Separation, solid-liquid thickening

Crystals suspended in liquors emerging from crystallizers are normally passed to solid-liquid separation devices such as gravity settlers or thickeners that may subsequently feed filters to remove yet more liquid prior to drying. Here the transport processes of particle motion and the flow of fluids through porous media are important in determining equipment size, the operation of which may be intensified by application of a centrifugal force. 

Osborne, D.G., 1990. Gravity thickening. In Solid-liquid separation, 3rd edition. Ed. L. Svarovsky. Oxford Butterworth-Heinemann. 

The solid-liquid separation of shinies containing particles below 10 pm is difficult by conventional filtration techniques. A conventional approach would be to use a slurry thickener in which the formation of a filter cake is restricted and the product is discharged continuously as concentrated slurry. Such filters use filter cloths as the filtration medium and are limited to concentrating particles above 5 xm in size. Dead end membrane microfiltration, in which the particle-containing fluid is pumped directly through a polymeric membrane, is used for the industrial clarification and sterilisation of liquids. Such process allows the removal of particles down to 0.1 xm or less, but is only suitable for feeds containing very low concentrations of particles as otherwise the membrane becomes too rapidly clogged.2,4,8... 

It provides better solids-liquid separation than a gravity thickener. 

Qu and Qc, and solids concentrations of Cu and Ca, respectively. The most desirable division of solids in a device for solid/liquid separation, such as a thickener (described in Chapter 5), is where all of the solids report to the underflow so that the overflow is clarified liquid and consequently Ca is zero. If the efficiency of separation (E) is defined as the mass ratio of solids (of all sizes) in the underflow to that in the feed, then a clarified overflow corresponds to an efficiency of 100 per cent. 

The solids-liquid separation of slurries containing particles below 10 xm is difficult by conventional filtration techniques. A conventional approach would be to use a slurry thickener in which the formation of a filter cake is restricted and the product is discharged continuously as a concentrated slurry. Such filters use filter cloths as the filtration medium... 

Sedimentation is also used for other purposes. For example, relative motion of particles and liquid increases the mass-transfer coefficient. This motion is particulady useful in solvent extraction in immiscible liquid—liquid systems (see Extraction, liquid-liquid). An important commercial use of sedimentation is in continuous countercurrent washing, where a series of continuous thickeners is used in a countercurrent mode in conjunction with reslurrying to remove mother liquor or to wash soluble substances from the solids. Most applications of sedimentation are, however, in straight solid—liquid separation. 

The recovery of valuable minerals and metals requires several stages of sequential processing operations. The mined ore must be crushed and ground to fine sizes prior to treatment by such bene-ficiation processes as heavy-medium separation, tabling, magnetic separation, electrostatic separation, flotation, selective flocculation, etc. Since most of these processes are carried out in aqueous media, solid-liquid separations by such operations as thickening and filtration are an integral part of the benefici-... 

Svarovsky L (2000), Gravity clarification and thickening, In Svarovsky L (ed), Solid-Liquid Separation, Butterworth-Heinemann, Oxford. 

Solid-Liquid Separators. Most of the SO2 transferred from the fine gas into the liquid in the scrubber is converted to solid sulfur compounds in the scrubber and in the reaction tanks. The solid sulfur compounds and the fly ash collected in the scrubber have to be separated from the liquid and disposed of in a way that does not cause water pollution. Clarifier-thickeners, vacuum filters, ponds, etc., are being used as the solid-liquid separators. The liquid is returned to the system, and the solids are removed as waste. 

The solids concentration of the underflow from the clarifier-thickeners is between 20 and 40 wt % and is not concentrated enough for transportation by either trucks or railroad cars to the disposal areas. For this reason vacuum filters are usually used to concentrate the underflow to 60-80 wt % solids. Ponds are also being used to store solids and to accomplish solid-liquid separation. C-E is doing developmental work to determine the leaching characteristics of the sulfur compounds present in the sludge and also to determine the most suitable filtration system for lime/limestone wet scrubber systems. 

Liquid waste streams with a high-suspended solids content can be cleaned up by solids removal in clarifiers, thickeners, and liquid cyclones and by accelerated settling by inclined Chevron settlers or the like [73]. For waste streams with very finely divided solids in suspension (i.e., less than about 100 pm) dissolved air flotation techniques have been shown to be more efficient than methods employing sedimentation. Final dewatering of the sludges obtained may be carried out on a continuous filter or a centrifuge. The clarified water product can be accepted for more potential options of reuse or final disposal options than untreated water, and the separated solids may be burned or discarded to landfill, as appropriate [74]. 

Solid-liquid separation systems generally consist of four stages, which are 1) pretreatment to increase particle size 2) solid concentration in thickeners 3) solid separation in filters and centrifuges and 4) posttreatment to remove solubles and reduce liquid content. Fig. 6 shows the relationship among these stages. 

Solid-liquid separation systems generally consist of four stages including pretreatment, solid concentration in thickeners, solid separation in filters or centrifuges, and post-treatment by expression and washing operations. There are different types of SLS equipment served for different functions in relation to the four stages. Product specification, characteristics of solid-liquid suspension, solid settling velocity, rate of cake... 

In general, flocculants are used in solid-liquid separation processes such as thickening and filtration. Inorganic salts are also used sometimes to aggregate fine particles. Flocculation technique has been developed further for special applications of selective flocculation, selective dispersion and agglomeration flotation. 

It is well Icnown that the solid-liquid separation characteristics of residues become a problem when zinc concentrates with high silica content are treated in a hydrometallurgical process. Once treated improperly, the silica dissolved in the sulfuric acid solution turns to a gel which significantly reduces the solid-liquid separation characteristic of the residue. As a consequence, larger capacity solid-liquid separation facilities such as thickeners, filter presses and others are required. In some cases, the slurry may completely turn into a gelatinous form, which cannot be treated. For this purpose, a technology to settle the dissolved silica in a form easy for solid-liquid separation becomes necessary. 

Solid-Liquid Separation Equipment (Thickeners, Clarifiers, FUters, Centrifuges, and Expression)... 

Solid-liquid separators Thickener, steel Settling area. A, ft 80-8,000 ft Cp = 2,650 A Carbon steel... 

A basic appreciation of slurry rheology, or flow behaviour, is in oitant in many solid-liquid separations, e.g. when feeding pressure filters, punq)ing thickener underflow, hydrocyclone feed and exit streams and during cross-flow filtration. This Appendix is designed to introduce some of the terminology and basic concepts. A more thorough text such as Wilkinson [1960] should be referred to for further details, if necessary. 

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