Hebei Messi Biology Co., Ltd. stated that the hydration rate and activity of magnesium oxide are mutually restrictive. The hydration rate refers to the ability of magnesium oxide to generate magnesium hydroxide. As an isolation layer before high-temperature annealing of oriented silicon steel, it is usually made of magnesium oxide and an appropriate amount of water into a slurry, which is applied on the steel plate and dried. The formed Mg-(OH)2 contains chemical water. This part of water cannot be removed through the drying furnace and will enter the high-temperature annealing furnace process, which will prolong the annealing time and cause oxidation of the steel strip, which is not conducive to the formation of magnesium silicate film.
Silicon steel grade magnesium oxide needs a hydration rate of less than 4%. In order to ensure that magnesium oxide has a low hydration rate, it is generally selected to be calcined at high temperature. The magnesium oxide obtained by calcining above 1300°C hardly undergoes hydration reaction at room temperature, but too high a temperature is easy to cause grain sintering and reduce activity. Therefore, a roasting temperature of about 950°C is usually selected. In addition to the calcination temperature, the cooling treatment method after calcination will also affect the hydration rate of magnesium oxide.
Hebei Messi Biology Co., Ltd.’s research shows that calcined magnesium oxide should be cooled to room temperature under vacuum conditions to avoid the influence of external factors, so that the magnesium oxide particles will shrink naturally as the temperature gradually decreases, forming a denser and freely spread crystal particle, thereby reducing its hydration rate. Through experiments, it was found that if the calcined magnesium oxide is cooled in the air, its hydration rate will increase greatly, generally reaching more than 10%. If it is cooled to room temperature under a closed condition, its hydration rate can be lower than 3%, which can meet the requirements of the hydration rate of silicon steel grade magnesium oxide.
Activity refers to the ability of magnesium oxide to participate in chemical or physicochemical processes. Highly active magnesium oxide is easy to react with silicon oxide to form magnesium silicate glass. Magnesium oxide with too low activity has poor suspension in water and is prone to uneven coating. Since the activity of a solid substance is proportional to its surface area, some people use the size of its surface area to express the activity, so the specific volume of the specific surface is introduced. Silicon steel grade magnesium oxide requires a specific volume of 3.7~4.5mL/g.
Research by Hebei Messi Biology Co., Ltd. shows that the activity of magnesium oxide is largely determined by its crystal properties. The formation mechanism of magnesium oxide is different, and the degree of perfection of its lattice is different. The activation energy required to destroy its lattice is different, which is manifested as different chemical activities of magnesium oxide. In addition, the calcination conditions have a great influence on the activity of magnesium oxide. The intermediate magnesium carbonate is calcined at 600-700℃ to obtain highly active magnesium oxide. Since the activity of magnesium oxide is related to the hydration rate, the greater the activity, the greater the hydration rate, so the selection of the hydration rate and activity of silicon steel grade magnesium oxide should be considered comprehensively.