Leaching is the process of extracting substances from a solid by dissolving them in a liquid, either naturally or through an industrial process. In the chemical processing industry, leaching has a variety of commercial applications, including separation of metal from ore using acid, and sugar from sugar beets using hot water.
In a typical leaching operation, the solid mixture to be separated consists of particles, inert insoluble carrier A and solute B. The solvent, C, is added to the mixture to selectively dissolve B. The overflow from the stage is free of solids and consists of only solvent C and dissolved B. The underflow consists of slurry of liquid of similar composition in the liquid overflow and solid carrier A. In an ideal leaching equilibrium stage, all the solute is dissolved by the solvent; none of the carrier is dissolved. The mass ratio of the solid to liquid in the underflow is dependent on the type of equipment used and properties of the two phases.
Leaching is the process by which inorganic, organic contaminants or radionuclides are released from the solid phase into the water phase under the influence of mineral dissolution, desorption, complexation processes as affected by pH, redox, dissolved organic matter and (micro)biological activity. The process itself is universal, as any material exposed to contact with water will leach components from its surface or its interior depending on the porosity of the material considered.
One such reaction is:
- Ag2S + 4NaCN → 2Na[Ag(CN)2] + Na2S
Leaching processes for biological substances
Many biological organic and inorganic substances occur in a mixture of different components in a solid. In order to separate the desired solute constituent or remove an undesirable solute component from the solid phase, the solid is brought into contact with a liquid. The solid and liquid are in contact, and the solute or solutes can diffuse from the solid into the solvent, resulting in separation of the components originally in the solid. This separation process is called liquid-solid leaching or simply leaching. Because in leaching the solute is being extracted from the solid this is also called extraction. In leaching, when an undesirable component is removed from a solid with water, the process is called washing.
In the biological and food processing industries, many products are separated from their original natural structure by liquid-solid leaching. An important process for example is the leaching of sugar from sugar beets with hot water. In the production of vegetable oils, organic solvents such as hexane, acetone, or ether are used to extract oil from nuts, beans, and seeds.
In the pharmaceutical industry, many different pharmaceutical products are obtained by leaching plant roots, leaves, and stems.
Leaching processes for inorganic and organic materials
Leaching is extensively used in metal processing industries. The useful metal may occur in mixtures with very large amounts of undesirable constituents, and leaching is used to remove the metals as soluble salts. The use of acids is prevalent in the metal processing industry. Sulfates are normally used to remove metals from the solid phase. These processes result in harmful sulfate-rich environmental byproducts. Because acids solubilize metals, large-scale release of heavy-metal-rich acidic mine drainage (AMD) can thereby occur.
As mentioned above when leaching a solid with a liquid the desired solid goes to the liquid phase while undesired solid remains. The removal of the solid as the liquid dissolves into the particle leads to a diameter of unleached core that shrinks with time. A mathematical model can be used for this process, derived using Fick's law of diffusion taking pore diffusion as rate limiting step:
Environmentally friendly leaching
Some recent work has been done to see if organic acids can be used to leach lithium and cobalt from spent batteries with some success. Experiments performed with varying temperatures and concentrations of malic acid show that the optimal conditions are 2.0 m/L of organic acid at a temperature of 90 °C. The reaction had an overall efficiency exceeding 90% with no harmful byproducts.
- 4 LiCoO2(solid) + 12 C4H6O5(liquid) → 4 LiC4H5O5(liquid) + 4 Co(C4H6O5)2(liquid) + 6 H2O(liquid) + O2(gas)
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