A METHOD OF RECOVERING BASE METALS FROM LOW GRADE ORES AND RESIDUES

Abstract

A method of recovering a metal from a low-grade ore which is subjected to cyanide leaching to produce a PLS which contains a metal cyanide which is removed from the PLS by ultrafiltration and nano-filtration, and then acidified and sulphidised to produce a metal sulphide from which the metal is extracted, and hydrogen cyanide which is recycled to the cyanide leaching step.

Claims

1. A method for the recovery of at least one metal from a low grade ore or residue, wherein the method includes the following steps: a) cyanide-leaching the low grade ore or residue, at an alkaline pH, to produce a pregnant leach solution which contains at least one metal cyanide; b) upgrading the pregnant leach solution by removing leached or barren solids from the pregnant leach solution; c) subjecting the upgraded pregnant leach solution to ultrafiltration to produce a clarified liquor; d) subjecting the clarified liquor to nano-filtration to produce a metal cyanide concentrate and a permeate; e) subjecting the metal cyanide concentrate to acidification and then to sulphidisation to produce a solution which contains a metal-sulphide and hydrogen cyanide; f) subjecting the solution formed in step e) to a solid/liquid separation step, thereby to recover at least some of the metal sulphide and at least some of the hydrogen cyanide; and h) recycling the recovered hydrogen cyanide to the cyanide leaching (step a)).

2. A method according to claim 1 which is conducted at a pH of between 10 and 11.

3. A method according to claim 1 wherein the solids removed in step b) are washed and separated wash water is then recycled to the upgraded pregnant leach solution.

4. A method according to claim 1 wherein, in step e), sulphuric acid is added to the concentrate to lower pH to a value between 2 and 6.

5. A method according to claim 1 which includes the step of recycling the permeate produced in step d) to the cyanide leaching step a).

6. A method according to claim 1 wherein the permeate produced in step d) is subjected to a reverse osmosis process to form a high quality water stream and a NaCN concentrate which is recycled to the cyanide leaching step a).

7. A method according to claim 1 wherein the metal is extracted from the metal sulphide.

8. A method according to claim 1 wherein the metal is a base metal or a precious metal.

9. A method according to claim 1 wherein the metal is copper, nickel, cobalt, zinc, gold or silver.

10. A method according to claim 2 wherein the solids removed in step b) are washed and separated wash water is then recycled to the upgraded pregnant leach solution.

11. A method according to claim 2 wherein, in step e), sulphuric acid is added to the concentrate to lower pH to a value between 2 and 6.

12. A method according to claim 3 wherein, in step e), sulphuric acid is added to the concentrate to lower pH to a value between 2 and 6.

13. A method according to claim 2 which includes the step of recycling the permeate produced in step d) to the cyanide leaching step a).

14. A method according to claim 3 which includes the step of recycling the permeate produced in step d) to the cyanide leaching step a).

15. A method according to claim 4 which includes the step of recycling the permeate produced in step d) to the cyanide leaching step a).

16. A method according to claim 2 wherein the permeate produced in step d) is subjected to a reverse osmosis process to form a high quality water stream and a NaCN concentrate which is recycled to the cyanide leaching step a).

17. A method according to claim 3 wherein the permeate produced in step d) is subjected to a reverse osmosis process to form a high quality water stream and a NaCN concentrate which is recycled to the cyanide leaching step a).

18. A method according to claim 4 wherein the permeate produced in step d) is subjected to a reverse osmosis process to form a high quality water stream and a NaCN concentrate which is recycled to the cyanide leaching step a).

19. A method according to claim 2 wherein the metal is extracted from the metal sulphide.

20. A method according to claim 3 wherein the metal is extracted from the metal sulphide.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The invention is further described by way of examples with reference to the accompanying drawings in which:

[0030] FIG. 1 is a flow sheet of the SART process and has already been described in the preamble to this specification;

[0031] FIG. 2 is a flow sheet of a modified SART process according to the invention; and

[0032] FIG. 3 illustrates a variation to the process in FIG. 2.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0033] FIG. 2 of the accompanying drawings illustrates steps in a method 10 according to the invention for the recovery of metals from low grade ores and residues 12. Typically the ores and residues are found in the tailings of sulphide and oxide ores and contain high levels of acid-consuming gangue materials such as calcite, dolomite, calcium, magnesium, aluminium, manganese and iron. The metals in these ores and residues may include precious metals such as gold and silver and base metals such as copper, nickel, cobalt and zinc.

[0034] In a first step 14 the ores and residues 12 are subjected to cyanide leaching at an alkaline pH by the addition of cyanide 16. Preferably the pH is between 10 and 11 pH values which are regarded as optimal. The outcome of the cyanide leaching step is a pregnant leach solution 18 which contains at least one metal cyanide i.e. a cyanide of a target metal which may be any one of the aforementioned metals.

[0035] The pregnant leach solution 18 is subjected to a solid/liquid separation process 20 which may be effected by means of counter-current decantation, filtration or the like. The process 20 removes barren or leached solids 22 from the solution and produces an upgraded pregnant leach solution 24 which contains the metal cyanide.

[0036] To optimise the recovery of the base metal and the free cyanide the solids 22 are washed in a step 26 and a resulting solution is filtered to produce a wash water 28 and washed solids 30.

[0037] The washed solids are subjected to a cyanide destruction step 32 to produce a detoxified product 34 which is sent to a slimes dam 36.

[0038] The wash water 28 which may contain some of the metal cyanide, and the upgraded pregnant leach solution 24 are subjected to an ultrafiltration or clarification step 38 which produces a clarified liquor 40 which contains the metal cyanide. This is followed by a nano-filtration step 44 during which the metal cyanide is removed from the clarified liquor 40. The nano-filtration step 44 is carried out using an appropriate membrane which has a chosen pore size which allows a permeate 46 of water and sodium cyanide (NaCN) to pass through the membrane. The permeate is then recycled to the step 12.

[0039] The pore size of the membrane prevents the metal cyanide from permeating the membrane. The metal cyanide which is thereby concentrated constitutes the retentate 48.

[0040] In an acidification step 50 sulphuric acid 52 is added to the retentate 48 to lower the pH of the retentate to a value of between 2 and 6. Once the pH is sufficiently low, the metal cyanide complex dissociates and forms a solution 54 which contains a metal sulphate complex and dissolved hydrogen cyanide.

[0041] In a subsequent sulphidisation step 56 hydrogen sulphide 58 (or sodium sulphide in a salt form) is added to the solution 54 to produce a solution 60 which contains a metal sulphide complex 62. This complex 62, which for example may be a copper, gold or silver sulphide product, is recovered from the solution 60 by means of a solid/liquid separation step 64. The metal content may be recovered from the complex using any suitable technique.

[0042] The pH of a liquid 66 produced by the step 64 is adjusted in a step 68, for example by the addition of lime 70 (CaO), to a value of between 10 and 11, ie. to the alkaline operating range of the cyanide leaching step 14. Hydrogen cyanide 74 emerging from the step 68 is recycled to the cyanide leaching step 14.

[0043] The invention provides a number of significant benefits which include the following: [0044] 1. the nano-filtration step 42 effectively removes the high acid consuming free cyanide, thereby reducing the amount of sulphuric acid (56) which is subsequently needed to acidify the retentate; [0045] 2. as the permeate 46 is recycled to the step 14 before the alkalizing step 48, the permeate remains at the correct alkaline pH. This reduces the amount of calcium oxide needed to alkalize the liquid 66 and thus to recover the hydrogen cyanide 74; [0046] 3. due to the decrease in chemical processing a substantial amount of sodium cyanide is recovered in the permeate, and is recycled to the step 14 to provide a significant reduction in the reagent. [0047] 4. the discharge of harmful waste waters in to the environment is reduced.

[0048] The aforementioned benefits produce a substantial saving in operating and capital costs. Table 2 shows the savings in operating expenditure (OPEX) produced by the process according to FIG. 2 when compared to a conventional SART process of the type shown in FIG. 1.

TABLE-US-00001 TABLE 2 Reagent Reagent prices, R/t H.sub.2SO.sub.4, 98% 1500 CaO 2500 NaCN 21000 Reagent costs, million R/a H.sub.2SO.sub.4 CaO NaCN Total No NaCN recovery 143 143 Base case NaCN recovery (FIG. 1) 10 14 14 38 NaCN recovery via nano-filtration 1.8 1.1 3.6 6.5 (FIG. 2 and 3)

[0049] FIG. 3 is a flow sheet of a process 10A, which is a modification of that shown in FIG. 2. The processes 10 and 10A have a substantial degree of identity and, as appropriate, like reference numerals are used to designate like materials, components and treatment steps.

[0050] In the process 10A the permeate 46 emerging from the nano-filtration step 44 is subjected to a reverse osmosis step 80. This produces a sodium cyanide concentrate 82 and high quality water 84. The concentrate 82 is recycled to the step 14. The modification embodied in the process 10A means that the production of waste water of an unacceptable quality is reduced.