1 Chemical iron removal process -- acid leaching method
The iron impurities in SIC powder mainly exist in the form of elemental iron and its oxide ferric oxide. Because elemental iron and its iron trioxide are soluble in sulfuric acid, nitric acid, hydrochloric acid, etc., soluble iron salt can be removed by adding water to wash.
By comparing the effect of hydrochloric acid concentration and reaction temperature on the iron removal rate, it is found that the concentration of hydrochloric acid is 180g /L and the reaction temperature is 80℃, and the iron removal rate can reach about 93% under this condition.
Wang Chunli with sulfuric acid, hydrochloric acid, hydrofluoric acid one or two as the leaching solution, the experimental results show that: for the median particle size of about 0.5μm of silicon carbide powder, with a volume ratio of 20% of hydrofluoric acid and 10% of hydrofluoric acid mixed acid, held at 70℃ for 3h, the removal rate of iron impurities in silicon carbide powder can reach 88%.
Sun Yi et al. declared the patent of "Treatment device and treatment method of high-speed iron silicon carbide powder". The ferrosilicon carbide powder obtained by electromagnetic iron removal is first pulped with a certain proportion of water at 80 ~ 90℃. After the slurry is stirred evenly, some sulfuric acid, nitric acid, or mixed acid is added. After soaking for 4 ~ 6 hours, the water is washed to neutral. The separation of SIC powder and impurity iron is realized.
The traditional method of chemical acid removal has high iron removal rate. However, the acid water produced by the removal of iron needs to be treated before discharge, such as improper treatment is easy to cause groundwater pollution. And after pickling silicon carbide powder is easy to produce aggregation, which is not conducive to the further treatment of silicon carbide powder, after pickling silicon carbide powder needs to be washed to neutral, so washing will also cause a large amount of pure water waste.
2. Physical iron removal process -- magnetic separation method
Magnetic separation method is to use the magnetic properties of metal and its oxides, adding magnetic raw materials around the metal atoms to exert an attractive or repulsive force to make the metal atoms move away from the silicon carbide powder method. According to the dry and wet of raw materials, it can be divided into: wet magnetic separation method and dry magnetic separation method.
The patents of iron removal devices for silicon carbide powder declared by Jing Donghui and others are all about the separation of iron impurities in silicon carbide powder and silicon carbide powder by primary or multi-level magnetic separation equipment.
Electromagnetic iron removal efficiency, high degree of automation, in line with the needs of industrial production, while avoiding the use of a large number of chemicals, in line with the national energy conservation, emission reduction policies. However, for iron impurities with high iron content or weak magnetism, the pure use of electromagnetic iron removal cannot achieve the purity of silicon carbide required by the process.
3 Physical-chemical synthesis method
Due to the limited iron removal capacity of the magnetic separator itself, the silicon carbide powder with high iron content is only through the physical method, and the obtained silicon carbide powder may not be able to achieve the desired purpose. Therefore, the magnetic separation method can be used for preliminary magnetic separation, and then the secondary iron removal method can be carried out by acid leaching.
Zhao Ping et al. first used a pulsating high-gradient electromagnetic magnetic separator to conduct preliminary magnetic separation of ceramic grade silicon carbide micro-powder, and then added four different types of acids, namely sulfuric acid, hydrochloric acid, oxalic acid, hydrochloric acid and oxalic acid as the leaching solution. The experimental results show that the removal rate of iron impurities in silicon carbide can reach about 95% after the primary magnetic separation of silicon carbide powder and the leaching solution of 12% hydrochloric acid and 1∶1 ratio of oxalic acid.
4 Other iron removal processes
Acid leaching and electromagnetic iron removal are widely used in silicon carbide powder industry. However, there are other iron removal methods and processes in other industries, and some processes are listed to provide theoretical basis for the silicon carbide micro-powder iron removal process.
4. 1 Oxidation leaching
The iron oxidation method uses strong oxidants (sodium hypochlorite, chlorine, etc.) to oxidize the insoluble Fe into the soluble Fe2+ in the water medium, so as to remove the iron impurities by washing.
4. 2 Leaching by reduction method
The insoluble trivalent iron ions are reduced to divalent iron ions, and then washed by adding pure water to achieve the removal of iron impurities.
