Utilization of fly ash and waste lime from pulp and paper mills in the Argon Oxygen Decarburization process

Fly ash and waste lime are solid wastes generated in pulp and paper mills. Due to high CaO contents, these secondary lime materials can be potentially utilized in the steelmaking process to partially replace primary lime. In this study, fly ash and waste lime from Swedish pulp and paper mills were made into briquettes and for the first time utilized as slag formers to replace primary lime in a pilot-scale Argon Oxygen Decarburization (AOD) stainless steelmaking process. The purpose was to evaluate how these CaO-containing materials could affect the AOD process and the steel quality. The results show that the S, P and CaO contents in the fly ash and waste lime are important concerns for metallurgical application. Firstly, due to lower CaO contents (61.5% CaO in fly ash and 90.6% CaO in waste lime versus 95.2% CaO in primary lime), utilization of these secondary lime materials could lead to higher slag amounts in the AOD converter. Secondly, due to higher S and P contents (0.40% S and 0.26% P₂O₅ in fly ash, 0.11% S and 0.75% P₂O₅ in waste lime versus 0.08% S and 0.01% P₂O₅ in primary lime), utilization of these secondary lime materials could increase the S and P contents in the steel. The S content in the steel can be controlled during the reduction and desulfurization stages; however, the P content in the steel increases with an increased use of these secondary lime materials. Thirdly, the metallurgical performance of fly ash and waste lime is as good as primary lime when considering the aspects that: (a) utilization of fly ash and waste lime doesn’t have negative effects on the decarburization stage nor on the reduction stage during the AOD process; and (b) fly ash and waste lime are as efficient desulfurization agents as primary lime. In future industrial applications, the alkali content and the variations of chemical compositions (especially with respect to S, P and CaO) of these secondary lime materials need to be carefully controlled to ensure a sound metallurgical operation.