Coarse solids, leaching

Fig. 8.40 Retention (R, defined as deposited solids per unit length of section/total added suspended solids, mm 0 of suspended solids in the soil subsurface (a) distribution of deposited solids in coarse sand and silt loam, (b) relative deposition (defined as -ln(a/a), where a denotes initial amount of applied suspended solids and a. denotes measured amount of deposited solid mass per unit length of section at each depth) of suspended solids in silt loam and coarse sand leached by filtered and unfiltered effluents. (Vinten et al. 1983)...

Continuous Percolators Coarse solids are also leached by percolation in moving-bed equipment, including single-deck and multideck rake classifiers, bucket-elevator contactors, and horizontal-belt conveyors. 

Leaching has in the past been carried out mainly as a batch process although many continuous plants have also been developed. The type of equipment employed depends on the nature of the solid—whether it is granular or cellular and whether it is coarse or fine. The normal distinction between coarse and fine solids is that the former have sufficiently large settling velocities for them to be readily separable from the liquid, whereas the latter can be maintained in suspension with the aid of only a small amount of agitation. 

The suspended solid particle size and the volume of effluent also must be considered in examining deposition in the subsurface. For example, under leaching of a waste disposal site or following irrigation with sewage effluent, the coarse fraction of suspended solids is retained in the upper layer, while the finer colloidal fraction is more mobile, and its transport is controlled by the porosity of the subsurface solid phase. 

Similar result was obtained for skeletal Si . The solubility of Si in the a -A1 solid solution is negligibly small so that the two phases a -A1 and relatively coarse Si existed in the conventionally prepared Al-Si alloy. By hydrochloric acid leaching, only the coarse stable Si remains after leaching, so that it is difficult to form fine Si particles by leaching. In this case, the rapid solidification increases the solubility to about 12 at%. The specific surface area of skeletal Si formed from RWA was in the range of 65-75mVg and the mean particles size was about 30-50nm by direct observation of the skeletal Si. 

The jarosite process separates icon(III) from zinc in acid solution by precipitation of MFe2(0H)g(S0 2 where M is an alkali metal (usuaUy sodium) or ammonium (see Fig. 2) (40,41). Other monovalent and hydronium ions also form jarosites which are found in the precipitate to some degree. Properly seeded, the relatively coarse jarosite can be separated from the zinc-bearing solution efficiently. The reaction is usuaUy carried out at 95 0 by adding ammonia or sodium hydroxide after the pH has been adjusted with calcine and the iron oxidized. The neutral leach residue is leached in hot acid (spent + makeup) with final acidity >20 g/L and essentiaUy aU the zinc, including ferrite, is solubilized. Ammonium jarosite is then precipitated in the presence of the residue or after separating it. If the residue contains appreciable lead or silver, they are first separated to avoid loss to the jarosite waste solids. Minimum use of calcine in jarosite neutralization is required for TnaxiTniiTn recovery of lead and silver as weU as zinc and other metals. 

The opposition of the factors suggests an optimum particle size for any particular extraction. This is determined to some extent by the physical nature of the solids. A dense, woody structure would be extracted as a fine powder. An example is given by the root of Ipecacuanha. A leafy structure, on the other hand, would be more satisfactorily leached as a coarse powder. 

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