Coreless vacuum induction furnaces for refining and recycling of metals 2

Zinc distillation plant; vacuum furnace (right) with zinc condenser (left)

Vacuum-type induction furnaces are becoming increasingly widespread for removing undesirable alloying elements, as well as for the production of high-purity, low-gas alloys and for melting special materials. Typical applications include the following:

• Deoxidation of high-grade steel melts

• Carbon reduction of molten steel

• Melt degassing

• Distillation of zinc and other low-melting alloys

The design of OTTO JUNKER’s vacuumtype coreless induction furnaces guarantees a high level of operating safety and reliability since only their actual melting chamber is evacuated whereas the induction coil with its power and water inputs remains outside the vacuum.

A cooling air system arranged between coil and crucible lining protects the furnace shell from overheating while also permitting continuous temperature monitoring of the refractory lining via the cooling air ducts distributed along the circumference of the crucible. This design prevents electric glow discharge and any hazardous contact between cooling water and molten metal in the case of a breakdown.

To illustrate the operation of a vacuum furnace for zinc distillation, let us look at the following specific project:
Overall, the plant essentially comprises a vacuum-type coreless induction furnace with the necessary ancillary equipment including a frequency converter and switchgear system, zinc condenser, filter unit and vacuum pump.

The distillation proceeds as follows:
The charge material is melted in the furnace under atmospheric pressure. Once it is molten, the furnace is sealed off with a hood and connected in vacuum-tight manner to the zinc condenser and filter unit arranged beside the furnace.
An appropriate vacuum is then generated by the vacuum pump and the melt temperature is raised to approx. 1,000 – 1,050 °C, causing the zinc to evaporate intensely. The resulting zinc vapours are passed into the unheated condenser and condense there. The condenser design ensures that the temperature exceeds the melting point of zinc at all times but will never rise too high. Accurate temperature management and partial cooling of the vessel make for an optimum process.

When the condensation step is completed, the condenser is disconnected from the furnace and the molten zinc is drained into a ladle. The alloy left in the melting furnace is removed in the same manner. This complex process calls for accurate monitoring and control of all operating parameters, especially temperatures, pressure and electric power. These functions are performed by a furnace control system designed specifically for this application.

At present, such vacuum-based zinc distillation systems are being built for customers in Germany, India and Canada. Some are in their manufacturing stage, others are being assembled.