Submerged Arc Furnace Production of Ferrochrome
Ferroalloy smelting employs various methods depending on the target product and quality specifications. The primary techniques include the carbothermal reduction method (blast furnace), the electrothermal reduction method (submerged arc furnace, SAF), the metallothermic reduction method, and the electrolytic method. For refining, processes like the electro-silicothermic process, oxygen blowing, and vacuum solid-state decarburization are used to convert high-carbon or high-silicon products into medium- and low-carbon ferroalloys.
This method uses a blast furnace, operating similarly to pig iron production. It is suitable for producing ferromanganese, mirror iron, low-silicon ferrosilicon, and ferronickel. However, its application is limited compared to the submerged arc furnace.
The submerged arc furnace (SAF) is the dominant equipment for producing the vast majority of ferroalloy tonnage, including ferrosilicon, carbon ferromanganese, silicon manganese, carbon ferrochrome, ferronickel, silicon chromium, and silicon calcium alloys.
Process Principle: In the SAF, a mixture of ore (e.g., chromite) and a carbonaceous reductant (coke) is heated electrically. During operation, the electrodes are buried in the charge. Heat is generated by a combination of arc heat at the electrode tips and, crucially, resistive (Joule) heating as the high current passes through the conductive charge and slag.
Product Characteristics: Elements like chromium and manganese readily form stable carbides. Therefore, direct reduction in an SAF typically yields high-carbon ferrochrome (HCFeCr) or high-carbon ferromanganese. In alloys like silicon-chromium or silicon-manganese, the presence of silicon lowers the carbon content. Due to the use of electricity as the primary heat source and carbon as the reductant, this method is termed the electro-carbothermal process.
Operation: The process is continuous or semi-continuous. Molten alloy and slag are tapped at intervals. For very high-melting-point alloys (e.g., ferrotungsten), special methods like the "iron accumulation" or "agglomeration" process are used within the furnace.
The Principle of Carbon Reductant Selection in SAF Smelting:
Optimizing the carbonaceous reductant is critical and involves two key concepts:
Chemical Carbon Requirement: The stoichiometric amount of carbon needed to reduce the metal oxides in the ore.
Physical Carbon Matching: The selection and preparation of reductant (type, resistivity, particle size) to ensure proper charge permeability (gas flow) and electrical conductivity, which directly impacts furnace stability and efficiency.
High-carbon ferrochrome from the SAF often requires further refining to produce medium-, low-, or micro-carbon grades. Key refining methods include:
Oxygen Converter Method: Similar to steelmaking. Liquid high-carbon ferrochrome is charged into a converter. High-purity oxygen is blown onto the melt (top, bottom, or combined blowing). The exothermic oxidation of carbon and silicon raises the temperature, enabling decarburization without external heating. This is the primary method for producing medium- and low-carbon ferrochrome (MC/LCFeCr).
Perrin (or Heat Exchange) Process: A silicothermic refining method. Liquid high-carbon ferrochrome is mixed with a slag rich in chromium oxides (from ore) and lime. The silicon in the alloy reduces the Cr₂O₃ in the slag, exchanging silicon for chromium. The heat from the exothermic reaction sustains the process. It is also known as the "shake ladle" method when used for silicon-manganese or silicon-chromium master alloys.
Vacuum Solid-State Decarburization: Used for producing micro-carbon ferrochrome. Solid high-carbon ferrochrome is ground, mixed with an oxidant (e.g., ferrochromium scale), pressed into briquettes, and heated under vacuum. Carbon in the alloy reacts with oxygen from the oxidant to form CO gas, which is removed by the vacuum system, thereby lowering the carbon content in the solid state.
Electro-Silicothermic Process: Conducted in a refining electric arc furnace. A silicon-rich alloy (e.g., silicon-chromium) is used to reduce chromium ore in the presence of lime. This method desiliconizes the master alloy while recovering chromium, producing refined ferrochrome with controlled carbon and silicon levels.
Conclusion:
The submerged arc furnace is the workhorse for the primary production of carbon ferrochrome and other bulk ferroalloys via electro-carbothermal reduction. Subsequent refining of this high-carbon product to meet stringent carbon specifications is achieved through secondary processes such as oxygen converter decarburization, silicothermic reduction (Perrin process), or vacuum treatment, each chosen based on the target grade, cost, and scale of production.
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