The mineral composition of fireclay series is a majority of kaolin (Al2O3·2SiO2·2H2O) and 6%~7% of impurities (oxides of potassium, sodium, calcium, titanium and iron). The process of sintering fireclay series mainly includes continuous dehydration of kaolin and decomposition to mullite (3Al2O3·2SiO2) crystallization. During sintering process, SiO2 and Al2O3 in fireclay products will combine with impurities, form as silicate cocrystallization with low melting point, and surround mullite crystals. Fireclay series, belonging to weak acid refractory products, is able to resist erosion of acid slag and acid gas but with relatively poor resistance to alkaline material. In addition, fireclay series has good thermal performance and is capable of withstanding rapid cooling and heating.
According to differences of producing process and application fields, fireclay series is usually divided into normal fireclay product, low-porosity fireclay product and casting fireclay product in glass industry.
Due to a low refractoriness under load of fireclay series, it will be shrinked under high temperature. Its thermal conductivity is 15%~20% lower than that of silica series and mechanical strength is also worse than that of silica series. Therefore, fireclay series is mainly used in secondary parts of glass furnace, such as walls of regenerators, lining blocks of small flues, regenerator packing, bottom pave (fireclay blocks), etc. Besides, the series can also be used in metallurgy field.
The refractoriness of fireclay series with 1690~1730ºC is very similar to that of silica series, however, its refractoriness under load is over 200ºC lower than that of silica series. The reason is that besides of the content of mullite crystal which owns high refractoriness, fireclay series also has nearly half of amorphous glass phase with low melting point.
As the materials of producing fireclay series is easy to obtain, and with low requirements and process of production, therefore, the price of this series is relatively low. It is widely used in external structure of high temperature furnace and the parts within relatively good working environment.
Physical and Chemical Indicators
6.1 Normal fireclay product is the refractory material with Al2O3 content of 30%~48%. It is the most productive and widely used refractory material, because of its low price and properties of high temperature resistance, thermal insulation and chemical corrosion resistance.
According to different Al2O3 content, normal fireclay series can be divided into: first class with Al2O3 content more than 40%, second class with Al2O3 content more than 35%, and third class with Al2O3 content more than 30%. Moreover, their corresponding refractoriness are not less than 1730ºC, 1670ºC and 1610ºC respectively. Refractoriness under load are all between 1250ºC and 1450ºC. Thermal shock resistance are all good and with low prices.
According to physical and chemical indicators of China national standards, fireclay refractory materials can be classified into eight types: N-1, N-2a, N1-2b, N-3a, N-3b, N-4, N-5 and N-6. The physical and chemical indicators of them are shown in table 1.
Table 1: Physical and Chemical Indicators of Fireclay Refractory Materials
6.2 Porosity, a basic technical indicator of most refractory materials, is capable of impacting on all properties of refractory products, especially on strength, thermal conductivity, slag resistance and thermal shock resistance. In addition, according to differences of apparent porosity, low-porosity fireclay products can be named as DN-12 (AP≤12%) and DN-15 (AP≤15%).
The obvious characteristics of low poro