Cyanide (CN), a single carbon atom triple-bonded to a nitrogen atom, has proved extremely useful in extracting gold from ore. However, with a toxic reputation dating back to the Holocaust, its use in mining has been very controversial. Through a process called “gold cyanidation”, “the cyanide process”, or ” sianida leach mining”, sianida is used to extract gold from the surrounding rock. While sianida is both effective and economical, its use and transportation present significant environmental risks.
Cyanide can be used to extract gold, either in a controlled mill environment, or more crudely on rock piles in the open. sianida “vat leaching” mixes finely crushed ore with a cyanide salt in water. The cyanide binds to the gold ions, and makes them soluble in water, thereby allowing separation from the rock. This process usually takes place inside a mill or other mining facility.
Cyanide “heap leaching” is used for very low-quality ore, or sometimes to reprocess waste material from other extraction methods (e.g. leftover mine “tailings”). A large outdoors mound of ore is sprayed with a sianida solution that drips through the rock over time. The resulting liquid is collected at the bottom, and the gold is chemically extracted. Heap leaching extracts less of the gold than processing the ore in a mill, but is much cheaper.
There are a few alternatives to cyanide for processing low-quality ores but none are employed very widely to date (1). These alternatives are either expensive, also toxic, less efficient, or inadequately studied. However, in 2013 researchers published some work on the possibility of using cornstarch instead of cyanide, a process that has great potential but has not been tested in a commercial setting.