NZ286459A - Use of an anti preg-robbing solvating agent to raise yield in leaching of metal from ore - Google Patents

Description

New Zealand Paient Spedficaiion for Paient Number £86459 T New Zealand No. 286459 International No. PCT/ TO BE ENTERED AFTER ACCEPTANCE AND PUBLICATION Priority dates: 27.04.1995; Complete Specification Filed: 26.04.1996 Classification:^) C22B3/04.24; C22B11/08 Publication date: 27 April 1998 Journal No.: 1427 (\io NEW ZEALAND PATENTS ACT 1953 COMPLETE SPECIFICATION Title of Invention: Controlled potential leaching Name, address and nationality of applicant(s) as in international application form: MICHAEL STOYCHEVSKl, an Australian citizen of 42 Moate Avenue, Brighton, New South Wales 2216, Australia 2 8 6 4 5 9 NEW ZEALAND PATENTS ACT, 1953 No: Date: COMPLETE SPECIFICATION CONTROLLED POTENTIAL LEACHING We, MICHAEL STOYCHEVSKl, an Australian citizen, of 42 Moate Avenue, Brighton, New South Wales 2216, Australia, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: 28 6 459 TECHNICAL FIELD This invention relates to the recovery of metals from their host materials. In particular it relates to the recovery of precious metals, such as gold, from metal bearing hosts.

The invention will be preferably described in relation to the recovery of preg-robbed gold from gold-bearing materials. However, it should be appreciated that the invention is also useful for the recovery of other metals from their host materials.

Gold which cannot be physically removed from gold-bearing materials must be dissolved in order to separate it from its surrounding matrix. This process generally requires the formation of an electrically charged gold complex which is soluble under the prevailing conditions. In the gold mining industry, the dissolution of gold is commonly achieved using the CN" ion to form a [Au(CN)2]" complex under alkaline conditions (ie. cyanidation). Since this process requires contact between metallic gold and CN", the ore must first be crushed to a size which liberates most of the gold to the liquid phase. Once dissolution has been achieved, gold recovery is relatively simple. The majority of gold plants utilise carbon to adsorb [Au(CN)2]"from the ore slurry in what is known as carbon-in-pulp (CIP) or carbon-in-leach (CIL) processing. Relatively large carbon fragments can be physically separated from a slurry and eluted (washed) to remove [Au(CN)2]'. The carbon can then be roasted to reactivate its surface. Although carbon is a very effective adsorbing agent, ore components which carry residual positive charges can also bind [Au(CN)2]" to their surface. These ore components are generally referred to in the mining industry as preg-robbers. Preg-robbers effectively compete with carbon for [Au(CN)2]\ Their competitiveness is dependent on their activity (tendency to adsorb gold) and their concentration. In general, the adsorption of particles is based on the dynamic equilibrium: S + [Au(CN)2]-aqu S[Au(CN)2]-ads The amount of adsorbed [Au(CN)2]" on the surface of the ore is therefore proportional to the concentration of aqueous [Au(CN)2]" ( [Au(CN)2]"aqu ) and the amount of active surface sites (S). Therefore during CIL or CIP, [Au(CN)2]" is in dynamic equilibrium with active sites. If the activity of S is high, then the equilibrium strongly favours the right hand side and, therefore, the adsorption BACKGROUND ART 2 .286459 © concentrations of carbon in the slurry, preg-robbing can be minimised. However, in some ores the prevalence and/or activity of preg-robbing species is high enough to significantly reduce leach recoveries.

In complex ore bodies, there exists a vast potential for preg-robbing agents. 5 These range from indigenous materials through to inorganic species with ion exchange properties. Almost all ore types exhibit some capacity to adsorb charged species, hence the need for CIP or CIL .

Potential preg-robbing agents include oxide structures which are inherently electrostatic and carry residual surface charges. Structures such as alumina (Al203), 10 silica (Si02), and a host of transition metal oxides (eg., Fe203, NiO, ZnO, Mn02, Cr203, Mo03, etc.) are widely used in industry as adsorbents (structure which adsorb molecules). Analogous oxide minerals which occur naturally in ore bodies in the form of clays are also well known preg-robbing agents. Naturally occurring carbonaceous material such as graphite, decaying vegetation or other organics can also act as preg-15 robbers by reactions alike to, and in opposition to, those of activated carbon, added for the purpose of recovering the metal. Thus for many processes relating to the recovery of metals there exists the potential for preg-robbing.

In Stoychevski M. and Williams L.R., "Sodium sulfide - assisted gold recovery from arsenopyrite and roasted arsenopyrite" Transactions of the Institution of Mining 20 and Metallurgy (Section C: Mineral Process. Extr. Metall.), Volume 102, pp C179-183, the addition of sodium sulfide during the cyanidation process was investigated. This study established that sodium sulfide addition during the cyanidation process effectively increased gold recovery by lowering the solution potential.

It was, however, noted that the effectiveness of this process was limited as that 25 as the S2" concentration was increased to sustain the solution potential at the lowest possible level, the recovery of gold was retarded possibly by the formation of a passivating layer on the gold. In conclusion, it was found that the effectiveness of S " in enhancing gold cyanidation appeared to be limited to oxidising ore slurries at solution potentials which resulted in unfavourably high cyanide consumption. 28 6 45 9 OBJECT OF THE INVENTION It is an object of this invention to provide conditions (ie: that preferably control the environment) which minimise or reverse the preg-robbing capacity of metal-bearing materials and thus enable the recovery of the metal.

It is also an object of this invention to enhance the recovery of gold by providing conditions either before or after a traditional gold leaching process which may liberate the gold from its surrounding sulfide matrix and thus enable greater gold dissolution.

It is a further object of the invention to overcome or alleviate at least one of the problems associated with prior art processes by minimising the preg-robbing capacity of metal-bearing materials.

DISCLOSURE OF THE INVENTION The present inventor has surprisingly found that the addition of particular agents (ie. anti preg-robbing agents) to a metal-bearing slurry either before or after a traditional metal leaching process can minimise the formation of preg-robbing oxides and/or minimise the activity of existing preg-robbing species. In particular for the recovery of gold , applicant has found that these anti pre-robbing agents, through their solvent effect on gold-bearing sulfide minerals, can also liberate gold and thus enhance gold dissolution during leaching In accordance with the present invention there is provided a method of recovering a metal from a metal-bearing ore, wherein said method includes the addition of an anti preg-robbing solvating agent to a metal-bearing material slurry either before or after a metal leaching step.

In a preferred embodiment of the invention there is provided a method for the recovery of a metal from a metal bearing slurry, wherein said method includes; a) adding an anti preg-robbing solvating agent to the metal bearing slurry, and b) subsequently subjecting the metal bearing slurry to a leaching step and recovering the metal.

In another embodiment of the invention there is provided a method for the recovery of a metal from a metal bearing slurry, wherein said method includes; a) subjecting the metal bearing slurry to a leaching step and recovering a first portion of said metal and collecting the tailings therefrom, b) adding an anti preg-robbing solvating agent to the tailings, and c) subsequently recovering a second portion of said metal from the tai 4 28 6 459 In the method of the invention any suitable anti preg-robbing solvating agent may be used. The selection of the anti preg-robbing solvating agent will necessarily depend on a number of factors including; whether the anti-preg-robbing agent will effect any other step involved in the overall leaching of the metal, availability arid cost. Suitable 5 anti preg-robbing solvating agents include sulfur compounds containing S" or S2" functional groups, or a group which results in the formation of HS' when the compound is placed in an aqueous solution.

For the leaching of gold, applicant has found that compounds having a sulfide ion (S2") are the preferred anti preg-robbing solvating agents, primarily due to their cost 10 effectiveness. In the leaching of gold the sulfide ion is preferably added to the process as Na2S. Na2S can be added as a solid or diluted to approximately 25% weight/weight in water. Applicant has found that Na2S can be utilised as an anti preg-robbing agent, either prior to and/or subsequent to gold leaching. It has been observed that certain ore types are dehydrated by the action of Na2S, resulting in low viscosity ore slurries. It 15 is speculated that this loss of water occurs after the deposition of sulfur onto adsorbents within the ore. For example, the tendency for clay minerals to hold well in excess of their own weight in water is widely known. This tendency arises from the electrostatic attraction between oxide minerals and the polar H20 molecule. The deposition of an insoluble sulfur layer(s) onto a charged surface neutralises the electrostatic attractions 20 which adsorb polar species such as H20, ie. it decreases preg-robbing. In addition to the anti preg-robbing capacity of Na2S, there is also strong evidence to suggest that Na2S effectively decomposes sulfide minerals and thereby exposes gold which may have been previously occluded in the mineral matrix.

In the leaching of gold, Na2S consumption is dependent on the nature of the ore. 25 Preferably at least, 5 Kg Na2S per tonne ore is sufficient to significantly enhance gold recoveries from ores with a high preg-robbing capacity. More preferably at least 10 Kg Na2S per tonne ore is used.

In the method of the invention any suitable metal leaching step may be usea. The selection of the variables in the leaching step such as the selection of components 30 and physical parameters will necessarily be dependent on a number of factors such as the type of metal to be recovered and the availability and reactivity of the various components. 286459 In a preferred embodiment of the invention the metal slurry is subjected to an oxidation step after the addition of the anti preg-robbing solvating agent. The oxidation step is desirable to promote the deactivation of preg-robbing species and/or decomposition of metal bearing sulfides. The type and extent of such oxidation step will be dependent on the ore used in the method of the invention. Oxidation typically involves air or oxygen sparging such that an optimum solution potential (Eh) level is reached. Peroxides and other non-gaseous oxidants may also be effective during the oxidation step.

When the anti preg-robbing solvating agent is added prior to the leaching of the metal slurry, it is particularly preferred to continue the oxidation until an optimum minimum Eh level is achieved so as to maximise deactivation of preg-robbing species and to minimise any carbon fouling during the leaching step. Identification and maintenance of the optimum Eh is preferred, since very lew Eh conditions have been found to have a negative effect on metal recovery during the leaching step.

Where the anti preg-robbing solvating agent is added after the leaching step then it is preferred to maintain the Eh level within an optimum range during the oxidation step to minimise the precipitation of dissolved gold as an insoluble sulfide during the recovery of the second portion of the metal. It has been found that if the Eh level is decreased to a level below the optimum range then this can result in precipitation of leached metal.

The preferred optimum minimum or optimum range for the solution potential will be dependent on a number of factors. For example the preferred Eh level will depend on the metal ore, the leaching system and the anti preg-robbing solvating agent.

In the embodiment where the anti preg-robbing solvating agent is added prior to the leaching step, the solution potential is preferably decreased to a level within the range of - 600mV to -700mV relative to a saturated calomel electrode during the addition of the anti preg-robbing solvating agent. The minimum solution potential is more preferably decreased to a level within the range of -600 to -650mV. During the oxidation step it is preferable to increase the solution potential to approximately -100 to +100mV prior to commencing the leaching step, more preferably 0 to +100rnV. It is particularly desirable to increase the Eh as the addition of the anti preg-robbing solvating agent generally causes the Eh to decrease. For example, in the opeferre^sw embodiment where Na2S is added to a gold leaching system the leach slujwvfe initially '^ 6 "V n r-r 28 6 459 m very sensitive to Na2S addition and the Eh decreases rapidly. However, Eh control below -400mV is greatly simplified by the fact that such a highly reducing environment becomes relatively insensitive to further reagent additions.

In the preferred embodiment, where the anti preg-robbing solvating agent is 5 added to the tailings after the leaching step, the solution potential is preferably maintained at an optimum level within the range of -550 to -650mV relative to a saturatod calomel electrode, during the addition of the anti preg-robbing solvating agent. The minimum solution potential is more preferably maintained within the range of -600 to -650mV. During the oxidation step it is preferable to increase the solution 10 potential to approximately -100 to +1 OOmV, more preferably 0 to +1 OOmV.

The solution potential may be measured and monitored by any suitable means. For example, the solution potential ma;, be measured and monitored by an inert sensing electrode and suitable reference electrode such as a saturated calomel electrode.

In the embodiment of the invention wherein the solution potential is controlled the process of the invention is defined as a Controlled Potential Leaching (CPL) process.

In the method of the invention wherein the anti preg-robbing solvating agent is added prior to the leaching step and CPL is also employed as a conditioning step (ie. 20 prior to any other leaching step), the function of the anti preg-robbing solvating agent is to alter the mineral surface and deactivate pre-existing or potential preg-robbing sites and/or expose gold by decomposing gold-bearing sulfide minerals. This is preferred where plant design does not afford tails treatment.

The addition of anti preg-robbing solvating agent is best employed, for some 25 instances, as a strip phase after gold has been essentially leached from a gold bearing material. In this way, the effect of the method can be measured and modified. Since ore deposits can vary dramatically in composition, process modifications need to be flexible. By adding the anti preg-robbing solvating agent after leaching, the integrity of the optimum Eh value can be monitored and reagent additions can be modified with 30 virtually instantaneous feedback. In addition, post-leach addition of anti preg-robbing solvating agents does not have the potential to reduce leach recoveries due to excessively low Eh levels. 28 6 45 9 In the embodiment of the invention where the anti preg-robbing solvating agent is added to the tailings after the leaching step, the recovery of the second portion of said metal may be achieved by any suitable method. For example, the recovery of the second portion of said metal may be achieved by adding an adsorbent, such as carbon, to the mixture of anti preg-robbing solvating agent and tailings.

The following examples illustrate preferred embodiments of the process of the invention and should not be construed as limiting on the scope of this application.

In the following examples pertaining to gold-bearing sulfide ores, Eh potentials during cyanidation were considered to be conducive to the formation of surface oxides and were recorded to be in excess of -100mV relative to a saturated calomel electrode (SCE).

Example 1 - Ores A and B The results recorded in table 1 show the effects of Eh on gold recovery from ore A based on CIL cyanidation of 500 g ore at 40 % solids using 5 Kg per tonne NaCN and 30 g per L carbon. CPL prior to and after cyanide leaching is referred to as CPL-cond and CPL-strip, respectively. The sulfidisation phase was conducted in a stirred beaker, whilst the leach phase was conducted in rolled bottles.

In table 1, CPL results are based on a 6 hour sulfidisation period in conjunction with CIL cyanidation over 48 hours. It should be noted that hydrolysis of S2" forms OH": S2' + H20 -» HS" + OH' As a result, CPL induces alkaline conditions. It should also be noted that further hydrolysis forms H2S gas which, has a disagreeable (rotten egg gas) odour: HS" + H2Q -> H2S + OH" The results shown in table 1 for CPL-cond therefore resulted in a highly alkaline leach environment (ie. pH 12-12.5) whereas CPL-strip was conducted after leaching at an initial pH of 10.5. In view of this, the results shown in table 1 for ore A suggest that pH does not significantly effect gold recoveries.

The gold recoveries shown in table 1 reveal that the optimum Eh for CPL of ore A is -600mV. The addition of Na2S enhanced gold recovery down to a potential of -600mV, but a dramatic decrease was observed at -650mV. The results for aqueous gold indicate that the equilibrium between carbon and adsorbed [Au(CN)2]" is advers 8 28 6 45 9 excessive sulfide concentrations. In terms of gold recovery, there appears to be no apparent benefit in sulfidising before as opposed to after CIL.

The free CN" results of table 1 clearly show that strongly reducing conditions result in CN" consumption. Although this did not effect CPL-cond results in this case, it 5 emphasises the advantage of sulfidising after CIL, where CN" consumption is irrelevant.

Table 1. Effect of controlled Eh sulfidisation over a 6 h period on gold recovery obtained after 48 h CIL cyanidation.

Eh (mV) gold recovery (%) Aqueous Au (%) free CN" (%) > -100 mV (CIL) 75.1 0 .0 -300 mV (CPL-cond) 76.4 0 26.0 -400 mV (CPL-cond) 78.5 0 18.3 -500 mV (CPL-cond) 84.6 0 7.3 -600 mV (CPL-cond) 86.8 0 0.05 -650 mV (CPL-cond) 51.6 0.45 <0.05 -300 mV (CPL-strip) 76.4 0 .5 -400 mV (CPL-strip) 79.2 0 16.6 -500 mV (CPL-strip) 84.5 0 6.2 -600 mV (CPL-strip) 87.0 0 0.8 -650 mV (CPL-strip) 51.0 0.50 <0.5 Gold recoveries determined by fire assay of leached residue and are based on a 10 head assay of 23.6 g gold.

Aqueous gold determined by atomic absorption spectroscopy.

Free CN' determined by titration with Ag(N03).

Y? ^ -J I 28 6 4 5 9 Na2S consumption over a 6 hour sulfidisation period at the optimum Eh of -600 mV was found to be in the 8-10 Kg per tonne ore range. Based on a head assay of 23.6 grams gold, this quantity of Na2S recovered an additional 2.8 grams of gold from the ore. This result was the most significant from a series of tests performed on at least 5 30 individual samples obtained from a sulfide ore deposit. CPL with Na2S also proved effective on ore samples that were relatively amenable to CIL. Ore B represents a sulfide obtained from the same deposit as ore A, but of a less refractory nature.

Table 2 compares CIL results obtained from ore B with CPL performed on the CIL tailing after leaching. Conditions were identical to those employed for ore A. 10 The results obtained for ore B are consistent with those shown for ore A in table 1. The optimum Eh was again found to be -600 mV and Na2S consumption was recorded at 10 Kg per tonne ore. In this example, sulfidisation of the leach tailing recovered an additional 1.1 gram of gold from the ore after CIL. This is an important result because it highlights the fact that CPL can be successfully employed on a 15 relatively tractable ore.

Table 2. Effect of controlled Eh sulfidisation over a 6 h period on leash tailing.

Eh (mV) gold recovery (%) Aqueous Au (%) free CN (%) > -100 mV (CIL) 85.7 0 38.1 -600 mV (CPL) 90.1 0 0.9 -650 mV (CPL) 48.4 0.7 <0.5 Gold recoveries determined by fire assay of leached residue and are based on a head assay of 27.6 g gold.

Aqueous gold determined by atomic absorption spectroscopy.

Free CN" determined by titration with Ag(N03).

The results shown in table 2 were obtained after batch leaching in fixed vessels. In order to validate these results, identical samples representing ore B were 25 through a continuous flow pilot plant. Continuous flow CPL performed on th 286459 tailing over a period of 10 days recovered an average of 1.0 grams of gold per tonne ore, consistent with the results presented in table 2.

Example 2 - Ore C The potential of CPL as a tailings treatment process was tested on a refractory pyrrhotite which was reported to be highly preg-robbing in nature. CIL cyanidation was carried out in stirred beakers at 40% solids using 2 Kg per tonne NaCN and at an initial pH of 10.5. Experimental results are summarised in table 3.

Table 3. Gold recovery from ore C.

Conditions gold recovery (%) free CN" (%) carbon-free leach 17.2 11.3 CIL 69.5 .2 -450 mV (CPL) 86.3 0.8 -500 mV (CPL) 55.7 <0.5 Gold recoveries determined by fire assay of leached residue and are based on a head assay of 2.3 g gold.

Free CN" determined by titration with Ag(N03).

Carbon-free cyanidation of ore C yielded only 17.2% gold recovery and revealed 15 the extent of the preg-robbing nature of this sample. CIL enhanced recoveries to 69.5%, but CPL of the tailing over a 6 hour period at the optimum Eh of -450 mV recovered a further 16.8% gold. Although this only represented a 0.4 g increase in gold recovery, Na2S consumption was only 4.4 Kg per tonne ore.

Example 3 - Ore D Ore sample D was leached at 40% solids using 2.5 Kg per tonne NaCN at an initial pH of 10.0. Results reported in table 4 are compatible with the results presented in tables 1-3 in that sulfidisation of the CIL tailing over a 6 hour period recovered a further 18% of gold, corresponding to an additional 19 grams of gold based per tonne head grade. Na2S consumption at the optimum Eh of-550 mV wa§frecorded 11 28 6 4 5 9 at 15.5 Kg per tonne ore. This particular example, involving a high grade sulfide concentrate, clearly illustrates the potential of CPL to recover large quantities of preg-robbed gold.

Table 4. Gold recovery from ore D.

Conditions gold free CN" recovery (%) (%) carbon-free leach 17.0 24.0 CIL 68.5 22.3 -550 mV (CPL) 86.4 0.5 -600 mV (CPL) 45.9 <0.5 Gold recoveries determined by fire assay of leached residue and are based on a head assay of 105 g gold.

Free CN' determined by titration with Ag(N03).

Example 4 - Ore E Ore sample E was conditioned for 30 minutes with 10 kg/t Na2S following by air sparging for 12 hours. After treatment the ore lost 15% of its mass, predominately in water. However, X-ray analysis of the sample revealed that a large percentage of the pyrite and arsenopyrite minerals had been decomposed, in view of these observations 15 and the relatively low preg-robbing nature of this ore (see table 5), it would appear that, in this particular case, an important mode of action of Na2S is to liberate gold from its host sulfide minerals.

Table 5. Gold recovery from ore E Conditions gold recovery (%) carbon free leach 61.2 CIL 74.5 kg/t Na2S condition 89.7 12

Claims (21)

2 8 ft A 5 9 Finally, it is to be understood that various alterations, modifications and/or additions may be made to the process of the invention as previously described without departing from the spirit or ambit of the invention. 286459 WHAT l/WE CLAIM IS: T1 IE CLAIMO DEriNINO Tl IE INVENTION ARE AS FOLLOWS:

1. A method of recovering a metal from a metal-bearing ore, wherein said method includes the addition of an anti preg-robbing solvating agent to a metal-bearing slurry either before or after a metal leaching step. 5

2. A method for the recovery of a metal from a metal bearing slurry wherein said method includes: a) adding an anti preg-robbing solvating agent to a metal bearing slurry, and b) subsequently subjecting the metal bearing slurry to a leaching step and recovering the metal. 10 '

3. A method according to claim 2, wherein the solution potential is monitored during the adding of the anti preg-robbing solvating agent and is maintained above an optimum minimum level.

4. A method according to claim 3, wherein the solution potential is decreased to an optimum minimum level within the range of -600mV to -700mV relative to a saturated 15 calomel electrode.

5. A method according to any one of claims 2 to 4, wherein the method also includes an oxidation step after the addition of the anti preg-robbing solvating agent and prior to the metal leaching step.

6. A method according to claim 5, wherein the oxidation step is continued until the 20 solution potential is increased to -100 to +100mV relative to a saturated calomel

7. A method for the recovery of a metal from a metal bearing slurry, wherein said method includes; a) subjecting the metal bearing slurry to a leaching step and recovering a b) adding an anti preg-robbing solvating agent to the tailings, and c) subsequently recovering a second portion of said metal from the tailings.

8. A method according to claim 7 wherein the second portion of metal is recovered by adding an adsorbent to the mixture of anti preg-robbing solvating agent and tailings. 30

9. A method according to claim 7 or claim 8, wherein the solution potential is monitored during the adding of the anti preg-robbing solvating agent and is maintained within an optimum range. electrode. 25 first portion of said metal and collecting the tailings therefrom, 14

10. A method according claim 9 wherein the solution potential is maintained within the range of -600 mV to -650mV relative to a saturated calomel electrode.

11. A method according to any one of claims 7 to 10, wherein the method also includes an oxidation step after the addition of the anti preg-robbing solvating agent and prior to the recovery of the second portion of metal.

12. A method accordingly to claim 11, wherein the oxidation step is continued until the solution potential is increased to 0 to +100mV relative to a saturated calomel electrode.

13. A method according to any one of claims 1 to 12, wherein the anti preg-robbing agent is a sulfur compound containing an S" or S?" functional group, or a group which results in the formation of HS' when the compound is placed in an aqueous solution.

14. A method according to claim 13, wherein the anti preg-robbing agent is Na2S.

15. A method according to claim 14, wherein sufficient Na2S is added such that an Eh potential is reached which results in the deactivation of preg-robbing species.

16. A method according to claim 14 or claim 15, wherein sufficient Na2S is added such that an Eh potential is reached which results in the decomposition of metal bearing sulfide materials.

17. A method according to any one of claims 14 to 16, wherein at least 5 kg of Na2S per tonne of ore is added to the metal-bearing slurry.

18. A method according to any one of claims 1 to 17, wherein the metal to be recovered is gold.

19. A method according to claim 1 substantially as hereinbefore described with reference to any one of the examples.

20. A method according to claim 2 substantially as hereinbefore described with reference to any one of the examples.

21. A method according to claim 7 substantially as hereinbefore described with reference to any one of the examples. END OF CLAIMS