SPENT CAUSTIC RECYCLE AT FARLEIGH MILL

CAUSTIC SODA is routinely used in most Australian raw sugar factories to clean evaporator vessels and other process equipment. After repeated use, the caustic soda becomes fouled with both soluble and insoluble contaminants. The end result is the efficacy of the caustic soda is diminished and requires complete replacement. A low cost method for the disposal of spent caustic is required. Spent caustic soda is commonly disposed of, if possible, by trickling into the mill’s effluent to control pH levels in the effluent treatment plant. The other practical option available at present is removal and disposal by specialist waste treatment companies. Other options for caustic soda regeneration and re-use are not regarded as viable. This paper examines the recycle of spent caustic soda to process in the 2012 and 2013 seasons and considers some impacts on process operations for Farleigh mill. Recycling of spent caustic soda at about 100 ppm sodium hydroxide on cane to primary mixed juice has the potential benefits of reducing scaling in the multiple effect evaporators and decreasing lime consumption. Lime consumption reduced by about 33 and 81 ppm on cane during periods of recycle in 2012 and 2013 respectively. The heat transfer coefficients for the multiple-effect evaporator at Farleigh mill during periods of spent caustic soda recycling compare favourably with previously published data. The costs for cleaning chemicals have reduced from about A$0.152/tonne cane for seasons when recycling was not undertaken to about A$0.083/tonne cane for the two seasons when recycling was undertaken. The reduction in cleaning chemical costs could be influenced by good quality cane, fine weather and good management of the evaporators at the time of the study.