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 isparticularly 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 limingof 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 resultsfor 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:
lReduction 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 breakout of the floc after a while and the particle will sink. Early methods of aeration used oversized centrifugal pumps with the impellers machined down so that they were inefficient and produced high turbulence (Rein et al., 1987). These pumps gave erratic performance and proved troublesome to maintain. They have since been replaced by sintered sparge tubes which have generally proved successfulprovided that they are of the correct construction and pore size, and that velocity of the syrup flow across the surface of the sparger is high enough to strip off sufficiently small bubbles. Syrup Heating and Cooling Raw syrup requires heating to 85°C prior to flotation. This should be a relatively simple operation but problems have been experienced with blockage of plate heat exchanger channelsin one case, and with steam hammer in a direct contact heater at another installation. Cooling of syrup after flotation clarification is necessary to avoid sugar loss and colour formation in subsequent storage. This is achieved simply and effectively using a small flash vessel whose vapour space is connected to the evaporator final effect. Scum Depth The flotation clarifier is provided with anadjustable weir that controls the level of the syrup (and therefore the scum) in the clarifier. The consistency and depth of the scum is examined by the operator from time to time and the level of the syrup set by adjusting the weir height if necessary. Automation of this process has been patented (Schumann, 1995) using infrared measurement of scum level, but life of the sensors immersed in the hotsyrup has been a problem. The use of fibre optics may provide the solution here. Reduction in Insoluble Matter by Flotation Clarification
l Further reduction of VHP sugar colour (trial basis only) l Low-cost purification of melt in a back-end refinery.
Following some comments on operation of the flotation process in general, this paper discusses applications of each of the above....