Ferric salts produced in these reactions were hydrolyzed and polymerized:

    2mFe(OH)SO₄ + (1-n/2)_mH₂SO₄ ＝ [Fe₂(OH)_n(SO₄)_3-n/2]_m + (2-n)mH₂O      (16)

or  Fe₂(SO₄)₃ + m×nH₂O ＝ [Fe₂(OH)_n(SO₄)_3-n/2]_m + nm/2 H₂SO₄                       (17)

    It can be seen from the reactions above that the mechanism of the oxidizing reaction of ferrous sulfate had been changed owing to the participation of NO which made the oxidizing reaction easy. The catalyzing oxidation time was shortened to 4~5 hours from original 17 hours. Even so, the time was still longer and the consumption quantity of catalyst was also more. Meanwhile, nitrogen oxides were discharged into air in the manufacturing process of PFS and this resulted in the environmental pollution.

    For the sake of shortening the reaction time, some measures such as heating and high pressure and strong stirring, were often adopted in the manufacturing process. Among them, the method of two steps oxidation could shorten the time. Firstly, ferrous sulfate was oxidized into ferric sulfate under the heating condition. Secondly, both ferrous sulfate and hydrogen peroxide were added in the solution of ferric sulfate made above according to proportion. Then, PFS was obtained after one hour reaction under the stirred condition.

    In order to avoid pollution, Shorten further reaction time, and reduce energy consumption, the manufacturing technology and the manufacturing method of PFS had been investigated thoroughly. Some progresses had been made^[16], for example, the catalysis oxidizing reaction taken place in a hermetic container and the proportion of SO₄^2-/Fe³⁺ was controlled in the reaction system. In addition, taking the place of air, oxygen was used as oxidizer. In such situation, PFS was obtaining under the condition of stirring strongly within 1.5~2 hours and 55~90℃ and 29.4-147´10⁴ Pa. Nevertheless these conditions of high temperature and high pressure and strong stirring will increase the difficulty of technology and the investment of equipments.

     Further improvement was that the speed of adding sulfuric acid was controlled and sodium nitrite catalyst was added in batches in the oxidizing process of ferrous sulfate. Under the conditions of normal pressure and above 40℃, the oxidizing reaction could achieved within 2-3 hours when oxygen was used as oxid

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