In this chapter, we continue with next step of cane sugar processing. In previous milling step, fresh cane juice is obtained. To obtained safe consumable sugar, clarification process shall be done before evaporation and crystallization process.
Clarification: In this step, impurities will be removed by the means of sedimentation. Fresh cane juice is mixed with slaked lime (Calcium hydroxide, Ca(OH)2) at around 70 oC. If sulphitation method is selected, SO2/SO3 are added to the solution at the equal stoichiometric amount to previously added slaked lime. Sulphur oxide amount shall be controlled tightly to avoid excess Sulphur present in the end-product. It is expected to form Calcium Sulphate (CaSO4) which can adsorb the impurities and colors from the solution. The solution is heated to around 105 oC where CaSO4 can be precipitated. Then, the solution enters a sedimentation tank which is designed 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.