Filtration ability of hollow fiber membrane for production of magnesium ammonium phosphate crystals by reaction crystallization

Abstract

Relationship between magnesium ammonium phosphate (MAP) crystal properties and the filtration ability of hollow fiber membrane (HFM) were investigated. Phosphorus recovery process by crystallization has a problem that it produces a large amount of fine crystals. So improvement of the crystallization process by combining with filtration was discussed. MAP crystals were obtained by batch reaction crystallization and the filtration characteristics were investigated. The filtration was evaluated by the specific filtration resistance (α_c) on HFM. Filtered slurry was prepared with each suspension density and crystal size distribution. The solution was filtered at constant pressure of 0.02 MPa and the filtration time on each filtrated volume was recorded. As a result, α_c decreases exponentially with suspension density increasing from 0.25 g/L to 0.5 g/L and α_c decreases moderately with suspension density increasing from 0.5 g/L to 1.5 g/L. α_c of large crystals decreases exponentially at less suspension density than α_c of small crystals does. Also, α_c increases as the ratio of the fractured crystals increases.

Key words： membrane separation; crystallization; MAP

Acknowledgments

Authors thank KURARAY Co. Ltd. for financial support.

Nomenclature

C0	Suspension concentration (kg∙m-3)
J	Filtration flux (m∙s-1)
K	Filtration constant (m2∙s-1)
⊿P	Filtration pressure (Pa)
V	Filtrated volume per membrane area (m)
Vm	Filtration volume till cake formed (m)
αc	Specific filtration resistance (m∙kg-1)
μ	Filtrate viscosity (Pa∙s)

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