Modern gold operations with grinding circuits include a SAG mill and Ball mills whose operation is critical for the gold recovery process (e.g. cyanidation, gravity, flotation). The control of the ore reclamation system from stockpile and grinding circuit at the gold mill are key aspects for the efficient performance of the operation. The purpose of the reclamation from stockpile and grinding circuit is to reclaim crushed ore from the stockpile and grind the ore to a size small enough to liberate the free gold and gold-bearing minerals in the crushed ore. The grinding is accomplished using a closed-circuit SAG mill followed by closed-circuit ball milling with hydrocyclone classification.
The term SAG mill is an acronym for semiautogenous grinding mill. The term autogenous grinding means that all of the grinding action is performed by the ore particles tumbling into one another. The size reduction is accomplished by the action of ore crushing and grinding other ore particles. In fully autogenous mills, there are no grinding media. In semiautogenous mills, a portion of the grinding is autogenous and a portion is performed by grinding media (steel balls); hence the term semiautogenous.
The crushed ore is reclaimed from one or two stockpiles and the number of stockpiles is based on plant size and complexity of ore body (i.e. ore types). Basically a stockpile is equipped with apron feeders that discharge onto a SAG mill feed conveyor. A dust suppression system is used in the reclaim system and the dust is suppressed at each transfer point of crushed ore from the apron feeder onto the feed conveyor. The crushed ore is fed into the SAG mill for grinding by the SAG mill feed conveyor. On its way to the SAG mill, the reclaimed ore passes over a belt scale installed on the SAG mill feed conveyor. The scale is important not only for plant metallurgical accounting but also because it is used in controlling the feed rate to the SAG mill.
The ore, grinding balls, and water cataract in the mill, and the ore size is reduced until it flows through the slotted mill discharge grates on the discharge end of the mill. SAG mill water is added at the feed chute and also to the mill discharge trommel. The combined water addition is made in proportion to the weight of ore being fed from the ore stockpile. This gives the desired slurry density or as percent solids in the tumbling mill. The discharge from the SAG mill trommel screen goes to a distribution box equipped with valves for feeding the hydrocyclone feed pump box. Reagents such as sodium cyanide and milk of lime are added to the SAG mill feed chute. If gold is recovered by flotation, some chemical reagents may be added at this point.
Hydrocyclone are fed with SAG mill discharge, ball mill discharge, and water used for density control from the hydrocyclone feed pump. The overflow from the hydrocyclone goes to the leaching process (e.g. Carbon-in-Pulp, Carbon-in-Leach) or flotation circuit. The underflow from the hydrocyclone goes to the ball mill feed box. The ball mill works in closed circuit with the hydrocyclone to grind the underflow.
The overflow stream must be must be monitored and some samples have to be taken for gold grade control, percent solids and particle size. The last one is the most critical parameter to control due the efficiency of the gold recovery process is influenced by this parameter.
SAG Mill