Slag treatment at Kardemir integrated iron and steel works

Slag treatment at Kardemir integrated iron and steel worksY .Topkaya *,N.Sevinc ß,A.Gu ¨naydınDepartment of Metallurgical and Materials Engineering,Middle East Technical University,Ankara 06531,TurkeyReceived 23July 2003;received in revised form 14August 2003;accepted 26August 2003AbstractIntegrated iron-and steelmaking plants generate large amounts of solid wastes,which are mainly blast furnace and steelfurnace slags,dusts,sludges,etc.The main problem faced at Kardemir integrated iron and steel works is with the steelmaking slags,i.e.open-hearth furnace slag and more recently basic oxygen furnaces (BOF)slag.Over the last 50years of operation,more than 5million tons of slag has accumulated in the waste stockyard.To make use of this slag,a slag-processing plant came into operation at Kardemir in 1999.In this plant,after crushing and classification of the slag to various sizes,the magnetic particles present in the slag are separated by electromagnets.The magnetic 10–50mm product of the slag-processing unit is charged to blast furnaces as raw material and the magnetic pieces in the range of 50–500mm are charged to BOF as scrap.This study was undertaken with the objective of determining the effects of addition of the above-mentioned magnetic products of the slag-processing unit on the operation and performance of the blast furnace and the BOF.No magnetic separation is applied to the À10mm fraction and this material is added into sinter mix as a source of iron and fluxing materials.A further objective of the study was determination of the effect of addition of the À10mm product of the slag-processing unit on the properties of the sinter and use of slag-added sinter on the operation of the blast furnace.The slag-processing unit has been found to be a good solution to the environmental problems created by the accumulated slag.D 2003Elsevier B.V .All rights reserved.Keywords:reducibility;RDI;sinter;steelmaking slag;recycling1.IntroductionIntegrated iron and steel plants generate large amounts of solid wastes (Szekely,1996).Various studies done in this field indicate that about 420kg of solid wastes consisting of blast furnace slag,steelmaking slag,dusts,sludges,mill scales,used refractories,etc.are produced per ton of steel (C ßamcıand Aydın,2000;Thakur,2000;Yadav et al.,2001).This study is related to about 5million tons of Kardemir steelmaking slags,which have accumulatedat the waste yard during the last 50years.Most of these slags were generated by the Siemens–Martin (open-hearth)steelmaking furnaces,which was in operation until 1998.Recently,basic oxygen furnaces (BOF)came into operation and the BOF slag pro-duced for the last few years is piled separately and planned to be processed at a later date.A slag-processing plant was designed and set-up at Kardemir in 1999to make use of the accumulated steelmaking slag.This plant has a capacity of 250,000tons/year when operated on one shift basis.In this plant after the separation of the skull by the use of lancing facilities and dropballs,the slag is crushed,classified and magnetically separated.Magnetic con-0301-7516/$-see front matter D 2003Elsevier B.V .All rights reserved.doi:10.1016/j.minpro.2003.08.005*Corresponding author.Fax:+90-312-2101330.E-mail address:topkaya@.tr (Y .Topkaya)./locate/ijminproInt.J.Miner.Process.74(2004)31–39centrates produced in the particle size ranges of 10–50and 50–500mm are charged to Kardemir blast furnace and converters,respectively.The amount of magnetic pieces separated from the open-hearth slag varies in the range of 5–8%of the slag processed.The 10–30mm non-magnetic pieces are sold as railroad ballast to Turkish Railroads and the 30–65mm non-magnetic fraction is marketed as aggregates for road construction and for other purposes.No magnetic separation is applied to the À10mm product of the slag-processing plant and this material is added into sinter mix as a source of iron and fluxes.Research related to the production of cement as well as grit material for shipping industry from the À10mm slag fraction is in progress.The effects of use of various products of the slag-processing plant at Kardemir integrated iron and steel works as blast furnace charge material and as scrap for the converters are given in this study together with the effect of slag addition into sinter mix on the physical,chemical,mineralogical and reduction properties of the sinter produced.es of slag concentrates at Kardemir ing slag concentrates as a blast furnace feed The magnetic concentrate in the size range of 10–30mm produced at the slag-processing plant was charged into the Kardemir No.3blast furnace.Chem-ical analysis indicated that the 10–30mm particle size magnetic concentrate contained 70–85%Fe.In the first group of tests which lasted 1month,700tons of this material was fed into the Kardemir No.3blast furnace in quantities of 20–25tons/day,which con-stituted f 1%by weight of the iron-bearing burden (sinter +pellets +lump ore).An increase of about 12tons/day was observed in the daily hot metal output of the furnace.In the second group of tests which lasted 2weeks,900tons of this material was used,which wascharged into the same furnace in quantities of f 60tons/day which constituted f 2.7%by weight of the iron-bearing burden.The hot metal production was increased from 1500to 1525tons/day during this period.No operational difficulty was encountered in these tests and it appeared that larger quantities could be used.A change was made in the slag-processing unit so that the slag was crushed and classified to the sizes of À10,10–50and 50–500mm instead of 10–30and 30–500mm sizes and the magnetic part of the 10–50mm was charged to the blast furnace.This change resulted in production of a larger quantity of magnetic material to be charged into the blast furna-ces.The Fe content of the 10–50mm magnetic concentrate was also found to be in the 70–85%Fe range.Two groups of tests were made with the 10–50mm size magnetic slag concentrate.The slag concen-trate constituted 4.5%by weight of the iron-bearing burden in the first group of these tests as shown in Table 1where the average percentages by weight of the slag concentrate,sinter and pellet used together with the average daily production rates,coke rates and tar rates are presented.The charge make-up could not be kept constant in this group of tests due to opera-tional reasons.In spite of lower quantities of sinter and pellet in the charge,addition of the slag concen-trate at a quantity of 4.5%into the charge resulted in a 70tons/day (f 5%)increase in the production rate,an f 20kg/ton hot metal (f 4%)decrease in the coke rate of the Kardemir No.3blast furnace when the tar rate was decreased from 42.8to 33.7kg/ton hot metal.Care was spent to keep the relative quantities of sinter and pellet in the charge in the second group of tests the data of which is presented in Table 2.The quantity of slag concentrate in the charge was 10.3%by weight of the iron-bearing burden in this group of tests and the effect of this larger quantity of the slag concentrate on the performance of the blast furnaceTable 1Average charge make-up,production rate,coke rate and tar rate in the first group of tests (Sevinc ß,2001)Slag concentrate in charge (%)by weight Sinter in charge (%)by weight Pellet in charge (%)by weight Daily production rate tons hot metal Coke rate kg coke/ton hot metal Tar rate kg tar/ton hot metal 0.05327140952542.84.54820147950533.7Y.Topkaya et al./Int.J.Miner.Process.74(2004)31–3932was larger:an 85tons/day (f 6%)increase in the production rate,a 49kg/ton hot metal (f 9%)de-crease in the coke rate when the tar rate was increased from 37.4to 42.9kg/ton hot metal.No important change was observed in the chemical analysis of the hot metal except for a slight (less than 0.01%)decrease in the sulfur content.ing slag concentrates as BOF feedBy processing the open-hearth furnace slag,metal-lic pieces in the size range of 50–500mm were also produced.These metallic pieces were used together with skull,which was obtained by dropping a steel ball on to the huge slag pieces.They were added in varying amounts to the converters at Kardemir.Dur-ing the production of steel at 100-ton capacity con-verters of Kardemir,normally 20tons of scrap is charged together with hot metal from the blast furna-ces.In a series of tests conducted in the melt shop,various amounts of 50–500mm scrap (CDH)and skull (OMS)obtained from slag were mixed with the normal scrap (S)and charged into the converter.The metallic yield,defined as the ratio of weight of steel produced to the weight of metallic material (hot metal +S +CDH +OMS)charged into the converter,was determined from the known weights of the charge materials and of the liquid steel produced.The weight of S +CDH +OMS used in these tests was kept constant at 20tons/heat.The results of these tests are presented in Fig.1where the metallic yield is plotted against the type of charged scrap.As it can be seen from Fig.1,the metallic yield was decreased with the use of skull and/or scrap obtained from slag.The results were not surprising,since both skull and magnetic pieces obtained from the slag-processing unit were not as good quality as normal scrap due to the presence of slag stuck onto these materials,which could not be separated completely at the skull sepa-ration,or the slag-processing plant.Costs of OMS and especially CDH are very low however as their costs consist of only transportation and operation costs for Kardemir and use of these materials decrease the cost of the metallic charge to the converter.At the timeFig.1.Metallic yield values with different types of scrap (Sevinc ß,2001).Table 2Average charge make-up,production rate,coke rate and tar rate in the second group of tests (Sevinc ß,2001)Slag concentrate in charge (%)by weight Sinter in charge (%)by weight Pellet in charge (%)by weight Daily production rate tons hot metal Coke rate kg coke/ton hot metal Tar rate kg tar/ton hot metal 05723141653337.410.35721150148442.9Y.Topkaya et al./Int.J.Miner.Process.74(2004)31–3933these tests were conducted,the price of scrap was f75%of the cost of the hot metal and the costs of OMS and CDH were f11%and f2%,respective-ly,of the cost of the hot metal,as determined by Kardemir officials.Unit costs for liquid steel(origi-nating from the metallic charge only),defined as the ratio of the cost of the metallic charge to the metallic yield,calculated and expressed as the percentage of the unit cost of the(hot metal+normal scrap)are also presented in Fig.1.Replacement of some of the scrap by OMS or CDH is seen to decrease the unit cost of the liquid steel in spite of decrease in the metallic yield and lowest cost is seen in the H+5000kg OMScharge;the unit cost is also low for the H+5000kg CDH charge.Based on these results replacement of some of the scrap by OMS and/or CDH has been decided to be useful by Kardemir officials and these materials are charged into the converters during nor-mal production.Visual observations indicate that the slag volume increases by use of OMS and/or CDH and handling of these materials is more difficult than that of normal scrap.Due to these,the quantity of OMS and/or CDH used singly or in combination in one charge is limited by5tons.ing slag concentrates as sinter feedThe chemical analysis of theÀ10mm slag fraction indicated that it contained25–30%Fe,17–22% SiO2,5–8%Mn,2.5–4.5%Al2O3,31–35%CaO and7.5–9%MgO.Since no magnetic separation was applied to this fraction,it was found to contain some metallic pieces of iron as well.As seen from the chemical analysis,the slag contained a large quantity of useful elements or fluxing materials such as Fe, Mn,CaO,MgO,etc.In order to utilize the useful components of theÀ10mm slag fraction,it was added to the sinter mix of Kardemir.The slag was added to the sinter mix up to4%by weight of the iron bearing charge.Chemical analysis of the sinters pro-duced,presented in Table3,indicated that as the amount ofÀ10mm slag addition to the sinter mix was increased from0%to4%in steps of1%,the iron content of sinter decreased and the CaO,MgO,Mn, P2O5contents increased(Sevincßand Topkaya,2001). There was also a slight increase in the basicity of the sinter product.Sinters having0–4%by weight of slag addition were charged into the Kardemir No.3blast furnace.No beneficial or detrimentral effect of use of slag-added sinters on the operation of the blast furnace could be detected except that the phosphorus content of the hot metal was increased.Based on these results, the addition ofÀ10mm slag into the sinter mix up to 4%by weight of the iron-bearing burden has been decided by Kardemir officials to be used as a standard practice at Kardemir.3.Properties of slag-added sinters produced at KardemirNo significant change in the operation of the Kardemir No.3blast furnace could be detected when slag-added sinters were used in the burden as stated above.Addition of slag into the sinter mix is expected to affect the properties of the product sinter,however. Based on this expectation,physical,chemical,miner-alogical and reduction properties of the sinters were determined.The results of these studies are presented in this section.3.1.Chemical and mineralogical propertiesChemical analyses of theÀ10mm slag fraction and the slag-added sinters produced were given above.TheÀ10mm slag fraction and sinter samples containing0%,1%,2%,3%and4%slag were subjected to X-ray diffraction studies and mineralog-ical investigation under an optical microscope with the objective of determination of phases existing in their structures.X-ray diffraction pattern of theÀ10 mm slag fraction is given in Fig.2together with the different phases detected to be present in its structure. Table3Chemical compositions of sinters with varying percentages of slag addition%Slag01234%Fe51.7151.1351.3350.8650.81 %FeO11.3111.5312.5712.6411.93 %Mn 1.27 1.25 1.27 1.34 1.39 %CaO12.1112.6412.4712.9112.69 %MgO 2.16 2.12 2.08 2.34 2.49 %SiO28.098.358.208.218.28 %P2O50.010.020.030.040.04 Basicity(%CaO/%SiO2)1.50 1.51 1.52 1.57 1.53Y.Topkaya et al./Int.J.Miner.Process.74(2004)31–39 34From this figure and mineralogical study under mi-croscope (C ßag˘atay,1999),the À10mm slag fraction was concluded to be composed of primarily FeO (wustite),2CaO ÁSiO 2(dicalcium silicate)and various CaO –FeO –SiO 2solid solutions like CaFeSiO 4(ferro-monticellite).X-ray diffraction patterns of sin-ter samples containing 0%,1%,2%,3%and 4%slag were found to be similar and were like that given in Fig.3for the sinter sample containing 2%slag.The results of the X-ray diffraction studies together with the results of mineralogical studies conducted onsinter samples containing 1%and 4%slag (C ßag˘atay,2001)indicated that addition of slag into the sinter mix at Kardemir did not change the mineralogical structure of the sinter and that all sinter samples contained magnetite (Fe 3O 4),hematite (Fe 2O 3),wus-tite (FeO),ferro-monticellite (CaFeSiO 4),dicalcium silicate (Ca 2SiO 4),calcium ferrites (CaFe 5O 7,Ca 4Fe 14O 25,Ca 2Fe 7O 11)and an amorphous glassy phase.Although slag addition into the sintermixFig.3.X-ray diffraction pattern of the 2%slag added sinter.()Fe 2O 3(hematite),()Fe 3O 4(magnetite),()FeO (wustite),(o )CaFeSiO 4(ferro-monticellite),()Ca 2SiO 4(dicalcium silicate),()CaFe 5O 7,()Ca 4Fe 14O 25,()Ca 2Fe 7O 11,(4)MgSiO 3.Fig.2.X-ray diffraction pattern of the À10mm open-hearth furnace slag.(o )CaFeSiO 4(ferro-monticellite),(5)MgFe 2O 4(spinel),()FeO (wustite),()Ca 2SiO 4(dicalcium silicate),(4)Ca 3Si 2O 7(kilchoanite),()K 2MgSi 5O 12,()K 2MgSi 5O 12.Y.Topkaya et al./Int.J.Miner.Process.74(2004)31–3935has been found not to result in the formation of new phases,increase in slag addition has been found to give rise to decrease in hematite and increase in Fe (II)contents of the sinters.The sinters were observed to be very heterogeneous,as can be seen from the microstructures of the sinter samples containing 1%and 4%slag presented in Figs.4–7,possibly due to the relatively coarse grain size of some raw materials,the limited diffusion possible during the short period at melt conditions and variations in the peak temper-ature reached.3.2.PorosityTrue and apparent densities of the sinter samples were measured to determine the total porosity.True densities were measured by use of crushed and ground sinter samples and water pycnometer and apparent densities of the of the sinter samples with the original size of 10–12.5mm were measured by the mercury pycnometer as described in the Turkish Standard TS 4379.Total porosity,e ,was then calculated from thetrue density,d T ,and apparent density,d A ,values by the formula (1)and the results are presented in Table 4:e ¼1Àd A d TÂ100:ð1ÞIn a previous study (Sevinc ßet al.,1987),a totalporosity of about 29%was found for Turkish Divrig˘i iron ore pellets and total porosities of the Turkish lump ores of Hekimhan,Koruyeri and Attepe were found to be in the 25.5–29.6%range,while total porosities of the Turkish lump ores of Kesikko ¨pru ¨and Avnik were found to be less than 6%.Pellets are known to have porous structures due to finely dis-persed micropores and Hekimhan,Koruyeri and Attepe ores are mostly composed of goethiteandFig.4.Magnetite crystals {1},hematite recrystallised along <111>directions {2},glassy material {3},(1%slag addedsinter).Fig.5.Magnetite crystals {1}and wustite in rod like form {5}surrounding magnetite crystals (1%slag addedsinter).Fig.6.Dendritic calcium ferrite crystals {4},below are magnetite {1}(4%slag addedsinter).Fig.7.Magnetite {1}and wustite {5}crystals are between each other;some calcium ferrite {4}is in the middle (4%slag added sinter).Y.Topkaya et al./Int.J.Miner.Process.74(2004)31–3936Table 4True density,apparent density and total porosity values of the Kardemir sinters %Slag addition to sinter1234Apparent density (g/cm 3)min. 3.27 3.11 3.27 3.16 3.17Apparent density (g/cm 3)max. 3.31 3.13 3.40 3.22 3.25True density (g/cm 3)average4.27 4.23 4.24 4.23 4.22Total porosity (%)max.23.4526.7022.6825.2824.89Total porosity (%)min.22.5126.2319.623.8622.99Fig.8.Gakushin reducibility test results.limonite type of iron oxides and are known to be porous.Kesikko ¨pru ¨and Avnik ores are magnetite and magnetite–hematite type ores and are known to be compact.The results presented in Table 4,according-ly,indicate that the Kardemir iron ore sinters had total porosities to provide sufficient surface area for gas-eous reduction. 3.3.ReducibilityThe Japanese standard JIS 8713known asGakushin test was used to determine the reducibilities of the sinters.500g (F 1particle)of sinter oven dried at 105j C for 1–1.5h was placed inside the test tube and reduced with a reducing gas composed of 70%N 2–30%CO with a total flow rate of 15l/min at 900j C.The reducing gas was passed through the system for 3h during which period the weight loss of the test tube was recorded.By the reduction reactions,oxygen in iron oxides combines with CO and leaves the system as CO 2giving rise to a loss in the weight of the system.Total weight of oxygen bound to iron can be determined from the chemical analysis of the sinters by stoichiometric calculations.The extent of reduction defined as (Eq.(2)):%Reduction ¼Weight of oxygen removed from iron oxides by the reducing gasTotal weight of oxygen bound to iron before reductionÂ100ð2Þcould thus be calculated as a function of time.The results of the reducibility experiments are presented graphically in Fig.8where %reductions for different sinter samples are plotted against time.The data presented in Fig.5was found to obey the porous solid model kinetic Eq.(3)(Hughes,1982):Àln ð1À%R =100Þ¼kt ð3Þwhere k is the reaction rate constant (min À1),also referred to as the reducibiliy index,and t is time in minutes.This model assumes that the reducing gas penetrates uniformly into the pores of the solid particles and that the reaction is rate-limited by gas–solid reaction on the pore walls.The reducibility indices for the different sinter samples determined from the slopes of the Àln(1À%R /100)vs.time plots are presented in Table 5.Y.Topkaya et al./Int.J.Miner.Process.74(2004)31–3937Fig.8and Table5indicate that reducibility of sinters decrease with increase in the quantity of slag added into the sinter mix.Reducibility depends on many factors like porosity,size of particles and phases found in the structure.Size of the samples was fixed in the experiments since the samples were taken in a size range of10–12.5mm and porosities of the sinter samples were not very different from each other as stated above.Decrease in reducibility of the sinters with increase in the quantity of slag added into thesinter mix is probably related to the mineralogical structures.Jasienska and Durak(1999)studied reduc-ibilities of different phases that may exist in iron sinters by reducing synthetically prepared specimens by pure CO gas at850j C and found that certain calcium ferrites like Ca3Fe15O25,CaFe4O4,CaFe3O5 and CaFe2O4are highly reducible while the reduc-ibility of fayalite is very low and that reducibility of magnetite is lower than that of hematite.A reducibil-ity value for wustite,FeO,is not given in his study but it is known that its reducibility is quite low.Highly reducible calcium ferrites and fayalite did not exist in Kardemir sinters and slag addition did not result in formation of new phases as stated in Section 3.1 above.Decrease in reducibility of Kardemir sinters with increase in quantity of slag added into the sinter mix probably is due to the accompanying decrease in hematite and increase in Fe(II),present in the system mostly as wustite and ferro-monticellite.3.4.Reduction–disintegration propertiesThe international standard ISO4696-1was chosen as the procedure for determining the reduction–dis-integration properties of the iron ore sinters in this study.Oven dried samples of500g(F1particle) weight were reduced with a reducing gas composed of70%N2–30%CO with a total flow rate of15l/min for30min at550j C.At the end of reduction,the system was cooled to below100j C and the sinter samples were taken out of the test tube,weighed and then placed in130mm inner diameter,200mm long tumbler drum.The tumbler drum was rotated at30 rpm for30min and the samples were taken from the tumbler drum for screening.Reduction–disintegration indices,RDI+6.7,RDI+3.35and RDIÀ0.5values,de-fined as the weights of the+6.7mm fraction,the +3.35mm fraction and theÀ0.5mm fraction, respectively,expressed as the percentage of the weight of the sample before tumbling were then determined. The results presented in Table6indicate that RDI+6.7 and RDI+3.35indices of the sinters increase slightly with increase in the quantity of slag added into the sinter mix and that the RDIÀ0.5index,referred to as the dusting index,is not very much influenced.A low reduction–degradation breakdown index, RDI+3.35mm,is considered bad for blast furnace operation as this causes formation of fines within the furnace,which may result in hanging,slipping, checking,reduced wind rates and generally irregular practice leading to decreased furnace productivity and variable hot metal analysis.The limiting level is debatable but plant experience has shown that levels below55–60%are undesirable(Bagnal,1977).The values found for Kardemir sinters,which are all higher than80%,can be accepted as good values. 4.ConclusionsThe slag-processing plant built at Kardemir has been found to be a good solution to process more than 5million tons of slag that has accumulated in the waste stockyard within the past50years.In this plant, after crushing and classification of the slag to various sizes,the magnetic pieces present in the slag areTable5Reducibility indices of Kardemir sinters%Slagadditionto sinter01234Reducibility indices, k(minÀ1)0.00680.00660.00590.00620.0055Table6Results of reduction–disintegration tests%Slag added to sinter01234RDI+6.7(+6.7mm part as%)58.7159.3962.1167.5562.47RDI+3.35(+3.35mm part as%)83.5282.7984.7286.2484.70RDIÀ0.5(À0.5mm part as%)3.624.51 4.45 3.23 3.74Y.Topkaya et al./Int.J.Miner.Process.74(2004)31–3938separated by electromagnets.The magnetic10–50 mm product of the slag-processing unit is charged to blast furnaces as raw material and the magnetic pieces in the range of50–500mm are charged to BOF as scrap.No magnetic separation is applied to theÀ10 mm fraction and this material is added into sinter mix as a source of iron and fluxing materials.Use of the magnetic10–50mm product of the slag-processing plant has been found to increase the pro-duction rate and decrease the coke rate of the Kardemir No.3blast furnace.This material has been charged into the Kardemir No.3blast furnace at quantities exceeding10%by weight of the iron-bearing burden and no operational problems have been encountered. The only problem with this material is its limited production at the slag-processing plant.Use of the magnetic50–500mm product of the slag-processing plant together with skull produced from open-hearth slag to replace scrap in the BOF has been found to decrease the metallic yield defined as the ratio of weight of steel produced to the weight of metallic material charged into the converter.The unit cost of the steel produced has been found to be decreased,however,by use of the magnetic50–500 mm product of the slag-processing plant in the BOF due to the very low cost of this material compared to scrap.Visual observations indicate that the slag vol-ume increases by use of OMS and/or CDH and handling of these materials is more difficult than that of normal scrap.Due to these,the quantity of OMS and/or CDH used singly or in combination in one charge is limited by5tons.TheÀ10mm fraction of the slag-processing plant was added into the sinter mix up to4%by weight of the iron bearing charge and sinters thus produced were charged into the Kardemir No.3blast furnace.No beneficial or detrimental effect of the use of slag-added sinters on the operation of the blast furnace could be detected.Experimental studies conducted on sinters produced at Kardemir indicated that physical,chemi-cal and mineralogical properties were not significantly affected by the quantity of slag added into the sinter mix and that the porosities,reducibilities,reduction–disintegration properties of the slag-added sinters were reasonably good.Based on these results use of4%by weight of the iron bearing charge ofÀ10mm slag in the sinter mix has become a standard practice at Kardemir iron and steel works. AcknowledgementsThis investigation was supported by the Scientific and Technical Research Council of Turkey(TU¨BI˙-TAK)and Kardemir Iron and Steel works. 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