This invention relates to the recovery of gold, copper, antimony and bismuth from the sulphidic concentrates and other residues. Such concentrates may for example be arsenopyrite, chalcocite, chalcopyrite and stibnite.

One prior art process is to leach gold bearing sulphidic ore material such as arsenopyrite, pyrite, chalcosite or chalcopyrite concentrates with ferric chloride or ferric sulphate solution under pressure above 100°c to oxidise the sulphidic material to elemental sulphur form and also to leach copper, if present in the material. Thereafter the gold bearing residue is subjected to a cyanidation process to recover the gold. In this process one of the major problems to arise is the long residence time and the numbers of stages which are required.

During this time, due to the complexity in solid handling system, some unnaccountable losses of gold is unavoidable. Furthermore, gold inventory will be very high.

The disadvantages of the above described prior art and most prior art processes are the requirement of the two subsequent stages for recovery for base metals and precious metals. Furthermore, the gold is recovered only from the second stage of the leach stages and therefore the precious metal inventory is very high.

The present invention seeks to ameliorate these disadvantages by providing a process where the majority of the gold is recovered in the first stage of the process and as a result the gold inventory in the plant is minimised.

In one broad form the invention comprises a process for extracting gold from copper, antimony, bismuth, arsenic and iron bearing mineral sulphides which comprises the step of: dispersing finely divided mineral sulphide in an aqueous ferric chloride/chlorine solution to form a slurry; adjusting the relative amounts of ferric chloride

in the slurry such that there are at least sufficient ferric ions present to oxidise the metal sulphide content to metal ions and sulphide to elemental sulphur and sulphate, with sufficient Hydrochloric acid present initially to combine with the metal ions; reacting the slurry in a closed reation vessel at a temperature above the boiling point of water and below the melting point of sulphur, with free chlorine gas to dissolve the major portion of the gold and also to maintain the ferric ion concentration at high level; and continuing the said reaction with active agitation to effect satisfactory extraction of gold.

Preferably the reaction is carried out at a pressure of between 15 to 50 p.s.i. The present invention will now be described by way of example with reference to figure 1 which sets out a flow diagram of one embodiment of the present invention.

The first stage of leaching is carried out by adding a mixture of ferric chloride and free chlorine under pressure at an elevated temperature. Preferred concentration of ferric chloride and chlorine is double the equivalent quantity and half the equivalent quantity respectively. The objective of the addition of chlorine is to maintain the high oxidation potential by re-oxidising the ferrous ions and also to dissolve the gold. Leach temperature of 105 - 110°c and the pressure of 20 PSI is optimum. Leaching was carried out for 1 hour. Eighty per cent of the sulphide was oxidised to elemental sulphur and the gold from the feed was extracted into the solution as the chloride complex. Gold extraction of 70% is achieved by this method.

The gold solution is separated from the gold leach residue at 2 and treated at a recovery stage 3 with activated carbon or by bubbling sulphur dioxide to extract the gold. Further residule solution is treated to regenerate ferric chloride at 5 for return to the 1st stage leaching.

Thereafter, second stage leaching at 4 with

chlorine in hydrochloric acid medium is conducted to dissolve the remaining gold in the leach residue. This stage is carried out at atmospheric pressure and the ambient temperature. Leaching time of 3 hours is sufficient. The residual solids being separted from the liquid for disposal at 6. The gold is then recovered at 7 by extraction with activated carbon or sulphur dioxide bubbling.

As a result of this invention most of the sulphide bearing materials containing gold can be treated with a single stage leaching to dissolve most of the gold; thus making the process of treating the complex gold concentrates very much more economical and simpler, than the conventional prior art methods. Test results for the first stage leaching were carried out on a feed with a solid density of 10% was added to a glass lined pressure reactor.

A feed containing arsenopyrite material with Au 300g/t, S - 30%, As - 12%, Fe 20%, Cu 2%.

Ferric chloride with the strength of 20 g/1 was added to the solution along with 5-10 g/1 varied concentrations of free chlorine. Then 5 g/1 of Hydrochloric acid was added to the reactor and it was sealed and heated to 110°c and stirred for one hour. The leach solution was analysed after the leach experiment and the results are summarised in the Table 1.

TABLE 1 LEACH EXPERTMEMT STAGE 1 SERIES SOLID Fe Cl ₃ Cl ₂ HC1 TEMP LEACH Au

EXTRACTIO g/i g/i g/i g/i °C TIME hr %

1 100 20 5 5 115 1 70

2 80 20 5 5 110 1 68

3 100 20 - 5 115 1 -

4 100 - 5 5 115 1 20

5 100 20 10 5 115 1 73

In the first stage of the leaching stages most of the sulphidic material is oxidised along with some major portions of the gold also being leached out. This is achieved by maintaining an oxidation potential which enhances the dissolution of gold from the sulphidic material. Most favourable conditions for such a dissolution are the combined oxidants of ferric chloride and chlorine under pressure at elevated temperature.

The leach residues from the before tests were filtered and

10 the next stage leaching was conducted with acidifed solutions of hydrochloric acid varying from 5-10 g/1, and the chlorine concentration being varied from 5-10 g/1. Leaching temperature was 25°c under atmospheric pressure, stirred for 1-4 hours. Extraction efficiency from these examples are summarized in the

15 Table 2.

TABLE 2 LEACH EXPERIMENT STAGE 2

SERIES SOLID Cl, HC1 TEMP LEACH Au EXTRACTION g/i g/i g/i g/i °c TIME hr.

1 100 5 5 25 1 95

2 80 10 5 25 4 80

3 100 10 10 25 2 85

^♦ Extraction is based on the gold content of the leach residue.

Thus advantages of embodiments of the present invention include:

1. A wide variety of mixed sulphide concentrates can be reated without generating gases or particulate emissions.

2. Generally improved metal recoveries result from the two stages of leaching.

3. Concentrates containing arsenic, antimony, mercury and large amounts of iron are all acceptable feed materials for the chlorine-oxygen leach process.

4. Iron, arsenic and antimony are discarded in the tail as insoluble oxides.