Solid/liquid separation washing

In the following section, important downstream processes as solid-liquid separation, washing, and reslurrying are discussed briefly with respect to their impact on final product purity. 

Staff profile page - the Engineering Faculty at Loughborough. .. Broad Interests and Expertise. Compressible cake filtration Selection, scale-up and process simulation of solid/liquid separation equipment Washing and. .. http //WWW. Iboro. ac. uk/departments/eng/research/staff/html/tarleton. html [More Results From www.lboro.ac.uk]... 

Wakeman, R.J., 1990b. Filter cake washing. In Solid-liquid separation, 3rd edition. Ed. L. Svarovsky. Oxford Butterworth-Heinemann. 

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. 

Crystal morphology (i.e., both form and shape) affects crystal appearance solid-liquid separations such as filtration and centrifugation product-handling characteristics such as dust formation, agglomeration, breakage, and washing and product properties such as bulk density, dissolution kinetics, catalytic activity, dispersability, and caking. 

If a screen is installed at the bottom of the tank, the solid-liquid separation can take place within the extraction equipment. The extract drains out of the tank, leaving the solids behind. Water can then be added to reslurry the solids, washing solvent from the marc. However, some means of removing the washed marc from a tank equipped with a screen is needed. Where external filtration or centrifuging equipment is used, solvent can be removed from the marc by water washing or conveying to an external dryer. 

As a uniform solvent flow is required for percolation operations, care must be taken in grinding the raw material to provide a particle size that allows adequate flow at an acceptable extraction rate. When the extraction is complete, the tank is drained of liquid, eliminating a separate solid-liquid separation step. In addition, the solvent can be removed from the marc within the extractor by washing with water or heating the marc with steam. Alternatively, the solvent-soaked marc can be removed from the extractor, and solvent can be recovered in an external dryer. 

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 solution crystallization product purity is controlled to a great extent by the efficiency of solid-liquid separation. Impurities dissolved in the ciystallizer liquid are normally removed by spinning liquor from the crystals and then washing the resulting cake. The residual impurity level is a function of factors listed in the following equation ... 

One of the most commonly used solid-liquid separation methods in crystallization processes is centrifugal filtration, such as continuous pusher and batchwise peeler centrifuges shown schematically in Figure 64.10. A manufacturer of centrifuges used in crystallization processes is KMPT AG [52]. In addition, Nutsche filters, frame pressure filters, and belt filters have also been used. Most of these filters have a possibility of cake washing which is important for the final purity as discussed earlier. 

The crystals have to be separated from the remaining melt (mother liquor) to achieve the intended purification. In case of layer crystallization, this is done by draining the remaining melt, collecting it separately, and melting down the crystal layer afterward (Fig. 8.2-11). In case of suspension crystallization the solid-liquid separation is done either by conventional filtration or by a sedimentation apparatus, with or without support of centrifugal forces. Another device repeatedly discussed in the context of solid-liquid separation is the wash column (Arkenbout 1995). 

Impure product because of surface contamination poor solid-liquid separation/ poor washing, see Sections 5.12-5.14. 

The term post-treatment refers to processes vdiich are used after the principal solid-liquid separation has been achieved. The final step in the latter operation is usually some type of cake filtration which will leave a cake that may be fully saturated with liquid or partially drained and it is extremely unlikely that the solids are in an acceptable condition for any subsequent operation such as thermal drying. Thus the post treatment processes are washing and deliquoring which are concerned req>ectively with removing the soluble solids from the liquid remaining in the cake and then purging the major proportion of the liquid fi om the cake pores. 

Many solid-liquid separation systems must recover a valuable solid product fi om a su ension which has undesirable constituents such as soluble dissolved solids in its liquid coirponent. Crystallisation and precipitation are obvious exanq>les where a solid phase has to be produced in a pure state firom a contaminant liquor. In such a Nation the cake must be washed with a clean wai fiquor to reduce the contaminants to an acceptable level. 

The semiconductor catalyst is generally in the form of powder suspended in a liquid medium. The inconvenience of this as a large-scale approach is the catalyst-recovery step from the solution at the end of operation. Thus, solid-liquid separation is an extremely important issue for the development of the photocatalytic technology indeed, the best possible recovery of particles must be ensured, in order to prevent their wash-out and consequently a decrease of their amount in the reactor system. 

Many of the procedures applied in this field are modifications of chemical engineering techniques, mainly in the area of solid-solid or solid-liquid separation. The more advanced techniques are washing procedures, of which the fine-grained residues are eventually deposited. With respect to acid extraction procedures, there are many experimental designs, but few practical applications. Electrochemical procedures are in progress, in particular for in-situ-reclamation of metal-rich areas. 

Solid-liquid separation methods in direct method include washing and nonwashing. Solid-liquid separation methods in indirect method include filtering and centrifuging. 

Solid/Liquid Separation Equipment Selection and Process Design 7.3.1.2 Washing phase... 

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