Gravity belt filter

Typical uses. Separation of free-draining solids requiring gentle handling, e.g. municipal waste water. 

Typical particle size and feed concentration range 100-10000 pm and 3% w/w. 

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Figure 1.55 Photograph of a gravity belt filter (Sernagiotto). The unit can be covered to prevent the escape of vapours and/or odours as required.

A belt filter press consists of a gravity drainage belt, foUowed by a series of roUer presses which squeeze out water. 

The feed rate is manually set as required for either pH or reactant stoichiometry control. Soda ash is added to the fourth reactor as sodium makeup. Reactor effluent slurry flows by gravity to the thickener centerwell. Clarified liquor overflows from the thickener to the forward feed hold tank from which it is pumped to the tray tower. A horizontal belt filter is used for further dewatering of the thickener underflow solids. 

Belt Filter Presses Belt presses (Fig. 18-193) have two filter belts that move around rollers of different sizes to dewater the slurry. A typical belt press may have one or more of the following stages a preconditioning zone, a gravity drainage zone, a linear compression zone (low pressure), and a roller compression zone (high pressure). 

In vacuum filtration, the driving force ( 20 in Hg = lOpsi, 1 psi = 6895 Pa) is slightly higher than the gravity. The vacuum operation is frequently tied with continuous equipment such as drum, disc or belt filters, in which cake is removed continuously. As the permeability of cakes diminishes, pressure becomes an important element in producing a satisfactory flow rate. 

Gravity filters depend upon atmospheric pressure to force the solids through the medium. The slurry is fed into the top of the filter and through the medium, with the clear filtrate emerging from the underdrain. Sand filters, travelling belt filters and rotary drum gravity filters are found in this group. 

With top-fed vacuum filters, this to some extent takes place naturally by gravity settling the resulting cake is somewhat coarser at the bottom, nearer the filtrate medium. This size stratification is the right way up and beneficial to the filtration process. Figure 13.14 shows this schematically for a horizontal belt filter. 

The principle of continuous vacuum filtration on a flat horizontal surface with filtration in the direction of gravity is utUised amongst others by belt filters and pan filters. 

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