Bohlen carbon ferrochrome smelting in foreign countries have been 50 or 60 years of history, the advantage of silicon thermal power can not be compared with products with low carbon content, high silicon utilization, and high recovery rate. Table 1           Chromium ore chemical composition% Cr 2 O 3 FeO MgO SiO 2 Al 2 O 3 H 2 O 47~50 9~12 12~20 3~8 5~8 5~7 Table 2 lime chemical composition table% CaO MgO SiO 2 P 89~95 0.7~1.5 0.3~2 0.003~0.006 The chrome ore particle size is less than 20mm, and the lime powder is less than 5mm. The roasting mixing temperature is generally 1000~1150°C, the kiln lining temperature is 1100~1200°C, the roasting time is 2~2.5h. The chrome ore is at the temperature of 1100~1200°C. When the lime interaction is fully integrated into the furnace, the power consumption per ton of melt is about 800~900kWh.[next] table 3           Jilin factory chrome ore chemical composition% Cr 2 O 3 SiO 2 MgO Al 2 O 3 CaO FeO H 2 O 52 1.68 12~13 11.41 0.4 16.38 7 Table 4             Jilin factory lime chemical composition% CaO SiO 2 P Raw burning lime powder 88.73 1.5 0.0035 5 5 There is no pretreatment process for chrome ore and lime into the furnace. All the raw materials for the furnace are fully enclosed and continuously fed. The time for each batch of materials is about 15 minutes. All the raw materials need about 50 minutes. Preheating with some raw materials is added. Dry, the loss of the flying part is concentrated and collected by a powerful exhaust fan. After washing with a shaker, the pure chrome ore is reused. table 5    Silicone chromium chemical composition and particle size for Swedish and Jilin plants Factory name Cr/% Si/% Particle size / m <5mm Swedish factory 40 45 5~25 0 Jilin factory 33 40 0~30 30 Third, alkalinity control In the heat exchange process, the following chemical equilibrium exists between Cr 2 O 3 and FeO and CrFe in the metal 2[Cr]+3(FeO)→(Cr 2 O 3 )+3[Fe] Figure 2 is a graph showing the change of FeO and Cr 2 O 3 content when silicon chromium is added in two stages. It can be seen from the curve that when the amount of silicon chromium added is 30%, the FeO in the melt is almost completely reduced. The reduced high-iron alloy (containing about 52% Fe) is then separated from the melt and the remaining 70% of the silicon-chromium alloy is added to the melt, at which point the alloy is a high-chromium alloy. Table 6 shows the average composition of the high chromium alloy, wherein the highest chromium is 74.28% and the lowest is 72.92%. [next] Table 6          High chromium alloy average composition% Cr Si C P S 73.59 0.37 0.3 0.025 0.005 Table 7      Relationship between liquidus temperature and chromium content in Fe-Cr binary system Cr% 50 60 70 80 90 92 T/°C 1580 1620 1650 1720 1780 1800 Table 7 lists the melting temperatures of ferrochrome with different chromium contents, which increases with increasing chromium content of the alloy. In order to obtain a high chromium alloy smoothly, it is necessary to increase the melt discharge temperature to increase the heat of reaction. Fishing Type Mask Making Machine
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Since Bohlen method has the above advantages, has been widely used in Western countries, in the 1990s, China's Shanghai, Yokoyama ferroalloy plant was finally able to introduce this technology in 1997, Jilin plant 504 # furnace started trial production at Bohlen law In the past two years, the Jilin factory has improved the unique Polen process based on the Balun process technology, including: raw materials, alkalinity, high chromium products, product quality upgrade, lining life, equipment, etc. "JL" method.
I. Principle of physics and chemistry of Polen method The production of micro-carbon ferrochrome by the Polen method is to add chrome ore and lime to the slag furnace in a certain proportion, melt it by arc high temperature, and then pour the melt into the reaction package. Add to the reaction package. Solid (or liquid) siliceous reducing agent (usually silicon-chromium alloy), through the reduction reaction to obtain the required micro-carbon ferrochrome.
Metallographic analysis showed that the chrome ore and lime melt consisted mainly of high melting point phase Ca 2 Cr 2 O 5 and Ca 2 Fe 2 O 5 phase. According to the phase analysis data, the Bowen reduction reaction equation was 2/3 (Ca 2 ). Cr 2 O 5 )+[Si]+2/3(CaO)→4/3[Cr]+( Ca 2 SiO 4 )
ΔH 1 =-216.3kJ/mol(1)
2/3(Ca 2 Fe 2 O 5 )+[Si]+2/3(CaO)→4/3[Fe]+(Ca 2 Si 2 O 4 )
ΔH 2 =-420kJ/mol(2)
The reactions (1) and (2) are exothermic reactions. The purpose of reducing the Cr 2 O 3 and FeO by melting the silicon-chromium alloy added to the melt by the reaction heat is divided into a one-step method and a two-step method: One-step method is to directly mix the melt with silicon chrome and do not produce intermediate alloy and intermediate slag. The two-step method is to add silicon-chromium alloy with different silicon content twice in the whole process, and produce high silicon or low. The intermediate alloy of silicon and the intermediate slag containing a certain amount of Cr 2 O 3 are more typical of the two-step method of the Tromichtan plant in Sweden, and the recovery rate is 88%.
The "JL" method uses a one-step wave process of cold silicon chromium and melt mixing. Since the process was put into production in 1997, various technical and economic indicators have been stable, with an average recovery rate of 81.66%. The highest reached 85.34%, except for the unit power consumption, other indicators are better than the electric silicon thermal method.
2. Raw materials <br> The raw materials used in the “JL†method mainly refer to chromium ore, silicon chromium alloy and lime.
Production of foreign PERRIN generally be pretreated feedstock, which process is a process of chromium ore, limestone (or lime) together with a length of firing coal or heavy gas inside the kiln 69 70m ~ chromium ore, lime chemical composition, see Table 1, Table 2.
See Table 3 and Table 4 for the chrome ore and lime components required for the “JL†method.
As a reducing agent for silicon-chromium alloy, the two-step heat exchange method generally adopts liquid silicon chrome, and the one-step method uses solid silicon chrome. Because it is convenient to operate, 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 difficult to control. .
By comparison, the thermal enthalpy of silicon chrome used in the Jilin plant is lower than that of the Swedish plant, but the raw material size range is wider than that of the Swedish plant. Table 5.
The alkalinity of the electric silicon thermal slag is 1.8~2.0, while the alkalinity of the hot slag slag is 1.2~2.7, which is very different. In the electrothermal method, it is necessary to use high alkalinity slag. This is because the slag is the carrier of heat transfer, and the electrode is operated by arcing. The heat generated by the arc can only be transmitted to the metal through the slag, while the micro-carbon ferrochrome The melting temperature is 1650~1700 °C. The slag must have a high melting point to increase the furnace temperature and make the alloy overheat. The slag temperature should be controlled at 1750~1800 °C. If the alkalinity of the slag is low, the slag has a high degree of superheat and good fluidity, which will aggravate the erosion of the furnace wall and the damage of the alkali lining. Moreover, due to the low melting point of the slag, it is difficult to separate the slag, and it is easy to cause slag iron.
In the "JL" method, Cr 2 O 3 in the silicon reduction 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, so that the reaction temperature increases the slag and The metal temperature difference is not too great, so the slag overheating is not too large. Through practice, when the “JL†method adopts the ratio of gray toe of 1.0, when the alkalinity is controlled at 1.7~1.8, it is cast by ingot mold, no slag inclusion occurs, the quality of the alloy is guaranteed, and the service life of the reaction package is Both refractory and refractory materials are beneficial. [next]
4. High chromium ferrochrome production According to the thermodynamic data provided in the relevant article, the relationship between the free energy of reaction and temperature is shown in Figure 1. 
ΔG=-728.6kJ/mol at T=1900K
Since iron is preferentially reduced by chromium, the proportion of Cr 2 O 3 /FeO in the slag will change as the reaction proceeds.
Fifth, improve the life of the reaction package
PERRIN carbon ferrochrome production of reacted magnesium refractory ladle lining, typically magnesia brick Jingzhen Cao during the entire heat exchange operation of the reactor to produce a chemical reaction of the lining damaging physical attack.
(I) Erosion of SiO 2 Although the melt contains higher CaO, in the reduction process of (Cr, Fe) 2 O 3 by Si, it is SiO 2 and thus there is chemical attack of SiO 2 on the basic refractory. Especially, the slag-iron interface area in the lower part of the reaction pack is more serious.
(2) High temperature erosion The whole reaction is carried out in the high temperature range of 1850~2000 °C, and the melt simultaneously produces strong reflux. Melting and erosion erosion of the liner causes the entire liner to be thinned.
(3) The hot-breaking heat-exchange process of the lining adopts the regular slagging operation, so the reaction package generally works continuously under the alternating environment of hot and cold, causing the lining to generate thermal cracking and natural flaking damage, and even the leakage phenomenon occurs. Natural flaking damage occurs during the phenomenon, and even leakage occurs.
In view of the characteristics of the above reaction packages, the use of magnesium brick masonry reaction packages has not adapted to the operational requirements of the hot-exchange process. The “JL†method has been tested and cooperated with refractory manufacturers to implement a mixing process for the reaction package lining by the combination of magnesia brick and magnesia-calcium slag. The method is to use the ramming material in the bottom and reaction zone. No need for additives and running stirring. Pour it into the bag and use a wind hammer to fight it. The beating process requires as much as possible to level and tamper. After the other parts are made of normal magnesia bricks, the moisture of the magnesium brick masonry binder is dried, and the reaction package can be used for normal production.
In the actual use process, through the melt reaction temperature transfer of about 1900 °C, the magnesium-calcium iron ramming layer can completely achieve the self-sintering effect. At present, the package has reached 100 furnaces. Compared with the normal magnesium brick masonry process, the number of 50 heats is more than doubled. Compared with foreign countries, no additional equipment is added. For example, only new materials are used for the slag slag machine, which effectively improves the service life of the reaction package.
Sixth, product quality "JL" method smelting micro-carbon ferrochrome, not only can produce C ≤ 0.03% of micro-carbon ferrochrome, it can also produce other grades of refined ferrochrome according to market needs. In addition, "JL" method has one The important feature is that it can upgrade the quality of the product.
At present, users have higher and higher requirements for the quality of ferroalloy products. Most users require small-grained products. In the process of small-grain processing, about 15% of the undersize products are produced. On the other hand, each ton of sieve material requires a certain packaging cost. After adopting the "JL" method, after simple repacking and remelting treatment, the funds can be activated to reduce the occupation of the site, and the low-grade undersize products can be converted into high-grade products to increase the added value of the products.
Seven, equipment conditions
The “JL†method micro-carbon ferrochrome production is based on the original 6.3MVA tilt-type “electric silicon thermal method†refining electric furnace, which is designed and modified by itself. It has the following characteristics:
(1) The exchange between the "electric silicon thermal method" and the "Poll method" can be freely realized by the star angle transformation of the transformer connection method.
(2) The tilting electric furnace can control the amount of melt per furnace to ensure the normal reaction.
(3) The fully enclosed electric furnace with cover reduces the noise, reduces the flying of the furnace material, and facilitates the operator to pick up the electrodes.
(4) The graphite carbon head replaces the copper head, which improves the service life of the carbon head and attaches a water-passing movable sleeve, effectively stopping the electrode from falling.
(5) The tubular combined water-cooled furnace cover ensures that the furnace cover will not leak water for many years and stabilize production.
VIII. Conclusions <br> (1) The “JL†method is based on the original “electric silicon thermal method†electric furnace, which has the characteristics of less investment, short cycle and quick effect.
(2) The "JL" method can achieve better technical and economic indicators under the premise that raw material conditions and equipment conditions are not perfect.
(3) The “JL†method produces micro-carbon ferrochrome, which has the advantages of flexible process conversion, high value-added products and long service life.
(IV) The next step is to improve the current raw material conditions, strive to enter the furnace with hot materials, and use the slag casting to realize the two-step process, which will be the further improvement and development of the “JL†method. 