High temperature dephosphorization behavior of monazite concentrate with charred coal

Availableonlineatwww.sciencedirect.comScience directTransactions ofNonferrous metalsSociety of ChinaELSEVIEr PressTrans. Nonferrous Met. Soc. China 20(2010)2392-2396www.nmsccnHigh temperature dephosphorization behavior ofmonazite concentrate with charred coalXING Peng-fei(邢鹏飞), ZHUANG Yan-xin(庄艳猷, TU Gan-feng(涂赣峰, GUO Jing(郭菁)School of Materials and Metallurgy, Northeastern University, Shenyang 110004, chinaReceived 17 December 2009; accepted 27 April 2010Abstract: Dephosphorization behavior of monazite concentrate with charred coal at high temperature was investigated. It is foundthat the roast temperature is the main factor for the dephosphorization of the monazite. The high dephosphorization efficiency can bereached at the temperatures ranging from 1 200 to 1 400C. when the monazite pellets, made by pressing mixture of the monazitecharred coal and water into mould, were roasted at 1 400C for 2 h, 98% of phosphorus was removed from the monazite pellets. Theroast time has little effect on the dephosphorization efficiency. Meanwhile, the particle size of the charred coal also has greatinfluence on the dephosphorization efficiency of the monazite, and it is better to control particle size around 150 um, while Fe andFe2O, have neglectable effect on the dephosphorization of the monaziteKey words: dephosphorization; monazite concentrate; charred coal; high temperatureearth concentrate with high-content phosphorus. WU et1 Introductional[10 investigatedof the mixed rare earthRare earth(RE)ferrosilicon alloys have been containing monazite. ZHANG et al[ll] explored thewidely used as intermediate alloys in iron and steel, dephosphorization techniques of high-content phosphormachinery, automotive and military industry[1-2]. The concentrate in electric arc furmace. TU et al[ 12] reportedrare earth intermediate alloys are mainly fabricated by the dephosphorization kinetics of lanthanum phosphatesilicon thermal reduction method(SRM)and carbon using carbon. However, to our knowledge there is nothermal reduction method(CRM)[]. The CRM can be report about the dephosphorization behavior of theregarded as a short, low-cost and environment-friendly monazite with charred coalprocess, which has great potential industrialDue to its high thermal stability, the rare earthapplication[4]. Nevertheless, up to now, the CRM is only phosphate is very difficult to decompose even at a highsuccessful in preparing RE-alloys from the bastnaesite temperature, and hereby it is infeasible to dephosphorizeore[ 5], and cannot be applied to the Baotou ore due to its through high temperature roasting. In chemical industryhigh phosphorus content (w(P2O5 )of 4%-10%) Baotou it has been well known that the phosphorus can beore is a mixed ore, which consists of 60% monazite and fabricated through reduction of calcium phosphate using40% bastnaesite. The main component of the monazite is carbon followed by evaporation and condensation of therare earth phosphate(REPO4) In the metallurgy process, phosphorus (Cay(PO4)2+5C-3Ca0+P2+5CO)[13-14the reaction between the phosphorus and RE will take similar mechanism can be usedand results in the formation of various phosphides, phosphorus from the monazite. We have reported thecauses pulverization of the RE-alloys[6-7]. dephosphorization behavior of the monazite using cokingTherefore, it is very important to remove the phosphorus coal as a reductant[ 15]. In this work, thefrom the ore and to decrease the amount of the dephosphorization of the monazite using charred coal isphosphorus entering into the alloys[ 8]. ZHAO et al[9] alsoue under diffestudied the carbon-thermal reduction process of rare condit中国煤化工 e of the charredCNMHGCorrespondingauthor:XINGPeng-fei;Tel:+86-24-83681320:E-mail:xingpf@smm,neu.edu.cnDoI:0.l016S|0036326(10)606607XING Peng-fei, et al/Trans. Nonferrous Met. Soc. China 20(2010)2392-23962393coal, roast temperature, roast time, Fe and Fe2O, on the (Rigaku-D/max)dephosphorization behavior were investigated. Theresults provide technical support for realization of 3 Results and discussionpreparing RE alloy from the Boutou ore using the CrMin the future3.1 High temperature dephosphorization of monaziteAfter being roasted at high temperature, the baked2 Experimentalpellets became light with unchanged volume. When theroasting temperature was higher than 1 200C, a largeThe monazite concentrate powder with the particleumber of white smokes generated after pellets weresize of 75 um was from Hunan Taojiang smelter, China,oaded into furnace. Afterwards. a lot of brown solidIt consisted of 62. 20% RE oxide(REO)and 9.74% P20powders, which are red phosphorus as determined byAmong the reO, the main materials were CeOz, La2O3XRD, have been found at the end of the furnacePrOu and Nd O3. The charred coal, used as a reductant,meaning that the phosphorus have been significantlyconsisted of 76.3%carbon, 17.2% volatile and 6. 5% ash. removed from the pellets at 1 200C. Fig. 1 illustrates theThe particle sizes of the charred coal were 75, 109, 150,dephosphorization efficiency of the monazite as afunction of the roasting temperature and time, whereted of 50 carbon and 50% volatile was added as particle size of the charred coal is 150 um. The roastingan adhesive to improve the compressive strength of temperature has great impact on the dephosphorizationpellets. The purities of Fe and Fe2 0, were larger than 99 efficiency The dephosphorization rate reaches 18.2%at% and the particle size was 75 um.1 100C for I h, suggesting that the dephosphorizationIn the process of CRM, the monazite is firstlyprocess starts at 1 100 C even though thereduced to RE2O3 by carbon in the submerged arcdephosphorization rate is relative low. Thefurnace, and the reaction can be expressed asdephosphorization ratio increases with the increase ofroasting temperature. Especially, at the temperature2REPO4+5C=RE,O3+P2+5c0(1) between 1 200oC to 1 400oC, the dephosphorizationThen the re,o further reacts with carbon to form rate increases rapidly with the increase of temperaturEC2 according to the following reaction:meaning that the temperature has stronger effect on thereaction rate. The dephosphorization rate reaches 98%atRE2O3+7C= 2REC+3CO1 400C for 2 h, indicating that most of the phosphorusThe total reaction between REPO4 and c isin the pellets has been removed and further higher2REPO4+12C= 2REC2+P2+8CO(3) roasting temperature will be unnecessary. ThephosphorizationThe theoretical mass ratio of materials used in the increasing the roasting time. The longer roasting time isdephosphorization process of the monazite can be helpful for removing phosphorus in the system. However,calculated from reaction(3). Considering the burning loss with increasing the temperature, the time becomes lessof carbon, the amount of carbon was actually 1. 2 times important. For example, at 1 400oC,further longerthe theoretical amount. The mass ratio of the materials of roasting time than 2 h is not necessary since the most ofmonazite, charred coal, water and residue oil was 100: 43: phosphorus has been removed after 2 h. It can be10: 5(The average molecular mass of RE used in the concluded that the temperature is a main factor for thecalculation is 140). The pellets were fabricated by dephosphorization of the monazite18 MPa. Then the pellets were heated up to 500C forbaking.To remove the phosphorus, the pelletsat1100,1200,1250,1300,1350,andl400C,respectively. The pellets were placed into a graphitecrucible, covered with charred coal powder, and thenloaded into a furnace that was preheated to the desiredtemperatures. After being kept at the temperature for agiven time, the pellets were taken out from theand cooled中国煤化工analysis. The content of phosphorus in the pellets wasG01400determined by quantitative chemical analysis methodCNMHThe structure of the samples was characterized Fig. 1 Relationship between deplby x-ray diffractometer(XRD) using Cu Ka radiation monazite and roasting temperature t and time2394XING Peng-fei, et al/Trans. Nonferrous Met. Soc. China 20(2010)2392-2390Fig 2 shows the XRd patterns of the charred coal theoretical initial reaction temperature is 1 231C for theand pellets of the monazite roasted at various reduction reaction between LaPO4 and C, 1 325C fotemperatures for 2 h. The main contents of the charred the reaction between CePO4 and C, 1 225C for thecoal are carbon and small amount of SiO2. After being reaction between PrPO4 and C, and 1 251C for theroasted at 1 100oC for 2 h, the pellets consists of rEpO reaction between NdPOA and C, respectively. However,and a small amount of La, 03, Ce,03, Pr O, and Nd, 03, the partial pressure of P2 and CO in the reaction systemindicating that only small amount of rEPO is reduced to has effect on the initial reaction temperature. Forthe reo and the reaction rate is relative low. However. example, in the reaction system, the CePO4 reacts with Cat I 300C, the most of REPOa in the pellets was to form CezO, the theoretical initial reaction temperatureincreases with the increase of temperature. At1 40e ate is 1 598K(1 325 C)as given in Table 1. Howeverreduced to REO, indicating that the reactionwhen the pressure ratio of P2 to CO is 5: 1, Gibbs freeonly the peaks of La2 03, Ce203, PT2O, and Nd, 0, have energy for the reaction isbeen found on the XRD pattem, no REPOa has been(PR pCo)found in the limitation of XRD detection, suggesting△G2=△G2+RTnthe REPOa in the pellets has been reduced towhich is in agreement with the conclusions above1822806-1140.687+8314TnTherefore, the charred coal is very effective to removephosphorus from the monazite in the temperature range=1822806-1163.16T1200-1400°CAssuming AG2=0, then the initial reactiontemperature is I 293C for the practical reaction system,meaning that the real initial reaction temperature is lowerthan the theoretice. The real initial reactionCharred coatemperatures at the condition of vCO): V(P2)5: 1 for all1100cthe five reactions are also given in Table 1forcompanison.∪M流迹线In the practical production, under the conditions of1300°Ctemperature larger than 1 000C and larger amount ofcarbon existing, most of carbon will become COMeanwhile, the pressure of P2 is very small in the1400°Creaction system, meaning that the volume ratio of co toP2 will be significantly larger than 5: 1. On the other hand,there are other impurities in the system, which cause2(°)other reduction gas such as H2O and Sio presenting athigh temperature. Considering all the practical condition,Fi -2 XRD pattens of charred coal and pellets of monazite the dephosphorization reaction of the monazite willafter being roasted at various temperatures for 2 hactually start at even lower temperature. This is thereason why the dephosphorization starts even at 1 100Cthe table 1 gives the standard Gibbs free energy and as found in the work. The higher temperature willretical initial reaction temperature of the readbetween REPOa with carbon, which are obtainedaccording to Eq 4. The results above suggest that thethe thermodynamic calculation at the standard state. The charred coal is an effective agent for dephosphorization.Table 1 Standard Gibbs free energy AG, theoretical initial reaction temperature to and real initial reaction temperature t atcondition of M(CO: V(P2)5: 1 for reaction of rare earth phosphate with carbor2LaPO4+5C-=La O+P2+5cO△G=1863930-12397T12012PrPO4+5C=Pr2Oj+P2+5CO△Ge=1856790-12392T中国煤化工11982NdPO4+5C-Nd2O3+P2+5COG4=1884160-1236.7TCNMHG 11962CeP04+5c=Ce0+P2+4CO△G=196640970T1336XING Peng-fei, et al/Trans. Nonferrous Met. Soc. China 20( 2010)2392-239623953.2 Effect of charred coal sizes on dephosphorizationSince the pellets madereductant powder were used in the process of the CRM,the contact area between the monazite and charred coali.e. the particle size of the reaction materials is veryimportant for the reaction rate. Fig 3 shows the一1300C,2hdephosphorization rate as a function of particle size of thecharred coal, where the particle size of the monazite▲-1500C,2hremains 75 um, and the pellets are roasted at 1 400Cfor 2 h. It can be seen that the dephosphorization rateincreases with the decrease of the particles size of thecharred coal. However, when the particle size is smallerthan 150 Hm, the particle size of the charred coal has lessw()%Therefore, the optimized particle sizes of the charred Fig 4 Influence of Fe mass fraction onphosphorization ratecoal should be about 150 um.of monazite under different conditionsFig5 illustrates the effect of Fe203 on the1400°C,2hdephosphorization efficiency of the monazite withcharred coal, where the particle sizes of the monazite andcharred coal are 75 and 150 um, respectively. It can bedephosphorization efficiency under the conditionsinvestigated, which is similar to Fe. However, thephosphorization efficiency of the monazite.Fig 3 Influence of particle size d of charred coal on1300°C.2h3.3 Effect of Fe and Fe:O3 on dephosphorizat二1400:2hIt is well known that the electrodes of thesubmerged are furnace are covered with a layer of ironDuring the reaction, the iron will be continuously meltedinto the reaction system. In the production process, someof iron will be unavoidably introduced into the alloysMeanwhile, there are a small amount of iron oxides inthe Baotou ore. Therefore, it is necessary to know theeffect of iron and iron oxide on the dephosphorizationrate of the monaziteFig 5 Influence of Fe2O, content on dephosphorization rate ofmonaziteFig 4 shows the dephosphorization rate as afunction of iron content, where the particle size of theFig 6 shows the XRD pattern of the monazite pelletsazite and the charred coal are 75 and 150 um,roasted at 1 500C for 2 h, where 5% Fe, O, has beenrespectively. It can be found that the iron has little effect added to the monazite pellets. No diffraction peaks ofon the dephosphorization of the monazite at 1 300, 1400 REPO, have been found on the XRD patten, suggestingand 1 500C. The dephosphorization rate increases from that all of the REPOA have been reduced to REO86%to 98% with the roasting temperature increasing Meanwhile, no Fe,03 has been found in the limitation offrom 1 300C to 1 400C. However, it increases only XRD中国煤化工 Fe has been found1% from 1 400 to 1 500C. This suggests that most ofAtCarbon exists in thephosphorus has been removed at 1 400C, and the systerCNMHtemperature is onemain factors to affect calculation, Fe203 in the systems will be reduced to Fedephosphorization efficiency of the monazite.by CO at temperature higher than 710C. This is the2396XING Peng-fei, et al/Trans. Nonferrous Met. Soc. China 20(2010)2392-2396reason why there is no Fe2O, detected in XRD patternsTherefore, the effect of Fe203 on the dephosphorization Referencesefficiency in the temperature region is the same as Fe. Italso can be found that Fe has no obvious effect on the [1] YU Zong sen. Application of rare earth in iron and steel [M]. 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Kinetics of the dephosphorization of lanthanum3)When the particle size of the monazite is 75 um.phospate with carbon [] Journal of Northeastern University, 1998,the particle size of the charred coal has a significant9(s1): 50-53(in Chinese)nfluence on the dephosphorization efficiency of the[13] Chemical Industry Books Editing Room. Phosphorus and phosphate[M].Beijing: Chemical Industry Press, 1963.(in Chinese)monazite. High dephosphorization efficbe [14] ABDEL A M. Physical and thermal treatment of phosphate ores-reached only when the particle size of the charred coal isOverview [] Inter Joumal of Mineral Proc, 85(4): 59-84.about 150[15] XING Peng-fei, TU Gan-feng. GAO Hao-jun, WU Wen-yuan.On4) Fe and Fe2O, have no obvious effect on thethe high-temperature dephosphorization of monazite concentrate withdephosphorization efficiency of the monazite using thecoking coal p]. Journal of Northeasten University, 2010, 31(4)536-539.(in Chinese)charred coal when the temperature is lower than(Edited by LI Xiang-qun)1500°C.中国煤化工CNMHG