Smelting Furnace

The Smelting (S) Furnace is the first stage of copper production in the Mitsubishi Process. It is a continuous-operation furnace that produces a high-grade copper sulphide matte as well as a fayalite (silaceous) slag. As depicted above, the circular smelting furnace uses a number of vertical top-blown lances to deliver silica flux, recycled C-slag (from the C-Furnace), coke, and O2-enriched air directly into the furnace bath.

The lances are constructed of two concentric pipes made from high-chrome steel manufactured by Kubota Metal Corporation. The outer pipes rotate to prevent unwanted accretions, and carry the blowing air and particulates, injected through a central feed pipe, onto the center of the bath. The feed meets the oxidizing gas at the exit of the inside pipe. The mixture reacts in the molten bath to form matte and slag which continuously overflows through a single melt outlet and is delivered by a sloping launder into the Slag Cleaning (CL) Furnace.

The smelting reactions are:

2 CuFeS₂ + 4 O₂ Cu₂S + 2 FeO + 3 SO₂

FeS + 3/2 O₂ FeO + SO₂

FeS₂ + 9/2 O₂ FeO + 2 SO₂

The lances, in addition to injecting the S-Furnace feed materials and oxygen-enriched air, are themselves slowly consumed. Their tips are adjusted daily to around 0.3-0.5 m above the melt surface. Pneumatically controlled hoppers pulse-feed the solids to the top of each lance. The air-oxygen blowing rate gives the gas/solids mixture at the lance tip an average linear velocity of about 200 m/s.

In the melt surface below the lances, transient cavities, or "jet holes", continuously form and collapse, creating a turbulent, high-intensity reaction zone in the center of the furnace bath. Here virtually all of the feed particles are captured and smelted with minimal dust carry-over into the gas cooling and cleaning equipment. The Mitsubishi S-Furnace is an example of "bath" smelting - the inter-phase reactions all occur in the melt, as opposed to "flash" smelting where the reactions take place in the gas zone above the furnace bath level. Flash smelting has a tendency to over-oxidise iron and copper, resulting in more magnetite formation and higher copper losses in discard slag.

The SO2-rich off-gases pass through the furnace uptake into a waste heat boiler, then through electrostatic precipitators where they are cooled and cleaned before delivery to the acid plant. In normal operations the S-Furnace is autogenous, taking advantage of the exothermic energy released by oxidation of iron and sulphur; only a small amount of coke is used to aid in the bath reactions.

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