Elyutin (1961) defined the optimum operation conditions for the production of ferrochromium. It was concluded that slag must have a liquidus temperature 100°C higher than alloy. High MgO to CaO ratio and low Al2O3 contents are the key factors to match with low viscosity requirements of a slag which may contain 45 to 50 wt. % of SiO2. During the production of ferrochromium, the silica can be partly reduced by the iron and chromium. The reduction of SiO2 and Cr2O3 proceed in parallel and the product of this reaction, silicon can take place in the further reduction of chromium oxides as shown in reaction (2.1).
[ ]
[ ] ( ) 3 4 ) ( 3 2 2 3 2O Si Cr SiO Cr + ⇔ + (2.1)Furthermore, the phosphorus of the charge is partially carried away with the off- gases, but some remains in the slag and the largest part (60 %) moved into the alloy. Due to the poor desulphurisation capacity of high carbon ferrochromium smelting slags, sulphur in the system is partly moved into alloy and the rest of the sulphur is carried away from the furnace as off-gas. Also sulphur and phosphorus mainly originate from the reducing materials such as coke, coal or char used in the production of ferroalloy (Elyutin, 1961).
Chapter 2- Literature Review 36 Nafziger (1983) investigated high carbon ferrochromium smelting by using
chromite concentrates. It was reported that the silicon concentration in the metal varied inversely with the amount of sulphur in the metal. In another study, Nafziger (1988) claimed that the sulphur can be removed as a volatile silicon sulphide gas at higher levels of silicon (4-5 wt. %) in the metal. However, the alloy’s phosphorus levels (0.12-0.19 wt. %) were significantly higher than the requirements of ASTM standards. The chromite concentrate was found to be the one of the main sources of phosphorus. The amount of phosphorus was in the range of 0.04-0.08 wt. % in the chromite concentrate. However, quartz was found to contain up to 0.24 wt. % phosphorus and assumed as another major source of phosphorus for ferrochromium alloy. According to the results, 60-80 % of initial phosphorus can be transferred into alloy phase (Nafziger, 1988). Nafizger indicated that to have a highly basic slag in the system with the low bath temperatures can limit the phosphorus transfer from the slag to metal phase. The optimum slag liquidus temperature was found to in the range of 1550-1 575°C. Also, a nominal slag basicity value of 1.04 was determined as optimum slag basicity in order to achieve high carbon ferrochromium smelting. The optimum physicochemical properties of slag such as low liquidus temperature, basicity and fluidity were achieved by the addition of CaO and SiO2 into smelting process (Nafziger, 1983).
Akyuzlu and Eric (1992) conducted a study on the equilibrium between carbon- saturated Fe-Cr-Si-C alloys and SiO2-CaO-MgO-Al2O3-CrOx-FeOy slags under argon and carbon monoxide atmospheres at temperatures of 1 500°C and 1 600°C. The silica reduction can be explained by using reaction 2.2, where the quantities in the round, square and parenthesis brackets refer to the slag,metal and gas phases, respectively. In the Eq. (2.3); K, ai and Pi represent the equilibrium constant, activity of the species and the partial pressure, respectively.
} { 2 ] [ ] [ 2 ) (SiO2 + C ⇔ Si + CO (2.2)
Chapter 2- Literature Review 37 2 2 2 . 2 2 . C SiO CO Si a a P a K = (2.3)
The carbon and silicon solubilities in metal phase tend to increase with temperature. The silicon content in the metal phase was found to be directly proportional to the silica content of the slag phase at 1 600°C. This can be a result of increase in K2.2. As the temperature increases, reaction moves to right direction. When CaO to Al2O3 ratio of the slag increased, the level of silicon in the slag showed a decline due to replacement of alumina by lime which caused a decrease in the activity of silica in the slag phase. With the network modification power of CaO in the slag, the silica network will break down and results an increase in the free oxygen ions which eventually leads to a decrease in the silica activity. According to the results, an increase in the MgO to CaO ratio of the slag causes a decrease in the silicon distribution (partition) ratio (wt. % of Si in the slag / wt. % Si of in the metal). This indicated that the replacement of MgO with CaO causes an increase in the activity of silica which results an increase in the silicon content of the metal phase. Furthermore, experimental results showed that the chromium content of the slag increased wi th the increase of iron content of the metal phase. On the other hand, at low basicity values, the chromium content of the slag phase decreased sharply as the basicity increased from 0.4 to 0.8 at 1 500°C under argon and carbon monoxide atmospheres, and from 0.7 to 1.0 at 1 600°C under carbon monoxide atmosphere. When the threshold values had been passed, the basicity did not influence the chromium solubility in the slag system which reached to a constant value around 0.15 wt. %. At relatively low oxygen potentials the chromium dissolves in the silicate melts as divalent and trivalent ratio, the concentration ratio Cr+2:Cr+3 being dependent on temperature and slag basicity. As the basicity increases the dissolving power of the slag with respect to Cr+2 ions decreases which proves that the depolymerisation of the melt with increasing basicity resulting in an increase in oxygen ion activity. Therefore, we can conclude that the chromous capacity (is the concentration of Cr in the slag assumed to be Cr+2) of the slag decreases as the basicity increases. Also, the chromic capacity (is the concentration of Cr in the slag assumed to be Cr+3) of the
Chapter 2- Literature Review 38 slag reduces as the basicity increases.In other words, the slags low in SiO2 (high
basicity slags) can dissolve less chromium. (Akyuzlu and Eric, 1992).
As a summary, the previous studies showed that physicochemical properties of slag such as low liquidus temperature, basicity and fluidity are the critical factors in the control of phosphorus, sulphur and chromium distribution between slag and alloy phases.