Solid-Liquid Mass Transfer During Leaching of Calcium from Dilute Slurries of Flyash

Abstract

Flyashes obtained by burning Curragh and Tarong coal were air elutriated into seven size fractions. Samples were then leached with deionised water in shaken and stirred vessels. All size fractions were leached in an orbital shaker at 1:25 ash water ratio. The results suggest that ions from a surface shell and from the particle interior make distinct contributions to the conductivity of the leachate. The fine fractions ( d 32 ≈ 2 microns) were also leached in an unbaffled turbine mixer at a minimum speed, so that the particles remained in suspension. The leaching of calcium as calcium hydroxide into the diffuse double layer which forms around the negatively charged particles was examined. A mass transfer model was developed, based on rate expressions derived for the transport of calcium as calcium hydroxide from the surface of the particle into the diffuse double layer, from the diffuse double layer into the boundary layer and from the boundary layer into the bulk solution. Changes in the thickness of the diffuse double layer were taken into consideration. The model assumes rapid leaching of ions from the surface shell and a uniform calcium concentration across the diffuse double layer. From the experimentally measured concentration data for calcium ions in the diffuse double layer and in the bulk solution, transport values for the model were deduced. It was found that mass transfer from the surface of the flyash into the diffuse double layer is the rate controlling step.