Preparation background and overview of calcium and magnesium carbonate
Calcium magnesium carbonate is also called dolomite. Dolomite is the main solvent raw material for preparing glass. Since it is a natural magnesium ore, it can be used to extract magnesium oxide raw materials. A mineral composed of calcium magnesium carbonate CaMg(CO3)2. Colorless, white or gray. Rocks containing more than 15% magnesium carbonate are called dolomite, those with less content are magnesia limestone, and those containing both dolomite and calcite are dolomite limestone. Dolomite occurs as primary sediment in metallic veins and in limestones whose properties have been modified by the dolomitization process. In this process, calcium carbonate is fully or partially converted into dolomite by magnesium-rich seawater or magnesium-rich solutions that seep into rock crevices.
Ideal dolomite is a solid solution composed of 56% calcium carbonate and 44% magnesium carbonate. Colors vary depending on impurities: off-white, white, yellow, light yellow, red, light red, green, blue, gray, brown or black. The crystal is a trigonal crystal system, the crystal is often rhombohedral, and the crystal face is bent into a saddle shape. It often forms a dense block with a density of 2.8 to 2.9 and a hardness of 3.5 to 4. It has a glass luster, some has a pearl luster, and the streaks are white. The decomposition temperature is 730-900°C. At 730-790°C, it decomposes into magnesium oxide and calcium carbonate. At about 900°C, all the CO2 in the dolomite is discharged, and the dolomite is burned into dolomitic lime (CaO·MgO). Dolomite tends to decompose at lower temperatures if an acidic flux with a lower melting point is present.
Dolomite is insoluble in water (solubility in water is 0.03218g/100g water) and soluble in hot hydrochloric acid, but does not interact with 10% dilute hydrochloric acid. This characteristic can be used to distinguish calcite and dolomite. The composition of dolomite is a rock intermediate between limestone (CaCO3) and magnesite (MgCO3). Although some of the ores contain a considerable amount of magnesium carbonate, it is 44% less than the magnesium carbonate in pure dolomite. This is usually considered The ore is also called dolomitic limestone. Dolomite mines are rich in reserves and are mainly distributed in Haicheng, Liaoning, Leyu, Qinghai, Yongdeng, Gansu, Daye, Hubei, Nanjing, Jiangsu and other places in my country. It is mainly used in ceramic blanks and glazes, alkaline refractory materials, and the glass industry. It is also used as a raw material for dolomite cement and building materials. It is a commonly used flux in the metallurgical industry.
Preparation of calcium magnesium carbonate and preparation of ultrafine dolomite
A method for preparing ultrafine dolomite. Dolomite undergoes processes such as crushing, pulping, peeling, drying, and crushing to finally produce 55-95 (2 μm 90%) grade ultrafine dolomite powder.
1. Dolomite crushing. After the dolomite is crushed by the crusher, 325 mesh dolomite powder is selected.
2.325 mesh dolomite powder pulping. Add grinding aid to the water, the amount of water accounts for 30% of the dolomite powder slurry, stir to dissolve the grinding aid, add 325 mesh dolomite powder to form a dolomite slurry with a solid content of 70%, stir thoroughly to make the slurry The material has good fluidity. The amount of grinding aid added is 1% (relative to dolomite dry powder). The grinding aid is sodium polyacrylate;
3. Ultra-fine peeling of dolomite slurry. Use a wet ultra-fine stripping machine (or stirring mill) to perform ultra-fine grinding of dolomite slurry. Using the multi-stage peeling method, each peeling machine is equipped with grinding media prepared in a certain proportion. The particle size ratio of the grinding media in each level of peeling machine is very important, which directly determines the grinding efficiency.
Under the stirring of the peeling disk, the dolomite slurry and the grinding medium undergo violent collision, extrusion, grinding, shearing and high pressure, causing the coarse dolomite particles to be ground and peeled off into ultrafine particles. The ultrafine slurry flows to the upper part under the pressure of the pump and the stirring action of the peeling disk, and is filtered by the screen and flows into the next level of peeling machine. Finally, more than 55%-95% of the slurry that is ultrafine to particles with a diameter less than 2 μm of dimethyl sulfoxide is collected together and enters the drying process. Since the slurry particles gradually become finer during the ultrafine process, the viscosity of the slurry gradually increases, which seriously affects the production efficiency. Therefore, the method of adding grinding aids dropwise into the peeling barrel from the upper part of the peeling machine is used to make the slurry Always maintain good fluidity, so that the grinding efficiency can be increased by 20%, and the operability is also enhanced.
4. Dry the dolomite slurry with 55%-95% particles with a diameter less than 2 μm, break it up, and collect the finished product. The principle is that when the wet material is fed into the machine, it is crushed powerfully at high speed (the speed can reach thousands of revolutions), and the hot air introduced dries the material at the same time. The granular materials running upward fall back to the crushing section under the action of the rotating classification device above and continue to be crushed and dried; the dry fine powder passes through the classification device and is collected by the receiving device at the back.
Preparation of calcium and magnesium carbonate
A method for preparing calcium and magnesium carbonate by direct carbonization of calcined ammonium salt leachate of medium and low-grade phosphate rock, including the following steps:
(1) Calcining medium and low-grade phosphate rock to obtain pyrolysis products and CO2 gas. The following reactions mainly occur during this process: CaCO3·MgCO3=CaO+MgO+2CO2;
(2) Mix and leach the obtained pyrolysis product with ammonium nitrate solution (or ammonium chloride solution) to obtain calcined medium and low-grade phosphate rock ammonium salt leachate containing calcium and magnesium salts and NH3 gas. In this process, the main The following reaction occurs:
CaO+MgO+2H2O=Ca(OH)2+Mg(OH)2;
Ca(OH)2+2NH4NO3=Ca(NO3)2+2NH3+2H2O;
Mg(OH)2+2NH4NO3���Mg(NO3)2+2NH3 polycarbonate+2H2O;
(3) Use the CO2 gas obtained in step (1) and the steps to use the obtained calcined low-grade phosphate rock ammonium salt leachate
(2) The obtained NH3 gas is passed together into the calcined low-grade phosphate rock ammonium salt leachate, so that the calcined medium-low-grade phosphate rock ammonium salt leachate is carbonized and precipitated calcium and magnesium to obtain calcium and magnesium carbonate. The main events in this process are as follows Reaction: Ca(NO3)2+Mg(NO3)2+4NH3+ 2CO2+2H2O=CaCO3+Mg CO3+4NH4NO3.
