Settling parameters determined during thickening and washing of red muds

This paper substantiates why it is necessary to reduce the concentration of sodium hydroxide in red mud before it can be transported to the mud disposal area and why the liquid-to-solid ratio should be maintained after each unit of the thickening and washing line. The main unit is a single-tier radial thickener comprised of flocculation, free settling, hindered settling and rake zones. The paper gives a brief overview of the basic mathematical formulas describing the hindered and free settling zones of the thickener, as well as the basic material balance equations for describing the countercurrent washers. At the studied site, no reagent is added at the washing stage and there is no flocculation zone in the washers. The first part of experimental study focused on red mud thickening with addition of dissolved flocculant that is normally used at the studied site, considering how it is prepared and injected. Sedimentation curves for red mud samples with different concentrations of solids in the feed stream were obtained. The authors substantiate the application of modified Kynch method for calculating empirical parameters based on sedimentation curves. The following parameters were calculated: settling rate, average Stokes diameter of a flocculated particle, critical concentration (gel point), hindered settling index. The Kynch flux density functions were calculated for different concentrations of solids in the initial slurry. The second part of experimental study focused on countercurrent washing of the earlier flocculated slurry. Following a series of periodic experiments on sedimentation of flocculated slurry involving 3, 4, 5 and 6 cycles of countercurrent washing, conclusions were drawn on the concentration of sodium hydroxide at each washing stage, and the washing efficiency was estimated. The described results will be used for building a mathematical model of a radial thickener at the thickening and washing stage.