On the whole, the manufacture of PFS by slags or waste residue is an economical method. This method is simpler than the catalysis oxidation method mentioned above. Certainly, there are still some problems that should be solved furtherly in the manufacturing process of PFS. For example, the dissolving rate of slags should be raised again, so as to raise the utilization rate of raw material. In addition, there is excessive acid generally in the solution of ferric sulfate obtained by dissolving slags in sulfuric acid. This will increase the consumption quantity of other assistants in the course of hydrolysis and polymerization. If there are some harmful heavy metals in slags and waste residue, there will be some heavy metal ions in PFS. This will restrict its application in drinking water treatment.

3 Manufacture of PFS with other methods

3.1 Method of biologic oxidation

    Using pyrite, coal, sulfur, and hydrogen sulfide as raw materials, PFS can be made with the method of biologic catalyzing oxidation under the influence of iron-sulphur bacteria, such as thiobacillus ferrooxidans and thiobacillus thiooxidans. Literature [29] investigated some factors which affecting the manufacturing of PFS with biologic oxidation. It reported that these factors were pH, the addition of pyrite, temperature, stirring, and reaction time. Under the conditions of pH2.0~2.3, 2.8g pyrite/L, 30℃, and 300 r.p.m., a mid-product was obtained in 10 hours reaction. Then PXT coagulant(that is PFS) was made through acidification, concentration, and aging. When the PXT was used to treat the high SS ash water of power plant, it could decrease not only SS but pH and F^-.

    In the process of biologic oxidation, both thiobacillus ferrooxidans and thiobacillus thiooxidans play an important role. They are the peculiar micro-organisms which can obtain carbon resources from CO₂ in air and live in inorganic environment. They can oxidize iron and reduce sulphur into ferric and sulfuric acid, respectively.

    The preparation of PFS with biologic oxidation is a very good way. The way does not need any other raw materials besides pyrite and a little dilute sulfuric acid used to adjust the pH of reaction system. It is characterized by simple proces, saving energy. Nevertheless, many studies should be done if the method of biologic oxidation is used in industrial production of PFS. At present, this method is still in the initial stage of study and development.

3.2 Method of electrolytic oxidation

    It should be pointed out that the electrolytic oxidation of ferrous sulfate is also a preparing methods of PFS. First, ferrous sulfate was oxidized into ferric sulfate with D.C. electricity in electrolytic bath. Then , P

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