THE MANUFACTURE OF RAW SUGAR

The details of the manufacture of raw sugar from cane and of sugar from beet roots differ, but there are several processes common to both. The operations necessary for making raw cane sugar are as follows:

Every mill has an extensive laboratory where skilled chemists are constantly engaged in sampling and analyzing cane, raw juices, syrups, sugars and molasses. In fact the chemical work is a most important feature in the raw-sugar house, beet-sugar factory or refinery. The superintendent should be an expert chemist, as the proper recovery of the sugar from the cane and beet juices is wholly dependent upon the technical control of manufacturing processes.

After passing the scales, the cars containing the cane are switched alongside the carrier which feeds the cane into the mills. Before the cane is unloaded, however, samples are taken from each car and sent to the laboratory, where they are carefully analyzed. The amount of sugar present is ascertained, as well as the quantity and quality of the juice in the cane. It is, however, impossible to get a fair average sample of the cane in this way, and therefore the efficiency of the mill work is determinedon the basis of an analysis of the juice and the fiber after it has passed through the crushers.

TRAIN-LOAD OF CANE READY FOR THE MILL

A MODERN MILL

The carrier just referred to is a wide slat conveyor, running alongside the railroad tracks in the yards to a point directly over the first set of crushers. The cane is taken from the cars by a mechanical unloader, the arms of which reach out and with distended fingers pull the cane stalks off and land them on the slow-moving carrier, which takes them onward and upward to the crusher.

The crusher consists of two large rolls, with immense interlocking, corrugated teeth on the circumference of each. These rolls are set close together, and the cane passing through is broken into short pieces and matted to an even layer. The juice squeezed out by this preliminary crushing runs through a metal trough into a large receptacle known as the juice tank.

From the crusher the mat of cane passes to the mills proper. These mills consist of from nine to eighteen rolls, about thirty-four inches in diameter and seventy-eight inches long, arranged in groups of three, set in the form of an isosceles triangle, one above and two below, one set following the other in a direct line. The lower rolls are parted enough to allow the expressed juice to fall through them, while the half-crushed cane is carried over by means of an iron bar called the returner. The faces of the rolls are more or less roughened, or grooved, so as to draw the cane through and give a better crushing action. They are turned slowly by powerful engines, which transmit the power to each set of rolls through a system of gears. The rolls are forced together by hydraulic rams exerting a pressure of from four hundred to six hundred tons. It is this tremendous pressure that squeezes the sugar-bearing juice out of the cane.

From the crusher the matted cane passes through the first set of rolls, where a large percentage of the remaining juice is liberated. This is caught in a metal trough and, after passing overa fine screen to remove the small pieces of cane, runs to the juice tank. The cane passes through the second set of rolls, thence to the third set, and so on to the end of the mill. In front of the last set of rolls, hot water is sprayed on the cane to soften the fiber and dilute the remaining juice, thus aiding the final extraction. The adding of hot water is termedmaceration. By the time the cane has passed through the last set of rolls, all the economically recoverable juice is out of it and delivered into the juice tank, with the exception of the juice and maceration water from the last set of rolls, which is always returned to the preceding set of rolls for maceration purposes. The juice or maceration water coming from the last set of rolls contains very little sugar, and the object is to secure greater concentration by using it for double maceration instead of adding that much additional water which would have to be evaporated later on in the process.

In well-designed, modern mills, with cane carrying not over twelve per cent of fiber, more than ninety-eight per cent of the sugar in the cane is extracted, the remainder being left in the fiber. This is almost perfection today. What it will be tomorrow no one can say.

The fibrous, woody part of the cane, or bagasse as it is called, is comparatively dry as it leaves the last rolls. It is conveyed from the mills to the boiler house on a wide slat conveyor, and fed directly into the furnaces under the boilers that generate the steam for power and boiling purposes. A modern raw-sugar mill requires practically no other fuel than that obtained as a by-product from the crushing of the cane.

The boiler plant is usually of large capacity, as a great deal of steam is required to drive the engines that run the crusher, the rolls, the electric lighting system, the pumps and other machinery. Besides, a large amount is needed to evaporate the water in the juice and to boil and dry the sugar. The ashes fromthe furnaces are returned to the fields as fertilizer, so that very little is lost.

CANE CARRIER AND MECHANICAL UNLOADER

ANOTHER TYPE OF CANE UNLOADER

The juice as it comes from the mills contains impurities such as dirt from the fields, small pieces of cane stalks and other foreign matter, besides salts, gum, wax and albumen. It is necessary to remove as many of these substances as possible, and this is where the chemist’s work begins.

So long as the juice is confined in the living cells of the cane it does not quickly ferment, but when liberated it rapidly undergoes such change. Therefore no time is lost in arresting this action. The juice is pumped to the top floor of the mill and there a solution of milk of lime is added in sufficient proportions to neutralize the acidity. The mixture is then heated in closed tanks under pressure to 215 degrees Fahrenheit. The heat causes the lime to combine rapidly with the gums and salts in the juice, and the albumen to coagulate.

The hot juice is then run into large settling tanks, where the insoluble solids and the albumen sink to the bottom, carrying with them vegetable and other matter suspended in the juice. Certain foreign substances of light specific gravity float to the surface in the form of scum.

After settling for a time the clear juice is drawn off and the scum, mud and cloudy liquor left in the tank. As a vast amount of liquor must be handled every hour, it is not practicable to have tank capacity great enough to admit of the liquor standing a sufficient length of time for every particle of foreign matter to settle, so as an adjunct to the settling tank, filters are used. These are cylindrical iron tanks, packed tightly with ordinary wood fiber, known as excelsior. The juice is conducted to these filters, and as it percolates through the excelsior, practically all of the remaining foreign matter is caught and retained in the fiber. The clear juice is then run to the receivingtanks for the evaporators and the mud and scum that remain are drawn off into mud tanks, where more lime is added and the mass stirred up. Finally it is delivered to the filter presses, where the mud and other impurities are taken out and the clear liquor containing sugar is sent to the evaporators.

Another method for cleaning, called “precipitation in motion,” is to carefully lime the juice and then heat it in closed vessels and under sufficient pressure to carry it through a pipe to large insulated settling tanks.

These settling tanks, usually of sheet steel, are made in the form of truncated cones with conical bottoms, the small diameter of the tank being at the top. Suspended in the center is a vertical cylinder somewhat less in diameter than the upper part of the tank. This cylinder extends downward about eight feet to a point opposite the largest diameter, which makes the area between the circumference of the suspended cylinder and the tank at that point very much greater than the area of the cylinder itself. This difference in area is necessary to retard the flow of the juice and allow the sediment, mud and insoluble solids to be deposited at the bottom of the tank.

The juice is delivered by a pipe into the top of the cylinder which projects a few inches above the edge of the surrounding settling tank. It passes slowly down the central passageway, turns at the bottom, where its speed is materially slackened, and goes out through a pipe line connected to the side of the tank just below the upper edge.

There are several other methods in general use, but in all of them the principle of settling, upon which the separation or cleaning depends, is the difference in specific gravity between the juice and the dirt. A high and even temperature should be maintained by preventing radiation, as lowering the temperature would increase the specific gravity and viscosity of the juice without increasing that of the dirt in equal proportion.

TWELVE-ROLLER MILL

MODERN CRUSHING PLANT—TWO FIFTEEN-ROLLER MILLS AND CRUSHERS. CAPACITY, ONE HUNDRED AND FIVE TONS PER HOUR

There are many different types of filter presses, but those at present in general use are long, oblong machines, set horizontally on the floors, with layers of corrugated iron plates, covered with canvas sheets, between which are hollow frames so arranged that the juice will pass from the hollow frames through the canvas to the corrugations in the plates.

In passing through the presses under pressure the sediment, scum and other impurities are caught on the canvas sheets and the clear juice passes through the canvas, down the corrugations and out through small holes in the plates controlled by valves on the outside of the presses, from whence it runs to the evaporator tanks. The sugar in the mud caught in the hollow frames is washed out of the mud with water and is sent to the evaporator, while the mud itself is finally returned to the field, to be used as a fertilizer.

The clarified juice from the settling tanks, filters or presses, is light brown in color, but is thin and watery, and must now be reduced to syrup point. All the suspended impurities have been removed, but some impurities in solution and the original coloring matter still remain. Some of these foreign substances are subsequently eliminated during the process of crystallization in the vacuum pans described later on.

The object to be attained in a raw-sugar house is the production of a sugar containing ninety-six per cent of sucrose, and there is little or nothing to be gained by carrying the process of manufacture beyond the stage that insures such result.

The final extraction of all the impurities and the conversion of the impure raw into pure white granulated sugar is the work of the refiner, which is dealt with in a subsequent chapter.

From the time the juice leaves the cane until it is crystallized it is kept at a high temperature, as cold juices or syrups are viscous and run slowly. High temperatures kill germs, prevent fermentation and expedite manipulation.

Under ordinary atmospheric pressure at sea-level, water boils at a temperature of 212 degrees Fahrenheit and sugar juice at a few degrees higher, according to its density. This temperature if long applied to sugar juice would tend to burn and destroy the sugar, but the juice can be heated to 250 degrees for a short time without deterioration.

The clarified juice contains about eighty-five per cent of water and fifteen per cent of solid matter. A large proportion of the water must be removed by evaporation. To accomplish this under ordinary atmospheric conditions would require heat increasing from 212 degrees Fahrenheit, as the solution increased in specific gravity above the standard of pure water. This would require a large amount of fuel, and the juice would also be more or less adversely affected by long maintenance of comparatively high temperature.

To obviate these conditions the juice is boiled in a multiple evaporator, the invention of Norberto Rillieux, whose first construction in New Orleans in 1840 was a double effect horizontal submerged tube apparatus which has since undergone many changes and improvements. The theory of evaporationin vacuowas extended to two or more cells or vacuum bodies, using the steam or vapor from the first to heat the juice or syrup in the second and so on. At the present time the quadruple effect, or four-cell evaporator, is most commonly in use, although sextuple effects are not rare. The ordinary practice is as follows:

The juice enters cell No. 1 and covers the heating tubes, to which is admitted sufficient steam—generally exhaust from the engines—to cause the liquid to boil. The steam or vapor liberated from this first boiling is conducted through the vapor pipe directly into the heating tubes of cell No. 2, while the juice from cell No. 1 is passed into the second, or cell No. 2, and surroundsthe heating surfaces which contain the hot vapor given off from the same juice in cell No. 1.

DELIVERING BAGASSE TO FIRE-ROOM

GENERAL INTERIOR VIEW OF MODERN RAW-SUGAR MILL

As there is little or no pressure above the liquid in the first cell, the juice boils at from 215 degrees to 220 degrees Fahrenheit. By maintaining a vacuum of five inches in the second cell, the temperature at which the liquid will boil is reduced to 203 degrees, and the vapor from cell No. 1 is hot enough to boil the juice in cell No. 2 without any addition of heat. The vapor from cell No. 2 in the same way enters the heating tubes of cell No. 3, while the juice entering this cell is exposed to a vacuum of fifteen inches, which reduces the boiling temperature to 180 degrees, so that the difference of 23 degrees between the conditions of cell No. 2 and cell No. 3 causes a third boiling and evaporation without any additional steam being added.

A vacuum of twenty-six inches in the last cell, No. 4, brings the final boiling temperature down to about 150 degrees. The vapor from this last cell enters a condenser, where it is exposed to a spray of cold water, is condensed and passes down a pipe not less than thirty-four feet long, terminating in a water seal, and called the Torricellian tube, after Torricelli, who discovered that mercury would rise thirty inches in a tube while water would rise thirty-four feet with a perfect vacuum.

The juice in passing through these evaporating cells is boiled to a syrup containing about thirty-five per cent of water and sixty-five per cent of solid matter. It is pumped out of the fourth cell into the receiving tank for the vacuum pan.

This quadruple system of boiling only requires about one-fourth the amount of heat that would be necessary to do the same work in a single vessel. As the evaporators operate continuously, a constant level of the boiling liquid is maintained in each cell, the juice being drawn from one to the other by increasing vacuum and controlled or regulated by means of valves.

A powerful vacuum pump draws the air and other incondensable gases from the condenser and maintains the vacuum, which is applied to the necessary extent in each cell. The heating tubes are connected to drain pipes, which remove the condensed vapors.

Vacuum, simply and concisely stated, is the absence of air or gas. It is usually obtained by pumps which suck the air or gas out of closed containers or pipes. No doubt many of the readers in their younger days have sucked on the end of a bottle and were amused to find the bottle hanging on the end of the tongue. It was the vacuum, or lack of air in the bottle, which caused it to hang thus. The outside atmospheric pressure (which at sea-level is fifteen pounds per square inch) was doing its best to gain an entrance through the tongue into the bottle from which the air had been extracted.

The pumps simply suck the air out of the containers or pipes and discharge it through valves, in much the same way that the air was sucked out of the bottle. It must be remembered, however, that in boiling water or juice, the vacuum is being continually broken or reduced by the liberation of air and gases from the juice, steam and condensing water. This action must be overcome by the constant work of the vacuum pump.

To determine the amount of vacuum carried in any container, a small mechanical contrivance, known as a vacuum gauge, is used. This, in its simplest form, is a bowl of mercury with a long glass tube leading from it. If the upper end of the glass tube is attached to the container from which the air is to be drawn, the mercury in the tube will rise in proportion to the amount of air extracted. When an absolute vacuum has been formed, the mercury in the glass will stand at a height of thirty inches.

FILTER PRESSES

SET OF QUADRUPLE EVAPORATORS

In commercial operations a vacuum greater than twenty-eight inches is seldom required, as this is sufficient for all practicalpurposes. The degree of vacuum for any container can be varied easily by mechanical manipulation, so that a vacuum anywhere from one to twenty-eight inches may be maintained.

From the receiving tanks the syrup is drawn into the pans by a vacuum ranging between twenty-five and twenty-seven inches. The pans are large cast-iron or copper cylinders, standing in a vertical position, with dome-like tops and conical bottoms, almost spherical in shape. Leading from the top is a large pipe through which the vapors from the boiling are drawn off and condensed. On the conical bottom is a large valve, which may be opened when the boiling is finished to allow themassecuite(a French term meaning cooked mass) to drop out.

At regular intervals in the height of the pan there is a series of copper coils, connected with a steam line at one end and a drain line at the other.

The general principle involved in boiling sugar is the separation of the sucrose contained in a solution from the impurities present in that solution, and this is accomplished by evaporation and concentration through the agency of heat. After the sugar is once formed in definite crystals these crystals attract and appropriate the sucrose in solution in the process of building up the crystal structure, while repelling or excluding the impurities, so that, as a consequence, the latter remain in solution. The crystals thus formed are subsequently removed from the solution by means of centrifugal machines. Crystallization, whether in a pure or impure solution, will proceed to only a certain extent, and will only partially remove the sucrose from the solution in one boiling, the limit of crystallization being governed by the amount and nature of impurities present.

The process of boiling is begun by drawing some of the concentrated juice into the pan and turning steam into the coils,which starts the boiling. This is continued until the supersaturation is such that minute crystals of sugar form or “grain out.” By properly timed admissions of fresh concentrated juice, drawn into the pan by vacuum as before, the crystals grow in size and at last the pan becomes filled with a mass of sugar crystals of regular shape and size, immersed in a thick “mother liquor” containing sugar and the impurities that were not removed by the filters or settling tanks.

The size of the grain may be varied at will by the operator in charge, who is known as the sugar boiler. After the grains are once formed, their number (if the sugar boiler is an expert) does not increase, but the size does, as the original grain continually builds up on itself from the outside.

The question may be asked, why is all the moisture not boiled out in the pan and the sugar dropped in a dry, crystallized state? There are several reasons why such a course is impracticable; first, because the impurities, which must be eliminated by crystallization and which are carried off in the mother-liquor, would be boiled into the sugar and make it unsalable; second, because to aid crystallization and prevent scorching or burning on the hot steam coils the mass must be kept in active circulation during the boiling process, or, long before all the moisture could be driven off, a large part of the contents of the pan would be burned on the coils; and third, even if it were practicable to boil the contents down to a solid state, the grains would stick to each other and become one solid mass, which would have to be removed from the pan with bars, picks or chisels. Enough moisture, or rather liquor, is left in the mass to enable it to flow from the pan by gravity. This liquor, with the impurities it carries, is subsequently removed from the sugar by a drying or separating process which will be explained later on.

Massecuite is a viscous, sticky, semi-fluid mass of the consistency of half-formed ice.

VACUUM PANS

CENTRIFUGAL MACHINES

The reason sugar “grains” is because the water in the juice has the power to hold in solution only so much sugar. As it goes into the pan, the juice is almost a saturated solution, and as the water is driven off by evaporation, the solids that up to this point have been in solution must of necessity crystallize.

When the sugar boiler decides that the “strike,” that is, the massecuite contained in the pan at one boiling, is satisfactorily grained, he breaks the vacuum by opening a valve on the top of the pan, thus allowing the air to enter. He then opens the valve at the bottom of the pan and the mass drops into a long tank with a rounded bottom, called the mixer, in which a shaft, equipped with paddles, is revolving. The paddles are for the purpose of keeping the mass agitated and in an even condition. The agitation prevents the grains from dropping to the bottom of the tank and forming a solid block, called concrete.

From the mixer the massecuite runs through spouts into the centrifugal machines. Centrifugal machines are cylindrical-shaped, perforated brass baskets, usually forty inches in diameter and twenty-four inches deep, hung on a central shaft suspended from beams overhead, and surrounded by a solid outside curb or casing.

On the shaft is a pulley, which is driven by a belt connected with an engine or an electric motor. The inside of the basket is lined with a fine-meshed brass screen, which retains the grains of sugar, but allows the liquor to escape freely into the outer casing.

As soon as the centrifugal machine is filled with massecuite from the mixer above, the power is turned on and the machine begins to spin around at an increasing speed until a velocity of one thousand revolutions per minute is reached. The centrifugal action forces practically all the liquor out through the screenand leaves in the machine all the grains of sugar that were formed in the pan. A little dry steam is sometimes turned in to assist in reducing the moisture in the sugar.

The centrifugal is then stopped, a valve in the bottom is opened, and the nearly dry crystallized raw sugar is dropped into bins. From the bins it is drawn off through spouts and packed in sacks containing about one hundred and twenty-five pounds each.

It has been demonstrated that raw sugar containing a large amount of moisture inverts or deteriorates more rapidly than that with a low-moisture content. It is apparent that as moisture adds to the weight, the transportation charges, which are based on tonnage, are greater in the case of wet sugar than in the case of dry. In many of the modern mills, therefore, a further treatment is given the sugar to reduce loss by inversion and lessen freight charges.

From the bins last mentioned the sugar is dropped into revolving drums six feet in diameter and twenty-six feet long, set at an incline so that as the drum revolves the sugar is carried round to the highest point on the circumference of the drum and dropped to the lower side, at the same time traveling from the receiving to the discharging end. The shape, motion and inclined position of the drum cause a perfect shower of sugar in the drum for its entire length and breadth. While it is revolving a current of hot, dry air is drawn through the drum by means of suction fans, and as a result the moisture in the sugar is absorbed by the air and carried out of the building. At this stage the product has a good hard grain of a yellowish-brown color; contains from ninety-six to ninety-seven per cent of pure sugar and about one-half of one per cent of moisture.

FILLING, WEIGHING AND SEWING SACKS

TRAIN-LOAD OF RAW SUGAR LEAVING MILL

From the end of the revolving drum the sugar is drawn off into sacks holding about one hundred and twenty-five pounds each. These sacks are sewed by machinery and put into railroadcars to be hauled to the docks at the shipping port, where the cars are switched under huge hoisting cranes or alongside speedy conveyors which carry the sugar into large seagoing steamers especially built for the trade. Some of these ships have a cargo capacity of two hundred and twenty thousand sacks, and they transport the sugar to the buyers on the mainland in San Francisco, New York or Philadelphia, as the planter directs.

The liquor thrown off by the centrifugals is not lost; it is taken back to the pans and reboiled. After this has been done several times and most of the sugar extracted, the purity is so low and the sugar content so small that it does not pay commercially to reboil further, and the residue is sold as molasses. It contains about thirty-five per cent of sugar and from twelve to fourteen per cent of invert sugar, or glucose, as it is generally called.

Some of the waste molasses is mixed with fodder and tender cane tops and fed to cattle and plantation stock, the sugar content proving of great value as a fattening agent and energy builder. Part of the molasses is sprayed on the bagasse as it leaves the crushers and serves, first, as a fuel under the boilers, and, second, as a fertilizing agent in the form of ashes after it has been burned. During the past few years much of it has been shipped in tank steamers to the mainland, where it is used for the manufacture of spirits and vinegar, and also as the principal ingredient in prepared stock foods which are much in demand today.

Every bag of sugar shipped from the plantation is marked to indicate the plantation from which it came. The net weight of the sugar in each bag is recorded, a sample of the sugar taken and its sucrose content ascertained, for it is on the basis of weight and sucrose content that raw sugar is bought and sold.

From the beginning to the end of the process of manufacture,chemists are vigilantly alert sampling, testing, analyzing and supervising the operations. Records are made of all analyses, temperatures, purities, densities, extractions, etc., and the results tabulated for future reference.

The average cost in Hawaii of preparing the fields, planting, irrigating, fertilizing, cultivating and cutting the cane, manufacturing the sugar and delivering it in the New York market, is about $56.00 per ton of two thousand pounds.

Back to Index Next