METHOD FOR PRODUCING CHROMIUM CONTAINING MOLTEN IRON WITH LOW SULPHUR CONCENTRATION

Abstract: Reduction of chromium oxide is performed by refining or reducing top-blown vessels. Chromium oxide is charged in the molten iron bath in the aforementioned vessel. Content of slag is adjusted to maintain the following equation: CaO/SiO₂ : 2.1 to 3.5 MgO/Al₂O₃ : 0.6 to 0.8

Description: The present invention relates generally to a method for producing chromium containing molten iron. More specifically, the invention relates to the reduction of chromium containing molten iron produced from chromium oxide, such as chromium ore, for desulphurization. Japanese Patent First (unexamined) Publication (Tokkai) Showa 60-9815 and Japanese Patent Second (allowed) Publication (Tokko) Showa 62-49346 disclose technologies for the melting reduction of chromium oxide, such as chromium ore utilizing top and bottom-blown converter. In the disclosed technology, chromium oxide and carbon containing reducing agent, which also serves as heat source, are charged in molten pig iron in a melting bath. An oxygen jet is injected into the molten melting bath for combustion of the carbon to perform reduction of the chromium oxide by the heal generated by combustion of the carbon. In such a melting reduction process, a large amount of carbon containing material, such as coal, is used as the heat source and reducing agent. Such carbon containing material generally contains sulphur in a content of 0.5 wt%. Therefore, the concentration of sulphur in the molten iron increases according to the increasing amount of carbon material. The relationship between the amount of carbon material and the concentration of sulphur (ratio versus overall molten iron amount = %S) is shown in Fig. 6 in the accompanying drawings. Therefore, desulphurization treatment has been required after the reduction process. For example, desulphurization treatment can be performed by flux injection after tapping the molten iron from the converter. This requires additional processes for desulphurization and thus clearly lowers production efficiency. In order to avoid the additional desulphurization processes after tapping, a desulphurization process is generally performed in the converter during the reduction period after decarbonization. However, such a process increases the load in the reduction process, creating the following problems: First of all, during the reduction period after the decarbonization process, a large amount of ferrosilicon which is known as an inexpensive reduction agent, is used for reduction. Therefore, in order to promote desulphurization, it is required to maintain the basicity at high level. Therefore, the required amount of calcium hydroxide is increased. Secondly, increasing of amount of calcium hydroxide, temperature of melting bath is raised for compensation of heat and for promoting desulphurization. This accelerates the damaging of the refractory in the converter wall. Furthermore, in order to maintain the oxygen potential in the melting bath during the desulphurization process, an additional amount of ferrosilicon as deoxidation agent becomes necessary. In addition, performing desulphurization in the converter necessarily expands the process period in the converter, causing shortening of the life of the refractory. Expansion of the process period in the converter also increases the amount of bottom-blown inert gas, such as Ar gas, which is expensive. Therefore, it is desirable to produce low sulphur concentration molten iron through a reduction process. Therefore, it is an object of the present invention to provide a novel reduction process for producing chromium containing molten iron which has a low sulphur concentration so as not to require an additional desulphurization process. It comes to the inventors' attention that effective melting reduction of chromium ore and semi-reduced pellet can be performed in a melting reduction furnace. Effective reduction which can be performed in the melting reduction furnace results in a low oxygen potential in the slag and molten iron to effectively promote a desulphurizing reaction. As is well known, in order to promote a desulphurizing reaction, it is necessary to raise the basicity level, to raise the temperature of the molten iron and to lower the oxygen concentration in the molten iron. In the case of the reduction of chromium oxide, an improvement of the yield and a minimizing of the damage caused on the refractory has to be achieved. After various experiments, the inventors have reached the idea of effective reduction of chromium oxide with satisfactorily high yield and with minimizing melting of the retractory under the following condition. According to the present invention, reduction of chromium oxide is performed by utilizing a refinement or reduction container having top and bottom blowing capability. Chromium oxide is charged in the molten iron bath in the aforementioned container. The content of slag is adjusted to maintain the following condition: CaO/SiO₂ : 2.1 to 3.5 MgO/Al₂O₃ : 0.6 to 0.8 In order to implement the reduction process according to the present invention, it is required to provide strong stirring ability for promoting reaction between the charge and molten iron bath. Therefore, the container to be used for the process according to the invention should have the capability of top and bottom blowing. Furthermore, the container should associate with a facility which can perform an intermittent or continuous charge of chromium containing oxide, such as chromium ore, semi-reduced chromium pellet, carbon containing material, dolomite, calcium hydroxide and other charges. According to one aspect of the invention, a process for producing chromium containing molten iron with low sulphur content, comprising the steps of: providing a container which has a top and bottom blowing injection capability; forming molten iron bath in the container with molten pig iron; preparing slag to provide CaO/SiO₂ in a range of 2.1 to 3.5 and MgO/Al₂O₃ in a range of 0.6 to 0.8; and charging chromium oxide containing material and reduction agent containing material to the molten iron bath in the container, and bottom blowing the molten iron bath. The process according to the invention, controls the content of sulphur in the final product of molten iron to less than or equal to 0.015 wt%. Also, the process according to the present invention is designed for producing molten iron containing chromium in a range of about 5 wt% to 35 wt%. Further preferably, the process comprises a step of continuously charging flux at a controlled amount so as to maintain CaO/SiO₂ in the range of 2.3 to 3.5 in order to control content of sulphur in the final product of molten iron to less than or equal to 0.008%. In the preferred process, the container comprises a top and bottom-blown converter. The chromium containing material and the reduction agent containing material from the top of the converter. The process may further comprises a step of continuously charging a melting promotion additive at a controlled amount so as to maintain CaO/SiO₂ in the range of 2.1 to 3.5 and MgO/Al₂O₃ in the range of 0.6 to 0.8. The melting promoting agent is lime, dolomite. The amount of the melting promoting agent may be determined according to the charge amount of the chromium containing material and the reduction agent containing material. In the practical implementation of the chromium reducing process, according to the present invention, set forth above, the process comprises the steps of: charging a chromium containing scrap and molten pig iron to a top and bottom-blown converter for forming molten iron bath; performing a scrap melting and heating stage operation in which top blowing of oxygen with charging of a carbon containing material and a slag forming agent through the top of the converter is performed for melting the chromium containing scrap and heating the molten iron bath to a predetermined temperature; and performing a reduction stage operation subsequent to the scrap melting and heating stage operation, in which top blowing of oxygen with charging of a carbon containing material and chromium oxide through the top of the converter is performed for reducing chromium and thus forming chromium containing molten iron. Further practically, the scrap melting and heating stage operation is performed for heating the molten iron bath at a temperature higher than or equal to 1500 °C. In addition, it is preferable that the scrap melting and heating stage operation is performed to establish a relationship between carbon concentration [C] and chromium concentration [Cr] satisfying the following formula: [C] ≧ 4.03 + 0.084 x [Cr] In addition, the scrap melting and heating stage may be separated into two series steps, in which a first scrap melting step is performed in advance of a second heating step, for melting the scrap and the second heating step is performed subsequent to the first scrap melting step for rising the temperature of the molten iron bath to a temperature higher than or equal to 1500 °C and adjusting carbon concentration [C] versus chromium concentration [Cr] to satisfy the following formula: [C] ≧ 4.03 + 0.084 x [Cr] A process may further comprises a step of monitoring a condition of molten iron bath and detecting a time of transition between the first scrap melting step and the heating step on the basis of the monitored condition. The present invention will be understood more fully from the detailed description given herebelow and from the accompanying drawings which illustrate results of experiments, which, however, should not be taken to limit the invention but are for explanation and understanding only. In the drawings: Fig. 1 is a graph showing the relationship between CaO/SiO₂ in the slag and the sulphur concentration (%S) in the molten iron; Fig 2 is graph showing the relationship between CaO/SiO₂ and the chromium reduction yield; Fig. 3 is a graph showing the relationship between MgO/Al₂O₃ in the slag and the sulphur concentration (%S) in the molten iron; Fig. 4 is a graph showing the relationship between MgO/Al₂O₃ in the slag and the melting index of MgO; Fig. 5 is a graph showing the relationship between MgO/Al₂O₃ and the T.Cr amount; and Fig. 6 is a graph showing the relationship between the carbon material amount and the sulphur concentration (%S) in the molten iron bath. As set forth above, according to the present invention, reduction of chromium oxide is performed by utilizing a refinement or reduction container having a top and bottom blowing capability. Chromium oxide is charged into the molten iron bath in the aforementioned container. The content of slag is adjusted to maintain the following condition: CaO/SiO₂ : 2.1 to 3.5 MgO/Al₂O₃ : 0.6 to 0.8 In order to implement the reduction process according to the present invention, it is necessary to provide strong stirring ability for promoting reaction between the charge and molten iron bath. Therefore, the container to be used for the process according to the invention should have the capability of top and bottom blowing. Furthermore, the container should be associated with a facility which can perform intermittent or continuous charge of chromium containing oxide, such as chromium ore, semi-reduced chromium pellet, carbon containing material, dolomite, lime and other charges. In order to implement that process according to the present invention, molten pig iron of 85 tons was filled in a top and bottom blown converter. The molten pig iron contains more than or equal to 3.5 wt% of C for forming the molten metal bath. The temperature of the molten pig iron was in a range of 1500 °C to 1600 °C. Semi-reduced Cr pellet of 250 kg/t to 400 kg/t and coke of 200 kg/t to 300 kg/t were charged. Melting reduction was performed for obtaining molten iron containing 10 wt% to 20 wt% of Cr . During the process, the relationship between CaO/SiO₂ and sulphur content in the molten iron was checked. The result is shown in Fig. 1 . As will be seen from Fig. 1 , by increasing CaO/SiO₂, desulphurizing efficiency is increased. When CaO/SiO₂ is smaller than 2.1, the sulphur content in the molten iron fluctuates at significant level and one cannot stably obtain a low sulphur concentration in the molten iron. In the same condition, the relationship between CaO/SiO₂ and Cr reduction yield was checked. The result is shown in Fig. 2 . Cr reduction yield was obtained from the following equation: Yield = {(Output Cr (kg))/(Input Cr (kg))} x 100 (%) As will be clear from Fig. 2 , the yield is lowered through increasing CaO/SiO₂. Lowering of the yield is considered to be caused by an increase of the slag volume, by splashing of the molten iron, by granulating loss and by a slow-down in the solidification of the slag, causing lowering of the reduction speed of Cr oxide. As seen from Fig. 2, the yield drops substantially when CaO/SiO₂ becomes greater than 3.5. Therefore, the preferred range of CaO/SiO₂ is in a range of 2.1 to 3.5. When reduction of Cr oxide is performed by adjusting CaO/Sio₂ in the slag in the range set forth above, the sulphur concentration in the molten iron bath still fluctuated in a range of 0.005 wt% to 0.020 wt%. In order more stably and more effectively to perform desulphurization, various attempts were performed. After various experiments, the inventors have found that MgO/Al₂O₃ was an effective parameter for stably obtaining chromium containing molten iron with low sulphur content. As is well known, MgO and Al₂O₃ are contained in Cr ore. Accordingly, when the amount of Cr ore charged in the molten iron bath is increased, concentrations of MgO and Al₂O₃ are naturally increased. This causes an increase of the total amount of Cr ( T.Cr ) contained in the slag to lower the Cr reduction yield. In the preferred process, since the CaO/SiO₂ is adjusted in the range of 2.1 to 3.5 in the slag, CaO is effective to dilute MgO and Al₂O₃. By maintaining CaO/SiO₂ in the range of 2.1 to 3.5, the relationship between MgO/Al₂O₃ was checked and result is shown in Fig. 3. As seen from Fig. 3, by adjusting MgO/Al₂O₃ in the range of 0.5 to 1.0, Cr containing molten iron with a low sulphur content of less than or equal to 0.015 wt% can be stably produced. On the other hand, when adjusting MgO/Al₂O₃ in the range of 0.5 to 1.0, charge to cause substantial melting of refractory can be created. Fig. 4 shows the relationship between the MgO melting amount as represented by melting the index and MgO/Al₂O₃. The MgO melting index is derived by calculating the slag amount on the basis of Al₂O₃ concentration and performing a balance calculation. The melting index in positive value (+) shows that MgO in the refractory is melting out and in negative value (-) shows that MgO is adhering on the refractory. As seen from Fig. 4, in order to maintain the MgO melting index less than or equal to 0.5, MgO/Al₂O₃ is to be adjusted in a ratio greater than or equal to 0.60. Fig. 5 shows the relationship between the T.Cr amount (wt%) and MgO/Al₂O₃. As seen from Fig. 5, MgO/Al₂O₃ is required to be set less than or equal to 0.8 for improving the Cr reduction yield. If MgO/Al₂O₃ is greater than 0.8, the reduction speed is lowered causing lowering of the Cr reduction yield. MgO/Al₂O₃ can be adjusted by adjusting the charge amount of dolomite and Al₂O₃ depending upon the MgO/Al₂O₃ amount contained in the Cr ore. In view of various factors set out above, the preferred range of MgO/Al₂O₃ is 0.6 to 0.8. By setting MgO/Al₂O₃ in the range set forth above and setting CaO/SiO₂ in the range of 2.1 to 3.5, Cr containing molten iron with satisfactorily low sulphur concentration, i.e. less than or equal to 0.015 wt%, can be stably produced without causing substantial damage of the refractory. [ Example 1 ] Utilizing a top and bottom-blown converter having capacity of 85 tons, the reduction process according to the present invention was performed to produce 14% chromium containing molten iron. Molten pig iron filled in the converter had a composition as set out in the following table I . (wt%) C Si Mn P S 4.15 Tr 0.07 0.011 0.032 The temperature of the molten iron was 1190 °C. The molten iron was filled in the converter in amount of 63.8 tons. Coke and semi-reduced Cr pellet are continuously charged. The semi-reduced Cr pellet had a content as shown in table II . (wt%) T.Cr T.Fe SiO₂ Al₂O₃ CaO MgO P S 32.18 22.23 4.72 17.28 0.45 9.10 0.024 0.128 The amounts of lime, dolomite were adjusted according to the charge amount of coke and semi-reduced Cr pellet so that the composition of the slag can be adjusted to be suitable for implementing the preferred reduction process according to the present invention. In the shown implementation CaO/SiO₂ was set at 2.5 and MgO/Al₂O₃ was set at 0.65. The amounts of lime, coke, semi-reduced Cr pellet and top-blown oxygen were as shown in the following table III. Cr Pellet Coke Lime Dolomite O₂ 36.09t 33.24t 6.55t 2.61t 23346 Nm³ The composition of the molten iron after the preferred reduction process according to the invention is shown in the following table IV and the composition of the slag is shown in the following table V. The results shown in the tables IV and V were obtained after the reduction process for a period of 87.6 minutes; the amount of tapped molten iron was 75.1 tons and Cr reduction ratio was 91.82%. (wt%) Tapping Temp. C Si Mn P S Cr Cr Yield 1556 °C 6.02 Tr 0.21 0.29 0.003 14.20 91.82 (wt%) T.Fe T.C SiO₂ MnO P₂O₅ S Al₂O₃ 0.7 0.23 14.5 0.1 0.01 0.50 25.97 CaO MgO T.Cr CaO/SiO₂ MgO/Al₂O₃ 35.0 16.65 0.6 2.41 0.64 As will be appreciated from the tables IV and V, by adjusting CaO/SiO₂ and MgO/Al₂O₃ in the ranges set forth above, chromium containing molten iron with satisfactorily low sulphur content can be effectively produced without causing damage on the refractory. [ Example 2 ] Utilizing a top and bottom-blown converter having capacity of 85 tons, a reduction process according to the present invention was performed to produce 14% chromium containing molten iron. The molten pig iron filled in the converter had a composition as set out in the following table VI. (wt%) C Si Mn P S 4.07 0.02 0.04 0.014 0.051 The temperature of the molten iron was 1235 °C. The molten iron was filled in the converter in amount of 65.3 tons. Coke and semi-reduced Cr pellet are continuously charged. The semi-reduced Cr pellet had content as shown in foregoing table II . The amounts of lime, dolomite were adjusted according to the charge amount of coke and semi-reduced Cr pellet so that the composition of the slag can be adjusted to be suitable for implementing the preferred reduction process according to the present invention. In the shown implementation, CaO/SiO₂ was set at 2.5 and MgO/Al₂O₃ was set at 0.65. The amounts of lime, coke, semi-reduced Cr pellet and top-blown oxygen were as shown in the following table VII. Cr Pellet Coke Lime Dolomite O₂ 37.38t 29.74t 5.28t 3.65t 21351 Nm³ The composition of the molten iron after the preferred reduction process according to the invention is shown in the following table VIII and the composition of the slag is shown in the following table IX. The results shown in the tables VIII and IX were obtained after the reduction process for a period of 75.5 minutes; the amount of tapped molten iron was 72.4 tons and the Cr reduction ratio was 91.14%. In this experiment, the MgO melting index was -0.36. (wt%) Tapping Temp. C Si Mn P S Cr Cr Yield 1562 °C 6.02 0.01 0.16 0.29 0.012 13.91 91.14 (wt%) T.Fe T.C SiO₂ MnO P₂O₅ S Al₂O₃ 0.5 0.04 13.7 0.1 0.01 0.531 28.64 CaO MgO T.Cr CaO/SiO₂ MgO/Al₂O₃ 29.2 19.96 0.5 2.13 0.696 [ Example 3 ] Utilizing a top and bottom-blown converter having capacity of 85 tons, a reduction process according to the present invention was performed to produce 14% chromium containing molten iron. The molten pig iron filled in the converter had a composition as set out in the following table X. (wt%) C Si Mn P S 4.09 0.02 0.05 0.016 0.049 The temperature of the molten iron was 1230 °C. The molten iron was filled in the converter in amount of 71.1 tons. Coke and semi-reduced Cr pellet are continuously charged. The semi-reduced Cr pellet had a content as shown in foregoing table II. The amounts of lime, dolomite were adjusted according to the charge amount of the coke and the semi-reduced Cr pellet so that the composition of the slag can be adjusted to be suitable for implementing the preferred reduction process according to the present invention. In the shown experiment, CaO/SiO₂ was set at 3.2 and MgO/Al₂O₃ was set at 0.75. The amounts of lime, coke, semi-reduced Cr pellet and top-blown oxygen were as shown in the following table XI. Cr Pellet Coke Lime Dolomite O₂ 38.90t 34.17t 5.87t 4.88t 24078 Nm³ The composition of molten iron after the preferred reduction process according to the invention is shown in the following table XII and composition of slag is shown in the following table XIII. The results shown in the tables XII and XIII were obtained after the reduction process for a period of 82.5 minutes; the amount of tapped molten iron was 85.5 tons and Cr reduction ratio was 96.2%. In this experiment, the MgO melting index was -0.17. (wt%) Tapping Temp. C Si Mn P S Cr Cr Yield 1574 °C 6.20 0.03 0.17 0.30 0.001 15.14 96.2 (wt%) T.Fe T.C SiO₂ MnO P₂O₅ S Al₂O₃ 0.6 1.69 12.2 0.1 0.01 0.606 25.44 CaO MgO T.Cr CaO/SiO₂ MgO/Al₂O₃ 39.0 19.29 0.6 3.20 0.76 [ Example 4 ] Utilizing a top and bottom-blown converter having capacity of 85 tons, the reduction process according to the present invention was performed to produce 14% chromium containing molten iron. The molten pig iron filled in the converter had a composition as set out in the following table XIV. (wt%) C Si Mn P S 4.15 0.01 0.05 0.009 0.034 The temperature of the molten iron was 1190 °C. The molten iron was filled in the converter in amount of 60.8 tons. Coke and semi-reduced Cr pellet are continuously charged. The semi-reduced Cr pellet had a content as shown in foregoing table II. The amounts of lime, dolomite were adjusted according to the charge amount of the coke and semi-reduced Cr pellet so that the composition of the slag can be adjusted to be suitable for implementing the preferred reduction process according to the present invention. In the shown experiment, CaO/SiO₂ was set at 2.5 and MgO/Al₂O₃ was set at 0.7. The amounts of lime, coke, semi-reduced Cr pellet and top-blown oxygen were as shown in the following table XV. Cr Pellet Coke Lime Dolomite O₂ 38.07t 29.88t 5.82t 3.74t 21778 Nm³ The composition of the molten iron after the preferred reduction process according to the invention is shown in the following table XVI and composition of the slag is shown in the following table XVII. The results shown in the tables XVI and XVII were obtained after the reduction process for a period of 79.3 minutes; the amount of tapped molten iron was 79.0 tons and the Cr reduction ratio was 92.73%. In this experiment, the MgO melting index was -0.15. (wt%) Tapping Temp. C Si Mn P S Cr Cr Yield 1556 °C 6.03 0.01 0.15 0.30 0.003 14.81 95.50 (wt%) T.Fe T.C SiO₂ MnO P₂O₅ S Al₂O₃ 0.5 0.18 13.4 0.1 0.01 0.583 28.45 CaO MgO T.Cr CaO/SiO₂ MgO/Al₂O₃ 32.40 20.75 0.6 2.42 0.73 [ Example 5 ] In order to practically implement the chromium reducing process according to the present invention, another experiment was carried out through the following process which comprises the steps of: charging a chromium containing scrap and molten pig iron to a top and bottom-blown converter for forming a molten iron bath; performing a scrap melting and heating stage operation in which top blowing of oxygen with charging carbon containing material and slag forming agent through the top of said converter is performed for melting said chromium containing scrap and heating said molten iron bath to a predetermined temperature; and performing a reduction stage operation subsequent to said scrap melting and heating stage operation, in which top blowing of oxygen with charging of carbon containing material and chromium oxide through the top of said converter is performed for reducing chromium and thus forming chromium containing molten iron. Further practically, the scrap melting and heating stage operation is performed for heating said molten iron bath at a temperature higher than or equal to 1500 °C. In addition, it is preferable that the scrap melting and heating stage operation is performed to establish a relationship between the carbon concentration [C] and the chromium concentration [Cr] satisfying the following formula: [C] ≧ 4.03 + 0.084 x [Cr] In the experiment, for the converter, stainless steel scrap in amount of 22.9 tons was charged by means of scrap shoot. After charging the stainless steel scrap, dephosphorized molten pig iron in amount of 41.3 tons was charged. Immediately after charging the molten pig iron, the converter is set at the vertical position and blowing was performed. The composition of the stainless steel scrap and amount of scraps are shown in the following table XVIII. C Si P S Cr Ni Weight (ton) Total Weight (ton) SUS304 Heavy Scrap 0.07 0.45 0.040 0.010 18.15 8.50 6.4 22.9 SUS304 Light Scrap 0.07 0.45 0.040 0.010 18.15 8.50 16.5 On the other hand, the composition of dephosphorized molten pig iron is shown in the following table XIX. Temp. °C C Si Mn P S Cr Ni 1170 4.41 tr 0.03 0.010 0.024 --- --- After blowing oxygen in an amount of 5500 Nm³, a sub lance was inserted into the molten iron bath for measuring the temperature thereof. The temperature was 1525 °C. In the scrap melting process and before measuring the temperature of the molten iron bath, 280 kg of lime was charged for compensating basicity for Si contained in the scrap. At the blowing of oxygen in amount of 5500 Nm³, the temperature rising coefficient k can be obtained from the following condition: molten pig iron temperature : 1170 °C measured molten iron temperature : 1525 °C amount of molten pig iron : 41.3 tons amount of scrap : 22.9 tons k = {(1525 - 1170)/5500} x (41.3 + 22.9) = 4.14 The molten iron temperature was again measured after blowing oxygen in an amount of 6200 Nm³. The measured molten iron temperature was 1565 °C. From this, the temperature rising coefficient k is derived from: k = {(1565 - 1525)/(6200 - 5500)} x (41.4 + 22.9) = 3.67 Judgement could be made that the scrap was melted at this time. In the shown experiment, a target temperature for performing Cr reducing process was set at 1575 °C. Therefore, in the heating step, a temperature increase of 10 ° was required. For raising the molten iron temperature by 5 °C, the required oxygen amount to be blown can be derived from: {(1575 - 1565)/3.67} x (41.3 + 22.9) = 175 Nm³O₂ Therefore, after blowing 180 Nm³ of oxygen in the heating step, the process moves to the second Cr reduction step. Through the scrap melting step and the heating step, the carbon containing material, i.e. coke was charged in a ratio of 1.8 kg/Nm³ O₂. The process time from the beginning of the process to the beginning of the Cr reduction process was 28.6 min. Here, %Cr of the scrap and charged weight of scrap, %Cr in the molten iron bath can be derived by: {(6.4 + 16.5) x 0.1815/(22.9 + 41.4)} x 100 = 6.47% From this, it is appreciated that %C has to be greater than or equal to 4.57. After blowing 6200 Nm³ of oxygen, the %C derived from the analysis of measured data by means of the sub lance was 4.60 which satisfies the formula of: [%C] ≧ 4.03 + 0.084 x [%Cr] In the reduction process, in order to maintain the molten iron bath temperature constant and in order to maintain heat balance, semi-reduced Cr pellet of 2.4 kg/Nm³ O₂ and carbon containing material of 1.3 kg Nm³ O₂ were charged. The composition of the semi-reduced Cr pellet is shown in the following table XX. After completing charging of semi-reduced Cr pellet set forth above and after blowing oxygen in an amount of 18000 Nm³, the process moves to a finishing reduction stage. In the finishing reduction stage, the oxygen blowing speed is reduced to perform top-blow in a ratio of 60 Nm³/min, and to perform bottom-blow in a ratio of 60 Nm³/min. After 10 min of the finishing reduction process, the resultant molten iron was tapped. The overall process period was 69.95 min. Immediately before entering into the finishing reduction process, the temperature of molten iron bath was measured by means of a sub lance. The measured temperature was 1570 °C. This proves that the temperature of the molten iron bath was maintained substantially in constant. The molten iron bath temperature and the composition of tapped molten iron are shown in the following table XXI. Temp °C C Si Mn P S Cr Ni 1554 6.06 --- 0.39 0.032 0.009 2.73 On the other hand, the composition of the slag at tapping is shown in the following table XXII. T.Fe T.C SiO₂ MnO P₂O₅ S Al₂O₃ CaO MgO T.Cr 0.7 2.31 14.7 0.1 0.01 0.569 23.71 37.5 16.34 0.6 The charge charged in the converter is shown in the following table XXIII Pig Iron Scrap Cr Pellet Coke Lime Dolomite O2 Pr N2 41.3t 22.9t 23.35t 29.02t 5.74t 2.28t 19575 245 1764 In the experiment set out above, the Cr reduction yield was 95.21%, the molten iron production yield was 92.72% and the Ni reduction yield was 100%. [ Example 6 ] Other experiments were performed for checking the efficiencies of production of chromium containing molten iron when the finishing reduction stage is performed and is not performed. In addition, conventional processes with a final reduction process and without finishing reduction process were performed in order to obtain comparative data. The results are shown in the following table XIV. Example 1 Example 2 Comp. 1 Comp. 2 Pig Iron (t) 41.3 40.7 38.7 39.7 Scrap (t) 23.4 22.9 23.0 23.0 Cr Pellet (t) 26.78 21.95 25.84 24.54 Coke (t) 29.74 27.42 26.19 26.60 Oxygen (Nm³) 21120 19440 21707 21087 Lime (t) 6.25 5.39 6.46 5.99 Dolomite t 2.34 2.31 2.15 2.14 Tap C (%) 5.46 5.57 5.54 5.21 Tap Cr (%) 16.43 15.26 14.90 14.77 Tap Temp. (oC) 1557 1562 1565 1576 Cr Yield (%) 93.5 96.0 80.4 84.2 Iron Production Yield (%) 92.3 93.5 88.9 90.5 Tapping Amount (t ) 73.2 70.6 67.4 68.8 Blowing Time (min ) 72 69 74 78 Finishing Reduction (min) -- 10 -- 10 Scrap Melting Step (min) 26.9 26.9 -- -- Reduction Step (min) 45.1 32.1 -- -- In the foregoing table VII, example 1 is the result obtained from the preferred process but without performing the finishing reduction, example 2 is the result obtained from the preferred process with the finishing reduction, comp. 1 is comparative example performed according to the conventional process and without performing a finishing reduction, and comp. 2 is comparative example performed according to the conventional process with a finishing reduction. As will be appreciated herefrom, the present invention enables production of the chromium containing molten iron to be performed with the converter with satisfactorily high yield. Furthermore, according to the present invention, damage on the refractory wall of the converter can be minimized.

Claims: A process for producing chromium containing molten iron with low sulphur content, comprising the steps of: providing a container which has a top and bottom-blowing capability; forming a molten iron bath in the container with molten pig iron; characterised by the further steps of adjusting slag to provide CaO/SiO₂ in a range of 2.1 to 3.5 and MgO/Al₂O₃ in a range of 0.6 to 0.8; charging chromium oxide containing material and carbon containing material to the molten iron bath in said container; and bottom blowing the molten iron bath. A process as claimed in claim 1 whereby the content of sulphur in the final product of molten iron is less than or equal to 0.015 wt%. A process as claimed in claim 1 or 2, which produces molten iron containing chromium in a range of about 5 wt% to 35 wt%. A process as claimed in claim 1, 2 or 3 which further comprises a step of continuously charging flux at a controlled amount so as to maintain CaO/SiO₂ in the range of 2.1 to 3.5 and MgO/Al₂O₃ in the range of 0.6 to 0.8. A process as claimed in claim 4, wherein said flux is lime and dolomite. A process as claimed in claim 4 or 5, which further comprises a step of continuously charging flux at a controlled amount so as to maintain CaO/SiO₂ in the range of 2.3 to 3.5 in order to control the content of sulphur in the final product of molten iron to less than or equal to 0.008%. A process as claimed in any preceding claim, which further comprises a step of charging a chromium containing scrap during formation of said molten iron bath. A process as claimed in claim 7, wherein the process for producing chromium containing molten iron, comprises the steps of: charging a chromium containing scrap and molten pig iron to a top and bottom-blown converter for forming molten iron bath; performing a scrap melting and heating stage operation in which top blowing of oxygen with charging carbon containing material and slag forming agent through the top of said converter is performed for melting the chromium containing scrap and heating the molten iron bath to a predetermined temperature; and performing a reduction stage operation subsequent to the said scrap melting and heating stage operation, in which top blowing of oxygen with charging of carbon containing material and chromium oxide through the top of said converter is performed for reducing chromium and thus forming chromium containing molten iron. A process as claimed in claim 8, wherein the scrap melting and heating stage operation is performed for heating the molten iron bath to a temperature greater than or equal to 1500 °C. A process as claimed in claim 8 or 9, wherein the scrap melting and heating stage operation is performed to establish a relationship between carbon concentration [C] and chromium concentration [Cr] satisfying the following formula: [C] ≧ 4.03 + 0.084 x [Cr] A process as claimed in claim 8, wherein the scrap melting and heating stage is separated into two series steps, in which a first scrap melting step is performed in advance of a second heating step, for melting the scrap and the second heating step is performed subsequent to the first scrap melting step for raising the temperature of the molten iron bath to a temperature greater than or equal to 1500°C and adjusting carbon concentration [C] versus chromium concentration [Cr] to satisfy the following formula: [C] ≧ 4.03 + 0.084 x [Cr] A process as claimed in claim 11, which further comprises a step of monitoring a condition of the molten iron bath and detecting a time of transition between the first scrap melting step and the heating step on the basis of the monitored condition.

Inventors: BADA, HAJIME, TADA, CHIKASHI, TAOKA, KEIZO, YAMADA, SUMO, Bada, Hajime, c/o Kawasaki Steel Corp., TADA, CHIKASHI, C/O KAWASAKI STEEL CORP., TAOKA, KEIZO, C/O KAWASAKI STEEL CORP., YAMADA, SUMO, C/O KAWASAKI STEEL CORP.
Applicants: KAWASAKI STEEL CORPORATION
Application year: 1989
Main classification: C21C5
IPC codes: C21B13, C21C5, C21B11, C22C33