The Belt Filter

The slurry is fed from above by a variety of feed-in systems onto a filter surface which is arranged horizontally. On here is where the whole filter cycle takes place, and where a number of process and technical engineering problems are more beneficially solved than is the case with the drum filter. This led, particularly after WWII, to a rapid expansion of use of the belt filter [17]. Typical applications today include the chemical industry (zeofite, various salts, the potash industry, aluminium hydroxide, fertilizers, etc.), minerals processing (ore sludge, minerals, coal, etc.) or protection of the environment (flue-gas gypsum, power-station fly-ash, ground remediation, etc.). 

The sealing of the stationary vacuum box against the rubber belt is accomplished by means of low-friction slip elements. The rubber belt shdes either over a sliding deck, which is fed with air or water to reduce the friction or over a system of low-friction, low-maintenance rollers. 

Beyond the drying zone the filter cloth is removed by a system of rollers and on its route back, it is washed on both sides through jet pipes. This guarantees a consistently-high filter capacity. Discharge of the filter cake is carried out on the discharge roller by a sharp reversal of the filter cloth. In this way, it is also possible to remove very thin cakes - with the assistance of a discharge-wire additionally - quite effectively. 

On a belt filter the filter cake can be washed in one step or several steps, according to the specification of the end product. 

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The belt filter also works on the vacuum principle, but this time two drums rotate (Figure 4.8). The cake is thus held in a horizontal position during filtration thereby potentially increasing its stability. 

Figure 11.11. Continuous horizontal vacuum filters especially suited to free settling and draining solids, (a) Principle of the conveyor belt filter units may operate up to 0.S m/sec with a cycle time up to 10 min and produce cake thicknesses up to 15 cm. (b) Showing the construction of a grooved rubber belt support for the filter cloth of the belt filter (Purchas, 1981). (c) Rotating horizontal vacuum filter the unit has readily accessible piping and is amenable to thorough washing of free draining solids (Dorr-Oliver Inc.).

Jajwashed 1n the belt filter (c]Active Na = TOS + Total Alkalinity, gmol/1 ( )Mass balance (d)Data not available... 

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. 

During the last decade, many successful belt filter installations have been made by Eimco, Fibres Philippe, Delkor, and Gaudfrin. Even in hemi service at Namhae, Indo-Jordan, and WMC, operating at 95°C or above, the belt filters appear to be successful. Because of their long narrow configuration, the belt filter is well suited as a supplemental filter and three have been installed in Florida for this purpose. 

The only disadvantage is the high cost for the investment and maintenance of the belt filter press, and for a drier to make the cake burnable. 

Washing and adsorption The belt filters are used for washing and dewatering of fine solids in the manufacture of catalysts, zeolites, alumina and other crystalline substances, and so on. The belt filter requires the associated equipment — several vacuum pumps and separation drums. Through-circulation tray driers and through-circulation rotary driers are used to dry solids. All these are continuous crosscurrent processes. These could be replaced with a moving-bed, Hildebrand (screw) conveyer or Kennedy extractor [18], each of which is a countercurrent process. Their potential for PI has not been explored. 

Figure 1.23 Photographs of a single horizontal belt vacuum filter with permission from Pannevis (top) and a large scale, multiple belt filter installation with permission from Delkor (bottom). A schematic of the belt filter cycle is shown in Figure 7.1.

The belt filter press continuously squeezes the sludge through a series of rollers, which apply increasing pressure and shear force on the sludge. 

In the late 1970s and the 1980s, belt filters became more readily accepted. The belt filter... 

Reciprocating tray machines have vacuum trays that are individually evacuated and clamped by suction onto the underside of the belt The trays move forward with the belt filter medium at the same speed, acting as a support for it. At the end of each stroke, with filtration and washing completed, the vacuum in the tray is released and the belt continues to move at a constant velocity. The tray is retracted downwards, away from the belt, to return to the start position, to be evacuated and for the process to be repeated. As the trays move with the belts, there are no sealing problems between belt and vacuum space, and so no air in-leakage, and there... 

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