GB2141737A - Dewatering of aqueous dispersions - Google Patents

Abstract

The invention provides a method and an apparatus for reducing the water content of flowable aqueous particulate dispersions such as slurries and flocs by electro-osmosis. The apparatus is an electro-extraction cell 10, comprising a chamber 36 into which the dispersion is pumped. The chamber 36 is bounded on one side by a stationary permeable membrane 30, and electrodes 24, 40 enable an electric-field to be applied to the dispersion to cause water to pass from the dispersion through the membrane 30. The extracted water passes out of the electro-extraction cell 10 through a duct 20, while the dewatered dispersion flows out of an outlet duct 50 from the chamber 36. <IMAGE>

Description

SPECIFICATION Dewatering of aqueous dispersions This invention relates to a method and to an apparatus for extracting water from a flowable aqueous particulate dispersion, i.e. for dewatering such a dispersion.

According to the present invention there is provided a method for dewatering a flowable aqueous particulate dispersion comprising, causing the dispersion to flow into a region adjacent to one side of a water permeable membrane, applying an electric field to the dispersion so as to cause water from the dispersion to pass through the membrane in preference to particulate matter, and removing the water thereby extracted, wherein the dispersion is maintained in a flowable state adjacent to the membrane and is caused to flow out of the region.

The flowable aqueous particulate dispersion may contain particles of solid matter over a wide range of particle sizes, and may have a wide range of concentrations as long as the dispersion remains flowable. For example the dispersion may be a sludge, a slurry, a floc or a colloid.

The particles may be charged positively or negatively, and the direction of the field must be such as to exert an electrophoretic force on the particles away from the membrane. In either case water is extracted from the dispersion by electro-osmosis.

Desirably the method includes adjusting the strength of the field so as to maintain the concentration of particulate matter in the extract water below a predetermined level.

The present invention also provides an apparatus for dewatering a flowable aqueous particulate dispersion comprising, a first chamber having an inlet port for the inflow of the dispersion and an outlet port for the outflow of the dispersion, a second chamber adjacent to the first chamber separated therefrom by a stationary water-permeable membrane and with an outlet port for the outflow of water from the second chamber, and means for applying an electric field to the dispersion within the first chamber so as to cause water from the dispersion to pass through the membrane in preference to particulate matter. This apparatus is referred to herein as an electro-extraction cell.

The electro-extraction cell may be used in a once-through or in a recirculating manner depending on the required degree of dewatering.

The invention will now be further described by way of example only and with reference to the accompanying drawing which shows a medial sectional view of an electro-extraction cell.

Referring to the drawing, an electro-extraction cell 10 comprises a cylindrical block 12 of polymethylmethacrylate defining a cylindrical cavity 14, open at one end 15. The other end 16 of the cavity 14 opens into a coaxial cylindrical extract chamber 18 of smaller diameter than the cavity 14, from which an extract duct 20 extends radially through the wall of the block 12. At the end 16 of the cavity 14 adjacent to the extract chamber 18 is a platinum mesh working electrode 24 between two rubber annular washers 26, a lead 28 through the wall of the block 12 and through one of the washers 26 providing electrical connection to the working electrode 24. At the side of the working electrode 24 facing away from the extract chamber 18 is a circular membrane 30.

The membrane 30 provides one face of a cylindrical dispersion chamber 36, whose sides are defined by a polymethylmethacrylate annular spacer 38 which fits into the cavity 14. The other face of the dispersion chamber 36 is defined by a platinum mesh counter electrode 40 backed by an annular washer 26, and by a circular polymethylmetharylate cap member 42 with a central raised portion 44 fitting into the open end 15 of the cavity 14. A lead 46 extending through the washer 26 and the cap member 42 provides electrical connection to the counter electrode 40. An inlet duct 48 and an outlet duct 50 are defined through the wall of the cavity 14 and through the spacer 38 from opposite directions into the dispersion chamber 36, so that a dispersion may be passed through the dispersion chamber 36.The inlet duct 48 and the outlet duct 50 lie in planes at a small angle to the membrane 30, and communicate with the dispersion chamber 36 adjacent to the membrane 30. The dispersion chamber 36 is of diameter 25mm and of width (determined by the width of the spacer 38) of 4mm.

In use of the electro-extraction cell 10, a flowable aqueous particulate dispersion is pumped through the inlet duct 48, into the dispersion chamber 36, and out of the outlet duct 50. A potential difference is applied between the working electrode 24 and the counter electrode 40 so as to set up an electric field through the dispersion, and extract liquid (almost pure water) is thereby caused to flow through the membrane 30 from the dispersion into the extract chamber 18 and out of the extract duct 20. Consequently the water content of the dispersion is reduced.

Turbulence within the dispersion chamber 36 due to the flow of dispersion prevents the dispersion settling out as a layer on the membrane 30 and so enables the dispersion to remain flowable throughout the dispersion chamber 36.

The operation of the electro-extraction cell 10 will now be further described with reference to the dewatering of various dispersions in the following examples.

Example I A magnesium-containing dispersion or sludge was made up as follows. A solution of Magnox alloy (an alloy of magnesium containing small amounts of aluminium and beryllium) in nitric acid containing 209 magnesium ions was added to sufficient sulphuric acid to provide 189 sulphate ions. Sodium hydroxide was added to bring the pH up to 1.5, and 0.29 of Ni2Fe(CN)6 added. Further sodium hydroxide was added to bring the pH to 8.5 and then 0.2g of Na2S, 2.09 of Ba(NO3)2, and sufficient cobalt sulphate to provide 0.29 cobalt ions were added. After adding a small quantity of polyelectrolyte Magnafloc 11 01, this was made up to one litre with water and well stirred. After standing, a sludge of pH about 10 and solids content of about 16% settled out.

A circular piece of cambric cotton was used as the membrane 30 for the electro-extraction cell 10, and before assembling the cell 10, the membrane 30 was soaked in the sludge for 20 minutes. The cell 10 was then assembled, as shown in the Figure, and the sludge pumped by a peristaltic pump (not shown) through the sludge chamber 36, being continuously recirculated.

The working electrode 24 was made positive relative to the counter electrode 40 and a steady current of about 100mA passed through the dispersion chamber 36 (i.e. a current density 20mA cm-2). During operation of the cell 10 the potential difference between the working electrode 24 and the counter electrode 40 gradually dropped from 100V to about 30V due to increasing conductivity of the sludge. A solids content of 35% was achieved in the sludge, the limiting factor being the ability of the pump to pump the sludge through the cell 10. No colloidal particles were detectable spectrophotometrically in the water extracted by electroosmosis through the membrane 30 into the extract chamber 18.

Example II An alumino-ferric floc or dispersion was made up as follows: c.HNO3 1849 H20 3009 Fe(N03)3.9H20 72.1g Al(N03)3.9H20 69g Mg(N03)2.6H20 10.5g Ca(N03)2.4H20 29.5g Cr(N03)3.9H20 23.1g c.H2S04 2g NH4NO3 50g The above ingredients were stirred together for 24 hours. Then ammonia solution (density 0.88g cm-3) was added, while stirring, to bring the pH to about 8. The mixture was made up to one litre with water, stirred, and left to settle. After standing, a floc settled out, of solids content 7.2%.

A circular piece of cambric cotton was used as the membrane 30, and, as in Example I, the membrane 30 was soaked in the floc for 20 minutes before the cell 10 was assembled. The floc was recirculated through the cell 10 by a peristaltic pump, the working electrode 24 being made positive and the current being about 100mA. During operation of the cell 10 the potential difference between the working electrode 24 and the counter electrode 40 gradually increased from 40V to 125V due to decreasing conductivity of the floc. A solids content of 13.5% in the floc was achieved. This value is lower than in Example I because the dispersion chamber 36 became progressively blocked by a dense floc deposit caused by the electrophoretic concentration and deposition of the floc at the counter electrode 40.

Example Ill A zirconia floc or dispersion was made up as follows. 2 kg of Zircaloy alloy (an alloy of zirconium and aluminium) was dissolved in 53 litres of 3M ammonium fluoride. To this was added 5 litres of water. 10 litres of ammonium hydroxide (density 0.889 c.m-3) was added, and a zirconia floc settled out. The floc is thought to have the composition NH4 [ Zq0H)2F3#H30. The settled floc was washed twice with pure water to reduce its electrical conductivity, and then, after settling, filtered with a Buchner funnel to give a floc of solids content 10.3%.

A circular piece of cambric cotton was again used as the membrane 30 and was, as in the previous examples, soaked in the floc for 20 minutes before the cell 10 was assembled. Before pumping the floc through the cell 10, it was ultrasonically stirred to improve its flow properties.

The floc was then recirculated through the dispersion chamber 36 of the cell 10, the working electrode 24 being made negative relative to the counter electrode 40, the potential difference between them being a constant 25V. During operation of the cell 10 the current varied in the range 200 to 400mA. A solids content of 24% was achieved in the dispersion.

Example IV An iron colloid or dispersion was made up as follows. A ferric chloride solution of concentration 0.01 M was boiled for 1 hour. The resulting liquid was pH 1.35, and contained colloidal particles too small to coat a cotton membrane, so instead experiments were carried out in which cellulose acetate/nitrate micropore membranes were used for the membrane 30 in the cell 10. In each case the membrane 30 was precoated by soaking in the colloid-containing solution for 20 minutes before the cell 10 was assembled.

The colloid was recirculated through the dispersion chamber 36 of the cell 10, the working electrode 24 being made positive relative to the counter electrode 40. For each micropore membrane 30 the cell was operated with the voltage set at zero, 10V, 20V and 30V, and in each case the separation factor (i.e. the ratio of the solids content of the dispersion to the solids content of the extract water) was measured, the results being given in the Table. Values above 2500 are above the limit of spectrophotometric detection.

Table Separation Factors for Membranes at Different Voltages Membrane Applied voltage/ V pore size/llm 0 10 20 30 0.22 3.0 290 > 2500 > 2500 0.45 1.7 10 42 870 0.65 1.2 4 5 12 For each of these membranes the rate at which water was extracted from the dispersion was approximately proportional to the electric current, being about 0.16 I h-1A-1. With a membrane of pore size 0.1 ym (not given in the Table) the rate of water extraction was much less, as the pores at the surface of the membrane became completely blocked by the colloid particles.The power consumption of the cell 10 is of course proportional to the square of the current, and so to economize on energy consumption the cell 10 should be operated at the lowest voltage (and current) consistent with the required throughout and the required purity of the extract water. It will thus be appreciated that if a second sample of a dispersion supplied to the cell 10 has a lower solid content than a first sample of the dispersion, then the same purity of the extract water may be achieved at a lower separation factor and so a lower voltage across the cell 10, thereby reducing the power consumption.

It has been found preferable to use a membrane 30 with pores comparable in size with the particles with which it is to be used, and it is desirable to precoat the pores of the membrane. In the examples given above this was brought about by soaking the membrane 30 prior to use, the floc or colloid particles spontaneously occupying the pores and voids in the membranes 30. An alternative technique is to apply, briefly, an electric field across the dispersion chamber 36 in the opposite direction to that required in normal operation of the cell 10 so as to force the particles onto the membrane 30 by electrophoresis.

Claims (7)

1. A method for dewatering a flowable aqueous particulate dispersion comprising, causing the dispersion to flow into a region adjacent to one side of a water permeable membrane, applying an electric field to the dispersion so as to cause water from the dispersion to pass through the membrane in preference to particulate matter, and removing the water thereby extracted, wherein the dispersion is maintained in a flowable state adjacent to the membrane and is caused to flow out of the region.

2. A method as claimed in Claim 1 which includes adjusting the strength of the field so as to maintain the concentration of particulate matter in the extract water below a predetermined level.

3. A method as claimed in Claim 1 or Claim 2 wherein the membrane has pores comparable in size with the particles in the dispersion.

4. A method as claimed in any one of the preceding Claims including the initial operation of precoating the pores of the membrane with the particles in the dispersion.

5. An apparatus for dewatering a flowable aqueous particulate dispersion comprising, a first chamber having an inlet port for the inflow of the dispersion and an outlet port for the outflow of the dispersion, a second chamber adjacent to the first chamber separated therefrom by a stationary water-permeable membrane and with an outlet port for the outflow of water from the second chamber, and means for applying an electric field to the dispersion within the first chamber so as to cause water from the dispersion to pass through the membrane in preference to particulate matter.

6. A method for dewatering a flowable aqueous particulate dispersion, substantially as hereinbefore described with reference to any one of the examples.

7. An apparatus for dewatering a flowable aqueous particulate dispersion, substantially as hereinbefore described and with reference to and as shown in the accompanying drawing.