Clay bricks

Clay brick is a refractory product with AL2O3 content of 30%~48% produced from refractory clay. Clay bricks are widely used in many thermal equipment such as blast furnaces, hot blast furnaces, steel drum linings and steel pouring systems. Its production accounts for more than half of the total production of refractory materials. Clay bricks are divided into varieties such as ordinary clay bricks, multi-clinker clay bricks, whole raw clay bricks, and high-siliceous clay bricks. 
Ordinary clay bricks refer to varieties with Al2O3 content between 36%~42%, with high yield and wide use. Ordinary clay bricks are made of soft clay with strong plasticity and large dispersion and some clay clinker. It is broadly divided into clay-clinker bricks and kaolin clinker bricks. Their main high-temperature performance is refractory, 1750 °C for special grades, 1730~1750 °C for first-grade products, 1670~1730 °C for second-grade products, and 1580~1670 °C for third-grade products. 
Multi-clinker clay bricks refer to products containing more than 80% clinker and less than 20% combined clay. Due to the fact that most of the clinker clay bricks are in a barren state, they are not easy to deform in the manufacturing process, which can ensure the overall size of the brick and make the fired products have ideal bulk density, mechanical strength, thermal shock resistance and high refractoriness. 
Whole raw clay bricks, also known as clinker-free clay bricks, are made of hard clay or pyrophyllite with low plasticity, weak dispersion, and little shrinkage. Because this raw material inherently binds less water, although it is given a certain amount of external water after crushing, it can still maintain the original particle composition, so no additional clinker can be added, and the finished product will not shrink because there is no clinker. 
The main raw materials for highly siliceous clay bricks are barren clay (calcined hard clay, kaolin or pyrophyllite) and natural quartz sand. According to different types of raw materials, high siliceous clay bricks can be divided into quartz-kaolinite high silica bricks and quartz-clay high silica clay bricks. The highest content of SiO2 in the former can reach 75%~80%, the refractory degree is greater than 1710°C, and the apparent porosity is generally greater than 25%. The latter is made of clay containing quartzite or in sintered hard 
Quartz sand is added to the clay to prepare. The refractory degree of the product is low, generally between 1610~1700 °C. The high siliceous clay bricks are stable in volume at a certain temperature, and do not produce excessive volume expansion or contraction after long-term use, and the load softening temperature is also higher than that of ordinary clay bricks, which is very suitable for masonry regenerators or steel barrel linings. 
The production of clay bricks mainly includes five processes: crushing and crushing, batching and mixing, moulding, drying and firing. Its moulding process has plastic method and semi-dry method, as follows. 
(1) Crushing and crushing. The clay clinker is coarsely crushed into lumps less than 50mm, and then crushed into aggregates of all levels of 5~2mm, and the sieve feeds into the mill to grind into a fine powder of less than 0.5mm. The combined clay is generally dried, then coarsely crushed, crushed, and finely ground into a fine powder of less than 0.5mm, which is made into a mixed fine powder with clinker. 
(2) Ingredients and mixing. The ratio of mud includes clinker, binding clay, water and admixtures between mass and particle ratio. The amount of clay and water added varies depending on the moulding method. Plastic moulding, the ratio is, clinker: clay is 7~5:3~5, moisture 16%~20%; Semi-dry moulding, the ratio is, clinker: clay is 9~7:1~3, moisture 3.5%~8.0%. All kinds of granules are added to the mixing equipment in order, and various binders are added to make a uniform mud. 
(3) Molding. The bulk density of clay products is 2.0~2.1g/cm3 when moulded by plastic method, and 2.1~2.4g/cm3 when moulded by semi-dry method. 
(4) Drying. It is required that the residual physical moisture content of the dried brick is less than 2%. 
(5) Firing. Firing takes place in an inverted flame kiln or tunnel kiln. Firing is broadly divided into four stages: 
1) Normal temperature to 200°C: At this time, the temperature should not be too fast to prevent cracking. When firing in a tunnel kiln, the temperature of the first No. 4 parking space should not exceed 200°C. 
2) 200~900°C: The temperature rise at this stage should be accelerated to facilitate the chemical reaction of organic matter and impurities in the billet. During the period of 600~900 °C, a strong oxidation atmosphere should be maintained in the kiln to avoid "black-hearted" waste. 
3) 900°C to 1380°C: The temperature rise should be stable in the high temperature stage, continue to maintain the oxidation atmosphere, make the green body evenly heated, and also prevent the bricks from cracking. Since the sintering shrinkage is very strong at high temperatures above 1100°C, with a shrinkage rate of up to 5%, it is important to keep the temperature gradient gentle and eliminate internal stresses. The fire-stopping temperature of clay products is generally higher than the sintering temperature of 100~150 °C, if the sintering temperature range of the sintered clay used is narrow, the fire-stopping temperature is appropriately lower, and it is more suitable to be about 50~100 °C. The firing temperature of clay products should ensure that the combined clay is fully softened, so that the reaction with the surface layer of clinker fine powder and coarse particles is fully carried out, so as to achieve the purpose of binding clinker particles and making the products obtain appropriate strength and volume stability. The firing temperature is generally 1250~1350 °C. If the content of Al2O3 is high, the firing temperature of the product should be appropriately increased, about 1350~1380 °C. The sintering holding time is generally 2~10h, so as to ensure that the reaction in the product is sufficient, and the quality of the product is consistent. 
4) Cooling stage: According to the change of the crystal lattice of the product in the cooling section, the cooling rate should be slowed down quickly in the high temperature stage above 1000 °C, and the cooling rate should be slowed down at 1000~800 °C. In fact, in the actual production, the actual cooling rate will not cause the risk of cold cracking to the product.