Filtration horizontal vacuum filters

Fihration - The function of the filtration step is to separate the gypsum (and any insoluble materials derived from phosphate rock or formed in the reaction) from the phosphoric acid product as completely, efficiently, and economically as possible. All modem plants use only continuous horizontal vacuum filters. 

Figure 5. Cross section of a rotary horizontal vacuum filter showing filtrate removal system, filter cloth and discharge scroll.

Figure 14-8. Rotary horizontal vacuum filter. (Courtesy of Filtration Engineers Division, American Machine and Metals Inc.)...

Vacuum filters are available in a variety of types, and are usually classified as either batch operated or continuous. An important distinguishing feature is the position of the filtration area with respect to gravity, ie, horizontal or non-horizontal filtering surface. 

Horizontal filter surfaces also allow a high degree of control over cake formation. Allowances can be made for changed feeds and/or different cake quality requirements. This is particularly tme of the horizontal belt vacuum filters. With these units the relative proportions of the belt allocated to filtration, washing, drying, etc, as well as the belt speed and vacuum quality, can be easily altered to suit process changes. 

Horizontal belt filters are well suited to either fast or slowly draining soHds, especially where washing requirements are critical. Multistage countercurrent washing can be effectively carried out due to the sharp separation of filtrates available. Horizontal belt vacuum filters are classified according to the method employed to support the filter medium. 

Some horizontal belt vacuum filter designs incorporate a final compression stage for maximum mechanical dewatering. This is achieved by another compression belt which presses down on the cake formed in the preceding conventional filtration stage. 

This family of filters consist of a vertical pressure vessel with a horizontal filter plate at the bottom. The filtrate from this equipment flows out a nozzle on the bottom of the filter. These devises are usually used for slurries where large amounts of solids are being collected. Variations of this equipment include equipment with removable lower heads for easy cake removal, ability to pressure or vacuum filter, ability to wash the filter cake, an agitator to break-up and rewash the filter cake, and heating or cooling jackets for the whole vessel. The Nutsche filter is the industrial version of the well known laboratory scale Buchner Funnel with the exception that it is designed to operate under either on vacuum or pressure. 

Horizontal Belt Vacuum Filter The final piece of equipment is the horizontal belt filter, in which the nnderflow from the hydroseparator (basically gypsum diluted with process liquor) will be filtered to produce marketable gypsum of -12% moisture. The filter cake will be washed to reduce the amount of dissolved solids and will be conveyed to a prodnct gypsnm storage area. Filtrate from this filter will be disposed of as an aqneons pnrge stream. 

The calcinate was quenched in water to produce a pumpable slurry. Carbon washing was carried out on a horizontal belt vacuum filter which reduced the salt content to 4 to 5%. To further reduce the salt content, if needed, additional slurrying and washing on the belt filter was required. In commercial operation the filtrate containing potassium salts could be processed to reconstitute the KOH for recycle. 

Like horizontal belt filters, the indexing belt filters are operated semicontinuously under vacuum or pressure. The belt is stationary during filtration and indexes forward for cake discharge. Dispos-... 

Although widely used in dust collection, nonwoven fabrics have found only limited use in liquid filtration because their thickness and density render them prone to blinding in many applications. One area where they have found some success, however, has been in the filtration of metal ore concentrates such as copper on horizontal vacuum belt filters. These applications tend to be very aggressive on the filter fabric, and hence a suitably designed and finished nonwoven fabric is often more cost effective than a considerably more expensive woven fabric, especially if required in lengths of around 80 m and widths up to 6 m. The solids which are captured in such applications quickly form a cake on the surface and, should some penetration occur, as with woollen spun yams, the bulky nature of the material provides scope for the particles to escape. For such arduous applications, nonwoven fabrics are generally in the area density range of 800-1000 gm- ... 

Continuous filtration is discussed by Coulson and Richardson.. It commonly employs rotary vacuum filters. These are usually cylindrical in shape, rotating about a horizontal axis with a filter medium on the curved surface and a vacuum applied from inside. Rotating disc filters are less frequently employed (see Fig. 2.4). These have a number of discs on the same shaft with vacuum applied from inside but the flat surfaces of the discs hold the filter medium. The methods of continuous discharge of the filter cake during operation, and of replacement of the filter medium at intervals, are less elegant than those applicable to rotating cylinders. 

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