The Horizontal Belt Filter

Description

Horizontal Belt Filters are, in broad terms, the most commonly used vacuum filters in the industry due to their flexibility of operation, adaptation to corrosive slurries and suitability to handle large throughputs.

The development of the Horizontal Belt Filters for the chemical process industries was closely associated with the progress in rubber technology since they incorporate an endless and thick rubber belt of a complex design to support the cake retained by the filter cloth.

The first known filters were the Landskrona and Lurgi built in the 20's and the Giorgini which was a belt filter but with attached trays. The belts were very narrow and short, with a 30 cm wide by 4-5 meters length, and were primarily applied to the washing of phosphate rock. Later, being top feed filters that facilitated multi-washing stages, they were applied in phosphoric acid plants to replace the chains of 3 or 4 internal feed rubber covered Drum Filters used for gypsum washing. As the demand for area has gone up filters were manufactured with three and four 30 cm wide belts running in parallel since the rubber manufacturers were unable to catch-up with the growth of the chemical plants. For this reason the main rivals over the years to belt filters were the Tilting Pan and Table Filters so when rubber belts were the constraint to filtration area growth these filters were in demand and vice versa. Nowadays it is high time for belt filters since rubber technology has made a big step forwards in the past 10 years. Belts 4 meter wide for 120 m2 filters weigh more than 10 ton and are manufactured in one piece from sophisticated rubber compounds. 

Belt speed is another parameter that sets forth a race among the designers of filters since for many applications a short cycle time is essential. The constraining factor on belt speed is purely mechanical and depends largely on the supporting method of the heavy belt with its cake on it. Belt filters are the fastest filters available today and the speed of modern filters can reach over 50 m/min and yield very short cycle times.

A typical flowscheme and its operational sequence with three counter-current wash  stages and separation of mother and wash filtrates may be viewed on the image below:

The filter consists of the following subassemblies:

The Belt

An endless rubber belt with traversing grooves drains the filtrate towards holes positioned along the belt. The sides of the belt have elastic rubber shrouds that contain the incoming slurry and then the cake as it moves towards the discharge end. Synthetic plies encapsulated in the rubber part below the grooves serve to withstand the longitudinal stresses to which the belt is subjected.

The weight of a 3 m wide belt is 125 Kg/m and this is the heaviest single component to be considered for the design of the hoisting facilities.

The Filter Cloth

The filter cloth retains the cake and moves together with the belt. Nowadays, with some exceptions, they are made from synthetic materials such polypropylene or polyester with monofilament or multifilament yarns and with sophisticated weaves. The entire subject of filter cloth and its selection will be discussed in a separate section that was not yet constructed.

The Vacuum Box and Wear Belts

A vacuum box below the belt that is mounted along the filter and collects the filtrate through a manifold to the receivers. The box at its topside has two lips covered with low friction synthetic strip liners that seal through intermediate wear belts between the bottom side of the belt and the surface of the strips. Since the belt is the most expensive part of the filter these endless narrow belts serve as a sacrificial component that takes the wear between the surfaces, protects the rubber belt and secures against vacuum leaks.

The Vacuum Box Lowering Mechanism

The Cake Discharge End

Once the belt reaches the end of the vacuum box the cake drying portion of the cycle terminates and the cloth leaves the rubber belt. The cloth continues moving, changes direction over the discharge roll and the cake drops through a chute for further handling.

The Belt Supporting Deck

A deck attached to the frame and mounted underneath the belt is designed to support the heavy rubber belt and the cake load. The friction between the surfaces is reduced by injecting water for lubrication and blowing air that floats the belt or by a moving floor constructed of narrow endless belts that move together with the main rubber belt.

The Cloth Tracking Mechanism

A pneumatic or electrical tracking mechanism controls the filter cloth from slipping sideways by guiding it to the left or to the right.

There are several types of mechanisms but the following are very common:

• Two pairs of rolls that pinch the cloth alternatively and are positioned on both sides.

• A roll is that spans across the cloth, is hinged at one end and swings forwards or backwards on the other end.

 Selection Criteria

Horizontal Belt Filters are selected in the following cases:

• For solids that are fast settling and cannot be kept as a homogenous slurry in bottom or side feed filters such as Drum or Disc Filters.

• When long drying time is required to reach asymptotic moisture in the cake. On Drum Filters, for example, the ratio of dry to form cannot normally exceed 1.5 since it is determined by its geometry and the number of circumferential compartments.

• When very short cycle times are required for fast dewatering cakes such as phosphate slurry.

• If a clear filtrate is required right from the start it is good practice to form a thin heel that serves as filter medium over the exposed cloth. This is done by either a "cloudy port outlet" that is recirculated or, if solids are settling fast, by allocating the first 20-30 cm to act as a "sedimentation pool" prior to entering the vacuum zone.

• When intensive cake washing is required since belt filters make it possible to apply counter-current washing. On Drum Filters, for example, the time available for washing is rather limited due to its geometry.

• When cakes tend to crack under vacuum measures such as a flapper, compression blanket or pressure roll may assist in sealing the cracks thus avoiding loss of vacuum. When such measures are used it is necessary to make sure that the belt supporting system can take these extra vertical loads.

• When scale formation due to flash evaporation is a problem or filtrate temperature must be maintained a vacuum box steam jacketing may be provided.

• When the cake tends to clog the cloth its continuous removal after cake discharge enables dislodging of particles by thorough washing of the cloth on both sides with high impact nozzles.

Operational Sequence

A filtration cycle on a Horizontal Belt Filter, as may be seen in the flowscheme above, consists of the following zones at maximum configuration:

• Cloudy Port Recycle or Sedimentation Pool.

• Cake Formation.

• Cake Predrying.

• First Washing.

• First Predrying.

• Second Washing.

• Second Predrying.

• Third Washing.

• Final Drying.

Maintenance

Horizontal Belt Filters are designed nowadays to meet a wide range of process requirements many of which are subjecting its components to severe and demanding conditions. Modern filters run at high speeds, handle thick and heavy cakes, operate at high temperatures and often in an unfriendly environment hence, they are of a sturdy design and made from sophisticated materials of construction.

The main points to observe are:

• Evidence of cracks in the rubber belt may cause separation of the plies which are encapsulated between the rubber layers. This weakens the belt and should be repaired on site without delay.

• The shrouds on both sides of the belt are subjected to high tension while going over the head and tail pulleys. Their duty is to contain the incoming feed and if the edges tear slurry may pour all over so inspection and their repair is essential.

• The vacuum box is hinged and swings to one side so as to enable the periodical cleaning of its internals from settled fines. The repositioning of the box is one of the main reasons for loss of vacuum and special care must be taken to seal the box's anti-friction liners against the sacrificial wear belts and the bottom side of the main belt.

• The endless wear belts must be inspected to ensure that they are in good condition otherwise the main belt may be damaged. Likewise, the wear belts should be checked if they seal properly between the stationary vacuum box and the moving belt.

• The life of the belt and the main drive depend largely on the water lubrication between the surfaces of the moving and stationary parts hence, the tubes leading to those parts must be kept clean.

• It is recommended that the alignment of the filter is inspected from time to time. This applies mainly to large filters since misalignment due to differential settling of the building foundations during the first years after start-up or any other reason may cause the following:
   

  • Along the filter, difficulties may arise in sealing the long and segmented vacuum box.

  • Across the filter, the thickness of the cake may taper in one direction causing uneven cake washing. The alignment across the filter is particularly important for thin cakes since a 0.5% slope on a 2 meter wide belt and a 20 mm cake reduces cake thickness on one side from 20 to 10 mm.

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