8th International Congress on Science and Technology of Ironmaking - ICSTI 2018

At SSAB Europe’s integrated steel plant in Luleå, Sweden, much effort is directed towards maximising the recycling of in-plant residues. As the blast furnace (BF) operates with a 100 % pellet burden, the main part of all dusts and sludge are recycled via a BF cold bonded briquette and by the BF-injection of its flue dust. However, BF sludge is not presently recycled due to concerns regarding BF zinc load. This has continuously risen with increased recycling, to a current level of 100-120 g/tHM. In order to investigate the possibilities to introduce the recycling of all or parts of the freshly generated BF sludge (0.8% Zn), or reclaiming previous year’s produced BF sludge from settling ponds (0.2-0.5% Zn), a system analysis case study has been performed. The main focus of the case study was to estimate the capacity and economic benefits for BF sludge recycling (back to the BF) via the by-product briquette, based on a set maximum allowed blast furnace zinc load. An increase of the BF zinc load will, of course, raise the zinc level in the total system. Therefore, the developed system analysis tool used in this case study models the influence of the total BF zinc load on all generated residues and, hence, on the zinc content of recycled briquettes and injected flue dust. The establishment of the system zinc balance has been made possible due to good in-plant knowledge of zinc content in input as well as output materials. The system analysis shows that, with a maximum allowed BF Zn-load of 150 g/tHM, a “closed loop” recycling of freshly generated BF sludge is limited to about 40% (or 2 of 5 kg/tHM produced). However, in the case of reclaiming BF sludge from settling ponds the amount of recycled sludge can be considerably larger as the Zn content of recycled sludge will not be affected by the obtained increase in Zn content in the freshly produced BF sludge. At a Zn content of 0.4%, the amount of reclaimed sludge that can be recycled will equal the generated amount (i.e., about 5 kg/tHM). This will result in a reduced need for iron ore pellet and coke (dependent on the sludge Fe and C content) of some 2 kg/tHM, respectively. Hence, recycling of reclaimed BF sludge should be preferred to a “closed loop” recycling. A prerequisite for optimising the recycling of reclaimed sludge is to perform a comprehensive mapping of the zinc distribution in settling ponds.