WO1991008168A1 - Process for producing reactive silicon dioxide phases - 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 C.

Description

 Process for the production of reactive silicon dioxide phases

The invention relates to a method for producing reactive silicon dioxide phases from quartz sand. The reactive phases consist of cristobalite, tridymite, amorphous silicon dioxide and alkali metal silicate and are characterized by a small proportion of quartz.

At normal pressure, three crystalline modifications of the silicon dioxide are known. These are quartz, tridymite and cristobalite. Quartz is the phase stable up to 870 ° C, then the tridymite, which changes into cristobalite above 1470 ° C (Hollemann-Wiberg, "Textbook of Inorganic Chemistry", 81st - 90th edition, De Gruyter, Berlin 1976, page 545). The conversion within these modifications is only possible by breaking the bond and forming new Si-O-Si bonds.

Tridymite and cristobalite have a more open structure than quartz, which is also reflected in the different densities (2.65 g / cm3 for quartz, approx.2.3 g / cπ.3 for tridymite and cristobalite) and in the increased reactivity, e.g. for the hydrothermal production of sodium polysilicates.

The aim of various works is the production of cristobalite, which because of its white color and for controlling the coefficient of expansion is used primarily as a raw material and filler, for example in ceramic products, but also for the production of colors. Cristobalite is produced by converting quartz sands in a rotary kiln at temperatures of approx. 1500 ° C with the addition of alkali (Ullmann's Encyklopadie der Technischen Chemie, 4th edition, Verlag Chemie, Weinheim, Volume 21 (1982), page 442).

EP-A-0283933 describes the production of cristobalite from amorphous silicon dioxide with a specific surface area at a temperature between 1000 and 1300 ° C. The amorphous silicon dioxide has to be produced for this purpose and is already characterized by an increased reactivity. Alkali metal compounds containing lithium, sodium or potassium are proposed as catalysts for this reaction. These compounds are used in very small amounts since they then have to be removed from the cristobalite again by treatment at temperatures above 1300 ° C. The entire process is characterized by very long reaction times.

The phase change from quartz to cristobalite without addition of catalyst has been investigated in many works. Schneider et al (Materials Science Forum, 1_, (1986) page 91 ff) describe such a phase transition that takes several hours to days. The reaction rates are very much determined by the crystallinity of the quartz and by the proportion of impurities. The investigations clearly show that the conversion takes place via amorphous intermediate phases. Ibrahim et al (La Ceramica, (1985) page 19 ff) describe the reaction of quartz with activated silicates in the temperature range of 1350 ° C and 1500 ° C. Thereafter, mixtures of tridymite and cristobalite form within three days. There is no agreement in the literature as to whether tridymite is stable at all without impurities. Larger quantities of accompanying elements (alkali metals, aluminum) favor the formation of tridymite, which often has many structural defects in the crystal structure. According to Novakovic et al (Interceram, (1986) pages 29-30), the conversion to tridymite (1350 ° C, 114 hours, Li catalysis) first takes place via cristobalite, which converts to tridymite in a second step.

The object of the present invention was to provide reactive silicon dioxide phases from quartz sand which are distinguished by a very low quartz content. The reaction temperatures and especially the reaction times should be shorter or shorter than those of known processes.

The object was achieved according to the invention by a process for the production of reactive silicon dioxide phases, which is characterized in that quartz sand is mixed with an alkali metal compound or its aqueous solution, the alkali metal compound being selected from the group of compounds which are used in the Heating into the corresponding alkali metal oxides, that the molar ratio of SiO 2 to alkali metal oxide is between 1: 0.0025 and 1: 0.1 and that this mixture is heated to a temperature between 1100 ° C. and 1700 ° C.

If a molar ratio of SiO 2 to alkali metal oxide is mentioned here and also below, this means the molar ratio of the silicon dioxide contained in the quartz sand to the alkali metal oxide, based on the alkali metal compound used in each case. The reactive silicon dioxide phases obtained according to this process consist of cristobalite, tridymite, amorphous silicon dioxide and alkali metal silicate and are characterized by a small proportion of quartz, as determined by X-ray diffraction analyzes.

The reaction times decrease with increasing reaction temperature and decrease especially from 1300 ° C. At 1400 ° C. and a catalyst addition of 5% by weight sodium hydroxide, corresponding to a molar ratio of silicon dioxide to alkali metal oxide of 1: 0.0375, after a reaction time of 30 minutes no residual quartz was detectable in the reaction product of this example .

The reaction temperatures can be increased still further, as a result of which the reaction time, based on a specific catalyst, can be shortened further. Lowering the temperature to 1200 ° C leads to an increase in the residual quartz content.

The residual quartz content changes with variation of the catalyst content. For example, with a reaction time of 60 minutes and using sodium hydroxide as a catalyst, the residual quartz content increases from 0% by weight (silicon dioxide: sodium oxide = 1: 0.0375) to 2% by weight (silicon dioxide: sodium oxide = 1: 0) , 0038). When using potassium hydroxide, even lower quartz residues are found at lower reaction temperatures (see Table 1).

Molar ratios of silicon dioxide to alkali metal oxide of 1: 0.0035 to 1: 0.05 have proven to be particularly preferred for carrying out the reaction, for example an addition of 0.45% by weight to 6.45% by weight in the case of sodium hydroxide speaks. Applied to the addition of potassium hydroxide, this corresponds to 0.63% by weight to 9.0% by weight.

According to the invention, alkali metal compounds which convert to the corresponding alkali metal oxides on heating can be used as catalysts. These are in particular lithium, sodium or potassium hydroxide, as well as the carbonates, nitrates, nitrites, sulfates, sulfites, oxalates or formates of these alkali metals.

A particularly uniform distribution of the catalyst over the quartz sand is achieved by applying a 5 to 50% strength by weight aqueous solution or slurry of the alkali metal compound to the quartz sand. Particularly suitable concentrations of these solutions are between 15 and 25% by weight.

As detailed in the examples, the quartz sand is mixed with the appropriate amount of an alkali metal compound or its aqueous solution and annealed in a muffle furnace, rotary kiln or shaft furnace for a defined time. The use of rotary kilns is particularly recommended for carrying out the process on a larger scale.

As the examples show (Table 1), the residual fractions of quartz decrease with increasing temperature in relation to a specific reaction time. The effectiveness of the alkali metal catalyst increases in the series of lithium, sodium and potassium oxide.

In Example 1 (ac) the samples were annealed at a temperature of 1400 ° C. With the addition of 0.5% by weight of sodium hydroxide in the form of an aqueous solution, 90% of the quartz has already reacted after 15 minutes. With a catalyst amount of 5% by weight sodium hydroxide, no quartz is left in the samples after 30 minutes Find. With 0.5% by weight sodium hydroxide addition, the reaction is complete within one hour.

The reaction proceeds more slowly at a temperature of 1300 ° C. (example 2a-c), but even then, with a catalyst amount of 5% by weight, 97% of the quartz has reacted within half an hour.

If 1% by weight 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 3a-c) does not lead to a complete conversion of the quartz into reactive phases within 3 hours, but conversions of more than 80% are also achieved here.

A reaction temperature of at least 1300 ° C, i.e. in the range from 1300 to 1700 ° C. is therefore particularly preferred. For the purposes of the invention, it is further preferred that the process is carried out at reaction times of 10 to 180 minutes, in particular less than 60 minutes - i.e. at reaction times in the range from 10 to 60 minutes.

Example 4 (a-c) shows the particularly high catalytic activity of potassium salts. Even the addition of 0.7% by weight of potassium hydroxide in the form of an aqueous solution (corresponding to a molar ratio of silicon dioxide: potassium oxide = 1: 0.00375) results in higher conversions at a temperature of 1300 ° C. compared to additives reached by 5 wt .-% sodium hydroxide.

When solid sodium carbonate is added to the quartz sand, the degree of phase change is not as high as when aqueous sodium hydroxide solution is added, but the trend is same (example 5). The reason for this is the less uniform distribution of the catalyst over the quartz sand, compared to the use of solutions of the alkali metal compounds.

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

With the help of differential thermal analysis (differential scanning calorimetry, "DSC") the residual quartz content of the samples can be determined. The proportions of the reaction products in the individual samples can be estimated by means of X-ray diffraction analysis ("RBA") and DSC. It shows that cristobalite is formed in a first reaction step, although all reactions have been carried out below the thermodynamic stability temperature for cristobalite. Tridymite is formed in a second reaction step only with longer reaction times and preferably with large amounts of catalyst. Table 2 shows the ratio of cristobalite to tridymite in rough values. No difference is observed between the sodium and potassium-catalyzed reactions.

When the proportions of the reaction components are added using the DSC, it can be seen that at moderate degrees of reaction (quartz conversion of approx. 60%) considerable proportions of amorphous phases are present in the samples, for example 40% in Example 2c after a reaction time of 30 Minutes. These amorphous phases consist of alkali metal silicates and amorphous silicon dioxide. The proportions of alkali metal silicate are due to the alkali metal compounds involved in the reaction. The following examples are intended to illustrate the process according to the invention without, however, being restricted thereto.

Examples:

The quartz sand reacted contained> 99.9% silicon dioxide and was of natural origin. For the reactions that were carried out with solutions of the alkali metal compounds, the respective alkali metal compound was dissolved in just enough water that the quartz sand was covered with the solution. Then it was slowly dried and the dry sand was mixed vigorously. In reactions in which the alkali metal compound was not mixed with the quartz sand in aqueous solution, the two solid components were intimately mixed with one another before the reaction and heated over a period of between 10 and 180 minutes, preferably less than 60 minutes. A glazed alumina crucible served as the reaction vessel.

Temperatures and reaction times for the individual examples are shown in Table 1 below. In addition to the proportion by weight of alkali metal compound, Table 1 also gives the molar ratio of S1O2 to alkali metal oxide.

Table 1:

Ex. Temp. Remaining catalyst content of quartz No. (° C) addition, wt .-% (in wt .-%) after reaction time (SiO 2: alkali metal - 15 min 30 min 60 min oxide)

la 1400 0.5% NaOH (1: 0.0038) 10 5 2 lb 1400 1 NaOH (1: 0.0077) 3 2 0 lc 1400 5 NaOH (1: 0.0375) 3 0 0

30 min 60 min 180 min

a 1300 0.5% NaOH (1: 0.0038) 55 22 1 b 1300 1% NaOH (1: 0.0077) 37 7 1 c 1300 5% NaOH (1: 0.0375) 14 3 0

a 1200 0.5% NaOH (1: 0.0038)> 50> 30 16 b 1200 1% NaOH (1: 0.0077)> 50 29 15 c 1200 5% NaOH (1: 0.0375)> 50 15 8th

a 1300 0.7% KOH (1: 0.0038) 7 2 0 b 1300 1.4% KOH (1: 0.0077) 5 2 0 c 1300 7% KOH (1: 0.0375) 3 0 0

1300 6.6% Na ₂ CO 3 (1: 0.0375) 23 16

1300 8.9% Na Sθ4 (1: 0.0375) 24 10

1300 0.6% LiOH (1: 0.0077) 50 12 Table 2:

Ratio of cristobalite: tridymite in tempered quartz sands (estimate from RBA diagrams)

The reaction parameters are listed in Table 1

Claims

Claims 1. A process for the preparation of reactive silicon dioxide phases, characterized in that quartz sand is mixed with an alkali metal compound or its aqueous solution, the alkali metal compound being selected from the group of compounds which are heated to the corresponding alkali metal oxides go that the molar ratio of SiÜ2 to alkali metal oxide is between 1: 0.0025 and 1: 0.1 and that this mixture is heated to a temperature between 1100 ° C and 1700 ° C. 2. The method according to claim 1, characterized in that the re¬ active silicon dioxide phases consist of cristobalite, tridymite, amorphous silicon dioxide and alkali metal silicate. 3. The method according to claim 1 and 2, characterized in that the alkali metal compounds are selected in particular from the group lithium, sodium or potassium hydroxide, and the carbonates, nitrates, Ni¬ trites, sulfates, sulfites, oxalates or formates of these alkali metals. 4. Process according to claims 1 to 3, characterized in that the molar ratio of S1O2 to alkali metal oxide is between 1: 0.0035 and 1: 0.05. 5. Process according to claims 1 to 4, characterized in that the mixture is heated to a temperature of at least 1300 ° C. 6. Process according to claims 1 to 5, characterized in that the reaction time is 10 to 180 minutes, preferably less than 60 minutes.