Light-calcined magnesia powder is produced by roasting magnesite at a temperature below 1000°C.
It plays a role in metallurgy, chemical industry, light industry, agriculture, mining, environmental protection and other industries. Light-burned magnesia is an important part of the raw materials required for the production of magnesite products, that is, activated magnesia produced by calcination of magnesite.
The ability of magnesium oxide to perform physical adsorption and chemical reaction is called the activity of magnesium oxide, which has a very significant impact on the production process and quality of the product. Sintered magnesia is calcined light-burned magnesia by two-step calcination method.
Light-burned magnesia is commonly known as light-burned powder, light-burned magnesia, also known as caustic magnesia, and it often appears in white and light yellow shapes. Its melting point and boiling point are 2852°C and 3600°C, respectively. It is easily soluble in acids and ammonium salts, but not easily soluble in organic solvents such as ethanol. It is a hard cementing material that is easily deliquescent in the air. Samples with a mass fraction higher than 98% are called high-purity magnesia, because they have excellent alkali resistance, insulation resistance, and light transmission performance in high-temperature environments, and have a relatively large thermal expansion coefficient and excellent Thermal conductivity and other advantages are widely used in electronics, ceramics, and chemicals.
At this stage, most of the production of light-burned magnesia is obtained by calcination at 700-1200°C with magnesite with a block size of 20-100mm. Due to the different decomposition rate of MgCO3 and the state of MgO development, the activity will also be different. Its activity lies in the content of active magnesia in light-burned magnesia, and its physical adsorption and chemical reactivity depend on this. Because of this, whether its activity can be accurately measured directly affects the production and application of light-burned magnesia in practice.
There are two main processes for preparing magnesia from magnesite. They are one-step calcination and two-step calcination, and the intermediate product of two-step calcination is light-calcined magnesia powder. The specific process is as follows:
(1) One-step calcination process: raw material (magnesite with certain requirements) → calcination (coke is used in a rotary kiln or a shaft kiln, and calcined at different temperatures) → different grades of magnesium powder
(2) Two-step calcination process: raw material (magnesite with certain requirements) → light calcination (in suspension roasting kiln, rotary kiln, reverberatory furnace or fluidized bed furnace, using ashless fuel, calcining at different temperatures) → fine Grinding (the finer the particle size, the higher the purity of the magnesium powder) → secondary calcination (calcination in a shaft kiln or a rotary kiln, at different temperatures) → different grades of magnesium powder
The two-step calcination is a calcination method extended by being inspired in the process of studying the enrichment effect of oxides of atomic energy. The first step is generally to seek a certain chemical reaction or to improve the reactivity of the raw material. The second step is to achieve the desired density and microstructure to achieve the sintering process. The sintering principle of the second step is to let the dead burnt system react in solid phase, thereby expanding its volume. In terms of dead burning temperature, compared with one-step two-step calcination, the temperature can be reduced by 100~200°C, and it has a higher bulk density of magnesia. The advantage of the two-step calcination process is that the calcination temperature can be reduced, so that the pure magnesia can obtain the best volume and density state of the mineral phase composition, but it consumes about 50% more fuel in comparison.
In the production and application of light-burned magnesia, the specific surface area, activity and grain size are all important indicators to measure the quality of light-burned magnesia, and a comprehensive evaluation is generally made from these aspects.
(1) specific surface area
The specific surface area is the sum of the surface areas per gram of light-burned magnesia powder calculated according to the BET adsorption theory, and is usually measured by the method of adsorbing nitrogen. The particle size of the raw material has a direct impact on the specific surface area of light-burned magnesia. However, the active surface of light-burned magnesia is related to the particle size, activity and the fine grinding method of MgO. For example, in environmental projects, generally only a particle size of 6-20mm is required to ensure a specific surface area of 10-60m2/g.
(2) Activity
Generally, the citric acid hydration method is used to measure the activity of light-burned magnesia powder. Activity refers to the reaction time of light-burned magnesia and standard acid solution, and is an important index for detecting the reactivity of magnesia. There are many factors affecting the activity of light-burned magnesia, including: the content of inert compounds, the change of temperature, time, the particle size of magnesite and the particle size of the product MgO, etc.
(3) Grain size
The grain size refers to the crystal size of the light-burned magnesium oxide powder, and the smaller the grain size, the higher the activity. If the calcination temperature is too high, the crystal grains will increase and the activity will decrease.