DC arc furnaces

DC arc furnaces - are probably the biggest innovation in the arc furnace technology in recent years. The concept of direct current furnaces is not new, but only recently the cost of rectifiers decreased to the point where the arc furnaces became economical. Such furnaces have a number of unique requirements that distinguish them from the AC furnaces, except the obvious differences in the power source.

DC arc furnaces have only one electrode arm and one graphite electrode. This electrode works  as a cathode. Thus, at the top of the arc furnace there are no piling elements, like in the AC furnace. DC furnaces, however, require a reverse current electrode (anode) for the electrical circuit closing. The anode is usually a bottom electrode, so named because it is located in the bottom of the arc furnace. There are several different designs of the bottom electrode: metal pin electrode with a non-conductive lining; rod electrode; a metal plate electrode; and conducting bottom lining.

In the case of using a conductive lining, the furnace bottom center serves as the anode. A round flange is built into the bottom which is located inside the circular channel, welded to the furnace shell. Inside the channel the flange is supported by reinforced ceramic blocks. The space between the channel, supporting blocks and the flange is filled by the refractory ramming mass. Thus, the bottom is isolated from the rest of the arc furnace shell.

Spherical bottom is made of heat-resistant steel. The round copper plate is fixed directly to the bottom by means of bolts. Four copper clamps extend from the copper plate down through a bottom and are connected to the cables that connect the tube bases. On the top of copper plates there is placed the conductive lining. The heat flow from the furnace bottom (usually about 15 kW ·m2) is removed by a forced air cooling. Due to the large surface area of ​​the bottom electrode, the current density is usually quite low, about 5 kA · m2. However, in some furnaces it was necessary to use a nonconducting material for bottom lining to cause the current distribution more evenly across the bottom. If you cannot obtain a correct current distribution, this leads to the appearance of hot spots in the center of the furnace.

Rod electrodes comprise from one to four large steel rods (10-15 cm in diameter, but can reach up to 25 cm in diameter) depending on furnace size. Typically, the structure is designed for a bottom electrode current of 40-45 kA. The rods are in contact with the bath in the upper surface and melted. The extent to which the rods are melted is controlled by water cooling. The rod is inserted into a copper housing, through which cooling water circulates. Due to a sufficient cooling, the rod is not melted completely. Thermocouples control the bottom electrode temperature and the cooling water temperature. The insulation coating insulates the copper shell from the rod. The rod is connected with a  copper base, which provides a connection to the power cable.

Specialists of UkrNIIElectroterm use the bottom electrodes produced by Private Enterprise Firm "Roud". These electrodes have high reliability and are recoverable during melting. The service life of the electrode is more than 7 years with proper maintenance.

The pin bottom electrode consists of several metal pins 2.5-5.0 cm in diameter, to allow reverse current flow. These pins are mounted vertically and penetrate the lining. The pins are drawn down to the furnace bottom where they are fixed by two metal plates. The lower ends of the pins are attached to the lower conductive plate. The lower contact plate is air-cooled and situated in the central part of the furnace bottom. The top of the pins are on the similar level with the working furnace lining. Pins are in direct contact with bath, and with the disappearance of the working lining, gradually melt. Reverse power cable is attached to the bottom conducting plate.

The advanced temperature control system is designed for monitoring the lining and the bottom electrode wear, which allows the scheduled replacement of the bottom electrode. Advanced built-in cartridge design allows you to change the bottom electrode rapidly in the eight-hours planned downtime for maintenance. Bottom electrode replacing Procedure is following:

1. After the last melt before replacement the slag is downloaded from furnace and water is sprayed onto the lining for cooling acceleration.
   
2. The electrical connections of the bottom electrode are disconnected and the thermocouples are shut off.
   
3. Built-in cartridge is pushed upwards by six hydraulic cylinders located on the perimeter of the bottom contact plate.
   
4. After the cartridge is removed from the bottom of the furnace, it may be removed by crane.
   
5. The new cartridge is lowered into place and the electrical connections are closed.
   
6. Small amount of scrap is loaded and arc lights to test the system. After a successful test, the scrap is charged into the furnace and the melt starts at the reduced current. Once the liquid heel is established, it is possible to resume normal operation.

The steel plate bottom electrode consists of steel plates, assembled to a ring in the furnace bottom and separated into several sectors. Each sector consists of a horizontal ground plate and several welded steel plates which extend upwardly through the lining. The plates have a thickness of about 0.16 cm and are spaced about 9 cm apart. Sectors are bolted to the air-cooled bottom shell, which is electrically isolated from ground and is connected to four copper conductors.

Most DC arc furnaces operate with long arcs – in two to three times longer than in conventional electric furnaces of ultra-high power. As a result such furnaces require higher water consumption for the panels cooling– approximately 160 l / min·m2 for the side panels and 180 l / min · m2 for roof panels.

Currently DC arc furnaces are used both at steelmaking and at ferroalloy plants, in particular for smelting of fines, scraps and slag-metal mixes. Also, the direct current is applied to the ore-smelting furnaces, for example, to produce ferrochrome, ferrotitanium, ferro-alloys based on rare and rare-earth metals.