Grog (clay)

As fireclay ( f ) (regional and in Austria exclusively Fireclay (m.) ) are referred to in common parlance refractory bricks and brick.

In technical terms is with fireclay only the one described in this article, rock -like, man-made, refractory material with 10 to 45 percent alumina content referred (Al2O3 ), but not other refractory materials. The word flock moth is said to have been formed in the 18th century by Italian porcelain workers in Thuringia ( sciarmotti, scarmotti ) with reference to " Schärm " or " Sharm ", the Thuringian pronunciation for " shards ".

It consists of:

  • Normal fire clay: Al2O3 content between 30 and 45 %
  • 3.1 raw Materials
  • 3.2 design
  • 3.3 Sintering of fireclay
  • 4.1 everyday applications
  • 4.2 Technical Applications
  • 5.1 Iron and Steel Material Specification 917

Properties

Fireclay is very inexpensive compared to other refractory bricks, therefore, the use takes place whenever it is possible by the chemical and thermal conditions. Rather low loads are typical. The material has no special fire-resistant or chemical properties.

Critical and therefore technically important are particularly strong CO-containing reducing atmospheres at temperatures below 800 ° C, as the Boudouard equilibrium then tends to form elemental carbon ( CO 2 ⇌ CO2 C). This carbon is deposited in the pores of rocks and leads to attrition of the structure. However, the activation energy for this reaction is not usually sufficiently below 800 ° C. Free iron oxide or iron acts catalytically, however, so the iron content must be kept low. This fireclay is not suitable as a lining for the smelting of iron.

The specific heat capacity is quite high at 1.00 kJ / (kg K) for masonry, roughly the same as concrete or plaster.

Location in the binary system SiO2 -Al2O3

In this excerpt from the binary system can be seen that the melting point of the compounds is steadily increasing. With the increase of the melting point with increasing Al2O3 content, other properties of the finished stones change:

  • The strength increases
  • The pore volume is reduced
  • The thermal shock resistance increases

It should be noted that at 30 percent of Al2O3 is at a maximum melt phase and thus be present other characteristics than expected. It thus enters a waste of good properties. From 30 percent of the good properties rise again.

Mineral phases

  • Mullite 25-50 %
  • Glass 25-50 %
  • Quartz and cristobalite to 30 %

The aim is to maximize the amount of mullite. At low Al2O3 contents, this is difficult to achieve. This results in inferior qualities of high amounts of SiO2 ( acidic fireclay). A high quality firebrick (higher application temperature) is characterized by the highest possible proportion of Al2O3 to form as much mullite 3 Al2O3 · 2 SiO2.

History

Ferdinand Didier, founder of Didier-Werke, was one of the first entrepreneurs in Germany who established fireclay. He owned a brick and lime kiln in Podejuch, later Szczecin district Podjuchy. He discovered in 1849 at Podejuch natural occurrence of quartz pebbles and quartz sand, which he used for the production of refractory materials, known as grog or chamotte. The fire was used in particular for the emerging Gaswerksbau.

Production

Raw materials

The actual raw material for the firebrick as the final product is sound. One speaks of fireclay, as long as only sound was used in the batch. However it can not be made directly from pure tone of the fire of the stone, as a massive variety in firing shrinkage occurs, which would make the finished stone little dimensionally stable and above all very susceptible to cracking.

In practice, chamotte grain is produced therefore first. These raw clay is plastic recycled, crushed and dried. Thereafter, it is fired to the maximum mullite, and then crushed and / or ground and fractionated, the intermediate product into coarse, medium and fine grains. This fireclay grain is now combined with further raw clay raw material for the final product, wherein the raw clay is added used both for binding the refractory grit and to complement of the structure.

The clay used for both steps must be as pure as possible. With bad qualities, a relatively large proportion of quartz to be included in the raw materials, thus you can then restore only the low quality acidic fireclay. It is also advantageous to have a high proportion of kaolinite in the clay as possible, since this brings no alkali.

Additions of periclase ( MgO) cause a cordierite formation, which strengthens the thermal shock resistance ( TSR ) due to the lower CTE, but the temperature resistance slightly decreases.

Shaping

Preparation is classic ceramic by producing casting slip, plastic mass or press granulate. Here, the ratio used chamotte grain to sound is different in each case. The pre-fired chamotte grain is no longer plastic, with complex shapes you need, therefore, a higher clay content.

The design itself takes place by slip casting, extrusion by extrusion press, dry pressing. The drier is the mass in the shaping, the better the refractory properties of the finished stone.

With decreasing water content in the preparation, the following properties change:

  • Density increases
  • Porosity decreases ( optimum at about 20%)
  • KDF ( cold crushing strength ) increases
  • T05 ( DE / printer switches) increases (higher application temperature)
  • TWB ( thermal shock resistance ) increases
  • Dimensional stability increases
  • Firing and drying shrinkage decrease

A dry-pressed brick is thus superior to the plastic molded or slip cast stones with respect to the refractory properties. It differs only from other manufacturing processes, where the form is to be created not be reached by dry pressing.

Sintering of fireclay

The aim is a maximum mullite under the assumption that all of the alumina participates in the raw material to the formation of mullite. This is done at temperatures between 1000 ° and 1400 ° C. First Schuppenmullit forms, from the forms at higher temperatures mullite needles. Through the needle-like shape of the mullite needles, the individual mullite crystals entangled with each other and obtain the strong bonding in the stone.

There is a need to balance:

  • Long burn time, maximum mullite, reaching a maximum heat resistance
  • Short burn time, save on energy costs
  • Carefully at high quartz content Jump heating due to modification

Application

Everyday applications

  • Stoves
  • Fireplaces
  • Pizza stones
  • Heat storage mass in electric building heating
  • For unambiguous identification of the ashes of the deceased in a cremation fire bricks are used with engraved number.

Technical Applications

  • Permanent linings of higher quality stones
  • Internal coating of containers, in which molten iron and molten iron is transported

Classification of firebricks

Classification from 1962 ( dated but still in use )

Here, the chemical composition is used as a classification feature, regardless of the characteristic features, which may be independent of the chemical composition.

Iron and Steel Material Specification 917

The Al2O3 content is to be regarded as a guide only, primarily for the classification is compliance with the minimum / maximum property values.

Boudouard reaction Masonry heater
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