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Leaching

Anam Saeed

engineering-resource.com

What is 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 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.


� 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 countercurrently to the adjacent stages. The solvent phase, called the extract, becomes more

concentrated as it contacts in stagewise fashion the increasingly solute-rich solid. The raffinate

becomes less concentrated in soluble material as it moves toward the fresh solvent phase.


Factors:

There are four 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

Solids" The solvent is contacted with the 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.


� 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.


� Extraction battery:

� Shanks process:

•Diffusion battery:


Moving Bed Leaching:


Dispersed Solid Leaching:


Principles Of Continuous

Counter Current Leaching:


Ideal Stages In Counter Current

Extraction:


�Equilibrium:

�Operating Line:

�Constant and Variable underflow:

�Number of Ideal stages for constant

underflow:


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