The plant has a capacity of 1200 tons (imperial) per day. The mode of the occurrence of the precious metals is complex, and the physical nature of the ore highly variable. The incidence of primary fines, at times up to 40%, is a major problem.
Immediately after grinding in circuit (cyanide) solution, a telluride concentrate is produced. The remainder of the main pulp stream is cyanided, filtered, washed, and repulped with fresh water. This pulp is then treated with roaster gas, containing sulphur dioxide, and after further conditioning, a sulphide flotation concentrate is produced.
Flotation of the tellurides is carried out immediately after grinding and classification. Three stages of flotation are employed and as grinding is conducted in circuit solution the presence of lime and cyanide effectively depress the major proportion of the sulphides present.
Testwork conducted prior to the introduction of telluride flotation, and early plant trials indicated that gold and silver recovery from telluride flotation concentrates presented a number of difficulties. Direct cyanidation of telluride concentrates after one stage of cleaning was attempted. Both cyanidation rate, and recovery were unacceptably low. Also residual reagents produced an uncontrollable froth condition.
These difficulties were overcome by roasting the concentrate, and cyanidation of the calcine. This separate treatment of the tellurides, as opposed to the previous procedure where they were bulk, floated with the sulphides, and calcined in Edwards roasters, had several advantages..
In testwork recently conducted, it was found that extremely fine grinding of the raw concentrates followed by cyanidation, gave improved gold recovery, 99% being achieved consistently but dissolution rate was prohibitively slow, up to fourteen days being required for some charges.
Of all the systems tested, the most successful was basic hypochlorite oxidation. The reagent combination is NaOH and Na2CO3/Ca(OCl)2. By controlling the tellurium/chlorine ratio, complete oxidation of the tellurides can be achieved in one hour. Cyanidation is normally complete after three to four hours contact time.
Initial attempts at tellurium extractions from roasted concentrates gave low recoveries as mentioned earlier. The extraction method employed was to leach the roasted concentrates prior to cyanidation with 20% NaOH. The leach liquor was neutralised with HCl, precipitating a tellurous acid/oxide mixture.
After much investigation, the Na2S/Na2SO3 process was chosen, as it had many advantages. In this method, the cyanidation residue (either a cyanided calcine or a cyanided chemical oxidation charge) is leached with Na2S solution. Elemental tellurium is then precipitated by the addition of Na2SO3. The precipitate is of high purity for while Na2S is an effective solvent of oxidized tellurium compounds, the major possible contaminents Fe, Pb & Cu remain predominately in the residue.