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A belt filter resembles an ordinary belt conveyor driven by one of the guide rollers. Slurry is fed onto the filter belt from above and the filtrate drains or is pulled by suction from the bottom of the belt. Belt filters are operated as simple grav-ity, vacuum and pressure filters. Whether oper-ation is continuous or intermittent (semicontin-uous), slurry is fed onto one end of the belt. The filtrate drains into vacuum tanks, which can be stationary or can move intermittently with the belt. The cake can be washed and dewatered in downstream zones. If necessary, the filter belt is washed during recycling. In another design, the belt is made up of a number of individual cells, which bear the filter cake. In this way, a more careful separation of the filtration stages (see also Fig. 1) is achieved.

In general, belt filters are applied for read-ily filterable suspensions that contain coarser, and therefore easily sedimented, solids (groups F and M in Table 2), where the filtrate and the wash liquor are to be collected separately and the solids must have gentle handling. The maximum

Table 2. Filterability of cakes

Filtration characteristics Group of suspension, symbol

Fast Medium Slow Dilute Very dilute

(F) (M) (S) (D) (VD)

Initial cake formation rate, min/cm 0.005 – 0.1 0.1 – 1 1 – 10 10 – 100 no cake

Slurry concentration, % > 20 10 to 20 1 to 10 < 5 < 0.1

Settling rate rapid medium slow slow

Leaf test rate, kg/m²h > 2500 250 to 2500 25 to 250 < 25 no test

Filtrate rate, m³/m²h > 10 0.5 to 10 0.0025 to 0.05 0.025 to 0.05 0.025 to 0.05

Typical slurry crystalline solids salts pigments wastewater water

filtration area is 120 m². Fluctuations in product call for control of the belt speed or slurry rate.

For especially sensitive filtrations, belt filters are used with filter media that remove the liq-uid from the cake by capillary action alone. The filter cloth runs over felt or similar filter media.

Vacuum Belt Filters. A continuous vac-uum belt filter consists of the moving filter cloth supported on a profiled elastomer transport belt (Fig. 37). To provide better sealing for the vac-uum space, sliding belts can be placed under the transport belt, or a specially shaped belt can travel along with the filter cloth. The difficul-ties met with in ensuring a good vacuum with moving seal systems are overcome when the fil-ter belt or filtrate box move infil-termittently. Fig-ure 38 shows an example. When the belt is sta-tionary, slurry is fed to the filter and at the same time the filter cake is washed, dewatered and, if necessary, pressed. In the intermittent move-ment of the belt, the discharge roller shifts the belt, while the retaining roller holds the belt in place. The movement is compensated by move-ment of the compensating roller in the opposite direction. The vacuum is let down when the belt is moving.

Another continuous vacuum belt filter has vacuum tanks of pan-shaped sections each 1.4 m long. Each section is divided into two parts with flexible connections to the vacuum system (vac-uum pumps). In this way the individual filtra-tion stages are divided into arbitrary lengths as needed. The vacuum tanks are designed with grids that allow the filtrate to drain freely. Dur-ing filtration, the tanks move along with the belt at the same speed, as the vacuum builds up. At the end of the co-moving path, the vacuum is broken and the tank vented, while the filter cloth

moves on. Slurry and wash liquors used are fed continuously.

Vacuum belt filters are usually installed in open frames, or in closed housings if no va-por is to be released into the environment. Good filtration capacities and washing effects can be achieved only with uniform loading of the filter medium or filter cake. For this reason, the feed and wash liquor are distributed over the width of the filter belt by flat nozzles or washing grooves.

Downstream dewatering of the filter cake is done by double-belt expression (see twin-belt press, page 36) or a plate press.

Because of the versatility of vacuum belt fil-ters, they have found use in nearly every field of liquid–solid separations where slurries are rela-tively easy to separate.

Even difficultly filterable slurries with com-paratively finely dispersed solids can be sep-arated on belt filters if suitable belt materials are employed and the solids do not immedi-ately block the medium. In such cases, pres-sure is most commonly used instead of vacuum (see page 36).

Belt filters for gravity and suction filtration offer low equipment costs. With a suitable filter medium (nonwovens are generally used), they can successfully clean up slurries that are not too highly concentrated (solids down to 50 mg/m³) and of the filterability group F in Table 2. Feeds include machine-tool cooling and cutting lu-bricants (emulsions), electrolysis cell slimes, beer (for removal of turbidity and yeast), waste-waters, and chemical solutions.

A gravity or hydrostatic filter with a contin-uous belt (Fig. 39) consists of a tank, in which a moving support belt made of coarse woven fabric or a flexible belt forms a depression. The filter belt proper rests on this support belt. Op-eration is intermittent: slurry is fed in until the

Figure 37. A), B) Vacuum belt filter (reproduced with permission of Dorr – Oliver)

a) Filter cloth take-up assembly; b) Feed box; c) Cake wash; d) Vacuum pan; e) Filter cloth; f) Filter cloth wash; g) Drip pan drain; h) Drainage belt; i) Tensioning device; j) Filter cloth aligning; k) Drainage belt

Figure 38. Jerking-type vacuum belt filter (BAS Sonthofen)

a) Filter cloth take-up assembly; b) Cake wash; c) Vacuum trays; d) Filter cloth; e) Movable discharge roller; f) Cloth wash assembly; g) Blocking roller; h) Tensioning assembly; i) Floating roller

increasing resistance to flow causes the liquid above the filter belt to reach a predetermined depth. The hydrostatic head promotes the tration rate. The belt is then advanced, the fil-ter cake carried out of the tank, and new filfil-ter medium transported into it. The filtrate flow rate is 0.2 m³m⁻²min⁻¹.

There are two designs of suction belt filters, also called flat-bed filters. One, used chiefly for wastewater treatment, consists of a receiving tank for slurry, with a drag chain conveyor. The perforated or slotted floor of the tank holds the filtrate receiver, which is connected to the pump.

The filter medium is guided between the scrap-ers and the tank floor. After a filter cake has built

Figure 39. Hydrostatic belt filter

a) Sludge bin; b) Float control; c) Filter trough; d) Filter medium; e) Carrier belt; f) Filtrate tank

up to a predetermined thickness, the increasing pressure drop causes the conveyor to begin mov-ing. It advances some 20 – 50 cm along with the filter cloth. These filters are made with filtration areas up to 21 m².

The other design consists of a vacuum tank with lid; the paper filter tape is fed between the tank and the lid. This type of filter also operates intermittently. At a filter area of 1 m²a through-put of 0.1 – 0.5 m³/h can be achieved, depending on the amount of solids in the suspension and the presence of filter aids.

Pressure Belt Filters. A pressure belt filter of the flat bed type consists of a filtrate receiver and a pressure chamber. The filter medium is guided between the two chambers, which are situated in a housing. Operation is necessarily intermittent, since the pressure chamber, which is provided with slotted gates, is pressurized dur-ing filtration. In smaller units, the use of slotted gates is replaced by pivoting up the pressurized section. Pressure belt filters are also made with expression devices (up to some 2 MPa).

A twin-belt pressure filter is a belt filter with a second, co-moving belt, which exerts an addi-tional mechanical compressive load on the filter cake with pressure rollers. Such a filter can be employed provided the cake is deformable. This condition holds especially for bulky flocculated slimes such as occur with filter cakes made up of relatively fine solid particles (mainly of organic origin) having a broad particle size distribution.

Twin-belt pressure filters have proved them-selves for the dewatering of sewage sludges that have been treated with polyelectrolytes (based on poly(acrylic acid) and polyacrylamide) to produce relatively large, stable flocs.

Figure 40. Schematic processing in twin-belt pressure fil-ters

This type of filter has made it possible to re-duce the relatively high 70 vol % residual mois-ture in municipal sewage sludges by as much as 10 vol %, corresponding to a decrease of 35.7 % in the moisture content per cubic meter of sludge. Further applications have also been tested, and this class of filter has been adopted for some dewatering jobs, such as slimes from coal washing, large-scale animal husbandry, cel-lulose and pulp production, electrolysis, and off-gas treatment. An average dewatering capacity for designs now current is 6m³ of municipal sewage sludge per meter of belt width and hour.

With sludges that are readily dewaterable, the influent rate can be increased to 15 m³/h.

Machines with multistage dewatering are used in nearly all these sample applications. Fig-ure 40 shows a simplified diagram. The floccu-lated sludge is put through preliminary dewater-ing in a simple screen drainage (rotatdewater-ing or flat screens), since part of the liquid bound up in the

Figure 41. Twin-belt pressure filter (reproduced with permission of Flottweg)

sludge is released in the flocculation step. Vac-uum filtration usually follows before the filter cake reaches the expression zone.

A special feature of twin-belt pressure filters is that the belts change direction several times, on rollers that shear and press the cake, thus promoting its compaction. Usually the filtrate produced in the several stages, which is not en-tirely clear, is recycled to the first stage, where the solids still dispersed in the filtrate are re-flocculated. The filtrate resulting here is largely free of solids and can be passed on to, for exam-ple, biological treatment.

Quite a variety of arrangements of the indi-vidual process stages and design features has come into being. The performance of these ma-chines thus varies widely, being influenced as well by the properties of the influent sludges.

Twin-belt pressure filters will find additional use in food treatment, for instance in juice ex-traction from fruits and vegetables. Figure 41 shows a typical arrangement of that twin-belt press filters including horizontal press section and several turns of the belts. Here the recov-ery of juice may reach 65 – 70 % in a 2500 mm machine and 7 – 10 t/h throughput.

In the modification of Figure 42 A, the sludge enters the expression zones from below, through a straining zone (gravity dewatering). Passing

over rolls that vary in diameter, the cake is sheared and compressed (up to somewhat over 100 kPa). The modification of Figure 42 B ap-plies a further increased compressive load with a third moving flat belt.

Figure 42 C shows a filter in which an over-flow (for gentle handling of the flocculated solids) directly connects the flocculation vessel to the gravity filtration and dewatering stages;

the filter cake passes downward through the suc-cessive expressing zones. The last stage is sup-plemented by a device that exerts a controlled linear pressure.

Cake compression in twin-belt pressure fil-ters is rather modest in comparison with the com-pressions achieved in membrane filter presses (see page 58). The twin-belt devices have the ad-vantage of continuous operation and relatively low cost. A recent development demonstrates the possibility of reaching pressures in twin-belt expressing filters that are similar to those in membrane filters (up to 2 MPa). A filtration zone with filter belts converging in wedge fash-ion is followed by compactfash-ion as the cake turns around a drum pressurized by hydraulically ac-tuated elements.

Experience and theoretical analyses (see Sec-tion 4.2) lead to the conclusion that optimal per-formance is attained in twin-belt pressure filters

Figure 42. Modifications of twin-belt pressure filters (A, B, and C)

a) Press zone; b) Gravity strainer zone; c) Pre-compression zone; d) Press belt high compression; e) Low compression zone;

f) Flocculation; g) Cake break-up; h) Line compression

when the cake is relatively thin, the filtrate drains on both sides, the expressing times are long (es-pecially for readily compressible cakes), and the expressing force is as great as possible.