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Rare Earth Extraction from Bastnaesite Concentrate byStepwise Carbochlorination-Chemical Vapor Transport-Oxidation

LI-QING ZHANG, ZHI-CHANG WANG, SHU-XUN TONG, PENG-XIANG LEI, and WEI ZOU

A stepwise carbochlorination-chemical vapor transport-oxidation process is developed for the green rareearth extraction from a bastnaesite concentrate using carbon as reductant, chlorine gas as chlorinationagent, SiCl4 gas as defluorination agent, AlCl3 as vapor complex former, and (O2H2O) mixed gas as

oxidant. Between 500 °C and 800 °C, the apparent activation energy of the carbochlorination within2 hours changed from 17 to 10 kJ/mole roughly for the initial 20 minutes and final 1.5 hours, respectively,in the absence of SiCl4, but these values reduced to 15 and 5.9 kJ/mole under 10 kPa of SiCl4 gas, whilethe rare earth chloride conversion for 2 hours was 43 to 81 mol pct in the absence of SiCl4 and 55 to99 mol pct under 10 kPa of SiCl4 gas. After carbochlorination at 550 °C for 2 hours in the (Cl2 SiCl4)atmosphere for efficient rare earth extraction and thorium-free volatile by-product release, thorium wasremoved by chemical vapor transport at 800 °C for 0.5 hours in the (Cl2 SiCl4 AlCl3) atmosphereand alkaline earths were separated from rare earths by oxidation at 700 °C to 1000 °C in the (O2H2O)atmosphere for 0.5 hours, followed by water leaching at room temperature. Their combination allows aclean and efficient rare earth extraction from the concentrate.

I. INTRODUCTION

RARE earth minerals always contain both rare earthsand non–rare earths including more or less radioactive ele-ments, and thus, the green metallurgy of rare earths is dif-ficult as compared with other metals.[1]

Bastnaesite is mainly a fluorocarbonate of light rare earthsand contains only 0.1 to 0.3 wt pct of thorium; thus, it is animportant rare earth source based on radioactivity safety con-siderations. [1] At present, bastnaesite concentrate is com-monly treated by calcination-acid dissolution or directly byacid dissolution or alkaline roasting for rare earth extraction, [1]

but these processes require complicated operations and resultin a large amount of low-concentration thorium-containingslag and wastewater, which cannot meet the requirement of

thorium effluent standard. The Goldschmidt process at1000 °C to 1200 °C for carbochlorination of bastnaesite con-centrate, using carbon as reductant and chlorine gas as chlori-nation agent,[2] has many advantages over the wet processes,but it is no longer in industrial use due to the high fluorinecontent in the rare earth product[2] and the thorium radioactivecontamination both for rare earth chloride product and fornon–rare earth volatile by-product.[3]

We[4] have recently developed a stepwise carbochlorination –chemical vapor transport (SC-CVT) process for the rare earthextraction and separation from a mixed bastnaesite-monaziteconcentrate, which is obtained from the world’s largest rareearth deposit located at Baiyunebo in China, based on thermo-dynamic and kinetic analysis, using carbon as reductant, chlorine

gas as chlorination agent, SiCl4 gas as defluorination agent, andAlCl3 as vapor complex former. The bastnaesite and monazite

minerals in the ore are about 0.01 to 0.04 mm in sizes.[5] It is

shown that the efficient rare earth chloride conversion andthorium-free volatile by-product release could be realized bycarbochlorination of the mixed concentrate at temperatures aslow as 500 °C under 2 kPa of SiCl4 gas, which is obviouslydifferent from the Goldschmidt process at 1000 °C to 1200 °C,and that the complete thorium removal from the rare earth chlo-ride product could be performed by the subsequent CVT reactionat 800 °C in the presence of AlCl3.

The bastnaesite concentrate investigated in this study isobtained from the Panxi rare earth deposit, which is the second-largest rare earth deposit in China. As compared with themixed bastnaesite-monazite concentrate investigated in Refer-ence 4, it contains a certain amount of barite (Table I), whichshould also be recovered. The bastnaesite mineral in the ore

is between 0.1 and 10 mm in size.[5] In this study, we triedto develop a SC-CVT oxidation process for the green rareearth extraction from the bastnaesite concentrate.

II. THERMODYNAMIC ANALYSIS

Thermodynamic analysis in Reference 4 has shown thatcarbochlorination is feasible for LnOF (where Ln rareearth elements) and non–rare earth oxides contained in themixed bastnaesite-monazite concentrate between 400 °C and1100 °C with the negative standard free energy change orderof CaO LnOF Fe2O3 ThO2 SiO2, that the reac-tion at low temperature in the presence of SiCl4 favors the

LnOF-to-LnCl3 conversion, and that ThCl4 has a negligibleequilibrium vapor pressure at the temperatures below 600 °C.On the other hand, as compared with the mixed concentrate,[4]

the bastnaesite concentrate has additional carbochlorinationreactions for the rich compound barite:

BaSO4 (s) C (s) Cl2 (g) BaCl2 (s or 1) SO2 (g) CO2 (g) [1]

BaSO4 (s) 2C (s) Cl2 (g) BaCl2 (s or 1) SO2 (g) 2CO (g) [2]

METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 35B, APRIL 2004—217

LI-QING ZHANG, formerly Graduate Student, Department of Chem-istry, Northeastern University, is Associate Professor, Shenyang Institute of Chemical Technology, Shenyang 110021, P.R. China. ZHI-CHANG WANG,Professor, SHU-XUN TONG, Associate Professor, PENG-XIANG LEI,Graduate Student, and WEI ZOU, Assistant Professor, are with the Depart-ment of Chemistry, Northeastern University, Shenyang 110004, P.R. China.Contact e-mail: [email protected] or [email protected].

Manuscript submitted September 16, 2002.



218—VOLUME 35B, APRIL 2004 METALLURGICAL AND MATERIALS TRANSACTIONS B

Table I. Composition of the Bastnaesite Concentrate (Weight Percent)

La2O3 CeO2 Pr6O11 Nd2O3 BaO CaO Fe2O3 ThO2 Al2O3 SiO2 CO2 SO3 F

22.8 28.3 2.16 6.43 9.1 0.35 0.27 0.15 0.20 1.34 16.1 4.8 7.9

Fig. 1—Standard free energy changes of carbochlorination of barite, con-tained in the bastnaesite concentrate, to form BaCl2 and SO2 together with

either CO2 (Reaction [1]) or CO (Reaction [2]).

[6]

Figure 1 shows the accompanying standard free energy changesbetween 400 °C and 1100 °C from Barin and Knacke.[6] It canbe seen that the thermodynamic carbochlorination ability of barite is close to LnOF between 450 °C and 550 °C but evenhigher than CaO above 800 °C. Therefore, carbochlorinationat the temperatures below 600 °C in the presence of SiCl4 maythermodynamically increase the rare earth chloride conversionand benefit the thorium-free volatile non–rare earth releasefrom the bastnaesite concentrate.

It has also been shown in Reference 4 that CVT at tempera-tures below 900 °C in the presence of AlCl3 may enhance

ThCl4 apparent vapor pressure due to the vapor complexThCl4AlCl3 formation. This should improve the productionof a thorium-free crude chloride mixture of LnCl3 and BaCl2from the bastnaesite concentrate. On the other hand, LnCl3may react with O2 and H2O to form oxides and oxychlorides:

CeCl3 (s or 1) (1/4)O2 (g) (3/2)H2O (g)

CeO2 (s) 3HCl (g) [3]

[4]

[5]

[6]

Figure 2 shows the accompanying standard free energy changesbetween 400°C and 1100 °C from Barin et al.[6,7] It can be seenthat the oxidation reactions are thermodynamically favorableat above 650 °C in the (O2H2O) atmosphere. Under the sameconditions, however, the standard free energy changes are posi-tive for the LnOCl-to-Ln2O3 and BaCl2-to-BaO conversion.[6,7]

They suggest that selective oxidation of the thorium-free chlo-ride mixture at above 650 °C in the (O2 H2O) atmosphere

LnCl3 (s or 1) (1>2)O2 (g) LnOCl (s) Cl2 (g )

CeCl3 (s or 1) O2 (g) CeO2 (s) (3>2)Cl2 (g )

LnCl3 (s or 1) H2O (g) LnOCl (s) 2HCl (g )

may transform LnCl3 into CeO2 and LnOCl, which enablesBaCl2, CaCl2, and trace RaCl2 to be removed by subsequentwater leaching at room temperature.

III. EXPERIMENTAL

The experimental apparatus and procedure used in thisstudy are very similar to those used in Reference 4 except

for the details noted subsequently.The Cl2 gas with a flow rate of 20 cm3min1 at standard

temperature and pressure was used as the chlorination agent.The AlCl3 gas, which was produced by heating AlCl3 in asealed glass ampoule with a small hole at 200 °C, was usedas vapor complex former for ThCl4. On the other hand, ithas been shown in Reference 4 that the reduction of carbo-chlorination temperature down to 600 °C is the key to avoidingThCl4 contamination for non–rare earth volatile by-product,which could be realized for the mixed bastnaesite-monaziteconcentrate at 500 °C under 2 kPa of SiCl4 gas, which actedas defluorination agent. However, our preliminary experimentsshowed that the rare earth chloride conversion for the bast-naesite concentrate was only 81 mol pct at 600 °C for 2 hours

under 2 kPa of SiCl4 gas, which may be caused by its largermineral size than the mixed concentrate mentioned previously.Thus, we increased the SiCl4 gas pressure up to about 10 kPain this study, which was obtained by the reaction of (SiO2

C) with Cl2 gas at 1100 °C. The (O2 H2O) mixed gas,which was formed by passing air or pure O2 gas with a flowrate of 20 cm3

min1 at standard temperature and pressurethrough water at room temperature, was used as oxidant.

The particle radius from Brunauer-Emmett-Teller (BET)is 21.6 m for the bastnaesite concentrate. A raw mixturewas formed by mixing powdered activated carbon with the

Fig. 2—Standard free energy changes of oxidation of rare earth chlorides,contained in the carbochlorination product of the bastnaesite concentrate, inthe (O2H2O) atmosphere to form cerium dioxide (Reactions [3] and [5])[6,7]

or noncerium rare earth oxychlorides (Reactions [4] and [6]),[6,7] taking lan-thanum as an example of the noncerium rare earth elements.




concentrate at a total atomic ratio of Ln:C 1:3 and put intoa graphite boat, which was placed in a cylindrical alumina tubereactor. The raw material was preheated in argon atmosphereup to the desired temperature before carbochlorination or SC-CVT oxidation. Isothermal carbochlorination kinetics was stud-ied within 2 hours at the temperature ranging between 450 °Cand 1000 °C in both Cl2 and (Cl2 SiCl4) atmospheres. TheSC-CVT reaction was performed at 550 °C for 2 hours in the(Cl2 SiCl4) atmosphere and then at 800 °C for 0.5 hours inthe (Cl2 SiCl4AlCl3) atmosphere. Following the SC-CVT

reaction, the oxidation was investigated between 700 °C and1000 °C for 0.5 hours in the (O2 H2O) atmosphere. Aftercarbochlorination or SC-CVT, the tube reactor was cooled toroom temperature in argon atmosphere and the product wasleached in 5 wt pct of hydrochloric acid. After SC-CVT-oxidation, the tube reactor was cooled to room temperature inair and the product was leached in water. The compositionswere determined using an inductively coupled plasma atomicemission spectrometer.

IV. RESULTS AND DISCUSSION

Figure 3 shows the rare earth chloride conversion, X , which

was defined as the total atomic ratio of rare earth elements inthe produced chlorides to those in the initial material, as afunction of time within 2 hours at the temperature rangingbetween 450 °C and 800 °C in the (Cl2 SiCl4) atmospheres

with 0 and 10 kPa of SiCl4, respectively. The carbochlorina-tion was inefficient at below 500 °C in the two cases, and thereaction rate increased with temperature but the temperatureeffect was reduced with the carbochlorination progress. Therare earth chloride conversion for 2 hours between 500 °C and800 °C was 43 to 81 mol pct in the absence of SiCl4 butincreased to 55 to 99 mol pct at 10 kPa of SiCl4. These results,together with the negligible equilibrium vapor pressure of ThCl4at below 600 °C, allow the carbochlorination of the bastnae-site concentrate at 550 °C under 10 kPa of SiCl4 gas for effi-

cient rare earth chloride conversion and thorium-free volatileby-product release. The main gaseous by-product is CO2. Thecontained gaseous compounds SiF4 and SO2 may be trans-formed into valuable fluorides and sulfuric acid, respectively.

As compared with pure compounds, carbochlorination of the concentrates containing many reacting solids often showssome complicated results such as different apparent activationenergies for initial and final states.[4,8] Thermodynamic analysishas shown that the most credible carbochlorination reactionsfor LnOF-to-LnCl3 conversion in the (Cl2 SiCl4) atmospheremay be LnOF (s) (1/2)C (s) Cl2 (g) (1/4)SiCl4 (g)LnCl3 (s) (1/4)SiF4 (g) (1/2)CO2 (g) at low tempera-tures and LnOF (s)C (s)Cl2 (g) (1/4)SiCl4 (g) LnCl3(s or 1) (1/4)SiF4 (g) CO (g) at high temperatures.[4] Inthe case of pure LnOF, these reactions may be assumed tobe controlled by the formation of an activated complex (e.g.,C-LnOF-(Cl)-Si) on the interface, thereby following the shrink-ing sphere model:[9,10]

[7]

where k constant and t carbochlorination time. In thecase of the bastnaesite concentrate, however, the non–rareearth compounds it contains, and their carbochlorinationproducts, may affect the reaction mechanisms. Figure 4shows isothermal carbochlorination data between 500 °Cand 800 °C under 0 and 10 kPa of SiCl4 gas, fitted based onEq. [7]. It can be seen that the linear relations of (1 X )1/3

vs time are valid roughly for the initial 20 minutes andfinal 1.5 hours respectively. Figure 5 shows the correspondingArrhenius relation:

[8]

where k ° pseudo-frequency factor and E a apparent acti-

vation energy. The apparent activation energy values werefound to be 17 and 10 kJ/mole for the initial and final stages,respectively, in the absence of SiCl4, but these values reducedto 15 and 5.9 kJ/mole under 10 kPa of SiCl4 gas. Figure 5also shows a clear break in the Arrhenius plots at the temper-ature ranging between 500 °C and 550 °C. Below this temper-ature, the apparent activation energy was about 50 to

100 kJ/mole. Such a stepwise phenomenon remains whenthe kinetics equations[9,10] other than Eq. [7] were used tofit the experimental data.

As mentioned previously, the conversion ratio X reportedin this study is for the rare earth chlorides only, which maybe affected by many side reactions. For example, the differ-ential thermal analysis (DTA) determinations in air of the bast-naesite concentrate[11] indicate an endothermic signal between320 °C and 580 °C with a strongest peak at 473.7 °C, and thecorresponding TGA curve[11] shows a total weight loss of 17.1 pct in this temperature range due to the decomposition

k k °exp ( E a>RT )

kt 1 (1 X )1/3

(a)

(b)

Fig. 3—Carbochlorination isotherms of the bastnaesite concentrate for (1)450 °C, (2) 500 °C, (3) 550 °C, (4) 600 °C, (5) 700 °C, (6) 800 °C, and (7)900 °C: (a) in the absence of SiCl4 and (b) under 10 kPa of SiCl4 gas.



220—VOLUME 35B, APRIL 2004 METALLURGICAL AND MATERIALS TRANSACTIONS B

Fig. 5—Arrhenius diagrams for the initial () and final () stages of isother-mal carbochlorination of the bastnaesite concentrate: (a) in the absence of SiCl4 and (b) under 10 kPa of SiCl4 gas.

(a)

(b)

of LnFCO3, which may be responsible for the Arrhenius plot

breaking at the temperature ranging between 500 °C and 550 °Cin both cases with and without SiCl4. On the other hand, acomplete carbochlorination of the rich barite in the bastnae-site concentrate may be achieved within 2 hours in the tem-perature range between 500 °C and 550 °C. The much highercarbochlorination ability of barite than LnOF may play a majorrole in suppressing the rare earth chloride conversion in theinitial stage and keeping the higher apparent activation energyvalues in the initial stage than that in the final stage at hightemperature.

After carbochlorination of the bastnaesite concentrate at550 °C under 10 kPa of SiCl4 gas for 2 hours, the carbochlori-nation temperature was increased to 800 °C and the subsequentcarbochlorination and CVT were performed at that temperature

for 0.5 hours in the (Cl2 SiCl4AlCl3) atmosphere, increas-ing the rare earth conversion up to 99 mol pct and completelyremoving thorium via vapor complex ThCl4AlCl3, which isthe same as those for the mixed bastnaesite-monazite concen-trate.[4] The thorium-free chloride mixture may contain 80 to85 wt pct of LnCl3, about 11 wt pct of BaCl2, and a smallamount of carbon, SiO2, and CaCl2. The subsequent oxidationof the chloride mixture with the wet air at 700 °C to 800 °Cfor 0.5 hours, followed by water leaching, may result in therare earth dissolution up to 1.2 mol pct. However, this valuemay decrease to 0.2 mol pct when replacing air with pure

oxygen or increasing the temperature to 1000 °C. The rare

earth oxide-oxychloride mixture contains about 0.5 wt pct of Si, which is a perfect material for individual rare earth separa-tion. Meanwhile, the valuable element barium could be recov-ered after removing trace radium from the aqueous solutionby coprecipitation with BaSO4.

V. CONCLUSIONS

The isothermal carbochlorination of a bastnaesite con-centrate in both Cl2 and (Cl2 SiCl4) atmospheres between500 °C and 800 °C follows the relation kt 1 (1 X )1/3

for the initial and final stages within 2 hours. Efficient rareearth chloride conversion and nearly complete thorium-free

volatile by-product release could be realized from the con-centrate by carbochlorination at 550 °C for 2 hours under10 kPa of SiCl4 gas. The subsequent carbochlorination andCVT at 800 °C in the (Cl2 SiCl4 AlCl3) atmosphere for0.5 hours via vapor complex ThCl4AlCl3 may yield com-plete ThCl4 removal, while the oxidation at 700 °C to 800 °Cin the (O2 H2O) atmosphere for 0.5 hours, followed bywater leaching at room temperature, may result in a rareearth oxide-oxychloride mixture almost without any non–rareearth impurity. The CO2 gas produced at 550 °C is the mainwaste by-product. Therefore, the SC-CVT-oxidation process

Fig. 4—Mathematical fitting of isothermal carbochlorination data of thebastnaesite concentrate using Eq. [7] for (1) 450 °C, (2) 500 °C, (3) 550 °C,(4) 600 °C, (5) 700 °C, (6) 800 °C, and (7) 900 °C: (a) in the absence of SiCl4 and (b) under 10 kPa of SiCl4 gas.

(a)

(b)



allows the green rare earth extraction from the bastnaesiteconcentrate.

ACKNOWLEDGMENT

This work was supported by the National Natural ScienceFoundation of China (Contract No. 59874008).

REFERENCES

1. C.K. Gupta and N. Krishnamurthy: Int. Mater. Rev., 1992, vol. 37,pp. 197-248.

2. W. Brugger and E. Greinacher: J. Met ., 1967, vol. 19, pp. 32-35.3. G.-C. Zhang and X.-W. Huang: Rare Met ., 1997, vol. 21, pp. 193-99.

4. Z.-C. Wang, L.-Q. Zhang, P.-X. Lei, and M.-Y. Chi: Metall. Mater.

Trans. B, 2002, vol. 33B, pp. 661-68.5. G.-X. Xu: Rare Earths, Metall. Ind. Press, Beijing, 1995, vol. 1, pp. 242

and 279.6. I. Barin and O. Knacke: Thermochemical Properties of Inorganic Sub-

stances, Springer-Verlag, Berlin, 1973, pp. 74, 78, 116, 162, 163, 319,394, 584, 656, and 744.

7. I. Barin, O. Knacke, and O. Kubaschewski: Thermochemical Properties

of Inorganic Substances, Supplement , Springer-Verlag, Berlin, 1977,pp. 156, 160, 295, 354, 451, 453, and 542.

8. K. Kanari, I. Gaballah, and E. Allain: Metall. Mater. Trans. B, 1999,vol. 30B, pp. 577-87.

9. S.S. Tamhankar and L.K. Doraiswamy: AIChE J ., 1979, vol. 25, pp. 561-82.10. J. Szekely, J.W. Evans, and H.Y. Sohn: Gas-Solid Reactions, Academic

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