Lead recycling from complex secondary materials has gained increasing interest in recent years. Around 80-85% of the total secondary lead is recycled from lead paste. Due to the high toxicity of lead and increasingly stringent environmental regulations, cleaner and efficient processing of such materials and development of industrial processes are urgently required. In the present work, an innovative and clean lead-acid battery paste recycling technique is proposed. Iron-containing waste materials are utilized as sulfur-fixing agents to capture sulfur as FeS, instead of generating excessive amount of SO2. Na2CO3 or Na2CO3-Na2SO4 salt mixture is added to the smelting system to promote the reactions and improve valuable metals' recovery and sulfur-fixation efficiency. Thermodynamic calculations and experimental determination of the reaction mechanisms in the PbSO4-Fe2O3-C, PbSO4-Na2CO3-C, and PbSO4-Fe2O3-Na2CO3-C systems are carried out to build the fundamental knowledge. Thermogravimetric analysis and quenching method coupled with a direct spectroscopic analysis of the phase and chemical compositions of the resulting phases, i.e. XRD and SEM-EDS, are employed to investigate the phase transformation mechanism. The results indicate that without Na2CO3 addition, lead in lead paste is shown to be extracted mainly through the sequence of PbSO4→PbS→PbO→Pb. Sulfur in PbSO4 is thus first transferred to PbS and finally fixed as FeS. In the presence of Na2CO3, at low temperatures and in weakly reductive atmospheres, lead is extracted mainly through the sequence of PbSO4→PbO→Pb. Na2CO3 helps to transform SO3 from PbSO4 to Na2SO4. At high temperatures and strong reducing atmospheres, PbSO4→PbS dominates the reactions, and lead and sulfur are conserved as PbS. Lead can not be effectively extracted from PbS without a sulfur-fixing agent. When Fe2O3 is presented, lead in PbS will further be extracted through the sequence of PbS→PbO→Pb. Finally, sulfur is fixed as FeS, NaFeS2 and Na2S. A series of bench-scale experiments are conducted to detect the influence mechanisms of smelting conditions on lead extraction and sulfur fixation efficiency. The optimum smelting parameters obtained are integrated to an industrial pilot campaign. Under the optimum conditions, three smelting products, crude lead, ferrous matte, and slag, are formed. 91%-96% lead in the initial raw materials is found to be enriched in crude lead bullion. 97%-99% sulfur is fixed in the sulfide matte and slag. Purity of the crude lead reaches 96%-98%. Lead concentrations in the matte and slag are below 2.4%-4.1% and 0.5-1.2%, respectively, without subsequent matter and slag cleaning. The addition of sodium salts, e.g. Na2CO3 and a Na2CO3-Na2SO4 mixture, is seen to promote the reductive sulfur-fixing reactions and improve extraction and sulfur-fixation efficiency.
|Julkaisun otsikon käännös||Experimental thermodynamic and kinetic studies on extraction and recycling of lead from spent lead-acid battery paste|
|Tila||Julkaistu - 2019|
|OKM-julkaisutyyppi||G5 Tohtorinväitöskirja (artikkeli)|