WO1984002148A1

WO1984002148A1 - Precious metal recovery

Info

Publication number: WO1984002148A1
Application number: PCT/AU1983/000176
Authority: WO
Inventors: Stuart Guy Clarke
Original assignee: Stuart Guy Clarke
Priority date: 1982-12-02
Filing date: 1983-12-01
Publication date: 1984-06-07
Also published as: PH20566A; ZA838949B

Abstract

A process for recovering precious metals from an alkaline cyanide process liquor containing a base metal component. The liquor is treated with precipitant for at least one base metal of said component, the precipitant being selected from mineral acids and reducing agents and being added in an amount sufficient to precipitate a major portion of said at least one base metal and to coprecipitate said precious metal values. The process is exemplified by cyanide leaching a Cu/Ag/Au tailing and a coprecipitating Cu/Ag/Au by adding H2SO4 to bring the pH to 2.5 to 1.0, or by using a sulphite as a reductant; with the base metal component being supplemented by adding Cu or Fe salts to the leach liquor. The precipitate product of precious metal and base metal salts (preferably both as cyanide salts) is filtered, roasted and sold for further refining.

Description

This invention relates to a method of recovery of precious metal values from bodies of material (hereinafter referred to as "ores") such as ores, tailings, slimes, sands and wastes which contain precious metal values.

Much effort has been made in the past to extract precious metal from ores which contain it, and frequently wh precious metal is accompanied by other metals, such as a bas metal, the process has proved uneconomic and at best only be partially successful. In the recovery of the precious metal gold and silver from their ores, the conventional procedure to crush and grind the ores; treat the ore with a dilute, alkaline cyanide solution in the presence of oxygen to leach out the precious metals; and to separate resultant pregnant solution from remaining solids and to extract the precious metals from the pregnant solution. The most successful metho for extracting the precious metals is to'agitate the pregnant solution, after deoxygenation, with fine zinc dust to precipitate metallic gold and silver. The precious metal precipitate, which contains zinc and other, co-precipitated base metals, is removed by filtration and melted with oxidizi fluxes to slag off the zinc and other base metals, leaving th precious metals to be cast as bars of dore bullion.

Base metals in the ores react with cyanide in the leach solution, and the concentration of free cyanide thus ha to allow for this in order that the precious metals are leached efficiently. This, in itself, is not a major factor; although another problem arises, particularly with the base metal copper. In the case of copper leached from the ores, that base metal is displaced from the pregnant solution on addition of zinc dust and coats the zinc particles, thereby inhibiting precipitation of precious metals and increasing consumption of zinc.

The present invention is concerned with an improved process enabling this problem due to displaced copper to be overcome while accommodating other base metals present. The invention principally is concerned with recovery of precious metals from ores containing at least one base metal but, as will be appreciated from the following, the invention can als be used with advantage with ores which are low in, or substantially free of, base metal such as copper.

According to the present invention, there is provided a process for recovery of precious metals, including gold and silver, from an alkaline cyanide process liquor containing a base metal component, wherein the liquor is treated with precipitant for at least one base metal of the component, the precipitant being selected from mineral acids and reducing agents and being added to said liquor in an amount sufficient to precipitate at least a major portion of said at least one base metal and to co-precipitate said precious metals.

The base metal component preferably comprises at least one of copper, iron, lead, and zinc, but may also include cobalt, arsenic, antimony, bismuth or a combination o these. Other metals also may be present. A base metal of principal importance in the present invention is copper.

The process of the invention is characterized by intentional co-precipitation of base metal and precious metal It is found that the base metal precipitate physically collects the smaller quantity of the precious metals and act as a carrier for the latter. By this means, substantial. FI complete extraction of precious metal values from the process liquor can be achieved; the base metal co-precipitat acting to strip the precious metals from the liquor.

The action of base metal precipitate is particularly pronounced with copper. In a, highly preferred form of the invention, copper is a significant part of the co-precipitate. Indeed, while the action of copper enables the invention to be used to recover precious metals from process liquor derived from cyanide leaching of ores relatively high in copper, it also enables use of the invention to recover precious metals from process liquors from cyanide leaching of ores relatively low or substantially free of copper. In the latter case, a suitable copper salt, most conveniently copper sulphate, can be added to the process liquor prior to its treatment with precipitant.

The base metal component preferably is present in the process liquor, prior to treatment with precipitant, at a level of at least 250 pp , irrespective of the concentration of precious metals. With two or more base metals present in an amount to contribute significantly to stripping precious metal values by co-precipitation, the has e metals on aggregate preferably are present at a level of at least 250 ppm. The aggregate maximum level of base metal is dictated by economics, but typically does not exceed 1,000 ppm.

While copper is a highly preferred base metal, it also is desirable that iron also is present. It is found that co-precipitation of iron aids in providing a bulky _r free settling co-precipitate. Thus, in one form, the invention provides for the addition of a soluble iron salt, such as ferrous sulphate in an amount to provide, or adjust the iron concentration to, a level of iron of about 50 ppm. Such addition would be made where the process liquor is low in, or substantially free of, iron.

In the case of the use of a mineral acid, any of sulphuric, hydrochloric, nitric, perchloric and sulphurous acid can be used. Nitric acid is the most effective of these, although sulphuric acid is the most cost effective. In each case, the sufficient quantity of acid is that necessary to effect the required precipitation of base metal. With the exception of sulphurous acid, this normally requires a quantity sufficient to reduce the pH of the cyanide liquor to at least 3.5, preferably to at least 2.5 - 1.0. However, in the case of sulphurous acid, it can be necessary only to reduce the pH of the liquor to 5.5 to 4.5, since the pronounced reducing property of that acid also provides a precipitating action additional to that of pH reduction per se.

In the case of reducing agents, it is found that the sulphite, bisulphite, metabisulphite and thiosulphate of sodium, potassium and calcium are most suitable. In each case, the sufficient quantity of the salt is that necessary to effect the required precipitation of base metal. This normall requires a quantity sufficient to reduce the pH of the cyanide liquor to 5.5 to 4.5.

The cyanide process liquor typically contains the precious metal values and the base metals in solution as their cyanide salts. Particularly where base metal such as copper, - iron or both of these are to be added to the process liquor prior to the latter being treated with precipitant, the leach liquor most conveniently has a free cyanide concentration sufficient to react with the added base metals- and convert these to cyanides. However, beyond that requirement for free cyanide, the process liquor preferably is depleted of free cyanide at the time of its treatment with precipitant.

The co-precipitation of precious metal values and base metal leaves a co-precipitate and a resultant barren liquor substantially depleted of precious metal values. In contrast to the above mentioned conventional procedure in which the precious metal and at least some base metal are precipitated, on addition of zinc dust, as the respective metal, very little metallic precipitate is provided by the present invention. Rather, the co-precipitate of the present invention is characterised by cyanide salts of the precious metal values and of the base metal. The salts appear not to be simple cyanides such as AuCN, CuCN or CuSCN, but to be complex cyanide salts. The invention departs significantly from the conventional procedure. Zinc dust is not added, although zin may be one of the base metals present in the process liquor. Additionally, the precious metals are recovered as a co-preci¬ pitated cyanide salt with base metal cyanide. This latter distinction leads to a further departure in that it is found that the co-precipitate is a commercially acceptable product from which to further recover the precious metal values. However, rather than slag off the base metals to provide precious metal bullion, the present invention provides for processing of the co-precipitate for base metal snn fir , with recovery of the precious metal values as a secondary aspect of the base metal recovery. Thus, where in the preferred form of the invention, copper is the principal base metal, the co-precipitate, can be included as feed to a copper converter for incorporation in a crude copper product for electrorefining; the precious metal values in that case being subsequently recovered by refining the anode mud resulting from the electrorefining.

In the conventional procedure, an alkaline, cyanide leach liquor is regenerated from the barren liquor resulting from precipitation by zinc dust addition. However, in the present invention, substantially all cyanide preferably reports in the co-precipitate, leaving a barren liquor which is able to be discarded without environmental hazard. As the cyanide is lost on subsequent processing of the precipitate, fresh cyanide process liquor has to be generated by use of a fresh cyanide leach solution.

The need for fresh-cyanide leach solution provides a slight negative cost factor in relation to the present invention. However, despite this, the present invention has an overall cost advantage compared with the conventional procedure, particularly when the process liquor is derived from leaching an ore having a relatively high copper content. A positive cost factor is avoidance of the need for zinc dust. A further positive cost factor is the ability for the process liquor to have little or no free cyanide; tailings resulting from production of the process liquor thus being able to be discarded with little or no treatment to remove cyanide in compliance with environmental safety requirements. A still further cost factor is that, in processing the co-precipitate, both the base metals and precious metal values are recover¬ able. The co-precipitate thus can be sold for both preciou metal and base metal value.

In the accompanying drawing, Figure 1 depicts a flowsheet illustrating the invention as applied to a'_'5,000 metric tonne pilot plant recovery of gold by cyanide leachi of tailings from a carbon in pulp gold recovery operation. The invention is particularly suited to such recovery since it enables relatively small scale operation. However, the invention also can be applied to a large scale operation, a to the recovery of precious metals from freshly mined ore.

With reference to Figure 1, 5,000 MT of the tail¬ ings are charged to a plastic lined dam 10 constructed for the purpose of heap leaching the tailings. Dam 10 has drainage pipes at 1.5 metre intervals across its full width and terminating in a manifold leading into a pregnant liquor sump. Leach solution containing approximately 0.025 - 0.05% sodium cyanide solution at pH 10 to 11 is charged to the dam The leach solution is flooded over the tailings in the conventional manner to percolate down through the bed of tailings, with the solution being oxygenated in the usual manner. However, it is found that excellent leaching also can be obtained, without oxygenation of the solution, by charging the solution to the base of the bed of tailings and causing it to percolate upwardly through the bed. In the latter case, it is found that sufficient air is present within the bed to provide the oxygen necessary for efficient leaching of the tailings.

On completion of leaching, the pregnant leach liquor is pumped from the sump to a 45 cubic metre treatment

LT∑EA tank 12. In treating tailings relatively high in base metal, the pregnant liquor then may simply be treated with a precipitant, preferably sulphuric acid or sodium etabi- sulphite. However, if the tailings are low in base metals, base metal is added to tank 12 prior to or simultaneously with addition of precipitant. In the latter case, the base metal preferably is copper sulphate monohydrate, 25 kg of that salt typically being added- Using sodium metabisulphite as precipitant, a quantity sufficient to drop the pH to between 6.0 to 5.0, typically 5..5, is added, 14 kg typically being added. Where sulphuric acid is used instead of the metabi¬ sulphite, sufficient commercial grade acid, typically 60 litres, is added to attain a pH of 3.0 to 1.5, typically 2.4. Addition of the precipitant generates a precipitate of salts of gold and base metal. This is allowed to settle, and clear barren liquor is decanted and pumped to a retreat- ment tank 14. A sludge, comprising the precipitate and residual barren liquor is pumped from tank 12 to filter 16; filtered solids from the latter then being dried and roasted in oven 18.

The barren liquor pumped to tank 14 can be discarde if all free cyanide was consumed on addition of the copper sulphate. However, as it can contain a slight amount of gold, it preferably is recycled for further leaching of fresh tailings in dam 10 after addition of caustic soda to adjust is pH to 10 and make up sodium cyanide-.

The roasted solids from oven 18 can be packed into drums for sale to a copper refiner. The solids can be sold for both their gold and copper content, as both metals can be recovered by conventional refining procedures, typically involving electrorefining for the copper.

In the pilot plant operation as outlined above, conditions were as follows: Leach Liquor: pH 10.0 - 11.0, cyanide, concentration 0.025 -

0.0.5. Tailings: Au 1.05 gm per tonne, Ag 0.5 gm per tonne,

Cu 800 gm per tonne. Total Solution loading: 300 MT. Circulating Solution Load: 40 MT/day. Pregnant Liquor: Au 0.8 - 3.0 gm per tonne, mean Au 1.04 g per tonne. Reject Tailings: after 108 days, Au mean value 0.15 gm per tonne. Gold extracted: 108 days, 4506 gm.

In a first series of experiments, the pregnant solution (head grade Au 2.0 - 3.0 gm per tonne) was treated with sulphuric acid alone to attain a pH between 1.5 to 3.0 (typically 2.4) ; the residual gold in the barren solution typically being 0.2 to 0.4 gm per tonne. The barren solution was re-adjusted to pH 10.0 with caustic soda, prior to addition of sodium cyanide and returned to the leaching dam.

In a second series of experiments (solution assays in the range Au 0.8 - 2.0 gm per tonne and Cu 75 - 250 gm per tonne) the pregnant solution was treated by adding 25 kg copper sulphate monohydrate to 45 MT of liquor, followed by 12.5 kg sodium etabisulphite. The final pH was in the range 5.0 - 6.0 and the gold in the resultant barren liquor was 0.1 - 0.3 gm per tonne. In each experiment, the level of gold in the barren liquor was not considered significant because it was recirculated. Closer overall control on acid or bisulphite and, in the second experiment, copper sulphate enables a further reduction of that gold level.

The original ore from which the tailings were derived, i.e. prior. to C.I.P. recovery, had Au 3.9 gm per tonne, Ag 1.4 gm per tonne and Cu 2100 gm per tonne. The host rock was mainly quartz and muscovite, with minor amounts of pyrite, chalcopyrite,. covellite and chalcosite.

The final product recovered by decanting, filtering, drying and roasting the precipitate comprised:

Copper oxide 70 - 80%

Gold 2000 GM/MT

Silver 550 GM/MT

Silica 2 - 10%

Iron 1.25%

Nickel 0.09%

Cobalt 0.07%

Manganese 0.01%

Magnesium 0.08%

Sodium 0.26%

Potassium 0.04%

Calcium 0.06%

Zinc 0.01%

Claims

Having thus described the invention, what I claim as new is :

1. A process for recovery of precious metals_ from an alkaline cyanide process liquor containing a base metal component, wherein the liquor is treated with precipitant for at least one base metal of said component, the precipitant being selected from mineral acids and reducing agents and being added in an amount sufficient to precipitate a major portion of said at least one base metal and to co-precipitate said precious metal values, a precipitate produce of precious metal and base metal cyanide salts being separated from a resultant barren liquor for recovery of precious metal values therefrom.

2. A process according to claim 1, wherein said base metal component comprises at least one of copper, iron, lead and zinc.

3. A process according to claim 2, wherein said base metal component also includes at least one of cobalt, arsenic antimony and bismuth.

4. A process according to any one of claims 1 to 3, wherein said base metal component is present in said process liquor at a concentration of at least 250 ppm.

5. A process according to any one of claims 1 to 4, wherein said process liquor is derived from leaching on ore containing said precious metal values and. said base metal component and, if said component in said process liquor is at a concentration of less than 250 ppm, at least one base metal is added to said process liquor prior to treatment of said process liquor with said precipitant.

6. A process according to any one of claims 1 wherein said precipitant is a mineral acid selected from sulphuric hydrochloric, nitric, perchloric and sulphurous acids, and said amount thereof is sufficient to reduce the pH of said process liquor to at least 3.5, preferably to at least 2.5 to 1.0.

7. A process according to claim 6, wherein said mineral acid is sulphuric acid.

8. A process according to any one of claims 1 to 5, wherein said precipitant is a reducing agent selected from the sulphite, bisulphite, metabisulphite and thiosulphate of a metal selected from sodium, potassium and calcium, and said amount of said reducing agent is sufficient to reduce the pH of said process liquor to below 5.5, preferably- to 5.5 to 5.0.

9. A process according to claim 8, wherein said reducing agent is selected from sodium and potassium metabisulphite.

10. A process according to any one of claims 1 to 9, wherein precipitation of said at least one base metal and precious metal values is effected in the absence of added zinc metal.

11. A process according to any one of claims 1 to 10, wherein said process liquor and said barren liquor each are characterised by a^" substantial absence of free cyanide.

12. A process according to any one of claims 1 to 7, wherein said precipitant is sulphuric acid and said amount thereof is sufficient to reduce the pΞ of said process liquor to 2.5 to 1.0, said base metal component comprising essentially copper at a concentration of at least 250 ppm.

13. A process according to any one of claims 1 to 5, 8 and 9, wherein said precipitant is sodium metabisulphite

said amount thereof is sufficient to reduce the pH of said process liquor to from.6.0 to 5.0, said base metal component comprising essentially copper at a concentration of at least 250 ppm.

14. A process according to any one of claims 1 to 13, wherein said precipitate produce is subjected to base metal refinement to recover both base metal and precious metal values therefrom.