This invention relates to synthetic clays.

The synthetic analogues of a number of clay minerals, for example the smectites, have been synthesised.

Smectite minerals may be defined as a group of minerals or phyllosilicates of the 2:1 layer type with the general formulae:

(Mm+ _(x+y) /- _n nH ₂ 0)(R ³⁺ 2-yR ²⁺ y) (Si ₄ -χAl _x )O ₁₀ (OH) ₂

(dioctahedral) (M ⁺ ( _X+ y)/mnH ₂ 0) (R2+3_. _y Li ⁺ _y ) (Si ₄ _ _x Al _x )O ₁₀ (OH) ₂

( rioctahedral) (M ^m+ ( _x -y)/mnH ₂ O)(R2+3_yR3+ _y)( si ₄ _ _χ Al _χ )O ₁₀ (OH) ₂

( rioctahedral)

where M ^m+ represents exchangeable cations, such as Ca++, Mg++, having a valency m, necessary to satisfy the negatively charged lattice, R2+ represents magnesium or iron and R3+ represents aluminium or iron. The smectite group of minerals includes the mineral sub-groups montmorillonite, beidellite, nontronite, saponite, hectorite and sauconite.

There exist a number of processes for producing synthetic smectite clays. British Patent Specifications Nos. 1054111 and 1213122 describe forming such synthetic clays by precipitation from an alkaline solution containing

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a magnesium salt, a sodium compound and a silicon compound and treating the precipitate under hydrothermal pressure conditions until the precipitated product has crystallised. British Patent Specification No. 1432770 describes forming synthetic smectite clays by forming an aqueous suspension of a water insoluble magnesium compound containing lithium and fluorine compounds, if desired, and forming in the suspension a precipitate of silica followed by treatment under hydrothermal pressure conditions to achieve crystallisation. In each process the crystalline product may be separated from the mother liquor, washed and dried. The disclosures of British Patent Specifications Nos. 1054111, 1213122 and 142770 in relation to methods for the production of synthetic smectite clays is incorporated herein by reference. Preferably the clay utilised in the practice of the invention described hereafter is a synthetic hectorite which may be produced according to the disclosures of any of the above identified patent specifications. A suitable synthetic hectorite is available from Laporte Industries Limited under the Trade Name Laponite. Any other synthetic clay, whether or not a smectite and whether or not manufactured according to the above identified disclosures may alternatively be utilised.

Synthetic clays are generally manufactured to an ultimate particle size of about 25 nanometers and supplied as a powdered product having a particle size (diameter) substantially wholly below 250 microns and generally at least 75% by weight below 75 microns in diameter. Synthetic smectite clays so manufactured can display rheological properties which are of a high order and can be relatively easy to disperse in aqueous media.

The present invention provides a method for processing synthetic clays which can lead to an improvement in certain properties. The invention also provides a form of synthetic clays which can possess said improved properties.

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The process according to the present invention comprises forming a compressed cake of particles of the synthetic clay, extruding the compressed cake through restricted diameter holes to form extrudates and drying the extrudates. The invention is preferably practised by dispersing the synthetic clay in water which may contain a proportion, preferably a minor proportion, of a polar organic solvent or other organic material, filtering the dispersion under a pressure of at least 4 bar to give a compressed filter cake having a solids content of at least 15% by weight and extruding the compressed filter cake material. The extrudates so formed may be dried and reduced preferably to a particle size of from 200 to 2000 microns to give a product dispersible in aqueous media. Alternatively, according to a further aspect of the invention, a mixture of the compressed filter cake material with particles of another material, for example of a zeolite having catalytic activity, may be extruded and the extrudates dried with or without particle size reduction and calcined to give aggregates of the other material for example catalytically active aggregates of zeolite.

The terms "synthetic smectite" or "synthetic clay" are used to include those clays which have been ion exchanged to a different cationic form, from that in which they were synthesised, for example to an alkali metal or alkaline earth metal form or to an organophilic form where the exchange cation may be an organic ammonium, phosphoniu or other onium cation.

Additives may be present in a synthetic clay product treated according to the invention. For example, a peptiser such as polyphosphate may be included to enable the synthetic clay to form a stable solution. Suitable polyphosphates are sold under the Trade Name "Tetron".

The filtration pressure is an important factor in the practice of the present invention and is preferably at least

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5 bar and particularly preferably at least 6 bar. An upper pressure limit of 10 bar or even 8 bar may suffice. The input to the filter press is preferably a dispersion of a washed crystalline synthetic clay product which may be washed but as yet undried synthesis product or which may be a redispersion of a previously washed and dried powdered product. The solids content achieved by filtration according to the invention is preferably at least 17% but maybe up to 21% or even 25% by weight or above consistant with the ability to extrude the filter cake material. The term 'filter' is used herein to encompass means of dewatering a slurry or aqueous dispersion by the application of pressure. The invention does not encompass the extrusion of a product made merely by mixing a dry synthetic clay with a quantity of water corrsponding to that in the compressed filter cake material.

The extrusion of the compressed filter cake material is preferably performed using hole diameters of from 2 to 10 mm preferably from 5 to 7 mm although a larger diameter product is required the upper limit of 10 mm may be exceeded. However, the pressure developed during extrusion is a material factor in the practice of the present invention and it is therefore preferred that the hole diameter be selected so as to cause the development of sufficient extrusion pressure to give the required product properties. In the practice of this invention a slit having a smaller dimension of 2 to 10 mm is regarded as being equivalent to a series of holes of the same diameter and the term "hole" is therefore not intended to be construed narrowly. The extrusion apparatus may include hydraulic rams as a means for developing a sufficient extrusion pressure and the extrudates may be received on a moving bed.

One effect of operating according to some or all of the indications given above is that the extrudates, on drying tend to disintegrate in such a manner that a product within the preferred size range may be achieved by using a

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relatively low energy size reduction means. Product in that size range can show improved dispersibility properties.

The extrudates may be dried in a tray drier, or other suitable drier, at a temperature preferably of from 80 to 150°C but particularly preferably of from 90 to 125°C. The dried extrudates may be reduced in size by means of a suitble mill such as a hammer mill for example a Raymond mill or a pin type mill for example an Attritor mill. The words Raymond and Attritor are Trade Names.

The reduced size extrudates are preferably in the size range of 200 to 2000 microns. Particularly preferably the extrudates are at least 212 microns for example, most suitably, at least 250 microns and particularly preferably not more than 1200 microns, for example very suitably not more than 1000 microns in size. The presence of a proportion of extrudates having sizes outside these ranges, for example 10% by weight or even up to 25% by weight or more while it may result in a reduced dispersion performance, does not depart from the spirit of this invention. The grinding time should preferably be kept to a minimum to achieve the desired particle size ranges to avoid undue fracture of the granules. Preferably a fraction of granules having the desired particle size range is removed on a sieve, and the oversize particles are recycled, on a continuous or discontinuous basis.

As foreshadowed above the synthetic clay, as extruded, may contain a proportion of a particulate active material such as a zeolite. Alternatively the particulate material may be any siliceous or metallosiliceous substance, for example a natural clay or ground rock, of which it may be desired to form aggregates.

The term "zeolite" is used broadly to include not only naturally occurring zeolites such as clinoptilolite but also both low silica and high silica families of synthetic

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zeolites such as, for example, zeolites A, X and Y, the ZSM series of zeolites and other families of zeolites competitive with, similar to or modifications of any of the above. The present invention is also particularly concerned with the zeolite mordenite which may be produced, for example, by the process described in United States Patent Specification No. 3436174 and other specifications of the Norton Company which zeolite is widely used in shaped form.

The proportion of synthetic clay in admixture with the zeolite is suitably from 5 to 30% but preferbly from 10% to 25% based on the dry weight of the clay and the zeolite. The mixture may be wetted using water or a mixture of water with a minor quantity of a polar organic solvent in a quantity to provide a weight loss on drying and calcination preferably of from 30 to 60% particularly preferably 40 to 50% by weight.

The shaping is preferably accomplished using an extruder, for example a Buss (Trade Name) extruder. Preferably only the co-kneader part of the Buss extruder is used.

The extrudates may be dried as above taught to remove free moisture at and are then preferably calcined. Synthetic clays may remain substantially stable to calcination temperatures up to about 700°C and this temperature is preferably not exceeded. Preferably the calcination is conducted at from 500 to 650°C for a time of from 30 minutes to 4 hours, particularly preferably of from 1 hour to 3 hours.

Certain features tend to augment the effect of the present invention and one or more of these are preferably used therein:

w'__τ _~; \Z _* - - - '—,- » --.- _-i - ._-._-- I

i) The synthetic clay contains fluorine included during synthesis or has been washed with a dilute alkali meal fluoride solution after synthesis and after washing free of salts formed during synthesis.

ii) The synthetic clay and/or any zeolite which is used in conjunction with it and particularly mordenite if the latter is used is in the H+ form which may be attained, for example, by treating with sulphuric or hydrochloric acid for example sulphuric acid having a concentration of 5 to 10% w/w, at an elevated temperature, for example from 25 to 50°C followed by washing to remove displaced sodium ion.

iii) The required water content of the mixture of synthetic clay and of zeolite particles to be extruded is achieved by adding an aqueous solution of a base for example of sodium hydroxide which solution may have a concentration of from 0.5 to 1.5 preferably 1.0 Molar.

Calcined aggregates of zeolites produced by including a zeolite in a synthetic clay granulation process according to this invention, for example granules containing hydrogen mordenite or sodium mordenite, show excellent physical integrity, a high pore volume comparable with that of the unpelleted zeolite and possess a high degree of purity with respect to possible contaminants such as iron which could interfere with catalytic applications. Such aggregates are preferably calcined at a temperature no greater than 600°C for a satisfactory combination of properties.

The invention will now be more specifically described by reference to the following Examples which are not intended to be limitative thereon.

A synthetic smectite clay product, available from Laporte Industries Limited under the Trade Name Laponite, in

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the form of a powder of which 100% was less than 212 microns wsa dispersed in water at a concentration of 10% wt and filtered in a Hoechst filter press using a pressure of 6 bar to give a filter cake having a solids content of 21% wt. This filter cake was extruded through a steel plate having a plurality of holes of 6 mm diameter and the extrudates so formed were dried at 105°C for 100 minutes. The dried extrudates were milled under varying conditions on a Raymond Mill to give a series of 4 samples having the size ranges <212 microns, 212 to <710 microns, 710 to <1200 microns and 1200 to <2000 microns. These samples were dispersed in demineralised water at a concentration of 1% wt and the optical density of the dispersions was measured by absorption spectrophotometry at various time intervals to determine the relative ease of dispersion of the samples.

The results of the optical density measurements are given in Table I below.

Table 1 Particle Size Distribution (microns)

212-710 μm 710-1200 μm 1200-2000 μm

In a similar manner the rate of dispersion of a powder of the same synthetic smectite having a particle size 100% below 212 microns which had not been processed according to this invention was compared with that of particles of the same synthetic smectite having a particle size in the range 250-1000 microns manufactured according to the invention, each at a concentration of 2% by weight in water. The variation of the optical density with time of each of these materials was measured and expressed in graphical form in Figure I attached hereto.

It is seen from Figure I that the dispersibility of the powder was the lower.

The rate of gel formation of samples of a synthetic smectite both processed according to this invention but having different particle size ranges was compared. The apparatus was a glass cylinder having a nominal capacity of 120 cπ.3 which was arranged for end over end rotation at a rate of 90 inversions/minute. The cylinder contained a glass sphere of 5 g mass and 15 mm diameter. 95 g of demineralised water containing 1.128 g of sodium sulphate/litre was introduced into the cylinder followed by 5 g of Laponite swelling synthetic smectite extrudates which had been processed according to this invention. The test was separately performed for product having a particle size of 100% below 250 microns with at least 75% below 150 microns and product having a particle size of 250 to 1000 microns and was repeated for 25 samples of each. The test was conducted by measuring the time taken for gel formation to become sufficiently marked to prevent the glass sphere striking the end of the cylinder as a result of inversion of the cylinder in the course of rotation. The times for the samples were averaged and the average for the smaller article size product normalised to 100. The larger particle size product (250-1000 microns) was found to achieve gel formation in 85% of the normalised time.

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