Caso flotation

SOME DEVELOPMENTS IN FLOTATION CLARIFICATION
IA SMITH, GT SCHUMANN AND DC WALTHEW Tongaat-Hulett Sugar Ltd

Abstract Flotation clarifiers are used on syrup in some southern African mills to improve raw sugar quality. Air bubble size is a critical performance factor, and sintered pipe sections have replaced mechanical aerators for the dispersion of air in the clarifier feed. The process is particularly effective in eliminating suspended material from direct consumption raws. Data are provided on removal of bagacillo and increase in filterability. The results of plant trials on the use of syrup sulphitation in conjunction with flotation are summarised. A purification process for raw melt has been developed using the same design of flotation unit as for raw syrup. This involves liming of the melt followed by a light carbonatation. The resultant precipitate is floated off and disposed of by returning to a syrup flotation clarifier in the raw house. The requirement for filters is reduced to a “check filtration” duty on clarified melt, eliminating the cost and complexity of the two-stage filtration and desweetening of carbonatation muds. Laboratory and preliminary plant results for this process are mentioned. Introduction Flotation clarification of syrup has been in operation at a few South African mills over the past 20 years. The original motivation was to enable production of VHP quality sugar at mills that received poor quality cane (Rein et al., 1987; Rein, 1988). Use of flotation clarification processes has subsequently been extended to meet the following needs: l Reduction of insoluble matter in direct consumption brown

Operation of Flotation Clarification. Importance of Air Addition The formation of a scum that is stable for several hours and is easily removed from the surface of the syrup depends on having air bubbles ranging in size from 5 to 20 microns, that are incorporated inside the floc formed by the flocculant. Bubbles that are too large break out