Gold Roasting

Roasting is used to convert base metals, such as zinc, iron, lead and copper, to their respective oxides so that they can be easily removed into a slag during subsequent smelting process. Any minor amount of sulphides in the roasted material is also oxidized. Oxidation is most effectively performed in the range 600-700 ^oC to enable conversion to the oxide within practical time scales, typically 12-18 hours, although temperatures well in excess of 700 ^oC have been used in practice to reduce roasting time. Above 600 ^oC most metals of importance have negative free energy of formation values, implying that the oxide should be formed under these conditions.

Slow roasting conditions are generally more efficient for converting these metals to their oxides; the reaction is usually limited by the mass transport of oxygen to the metal surfaces and an adequate flow of oxygen, as air, must be provided to all regions of the material. Thin layers of material (40-80 mm thick) in roaster-calciner trays should be used and care should be taken not to pack the material.

Smelting: The purpose of smelting is to remove base metals and other impurities from high grade concentrates to produce a gold-silver bullion containing typically more than 95% precious metals. The smelted product, called dore bullion, is suitable for direct sale and or further refining.

Smelting with fluxes: The smelting process is achieved by heating the material in the presence of slag-forming fluxes at temperatures en excess of the melting point of all the components of the charge, typically between 1200-1400 ^oC. This maximum temperature is maintained for approximately 1.5 hours to ensure complete separation of impurities into the slag. The molten gold and silver form an alloy that is heavier than the slag and sinks to the bottom of the smelting vessel.

The efficiency of separation depends on the quality of the slag that is formed, measured in terms of gold grades in the slag and the recovery of base metals and other impurities to the slag. The performance depends on the nature of the gold-bearing materials smelted and the properties of the fluxes used. Silica usually forms the basis for the flux as it has the capability of dissolving most metals oxides. The metal oxides break up the silica lattice and are incorporated into the modified structure.

Silica has a high melting point (1723 ^oC) and tends to form a highly viscous slag which may entrain precious metals. The addition of sodium and boron oxides (sodium borate or borax) reduces both the melting point and viscosity. For example a 1:1 molar ratio of sodium borate and silica has a melting point of approximately 800 ^oC, which forms a much less viscous, but still strongly acid slag. The reaction of sodium borate with metal oxides is analogous to silica. Consequently, silica-sodium borate mixtures are commonly used as fluxes.

Fig 6.3 Gold smelting using a tilting furnace