Filtration polishing filters

Cake Filters Filters that accumulate appreciable visible quantities of solids on the surface of a filter medium are called cake filters. The slurry feed may have a solids concentration from about 1 percent to greater than 40 percent. The filter medium on which the cake forms is relatively open to minimize flow resistance, since once the cake forms, it becomes the effective filter medium. The initial filtrate therefore may contain unacceptable sohds concentration until the cake is formed. This situation may be made tolerable by recycling the filtrate until acceptable clarity is obtained or by using a downstream polishing filter (clarifying type). 

One of the differences between polishing and cake filtration is the space between the plates. For polishing applications the clearances are about 20 mm as opposed to cake filtration applications where, depending on the percentage of solids and cake build-up properties, clearances may reach 100 mm. Hence, polishing filters accommodate more plates than cake filters so for the same vessel size more effective area is available with polishers. 

Granular bed filters are used in porcelain enameling wastewater treatment to remove residual solids from clarifier effluent (sedimentation effluent or flotation effluent). Filtration polishes the effluent and reduces suspended solids and insoluble precipitated metals to very low levels. Fine sand and coal are media commonly utilized in granular bed filtration. The filter is backwashed after becoming loaded with solids and the backwash is returned to the treatment plant influent for removal of solids in the clarification step.10-12... 

Figure 7.13 Typical in-line filtration operation using two cartridge filters in series. The prefilter removes all of the large particles and some of the smaller ones. The final polishing filter removes the remaining small particles...

Like primary filters, polishing filters must be taken off line for cleaning when flow rate deteriorates, differential pressure increases, or the body of the filter fills with solids. The emphasis in fundamental analysis of the filtration of brine is on the production of clear brine at a defined rate. The efficient removal of cake from the filter, however, is of equal importance. The removal of occluded brine from the cake for recovery or for waste minimization may also be necessary. Before cleaning, the brine inside the filter body is usually removed. This involves the use of compressed air to blow the brine back to its... 

Backpulse Filtration. The polishing filters described in the preceding section rely on filter aids for efficient removal of solids. This is necessary because the pores in the filter cloths are larger than some of the soUds that are to be filtered. It is necessary first to build up a surface that will retain these solids and then sometimes to continue to renew the surface to prevent its blinding with small or compressible particles. At least in principle, the operation might be simplified if the pores in a filter medium could be made reliably smaller than the suspended solids in the feed liquor and yet controlled to... 

At least conceptually, the potential of these filters goes far beyond replacement of the polishing filters. The ultimate achievement in the chlor-alkali brine process would be to take treated brine from the precipitation reactors and filter it directly in a single step to produce clear filtrate. This would combine the functions of the clarifier, the primary filters, and the polishing filters in a single step. Further discussion is in Section 17.3.1. 

Decolourizing charcoal is often added to the hot, saturated solution prior to cooling. After agitating the suspension for a short time the charcoal is removed by filtration and the crystallization is completed in the normal manner in another vessel. If charcoal is not used, the hot solution is normally passed through a polishing filter to remove small quantities of undissolved impurities or dirt and obtain a clear, bright solution (p. 49). 

From this it follows that one must first of aU look at sedimentation. In this respect it is important to note that even if sedimentation does not achieve exactly the separation required it may still be a valuable first step in a process and may be followed by filtration. A typical example of this is the well-known clarification of water in water works. Here sedimentation with or without flocculation is first carried out, and the still somewhat hazy overflow is then passed through polishing filters. To achieve the end result purely by sedimentation or purely by filtration would be prohibitively expensive. The combined procedure, however, is both practical and reliable. 

The polishers operate as filters to remove the oxide of metals and undissolved solids from the condensate. As iron oxide is particulate matter, it can be removed by filtration. The filter is a mixed cation and anion exchange resin. The filter removes the insoluble particles and ion exchange removes the dissolved contaminants against the formation of calcium scale while at the same time avoiding the formation of harmful free caustic in water. 

Depth filters are usually preferred for the most common type of microfiltration system, illustrated schematically in Figure 28. In this process design, called "dead-end" or "in-line" filtration, the entire fluid flow is forced through the membrane under pressure. As particulates accumulate on the membrane surface or in its interior, the pressure required to maintain the required flow increases until, at some point, the membrane must be replaced. The useful life of the membrane is proportional to the particulate loading of the feed solution. In-line microfiltration of solutions as a final polishing step prior to use is a typical apphcation (66,67). 

Vertical Pressure Leaf Filters are essentially the same as Horizontal Plate Filters except for the orientation of the filter elements which are vertical rather than horizontal. They are applied for the polishing slurries with very lov solids content of 1-5% or for cake filtration with a solids concentration of 20-25%. As with the horizontal plate filter the vertical leaf filters are also well suited for handling flammable, toxic and corrosive materials since they are autoclaved and designed for hazardous environments when high pressure and safe operation are required. Likewise, they may be readily jacketed for applications whenever hot or cold temperatures are to be preserved.The largest leaf filters in horizontal vessels have a filtration area of 300 m and vertical vessels 100 m both designed for an operating pressure of 6 bar. 

---