Use of Chromium Ore Briquettes in the Manufacture, of Ferrochrome Silicon

Transcription

1 174 Electric Furnace Proceedings, 1970 Use of Chromium Ore Briquettes in the Manufacture, of Ferrochrome Silicon by Ivo Madronic The. worldwide and persistent shortage of lump, highgrade metallurgical chromium ores, aggravated by the United Nations economic sanctions against the Smith government in Rhodesia, and the relative availability of chromium ore fines and concentrates, have encouraged the producers of chromium ferroalloys in the United States and in other countries to seek methods which would allow a partial or total replacement of high-grade, lump.metallurgical ores with chromium ore fines or concentrates.. Globe Metallurgical, a Division of Interlake, Inc., one of the major producers of chromium ferroalloys in the United States, has approached this acute problem by using chromium ore briquettes as a replacement for, natural lump ore. This paper describes an industrial trial made in an electric furnace producing ferrochrome silicon. Results of this trial operation are compared with 'the standard operation. INTRODUCTION Everyone involved in the production of ferroalloys knows how significant are the chemical and ~hvsical characteristics i f the ores on furnace operation: 0"ne of these important characteristics is the size of the ores. Many years of operating practice have established the criteria concerning the ore size for each type of ferroalloy. It has been found that a sound operation requires a,certain minimum amount of lump ore in the burden. This amount, below which unfavorable furnace operation results, will vary from alloy to alloy. At Beverly, the proportion of lump ore required for ferrochrome silicon is 50-60% of the ore burden. The production of high carbon ferrochrome requires as much as 70-80% of lump ore. Until 3-4 years ago there was a fairly good balance between lump ore consumption and supply. The availability of metallurgical grade lump chromium ore on the world market was sufficient to cover the needs of the chromium alloys industry in the United States and in the other countries of the world. It is quite understandable that in such a balanced lump ore situation there was no real need and not enough economic incentive to find a substitute for the lump ore. In the United States ferroalloys industry, there were some trials in the use of chromium ore briquettes as a replacement for the lump ore during previous instances of short supply, but it appears that these attempts were not pursued with persistence. As soon as the high grade metallurgical ore IVO MADRONIC is Chief Metallurgist, Globe Metallurgical, a Division of lnterlake Inc., Beverly, Ohio. Yi C V-( Yi : m.ohmm m u n m m -nor-om rnar.com YEAR \o\oulrua \oaa\o\o ulouloa o\oaaa Rhodesia U. S. S. R Turkey Other Pig. 1-Chrome Iron Age... ore, U.S. imports. Metallurgical grade only. Source: became available on the market and abundant reserves were stockpiled, there was no need to add to the ever present difficulties of furnace operations by experimenting with the other types of ores. The United States has no deposits of high quality metallurgical chromium ore. Until 1965 the major part of the metallurgical grade chrome ores was imported from Rhodesia. As a direct result of the economic sanctions against Rhodesia, the ore balance has been heavily changed, and the ferroalloys producers in the United States have found themselves cut from a vital supplier. Despite a sharp increase in imports from the USSR and Turkey (Fig. 1) during the past five years, the imports of metallurgical grade ore were short in 1969 by about 100,000 tons. This shortage was met from plant inventories and United States Government Supplemental Stockpiles. For 1970 the estimated shortage will be even more acute. According to the experts, only 400,000 tons of high grade lump metallurgical ore will be available to the United States ferroalloy industry against a requirement of 605,000 tons. Two other problems have been added to the existing problem of ore shortage. First, the quality of the high grade metallurgical Russian ore has shown substantial deterioration recently. The size is not uniform and much of the ore contains an increased amount of fines. Ores are showing lower chromium. oxide content and lower chromium-to-iron ratio. Physical size of metallurgical lump ore imported to the United States remained satisfactory, but a substantial part has only 40% chrome oxide against a normal 48-49% content. The second problem pressing on chrome alloys producers is

2 Arc Furnace Operations 175 Table I. Price Quotations of Various Grades of Foreign Chromite ($/Long Dry Ton)* G8 1969, 1970 Rhodesia 48% CrzOa, 3:l Cr/Fe ratio Turkey n.a. n.a. n.a. 48% Crz03, 3: 1 Cr/Fe ratio SouLh Africa 44% Crz n.a. USSR 55% Crz0~. 4:l Crpe ratio c Source: U.S. Bureau of Mines. - Table II. Chromium Ores and Briquettes Used in Beverly Plant During the Trial Run on Ferrochrome Silicon Production, Furnace No. 2, MgO/ ment Type Size CrsOa Cr Fe Si02 ~ 1 ~ MgO 0 ~ CaO &/Fe AlsOa Me0 Consign- Russian Rhodesian Fethiye Regular 4% Coke - R/M R/M Conc. Briq. Briq N.D N.D N.D N.D N.D N.D Fethiye concentrates briquetted in Baltimore. the high cost of the chrome ore. There is a continuous trend to raise prices. The ores priced in 1965 in the range of $30-35 per long ton cost about $55 per long ton in 1970 (Table I). With such a critical shortage of metallurgical lump chrome ore, the ferroalloys producers could no longer remain indifferent in searching for a way to overcome the existing situation. It has been proposed that steelmakers change their specifications, which would permit the blending of high grade ores with the low grade ores and allow a higher proportion of fine ore to be used in the ore burden. From private communications we are aware that some European ferroalloys producers are involved in a testing program with chromium ore pellets. Similar attempts are being made recently in the United States. Globe Metallurgical, a Division of Interlake, Inc., with a ferroalloys plant in Beverly, Ohio, has been faced with all of the problems caused by the critical ore situation. In order to ease the lump ore situation, a test program was established substituting a portion of lump ore with chrome ore briquettes manufactured from chrome ore concentrates. The program consisted of two phases: the first involved the use of briquettes in the manufacturing of ferrochrome silicon; the second used briquettes in the production of high carbon ferrochrome. Until now, only the first phase has been completed and evaluated. Considering that the results obtained during this trial run may be of general interest, the management of the Globe Metallurgical decided to make them public. CHROME ORE BRIQUETTES Manufacture For the industrial test, a total of 2,346 net tons of chrome ore briquettes was manufactured by the International Briquetting Corp., Baltimore, Maryland. The briquettes were made from Fethiye chrome ore concentrates (Table 11) by adding about 5% binder consisting of lime and molasses. From the above quan- tity, 1,544 net tons were regular briquettes and 802 net tons were briquettes with an addition of 4% metallurgical coke. Unfortunately, the coke breeze used for the addition was too coarse and not too well distributed. Upon arrival at the Beverly plant, the briquettes were stored in the open yard and after 2-5 winter months, showed no substantlal degradation. Transportation of briquettes and other in-plant handling generated a certain amount of fines. A screen test of briquettes taken from the vibrator feeding the batch car showed an average of 82% retained on 1-inch screen, 89% on %-inch screen and 91% on 8-mesh screen. When charged to the electrlc furnace, the amount of fines smaller than Y4-inch has probably increased to approximately 15%. We think that with more careful handling and by keeping the bins as full as possible we will be able to reduce the generation of fines. The briquettes have a length of 5%-inches and an elliptical cross section of 3% by 2% inches. The average weight of one briquette is 2.3 pounds. Laboratory Testing Prior to use in the electric furnace, the briquettes were tested at Interlake's Research Center in Chicago and at the Illinois Institute of Technology at Chicago. This testing consisted of a reducibility test in a carbon monoxide atmosphere, in molten ferrochrome silicon, and in molten high carbon ferrochromium. A tumble test and spalling test were also run in order to establish the behavior of the briquejtes during handling and later in the electric furnace. The results of this preliminary laboratory testing can be summarized as follows: Comparative reducibility tests were performed on natural lump chrome ore, regular chrome ore briquettes, and 4% coke addition briquettes in a carbon monoxide atmosphere at 125O0C, in molten ferrochrome silicon at 1650" to 179O0C, and finally in molten high carbon ferrochromium at 1540" to 1760 C. These tests showed that the 4% coke modified briquettes have the greatest reducibility and that the regular briquettes are reduced more rapidly

3 176 Electric Furnace Proceedings, Table Ill. Tumble Test on Chromium Ore Briquettes Briquettes Time % Remaining on % Remaining on Type Minutes 1-Inch Screen 6-Mesh Screen Regular % Coke Regular % Coke Regular % Coke than the natural lump ore in both the carbon monoxide and high carbon ferrochrome tests..tumble tests were run in equipment conforming to the ASTM tumble test for coke-d Testing procedures were modified to include test periods of 5, 30, and 60 minutes. Results are shown in Table 111..The spall test performed in a muffle oven at 870 C and at 1290 C for a period of 2 hours has shown that the briquettes did not spall or disintegrate. Similar stability of briquettes was noted during the reducibility testing. On the other hand, the natural lump ore spalled violently in the course of the reduction tests. THEORETICAL EVALUATION.Let us examine the results of the laboratory testing of briquettes in the light of their suitability for the manufacturing of ferrochrome silicon and high carbon ferrochrome. Ferrochrome Silicon j The ease of reduction of chromium ore regular-briquettes and particularly coke-m-odified briquettes should be considered as a favorable factor in the manufacturing of ferrochrome silicon. The lump size and good resistance against spalling should further increase the permeability of mix and improve the furnace conditions on the top. According to the technical literature1-' describing the mechanisms occurring in the various temperature zones in the crucible of the electric furnace producing ferrochrome silicon, it appears that the smelting process should be conducted in a manner that would allow a major part of the Cr-0, to be reduced in the upper, relatively low temperature zone. If that is not the case, the unreduced Cr,O,, will reach the temperature zone where the reduction of silicon from SiO, is taking place. Reduced silicon will react with the un-,reduced portion of Cr,O, forming SiO, which passes irito the slag and makes it very acid and viscous. Easily reducible chromium ores should therefore be considered favorable for the production of ferrochrome silicon. High Carbon Ferrochrome In the production of high carbon ferrochrome, the requirements for the chromium ore are quite different from those for ferrochrome silicon. This is particularly 'true in the case of the production of high carbon ferrochrome with 5% max. carbon content. This particular grade requires a burden with a high portion (about 80%) of lumpy ores that are refractory and are hard to reduce. The increase of small and fine ore fractions will result in higher carbon content. Technical literature' has explained why the size and the refractoriness of the ores has such paramount importance in the control of the carbon content. The analysis of the first alloy formed in the upper zone of an electric furnace making high carbon ferrochrome corresponds to the formula (CrFe),C,. The carbon content of this first alloy is extremely high, about 9%. Droplets of this alloy flow down and pass through a layer which is a mixture of unreduced ore and slag. On the,way down the droplets are partially refined from carbon by virtue of the unreduced ore. Such oxidizing ore layers can be formed only by high grade refractory lump ores. Because of the lump size and low reducibility, such ores will reach the lower furnace zone before being completely reduced and will have the ability to perform the refining action. If the burden contains a larger portion of easily reducible and fine ore, the ore will be reduced completely in the upper zone and there will be no oxidizing layer in the lower zone. For this reason, a large portion of lump, hard-to-reduce ore is required in the ore blend for high carbon ferrochrome. In the case of chromium ore briquettes, we have a good lumpy sized material, but one which is relatively easily reduced. It is therefore impossible to predict, without an industrial test, which of the two characteristics, size or reducibility, will be more influential. TESTING OF BRIQUETTES Testing Program The testing program consisted of charging chromium ore briquettes into an electric furnace together with the natural lump and R/M chromium ores, quartzite, metallurgical coke, and woodchips. In a total of seven different trial phases, the proportion of ore briquettes in the ore blend was gradually increased from 20% to 100%. Tested were 4% coke briquettes and regular briquettes. The duration of each trial phase was from a minimum of 6 days to a maximum of 14 days. Throughout all the testing period, our standard practice was followed with respect to mix preparation, furnace operation; casting, and analytical control. Briquettes were tested in the manufacture of two commercial ferrochrome silicon grades: 41/41 (41 Cr/41 Si/0.05 C) and ELC (37 Cr/48 SY0.02 C.) Electric Furnace Testing of chromium ore briquettes was made on Beverly No. 2 furnace. This is a relatively obsolete, 15-year-old open top smelting furnace producing ferrochrome silicon most of the time and occasionally high carbon ferrochrome. A description of this furnace and operation was given in papers presented to this conference in 1960 and 1965."' For this reason I will limit myself only to those most important characteristics and parameters, without which I think this presentation would be incomplete. Furnace load is 10,000 kilowatts. The transformer is 13,500 kilovolt-amperes, three phase, 60 cycles. Primary voltage is 13,800 volts, secondary voltage is 136 to 216 in 9 steps. Design voltage is 177 volts. Furnace shell is stationary and of triangular shape. Inside dimension of the shell is 23 feet; the depth of the crucible is 92 inches. Three 35-inch diameter amorphous carbon electrodes are disposed on a 104-inch diameter electrode circle. The electrode spacing face-to-face is 55 inches. Crucible lining consists of 40-inch carbon blocks bottom and 24 by 30-inch carbon block walls. Operation The mix is charged to the furnace through eight chutes disposed around the electrodes and one chute in the center. A light electric stoker is used for the leveling and stoking of the furnace charge. Electrode penetration aim for the ferrochrome silicon operation is 54 inches below the mix level. The penetration is plotted on a chart each hour and is a very valuable instrument for the furnace operator. Furnace tapping and casting schedule is based on 14 taps per day. Both alloy and slag are tapped into a side tap ladle with a spout for the overflow of slag. This ladle, containing the alloy and some slag, is held for a minimum of 45 minutes in order to allow the silicon carbide to rise to the top. The alloy is retapped into another smaller ladle, poured into a cast iron mold, and properly skimmed. It has been our experience that good carbon control can be achieved at a given silicon content only if combined with a sufficient holding time, proper casting temperature, and a careful skimming practice. We.

4 Arc Furnace Operations 177 do not normally have difficulty draining slag from the furnace, but occasionally we do transfer a Gradall machine from our new silicon metal shop and use it very effectively for slag drainage. Slag from this process contains some metallics, so it is collected, crushed, and processed in a concentrating plant. The metallic concentrates are recycled back to the ferrochrome silicon operation. Table IV. Operating Data for 41/41 Grade Ferrochrome Silicon Trial with Chromium Ore Briquettes, Furnace No. 2 Phase A I I1 Period Dee , Feb. 5-16, Feb , 1060 l0io 1070 Briquettes: Type 456 Coke 4% Coke % In Ore Blcnd Operating Days Typlcal Ore Blend (Ib/mix) Turk Russ. R/M 100 A Turk. Lumo hod. R/M& loo B-1 4% Coke Briq. 115 B Total Lb Ores/Mix Allov Produced: ~dtal-period, NT Day, NT % Oper. T~mc l'ower consumption: : Per Day, kw-hr , ,000 Per NT Allov. kw-hi, P& NT silicbn, kghr Actual Load, kw Percent Operating Timc Alloy Composltion: % Chromium % Silicon 42.2: A Chrome Recovel.? Avg. Elect. Penetration, in Elect. Spacing IF/FJ, in. '?h Carbon ~ - -Theorv - ~ ~vg. Voltage, ~ ~ i t s MgO/Alr03 Ratio: In Ores In Slag As a reducer we use metallurgical coke, which is byproduct from blast furnace coke operations. We regularly screen this coke to 1%- by %-inch size in order to maintain a constant and uniform size and consequently better carbon theory control. We do not dry the coke, but we analyze moisture twice per shift. For each 1,000 pounds of quartzite in the burden we use 600 pounds of woodchips. In our normal ferrochrome silicon operation, we recycle to the electric furnace metallic concentrates recovered from the slag in the concentrating plant. During the trial run with briquettes, we purposely avoided the recycling of these concentrates in order to eliminate their influence on the productivity, recovery, and electric power consumption. EVALUATION OF THE OPERATING RESULTS Ore blend, production, electric power consumption, alloy analyses, and other important operating data are shown separately.for 41/41 and ELC grade in Tables IV and V. The same data are tabulated for comparison for the two selected 10-day periods where no briquettes and no remelts were used. Results on 41/41 Grade On this grade, where the portion of 4% coke briquettes in the ore blend amounted to only 20% and 30%, the operating results are practically identical with the results obtained with a regular blend of the lump and run of mine ores. There is no evidence of any significant difference between the two runs. A deeper electrode penetration observed during the trial run with briquettes should probably be attributed to a closer surveillance of the furnace throughout the test periods. The furnace productivity was about 15% above our standard for this grade. The power consumption per one net ton alloy was 11% below the standard. Results on ELC Grade The furnace productivity on this grade was in all phases better with the use of briquettes than in phase B, where no briquettes were used. Table V. Operating Data for ELC Grade Ferrochrome Silicon Trial with Chromium Ore Briquettes Furnace No. 2 Phase B* I1 a 111 IV V VI Period Jan. 2 6 Feb. 25- Mar. 3-15, Mar. li-zd, Mar. 31- Apr , Feb. 4, 19iO Mar. 2, 19;U Apr. 12, 19iU 19iU Briquettes: Type 4% Coke 4% Coke Regular Regular Regular % In Ore Blend Operating Days Typical Ore Blend (Ib/mlx) Turk. 109 A Turk Rhod. R.M 100 B Russ. R/M 100 A Turk I 4% Coke Briq. 115 B Reg. Briq Total Lb Ores/Mix Alloy Produced: Total-Period, NT Day, NT % Opcr. Time Ponrer consumption: Per Day, kw-hr 213, , , , , ,200 Per NT Alloy, kw-hr 8,089 8,134 8,001 8,095 8,046 7,887 Per NT Silicon, kw-hr 17,148 16,476 16,793 16,891 16,946 16,738 Actual Load, kw 9,557 9,950 9,858 9,934 10,137 10,250 : Percent Operating Time Alloy Composltion ! % Chromium % Silicon % Chrome Recovery Avg. Elect. Penetration, in Elect. Spacing (F/F), in. % Carbon Theory Avg. Voltage, volts MgO/A1103 Ratio: In Ores In Slag No. 3 Furnace.

5 178 Electric Furnace Proceedings, 1970,This better performance is basically due to the higher actual load during the trials with briquettes. This is particularly true for the period with 100% briquettes (Phase VI), where a high actual load of 10,250 kilowatts was maintained. In this period, a combination of a high load and a high operating time resulted in a daily power input of 235,000 kilowatt-hours. It appears that the use of 4% coke briquettes did not show any discernible difference when compared with the period where regular chromium ore briquettes were used. As a matter of fact, the operating results in Phase I11 (50% of ore burden containing coke modified briquettes) and Phase IV (50% of ore burden containing regular briquettes) are practically identical. However, such similarity in the behavior of the two types of Ixiquettes was somewhat expected due to the fact that the coke added in the briquetting process was too coarse and therefore not suitable for the purpose we wanted to achieve. The furnace productivity with 50% and 7570 of the ore burden com~rised of briauettes was about 12% higher t$an our standard productivity for this ELC At 100% briquettes, the productivity was 18.5% above the stpndard. lthe average electric power consumption per one net ton alloy was 8,050 kilowatt-hours when 50% and 75% of the ore blend was made up of briquettes. This consumption was 170 lower than our standard consumption. At 100% briquettes, the power consumption decreased to about 3% below the standard. FINAL WORD When evaluating the above achievements, we should, for the sake of objectivity, admit that our very recent production figures obtained on the same.furnace with _regular chromium ores differ very llttle from the figures observed during the test run with briquettes. 1 Without a doubt, the recent improvements in productivity reflect the extra effort of all concerned personnel and a better understanding of factors influencing the furnace operation. It would, therefore, be unjust to claim that the briquettes are superior to the regular lump ore. But, and this is important, neither are they inferior. The briquettes have therefore fulfilled the goal we wanted to achieve: the substitution for deficit natural lump ores with other, more easily available materials. From this point of view, the possibility of efficiently using briquettes in the manufacture of ferrochrome silicon represents for us, and I am sure for other ferroalloys producers, an important balancing tool against chromium ore shortages, skyrocketing prices, and a situation where the prospects for the future are very dim. This report has been limited to the test results of the use of chromium ore briquettes in the manufacture of ferrochrome silicon. At this writing, 5,000 long tons of briquettes are being tested in the manufacture of high carbon ferrochrome. However,,the results are not yet available for publication. ACKNOWLEDGMENT The author expresses his gratitude to the management of Globe Metallurgical for permission to. present this paper and his appreciation to all plant and research personnel for their contributions. REFERENCES 1 Elyutin, V. P., et al., "Production of Ferroalloys," pp. 180 and 206. State Scientific and Technical Publishine Housc for Literature on Ferrous and Non-Ferrous Metallurgy, ~osrow, (Translation, National Science Foundation, Washington, D. C., 1961.) "Bezobrazov. S. V.. et al., "Investigation of the Bath after Experimental Smelting of Ferrosilicochromium from Ore and Quartzite." Stal in Enalish., Vol on ~adarmetov, Kh. ~:,-et al:.' "1n;estigating the Proccss of Ferrosilicochrome Production," Stal in English, Vol. 8, 1964, pp L Mertdogan, A. and Keyser, N. H.. "Theoretical Considerations in the Manufacture of Low-Carbon Ferrochromium Silicon," Electric Furnace Proceedings, TMS-AIME, Vol. 26, 1968, pp Meredith, W..R.,"Single-Stage Manufacture of Low Carbon Ferro- chromium Silicon." Electric Furnace Proceedihos. -. TMS-AIME. Vol. 18, 1960, pp "Leeper, R. A. and Dyrdek, T. J., "Smelting,of High Carbon Ferrochromium in a Three-phase Electric Furnace, Elect~ic Furnace Proceedings, TMS-AIME, Vol , pp