Investigation into the Filtering Quality of Raw Sugar

The investigation into the filterability of raw sugar was In response to occasional complaints from customers, attention has focused on the filtration behaviour of raw sugars. Offending sugars, together with several others, were extensively analysed to establish the probable causes of the problem. Parameters correlating most strongly with poor filterability were turbidity and soluble phosphate (r = -0,83; n = 32) and (r = -0,76; n = 32) respectively. Starch (r = -0,67; n = 32), suspended solids (r = -0,66; n = 32), gums (r = -0,60; n = 32) and dextran (r = -0,50; n = 32) showed lesser correlations. When ranked according to phosphate levels, all the complaint sugars with the exception of one shipment were grouped at the top. Introduction To the refiner the filterability of raw melt is important because it influences refinery throughput. Processing of good quality raw sugars results in considerable savings in the refinery operation. Most raw sugar manufacturers are willing to do their utmost to provide sugar of acceptable quality as long as there are no great deviations from the normal methods employed in the manufacture of raw sugar. The findings presented here, although not novel, reflect the current situation with regards to certain quality parameters, in particular filterability, associated with South African raw sugar. They are certain to stimulate debate as to whether wholesale changes need to be made to the process currently employed in raw sugar production, or whether it is simply sufficient to ensure good process control with existing methods. Changes in clarifier design or a move to saccharate liming or sulphitation may effect the desired changes in sugar quality. A change in liming procedure would not involve considerable expense. Based on results to-date it is evident that poor filterability is a sporadic problem. It is not clear, however, whether it is related to particular mills or is a consequence of seasonal or geographic influences. For a complete picture one would also require an investigation into the effects which cane variety and harvest techniques have. The investigations into these issues present a complex task made even more difficult by the fact that it is in all likelihood no one single factor that would impact on raw sugar filterability but rather a combination of several factors. Many of these factors are beyond the control of the mill which must therefore develop procedures to cope with the problem or optimise existing procedures. initiated as a result of occasional complaints received from overseas customers. The complaints originated from predominantly one source, a new customer, operating a carbonatation refinery. Although some of the observations resulting from the investigation are interesting in so far as they have highlighted potential deficiencies in sugar quality, it must be borne in mind that the aim was to provide a 'quick fix' to a problem. The work may be viewed as incomplete in terms of a thorough investigation into the filtration impeding impurities in raw sugar. Retained samples of the problem sugar were analysed together with a number of other shipment sugars selected in such a way that a cross-section of good and bad filtering sugars made up the database. In addition samples of Komati sugar which had been analysed for filterability by the South African Sugar Terminals (SAST), and exhibited above average results, and two sugars (Benchmark 1 and Benchmark 2) provided by the complainant, and said to have good filterability were included as benchmark samples. A local carbonatation refinery (Tongaat-Hulett Refinery) (HR) had experienced filterability problems with certain sugars drawn from the South African Sugar Terminals and these samples 2 , 4 and 7 complete the sample set. Several analyses are routinely performed either by the SAST or by the Sugar Milling Research Institute (SMRI) and these together with the analytical results for suspended solids, bagacillo, turbidity, pH, elemental analysis and phosphates (P205 ppm) are given in Table 1. The additional analytical parameters are some of those which have been reported in previous publications as having an impact on sugar filterability. All refiners do not employ the same method of melt clarification and a sugar that behaves well in a carbonatation refinery may not necessarily do so in a phosphatation refinery. How the individual impurities impact on filterability with particular reference to a carbonatation refinery is discussed later. Experimental Thirty-two sugars (unaffinated) were analysed for those parameters indicated in Table 1. The filterability test is routinely performed by SAST and is essentially the method outlined by Nicholson and Horsley (1956). Test methods for the other analyses were taken from the ICUMSA Methods Book and the Laboratory Manual for South African Sugar Factories (Anon, 1985). Dextran levels were measured using the Roberts (1982) method. 242 Proc S A@ Sug Technol Ass (1998) 72 Table 1. Analytical results for raw sugar tested.