Leaching Equipments PDF

Leaching Leaching is a solid-liquid operation. The two phases are in intimate contact, the solute(s) can diffuse from the solid to the liquid phase, which causes a separation of the components originally in the solid. A special leaching process, when an undesirable...

Leaching Leaching is a solid-liquid operation. The two phases are in intimate contact, the solute(s) can diffuse from the solid to the liquid phase, which causes a separation of the components originally in the solid. A special leaching process, when an undesirable component is removed from a solid with water, is called washing.  Leaching Applications:  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.  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.  Principles of Leaching 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.  Types of Leaching Unsteady state operations Steady state (Continuous) – In Situ (In-Place) Leaching operations – Heap Leaching – Agitated Vessels – Percolation Tanks – Thickeners – Shanks System – Continuous Countercurrent – Filter press leaching Decantation (CCD) – Agitated vessels – Hydrocyclones – Filter leaching  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.  Unsteady State Leaching 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.  Unsteady State Leaching 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.  Unsteady State Leaching Percolation tank Leaching:  Steady State Leaching 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. 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.  Steady State Leaching Continuous Countercurrent Decantation (CCD): a) Simple flowsheet a) Flowsheet with intermediate agitation and filtration of washed solids  Steady State Leaching BOLLMAN EXTRACTOR: Contains a bucket elevator in a closed casing. The buckets are loaded with flaky solids such as soybeans. The solids are sprayed with appropriate amount of half miscella as they travel downward. Half miscella is the intermediate solvent containing some extracted oil and some small solid particles. As solids and solvent flow concurrently down the right-hand side of the machine, the solvent extracts more oil from the beans.  Steady State Leaching BOLLMAN EXTRACTOR:  Steady State Leaching ROTOCEL EXTRACTOR: 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. Solid are admitted to each compartment at feed point. The compartments then pass a number of solvent sprays, a drainage section and a discharge point. To give countercurrent extraction, the fresh solvent is fed only to the last compartment before the discharge point.  Steady State Leaching ROTOCEL EXTRACTOR:  Steady State Leaching KENNEDY EXTRACTOR:  Steady State Leaching CONTINUOUS HORIZONTAL EXTRACTOR: