FORM 2 THE PATENTS ACT, 1970 (39 of 1970) COMPLETE SPECIFICATION (See Section 10, rule 13) TREATING KAOLINITIC CLAYS IMERYS MINERALS LIMITED of JOHN KEAY HOUSE, ST. AUSTELL, CORNWALL, PL25 4DJ, ENGLAND, BRITISH Company The following specification particularly describes the nature of the invention and the manner in which it is to be performed : - 1-1 FR0M:ECC IP DEPARTMENT 017S681B075 TO:0091E28380737 PfiGE:005.-039 WO 00/50359 PCT/GB00/00693 TREATING KAOLINITIC CLArS This invention is concerned with a method of treating kaolinitic clays, especially to improve the 5 properties of such clays for use as ingredients for ceramic forming compositions, especially compositions which are to be used for preparing ceramic articles, eg whiteware articles such as tableware and the like. Ceramic articles, eg tableware for use in the 10 home and in the catering industry, are generally . formed from a wet high solids composition which comprises a blend of various particulate ingredients which include kaolinitic clays, ie clays which contain the mineral kaolinite, such as kaolin or 15 china clays and/or ball clays. Usually fluxing materials such as china stone, feldspar or nepheline syenite, and at least one silica-containing material, such as quartz or flint are also included in such compositions. If it is desired to produce articles 20 of bone china, the composition will also contain a substantial proportion of ground, calcined animal bone, especially from cattle, or bone ash. The composition may also include minor proportions of other ingredients such as calcium carbonate, dolomite 25 and talc. The proportion's of the various ingredients used in the composition vary according to the properties required in the fired ceramic article. Many different types of ceramic tableware are • produced in various parts of the world, including, 30 fine earthenware, semi-vitreous china, semi-vitreous SUBSTITUTE SHEET (RULE 26) WO 00/50359 PCT/GBOO/00693 -2- porcelain, hotel china, household china, bone china, hard porcelain and stoneware. Ceramic tableware articles, are generally formed from the wet ceramic forming composition by a process 5 which i$ based on the ancient technique of hand throwing on a potter's wheel. The technique of free-hand throwing is still used for shaping individual act pieces, but when a large number of substantially identical articles are required to be formed, a 10 degree of automatic mechanisation is incorporated into the process. In this latter case, a mould of a suitable material, for example plaster or a synthetic resin, may be employed fixed to a wheel which is capable of high speed rotation in a horizontal plane. 15 A suitable amount of the ceramic composition is then introduced onto or into this mould. If the mould is substantially convex, and is used for shaping, for example, the inside of a plate or dish, the process is widely known as "jiggering". If, however, the 20 mould is concave, and is used for shaping the outside of a cup or jug, the term vjolleying" is often used. The second surface of the article, which is not in contact with the mould, is generally shaped by means of a profiling tool, most commonly of metal, which 25 is brought into contact with this surface, whilst the article being shaped is rotated on the wheel. The shaping process has recently been rendered faster and more efficient through the introduction of roller head machines. In these machines the profiling tool 30 is replaced by a heated rotating die, and both die SUBSTITUTE SHEET (RULE 26) WO 00/50359 3 and mould rotate continuously at appropriate speeds during the shaping of an article. In order to perform satisfactorily in a shaping process, eg of the type described above it is 5 necessary for the ceramic forming composition to have sufficient plasticity to enable it to flow and deform under the action of compressive, tensile and shear stresses. The shaped article must also possess sufficient strength in its unfired or "green" state, 10 to permit a certain amount of handling without loss of its integrity and shape. The green strength of a ceramic forming composition is generally determined by measuring the modulus of rupture (MOR) of dried extruded bars formed from the composition under 15 certain standard conditions described later. Some ceramic tableware is formed by a slip casting process. In this case the clays and other ingredients of the composition are mixed with a larger quantity of water, optionally with one or more 20 additives, eg one or more dispersing agents, to form a fluid suspension, slurry or "slip". The slip is poured into a porous mould where a shaped article is formed by a process which is similar to that by which a filter cake is formed in a filter press. Partial 25 dewatering of the shaped article occurs as water passes from the composition through the porous walls of the mould, until the article is sufficiently formed, in a dry and firm state, to be removed from the'mould. 30 A further shaping process used for forming articles of ceramic tableware is that of dust SUBSTITUTE SHEET (RULE 26) WO 00/50359 PCT/GBO0/0O693 -4- pressing. In this process a'ceramic composition in the form of an aqueous suspension containing a relatively high concentration of, solid material, together with one or more dispersing agents for the 5 solid material, is subjected to spray drying to form substantially dry hollow microspheres of diameter of the order of about 0.1mm. A charge containing an appropriate quantity of these microspheres is introduced into a suitable mould to which pressure is 10 applied to compact the charge to form the desired ceramic article. Again, when articles are formed by dust pressing, it is necessary for the ceramic composition to possess sufficient green strength to enable the shaped article to be handled without undue 15 risk of breakage. Subsequent to the shaping process, whatever shaping method is used, the shaped body produced in its green state is dried before firing one or more times to a suitable temperature in a kiln, to produce 20 a ceramic article of the type desired. Glazes and decoration may also be applied at this stage. An object of this invention is to improve the properties of kaolinitic clay components of ceramic forming compositions in order to increase the 25 strength of green shaped articles formed from the compositions. According to the present invention there is provided a method of treating a kaolinitic clay which is intended for use as an ingredient in a ceramic 30 forming composition which method comprises the steps of (a) mixing with the kaolinitic clay from 0.1% to SUBSTITUTE SHEET (RULE 26) WO 00/50359 PCT/GBOO/00693 -5- 15.0% by weight, based on the dry weight of the kaolinitic clay, of a smectite clay; and (b) subjecting a moist mass in a plastic state of the mixture formed in step (a) to mechanical working 5 under conditions such that there is dissipated in the moist plastic mass at least 5kJ of energy per kilogram of the clay mixture on a dry weight basis. The amount of energy dissipated in step (b) may be in the range of from 5kJ to 300kJ of energy per 10 kilogram of the clay mixture on a dry weight basis. The kaolinitic clay used in step (a) may already have been subjected to known preliminary processing or refining steps, eg steps selected from degritting, washing, magnetic separation of impurities and one or 15 more particle size separation steps. The moist plastic state mass treated by mechanical working in step (b) preferably contains between 20% and 30% by weight of water. The mixture of clays produced in step (a) may 20 have a water content which is suitable for use in step (b). Alternatively, the water content of the clay after production may be adjusted to provide a suitable moist mass in a plastic, workable state. The water content adjustment may be by addition of an 25 aqueous liquid or by concentration, depending on the water content of the mixture produced in step (a). Where the clay mixture produced in step (a) is in the form of a dry powder the required moisture content may be adjusted simply by addition of water 30 and mixing, SUBSTITUTE SHEET (RULE 26) WO 00/50359 PCT/GE00/00693 -6- Where the clay mixture produced in atop (a) lu in the farm of a dilute slurry or suspension the required moisture content may,be obtained by one or more known dewatering processes, eg filtering and/or 5 pressing and/or partial drying and/or adding already dried material', ie using a dry feedback or dry-return supply loop from a subsequent dryer output. The kaolinitic clay used in step (a) may comprise one or more kaolin clays of primary or 10 secondary origin. Kaolinitic clays were formed in geological times by the weathering of the feldspar component of granite. Primary kaolin clays are those which are found at the site at which they were formed, and are generally present in a matrix of 15 undercomposed granite which must be separated from the clay during the refining process for the clay. Secondary kaolin clays, which are alternatively known as sedimentary kaolin clays, are those which were flushed out in geological times from the granite 20 matrix in which they were formed, and were deposited in an area remote from their site of formation, generally in a basin formed in the surrounding strata. Kaolin clays are generally found in association with relatively small proportions of 25 impurities, such as mica, feldspar, quartz, titanium compounds and the like, and may also include a trace of smectite clays. The kaolinitic clay may alternatively comprise one or more ball clays, or a mixture of one or more ball clays with one or more 30 kaolin clays. Ball clays are sedimentary clays which are very finely divided, in that they have a particle WO 00/50359 -7- size distribution such that the particles predominantly have an equivalent spherical diameter smaller than 2unu However, ball clays tend to have a higher proportion of impurities than kaolin clays, 5 and to be" proportions of fins silica, together with minor amounts of compounds of iron and titanium and also organic matter such as lignite. 10 Smectite clays are formed predominantly of smectite mineral particles which are sheet silicates with a high cation exchange capacity arising from charge imbalance due to substitutions within the crystal lattice. This charge imbalance is 15 compensated by cations adsorbed from solution, known as exchangeable ions because they can easily be exchanged with ions of a different type. For most naturally occurring smectites, the exchangeable ion is a divalent cation, principally calcium, although a 20 few smectites are found with a monovalent ion, principally sodium, as the exchangeable cation, notably smectites from Wyoming, USA, In water, those smectites with divalent calcium cations disperse to a lesser degree than those with 25 monovalent cations. This is due to the greater effect of the divalent cation in compressing the so-called electrostatic double layer around the particles that causes them to repel each other, compared with the monovalent cation. 30 Monovalent ion exchanged smectites are relatively easily dispersed ih water to give SUBSTITUTE SHEET (RULE 26) WO 00/50359 PCT/GB00/O0693 -8- individual plates or crystallites, whereas the divalent ion exchanged smectites tend only to disperse to "packets" or three or four crystallites. Monovalent ion exchanged smectites, especially sodium 5 smectites, are generally more effective in user applications. It is a relatively simple matter to convert a calcium smectite to a sodium smectite, by adding a small amount of a sodium ion containing solution, eg 10 sodium carbonate, typically about 4% to 5% by weight. When dispersed in water, the exchangeable calcium ions are precipitated as calcium carbonate and the sodium ions become the exchangeable ions. The smectite is then said to be "sodium activated'". 15 However, the term "activated" should be used with caution, as smectites can also be "acid activated" for use in decolouring vegetable oils, which is an entirely different activation process. In the method of the present invention, the 20 smectite clay is preferably a montmorillonitic clay such as a bentonite, and preferably has a monovalent ion such as sodium as the predominant exchangeable cation. Such a clay can be prepared fox example by activating a calcium bentonite with sodium carbonate. 25 Other smectite clays such as hectorite, saponite and beidellite may be suitable for use in the method of the present invention. The amount of the smectite clay mixed with the kaolinitic clay in the method of the invention is 30 preferably in the range of from 0.5% to 7.0% by SUBSTITUTE SHEET (RUIE 26) WO 00/50359 -9- weight, based on the dry weight of the kaolinitic clay. The smectite clay when added to the kaolinitic clay may be in the form of a powder or a slurry, ie 5 an aqueous suspension, Likewise, the kaolinitic clay may be in powder or slurry form. The clays after being added together are preferably mixed thoroughly together for a period of time, eg at least 1 minute, preferably at least 2 minutes. Desirably, the 10 mixture of the two clays is moist, eg contains at least 10% by weight water, in some cases from 10% to 90% by weight water, when the clays are being mixed together. The clays may be mixed together in moist form in 15 a mixing or compounding device. The individual clays may be added together on an inlet conveyer to such a device or conveyed separately for addition and mixing in the device. In step (b) of the method of the invention, the 20 water content of the treated moist plastic state mass is preferably from 23% to 28% by weight based on the dry weight of the clay mixture. The mixture of clays may be treated by one or more additional procedures, in addition to any water 25 content adjustment required, prior to step (b) . For example, so called "tramp iron" or large pieces of iron may be removed prior to step (b) by a permanent magnet. ' In step (b), of the method of the invention, the 30 mechanical working may be exerted upon the plastic state mass by means of an extrusion device, such as SUBSTITUTE SHEET (RULE 26) WO 00/50359 lo an auger-type pug mill, a Z-blade mixer, an edge runner mill or a similar device known for working masses of particulate material in a moist plastic state. The device is preferably an auger-type pug 5 mill, which is a known device, eg as described in GB1,194,866 the contents of which are incorporated by reference and is conveniently provided with known means for adjusting the size of the outlet nozzle(s) in order to control the throughput rate of material 10 passing through the device, and the pressure built up inside the device, and thus the amount of energy dissipated in the plastic state clay mixture. The amount of energy dissipated in the plastic state mass is preferably in the range of from l0kJ to 250kJ, and 15 most preferably from 20kJ to 175kJ, per kilogram of the clay mixture treated on a dry weight basis. After treatment by steps (a) and (b) of the method of the invention, the resulting clay mixture product may optionally be further processed by one or 20 more known refining processes. The resulting product (with or without further processing) may be delivered to a user in wet slurry form or in dry powder form, eg by thermally drying the resulting product prior to delivery. The 25 resulting product may be'employed in the production of compositions to make ceramic articles in a known way, eg using one of the prior art methods described earlier. 'The resulting product of the method according to 30 the invention can, when extruded and dried in a standard manner, show an improved modulus of rupture SUBSTITUTE SHEET (RULE 26) WO 00/50359 11 and can also provide improved plasticity and higher solids castable compositions all of which are beneficial properties when the product is used in compositions for shaping to form ceramic articles. 5 These benefits are demonstrated in the following Examples. Embodiments of the present invention will now be described by way of example in the following Examples. 10 EXAMPLE 1 An English kaolin clay intended for use in the ceramic tableware industry was obtained having a particle size'distribution such that 60% by weight 15 consisted of particles having an equivaleni: spherical diameter smaller than 5pm and 38% by weight consisted of particles having an equivalent spherical diameter smaller than 2um. samples of the kaolin clay were prepared for treatment in a pug mill by being mixed 20 with sufficient water to form a paste containing 253 by weight of water using a high speed mixer. The samples were divided into two batches "A" and "B". To the samples in batch A in moist form there was added before treatment in the pug mill 1,5% 25 by weight, based on the weight of dry kaolin clay, of a Wyoming sodium bentonite, while no bentonite was added to the samples in batch B. Samples of batches A and B were then subjected to mechanical working in a pug mill of the kind 30 described in GB 1,194,866 equipped with a pressure sensitive transducer and integrator to enable the SUBSTITUTE SHEET (RULE 26) WO 00/50359 -12- energy dissipated in the clay to be calculated. The amount of energy dissipated could be varied either by selecting a larger or smaller outlet nozzle for the pug mill, or by passing the clay several times 5 through the pug mill. Samples of each of batch A and batch B were retained for testing without being subjected to mechanical working in the pug mill. The samples of batch A and batch B passed 10 through the pug mill experienced a range of different amounts of energy dissipated therein. Each sample was then tested for modulus of rupture at 80% relative humidity by the method described as follows. 15 In order to measure the modulus of rupture, or green strength, of the kaolinitic clay, a sample of each batch of clay in the plastic state was extruded by means of a piston type extruder through a circular aperture to form a cylindrical rod of diameter 6mm. 20 The extruded rods were then cut into bars of length 150mm. The bars were allowed to dry in the air, and were then dried at 60°C in an oven, overnight. The bars were then cut in half and placed in a controlled atmosphere of 80% relative humidity for several 25 hours, before being broken on a three-point flexure jig with a 50mm span mounted on a universal testing machine or similar equipment. The diameters of the bars were measured at the point of fracture, and the modulus of rupture calculated from the diameter and 30 the force required to break the bar,. At least ten SUBSTITUTE SHEET (RULE 26) WO 00/50359 PCT/GBQO/00693 13 bars were tested in this way for each batch of clay, and the average modulus of rupture was calculated. The results are set forth in Table 1 as follows in which individual samples are designated Al. , ., 5 Bl..., etc according to the batches A or B from which they were selected. TABLE 1 Sample % by weight of added bentonite Energy dissipated (kJ.kg-1) Modulus of rupture (MPa) Al 1.5 0 0.882 A2 1.5 63 1.226 A3 1.5 114 1.344 A4 1.5 161 1.569 A5 1.5 230 1.500 Bl 0 0 0.490 B2 0 53 0.922 B3 0 98 0.951 B4 0 151 1.069 B5 0 206 1.059 10 The results in Table 1 show that the mechanical working of both batches A and B of kaolinitic clay caused the modulus of rupture to undergo modest increases. However, where 1.5% by weight of sodium bentonite was added to the kaolinitic clay for 15 samples in batch A the modulus of rupture, unexpectedly and beneficially, was significantly increased. Table 1 shows that increase continued as SUBSTITUTE SHEET (RULE 26) WO00/503S9 14 the amount of energy dissipated in the mechanical working is increased until the amount of energy reached about 175kJ.k1. There was seen to be little benefit in increasing the amount of energy dissipated 5 in the kaolinitic clay during the mechanical working step above about 175kJ.kg_:1 EXAMPLE 2 A second English kaolin clay intended for use in 10 the ceramic tableware industry had a particle size distribution such that 52% by weight consisted of particles having an equivalent spherical'diameter smaller than 5um and 32% by weight consisted of particles having an equivalent spherical diameter 15 smaller than 2um. Samples of this kaolin clay were subjected to working treatment in the same pug mill as was used in Example 1, Each sample of the kaolin clay was prepared for treatment in the pug mill by mixture with sufficient water to form a paste 20 containing 25% by weight of water in a high speed mixer. The samples were divided into two batches "C" and "D". To the samples in batch C there was added before treatment in the pug mill 0.9% by weight, 25 based on the dry weight of the kaolin clay, of a Wyoming sodium bentonite, while no bentonite was added to the samples in batch D. Samples of each of batch C and batch D were retained for testing without being subjected to mechanical working in the pug 30 mill. The remaining samples of batch C and batch D were passed through the pug mill under varying SUBSTITUTE SHEET (RULE 26) WO00/503S9 PCT/GBOO/00693 -15- conditions, so that samples from each batch had a range of different amounts of energy dissipated therein. Each sample was then tested for modulus of 5 rupture as described in Example 1 above, and the results obtained are set forth in Table 2 as follows wherein the individual samples are designated CI..,, Dl,,., etc according to the batch C or D from which they were obtained. 10 SUBSTITUTE SHEET (RULE 26) WO 00/50359 PCT/GB0O/0O693 -16- TABLE 2 Sample % by weight of added bentonite Energy dissipated (kJ.kg-1) Modulus of rupture (MPa) CI 0.9 0 0.892 C2 0-9 79 1.275 C3 0.9 95 1.344 C4 0.9 190 1.589 Dl 0 •0 0.706 D2 0 37 0,892 D3 0 77 0.991 D4 0 135 1.069 D5 0 278 1.177 The results in Table 2 show the same trends as observed in Example 1 (Table 1). 5 EXAMPLE 3 A blend of kaolinitic clays (batch E) was obtained which consisted of a mixture of two intermediate products from kaolin refining plants and 10 the coarse fraction from a particle size separation process which produced coarse and fine fractions from an intermediate kaolin product. The blend had a particle size distribution such that 23% by weight consisted of particles having an equivalent spherical 15 diameter larger than lOum and 25% by weight consisted of particles having an equivalent spherical diameter smaller than 2pm. The blend was prepared by suspending each component clay in sufficient water to SUBSTITUTE SHEET (RULE 26) WO 00/50359 PCT/GBOO/00693 17 form a suspension containing 10% by weight of clay on a dry weight basis, and mixing the suspensions in the proportions required to provide the required blend. The blended suspension thus formed was divided into 5 six samples. To two of the six samples there was added 1% by weight of a montmotrillonite derived from the kaolin producing area of Cornwall, England, To another two of the samples 2% by weight of the same montmorillonite was added. No montmorillonite was 10 added to the remaining two samples of the suspension. Each of the samples of the suspension was then dewatered by filtration to form a cake containing about 25% by weight of water. One of each pair of samples (Samples El, E2 and E3) was reserved for 15 testing for modulus of rupture as described in Example 1 earlier. The other Sample of each pair (Samples E4, E5 and E6) was subjected to mechanical working in a pug mill as described in Example 1, the conditions being such that the amount of energy 20 dissipated in the clay mixture was in the region of about 165kJ per kilogram of clay (on a dry weight basis). The samples which had been subjected to mechanical working were also tested for modulus of rupture. 25 The results are set forth in Table 3 below. TABLE 3 Sample % by wt of Energy Modulus of r added dissipated xupture (MPa) moatmorillonito (fcj.kg-1) SUBSTITUTE SHEET (BtfLE 26) WO 00/50359 PCT/GBOO/00693 18 El 0 0 0.374 E2 i 0 0.452 E3 . 2 0 0.451 E4 0 159 0.735 E5 1 143 0.814 E6 2 164 1.138 These results show that the addition of a small amount of montmorillonite to the kaolinitic clay causes an increase in the green strength of the clay. 5 When the samples of kaolinitic clay are subjected to mechanical working, the green strength is further substantially increased. EXAMPLE 4 10 A kaolinitic clay (Sample F) which was an intermediate product from a kaolin refining plant had a particle size distribution such that 9% by weight consisted of particles having an equivalent spherical diameter larger than 10pm and 57% by weight consisted 15 of particles having an equivalent spherical diameter smaller than 2um. The clay was made available in the form of a relatively dilute aqueous suspension. The suspension of clay was divided into four portions. To three of the four portions there were added 1% by 20 weight, 2% by weight and 3% by weight, respectively, of a bentonite derived from Texas. No bentonite was added to the other portion of clay. Each portion of suspension was then dewatered by filtration to form a cake containing about 30% by 25 weight of water, A fraction of this moist cake was SUBSTITUTE SHEET (BULK 26) WO00/50359 PCT/GB00/00693 -19- removed and thermally dried and then mixed back into the remainder of the moist cake to decrease the water content to 22% by weight. Each portion of cake was then divided into two halves. One half of each 5 portion (Samples Fl - EM) was reserved for testing for modulus of rupture as described in Example 1 above. The other half (Sample F5 - P8) was subjected to mechanical working in a pug mill as described in Example 1. The working conditions in the pug mill 10 were such that the amount of energy dissipated in the kaolinitic clay mixture was about 40kJ per kilogram of clay (on a dry weight basis) in the case of Samples F6 to F8f to which bentonite had been added, and about lOOkJ per kilogram (on a dry weight basi3) 15 in the case of Sample F5, which was free of bentonite. A further sample of the cake formed from the portion of the suspension which had not been treated with bentonite (Sample F9) was subjected to mechanical working in the pug'mill under conditions 20 such that about 230KJ per kilogram of energy was dissipated therein. The portions of clay which had been subjected to mechanical working were also tested for modulus of rupture. The results are set forth in Table 4 as follows. 25 TABLE 4 Sample i. % by wt of added bentonite Energy dissipated (kj.kg1) Modulus of rupture