DE4242610A1 - Mixt. for high value silicon carbide prods. - comprises silicon carbide, dextrin soln., alumina, corundum, amorphous silicic acid and refractory binding clay - Google Patents

Abstract

The new mixt. (I) comprises: (a) 70-75% SiC of grain size less than 4 mm; (b) 3.5-4.5% of a 20-25% aq. dextrin soln.; and (c) a dry binder contg. 13.5-16% alumina of grain size less than 0.05 mm, 5.5-7% precious corrundum of grain size less than 0.25 mm, 3.5-4% amorphous silicic acid of grain size less than 0.05 mm, and 2.5-3% refractory binding clay of grain size less than 0.5 mm. Prodn. of (I) is also claimed. USE/ADVANTAGE - (I) can be used for mullite-bound SiC products, pref. as thin-walled auxillary combustion aids

Description

The invention relates to a mixture for mullite-bound SiC products containing 70 to 75% SiC and a process for producing these products with a mullite-forming binders based on Al ₂ O ₃ and SiO ₂ . These products are preferably used as thin-walled kiln furniture.

SiC-containing products, in particular for use as kiln furniture, can be described in terms of their binding in
to silicate,
mullitgebundene
and nitride-bonded or recrystallized SiC products.

This results in different conditions in Substantive and procedural different Quality properties and fields of application.

Although nitride-bonded or recrystallized SiC products from this series require the highest effort and the have the highest utility value, this use value, For example, the very high hot bending strength in the Practice insufficiently exploited. It is therefore the tendency to record the utility value of the more cost-effective constantly increase mullite-bound SiC products. Mullite-bound SiC products are classified according to the type of their Mullite formation distinguished.

One possibility is for the required Mullite formation a the stoichiometric ratio of Mullits corresponding proportion of SiC used after the reaction equation

SiC + 2 O ₂ → SiO ₂ + CO ₂ ,

to oxidize to silica, according to the reaction equations

3 Al ₂ O ₃ + 2 SiO ₂ → 3 Al ₂ O ₃ · 2 SiO ₂ ,
2 Al ₂ O ₃ + SiO ₂ → 2 Al ₂ O ₃ .SiO ₂ ,

the desired and binding-producing mullite formation achieve. For this Mullitbildung are firing temperatures of well over 1400 ° C required.

Technical solutions for this type of mullite formation are described in patents DD 2 96 800 and DD 2 99 465. To be able to carry out the product firing and thus the mullite formation at the lowest possible temperatures, it is known from the patent DD 2 96 800 that activated alumina, oxide oxide and hydroxide containing alumina are used as binders. These phases show in the transition to the oxide form in conjunction with the resulting by the burning of SiC SiO ₂ in the product fire a high responsiveness to mullite formation. The firing temperatures are above 1400 ° C.

In contrast, patent GB 2 99 465 describes a binder which effects mullite formation in the form of a suspension whose solid consists of alpha-Al ₂ O ₃ and / or its hydrates, which preferably has a particle size of less than 20 μm and dry mechanical less than 0.5 mm prepared binding kaolin contains. The advantage of this solution is that due to the high activity of the suspension and its excellent distribution possibility, an optimal reaction with the SiO ₂ formed by oxidation of the SiC to mullite in the product fire (PK 148) takes place.

In order to minimize the burnup (oxidation of the SiC to SiO ₂ ), it is known from the patent DD 2,949,931 to add SiO ₂ in the form of pyrogenic silica (so-called FeSi dust) and / or silicic acid or else as fine-grained Si. The SiO ₂ reacts at the sintering temperature (PK 146-156) with the finely ground clay to form mullite, wherein the finely ground alumina having an alpha-Al ₂ O ₃ - content of between 85 and 95% through the remaining content of transition aluminas is so reactive that a good Mullitausbildung is guaranteed at sintering temperature. The fine-grained Si is oxidized to SiO ₂ during sintering. Although the burnup of the SiC is largely avoided by this method, this is at least 2.7% (Example 5), but mostly over 10%.

Furthermore, a solution is described in the patent DD 2 96 268 and in its prior art, in which the mullite is caused exclusively by the addition of Al ₂ O ₃ and SiO ₂ , ie without burning of the SiC. Hereafter, previously activated alumina, amorphous SiO ₂ , slaked kaolin, organic binder and hexametaphosphate are present as individual components of the binder in two separate process steps (suspension consisting of water, dextrin, hexametaphosphate, activated calcined clay, FeSi dust, and dry binder of activated calcined clay, slaked kaolin and refractory binding clay) in the total offset. The prior art advancement described in patent DD 2 96 268 involves the use of a uniformly composed binder (sprayed grain) incorporated as both a suspension and a dry binder. Although the product firing is carried out at 1300 ° C to 1600 ° C, it can not be ruled out that burning occurs at higher temperatures.

The binders responsible for mullite formation are known to consist of SiO ₂ and Al ₂ O ₃ . These materials are, unless the SiO ₂ is obtained by burning of SiC, used in various forms and modifications. For example: (calcined) alumina, partially in activated form as oxide, hydrated oxide, hydroxide, (leached) kaolin, clay (binder clay), silicic acid (FeSi dust, silica sol). In addition to these main constituents, further additives are added to the binders which fulfill their function as dispersing, grinding or activating auxiliaries. The binders known hitherto also differ with regard to their pretreatment and their introduction into the offset.

It has already been mentioned that binders based exclusively on Al ₂ O _{3 are} subjected to dry or wet mechanical activation or fine grinding, which likewise effects partial activation. The binder is then used as a suspension and / or spray grain. This procedure also applies to binders which additionally contain SiO ₂ . Either the SiO _{2 is} already used in an activated form or it is also subjected to activation.

All these methods adhere to the disadvantages that cost, Time and energy consuming processes for the production of Binders are required.

The SiC used is used as a mixture of granules, predominantly with a defined grain spectrum used. Also known from the patent DD 2 96 800, the grain mixture before the addition of the binder water and sulfite waste liquor add. This addition is required in order for the non-plastic presses required moisture and adhesive strength too receive. The product firing subsequent to the shaping becomes crucial on the type and extent of mullite formation certainly. A predominantly oxidative atmosphere and high Firing temperatures cause burnup of the SiC used.

The invention is based on the problem, on the one hand high quality SiC products, especially thin-walled ones Products with a high mullite content and lowest Glass phase and thus as a result with a high Dry flexural strength and hot bending strength and on the other cost, time and energy consuming Processes in the manufacture of such products avoid. Also dry mechanical meal and Activation processes leading to high dust pollution lead, be turned off.

According to the invention, the problem is solved by the in the characterizing part  the claims disclosed disclosed means.

For the successful realization of the invention is the Consistent compliance with the material and procedural conditions of more crucial Meaning. Minor deviations cause one sudden deterioration of the positive effects of solution according to the invention.

For example, patent DD 2 96 800 describes SiC mixture directly water and sulfite waste liquor add. Other variants are described. According to inventive solution, however, in combination with the other conditions the exclusive addition of the aqueous Dextrin solution to SiC grain possible. Other variants in the Meaning of the patent DD 2 96 800 are in the inventive Solution is not possible, as it does not have the positive effects be achieved.

Also had to be overcome by the invention of the contradiction be that to achieve a high Mullitgehaltes the Addition of Bindeton, especially for the hot bending strength reducing glass phase formation results as low as possible is. But a low content of binding clay brings again as a disadvantage, a small green and Dry bending strength, so that as a result of a high Cost-causing breakage arises or the Production of thin-walled products is not possible. Also, it is extremely surprising for a person skilled in the art that the Addition of dry, fine binding clay less than 0.5 mm in small shares at the end of the offsetting sufficient mixing - without complex mixing processes - leads and has an excellent "adhesive power".

To the substantial progress of this invention prove a comparison of the product parameters compared to those represented by the prior art Products are shown:  

For repeated use, for example in a porcelain Quick firing furnace, is another improvement of Product parameters, which are based on the progress of the Mullitization is due. Values after approx. 50 uses:

Christobalite (%) 1-2 Mullite (%) 13-15 Corundum (%) 4-5 OP (%) <15 HBF 1400 ° C (MPa) 15-25 KBF (MPa) 35-40

It was also found that the use of spray grain according to patent DD 2 96 268 with an associated Improvement of the flowability the production of the least Shards of the compacts are not relieved. Rather, this is due to the process of the invention achieved high green and dry flexural strength (5-6 MPa responsible.

The result clearly proves that when adding only 3% of binding clay a maximum amount of mullite at the lowest Glass phase arises and together with a high TBF a high  HBF is achieved, the previously known methods and Is superior to offsets.

The invention will be described in more detail in 3 examples be explained.

The production of the products takes place in a 3-stage process. In the 1st stage, the mixture of the SiC grain with the dextrin solution is mixed in an Eirich mixer. In a separate process in a vortex mixer, the preparation of the dry binder of alumina, aluminum oxide and SiO ₂ . This dry binder is added in a second stage of wet SiC grain and mixed again. Finally, in the third stage, the addition of the dry Bindetones, wherein after mixing a compressible mixture is formed. The unplastic mixture is pressed with a pressure of 70 MPa and fired after drying at 1380 ° C with a holding time of 6 hours in an oxidizing atmosphere.

The achieved product parameters are in the table above shown.

Claims (2)

1. Mullite-bound SiC products containing 70 to 75% SiC, consisting of a SiC grain, an organic binder and mullite-forming binders based on Al ₂ O ₃ and SiO ₂ , characterized in that the mixture

• - 70 to 75% SiC with a grain size of less than 4 mm
• - 3.5 to 4.5% (based on the dry mixture) of a 20 to 25% aqueous dextrin solution
• - a dry binder consisting of
• - 13.5 to 16% clay with a grain size of less than 0.05 mm
• - 5.5 to 7% corundum with a grain size of less than 0.25 mm
• - 3.5 to 4% amorphous silicic acid with a grain size of less than 0.05 mm
• 2.5 to 3% refractory binding clay with a grain size of less than 0.5 mm
  consists.

2. A process for producing mullitgebundener SiC products containing 70 to 75% SiC and with a mullite causing binders based on Al ₂ O ₃ and SiO ₂ and using an organic binder, characterized by compliance with subsequent material and process technology Conditions:

• - 70 to 75% SiC with a grain size less than 4 mm are first mixed with 3.5 to 4.5% (based on the dry mixture) of a 20 to 25% aqueous dextrin solution
• - Addition of a dry binder mixture consisting of
• - 13.5 to 16% clay with a grain size of less than 0.05 mm
• - 5.5 to 7% corundum with a grain size of less than 0.25
• - 3.5 to 4% amorphous silica with a grain size of less than 0.05 mm, followed by homogenization
• - Submixing of 2.5 to 3.5% refractory binding clay
• - Forming the free-flowing mixture under a pressure of greater than 70 MPa with subsequent drying and
• - Product firing at 1380 ° C with a holding time of 6 hours in an oxidising atmosphere.