Leaching percolation type

Moving-bed percolation systems are used for extraction from many types of ceUular particles such as seeds, beans, and peanuts (see Nuts). In most of these cases organic solvents are used to extract the oils from the particles. Pre-treatment of the seed or nut is usually necessary to increase the number of ceUs exposed to the solvent by increasing the specific surface by flaking or rolling. The oil-rich solvent (or misceUa) solution often contains a small proportion of fine particles which must be removed, as weU as the oil separated from the solvent after leaching. 

Screw-Conveyor Extractors One type of continuous leaching equipment, employing the screw-conveyor principle, is strictly speaking neither a percolator nor a dispersed-solids extractor. Although it is often classed with percolators, there can be sufficient agitation of the solids during their conveyance by the screw that the action differs from an orthodox percolation. 

Infiltration and percolation rates also determine which salts have been leached out of the soil. For instance, high infiltration and percolation rates leach calcium and magnesium out of soil and they become acidic. Where calcium and magnesium are not leached out, the soils are neutral or basic. Thus, the type and amount of salts present will affect a soil s pH, which will in turn affect the solubility and availability of both natural and contaminating inorganic and organic compounds. 

The different systems of leaching may be listed as follows (1) Leaching by percolation which means flowing the solution past the stationary solids and separation of the solution thus obtained from the residues or undissolved material. (2) Leaching by agitation in which dissolution is obtained while the solids are held in suspension in the solvent and a certain amount of relative motion is maintained and separation of the solution later by decantation or filtration. Each type listed above will be discussed showing the equipment involved, comparative costs, and its applications in industrial problems. 

The Kennedy extractor (Fig. 18-87), also requiring little head-room, operates substantially as a percolator that moves the bed of solids through the solvent rather than the conventional opposite. It comprises a nearly horizontal line of chambers through each of which in succession the solids being leached are moved by a slow impeller enclosed in that section. There is an opportunity for drainage between stages when the impeller lifts sohds above the liquid level before dumping them into the next chamber. Solvent flows countercur-rently from chamber to chamber. Because the solids are subjected to mechanical action somewhat more intense than in other types of... 

Normal superphosphate or triple superphosphate (common commercial fertilizers) are cheap sources of water-soluble phosphate. Normal superphosphate is primarily a mixture of monocalcium phosphate and calcium sulfate (gypsum), while triple superphosphate is essentially all monocalcium phosphate. Monosodium phosphate was prepared from superphosphate by first leaching superphosphate with sea water until a saturated solution of monocalcium phosphate was obtained. Then the monocalcium phosphate solution in sea water was percolated through a column of Dowex 50 (strongly acidic type resin) in the sodium form. The effluent from the column was a solution of monosodium phosphate in sea water and the resin was converted to the calcium form as shown by Equation 6. 

There are few data that show accurately the amount of nitrogen that leaches through soils under field conditions. Most of the data that are available have necessarily been obtained from lysimeters. These are of two kinds, the filled-in and the monolith types. In the filled-in type the soil is removed from the field, thoroughly mixed by horizons, and placed in tanks in the natural order. The tanks commonly vary from 1 to 5 ft. in diameter and may be 1.5 to 6 ft. in depth. In the other less common monolith lysimeter the sod profile is left undisturbed and is surrounded by a metal or concrete wall. The soil may be removed intact from its original location, or it may merely be surrounded by a wall and left in place. In both types of lysimeters the percolation is collected and analyzed at frequent intervals. The lysimeter method is obviously an artificial system that has been subject to many criticisms. Nevertheless, it has furnished much valuable information, especially on the effect of comparative treatments, but seldom can the data be considered in a quantitative way as a measure of what is happening under field conditions. Since Allison (1955, 1965) has summarized the results obtained in several lysimeter experiments, the subject need not be considered further in the present discussion. 

Various types of leaching can be undertaken - direct extraction, percolation, pulsed flow extraction, and countercurrent extraction. 

The BoHman extractor [38] (Fig. 13.20) is one of several basket-type machines. Solids are conveyed in perforated baskets attached to a chain conveyor, down on the right and up on the left in the figure. As they descend, they are leached in parallel flow by a dilute solvent-oil solution ha f miscelld) pumped from the bottom of the vessel and sprayed over the baskets at the top. The liquid percolates through the solids from basket to basket, collects at the bottom as the final strong solution of the oil (/ // miscella)y and is removed. On the ascent, the solids are leached countercurrently by a spray of fresh solvent to provide the half miscella. A short drainage time is provided before the baskets are dumped at the top. There are many variants of this device, e.g., the horizontal arrangement of Fig. 13.21. 

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