Leaching reagents

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Leaching agents are chosen based on a number of factors such as, chemical and physical properties, cost, corrosion properties, selectivity, and possibility to regenerate. Commonly used reagents are water, acids and alkali. Sometimes metal salts and oxidants like oxygen are used to facilitate dissolution.

  • Water: Is the cheapest of all reagents but has limited use since not many minerals are water soluble. It is mainly used for dissolving naturally occurring sodium and potassium salts as carbonates, chlorides, sulfates and nitrates. It is also used for leaching of calcines obtained after sulfatizing and chloridizing roasting.
  • Acids: Are the most commonly used leaching agents and especially sulfuric acid (H2SO4) is used due to low cost and possibilities to regenerate during electrolysis. Other used acids include hydrochloric acid (HCl), nitric acid (HNO3) and hydrofluoric acid (HF). Aqua regia, a 3:1 mixture of concentrated HCl and HNO3, is used in gold refining because it can solubilise gold.
  • Bases: Sodium hydroxide (NaOH) is mainly used for leaching of bauxite in aluminium production. Ammonia (NH3) is sometimes used for leaching of copper, nickel and cobalt due to their tendency to form soluble ammonia complexes.
  • Salts: Salts are commonly used in leaching due to their complexing abilities. Examples are sodium cyanide (NaCN), sodium and calcium chloride (NaCl, CaCl) and sodium carbonate (NaCO3).
  • Oxidants: Air or pure oxygen, hydrogen peroxide (H2O2), ferric ion (Fe3+) and chlorine gas (Cl2) are used when reduced compounds are needed to be oxidised during leaching.
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