The second is oxidation, hydrolysis and polymerization:

    FeSO₄＋2H₂SO₄＋O₂ ＝ 2Fe₂(SO₄)₃＋2H₂O                     (20)

    Fe₂(SO₄)₃ + nH₂O ＝ Fe₂(OH)_n(SO₄)_3-n/2 + (n-2)H₂SO₄                (21)

    mFe₂(OH)_n(SO₄)_3-n/2 ＝ [Fe₂(OH)_n(SO₄)_3-n/2]_m                       (22) 

   In the second step, ferrous sulfate may be oxidized with different methods. One is directed oxidation^[19]. Another is catalysis oxidation^[41]. Moreover, when the content of ferrous sulfate is very low in the filtrating, the oxidation may be left out and both the hydrolysis and polymerization can take place directly^[21,22].

   In the manufacturing process, it is important to improve the dissolving rate of pyrite cinder furtherly in sulfuric acid. Author's studies indicated^[23,24] that the dissolving rate is concerned with sulfuric acid concentration, temperature, and the ratio between solid and liquid. Because the dissolving course of pyrite cinder is an exothermic reaction, the dissolving rate may achieve 95% under the effect of the reaction heat. The content of ferrous only occupies 3% of the total iron content in the filtrating. PFS can be obtained through the hydrolyzing and the polymerizing of ferric sulfate under certain condition. Oxidizer is not needed in the manufacturing process. This has not only eliminated the pollution of nitric oxides, and also saved spending and cut down further the cost of product.

    When PFS made by ourselves was used in the treatment of papermaking waste water (COD_Cr = 1000mg/L), the removal rate of COD_Cr was more than 90% as the addition of PFS was 150mg/L(that is, [Fe³⁺]=150mg/L). In addition, the removal rate of COD_Cr was more than 60% in the treatment of saccharin sodium slat waste water(COD_Cr = 15000mg/l) as the addition of the PFS was 800mg/L.

    In order to raise the dissolving rate furtherly, pyrite cinder may be preprocessed^. First,  ferric oxide in pyrite cinder was red

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