GB2151511A

GB2151511A - Method of magnetically filtering radioactive particles from air or other fluids

Info

Publication number: GB2151511A
Application number: GB08334143A
Authority: GB
Inventors: Dr James Henry Peter Watson
Original assignee: Individual
Current assignee: Individual
Priority date: 1983-12-22
Filing date: 1983-12-22
Publication date: 1985-07-24
Also published as: GB2151511B

Abstract

A method of filtering air or other fluid to remove suspended radioactive particles using a filter comprising (preferably permanently) magnetised ferromagnetic material in a divided form (preferably perforated or expanded sheet) which generates magnetic field gradients adjacent surfaces of ferromagnetic material. The invention is applied especially to the air intakes of fall-out shelters, warships, tanks and the like.

Description

SPECIFICATION Method of filtering radioactive particles from air or other fluids This invention relates to a method of filtering air or other fluids to remove suspended radioactive particles and has applications in the protection of military and civilian personnel from radioactive fall-out.

The invention is based on the realisation that all significant natural radionuclides and nearly all radioactive fission products are transition metals (using that expression to include the "inner transition" groups consisting of the lanthanides and the actinides) and in consequence are paramagnetic.

In accordance with the present invention, a method of filtering air or other fluid to remove suspended radioactive particles comprises passing the fluid through a filter comprising magnetised ferromagnetic material in a divided form which generates magnetic field gradients adjacent surfaces of ferromagnetic material.

In a few cases a soft ferromagnetic material magnetised by an associated electromagnet or permanent magnet may be acceptable, but in all cases a hard ferromagnetic material permanently magnetised is to be preferred.

The preferred form of the ferromagnetic filter material is a perforated or expanded metal sheet, but alternatives include woven cloth, randomly or regularly packed wires (either straight or crimped), packed chopped wires, grains or spheres.

Normally a material with a coercivity greater than 100 Oersted (preferably at least 200) and a remanent flux density of at least 0.3 Tesla (preferably at least 0.5) will be required.

For perforated or expanded metal and wirebased forms the preferred materials are ductile alloys of composition (% by weight) Chromium 30, Cobalt 10, Iron 60, and Vanadium 11, Cobalt 51, Iron 38, both sold by Telcon Metals Limited of Manor Royal, Crawley, Sussex under the designations Chromindur and Vicalloy respectively.

If required the radioactive particles can be eluted by washing with a suitable fluid (a) with the magnetisation substantially eliminated by application of a reverse field (or in the case of a soft magnetic material by removing or turning off the associated magnet) or preferably (b) chosen to have a magnetic permeability comparable to that of the particles, as more fully-described and claimed in another application filed by me on the same day as this application.

In many cases it will be desirable to use a HEPA filter downstream of the magnetic filter, but since the bulk of particles will be removed by the magnetic filter the HEPA filter can be smaller and/or last very much longer than would otherwise be the case. The pressure drop across the magnetic filter can be very low, and so pressure maintenance is not a significant problem.

The invention is illustrated by the following example.

A magnetic filter unit comprises two stacks each of 30 Chromindur sheets 300 mm long by 200 mm wide by 0. 1 mm thick, perforated with 0.8 mm diameter round holes spaced to occupy about 50% of the area of the sheet.

Spaces or dimples are used to maintain a gap of about 0.1 mm between adjacent sheets in the stack, so that each stack is 6 mm thick.

The two stacks are spaced 6 mm apart and enclosed in a chamber, closely fitting the sheet edges and about 28 mm deep. A median slot 6 mm wide along one of the shorter side faces of the chamber communicates with the space between the stacks to form an air inlet and two lateral slots along the opposite shorter side face each 4mm wide communicate with respective spaces on the outside of the stacks to form outlets for filtered air. The filter unit is magnetised to remanence in its length direction by application of a saturating field.

In a minor modification to the design described, the depth may be reduced to 22 mm (measured inside the chamber wall) by tapering the space between the two stacks so that they practically touch along the the edge remote from the inlet slot.

In either case the unit is capable of filtering 2.8 m3 of air per hour.

An appropriate number of units of either design is placed upstream of a conventional "HEP" paper filter to provide initial filtration, and a degree of protection against flash and blast, to the air intake of a fall-out shelter, warship, tank or other manned vehicle or installation. Eight units per head of personnel are suggested for any long-term exposure situation, though a smaller number may be adequate for short-term missions.

The magnetic filter has an estimated capture efficiency for radioactive fall-out particles of 80% (and also collects some innocuous dust particles); its operational life is expected to be better than 200 hours.

1. A method of filtering air or other fluid to remove suspended radioactive particles comprising passing the fluid through a filter comprising magnetised ferromagnetic material in a divided form which generates magnetic field gradients adjacent surfaces of ferromagnetic material.

2. A method as claimed in Claim 1 comprising using a hard ferromagnetic material permanently magnetised.

3. A method as claimed in Claim 1 or Claim 2 in which the ferromagnetic filter material is a perforated or expanded metal sheet.

4. A method of filtering air to remove

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Method of filtering radioactive particles from air or other fluids This invention relates to a method of filtering air or other fluids to remove suspended radioactive particles and has applications in the protection of military and civilian personnel from radioactive fall-out. The invention is based on the realisation that all significant natural radionuclides and nearly all radioactive fission products are transition metals (using that expression to include the "inner transition" groups consisting of the lanthanides and the actinides) and in consequence are paramagnetic. In accordance with the present invention, a method of filtering air or other fluid to remove suspended radioactive particles comprises passing the fluid through a filter comprising magnetised ferromagnetic material in a divided form which generates magnetic field gradients adjacent surfaces of ferromagnetic material. In a few cases a soft ferromagnetic material magnetised by an associated electromagnet or permanent magnet may be acceptable, but in all cases a hard ferromagnetic material permanently magnetised is to be preferred. The preferred form of the ferromagnetic filter material is a perforated or expanded metal sheet, but alternatives include woven cloth, randomly or regularly packed wires (either straight or crimped), packed chopped wires, grains or spheres. Normally a material with a coercivity greater than 100 Oersted (preferably at least 200) and a remanent flux density of at least 0.3 Tesla (preferably at least 0.5) will be required. For perforated or expanded metal and wirebased forms the preferred materials are ductile alloys of composition (% by weight) Chromium 30, Cobalt 10, Iron 60, and Vanadium 11, Cobalt 51, Iron 38, both sold by Telcon Metals Limited of Manor Royal, Crawley, Sussex under the designations Chromindur and Vicalloy respectively. If required the radioactive particles can be eluted by washing with a suitable fluid (a) with the magnetisation substantially eliminated by application of a reverse field (or in the case of a soft magnetic material by removing or turning off the associated magnet) or preferably (b) chosen to have a magnetic permeability comparable to that of the particles, as more fully-described and claimed in another application filed by me on the same day as this application. In many cases it will be desirable to use a HEPA filter downstream of the magnetic filter, but since the bulk of particles will be removed by the magnetic filter the HEPA filter can be smaller and/or last very much longer than would otherwise be the case. The pressure drop across the magnetic filter can be very low, and so pressure maintenance is not a significant problem. The invention is illustrated by the following example. A magnetic filter unit comprises two stacks each of 30 Chromindur sheets 300 mm long by 200 mm wide by 0. 1 mm thick, perforated with 0.8 mm diameter round holes spaced to occupy about 50% of the area of the sheet. Spaces or dimples are used to maintain a gap of about 0.1 mm between adjacent sheets in the stack, so that each stack is 6 mm thick. The two stacks are spaced 6 mm apart and enclosed in a chamber, closely fitting the sheet edges and about 28 mm deep. A median slot 6 mm wide along one of the shorter side faces of the chamber communicates with the space between the stacks to form an air inlet and two lateral slots along the opposite shorter side face each 4mm wide communicate with respective spaces on the outside of the stacks to form outlets for filtered air. The filter unit is magnetised to remanence in its length direction by application of a saturating field. In a minor modification to the design described, the depth may be reduced to 22 mm (measured inside the chamber wall) by tapering the space between the two stacks so that they practically touch along the the edge remote from the inlet slot. In either case the unit is capable of filtering 2.8 m3 of air per hour. An appropriate number of units of either design is placed upstream of a conventional "HEP" paper filter to provide initial filtration, and a degree of protection against flash and blast, to the air intake of a fall-out shelter, warship, tank or other manned vehicle or installation. Eight units per head of personnel are suggested for any long-term exposure situation, though a smaller number may be adequate for short-term missions. The magnetic filter has an estimated capture efficiency for radioactive fall-out particles of 80% (and also collects some innocuous dust particles); its operational life is expected to be better than 200 hours. CLAIMS

4. A method of filtering air to remove suspended radioactive particles substantiaily as described with reference to the example.

CLAIMS Amendments to the claims have been filed, and have the following effect: Claims 1 above have been deieted or textually amended as follows; 1,45678 Claims 4 above have been re-numbered as 9 and their appendancies corrected.

1. A method of filtering air or other fluid to remove suspended radioactive particles comprising passing the fluid through a filter comprising magnetised ferromagnetic material in a divided form which generates magnetic field gradients adjacent surfaces of ferromagnetic material, and eluting the radioactive particles by washing with a fluid chosen to have a magnetic permeability comparable to that of the particles.

4. A method according to any preceding claim and in which said filter is placed upstream of a paper filter.

5. A method according to claim 4 and in which said paper filter is a HEVA filter.

6. A method according to any preceding claim and in which the first mentioned filter is made from a material having a coercivity greater than 100 Oersted and a remanent flux density of at least 0.3 Tesla.

7. A method according to any preceding claim and in which the first mentioned filter is made of expanded metal or wire.

8. A method according to claim 7 and in which the expanded metal or wire is fabricated from Chromindur or Vicalloy.