Solid-Liquid Separation and Sweating

The process of solid-liquid separation controls the content of impurities via the quantity of mother liquor left in the solid. This refers to impurities being adsorbed on the crystal surface or entrapped between crystals. Obviously, the more efficient the solid-liquid separation, the lower the content of mother liquor remaining in the solid product and the higher the purity achievable by this process step. 

Generally, there are different techniques to separate the solid phase, the crystals, from the mother liquor. In most cases, pressure filtration or centrifugation is used sedimentation rather rarely. Furthermore, different technical equipment are available to apply the individual techniques for solid-liquid separation. A major characteristic of these techniques is the residual moisture content, which directly determines the amount of impurity in the solid separated. 

As shovm in Table 7.3, the residual moisture content decreases significantly from sedimentation to filtration, with best results for centrifugation, where still 4-8% residual moisture remain in the product. These data stem from experiences of the potash industry, a very optimistic case in particular with regard to the filtration results. Since the quality of filtration also depends on particle properties (size and shape), the residual moisture content can vary in a wide range. For example, there are also references to residual moisture contents between 30% and 50% for nutsche or pressure filters. For highly disperse materials such as Al(OH)3 or Fe(OH)3 the residual moisture content can reach 70-90%. 

The impurity content decreases clearly from about 7 wt% for simple draining to 3 wt% for filtration and below 1 wt% for centrifugation. Further improvement was achieved by sweating (partial melting) and washing in the centrifuge reaching a purity of 99.8 wt%. 

Reslurrying and washing the crystalline product are aimed at a further removal of impurities that remained after solid-liquid separation. In fact, a further depletion of impurities originating from bulk phase inclusion of mother liquor, surface adhesion to the crystals, and also liquid inclusions in the crystals can be achieved 

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