Oxidation aspects of the lime-concentrate-pellet roasting process
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I.
INTRODUCTION
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INCREASING anthropogenic emission of gaseous sulfur compounds into the environment has resulted in a spate of developments for handling the "sulfate" problem. To meet abatement standards in the extraction of copper, combined pyrometallurgical and hydrometallurgical processing have featured prominently. 1~2,3One such development, the LimeConcentrate-Pellet-Roast Process (L.C.P.R.-Process), 4-s involves fixation of sulfur in copper sulfide concentrates at the source in an innocuous form using lime:
I
.
0
I
I
l
I
Co(oHb= c ~ ( s 6 )
2o ~,0 6O
CuFeS2 +
_~
3 02 + ~Ca(OH)2
m
~80 1 F e203 -~- ~-CaS 3 O 4 -}- s3H 2 0 1 C u O ' C u S O 4 -~ -~-
,,r
[1]
1
100
Copper concentrates and lime in granulated and ungranulated form were first investigated by the Rumanians at temperatures in the range 973 to 1123 K . 9 Subsequent application of alkalies to the sulfatizing step of the RoastLeach-Electrowinning (R.L.E) Process was proposed by the United States Bureau of Mines.l~ Ca(OH)2 was found to be most effective in retaining up to 98 wt pct of the sulfur as anhydrite (Table I). Bartlett and Haung 4 adapted pelletized feed which improved both sulfur retention and temperature control within the system. However, later examinations of the L.C.RR.-Process using sulfuric acid leaching 5-8 (Table I) have had difficulties in achieving satisfactory metal recoveries. The present investigation was undertaken to examine oxidation rates and to attempt to establish those factors affecting copper recoveries. II.
T H E R M O D Y N A M I C CONSIDERATIONS
Figure 1 shows an Ellingham type diagram 12for a number of sulfation reactions in the L.C.P.R.-Process (per two moles of SO2). Disregarding trace impurities, the process may be regarded as a five-component system. Figure 1 indicates that CaO will be sulfated in preference to iron and copper oxides at all temperatures of interest. This postulates that the presence of CaO in the roast system would result in a deficiency of SO2 for the sulfation of other metals. H. AHMADZAI and L-I STAFFANSSON are Research Assistant and Professor, respectively, at the Department of Theoretical Metallurgy, Royal Institute of Technology, S-100 44, Stockholm, Sweden. S. BLAIRS and B. HARRIS are both Senior Lecturers at the School of Metallurgy, The University of New South Wales, P. O. Box 1, Kensington, New South Wales 2033, Australia. Manuscript submitted May 7, 1982. METALLURGICAL TRANSACTIONS B
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i 120 i
s .,0
lg0 750
so2_::, Loo..
- - I ~
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BOO
850
900
l
950
1000
1050
Temperature (K) Fig. l - - E l l i n g h a m diagram for sulfation reactions in the Cu-Fe-Ca-S-O system with respect to 2 moles of SO2. Numbers on the lines represent data sources.
Predominance area diagrams 13'14 of separate M-O-S systems may be superimposed to obtain an indication of the most stable compound of each metal.15'16 For the L.C.P.R.Process, Cu-Fe-Ca-S-O predominance area diagrams have been delineated from the respective M-S-O diagrams at 800 K and 1000 K (Figures 2(a) and 2(b))
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