Here we describe a cyanide leaching-carbon adsorption plant designed to recover the gold from the molybdenum concentrate.
Process Design: The molybdenum concentrate assays typically, 45 percent molybdenum, less than 1 percent copper (chalcopyrite) and 3.5-5.0 percent iron (chalcopyrite-pyrite). Gold and silver average 8 and 40 ppm respectively which at an average molybdenum concentrate production of 10 tonnes per day translate into an annual potential of approximately 950 ounces of gold and 4700 ounces of silver. These preliminary calculations indicate that silver recovery was of marginal value to this project.
A characterization study revealed that over 80 percent of the gold in the molybdenum concentrate was either liberated or exposed and would therefore be amenable to cyanidation. Extensive testwork conducted at Island Copper’s metallurgical laboratory confirmed that gold recovery from the molybdenum concentrate using established cyanide technology was economically feasible.
To achieve the desired gold leach extraction at economical dissolution rates, a variation of the standard leach cyanidation procedures involving the use of hydrogen peroxide and other reagents appeared optimum at a bench- scale and was therefore proposed for the full scale facilities. Based on the laboratory testwork it was determined that four hours would be required to obtain a target gold extraction of 80 percent.
Since the molybdenum concentrate is normally recovered from the flotation slurry by filtration, a decision to cyanide leach the molybdenum concentrate before filtration and recover the gold from the filtrate solutions was made.
For gold recovery from the pregnant leach solutions adsorption on activated carbon was studied. In view of the small quantity of loaded activated carbon that could be annually produced, it was decided that gold would be marketed as loaded carbon. Gold loading and the price of the activated carbon were then the basic criteria for adsorption data evaluation.
Lab column adsorption tests with three pre-selected activated carbons were carried out. The best adsorption performance, in terms of loading and recovery, was achieved with Norit Pkl-3. This is a 8 x 20 mesh peat based steam-activated granular carbon. In addition, this is the cheapest carbon in the market (Cu 2.2/kg) because it is very difficult to strip.
The only disadvantage of Norit Pkl-3 to Island Copper’s application appeared to be its lower bulk density (0.24 g/cm³ against 0.45-0.50 g/cm³ for most carbons). As a result of its lower density, this carbon has a greater tendency to break down in fines which if loaded would mean gold losses. Pre-screening and in-column pre-washing (optional) were proposed to minimize potential losses.
The thickened concentrate is then cyanide-leached in a series of 5 enclosed leaching tanks. Each leach tank is 34 inches in diameter and 45 inches high; agitation is provided by a 1.5 HP dual-propeller mixer. Due to the possibility of CN gases generation, overhead ducting with positive suction from a fiberglass wall fan is used for ventilation.
Leached concentrate is transferred by gravity to mechanically agitated storage tanks (50 m capacity) and from there pumped to the vacuum filter. Water sprays to wash the already soluble gold out of the filter cake are installed. Filter cake is treated as usual while the filtrate solution which is the pregnant leach solution is stored in a 1000 gallon capacity tank. Pregnant leach solution is pumped via a rotameter to a series of six carbon adsorption columns.
