Concentration of ferric hydroxide suspensions in saline medium by dynamic cross-flow filtration

We have investigated the concentration of ferric hydroxide suspension in sa line solution by high shear cross-flow filtration using a prototype unit wi th a disk rotating parallel to a circular membrane at speeds up to 1500 RPM . Three different membranes were used in the tests: two organic (nylon with 0.2 mu m pores, PVDF with 40 kDa cut-off) and a mineral one (zirconium oxi de, 0.1 mu m pores). The system operates in the laminar boundary layer regi me with a core fluid rotating at about 42% of the disk angular speed. The l ocal permeate flux increases with increasing radius and is twice as large i n the external ring as in the central part of the membrane within a 3 cm ra dius. With the nylon membrane at a speed of 1500 RPM, the permeate flux ave raged over the total membrane area decays from 520 l/h m(2) at a Fe3+ conce ntration of 15 g/l to 250 l/h m(2) at 100 g/l, values which are much higher than those obtained with the same suspension in tubular membranes. With th e zirconium oxide membrane which has a lower permeability, the permeate flu x at a concentration of 9 g/l rises linearly with pressure and reaches 450 l/h m(2) at 110 kPa; which is exactly the value given by hydraulic permeabi lity, confirming the absence of fouling. A particular feature of this device is that the permeate flux actually rise s when concentration increases up to 30 g/l because transmembrane pressure increases with suspension density and viscosity. This work confirms that dy namic filtration is well suited to the concentration of ferric hydroxide su spensions up to concentrations of 130 g/l. (C)2000 Elsevier Science B.V. Al l rights reserved.