In this chapter, we continue with next step of cane sugar processing. In previous milling step, we obtain fresh cane juice as final product. To obtained safe consumable sugar, fresh cane juice shall undergo a clarification process before evaporation and crystallization process.
This step contains mainly impurities removal by the means of sedimentation. Fresh cane juice is mixed with slaked lime (Calcium hydroxide, Ca(OH)2) at around 70 ºC. For sulphitation method, add SO2/SO3 to the solution at the equal stoichiometric amount to previously added slaked lime. In this process, controls tightly Sulphur oxide amount to avoid excess Sulphur present in the end-product. By the addition of calcium hydroxide and sulphur, formation of Calcium Sulphate (CaSO4) may occur which can adsorb the impurities and colors from the solution.
The solution is heated to around 105 ºC where precipitation of CaSO4 may occur. Then, the solution enters a sedimentation tank to let CaSO4 solid particle settle down to the bottom of the tank. Clear juice is obtained from the top part of the tank with standard ICUMSA target of 100 RBU.
The settled solid particle is collected at the bottom of the tank and transferred to a vacuum rotary filter to recover sugary liquid. The liquid is recycled back to the clarifier, while the solid can be used as a fertilizer for several crops, particularly for cane plantation itself.
Other methods available for this step are phosphatation and carbonatation process. The differences are mostly at the reactant used. Instead of Sulphur oxide substance, phospatation use phosphoric acid while carbonatation uses carbon dioxide. Regarding the clarification process flow, phosphatation produce floating particles that adsorb impurities. Thus, the clear juice is obtained below. Meanwhile, carbonatation process is very similar to sulphitation method. The selection of this process is mainly depending on the availability of the raw material near the factory. In the mountainous region which Sulphur can be obtained cheaply, most sugar factory use sulphitation method.
Why is clarification an important process? In technical standpoint, the impurity in fresh juice can hinder crystallization process. Once we get clear juice, we can begin crystallization process efficiently in the next chapter.