Optimum Particle Ratio Range for Burnt Magnesia Bricks
Production practice shows that expanding the critical particle size of sintered magnesia from 2mm to 3mm will improve the forming performance of the material and the thermal shock stability of the product. Therefore, the particle part in the particle composition, that is, the 3~0.5mm part, is controlled at 30~50%, and it is advisable to be close to 50%. Wherein particle size is 3～2mm part, it is advisable to be controlled at 18～20%.
The fine powder part in the batching, namely <0.088mm part is generally controlled at 35～42%.
The intermediate particles in the ingredients can improve the normal temperature strength and high temperature strength of the product, and reduce the porosity. Intermediate particles are generally controlled at about 10%.
To determine the particle composition of the ingredients, it should be based on the specific conditions of production, the degree of sintering of raw materials, molding pressure, firing temperature and other process parameters. The degree of sintering is good, the molding pressure is high, the firing temperature is high, and the fluctuation range of the particle ratio of each level can be appropriately relaxed. In addition, an appropriate amount of waste bricks or fired waste products of the same type can be added to the ingredients.
Preparation of mud for sintered magnesia bricks
The sintered magnesia crushed to meet the particle size requirements is placed in a wet mill according to the proportion of the ingredients, and the sulfurous acid pulp waste liquid with a density of 1.2-1.22g/cm3 is added as a binder, and the addition amount is 3-4%. Then add magnesia fine powder, mix 15～30min. Generally, the moisture content of the mud is controlled at 2 to 3%. Sometimes MgCl2 can also be used as a binding agent. When MgCl2 is used as the binder, the strength of the green body is very high after drying.
If the CaO content in the raw material is high, it is necessary to trap the material. Trapped material temperature 10 ~ 20 ℃, trapped material time 3 ~ 4d.
adobe
Forming magnesia bricks should be carried out under high pressure. Usually, a friction brick press of more than 300t is used for molding, and the bulk density of the adobe is controlled at 2.90~3.00g/cm3. It can also be formed by hydraulic press.
Key points of molding operation: put the mud into the model, spread the material evenly, flatten the four corners, and start to pressurize. First light and then heavy, the number of times of pressing varies depending on the size and shape of the adobe, generally 3 to 12 times. In production, the bulk density of adobe is usually controlled by controlling the unit weight and external dimensions of adobe.
drying and firing
The drying temperature of the adobe should not be too high, generally below 120°C. Drying is usually carried out in a tunnel dryer. The hot air inlet temperature is 100~150°C, the outlet temperature is 60~90°C, and the residual moisture is controlled at 1.5~0.6%.
Firing should be carried out in a weak oxidizing atmosphere. In the low temperature stage, the elimination of structural water is accompanied by volume effect, which can cause cracks in the green body, so the heating rate should not be too fast. After 1300°C, the liquid phase begins to appear, the strength increases, and the heating rate can be accelerated; when it is close to the firing temperature (1550~1600°C), the liquid phase increases, the strength decreases, and the heating rate slows down.
During the cooling process, the temperature can be rapidly lowered before the green body is solidified. After solidification, the “plasticity” of the green body disappears. In order to avoid cracks, the cooling rate should be properly controlled. Below 800℃, it can cool down quickly.
