Leaching

(Redirected from Leaching (mobilization))

Definition

1. Leaching (mobilization) is the process when a solid material is dissolved into an aqueous solution. In other words, metals bound in minerals are transformed into metal ions that are released into an aqueous solution, i.e. immobilized metals become mobilized.
2. Extraction of metals from ores to aqueous solutions by means of chemicals.

Leaching occurs naturally but may also be applied as a preparatory step for metal recovery.

Rock which is unexposed and exposed to leaching. By courtesy of Barrie Johnson

Applied leaching

The process of extracting a component from a mixture by treating the mixture with a solvent which will dissolve the component but has no effect on the remaining portions of the mixture. Acid is commonly used to dissolve soluble metals contained in mineral ores. The resulting solution is subjected to further treatment to recover the desired metals. The non-soluble residue obtained is usually discarded.

Leaching can be done on primary raw materials like metal containing ores and concentrates or on secondary resources like metallic scrap or metal containing by-products. In many cases the raw materials are pre-treated by grinding, flotation, roasting etc. to enhance leaching yields and rates. Leaching is done with a number of different leaching reagents and methods. The metal rich aqueous solution obtained is subsequently processed for recovery of the metal.

Leaching can broadly be classified into two types, namely non-oxidative and oxidative leaching.

Non-oxidative leaching involves a chemical dissolution process using water, acid or an alkali as reagent. Some examples are the dissolution of oxidized copper ore (CuSiO3.2H2O) or zinc calcine (ZnO) using sulfuric acid (H2SO4) as reagent and the leaching of bauxite with sodium hydroxide (NaOH):

$CuSiO_3 . 2H_2O + H_2SO_4 \rightarrow CuSO_4 + SiO_2 + 3 H_2O$
$CuSiO_3 . 2H_2O + H_2SO_4 \rightarrow CuSO_4 + SiO_2 + 3 H_2O$
$ZnO + H_2SO_4 \rightarrow ZnSO_4 + H_2O$
$Al(OH)_3 + NaOH \rightarrow Na^+ + Al(OH)_4^-$

Oxidative leaching involves the use of oxidizing agents such as O2, Cl2, Fe3+, Cu2+ etc. For example Fe3+ is used as oxidant in the leaching of uranium from uraninite (UO2) and copper from chalcocite (Cu2S) respectively:

$UO_2 + 2Fe^{3+} \rightarrow UO_2^{2+} + 2Fe^2+$
$Cu_2S + 4Fe^{3+} \rightarrow 2Cu^{2+} + 4Fe^{2+}+S$

Under suitable conditions the ferric iron (Fe3+) can be regenerated by iron oxidising microorganisms as is practised in bioleaching operations.

Natural leaching

Leaching also occurs naturally, for example when rocks weather in contact with water resulting in the release of metals into the surroundings. One special case is the natural leaching of sulfide minerals. Leaching of sulfide minerals may be enhanced by microbes up to 100 000 times (Singer & Stumm, 1970)[1]. Without the microbes, natural leaching of sulfide minerals is a slow process.

Leaching Theory and Kinetics

Leaching is a heterogeneous reaction that takes place at the interface between a solid and liquid phase and sometimes also a gaseous phase. At the boundary between the two phases a diffusion layer is formed. In the case of a solid in an aqueous phase this layer consists of a stationary aqueous layer. The diffusion layer can be thinned by vigorous stirring but never be completely removed. Typical thickness of the diffusion layer in a well stirred system is in the range of 1-10 μm.

1. Diffusion of reagent through the diffusion layer
2. Adsorption of reagent on surface
3. Reaction on the surface
4. Desorption of product from surface
5. Diffusion of product through the diffusion layer

The slowest step in the leaching reaction is the rate-controlling step. Depending on which process is rate-controlling, three different type reactions may be obtained, i.e. reaction controlled leaching, diffusion controlled leaching and intermediate controlled leaching:

Factors affecting leaching kinetics

The equations arrived at for reaction controlled and diffusion controlled leaching have implications on operating costs. Such costs are for example size reduction by grinding, leaching temperature and agitation rate. Depending on the leaching mechanism, i.e. if the leaching process is reaction- or diffusion-controlled the leaching kinetics are influenced differently by variations of these parameters.

References

1. SINGER, P. C. & STUMM, W. (1970) Acid mine drainage: The rate determening step. Science, 167, 1121-1123.