Matte Snielting Fundamentals 67 oxidize, Eqn (4.11) The reactions are exothermic, and the energy they generate heats and melts the products The contact time between concentrate particles and the gas is short (a few seconds), so ensuring good reaction kinetics is essential Nearly all smelters accomplish this by mixing the concentrate with the gas prior to injecting it into the smelting furnace The use of oxygen+nriched air instead of air also improves reaction kinetics, and is increasingly popular Use of oxygen-enriched air or oxygen also makes the process more autothermal Because less nitrogen is fed to the furnace, less heat is removed in the offgas This means that more of the heat generated by the reactions goes into the matte and slag As a result, lcss (or no) hydrocarbon fuel combustion is required to ensure the proper final slag and matte temperature, -1250°C A new method for contacting concentrate and O2 is being used in submerged tuyere smelting furnaces In these furnaces, concentrate is blown into a mixture of molten matte and slag, and the oxidation process takes place indirectly This is discussed in Chapters and (b) Letting the matte settle through the d a g luyer into the matte layer below the slag Most smelting furnaces provide a quiet settling region for this purpose During settling, FeS in the matte reacts with dissolved CuzO in the slag by the reverse of Reaction (4.12): FeS + in matte CuzO in slag + FeO in slag + Cu2S in matte (4.15) This further reduces the amount of Cu in the slag The importance of low slag viscosity in encouraging settling has already been mentioned Keeping the slag layer still also helps A trade-off is at work here, too Higher matte and slag temperatures encourage Reaction (4.15) to go to completion and decrease viscosity, but they cost more in terms of energy and refractory wear (c) Periodically tapping the matte and slag through separate tap holes Feeding of smelting furnaces and withdrawing of offgas is continuous Removal of matte and slag is, however, done intermittently, when the layers of the two liquids have grown deep enough The location of tap holes is designed to minimize tapping matte with slag 4.5 Smelting Products: Matte, Slag and Offgas 4.5 I Matte In addition to slag compositions, Table 4.2 shows the composition of mattes 68 Extractive Metallurgy of Copper tapped from various smelters The most important characteristic of a matte is its grade (mass% Cu), which typically ranges between 45 and 75% Cu (56-94% Cu2S equivalent) At higher levels, the activity of CuzS in the matte rises rapidly, and this pushes Reaction (4.12) to the right Fig 4.6 shows what happens as a result The rapidly increasing concentration of Cu in slag when the matte grade rises above 60% is a feature many smelter operators prefer to avoid However, producing higher-grade mattes increases heat generation, reducing fuel costs It also decreases the amount of sulfur to be removed during subsequent converting (decreasing converting requirements), and increases SOz concentration in the offgas (decreasing gas-treatment costs) In addition, almost all copper producers now recover Cu from smelting and converting slags, Chapter 11 As a result, production of higher-grade mattes has become more popular Most of the rest of the matte consists of iron sulfide (FeS) Table 4.3 shows the distribution of other elements in copper concentrates between matte, slag and offgas Precious metals report almost entirely to the matte, as most Ni, Se and Te 4.5.2 Slag As Table 4.2 shows, the slag tapped from the furnace consists mostly of FeO and SO2, with a small amount of ferric oxide Small amounts of AI2O3,CaO and MgO are also present, as is a small percentage of dissolved sulfur (typically less than one percent) Cu contents range from less than to as high as percent Higher Cu levels are acceptable if facilities are available for recovering Cu from smelter slag Si02/Femass ratios are usually 0.7-0.8 4.5.3 Offgas The offgas from smelting contains SOz generated by the smelting reactions, N2 from the air used for oxidizing the concentrate and small amounts of COz, H and volatilized impurity compounds The strength of the offgas is usually 10 to 60 vol% SOz The strength depends on the type of O2