Solid-liquid mass transfer during leaching of calcium from dilute slurriesof flyash

Flyashes obtained by burning Curragh and Tarong coal were air elutriated in to seven size fractions. Samples were then leached with deionised water in shaken and stirred vessels. All size fractions were leached in an orbital s haker 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 co nductivity of the leachate. The fine fractions (d(32) approximate to 2 micr ons) were also leached in an unbaffled turbine mixer at: a minimum speed, s o that the particles remained in suspension. The leaching of calcium as cal cium hydroxide into the diffuse double layer which forms around the negativ ely charged particles was examined. A mass transfer model was developed, ba sed on rate expressions derived for the transport of calcium as calcium hyd roxide from the surface of the particle into the diffuse double layer, from the diffuse double layer into the boundary layer and from the boundary lay er into the bulk solution. Changes In the thickness of the diffuse double l ayer were taken into consideration. The model assumes rapid leaching of ion s from the surface shell and a uniform calcium concentration across the dif fuse double layer. From the experimentally measured concentration data for calcium ions in the diffuse double layer and in the bulk solution, transpor t values for the model were deduced. It was found that mass transfer from t he surface of the flyash into the diffuse double layer is the rate controll ing step.