Download Source Reduction Measures in the Metal Finishing Sector

Source Reduction Measures Employed in the
Metal Finishing Sector
Report supplementing WRC Report TT 645/15:
Natsurv 2 – Water and Wastewater Management in the Metal
Finishing Industry
by
HA Ally, W Kamish and T van der Spuy
i
ACKNOWLEDGEMENTS
The research in this report emanates from a project that was undertaken by the Water Research
Commission, entitled:
NATSURV 2: Water and Wastewater Management in the Metal Finishing Industry (Ed 2)
The Steering Committee is thanked for contributing their knowledge and insights to the project and the
content of this guide.
The project team would like to extend their thanks to the following people:
•
All the metal finishing companies who gave of their time to complete the survey and participate in
the site visits and interviews
•
The regulators who provided input into the development of the guide
ii
TABLE OF CONTENTS
Page No
1.
BACKGROUND .......................................................................................................... 1
1.1
1.1.1
INTRODUCTION ...................................................................................................... 1
Industry overview ...................................................................................................... 1
1.2
THE OVERALL OBJECTIVES OF THE PROJECT ................................................. 2
1.3
DESCRIPTION OF THE RESEARCH PRODUCTS................................................. 3
2.
WASTEWATER TREATMENT IN THE METAL FINISHING SECTOR ..................... 4
2.1
3.
WASTEWATER TREATMENT OF POTENTIAL RELEASES TO THE ENVIRONMENT
........................................................................................................................ 4
CLEANER PRODUCTION INITIATIVES FOR THE METAL FINISHING SECTOR .. 8
3.1
4.
MANAGEMENT AND HOUSEKEEPING ................................................................. 8
BEST PRACTICES APPLICABLE TO JIG AND BARREL PLATING OPERATIONS10
4.1
REDUCING DRAG-OUT ........................................................................................ 10
4.2
IMPROVEMENTS IN RINSING .............................................................................. 11
4.3
CONTROLS ON RINSE WATER FLOWS:............................................................. 11
4.4
PREVENTING “BACK-MIXING” OR “SHORT-CIRCUITING” ................................ 12
4.5
MINOR PROCESS BATH IMPROVEMENTS ........................................................ 13
4.6
IMPROVE MAINTENANCE PROCEDURES ......................................................... 13
4.7
PROCESS CONTROL ............................................................................................ 14
4.8
PRODUCT STORAGE ........................................................................................... 14
4.9
SOLVENT AND ALKALINE DEGREASING SYSTEMS ......................................... 15
4.10
IMPROVE THE OPERATION OF CONVENTIONAL VAPOUR DEGREASERS ... 17
4.11
USE A LOW-EMISSION VAPOUR DEGREASER ................................................. 18
4.12
PLATING................................................................................................................. 18
4.13
CHEMICAL AND ELECTROCHEMICAL CONVERSION COATINGS (CHROMATING,
PHOSPHATING, ANODISING, AND COLOURING) ................................... 19
4.14
ALTERNATIVE STRIPPING PROCESSES ........................................................... 20
5.
DESIGN METHODOLOGY FOR COUNTERFLOW RINSING ................................. 21
5.1
THEORY OF COUNTERFLOW RINSING SYSTEMS ........................................... 21
5.2
DESIGN MODEL ASSUMPTIONS ......................................................................... 23
5.3
DESIGN MODEL BASIS ......................................................................................... 23
5.4
5.4.1
5.4.2
5.4.3
DESIGN MODEL OUTPUT .................................................................................... 24
Additional design considerations ............................................................................ 24
Equipment sizing..................................................................................................... 24
Rinse tank operation ............................................................................................... 24
iii
5.4.4
5.4.5
Process control ....................................................................................................... 24
Drag-out control ...................................................................................................... 24
5.5
5.5.1
5.5.2
5.5.3
5.5.4
5.5.5
CALCULATIONS AND FINDINGS ......................................................................... 25
Water consumption ................................................................................................. 25
Equipment sizing..................................................................................................... 25
Process operation ................................................................................................... 26
Process control automation .................................................................................... 26
Drag-out control ...................................................................................................... 26
5.6
SUMMARY OF THE DESIGN ................................................................................ 27
6.
COSTING OF A CP INITIATIVE IN THE METAL FINISHING INDUSTRY .............. 28
6.1
6.1.1
7.
THEORY OF THE COSTING APPROACH ............................................................ 28
Study estimate ........................................................................................................ 28
REFERENCES .......................................................................................................... 31
LIST OF FIGURES
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Design for preventing "back-mixing" in tanks ............................................... 12
Schematic drawing of an Eductor ................................................................. 15
Operation of Eductors................................................................................... 16
Diagram demonstrating the setup of an eductor array in a cleaning tank .... 16
Counterflow rinsing (Hyder Consulting and Hemsley 1999) ........................ 21
Counterflow rinsing with 3 stages ................................................................. 23
Air lift pump (Van Der Spuy, 2004) ............................................................... 26
Design and dimensions of a 5-stage rinsing unit.......................................... 28
LIST OF TABLES
Table 1
Table 2
Table 3
Table 4
Typical treatment regimens for effluent treatment .......................................... 4
Possible treatment(s) specific to each constituent of concern ....................... 6
Typical components of an Environmental Management Plan (EMS) ............. 8
Metal concentrations in Rinse tanks 1 to 3................................................... 25
iv
GLOSSARY
Anode
is an electrode through which an electric current flow into a polarised electric
device
EPA
environmental protection agency
ISO
international standards organisation
METSEP
METals SEParation is the Johannesburg based company that recycles acids for
industry. www.metsep.co.za
mg/m3
milligrams per cubic metre (concentration)
Micron
1 millionth of a metre
MSDS
materials safety and data sheet
ppm
parts per million (concentration)
SABS
South African Bureau of Standards
Tensides
is a substance which lowers the surface tension between two liquids and
supports the formation of dispersions. They are also known as emulsifying
agents
UNEP
United Nations Environment Program
zamak
often referred to as mazak, is an alloy of zinc aluminium magnesium and copper
and is used extensively in die casting industry
v
1.
BACKGROUND
1.1
INTRODUCTION
The last National Survey relating to the management of resources in the Metal Finishing Industry was
completed in October 1987, 25 years ago. Since then there has been concerted effort to develop
coating systems that rely on more environmentally friendly chemicals along with scientific evaluation of
process lines that has resulted in significant improvements to layout and handling methods.
Since the last NATSURV, a portion of the South African metal finishers have benefitted from financial
and technical assistance from DANIDA (the Danish Foreign Aid agency) with the implementation of
our own Resource Conservation program. This initiative, the CPMFI project [Cleaner Production in the
Metal Finishing Industry] ran for a period of three and a half years, officially ending in 2004 and
culminating in the establishment of SAMFA, the South African Metal Finishing Association.
Ultimately eighteen companies rebuilt their plants with partial subsidies from DANIDA totalling 1.2
million rands. Together they achieved combined savings of around 3.8 million Rands in year one and
on average recouped their investments through savings in an average period of 18 months. Scores of
others made smaller but measurable improvements using their own resources.
To quantify the extent to which the metal finishing industry in South Africa has changed since the last
NATSURV conducted in 1987, it was again necessary to collate up-to-date information such that
meaningful conclusions could be drawn.
The major outcomes expected from this study are as follows:
•
A review of current state of the metal finishing sector which would provide the platform for
further initiatives aimed at positively transforming the sector.
•
The development of a product that will allow more cost effective planning for the sector,
resulting in better run factories with the potential to become more profitable.
•
An assessment of the extent of compliance with ISO 14000 and Chemicals Management
Action Plans (CMAP) to provide an indication of the protective measures currently provided to
workers in this sector.
•
To provide an indication of the varying degrees of training required for general worker safety
awareness, the use of personal protective gear, correct operational procedures, improving the
state of poor housekeeping and to rectify poor administration.
1.1.1
Industry overview
It is often necessary to use metals or metal alloys which have certain surface properties, e.g.
resistance to corrosion, hardiness, high temperature tolerance, etc. Generally, no one metal
will possess all the necessary properties. For example, steel has many good properties. It is
inexpensive, abundant and a strong material that can easily be worked into all manner of
shapes and forms. However, its major flaw is poor resistance to corrosion, causing it to rust
severely in damp atmospheres. It needs to be coated with another material to provide
corrosion resistance. The term metal finishing refers to a range of techniques that treat metal
surfaces for its intended purpose. Metal Finishing enables the engineer to combine the good
mechanical properties of one metal with the desirable surface properties of another.
1
Electroplating thus offers the engineer a wide choice in combining various metals in
determining a specific objective.
Currently, metal finishing operations take place in two broadly defined divisions:
1. The jobbing shop sector (or commercial finishers): This sector includes
businesses that offer metal finishing services to companies that elect not to install an
in-house finishing capability to deal with their finishing requirements.
2. In-house finishing operations: It is possible that this sector has experienced growth
since the last NATSURV, particularly amongst OEM (original equipment
manufacturers) and second tier automotive component manufacturers (SAMFA,
2013). Some of the reasons for this growth are:
•
•
•
Inventory control is less complicated as parts are not transported to off-site
locations;
The manufacturer is able to concentrate his resources on dealing with specific
component configurations and finishes; and
The manufacturer is better able to manage finishing quality control, with no
third party involvement
Even with a national survey, an estimate of the number of metal finishing installations in South
Africa is difficult to quantify as a significant percentage of the work is performed in-house as
an essential part of an overall manufacturing activity. The remainder of the metal finishing
work is undertaken on a contract basis by specialist companies that include some large and
dominant companies along with a greater number of smaller operators. Not all of the latter
necessarily adhere to desirable industry standards, local and national legislation.
1.2
THE OVERALL OBJECTIVES OF THE PROJECT
The project objectives as formulated in the Agreement with the Water Research Commission
were to:
1. Provide a general overview of the metal finishing industry in South Africa, its changes
since 1980 and its projected change.
2. Evaluate and document the generic industry processes
3. Determine the water consumption and specific water intake
4. Determine the wastewater generation and typical pollutant loads
5. Determine local electricity, water and effluent prices and by-laws within which these
industries function.
6. Critically evaluate the water (inclusive of wastewater) management processes
adopted and provide recommendations
7. Evaluate the industry adoption of the following concepts: cleaner production, water
pinch, energy pinch, life cycle assessments, water footprints, and ISO 14 000 to name
a few.
8. Provide recommendations for best practice
The major findings obtained in meeting the abovementioned objectives are described in this
report in the traditional format of previous NATSURV documents. Other information which
2
could support this document, but cannot necessarily be included within the structure of the
NATSURV document have been completed as supporting documents and will be discussed
further in the ensuing sections.
1.3
DESCRIPTION OF THE RESEARCH PRODUCTS
In the original proposal one major research product and two supporting documents were
envisaged for this Project:
•
NATSURV 2 : Water and Wastewater Management in the Metal Finishing Industry
(Ed 2)
•
An overview of the status quo of the metal finishing sector in South Africa
•
An overview of source reduction methods which are or can be employed in the metal
finishing sector.
The project yielded three deliverables in the form of reports:
•
Ally, S.H., Kamish, W. and van der Spuy, A. (2015). NATSURV 2 : Water and
Wastewater Management in the Metal Finishing Industry (Ed 2). WRC Report No.
K5/2224, Water Research Commission, Pretoria.
•
Ally, S.H., Kamish, W. and van der Spuy, A. (2015). Status Quo of the Metal Finishing
Sector in South Africa. WRC Report No. K5/2224, Water Research Commission,
Pretoria.
•
Ally, S.H., Kamish, W. and van der Spuy, A. (2015). Source Reduction Measures
Employed in the Metal Finishing Sector. WRC Report No. K5/2224, Water Research
Commission, Pretoria. (This report)
The major findings from the survey on industries in the metal finishing sector are described in
this report. The updated NATSURV 2: Water and Wastewater Management in the Metal
Finishing Industry (Ed 2) can provide support to professionals in the plating industry, officials
of the DWS as well as practitioners who supply support services in the field of cleaner
production in the metal finishing sector.
The supporting documents provide a more detailed account of the status quo and the extent of
implementation of cleaner production (CP) technologies in the metal finishing sector. Once
again, the documents can be used by the sector as well as those providing support services to
the sector as a point of departure for what is practically possible in term of CP implementation.
3
2.
WASTEWATER TREATMENT IN THE METAL FINISHING SECTOR
2.1
WASTEWATER TREATMENT OF POTENTIAL RELEASES TO THE ENVIRONMENT
Constituents of concern which are potentially released during metal finishing processes include
metals, organic material, cyanides, hypochlorite, chlorine, AOX, surfactants, complexing agents,
acids/alkalis, ions, solvents, dusts and general wastes. The emission of pollutants in wastewaters and
the production of waste are considered more significant than emissions to air.
Since most surface treatments by metal deposition are water-based, significant quantities of water is
used as well as copious amounts of wastewater is generated. The most effective method for
preventing pollutants entering the water environment is in the minimisation of the loss of materials,
where these materials are lost by drag-out into rinse waters. Furthermore, wastewater treatment
serves to remove the constituents of concern before discharge into the surrounding water bodies.
These treatments include neutralisation and precipitation, oxidation and reduction, filtration, absorption
techniques, crystallisation, atmospheric evaporation, vacuum evaporation, electrolysis, ion exchange,
electodeionisation, acid sorption, membrane filtration, reverse osmosis, diffusion dialysis, membrane
electrolysis and electrodialysis. These common techniques are listed in Table 1 below.
Table 1 Typical treatment regimens for effluent treatment
Treatment type
Oxidation
Description of treatment
•
•
•
•
Reduction
•
•
Neutralisation and
precipitation
•
•
•
Filtration
•
•
•
•
Absorption techniques
•
Crystallisation
This is specifically for cyanides
Cyanides are oxidised to cyanates or completely
destroyed using sodium hypochlorite (NaOCl) or calcium
hypochlorite [Ca(OCl)2]
The process is usually batch processing
Wastewaters containing hexavalent chrome are treated
with sodium bisulphite or sodium metabisulphite
This produces a trivalent chrome compound which is
treated to produce chromic hydroxide which precipitates
out as sludge
Caustic soda is usually used to produce nickel hydroxide,
copper hydroxide and sodium sulphate.
The hydroxides precipitate out and form sludge
The operation is at pH 8.5 to 9.5 so that other metals
present also react and precipitate out
Sand filters are used for cleaning raw water or polishing
effluents
Belt filters or filter presses are used with higher solids
applications such as wastewater sludges, often in
conjunction with coagulants
The filter medium with the filtrate is usually disposed of as
a waste
Activated carbon is used to adsorb unwanted organic
substances formed from breakdown products in a solution
Activated carbon will also remove a portion of the useful
organic chemical additives (e.g. brighteners), which will
need replacing
The absorbent material along with the retentate and filter
medium is usually disposed of as a waste, although
precious metals may be recovered
Various evaporation and cooling systems are used to bring
solutions to a super-saturation point where solid crystals form and
can be separated from solution
4
Treatment type
Description of treatment
•
Atmospheric evaporation
•
Atmospheric evaporation occurs when solution are
heated. It reduces the volume of process solutions and
allows drag-out to be returned of fresh chemicals to be
added to the process solution
Evaporators are often used with a condenser to recover
distilled water
Vacuum evaporation
Reduced pressure and elevated temperature combine to separate
constituents with relatively high volatility from constituents with
lower volatility
Electrolysis – plating out
Transition metals can be removed from wastewater streams by
plating out on high surface area electrodes in metal recovery cells
Electrolysis – oxidation
It is possible to oxidise both unwanted organic by-products and
metals in solutions, such as trivalent chromium to hexavalent
chromium
Ion exchange – resin
Ions in solution are selectively removed by exchanging positions
with resin-functional groups
The direct ion exchange treatment of wastewater provides a
means of concentrating multivalent cations for subsequent
treatment on column regeneration or by plating out
Water from ion exchange can be recycled (Dahl)
Electrodeionisation
Ions are removed using conventional ion exchange resins
•
Acid (resin) sorption
•
•
Ion exchange – liquid/liquid
•
•
•
•
•
Membrane filtration
•
•
•
Acid sorption is configured similarly to ion exchange.
Resins are designed to selectively adsorb mineral acids
while excluding metal salts (adsorption phase)
Purified acid is recovered for re-use when the resin is
regenerate with water (desorption phase)
Ionic contaminants are removed from process solutions
into immiscible primary liquid extraction solutions
Secondary liquid extraction solutions are sued to remove
the contaminants and to regenerate the primary extraction
solution
Water from ion exchange can be recycled (Dahl)
Membrane filtration can be used for the purification and
recirculation of oily water (Dahl)
Various types of membrane filtration exist that are
dependent on the pore size
Microfiltration is a membrane filtration technology that
uses low applied pressures with pore sizes in the range of
0.02 to 10 microns, to separate relatively large particles in
the macromolecular and micro particle size range
Ultrafiltration passes ions and rejects macromolecules of
0.005 to 0.1 microns and removes organics from process
solutions
Nanofiltration is used for larger size rejection reverse
osmosis (rejects molecule larger than 0.001 to 0.008
microns) – for partial desalination of rinse water, removal
of aluminium from pickling baths and concentration of
chromating chemical in rinse water
Purified water can be re-used for the degreasing bath or
rinsing
5
Treatment type
Description of treatment
•
Reverse osmosis
•
•
•
•
Diffusion dialysis
•
•
Membrane electrolysis
•
•
Electrodialysis
•
Reverse osmosis is effectively a filtration of ions through a
semi-permeable membrane at high pressure which
desalinates chemically treated wastewater
It provides an alternative means of concentrating metal
impurities for subsequent removal
This approach can be capital intensive and any solids or
organics have to be removed prior to treatment
The chemicals from zinc-, chrome- and copper plating
baths can be re-circulated
Diffusion dialysis is a membrane separation process that
typically uses an anionic exchange membrane to transport
acid anions and protons from waste acid solutions into
deionised water streams
The anions and protons are treated in wastewater
treatments plants and the acid is recovered
Membrane electrolysis used one or more ion-selective
membranes to separate electrolyte solutions within an
electrolysis cell
The membranes are ion-permeable and selective
Anions and cations are removed from solutions with an
applied electric field in cells with alternation anion- and
cation-permeable membranes
Various acids from electrolyte solutions can be recycled
The possible treatment(s), specific to each constituent of concern can now be listed (Table 2).
Table 2 Possible treatment(s) specific to each constituent of concern
Constituent of concern
Immiscible organics:
Non-halogenated (oils, greases,
solvents) and halogenated (oils,
degreasing solvents, paint solvents)
Possible treatment
•
Reduced to solubility limit by physical separation
(e.g. flotation) or by liquid/liquid phase
separation
Followed by either air stripping (activated
carbon) or oxidation to carbon dioxide (using UV
irradiation and hydrogen peroxide addition)
Soluble organics increase the difficulty in
removing metals by flocculation
Concentration may be reduced by oxidation (by
UV irradiation and hydrogen peroxide addition)
Dissolved organics increase COD; biological
treatment may be necessary
pH adjustment is usually required
pH may be partially neutralised by mixing with
other streams
May be removed by settling or filtration
Filtration uses a filter- or belt press to produce a
cake manageable as a solid
•
•
Soluble organics:
Wetting agents, brighteners, organic
ions and ligands
•
Acids and alkalis
•
•
Particulate material:
Metal hydroxides, carbonates,
powders, dusts, film residues,
metallic particles
•
•
•
•
Metals
For soluble anions the capture of precious
metals for re-use, e.g. platinum, gold, silver,
rhodium and ruthenium may be achieved by
electrochemical recovery or ion exchange
In some cases it may be necessary to reduce the
oxidation state of the metal ion as the higher
oxidation state may not be readily flocculated
and precipitated by pH change
•
6
Constituent of concern
Possible treatment
•
Complexing agents:
Sequestering and chelating agents
Nitrogenous materials:
Ammonia, Nitrites
Cyanides
Sulphide
Fluorides
Phosphate compounds
Other salts
Multivalent ions are most conveniently removed
by precipitation and pH adjustment
• May be removed by precipitation with the
predecessor of activated carbon process
• Microbiological oxidation is also a possibility for
removal of complexing agents
• Ammonia can be removed by steam stripping or
by oxidation to nitrogen with sodium hypochlorite
• Nitrites can also be oxidised with sodium
hypochlorite
• Note that AOX may be formed when using
hypochlorite solutions
Cyanide from degreasing may be oxidised by using
hypochlorite or chlorine gas at high pH
When in excess sulphide can be precipitated out as
elemental sulphur on oxidation with hydrogen peroxide or
iron (III) salts.
Is readily precipitated out as calcium fluoride at a pH
above 7
Precipitated out as calcium hydroxide phosphate
• Other ions such as Cl-, SO42-, K+, Na+ and Ca+
• Sulphate can be readily precipitated as calcium
sulphate
• Ion exchange, reverse osmosis or evaporation to
remove other ions
Sludge waste is produced when effluent from metal finishing operations is treated to create insoluble
metal compounds like metal hydroxides or carbonates. These precipitate to create a watery sludge
which needs to be dewatered by various methods such as filtering through a filter press, belt press or
centrifuge to produce a cake manageable as a solid. When operating at pressures above 15 bar the
final cake can have 15 to 35% solids. The filter can then be dried further to lower the water content.
Some waste solutions can be disposed of as liquid- or hazardous wastes, or can be recovered or
recycled. Examples of wastes that have the potential to be recovered or recycled include autocatalytic
plating, spent etchants and sludge from anodising.
Further steps to abate potential releases to the environment include cleaner production initiatives,
which aim to minimise the use of water and material discharged from the processes. It is important to
note, however, that the minimisation of water usage can increase the concentration of dissolved salt
and various metals, which increases the solubility of metals in the wastewater. Further, it can become
difficult to maintain a stable pH in the narrow margins required to minimise the solubility of individual
metals when dealing with a mixture. This suggests that when dealing with a mixture of metals it will
become increasingly difficult to optimise every parameter.
7
3.
CLEANER PRODUCTION INITIATIVES FOR THE METAL FINISHING
SECTOR
3.1
MANAGEMENT AND HOUSEKEEPING
The most practical way of improving plant operation to reduce the wastewater generated is by
improving management and housekeeping.
The best environmental performance is usually achieved by the installation of the best technology and
its operation in the most effective and efficient manner. An Environmental Management System
(EMS) is a tool that operators can use to address design, construction, maintenance, operation and
decommissioning issues. Environmental management systems typically ensure the continuous
improvement of the environmental performance of the installation. Components of the EMS typically
include:
•
•
•
•
•
•
•
•
•
•
A definition of the environmental policy
Planning and establishing objectives and targets
Implementation and operation of procedures
Checking and corrective action
Management review
Preparation of a regular environmental statement
Validation by certification body or external EMS verifier
Design considerations for end-of-life plant decommissioning
Development of cleaner technologies and
Benchmarking
These factors are summarised in Table 3.
Furthermore, if workpieces are treated incorrectly, in terms of incorrect specification or faulty
application of the correct specification, the outcome will be significant amounts of metal stripping or
scrapping of the workpiece. Implementation of a Quality Management System (QMS) can reduce the
reworking and scrap. Avoiding rework minimises losses in raw material, energy and water inputs, as
well as minimising wastewater treatment and the generation of sludge and liquid acid wastes.
Table 3 Typical components of an Environmental Management Plan (EMS)
Components of EMS
Description
•
Definition of environmental
policy
•
•
Planning
Implementation and
operation of procedures
•
Top management are responsible for defining an
environmental policy that includes a commitment
pollution prevention and control and to comply with all
relevant applicable environmental legislation
The policy must provide a framework for setting and
reviewing environmental objectives and targets, must be
documented and available to all interested parties
Procedures to identify the environmental aspects of the
installation and the legal and other requirements to which
the organisation subscribes
Establishing and reviewing documented environmental
objectives and targets, and establishing and regularly
updating an environmental management programme
Effective environmental management must ensure that
procedures are known in areas of
• Structure and responsibility
8
Components of EMS
Checking and corrective
action
Description
•
•
•
•
•
•
•
Training, awareness and competence
Communication
Employee involvement
Documentation
Efficient process control
Maintenance programmes and
Emergency preparedness and response
•
Establishing and maintaining documented procedures to
monitor and measure key characteristics of operation
and activities
Establishing and maintaining procedures for defining
responsibility and authority for handling and investigating
non-conformance with permit conditions
Establishing records, EMS audits and periodic evaluation
of legal compliance
•
•
•
Management review
•
Reviewing the EMS at regular intervals to ensure its
suitability, adequacy and effectiveness
Ensure that necessary information is collected to carry
out evaluation and documentation of the review
Preparation of a regular
environmental statement
This statement compares the results achieved by the installation
to the environmental objectives and targets
Validation by certification
body or external EMS
verifier
Having the EMS validated by an accredited certification body or
external EMS verifier to enhance the credibility of the system
Design considerations for
end-of-life plant
decommissioning
Giving consideration the environmental impact from the eventual
decommissioning of a unit
Development of cleaner
technologies
Giving consideration to the development of cleaner technologies
that incorporate techniques at the earliest possible design stage
to be more effective and cheaper
Benchmarking
Carrying out systematic and regular comparisons with the sector
on a national or regional scale
Other management techniques include a reduction in reworking by process specification and quality
control which minimises wastewater treatment and the generation of sludge and liquid acid wastes. A
theoretical optimisation of the process line improves the consumption of water, energy and
conservation of raw materials, as well as minimising emissions to water. Real time process control
systems that collect data and react to maintain predetermined process values improve plant efficiency
and product quality as well as lowering the emissions.
9
4.
BEST PRACTICES APPLICABLE TO JIG AND BARREL PLATING
OPERATIONS
4.1
REDUCING DRAG-OUT
•
Better positioning of work-pieces on racks: parts should be placed on the rack so that the
largest surface or plane is nearly vertical and the longer dimensions are horizontal. The lower
edge should be inclined, so that run-off will occur at a corner rather than the entire edge.
Avoid positioning parts directly over one another where possible.
•
Optimise the design of racks: The rack should be positioned/inclined so that horizontal
surfaces on the rack are minimised.
•
Better operation of barrels: Barrel holes should be kept clear to permit maximum drainage.
Make sure barrel doors face upwards during withdrawal (usually less holes on doors).
Intermittently rotate the barrel during draining and hold for 10 seconds in each position of
rotation
•
Slower work-piece withdrawal from tanks. The faster the work-piece is withdrawn from the
process bath, the thicker the film on the work-piece surface and the greater the drag-out
volume. The effect is so significant that most of the time allowed for draining a rack could
instead be used for withdrawal only. This is obviously easier to achieve with electrically
operated systems than manual systems.
•
Drainage time. Allow as much time as is feasible to drain above the process tank, except in
the case of processes that require a rapid cessation of a reaction on the surface, such as
passivation. Some guide values for minimum withdrawal and drainage times are:
•
Choice of barrel: Use barrels with as high a proportion of the surface covered in perforations
as is possible. The size of the holes in a barrel are important. If one is plating fine pins, the
holes have to be small. This also means above-tank drainage will be slow. It also reduces
plating speed. Using the same barrel for M20 bolts and nuts is inadvisable because such
small holes are unnecessary.
•
Spray rinses can be installed over heated process tanks that will remove drag-out and make
up for evaporative losses. These rinses can be automated to only be in operation when the jig
is being lifted, or for manual systems a switch with a timer could be used.
•
Choice of solution chemistry. The higher the concentration of the solution, the larger the
amount of drag-out (in terms of volume and concentration). Experiment to establish if it is
possible to achieve the desired results with a less concentrated solution.
•
Wetting agents can also reduce drag-out. Ensure that the level of wetting agent in the
solution is at the optimum level.
10
•
4.2
Operating temperature. A higher operating temperature will reduce drag-out as viscosity will
be lower and create space for the effective use of dragout solution. This has to be off-set
against increased energy costs.
IMPROVEMENTS IN RINSING
•
Where there is no space or capital available for additional rinse tanks, a simple but very
effective improvement in rinsing is to dip twice in the same rinse tank.
•
Use drag out tanks effectively (also referred to as static rinse tanks.) These are rinse tanks
without a flow of water. Water from the dragout tank can be added back into the process tank
to make up for evaporative losses. They must be used in combination with running rinses. For
hard chrome plating solutions in particular and other concentrated heated solutions like acid
nickel plating, two, or even three dragout tanks can be used.
•
Use the drag out tank in both directions.
Use the dragout tank as the pre-dip before parts go into the process bath as well as when they
come out of the process bath. This is because clean rinse water clinging to parts will be
carried into the dragout tank from the proceeding rinse, diluting the drag out slightly. Then at
the next step dragout solution will be carried into the process tank, effectively replenishing it.
After the process tank the jig (or barrel) is returned to the dragout tank, increasing its
concentration. This will help stabilise the concentration of chemicals in the dragout tank at
approximately 50% of the bath concentration.
This method is particularly suitable for ambient process tanks as it also prevents the volume
from growing, stopping the return of solution to it. This system is sometimes referred to as an
eco-rinse. If preferred an additional tank can be included prior to the process tank, and
coupled to the dragout tank in such a way that the dragout is circulated through this “drag-in”
tank, effectively equalising the strength of the “drag-in” and dragout tank. This eliminates the
necessity for the jig to have to overshoot the process tank to be immersed in the dragout tank
before returning to the process tank. We cannot refer to this “drag-in” tank as a pre-dip or
conditioning step which still has to be included in the line. As an example, for nickel plating a
weak solution of sulphuric acid, a so-called “sour dip” has to be present before the plating
process tank to condition the surface and combat any slight oxidation that may develop as the
jig travels towards the process tank. Creating a “drag-in” tank rather than simply overshooting
the process tank, immersing into the dragout tank and then back-tracking to the process tank,
will add the cost of one extra tank to the line (including the cost of extra space, overhead lift
equipment, plumbing, etc.).
4.3
CONTROLS ON RINSE WATER FLOWS:
•
Flow restrictors on flow-through rinses. These simple mechanical flow control valves give
constant flow independent of pressure and are very cheap. This eliminates variations in water
flow rate arising from water line pressure changes or operators adjusting valves in error.
•
Control valves on rinse water flows with timers delivering a fixed amount of water triggered
by programming or a switch.
11
•
Conductivity control in the flow-through rinse tank. This will deliver fresh rinse water only
when a preset level of conductivity, indicating contamination, has been exceeded.
•
Counterflow rinsing. If there is room for more than one rinse tank set up a counterflow rinse
system, i.e. have the fresh water entering the last tank that the work-pieces go through. Let
the overflow from this last tank be the feed for the earlier rinse tank. A two-stage countercurrent rinse system will drastically reduce the amount of water needed to clean a part to the
same degree of cleanliness – water reduction can be 90% or more. All new plant builds should
incorporate counterflow rinsing.
4.4
PREVENTING “BACK-MIXING” OR “SHORT-CIRCUITING”
The diagram below demonstrates a design that prevents the phenomenon of “back-mixing” or “shortcircuiting” in a counter flow rinse system.
When the barrel is immersed in Tank 1 is should not displace enough water to rise up and cascade
into Tank 2, as this will work against the counter current principle. Water should naturally flow from
Tank 2, the cleanest water, to Tank 1, the first port of call, with the most contaminated water. If water
from Tank 1 contaminates the water in Tank 2, then the system is compromised and will not deliver
the theoretical results.
Flow of Water
Immersed Plating Barrel
Baffle
1
2
Rinse
Water
In
Flow of Work
Figure 1
Design for preventing "back-mixing" in tanks
•
Closed loop counterflow rinsing. With a multiple counter-current rinse tanks it may be
possible to reduce rinse water flow such that the rinse water can be used to make up for
evaporative losses.
•
Spray rinses. Spray rinses can be installed over rinse tanks. These act like a second rinse
step. These rinses can be automated to be activated only when the jig is being lifted. For
manual systems a switch with a timer could be used. It is also possible to design a process
tank to incorporate a spray manifold length wise along the top of the tank on both sides. This
may be activated to direct a fog spray onto the workpieces, rinsing dragout directly back into
the process solution as soon as the jig moves past it on removal. The system is ideal for hot
process solutions, and for parts that do not have recessed or internal surfaces that the spray
cannot access. It is not easy to retrofit as a bit of extra freeboard is needed at the top of the
tank, and this also affects the distance that the carrier device has to lift and lower at that
station.
12
•
An overall master control valve on the main water supply to the finishing process with a
switch easily accessible to the operator for stopping water loss during breaks, over nights and
weekends. This avoids having to change settings on individual valves on tanks.
•
Air agitation. This keeps the rinse tank stirred and also has a scrubbing effect in removing
process solution from parts.
•
Photo sensor on an automated line. This can detect when dripping stops.
4.5
MINOR PROCESS BATH IMPROVEMENTS
Some relatively inexpensive modifications can be carried out on existing process baths:
4.6
•
Drain boards. These can be installed between tanks and angled to allow drips to return to the
process bath when parts are transferred between tanks. They also help to keep the area in
and around the tanks clean.
•
Drip trays. These can be inserted either manually or automatically under the parts suspended
from the jig to prevent contamination of other process baths when the jig is being moved.
IMPROVE MAINTENANCE PROCEDURES
Routine removal of sludge, soils and other contaminants from baths should be carried out. Most
plating systems benefit from continuous filtration which prevents sludge build up. However, in the
event that this option is considered too costly, then periodic de-sludging and filtration should be carried
out. Oil can be removed from process baths and degreasing solvent or solutions by skimmers, gravity
separators and centrifuges. Ion exchangers can be used in some applications to remove various
contaminants (for example, removal of copper/zinc/nickel contaminants from trivalent chromium
plating baths).
Certain salt contaminants can be removed by cold crystallisation – the bath is cooled down and the
salts precipitate out (such as sodium carbonate from plating baths that use caustic). Such a process
can be incorporated into a normal shut-down when baths will cool anyway.
Carbon filtration of organic contaminants can be achieved by either adding carbon to the process bath
and removing it in the filter or else having a carbon filter installed in the same line as the solids filter
(for example, removal of organic contaminants which can cause dullness in the finish from a nickel
plating bath).
Minimising drag-out means that more systematic methods for regular removal of contaminant build up
will be required. This is because drag-out does assist in the removal of contaminants (along with the
valuable process solution).
Maintenance of racks/barrels to prevent products of corrosion entering the process baths:
Check barrels regularly to see that holes are not being closed off through the tumbling action during
processing. Any holes in polypropylene barrels that are starting to close off can be drilled back to
normal size.
13
Periodic checking of jig insulation (if present) for cracks which would trap process solution.
4.7
PROCESS CONTROL
The process solution should be regularly checked to see that it is maintained at the desired strength.
The correction of solution strength by making small and frequent additions is much more effective than
making a few large additions.
An accurate log of all checks and additions should be kept.
Make use of chemical suppliers to advise on addition rates and which parameters should be used for
control purposes.
A simple Twaddel or Baumé hydrometer test will easily identify whether a solution has become
severely weakened as a result of solution loss or depletion through lack of anodes. The test will work
on many solutions including acid copper, acid nickel, decorative and hard chrome solutions, etc. It
does not work on solutions that build up carbonates or that become loaded with dissolved metal.
Conductivity probes may assist in maintaining a constant concentration in the case of some process
solutions.
Use a deioniser on your water supply for additions to process baths as well as for rinse water used in
static baths and in closed loop rinses. This will help to reduce contaminant build-up. Standard
deioniser packages can be fairly easily incorporated into your water supply. It should be noted that
deionised water is not recommended for the rinsing step directly after nickel and before chrome plating
as it can have the effect of passivating the nickel surface, hampering chrome receptivity.
4.8
PRODUCT STORAGE
Try to store products prior to processing away from the humid and acidic conditions of processing
areas for as long as possible. This will reduce corrosion formation. Corrosion on products might need
additional cleaning before they are processed.
14
4.9
SOLVENT AND ALKALINE DEGREASING SYSTEMS
•
Reduce or Eliminate the Need for Cleaning
The subsequent processes a work-piece is to be subjected to will have a bearing on what degree
of cleaning is required. Careful storage may reduce cleaning requirements. Protective packaging
may help reduce the amount of cleaning required. This does need to be considered against the
additional resource use as a result of such packaging. Reusable packaging or coverings may be
applicable.
•
Use Alternative Cleaning Processes
If you operate a degreaser or use solvent to clean parts by cold soaking, the need to use
these processes in every case should always be assessed. In many cases they are used all
the time just because they are there. Consider the following alternatives for cleaning:
•

Carry out some initial manual cleaning with dry rags or paper for larger parts prior to use
of the degreaser. This will extend the degreaser’s working life (although it will generate
waste soiled rags/paper). Mechanical cleaning such as power wire brushing or shot
blasting may be applicable.

Use water-based solutions. There are a wide variety of cleaners available for different
types of soiling, different types of substrate and for the different types of follow on
processes. Work-pieces are soaked in the solution. The solutions do require regular
replenishment. Exhausted solutions will need to be either treated or properly disposed.
Specialised equipment is available for using such solutions but is not always necessary.
Some such equipment would be relatively simple and inexpensive. Some have integral
brushes and heating to assist the cleaning process.
Use Eductors in the Alkaline Cleaning Tank
Eductor nozzles use the venturi principle to amplify
and direct solution flow from a pump to the
required area of the tank. For 1 litre of solution
pumped through an eductor at required pressure,
discharge flow from nozzle will be 5 litres.
Therefore it becomes economically viable to have
a smaller pump perform agitation!
Figure 2
15
Schematic drawing of an
Eductor
Figure 3
Operation of Eductors
1. Vigorous solution agitation focussed around, across or directly at work and sweeping the tank
floor
2. More efficient cleaning, lower rejects without air
3. More uniform temperature
4. Better heat transfer, possibility to lower temperature and reduce heating costs
5. Improved cleaning efficiency of brass sand castings, eliminating roughness rejects after
plating
6. Ability to lower cleaner temperature from 80°to 60° whilst maintaining cleaning efficiency.
Achieving very positive heat savings up to 40%.
Figure 4
•
Diagram demonstrating the setup of an eductor array in a
cleaning tank
Biological Cleaning Systems.
Use a biological degreasing system. Specialised equipment is needed. Uses microorganisms
to break down the oil and grease on the surface of work-pieces. These are closed systems so
only very small amounts of sludge need occasional disposal. Only very small amounts of
cleaning chemicals need to be added occasionally. It has the advantage of there being no oil
or solvent for disposal. The chemistry may not be suitable for all types of oil. Such systems
are commercially available.
16
•
Ultrasonic Cleaning
Ultrasonic cleaning can be used to enhance cleaning systems, but specialised equipment is
needed. Usually this is considered to be an expensive option for anything but the smallest of
operations such as manufacturing jewellers, or manufacturers of costume jewellery, fashion
trim and the like. An ultrasonic cleaning unit is tank that has an attached transducer capable
of generating sound waves. High frequency sound waves are transmitted through the cleaning
solution, causing formation and collapse of small vapour bubbles at the solid surface, called
micro-cavitation. This agitation assists in the removal of soiling, especially in hard to reach
areas. Hence the efficiency of the cleaning process is enhanced. Tanks are usually operated
with aqueous and other non-volatile media.
4.10
IMPROVE THE OPERATION OF CONVENTIONAL VAPOUR DEGREASERS
The use of solvent degreasing is strongly discouraged in the modern environmentally conscious world.
If it is considered that it is absolutely necessary to use a solvent degreasing system, then there are a
number of operational procedures that should be observed to minimise the loss of vapour from
conventional vapour degreasers. When inserting or withdrawing components avoid speeds greater
than 3 metres per minute to minimise the amount of solvent vapour lost. The use of a power-operated
hoist is recommended for all degreasing plants to control the speed of entry and exit of the workpieces.
Avoid heavy loads that will result in the collapse of the vapour blanket and infiltration of air into the
unit. This solvent saturated air will then be expelled when the vapour layer re-establishes. Ensure that
the parts have reached the temperature of the vapour before removal so that condensation has
ceased. Look to see that there is no liquid on the parts.
Rim ventilation should be high enough to protect operators, but excessive extraction results in
unnecessary solvent consumption. An extraction rate of 640-915 m3/hr per m2 of bath surface is
recommended. For any degreaser with a specific rim vent slot design, extract fan specification and
ductwork configuration, there will be a specific rim vent slot velocity. Some users may find it easier to
check this measurement rather than the total volume of air extracted. The degreaser supplier should
be contacted for the appropriate figure.
To ensure the degreaser functions efficiently, the solvent temperature should be maintained at a level
adequate for vapour production. Improvements that can be made to conventional vapour degreasers
that will reduce solvent emissions are as follows:
Locate the degreaser away from drafts or use baffles to prevent upset. Proper location can reduce
solvent loss by up to 30%. Features that create air currents that can disturb the vapours include doors,
windows, heating and ventilation systems and busy passages.
Install support frames within the condensation zone to allow work that is mounted on jigs to be
supported while degreasing is in progress. This enables the lifting device to be raised and the lid
closed over the work during the degreasing process.
Install covers to eliminate drafts and reduce diffusion. These lids should be fitted below the rim
ventilation slot, otherwise this can allow the extraction system to pump the system dry, and should be
17
fitted at the top of the freeboard zone. A roller or a slide design should be used rather than lift-out
panels as this is less likely to disturb the vapour. Double-door systems can be used, which are more
expensive to install, but reductions in solvent consumption of up to 80% have been claimed. Such
systems can have timed interlocks. The use of baskets having an area less than 50% of the
degreaser opening will limit vapour dragout due to the piston effect.
Increase freeboard height. A freeboard ratio (freeboard height divided by the width of the tank) of at
least 0.75:1 and preferably 1:1 is recommended.
Install fixed pipework, connected to the sump, for topping up with new solvent, rather than manually
pouring new solvent into the degreaser from drums or buckets.
Install refrigerated coils, which will condense solvent from the air leaving the degreaser and return it to
the unit.
A rapid-response temperature sensor installed immediately below the condensation zone acts as an
energy saving device. The sensor cuts the energy input to the sump in response to the vapour
temperature. The cooling effect of the new load being placed in the unit reactivates the main heating
system.
4.11
USE A LOW-EMISSION VAPOUR DEGREASER
The Low-Emission Vapour Degreaser, by contrast to conventional degreasers, is a completely sealed
unit. It incorporates both refrigerated condensation and carbon adsorption/desorption to trap and
regenerate solvent.
Alternative solvents that are less hazardous are available which could be used instead of chlorinated
solvents and aromatic hydrocarbons. These include terpenes, aliphatic hydrocarbons, alcohols, esters
and glycol ethers.
4.12
•
PLATING
Electrolytic Metal Recovery
Electrolytic recovery of metals from static or closed loop rinse systems can be used for
precious metals as well as nickel, copper, zinc, and chromium. Commercially produced
packages are available in different sizes. Electrolytic recovery is usually used as one step in a
treatment system as recovery from very dilute solutions cannot be achieved using this
method. A heated solution is desirable to overcome electrode polarisation and low diffusion
rates.
Mechanical mixing is also necessary to improve plating, this is achieved by a moving or
rotating cathode or a high solution velocity over fixed cathodes. In some cases a high surface
area is used where the metal is deposited onto a fibrous or filamentous substrate. This can be
sold as a low volume residue or the deposited metal can be stripped chemically or
electrochemically so that the end result is a concentrated solution of the metal which is
recovered for reuse.
18
If inert anodes are used the metal can be reused as anode material. These anodes can be
reused several times as the inert base metal will not be dissolved during plating. The process
also has the advantage of destroying any cyanide in the solution in parallel with the electrolytic
recovery of the metal.
•
Alternative Plating Processes
The use of alternative coatings that will provide the same degree of protection, decoration, etc.
should always be assessed. Developments in the area of alternative plating processes were
accelerated by Directive 2002/95/EC on the restriction of the use of certain hazardous
substances in electrical and electronic equipment (ROHS Directive). This Directive required
the removal of lead, mercury, cadmium, and hexavalent chromium in such equipment, from 1
July 2006, except for certain applications where there are no feasible alternatives (for example
cadmium plating).
•
Non Cyanide Plating Baths
There are several alternatives available in relation to the replacement of cyanide plating baths:
•

Acid zinc plating in place of cyanide zinc plating. Has the added benefit of reduced energy
requirements. Needs good process control and work-pieces must be very clean for this
process.

Alkali cyanide-free zinc plating. Has disadvantage of a higher energy requirement. As for
acid zinc plating, this process needs good control and well pre-cleaned work-pieces. It has
been reported to achieve better metal distribution than cyanide plating.
Alternative to Hexavalent Chromium Plating Baths
Earlier problems with trivalent chromium as an alternative to hexavalent chromium plating
such as colour differences and variations have been overcome with newer products. Such
processes require a higher degree of control and some modifications will be needed to
equipment. Product quality is reportedly better than with hexavalent chromium plating.
Trivalent chromium cannot replace hard chromium processes. There are very few, if any,
operational decorative trivalent chromium systems in RSA at the time of the drafting of this
report. Despite the claims of manufacturers it would seem that industry operators have
decided that the system is more expensive to install and operate as it requires far more
sophisticated equipment and controls.
4.13
CHEMICAL AND ELECTROCHEMICAL CONVERSION COATINGS (CHROMATING,
PHOSPHATING, ANODISING, AND COLOURING)
Trivalent chromium conversion coatings are available as an alternative to hexavalent chromium
conversion coatings. These solutions can be used in existing equipment with only minor modifications,
so they are relatively easy to retrofit. There are chrome-free conversion coatings available but workpieces may need additional coating to provide corrosion protection.
19
4.14
ALTERNATIVE STRIPPING PROCESSES
There are alternatives available for cyanide-free nickel stripping solutions. Tank lining may
needed.
be
During the course of the survey several companies in the metal finishing sector were visited to
establish the current practices in terms of chemicals management regarding handling, processing,
storage and transportation with particular attention being focused on water usage and wastewater
practices. The sector types that were visited included:
• Electroplaters
• Powder coaters
• Chemical conversion
The information presented in the ensuing sections was abstracted by emailing the selected companies
with the questionnaire as well as conducting interviews with the appropriate staff members of the
various companies.
20
5.
DESIGN METHODOLOGY FOR COUNTERFLOW RINSING
Counterflow rinsing is renowned as a CP technique that reduces the consumption of water used by
any electroplating facility. The design methodology used for counterflow rinsing unit is discussed in
the ensuing sections which includes:
•
•
•
•
•
5.1
An evaluation of the mathematical theory used in counterflow rinsing techniques;
The list of the assumptions made for the design;
The list of model inputs and motivation for its chosen quantities;
The model outputs to be calculated; and
Other process parameters which may affect the design.
THEORY OF COUNTERFLOW RINSING SYSTEMS
Rinsing takes place by the thinning of the liquid layer on the surface of an object. The more times the
liquid layer is thinned the greater the cleanness of the object’s surface. Rinsing can be carried out in
one or more stages by dipping, flushing or a combination of both. A method which significantly
reduces the volume of water required for rinsing is the counterflow rinsing method (Hyder Consulting
and Hemsley 1999). In counterflow rinsing the water flows between the rinsing stages in a direction
counter to that of the object flow, which can be either continuous or discontinuous. In electroplating
the workpiece can therefore be dipped into the first tank (Rinse A) and then the second and so forth
until it exits the final rinse tank (Rinse D), while fresh water flows from the final rinse tank and is let out
of the first rinse tank (see Figure 5). The overflow from Rinse A has the highest metal concentration
and can either be sent to the wastewater treatment plant or be recycled to the process tank if this is
practical.
Figure 5
Counterflow rinsing (Hyder Consulting and Hemsley 1999)
The water required to effectively rinse workpieces can be improved by extending the rinsing process
with more rinsing tanks/stages. The freshwater consumption in counterflow rinsing is calculated from
the following formula.
Qn = D ⋅ n
C0
Cn
= D⋅n F
Equation 1
Where,
• n is the number of rinsing stages;
21
•
•
•
•
•
Qn is the consumption of fresh rinsing water (volume/time) when using counter-current rinsing
with n stages;
D is the drag-out from the cleaning vat (volume/time);
Co is the concentration of the constituent in the plating bath (mass/volume);
Cn is the concentration of the constituent in the nth rinsing tank (mass/volume); and
F is the dilution factor (dimensionless).
It is seen from Equation 1 that the amount of freshwater consumption (Qn) will decrease for a
decrease in drag-out (D), an increase in the number of rinsing stages (n), and a decrease in the
dilution factor (F). The dilution factor is dependent on the metal concentration that can be tolerated in
a rinse without having adverse effects on the rinsing quality (Van Der Spuy, 2004) while the drag-out
is dependent on the workload and dripping time of workpieces over the process tank. Since the
dilution factor has a limit that varies dependent on which process tank it serves, the freshwater
consumption can only be minimized by employing methods which reduce drag-out and by increasing
the number of rinsing stages.
The use of a five-stage counterflow rinse has been reported as an ideal situation design if a closed
loop rinse system is being considered but a cost benefit analysis show that for most requirements a
three stage rinse is most adequate and is therefore considered for the design of this CP initiative (Van
Der Spuy 2004). It is also very plausible that not all facilities have the available space or capital for a
5-stage unit. The required freshwater flow for a 5-stage configuration is expected to be low enough to
keep up with the drag-out and evaporative losses in the process tank so that the concentrated rinse
water (in Rinse A) may be recycled to the process tank. A closed loop system is therefore formed,
achieving zero discharge. This system is most applicable to chrome and nickel plating installations
where there is considerable evaporation.
A counterflow rinse with 3 stages is shown in Figure 6. Concentrations C1, C2, and C are the mixed
concentrations in Rinse tanks 1, 2 and 3 respectively. The mass conservation principle applied for
metal concentrations in the rinse tanks is as follows:
•
A workpiece coated with a solution of the original process solution (C0) mixes with the
overflow from Rinse tank 2 (C2) in Rinse tank 1 to achieve a final concentration,C1 An
similar approach would be applicable for determining the drag-out and tank concentrations
in the subsequent tanks.
Furthermore, since drag-out (D) and the flow of rinse water (Q3 for 3 stages) is assumed to be
constant over all the rinse tanks, the flow rate of rinse water between rinse tanks is determined as
Q3+D.
22
Co, D
C1, D
C2, D
Rinse tank
1
(C1)
Process
tank
C1, Q3+D
Figure 6
Rinse tank
2
(C2)
Rinse tank
3
(C3)
C3, Q3+D
C2, Q3+D
Counterflow rinsing with 3 stages
C0
With a predetermined value of the dilution factor F, C3 can be calculated as
concentrations can then be solved with simple mass balances:
Mass balance of metal over Rinse tank 3:
Q3+C2D=C3x(Q3+D)
Equation 2
Mass balance of metal over Rinse tank 2:
C1D+C3x(Q3+D)=C2x(Q3+D)+C2D
Equation 3
Mass balance of metal over Rinse tank 1:
CoD+C2x(Q3+D)=C1x(Q3+D)+C1D
Equation 4
5.2
F . All other metal
DESIGN MODEL ASSUMPTIONS
The design of this counterflow rinsing unit is based on the following assumptions:
•
•
•
•
•
•
5.3
The counterflow rinsing unit is intended for rinsing of workpieces that have just been
immersed in an electroplating bath
Only one electroplating solution enters the rinsing tanks;
The flows of drag-out across each tank are equivalent;
The overflows of rinsing water from each tank are equivalent;
Perfect mixing occurs in each rinsing tank ensuring that the concentration of solutions leaving
the tank are equivalent;
The flow rate of the overflow from the first rinsing tank does not significantly disturb the
process tank volume because it is compensated for by the sum of the evaporation- and dragout rate.
DESIGN MODEL BASIS
Based on the presented theory, a few parameters that affect the process are required to be predetermined before the three stage counterflow rinsing unit can be designed. These include
determining the drag-out rate (D), metal concentration of the process tank (C0), and dilution factor (F).
23
Q3
For this design, a drag-out rate of 1 ℓ/h has been assumed as a rough estimation based on a Danish
study (Dahl, 1997).
For zinc plating, the metal concentration of the process tank has been assigned a value of C0 = 55x103
mg/ℓ. This is the maximum zinc concentration that can be present in the process solution when an
acid zinc electroplating method is used (IPPC, 2006).
Finally, the required dilution factor is estimated at F = 1000, which is at the more concentrated end of
the proposed range for rinsing processes implemented after electroplating baths (Van Der Spuy,
2004). This means that the concentration required to effectively rinse the workpiece in the third tank
(C3) must be equivalent to one thousandth of the metal concentration in the process tank, i.e. 55 mg/ℓ.
5.4
DESIGN MODEL OUTPUT
The design model inputs can be used to determine the required freshwater flowrate (Q3) with Equation
1. The final concentration of the rinse exiting Rinse tank 1 (C1) can then be calculated using
Equations 2 to 4.
5.4.1
Additional design considerations
In addition to the aforementioned parameters there are also other considerations for the unit design,
namely equipment sizing, rinse tank operation, process control, drag-out control, and power
requirements.
5.4.2
Equipment sizing
Recommendations on the size of a rinsing tank should be based on the maximum expected size of the
workload. In most cases, the counterflow rinsing unit will consist of a single tank divided into a series
of compartments.
5.4.3
Rinse tank operation
If there is no way to monitor or control the flow of water through a rinsing system it would not make
any meaningful difference whether one operated a single static rinse or a three stage counterflow
rinse (Van Der Spuy, 2004). An effective system will therefore need to be used to ensure that the
rinse water is transported from the one tank to the next at a controlled flow rate. This can be achieved
by implementing a gravity feed whereby tanks are installed at different levels to provide the required
head. However, this is not always practical, for example, when auto lines are used to transport
workpieces with fixed travel on lifts. In that case, pumps can be used to circulate the water.
Furthermore, proper mixing of the drag-out and rinse water should be ensured.
5.4.4
Process control
To ensure efficient control of the rinse water flow, the flow rate should depend on and be adjusted
according to parameters that are monitored in the rinse tank. It is impractical to manage that which is
not measured.
5.4.5
Drag-out control
The drag-out rate is dependent on the workload and dripping time of workpieces over the process tank
from which it has emerged. However, where electroplating plants are manually operated, the drag-out
rate can fluctuate if the workpiece is not drained for a consistent length of time. It is therefore
24
imperative that consistent and effective drag-out reduction methods are implemented in conjunction
with a counterflow rinsing unit so that the unit may operate efficiently.
5.5
CALCULATIONS AND FINDINGS
5.5.1
Water consumption
Based formulas defined in previous sections the freshwater consumption rate (Q3) was calculated as
10 ℓ/h ignoring evaporation. If a single stage static rinse tank was used, 1000 ℓ/h freshwater would be
required for this design. Using a three stage counterflow rinsing unit therefore enables a 99% water
saving when rinsing workpieces that have been electroplated.
By solving Equations 2 to 4, the metal concentration rinse tank 2 can be determined. The results are
shown in Table 4.
Table 4 Metal concentrations in Rinse tanks 1 to 3
Tank number
Concentration (mg/l) Rinse tank concentrations of
of Zn
Zn (mg/l)
Process tank
55000
Tank 1
5887
10.7% of Co
Tank 2
541
0.98% of Co
Tank 3
55
0.1% of Co
The first rinse tank therefore has a zinc concentration of 5887 mg/ℓ, which represents 10.7% of the
initial zinc concentration in the process tank. Ideally, this rinse water should be concentrated before
adding back to the process tank thereby not diluting it.
5.5.2
Equipment sizing
On a line where components are transported either manually or automatically over the process line
tanks in sequence, the rinse tanks have to be sized to accommodate the same size load as the main
plating tank. As these tanks do not have to include anodes or heaters, they can be a bit narrower to
save on materials. For three stage counterflow rinses, 3 similarly sized tanks are required. These can
be individual tanks or a large tank divided into compartments.
On totally manual lines where individual smaller jigs are removed from the plating bath and the
operator physically transports them to rinses, one or two at a time, these tanks can be a lot smaller
than the process tank. Nonetheless, 3 equally sized tanks are required, usually comprised of one tank
divided into three compartments. The same applies on manual barrel plating lines where components
are ejected from the barrels and transported to rinse stations in perforated baskets
Sizing will depend on the volume the jigs (or baskets) will occupy allowing for adequate space all
around for convenience. Usually, the material of construction dictates sizes as polypropylene sheets
(and other materials) are manufactured in standard sizes such as 1.2 x 2.4 m or 3 x 1.5m and
divisions of these sizes will make up panels of the tanks.
It can also be understood that provided that rinse tanks are sized appropriately to adequately contain
the volume of components that is expected, no specific size is dictated. What is important is the
dilution factor and that is determined by the flow rate.
.
This explains why the cost of installing a counterflow rinse system is much more expensive on
automatic and semi-automatic lines. Of necessity the tanks are much bigger than those used on the
25
more antiquated totally manual lines where operators physically move jigs and baskets in and out of
rinses.
5.5.3
Process operation
If it was preferred not to design the system so that the water cascades (effectively runs downhill) then
it would be possible to position all the tank overflows at the same height and rely on an airlift pump to
transport rinse water. This pump has no moving parts and thus suited to the potentially corrosive
environment nor is it possible to accidentally empty a tank through a leak or siphon action, and it is
inexpensive (Van Der Spuy, 2004). The mechanism of the airlift pump is illustrated in Figure 7.
Figure 7
Air lift pump (Van Der Spuy, 2004)
To ensure proper mixing, the use of air agitation must be implemented. This will help remove any
plating solution clinging to the surface of the workpiece as well as mix the contents of the tank. Air
can be introduced into the tank by an aerator placed diagonally across the bottom of the tank (Water
conservation for electroplaters, http://www.p2pays.org/ref/01/00050.htm).
5.5.4
Process control automation
The freshwater flow can be adjusted accordingly by monitoring conductivity readings. Sensors in the
tank relay the conductivity reading to a control limit (in this study, a conductivity relating to 55 mg/ℓ
zinc), which opens the valve allowing freshwater in until the acceptable limits are reached.
5.5.5
Drag-out control
To maintain a constant drag-out volume the dripping time for all workpieces should be identical. On
automatic and semi-automatic plants, when the carrier lifts the components out of solution, it must wait
for the appropriate time (10 to 30 seconds) to allow for dripping before moving on. On totally manual
plants where operators lift and lower components into the process tanks, the process tanks should be
equipped with a rail directly overhead from which operators can temporarily suspend the jigs to allow
26
for drip off. Alternatively, a ledge of perforated material can be provided at the edge of the tank on
which operators can rest a jig to allow for drainage back to the process tank.
5.6
SUMMARY OF THE DESIGN
Based on the aforementioned discussion it can be concluded that the installation of a 3-stage
counterflow would result in a water consumption reduction of 990 l/h from the initial single stage rinse
setup of 1000l/h. The loss of plating solution is also prevented by returning concentrated rinse water
from a dragout tank to the plating bath so that the running rinse system deals with substantially lower
concentrations of process solution than would be the case if no dragout tank was in place.. Savings
can therefore be realised in terms of water costs, effluent treatment costs and the cost of plating
solution.
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6.
COSTING OF A CP INITIATIVE IN THE METAL FINISHING INDUSTRY
The objective of this part of the survey was to present a study estimate of installing the prioritized
cleaner production initiative, a counterflow rinsing unit, for the metal finishing industry.
6.1
THEORY OF THE COSTING APPROACH
6.1.1
Study estimate
A diagram of a proposed dual dragout/three stage counterflow rinse was submitted to a respected
plant builder in RSA for a quotation. The most important factor in rinse tank design is to ensure that
the rinse water is completely mixed. A rinse tank must include a feed water distribution line, air
agitation, and a flow control valve. The flow control valve can be controlled by measuring the
conductivity in the rinse water. The major equipment that was identified for the 3-stage counterflow
unit was therefore the tanks, aerators, flow control valves, and the conductivity control system. The
contractor is highly experienced and full aware of all these requirements.
Tanks
The material chosen for the rinse tanks was polypropylene. At the contractors suggestion, a material
thickness of 10 mm was identified as being adequate taking into account operating temperature and
density of the solutions. Appropriate ribbing would be added to reinforce the tanks.
The unit designed included two dragout tanks followed by a three stage counterflow system, and
would be ideal for a system like acid nickel plating, but would be equally effective for any other type of
plating as well.
Figure 8
Design and dimensions of a 5-stage rinsing unit
Each compartment therefore has the dimensions of 1 m x 1 m x 1.2 m.
compartment was raised to 1.2 m to allow for a 20% disengaging space.
The height of each
Price: Such a tank with reinforcing and valves with cascade overflow and down pipes in the next tank
to ensure proper counterflow will cost about R60 000 for 5 stations.
Aerators
Air agitation mixes the contents of the tank and helps to remove any plating solution clinging to the
surface of the work piece. Air can be introduced into the tank by an aerator placed diagonally across
the bottom of the tank. A suitable blower to aerate all 5 tanks was quoted at R9000 with an additional
R500 per tank for the air sparge in each. Total for Aerators – R11,500
Flow control valves
A flow control valve is used to restrict the fresh water feed rate to an optimum level. The conductivity
control system uses a conductivity probe to measure the level of dissolved solids in the rinse tank.
28
When the level exceeds a predetermined maximum value, the controller will open the fresh water feed
valve. When the level reaches a pre-set minimum value, the water flow is stopped. This equipment
was quoted at an additional R16,000.
Total cost from study estimate
The study estimate conducted therefore revealed that the total cost for the dual dragout 3-stage
counterflow rinsing unit amounts to R87 500. [R60 000 + R11 500 + R16 000] This is estimated to be
the price that would have to be paid for a well-constructed item of plant of this nature in RSA today.
Payback period
The payback period was determined by estimating how long it would take the electroplater to pay off
the capital costs incurred to implement a rinse system that incorporates a dual dragout followed by a
3-stage counterflow rinse. Capital costs would be paid off due to savings in water and metal solution.
In designing the 3-stage counterflow rinse it was reported that water savings of 990 ℓ/h would be
achieved when compared to a single-stage static rinse.
However, in these calculations the impact of an effective dragout recovery system has not been
factored into the equation. It is known that up to 95% of metal would be trapped in the dragout tanks
before the workpieces move on into the three stage counterflow system. Depending on how well the
dragout system is managed, the concentration of the dragout can obviously be maintained at levels of
a fraction of the strength of the process tank itself. This means that workpieces entering the three
stage counterflow arrive there with very much lower levels of the process solution clinging to them
than would be the case if they entered the rinse system directly from the process tank. There are
several variables at play here. The amount of dragout that can be returned to the process tank will
depend on:
•
•
•
•
The temperature and evaporation rate of the solution in the process tank
How much volume is being dragged into the process tank when parts are introduced for
plating
Whether the pre-dip and the dragout tank can be the same tank – i.e. the drag in equals the
drag out.
How well the parts are drained over the process tank before moving into the dragout tank
The latter consideration is of some importance, because good jig design and excellent drainage
ensure that the dragout does not become concentrated too quickly, so too reducing the load on the
dragout tank and ultimately the rinse system.
All of the above translates to the fact that the flow rate can in effect be much lower than the calculation
that follows, resulting in even bigger savings on water. It also means that the operator may consider
working with two stage rinse tanks, rather than three, saving money on plant design without
compromising on rinse quality. This explains why there are not more three stage rinse units in use.
A plant working a 40 hour week and saving 990 l/h would reduce their water consumption by 39.6 kl
per week. At City of Cape Town rates of R22.13 per kl this represents a saving of R876 per week.
[Rate is comprised of Cost of Water R12.51/kℓ and Sewage Surcharge R9.62/kℓ combined = R22.13]
Thus if the price of an effective dragout and counterflow rinse system is R87,500 then the system
would pay for itself in 100 weeks based on water savings alone. [1.92 years]
29
Savings on Process Chemicals
Savings on process chemicals depends very much on the system under consideration. We will
consider two processes here.
Nickel Plating Solution – Standard Watts formulation and brighteners
An acid nickel plating solution costs about R35.00 per litre [March 2015] to make up inclusive of
proprietary specialised additives. Thus if 1 litre per hour of process solution was being dragged out of
the tank, and not recovered, then in a 40 hour week solution to the value of R1 400 would be lost.
If this could be restricted to 10% of that figure by the correct use of a dual drag out system, then
savings of R1 260 per week would be made on the price of the process solution.
Adding the saving on process chemicals to the savings on water [R876 + R1 260 = R2 136] the
payback period reduces to 41 weeks
Because an acid nickel plating system typically operates at 60-70oC the level of the operating solution
will drop due to evaporation making way for the return of dragout solution. Thus it is more feasible to
operate a dragout recovery system on heated process solutions. In addition, the value of the nickel
solution is 7 times higher than that of the acid zinc solution, further incentivising the operator to
consider this CP option.
Acid Zinc Plating Solution
An acid zinc plating solution costs about R5.00 per litre [March 2015] to make up inclusive of
proprietary specialised additives. Thus if 1 litre per hour of process solution was being dragged out of
the tank, and not recovered, then in a 40 hour week solution to the value of R200 would be lost.
If this could be restricted to 10% of that figure by the correct use of a dual drag out system, then
savings of R180 per week would be made on the price of the process solution.
Adding the saving on process chemicals to the savings on water [R876 +R180 = R1056] the payback
period reduces to 83 weeks [1.6 years]
Acid Zinc plating solutions, depending on whether rack or barrel plating and on the particular
chemistry traditionally operate in a temperature band between 20-45oC. A plant operating at the lower
temperature levels will not experience much evaporation, and if over-tank dripping is optimised there
won’t be room for return of dragout.
If one wanted to make use of the dragout solution, it would be necessary to concentrate this using one
or another technology. Dragout concentration devices have proposed, but with potential savings of
only R200 per week, in this scenario, operators are not inclined to spend money on a fairly expensive
device. At this time, no devices are being offered in the South African market.
30
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33