Lime-enhanced hydrogen reduction of molybdenite
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I.
INTRODUCTION
TRADITIONAL routes for molybdenum extraction consist of a large number of processing steps and the generally used practice I comprises roasting of molybdenite concentrate, followed by chemical upgrading/purification of the resultant M o O 3 calcine, and finally hydrogen reduction of the trioxide to the metal in two/three stages. In such pyro-cumhydrometallurgical practices, the overall recovery is not high and one has to contend with SO2 pollution of the environment. In the last two decades numerous alternative approaches have been proposed for the production of molybdenum from MoS2 concentrates and these include (a) direct dissociation of molybdenite2'3'4 (at high temperature and under high vacuum), (b) metallothermic reduction5-9 (e.g., MoS2 reduction by Sn, Pb, Na, or AI under suitable conditions), (c) fused salt electrowinning 1~ directly from MoS2 or via suitable Mo intermediates (like sesquisulfide, trioxide, or carbide), and (d) chlorination. 13So far, none of these methods has gained any commercial acceptance. In recent years yet another new concept receiving serious attention for the winning of metals from their sulfides is their direct reduction by hydrogen in the presence of powerful sulfur acceptors like lime (e.g., lime-hydrogen reduction of chalcopyrite, 14.15.16 pentlandite, 17 stibnite18). The potentialities of such an approach have recently been reviewed by the present authors.19 The hydrogen reduction of MoS: in the presence of lime may be represented by 0.5MoS2(s) + CaO(s) + H2(g)-~0.5Mo(s) + CaS(s) + HEO(g), [ 1] which is, in fact, a sum of the following two individual reactions: T. R. MANKHAND, Lecturer in Metallurgy, and P. M. PRASAD, Professor of Process Metallurgy, are both with the Department of Metallurgical Engineering, Institute of Technology, Banaras Hindu University, Varanasi221005, India. Manuscript submiued April 27, 1981. METALLURGICAL TRANSACTIONS B
0.5MoSE(S) + H2(g)---~0.5Mo(s) + H2S(g)
[2]
CaO(s) + HES(g)---~CaS(s) + HEO(g).
[3]
The direct hydrogen reduction of MoS2 as per reaction [2] is thermodynamically unfavorable. The incorporation of lime, which is an effective H2S absorbent, in the reaction charge (MoS2) tremendously enhances the possibility of reduction. This is so because the equilibrium constant (KE)for the direct hydrogen reduction of MoS2 at 1200 K is only 2.76 • 10 -3, whereas that for lime-hydrogen reduction (scavenged reduction) of MoS2 as per reaction [1] is three orders of magnitude higher (Kl = 1.40 at 1200 K), owing to the high HES fixation ability of lime (K3 = 5.08 • 102 at 1200 K).* Compared with direct reduction, another interesting aspect of scavenged reduction of MoS2 is that it possesses favorable energetics as evidenced from the heat of reaction values;* e.g., at 1200 K, A/-/~ = + 39.5 kJ as *Calculated from References 20 and 21.
against ~ = + 105.3 kJ. A further noteworthy feature of the said approach is that the offgas from the reactor is expected to consist essentially of water vapor, which is a nonpolluting fluid. In a pione
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