IE904222A1 - Process for preparing reactive silicon dioxide phase - Google Patents

Abstract

The invention relates to a process for producing reactive silicon dioxide phases in which quartz sand is mixed with an alkaline metal compound or its aqueous solution where the alkaline metal compound is selected from the group of compounds which are converted into the corresponding alkaline metal oxide on heating, the mol ratio of SiO2 to the alkaline metal oxide is between 1:0.0025 and 1:0.1 and this mixture is heated to a temperature of between 1100 and 1700 DEG C. [DE3938730A1]

Description

Processes for Preparing Reactive Silicon Dioxide Phase BACKGROUND OF THE INVENTION 1) Field of the Invention The invention relates to processes for preparing reactive silicon dioxide phases. The starting material for these processes is quartz sand, and the reactive phases thus obtained are cristobalite, tridymite, amorphous silicon dioxide and alkali metal silicate, which are characterised by their low quartz content. .

At standard pressure three crystalline modifications of silicon dioxide are known, namely quart2, tridymite and cristobalite. Quartz is the phase stable up to 870°C, after which comes tridymite, which finally converts into cristobalite above 1470°C (see Hollemann-Wiberg, Lehrbuch der Anorganischen Chemie, 81st - 90th Edition, Verlag De Gruyter, Berlin (1976), page 545), The conversion within these modifications is only possible via bond-breaking and formation of new Si-G-Si bonds.

The structure of tridymite and cristobalite is more open than that of quartz; this is also indicated by the differing densities (2.65 g/cm3 for quartz, approx. 2.3 g/cm3 for tridymite and cristobalite) and by their increased reactivity, e.g. in the hydrothermal preparation of sodium polysilicates. 2) Background of the Invention Various studies have been carried out with the aim of preparing cristobalite, which is used, on account of its white colour and as a means of controlling the expansion coefficient, predominantly as a raw material and filling agent, e.g. for ceramic products and also in the manufacture of paints.

Cri3tobalite is prepared by converting quartz sands, with the addition of an alkali, in a rotary kiln at temperatures of approx. 1500 °C (see Oilman's Encyklopaedie der Technischen Chemie, 4th Edition, Verlag Chemie, Weinheim, Volume 21 (1982), page 442).

The preparation of cristobalite from amorphous silicon dioxide with a set specific surface at a temperature between 1000 and 1300°C has been described in European Patent Specification No. EP 0,283,933 A. For this process it is necessary first to prepare amorphous silicon dioxide, which even at this stage is characterised by incritesed reactivity. The suggested catalysts for this reaction are alkali metal compounds containing lithium, sodium or potassium, but these compounds are used in very small quantities because they must afterwards be removed from the cristobalite by treatment at temperatures above 1300°C, and the whole process requires very long reaction times.

The phase conversion from quartz to cristobalite in the absence of a catalyst has been the subject of much research. Schneider et al. (see Materials Science Forum, 7 , (1 986) pages 91 onwards) describe such a phase conversion, which takes upwards from several hours to days. The reaction velocities achieved are very markedly dependent upon the crystallinity of the quartz and the level of impurities. Research has clearly shown that the conversion proceeds via amorphous intermediate phases. Ibrahim et al. (See La Ceramica, (1985) pages 19 onwards) describe the reaction between quartz and activated silicates in the temperature range 1350°C to 1500°C, after which mixtures of tridymite and cristobalite are formed in the course of three days.

There is no agreement In the literature on whether tridymite is stable without any impurities. The presence of larger quantities of companion elements (alkali metals, aluminum) promote the formation of tridymite, which frequently has many structural defects in its crystalline lattice. According to Novakovic et al. (see Interceram, (1 986), pages 29-30) the conversion of quartz to tridymite (1350°C, 114 hours, Li-catalysis) takes place initially via cristobalite, which then changes to tridymite in a second stage.

BRIEF SUMMARY OF THE INVENTIVE CONCEPT It is an object of the present invention to devise processes starting from quartz sand for preparing silicon dioxide phases, which are characterised by a very low quartz content, and yet to do so employing reaction temperatures . fc· which desirably should be lower and reaction times which above all should be shorter than those for the known processes.

According to this invention there are provided processes for preparing reactive silicon dioxide phases from quartz sand in which the quartz sand is mixed with at least one alkali metal compound, optionally but desirably in aqueous solution, which on being heated changes into the corresponding alkali metal oxide, in amounts such that the molar ratio of SiO2 to alkali metal oxide lies between 1 : 0.0C25 and 1 : 0.1, and this mixture is tempered by heating to a temperature between 1100°C and 1700°C, Reference above and subsequently to a molar ratio of SiC^ to alkali metal oxide is to be understood as the molar ratio of the silicon dioxide contained in the quartz sand relative to the alkali metal oxide corresponding to the alkali metal compound used in each case.

The reactive silicon dioxide phases which are thus obtained consist of cristobalite, tridymite, amorphous silicon dioxide and alkali metal silicate, and all of them are characterised by a low quartz content, as determined by Xray diffraction analysis.

The appropriate reaction times become shorter as the reaction temperature rises and above 1300° C there is a marked reduction in the necessary reaction times. At a temperature of 1400°C and when 5 % by wt. of sodium hydroxide is added as a catalyst, corresponding to a molar ratio of silicon dioxide to alkali metal oxide of 1 : 0.0375, no residual quartz could any longer be detected in the reaction product in this case after a reaction time of 30 minutes.

The reaction temperatures can be still further increased if desired, and the reaction time (relative to any specific catalyst) can be thereby reduced even further*. Lowering the reaction temperature to 1200°C however results in an increase in the residual quartz content.

The residual quartz content moreover also changes as the amount of catalyst is varied. Thus for example, with a reaction time of 60 minutes and using sodium hydroxide as catalyst, the residual quartz content rises from 0.0 % by wt. (corresponding to a molar ratio silicon dioxide : sodium oxide » 1 : 0.0375) up to 2 % by wt. (corresponding to a molar ratio (silicon dioxide : sodium oxide * 1 : 0.0038). The residual quartz content tends to be lower (cf. Table 1) when potassium hydroxide is used instead of sodium hydroxide, even indeed when the process is operated at lower reaction temperatures.

In carrying out the processes of this invention it has proved especially advantageous to employ molar ratios of silicon dioxide to alkali metal oxide in the range of from 1 : 0.0035 to 1 : 0.05. In the case of sodium hydroxide, for example, this is equivalent to adding 0.45 * by wt. to 6.445 % by wt., while in the case of potassium hydroxide, this is equivalent to adding 0.63 % by wt. to 9.0 % by wt.

The catalysts for use in the process of the invention are - 6 alkali metal compounds which change into the corresponding alkali metal oxides on being heated, and these in particular can be lithium, sodium or potassium hydroxide, as well as the carbonates, nitrates, nitrites, sulfates, sulfites, oxalates or formates of these alkali metals.

The catalyst should of course be distributed throughout the quartz sand, and a particularly even distribution can readily be achieved by applying a 5 to 50 % by wt. aqueous solution or suspension of the alkali metal compound to the quartz sand. Particularly suitable concentrations for such catalyst solutions lie between 15 and 25 % by wt.

At the operational level, broadly-speaking (but as further illustrated in the Examples) the quartz sand should be mixed with the corresponding amount of an appropriate alkali metal compound (or an aqueous solution thereof) and then tempered for a defined period of time in a muffle furnace, rotary kiln or shaft furnace. The use of rotary kilns is recommended, especially if the process is to be carried out on a large scale.

For reasons which appear more clearly from the subsequent Examples, it is preferred to employ a reaction temperature of at least 1300°C, and one in the range of from 1 300 to 1700°C is especially preferred. It is also much preferred to carry out the process with reaction times in the range of from 10 to 180 minutes, and preferably lees than 60 minutes - thus especially with the reaction times in the range of from 10 to 60 minutes.

The residual quartz content of the samples was determined with the aid of differential thermal analysis (Differential Scanning Calorimetry, DSC), and by using both X-ray diffraction analysis (XRA”) and DSC it was possible to assess the quantities of the reaction products in the individual samples. This assessment shows that cristobalite is formed in a first reaction stage, although all the - 7 reactions were carried out below the thermodynamic stability temperature for cristobalite; and it is only when the reaction times are longer, and preferably when there are large quantities of catalyst present, that tridymite is formed in a second reaction stage. The rough values for the ratio of crtstobalite to tridymite are given in Table 2 below, which shows that there is no difference in this respect between the sodium- and lithium-catalysed reactions.

DETAILED DESCRIPTION OF. PREFERRED MODES OF OPERATION In order that the invention may be well understood it will now be described in more detail, though only by way of illustration, by means of the following wor&Lng Examples.

EXAMPLES 1 TO 7 In all the Examples the reacted quartz sand contained > 99.9 % silicon dioxide, and was of natural origin. In some cases the alkali metal compounds used as catalyst were incorporated in the form of solutions containing the alkali metal compound dissolved in just enough water for the quartz to be covered by the solution, after which the quartz sand was slowly dried and the dry sand was thoroughly mixed. In other cases the two solid components were thoroughly mixed and heated together for a period of between 10 and 180 minutes (preferably less than 60 minutes) before subjection to the main reaction. The reaction vessel used was a glazed aluminum oxide crucible.

The various temperatures and reaction times used in the individual Examples are summarized in tabular form below, in Table 1 - which moreover shows not only the percentage by weight of the alkali metal compound content, but also the molar ratio of S1O2 to alkali metal oxide.

•E 904222 - 8 TABLE_1 Ex. Temp. Catalyst added in Residual Quartz NO. ( °C ) % by wt. (SiO2 : Content (in % by Alkali Metal Oxide) weight) after a Reaction Time of: 1 5 min. 30 min. 10 mip. 1 a 1400 0.5% NaOH (1 :0.0038) 10 5 2 1b 1 400 1 % NaOH (1 :0.0077) 3 2 0 1 c 1 400 5 % NaOH (1 :0.0375) 30 3 min. %0 0 min. 180 0 min. 2a 1 300 0.5% NaOH (1 :0.0038) 55 22 1 2b 1 300 1 % NaOH (1 :0.0077) 37 7 1 2c 1 300 5 % NaOH (1 :0.0375) 1 4 3 0 3a 1 200 0.5% NaOH (1 :0.0038) > 50 > 30 16 3b 1 200 1 % NaOH (1 :0.0077) > 50 29 1 5 3c 1 200 5 % NaOH (1 :0.0375) > 50 1 5 8 4a 1 300 0.7% KOH (1 :0.0038) 7 2 0 4b 1 300 1 .4% KOH (1 :0.0077) 5 2 0 4c 1 300 7 % KOH (1 :0.0375) 3 0 0 5 1 300 6.6% Na2CO 3 (1:0.0375) 23 16 1 6 1300 8.9% Na2SO 4 (1:0.0375) 24 10 1 7 1 300 0.6% LiOH (1:0.0077) 50 1 2 3 As appears from the Examples summarized in Table 1, the residual quartz content falls as the temperature rises, relative to a specific reaction time. The results also show that the effectiveness of the alkali metal catalyst increases in the sequence lithium oxide, sodium oxide and potassium oxide.

In Example 1 (a-c) the samples were tempered at a temperature of 1400 °C. when 0.5 % by wt. of sodium hydroxide is added in the form of an aqueous solution, 90% of the quartz has been consumed by reaction after only 15 minutes. When the amount of catalyst is 5 % by wt. of sodium hydroxide, there is no longer any quartz remaining in the samples after 30 minutes, When 0.5 % by wt. of sodium hydroxide is added, the reaction i9 complete within one hour.

At a temperature of 1300°C (Example 2 a-Λ the reaction takes longer; nevertheless, with 5 % by wt. of catalyst, 97 % of the quartz is consumed by reaction within half an hour.

When 1 % by wt. of sodium hydroxide is added, only traces of quartz can be detected in the reaction mixture after 3 hours.

A reaction temperature of 1200°C (Example 3 a-c) does not result in complete conversion of the quartz into reactive phases within 3 hours; but even here conversion rates of over 80 % are achieved.

Example 4 (a-c) shows the especially high catalytic activity of potassium salts. Even the addition of as little as 0.7 % by wt. of potassium hydroxide in the form of an aqueous solution (corresponding to a molar ratio of silicon dioxide : potassium oxide » 1 : 0.00375) at a temperature of 1 300°C produces higher conversion rates as compared with additions of 5 % by wt. of sodium hydroxide.

As appears from Example 5, when solid sodium carbonate is added to the quartz sand the degree of phase conversion achieved is not as high as with the addition of aqueous sodium hydroxide solution; however, the trend is the same. •Ε 904222 - 10 The reason for this poorer performance is that the catalyst in this case is not distributed so evenly throughout the quartz sand as when solutions of the alkali metal compounds are used.

Example 6 shows the results of the reaction catalysed by addition of sodium sulfate - in aqueous solution. Example 7 shows the results of the reaction catalysed by lithium hydroxide - likewise in aqueous solution.

TABLE2 Cristobalite : tridymite ratios in tempered quartz sand (assessment from XRA diagrams) Example Reaction time Ratio of No. (min) cristobalite to tridymite 1a 15 - 60 cristobalite only 1 b 1 5 cristobalite only 1b 30 10 : 1 1b 60 5 : 1 1 c 1 5 5 : 1 1 c 30 1 : 1 1 c 60 1 : 3 2a 60 cristobalite only 2a 180 10 : 1 2b 30 10 : 1 2b 60 5 : 1 2b 1 80 3 : 1 2c 30 5 : 1 2c 60 1 : 1 2c 1 80 1:10 [The reaction parameters are set out in Table 1 above] With the aid of DSC, when the reaction components are added up, it can be seen that for average degrees of reaction (quartz conversion approx. 60%) considerable quantities of amorphous phases are present in the samples, e.g. 40 % in Example 2c after a reaction time of 30 minutes. These amorphous phases consist of alkali metal silicates and amorphous silicon dioxide. The quantities of alkali metal silicate are due to the alkali metal compounds which take part in the reaction.

Claims (6)

CLAIMS What is claimed is:

1. A process for the preparation of reactive silicon dioxide phases, in which quartz sand is mixed with an alkali metal compound selected from the group consisting of those which change into the corresponding alkali metal oxides on being heated, the molar ratio of SiO 2 to alkali metal oxide in the mixture being between 1 : 0.0025 and 1 : 0.1, and this mixture is tempered by heating it to a temperature between 1110°C and 1700°C.

2. Processes according to claim 1, in which the reactive silicon dioxide phases thus prepared consist of cristobalite, tridymite, amorphous silicon dioxide and alkali metal silicate.

3. Processes according to claim 1, in which the alkali metal compounds are selected from the group consisting of lithium, sodium and potassium hydroxides, carbonates, nitrates, nitrites, sulfates, sulfites, oxalates and formates.

4. Processes according to claim 1, in which the molar ratio of SiO 2 to alkali metal oxide in the mixture lies between 1 : 0.0035 and 1 : 0.05.

5. Processes according to claim 1, in which the mixture ls tempered by heating it to a temperature of at least 1300°C. 6. Processes according to claim 1 , in which the reaction time is in the range of from 1 0 to 180 minutes. 7. Processes according to claim 6, in which the reaction time is in the range of from 10 to 60 minutes. •Ε 904222 - 13 8. A process for the preparation of reactive silicon dioxide phases substantially as hereinbefore described by way of Example.

6. 9. Reactive silicon dioxide phases whenever prepared by a process as claimed in any of the preceding claims.