ISIJ International, Vol. 57 (2017), ISIJ International, No. 7 Vol. 57 (2017), No. 7, pp

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1 ISIJ Internatonal, Vol. 57 (207), ISIJ Internatonal, No. 7 Vol. 57 (207), No. 7, pp Meltng Separaton Process of Hgh Chromum Vanadum-bearng Ttanomagnette Metallzed Pellet and ts Optmzaton by Mult- Index Synthetc Weghted Scorng Method Jue TANG, Mansheng CHU,* Cong FENG, Feng LI, Yatng TANG and Zhenggen LIU Insttute of Ferrous Metallurgy, Northeastern Unversty, 3- Wenhua Road, Hepng Dstrct, Shenyang, 089 Chna. (Receved on December 7, 206; accepted on March 2, 207; J-STAGE Advance publshed date: May 26, 207) Based on the gas-based drect reducton followed by meltng separaton process, the meltng separaton process of hgh chromum vanadum-bearng ttanomagnette metallzed pellet and ts optmzaton by mult-ndex synthetc weghted scorng method are studed n the present work. The optmal meltng separaton parameters nclude a meltng temperature of 650 C, a meltng tme of 45 mn, and a bascty of.0. Under these condtons, the recoveres of Fe, V, Cr, and TO 2 reach 99.87%, 98.26%, 95.32%, and 95.04% respectvely; the mass fracton of Fe, V, Cr, and TO 2 are 94.6%, 0.94%, 0.76%, and 38.2% respectvely. The bascty has the strongest effect and ts effect on the meltng separaton knetc s more sgnfcant than thermodynamc. As ncreased bascty from 0.6 to., the slag vscosty decreases and surface tenson ncreases, whch are both attrbuted to smooth meltng separaton and mproved ndexes. But further ncreasng bascty to.2, the amount of CaTO 3 and slag meltng pont ncrease, and the slag amount s relatvely excessve, then all the meltng separaton ndexes decrease nstead. The meltng separaton contans four key behavors: Fe C melt formaton and Fe(l) generaton; slag meltng ntaton and slag(l) generaton; small ron droplets formaton and start of ron-slag separaton; contnuous aggregaton and growth of ron and accomplshment of ron-slag separaton. The ron aggregaton and growth should go through ron crystal nucleus formaton, reacton nterface formaton and enlargement, and subsequent reacton nterface decrease. KEY WORDS: hgh-chromum vanadum-bearng ttanomagnette; meltng separaton; recovery; optmzaton; mult-ndex synthetc weghted scorng method.. Introducton Vanadum-bearng ttanomagnette (VT) s a polymetallc ntergrowth ron ore that owns a wde dstrbuton throughout the world such as Russa, Chna, North Afrca, and Amerca etc. ) In VT, there are varous valuable elements ncludng ron, ttanum, vanadum, and chromum whch are typcal strategc resources due to ts extensve applcaton n metallurgy, chemcal ndustry, and aerospace etc. 2 4) Based on the mass fracton of Cr 2 O 3, VT s classfed nto two categores: ordnary VT and hgh chromum vanadum bearng ttanomagnette (HCVT), and HCVT has a hgher comprehensve utlzaton value. 2) At present, VT s smelt n blast furnace (BF) but the recovery rate of Fe, V, and TO 2 s only reach 70%, 39% and 3% respectvely. 5 7) Recently, the technology of drect reducton followed by meltng separaton applyng to dspose specal metallurgcal resources has attracted ncreasng attenton. 8 ) Jun et al. 8) studed the carbothermal reductonmeltng separaton process for copper slag. In Guang et * Correspondng author: E-mal: chums@smm.neu.edu.cn DOI: al. s 9) work, both the boron-bearng nugget and boron-rch slag were obtaned from ludwgte through carbon bearng pellet reducton-meltng technology. No matter BF process and present coal based drect reducton-meltng separaton process, t should be noted that the reducton by carbon s relatvely slow and consumes large amounts of energy. But the gas-based reducton seems to be a novel and effectve method to process VT due to ts rapd reducton and clean producton. 2 4) The mult-ndex synthetc weghted scorng method s one of scentfc evaluaton methods used to optmze parameters for mult-ndex system. It contans three parts: () the weght of each factor s determned by ts mportance for the whole test; (2) the mult-ndex results s changed to a fnal sngle result that s synthetc scorng value; (3) the synthetc scorng value s analyzed by the normal sngle ndex analyss method. Here, the weght of each factor s a key pont. To ensure a true and effectve result, both the subjectve cognton (experence) on the factor mportance by tester and the objectve nformaton from test results are consdered n ths method. Fnally the optmzaton model of synthetc scorng value s establshed based on the optmzaton theory, and ts exact soluton s gven. 5,6) So the 207 ISIJ 56

2 analyses are reasonable and credble, and t has been wdely appled n lots of felds such as agrculture, pharmacy, and manufacturng ndustry. 8,9) Due to the prevously dscussed low recovery rate, hgh energy consumpton, and slag polluton by sold reductant durng BF or coal-based drect process, a new gas based reducton-meltng process for HCVT has been proposed and researched by authors team. 2,20,2) From the prevous work by author, 2) t has been proved that the ron contanng V and Cr together wth the slag bearng T can be obtaned by the new process. Also, the effect rules of key parameters on hgh chromum vanadum-bearng ttanomagnette metallzed pellet (HCVTMP) meltng separaton are nvestgated by the sngle ndex test. However, for a mult-ndex evaluaton system, the optmal parameters obtaned from sngle ndex test are not enough and mprecse, the scentfc test should be desgned and carred out. Subsequently, the proper and credble data analyss method s also dstnctly necessary. In addton, tll now there s no clear descrpton on the detaled HCVTMP meltng separaton process, such as the behavors on the aggregaton, growth, and separaton of reduced metals. Therefore n ths work, frstly the orthogonal test s conducted and the key HCVTMP meltng separaton parameters are optmzed by mult-ndex synthetc weghted scorng method. And then the detaled process of HCVTMP meltng separaton s dscussed wth the help of n-stu confocal laser-scannng mcroscope (CLSM) observaton. 2. Expermental The HCVTMP wth a metallzaton rate of about 95% obtaned from gas-based furnace drect reducton s appled n the work and the chemcal composton analyzed by ICP- OES s lsted n Table. The amount of HCVTMP sample s 00 g. The analytcally pure actvated carbon, CaO, and CaF 2 wth the partcle sze less than mm are used n the work. The C/O s defned as the mole rato of carbon to reducble oxygen n ron, vanadum, chromum, and ttanum oxdes. Accordng to the early work by author, 2) the amount of carbon n the work s based on C/O of.20 for suffcent deep reducton and carburzaton. The bascty defned as CaO/SO 2 s the mass fracton rato of CaO to SO 2 n raw materal. Through adjustng the mass of CaO, a seres of bascty n desgned tests s obtaned. Takng the bascty of.0 as an example, f the bascty s adjusted to.0, the mass of addton CaO s 3.08 g ((0.34+x)/3.=.0, x=3.08 g). And consderng the slag wth good fludty and the eroson to brusque based on the early work by author, 2) the addton of CaF 2 n the work s 2% (00 g HCVTMP, 2 g CaF 2 ). The meltng separaton of HCVTMP s carred out n a medum frequency nducton furnace (XZ-40B), and the sketch map s shown as Fg.. The temperature s measured by an nfrared thermometer (DT-8869h) wth a measure range of 50~2 200 C and a accuracy of 0. C. The HCVTMP s crushed to below mm. Then the materals ncludng powder samples, actvated carbon, CaO, and CaF 2 are weghted, mxed homogeneously, and loaded n a hgh pure graphte crucble. Next, the crucble s placed n the effectve temperature secton of furnace and heated at Table. Chemcal composton of HCVTMP (wt%). TFe MFe V 2O 5 TO 2 Cr 2O 3 CaO SO 2 MgO Al 2O Fg.. Sketch map of medum frequency nducton furnace. hgh temperatures to realze the ron and slag separaton. Argon atmosphere s keepng throughout the process. After meltng separaton, the furnace s shut down and the sample s rapdly cooled down n argon atmosphere. To ensure the sgnfcance of result, each test s checked for three tmes and the mean s taken as the fnal result. The samples are analyzed by ICP-OES, SEM-EDS, and Factsage 7.0. In all calculatons by Factsage, the system pressure s Pa, and the atmosphere s n Ar. And the equlbrum calculatons are assumptve to explan the reactons and behavors occurrng durng the experment (non-equlbrum). The recovery rates of elements are calculated accordng to the mass balance and defned as: W2 MS RTO2 = 00%... () W MP Where, R TO2 s the recovery rate of TO 2 n slag, %. W and W 2 s the mass fracton of TO 2 n HCVTMP and slag respectvely, %. M P and M s s the mass of HCVTMP and slag respectvely, g. W4 MI Rx = 00%... (2) W3 MP Where, R x s the recovery rates of elements (Fe, V and Cr) n ron, %. W 3 and W 4 s the mass fracton of element n HCVTMP and ron respectvely, %. M I s the mass of ron, g. CSLM s help for n-stu observaton of HCVTMP meltng separaton behavor. A pressed pellet wth the mxed materal (crushed HCVTMP, C, CaO, and CaF 2 ) s put n a platnum crucble and placed to the heatng range just under the laser mcroscope. The nner chamber s sealed and purged wth hgh purty Ar flow of 200 ml/mn durng observaton. The sample s rapdly heated up to 050 C at the heatng rate of 50 C/mn, and then mantaned for 30 s at 050 C. After stablzaton, the sample s heated to 600 C wth the heatng rate of 24 C/mn, and then mantaned for 40 mn at 600 C. After the test, the sample s ISIJ

3 cooled down. 3. Results and Dscusson 3.. Orthogonal Test From the prevous sngle ndex test, 2) the rough ranges of HCVTMP meltng separaton parameters are obtaned: meltng temperature (the set temperature of meltng separaton test ) s around 625 C, meltng tme (the holng tme under the set meltng temperature) s about mn, and bascty (CaO/SO 2 ) s approxmately kept at.0. But for a mult-ndex evaluaton system, the optmal parameters gotten from sngle ndex test are not enough and mprecse, the scentfc test should be desgned and carred out. So based on these results, the scheme of orthogonal test s detaled n Table 2. Three key process factors and three varyng levels are used, ncludng a meltng temperature of C, a meltng tme of mn, and a bascty from.0 to.2. In the test, the recovery rate of Fe, V, and Cr n ron (R Fe, R V, R Cr ) together wth the recovery rate of TO 2 n separaton slag ( R TO2 ) s consdered as the four evaluaton ndexes. The results ncludng ndex data and morphologes of ron and separaton slag are gven n Table 2 and Fg. 2, respectvely. The consdered ndexes show relatvely good values, meanwhle, the surface of ron s smooth whch ndcates that the separaton proceeds successfully. In other words, the optmzaton n the gven range s reasonable and credble. The orthogonal range analyss s lsted n Table 3. The change rule of HCVTMP meltng separaton ndexes under seres factor level s descrbed n Fg. 3. These results quanttatvely ndcate the sgnfcance of each key process factor Fg. 2. Morphology of the separaton ron and slag for each orthogonal test. Table 2. Scheme and result of the orthogonal test. No Factor Results A/ C B/mn C/- R Fe/% R V/% R Cr/% R TO2/% Note: A-Meltng temperature, B-Meltng tme, C-Bascty Fg. 3. Change rule of HCVTMP meltng separaton ndexes under seres factor level. Table 3. Range analyss of orthogonal test. Index R Fe/% R V/% R Cr/% R TO2/% Factor A B C A B C A B C A B C k k k R Sgnfcance R B>R A>R C R C>R A>R B R C>R A>R B R A>R C>R B Optmum A B C A 3 B C 2 A 3 B 3 C 3 A 2 B 3 C 2 Note: A-Meltng temperature, B-Meltng tme, C-Bascty 207 ISIJ 58

4 on the HCVTMP meltng separaton ndexes and the optmal condtons for dfferent factors. The factor affectng R Fe n descendng order of sgnfcance s meltng tme>meltng temperature>bascty; the optmal condton for better R Fe s 600 C, 45 mn, and bascty of.0. The order of each factor affectng R V s bascty>meltng temperature>meltng tme; the optmal condton for hgher R V s 650 C, 45 mn, and bascty of.. The order of each factor affectng R Cr s bascty>meltng temperature>meltng tme; the optmal condton for hgher R Cr s 650 C, 55 mn, and bascty of.. The order of each factor affectng R TO2 s meltng temperature>bascty>meltng tme; the optmal condton for hgher R TO2 s 625 C, 55 mn, and bascty of Optmzaton of HCVTMP Meltng Separaton by Synthetc Weghted Scorng Method The mult-ndex synthetc weghted scorng method s used to determne the optmal HCVTMP meltng separaton parameters. Consderng the mportance of subjectve cognzance and makng the most use of objectve test results, the weght value of ndexes should accomplsh the unfcaton of the subject and object, then the scorng s objectve, true, and effectve Determnaton of Standardzed Evaluaton Matrx In a mult-ndex test, there are n schemes marked as I = {, 2, 3,, n} and m ndexes marked as J = {, 2,, m}. The correspondng expermental data are x j (=, 2, 3,, n; j=, 2, 3,, m) whch make up an evaluaton matrx marked as X = (x j ) n m. To unfy the tendency requrement of each ndex and remove ts ncommensurablty, the evaluaton matrx should be standardzed. Assumng, I = {the ndexes wth the lower, the better }, I 2 = {the ndexes wth the hgher, the better }, I 3 = {the ndexes wth a stable and desred value}, I I 2 I 3 = I. When the judgment standard of the weght scorng value s the lower, the better or the hgh, the better, the dfferent forms of y j are gven as Eqs. (3) and (4), respectvely. Where, x * j ( j I 3 ) s the desred value for ndex; xjmax = max{ xj}. n For unfyng the ndex s magntude order and removng ts dmenson, the z j s commanded as Eq. (5). Where, y jmn = mn{y j =, 2, 3, n}; y jmax = max{y j =, 2, 3, n}. Z = (z j ) s the fnal standardzed evaluaton matrx. y y j j x j = xjmax xj * xj xj x = xj x jmax j x x j * j j I j I2... (3) j I 3 j I j I2... (4) j I 3 zj = 00 ( yj yjmn )/( y jmax yjmn ),... (5) = 23,,..., n; j =, 23,..., m In the present HCVTMP meltng separaton process, the recovery rates of Fe, V, Cr, and TO 2 are the four evaluaton ndexes whch are all expected to be hgher as much as possble. The evaluaton matrx X = (x j ) can be ganed from expermental data lsted n Table 2. Accordng to the judgment standard that the hgher, the better as Eq. (4), the fnal standardzed evaluaton matrx Z(z j ) s obtaned. X = ( x 94 )= Z = ( z 94 )= Determnaton of Synthetc Weght of Each Index The HCVT used n the work s mported from Russa, and t s a remnant after the ron ore benefcaton ttanum. The content of TO 2 s relatvely low. On the premse of ensurng the relatvely hgh recovery of T, the man am for comprehensvely utlzng ths HCVT s to mprove the recoveres of Fe, V and Cr as much as possble. As a result, frstly based on the prevous experence and requrement durng the optmzaton of HCVTMP meltng separaton process, the expert nvestgatng method s used to obtan the subjectve weght for each ndex. 5,6) The subjectve weght of Fe, V, Cr, and TO 2 recovery rate n ths work s α = 0.3, α 2 = 0.3, α 3 = 0.3, and α 4 = 0., respectvely. It means that the matrx of subjectve weght s α = [0.3, 0.3, 0.3, 0.] T. Secondly, from the entropy evaluaton method, 6,22) m j k k= n j = (ln n) j ln j = β = ( h )/ ( h ) ( j =, 2, 3,, m; k j), where, n j j j = h p p, and p = z / z. Notes, p j ln p j = 0 ( =, 2, 3,, n; j =, 2, 3,, m) when p j = 0. Then the objectve weghts are also gven as β = [0.3, 0.27, 0.24, 0.8] T. Fnally, n order to acheve the unty of the subjectve and objectve, the optmzaton decson model s establshed as Eq. (6). Where, μ (0 μ ) s the nclnaton coeffcent that reflects the preference degree for the subjectve and objectve weghts. Here, μ = 0.5. The fnal synthetc weght for each ndex s obtaned as w = [0.30, 0.28, 0.27, 0.4] T ISIJ

5 mn F( w) = µ ( w α ) z ( µ ) ( w β ) z m w j = st j= wj 0, j =, 2,..., m n m { [ j j j ] 2 + [ j j j ] 2 } = j=... (6) Calculaton of Synthetc Weghted Score From the formula descrbed as Eq. (7), the synthetc weghted score for each ndex s calculated as: f = [94.53, 94.40, 95.09, 96.9, 95.4, 93.79, 95.8, 94.50, 96.27] T n f = w z ( = 2,,..., n, j = 2,,..., m)... (7) j j = Sngle Index Analyss Evaluaton Subsequently, the optmzaton of HCVTMP meltng separaton s nvestgated by the sngle ndex analyss evaluaton and the results are lsted n Table 4. Through the whole mult-ndex synthetc weghted scorng method, the optmal HCVTMP meltng separaton parameters are obtaned as follows: a meltng temperature of 650 C, a meltng tme of 45 mn, and a bascty of.. The bascty most strongly affects the HCVTMP meltng separaton, followed successvely by the meltng temperature and meltng tme. In addton, the effect of nclnaton coeffcent (μ) on the optmzaton s also dscussed n Table 5. All the sgnfcance order and optmum are the same under any nclnaton coeffcent. The bascty stll has the most sgnfcant effect on HCVTMP meltng separaton. The HCVTMP meltng separaton s appled under the optmal condtons. The recovery amount and compostons Table 4. Result and concluson of the optmzaton for HCVTMP meltng separaton. of slag and metal are lsted n Table 6. The mass fracton of Fe, V, Cr, and TO 2 reach 94.6%, 0.94%, 0.76%, and 38.2%, respectvely. And the recovery of Fe, V, Cr, and TO 2 s hgh to 99.87%, 98.26%, 95.32%, and 95.04%, respectvely. Compared wth the sngle ndex test results, 2) all the recoveres and mass fracton are mproved obvously. The cross secton of separaton slag and ron (rapdly cooled down n argon atmosphere) under the optmum are analyzed by SEM-EDS, shown n Fg. 4. In (a) and (c), the Table 5. Optmal parameters under dfferent nclnaton coeffcent μ. μ Synthetc weght w w 2 w 3 w 4 Sgnfcance Optmum C>A>B A 3B C C>A>B A 3B C C>A>B A 3B C C>A>B A 3B C C>A>B A 3B C C>A>B A 3B C C>A>B A 3B C 2 Note: A-Meltng temperature, B-Meltng tme, C-Bascty Table 6. Recovery amount and compostons of slag and metal under the optmal condtons. M I/g 8.26 M S/g C I/% TFe V Cr T S S C S/% TFe V Cr TO 2 CaO SO 2 MgO Al 2O Note: M P, M I, M S-Mass of HCVTMP, Iron, Slag; C I, C S- Composton of ron, slag No Index Factor A * / C B * /mn C * /- R Fe/% R V/% R Cr/% R TO2/% f K w = 0.30, w 2= 0.28, w 3= 0.27, w 4= 0.4 K Sgnfcance C >A>B K Level C 2 A 3 B R Optmum A 3 B C 2 Note: A-Meltng temperature, B-Meltng tme, C-Bascty Fg. 4. SEM-EDS analyses of the separaton slag and ron under optmal condtons. 207 ISIJ 60

6 two phases that the lght gray (Pont A) has more calcum whle the dark gray (Pont B) contans more ttanum. Seen from (b) and (d), there are also two phases n ron, the brght whte of pure metallc ron (Pont C) and the gray whte of metallc ron contanng V and Cr (Pont D) Functon Mechansm of Bascty on the HCVTMP Meltng Separaton From the optmzaton by mult-ndex synthetc weghted scorng method, t s evdent that bascty s really the most sgnfcant factor that affects HCVTMP meltng separaton, t s qute necessary to further confrm ts functon mechansm. Therefore the detaled sngle ndex tests are supplemented and shown n Fg. 5. When holdng the HCVTMP orgnal bascty of 0.3, the meltng can t be acheved at all. Slghtly ncreasng bascty to 0.60, the relatvely bg ron s gathered at the bottom of crucble, but the separaton s stll not done successfully. So the data under the two bascty are naccurate and not gven. Further ncreasng bascty contnuously, all the ndexes show the tendency of frst ncrease and then decrease, and have the hghest value at the bascty of.0. Effect of bascty on the thermodynamc equlbrum of HCVTMP meltng s evaluated by the Equlb module n FACTSAGE 7.0 package. The databases ncludng FactPS, FToxd, FTstel, and Fe 2 VO 4 23) (establshed by author) are appled. The system pressure s Pa, and the ntal atmosphere s n Ar. The possble products are consdered: lqud metal, metal carbde, metal oxde, lqud carbon, sold carbon, spnel, anosovte, monoxde, clnopyroxene, wollastonte, anorthte, corderte, mellte, olvne, mullte, corundum, pseudobrookte, ulvaespnel, rutle, and perovskte etc. 00 g HCVTMP samples wth C, CaO, and CaF 2 are analyzed wth the temperature range of C and the bascty range of The possble lqud products ncludng Fe (l), V (l), Cr (l), and T (l) are emphaszed n Fg. 6. As ncreased bascty, the amount of T (l) has a slght decrease whch s good for ttanum oxde enrchment n slag. Beyond that, the major lquds of Fe (l), V (Cr), and V (l) are almost the same despte of the obvous ncrease of bascty. However, the result from experment (Fg. 5) shows that all the ndexes are senstve to bascty. It s speculated that the nfluences of bascty on meltng separaton knetc are greater than thermodynamc. Vscosty of the slag formed n HCVTMP meltng separaton wth dfferent bascty s calculated by Vscosty module n FACTSAGE 7.0 package. The Melts database s appled. The evaluated temperature range s C. The vscosty calculaton results are descrbed n Fg. 7. It s obvous that the vscosty decreases gradually as ncreased bascty. Generally, the slag wth a low vscosty s of great beneft to good dffuson and mass transfer due to ts good fludty. So by rsng bascty, the meltng separaton knetc s enhanced due to the decreasng slag vscosty, the successful separaton of ron and slag s promoted. Besdes vscosty, the surface tenson of slag s another key pont n meltng separaton process for controllng varous surface and nterfacal phenomena. The bgger surface tenson of molten slag wll lead to the decreasng adheson work between ron and slag whch makes the slag dffcult to Fg. 6. Thermodynamc equlbrum analyss by FACTSAGE 7.0 package under dfferent bascty. Fg. 5. Meltng separaton ndexes n ron (a) and separaton slag (b) under dfferent bascty. Fg. 7. Vscosty of the slag wth dfferent bascty ISIJ

7 be brought to ron, and then the separaton of ron and slag s mproved. 24) Therefore, the effect of bascty on HCVTMP meltng separaton s consdered from the aspect of slag surface tenson. Accordng to the predcton model derved by Tanaka et al. 25) and the research on the surface tenson of molten blast furnace slag bearng TO 2, 26) the surface tenson of HCVTMP meltng separaton slag (CaO SO 2 MgO Al 2 O 3 TO 2 CaF 2 ) s evaluated based on the Bulter s equaton. The surface tenson s calculated by Eqs. (8) (0). σ = σ Pure RT + A ln M M M P Surf Bulk... (8) 3 A = N / 23 V /... (9) = 0 R R R R Caton Anon Caton Anon N P N P... (0) Where, σ s the surface tenson of molten slag, mn/m; refers to the followng components: CaO, SO 2, MgO, Pure Al 2 O 3, TO 2, CaF 2 ; σ s the surface tenson of pure molten component, mn/m; R s the gas constant, -; T s the absolute temperature, K; A s the molar surface area n a mono layer of pure molten component, m 2 ; N 0 s Avogadro number, -; V s the molar volume of pure molten component, m 3 /mol; R Caton and R Anon s the radum of caton and anon respectvely, m; N P s the mole fracton of component n phase P, -; P s Surf (surface) or Bulk (bulk). In 4 addton, SO 4 s thought to be the mnmum anonc unt n SO 2 and the rato of R S 4+ to R 4 SO4 s ,28) The data ncludng the onc rad together wth the temperature dependences of molar volume and surface tenson for pure component used n the present evaluaton are obtaned from prevous works 26 3) and lsted n Tables 6 and 7. The calculated surface tenson wth dfferent bascty at 650 C s gven n Table 8. Meanwhle, the detecton test s carred out by usng RTW-0 melt physcal property comprehensve testng nstrument (degned by Northeastern Unversty, CHINA), and the data s also added nto Fg. 8. The calculated values based on the present model are n good agreement wth the expermental. As bascty ncreased, the surface tenson goes up resultng n decreasng adheson work between ron and slag, then the separaton of ron and slag s enhanced and all the ndexes are mproved. The metal-separaton temperature usually depends on the slag meltng temperature. The effects of bascty on slag ntal meltng temperature and slag meltng temperature are evaluated by Equlb module n FACTSAGE 7.0 package and presented n Fg. 9. The evaluated bascty range s When calculatng the slag ntal meltng temperature, the FToxd-Slag A s set as F (Formaton target phase). And the FToxd-Slag A s set as P (Precptate target phase) f calculatng slag meltng temperature. The other calculaton condtons are the same to the condtons of Fg. 6. When the bascty rsng from the orgnal (0.3) to 0.60, t s obvous that the ntal meltng temperature has a slght decrease but the slag meltng temperature dramatcally decreases from 536 C to 353 C accordngly. That s a pvotal reason why the HCVTMP meltng separaton can t be acheved successfully wth the bascty below 0.6. Further ncreasng bascty, the slag ntal meltng tem- Table 7. Rad of caton and anonc. Rad of ons/a Oxde Caton Anon CaO Ca O 2.44 SO 2 S SO MgO Mg O 2.44 Al 2O 3 Al O 2.44 TO 2 T O 2.44 CaF 2 Ca F.33 Fg. 8. Effect of bascty on the 6-component slag surface tenson. Table 8. Temperature dependences of surface tenson and molar volume of pure oxde. Oxde Surface tenson/(mn/m) Molar volume/(m 3 /mol) CaO T 20.7 [+0 4 (T 773)] 0 6 SO T [+0 4 (T 773)] 0 6 MgO T 6. [+0 4 (T 773)] 0 6 Al 2O T 28.3 [+0 4 (T 773)] 0 6 TO T 22.2 [ (T 023)] 0 6 CaF T 3.3 [+0 4 (T 773)] 0 6 Fg. 9. Effect of bascty on ntal meltng separaton temperature and meltng temperature of the slag. 207 ISIJ 62

8 Fg.. Amount of slag formed durng HCVTMP meltng separaton under dfferent bascty. Fg. 0. Temperature dependence of equlbrum phase fracton n the slag wth dfferent bascty. perature has no obvous changes. Instead, the slag meltng temperature shows an ncreasng tendency whch wll brng some dsadvantages to the separaton of ron and slag. To better understand the effect of bascty on HCVTMP meltng separaton behavor, the phase fracton changes of the slag wth dfferent bascty as a functon of temperature are calculated by Equlb module n FACTSAGE 7.0 package and shown n Fg g HCVTMP samples wth C, CaO, and CaF 2 are analyzed wth the temperature range of C and the bascty range of The other calculaton condtons are the same to the condtons of Fg. 6. When the bascty s 0.3 (Fg. 0(a)), TO 2 s observed to be n the forms of less anosovte MgT 2 O 5 () and more TO 2 (2); CaO s prmtvely combned wth SO 2 and Al 2 O 3 to generate anorthte CaAl 2 S 2 O 8 (3); Mg s manly dssolved n corderte Mg 2 Al 4 S 5 O 8 (4) and sapphrne Mg 4 Al 0 S 2 O 23 (5); and a large amount of lqud phase slag (7) suddenly forms at around 400 C. Increasng bascty to 0.60 (Fg. 0(b)), part of CaO s stll n CaAl 2 S 2 O 8, but the excessve CaO s combned wth TO 2 and the perovskte CaTO 3 (8) wth a hgh pont of 960 C appears; Mg s manly combned wth about half of TO 2 to form MgT 2 O 5 and the new phase of MgT 2 O 4 s also observed; meanwhle, the temperature that much lqud phase wll generate decreases substantally. Further rsng the bascty (Fgs. 0(c) 0(f)), the temperature generatng much lqud slag has no obvous changes, but the amount of CaTO 3 ncreases evdently resultng n the ncreasng of slag meltng temperature that has been shown n Fg. 9. Drectly, the smooth HCVTMP meltng separaton s hndered by excessve bascty and all the ndexes decrease more or less. For mprovng the HCVTMP meltng separaton ndexes, the controllng of slag amount s another key step. The proper more lqud slag amount can supply a relatvely good envronment for ron-slag meltng separaton. 9) But the excessve amount slag can prevent the agglomeraton of ron. Then the meltng separaton can t proceed perfectly. The un-agglomerate ron partcle wth lttle sze may be wrapped by slag, whch wll decrease the recovery of Fe, V, and Cr n ron. 32) As seen from the slag amount under dfferent bascty n Fg. (calculated by Equlb module n FACTSAGE 7.0 and the calculaton condtons are the same to the condtons of Fg. 6), t ncreases wth rsng bascty. And from the expermental result n Fg. 5, all the ndexes decrease when the bascty s further more than.0. Therefore, from the aspect of slag amount, HCVTMP meltng separaton s enhanced by rsng bascty should be appled n a proper range. Accordng to the above analyses, ncreasng bascty wthn a certan bascty range s attrbuted to decreasng vscosty, ncreasng surface tenson, decreasng slag meltng pont, and mprovng meltng separaton knetc. But the excessve bascty wll result n rsng slag meltng temperature nstead and generatng a large amount of slag, whch are bad for mprovng meltng separaton ndexes. As a result, the proper bascty for HCVTMP meltng separaton should be mantaned at a compromsed value, and for the expermental condtons, the compromsed value s around Process of HCVTMP Meltng Separaton The observed HCVTMP meltng separaton behavors wth the bascty of.0 are seen n Fg. 2. The camera lens s focused on the slde. They show (a) Fe C melt formaton and Fe(l) generaton for carburzaton, (b) slag meltng ntaton and slag(l) generaton, (c) start of ron-slag separaton (startng of aggregaton of small ron droplets or slag droplets), (d) contnuous aggregaton and growth of ron or slag and then accomplshment of ron-slag separaton (relatvely bg sze ron droplets formaton). In order to observe the detaled behavors of ron aggregaton and growth, the camera lens s focused on the sample, and the observaton results are gven as Fg. 3. Frstly, n Fg. 3(a), t s spontaneous crystal nucleus formaton of ron droplet at a certan pont of low potental barrer, the carburzaton s relatvely slow n ths stage, wth the ncreasng temperature, the carburzaton s accelerated and the amount of slag also ncreases, then the perod for ron crystal nucleus formaton s shortened and the spontaneous nucleaton rate s enhanced. Secondly, n Fgs. 3(b) and 3(c), t s reacton nterface formaton and enlarge ISIJ

9 Fg. 2. In-stu CLSM observaton mages for HCVTMP meltng separaton behavors. (a)- 43 C; (b)- 358 C; (c)- 492 C; (d)- 547 C. Fg. 3. In-stu CLSM observaton mages for metal aggregaton and growth. (a)- 58 C; (b)- 537 C; (c)- 543 C; (d)- 546 C; (e)- 548 C; (f)- 549 C. Fg. 4. Illustraton of several notceable ponts durng HCVTMP meltng separaton. ment based on the exsted ron crystal nucleus, and the carburzaton s accelerated contnuously. Thrdly, n Fgs. 3(d) 3(f), t s reacton nterface decrease, the reacton nterfaces that formaton from each crystal nucleus are assembled together, the reacton nterface wll decrease after the maxmum generaton, then ron droplets are combned and grown, the partcle sze of ron s becomng larger and the nterfacal tenson between ron and slag s bgger, and then the separaton of ron and slag wll be acheved. Durng the HCVTMP meltng separaton, the excellent slag propertes ncludng less vscosty, good fludty, and bgger surface tenson are conductve to ron crystal nucleus formaton, reacton nterface formaton and enlargement, and followng reacton nterface decrease and ron aggregaton. Addtonally, the rato of σ metal-slag (nterfacal tenson between metal and slag) to η slag (slag vscosty) also has sgnfcant effects on the separaton of ron and slag. If σ metal-slag /η slag s large, 33) the separaton s accomplshed smoothly. At last, the several notceable ponts durng HCVTMP meltng separaton are llustrated n Fg. 4. The carburzaton phenomena s gven n Fg. 4(), there are about three possbltes for ron carburzaton durng HCVTMP meltng separaton, ncludng drect carburzaton, gas carburzaton, and molten slag carburzaton (durng meltng). As the carburzaton reacton, the meltng pont of reduced ron decreases, and the dffuson together wth agglomeraton s accelerated, therefore, the carburzaton s the prerequste for HCVTMP meltng separaton. In addton, the molten slag also plays an mportant role n HCVTMP meltng separaton. Km et al. have ponted that the molten slag has a good wettng wth ron but bad wettng wth carbon. Ths good wettng and slag spreadng wll ntroduce an attractve force between ron and slag so as to make them come nto contact wth each other. 34) Wth the developng of carburzaton, the lqud ron wll form between sold ron and carbon and ts amount ncreases wth tme. These processes when molten slag carburzaton durng meltng are clearly llustrated n Fg. 4(2): (a) sold ron and carbon wll come to close after molten slag formaton, (b) and they contact wth each other by the attractve force from good wettng between ron and molten slag, (c) then the lqud ron generates for carburzaton, (d) fnally the contactng area between lqud ron and carbon s enlarged and the amount of lqud ron ncreases. Combnng wth the CLSM observaton, the ron aggregaton and growth process s smply descrbed as Fg. 4(3), t contans (a) ron crystal nucleus formaton, (b) reacton nterface formaton and enlargement, (c) and (d) reacton nterface decrease together wth ron droplets aggregaton and growth. After ron droplets of bg sze formaton, the ron droplets sedmentate to the bottom but the slag rse to the surface for dfferent densty and surface tenson, and then the separaton of ron and slag s acheved naturally. It s worth notng that the reducton of the oxdes bearng ron, vanadum, chromum, and ttanum n HCVTMP s run through the whole deep reducton meltng separaton process, but the restrctve step of reducton s dfferent durng each stage. 4. Conclusons () The optmal HCVTMP meltng separaton parameters are obtaned by mult-ndex synthetc weghted scorng method and ncluded a meltng temperature of 650 C, a meltng tme of 45 mn, and a bascty of.0. Under these condtons, the recoveres of Fe, V, Cr, and TO 2 reach 99.87%, 98.26%, 95.32%, and 95.04% respectvely; the mass fracton of Fe, V, Cr, and TO 2 are 94.6%, 0.94%, 0.76%, and 38.2% respectvely. The bascty most strongly affects the HCVTMP meltng separaton, followed successvely by the meltng temperature and meltng tme. And the optmzaton s credble and vald. (2) The effect of bascty on the HCVTMP meltng 207 ISIJ 64

10 separaton knetc s more consderable than thermodynamc. As ncreased bascty from 0.6 to., slag vscosty decreases, surface tenson ncreases, and meltng temperature decreases, whch are all attrbuted to smooth meltng separaton and mproved ndexes. But further ncreasng bascty to.2, the amount of CaTO 3 ncreases and slag meltng pont rses nstead, meanwhle, the amount of slag s relatvely excessve, then all the HCVTMP meltng separaton ndexes show a decrease tendency. (3) HCVTMP meltng separaton process contans four key behavors: Fe C melt formaton and Fe(l) generaton, slag meltng ntaton and slag(l) generaton, start of ronslag separaton, and contnuous aggregaton and growth of ron or slag and then accomplshment of ron-slag separaton. As for ron aggregaton and growth, t should go through ron crystal nucleus formaton, reacton nterface formaton and enlargement, and followng reacton nterface decrease. Acknowledgments The authors are especally grateful to Natonal Natural Scence Foundaton of Chna (Grant No ). REFERENCES ) L. J. Xao: Met. Mne, (200), No. 295, 28. 2) J. Tang, M. S. Chu and X. X. Xue: Int. J. Mner. Metall. Mater., 22 (205), 37. 3) G. C. Du: Iron Steel Vanadum Ttan., 36 (205), 49. 4) H. X. Fang, H. Y. L and B. Xe: ISIJ Int., 52 (202), ) T. Hu, X. W. Lv, C. C. Ba, Z. G. Lun and G. B. Qu: ISIJ Int., 53 (203), ) X. W. Lv, Z. G. Lun, J. Q. Yn and C. G. Ba: ISIJ Int., 53 (203), 5. 7) S. Y. Chen and M. S. Chu: Int. J. Mner. Metall. Mater., 2 (204), ) J. Zhang, Y. H. Q, D. L. Yan, X. L. Cheng and P. He: J. Iron Steel Res. Int., 22 (205), 2. 9) G. Wang, Q. G. Xue, X. F. She and J. S. Wang: ISIJ Int., 55 (205), 75. 0) C. R. Borra, S. Dwarapud, G. Kapure, V. Tathavadkar and M. B. Denys: Ironmakng Steelmakng, 40 (203), 443. ) Y. P. Zhang, Z. L. Xue, Z. B. L, J. W. Zhang, H. S. Yang and Y. S. Zhou: Iron Steel, 40 (2005), 7. 2) E. Park, S. B. Lee, O. Ostrovsk, D. J. Mn and C. H. Rhee: ISIJ Int., 44 (2004), 24. 3) E. Park and O. Ostrovsk: ISIJ Int., 44 (2004), ) Z. C. Wang, S. Y. Chen, M. S. Chu, Z. W. Han and X. X. Xue: Iron Steel Vanadum Ttan., 33 (2006), 34. 5) J. M. Wu: Trans. Chn. Soc. Agrc. Mach., 24 (993), 66. 6) J. C. Tao and J. M. Wu: Syst. Eng. Theory Pract., 2 (200), No. 8, 43. 7) Y. Zhang, J. Tang, M. S. Chu, Y. Lu, S. Y. Chen and X. X. Xue: J. Iron Steel Res. Int., 2 (204), 44. 8) X. Wang, L. Q. Jao, Y. Mn and C. M. Yn: Chn. Tradt. Herb. Drugs, 46 (205), ) S. B. Yao and C. Y. Yue: Syst. Eng. Electron., 27 (2005), ) J. Tang, M. S. Chu, Z. G. Lu, S. Y. Chen and C. Feng: Proc. 8th Metallurgcal Reacton Engneerng, The Chnese Socety for Metals, Chongqng, (204), 04. 2) J. Tang, M. S. Chu, C. Feng, Y. T. Tang and Z. G. Lu: ISIJ Int., 56 (206), ) F. X. Zhu and Y. H. Chen: J. Anhu Unv., 30 (2006), 4. 23) Y. Wang, T. Guan, H. L. Cao, S. Q. Zhang, Z. M. Zhang and B. Xe: J. Iron Steel Res., 23 (20), ) H. Y. Chen, Y. Wang, D. K. L and M. Hu: Contn. Cast., 4 (2008), 42, (n Chnese). 25) T. Tanaka, K. Tomoko and A. B. Ida: ISIJ Int., 46 (2006), ) Y. H. Lu, X. W. Lv, C. G. Ba and B. Yu: ISIJ Int., 54 (204), ) M. Hanao, T. Tanaka, M. Kawamoto and K. Takatan: ISIJ Int., 47 (2007), ) M. Nakamoto, A. Kyose, T. Tanaka, L. Holappa and M. Hämälänen: ISIJ Int., 47 (2007), ) R. Bon and G. Derge: J. Met., 8 (956), ) R. Shannon: Acta Crystallogr. A, 32 (976), 75. 3) I. Sohn and D. J. Mn: Steel Res. Int., 83 (202), 6. 32) K. Ohno, M. Kamoto, T. Maeda, K. Nshoka and M. Shmzu: ISIJ Int., 5 (20), ) B. Ja and B. M. Q: J. Iron Steel Res., 3 (996), 4. 34) H. Km, J. G. Km and Y. Sasak: ISIJ Int., 50 (200), ISIJ