What is Leaching: Leaching: Leaching generally refers to the removal of a substance from a solid via a liquid extraction media. The desired component diffuses into the solvent from its natural natural solid form. Examples of leaching include the removal of sugar from sugar beets with hot water and the removal of nickel salts or gold from their natural solid beds with sulfuric acid solutions.
Process:
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.
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This leaching is essentially countercurrent. In industrial leaching, solvent and solid are mixed, allowed to approach equilibrium, and the two phases are separated. Liquid and solids move counter currently to the adjacent stages. The solvent phase, called the extract, becomes more concentrated as it contacts in stage wise fashion the increasingly solute-rich solid. The raffinate becomes less concentrated in soluble material as it moves toward the fresh solvent phase.
Leaching Types
Leaching (agriculture): The loss of water-soluble plant nutrients from the soil; or applying a small amount of excess irrigation to avoid soil salinity.
Leaching (chemistry): The process of extracting minerals from a solid by dissolving them in a liquid.
Leaching (metallurgy): A widely used extractive metallurgy technique which converts metals into soluble salts in aqueous media
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Leaching (pedology): The loss of mineral and organic solutes due to percolation from soil. Bioleaching: The extraction of specific metals from their ores through the use of bacteria.
Factors: There are few important factors that aid in leaching: Temperature Contact Time/Area Solvent Selection Particle size
Temperature is adjusted to optimize solubility and mass transfer. Liquid-to-solid contact is essential for the extraction to take place and maximize contact area per unit volume reduces equipment size. Solvent selection plays an important role in solubilities as well as the separation steps that follow leaching. Nearly all leaching equipment employs some type of agitation to aid in mass transfer and to ensure proper mixing.
There are many different types of equipment used for leaching. Most of these pieces of equipment fall into one of two categories:
Percolation....."Liquid Added to The solvent is contacted with the Solids" solid in a continuous or batch method. This method is popular for in-place ore leaching or large scale "heap" leaching. Popular for extreme amounts of solids.
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Dispersed Solids....."Solids Added to Liquid" The solids are usually crushed into small pieces before being contacted with solvents. This is a popular leaching method when an especially high recovery rate can economically justify the typically higher operating cost (Ex/ gold extraction)
Leaching Equipments: Two types on the basis of nature of solids: When the solid form an open permeable mass throughout the leaching operation. Solvent may be percolated through an unagitated bed of solids. With impermeable solids or materials that disintegrate during leaching, the solids are dispersed into the solvent and later separated from it.
Leaching by percolation: Two types: Through stationary solid beds Through Moving bed leaching
Through stationary solid beds
Is done in a tank with a perforated false bottom to support the solids and permit drainage of the solvent. In some cases the rate of solution is so rapid that 1 passage of solvent is sufficient otherwise battery of tanks is used.
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Extraction battery: Shanks process:
Diffusion battery:
Moving Bed Leaching:
Dispersed Solid Leaching:
Principles Of Continuous Counter Current Leaching:
Ideal Stages In Counter Current Extraction:
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Equilibrium: Operating Line: Constant and Variable underflow: Number of Ideal stages for constant underflow:
