(222d) Effect of Preparation Variables On Morphology and Water Flux of PVDF Hollow Fiber for Membrane Distillation

Metallurgy is one of the pillar industries of China. The conventional pyrometallurgy suitable for high grade ore reserves is facing the significant emission problem. Hydrometallurgy can provide wide and varied techniques and their combinations to process the lean and complex ores. Nevertheless it is demanding in process materials and energy with great amounts of liquid and/or solid wastes as side effect.

Due to its mechanisms, membrane-based separation possesses the advantages of energy efficiency and bio-friendliness. Application of membrane-based separation in metallurgical industry does not have a long history, but seems to be a growing prospect as it has achieved obvious economic and environmental benefits against the aforementioned background.

This seminar will firstly present the current status and potential of applying membrane separation in the hydro- and pyro-metallurgical field with detailing the flowsheets, environmental influence and economic analysis. Our research and development work in the fabrication of micro-porous hydrophobic membranes for membrane distillation (MD) will also be highlighted.

Porous poly (vinylidenefluoride) (PVDF) hollow fiber membranes were fabricated in current work by the dry/wet spinning and phase inversion. Effects of PVDF concentration, glycerol content and external coagulant composition on membrane morphology and performance were investigated. Taguchi experimental design with a set of the nine experiments was used to elucidate the relationship between the above conditions and water flux in DCMD. The highest flux in desalinating 3.5wt.% NaCl aqueous solution was 23kg/m²-hr at average feed temperature of 62.5^oC with 99.9% rejection. Analysis of variance revealed that glycerol content was the more influential factor than the other two. Predictions regarding the performance of membranes involving other preparation conditions were in agreement with experimental results.

The FESEM study showed that finger-like macrovoids were developed underneath the inner surface of all the fibers, which indicated rapid phase demixing occurring at the lumen skin of the membrane. Increasing the ethanol content in external coagulant facilitated the macrovoids growth at inner skin but depressed those at the shell side; higher PVDF content in dopes inhibited the macrovoids expansion; the macrovoids in fibers prepared with 5% glycerol was bigger as compared with those in fibers containing no glycerol and 10% glycerol. The inner skin of the fibers prepared with glycerol in dopes was porous. Gas permeation test demonstrated that the pore size of the shell skin ranged from 2nm to 50nm. This range corresponds well with that revealed by FESEM. Mechanisms for the membrane morphologies evolved were discussed using XRD characterization and the characteristics of phase inversion.

 The membrane surface morphology (i.e. pore size) correlates positively with both gas flux and DCMD flux, with the latter being influenced more obviously. It was also found that the surface morphology influenced membrane long-term performance in desalination of waste acid containing dissolved salts discharged from metallurgical process using both DCMD and vacuum membrane distillation (VMD).