Prevention and removal of iron ions in sulfite, the desulfurization product

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We have a small coal-fired boiler system. Since the coal used is of poor quality and contains high levels of sulfur, it needs to be desulfurized. A factory nearby requires sulfurous acid, but it has strict restrictions on the levels of heavy metals such as iron ions in this substance. We plan to use ammonia water for desulfurization in order to produce sulfurous acid. The flue gases from the boiler pass through an electrostatic precipitator with four electrodes, followed by wet washing, and then through an absorption tower where ammonia water is used to produce sulfurous acid. However, the problem is that the iron ion content in the resulting sulfurous acid is quite high, at 50 ppm, while the customer requires it to be below 5 ppm. Is there any way to remove as many iron ions as possible during this process? Of course, the dust removal method should be as simple as possible, as otherwise the investment or operating costs would be too high, making it uneconomical. Additionally, if it is difficult to remove iron ions through dust removal, are there any simple methods that do not affect the quality of the sulfurous acid and can still effectively remove iron ions from the final product? Thank you!