Coal ash induced ring formation in a pilot scale rotary kiln for low-grade iron ore direct reduction process: Characterization and mechanism

•Direct reduction of low-grade iron ore was performed by a coal fired rotary kiln.•Actual running status and furnace burden of rotary kiln were in-situ investigated.•Ring formation mechanism was revealed by given the real physicochemical condition.•Hedenbergite and fayalite developed by coal ash and ore induced key molten phase.•Corresponding solutions were suggested for practical rotary kiln operation.