• A scale-up rotary kiln process achieved a boron leaching efficiency of 70.23%, an iron grade of 94.12wt%, and a recovery of 93.35% from boron–iron mixed concentrate.
• The integrated process combines reductive soda-ash roasting, wet-grinding, magnetic separation, and fractional crystallization to produce reduced iron powder suitable for short-process steelmaking.
• High-purity boric acid (>99wt%) with regular morphology was prepared from boron-rich liquor by adjusting pH with sulfuric acid, and the solution chemistry was clarified.
• Scale-up validation in a rotary kiln addresses lab-scale limitations by replicating industrial dynamic thermal fields and mass/heat transfer conditions, confirming industrial feasibility.