Process for preparing a coloured granular alkali metal silicate

Abstract: The invention provides coloured granules (detergent speckles) which comprise an alkali metal silicate, which granules are coated with a colourant material. Preferably the coloured granules comprise a sodium silicate, in particular a metasilicate. Very good results are obtained with silicate hydrate granules, in particular a hydrate containing 5-6 water molecules per molecule of silicate in the crstal. The invention also provides a process for manufacturing the coloured granules.

Description: The invention relates to a process for preparing a coloured granular alkali metal silicate, at least comprising the steps of spraying the granular material with a dye or a pigment and a solvent in such a way that the granules are kept in motion whilst spraying, and drying the granules. Coloured granules in detergent compositions (detergent speckles) are well-known in the art, especially in phosphate containing detergent compositions and the detergent speckles therefore are usually mainly based on soluble inorganic salts like phosphates, especially sodium triphosphate, sodium sulphate or mixtures therefrom, which granules have been coated/impregnated with a solution or dispersion of a colourant i.e. a dye or pigment. Preferable the speckles are green or blue. Speckles give the detergent composition a more attractive consumer appeal, moreover it is possible to incorporate special purpose ingredients via the speckles in the detergent composition. It is desirable that the amount of colourant in the granular material is so low as not to interfere with the washing process, moreover the speckles should be bright coloured and if possible the detergent speckle base material should serve a useful and practical purpose in the detergent composition. For "zero-phosphate detergent compositions" of course no phosphate-based speckles can be used. Consequently there was the problem of providing a suitable coloured detergent speckle which show a uniform, bright colour whilst comprising only a low percentage of pigment or dye, which granules remain stable as to colour, which are low in attrition and non, or low, dusting, which are based on a functional base material, which have a suitable bulk density as to avoid segregation from the detergent powder upon handling/storage and which are non-caking and dissolve fast in the washing liquor. The invention provides a process for preparing a coloured granular alkali metal silicate satisfying most of the above requirements, which is characterized by preheating said granular material to a temperature below the melting temperature thereof and spraying a solution of a dye or a suspension of a pigment on the granular material, said granular material being dried and cooled afterwards. Preferably the granular material is preheated by blowing with a gas with a temperature of between 25 and 70 °C and more preferably with a temperature of between 30 and 40 °C. In the art numerous references disclose the preparation of coloured speckles for use in detergents. US-A-3 679 610 relates to tinted granules formed by colouring water-soluble alkali granules by treating the surfaces thereof with a water-soluble, volatile, organic hydrogen bonding polar solvent or a mixture of such solvent to provide coloured speckles with a low percentage of colourant. DE-B-1 938 756 is directed to a porous highly soluble product based on hydrated, crystalline sodium silicate, which is produced by crystallisation from a mother liquor. Said product may be coloured by adding colourant to a detergent suspension after which the suspension is spray-dried or the colourant may be added to the mother liquor before crystallisation. Also the colourant may be sprayed on the crystallisation product. No mention is made of preheating the granules before spraying with a colourant. DE-A-2 626 383 describes a process for the preparation of modified sodium silicate hydrates, the crystallisation of the sodium silicate hydrate being executed in the presence of fine pigments, which are not soluble in sodium silicate hydrate and/or fine zeolites. Colourant is added to the mother liquor from which the sodium silicate particles are crystallised. The particles are therefore coloured throughout. EP-A-0 060 728 relates to a process for manufacturing detergent speckles in which particular material is sprayed with an aqueous solution of colourant, which also comprises a hydratable salt. The intention is to use only a small amount of colourant, which is achieved by incorporating a hydratable salt in the solution of colourant. No preheating of the granules is disclosed. GB-A-808 668 is directed to providing granular detergent compositions, the detergent being a soap and/or a synthetic detergent having a distinctive and readily recognisable appearance. This is achieved by incorporating 2-30 % by weight of coloured granules substantially uniformly distributed throughout the composition. Said coloured granules are prepared by separating part of the detergent granules and spraying these with a colourant. No alkali metal silicate granules which are preheated before spraying with a colourant are described. For the preparation of alkali metal silicate granules 〈〈per se〉〉 reference is made to FR-A-2 405 091, which relates to a process for the preparation of granules without fines and which flows easily. Said process is characterized in that solid starting materials which comprise water and which release water below their melting point, are subjected to granulation at a temperature which is sufficient to cause the elimination of water. As solid starting materials alkali silicates containing water and phosphates containing water are mentioned. No colouring of such granules is however described. In particular the alkali metal silicate is sodium silicate, more preferably metasilicate and/or disilicate. Silicates have the functions of improving the washing effect and of corrosion inhibitor in detergent composition and consequently are a functional additive. Especially preferred are silicate hydrates as the base material for the speckles according to the present invention and sodium silicate penta- and/or hexahydrate give optimal results. In view of the well-known high alkalinity of alkali metal silicates it was quite unexpected that these silicates could serve as the base material of uniformly, bright coloured of a deep hue and especially colour-stable detergent speckles. Advantageously, the alkali metal silicate is an alkali metal silicate hydrate, which is preferably a hydrate containing 5-6 water molecules per molecule of silicate in the crystal. Further it is advantageous for the granules to have a spherical shape. Although it is recommended that the base material for the speckles consists substantially of sodium silicate it is, however, also possible to use as the base material for the speckles a mixture of materials in which at least 50% of the dry weight consists of alkali metal silicate, the remainder being other inorganic materials like eg. zeolite, sodium sulfate or sodium phosphate if the latter can be tolerated in the eventual detergent composition. The coloured silicate granules produced according to the present invention usually have an average particle size of 0.2 - 2mm, preferably from 0.8 to 1.6 mm, diameter. Granules with an spherical shape have advantages and then the diameter is taken to determine the size. Of course the actual size of the speckles desired depends on that of the detergent powder in which the speckles are incorporated. The colourant, i.e. dye or pigment, which forms a coating on the alkali metal silicate granules is normally present in an amount of 0.1 to 2.0% (w.w.) of the granules. The colourant usually is a commercially available dye and compounds like Monastral Blue(CI no. 74160 ) and Monastral Green (CI no. 74260 ) + Unidispers Blue (CI no. 74160 ) (ex CIBA-Geigy) are preferred. (Monastral dyes are ex ICI, England.) The coloured detergent speckles are usually incorporated in detergent powders in a level between 0.1 - 5%, preferably between 0.2 and 2.0%, by weight of the total detergent composition. The invention extends particularly to the use of sodium silicate having Na₂O:SiO₂ ratios from 1:1.2 to 1:3.3, preferably 1:1.5 to 1:3.0. The higher ratios are generally less preferred because of slower solution. The granules of alkali metal silicate can be prepared by a number of suitable procedures, for example metasilicate (Na₂O:Silica ratio 1:1) can be granulated in a rotary barrel as described in GB-A-1149859 (Baker-Holloway). Silicates, for example sodium disilicate, can be prepared in compacted form of increased bulk density by use of the procedures described in US-A-3875282 (Stauffer Chemical Co) and US-A-3931036 (PQ). The use of silicates in compacted form is particularly valuable for speckles intended for the higher density detergent powder compositions, that is having a bulk density above 700 g/l, preferably above 800 g/l. Sodium disilicate can be prepared with a bulk density between 700 and 900 g/l by compaction between rollers. Sodium metasilicate can be prepared with a bulk density between 880 and 1,000 g/l using granulation in a rotary barrel. At the lower density ranges the use of a binder, eg. silicate solution, may be necessary to provide integrity in the compacted mass obtained by twin rollers. Traditionally detergent speckles are manufactured by a number of methods. One method involved the spray-drying of a special coloured slurry but the obvious disadvantage is that thorough cleaning is required before normal detergent powder can be produced again. Another method involved spraying a coloured solution onto a spray-dried detergent powder, but this requires, as the previous method, a large amount of coloured solution. Also it has been suggested to spray onto granules a solution of a colourant that also comprises a hydratable salt, preferably a warm and concentrated solution, to facilitate subsequent crystallisation. For zeolite containing detergent compositions this method was found not quite practical because a high percentage of colourant was required. The coloured detergent speckles, which are based on alkali metal silicate, can be prepared according to the present invention by spraying under suitable conditions a solution of a colourant on pre-heated granular alkali metal silicate, which is kept in constant motion so as to avoid caking. This can be done e.g. by carrying out the operation in a fluidized bed, or in a mixer, followed by drying while keeping the granules in constant motion. Suitable equipment for this technique comprises fluidized beds, nozzles for the coloured liquid, for hot and cool air, Tellschig*-type mixers consisting of rotating drums and fitted with a spray-nozzle, Loedige-type mixers consisting of fixed drums with an internal scraper fitted with a spray-nozzle and Nauta*-mixers of a type consisting of a conical vessel with a rotating scraper fitted with a spray-nozzle. (*stands for tradename). It is preferred to carry out the process in 4 steps to wit: pre-heating the granules, spraying on the colour, drying and cooling. The amount of dye solution or pigment suspension sprayed on the alkali metal silicate granules normally lies between 0.1 and 5.0% by weight, preferably between 0.5 - 2.0%, calculated on the alkali metal silicate. The particle size distribution of the starting granules determines the size of the coloured speckles and most alkali metal silicate granules are available in various, often uniform or narrow particle size ranges which can be chosen dependent on the size desired in the detergent composition. The solution which is sprayed onto the granular alkali metal silicate material usually contains from 0.05 to 50% of colourant preferably completely dissolved. Of course there is also the limitation of the solubility of e.g. the dye in the solvent to be observed. Water is preferred as the solvent but it is sometimes advantageous to use (aqueous) methanol to increase the solubility of the dye. The quantity of liquid sprayed onto the granules is normally between 0.1 and 5%, preferably between 0.5 and 2%, by weight calculated on the silicate and spraying normally is completed within a few minutes to half an hour, 2 to 10 minutes being typical. Pre-heating the alkali metal silicate granules before starting spraying with colourant solution leads to the obtention of very good results. In any case it should be avoided to heat the alkali metal silicate granules to a temperature above their melting points, which is, in the case of sodium metasilicate pentahydrate, around 72°C. The temperature of the gas (air) blown onto the still wet granules normally varies between 25 and 70, preferably between 30 and 40°C. In order to ensure that certain silicate hydrates are substantially formed or retained, it is necessary to monitor the water content of the granules during the drying operation. The percentage of water in the granule also determines its bulk density. Cooling whilst keeping the detergent speckles in constant movement also helps solidification of the surface and to prevent caking. The detergent speckles so obtained are distinctive, of uniform colour, resistant to attrition and have virtually the same particle size distribution as the alkali metal silicate starting granules have. The use of alkali metal silicates as a core for speckles is particularly efficient because, for one or more of the reasons, i) silicates are an active component of detergent compositions and thus no superfluous ingredients are introduced, ii) the silicate cores are utilised in a form in which they would, in any case, be added to a powder composition, iii) the silicate speckles dissolve rapidly so as to provide rapid dissolution and dilution of the surface colourant, iv) the silicate surface does not absorb the colourant, so that the latter is used efficiently, but retains the colourant so that it does not become dispersed throughout the powder, v) the silicate speckles have a uniform and acceptable appearance, and vi) they can be prepared with a bulk density suitable for the higher density powders. The invention is further described and illustrated in the following examples. EXAMPLE 1 1.5 kg sodium metasilicate pentahydrate granules with a particle size between 0.2 and 1.2 mm was placed in a Loedige mixer of 5 litres. The mixer was switched on and the contents were pre-heated to 35 - 45°C by blowing in hot air with a temperature of 50°C. An aqueous solution containing 25% by weight of Monastral Blue was sprayed onto the granules for 5 minutes. The amount of solution sprayed onto the granules was 1% by weight calculated on the metasilicate. Consequently only 0.25% of dye was used relative to the metasilicate. Drying was also carried out at a temperature of 35-45°C for a period of 20 minutes. The water content of the metasilicate was monitored during the drying and kept at around 43% by weight. The detergent speckles were then cooled to 20°C by blowing in cool air. The detergent speckles thus obtained showed excellent brightness, a deep hue, were uniformly coloured, colour-stable and the original particle size distribution had substantially been retained. When mixed in a detergent powder at a level of 1% by weight they imparted an excellent consumer appeal to the product. EXAMPLE 2 Sodium disilicate (Na₂O:SiO₂ ratio of 1:2) was spray-dried to provide granules. These were fed to a two roller press by a feed screw and compressed to a thin, ca 1mm, sheet under pressure. The sheet was then broken up by passage between intermeshing toothed wheels and sieved to provide a granular product having particle size distribution between 0.3 and 1.2 mm. The product had a bulk density of about 850 g/litre. These granules were coated with Monastral blue using the process described in Example 1 but using a 50% aqueous suspension of the pigment to provide a coating level of 0.25% by weight of silicate. This product was then used as speckles in a detergent formulation.

Claims: Process for preparing a coloured granular alkali metal silicate, at least comprising the steps of spraying the granular material with a dye or a pigment and a solvent in such a way that the granules are kept in motion whilst spraying, and drying the granules, characterized by preheating said granular material to a temperature below the melting temperature thereof and spraying a solution of a dye or a suspension of a pigment on the granular material, said granular material being dried and cooled afterwards. Process according to claim 1, characterized in that , the granular material is preheated by blowing with a gas with a temperature of between 25 and 70°C. Process according to claim 2, characterized in that , the granular material is preheated by blowing with a gas with a temperature of between 30 and 40°C. Process according to one or more of the preceding claims, characterized in that , the granular material comprises a sodium silicate. Process according to one or more of the claims 1-3, characterized in that , the alkali metal silicate is meta-silicate. Process according to claim 4 or 5, characterized in that , the alkali metal silicate is an alkali metal silicate hydrate. Process according to one or more of claims 4-6, characterized in that , the sodium silicate is a hydrate, containing 5-6 water molecules per molecule of silicate in the crystal. Process according to one or more of the preceding claims, characterized in that , the granules have a spherical shape. Process according to one or more of the preceding claims, characterized in that , the pigment and/or dye is chosen amongst the class consisting of Monastral Blue (CI no. 74160) and Monastral Green (CI no. 74260) + Unidispers Blue (CI no. 74160) (ex CIBA-Geigy).

Inventors: OSINGA THEO JAN, THEUNISSEN JOSEPH PIERRE HUBER, OSINGA, THEO JAN, THEUNISSEN, JOSEPH PIERRE HUBERT
Applicants: UNILEVER NV, UNILEVER PLC, UNILEVER N.V.
Application year: 1989
Main classification: C11D11
IPC codes: C11D3, C11D17