Cane Sugar Production at Millaquin Mill, Bundaberg, Australia Content from the guide to life, the universe and everything

Cane Sugar Production at Millaquin Mill, Bundaberg, Australia

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Sugar cane is one of Australia's largest farm crops, with up to 30,000,000 tonnes crushed annually, between June and December. Most cane is grown in Queensland along the east coast, from Nambour (90 minutes north of Brisbane) to the Atherton Tablelands. The production of sugar, can be quite complicated, but can be broken down into four stages:

Stage One - Crushing

The production of sugar begins with the harvesting of sugar cane - a relative of bamboo. The cane stalks are cut into lengths of approximately 20cm and are called 'billets'. The billets are transported to the mill, either by truck or light rail.

On reaching the mill they are crushed through a series of large rollers. This produces raw juice (brown in colour) and cane fibre called 'bagasse' (pronounced 'bar-gas'). Very fine bagasse is called 'bagacillo' (pronounced 'bag-a-sillo'). This bagasse is burnt by the mill in large boilers, to produce high-pressure, super-heated steam to run turbines and produce electricity. The exhaust steam is then used in other stages of production.

Stage Two - Clarification and Evaporation

The raw juice is tested for sugar content and this reading is used in an equation that provides a relative sum to pay the farmers for their cane.

The raw juice is heated to 75°C to melt any starch and is allowed to sit, or 'incubate' for about 30 minutes. Once incubation has taken place, a lime solution is added to the juice to change its pH. Sugar is naturally acidic and breaks down quickly so it must be made alkaline. The juice is again heated, this time to over 100°C. This juice is now called 'secondary juice'. It is briefly exposed to atmosphere to 'flash' - this allows any gases (usually nitrogen) to escape the juice.

The juice is now allowed to settle. Adding small amounts of flocculant (a binding agent) helps the dirt to settle out. Clear pale yellow juice is taken from the top of the tank. The 'mud' is taken out of the bottom of the tank and is processed further. Mud is a mix of dirt, sand and sugar juice. The mud is mixed with bagacillo and put over a filter. 'Filters' are large rotating drums. By using a vacuum, the mud is picked up, water is sprinkled over the filter and washes away any sugar left in the mud. This liquid is run back to the secondary juice and is reprocessed.

The clear juice is now called 'ESJ' or 'Evaporater Supply Juice'. The ESJ is passed through a series of vessels called 'effets' (pronounced 'ef-ays').

These are large upright cylinders, with a series of tubes in the bottom third. As the juice boils through the tubes, vacuum and steam are applied to the vessel to lower its temperature so as not to burn the sugar in the juice. When the juice boils, steam (water) is given off and the juice begins to concentrate. After passing through four or five effets, the juice, now called 'syrup', is stored in a tank, ready to be boiled into sugar.

Now things get complicated.

Stage Three - Sugar Boiling

Not all of the sugar in the syrup comes out in one go, therefore the sugar is extracted in stages: taking out some sugar from the syrup, then taking more from the resultant molasses and finally a third boiling occurs to remove more. Each stage takes approximately twice as long as the previous one, as there is less sugar available in the resultant molasses.

There are three types of sugar produced at the pan stage:

  • A-sugar - This is made by 'footing' a pan - under a vacuum a set amount of magma (a thick paste of small sugar crystal and water) is drawn into the pan. The footing is then fed with syrup until the crystals grow to the right size (0.8 - 0.9mm).

    The crystals are in a mix with molasses. This mix is called 'massecuite' (pronounced 'mass-e-kweet'). The massecuite is dropped out of the bottom of the pan into receivers, to await separation.

    A-sugar produces A-sugar crystal and A-molasses

  • B-sugar - This is the same as A-sugar except the pan is fed on A-molasses and takes twice as long to grow the crystal, because we have already removed some of the sugar.

    B-sugar produces B-sugar crystal and B-molasses.

  • C-sugar - A set amount of A-molasses is drawn into a pan and boiled until it is very thick, this is then seeded with minute sugar particles and allowed to grow, fed on B-molasses, until the crystal reaches 0.2 - 0.3mm. This can take up to seven hours.

    Once the C-sugar has been dropped into receivers, it is allowed to continue to growing for 24 hours by pumping it into 'crystalisers' - large stirred holding pans. As the C-massecuite cools, it concentrates, helping the crystal to develop.

To separate the crystal from the molasses, the sugar is 'fugalled' (pronounced 'few-galled').

Stage Four - Fugalling

Massecuite from the A- and B-receivers is fed into 'fugals' in batches.

A fugal is similar to an upright spin-dryer. The massecuite is spun against fine screens, the molasses is spun out and piped to holding tanks. The sugar crystal is dropped out of the fugal and taken to a large tumble-dryer and dried.

A- and B-sugar, mixed and now called shipment sugar, are now ready to be stored for further processing in a refinery.

C-sugar is made by briefly reheating C-massecuite from the crystalisers and feeding it into continuous fugals. The molasses spun off is called C-molasses - or final molasses - and is used in making rum or for stockfeed. The crystal or C-sugar is mixed with a little water, to form a thick crystalline paste, and pumped back to the pan stage to be used as footings for A-and B-sugar. This paste is called magma.

And Finally

Shipment sugar is transported to bulk storage, to await transport, for export or to refineries for further processing into white sugar, such as table sugar or caster sugar, and fancy sugars, such as golden syrup or coffee crystals.

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