Continuous rotary vacuum filtration

Once the proper pH is reached, the slurry can be filtered continuously via a rotary vacuum filtration. A clear KF filtrate can be fed directly to the AQUATECH cell stack. The collected cake is a valuable byproduct. For this case the precipitate is a titanium dioxide which is easily upgraded via washing, drying, grinding and purification to the high value pigment grade Ti0-- Even in the raw form, this byproduct will draw credits of up to 30 /T. 

Continuous rotary vacuum filters are sometimes adapted to operate under positive pressures up to about 15 atm for situations in which vacuum filtration is not feasible or economical. This may be the case when the solids are very fine and filter very slowly or when the liquid has a high vapor pressure, has a viscosity greater than 1 P, or is a saturated solution that will crystallize if cooled at all. With slow-filtering slurries the pressure differential across the septum must be greater than can be obtained in a vacuum filter with liquids that vaporize or crystallize at reduced pressure, the pressure on the downstream side of the septum cannot be less than atmospheric. However, the mechanical problems of discharging... 

Several of the eontinuous filtration units discussed below may be operated in pressure and vacuum conditions thus the continuous rotary vacuum drum, disc or hnmimtsjl belt filter can be enclosed in a pressure v seL Separation may then ensue by use of a gas pressure blanket in the vessdL This technique is oflen used in solvent fihrations. Some processes m the simultaaeous use of pressure upstream, with vacuum downstream of tire filter in this maimer, the continuous filtration characteristic can be coiqiled with fairer filtration dif ential pressures. 

The operation of the continuous rotary vacuum filter is illustrated in Figure 4.17. The filter drum is immersed in a bath of slurry from where the liquor is drawn through the filter medium by the pressure drop caused by the application of a vacuum within the drum. During filtration cake builds up on the outside surface of the drum between points A and B as if by constant pressure batch operation. It then travels to point C where it is removed and the cycle repeated. 

Filtration is the separation of two phases, particulate form, ie, soHd particles or Hquid droplets, and continuous, ie, Hquid or gas, from a mixture by passing the mixture through a porous medium. This article discusses the more predominant separation of soHds from Hquids. Filtration of soHd particles or Hquid droplets from gases is dealt with elsewhere (see Airpollution controlmethods). The oldest recorded appHcations of filtration are the purifications of wine and water practiced by the ancient Greeks and Romans. Cake filters, such as the rotary vacuum filter and the filter press, were developed much later from the necessity to filter sewage. 

Most continuous pressure filters available (ca 1993) have their roots in vacuum filtration technology. A rotary dmm or rotary disk vacuum filter can be adapted to pressure by enclosing it in a pressure cover however, the disadvantages of this measure are evident. The enclosure is a pressure vessel which is heavy and expensive, the progress of filtration cannot be watched, and the removal of the cake from the vessel is difficult. Other complications of this method are caused by the necessity of arranging for two or more differential pressures between the inside and outside of the filter, which requires a troublesome system of pressure regulating valves. 

A continuous rotary filter is required for an industrial process for the filtration of a suspension to produce 0.002 m3/s of filtrate. A sample was tested on a small laboratory filter of area 0.023 m2 to which it was fed by means of a slurry pump to give filtrate at a constant rate of 12.5 cm3/s. The pressure difference across the test filter increased from 14 kN/m2 after 300 s filtration to 28 kN/m2 after 900 s, at which time the cake thickness had reached 38 mm. What are suitable dimensions and operating conditions for the rotary filter, assuming that the resistance of the cloth used is one-half that on the test filter, and that the vacuum system is capable of maintaining a constant pressure difference of 70 kN/m2 across the filter ... 

FIGURE 7.7 (a) Cross section of a rotary vacuum filter (b) flow sheet for continuous vacuum filtration. 

Continuous filters are most attractive when the process application is a steady-state continuous one, but the rate at which cake forms and the magnitude of production rate are sometimes overriding factors. A rotary vacuum filter, for example, is a dubious choice if a 3-mm (0.12-in) cake will not form under normal vacuum in less than 5 min and if less than 1.4 m /h (50 ft /h) of wet cake is produced. Upper production-rate limits to the practicality of batch units are harder to establish, but any operation above 5.7 m /h (200 ft /h) of wet cake should be considered for continuous filtration if it is at all feasible. Again, however, other factors such as the desire for flexibility or the need for high pressure may dictate batch equipment. 

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