The electric arc furnace has characteristics which make it attractive for a number of metallurgical applications. Some of these characteristics are: high thermal efficiency, the possibility of attaining very high temperatures, low off-gas volume compared to fuel-fired furnaces, no impurities are introduced by the heat source, and impurities in only minor amounts are introduced by the electrodes. It also has disadvantages, the chief ones being the relatively high cost of electrical energy compared to fossil-fuel energy and the fact that, when oxide ores are smelted using carbon as reductant, the off-gases emitted are rich in carbon monoxide.
The vital components of the shaft-electric furnace are: the arc furnace, the feed-control mechanism between the arc furnace and the shaft furnace, the shaft furnace and its associated feed system, and an exhaust system capable of exhausting all the gaseous products from both the arc furnace and shaft furnace. In order to maintain countercurrent flow in the shaft, with solid charge descending and gases ascending, it is necessary that the exhaust system maintain a reduced pressure at the top of the shaft, thus it must be connected to the top of the shaft, adjacent to the incoming feed.
The 250-kVA arc furnace was already available in the laboratory; the only significant alteration made to it during this project was the installation of a rotating hearth to facilitate the even distribution of the hot material as it was transferred from the shaft furnace to the arc furnace. The mechanism for controlling this transfer of hot solid material from the shaft furnace to the arc furnace consists of a refractory-lined horizontal section, 18 in. long, with the same cross-sectional dimensions as the shaft furnace (6 in. x 20 in.). The various configurations used will be outlined later when descriptions are given of specific smelting or melting experiments. The feed and exhaust systems were common to all experiments. A tubular vibrating conveyor is used to introduce the solid feed material at the top of the shaft because it performs this function satisfactorily while simultaneously providing considerable resistance to the in-flow of air at this point. The balance of the feed system was assembled using standard bucket elevators, vibrating conveyors, etc.
About 100 tons of commercial iron-oxide pellets have been smelted in the unit to produce molten iron containing about 2.5% carbon. Because this involves the reduction of iron oxide by carbon, a plentiful supply of CO-rich gas is available from the reaction zone in the arc furnace. When this CO-rich gas is drawn up through the shaft and used to prereduce the incoming oxide, the supply of CO from the reaction zone in the arc furnace will be diminished according to the degree of prereduction that has been achieved in the shaft.
Iron oxide pellets were smelted both with and without the shaft furnace in operation to evaluate the benefits gained when the ore was preheated and prereduced in the shaft. The results indicated that the throughput of this pilot-plant arc furnace could be approximately doubled, that the electrical energy required could be reduced by about 40%, and that the coke and limestone requirements could be significantly lowered by preheating and prereducing the oxide pellets in the shaft furnace.
When prereduced iron pellets are melted in this unit, the operating conditions in the shaft furnace must be considerably different than when an oxide ore is being smelted. Because the pellets are almost completely reduced, only a minor amount of reduction remains to be completed in the arc furnace and, therefore, only a very small quantity of CO is generated there.
The making of pig iron in a continuously charged blast furnace has been normal practice for many years. The making of steel in a continuously charged furnace, or system of furnaces, has long been discussed and, in recent years, has been investigated to a limited extent; but to date no commercial installation exists. The results achieved when high-carbon iron was made from metallized pellets in this shaft-electric furnace suggested that it might be a desirable unit, or at least a component of a system, for a continuously fed steelmaking facility.
Two relatively stable periods of continuous operation were obtained during this experiment. The first of these had a duration of 42 hr, and the average carbon content of the metal produced was 0.03%,between a maximum of 0.06% and a minimum of 0.01%. The second period was similar to the first, but 5 lb of iron oxide pellets were added to each 100 lb of metallized pellets to simulate the slight oxidation of the pellets in the shaft furnace which was mentioned earlier (when the % metallization decreased from 95.2 to 94.7%).