Dolomite is a 1:1 double salt composed of magnesite (MgCO3) and calcite (CaCO3). It is the second largest alkaline refractory material after magnesite. It is mainly used to prepare dolomite refractory materials.
Dolomite ore is first calcined (or fused) to obtain magnesia-calcium raw materials (such as dolomite sand/magnesia dolomite sand, magnesia-calcium sand, magnesia-calcium iron sand), and then prepared into various dolomite refractory materials. In addition to the advantages of high refractoriness, slag erosion resistance, and thermal shock resistance of dolomite refractories, CaO can adsorb S, P, Al2O3, SiO2 and other non-metallic inclusions in molten steel, and also has the function of purifying molten steel. Therefore, it is widely used in stainless steel refining furnaces; in addition, because dolomite bricks contain a large amount of free CaO, they can easily react with C2S in cement kiln materials to form C3S and produce stable kiln shells. Therefore, they are suitable for use in cement rotary kilns. Bring it.
Dolomite bricks are made of dolomite sand that is calcined from natural dolomite. Dolomite brick contains w (CaO) ≥ 40%, w (MgO) ≥ 35%, and also contains a small amount of SiO2, Al2O3, Fe2O3 and other impurities. If the mass ratio of CaO to MgO in the brick is less than 1.39, it is called magnesia dolomite brick.
In order to improve thermal shock resistance, ZrO2+ dolomite bricks and ZrO2+ magnesia dolomite bricks have been developed. Dolomite bricks are mainly used in stainless steel refining furnaces (such as AOD, VOD furnaces, etc.) and cement kiln firing zones.
For example, in the AOD furnace lining, currently, Germany uses fired dolomite bricks, some other European countries use pure dolomite bricks, and Japan uses magnesia dolomite bricks. In addition, dolomite brick is the most commonly used refractory material in cement kiln firing zones in Europe and the United States. Among them, 86% of cement kiln firing zones in the United States use this product. However, dolomite bricks tend to absorb moisture and hydrate during transportation, storage and shutdown of the kiln, and have poor thermal shock resistance. They are generally used in kilns with an annual operating rate of over 80%. Only a few manufacturers in my country use cement rotary kilns. Use imported dolomite bricks.
Magnesia-calcium bricks include two types: one is a magnesia-calcium brick produced with synthetic magnesia-calcium sand as the main raw material; the other is a magnesia-calcium brick produced with dolomite sand (or magnesia dolomite sand) and magnesia as the main raw material , Also called magnesium dolomite brick. At present, the fired magnesia calcium brick/magnesia dolomite brick with w(CaO) of 20% is the largest production and consumption of synthetic magnesia calcium refractory products in China. It is mainly used as the lining of stainless steel refining furnaces (AOD furnaces, VOD furnaces, etc.) The material can replace the magnesia chrome brick which is extremely harmful to the environment and human body, and the use effect is good.
For example, the linings of the AOD furnaces used for stainless steel smelting in Taiyuan Iron and Steel Company have been used for long-term burning of magnesia-calcium bricks. my country's fired magnesia-calcium brick has become the main refractory material in the stainless steel industry. In addition, my country also produces a certain amount of fired magnesia-calcium bricks/magnesia dolomite bricks with w (CaO) ≥ 30%.
The main raw materials are sintered dolomite sand, magnesia sand and/or graphite of different particle sizes. It is a refractory material that can be used directly without firing. It has energy saving, good thermal shock stability, simple production process, and good slag resistance. .
At present, the unburned magnesia-calcium (carbon) bricks produced in my country have a w (CaO) of 10% to 30%. They are magnesia-calcium products with medium and low calcium, which are mainly used in refining furnaces such as LF-VD or LF, VOD furnaces. The bottom and the molten pool, and the bottom of the GOR furnace, have good results. However, my country's unburned magnesia-calcium (carbon) bricks have not been widely promoted and applied. The first reason is that the price is high; the second is that the performance needs to be improved.
Dolomite is also used to prepare a variety of magnesium-calcium amorphous refractory materials, such as electric furnace bottom ramming material, tundish coating material, ladle gunning material, converter gunning material, RH furnace gunning material, artesian hot filling material, etc. Refractory materials. Among them, the most used magnesia-calcium-iron dry ramming material for the bottom of the electric furnace is prepared by using iron-containing magnesium dolomite synthetic sand (magnesia-calcium iron sand) as raw materials, and the use effect is good, such as Shanghai Baosteel 150tUHP The bottom life of the electric furnace reaches 1087 times; the bottom life of the 90t UHP electric furnace of Wuyang Iron and Steel Company is also more than 1500 times.
At present, my country encourages the development of electric furnace steelmaking, the number of UHP electric furnaces continues to increase, and the demand for various dry bottom ramming materials will also continue to increase. In addition, the gunning material developed by using dolomite is used for the repair of converters, electric furnaces, ladle slag lines, RH insertion pipes, etc. The effect is good, and the current application volume is gradually expanding; using dolomite as the main product can also produce tundish Spread material, dry material, excellent performance.
At present, dolomite is mainly used to synthesize magnesia-calcium refractories in refractories. In order to expand the application of dolomite resources in refractories, research has been conducted on the use of natural dolomite ore to synthesize calcium aluminate cement containing magnesia alumina spinel. Calcium aluminate cement (CMA) containing microcrystalline magnesia alumina spinel is a new type of binder for refractory materials. Its CaO content is reduced and contains magnesia alumina spinel. Compared with ordinary aluminate cement, it has more It has excellent high temperature volume stability, thermal shock resistance and slag erosion resistance.
Researchers at Xi’an University of Architecture and Technology used dolomite and industrial aluminum oxide as raw materials to sinter to synthesize calcium aluminate cement containing magnesia-aluminum spinel. This cement has high bonding strength. Researchers at Wuhan University of Science and Technology Natural dolomite of 5~50μm and industrial alumina (purity ≥95.0%) of 5~44μm are used as raw materials, and the two are mixed in a certain proportion, after ball milling, sintering and crushing processes, you can get nano-magnesium aluminum Spinel aluminate cement (with a particle size of only 50~500nm). Since natural dolomite resources are abundant and the cost is low, the preparation method of the aluminate cement containing nano-magnesia-aluminum spinel has low cost and is suitable for industrial production.
There are also researchers using natural dolomite ore and different aluminum sources to synthesize calcium aluminate-magnesia alumina spinel lightweight refractory materials, and calcium dialuminate-magnesia alumina spinel composite materials resistant to cement clinker erosion. Good progress.
Nowadays, the steel industry is developing towards the goal of super steel technology of "high cleanliness, high uniformity, and ultrafine grain". The output of clean steel and pure steel is increasing, and dolomite refractories have a good effect on slag and metal. Chemical stability, especially free CaO, has the effect of purifying molten steel. Accordingly, the demand for dolomite refractories will increase, and dolomite refractories have great development prospects.
In the field of cement industry, for some reason, some enterprises in our country still use magnesia-chrome bricks in the firing zone and transition zone of cement rotary kilns, while dolomite bricks have excellent high temperature resistance, kiln-hanging skin and resistance to alkaline substances. It can be used in the burning zone and transition zone of cement kilns to replace magnesia-chromium refractory bricks. At present, only a few manufacturers in my country use imported dolomite bricks in cement rotary kilns. Dolomite bricks are used in the burning zones of cement rotary kilns in my country. There is a lot of room for development. However, dolomite refractories are difficult to sinter. At the same time, refractories using dolomite sand as raw materials can easily absorb water and hydrate to form hydroxides during storage and repeated use, resulting in volume expansion, resulting in product cracking and powdering. Acute weaknesses limit the application and development of dolomite refractories.
Therefore, in the future, we will increase research in this area to increase the application of dolomite resources in refractory materials, which can save relatively small reserves of magnesite. Use dolomite to develop new refractory materials, such as new aluminate cement, calcium hexaaluminate-spinel lightweight refractories, etc.