After the complexed iron is consumed, in what form does the complexing agent exist?

The complexed iron is not consumed; ferric iron is reduced to ferrous iron, and through oxidation by oxygen, ferrous iron is again oxidized to ferric iron. So it can be recycled. Chelated iron has certain advantages in desulfurization in this regard. The consumption of complexed iron may be due to the complexity of coking exhaust gases, which can cause some of the iron to settle; any circulating liquid will inevitably produce solid residues. As the concentration increases and a certain level of saturation is reached, solid residues will form, thereby removing some of the complexed iron. It is not as good as PDS desulfurization in this regard. Because in the complexed iron process, it is not worthwhile to recover salts from either the waste liquid or the solid residues; at present, there is no way to process them, other than treating them as solid waste.