Mass transfer operation Topic : Leaching

Leaching

Leaching is a preferential solution of one or more constituents of solid mixture by contact with a liquid solvent. This unit operation is one of the oldest in the chemical industries.

 

Industrial application Leaching is widely used in the biological and food processing

industries, such as the separation of sugar from sugar beets with hot water.

The extraction of oils from peanuts, soybeans, sunflower seeds, cotton seeds, and halibut livers.

In pharmaceutical industry, many products are obtained by leaching plant roots, leaves, and stems.

In the metals processing industry, leaching is used to remove the metals from their ores, which contains many undesirable constituents, as solute salts.

In gold leaching, gold is leached from its ore using an aqueous sodium cyanide solution.

Shank system

Leaching and washing of the leached solute from percolation tank by crosscurrent methods results in weak solutions of the solute. The strongest solution will result if counter current scheme is used, wherein the final withdrawn solution is taken from contact with the freshest solid and the fresh solvent is added to solid from which most of the solute has been already leached or washed.

Shanks system

This shank system is operated in following manner:1. Assume at the time of inspecting the system at figA

that it has been in operation for some time. Tank 6 is empty, tanks 1 to 5 are filed with solid and with leached liquid and tank 5 is n contact with freshest solid. Fresh solvent is added to tank 1. 

2. Withdraw the concentrated solution from tank 5, transfer the liquid from tank 4 to tank 5, 3 to 4, 2 to 3, 1 to 2. Add fresh solid to tank 6.

3. Refer figB . Discard the spent sold frm tank 1. Transfer the liquid from tank 5 to 6, 4 to 5, 3 to 4, 2 to 3,. Add fresh solvent to tank 2.

4. Continue the operation in the same manner as before. 

Leaching equipments

Agitated vessels Dorr thickeners Continuous counter-current decantation Hydrocyclones Rotocel Kennedy extractor Bollman extractor Continuous horizontal extractor

Kennedy extractor

Kennedy extractor

The kennedy extractor, a modern arrangement of which is indicated schematically in figure is another stagewise device which has been in use since 1927,originally for leaching tannins from tanbark.it is now used for oilseed and other chemical leaching operations.

The solids are leached in a series of tubs and are pushed from one to the next in the cascade by paddles, while the solvent flows in counter-current.

Kennedy extractor

Perforation in the paddles permit drainage of the solids between stages and the solids are scraped from each paddle as shown.

As many tubs may be placed in a cascade are as require.

Continuous Countercurrent Decantation :

A simple arrangement is shown in figure.

Continuous Counter-current Decantation :

The solids to be leached, together with solution from the second thickner are introduced into the leaching agitators at the left and the strong solution thus produced is decanted from the solids by the first thickener.

The agitators together with the first thickener then constitute a single stage.

The sludge is passed through the cascade to be washed by the solvent in true counter-current fashion and the washed solids are discharge at the right

Continuous Counter-current Decantation :

There may, of course be more or fewer than the four stages shown, and the agitators may be replaced by any continuous leaching device, such as a grinding mill.

Many variations in the flowsheet are regularly made.

Bollman extractor

Bollman extractor

Bollman Extractor is mainly used for the recovery of additional oil from the residues obtained after mechanical pressing of solids.

The Bollman Extractor essentially consists of a vapour tight vertical chamber in which a series of perforated baskets are attached to a chain conveyor.

Bollman extractor

The baskets are provided with perforations at the bottom.

At the top right hand corner of the extractor solids are conveyed in to the perforated baskets. The baskets are loaded with flaky solids.

These fully loaded baskets are then sprayed with Half Micelle as they travel downward through the right hand side.

Half Miscella is the intermediate solvent containing some extracted oil and some small solid particles.

Bollman extractor As the solids and solvent flow co-currently down the

right hand side of the machine, the solvent extracts more oil.

Simultaneously the fine solids are filtered out of the solvent, so that the clean full miscella can be pumped from the outlet provided at the right hand bottom.

Then as these partially extracted solids rise through the left hand side of the machine a stream of pure solvent is sprayed on them to obtain a dilute solution of the oil (half miscella) at the bottom of the chamber.

Bollman extractor

This pure solvent percolates counter-currently through them and collects in the left hand sump which is then passed to the Half Miscella storage tank.

The fully extracted solids are then dumped from the baskets at the top of the elevator to the hoppers from which they are removed by Screw Conveyors.

 Usually the conveyor speed is one revolution per hour.

Rotocell extractor

In the Rotocel extractor,illusrated in figure b,

A horizontal basket is divided into walled compartments with a floor that is permeable to the liquid.The basket rotates slowly about a vertical axis.

Rotocell extractor

Solids are admitted to each compartment at the feed point;the compartments then successively pass a number of solvent sprays, a drainage section, and a discharge point at which the floor of the compartment opens to discharge the extracted solids.The empty compartment moves to the feed to point to receive its next load of solids.

To give countercurrent extraction, fresh solvent is fed only to the last compartment before the discharge point, and the solids in each preceeding compartment are washed with the effluent from the succeeding one.

Thank you