Refining Process

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Project Description

The fundamental objective of the raw sugar refining process is to capture all of the sucrose as free‑flowing shiny white sugar crystals, and to leave behind all of the impurities that are normal constituents of unrefined raw sugar. The goal is to create a food ingredient of the highest quality, eminently suitable for direct consumption, or as a component of many other products in the food and beverage industries. The removal of color, non-sucrose solids, fine particulates and other impurities is accomplished via a series of processes throughout the refining operation, briefly described below:

Melting:

The raw sugar crystals are dissolved in hot water and mixed with other concurrent sugar liquors circulating in the overall refining process, to produce raw melt liquor with a sugar purity of around 97%. The melt liquor is then screened to capture the major insoluble impurities.

Carbonatation:

In this process stage, milk of lime is added to the raw melt liquor and then carbon dioxide gas is bubbled through this mixture in large carbonatation vessels. All flows and process conditions are carefully controlled to achieve an optimum temperature, pH and alkalinity, at which time a calcium carbonate precipitate is formed. This precipitate attracts and absorbs many of the impurities present in the mixture, including some of the color and all of the waxes and gums left over from the original sugar cane processing. The milk of lime ingredients are locally manufactured, and the carbon dioxide gas is extracted from boiler flue gases produced in ASC’s own power house.

Filtration:

The limed and carbonatated melt liquor is passed through 5pressure filters working in parallel which remove the calcium carbonate precipitate, and all of the impurities which it has attracted and captured. After further processing to recover any trapped sucrose, the precipitate leaves the refinery together with its impurities as a pressed semi-dry filter cake for use on agricultural land as a soil improver.

Decolorization:

The filtered liquor now passes through columns filled with ion exchange resin which capture color molecules from the liquor. When the resin in any column approaches its exhaustion point, the columns are taken off line in turn, and the resin is regenerated with brine solution. The resin is reactivated to its maximum potential by the brine solution, and the color molecules are taken away at the same time. The colored brine is then treated in a process known as nano filtration, where the color molecules are removed and concentrated, and the brine is recirculated to be used again. The color molecules finally end up in molasses.

Evaporation:

Up to this point in the refining process, all prior stages have been undertaken with liquors that are 60-67% concentration (measured as dissolved solids). Decolorized fine liquor now passes to the evaporators where the liquor is concentrated by the removal of water (by evaporation) to produce concentrated fine liquor at 74%.

Crystallization:

The concentrated fine liquor is then boiled under reduced pressure in the vacuum pans, to concentrate it further to the point where a known number of very fine sugar crystals (around 5 microns in size) can be stably introduced into the liquor. These small crystals are then grown over the next few hours to their full size of 700 microns (0.7 mm). The crystallization process is controlled with the use of computerised instrumentation which constantly monitors and adjusts each aspect of the crystallization process.

Centrifugation:

When the sugar crystals are fully grown, the mixture of crystals and liquor(now known as massecuite) is passed through centrifuges, where the crystals are separated from the mother liquor in which they were grown. The separated liquor is still rich in sugar, so it is passed to a second and then a third crystallization stage to maximize the sugar crystal yield. The separated crystals are washed whilst still in the centrifuge, and then passed to the drying stage.

Drying& Cooling:

The white sugar crystals from the centrifuges still retain up to 1% of their weight as water from the washing. This is removed in a rotating dryer, where the sugar crystals pass counter-currently through a flow of filtered, hot, dry air to reach a moisture level of 0.04%. The sugar crystals then pass to a fluidized bed cooler.

Sugar Conditioning:

Cooled sugar is conveyed to tall maturation silos where filtered warm dry air is passed upwards through the stored sugar for up to 72 hours. This removes any residual moisture retained within the sugar crystal structure, to ensure the sugar will remain perfectly free flowing in the future.