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Raw Sugar Receiving
Some refineries are attached to cane mills while others are stand alone facilities. In a stand alone sugar refinery, raw sugar will arrive in bulk by barge, rail, or truck for unloading into the raw sugar warehouse. The raw sugar will undergo metal and debris removal and large lumps will be crushed prior to transfer to the mingler to begin remelting and refining.
Raw sugar is mingled with hot affination syrup which melts the outermost layer of the crystal. This outer layer contains the largest concentration of color. Remaining syrup is separated from the sugar crystals in a centrifuge. The bulk of the colorants are removed during the affination step (about 50% of raw sugar color) and then during the clarification step (about 40% of melt liquor color).
The affinated sugar is dissolved with hot condensate to a liquid concentration of approximately 72 ºBrix at 75 ºC prior to defecation.
Approximately 40% of the remaining colorants are removed in the clarification step. There are two alternative types of defecation processes used in cane refineries; carbonatation and phosphatation.
Carbonatation, also called carbonation, begins by adding lime (CaO) to the melt liquor produced during affination. This juice then passes through a carbonation vessel. The reaction between the carbon dioxide and the lime produces a calcium carbonate precipitate. Color bodies are entrapped in the precipitate and are removed during filtration of the solids. In the limed melt liquor, destruction of invert sugars occurs due to the high pH produced by the lime. The closing step takes place by adding polymers to the juice to assist in the formation of a precipitate floc which is more easily settled and filtered.
During phosphatation, lime (CaO) and phosphoric acid (H3PO4 or P2O5) are added to the melt liquor. This results in the formation of a calcium phosphate precipitate. Color bodies adsorb onto the calcium phosphate precipitate and are removed during the subsequent clarification and filtration. Polymers are added once again to aid in the formation of a precipitate floc which is more easily filtered.
The clarified juice from carbonatation or phosphatation contains suspended solids that plug the interstitial spaces and blind the pores of the decolorization resin, bone char, or carbon. Precoat vacuum filters, precoat pressure filters, deep bed multimedia filters, or some combination of these are used to produce a filtered syrup that flows through the decolorization columns without causing a pressure buildup. The next step involves decolorization of the sugar juice.
Several techniques can be used for removing color from the sugar juice and they are subject to continuous developments. The main ones being:
Activated carbon: Numerous types of activated carbon are available in the marketplace according to the precursor carbonaceous material (coal, wood, coconut, etc.) and their size. The most common types used for sugar juice decolorization being powdered activated carbon (usually termed as PAC) and granular activated carbon (GAC).
Bone char: Pyrolyzed ground animal bones have a high surface area to adsorb color and remove some ash.