Two Processes for Producing Ferrosilicon Chromium Alloy
There are two processes for producing ferrosilicon chromium alloy from chromite ore: the one-step method and the two-step method.
The one-step method involves directly smelting chromite ore, silica stone, coke, etc., in a submerged arc reduction electric furnace to produce ferrosilicon chromium alloy. This method is also known as the direct method or slag-containing method. However, it results in a large amount of slag and significant chromium losses in the slag. The partial reduction and volatilization of MgO, Al2O3, and CaO increase energy consumption. As a response, the two-step method was developed. In this method, chromite ore, coke, and some silica stone are first melted to produce high-carbon ferrochromium in one submerged arc reduction electric furnace. This product is then granulated or crushed into granules. In the second step, these high-carbon ferrochromium granules are mixed with silica stone and coke and smelted in another submerged arc reduction electric furnace to produce ferrosilicon chromium alloy. Since the alloy is obtained through two steps, this method is also called the two-step method or the indirect method. After smelting, the high-carbon product needs decarburization treatment outside the furnace to obtain low-carbon ferrosilicon chromium alloy.
In comparison, the one-step method requires fewer electric furnaces, reducing investment costs, and allows for the direct production of low-carbon products with a shorter process. The one-step method also has a higher chromium recovery rate and lower overall smelting energy consumption after metal recovery from slag. However, it is challenging to control due to the high viscosity of the slag, particularly in facilitating smooth slag discharge from the furnace. Other methods include the shaking ladle decarburization and slag washing decarburization methods.
One-Step Method of Ferrosilicon Chromium Alloy:
Chromite ore, silica stone, coke, and steel scraps are uniformly mixed and added to the submerged arc reduction electric furnace. A suitable power supply system is chosen, with electrodes deeply inserted into the furnace charge to maintain a high slag temperature, facilitating reduction reactions and refining the alloy. This process also disrupts SiC, making it easier for slag to be discharged from the furnace. The composition of the slag is adjusted to have a MgO/Al2O3 ratio less than 2 and SiO2 content between 40% and 50%. This composition helps to keep the carbon content in the alloy below 0.04% and facilitates efficient slag discharge. The presence of a thick slag layer in the molten pool is essential. The upper layer is similar to the smelting of high-carbon ferrochromium, with SiO2 content of 44% and SiC of 23% in the slag. The lower part consists of ferrosilicon chromium alloy and final slag, with SiO2 content of 30% and SiC of 2%. The main challenge is the discharge of slag from the furnace, which is addressed by installing a slag-pulling machine before the furnace. This machine uses steel rods to pull the adhered slag out of the furnace, which is then separated using gravity separation methods, such as a jigging machine, to increase chromium recovery by approximately 5%.
Two-Step Method of Silicochromium:
The first step involves producing high-carbon ferrochromium. In the second step, the production of ferrosilicon chromium alloy is similar to smelting 45% silicon iron. The difference lies in using high-carbon ferrochromium granules instead of steel scraps. The particle size of high-carbon ferrochromium significantly affects the destruction of (Cr, Fe)7C3. Smaller particle sizes, when mixed uniformly with the charge, increase the contact of Si with (Cr, Fe)7C3, leading to its thorough destruction before entering the molten pool. Practical production experience shows that using larger chunks of high-carbon ferrochromium results in a carbon content of up to 0.13% while using particle sizes less than 20mm reduces the carbon content to less than 0.06%. The silicon content in ferrosilicon chromium alloy decreases as the silicon content increases, with SiC forming when Si > 34%. When Si > 43%, the alloy’s carbon content decreases less significantly. Therefore, maintaining a silicon content between 43% and 53% is suitable for producing ferrosilicon chromium alloy. The solubility of SiC in the alloy is minimal, and under appropriate conditions, SiC can be separated from the alloy. Floating SiC requires a high pouring temperature, such as 1650–1750°C, a prolonged holding time in the ladle for over 60 minutes, and keeping the chromium content in the alloy below 34% to reduce viscosity. By maintaining insulation, the carbon content in the alloy can be reduced from 0.15%–0.30% to 0.04%. However, there is a significant difference in carbon content between the upper and lower layers and the center and edges of the alloy. The alloy obtained from the furnace contains 0.4%–0.8% carbon and requires decarburization treatment outside the furnace. The industry uses shaking ladle decarburization and slag washing decarburization methods.
Shaking Ladle Decarburization Method of Silicochromium:
The iron ladle containing molten ferrosilicon alloy and decarburization agent is placed on a ladle-shaking frame, causing eccentric motion. The eccentric motion creates a “wave” movement in the molten pool due to the mass points moving up and down, promoting mixing and stirring for effective mixing of ferrosilicon chromium alloy and decarburization agent. SiC is absorbed by the decarburization agent after precipitating from the alloy. The decarburization agent can be ferrosilicon alloy slag or a mixture of lime and fluorite slag, with a usage rate of 5%–8% of the ferrosilicon chromium alloy. The shaking time is typically 5–10 minutes. After shaking ladle decarburization treatment, the carbon content in ferrosilicon chromium alloy can be reduced to 0.02%.
Slag Washing Decarburization Method of Ferrosilicon Chromium Alloy:
Liquid ferrosilicon alloy is directly poured into the molten low-carbon ferrochromium slag. The slag liquid is dispersed and mixed with the alloy liquid, and as the alloy rises, the slag adsorbs most of the silicon carbide. During cooling, the alloy continues to precipitate silicon carbide, which floats to the slag-alloy interface and enters the slag. Through slag washing, the carbon content in the slag can reach 4%, and the carbon content in the alloy can be reduced to 0.02%. Slag washing not only achieves a high decarburization rate but also allows for the recovery of chromium from low-carbon ferrochromium slag. The chromium content in the slag can be reduced to around 0.5%. Through slag washing, the phosphorus content in ferrosilicon chromium alloy can be reduced by 75%–90%.