The smelting of the nickel ore for a crude matte

The partially roasted ore is smelted with the correct proportions of coke and siliceous fluxes in blast-furnaces so as to yield a matte of iron, copper, and nickel sulphides. In some cases, the roasted ore is self-fluxing. In the smelting the pyrrhotite in these ores is almost entirely converted into iron silicate in the smelting furnace. The ferric oxide present is reduced in the furnace by the sulphur of the pyrites to form ferrous oxide, which, in the presence of silica, forms a slag: FeS + 3Fe₂O₃ + nSiO₂ = SO₂ + 7FeO.nSiO₂. Any nickel monoxide which may be present reacts with an equivalent amount of ferrous sulphide to form nickel sulphide and ferrous oxide, which in turn passes into slag.

The copper and nickel in the slag range up to about 0.4 per cent. The slags may be rejected, or part may be used again in similar smeltings, or in later stages of the concentration process. The matte must contain sufficient iron to prevent nickel passing into the slag.

The precious metals in the ore accumulate in the matte, and in the latter case, there were present 1.90 ozs. of silver, 0.35 oz. of platinum, and 0.35 oz. of palladium per ton. According to G. P. Schweder, the sulphur in the matte is present as mono-sulphides of silver, copper (ous), nickel, and iron; and if insufficient sulphur is present to form NiS, and FeS, the excess of metal dissolves in the molten sulphide. Any nickel silicate which may be formed is decomposed by the iron sulphide to form iron silicate and nickel silicate, and so long as enough iron sulphide is present, only a very small proportion of nickel can pass into the slag - prills of matte may be imprisoned in the slag if its viscosity in the furnace is too great. If the ores have been over-roasted nickel will appear in the slag, and in that case some unroasted ore is mixed with the furnace charge. Copper silicate behaves like nickel silicate, but the cobalt silicate does not react so easily with the iron sulphide, and when cobalt silicate is produced, it will pass into the slag. Any antimonates or arsenates are reduced and volatilized in the furnace; if only small proportions of arsenic and antimony are present, they pass into the matte. Gypsum in the ore is reduced to sulphide, and it acts like iron sulphide on iron and copper oxides. V. Tafel and F. Klewata discussed the reaction.