RE: RE: RE: gold production from the heap As I said, no ‘new’ ore is being mined, - - -but, the existing heap leach operation carries on.
In that heap leaching involves stacking of metal-bearing ore into a "heap" on an impermeable pad, irrigating the ore for an extended period of time (weeks, months or years) with a chemical solution to dissolve the sought-after metals, and then collecting the leachant ("pregnant solution") as it percolates out from the base of the heap, - heap leaching is really a form of milling.
Alhambra’s “Heap Leach Method” is as follows: oxide gold mineralization (containing between 32,000 and 36,000 ounces of ‘recoverable gold’) has already been stacked onto one of five leach pads; with the leach pad being an impermeable structure designed specifically to retain the solution used in the leaching process.
- To prepare a heap for leaching, the stacked material is saturated with water followed by the addition of alkali to achieve a pH of between 10.5 and 11.0 in the heap. A solution containing very low concentrations of cyanide is applied to the heap utilizing a drip emitting system. (The drip emitting system is preferred over other available sprinkler systems as it minimizes exposure of the solution to sunlight and prevents dispersion of the solution due to wind).
- As this solution seeps down through the heap, it dissolves the gold; and the gold bearing solution is then collected in a berm located around the foot of the heap to be pumped to the gold recovery plant. Once there, resin is used in the gold recovery plant to recover gold from the solution (by way of a chemical reaction); and after the resin is loaded with gold, it is removed from the resin columns and shipped to a stripping plant operated by an independent third party.
Leaching operations are performed on a year round basis, and the heap leach operation conducted by Alhambra is similar to other heap leach mining operations around the world.
- The efficiency of the heap leaching process (as measured by gold recovery rate) is significantly affected by weather conditions. Temperatures in Northern Kazakhstan range from +30_C in the summer to -30_C in the winter; and the recovery rate (of the heap leaching process) is significantly lower than average during February-March and significantly higher than average during July, August and September, depending on actual temperatures. This is primarily due to the cyanide solution freezing as the temperature drops.
- Low temperature can be a problem since the rate of reaction is slowed as solution temperature approaches freezing; and comparative laboratory column tests show that recovery rate drops significantly when the heap temperature drops below 5oC. This is because solution viscosity increases significantly as temperature drops, affecting both the heap and the process plant; and cold heaps tie up more process solution (and more gold inventory) than warm heaps.
Besides the weather, factors such as tons of material under leach, temperature of solution from the heap, and daily circulating solution volumes also affect gold production from a heap leach operation.
- Ponds are installed downslope from the heap to provide storage of process solution, and the ideal padsite begins as a uniformly sloping area with a grade of 0.5% to 2.0% in the direction of the process ponds. A drainage base of crushed rock and embedded perforated pipes is installed above the plastic leach pad and below the ore heap; and (ideally) the solution should percolate vertically downward through the entire ore column, and then enter a solution removal system with zero hydraulic head. If the drainage base cannot take the entire flow then solution builds up in a stagnant zone within the heap, and leaching within this stagnant zone can be very slow. In principal, this is not a difficult engineering accomplishment, and flow is carried in pipes within the base.
- With regard to sprinkling rate, the timing of gold recovery is a function of five factors: (1) the rate at which the gold dissolves (coarse gold particles dissolve very slowly, and may not fully dissolve for several months in a heap leach environment), (2) the percentage of gold that exists as free or exposed particles, (3) the rate of diffusion of the cyanide solution into rock fractures, and of gold cyanide back out of the rock fractures (- where the gold occurs on tight fractures or in unfractured rock, the rock must be crushed into fine particles to achieve target rates and levels of recovery), (4) the effect of chemical reactions within the heap, or within rock particles, which destroy cyanide and alkalinity or which consume oxygen, and (5) the rate of washing of gold off of the rock surfaces and out of the lift of ore under leach. Achieving success is a complex undertaking, which depends on the overall permeability of the lift.