Pressure filtration cake squeezing

Batch Expression Equipment In batch expression equipment, the cake is initially formed by pressure filtration just as in other pressure filters. After the filtration stage, a squeezing device such as a diaphragm is inflated with gas or liquid to compress the cake. Batch expression equipment allows longer compression time and higher compression pressure. The cake can be very dry. 

A laboratory pressure filter (Fig. 18-189) equipped with a piston can provide a simple feasibility test. In this kind of device, the suspension is poured into the filter cylinder, and the first stage of the test is just like a pressure filtration test. After the filtration, compressed air or water is used to push the piston down to squeeze the filter cake. The filtration rate, final cake thickness and dryness are recorded for evaluation and comparison with the same test without the compression by the piston. 

Tuhtdar Presses As the name implies, this press is composed of a candle filter inside a cyhndrical hydrauhc casing (Fig. 18-192). The filter cloth is wrapped around the filter candle, and a diaphragm is attached to the inner side of the outer casing. During the filtration step, the space in between two cylinders is filled with slurry, and pressure filtration is conducted. At the end of the filtration step, the diaphragm is inflated to squeeze the cake around the filter candle. At the end of expression, the bottom of the hydrauhc casing tube is opened and the filter assembly is lowered. Air is then introduced to pulse the cake off the candle. Alter the cake is discharged, the inner filter candle moves back, and the bottom is closed for the next filtration cycle. 

In the units descnbed below, fluid pressure necessary for filtration is g erated by pim]ping. Pressures are usually below 600 kN/ni but in some machines, hi er values are used, e.g. iq> to 140 bar in those madunes fitted with fiicilities for cake squeezing afiier filtration. 

The driving force for filtration in pressure filters is usually the liquid pressure developed by pumping or by the force of gas pressure in the suspension feed vessel. Alternatively, or in addition, the liquid may be squeezed through and out of the cake by the mechanical action of an inflatable membrane, a piston or a porous medium pressed on top of the cake. Pressure filtration is, therefore, defined here as any means of surface filtration where the liquid is driven through the medium by either hydraulic or mechanical pressure, greater than atmospheric. The solids are deposited on top of the filter medium (as in all surface filters), with the possible exception of some cartridge filters which also use a certain amount of depth filtration. In this chapter, the suspension is assumed to approach the medium at 90° and this excludes the so-called dynamic fUter/thickeners or cross-flow filters (also driven by pressure) which are dealt with in a separate chapter (11). 

The compressibihty of many industrial filtration cakes, an unwanted phenomenon in filtration by hydrauhc pressure, may be turned into an advantage by considering mechanical squeezing of the pre-formed cake as an alternative or additional dewatering method. The fact that a cake has a reasonable or high... 

In the past decade or so, a number of new filters have appeared on the market, utilizing some form of mechanical compression of the filter cake, either after a conventional pressure filtration process or as a substitute for it. In most designs the compression is achieved by inflating a diaphragm which presses the slurry or the freshly formed filter cake towards the medium, thus squeezing an additional amount of liquid out of the cake—see Figure 12.11. 

As much as the vacuum horizontal belt filters have made a considerable impact in vacuum filtration, one would expect that the same principle may be used in pressure filtration, by enclosing the whole or part of the belt in a pressure vessel. This idea has to be seen as entirely separate from the so-called belt presses (section 12.3.5.1) which use the squeezing of cake by a second belt and do not use the pressure of the liquid or air to drive the filtrate through and out 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). 

Belt Presses Belt presses were fiiUy described in the section on filtration. The description here is intended to cover only the parts and designs that apply expression pressure by a mechanism in adchtion to the normal compression obtained from tensioning the belts and pulling them over rollers of smaller and smaller diameters. The tension on the belt produces a squeezing pressure on the filter cake proportional to the diameter of the rollers. Normally, that static pressure is calculated as P = 2T/D, where P is the pressure (psi), T is the tension on the belts (Ib/hnear in), and D is the roller diameter. This calculation results in values about one-half as great as the measured values because it ignores pressure created by drive torque and some other forces [Laros, Advances in Filtration and Separation Technology, 7 (System Approach to Separation and Filtration Process Equipment), pp. 505-510 (1993)]. 

A vertically oriented press where two endless, slightly off-vertical, filter cloths move continuously over a series of rollers. These moving belts are sealed at their edges by two other stationary belts in a manner that allows the feed suspension to be mechanically squeezed and cake filtration to occur. The maximum squeeze pressure is restricted to -250 kPa and controlled as appropriate via the gap between the moving cloths at the base of the unit. Typical cake discharge thickness ranges between 6 and 8 mm. The tower press as described here, which should not be confused with the similarly named, but very different, unit described in Section 1.4.2.5, has now been largely superseded by the belt press (see earlier in this section). 

There have been some relatively recent developments of continuous pressure filters, of both the hydraulic and squeezing variety. The different parts of the filtration surface in these units at any point in time are undergoing different stages of the filtration and dewatering cycle, with one part continuously discharging the cake. The cake and filtrate production is continuous. 

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