One step'technology to separate copper, zinc, lead from iron in metallurgical slag and pyrite cinder

‘One step’technology to separate copper, zinc,lead from iron in metallurgical slag and pyrite cinderPart1–Laboratory scale testD.-Q.Zhu,D.Chen*,J.Pan,Y.Cui and X.L.ZhouIn this paper,a study of processing pyrite cinder and metallurgical slag using a‘one step’technology in strand grate–rotary kiln at a laboratory scale is presented,which aims to recover iron,lead,zinc and copper.The compressive strength of preheated pellets reached527N per pellet and the removal rates of copper,lead and zinc reached36?36,92?86and20?0% respectively,when the green balls were preheated at1125u C for10min.Reduction of preheated pellets was followed,at the carbon/iron ratio of3?0and1100u C,for the reduction time of70min, and the metallisation degree of86?31%,the compressive strength of1777N per pellet and the copper,lead and zinc removal rates of36?36,99?12and71?00%were achieved respectively. These prereduced pellets with80?58%Fe made from pyrite cinder and metallurgical slag can be widely applied to enhance the operation of blast furnace.Keywords:Pyrite cinder,Metallurgical slag,Chloridising roasting,Reduction roasting,Prereducted pellets,Non-ferrous metalsIntroductionWith the rapid development of national economy,steel, chemical and non-ferrous metal industries have been fast expanding.China has become the biggest country in production and consumption of iron,copper,lead and zinc(Xiong,2009).But most of iron ore resources are characterised by low grade,coexistence of many valuable metals and being difficult to upgrade and smelt in China.More than85%of non-ferrous metal resources are complex ores,and y31%of total reserves are closely associated with iron(Li,1994).Therefore,iron,copper, lead and zinc resources are severely dependent on import.About69%of iron ores were imported abroad in2009in China,and moreover,the price of iron ore soared.The price of iron ore in2009was y4?5times more than that in2004.Therefore,it results in a great cost pressure for Chinese steelmills.In the meantime, more than10million tons of copper,lead and zinc ores are also dependent on import(Ministry of Industry and Information Technology of the People’s Republic of China,2010).Shortages of iron,copper,lead and zinc ores have become a bottleneck for the sustainable development of Chinese metal industry.Hence,it is of great significance for Chinese metal industry to develop the refractory resources efficiently,economically and friendly.Tens of millions of tons of metallurgical slag and pyrite cinder are discharged in non-ferrous metal and chemical industries annually(Liu et al.,2002;Guo et al.,2007).In addition,the accumulated waste slag in storage is.100million tons which not only occupies much land but also pollutes the environment.However, metallurgical slag and pyrite cinder cannot be efficiently utilised and only a small amount is used as additives in cement,paving,brickmaking and auxiliary additives (Zhou et al.,2006;Shi et al.,2008;Yang et al.,2008). They contain both abundant iron and considerable valuable metals such as copper,lead and zinc.After the improvement of the iron grade by mineral processing processes,pyrite cinder and slag containing lower non-ferrous metal can be used as the blast furnace burden (Xiang et al.,2006;Zhu et al.,2007).However,if the upgraded iron concentrates containing high copper,lead and zinc are used as raw materials for the blast furnace, lead and zinc will harm blast furnace lining and shorten blast furnace life,and moreover,zinc will cause nodulation in the blast furnace.Copper is easily reduced and enters into hot metal,which adversely impacts the quality of hot metal and molten steel.It is important for copper,lead and zinc being removed and separated from iron to produce pure iron ore concentrate via hydro-metallurgy,pyrometallurgy or combined process of hydrometallurgy–pyrometallurgy technologies(Arslan and Aralan,2002;Peng and Wu,2002;Sun et al., 2005).Thus,comprehensive utilisation of metallurgical slag and pyrite cinder not only eliminates the environ-mental pollution,but also provides high quality raw materials for ironmaking and recovers considerable copper,lead and zinc.School of Minerals Processing and Bioengineering,Central South University,Changsha410083,China*Corresponding author,email chendong_csu@ß2012Institute of Materials,Minerals and Mining and The AusIMMPublished by Maney on behalf of the Institute and The AusIMM Received4November2010;accepted13July2011DOI10.1179/1743285511Y.0000000022Mineral Processing and ExtractiveMetallurgy(Trans.Inst.Min.Metall.C)2012VOL121NO279Valuable metals are too low in metallurgical slag and pyrite cinder to be recovered by the traditional methods reported in the literature,where high production cost,poor economic benefit and serious secondary pollution were confronted.High temperature chloridising roasting technology has been successfully applied to remove valuable non-ferrous metals.But the conventional high temperature chloridising roasting is mainly used for processing high iron grade ores,and it heavily erodes equipment,especially rotary kilns (Li,1992;Fraissler et al.,2009).Furthermore,reduction roasting in the rotary kiln is a significant method and has been successfully applied in industry to recycle lead and zinc from metallurgical solid wastes (Jiang et al.,2006;Forton et al.,2007).Reduction roasting is also an effective technology to improve the iron grade of blast furnace burden.The traditional grate–kiln technology (two step technology),which produces reduced pellets with oxide pellets,includes productions of both oxide pellets and reduced pellets.In contrast,‘one step’technology,which produces reduced pellets with pre-heated pellets,possesses a shorter flow (Li,2007).Therefore,comprehensive recoveries of iron and valu-able non-ferrous metals from metallurgical slag and pyrite cinder were investigated by simulating grate–kiln process (one step technology)including high tempera-ture chloridising roasting and prereduction at a labora-tory scale.ExperimentalRaw materialsThe raw materials used consist of iron bearing materials,bentonite,chemical grade CaCl 2and bituminous coal.The iron bearing materials were offered by Jinyuan Minerals Limited,and their chemical compositions and size distributions are listed in Tables 1and 2.Pellet feed containing low grade iron and high content of non-ferrous metals is not suitable for ironmaking.Therefore,copper,zinc and lead must be removed and the grade of iron should be further upgraded before ironmaking.In addition,the content of sulphur in the pellet feed is slightly high,which may lead to a high content of sulphur in the preheated pellets.Although the high content of SiO 2in the pellet feed is unfavourable to ironmaking,it favours chlorination by promoting the formation of hydrogen chloride (Metallurgical Laboratory of Central South College of Mining and Metallurgy,1978).The pellet feed is coarse for pelletis-ing with the size of 76?57%passing 0?075mm and 58?91%passing 0?045mm.In order to improve the pellet quality,it is essential to pretreat pellet feed before balling.The chemical phase analyses of zinc,copper and lead in pyrite cinder and metallurgical slag are shown in Tables 3–5.The occurrences of zinc,copper and lead arecomplicated in pyrite cinder and metallurgical slag.Zinc mainly exists in the form of zinc ferrite.Most of the copper exists in the magnetic iron oxide.Lead mainly exists in the form of lead silicate,lead oxide and metallic lead in metallurgical slag and exists in the form of lead silicate in pyrite cinder.The industrial analysis of the bituminous coal is given in Table 6.The bituminous coal containing 37?26%ash easily leads to the formation of accretion inside the rotary kiln.The total sulphur is also too high (1?35%),which easily results in the high content of sulphur in prereduced pellets.Therefore,it is necessary to wash the bituminous coal to remove sulphur and reject gangue to reduce ash and improve fixed carbon content in industrial operation.MethodsThe test flow chart is provided in Fig.1to simulate the grate–kiln process.The tests covered proportioning of raw materials,blending and homogenising,balling in a disc pelletiser,chlorinising roasting in pot grate to remove non-ferrous metals and prepareing preheated pellets for prereduction,prereduction by the bituminous coal in a rotary kiln to make prereduced pellets and further remove lead and zinc.Chloridising roasting and preheating of pelletsPart removal of non-ferrous metals and hardening of green balls will be achieved by preheating green balls containing chlorides in a strand grate,and imparting enough strength to preheated pellets to withstand reduction in a rotary kiln is provided.Before pilot tests,the laboratory scale preheating tests were performed in a tube furnace,and both ends of the tube furnace were open (Li,2007).Ten dry pellets loaded in a corundum crucible were pushed into a corundum tube (W 506600mm)when the temperature of the tube furnace centre reached the target value.Five minutes were required for moving the corundum crucible into the centre of tube furnace in order to prevent too fast heating rate and obtain high compressive strength of preheated pellets.After staying in the centre for the setting time,the corundum crucible loaded with pellets was drawn out during 5min and cooled in air.The compressive strength of preheated pellets was measured by using an intelligent compressive strength instrument (type:ZQYC)according to the standard of ISO 4700.Table 1Chemical compositions of iron bearing materials/mass-%Materials Fe total FeO Fe 2O 3SiO 2Al 2O 3CaO MgO P S Cu Pb Zn Magnetite 69.0827.5368.171.200.740.281.390.00860.150.0120.0320.064Pyrite cider62.9511.5777.155.622.080.450.310.0350.390.320.0720.32Metallurgical slag 57.0821.5057.659.00.880.630.0760.0750.240.243.520.18Pellet feed *61.5019.7965.906.441.220.510.410.0500.270.221.790.20*The feed comprises 20%magnetite concentrate,30%pyrite cinder and 50%slag.Table 2Size distributions of iron bearing materials/mass-%Size fractions/mm z 0.1060.106–0.0750.075–0.04520.045Magnetite 212145022836055Metallurgical slag 9.4517.9716.5256.06Pyrite cider 6.5514.8516.0562.55Pellet feed 7.1116.3217.6658.91Zhu et al.Technology to separate copper,zinc,lead from iron:Part 180Mineral Processing and Extractive Metallurgy (Trans.Inst.Min.Metall.C)2012VOL121NO2The removal rates of non-ferrous metals were calculated by equation(1)R m~1{m b i m0x i|100%(1)where R m is the metal removal rate(%),m0is the mass of dry green balls(g),x i is the mass content of metal i in dry green balls(%),m is the mass of preheated or prereduced pellets(g)and b i is the mass content of metal i in preheated or prereduced pellets(%).Reduction of preheated pelletsThe laboratory scale reduction tests were performed in a shaft furnace(Li,2007).The upper end of corundum tube was covered with a refractory block during reduction in order to maintain enough CO concentration in the shaft furnace(W806200mm).The stainless steel jar loaded with15cool preheated pellets was put into the centre of shaft furnace to be preheated for5min and then taken out.After being charged with the bituminous coal,the jar was moved into the shaft furnace immediately again for reduction.The reduced pellets were cooled by nitrogen gas to prevent reoxidising when the reductionfinished. The calculation of the removal rate of non-ferrous metals during reduction is also presented in equation(1).The quality of prereduced pellets was also valued by using metalisation degree.Results and discussionVolatilising of non-ferrous metals by preheating pelletsThe effect of preheating temperature on the compressive strength of preheated pellets and the removal rates of non-ferrous metals are illustrated in Fig.2.It can be seen that the compressive strength of preheated pellets obviously increases and the removal rates of copper and zinc slightly increase with an increase in preheating temperature;whereas,the removal rate of lead always clearly decreases.However,the removal rates of copper and zinc begin to reduce when the preheating tempera-ture is beyond1125u C.The highest removal rates of copper,lead and zinc are36?36,92?86and25?00% respectively.The thermodynamics calculations(Metallurgical Laboratory of Central South College of Mining and Metallurgy,1978;Liang et al.,1993)(Fig.3)demon-strate that the higher the preheating temperature,the less the required concentration of hydrogen chloride in the chlorination of copper and zinc;whereas,the removal of lead is opposite within the tested tempera-tures.The experimental results(Fig.2b)agree with the thermodynamic properties at the temperature range of 1025–1125u C.However,preheating at a higher tempera-ture dramatically raises the saturated vapour pressures of calcium chloride and hydrogen chloride,which accelerates the volatilisation of them.Therefore,the hydrogen chloride for chlorination reactions decrease, leading to a drop in removal rates of copper,lead and zinc.High amounts of Cl addition,low concentration of H2O in the reaction atmosphere,high heating rate of pellets and high Si content in the pellets may further increase the removal of copper and zinc(Fraissler et al., 2009).Preheating at a higher temperature accelerates the solid phase diffusions,reactions and consolidation, which strengthens the compressive strength of preheated pellets(Fu et al.,1995).After the optimisation of preheating temperatures,contents of non-ferrous metals in preheated pellets were still too high,so the preheating time should be further extended.The effect of preheating time on the compressive strength of preheated pellets is presented in Fig.4a.The compressive strength of preheated pellets slightly in-creases with an extension of preheating time.PreheatingTable3Chemical phase analysis of zinc in pyrite cinder and metallurgical slag/mass-%Materials Zinc sulphate Zinc oxide Zinc silicate Zinc sulphide Zinc ferriteMetallurgical slag012756111706Pyrite cider6.28.02.15.678.1Table4Chemical phase analysis of copper in pyrite cinder and metallurgical slag/mass-%Materials Copper sulphate Copper oxide Copper sulphide Magnetic iron oxide Copper silicate Metallurgical slag0051296023Pyrite cider3.82.23.184.36.6Table5Chemical phase analysis of lead in pyrite cinder and metallurgical slag/mass-%Materials Lead oxide Lead sulphide Metallic lead Lead silicateMetallurgical slag37354159414Pyrite cider1.83.63.691.0Table6Industrial analysis of bituminous coalFCad/mass-%Vdaf/mass-%Ad/mass-%Mad/mass-%Coking index St/mass-%38.8122.4037.261.5311.35*St:total sulphur in coal.Zhu et al.Technology to separate copper,zinc,lead from iron:Part1 Mineral Processing and Extractive Metallurgy(Trans.Inst.Min.Metall.C)2012VOL121NO281at a longer time favours the solid phase diffusions and reactions fully carrying out inside pellets (Fu et al.,1995),which results in higher compressive strength of preheated pellets.The removal rate of zinc slightly augments with an increase in preheating time;whereas,the removal rates of copper and lead keep steady at 36?36and 92?86%respectively (Fig.4b ).It is clear that chlorination reactions and volatilisation of metal chlorides take place more rapidly in this experiment than that in the literature (Mattenberger et al.,2008)and finish in ,10min.Therefore,the fast volatilisation of non-ferrous metals is provided and hardening of green balls can be accomplished in the strand grate synchronously.The highest removal rates of copper,lead and zinc are 36?36,92?86and 25?00%respectively,by the optimisa-tion of preheating pellets containing 2%calcium chloride that is 1?32times of the theoretical dosage.However,most of the copper and zinc still remain in the preheated pellets.On the one hand,dosage of calcium chloride is presumably so low that it cannot keep enough high concentration of hydrogen chloridefor1Test flowchart2Effect of preheating temperature on a compressive strength of preheated pellets and b removal rates of non-ferrousmetals (2%CaCl 2and preheating for 20min)Zhu et al.Technology to separate copper,zinc,lead from iron:Part 182Mineral Processing and Extractive Metallurgy (Trans.Inst.Min.Metall.C)2012VOL121NO2monly,the calcium chloride added in pellets should be 1?86times of theoretical calculation (Metallurgical Laboratory of Central South College of Mining and Metallurgy,1978).On the other hand,the presence of gangues (Al 2O 3and SiO 2)contributes to the formation of more stable aluminate and silicate,which are difficulty to remove by hydrogen chloride.According to the results of the preheating procedure,the remained zinc and lead must be removed in the next prereduction procedure.Reduction of preheated pelletsIn the preheating of pellets,lead oxide,metallic lead and most of lead silicate were chloridised and volatilised.However,most of zinc ferrite and zinc oxide and a spot of lead silicate remained in preheated pellets.In the reduction,the process of zinc ferrite to zinc oxide is liable to happen,so the restrictive line of the reduction of zinc ferrite to metallic zinc is the reaction of zinc oxide to metallic zinc (Xu,1979).With regard to lead silicate,the reduction of lead silicate to metallic lead is very difficult.The reduction of lead silicate to lead oxide will be very prone to occur if ferrous oxide takes part in the reduction of lead silicate (Peng et al.,2003).Therefore,zinc oxide and lead oxide are considered in the reduction.Figure 5depicts the effect of reduction temperature on the metallisation degree and the compressive strength of prereduced pellets and the removal rates of non-ferrous metals respectively.When the reduction tem-perature increases to 1100u C,the metallisation degree,the compressive strength and the removal rates of lead and zinc reached the peak values of 75?59%,1563N per pellet,98?90%and 62?50%respectively.The main rea-son is that Boudouard reactions react more violently with increasing reduction temperature,which promotes the reduction of Fe x O y ,ZnO ?Fe 2O 3,ZnO and PbO.Another reason is that the reductions of Fe 3O 4and ZnO are endothermic reactions.Therefore,it is beneficial to reduce Fe 3O 4and ZnO by elevating the reduction tem-perature.In the meantime,metallisation is favoured,metallic bond is fully developed and microstructure is densified inside prereduced pellets with increasing re-duction temperatures.All of these contribute to the im-provement of the compressive strength of prereduced pellets.In the reduction,most of zinc is removed,copper is difficult to remove and the left lead is further removed although most of lead has been removed in the chlori-nation.Furthermore,copper oxide is easily reduced to metallic copper in the reduction atmosphere,and me-tallic copper remains in the prereduced pellets.That is why the total removal rate of copper is only 36?36%.With the increasing ratio of carbon to iron (C/Fe),the compressive strength and the metallisation degree significantly increase,the removal rate of zinc slightly increases and the removal rate of lead is kept at high value while the removal rate of copper is kept steady at lower value (Fig.6).The concentration of CO increases with the increasing ratio of C/Fe,which promotes the reduction reactions.When the ratio of C/Fe increases to 3?0,the metallisation degree,the compressive strength and the removal rate of zinc reach 86?31%,1777N per pellet and 71?0%respectively.According to the reduc-tion thermodynamics calculations (Liang et al.,1993)(Fig.7),the theoretically required CO concentration for the reduction of FeO to Fe is .71?5%at 1100u C and the theoretically required CO concentration for the reduc-tion of ZnO to Zn is .34?4%at 1100u C,leading to the high ratio of C/Fe in the reduction of FeO and ZnO.However,required CO for the reduction of PbO to Pb is just ,5%.About 93%of lead has already been removed in the chlorination,and the reduction of PbO to Pb happens quickly,so the removal rate of lead is kepthigh.3Equilibrium diagram of reactions between CuO,PbO,PbO ?SiO 2,ZnO and HCl (in pellet feed,Cu,Zn and Pb mainly exist in form of oxides,ferrites andsilicates)4Effect of preheating time on a compressive strength of preheated pellets and b removal rates of non-ferrous metals(2%CaCl 2and preheating at 1125u C)Zhu et al.Technology to separate copper,zinc,lead from iron:Part 1Mineral Processing and Extractive Metallurgy (Trans.Inst.Min.Metall.C)2012VOL121NO283As presented in Table 7,lead and zinc can be removed effectively,and high iron grade can be obtained.However,copper cannot be removed effectively in thechlorination,and the mass loss of prereduced pellets leads to enrichment of copper content.Therefore,the content of copper reaches up to 0?14%.In order to control the copper content below 0?2%,the content of copper in green balls and the removal of copper in the preheating should be further considered.Conclusion1.The ‘one step’grate–kiln process including the preheating (chloridising roasting)in the grate and the prereduction in the rotary kiln was developed to utilise these refractory iron bearing materials.CaCl 2was added in the pellet feed to partly remove non-ferrous metals,and qualified preheated pellets were obtained for the next prereduction process by preheating pellets.Re-mained lead and zinc were further removed,and high quality prereduced pellets,which can amelioratethe5Effect of reducing temperature on a metallisation degree and compressive strength of prereduced pellets and bremoval rates of non-ferrous metals (2%CaCl 2;preheating at 1125u C for 10min;reducing at C/Fe ratio of 2?5for 70min;solid curve:total removal rates of non-ferrous metals including chlorination and reduction process;dot curve:non-ferrous metals just removed byreduction)6Effect of ratio of C/Fe on a metallisation degree and compressive strength of prereduced pellets and b removal ratesof non-ferrous metals (2%CaCl 2;preheating at 1125u C for 10min;reducing at 1100u C for 70min;solid curve:total removal rates of non-ferrous metals including chlorination and reduction process;dot curve:non-ferrous metals just removed byreduction)7Relationship between reduction temperature and equili-brium phase composition and CO concentrationTable 7Chemical compositions of prereduced pellets */mass-%TFeFeO MFe Cu Pb Zn 8058747695501400160058*Reducing at 1100u C and the C/Fe ratio of 3?0for 70min.Zhu et al.Technology to separate copper,zinc,lead from iron:Part 184Mineral Processing and Extractive Metallurgy (Trans.Inst.Min.Metall.C)2012VOL121NO2operation of blast furnace,were acquired by reduction of preheated pellets.2.High temperature chloridising roasting is one of effective ways to remove non-ferrous metals from iron materials.When green balls containing2%CaCl2were preheated at1125u C for10min,the compressive strength of the preheated pellets reached517N per pellet and removal rates of copper,lead and zinc were 36?36,92?86and20?0%respectively.The remained non-ferrous metals in preheated pellets would be further removed in reduction.3.Reduction in the rotary kiln is an efficacious measure to not only further remove lead and zinc but also dramatically heighten the grade of iron.When the preheated pellets were reduced at the C/Fe ratio of3?0 and1100u C for70min,the metallisation degree,the grade of iron and the compressive strength of the prereduced pellets were86?31%,80?58%and1777N per pellet respectively.The total removal rates of copper, lead and zinc reached36?36,99?12and71?00% respectively.The prereduced pellets,which contained 0?14%Cu,0?016%Pb and0?058%Zn,could be used as the blast furnace burden with high quality.However, more work has to be done to further remove copper in the one step process.ReferencesArslan,C.and Aralan,F.2002.Recovery of copper,cobalt,and zinc from copper smelter and converter slags,Hydrometallurgy,67,1–7.Forton,O.T.,McGrady,L.,Singh,M.M.,Taylor,E.R.M.,Moles, N.R.and Harder,M.K.2007.Characterisation of rotary kiln residues from the pyrolysis of shredder residues:issues with lead, J.Anal.Appl.Pyrolysis,79,396–402.Fraissler,G.,Joller,M.,Mattenberger,H.,Brunnera,T.and Obernberger,I.2009.Thermodynamic equilibrium calculations concerning the removal of heavy metals from sewage sludge ash by chlorination,Chem.Eng.Process:Process 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