Inside a sugar factory • Learning objective: • To understand the key stages in the production of sugar
At the sugar factory The extraction and production of sugar from sugar beet takes place at factories on a large scale. Other products are also produced such as icing sugar, caster sugar, Demerara sugar, dark and light brown sugar and syrups. Co-products from the production of sugar are also produced including animal feed, soil, stones, tomatoes and electricity.
1. Weighing and sampling A sample from each delivery of sugar beet entering the factory is weighed and then tested to determine its sugar content. On average one hectare of sugar beet crop yields about 41 tonnes of clean, topped roots from which seven tonnes of sugar can be extracted.
2. Cleaning In the cleaning stage of the process the sugar beet is washed in large quantities of water, allowing it to pass through weed and stone separators, before being separated from the water by a vibrating screen, known as a dewatering screen. The sugar beet flows through the separators by the force of gravity. The co-products of this process are topsoil and stones, which are removed for separate processing at this stage.
3. Slicing and diffusion The slicers slice up to 520 tonnes of sugar beet an hour and work in a similar manner to a kitchen grater. They cut the sugar beet into thin ‘v’ shaped strips called cossettes. These are then pumped into three diffusion towers where they are mixed with hot water to extract the sugar.
3. Slicing & diffusion The water temperature in the diffusers is about 70°C; a little cooler than water from a household kettle. The sugar passes from the plant cells into the surrounding water. From this process two important substances remain – the pulp and the sugar that is in the water, which is known as raw juice.
What happens to the pulp? The pulp is mechanically pressed to extract as much remaining sugar and water as possible. Molasses is added, before it is dried at 880°C (about nine times hotter than a household kettle) before being formed into pellets. The pellets are sold in bulk as animal feed for cattle, sheep, horses and other livestock.
4. Purification The raw juice from the diffusion process passes through an important purification stage called carbonatation. Milk of lime (calcium hydroxide) and CO2 (carbon dioxide) gas are added. During this process, the CO2 and milk of lime re-combine to produce calcium carbonate which precipitates out, taking most of the impurities with it.
What happens to the lime? The solid waste is removed through a filtration system which uses a series of frame/plate presses with fine grade cloth filters. Pressure is applied, which squeezes out the juice, leaving lime solids, trapped by the cloth. This lime contains trace elements such as v potassium and magnesium, which is then sold to farmers as a soil improving fertiliser.
5. Evaporation The juice that remains from the purification process is called thin juice. The next stage in the process, evaporation, is where the water is boiled off in a series of six evaporator vessels, known as multiple effect evaporators. This process increases the solids content of the juice from 16% to 65%. The liquid that remains is known as thick juice which then goes through the crystallisation process.
6. Crystallisation The thickjuice is placed in pans which boil the juice under pressure (vacuum), to lower the boiling point. The thickjuice is seeded with tiny sugar crystals which provide the nucleus for larger crystals to grow. A solution which is about to crystallise is known as super saturated.
6. Crystallisation Once the crystals have reached the desired size, the process is stopped. The resultant mixture of crystal sugar and syrup, known as massecuite, is spun in centrifuges to separate the sugar from the mother liquor. Once the sugar crystals have been removed the remaining juice is returned to the process to be spun again. The sugar crystals are washed, and after drying and cooling, are conveyed to storage silos at a rate of 1,200 tonnes on an average day.
7. Packaging The sugar that is produced in the factory is sent to the packaging complex where it is packed via a series of automated machinery. A machine automatically channels and measures the amount of bags going through and pulls them onto a pallet. Once on pallets, the bags of sugar are shrink wrapped to protect them from moisture.
Summary The stages of sugar production include: • Weighing and sampling • Cleaning • Slicing and diffusion • Purification • Evaporation • Crystallisation • Packaging