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The role of electrochemistry in the extraction of gold
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THE.ROLE
OF ELECTROCHEMISTRv; IN THE EXTRACTION . . .
. R-L.
*
:
~..
OF-GOLD
PAUL
Council
for
#ineral
Technology,
Private
Bag X3015,
Randburg
2125
(South
Africa)
ABSTRACT The flow sheet for the recovery of gold from an ore consists of several unit opet-ati on5 , of which a high percentage are based upon electrochemical principles. The unit operations are discussed in this paper to illustrate the diverse role played by electrochemistry in the overall recovery process. The discussion is based,upon the results of recent research carried out at the Counci 1 for Mineral Technology (Xintek) _
INTROIIUCTICN
sheet for a ‘conventional ’ gold plant consists of crushing and of the ore, followed by cyanidation, filtration, and recovery of gold
The flow
milling
by cementation
onto
large cyanided _ To reduce many gold existing is
operating
producers unit
costs
while
of
gold can
increasing
the
of
occluded
, which
process
roasting
without
permits
the
In view of of
gold
interest are
that,
clearly
cementation, trochemical
(viz.
these from of
recent the eight
identifiable
as having
though
material
different
and electrowinning). basis,
this
unit
A fourth is
0022-0728/84/$03.00
the role
of
is
flow
operations
perceived,
techniques
electrochemistry
0 1984 Elsevier Sequoia S.A.
basis
for
in hematite of
has had
and slimes sheet
for 1.
depicted
flotation)
(viz.,
not generally
recovery
shown in Fig.
an electrochemical
, CIY) makes. use of- electrochemical
optimization. In this paper,
a ‘typical’
developments,
some pyritic,feed
the ore_
lost to
filtration,
such as calcines a major impact on the processing of pulps, which are not easily filtered_ treatment of dump material, tion
of
pyrite
the
for
need
gold,
flotation
content
the
~-to
is
of
(and thus
of
used
or replaced
The use of (FeS2)
the pyrite
(CIP)
pulp
steps
techniques. the pyritic
within
is
the pulp recovery
processing
by
leached
separation
before
the overall
additional
be liberated
The carbon-in-pulp from
gravity
by milling
by newly developed
processes)
gold
some plants,
liberated
common in recovery
particles
dissolved
In
have introduced
fairly
conventional
(Fe203).
dust_
of gold
operations
therefore
Microscopic
zinc
particles
recover
It
the
the extracis
in Fig.
(viz.,
from
of 1,
three
cyanidation.
also.
has an elec-
whereas. a fifth. process
in the extraction
control of
gold
i
and. is
dis-
...
148 cussed
in terms
of recent
of both
the
schematic
flow
sheet shown in Fig.
1 and .the
results
research conducted at the Council for Mineral Technology (Man&k).
Au Fig. 1.
A 'typiealt fiaw
sheet far the reesvery 6f gsld.
PhCifAflBN8F PYRITE The @arl;it%&deeumanixd prae@oo f6r the s@parat46n 6F minerals by s@l@et;tve appears t6 be a pat@nt fil@d by B@ssel and B@ssal fn i877 [I]. fn
flstatien
&hi5 preeasg, graphdta partlalas w@r@ flaafod fram the gangue matrix by the additI6n 6f 641 t6 tha gulp, which ws
then beiled. hater werk@rs appaared t6
cbneentrate 6~ imprevfng the methed fer g@n@rating gas withIn the pulp; t@eh= nlques roqu+rlng the add;i%fer, ef earbenatae and aelds, the applIeatt@n of rsdue@d pr@s~ur@~, and @WI
fh@ @f@e&~fy~~s @f wat@r, a11 m@t wjth varying
d@gr@@s 6f ~uee@f~. CIad@rnflotatjon ealla ut~l~x@ a aubm@rg@d lmpellar to guek air down chann@l~ ;Inthe shaft, Thfr air am@rgesas a flno eurtalnaf
bubblesfram staf@gIeally plae@d ordf;ie@eereund tha imp@ll@r, 1~ lat@r y@ars, Plotallon r@s@areh has eaneentratad 6~ th@ @r-genier@agents (@all@d 'eelleetors')that ar@ requtrad to Interaeg a@l@ef~v@ly with th@ sur= Pac@s of th@ partIel@e t@ ba #l@at@d, Thle interestlan r@nd@ra th@ ~urfae~ ef th@ d@slr@d partfef@s hydropheble, and this faeilltat@e bubble attachment and
fletatien, In 1%3,
%alaw and Nlxen [2] prapes@d that *ha ehamleal #areas
aetlng b@tw@@nth@ surfaea and the eel'taeterw@r@ @l@etr@eh@mleal In @rIgIn, a prepesal that app@ar% te hav@ w@n general aecepfanee f&5] by wsrkars In fhls fjold.
eell@etoremest ssnnnenly utiadin tha flotatfon of pyrlta in n@utrel te mltdly alkellnepulps [6] ar@ tha xanthates(typIeal'iy,!a@-prepyl and s@c= The
butyl xanthatas). A'lllsens8 aZ, [P] measurad tha open-elreult potentials of sevaralmetal sulphidas in th@ presort@@of potassdum @thy1 xanthate (6.2% x
:’
19%)
ata
pH value
and pyrrhotite the
.exhibited
which, all
to the reversible
dixanthogen 2EtOCS;
potential
in-the
in this of 0.13
te -(FeAsS) ; chal copyri to paper
range-of are
V calculated
0.14
quoted), for
(CuFeS2) -
to 0.22.V which
the ethyl
.(u,s
are xanthate-
:
+ Ze-
(1)
dixanthogen
(att potentials in this i.e.
potentials
potentials
couple (E' = -0.06-V);
+ (EtOCS2)2
xanthate
arsenopyri
of 7..Pyrite,‘
-(Fe+*)
HljE, against
anodic
.1&a
paper
have signs
Ox + ne- + R, irrespective
consistent
with
reduction
reactions,
of the manner with which the reactions are
displayed in-the text). Bornite (Cu5FeS4) and galena (PbS)
displayed
open-
circuit potentials of 0.06 V that are cathodic tc the xanthate-dixanthogen redex petent~al. Analysis
of
the chemical campounds present cn the mineral
surfaces revealed the presence of dIxanthagen on the first group of four sulphides, whereas metal xanthate was detected on the surface of the miinerals in the second group. formation of the metal vanthatas can cccur via the react$on
although reactions producing oxfdized sulphur apec'ies(such as S,&‘, S40t-, etc.) are also feasible, Evidence supparttng reactions s=Imitarto (2), in pr@faranee to the simple Ian=axchange displacement reactIon
of dlsselved oxygen [S], i.e., the cathodic current ex&Pn (4) 4s requ4~ad
te suste-inthe anedje
current ee ranetlan (1) or (2). Woods [8]
studded the eleetreehem~eal eharaefar~st~ea oQ galena and noble-metal ala+ Wades
!n tka presanea of ethyl xanthata, and eencludad that the ehamiserptisn
oP xentheta i,a,,
occurs en the gelene surface at the open-circuit potential. Zn aeidle pulps (#.g, those obtalned in the treatment of material from slimes,
160
dams or of leach residues from uranium plants), co77ectors such as diethyl dithiophoophate (3TP) or mecaptobenzothiozole (IIBTjare commonly used 161. and Finkelstein [3] identified the products of reaction between.a number of sulphide minerals and 3TP as well as MBT. The dithiolates (DTP)2 and (MDT)2 Cioold
were positively identified as being present on the surface of pyrite, although the ferric salt of MBT, Fe(MBT)3 , was also found to be present. In both instances, the open-circuit potentials of a pyrite electrode in the presence of thiols (100 mg 7-l) was 'anodic to the reversible potentials calculated for the reactions: Eco, += %/Et+ P Etd 'S-S' 'OEt
2e___,2 E?/
(OTW 2
IMBT)
EtO' 'SDTP-
EO-0.25 V
MBT-
2
Fuerstenau et al. [lOI agreed that (0TP)g is the major product formed on pyrite surfaces in the presence of DTP, whereas Yitrofanov and Kusnikova [ll] favoured the insoluble metal dithiophosphate. The weakness in the research described above is that there is no certainty that the identification of a product adsorbed onto the mineral surface proves that flotation is indu.ced by that particular chemical compound. A recent study at Mintek 1121 of the electrochemical
behaviour of platinum, gold, and pyrite
electrodes in the presence of OTP and MBT made significant progress in identification of the possible surface products involved in the flotation of pyrite, though, at this stage, the results cannot be regarded as conclusive. Cyclic voltammograms
illustrating the i-E
response of a platinum eleitrode
(electrolyte NaCIOq at 0.1 M and pH 5) in the presence of DTP (50 mg 1
, i.e.,
2.23 x ?o-4 M, added as the potassium salt, KDTP), and in its absence, are shown in
Fig. 2. From the evolution of hydrogen in an anodic direction, the presence
of DIP is seen to inhibit the small current peaks associated with the desorption of hydrogen. The heights of the well-defined peak at -0.15 V and the poorly defined shoulder at about 0.15 V in the presence of DTP are linearly related to the potential sweep rate, indicating the possible adsorption of DTP anions. -2 measured at 0.1 V in the Interfacial capacitance values of 38 and 60 PF cm presence of OTP. and in its absence, provide confirmatory evidence supporting the adsorption of DTP. Also shown in Fig. 2 are contact angles measured on a platinum electrode held at
various
potentials
in
the presence of DTP. Contact angles greater than 20’
---
Pt
Pt t OTP-
I
0.05 rnAcia-3
i /
E/V I
-0.8
V
0
-
*o
e 0
l
e
0
e
1 40
.*
l l
0
20
Fig. 2. Cyclic voltamnograms (10 mV s-l) and contact angle measurements for Pt electrod in NaC104 (0.1 M) at a pH va'lue of 5 in the presence of DTP (2.23 x 10Bz M) and in its absence.
a
(indicating that the platinum surface is becoming hydrophobic) are obtained at potentials anodic to about 0.1 V. Because this'potential is cathodic to the equilibrium potential of the (DTP)2-DTP- reaction (0.47 V for DTP at 2.23 x 10W4 M), the results demonstrate
that species other than (DTP)2 can induce hydro-
phobicity on a platinum surface. Xn view of the results presented above, it is suggested that adsorbed OTP anions are responsible for the increased contact angle at these low potentials. The contact angle increases rapidly with increasing potential, reaching a maximum value of 57' at 0.83 V. If the potential is held at this value for some hours, the surface becomes covered with an oily film, thought to be the dithiolate (DTP)2. Chandler and Fuerstenau salt, platinum-DTP,
1131 suggested that the ins@luble platinum
is formed in this potential region. If the potential is-
increased beyond 0.83 V, a steady decrease in the contact angle is observed. In _
the region of oxygen evolution, the surface of the electrode is once again
hydrophilic. This decrease in the contact angle is cdnsidered to be due to oxidation of the dithiolate to.form further oxidation products, because
a
platinum electrode held for extended periods at potentials above 1.0 V does not
appear to generate the oily film of (OTP)3 that is observed at lower potentials. Attempts to study.the electrochemical
behaviour of DTP on pyrite electrodes
were frustrated by the high background-current& of-this mineral (due to oxidation and reduction of the pyrite) in relation to the low currents associated with the collector. Several microflotation
studies were therefore carried out so
_.
.-
:
152.
:
that the data generated actual
flotation
with several
electrodes
correlation.
by the use of platinum.electrodes
practice_
A Partridge
(platinum
of the efficiency
could-be
and Smith microflotition
of flotation
with some. electrochemical of each float
or oxygen as the carrier
air
various
(such as Na2S, KCN, or Na2S03) to alter were ontained
gas,
could
use of nitrogen, reagents
to
vtas .fitted
as tie11 as pyrite electrodes were.used) for
of the flotation system. The conditions
the pulp. Samples of pyrite
related
cell
be altered
or by the addition
property by the of
the redox- Potential
of
from Custer in South Dakota via Gfard’s
Natural Science
Establishment , Inc. , and from the Durban Roodepoort Beep (DRD) Gold Mine in South Africa. The samples were screened to a particle size between 106 and 36 Pm, and were tested for total floatability with amyl xanthate. Values of 94 and 90 % were obtained for the Custer and DRD pyrite, respectively. All flotation tests were conducted on 2 g of sample in DTP (2.23
The extent tial
of flotation
of the pyrite
electrode
appears
to correlate
reasonably
made from Custer pyrite
x 10v4 14) at ptl 5.
well
(Fig.
3).
with the potenBoth pyrite
samples exhibit increasing flotation recoveries with increasing potential, though the Custer pyrite is inherently more floatable than the JRD material. Although the potentials at which flotation occurs are cathodic to the reversible potential
of the (DTP)2-DTP- couple
enough for
formation
(0.47
of the dithiolate
V),
the difference
to be discounted.
is not large
Furthermore,
when
several large and seemingly homogeneous samples of Custer pyrite were crushed to a particle size of about 1 mm, the individual particle potentials (measured with
a small
platinum
values as large
probe
with
a pointed
tip)
were found
to
have a spread
of
as 150 mV. The average potential for 30 such particles in an
oxygen-purged electrolyte was 0.53 V. and only two particles failed to attain potentials higher than 0 -47 V. If it is assumed that a17 particles exhibiting potentials anodic to the (DTP)2-DTP- equilibrium potential will float, a flotation
recovery
of 93 % can be predicted.
This predicted
value is to be expected
from the results in Fig. 3, if it is borne in mind that the total floatability of the Custer pyrite (as measured with amyl xanthate) was 94 %. This experiment was repeated
in a nitrogen-purged
of 0.42 V and a predicted flotation the range 80 to 90 % were obtained
electrolyte-,
and yielded
an average
potential
recovery of only 7 %. However, values experimentally (Fig. 3).
in
The conclusion to be derived from these results is that good floats can be obtained even when the potentials of the individual particles in the pulp are
cathodic
to the equilibrium
potential
of the (DTP)2-DTP- couple.
This floata-
bility is attributed to the presence of adsorbed DTP anions, which apljear to be capable of inducing contact angles of 20’ to 30’ (Fig. 2). The positive identification of (DTP)2 on the surface of floated pyrite reported~by Goold and Finkelstein
191 could
arise
from the fraction
of all
pyrite
particles
that had
-.
-:.
153. '. _ioo_ : -. m
:
l
Extent of flotation of surface potential
potentials
sufficiently
problem can provide the more classical
0.44
Custer and Durban ?oodepoort Deep pyrites (collector 2.23 x 10-4 M DTP).
of
anodic for the oxidation
flotation of the bulk of the material DTP anions. Although there field of flotation,
0.42
0.40 0.38 Potential/V.
0.36
0.34
Fig. 3. function
:-
Custer Pyrite Durban Oeep Pyrite
of DTP to occur.
is attributed
as a
However,
to the presence
of adsorbed
is clearly much mdre fundamental research required in the it is apparent that anelectrochemical approach to the an insight
into
the phenomenon of flotation
that complements
approach.
CYANIDATION OF GOLD The leaching
of gold by oxygen in the presence
of cyanide proceeds
via the
reacti on 4Au -i- o2 + SCN- + 2H20 +
4Au(CN); + 40H-
(6:
although it is still uncertain whether the reduction of oxygen stops at hydrogen peroxide [14]. Because of
efficiencies South Africa
cal cined
the difficulty involved in the achievement of high is leached from pyrite (a problem that Was recognized -in over 80 years ago [15]), it is comnon practice ~for the pyrite to be
when gold (producing
hematite ; Fe20B , which is subsequently-subjected
or for the pyrite to be milled to a very fine particle tion), dation; The poor leaching characteristics of gold from pyrite attributed to one or more of the following factors:
(ij
.-the presence ‘trapped milling
of very fine
within
the.larger
to liberate
size
to cyanidabefore
parti cl es of gold, -which are occluded Pyrite
the gold;
particl&.necessitating 1
cyani-
are generally
fine
or
_
164_. ... (ii)
:
..
.’
reduction.
present
(iv)
sulphide
aeration
is interesting,
of free
which- -ii u&ally
cyanide
due to the~formation
of
and.
of sulphide
ions (a particularly effective poison in into the pulp, above are trivial, since they can be eliminated-merely
and the addition because
of sufficient
the practical
addition
of lead
(usual ly in the form of
leaching
vessels
[17].
cementation
pyrrhotite~,
1
and ferri-cyanides;
the liberation
(iv)
(particularly
in the concentration
cyanidation [I6]) Points (ii) and (iii) by adequate
minerals
with pyrite);
reduction ferro-
.
in .the ckmcentrtition .oi -oxygen. in the -pulp due to ~&idat>on
-of- the iron (iii)
.-_
This effect
of gold onto zinc
cyanide
solution
Point
to the problem involves
lead nitrate
is discussed
to the pulp.
or li tharge,
later
in relation
the.
PbO) to the
to the
dust.
By far
the most interesting problem is that of the occlusion of gold within necessitating either fine milling or calcination. The the pyrite particles) question arises as to whether the problem is totally physical in nature (as it involvement_ Evidence appears to be), or whether there is some electrochemical that strongly supports the role of electrochemistry was recently presented by Filmer [181. In Fig. 4, the i-E curve
(as measured in this
laboratory)
is shown for
oxida-
tion lyte
of a gold rotating-disc (500 rpm) electrode in a nitrogen-purged electroof 0.01, 0 -02. and 2 x containing NaOH, NaCN, and Pbzt at concentrations Also shown are the curves for the reduction of oxygen on IO-5 M, respectively_ the same gold electrode and on a pyrite rotating-disc electrode (air-saturated electrolyte gold will
containing corrode
0.01 M NaOH). It is clear
spontaneously
in an aerated
from these
5-L” curves
al kal ine cyanide
solution
that at a
mixed potential of approximately -0.41 V and at a rate (limited by the mass transport of oxygen to the electrode surface) of about 30 PA (approximately 0.4 mA cmm2). Although for
the reduction
measured currents
the current
density
(which is
of oxygen on the pyrite
electrode
are considerably
because
higher
limited is also
this
by mass tpnsport) 0.4 mA cm
, the
electrode
has a much cm2). Since the
larger surface area (0.58 cm2) than the gold electrode (0.071 peak current (at -0.28 V) for the oxidation of gold (0.23 mA) is lower than that for the reduction of oxygen on pyrite (-0.25 mA at -0.28 V), short-circuiting
of the INO electrodes
should
result
in immediate passivation
of the
gold electrode. When the experiment was carried out, the open-circuit potential of the paired electrode increased to a value of 0.01 V in a matter of seconds. At this potential , the corrosion rate of the gold electrode is only 4 PA (approximately 0.06 mA cmq2). The practical implication-of any gold particle embedded in the surface of a much larger
this result is that pyrite particle, .but
of gold and Current-potential curves (1 mV S-l) for the oxidation Fig. 4. (Gold surface area 0 -071 cm2 ; reduction of oxygen on gold and pyrite electrodes. FeS2 surface area 0.58 cm*_) nevertheless exposed retarded rate.
to oxygen and cyanide,
wi 11 be leached
at a -greatly
Experimental results supporting the above electrochemical interpretation were reported by Filmer [18] . In a 24-h leach, the extraction of gold from milled pyrite (with a BET surface area of 0.3 m2 g-l) was 17 %, compared with 82 % from the calcine (with a surface area of 3.0 m2 g-l) produced by roasting of the pyrite. Extension of the leaching period to 21 days increased the recovery of gold from the pyrite (28 %) , but had no effect upon the calcine-. However, partial reduction (by hydrogen at 7OO’C) of’ the calcine to yield some magnetite reduced the -extractibn of gold (in (FegD4), which is an electronic conductor, a 24-h.leach) from 82 to 60 %, though no change in the surface area was observed. These results indicate the beneficial effects of roasting of the pyrite to liberate occluded.gold, as well as the retarding effects of large surface areas for
the reduction
of oxygen on the rate .at which gold
CARBON-IN-PULPPROCESSFOR GOLDRECOVERY In a conventional gold plant, a significant
is leached.
percentage
and 1 %) of the go1.d dissolved during the cyanidation the filtration stage that yields a clarified solution
(varying
6etween 0.2
process is ‘lost’ during for the -cementation step.
This loss arises .because of the large tonnage treated (a typical plant processes 200 OODt of ore per month) and a constraint on the amount of water that can be used to wash t.he ioluble gold from the filter cake. This problem is exacerbated by the difficulty the retreatment of their
with which. certain materials (such ascalcines and. slimes from of old dumps) are f i 1tered and washed, owing ‘to the small size
particles.
.:&+:
--2.
., -. -, ..-. ..~:
-;-
y,‘:.
._. In .the. CI?_process
;19,20]
.r ._:;::.:- __._ -:,:.. ;;;.::.._. . .. .: -.. .:_ :.
i the entirefiltration
,tep
js
:
.,-
ei<.mfnat&~.by:
the-..
’ -___ ._ : is .added to the l&t_ tank (usually -referred to as a. ‘stage’) in a-‘series of .. ‘~. absorption stages (typically four to six stages are-used). .The pulp, containing.
addition.
of granules
dissolved all
gold;
the
of activated.carbontol’the
is pumped into at a ‘flow
stages
in each stage.
Screens
the first
rate designed
on the
peripheries
cyanidation:pulp’.
stage , and flows
The carbon
under gravity
through.
to permit a residence. time of about i-h of the tanks restrain the.carbon.
granules within each tank while permitting .the pulp to flow to the next stage. At regular intervals, a percentage of the pulp and the carbon in the nth stage is
pumped back to the
A certain
(n-1)th
stage,
thus maintaining
a carbon
flow that is
each stage
is adsorbed
to the overall flow of the pulp.
counter-current
percentage
of the soluble
gold entering
onto the carbon. If six stages are used, and 70 % of the dissolved aurocyanide ions are adsorbed onto the carbon in each stage, an overall recovery higher than
99.9
eluted ratures
The loaded carbon emerging from the first
% fs attained.
stage
is
[Zl] by treatment with a solution of NaOHand NaCNat elevated tempe, and the gold is finally recovered by electrowinning [ZZ]. The eluted
carbon is reactivated for 2 to 3 min, after
by being heated to about 700 ‘C in the presence of steam which it is returned to the last stage of the absorption
circuit. To optimize operation developed a mathematical various
operational
of the adsorption circuit, Fleming et aZ. (231 model for the process, which relates the effects
parameters
(such as the flow rates
of
of the Pulp and the
carbon, and the mass of carbon in each stage) to the efficiency of adsorption. One of the interesting conclusions to emerge from this study was that each stage should contain
equal. masses of carbon.
However,
for
various
reasons
(such
as
surges
in the flow rate of the pulp, blocked screens, etc.), large fluctuations in the carbon concentrations in each stage are frequent. A typical variation in
the carbon concentration in the last stage of a South African CIP plant is shown in Fig. 5. The data points were obtained by weighing of the carbon obtained in a IO-litre
‘grab sample’
of pulp from the adsorption
tank.
Also shown in Fig. 5 is a continuous curve of the carbon concentration as determined by an electronic ‘carbon meter’ [24]. This instrument, which is still under development to-date
at Mintek,
measurements
provides
the
of the concentration
plant
operatorwith
of carbon
into
and generally
the pulp,
and carbon particles
have surface
die to the potential the open-circuit to 5 mV in height.
potentials
of the probe)
potential
that
strike
of the electrode,
earthed. The probe
(which are electrically are
and up-
in each stage. The measuring
probe consists of two metal electrodes, one of which is
inserted
accurate
several
the indicator producing
hundred
millivolts
electrode. small
is conducting
voltage
catho;
This affects pulses
1
The pulses are amplified and counted if they are above the
5. Comparison between the measurements obtained manually and by the Mintek carbon meter of the carbon concentration in a CIP absorption tank.
Fig.
random, backgrcund
noise
level . The recorder curve shown in Fig. 5 is the output from an electronic ratemeter, i .e., a voltage that is proportional to the rate at which pulses are detected. This, in turn, is proportional to the concentration of carbon in the pulp. Perhaps even more important than maintenance of the correct concentration of carbon in each stage is the activity of the carbon that is returned to the adsorption circuit. If the eluated carbon has not been sufficiently reactivated thermally, the capacity and the kinetics of the adsorption process are affected_ The activity of the carbon can be determined in the laboratory as follows. A known mass of carbon is added to a-stirred beaker containing gold (as potassium aurocyanide) at a concentration of 100 mg 1-l , and the decrease. in the concentration of gold with time as it-is adsorbed by the carbon is monitored. However, this
technique
is labour-intensive
using an_ atomic-absorption gold
in alkaline
cyanide
and requires
spectrophotometer. electrolytes
is readily
capable
the analysis
accomplished
of
of
by anodic-strip-
is well
ping vol tammetry ) and this
technique
processor-based
is currently
instrument
a skilled.dperator In contrast.
suited to.automation. A microbeing tested at :?intek. Once activated,
the instrument continuously measures the.gold concentration during its adsorption onto the~carbon, each data point (i.e., concentration US time of. measurement) being automatically plotted on an X-Y recorder. Apart from the-addition of the test solution to .the stirred beak~er?, no further .attention- is required from the operator, and a complete the end of the. test. :
carbon-adsorption .: -’ ._ . .
profile
is available
at -.
-_
l$s
ELE&JwINNINGOF &jLD.
:
‘~
-The ele&rowinning-of
,,: ..
-
_’
gold
-_
:
has become ‘a- familiar
unjf
-’
-.
._
--. ::-
operation
-.
._..
:’
in~,South :~:.‘.‘_
African gold mink, the technique being -used in the treatment of GIP eluates and of .leach liquors resulting from intensive -cyanidati’oh [25] _’ These two-. ‘1 -.. -1 .types of electrolytes contain gold 1;: the concentrationrange 100. to .lOOO’mg after el ectrowinning _ An interesting. before electrolysis, and 5 to 10 mg new development
[26]
is the elution,
and simultaneous-
recovery
by eleitrowin-
resins that have. been used .in the ning, of gold from poisoned anion-exchange extraction of uranium. In this process, the eluate (acidic thiourea or.neutral ammonium thiocyanate) is circulated from a tank containing-the resin through the electrowinning cell and back to the tank. As the eluted gold is recovered during the elution procedure, the concentration of gold entering t&cell -1 rarely rises above 10 mg 1 . Even at these low concentrations. use of the Mintek packed-bed electrowinning cell [22] results in recoveries of 50 to 70 % of the gold in the eluate per pass through the celll?t -1 O-4 m min (i.e., a volume flow rate of 200 1 min ). CEMENTATION OF GOLD .The recovery of metallic
gold
by cementation
a linear
onto zinc
flow
rate
of
dust is an electro-.
chemical process [27] involving the oxidation of zinc and the reduction of aurocyanide ions. The overall stoichiometry of the process is as follows: Zn f 2Au(CN)i
+
Zn(CN)i
-e
(7)
~Au
Pregnant solution from the filter plant (typically containing 5 to 10 mg of dissolved gold per litre) is usually stored in a surge tank before being pumped via a Crowe tower (for de-aeration) into an emulsifier tank, where lead nitrate and zinc
dust
(of
particle
size
between 110 and 70 pm) are added.
After
a
mixture is pumped residence time no longer than 1 or 2 min, the solid-liquid into a Stellar or a Merrill filter unit. The zinc dust forms a continuous bed of reducing
agent
on the
filter
medium through
which
the
pregnant.
solution
must
flow. Owing to the cathodic potential of the zinc (typically -1-l V , which causes the rate of go?d deposition to be 1 imited by the mass transfer of aurocyanide
ions to the surface
namic conditions’within
the
very high, barren solutions usually being obtained. Poor cementation following (i)
of the zinc bed,
particles)
the efficiency
of the cementation
with a gold content
efficiencies
can usually
and the excellent
hydrodyprocess -1
is
lower than 0.01 mg 1
be attributed
to one of the
effects: inefficient
de-aeration;;
to the reduction
resulting
of oxygen;
in excessive
corrosion
of zinc
due
1 ._. ~-___;: :.~I;
,,
:
-._ ._I .; ,i;;,_.. -, (iii)
-._
y..’
.- -:.
_
1:.
_
-.
._ :..
__.._ ,_..-.,:
;
‘.
-_I_
-
~.
:
_.
. .
._
__,i.
.
-.
..
:
:..
..-. ... _
_ -ticin .hydrbxide; Ghiih . . . .of.iinc .;.. passivates . . the-anodit.dissolution-of ~_ _‘::zinc’, and__, .’ -.. ‘1. .I. _ :‘;I 2: ._‘ _-:_.:‘-:‘ .:. 1..the: p&.&e
of::poisons-;-
-.‘.whi&i are reported’&
.-
::
..
159 ,. .. ._ .- _: _._ : .:~ ,_ :. ; .-. ;:‘-:-::.-:._:: ..; -;- ::-. ‘--.f_;.._ _- ,-; .’ -: :_:. :__. ” pregnant-solution; resulting---in the forma-. ~.. ‘.;I- insufficient..-cyanide:*:!.n-;:the ‘_ _,:
.. -. cy at concentrations .During.recent pilot-plant
such as sulphide
h&e .a deleterious
ions-, in _the pregnant solution, -effect-
on cementation
-as 1ow as. 0.03 mg 1-I [14]. testwork at Mintek on the cyanidation
efficien-
and .subse-
quent cementation of gold from the clarified. leach liquor, the control experiment (which duplicated the procedure used on the gold plant from which the milled ore was obtained) yielded extremely poor cementation results (barren solutions .of ._0.5 mg 1-1 from a head-value of 5 mg 1-l). However, additional cementation tests performed several tailings of less .than 0.002 mg 1-l.
days later
on the same leach liquor
yielded
Close inspection of the gold‘plant revealed that the clarified leach liquor from the filter plant is stored in a 1200 m3 surge tank before being pumped to the Merrill tion
filters
in this
for
cementation.
tank is approximately
The average
residence
1 h. A concentrated
time
solution
for
the
solu-
of lead nitrate
-is continuously pumped into the surge tank to give a concentration lead of about 5 mg 1-I.- Additional lead nitrate is added directly -1 2+ .. emulsifiers to raise the Pb valueto8mgl
of dissolved to the
During leaching of a further 400 kg of pulp at Mintek, a sample of the pregnant solution was subjected to cementation immediately after the addition of Pb2+ at- a concentration of 8 mg 1 -1 _ Lead nitrate was added to a second sample of 5 mg 1-l) which was of the pregnant solution (to give a Pb2+ concentration aerated resulting
for
2 h. Further Pb2+ at a concentration
solution
cementation
yielded
Although the results sulphide
ions,
of 3 mg 1-l was added, and the
was pumped through the cementation barren solutions
cell _ The two tests
of 0.2 and 0.002 mg .1-I,
appeared to point
to the presence
in the pregnant electrolyte
on
respectively.
of a poison,
,- which is subsequently
such as oxidized
on
contact with atmospheric oxygen, confirmatory evidence was not easily obtained owing to the difficulty involved in‘the analysis of sulphide ions in solutions containing cyanide ions. Nevertheless, this investigation led to a study of the rate of oxidation of sulphide ions in an air-sparged electrolyte containing NaOH(0 .OI M) and S2- (50 mg 1-l). (A sulphide ion-selective electrode was used since
no cyanide
ions were present.)
The rate of oxidation-of
sulphide
ions was
found to be very slow in the absence of dissolved lead, no measurable drop in the concentration of-sulphide being observed over a 3-h period. However, the -1 -addition of Pb2? at a concentration of 4 mg 1 considerably enhanced the rate of oxidation;
the concentration
over the 3-h period. 1-I is theoretically
of sulphide
ions declined
(It should be noted that Pb? capable
of precipitating
from 50 to 2 mg l-1.
at a.concentration of 4-mg only 0.6 mg -1-l iof 52- as PbS.)
..:-... ‘160. ~: :
_
_:
_.
Similar
effects
were -noted with Cupric,
The .insoluble. electronic solids
metal sulphides
Conductors.
on the rate
1 g of coppe;
It
powder (BF-surface
and ..ferrous
_.
ions-.. --:.
of all, the .metal ions discu&ed
was therefore
of sulphide
ferric
. .
oxidation
.decided
thatthe
should-be
above. are good
effects-
area 0;19 m2 g-l)
to 1 1itre
.. 1
of various
measured...The
addition
of
of 0.01~ M NaOH
to below 0.1 mg containing S - (58 mg 1 ) reduced the sulphide concentration 1-I in approximately.15 min, whereas-l g of natural galena and pyrite (surface required approximately 50 min to areas of 9.67 and 0.54.1s~ g-l. respectively) effect a similar reduction. Chalcopyrite (surface.area. 0.58 m* g-l)- decreased -1 the rulphide concentration from 50 to 20 mg 1 in 3 h. In Fig. 6, the current-potential .curves (10 mV s-I) are shown for the oxidation of S2- (50 mg 1-l) in nitrogen-purged 0.01 M NaOHirith rotating-disc electrodes (500 rpm) fabricated from the minerals potentials recorded for the copper electrodes 2cu + s*-
mentioned above. The very negative are due to the reaction
-+ Cu2S f 2e-
(3)
2which has an equiliirium potential of -0.83 V in an electrolyte with an S content of 50 mg l-I_ The overpotentials for the oxidation of sulphide ion (the equilibrium potential for the S-S*- couple is -0.43 V) increase in the order
PbS c FeS2 c
CuFeS2.
Also shotin in Fig.
of oxygen (as well
6 are the corresponding
as reduction
cathodic
of the mineral
curves
for
in some instances)
sparged electrolyte of O.OL M NaOH. The overpotentials FeS2 c Cu -Z PbS -= CuFeS2. Xand [28] also found pyrite
the reduction from an air-
increase in the order to possess the greatest
activity for the-reduction of oxygen, though he reported chalcopyrite to be more active than galena. Combination of the various anodic and cathodic processes give the electroihemical cm
-2
rates
for the reaction of sulphide as 0.66, 0.11, 0.070 and 0.014 mA
for the Cu, FeS2, PbS and CuFeS2 eTectrodes, respectively. These rates
follow the same order as that observed for the experiments in which 1 g samples of the powdered minerals were used. The implication of these results is that the homogeneous rate of sulphide oxidation by oxygen is very slow, and that the heterogeneous rate at the surface of an electronic conductor is considerably faster. The addition of lead nitrate in gold
plants where there is a sulphide problem results in the of lead sulphide, which acts as a site for the reduction of oxygen and the,oxidation of sulphide ions. Furthermore, the presence of a precipitation
surge tank for storage of the pregnant solution from the filter plant provides sufficient residence time for oxidation of the residual su_lphide ions. The common practice
of 1i tharge or lead nitrate.
addition
to the leaching
vessels
Anodic and cathodic i-E curves for the oxidation of sulphide ions and the reduction of. oxygen in -0 .Ol :J NaOHwith copoer , galena , pyrite , and chalcopyrite rotating electrodes (500 rpm, 10 mV s-1).
Fig. 6.
when sulphidic i.e.,
ores
acceleration
are
being
of the
rate
treated
undoubtedly
at-which
sulphide
serves is
the
same purpose,
oxidized.
CONCLUSIONS
There is little doubt that the evidence clearly justifies the statement that the recovery of gold is predominantly an eiectrochemical process. In addition to -and cementation, electrochemisits obvious role in iryanidation. electrowinning, try plays a far more subtle part in other.operakions
CIP process. Although metallurgy
is ersentia7ly
an applied
such as flotation
practice,
and the
the application
of
theoretical -electrochemical principles to the subject has facilitated the optimization of many of the process stages. Further developments and improvements can be expected in this field as a better of the -various operations is developed.
understanding
of the electrochemistry
ACKNWLEOGEMENT
This paper is published
by permission
of the Council
for Yineral Technology.
REFERENCES 1 Bessel and Bessel, Ger. Patent no. 42, Class 22 (1877). 2 S.G. Salamy and J.C. Nixon, Recent Developments in 14ineral Dressing, 3
4 5
Institution of Mining and Metallurgy,, London, 1953, pp_ 503-516. S.X. Ahmed, in N.Li. Norman (Ed-), Recent Developments in Separation Science. Vol. .V, CRC Press, Cleveland, 1972, pp. 95-133. R. Woods, in M.C. Fuerstenau (Ed.). Flotation, A.M. Gaudin Memorial Volume,_-Vol. 1, AIME, New York, 1976; pp. 298-333.
G-W. Poling._in
M.C.
Fuerstenau
(Ed.),
Flotation,
A.M. Gaydin Memorial
162
_.
,.
Volume, Vol. 1,. AIME New. York 1976. pp. 334-363.. -, Vacation School , Johannesburg, P.-J. Lloyd, in Principles of &a .Flo&ition, 1978, University of the Witwatersrand, Johannesburg, 1978, pp. 57-107. 1-A. Goold, N.J. Nicol, and A..Granville, Metall. Trans., 7 S.A. Allison, 3 (1972) 2613-2618. 354-362.’ 8 R_ Woods, J. Phys. Chem., 75 (1971) Report 1439, National Institute for 9 L.A_ Goold and N-.P.Finkelstein, Metal 1urgy , Johannesburg, (1972). J .L. Huiatt, and M.C. Kuhn, Trans. Metal1 . Sot. AIME, 10 M.C. Fuerstenau, 250 (1971) 227~231. Mitrofanov and V.G. Kusnikova, in Svenska Gruvfareningen and 11 S.I. Jernkontoret (Eds.), Progress in :4ineral Dressing, Transactions; 4th 1957, Almqvist and International :4i neral Processing Congress i Stockholm, Wiksel l , Stockholm, 1958, pp. 461-473. at MINTEK 50, Sandton, 1984. i2 D. Groot, Paper presented J. Electroanal . Chem., 56 (1974) 217-247. 13 S. Chandler and D.W. Fuerstenau, Gold Metallurqv in South Africa, 14 N.P. Finkelstein. in R.J. Adamson (Ed.). Cape and Transvaa? Printers Ltd, Cape Town, 1972, pp. %4-351_ in R. Stokes (Ed.), A Text-book of Rand :4etallurgical 15 W.A. Caldecott, Practice, Charles Griffin and Co., London, 1912, pp. 380-398. 16 C.G. Fink and G.L. Putnam, Trans. Tiletall. Sot. AIHE, 187 (1950) 952-955. Dressing Notes no. 23, American Cyanamid 17 N. Hedley and H. Tabachnick , Mineral Co. 1968. p. 36_ 18 A-0. Filmer, J.S. Afr. Inst. Min. !4etall., 82 (1982) 90-94. F.:4. Howell, Northern !4iner., 20th April (1970). Holtum-and R. Rubin, in H.W. Glen (Ed.), :z P .A_ Laxen, C.A. F7eming;D.A. Proc. Twelfth Congress of the Council for Ilining and Metallurgical Institutions, The South African Institute of Mining and Metal turgy, Johannesburg, 1982, Vol. 2, pp. 551~5:1, inst. 14in.frletall-, 77 (1977) 21 R-3. Davidson and D. Duncanson, J-S. Afr.
6
254-261. in K. Osseo-Asare and J.D. lililler 22 R.L. Paul, A-0. Filmer and M.J. Nicol. Development and Plant Practice, Proc. 3rd (Eds.), Hydrometallurgy-Research, International Symposium on Hydrometal lurgy , Atlanta, 1983, pp. 689-704. M.J. Nicol and D.I. Nicol, Symposium: Ion Exchange and Solvent 23 C.A. Fleming, Extraction in Mineral Processing, National Institute for Metallurgy, Randburg, 1980. Paper presented at MINTEK and M-J. Ohlson de Fine, 24 N.D. Hulse, B. Fitzgerald 50, Sandton, 1984. G.A. Brown, C.G. Schmidt, N-W. Hanf, 3. Duncanson *and 25 R.J. Davidson, 5.3. Tavlor, J. S. Afr. Inst. Min. Hetall., 78 (1978) 146-165. C.A. Fleming, Council for Mineral Technology, Private Communication. :67 :4-J. Nicol, E. Schalch and P. Balestra, J. S. Afr. Inst. Min. Metal1 ., 79 (1979). 191-197. Chem., 83 (1977) 19132. 28 3-A-J. Rand, J. Electroanal.
-.
1,.
-_ -_:
._
-_
:
_~
: _-
.-
j.-Eieccioo;;or;-Chern,;~~~ &Y+&~-~&~. ._.-
-
-’
-
..
-147
_-
.- Elsevier S&uoia SiA., Lai&nne Y Printed.in The NethMands
:
1.
- _. -. .: :
‘~,_ .’
:
‘.
._.
THE.ROLE
OF ELECTROCHEMISTRv; IN THE EXTRACTION . . .
. R-L.
*
:
~..
OF-GOLD
PAUL
Council
for
#ineral
Technology,
Private
Bag X3015,
Randburg
2125
(South
Africa)
ABSTRACT The flow sheet for the recovery of gold from an ore consists of several unit opet-ati on5 , of which a high percentage are based upon electrochemical principles. The unit operations are discussed in this paper to illustrate the diverse role played by electrochemistry in the overall recovery process. The discussion is based,upon the results of recent research carried out at the Counci 1 for Mineral Technology (Xintek) _
INTROIIUCTICN
sheet for a ‘conventional ’ gold plant consists of crushing and of the ore, followed by cyanidation, filtration, and recovery of gold
The flow
milling
by cementation
onto
large cyanided _ To reduce many gold existing is
operating
producers unit
costs
while
of
gold can
increasing
the
of
occluded
, which
process
roasting
without
permits
the
In view of of
gold
interest are
that,
clearly
cementation, trochemical
(viz.
these from of
recent the eight
identifiable
as having
though
material
different
and electrowinning). basis,
this
unit
A fourth is
0022-0728/84/$03.00
the role
of
is
flow
operations
perceived,
techniques
electrochemistry
0 1984 Elsevier Sequoia S.A.
basis
for
in hematite of
has had
and slimes sheet
for 1.
depicted
flotation)
(viz.,
not generally
recovery
shown in Fig.
an electrochemical
, CIY) makes. use of- electrochemical
optimization. In this paper,
a ‘typical’
developments,
some pyritic,feed
the ore_
lost to
filtration,
such as calcines a major impact on the processing of pulps, which are not easily filtered_ treatment of dump material, tion
of
pyrite
the
for
need
gold,
flotation
content
the
~-to
is
of
(and thus
of
used
or replaced
The use of (FeS2)
the pyrite
(CIP)
pulp
steps
techniques. the pyritic
within
is
the pulp recovery
processing
by
leached
separation
before
the overall
additional
be liberated
The carbon-in-pulp from
gravity
by milling
by newly developed
processes)
gold
some plants,
liberated
common in recovery
particles
dissolved
In
have introduced
fairly
conventional
(Fe203).
dust_
of gold
operations
therefore
Microscopic
zinc
particles
recover
It
the
the extracis
in Fig.
(viz.,
from
of 1,
three
cyanidation.
also.
has an elec-
whereas. a fifth. process
in the extraction
control of
gold
i
and. is
dis-
...
148 cussed
in terms
of recent
of both
the
schematic
flow
sheet shown in Fig.
1 and .the
results
research conducted at the Council for Mineral Technology (Man&k).
Au Fig. 1.
A 'typiealt fiaw
sheet far the reesvery 6f gsld.
PhCifAflBN8F PYRITE The @arl;it%&deeumanixd prae@oo f6r the s@parat46n 6F minerals by s@l@et;tve appears t6 be a pat@nt fil@d by B@ssel and B@ssal fn i877 [I]. fn
flstatien
&hi5 preeasg, graphdta partlalas w@r@ flaafod fram the gangue matrix by the additI6n 6f 641 t6 tha gulp, which ws
then beiled. hater werk@rs appaared t6
cbneentrate 6~ imprevfng the methed fer g@n@rating gas withIn the pulp; t@eh= nlques roqu+rlng the add;i%fer, ef earbenatae and aelds, the applIeatt@n of rsdue@d pr@s~ur@~, and @WI
fh@ @f@e&~fy~~s @f wat@r, a11 m@t wjth varying
d@gr@@s 6f ~uee@f~. CIad@rnflotatjon ealla ut~l~x@ a aubm@rg@d lmpellar to guek air down chann@l~ ;Inthe shaft, Thfr air am@rgesas a flno eurtalnaf
bubblesfram staf@gIeally plae@d ordf;ie@eereund tha imp@ll@r, 1~ lat@r y@ars, Plotallon r@s@areh has eaneentratad 6~ th@ @r-genier@agents (@all@d 'eelleetors')that ar@ requtrad to Interaeg a@l@ef~v@ly with th@ sur= Pac@s of th@ partIel@e t@ ba #l@at@d, Thle interestlan r@nd@ra th@ ~urfae~ ef th@ d@slr@d partfef@s hydropheble, and this faeilltat@e bubble attachment and
fletatien, In 1%3,
%alaw and Nlxen [2] prapes@d that *ha ehamleal #areas
aetlng b@tw@@nth@ surfaea and the eel'taeterw@r@ @l@etr@eh@mleal In @rIgIn, a prepesal that app@ar% te hav@ w@n general aecepfanee f&5] by wsrkars In fhls fjold.
eell@etoremest ssnnnenly utiadin tha flotatfon of pyrlta in n@utrel te mltdly alkellnepulps [6] ar@ tha xanthates(typIeal'iy,!a@-prepyl and s@c= The
butyl xanthatas). A'lllsens8 aZ, [P] measurad tha open-elreult potentials of sevaralmetal sulphidas in th@ presort@@of potassdum @thy1 xanthate (6.2% x
:’
19%)
ata
pH value
and pyrrhotite the
.exhibited
which, all
to the reversible
dixanthogen 2EtOCS;
potential
in-the
in this of 0.13
te -(FeAsS) ; chal copyri to paper
range-of are
V calculated
0.14
quoted), for
(CuFeS2) -
to 0.22.V which
the ethyl
.(u,s
are xanthate-
:
+ Ze-
(1)
dixanthogen
(att potentials in this i.e.
potentials
potentials
couple (E' = -0.06-V);
+ (EtOCS2)2
xanthate
arsenopyri
of 7..Pyrite,‘
-(Fe+*)
HljE, against
anodic
.1&a
paper
have signs
Ox + ne- + R, irrespective
consistent
with
reduction
reactions,
of the manner with which the reactions are
displayed in-the text). Bornite (Cu5FeS4) and galena (PbS)
displayed
open-
circuit potentials of 0.06 V that are cathodic tc the xanthate-dixanthogen redex petent~al. Analysis
of
the chemical campounds present cn the mineral
surfaces revealed the presence of dIxanthagen on the first group of four sulphides, whereas metal xanthate was detected on the surface of the miinerals in the second group. formation of the metal vanthatas can cccur via the react$on
although reactions producing oxfdized sulphur apec'ies(such as S,&‘, S40t-, etc.) are also feasible, Evidence supparttng reactions s=Imitarto (2), in pr@faranee to the simple Ian=axchange displacement reactIon
of dlsselved oxygen [S], i.e., the cathodic current ex&Pn (4) 4s requ4~ad
te suste-inthe anedje
current ee ranetlan (1) or (2). Woods [8]
studded the eleetreehem~eal eharaefar~st~ea oQ galena and noble-metal ala+ Wades
!n tka presanea of ethyl xanthata, and eencludad that the ehamiserptisn
oP xentheta i,a,,
occurs en the gelene surface at the open-circuit potential. Zn aeidle pulps (#.g, those obtalned in the treatment of material from slimes,
160
dams or of leach residues from uranium plants), co77ectors such as diethyl dithiophoophate (3TP) or mecaptobenzothiozole (IIBTjare commonly used 161. and Finkelstein [3] identified the products of reaction between.a number of sulphide minerals and 3TP as well as MBT. The dithiolates (DTP)2 and (MDT)2 Cioold
were positively identified as being present on the surface of pyrite, although the ferric salt of MBT, Fe(MBT)3 , was also found to be present. In both instances, the open-circuit potentials of a pyrite electrode in the presence of thiols (100 mg 7-l) was 'anodic to the reversible potentials calculated for the reactions: Eco, += %/Et+ P Etd 'S-S' 'OEt
2e___,2 E?/
(OTW 2
IMBT)
EtO' 'SDTP-
EO-0.25 V
MBT-
2
Fuerstenau et al. [lOI agreed that (0TP)g is the major product formed on pyrite surfaces in the presence of DTP, whereas Yitrofanov and Kusnikova [ll] favoured the insoluble metal dithiophosphate. The weakness in the research described above is that there is no certainty that the identification of a product adsorbed onto the mineral surface proves that flotation is indu.ced by that particular chemical compound. A recent study at Mintek 1121 of the electrochemical
behaviour of platinum, gold, and pyrite
electrodes in the presence of OTP and MBT made significant progress in identification of the possible surface products involved in the flotation of pyrite, though, at this stage, the results cannot be regarded as conclusive. Cyclic voltammograms
illustrating the i-E
response of a platinum eleitrode
(electrolyte NaCIOq at 0.1 M and pH 5) in the presence of DTP (50 mg 1
, i.e.,
2.23 x ?o-4 M, added as the potassium salt, KDTP), and in its absence, are shown in
Fig. 2. From the evolution of hydrogen in an anodic direction, the presence
of DIP is seen to inhibit the small current peaks associated with the desorption of hydrogen. The heights of the well-defined peak at -0.15 V and the poorly defined shoulder at about 0.15 V in the presence of DTP are linearly related to the potential sweep rate, indicating the possible adsorption of DTP anions. -2 measured at 0.1 V in the Interfacial capacitance values of 38 and 60 PF cm presence of OTP. and in its absence, provide confirmatory evidence supporting the adsorption of DTP. Also shown in Fig. 2 are contact angles measured on a platinum electrode held at
various
potentials
in
the presence of DTP. Contact angles greater than 20’
---
Pt
Pt t OTP-
I
0.05 rnAcia-3
i /
E/V I
-0.8
V
0
-
*o
e 0
l
e
0
e
1 40
.*
l l
0
20
Fig. 2. Cyclic voltamnograms (10 mV s-l) and contact angle measurements for Pt electrod in NaC104 (0.1 M) at a pH va'lue of 5 in the presence of DTP (2.23 x 10Bz M) and in its absence.
a
(indicating that the platinum surface is becoming hydrophobic) are obtained at potentials anodic to about 0.1 V. Because this'potential is cathodic to the equilibrium potential of the (DTP)2-DTP- reaction (0.47 V for DTP at 2.23 x 10W4 M), the results demonstrate
that species other than (DTP)2 can induce hydro-
phobicity on a platinum surface. Xn view of the results presented above, it is suggested that adsorbed OTP anions are responsible for the increased contact angle at these low potentials. The contact angle increases rapidly with increasing potential, reaching a maximum value of 57' at 0.83 V. If the potential is held at this value for some hours, the surface becomes covered with an oily film, thought to be the dithiolate (DTP)2. Chandler and Fuerstenau salt, platinum-DTP,
1131 suggested that the ins@luble platinum
is formed in this potential region. If the potential is-
increased beyond 0.83 V, a steady decrease in the contact angle is observed. In _
the region of oxygen evolution, the surface of the electrode is once again
hydrophilic. This decrease in the contact angle is cdnsidered to be due to oxidation of the dithiolate to.form further oxidation products, because
a
platinum electrode held for extended periods at potentials above 1.0 V does not
appear to generate the oily film of (OTP)3 that is observed at lower potentials. Attempts to study.the electrochemical
behaviour of DTP on pyrite electrodes
were frustrated by the high background-current& of-this mineral (due to oxidation and reduction of the pyrite) in relation to the low currents associated with the collector. Several microflotation
studies were therefore carried out so
_.
.-
:
152.
:
that the data generated actual
flotation
with several
electrodes
correlation.
by the use of platinum.electrodes
practice_
A Partridge
(platinum
of the efficiency
could-be
and Smith microflotition
of flotation
with some. electrochemical of each float
or oxygen as the carrier
air
various
(such as Na2S, KCN, or Na2S03) to alter were ontained
gas,
could
use of nitrogen, reagents
to
vtas .fitted
as tie11 as pyrite electrodes were.used) for
of the flotation system. The conditions
the pulp. Samples of pyrite
related
cell
be altered
or by the addition
property by the of
the redox- Potential
of
from Custer in South Dakota via Gfard’s
Natural Science
Establishment , Inc. , and from the Durban Roodepoort Beep (DRD) Gold Mine in South Africa. The samples were screened to a particle size between 106 and 36 Pm, and were tested for total floatability with amyl xanthate. Values of 94 and 90 % were obtained for the Custer and DRD pyrite, respectively. All flotation tests were conducted on 2 g of sample in DTP (2.23
The extent tial
of flotation
of the pyrite
electrode
appears
to correlate
reasonably
made from Custer pyrite
x 10v4 14) at ptl 5.
well
(Fig.
3).
with the potenBoth pyrite
samples exhibit increasing flotation recoveries with increasing potential, though the Custer pyrite is inherently more floatable than the JRD material. Although the potentials at which flotation occurs are cathodic to the reversible potential
of the (DTP)2-DTP- couple
enough for
formation
(0.47
of the dithiolate
V),
the difference
to be discounted.
is not large
Furthermore,
when
several large and seemingly homogeneous samples of Custer pyrite were crushed to a particle size of about 1 mm, the individual particle potentials (measured with
a small
platinum
values as large
probe
with
a pointed
tip)
were found
to
have a spread
of
as 150 mV. The average potential for 30 such particles in an
oxygen-purged electrolyte was 0.53 V. and only two particles failed to attain potentials higher than 0 -47 V. If it is assumed that a17 particles exhibiting potentials anodic to the (DTP)2-DTP- equilibrium potential will float, a flotation
recovery
of 93 % can be predicted.
This predicted
value is to be expected
from the results in Fig. 3, if it is borne in mind that the total floatability of the Custer pyrite (as measured with amyl xanthate) was 94 %. This experiment was repeated
in a nitrogen-purged
of 0.42 V and a predicted flotation the range 80 to 90 % were obtained
electrolyte-,
and yielded
an average
potential
recovery of only 7 %. However, values experimentally (Fig. 3).
in
The conclusion to be derived from these results is that good floats can be obtained even when the potentials of the individual particles in the pulp are
cathodic
to the equilibrium
potential
of the (DTP)2-DTP- couple.
This floata-
bility is attributed to the presence of adsorbed DTP anions, which apljear to be capable of inducing contact angles of 20’ to 30’ (Fig. 2). The positive identification of (DTP)2 on the surface of floated pyrite reported~by Goold and Finkelstein
191 could
arise
from the fraction
of all
pyrite
particles
that had
-.
-:.
153. '. _ioo_ : -. m
:
l
Extent of flotation of surface potential
potentials
sufficiently
problem can provide the more classical
0.44
Custer and Durban ?oodepoort Deep pyrites (collector 2.23 x 10-4 M DTP).
of
anodic for the oxidation
flotation of the bulk of the material DTP anions. Although there field of flotation,
0.42
0.40 0.38 Potential/V.
0.36
0.34
Fig. 3. function
:-
Custer Pyrite Durban Oeep Pyrite
of DTP to occur.
is attributed
as a
However,
to the presence
of adsorbed
is clearly much mdre fundamental research required in the it is apparent that anelectrochemical approach to the an insight
into
the phenomenon of flotation
that complements
approach.
CYANIDATION OF GOLD The leaching
of gold by oxygen in the presence
of cyanide proceeds
via the
reacti on 4Au -i- o2 + SCN- + 2H20 +
4Au(CN); + 40H-
(6:
although it is still uncertain whether the reduction of oxygen stops at hydrogen peroxide [14]. Because of
efficiencies South Africa
cal cined
the difficulty involved in the achievement of high is leached from pyrite (a problem that Was recognized -in over 80 years ago [15]), it is comnon practice ~for the pyrite to be
when gold (producing
hematite ; Fe20B , which is subsequently-subjected
or for the pyrite to be milled to a very fine particle tion), dation; The poor leaching characteristics of gold from pyrite attributed to one or more of the following factors:
(ij
.-the presence ‘trapped milling
of very fine
within
the.larger
to liberate
size
to cyanidabefore
parti cl es of gold, -which are occluded Pyrite
the gold;
particl&.necessitating 1
cyani-
are generally
fine
or
_
164_. ... (ii)
:
..
.’
reduction.
present
(iv)
sulphide
aeration
is interesting,
of free
which- -ii u&ally
cyanide
due to the~formation
of
and.
of sulphide
ions (a particularly effective poison in into the pulp, above are trivial, since they can be eliminated-merely
and the addition because
of sufficient
the practical
addition
of lead
(usual ly in the form of
leaching
vessels
[17].
cementation
pyrrhotite~,
1
and ferri-cyanides;
the liberation
(iv)
(particularly
in the concentration
cyanidation [I6]) Points (ii) and (iii) by adequate
minerals
with pyrite);
reduction ferro-
.
in .the ckmcentrtition .oi -oxygen. in the -pulp due to ~&idat>on
-of- the iron (iii)
.-_
This effect
of gold onto zinc
cyanide
solution
Point
to the problem involves
lead nitrate
is discussed
to the pulp.
or li tharge,
later
in relation
the.
PbO) to the
to the
dust.
By far
the most interesting problem is that of the occlusion of gold within necessitating either fine milling or calcination. The the pyrite particles) question arises as to whether the problem is totally physical in nature (as it involvement_ Evidence appears to be), or whether there is some electrochemical that strongly supports the role of electrochemistry was recently presented by Filmer [181. In Fig. 4, the i-E curve
(as measured in this
laboratory)
is shown for
oxida-
tion lyte
of a gold rotating-disc (500 rpm) electrode in a nitrogen-purged electroof 0.01, 0 -02. and 2 x containing NaOH, NaCN, and Pbzt at concentrations Also shown are the curves for the reduction of oxygen on IO-5 M, respectively_ the same gold electrode and on a pyrite rotating-disc electrode (air-saturated electrolyte gold will
containing corrode
0.01 M NaOH). It is clear
spontaneously
in an aerated
from these
5-L” curves
al kal ine cyanide
solution
that at a
mixed potential of approximately -0.41 V and at a rate (limited by the mass transport of oxygen to the electrode surface) of about 30 PA (approximately 0.4 mA cmm2). Although for
the reduction
measured currents
the current
density
(which is
of oxygen on the pyrite
electrode
are considerably
because
higher
limited is also
this
by mass tpnsport) 0.4 mA cm
, the
electrode
has a much cm2). Since the
larger surface area (0.58 cm2) than the gold electrode (0.071 peak current (at -0.28 V) for the oxidation of gold (0.23 mA) is lower than that for the reduction of oxygen on pyrite (-0.25 mA at -0.28 V), short-circuiting
of the INO electrodes
should
result
in immediate passivation
of the
gold electrode. When the experiment was carried out, the open-circuit potential of the paired electrode increased to a value of 0.01 V in a matter of seconds. At this potential , the corrosion rate of the gold electrode is only 4 PA (approximately 0.06 mA cmq2). The practical implication-of any gold particle embedded in the surface of a much larger
this result is that pyrite particle, .but
of gold and Current-potential curves (1 mV S-l) for the oxidation Fig. 4. (Gold surface area 0 -071 cm2 ; reduction of oxygen on gold and pyrite electrodes. FeS2 surface area 0.58 cm*_) nevertheless exposed retarded rate.
to oxygen and cyanide,
wi 11 be leached
at a -greatly
Experimental results supporting the above electrochemical interpretation were reported by Filmer [18] . In a 24-h leach, the extraction of gold from milled pyrite (with a BET surface area of 0.3 m2 g-l) was 17 %, compared with 82 % from the calcine (with a surface area of 3.0 m2 g-l) produced by roasting of the pyrite. Extension of the leaching period to 21 days increased the recovery of gold from the pyrite (28 %) , but had no effect upon the calcine-. However, partial reduction (by hydrogen at 7OO’C) of’ the calcine to yield some magnetite reduced the -extractibn of gold (in (FegD4), which is an electronic conductor, a 24-h.leach) from 82 to 60 %, though no change in the surface area was observed. These results indicate the beneficial effects of roasting of the pyrite to liberate occluded.gold, as well as the retarding effects of large surface areas for
the reduction
of oxygen on the rate .at which gold
CARBON-IN-PULPPROCESSFOR GOLDRECOVERY In a conventional gold plant, a significant
is leached.
percentage
and 1 %) of the go1.d dissolved during the cyanidation the filtration stage that yields a clarified solution
(varying
6etween 0.2
process is ‘lost’ during for the -cementation step.
This loss arises .because of the large tonnage treated (a typical plant processes 200 OODt of ore per month) and a constraint on the amount of water that can be used to wash t.he ioluble gold from the filter cake. This problem is exacerbated by the difficulty the retreatment of their
with which. certain materials (such ascalcines and. slimes from of old dumps) are f i 1tered and washed, owing ‘to the small size
particles.
.:&+:
--2.
., -. -, ..-. ..~:
-;-
y,‘:.
._. In .the. CI?_process
;19,20]
.r ._:;::.:- __._ -:,:.. ;;;.::.._. . .. .: -.. .:_ :.
i the entirefiltration
,tep
js
:
.,-
ei<.mfnat&~.by:
the-..
’ -___ ._ : is .added to the l&t_ tank (usually -referred to as a. ‘stage’) in a-‘series of .. ‘~. absorption stages (typically four to six stages are-used). .The pulp, containing.
addition.
of granules
dissolved all
gold;
the
of activated.carbontol’the
is pumped into at a ‘flow
stages
in each stage.
Screens
the first
rate designed
on the
peripheries
cyanidation:pulp’.
stage , and flows
The carbon
under gravity
through.
to permit a residence. time of about i-h of the tanks restrain the.carbon.
granules within each tank while permitting .the pulp to flow to the next stage. At regular intervals, a percentage of the pulp and the carbon in the nth stage is
pumped back to the
A certain
(n-1)th
stage,
thus maintaining
a carbon
flow that is
each stage
is adsorbed
to the overall flow of the pulp.
counter-current
percentage
of the soluble
gold entering
onto the carbon. If six stages are used, and 70 % of the dissolved aurocyanide ions are adsorbed onto the carbon in each stage, an overall recovery higher than
99.9
eluted ratures
The loaded carbon emerging from the first
% fs attained.
stage
is
[Zl] by treatment with a solution of NaOHand NaCNat elevated tempe, and the gold is finally recovered by electrowinning [ZZ]. The eluted
carbon is reactivated for 2 to 3 min, after
by being heated to about 700 ‘C in the presence of steam which it is returned to the last stage of the absorption
circuit. To optimize operation developed a mathematical various
operational
of the adsorption circuit, Fleming et aZ. (231 model for the process, which relates the effects
parameters
(such as the flow rates
of
of the Pulp and the
carbon, and the mass of carbon in each stage) to the efficiency of adsorption. One of the interesting conclusions to emerge from this study was that each stage should contain
equal. masses of carbon.
However,
for
various
reasons
(such
as
surges
in the flow rate of the pulp, blocked screens, etc.), large fluctuations in the carbon concentrations in each stage are frequent. A typical variation in
the carbon concentration in the last stage of a South African CIP plant is shown in Fig. 5. The data points were obtained by weighing of the carbon obtained in a IO-litre
‘grab sample’
of pulp from the adsorption
tank.
Also shown in Fig. 5 is a continuous curve of the carbon concentration as determined by an electronic ‘carbon meter’ [24]. This instrument, which is still under development to-date
at Mintek,
measurements
provides
the
of the concentration
plant
operatorwith
of carbon
into
and generally
the pulp,
and carbon particles
have surface
die to the potential the open-circuit to 5 mV in height.
potentials
of the probe)
potential
that
strike
of the electrode,
earthed. The probe
(which are electrically are
and up-
in each stage. The measuring
probe consists of two metal electrodes, one of which is
inserted
accurate
several
the indicator producing
hundred
millivolts
electrode. small
is conducting
voltage
catho;
This affects pulses
1
The pulses are amplified and counted if they are above the
5. Comparison between the measurements obtained manually and by the Mintek carbon meter of the carbon concentration in a CIP absorption tank.
Fig.
random, backgrcund
noise
level . The recorder curve shown in Fig. 5 is the output from an electronic ratemeter, i .e., a voltage that is proportional to the rate at which pulses are detected. This, in turn, is proportional to the concentration of carbon in the pulp. Perhaps even more important than maintenance of the correct concentration of carbon in each stage is the activity of the carbon that is returned to the adsorption circuit. If the eluated carbon has not been sufficiently reactivated thermally, the capacity and the kinetics of the adsorption process are affected_ The activity of the carbon can be determined in the laboratory as follows. A known mass of carbon is added to a-stirred beaker containing gold (as potassium aurocyanide) at a concentration of 100 mg 1-l , and the decrease. in the concentration of gold with time as it-is adsorbed by the carbon is monitored. However, this
technique
is labour-intensive
using an_ atomic-absorption gold
in alkaline
cyanide
and requires
spectrophotometer. electrolytes
is readily
capable
the analysis
accomplished
of
of
by anodic-strip-
is well
ping vol tammetry ) and this
technique
processor-based
is currently
instrument
a skilled.dperator In contrast.
suited to.automation. A microbeing tested at :?intek. Once activated,
the instrument continuously measures the.gold concentration during its adsorption onto the~carbon, each data point (i.e., concentration US time of. measurement) being automatically plotted on an X-Y recorder. Apart from the-addition of the test solution to .the stirred beak~er?, no further .attention- is required from the operator, and a complete the end of the. test. :
carbon-adsorption .: -’ ._ . .
profile
is available
at -.
-_
l$s
ELE&JwINNINGOF &jLD.
:
‘~
-The ele&rowinning-of
,,: ..
-
_’
gold
-_
:
has become ‘a- familiar
unjf
-’
-.
._
--. ::-
operation
-.
._..
:’
in~,South :~:.‘.‘_
African gold mink, the technique being -used in the treatment of GIP eluates and of .leach liquors resulting from intensive -cyanidati’oh [25] _’ These two-. ‘1 -.. -1 .types of electrolytes contain gold 1;: the concentrationrange 100. to .lOOO’mg after el ectrowinning _ An interesting. before electrolysis, and 5 to 10 mg new development
[26]
is the elution,
and simultaneous-
recovery
by eleitrowin-
resins that have. been used .in the ning, of gold from poisoned anion-exchange extraction of uranium. In this process, the eluate (acidic thiourea or.neutral ammonium thiocyanate) is circulated from a tank containing-the resin through the electrowinning cell and back to the tank. As the eluted gold is recovered during the elution procedure, the concentration of gold entering t&cell -1 rarely rises above 10 mg 1 . Even at these low concentrations. use of the Mintek packed-bed electrowinning cell [22] results in recoveries of 50 to 70 % of the gold in the eluate per pass through the celll?t -1 O-4 m min (i.e., a volume flow rate of 200 1 min ). CEMENTATION OF GOLD .The recovery of metallic
gold
by cementation
a linear
onto zinc
flow
rate
of
dust is an electro-.
chemical process [27] involving the oxidation of zinc and the reduction of aurocyanide ions. The overall stoichiometry of the process is as follows: Zn f 2Au(CN)i
+
Zn(CN)i
-e
(7)
~Au
Pregnant solution from the filter plant (typically containing 5 to 10 mg of dissolved gold per litre) is usually stored in a surge tank before being pumped via a Crowe tower (for de-aeration) into an emulsifier tank, where lead nitrate and zinc
dust
(of
particle
size
between 110 and 70 pm) are added.
After
a
mixture is pumped residence time no longer than 1 or 2 min, the solid-liquid into a Stellar or a Merrill filter unit. The zinc dust forms a continuous bed of reducing
agent
on the
filter
medium through
which
the
pregnant.
solution
must
flow. Owing to the cathodic potential of the zinc (typically -1-l V , which causes the rate of go?d deposition to be 1 imited by the mass transfer of aurocyanide
ions to the surface
namic conditions’within
the
very high, barren solutions usually being obtained. Poor cementation following (i)
of the zinc bed,
particles)
the efficiency
of the cementation
with a gold content
efficiencies
can usually
and the excellent
hydrodyprocess -1
is
lower than 0.01 mg 1
be attributed
to one of the
effects: inefficient
de-aeration;;
to the reduction
resulting
of oxygen;
in excessive
corrosion
of zinc
due
1 ._. ~-___;: :.~I;
,,
:
-._ ._I .; ,i;;,_.. -, (iii)
-._
y..’
.- -:.
_
1:.
_
-.
._ :..
__.._ ,_..-.,:
;
‘.
-_I_
-
~.
:
_.
. .
._
__,i.
.
-.
..
:
:..
..-. ... _
_ -ticin .hydrbxide; Ghiih . . . .of.iinc .;.. passivates . . the-anodit.dissolution-of ~_ _‘::zinc’, and__, .’ -.. ‘1. .I. _ :‘;I 2: ._‘ _-:_.:‘-:‘ .:. 1..the: p&.&e
of::poisons-;-
-.‘.whi&i are reported’&
.-
::
..
159 ,. .. ._ .- _: _._ : .:~ ,_ :. ; .-. ;:‘-:-::.-:._:: ..; -;- ::-. ‘--.f_;.._ _- ,-; .’ -: :_:. :__. ” pregnant-solution; resulting---in the forma-. ~.. ‘.;I- insufficient..-cyanide:*:!.n-;:the ‘_ _,:
.. -. cy at concentrations .During.recent pilot-plant
such as sulphide
h&e .a deleterious
ions-, in _the pregnant solution, -effect-
on cementation
-as 1ow as. 0.03 mg 1-I [14]. testwork at Mintek on the cyanidation
efficien-
and .subse-
quent cementation of gold from the clarified. leach liquor, the control experiment (which duplicated the procedure used on the gold plant from which the milled ore was obtained) yielded extremely poor cementation results (barren solutions .of ._0.5 mg 1-1 from a head-value of 5 mg 1-l). However, additional cementation tests performed several tailings of less .than 0.002 mg 1-l.
days later
on the same leach liquor
yielded
Close inspection of the gold‘plant revealed that the clarified leach liquor from the filter plant is stored in a 1200 m3 surge tank before being pumped to the Merrill tion
filters
in this
for
cementation.
tank is approximately
The average
residence
1 h. A concentrated
time
solution
for
the
solu-
of lead nitrate
-is continuously pumped into the surge tank to give a concentration lead of about 5 mg 1-I.- Additional lead nitrate is added directly -1 2+ .. emulsifiers to raise the Pb valueto8mgl
of dissolved to the
During leaching of a further 400 kg of pulp at Mintek, a sample of the pregnant solution was subjected to cementation immediately after the addition of Pb2+ at- a concentration of 8 mg 1 -1 _ Lead nitrate was added to a second sample of 5 mg 1-l) which was of the pregnant solution (to give a Pb2+ concentration aerated resulting
for
2 h. Further Pb2+ at a concentration
solution
cementation
yielded
Although the results sulphide
ions,
of 3 mg 1-l was added, and the
was pumped through the cementation barren solutions
cell _ The two tests
of 0.2 and 0.002 mg .1-I,
appeared to point
to the presence
in the pregnant electrolyte
on
respectively.
of a poison,
,- which is subsequently
such as oxidized
on
contact with atmospheric oxygen, confirmatory evidence was not easily obtained owing to the difficulty involved in‘the analysis of sulphide ions in solutions containing cyanide ions. Nevertheless, this investigation led to a study of the rate of oxidation of sulphide ions in an air-sparged electrolyte containing NaOH(0 .OI M) and S2- (50 mg 1-l). (A sulphide ion-selective electrode was used since
no cyanide
ions were present.)
The rate of oxidation-of
sulphide
ions was
found to be very slow in the absence of dissolved lead, no measurable drop in the concentration of-sulphide being observed over a 3-h period. However, the -1 -addition of Pb2? at a concentration of 4 mg 1 considerably enhanced the rate of oxidation;
the concentration
over the 3-h period. 1-I is theoretically
of sulphide
ions declined
(It should be noted that Pb? capable
of precipitating
from 50 to 2 mg l-1.
at a.concentration of 4-mg only 0.6 mg -1-l iof 52- as PbS.)
..:-... ‘160. ~: :
_
_:
_.
Similar
effects
were -noted with Cupric,
The .insoluble. electronic solids
metal sulphides
Conductors.
on the rate
1 g of coppe;
It
powder (BF-surface
and ..ferrous
_.
ions-.. --:.
of all, the .metal ions discu&ed
was therefore
of sulphide
ferric
. .
oxidation
.decided
thatthe
should-be
above. are good
effects-
area 0;19 m2 g-l)
to 1 1itre
.. 1
of various
measured...The
addition
of
of 0.01~ M NaOH
to below 0.1 mg containing S - (58 mg 1 ) reduced the sulphide concentration 1-I in approximately.15 min, whereas-l g of natural galena and pyrite (surface required approximately 50 min to areas of 9.67 and 0.54.1s~ g-l. respectively) effect a similar reduction. Chalcopyrite (surface.area. 0.58 m* g-l)- decreased -1 the rulphide concentration from 50 to 20 mg 1 in 3 h. In Fig. 6, the current-potential .curves (10 mV s-I) are shown for the oxidation of S2- (50 mg 1-l) in nitrogen-purged 0.01 M NaOHirith rotating-disc electrodes (500 rpm) fabricated from the minerals potentials recorded for the copper electrodes 2cu + s*-
mentioned above. The very negative are due to the reaction
-+ Cu2S f 2e-
(3)
2which has an equiliirium potential of -0.83 V in an electrolyte with an S content of 50 mg l-I_ The overpotentials for the oxidation of sulphide ion (the equilibrium potential for the S-S*- couple is -0.43 V) increase in the order
PbS c FeS2 c
CuFeS2.
Also shotin in Fig.
of oxygen (as well
6 are the corresponding
as reduction
cathodic
of the mineral
curves
for
in some instances)
sparged electrolyte of O.OL M NaOH. The overpotentials FeS2 c Cu -Z PbS -= CuFeS2. Xand [28] also found pyrite
the reduction from an air-
increase in the order to possess the greatest
activity for the-reduction of oxygen, though he reported chalcopyrite to be more active than galena. Combination of the various anodic and cathodic processes give the electroihemical cm
-2
rates
for the reaction of sulphide as 0.66, 0.11, 0.070 and 0.014 mA
for the Cu, FeS2, PbS and CuFeS2 eTectrodes, respectively. These rates
follow the same order as that observed for the experiments in which 1 g samples of the powdered minerals were used. The implication of these results is that the homogeneous rate of sulphide oxidation by oxygen is very slow, and that the heterogeneous rate at the surface of an electronic conductor is considerably faster. The addition of lead nitrate in gold
plants where there is a sulphide problem results in the of lead sulphide, which acts as a site for the reduction of oxygen and the,oxidation of sulphide ions. Furthermore, the presence of a precipitation
surge tank for storage of the pregnant solution from the filter plant provides sufficient residence time for oxidation of the residual su_lphide ions. The common practice
of 1i tharge or lead nitrate.
addition
to the leaching
vessels
Anodic and cathodic i-E curves for the oxidation of sulphide ions and the reduction of. oxygen in -0 .Ol :J NaOHwith copoer , galena , pyrite , and chalcopyrite rotating electrodes (500 rpm, 10 mV s-1).
Fig. 6.
when sulphidic i.e.,
ores
acceleration
are
being
of the
rate
treated
undoubtedly
at-which
sulphide
serves is
the
same purpose,
oxidized.
CONCLUSIONS
There is little doubt that the evidence clearly justifies the statement that the recovery of gold is predominantly an eiectrochemical process. In addition to -and cementation, electrochemisits obvious role in iryanidation. electrowinning, try plays a far more subtle part in other.operakions
CIP process. Although metallurgy
is ersentia7ly
an applied
such as flotation
practice,
and the
the application
of
theoretical -electrochemical principles to the subject has facilitated the optimization of many of the process stages. Further developments and improvements can be expected in this field as a better of the -various operations is developed.
understanding
of the electrochemistry
ACKNWLEOGEMENT
This paper is published
by permission
of the Council
for Yineral Technology.
REFERENCES 1 Bessel and Bessel, Ger. Patent no. 42, Class 22 (1877). 2 S.G. Salamy and J.C. Nixon, Recent Developments in 14ineral Dressing, 3
4 5
Institution of Mining and Metallurgy,, London, 1953, pp_ 503-516. S.X. Ahmed, in N.Li. Norman (Ed-), Recent Developments in Separation Science. Vol. .V, CRC Press, Cleveland, 1972, pp. 95-133. R. Woods, in M.C. Fuerstenau (Ed.). Flotation, A.M. Gaudin Memorial Volume,_-Vol. 1, AIME, New York, 1976; pp. 298-333.
G-W. Poling._in
M.C.
Fuerstenau
(Ed.),
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