This article introduces the characteristics, development status and manufacturing process of a widely used magnesium-calcium refractory material. Some measures were also proposed to address the problem of waterproofing free CaO in magnesia-calcium refractory materials.
Keywords: magnesia-calcium refractory materials; waterproofing; development status; manufacturing process
Preface
At the beginning of its development, magnesia-calcium refractory materials were mainly used in alkaline oxygen converters, and refractory materials containing free CaO were developed earlier. By the late 1980s, due to the popularization and application of magnesia-carbon bricks, the amount of magnesia-calcium refractory materials was drastically reduced. Beginning in the 1980s, Japan developed CaO bricks. Since then, many research results and patents have appeared. Especially in recent years, the development of CaO-containing refractory materials has received widespread attention. With the rapid development of stainless steel and clean steel production in my country, the application fields of magnesia-calcium refractory materials have been transferred to external refining furnaces and different types of ladles. Because the CaO in it has good creep properties, the magnesia-calcium refractory material has good high-temperature toughness, excellent spalling resistance and impact resistance. Moreover, the free CaO has wide adaptability to slag. It can not only resist the corrosion of highly alkaline slag, but also can increase the slag alkalinity on the surface of the refractory material and reduce the chemical corrosion of the slag when the slag alkalinity is low. Various magnesia-calcium refractory materials have high performance and low price, can meet environmental protection requirements, and have certain competitiveness. Currently, their manufacturers are increasing and their application fields are becoming wider and wider.
Characteristics of magnesia-calcium refractory materials
MgO and CaO alkaline materials are currently widely recognized as efficient and high-quality materials. It is mainly used in converters, electric furnace steelmaking, continuous casting tundish and refining furnaces. It is also widely used in smelting special steel and clean steel. Magnesia-calcium refractory materials have good physical and chemical properties.
Good purification and anti-corrosion effects
The free CaO in magnesia-calcium bricks has a very low oxygen partial pressure and has very little solubility in molten steel. It is an oxide that can effectively prevent re-oxidation of molten steel. Free CaO can adsorb [S], [P], [O] and non-metallic inclusions such as Al2O3 and SiO2 in molten steel to purify the molten steel. Since the invading slag reacts with MgO-CaO in the refractory material to generate C2S with high refractory degree, the slag wetting angle increases and the viscosity increases. It can prevent the slag from intruding into the refractory material and improve the unfavorable working conditions of thickening of the deteriorated layer. The magnesia-calcium brick can also improve the desulfurization effect. Various performance indicators of magnesia-calcium bricks containing a certain amount of ZrO2 can reach or even exceed the standards of magnesia-chrome bricks. In addition, the ability of magnesia-calcium bricks to resist high alkalinity (R > 1.5) slag is better than that of magnesia-chrome bricks. Steelmakers can carry out smelting operations under high-alkalinity slag conditions, which increases the life of the furnace lining.
Low cost and no pollution
The main raw material for the production of magnesia-calcium bricks is natural dolomite. Dolomite reserves are abundant in my country and the mining cost is low. In addition, the hexavalent chromium produced by magnesia-chrome bricks pollutes the environment, but this problem does not exist in the production of magnesia-calcium bricks. It is precisely because of the advantages in the above aspects that fired magnesia-calcium bricks gradually replace magnesia-chrome bricks in many application fields. In terms of raw materials for MgO-CaO refractory materials in my country, the past one-step calcined dolomite clinker in coke shaft kilns has developed into two-step calcined dolomite clinker, artificially synthesized two-step calcined dolomite clinker and fused magnesium dolomite clinker. Materials etc. In terms of products, the past single asphalt-bonded dolomite bricks have been developed into asphalt-bonded magnesia dolomite bricks, light-burned oil-impregnated magnesia dolomite bricks, fired magnesia dolomite bricks, and fired oil-impregnated magnesia dolomite bricks. A variety of high-quality MgO-CaO refractory materials such as magnesium dolomite carbon bricks combined with anhydrous resin and composite binders.
Manufacturing process of magnesia-calcium refractory materials
my country’s manufacturers of unburned magnesium calcium (carbon) bricks are mainly distributed in Liaoning, Shandong, Hubei, Jiangsu and Zhejiang provinces. At present, my country’s production of magnesia-calcium (carbon) bricks uses various magnesia-calcium sands, fused magnesia sand, and flake graphite as the main raw materials, and uses anhydrous resin as the binding agent. The preparation of bricks generally uses magnesia-calcium sand as the aggregate. , fused magnesia and graphite are made into fine powder. The fused magnesia powder and graphite should be pre-mixed before mixing. In order to reduce the viscosity of the binder and facilitate mixing, it is best to use the binder at a temperature of about 50°C. The binder should be heated during low temperature seasons. After forming, the unfired bricks must be heat treated at 200 to 250°C to dissolve the binding agent and improve the strength of the bricks, and then use organic matter for surface treatment to prevent hydration.
Waterproofing of Free CaO
The biggest shortcoming of MgO-CaO refractory materials is the hydration problem of free CaO. CaO expands when it meets H2O to form Ca(OH) 2, which pulverizes the MgO and CaO sand and causes cracks or cracks in the bricks. Since CaO reacts easily with water, the actual production and application of this type of material are greatly restricted. Therefore, in order to fully promote its application, the key technical issue of hydration must be solved.
Preventing hydration of magnesia-calcium refractory materials requires comprehensive waterproofing measures in all aspects of manufacturing and production. For example :
1) Select raw materials with high density, low porosity and appropriate purity;
2) Use anhydrous binder and reasonable particle gradation;
3) Dry the bricks and adobe, and soak them in lightly burned oil;
4) The finished bricks are packaged in thermoplastic and vacuum packaging to minimize the contact between the surface of the bricks and the atmosphere. This ensures long-term storage and is suitable for long-distance transportation;
5) High-temperature firing or electric fusion makes CaO grains grow, fully sintered, densified, reduced grain boundary area, and improved its hydration resistance.
Some manufacturers in Germany and France are keen on sealing packaging measures. For example, the dolomite bricks produced by the German Dolomite Company are vacuum-sealed and packaged with aluminum foil, which can achieve the goal of never hydrating; Japan pays more attention to the use of surface treatments and additives to improve hydration resistance. Our country mainly adopts the method of adding additives, including rare earth oxides, zircon sand, etc. In industry, methods such as high-temperature firing, secondary calcination and light burning oil immersion are mainly adopted. Of course, no matter what kind of waterproofing measures are taken, the cost of the product must be taken into consideration.
For the surface treatment methods commonly used in industry, the following are mainly introduced:
1) Treat the sand with phosphoric acid, and the phosphate compounds generated in the grain boundaries and gaps of the magnesia-calcium particles will cover the surface to isolate the air and thus play a waterproofing role.
2) Use a certain concentration of silicone solution to wet the sand surface, and then dry it to form a covering layer of silica film on the sand surface to improve its hydration resistance.
3) Spray a layer of protective film such as anhydrous organic matter, dehydrated asphalt, anhydrous resin, etc. on the brick surface to prevent the brick surface from contacting the atmosphere.
4) Pass carbon dioxide and water vapor through the sand surface to form a carbonate compound covering layer on the sand surface, which is the carbonation treatment method.
When manufacturing magnesium calcium sand, a small amount of additives such as BaO, SrO, Al2O3, SiO2, ZrO2, CaF2 and rare earth oxides are added. These additives interact with CaO to generate low melts or solid solutions, which can improve the microstructure, promote sintering, and play a waterproofing role.