Nowadays precious metal heap leaching technology has largely developed over the past decade, although the principles of heap leaching per se, as well as those of gold extraction through Cyanidation have a long history. Nevertheless, it is in the last 20 years that heap leaching has developed into an efficient method of treating oxidized gold and silver ores. It has proven to be both an efficient way to extract precious metals from small, shallow deposits, as well as an attractive way to treat large, low grade, disseminated deposits. Heap leaching has several advantages compare to conventional milling (i.e., crushing, grinding, and agitating leaching). In general, these advantages include simplicity, lower capital and operating costs, shorter start-up times, and less intensive environmental regulatory concerns. A possible disadvantage of currently existing heap leaching technology is a potentially lower percentage of metal extraction being obtained from ore than would be the case with conventional milling. The actual principle of heap leaching has a long history, as inferred previously. For example mines in Hungary recycled copper-bearing solutions through waste heaps in the mid-sixteenth century, and Spanish miners percolated acid solutions through large heaps of oxide copper ore on the banks of the Rio Tinto in 1752. By 1900, leaching operations were employing such techniques as leach/rest cycles to maximize copper recovery. Copper dump leaching is currently practiced worldwide for low grade ores. Additionally, heap leaching using both acid and alkaline solutions has been practiced by uranium producers since the late 1950’s.
Heap leaching can be considered as a percolation process on ore piles. The process is characterized for its economic cost against the agitated leaching process when the ore deposit has a low gold content. Typically the process can process ores whose gold content is 0.8-1.1 g/t. The leaching solution is spread at the top of the pile and the pregnant solution will have to percolate through the pile. The design considers special drainage pads for pregnant liquid collection. The design is influenced by the manner of occurrence of the gold or gold host minerals, geological characteristics of the ore, and the volume of the deposit. Operations are conducted on ore stacked on impermeable pads. Special pad and piping systems are required to collect and transport de pregnant solution so that the losses of gold to the ground can be eliminated. Commonly, the materials used for constructing the pads are rock mixed with bentonite, asphalt mixed with gravel, reinforced concrete pads, and rubber covers on an excavated area.
There are two popular heap leaching designs, one is called short-term leaching of crushed ore, and the other is a long term leaching of run of mine ore. The first one comprises one or two crushing stages previous to form the ore pile. Cyanide solution percolates through the heap leaching the gold and silver and is collected on the pregnant solution pad. The ore is crushed to a size that will give good liberation of gold minerals. In most operations with this crushing stage, the ore is crushed until two-three inch. A finer size can be considered if the ore needs fine liberation. The leach cycle is normally from 20 to 30 days. When the leaching cycle is complete, a new crushed ore is piled.
The second option is the most economical option and the ore is taken directly from the blasting. A typical ore size is six inch. A heap constructed under this design can treat high tons of material. Most heaps look like truncated pyramid 25 to 40 feet high. The height is in based on ore permeability, residual alkalinity, cyanide strength and dissolved oxygen.
For efficient leaching (i.e. in order for the reactions defined above to occur rapidly), the gold should be as free, fine size, clean particles in an ore that contains no cyanicides or impurities that might destroy cyanide or otherwise inhibit the dissolution reaction. An adequate supply of dissolved oxygen must be present in the cyanide solution throughout the reaction period. Only certain kinds of ore have the general characteristic required. The major ore types suitable for cyanidation and heap leaching cyanidation are:
· Oxidized disseminated ores,
· Sulfide ores where the precious metals are not intimately associated with the sulfide minerals, and
· Certain lode or placer materials which contain fine particles of gold or particles with a high surface area to weight ratio.
More specific characteristics required of ores if they are to be amenable to heap leaching process include:
· Ore with contained precious metals leachable by cyanide
· Ore with extremely small or flattened gold particles
· Ore occurring in porous and permeable host rock (precious metals in ores of low porosity can be liberated by fracturing and crushing)
· Ore free of carbonaceous or pre-robbing material which causes premature adsorption or precipitation of dissolve values
· Ore relatively free of cyanicides that consumes cyanide or interfere with the dissolution reactions
· Ore relatively free of fines or clays that impede uniform solution percolation , an agglomeration pretreatment is required if excessive fines or clays are contained in the feed)
· Ore free of acid forming constituents that cause high cyanide or base consumption.
Pad Construction
The leaching solution is introduced onto the heaps by spraying from perforated plastic tubes, and by sprinkling using plastic sprinkler heads. The pregnant solution can be treated by activated carbon or zinc powder (Merrill-Crowe process). The second is applied in many operations, but the final decision is based on the silver/gold ratio and economical considerations. Activated carbon does not recover much of the silver, but the gold recovery is high. In other words, the efficiency of the recovery process is influenced by the silver content.
Leach solution is sent from the barren pond to the heap through a piping system. A pump system is usually required to give sufficient pressure for sprinkler application of the leachate. Few operations use ponding on the heaps as a means of solution application. Application can be done with sprinklers, wigglers, or similar equipment. The major requirement is even distribution of the leachate. Typical application rates are from 0.003 to 0.005 gpm/ft2 (0.002 to 0.003 l/s/m2).Application rates normally result in the partially saturated flow of leachate through the heap material. The corollary of the above application rates is that a minimum permeability of 10-4 cm/sec is required for the ore within the pad. The chemical reaction in which the metals are dissolved by the cyanide requires oxygen. The unsaturated condition of the ore allows oxygen to be available for the dissolution process, while sprinkling also enhances oxygen entrainment in the leach solution. Some applications techniques have been adopted in which introduction of leach solution is performed by buried pipe networks or beneath a heap cover. Such techniques can alleviate problems of freezing or high evaporation losses in the sprinkling process.
The flow of solution through the heap is essentially vertical from the surface to the base of the heap. Some methods of heap operation are designed to store solution within the heap itself. When this is the case, the solution simply progresses vertically to the level of the store solution.
Heap leaching process
