The Effect of Different Extraction Processes on the Nutrient Content of Filtercake

Samples of filtercake from all mills in the South African and Swaziland sugar industries were analysed for nutrient composition at regular intervals during 1979. It was found that at those mills using the cane diffusion process average P, Ca, Mg, Zn and Mn percentage contents of filtercake were considerably higher than those from factories where tandem rollers were used to crush cane. This can probably be attributed to the fact that filtercake percent cane from 8 mills with diffusers was on average about half that from facaverage tories where mill tandems only were used, the latter producing a filtercake containing a larger amount of extraneous matter, which has a diluting effect on nutrient content. Because a cane producer's allocation of filtercake is based on the amount of cane he delivers to the mill and the average filtercake percent cane for the mill, the amount of nitrogen in I filtercake allocated to a cane producer supplying a factory 1967-68 69-70 71-72 73-74 75-76 77-78 79-80 with diffusers was shown to be only half to three quarters of that contained in the from factories with FIGURE I The effect of different extraction processes on filtercake tandems. The percentage of N was also substantially reduced percent cane. by incorporation of fly ash with filtercake. Introduction The last comprehensive survey of the nutrient content of Filtercake from all factories in the South African sugar industry including Swaziland was conducted in 197 1 by Alexander2. The average total nitrogen content of filtercake was then 1,93% and only two mills produced filtercake containing less than 1,60% N. Today the N content of filtercake is often much lower, and in several instances has been close to 1,00%. It was thought that the lower nitrogen content might be zssociated with the diffusion process which has been introduced in a nu'mber of South African mills for extracting sucrose from sugarcane. Figure 1 shows that the amount of filtercake in relation to the amount of cane milled has fallen considerably at those mills which have changed over to the diffusion process. The figures for the Amatikulu and Pongola mills illustrate this reduction particularly clearly. However, where cane is still crushed with mill tandem rollers, the filtercake percent has not declined markedly. Lower nitrogen values may be due in some instances to the addition of flue gas scrubber waste (fly ash) to the filtercake. When the last survey was made relatively few factories extracted sucrose by the diffusion process or cleaned the boiler furnace flue gases. Therefore it was decided that during the 1979-80 milling season samples of filtercake from all mills would be analysed at regular intervals, with a view to determining what changes, if any, had occurred in nutrient content since the last survey, and to examine the agricultural implications of such changes. Procedure At each mill composite samples were taken during July, August,, October and November 1979 using the following procedures. Over a two-day period whenever a filtercake sample was collected directly from the filters for testing in the mill laboratory, a small portion (100-200 g) was set aside in a container, all portions being placed in the same container. At the conclusion of each 48-hour period the resultant composite sample was oven dried to constant mass at approximately 85OC, and the dried material transferred to a plastic bag. The sample was then forwarded to the Fertilizer Advisory Service laboratory at the Experiment Station for analysis. Results and Discussion Table 1 shows the average nutrient composition of the four composite filtercake samples taken from each factory between July and November 1979, together with average filtercake percent cane figures for all mills during the 197980 milling season. Also shown are the average nutrient values for all mills together with those obtained for the same factories during the 1970 and 1971 filtercake surveys1* 2. Comparisons should be made with reservation because the 1970 average values were based on samples taken off the filters in July and October only, whilst the 1971 average values refer to wmposite samples collected over a single week during September 1971. Probably the most meaningful comparison that can be made, therefore, is that between the 1970 average values and the meaned values for July and October 1979, which are also shown in Table 1. Proceedings of The South African Sugar Technologists' Association June 1981 TABLE 1 Average nutrient composition of filtercake samples taken between July and November 19791 (mean of four samples on an oven-dry basis) * CD cane diffuser t MTR mill tandem rollers $ BD bagasse diffuser 8 3 samples only 7 2 samples only It appears that the average nitrogen content of filtercake has declined slightly whilst phosphorus, calcium, magnesium zinc and manganese contents have risen quite significantly since 1970. Table 2 shows that these changes could be largely attributed to those factories now using the diffusion process for sucrose extraction and in particular cane diffusers. In the table the mean nutrient composition of filtercake from three factories which operate cane diffusers (Amatikulu, Malelane, and Union Co-op) and three factories with bagasse diffusers (Empangeni, Entumeni and Pongola) is compared with that from nine factories where mill tandems only were used in 1979 (Darnall, Felixton, Glendale, Illovo, Mount Edgecombe, Noodsberg, Ubombo, Umfolozi and Umzimkulu). Results from three factories where both diffusers and mill tandem rollers were used (Gledhow, Sezela and Tongaat) have been included together with average filtercake percent cane and moisture percent filtercake values. Of significance in helping to explain the higher nutrient content of filtercake from factories with diffusers is the average filtercake percent cane value which, as shown in Table 2, was only half that obtained from factories where mill tandems only were used (see also Figure 1). It would be appropriate at this stage to consider briefly the three processes currently used to extract sucrose in South African sugar mills, na-mely cane diffusion, bagasse diffusion or mill tandem rollers. The difference between these processes is shown diagrammatically in Figure 2. P % avail 2,07 0,74 0,62 0,79 0,67 0,87 0,78 0,71 2,21 0,92 0,58 0,44 1,15 0,77 0,91 0,70 1,04 0,54 1,12