The Horizontal Belt Filter

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Horizontal Belt Filter


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.

Typical flowschemes and their operating sequence is shown below:




Belt Filter without Washing

This shows a basic flowsheet existing in all applications that require straight forwards dewatering. In these applications the objective is to produce a cake with the lowest moisture and there is no importance that remaining liquid in the cake retains its original quality.

Belt Filter with Washing

This flowsheet shows the addition of a cake washing stage at some point downstream cake formation. In this application water, or any other wash liquid, is used to displace the mother liquid whenever the process requires a cake that is free of substances that contaminate the discharged cake.




Belt Filter with Counter-Current Washing

This flowsheet shows a counter current wash system that better utilizes the wash water than a co-current system. In this arrangement solids move in the direction of belt travel and the wash liquid in the opposite direction. For efficient washing and sharp separation between the wash filtrates the wash boxes are positioned close to the partitions that are inside the vacuum box. The wash efficiency is defined as a percent of remaining contaminants in the final cake to the contaminants prior to wash.


Belt Filter with Counter-Current Washing and Cloudy Recycle

When a slurry is applied onto the permeable filter cloth a small amount of solids passes through the pores and finds its way to the mother filtrate. This can be avoided by inserting a partition in the vacuum box just at the point where the slurry feed meets the filter cloth. It requires incorporating a small vacuum receiver with a seal tank the removes this  fraction of "cloudy" filtrate that contains the solid particles. The top of this receiver has a valve set to low vacuum so that a thin heel of cake forms on the filter cloth that serves as a filter medium over the porous cloth and produces a solid free mother filtrate.

This flowsheet is applied to the production of phosphoric acid and the cloudy filtrate is recycled to the upstream reactor or back onto the filter cake as shown in the diagram.



The following animation shows the operation of a belt filter and its components:

Main Belt
Filter Cloth
Feed Box
Wash Box
Vacuum Box
Cloth Wash Box
Discharge Roll
Aligning Roll
Take-Up Roll
Cloth Form Roll

Move mouse pointer over the menu to view the components

Belt filter



The filter consists of the following components and subassemblies:

The Drainage 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 curbs that contain the incoming slurry and then the cake as it moves towards the discharge end. Synthetic heavy duty polyester plies are encapsulated in the rubber part below the grooves serve to withstand the longitudinal stresses to which the belt is subjected during its travel.

Drainage belts are available in 2, 3 and 4.2 meter widths and thicknesses of 28, 32 and 39 mm. The belts may be supplied in SBR or EPDM rubbers and both are elastomers characterized by a wide range of applications.

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 as polypropylene or polyester with monofilament or multifilament yarns and with sophisticated weaves and layers. The images on the right show an ultrasonically welded joint and a clipper joint of the cloth ends. With clipper joints, as may be seen on the right, it is necessary to thread multifilament strings across the entire cloth width to retain the fines from passing through to the filtrate.

The entire subject of filter cloth and its selection will be discussed in a separate section that was not yet constructed.

Cloth End Joints Cloth Trajectory

The Vacuum Box and Wear Belts

Vacuum box

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

Vacuum box lowering  

A special mechanism allows parallel lowering or swinging of the vacuum box for cleaning from fines that may have settled inside. The mechanism is designed to accurately seal between the underside of the main belt and the two narrow wear belts that move together along the slide strips attached to the top shoulders of the vacuum box.




The Feed, Wash Boxes and Spray Manifolds 

A feed box and one or more wash boxes are mounted over the filter and designed to distribute evenly the slurry and wash water across the belt.


 Feed and wash boxes


Spray washing as shown in the clip is also used quite often.


The Cake Discharge End

To watch the discharging cake please play the clip below:

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.

Mother Filtrate
Wash Filtrate 1
Wash Filtrate 2
Filtrate Manifold
Filtrate Receiver
Filtrate Pump
Vacuum Pump
To view the components move mouse pointer over the menu Filtrate manifold


The Filtrate Manifold

A filtrate manifold collects the mother and wash liquids to one or more vacuum receivers. It should be kept in mind that a short path of filtrate between the vacuum box and the receivers reduces to a minimum the losses of vacuum for both the single phase flow of the mother filtrate and the two phase flow of air and wash filtrates.

In the picture all filtrate outlets are connected to a common manifold with a single receiver so both mother and wash filtrates are mixed. However, as may be seen in the flowscheme, mother and wash filtrates may be delivered separately.

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:


Selection Criteria

Horizontal Belt Filters are selected in the following cases:


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: