Typically, the refined silver crystals are either granulated or cast into 1000 troy ounce ingots. Metso provides equipment for both process solutions.
The purified material is melted in a smelting furnace. When the batch of the bullion is ready for casting the furnace is tilted and the molten metal is poured either via a dosing tundish or directly to the mold positioned on the casting wheel in front of the furnace spout – all depending on the furnace operability.
To avoid the risk of the ingots sticking but not introducing any material that might contaminate the product, heat and a layer of soot is applied in the molds. This is done by a fixed torch and burner system, which is also assisting during the initial solidifying part of the ingot creation, minimizing the surface oxidation.
After the mold is filled with the bullion the wheel is rotated to place the next empty mold in the filling position. Rotation of the wheel is performed from the control panel by the same operator who operates the induction furnace. The casting process is repeated until all the bullion in the furnace has been transferred into the molds.
The molds filled with silver slowly passes the cooling section of the casting wheel. Here the mold and ingot are air- and water cooled to effectively reduce the temperature of the bullion. Vapors from this process is collected and removed by the ventilation hood. After this stage the bar has taken its solid form.
When completely solidified, the ingot is separated from the mold. The operation is performed by means of a mechanical ingot ejector and the ingot is caught by a receiver device.
From the receiver station several solutions for continuous cooling and handling are available. Metso promotes the use of the PWMS robot station which is designed to carry out the final quality control and traceability operations in a manner that is both safe and user-friendly.
To simplify maintenance of the furnace, the casting equipment may need to be moved from the position in front of the furnace. The casting equipment can be installed on a movable subframe with an independent drive system to simplify repositioning.
