AU717232B2 - Process for removing fine iron containing particles from liquids containing the same - Google Patents

Description

WO 98/11183 PCT/GB97/02390 PROCESS FOR REMOVING FINE IRON CONTAINING PARTICLES FROM LIQUIDS CONTAINING THE SAME This invention relates to the removal of fine ironcontaining particles from liquids containing same, for example for removal of scale from cooling liquids used to cool the rolls of a rolling mill or to cool continuous casters.

In the operation of a rolling mill for rolling metal ingots, billets or strip, the rolls are conventionally kept cool by spraying them with a cooling liquid, such as water.

Since the surface of the metal ingot, billet or strip is usually hot, it becomes oxidised through contact with air.

As a result the surface of the ingot, billet or strip is normally somewhat contaminated with a metal oxide layer.

Hence when rolling steel ingots, billets or strip the surface of the ingot, billet or strip becomes contaminated with a more or less continuous thin layer of iron oxides, sometimes only a few atoms thick. This layer becomes detached in passage through the roll nip with the result that the cooling liquid becomes contaminated with small particles of metal oxide or oxides, e.g. iron oxides. Such contaminant particles are often termed mill scale. They are of variable size, typically in the range of from about 1 Am up to about 300 Am.

In addition it is also usual to utilise a lubricant to lubricate the mill rolls. This also contaminates the coolant liquid.

Coolant liquids, such as water, are also used to cool continuous casters and, in so doing, become contaminated with scale particles, i.e. small iron-containing particles.

It would be undesirable in most cases to discharge such contaminated cooling liquid directly to the environment.

Hence the contaminated water or other cooling liquid requires treatment before it can be discharged to the WO 98/11183 PCT/GB97/02390 2 environment. Alternatively, since it is usually uneconomic to discharge the cooling liquid, e.g. water, to the environment even after preliminary treatment to remove at least some of the scale particles contained therein, it is expedient to recycle the cooling liquid for further use.

However, before the cooling liquid can be recycled, it is necessary to treat the cooling liquid to effect removal of at least the major part of the scale therefrom so as to reduce the solids content thereof to less than about 50 ppm by weight. In this way the danger is minimised that the coolant spray nozzles shall become blocked. In the case of a rolling mill it is further ensured that the risk that the surface finish of the rolled product shall be marred as a result of mill scale particles becoming embedded therein is reduced as far as possible.

In a typical rolling mill plant the contaminated water or other liquid coolant is pumped to a coarse settling pit and then through one or more secondary settling ponds. In the coarse settling pit essentially all of the mill scale particles with a particle size of about 200- m or greater settle out relatively rapidly under gravity and form a sludge on the bottom of the pit. Particles of less than about 200 gm in particle size, however, tend to settle out relatively slowly from suspension in the cooling liquid and so the residence time in the secondary settling ponds has to be relatively long. Hence the secondary settling ponds typically each have an area of about 10 m wide by about 30 m long. Typically they are about 3 m deep.

Settling ponds of a similar size are conventionally used in the treatment of cooling waters from continuous casters.

Various alternative or additional measures have been adopted in practice to improve the recovery of mill scale WO 98/11183 PCT/GB97/02390 3 particles from rolling mill coolant liquids. Thus passage of the contaminated coolant liquid through one or more hydrocyclones has been proposed. Another device that is marketed for this purpose is the LAKOS TM separator. It has also been proposed to use tilted plate separators to improve separation of mill scale particles.

Filtration is another expedient that has been suggested. Thus it has been proposed to use NECKAR TM filters which comprise a tank filled with bales of hay. However, any filter will in time become blocked, whereupon it then becomes necessary to backwash it. This is an inconvenient procedure and there is a danger that, if the need to backwash the filter should be overlooked, the filter can become totally blocked with serious consequences to safe operation of the rolling mill.

The use of magnetic separators for the removal of relatively large metal-containing particles from aqueous streams has been proposed on numerous occasions in the prior art. However, in such applications the particle size of the iron-containing particles is generally large, typically being in excess of about 200 Am. Moreover the concentration of the iron-containing particles in the liquid stream is normally fairly high, typically in excess of about 1% weight by volume.

In US-A-2607492 there is described an apparatus for removing small amounts of iron sulphides from treating solutions, such as a solution in water of diethanolamine or of tripotassium phosphate, where used for the treatment of fluid hydrocarbon materials as for the removal of hydrogen sulphide therefrom. A portion of the solution is passed along a pipe through a separator between field elements of a core having a coil connected to a source of electrical energy. Small particles of iron impurities are retained in WO 98/11183 PCT/GB97/02390 4 the separator by magnetism induced by the electro-magnet.

Flushing the separator of iron impurities from time to time is envisaged, using water with the electro-magnet disconnected.

A method and apparatus for separating ores or similar materials is disclosed in US-A-2954122. A mass of material to be treated, such as a relatively thick slurry formed from finely divided tungsten ore, is thoroughly mixed with a multiplicity of relatively large bodies of highly magnetisable material such as balls or shotlike bodies of alnico and the resulting mixture is passed through a conventional magnetic separator to remove the alnico bodies from the moving mass of the ore. The magnetic bodies pick up finely divided weakly magnetic material within the mass and carry it with them through the magnetic separation process so that the magnetic bodies are removed with the finely divided weakly magnetic material adhering to them.

US-A-3412638 teaches a process for separating solids from sewage in which an iron ore weighting agent having a specific gravity grater than 1 and a particle size less than mesh (US Standard Sieve), i.e. a particle size less than 300 Im, and a water dispersible synthetic organic cationic polyelectrolytic flocculating material are admixed with the sewage and the resulting mixture is then subjected to accelerated sedimentation of solids in an artificially imposed magnetic field.

It has also been proposed to remove swarf from coolants and cutting oils using a magnetic separator of the type in which horseshoe magnets are mounted in longitudinally spaced rows arotnd the inner periphery of a drum and the coolant flows through an arcuate passageway surrounding a portion of the drum, the swarf adhering to the drum in the form of brushes. Such a proposal is described in US-A-3017031.

WO 98/11183 PCT/GB97/02390 Another document concerned with separation of swarf admixed with cutting oil or other coolant liquid used in metal cutting machines is US-A-3094486. This design utilises a rotating drum with a plurality of magnets inside it; adhering swarf is scraped from the surface of the drum and passes down the upper surface of the scraper together with liquid that has not already drained away. The metal swarf passing down the scraper is separated from the liquid by a second drum also provided with internal pairs of magnets.

Removal of suspended particulate matter from an effluent from a biological or chemical process such as sewage treatment is described in US-A-3351203. This proposal involves filtering the effluent through a bed of ferromagnetic particles, such as magnetite, contained in a vessel whose bottom slopes towards an outlet pipe at the lowest point of the vessel. A pumparound system with an electromagnet positioned around an upwardly sloping length of pipe enables the particulate filter to be built up to the required density and thickness.

US-A-3536198 proposes combination of magnetic flocculation with a chemical method of flocculation by adding blast furnace dust or steelmaking dust to carry the magnetic charge into the suspension. Passage of the magnetised slurry through a flattened stainless steel tube where the magnet is attached so that it can produce a magnetic field of at least 800 gauss is described. It is said that the normal proportion of chemical flocculant can be reduced by approximately one half in the combined process.

US-A-3042211 suggests a filter for liquids containing magnetic sludge and solids comprising a tank for receiving and holding a quantity of liquid to be filtered, a magnetic WO 98/11183 PCT/GB97/02390 6 filter assembly having a plane face collecting surface positioned within the tank, a baffle plate positioned closely in front of the collecting surface and defining a narrow space open to liquid along the sides and bottom, an outlet pipe having its mouth opening into the narrow space generally at the centre thereof, and scraper blades movable linearly across the collecting surface of the filter assembly to remove material clinging thereto. The magnetic field developed by the assembly is non-reversing along the line of movement of the blades. The filter assembly also includes a plurality of permanent magnets positioned on the rear side of the surface with their north poles only positioned along at least one line parallel to the line of movement of the blades and with their south poles only positioned along at least another line parallel to the line of movement.

US-A-3062376 teaches a magnetic separator with a filter element which comprises a circular framework of substantially non-magnetic material which includes an outer ring defining the periphery of the framework, an inner ring and a plurality of radial members secured to the rings, together with a plurality of part-annular magnetic members each being arranged to fit within the framework in a space bounded by a portion of the inner ring, a portion of the outer ring and two of the radial members.

Treatment of waste water from a steel works by magnetic flocculation is described by K. Otsubo et al., Yosui To Haisui (1976), 18(5), pages 597 to 600 (see Chem. Abstr.

86:21445q). It is taught that conventional precipitation processes cannot be used for such types of waste water because of the large amount of time required for obtaining low suspended solids values and that the use of more coagulant for achieving higher precipitation rates is not WO 98/11183 PCT/GB97/02390 7 practical. These authors proposed oxidation of the iron compounds suspended in the waste water to FeOOH which then tends to absorb colloidal graphite. Application of this magnetic flocculation procedure is said to give high coagulation rates and precipitates with less water.

Cleaning the surface of the suspended material by reduction before the oxidation apparently accelerates the coagulation.

In Iron Steel Eng. (1969), 106, 108 there is described a way of separating fine solids from liquids which uses both magnetic and chemical flocculation.

In US-A-5534155 it is proposed to separate coarse rolling mill scale from the water used in rolling mills by gravity sedimentation, tosubsequently remove the mid-size and fine rolling scale particles from the process water prepurified in this manner by magnetic separation, to divide by flotation the thus finished clarified or purified process water into fractions containing oil concentrate, water and the finest rolling scale particles carrying little oil, and reusing the purified water as a cooling agent. To effect magnetic separation in this process a magnetic separator is used that has an endless belt to which the mid-size and fine rolling scale particles are attracted and from which such particles are scraped or wiped off the belt above the water level.

It would be desirable to provide an improved method of removing scale from cooling fluids for rolling mills or continuous casters so as to reduce their solids content sufficiently to obviate the danger of the coolant spray nozzles from clogging up in use. It would also be desirable to provide an improved method of removing mill scale from coolant fluids used to cool rolling mill rolls, thereby to minimise the danger of mill scale particles present in recycled coolant fluid becoming embedded in the surface of WO98/11183 PCT/GB97/02390 8 the rolling mill rolls and hence damaging the rolled product from the mill. It would further be desirable to provide a simple and reliable method of reducing the solids content of the coolant sufficiently during recycle thereof in order to permit reuse of the resulting treated coolant liquid.

Another desirable objective is to minimise the area of the settling tanks or ponds required for settlement of the finer particles of scale from contaminated cooling liquids used for cooling rolling mill rolls or for cooling continuous casters.

The present invention accordingly seeks to provide a simple and reliable method of removing scale from cooling fluids for rolling mills or continuous casters so as to reduce their solids content sufficiently to obviate the danger of the coolant spray nozzles from clogging up in use.

It also seeks to provide a simple and reliable method of removing mill scale from rolling mill cooling liquids so as to obviate the risk of particles of mill scale in recycled cooling liquid becoming embedded in the surface of the rolling mill rolls. It further seeks to provide an improved method of reducing the solids content of coolant sufficiently during recycle thereof in order to permit reuse of the resulting treated coolant liquid. Another objective that the present invention seeks to achieve is to provide a method for economically and simply reducing the level of scale particles in a contaminated cooling liquid to less than about 50 ppm. Yet again the invention seeks to provide a method of treating contaminated rolling mill or continuous caster cooling liquids which minimises the ground area required for construction of settling tanks or ponds. It is also an object of the invention to provide a method of handling increased quantities of coolant liquids containing scale particles arising from addition of further rolling WO 98/11183 PCT/GB97/02390 9 mill capacity without the expense and inconvenience of constructing new settling tanks.

According to the present invention there is provided a method of treating a coolant liquid used for cooling rolls of a rolling mill or for cooling a continuous caster and contaminated with particles of scale which comprises passing the contaminated coolant liquid through a primary settling zone, retaining the cooling liquid in the primary settling zone for a period sufficient to permit substantially all particles of scale having a particle size greater than about 200 Am to settle out under gravity from suspension, recovering from the primary settling zone an intermediate liquor which is substantially free from particles of scale having a particle size greater than about 200 Am, passing the intermediate liquor along a flow path which includes a portion wherein the intermediate liquor is flowing under substantially streamline flow conditions, said portion comprising a length of conduit made of non-magnetic material adjacent to which is or are positioned one or more magnets arranged to produce a substantially constant magnetic field within said length of conduit, subjecting the intermediate liquor in passage through said portion to said substantially constant magnetic field thereby to coagulate iron-containing particles in the intermediate liquor and enhance sedimentation of resulting coagulated particles in the intermediate liquor, and collecting resulting treated liquor now containing less than about 50 ppm solids by weight for reuse for cooling the rolls of a rolling mill or for cooling a continuous caster. Preferably the method includes the further steps of passing the treated liquor to at least one secondary settling zone and retaining the liquor therein for a period sufficient to permit the solids content thereof to drop to less than about 20 ppm.

WO 98/11183 PCT/GB97/02390 The primary settling zone may comprise a settling tank or pond, a lamellar thickener, a hydrocyclone, or a combination of two or more thereof. Preferred forms of primary settling zone comprise a settling tank or pond or a lamellar thickener or hydrocyclone followed by a settling tank or pond.

Conveniently the portion of the flow path wherein the intermediate liquor is subjected to a magnetic field comprises a length of conduit made of non-magnetic material adjacent to which is or are positioned one or more permanent magnets. Stainless steel or a plastics material such as polypropylene are suitable materials from which to construct such a conduit. Such a length of conduit is conveniently substantially rectangular in section and at least one pair of magnets is mounted adjacent the rectangular section conduit with the south pole of one of the pair of magnets adjacent one of a pair of opposed faces of the conduit and with the north pole of the other magnet of the pair adjacent the other opposed face of the conduit. Typically the magnet or magnets is or are chosen so as to produce within said length of conduit a magnetic field of from about 600 to about 800 gauss. Field strengths of this order are readily obtainable using permanent ferritic magnets. However, rare earth magnets which enable higher field strengths to be achieved can alternatively be used. It is alternatively possible to utilise an electromagnet or electromagnets.

The residence time in the magnetic field can be quite short, for example, from about 0.05 seconds up to about 0.1 seconds. However, the use of longer residence times up to, for example, about 5 seconds is not excluded.

The portion of the flow path wherein the intermediate liquor is subjected to a magnetic field is designed to provide substantially streamline flow conditions therein.

WO 98/11183 PCT/GB97/02390 11 Desirably the flow path for the intermediate liquor is also designed so that the flow conditions in the region immediately upstream from said portion are also substantially streamline conditions. It is also desirable for the flow conditions in the region immediately downstream from said portion also to be substantially streamline conditions.

In one embodiment the flow path for the intermediate liquor comprises a pipe leading from the primary settling zone, e.g. the primary settling pit, to a secondary settling tank. This pipe has an inside surface that is preferably as smooth as possible. Moreover the pipe and any bend therein is designed so that, at the design flow rate, there is no significant risk of turbulent flow developing therein.

Hence any bend is designed to be as gradual as possible within the design limits of the available site. It is further desirable to mount the magnet or magnets as close as possible to the point of discharge of the intermediate liquor into the secondary settling tank.

Typically the secondary settling zone comprises a settling tank or pond.

In order that the invention may be clearly understood and readily carried into effect a preferred method of treatment of contaminated coolant liquid from a rolling mill conducted in accordance with the invention, and an apparatus for use therewith, will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, wherein:- Figure 1 is a flow diagram of the flow of cooling liquid and lubricant to and from a rolling mill and the treatment of the resulting contaminated cooling liquid by a preferred method according to the invention; Figure 2 is a diagram of the magnetic field used in the WO 98/11183 PCT/GB97/02390 12 method of the invention; and Figure 3 summarises the results of experiments utilising magnetic flocculation for treatment of cooling water used to cool the rolls of a plate coil rolling mill.

Referring to Figures 1 and 2 of the drawings, which are not to scale, a rolling mill for rolling steel strip 1 includes a pair of rolls 2, 3 which are positioned to form a nip 4 through which the steel strip 1 is passed in the direction of the arrow A. Lubricant is sprayed on the upper roll 2 from nozzles 5 from a supply line 6. Coolant liquid, e.g. water, is sprayed on the upper roll 2 from nozzles 7 which are fed by line 8.

Contaminated coolant liquid, which contains lubricant and mill scale particles, is collected in tank 9 from which it is pumped through line 10 to a primary settling pond 11.

In primary settling pond 11 substantially all particles of mill scale having a particle size greater than about 200 Am settle out under gravity and collect as a sludge in the bottom of pond 11. The size of pond 11 is so selected in relation to the flow of contaminated coolant liquid in line as to give a residence time in pond 11 sufficient for such settlement to occur. Typically the volumetric loading rate in the primary settling tank or pond 11 is from about to about 35 m 3 /m 2 /hr.

From primary settling pond 11 the partially clarified coolant liquid is pumped via line 12 to magnetic flocculator 13.

Magnetic flocculator 13 includes a cylindrical section 14 made of conventional steel with a flange 15 by means of which it is connected to a conventional circular section steel pipe forming line 12. It further includes a transition section 15 so as to permit substantially streamline flow conditions to occur therein. This is in WO 98/11183 PCT/GB97/02390 13 turn connected to a rectangular section conduit 16 made of non-magnetic stainless steel, the free end of which is downwardly turned at 17 and ends in a downwardly pointing section 18. As will be explained further below the coolant liquid passes through a constant magnetic field in passage through rectangular section conduit 16 which is generated by a pair of magnets (not shown, for the sake of clarity, in Figure As a result particles of mill scale in the coolant liquid are attracted towards each other and coagulate to form larger particles which tend to sink towards the lower wall of the rectangular section conduit 16. The shape of the turning portion 17 and of the downwardly pointing section 18 is selected in each case so as to ensure as far as possible that the flow pattern in these sections remains streamline.

From section 18 the treated coolant liquid passes as indicated by line 19 into a secondary settling pond 20 in which the loading rate is from about 2 to about 8 m 3 /m 2 /hr.

After a period of residence in secondary settling pond the coolant liquid is pumped back to nozzles 7 by way of line 21 and line 8. Make-up water, or other coolant liquid, can be supplied by way of line 22.

Figure 2 shows the arrangement of magnets 23 and 24, magnet 23 being positioned immediately above rectangular section conduit 16 and magnet 24 being positioned under and immediately adjacent rectangular section conduit 16. As indicated in Figure 2 the south pole of magnet 24 is positioned at the top and the north pole of magnet 23 at its underside. The lines of magnetic force are indicated diagrammatically in Figure 2.

The invention is further illustrated in the following Examples.

Example 1 WO 98/11183 PCT/GB97/02390 14 A plate coil mill water treatment plant was utilised for the experiments described in this Example. The cooling water used to cool the rolls of the plate coil rolling mill was discharged into a primary settling pond in which the volumetric feed rate was about 30 m 3 /m 2 /hr. After passage through the primary settling pond, the water was pumped through a conduit into a series of three rectangular plan secondary settling ponds (designated herein ponds A, B and C) connected in parallel. This conduit was arranged to discharge into each secondary settling pond through a respective rectangular section outflow section with a downwardly curved discharge outlet so as to preserve streamline flow therein as far as possible. The rectangular section outflow section was in each case made from stainless steel. Bar magnets were removably mounted, one above and one below, the rectangular section outflow section for one of the ponds (pond A) with the north pole of one magnet and the south pole of the other magnet in each case lying adjacent the rectangular section outflow section so as to produce therein a magnetic field of about 700 gauss.

250 ml samples of the feed water were taken from the conduit supplying the three secondary settling ponds for measurement of the millscale content thereof. Samples were taken from the three secondary settling ponds at points X and Y along the pond corresponding to approximately one third and approximately three quarters of the distance respectively along the pond from the inlet at an end to the outflow point at the other end of the pond in each case.

Samples were taken with all of the flow being divided in the ratio 25/25/50 between the three ponds A, B and C ("low flow" conditions) and with one of the secondary settling ponds (pond C) taken out of service so that only two secondary settling ponds A and B were used ("high flow" WO 98/11183 PCT/GB97/02390 conditions). The effluents from the ponds were in each case combined for recycle to the mill. In each case the millscale constant of the combined stream was measured.

Each sample was shaken and its contents poured into a beaker and boiled down. 10 ml of concentrated hydrochloric acid was then added to each bottle and any residual solids allowed to dissolve and the acid solution was then poured into the beaker. The bottle was then washed again with w/v HC1 and the washings added to the beaker whose contents were then boiled down to about 50 ml. The resulting solution was made up to 100 ml with deionised water and ml of the resulting solution was diluted to 100 ml for analysis for ion by atomic adsorption spectrophotometry.

Standards were incorporated into the series of experiments and a sample of filtered water was also analysed as a blank by the same technique. The results were converted to ppm millscale using a factor of 1.38 on the assumption that the millscale was magnetite (Fe30 4 The results for "low flow" conditions are presented in Table 1 and those for "high flow" conditions in Table 2. X and Y in Tables 1 and 2 designate the sample points X and Y mentioned above.

TABLE 1 Millscale Concentration Percentage Removal mg/l Pond A and Pond A and Day Inlet Magnet Pond B Magnet Pond B Pond C Pond C Y Y Start Finish Average X Y X Y X Y X Y 1 38.6 33.7 36.1 22.1 14.4 25.4 21.6 27.6 38.8 60.1 29.6 40.2 23.5 2 50.3 56.4 53.4 29.3 22.7 45.3 35.4 39.8 45.1 57.5 15.2 33.7 25.5 3 85.6 90.5 88.1 40.3 29.3 59.1 45.3 61.3 54.3 66.7 32.9 48.6 30.4 4 56.3 50.8 53.5 27.6 19.3 40.8 37.0 38.6 48.4 63.9 23.7 30.8 27.9 53.5 55.8 54.7 25.4 15.5 34.2 32.6 45.8 53.6 71.7 37.5 40.4 16.3 Average 57.2 28.9 20.2 41.0 34.4 42.6 48.0 63.9 27.8 38.7 24.7 TABLE 2 Millscale Concentration Percentage Removal mg/l Pond A and Pond A and Day Inlet Magnet Pond B Magnet Pond B Start Finish Average X Y X Y X Y X Y 1 3.3 3.9 3.6 3.3 3.9 5.0 6.1 2 55.2 66.2 60.7 38.6 38.1 55.8 51.9 36.4 37.2 8.1 14.5 3 87.2 79.5 83.4 48.6 41.4 59.6 54.6 41.7 50.4 28.5 34.5 4 86.7 85.0 85.8 45.8 40.8 68.4 59.6 46.5 52.4 20.3 30.5 24.8 22.1 23.5 14.4 11.0 19.9 16.6 38.7 53.2 15.3 29.4 Average 63.4 36.9 32.8 50.9 45.7 40.9 48.3 18.1 27.2 WO 98/11183 PCT/GB97/02390 18 The results are summarised in Figure 3.

Example 2 The plate coil mill water treatment plant used in Example 1 is modified by fitting with magnets, similar to those used for the inlet to pond A, the corresponding inlets to pond B and also that to pond C. The following results set out in Table 3 are obtained.

TABLE 3 Millscale content of Magnetic flocculation added combined treated water stream mg/l Flow Pond A Pond B Pond C Low High No No No 38.5 50.6 Yes No No 35.0 47.4 Yes Yes No 31.4 44.2 Yes Yes Yes 24.2 37.8

Claims (12)

1. A method of treating a coolant liquid used for cooling rolls of a rolling mill or for cooling a continuous caster and contaminated with particles of scale which comprises passing the contaminated coolant liquid through a primary settling zone, retaining the cooling liquid in the primary settling zone for a period sufficient to permit substantially all particles of scale having a particle size greater than about 200 Am to settle out under gravity from suspension, recovering from the primary settling zone an intermediate liquor which is substantially free from particles of scale having a particle size greater than about 200 Am, passing the intermediate liquor along a flow path which includes a portion wherein the intermediate liquor is flowing under substantially streamline flow conditions, said portion comprising a length of conduit made of non-magnetic material adjacent to which is or are positioned one or more magnets arranged to produce a substantially constant magnetic field within said length of conduit, subjecting the intermediate liquor in passage through said portion to said substantially constant magnetic field thereby to coagulate iron-containing particles in the intermediate liquor and enhance sedimentation of resulting coagulated particles in the intermediate liquor, and collecting resulting treated liquor now containing less than about 50 ppm solids by weight for reuse for cooling the rolls of a rolling mill or for cooling a continuous caster.

2. A method according to claim 1, which includes the further steps of passing the treated liquor to at least one secondary settling zone and retaining the liquor therein for a period sufficient to permit the solids content thereof to WO 98/11183 PCT/GB97/02390 drop to less than about 20 ppm.

3. A method according to claim 2, in which the volumetric feed rate to the secondary settling zone is from about 2 to about 8 m 3 /m 2 /hr.

4. A method according to any one of claims 1 to 3, in which the length of conduit is made of stainless steel or a plastics material.

A method according to claim 4, in which the length of conduit is made of polypropylene.

6. A method according to any one of claims 1 to 5, in which the length of conduit is substantially rectangular in section and at least one pair of magnets is mounted adjacent the rectangular section conduit with the south pole of one of the pair of magnets adjacent one of a pair of opposed faces of the conduit and with the north pole of the other magnet of the pair adjacent the other opposed face of the conduit.

7. A method according to claim 6, in which the magnets are chosen so as to produce within the length of conduit a magnetic field of from about 600 to about 800 gauss.

8. A method according to claim 8, in which the magnets are permanent ferritic magnets.

9. A method according to any one of claims 1 to 8, in which the residence time in the magnetic field is from about 0.05 seconds up to about 0.1 seconds.

A method according to any one of claims 1 to 9, in which the portion of the flow path wherein the intermediate liquor is subjected to a magnetic field is designed to provide substantially streamline flow conditions therein.

11. A method according to any one of claims 1 to 10, in which the flow path for the intermediate liquor comprises a WO 98/11183 PCT/GB97/02390 21 pipe leading from the primary settling zone to a secondary settling tank.

12. A method according to any one of claims 1 to 11, in which the primary settling zone comprises a primary settling tank or pond and in which the volumetric feed rate to the primary settling tank or pond is in the range of from about to about 35 m 3 /m 2 /hr.