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Cyanide-free AARL elutions are feasible
Minerals Engineering, Vol. 7, Nos 2/3, pp. 251-264, 1994
0892-6875/94 $6.00+0.00 © 1993 Pergamon Press Ltd
Printed in Great Britain
CYANIDE-FREE AARL ELUTIONS ARE FEASIBLE
P.T.E. BOSHOFF Anglo American Research Laboratories (Pty) Ltd, P.O. Box 106 Crown Mines 2025, South Africa (Received 15 July 1993; accepted 1 September 1993)
ABSTRACT
A major operating costcommon to elution processes has been the use of sodium cyanide. In recent years plants operating the universally used Zadra mwl AARL elution processes have been reporting satisfactory elution results with greatly reduced cyanide additions atut attendant cost savings. The Anglo American Research Laboratories (AARL) embarked on a testwork campaign aimed at establishing under what conditions cyanide-fi'ee AARL elutions were possible. The testwork demonstrated that cyanide-jS"ee elutions of carbons containing low amounts of copper (less than 1 200 g/t), are indeed possible and that under these conditions the severity of the acid wash conditions (acid wash temperature and acid concentration) could be reduced without a detrimental effect on gold elution e~ciency. It was suggested, however, that the type and quantity offoulants to be stripped from the carbon during acid washing would dictate the optimum conditions employed during the acid wash. bt the case where copper presents a problem, alternative methods of processing have been highlighted. These include - minirnisation of copper dissolution and adsorption, copper pre-leaching and selective copper elution using ambient temperature cyanide solutions. Keywords Activated - carbon, copper, elution, zero-cyanide, AARL-elution
INTRODUCTION The technology of gold elution is dominated by two systems, the first developed by Zadra at the United States Bureau of Mines in 1950 and the second, the AARL system patented by the Anglo American Research Laboratories (AARL) in 1973. Since every major CIP plant throughout the world uses one or other of these two systems there has been vigorous debate as to their relative merits. Advantages such as ease of operation, better security, lower capital cost, lower power requirements and lower reagent consumption have all been advanced in favour of the two systems in turn. A major cost common to both processes has been the use of sodium hydroxide and more notably the use of sodium cyanide. However, in recent years, plants using the Zadra elution process have reported that cyanide free elutions could be performed with no significant decrease in circuit performance. This resulted in substantial savings in reagent costs and hence in total running costs. 251
252
P.T.E.
BOSHOFF
In parallel with these developments, operators of the AARL system were also reporting satisfactory elution results with greatly reduced cyanide additions and attendant cost savings. As a result, investigations were initiated at the AARL to establish under what conditions cyanide could be omitted from the elution operation. In particular, it was attempted to establish a relationship between the efficiency of gold elution and the base metal (particularly copper) loadings on the carbon. Additional testwork was aimed at optimising reagent requirements during the elution sequence when it became apparent that cyanide-free elution was indeed possible. THE EFFECT OF BASE METALS IN THE CARBON CIRCUIT During cyanide leaching of gold ores, a number of base metals are also leached, notably copper, nickel, cobalt, iron and zinc. The presence of various base metals in CIP and CIL circuits can have a detrimental effect on efficiency and economics due to high cyanide consumption and a reduction in leaching and adsorption rates. Furthermore, the presence of readily loaded and adsorbed metals such as copper can significantly reduce the carbon loading capacity for the precious metals, thereby necessitating a higher carbon inventory and more frequent elution with higher NaCN concentrations. However, unlike copper, and to a lesser extent nickel, the cyanide complexes of cobalt, iron and zinc are not loaded well onto activated carbon, and their presence in solution has little effect on the extraction efficiency of gold. For Witwatersrand gold ores, copper and nickel are the predominant base metals in solution with copper being by far the most problematic. Copper is particularly important, in that the loading of its cyanide complexes onto carbon is greatly influenced by the chemical conditions prevailing in solution. Under certain conditions, it is loaded more strongly than the aurocyanide species and tends to displace gold from the activated carbon; under other conditions it is hardly loaded at all. Copper dissolves in cyanide solution during leaching as a series of copper (I) complexes depending on the ratio of cyanide to copper. The following complexes are present in solution: Cu(CN)2- Cu(CN)32and Cu(CN)43- with Cu(CN)32- being the predominant form in typical plant solutions. If during the adsorption stage, conditions of low pH and low free cyanide in the pulp prevail, the above copper complexes are adsorbed onto the carbon. It has been shown that the Cu(CN)2- species is loaded very strongly, whereas Cu(CN)32- and Cu(CN)43- are hardly loaded at all [1]. By maintaining a high free cyanide level in the pulp along with high pH, the adsorption of copper onto activated carbon can be minimised. However, this is counter-productive to minimising cyanide utilisation and hence in CIP circuits treating copper bearing ores, carbon tends to load copper to a significant degree. This has major implications on the amount of cyanide subsequently required to elute gold effectively.
EXPERIMENTAL The elution testwork was conducted in a laboratory elution column as shown in Figure 1. The method of gold elution used in the investigation was as follows: 500 ml of wet-settled gold-loaded carbon was placed in an oil-jacketed stainless steel elution column of 38 mm internal diameter and 460 mm length. Oil was heated in a separate vessel and then circulated through the jacket to maintain the contents of the column at the desired elution temperature. Before entering the elution column, the eluant was heated to elution temperature by being pumped through a stainless steel spiral placed inside the oil-jacket. The eluant was pumped through the column using a metering pump. Pressure within the system was maintained by means of a manually operated valve on the effluent line of the column. Column eluate was cooled in an air-cooled stainless steel spiral condenser before being collected in 500 ml volumetric flasks. The elution was stopped once 5 L of eluate (10 bed volumes) had been collected.
253
Cyanide-free AARL elutions
i co=o==so= ]
I NaOH NaCN
I I AGIDWASH PERFORMED IN ~EPARATE VESSEL
WATER
OIL JACKETED EL U TION COL UMN
OIL BATH AND TEMPERATURE REG UL ATOR
J
S
ELUATE
1I
Fig. 1 Diagrammatic Representation of the Laboratory Scale Elution Column The elution procedure used in the investigation included acid washing of the loaded carbon in a glass beaker for 15 minutes using I bed volume of a hydrochloric acid solution, water washing using 10 bed volumes of ambient temperature tap water, pretreatment with 1 bed volume of a caustic cyanide reagent followed by elution using deionized water at a flowrate of 2 bed volumes per hour. Variations of the highlighted items in the elution procedure summarised below were used to establish the effect of acid wash temperature, hydrochloric acid concentration and cyanide concentration.
ACID WASH Volume Acid Concentration Temperature Time Water Wash
: : : : :
1 bed volume 1 and 3 percent (v/v) 25 and 90"C 15 minutes 10 bed volumes
PRETREATMENT Caustic
:
2 percent (m/v)
Cyanide
:
0 and 2 percent (m/v)
Temperature Volume Flowrate Soaking Time
: : : :
125"C 1 bed volume 2 bed volumes/hr 30 minutes
254
P . T . E . BosrloFF
ELUTION Eluant Temperature Volume Flowrate
: : : :
deionised water
125"C 10 bed volumes 2 bed volumes/hr
The loaded carbons used throughout the testwork, (identified as carbon 1, carbon 2 and carbon 3) were obtained from three South African CIP circuits. The carbons were selected because the differences in their gold and copper loadings made it possible to assess the effects of high copper loadings on the feasibility of cyanide-free elutions. Elution efficiency was defined as: Cumulative gold eluted per stage xl00 Total gold eluted + residual gold on carbon This efficiency was determined for each half hour period during the elution.
DISCUSSION The metallic loadings for the carbons used during the testwork are presented in Table 1. All carbon samples were obtained from the loaded carbon catch screens. However, although samples A, B and C of carbon 1 were obtained from the same place within the carbon circuit, they were taken a month apart for three months. It is evident that the gold and copper loadings of carbon 1 vary significantly over the three month period which highlights the extreme variability of the ore being fed to the plant.
TABLE 1 Metallic ioadings of carbon used during elution testwork Loading (g/t) Carbon 1
Element
Carbon 2
Carbon 3
A
B
C
Au
3 200
9 830
4 500
420
2 900
Ag
345
117
230
50
190
Cu
1 200
8 970
12 000
270
95
Ni
2 950
1 300
2 500
7 100
2 200
Fe
850
905
1 200
650
330
Zn
145
100
19
155
30
Ca
2 300
1 270
2 900
17 600
10 000
SiO 2
12 500
27 700
13 000
5 800
5 500
Cyanide-free AARL elutions
255
Carbon 1 Table 2 presents the gold elution efficieneies, after a five hour elution using deionised water, for a low copper loaded carbon (Carbon 1A), after pretreatment under different conditions of acid wash temperature, hydrochloric acid concentration and sodium cyanide concentration. The influence of these variables on the kinetic response is shown in Figure 2.
TABLE 2 Elution efficiencies for low copper loaded carbon Acid Wash Conditions HCI Cone
Pretreatment Conditions
Gold Elution Efficiency
(%)
Temperature
NaOH Cone
NaCN Cone
(%)
(*c)
(%)
(%)
3
90
2
2
98.3
3
90
2
0
94.3
3
25
2
2
99.0
3
25
2
0
96.6
1
25
2
0
96.5
Copper Loading = 1 200 g/t Au Elution Efficiency
(%)
100
80
60 TEST
40 ¢~
20
~r r7
0 0
2
ACID TEMP
ACID CONC
NaCN CONC O% O % O% 2 2~ 2 ~
25C 19~ 25C [ 3~ NO ACID WASHING 25C 3~ 90C 3 ~
I
I
I
I
4
6
8
lO
Number
of Bedvolumes
Eluant
Copper Loading : 1 2 0 0 g / t Gold Loading : 3 2 0 0 g / t Fig.2 Gold Elution Efficiency Curves for Carbon 1A
12
A.A.R.L.
256
P.T.E.
BOSnOFF
With reference to Table 2 and Figure 2 the following can be deduced:
i)
Provided cyanide is present during the pretreatment stage, hot acid washing may be replaced with an ambient temperature acid wash with no detrimental effect on the overall gold elution efficiency. Although initial elution kinetics are slightly retarded, efficiencies become comparable to those obtained with hot acid washing, after approximately 2.5 hours, i.e. well within normal plant elution times. Cold acid washing removes 78 percent of the loaded calcium during a 15 minute acid wash, while a 90°C acid wash removes 85 percent. Conceivably the decrease in calcium removal during cold acid washing may eventually have a detrimental effect on elution performance due to fixation of calcium on the carbon during regeneration. The implications of excessive calcium loadings on the regenerated carbon recirculated to adsorption are two-fold. Firstly, the presence of a layer of calcium carbonate on the surface of the carbon significantly reduces the carbon's affinity towards gold in solution due to blocking of the macropores within the carbon. Secondly, repeated regeneration of excess calcium carbonate would possibly result in the formation of low melting point glasses within the porous structure of the carbon. These glasses have vastly different coefficients of thermal expansion from carbon and therefore the uneven expansion of the carbon during regeneration would alter the internal pore size distribution of the carbon. This would seriously affect the kinetic and equilibrium characteristics of the carbon.
ii)
When cyanide was omitted from the pretreatment operation, elution efficiency dropped 2.5 - 4 percent. However, gold elution efficiencies remained high and ranged between 94% (72 g Ault on eluted carbon) and 97 % (36 g Au/t on eluted carbon). It is suggested that an increase in the eluted carbon values from 36 to 72 g/t Au would not seriously affect the adsorption efficiency.
iii)
Overall elution efficiency improved from 94 % to 97 % when hot acid washing was replaced with a cold acid wash. In addition, elution kinetics were faster. A further benefit of cold acid washing was that a decrease in hydrochloric acid concentration from 3 % to 1% had no detrimental effect (elution efficiencies were approximately 97 % for both concentrations). Initial elution kinetics were only slightly retarded at the low HCI concentration while calcium removals were in the order of 77%. However it should be noted that the calcium loading on this carbon was only 2.3 kg/t. Stoichiometrically, 73 g HCI (100%) is required to remove 40 g of calcium [2]. On this basis it can be calculated that to remove 2.3 kg/t Ca from the carbon 4.2 kg/t HCI would be required. If the calcium loading increased to 15 kg/t the theoretical HCI requirement would increase to 27.4 kg/t. It is therefore evident that acid requirements are largely dependent on the extent of calcium fouling.
Table 3 presents the nickel and gold elution efficiencies in the absence of cyanide. It can be seen that in the absence of cyanide, nickel elutions decreased from 89% when cold acid washing was employed to 52% when a 90"C acid wash was used prior to the elution. Under the same conditions, gold elution effieiencies were 97% and 94% respectively. It is therefore suggested that the nickel loading (approximately 3 kg/t) is unlikely to have as marked an effect on gold elution as the copper loading. A similar, but less pronounced trend was also observed when eluting with cyanide. Figure 3 shows the gold elution efficiencies for Carbon IB which had gold and copper loadings of approximately 9.8 kg/t and 9.0 kg/t respectively.
Cyanide-free AARL elutions
257
TABLE 3 Nickel and gold elution efficiencies in the absence of cyanide
ACID WASH CONDITIONS
ELUTION EFFICIENCY
HCI C O N C
TEMPERATURE
(%)
(*c) 90 25
Ni
Au
(%)
C%)
52.4 89.2
94.3 96.6
i) Ni Loading ffi 3 000 g/t ii) Au Loading = 3 200 g/t iii) Prior to the elution using deionised water the carbons were pretreated with 2 percent NaOH.
NOTES:
Au Elution Efficiency (%) 100
80 60
40 D TEMP
/ f
20
z
s%
9oc
3 •
,25C
NaCN CONC
2 %
0 0
2
4
6
8
10
12
Number of Bedvolumes Eluant
Copper Loading : 8 970 g / t Gold Loading : 9 830 g / t
A.A.R.L.
Fig.3 Effect of Acid Wash Conditions on Gold Elution Efficiency for a High Copper/High Gold Loaded Carbon Despite the excessive copper loading and uncharacteristically high gold loading of 9.8 kg/t (CIP plants in South Africa load gold to between 3 and 5 kg/t) efficient elution was achieved (approximately 97 %) after a 5 hour elution when 2% cyanide was added during the pretreatment stage. Without the addition of cyanide, elution efficiency dropped to 68 % when cold acid washing was employed prior to the elution. A further reduction in gold elution efficiency to 63 % occurred when hot acid washing was used prior to a cyanide-free elution. Figure 4 illustrates the gold elution efficiencies for Carbon IC which had an extremely high copper loading of 12 kg/t and a gold loading of 4.8 kg/t. The elution in which cyanide was present gave a 98% gold elution efficiency while in the absence of cyanide elution efficiencies dropped to 46 %. Both these elutions were preceded witti an ambient temperature (25°C) acid wash using a 3% solution of
P.T.E. Bosxow
258
hydrochloric acid. Elution efficiency increased slightly to 53 % when the carbon was not subjected to an acid wash prior to the cyanide-free elution.
lOO
Au Elution E f f i c i e n c y (%) 7.
/,//~TEST
ACID CONC
80 -
/
/
I
I I
I
60
i
~
a%
o []
3~ No Ac,o w a s .
/
[]
°
O
NaCN CONC 2% 0% 0% F1
40 2 0 _¢~1.
v
i
i
]
iO
2
4
6
8
0
0
10
N u m b e r o f Bedvolumes Eluant Copper Loading : 12 000 g / t Gold Loading : 4 800 g / t
12
A.A.R.L.
Fig.4 The Effect of High Copper Loadings on Gold Extraction Under Various Acid Wash Conditions Table 4 provides a summary of the results achieved on Carbon 1 (A, B and C) and presents the differences in gold elution efficiencies for elutions conducted in the presence or absence of cyanide. TABLE 4 Elution efficiencies in the presence/absence of cyanide as a function of copper loading
Sample
Copper Loading (g/t)
Gold Loading (g/t)
Acid Wash
NaCN Pretreatment
A
1 200
3 200
Yes Yes
Yes No
99.0 96.6 (2.4)
B
8 970
9 830
Yes Yes
Yes No
97.2 68.2 (29.0)
C
12 000
4 800
Yes Yes No
Yes No No
97.6 45.9 (51.7) 53.4 (44.2)
NOTES:
Au Elution Efficiency (%)
i)
For the tests in which acid washing was conducted prior to the elution, a 25°C and 3% HC! acid wash was used.
ii)
During pretreatment, 2 percent NaOH was used in addition to 2
iii)
percent NaCN. The figures in brackets are the differences between the gold elution efficiencies achieved for elutions with and without cyanide.
Cyanide-free AARL elutions
259
It is obvious that, as the copper loading on the carbon increases, the success of cyanide-free elutions is reduced. It appears that, in the presence of very high copper loadings, cyanide addition becomes necessary to convert the copper to a form that can be eluted. Figure 5 illustrates the effect of copper loading on the elution efficiency of gold and copper in the absence of cyanide. Efficient gold elution is largely dependent on the efficiency with which copper is eluted which in turn is related to the copper loading of the carbon. Any increase in the amount of copper loaded on the carbon will significantly reduce gold elution efficiency in the absence of cyanide. Above a certain copper loading (3 kg/t), efficient gold elution can only be achieved by the addition of cyanide. It is also suggested that during acid washing, partial formation of an insoluble copper species occurs, thus reducing gold efficiencies even further due to macropore blockage. It is believed that the formation of this undesirable copper species will be minimised as acid washing conditions become less aggressive, e.g. ambient temperature acid washing at reduced HCI concentrations.
Au Elution E f f i c i e n c y (%) 100 r
Cu Elution E f f i c i e n c y (%) 100
_
1 2 0 0 g / t Cu
80
80
60
-
60
40
-
40
20
-
20
12 0 0 0 g / t Cu
0 ~!W-"~-
0
I
q
I
I
I
I
2
4
6
8
10
12
0 14
N u m b e r o f Bedvolumes E l u a n t []
Gold
~
Copper
Fig.5 Effect of Copper Loading on Gold and Copper Elution in the Absence of Cyanide.
Carbon 2 The carbon used for this phase of the testwork was a loaded carbon obtained from a carbon-in-leach plant treating low-grade dam material. The gold and copper loadings for this carbon were 420 g/t and 270 g/t respectively. Figure 6 shows that while the initial gold elution rates were retarded in the absence of cyanide, overall elution efficiencies were little affected after approximately 3 hours (6 Bed volumes of eluant). Again it can be seen that a reduction in the severity of the acid washing conditions employed prior to the cyanidefree elution of a low copper loaded carbon had no effect on either the rate of elution or the final elution efficiency.
P . T . E . BOSHOFF
260
Au Elution Efficiency (~) 100
m
am
~
80 60
TEST
40 20 Oe
0
2
ACID TEMP
ACID CONC
NaCN CONC
90 C
3~
2~
A
90 C
3~
O~
X
25 C
3~
2%
o
25 C
3%
0%
[]
NO ACID WASHING
I
I
4
6
0% 1
_l
8
Number of Bedvolumes Eluant Copper Loading : 270 g / t Gold Loading : 420 g / t
10
12
A.A.R.L.
Fig.6 Graph of Au Elution Efficiencies Demonstarting the Feasibility of Cyanide-free Elutions for Carbons Loaded with Low Levels of Copper. Since this carbon had a low gold loading of 420 g/t it might be expected that this small amount of gold would be eluted irrespective of the presence or absence of cyanide. Figure 7 presents the gold elution efficiencies in the absence and presence of cyanide, for a carbon with 155 g/t gold and 22.5 kg/t Cu. Despite the addition of 2 % cyanide during pretreatment, the efficiency of gold elution was unsatisfactorily low at 74%. A cyanide-free elution of this carbon resulted in an elution efficiency of only 25 %. Although these elutions were conducted subsequent to a hot (90°C) acid wash it is unlikely that the use of cold acid washing would have produced significant improvements in the gold elution efficiency. It is concluded that the success or failure of cyanide-free elutions is largely dependent on the extent of copper loading and practically independent of the amount of gold on the carbon. Carbon 2 had a nickel loading of 7.1 kg/t which made it possible to assess the effect of high nickel loadings on gold elution efficiencies in the absence of cyanide. Table 5 presents gold and nickel elution efficiencies in the absence of cyanide using different acid washing temperatures. Nickel elution dropped from 91% to 82% when hot acid washing was used as opposed to cold acid washing. It was expected that a similar decrease in gold elution efficiency would occur if high nickel loadings affected gold elution efficiency to the same degree as copper. However, since gold elution efficiencies remained unchanged at 99%, nickel does not play 'a significant role in governing the success of cyanide-free elutions. Carbon 3 The final carbon used in the investigation was one obtained from a low grade CIP plant that has recently converted successfully to a cyanide-free AARL elution. Gold and copper loadings were 2.9 kg/t and 95 g/t respectively. Figure 8 shows that cyanide-free elutions subsequent to a hot or ambient temperature acid wash gave extractions in excess of 99 %. The corresponding extraction tbr the elution where cyanide was added was
Cyanide-free AARL elutions
261
also above 99 %. This Figure serves to highlight the fact that dution of carbons that contain normal amounts of gold with low copper values can be effectively eluted with no cyanide addition. Although the acid washing conditions do not play a significant role in the final elution efficiencies, a reduction in the acid wash temperature to 25°C would be preferred as this would reduce the thermal requirements and hence operating costs of the process. It must, however, be borne in mind that many elution circuits conduct the acid washing stage in the elution column, and therefore hot acid washing would be the preferred option as this could be included as part of the heating cycle for the elution process.
Au Elution Efficiency (%) 100 80 r3
m
60
40 © kJ
20
0
0
2
4
6
I
I
8
10
N u m b e r of Bedvolumes Eluant
CN Present
[]
CN Absent
c,
C o p p e r Loading : 2 2 5 0 0 g / t Gold Loading : 155 g / t Fig.7 Graph Illustrating the Success of Cyanide-free Elutions is Largely Dependent on the Extent of Cu Loading and Relatively Independent of Gold Loadings.
TABLE 5 Nickel and gold elution efficiencies in the absence of cyanide
ACID WASH CONDITIONS
ELUTION EFFICIENCY
TEMPERATURE (°C)
HCI CONC
Ni
Au
(%)
(%)
(%)
90 25
3 3
81.7 91.3
98.7 98.8
NOTES:
i) ii) iii)
Ni Loading = 7 100 g/t Au Loading = 420 g/t Carbons pretreated with 2 percent NaOH.
12
262
P . T . E . BOSSOFF
Au Elution Efficiency (~)
100 8060
NaCN CONC
40
p
//
c
y O
x t
0
2
I
90C I
,,
o,
3~
2%
I
4 6 8 Number of Bedvolumes Eluant
~__ ......
10
C o p p e r Loading : 9 5 g / t Gold Loading : 2 9 0 0 g / t
12
A.A.R.L.
Fig.8 Elution Efficiency Curves for a Carbon that Contains Low Copper and a Gold Loading Typical of Many CIP Loaded Carbons.
PLANT IMPLICATIONS This investigation has demonstrated that cyanide-free AARL elutions are feasible provided copper loadings on the carbon are low (less than 1.2 kg/t) and seen in isolation there might be motivation to eliminate cyanide from the elution system. However, elution is linked to various other operations and it would be unwise to optimise it in terms of cyanide addition, acid wash temperature, acid concentration and elution time in isolation from the other processes. The adsorption of copper onto carbon in a CIP circuit can be reduced by maintaining high cyanide concentrations in solution and operating at around pH 12, this however results in higher operating costs. An alternative remedy is to leach with ammonia cyanide solutions. La Brooy [3,4] showed this to enhance the dissolution of gold relative to copper while reducing copper adsorption on carbon. At the Paris Mine in Western Australia where the process was applied, loaded carbon containing 3.3 kg/t Au and 1.5 kg/t Cu was produced from a solution assaying 1 g/t Au and 100 g/t Cu [5]. Sceresini and Richardson [6] developed a process whereby copper is selectively leached from gold, adsorbed onto carbon, eluted and precipitated to produce a saleable copper salt. Secondary recovery of the gold is then achieved under improved conditions in the absence of a large proportion of the copper. Yet another alternative is to allow copper and gold to load during adsorption and solve the copper problem either prior or subsequent to elution. Once copper has loaded onto carbon it is often desirable to selectively elute it. During electro-winning copper plates on the cathodes and hinders the recovery of the gold cement. During zinc precipitation the presence of copper results in excessive zinc utilisation in addition to contaminating the dor4.
Cyanide-free AARLelutions
263
In a study conducted by McArthur [7] copper removals in excess of 95 % were achieved for a carbon containing 19 kg/t Cu using a 0.5 % NaCN solution at ambient temperature. The associated gold removal was negligible at 0.2 ~. In practice, the majority of plants return the ambient eluant to the process solution with no significant build-up of copper on the carbon. The other approach is to tackle the problem at the electro-winning stage. Testwork conducted by Avraamides [8] has shown that maintenance of high free cyanide concentrations in the electro-winning circuit might be a viable approach to retaining copper in solution. On the Witwatersrand such measures are generally not required, in other areas the most appropriate solution would depend on the severity of the problem and on local conditions.
CONCLUSIONS The testwork has demonstrated that provided copper loadings are low, cyanide-free AARL elutions are indeed possible. It is suggested that cyanide-free elutions are certainly possible for copper loadings less than 1.2 kg/t, may be possible for loadings between 1.2 kg/t and 3.0 kg/t and are highly unlikely if the copper loading exceeds 3.0 kg/t. The difference between the elution efficiencies obtained for elutions conducted in the presence and absence of cyanide were 2%, 29% and 52% for carbons with copper loadings of 1.2 kg/t, 9 kg/t and 12 kg/t respectively. Furthermore, it has been shown that for carbons containing lower copper values the hydrochloric acid concentration and temperature could be reduced with no significant reduction in the elution efficiency of gold. While the investigation demonstrated the significant potential savings in reagent utilisation likely to be incurred when eluting lower copper loaded carbons, these savings can only be realised if elution is considered in isolation to other processes. It would be unwise to optimise elution in isolation from other processes and therefore alternative methods for coping with problematic copper need to be considered.
ACKNOWLEDGEMENTS The author wishes to thank the Anglo American Corporation of South Africa, Gold Division for the financial support and the Anglo American Research Laboratories (Pty) Limited for permission to publish the paper.
REFERENCES .
.
.
4. 5.
Fleming, C.A. & Nicol, M.J., The Adsorption of Gold Cyanide onto Activated Carbon: Part III. Factors Influencing the Rate of Loading and Equilibrium Capacity. J.S. Afr., Inst. Min. Metall., 84, 85-93 (1984). Rogans, D.M., Temperature and Acid Requirements for Calcium Removal from CIL Loaded Carbon. East Rand Gold and Uranium Company Limited. Internal Report. Ergo Project No. 88/55. (1988). La Brooy, S., Use of Ammonia/Cyanide in AMMTEC Colloquium on Processing of GeM-Copper Ores (Practical Aspects), Perth, Australia, (July 1991). La Brooy, S., Copper-Gold Ore Treatment Options and Status. Randol GoMForum VANCOUVER '92. 173-177 (1992). Ruane, M., Gold Recovery from the Paris Mine Tailings using Ammoniacal Cyanide Leachant. in AMMTEC Colloquium on Processing of Gold-Copper Ores (Practical Aspects), Perth, Australia (July 4 1991).
264
.
°
8.
P.T.E.
BosHo~
Sceresini, B. & Richardson, P., Development and Application of a Process for the Recovery of Copper and Complexed Cyanide from Cyanidation Slurries. Randol Gold Forum. CAIRN$ '91, Cairns, Australia, (April 16-19, 1991). McArthur, D & Christie, T.L., Elution and Gold Recovery. Gold Elution Subsequent to Copper Removal. AARL Project No. R/83/123. Report No. 5. (1982). Avraamides, J., Jones, K., Staunton, W.P. & Sceresini, B., Gold Hydrometallurgy Research at the Mineral Processing Laboratory of the Department of Mines, Western Australia. Hydromet~llurgy, 30, 163-175. (1992).
0892-6875/94 $6.00+0.00 © 1993 Pergamon Press Ltd
Printed in Great Britain
CYANIDE-FREE AARL ELUTIONS ARE FEASIBLE
P.T.E. BOSHOFF Anglo American Research Laboratories (Pty) Ltd, P.O. Box 106 Crown Mines 2025, South Africa (Received 15 July 1993; accepted 1 September 1993)
ABSTRACT
A major operating costcommon to elution processes has been the use of sodium cyanide. In recent years plants operating the universally used Zadra mwl AARL elution processes have been reporting satisfactory elution results with greatly reduced cyanide additions atut attendant cost savings. The Anglo American Research Laboratories (AARL) embarked on a testwork campaign aimed at establishing under what conditions cyanide-fi'ee AARL elutions were possible. The testwork demonstrated that cyanide-jS"ee elutions of carbons containing low amounts of copper (less than 1 200 g/t), are indeed possible and that under these conditions the severity of the acid wash conditions (acid wash temperature and acid concentration) could be reduced without a detrimental effect on gold elution e~ciency. It was suggested, however, that the type and quantity offoulants to be stripped from the carbon during acid washing would dictate the optimum conditions employed during the acid wash. bt the case where copper presents a problem, alternative methods of processing have been highlighted. These include - minirnisation of copper dissolution and adsorption, copper pre-leaching and selective copper elution using ambient temperature cyanide solutions. Keywords Activated - carbon, copper, elution, zero-cyanide, AARL-elution
INTRODUCTION The technology of gold elution is dominated by two systems, the first developed by Zadra at the United States Bureau of Mines in 1950 and the second, the AARL system patented by the Anglo American Research Laboratories (AARL) in 1973. Since every major CIP plant throughout the world uses one or other of these two systems there has been vigorous debate as to their relative merits. Advantages such as ease of operation, better security, lower capital cost, lower power requirements and lower reagent consumption have all been advanced in favour of the two systems in turn. A major cost common to both processes has been the use of sodium hydroxide and more notably the use of sodium cyanide. However, in recent years, plants using the Zadra elution process have reported that cyanide free elutions could be performed with no significant decrease in circuit performance. This resulted in substantial savings in reagent costs and hence in total running costs. 251
252
P.T.E.
BOSHOFF
In parallel with these developments, operators of the AARL system were also reporting satisfactory elution results with greatly reduced cyanide additions and attendant cost savings. As a result, investigations were initiated at the AARL to establish under what conditions cyanide could be omitted from the elution operation. In particular, it was attempted to establish a relationship between the efficiency of gold elution and the base metal (particularly copper) loadings on the carbon. Additional testwork was aimed at optimising reagent requirements during the elution sequence when it became apparent that cyanide-free elution was indeed possible. THE EFFECT OF BASE METALS IN THE CARBON CIRCUIT During cyanide leaching of gold ores, a number of base metals are also leached, notably copper, nickel, cobalt, iron and zinc. The presence of various base metals in CIP and CIL circuits can have a detrimental effect on efficiency and economics due to high cyanide consumption and a reduction in leaching and adsorption rates. Furthermore, the presence of readily loaded and adsorbed metals such as copper can significantly reduce the carbon loading capacity for the precious metals, thereby necessitating a higher carbon inventory and more frequent elution with higher NaCN concentrations. However, unlike copper, and to a lesser extent nickel, the cyanide complexes of cobalt, iron and zinc are not loaded well onto activated carbon, and their presence in solution has little effect on the extraction efficiency of gold. For Witwatersrand gold ores, copper and nickel are the predominant base metals in solution with copper being by far the most problematic. Copper is particularly important, in that the loading of its cyanide complexes onto carbon is greatly influenced by the chemical conditions prevailing in solution. Under certain conditions, it is loaded more strongly than the aurocyanide species and tends to displace gold from the activated carbon; under other conditions it is hardly loaded at all. Copper dissolves in cyanide solution during leaching as a series of copper (I) complexes depending on the ratio of cyanide to copper. The following complexes are present in solution: Cu(CN)2- Cu(CN)32and Cu(CN)43- with Cu(CN)32- being the predominant form in typical plant solutions. If during the adsorption stage, conditions of low pH and low free cyanide in the pulp prevail, the above copper complexes are adsorbed onto the carbon. It has been shown that the Cu(CN)2- species is loaded very strongly, whereas Cu(CN)32- and Cu(CN)43- are hardly loaded at all [1]. By maintaining a high free cyanide level in the pulp along with high pH, the adsorption of copper onto activated carbon can be minimised. However, this is counter-productive to minimising cyanide utilisation and hence in CIP circuits treating copper bearing ores, carbon tends to load copper to a significant degree. This has major implications on the amount of cyanide subsequently required to elute gold effectively.
EXPERIMENTAL The elution testwork was conducted in a laboratory elution column as shown in Figure 1. The method of gold elution used in the investigation was as follows: 500 ml of wet-settled gold-loaded carbon was placed in an oil-jacketed stainless steel elution column of 38 mm internal diameter and 460 mm length. Oil was heated in a separate vessel and then circulated through the jacket to maintain the contents of the column at the desired elution temperature. Before entering the elution column, the eluant was heated to elution temperature by being pumped through a stainless steel spiral placed inside the oil-jacket. The eluant was pumped through the column using a metering pump. Pressure within the system was maintained by means of a manually operated valve on the effluent line of the column. Column eluate was cooled in an air-cooled stainless steel spiral condenser before being collected in 500 ml volumetric flasks. The elution was stopped once 5 L of eluate (10 bed volumes) had been collected.
253
Cyanide-free AARL elutions
i co=o==so= ]
I NaOH NaCN
I I AGIDWASH PERFORMED IN ~EPARATE VESSEL
WATER
OIL JACKETED EL U TION COL UMN
OIL BATH AND TEMPERATURE REG UL ATOR
J
S
ELUATE
1I
Fig. 1 Diagrammatic Representation of the Laboratory Scale Elution Column The elution procedure used in the investigation included acid washing of the loaded carbon in a glass beaker for 15 minutes using I bed volume of a hydrochloric acid solution, water washing using 10 bed volumes of ambient temperature tap water, pretreatment with 1 bed volume of a caustic cyanide reagent followed by elution using deionized water at a flowrate of 2 bed volumes per hour. Variations of the highlighted items in the elution procedure summarised below were used to establish the effect of acid wash temperature, hydrochloric acid concentration and cyanide concentration.
ACID WASH Volume Acid Concentration Temperature Time Water Wash
: : : : :
1 bed volume 1 and 3 percent (v/v) 25 and 90"C 15 minutes 10 bed volumes
PRETREATMENT Caustic
:
2 percent (m/v)
Cyanide
:
0 and 2 percent (m/v)
Temperature Volume Flowrate Soaking Time
: : : :
125"C 1 bed volume 2 bed volumes/hr 30 minutes
254
P . T . E . BosrloFF
ELUTION Eluant Temperature Volume Flowrate
: : : :
deionised water
125"C 10 bed volumes 2 bed volumes/hr
The loaded carbons used throughout the testwork, (identified as carbon 1, carbon 2 and carbon 3) were obtained from three South African CIP circuits. The carbons were selected because the differences in their gold and copper loadings made it possible to assess the effects of high copper loadings on the feasibility of cyanide-free elutions. Elution efficiency was defined as: Cumulative gold eluted per stage xl00 Total gold eluted + residual gold on carbon This efficiency was determined for each half hour period during the elution.
DISCUSSION The metallic loadings for the carbons used during the testwork are presented in Table 1. All carbon samples were obtained from the loaded carbon catch screens. However, although samples A, B and C of carbon 1 were obtained from the same place within the carbon circuit, they were taken a month apart for three months. It is evident that the gold and copper loadings of carbon 1 vary significantly over the three month period which highlights the extreme variability of the ore being fed to the plant.
TABLE 1 Metallic ioadings of carbon used during elution testwork Loading (g/t) Carbon 1
Element
Carbon 2
Carbon 3
A
B
C
Au
3 200
9 830
4 500
420
2 900
Ag
345
117
230
50
190
Cu
1 200
8 970
12 000
270
95
Ni
2 950
1 300
2 500
7 100
2 200
Fe
850
905
1 200
650
330
Zn
145
100
19
155
30
Ca
2 300
1 270
2 900
17 600
10 000
SiO 2
12 500
27 700
13 000
5 800
5 500
Cyanide-free AARL elutions
255
Carbon 1 Table 2 presents the gold elution efficieneies, after a five hour elution using deionised water, for a low copper loaded carbon (Carbon 1A), after pretreatment under different conditions of acid wash temperature, hydrochloric acid concentration and sodium cyanide concentration. The influence of these variables on the kinetic response is shown in Figure 2.
TABLE 2 Elution efficiencies for low copper loaded carbon Acid Wash Conditions HCI Cone
Pretreatment Conditions
Gold Elution Efficiency
(%)
Temperature
NaOH Cone
NaCN Cone
(%)
(*c)
(%)
(%)
3
90
2
2
98.3
3
90
2
0
94.3
3
25
2
2
99.0
3
25
2
0
96.6
1
25
2
0
96.5
Copper Loading = 1 200 g/t Au Elution Efficiency
(%)
100
80
60 TEST
40 ¢~
20
~r r7
0 0
2
ACID TEMP
ACID CONC
NaCN CONC O% O % O% 2 2~ 2 ~
25C 19~ 25C [ 3~ NO ACID WASHING 25C 3~ 90C 3 ~
I
I
I
I
4
6
8
lO
Number
of Bedvolumes
Eluant
Copper Loading : 1 2 0 0 g / t Gold Loading : 3 2 0 0 g / t Fig.2 Gold Elution Efficiency Curves for Carbon 1A
12
A.A.R.L.
256
P.T.E.
BOSnOFF
With reference to Table 2 and Figure 2 the following can be deduced:
i)
Provided cyanide is present during the pretreatment stage, hot acid washing may be replaced with an ambient temperature acid wash with no detrimental effect on the overall gold elution efficiency. Although initial elution kinetics are slightly retarded, efficiencies become comparable to those obtained with hot acid washing, after approximately 2.5 hours, i.e. well within normal plant elution times. Cold acid washing removes 78 percent of the loaded calcium during a 15 minute acid wash, while a 90°C acid wash removes 85 percent. Conceivably the decrease in calcium removal during cold acid washing may eventually have a detrimental effect on elution performance due to fixation of calcium on the carbon during regeneration. The implications of excessive calcium loadings on the regenerated carbon recirculated to adsorption are two-fold. Firstly, the presence of a layer of calcium carbonate on the surface of the carbon significantly reduces the carbon's affinity towards gold in solution due to blocking of the macropores within the carbon. Secondly, repeated regeneration of excess calcium carbonate would possibly result in the formation of low melting point glasses within the porous structure of the carbon. These glasses have vastly different coefficients of thermal expansion from carbon and therefore the uneven expansion of the carbon during regeneration would alter the internal pore size distribution of the carbon. This would seriously affect the kinetic and equilibrium characteristics of the carbon.
ii)
When cyanide was omitted from the pretreatment operation, elution efficiency dropped 2.5 - 4 percent. However, gold elution efficiencies remained high and ranged between 94% (72 g Ault on eluted carbon) and 97 % (36 g Au/t on eluted carbon). It is suggested that an increase in the eluted carbon values from 36 to 72 g/t Au would not seriously affect the adsorption efficiency.
iii)
Overall elution efficiency improved from 94 % to 97 % when hot acid washing was replaced with a cold acid wash. In addition, elution kinetics were faster. A further benefit of cold acid washing was that a decrease in hydrochloric acid concentration from 3 % to 1% had no detrimental effect (elution efficiencies were approximately 97 % for both concentrations). Initial elution kinetics were only slightly retarded at the low HCI concentration while calcium removals were in the order of 77%. However it should be noted that the calcium loading on this carbon was only 2.3 kg/t. Stoichiometrically, 73 g HCI (100%) is required to remove 40 g of calcium [2]. On this basis it can be calculated that to remove 2.3 kg/t Ca from the carbon 4.2 kg/t HCI would be required. If the calcium loading increased to 15 kg/t the theoretical HCI requirement would increase to 27.4 kg/t. It is therefore evident that acid requirements are largely dependent on the extent of calcium fouling.
Table 3 presents the nickel and gold elution efficiencies in the absence of cyanide. It can be seen that in the absence of cyanide, nickel elutions decreased from 89% when cold acid washing was employed to 52% when a 90"C acid wash was used prior to the elution. Under the same conditions, gold elution effieiencies were 97% and 94% respectively. It is therefore suggested that the nickel loading (approximately 3 kg/t) is unlikely to have as marked an effect on gold elution as the copper loading. A similar, but less pronounced trend was also observed when eluting with cyanide. Figure 3 shows the gold elution efficiencies for Carbon IB which had gold and copper loadings of approximately 9.8 kg/t and 9.0 kg/t respectively.
Cyanide-free AARL elutions
257
TABLE 3 Nickel and gold elution efficiencies in the absence of cyanide
ACID WASH CONDITIONS
ELUTION EFFICIENCY
HCI C O N C
TEMPERATURE
(%)
(*c) 90 25
Ni
Au
(%)
C%)
52.4 89.2
94.3 96.6
i) Ni Loading ffi 3 000 g/t ii) Au Loading = 3 200 g/t iii) Prior to the elution using deionised water the carbons were pretreated with 2 percent NaOH.
NOTES:
Au Elution Efficiency (%) 100
80 60
40 D TEMP
/ f
20
z
s%
9oc
3 •
,25C
NaCN CONC
2 %
0 0
2
4
6
8
10
12
Number of Bedvolumes Eluant
Copper Loading : 8 970 g / t Gold Loading : 9 830 g / t
A.A.R.L.
Fig.3 Effect of Acid Wash Conditions on Gold Elution Efficiency for a High Copper/High Gold Loaded Carbon Despite the excessive copper loading and uncharacteristically high gold loading of 9.8 kg/t (CIP plants in South Africa load gold to between 3 and 5 kg/t) efficient elution was achieved (approximately 97 %) after a 5 hour elution when 2% cyanide was added during the pretreatment stage. Without the addition of cyanide, elution efficiency dropped to 68 % when cold acid washing was employed prior to the elution. A further reduction in gold elution efficiency to 63 % occurred when hot acid washing was used prior to a cyanide-free elution. Figure 4 illustrates the gold elution efficiencies for Carbon IC which had an extremely high copper loading of 12 kg/t and a gold loading of 4.8 kg/t. The elution in which cyanide was present gave a 98% gold elution efficiency while in the absence of cyanide elution efficiencies dropped to 46 %. Both these elutions were preceded witti an ambient temperature (25°C) acid wash using a 3% solution of
P.T.E. Bosxow
258
hydrochloric acid. Elution efficiency increased slightly to 53 % when the carbon was not subjected to an acid wash prior to the cyanide-free elution.
lOO
Au Elution E f f i c i e n c y (%) 7.
/,//~TEST
ACID CONC
80 -
/
/
I
I I
I
60
i
~
a%
o []
3~ No Ac,o w a s .
/
[]
°
O
NaCN CONC 2% 0% 0% F1
40 2 0 _¢~1.
v
i
i
]
iO
2
4
6
8
0
0
10
N u m b e r o f Bedvolumes Eluant Copper Loading : 12 000 g / t Gold Loading : 4 800 g / t
12
A.A.R.L.
Fig.4 The Effect of High Copper Loadings on Gold Extraction Under Various Acid Wash Conditions Table 4 provides a summary of the results achieved on Carbon 1 (A, B and C) and presents the differences in gold elution efficiencies for elutions conducted in the presence or absence of cyanide. TABLE 4 Elution efficiencies in the presence/absence of cyanide as a function of copper loading
Sample
Copper Loading (g/t)
Gold Loading (g/t)
Acid Wash
NaCN Pretreatment
A
1 200
3 200
Yes Yes
Yes No
99.0 96.6 (2.4)
B
8 970
9 830
Yes Yes
Yes No
97.2 68.2 (29.0)
C
12 000
4 800
Yes Yes No
Yes No No
97.6 45.9 (51.7) 53.4 (44.2)
NOTES:
Au Elution Efficiency (%)
i)
For the tests in which acid washing was conducted prior to the elution, a 25°C and 3% HC! acid wash was used.
ii)
During pretreatment, 2 percent NaOH was used in addition to 2
iii)
percent NaCN. The figures in brackets are the differences between the gold elution efficiencies achieved for elutions with and without cyanide.
Cyanide-free AARL elutions
259
It is obvious that, as the copper loading on the carbon increases, the success of cyanide-free elutions is reduced. It appears that, in the presence of very high copper loadings, cyanide addition becomes necessary to convert the copper to a form that can be eluted. Figure 5 illustrates the effect of copper loading on the elution efficiency of gold and copper in the absence of cyanide. Efficient gold elution is largely dependent on the efficiency with which copper is eluted which in turn is related to the copper loading of the carbon. Any increase in the amount of copper loaded on the carbon will significantly reduce gold elution efficiency in the absence of cyanide. Above a certain copper loading (3 kg/t), efficient gold elution can only be achieved by the addition of cyanide. It is also suggested that during acid washing, partial formation of an insoluble copper species occurs, thus reducing gold efficiencies even further due to macropore blockage. It is believed that the formation of this undesirable copper species will be minimised as acid washing conditions become less aggressive, e.g. ambient temperature acid washing at reduced HCI concentrations.
Au Elution E f f i c i e n c y (%) 100 r
Cu Elution E f f i c i e n c y (%) 100
_
1 2 0 0 g / t Cu
80
80
60
-
60
40
-
40
20
-
20
12 0 0 0 g / t Cu
0 ~!W-"~-
0
I
q
I
I
I
I
2
4
6
8
10
12
0 14
N u m b e r o f Bedvolumes E l u a n t []
Gold
~
Copper
Fig.5 Effect of Copper Loading on Gold and Copper Elution in the Absence of Cyanide.
Carbon 2 The carbon used for this phase of the testwork was a loaded carbon obtained from a carbon-in-leach plant treating low-grade dam material. The gold and copper loadings for this carbon were 420 g/t and 270 g/t respectively. Figure 6 shows that while the initial gold elution rates were retarded in the absence of cyanide, overall elution efficiencies were little affected after approximately 3 hours (6 Bed volumes of eluant). Again it can be seen that a reduction in the severity of the acid washing conditions employed prior to the cyanidefree elution of a low copper loaded carbon had no effect on either the rate of elution or the final elution efficiency.
P . T . E . BOSHOFF
260
Au Elution Efficiency (~) 100
m
am
~
80 60
TEST
40 20 Oe
0
2
ACID TEMP
ACID CONC
NaCN CONC
90 C
3~
2~
A
90 C
3~
O~
X
25 C
3~
2%
o
25 C
3%
0%
[]
NO ACID WASHING
I
I
4
6
0% 1
_l
8
Number of Bedvolumes Eluant Copper Loading : 270 g / t Gold Loading : 420 g / t
10
12
A.A.R.L.
Fig.6 Graph of Au Elution Efficiencies Demonstarting the Feasibility of Cyanide-free Elutions for Carbons Loaded with Low Levels of Copper. Since this carbon had a low gold loading of 420 g/t it might be expected that this small amount of gold would be eluted irrespective of the presence or absence of cyanide. Figure 7 presents the gold elution efficiencies in the absence and presence of cyanide, for a carbon with 155 g/t gold and 22.5 kg/t Cu. Despite the addition of 2 % cyanide during pretreatment, the efficiency of gold elution was unsatisfactorily low at 74%. A cyanide-free elution of this carbon resulted in an elution efficiency of only 25 %. Although these elutions were conducted subsequent to a hot (90°C) acid wash it is unlikely that the use of cold acid washing would have produced significant improvements in the gold elution efficiency. It is concluded that the success or failure of cyanide-free elutions is largely dependent on the extent of copper loading and practically independent of the amount of gold on the carbon. Carbon 2 had a nickel loading of 7.1 kg/t which made it possible to assess the effect of high nickel loadings on gold elution efficiencies in the absence of cyanide. Table 5 presents gold and nickel elution efficiencies in the absence of cyanide using different acid washing temperatures. Nickel elution dropped from 91% to 82% when hot acid washing was used as opposed to cold acid washing. It was expected that a similar decrease in gold elution efficiency would occur if high nickel loadings affected gold elution efficiency to the same degree as copper. However, since gold elution efficiencies remained unchanged at 99%, nickel does not play 'a significant role in governing the success of cyanide-free elutions. Carbon 3 The final carbon used in the investigation was one obtained from a low grade CIP plant that has recently converted successfully to a cyanide-free AARL elution. Gold and copper loadings were 2.9 kg/t and 95 g/t respectively. Figure 8 shows that cyanide-free elutions subsequent to a hot or ambient temperature acid wash gave extractions in excess of 99 %. The corresponding extraction tbr the elution where cyanide was added was
Cyanide-free AARL elutions
261
also above 99 %. This Figure serves to highlight the fact that dution of carbons that contain normal amounts of gold with low copper values can be effectively eluted with no cyanide addition. Although the acid washing conditions do not play a significant role in the final elution efficiencies, a reduction in the acid wash temperature to 25°C would be preferred as this would reduce the thermal requirements and hence operating costs of the process. It must, however, be borne in mind that many elution circuits conduct the acid washing stage in the elution column, and therefore hot acid washing would be the preferred option as this could be included as part of the heating cycle for the elution process.
Au Elution Efficiency (%) 100 80 r3
m
60
40 © kJ
20
0
0
2
4
6
I
I
8
10
N u m b e r of Bedvolumes Eluant
CN Present
[]
CN Absent
c,
C o p p e r Loading : 2 2 5 0 0 g / t Gold Loading : 155 g / t Fig.7 Graph Illustrating the Success of Cyanide-free Elutions is Largely Dependent on the Extent of Cu Loading and Relatively Independent of Gold Loadings.
TABLE 5 Nickel and gold elution efficiencies in the absence of cyanide
ACID WASH CONDITIONS
ELUTION EFFICIENCY
TEMPERATURE (°C)
HCI CONC
Ni
Au
(%)
(%)
(%)
90 25
3 3
81.7 91.3
98.7 98.8
NOTES:
i) ii) iii)
Ni Loading = 7 100 g/t Au Loading = 420 g/t Carbons pretreated with 2 percent NaOH.
12
262
P . T . E . BOSSOFF
Au Elution Efficiency (~)
100 8060
NaCN CONC
40
p
//
c
y O
x t
0
2
I
90C I
,,
o,
3~
2%
I
4 6 8 Number of Bedvolumes Eluant
~__ ......
10
C o p p e r Loading : 9 5 g / t Gold Loading : 2 9 0 0 g / t
12
A.A.R.L.
Fig.8 Elution Efficiency Curves for a Carbon that Contains Low Copper and a Gold Loading Typical of Many CIP Loaded Carbons.
PLANT IMPLICATIONS This investigation has demonstrated that cyanide-free AARL elutions are feasible provided copper loadings on the carbon are low (less than 1.2 kg/t) and seen in isolation there might be motivation to eliminate cyanide from the elution system. However, elution is linked to various other operations and it would be unwise to optimise it in terms of cyanide addition, acid wash temperature, acid concentration and elution time in isolation from the other processes. The adsorption of copper onto carbon in a CIP circuit can be reduced by maintaining high cyanide concentrations in solution and operating at around pH 12, this however results in higher operating costs. An alternative remedy is to leach with ammonia cyanide solutions. La Brooy [3,4] showed this to enhance the dissolution of gold relative to copper while reducing copper adsorption on carbon. At the Paris Mine in Western Australia where the process was applied, loaded carbon containing 3.3 kg/t Au and 1.5 kg/t Cu was produced from a solution assaying 1 g/t Au and 100 g/t Cu [5]. Sceresini and Richardson [6] developed a process whereby copper is selectively leached from gold, adsorbed onto carbon, eluted and precipitated to produce a saleable copper salt. Secondary recovery of the gold is then achieved under improved conditions in the absence of a large proportion of the copper. Yet another alternative is to allow copper and gold to load during adsorption and solve the copper problem either prior or subsequent to elution. Once copper has loaded onto carbon it is often desirable to selectively elute it. During electro-winning copper plates on the cathodes and hinders the recovery of the gold cement. During zinc precipitation the presence of copper results in excessive zinc utilisation in addition to contaminating the dor4.
Cyanide-free AARLelutions
263
In a study conducted by McArthur [7] copper removals in excess of 95 % were achieved for a carbon containing 19 kg/t Cu using a 0.5 % NaCN solution at ambient temperature. The associated gold removal was negligible at 0.2 ~. In practice, the majority of plants return the ambient eluant to the process solution with no significant build-up of copper on the carbon. The other approach is to tackle the problem at the electro-winning stage. Testwork conducted by Avraamides [8] has shown that maintenance of high free cyanide concentrations in the electro-winning circuit might be a viable approach to retaining copper in solution. On the Witwatersrand such measures are generally not required, in other areas the most appropriate solution would depend on the severity of the problem and on local conditions.
CONCLUSIONS The testwork has demonstrated that provided copper loadings are low, cyanide-free AARL elutions are indeed possible. It is suggested that cyanide-free elutions are certainly possible for copper loadings less than 1.2 kg/t, may be possible for loadings between 1.2 kg/t and 3.0 kg/t and are highly unlikely if the copper loading exceeds 3.0 kg/t. The difference between the elution efficiencies obtained for elutions conducted in the presence and absence of cyanide were 2%, 29% and 52% for carbons with copper loadings of 1.2 kg/t, 9 kg/t and 12 kg/t respectively. Furthermore, it has been shown that for carbons containing lower copper values the hydrochloric acid concentration and temperature could be reduced with no significant reduction in the elution efficiency of gold. While the investigation demonstrated the significant potential savings in reagent utilisation likely to be incurred when eluting lower copper loaded carbons, these savings can only be realised if elution is considered in isolation to other processes. It would be unwise to optimise elution in isolation from other processes and therefore alternative methods for coping with problematic copper need to be considered.
ACKNOWLEDGEMENTS The author wishes to thank the Anglo American Corporation of South Africa, Gold Division for the financial support and the Anglo American Research Laboratories (Pty) Limited for permission to publish the paper.
REFERENCES .
.
.
4. 5.
Fleming, C.A. & Nicol, M.J., The Adsorption of Gold Cyanide onto Activated Carbon: Part III. Factors Influencing the Rate of Loading and Equilibrium Capacity. J.S. Afr., Inst. Min. Metall., 84, 85-93 (1984). Rogans, D.M., Temperature and Acid Requirements for Calcium Removal from CIL Loaded Carbon. East Rand Gold and Uranium Company Limited. Internal Report. Ergo Project No. 88/55. (1988). La Brooy, S., Use of Ammonia/Cyanide in AMMTEC Colloquium on Processing of GeM-Copper Ores (Practical Aspects), Perth, Australia, (July 1991). La Brooy, S., Copper-Gold Ore Treatment Options and Status. Randol GoMForum VANCOUVER '92. 173-177 (1992). Ruane, M., Gold Recovery from the Paris Mine Tailings using Ammoniacal Cyanide Leachant. in AMMTEC Colloquium on Processing of Gold-Copper Ores (Practical Aspects), Perth, Australia (July 4 1991).
264
.
°
8.
P.T.E.
BosHo~
Sceresini, B. & Richardson, P., Development and Application of a Process for the Recovery of Copper and Complexed Cyanide from Cyanidation Slurries. Randol Gold Forum. CAIRN$ '91, Cairns, Australia, (April 16-19, 1991). McArthur, D & Christie, T.L., Elution and Gold Recovery. Gold Elution Subsequent to Copper Removal. AARL Project No. R/83/123. Report No. 5. (1982). Avraamides, J., Jones, K., Staunton, W.P. & Sceresini, B., Gold Hydrometallurgy Research at the Mineral Processing Laboratory of the Department of Mines, Western Australia. Hydromet~llurgy, 30, 163-175. (1992).