Heap leaching is an
industrialAn industry is the manufacturing of a good or service within a category. Although industry is a broad term for any kind of economic production, in economics and urban planning industry is a synonym for the secondary sector, which is a type of economic activity involved in the manufacturing of raw...
miningMining is the extraction of valuable minerals or other geological materials from the earth, usually from an ore body, vein or seam. Materials recovered by mining include base metals, precious metals, iron, uranium, coal, diamonds, limestone, oil shale, rock salt and potash...
process to extract precious metals and
copperCopper is a chemical element with the symbol Cu and atomic number 29.It is a ductile metal with very high thermal and electrical conductivity. Pure copper is rather soft and malleable and a freshly-exposed surface has a pinkish or peachy color...
compounds from
oreAn ore is a type of rock that contains minerals such as gemstones or metals that can be extracted through mining and refined for use. Samples of ore in the form of exceptionally beautiful crystals, exotic layering visible when sectioned or polished or metallic presentations such as large nuggets or...
.
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
solutionIn chemistry, a solution is a homogeneous mixture composed of two or more substances. In such a mixture, a solute is dissolved in another substance, known as a solvent. Gases may dissolve in liquids, for example, carbon dioxide or oxygen in water. Liquids may dissolve in other liquids. Gases can...
to dissolve the valuable metals. Either sprinklers, or often drip irrigation, are used to minimize
evaporationEvaporation is the vaporization of a liquid and the reverse, of condensation. A type of phase transition, it is the process by which molecules in a liquid state spontaneously become gaseous . Generally, evaporation can be seen by the gradual disappearance of a liquid from a substance when exposed...
. The solution then percolates through the heap and leaches out the precious metal.
Heap leaching is an
industrialAn industry is the manufacturing of a good or service within a category. Although industry is a broad term for any kind of economic production, in economics and urban planning industry is a synonym for the secondary sector, which is a type of economic activity involved in the manufacturing of raw...
miningMining is the extraction of valuable minerals or other geological materials from the earth, usually from an ore body, vein or seam. Materials recovered by mining include base metals, precious metals, iron, uranium, coal, diamonds, limestone, oil shale, rock salt and potash...
process to extract precious metals and
copperCopper is a chemical element with the symbol Cu and atomic number 29.It is a ductile metal with very high thermal and electrical conductivity. Pure copper is rather soft and malleable and a freshly-exposed surface has a pinkish or peachy color...
compounds from
oreAn ore is a type of rock that contains minerals such as gemstones or metals that can be extracted through mining and refined for use. Samples of ore in the form of exceptionally beautiful crystals, exotic layering visible when sectioned or polished or metallic presentations such as large nuggets or...
.
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
solutionIn chemistry, a solution is a homogeneous mixture composed of two or more substances. In such a mixture, a solute is dissolved in another substance, known as a solvent. Gases may dissolve in liquids, for example, carbon dioxide or oxygen in water. Liquids may dissolve in other liquids. Gases can...
to dissolve the valuable metals. Either sprinklers, or often drip irrigation, are used to minimize
evaporationEvaporation is the vaporization of a liquid and the reverse, of condensation. A type of phase transition, it is the process by which molecules in a liquid state spontaneously become gaseous . Generally, evaporation can be seen by the gradual disappearance of a liquid from a substance when exposed...
. 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
cyanideA cyanide is any chemical compound that contains the cyano group , which consists of a carbon atom triple-bonded to a nitrogen atom. Inorganic cyanides are hydrogen cyanide salts in which cyanide is generally the anion CN
-. Organic compounds that have a -C≡N functional group bonded to...
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 solutionPregnant Leach Solution is acidic copper-laden water generated from stockpile leaching and heap leaching. Pregnant Leach Solution is used in the SX/EW process....
") 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 treatmentIndustrial Water Treatment can be classified into the following categories:* Boiler water treatment* Cooling water treatment* Wastewater treatment...
facility where the residual cyanide is treated and residual metals are removed. The water is then discharged to the environment, posing possible
water pollutionWater pollution is the contamination of water bodies such as lakes, rivers, oceans, and groundwater. All water pollution affects organisms and plants that live in these water bodies and in almost all cases the effect is damaging either to individual species and populations but also to the natural...
.
During the extraction phase, the gold ions form complex ions with the cyanide:
Recuperation of the gold is readily achieved with a
redoxRedox describes all chemical reactions in which atoms have their oxidation number changed....
-reaction:
Copper method
The method is similar to the cyanide method, above, except
sulfuric acidSulfuric acid, , is a strong mineral acid. It is soluble in water at all concentrations. Sulfuric acid has many applications, and is one of the top products of the chemical industry. World production in 2001 was 165 million tonnes, with an approximate value of US$8 billion...
is used to dissolve
copperCopper is a chemical element with the symbol Cu and atomic number 29.It is a ductile metal with very high thermal and electrical conductivity. Pure copper is rather soft and malleable and a freshly-exposed surface has a pinkish or peachy color...
from its ores. The acid is recycled from the
solventA solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution.The most common solvent in everyday life is water. Most other commonly-used solvents are organic chemicals. These are called organic solvents...
extraction circuit (see solvent extraction-electrowinning, SX/EW) and reused on the leach pad. A byproduct is
iron(II) sulfateIron sulfate or ferrous sulfate is the chemical compound with the formula , known since ancient times as copperas...
,
jarositeJarosite is a basic hydrous sulfate of potassium and iron with a chemical formula of KFe362. This sulfate mineral is formed in ore deposits by the oxidation of iron sulfides...
, which is produced as a byproduct of leaching
pyriteThe mineral pyrite, or iron pyrite, is an iron sulfide with the formula FeS
2. This mineral's metallic luster and pale-to-normal, brass-yellow hue have earned it the nickname fool's gold due to its resemblance to gold...
, 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
profitableIn economics, economic profit is the difference between a company's total revenue and its opportunity costs. It is the increase in wealth that an investor has from making an investment, taking into consideration all costs associated with that investment including the opportunity cost of...
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.
See also
- Gold cyanidation
Gold cyanidation is a metallurgical technique for extracting gold from low-grade ore by converting the gold to water soluble aurocyanide metallic complex ions. It is the most commonly used process for gold extraction...
- Gold extraction
Gold extraction or recovery from its ores may require a combination of comminution, mineral processing, hydrometallurgical, and pyrometallurgical processes to be performed on the ore....
- In-situ leach
In-situ leaching , also called in-situ recovery or solution mining, is a process of recovering minerals such as copper and uranium through boreholes drilled into the deposit. The process initially involves drilling of holes into the ore deposit. Explosive or hydraulic fracturing may be used to...
- Tailings
Tailings are the materials left over after the process of separating the valuable fraction from the worthless fraction of an ore....
- Mineral processing
Mineral processing, otherwise known as mineral dressing, is the practice of beneficiating valuable minerals from their ores.Industrial mineral treatment processes usually combine a number of unit operations in order to liberate and separate minerals by exploiting the differences in physical...
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