Download Chemistry and applications of chelating agents in flotation and

Chemistry and applications of chelating agents in flotation and
flocculation
P. Somasundaran and D. R. Nagaraj
Synopsis
Chelating agents, because of their metal specificity or
selectivit)", can function as good flotaids and selective
flocculants. In addition to the metal selectivity, chelating
agents offer certain advantages over conventional
mineral processing reagents even from the synthesis
point of,-je,,". With the major donor atoms-S, N, 0 and,
to some extent, P-numer<KJs possibilities exist to tailormake reagents for specific applications. For polymers the
backbone offers funher possibilities for the
incorporation
of the required propenies.
In general, the choice of a chelating agent or a group is
made on the basis of its function in well-known analytical
metal separations. Such a choice is limited in that the
number orwell-known
analytical separations is also
limited. In the present work, a new and effective do"or
atom approach is discussed for the design of mineralspecific chelating groups. This generalized approach is
focused on the chemical properties of the imponant
donors both in the reagent and on the mineral. Data from
many chemical S<KJrceshave been summarized and
applied to systems of relevance to mineral processing.
Understanding of the chemical behaviour of donors is of
param<KJnt imponance in predicting the propenies of
chelating groups.
Numerous investigations have been made of the
application of chelating agents in mineral processing.
The various fundamental aspects of the interactions
between a mineral and a chelating group are, however,
not clearly understood, although there is evidence to
suggest that some form of a metal complex is formed at
the surface.
The separation of mineraJsfrom one another by flotation and
floccu1ation depends mainly on the sdeaive 8dsorption of
surfactants and polymers on them. Although many reagents have
~ng hem used in practi~ for the flotllion of mioera1s.most arc
not as selecti\'e E is required for the efficient separation, in
particular, of mineral fmes and ultrafmes. Reagents with
chelating functional groups have received increased attention for
this purpose in recent years, being known to exhibit exceUent
metal selectirity in analytical separllions. MInY chelating agents
have been tried E coUeCton for various mineral systems and
excellent seper8tions have been obtained in cenain czcs. The
mechanism~ that involve their colleCting action arc, however, not
adequate~' understood. In particular, the differences in their
mode of action at the mineral surface venus that in the bulk are
oot fully ~
and, as a resuh, the dcve~pment of
collecton based on their use in analytical separations is not
ach~'ed easily. It is to be noted in this regard tha chelating
coUecton are seldom ~tal-specific or mineral-specific and the
propenies of both the chelating agents and the mineral are
imponant in determining their colleCting action. The dIXlor
atoms on the chelating agents, as well as those associatedwith the
mineral species, playa governing role in their intenctions on the
surface. In this paper the application of chelating agents in
mineral processing systems is re\'iewed on the blSis of the donor
properties of the c1'w:latinggroups and the metal species, and
recent approaches to understand chelation are discussed briefly.
Emphasis is given to the criteria for selecrioo of chelating agentS
IS colleaors for various minerals and the prediCtabilit)" of the
behaviour of a chelating agent for a given mineral system.
Chelating agents
Ch~latingagentsarecompoundsthat fonn metalcomplexes
characterizedb)' ring struaurcs ilIusuatedin Fig, I.I.~.j In type I
th~ metalis coordinatedto th~ four nitrogcnsof two moleculesof
ethylcn~diamine,giving rise to a d1argedcmubleringed compla
,,'ith cbIorid~nNtralizing th~ two charges,I Other typesof
dldating oomplcxesindudc t}"peII. "ith 81 inm-molccular
hydrogenbridge.and type III. involving polynuclearhalogen
bridges.
.NH2-CHz
Cf2
.W
I
'NH2-C"z
H3C\
I'c~o ..
H
HC
"
I
(D)
~-O
H,C
.CI-
...:,
(ID)
,ci.
B
FiB- I
B
GeIxr8I sUUCtUresof clr1ating -cents I
Chd~ng agentscm be cl~sirlCdon the b85isof donor atoms
involvrd (0-0, N-O, N-N, 5-0, 5-5, 5-N)or ring size(4-,5or 6-memberrd)or chlrg~ on th~ complex(Inion, cation,
neutral)or numberof bondsto the meuI for everydteJating
molecule(bidentat~I: 1
/A;
M
)
'A
bidentatc1:2
( A 'M/A)
A/
'A
;
A
terdentBte
M~)
'A)
Specificityof chelatingagentsdependson the int~on
209
Weal ckIDOratoUD Ind sunKe species on the panicle. A list of
/lOr atoms Ind funaional groups a>ntlining the four major
lIDS(N, O. P and $) are given in Table 1. Halogen atoms, a.
and I, panicipate in chelate ring formation in bridged
S.illa"
HO-CH-COOH
I
HO-CH-COOH
UmuuDble
CH2-COOH
I
CH2-COOH
Table 1 Donor atomsand functional groups containing
major donor atoms)
t:
rI N
OH
H
---"'t
0
I
, P
,
s
: I_!_~
~
IAs
L__-
r
I
a:
Br:
I
I
I I I
Tt Ii-.L_-J
Sb
-NH,
0
s
-rooM
-OH (mol 01' phenol)
=0
-08
-P(O:.(OHh
-5H
-S-R
(ajQ)1XII5
polynuclearcomplexesthat an of minimal imponancein
mineralprocessing.As and Sefonn only a few usefulcomplexes
and.tberef~, an seldomused.
Requirements for c:helatinl8JeDU
Thcr~ Ire. essentially, tWObasic:requir~mcnts that c:h~lating
agents must satisfy to form metal cbclates: the molecules should
hav~ suitabl~ functional groups and the functional groups must
be situated to permit the formation of a ring with a mctaI a the
closing m~mber.
These t"-o conditions are ncces~', but not sufficient for the
formation of a d1elat~ ring. For aample, under ccnain
conditions and under sufficiently to.. pH valum, a potential
chelating molecule may attach itself to a mcta1atom through only
on~ ligand atom:
"/
"
a
H ooc. CH zNH z
,0
Pt
NH~2COOH
or
HOOC.CH:S-M
I t hE been generaUyobserved that certain Iddioonal
requirements must be satisfied b}. chelating agents for mineral
processing applications: the chelates should, preferably. be
neutral, if the chelating agent is to function IS a collector, and the
dlelates should, preferably, be charged and very hydrophilic if
the chelating agent is to function as a depressant. On a pol}'Dleric
depressant or a flocculant this require~t
is not stringent so
long IS the backbone hE other hydrophilic groups (seebelow).
Rcquirmtcuu for dr.pressanu:ionic chr.lMes,r..g.
lCo(DMG)~HzO~'. and bydrophilizinggroupson moJecuJr.
210
H~H
OH
rt:JfH
Neutral complexes arr usuallyinsolublr in wltrr and.
therefore,promotr hydrophobicit},.For insolubilit}, in wltrr,
howr\"er,it is not sufrlcimt that thr chrlltr br a neutral complex.
A classicrxamplr is that of chrlatesof dimethylglyoximes
(DMG) (5« Fig. 2). The 1:2complrx of nickelwith DMG is
insolublr in wltrr. Thr coppercomplrx is, on the othrr hand,
rdai,'rly ~'.rr-soluble. This resultsfrom the subtlr differences
in thc shapesof the tWoIOOlecutcS
and consequmtdiffrrmces in
the IOOdrof packingof thc tWomolecuJes
in their resprctivr
crystals(Fig. 2). The nickel compoundis planarand the
molrculesarr stackrdin thr crystal,sothr Ni atomsarr co-linrar
md havr a wrak bond brtWrcn them. In the coppercompound,
howr\.er,thc chelaterings arenot co-planarbut makea small
angleof 28° with onr anothrr andthere is no metal-metal
bonding.I The tetragonalpyramidalcopprr chrll(r molrculesarr
pairrd in thr crystalin sucha waythat ram copperatomhz an
0 atomirom itSIdjKent molrcuk a onr of its fivr immediate
neighboun, In thr Ni complexthr -OH md thr -0- are
strOnlly internallyhydrogenbondrd Ind, bmce, lessreadily
sol"atrd. The coppercomplrx aCtUallyhasa highrr stability
constantthanthe Ni complexbut, sincr no intrrnal hYdrogrn
bondingis po5siblr,thr complexis easilysolvatrd.
Specificity and selectivity
Most of the chelatinggroups form complexeswith almostall of
the transition and many non-transition metalsand, therefore,
specificity is not asabsoluteasis required for their selective
adsorptionon minerals.In practiceselectivity is achievedby
making useof differencesin stability constantsand chelate
formation under various solution conditions. As wasindicated
earlier, contribution from the donorsasthey arelocatedin the
minerallanice hasto be consideredin obtaining the selectivity.
Also, the solubility of the mineral, in addition to that of the
metal chelate,hasa pronouncedinfluenceon the selectivityand
the collectionpower of the chelatingagent.
Stability
of complexes
_\1ajorfactorsthat determinethe stabilit~.constantare,in order
of decreasingimponance, pK. of the ligand molecule,
substin:ents,natureof donor atoms,central metalatom,and
ring size,number of rings.
The fIrStfour factorsarevery imponant in that they
determi~ the natureand strengthof bonds.pi(. of the che~
agenthasa direct influenceon the chelateformation sinceit
representsthe tendenC)'of the donor atomsto donateelectronsto
metalatoms(or acceptfrom ~a1s, in cenain cases)andthus
Ionn a chelate.Substitution on the chelatingmoleculehastWo
imponant effects:J it altersthep?<.and/orit introduces
steric
Iactorsfor the formationof a c~late.
The influenceofthc natureof donor atom and the central
metal atomshouldbe consideredtogetherbecauseof the interrclatio:nshipbetweentheir behaviour(seelater).
Properties of acceptors
Acceptorscanbe divided convenientlyinto two maingroups
basedon the unidentateligands:(a) thosewhidl form most
stablecomplexeswith N, 0 and F, Illd (b) thosewhidl form
moststabk complexeswith P, 5, and a (fable 2).
The stability constantsof metalcomplexesthat beJongto the
two groupsfollow the generalordersN»P>As>5b; 0>5;
F>CI>Br>I; Illd for similar liglllds N>O>F; differencesin the
seriesP>5>CI arefound to be lesspronounced.4It is seenthat a
vastmajority of elementsbelongto group (a), i.e. they
preferentiallycomplexwith 0- and N-containing liglllds.
A given metal, funhermore, may belongto group (a) or (b),
dependingon its oxidationstate:for example,Cu(I) belongsto
group (b), wherasCU(II)belongsto the overlap regionof (a)
and (b).
A major shoncomingof the abovegrouping is that it is based
on unidmtatt liglllds; the situation is complicatedwhenchelates
(or bidentates)areinvolved. Considerationmust be given in this
caseto chelatingagentsin which the donor atomsarethe same
(examplesareethylenediamine,oxalateor thiooxalate)Illd
chelatingagentsin which the donor atomsaredifferent
(examplesareo-aminothiophenoland o-arninophenol).For
nickel, tbiooxalateion is Believedto form a more stablenickel
chelatethlll the oxalateion. For zinc, stability constlllts of the 0aminophenoland o-aminothiophenolcomplexesshowthat the
substitutionof 5 for 0 doesha\'~a markedinfluenceon stability.
A moreusefulgroupingis that basedon the electronic
configurationof the metals.Thus metalscanbe classifiedinto
threegroups-those which contain (a) inen-gastype
configuration, (b) panially filled d or f orbitals and (c) ions of
metals~;th filled d orbitals (Ag., Zn2+,etc).
Metalsof group (a} reactprt:f~rentia1Jy
~;th O-containing
ligands,suchascarboxylateions and th~ anionsof quinalizarin,
Illd the p-diketones.
Table 2 Classificationof acceptorsbasedon unidentateligands
I
';:V/ ',"
8/1';(
NO
F
Ir,'
Mg
Sc Ti
D
~
D
",
' G~
,
V
CLASS
A
BORDER
Form stable
complexes
with
K,O,F
REGION
Class B:
Form stable
complexes with
P.S.Cl
211
0
J.
~-
a
1f~
O'
O'
..,..C
Transition-metalionsform morestablecomplexeswith
ligandsthat containpolarizableponions, suchasaminogroups
andheterocyclicnitrogenatoms.
Metals in group(c) alsoprefer highly polarizableligands,
especiallyif the latter havesuitablevacantorbitals into which
someofthc d electronsfrom metalscanhc 'back-bonded'.
Examplesof suchligandsare thosewhich contain S asthe donor
which is highly polarizableand basvacant3dorbitals.Thus,
thioanalideforms insoluble complexeswith Cu, Ag, Cd, Hg(ll),
11, Sn(ll), Pb, As, Sb, Bi, Pt and Pd.
0
~""I/::"T N H -
~...J,,~
II
C
SH
-
I
CH
2
Thioanalide
A reagent, such as dithizone, which oontains N and S dono~
therefore reactSwith a large part of the transition metal and filled
d orbital series.To achieve the degree of selectivity that is
desirable in praCtical applications it is usually necesSar).to arrive
at the appropriate oombinations of dono~.
Donor-acceptor
relationships
A useful classification of acceptors as well as donors is that based
on Pearson's concept of hard and soft acids and bas6 (rable
3).S.6.7
The acids (or electrophiles) are mostly (metal) cations.
This group also includes such electrophiles as CO2' 0, a and N
and metals iI\ the zero oxidation state. The bases(or
nucleophiles) are non-metal anions, neutral atoms and
molecules. It should be noted that the donors 0 and N are hard
basesand donor S and P are soft bases.
Table 3 Characteristicsof bard andsoft acidsand bases
Hard
(acid or baK)
Soft
(acid or base)
Smallsize
Orbitals involved far apan
in ener~'
Ionic bonding coulombic
attraction
Large size
Orbitals closein ~rgy
High chargeions
High chargeions
Non-polarizabk
Hard likes hard
Polarizable
Baseshave10"-proton
affmity
Soft likes soft
High elecrronegativity
Lo'" elcctroncgativity
Co,"alentbonding
The bondingcharacteristicsof thesegroupsaregivenin Table
3..ln general,hard basespreferentiallyreactwith hard acids,and
similarly for soft bases.The former interactionis characterized
by ionic bondingsincethe orbitals in"olved arefar apartin
energ}',therebypromoting coulombicattraction.On the other
hand,the interactionbetweensoft basesand soft acidsis
characterizedby covalentbondingsincethe orbitalsinvolvedare
closein energy.
It is questionablewhether this sort of classificationwill be
valid for mineralspecies,althoughthere is someevidencein the
literature that the metalcationsthat prefer to reactwith 0
donorsalsotend to preferentiallyadsorbO-containing
212
coUccton on their min~ral surfaces.7
Similarly, S donorsreact
pr~ferentiallywith sulphid~min~ra1s.It hasbeenpoint~dout
that a bond betweenm~tal ions and soft basesmakesth~ metal
ions soft acidsand vice v~na.8This is ~speciallyimportant to
bord~rlin~ cations.From this viewpoint sulphidizationof
oxidized min~ralsshould leadto an incr~asein the capacityof
th~ir reactiv~centresto reactwith sulphydryl coUccton.7Thus,
th~ imponant contribution of donon on th~ min~ralsurfacesto
int~ractionsbetweenmineralsand ch~latingag~ntsmust be
noted.Oxjd~ min~rals(which have0 donon) of soft-acid metal
ions would preferentiallyadsorb0- and N-containing
collccton, wh~reasth~ir correspondingsulphideswould
pref~rentiallyadsorbS-containingcollectors.
Properties of donor atoms
The donors o and N belong to the first row of elements in the
periodic table and follow the octet rule'-4,fo9"o,'1with valencies 2
and 3. The donors P and S belong to the second row, and these
also follow the octet rule with valencies 3 and 2; however, P and
S can exhibit higher valence states also since they have easily
accessible vacant 3d orbitals. It is the electrons at or near the
surface of atoms and ions that are the most important in
determining their chemical andph)osical properties. This is
especially true of donors P and S and transition metal ions. The
laner have incompletdy filled d orbitals, which have quite large
fractions of their tOtal volumes near the outsides of the ions, so
they are easily accessible for bond formation. Additionally, the
J orbitals are much more easily polarized, promoting a much
more favourable orbital overlapping.
The important properties of the four donors 0, N, S and Pare
summarized in Table 4.
o and N have 2p electrons and no accessiblevacant d orbitals.
S and P have 3p electrons in their outermost shells, but they
have, in addition, easily acce:ssiblevacant 3d orbitals. The
electronegativities decreasein the order
0
,3.5)
N
3.07}
s
P
:2.44: (2.06)
Consequently, 0 invariably forms ionic bonds with a majority of
elements in the periodic table. Thus, chelating Igents with 0-0
donors often form chelates with a large number of metals and
are, therefore, lessselective. The selectivit). in general should
increase from 0 to P.
The normal ,'alencies are 2, 3, 2 and 3 for 0, N, S and P.
respectively. 0 has tWo unpaired electrons or two elearon pairs
to donate, but it seldom donates ooth pairs; only one pair is
active. It can form a maximum of four oonds. but seldom attains
all four (three oonds are common). 0 forms multiple oonds, N
has five valence electrons. but only four orbitals and, therefore, a
maximum of four oonds can be formed, It has one lone pair of
electrons to donate when three electron-pair bonds are formed
Like its neighoours C and 0, N also readily forms multiple
oonds. In this respect N differs from P. S. As, Sb and Bi, S has
rn'o valence electrons and its normal valency is 2, But it has four
orbitals and easily accessibled orbitals. Consequently, S can
torm two to six bonds. Similarly, P forms three to six oonds,
although its normal valency is 3 and it has five valence electrons.
.0\11
four donors have one active lone pair of electrons. Although
0 and N show strong prr-pir bonding (becauseof the pouter
orbitals and no d orbitals). S and P show ver)'little or no
tendency for P70-p1T.On the other hand, only S and P show
strong d1T-d1Tbonding. In addition. S and P sho,,' d1T-p1T
bonding (or back-oonding) since they can accommodate
ela.~rons from metals in their vacant d orbitals, This ability for
back-oonding for P and S, indeed, is an important distinction
that puts these donors in a special class. The polarizability of the
lone-pair electrons on these donors follo""5 an almost rC"'erse
Table 4 Major propenicsof donor atoms0, N, S and P
0
CONFIGURATION
lS22S22pl&
2
3
OF ORBITALS
II
4
2
4+D
OF BOUDS
3
II
2-6
ELECTRONS
NORMAL VALENCY
(VALENCY
EXPAaSION)
p
[Nij 3S23PQ3Do [HE]3SQp3300
2.11i
2.44
5
2
3.07
5
VALENCE
II
lS22S22P3
3.5
2
ELECTRONEGATIVITY
Ii
s
tl
3
/I + 0
1
1
2
1
pw-p.
STRONG
STRONG
POOR
NONE
Ow-p.
NONE
NONE
STRONG
$TRatG
POLARIZABILITY
NIL
GOOO
STRONG
GOOD
H-BONDS
STRONG
STRONG
VERY WEAK
NONE
BONDS
MORE IONIC
LESS IONIC
COVALENT
COVALENT
STERIC
ACCESSIBILITY
LOW
LOW
HIGH
HIGH
LONE PAIRS
{BACK-BONDING)
ord~r ofth~ ~lectron~gativities.
Thus, for polarizability
~«.s-p
0 and N have the distinction of having the ability to fonn very
strong H bonds, whereas S and P show little or no such
tenden~'. Since P and S h8\'e d orbitals, and large size, these
donors Ire much more sterically accessiblefor bond formation
than90rN.
S is unique in its abilit). for catenation (forming bonds with
itself); its common OCCUiTence
is in the fonn of an eightmembered ring. S is also knO\\"Dto fonn higher polymers. This
unique featUre of S is Ver)' important in all its interactions.
0 and S can enter into chelation either through the ether or
thiOetber fonn R-O-R, R-S-R, or the R-OH, R-SH. The -SH
group is much more acidic than -OH and is highly polarizable,
but not an effective acceptor. The R-S-R group forms
~'ramidal bonds and is an effective drr acceptor. The R-SH
sho\\'Sa strong tendent1' for unidentate bonds, and the R-S-R
sho"'S a strong tendenC}' to form chelate rings. The C=S is more
ionizable and has a greater volume than C=O. S is a better donor
than 0 with reglrd to sharing or donating the lone pair of
electrons. Similarly, N is also a better donor than O.
It can be readily seen from the foregoing discussion that each
donor has some unique properties. The acceptors also fall into
certain \\'ell-recognized patterns. A judicial combination of
donors for any given acceptor, taking into account the properties
of both acceptors and donors (on mineral as well as the chelating
agent), should provide the required selectivity in minerals
processing applications,
The "\\olecullr Orbital treatment is a powerful tool in
understanding the donor-acceptor interactions. In this
approachI '.10.12.13 the individual atoms \\'ith their nuclei are all
placed in position and all the electrons concerned in bond
formation are allotted to the various molecular orbitals, This
treatment, however, has scarcely been used in understanding
the mechanism of adsorption on minerals,
Substituent effects
The electron-donating or \\'ithdrawing tendencies of organic
groups \\'ill influence the electron densities on donor atoms and
the pKa of the molecule. ',1'.14-1"The inductive effect results from
electronegativity differences, Much stronger changes in
electron density and delocalization of electrons arise from the
resonance or mesomeric effects. With alkyl amines, for example,
the aJI,.-y1
group exertS an inductive effect on the nitrogen, making
its unshared electron pair more available for bonding and
thereby increasing the basicity.
H
H
R-N:+H'~
R- N
H'
H
H
In carboxylicacidsthe inductive effectof analkyl group
decreases
the acidity; for formic acidtbe dissociationconstantK.
= 17.7x 10-5,whereasforacenc acidK. = 1.75x 10-5-almost
onc-tenthof the valuefor formic acid
#0
~, 0
R -+- C'
{-]
[K. decreases)
If, on the other hand. the R group exerts Ul oppositeinductive
effect owing to certain substitUents on R such IS Cl or NOl' the
acid strength of the carboxylic acid incrcascs.
R -+
~'
C?'
~,
0
Q
(K. increzes]
The electroniceffectsaremuch strongerin aromatic
compounds.A benzenering oouldexertooth electron-donating
andelectron-withdrawingeffects.In benzoicacidthe aromatic
ring exertsan electron-withdra~ingeffectand,hence,it hasa
much higherK. than that of aceticacid.An alkyl group in the
para positionon the ring exertsan electron-releasing
effect.
Groups suchas-OH or -OCH} areelectron-releasing
o~.jng
to resonance,in spiteof the electronegative
oxygenand the
electron-withdrawinginductiveeffectsthat the groupscanbe
soownto ~sess.
The effectof electron-activesubstitUentson an aromaticring
is well exemplifiedby aromatichydroxy oximes.For anelectronreleasingR group,suchz -CH) or -OCH)J the acidstrength
decreases,
andtih. increaseS,
the releze of electrons
destabilizingthe phenolateion (the electrondensityon N may
alsoincrease):
R
R
!
(;;"1RI
-y ~/.--o."
!
H~
OHNOH
/
OH NOH
SALO
10.70
II
-o-C-'-NH.
Thus,
s
RO'
Table S pK. of SALO II1d ligand field stabilizations 18
CH,.SALO
11.06
s
RI
SubstitucntsR = 0, NO2ttc., ha\'eoppositeeffects.The
incrc~e in electrondensityon donorscan(a) enhanceb~icity
and(b)thereforeenhancea bond stability, but (c)it may also
decreaserr acceptorability of ligand and (a')thereforedecre~e
donor rr-bond stability,
Effects(c)and (a')could be pronouncedfor donorssuchasS
sincethe donor-rr bond(or dativebond) is formedbetweenthe
~tal andthe organicreagent(in additionto the coordinativeor
I-bond) ~ a result of tranSferof electronsfrom d orbitalsof metal
into vacantd orbitalsof the donor.The formationof a dative
bond,whidl reinforcesthe 0 bond, is moreprobablefor mttals
in their IIjweroxidationstatesincethesewould havemore
electronsthan thosein the higheroxidationstate.Another
importantrequirementis that the donor atomshouldhaveeasily
accessible
vacantd orbitals(S). 0 and N, whidl do not have
thesed orbitals,do not form a dativebond.The nitrogenin
oximes,however,forms a donor-rr bond and,therefore,oximes
arean exception.This explainsthe order of the ligand field
stabilizationeffectsobservedfor the chelatesbetweeen
substitutedsaliC)'ialdoximes
(SALO) and Cu. Ni and Co (Table
5).
pK.
:to.OS
Simultaneouslythe groupsRO and R INH could exert an
electron-donatingefTect
sincethe C+-S- is more stablethan
-C=S. This tautomerism,togetherwith the inductive effect,
would delocalizcelectronsover the entire activegroup
Substituents introduced into the molecule influence the
electron distribution and, therefore, produce some interesting
effects. Russian investigators7.\) have done some extensive StUdy
on substitUted thionocarbamates-especially phenyl
substitUents. The. V-benzoyl thiOnocarbamate has been stUdied
the most:
S
Ii
0.8
2.6
2.4
3.8
3.5
s
,
c-o-c
CH:
The aromaticring accentuates
the eleCtroniceffectscompared
with an alkyl analogue
s
II
CH) CHzO-
C
214
NH
0
II
c -CH;
In addition,the 0 of the C=O exertssomeelectron-withdrawing
effect,beingstronglyelectronegative.
The delocalizationof
electronsis morepronounced.There is a decreasein electron
dcnsi~' on S andthe pi(..
s
0
'I;~~~
NH =Jt.'V
~ substitutionof a phenylgroup insteadof a benzOylgroup
mayleadto a different electroniceffect,sincethe phenylgroup
caneither beeJectron-donating
or electron-withdrawing.
For thiol collectors(dithiophosphates,
xanthates,and
dithiocarbamates'the electrondensitieson tbe activeS donors
dependon the inductiveeffectsin the molecules.
s
RO,"
P
S- RO
RO/
"""'S- 1(/
DTP
Xanthate
C;5
'So
DTC
II
NH
.CH,CH)
/
0 andN exen an electron-withdrawinginductive effect.
R
c-<Q)
10.1
The ligand field stabilization(LFS) increasesfor eachmetal
asthe pK. decreases,
i.e. asthe electrondensityon the donor
atomsdecreases.
This increasein LFS is attributed to the effect
of substituentson the 1T-acceptor
capabilityof the ligands.18
The thionocaroonates(Z-200 t).pe)arenot only excellent
sulphidecollecton but alsootTeran interestingcasefor study.
The moleculecontainsthe three iml'Ortant donors0, N and S.
o andN arehard basesand S is a soft base.
CH,
0
II
CH, CH2O- C -NH-
CLSALO NOrSALO
10.2S
8.72
8.4
1.5
1.1
7.S
NHR
The dectron-density order may follow S>O>N and,
considering the sizes and polarizability of the donors, the steric
accessibility would follow the order S>N,O. S may also show a
tendenC)' to bond with S on the sulphide surface. Thus, S is a
ve~' active donor in the thionocarbamate molecule. 7
,',
CH} CH20dt'=
0.6
"C
S
II
O-E-C~NHRJ
In DTP the {\\'o RO groupsexen an electron-withdrawing
inductive effectbecauseof the higherelectronegativityof the 0
atoms.This will delocalizethe electronandstabilizethe anion.
Funhermore,P is moreelectropositivethan C in xanthate.The
electrondensityon S in DTP would be decreased
and,
consequently,DTP is a strongeracid,weakercollectorand more
selectivethan xanthate.Theseelectroniceffectsareless
pronouncedin xanthatesincethereis one lessRO group andC is
lesseleCtropositive
than P. In DTC N is lesselectronegative
than
0 and has. highertcndenq to donateelearons. As a resultthe
DTC would be . strongeroollector,weakeracid andless
selectivethan DTP (DTC<X <DTP).
6
SALICYLALDOXIME
O-HYOROXY ACETOPHENONE OXIME
O-HYDROXY BUTYROPHENONE OXIME
0 Z-HYDROXY-5-METHOXY-BENZALDOXIME
() Z-HYDROXY-5-METHYL-ACETOPHENONE
~ 2-HYDROXY-)-NAPHTHAL:>OXIME
O-HYDROXY CYCLOHEXANONE OXIME
a-HYDROXY BENZOPHENONE OXIME
0
0
Table 6 StructUreand water solubility of various
hydroxyoximesl9
.
OXIME
.
pH- 4.8
1001
p~?
? p
80
at
>-
~
r 1
a:
!oj
~
60
u
!oj
a:
z
9;
f
/
/
f
f
p
I
""
~
~
~40
0
-
.J
"-
20
~
-.
1-11-
165
INITIAL
10-4
103
COLLECTOR CCNC::NTR~7,ON.M
Fig. 3 Flotation of c~'socolla ~'ith sa1i~'laldoximcand various
subsutut~doxi~I'
A recentdetailedstudy \\ith severalwater-solublechelaIing
agents(fable 6) of the classof aromatichydroxy oxi~s showed
5e\'eralinterestingeffeCtS
of substitution.Flotation results
obtainedwith thesechemicals,which canbe representedb). the
generalformula
RI
aregiven in Fig. 3. IfR~=H andR.=-H, -CH)or -CHzCHJ
CH}) the effea on the oolle-.""tor
efficient)' wasobservedto
increasein the order -H«-CH)<-CH~CH:CH,.
The introductionofth~ fl.'"St-CH2 in SALO resultsin a
much largerchangein polari~' of the moleculein comparison
with funher additionoft\\.o -CH~ groups.This is reflectedalso
in the water-solubilit).of the oximes-SALO, OHAPO and
OH-BuPO. Funhermore,the electron-donatingtendenC)'of
-CH: group mayalsofavour chelationr~ction.
Substitutionof a phenyl group on R. CRz=H)will makethe
tDOlecule
much lesspolar than SALO, asreflectedin the ~'atersolubili~.. On this basisOHBePOcanbe predictedto havea
highercollectorefficien~.than SALO. The actualfmding ,,,'as,
ho~'ever,cont~. to the prediction,suggestedasbeingdue to
tbe possiblesterichindranceto chelationofferedby the second
benzene ring and the slower kinetics of adsorption of OHBePO,
Adsorption tests, ho\\'ever, snowed the adsorption of OHBePO
on dlrysocolla to be significantly higher than that of SALO at
the same oxime concentration, thereby suggesting that Steric
hindrance or slow chelation kinetics m~' not be responsible for
the lo~'er collector efficienC}' of OHBePO than that of SALO,
Bubble size reduCtion observed during flotation may be the
major faCtor in this case,
Substitution in R2 by -CHJ(R,=CH;
or H, respeCtively) not
only decreasedthe \\'ater-solubility of the parem oximes but also
decreasedthe collector efficienC}', The decteasein "'atersolubili~' is to be expected from the decreasein ~larity of the
molecule as a resuh of substitution, The decreasein collector
efficien~' could be attributed to the decreasein acid strength of
the molecule o,,-ing to the increase in electron densi~' on the
phenolic oxygen caused by the nucleophilic substituents,20This
can, howe\'er, increase the metal-ligand stability, as is generally
observed, 3The effeCt of eleCtron-releasing groups on the ring is,
therefore, generally favourable for chelation, Again, the
reduCtion in bubble size appears to be the major faCtor in regard
to the lo"-er colleCtor efficienC}', This appears to hold for the low
collector efficiency obsen'ed in the caseof OHNAO, although
the solution concentrations stUdied \\'ere severely limited by its
low solubility,
Surface chelation
I t is now generallybeli~'ed that the adsorptionof xanthateon
galenaproceedsby the formation of a chelatecompoundPbX
on the surfaceof galena- asopposedto PbX2in the bulk.21
Thus, the fint layer on galenais belie,.edto be PbX (with a high
215
~
tenacity)overwhich iayen of PbX1build up by 'sheerphysical
attachment'.n
Taggartand co-workers23
proposedthat surfacecompounds
differ greatlyin their propertiesfrom nortnally expectedbulk
compounds.This is becausethe lattice ionsor atomson the
sunlCehaveonly a part of their coordinationsphereto
contributeto the formation of a surfacecompound In addition,
stericeffectsassumea particularly significant role for reactions
on surfaceof mineral asopposedto thosein the bulk. It is
thereforeconceivablethat the first-layer compoundon mineral
maybe different from the compoundformed by a reactionin
the bulk. If it provesthat the bulk compoundis energetically
morefavourable(e\'enafter taking into accountthe fact that
adsorptionof collector is, in general,energericallyfavourable),
eitherthe collectormoleculesor the surfacecompound
comprisingthe collectormoleculesand latticeion or atom will be
scaled-offor detached.Of course,contribution to this
detachmentcould alsocomefrom physicalfactors.
It is not known whetherthere is a direct relationshipbetween
surfaceandbulk compounds.In manystudiesa I: I surface
chelatehasbeentacitly assumedwith little experimental
evidenceto suppon the assumption.Often elaboratestructures
for surfacechelateshavebeenproposed24.2S
(tWoexamplesare
shownin Fig. 4).
The imponanceof distinguishingbetWeensurfaceand bulk
chelatesandquantitativelydeterminingthem in the samesystem
underflotation conditionswasclearlyshownby Nagarajand
So~undaran,19.2~.27
Surfacechelatewasfavouredunder certain
conditionsandtheseconditionscoincidedwith thosein which
flotationof tenoritewasobtainedwith SALO (Fig. 5). The bulk
chelateformedunder a ,,'ide rangeof conditionsand this chelate
wasineffectivein causingflotation whendispersedin the bulk
aqueousphase.This chelate,howe\'er,could aid flotation ,,'henit
is still attachedto the first layeron the mineral, asfor thiol
collectorson sulphides.Chanderand Fuerstenau28
discussed
the rolesof surfacereactionand bulk reactionin the system
chalcocite-DTP. Ananathapadmanabhan
and Somasundaran,29
basedon detailedcalculationsof relevantmineral-collector
equilibria, haveclearlyshownthat much of the flotation results
in the literature canbe explainedon the basisof surface
reactionsor precipitation (asopposedto bulk reactionor
precipitation).
The influenceof bulk chelationasopposedto surfacechelation
hasnot beentakeninto considemionin the pastfor most
systems,evenwhenthe bulk chelationis Vcr)'pronounced.For
mineralsthat havefinite solubility (or speciation)in water, bulk
chelation(or precipitation)is inevitableif the kineticsof metal
chelation(or other complexformation)arereasonablyrapid Any
collectorassociated
,,'ith this chelatecanbe considered
essentiallywastedin flotation s)"Sterns.
Selective flocculation
The useof chelatingagentsfor selecti"eflocculation and
dispersionhasbeenclearly illustrated by several
:~"co.t4
HCi
~.ro,.
!'
~ ".}=.I
,0"-/0\
~'C
IC)
H
+
COZ + H2O
"C"-ro,-H
-1 .
~
~
3~
~ P
J,o o
.~,
J ..-c
I
0
~
l'
r-""\
.;
--1
'..j
-'..1
H
~
d .tC'"
,~-Cu
~,
A
4
"'-to
.~~
+COz+HaO:?
-
~ 0
+COz+~O'
:j
"
---
~
(';::.yA
~ ~
('~OH
fv.D~~
CuC03gq+
H
I~~
I
I
+(01
+
HaO
Fig. 4 Schematic illustrations «a)(l,ft}2' and (b) (abot.,f') of surfa~
chtlations
216
invesngators.-- -- 1.nsome 01 these WOrkSCneJatlQggroups were
incorporated into a polymeric-type molecule) which acted IS
selective flocculants. For example, Sresty and
Somasundarann.)7 observed hydroxy propyl cellulose xantha1e
containing an active thiol group to produce good flocculation of
chalcopyrite with little effect on quartz (Fig. 6).
0
...
"
u
~
..
Reagcots
0-0 t~
Minerals
Cupferron
Salicylaldehyde
Q-Nitroso~-naphthol
Cassiterite,uraninite, hematite
Cassiterite
Q)baltite
Acctylaccton~
.\1kylhydroxamicacid (IMSO)
Malachite, chrysocolla
Chrysocolla,hematiteand minerals
aIrItainingTi, Y, La, Nb, Sn andW
Cassiterite
Pbospbonic acids
.\'-<.JIy~
P-HydroX}'oximes
«-Hydroxyoximes
8-Hydroxyquinoline
Cu oxide minerals
Cu oxide minerals
Cerussite, pyrochiorc, chr}~ocoUa
.\' , Iypc
Dipbenyiguanidine
Dimetbylglyoxime
Benzotriazo Ie
Cu minerals
Ni mi~ra1s
Cu minerals
S-S r_vpe
Xanthates
Dithiophosphates
Dithiocarbamate5
Dithizone
Applications
Sulphideand tarnished
sulphides
Sulphides
S -0 ',vl'"
X-benzoyl a-alkyl
tbionoclrbamac
TestS "lth synthetic mixtUres of these minerals showed both
grade and recover}- of chalcopyrite in the sedimcnt portion to
improve with increase in the concentration of the above polymer,
but at Ver)- high dosagesentrapment of the quartz by the bulky
cha1cop~-riteflocs caused a decreasein the grade- This problem
can, however, be overcome easily by cleaning the product by
redispersion follo,,-ed by settling-
All sulphid~ minerals
.\"-S type
.\ \crcaptobenzothiazole
Fig. 6 Flocculation of chalcopyrite-andquartz fmcs in terms of per
cent solidsscttJedin 45 s asa function of hydrox}-propylcellulosc
xanthate(re.ntizing time, 30sy'
Sulphides
of chelating agents
Even though the relativeproportion of chelating-typereagents
amongthe industrial non-thio collectorsis not significant, the
actualnumber of chelatingagentsthat havebeentested
successfullyascollectors,depressantsand flocculantsfor
variousmineral systems,at leaston a laboratoryscale,is large
(Tables 7,8 and 9). It is e,-identthat the actualuseof these
t).pcsof reagentsin the processingof mineralsis likely to
undergoa significant rise in the coming decades.
217
~
TableS Examplesof chelatingagentsasdepressants
Reagent
Tanaric acid
Gallic acid
Alizarin red S
Starchxanthates
CtlIu~ xantbates
Mineral
Reagents
Starchxanthates
Cellulosexanthates
Selective flocculation and
depression of sulphides
Poly (4- and S-acr)'lamidosalyclic
Cation-exchange polymer for
Fe", Cu", Crt- and VOl"
acids)
Resincontaininghydroxy oximesgroup
"
.,
"
~l}:h~~xamic acids
..
"
,.
Concluding remarks
I t is clearthat althoughchelatingagentscanbe usedeffectively
for the flotation of ores,they do not possessabsolutespecificity
towardsmineralspeciesand it is only the judiciouschoiceof the
chelatingagentsand conditionsfor selectiveseparationthat will
maketheir us~possiblefor the beneficiationof many
problematicores.In this regarda full understandingof the basic
mechanismsinvolved in their chemicalinteractionwith mineral
speciesin the bulk and on the surfacebecomesessential,even
though thosein the interfacial regionmight be understood
accuratelyonly by useof newexperimentalapproachesthat will
permit direct probing of this region.
The role of surfacechelationversusbulk chelationhasto be
takeninto accountin the developmentof any mechanismfor it
to be of significant usein flotation.
Also, more importantly, the role of surfacechemical
alterationsdueto either changein oxidation stateof surface
speciesor precipitation of variousdissolvedmineralspecieswill
haveto be consideredfor applicationof the aboveinformation
in aaual mineralprocessingsystems.
Acknowledgement
The authorswish to thank the mineralsand primar}' materials
programof the National ScienceFoundationfor the supportof
this work.
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