What Is Heap Leaching?

Heap leaching is a method of spraying a ore heap with a leaching solution to selectively extract the useful components in the ore during the infiltration process and recover the useful components from the rich liquid flowing out of the bottom of the heap. According to the ore grade, it can be divided into ore heap leaching and waste rock heap leaching; according to the location of the yard, it can be divided into surface heap leaching and underground heap leaching. The heap leaching method is mainly used in the mining of copper, uranium, gold and silver ore, as well as the treatment of smelters and tailings of smelters containing ingredients. The heap leaching method has simple process, less equipment, and low energy consumption, so the capital investment and production cost are also low. Its main disadvantage is that the leaching rate of useful components in the ore is low. ^[1]

A mineral leaching process. This method allows the leaching agent to penetrate into the ore heap and dissolve the useful components. Spain is the earliest country to use heap leaching technology. In 1752, copper oxide ore was leached by this method. China used this process to process low-grade uranium ore in the late 1950s. In 1967, the US Bureau of Mines began to process low-grade gold ore with heap leaching. Heap leaching mainly refers to ore heap leaching and waste rock heap leaching. In situ leaching also belongs to the heap leaching category. Industrial heap leaching is mainly used for leaching low-grade uranium ore, copper oxide ore and gold ore.

In situ leaching is a method of directly leaching the target component in the ore body using diafiltration. During the operation, drilling holes (divided into injection holes and recovery holes) are divided on the surveyed mining surface. The leaching agent is injected into the ore body from the injection hole, and the useful components in the ore are leached by the pore fissure capillary action of the ore body. The recovery hole pumps the leachate to the ground for further processing. In situ leaching can save operations such as well construction, mining, transportation, crushing, grinding, physical beneficiation, and solid-liquid separation. The leaching operation can be moved to the underground ore body, and tailings can be discarded and stored in situ, which has obvious economic benefits and Protecting environmental benefits. However, the in-situ leaching has strict requirements on the orebody formation conditions. The orebody should have good permeability. There should be a corresponding impermeable layer around the orebody. The bedrock is stable and the groundwater level is low, which is conducive to the recovery of the leachate. Due to the many constraints of in-situ leaching, it is mainly used to recover copper and uranium from the residual mines in underground goafs. In the mid-1980s, China used the in-situ leaching method to leaching uranium ore bodies after blasting, which greatly reduced production costs.

The leaching solution sprayed intermittently on the ore pile did not fill the gap between the ore blocks, and only formed a thin film of leaching solution on the surface of the ore blocks. During heap leaching, useful components in the ore are leached through convection diffusion (when spraying) and molecular diffusion (when spraying is stopped), merge into the rich liquid stream, and flow out from the bottom of the heap. The leaching process can be divided into three stages: in the first stage, the sprayed leachate adheres to the surface of the ore block and diffuses inward along the crevices and pores of the ore, extruding the fissures and pore water; in the second stage, the leaching solution and When the useful components in the ore come into contact, a chemical reaction occurs to form a soluble compound or complex; in the third stage, because its concentration is higher than that on the surface of the ore block, it moves toward the surface of the ore block along cracks and pores through molecular diffusion, and Under the action of gravity and convection diffusion, it moves downward from the surface of the ore and sinks into the rich liquid stream.

The heap leaching process includes preparation of ore, construction of leaching mats, stacking, preparation of leaching solution, distribution and collection of liquid, and processing of rich liquid (noble liquid).

Heap leaching was first used for copper ore. As early as the Western Han Dynasty, there were records of oxidizing acid copper ore with water and then replacing copper with iron. In 1752, the Rio Tinto copper mine in Spain performed a heap leaching production test. In 1941, the United States began to mine copper-depleted ore with heap leaching; since the 1960s, heap leaching of uranium and gold and silver mines. In 1982, gold and silver produced by the heap leaching method in the United States accounted for 20% and 10% of the total output. Most of France's uranium comes from heap leaching.

With the gradual reduction of rich ore suitable for conventional mining methods, because the production cost of heap leaching is low, and its process technology is increasingly perfect, the scale of heap leaching production will continue to expand, and the scope of application of heap leaching will also be more and more. widely. ^[2]