Cake moisture correction

In some cases, particularly in the filtration of highly concentrated suspensions, when working out the mass of dry cake from filtrate volume collected a cake moisture correction has to be applied, which accounts for the presence of moisture in the cake. The relationship that replaces equation 9.9 in such cases is obtained from solids concentration c (kg m ) in the original suspension and from the moisture content of the cake... 

There is hidden assumption in equation 4 that the volume of the soHds and Hquid retained in the cake is negligible. This is reasonable at low concentrations but can lead to errors at high soHds concentrations and moisture contents of cakes. A corrected value of the concentration c can be used in equation 4 to reduce the errors, ie. 

Pretreatment Chemicals Even though the suspended solids concentration of the slurry to be tested may be correct, it is frequently necessary to modify the sluriy in order to provide an acceptable filtration rate, washing rate, or final cake moisture content. The most common treatment, and one which may provide improvement in all three of these categories, is the addition of flocculating agents, either inorganic chemicals or natural or synthetic polymers. The main task at this point is to determine which is the most effective chemical and the quantity of chemical which should be used. 

The discussion of chemical and physical pretreatments given in this chapter highlights the potential benefits (and pitfalls) of artificially changing the particle size present in a suspension. Chemical pretreatments in particular can enhance performance in all phases of a filter cycle and provide for improved filtration rates and reduced cake moistures. Some separation devices such as the deep cone thickener and belt filter press rely on the satisfactory performance of flocculants to function correctly and their use is a necessary prerequisite. The use of coagulants and flocculants with other separation devices must be assessed on an individual basis. Their introduction generally increases the operating cost of a separation which must be balanced by the potential improvements in separation characteristics. 

The cake moisture content at the end of washing is the same as at the end of consolidation (i.e. = 46.41%). Although no corrections have been applied... 

Correction of the slurry concentration for cake moisture (equation 9.39) ... 

Appearance the cake should have the correct appearance and color (i.e., no back melting or pockets of moisture or particulate matter). 

Hydrocyclones are also used as classifiers simply to improve the performance of other filtration or separation equipment. A good example is in applications where the cyclone separates the feed into coarse and fine particles, and the coarse material is fed onto the horizontal belt filter first, as a precoat, with fines to follow (see Figure 6.25). This will, if the cut point of the cyclone is set correctly, give good filtrate clarity and extend the usefulness of vacuum filters to finer feeds without necessarily adversely affecting the moisture content of the cake. 

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