The Horizontal Belt Filter

This devise consists of a continuous traveling horizontal belt that looks like a conveyor belt. Slurry is loaded at one end and vacuum is pulled on the underside of the belt. The filtrate passes through the belt and the solid cake remains on top. As the belt moves the cake is washed and dried and is then discharged off the end of the belt as is wraps around under the machine. This equipment is usually used for slurries that have solids that are not easily suspendible in solution. Although there are many of these industrial-type cake filters in operation outside biotechnology, few have been specifically optimized or "scaled-down" use in the Biotech lab or pilot plant. Some manufacturers do provide small scale versions of their equipment, but typically these units are used to run pilot tests on a customer's slurry for the purpose of designing the proper large scale equipment. A photograph of an installation is shown in Figure 17 and a schematic of the operation is given in Figure 18.

Horizontal belt filters are perhaps the most commonly used vacuum filters in the chemical processing industry due to their flexibility of operation, adaptation to corrosive slurries and suitability to handle large throughputs. Applications to water treatment have been liomited, but still this is an important machine worth mentioning. 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.

Figure 17. Photo of a horizontal vacuum belt filter installation

The first known filters were the Landskrona and Lurgi built in the 1920'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. Belts 4 meter wide for 120 m2 filters weigh more than 10 tons and are manufactured in one piece from sophisticated rubber compounds are common in the industry. The filter cloth retains the cake and moves together with the belt. With some exceptions, they are made from synthetic materials such polypropylene or polyester with monofilament or multifilament yarns and with sophisticated weaves. 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

Continuous Feed

Continuous Cake Wash coiitwiuùus» cake Qi^vhjl Overflow Weir {stationary! by Scrap: r '»Vire "X Rsiter

Continuous Feed

Continuous Cake Wash coiitwiuùus» cake Qi^vhjl Overflow Weir {stationary! by Scrap: r '»Vire "X Rsiter

V.otîicf Liquor

Wash Filtrato

Figure 18. Schematic of operation.

V.otîicf Liquor

Wash Filtrato

Htti-ytlü oi Ckj'.ti Wölfl iVljy Contain SflJwJt)

Figure 18. Schematic of operation.

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. A special mechanism allows parallel lowering or swinging of the vacuum box for

Select a Horizontal Belt Filter:

<> 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.

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

These machines are designed to meet a wide range of process requirements many of which are subjecting its components to severe and demanding conditions. Systems 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. From a maintenance standpoint, the following are areas that often require attention: (1) 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; (2) 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; (3) The vacuum box is hinged and swings to one side so as to enable the periodical cleaning of its internals from settled fmes. 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 antifriction liners against the sacrificial wear belts and the bottom side of the main belt; (4) 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; (5) 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; (6) 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 can cause difficulties in sealing the long and segmented vacuum box.. Also, 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.

Select a Horizontal Belt Filter:

O When intensive cake washing is required since belt filters make it possible to apply countercurrent washing. O 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. O When scale formation due to flash evaporation is a problem or filtrate temperature must be maintained a vacuum box steam jacketing may be provided.

O 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.

Waste Management And Control

Waste Management And Control

Get All The Support And Guidance You Need To Be A Success At Understanding Waste Management. This Book Is One Of The Most Valuable Resources In The World When It Comes To The Truth about Environment, Waste and Landfills.

Get My Free Ebook


Post a comment