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ROTATING ARC FURNACE

It is well known that ferro-alloy furnaces and other electric melting furnaces, operating at high temperatures, will in time develop craters which result in a less efficient operation of the furnace. These craters lead to uneconomical and difficult operation.

The furnace gases cannot penetrate the crater walls, and accordingly have to force their way along the surface of the electrodes to the top of the furnace. The gases thus become overheated and leave the furnace at an unusually high temperature, and will often, with great violence, break through the slag covering at certain sports, and simultaneously fine white flames of evaporated metal will appear. In this way serious losses of material from the furnace may result, due not only to evaporation of the constituents of the charge but also due to dust caused by the violent escape of gas. Consequently, there may also be large losses of heat from the furnace. The hot gases will overheat the electrode sections outside the furnace so that the wear and tear on the electrode holders will be very serious.

New rotating electric melting furnaces effectively eliminate the formation of craters. Ordinarily this furnace is designed as a three phase furnace with the electrodes suspended at the three points of an equilateral triangle. The furnace pot is round and rotates, so that the melting zones constantly move sideways relative to the electrodes. By this rotation the whole furnace charge is successively drawn into the melting zone, and the bath is kept clean and free from craters.

At first there was put in operation a 700-kw furnace of 190 tons weight on the turnable. This furnace proved very satisfactory and now many new furnaces of this type are build in metallurgical works.

At one metallurgical works such a furnace and an ordinary non-rotating furnace were operated simultaneously, producing 75 percents ferro-silicon with the same class of raw materials. The result showed that the rotating furnace had about 17 percents larger production with the same consumption of power and the consumption of materials per ton of ferro-silicon produced was about 10 percents less.

The operation and maintenance costs for the rotary equipment are very little.

 

HIGH FREQUENCY FURNACES

The three main types of electric melting furnaces are resistance, arc, and induction. Resistance furnaces are very simple, but high temperatures cannot be obtained. Arc furnaces contaminate the charge with carbon from the electrodes and volatile elements may be lost from the melt. Induction furnaces may be constructed with or without a magnetic core.

The induction furnaces operate with a current that may be generated by an alternating current (a. c.) generator, which has a relatively low fixed frequency, or by an electronic tube, where a triode tube functions as the source of the alternating current.

Electric tube induction furnaces have many advantages for small charges, especially of expansive metals. The frequency may be as high as several million cycles per second and is easily regulated. An adjustment of the circuit when the charge melts is not so important as it is with furnaces using an a. c. generator. Melting is very rapid. The operation of these furnaces is very flexible and uniform. The crucible is deep to minimize oxidation. Moreover, the melting can be conducted in vacuum or under a controlled atmosphere if desired.

Since the heat is generated in the metal, there is no danger of carburizing the melt. The equipment is easy to maintain as there are no moving parts. The crucibles are cheap and have a long life. However, the equipment is more expensive than an a. c. generator and is justified only for small charges.

One of the most powerful furnaces of this type has the following characteristics. It is a 2-crucible, 250 kw. installation of the electronic tube type. The nominal capacity is 110 lbs., but actually 287 lb. may be melted. The average melting times are 21 min. for 192 lb. of steel and 32 min. for 287 lb. However, some heats are much shorter. Each pound of steel melted requires 0.57 kwh. (kilowatt-hour). With 176 lb. heats of steel, each crucible will last about 100 heats. Particular attention is given to the simplicity of operation and to the safety provisions.