Percolation rates

The sohd can be contacted with the solvent in a number of different ways but traditionally that part of the solvent retained by the sohd is referred to as the underflow or holdup, whereas the sohd-free solute-laden solvent separated from the sohd after extraction is called the overflow. The holdup of bound hquor plays a vital role in the estimation of separation performance. In practice both static and dynamic holdup are measured in a process study, other parameters of importance being the relationship of holdup to drainage time and percolation rate. The results of such studies permit conclusions to be drawn about the feasibihty of extraction by percolation, the holdup of different bed heights of material prepared for extraction, and the relationship between solute content of the hquor and holdup. If the percolation rate is very low (in the case of oilseeds a minimum percolation rate of 3 x 10 m/s is normally required), extraction by immersion may be more effective. Percolation rate measurements and the methods of utilizing the data have been reported (8,9) these indicate that the effect of solute concentration on holdup plays an important part in determining the solute concentration in the hquor leaving the extractor. 

Immersion extraction systems are usefiil in handling finely ground material or when the percolation rate through the material to be extracted is too rapid to aUow effective diffusion from the soHds. These systems are appHed extensively in the sugar industry, in extraction from oilseeds having a high oil... 

Whatever the mechanism and the method of operation, it is clear that the leaching process will be favored by increased surface per unit volume of sohds to be leached and by decreased radial distances that must be traversed within the solids, both of which are favored by decreased particle size. Fine solids, on the other hand, cause slow percolation rate, difficult solids separation, and possible poor quahty of sohd product. The basis for an optimum particle size is estabhshed by these characteristics. 

Batch Percolators The batch tank is not unlike a big nutsche filter it is a large circiilar or rectangiilar tank with a false bottom. The solids to be leached are dumped into the tank to a uniform depth. They are sprayed with solvent until their solute content is reduced to an economic minimum and are then excavated. Countercurrent flow of the solvent through a series of tanks is common, with fresh solvent entering the tank containing most nearly exhausted material. In a typical ore-dressing operation the tanks are 53 by 20 by 5.5 m (175 by 67 by 18 ft) and extract about 8200 Mg (9000 U.S. tons) of ore on a 13-day cycle. Some tanks operate under pressure, to contain volatile solvents or increase the percolation rate. A series of pressure tanks operating with countercurrent solvent flow is called a diffusion battery. 

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. 

Table 2.1 Effect of puddling on percolation rates in a range of flooded Philippine soils...

Water regime, particularly where there is mid-season drainage allowing escape of entrapped CH4 but also oxidation of the soil. The water regime is also affected by soil texture as this affects percolation rates high percolation rates tend to decrease emissions because less reducing conditions are maintained in the soil. Texture may also affect gas entrapment and ebullition. 

The various factors that influence percolation leaching were examined by Seidel (S6). The percolation flow rate through the ore bed is very dependent on the permeability or texture of the bed. It decreases with increase in air or gas bubbles present in the bed and can be increased by increasing the temperature of the leaching solution. The size distribution of the particle, the porosity, temperature, leaching solution concentration, and percolation rates influence the mass-transfer rate. 

The perception of glacially slow movement is not shared by USDA soil chemist Robert Bowman (26). His research in Arizona of pesticide percolation rates indicates that pesticides applied to both wet and dry soils move through these soils from three to five times faster than previous studies indicated. 

U S. Bureau of Mines researchers pioneered the commerical implementation of heap leaching applied to low-grede gold-silver ores and heve demonstrated ihet agglomeration by the use of Portland cement, added in amounts of up to L0 ]bm per ton of feed, can improve the percolation rate of leach liquors... 

The recovery of OPPs by SPE depends on the physicochemical properties of the analytes, the nature of the water (pH, organic matter load, ionic strength, etc.), the volume of sample percolated, the percolation rate, the type and mass of sorbent used, and the desorption method (eluent, contact time, flow rate). ... 

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