Kinetics of Voluminal Reduction of Chromium Ore Fines Containing Coal by Microwave Heating

Availableonlineatwww.sciencedirect.com° ScienceDirectJOURNAL OF IRON AND STEEL RESEARCH, INTERNATIONAL. 2008, 15(6): 10-15Kinetics of Voluminal Reduction of Chromium Ore FinesContaining Coal by Microwave HeatingCHEN Jin', WANG She-bin, ZHANGLiU Jin-ying, ZHOU Jian-xiong(1. College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China;2. 12th Research Institute, China Electronics Technology Group Corporation, Beijing 100016, Chinat3. Institute of Mineral Resources, Chinese Academy of Geological Science, Beijing 100037, ChinaAbstract: The kinetics of voluminal reduction of chromium ore fines containing coal (COFCC)by microwave heatingwas studied. When the molar ratio of carbon to oxygen was 0. 84 and that of Cao to SiOz was 0. 39 in COFCC, theemperature-rising rate of COFCC by microwave heating was 62.5 C/min, 68 75 C/min, 70. 59 C/min, and72.22℃/ min at1000℃,1100℃,1200℃,and1300℃, respectively. The results show that the voluminal reduction of COFCC by microwave heating at solid-solid phase is first order reaction, with the apparent activation energyof 51. 480 kJ/mol. The limiting step of reaction rate for the overall reaction is the mass transfer of Co in the reducedproduct layer between dielectric particles of chromium ore and coal.Key words: kinetics; microwave heating; voluminal reduction; chromium ore fineThe main kind of chromium-bearing ores in the step is carbon gasification Boudouard )reaction.earth crust is chromite ores. Approximate 60% of According to the unreacted core model, there arehe mined chromium ores is used for production of many factors influencing the reaction rate such asferrochromium in metallurgical industry, 20% for diffusion of CO inside the reactive particles, massrefractory materials, 12% for chemical industry, transfer on the boundary layer, interface chemicaland the other 8% for casting.2. Despite the rela- reaction, and so on. As is well known, the micro-tively stable development of global steel output, the wave heating has the characteristics of voluminalstainless steel products have increased rapidly since heating. For the chromium ore fines containing coaltries is 14% and it is expected that the average annual part in the chemical reaction, but also are taken 980.The annual growth rate in the developed coun- (COFCC), all the ore-coal particles not only act asgrowth rate of global stainless steel is 5%-6%33. internal heat resources to provide enough heat forWith the increase in the exploitation of chromium the reduction reaction. Consequently, the reductionore resources and the gradual decrease in rich ores or process is related to the mass transfer as well as tolump ores, fine chromium ores and chromium ore ore-coal particles and dielectric property of the prod-concentrates produced by mineral dressing increase uct. So this process is defined as voluminal reduc-year after year, which has attracted increased atten- tion. The microwave heating can induce the solid-tion for the preprocess of chromium ore fines 4Jsolid phase voluminal reduction in COFCC, whichBeing a kind of refractory ore, chromic oxides reduces the resistance of the transferring heat andcan be reduced to the metal or the metal carbide in the transferring mass on the interface of the ore-coalsolid states. Generally, there are three steps dur- granules, accelerates the diffusion of CO inside theing the solid-solid phase reduction by carbon: the ore granules, and increases the interfacial chemicalfirst step is the direct reduction of carbon; the sec- reaction rate. Therefore, it is significant to furtherond is gas-solid phase reduction of CO; and the thirdresea中国煤化工 reduction on COF-Foundation Item: Item Sponsored by National Natural Science Foundation of China aCNMHGChinaBiography:CHENJin(1955-),Male,Doctor,Professor:E-mail,chenjinty@sohu.com:RevisedDate,September15,0a0474083)Issue 6Kinetics of Voluminal Reduction of Chromium Ore Fines Containing Coal by Microwave Heating11CC by microwave heating.grams of COFCC is a mixture ofI Experimental Methodore fines from India (shown in Table 1),150 g ofanthracite fines(shown in Table 2), and 50 g of cal1.1 Test materialscic lime fines (shown in Table 3). The atomic molarAll the raw materials in the experiment are de- ratio of carbon to oxygen is 0. 84 and the molecularrived from the metallurgical industry. Thousand molar ratio of CaO to SiO2 is 0. 39.Table 1 Chemical composition in mass percent of chromium ore fines from IndiaCrnO SiOz Al2, TiO Feo Fe?Oo MgO Mno K0 Nago s POs Loss on ignition(L O 1) Total49.279.2012.080.124.3212.920.355.440.360.360.260.0290.0183.8198.537Table 2 Proximate analysis of anthracite powderIndustrial analysisAsh in mass percentcad Total SiOz Al,O, TiOz Fez, Cao MgO P Total21.125.903.760.3469.22100.3449.5018.371.2011.4611.892.400.4995.31Noter Ad is ash in the air dried sample v,d is volatile matter in the air dried sample: Mad is moisture in the air dried sampleSud is total sulfur content in the air dried sample: and Fad is fixed carbon in the air dried sampleTable 3 Chemical composition in mass percent of calcic lime powderSiOzK,0L.O.1 Total3.800.7667.030.540.200.0300.0780.03027.00100.1281.2 Test procThe crucible used in this experiment is made oflight firebricks with an inner diameter of 65 mm anda height of 260 mm (shown in Fig. 1). All the rawmaterials were separately ground to 0. 074 mm andLOooJmixed together according to the proportion. One kilogram of COFCC in the crucible was put into the microwave-metallurgical test furnace at atmosphericpressure without protective gas(shown in Fig. 2).Thesample was heated by an appropriate microwave power(a frequency of 2. 450 GHz) to the given temperatures.1-Magnetron; 2-Platinum-rhodium thermocouple andtemperature measurement device; 3-AC power: 4-Fuchamberi 5-Crucible with powder sample: 6-Baseig 2 Schematic diagram of microwave metallurgical ovenThe sample is taken out of the furnace after it iscooled at atmospheric pressure without protective gas.1.3 Estimate of reduction degreeIn the chemical analysis of chromium, there haslutical mathod to determine the to-tal中国煤化工 ores or ferrochCNMH Naterials, currently,Fig. 1 Schematic diagram of crucible for chromite finesit is impossible to use a conventional chemicalcontaining coal heated by microwavesis to get the accurate relative content of the re12·Journal of Iron and Steel Research, InternationalVol. 15materials because of the coexistence of metallic sub- wave heating, it is carried out as the following threestance and oxide. So a physical analysis method is steps:applied in this experiment. A surface scanning anaFeO. Cr, O.+4C=Fe+2 Cr+4C0ysis of electron microprobe is used in this paper toInitial reaction)determine the reduction degree of the reduced mate-FeO. Cr2 O3+4C0Fe-+2Cr+4c02ls. The expressions for calculating reduction de-(leading reaction)gree is shown as follows4C02+4C=8CO(leading reaction)7-100-/100-WAM X100%=100-wAoIx100% Here, the Boudouard reaction is100-wMCO,=2CO△G°=166550-171TJ·mol(5)where n is the reduction degree of the reduced mate-On the basis of the above reaction processes, itrials: wam is the actually total content of metal in the can be concluded that the voluminal reduction ofreduced sample; WM is the initially total content of CoFCC by microwave heating is conducted accordingetal in the chromium ore; wao is the actual content to the following steps: (1)the initial direct reduc-of oxygen in the reduced sample; and wo is the ini- tion of solid-solid phase between chromium ore finestial content of oxygen in the chromium ore.(FeO. Cr2 O3)and coal powders(C);(2)the difftInput the experialue into eqn. (1), and sion of initial product co into the center of chromi-theum ore grains through the reduced product layer;7=100100—wAM1×100%(2)(3)the indirect reduction of CO in gas-solid phase100-52.37on the interface of chromium ore fines:(4)the out-here no is the reduction degree of COFCC. % and ward diffusion of CO, in chromium ore grains52. 37 is the average metal amount of overall metal through the reduced product layer;(5)the gasifica-oxide in India chromium ore fines,tion of carbon between CO, and coal in COFCC to2 Results and Discussionproduce CO; and (6)inward and outward diffusionof CO until the end of the carbon gasification reac-The thermodynamic conditions of chromium ox- tion, owing to its partial pressure.ides and iron oxides directly reduced by carbon areThe voluminal reduction rate of COFCC can begiven in Fig. 36). For Cr2 O, existing as ferrichrome- expressed by the change rate of the percent oxygenspinel(FeCr: O), its reduction reaction goes as fol- loss of the reactant in the reaction [Egn. (3)]withchange of time. The differential expression of theFeO.Cr2 03+C-Fe+Cr, O3+COvoluminal reduction rate is△G°=163830-138.43TJ·mol-1(3)Production of the metal chromium in the reduc-dC=k·Ceo,49·C=k·C(6)tion reaction is given bywhere C is the oxygen content of sample at the givenFeO. Cr, O3+4C=Fe+2Cr+4c0time of r, mol.m-r is the time, s; dC/dr is the△G°=983772-679.6TJ·mol-1(4) change rate of the oxygen loss of sample with changeIn the voluminal reduction of CoFCC by micro of time, mol.m 3.s":k is the constant of tparent voluminal reduction rate, s"; and n is the reFeo-Cr O3+4C-Fe+ 2Cr+400action order (n=a+6)At different times, the temperature of volumi-nal reduction in Cofcc by microwave heating is:o-Cr, O3+C=Fe+ Cr O +Coshown in Fig 4. Thepercent oxygenloss of samplesFeo+C-Fecois calculated according to egn.(2)(shown in Table 4).The residual oxygen content of the sample at differ-l 11 c1242℃ent temperatures and times is calculated and shown100014001800中国煤化工 linear with,proCNMHGn in solid-solid phaseFlg. 3 State diagram of direct reduction thermodynamies of of COFCC by microwave heating is the first order re-chromium, iron, and manganese oxide by carbonaction [Egn. (4)]. The integral formula of the reduc-Issue 6Kinetics of Voluminal Reduction of Chromium Ore Fines Containing Coal by Microwave Heatingtion rate is given as followHolding stage(7)▲人A△where Co is the initial oxygen content of sample, Coa100014134.56mol/m3Table 6 shows the average of k for the apparenta600voluminal reduction rate at different temperatures1000℃The oxidation at 1 300 C leads to an increase in the·1100℃residual oxygen content (C)and decrease of value of▲1200℃v1300℃k because of the absence of the protective atmos-phere during the experiment ( Fig. 5 and Table 6).Therefore, only the average of k at 1 000 C, 1 100 Cand 1 200 C is used for the regression fit(Fig. 5)Fig 4 Temperature rise curve of COFCC voluminallywith the correlation coefficient being 0. 999. So thereduced by microwave heatingrelational expression is given as follows:Table 4 The reduction degree of COFCC and its temperature-rising ratePercent reduction at different temperatures andInitial temperature of Heating timisothermal holding time/%Temperature-risingsample/100c18.3420.462.9024,2426.0361200℃30.1432427.9728866727.43171300℃33.5435.4837.72Table 5 The residual oxygen content of COFCC at different temperaturesInitial temperature ofOxygen conccntration at different temperatures and isothermal holding time/(mol m-3)sample/KHeating time/s960273K11542.2811242.63108977510708.3410455.331373K10364.8710257.451018l.1210055.332981020473K9874.409552,141573K9393.839119.62lgk=-268651.665(8) tivation energy of carbothermal reduction reactionwith Tibet chromium ore in solid-solid phase by con-The apparent activation energy of voluminal re-duction e isE=(-19.147)×(-2688.65)=51.480kJ/mol273K(9)30Lin Qin et alls) made a study of the apparentI373Kactivation energy on carbothermal reduction of thesynthetic chromite in solid-solid phase by the con-ventional heating method. The reduction reaction is1473Kthe first order reaction and its apparent activationl573Kenergy is 270 kJ/moL. Because the synthetic chro-mite has less refractory impurities and is easy to be中国煤化工035reduced, the rate-limiting step in the whole reduc-CNMHGtion reaction is the reaction of carbon gasificatioHKerauonsnp Between microwaveXU Rong-jun et als) also researched the apparent ac-heating time and Inc14Journal of Iron and Steel Research, Internationalvo.15Table 6 Average of reaction rate constant (k)atonly on chemical reaction conditions(temperature,different temperaturesconcentration,etc. )but also on the physical properTemperature/c 1000 1 100 1 200 1 300 ty of sample (electromagnetic property, etc.Mean value ofEqn.(4), which is the first order reaction, indi0.0001680.0002350.003250.00022cates that CO adsorbed on the active particles on the-3.775 -3.629 -3.488 -3.651 surface of chromium ore grains fails to reach the ad(1/T)×10-4/7.8557.2836.7896.357sorptive saturation and the mass transfer of CO isblocked. When the temperature is above 900 C, thelimiting step of reaction rate for the overall reactionventional heating method. The reduction reaction is is neither the carbon gasification reaction nor the inalso the first order reaction and its apparent activa- terface reduction reaction between dielectric particlestion energy is 513 kJ/mol. It is pointed out that the of ore and coal. It is believed that the mass transferdiffusion of co in the product layer formed by the of CO in the reduced product layer is the rate-limit-efractory impurities in the chromium ore is the rate-tep for the overall reaction, which determineslimiting step in the whole reduction. ZHANG Li- the reaction rate of the voluminal reduction in COF1410) studied the reduction kinetics of chromi- CC by microwave heating.um ore reduced in the liquid-solid phase by carbon inSo the reaction rate of the voluminal reductionthe resistance furnace. The initial stage of the reduc- in COFCC by microwave heating is also expressed as:tion reaction is zero order reaction and its apparentactivation energy is 316 kJ/mol. The melt of chromi- where, R is the voluminal reaction rate of microwaveum ore and mass transfer is considered as the limit- heating; and Poo is the partial pressure of carboning step of the reaction rate.monoxideHowever, the kinetic conditions of voluminalThe comparison of the apparent activation ener-reduction for CoFCC by microwave heating are bet- gy for carbothermal reduction of chromium ore be-ter than those by conventional heating method, be- tween microwave heating and conventional heating iscause of the importance of electromagnetic features given in Table 7.of the sample and the molecular turbulence of micro-In addition, chromium ore fines from India arewave field. It is determined not only by the constant easy to be voluminally reduced by microwave heatingof reaction rate but also by the carbon content in the because of its small granularity, fusibility, high spe-sample. So the voluminal reduction of COFCC in cific resistance, high content of Al2 O,, and low ratiosolid-solid phase is the first order reaction with low- of MgO/Al2, 11er apparent activation energy of 51. 480 kJ/ moL.Generally, the apparent activation energy of carbon 3 Conclusionsgasification is relatively higher (230-355 kJ/mol)India chromium ore fines containing coal is easy towhereas that of the interface reduction reaction is be voluminally reduced by microwave heating. At therelatively lower(48. 5-175 kJ/mol) o. The appar- molar ratios of carbon to oxygen of 0. 84 and Cao toent activation energy of voluminal reduction by mi- SiO, of 0. 39 in COFCC, heating rate of COFCC by microwave heating drops because of the inter-reaction crowave heating is 62. 5 C/min, 68. 75 C/min,of the molecular orbits of reacting matter in micro- 70. 59 C/min, and 72. 22 C/min at 1 000 C, 1 100 Cave field. Its apparent reaction rate depends not 1 200 C, and 1 300 C, respectively. The results showTable 7 Apparent activation energy for microwave heating and conventional heatingLiN Qin et al.[旬XU Rong-jun et al. [9JZHANG Li- juan et aL I oJThis experiigh temperaturermogravimetric analyzer thermogravimetric analyzerType of chromium oreSynthetic chromium oreTibet chromium ore finesNatural chromium ore fines India chromium ore finesActivation energy/(kJ. mol-1) 270中国煤化工5480First orderFirst ordLimiting stepCarbon gasification reactionDiffusion of gas in theCNMHGMass transfer of Co in theIssue 6Kinetics of Voluminal Reduction of Chromium Ore Fines Containing Coal by Microwave Heating 15hat the voluminal reduction of coFcc by microwavePelletizing, 2004, 29(2)t 27(in Chineseheating at solid-solid phase is a first order reaction with [5] Niayesh MJ. Chromite Ore Solid- State Reduction D)J. Ferro-the apparent activation energy of 51. 480 k/mol. The [6] ZHANG You ping, LI Zheng-bang, XUE Zheng-liang. 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