Furnaces operations

FURNACE OPERATION Feed preparation and feeding methods 

The formation of accretions within the charge and particularly on the walls of the furnace is a major issue and can restrict gas flow and hence furnace throughput, as well as causing gas channelling and surface eruptions or gas blows. Accretion formation is discussed above and the main areas of concern are the wall accretions, the central acaetion or deadman or sow and the hearth accretion. 

One factor contributing to the formation of the central acaetion or deadman is the lack of adequate penetration of the blast. This can be corrected by increasing blast velocity through reduction in the diameter of the tuyeres and also by reducing the width of the furnace at the tuyere level. The Naoshima Smelter in Japan reported an effective removal of the central accretion by these measures, including reduction of furnace width from 1.66 to 1.42 m, and significantly increased furnace capacity (Moriya, 1989). 

As indicated above, the hard hearth acaetion forms relatively slowly and eventually causes interruption to the tapping of the furnace. It requires a complete shutdown for removal. 

Furnace capacity is ultimately the lead production rate as tonnes per day per square metre of hearth area, but is basically controlled by the carbon burning capacity in tonnes of carbon burned per day per square metre of hearth area. This in turn relates to the air blast or oxygen throughput that can be achieved, as influenced by the permeability of the charge. Hence the importance of uniform sizing of coke and sinter, and the retention of sinter stracture without degradation. 

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W. Trioks, Industrial Furnaces, Fuels, Furnace Types and Furnace Equipment—Their S election and Influence Upon Furnace Operation, 4th ed., Vol. II, John Wiley Sons, Inc., New York, 1967, 358 pp., emphasis placed on heating furnaces (fuel-fired and electric) rather than melting furnaces. 

Table 1 shows some of the typical electrode consumption figures for various submerged-arc furnace operations. 

The efficiency of an induction furnace installation is determined by the ratio of the load usehil power, P, to the input power P, drawn from the utihty. Losses that must be considered include those in the power converter (transformer, capacitors, frequency converter, etc), transmission lines, cod electrical losses, and thermal loss from the furnace. Figure 1 illustrates the relationships for an induction furnace operating at a constant load temperature with variable input power. Thermal losses are constant, cod losses are a constant percentage of the cod input power, and the usehd out power varies linearly once the fixed losses are satisfied. 

The quantity of coproduct acetylene produced is sensitive to both the feedstock and the severity of the cracking process. Naphtha, for example, is cracked at the most severe conditions and thus produces appreciable acetylene up to 2.5 wt % of the ethylene content. On the other hand, gas oil must be processed at lower temperature to limit coking and thus produces less acetylene. Two industry trends are resulting in increased acetylene output (/) the ethylene plant capacity has more than doubled, and (2) furnace operating conditions of higher temperature and shorter residence times have increased the cracking severity. 

The principal U.S. lead producers, ASARCO Inc. and The Doe Run Co., account for 75% of domestic mine production and 100% of primary lead production. Both companies employ sintering/blast furnace operations at their smelters and pyrometaHurgical methods in their refineries. Domestic mine production in 1992 accounted for over 90% of the U.S. primary lead production the balance originated from the smelting of imported ores and concentrates. 

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. 

Standardized appHcations of different refractory types are Hsted ia an excellent series compiled by the ASTM (36). These surveys cover the principal iadustrial appHcations of refractories and furnish a description of furnace operations and destmctive influences such as slagging, erosion, abrasion, spalling, and load deformation. 

A 75% ferroshicon furnace operating at 90% shicon recovery would give 0.24 kg of by-product shica fume per kg of shicon produced. A shicon metal furnace operating at 85% recovery (typical of the iadustry) would give 0.38 kg of fume shica per kg shicon produced. 

Electric-Arc Furnace. The electric-arc furnace is by far the most popular electric steelmaking furnace. The carbon arc was discovered by Sir Humphry Davy in 1800, but it had no practical appHcation in steelmaking until Sir William Siemens of open-hearth fame constmcted, operated, and patented furnaces operating on both direct- and indirect-arc principles in 1878. At that early date, the avadabiHty of electric power was limited and very expensive. Furthermore, carbon electrodes of the quaHty to carry sufficient current for steel melting had not been developed (see Furnaces, electric). 

St. Joe Minerals Corporation uses a fluid-bed roaster to finish the roasting at 950°C of material that has been deleaded in a modified multiple-hearth furnace operated with insufficient oxidation (34). First, sulfur is reduced from 31 to 22% and lead from 0.5 to 0.013%. Somewhat aggregated, the product is hammer-milled before final roasting. Half of the calcined product is bed overflow and special hot cyclones before the boiler remove the other half total sulfur is ca 1.5%. Boiler and precipitator dusts are higher in sulfur, lead, etc, and are separated. 

The use of potassium hexafluorosihcate is preferred over sodium hexafluorosihcate because of the lower tendency of the potassium compound to dissociate the lose sihcon tetrafluoride by sublimation. The addition of potassium carbonate or chloride to the fusion mix further reduces this tendency and promotes completion of the reaction. The reaction is conducted in a rotary furnace operating at 700°C. The product is cmshed prior to leaching with acidified hot water. The hot slurry is filtered to remove the sihca, and potassium hexafluorozirconate crystallizes as the solution cools. 

The carbide impurities tend to distill and are reoxidized in the upper, cooler region of the charge. This process forms cmsts near the top of the charge or around the cooler part of the reaction cmcible and causes trouble in furnace operation. Large amounts of dissolved calcium siUcate and alurninate may form a viscous melt and impede the tapping process. FerrosiUcon is commonly removed from the cmshed carbide by electromagnets. 

Phosphate rock. In this application, the rotary Idln is used to nodulize the fines in the ore and prepare them for electric-furnace operation. Ore under 5 cm in size and containing 50 percent or more minus 100 mesh is calcined. Ore nodulizes at approximately 1475 to 1500 K. 

The lead blast furnace operates at a lower temperature than the iron blast furnace, die temperature at the tuyeres being around 1600K as opposed to 1900K in the ironmaking furnace (see p. 333) and this produces a gas in which die incoming air is not completely reduced to CO and N2, as much as one per cent oxygen being found in the hearth gas. 

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