At present, the major manufacturing method of PFS is the oxidation method of ferrous sulfate. Ferrous sulfate and sulfuric acid are important raw materials in this method. PFS can be made by the oxidation of ferrous sulfate and the hydrolysis and the polymerization of ferric sulfate. Another method is that some iron ore dregs (or waste residue) and sulfuric acid are used as the raw materials of PFS. Iron ore dregs are dissolved in sulfuric acid, then hydrolysis, polymerization.

1  Manufacture of PFS with ferrous sulfate

    In general, ferrous sulfate, which is a by-product produced in the production process of titanium white(titanium dioxide), and sulfuric acid are used as the raw materials of PFS. Besides, acid washing waste liquids, scrap-iron, and sulfuric acid can also be used. In the manufacturing  process of PFS, the reactions of oxidation, hydrolysis, and polymerization are all existential at the same time. Because of the cause of dynamics, the oxidizing reaction of ferrous sulfate is slower under acidity condition. So the rate controlling step of the reaction system is the oxidizing reaction. This reaction may be directed oxidation or catalyzed oxidation according to different oxidizing process.

1.1 Method of directed oxidation

    The method of directed oxidation is that ferrous sulfate was oxidized directly into ferric sulfate by oxidizer. The oxidizer used here was hydrogen peroxide^[7], manganese dioxide, potassium chlorate^[8], sodium persulfate, sodium hypochlorite or nitric acid and so on. The oxidizing reaction can be completed within two hours under the condition of room temperature when hydrogen peroxide is used as oxidizer:

    2FeSO₄ + H₂O₂ + H₂SO₄ = Fe₂(SO₄)₃ + 2H₂O                   （1）

Then, hydrolysis and polymerization:   

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

    m[Fe₂(OH)_n(SO₄)_3-n/2] = [Fe₂(OH)_n(SO₄)_3-n/2]_m                   （3）  

The ti

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