Preparation process of unburned magnesia-calcium refractory materials

Magnesium-calcium refractory materials are a type of alkaline refractory materials, the main components of which are magnesium oxide and calcium oxide, with the calcium oxide content generally ranging from 20% to 50%. The advantages of magnesium-calcium refractory materials are summarized as follows:

(1) The unique advantage of magnesium-calcium refractory materials is that they can purify molten steel, which is an advantage that other refractory materials cannot match. The reason why magnesium-calcium refractory materials can purify molten steel is that the calcium oxide contained in magnesium-calcium refractory materials can absorb impurities such as S and P in molten steel.

(2) Magnesium-calcium refractory materials have high corrosion resistance in alkaline environments. In high-temperature environments, calcium oxide can react with silicon dioxide in slag to form dicalcium silicate, forming a high-melting-point protective film on the surface of the bricks to prevent further erosion by the slag.

(3) The raw material reserves of magnesium-calcium refractory materials are abundant. At present, the main raw material for the production of magnesium-calcium refractory materials in China is artificial magnesium-calcium sand, which can be prepared by high-temperature calcination of dolomite. my country has abundant dolomite reserves and low mining difficulty, so the raw material reserves of magnesium-calcium refractory materials are abundant. (4) Magnesium-calcium refractory materials do not cause pollution to the surrounding environment during production and use. Magnesium-calcium refractory materials have a broader application prospect than magnesium-chromium refractory materials.

Classification of magnesia-calcium refractories

Magnesium-calcium refractory materials can be classified into the following categories according to the preparation method and raw materials:

(1) Sintered magnesium-calcium refractory materials

The most significant process in the preparation of sintered magnesium-calcium refractory materials is high-temperature sintering. The characteristics of sintered magnesium-calcium refractory materials are high density and excellent hydration resistance among magnesium-calcium refractory materials.

(2) Regenerated magnesium-calcium refractory materials

Regenerated magnesium-calcium refractory materials are made by crushing and screening magnesium-calcium refractory materials after use in kilns, and adding a certain proportion of sintered magnesium-calcium sand and fused magnesium sand to prepare magnesium-calcium refractory materials through mixing, pressing, drying, sintering and other processes. They can achieve the purpose of solid waste recycling and green environmental protection.

(3) Unburned magnesia-calcium refractory materials

Compared with the preparation process of other magnesia-calcium refractory products, the preparation of unburned magnesia-calcium refractory products does not require a high-temperature sintering process. They can be put into use after only low-temperature drying. The preparation process is simple and easy to operate, with a short production cycle, low cost, high efficiency, and environmental protection.

In the actual production process, the contact between magnesium-calcium refractory products and water is minimized. Therefore, when selecting binders, organic anhydrous binders such as paraffin, hydraulic oil, tar, asphalt, and resin are generally used. In the processing of finished products, manufacturers generally vacuum-pack, wax-impregnate, and modify the surface of unburned magnesium-calcium refractory products to protect the bricks from hydration.

Application of Unburned Magnesia-Calcium Refractories

Magnesia-calcium refractories are primarily used in AOD furnaces, VOD furnaces, and kilns in the nonferrous metallurgy and construction industries. Figure 2 is a schematic cross-sectional view of an AOD furnace, which clearly illustrates the range of materials available for its lining. An AOD furnace is a type of refining equipment that decarburizes, desulfurizes, dephosphorizes, and adjusts the steel's composition by introducing oxygen into the molten steel in a vacuum or reduced pressure atmosphere. Figure 3 is a schematic diagram of the material selection for a VOD furnace lining. The material selection for a VOD furnace differs slightly from that for an AOD furnace. Magnesia-chromium refractories are generally used in the slag line, but due to environmental protection requirements, magnesia-calcium refractories are being used in place of magnesia-chromium refractories. Magnesia-calcium refractories can also be used in place of magnesia-chromium refractories for the sidewalls and floor.

Difficulties faced by unburned magnesium-calcium refractory materials

The main difficulties faced by unburned magnesium-calcium refractory materials can be divided into the following two points:

(1) The early strength of unburned magnesium-calcium refractory materials is not high, and they are easily damaged during transportation, resulting in a high scrap rate. In addition, during the transportation of finished bricks, there is also damage to the brick body due to stress impact such as collision and friction. Therefore, in the process of preparing unburned magnesium-calcium refractory products, special attention should be paid to the early strength of unburned magnesium-calcium refractory products. The main solutions adopted by manufacturers are: ① Increasing the pressure during the pressing process to make the unburned magnesium-calcium refractory products more dense, but the effect is not ideal, because increasing the pressure requires the manufacturer to increase the investment in mechanical equipment, and the wear rate of the mechanical equipment also increases. Excessive pressure in the pressing process will destroy the internal particle grading of the product, which is not conducive to the densification of the brick body, so the pressure can only be increased within a certain range; ② Modifying the binder. The method of modifying the binder can also increase the density of the unburned magnesium-calcium refractory products and further improve the early strength of the unburned magnesium-calcium refractory materials. However, there are certain difficulties in modifying the binder and the cost is high.

(2) The hydration resistance of unburned magnesium calcium refractory materials is not high. Due to the properties of the main raw materials in unburned magnesium calcium refractory materials, calcium oxide and magnesium oxide can spontaneously react with water vapor in the air at room temperature to form hydroxides, causing cracks or even powdering of unburned magnesium calcium refractory products. Taking the hydration of calcium oxide as an example, its thermodynamic reaction equation (1) is shown as follows:

From equation (1), it can be calculated that calcium oxide can spontaneously react with water vapor in the air at a temperature below 508℃ to generate 2)(OHCa. In fact, the generation of 2)(OHCa will cause a certain volume expansion, causing the unfired magnesium-calcium refractory products to crack or even pulverize, ultimately leading to the damage of the unfired magnesium-calcium refractory products.

The corresponding measures that manufacturers can take are to use sealed packaging to treat them, or to use surface modification to form a protective film on the surface of the magnesium-calcium refractory materials to prevent the entry of water vapor and thus protect the magnesium-calcium refractory products from being hydrated.