Carbochlorination of argentinian tantalo-columbites

Canodmn

Merallurgrca/

Pergamon

Quarrerly, Vol. 36, No. 2, pp. 103-I IO, 1997 0 1997 Pubhshed by Elsevier Science Ltd Printed m Great Britain. All rights reserved 00084433/97 s17.00+0.00

PII: s0008-4433(96)0004&7

CARBOCHLORINATION OF ARGENTINIAN TANTALO-COLUMBITES M. DEL C. RUIZ, J. A. GONZALEZ

and J. B. RIVAROLA

Institute de Investigaciones en Tecnologia Quimica (INTEQUI), Universidad National de San Luis-CONICET, Casilla de Correo 290, 5700 San Luis Argentina (Received

6 May

1996; in revisedform

14 November

1996)

Abstract-The extraction of Nb and Ta from tantalo-columbites of San Luis, Argentina has been studied at the laboratory scale. The ore mixed with carbon was chlorinated in a fixed bed reactor operated with downcoming flow and pressures close to atmospheric. The variables investigated were: ore/carbon ratio, mixing time and procedure; temperature and reaction time; particle size and previous ore treatment with HCl, EDTA and NaOH solutions. The results obtained showed that: the optimum percentage of carbon varies with temperature; Nb and Ta recovery is hardly affected by the mixing time; the influence of the mixing procedure is important; recovery increases both with higher temperature and with longer reaction times, and decreases with larger particle size; in general, ore leaching led to a decrease in the solid reactivity to the chlorination process. The solids were characterised by BET, XRD, XRF, SEM and optical microscopy. Quantitative chemical analyses were carried out by XRF, ICPAES, gravimetry and UVVisible spectrometry. 0 1997 Published by Elsevier Science Ltd R&urn&On a Ctudie, B I’tchelle du laboratoire, l’extraction du Nb et du Ta des tantalo-columbites de San Luis, en Argentine. Le minerai mClang& avec du carbone a tt8 chlorC dans un rkacteur g lit fixe optrC avec courant decendant et B des pressions proches de l’atmosph&re. On a Btudit les variables suivantes: proportion de minerai/carbone, durt?e de mtlange et pro&d&, tempCrature et durCe de r&action; taille des particules et traitement ant&dent du minerai avec des solutions d’HC1, d’EDTA et de NaOH. Les rCsultats obtenus ont montrk ce qui suit: le pourcentage optimum de carbone varie avec la tempkrature; la rtcupCration de Nb et de Ta est trts peu affect&epar la dun&e de mtlange; l’influence du pro&d& de mtlange est importante; la r&up&ration augmente & la fois avec de plus hautes tempkratures et avec de plus longues durkes de r&action et diminue avec l’augmentation da la taille des particules; en g&n&al, le lessivage du minerai a conduit a une diminution de la rCactivitt du solide au pro&d& de chloration. Les solides ont tt& caractCris& par BET, par XRD, par XRF, par SEM et par microscopic optique. On a cornpI& des analyses chimiques quantitatives par XRF, par ICPAES, par gravimetrie et par spectromirtrie g UVVisible.

poundsinvolvesbeneficiationmethodsto removegangueminerals. This is done by different processes, among which Due to their physicochemical properties, Nb and Ta aremetals chlorination is one of growing use,sinceit producesa mixture of growing use in the aeronautic, space, electronic, superof the chloridesof theseproducts, which is suitable for the conductorandmetallurgicalindustries[ 1,2]. This has promoted eventualseparationof Nb and Ta [4-81. a marked increasein the production of both thesemetalsand Several studieshave been carried out on the chlorination their oxides. of different materialscontaining Nb and Ta, suchas oxides, In nature, Nb and Ta occur in combination with oxygen carbides,slags,oresandconcentrates.The mostfrequently used and other metalssuchas Fe, Mn, V, U, as their niobatesand chlorinatingagentshave beenchlorinein the presenceof carbon tantalates.The most abundant ores are tantalite, columbite, [6, 9-181 and chlorine at high temperatures [15, 18-241; while pyrochlore, microlite and samarskite,thesebeing mainly pro- lessfrequently usedonesarecarbontetrachloride[25-271,chlorduced in Congo, Nigeria, Norway, Brasil, Malaysia and ineandcarbonmonoxidemixtures[28,29], phosgene,hydrogen Canada.In Argentina, Nb and Ta are found asaccessorymin- chlorideand sulfur dioxide [15, 251. erals in small proportions in pegmatitesin the provinces of Studieshave also been performed on the effect of adding Salta, Cbrdoba, Catamarca,La Rioja and San Luis; and the sodiumchlorideto solidscontainingNb and Ta [10, 11, 301;or most frequently found speciesare thosebelongingto the tan- their leaching with acids or alkalis before chlorination [16, 311, talite-columbite series,which includesthe membersof the iso- in order to decreaseimpurity concentrationin the chlorinated morphousseriesniobite (Fe, Mn) Nb206andtantalite (Fe, Mn) Nb and Ta products. Ta,O,. Suchdepositsare not exploited for themselves,but as In the presentpaper,we have studiedthe influenceof various by-products of feld spar, quartz, mica, Li and Be ore mining working conditionson Nb and Ta extraction on the laboratory scale,by meansof chlorination in the presenceof carbon from 131. The first stagein the preparation of pure Nb and Ta com- tantalo+olumbites of SanLuis, Argentina. The trials wereoriINTRODUCTION

103

M. DEL C. RUIZ et al.: CARBOCHLORINATION

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b

HCI

NaOH

Fig. 1. Schematic representation of the apparatus used in chlorination trials. 1. Drying unit; 2. Flowmeters; 3. Gas scrubbing trap; 4. Exhaust; 5. Digital thermometer; 6. Thermocouple; 7. Furnace; 8. Gas preheater; 9. Sample; 10. Condenser; 11. Collecting system. ented to the determination of working conditions that allow optimum Nb and Ta recovery, expecting the results obtained to be useful in large scale extractive metallurgical procedures.

EXPERIMENTAL Apparatus Experiments were carried out in the equipment 1, including the following:

shown in Fig.

(a) A quartz reactor with three heating zones: (a gas preheater; a sample holder and a tube where reaction products condense). (b) A system providing the gaseous reagent, commercial quality chlorine, and the purging gas, 99.99% purity nitrogen. The chlorine passes through a concentrated sulfuric acid trap before entering the reactor and the nitrogen through silica gel. (c) A collecting system to retain products and neutralise chlorine excess.

10

20

30

40

50

60

D/Max-3C diffractometer operated at 35 kV and 30mA by employing Ni-filtered Cu Ka radiation, 3.= 0.154 18 nm) showed the presence of columbite, muscovite, felde spar and quartz [Fig. 2(a)]. The ore particle morphology was studied by SEM, using a Philips 515 equipped with an EDAX 9900 system. In Fig. 3 it can be observed that the particles present an irregular shape with several flat faces. The ore was ground in a disk grinder and then separated into fractions of different particle size, from - 50+80 to -325 mesh. The Nb, Ta and Ti content of the different fractions was determined by XRF and gravimetry followed by molecular absorptiometry in the UV-Visible zone of the spectrum [32]. Their surface area was determined by the BET method at 77 K with Kr adsorption. Results obtained are shown in Table 2.

Materials The ore used was a tantalite
ore sample

Compound

wt%

Compound

wt%

NW5 ‘MA TiO, MnO Fe0 SiO, AdA CaO

41.20 36.80 1.16 6.83 9.58 1.48 1.13 0.38

K,O U Zn Sn V Pb Ce La

0.143 0.220 0.060 0.020 < 0.020 0.015 0.005 0.003

70

28 Fig. 2. XRD spectra of ore sample and chlorination residues; (a) ore; (b) residue at 870 K and (c) residue at 1050 K.

Fig. 3. Morphology of tantalocolumbite

particles.

M. DEL C. RUIZ et al.: CARBOCHLORINATION Table 2. Surface area and Nb, Ta and Ti contents in the ore fractions of various particle sizes Size (mesh) -50+80 -100+140 -200 -325

NW, (w%)

Ta205 (w%)

Ti02 (w%)

Surf. area (m*/s)

41.4 41.4 41.2 41.3

35.6 35.8 36.0 35.9

1.19 1.16 1.06 1.18

0.103 0.455 0.9 1.49

For some experiments the ore was subjected to a leaching process prior to mixing with carbon. The leaching agents used were HCl, EDTA and NaOH, all those reagents were prepared in water solution, and their concentrations were 11.6, 0.27 and 5 mol/l, respectively. Four samples were prepared: one was leached with HCl; the second one was leached with EDTA; the third one was treated with HCl and the residue was then subjected to a second leaching with EDTA; and the fourth one was leached with NaOH and then HCl. In all cases, the leaching conditions were the following: particle size: - 200 mesh; temperature: 80°C; time: 5 h; solid concentration: 50 g/l; shaking: 110 rpm. After the leaching step, the solid was filtered, washed and dried. The surface area and the Nb, Ta and Ti content of the residues were determined, obtaining the results given in Table 3. The samples subjected to chlorination were a mixture of the ore (with no previous treatment in some cases and subjected to leaching in others) and carbon black, manufactured by Cabot Corporation, with a surface area of 13.65 m’/g, determined by BET at 77K with Nz adsorption. The NMR analysis showed the presence of paraffinic oils and aromatic compounds. Three different procedures were used for preparing this mixture: in an agate mechanical mortar with an agate ball (I); in a glass rotatory mixer (II); and by preparing pellets through pressing at 22 133 kPa of the mixture II, which were then ground and sieved to a specific particle size (III). Although the ore/ carbon ratios varied, the total mass mixed was kept the same in all cases. Different mixing times and ore/carbon ratios were tried. For the study of the other variables the ore-carbon mixtures were prepared by method I. All the other reagents used were of analytical quality. Experimental

105

TANTALO-COLUMBITES

Cl, was connected (with a previously regulated flow, F, at 0.09 l/min) for the time fixed for the experiment; after this N, was circulated again. At the end of each trial, the reaction products were qualitatively collected with concentrated HCl to avoid hydrolysis of the Nb and Ta compounds. The quantitative determination of these products was carried out, in some cases, by two different analytical methods, so as to the ensure reliability of the results [33]. The qualitative composition of the reaction products was established by XRF.

RESULTS AND DISCUSSION The results obtained have been interpreted in terms of the Nb and Ta conversion (expressed as pentoxides) present in the ore-carbon mixture, which was defined as: reacted oxide mass initial oxide mass > ’ loo

(1)

Effect of the ore/carbon ratio The effect of the ore/carbon ratio was studied in the range between zero and 40% (w/w). Experiments were performed at two temperatures, 773 K and 903 K for 90min and with a Cl, flow of O.O9l/min. From the results shown in Fig. 4, the following can be noted: 1) there is an optimum ore/carbon ratio, beyond which the conversion values decreased; 2) this optimum relationship changes with temperature, being between 25% and 30% of carbon at 773 K and between 15% and 20% at 903 K. The fact that the optimum value for the ore/carbon ratio for ores containing Nb changes with temperature has already been reported by other authors [15, 161. Biceroglu and Gauvin [34]

100 x”/

80

60

procedure

After placing the sample in the reactor (approximately for all trials) a N, stream was passed until the working perature was reached; the N, flow was then interrupted

0.2 g temand

Table 3. Surface area and Nb, Ta and Ti contents in the leached ore. (a) 11.6 M HCl (b) 0.27 M EDTA; (c) 11.6 M HCI and 0.27 M EDTA, successively; (d) 5 M NaOH and 11.6 M HCl, successively Leaching reagent HCl’“’ EDTA@” HCl + EDTA’“’ NaOH + EDTA@’

OF ARGENTINIAN

NW4 (wt%)

TazOs (wt%)

Ti02 (wt%)

38.3 37.4 40.1 39.3

40.0 42.2 44.3 43.2

1.7 1.85

1.0 1.11

40

20

Surf.area (m’/g) 2.724 0.497 0.923

1.305

0 U

10

20 30 % of carbon [w/w]

40

50

Fig. 4. Effect of the ore/carbon ratio (X of carbon) upon conversion (X%) of the Nb and Ta oxides. Dotted line: T= 773 K; dashed line: T=903 K.

M. DEL C. RUIZ et al.: CARBOCHLORINATION

106

showed the effect of distance between these solids in the original mixture, which allows maximum chlorination rate. On the other hand, the existence of such an optimum value and the fact that it changes with temperature relates to the stoichiometric quantities of carbon needed for the carbochlorination reaction to occur as well as to the Cl,-C interaction, which is a function of temperature and can lead to HCl release by interaction between Cl, and the C-H surface bonds [35] and to the formation of atomic chlorine (where the carbon surface could act as a catalyst) [36]. The decrease of conversion from ore/carbon ratios higher than 30% can be accounted for by a higher resistance to diffusion of the gaseous reagent to the ore surface. Such resistance to diffusion is due to the fact that this type of carbon totally covers the ore particles, this covering becoming greater as its proportion in the ore-carbon mixture increases. Effect of mixing procedure The three mixing techniques tried have been above described and identified as I, II and III. Mixing time was set at 60 min and the amount of carbon to 25%. Results and other experimental conditions are reported in Fig. 5. The mixing procedure giving the highest reaction rate was the mechanical mortar. The other two techniques led to similar lower rates. The determination of particle size distribution was carried out in order to determine whether it was affected by the mixing procedure. The results obtained showed no significant difference. The results obtained in the chlorination trials can be explained taking into account what has been postulated by other authors [34, 37, 381, who suggest that the chlorination rate is accelerated when there is a gaseous phase between the carbon and the oxide, and when the distance between the two solids is

hww

7

r Mesh -100 +I40 % carbon 25 Irn 60 ml” f 90 man T 773 K F 009 llm~n

20

t

10

“td I

(1)

(11) (WI Mixing method Fig. 5. Effect of mixing procedure upon conversion of the Nb and Ta oxides.

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TANTALO-COLUMBITES

less than about 200 microns [37] or approximately 40 microns [38]. An appropriate distance between the two solids, besides increasing the contact area between ore and carbon, would prevent the active chlorine species such as atomic chlorine [36] (which are formed on the carbon surface and desorbed in the gaseous phase) from recombining and reaching the mineral surface. For active species not to recombine and reach the mineral surface, the species mean free path should be somewhat larger that the distance between the ore and the carbon. According to theoretical calculations [39] the mean free path of the gaseous species formed is within the limits mentioned 137, 381. Such conditions would be the most favourable when the mechanical mortar is used. This is supported by the results shown in the micrographs of Fig. 6 (obtained by reflected light microscopy, in a Carl Zeiss Pol II microscope). Figure 6 which corresponds to columbite-tantalite, shows moderate reflectivity and is grey with a brown tint in polished sections. The anisotropic effects under+nicol are quite weak. Figure 6(b) corresponds to carbon and is isotropic. In Fig. 6(c), (d) and (e) it is observed that the degree of mineral covering by carbon is greater in the mixture obtained by method I. A probable chlorination mechanism could be the following: chlorine reacts with carbon generating active species which diffuse to the ore surface where they react yielding the corresponding chlorides or oxichlorides and oxygen, which in turn reacts with carbon to form CO or CO,. Effect of mixing time (t,) The effect of mixing time (tm) was studied in the range from 30 to 120 min, for two different mixing procedures. Experimental results and other working conditions are shown in Fig. 7. The conversion of the Nb and Ta oxides present in the ore varies slightly with mixing time when the mechanical mortar is used; while no changes are observed when using the rotatory mixer. These results indicate that the variation of mixing time during the studied period, does not significantly modify either the contact area between ore and carbon, or the distance between both solids. Hence, the formation of active chlorine species on the carbon surface is not affected. The probability of these species reaching the ore surface and reacting to form their respective Nb and Ta chlorides does not change either. Effect of particle size The particle sizes as well as the ore composition and specific surface are reported in Table 2. The working conditions and chlorination results are shown in Fig. 8. A marked influence of particle size upon Nb20, and Ta205 conversion is observed. This can be explained if it is taken into account that the specific surface change between the two extreme particle sizes is one and a half orders of magnitude, and that an increase in the specific surface increases the orecarbon contact area, which favours the reaction rate, as it has already been noted when studying the ore/carbon ratio effect. Effect of temperature (T) The effect of temperature (r) was studied within the range 673-1273 K. Working conditions and results are presented in Fig. 9. The following can be observed from this figure: 1) the chlorination of Nb and Ta oxides present in the ore starts above

M. DEL C. RUIZ et al.: CARBOCHLORINATION

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107

TANTALO-COLUMBITES

Fig. 6. Photomicrographs of ore, carbon and ore-carbon mixtures obtained by different methods. (a) ore, (b) carbon, (c) method I, (d) method II and (e) method III.

750K;

2) at a given temperature,

the attack

on Nb oxide is

higher than on Ta oxide; and 3) in the temperature range between850 and 1050K the influenceof this variable on the reaction rate is lower than in the 750-850and 105&1250K

1/5Ta,O,(s)+C1Z(g)+C(g) --f 2/5TaCLk)+CO(g) Figure 2(b) and (c) show the XRD spectra of the chlorination

intervals. The higher reactivity of Nb oxide compound to Ta oxide is in agreementwith the standard Gibbs energy changevalues

(Table 4) calculatedfrom the formation AG” [39] for the carbochlorination reactionsof theseoxides, which are nossible underthe conditionsstudied[41, 421: l/SNb,O,(s) + Cl,(g) +C(s) + 2/5NbCl,(g)+ CO(g)

(2)

1/3Nb,O&)+ Cl,(g) + C(s) + 2/3NbOCl,(g) + CO(g) (3)

(4)

Table 4. AG” values for reactions (2). (3) and (4) AG” (kJ/mol Clz)

T WI

Reaction (2)

Reaction (3)

Reaction (4)

700 1000 1300

- 84.60 -115.37 - 145.37

-91.782 - 138.587

-81.544 - 112.850 - 143.31

M. DEL C. RUIZ et al.: CARBOCHLORINATION

OF ARGENTINIAN

TANTALO-COLUMBITES

1ocI-

5c L

.

.

.,.:

-.

X%

..-.-

40

..

Nb205

X% 80

I-

Ta2 05 . Mesh -100 +,4O % carbon 25 I 90 ml” T 773 K F 0 09 llmln

30

:::::::::

Ta2 05

Mesh -100 +140

60

% carbon

25

tm 60 InIl. t 90 nl,”

20

40

F 009

llmln

10

0 0

30

60

90

120

Mixing time [mln] Fig. 7. Effect of mixing time upon conversion of Nb and Ta oxides. Dotted line: mechanical mortar mixing; dashed line: rotatory mixer.

650

750

850

950

1050

1150

1250

Temperature [K] Fig. 9. Effect of temperature upon conversion of Nb and Ta oxides. and it therefore

completely

“bathes”

the ore particles,

gen-

residues at 870 and 1050 K, respectively. It can be observed that

erating resistanceto diffusionhigher than in the previousstage, MnCl, is only presentin the residueof chlorination at 870K sincesolid MnCl, presentsa certain degreeof porosity which (hydration is dueto samplehandling). permits the chlorinating agent to reach the ore surfacemore Taking the obtained resultsinto account, the reaction rate easily.At highertemperatures,the pressureof manganese chlordecreaseobservedbetween850and 1050K can be interpreted ide vapour is considerable,thus it is in the gaseousphaseand asfollows: at temperaturesbetween750and 850K Nb, Ta and its formation doesnot affect the chlorination of niobium and Fe chloridesvolatilize, while MnC& doesnot, sinceit is solid tantalum oxides. under suchconditional Above 850K and up to approximately 1050K, MnCl, is in liquid state with low vapour pressure [18] Effect of reaction time (t) Figure 10 showsthe effect of reaction time between30 and 120 min at 773K; other working conditionsare alsoindicated. IOC 1 Beyond 90min reaction time no appreciableincreaseis observedin the extraction rate. This can be attributed to the X% % carbon 25 fact that the conversiondegreereachedincreasesthe distance tm 60 ml” betweenthe ore and the carbon due to a decreaseof the ore 8C I 90 ITI” particle size causedby the formation of volatile products. T 773 K Hence, on the one hand the ore-carbon contact area is F 0.09 llmin decreased,and on the other, it permits the recombination of 60 active chlorine speciesbeforethey reachthe ore surface. To obtain greaterconversionsin relatively short times it is necessaryto work at elevatedtemperatures.

‘r

r

40

20

0 .I -50

+80

-100

+140

200

I -325

Partrcle size [mesh] Fig. 8. Effect of particle size upon conversion of Nb and Ta oxides.

Effect of leaching

Leachingthe ore with different solutionsbefore mixing with carbon led to somevariations in specific surface and composition of the sampleswith respectto the original ore (Table 3). Figure 11(b) and (c) showthe XRD spectraof the mineral after it hasbeentreated with 35% HCl and with 10% EDTA, respectively, where the disappearanceof muscovite can be observed.The XRD spectra of the mineral subjectedto the other leachingtreatmentswere similarto thoseshownin Fig. 11(b) and (c). The resultsof the chlorination reaction are in Table 5. Analysisof the experimentaldata showsthat:

M. DEL C. RUIZ et al.: CARBOCHLORINATION

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109

Table 5. Effect of previous ore leaching. Particle size: -200 mesh; %C = 25; t,,,= 60min; T = 773 K; I = 90min; F = 0.09 l/mm

6C ‘r

Leaching reagent

Mesh -100 +140 96 carbon

Nbz O5

25

tm 60 mr

no leaching HCI EDTA HCl + EDTA NaOH

*

f

%Nb>O,(extracted)

%Ta,O,(extracted)

59 24

64 9 66 35

15

13

73

21

-

Obviously, the decrease in the leachedore reactivity is dueto a surface deactivation of the reacting sites of the solid in contact with the liquid phaseaswell asto a poisoningof the surfaceby products of chemical reactions taking place between the mineral

and the leachingliquid. CONCLUSIONS 0

30

90

60 Reaction

time

120

[min]

Fig. IO. Effect of reaction time upon conversion of Nb and Ta oxides at 773 K. (1) In general, leaching decreases the ore reactivity, even in those cases in which surface area changes might lead the opposite to be expected.

(2) Muscovite disappearance is not directly related to changes in the solid reactivity, since addition of muscovite to the leached ore did not elevate the Nb and Ta quantities

extractedby chlorination.

The present laboratory-scalestudy of the chlorination of tantalite
of Nb and Ta from the above mentioned

ores.The resultsobtained,together with studieson separation and purification of theseelements,can be usedfor an eventual adoption of chlorination by industriesinvolved with extractive metallurgy of Nb and Ta in Argentina. Further studiesare being carried out to postulatethe most probablekineticsand reaction mechanism. Acknowledgements-The authors wish to thank support given by Universidad National de San Luis and Consejo National de Investigaciones Cientificas y Tecnicas (Argentina), as well as advice from the Organic Chemistry Department with regard to carbon characterization and the Analytical Chemistry Department for qualitative and quantitative determinations of the samples. They thank also Dr Fathi Habashi for fruitful discussion.

REFERENCES

1

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50

a

365-368.

_I

10

20

30

40

50

60

70

20

Fig. 11. XRD spectra of ore sample and leaching ore: (a) ore; (b) ore leaching with 11.6 M HCl and (c) ore leaching with 0.27 M EDTA.

Kirilova, G. F., Meerson, G. A. and Zelikman, A, N., Kinetics of Chlorination of Titanium and Niobium Carbides, Zzvest. Vysshikk Ucheb, Zavedenii Met., 1960, 3(3), 90-96. 10. Mathur, B. S., Sastri, V. S. and Gokale, Y. W., Use of Sodium 9.

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OF ARGENTINIAN

TANTALO-COLUMBITES

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