Rotary & Centrifugal Filter

it just covers a basic info of rotary drum filter and basic aspects of centrifugal filter.

1. Rotary & Centrifugal Filters By Nofal Umair

2. FILTRATION * F iltration may be defined as the separation of solid from a fluid by means of a porous medium that retains the solid but allows the fluid to pass. *The suspension of solid and liquid to be filtered is known as the slurry. The porous medium used to retain the solids is described as the filter medium; * The accumulation of solids on the filter is referred to as the filter cake, while the clear liquid passing through the filter is the filtrate. Factors Affecting Filtration Filtration is affected by the characteristics of the slurry, including: 1- The properties of the liquid , such as density, viscosity, and corrosiveness. 2- The properties of the solid, for example, particle shape, particle size, particle size distribution, and the rigidity or compressibility of the solid. 3- The proportion of solids in the slurry. 4- Whether the objective is to collect the solid, the liquid ,or both. 5- Whether the solids have to be washed free from the liquid or a solute.

3. Rotary vacuum filter (Rotary filter) In large –scale operation, continuous operation is sometimes desirable and it may be necessary to filter slurries containing a high proportion of solids. The rotary filter is continuous in operation and has a system for removing the cake that is formed , so, it is suitable for use with concentrated slurries. It is a metal cylinder mounted horizontally, the curved surface being a perforated plate, supporting a filter cloth. Internally, it is divided into several sectors and a separate connection is made between each sector and a special rotary valve.

4. Operation : The drum is immersed to the required depth in the slurry, which is agitated to prevent settling of the solids, and vacuum is applied to those sectors of the drum which is submerged. A cake of the desired thickness is produced by adjusting the speed of rotation of the drum. Each sector is immersed in turn in the slurry and the cake is then washed and partially dried by means of a current of air. Finally, pressure is applied under the cloth to aid the removal of the cake. Removal of the washed and partially dried cake is affected by means of a doctor knife.

5. Summary of the process: The drum is dipped into the slurry and vacuum applied to the outlet, which is connected to the filtrate receiver. When the cake has formed, the cake drained or partially dried by vacuum. The drum is sprayed with water to wash the cake. Retaining the vacuum connection drains the cake and produces partial dryness then, removed by a doctor knife. When the solids of the slurry are too much that the filter cloth becomes blocked with the particles, a pre-coat filter may be used. A pre-coat of filter aid is deposited on the drum prior to the filtration process.

6. Advantages & disadvantages of rotary drum filter Advantages: 1- The rotary filter is automatic and is continuous in operation, so that the labour costs are very low. 2- The filter has a large capacity , so it is suitable for the filtration of highly concentrated solutions. 3- Variation of the speed of rotation enables the cake thickness to be controlled. 4- Pre-coat of filter aid could used to accelerate the filtration rate. Disadvantages: 1- The rotary filter is a complex piece of equipment , with many moving parts and is very expensive,. 2- In addition to the filter itself, some accessories are connected , e.g , a vacuum pump, vacuum receivers , slurry pumps and agitators are required . 3 - The cake tends to crack due to the air drawn through by the vacuum system, so that washing and drying are not efficient .. 4 - It is suitable only for straight- forward slurries

7. Uses: 1- The rotary filter for continuous operation on large quantities of slurry. 2- Suitable for slurry contains considerable amounts of solids in the range 15-30%. Examples of pharmaceutical application include the collection of calcium carbonate, magnesium carbonate, and starch, and the separation of the mycelium from the fermentation liquor in the manufacture of antibiotics

8. String-discharge rotary drum filter

9. Centrifugal Filters A centrifuge consists of a basket in which mixture of solid and liquid , or mixture of two liquids is rotated at high speed so that it is separated into its constituents by the action of centrifugal force. Types of baskets : A - Imperforated, in which the liquid is removed through a skimming tube , while the solid particles, sediment to the wall. In pharmacy, the centrifuge is commonly used for drying crystals and for separating emulsions into their constituent liquids. B- Perforated basket , in which the liquid passes out through the holes. 1-The perforated basket centrifuge: A vessel about 1m. in diameter and its outer wall is perforated. It is mounted on a vertical shaft by means it can be rotated at a high speed. An outer casing with an outlet collects the liquid thrown out from the basket. The drive motor may be below the centrifuge and it is called under-driven, Other form is over-driven,.

10. Advantages & Disadvantages: Advantages of a centrifuge: 1- It is very compact, occupying very little floor space, 2- It is capable of handling slurries with high proportions of solids . 3- The final product has generally, a very low moisture content if compared to a filter cake of a similar material. Disadvantages: 1- Batch process 2- It involves a considerable labor cost, making the process expensive.