are taught in departments and sub-departments with names
such as: Mineral Processing, Mineral Engineering, and Metallurgical Engineering. Ceramic applications
of aqueous processing are confined to departments of Ceramic Science and Engineering and
departments of Materials Science and Engineering. Aqueous chemical aspects of cement technology are
taught in Ceramic Science and in Civil Engineering programs, catalyst preparation is taught in Chemical
Engineering, and waste treatment in departments of Environmental Engineering. Wet chemical
processing in semiconductor device microfabrication is found in departments of Electronic and
Electrical Engineering.
Figure 1.1 Aqueous processing technologies
One advantage of the established situation is that the student is exposed to aqueous processing in
context, i.e., there can be immediate connection with the student's chosen field. Unfortunately, however,
this approach to technical education is not without some disadvantages. First of all, many aqueousoriented students may be completely unaware that in other academic departments (sometimes, housed
literally next door) fellow students are tackling problems that are quite similar to theirs. The similarity
may be in terms of materials, reagents, equipment, or process chemistry. So what happens then?
Someone loses, who might otherwise benefit from the insights of the other group. There is unnecessary
reinvention of the wheel; progress is slowed, while the answer to our problem lies idle next door; we
congratulate ourselves for achievements, which are really no breakthroughs. Secondly, university
educators have to come to grips with the fact that we are not just educating for today but for tomorrow.
What kind of tomorrows do we see? In the future that is before us, an increasingly important
requirement is flexibility; that is, a student's education should make him/her flexible in approaching
problems. Part of this flexibility is acquired by gaining an early appreciation for the variety in the kinds
of problems that a particular tool can tackle, and the variety in the tools (i.e., alternative
tools/concepts/strategies/technologies) that can be brought to bear on a particular problem. We become
acquainted with this variety, in part, by learning to look over our shoulders at our colleagues in other,
but related majors. As we move towards the 21st century we have to consider the real possibility that
the needs of the future may require that we reconfigure the traditional majors of technical education.
The need for realignment, of course, is not a new phenomenon, and neither is it, at present, confined
only to what one might call the applied fields. The pure sciences are confronted with a similar situation;
for example, the traditional distinctions between solid-state chemistry and solid-state physics are
becoming increasingly diffuse:
For many years the study of the electronic structure and properties of solids has been regarded as the
preserve of physicists. Chemists, who spend a good deal of time and effort looking at electronic structures
of molecules, have tended to avoid these aspects of solids. The situation is changing, however, and there
has recently been an upsurge of interest in those aspects of solid-state chemistry that have to do with
electronic properties and their relation to chemical bonding. In part, this change is due to the discovery of
new classes of solids with unusual and surprising properties. Many of these investigations have been
carried out by teams of chemists and physicists working together, and such collaboration is slowly helping
to break down the traditional barriers between these two disciplines. (Cox, 1987, p.v).
Thirdly, and on a more pragmatic level, we should also be concerned about the employability of the
students that pass through our educational factories. In the U.S., companies lay off their employees
when the economic situation gets tough. When you are laid off, can you dust off your resume and
repackage it attractively and solidly for a new prospective employer? That is, are you aware of the
variety in the kinds of problems that your tools can tackle?
AQUEOUS PROCESSING
M INERAL
PROCESS.
HYDRO- CERAM IC ENVIRON- CHEM .
AQUEOUS M ENTAL
M ET
ENG.
PROCESS. ENG.
CIVIL
ENG.
ELECTRON.M AT. SCI
ENG.
& ENG.
Figure 1.2 The many mansions of aqueous processing.
This book is an invitation to join in breaking down the walls of our academic ghettos. Jesus said: In
God's house there are many mansions. They are all yours: enter by the main gate above and all the
rooms will be open before you (see Figure 1.2). Insist on entering by the individual doors below, and
you are in danger of being boxed in by your major.
1.2.2 Mineral Processing
In mineral processing technology (Figure 1.3), the overall objective is to use physical and chemical
techniques to separate a solids feed of ores into a concentrate (which contains the minerals of
commercial value) and the gangue (which is a collection of the waste solids of little immediate
commercial value). Typically, no bulk chemical changes are effected, so that any chemical
transformations that occur tend to be confined to the solid surfaces. Three processes used in mineral
processing, and which fall within the scope of our present discussion of aqueous processing are
flotation, flocculation, and dispersion (Table 1.1). All three may be viewed as involving interfacial or
adsorptive processing.
Flotation is a particulate separation method in which particles with appropriately prepared surfaces
attach themselves to gas bubbles, and are thereby parachuted upwards (floated) from the aqueous phase.
Three main types of reagents are used: collectors, frothers, and regulators. Collectors and frothers are
members of a class of molecules called surfactants. Table 1.2 shows the structures of selected flotation
surfactants. The molecular structure of a surfactant is characterized by the simultaneous presence of both
a nonpolar (i.e., hydrophobic or water-hating) group and a polar (i.e., hydrophilic or water-loving)
group (e.g., -NH2, -SO3). By adsorbing at the solid/aqueous interface, a collector imparts an oil-like
character (hydrophobicity) to the surface
ORE
COM MINUT ION
CL ASSIFICAT ION
RE GULAT ORS, COLLE CT ORS,
CONDIT IONING
DISPERSANT S
FROT HERS, AIR
FLOT AT ION
FLOCCUL ANT
SOLID/L IQ. SEP'N
SOLID/L IQ. SEP'N
CONCENT RAT E
Figure 1.3 A schematic flowsheet for mineral processing
GANGUE
Table 1.1 Unit processes in aqueous processing systems
Mineral Processing
Flotation
Flocculation
Dispersion
Conditioning
Semiconductor Processing
Polishing, Wet Chemical
Cleaning, Etching,
Chemical-Mechanical
Planarization
Hydrometallurgy
Leaching
Flocculation
Precipitation
Crystallization
Adsorption
Ion Exchange
Solvent Extraction
Electrowinning
Electrorefining
Surface Finishing
Alkaline Cleaning
Emulsion Cleaning
Electropolishing
Electroplating
Chemical (Electroless) Plating
Anodization
Pickling, Phosphatizing
Chromatizing
Ceramic Processing
Precipitation
Dispersion
Flocculation
Coagulation
Slip Casting
Gel Casting
Tape Casting
Leaching
Environmental Eng.
Soil Washing
Precipitation, Ion Exchange
Coagulation, Flocculation
Leaching
Battery Technology
Synthesis of Cathode/Anode
Materials
Discharging, Recharging
Precipitation
Adsorption
Infiltration
Photographic Technology
Precipitation of Silver Halides
Fixing, Developing
Catalyst Preparation
and this facilitates the removal of collector-coated particles from the aqueous phase (Figure 1.4). The
contact angle at the solid/water/air boundary provides a measure of the hydrophobicity/hydrophilicity of
a surface (Figure 1.4). Frothers adsorb at the gas/aqueous interface and thereby exercise a major
influence on the rate of particle-bubble attachment. Table 1.3 (Leja, p.225) provides a collection of
common flotation reagents, with corresponding applications. As indicated in Table 1.3, regulators tend
to be inorganic reagents, although some organics, such as starch and tannic acid are also utilized.
Regulators are used for a variety purposes, e.g., to control pH, modify the solid/aqueous interfacial
charge, alter oxidation states of surface or aqueous phase atoms, and alter concentrations via
precipitation or complexation. Regulators are called activators if they enhance collector adsorption and
depressants if they have the opposite effect.
Table 1.2 Structures of typical flotation surfactants
Solid particles in water usually carry surface charges. These charges are acquired through the
ionization of surface groups or adsorption of charged species. For example, in the case of a metal oxide
surface (-MO), the surface charging process involves the following steps: (a) surface hydration,
resulting in the formation of surface hydroxyl groups -MOH(s) (Equation 1.1), (b) surface
deprotonation, resulting in the formation of negatively charged surface groups (Equation 1.2), and
surface protonation, giving positively charged surface groups (Equation 1.3):
Figure 1.4
-MO(s) + H2O = -M(OH)2(s)
-MOH(s) = -MO-(s) + H+(aq)
-MOH(s) + H+(aq) = -MOH2+(s)
(1.1)
(1.2)
(1.3)
When a suspension of particles is allowed to stand, the larger particles tend to settle out, under the
influence of gravitational forces. However, the particles of colloidal dimensions (~ 1m) will remain
suspended. It is then said that we have a stable dispersion (Figure 1.5). Also, dispersants, compounds
which promote particle dispersion (by increasing surface charge or by providing adsorbed films which
act as steric barriers) may be added. A dispersion may be destabilized by introducing coagulants,
typically inorganic salts or flocculants, long-chain organic molecules with molecular weights in the
range of 104-107. These molecules have repeat units which carry functional groups that are capable of
adsorbing on particle surfaces. The multiple attachments produce particle aggregates which settle out of
the liquid phase. Table 1.4 presents a classification of some of the common flocculants and Table 1.5
shows structures of some flocculants.
Table 1.3 Typical flotation reagents and their applications (Leja, 1982)
Type
Collectors
Classification and
Composition
Usual form of additions
Typical Applications and Some
Properties
Xanthates
1.
Ethyl-Na
Ethyl-K
10% solution
10% solution }
For selective flotation of Cu-Zn, Cu-PbZn sulfide ores
2.
Isopropyl-Na
Isopropyl-K
10% solution
10% solution }
3.
Amyl-K secondary
Amyl-K
10% solution
10% solution }
More powerful collector than ethyl for
Cu, Pb, and Zn ores, Au, Ag, Co, Ni, and
FeS2
Most active collector but not very
selective; used for tarnished sulfides
(with Na2S), Co-Ni sulfides
Dithiophosphates, Aerofloats
1.
Diethyl-Na
5-10% solution
For the Cu and Zn (but not Pb) sulfide
ores; selective, nonfrothing
2.
Dicresyl, 15% P2S5
Dicresyl, 25% P2S5
Dicresyl, 31% P2S5
Undiluted
Undiluted
Undiluted
Ag-Cu-Pb-Zn sulfides, selective, froths
Ag-Cu-Pb-Zn sulfides, selective, froths
Mainly used for PbS and Ag2S ores
3.
Di-sec-butyl-Na
5-10% solution
Au-Ag-Cu-Zn sulfides; not good for Pb
Thionocarbamate,
ethylisopropyl (Z-200)
Liquid emulsion
Selective collector for Cu sulfides (or
Cu-activated ZnS) in the presence of
FeS2
Mercaptobenzothiazole
Solid
Floats FeS2 in acid circuits (pH 4-5)
Fatty acids
1.
2.
3.
Tall oil (mainly oleic
acid)
Refined oleic acid
Na soap of fatty acids
Alkyl sulfates and sulfonates
C12-C16 (dodecyl to cetyl)
Collectors for fluorspar, iron ore,
chromite, scheelite, CaCO3, MgCO3,
apatite, ilmenite; readily precipitated by
"hard" waters (Ca2+ and Mg2
+
)
5-20% solution
Collectors for iron ores, garnet, chromite,
barite, copper carbonates, CaCO3, CaF2,
BaSO4, CaWO4
In kerosene
Used to separate KCl from NaCl and to
float SiO2
Used to float quartz, silicates,
chalcopyrite
Used to float quartz, silicates,
chalcopyrite
Cationic reagents
1.
2.
3.
Primary and
secondary amines
Amine acetates
Quaternary
ammonium salts
5-10% solution
5-10% solution
Frothers
Pine oil (-terpineol)
Undiluted
Provides most viscous stable froth
Cresylic "acid" (cresols)
Undiluted
Less viscous but stable froth; some
collector action
MW ~ 200 (D-200)
Solutions in H2O
Fine, fragile froth; inert to rubber
MW ~ 250 (D-250)
Solutions in H2O
Slightly more stable froth than with
D-200
MW ~ 450 (D-450)
Solutions in H2O
Slightly more stable froth than with
D-250
Polyoxyethylene
(nonyl phenol)
Undiluted
Used as calcite dispersant in apatite
flotation
Aliphatic alcohols: e.g.,
MIBC, 2-ethyl hexanol
Undiluted
Fine textured froth; used frequently with
ores containing slimes
Ethers (TEB, etc.)
Undiluted
Lime (CaO) or slaked lime
Ca(OH)2
Slurry
pH regulator; depresses FeS2 and
pyrrhotite
Soda Ash, Na2CO3
Dry
pH regulator; disperses gangue slimes
Caustic soda, NaOH
5-10% solution
pH regulator; disperses gangue slimes
Sulfuric acid, H2SO4
10% solution
pH regulator
Cu2+ (CuSO4 · 5H2O)
Sat. solution
Activates ZnS, FeAsS, Fe1-xS, Sb2S3
Pb2+ (Pb acetate or nitrate)
5-10% solution
Activates Sb2S3
Zn2+ (ZnSO4)
10% solution
Depresses ZnS
S2- or HS- (Na2S or NaHS)
5% solution
In sulfidization, activates tarnished
(oxidized) sulfide minerals and
carbonates
CN- [NaCN or Ca(CN)2]
5% solution
Depresses ZnS, FeS2, and when used in
excess, Cu, Sb, and Ni-S
Cr2O72-, CrO42-
10% solution
Depresses PbS
SiO2 (Na2SiO3)
5-10% solution
Disperses siliceous gangue slimes;
embrittles froth
Polypropylene glycols
Regulators
Starch, dextrin
5-10% solution
Disperses clay slimes, talc, carbon
Quebracho, tannic acid
5% solution
Depresses CaCO3, (CaMg)CO3
SO2
3% solution
Depresses ZnS and Fe1-xS and, with
CN-, depresses Cu sulfides
_____________________________________________________________________________________________
+ +
+
+
++
+
+
+
+
+ +
+ +
+
+
+ +
+ +
+
+
++
+
+ +
+
+ +
++
+
+
+ +
Po sitiv ely ch arged p articles
- -- - - -- - - -- -- -- -- - -
- -- -- -- --
Negatively charged p articles
(a) Stable dispersio ns
- -- -+ ++ - +- +
++ - + +
+ -- -- +++
+ - -+
+
++ + - - -+ +
+- - ++ - - Op po sitely ch arged p articles
Neutral charge particles
(b) Un stable dispersio ns: Coagulation
-
+ +
+
+
+ +
-
-
A p olymer chain with anion ic functio nal gro up s
+
+
+
+
+
+ +
+ +
+
+ +
+ +
+
+
++
-
+
+ +
+
+ +
-
-
+ +
+
+
+ +
++
+ +
++
++
+
+
+ +
-
+
+ +
+
+ +
M ultiple particle attach men ts
(c) Flo cculatio n by po ly mer bridgin g
Figure 1.5 Coagulation and flocculation processes.
Table 1.4 Classification of some flocculants*
Origin
Natural products
Non-ionic
Gum guar; starch
Anionic
Derivatives of
natural products
Dextrin
Sodium alginate;
phosphated starch;
sodium carboxy
methyl cellulose
Synthetic polymers
Polyacrylamide;
Polyethylene oxide;
Polyvinylalcohol
Hydrolyzed
polyacrylamide
Cationic
Amphoteric
Gelatine; albumen
Cross-linked gelatin;
aminated tannin
Polyethylenimine;
vinyl pyridineacrylamide
copolymers
*J. A. Kitchener, "Principle of Action of Polymeric Flocculants", Br. Polym. J., 4, 217-229 (1972).
Table 1.5 Structures of typical synthetic flocculants
1.2.3 Hydrometallurgical Extraction
The main technologies available for converting ores and concentrates into refined metals and metal
compounds are pyrometallurgy which exploits high temperature nonaqueous chemistry (e.g., gas/solid
reactions, molten salt reactions, and liquid metal reactions), and hydrometallurgy, which is a waterbased chemical technology. Table 1.6 presents a collection of the main metals which are extracted
hydrometallurgically.
FEED SOLIDS
SOL IDS
PREPARAT ION
WAST E SOL IDS
LIXIVIANT S
LE ACHING
RE AGENT S
SOLID/L IQUID
SEP ARAT ION
LE ACH RE SIDUE S
RE AGENT S
SOLUT ION
PURIFICAT ION
IM PURIT IES
RE AGENT S
MET AL
PRODUCT ION
AQUEOUS SOLUT ION
M ET AL/M ET AL COMP OUND
Figure 1.6
Hydrometallurgical extraction
A hydrometallurgical plant typically is organized around subunits which provide the following five
main functions: (a) solids preparation, (b) leaching, (c) solid/liquid separation, (d) solution
purification, and (e) metal production, as illustrated in Figure 1.6. Solids preparation may involve
physical operations such as grinding and magnetic separation, interfacial separation processes such as
flotation, and pyrometallurgical processses such as roasting (a gas-solid reaction, such as the conversion
of a sulfide to an oxide: MS(s) + 3/2O2(g) = MO(s) + SO2(g)). Leaching is a dissolution process in
which the prepared solids are treated with reagents called lixiviants or leachants. Table 1.7 presents a
listing of selected hydrometallurgical reagents. Lixiviants include acids, such as sulfuric acid or
hydrochloric acid, and bases, such as caustic soda and ammonium hydroxide. The acid provides
hydrogen ions (H+) which react with the solid, causing its decomposition and the release of metal ions
into the aqueous solution:
MO (s) + 2H+ (aq) = M2+ (aq) + H2O
(1.4)
On the other hand, a base provides hydroxide ions (OH-) which effect dissolution by binding with metal
ions to form water-soluble complexes:
M(OH)2 (s) + OH- (aq) = M(OH)3- (aq)
(1.5)
A complexant binds chemically to a metal ion, thereby enhancing its aqueous solubility:
Cu2+ (aq) + 4NH3 (aq) = Cu(NH3)42+ (aq)
(1.6)
Ag+ (aq) + 2CN- (aq) = Ag(CN)2- (aq)
(1.7)
The leaching stage produces a metal-containing aqueous solution called the pregnant solution and
unreacted solids, i.e., leach residues. The purpose of the solid/liquid separation step is to separate the
pregnant solution from the leach residues. Frequently, flocculants are used as settling or filtration aids.
Typically the pregnant solution contains a wide variety of metal ions. The solution purification step
uses techniques such as precipitation, ion-exchange, adsorption, and solvent extraction to eliminate
metallic impurities from the aqueous liquor and to concentrate the metal values in solution.
Precipitation reagents include bases, acids (e.g., H2S), nonmetallic reducing agents (e.g., H2), and metals
such as zinc and iron:
Fe3+ + 3OH- = Fe(OH)3(s)
(1.8)
Ni2+ + H2S = NiS (s) + 2H+
(1.9)
Co2+ + H2 (g) = Co (s) + 2H+
(1.10)
Au(CN)2- + Zn (s) + CN- = Au (s) + Zn(CN)3-
(1.11)
Cu2+ + Fe (s) = Cu (s) + Fe2+
(1.12)
Precipitation reactions which use elemental metals as precipitants (e.g., Equations 1.11 and 1.12) are
called cementation or contact reduction processes.
Table 1.6 Aqueous Processing in Metal Extraction and End-Use of Extracted Metals and Metal Compounds
Metal
Aluminum (Al)**
Beryllium (Be)**
Boron (B)**
Cadmium (Cd)**
Cobalt (Co)*
Copper (Cu)*
Germanium (Ge)**
Gold (Au)**
Hafnium (Hf)**
Lithium (Li)**
Hydrometallurgical
Extraction Technologies
Leaching of Al from bauxite
ores; production of alumina
trihydrate
Leaching of Be from bertrandite
and beryl ores; precipitation
Solvent extraction of B from
natural brines containing
dissolved borax (i.e., sodium
tetraborate, Na2B4O7 · 10H2O);
production of boric acid
(H3BO3)
Leaching, precipitation,
electrowinning
Leaching of oxide, and sulfide
ores, solution purification,
electrowinning, electrorefining
Leaching and precipitation;
byproduct recovery from Al and
Zn extraction
Leaching, precipitation, solvent
extraction
Leaching, separation with
carbon, cementation on zinc,
electrowinning, electrorefining,
precipitation
Coproduct Zr-Hf extraction by
leaching of zircon concentrates;
precipitation; solvent extraction
Leaching from spodumene
(LiAlSi2O6) ores and
concentrates; precipitation of
hydroxide and salts
Refined
Products
Alumina
trihydrate, Al2O33H2O
Calcined alumina
(Al2O3)
Be(OH)2, BeSO4
· 4H2O, BeO
Boric acid
(H3BO3);
Boric oxide,
B2O3
Cd metal
Uses of Refined Products (2)
Al2O3 consumed by Primary Al smelters (84%); chemical,
abrasive, refractory, advanced ceramic and other industries
(16%)
Wastewater Treatment
Technologies
Chemical Precipitation
BeO consumed in production of Be metal;
Be-Cu alloys (0.5-2.0% Be); BeO oxide ceramics.
Synthesis of boron metal from B2O3; Synthesis of borates,
used in glass fibers;
Borosilicate glass; Cleaning and bleaching; Metallurgical:
fluxes, electroplating baths, alloys; Neodymium-iron-boron
magnets.
Batteries (32%), coatings and plating (29%), pigments
(15%), plastic stabilizers (15%), alloys and other uses (9%)
Super alloys, cutting and wear-resistant materials, magnetic
alloys, chemicals and ceramics.
Chemical precipitation;
evaporation ion-exchange
GaOH3, Ga2O3
Mainly consumed as GaAs for opto/electronic applications
Chemical precipitation, ionexchange, evaporation,
electrolysis
Au metal
Infrared systems (65%); fiber optics (10%); gamma-ray, Xray, and i.r. detectors (6%); semiconductors (5%); other
(14%)
Aerospace and electronics industries; jewelry.
Hf metal
Major metallurgical applications in nuclear industry.
Lithium
hydroxide
monohydrate
(LiOH·H2O),
lithium carbonate
(Li2CO3)
Synthesis of Li metal by fused salt electrolysis; Al industry:
Li2CO3 used to synthesize LiF in cryolite bath; ceramic and
glass manufacturing industries; chemicals, e.g., Li-based
greases; Al-Li alloys.
Co metal, oxides
Table 1.6 Aqueous Processing in Metal Extraction and End-Use of Extracted Metals and Metal Compounds
Metal
Magnesium (Mg)
Hydrometallurgical Extraction
Technologies
Precipitation, dissolution,
evaporation, crystallization of
Mg salts from seawater, and well
and lake brines
Refined
Products
MgCl2
Nickel (Ni)*
Leaching of oxide and sulfide
ores; solution purification,
electrowinning, precipitation,
electrorefining
Niobium (Nb)**
Leaching of niobite-tantalite
ores; precipitation; solvent
extraction
Leaching of metallurgical
byproducts and scrap; solution
treatment
Nb metal; Nb2O5
Leaching of monazite and
bastanasite ores and
concentrates; solvent extraction;
precipitation
Oxide salts
Byproduct recovery from
tungsten concentrates, fluorite
tailings, uranium solutions;
leaching, precipitation
Byproduct of Cu electrorefining
Sc oxide
Leaching of ores, concentrates,
metallurgical intermediate
products and scrip, adsorption,
electrowinning, electrorefining
Ag metal
Platinum Group**
Metals (Pt, Pd, Ir,
Os, Rh, Ru)
Rare earths**
(La, Ce, Pr, Nd,
Pm, Sm, Eu, Gd,
Tb, Dy, Ho, Er,
Tm, Vb, Lu)
Scandium (Sc)**
Selenium (Se)**
Silver (Ag)**
Uses of Refined Products
Synthesis of Mg metal by fused salt electrolysis;
applications of Mg metal in metallurgical industry; AlMg alloys, wrought foundry products, castings;
applications of magnesia: refractories, animal feed,
chemicals, pulp and paper, ceramics.
Forms of primary Ni consumed: Ni cathodes and pellets
(56%), Ni briquets and powders (20%), ferronickel
(13%), nickel oxide (8%), nickel salts (2%), other forms
(2%); end-use consumption: stainless and heat-resisting
steel (42%), non-ferrous alloys (excluding super alloys)
(17%), electroplating (17%), super alloys (14%), other
(10%).
Steel and alloys.
Wastewater Treatment
Technologies
Chemical precipitation
(hydroxide, carbonate,
sulfide), ion-exchange,
evaporation, reverse
osmosis
Automotive, catalyst for emission control (42.6%);
electrical (21%); dental and medical (18.2%); chemical
(5.3%); petroleum (3.4%); jewelry and decorative
(1.1%); glass (0.6%); miscellaneous (7.9%).
Catalysts (53%); metallurgical (22%); ceramics and
glass (18%); miscellaneous (7%).
Major use as laser crystals of gandolinium-scandium
gallium garnets (GSGG); other, e.g., high-intensity
mercury vapor lights.
Electronic and photocopier components (43%);
pigments and chemicals (20%), glass manufacturing
(20%); other, e.g., agriculture and metallurgy (17%)
Photographic materials (46%); electrical and electronic
products (19.4%); coinage (11.3%); electroplated ware,
sterlingware, jewelry (8.6%); brazing alloys and solders;
coins, medallions, commemorative objects catalysis;
dental and medical supplies; miscellaneous
Chemical precipitation,
activated carbon, ionexchange, reverse osmosis,
coagulation/coprecipitation
Precipitation, ion-exchange,
electrodeposition,
electrodialysis, reverse
osmosis
Table 1.6 Aqueous Processing in Metal Extraction and End-Use of Extracted Metals and Metal Compounds
Metal
Hydrometallurgical
Extraction Technologies
Refined
Products
Sodium (Na)**
Strontium (Sr)**
Leaching of celestite ores;
precipitation
Tantalum (Ta)**
Leaching of niobite-tantalite
ores; precipitation; solvent
extraction
Byproduct of Cu electrorefining
Tellurium (Te)**
Thorium (Th)**
Tungsten (W)**
Uranium (U)
Vanadium (V)**
Zinc (Zn)**
Zirconium (Zr)**
Sr salts (e.g.,
carbonate,
chloride,
chromate)
Ta metal; Ta2O5
Leaching of monazite ores and
concentrates; solvent extraction;
precipitation
Leaching of scheelite and
wolframite ores; solvent
extraction, ion-exchange;
precipitation
Leaching, precipitation, solvent
extraction, ion-exchange
Leaching, byproduct of uranium
ore processing; precipitation;
solvent extraction
Leaching of ores, concentrates
and roasted concentrates; solvent
extraction; electrowinning;
electrorefining
Th metal, oxide,
nitrate
Leaching of zircon concentrates;
solvent extraction; precipitation
Zr metal; oxide
**Primary extraction via hydrometallurgy
Oxide, tungsten
metal
Uses of Refined Products (2)
Wastewater Treatment
Technologies
Glass (50%); chemicals (21.2%); soaps and detergents
(9.1%); flue gas desulfurization (4.4%); water treatment
(3.6%); pulp and paper (2.9%)
TV picture tubes (63%); pyrotechnics and signals (13%);
ferrite ceramic magnets (11%); electrolytic production of
zinc (5%); pigments and fillers (5%); other (3%)
Alloys.
Iron and steel (58%); nonferrous metals (20%); chemicals
and rubber (15%); other, e.g., xerographic and electronic
applications (7%)
Refractory application (57%); lamp mantles (18%);
aerospace alloys (15%); welding electrodes (5%);
miscellaneous, e.g., ceramics, lighting (5%)
Tungsten carbide powder (54%); tungsten metal powder
(28.7%); ferrotungsten (3.5%); other.
Nuclear industry
V metal; oxide
Steel (85%); other (e.g., super alloys, chemicals, ceramics,
catalysts).
Zn metal; oxide;
Zinc sulfate
Zn metal: construction industry (43%); transportation
(22%); machinery (10%); electrical (10%); chemical and
other (15%). ZnO powder: rubber industry (58%);
chemicals, paint, ceramics, photocopying. ZnSO4 powder:
agriculture (83%).
Zr metal: major use in nuclear power industry; chemical
industry (corrosion resistant alloys); other (e.g., electronic
industry). Zr oxide: ceramics, abrasives, chemical
manufacture.
Chemical precipitation; ionexchange evaporation
Table 1.7 Typical hydrometallurgical reagents
Reagent Type
Name
Chemical Formula
Acid
Sulfuric acid
Hydrochloric acid
Nitric acid
Hydrofluoric acid
H2SO4
HCl
HNO3
HF
Base
Caustic Soda (Sodium hydroxide)
Ammonium hydroxide
NaOH
NH4OH
Complexant
Ammonium hydroxide
Sodium cyanide
Sodium Hydroxide
Hydrogen chloride
Hydrogen fluoride
NH4OH
NaCN
NaOH
HCl
HF
Oxidant
Oxygen
Hydrogen peroxide
Sodium hypochlorite
Ferric sulfate
O2
H2 O2
NaOCl
Fe2(SO4)3
Reductant
Hydrogen
Sulfur dioxide
Ferrous sulfate
Metallic zinc
Metallic iron
H2
SO2
FeSO4
Zn
Fe____________________
Adsorption, ion-exchange, and solvent extraction are two-phase separation processes which remove
dissolved species from the bulk aqueous phase in the form of ions, complexes, or molecular species. In
all three processes, the aqueous solution constitutes one of the two phases. In the case of adsorption
separations, the second phase is a particulate solid material (the adsorbent) which possesses surface
functional groups that can interact with aqueous species.
The solid material may be organic or
inorganic. In hydrometallurgy the most common adsorbent is activated carbon. Activated carbons are
high surface area carbon-containing materials prepared by the thermochemical decomposition of natural
(e.g., wood and vegetable matter) or synthetic (e.g., organic polymers) carbonaceous solids. Table 1.8
provides examples of the surface functional groups that are carried by activated carbons.
Organic ion-exchange materials are solid polyelectrolytes which are characterized by the presence of
three main constituents; (a) an insoluble skeleton or matrix, which is a hydrophobic three-dimensional
network of hydrocarbon chains, (b) the fixed ionic groups, which are inorganic hydrophilic functional
groups attached to the hydrocarbon matrix, and (c) the counterions, which are absorbed into the ion-
exchange material in order to establish electroneutrality with the fixed charges. Table 1.9 presents
typical functional groups encountered in organic polymer-based ion-exchange systems.
Solvent extraction is based on organic (oil)/aqueous systems. The main components of the organic
phase are: (a) an extractant, which is the chemical reagent which carries the functional groups that bind
(extract) aqueous species, and (b) a diluent, the organic solvent in which the extractant and the extracted
complex are retained as soluble species. Sometimes a third component, termed a modifier, is added to
enhance the organic phase stability. Table 1.10 presents a collection of selected solvent extraction
reagents. For an acidic extractant RH, the extraction of a metal ion into the organic phase may be
represented as:
RH(org) + Mz+(aq) = MRz(org) + zH+
(1.13)
With solution purification accomplished, the metal-containing solution can be further processed to
give a final product of elemental metal (e.g., Au, Ni, Cu, Zn) or metal compound (e.g., Al(OH)3(s)).
The techniques used in the metal/metal compound production stage include electrowinning,
electrorefining, hydrogen reduction, and crystallization. In both electrowinning and electrorefining,
dissolved metal is precipitated out of solution by the direct application of electric current:
Cu2+ + 2e-
= Cu (s)
Au(CN)2- + e- = Au (s) + 2CN-
(1.14)
(1.15)
In hydrogen reduction, the electron needed for the precipitation is generated from molecular hydrogen:
H2(g) = 2H+ + 2e-
(1.16)
Table 1.8 Examples of surface functional groups on activated carbons
Name of Functional Group
Structure
O
Carboxylic
C
OH
OH
Pheno lic hydroxyl
O
Normal lacton e
O
Quinone-type carbonyl
O
Table 1.9 Ion-exchange functional groups
Type of Ionic Group
Structure
O
Strong acid
P
O- H +
S
Sulfon ic acid
O
O
Weak acid
P
Stron g base
P
C
O- H +
Cl + CH 3
N
Carbox ylic acid
Quatern ary ammo nium salt
CH 3 CH 3
Weak base
P
N
CH 3
CH 3
Dimethy l amine
H+ Cl
Ch elating
COOH
P
CH 2
N
COOH
Imino diacetic acid
P
represents th e resin matrix
Table 1.10 Selected solvent extraction reagents
T y pe o f Ex tractant
An ion ic
Name
Carbox ylic acids
Structure
R1
COOH
C
R2
Ph osp ho ric acids
CH3
R
O
R
O
O
R1 = R2 = C6
(Versatic 1 0)
R = CH3 (CH2 )3 CHCH2
P
OH
CH2 CH3
(Di (2-ethy lh ex yl) ph osp ho ric acid)
Hy dro xy ox imes
R2
R 1 = H, R2 = 9C H1 9
OH
R1
(Acorga P5 0)
R 1 = H, R2 = 1C2 H2 5
C
N
OH
(LIX 86 0)
R 1 = CH3 , R2 = 9C H1 9
(LIX 84 , SME 25 9)
Catio nic
Primary amines
R
N
R = (CH3 3) C(CH2 C(CH3 2) )
4
(Primine JM T )
H
H
Seco ndary amin es
R1
N
R1 = C9 H1 9 CH
H
CHCH2
R2 = CH3 C(CH3 2) (CH
3 2) 2)
2 C(CH
R2
(Amberlite LA-1 )
T ertiary amin es
R1
N
R3
R 1 = R 2 = R 3 = CH3 (CH2 )7
R2
Quatern ary ammon ium salts
Neutral
(Alamin e 30 0)
R3
N
R 2 + (CH 3) Cl R1
RO
RO
RO
P
O
R = R 2 = R = C - C1 0
1
3
8
(Aliquat 3 36 )
R = C4 H9
T ributyl ph osp hate (T BP )
1.2.4 Ceramic Aqueous Processing
Aqueous processing is increasingly becoming an important technology in the manufacture of
ceramic materials. The main applications of aqueous processing are in materials synthesis and in
suspension/colloidal processing. A related concern is the corrosion of ceramic materials. Traditionally,
the manufacture of ceramic components relied primarily on dry powder-based techniques. For example,
to prepare a complex oxide, such as barium titanate (BaTiO3), one would mix dry powders of barium
oxide and titanium oxide, hot press the mixture, and then sinter. In an alternative aqueous-based
synthesis method, a complex barium titanyl oxalate salt is precipitated from solution, and this then
serves as the precursor for BaTiO3 synthesis via thermal decomposition.
Aqueous synthesis offers a number of advantages over the conventional dry methods. In aqueous
systems, mixing of the starting materials is typically achieved at the molecular level, and thus, chemical
homogeneity is better achieved. Sintering is favored by small particle size and the necessary fine
particles can be prepared by growth control during aqueous precipitation. The usual dry approach also
requires grinding, which runs the risk of introducing impurities from the grinding media. The materials
synthesized from solution include simple oxides and sulfides, such as ZnO and ZnS, as well as complex
oxides, silicates, fluorides, and chalcogenides, as summarized in Table 1.11. The solid products may be
in the form of gels, powders, and films.
Materials synthesis in aqueous systems typically involves three main steps: (a) solution preparation,
(b) solvent removal, and (c) salt decomposition, as illustrated in Figure 1.7. On the basis of the method
of water removal, materials synthesis techniques may be divided into three major categories: (a)
precipitation-filtration (or-sedimentation), (b) solution volatilization, and (c) solvent-assisted
dehydration. The most common technique is precipitation-filtration. Table 1.12 presents a summary of
various synthesis techniques. Schematic illustrations of many of these processes, are provided in Figure
1.8.
Aqueous-based techniques are also exploited in ceramic forming processes. Forming converts a
particulate feed material into a consolidated form characterized by a definite geometry and
microstructure. Figure 1.9 presents flowsheets illustrating the three main powder-forming processes,
i.e., pressing, casting, and injection. In pressing, a powder or granular feed material, contained in a die
or a mold, is subjected to pressure. The casting technique is applied to slurries and among the main
variations of this method are slip casting, where a porous mold is used, gel casting, which relies on a
polymerizable binder, and tape casting, where a thin coating of the slurry is deposited on a carrier film.
As can be seen from Figure 1.9 all three forming processes first start by combining powders, water,
and certain processing additives. The proportions of these components differ, however, for the various
forming processes. Table 1.13 presents a collection of various types of processing additives.
Table 1.11. Materials synthesis in aqueous systems
Form of Material
Metal and Metal Compound
Method ________________
Metal Powder
Au, Ag, Pt, Pd, Cu, Ni, Co Chemical Pptn
Fe
Electrolysis
Simple Oxide Powder
MnO2
Fe2O3, Fe3O4, ZrO2, SiO2, Cr2O3, ZnO
Fe2O3
Electrolysis
Chemical Pptn,
Spray Pyrolysis
Complex Oxide Powder
BaTiO3, PbTiO3, LiTaO3, YBa2Cu3O7
Chemical Pptn.
Metal Films
Au, Ag, Pt, Pd, Cu, Ni, Co, Cr
Electroplating,
Chemplating
Oxide Films
Al2O3, TaO2
Anodizing, Pptn.
Chalcogenide Powders and
Films
CdS, ZnS, ZnSe, ZnTe, CdTe
Pptn., Spray pyrol.
Salt Powders
AgI, AgBr, AgI, etc.
Chemical Precipitation
H 2O
Metal
Salts
Solution
Preparation
Solvent
Removal
Acids/Bases
H 2O
Metallic
Phas es
Salt
Decompos 'n
Product
Figure 1.7 The main steps in materials synthesis
Ceramic and glass materials may be categorized in terms of the properties that are exploited in
practical applications, i.e., electrical and electronic, magnetic, optical, chemical, thermal, mechanical,
and biological. Tables 1.14 - 1.16 present a list of materials and corresponding applications, with
emphasis on those which are or can be prepared by solution techniques.
Table 1.12 Types of materials synthesis techniques
Water Removal Method
Precipitation Technique
Process Description
Precipitation-Filtration
Simple Precipitation
Simultaneous addition of reactant solutions
containing salts MA and KB, leading to
reaction to form the single salt NB(s).
Coprecipitation
Addition of cation-containing solutions (MA,
NY) to an excess of precipitant solution (KB),
leading to simultaneous precipitation of MB(s)
and NB(s).
Homogeneous Precipitation
In-situ generation of precipitant.
Alkoxide hydrolysis
(sol-gel process)
Reaction of an alkoxide (ROM) with water to
produce a hydrous metal oxide.
Hydrothermal Synthesis
Use of hot solutions (frequently above the
boiling point of the aqueous phase) to increase
reaction rates and/or obtain solution conditions
that favor precipitation.
Electrolytic Precipitation
Electrolytically generated reactants (e.g.,
electrons, hydroxyl ions) are used as
precipitants.
Direct (Simple) Evaporation
Water is removed slowly be evaporation.
Spray Drying
Salt solution is atomized and passed into a
heated drying chamber.
Spray Roasting (Evaporative
Decomposition)
Similar to spray drying, except that higher
temperatures are used in order to achieve
drying (salt production) and salt
decomposition in one step.
Fluid-bed Drying
Feed salt solution is passed through a nozzle
and is received as droplets by a fluidized bed
of the solid particles contained in a furnace.
Emulsion Drying
Small droplets of aqueous salt solutions are
produced by using surfactants to stabilize
water-in-oil (w/o) emulsions. Water
volatilization is then accomplished via e.g.,
hot kerosene drying, freeze-drying.
Freeze Drying
Liquid drops generated via spraying are
rapidly frozen. Water is subsequently
eliminated from the frozen drops by
sublimation.
Hot Kerosene Process
The aqueous salt solution is sprayed into a hot
water-immiscible liquid (e.g., kerosene),
resulting in the evaporative elimination of
water.
Solution Volatilization
Solvent-assisted
Dehydration
Glassy Solid Process
Salt solution containing an organic
polyfunctional acid (e.g., citric acid) is
subjected to rapid evaporative dehydration,
followed by vacuum drying.
Liquid Drying
An aqueous salt solution is sprayed into a
polar organic solvent (e.g., acetone, ethanol).
The lower solubility of the mixed organicaqueous solvent leads to salt precipitation.
Gel-microsphere Process
Preferential extraction of water by a polar
organic liquid results in salt precipitation.
H2O
MA
H2O ,KB
KB
NY
M+ + A + K ++ B
+
= MB (s ) + K + A -
MIXING
H2O
MA
FILTRAT'N
MA + KB = MB(s)
NY + KB = NB(s)
MIXING
H2O
FILTRAT'N
MB(s )
MB(s ) + NB(s)
(i) Simple Precipitation : Simu ltan eou s
ad ditio n of reactant s olutions
(ii) Cop recipitation : Addition of catio n
so lutio ns to an exces s of precipitant s olution
H2O
MA
KX
MIXING
H2O
ROM
+ Alcoho l
KX = NB
MA + NB
= MB(s) + KN
FILTRAT'N
MB(s )
MIXING
H2O
FILTRAT'N
Alco hol
(iii) Homog eneous Precipitation via
in-s itu gen eration of precipitant, NB
MA
Heat
H2O
H2O
H 2O + Catalys t
ROM + H 2O = ROH + MOH
MOH + MOH = M 2O + H O2
MOH + ROM = M O
2 + ROH
M2 O
(iv ) Alko xide Hy droly sis
KB
KB
MIXING
M +B
= MB(s) + e FILTRAT'N
ELECTROLYSIS
MB (s )
MB(s )
(v) Hydroth ermal Precip itation: Solubility
prod uct of MB (s ) exceeded at hig h temp erature
(vi) Electrolytic Precipitation
Figure 1.8a. Precipitation-filtration processes.
+ M ,B
H2O
+
M
B
Heat
o
oo
o o
o
o oo
o
Heat
MB(s )
(ii) Spray Drying
(i) Direct (simp le) Evaporation
+ M ,X
H2O
o
oo
ooo
o oo
o
Heat
oo
oo oo ooo o
MB(s )
ooo oooooo ooo oooo
oo oo o o (Dep osited on
oooooo
oo oo oo o ooo flu idized
o o
o
o o oo oo oo particles )
Heat
MB(s )
M ++ X -= MX(s)
MX(s) = MB(s) (Deco mp osition)
M + , B- , H2O
(iii) Sp ray Roas ting
Su rfactant, Oil
(iv ) Flu id-bed Drying
+
M , B - , H2O
M+ , B - , H 2O
EMULSIFICATION
Emulsio n
o
oooo o
o oo o
Heat
Kero sene
MB(s )
+ M , B , H2O
H2O
(vii) Freeze Dryin g
o
Kero sene
oo ooo
MB(s )
o oo
o
+
M , B , H2O
o
Sp raying/
oo oo o
Freezin g
o oo
o
Fro zen Drop s
SUBLIMATION
Heat
(vi) Hot Kero sene Dry ing
(v) Emuls ion Dry ing
Coo ling
Sp raying
Chamber
MB(s )
MB(s )
Organic
Po lyfun ctio nal
Acid
GELATION
Gel
DEHYDRATION
H2O
Glass y Material
(viii) Glas sy Solid Process
Figure 1.8b. Solution-volatilization processes.
POWDE RS, ADDIT IVE S,
WAT ER
POWDE RS, ADDIT IVE S,
WAT ER
POWDE RS, ADDIT IVE S,
LIQUID
MIXING/MIL LING
MIXING/MIL LING
MIXING/MIL LING
GRANUL AT ION
CAST ING
GRANUL AT ION
DRYING
INJECT ION
SINT ERING
SINT ERING
DEBINDING/
SINT ERING
PRODUCT
PRODUCT
PRODUCT
(a) Pressin g
(b) Castin g
(c) In jection
PRE SSING
Figure 1.9 Powder-forming processes.
Table 1.13 Processing additives and their functions*
Additive
Deflocculant
Coagulant
Binder/Flocculant
Plasticizer
Lubricant
Wetting agent
Antifoaming agent
Foam stabilizer
Chelating agent/sequestering
agent/precipitant
Antioxidant
Functions
Particle charging, aid and maintain dispersion
Uniform agglomeration after dispersion
Modify rheology, retain liquid under pressure, yield strength,
green strength, adhesion
Change viscoelastic properties of binder at forming temperature,
reduce Tg of binder
Reduce die friction (external), reduce internal friction, mold release
Improve particle wetting by liquid, aid dispersion
Eliminate foam
Stabilize foam
Inactive undesirable ions
Retard oxidative degradation of binder
*After J. S. Reed, Principles of Ceramics Processing, 2nd ed., 1995, p. 400)
Table 1.14 Industrial Applications of Single-Component Oxide Ceramics*
Functional Group
Material
Property Exploited
Application
Electrical/Electronic
Al2O3,
BeO
Fe3O4
SnO2
ZnO-Bi2O3
TaO2
b-Al2O3
Stable ZrO2
Insulator
Insulator
Magnetism
Semiconductivity
Semiconductivity
Ionic Conductivity
Ionic Conductivity
IC Substrates
IC Substrates
Magnetic Core
Gas-sensor
Varistor
Capacitor
NaS battery
Oxide-Sensor
Mechanical
Al2O3, ZrO2
Wear resistance
Polishing materials Grindstones
Optical
Y2 O 3
Al2O3
Fluorescence
Transparency
SiO2
Light conductivity
Sodium lamp
Mantle tube
Optical Fibers
Thermal
Al2O3
ZrO2
BeO
Heat resistance
Heat insulation
Heat conductivity
Structural refractories
Heat-insulating materials
Substrates
Nuclear
UO2
BeO
Nuclear properties
Nuclear fuel
Moderator
Chemical
TiO2
Color, chemical
inertness, high refractive
index
Color
Chemical inertness
Paint Pigments
Fe2O3
Al2O3
Biological
Al2O3
Ca10(PO4)6(OH)2
Paint Pigments
Catalyst Support
Artificial teeth and bones
*After N. Ichinose, ed., Introduction to Fine Ceramics, 1987, pp. 4-6.
Table 1.15 Industrial Applications of Complex Oxide Ceramics*
Functional Group
Electrical
Material
.
BaTiO3
BaTiO3
Pb(Zr, Ti)O3(PZT)
LiNbO3
MgAl2O4
MFe2O4
YBa2Cu3Ox
La2-xSrxCuO4
Property Exploited
Dielectric
Semiconductivity
Piezoelectric
Piezoelectric
Insulator
Magnetic
Conductivity
Conductivity
*After N. Ichinose, ed., Introduction to Fine Ceramics, 1987, pp. 4-6.
Application
.
Capacitor
Resistance junction
Oscillators, ignition junction
Piezoelectric oscillators
IC substrates
Superconductors
Superconductors
Table 1.16 Industrial Applications of Chalcogenide and Related Ceramics*
Functional Group
Optoelectronic
Chemical
Material
ZnS, CdS,
CdSe, GaAs,
PbS, PbSe
CdS, Cu2S
CdTe, CuInSe2
(Cd, Zn)S
CuInS2
.
Property Exploited
Optical/electronic
Application
Photodetectors
Optical/electronic
Phtovoltaic devices
MoS2,MoS3
Co9S8
*After N. Ichinose, ed., Introduction to Fine Ceramics, 1987, pp. 4-6.
Catalysts
Catalysts
.
1.2.5 Catalyst Preparation
Many industrially important reactions occur at economically acceptable rates only in the presence
of catalysts. It is relatively easy to separate solid catalysts from soluble reaction products and therefore
solid catalysts are the preferred choice in industrial practice. Table 1.17 presents a collection of some
important commercial catalysts and their applications. For reactions occurring at surfaces, the key
concern is to ensure that the surface has the desired activity, selectivity, and accessibility. This latter
requirement means that the transport of reactants to the active surface should be as free as possible.
Table 1.17 Selected commercial catalysts and their applications (after Brunelle, 1978)
DISPERSED
TYPE OF CATALYST
METAL
CARRIER
Auto-exhaust gases Post-combustion
Oxidation catalysts
Three-way catalysts
Yes
Yes
Pt+Pd
Pt+Rh
Alumina or
Alumina coated
Cordierite
Selective hydrogenation of:
Olefins streams in ethylene plants
Pyrolysis gasoline in ethylene plants
Yes
Yes
Pd
Pd
Alumina
Alumina
Catalytic Reforming
Yes
Pt (+Re,Ir,Au..)
Alumina + Cl
Hydrocracking
Yes
Pd
Y zeolite based
Isomerization of:
Paraffins
Yes
Pt
Yes
Pt
Alumina + Cl or
mordenite
Alumina + Cl or
Alumina-Silica
Dismutation of Toluene
Yes
Cu, Ni
Zeolite
Fine organical chemistry
Yes
Pt, Rh, Pd, Ru, Ni Raney
Charcoal
Off-gas Treatments
Yes
Pt, Pd
Alumina
Fuel Cells
Yes
Pt, Pd
Charcoal
Ammonia Oxidation
No
Pt + Rh
-
Selective oxidation of ethylene in
ethylene oxide
Selective oxidation of methanol in formol
No
Ag
 Alumina
No
Ag
Carborundum
PROCESS
Xylenes
Catalytic reactors may be fixed bed or fluid bed. In a fixed bed reactor, the pellet size is typically of the
order of 1 mm, while it is much larger for fluid bed reactors, of the order of 30 mm. Particles in the size
range of 1-30 mm do not provide enough surface area per volume of reactor for reaction to proceed
satisfactorily. Thus there is a need for the catalysts to have considerable porosity. In general, it is
impractical to prepare the catalytically active materials in the form of pellets or particles, while at the
same time securing the desired porosity. Hence there is a need for catalyst supports.
Two main types of catalytic materials are used commercially. These are noble metals, and less
expensive metals, oxides, and sulfides. Since the noble metals are relatively expensive, they must be
used sparingly (typically loadings of ~ 1 wt%). This means that they must be practically atomically
dispersed at the support surface. In the case of the less expensive catalytic materials, it is possible to use
higher loadings (e.g., up to 40 % and above). In the production of supported catalysts, two main
methods are used: (a) Deposition of the active precursor onto a previously prepared support. (b)
Preferential separation of a component or more from previously prepared fine particles.
1.2.6 Emulsion-based Photographic Technology
The silver-halide-based photographic process involves a number of steps: (a) exposure of the
photo-sensitive material to light, thereby forming the latent image, (b) treatment of the exposed film,
resulting in the negative, a process termed development, (c) stopping of the development process, (d)
fixing.
The active material in a photographic film is the silver halide, which is typically silver bromide
(AgBr). Silver bromide is prepared by combining two water-soluble reactants, e.g., silver nitrate
(AgNO3) and potassium bromide (KBr):
AgNO3 + KBr  AgBr(s) + KNO3
(1.17)
Silver bromide particles are deposited as a thin layer on a transparent plastic substrate. Adhesion of the
particle layer to the substrate is achieved by embedding the particles in gelatin, which is a protein-based
material with the following general structure:
H
H
N
O
R
H
O
C
CH
N
C
CH
N
C
R
H
O
CH
n
OH
R
where the repeat unit is identified by the parenthesis and n ≈ 100-300. Photographic gelatin is
transparent to light (thus, it does not interfere with the reception of light by the halide particles), it
immobilizes the silver halide particles (particle movement would cause the image to become blurred); it
has sufficient porosity to permit processing chemicals access to the halide particles. Table 1.18 presents
a list of selected photographic processing chemicals and their applications.
In the exposure step, a photochemical reaction occurs:
AgX + Light ---> Ag + X
(1.18)
Table 1.18 Photographic chemicals and their applications
Bath
Type of Reagent
Example
Function
Developer
Reducing Agent
Activator
Restrainer
Preservative
Hydroquinone
Hydroxide Ion
Potassium Bromide
Sodium Sulfite
Reduce silver ions to metallic state
Facilitates deprotonation of hydroquinone
Minimize reduction of unexposed silver halide (fogging)
Inhibit decomposition of hydroquinone
Stop Bath
Weak Acid
Acetic Acid,
Boric Acid
Terminate the reduction reaction
Fixing Sol'n
Sodium Thiosulfate
Dissolve residual silver halide
__________________________________________________________________________________________
This reaction results in the formation of small silver particles -so small that they are invisible to the
naked eye, hence the designation as latent. The development step serves to enhance this latent image.
This is accomplished via an aqueous-based chemical reaction which causes additional silver to be
deposited around the initial ultrafine silver particles.
The silver deposition is made possible by the presence of a reducing agent (i.e., a reagent that
can release electrons) in the developer. A common reducing agent in photographic developers is
hydroquinone:
OH
O
+ 2H+ + 2e -
=
OH
O
(Hydroquinone)
Ag+ + eAgBr + e-
(1.19)
(Benzoquinone)
= Ag
= Ag + Br-
(1.20a)
(1.20b)
The developer contains additional reagents, as listed in Table 1.18. The activator, the hydroxide ion
(OH-), participates in the reduction process by facilitating the removal of the hydrogens from
hydroquinone:
OH
+ 2OH - =
OH
OO-
O-
+ 2H2O
(1.21)
+ 2Ag
(1.22)
O-
+ 2Ag+ =
O
O
(2AgBr)
(+ 2Br -)
This silver reduction is a selective process. The silver ions nearest to the light-generated silver atoms
respond more rapidly to the attack by the reducing agent. It is this selectivity that underlies the very
possibility of latent image enhancement.
Following film development, it is typically observed that the entire film surface acquires a slightly
dark coloration, termed a fog. This fogging reflects the fact that there is a general silver reduction over
the entire film surface. The role of the restrainer is to minimize fogging by preventing the
underexposed silver halide from undergoing the reduction reaction. Potassium bromide (KBr) serves as
a common restrainer. This reagent functions by slowly dissolving AgBr via complex formation:
AgBr(s) + Br-  AgBr2-
(1.23)
Thus, now, the AgBr is subject to two competing reactions in the developer, i.e., the reduction reaction
(Equation 1.20) and complex formation leading to dissolution (Equation 1.23). In the neighborhood of
the exposed AgBr sites, the reduction reaction proceeds more rapidly than the complexation reaction and
therefore darkening occurs readily. In contrast, for the unexposed AgBr, the dissolution proceeds me
rapidly and therefore darkening is minimized.
Activated hydroquinone is unstable in the presence of air. This is because the molecular oxygen in
air reacts with it:
O-
O
+ O2 + 2H2O =
O-
+ 4OH -
(1.24)
O
It can be seen that, as in the reaction with silver, benzoquinone is formed here too. It turns out that
benzoquinone can accelerate the oxygen-hydroquinone reaction. In order to minimize the deleterious
effects of this reaction, sodium sulfite (Na2SO3) is introduced into photographic developers as a
preservative. This reagent acts by sulfonating benzoquinone, thereby eliminating it from the reaction
solution:
OH
O
O
+ SO 23
+ H 2O =
-
SO3
+ OH-
(1.25)
OH
It is considered that the film has been sufficiently developed when the reduction of silver is high
enough to yield an acceptable image density. At this point, the film is immersed in a stop bath, whose
function is to terminate the reduction reaction. The active ingredient in a stop bath is a weak acid, e.g.,
acetic acid (CH3COOH) or boric acid (B(OH)3). The stopping process is based on the destruction of the
activator (hydroxide ion) via a neutralization reaction:
H+ + OH-  H2O
(1.26)
The hydrogen ion (H+) shown in Equation 1.26 is generated through the ionization of the weak acid
(HA):
HA  H+ + A-
(1.27)
Following development and stopping, the photographic film acquires two kinds of silver, i.e.,
reduced silver (Ag) responsible for the enhanced image, and the residual silver bromide (AgBr) residing
in the unexposed regions of the film. In the fixing process, the remaining silver halide is eliminated
from the film. The fixing solution typically contains sodium thiosulfate (Na2S2O3). The thiosulfate ion
2-
(S2O3
) can form water-soluble complexes with silver ions:
3-
AgBr(s) + 2S2 Ag(S2O3)2
+ Br-
(1.28)
1.2.7 Surface Finishing
The term surface finishing is the general name given to a variety of surface treatment processes
which serve to impart certain desired properties to the surface of a basis material. The process is termed
metal finishing when the basis material is a metal. The term metal finishing is also applied to surface
treatments in which metallic films are deposited on plastic basis materials. Examples of desired surface
properties include enhanced corrosion resistance, improved electrical conductivity, and a heightened
aesthetic appeal. The surface treatment induces physical and/or chemical changes which result in
cleaning, hardening/softening, smoothing/roughening, and coating of the basis material. Table 1.1 lists a
number of surface finishing processes. Before coatings are applied, the surface of the workpiece must
be cleaned appropriately. Table 1.19 lists the kinds of soils that are frequently encountered and Table
1.20 provides a summary of the typical cleaning methods.
Table 1.19. Types of soil
Soil Type
Pigmented drawing lubricants
Common Constituents
Bentonite, flour, graphite, lithopone, metallic soaps,
mica, molybdenum disulfide, white lead, whiting, zinc
oxide
Unpigmented oil and grease
Drawing lubricants, rust-preventive oils, quenching oils
Chips and cutting fluids
Plain or sulfurized mineral and fatty oils, chlorinated
mineral oils, soaps, amines, sodium salts of sulfonated
fatty alcohols, alkyl aromatic sodium salts of sulfonates,
metal chips generated by machining.
Polishing and buffing compounds
Admixtures of burned-on grease, metallic soaps, waxes,
metal and abrasive particles.
Rust and scale
Metal oxides
Electroplating. In this technique, a metallic coating is deposited on the surface of a workpiece by
using electrons generated with the aid of an externally applied voltage:
M2+ + 2e- = M
(1.29)
Table 1.21 presents examples of metallic coatings and their applications. Different kinds of
electroplating baths are used, as summarized in Table 1.22.
Figure 1.10 presents a simplified diagram showing the sequence of steps involved in an
electroplating operation. The surface of the basis metal can have a significant influence on the
physical and chemical characteristics of the electrodeposits. Therefore, prior to electroplating
proper, it is necessary to subject the basis metal to various surface preparation processes. The
surface of the basis metal is given a preliminary pretreatment which serves to remove oily materials
(e.g., grease, buffing compounds, drawing lubricants), scale, and heavy rust. Table 1.23 gives
examples of such preliminary pretreatments.
Table 1.20 Cleaning methods
Method_______________________
Constituents of Cleaning Medium
Cleaning Principles______________
Solvent cleaning
Hydrocarbon and chlorinated
hydrocarbon solvents, e.g., benzene,
toluene, methylene chloride,
trichloroethylene
Dissolution of contaminants
Emulsion cleaning
Hydrocarbon solvents, e.g.,
kerosene, water, emulsifiable
surfactants
Wetting of particulate contaminants,
flotation of particulates by emulsion
drops, solubilization by organic
solvents
Alkaline cleaning
Alkali metal hydroxides, surfactants
(wetting agents, emulsifiers),
sequestering agents, saponifiers
Hydroxide ions impart negative
charge to surfaces, leading to
particle detachment via electrostatic
repulsion; oily soils removed via
emulsification.
Acid cleaning
Most common mineral acids (e.g.,
HCl, H2SO4, HNO3), phosphoric
acid, organic acids (e.g., citric,
acetic, oxalic, gluconic, tartaric
acids), surfactants
Dissolution, especially, of oxides;
organic acids function as
complexants.
Electrolytic cleaning
Gas (O2 or H2) generated by
electrolysis of alkaline cleaning
solutions
Tiny bubbles produced by
application of electric current
contribute scrubbing action and
promote flotation of soil particles.
Abrasive cleaning
Small abrasive particles, water jets,
air streams
Under the influence of impact force
provided by water jets and air
streams, abrasive particles dislodge
contaminant particles.
Ultrasonic cleaning
Liquid cleaners plus high frequency
sound waves
Tiny gas bubbles generated by
ultrasonic waves provide scrubbing
action.
Table 1.21 Types of metallic coatings and their functions
(after J. Hyner, in Electroplating Engineering Handbook, 4th ed., p. 50)
Surface Coating____________
Basis Material___________
Desired Property____________________
Cu, Ni, Cr
Steel, zinc die castings
Corrosion resistance
Zn, Cd
Steel
Corrosion resistance
Cu, Ni, Cr
Steel
Decorative appeal
Ni, Ag ,Au
Brass
Decorative appeal
Cr, Ni
Steel
Superior hardness and better wear
resistance
Au
Brass, Cu
Lower contact resistance and increased
reliability for electrical contacts
Sn
Brass
Improved solderability and/or weldability
Ni
(Under Au or Cr)
Better base for other finishes
Ag
Bronze
Improved lubricity under pressure
Cu, Ni, Cr
Plastics
To strengthen the basis material and
render it more temperature resistant
Cu
Steel (for carburizing)
To act as a stop-off in heat-treating
Bronze
Steel (for nitriding)
To act as a stop-off in heat-treating
Table 1.22 Electroplating baths
Bath Type
Dissolved Metals
Sulfuric Acid
Rh, Cu
Phosphoric Acid
Rh
Sulfate
In, Fe, Ni
Chloride
Fe,Ni
Cyanide
Cd, Cu, Zn, Au, Ag
Fluoborate
Cu, Cd ,In, Fe, Pb, Sn, Ni
Pyrophosphate
Cu
Chromic acid-sulfuric acid
Cr
Sulfamate (SO3NH2)
Ni
WORKPIECE
PRELIM. TREATMENT
RINSE
ELECTROCLEAN
RINSE
ACTIVATE
PLATE
PLATED PART
Figure 1.10. Electroplating processing sequence
Table 1.23 Types of pretreatments for electroplating
Reagents_______________________
Basis Material _________________
Removal of Oily Materials
Alkali or emulsion-type cleaners
Solvent degreasers (e.g., trichloroethylene or perchloroethylene)
For all basis metals
Scale Removal
Alkaline bath (e.g., NaOH (180 g/L),
NaCN (120 g/L), chelating agents (80
g/L) T = 40˚C)
Ferrous metal parts
Hydrochloric acid
Low carbon steel; stainless steels
Sulfuric acid
Low carbon steel; stainless steels;
copper and copper-base alloys
Hydrochloric acid-nitric acid
High-carbon, case-hardened steels,
stainless steels
Sulfuric acid-hydrofluoric acid
Cast irons
Nitric acid-hydrofluoric acid
Stainless steels
Type of _
Pretreatment
____________
Pickling. Scale and rust are oxidic materials (e.g., Fe2O3, FeO) and their removal generally
involves reaction with acidic solutions, a process termed pickling. In the case of steel, one can write:
Fe2O3 + 6H+ = 2Fe3+ + 3H2O
(1.29)
An important contribution to scale removal is the hydrogen gas released when the acid travels through
cracks in the scale and attacks the underlying metal:
Fe + H2SO4 = H2(g) + FeSO4
(1.30)
Fe + 2HCl = H2(g) + FeCl2
(1.31)
The subsequent pressure buildup dislodges the scale from the metal surface. Aside from its use as a
pretreatment step in electroplating, pickling is used extensively for scale-removal in metal-production
operations (e.g., steel making). Due to its low cost, sulfuric acid is the reagent of choice. However, a
number of other acids are also used, e.g., nitric acid-hydrofluoric acid (for stainless steels), hydrochloric
acid, and phosphoric acid.
Electropolishing. Where there is a need to produce a smooth and polished basis metal surface,
this can frequently be accomplished by means of electropolishing. This is an electrochemical process
in which metal is removed from the surface of the basis metal:
M = Mz+ + ze-
(1.32)
The workpiece serves as the anode of an electrochemical cell. Upon the application of voltage, the
projections on the rough surface dissolve faster than the depressions and this results in a smoothing of
the surface. Table 1.24 presents a list of commercial electropolish baths. Typically acidic baths are
used, with phosphoric acid combined with one more other acids.
Table 1.24 Types of commercial electropolishing baths and their applications
Bath Types
Basis Metals
Fluoboric acid
Al
Sodium phosphate-carbonate
Al
Phosphoric acid
Cu, Stainless Steels
Phosphoric-chromic acid
Brass, Cu, Nickel-Silver, Steels, Stainless Steels
Phosphoric-sulfuric chromic acid
Al, Ni, Stainless Steels, Steels
Phosphoric-sulfuric acid
Ni, Stainless Steels, Steels
Phosphoric-sulfuric-hydrochloric
Ni, Monel
Sulfuric acid
Ni
Sulfuric-citric acid
Stainless Steels
Potassium cyanide
Ag
Caustic soda
Zn
Phosphate Coating. When certain metals are subjected to controlled corrosion in a phosphate
bath, a metal phosphate coating may develop on the metal surface. The physical and chemical
characteristics of phosphate coatings are exploited in a number of ways, e.g., to facilitate the adhesion of
organic coatings to metal substrates, to serve as corrosion barriers, and (upon lubrication) to aid cold
deformation processing (wire drawing, etc).
Surfaces typically phosphate-coated are: iron, steel, galvanized steel, aluminum, electrodeposited
zinc, and electrodeposited cadmium. The types of coating include manganese phosphate, zinc
phosphate, iron phosphate, and aluminum-chromium phosphate.
Chromate Coating. Metallic chromate coatings are produced by subjecting the substrate metal to
controlled corrosion in an aqueous chromate solution. Chromate coatings are given to electropolated
metals (e.g., Zn, Cd, Cu, Sn, Cr, brass, Ag), zinc base die castings, copper and copper alloys, Al, Mg,
316 stainless steel, and galvanized zinc coatings. Applications include decoration, corrosion protection,
and paint base.
Anodizing. When an aluminum workpiece is made the anode of an electrolytic cell filled with an
acidic aqueous electrolyte, an adherent film of aluminum oxide is produced on the aluminum surface. In
this process, the electric current drives oxide and hydroxide ions generated by the water molecules to the
aluminum surface where reaction occurs to form the oxide coating:
2Al + 3H2O = Al2O3(s) + 6H+ + 6e-
(1.33)
Electroless plating refers to the deposition of a metal coating on a substrate through the action of a
chemical reductant. Metals such as Ni, Co, Pd, Pt, Cu, Au, Ag are plated commercially by electroless
processes. A catalytic surface is required to initiate the deposition process and subsequent deposition
depends on the catalytic ability of the deposited metal. Among the advantages of electroless plating that
may lead to the preference of this deposition technique over electroplating are the following:
1.
2.
3.
4.
5.
The ability to deposit uniform coatings on complex substrates
The formation of relatively less porous coatings
The elimination of the need for electrical energy and related equipment
The ability to deposit coatings onto nonconductors
The ability to produce deposits with special physical-chemical (e.g., magnetic) properties.
A plating bath is a complex aqueous solution. Table 1.25 provides a summary of the various
reagent types and their functions. Typical reductants are hypophosphite (H2PO), formaldehyde
(HCHO), hydrazine (H2NNH2), borohydride (BH4 ) and amine borane (R2NHBH3).
Table 1.25. Types of Plating Bath Reagents
Types of Reagents
Function
Examples
1. Metal salts
Provide the metal ions which are
deposited
Metal chlorides and
sulfates
2. Reducing agents
Provide the electrons needed for the
reaction Mz+ + ze- = M
Sodium,
hypophosphite,
formaldehyde, hydrazine,
sodium borohydride,
dimethylamine borane
3. Complexing agents
Maintain the metal ions as
soluble complexes thereby counteracting the tendency to
precipitate insoluble compounds
Hydroxyacetate, lactate,
EDTA, ammonia,
citrate
4. Acids and bases
Provide the appropriate pH regime
needed by the reduction reaction
and the stability of the bath reagents.
NaOH, KOH, NH3
Na2CO3
5. Buffering agents
Counteract drastic decreases in
bath pH and the resulting decrease
in deposition rate
Lactate,
hydrocyacetate,
citrate
6. Deposition catalysts
Enhance deposition rates by
catalyzing the reaction of the
reducing agent.
7. Stabilizers
Counteract spontaneous bath
decomposition. By adsorbing
preferentially on catalytic
nuclei in the plating bath,
stabilizers inhibit metal
deposition within the bath.
8. Brighteners
Enhance the brightness of the deposit.
Sodium benzene disulfonate
______________________________________________________________________________________________
1.2.8 Semiconductor Device Microfabrication
Integrated circuits are based primarily on silicon technology; however, compound semiconductors,
e.g., GaAs, are exploited for specialized applications. Wet chemical processing is used in three main
areas of solid state device manufacture: (a) Wafer cleaning, (b) thin film etching for pattern definition,
and (c) planarization of metallization structures. Another area of aqueous processing application is in
electronic packaging, where the manufacture of printed circuit boards (PCBs) (also termed printed
wiring boards (PWBs)) relies extensively on etching and plating techniques.
The production of an integrated electronic component involves several steps, as illustrated in Figure
1.11. In the first step, wafer preparation, a number of operations are performed, including, sawing of
the single crystal Si boules into wafers, followed by mechanical polishing, chemical polishing, and
chemical-mechanical polishing of the wafers. The wafer processing operation consists of the following:
Thin film deposition, oxidation, diffusion, and implantation. The manner in which wafer processing is
combined with the masking, and etch/clean circuits is illustrated in Figure 1.12 (Moreau, p. 4). In the
wafer masking operation, a radiation-sensitive coating, called a resist, is applied to the wafer surface.
The photolithographic technique for pattern definition is illustrated in Figure 1.13 (Speight, Fig. 1.3b).
Tables 1.26 and 1.27 provide a listing of typical etchants used in pattern definition.
SINGLE CRYSTAL
SILICON BOULES
CHEM ICALS/M ATERIALS
M ASK GENERATION
WAFER PREP ARATION
WAFER M ASKING
WAFER PROCESSING
ETCH AND CLEAN
PASSIVATION
ASSEM BLY AND TEST
INTEGRATED
CIRCUITS
Figure 1.11 Processing steps in semiconductor device fabrication
(after Moreau, p.3).
Table 1.26 Etchants for silicon and GaAs (Kern and Schnable, p.256).
Etchant
Semiconductor
CP-4A
Si
Planar
etch
White
etch
Si
Polysilicon
etch
Alcohol-KOH
Poly-Si
Br2-methanol
GaAs
Peroxidesulphuric
GaAs
Si
Si
Composition
Comments
3:5:3
HF-HNO3-CH3COOH
2:15:5
HF-HNO3-CH3COOH
1:3
HF-HNO3
3:50:20
HF-HNO3-H2O
50g KOH, 200g
n-propanol,
800g H2O
1:9
Br-CH3OH
5:1:1
H2SO4-H2O2-H2O
Polishing,
80 µm min-1
Polishing,
5 µm min-1
Polishing
0.8 µm min-1
Anisotropic
Polishing
Polishing
Table 1.27 Etchants for dielectrics and insulators used in silicon microelectronic devices
(Kern and Schnable, p. 245)
Material
SiO2
Form
Amorphous or
crystalline
SiO
Amorphous
Al2O3
Amorphous
Crystalline
Sapphire
Ta2O3
Amorphous
TiO2
Amorphous
Crystalline
AsSG
Amorphous
BSG
Amorphous
BPSG
Amorphous
PSG
Amorphous
Si3N4
Amorphous
SiNxHy
Amorphous
SiOxNyHx
Amorphous
Etch rates: H, high; M, medium; L, low; 0, nil.
HFH2 O
NaOH
(30%
hot)
BHF
H3PO4
(hot)
H2SO4
(hot)
H
H
L
0
L
0
H
0
0
M
H
L
H
H
H
H
L
H
H
0
H
0
0
L
M
0
M
L
H
H
L
M
M
0
M
M
0
0
0
0
0
0
0
0
L
L
L
L
L
L
L
M
H
L
0
0
0
0
0
0
L
L
L
0
0
0
Figure 1.12 Fabrication of metal-on-silicon (MOS) devices (Moreau, p.4).
Figure 1.13 The photolithographic technique for pattern definition
(Speight, Fig. 1.3b).
Table 1.27 Etchants for dielectrics and insulators used in silicon microelectronic devices
(Kern and Schnable, p. 245)
Material
SiO2
Form
Amorphous or
crystalline
SiO
Amorphous
Al2O3
Amorphous
Crystalline
Sapphire
Ta2O3
Amorphous
TiO2
Amorphous
Crystalline
AsSG
Amorphous
BSG
Amorphous
BPSG
Amorphous
PSG
Amorphous
Si3N4
Amorphous
SiNxHy
Amorphous
SiOxNyHx
Amorphous
Etch rates: H, high; M, medium; L, low; 0, nil.
HFH2 O
NaOH
(30%
hot)
BHF
H3PO4
(hot)
H2SO4
(hot)
H
H
L
0
L
0
H
0
0
M
H
L
H
H
H
H
L
H
H
0
H
0
0
L
M
0
M
L
H
H
L
M
M
0
M
M
0
0
0
0
0
0
0
0
L
L
L
L
L
L
L
M
H
L
0
0
0
0
0
0
L
L
L
0
0
0
Solid-state device processing is a thin-film-based technology. In this kind of technology surface
contamination constitutes a major problem. The presence of contaminants (ions, molecules, particles)
on the surface of the substrate can have adverse effects on the physical, chemical and electronic
characteristics of the deposited films. The most widely accepted wafer cleaning technique is the RCA
method. This is a two-step operation: Standard Clean 1 (SC-1) NH4OH/H2O2/H2O solution (typical
volumetric ratio: 1/1/5); also referred to as ammonia peroxide mixture (APM). T = 70-90 C, t = 10-15
min. Standard Clean 2 (SC-2) HCl/H2O2/H2O (1/1/5).
An integrated circuit consists of several millions of devices (transistors, capacitors, resistors) on
a given chip. These devices are linked together electrically by metal wires called interconnects. In
order to minimize the length of the wiring (and therefore minimize signal delays) interconnect systems
consist of multiple layers of metals. Figure 1.14 presents a schematic illustration of the cross-section of
a four-layer multilevel metallization (MLM) interconnect system. The main structural components are
described in Table 1.28.
MLM interconnect systems are built up layer by layer by using a combination of processing
techniques, such as etching, deposition, and planarization. The surface of each layer must exhibit a high
degree of global planarity before the next layer is added. An increasingly important planarization
technique is chemical-mechanical polishing (CMP). In the CMP process an aqueous slurry containing
an abrasive (e.g., alumina, silica) and chemicals which can attack the dielectric material (e.g., NH 4OH
for SiO2) or the metal (e.g., an oxidant plus a base, acid or a complexant) is used with a polishing tool as
illustrated in Figure 1.15. The slurry is formulated with a dispersant; particle aggregation has an adverse
effect on the surface roughness of the polished films.
Figure 1.14 Schematic illustration of a four-layer multilevel metallization (MLM) interconnect
system (After Wilson, Tracy, and Freeman, p. 2).
Figure 1.15 Chemical-mechanical polishing (Olsen and Moghadam, in Handbook of Multilevel
Metallization)
Table 1.28 Principal structural components of multilevel metallization systems*
Structural Component
Function_________________________
Material______________________
_
ILD0
Provide dielectric insulation between
silicon device components and the metal
interconnect layers
Boron-doped and phosphorus-doped
silicon glass (BPSG)
Contact
Provides electrical connection from the
interconnect to silicon components
through ILD0
Ti + TiN/W
Metal 1 (M1)
First layer of metal interconnect
W or Al (Cu
Metal 2 (M2)
Second layer of metal interconnect
Al(Cu)
Metal 3 (M3)
Third layer of metal interconnect
Al(Cu)
Metal 4 (M4)
Fourth layer of metal interconnect
Al(Cu)
ILD1
Dielectric layer between M1 and M2
PETEOS
ILD2
Dielectric layer between M2 and M3
PETEOS
ILD3
Dielectric layer between M3 and M4
PETEOS
Via 1
Provides electrical connection between
M1 and M2 through ILD1
W
Via 2
Provides electrical connection between
M2 and M3 through ILD2
W
Via 3
Provides electrical connection between
M3 and M4 through ILD3
W
Passivation
Final dielectric layer, provides protective
physical, chemical barrier to the circuit
PSG + Si3N4
*Modified from S. R. Wilson, C. J. Tracy, and J. L. Freeman, Jr., eds., Handbook of Multilevel Metallization for
Integrated Circuits, 1993, p. 4.
Once the silicon-based devices have been fabricated, they are organized into various functional
and operating systems. This process is termed electronic packaging. In printed circuit boards (PCBs)
metallic films (primarily copper) provide the electrical conductor paths needed to link the various
mounted electronic components. Figure 1.16 illustrates the basic manufacturing steps used to lay down
the metallic materials (Figure 3.6, p. 3.12, Coombs). Table 1.29 lists various etching solutions used in
PCB manufacturing.
Figure 1:16 Basic steps in the manufacture of printed circuit boards (after H. Nakahara, in C. F.
Coombs, Jr., ed., Printed Circuits Handbook, 4th ed., 1996, p. 3.12).
Table 1.29 Printed circuit board copper etchants
Etching Solution___________________
Chemical Reactions______________________
Alkaline Ammonia
Cu + Cu(NH3)42+ = 2Cu(NH)2+
4Cu(NH3)2+ + 8NH3 + O2 + 2H2O
= 4Cu(NH3)42+ + 4OH2Cu + 8NH3 + ClO2- + 2H2O
= 2Cu(NH3)42+ + Cl- + 4OH-
Sulfuric Acid - Hydrogen Peroxide
Cu + H2O2 + H2SO4 = CuSO4 + 2H2O
Cupric Chloride
Cu + CuCl2 = Cu2Cl2
Cu2Cl2 + 4Cl- = 2CuCl32-
Persulfate
Cu + (NH4)2S2O8 = CuSO4 + (NH4)2SO4
Ferric Chloride
Cu + FeCl3 + 2HCl = CuCl32- + FeCl2 + 2H+
Chromic-Sulfuric Acid
3Cu + 2HCrO4- + 14H+ = 3Cu2+ + 2Cr3+ + 8H2O
Nitric Acid
3Cu + 2NO3- + 4H+ = 3Cu2+ + 2NO2 + 2H2O
1.2.9 Cement Technology
The main use of cement is in the construction industry, where it is used in buildings and in civil
structures such as motorways, bridges, garages, and dams. Other fields of application include the
petroleum industry, where it is used as a liner for oil wells. Cement powder is a mixture of pulverized
gypsum (CaSO4.2H2O) and clinker particles. Clinker, in turn, is obtained by subjecting a mixture of
limestone (source of calcium), and clay or shale (source of silicon, aluminum, and iron) to a high
temperature treatment. Typically, the particles in cement range in size from under 1mm to 100mm.
The practical utility of cement in construction stems from its ability, in the presence of water,
to transform into a porous solid. The stage at which this transformation is observed is termed the set
point. In order for setting to occur, the cement-water slurry must undergo a complex suite of chemical
reactions, which together, are described as cement hydration. Figure 1.17 presents an illustration of the
complexity of the hydration process, using the language of cement chemistry. The following notations
are used: A = Al2O3, C = CaO, S = SiO2; thus C3S = Ca3SiO5 (tricalcium silicate), CH = calcium
hydroxide (CaOH), CSH = amorphous or poorly crystalline calcium silicate hydrate. Typically,
tricalcium silicate constitutes 50-70% of the cement particles and it is believed that the main reaction
responsible for the setting of cement is the following:
C3S + nH2O = CSH + CH
(1.34)
The reaction product CSH is a hydrated gel, and the setting of cement is associated with the progressive
dehydration of this material. Another cement component which can undergo hydration is calcium
aluminate (C3A). In fact, this material reacts much more rapidly than C3S and this can lead to
undesirable premature flash set of cement. An important role of gypsum in cement is to inhibit this
reaction; The dissolution of gypsum releases sulfate ions which react with calcium aluminate to give the
mineral ettringite:
C3A + Gypsum --> Ettringite
Figure 1.17 The main chemical reactions associated with cement hydration
(After Billingham and Coveney, 1993)
(1.35)
1.2.10 Battery Technology
Batteries are devices that exploit electrochemical reactions to generate electricity. The active
unit of a battery system is the electrochemical cell. A cell has three key components: a positive
electrode (cathode), a negative electrode (anode) and an electrolyte. When the anode is immersed in the
electrolyte, it undergoes an electron-producing (oxidation) reaction with one or more of the aqueous
phase constituents. On the other hand, the cathode undergoes an electron-consuming (reduction)
reaction. When an external electrical connection is made between the two electrodes, electrons flow
through this connection from the anode to the cathode. This electron flow constitutes an electrical
current. Within the electrolyte, the current is carried by the transport of ions.
The electrochemical reactions consume the anode and cathode materials, and the cell becomes
completely discharged when one of the electrodes is used up. In some cases, it is possible to regenerate
the active materials by applying an external source of current but in the opposite direction. This process
is termed recharging. We can, therefore, distinguish between primary
cells, which are not rechargeable, and secondary cells, which are rechargeable. Tables 1.30 and 1.31
respectively present lists of typical primary and secondary cells. It can be seen that aside from oxygen,
the cathodes are all metal oxides, while all the anodes are metals. The preferred electrolytes are those
based on strong mineral acids (e.g., sulfuric acid, H2SO4), caustic alkalis (e.g., KOH), and salts of
strong acids (e.g., ZnCl2).
Table 1.30 Primary cells (after M. Barak, ed., Electrochemical Power Sources, pp. 3, 99)
Cell
Electrodes
Cathode
Anode
Electrolyte
Aqueous Solution
Open-circuit
EMF(V)
Overall Cell Reactions
Leclanché dry
cell
MnO2(C)
Zn
NH4Cl/ZnCl2
1.60
2MnO2(s) + 2NH4Cl + Zn 
2MnOOH(s) + Zn(NH3)2
Cl2(s)
8MnO2(s) + 8H2O + ZnCl2 +
4Zn  8MnOOH + ZnCl2 .
4Zn(OH)2(s)
Alkaline
Manganese cell
MnO2(C)
Zn
KOH
1.55
2MnO2(s) + H2O + Zn 
2MnOOH(s) + ZnO(s)
Mercury cell
HgO
Zn
KOH
1.35
HgO(s) + Zn  Hg + ZnO(s)
Lalande cell
CuO
Zn
KOH
1.10
CuO(s) + Zn  Cu + ZnO(s)
O2(C)
Zn
KOH or NaOH
or NH4Cl
1.45
O2 + 4NH4Cl + 2Zn  2H2O
+ 2Zn(NH3)2Cl2(s)
Air-depolarised
cell
Table 1.31
Cell
Secondary cells (after M. Barak, ed., Electrochemical Power Sources, pp. 4, 328)
Electrodes
Electrolyte
Cathode
Anode
Aqueous
Solution
Open-circuit
EMF(V)
Overall Cell Reactions
PbO2
Pb
H2SO4
2.10
PbO2(s) + H2SO4 + Pb 
PbSO4(s) + 2H2O
Nickel/cadmium
NiOOH
Cd
KOH
1.29
2 NiOOH(s) + Cd + 2H2O 
2 Ni(OH)2(s) + Cd(OH)2(s)
Nickel/iron
NiOOH
Fe
KOH
1.37
2 NiOOH(s) + Fe + 2H2O  2
Ni(OH)2(s) + Fe(OH)2(s)
Silver/zinc
AgO
Zn
KOH
1.85
AgO(s) + Zn + H2O Ag +
Zn(OH)2(s)
Ag2O
Zn
KOH
1.60
Ag2O(s) + Zn + H2O  2 Ag
+ Zn(OH)2(s)
AgO
Cd
KOH
1.38
AgO(s) + Cd + H2O Ag +
Cd(OH)2(s)
Ag2O
Cd
KOH
1.15
Ag2O(s) + Cd + H2O  2 Ag
+ Cd(OH)2(s)
NiOOH
Zn
KOH
1.74
2 NiOOH(s) + Zn + 2H2O  2
Ni(OH)2(s) + Zn(OH)2(s)
Lead/acid
Silver/cadmium
Nickel/zinc
1.2.11
Solar Energy Conversion and Photoelectrochemical Processing
The amount of solar energy received on the earth's surface in two weeks is comparable with the
total (past and present) fossil fuel resources in the world. With the continuing depletion of these fossil
fuel resources, there is some interest in developing ways to tap into the sun's energy for the generation of
electricity as well as the production of chemicals. An aspect of solar energy conversion of direct
relevance here concerns the use of photosensitive electrodes in aqueous systems. An especially
attractive system is the conversion of water to hydrogen and oxygen:
H2O + Sunlight H2 + 1/2O2
(1.36)
Large amounts of water are readily available and as a fuel, hydrogen has the advantage that it is light
and easy to transport, and its combustion produces no troublesome chemicals.
The operation of a photoelectrochemical (PEC) cell is illustrated schematically in Figure 1.16.
When subjected to illumination of the proper energy, electron-hole pairs are created in the
semiconductor (MA) (a hole, h+, is simply the absence of an electron):
MA + Light  MA + h+ + e-
(1.37)
In the case of the photochemical conversion of water, the electron travels through the external circuit to
the counter electrode where it reacts with protons to give molecular hydrogen:
2H+ + 2e- + = H2
(1.38)
At the semiconductor/electrolyte surface, the hole reacts with hydroxyl ions to give molecular oxygen:
2OH- + = H2O + 1/2O2
(1.39)
Examples of the relevant photosensitive materials are semiconductors such as cadmium sulfide
(CdS), titania (TiO2), and gallium arsenide (GaAs). The nature of the material determines which region
of the solar spectrum is accessible for the PEC process. A major issue in the operation of
photoelectrochemical systems concerns the chemical stability of the electrode material. In the absence
of light, the solid may dissolve in the aqueous solution. Also, a hole is capable of decomposing a solid
unless there is an aqueous phase species which can react sufficiently rapidly with it:
MA + 2h+ = M2+ + A
(1.40)
e-
Light
h+
Semiconductor
Electrode
Figure 1.18
Counter
Electrode
Schematic illustration of a photoelectrochemical (PEC) cell.
1.2.12
Waste Treatment and Control
It can no longer be said that the work of the metallurgical or materials processing engineering is
over, once the refined metal has been produced, the workpiece has been plated, or the appropriate
powder has been synthesized. We now live in an era where the job of the metallurgical/materials
processing engineer extends to the treatment of the effluents generated while preparing the saleable
product. In the U.S., for example, there are federal regulations on pollution control and hazardous waste
treatment which have a direct bearing on the plant-level activities of industries engaged in metal
extraction and materials synthesis and processing. These federal laws include: the Federal Water
Pollution Control Act, the Toxic Substance Control Act (TSCA), the Resource Conservation and
Recovery Act of 1976 (RCRA), and the Comprehensive Environmental, Compensation and Liability Act
of 1980 (CERCLA). There are strict EPA definitions of hazardous materials based, on certain
ignitability, corrosivity, reactivity, and toxicity criteria.
According to one report, the total volume of hazardous waste produced in the U.S. in 1986
amounted to 747 million tons, 58.8% of which was generated by the chemical and petroleum industries,
32.7% by the metal-related industries, and 8.5% by other industries (Brooks, p. 3). The metal-related
industries include the primary metals industries, the fabricated metals industries, the electrical and
electronic machinery industries, and the transportation equipment industries. Metallic wastes that have
been classified as hazardous include those listed in Table 1.32.
Various waste treatment and control options are available and these can be categorized as: (a)
waste volume minimization, (b) conventional technologies, (c) alternative technologies, (d) materials
recovery, and (e) reagent substitution. Brief characteristics of these options are presented in Table 1.33.
Figure 1.17 illustrates the manner in which waste treatment and materials recovery can be combined
with conventional mineral processing, hydrometallurgical extraction, ceramic processing, and surface
finishing.
Table 1.32
Selected hazardous metallic wastes (from Brooks, Metal Recovery from
Industrial Waste, pp. 14-16).
Industry and EPA
Hazardous Waste No.
Hazardous Waste
F006
Wastewater treatment sludges from electroplating
operations.
F019
Wastewater treatment sludges from the chemical
conversion coating of aluminum
F007
Spent cyanide plating bath solutions from electroplating
operations.
K002
Wastewater treatment sludge from the production of
chrome yellow and orange pigments.
K003
Wastewater treatment sludge from the production of
molybdate orange pigments
K004
Wastewater treatment sludge from the production of zinc
yellow pigments.
K007
Wastewater treatment sludge from the production of iron
blue pigments.
K061
Emission control dust/sludge from the primary
production of steel in electric furnaces
K062
Spent pickle liquor generated by steel-finishing
operations of plants that produce iron or steel.
K069
Emission control dust/sludge from secondary lead
smelting.
K100
Waste leaching solution from acid leaching of emission
control dust/sludge from secondary lead smelting.
_____________________________________________________________________________
Table 1.33
Processing options for waste and control
Waste Treatment/Control Option
Remarks
Waste Volume Reduction
In-plant changes, e.g., modifications in the mode and
amount of water and chemical additions, manipulation of
equipment configuration, improved instrumentation.
Conventional Treatment Technologies Pollution control technologies currently well established
in specific metals/materials processing industries, e.g.,
sulfur dioxide (SO2) for conversion of hexavalent to
trivalent chromium, alkaline chlorination for cyanide
destruction, hydroxide precipitation for heavy metal
removal, coagulation/flocculation to enhance solids
settling rates, filtration/gravity settling (thickening) for
solid/water separation.
Alternative Technologies
Necessitated by limitations in conventional technologies,
e.g., in ability to meet effluent standards with hydroxide
precipitation (replace with sulfide precipitation),
generation of prohibitively high sludge volumes by
conventional hydroxide precipitation, inefficient removal
of finely divided solids, adverse environmental effects of
waste treatment reagents (e.g., chlorine).
Materials Recovery
Reclamation of process water and reagents for recycle;
motivated by increasing costs of raw materials and waste
disposal.
Reagent Substitution
Use new formulations of process solutions with less
toxic chemicals, e.g., replace cyanide solutions with noncyanide solutions.
_______________________________________________________________________________
1.3 Common Denominators in Aqueous Processing
1.3.1 Integrated Processing
In practice, there is a certain amount of linkage between the various aqueous processing
application fields. In Section 1.2.12, we noted the respective links between waste treatment and
hydrometallurgical extraction, and minerals/materials processing.
Also, traditionally, mineral
processing operations and hydrometallurgical plants are frequently located adjacent to each other and
are linked by materials flow. Integration of mineral processing, hydrometallurgical extraction, ceramic
processing and waste treatment is also possible, as illustrated in Figure 1.20.
M INERAL
PROCESSING
ORE
RECYCLE
WAST ES
CHEM ICALS
WAST E
T REAT M ENT
(a)
PRE CURSORS
ENVIRONMENT AL
DISPOSAL
CERAM IC POWDE RS &
FIL MS
PROCESSING
WAST ES
CHEM ICALS
CONCE NT RAT E
RECYCLE
WAST E
T REAT M ENT ENVIRONMENT AL
DISPOSAL
(c)
HYDROM ET REFINED M ET ALS &
EXT RACT 'N
CONCE NT RAT E
ME T AL COM POUNDS
ORE &
WAST ES
CHEM ICALS
RECYCLE
WAST E
T REAT M ENT
(b)
PART &
CHEM ICALS
SURFACE
FINISHING
PLAT E D PART
RECYCLE
WAST ES
CHEM ICALS
ENVIRONMENT AL
DISPOSAL
WAST E
T REAT M ENT
ENVIRONMENT AL
DISPOSAL
(d)
Figure 1.19 (a) Combined mineral processing-waste treatment; (b) combined hydrometallurgical
extraction-waste treatment; (c) Combined ceramic processing-waste treatment, (d)
combined surface finishing-waste treatment.
1.3.2 Reagents, Solutions, and Materials
It can be seen from the above discussion that the applications of aqueous processing are many and
diverse. However, these different aqueous processing systems have many things in common. First, we
recognize that water is the common solvent. Secondly, as illustrated in Table 1.34, a given reagent may
have several different applications. Thus, for example, cyanide solutions are encountered in the
hydrometallurgical extraction of gold and silver; cyanide baths are also exploited in the electroplating of
these same metals.
From a chemical standpoint, similar solids are encountered in the various aqueous systems as
illustrated in Table 1.35. Thus the silica/water interface is important in mineral flotation,
hydrometallurgical
extraction,
catalyst
preparation,
semiconductor
device
microfabrication,
micromachining of micromechanical devices, and waste treatment. Manganese oxides are leached in
hydrometallurgy, electrolytic manganese dioxide is prepared for the battery industry, and reactions at the
MnO2/aqueous interface are exploited in batteries.
1.3.3 Unit Processes
In a given aqueous processing operation, the feed (raw) materials and the products are typically
linked by a series of processing steps. These steps, termed unit operations or unit processes, include
grinding, dissolution, flocculation, coagulation, dispersion, filtration, gravity settling, evaporation,
distillation, absorption, adsorption, ion-exchange, solvent extraction, chemical precipitation, and
electrodeposition, as summarized in Table 1.36. A given unit process may find application in several
different aqueous processing systems, as illustrated by the examples presented in Table 1.37.
1.3.4 Physicochemical Processes
Underlying each unit process is a mechanism of action. For example, both leaching and
precipitation rely on solubility phenomena. These mechanisms of action, in turn, are made possible by a
variety of physicochemical processes, which are molecular-level events (such as collision of molecules,
bond-formation, bond-breakage, electron transfer, molecular diffusion, ion migration) and colloidal and
interfacial interactions such as aggregation, dispersion, and particle-fluid dynamics. Figure 1.19 presents
a schematic illustration of selected interfacial and colloidal physicochemical processes. A major focus
of the material in the following chapters is on the roles of these physicochemical processes in various
aqueous processing systems.
Figure 1.20 Integrated processing: Combined mineral processing-hydrometallurgical extractionceramic processing-waste treatment.
Table 1.34 Reagents in aqueous processing
Reagents_______
Industries______
Applications_______
Sulfuric Acid
Hydrometallurgy
Leaching
Solution
Metal production
Metal Finishing
Pickling (steels)
Batteries
Pb-acid cell
Hydrometallurgy
Leaching
Solution purification
Metal production
Metal Finishing
Pickling
Electroplating
Hydrometallurgy
Leaching(Au, Ag)
Solution purification
Metal production
Metal Finishing
Electroplating, Pickling
Metal Finishing
Electroplating (Rh), Electropolishing
Semicond. Microfab.
Etching
Caustic Soda (NaOH)
Hydrometallurgy
Leaching (e.g. Al from bauxite ores)
Precipitation
KOH
Semiconductor
Microfabrication
Etching (SiO2, Si)
Micromachining
Etching (SiO2, Si)
Batteries
Electrolyte (alkaline storage
batteries)
Hydrometallurgy
Leaching (Cu, Ni, Co)
Solution purification
Metal production
Metal Finishing
Electroplating
Semiconductor
Microfabrication
Etching (SiO2, Si)
Micromachining
Etching (SiO2, Si)
Hydrometallurgy
Precipitation (e.g., rare earths)
Ceramic Processing
Powder synthesis
Hydrochloric Acid
Cyanide
Phosphoric Acid
NH4OH
Oxalate
_
Table 1.35 Materials in aqueous processing
Materials
Industries
Applications
Silicon dioxide
Mineral Processing
Flotation, Coagulation, Flocculation
Hydrometallurgy Leaching, Precipitation
Alumina
Metal Sulfides
Ceramic Processing
Powder/Film synthesis, suspension
processing
Catalyst Preparation
Synthesis of substrates
Cement Technology
Cement hydration
Semiconductor Device
Microfabrication
Etching for cleaning and pattern
definition
Micromachining
Etching for microstructure development
Waste Treatment
Silica precipitation, coagulation,
flocculation
Mineral Processing
Flotation, Coagulation, Flocculation
Hydrometallurgy
Leaching, Precipitation
Ceramic Processing
Powder/Film synthesis, suspension
processing
Cement Technology
Cement hydration
Surface Finishing
Anodic oxide coating on aluminum
Waste Treatment
Adsorbent
Mineral Processing
Flotation
Hydrometallurgy
Leaching, precipitation
Ceramic Processing
Powder Synthesis (e.g., ZnS)
Catalyst Preparation
Synthesis of metal sulfides
Waste Treatment
Metal removal as sulfide precipitate
Table 1.36. Types of Unit Processes and Unit Operations (1).
Name
Evaporation
Liquid
Feed
Change Agent
Heat
Products
Liquid + Vapor
Principle of Action
Difference in
volatilities
Evaporation of water
Practical Example
Concentration of
metal salt solutions
Drying of precipitates
Drying
Moist solid
Heat
Crystallization
Liquid
Cooling or heat +
evaporation
Dry solid + humid
vapor
Liquid + solids
Temperature
dependence of
solubilities
Sublimation of water
Preparation of metal
salts
Freeze drying
Frozen watercontaining solid
Heat
Dry solid + water
vapor
Absorption
Solvent extraction
Gas
Liquid
Nonvolatile liquid
Extractant in
Immiscible Liquid
Liquid + vapor
Two liquids
Ion exchange
Liquid
Solid resin
Liquid + solid resin
Adsorption
Dissolution or
Leaching
Liquid
Solids
Solid adsorbent
Aqueous-soluble
chemical reactant
Liquid + solid
Liquid + solid
Preferential solubility
Referential organic
phase solubility of
metal-extractant
complex
Complex or ion-pair
formation
Surface complexation
Preferential solubility
Washing
Solids with sol'n
entrainment
Aqueous sol'n
Liquid + solid
Dilution
Precipitation
Liquid
Chemical reactant
Liquid + solid
Conc'n. dependence
of solubility
Preparation of
unagglomerated
powders
Solution purification
Solution purification
Solution purification
Gold extraction from
ones; removal f
coatings from
substrates.
Recover entrained
valuables; purify
precipitates
Solution purification;
metal recovery;
powder synthesis
(Table 1.36 contd.)
Name
Electrodeposition
Liquid
Feed
Change Agent
Electricity
Products
Liquid + solid
Principle of Action
Electrochemical
reaction
Dialysis
Liquid
Selective Membrane;
solvent
Liquids
Electrodialysis
Liquid
Anionic and cationic
membranes; electric
field
Liquids
Liquid membrane
Liquid
Solvent liquid layer
Liquids
Reverse osmosis
Liquid sol'n.
Pressure gradient
(pumping power) +
membrane
Two liquid sol'ns.
Ultrafiltration
Liquid sol'n.
Pressure gradient
(pumping power) +
membrane
Two liquids
Froth flotation
Mixed powdered
solids
Added surfactants;
rising air bubbles
Two solids
Ion flotation
Liquid sol'n.
Added surfactants;
rising air bubbles
Solid + liquid
Coagulation
Colloidal dispersion
Electrolyte
Solid aggregates +
liquid
Flocculation
Colloidal dispersion
Polymer
Solid aggregates +
liquid
Different rates of
diffusional transport
through membrane
(no bulk flow)
Tendency of anionexchange membranes
to pass only anion,
etc.
Different rates of
permeation through
liquid layer
Different combined
solubilities and
diffusivities of species
in membrane
Differential
permeabilities through
membrane (molecular
size)
Selective adsorption
leading to
hydrophobic solids
Selective electrostatic
and/or complexation
interactions leading to
hydrophobic species
Modification of
surface charge
through adsorption
Particle bridging
through adsorption
Practical Example
Electrowinning of
metals;
electrorefining;
electroplating
Recovery of NaOH
in rayon
manufacture
Desalination of
brackish waters
Waste-water
processing; metal
separations.
Seawater desalination
Waste-water treatment
Ore flotation
Ion separations
Solid/liquid sep'n.
Solid/liquid sep'n.
(Table 1.36 contd.)
Name
Dispersion
Feed
Colloidal aggregates
in liquid
Change Agent
Electrolyte; polymer
Products
Colloidal dispersion
Calcination
Solids
Heat
Solid (+ vapor)
Roasting
Solids
Solid (+ gas)
Sintering
Porous Solid
Heat + chemical
reactant
Heat
Comminution
Solid particles
Ground particles
Size classification
Solid particles
Mechanical energy +
grinding media (e.g.
balls)
Screen
Settling
Gravity
Filtration
Liquid + solid or
another immiscible
liquid
Liquid + solid or
another immiscible
liquid
Liquid + solid
Liquid + solid or
another immiscible
liquid
Liquid + solid or
another immiscible
liquid
Liquid + solid
Mixing
Solid + liquid
Centrifugation
Centrifugal force
Pressure reduction
(energy); filter
medium
Mechanical energy
Densified solid
Two or more solids
Principle of Action
Modification of
surface charge and
surface layers through
adsorption leading to
destabilization to
aggregates
Temperature
dependence of
chemical stability
High temp. chemical
reaction
Interparticle diffusion
Practical Example
Stabilization of
suspensions
Production of oxides
from metal
hydroxides
Production of metal
oxides from sulfides.
Preparation of
ceramics
Grinding of ores and
ceramic powders
Particle breakage due
to collision of solids
with grinding media
Size-selective passage Feed preparation for
through screen
leaching; sorting of
openings
ceramic powder
products into size
fractions.
Density difference
Solid/liquid separation
Density difference
Solid/liquid separation
Size-selective passage Solid/liquid separation
through pores of filter
medium
Solid-liquid
suspension
_____________________________________________________________________________________
(1) Based in part on Table 1-1, C. J. King, Separation Processes, 2nd ed., McGraw-Hill, New York, 1980.
Table 1.37 Unit Processes in Aqueous Processing
Unit Processes
Dissolution
Industries
Mineral Processing
Hydrometallurgy
Ceramic Processing
Precipitation
Ion Exchange
Electrodeposition
Applications/Roles
Surface modification in
mineral flotation
Metal extraction
Fabrication of specialty glasses
Glass corrosion
Cement Technology
Cement hydration
Surface Finishing
Stripping of defective deposits
Batteries
Discharging of cells
Semiconductor Device
Microfabrication
Etching for cleaning and pattern
definition
Micromachining
Etching for microstructure
development
Waste Treatment
Leaching (instability) of solid
wastes
Mineral Processing
Surface modification in mineral
flotation
Hydrometallurgy
Solution purification, metal/metal
compound production
Ceramic Processing
Powder/Film/Fiber synthesis
Cement Technology
Cement hydration
Semiconductor Device
Microfabrication
Surface contamination
Surface Finishing
Chemical plating
Batteries
Electrode/electrolyte reactions
Waste Treatment
Metal/anion removal as precipitates
Hydrometallurgy
Ion separations
Surface Finishing
Process water purification
Waste Treatment
Ion removal
Hydrometallurgy
Electrowinning, electrorefining
Ceramic Processing
Electrosynthesis
Surface Finishing
Electrodeposition, anodization
Batteries
Recharging reactions
Waste Treatment
Metal recovery
Figure 1.21
Interfacial and colloidal physicochemical processes.
FURTHER READING
1.
P. A. Cox, The Electronic Structure and Chemistry of Solids, Oxford, New York, 1987.
2.
J. Billingham and P. V. Coveney, "Simple Chemical Clock Reactions: Applications to Cement Hydration", J. Chem.
Soc., Farad. Trans., 89, 3021-3028 (1993).
3.
F. N. Kemmer, ed., The Nalco Water Handbook, McGraw-Hill, New York, NY 1979.
4.
D. M. Considine, Ed., Chemical and Process Technology Encyclopedia, McGraw-Hill, New York, NY, 1974, pp. 926929.
4a.
Kirk-Othmer, Encyclopedia of Chemical Technology, Wiley-Interscience, New York, 1983.
5.
Minerals Yearbook 1993, Volume 1, Metals and Minerals, U. S. Bureau of Mines, Washington, D.C. 1989.
6a.
J. Leja, Surface Chemistry of Froth Flotation, Plenum, New York, 1982.
6b.
F. Halverson and H. P. Panzer, "Flocculating Agents", in Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 10,
3rd. ed., 1980, pp. 489-523.
7.
C. K. Gupta and T. K. Mukherjee, Hydrometallurgy in Extraction Processes, Vols. I & II, CRC Press, Boca Raton,
FL, 1990.
8.
A. R. Burkin, The Chemistry of Hydrometallurgical Processes, Spon, Lond, 1966.
9.
F. Habashi, Extractive Metallurgy. Vol. 2 Hydrometallurgy, Gordon and Breach, New York, NY, 1970.
9a
F. Habashi, A Textbook of Hydrometallurgy, Métallurgie Extractive Québec, Enr., Sainte Foy, Quebec, 1993.
10.
E. D. Jackson, Hydrometallurgical Extraction and Reclamation, Ellis Horwood, Chickester, U. K., 1987.
11.
M. Streat and D. Naden, eds., Ion Exchange and Sorption Processes in Hydrometallurgy, W
12.
J. Rydberg, C. Musikas, and G. R. Choppin, eds., Principles and Practices of Solvent Extraction, Marcel Dekker, New
York, 1992.
12a.
T. C. Lo, M. H. I. Baird, and C. Hanson, eds., Handbook of Solvent Extraction, Wiley, New York, 1983.
12b. G. M. Ritcey and A. W. Ashbrook, Solvent Extraction. Principles and Applications to Process Metallurgy. Elsevier,
New York, 1979.
13.
D. W. Johnston, Jr., "Nonconventional Powder Preparation Techniques", Ceram. Bull., 60(2), 221-224, 243 (1981).
14.
L. S. Millberg, "The Synthesis of Ceramic Powders", J. Metals, August 1987, pp. 9-13.
15.
D. W. Johnson, Jr., and P. K. Gallacher, "Reactive Powders from Solution", in Ceramic Processing Before Firing, G.
W. Onoda, Jr., and L. L. Hench, eds., Wiley, New York, NY, 1978, pp. 125-139.
16.
L. H. Yaverbaum, ed., Technology of Metal Powders. Recent Developments, Noyes Data Corp, Park Ridge, NJ, 1980.
16c.
M. C. Fuerstenau, ed., Flotation, A. M. Gaudin Memorial Volume, AIME, New York, 1976.
16d. K. J. Ives, ed., The Scientific Basis of Flocculation, Sijthoff & Noordhoff, Alphen aan den Rijn, The Netherlands,
1978.
17.
N. Ichinose, ed., Introduction to Fine Ceramics, Wiley, New York, 1987.
17a.
J. S. Reed, Principles of Ceramics Processing, 2nd ed., Wiley, New York, 1995.
17b. M. N. Rahaman, Ceramic Processing and Sintering, M. Dekker, New York, 1995.
18.
D. E. Clark and B. K. Zoitos, eds., Corrosion of Glass, Ceramics and Ceramic Superconductors, Noyes, Park Ridge,
NJ, 1992.
19.
H. H. Kung, Transition Metal Oxides: Surface Chemistry and Catalysis, Elsevier, New York, 1989.
20.
O. Weisser and S. Landa, Sulfide Catalysts, Their Properties and Applications, Pergamon, New York, 1973.
21.
J. P. Brunelle, "Preparation of Catalysts by Metallic Complex Adsorption on Mineral Oxides", Pure Appl. Chem., 50,
1211-1229 (1978).
22.
J. W. Geus, "Production and Thermal Pretreatment of Supported Catalysts", in Preparation of Catalysts III, G.
Poncelet, P. Grange, and P. A. Jacobs, eds., Elsevierr, New York, 1983, pp. 1-33.
23.
B. H. Carroll, G. C. Higgins, and T. H. James, Introduction to Photographic Theory. The Silver Halide Process,
Wiley-Intersciece, New York, 1980.
24.
T. H. James, ed., The Theory of the Photographic Process, 4th ed., Macmillan, New York, 1977.
25.
R. K. Bunting, The Chemistry of Photography, Photoglass Press, Normal, IL, 1987.
26.
A. T. Kuhn, ed., Industrial Electrochemical Processes, Elsevier, New York, 1971.
27.
D. Pletcher, Industrial Electrochemistry, Chapman and Hall, New York, NY, 1982.
28.
F. A. Lowenheim, ed., Modern Electroplating, 3rd. Ed., Wiley, New York, 1974.
29.
L. T. Durney, ed., Electroplating Engineering Handbook, 4th Ed., Van Nostrand, Reinhold, New York, 1984.
29a.
Metals Handbook, 9th ed., Vol. 5. Surface Cleaning, Finishing and Coating, American Society for Metals, Metals
Park, OH, 1982.
29a.
G. Mallory and J. B. Hajdu, eds., Electroless Plating: Fundamentals and Applications, American Electroplaters and
Surface Finishers Society, Florida, 1990.
30.
J. Skalny and S. Mindess, Materials Science of Concrete II, The American Ceramic Society, Westerville, OH, 1991.
31.
H. F. W. Taylor, Cement Chemistry, Academic, New York, 1990.
32.
W. Kern, "The Evolution of Silicon Wafer Cleaning Technology", J. Electrochem. Soc., 137, 1887-1892 (1994).
33.
J. Speight, "Overview", in The Chemistry of the Semiconductor Industry, S. J. Moss and A. Ledwith, eds., Chapman
and Hall, New York, 1987, pp.1-16.
34.
W. Kern and G. L. Schnable, "Wet Etching", in The Chemistry of the Semiconductor Industry, S. J. Moss and A.
Ledwith, eds., Chapman and Hall, New York, 1987, pp.225-281.
35.
W. Kern and C. A. Deckert, "Chemical Etching", in Thin Film Processes, J. L. Vossen and W. Kern, eds., Academic,
New York, 1978, pp.401-496.
36.
W. M. Moreau, Semiconductor Lithography. Principles, Practices, and Materials, Plenum, New York, 1988, pp.1-27.
36a.
J. M. Steigerwald, S. M. Murarka, and R. J. Gutmann, Chemical Mechanical Planarization of Microelectronic
Materials. Wiley, New York, 1997.
36b. S. R. Wilson, C. J. Tracy, and J. L. Freeman, Jr., Handbook of Multilevel Metallization for Integrated Circuits.
Materials, Technology, and Applications, Noyes Publications, Park Ridge, NJ, 1993.
36c.
W. Kern, ed., Handbook of Semiconductor Wafer Cleaning Technology, Noyes Publications, Park Ridge, NJ, 1993.
36d. C. F. Coombs, Jr., ed., Printed Circuits Handbook, 4th ed., McGraw-Hill, New York, 1996.
37.
J. W. Patterson, Industrial Wasterwater Treatment Technology, 2nd Ed., Butterworth, Stoneham, MA, 1985.
38.
J. W. Patterson and R. Passino, eds., Metals Speciation, Separation, and Recovery, Lewis Publishers, Chelsea, MI,
1987.
39.
J. W. Patterson and R. Passino, eds., Metals Speciation, Separation, and Recovery, Vol. II, Lewis Publishers, Chelsea,
MI, 1990.
40.
A. J. Rubin, ed., Aqueous-Environmental Chemistry of Metals, Ann Arbor Science Publishers, Ann Arbor, MI, 1974.
41.
L. D. Benefield, J. F. Judkins, and B. L. Weand, Process Chemistry for Water and Wasterwater Treatment, PrenticeHall, Englewood Cliffs, NJ, 1982.
42.
J. N. Lester, ed., Heavy Metals in Wastewater and Sludge Treatment Processes, Vol. I Sources, Analysis, and
Legislation; Vol. II. Treatment and Disposal, CRC Press, Boca Raton, FL, 1987.
43.
B. A. Bolto and L. Pawlowski, Wastewater Treatment by Ion Exchange, Spon, New York, 1987.
44.
W. J. Eilbeck and G. Mattock, Chemical Processes in Waste Water Treatment, Wiley, New York, 1987.
45.
S. D. Faust and O. M. Aly, Chemistry of Water Treatment, Butterworths, Boston, 1983.
46.
S. D. Faust and O. M. Aly, Adsorption Processes for Water Treatment, Butterworths, Boston, 1987.
47.
W. W. Eckenfelder, Jr., Industrial Water Pollution Control, 2nd ed., McGraw-Hill, New York, 1989.
48.
P. Côté and M. Gilliam, eds., Environmental Aspects of Stabilization and Solidification of Hazardous and Radioactive
Wastes, ASTM, Philadelphia, PA, 1989.
49.
C. Calmon and H. Gold, eds., Ion Exchange for Pollution Control, Vols I and II, CRC Press, Boca Raton, FL, 1979.
50.
G. C. Cushnie, Jr., Electroplating Wasterwater Pollution Control Technology, Noyes., Park Ridge, NJ, 1985.
51.
C. S. Brooks, Metal Recovery from Industrial Waste, Lewis Publishers, Chelsea, MI, 1991.
52.
M. R. Watson, Pollution Control in Metal Finishing, Noyes., Park Ridge, NJ, 1973.
53.
J. H. L. van Linden, D. L. Stewart, Jr., and Y. Sahai, Recycling of Metals and Engineered Materials, Proceedings of
Second International Symposium, TMS, Warrendale, PA, 1990.
54.
D.J. Hurd, D.M. Muchnick, M.F. Schedler, and T. Mele, Recycling of Consumer Dry Cell Batteries, Noyes, Park
Ridge, NJ, 1993.
55.
M. Barak, ed., Electrochemical Power Sources, Peter Peregrines, New York, NY, 1980.
56.
K.V. Kordesch, ed., Batteries, Vol. 1, Manganese Dioxide, Marcel Dekker, New York, NY 1974.
57.
K.V. Kordesch, ed., Batteries, Vol. 2, Lead-Acid Batteries and Electric Vehicles, Marcel Dekker,
New York, NY, 1974.
58.
D. Segal, Chemical Synthesis of Advanced Ceramic Materials, Cambridge, New York, 1989.
59.
W. Kern, ed., Handbook of Semicondutor Wafer Cleaning Technology, Noyes, Park Ridge, NJ, 1993.
60.
Y. V. Pleskov, Solar Energy Conversion. A Photoelectrochemical Approach, Springer-Verlag, New York, 1990.
61.
M. D. Archer, "Electrochemical Aspects of Solar Energy Conversion", J. Appl. Electrochem., 5, 17-38 (1975).
62.
K. Rajeshwar, P. Singh, and J. DuBow, "Energy Conversion in Photoelectrochemical Systems - A Review",
Electrochim. Acta, 23, 1117-1144 (1978).
63.
A. J. Novak, "Photoelectrochemistry: Applications to Solar Energy Conversion", Ann. Rev. Phys. Chem., 29, 189-222
(1978).
64.
M. A. Butler and D. S. Ginley, "Principles of Photochemical Solar Energy Conversion", J. Mater. Sci., 15, 1-19
(1980).
65.
A. K. Kuczkowski, "Photoelectrochemical Processes at Semiconductors", in Photochemistry and Photophysics, Vol.
III, pp. 103-136 (1990?).
66.
H. Kisch and R. Kunneth, "Photocatalysis by Semiconductor Powders - Preparative and Mechanistiv Aspects", in
Photochemistry and Photophysics, Vol. IV, pp. 103-136 (1990?).
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