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

Abstract

We have investigated the concentration of ferric hydroxide suspension in saline solution by high shear cross-flow filtration using a prototype unit with 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 μm pores, PVDF with 40 kDa cut-off) and a mineral one (zirconium oxide, 0.1 μm pores). The system operates in the laminar boundary layer regime with a core fluid rotating at about 42% of the disk angular speed. The local permeate flux increases with increasing radius and is twice as large in the external ring as in the central part of the membrane within a 3 cm radius. With the nylon membrane at a speed of 1500 RPM, the permeate flux averaged over the total membrane area decays from 520 l/h m 2 at a Fe 3+concentration 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 the zirconium oxide membrane which has a lower permeability, the permeate flux at a concentration of 9 g/l rises linearly with pressure and reaches 450 l/h m 2at 110 kPa, which is exactly the value given by hydraulic permeability, confirming the absence of fouling. A particular feature of this device is that the permeate flux actually rises when concentration increases up to 30 g/l because transmembrane pressure increases with suspension density and viscosity. This work confirms that dynamic filtration is well suited to the concentration of ferric hydroxide suspensions up to concentrations of 130 g/l.