CS248912B1 - Mass for cordierite burning aids - Google Patents

Abstract

The material is intended for the production of firing aids for firing ceramics. It consists of a magnesium oxide carrier /MgO, MgCO^, talc/ and a grinding stone of such a composition that it stoichiometrically complements the MgO composition of cordierite. The latter is then formed by the reaction of the fine grinding stone fraction with the MgO carrier. The fine grinding stone fraction must be in excess in relation to the MgO carrier. Extrusions from the material can be strengthened with temporary binders.

Description

The invention relates to a material for cordierite burning aids.

Cordierite firing cups for firing ceramics, such as plates and cases, are characterized by sufficient mechanical strength and high resistance to sudden temperature changes due to the low coefficient of thermal expansion of cordierite and thus of cordierite-containing ceramic materials.

Until now, cordierite firing aids have been made from materials of classical ceramic composition - that is, plastic components of kaolin or clay, aluminum oxide and grit of various grain sizes; the magnesium oxide required for the formation of cordierite is usually introduced into the materials with talc, sometimes also with carbonate or magnesium oxide.

In the case of using talc, the addition of alumina is necessary. Due to the limited reactivity of these starting components, the reaction does not occur in stoichiometric proportions, a limited amount of cordierite is formed in addition to the glass phase and the remains of unreacted components. The resulting low-viscosity glass phase does not allow the operational use of the necessary firing temperature, because at temperatures above 1,350—1,380 °C these materials become pyroplastic and are deformed even by the action of small forces.

The glass phase remains present in these products even after firing and causes further deformation even with repeated firings at application temperatures. The listed negative effects also deteriorate other properties of these materials, especially resistance to temperature changes.

These main shortcomings, i.e. deformation of the materials during production firing and repeated firings during use and the content of undesirable phases at the expense of cordierite, which reduce resistance to temperature changes, are eliminated according to the invention by the fact that the material is made up of 25 to 40% by weight of magnesium oxide carrier and 60 to 90% by weight of mullite aggregate with a ratio of silica to alumina in the case of using talc of 0.6 to 0.8:1 and in the case of using magnesite or magnesium oxide of 1.3 to 1.5:1.

Coarser fractions of the ash above 60 µm may have a different composition, that is, a different ratio of silicon dioxide to aluminum oxide, because they do not participate in the reaction.

Depending on the magnesium oxide carrier, kaolin enriched with silica or alumina fired to temperatures above 1,400 °C can be used as a grinding medium.

If, for handling reasons, it is necessary to increase the strength of the moldings immediately after molding or after drying, it is possible to use known temporary binders such as polyvinyl alcohol, cellulose derivatives, starches, dextrin, sulfite liquor, molasses and others, usually in amounts up to 3%.

Compared to the classic ceramic materials used so far, materials composed in this way also have the advantage that they do not become airy during pressing.

The main advantages of such composite materials are the fact that they are highly intimate and more reactive mixtures, because fine grinding stone is more energetically rich than unfired mixtures of kaolinite with low-reactive alumina or silica.

When firing such materials, there is no accumulation of a glassy phase. If it does form, it immediately reacts to crystalline cordierite, i.e. to a solid phase again. In addition, these materials allow the use of such a high content of shot that its grains, which are not pyroplastic, lean against each other and form reinforcement. The reactions then take place mainly in the spaces between the shot grains.

Therefore, it is possible, and with regard to the formation of cordierite, appropriate to use higher production firing temperatures of 1,410 °C, preferably 1,420 to 1,430 °C, which will create a higher proportion of cordierite. This results in improved ceramic-technological properties - increased strength and reduced coefficient of thermal expansion. Limited presence of glass phase in fired products

then it also guarantees less deformation even under load during subsequent firings during the use of firing aids and their use at temperatures higher than 1,280 °C, which is the limit temperature of use for common materials.

Example 1

Talc EK - I is used as a carrier of magnesium oxide. Then it is necessary to use a grinding stone with a SiO ₂ : Al ₂ 0 ₃ ratio of approximately 0.7. Such a grinding stone is the material produced in SKZ - Děčín, namely SO - 55, which contains 55% Al ₂ O ₃ ^{and 40} * ^Si0 2' ^For the selected final composition of the material: 50% grinding stone, 50% cordierite, the mass has the following composition:

talc EK - I ................21%

SO - 55-40.06mm..............29%

SO - 55 0.06 - 1.5 mm........50 *

To increase the strength of the moldings, 3% dextrin is added above 100% and the mass is moistened to the required pressing moisture of 5 i. .

After firing at 1,420 °C, the resulting material has a coefficient of thermal expansion of ¢^.20-800 2/8-1° 6.K ¹ and a flexural strength of 25 - 30 MPa.

Example 2

Talc EK - I is used as a carrier for magnesium oxide, and SO - 55 as a grinding stone. For the selected final composition of the material: 80% grinding stone and 20% cordierite, the mass has the following composition:

talc EK-I................... 8.6%

SO - 55 < 0.06 mm.............11.4%

SO - 55 0.06 - 1.5 mm....... 80.0%

To increase the strength of the moldings, 3% dextrin is added above 100% and the mass is moistened to the required pressing moisture of 5%.

After firing at 1,420 °C, the resulting material has a coefficient of thermal expansion <0 20-800 3.3.10 ⁶ .K ¹ and a flexural strength of 15 - 18 MPa.

Example3

Magnesite is used as a carrier of magnesium oxide. In this case, it is necessary to use a grinding stone with a SiO ₂ : AL ₂ O ₃ ratio of approximately 1.4. Such a grinding stone is fired shale from the Nové Strašecí ČLUZ, which contains 40% Al ₂ 0 ₃ and 55% S10 ₂ · For the selected final material composition: 50% grinding stone and 50% cordierite, the mass has the following composition:

magnesite......... 15.9% schist <0.06 mm..............34.1% schist 0.06 - 1.5 mm........50.0%

The strength is increased by adding 3% polyvinyl alcohol. The molding compound is moistened to 4% moisture.

After firing at 1,420 °C, the resulting material has a coefficient of thermal expansion ^20-800 ” of 2.6.10 ^-6 .K ^{_1 and} a flexural strength of 15 - 20 MPa.

As can be seen from the examples given, in order to achieve the desired properties of the resulting material, it is necessary to subordinate the type of magnesium raw material used to the composition of the mullite aggregate used.

The grain size of the used shot can be determined by the method of production. However, it is necessary to maintain the content of the shot fraction <0.06 mm resulting from the selected final composition of the fired material, i.e. from the content of the magnesium raw material with which this fraction reacts to form cordierite.

Claims (1)

SUBJECT OF THE INVENTION Mass for cordieritic firing aids, characterized in that it is composed of 5 to 40% by weight. % magnesium oxide carrier and 60 to 95 wt. mullite slag with a silica to alumina ratio in the case of using talc of 0.6 to 0.8: i and in the case of using magnesite or MgO of 1.3 to 1.5: 1.