The separation of the accompanying elements takes place through selective oxidation with oxygen. This is possible because arsenic, tin and antimony oxidise before lead during the process.
There are several possibilities available for the addition of oxygen:
On the one hand, the oxidising agent, in the form of an oxygen and nitrogen gas mixture, can be introduced directly into the refining kettle by means of lances. The Oxijet- basic from Messer is suitable for this. The system consists of one or more Oxijet lances, a control loop for O2/N2 as well as an electrical control unit which is optionally equipped with temperature control. The Oxijet lances are mounted on the lid of the refining kettle and protrude into its interior. The temperature in the kettle is monitored continuously to prevent over-heating. During the process tin-containing dross is first skimmed off, followed by dross containing arsenic and then antimony. Any resulting gas is sucked out via an exhaust hood.
With the VARTA / Messer process, lead from the refining kettle is pumped into a second, cylindrical flushing vessel with spindle plugs, which is accommodated in the refining kettle. A mixture of oxygen and nitrogen is introduced to the liquid lead in the flushing vessel using lances. The resulting dross is removed via an overflow and circulated to the melting process, the resulting gas is sucked out using an exhaust hood.
Lance, gas feed line and an electrical control device, which can optionally be implemented with temperature control, are similar to the system for Oxijet-Lancing. A significant advantage of the process is that the dross does not have to be skimmed off manually.
Still at the development stage is the Oxipot process. Similar to the VARTA/Messer process, the lead is pumped to a separate cylinder-shaped vessel. However, the gas mixture consisting of oxygen and nitrogen is not added using lances but through purging plugs in the bottom of the flushing vessel. The resulting dross is removed via an overflow and the resulting gas is sucked out using an exhaust hood.
In comparison to the Harris Process which uses caustic soda and nitric acid as oxidising agents, the selective oxidising process offers, amongst others, the following advantages:
- Environmentally friendly
- Little or no additives
- Low investment costs
- Easy to use
- Faster oxidation
- Lower waste disposal costs
- Lower production costs