4 Manufacture of solid PFS

    In general, the product of PFS is liquid state. This product has the merit of being easy to use in waste water treatment process, but its shortcoming is that the cost of transportation is higher than solid state because of its valid component lower than solid. So some studies about the manufacture of solid PFS had been made.

    Solid product had been developed though concentrating liquid PFS^[9,12,15,27], such as heating evaporization, stoving, and vacuum evaporation and drying. We call these methods the two-step process as it includes both the preparation of liquid PFS and the concentration.

Kreplka J^[30,31] reported that solid PFS could be manufactured by oxidizing the solution of ferrous sulfate with air under the conditions of 120~180℃ and 0.3~2.5 MPa. In addition, Prasak T P^[32] developed a simple method of manufacturing solid PFS: firstly, preparing solid basic ferric sulfate by oxidating ferrous sulfate with air in a revolving blast furnace under the conditions of 200℃ and 3 hours, and then obtaining solid PFS by making the solid basic ferric sulfate reacts with sulfuric acid. The shortcoming of the method is that the oxidation of ferrous sulfate is not complete in the first step. Using solid raw materials, Li Kechang^[33] also succeeded in manufacturing solid PFS through solid phase reaction.

    Recently, Wang Duoren^[34] reported that solid PFS could also be made by one-step process. First, mixturing the solution of potassium chlorate and the suspension of ferrous sulfate. Then, obtaining solid granulated PFS under the conditions of 100℃and pH2-3 after 1 hour reaction.

    Although having overcome the shortcoming of the high transportation cost of liquid PFS, solid PFS consumes enormous heat energy in manufacture process, and it have to be dissolved again before being used in flocculation process. So the suitable method of manufacturing PFS should be to make high concentration liquid PFS. 

5 Stability of PFS

    If liquid PFS is long-term stored, Fe(OH)SO₄ sediment will be produced in PFS solution, that is, liquid PFS is unstable. The stability of PFS is a very universal and very important problem, and it must be considered in the manufacture and the application of PFS, but this problem is reported seldom in most literatures about PFS.

    Liu Changrang et al.^[9] had studied the stability of PFS and pointed out that liquid PFS, which was made from ferrous sulfate with catalysis oxidation method, was unstable. One stabilizer was added in liquid PFS in orde

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