During this chemical reaction, the silicon in the chemical compounds SiO2, SiO and SiC are converted to pure silicon, which collects and is finally tapped from the furnace as silicon metal.
The overall chemical reaction is never absolutely complete, but silicon production can achieve up to 95% with optimum control of the processes. The right scaling and installation of furnaces, auxiliary and side equipment and skilled people in production control are essential for the utilization of the plant.
Figure 5 shows a typical submerged arc furnace, similar to the one that will be used in PCC’s plant.
The processes of silicon metal production are shown in Figure 2.
Figure 2: A simplified diagram of silicon metal production
The submerged arc furnace is divided into two parts: the upper portion above the smoke hood, and the lower portion, where the actual chemical reaction takes place and silicon metal is produced.
Pre-baked electrodes are located in the upper portion of the furnace and go through the smoke hood down to the lower portion, where the chemical reaction takes place. Exhaust gas is collected into the smoke hood and conducted to the gas cleaning plants (GCP) with bag filters. Heat is also conducted out of the hearth through the smoke hood.
Molten silicon metal is regularly tapped from the furnaces into crucibles and transferred to the casthouse where it is refined and cast into ingots. After cooling, the ingots are pre-crushed and stored temporarily within the furnace building. The product is later taken to crushing. After crushing the material is graded, packaged and transferred to the warehouse where it awaits sea transport.
The production process also generates MicroSilica, which is separated from the exhaust in bag filters in the GCP. During the production process, a very small amount of solid by-products are generated, as the majority of the raw materials is converted into silicon metal; a small part of them turns to slag. All exhaust from the furnaces and the crucible (i.e. the refining process) is treated.