Cake cracking

Patient age 60-75 Actinic keratoses Prior skin cancers Wrinkling throughout Makeup cakes cracks Severe acne scarring... 

Prior to and between washes, the green filter cake cracks and should be pushed down" with a spatula to form a uniform surface prior to any subsequent washes. 

It is recommended that optimization be carried out by developing three different cake thicknesses. From this, a capacity versus cake thickness curve can be developed. Additional parameters that have to be evaluated are vacuum level, wash requirements, slurry concentration, and slurry temperature. If cake cracking occurs, the wash should be introduced earlier to avoid channeling. 

A phenomenon called cake cracking can occur and will be evident in this simple test. Not all materials exhibit this. It depends upon the surface tension of the product and its tendency to shrink as dewatering occurs. Amorphous, thixotropic materials will exhibit this more than rigid solids. 

Washing efifidendes on large-scale fitters vary fi-om 35 to 90% the main causes for such inefficiencies are uneven cakes, maldistribution of wadi water, and cake cracking. 

A series of practical pilot plant measuremmts using 0.1, 0.3 and 1.0.m rotary vacuum filters indicated that, in many systems, smous deviations can occur between theoretical predictions using leaf tests obtained at boidi scale, and practical results [Rushton Wakeman, 1977]. Practical yields are affected by fiictors not considered in the theoretical model, whkh assumes that the mass of filter cake dqrosited by the passage of V volumes of filtrate may be calculated fi om w = cV, where c is is gtvm by Equation 11.3. Furtiier more, cake cracking may occur, in which case the effective vacitum fi)r particle pick-19 reduced. 

Cake cracking has received limited attention [Rushton Hameed, 1969 Wakeman, 1974] in the literature. Recent contributians [Yagi a et al, 1990] list the methods norma% suggested to avoid cake cracking ... 

Whilst cake cracking and drop-off are piinc al sources of differences in the yield of actual plant, conq)ared with laboratory-scale tests, other considerations include (1) equality of the pressure differential available for cake formation and (2) equivalence of media condition and resistance. The latter will depend on the age of the medium and the effectiveness of cloth wadung operations. Filtration equipment is generally poorly instrumented. The location of the AP measuring device and the pressure (vacuum) losses present between the measurement and point of filtration will vary ftom station to station. 

EfiScient cake discharge can be effected by separation of the belt fi om the cloth and directing the latter over a set of discharge roUws. Here a sharp turn in cloth travel, over a small diameter roller produces cake cracking and discharge. Another area which has received attention, is that of the control of the down-stream pressure drops which ensue with the two-phase flow of air and water through the vacuum system. 

Successfiil industrial fications have been reported [Groh Van der Werfi 1982], particular in the pressnig of solvmts, which would preclude the use of elastomer belts. Noxious, corrode vapours are handled by closing the entire filter in vapour-ti enclosures. A further important advantage over other SLS systems is the control of cake cracking by minimisation of the area between flooded filtration and wash zones on the belt. 

In the second dewatering zone, the cake is also in contact with the atmosphere, and the compartment is connected with the port-6. Consequently, the washing liquid is displaced from the cake pores and delivered b the collector. To avoid cake cracking during washing and dewatering, an endless belt-7 is provided, which moves over a set... 

Smaller Nutsche filters require a manual cake discharge while larger machines generally employ mechanical ploughs or rakes. Fully automated versions of the vacuum Nutsche filter are available and some include cake smoothing devices to minimise the problems of cake cracking. A range of filter media can be accommodated with filter areas up to a maximum of 10 m. ... 

Restricted to prevent excessive, and process iimiting, cake cracking. 

Wakeman R.J., 1974. The role of internal stresses in filter cake cracking, Filtr. Sep., 11, 357-360. 

Gas deliquoring Over extended application of pressure Cake cracking a. Difficulty in maintaining air pressure b. Possibly wet cake (in parts)... 

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