DE1792551A1 - Process for the production of synthetic wollastonite - Google Patents

Description

RHEINISCHE LIMESTWORKS 1792551

G. M. B. H. September 18, 1968

Wülfrath / RhId.

7experienced_for_ the_production_of_synthetic full astonite

The present invention relates to a process for the production of synthetic wollastonite CaOSiO ₂ from lime and silica.

Solid astonite, in particular the naturally occurring ß-solid astonite, is increasingly finding its way into the ceramic industry, as its chemical and physical properties improve ceramic bodies and glazes. The advantageous properties of solid astonite are particularly evident in wall tile ceramics and electrical ceramics. A ceramic raw mass of, for example, 70 % solid astonite and 30 % clay allows so-called "fired" products to be produced, in which the glaze is already applied to the blanks and bonded to the base mass in a firing process. Solid elastonite reduces the porosity and increases the contraction and dielectric strength of electroporcelain, so that it is possible to manufacture particularly low-loss insulators. Full aetonite is also very suitable for glazes. With the addition of full elastonite, the glazes remain permeable to the gases given off by the broken glass and glaze almost until the end of the fire and only melt tightly at the end of the fire.

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A wide use of wollastonite in ceramic However, industry has hitherto been opposed to the relatively high price of the natural material. There are therefore a number of procedures have already become known, according to which wollastonite is obtained from the raw materials or from suitable waste materials by melting them together. However, these processes have the disadvantage that the raw materials at temperatures above the formation temperature of wollastonite must be heated for a long time to free lime or free silica in the reaction product to avoid. Intensive previous grinding of the raw materials shortens the burning time on the one hand but on the other hand considerable fine grinding costs.

These disadvantages are avoided according to the present invention, through the production of calcium hydro-

Silicates as intermediates.

It has been found that it is very easy to make synthetic wollastonite by hydrothermal conversion of lime and silica in approximately equivalent amounts and subsequent dewatering of the Hydrosilicates to ß-wollastonite.

The preferred temperature range for the hydrothermal reaction is between 170 and 200 ^° C., with the water pressure dropping at this temperature

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is being worked on. These temperature and pressure conditions can easily be achieved using the hardening kettles commonly used in the sand-lime brick industry. During the hydrothermal reaction, xonotlite and tobermorite are essentially formed, which split off water when heated and easily convert to finely crystalline β-wollastonite ^{at around 800 ° C.}

As starting materials for the process according to the invention, burnt lime and quartz sand are expediently used, the quartz sand advantageously being used as fine grain in order to achieve a smooth and rapid conversion. The lime should contain at least 80% CaO. Of course, instead of quicklime, slaked lime can also be reacted with silica, but it is better to use the heat generated during the slaking process to heat the lime-silica mixture for the hydrothermal reaction.

According to an advantageous variant of the method of the invention the hydrothermal conversion of the starting materials to calcium hydrosilicates with simultaneous crushing or grinding of the reaction mixture carried out. It has been sawn that in this way a much faster one Reaction is achievable and that you can therefore use normal coarse-grained quartz sand. The hydrosilicate layers on the quartz grains are made by the during

-Λμ.

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BATH ORIGINAL

the implementation taking place bald removed, so that the Reaction is accelerated considerably. The Mahlarbbii 'istt / significantly less than a pre-grinding of the quartz, since the hydrosilicates are considerably softer than quartz sand.

The hydrothermal hardening with simultaneous grinding of the reaction product can according to the invention in a simple Way in a steam pressurized mill, e.g. a tube mill.

For some areas of application, the ß-wollastonite obtained by dewatering the hydrosilicates is only suitable to a limited extent because it is too fine-grained. Therefore, this beta-wollastonite is then heated to produce a coarsely crystalline wollastonite to temperatures above the <A / ß transformation point, which is at about 1200 ⁰ C, preferably, wherein it is transformed into a relatively short time at <* wollastonite. This σ (wollastonite can then by heating to temperatures around 1200 ⁰ C or by slow cooling in this temperature range are converted in the known coarse crystalline beta-wollastonite.

The dehydration of the hydrosilicates to ß-wollastonite, its conversion into C ^ -wollastonite and finally the regression to ß-wollastonite is expediently carried out in one operation in a rotary kiln. It is advantageous if the reaction mixture expediently mineralizers known per se, such as boric acid, even before the hydrothermal reaction,

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BADORLGiNAL

Alkali carbonates or calcium phosphate are added in amounts between 0.5 to 1%. These mineralizers accelerate in particular the regression of the ß-wollastonite from the cX modification.

The method according to the invention is based on the drawing with an exemplary embodiment on a technical scale ^ explained:

The SiOp components, for example quartz sand, from the silo 1 and quicklime from the silo 2 are in a weight ratio of 1: 1 with the addition of small amounts (approx. 0-1%) mineralizers from the silo 3 via metering elements 4 and a container 5 designed as a mill autoclave 6 abandoned. Water and steam are introduced into the closed "autoclave mill". The reaction process is conducted in such a way that the starting components are completely converted into a mixture of calcium hydrosilicates. The reaction process is initiated by the heat generated when the lime is extinguished. The reaction temperature in the “autoclave mill” ¹ is reached and maintained by additional heat input, for example by steam or other heat transfer media. The simultaneous grinding process creates the prerequisite for rapid conversion of the lime-silica mixture because the quartz grain forms the hydrosilicate reaction layer is repeatedly removed by abrasion. In this way it is possible in proportion

-6-

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BATH ORIGINAL

a mixture of calcium hydrosilicates for a short time obtain.

The temperature, pressure, circumferential speed, grinding media filling and grinding time of the "Autokl4.vmühle" depend on the grain size and reactivity of the SiOo component. After completion of the autoclave process, the lamb-shaped calcium hydrosilicate mass is collected in a container 9 and conveyed in the form of a sludge by means of a conveying device 10 to a consistency regulator 11. From there the set to the required water content of sludge on the transport path 12 passes into the storage silo 13 x Upper the metering element 14 enters the sludge in the rotary kiln 15. In the firing process, the mass is heated up to 1200-1400 ⁰ C and then 1 ".To 2 .. * Frited hours. At about 1200 ⁰ C an intensive solid-state reaction takes place, which leads to the formation of O ^ -Wollastonit. By targeted cooling in the cooler 16 with previously added mineralizers, the C *-wollastonite can be converted into β-wollastonite. The firing process can also be carried out in tunnel ovens 15 & or in chamber ovens 15b. Of course, Biee requires the sludge-like calcium hydrosilicate mass to be dewatered, for example in a filter 11a.

- patent claims -

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Claims (6)

1. A process for the production of synthetic wollastonite CaO-SiO ₂ from lime and silica, characterized in that lime and silica are first converted hydrothermally to calcium hydrosilicates in approximately equivalent amounts, whereupon the hydrosilicates are dehydrated to β-wollastonite. In that the hydrothermal reaction to calcium hydrosilicates at temperatures of 170 2. The process according to claim 1, characterized in that - 200 ⁰ C. 3. The method according to claims 1 or 2, characterized in that that the hydrothermal conversion to calcium hydrosilicates with simultaneous comminution or Grinding of the reaction mixture takes place. 4. The method according to claims 1 to 3 »characterized in that the obtained by dehydration from the hydrosilicates ß-wollastonite is converted into <Λ-wollastonite by heating to temperatures above the transformation temperature. 5 × The method according to claim 4, characterized in that the c £ wollastonite is converted by heating to temperatures around 1200 ⁰ C and by slow cooling in this temperature range in beta-wollastonite. 6. Process according to claims 1 to 5, characterized in that mineralizers known per se are added. 109808/1689 Le e rs e ite