Thermal decomposition of 1. ammonium sulfate-sodium dichromate
This method is the basic method for producing chromium oxide in the United States, Britain, Germany and other countries, and has become the production method of chromium oxide with the largest output, the best quality and the most varieties abroad. Its advantages are simpler production process than liquid phase reduction method, lower cost than chromic anhydride decomposition method, and wide adaptability (making pigments, abrasives, refractories and metallurgical chromium oxide), which is suitable for large-scale production in rotary kiln.
No harmful gas is generated in the production process. Therefore, it replaces the early thermal decomposition method of sodium dichromate-ammonium chloride. Almost all commercial chromium oxide is made directly or indirectly from sodium chromate, and its output accounts for about 20% of sodium chromate consumption.
The total production capacity of chromium oxide in the world is about 654.38 million tons/year.
2.？ Direct thermal decomposition of chromic anhydride
Thermal decomposition method of chromic anhydride: the thermal decomposition of chromic anhydride is carried out at a high temperature of 900℃, and the finished product is obtained after a little cooling. In recent years, the chromium oxide produced by this method has developed rapidly in China. 1999 The output of chromic anhydride is about 13000 tons, which is twice that of metallurgical-grade chromic oxide produced by sodium chromate reduction in Tietai Gold Plant. The thermal decomposition process of chromic anhydride is complicated. With the increase of temperature, chromic anhydride will be decomposed into four kinds of chromic oxide. Because chromic anhydride melts at about 200℃ and begins to decompose, oxygen and chromium oxide are separated. The crystal grown by this method has few defects, can maintain many excellent properties of chromium oxide single crystal, and has high product quality, so it is widely used. The results show that when the temperature rises to 470℃, cr2o 3 is generated, and when the temperature rises to 550V, Cr2O3 is completely converted into Cr2O3. However, it is found in the experiment that the actual decomposition temperature is higher than this temperature. The reason is that during the decomposition of Cr2O3, the generated Cr2O3 film is coated on the surface of unconverted chromium oxide, and the melting point of Cr2O3 is very high (2266 25)℃, which hinders the further decomposition of chromium oxide. Therefore, the process of adding a small amount of water can be used to reduce the reaction temperature. On the one hand, Cr2O3 is easily soluble in water, and on the other hand, additives can be evenly mixed with Cr2O3 raw materials. The results of sample analysis show that the mass fraction of Cr2O3 is over 99%. The reaction temperature and time also have great influence on the decomposition of chromic anhydride.
3. Preparation of chromium trioxide from chromium hydroxide
Up to now, although there have been many methods to produce chromium oxide by neutralization and separation from chromium sulfate solution, the generated chromium hydroxide is a fine colloid, which is not only difficult to handle, but also low in purity and will not dissolve in acid and alkali after long-term storage. In addition, if alkali metal hydroxide or carbonate is used as neutralizer, insoluble or insoluble by-products will be formed, which limits the use of neutralizer and the popularization of this method. In order to solve this problem, German patent 4 18050 proposes a method for producing hydrogen and chromium oxide through the following reactions:
However, this method also has some disadvantages: the operation is complicated and iron is easily mixed with chromium hydroxide towels. Therefore, chromium hydroxide is prepared by neutralization with a simple aqueous solution of chromium chloride.
In addition, chromium oxide is prepared from water-soluble trivalent chromium salt by chromium hydroxide or chromium hydroxide (CrOOH); Preparing chromium oxide from chromium-containing waste; Preparing pigment grade chromium oxide from non-pigment grade chromium oxide; Direct preparation of melt-blown chromium oxide by aluminothermic method or silicothermic method.