Refinement of SiO2 Content in Silica Fume

Silica fume is commonly used as a concrete admixture and an additive for refractory materials, with an industry requirement that the content of SiO₂ should be above 85%. The SiO₂ content of silica fume collected from silica metal (98%) is ranging from 87% to 98%. For ferrosilicon (75%) production, that is 86% – 90%, normally 88%. The higher the SiO₂ as well as the lower the content of impurities, the larger the specific surface area and the better activity. So the improvement methods below are used and the SiO₂ content of silica fume from ferrosilicon production has been increased to 91%.

The process of collecting silica fume from ferrosilicon furnace:

The raw materials undergo a series of reactions at a high temperature, resulting in a large amount of gases containing dust, in which particles with relatively large sizes are separated by cyclone separators, most of which are carbon powder and large agglomerates. And those extremely fine silica fume particles condense in the baghouses via positive pressure fans.

furnace A and furnace B are selected for the comparative test. The two furnaces have the same smelting parameters, electrical parameters and dust removal device. The refined silica fume is produced from furnace B.

Refinement of raw materials

The main raw materials in the ferrosilicon production are quartz, coke, and ferrous materials. Steel scraps with ∑Fe content over 97% are added, which though have little effect on the silica fume as their content in the materials is only 6%. The result of high content of Mg0, CaO, Al₂O₃, and LOI (loss on ignition) is mainly due to the ash content of coke.

For furnace B, quartz materials are all cleaned before entering the furnace, and those containing soil are not used. The ash content of coke is controlled to no more than 6.5%. The coke materials with sizes below 3mm and impurities in the steel scraps below 5mm are screened out. After the refinement of raw materials, the SiO₂ content of silica fume is improved to 90% – 91.5%. The average content values can be seen in the following table.

Retrofit of cyclone separators

Even though the content values have been improved by strict control of raw materials, they are still quite unstable, especially the high LOI value caused by some large particles and agglomerates not being fully separated by the cyclone separators.

So a control valve is designed at the inlet of the small cyclone to adjust the amount of gases entering. In the original design, only the tangential direction of the gases entering the small cyclone is considered. In actual use, some large particles collide with the wall of the cyclone, changing their direction and moving to the vortex area, thereby entering into the baghouse. By adjusting the valve, when the amount of gases is large, more large particles can be separated.

In the original design, the central opening of the top wall of the small cyclone is connected to the recirculation system. In the new design, an exhaust pipe is used to optimize the formation of the vortex. The results show that the LOI and SiO₂ of silica fume have both been improved.