Chamotte

from Wikipedia, the free encyclopedia
Fireclay in a night storage heater

As a chamotte f. (regionally and in Austria the Schamott m. ) are generally used to refer to refractory bricks and brickwork.

In technical terms , chamotte is only used to describe a stone-like, artificially produced, refractory material with a proportion of 10 to 45 percent aluminum oxide (Al 2 O 3 ), but not other refractory building materials.

The word chamotte is said to have been formed in the 18th century by Italian porcelain workers in Thuringia ( sciarmotti , scarmotti ) with reference to “Schärm” or “Scharm”, the Thuringian pronunciation for “ shards ”.

A distinction is made according to the Al 2 O 3 content:

  • sour chamotte: 10 to 30%
  • Normal chamotte: 30 to 45%.

properties

Fireclay is cheaper than other refractory bricks . It is usually only able to withstand low mechanical loads and is protected from mechanical damage on the side facing away from the fire, usually by sheet metal, tiles, clinker or other resistant materials.

When burning coal and wood, there is often a CO -containing, reducing atmosphere at temperatures below 800 ° C, at which the Boudouard equilibrium tends to form elemental carbon (2 CO ⇌ CO 2 + C). This carbon is deposited in the pores of the stone and causes the structure to wear down. However, the activation energy for this reaction is usually insufficient below 800 ° C. However, free iron oxide or iron has a catalytic effect , so the iron content must be kept low. This means that chamotte is not well suited as a lining for smelting iron ore.

The specific heat capacity (ability to store heat) is 1.00 kJ / (kgK) for masonry, roughly on par with concrete or plaster, while the thermal conductivity is 0.8 W / (mK) at 100 ° C and 1 W / (mK) at 1000 ° C is rather low. The melting point is around 1,780 ° C with an aluminum oxide content of 30%.

The water absorption should be less than 7%. The linear thermal expansion coefficient is temperature dependent and at room temperature is about 10 · 10 -6 1 / K and decreases at 1000 ° C to about 1 × 10 -6 1 / K from.

Two- component system SiO 2 - Al 2 O 3

The horizontal line at almost 1600 ° C is at the level of the eutectic point . Below this temperature, the entire entire melt has solidified. Between this line and the curve above there is a mixture of liquid and solid phase, since Al 2 O 3 crystallizes out first with an Al 2 O 3 content above the eutectic point . This reduces the Al 2 O 3 content in the melt, so that it approaches the composition at the eutectic point. With further heat emission, this eutectic composition crystallizes out at a constant temperature until the melt has completely solidified. The upper curve marks the point at which the mass has completely melted.

With the content of alumina

  • the melting point of the mixture increases,
  • the strength increases,
  • decreases the pore volume and
  • the resistance to temperature changes increases.

Mineral phases

The goal is usually to maximize the amount of mullite. This is difficult to achieve with low Al 2 O 3 contents. This results in high proportions of SiO 2 (acidic chamotte) with inferior qualities . A high quality fireclay brick (higher application temperature) is characterized by the highest possible Al 2 O 3 content in order to form as much mullite 3 Al 2 O 3 · 2 SiO 2 as possible.

history

The oldest manufacturer of refractory products in Germany are the Wolfshöher Tonwerke ; founded in 1856 by Lorenz Wolf. As a brickyard owner, he supplied the gas works in Nuremberg, among other things, in the middle of the 19th century. Here he got to know refractory stones for the first time. These were needed to a large extent for the construction of furnaces, but until then they had to be obtained from England at very high prices. Since he had often come across traces of refractory clay on his way to Nuremberg, which is needed for the production of refractory materials (so-called chamotte or chamotte), he recognized his opportunity and founded a chamotte production on the Rollhofener Höhe.

Manufacturing

raw materials

The raw material for the firebrick are clay minerals . One speaks of chamotte as long as only clay was used in the offset. However, the green bodies cannot be formed from pure clay, as this would result in enormous firing shrinkage, which led to cracks and deformations when the stones were fired.

Fireclay grain is therefore first produced. For this purpose, the raw clay is plastically processed, roughly chopped and dried. Then it is burned until the maximum formation of mullite, then broken or ground and the intermediate product is fractionated into coarse, medium and fine grains by sieving. This chamotte grain is then mixed again with a certain proportion of unfired raw clay, which is used to bind the chamotte grains and to supplement the structure.

The clay used should be as pure as possible. Plain clay usually contains a relatively large proportion of quartz , which results in an inferior quality acidic chamotte. The highest possible proportion of kaolinite in the clay is an advantage, since it does not contain any alkalis .

Additions of periclase ( MgO ) cause cordierite to form , which strengthens the thermal shock resistance (TWB) due to the lower CTE , but slightly lowers the temperature resistance.

Shaping

The shaping takes place through the production of casting slip, plastic mass or pressed granulate . The ratio of chamotte grains to clay varies. The pre-fired chamotte grains are not malleable. The production of complex shapes therefore requires a higher proportion of clay.

Shaping is done by slip casting , extrusion using an extrusion press or dry pressing. The drier the mass is when it is shaped, the better the refractory properties of the finished brick.

The lower the water content of the raw mass, the more

  • higher the bulk density,
  • lower the porosity (the optimum is approx. 20%),
  • higher the KDF (cold compressive strength),
  • higher the pressure softening T 05 (higher application temperature),
  • higher the thermal shock resistance TWB ,
  • higher the dimensional accuracy and
  • lower the burning and drying shrinkage.

A dry-pressed brick is superior to plastically formed or slip-cast bricks in terms of refractory properties. The latter manufacturing processes are therefore only used if the desired shape cannot be achieved by dry pressing.

Sintering of chamotte

The aim is to achieve maximum mullite formation on the assumption that all of the aluminum oxide in the raw material takes part in the mullite formation. This happens at temperatures between 1000 and 1400 ° C. First, flaky mullite forms, from which needle mullite is formed at higher temperatures. The needle-like shape allows the mullite crystals to become entangled, thereby solidifying the stone.

There is a tradeoff between

  • long burning time and maximum mullite formation to achieve maximum heat resistance, as well
  • short burning time and saving energy costs.

High quartz contents require careful heating (due to modification changes).

application

A firebrick used in a cremation

Everyday applications

Technical applications

  • Internal coating of containers in which metal is melted or liquid metal is transported
  • Lining of furnaces (possibly also for the backing of high quality refractory material )

Classification of the firebricks

designation Al 2 O 3 Sailing cone number Max. Application temperature
A0 > 42% 34 1400 ° C
AIS 40-42% 33-34 1350-1400 ° C
AI 37-40% 33 1300-1350 ° C
AII 33-37% 32 1250-1300 ° C
AIII 30-33% 30th 1200-1250 ° C
designation SiO 2 Sailing cone number Max. Application temperature
BI approx. 78% 32-33
BII approx. 72% 30-31
BIII approx. 67% 28-29

Classification from 1962 (outdated, but still in use)

Here, the chemical composition is used as a classification characteristic, regardless of the property characteristics, which can be independent of the chemical composition.

Steel-iron material sheet 917

variety Al 2 O 3 content Fe 2 O 3 content Bulk density Open porosity KDF DFB a
[%] [%] [g cm −3 ] Standard dev. [%] Standard dev. N mm −2 Standard dev. x min [° C] Standard dev.
A40t > 40 <2.5 > 2.15 0.05 <20 2 > 30 15th 20th > 1450 40
A40h > 40 <2.5 > 2.10 0.05 <21 2 > 25 15th 15th > 1420 40
A40p > 40 <2.5 > 1.90 0.06 <26 2 > 15 10 10 > 1380 40
A35t 35-40 <2.5 > 2.10 0.05 <19 2 > 30 15th 20th > 1400 40
A35h 35-40 <2.5 > 2.05 0.05 <20 2 > 25 15th 15th > 1380 40
A35p 35-40 <2.5 > 1.90 0.06 <25 2 > 15 10 10 > 1350 40
A30t 30-35 <3 > 2.05 0.05 <19 2 > 30 15th 20th > 1370 40
A30h 30-35 <3 > 2.05 0.05 <20 2 > 25 15th 15th > 1350 40
A30p 30-35 <3 > 1.90 0.06 <24 2 > 15 10 10 > 1320 40
A25t <30 <3 > 2.05 0.05 <18 2 > 35 15th 25th > 1340 40
A25h <30 <3 > 2.05 0.05 <19 2 > 30 15th 20th > 1320 40
A25p <30 <3 > 1.90 0.06 <22 2 > 20 10 15th > 1300 40

The Al 2 O 3 content is only to be regarded as a guideline value; compliance with the minimum / maximum property values ​​is paramount for the classification.

literature

  • Paul Werner: The refractory industry. Hartleben, Vienna 1911 ( chemical-technical library . Volume 334)
  • Gerald Routschka, Hartmut Wuthnow (Hrsg.): Refractory materials. 4th edition. Vulkan, Essen 2007, ISBN 3-8027-3157-3
  • Ceramic refractory materials. Stahleisen, Düsseldorf 1984 ( steel-iron material sheets of the Association of German Ironworkers. 917)

Individual evidence

  1. ^ Meyer's Large Conversation Lexicon 1905
  2. ↑ The boiling point is sometimes given as over 9,000 ° C. However, this value is questionable, since compounds with the highest boiling point such as tungsten (IV) carbide boil at around 6000 ° C and Al2O3 at 3000 ° C. See also the entry Discussion: Fireclay # Alleged boiling point of over 9000 ° C from user: Eheran  on the discussion page .
  3. ↑ Thomas Hermann Funke: Temperature and voltage calculations for the analysis and optimization of the heating and cooling phases when firing firebricks . P. 96. Dissertation, 2007, urn : nbn: de: hbz: 464-20080117-153630-3 .
  4. ↑ explanations of users: Eheran  on the talk page .
  5. ^ History. In: Wolfshöher Tonwerke. Retrieved on October 23, 2019 (German).