EP0119685B1

EP0119685B1 - Hydrometallurgical arsenopyrite process

Info

Publication number: EP0119685B1
Application number: EP84300292A
Authority: EP
Inventors: Morris John Vreugde Beattie; Rein Raudsepp; Ernest Peters
Original assignee: Peters Ernest; Raudsepp Rein
Current assignee: BEATTIE, MORRIS JOHN VREUGDE; Peters Ernest; Raudsepp Rein
Priority date: 1983-01-18
Filing date: 1984-01-18
Publication date: 1988-08-03
Also published as: ZA84153B; EP0119685A1; CA1219132A; AU2351584A; AU566135B2; DE3473163D1

Description

Field of the invention

This invention is directed to a novel environmentally amicable hydrometallurgical process for the recovery of gold from arsenical pyrite concentrate.

Background of the invention

The mineral arsenopyrite is known to contain gold which is in solution in the mineral matrix or is present as fine inclusions. This gold is not available for extraction by hydrometallurgical processes which treat only the mineral surfaces, for example, cyanidation. The mineral pyrite is often associated with arsenopyrite and may contain in its matrix finely dispersed gold which is difficult to extract. Arsenopyrite and pyrite are the main constituents of arsenical pyrite concentrates.
The conventional means of liberating gold from arsenical pyrite concentrates is to roast the material and then treat the calcine by cyanidation. This process generates environmental pollution problems due to the airborne emission of sulphur and arsenic oxides. The tailings from the calcine cyanidation contain arsenic which is also a potential environmental contaminant.
Arsenical pyrite concentrates may also be treated for gold recovery through conventional pyrometallurgical processes which include copper smelting, lead smelting and zinc roasting. These processes also produce potentially harmful airborne arsenic emissions from the treatment of these concentrates. Problems associated with the added arsenic burden in the process flows also arise.
Two hydrometallurgical processes exist which could potentially be used to decompose arsenical pyrite concentrates though they are not specifically used for this purpose. These are the Sill and Calera processes which are both used for the treatment of cobalt and arsenic-bearing materials. In the Sill process, the concentrate is solubilized by the action of a caustic substance and oxygen under elevated temperatures and pressures. In the Calera process sulphuric acid and oxygen at high temperature and pressure are the active agents. Neither process, as far as is known, is commercially operated at the present time.
U.S. 3,793,429 discloses a nitric acid leaching process for extracting gold, iron and copper sulphide ores.
U.S. 2,805,936 discloses a hydrometallurgical process for the recovery of valuable metals as Cu, Ni, Co, As, Au or the like from an arsenical sulphide concentrate, said process comprising the steps:

a) treating the concentrate with an acidic solution containing oxidized nitrogen species to leach decompose, said concentrate, and
b) treating the resulting liquid fraction to precipitate dissolved arsenic species formed in step a), said oxidized nitrogen species being regenerated by reaction with oxygen or oxygen-containing gas. In this process, arsenic is precipitated as AS ₂0₃.

An object of the present invention is to provide an environmentally amicable process for decomposing gold-bearing arsenical pyrite concentrates.
The invention is characterised in that the arsenical sulphide concentrate is arsenical pyrite concentrate containing gold, the acidity of said acidic solution is sufficient to cause arsenic in said concentrate to be oxidized to the +5 oxidation state and to cause nitrogen of said oxidized nitrogen species to be reduced essentially to nitric oxide, gold is extracted from a solid residue formed in step a), arsenic in the precipitated arsenic species of step b) is in the +5 oxidation state and the liquid fraction treated in step b) is separated from the precipitated arsenic species and re-used in step a).
Preferably, in step a), iron in said concentrate is oxidized to the +3 oxidation state and sulphide in said concentrate is oxidized to sulphate. Preferably, said solid residue formed in step a) is separated from said acidic solution before said dissolved arsenic species is precipitated in step b).
Arsenopyrite and pyrite are decomposed in acid solutions where the pH is less than 2 by the action of oxidized nitrogen species where the nitrogen has an oxidation state of +3 or greater. These species include nitric acid, nitrous acid and nitrogen dioxide. The main products from the decomposition are soluble ferric iron species, soluble arsenate species, soluble sulphate species, elemental sulphur and nitric oxide, as well as nitrogen dioxide. Nitrogen dioxide becomes increasingly abundant as a product in the gas phase as the nitric acid concentration increases: see Canadian Patent No. 995,468, Paul B. Queneau et al., August 24, 1976. The minor products are arsenic trioxide and nitrous acid. The gold contained in the concentrate remains in the solid residue which is composed of elemental sulphur and insoluble gangue minerals. Any silver present in the concentrate would also report to the residue.

Drawing

Figure 1 illustrates arsenic concentration as a function of time for three similar experiments with solution composition as a variable.

Detailed description of the invention

The decomposition of arsenopyrite and pyrite by nitric acid occurs according to the following reactions.
In the reaction with arsenopyrite, it has been found that 60-90% of the mineral's sulphur is converted to soluble sulphate species. In the reaction with pyrite, the degree of conversion is 80-100%.
The gold in the decomposition residue may be readily extracted by conventional techniques such as cyanidation, following leaching of the residue with sodium hydroxide to dissolve sulfur prior to cyanidation, or treatment with oxidizing chloride lixiviants, such as aqua regia. Silver may also be extracted by these techniques.
It is important that the decomposition solution does not contain significant quantities of species which complex gold, for example, chloride ions. These would put the gold into solution and a separate additional process step would have to be included to extract it.
The active nitrogen oxides are required only to decompose the minerals in the concentrate. The oxidizing nitrogen species should be present in sufficient concentration in the solution to provide an adequate rate of dissolution. Any suitable acid may be used to form the soluble ferric iron species. An adequate rate of dissolution is about 10 to 30 minutes.
In the reaction detailed below, nitrogen dioxide is the decomposition agent for arsenopyrite with sulphuric acid present.
In the reaction detailed below, the sulphuric acid is formed from the decomposition of pyrite.
In the preceding reactions, the active nitrogen oxides are reduced to nitric oxide which may then be regenerated by an oxidant. A useful oxidant is oxygen which reacts with nitric oxide in the presence of water to form nitrogen dioxide, nitrous acid and nitric acid as shown in the reactions set forth below.
The regeneration of nitric oxide to the higher valence states may be done concurrently with the decomposition or as a separate operation.
Nitrous oxide is formed by the decomposition of nitric oxide according to the side reaction shown below.
When the regeneration step is carried out with oxygen concurrently with the decomposition reactions, the overall stoichiometry of arsenopyrite reacting with nitric acid and oxygen to produce sulphuric acid as the sulphur product is illustrated by the reaction below.
Since the active nitrogen oxides can be regenerated during the decomposition step, the quantity of these oxides present at any time may be quite small. The criterion is that there must be sufficient acid present in solution to form the soluble ferric iron species. It must be emphasized that it is the oxidized nitrogen species rather than oxygen that are the active decomposition agent. The presence of oxidized nitrogen species with sulphuric acid differentiates the decomposition step described above from the Calera process.
An important feature of decomposition using oxidized nitrogen species is the high speed of reaction. If a solution which is three molar in nitric acid is reacted with fine arsenical pyrite flotation concentrate, it has been found that the reaction is complete within ten minutes. This is significantly faster than rates claimed by other processes at similar conditions.
Figure 1 shows the arsenic concentration as a function of time for three similar experiments in which the only variable is the composition of the solution. The three compositions are 3 M acid as HN0₃; 2.5 M acid as H₂SO₄; 0.5 M acid as HN0₃; and 3.0 M acid as H₂SO₄. The other conditions are given on Figure 1. It is apparent from the data that the presence of nitric acid greatly speeds the rate of reaction.
The decomposition and regeneration steps are both exothermic. When a solution which is three molar in nitric acid is reacted with fine arsenical pyrite concentrate at 15% solids without oxygen present for regeneration, the temperature increase of the slurry is 40°C. With oxygen present for generation, the temperature increases is 130°C. Since the rates of the decomposition and regeneration reactions increase with temperature the overall reactions appear to accelerate as they proceed. It is possible that controlled cooling may be required to prevent the melting of elemental sulphur and to prevent the precipitation of salts.
The decomposition step proceeds at any temperature above ambient. However, on a practical basis, the reaction is preferably carried out at temperatures of between 80° and 120°C. It is desirable that sufficient acid be present to form the soluble ferric iron species. Without this acid, compounds will precipitate from solution. If oxygen is used for regeneration, any oxygen pressure above ambient is adequate. Agitation increases the speed of the reactions and improves the quality of the final sulphur-bearing residue.
The decomposition leach can be carried out over a wide range of solid-liquid ratios. Increasing the ratio of solids to liquids provides economic benefits, but the upper limit of this ratio is reached when the solubility limit of dissolved species is reached.
When the decomposition reactions are complete, a solid-liquid separation is carried out to produce a residue containing all the gold and a clarified solution which is recycled in the process.
To enable the solution to be reused for the decomposition step, the soluble arsenic, iron and sulphur must be removed from solution.
Arsenic in the pentavalent state as ferric arsenate can be removed from solution with ferric iron. The following reaction shows the formation of ferric arsenate from ferric nitrate and arsenate.
Ferric arsenate is produced virtually quantitatively from an equimolar solution of ferric nitrate and arsenic acid at all temperatures above ambient. However, the rate of precipitation can be controlled by temperature. At room temperature, complete precipitation requires several months; at 100°C precipitation requires one to two hours; and at 200°C precipitation occurs in less than one hour.
Ferric arsenate can be precipitated rapidly at low temperatures by the neutralization of the acid in the solution. At 25°C the solubility of ferric arsenate between pH 3 and pH 7 is very low. The solids produced at low temperature tend to be colloidal and difficult to filter. The solids can contain ferric hydroxide which also tends to be colloidal,
The presence of sulphate in solution raises the solubility of ferric arsenate. A solution which is 1 M in ferric nitrat and arsenic acid is stable at room temperature in the presence of 0.5 M H₂SO₄. At higher temperatures, the effect of sulphate is less pronounced.
A calcium-bearing neutralizing agent, such as calcium oxide or calcium carbonate, can be used to neutralize excess acid in solution and to remove sulphate in order to improve ferric arsenate precipitation.
A small portion of the extracted arsenic is present as arsenite and thus arsenic trioxide can precipitate when the filtered decomposition solution is cooled.
If insufficient arsenate or ferric is present in solution to bring about complete precipitation of the appropriate species, then arsenate or ferric compounds may be added to the solution.
Sulphate is removed from solution by the addition of calcium-bearing materials to form calcium sulphate. The reaction between calcium carbonate and sulphuric acid is as follows.
There are two forms of calcium sulphate which may be formed. Gypsum (CaS0₄- 2H₂0) has a low solubility which is virtually unaffected by temperature. In a 1 M solution of ferric nitrate and arsenic acid, the solubility ofCaS04 - 2H ₂0 is approximately 0.1 M. Anhydrite (CaS0₄) forms at temperatures above 60°C (although the crossover point from gypsum may be as high as 110°C due to supersaturation). The solubility of anhydrite drops rapidly with temperature. Solubility data for anhydrite in water gives a solubility of 0.02 M at 60°C and .0015 M at 160°C.
Ferric iron can be removed from solution by the formation of insoluble iron compounds.
When a ferric iron solution is slowly neutralized at low temperatures, ferric hydroxide (Fe(OH)₃) is formed. This material may be undesirable as it is colloidal and very difficult to filter. As the temperature is raised to 100°C, the precipitate is transformed to goethite, a more crystalline ferric iron compound; and as the temperature is raised further to 130°C, hematite (Fe₂0₃) is produced. The exact nature of the precipitate is dependent on neutralization history and the duration at temperature.
Higher acid concentrations are permitted for a given iron rejection as the temperature is raised.
In the production of hematite, a residual iron concentration of 5 g/I can be achieved in the presence of 60 g/I H₂SO₄ at 150°C. At 200°C, the same residual can be achieved in the presence of 90 g/I H₂SO₄.
With sulphate in solution, various basic sulphate salts are stable. In the range of temperature and solution compositions expected in the iron precipitation stage, hydronium jarosite ((H₃0)Fe₃(SO_Q)₂(OH)₆) and fibroferrite (Fe(OH)(S0₄)) are expected to form. Hydronium jarosite is the most significant below 150°C. Fibroferrite is most significant above 150°C.
Other forms of jarosite may be formed by the addition of alkali salts where the alkali metal or radical is NH₄, Na, K, Ag or Pb. Jarosites are typically formed at 90 to 150°C at a pH of 1.0 to 1.5.
The reaction below shows the formation of ammonium jarosite.
The exact nature of ferric-sulphate compounds precipitated is difficult to specify as many different species are possible and the factors which govern their formation are complex.
Various trace elements such as bismuth or tellurium may be present in the concentrate being treated. While some of these trace elements will report to the leach residue or waste precipitation residues, some may build up in solution and have to be bled-off. When trace elements are present in sufficient concentration, their recovery may be warranted.
The operations described can be combined to create processes which will effectively decompose arsenical pyrite concentrates of varying compositions producing a residue which can be treated for gold recovery and a solution from which the soluble arsenic, iron and sulphur species can be removed. This solution can then be reused in the decomposition step.
. With a concentrate that is primarily arsenopyrite one possible process is a decomposition step with a recycled nitric acid solution using oxygen for regeneration, followed by a solids-liquid separation. The solids go to gold recovery, while calcium carbonate is added to the liquid and C0₂ is evolved. The solution is then heated to a high temperature at which ferric arsenate and calcium sulphate co-precipitate. Another solids-liquid separation provides liquid for re-use in the decomposition, with nitric acid and water being added to account for losses.
Another possible process for a concentrate that is primarily arsenopyrite is a decomposition step with recycled nitric acid solution containing soluble calcium, using oxygen for regeneration. After leaching, the solution is cooled to precipitate calcium sulphate and a solid-liquid separation is made. The liquid is heated to precipitate ferric arsenate and another solid-liquid separation is made to give a solution to which calcium carbonate is added before reuse.
With a concentrate that contains significant quantities of pyrite, one possible process is a decomposition step with a recycled solution, nitric oxide gas and oxygen being used for regeneration. This is followed by another decomposition step without oxygen to convert all the nitrogen oxides to nitric oxide, which is bled off. A solid-liquid separation produces a residue for gold treatment. Calcium carbonate is added to the liquid which is then heated to a high temperature to precipitate ferric arsenate, calcium sulphate and hematite. Another solids-liquid separation provides liquid for decomposition.
Other processes within the scheme of the invention can be proposed from the steps described. Some processes are illustrated in the following examples.

Example 1

A test was done to demonstrate the decomposition of an arsenical pyrite concentrate containing a large fraction of arsenopyrite (44.7% As, 31.7% Fe, 17.3% S, 7.34 oz/ton (204,8 g/t) Au). This example demonstrates the basic decomposition step with a nitric acid solution. The following conditions and results were noted.
The decomposition was rapid and complete.

Example 2

. A series of tests were run to demonstrate the decomposition of an arsenopyrite concentrate (as in Example 1) using a nitric acid solution and oxygen to regenerate the active nitrogen oxides.
The conditions and results from a typical test in this series are shown below.
The use of oxygen permits the pulp density to be raised with respect to the nitric acid concentration.

Example 3

In another test under conditions similar to those shown above, it was found that 99% of the gold in the concentrate reported to the decomposition residue.

Example 4

The residue from another test conducted under conditions similar to those outlined in Examples 1 and 2 was treated with an aqua regia solution composed of 2 parts HCI, 1 part HN0₃ and 3 parts H ₂0 at 60°C. The gold extraction was 98% based on the initial concentrate.

Example 5

Another residue prepared along the lines of Examples 1 and 2 was treated with an alkaline cyanide solution. The gold extraction was 86% based on the initial concentrate. Cyanide use was heavy due to the formation of thiocyanate from the reaction of sulphur and cyanide.

Example 6

A test was done to demonstrate the decomposition of an arsenical pyrite concentrate containing a large fraction of pyrite (4.9% As, 36.9% Fe, 36.2% S). A nitric acid solution was used with oxygen to regenerate the active nitrogen oxides.
The decomposition of this material was observed to be rapid and eomplete.

Example 7

A test was conducted to demonstrate the decomposition of a pyrite-rich concentrate (as in Example 2) using a ferric nitrate and sulphuric acid solution. This example simulates a decomposition using the product solution from Example 6.

Example 8

A test was performed to demonstrate the decomposition of an arsenopyrite concentrate (as in Example 1) using nitric oxide gas. Oxygen was added to regenerate the active nitrogen oxides. The nitric oxide gas was produced by the reaction of arsenopyrite with nitric acid as in Example 1.
A pyrite-rich concentrate can be reacted in a similar manner.

Example 9

A test was done to demonstrate the precipitation of ferric arsenate by raising the solution temperature. No sulphate was present in the solution.
Similar results were obtained at 150°C.

Example 10

A series of tests were conducted to demonstrate the effect of neutralization and sulphate removal on the precipitation of ferric arsenate. To remove sulphate, calcium carbonate was added to the solution and the evolved CO₂ was released.

Example 11

A series of tests were performed to demonstrate an entire process which would treat an arsenical pyrite concentrate containing a large fraction of arsenopyrite (as in Example 1).
The decomposition step was the same as for Example 2 using a nitric acid solution and oxygen for regeneration.
After filtration, calcium carbonate was added to the solution, the C0₂ evolved was released and the mixture was heated as in Example 10. The removal figures shown are relative to the starting solution.
The solution was then used for a second decomposition step with arsenopyrite concentrate. The extraction figures are relative to the added concentrate.

Example 12

A series of tests were done to demonstrate an entire process which would treat an arsenical pyrite concentrate containing a large fraction of arsenopyrite (as in Example 1).
The decomposition step was carried out with a nitric acid and calcium nitrate solution. After decomposition, the slurry was cooled to reduce the solubility of calcium sulphate.
After filtration, the solution was heated to precipitate ferric arsenate.
After filtration, calcium carbonate was added to the solution and the C0₂ evolved was released. The solution was then used in a second decomposition stage similar to the first.
^* In the second decomposition step, more sulphate was removed than was produced by the decomposition.

Example 13

A series of tests were run to demonstrate an entire process which would treat an arsenical pyrite concentrate containing a large fraction of pyrite (as in Example 6).
The decomposition step was as Example 6 using nitric acid solution and oxygen for regeneration.
A second decomposition step was conducted as in Example 7 using the filtrate from above. Oxygen was not used.
Calcium carbonate was added to the filtrate obtained from the previous step and C0₂ was evolved. The solution was then heated to precipitate a mixture of ferric arsenate, basic iron compounds and calcium sulphate.
After filtration, the solution from the precipitation stage could be reused by the addition of nitrogen oxides, for example, the addition of nitric acid or the addition of nitric oxide and oxygen.

Claims

1. A hydrometallurgical process for the recovery of metals from an arsenical sulphide concentrate, said process comprising the steps:

a) treating the concentrate with an acidic solution containing oxidized nitrogen species to leach decompose said concentrate, and

b) treating the resulting liquid fraction to precipitate dissolved arsenic species formed in step a), said oxidized nitrogen species being regenerated by reaction with oxygen or oxygen-containing gas;

characterized in that said arsenical sulphide concentrate is arsenical pyrite concentrate containing gold, the acidity of said acidic solution is sufficient to cause arsenic in said concentrate to be oxidized to the +5 oxidation state and to cause nitrogen of said oxidized nitrogen species to be reduced essentially to nitric oxide, gold is extracted from a solid residue formed in step a), arsenic in the precipitated arsenic species of step b) is in the +5 oxidation state and the liquid fraction treated in step b) is separated from the precipitated arsenic species and re-used in step a).

2. A process as claimed in Claim 1 wherein, in step a), iron in said concentrate is oxidized to the +3 oxidation state and sulphide in said concentrate is oxidized to sulphate.

3. A process as claimed in Claim 1 or Claim 2 wherein said solid residue formed in step a) is separated from said acidic solution before said dissolved arsenic species is precipitated in step b).

4. A process as claimed in any preceding claim wherein said precipitated arsenic species consists essentially of ferric arsenate.

5. A process as claimed in any preceding claim wherein the total amount oxidized nitrogen species plus nitric oxide in the system is substantially less than the amount required to react stoichiometrically with said concentrate in step a).

6. A process as claimed in any preceding claim wherein said oxidized nitrogen species is nitric acid, nitrous acid or nitrogen dioxide.

7. A process as claimed in any preceding claim wherein the decomposition leach is conducted at a slurry temperature between 60°C and 120°C.

8. A process as claimed in any preceding claim wherein the decomposition leach is conducted at a pH of less than 2.

9. A process as claimed in any preceding claim wherein regeneration of higher valence nitrogen oxides is conducted concurrently with the decomposition leach of step a).

10. A process as claimed in any of Claims 1 to 8 wherein the nitric oxide is reacted with oxygen external to the leach to provide the oxidized nitrogen species of step a).

11. A process as claimed in any preceding claim wherein dissolved arsenic species is precipitated from the liquid fraction by elevating the temperature thereof.

12. A process as claimed in any preceding claim wherein iron is removed from the liquid fraction by the formation of insoluble iron compounds caused by the combined action of neutralization and temperature elevation of the fraction.

13. A process as claimed in any preceding claim wherein sulphate is removed from the liquid fraction by the addition of calcium bearing materials to form calcium sulphate.

14. A process as claimed in any preceding claim wherein gold is recovered from said solid residue by utilizing a cyanidation process.

15. A process as claimed in any of Claims 1 to 13 wherein gold is recovered from said solid residue by treating the residue with an oxidizing chloride lixiviant.

16. A process as defined in Claim 13 wherein said calcium bearing material is selected from the group consisting of calcium oxide and calcium carbonate.

17. A process as defined in Claim 11 or 12 wherein the temperature of the liquid fraction is elevated to a temperature between 100°C and 200°C.

18. A process as claimed in any preceding claim wherein said oxidized nitrogen species are present in sufficient concentration in said acidic solution that the dissolution of step a) occurs in a time of less than 30 minutes.

19. A process as claimed in Claim 18, wherein said oxidized nitrogen species are present in sufficient concentration in said acidic solution that dissolution occurs in a time of 10 minutes or less.

20. A hydrometallurgical process as claimed in any preceding claim comprising the additional step, after step a) but before step b), of cooling the solution to precipitate calcium sulphate.

21. A hydrometallurgical process as claimed in any preceding claim, comprising the additional step, after step a) but before step b), of:

subjecting the decomposed concentrate to a second leach decomposing step using a leach composed of higher valence nitrogen oxides present in acidic solution whereby the solution has been regenerated using nitric oxide gas in the absence of oxygen to thereby convert all the nitrogen oxides to nitric oxides and bleeding off the nitric oxides.

22. A process as claimed in any preceding claim wherein said oxidized nitrogen species are regenerated by treatment with oxygen under high pressure.