GB2059811A - Beneficiation of minerals - Google Patents

Abstract

A process for beneficiating a crude mineral in which a deflocculated aqueous suspension of the crude mineral is treated with a flocculating agent to selectively flocculate the desired mineral or the impurities, and then the flocs of the desired mineral or impurities are passed countercurrent to a stream of an aqueous washing medium in order to wash the flocs and remove therefrom entrapped deflocculated particles.

Description

SPECIFICATION Improvements in or relating to the bsgiefaciation of miMSralS This invention relates to the beneficiation of minerals and, more particularly, is concerned with a process for separating from a crude mineral, such as a crude clay mineral or a crude alkaline earth metal mineral, a substantial proportion of the impurities associated therewith.

Various impurities exist in crude minerals and are undesirable in the finished product for a number of reasons. For example, mica, tourmaline and titania are often associated with kaolinitic clay minerals and they frequently contain iron atoms in their crystal lattice or have iron compounds associated with them, and as a result are dark coloured and impair the whiteness of the kaolinitic clay in which they are present; kaolinitic clay minerals may also contain quartz and feldspar which are relatively hard minerals and a kaolinitic clay containing them tends to be abrasive when used as a filler or coating pigment for paper, thus causing wear of paper-making or printing machinery; and kaolinitic clay minerals also contain minerals, such as montmorillon- ite, which may have the property of expanding their crystal lattice in water and thus influence the overall particle size distribution and the particle packing of a kaolinitic clay in such a way that the viscosity of a concentrated aqueous suspension of the keolinitic clay is increased.

There are known many processes for beneficiating crude minerals by separating impurities from the crude minerals. These processes include magnetic separation, froth flotation, particle size classification, two liquid phase separation techniques, and differential or selective flocculation. However, none of these processes will separate from a crude kaolinitic clay mineral all of the different types of undesirable impurity which may be present in associatin with the kaolinite in the crude kaolinitic clay mineral. Also, some impurities, notably mica, are extremely difficult to separate from kaolinite.

According to the present invention, there is provided a process for separating from a crude mineral a substantialy proportion of the impurities associated therewith, which process comprises forming a deflocculated aqueous suspension of the crude mineral, selectively flocculating either the desired mineral particles or the impurity particles in the deflocculated aqueous suspension of the crude mineral, and then causing the flocs of the desired mineral particles or impurity particles to flow countercurrent to a stream of an aqueous washing medium in order to wash the flocs and separate therefrom entrapped deflocculated particles.

The process of the invention will now be described in more detail by reference to the application of the process to clay minerals, but it is to be understood that the process can also be applied to other minerals, for example alkaline earth metal minerals such as those comprising calcium carbonate, strontium carbonate, strontium sulphate or barium sulphate.

In one method of applying the process of the present invention to crude clay minerals the following steps are carried out: a) the crude clay mineral is mixed with water to form a suspension: b) a dispersing agent is added to the suspension is order to deflocculate the clay mineral particles and any impurity particles associated therewith; c) the pH of the suspension, if not at a value in the range of from 7 to 11, is adjusted to a value in said range; d) an anionic polyelectrolyte flocculant is added to the suspension to flocculate the clay mineral particles whilst leaving the impurity particles in a substantially deflocculated state; and e) the flocs of clay particles are passed countercurrent to a rising stream of an aqueous washing medium having a velocity sufficient to convey substantially all the deflocculated impurity particles away from the flocs of clay particles.

The process of the invention has been found to be particularly useful for the treatment of crude kaolinitic clay minerals, and a more detailed discussion of each of steps (a) to (e) set out above, is given below with reference to the treatment of a crude kaolinitic clay mineral, e.g.

kaolin.

In step (a) the crude kaolin is conveniently obtained from the ground by hydraulic mining, and the dilute aqueous suspension of crude kaolin thus formed is thickened by gravitational sedimentation, and then subjected to a particle size classification step to remove particles of grit and oversize keolinite. | To improve the efficiency of the particle size classification step it may be desirable to add a dispersing agent to the suspension prior to carrying out the particle size classification step, but the amount of dispersing agent used should not significantly exceed the minimum necessary to effect substantially complete dispersion of the particles of impure kaolin.

The amount of dispersing agent required varies from one dispersing agent to another. The dispersing agent can be, for example, a water-soluble organic polymer, a water-soluble condensed phosphate, a water-soluble salt of a polysilicic acid, or a hydroxide or carbonate of an alkali metal or ammonium. If the dispersing agent is an organic polymer the amount thereof added should preferably not exceed about 0. 1% by weight. After thickening, the solids content of the suspension may be as high as 75% by weight, but usually it is preferably thickened to give a solids content in the range of from 10% to 40% by weight although in some circumstances it may be advantageous to operate the process of the invention at a higher solids content (say about 70% by weight).

In step (b) the dispersing agent used may be, for example, a water-soluble organic polymer, a water-soluble condensed phosphate, a water-soluble salt of a polysilicic acid, or a hydroxide or carbonate of an alkali metal or ammonium. The quantity of dispersing agent used will usually be such that the total amount of dispersing agent added in steps (a) and (b) is in the range of from 0.01% to 1.5% by weight, based on the weight of dry crude kaolin, but the optimum quantity varies from one kaolin clay to another. The process is sensitive to the dispersing agent dose in that, as the dispersing agent dose increases, the quality of the product improves but the recovery of the product is reduced. The nature of the dispersing agent does not appear to be critical and no technical advantage or disadvantage is apparent if the dispersing agent used in step (b) differs from that used in step (a).The choice of dispersing agent(s) is generally made on the basis of cost effectiveness. If a dispersing agent is used in step (a), the nature and amount of the dispersing agent used in step (b) are chosen to give good deflocculation for the least cost.

In some cases no extra dispersing agent will need to be added in step (b). It has been found that the dispersing agent used in step (b) is conveniently a sodium polysilicate, and the amount required is usually in the range of from 1 to 40 kg of "P60" sodium silicate solution per tonne of dry impure kaolin (60t Twaddell or 30% by weight of solid silicate having a molar ratio of SiO2: Na20 of 3.4: 1). This amount is equivalent to from 0.06 to 1.2% by weight of dry sodium silicate based on the weight of dry crude kaolin clay.Advantageously, in carrying out step (b) there is dissipated in the suspension during the mixing or the dispersing agent with the suspension an amount of energy in the range from 5 to 50 horsepower hours per ton of dry impure kaolin (13-130 kJ.kg-1).

In step (c) the pH of the suspension is preferably adjusted to lie in the range of from 8.5 to 10.

In step (d), the anionic polyelectrolyte flocculant is preferably a water-soluble salt of either a partially hydrolysed polyacrylamide or a copolymer of acrylamide and acrylic acid, which has a number average molecular weight of at least 100,000, and most advantageously has a number average molecular weight in excess of 106. The amount of such an anionic polyelectrolyte added is preferably in the range from 50 to 450 parts by weight of polyelectrolyte per million parts by weight of dry crude kaolin clay. Many different types of apparatus may be used for mixing the anionic polyelectrolye with the deflocculated aqueous suspension of crude kaolin clay, but a shrouded impeller mixer or a centrifal pump has been found to be especially suitable.The speed of the mixer or pump should be set so that the kaolin flocs which are formed are of generally uniform size and are not broken doen to an undesirable extent. High mixer or pump speeds tend to cause the chains of the polymeric flocculant to break with a resultant decrease in molecular weight and reduced effectiveness as a flocculant. A flocculant of higher initial molecular weight is better able to withstand the higher mixer or pump speeds because, even after breaking, the polymer chains are of adequate length.

In step (e), the flocs of kaolinitic clay are preferably washed by suspending them in a continuous rising current of an aqueous washing medium having a velocity which is sufficient to convey all the deflocculated impurity particles present, from the finest to the coarsest, upwards and away from the suspended flocs, but not so high that the smallest flocs of purified kaolinitic clay are conveyed upwards. It may be advantageous to provide a pulsed rising current of an aqueous washing medium as the pulsing action helps the washing process. The pulsing action can be introduced by using a reciprocating pump to supply the aqueous washing medium. Most preferably, the washing of the flocs is performed in apparatus comprising a plurality of stages in which the flocs fall from a higher stage to the next lower stage countercurrent to a rising current of the aqueous washing medium.The aqueous washing medium can be water, an aqueous solution or a dilute aqueous suspension containing particulate solid material. The pH of the aqueous washing medium is preferably at least 6.5 and most preferably at least 8.5. The aqueous washing medium entrains the deflocculated impurity particles and carries them out of the apparatus in which the washing step is performed. It is advantageous to recycle the aqueous washing medium after subjecting the suspension of deflocculated impurity particles which overflows at the top of the apparatus to gravitational or centrifugal sedimentation in order Xo separate impurity particles from the aqueous washing rredium. this latter operation can conveniently be performed in a scroll discharge centrifuge or in a hydrocyclone.

An apparatus suitable for use in carrying out step (e) is the subject of our copending British Patent Application No. to which reference should be made for details of the construction of the apparatus. Briefly, the apparatus comprises an upright vessl provided at its upper end with an upper inlet and an upper outlet, at its lower end with a lower outlet, and at a location intermediate its upper and lower ends with an intermediate inlet, the vessel being further provided with a tray, or with a plurality of trays arranged above one another, the or the lowermost of said plurality of trays being disposed above the intermediate inlet and the or each of the trays extending across the vessel and being provided with a perforated, downwardlyinclined surface which at its lower extent communicates with a downpipe. in use, the upper inlet of the apparatus is supplied with a suspension containing a mixture of flocculated mineral and deflocculated impurities. The lower outlet is used for the discharge of the flocculated mineral and the upper outlet is used for the discharge of the deflocculated impurities. The intermediate inlet is supplied with the aqueous washing medium which flows upwardly through the tray or trays and entrains therein the deflocculated impurities. The lower end of the apparatus is constructed in a manner promoting sedimentation and thickening of the flocculated mineral which travels downwards through the tray or trays, countercurrent to the aqueous washing medium.

For a better understanding of the invention, and to show more clearly how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawing which shows a flow diagram of a plant for carrying out the process of the invention.

Referring to the drawing, an aqueous suspension of crude kaolin from a previous processing plant containing not more than 0.1 % by weight, based on the weight of dry crude kaolin, of a sodium polyacrylate dispersing agent is supplied through a conduit 1 to a mixing tank 2. There is also supplied to a mixing tank 2, through a conduit 3, a solution of a sodium silicate dispersing agent.A stirrer 4 is provided in tank 2 to enable there to be dissipated in the suspension from 5 to 50 horsepower hours of energy per ton of dry crude kaolin (13 to 1 30 kJ kg-'). The resulting deflocculated suspension is drawn through a conduit 5 by a variable-speed centrifugal pump 6 to the inlet port of which there is also supplied, through a conduit 7, by means of a peristaltic dosing pump 8, a solution of an anionic polyelectrolyte flocculant which is prepared in a stirred tank 9.The mixture of flocculated kaolin and deflocculated impurities which is thereby formed is passed by the pump 6 through a conduit 10 to an inlet 11 at the top of an upright separating vessel 1 2 which is constructed in the form of a tower of square crosssection and internal dimensions 520 mm X 620 mnn, having a bottom portion 1 3 in the shape of an inverted pyramid in order to promote sedimentation and thickening of the flocculated material.Washing water is introduced into the separating vessel 1 2 through an intermediate inlet 14 which is positioned intermediate the upper and lower ends of the separating vessel 1 2 just above the top of the bottom portion 1 3. The washing water is supplied via a conduit 1 5 and its flow rate is controlled by a valve 1 6 and monitored by a rotameter-type flowmeter 1 7.

Four trays 1 8 are mounted in the separating vessel 1 2 above the washing water inlet 1 4. Each tray comprises a square of marine plywood in which are formed nine circular holes each of which accommodates a polyethylene funnel 21. Each funnel 21 has a diameter at its mouth of 1 65 mm and a straight-sided downpipe 22 of internal diameter 22mm and length 1 25 mm.

The frustoconical portion of the funnel has sides inclined at an angle of 60 to the horizontal and is provided with eighty four evenly spaced holes of diameter 3.5 mm through which the upcurrent of washing water passes. The washing water with entrained deflocculated impurity particles rises to the top of the separating vessel and overflows an upper outlet, in the form of a weir 25, into a collecting trough and then through a conduit 26 to a tailings dam or treatment plant yielding water for recirculation. The flocculant product, consisting of substantially pure kaolin, is discharged through a lower outlet 27 and a conduit 28 at the bottom of the separating vessel 12, the flow being regulated by a valve 29.

The invention is further illustrated by the following Examples: EXAMPLE 1 A suspension of a crude kaolin from South Devon which had been partially refined to give a product having a particle size distribution such that 56% by weight consisted of particles having an equivalent spherical diameter smaller than 2ym, and 7% by weight consisted of particles having an equivalent spherical diameter larger than 1 Osslm) contained 16.6% by weight of solids and not more than 0. 1% by weight, based on the weight of crude kaolin, of a sodium polyacrylate dispersing agent having a number average molecular weight of 1 650. The suspension was divided into four portions which were treated in the manner described below using the plant described above with reference to the accompanying drawing.

Each of the four portions was treated with a different amount of a solution of a sodium silicate dispersing agent. Each portion was mixed in the mixing tank 2 with the sodium silicate solution for ten minutes so that five horsepower hours of energy per ton of dry crude kaolin (13 kJ.kg - t) were disspated in the portion of suspension. In each case the pH of the suspension was adjusted to 9.5 with sodium hydroxide. Each portion was then dosed by means of the peristaltic dosing pump 8 with 100 parts by weight of an anionic polyelectrolyte flocculant per million parts by weight of dry crude kaolin. The polyelectrolyte had a number average molecular weight of 3 x 1 06 and consisted of polyacrylamide which was 40% hydrolysed to polyacrylic acid. The polyelectrolyte flocculant was mixed with the deflocculated suspension in the variable speed centrifugal pump 6 rotating at 630 rpm., the pump also feeding the mixture of flocculated kaolin and deflocculated impurities to the upper inlet of the separating vessel 1 2 at the rate of 10 litres per minute. The flocculated kaolin clay was discharged through lower outlet 27, and the impurities were entrained in the washing water and discharged through upper outlet 25. In each case the recovery or percentage by weight of solid material in the feed reporting to the flocculated fraction was measured and the flocculated material was analysed by X-ray diffraction for kaolinite, mica and feldspar.The results obtained are set forth in Table 1 below: TABLE 1 % by wt. of sodium silicate based on % % by weight of Portion wt. of crude kaoin recovery kaolinite mica feldspar Feed - - 78 14 7 1A 0.07 87.1 89 9 2 1B 0.13 86.1 86 11 3 1C 0.27 84.0 88 10 2 iD 0.40 78.6 90 8 2 EXAMPLE 2 Four further portions of the same suspension of crude kaolin as was used in Example 1 were each deflocculated with 0.27% by weight, based on the weight of dry crude kaolin, of sodium silicate but different amounts of sodium hydroxide were added to each portion to give a different pH in each case. Each portion was mixed with the sodium silicate solution for 10 minutes so that 5 horsepower hours of energy per ton of dry crude kaolin (13 kJ.kg-1) were dissipated in the suspension.Each portion was then dosed with 100 parts by weight of the same anionic polyelectrolyte as was used in Example 1 per million parts by weight of dry impure kaolin. The mixture of flocculated kaolin and deflocculated impurities was in each case fed to the separating vessel by a variable speed centrifugal pump rotating at 630 rpm. The feed flow rate and solids content and the washing water flow rate were the same as in Example 1. The washing water had a pH of 7.0. In each case the recovery of flocculated material and the percentage by weight of keolinite, mica and feldspar in the flocculated material were measured as in Example 1. The results obtained are set forth in the Table II below.

TABLE II % % by weight of Portion pH recovery Kaolinite Mica Feldspar 2A 7.25 83.0 88 9 3 2B 8.5 81.9 90 8 2 2C 9.5 84.0 88 10 2 2D 10.5 83.7 85 12 3 EXAMPLE 3 Four further portions of the same suspension of crude kaolin as was used in Example ' were each deflocculated with 0.27% by weight, based on the weight of dry crude kaolin, of sodium silicate and sufficient sodium hydroxide was added to adjust the pH to 9.5 in each case. Each portion was mixed with the sodium silicate solution for 10 minutes so that 5 horsepower hours of energy per ton of dry crude kaolin (13 kJ.kg-') were dissipated in the suspension. Each portion was then dosed with a different amount of the same anionic polyelectrolyte flocculant as was used in Example 1.The mixture of flocculated kaolin and deflocculated impurities was in each case fed to the separating vessel by a variable speed centrifugal pump rotating at 630 rpm. The feed flow rate and solids content and the washing water flow rate and solids content and the washing water flow rate was the same as in Example 1. The washing water had a pH of 7.0. In each case the recovery of flocculated material and the percentage by weight of aolinite, mica and feldspar in the flocculated material were measured as in Example 1. The results obtained are set forth in Table III below.

TABLE 111 Flocculant dose (parts by wt per million parts by wt of dry crude % Portion kaolin recovery kaolinite mica feldspar 3A 50 70.6 92 7 1 3B 100 84.0 88 10 2 3C 150 84.5 89 8 3 3D 200 89.4 88 9 3 EXAMPLE 4 Four further portions of the same suspension of crude kaolin as was used in Example 1 were each deflocculated with 0.27% by weight, based on the weight of dry crude kaolin, of sodium silicate and sufficient sodium hydroxide was added to adjust the pH to 9.7 in each case. Each portion was mixed with the sodium silicate solution for a different length of time so that different amounts of energy were dissipated in the suspensions.Each portion was then dosed with 1 50 parts by weight of the same anionic polyelectrolyte flocculant as was used in Example 1 per million parts by weight of dry crude kaolin. in each case the mixture of flocculated kaolin and deflocculated impurities was fed to the separating vessel by Ithe variable speed centrifugal pump rotating at 630 rpm. The feed flow rate and solids content and the washing water flow rate were the same as in Example 1. The washing water had a pH of 7.0. In each case the recovery of flocculated material and the percentage by weight of kaolinite, mica and feldspar in the flocculated material were measured as in Example 1.The results obtained are set forth in Table IV below: .ABLE IV Energy dissipated in the suspension % % by weight of Portion (kJ.Kg-1) recovery kaolinite mica feldspar 4A 13 84.0 88 10 2 4B 26 85.7 89 8 3 4C 40 86.0 89 8 3 4D 79 85.2 89 9 2 The amount of energy dissipated in the suspension is not critical provided that it is not substantially less than about 13 kJ.kg-1. If the amount of energy dissipated is substantially less than this figure the deflocculation and dispersion of the feed is incomplete and very little separation of the feed mixture into its mineral components will be achieved.

EXAMPLE 5 Five further portions of the same suspension of crude kaolin as was used in Example 1 were each deflocculated with 0.27% by weight, based on the weight of dry crude kaolin, of sodium silicate and sufficient sodium hydroxide was added to adjust the pH to 9.5 in each case. Each portion was mixed with the sodium silicate solution for 10 minutes so that 5 horsepower hours of energy per ton of dry impure kaolin (1 3kJ.kg - l) were dissipated in the suspension. Each portion was then dosed with 1 50 parts by weight of the same anionic polyelectrolyte flocculant as was used in Example 1 per million parts by weight of dry crude kaolin. The mixture of flocculated kaolin and deflocculated impurities was in each case passed through the variable speed centrifugal pump, the speed of rotation of the pump being different in each case, and samples of each portion were tested for sedimentation by determining the distance fallen by the interface between flocs and supernatant suspension in 1 minute and the specific gravity of the supernatant suspension in a 2 litre glass cylinder. The results are set forth in Table V below.

TABLE V Specific Speed of Depth of interface gravity of centrifugal below surface after supernatant Portion pump (rpm) 1 minute (cm) suspension 5A 1500 No flocs 1.000 5B 1230 0 1.025 5C 930 10.0 1.019 5D 630 18.4 1.015 5C 320 6.4 1.033 The speed of 630 rpm for the centrifugal pump gives the best combination of fast sedimentation rate and low specific gravity for the supernatant suspension, indicating that the supernatant suspension contains substantially only deflocculated impurities EXAMPLE 6 A suspension of a crude kaolin from South Devon (which had been partially refined to give a product having a particle size distribution such that 49% by weight consisted of particles having an equivalent spherical diameter smaller than 2 ym and 13% by weight consisted of particles having an equivalent spherical diameter larger than 101lem) contained 11.4% by weight of solids and not more than 0.1% by weight, based on the weight of crude kaolin, of the sodium polyacrylate dispersing agent described in Example 1.

Six portions of the suspension were passed continuously through the plant described above with reference to the accompanying drawing. Each portion was deflocculated with 0.27% by weight, based on the weight of dry crude kaolin, of sodium silicate, and the pH was adjusted to 9.6 with sodium hydroxide. In each case the suspension was passed through the mixing tank 2 at a rate such that the average residence time in the tank was 50 minutes so that 25 horsepower hours of energy per ton of dry crude kaolin (66kJ,kg-1) were dissipated in the suspension.Each portion was then dosed with 1 75 parts by weight, per million parts by weight of dry crude kaolin, of a partially hydrolysed polyacrylamide flocculant having a number average molecular weight of 6 X 1 O6 but in each case a different speed of rotation was selected for the variable speed centrifugal pump 6.

Each portion of feed mixture was supplied to the upper inlet 11 of the separating vessel 1 2 at the rate of 67kg of dry crude kaolin per hour (approximately 9.1 litres of feed suspension per minute) and the washing water was supplied to the intermediate inlet 14 of the separating vessel at the rate of 45.5 litres per minute. In each case the recovery of the flocculated material and the percentage by weight of kaolinite, mica and feldspar in the flocculated material were measured as in Example 1. The Valley Abrasion value of the flocculated material was also determined in each case by the method described in British Patent Application No. 7977/77.

The results obtained are set forth in the Table VI below: TABLE VI Speed of Valley centrifugal % % by weight of Abrasion Portion pump (rpm) recovery kaolinite mica feldspar (mg) 6A 320 86 85 7 7 268 6B 480 82 83 8 8 221 6C 630 81 87 8 5 202 6D 860 80 88 8 3 181 6E 1080 74 90 8 2 128 6F 1380 74 88 9 3 126 These results show that, under the conditions of this experiment, increasing the speed of the mixing pump brings about a reduction in the feldspar content and the abrasiveness of the flocculated material, although at the expense of a reduction in recovery.

It will be observed that good flocculation is obtained even at a mixing pump speed of 1 380 rpm and this is believed to be the result of using a polyelectrolyte flocculant of higher molecular weight than was used in Examples 1-5, and thereby producing stronger flocs which are more resistant to high shear rates.

EXAMPLE 7 Two further portions of the same suspension of crude kaolin as was used in Example 6 were each deflocculated with 0.27% by weight, based on the weight of dry crude kaolin of sodium silicate and sufficient sodium hydroxide was added to adjust the pH to 9.6 in each case. Each portion was mixed with the sodium silicate solution under the same conditions as were described in Example 6. Each portion was then dosed with 1 75 parts by weight of the same anionic poly lectrolyte flocculent as was used in Example 6 per million parts by weight of dry crude kaolin. In each case the mixture of flocculated kaolin and deflocculated impurities was fed to the separating vessel by the variable speed centrifugal pump rotating at 630 rpm. The washing water used in this case was recycled works water which contained a significant amount of acid and had a pH of 3.5.In the case of Portion 7A the water was used without any treatment but in the case of Portion 7B the pH of the washing water was adjusted to 9.1 with sodium hydroxide. In each case the flow rate of the washing water was 45.5 litres per minute.

The feed flow rate in each case was adjusted until the level of the layer of flocs on the top tray 1 8 remained contant. In each case the recovery of the flocculated material, the percentage by weight of kaolinite, mica and feldspar and the Valley Abrasion of the flocculated material were measured, and the results obtained are set forth in Table VII below: : TABLE Vll pH of Feed flow Valley washing rate % Abrasion Portion water (kg/hour) recovery kaolinite mica feldspar (mg) 7A 3.5 58 70 84 9 5 197 7B 9.1 75 78 86 8 5 197 It will be observed that the flocculated products in the two cases have similar properties, but when the pH of the washing water is raised by adding sodium hydroxide the flow rate of feed material through the apparatus may be increased by 29% and the recovery of the flocculated material is improved. Also the flocs which were formed in Portion 78 were seen to be larger than those which were formed in Portion 7A.

Claims (9)

1. A process for separating from a crude mineral a substantial proportion of the impurities associated therewith which process comprises forming a deflocculated aqueous suspension of the crude mineral, selectively flocculating either the desired mineral particles or the impurity particles in the deflocculated aqueous suspension of the crude mineral, and then causing the flocs of desired mineral particles or impurity particles to flow countercurrent to a stream of an aqueous washing medium in order to wash the flocs and separate therefrom entrapped deflocculated particles.

2. A process according to claim 1, wherein said crude mineral is a crude clay mineral, and wherein the clay particles in the deflocculated aqueous suspension of the crude clay mineral are selectively flocculated.

3. A process, according to claim 2, comprising the steps of: a) forming an aqueous suspension of the crude clay mineral; b) adding a dispersing agent to said aqueous suspension in order to deflocculate the clay mineral particles and any impurity particles associated therewith; c) adjusting the pH of the aqueous suspension to a value in the range of from 7 to 11, if it is not already at a value in said range; d) adding an anionic polyelectrolyte flocculant to the suspension to flocculate the clay mineral particles whilst leaving the impurity particles in a substantially deflocculated state; and e) passing the flocs of clay particles downwardly countercurrent to a rising stream of an aqueous washing medium having a velocity sufficient to convey substantially alll the deflocculated impurity particles away from the flocs of clay particles.

4. A process according to claim 3, wherein the solids content of the aqueous suspension of the crude clay mineral is in the range of from 10% to 40% by weight.

5. A process according to claim 2, 3 or 4, wherein the clay particles in the deflocculated aqueous suspension are flocculated with an anionic polyelectrolyte which is a water-soluble salt of either a partially hydrolysed polyacrylamide or a copolymer of acrylamide and acrylic acid, having a number average molecular weight in excess of 1 06.

6. A process according to claim 5, wherein there is added to the suspension containing the deflocculated clay mineral particles and impurity particles from 50 to 450 parts by weight of polyelectrolyte per million parts by weight of dry crude clay mineral.

7. A process according to any one of claims 1 to 6 wherein the flocs are washed by suspending them in a continuous rising current of an aqueous washing medium having a velocity which is sufficient to convey all the deflocculated particles present, from the finest to the coarsest, upwards and away from the suspended flocs, but not so high that the smallest flocs are conveyed upwards.

8. A process according to any one of claims 1 to 7, wherein said aqueous washing medium has a pH of at least 6.5.

9. A process, according to claim 1, substantially as described in any one of the foregoing Examples.