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INDUSTRIAL A N D ENGINEERING CHEMISTRY
Vol. 17, No. 10
Ten Years of Sugar Refining' By L. A. Wills A M ~ R I C ASUGAR N REPINING Co.,NEWYORK,N. Y .
EN years ago, and even long before that, the process of refining raw cane sugar had already reached a highly perfected state; so much so that refined granulated sugar WAS a t that time, as it is now, one of the most nearly chemically pure of all commodities. Neither were the principles involved in the refining process materially different from what they are today. The bulk of the raw sugar received consisted, as it does now, of high-grade centrifugals which were washed to a purity of approximately 99 degrees. The use of animal blood for clarification had long since been abandoned in favor of phosphoric acid compounds. The best refiners attached the same importance to perfect mechanical filtration as we now know to be essential for the production of white granulated sugar. Bone black was used for epuration in precisely the same manner as today, and was at least as effective in decolorizing and otherwise purifying the liquors; in fact, many refiners believe that the bone black manufactured up until a few years ago was of better quality and less friable, by reason of a more plenteous supply of bones suitable for its manufacture, than is the present-day product. The spinning, washing, and drying of refined sugars have not changed materially in principle. I n methods and equipment, however, much important progress has been made. There have been considerable improvements in labor-saving and other cost-reducing devices.
T
Raw Sugar Washing
As to the sffination process, the self-discharging centrifugal basket used in several modern installations has fully proved its value. By its means the number of operators required a t the washing station is materially reduced and with careful control the quality of the washed sugar is quite as good as ever. The basket consists of a shell, the top portion of which is cylindrical in form, with a lower section narrowing in diameter to form an inverted, truncated cone open a t the bottom. The magma to be spun drops on a horizontal disk mounted on the spindle within the basket and is thrown out against the side of the basket by centrifugal force. The angle formed by the conical section is steeper than the angle of repose of the load and, as the machine is braked, the charge falls through the open bottom. Direct motor drive has made it possible to bring modern machines to full speed in remarkably short time and improved brakes have greatly shortened the braking period. One 48-inch machine of the type described will handle a quarter of a million pounds of raw sugar in 24 hours. It is all important for perfect and efficient operation so to train the operators that they perform each step with the regularity of a pendulum. The proper cycle of operation must first be worked out by intelligent study for each installation and for varying amounts of melt. When this is once established and each man is assigned to the proper number of machines, it is only necessary for each to charge, wash, and brake his machines in rotation at a measured pace, neither hurried nor sluggish. Only by such regularity can uniformly good washing be maintained. Much has been done in the development of washing devices for centrifugals. Most of these deliver a measured quantity of water when a lever is turned by hand and are I Presented before the Division of Sugar Chemistry at tne 60th Meeting of the American Chemical Society, Baltimore, Md., April 6 to 10, 1925.
refilled as the lever is reversed. Some distribute the water in a fanlike spray over the surface of the sugar, whereas others have an oscillating spray which directs a heavier stream of water progressively from top to bottom of the basket. Most centrifugals for both raw and refined sugar are now equipped with one or another of the many measuring devices and sprays. One refinery has worked out an elaborate and ingenious system of automatic operation for its centrifugals by means of electrical control. In a battery of machines every operation except charging is electrically timed and operated. The writer saw in this plant one machine experimentally equipped which was even charged automatically, so that practically the only labor required for a battery of such was for supervision and maintenance. Filtration
Leaf filters began to replace the old bag filters in 1914. Infusorial earth as a filter aid was substituted for compounds of phosphoric acid wherever leaf filters were installed. The latter are operated under high pressure and phosphate of lime does not form a sufficiently porous cake under these conditions. On account of the cost of infusorial earth, which largely balances the savings in labor and filter cloth, both large items for the old bag filters, the economy of leaf filters has been questioned by some refiners. I n fact, so great has been the conservatism with respect to leaf filters, especially for the filtration of affination sirup, that not only have many existing bag installations been allowed to stand, but a new refinery has recently been equipped with bag filters. I n the writer's opinion, however, there is no doubt of sufficient net saving in the combined items of labor, cloth, and filter aid, to render an installation of leaf filters, properly operated, a profitable investment. Furthermore, the cleanliness attending the operation of leaf filters, and the rapidity with which the liquors pass through the process, both factors reducing the liability of sucrose losses through inversion, are features not to be overlooked. Devices for sluicing the leaf filters automatically have decreased the labor required a t the filter station in several refineries. Rotary suction filters with continuous, automatic removal of the cake have replaced in some plants the usual types of filter presses for the filtration of sluicings from leaf filters. An improved form of infusorial earth, prepared by a process in which calcination is understood to play a part, has been placed on the market within the last two years. This material at first gave a greatly increased liquor flow, but left something to be desired in clarity of filtrate. It has been the effort of the manufacturers so to modify the prepared earth as to give perfect filtration without sacrificing rate of flow, and the product now being turned out seems to give promise in that direction. Regeneration of infusorial earth has been discussed ever since the material came into general use in connection with leaf type filters. Many laboratory and some plant scale experiments showed that the earth could be efficiently revivified by burning, but several years elapsed before regeneration was put into actual practice. One refinery is now burning earth in the form of cake from suction filters in rotary kilns; another has demonstrated the effectiveness of a station-
October, 1925
INDUSTRIAL AND ENGINEERING CHEMISTRY
ary kiln of the type used for burning lime, for calcining earth recovered as filter press cake. I n both cases the earth must be reduced to a finely divided state, for which purpose wet grinding is preferred. About three years ago the use of paper pulp as a filter aid was proposed. It was first tried in leaf filters of the rotary type, which has since proved to be the only type applicable to this form of filtration. Several attempts were made to adapt the stationary leaf filters to pulp filtration, but, so far as the author knows, the project has been abandoned. The pulp filter cake is extremely porous. is bulky, and must be quite evenly coated on the cloth. This is difficult to accomplish in any but the rotary type of filter. Removal of the cake is also accomplished more easily with the latter type. The used pulp is regenerated by merely washing out the impurities with hot water, for which purpose a form of washer common t o the paper manufacturing industry is employed. A large quantity of water is required, but since waste water from the washing of char filters can be used the expense is small. Paper pulp is furnished by the manufacturers in the form of thick sheets. Soda pulp, or a mixture of same with ground wood, is generally used, though it is said that old newspapers work equally well. New pulp is fed to the beater, a device also known to the paper industry, where it is beaten to a thick cream with sufficient water. The make-up of new pulp required is said to be surprisingly small, since mechanical loss through the washing screens is not great and the quality of the pulp does not appear to suffer after repeated use. Pulp filtration is characterized by a high rate of flow. There still exists, however, a difference of opinion as to whether sufficiently clear liquors can be obtained a t all times in refinery practice. It was early found that double or triple filtration through a heavy coating of pulp is necessary, and that the pulp must be augmented with a small proportion of some coagulant, such as phosphoric acid. This decreases the rate of flow, which remains, however, still relatively great. I n the case of filtration with infusorial earth, if a trace of the filter aid passes into the filtered liquor, because of broken cloths or otherwise, it is caught in the char filters. I n pulp filtration, on the other hand, if the filtrate is ever so slightly contaminated, it is sure to appear hazy after char filtration, and the fine suspended or colloidal matter persists and becomes more concentrated in the granulated sirups, to the detriment of the refined product. It is therefore essential in pulp filtration to obtain highly brilliant filtrates, and apparently all refiners who have tried i t have not met with the same degree of success in the accomplishment. Bone black filtration presents little that is new. The continuous system of filtration has almost entirely replaced the old battery method, however, in line with the tendency of the times to cut down to the absolute minimum the amount of material carried in process. Oil firing of the kilns has become more widely used. A retort made in two sections is expected to cut reneval costs, since either top or bottom section can be replaced, separately, and the scrapping of the entire retort thus avoided. Evaporation
I n the pan house a notable innovation has been the installation of multiple-effect evaporators for concentrating liquors previous to entering the pans. Sugar boilers have been able to accommodate themselves to handling pan liquors a t 68" Brix, so that if the liquor runs from the bone black filters a t 55" Brix it is possible to accomplish about one-half the total evaporation in multiple effect, and largely with exhaust steam. The saving effected is, of course, very great.
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Another medium for the utilization of exhaust steam for evaporation is the calandria pan. Widely used in the raw cane sugar factories of Cuba, it has been adopted in some of our modern refineries. Refined-Sugar Machines Improvements in centrifugal machines have been discussed in connection with raw sugar washing. Although it has not been possible to apply the self-discharging principle to refined sugar machines, direct motor drive and various mechanical improvements have greatly increased their efficiency and eliminated labor. Separation of wash sirups from greens is now pretty generally applied to the lower granulated and 1 emelt sirups. Drying and Screening Methods of drying and screening sugar have remained practically unchanged during the last decade, except for the introduction to a very limited extent of air drying for the coarsegrained sugars to improve their brilliance. This has been accomplished by spreading sugar from the centrifugals in a thin layer on a slow moving belt, or, as in one case that has come to the writer's notice, by leading a current of warm, dry air through the housings of the screens used for sizing. At least in the former method it is essential to heat the sugar first by introducing steam into the centrifugal basket. Packing and Other Handling
The packaging of sugar has received a liberal share of attention and some wonderfully ingenious devices have been perfected in this field of endeavor. Automatic feeders for the carton-filling machines, container-sealing machines, bag-filling equipment, and a variety of ingenious arrangements for filling cartons of soft sugar and for the tight packing of tablets and cubes are but a few of the many interesting improvements. Methods of conveying, elevating, pumping, trucking, and otherwise handling the enormous quantity of material that daily passes through a modern refinery have kept pace with the engineering progress of the times, as have also the means of generating the prodigious quantities of steam and electric power required. Progress in the Laboratory Last but not least, our chemical laboratories, guardians of the refining process and insurers of quality in the product, have kept in the vanguard of progress. Control of the refining process is being assisted by modern methods of hydrogen-ion determination and by systematic color readings according to one or another method for expressing color as a numerical value. Screen analysis has been placed on a scientific basis Apparatus has been devised for comparing the filtrability of raw sugars and the efficiency of different filter aids, so that these variables can now be expressed in figures. Routine purity determinations have been shortened by improved methods and equipment. The sugar refining industry is, by the very nature of the vast scale of its operations, a conservative industry. A change in method usually means a change in equipment, and, since most of the equipment is large and costly, a considerable outlay of money is generally involved. The foregoing summary of a decade's progress, however, will indicate that the industry's technical progress is steady and material, even though along conservative lines. An investigation which is being conducted by the Bureau of Mines to determine possible improvements in the milling of fluorspar indicates that considerable improvement in concentration and more economical recovery may be effected.
