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Heap leaching

Heap leaching is an industrial mining process to extract precious metals and copper compounds from ore.

Process

The mined ore is crushed into small chunks and heaped on an impermeable plastic and/or clay lined leach pad where it can be irrigated with a leach solution to dissolve the valuable metals. Either sprinklers, or often drip irrigation, is used to minimize evaporation. The solution then percolates through the heap and leaches out the precious metal. This can take several weeks. The leach solution containing the dissolved metals is then collected.

Precious metals method

The crushed ore is irrigated with a dilute cyanide solution. The solution percolates through the heap and leaches out the precious metal. This can take several weeks.

The solution containing the precious metals ("pregnant solution") continues percolating through the crushed ore until it reaches the liner at the bottom of the heap where it drains into a storage (pregnant solution) pond. After separating the precious metals from the pregnant solution, the dilute cyanide solution (now called "barren solution") is normally re-used in the heap-leach-process or occasionally sent to an industrial water treatment facility where the residual cyanide is treated and residual metals are removed. The water is then discharged to the environment, posing possible water pollution.

During the extraction phase, the gold ions form complex ions with the cyanide: $Au^+ \left(s\right) + 2CN^- \left(aq\right) rightarrow Au\left(CN\right)_2^- \left(aq\right)$

Recuperation of the gold is readily achieved with a redox-reaction:

$2 Au\left(CN\right)_2^- \left(aq\right) + Zn \left(s\right) rightarrow Zn\left(CN\right)_4^- \left(aq\right) + 2 Au \left(s\right)$

Copper method

The method is similar to the cyanide method, above, except sulfuric acid is used to dissolve copper from its ores. The acid is recycled from the solvent extraction circuit (see solvent extraction-electrowinning, SX/EW) and reused on the leach pad. A byproduct is iron(II) sulfate, jarosite, which is produced as a byproduct of leaching pyrite, and sometimes even the same sulfuric acid that is needed for the process.

Although the heap leaching is a low cost-process, it normally has recovery rates of 60-70%, although there are exceptions. It is normally most profitable with low-grade ores. Higher-grade ores are usually put through more complex milling processes where higher recoveries justify the extra cost. The process chosen depends on the properties of the ore.

Sulfuric acid heap leaching of nickel

The method is an acid heap leaching method like that of the copper method in that it utilises sulfuric acid instead of cyanide solution to dissolve the target minerals from crushed ore. The method has been developed by European Nickel PLC for the rock laterite deposits of Turkey and the Balkans.