Smelting furnace

Electrodes. Because of the numerous different processes, there are many different types of electrodes in use (9), eg, prefabricated graphite, prefabricated carbon, self-baking, and composite electrodes (see Carbon). Graphite electrodes are used primarily in smaller furnaces or in sealed furnaces. Prebaked carbon electrodes, made in diameters of <152 cm or 76 by 61 cm rectangular, are used primarily in smelting furnaces where the process requkes them. However, self-baking electrodes are preferred because of thek lower cost. 

Because intense heat is generated in these furnaces it is understandable that the arc volatilizes such metals as tin, zinc, lead, cadmium, and the like. In addition, both melting and smelting furnaces may generate large amounts of carbon monoxide. As a result all new furnace installations require pollution... 

Fig. 4. Schematic illustration of an Imperial smelting furnace plant. LHV = low heating value.

The high lead slag from the smelting furnace is tapped continuously and transferred down a heated launder directly into the reduction furnace through a port in the side of the vessel. Lump coal for reduction is fed continuously to the furnace by conveyor and dropped direcdy into the bath. Heating for the endothermic reduction reactions is provided by oil injected down the lance. The combustion air stoichiometry is set at 95% of that required for complete oil combustion. Air is injected into the top of the furnace to afterbum the volatile materials from the coal and provide additional heat to the top of the furnace. Reduction temperatures range from 1170 to 1200°C to maintain slag duidity. 

The off-gas from each furnace is cooled in an evaporative gas cooler and cleaned in a reverse pulse baghouse before being either vented to atmosphere or used in manufacturing sulfuric acid. The baghouse dust from both the smelting and reduction furnaces is combined and recycled through the smelting furnace. 

Ore Size. The particle size of manganese ores is an important consideration for the smelting furnace. In general, the ore size for the furnace charge is —75 mm with a limit to the amount of fines (—6 mm) allowed. Neither electric furnaces nor blast furnaces operate satisfactorily when excessive amounts of fines are in the charge. 

Reduction to Liquid Metal. Reduction to Hquid metal is the most common metal reduction process. It is preferred for metals of moderate melting point and low vapor pressure. Because most metallic compounds are fairly insoluble in molten metals, the separation of the Hquified metal from a sohd residue or from another Hquid phase of different density is usually complete and relatively simple. Because the product is in condensed form, the throughput per unit volume of reactor is high, and the number and si2e of the units is rninimi2ed. The common furnaces for production of Hquid metals are the blast furnace, the reverberatory furnace, the converter, the flash smelting furnace, and the electric-arc furnace (see Furnaces, electric). 

Fuming is also an alternative to roasting in the processing of low grade concentrates (5—25 wt % tin). This procedure yields a tin oxide dust, free of iron, which is again fed back to a conventional smelting furnace. 

Roasting . Roasting has been largely abandoned ia modem copper smelters, ia which this function is combiaed with the smelting furnace. In older systems, the multiple-hearth roaster is a brick-lined tower having horizontal brick hearths. The concentrate is iatroduced at the top hearth, where rotating arms with rabble blades turn it over and move it to holes ia the hearth. The concentrate is transferred successively to lower hearths and finally... 

The sihca dux combines with iron(II) sulfide and iron(II) oxide to form slag. The duidity of the slag, in which unwanted impurities dissolve, is controlled by the addition of limestone. Reverberatory furnaces have been largely replaced by more advanced smelting furnaces, which require lower energy input, have higher capacity, and produce higher sulfur dioxide content off-gas. 

Fig. 1-1. Lead smelting furnace. Source G. Agricola, "De Re Metallica," Book X, p. 481, Basel, Switzerland, 1556. Translated by H. C. Hoover and L. H. Hoover, Mining Magazine, London, 1912. Reprinted by Dover Publications, New York, 1950.

Reducing smelting furnaces that produce a high-sulfidity, kraft-like green liquor are now employed at sodium-based sulfite mills. U.S. EPA anticipates that it would be necessary to replace the existing recovery boilers at ammonia-based mills if chemical substitution to a sodium base were employed. Additionally, it is likely that, because the heat value of sodium spent liquor is lower than ammonia spent liquor, evaporator modification may he required if excess capacity does not already exist. 

---