The Dross Recovery Furnace (DRF) reduces the Iron content of the zinc melt to almost zero by causing dissolved Iron in the zinc to react with a metallic reagent to form a precipitate. The precipitate formed is lighter than zinc and therefore floats on top of the melt in the furnace. It is manually skimmed off the surface of the liquid zinc by the furnace operator using hand tools or more commonly a paddle arrangement on the end of a fork lift truck. The purified zinc left in the furnace can be used directly in the vaporizing furnace.
The precipitate can be collected, stored and sold for reprocessing by others as it still retains a saleable quantity of zinc. The skimmings at this stage can contain as much as 75% recoverable low-iron zinc and therefore Burns Energy Systems has come up with an option to reclaim the zinc and return it to the process.
When the iron rich precipitate forms it tends to grow around beads of liquid zinc. When fully formed, the solid precipitate encapsulates the zinc, which is the reason for the high zinc content of the skimmings. Skimmings that have originated from Top Dross tend to be ashy and friable, and are generally easy to handle and produce a better yield. Bottom dross precipitate tends to be a low ash, high viscosity, spongy inter-metallic compound that floats on the surface of the zinc in the DRF.
The reclaimer has been designed to mechanically break down the encapsulating precipitate without the use of the flux. The reclaimer works by applying a force to the skimmings, of sufficient strength to break up the thin solid surface of precipitate surrounding the droplets of liquid. Zinc is released when these solid precipitate surfaces are disrupted. Free zinc collects in the bottom of the reclaimer crucible and the fragmented precipitate floats on the top of the liquid zinc.
It is possible to achieve 80% extraction efficiency with Top Dross skimmings and 60% with Bottom Dross skimmings. The lower yield from Bottom Dross precipitate is due in large part to the higher viscosity of the Bottom Dross Precipitate.
Recovery rates are improved if carbon powder and flux is added to assist the process. It is more difficult to mix Bottom Dross precipitate with flux and so yields tend to be lower. The quantity of flux and Carbon used in the reclaimer is significantly less than the quantities that would be used in the furnace where a flux only method is applied.
The Reclaimer is supplied in 2 parts; the base is a crucible furnace which is electrically heated and the top section is a cover to which a high power agitator is fitted. The cover has an built-in fume extraction vent which may be hooked up to the client's hygiene system when in operation.
The low iron zinc is recovered by tilting the furnace to discharge into a ladle where it can be put back into the DRF.