The lead sulphate or the lead carbonate obtained from the desulphurization process can be reduced to metallic lead in a rotative furnace at reduced atmosphere and in presence of carbon monoxide (CO) at temperatures that vary from 1300°C in case of lead sulphate to 1000°C in case of lead carbonate. The pyrometallurgical process is up to now the only process used in the lead industry worldwide .
This process is carried out in short rotative furnaces burning methane gas or other available fuel in presence of pure oxygen.
The choice to use oxygen as combustive agent arises from the well-established experience of the last 20 years in many plants all over the world. The advantages of the use of oxygen as a combustive agent are the following:
- a reduction of energetic consumption due to the absence of nitrogen. If air was used in the rotative furnace as a combustive agent, the nitrogen, 80 % of the air, would have to be heated up as well.
- the absence of nitrogen in the combustion reduces the emission of NOx.
- the absence of nitrogen fosters the reduction of the exhausted gases up to 70%. This method therefore makes it possible to reshape filters and other treatment plants for exhausted smoke.
- possibility to use the STC system for the combustion of pyrogas produced from the separators of the batteries. The high temperature that is reached in the combustion flame permits to degrade all the toxic substances which could develop from the combustion of possible PVC parts, accidently present in the plastic.
- shorter treatment times due to higher temperatures reached in less time.
- possibility to regulate the quantity of fed oxygen in order to foster the creation of a reductive atmosphere in some phases of the process.
One of the innovations proposed by STC is a system for the preparation of the charge to be loaded into the rotative furnace which is crucial for a successful metallurgical reduction. As loading system STC proposes a pelletizer that, using bituminous residuals, permits to create compact pellets which aggregate not only the paste to be treated but also the necessary additives for the reduction reaction. This system reduces the creation of volatile powder of 50% that is normally developed during a standard pyrometallurgical process.