GB2156794A

GB2156794A - Gold recovery

Info

Publication number: GB2156794A
Application number: GB08508164A
Authority: GB
Inventors: Elizabeth Anne Crathorne; Ronald David Hancock
Original assignee: BP PLC
Current assignee: BP PLC
Priority date: 1984-03-31
Filing date: 1985-03-28
Publication date: 1985-10-16
Also published as: GB8508164D0; AU4015985A; AU568727B2; GB2156794B; BR8501447A; GB8408352D0; ZA852104B

Abstract

A process for recovering gold from carbonaceous ores in which the ore is contacted at ambient temperature with an alkaline cyanide solution. The alkaline cyanide solution contains at least 0.3% by weight of an alkali metal hydroxide. The ore may be pre-treated with alkali metal hydroxide and the total quantity of alkali metal hydroxide contacted with the ore is sufficient to supply at least 5 kg of alkali metal hydroxide per tonne of ore in excess of the quantity required to neutralise any acidic components in the ore.

Description

SPECIFICATION Gold recovery The present invention relates to the leaching of gold from carbonaceous ores.

Gold is conventionally recovered from its ores by treatment with solutions of cyanide salts.

Cyanide solutions used for gold extraction are kept alkaline by the addition of a suitable alkali in order to avoid any risk of formation of HCN, which might occur if the cyanide solution became acidic. However the quantities of alkali required to maintain the necessary level of alkalinity are not very great. Generally about 0.5 kg of lime or NaOH per tonne solution is sufficient to keep the pH at the desired level of 9.5 to 11.

The alkali usually used is lime. The use of sodium hydroxide for this purpose is mentioned in for example "Chemistry of Cyanidation" June 1958, published by American Cyanamid Company, pages 8 and 9.

It is known that it is difficult to recover gold from carbonaceous ores (see US 4259107, US 4289532, US 4188208 and pre-print 81-34 of the Society of Mining Engineers, Littleton, Colorado, USA). The methods disclosed are generally complex and involve pretreatment of finely ground ore with reagents additional to those normally used in gold recovery, and/or heating. All these pretreatments result in increased processing costs.

US 4188208, mentioned above, discloses a process in which highly alkaline conditions are used in the cyanidation of gold ores.

However the process is carried out at temperatures of 167"F (75C)to 212'F (100"C). The purpose of the high alkali concentration is said to be to protect the cyanide complexes from decomposition at the temperatures used.

There is nothing to suggest that the addition of additional alkali will give any benefit at lower temperatures. The process of US 4188208, because of the need to heat large quantities of ore, will be an expensive one to operate.

Pre-print 81-34 mentioned above states that treatment of the carbonaceous ore with sodium hydroxide prior to treatment with cyanide increases the gold extraction significantly.

We have found that alkali pre-treatment of carbonaceous gold-bearing ore can have beneficial effects on gold recovery, but it is not in itself sufficient to give satisfactory recoveries with all ores.

It would be advantageous to find a method of recovering gold from carbonaceous ore which did not require the ore to be finely ground, which enabled gold to be recovered by conventional vat or heap or dump leaching techniques, as well as in agitated tanks, and which can be used at ambient temperatures and at atmospheric pressure.

According to the present invention the process for leaching gold from carbonaceous ores by contacting the ore at ambient temperature with an alkaline cyanide solution in which the alkaline cyanide solution contacted with the ore contains at least 0.3% by weight of alkali metal hydroxide and that the total quantity of alkali metal hydroxide contacted with the ore is sufficient to supply at least 5 kg of alkali metal hydroxide per tonne of ore in excess of the quantity required to neutralise any acidic components in the ore.

The quantity of alkali metal hydroxide added to the ore is well in excess of that merely required to give an alkaline pH for the cyanide solution.

The preferred alkaline cyanide is sodium cyanide and the preferred alkali metal hydroxide is sodium hydroxide. The concentration of sodium cyanide solution is preferably 0.025 to 1% by weight.

A defined quantity of alkali metal hydroxide is added in excess of that required to neutralise any acidic components in the ore. The quantity of alkali metal hydroxide required to neutralise the ore can be determined by contacting the ore with alkali metal hydroxide solution in the absence of cyanide for 24 hours and measuring the amount of alkali metal hydroxide consumed by the ore.

The process of the present invention may also be used to leach finely ground ore in agitated tanks. It is an advantageous feature of the invention that it may be applied to either a prelocation vat leaching process or a heap leaching process, in which processes the ore is used in coarsely ground form eg greater than 1.6 mm particle size.

It is necessary to bring a defined minimum quantity of cyanide solution containing alkali metal hydroxide into contact with the ore in addition to any alkali metal hydroxide required to neutralise the ore. Thus the total quantity of alkali metal hydroxide fed into contact with the ore may be in a range 5-120 kg/tonne ore, depending on the acidity of the carbonaceous ore, preferably at least 10 kg/tonne ore.

For a given quantity of alkali metal hydroxide brought into contact with the ore, the concentration of alkali metal hydroxide in solution will depend on the pulp density when using an agitated tank leaching system. Thus for a suspension containing 30% wt solids the use of 10 kg of alkali metal hydroxide per tonne of ore is equivalent to 0.43% wt in solution. At a pulp density of 50% wt solids the solution concentration of alkali metal hydroxide would be 1.0% wt.

Thus for an agitated tank leaching process the requirement for the use of at least 5 kg of alkali metal hydroxide per tonne of ore (in excess of that required to neutralise any acid components) will normally ensure that the requirement for a concentration of at least 0.3% wt alkali metal in the solution brought into contact with the ore is also met, providing the pulp density is not too low.

In vat heap or dump leaching in which the cyanide solution is percolated through a bed of ore and the conditions are not necessarily uniform throughout the bed, the requirement for a minimum alkali metal hydroxide concentration helps to ensure that the pH of the solution recovered from the leaching process does not fall below 12.8, which would impair gold recovery.

The concentration of alkali metal hydroxide in the solution is preferably at least 0.5% by weight. For economic reasons it is not normally desirable to use solutions with a concentration of more than 5% by weight of alkali metal hydroxide.

The leaching step is carried out under normal ambient temperatures eg from 5-40"C, preferably 10-30"C.

It is desirable to pre-treat the ore with alkali before treatment with the cyanide solution. It is not necessary to use alkali metal hydroxide for this purpose, and lime and sodium carbonate, for example, can be used. The purpose of the pre-treatment is to neutralise acidic materials which would otherwise take up alkali from the cyanide solution.

It is believed that an equilibrium is set up between the solubilised gold cyanide and gold adsorbed on the carbonaceous matter ie [AU]soln = I Au]carbon This equilibrium is strongly dependent on the concentration of alkali metal hydroxide in solution. In strongly alkaline solutions the equilibrium is strongly in favour of gold in solution. If the pH is allowed to fall below about 12.8 (eg by neutralisation of the leaching solution by acidic materials in the ore) then gold is adsorbed onto the carbonaceous matter from which it is very difficult to remove.

Thus it is particularly desirable to add alkali to the ore before the ore is vat or heap leached with cyanide otherwise, the pH of the leaching solution passing through the ore may easily fall below 1 2.8 if the ore contains acidic material.

The amount of alkali required in the pretreatment stage can be reduced by pre-washing the ore with water to remove soluble acidic material.

Ore to be heap leached is frequently agglomerated with water or cyanide solution to improve permeability. The additional alkali for the pretreatment can be incorporated at this stage.

Where an agglomeration step is not used the pre-treatment step may be carried out by allowing a solution of alkali to pass through the ore until the pH of the liquid leaving the ore is at least pH 12.8.

Similarly, finely ground carbonaceous ore to be leached in agitated tanks can be neutralised prior to cyanidation by stirring with a solution of an alkali.

Leaching techniques are well known and it is therefore not necessary to describe them in detail.

The gold contained in the cyanide solution may be recovered in various ways eg by zinc cementation or by adsorption on activated carbon. Adsorption on activated carbon can be operated satisfactorily even though high alkali concentrations are present.

The process of the present invention may be applied to any carbonaceous gold ore. It is particularly useful for ores containing substantial quantities (0.1-1%) of organic carbon.

By a carbonaceous ore, it is meant to be an ore containing organic carbon or carbon in an organic form.

The invention will now be illustrated by the following experiments in which examples of the invention are identified by numbers and comparative tests not according to the invention are identified by letters.

Test A A carbonaceous gold ore containing 12.3 ppm gold was leached for 24 hours in a continuously rolled bottle. The cyanidation test was carried out using a 0.1 % wt sodium cyanide solution, at a pulp density of 35%, keeping the pH between 10-11.5 by the addition of sodium hydroxide. The total amount of sodium hydroxide added was 3 kg/tonne ore. 9.5% of the gold was extracted from the ore.

Example 1 The ore referred to in Test A was leached under the same conditions but using a sodium hydroxide concentration of 40 kg/tonne ore.

(2.2% wt solution at 35% pulp density). After 24 hours 67% of the gold in the ore had been extracted, 1 6 kg NaOH/tonne ore having been consumed.

Example 2 The ore referred to in Test A was agitated with a solution of 2.2% wt sodium hydroxide for 24 hours before cyanidation. After filtering, washing and drying it was found that 22.5 kg NaOH/tonne ore had been consumed. The treated ore was then cyanided under the same conditions as above with a sodium hydroxide concentration of 10 kg/tonne ore (0.55% wt at 35% pulp density). After 24 hours 78% of the gold in the ore had been extracted, a further 2.7 kg NaOH/tonne ore being consumed by the ore.

Comparative Test B A carbonaceous gold ore was crushed to 100% passing 13.2 mm and 14 kg leached in a 6" (0.15 m) diameter glass column after agglomeration with 1066 mls of dilute cyanide solution (0.025% wt NaCN + 0.025% wt NaOH; pH = 10.6) to improve percolation.

The ore was then sprayed with the leach solution (0.025% wt NaCN + 0.025% wt NaOH) at a rate of 300 mis/hour and the product solution analysed for gold by AAS (Atomic Absorption Spectroscopy). The product solution had a pH of 6.8 after 1 day which had risen to 9.8 after 6 days leaching when 11 % of the gold in the gold had been extracted and no more gold was being found in solution (0.75 kg of sodium hydroxide had been passed per tonne of ore).

Example 3 The experiment described in Test B was continued but at this point the leach solution was changed to 0.025% wt NaCN + 0.5% wt NaOH. The pH of the product solution rose to 1 2.8 and gold extraction resumed. After a further 7 days leaching 36.9% of the gold in the ore had been extracted. A total of 18 kg NaOH had been passed per tonne of ore.

Comparative Test C A carbonaceous gold ore was crushed to 100% passing 13.2 mm and about 14 kg leached in a glass column after agglomeration with 815 ml dilute cyanide solution (0.05% wt NaCN + 0.05% wt NaOH) and five kg lime/tonne ore. The ore was sprinkled with a leach solution containing 0.05% wt NaCN and 0.05% wt NaOH (pH = 10.6) at a rate of 100 mls/hour. After 5 days only 1% of the gold had been extracted from the ore even though the pH of the product solution was between 10.1 and 10.8. At this point a total of 0.75 kg/tonne ore NaOH had been passed over the ore. The leach solution was changed to 0.05% wt NaCN and 0.25% wt NaOH (pH = 12.8). After a further 2 days leaching no further gold has been extracted from the ore.

Example 4 The leaching test described in Test C was continued but the leach solution then changed to 0.05% wt NaCN and 0.5% wt NaOH (pH 13). Gold began to be leached from the ore and after a further 31 days of leaching 32% of the gold in the ore had been recovered. In Example 4 the quantity of sodium hydroxide passed was 24 kg/tonne ore.

Comparative test C shows that alkali pretreatment of an ore, prior to leaching with a dilute sodium cyanide solution with a pH of 10.6 does not provide satisfactory extraction of the gold and even increasing the sodium hydroxide concentration of the solution to 0.25% wt (pH = 12.8) does not improve gold extraction.

Example 4 shows that a high sodium hydroxide concentration in the leach solution is necessary to extract the gold from the ore.

The figure shows the change of extraction against time and illustrates the effect of leaching of a gold containing carbonaceous ore under various conditions. Curves A, B and C shows the relationship of gold extraction and leach time with Curve A being ore agglomerated with lime, Curve B being ore agglomerated with no added alkali and Curve C being ore agglomerated with sodium hydroxide. The leach solutions for A and B were 0.025% NaCN + 0.025% NaOH and for C were 0.05% NaCH + 0.05% NaOH. In Curve C, the leach solutions was changed after 6 days to 0.025% NaCH + 0.5% NaOH and clearly indicates the advantageous extraction achieved by increasing the pH of the leaching solution.

In curve A, 5 kg/tonne of lime was used in agglomeration and 0.7 kg/tonne of NaOH was passed over the ore. In Curve C, 9.8 kg NaOH/tonne was added at the agglomeration stage and 20.5 kg/tonne ore of NaOH was passed over the ore during leach of which 12.3 kg/tonne was recovered.

Claims

1. Process for leaching gold from carbonaceous ores by contacting the ore at ambient temperature with an alkaline cyanide solution in which the alkaline cyanide solution contacted with the ore contains at least 0.3% by weight of alkali metal hydroxide and that the total quantity of alkali metal hydroxide contacted with the ore is sufficient to supply at least 5 kg of alkali metal hydroxide per tonne of ore in excess of the quantity required to neutralise any acidic components in the ore.

2. Process according to claim 1 in which the alkaline cyanide solution is sodium cyanide.

3. Process according to claim 2 in which the concentration of sodium cyanide solution is 0.025 to 1.0% by weight.

4. Process according to any of claims 1 to 4 in which the alkali metal hydroxide is sodium hydroxide.

5. Process according to any of the preceding claims in which the leaching process is carried out at 10 to 30"C.

6. Process according to any of the preceding claims in which the ore is pre-treated with alkali metal hydroxide prior to treatment with the alkaline cyanide solution.

7. Process according to any of the preceding claims in which the gold contained in the resultant cyanide solution is recovered.

8. Process according to claim 7 in which the gold is recovered by adsorption on activated carbon or by zinc cementation.

9. Process for leaching gold from carbonaceous ores as hereinbefore described and with reference to the examples.

1 0. Gold whenever recovered by processes according to claim 7 to 9.