PHOSPHATE REMOVAL IN A FLUIDIZED-BED .2. PROCESS OPTIMIZATION

The aggregation of fine primarily formed calcium phosphate particles w ith sand grains in a fluidized bed for phosphate removal was studied e xperimentally by means of a set-up which isolated aggregation from oth er processes during calcium phosphate precipitation, as well as throug h experiments under normal operation of the fluidized bed. The net agg regation process was described by means of a mathematical model which takes into account two competing mechanisms: orthokinetic aggregation and breakage. The net aggregation process was found to account for sim ilar to 60% of the phosphate removed by the fluidized bed. It was foun d that the orthokinetic aggregation can be improved by spreading the s upersaturation more evenly throughout the reactor, and breakage can be diminished by a low energy dissipation rate in the bed. Optimization of the phosphate removal efficiency was therefore achieved by selectin g sand grains of small sizes (0.1-0.3 mm) and a low superficial veloci ty (7 . 10-3 m/s), and by spreading the addition of the NaOH solution (reactant) over two dosage points. Under these conditions the phosphat e removal efficiency was similar to 80%. Copyright (C) 1996 Elsevier S cience Ltd