Factors which control the formation of metallic lead in a sinter bed have been investigated, and the results obtained have been used to derive a general theory for lead formation from heat balance considerations.
A study of the roasting reactions of PbS, ZnS, FeS and FeS2 was carried out in a tube furnace under controlled conditions. Products of oxidation were identified by x-ray analysis. Roasting mechanisms were derived from the results with the aid of equilibrium diagrams which were constructed from available thermodynamic data. The results of these studies were verified experimentally in a laboratory sintering unit using a bed quenching technique. It was found that PbS oxidises mainly to PbO, ZnS to ZnO and both FeS and FeS2 to Fe304. Analyses of exhaust gases in tests where coke was the only fuel used showed that the combustion ratio (—/CO+CO2) for the carbon was 0.27.
Further sintering experiments showed that the only reaction which could produce metallic lead in a normal sinter bed was PbS(g) + 2PbO = 3Pb + S02
The PbO taking part in this reaction was the PbO in the return sinter. It was also shown that this PbO would react only with gaseous sulphides and not solid or liquid ones. When coke was used in large amounts as the only fuel, some lead was formed at temperatures in excess of 1200oC by the reaction CO + PbO = Pb + CO2
Since variation in air flow rate and flame front velocity could alter conditions in the reaction zone, the factors which govern flame front velocity were investigated. It was found that an equation for flame front velocity which was derived from the work of Voice and Wild (l956, 1957 and 1958) applied to lead sintering as well as to iron sintering. In this expression, the velocity of the flame front is a function of air flow rate, bed porosity and the heat capacities of the exhaust gases and charge components. It can be deduced that gas composition in the reaction zone is independent of air flow rate.
Metallic lead formation during sintering was investigated experimentally using a wide range of charge compositions. The results were used in conjunction with a general theory derived from heat balance considerations to explain the factors which control lead formation. It was found that there was a definite value of fuel content in the bed at which lead started to form. As the fuel content was increased above this value, the weight of lead formed increased linearly with fuel content.