The invention relates to a method of operating a pelletizing plant for additional treatment of iron making, steelmaking and pellet plant residues. The pelletizing plant comprises an indurating furnace in which green pellets are charged and fired to produce hard pellets. The method includes: preparing iron ore green pellets; and charging and firing said green pellets in an indurating furnace. The method is remarkable in that agglomerates of by-products are charged and fired with the green pellets in the indurating furnace, thereby producing hardened pellets and hardened agglomerates of by-products. A pelletizing plant is also presented, which comprises a pelletizing section (16), an indurating furnace (12), an agglomerating section (40) for producing agglomerates of by-products. A feeding arrangement is configured to charge agglomerates of by-products into said indurating furnace, together with the green pellets.