Magnesia is an alkaline refractory raw material made from magnesite through electric fusion or high-temperature sintering or magnesium oxide extracted from sea water or salt lakes. It can be mainly divided into three major types, namely fused magnesia, light burnt magnesia, Sintered magnesia. Perclase crystals are the main mineral composition of magnesia, and MgO is the main chemical component of magnesia. The content of MgO and periclase and their crystallinity determine the quality of magnesia.
my country’s magnesia is mainly produced from magnesite, which is composed of magnesium carbonate (MgCO3). After processing, it is called magnesite. This material can only be used after being melted or sintered in an electric furnace, rotary kiln or shaft kiln. Its reaction formula is as follows:
When designing cementless corundum spinel castables, the selection of magnesium oxide raw materials must be considered, which is one of the key factors affecting the performance of refractory castables. Due to the hydration of MgO in magnesia, cracks will form in the material and the fluidity will decrease rapidly. This is the main problem that occurs in the construction, hardening and drying stages of refractory castables; factors affecting the hydration of MgO and the formation of spinel There are the purity, particle size and activity of magnesia. At the same time, the addition amount of magnesia also plays an important role in the performance of the castable. If the addition amount of magnesia is relatively low, the castable will produce various forms of cracks due to firing shrinkage at high temperatures, which will affect the resistance of the refractory castable. The slag performance will be adversely affected; on the contrary, if the addition amount of magnesia is relatively high, the amount of spinel generated in the castable at high temperature will increase with the increase of MgO content, and the accompanying volume expansion will increase. Excessive volume expansion produced during this process will produce higher apparent porosity inside the refractory castable, and at the same time reduce the strength of the castable sample. This will prevent the stress in the refractory castable from being released in a high temperature environment, and This will easily cause the material to swell, causing damage in the form of varying degrees of peeling.
The relationship between the amount of magnesia added and the density and porosity of corundum spinel castables was studied. It is believed that the addition of magnesia can reduce the porosity of the castables after low-temperature drying and increase their density; after high-temperature heat treatment, The spinelization of the castable material is violent, which makes the material structure loose, the volume expansion increases, the porosity increases, and the density and strength decrease.
Research shows that when the magnesium-aluminum ratio is close to the theoretical composition of MA 0.42, the volume density of the burned sample is larger, the linear change rate is smaller, the strength and thermal shock stability are the best, and the overall performance of the castable sample is the best at this time. .
Foreign studies believe that in aluminum-magnesium castables, when the mass ratio of MgO to Al2O3 is 25:75 (the actual mass ratio should be 28.3/71.7), the spinel reaction has the highest efficiency and the largest amount of production. Based on the MgO/Al2O3 ratio The formulated castable has the best high temperature performance.
As the particle size of magnesia increases, both the residual line change and the open porosity of the specimen increase. At the same time, the particle size of magnesia will also directly affect the production rate of MA in the sample. When the size of the magnesium oxide particles is small, the formation temperature of secondary spinel in the sample is also relatively low, and it is evenly distributed in the castable matrix. , which can improve the thermal shock resistance of the specimen.
Research shows that when MgO is introduced into the sample in an amount that theoretically constitutes spinel, the average linear expansion coefficient of the sample is the smallest, and the effect of MgO content on it gradually decreases as the heat treatment temperature increases.
In traditional refractory materials using cement, because the cement content is relatively high, the normal temperature strength of the sample can be guaranteed to a certain extent. However, at medium temperatures, the strength will be greatly reduced. This is because the cement in the sample will undergo a crystal transformation; at the same time, the cement will bring in a certain amount of calcium oxide, which can interact with the alumina and silica in the sample. The reaction will produce calcium-aluminum feldspar (C2AS) or anorthite (CAS2) with a lower melting point, which will reduce the high-temperature strength and resistance to slag erosion and penetration of the castable, which will affect the quality and production of steel.
MgO-SiO2-H2O bonding is a typical example of using cohesive bonding in cementless castables. This system has high bonding strength. The use of MgO-SiO2-H2O condensation combination has four characteristics:
(1) Due to the use of silica powder in the material, the castable will have excellent compactness and fluidity;
(2) It can avoid impurities introduced by the addition of other types of binding agents, such as CaO and sodium polyphosphate introduced by calcium aluminate cement or Na2O introduced by water glass, and MgO and SiO2 can generate high melting point 2MgO at high temperatures ·Chemical reaction of SiO2;
(3) Because the bound water content in the MgO-SiO2-H2O structure is relatively small, this will reduce the decomposition of bound water in the sample, making it possible to bake quickly without obvious cracking;
(4) During the heating process, the MgO-SiO2-H2O system can gradually lose water over a wide temperature range, which will make the sample have better burst resistance during the first heating process.