Washing filter cloths

Filtration. The separation of phosphogyp-sum or hemihydrate from its mother liquor has always been a difficult operation. The process has been subject to the formation of calcium sulfate, sodium fluosilicate, and other types of scale that clog the cloth and necessitate periodic filter washing. Filter cloth wear is severe, requiring cloth changes as often, in some cases, as two or three weeks apart. 

Another type of horizontal belt vacuum filter uses reciprocating vacuum trays mounted under a continuously traveling filter cloth. The trays move forward with the cloth as long as the vacuum is appHed and return quickly to their original position after the vacuum is released. This overcomes the problem of friction between the belt and the trays because there is no relative movement between them while the vacuum is being appHed. The mechanics of this filter are rather complex, and the equipment is expensive and requires intensive maintenance. A range of solvents can be used. Widths up to 2 m and areas up to 75 m are available. The cloth can be washed on both sides. 

Despite their theoreticaUy poor washing performance, due to uneven wash distribution and excessive mn-off because the filter surface is not horizontal, many multicompartment dmm filters continue to be used as cake washing filters. Effective washing of the filter cloth can be done only with the belt discharge type, where the cloth leaves the dmm for a brief period and can thus be washed on both sides. 

The automation of filter presses has affected several other advantages and developments. Plate shifting mechanisms have been developed, allowing the cloths to be vibrated filter cloth washing, on both sides, has been incorporated to counteract clogging from the expression and downtimes have been reduced with automation, thus increasing capacities. 

B. Phenylsuccinic acid. To the crude ester obtained above is added 500 ml. of concentrated hydrochloric acid (sp. gr. 1.19). The flask is fitted to a condenser (Note 3), and the mixture is heated under reflux for 18 hours (Note 4). At the end of this time only a small amount of red oil remains (Note 5). The mixture is cooled, and the nearly solid cake which forms is broken up and collected on a glass filter cloth (Note 5). The crude tan-colored phenylsuccinic acid is washed with 300 ml. of cold water and... 

Figure 11.11. Continuous horizontal vacuum filters especially suited to free settling and draining solids, (a) Principle of the conveyor belt filter units may operate up to 0.S m/sec with a cycle time up to 10 min and produce cake thicknesses up to 15 cm. (b) Showing the construction of a grooved rubber belt support for the filter cloth of the belt filter (Purchas, 1981). (c) Rotating horizontal vacuum filter the unit has readily accessible piping and is amenable to thorough washing of free draining solids (Dorr-Oliver Inc.).

A second problem encountered with the filtration step was the additional water introduced in the system most likely as a result of the continuous washing of the filter cloth. This problem would probably be minimized with larger-scale equipment. 

Centrifugation is one of the most widely employed types of filtration, notable for operational speed (assuming the solid packs well on the filter cloth, or sintered metal bowl, and the porosity of the solid cake is maintained to allow thorough washing) and for producing a relatively dry cake—it is not uncommon for a centrifuged filter cake to be dried out to a solvent content of approximately 20%. Sometimes... 

A valuable variant of the conventional centrifuge is the horizontal spindle centrifuge, especially the Heinkel type. This centrifuge has a unique offloading feature in that the filter cloth loaded with washed filter cake is pulled out to allow a reversed centrifugal removal of the solid as illustrated in Figure 10. 

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