UTILIZING A REDUCING GAS INJECTION IN CONVERTER SLAG FOR AVOIDING BOTTOM BUILD-UP IN REVERBERATORY FURNACES

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1 UTILIZING A REDUCING GAS INJECTION IN CONVERTER SLAG FOR AVOIDING BOTTOM BUILD-UP IN REVERBERATORY FURNACES Sh. Saeid and H.R. Hassani Research and Development Center, Sarcheshmeh Copper Complex, Sarcheshmeh, Kerman, Iran nicico.com R. Vaghar and S.R. Allah karam Professor of Tehran University and R & D Consultants, Sarcheshmeh Copper Complex, Sarcheshmeh, Kerman, Iran Abstract During the conversion of the matte into blister copper, a slag with high copper content and 20-24% magnetite is formed. In the Sarcheshmeh Copper Complex of Iran, copper from this slag is recovered by returning it to the reverberatory furnace. The problem associated with the collection of magnetite in the reverberatory furnace can be dealt with in several ways. In this research, magnetite in the converter slag has been reduced by the injection of reducing gas. Reducing gas was a mixture of propane and butane. Gas blowing was carried out through a lance in a ladle containing 50 kg of molten converter slag, for various periods and at different flowing rates. Results indicated that, with a high efficiency of about 85-90%, the amount of the magnetite could be reduced to less than 3-4%. In addition, a few tests were carried out in semi-industrial scales. The results for these tests showed that with regards to the stoichiometric volume of the reducing gas in accordance to the reduction reactions, the percentage of the magnetite in the converter slag could be reduced to as little as 9%. This reduction in the magnetite content of the converter slag was equivalent to the percentage of the magnetite in the reverb slag. 1. INTRODUCTION The production of molten matte in the reverberatory furnaces and the conversion of it to blister copper via converters is probably the most common way in producing copper from sulfide minerals. At present, the process of melting copper matte in commercial standards is carried out by two reverberatory furnaces in the Sarcheshmeh Copper Complex of Iran. Therefore, increasing efficiency for these furnaces and eliminating any factors which may reduce production in any ways are of paramount importance. The presence of magnetite crystals in the slag can in turn increase viscosity and prevent complete separation of matte from the slag. As a result of this, reduction in copper production via absorption of enriched minerals and matte by the slag can take place. Solid magnetite with higher density than matte, can precipitate in the bottom of the reverberatory furnace and cause the bottom build-up in the furnace and can adversely result in production capacity (1). The precipitation of Fe 3 O 4 can also cause a reduction in the life expectancy of the refractories due to the concentration of temperature above the melt, and the effect of the burner flame on the sensitive areas of the furnace which in turn hampers the feed charge to the furnace and the melt discharge from the furnace. On the other hand magnetite can increase the melting point of the slag and as a result of this the energy consumption will also increase.

2 Preventing furnace bottom build-up will require special treatments and a higher energy consumption. However the bottom build-up will not be avoided completely and the elimination will not be usually uniform. Therefore, it is essential to reduce the sources of producing Fe 3 O 4 in order to avoid the above mentioned problems with the reverberatory furnace. The main source of Fe 3 O 4 in the reverberatory furnace comes from the converter slag. The two reactions taking place in the converter are as follows (2) : FeS + 3/2O 2 = FeO + SO 2 (1) 3FeS + 5O 2 = Fe 3 O 4 + 3SO 2 (2) Thus, the amount of magnetite produced in the converter slag remains quite high and its amount for the Sarcheshmeh Copper Complex is about 20-24%. The presence of FeS in the melt together with the temperature of the melt determine the right conditions for the reaction no. 3 to occur. The conversion of Fe 3 O 4 to FeO and the subsequent reaction which takes place between FeO and SiO 2 results in the production of fyalite (reaction no. 4). The FeO.SiO 2 will remain in the slag and does not create any detrimental effect, which would otherwise be accompanied by the presence of magnetite. There is usually an adequate amount of SiO 2 present in the melt. If that is not the case SiO 2 may be added in order to obtain the right concentration. It might be interesting to know that during the period of operation the temperature reaches 1320 C in the converter. This can be very effective on the reaction processes and in particular reaction no. 3 (3). 3Fe 3 O 4 (s) + FeS(l) = 10FeO(l) +SO 2 (3) 2FeO + SiO 2 = 2FeO.SiO 2 (4) But, due to the high oxygen potential together with a small amount of FeS in the process of conversion, the amount of magnetite will remain high (4). Hence, after entering to the reverberatory furnace, this oxide will precipitate and cause bottom build-up. There are different methods of preventing or reducing bottom build-up in the reverberatory furnace. One method is to eliminate or reduce the magnetite content in the converter slag. The aim of this research is to prevent bottom build-up in the reverberatory furnace by using a reducing gas in order to reduce the magnetite content of the converter slag in the ladle before returning to the reverberatory furnace. 1.1 Reducing gas Gases such as propane and butane mixture which are contained in domestic gas cylinders, are considered as a reducing factor due to the presence of C and H in them. These natural gases can be obtained cheaply and hence their usage are economical. For the purpose of carrying out these experiments, domestic gas cylinders were used due to the difficulty of obtaining natural gas. By gas injection into the converter slag it is possible to have the following reactions in the melt. C 3 H 8 + 7Fe 3 O 4 = 21FeO + 3CO + 4H 2 O (5) C 4 H Fe 3 O 4 = 27FeO + 4CO + 5H 2 O (6)

3 2. EXPERIMENTAL A sample was obtained from about 1000 kg of homogeneous converter slag which had been mixed thoroughly. The analytical results indicated that the amount of magnetite in slag was between 22-24%. For each experiment, an amount of 50 kg of this slag was poured into a ladle which was then placed inside an oven. After about 3 hours the slag was melted and its temperature reached to about 1200 C. At this point the oven was switched off and gas injection was commenced. The pressure of the gas was controlled by a regulator which was connected to two gauges. One of the gauges showed the pressure of the cylinder and the other one showed the consumption rate. Together with these a flow-meter was used in order to indicate the amount of gas discharge in liters per minute. Gas injection into the melt was done by using a lance on top of the melt. When the gas pressure caused disturbance of the melt surface, the lance was immersed inside the melt while the gas injection continued (5). After completing the test, the melt was poured inside a mold in order to cool down. Samples were taken from different areas of the cooled slag (at least from 3 different areas). Experiments were repeated for various periods of injection and at different rate of blowing gas. 2.1 Pilot scale tests In the case of semi-industrial scale tests, 200 kg of the converter slag melt was placed in a large container and the gas blowing was carried out with the same procedure as described earlier. 3. DISCUSSION It is important to know that the main objective of these experiments was the reduction of magnetite in the converter slag before returning it to the reverberatory furnace. An optimum condition was achieved with regards to the gas consumption and time of the experiment. These experiments showed that it is possible to prevent bottom build-up in the reverberatory furnace by treating the slag in a ladle whilst being transferred from the converter to the reverberatory furnace. 3.1 Experimental scale results The results obtained for these tests are shown in table 1. Table 1. Conditions of the Reduction Tests and Their Results Test No. Initial Fe 3 O 4 (%) Blowing Time (min) Gas Flow-Rate (lit/min) Gas Volume (lit) Fe 3 O 4 After Reduction (%) Reduction Efficiency (%) Temperature at End of Test ( C)

4 It can be seen from the results that, there is an increased efficiency as the gas volume is increased. This is indicated in the figure 1. Reduction Efficiency Volum e ofreducing G as (lit) Figure 1 - The Effect of Reducing Gas Volume on the Reduction Efficiency It is also true to say that, at equal periods of the tests, a larger volume of the gas can produce a higher efficiency. This can be seen in figure 2. Reduction Efficiency Volum e ofreducing Gas (lit) Figure 2 - Effect of Reducing Gas Volume on the Reduction Efficiency at 5 min. Injection Period From the results obtained, it can also be deduced that, for an equal volume of the gas the best condition is achieved when the time of injection is increased. Accordingly, for a fixed gas volume of 50 liters, when the time of injection (blowing time) was increased from 5 to 10 minutes, the reduction efficiency raised from 85% to 88%. The reason for this can be due to an extended period when the gas is in contact with the melt, and there is more available time for the reactions occurrence.

5 The presence of sulfur in the slag was also shown to be beneficial. This can be observed by an increased efficiency obtained for the tests based on the gas volume consumption with regards to the stoichiometric reduction reactions no. 5 and Semi-industrial scale results Furthermore, the results obtained for the semi-industrial scale tests are indicated in table 2. Table 2. Semi-Industrial Scale Tests Test No. Initial Magnetite (%) Time (min) Gas Rate (lit/min) Gas Volume (lit) Final Amount of Magnetite (%) These tests were carried out in order to reduce the percentage of the magnetite in the converter slag to as little as the amount of magnetite in the reverb slag (9% ± 1% in Sarcheshmeh Copper Complex reverb slag). It might be worth mentioning that prior to this treatment, from the total amount of the magnetite which entered the reverb, some would enter into the matte, some would go to the slag and the remains would precipitate in the bottom of the furnace. Therefore, by carrying out the above treatment, it was possible to reduce the amount of magnetite in the converter slag and thus prevent bottom build-up in the reverberatory furnace. In addition, for an optimum treatment, it may be worth considering design variation of the ladle containing the converter slag in order to facilitate gas injection, and hence increase the efficiency of the system. 4. CONCLUSIONS In an industrial scale, achieving a high efficiency in the shortest period of time for a reduction operation, determines the best condition. Hence, in this research the best conditions for magnetite reduction in the converter slag were obtained as follows : 1. Five minutes blowing time of the gas mixture at flow-rate of 10 liters per minute. 2. An efficiency of 85% is obtained, due to a reduction of Fe 3 O 4 from 24 to 3.5%. 3. The temperature of the melt reaches 1050 C at the end of the test. Therefore, the melt can not be solidified at this temperature. 4. If there is no time limitation for the reduction operation, the best condition is achieved when using 10 minutes blowing time of the gas mixture at a flowing rate of 2.5 lit/min. Since the rate of gas consumption is reduced, a higher efficiency of 86% is obtained. 5. Bottom build-up can be prevented via reduction of magnetite in converter slag by injection of reducing gas before returning the slag to the reverberatory furnace. 6. Preventing bottom build-up by using a natural gas is very economical.

6 7. During reduction operation, there is a settling time for the reduced copper droplets or entrapped matte in the ladle of the slag. The upper layer of the slag can be disposed off completely, due to a substantial decrease in the copper content in this layer. Thus, a larger available volume of the reverberatory furnace may remain free for operation. 8. The semi-scale industrial results prove that, this procedure is an efficient method for preventing bottom build-up in the reverberatory furnaces. 5. ACKNOWLEDGEMENTS We hereby express our sincere gratitude to all the personnel of the Sarcheshmeh Copper Complex R&D Center in Iran, who helped us during this research. 6. REFERENCES 1. A.K. Biswas and W.G. Davenport, "Extractive Metallurgy of Copper", 3rd Edition, Pergamon Press, New York, NY, USA,1994, pp L.S. Diaz and et al., "Sarcheshmeh Copper Project-Smelter Calculations-Converter Study", Private Communication, The Ralph M. Parsons Company, USA, E. Nicknejad, "Mass and Heat Balance in Peirce-Smith Converter at Sarcheshmeh Copper Complex", MSc. Thesis, Sharif Technical University, Tehran, Iran,1998, pp T. Rosenqvist, "Thermodynamics of Copper Smelting", Advances in Sulphide Smelting- Vol. I-Basic Principles, H.Y. Sohn, D.B. George and A.D. Zunkel, Eds., The Metallurgical Society of AIME, Warrendale, PA, USA, J.M. Floyd, "Submerged Injection of Gas into Liquid-Pyrometallurgical Bath", United States Patent, No , 17 Feb 1981, pp. 1-3.