GB2164265A

GB2164265A - Waste immobilisation

Info

Publication number: GB2164265A
Application number: GB08520356A
Authority: GB
Inventors: Martin James Thomas; Richard Hope Knibbs
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1984-09-13
Filing date: 1985-08-14
Publication date: 1986-03-19
Also published as: GB8520356D0; GB2164265B; GB8423170D0

Abstract

Waste material containing low or medium levels of radioactivity and in the form of an aqueous slurry is blended with cement slurry, and the resulting mixture is fed into a disposable drum (16). Excess water is then extracted by immersing an array of edge filter elements (24) in the mixture and applying a pressure difference across each filter element. The filter elements are then removed; the drum is vibrated to compact the mixture and then left to allow the mixture to set solid. The filter elements may be precoated with diatomaceous earth. <IMAGE>

Description

SPECIFICATION Waste immobilisation This invention relates to a method for immobilising waste materials containing low or medium levels of radioactivity.

According to the present invention, a method of immobilising a waste material comprises the steps of blending a slurry containing the waste material in particulate form with a binding material, feeding the blended slurry into a disposable container, arranging a filter in contact with the blended slurry in the container and applying a pressure difference to the filter so as to extract liquid from the slurry, removing the filter and leaving the contents of the container to set solid.

Preferably the slurry is an aqueous slurry and the binding material is cement. The filter is preferably an edge filter comprising a stack of rings spaced apart by about 0.10 mm by ridges on one face of each ring. Depending on the particle size of the blended slurry the filter may be used with or without a precoat.

Preferably the pressure difference is applied by decreasing the pressure within the filter to below atmospheric pressure; alternatively the container may be sealed and a pressure above atmospheric applied within the container, the pressure within the filter being at atmospheric. The latter method enables a larger pressure difference (e.g. 3 atmospheres) to be applied across the filter.

After removing the filter, the contents of the container may be compacted by vibrating the container, before being left to set.

The invention will now be described by way of example only and with reference to the accompanying drawing, which shows diagrammatically an apparatus for performing waste immobilisation.

As shown in the drawing, the apparatus includes a mixing chamber 10 containing a perforated mixing plate 12, and a duct 14 leading from the base of the chamber 10. The duct 14 includes a valve 15, and the other end of the duct 14 is located just above an open-topped disposable drum 16. In operation of the apparatus, an aqueous slurry containing low or medium level particulate radioactive waste is fed into the chamber 10 through a pipe 18 while a cement slurry is fed into the chamber 10 through a pipe 20. The slurries are mixed by vibrating the plate 12, and then flow under gravity through the duct 14, when the valve 15 is open, into the drum 16.

Inserted into the drum 16 is a filter 22 consisting of seven 22 mm diameter stainless steel edge-filter elements 24 (only five are indicated in the figure), each 640 mm long, and arranged in a hexagonal array on a 90 mm pitch. The elements 24 are interconnected at their upper ends by a manifold 26.

Each filter element 24 consists of a stack of rings, each with a series of small ridges on one side separating it from the next ring. The ring separation is determined by the height of the ridges, but is preferably 0.10 mm. The stack of rings is held together by a fluted central drainage-and-clamping rod, with a nut at the lower end and a boss on the rod at the upper end. Suitable edge-filter elements 24 are obtainable from Stella-Meta Filters, Laverstoke Mill, Whitchurch, Hampshire.

The manifold 26 is connected by a pipe 28 to a vacuum reservoir 30 within which the pressure can be maintained at a low value by an air pump 32. A pipe 34 with a valve 36 is connected to the base of the reservoir 30 to allow removal of filtrate from the reservoir 30.

In operation, the filter 22 is first precoated with a thickness of about 1 mm of precoating material, by filling the drum 16 with a 2% slurry of diatomaceous earth, and extracting water from this precoat slurry through the filter 22. Application of low pressure within the filter elements 24 extracts water from the contents of the drum 16, the water passing through the manifold 26 and the pipe 28 to the reservoir 30.

After deposition of a precoat, the valve 15 is opened to allow the mixed slurry in the chamber 10 to flow into the drum 16, while extraction of water through the filter 22 continues.

As the water is extracted from the mixed slurry in the drum 16, fresh mixed slurry is added through the duct 14,the valve 15 being controlled in response to the level of slurry in the drum 16 to maintain the desired level in the drum 16. When the drum 16 contains sufficient dewatered mixture the valve 15 is closed and the filter 22 is withdrawn. The drum 16 is then vibrated to compact the mixture, and the mixture is left to set and cure in the drum 16.

The waste slurry typically contains about 10% by weight of solid matter, while the dewatered mixture typically might contain waste solids, cement solids, and water in the proportions 2:5:3 by weight.

Experiments carried out using magnesium hydroxide slurry as the waste slurry, and ordinary Portland cement, have produced cured blocks weighing 30 kg and with compressive strengths between 17 MPa and 30 MPa.

Although as described above the cement may be added as a slurry to the waste slurry, alternatively the cement may be added as a powder to the waste slurry. It will also be appreciated that the mixed slurries might alternatively be pumped through the duct 14 rather than relying on the valve 15 and gravity feed.

After use of the apparatus, water is passed through the mixing chamber 10, the duct 14 and the valve 15, to ensure no deposits of cement set within the apparatus.

1. A method of immobilising a waste material comprising the steps of blending a slurry containing the waste material in particulate form with a binding material, feeding the blended slurry into a disposable container, arranging a filter in contact with the blended slurry in the container and applying a pressure difference to the filter so as to extract liquid from the slurry, removing the filter and leaving the contents of the container to set solid.

2. A method as claimed in Claim 1 wherein the slurry is an aqueous slurry and the binding material is cement.

3. A method as claimed in Claim 1 or Claim 2

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Waste immobilisation This invention relates to a method for immobilising waste materials containing low or medium levels of radioactivity. According to the present invention, a method of immobilising a waste material comprises the steps of blending a slurry containing the waste material in particulate form with a binding material, feeding the blended slurry into a disposable container, arranging a filter in contact with the blended slurry in the container and applying a pressure difference to the filter so as to extract liquid from the slurry, removing the filter and leaving the contents of the container to set solid. Preferably the slurry is an aqueous slurry and the binding material is cement. The filter is preferably an edge filter comprising a stack of rings spaced apart by about 0.10 mm by ridges on one face of each ring. Depending on the particle size of the blended slurry the filter may be used with or without a precoat. Preferably the pressure difference is applied by decreasing the pressure within the filter to below atmospheric pressure; alternatively the container may be sealed and a pressure above atmospheric applied within the container, the pressure within the filter being at atmospheric. The latter method enables a larger pressure difference (e.g. 3 atmospheres) to be applied across the filter. After removing the filter, the contents of the container may be compacted by vibrating the container, before being left to set. The invention will now be described by way of example only and with reference to the accompanying drawing, which shows diagrammatically an apparatus for performing waste immobilisation. As shown in the drawing, the apparatus includes a mixing chamber 10 containing a perforated mixing plate 12, and a duct 14 leading from the base of the chamber 10. The duct 14 includes a valve 15, and the other end of the duct 14 is located just above an open-topped disposable drum 16. In operation of the apparatus, an aqueous slurry containing low or medium level particulate radioactive waste is fed into the chamber 10 through a pipe 18 while a cement slurry is fed into the chamber 10 through a pipe 20. The slurries are mixed by vibrating the plate 12, and then flow under gravity through the duct 14, when the valve 15 is open, into the drum 16. Inserted into the drum 16 is a filter 22 consisting of seven 22 mm diameter stainless steel edge-filter elements 24 (only five are indicated in the figure), each 640 mm long, and arranged in a hexagonal array on a 90 mm pitch. The elements 24 are interconnected at their upper ends by a manifold 26. Each filter element 24 consists of a stack of rings, each with a series of small ridges on one side separating it from the next ring. The ring separation is determined by the height of the ridges, but is preferably 0.10 mm. The stack of rings is held together by a fluted central drainage-and-clamping rod, with a nut at the lower end and a boss on the rod at the upper end. Suitable edge-filter elements 24 are obtainable from Stella-Meta Filters, Laverstoke Mill, Whitchurch, Hampshire. The manifold 26 is connected by a pipe 28 to a vacuum reservoir 30 within which the pressure can be maintained at a low value by an air pump 32. A pipe 34 with a valve 36 is connected to the base of the reservoir 30 to allow removal of filtrate from the reservoir 30. In operation, the filter 22 is first precoated with a thickness of about 1 mm of precoating material, by filling the drum 16 with a 2% slurry of diatomaceous earth, and extracting water from this precoat slurry through the filter 22. Application of low pressure within the filter elements 24 extracts water from the contents of the drum 16, the water passing through the manifold 26 and the pipe 28 to the reservoir 30. After deposition of a precoat, the valve 15 is opened to allow the mixed slurry in the chamber 10 to flow into the drum 16, while extraction of water through the filter 22 continues. As the water is extracted from the mixed slurry in the drum 16, fresh mixed slurry is added through the duct 14,the valve 15 being controlled in response to the level of slurry in the drum 16 to maintain the desired level in the drum 16. When the drum 16 contains sufficient dewatered mixture the valve 15 is closed and the filter 22 is withdrawn. The drum 16 is then vibrated to compact the mixture, and the mixture is left to set and cure in the drum 16. The waste slurry typically contains about 10% by weight of solid matter, while the dewatered mixture typically might contain waste solids, cement solids, and water in the proportions 2:5:3 by weight. Experiments carried out using magnesium hydroxide slurry as the waste slurry, and ordinary Portland cement, have produced cured blocks weighing 30 kg and with compressive strengths between 17 MPa and 30 MPa. Although as described above the cement may be added as a slurry to the waste slurry, alternatively the cement may be added as a powder to the waste slurry. It will also be appreciated that the mixed slurries might alternatively be pumped through the duct 14 rather than relying on the valve 15 and gravity feed. After use of the apparatus, water is passed through the mixing chamber 10, the duct 14 and the valve 15, to ensure no deposits of cement set within the apparatus. CLAIMS

3. A method as claimed in Claim 1 or Claim 2 wherein the filter is an edge filter comprising a stack of rings spaced apart by about 0.10 mm by ridges on the face of each ring.

4. A method as claimed in Claim 3 wherein the filter is coated with a precoat before being arranged in contact with the blended slurry.

5. A method as claimed in any one of the preceding Claims wherein the pressure difference is applied by decreasing the pressure within the filter to below atmospheric pressure.

6. A method as claimed in any one of the preceding Claims including the further step of vibrating the container to compact the slurry after removing the filter.

7. A method of immobilising a waste material substantialiy as hereinbefore described with reference to, and as shown in, the accompanying drawing.

8. An apparatus for immobiiising a waste material comprising, blending means for blending a slurry containing the waste material in particulate form with a binding material, a duct for feeding the blended slurry into a disposable container, and a filter adapted to be arranged in contact with the blended slurry in the container for extracting liquid therefrom.

9. An apparatus for immobilising a waste material substantially as hereinbefore described with reference to, and as shown in, the accompanying drawing.