Oxidation Mechanism of Molybdenite Concentrate
- PDF / 681,801 Bytes
- 7 Pages / 593.972 x 792 pts Page_size
- 74 Downloads / 206 Views
BACKGROUND
FOR the implementation of a roasting process of molybdenite concentrates at the Altonorte smelter in Chile, basic research on the kinetics and mechanisms of oxidation has been carried out. Industrially, the objective of the multihearth roasters is to produce molybdenum trioxide (MoO3) with low sulfur contents. Marin et al.[1] carried out MoS2 roasting on this concentrate using small disks with a thin layer of sample in a muffle furnace in air in order to simulate reactions in the multihearth roaster. It was found that the rate of oxidation is fairly slow, below 440 C, and then increases and remains fairly constant from about 540 C to 640 C. A two-step reaction mechanism involving the intermediate formation of MoO2 was proposed. The roasted samples tended to agglomerate somewhat at all temperatures, while unreacted concentrate remained as loose powder. At and above 620 C, melting/glazing started to take place and difficulties were experienced in removing the samples from the crucible bottoms. MoO3 sublimation only became significant above 650 C. A kinetic model involving various molybdenum species was proposed: MoS2 MoO2 MoO3 Coudurier et al.[2] carried out MoS2 concentrate roasting in a pilot multihearth roaster with an inner diameter of 46 cm and with 4 hearths. Based on their work, they found that the species MoS2, MoO2, and MoO3 were all involved. In tests lasting 4 hours, they found that the MoS2 concentration drops, nearly
T. UTIGARD, Professor, is with the Materials Science and Engineering Department, University of Toronto, Toronto, Canada M5S 3E4. Contact e-mail: [email protected] Manuscript submitted January 13, 2009. Article published online May 12, 2009. 490—VOLUME 40B, AUGUST 2009
linearly for the first 2 hours to a low value, while the MoO2 concentration increases for about the first 80 minutes, then reaches a maximum and drops. The MoO3 content first only slowly increases, before it suddenly starts to increase very rapidly between 80 and 120 minutes. They concluded that the roasting rate depended on oxygen gas-phase diffusion when the sulfur content was high, and then switched to some surface reaction control. Ammann and Loose[3] carried out molybdenite concentrate roasting by placing a small sample of concentrate on a porous stainless steel plate and flowing air at a high flow rate downward through the sample. They carried out measurements between 525 C and 635 C and concluded that chemical rate control dominated up to 70 to 80 pct conversion. At higher conversions, diffusion was assumed to be the rate limiting step. They also concluded that, in the early stages, MoO2 was formed preferentially to MoO3, as previously also suggested by other investigators.[1,2,4,5] They also showed that the rate was nearly proportional to the pct O2 in the gas, and that the slowdown in rate started at about the same conversion independently of the oxygen content. Wilkomirsky et al.[6] carried out oxidation of molybdenum concentrate oxidation in a hot-stage microscope and visually observed the rapid gr
Data Loading...
