Hydrometallurgy refers to the method of crushing metallurgical silicon and immersing it in an acid solution (or other substance solution) to remove metal impurities in the metal silicon. Hydrometallurgy needs to crush industrial silicon into powder with a suitable particle diameter, otherwise it will not be easy In addition to miscellaneous. Prepare a certain concentration of HCI, HF, H2SO4 or their mixture, soak the polycrystalline silicon powder in an acid solution and keep the acid solution at a suitable temperature, and filter out after a certain period of time. At this time, the concentration of metal impurities in industrial silicon It can be reduced by one to two orders of magnitude. The particle size of the pure intermediate silicon lost in the wet smelting, the acid concentration, the temperature of the pickling treatment and the length of the treatment time have an important influence on the removal of impurities. Fe, Al and Ca impurities contained in silicon are easier to remove than Mg, Ti, Zr and Ni impurities. Usually, if only acid is used, whether it is hydrochloric acid, sulfuric acid or nitric acid, the effect of removing metal impurities such as iron is better, but the effect of removing boron and phosphorus is not obvious.
The process route of wet purification of metallurgical grade silicon powder to produce super metallurgical grade silicon has the following advantages: low equipment investment, low operating temperature, low energy consumption, and large processing capacity. Therefore, many scholars began to carry out acid leaching and purification studies on metallurgical grade silicon from different sources in the 1960s and 1970s. For example, under the action of sulfuric acid, aqua regia, hydrofluoric acid and other acids, microwave treatment is used, and for example, the particle size is not larger than 50μm, and acid leaching in aqua regia at 75°C is used.
The following factors should be paid attention to during the leaching process, and appropriate measures should be taken. For example, when the process is controlled by a chemical reaction, the temperature and the concentration of the leaching agent should be appropriately increased to reduce the particle size of the raw material: if it is controlled by external diffusion, in addition to reducing the particle size, stirring should be strengthened; if it is In internal diffusion control, the particle size should be reduced, and the temperature should be increased. If necessary, the ball mill leaching method should be used to strengthen the leaching. In addition, in order to strengthen the leaching process, the following general methods of strengthening leaching can also be considered. ① Perform mechanical activation pretreatment on the mineral raw materials, that is, under the action of mechanical force, various defects are generated in the mineral crystals, so that they are in an unstable state with higher energy levels, and the activity of their chemical reactions is correspondingly increased. ②Ultrasonic activation strengthens the diffusion process. The mechanism of this method is still under study. Because ultrasound may cause local high temperature and high pressure (cavity effect), many scholars believe that ultrasound makes the water phase have turbulent hydraulic characteristics. The test results show that it has a more obvious strengthening effect on the leaching process. ③ Thermal activation. Preheating the mineral raw materials to a high temperature and then quenching may also increase the leaching efficiency. The principle is mainly due to the rapid cooling and heating of the solid phase itself, which produces thermal stress and defects in the crystal lattice, and at the same time, cracks are generated in the particles. ④Radiation activation. Under a certain amount of radiation, the mineral raw materials will produce various defects in the crystal lattice. At the same time, some molecules in the aqueous solution may be dissociated into more active atomic groups or ion groups, thereby accelerating the reaction. ⑤The application of the catalyst is mainly to strengthen the process of the oxidation-reduction reaction.
Some people also use acids, alkalis, complexes, ion exchange resins and other chemical agents to remove aluminum, phosphorus, and boron from silicon through ion exchange. Usually, the smallest particle size is 200-400 mesh. Meanwhile, these normal temperature chemical methods can only remove the impurities on the surface of the silicon powder, and basically have little effect on the impurities inside the particles. However, because the metal impurities are on the surface of the grain boundary during directional condensation, this method can be used as a physical method. A valuable intermediate process for polysilicon.