GB2038196A

GB2038196A - Filter aid

Info

Publication number: GB2038196A
Application number: GB7942913A
Authority: GB
Original assignee: GEC Diesels Ltd
Current assignee: MAN Energy Solutions UK Ltd
Priority date: 1978-12-19
Filing date: 1979-12-12
Publication date: 1980-07-23

Abstract

In a process for the filtration of industrial slurries, in which a precoat bed consisting of a particulate filter aid is deposited on the filter cloth or gauze before the slurry is applied thereto, the filter aid consists of fibrous vegetable matter composed of particles substantially all of which are smaller than 500 microns in diameter. The preferred fibrous matter is bagacillo dust. The precoat can be used with a rotary vacuum drum filter, or with any type of pressure filter in which the precoat technique is normally employed.

Description

SPECIFICATION Filtration of slurries This invention relates to the filtration of slurries which are produced in many industrial processes, including the chemical, food, brewing, man-made fibres, pharmaceutical and effluent treatment industries, and also in refineries of mineral oils and in the production of pigments and dyestuffs.

In a conventional technique, known as the "precoat" technique, for filtering slurries and clarifying filtrates, a coating of a particulate filtration medium is deposited on the surface of the liquid-pervious component of filtration apparatus, through which component filtration is to be effected, for example on the external surface of a filter cloth or gauze constituting the peripheral surface of a rotary vacuum drum.

Such a filtration medium or precoat retains the insoluble matter of the slurry on its surface and the liquid phase passes through it and is clarified, and flows away through the cloth or gauze which is the supporting medium for the precoat. Particulate materials used for this purpose include, for example, diatomaceous earth, kieselguhr, perlite and starch. It is an object of the present invention to provide an improved slurry filtration process in which a particulate filtration medium or precoat is employed, and which is applicable to rotary vacuum drum filters and to other types of filters in which the precoat technique is used, notably pressure filters of various types.

According to the invention, in a process for the filtration of a slurry, which includes the steps of depositing a coating of a particulate filtration medium over a surface of a liquid-pervious filter component, and applying the slurry to the coated surface, the said particulate filtration medium consists of fibrous vegetable matter composed of particles substantially all of which aresmallerthan 500 microns in diameter.

Preferably from 55% to 70% by weight of the said filtration medium consists of particles in the size range between 63 microns and 250 microns, that is to say particles capable of passing a 60 mesh BSS sieve (a sieve having 60 meshes to the linear inch) and retained on a 240 mesh BSS sieve. The preferred fibrous vegetable matter is bagacillo dust, which is derived from bagasse, the sugar cane fibre byproduct of the sugar manufacturing process, but other fibrous vegetable materials in finely divided form are also suitable. Bagacillo dust usually includes particles of larger sizes than the range required for the filtration medium and it is therefore screened to obtain material of the required particle size distribution, if necessary also being ground to reduce the larger particles.

The fibrous vegetable matter filtration medium may be employed with a vacuum drum filter of the type on which a particulate filtration medium is normally used, or with any of the known types of pressure filter in which such a medium is used for clarification purposes.

In use, the powdered fibrous material of the required particle size range is mixed with water or any other desired liquid to form a suspension containing, typically, from 2% to 5% of solid particulate material by weight. To prepare the filter for carrying out the filtration process, this suspension is deposited on the liquid-pervious filter component by means of a suitable applicator, which may be of a standard type employed for the application of conventional filtration media, and during the deposition a pressure differential is maintained through the filter component so that the liquid is withdrawn from the deposited suspension and the residual particulate material is compacted to form an adherent coating on the fiber surface.The coating of powdered fibrous filtration medium so formed may be of any suitable thickness, for example from 1 mm to 100 mm, depending upon the type of filter equipment used and the operating requirement. The slurry to be filtered is then applied to the coated filter in the usual manner, the suspended solids being retained on the coated surface and clear liquid passing through the bed of fibrous vegetable matter. During the filtration process, the retained solids may be removed from the filter either continuously or batchwise, by means of a scraper knife formed of hardend steel or ceramic material, or by a driven fluted roller with suitable cutting edges.

In a specific example of a process in accordance with the invention, for the filtration of a yeast slurry, the particulate filtration medium employed is bagacillo dust which has been ground and screened to give a powder of the following particle size distribution, in percentages by weight:~ > 500 microns (retained on a 30 mesh BSS sieve) 0.05-0.20% > 250 microns (retained on a 60 mesh BSS sieve) 5-10% > 125 microns (retained on a 120 mesh BSS sieve) :30-35% > 63 microns (retained on a 240 mesh BSS sieve) :25-35% < 63 microns (passing a 240 mesh BSS sieve) 18-30% This powder is mixed with water to form a suspension containing from 2% to 5% of bagacillo, by weight.This suspension is applied to the exterior of the peripheral surface, formed of filter cloth, of a rotary vacuum drum, which may be of the form described in Patent Specification No. 1,443,432, the drum being evacuated to draw the water of the suspension through the filter cloth and to compact the bagacillo powder on the cloth as a precoat bed: the bagacillo coating so formed is usually 25 to 100 mm thick.

The yeast slurry is then filtered in the usual manner, and during the filtration the deposited solids are removed continuously from the bagacillo-coated surface of the filter cloth by means of a scraper knife, which is advanced typically from 0.01 to 0.5 mm per revolution of the drum. As a result of this advancement of the scraper, a thin layer of the bagacillo coating is removed with each layer of deposited solids: this is necessary to ensure complete removal of the slurry solids. When most of the bagacillo coating has been removed in this way, the advancing scraper is automatically stopped, to avoid possible mechanical damage to the filter cloth which might occur if the coating were completely removed, and the rotation of the drum and filtering operation are also stopped.

The residual thin layer of bagacillo powder is then removed from the filter cloth and the latter is washed and then re-coated with bagacillo powder in readi nessforthe next filtration cycle.

If the slurry to be filtered is of a type requiring a clean environment, for example for the production of a pure, clean liquid, the powdered fibrous filtration medium can be sterilised by boiling the suspension thereof before it is applied to the filter, or alternatively a sterilising chemical, such as a dilute solution of sodium hypochlorite, can be mixed with the precoat suspension.

Claims

1. A process for the filtration of a slurry, which includes the steps of depositing a coating of a particulate filtration medium over a surface of a liquidpervious filter component, and applying the slurry to the coated surface, wherein the said particulate filtration medium consists of fibrous vegetable matter composed of particles substantially all of which are smaller than 500 microns in diameter.

2. A process according to Claim 1, wherein from 55% to 70% by weight of the said filtration medium consists of particles in the size range between 63 microns and 250 microns.

3. A process according to Claim 1 or 2, wherein the said filtration medium consists of bagacillo dust.

4. A process according to Claim 1, 2 or 3, wherein the coating of said filtration medium is deposited on the said filter component in the form of a suspension of 2% to 5% by weight of the particulate material in a liquid, and during such deposition a pressure differential is maintained through the filter component so that the liquid is withdrawn from the suspension and the residual particulate material is compacted to form an adherent coating on the filter component.

5. A process according to Claim 1, carried out substantially as hereinbefore described in the specific example.