What is the “JL” method for smelting micro-carbon ferrochromium?

The Bolan method has a history of 50 or 60 years in foreign countries in smelting micro-carbon ferrochromium. Its advantages are unmatched by the electrosilicothermal method. It has the characteristics of low carbon content in the product, high silicon utilization rate, and high recovery rate. Due to the above-mentioned advantages of the Pollan method, it has been widely used in Western countries. In the 1990s, my country’s Shanghai and Hengshan Ferroalloy Plants gradually introduced this process. In 1997, Jilin Plant 504# furnace began trial production of the Pollan method. Two In recent years, the Jilin plant has improved the unique Pollan process based on the summary of the Pollan process technology, which mainly includes: raw materials, alkalinity, high chromium products, product quality upgrades, lining life, equipment, etc., collectively referred to as “JL “Law.

1. The physical and chemical principles of the Pollan method The production of micro-carbon ferrochromium by the Pollan method is to add chromium ore and lime into the slag furnace in a certain proportion, melt them at high temperature through arc, and then pour the melt into the reaction bag. Add to the reaction bag Solid (or liquid) silicon reducing agent (commonly used silicon-chromium alloy), through reduction reaction, obtain micro-carbon ferrochromium that meets the requirements.

Metallographic analysis shows that chromium ore and lime melt are mainly composed of high-melting-point phases Ca2Cr2O5 and Ca2Fe2O5. According to the phase analysis data, the Bolan method reduction reaction equation is 2/3(Ca2Cr2O5)+[Si]+2/3(CaO )→4/3[Cr]+(Ca2SiO4) ΔH1=-216.3kJ/mol(1) 2/3(Ca2Fe2O5)+[Si]+2/3(CaO)→4/3[Fe]+(Ca2Si2O4) ΔH2=-420kJ/mol(2) Reactions (1) and (2) are both exothermic reactions. The silicon-chromium alloy added to the melt is melted by the reaction heat, thereby achieving the purpose of reducing Cr2O3 and FeO. The Pollan method is divided into one step Method and two-step method: The one-step method is to directly mix the melt and silicon chromium at once without producing intermediate alloy and intermediate slag. The two-step method is to add silicon chromium alloys with different silicon contents in two times during the whole CNC Lathing. And produce high-silicon or low-silicon master alloy and intermediate slag containing a certain amount of Cr2O3. The more typical one is the two-step method of the Tromihtan plant in Sweden, with a recovery rate of 88%. The “JL” method uses The one-step Pollan process of mixing cold silicon chromium and melt has been put into production in 1997. Various technical and economic indicators have been stable, with an average recovery rate of 81.66%. The maximum reached 85.34%. Except for the unit power consumption, other indicators are better than the silicon thermal method. ​

2. Raw materials The raw materials used in the “JL” method mainly refer to chromium ore, silicon-chromium alloy, and lime. Foreign Bolan process production generally involves raw material pretreatment. The treatment process is to roast chrome ore and limestone (or lime) together with gas or heavy oil in a 69~70m long rotary kiln. The chemical composition of chromium ore and lime
Powders with a particle size of chromium ore less than 20mm and lime less than 5mm must be screened out. The roasting and mixing temperature is generally 1000~1150℃, and the kiln lining temperature is 1100~1200℃. The roasting time is 2~2.5h. At the temperature of 1100~1200℃, the chromium ore and After the lime interaction is fully entered into the furnace, the power consumption per ton of melt is about 800~900kWh. The components of chromium ore and lime required by the “JL” method
There is no pretreatment process for chromium ore and lime entering the furnace. All raw materials entering the furnace are fully enclosed and continuous centralized feeding. Each batch of materials takes about 15 minutes to enter the furnace, and all raw materials take about 50 minutes. As the raw materials are added, some of the raw materials are preheated and baked. The dry and flying losses are recovered centrally by a powerful exhaust fan, and the pure chromium ore is obtained for reuse after washing with water in a shaking table. As a reducing agent for silicon-chromium alloy, liquid silicon-chromium is generally used in the two-step hot mixing method, and solid silicon-chromium is often used in the one-step method. , because it is more convenient to operate and the end point is easy to control, the two-step operation process is more complicated than the one-step method, and the end point is also difficult to control. By comparison, the heat enthalpy value of the silicon chromium used in the Jilin plant is lower than that of the Swedish plant, but the raw material particle size range is wider than that of the Swedish plant. .
3. Alkalinity control

The alkalinity of the slag of the electrosilicothermal method is 1.8~2.0, while the alkalinity of the slag of the thermal mixing method is 1.2~2.7. There is a big gap between the two. In the electrosilicothermal method, it is necessary to use high alkalinity slag. This is because the slag is the carrier of heat transfer. The electrode is an arc-off operation. The heat energy generated by the arc can only be transferred to the metal through the slag, and micro-carbon ferrochrome The melting temperature is 1650~1700℃. The slag must have a higher melting point to increase the furnace temperature and overheat the alloy. The slag temperature needs to be controlled at 1750~1800℃. If the alkalinity of the slag is low, the slag is highly superheated and has good fluidity, it will intensify the erosion of the furnace wall and damage to the alkaline furnace lining. And because the melting point of slag is low, it brings certain difficulties to slag separation, which can easily lead to the indistinguishability of slag from iron. In the “JL” method, the silicon reduction of Cr2O3 in the slag is an exothermic reaction. The reaction occurs at the slag-metal interface. The heat is transferred from the slag-metal interface to the slag and the metal respectively, causing the reaction temperature to increase with no difference in temperature between the slag and the metal. It will be too big so that the slag will not be overheated. Through practice, when the “JL” method adopts a gray ore ratio of 1.0 and the alkalinity is controlled at 1.7~1.8, casting with an ingot mold will not cause slag inclusions, the quality of the alloy is guaranteed, and the service life of the reaction package is greatly improved. and refractory material consumption are beneficial.

4. Production of high chromium ferrochrome

The relationship between reaction free energy and temperature is given based on the thermodynamic data provided in relevant articles.
During the hot mixing process, Cr2O3 and FeO have the following chemical equilibrium with CrFe in the metal

2[Cr]+3(FeO)→(Cr2O3)+3[Fe] At T=1900K ΔG=-728.6kJ/mol

Since iron is reduced prior to chromium, the ratio of Cr2O3/FeO in the slag will change as the reaction proceeds.
When silicon chromium is added in two stages, the content change curve of FeO and Cr2O3. It can be seen from the curve that when the silicon chromium addition amount is 30%, almost all the FeO in the melt has been reduced. Then the reduced FeO The high-iron alloy (containing about 52% Fe) is separated from the melt and the remaining 70% silicon-chromium alloy is added to the melt. At this time, the alloy obtained is a high-chromium alloy. Table 6 shows the average composition of high-chromium alloys, with the highest chromium being 74.28% and the lowest being 72.92%. [next] The melting temperatures of ferrochromium for different chromium contents are listed, and this temperature increases as the chromium content of the alloy increases. In order to successfully obtain high-chromium alloys, the melt exit temperature must be increased to increase the reaction heat. ​

5. Increase the service life of the reaction package

The Bolan method produces micro-carbon ferrochromium. The lining of the reaction bag is made of magnesia refractory material, usually magnesia bricks. The entire heating reaction operation process will produce the following chemical and physical erosion and destruction effects on the reaction bag lining. ​

(1) Erosion of SiO2 Although the melt contains a high amount of CaO, during the reduction process of (Cr, Fe)2O3 by Si, SiO2 is first generated, so there is a chemical corrosion effect of SiO2 on alkaline refractory materials, especially on the reaction The slag-iron interface area in the lower part of the package is more serious.

(2) High temperature corrosion The entire reaction is carried out in the high temperature range of 1850~2000℃, and the melt also produces a strong reflow effect. Melting and erosion of the lining will occur, making the entire lining thinner. ​

(3) Thermal cracking of the lining. The hot mixing process adopts regular slag discharging operations. Therefore, the reaction bag generally works continuously in a hot and cold environment, causing the lining to undergo thermal cracking and natural peeling damage, and even leakage and cracking. Natural peeling and damage may occur due to the phenomenon, and even package leakage may occur. In view of the above characteristics of the reaction bag, the use of magnesia bricks to build the reaction bag is no longer suitable for the operation requirements of the hot mixing process. The “JL” method passed tests and cooperated with refractory material manufacturers to implement a masonry process of mixing magnesia bricks and magnesium calcium iron ramming material for the lining of the reaction bag. The method is to use ramming material at the bottom of the bag and the reaction zone during operation. There is no need for additives and running stirring. Just pour it directly into the bag and pound it with a pneumatic hammer. During the pounding process, it is required to pound it as flatly and firmly as possible. After other Medical parts are built with normal magnesia bricks, the reaction bag can be used for normal production after drying the moisture of the binder of the magnesia bricks. During actual use, through the transfer of melt reaction temperature of about 1900°C, the magnesia-calcium-iron ramming material layer can achieve its own sintering effect. Currently, the package has been used for 100 furnaces. Compared with the 50 heats of the normal magnesia brick masonry process, it is more than doubled. Compared with foreign countries, no additional equipment is added, such as a slag hanging machine, etc. Only new materials are used, which effectively increases the service life of the reaction package. ​

6. Product Quality The “JL” method of smelting micro-carbon ferrochromium can not only produce micro-carbon ferrochromium with C≤0.03%, it can also produce other brands of refined ferrochromium according to market needs. In addition, the “JL” method also has a The important feature is that it can upgrade product quality. At present, users have increasingly higher requirements for the quality of ferroalloy products. Most users require small-grained products. During the processing of small-grained products, about 15% of the undersize products are produced. These On the one hand, the sieve waste requires a backlog of funds and takes up a lot of space; on the other hand, each ton of sieve waste requires a certain packaging fee. After adopting the “JL” method, after a simple repacking and remelting process, funds can be activated to reduce site occupation, and low-grade sieve products can be converted into high-grade products, increasing the added value of the product.

7. Equipment Conditions “JL” method micro-carbon ferrochromium production is based on the original 6.3MVA tilting covered “electrosilicothermal method” refining electric furnace, which is designed, modified and installed by itself. It has the following characteristics:

(1) Through the star-angle transformation of the transformer connection method, the interchange between the “electric silicon thermal method” and the “Pollan method” can be freely realized. ​

(2) The tilting electric furnace can control the amount of melt produced in each furnace to ensure normal reaction needs. ​

(3) The fully enclosed electric furnace with cover reduces noise, reduces the flying of furnace materials, and makes it easier for operators to connect and discharge electrodes. ​

(4) The graphite carbon head replaces the copper head, which increases the Prototy ping services of the carbon head. The additional water-passing movable sleeve effectively prevents the electrode from sliding down. ​​

(5) The tubular combined water-cooled furnace cover ensures that the furnace cover does not leak for many years and stabilizes production. ​

8. Conclusion

(1) The “JL” method is improved on the basis of the original “electrosilicothermal method” electric furnace. It has the characteristics of low investment, short cycle and quick results.

(2) The “JL” method can achieve better technical and economic indicators under the premise that raw material conditions, equipment conditions, etc. are not very perfect. ​

(3) The “JL” method produces micro-carbon ferrochrome, which has the advantages of flexible process conversion, the ability to produce high value-added products and long lining life. ​

(4) The next step is to improve the current raw material conditions, strive to put hot materials into the furnace, use slag capping for pouring, and realize the two-step process, which will be the further improvement and development of the “JL” method.