GB2306461A

GB2306461A - Treatment of liquid effluents

Info

Publication number: GB2306461A
Application number: GB9522229A
Authority: GB
Inventors: Ian Stuart Bleakley; Hannu Olavi Ension Toivonen
Original assignee: ECC International Ltd
Current assignee: Imerys Minerals Ltd
Priority date: 1995-10-31
Filing date: 1995-10-31
Publication date: 1997-05-07
Anticipated expiration: 2015-10-31
Also published as: GB9522229D0; GB2306461B

Abstract

A method for treating liquid effluent having fine solid material suspended therein to separate and to facilitate recovery therefrom of liquid and fine solid material suspended therein, the method including producing in the liquid of the effluent a substantially white precipitate in which the fine solid material becomes entrained, wherein the precipitate is formed by a reaction in the effluent of at least two reactants which are introduced separately into the effluent each in the form of a solution comprising the reactant in a liquid, whereby mixing and reaction of the reactants occurs in the liquid phase in the effluent.

Description

Treatment of Liquid Effluents The present invention relates to the treatment of liquid effluents. In particular, it relates to treatments to recover water and fine solid material from aqueous effluents.

Many naturally occurring mineral materials are subjected to particle size separations in order to select those particles which have the most desirable distribution of sizes for a particular application. In many cases the natural mineral material contains a significant proportion of particles which are undesirably fine for the particular application for which the mineral material is being prepared, and it is necessary to remove these excessively fine particles. When the particle size separation is performed on a mineral material in suspension in a liquid, which would most commonly be water, and when the undesired fine particles have an equivalent spherical diameter of about 0.5 m or less, the suspension of the undesired fine particles is often recovered in the form of a dilute suspension which is extremely difficult to dewater by conventional methods.It is generally unacceptable, for environmental reasons, to aGlow such dilute suspensions of fine mineral particles to be discharged to rivers or lakes, and, as a result, such unwanted suspensions of very fine particles are often retained in lagoons, thus occupying large areas of land which could more profitably be used for other purposes.

Sheet cellulosic products, for example paper and board, are generally manufactured on machines of the type in which a dilute suspension of finely divided solid materials in water is spread evenly over the surface of a moving wire mesh belt, which is generally referred to in the art as the "wire", and water is drawn through the wire by gravity and by suction to leave a thin felt-like mat of the solid materials on the wire.

When the web of sheet material formed in this way is partially dewatered it is transferred from the wire to a moving felt band which provides it with support while further dewatering is carried out.

The solid material used in the formation of paper and board products generally consists predominantly of fibres which are most commonly of cellulose, but which may contain a proportion of synthetic fibres. The fibrous material may be prepared, for example, by subjecting wood to a series of mechanical and/or chemical processes which separate the fibres substantially one from another and make them available for the sheet forming process in lengths ranging from about lOum to several millimetres. The solid material will often also include a particulate mineral material as a filler, the particles ranging in size from a fraction of a micrometer to about 50 m.

In order to manufacture a sheet material of homogeneous composition and uniform thickness, it is generally necessary to apply the solid material to the wire of the paper machine in the form of a very dilute aqueous suspension containing from about 0.5% to 1.0% by weight of solid material. This means that a very large quantity of water is required for the manufacture of paper and board; in fact the weight required is approximately two hundred times the weight of solid material used. It is therefore essential in most cases for environmental and economic reasons that as much as possible of the water which passes through the wire or is removed from the web of sheet material at a later stage is recovered for further use.

The water passing through the wire generally carries with it a substantial amount of fibrous or particulate material which is too fine to be retained by the mat of sheet material formed on the wire. This solid material is generally referred to as "fines". A useful definition of this terms is given in the TAPPI Standard No. T 261 cm-90 "Fines fraction of paper making stock by wet screening". This document describes a method for measuring the fines content of paper making stock or of pulp samples, and specifies that fines are those particles which will pass a round hole of diameter 76um.

Generally up to about 50% by weight of the solid material in the aqueous suspension which is fed to the head box of the sheet forming machine passes through the wire, and must be recovered for re-use. From about 1% to about 5% by weight of the solid material which is fed to a paper or board making process is finally rejected. Of this material, about 5% by weight is rejected because it is too coarse to be incorporated, and the remainder consists of fines.

The water which passes through the wire is generally referred to as "white water" on account of its high content of fine solids which gives it a high turbidity. Almost all of this white water is recirculated to the plant in which the paper making stock is prepared in what is called the "primary circulation loop". However, not all the white water can be recirculated in this way because less water is carried away from the sheet material forming machine in moist web than is introduced with the new solid material. The excess white water is withdrawn from the primary circulation loop and is processed in a secondary circulation loop which separates as completely as possible the solid materials from the suspending water, so that the solid material can either be re-used in the stock preparation process, or discharged as waste.The water which is then substantially free of suspended solids can then either be re-used in the sheet material forming plant, for example in sprays or "showers", or as pump sealing water, in various parts of the process, or may be discharged to a convenient natural water course.

The secondary circulation loop makes use of various pieces of apparatus which are known generically as "savealls". These generally operate on one of three principles, namely sedimentation, or filtration or flotation. In the sedimentation type of save-all the white water flows very slowly through a large tank so that the solid material sinks to the bottom and substantially clear water overflows at the top. It is usually necessary to add a chemical flocculant to the white water so that the solid material is present in the form of clusters and particles, rather than as discrete particles. Also the sedimentation type of save-all is rarely adequate on its own, but needs to be used in conjunction with additional separation equipment.The filtration type of saveall is operated by passing the white water through a filter medium, which may conveniently be a fine wire mesh, which is generally precoated with a layer of fibres to improve filtration. Again it is usually necessary to add a chemical flocculant to improve the separation of the solid particles from the water. In the flotation save-all process, the white water is introduced into a vessel in which a rising stream of fine air bubbles is provided. The solid particles become attached to the bubbles and rise to the surface where they are skimmed off by rotating paddles. It may in certain cases be necessary to introduce a chemical which renders the surface of the solid particles hydrophobic, and therefore increases their affinity with the air bubbles.It is usually necessary to use two or more save-alls in series to achieve acceptable separation of solid material from the water.

European patent specification No.0604095 concerns a process for treating an aqueous suspension of particulate waste material wherein an alkaline earth metal carbonate is precipitated in the aqueous suspension so that the particulate material present at the start of the process becomes entrained in the alkaline earth metal carbonate precipitate. The particulate waste material may comprise waste organic fibres such as those present in paper mill effluent. In the preferred example described in this prior art specification, an alkaline earth metal oxide or hydroxide is introduced into the aqueous suspension of the particulate waste material and a carbon dioxide containing gas is passed through the suspension to precipitate the alkaline earth metal carbonate.

British patent specification No. 2265916 concerns a composite product which comprises a plurality of fibres of expanded specific surface area and of hydrophilic character, having a substantial quantity of microfibrils on their surface, and crystals of precipitated calcium carbonate organised essentially in clusters of granules, which trap the microfibrils and are attached thereto by mechanical bonding.

A process is also described in which the microfibrillated fibres in suspension are contacted with calcium ions in the form of lime, and with carbonate ions introduced by the injection of carbon dioxide gas. The suspension of fibres and lime contains not more than 552 by weight of solids, and, during the admission of carbon dioxide, the temperature of the suspension is maintained in the range of from lOC to 50C.

European patent specification No. 0658606 concerns a process for separating fine solids from water in the used water recovery system of a sheet forming mill, wherein the used recovery system includes at least one stage in which an alkaline earth metal carbonate is precipitated in the aqueous suspension constituting the used water whereby the particulate material present in the used water becomes entrained in the alkaline earth metal carbonate precipitate. By the process in this specification it is possible to recover the water and the fine solid materials which pass through the wire mesh belt of a paper or board forming machine, and optionally recycle those recovered materials.In the preferred example described in this prior art specification the alkaline earth metal carbonate is again precipitated by adding an alkaline earth metal oxide or hydroxide to the suspension and passing a carbon dioxide containing gas therethrough.

Formation of precipitates by the processes described in the above prior art specifications is not ideal. In particular, the reaction kinetics is complex because of the different phases of matter involved. Ensuring an efficient, localised reaction between carbon dioxide in the gas phase and calcium hydroxide, generally in the form of a suspended solid which enters a liquid phase solution as carbon dioxide reacts with the solution, is not easy to achieve. The quantities of the reactants taking part in the precipitate forming reaction are not easy to meter accurately. There is a mass transfer hindrance to the delivery of gas to take part in the reaction, and the precipitation reaction can take as long as several hours to complete.

According to the present invention in a first aspect there is provided a method for treating liquid effluent having fine solid material suspended therein to separate and facilitate recovery therefrom of liquid and solid material suspended therein, the method including producing an insoluble, substantially white precipitate in the liquid of the effluent in which the fine solid material in the effluent becomes entrained, wherein the precipitate is formed by a reaction in the effluent of at least two reactants which are introduced separately into the effluent each in the form of a solution comprising the reactant in a liquid, whereby mixing and reaction of the reactants occurs in the liquid phase in the effluent.

The liquid effluent may be an aqueous suspension or a suspension in an organic liquid or a suspension in a liquid which comprises a mixture of aqueous and organic phases eg., oil in water.

The reactants may conveniently be delivered in separate streams at the same time in the method according to the invention. The liquids in which they are delivered may be aqueous solutions, although either or both may comprise, for example, non-miscible components, eg., a water-in-oil suspension, which incorporates the reactant in an aqueous component, eg., present in the form of droplets. The reactants may for example be present in aqueous solutions containing the required reactants as dissolved salts. The reaction providing the required precipitation may be a double decomposition reaction.

The fine solid material entrained in the particles of the precipitated material may include fine particles and/or fine fibres and/or particles of a species originally dissolved in the effluent solution but precipitated as a result of the reaction between the said reactents.

Desirably, the liquids containing the required reactants are fed into a portion of the effluent solution to be treated in a manner in which local reaction between them in the effluent is facilitated. The effluent is conveniently in a flowing state whilst the reactants are introduced therein, although the treated portion of effluent could alternatively comprise a static sample.

The reactant-containing liquids are preferably introduced into a flow of the effluent to be treated by one or more inline mixers, e.g.mixers injecting the liquids from different sides of a mixing region in the delivery vessel through which the effluent flows.

The mixer or mixers may conveniently be one or more devices providing mutual mixing of the reactants and the effluent by a rapid swirling action. Such devices are known as "power fluidics" mixing devices and operate with no moving parts the swirling being caused by vortexes formed by the construction of the device.

Such a device is described for example in "Mixing in the Process Idustries" by N. Harnby, M.E. Edwards and S.A.W.

Nienow; Butterworths 1985. The method according to the present invention may include the preliminary step of sampling the effluent to be treated to determine the characteristics of the effluent, e.g. dry weight of suspended fine solids in the effluent, the types of solids suspended and the effluent pH.

From this sampling procedure, the amount of precipitate required to be produced per unit amount of effluent to be treated can be calculated.

The amount of precipitate produced to entrain the fine solids in the effluent can be selected by the process operator between wide limits depending on the costs of further treating the precipitate-containing suspension. The dry weight ratio of precipitate produced to the fine solids in the effluent to be entrained thereby may for example in the range from 1:10 to 10:1.

The aggregated solid material comprising the precipitated material and entrained solids may be separated from the water or aqueous solution in which it is contained by a conventional separation process, e.g. filtration. The separated material may thereafter be washed and dried.

The aqueous solution remaining after separation of the aggregated solid material may be treated by one or more further treatment steps and thereafter recycled for re-use as clean water in the plant from which the effluent has issued.

For example, where the reaction providing precipitation comprises a double decomposition reaction, a second compound is formed in the method of the first aspect together with the precipitate in which the fine solids are entrained. For example, when calcium carbonate is precipitated by the double decomposition of sodium carbonate and calcium chloride, the reaction additionally produces sodium chloride. This byproduct may be separated from the resulting solution in a known way, e.g. by use of a subsequent evaporation step.

The aqueous effluent treated by the method according to the present invention may be one which is difficult to dewater because of the presence therein of fine particles and/or fibres. The aqueous effluent may be derived, for example, from a wet mineral beneficiation process or from a process for manufacturing a cellulosic sheet material such as paper or paper board.

If the effluent to be treated has issued, for example, from a paper mill plant the method according to the present invention allows the level of fine solids material comprising particles and fibres in the effluent to be substantially reduced whereby clear water may be recovered therefrom and subsequently recycled and re-used.

By "fine solids material" we mean, generally, that the material has a particle size distribution such that, when the material is incorporated in a dilute aqueous suspenslort containing not more than about 10% by weight of the material, the suspension is difficult to dewater by conventional means such as filtration or by means of a centrifuge. As a guide, the material may comprise mineral particles of size about 0.5um or less, and/or organic fibrous particles of size such that they pass through a circular hole of diameter 75 m (300 mesh).

The aqueous solution separated from the precipitate formed in the method according to the present invention may be further treated by known steps for the treatment of effluent.

For example, water obtained from paper mill effluent may be further treated by clarification.

The effluent may be'treated by one or more procedures in a known way prior to treatment by precipitation. For example, the effluent may be treated by a process to concentrate the fine solids therein, e.g. by sedimentation and/or filtration and/or flotation such as dissolved air flotation.

Treatment of aqueous effluent by the method according to the present invention can be carried out in a more efficient and controlled manner than by using the prior art precipitation methods referred to above. The time and cost of the precipitate forming reaction can be considerably reduced.

Use of liquid reactants to provide a double decomposition reaction allows efficient mixing of the reactants in a manner in which the reactants can be accurately metered. The method is versatile and allows a range of precipitates to be produced. Some of these precipitates may beneficially show enhancements over the precipitated calcium carbonate produced in the prior art methods.

According to the present invention in a second aspect there is provided a precipitated material comprising one or more insoluble, substantially white compounds having fine solids material entrained therein, wherein the precipitated material is produced by the method according to the first aspect.

The entrained solids material may comprise fine particles and/or fine fibres. The particles and fibres may be fines of materials used in processes for making cellulosic sheet material such as paper or board. The particles may for example comprise paper pigment or filler particles such as one or more of titanium dioxide, calcium sulphate, talc, calcium carbonate or clay. Fine fibres entrained may include organic, e.g. cellulosic fibres or synthetic fibres. Such fine materials may generally be "fines" as defined herein before, eg., training sizes in the range O.lpm to 10pm.

The substantially white compound or compounds of the precipitated material may comprise an inorganic compound or compounds having one or more cations chosen from the group consisting of calcium, aluminium and magnesium and one or more anions chosen from the group consisting of carbonate, phosphate and silicate. Preferably, the inorganic material is chosen from the group consisting of aluminium phosphate, magnesium phosphate, calcium phosphate, calcium silicate, aluminium silicate and mixtures thereof.

Beneficially, phosphates and silicates are less soluble than calcium carbonate in water and may also be more tolerant to an aggressive, e.g. acidic, aqueous effluent environment.

Phosphates and silicates may give better filtration rates as demonstrated below thereby facilitating easier separation from the solution containing them. The insolubility of the precipitated material in water may therefore have a pS value of substantially greater than 8, eg. greater than 10, 8 representing the solubility of calcium carbonate in water on a logarithmic scale. The parameter pS is the same as-log S where the S is the solubility product of the compound in water.

As noted above, the amount of fine solids entrained in the precipitated material may be varied within wide limits e.g. the dry weight ratio between the two may be in the range 1:10 to 10:1.

The precipitated material according to the second aspect may be suitable for recycle and re-use in the plant from which the effluent was issued or in some other application. For example, the material may be used as a pigment or filler in paper or board coating materials or as a filler or extender in paints, resins, rubbers, plastics and like materials.

Embodiments of the present invention will now be described by way of example.

EXAMPLE 1 Used water from the water recovery system of a paper mill contained 2% by weight of a mixture comprising 46.4% be weight of fine organic fibres and 53.6% be weight of fine particulate inorganic filler particles, approximately 13.5% be weight being calcium carbonate and 40.1% by weight kaolin. To a 700ml sample of this used water there were added aqueous solutions containing, respectively 34.6g of aluminium sulphate, A12(SO4)3, and 41.8g of sodium orthophosphate, Na3PO4. These were the amounts which were calculated to provide 14g of aluminium phosphate precipitate, which was an amount which was equal to the weight of solid material suspended in the sample of the used water.Aluminium phosphate precipitate was produced by double decomposition and an aggregated co-precipitate of the aluminium phosphate with the solid component of the waste water was formed.

The aggregated solid material was separated from the aqueous medium in which it was suspended by filtration and the filter cake was dried. The brightness of the dry material was determined by measuring the reflectance of a prepared surface of the dried solids to light of wavelength 457nm with a Carl Zeiss "ELREPHO" brightness meter and comparing the result with the reflectance to light of the same wavelength from an ISO standard brightness surface. The filtration rate of the suspension of the aggregated material was measured by the following procedure: A small sample of the suspension of the aggregated material produced in each case was poured lnto a Buchner filter funnel provided with a piece of standard filter paper, the side arm of the filtrate flask being connected to the laboratory vacuum source.The filtrate was collected in a measuring cylinder inside the filtrate flask, and at intervals the volume of filtrate collected and the time which had elapsed since the start of filtration were recorded. The square of the volume collected was plotted graphically against the elapsed time, and a curve was obtained which had a large central straight line portion. The slope of this straight line portion was recorded in each case.

The relationship between the square of the volume of filtrate collected and the elapsed time is given by the Carmen-Kozney equation: Q2 2.A2.P.E3.(y - 1) T 5.v.S2.(1 - E)2.d2 where: Q is the volume of filtrate collected; T is the elapsed filtration time; A is the area of the filter medium; P is the differential pressure across the filter medium; E is the fraction of voidage in the filter cake; v is the viscosity of the suspending medium; S is the specific surface area of the particulate phase; and d is the specific gravity of the particulate phase.

The slope Q2/T of the straight line portion of the graph plotted for each suspension gave a measure of the filtration rate in each case and, since A, P, v, S and d can be assumed to be constant under the conditions of the experiment, a standardised filtration rate F can be found to be given by: F = Q2.R T where: 1 Wc + d Sc R = Ws Wc Ss 5c where: Wc is the weight fraction of water in the cake; 5c is the weight fraction of solids in the cake; W5 is the weight fraction of water in the suspension; and Ss is the weight fraction of solids in the suspension.

The turbidity of the filtrate was also determined by measuring the attenuation of visible light passed through a sample of the filtrate.

As a comparison, a sample of the untreated water was also dewatered by filtration and the filtration rate, brightness of the dry solids and trubidity of the filtrate were measures by the procedures described above.

The results are shown in the Table set forth below.

EXAMPLE 2 To a further 700ml sample of the same used water as was tested in Example 1, there were added aqueous solutions containing, respectively 39.9g of magnesium sulphate, MgSO4.7H2O, and 39.5g of sodium orthophosphate, Na3PO4. These were the amounts which were calculated to provide 14g of magnedium phosphate precipitate, which was an amount which was equal to the weight of solid material suspended in the sample of the used water. Magnesium phosphate precipitate was produced by double decomposition and an aggregated co precipitate of the magnesium phosphate with the solid component of the waste water was formed.

The aggregated material was separated from the aqueous medium in which it was suspended by filtration, and the filtration rate of the suspension, brightness of the dry solids and turbidity of the filtrate were determined by the procedures described in Example 1 above. The results are shown in Table 1 set forth below.

EXAMPLE 3 To a further 700ml sample of the same used water as was tested in Example 1, there were added aqueous solutions containing, respectively, 19.8g of calcium chloride, CaC12, and 34.2g of sodium othophosphate, Na3P04. These were the amounts which were calculated to provide 14g of magnesium phosphate precipitate, which was an amount which was equal to the weight of solid material suspended in the sample of the used water Magnesium phosphate precipitate was produced by double decomposition and an aggregated co-precipitate of the magnesium phosphate with the solid component of the waste water was formed.

EXAMPLE 4 To a further 700ml sample of the same used water as was tested in Example 1, there were added aqueous solutions containing, respectively, sufficient calcium chloride, CaC12, to provide 7g of calcium and 7g of colloid silica SiO2. These were the amounts which were calculated to provide 14 g of calcium silicate precipitate, which was an amount which was equal to the weight of solid material suspended in the sample of the used water. Calcium silicate precipitate was produced by double decomposition and an aggregated co-precipitate of the calcium silicate with the solid component of the waste water was formed.

EXAMPLE 5 To a further 700ml sample of the same used water as was tested in Example 1, there were added aqueous solutions containing, respectively, sufficient aluminium sulphate, A12(S04)3. 16H20, to provide 7g of aluminium and 7g of colloidal silica, SiO2. These were the amounts which were calculated to provide 14g of aluminium silicate precipitate, which was an amount which was equal to the weight of solid material suspended in the sample of the used water. Aluminium silicate precipitate was produced by double decomposition and an aggregated co-precipitate of the aluminium silicate with the solid component of the waste water was formed.

The aggregated material was separated from the aqueous medium in which it was suspended by filtration, and the filtration rate of the suspension, brightness of the dry solids and turbidity of the filtrate were determined by the procedures described in Example 1 above.

The results obtained for the precipitated materials obtained respectively in Examples 1 to 5 are shown in Table 1 set forth below.

Table 1 Example Precipitated Brightness Filtration Turbidity Material Rate 1 aluminium 69.1 0.75 10.6 phosphate 2 magnesium 71.2 8.67 36.1 phosphate 3 calcium 54.7 0.43 36.9 phosphate 4 calcium 53.4 2.43 53.4 silicate 5 aluminium 50.2 1.56 14.1 silicate Control untreated 48.5 0.0018 58.5 used water Comparative calcium 69.7 0.39 20.3 carbonate In Table 1, filtration rate and turbidity are relative measures. Filtration rate is for water containing a fixed amount of the precipitated material and turbidity is for the filtrate solution. In Table 1 the units of brightness are ISO units measured as described above.

These results show that, in each case, the treatment by the method embodying the invention has resulted in a useful increase in the filtration rate of the suspension, and in a significant reduction in the turbidity of the filtrate.

Compared with the control (conventional) untreated used water process and a useful improvement in brightness of the solid material. In all of the cases the treatment embodying the invention achieves a useful improvement in filtration rate compared with that obtained by the comparative process using calcium carbonate as the precipitated material.

Claims

1. A method for treating liquid effluent having fine solid material suspended therein to separate and to facilitate recovery therefrom of liquid and fine solid material suspended therein, the method including producing in the liquid of the effluent a substantially white precipitate in which the fine solid material becomes entrained, wherein the precipitate is formed by a reaction in the effluent of at least two reactants which are introduced separately into the effluent each in the form of a solution comprising the reactant in a liquid, whereby mixing and reaction of the reactants occurs in the liquid phase in the effluent.

2. A method as in claim 1 and which includes separating the precipitated material from the liquid for further use as a product.

3. A method as in claim 1 or claim 2 and which includes separating liquid from the effluent for recycle and re-use.

4. A method as in any one of the preceding claims and wherein the effluent which is treated is one which contains fine solid material which comprises fibrous material and particulate material.

5. A method as in claim 4 and wherein the aqueous effluent is derived from a process of making cellulosic sheet material.

6. A method as in any one of the preceding claims and wherein the liquid comprises an aqueous solution.

7. A method as in any one of the preceding claims and wherein the reaction is brought about by the addition to the effluent of two liquids containing solutions comprising the reactants to provide the required reaction.

8. A method as in any one of the preceding claims and wherein the effluent when treated is in a flowing state.

9. A method as claimed in claim 8 and wherein the effluent when treated is in a flowing state,the reactant containing liquids being introduced into the effluent flow by one or more in-line mixers.

10. A method as in any one of the preceding claims and wherein the effluent is sampled to measure one or more of its properties prior to treatment to provide the required precipitation reaction.

11. A method as in any one of the preceding claims and wherein the dry weight ratio of the precipitate formed by the said reactants to the fine solid material in the effluent before the said treatment is in the range from 1:10 to 10:1.

12. A precipitated material comprising one or more insoluble substantially white compounds having fine solid material entrained therein, wherein the precipitated material is produced by the method as claimed in any one of claims 1 to 10.

13. A precipitated material as in claim 12 and wherein the substantially white compound, or one or more of such compounds, comprises a compound having one or more cations chosen from the group consisting of calcium, aluminium and magnesium and one or more anions chosen from the group consisting of carbonate, phosphate and silicate.

14. A precipitated material as in claim 12 and wherein the substantially white compound or compounds is selected from aluminium phosphate, magnesium phosphate, calcium phosphate, calcium silicate, aluminium silicate and mixtures thereof.

15. A precipitated material as in claim 12, 13 or 14 and wherein the entrained fine solid material comprises one or more of Tic2, calcium sulphate, talc, calcium carbonate or clay optionally together with organic or synthetic fibrous material.

16. A precipitated material as in any one of claims 12 to 15 and wherein the dry weight ratio of the precipitated compound or compounds to the fine solid material entrained therein is in the range from 1:10 to 10:1.

17. A method as in claim 1 and in which the treatment to remove fine particulate materials is carried out substantially as hereinbefore described with reference to any one of Examples 1 to 5.