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Fireclay refractories are prepared from refractory clays whose principal constituent is kaolinite, Al2SO3.2SiO2.2H2O, with additions of ferric oxides, alkali and alkaline earth elements as impurities. The exact chemical composition of any particular clay deposit or sample is revealed by chemical analysis. The alkali metals (Na, K) can be removed or reduced by washing the clay.

The most characteristic properties of clays important for brick making are plasticity, binding capacity (workability index) and sintering capacity. Plasticity ensures that particles can slip relatively to one another and at the same time the bonding between them is retained. Binding capacity of clays, i.e., capacity of combining non-plastic or inert materials into a uniform plastic mass, to a great extent depends on their plasticity.

It has been noted that clays lose their plasticity on drying at 110⁰C but restore this property upon addition of water. The plasticity lowers with increasing temperature and disappears entirely on heating above 450⁰C. Thus fireclay can be calcined to set a non-plastic material called chamotte or grog. Again, clays are sintered into a stone-like crock when heated up to 800⁰C. Hence, brick can be produced from them.

The mixture for making fireclay brick is made up of refractory clay, chamotte and crater. The clay is preliminarily dried, finely ground and screened to obtain a

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fraction under 0.5 mm (500µm). The calcined clay (chamotte) is also grouped-up and screened.

Refractory clay shrinks substantially on roasting, which results in cracking. The shrinkage of fireclay bricks is diminished by the addition of chamotte or grog, which is a lean substance. The smaller the particles of clay, the better they can envelop the larger particles of the leaner, which is needed to obtain a mass of a good moulding ability. The proportion of clay and chamotte in moulding mass is determined by the plasticity of the clay. (Mark, 2007).

According to Gilchrist (1977), each kind of brick is made from a different raw material and its treatment usually involves some very special features. However, for general principles, the following procedures are involved in the process of brick production.

2.15.1 Identification and collection of raw materials. The raw materials for the production of refractory bricks are mineral deposits – clays, sand, ores, and rocks – which must be mined or quarried and then crushed. The materials if imported, is usually bagged to maintain purity.

2.15.2 Crushing. Crushing is carried out by simple ore-dressing equipment and materials are graded and stored by size. The composition of the brick is adjusted by blending the materials along with any flux or bond additions that may be required.

Blending is however; not only by composition but also by size and the properties

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of the product depend very much on this stage. Low porosity can be attained by using a “close packing grading” such that interstices between the largest particles are filled by a smaller grade and residual interstices filled by a smaller grade again.

Thus, there is some control on the ultimate porosity and on strength, for the maximum number of contacts will also be made with the closest packing.

In basic bricks the coarse and fine fractions are sometimes of different composition and the fine fraction ultimately acts as the bond. “Grog”, or prefired materials, may also be mixed in at this stage. This may consist of broken and crushed scrap brick but sometimes specially hard fired material is prepared for the purpose.

2.15.3 Blending

Blending is carried out in a paddle mill with a kneading action where addition of water and bonds (sometimes temporary) are added. Clays attain some plasticity at this stage. Fireclay bricks can be moulded by plastic or semi-dry method. With the plastic (soft mud) method, the moulding mass may have moisture content of 16 to 25 percent depending on plasticity and sintering capacity of the clay. Also, the moulding mass is made up of 50 to 60 percent chamotte and 50 to 40 percent kaolinite clay. The content of chamotte is taken at the upper limit if the refractory clay used has a good plasticity. The mixed mass is pressed in a mould at a pressure of 295-590N/cm² into raw bricks. The shrinkage of bricks on firing is 3-4%

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Gilchrist (1977) added that hand-moulding is most successful with plastic mixes usually rather wet (14-20% water), which can be “thrown” into the wooden box-type mould and relied on to fill it. He also noted that it’s cheaper than machine-moulding on a jobbing basis, but the machine is preferred for mass production.

Another forming process is slip casting, applicable mainly to clays which can be formed into a colloidal suspension with water and poured into a mould of plaster of paris which absorbs the water and causes a uniform deposit of clay to build up in the inside of the mould. This is useful for awkward shapes and for hollow wares. It is also used for special refractories which are prepared from very fine powders.

2.15.4 Drying

Bricks must be dried. This is conducted either on large drying floors (heated by waste heat from kilns) where the bricks are laid out in open array, or in tunnel kilns where they are stacked on bogeys and passed through a tunnel against a stream of hot air. This is faster and more compact process but cannot readily be adopted where sizes and shapes are not fairly constant (Gilchrist, 1977). Drying is done to eliminate the hygroscopic water and increase green strength of bricks.

2.15.5 Firing

The final temperature of firing does not exceed 1400⁰C. the firing schedule should be carefully planned. It is recommended that a low heat be applied first to the green bricks in order to drive off any residual moisture – a process known as water

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smoking. Heating and soaking (holding at temperature for some time) should be applied to allow complete heat diffusion and phase transformations. Once heating is discontinued, the bricks should be allowed to cool with the furnace/kiln to avoid rapid cooling rate (Mark, 2007).