Reductant selection in ferro-alloy

Reductant selection in ferro-alloy - Reductant selection in...

Info iconThis preview shows pages 1–2. Sign up to view the full content.

View Full Document Right Arrow Icon
33 The Journal of The South African Institute of Mining and Metallurgy JANUARY/FEBRUARY 2002 Introduction In the production of ferroalloys in submerged- arc furnaces, the primary function of the carbonaceous material (mainly coke and coal) is to act as reductant: to react with the metal oxide to form metal and carbon monoxide. For example, in the production of ferromanganese the overall manganese reduction reaction can be written as [1] Standard free energy changes for the reduction of Cr 2 O 3 , MnO, SiO 2 and FeO by carbon are shown in Figure 1. Figure 1 suggests that the only real condition for the carbothermic reduction of these oxides is to heat the oxide-reductant mixture to a high enough temperature. Viewed in this simplistic way, the only requirement of a reductant for ferroalloy production (apart from the requirement that the reductant must contain carbon!) would be that it has an acceptably low level of impurities (such as sulphur and phosphorus) which would otherwise contaminate the product. While the chemical composition of the reductant is no doubt important 1 , the practical use of a mixture of reductants and of considerable amounts of relatively expensive coke indicates that there is more to the choice of reductant than simple compositional considerations. (Typically, 40% of the reductant is coke in the case of ferrochromium production, with the cost of the reductant sometimes more than that of the chromite ore 1 .) The reasons for choosing reductants based on more than composition arise from the actual steps which make up the overall reduction reaction, and from the electrical function. These are briefly reviewed below. Reaction steps in ferroalloy production The reaction sequences in the production of ferromanganese, ferrosilicon and ferrochromium were well studied in a remarkable series of local projects, mainly in the late 1970s 3–8. I cannot give justice to the full range of that work here. Rather, I restrict myself to summarizing the reactions which the reductant is likely to undergo. While this paper aims to identify general themes, it is clear that the reaction sequences in the production of different ferroalloys are quite different, and hence I mention these under separate headings (covering only three ferroalloys which I consider typical). Ferromanganese Excavation of a 75 MVA furnace following shutdown and water quenching 3,6 revealed several different reaction zones within the furnace (Figure 2). In the upper part of the ‘reaction cone’ around the electrodes, gaseous reduction (by CO) of mainly the FeO in the ore occurs, yielding porous ore particles containing solid metallic iron. Formation of a primary slag temporarily halts this reaction. Reduction resumes at higher temperatures, through reaction with carbon dissolved in the metal, and—lower in the furnace—with solid carbon.
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Image of page 2
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 10/08/2009 for the course CME MAT E 630 taught by Professor Dr. during the Fall '09 term at University of Alberta.

Page1 / 4

Reductant selection in ferro-alloy - Reductant selection in...

This preview shows document pages 1 - 2. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online