Effect of a catalyst on the kinetics of reduction of celestite (SrSO 4 ) by active charcoal
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I. INTRODUCTION
CELESTITE ore is a unique starting material for the manufacture of a variety of Sr chemicals such as SrCO3, SrCl2, Sr(NO3)2, Sr(OH)2, SrO, etc. In the so-called “blackash process,” the reduction of celestite is carried out in a rotary kiln at 1200 8C to 1300 8C in the presence of carbon/ coke as a reducing agent. The present invention is related to an improvement in the black-ash process for the recovery of water-soluble SrS from celestite ore (SrSO4) with the help of cheap catalysts. In the course of reduction, the prime reducing agent is carbon monoxide, which, in turn, is generated in situ by the reduction of carbon dioxide by carbon. The reactivity of coal is generally co-related by its reactivity with CO2 to generate CO. By using highly reactive coal/coke, it is possible to bring down the reduction temperature, resulting in greater energy savings. Most of the coal/coke used for the reduction process is less reactive, and the reactivity can be increased to a great extent by incorporating alkali and alkaline-earth metal salts. The reaction of CO2 with coal (impregnated with inorganic salts) is enhanced considerably, and this imparts a high reactivity. The reduction of barite and phosphogypsum using carbon with impregnated inorganic salts was studied extensively by Kale et al.[2] Earlier studies on the reduction of celestite by carbon have been very limited. Few scientists have carried out mechanistic studies of the reduction of SrSO4 by carbon. It is accepted that the initial reduction of celestite with carbon takes place according to SrSO4 1 2C { SrS 1 2CO2
[1]
where both celestite and carbon are in contact. The CO generated diffuses and reacts with celestite, which is not in contact with carbon, according to SrSO4 1 4CO { SrS 1 4CO2
[2]
R.S. SONAWANE and S.K. APTE, SST-IVs, and B.B. KALE, ST-IV, are with the Centre for Materials for Electronic Technology, C-MET, Pune411 008, India. M.K. DONGARE, Scientist-E, is with the National Chemical Laboratory, Pune-411 008, India. Manuscript submitted March 1, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS B
The CO2 diffuses back into the carbon to generate more CO, according to the Boudouard reaction: CO2 1 C { 2CO
[3]
Thus, in the solid-state reaction of celestite, CO is a gaseous intermediate. Pedak et al.[8] studied the reduction of celestite, using a reducing agent like H2 and a mixture of H2 and CO, at high temperatures. This reduction suffers from two main disadvantages: (1) a large excess of gaseous reductant is required, and (2) the nature of the solid and gaseous product is quite varied, depending upon the composition of the reducing gases and the reaction condition. Gitis et al.,[1] Lepsin et al.,[7] and Erdemoglu et al.[10] studied the reduction reaction using a stoichiometric amount of carbon and through intermediate temperatures ranging from 600 8C to 700 8C at the loading zone and from 1200 8C to 1300 8C at the discharging zone of a rotary kiln. Due to the high discharging-zone temperature, the process has some disadvantages: (1) a hi
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