Gold beneficiation is a process that involves two and a half stages of closed-circuit crushing, followed by two stages of closed-circuit grinding and flotation. Afterward, the selection process involves sweeping the tailings away through rough sorting operations. The selected tailings and sweep are combined with ore concentrate for regrinding. This process is highly efficient, with a high crushing capacity, and a processing capacity of 53.84 tons per hour. Its working time is short, and its sorting effect and high-technology index are impressive. Furthermore, it can improve the concentrate grade and recovery rates, increasing the recovery rate targets by 3.03%, the concentrate grade by 16.57 g/t, while reducing power consumption by 21.5% and costs by 18.9%, resulting in an annual cost increase of 8.6012 million yuan.
Gold has been recovered from its ores using various methods throughout history, ranging from the rocker or long tom of the California Forty-Niner to modern leaching techniques using cyanide. The cyanide process is very efficient, allowing for the recovery of 95% to 98% of the gold in the ore. Lime and cyanide are added to the ore pulp in the grinding circuit, with lime serving several functions, including protecting the cyanide from naturally occurring chemicals called cyanicides and improving the pulp’s settlement rate in the thickening stage. The actual dissolution of gold begins in the grinding step, where cyanide and lime solutions are introduced, and newly liberated gold particles are polished constantly by the grinding action while the solutions are heated by friction. Up to 70% of the gold may be dissolved during the grinding process, while additional time is needed to put the remaining liberated gold into solution. This is done by pumping the gold-bearing pulp to a number of mixing tanks known as agitators, where the pulp is aerated by mechanical or compressed air for a predetermined period of time.
In the 1980s, gold production from low-grade oxide deposits worldwide saw a rapid expansion that was made possible by the development of a new, low-cost method of recovering the gold known as heap leaching. Heap leaching avoids most of the above steps and does not require the construction of a mill, making it a very low-cost method of processing ore. In this process, broken ore is heaped onto a thick polyethylene sheet called a liner, and then a dilute cyanide solution is sprinkled on top of the heap. As the solution trickles down through the ore, the gold dissolves. Before constructing the heap, the polyethylene liner is laid down to ensure that the cyanide solution will drain to a central point. The gold-laden solution is then channeled into a man-made pond. However, one downside of heap leaching is its lower recovery rate of only 65% to 85% of the gold in the ore, which ends up in the gold bars a heap-leach mine produces.
Traditionally, recovering gold from cyanide solution involved separating the gold-laden, or pregnant, solution from the barren solids present and precipitating the gold. This approach is called the Merrill-Crowe method, which begins by moving the pulp from the agitators to one or more thickeners, large shallow tanks. The solution flows over the top of the tank and is collected in a launder around the tank’s perimeter, while the worthless rock particles sink to the bottom and are raked to the center by mechanical arms operating continuously. The material is discharged through a pipe at the bottom of the tank, but it contains too much valuable material to be discarded, so it is filtered to recover additional gold.