Sulfate Formation and Decomposition of Nickel Concentrates
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VALE Inco’s smelter in Thompson, Manitoba and Xstrata Nickel’s smelter in Falconbridge, Ontario produce nickel mattes from nickel sulfide concentrates. The first step in their processes is fluid bed roasting where a significant portion of the sulfur in the feed is oxidized to SO2. The resulting, partially oxidized concentrate, which is also called calcine, is collected in the off-gas handling system and transferred to electric furnaces together with a silica flux for smelting. The matte produced in the electric furnace is transferred by ladle to Pierce-Smith converters, where most of the remaining iron sulfide is removed by further oxidation, forming a Bessemer matte, which is the product from the smelters.[1–4] A schematic of a typical roast-smelting-converting process is given in Figure 1. During roasting and smelting of the nickel sulfide concentrates, a large amount of SO2 is formed. One of the main oxidation reactions is 3FeS þ 5O2 ¼ Fe3 O4 þ 3SO2
½1
The SO2 generated is normally collected and used to produce sulfuric acid (H2SO4) and sold as a by-product. The calcine leaving the fluid bed roasters is commonly collected in cyclones, wet scrubbers, and electrostatic precipitators, before being transferred to electric furnaces for smelting. During these steps, the calcine cools and is exposed to gases of different compositions depending on the extent of ingress air. Based on the
R. PANDHER, Metallurgical Engineer, is with Xstrata Nickel, Toronto, Ontario M5X 1E3, Canada. S. THOMAS and D. YU, Graduate Students, M. BARATI, Assistant Professor, and T. UTIGARD, Professor, are with the Department of Materials Science and Engineering, University of Toronto, Toronto, Ontario M5S 3E4, Canada. Contact e-mail: [email protected] Manuscript submitted May 21, 2010. Article published online January 5, 2011. METALLURGICAL AND MATERIALS TRANSACTIONS B
conditions and on the particle size distribution of the calcine, metal sulfates may form during these steps. Iron sulfates could form by the following reactions: FeS þ 2O2 ¼ FeSO4
½2
2FeS þ 5O2 þ SO2 ¼ Fe2 ðSO4 Þ3
½3
Nickel and copper sulfides may also form their respective sulfates (NiSO4 and CuSO4).[4–7] This means that the calcine being transferred to the electric furnaces consists of a mixture of unreacted sulfides in addition to various oxides and sulfates.[8–12] One important aspect of this is that during the subsequent heating in the calcine banks of the electric furnace, these sulfates will decompose releasing SO2 according to reactions such as Fe2 ðSO4 Þ3 ¼ 2FeS þ 5O2 þ SO2
½4
2FeSO4 ¼ Fe2 O3 þ 0:5O2 þ 2SO2
½5
In addition, SO2 is also formed by the solid-state reaction FeS þ 10Fe2 O3 ¼ 7Fe3 O4 þ SO2
½6
All these reactions will proceed as the temperature increases and SO2 gas is released into the freeboard of the electric furnace. Because of the large amount of other gases formed or ingressed into the furnace, the furnace freeboard gas is dilute in SO2, which makes it very difficult to collect economically and trap this SO2. Therefore, the normal practice is t
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