Sintered Fireclay Refractories

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Introduction of Sintered Fireclay Refractories

fireclay refractories

Fireclay refractories refer to weakly acidic refractory products with less than 44% Al2O3 content. It can resist the erosion of acid slag and acid gas, and the resistance to alkaline substances is slightly worse. Fireclay refractories have good thermal properties and are resistant to rapid cold and heat.

The variety of clays and manufacturing techniques allows the production of numerous brick types appropriate to particular applications. The usefulness of fireclay refractory bricks is largely due to the presence of mineral mullite, which forms during firing and is characterized by high refractoriness and low thermal expansion.

Classification of Fireclay Refractories

There are five standard classes of fireclay refractories. These classes are:

  • Super duty bricks.
  • High duty bricks.
  • Medium duty.
  • Low duty.
  • Semi silica.
fireclay refractories
super duty fireclay refractories

Super Duty Fireclay Refractory

  • SiO2 Content 49%-53%.
  • Al2O3 Content 40%-44%.
  • Other Content 5%-7%.
  • Working Temperature 1740 °C – 1760 °C.

Super-duty fireclay refractories have good strength and refractoriness. They have good volume stability at high temperatures and an alumina content of 40 % to 44 %. Super-duty bricks have superior resistance to cracking or spalling when subjected to rapid changes in temperature. Their refractoriness, in terms of their PCE values, may not be less than 33.

There are several possible modifications in the super-duty fireclay bricks, including bricks fired at temperatures several hundred degrees higher than the usual product. The high-temperature firing enhances the high-temperature strength of the brick, stabilizes their volume and mineral composition, increases their resistance to fluxing, and renders them practically inert to disintegration by carbon deposition in atmospheres containing carbon monoxide gas.

high duty fireclay refractories

High Duty Fireclay Refractory

  • SiO2 Content 55%-60%.
  • Al2O3 Content 35%-40%.
  • Other Content 5%-9%.
  • Working Temperature 1690 °C – 1740 °C.

High-duty fireclay refractories are used in large quantities and for a wide range of applications. Because of their greater resistance to thermal shock, high-duty fireclay bricks can often be used with better economy than medium-duty bricks for the lining of furnaces operated at moderate temperatures over long periods but subject to frequent shutdowns. The PCE value of the high-duty brick may not be less than 31.5 and usually varies from 31.5 to 33.

medium duty fireclay refractories

Medium Duty Fireclay Refractory

  • SiO2 Content 60%-70%.
  • Al2O3 Content 26%-36%.
  • Other Content 5%-9%.
  • Working Temperature 1630 °C – 1690 °C.

Medium-duty fireclay refractories are appropriate in applications where they are exposed to conditions of moderate severity. These bricks, within their serviceable temperature ranges,  can withstand abrasion better than many bricks of the high-duty class. Medium-duty fireclay bricks have PCE values in the range of 29 to 31.

low duty fireclay refractory

Low Duty Fireclay Refractory

  • SiO2 Content 60%-70%.
  • Al2O3 Content 23%-33%.
  • Other Content 6%-10%.
  • Working Temperature 1500 °C – 1590 °C.

These refractories find application as backup bricks for bricks with higher refractoriness. They are used for services where relatively moderate temperature prevails. The PCE values of low-duty fireclay bricks cover the range of 15 to 27-29. Low-duty fireclay refractories are mainly used for fireplaces, and chimneys, and are also used as backup bricks for bricks with higher refractoriness.

semi silica fireclay refractory

Semi-silica Fireclay Refractory

Semi-silica fireclay refractory is a kind of quartz clay brick, which is an aluminous silicate refractory product with an alumina content of 15-30%. It is generally made of refractory clay containing quartz sand, chlorite clay, or tailings of kaolinite beneficiation as raw material. Its refractoriness is 1670 ℃, and the load softening point is not less than 1250 ℃.

Semi-silica fireclay refractories have better resistance to rapid cold and heat than silica bricks, and the volume stability is better than silica bricks. It is mainly used as the refractory layer of steel drum lining and steel pouring system. In addition, it can also be used for hot furnace roofs, heat storage chamber lattice brick, iron furnace lining, various kiln furnace bottom and flue, and other parts.

Application of Fireclay Refractories

The application of fireclay bricks is influenced by several other properties in addition to the refractoriness.  These properties are dimensional accuracy, crushing strength, porosity, and refractoriness under load. Machine-pressed, fired fireclay refractories are used for many applications. The stress on the materials differs widely. For special applications, It is customary to manufacture bricks that are tailored to meet specific requirements. Fireclay refractory bricks are used in the steel industry in coke oven batteries, blast furnaces, hot blast stoves, and various other furnaces used in the steel industry.

FIREclay refractories applications

Fireclay Refractories Features and Advantages

  • High refractoriness.
  • High temperature is endurable.
  • Good corrosion resistance.
  • Good spalling resistance and wear resistance.
  • Good thermal shock resistance.
  • High mechanical strength.
  • High-temperature creep rate is low.
  • Good volume stability at high temperatures. 

The Firing Process of Fireclay Refractory

Room temperature to 200: At this time, the heating should not be too fast to avoid cracking the green body. When the tunnel kiln is fired, the temperature of the first 4 parking spaces should not exceed 200°C.

200-900°C: Heating should be accelerated at this stage. It is conducive to the chemical reaction of organic matter and impurities in the green body.

During the period of 600-900 °C, a strong oxidizing atmosphere should be maintained in the kiln to avoid becoming a waste product.

900°C to firing temperature: The high-temperature stage should be heated stably, maintain the oxidation atmosphere, make the green body heated evenly, and prevent the brick from cracking. Since the high temperature is above 1100 °C, the sintering shrinkage is very strong, and the shrinkage rate reaches 5%, so it is very important to maintain the temperature gradient slowly and eliminate internal stress.

Cooling stage: According to the change of the crystal lattice of the product cooling stage, the high-temperature stage above 800-1000 °C should be cooled rapidly, and the cooling rate should be eased below 800 °C, and the actual cooling rate used in actual production will not bring the risk of cooling to the product.

process of fireclay refractories
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