EP1387881A1

EP1387881A1 - Granular carbonate and silicate-containing compositions

Info

Publication number: EP1387881A1
Application number: EP02716953A
Authority: EP
Inventors: Maurice Newton; Remo Kanders; Pierre Theunissen; Alan Minihan
Original assignee: Ineos Silicas Ltd
Current assignee: Ineos Silicas Ltd
Priority date: 2001-05-08
Filing date: 2002-04-23
Publication date: 2004-02-11
Also published as: BR0209504A; WO2002090487A1; GB0111043D0

Abstract

A composition for use as a builder in detergent compositions comprises a granular material containing an alkali metal silicate and an alkali metal carbonate characterised in that the alkali metal silicate has a molar ratio, SiO2 to M2O, where M is an alkali metal, in the range 2.4 : 1 to 3.0 : 1, the amount of silicate in the granules, calculated as SiO2, is at least 30 per cent by weight, the amount of alkali metal carbonate in the granules, expressed as anhydrous material, is less than 35 per cent by weight and the amount of water in the granules is less than 25 per cent by weight. A preferred process for manufacture of such compositions comprises the steps of (a) preparing an aqueous mixture of an alkali metal silicate and an alkali metal carbonate, (b) spray-drying said aqueous mixture and (c) compacting the spray-dried powder thereby produced to form granules of said composition. In an alternative process the aqueous mixture of silicate and carbonate is dried and optionally granulated in a drum containing a multiplicy of rotating arms and into which a heated gas is introduced.

Description

GRANULAR CARBONATE AND SILICATE-CONTAINING COMPOSITIONS

This invention relates to granular compositions suitable for use as builders in detergent compositions and in particular to compositions containing alkali metal silicates and alkali metal carbonates.

Alkali metal silicates are included in wash formulations for both dish washing and fabric washing because of their properties, which include, for example, anti-corrosion, building, soil suspension and bleach stabilising. Generally, a high dose of Si0₂ is preferred in order to optimise these properties. Alkali metal silicates in which the molar ratio Si0₂ : Na₂0 is low, such as metasilicates are classified as corrosive due to their high alkalinity. Silicates with a higher molar ratio (e.g. above 1.5) are considered irritant and, therefore, silicates with a higher molar ratio are now preferred. However, silicates with a higher ratio generally have a lower solubility and frequently they are used in combination with a soluble salt such as an alkali metal carbonate. The lower solubility of high ratio silicates is generally observed as both a slow rate of dissolution and as a higher proportion of insoluble material. In addition, a low rate of dissolution of the silicate adversely affects the bleach stability of a detergent composition. Some silicates have a tendency to cake on storage and/or cause caking in a detergent containing them. It is therefore desirable that useful silicates have a low tendency to cause caking either alone or when incorporated into a detergent composition.

It has now surprisingly been found that by closely controlling the composition of silicate/carbonate co-granules, a product having a high silica-equivalent content, good dissolution properties and a low tendency to cake can be formed.

According to the invention, a composition for use as a builder in detergent compositions comprises a granular material containing an alkali metal silicate and an alkali metal carbonate characterised in that the alkali metal silicate has a molar ratio, Si0₂ to M₂0, where M is an alkali metal, in the range 2.4 : 1 to 3.0 : 1 , the amount of silicate in the granules, calculated as Si0₂, is at least 30 per cent by weight, the amount of alkali metal carbonate in the granules, expressed as anhydrous material, is less than 35 per cent by weight and the amount of water in the granules is less than 25 per cent by weight.

The alkali metal silicate of the composition can be any alkali metal silicate but potassium or sodium silicate is generally preferred for economic reasons with sodium silicate being the most preferred. Usually, the alkali metal silicate will be an amorphous silicate, since such silicates are generally more readily soluble than crystalline silicates. Preferably, the molar ratio, Si0₂ to M₂0, where M is an alkali metal, of the alkali metal silicate is in the range 2.6 : 1 to 3.0 : 1 and more preferably in the range 2.6 : 1 to 2.9 : 1.

Generally, it is desirable to have the silicate content of the granules as high as possible, because such a product allows the user of the granules more flexibility in formulating the final detergent compositions. Consequently, preferred compositions of the invention contain an amount of silicate, calculated as Si0₂, greater than 35 per cent by weight and more preferred compositions contain more than 40 per cent Si0₂ by weight. Usually, compositions according to the invention contain an amount of silicate, calculated as Si0₂, which is less than 50 per cent by weight, since higher amounts of silicate tend to increase the amount of caking observed both in the granules and in detergent compositions in which the granules have been incorporated.

In general, the alkali metal carbonate in the composition of the invention can be any alkali metal carbonate but sodium and potassium carbonates are preferred, with sodium carbonate being more preferable. The amount present is expressed with respect to anhydrous material, although hydrated carbonates can be used to prepare the compositions of the invention, depending on suitability in view of the production method used and the availability of materials. The amount of alkali metal carbonate present is preferably less than 30 per cent by weight, expressed as anhydrous material. Normally, at least 10 percent by weight of alkali metal carbonate, as anhydrous material, is present and, preferably, at least 15 per cent by weight is present.

The granular material contains less than 25 per cent water by weight. Preferably, there is less than 20 per cent water by weight present. Usually the amount of water present will be at least 10 per cent by weight and preferably is at least 15 per cent by weight.

A particularly useful composition of the invention consists essentially of alkali metal silicate, alkali metal carbonate and water but other useful compositions further comprise other materials which are conventionally used in preparing detergent compositions. Such materials include polymers, fluorescers and enzymes, which are typically incorporated in an amount up to 10 per cent by weight of the granules.

The composition of the invention is in the form of granules. Preferably, these granules have an average size in the range 150 to 1400 μm. More preferably, the average size is in the range 200 to 1250 μm. Frequently, it is desirable to control the particle size distribution and a preferred composition has a particle size distribution such that at least 95 per cent by weight of the granules are in the range 200 to 1250 μm, as determined by sieving. More preferably, at least 95 per cent of the granules have a size in the range 200 to 1000 μm, as determined by sieving.

The granules of the invention are particularly useful in detergent compositions with a high bulk density and, consequently, granules with a high bulk density are preferred. Preferably, the granules have a bulk density of at least 750 kg m^"3 and, more preferably, at least 800 kg m^"3, when determined using the test described hereinafter. Usually, the granules have a bulk density below 1400 kg m^"3 and preferably the bulk density is below 1100 kg m^"3. For use in preparing detergent compositions useful for fabric washing, the granules of the invention preferably have a bulk density in the range 750 to 1000 kg m^"3. When the granules are used to prepare detergent compositions for use in machine dish washing, they preferably have a bulk density above 800 kg m^"3 and, more preferably, above 850 kg m^"3.

The composition of the invention can be prepared by any means which provides a uniform mixture of the essential components in a granular form. It is preferred that the alkali metal silicate and alkali metal carbonate are intimately mixed and that the water is uniformly distributed throughout the granules. One preferred process for manufacture of the composition according to the invention therefore comprises the steps of (a) preparing an aqueous mixture of an alkali metal silicate and an alkali metal carbonate, (b) spray- drying said aqueous mixture and (c) compacting the spray-dried powder thereby produced to form granules.

The aqueous mixture of alkali metal silicate and alkali metal carbonate will generally be chosen to be as concentrated as possible to reduce the amount of water which needs to be reduced in the spray-drying step. It is not necessary that all the components are fully dissolved and a preferred aqueous mixture comprises a slurry of the components. A preferred method of preparing such a slurry is to mix an alkali metal carbonate with an aqueous solution of an alkali metal silicate. The alkali metal carbonate can be a hydrated form of carbonate such as sodium carbonate decahydrate or sodium carbonate monohydrate, but an anhydrous form is usually preferred to minimise the amount of water in the aqueous mixture. Aqueous solutions of alkali metal silicates are readily available and any are generally suitable for use in the process of the invention. The molar ratio, Si0₂ : M₂0 (M is an alkali metal), is chosen as desired for the final composition and the solids content will normally be as high as possible to reduce the need to remove water during the spray-drying step. Normally the solution will contain at least 27 per cent solids by weight and, preferably, the solids content of the alkali metal silicate solution is at least 32 per cent by weight. However, it is more difficult to prepare a slurry using a silicate solution with a high solids content. Therefore, a practical maximum concentration of silicate solution is 42 per cent by weight solids. Preferably, the silicate solution contains not more than 40 per cent solids.

Alternative methods for preparing an aqueous mixture of silicate and carbonate include heating a mixture of cristobalite, alkali metal carbonate and alkali metal hydroxide or a mixture of solid alkali metal silicate (waterglass), alkali metal carbonate and water in an autoclave.

The aqueous mixture of silicate and carbonate prepared as above is spray-dried using any suitable equipment. An important consideration is the residual amount of water in the spray-dried product. In principle, the amount of water in the spray-dried product is closely related to the desired water content of the final, compacted granular product. However, it is possible to adjust, to some extent, the water content of the mixture during granulation as explained hereinafter. A typical inlet temperature on the spray dryer is in the range 190 to 300° C and an inlet temperature in the range 190 to 270° C is preferred. Normal outlet temperatures are in the range 90 to 120° C and, preferably, in the range 95 to 115° C.

In an alternative method of preparing the granules of this invention an aqueous mixture of an alkali metal silicate and an alkali metal carbonate is introduced into a drum containing a multiplicity of rotating arms proximate its internal surface, which is heated and a heated gas is introduced into the drum. Preferably, the internal surface of the drum is heated to a temperature in the range 175 to 250° C and the gas is heated to a temperature of about

200 to 300° C at the point it is introduced into the drum. Preferably, the gas is introduced into the drum at a location close to where the silicate/carbonate aqueous mixture is introduced. In this way, the liquid and gas enter the drum cocurrently, which provides an advantageous temperature distribution throughout the drum. Preferably the aqueous mixture is introduced in the upper region of one side of the drum and the gas is introduced into the lower region of the drum. The gas used is not critical, provided it does not interact with the aqueous mixture, and air is a preferred gas. The material dried in this manner may be in the form of a powder or granules. In this alternative method, the preferred composition of the aqueous mixture is similar to that used for spay-drying.

The spray dried material or powdery material dried in a drum with rotating arms, as described hereinbefore can, in general, be compacted to form granules by any suitable means, but a preferred method for compacting the material is to subject said material to a roller compacting process. In this process the material is compressed between rollers. Suitable roller compactors include the Alexanderwerk compactors manufactured by Alexanderwerk AG, Remscheid, Germany, the Chilsonator compactors available from Fitzpatrick Company, Elmshurst, Illinois, USA and a roller compactor from Hosokawa Bepex of Minneapolis, USA. A roller pressure setting is selected according to the strength of granule desired, higher pressures leading to stronger granules. Generally, roller pressure is between 4 and 25 MPa. If desired, the compacted material from the compactor is fed into a granulator, which forms part of the machine, and forced through a mesh.

The water content of the compacted granules can be adjusted by adding some water before the material is compacted on the roller compactor. Typically, up to 4 per cent water by weight with respect to spray-dried material can be added. Preferably, the amount is up to 2 per cent by weight of the material. Surprisingly, the addition of water within these limits at this stage of the process has been found to desirably increase the friability of the granules produced.

If desired, after the composition has been compacted, the particle size distribution can be adjusted by removing fines and oversize particles using sieves.

Material prepared by spray-drying followed by compaction has been found to have a good Liquid Carrying Capacity (LCC), as defined by the test described herein. In particular, the LCC has been found to be higher than other, commercially available, silicate compositions. A high Liquid Carrying Capacity is desirable because it assists in the production of concentrated detergents. Currently, it has been possible to produce silicate/carbonate granules by spray-drying followed by compaction having an LCC determined according to the test described herein in the range 6 to 10%.

The method according to the invention, wherein the aqueous mixture is introduced into a drum equipped with rotating arms can be operated so as to produce a granular material directly. European Patent Application, published as EP 0 526 978A (equivalent to US patent 5 340 559) describes such a process for the single stage granulation of alkali metal silicates. Granular material according to this invention can be produced by an analogous process and the disclosures of this patent application are hereby incorporated by reference.

The following tests were used to characterise the compositions according to this invention. 1. Solubility of Granules

5 g silicate granules was accurately weighed into a 100 cm³ beaker. 500 cm³ of demineralised water was added to a 600 cm³ beaker and held at the test temperature (20°C or 60°C). The 600 cm³ beaker was placed on a magnetic stirrer and the stirring speed was adjusted to give a vortex of about 4 cm. The sample of silicate granule was added to the water, the solution was removed from the heat and stirring of the solution was stopped exactly 2 minutes after the silicate sample was added. The insoluble material in the solution was allowed to settle to the bottom of the beaker over a further 2 minutes. Then the solution was filtered using a weighed filter crucible G4 and a vacuum suction flask. The filter crucible was dried by heating for 2 hours in a drying oven at 105° C. After cooling in a desiccator the filter crucible was reweighed.

The solubility was expressed as percentage insoluble material in the granule calculated from:

Gp - Gp

% Insoluble material = — ^{u r} x 100

G_s

where G_D = weight of filter crucible + contents - after drying, G_F = weight of filter crucible empty and G_s = weight of silicate granule sample.

Preferred compositions according to this invention have a solubility of at least 90 per cent by weight when tested according to this test at 20° C.

2. Rate of Dissolution of Granules

The test is based on the fact that while the silicate granules are dissolving the conductivity of the solution will increase. The time needed to reach a constant conductivity value is used as a measure of rate of dissolution of the silicate granule at 20° C.

An 'Orion' Microprocessor Conductivity meter, model 120 was used to measure conductivity. This was calibrated using 0.1 M KCI solution which has a conductivity of approximately 12880 μS/cm (max. deviation of ± 200 μS/cm) at a solution temperature of 20° C.

2.50 gram of the silicate granule sample was weighed, accurate to 0.01 gram, in a 100 cm³ beaker. 1000 cm³ demineralised water was added to a 1000 cm³ tall-form beaker and the temperature was adjusted to 20°C. The electrode of the conductivity meter was placed in the demineralised water which was stirred on a magnetic stirrer at a stirring speed which produced a vortex of approximatey 4 cm. The weighed sample of silicate granules was added to the water and the conductivity of the solution was noted every 10 seconds until the conductivity remained constant.

The conductivity values were plotted against time and the time at which a constant conductivity was achieved was determined. This value (in seconds) is a measure of the rate of dissolution of the silicate granules.

Preferred compositions according to the invention have a rate of dissolution, as measured by this test of less than 240 seconds.

3. Bulk Density

(a) Powders

The bulk density was determined using a two-part bulk density cylinder with a known capacity which was filled with sample.

The lower part of the bulk density cylinder was placed on the balance and weighed to an accuracy of 0.01 g. The two parts of the bulk density cylinder were put together and the shutter was closed. The upper part of the cylinder was filled with test sample, the shutter was opened and the sample allowed to flow into the lower part, until the lower part overflowed. When necessary, the powder was stirred gently with a stirring rod. The shutter was closed again and the upper part of the cylinder removed. The piled up material was levelled with the top brim of the lower part of the cylinder and any surplus material was wiped off with a brush. The filled lower part of the bulk density cylinder was re-weighed to determine the sample weight.

The Bulk Density was obtained by dividing the sample weight by the volume of the lower part of the bulk density cylinder.

(b) Granules A 500 cm³ measuring cylinder was placed on a balance and the weight measured to 0.01 g. The measuring cylinder was then filled to the 500 cm³ line with the granule sample, using a funnel with a wide neck. The filled measuring cylinder was re-weighed to determine the weight of the sample. The Bulk Density was obtained by dividing the sample weight by the volume of the sample (500 cm³).

4. Caking of Granules

A sample of silicate granules was sieved to a size between 200 and 1000 μm before being placed in a humidity cabinet. 50 g of silicate granules were weighed into a polystyrene cup (approximately 10 cm diameter) which was fitted with perforated lid containing about 150 perforations and then this was placed in a humidity cabinet for 24 hours at 37° C and 70% humidity. The weight difference of the sample before (w- and after storage (w₂) was determined in order to calculate the moisture uptake. The degree of caking was determined by gentle sieving of the caked sample (after storage) over a pre-weighed 1600 μm sieve, diameter 20 cm, height 5 cm, for 2 minutes with an amplitude of 1.5 mm using a Retsch AS 200 sieve shaker. The fraction of the product which remained on the 1600 μm sieve was expressed as a percentage of the weight w₂.

5. Caking of Detergent Compositions containing Granules

The silicate granules were sieved to a size between 200 and 1000 μm before placing them in the humidity cabinet. In addition the detergent base powder was sieved to a size between 400 and 1000 μm. The silicate granules were thoroughly mixed at a level of 1.9% by weight or 3.8% by weight in the final mixture with a standard detergent base powder, as used in a European detergent formulation, but without minor additives, such as perfume.

Caking was determined by weighing 50 g of the detergent powder, mixed with the silicate granules, in a polystyrene cup (approximately 10 cm diameter) with perforated lid (containing about 150 perforations) and then placing this in a humidity cabinet for a predetermined period at 37° C and 70% humidity. The weight difference of the sample before (w-i) and after storage (w₂) was then determined in order to calculate the moisture uptake. The degree of caking was determined by gentle sieving of the caked sample (after storage) over a pre-weighed 1600 μm sieve, diameter 20 cm, height 5 cm, for 10 minutes with an amplitude of 0.75 mm using a Retsch AS 200 sieve shaker. The fraction of the product which remained on the 1600 μm sieve was expressed as a percentage of the weight w₂.

The absolute results obtained on this test can vary with the precise composition of the detergent used and laboratory conditions. It is therefore normally used to compare the relative properties of different silicate compositions. However, as an indicator of acceptable performance, it has been found that preferred compositions according to this invention, when incorporated into the detergent composition used and tested according to this test, show a retention of less than 5 % by weight on the sieve for compositions containing 1.9% granules after 3 and 7 days residence in the humidity cabinet. 6. Liquid Carrying Capacity

The liquid carrying capacity is determined using an arrangement of funnels shown in Figure 1 , wherein the dimensions marked are expressed in cm. 100 g_of material is placed in a mortar and 1 g of a non-ionic oxo alcohol (Lutensol A07 from BASF, a C 3-C₁₅, 7 oxyethylene oxo alcohol) is added and mixed in gently by hand, taking care to maintain the granular structure. The sample is then poured into the top funnel, a, with the rotating seal, c, closed. The rotating seal is then opened, allowing the material to flow into the lower funnel, b, tapping, if necessary, to start the flow. The diameter of the cone of material formed below the lower funnel, b, is then measured and recorded. The material is allowed to flow through the funnels as described, a further twice and an average of the three cone diameters is calculated. The material is then returned to the mortar and a further 1 g of Lutensol A07 is added and carefully mixed in by hand. An average of three cone diameters is again calculated using the method described above. The addition of Lutensol A07 in 1 g increments is repeated until a total of 15 g has been added and the average cone diameter is determined for each of the samples thus produced. The average cone diameters, thus obtained are plotted against the amount of Lutensol A07 added. A marked decrease in cone diameter is observed on the curve thus produced and the amount of Lutensol A07 added at this point (expressed as a percentage of the original material) is taken as the Liquid Carrying Capacity of the material.

The invention is exemplified by the following, non-limiting examples.

EXAMPLES

Example 1

The general method used to prepare examples of the invention was as follows.

A solution of sodium silicate having the desired mole ratio, Si0₂ to Na₂0, was prepared by blending appropriate amounts of a sodium silicate with a molar ratio of 3.3 and sodium silicate with a molar ratio of 2.0 in an agitated vessel. After mixing the silicate solutions, water was added to adjust the solids content of the mixture to 37% by weight. This silicate solution was then heated to 60° C and an appropriate amount of anhydrous sodium carbonate was then added to the solution as a powder to produce the desired composition. This solution was then dried in a Niro spray dryer having a diameter of 1.3 metres and a length of 1.3 metres. The inlet temperature of the air was maintained between 210 and 220° C and the outlet temperature varied between 100 and 110° C. The solution was fed to the dryer at a temperature between 60 and 80° C. The nozzle used in the spray drying tower was 2.0 mm diameter and the air flow was set at a pressure between 0.2 and 0.4 MPa. The dried powder from the tower had a moisture content between 13 and 20% by weight.

The dried powder was then charged to a Lodige M5R mixer and mixed with sufficient water to produce the desired final water content. The wetted powder was then compacted using a roller compactor type WP50 from Alexanderwerk. The pressure of the rollers was either 8 or 15 MPa. The formed ribbon produced on the roller compactor was milled using a hammer mill and sieved to a particle size range of 200 to 1250 μm.

The following results were obtained.

n/a = not available

Samples AA, BB, CC and DD are comparative examples.

Example 2

The solution prepared as described in Example 1 with a Si0₂ to Na₂0 ratio of 2.65 and an amount of sodium carbonate calculated to produce 30 weight percent in the final granule was spray dried in a Niro tower with a diameter of 7.6 metres and a length of 7.6 metres. The inlet temperature of the air varied between 210 and 220° C. The outlet temperature varied between 99 and 115° C. The solution was fed to the dryer at a temperature between 60 and 80° C. The atomizer used was of the radial type and the solution was fed to the tower at about 2 m³ per hour. The resulting powder had a moisture content in the range 12 to 20% by weight. This dried powder was compacted using an Alexanderwerk compactor, model WP480X260. During compaction sufficient water was sprayed onto the powder before it entered the compactor rollers to produce a final water content between 14 and 18% with an average of 16% in the granules. The pressure of the rollers was 17 MPa. The ribbon produced was milled using a hammer mill and classified with sieves to a size range of 200 to 1250 μm.

The final product had a Si0₂ content of 39.0% by weight a rate of dissolution of 50 seconds and an insoluble content of less than 5% at 20° C on the hereinbefore described tests. The caking of the granules was found to be 4.0 % after 24 hours on the aforementioned test. In the detergent formulation the granules had a caking level of 3.0 % after 3 days and 9.0% after 7 days.

Claims

1. A composition for use as a builder in detergent compositions comprising a granular material containing an alkali metal silicate and an alkali metal carbonate characterised in that the alkali metal silicate has a molar ratio, Si0₂ to M₂0, where M is an alkali metal, in the range 2.4 : 1 to 3.0 : 1, the amount of silicate in the granules, calculated as Si0₂, is at least 30 per cent by weight, the amount of alkali metal carbonate in the granules, expressed as anhydrous material, is less than 35 per cent by weight and the amount of water in the granules is less than 25 per cent by weight.

2. A composition according to claim 1 characterised in that the alkali metal silicate is sodium silicate or potassium silicate.

3. A composition according to claim 1 or 2 characterised in that the molar ratio Si0₂ to M₂0, where M is an alkali metal, is in the range 2.6 : 1 to 2.9 : 1.

4. A composition according to any preceding claim characterised in that the amount of silicate, calculated as Si0₂, is greater than 40 per cent by weight.

5. A composition according to any preceding claim characterised in that the alkali metal carbonate is sodium carbonate or potassium carbonate.

6. A composition according to any preceding claim characterised in that the amount of alkali metal carbonate, expressed as anhydrous material, is in the range 10 to 30 per cent by weight.

7. A composition according to any preceding claim characterised in that the granules have an average size in the range 150 to 1400 micrometers.

8. A composition according to any preceding claim characterised in that at least 95 per cent of the granules by weight have a size in the range 200 to 1250 micrometers.

9. A composition according to any preceding claim characterised in that the granules have a bulk density in the range 750 to 1400 kg nrf³.

10. A process for the manufacture of a granular material comprising the steps of

(a) preparing an aqueous mixture of an alkali metal silicate and an alkali metal carbonate,

(b) spray-drying said aqueous mixture and (c) compacting the spray-dried powder thereby produced to form granules characterised in that said granular material comprises an alkali metal silicate having a molar ratio, Si0₂ to M₂0, where M is an alkali metal, in the range 2.4 : 1 to 3.0 : 1, an amount of silicate, calculated as Si0₂, of at least 30 per cent by weight, an amount of alkali metal carbonate, expressed as anhydrous material, of less than 35 per cent by weight and an amount of water of less than 25 per cent by weight.

11. A process for the manufacture of a granular material comprising introducing an aqueous mixture of an alkali metal silicate and an alkali metal carbonate, into a drum containing a multiplicity of rotating arms proximate its internal surface which is heated and introducing a heated gas into the drum, characterised in that said granular material comprises an alkali metal silicate having a molar ratio, Si0₂ to M₂0, where M is an alkali metal, in the range 2.4 : 1 to 3.0 : 1 , an amount of silicate, calculated as Si0₂, of at least 30 per cent by weight, an amount of alkali metal carbonate, expressed as anhydrous material, of less than 35 per cent by weight and an amount of water of less than 25 per cent by weight.

12. A process according to claim 11 characterised in that dried material formed in said drum is in the form of a powder and is subsequently compacted to form granules.

13. A process according to any one of claims 10 to 12 characterised in that the aqueous mixture is prepared by adding an alkali metal carbonate to an aqueous solution of alkali metal silicate having a silicate solids content of at least 27 per cent by weight.

14. A process according to claim 10 or 13 characterised in that the aqueous mixture is spray-dried using a spray dryer inlet temperature in the range 190° C to 300° C and an outlet temperature in the range 90° C to 120° C.

15. A process according to any one of claims 10 to 14 characterised in that the dried material is compacted using a roller compactor.

16. A process according to any one of claims 10 to 15 characterised in that the water content of the compacted granules is adjusted by adding water to the spray-dried material before it is compacted on a roller compactor.

17. A process according to claim 11 characterised in that dried material formed in said drum is in the form of granules.