Carbothermic reduction of mechanically activated celestite

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Abstract

Effects of dry milling on the carbothermic reduction of celestite were examined. Celestite and coke mixture was milled up to 120 h in a planetary ball mill. Unmilled and milled mixtures and their black ashes from carbothermic reduction were characterized by a combination of X-ray diffraction (XRD) analysis, scanning electron microscope analysis, thermogravimetric analysis, particle size analysis and leaching tests. XRD diffraction peaks for celestite in the milled mixtures are lower and broader than those for the unmilled mixture, mainly due to a disordering of celestite crystal structure. Size of the particles significantly decreased by 1 h of milling, so that d50 decreased from 79.4 to 6.1 µm. But, milling for longer periods shifted the particle size distribution to coarse size region, mainly due to the agglomeration of fine particles. By milling the celestite–coke mixture for 1 h, formation temperature of SrS decreased from 957° to 900 °C, whereas 24, 72 and 120 h of milling decreased nearly to 700 °C. At low roasting temperatures, the mass loss by reduction increased with extending of the milling and the milled mixtures gave a high degree of soluble strontium in the black ash. However, sintering process prevailing at higher temperatures during the roasting of the milled mixtures became active, and the effects of disordering of the celestite structure vanished and subsequently the reduction reaction was retarded. It was concluded that although milling in a planetary ball mill produces more X-ray amorphous SrS, together with SrC2, carbothermic reduction reactions occur at lower temperatures as the milling time increases.

Introduction

Strontium carbonate (SrCO3) is an additive in the production of faceplate glass of color TV picture tubes and a constituent of ferrite (i.e., SrFe2O4) ceramic magnets for small DC motors. Other end uses are in the production of iridescent and special glasses, pyrotechnics, pigments, paints, driers and the production of strontium metal, and all other strontium compounds. However its consumption for faceplate glass is getting to decline because of the increased popularity of flat-panel television monitors.

The most common commercial process currently employed for the production of strontium carbonate from celestite ores is the black ash process. In this technique, celestite is converted to strontium sulphide (SrS) nearly at 1100°–1200 °C in the presence of metallurgical grade coke or petroleum coke. Chemical reactions occurred in the kiln result in carbothermic reduction of celestite. Final product of the roasting is a black ash and it contains mainly SrS which easily decomposes in hot water. After leaching the black ash with hot water and removal of insoluble metal sulphides, and other impurities, pregnant solution from leaching is finally reacted either with carbon dioxide or sodium carbonate to precipitate strontium carbonate. Details of the water leaching of black ash and the precipitation of SrCO3 have been extensively discussed by Erdemoğlu and Canbazoğlu (1998) and Owusu and Litz (2000).

Fundamental conditions of the carbothermic reduction have been investigated and discussed in detail by Erdemoğlu et al. (1998) and Sonawane et al. (2000). The first chemical reaction occurring at low temperatures (> 400 °C) is the solid-state reaction given as,SrSO4 + 4C  SrS + 4CO

Since the mineral celestite and coke are solids which do not easily fuse, contact between them is poor and the reaction is slow. With the formation of product SrS at the mineral surface, the solid–solid reaction is inhibited. The CO generated diffuses and reacts with celestite which is not in contact with carbon, according toSrSO4 + 4CO  SrS + 4CO2

The CO2 diffuses back into the carbon to generate more CO, according to the “Boudouard” reaction:CO2 + C  2CO

In the solid-state reaction of celestite, CO is a gaseous intermediate and since the interface between gaseous CO and solid SrSO4 takes place readily, CO is the principal reducing agent. Thus, the solid-state reaction between SrSO4 and C is of little importance. Plewa et al. (1989) has concluded that SrSO4 can be transformed into SrS at temperatures above 777 °C under the influence of CO and the outcome of this reduction depends on the CO2  CO concentration ratio in the gas phase. Erdemoğlu et al. (1998) and then Sonawane et al. (2000) studied the reduction reaction using a stoichiometric amount of carbon and through intermediate temperatures ranging from 600 °C to 700 °C at the loading zone and from 1200 °C to 1300 °C at the discharging zone of a rotary kiln. Due to the high discharging-zone temperature, the process has some disadvantages: (1) high electric energy requirement, (2) sintering of the reactants and products, and (3) difficulties in the selection of the material of construction for the reactor system for a commercial plant.

The rate of carbothermic reduction reactions can be greatly increased by premilling the mineral and carbon together when compared to with powders milled separately and then mixed (Welham, 1998, Welham, 2000, Welham, 2002). It has been also shown by Baláž and Ebert (1991) and Tkáčová et al. (1990) that mechanical activation is established to be successful for intensification of the thermal processes of sulfide minerals, such as oxidation, decomposition in inert atmosphere, and sublimation. The lowering of reaction temperatures, the increase rate and amount of solubility, preparation of water soluble compounds, the necessity for simpler and less expensive reactors and shorter reaction times are some of the advantages of mechanical activation. For example, the effect of extended milling on carbothermic reduction of manganese ore was investigated by Welham (2002). Milling of manganese ore with graphite led to enhanced reduction at decreased temperatures. The study of carbothermic reduction of hematite in air revealed that milling at ambient temperature increases the rate of reaction (Raygan et al., 2002). As stated by Welham (2002) that raise in the rate leads to a larger amount of material reduced per unit time and therefore increases the throughput of the thermal reduction stage. For that reason, if the mechanical activation of celestite–coke mixture intensifies the reduction in the kiln; existing plants may simply incorporate an appropriate mill prior to the kiln, with no any replacement of their current devices.

In a preliminary study, effect of mechanical activation on the X-ray diffraction traces of roasted celestite which was previously milled together with anthracite in a centrifugal mill was examined. Only depending on the results of XRD analysis of the black ash samples obtained by roasting, it was found that SrS begins to appear in the black ash at 600 °C, while 800 °C is required for the unmilled mixture.

In this circumstance, the present study is intended to contribute to the understanding of the effects of mechanical activation on the carbothermic reduction of celestite. For this purpose, fundamental characteristics of the intensively milled celestite–coke mixtures and their black ashes were examined.

Section snippets

Materials and methods

Samples of a commercial celestite concentrate and metallurgical grade coke were used in the study. Chemical analysis showed that the celestite concentrate contains about 95.50% SrSO4 (celestite), 3.25% CaSO4·2H2O (gypsum), 0.77% BaSO4 (barite) and 0.31% Fe2O3. Particle size distribution of the concentrate is as cumulative undersize %: 99.60, 2 mm; 9.28, 0.106 mm. d50 value is 0.380 mm. The chemical composition of the coke is as %: 82.97 C; 0.46 S and 15.92 ashes. Particle size distribution of

Results and discussion

The celestite–coke mixtures were first milled by a planetary ball mill for different periods and then fundamental characteristics of each milled mixture was determined by investigating the structural, morphological and thermal changes arising from the milling. Subsequently, milled mixtures were isothermally heated to obtain their black ashes that will be subjected to leaching tests for determination of the degree of conversion from celestite to SrS. For comparison, a well-mixed and unmilled

Conclusions

Celestite and coke mixture was milled up to 120 h in a planetary ball mill, in order to determine the effects of extended dry milling on the carbothermic reduction of celestite. Milling promoted partial amorphisation of solids along with some structural distortions in celestite. Hence, carbothermic reduction of celestite was improved. SEM and particle size analysis revealed that extended milling resulted in the formation of particle agglomerates. Rates of mass losses for the TGA patterns showed

Acknowledgements

The author would like to thank Prof. Dr.-Ing. habil. Eberhard Gock for his principal contribution to perform this subject. Special thanks to Dr. N. Metin Can, Dr. Meltem Asiltürk, Petra Sommer and Mustafa Birinci for their unlimited help in the laboratory.

References (28)

  • WelhamN.J.

    Activation of the carbothermic reduction of manganese ore

    Int. J. Miner. Process.

    (2002)
  • BalážP.

    Extractive Metallurgy of Activated Minerals

    (2000)
  • BalážP.

    Mechanochemistry in Nanoscience and Minerals Engineering

    (2008)
  • CankutS.

    Extractive Metallurgy

    (1972)
  • Cited by (0)

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