Raw Sugar Production Process
1. WEIGHBRIDGE TIPPLER Cane is transported to the factory on Tully Sugar Limited’s highly energy efficient cane railway network consisting of some 280 km of 610mm gauge track. Bin units have a nominal capacity of eight tonnes. Each rake of bins is accompanied by a consignment note from the grower. When the bins reach the cane marshalling yards the consignment notes are collected and the details entered into the weighbridge computer system. When the bin/unit reaches the tippler it is weighed, tipped into the hopper below then weighed again when empty. The net weight of cane is allocated to the particular grower by the computer system.
2. CRUSHING MILLS After tipping the cane passes through a shredder, which reduces the billets of cane to a fibrous mass. The shredded cane then passes through a series of five crushing mills to extract as much sugar juice as possible. Most of the crushing mills are driven by steam turbines. The juice from the first crushing mill is analysed in the laboratory for sugar content (CCS). Payment to growers is based on weight of cane and CCS value.
3. STEAM GENERATING PLANT The fibre remaining after the cane has passed through the milling train is called "bagasse". Bagasse is used as fuel for the boiler plant, which provides steam to power the entire factory including our electricity generating turbines. Bagasse is also stored for use in the lead up to the start of the crushing season and when wet weather disrupts harvesting operations. This makes our factory virtually independent of external energy needs during the crushing season.
4. “GREEN” POWER The vast majority of what you see coming out of our chimney stacks is carbon dioxide and steam. The carbon dioxide is released when our bagasse is burnt. However, it was removed from the atmosphere over the past growing year as the sugar cane used the CO2, water and solar energy to grow. Bagasse that is excess to the factory needs is used to make additional steam for generating “green” electricity, which is then exported to the state grid (enough to power about 5,000 homes continuously). Tully Sugar Limited has been exporting renewable “green” power for use by Queenslanders since 1997. The steam from the chimney stacks is the result of water sprays that clean the exhaust gases by removing particulate matter.
5. CLARIFICATION The cane juice, as delivered from the milling train, contains some soil and other undesirable impurities. To remove these the juice is heated and lime is added to settle the unwanted material. The impurities settle out in the clarifier and then go to the rotary vacuum filters, which filter out any remaining juice. The filter mud from the vacuum filters is rich in nutrients and is recycled back to cane fields. Clear amber coloured clarified juice containing about 15% sugar is decanted from the clarifiers to the evaporators.
6. EVAPORATORS (EFFETS) The clarified juice is concentrated to a thick syrup by boiling off water in the evaporators. The juice passes continuously from vessel to vessel until it is concentrated to a syrup containing approximately 70% sugar. The evaporators are operated under vacuum and in "multiple effect" configuration whereby the vapour space of one vessel is connected to the heating surface of the next throughout the set. This gives maximum efficiency in evaporation and steam usage. Boiling under a vacuum is very energy efficient because the boiling point of the liquid is much lower than at normal atmospheric pressure.
7. PAN STAGE At the pan stage the syrup from the evaporators is converted to crystal sugar. A charge of syrup is taken into a vacuum pan and again boiled under a vacuum. A quantity of very fine “seed” crystal is introduced. As water is evaporated fresh syrup is added and sugar is deposited on the seed crystal. This process continues until the crystals have reached the desired size (approx. 1 mm square). The resulting semi liquid mass of sugar crystals and molasses is called ‘massecuite'.
8. CENTRIFUGALS Sugar crystals are separated from the syrup in the centrifugals. The centrifugals are perforated metal baskets spun at high speed by an electric motor. While the sugar is spinning in the baskets it is given a short burst of hot water to help wash the sticky molasses off the sugar crystals.
9. SUGAR DRIERS Sugar has to be dried and cooled to precise levels to enhance its storage life. This is done in the sugar driers. Evaporation of excess water from the surface of the crystals results in the desired cooling of the sugar. In the generally wet conditions at Tully we use large airconditioning units to provide the cool dry air to the sugar driers. The dried and cool sugar is now ready for shipment.
10. SUGAR SHIPMENT The sugar is transported by road to the bulk sugar terminal at Mourilyan Harbour. Each truck carries 40 tonnes of sugar. The sugar is stored at the harbour in large sheds before being loaded on ships and transported to our overseas customers. All Tully sugar is exported and is widely recognised for its high and consistent quality.