From raw material which consists of sugar canes perhaps 10 feet long, modern mills produce sugar of high quality, the chief waste product being used as fuel for the boilers which supply power for the manufacturing processes
SET OF CENTRIFUGALS for a sugar mill. Centrifugals perform the final purification of massecuite, as the mixture of sugar crystals and mother liquor is called. Each centrifugal consists of a drum, with a diameter of about 40 in., revolving on a vertical shaft at a speed of between 800 and 1,200 revolutions a minute.
THE cultivation of the sugar cane is known to have been practised in India before the Christian era. The cultivation gradually extended to Egypt, Spain and the Canary Islands. Columbus carried some sugar cane plants from the Canaries to Santo Domingo in the West Indies. The plants thrived so well there that their cultivation spread rapidly to Cuba, Mexico, Brazil and other parts of the New World. At the present day the production of sugar from the cane is carried out on a large scale also in Java, the Philippine Islands, Hawaii, Formosa and Australia. Natal produces sugar on a smaller scale.
One method of extracting sugar from the cane used by the earlier civilizations was by pounding the canes with a pestle in a mortar. Others methods required small wooden roller mills, driven by cattle, for pressing out the juice. This was collected and placed in open pans to settle; then it was boiled and cooled in various ways. By these crude methods only about half the available sugar was extracted.
The systems now adopted in the manufacture of sugar from sugar cane and from sugar beet are somewhat similar. For cane sugar the canes are crushed and the juice is pressed out of them by powerful rollers. For beet sugar the beet is sliced up and placed in a battery of diffusers which leach out (drain away) the sugar with warm water in stages. This chapter describes the manufacture of cane sugar by what is known as the sulphitation process.
Before the canes leave the plantation the tops, trash (leaves) and roots of the canes are cut off. The cane stalks, about 10 feet long and up to some 2 in. in diameter, are then transported to the factory by such means of conveyance as are used in the locality. On arrival at the factory the cane stalks are fed on to the carrier, sometimes by men and sometimes by mechanical methods.
The cane carrier is a form of conveyer consisting of endless chains, across which are fixed steel, or sometimes wooden slats, preferably with the edges overlapping to prevent bits of cane from getting jammed in the intervening spaces. The mass of long stalks, as it passes up the carrier, is from 3 feet to 5 feet deep, and in that, condition would be difficult to pass through the roller mills. The first operation, then, is to cut the stalks up into suitable lengths. This is done by a machine consisting of numerous knives which are fixed on a shaft that is rotated at a high speed. Sometimes two sets of knives are used. The first set levels off the cane in the carrier and the second set, which is run at a higher speed, cuts the stalks into short lengths.
PROCESSES IN THE MANUFACTURE OF WHITE SUGAR FROM SUGAR CANE
Key to Flow Diagram: 1. Return Juice Pumps; 2. Juice Heater; 3. Juice Liming and Sulphuring Tank; 4. Sulphured Juice Tank; 5. Juice Eliminators; 6. Juice Subsiders; 7. Clear Juice Tank; 8. Mud Blow-Up; 9. Filter Press; 10. Air Compressor; 11. Sulphur Ovens; 12. Syrup Eliminators; 13. Syrup Subsiders; 14 Sulphured Syrup Tank; 15. Syrup Supply Tank; 16. Molasses Supply Tank; 17. Massecuite Gutters;
18. Molasses Pumps. The flow of sugar is shown by the black lines.
The canes are then passed through a shredder which still further reduces their size. One type of shredder contains a drum revolving at about 1,200 revolutions a minute. On the periphery of the drum are mounted from 100 to 300 steel beaters, which may be pivoted at the inner end so that, should they meet with a serious obstruction, they will yield to it, the centrifugal action due to the high speed normally keeping them straight out radially. Just beyond the beater tips is a row of steel anvil bars with bevelled edges. As the canes pass into the machine, they are sheared off and pulverized small enough to go through between the anvil bars to a chute which feeds the first set of rollers, called a crusher. In some instances, the shredder is placed after the crusher.
The crusher used for many years, before the introduction of knives and shredders, was of the Krajewsky type. This consists of a pair of strong cast-iron rollers, each with zigzag grooves about 2 in. deep. The ridges of the one roller worked in the corresponding furrows of the other, but without them. By this means the canes were torn up and partly crushed. The two rollers were placed one above the other, and slightly inclined forwards so that the juice which was squeezed out of the cane would tend to fall from the top of the bottom roller rather than run down the front of it.
This machine takes a great deal of power, in some instances 300 horsepower, and it has been sometimes questioned whether the results obtained make its use worth the expenditure of so much power. It does the hardest part of the whole process, however, an operation which would otherwise have to be distributed in succeeding mills.
Powerful Nests of Springs
Another advantage of the crusher is that the speed at which it is run may be varied independently of the mills, and thereby provides the necessary quantity of canes to meet the demands of the mill as they arise. Crushers are now generally made with three rollers spaced in a triangular form — one above and the other two below.
From the shredder the cut-up canes pass through the mills. Each mill consists of a set of three rollers, also mounted in triangular form in a strong cast-steel housing. The rollers are made of a special hard cast iron, and have small circumferential grooves cut in them. A trash bar fixed between the bottom two rollers directs the cane from the front and top rollers to the top and back rollers, so that none will pass through downwards between the bottom two rollers. The setting of the rollers is an important matter, as the amount of space between any pair has a direct bearing on the efficiency of the mill.
These spaces vary according to the stage of rolling and the size of the mills. Roughly, the gap between the top and front bottom rollers of the first mill may vary from 7/8 in. to 1½ in., with from 5/16 in. to 7/16 in. between the top and back bottom rollers. By the time the fourth mill is reached the gaps are reduced to about 1/16 in. only. These dimensions are arranged according to the percentage of fibre, or woody substance, in the cane, according to the quantity to be crushed, and according to the surface speed of the rollers. The percentage of fibre in the cane may vary from 10 to 20 per cent, according to the nature of the local growth.
Great pressure is required to keep the rollers down to their work. The pressure may be provided either by powerful nests of springs pressing direct, or through toggles, on the top roller (as sometimes adopted for crushers), or else by hydraulic pressure from a hydraulic accumulator and pumps. This pressure is from about 60 to 80 tons to every foot of the roller’s length. Thus a mill with rollers 3 feet in diameter and 7 feet long would have about 500 tons on its top roller. The speed at which the rollers are run varies from three to five revolutions a minute in the first mill, with a gradual increase of about 5 per cent in each successive mill.
Between each two mills is a slat conveyer to transfer the cane from one mill to the next in series. During the process of crushing in the mills a certain quantity of water has to be added, depending on the value of the sugar extracted.
THREE-ROLLER CRUSHER, which prepares the cut cane for the mills. The rollers are circumferentially grooved, and are made of a special hard cast iron. Spring or hydraulic pressure is applied to the top roller.
Most of the water is added before the fourth mill (assuming the installation to consist of four mills). The dilute juice extracted from the fourth mill is pumped back to the third, and juice from the third mill is pumped to the front of the second mill, where the water mixes with the juice from the first mill. When there is a great deal of liquid — either juice or water, or both — there is a tendency for the rollers to slip and polish. One method of avoiding this is to provide a few deep circumferential grooves in the front rollers, in addition to the more numerous small grooves. The deep grooves form a convenient channel for the liquid to run away more freely and also for more cane to enter the mill without slipping.
Having passed through the full set of mills, by which time all the sugar has been pressed out of the cane, with the exception of perhaps 2 to 3 per cent, the remaining fibrous substance, called bagasse, is carried off by an inclined conveyer to the boilers for fuel. Bagasse contains about 50 per cent of moisture, and thus requires special furnaces and firegrates for the boilers, with large combustion spaces. Further, as the fuel value of the substance may vary, it is necessary to provide ample heating surface to the boilers, and at the same time to supply engines large enough to develop their full power even should the boilers be working with a less satisfactory type of fuel.
The power generally used to drive cane sugar mills is steam. Even when the mills are driven by electric motors, the electricity is generated by steam turbo-generators in the factory. Enormous quantities of steam are required for the boiling processes, and it is therefore advantageous to use it for power production also. Only about 8 to 10 per cent of the heat value of the steam generated is required for engine power, the- rest of it being used for heating the juice in heaters, and for the evaporation of the juice in the evaporators and the vacuum pans. Each pound of steam used in an evaporator will evaporate about three and a half times its own weight of water extracted from the juice.
Mixed with Milk of Lime
From the crushing mills the mixed juice, as it is called at this stage, having been strained and weighed, is pumped through a first-stage heater. This consists of a nest of long straight copper or brass tubes, expanded at either end into steel plates. They are enclosed in a cylindrical casing of steel or cast iron. The juice flows through the tubes, which are themselves surrounded by low-pressure exhaust steam at about 8 lb. per sq. in. This raises the temperature of the juice to about 180° F.
From the heater the juice passes to a juice liming and sulphuring tank, vapours and air being released in a small vapour flash tank on the way, to avoid air disturbance in the settling tanks. In the liming and sulphuring tank the juice is mixed with a measured solution of milk of lime, produced in a liming mixer. This is a cylindrical tank containing a steel basket for holding the quicklime. A slowly revolving stirrer in the tank stirs up the liquid, dissolving the lime and making milk of lime. The unburnt lime remains in the basket and is removed as required. The object of introducing this lime water is to neutralize any acid present in the juice. The lime water and juice are kept agitated by stirrers in the juice liming tank, or the agitation may be done by compressed air.
UNLOADING SUGAR CANES from ox-carts in Jamaica. The canes from the plantations are brought to the railway by ox-carts and loaded into wagons for transit to the factory. Sugar cane stalks are generally about 10 feet long, and have a diameter of about 2 in. Before the canes leave the plantation, the tops, trash (leaves) and roots are lopped off
Sulphur, produced in a special sulphuring plant using sulphur stoves, is admitted into this tank in measured quantities. The action of the sulphur is to bring the condition of the juice back to neutral. The limed and sulphured juice then passes into a sulphured lime juice tank, and from there is pumped through second-stage heaters — generally two — in which the juice is raised to a temperature of about 230° F. The combined action of the lime, sulphur and heat is to coagulate and form the suspended gums and dirt into a dense solid precipitate which will settle and leave a clear amber-coloured liquid.
From here the juice goes through a clarification process. The vapours and air having again been allowed, to escape from a vapour flash tank, the juice is delivered into eliminators. These are tanks, heated by steam coils. In these tanks any gums which may be present rise to the surface and are brushed off into gutters provided round the edge of the tank. Sometimes phosphoric acid, or some other clarifying agent, is added in the eliminators.
The juice then goes through two stages of settling tanks called subsiders. Here it is allowed to stand for two or three hours, in which time the muddy matter and other solids are precipitated to the bottom of the tanks and the clear juice is decanted off into the clear juice tank. The muddy matter deposited in the subsiders is drawn off into a “mud blow-up”, where further lime may be added to give a denser precipitate, and the muds are heated by steam jets.
Filter Press
As the muds still contain a certain amount of juice which might be extracted, they are made to pass through a strainer, and are then pumped into a filter press. This consists of a large number of plates covered with filtering cloth through which the muds are pumped under great pressure. The juice passes through the cloth and, having become separated from the muds, is delivered into another clear juice tank, from which it is pumped back into the clarified juice tank after the subsiders. The mud from the filter press is removed and disposed of as a fertilizer.
The clear juice from the subsiders and filter press is now pumped into the evaporator supply tank, from which it goes through the important process of evaporation. This process is generally carried out in three or four stages. The clarified juice now contains the normal cane juice, added maceration (softening) water, milk of lime water, filter press washings, and water used in thinning scums. These together almost equal the weight of the original canes, and contain in solution about 16 per cent of solids. The temperature is now about 200° F.
The evaporator is a cylindrical vertical vessel from 3 feet to as much as 15 feet in diameter, and about half as high again. At the bottom of each vessel is a nest of brass or copper tubes occupying nearly half the height of the vessel. The tubes have a diameter of about 2 in. and a length of 5 feet; they are expanded at the ends into brass or copper tube plates. The nest of tubes is known as a calandria. Steam exhausted from the mill engines at a pressure of about 8 lb. per square inch is admitted to the space round the tubes of the calandria.
CALANDRIA, OR NEST OF TUBES, which is placed at the bottom of a vertical cylinder known as an evaporator, in the manufacture of cane sugar. The tubes have a diameter of about 2 in. and a length of 5 feet. They are expanded at the ends into brass or copper tube plates.
The juice is delivered into the bottom end of the vessel, and passes up through the tubes to an outlet pipe placed just above the top. tube plate, and the temperature is raised to boiling point by the surrounding steam. The vapours emanating from the juice boiling in the first vessel' rise to the top and pass over into the steam space of the calandria in the second-stage evaporator, where they also serve to boil the juice but at a lower temperature. Although the temperature at which water boils at normal atmospheric pressure is 212° F., that temperature steadily rises as the pressure increases, as in a boiler. Similarly, as the pressure decreases below atmospheric pressure, or when vacuum conditions hold, the boiling-point temperature decreases. Thus, with successive stages in this process, and a falling absolute pressure — or as we might say, an increasing vacuum — a gradually falling temperature in the vapour produced by the boiling juice will still in each successive stage be capable of boiling the juice in the corresponding stage.
The temperature of the steam in the calandria of the first evaporator is that corresponding to about 5 lb. pressure above atmospheric pressure, or 228° F. The vapour from the boiled juice in that vessel would leave that vessel at about 216° F. and enter the steam space of the calandria of the second vessel at about that temperature. The same operation would be repeated in the third vessel and the fourth. The final temperature of the calandria steam space would be about 180° F., and that of the
vapour passing out about 126° F., whence it would pass off to the condenser below atmospheric pressure.
By this time the juice has concentrated to a clear syrup, which contains 60 lb. of sugars in solution for every 100 lb. of liquid. When the juice entered the first evaporator it contained only, from 12 to 18 per cent of solids.
During these stages the steam which entered the space round the tubes of the first evaporator has, in its process of heating the juice, given up some of its heat and been condensed into water. This warm water is drained off and, being pure distilled water, is fed into the boilers. The condensed steam from the succeeding stages has, however, absorbed certain acids, due to the presence of organic and other acids, and has to be.. drawn off separately as it would not be safe to use this acid-laden water in the boilers.
From the evaporators the juice, which has now become syrup, is pumped into the syrup eliminators and from these it passes to the syrup subsiders. The process of separation from the solid contents still remaining is similar to that undergone in an earlier stage by the mixed juice. The muds or other solid contents left in the subsiders are again treated by the mud blow-up and filter press.
The syrup is delivered into the clear syrup tank, from which it is pumped into the sulphured syrup tanks. Here its colour may be improved by further treatment with sulphur dioxide gas from the sulphur ovens. This plant comprises a small air compressor which blows air through the burning sulphur in ovens producing sulphur dioxide gas, and then into the sulphuring tanks. The syrup is then pumped into large overhead supply tanks alongside the vacuum pans which are fed from them.
Formation of Crystals
The vacuum pans, of which there are generally three, are vertical cylindrical vessels of great size. Of the three generally used, two, or sometimes all three, are fitted with a calandria at the bottom similar to those in the evaporators. The other vacuum pan may be fitted with several copper tubular coils placed in the body of the vessel. During the boiling process a vacuum of about 27 in. is maintained in the pans, these being connected to the condenser. The steam pressure in the coil of the pan so fitted varies from 5 lb. to SO lb. per sq. in., the lower pressure being used in the steam space of the calandria vessel. Vacuum and temperature gauges are fitted to all the pans, with sight glasses to show the height of the massecuite (a mixture of sugar crystals and mother liquor). Apparatus known as proof-sticks is provided for withdrawing samples, and other fittings are installed so that the conditions inside the pans may be carefully watched.
SUGAR MILLS AND GEARING driven by two engines. The cut canes come down the slat conveyer, through the crushing rollers and into a series of four roller mills.
The boiling operation for each strike, or filling, of syrup is from three to four and a half hours. As the syrup boils it thickens and crystals are formed. An ordinary method of boiling a pan of sugar is to draw in sufficient syrup (containing 50-60 per cent of solids) to produce a quantity of minute crystals, which will grow and fill the pan. The vacuum having been established, steam is turned on to the steam space of the calandria, or the coils, and as the syrup thickens steam is turned off the coils, which become uncovered. After granulation, syrup is drawn in, in amounts so regulated that the crystals grow in size, but no fresh crystals are formed. If the charges are excessive and the crystals are small they may be remelted. If the crystals are large, a secondary growth of crystals may take place, called false grain. This causes loss and interferes with the separation of the mother liquor in the centrifugals.
As the depth of the charges increases in the pan additional coils are gradually brought into service, until the pan is finally filled with a mixture of crystals and mother liquor containing about 94 per cent solids. The vacuum is then destroyed, the discharge valve at the bottom is opened and the contents are dropped into the mixer, or crystallizer, as it is called.
The crystallizer is a closed cylindrical tank, or it may be open at the top. The crystallizer has a revolving stirrer which slowly turns the massecuite over and over for from two to six days.
The centrifugal performs the final purification of the sugar at this stage. The machine, embodying a circular steel drum or basket, with a diameter of about 40 in., is fixed to a vertical spindle rotated at a speed of from 800 to 1,200 revolutions a minute.
The centrifugal action causes the molasses contents of the massecuite to fly outwards through the graded wire meshing of the basket into the annular space between it and the fixed cylinder in which it is contained. The massecuite is fed into the basket through hoppers while running, and the molasses is discharged from the annular space surrounding the basket. The machine is stopped and the sugar drops through an opening in the bottom of the basket to a central hopper.
Rotary Sugar Dryer
There may be two sets of centrifugals, each serving a somewhat different process. The discharge of massecuite from the crystallizers can be delivered to what are known as the foreworker centrifugals, in which the sugar is separated from the mother liquor and then discharged to the magma mixer. Here it is mixed to a magma (or thin paste) with syrup taken from the vacuum pan supply tanks, and then pumped to the other set of centrifugals, or to the crystallizers for further mixing. The magma formed in the magma mixer may be dried in the centrifugal machines to give a second-quality sugar, or it may be mixed with first-quality massecuite and treated with it in the usual way. The second-quality sugar is of a light brown colour, because of the greater percentage of impurities. The dry sugar from the centrifugals drops into some form of conveyer, of belt or canvas, or having overlapping slats attached to endless chains. When the sugar is in a free state the cleanest and most satisfactory method of handling is by a grasshopper conveyer. This consists of a trough carried by springs which create a forward and return movement, so that the sugar is made to travel forward by a series of jumps.
From the conveyer the sugar is discharged into a rotary sugar dryer. This machine raises the sugar on a number of shelves, and by a slowly rotating movement the sugar is showered across a current of warm air which is blown through it. By the time the sugar has reached the end of the slightly inclined machine it is in a fit condition for consumption.
The by-products of the cane sugar sulphitation process are the bagasse, which is generally used for fuel; the filter press discharge, which may be made into fertilizers; and the molasses, which is used either for the manufacture of alcohol or rum, or for making food cake for animals, and in the coarser grades for fertilizers. It requires from eight to ten tons of sugar cane to produce one ton of finished sugar.
VACUUM PANS are large vertical cylinders in which the cane juice is boiled in a vacuum of about 27 in., to form massecuite. Vacuum and temperature gauges are fitted to all the pans, in addition to sight glasses which show the height of the mixture inside.
