An analysis of slag stratification in nickel laterite smelting furnaces due to composition and temperature gradients
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INTRODUCTION
In the production of ferronickel from nickelcontaining lateritic ores, the ore is calcined at temperatures up to 1000 ~ before being smelted in large electric furnaces. The required reduction may take place during the calcination and smelting stages or only during smelting. From a smelter feed of 1.5 to 3 wt pct Ni, a ferronickel product with 20 to 42 wt pct Ni is formed together with large amounts of slag. Generally, more than 90 pct of the feed to the electric furnaces reports as slag. rlj Because of the large tonnage treated, it is not economical to add fluxes to modify the slag composition. Therefore, the ore grade and composition are the main factors determining the feasibility of smelting an ore. t2~ The main components of the slags formed are FeO, MgO, and SiO:. Although the ore contains other compounds such as CaO and A1203, it is convenient to refer to the phase diagram shown in Figure 1 where only the main components (FeO, MgO, and SiO2) are considered.t2] The plant and pilot plant data show that the compositions may vary over a large composition range, with FeO contents from less than 10 wt pct to more than 60 wt pct and with liquidus temperatures varying from less than 1300 ~ to approximately 1700 ~ Table I shows some key operating parameters of major Fe-Ni producing facilities, ~3j including information on both slags and the metal/matte products. The energy needed to melt and reduce the calcine is liberated within or above the liquid slag, and heat has to be transported vertically downward into the metal/matte layer in order to maintain a liquid product. Therefore, the slag must he maintained hotter than the metal/matte 9 For slags of low liquidus temperatures, this may mean that they have to be superheated to a large extent, making them difficult to contain within a refractory-tined T. UTIGARD, Assistant Professor, is with the Department of Metallurgy and Materials Science, University of Toronto, Toronto, ON, Canada M5S 1A49 Manuscript submitted June 23, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS B
furnace. From Table 1, it is observed that the metal/ matte tapping temperature can be 160 ~ below the slag liquidus temperature. Because the thermal conductivity of the metal is more than 10 times higher than that of the liquid slag, we expect that a fairly steep temperature gradient may develop through the slag layer. This is supported by the results shown in Figure 2 from a study on a 19.5 MW furnace with six in-line electrodes used for the smelting of copper-nickel concentrates, t41 A temperature gradient of approximately 100 ~ is observed through the slag layer. For this furnace, it has been reported 141 that buildup of a chromite layer at the slag/ matte interface was a problem. By the use of radioactive tracers, Urquhart e t al. m found that approximately 34 pct of the slag behaves as a dead volume. In a similar study on a reverberatory furnace, Themelis and Spira t51 found a dead volume of 80 pct. These observations indicate that the slag in the vicinity of the metal/matte int
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