Flocculation pressure filters

Pressure filters can treat feeds with concentrations up to and in excess of 10% sohds by weight and having large proportions of difficult-to-handle fine particles. Typically, slurries in which the sohd particles contain 10% greater than 10 ]lni may require pressure filtration, but increasing the proportion greater than 10 ]lni may make vacuum filtration possible. The range of typical filtration velocities in pressure filters is from 0.025 to 5 m/h and dry sohds rates from 25 to 250 kg nY/h. The use of pressure filters may also in some cases, such as in filtration of coal flotation concentrates, eliminate the need for flocculation. 

It should be noted that the total loss of head of a filter bed is in inverse ratio to the depth of penetration of the matter in suspension. In a normal wastewater treatment plant, the water is brought onto a series of rapid sand filters and the impurities are removed by coagulation-flocculation-filtration. Backwashing is typically performed in the counterfiow mode, using air and water. One type of common filter is illustrated in Figure 6, consisting of closed horizontal pressurized filters. 

Where present in boiler MU water, both iron and manganese may present fouling and deposition problems in the pre-boiler section. These problems may extend to the boiler section, and therefore these metals must be removed at source. Typically, this is achieved by oxidation followed by filtering off the flocculated iron. (Process examples are aeration towers, contact with chlorine, pressure filters with BIRM media, manganese greensand filters, etc.)... 

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. 

If the vacuum tests are not successfiil because the rate is too slow or, if the tests are successful but the equipment for vacuum filtration would be too large or too costly, or simply because filtration is not achieved, then one has to look at four possible alternatives. These are a bottom feed vacuum filter with precoating flocculating the feed or adding body feed, either for bottom feed or top feed vacuum filters or pressure filtration in all its ramifications, either with or without flocculation or body feed. In certain cases one can even look at precoating pressure filters. In this respect, of course, pressure filters can equally well be centrifugal filters, or compression filters. 

Eor evaluation of flocculants for pressure belt filters, both laboratory-scale filters and filter simulators are available (52,53) in many cases from the manufacturers of the full-scale equipment. The former can be mn either batchwise or continuously the simulators require less substrate and are mn batchwise. The observed parameters include cake moisture, free drainage, release of the cake from the filter cloth, filter blinding, and retention of the flocculated material during appHcation of pressure. 

The second filtration is carried out under a constant pressure difference (normally by applying suction) and the time to filter a known volume is noted. This re-filtration time is directly related to the permeability of the filter cake and can be greatly reduced by flocculation. 

Pressure belt filters, flocculants for, 77 639 Pressure cells, 73 430 Pressure cycle, change related to, 73 402 Pressure cycle data, 73 410-411 Pressure-dependent regioselectivity, 73 439-440... 

Step 3—In a separate step, styrene-acrylonitrile (SAN) resin is prepared by emulsion, suspension, or mass polymerization by free-radical techniques. The operation is carried out in stainless-steel reactors operated at about 75°C (167°F) and moderate pressure for about 7 hours. Tlie final chemical operation is the blending of the ABS graft phase with the SAN resin, plus adding various antioxidants, lubricants, stabilizers, and pigments. Final operations involve preparation of a slurry of fine resin particles (via chemical flocculation), filtering, and drying in a standard fluid-bed dryer at 121-132°C (250-270°F) inlet air temperature. 

---