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, are 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.
The production of one gold ring, through this method, generated 20 tons of waste material.
During the extraction phase, the gold ions form complex ions with the cyanide:
Recuperation of the gold is readily achieved with a redox-reaction:
The method is similar to the cyanide method, above, except sulfuric acid is used to dissolve copper from its ores.
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.
^ Industrial England in the Middle of the Eighteenth Century, Nature, Vol, 83, No. 2113, Thursday, April 28, 1910; page 267.
^ Gage B. & G. 2008. American Outrage. USA: Human Rights Watch.
Heap leaching into groundwater is a major health concern from Rensselaer Polytechnic Institute school of engineering
European Nickel PLC official website
USGS 2005 Minerals Yearbook - Nickel
Categories: Metallurgical processesHidden categories: All articles with unsourced statements | Articles with unsourced statements from March 2008