THIS INVENTION relates to gold recovery. More particularly, it relates to a method of recovering metal values from refractory sulphide ores and concentrates.

According to the invention, there is provided a method of recovering metal values from refractory sulphide ores and concentrates, which method includes the steps of comminuting a refractory sulphide ore or concentrate; treating the ore or concentrate in an alkaline solution with a pH of at least 11 ; and recovering the metal values from the treated ore or concentrate.

The alkaline solution may have a pH of at least 11.

Preferably, the alkaline solution has a pH of about 11. 5.

Typically the metal value to be recovered will be gold, the metal values then being recovered from the treated ore or concentrate by conventional gold extraction methods.

By"sulphide ore or concentrate"is to be understood to include ores or concentrates including at least one mineral selected from the group consisting of pyrite, marcasite, pyrrhotite, realgar, orpiment, stibnite, and arsenopyrite.

Comminuting the refractory sulphide ore or concentrate typically includes finely grinding the refractory ore or concentrate. More particularly, the refractory ore or concentrate may be ground into a particulate material such that about 80 % by weight of the particulate material has a particle size of less than about 20 micrometres, preferably such that about 80% by weight of the particulate material has a particle size of less than about 11 micrometres, more preferably such that about 80% by weight of the particulate material has a particle size of less than about 8 micrometres.

Comminution of the ore or concentrate may be by way of a wet grinding method. Comminution of the ore or concentrate may occur by use of a tumbling mill, a vibratory mill, a tower mill, an ultra fine grinding mill, such as that available from Metprotech, Inc., or an IsaMill.

Treating the ore or concentrate in alkaline solution may include forming a fluid mixture/pulp of the ground ore or concentrate with an alkaline solution having a pH of at least 11. More particularly, treating the ground ore or concentrate may include forming a mixture of the ground ore or concentrate with water and adding a pH modifier to the water to yield an alkaline solution of pH at least 11. The ore or concentrate may be treated in alkaline solution for between about 2 hours and about 24 hours. It is to be noted that the alkaline treatment does not

necessarily, and typically does not in fact, lead to the complete destruction of the sulphide mineral. In other words, the ore or concentrate is typically treated in the alkaline solution partially to dissolve the sulphide minerals in the alkaline solution.

The pH modifier may comprise at least one alkali selected from the group consisting of sodium hydroxide, sodium carbonate, sodium bicarbonate, sodium hypochlorite, and calcium hydroxide in combination with any of the aforementioned alkalis. Preferably the pH modifier comprises sodium hydroxide.

Treating the ore or concentrate in alkaline solution may further include agitating the fluid mixture/pulp. It is to be appreciated that agitation serves to inhibit settling of and maintain in suspension the solid particulate material.

The method may include introducing oxygen into the pulp during agitation thereof. Oxygen may be introduced into the pulp as oxygen gas or air injected into the pulp, by the addition of hydrogen peroxide to the pulp or by the diffusion of atmospheric oxygen into the pulp at a surface of the pulp exposed to atmosphere.

Treating the ore or concentrate in an alkaline solution may take place at atmospheric pressure. Treating the ore or concentrate may occur at a temperature between the boiling point and the freezing point of the alkaline solution.

Recovering the metal values from the treated ore or

concentrate may include leaching the treated ground ore or concentrate.

Leaching the ore or concentrate may, for example, occur by cyanidation.

Instead, leaching the ore or concentrate may be by the addition of any other leaching agent to the treated ground ore or concentrate.

The invention will now be described, by way of example, with reference to the accompanying Examples : Example 1 Refractory sulphide ore encapsulating gold was finely ground by use of an IsaMill such that about 80% of the ground ore particulate material had a particle size of 13.7 micrometres. The finely ground ore was treated in a sodium hydroxide solution for four hours. Thereafter the treated ore was cyanide leached. Test results showed gold recovery to be around 72.22%.

Example 2: Refractory sulphide ore encapsulating gold was finely ground by use of an IsaMill such that about 80% of the ground ore particulate material had a particle size of 13.7 micrometres. The finely ground ore was treated in a sodium hydroxide solution for eight hours. Thereafter the treated ore was cyanide leached. Test results showed gold recovery to be around 72. 91 %.

Example 3: Refractory sulphide ore encapsulating gold was finely ground by use of an IsaMill such that about 80% of the ground ore particulate material had a particle size of 13. 7 micrometres. The finely ground ore was treated in a sodium hydroxide solution for twelve hours. Thereafter the treated ore was cyanide leached. Test results showed gold recovery to-be around 77.00%.

Example 4: Refractory sulphide ore encapsulating gold was finely ground by use of a tower mill such that about 80% of the ground ore particulate material had a particle size of 10.9 micrometres. The finely ground ore was treated in a sodium hydroxide solution for four hours. Thereafter the treated ore was cyanide leached. Test results showed gold recovery to be around 81.88%.

Example 5: Refractory sulphide ore encapsulating gold was finely ground by use of a tower mill such that about 80% of the ground ore particulate material had a particle size of 10.9 micrometres. The finely ground ore was treated in a sodium hydroxide solution for eight hours. Thereafter the treated ore was cyanide leached. Test results showed gold recovery to be around 86.08%.

Example 6 : Refractory sulphide ore encapsulating gold was finely ground by use of a tower mill such that about 80% of the ground ore particulate material had a particle size of 10.9 micrometres. The finely ground ore was treated in a sodium hydroxide solution for twelve hours. Thereafter the treated ore was cyanide leached. Test results showed gold recovery to be around 93.53%.

Example 7: Refractory sulphide ore encapsulating gold was finely ground by use of a vibratory mill such that about 80% of the ground ore particulate material had a particle size of 7.6 micrometres. The finely ground ore was treated in a sodium hydroxide solution for four hours.

Thereafter the treated ore was cyanide leached. Test results showed gold recovery to be around 88. 67%.

Example 8 : Refractory sulphide ore encapsulating gold was finely ground by use of a vibratory mill such that about 80% of the ground ore particulate material had a particle size of 7.6 micrometres. The finely ground ore was treated in a sodium hydroxide solution for eight hours.

Thereafter the treated ore was cyanide leached. Test results showed gold recovery to be around 92.70%.

Example 9: Refractory sulphide ore encapsulating gold was finely ground by use of a vibratory mill such that about 80% of the ground ore particulate material had a particle size of 7.6 micrometres. The finely ground ore was treated in a sodium hydroxide solution for twelve hours.

Thereafter the treated ore was cyanide leached. Test results showed gold recovery to be around 95.89%.

Discussion The above results indicate that a combination of fine grinding and alkaline treatment prior to leaching enhance gold recovery compared with typical sulphide ore whole-of-ore recoveries of about 45%.

Further testing showed that best recovery with no alkaline pre-treatment of the sulphide ore was obtained for the finest grind size of 7.6 micrometres, with recovery of 78.85% (whole-of-ore).

The Applicant believes that the method of the invention, provides an effective method for the recovery of gold with improved gold recovery from refractory sulphide ores over prior art methods. It is believed that the alkaline pre-treatment attacks and dissolves sulphide minerals in the ore at surfaces under which gold values are located and where there are distortions in the crystal lattice. These effects are greatest for fine particle sizes. It is believed that the method, performed as it is at atmospheric pressure and temperature, will not require

specialised equipment nor material for construction of such equipment nor will it require the use of chemicals not ordinarily available on a gold leaching plant. Further, as no sulphuric acid or sulphur dioxide is generated during the method of the invention, as is the case with pressure autoclaving, bacterial oxidation, roasting and other prior art methods, it is believed that no washing or neutralisation of the treated gold ore and concentrate will be required before leaching. The Applicant believes that the method of the invention provides a simple methodology for gold recovery, not requiring skilled labour for operation or supervision.