Compression characteristics, pressure filters

These general comments and the theoretical developments provided in Chapter 2 point to the need for information on the effect of pressure differentials on filter c e resistance, porosity, moisture content, etc. Test work must be designed to quantify the compressibility characteristics of the suspension of interest. This information fedlitates the estimation of filter size and productivity in various operating conditions. Quite ofi en, the level of pressure used in plant-scale separations are quite different to those available in the test laboratory. 

Since the precipitates we deal with typically are compressible and do not form permeable cakes with desirable characteristics, it is standard practice to add a filter aid. Materials such as the flocculants that are used in brine treatment are sometimes referred to as filter aids because they improve the filterability of the particles that are to be removed. In our usage, however, a filter aid is a solid material added to the process to improve the characteristics of a filter cake. It should provide a noncompressible surface that does not quickly lose its porosity as solids accumiflate. Without such a material, the surface in a pressure filter would soon become blinded. The filter aid is applied to the filtering surface before flow of brine begins in order to provide a good foundation. A slurry is circulated from a tank through the filter until enough filter aid, or precoat, has deposited. 

The principal objective of an expression test is to determine the compression deliquoring characteristics of a cake. However, the nature of the test allows both filtration and compression characteristics to be determined when the starting mixture is a suspension (i.e. where the solids are not networked or they are interacting to a significant extent). Cake formation rate, specific resistance and solids volume fraction data can be determined for the filtration phase while analysis of a subsequent consolidation phase allows the calculation of parameters such as consolidation coefficient, consolidation index and ultimate solids concentration in the cake. Repeated use of the expression test over a range of constant pressures allows the evaluation of scale-up coefficients for filter sizing and simulation as described in Section 4.7. 

The resistivity of the cake, however, very often increases with time. Under the pressure of filtration or the friction of the fluid that continues to pass through the cake, the filtered particles continue to compact and reduce the area available for flow. Section 7.S.4.2 also describes the empirical methods that are used to characterize the compressibility of the cake. It is significant that the particles produced by precipitation from the brine are often highly compressible. At the same time, they have a tendency to become lodged in the pores of the filter medium. Filtration of these particles can become very difficult. The usual solution to this problem is the use of a filter aid. The filter aid is applied to the surface of the filter medium before introducing the brine. This is the precoating operation. Filter aids are selected for their desirable characteristics, and the precoat material protects the pores from penetration by filtered solids while at the same time offering little resistance to the flow. Compression of the precipitates still occurs, and so filter aid ( admix ) is also added in small quantities to the brine to improve the characteristics of the cake. 

The SulFerox sulfur-cake matrix tends to be quite compressible and reportedly lends itself well to filtration via plate-and-fiame filters or automated batch filter presses with recessed membranes for post-filtration sulfur-cake squeezing. It is claimed that only small amounts of wash water are needed to displace the residual process solution from the filtercake. Rotary drum vacuum filters were used in all early SulFerox applications, but variations in feed slurry characteristics due to differing inlet gas contaminants and required additive levels made their performance inconsistent. It is repotted that filtration by pressurized-feed filter presses has eliminated the problem of cake quality variation and has substantially reduced iron chelate losses (Anon., 1994). The sulfur filtercake from pressurized-feed filter presses is reported to contain 10 to 23 wt% moisture (Allen, 1995). 

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