THE ELECTRICAL CHARACTERISTICS OF COPPER SLAGS IN A 270 KVA DC ARC FURNACE

Transcription

1 THE ELECTRICAL CHARACTERISTICS OF COPPER SLAGS IN... MK THE ELECTRICAL CHARACTERISTICS OF COPPER SLAGS IN A 270 KVA DC ARC FURNACE Bora Dern, Fhz Cnar ahn and Onuralp Ycel * Istanbul Techncal Unversty, Faculty of Chemstry and Metallurgy, Department of Metallurgcal Engneerng, Maslak, Istanbul-TURKIYE Abstract: The electrcal resstance of slags s the man crtera to determne the desgn and the operaton condtons of slag resstance furnace (SRF) dependng on temperature and composton. In ths study, a 270 kva DC electrc arc furnace were used to determne the electrcal characterstc of molten ancent copper slags. The specfc conductvty of the slag was estmated by usng furnace geometrc factor gven n the lterature as an emprcal formula and by usng furnace resstance durng smeltng of the copper slag wth or wthout dfferent addtves such as coke, CaO and AI 2 O 3. INTRODUCTION In pyrometallurgcal furnaces, heatng s suppled from fuel, electrcal energy or ore tself. In practce, the electrc arc furnaces, where the electrcal energy s used, are classfed nto three types: open arc, submerged arc and slag resstance furnaces. Open arc furnaces, where the electrcal energy s converted to heat by the resstance of ar, are generally used for smeltng processes. In submerged arc furnaces, electrodes are mmersed n a mxture of ore feed or coke The slag resstance furnaces (SRF), where the resstance of slag layer converts the electrcal energy to heat, are used for smeltng and also reducton purposes [1]. The electrc resstance between two electrodes n SRF s mportant factor for furnace desgn and performance [1-3] and can be defned as followng equaton: <y Q l.cm r /, cm' 1 where R: furnace resstance, f g : geometrc factor, and o: slag electrcal conductvty. The furnace geometrc factor, beng lnearly proportonal to (R), s a functon of dameter of the electrode (d), the depth of mmerson (h), electrode spacng (s) and the depth of slag layer (H). f g =f(d,h,s,h) (2) Accordng to the calculatons of Nelsen on salt solutons [2,4], the furnace geometrcal factor of a system s descrbed n Eq. 3 as an emprcal formula and t can be appled to such an arc furnace system shown n Fg 1. Js = 0,3034 x [(ff-/)/</] - 45 ^ d Proceedngs of 3 rd BMC-2003-Ohrd, R. Macedona 265

2 THE ELECTRICAL CHARACTERISSTICS OF COPPER SLAGS IN... Top ^electrode Bottom electrode Fgure 1. Schematc dagram of a drect current slag resstance furnace (DC-SRF) An approxmate amount of 2 mllon tons of copper smeltng slag has been lyng on the northern part of Turkey-Kure datng back Genoese Tmes. Ths nckel free slag contans 0.4 % Co and 0.82 % Cu. OUE man studes were about recoveres of valuable metals such as cobalt, copper and ron from the ancent slag va pyro and hydrometallurgcal methods [5-14]. Whle smeltng the slag usng DC arc furnace, we also gathered electrcal nformaton va computer and data acquston systems attached to the furnace. Present study s subjected to determne the electrcal characterstcs of the molten ancent kre slag, and of whch compostons were changed wth flux and coke addtons at dfferent temperature. The results were also compared wth prevous publshed lterature. EXPERIMENTAL STUDY RAW MATERIALS The chemcal composton of the ancent kre copper slag whch was used as a raw materal n our studes was gven n Table 1. In the experments, coke contanng 78.5 % fxed carbon, 3.64 % volatle substances, and 17,6 % ash was used as a reductant materal where as, CaCCh and AI2O3 (99.5% purty) were used as fluxng addtves. Table 1: Chemcal composton of the ancent Kure copper slags Components Co Cu Zn Fe S Weght (%) Components SO 2 A1 2 O 3 CaO MgO NO Weght (%) EQUIPMENTS Expermental works were carred on a laboratory scale, mono phase, submerged drect current (DC) electrc arc furnace wth 270 kva. Computer and data acquston systems were used to measure voltage, current, temperature and the precson ruler. Schematc dagram of a DC arc furnace nstalled n one of ITU laboratores are gven n Fgure 2. Whle graphte bottom electrode s fxed n the refractory materal of the crucble, top graphte electrode has the ablty to move n vertcal axs. Top electrode postoned nto molten slag where as molten metal n the crucble contacts wth the bottom electrode. MgO monolthc refractory was used for protecton of furnace sdewalls from slag eroson. 266 Proceedngs of 3 rd BMC-2003-Ohnd, R Macedona

3 THE ELECTRICAL CHARACTERISTICS OF COPPER SLAGS IN... Power Supply Top electrode " Ruler Removable Crucble Computer and acquston systems Bottom electrode Fgure 2. The Schematc of DC Electrc Arc Furnace wth 270 kv A. EXPERIMENTAL PROCEDURE The 25, 30 and 150 kg-weght samples of Kre slag wth or wthout addtves were charged nto EAP crucble for reducton smeltng. Tap to tap tme was select as at most 50 mn. At selected ntervals, submerged type of Pt-PtRhlO thermocouple was ntroduced to the crucble to measure the temperature of the molten slag. In the tapng stage, the metal and slag temperatures were also wth an optcal pyrometer. Chemcal compostons of the metal and slag samples were determned by usng Perkn-Elmer 1100B ASS. After the charge were completely melted, the heght of the molten slag layer (H) and the depth of mmerson of the top electrode wth 10 cm-dameter (d) were wth a precson ruler connected to the movement apparatus of the top electrode. Heght of the slag layer (H) was determned by submergng the top electrode to the level where the voltage s zero (where the top electrode connects wth bottom electrode or metallc phase) and pullng back the top electrode to the level where the open arc released (where the tp of the top electrode contacts wth ar). After these measurements, the experments were contnued nto normal smeltng condtons keepng the mmerson level of the electrode constant (h). RESULTS AND DISCUSSION The geometrc factor and the specfc conductvty of the slags were calculated usng Equaton (1) and (3), respectvely. Electrcal values durng smeltng of 120 kg of slag wthout addtves and the calculated values were gven n Table 2. In ths experments (Run: 106), no metallc or matte phase were observed n the crucbles. The slag temperatures were as C. Mean specfc conductvty of the slags was calculated as Q'Vcm" 1. The relatonshp between currentvoltage varatons and the dstance between the electrodes were plotted n Fgure 3. As can be seen, when the dstance between electrodes s zero, the short-crcut occurs at 15 V and 2000 A. The power appled to the furnace s kept constant at 60 kw usng a potentometer. Movng the electrode upwards ncreases the voltage and decreases the current. At the surface of the slag layer, the voltage reaches ts maxmum and the current reaches ts mnmum values. Proceedngs of 3 rd BMC-2003-Ohrd, R. Macedona 267

4 THE ELECTRICAL CHARACTERISTICS OF COPPER SLAGS IN Table 2. cm) H, cm The Measured and Calculated Electrcal Values of Ancent Kre Slag (Run=106, d=10 H, cm V A R,n f g, cm' 1 calculated c, ST'cm" 1 calculated Omean, JrW calculated Fgure 3. too 90 - <("-* Open Arc > 60 « Electrode Immerson 0 Depth, cm. 10 Short Crcut n Current, A The relatonshp between current-voltage varatons and the dstance between the electrodes The and calculated electrcal values of the ancent kre slag wth addtves were gven n Table 3., Snce metallc phases were obtaned n the bottom of the crucbles n each experments, n addton to electrodes and conjuncton cables, effect of metallc phase were also neglected n the calculatons because of ther hgh electrcal conductvtes. Table 3. The Measured and Calculated Electrcal (d=10cm) Run Addtves Slag Composton, ( %) h %, 2 CaO, 4 CaO, 6 CaO, 5 AI 2 O 3, 20 A1 2 O 3 SO FeO A1 2 O CaO cm Values of Ancent Kre Slag H cm f- cm" R n wth addtves a Q-'cm" The relatonshp between slag temperature and conductvty were also plotted n Fgure 4. As can be seen, ncreasng temperature also ncreases the specfc conductvty of the slag. In Fgure 5, the results of the entre study were compared to those of prevous publshed data. It can be seen that electrcal conductvty of the slags that we obtaned wth/wthout addtves are strongly smlar to those of fayalte-based slags. T C Proceedngs of 3 rd BMC-2003-Ohrd, R Macedona

5 THE ELECTRICAL CHARACTERISTICS OF COPPER SLAGS IN Temperature, C Fgure 4. The Temperature vs. Slag Conductvty Dagram of the Copper Slags Temperature, C fe 0:94.5, SI02: 5.5 ^^OO n g o Fgure 5. The Varaton of Electrcal Conductvty of Dfferent Slags wth Temperature. CONCLUSIONS The electrcal resstance values of the molten ancent Kre copper slags were durng smeltng condtons wth or wthout addtves n a DC electrc arc furnace. The specfc conductvtes of the slags were calculated usng emprcal formula known as "furnace geometrc factor". Mean specfc conductvty of the slag was determned as 1,85 Q'^cm 1 at 1440 C. Addton of coke, AI2O3 and CaO resulted to change n specfc conductvty between 0.78 and 1.56 ff'.cn" 1 at temperatures of 1250 to 1470 C. The specfc conductvty of the slag tends to ncrease wth ncreasng temperature. Proceedngs of 3 rd BMC-2003-Ohrd, R. Macedona 269

6 THE ELECTRICAL CHARACTERISTICS OF COPPER SLAGS IN Acknowledgements The authors express ther apprecaton to Prof. Adnan Tekn, Prof. Okan Addemr, Blent rn and Nevzat Oren. Fnancal support from the Research Foundaton of Istanbul Techncal Unversty s also gratefully acknowledged References [I] R.M. Westcott and N. J. Themels, "An Expermental Slag Resstance Furnace" (Paper presented atepd Congress, TMS, 1994), [2] Q. Jao, and N. J. Themels, "Correlaton of Electrcal Conductvty to Slag Composton and Temperature" Metallurgcal Trans. B, Vol 19B, (1988), [3] Q. Jao, and N. J. Themels, "Correlaton of Geometrc Factor for Slag Resstance Electrc Furnaces", Metallurgcal Trans. B, Vol 22B, (1991), [4J R.C. Urquhart et al., "The smeltng of Copper-Nckel concentrates n an Electrc Furnace" n Extractve Metallurgy of Copper, J.C. Yannopoulos and J.C. Agarwall, eds., TMS-AIME, Vol 1. (1964), 275-5, [5] B.Dern and O. Ycel, "The Dstrbuton of Cobalt between Co-Cu alloys and Al 2 O 3 -FeO-Fe 2 O3- SO 2 slags" Scandnavan Journal of Metallurgy, 31, (2002),12-19,. [6] O. Ycel et al, "Recovery of Cobalt from Copper Slags" Mneral Processng and Extractve Metallurgy Revew, 10 (1992), [7] O. Ycel et al., "A Reducton Study of Copper Slag n a DC Arc Furnace" Scandnavan Journal of Metallurgy, 28 (1999), [8] O. Ycel, "Effects of Addtves and Furnace Type on Reducton Smeltng of Copper Slags n D.C. Arc Furnace" Bull. Tech. Unv. Istanbul, 49 (1996), [9] E. Acma, "The Effect of Sulfur and Reducton Temperature on Cobalt Dssoluton durng Sulfurc Acd Leachng of Metallc Matte". Canadan Metallurgcal Quarterly, 36 (1) (1997), 25-. [10] O. Addemr et al., "Bakr Curuflanndan Uretlen Metalk Mattak Degerlern Selektf olarak Kazanlmas Uzerne br Cah ma",(paper presented at the 9th Internatonal Metallurgy and Materals Congress, Istanbul Turkye, 1997) [II] E. Acma et al., "Benefcaton of Copper Slag for the Producton of Magnetc Oxde" (Paper presented at Internatonal Conference on Recyclng of Metals, ASM, Dsseldorf/Neuss- Germany 1992) [12] O. Ycel, "Cobalt Recovery from Ergan Converter Slag n an Iron-Copper-Cobalt-Alloy" (Ph D thess, Istanbul Techncal Unversty, June 1986) [13] H. Hensgen et al., "Recovery of Cobalt from Ergan, Turkey, Converter Slag by Reducton Smeltng n a Laboratory-Scale Electrc Arc Furnace", Trans, of the Insttuton of Mnng and Metallurgy, Sec. C. Mn. Proc. and Extr. Met., (1989), CI [14] B. Dern et al. "A Process Desgned for The Ancent Copper Smeltng Slags", (Paper presented at Yazawa Internatonal Symposum on Metallurgcal and Materals Processng, TMS, 2003 San Dego, Calforna, USA, Vol:2), Proceedngs of 3 rd BMC-2003-Ohnd, R. Macedona