leaching equip2.pdf

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 proc...

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 The strongest solution will result if counter current scheme is used, wherein the final withdrawn solution is taken from contact with the freshest solid and the fresh solvent is added to solid from which most of the solute has been already leached or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. Shank System Continue….  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  Kennedy extractor  Continuous Countercurrent Decantation  Bollman extractor  Rotocell 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. Continue….  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 Decantaton  A simple arrangement is shown in figure. Contiue…  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 Continue…  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 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. Continue…  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 Continue…  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. Continue…  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 Extracter  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. Continue…  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  Single stage leaching Process flow Overflow solution Solvent Feed V1, x1 V 2, x 2 Feed Slurry Underflow solution L0, N0, y0, B L1, N1, y1, B V Mass of overflow solution xA Composition of A at overflow solution L Mass of liquid in slurry solution yA Composition of A at slurry solution B Mass of dry, solute – free solid. N Mass of dry,solute (B)/Mass of solution retained (L) Material balance is divided into 3 parts: L0  V2  L1  V1  M Totalsolution balance L0 y A0  V2 x A2  L1 y A1  V1 x A1  MxAM Comp. A balance B  N 0 L0  0  N1 L1  0  N M M Solid balance  Multi – stage counter current Leaching Process flow Overflow solution Solvent Feed V1, x1 VN+1, xN+1 Feed Slurry Underflow solution L0, N0, y0, B LN, NN, yN, B V Mass of overflow solution xA Composition of A at overflow solution L Mass of liquid in slurry solution yA Composition of A at slurry solution B Mass of dry, solute – free solid.  Multi – stage counter current Leaching The ideal stages are numbered in the direction of the solids or underflow stream. The solvent (C) – solute (A) phase or V phase is the liquid phase that overflows continuously from stage to stage countercurrently to the solid phase, and it dissolves solute as it moves along. The slurry phase L composed of inert solid (B) and liquid phase of A and C is the continuous underflow from each stage. Composition of V – denoted by x Composition of L – denoted by y Assumption: The solid B is insoluble and is not lost in the liquid V phase. The flow rate of solid is constant throughout the process Multi – stage counter current Leaching L0  VN 1  LN  V1  M Totalsolution balance L0 y A0  VN 1 x AN 1  LN y AN  V1 x A1  MxAM Comp. A balance B  N 0 L0  N N LN  N M M Solid balance