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Presentation On Leaching

1. Presentation report Name : Javiya Smit A. Enrollment : 180470105019 Department : Chemical Subject : Mass Transfer Operation Report on Leaching

2. Leaching Leaching is a solid-liquid operation The two phases are in intimate contact, the solute(s) ca n d i f fu s e f r om t h e s o l id t o t he liquid p h ase, whi c h cau s es a s e parati o n o f t h e c o m p o nen t s o r i g i nally in t h e s o li d . A special leaching process, when an undesirable component is removed from a solid with water, is called Washing

3. Application It is used in the biological and food processing industries . Ex : the separation of sugar from sugar beets with hot water, the extraction of oils from peanuts, soybeans, sunflower seeds, cotton seeds. 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. Ex : gold leaching, gold is leached from its ore using an aqueous sodium cyanide solution.

4. Principles of Leaching The solvent must be transferred from the bulk solvent solution to the surface of the solids. Next, the solvent must penetrate or diffuse into the solids. The solute then diffuses through the solid solvent mixture to the surface of the particle. Finally, the solute is transferred to the bulk solution. The rate of the solvent transfer from the bulk solution to the solid surface is quite rapid. However, the rate of transfer of the solvent into the solid can be rather slow or rapid. This solvent transfer usually occurs initially when the particle are first contacted with the solvent.

5. The rate of diffusion of the solute through the solid and solvent to the surface of the solid is often the controlling resistance in the overall leaching process and can depend on a number of different factors. If the solid is made of porous the diffusion through the porous solid can be described by an effective diffusivity. The resistance to mass transfer to the solute from the solid surface to the bulk solvent is generally quite small compared to the resistance to the diffusion within the solid itself.

6. Types of leaching Unsteady state operations In Situ (In-Place) Leaching Heap Leaching Percolation Tanks Shanks System Filter press leaching Steady state (Continuous ) operations Agitated Vessels Thickeners Continuous Countercurren Decantation (CCD) Hydrocyclones Filter leaching

7. Unsteady State Leaching In Situ (In-Place) Leaching : • Also called as solution mining. • The percolation leaching of minerals in place at the mine, by circulation of solvent over the ore. • Removal of salts from deposits below earth surface by the solution of salt water which pumped into the deposit. • Example: Leaching of Low-grade copper ores, uranium ores .

8. 2) Heap Leaching : Low-grade ores whose minerals values do not warrant the expense of crushing or grinding can be leached in the form of run of mine lumps into huge piles. Examples: Copper from pyritic ores, uranium. 3) Percolation tank Leaching : When the pressure drop for flow of liquid is too high for gravity flow, closed vessels must be used. (Diffusers) Closed tanks are necessary to prevent evaporation losses when the solvent is volatile. Examples: Sugar from sugar-beet slices.

9. Percolation tank Leaching

10. 4) 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.

11. Applications of Shanks system (Extraction Battery ): Recovery of tannins from tree barks and woods. Leaching of Sodium-nitrate from Chilean-nitrate bearing rock. Used in Metallurgical industries

12. This shank system is operated in following manner: 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. 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. 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. Continue the operation in the same manner as before .

13. Leaching equipments Agitated vessels Dorr thickeners Continuous counter-current decantation Hydrocyclones Rotocel Kennedy extractor Bollman extractor Continuous horizontal extractor

14. Agitated Vessels Finely ground solids can be readily suspended in liquids by agitation be continuously leached in any type of agitated tanks or vessels. There is a continuous flow of liquid and solid into and out of the tank, so that no accumulation of solid occurs. Turbine type agitator is used for effective operations. Types: Pachuca tanks and Dorr agitators (air-lift and mechanical principle). Central shaft acts as an air lift and revolves slowly.

15. Arms attached to the shaft bottom moves the settled solids towards the center, where they lifted by air through the shaft to the revolving launders attached to the top.

16. Kennedy extractor

17. 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 leachingoperations . 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 . 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.

18. Continuous counter-current decantation A simple arrangement is shown in figure:

19. 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 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.

20. Bollman Extractor

21. 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 . 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 .

22. 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. 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 .

23. 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. 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.

24. Rotocel 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 .

25. 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.