Roasting of refractory gold minerals: Test Procedure VS Cyanide Leaching

It is the classical process to be considered when there are refractory material such as pyrite and arsenopyrite containing precious metals. The main objective is to get a complete oxidation of the material and remove all the arsenic and sulphur. At the end, there will be a porous product which can be treated by cyanidation. The final product normally contains a good amount of hematite.

When a pyritic material is roasted, there is a transformation of phases towards pyrrhotite, hematite, and magnetite. The final products are influenced by the presence of oxygen into the furnace. Oxidation is done in the range of 450 – 750 ^oC. An appropriate formation of phases such as 75/25 hematite/magnetite let a good recovery of precious metals by cyanidation, but also a low consumption of cyanide.

Treatment of arsenopyrite concentrates by roasting is complex due to the toxic compounds that arsenic produces. These compounds are As₂O₃, As₂O₅, and ferric arsenate. The latter one must not be formed because recoveries of gold and silver by cyanidation will be low. In order to get a drastic reduction of arsenic, residence time and oxygen concentration must be evaluated and monitored with the most possible exactitude.

The reactions obtained during roasting are the following.

Pyrite.

• 4FeS₂ + 11O₂ = 2Fe₂O₃ + 8SO₂
• 3FeS₂ + 8O₂ = Fe₃O₄ + 6SO₂

Arsenopyrite.

• 2FeAsS + 6O₂ = 2FeAsO₄ + 2SO₂
• 3FeAsS + 5O₂ = As₂O₃ + Fe₂O₃ + 2SO₂

Roaster Testing: Temperature was changed, and other variables didn’t change its initial values. Conditions are listed below.

• Sample weight: 300 g.
• Temperature: 500 -700 ^oC
• Furnace: electric
• Bed: static
• Time: 2 hours

The next tables show results obtained.

 Oxidation

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