GB2163676A

GB2163676A - Magnetic filter

Info

Publication number: GB2163676A
Application number: GB08520401A
Authority: GB
Inventors: David Huw Prothero
Original assignee: International Research and Development Co Ltd
Current assignee: International Research and Development Co Ltd
Priority date: 1984-08-14
Filing date: 1985-08-14
Publication date: 1986-03-05
Also published as: GB8420668D0; GB8520401D0

Abstract

For removing ferromagnetic or paramagnetic particles from a flow stream the filter comprises permanently magnetised rods 12 of high coercivity extending across the flow path and forming an array supported by non-magnetic plates 10 and 11. In an alternative construction the array is formed of perforated plates of permanently-magnetised material disposed parallel to the direction of flow and defining flow passages therebetween. Preferred materials are given. The use of strongly permanently magnetised material obviates the need for a permanent magnet or electromagnet external to the stream. Cleaning of the filter is effected by back-flushing or by removing the filter from the flow stream and using a low-pressure jet of liquid or air. <IMAGE>

Description

SPECIFICATION Magnetic filter The present invention relates to a magnetic filter.

In its normal form such a filter consists of a matrix of ferromagnetic material (for example ferritic stainless steel to AISI 430) in the form of wire wool or balls. When placed between the poles of a permanent magnet or an electromagnet, the matrix becomes strongly magnetised and can attract strongly magnetised material such as iron or magnetite, and even much more weakly magnetic material such as chromium or copper oxide.

Such a filter has been used, for example in the chemical industry for removing magnetic materials from a process stream. It can be used for removing material either from slurries (liquid suspension) or from gas suspension.

The present invention differs from this normal pattern in that the filter consists of a matrix of strongly permanently magnetised material. In this case, the filter will be able to remove magnetic species without the need for any permanent magnet or electromagnet external to the process stream. This obviously leads to a considerable saving in the space occupied by a magnetic filter and also the cost of the filter unit.

For a magnetic filter in accordance with the invention strongly magnetised material is necessary. Ordinary magnetic materials such as mild steel or ferritic stainless steel, with coercive forces of the order of 0.8 kAm ' (10 oersted) are not suitable since for effective trapping of even strongly magnetic particles in a flow stream a coercivity of at least 8kAm (100 oersted), preferably 16 to 32kAm ' (200 to 400 oersted) is required. This is obtainable with known magnetic alloys such as the iron, cobalt and chromium alloy available under the name "Chromindur". Too high a coercivity is not usually suitable for use in trapping ferromagnetic particles such as iron particles because it is then very difficult to remove the particles trapped on the filter.However coercive forces of the order of 1000 oersted, which are possessed by various ferrite material and also by rare earth alloys (such as samarium-cobait), can be successfully used for trapping paramagnetic particles.

The filter need not be a tighly packed bed of magnetised material. On the contrary an arrangement in which the magnetised material occupies less than 5% of the space within the filter has been found to be particularly effective in removing finely divided iron in suspension in water. Hence it would be particularly suitable for a wide range of processes where it is required to remove steel swart or debris from a process stream.

Two forms of filter which are suitable for trapping iron particles are shown in the accompanying drawings, in which: Figures 1 and 2 are, respectively, a plan view and an end elevation of a filter assembly using magnetised rods, and Figures 3 and 4 are, respectively, a plan view and an end elevation of a filter assembly using magnetised perforated plates.

In the filter assembly of Figs. 1 and 2, two spaced perforated sheets 10 and 11 support a plurality of magnetised rods 12 which extend through aligned apertures in the sheets and are arranged in staggered rows with a row 13 of blank holes between each two rows of rods and with a blank hole 14 between each pair of rods in each row. The perforated sheets which form a supporting grid can be made of a wide range of materials providing they are not attacked by the process stream. Examples are austenitic or ferritic stainless steel, mild steel, brass, etc.

In the filter assembly of Figs. 3 and 4 perforated sheets 15 are themselves formed of magnetised material and are held in a stack with a suitable spacing between the plates by screwed rods 16. Alternatively sheets of expanded metal may be used as spacers. The perforations 17 in the magnetised sheets are necessary because the magnetic particles are trapped at the edges of the perforations.

In both types of filter assembly the magnetised elements can be fabricated in Chromindue alloy, ferrite or rare earth alloy. The assembly in each case is mounted in a cylindrical caniester through which the process stream flows.

Flushing of the filter by means of a backflow of the water through the canister is satisfactory provided that the particles which have been trapped are relatively small in size (i.e.

much less than 50 micrometres in mean diameter) or are not strongly magnetic. Backflushing however is not very satisfactory for removing iron particles of mean size 50 micrometres or more. Effective cleaning can be achieved in this case by removing the filter assembly from the canister and directing a low-pressure jet of water or other liquid, or alternatively compressed air, over the filter assembly.

1. A magnetic filter for the removal of magnetic material from a flow stream, the filter comprising an array of rods, bars, or plates of permanently-magnetised material hav ing a coercivity of at least 8kAm-' (100 oersted) which extend across the flow path, leaving spaces therebetween for the passage of the flow stream.

2. A filter as claimed in claim 1 in which the magnetised material occupies less than 5% of the space within the filter.

3. A filter as claimed in claim 1 or 2 in which the filter comprises perforated plates of magnetised material arranged in a parallel array.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Magnetic filter The present invention relates to a magnetic filter. In its normal form such a filter consists of a matrix of ferromagnetic material (for example ferritic stainless steel to AISI 430) in the form of wire wool or balls. When placed between the poles of a permanent magnet or an electromagnet, the matrix becomes strongly magnetised and can attract strongly magnetised material such as iron or magnetite, and even much more weakly magnetic material such as chromium or copper oxide. Such a filter has been used, for example in the chemical industry for removing magnetic materials from a process stream. It can be used for removing material either from slurries (liquid suspension) or from gas suspension. The present invention differs from this normal pattern in that the filter consists of a matrix of strongly permanently magnetised material. In this case, the filter will be able to remove magnetic species without the need for any permanent magnet or electromagnet external to the process stream. This obviously leads to a considerable saving in the space occupied by a magnetic filter and also the cost of the filter unit. For a magnetic filter in accordance with the invention strongly magnetised material is necessary. Ordinary magnetic materials such as mild steel or ferritic stainless steel, with coercive forces of the order of 0.8 kAm ' (10 oersted) are not suitable since for effective trapping of even strongly magnetic particles in a flow stream a coercivity of at least 8kAm (100 oersted), preferably 16 to 32kAm ' (200 to 400 oersted) is required. This is obtainable with known magnetic alloys such as the iron, cobalt and chromium alloy available under the name "Chromindur". Too high a coercivity is not usually suitable for use in trapping ferromagnetic particles such as iron particles because it is then very difficult to remove the particles trapped on the filter.However coercive forces of the order of 1000 oersted, which are possessed by various ferrite material and also by rare earth alloys (such as samarium-cobait), can be successfully used for trapping paramagnetic particles. The filter need not be a tighly packed bed of magnetised material. On the contrary an arrangement in which the magnetised material occupies less than 5% of the space within the filter has been found to be particularly effective in removing finely divided iron in suspension in water. Hence it would be particularly suitable for a wide range of processes where it is required to remove steel swart or debris from a process stream. Two forms of filter which are suitable for trapping iron particles are shown in the accompanying drawings, in which: Figures 1 and 2 are, respectively, a plan view and an end elevation of a filter assembly using magnetised rods, and Figures 3 and 4 are, respectively, a plan view and an end elevation of a filter assembly using magnetised perforated plates. In the filter assembly of Figs. 1 and 2, two spaced perforated sheets 10 and 11 support a plurality of magnetised rods 12 which extend through aligned apertures in the sheets and are arranged in staggered rows with a row 13 of blank holes between each two rows of rods and with a blank hole 14 between each pair of rods in each row. The perforated sheets which form a supporting grid can be made of a wide range of materials providing they are not attacked by the process stream. Examples are austenitic or ferritic stainless steel, mild steel, brass, etc. In the filter assembly of Figs. 3 and 4 perforated sheets 15 are themselves formed of magnetised material and are held in a stack with a suitable spacing between the plates by screwed rods 16. Alternatively sheets of expanded metal may be used as spacers. The perforations 17 in the magnetised sheets are necessary because the magnetic particles are trapped at the edges of the perforations. In both types of filter assembly the magnetised elements can be fabricated in Chromindue alloy, ferrite or rare earth alloy. The assembly in each case is mounted in a cylindrical caniester through which the process stream flows. Flushing of the filter by means of a backflow of the water through the canister is satisfactory provided that the particles which have been trapped are relatively small in size (i.e. much less than 50 micrometres in mean diameter) or are not strongly magnetic. Backflushing however is not very satisfactory for removing iron particles of mean size 50 micrometres or more. Effective cleaning can be achieved in this case by removing the filter assembly from the canister and directing a low-pressure jet of water or other liquid, or alternatively compressed air, over the filter assembly. CLAIMS

4. A method of removing suspended magnetic particles from a flow stream wherein the flow stream is passed through a filter as claimed in any of the preceding claims so that the magnetic particles are retained by the elements of the array.

5. A method as claimed in claim 4 in which the filter is subsequently cleaned by back-flushing with water.

6. A method as claimed in claim 4 in which the filter is removed from the flow stream and the retained particles are removed by means of a jet of liquid or air.