Carbonatation is a robust and efficient process to clarify cane sugar refining liquors. In this process calcium hydroxide reacts with carbon dioxide forming a calcium carbonate precipitate. Sugar liquor is separated by filtration for further processing, and carbonatation sludge is discharged. This sludge represents 1.6 ton (dry basis) per 100 ton of raw sugar and, in certain parts of the world, disposal presents an environmental problem. Some solutions have been presented since the beginning of the century. Until now, solutions found include calcination for reuse in sugar industry, utilisation for agriculture purposes (pH soil correction) and application in ceramic and paint industry.
One possible utilization of carbonatation sludge is its reuse in the Carbonatation process. Carbonatation sludge is heated to destroy organic matter and further heating causes calcium carbonate calcination. In laboratory tests it was observed that calcined carbonatation sludge may substitute fresh lime, maintaining the same decolourisation efficiency, with some extra advantages.
A process to minimize solid wastes quantity and decrease liquid effluents in gas washing was studied (Bento et al., 1999). In this process Carbonatation mud was dried and calcinated, to be used in gas neutralization and in carbonatation reaction.
Dried mud, in powder form, was reacted through cyclones with boilers gas. It was observed a reduction of SOx and NOx in the gas. After this treatment, gas does not need a neutralization wash and a reduction of water and sodium carbonate consumption will be observed. Quantity of liquid effluents will also be reduced.
In the same studied, part of dried mud was calcinated at 900ºC. At this temperature calcium oxide is formed and it can be used in carbonatation reaction.
In laboratory, carbonatation sludge was treated in a muffle at temperatures between 400 and 1000 ºC during 30, 40 and 60 min. This treated carbonatation sludge was used to carbonate Affination Liquor, previously diluted at 40 ºBrix, in a 2000 ml vessel. Refinery gas with 12 % of CO2 was used for gassing. Lime or treated carbonatation sludge was added to diluted liquor in a quantity equivalent to 0.60% of CaO (dry basis). Gas was bubbled until final pH of 8.5 (20ºC).
The results obtained are presented in Table 1.
Table 1 – Carbonatation decolourisation using sludge
treated at different temperatures
| Decolourization | |||
| Temperature | Heating Time | ||
| 30 min | 40 min | 60 min | |
|
400 C |
41.4 % |
|
45.3 % |
|
00 C |
45.6 % |
|
46.7% |
|
600 C |
46.5 % |
|
47.4 % |
|
700 C |
46.0 % |
|
48.4 % |
|
800 C |
|
50.8 % |
|
|
900 C |
|
57.8 % |
|
|
1000 C |
59.8 % |
It is known that during carbonatation high molecular material is partially removed and invert sugar are almost entirely destroyed. To observe the effect on these compounds, carbonatation of Affination Liquor was done, as described earlier, using calcined sludge at 1000 ºC in parallel with fresh lime. Starch, dextran and invert sugar variation was measured. Results are presented in Table 2. ICUMSA colours are also included.
Figure 2 - Comparison of utilisation of fresh and calcinated lime
| Affination | Carbonated | Liquor | |||
| Liquor | Fresh Lime | Calcinated | |||
| Reduction | 1000 C | Reduction | |||
|
Colour,IU |
1160 | 480 | 59.3 % | 470 | 59.5 % |
|
Starch, ppm |
140 | 55 | 60.7 % | 48 | 65.7 % |
|
Dextran, ppm |
558 | 432 | 22.6 % | 352 | 36.9 % |
|
Invert Sugar |
0.15 % | 0.05 % | 66.7 % | 0.08 % | 46.7 % |
The results show that carbonatation sludge calcined at 1000 ºC, when used in Carbonatation, has the same decolourisation effect as observed in the Refinery with fresh lime. Calcination at this temperature produces a calcined sludge with 80 to 95% of CaO. The variation in CaO content may result from filter aid usage in refinery security filtration.
Bibliography
Bento L.S.M., A. Ferraz, F. Bartolo, F. Rocha, M.Mota, 1999, Utilisation of cane sugar refinery sludge for
recycling and boiler flue gas desulphuration - preliminary studies, Proc. of S.I.T. Conf, 143-160
E 0024/29