this process reduces the soluble iron found in the final effluent of a plant making iron oxide by the penniman - zoph scrap process . the finished oxide product slurry is separated from the scrap iron , and the slurry is aerated while the ph is maintained between about 3 . 0 and 5 . 0 . as the soluble iron is converted to product oxide , the ph of the system rises toward the higher value .