The plasma arc furnace is a device to melt a substance by low-temperature plasma flow, typically created by an electric arc heater (plasmatron). The main field of application of the plasma furnace is electrometallurgy. The main advantages of a plasma furnace are:
high stability of the working process and an opportunity of continuous temperature adjustment by changing the plasmatron electric conditions;
the opportunity of operation in practically any desired atmosphere;
high degree adoption of alloying additions;
a decrease of the impurities content and, consequently, an opportunity of smelting low-carbon steels and alloys;
an opportunity of smelting the nitrided steels by usage of gaseous nitrogen;
a relatively small pollution of ambient air.
Various direct current (DC) and alternating current (AC) plasmatrons are used in plasma furnaces. In large-scale plasma furnaces, several plasmatrons are used to provide more homogeneous heating.
There are three types of the plasma furnaces: plasma furnaces for melting in a ceramic crucible; plasma furnaces for melting in a crystallizer; plasma furnaces for melting in a scull.
The ceramic crucible plasma furnaces are used mainly for melting steel, nickel-based alloys and waste metals with alloying additions. Argon is usually used as the plasma forming gas. Plasma furnaces are used also for gray cast iron smelting. The main advantage of a plasma furnace in comparison with a usual blast cupola is in that the plasma furnace does not need intensive air blasts to support the burning process. Besides in the case of gray cast iron smelting in a plasma furnace, the consumption of electric power and the charge decrease.
Plasma furnaces with a crystallizer are used mainly for the metal refining process. By contrast to electroslag, vacuum-arc and electron-beam refining processes, the main technological means of action on the liquid metal is the gas phase. In particular, a reaction of nitrogen dissolving in liquid iron and its alloys runs faster than in alternative melting devices. Plasma furnaces with a crystallizer can be used in a wide pressure range (106 —10−1 Pa). If necessary, pressure can be varied during a plasma furnace cycle.
A typical plasma furnace with crystallizer includes a remelting blank, two (or more) plasmatrons, a copper water-cooled crystallizer and a remelted ingot, all placed in a common chamber. The cylindrical blank (at the chamber top) and a crystallizer (at the chamber bottom) have mutual vertical axes. The plasma jets fall at an angle on a crystallizer bath and simultaneously flow about the blank, melting it. The melt drops into to a bath. The blank is driven downwards and is rotated to provide uniform melting. A solid ingot is extracted from the crystallizer by a power drive. The coordinated operation of blank drive and ingot extraction devices can provide a continuous remelting process.
The plasma furnaces for melting in a scull are designed to make steel castings, high-temperature alloys and refractory metals. These furnaces can operate in a wide pressure range. Their advantages are high metal purity, an opportunity of scrap charge and waste melting, a prolonged time of liquid metal bath holding for performing of such operations as alloying, mixing, taking a sample, etc.
The plasma furnaces are used also for ceramic melting. Their advantages are high purity of product and an opportunity to provide a ceramic casting process.