Sugar Manufacturing Operations

2.1 Cane handling
2.2 Milling
2.3 Clarification/evaporation
2.4 The pan stage
2.5 The fugal stage
2.6 Final sugar
2.7 Energy supply systems
2.8 Associated operations

A sugar mill is a large factory used to produce raw sugar and other products from sugar cane. Mills are made up of a range of industrial plant such as boilers, storage and processing vessels, crushing and hammer mills and a large range of maintenance equipment. Mills operate in two distinct modes, crushing and non-crushing, both of which introduce a range of specific and general hazards to employers, workers and others. In essence, a sugar mill can be broken into the following processes (see Figure 2 for a diagram that shows the sugar milling process).

2.1 Cane handling

Cane handling describes the methods used to move cane billets into the milling section of the process. Billets are transported and stored using items such as:

cane railway bins
road transport systems (such as multi-lifts and semi trailers)
in field transporters.

The cane billets are then transferred into the milling system by:

trans-loaders (such as from road to rail)
tipplers (tipping cane bins into carriers)
direct tip into the carrier (by infield transporters and road transport).

Rail transfer methods use large hydraulic systems to push or pull rakes of bins into the tippler which tips them onto a ‘carrier’ (a moving floor conveyor). Most mills have storage yards for excess bins. Tipplers are a rotary device which hold the rail bin in place and turn it 180 degrees to empty its contents into the main conveyor (carrier).

2.2 Milling

The milling process involves the initial breakdown of cane into its primary fibres by a large hammer mill (shredder). Shredders consist of a number of large hammers (usually around 12 kg in weight) attached to a rotor by swing rods which are then driven at around 1200 revolutions per minute (rpm) by mechanical means (either by steam turbine or electric motor). The billets are shredded by smashing them between the hammers and the grid bar (a hard set of plates on one side of the shredder) breaking them into individual strands of fibre. This fibre is then processed through a series of crushing mills to extract juice. Mill rollers exert huge forces on the shredded cane which is fed through them via a vertical chute. The pressure between the rollers is large enough to break down the cell structure of the fibres so that the sucrose can be extracted within the juice. Juice contains a large amount of water which is removed or reduced in subsequent processes. The remaining fibre is then burned in a boiler to produce steam which drives most mill processes in a typical factory.

Extraction of as much of the sucrose as possible is a key element in milling. Mills use a number of methods to aid sugar extraction which include the application of hot water (around 95ْ C) to the fibre within the mill set, a series of crushing mill sets (the milling train) and reapplication of mixed juice and water (maceration) throughout the milling process.

2.3 Clarification/evaporation

The clarification/evaporation stage executes a number of functions such as:

mixed juice incubation
adjusting PH by addition of lime
heating
addition of flocculant (a product which assists contaminants to subside)
addition of anti-scale chemicals
removal of mud and heavy contaminants
reduction of water levels in the juice.

Heating is completed using shell and tube heaters that are normally either cylindrical units with multiple passes for juice in tubes surrounded by steam (allowing thermal transfer between the two products) or multi-path plate and frame commercial units that are smaller than conventional heaters and are constructed from pressed SS sheets separated by gasket material.

Lime and flocculant are usually added to the juice as a slurry. A subsider then removes heavy contaminants from the juice. Subsiding, the process of allowing heavy materials to sink or fall to the bottom, usually removes the majority of dirt and the chemical mud formed from the reaction between the phosphate in the juice and the added lime from juice. The mud is then spread across a moving filter (a rotary drum filter) and ‘washed’ to leech out any remaining sucrose before removal from the factory. Mill mud is a nutrient rich product which is normally returned to the field.

The effet stage consists of a number of evaporators (large kettles) in series that boil the juice to reduce the water content. Effets are constructed in a particular pattern using multiple effet evaporation. Vapour produced from each vessel is used to boil the juice in the subsequent vessel at a lower pressure making maximum use of the energy initially put into the first vessel as low pressure steam. The latter effets in the set are operated at a vacuum in order to reduce the boiling point. The final product from the effet stage is usually known as ’liquor’ or ‘syrup’ and is a dark gold coloured liquid.

Dependent on juice properties heating surfaces within the effets and contact heaters are prone to contaminant build up (scale) which reduces heating efficiencies and after a period needs to be removed. Most factories use a chemical process to remove scale build up, normally by boiling caustic soda in the vessels or other chemical means such as sulphamic acid or rarely EDTA. On some occasions manual cleaning is required and is completed by blasting with high pressure water or mechanical brushing.

2.4 The pan stage

The pan stage is a similar process to the effets in that a pan boils off additional water. The main function of the pan stage is to produce sugar crystal from the liquor. In order to increase the speed of this process the pan stage operates in a manner which utilises ’seed crystal’ and a combination of products with varying levels of sugar content to produce a range of crystal sizes and hence qualities. The pan stage has many storage tanks such as receivers (tanks which receive product from the pans), crystallisers (a series of tanks and stirrers which cool the product from the pan stage resulting in additional crystal growth before fugaling) and large transfer pipes and valves.

2.5 The fugal stage

A fugal is a large electric centrifuge which spins up to 1200 revolutions per minute (rpm) dependent on its function and stage of operation (while filling batch fugals only turn at around 50 rpm). There are two types of centrifuge in use within sugar mills, high grade centrifuges (usually batch, but sometimes continuous) and low grade centrifuges which are continuous. Continuous fugals maintain a constant flow of product through them while batch fugals fill, operate and then discharge the final product. The fugal stage removes the remaining liquid product which surrounds the crystal, washes the crystal and delivers it into the final sugar system through a series of conveyors and a drier. The material removed during the centrifuge process is known as molasses and has a range of uses including sale as stock feed, fermentation for distillery production and as a component of cattle licks.

2.6 Final sugar

Finally, the sugar crystal is dried and moved to large storage bins awaiting transport to sugar terminals or other areas (such as refineries). Driers are large cylinders which are fluted and rotate to pass the crystal through at an even rate whilst dry air is applied via ducted fans or large air conditioners. Moisture levels and sucrose purity are important measures for sugar quality. Storage bins hold large amounts of raw sugar and the conveyor system supplying them can be directed into different bins dependent on the product type. Low moisture levels in final sugar product and atmospheric conditions can create a risk of sugar dust explosion. Sugar dust explosions are rare, however, they have caused significant damage and loss of life in sugar mills overseas.

2.7 Energy supply systems

Mills are usually powered by steam and subsidised by electrical devices, however in recent years a number of factories are moving to predominantly electric powered equipment. A standard sugar mill will still include equipment such as suspension or multiple fuel boilers, steam turbines, electrical generators and all of the associated distribution equipment for electric and steam power. A range of equipment is associated with steam and electric energy including transformers, high and low voltage distribution systems, protection devices such as circuit breakers, steam relief valves, expansion joints and water traps.

Mills also have extensive air distribution systems supplying general and instrument air.

2.8 Associated operations

A range of facilities associated with sugar production are located on site including:

laboratory and associated processes
packaging lines
engineering workshops covering areas such as rolling stock repair, general engineering and fabrication, and electrical
administration areas
molasses storage and distribution systems
water supply and effluent systems
mud, ash, bagasse and other by-product handling and storage.

Figure 2 The sugar milling process

Figure 2 The sugar milling process