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includes and occludes organic impurities such as the gums, amino acids and colour
components mentioned earlier, removing them from the sugar syrup. The carbonatation
process is carried out in two stages to obtain an optimum quality precipitate for filtration, i.e.
a suitable size and distribution of precipitate particles. Measurement of the electrical
resistance of the solution indicates the residual lime content. Eighty to ninety percent of
precipitation is sought in the first stage. The second stage is controlled by the measurement
of the pH of the solution which is important throughout the process and ensures complete
precipitation of the lime.
pH control
The pH of factory liquors is of considerable importance. Below pH 7 sucrose is hydrolysed
to the reducing sugars glucose and fructose, while above pH 9, alkali destruction of sugars
occurs and coloured components are formed.
Filtration
The calcium carbonate precipitate, including the impurities, is now removed in a pressure
filtration stage using polypropylene filter cloth as supporting media and utilising the calcium
carbonate as a filter aid. The filter mud is later subjected to water washing to remove sucrose
residue and this mud is a waste material. Sweet sugar recovered by washing the mud is used
for melting in an earlier stage.
Step 3 - Char Filtration
The relatively pure honey coloured liquor from the filtration stage, "raw liquor", is then
subjected to final decolourisation by contact with bone charcoal. The bone char consists of
active carbon on a calcium phosphate skeleton. It has a high surface area and the the unique
ability to absorb colour and inorganic ash impurities from the sugar. The bone char used at
the Chelsea refinery is imported from Portugal or Scotland. Following the decolourisation
cycle the bone char is revivified first by water washing, to remove inorganic impurities, and
then heating in the absence of air to 650oC to volatilise organic impurities. The decolourised
'fine liquor' is now ready for the final refining and recovery step, which is achieved by
crystallisation in vacuum pans.
Step 4 - Crystallisation
Crystallisation is not only a means to convert the sucrose to a more usable form, but also an
important refining step, since pure sucrose tends to crystallise out of the solution, leaving
most of the impurities in the associated syrup. The process is carried out under a reduced
pressure of 75 - 90 kPa to allow a reduced boiling temperature (60 - 70 oC), so avoiding the
further formation of colour compounds. Vacuum is achieved by the use of an air pump and
vapour contact condenser. These condensers utilise salt water for cooling drawn from the
nearby Waitemata Harbour. Fine liquor is concentrated until it is super-saturated then stage
is shock-seeded by the addition of a small quantity of sucrose to give spontaneous nucleation
of fine sucrose crystals. The crystals thus formed are grown under automatically controlled
conditions until the desired final grain size is achieved. The pan contents, known as
massecuite, are then discharged to receivers prior to separation on automatic centrifugal
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machines. The sugar thus recovered is then dried and graded prior to packing, the syrup
being recycled for three further recovery boilings. The final syrup is used as the starting
material for other sugar products, such as soft brown sugar, coffee crystals, golden syrup and
treacle.
General information
The Chelsea Refinery, Auckland, has a production capacity of 4 500 tonnes per week and
normally operates on a five day, 24 hour, basis. The main power source is from steam
produced in an natural gas fired boiler station. The process represents an efficient heat usage
as high pressure steam is used in expansion engines to produce electrical power for pumping
cooling water. The exhaust steam from these engines is then utilised for heating and
evaporation. Process control instruments are basically electro-pneumatic.
REFINED PRODUCTS
Granulated sugar containing 99.93% sucrose and sold as castor sugar (fine crystals) and 1A
sugar is the major refined product. Soft brown sugar is a specialty product with a
characteristic flavour, produced by crystals used from a selected syrup with a high reducing
sugar and ash content. Coffee crystals are made from the same syrup as brown sugar, but are
crystallised over a much longer period of time giving much larger crystals.
Liquid sugars
Several grades are produced in liquid form with quality to meet customer requirements. This
sugar is produced at 67o Brix (% solids) at which density it is unlikely to crystallise. Liquid
sugar is utilised in industry.
Inverted syrups
Golden syrup and treacle are produced from selected syrups by inverting1 a portion of the
syrup using invertase (an enzyme) or acid hydrolysis to split sucrose into glucose and
fructose. Golden syrup contains about 27% sucrose and 47% reducing sugar with 3% ash
and 18% water. The high reducing sugar component inhibits crystallisation and allows a
table syrup to remain in liquid form. Treacle is produced in the same manner from a similar
syrup except that char decolourisation is not used.
The various products manufactured or packaged by the New Zealand Sugar Company are
summarised in Table 1.
1
Sucrose and glucose are dextrorotatory sugars ([±]D = +66o and +52o respectively), which means that they
rotate plane polarised light in a clockwise direction by the number of degrees given above. Fructose, on the other
hand, is laevorotatory ([±]D = -92o), i.e. it rotates plane polarised light anticlockwise. Thus on complete hydrolysis,
the rotation of plane polarised light passing through the sugar solution changes from positive to negative.
Hydrolysed sucrose is thus often referred to as 'invert sugar' as its rotation has been inverted.
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 Table 1 - Composition of refined products
Sucrose % Reducing Inorganic ash Water %
sugars % %
1A sugar 99.93  0.07 
Castor sugar 99.93  0.07 
Coffee sugar
Brown sugar
Treacle 27 47 3 18
Golden syrup 27 47 3 18
Raw sugar 97.5  2.5 
Liquid sugar 67   33
ROLE OF THE LABORATORY
Most processes are continuous and semi-automatically controlled with direction from a
process control laboratory. Important criteria for control are density, pH and impurity
content (reducing sugars and inorganic ash), temperature and production rate.
ENVIRONMENTAL IMPLICATIONS
Pollution control is given a high priority in the refining process. Atmospheric emission from
the boiler station is strictly controlled using air/fuel ration mixing equipment to ensure
complete combustion with continuous monitoring of smoke emission density. It is
anticipated that natural gas fuel will eventually replace the fuel oil presently used, thus
eliminating sulfur oxide emission. Carbonation mud is used as dry land fill on the refinery
grounds. The material has little recycling potential. Liquid effluent is closely monitored by
laboratory staff.
Written by Mrs. H. Armishaw (Seddon High School). Revised by Heather Wansbrough from
information provided by Peter Simpson (New Zealand Sugar Company Ltd.).
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