SILICATES

Abstract: Potassium silicates can be prepared in the form of clear solutions by reacting potassium hydroxide solution under elevated temperature and pressure with cristobalite as the silica source.

Description: Silicates Field of the Invention The invention relates to a process for the preparation of potassium silicates by reacting silica, potassium hydroxide and water at elevated temperature and pressure. The invention further relates to a process for the growth of e.g. vegetables and flowers by adding to the growing media potassium silicate. Background to the Invention In principle two general processes are known for the preparation of potassium silicates: a) The furnace process by using sand and K-CO-. Then potassium silicate is obtained that is dissolved in water. This is an expensive process because one has to maintain high temperatures i.e. above 1000°C. b) The hydrothermal process in which silica is directly dissolved in a concentrated aqueous solution of potassium hydroxide. In this process potassium SUBSTITUTESHEET hydroxide and sand are reacted at a temperature of e.g. 240°C and at a pressure of 30 bar, during which a molar ratio SiO /K_0 is obtained of max 1.5. In this process it is very difficult to obtain molar ratios above 1.5. General description of the Invention Now it is possible to prepare potassium silicates through the hydrother al process, so that it is possible to vary the molar ratio SiO /K_0 between 0.5 and 4.0, which process is characterised by the use of cristobalite as the silica source. Cristobalite can, as such, be found in nature or can be prepared from sand or quartz by heating it at a temperature of 1000-1500°C with a small amount of catalyst. The quartz or sand used as starting material, is completely or partly converted into cristobalite. From Dutch patent application 8802446 (Unilever) a process is known for preparing sodium silicate in which cristobalite is used. From the data of this Dutch patent application 8802446 it could however not be derived that cristobalite could also be used, with a good result, for the preparation of potassium silicates. One of the main advantages of the process according to the invention is that one obtains an end product, being potassium silicate, which is crystal clear and not coloured. For this reason the product needs not be bleached with chlorine and avoiding the treatment with chlorine has strong advantages in connection with environmental problems, because one fears that during bleaching with chlorine this might cause the production of dioxin. In connection with environmental measures it is very desirable to use an improved process for the preparation of potassium silicates, which end product can especially be used for e.g. growing vegetables and flowers, to be used in the paper industry or for the production of detergents. Further advantages to be obtained by using the process according to the invention will be clarified in the following examples and can be summarised as follows. The production time for potassium silicate can be decreased from about 6 hours to about 2 hours by using cristobalite instead of the sand as normally used. The reaction is exothermic and starts at about 130°C. The further development of heat is such that the supply of energy can be stopped and the reaction heat is sufficient to heat the reactor to 150-165°C within 10-20 minutes. Cristobalite dissolves completely in the reaction mixture. This means that one obtains a clear solution a short time after the addition of cristobalite. For this reason filtration of the end product can be avoided. On basis of safety considerations a filtration will be carried out, but the process according to the invention gives little waste. Contrary to this a hydrothermal process based on sand, in which the solution of sand is incomplete, gives a sand residue that is not dissolved. This causes the lose of starting material, such as sand and alkaline material, that has to be dumped together with the sand residue. The known process also requires high costs for filtration and operation costs in connection with pumps, valves and such means, which can be attacked by the discharge of sand. The process according to the invention can be carried out continuously or as a batch process. The process of the invention can provide potassium silicates with a molar ratio of 0.5 to 4.0, preferably to 2.0. It is of value in the preparation of metasilicates in which the ratio is 0.95 to 1.05. During the supply of a solution of potassium silicate to the growing medium for e.g. vegetables and flowers is might be a problem when the remaining part of silica is present in the solution because the supply system can be blocked. It has been found that by using potassium metasilicate having a molar ratio Si0_/K ? 0 of 0.95 to 1.05, preferably about 1.0 the blocking of the nozzles, for feeding the nutrients, can be avoided. Potassium metasilicate can be prepared according to the invention with a good result. Up to now it was impossible to prepare potassium silicates through the hydrothermal process if the potassium silicates had a molar ratio Si0_/K 2 0 higher than 1.5, because the reactivity between the concentrated solution of potassium hydroxide and sand is low, even after 10 hours and 195°C. At a temperature of 240°C and a pressure of 20 bar the reaction still is relatively slow by using potassium hydroxide and sand. After 6 hours at such an elevated temperature and pressure the molar ratio Si0 2 /K 2 0 is not higher than 1.5, so that the reaction rate then is so low that it is impractical to continue the reaction. Specific Description of the Invention The invention will now be described in detail with reference to the following examples, in which fig. 1 indicates graphically the molar ratio of potassium metasilicate by using cristobalite and sand as a function of the reaction time in example 1, fig. 2 illustrates the reaction for the preparation of potassium silicate, in which the pressure has been indicated as a function of the time (example II) , and fig. 3 is comparable with the graph of fig. 2 in which the temperature has been indicated after a certain time. Example I The preparation of potassium silicate was performed using quartz sand for one preparation and cristobalite for a second preparation in an electrically heated laboratory autoclave. The distribution of the particle size of the cristobalite was such that 82% had a particle size between 250 and 500/xm, 16% of the particle size is between 100 and 250μm, 1.9% is above 500 μm and 0.1% is smaller than lOOμm. Cristobalite or sand was added in an amount of 1018g to 3646g K0H (50%) and 384g distilled water in the autoclave. The amounts were chosen such that a molar ratio SiO./K-O of 1.05 was obtained in the end product. Within 10 hours at a temperature of 195°C potassium metasilicate had been obtained having an Si0 2 / 2 0 ratio of 0.8 by using quartz sand. By replacing sand by cristobalite the molar ratio of 1.0 was already obtained within 30 minutes so that the reaction could be ended after 30 minutes. These results are given in fig. l. Example II An autoclave was used with a capacity of 20 tonnes. The starting materials were used in the same concentrations as indicated in example I with the exception that the amount of SiO as cristobalite was slightly lower, so that a potassium metasilicate composition was obtained with a molar ratio of 1.0 in the form of a 50 wt.% solution. The mixture was heated slowly from about 100°C. The reaction took place at about 120°C. The it appeared that the reaction was exothermic and further heating was stopped at a temperature of about 125°C. The exothermic heat development elevated the temperature by about 50°C, dependent on the concentration of the reacting substances and the heat dissipation to the environment. When the end temperature of 180°C was reached, all the cristobalite had been dissolved. The complete reaction time during which the temperature was elevated from 120°C to about 175°C was 40 minutes. The pressure had been measured in the autoclave and the development of pressure and temperature during the reaction is indicated in figures 2 and 3 respectively. During the reaction the pressure was elevated from 0.5 to 6 bar.

Claims: Claims 1. A process for the preparation of potassium silicates by reacting silica, potassium hydroxide and water at elevated temperature and pressure and separating the obtained reaction product from the reaction mixture, characterised in that cristobalite is used as the silica source. 2. A process according to claim 1, in which the reaction iiss ccaarrrriieedd oouutt ssoo tthhaatt tthhee molar ratio Si0 2 /K 2 0 in the end product is from 0.5 to 4.0 3. A process according to claim 2, in which the ratio of S i O 2 / K 2 ° i s from °- 5 to 2 • • *• • • 4. A process according to claim 3, in which the ratio of Si0 2 /K_0 is from 0.95 to 1.05. 5. A process for growing e.g. vegetables and flowers by adding to the growing medium potassium silicate, characterised in that one uses as potassium silicate potassium metasilicate having a Si0 ? /K_0 molar ratio of 0.95 to 1.05, preferably about 1.0, obtained according to the process of any of claims 1 to 4.

Inventors: OSINGA THEO JAN, OSINGA, THEO, JAN
Applicants: CROSFIELD JOSEPH & SONS, JOSEPH CROSFIELD & SONS
Application year: 1991
Main classification: C05D1
IPC codes: C01B33, C05D1