DE4338159A1 - Binder for mullite-bonded SiC-containing products and process for their manufacture - Google Patents

Abstract

It is an object of the invention to propose a versatile binder using generally available and inexpensive raw materials while avoiding ineffective process steps with a simultaneous increase in the mullite content and a significant rise in the hot flexural (bending) strength HFS at 1400 DEG C. This object is successfully achieved by the binder raw materials used being subjected to a combination of wet mechanical activation, spray drying and subsequent comminution. The subsequent comminution is carried out to at least 60% < 0.1 mm, of that at least 40% < 0.063 mm. The ratio of Al2O3 to SiO2 of the raw materials used is 3...4 to 1 and is to be adhered to strictly. The binder of the invention is used for producing a wide variety of high-value products.

Description

The invention relates to a binder and a Process for its manufacture. This binder takes place Use in the production of mullite-bound SiC-containing Products in different areas of application.

Such binders differ in their Composition, pretreatment of the raw materials used and the type of insertion into the offset.

From the patent DD 2 96 800 A7 it is known that that calcined alumina in a binder Wet drum mill with the addition of carboxymethyl cellulose and Sulphite waste liquor is subjected to wet activation. At this activation process form on the surface of the Primary particles from hydrate phases, which in the transition to the Oxide form in connection with the resulting SiO₂ Oxidation of SiC in a later product fire high reactivity in the formation of mullite in the Show matrix.

The extraction of the aluminum oxide and the resulting Hydrate phases from the suspension take place according to known Drainage process (spray drying).

In connection with the task for the invention a hot bending strength HBF at 1400 ° C of greater than 80 kp / cm² reached. No information is given on the mullite content. In the further development of this process is in the patent DD 3 01 635 A8 described a solution, after which the Spray drying is dispensed to with an aqueous Composition of the binder as a suspension material advantages of the high-purity bond to transfer complicated moldings. In both cases the product fire takes place at or above 1400 ° C to get the required SiO₂ by the SiC burnup.

Both patents have disadvantages. On the one hand is  the binder on the use of calcined alumina limited and on the other hand is an energy intensive one Product fire associated with at or above 1400 ° C an uncontrolled SiC burn.

To keep the product fire as low as possible, so only a defined proportion of SiC by oxidation in SiO₂ is transferred is a method in patent DD 2 99 465 A7 described that a suspension, whose solid from alpha-Al₂O₃ and / or their hydrates exists, which preferably has a grain size less than 20 microns has and mechanically processed less than 0.5 mm Bindekaolin is used.

The advantage is that by using the wet mechanical activation reaction kinetic advantages occur when used as a binder suspension remain. As a result, already at mullite formation occurs at lower firing temperatures. The disadvantage is that the fire is still above 1400 ° C is carried out and is associated with an SiC oxidation. About the decisive product parameters (mullite content and HBF) the patent does not contain any information.

In order to largely reduce the SiC burnup, the patent DD 2 94 931 A5 proposed, additionally pyrogenic Silicic acid in the form of the product of the Dust collector for ferrosilicon production (so-called FeSi dust) and / or silica sol. The SiO₂ reacts with the at sintering temperature (PK 146 to 156) finely ground alumina to mullite.

Although the SiC is largely due to the dense binding matrix is protected against oxidation Embodiments a very different SiC erosion (example 5: 2.7%, example 1: 13.7%) by the high product fire temperatures is due. Although The patent does not mention high HBF quantitative information.

With the goal of intensive mullite formation, a high To achieve HBF and thermal shock resistance TWB, according to DD 3 01 604 A7 are those for mullite formation  required components alumina and silica already in highly active form introduced into the SiC grain mixture. Of the The addition of the finest, highly active silica saves you otherwise necessary decomposition of SiC to form SiO₂ and thus also allows a firing temperature that is below the Decomposition temperature of SiC is.

The disadvantage is that the binder in two different Compositions (activated calcined alumina, FeSi- Dust, dextrin, hexametaphosphate, water or activated calcined alumina, kaolin, binder clay) and in two various forms as a suspension and as a dry binder is used.

The one achieved by this method is surprising Mullite content of 13-15%. As expected, the HBF is at 1300 ° C greater than 30 MPa.

In contrast, the patent DD 2 96 268 A5, in largely identical in chemical and material terms DD 3 01 604 A7, a process for the production of mullite-bound refractory products with increased Use value based on SiC with a wet activated Binder made from calcined alumina, slurried Kaolin and / or refractory binding clay, FeSi dust and / or silica sol, organic binder and hexamethylene phosphate consists.

After a joint wet activation, the obtained Dehydrated suspension in a spray dryer. The emerging Secondary grain has a predominant grain diameter of 0.2-0.5 mm. For further processing of this spray Part of the binder is whisked into a suspension and homogenized with the SiC grain mixture and in the next stage the remaining part of the dry spray grain mixed in.

The product is fired at 1300 to 1600 ° C with a Holding time of 4-10 hours.

The mullite content is 5-8% and the HBF at 1400 ° C after the product fire 7.4-9.8 MPa.

This process does not bring any decisive improvement the HBF compared to the patent OD 3 01 604 A7 (without Spray drying):

The patent proceeds from the knowledge of this fact DE 42 42 610 A1 a completely different way and does without an activation of the feed materials for the binder and spray drying at the same time.

The dry binder used, consisting of

• - 13.5 to 16% alumina with a grain size of less than 0.05mm
• - 5.5 to 7% corundum with a grain size less than 0.25mm
• - 3.5 to 4% amorphous silica with a grain size smaller 0.05mm

is added to a moist SiC grain. By using highly active and high-purity binder Activation is not required for raw materials. With this binder offset, surprising Product parameters achieved:

Mullite content: 11-12%
HBF at 1400 ° C: 12-14 MPa

The disadvantage is that only high quality and expensive Raw materials must be used.

The invention is based on the problem, a versatile applicable binder for the production of high quality SiC Products with a high mullite content and a high HBF hot bending strength with minimal SiC erosion propose and at the same time on the use of highly active Binder raw materials and therefore the use generally more available and cheaper raw materials enable.

At the same time, the unproductive form of using the Avoid binder as a suspension and dry binder.

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

For the successful implementation of the invention Combination on the one hand of the material and on the other hand the procedural conditions of importance.

After the product fire using the The binder according to the invention becomes the following quality values receive:

Mullite content: 18-20%
HBF at 1400 ° C: 18-20 MPa.

These values improve after approx. 50 furnace runs in a porcelain rapid firing furnace with 22-24% mullite content and 21-23 MPa HBF at 1400 ° C. These values are significantly higher than those of the patent DE 42 42 610 A1.

According to the knowledge from the prior art DD 3 01 604 A7 without using spray drying received high quality SiC product. The application of the Spray drying in patent OD 2 96 268 A5 leads under comparable conditions to an essential Deterioration in quality values. Hence one would Experts had expected that this trend would also adjusts the binder according to the invention. This The surprising result can therefore only be combined Effect of the features of the invention can be attributed.

The well-known difficulties of effective Shredding of such systems, for example that Baking on the grinding balls or the completely inadequate Shredding of the "hardened" by dextrin Secondary particles do not occur according to the invention. The disadvantageous and time-consuming addition of Binder both as a suspension and as a dry binder according to DD 2 96 268 A5 and DD 3 01 604 A7. In both The separate addition is mandatory for patents, because when stirring the necessary total amount of binder a suspension for the mixture produced from it high water content arises, which is essential for this quality absolutely necessary plastic compaction with high Would exclude pressing (DD 2 96 268, page 2, line 24 to 27).

The invention is based on two exemplary embodiments are explained in more detail.

example 1

First, 1% sulfite waste powder is left in  dissolved in hot water. Then stir in 21% calcined alumina with a grain size <0.05 mm and 6% amorphous silica with a grain size <0.01 mm. The exact Dosage of the ratio of A1₂O₃ to SiO₂ (3... 4 to 1) is vital. The suspension contains about 1/3 of water. The pumpable suspension becomes a wet mechanical one Activated in a wet drum mill. Now spray drying takes place, resulting in a spray grain with a grain size <0.5 mm, the main part is between 0.2 and 0.5 mm. The moisture of the spray grain is <2%. This spray grain is now one Subsequent shredding. The grain size is then at least 60% <0.1 mm, of which at least 40% <0.063 mm. This binder is processed further after known technologies.

70% SiC grain size is placed in a mixing unit, 3-4%, based on 100% dry matter, an aqueous Dextrin solution admixed and 27% of the invention Binder added in the 3rd stage. Finally, the Mix in 3% refractory binding clay. After pressing and drying, the product is fired at 1380 ° C and a holding time of 6 hours. The products produced as kiln furniture have following quality values:

Mullite content: 19.5%
HBF at 1400 ° C: 19.8 MPa.

After 50 runs in a porcelain rapid firing furnace the following values are measured:

Mullite content: 23.5%
HBF at 1400 ° C: 22.9 MPa.

Example 2

Analogously to Example 1, a binder is used using 21.5% calcined alumina and 5.5% amorphous silica manufactured. Using this binder becomes an article obtained, which has the following properties:

Mullite content: 18.5%
HBF at 1400 ° C: 19.0 MPa

After also 50 runs in a porcelain The following values are measured for rapid firing furnaces:

Mullite content: 23.0%
HBF at 1400 ° C: 22.5 MPa.

Claims (2)

1. Binder for mullite-bound SiC-containing products consisting of calcined alumina, SiO₂ and aqueous auxiliaries, characterized in that the binder in a combination of wet activated, spray-dried and to at least 60% <0.1 mm, of which at least 40% <0.063 mm, comminuted form, the ratio of Al₂O₃ to SiO₂ as 3rd . . 4 to 1 is used. 2. Process for the preparation of a binder for mullite-bonded SiC products consisting of calcined Alumina, SiO₂ and aqueous auxiliaries, thereby characterized in that the offset has a ratio of Al₂O₃ to SiO₂ as 3. . . 4 to 1 a combination of wet mechanical activation, spray drying and Post-shredding to at least 60% <0.1 mm, of which at least 40% <0.063 mm.