Some gold and silver ores are difficult to successfully heap leach because of the high percentage of clay, and/or fines which are intrinsic in the material or generated by the crushing or ore preparation. This type of ore can result in extremely slow percolation during heap leaching. Channeling can result which promotes dry areas within the heap, leach solutions may run off the sides instead through the bed, and extraction of precious metals is low. Agglomeration of the crushed ore has proven to be the answer in treating certain of these ores. Rotating inclined tubes, concrete mixers, slanted grooved rotating plates or discs, inclined rocking chutes or conveyor belts which move up an incline as the ore tumbles counter currently all have been used to produce competent agglomerates.
Some operators use measured additions of lime, cement or mixtures of these two components to insure permeable agglomerates. At some properties, milk of lime, strong cyanide solution, or barren cyanide liquors are used to supply the required moisture for agglomeration.
In most instances, the key to successful formation of a durable agglomerate is curing. The individual agglomerates require sufficient time to dry and harden. In the tropics, air drying of agglomerates is difficult. Consideration has been given to heat introduction during agglomeration through combustion gasses or by chemical reaction.
If cyanide solution is used as the wetting agent, conditions for commencing precious metal dissolution are ideal. Aeration is thorough during curing and after 2 or 4 days the initial pregnant solution is usually very high-grade.