Carbothermic reduction of high-grade celestite ore ta ...

Carbothermic reduction of high-grade celestite ore ta manufacture strontium carbonate M. Erdemoğlu, M. Canbazoğlu and H. Yalçı n

Synopsis The conversion of celestite (SrSO4) to strontium sulphide (SrS) by the carbothermic reduction process was investigated as a route for the production of SrCO3 from high-grade Turkish celestite ores. Experiments were conducted in the temperature range 900-1400°C and metallurgical-grade coke was used as the reducing agent. The efTects of reduction temperature, amount of carbon and roasting time were studled. The products of reaction (black ash) were examined, mainiy with the use of X-ray dift'raction and fiame atomic absorption spectrometry. The results include data on recoverable Sr obtained as strontium carbonate precipitate. The conversion reaction is complete at 1300°C, and it begins at lower temperatures. İn addition to sulphate—sulphide conversion, ail alkali earth metais found in the ore can be transfornıed to carbides in the process of reduction, especially when the roasting time and amount of carbon added to concentrate are increased. The reduction reaction seems to be very fast and it is concluded that the manufacture of SrCO3 is feasible from the roasted product.

Celestite (SrSO4) is the oniy commercialiy exploited strontium mineral that can be regarded as an important primary source of manufactured strontium carbonate (SrCO3), although investigations have been conducted into the possible use of the natural and less 'common strontium carbonate mineral strontianite.1-3 The largest use of strontium carbonate is in the manufacture of glass face-plates for colour television or computer tubes. İt is added to glass material at the level of 12-14 wt% (strontium oxide basis).4 Strontium carbonate is an eflective X-ray barrier because strontium has a large atomic radius and its presence is required in high-voltage television sets. SrCO3 is also consumed in the manufacture of ferrite powder for small dc motors in the automotive industry, high-purity, low-Iead electrolytic zinc, some high-technology ceramic materials and other strontium chemicals.4 To manufacture strontium carbonate celestite is processed either by the black ash process or by the double decomposition process, the former proving the more economic in most of the SrCO3 plants in the world.5 İn the black ash process celestite ore is roasted with coke to obtain black ash containing a soluble Sr compound, SrS. The product of roasting is then decomposed in water to give strontium hydroxide and strontium hydrosulphide. The solution from decantation is passed through carbonation columns, in which either soda ash or carbon dioxide gas is used to cause precipitation of

Manuscript first received by the Institution of Mining and Metallurgy on 11 October, 1996; revised manuscript received on 27 April, 1998. Paper published in Trans. 1mm Mm. Metal!. (Sea. C: Mineral Process. Extr. Meta.IL), 107, May-Aı.ıgust, 1998. © The Institution of Mining and Metallurgy 1998.

strontium carbonate crystals. İn the other process celestite ore or concentrate is reacted directly with soda ash 8 or ammonium carbonate9 in solution at temperatures of 80-90°C to produce impure strontium carbonate as a solid phase and a by-product solution containing sodium sulphate. or ammonium sulphate. The impure strontium carbonate is then dissolved to remove the insoluble gangue material. The new solution, containing mainly SrCİ2, is reacted with soda ash to reproduce a technical-grade strontium carbonate precipitate. The products derived from the two processes are of similar quality; usualiy, a SrCO3 purity of more than 97% can be obtained with the black ash process.5 The Sivas basin in Turkey contains many large and small celestite occurre'nceslo and it is one of the important celestite producers in the world. The main purpose of the present work was to investigate the carbothermic reduction conditions of high-grade Turkish celestite ore with a view to the manufacture of high-grade strontium carbonate by the black ash process.

Experimental Materials The reduction experiments were conducted on ore from the largest deposit worked in Turkey, at the mine and concentrator near Sivas operated by Bant Maden TAŞ. X-ray powder difiracdon patterns show that the sample is mainiy celestite (Fig. 1);h1 other components are gypsum and barite.

cc 15

25

35

C

45

c 55

C

c 65

2e, degrees

Fig. 1 X-ray powder diraction patterns of celesme concentrate sample used in experiments (B, barite; C, celestite; J, gypsurn) Chemical anaiysis yielded SrSO4, 95.5%; CaSO4, 3.25%; BaSO4, 0.77%; and trace Fe203. As a reducing agent metallurgical-grade coke, comprising 82.97% C, 0.46 S and 15.92% ash, was used. The grain size of the celestite used was finer than 2 mm. The results of partide-size analyses on ASTM E-1 1 test sieves indicated that 80% of the sample was finer than 800 pm and 9.28% finer r.han 106 ım. The coke was ground to —2 mm also, and the —800-ım and —106--.tm fractions were 75.65% and 15.18%, respectively. Before mixing both the sample and coke were dried at about 10°C for 4 h to remove any rnoisture. C65

Method Reduction experiments were carried out in a high-temperature horizontal-tube furnace (Carbolite STF 15/50/450 type; maximum heating temperature, 1500°C) in an inert atmosphere of N2 gas. Both sides of the roasting tube were covered to prevent exposure to air. The use of stainless-steel pipes of 5-mm internal diameter enabled the nitrogen gas to be admitted through one end of the tube and end-gases to be exhausted from the opposite end. About 20 g of celestite concentrate and various amounts of coke were mixed at ratios determined by the experimental conditions. The mixture was then transferred to an aluminium oxide crucible and introduced into the furnace. At the end of the run time the roasting furnace was cooled to 300°C and the roasted material was removed. The furnace was cleaned by passing N2 gas, before and during the heating and cooling. The leaching experiments were carried out in a 0.25-1 flask immersed in a temperature-controllec water bath. When the solution temperature reached the required value 1 g of black ash sample ground to —212 jim was introduced into the solution. The experiments were performed for a predetermined time, after which the solution was vacuum-fıltered at temperatures in excess of 75°C. The precipitation of Sr as SrCO3 was carried out in a flask with use of a slight stoichiometric excess of sodium carbonate at moderate temperatures. After the SrCO3 crystals had been filtered and the water content of the fiitrate evaporated it was observed that hydrous Na2S crystallized. The XRD patterns of SrCO3 are shown in Fig. 2, and Table 1 lists the components of the precipitate.

and 3.2 x i0, at 25°C, respcctively,'3"4 according to the reactions

- SrCO3 + NaOH Sr(HS)2 + Na2CO3 - SrCO3 + NaFIS NaHS + NaOH - Na2S + H20 Sr(OH)2 + Na2CO3

SrS(5) + Na2CO3(aq)

Weightloss, %=

WOWBA

wo

Recoverable Sr, % =

p 25

- SrCO3 + Na2S

(5)

The reduction products were anaiysed by fiame atomic absorption spectrometry (using a Philips PU9İ OOX model interfaced with a computer so as to be capable of simultaneous sampiing and calculation) for determination of the total amounts of strontium, calcium and barium; with a high-temperature oxygen combustion anaiyser (L.eco model SC 444) for total sulphur and carbon; by wet-chemical or gravimetric anaiysis for determination of the precipitable strontium content; and by X-ray powder diffraction anaiysis (Rigaku D-Max 111 Series model with Cu tube and Ni filter) to identify the phases in the various reduction conditions. JCPDS" fı les were used to determine the natural and artificial phases. The reaction of SrSO4 and carbon was studied at various temperatures, SrSd4/C ratios and roasting times. The weight loss is defined as

pP

15

(4)

The overali reaction can be written as

'cD

p

(3)

(6)

xlOO

where W0 is original weight of celestite concentrate and coke mixture and WBA is weight of black ash under giyen conditions. The recoverable Sr is defıned as

F

c

(2)

35

Tr

P P p Pp

45

55

65

29, degrees

Fig. 2 XRD traces of SrCO3 (P, precipitate)

(7)

x 100

where Sr is weight of Sr found in final precipitate containing mainiy SrCO3, afld T8r is the weight of total Sr found in black ash obtained from each set of roasting conditions. The term denotes per cent metal recovery. The reduction reaction assumed in the calculation of the stoichiometric quantities in the reaction mixtures is

Table 1 Chemical anaiysis of bulk SrCO3 powder SrSO4(8) + 4C(S) - SrS(S) + 4CO(g) SrCO3 BaCO3 CaCO3 Na2CO3 MgCO3 Fe203

98.50 ±0.55 0.97 ±0.03 1.63 ±0.75 1.16 ±0.77 not detected A 55

AA 65

20, degrees

Fig. 7 XRD traces of celestite (C), strontium sulphide (A), stron-

tiurn carbide (D), barium carbide (B), carbon (K) and calcium carbide (G) in black ash obtained at vanous amounts of excess coke C68

60

Excess Coke (%)

C. C D 25

Rccovezab4c Sr

0

A

A

15

go

0

A

5

100

Roasting time The efTect af raasting tinıe was investigated with a mixture of celestite and cake that contained 50% excess carbon. İt was roasted at 1300°C for various times in static conditions (Fig. 9). İt can be seen clearly from the results that the raasting time has a negiigible effect an the weight lass of the material. The sulphur and carbon cantents are alsa hardiy afiected.

100

604020

0

0 Cb

80

6

bon under any of the roasting conditions. After roasting for 120 mm traces of carbides of Sr and Ca become visible in the black ash (Fig. 10). Roasting with 100% excess carbon, however, increases the relative intensity of the XRD peaks of SrC2 .

25 * -20

A

,ISuiPhur A

0

0 0 Wg Ioss

-I l 0 0

.s

- 10

100

100

.5 0

.80

cbleSr *0

0

30

60

120

180

240

Z40

Fig. 9 Effect of roasting time on weight loss and carbon and su!phur contents of black ash

İt is observed from X-ray diffraction patterns of the black ash products from this group of experiments that the conversion rate of SrSO4 to SrS is very fast (Fig. 9), occurring under the giyen conditions in only 30 mm of roasting time. There were no traces of celestite, gypsum, barite or unbumed car-

0

0 0 Tc(aİ Sr

A

60 mm

A A

40

1.

20-

r2J

0

0

30

60 120 180 Roasting Time (ıninutes)

240

Fig. 11 Effect of roasting time on strontium recoverable as SrCO3

Discussion

JL

The reduction reactions that occur during the roasting of celestite ore with coke can be assumed to be

120 mm

SrSO4(S) + 4C() — SrS(S) + 4CO(g) (400-900°)

A

k.

SrSO4() + 4C(5) — SrS(5) + 4CO ) (>1000°C) 180 mirı A

liG_Ur_L 240 mm k

1Lk

35

(12)

SrSO4() + 4CO(g) — SrS(S) + 4CO2(S ) (900-1000°C) (13)

A

25

1.

0

Gravimetric anaiysis of black ash samples yielded the results summarized in Fig. 11. As the roasting time increased the total strontium content of the black ash varied oniy marginaliy, from 48.76 to 50.69%. The recovery of strontium also increased, from 75.55 to 87.34% with an average standard deviation of ±2.316%. The variation in the Sr grade of the black ash was, however, confined to a narrow range and the Sr compounds found in black ash were readiiy soluble/decomposable, and consequent]y precipitable as SrCO3. According to equation 11 SrC promotes the formation of Sr(OH)2, resulting in the increase in Sr recovery.

30mm

15

0 0

0V

0-

Roasing Time (mrnutes)

:"

-

60-

45

_ 55

65

20, degrees

Fig. 10 XRD traces of strontium sulphide (A), strontium carbide (D) and calcium carbide (G) in black ash obtained at various roasting times

(14)

Solid—solid contact reduction plays an important role in the low-temperature region also. With increase in temperature (>1100°C) the diminishing role of reaction 13 is due to the decrease in the contact area caused by the formation of product SrS on the SrSO4 core; even the 100% CO gas phase ensured by the oxidation of carbon cannot reduce the maccessible SrSO4 further. At temperatures greater than 1100°C the rate of regeneration of CO is greater than that of reaction 12, but the latter again proceeds to produce more SrS. İn this temperature range (1100-1400°C) solid carbon reacts with so!id sulphates, but the reduction rate is very slow until al! the sulphates are converted to sulphides. When up to 50% excess coke is added the recovery of Sr is approximately 90%. At greater coke additions Sr recovery decreases owing to the decrease in the total Sr content of the black ash. İt is proposed that the appearance of carbides depends mainiy on the roasting time. However, excess CO that does not reduce any material may yield CO2 by disproportionation at the bigher temperatures15 and cause partial reaction of free C69

carbon with SrS to give carbides and element sulphur: SrS + 2C - SrC2 + S°

(15)

CaS + 2C - CaC2 + S°

(16)

Nevertheless, no trace of sulphur was observed in the XRD analyses. Preil and co-workers have reported16 that when ali Group İlA elements are present in a system that also contains H20 and carbon monocarbides and dicarbides form according to the equations SrO+3C. .ı5o°C >SrC2+CO (17) CaO+3C '16O°C >CaC2+CO

(18)

İt was also reported by the same authors that the presence of calcium enhances the formation of iow-temperature SrC2 by the reaction CaC2 +SrO

=15OC

>SrC2 +CaO

(19)

There were no traces of monoxides of Sr and Ca in the biack ash samples obtained under any of the conditions investigated. At higher temperatures—lower than the atomization temperature and at ca 1230°C—Sr ion reacts with carbon to form hightemperature carbides: Sr + 2C - SrC2

(20)

This phenomenon suggests that a certain amount of the carbon that is added to the celestite concentrate is spent for the carbides. The formation of strontium carbide with the other carbides of Ca and Ba may contaminate the bulk SrCO3, which may be destined for high-value appiications. The maximum strontium recovery of approximately 90% is attributable to the low solubility of Sr(OH)2. Sudden decrease in temperature during the handiing of pregnant solution at the precipitation stage results in the rapid crystallization of Sr(OH)2 on the coid surfaces, causing metal losses in the SrCO3 precipitate.

Conclusions (1)The solid-state reaction of SrSO4 with carbon is initiated at the contacts at iow temperatures. With the formation of product SrS at the mineral interface the reaction of solid SrSO4 with solid carbon is inhibited. At temperatures greater than 900°C reduction is effected mainiy by gaseous CO. The change in the reaction rate is due to a high rate of CO generation. The rate of gaseous reaction siows at about 1100°C, and at high temperatures solid SrSO4-solid C reaction takes piace very slowly. (2)The amount of carbon in the mixture is the main contributor to the impurity content of the biack ash. Complete conversion of SrSO4 to SrS requires some excess carbon— typically 50% more than that stoichiometrically sufficient to convert 1 mole of SrSO4 to 1 mole of SrS according to reactions 12 and 13. Further carbon additions cause more carburization of the alkali earth metais found in celestite ore. (3)The conversion of SrSO4 to SrS is a very fast reaction at 1300°C. Reduction is compiete in 30 mm. İncrease in the roasting time causes the formation of carbides after 120 mm under static conditions. Partial conversion of sulphates to carbides, such as SrC2, CaC2 and BaC2, is dependent on the roasting time rather than on the amount of carbon added. C70

(4)The presence of dicarbide contributes to the Sr recovery in the form of SrCO3 since it hydroiyses to give Sr(OH)2. (5)Unavoidable Sr(OH)2 crystailization is responsible for metal losses during the transfer of pregnant Sr solution to the precipitation coiumn. (6)Celestite concentrate containing 95.5% SrSO4 can be advantageously treated by the black ash process to manufacture high- or chemical-grade SrCO3. References 1.Chen R. and Li D. Preparing saontium carbonate from strontianite by catalytic decomposition with reducing gases. Peop. Rep. China Patent CN 1,031,983. 1989, 5 p. 2.Zou X., Qiao X. and Fang J. Method for production of strontium salt using strontianite. Peop. Rep. China Patent CN 87,104,559. 1988, 7 p. 3. An Z. Process for producing strontium carbonate by calcination. Peop. Rep. China Patent CN 87,101,234. 1988, 6 p. 4. Griffiths J. Celestite and strontium chemical trade, the Mexican wave.Ind. Minerais, 301, 1992, 2 1-33. 5.Hong W. Celestite and strontianite: review of ore processing and exploitation. ind. Minerais, 309, 1993, 55-69. 6.Iwai M. and Toguri J. M. The leaching of celestite in sodium carbonate solution. Hydrornetall., 22, 1989, 87-1 00. 7.Kalafatoğlu 1. E., Ors N. and Kocakuşak S. Türk selestit cevherinden stronsiyum karbonat üretimi. İn Ikizler A. ed. Proceedings of IXth Symp. Chem. Chem.Eng.(Trabzon: Karadeniz Teknik Universitesi, 1993), 516. 8.Castillejos E. A. H., de la Cruz del B. F. P. and Uribe 5. A. The direct conversion of çelestite to strontium carbonate in sodium carbonate aqueous media. Hydrometall., 40, 1996, 207-22. 9.Cheng Z. and Jiang T. Production of SrCO3 by NH4HCO3 method without removing barium. Huadong Huagong Xueyuan Xueba., 18(6), 1994, 723-8. ChemicalAbsracts no. 120: 1 1079m. 10.Yalçın H. Ciay mineralogy and geochemistry of Sivas (Hafık district) evaporite basin, eastern interior Anatolia. Proc. VJlth Euroclay Conf., Dresden, 1991, 1185-90. 11.JCPDS Powder Diffracıion File. Aiphabetical İndexes: Inorganic Phases. Swarthmore, U. S. A., 1990. 12.Preisman L. Strontium compounds. İn Encyc. Chemical Technology (Wiley), 19, 1970, 50-4. 13.Meites L. An introduction to chemical equiibrium and kinetics (New York: Pergamon, 1981), 613. 14.Mortimer C. E. Chemistıy: a conceptual approach (New York: Van Nostrand, 1979), 797. 15.Cankut S. Ekstraktf Metalürji (Istanbul: Teknik Universite Matbaası,1972), 47. 16.Preil L. J., Sytris D. L. and Redfıeld D. A. Comparisorı of atomization mechanisms for Group İlA elements in electrothermal AAS.JAnalyiicalAtomic Spectrometry, 6(1), 1991, 25-32. Authors M. Erdemoğlu gained the degree of B.Sc. in mining engineerirıg from Hacettepe University, Ankara, in 1990. He was awarded a Ph.D. in mineral processing/hydrometallurgy by Cumhuriyet University, Sivas, Turkey, in 1996. Hc is currentiy employed as arı assistant professor in the Department of Mining Engineering at Inönü University, Malatya. Address: Department of Mining Engineering, Faculty of Engincering, University of Inönü, Malatya 44069, Turkey. M. Canbazoğlu gained the degree of B.Sc. in mining engineering from İstanbul Technical University in 1972. Hc was subsequently awarded a Ph.D. in hydrometallurgy by the Institut National Polytechnique de Lorraine, Nancy, France, in 1978. Hc was previously employed as metallurgist by ETİBANK and MTA in Ankara. Hc is Professor of Mineral Processing and Hydrometallurgy at Cumhuriyet University, Sivas. Hc is currently Dean of the Engineering Faculty at the same university. H. Yalçın gairıed the degrees of B.Sc., M.Sc. and Ph.D. in geological cngineering from Hacettepe University, Ankara, in 1980, 1984 and 1988, respectively. Hc has taught sedimentary petrology and gcochemistry as an associate professor in Cumhuriyet University since 1988. Hc was awarded research scholarships by the French Government in 1983 and 1993.