A Comprehensive Static Model of an Iron Bath Smelting Reduction Process with Thick Slag for Alumina-Rich Iron Ore

Transcription

1 ISIJ Internatonal, Vol. 55 (2015), ISIJ Internatonal, No. 10 Vol. 55 (2015), No. 10, pp A Comprehensve Statc Model of an Iron Bath Smeltng Reducton Process wth Thck Slag for Alumna-Rch Iron Ore Ybo HE, Bao TANG, Qang LI and Zongshu ZOU* Northeastern Unversty, School of Materals and Metallurgy, Shenyang, Laonng, Chna. (Receved on May 4, 2015; accepted on June 19, 2015) In order to make comprehensve utlzaton of alumna-rch ron ore, a two-step three-vessel smeltng reducton process s proposed, ncludng the Rotary Hearth Furnace (RHF) for the pre-reducton of alumnarch ron oxde pellet, the Smeltng Reducton Vessel (SRV) wth a thck slag bath for the fnal reducton and meltng of pellet, and the Gas Reformng Furnace (GRF) for reformng the exhaust gas from SRV. A three-level lances arrangement s desgned to mprove the energy utlzaton and producton condton of the SRV. An overall process model s establshed for the whole process to calculate the mass and heat consumptons of all sectons of the process, and to nvestgate the effect of operaton condtons on the process performance. Wth 80% heat transfer effcency of post combuston, the recommended post combuston rato (PCR) of the SRV gas s 55% and the pellet metallzaton rate (PMR) s 80%. A zoned model of the SRV s developed to calculate the mass and heat balances of each reacton zones and especally to optmze the operaton condtons. The recommended PCRs n the upper slag and lower slag are 15% and 0% respectvely. The coal-only njecton method s recommended n the lower slag. KEY WORDS: alumna-rch ron ore; ron bath smeltng reducton; thck slag bath; process model; zoned model. 1. Introducton Alumna-rch ron ore s a wdely exsted polymetallc depost n the nature, and t has a hgh comprehensve utlzaton value. Because the alumna and ron contents of alumna-rch ron ore vary from each other, there are many dfferent measures and processes to recover ron and alumna from the varous knds of alumna-rch ron ore. In the blast furnace (BF) ronmakng process, the alumna content n the ron ore fnes used n snterng makng all over the world s mostly less than 2%, but % n Inda. 1 3) As the less-slag operaton s promoted n the recent blast furnace operaton, the Al 2 O 3 n the BF slag ncreases and thus would cause troubles such as ncreasng pressure drop n the drppng zone and ncreasng the gas flow resstance n the cohesve zone. The study by Sunahara 4) ndcates that the optmum composton of hgh Al 2 O 3 slag s hgh MgO and low CaO/SO 2. The recoverng of Al 2 O 3 from slag wth hgh content of MgO s dffcult due to the appearance of 20CaO 13Al 2 O 3 3MgO 3SO 2 (Q phase) n the slag, 5) so the slag s used as raw materal for other ndustral producton, such as cement producton, whch would brng certan resource waste of Al 2 O 3. Alumna-rch ron ore wth 40% or more Al 2 O 3 s frst processed nto Al 2 O 3 and red mud by combned Bayer process or pure Bayer process, 6) and then the ferrc oxde and the resdual Al 2 O 3 n the red mud s to be recovered by other * Correspondng author: E-mal: zouzs@mal.neu.edu.cn DOI: methods. Wanchao Lu 7) obtaned an ron recovery rate of 81.4% wth drect roastng reducton process from a red mud wth 27.93% Fe 2 O 3 and 22.00% Al 2 O 3 and the resduals wth Al 2 O 3 were used for buldng materal producton. Xaobn L 8) adopted a process of reducton-snterng of a red mud wth 32.52% Fe 2 O 3 and 18.42% Al 2 O 3 and the recovery rates of ron and alumna were 60.67% and 89.71%, respectvely. Compared wth these two results, the ron and alumna content of the red mud were not fully recovered, the former hgh ron recovery rate but low alumna recovery rate and the latter on the contrary. Other alumna-rch ron ore wth Al 2 O 3 content between % cannot be used n BF process or Bayer process. So the smeltng reducton processes whch are used as economc and envronmental frendly alternatve ronmakng processes are consdered to utlze these polymetallc resources. A two-step three-vessel smeltng reducton process for processng alumna-rch ron ore s proposed n ths study, and the schematc dagram s shown n Fg. 1. The pelletzed ron ore wth coal s frstly preheated n a pre-heatng furnace and pre-reduced n a Rotary Hearth Furnace (RHF) to a pellet metallzaton rate (PMR) of 80%. The heat carred by exhaust gas of RHF s used to pre-heat oxygen-rch ar and pellet. The metallzed alumna-rch ron oxde pellet at 900 C s charged drectly nto the Smeltng Reducton Vessel (SRV) wth a thck slag bath. The fnal reducton and meltng of pellet take place n the thck slag bath of the SRV, whch can not only separate the meltng and reducton zones gradently and prevent the reoxdaton of ron droplet (pure ron reduced from ron oxdes) n the ISIJ

2 ISIJ Internatonal, Vol. 55 (2015), No. 10 Fg. 1. Schematc dagram of the two-step three-vessel smeltng reducton process. Table 1. Chemcal composton of alumna-rch ron ore (mass%). TFe Fe 2O 3 CaO MgO SO 2 Al 2O 3 MnO P 2O 5 S Rest slag, but also make the heat transfer more effcent from the post combuston of top gas to slag. The top gas generated by the SRV wth a post combuston rato (PCR) of 55% s reformed n the Gas Reformng Furnace (GRF) and the requred heat comes from ts sensble heat. The reformed gas s used n the RHF to produce heat for the reducton of the pellet. An overall process model was establshed n ths study and the mass and heat balance calculatons were carred out to nvestgate the consumptons of raw materal and energy of the entre process, ncludng RHF, SRV, and GRF. The PCR n SRV and the PMR n RHF were optmzed to balance the raw materal consumpton and heat loss. As the SRV s a complcated reactor and zoned model 9) can get more nsght nto the process, a zoned statc model was also establshed to make further exploraton of the SRV operaton condtons. Table 2. Chemcal composton of coal (mass%). H 2O Ash Vol C fx C H O N S Rest Table 3. Chemcal composton of hot metal (mass%). Fe C S Mn P S Rest Establshment of the Models The basc prncple of the models s that, on the bass of the mass balances, the result was obtaned from the couplng calculaton of mass and heat consumpton. The thermodynamc data used n the calculaton are generally acknowledged data from lterature ) The chemcal compostons of the alumna-rch ron ore, coal and hot metal are lsted n Tables 1, 2 and 3, respectvely The Overall Model for the Entre Process The overall model conssts of three calculaton modules ncludng RHF calculaton module, SRV calculaton module and GRF calculaton module whch were llustrated n the flow chart n Fg. 2. The composton of the metallzed pellet as an output data of RHF s used n the SRV module, the composton of exhaust gas of SRV s used n Fg. 2. Flow chart of the overall model. the GRF module, and the reformed gas s used n the RHF module. The Gas++ n Fg. 2 means that the mass of gas s an assgnment and progressvely ncreases n the teraton, Coal means that the mass of coal progressvely decreases 2015 ISIJ 2126

3 ISIJ Internatonal, Vol. 55 (2015), No. 10 on the reverse, and Coal ++ means that the mass of coal progressvely ncreases n the teraton RHF Calculaton Module The RHF s used for the reducton of carbon-bearng alumna-rch ron oxde pellet, because the drect reducton of alumna-rch ron oxde pellet by gas s very dffcult to reach a hgh reducton degree. 13) The man reactons n the RHF are the reducton of ron oxde by sold carbon, post combuston of gas generated by reducton, and the gas combuston from the tuyere. As the metallzaton rate and resdual carbon of the pellet are set, ts coal consumpton can be determned drectly. Combnng the assumpton of 100% PCR of the exhaust gas and the heat balance Eq. (1), the oxygen and gas consumptons can be calculated and other mass and heat changes n the RHF can be calculated from the mass balance wth teraton. QR IS + QR Cg1 + QR PCg2 + QR Cv = QR-OSj + QR-Rk + QR-HL... (1) Here, n the Eq. (1), donates the raw materals ncludng gas, oxygen-rch ar and alumna-rch pellet. QR IS represents the sensble heat of raw materal fed nto the RHF. Q R Cg 1, Q R PCg and Q 2 R Cv donate the combuston heat of gas from the tuyere, gas generated by reducton and the volatles of coal n the pellet. j donates the products of RHF ncludng metallzed pellet and exhaust gas. QR-OS j represents the sensble heat of products out of RHF. k donates the reactons takng place n the RHF ncludng the reducton of ron oxdes and the decomposton of lmestone. QR-R k represents the reacton heat. Q R-HL donates the heat loss n the RHF. The basc operaton condtons of RHF were set accordng to the stuaton of current RHFs n Chna, whch were 300 C of pellet pre-heatng temperature, 800 C of oxygenrch ar, 30% of ar oxygen enrchment, C of exhaust gas temperature, and 20% of heat loss. The temperature of gas from tuyere was determned by the follow-up process GRF due to ts characterstcs whch wll be dscussed n the latter paragraph. The metallzaton rate of pellet s one of the most mportant factors n practcal producton, whch wll be nvestgated n dfferent values to study the mass and heat consumpton of the whole process, so as to obtan the optmum metallzaton rate for practcal applcaton SRV Calculaton Module The value of coal consumpton s an assgnment for the entre teraton and wth whch the consumptons of lme and pellet can be calculated through Eqs. (2) (3). Fe = Fe H + FeS... (2) Al2O 3 (CaO1 AlO 2 3) + SO 2 (CaO2 SO2)... (3) = CaO S In the Eqs. (2) and (3), donates the raw materals ncludng lme and pellet, and Fe represent the mass of Fe n the raw materal. Fe H and Fe S donate the mass of Fe n the hot metal and the slag, respectvely. Al O 2 3, SO 2, CaO and S represent the masses of the Al 2 O 3, SO 2, CaO and S n the raw materal. (CaO 1 /Al 2 O 3 ) and (CaO 2 / SO 2 ) represent two mole ratos. The frst s the rato of part of CaO and all the Al 2 O 3 n the raw materal; the second s the rato of the rest part of CaO and all the SO 2 n the raw materal. These two values are taken as 1.4 and 2 respectvely accordng to the requrement for Al 2 O 3 leachng. The volume and composton of gas released from the SRV are calculated by four Eqs. (4) (7) as the C and H balance equatons, the PCR equaton, and the equlbrum of water gas shft reacton. In these equatons, donates the raw materals ncludng coal, pellet and lme, C represents the mass of C n the raw materals such as fx carbon n the coal, resdual carbon n the pellet, carbonates n the lme, etc. C HM donates the mass of C n the hot metal, C G donates the mass of C n the gas, H represents the mass of H n the raw materal, H G donates the mass of H n the gas, and PCR donates the PCR of gas. C= C HM + CG... (4) H = H G... (5) ( CO2 + HO 2 ) (CO+ H 2 + CO2 + HO 2 ) = PCR... (6) Θ CO + HO 2 = CO2 + H2 G = T J/ mol... (7) The man energy ncomes of the SRV are the sensble heat of pellet, combuston of Coal, post combuston of CO and H 2, and the latent heat of slag-formng. The heat generated by post combuston goes to three drectons ncludng the sensble heat of gas, heat transferred to the slag, and the heat loss. The percentage of the heat transferred to the slag s defned as the heat transfer effcency of post combuston and t s calculated by Eq. (8). Heat transferred tothe slag η PC = 100%... (8) Heat generated by post combuston The slag phase dagram s used for the calculaton of slagformng latent heat. Wth the composton of slag, the poston of the slag n the phase dagram can be located and the component trangle around t gves the phase composton of the slag. The latent heat of slag-formng s expressed by the sum of formaton heat of the three components on the trangle vertces. Fgure 3 shows part of the CaO SO 2 Al 2 O 3 phase dagram that contans the most of BF slag and hgh alumna slag. The C, S and A n Fg. 3 represent CaO, SO 2 and Al 2 O 3, respectvely. For the convenence of calculaton, three calculaton unts are establshed for three component trangle as 1: CS C 3 S 2 C 2 S C 2 AS, 2: C 2 S C 2 AS CA and 3: C 2 S C 3 A C 12 A 7 CA, as shown n Fg. 3. Once the composton of slag s worked out, the judgment statement wll lead the program nto the correct unt to calculate, and the slag-formng latent heat s then obtaned. The sensble heat of the slag s also calculated n the same way. The operaton condtons of the SRV are determned by the reference to the data of the current Blast Furnace. The slag temperature s set as C, the hot metal temperature ISIJ

4 ISIJ Internatonal, Vol. 55 (2015), No. 10 Fg. 3. Part of CaO SO 2 Al 2O 3 phase dagram. s set as C, and the exhaust gas temperature s set as C. The pellet temperature s set as 900 C n the consderaton of heat loss durng the transfer form RHF to SRV. Pure oxygen s used as the combuston supportng gas. The PCR s nvestgated as a man process parameter to optmze the energy consumpton of the SRV and then the optmum PCR can be obtaned GRF Calculaton Module The energy ncome of GRF s the sensble heat of exhaust gas from SRV. Because the man gas reformng reacton,.e. the soluton loss reacton n the Eq. (9) does not proceed sgnfcantly below 900 C, 14) the gas temperature out of GRF s set as 900 C and the composton of the GRF gas and the coal consumpton are calculated by the judgment of equlbrum of Eq. (7) wth the temperature and composton of the SRV gas, durng whch the coal ncreases progressvely n the teraton tll the reacton reachng ts equlbra. The results agree well wth the experment results from the lterature, 14) then the calculaton results can be used to optmze other operaton parameters. Θ C+ CO = 2CO G = T J/ mol... (9) Zoned Model of the SRV The SRV s desgned for not only the fnal reducton and meltng of the pre-reduced pellet but also the composton adjustng of slag that can be used n the later Al 2 O 3 leachng process. A schematc dagram of the SRV s shown n Fg. 4 where the three-level lance arrangement and four dfferent reacton zones are also llustrated. The oxygen lances on the top level s nserted nto the free space on top of slag (zone 1). They are desgned to supply pure oxygen for the post combuston of gas and the hgh flow velocty wll also promote the heat transfer effcency of post combuston. Both the oxygen-coal lances are of coaxal double ppes. The lances n the mddle are nserted nto the upper slag (zone 2) to provde suffcent energy for the meltng of pellet. The strrng acton of the mddle lance n the upper slag wll enhance the heat convecton between post combuston gas and slag and ncrease the heat transfer effcency. The lances at the bottom are nserted nto the lower slag (zone 3) and manly to supply reducng agent for the reducton of the ron oxdes, and oxygen wll be njected f the heat transfer to the lower slag s not suffcent from the sensble heat of upper slag or the temperature of hot metal (zone 4) becomes too low. For the SRV wth three-level lances, each reacton zone has dfferent functons and the process condton s not the Fg. 4. Iron bath smeltng reducton vessel wth thck slag. Fg. 5. Flow chart of zoned statc model of SRV. same as well, so the overall statc model cannot provde full-scale techncal data to gude the operaton of SRV. A zoned model, as schematcally llustrated n Fg. 5, was establshed for further study of the process condton of each reacton zone. The model conssts of three modules ncludng zone 1, the top free board space for gas post combuston; zone 2, the upper slag zone for meltng; and zone 3, the lower slag zone for reducng. The calculaton starts wth zone 2 and gves the ntal gas composton of zone 1, then the teraton begns tll the heat calculaton reachng ts balance. The calculaton of zone 3 starts wth the ntal data of slag and ron droplets and gves back gas composton to zone 2, then the whole system starts ts teraton tll all the heat calculatons reachng ther balances. 3. Results and Dscusson Fgure 6 shows the results of the overall statc model under the condton of 80% PMR, 55% PCR, and 80% heat transfer effcency of post combuston. To produce 1 t hot metal, kg alumna-rch metallzed pellet s needed, along wth kg lme, kg coal and kg oxygen. Because the Fe grade of the alumnarch ron ore s low, and the slag-adjustng requres a large amount of lme to make the composton sutable for Al 2 O ISIJ 2128

5 ISIJ Internatonal, Vol. 55 (2015), No. 10 Fg. 7. Effect of heat transfer effcency of post combuston on SRV coal and oxygen consumptons. Fg. 6. A typcal result of the overall statc model. leachng, the coal consumpton of the SRV s relatvely hgh. About 4.1 t slag s also obtaned, from whch about % 1.2 t Al 2 O 3 may be expected. The sensble heat of the RHF exhaust gas s a valuable energy resource to recycle for the pre-heatng of the pellet and ar. The sensble heat of the C, kg exhaust gas s kj, of whch kj s used for 300 C pre-heatng of the kg pellet, and kj s used for 800 C pre-heatng of the kg oxygen-rch ar. The sensble heat can be fully used f the heat exchange effcency of the exchanger s 70%. There s a huge amount of reformed gas from GRF wth hgh reducton potental and temperature whch s far more than the requrement of RHF, the extra gas can be used for energy storage or electrcty generaton n order to make full use of the energy of the process. The slag wth hgh alumna content produced n SRV contans kj sensble heat whch can be effcently recovered durng the coolng process. Because the phase composton of the alumna-rch slag has specal requrements for Al 2 O 3 leachng, the coolng process of the slag should be slow rather than quenchng by water or gas. Then the heat recovery wll be more effcent. Assumng that 50% of the sensble heat can be recovered, kwh wll be avalable for electrcty generaton. As the raw materal for alumna extracton wth leachng rate of 86% (former experment result 15) ), about 1 t Al 2 O 3 can be obtaned from the slag. After Al 2 O 3 leachng, the resdual wll be a good materal for cement producton Effect of Heat Transfer Effcency of Post Combuston n SRV Fgure 7 shows the effect of heat transfer effcency of post combuston on SRV coal and oxygen consumptons under the condton of 80% PMR and 55% PCR of SRV gas. The coal and oxygen consumpton ncrease rapdly wth the decrease of the heat transfer effcency. The mechansms of heat transfer n the smeltng reducton process wth a thck slag layer n a top and bottom blown converter was studed by H. Katayama, et al ) The study ndcates that f the gas temperature s below C, the 2129 heat transfer by radaton and gas convecton can account for only 30% or less of the total heat transfer, but suffcent amount of carbonaceous materal and moderate ntensty of strrng n the slag contrbute greatly to the heat transfer. The heat transfer effcency s about 90% n ther study. For the smeltng reducton process wth thck slag layer for alumna-rch ron ore proposed n the present work, the oxygen lances n the free space ensure the heat transfer by radaton and gas convecton, and the oxygen and coal lances n the upper slag supply coal and enhance the strrng acton of the slag, and the gas generated by the reducton n the lower slag also takes a part n the strrng, so the heat transfer effcency of the process can be mproved by all these measures. The heat transfer effcency of the post combuston n the top free space n the model s assumed to be 80% for the conservatve calculaton of the process and used n the followng calculatons Effect of PCR n SRV The gas post combuston n SRV plays an mportant role n the materals and partcularly heat balances, and t has sgnfcant nfluence on the coal consumpton of each part of the process. Fgure 8 shows the effect of gas PCR on the coal consumpton of RHF, SRV and GRF. The coal consumpton of SRV decreases wth the ncrease of PCR, but the decreasng rate slows down gradually. When the PCR gets hgher, more heat wll be generated by the same amount of coal and suppled to the SRV, so less coal s needed for the requred energy. But the 10% absolute heat loss of total output heat assumed n the model lmts the coal savng. The coal consumpton of the GRF decreases wth the ncrease of PCR. Ths s because that hgh PCR reduces the SRV gas generaton as shown n Fg. 9. And the SRV oxygen consumpton also decreases wth the ncrease of PCR. So, for the GRF, less gas generaton and hgh PCR make the equlbrum condton of the gas reformng reacton dfferent from each other. The reducton potental of the reformed gas s decreased wth the ncrease of PCR as shown n Fg. 10. When the metallzaton rate of the pellet s set as constant of 80% whle the PCR changes, the coal consumpton s the same for producng the same amount of metallzed pellet. The total coal consumpton of the process decreases wth 2015 ISIJ

6 ISIJ Internatonal, Vol. 55 (2015), No. 10 Fg. 8. Effect of SRV PCR on coal consumptons. Fg. 11. Effect of SRV PCR on heat loss. Fg. 9. Effect of SRV PCR on SRV gas generaton and oxygen consumpton. Fg. 12. Effect of PMR on coal consumptons. Fg. 10. Effect of SRV PCR on gas reformng. the ncrease of PCR. Heat loss exsts n each part of the process ncludng the heat loss of the furnace tself and the heat loss durng the heat recovery of the RHF exhaust gas and SRV slag. The values of absolute heat loss are 20%, 10% and 10% of the total output heat of RHF, SRV and GRF, 30% of sensble heat of RHF exhaust gas, and 50% of sensble heat of SRV slag. The relatve heat loss s the percentage that heat loss takes from the total output heat. The total absolute heat loss decreases wth the ncrease of SRV PCR as shown n Fg. 11 where ce s the abbrevaton of coal equvalent and 1 kg ce corresponds to kj. Fgure 11 also ndcates that the relatve heat loss ncreases wth the ncrease of PCR. As a result, the absolute heat loss s reduced, but the heat utlzaton rato s also reduced. In concluson, the recommended PCR s 55% and t s used n the followng calculatons. The relatve and the absolute heat loss are at moderate levels. Wth further ncrease of PCR, the decreasng rate of coal consumpton slows down, and so as those of SRV gas and oxygen consumpton. The qualty of the reformed gas wth low reducton potental also becomes unsutable for other applcatons. As a reference for the actual producton, the recommended 55% of PCR can be adjusted accordng to the practcal stuaton Effect of PMR n RHF The effect of PMR on coal consumptons s shown n Fg. 12. As t can be seen n Fg. 12, more coal s needed for the 2015 ISIJ 2130

7 ISIJ Internatonal, Vol. 55 (2015), No. 10 reducton of the pellet n the RHF and the coal consumpton ncreases wth the ncrease of PMR. Hgher metallzaton rate reduces the energy requrement of SRV for the meltng and reducng of pellet. So less coal wll be needed n the SRV and the SRV gas generaton and oxygen consumpton are reduced n the meanwhle as shown n Fg. 13. Wth the same gas PCR and temperature, less SRV gas leads to less coal consumpton n GRF. Thus, the total coal consumpton s reduced wth the ncrease of PMR. Fgure 14 shows that hgh PMR reduces the absolute quantty of heat loss but ncreases the relatve heat loss. So the recommended PMR s 80% to balance the raw materal consumpton and heat loss Zoned Model of SRV SRV s nstalled wth coaxal oxygen-coal lances n the upper and lower slag zones separately to produce gas wth certan PCR durng operaton. The metallzed pellets melt n the upper slag, and then ron oxdes and molten ron spread n the slag and move to the next slag layer for fnal reducton. So the gas PCR should be lmted n order to avod the reoxdaton of metallc ron n the slag. Fgure 15 shows the equlbrum dagram of ron oxdes reducton whch s calculated based on the data from the lterature ) One can see from the fgure that, n the SRV temperature range Fg. 13. Effect of PMR on SRV gas generaton and coal consumpton. (<1 650 C), the reoxdaton wll not happen when the gas PCR s lower than 15%. The heat transfer effcency of the post combuston n the top free space s 80%, but that nsde the upper or lower slag zones s 100%. So the zoned model can be verfed by the overall statc model by settng the PCR of upper and lower slag as 0%. The temperature gradent should be exsted n the slag, so the temperature of the upper slag s set as C, and the lower slag temperature s C. Because the output temperature of the slag s determned by the lower slag temperature, the temperature of the upper slag has no effect on the total coal consumpton. The PCR of the gas n the top free space s set as constant of 55%, and the PMR s 80%. Fgure 16 shows the results of the verfcaton and the values n the brackets are the dfferences between the zoned and overall model. It can be seen that the calculaton results of the zoned and overall model have acheved good agreement. The dfferences are the accumulaton error brought by the couplng calculaton between modules and teraton n the modules. In Fg. 16, US and LS represent the upper slag and lower slag respectvely. The pellet n Fg. 16 represents the metallzed pellet whose chemcal composton s shown n Fg. 6. The chemcal compostons of the upper and lower slags are lsted n Table 4. Calculatons are made to nvestgate the effect of PCR on the coal and oxygen consumpton of each reacton zone under the condtons of 1: PCR n the lower slag as constant of 0% and PCR n the upper slag changng from 0 15%; 2: PCR n the upper slag as constant of 15% and PCR n the lower slag changng from 0 15%. Fgure 17 shows the effect of PCR on the SRV coal consumpton. When the PCR n the lower slag s constant, the coal consumpton of the lower slag zone s nearly the same as the sensble heats of the slag and ron droplet from upper slag are fxed. Hgh PCR of upper slag ncreases the total heat generaton of post combuston. Thus the coal consumpton n the upper slag decreases wth the ncrease of PCR n the upper slag, and so as the total coal consumpton of SRV as shown n Fg. 17. The SRV coal consumpton does not change wth the ncrease of PCR n the lower slag whle settng the upper slag PCR as constant of 15% as shown n Fg. 17. Ths s because that, the total heat requred n the SRV s fxed, and the heat generated from coal s determned by the total PCR Fg. 14. Effect of PMR on heat loss. Fg. 15. Equlbrum dagram of ron oxdes reducton wth CO ISIJ

8 ISIJ Internatonal, Vol. 55 (2015), No. 10 Fg. 16. Calculaton result of zoned model verfcaton. Fg. 17. Effect of PCR on SRV coal consumpton. Fg. 18. Effect of PCR on percentage of coal consumpton. Table 4. Chemcal compostons of slag n upper and lower slag (mass%). Slag CaO SO 2 Al 2O 3 MgO FeO MnO CaS Rest Upper Lower n the upper slag before the gas gong to the top free space. As the PCR n the upper slag s constant, the same amount of coal wll be needed to meet the heat requrement of SRV. It s just attrbuted to ths causalty that the two-level lances n the slag can flexbly realze the smeltng reducton when the heat transfer s not suffcent from the upper slag,.e. the PCR n the lower slag can be ncreased to supply heat drectly wth no change n the total coal consumpton. The effect of PCR on the percentage of coal consumpton n upper and lower slag zones of SRV s shown n Fg. 18. When PCR n the lower slag s constant, the percentage of the coal consumpton n lower slag ncreases slghtly due to the slght decrease of coal consumpton n the upper slag. When PCR n the upper slag s constant, the percentage of the coal consumpton n lower slag decreases sgnfcantly wth the ncrease of the PCR n the lower slag. When the PCR n the lower slag ncreases, less coal s needed for the requred heat n the lower slag and less gas s suppled to upper slag. So more coal wll be needed n the upper slag to meet the heat requrement of the upper slag, and the percentage of the coal consumpton n the upper slag ncreases wth the ncrease of the PCR n the lower slag. Fgure 19 shows the effect of PCR on the percentages of oxygen consumpton n top free space, upper and lower slags. When PCR n the lower slag s constant, the percentage of oxygen consumpton n the lower slag s also roughly constant, and that n the upper slag ncreases wth the ncrease of the PCR n the upper slag. When PCRs n the upper slag and n the top free space are constant, the change rule of the percentage of oxygen consumpton n the upper 2015 ISIJ 2132

9 ISIJ Internatonal, Vol. 55 (2015), No. 10 Fg. 19. Effect of PCR on partton of oxygen consumpton. and lower slags s bascally the same as the change of the percentage of coal consumpton. So the effect of PCR n the upper slag s manly on the percentage of oxygen consumpton n the top free space and upper slag, and the effect of PCR n the lower slag s manly on the percentage of coal consumpton n the upper and lower slags. Hgh PCR n the upper slag reduces the coal consumpton of the SRV and ncreases percentage of oxygen consumpton n the upper slag whch guarantees the heat transfer effcency of post combuston n the top space by enhancng the strrng of the upper slag. So the recommended PCR n the upper slag s 15%. Although the ncrease of PCR n the lower slag can ncrease the percentage of coal and oxygen consumpton n the upper slag whch wll enhance the strrng acton of the upper slag, the CO 2 and H 2 O n the lower slag wll ncrease the rsk of the reoxdaton of the ron droplets n slag and the hot metal. So the recommended PCR n the lower slag s 0%. Under such condtons, the coal consumpton of the SRV s kg; oxygen consumpton s kg, gas generaton s kg; the percentages of coal consumpton n upper and lower slag are 67% and 33% respectvely; and the percentage of oxygen consumpton n top free space, upper and lower slag are 39%, 47% and 14%, respectvely. Comparng wth the overall model, the coal consumpton s decreased by 93.4 kg; oxygen consumpton s decreased by kg; and gas generaton s decreased by kg. As the basc operaton condtons n the SRV are not changed, the result of the zoned model s a modfcaton for the result of SRV n the overall statc model, and t s more accurate to descrbe the mass and heat consumpton of the SRV. When the PCR of the lower slag s 0%, there s 14% oxygen need to be blown nto the lower slag. In order to avod the reoxdaton n the lower slag, the coal-only njecton s desrable n the lower slag. In such a case, the requred heat n the lower slag need to be suppled by ncreasng the upper slag temperature and decreasng the lower slag temperature. The effect of slag temperature on the oxygen consumpton n the lower slag s calculated and the results are shown n Fg. 20. The oxygen consumpton decreases wth the decrease of lower slag temperature and the ncrease of the upper slag temperature. The data all fts well as lnear relatonshp, so the temperature of the slag wth 0 kg oxygen Fg. 20. Effect of slag temperature on oxygen consumpton of lower slag. Fg. 21. Slag temperature when oxygen consumpton of lower slag s 0 kg. consumpton n the lower slag can be calculated by the ftted functon and the results are shown n Fg. 21. The relatonshp between the upper slag temperature and the lower slag temperature s lnear, so the temperature can be predcted by the lnear equaton when one of the temperature s gven. The predcted data are verfed by the zoned model and the correspondng mass and heat consumpton n each zone can be calculated. As t can be seen n Fg. 21 that the temperature gradent between the upper and lower slag ncreases to about 133 C, and t s a necessary condton to realze the coal-only njecton operaton. Accordng to the calculaton results of the Factsage TM 6.1, the temperature of the slag n ths paper should be hgher than C to mantan a stable lqud phase. So f the temperature of the lower slag s set to be C, the upper slag temperature should be C to realze the smeltng reducton process wth no oxygen n the lower slag. Then, the total coal consumpton of SRV s kg; oxygen consumpton s kg; gas generaton s kg; the percentage of upper slag coal s 89%; and the percentages of oxygen consumpton n the top free space and upper slag are 39% and 61% respectvely. Comparng wth the results of upper slag temperature of C and lower ISIJ

10 ISIJ Internatonal, Vol. 55 (2015), No. 10 slag temperature of C, the SRV coal consumpton ncreases by 12.5 kg; oxygen consumpton ncreases by kg; and gas generaton ncreases by 34.3 kg. 4. Concluson An overall model was establshed to calculate the mass and heat consumptons of the entre process. The total coal consumpton and gas generaton are both decreased wth the ncrease of PCR and PMR, but the relatve heat loss ncreases at the same tme. The temperature and reducton potental of the reformed gas decreases wth the ncrease of PCR. The recommended values of PCR and PMR are 55% and 80% respectvely, to balance the raw materal consumpton and heat loss. To produce 1 t hot metal, kg alumna-rch metallzed pellet s needed, along wth kg lme, kg coal, and kg oxygen, and the exhaust gas generaton s kg. About 1 t Al 2 O 3 can be recovered from the slag. A zoned model of SRV was establshed to make further optmzaton of ts operaton condton. The SRV coal consumpton decreases wth the ncrease of PCR n the upper slag layer, and does not change wth the ncrease of PCR n the lower slag. The percentage of oxygen consumpton n the top free space and upper slag s determned by PCR n the upper slag, and percentage n the upper slag ncreases wth ncrease of PCR n the upper slag. The percentage of coal consumpton n the upper and lower slag s determned by the PCR n the lower slag, and the percentage n the upper slag ncreases wth the ncrease of PCR n the lower slag. When the PCR s lmted below 15% n the slag, and the heat transfer effcency of the post combuston n the top free space s 80%, the recommended PCRs n the upper and lower slag are 15% and 0% respectvely, whch wll decrease the coal consumpton by 93.4 kg, oxygen consumpton by kg, and gas generaton by kg comparng wth the overall model. The coal-only njecton n the lower slag s recommended. One possble condton s that the temperatures of the upper and lower slag are C and C respectvely, and the coal consumpton ncreases by 12.5 kg, oxygen consumpton by kg, and gas generaton by 34.3 kg comparng to the zoned model results wth the upper slag temperature of C and lower slag temperature of C. Acknowledgements The authors are grateful for the fnancal support by the translaton of the fund was not accurate and the correct translaton s Fundamental Research Funds of the Central Unverstes of Chna (grant no. N ). REFERENCES 1) M. K. Choudhary, D. Bhattacharjee, P. S. Bannerjee and A. K. Lahr: ISIJ Int., 48 (2008), ) P. Pradp: Int. J. Mner. Metal. Mater. Eng., 59 (2006), ) M. Snha and R. V. Ramna: ISIJ Int., 49 (2009), ) K. Sunahara, N. Kaoru, H. Masahko, I. Takanobu, K. Shusaku and Y. Takaku: ISIJ Int., 48 (2008), ) B. Wang, H. Y. Yu, H. L. Sun, S. W. B and G. F. Tu: Chn. J. Nonferrous Metal., 19 (2009), ) Y. Lu, C. X. Ln and Y. G. Wu: J. Hazard. Mater., 146 (2007), ) W. C. Lu, J. K. Yang and B. Xao: J. Hazard. Mater., 161 (2009), ) X. B. L, W. Xao, W. Lu, G. H. Lu, Z. H. Peng, Q. S. Zhou and T. G. Q: Trans. Nonferrous Metal. Soc. Chna, 19 (2009), ) Y. X. Qu, Z. S. Zou and Y. P. Xao: ISIJ Int., 52 (2012), ) I. Barn: Thermochemcal Data of Pure Substances, Wley-VCH Verlag GmbH, Germany, (1995), 1. 11) O. Knacke, O. Kubaschewsk and K. M. Hesselman: Thermochemcal Propertes of Inorganc Substances, Sprnger-Verlag, Berln, (1991), 1. 12) M. W. Chase: NIST-JANAF Thermochemcal Tables, Amercan Insttute of Physcs, New York, (1998), 1. 13) Z. L. Zhang, Q. L and Z. S. Zou: Ironmakng Steelmakng, 41 (2014), ) W. H. Chen and B. J. Ln: Appl. Energy, 101 (2013), ) B. Wang, H. Y. Yu, H. L. Sun and S. W. B: J. Northeastern Unv. (Nat. Sc.), 29 (2008), ) H. Katayama, T. Ohno, M. Yamauch, M. Matsuo, T. Kawamura and T. Ibarak: ISIJ Int., 32 (1992), ) M. Matsuo, C. Sato, H. Katayama, H. Hrata, M. Kanemoto and T. Ibarak: Tetsu-to-Hagané, 76 (1990), ) M. Matsuo, C. Sato, H. Katayama, H. Hrata and Y. Ogawa: Tetsuto-Hagané, 76 (1990), ISIJ 2134