1946_Rault_Working With the Carbon at at Ion

8/4/2019 1946_Rault_Working With the Carbon at at Ion

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40WORKING,WIT'H THE CARBONATATION, PROCESS

UNDER SOUTH AFRICAN CONDITIONSBy J. RAULT.

In 1920 Mr. William A. Campbell, managingdirector of the Natal Estates Ltd., writing in theJournal of Industries,2 spoke of the first applicationof a process of sugar manufacture new to Natal,namely, the carbonatation process of cane juiceclarification. I t was stated that :-1. The viscous impurities naturally present in juicesfrom the Uba cane grown under Natal climaticconditions were extremely refractory to clarification, slowed down all factory operations, requireda disproportionate plant for the work done, andcaused heavy sugar losses, with consequent poorcrystal recovery; these losses being further aggravated when attempting to produce a white sugarfor direct consumption.2. Most of the difficulties encountered in the treatment of these refractory juices had been eliminated or at least minimised by the new process.3. A higher percentage of sugar and a better qualitysugar than previously was now obtained from agiven quantity of juice.4. South African material, namely, limestone andcoke, was used instead of the previously employedlime, sulphur and phosphoric acid, i.e. an important consideration in the national economy andan insurance in war-time against short supplyfrom overseas.The high cost of the chemical bill, as a result ofheavy transport cost, was unfavourably commentedupon and declared to be the greatest deterrent to theadoption of the process by other milling concerns.In 1923, F. Maxwell" commented favourably onthe carbonatation process for white sugar production, based his case for its adoption in South Africaon the experience of Java with the de Haan's modification, backed by a few results of the Mount Edgecombe factory communicated to him by the presentwriter.

from the annual summary of factory results, or occasional answers to questionnaires under the anonymous designation of factory No. 1.The reason for this mild "blackout" may be explained by the dissimilarity of the process from theusual South African processes offering no commonbasis for discussion and forcing us to solve ourparticular problems by a slow process of self-helpdevoid of the benefit of exchange of experience withfriendly neighbours.Over 25 years have elapsed since the decision toadopt the carbonatation process was taken by Mr.W. A. Campbell, at a time when this move wasregarded as a risky undertaking, in view of the lackoftechnical data, the cost of the process, and the veryinadequate plant for its efficient practice.During that period a revolution in cane plantinghas been accomplished by the introduction of newvarieties. Our agricultural practice has considerablyimproved through increased mechanization, intensive

fertilization and green manuring, irrigation, etc.South African factories have modernized their equipment. Scientific methods of control in all its variousaspects and co-operative investigational work fromthe South African technologists have enriched in amarked degree the pool of knowledge of sugar technology and consequently contributed to the efficiencyof the industry.A glance at the accompanying graph of the NatalEstates Ltd., Mount Edgecombe, results, shows theupward trend of progress, coincident with the adoption of the carbonatation process.The following table shows a few landmarks ofseasonal progress for field returns, hourly throughputof the mills, mill extraction, boiling house recovery,overall recovery, tons of cane to sugar and output ofsugar.

Per Tons Tons Boiling Over- Tons SugarFarnell," in a paper resuming his investigations of cent cane cane Mill house all cane outputYear. Uba per crushed ex- re- re- to forthe colloids of cane juices and syrups in Natal and cane. acre. per hour. tracti on. covery. covery. sugar. season.Mauritius, refers to the carbonatation process and the 1920 100.0 20.8 40.2 88.10 77.91 68.63 11.18 12,891work done by Rault on the comparative removal of 1923 100.0 21.6 52.9 91.91 85.20 77.77 9.47 20,0931928 100.0 18.6 64.7 93.70 83.65 79.63 9.13 30,220gum, wax and nitrogenous matter by carbonatation 1933 100.0 24.9 102.7 94.21 85.81 80.84 8.62 40,481versus sulphitation. 1934 95.1 29.8 109.4 94.20 87.57 82.49 9.29 48,818Very little has been published on the process in our, 1937 66.7 30.5 119.6 94.80 89.33 84.69 8.11 55,3491943 7.6 44.0 143.3 94.96 90.52 85.88 8.87 66,236Proceedings of the' South African Sugar Technolo- 1944 6.4 39.6 143.7 94.86 89.72 85.11 8.45 69,265gists' .Association, apart from what can be gathered 1945 4.9 -36.7 139.3 94.78 90.06 85.36 8.31 65,964


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130 86

120 70.000 85110 84 I I I l I I I I I I I I I I I I I ,IIII'I'''l'II,.'!t',

100 60;000 83

90 I 82 BOILING HOUSE RECOVERY roo- rr -- T"""80 50.000 81 t- , ...-r--tt- , "'".. ;-, ., --l--J.-W-I-.W- l-l- '-I- I I I I I -JIl -' L , ,70 80~ O 79

50 I 78 .. . ,40 130,000 TONS OF CANE CRUSHED77 ,.

PER HOUR30 76

20 )0.000

10.000 TONS OF SUGAR MADE PER SEASON

1920 1921 1922 1923 1924 1925 1926 1927 1928 1929 1930 1931 1932 1933 1934 1935 1936 1937 1938 1939 /9'lO 1941 1942 /943 1944 1945

26 Years results , worklng with the Carbonatatlon Process at Natal Estates Ltd., Mount Edgecombe, Natal


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Quality of Product.It is now generally admitted that the carbonatation process, by the energetic elimination of a largepart of the juice impurities, puts on the market a

final product of .outstanding value, possessingqualities of its own such .as crystalline structure, clarity,brilliancy, very low sulphur dioxide and thermophilicbacteria content, fitness for aerated waters, keepingquality on storage, unrivalled -by the product ofotherjuice process-es.A compliment to the value of the process comesfrom a source not 'usually favourably disposed towards methods of refiningother thanbonechar, whenwe read in one of the most candid and informativebooks on sugar refining, from the penof Oliver Lyle,?that the incomparable high standard ofTate &Lyle's

granulated, which has set a model to the sugar world,could not be maintained if it did not use a mild formof carbonatation as part of its process..On the other hand, Honig of java" maintains thatby carbonatation methods, a granulated sugar equalin quality to standard refined, can 'be made withoutfurther treatment with bone-black or vegetablecarbon. 'With our presentsystem.and the conditionsof theSouth African market, the production ofNo.1 Edgecombe refined constitutes an attempt to refine sugarliquors right at the earliest stage possible, as it comesfrom the cane, together with a reprocessing of ourown washed raw sugars from the second and thirdboilings. :

The Process.Carbonatation relies on the mass action of limedispersed in the juice at variousconcentrations orpH, subsequently neutralised by CO2 gas for theflocculation, absorption and precipitationof organicand mineral impurities present in the juice in thestate of coarse suspensions, colloidal dispersion 'ortrue solutions, which are partly or wholly removedin the lime carbonate filter cake.A limekiln working continuously is an essentialpart of acarbonatation plant. We will not touchonthis part of our process, which is interesting enoughfor a comprehensive paper, and much of it can befound in text books.Filtration rates become the touchstone of successful juice treatment, and a carbonatation factory, inwhich all the juice must be filtered faultlessly at 'somestage previous to evaporation, develops consequentlya "filtration complex" where the advantages derivedfrom a high juice handling capacity is of greater andmore immediate economic importance than thestriving after the optimum conditions laid down byobscure theories on the exhaustibility of finalmolasses.

42In the successful adaptation of the process to ourspecial South African conditions.the following factorshave had a far-reaching influence on the techniqueof our factory and consequently on the ultimate per

centage recovery of our final product.1. Aninstability in the hourlythroughput ofcanes,which, far from being static and providing a basisfor standardising our equipment for an optimum'capacity, has by successive leaps and bounds increased by 300 per cent. and thrown out of balanceone or other station of the manufacturing process inthe course of the past 25 years.2. Fluctuations in the ratio ofrefined sugars to rawsugars for export or for the local market, viz., in thecourse of-the same campaign a 100 per 'cent. refinedsugar production was carried out for a continuous

period of seven weeks, for supplying a depletedmarket in June, to be followedby months of 70 percent. refined and 30 per cent. raws; whilst on a fewoccasionsour refined juices had to be turned out into, a cargosugarforexport. For the 1'942 season, 85percent. of our whole production was a No.1 refined.3. A rising standard in the quality of the No. 1refined sugar, with special reference to colour, as set

by the bone-char and vegetable-char refineries.4. The general trend towards expensive labour andthe necessity of simplifying our process of manufacture in view of economising labour.That our peculiar climatic conditions, with itsrecurrent dry and cold spells, rather than the muchmaligned Uba variety, is the cause of our processingdifficulties, is admitted by all technologists who havehad Jccalexperience. For this reason, so far nofactory has succeeded or is willing to clarify theirjuices by the simple lime defecation process of othercane' sugar lands, notwithstanding the practicalelimination of the Uba from our fields.The rawsugar factories, in order to work smoothlyand alsoto avoid throwing difficultiesat a later,stageon the refinery, are commonly using amounts oflime, sulphur and phosphoric acid that would beconsidered excessivefor sulphitation factories manufacturing plantation whites.The.samebasic cause, togetherwith the four factorsenumerated above, have contributed to put up thelimestone consumption ofour factory to a level whichwould be judged wasteful by ]avan technologistsexperienced in the treatment of tropical cane juices,or by beet technologists who inthe last decade"havesuccessfully reduced their chemical bill by adoptingpre-liming methods.Laboratory investigation on the treatment of rawcane juice with increasing amounts of lime, has givena curve of pH where two definite zones of improvedclarity are noticeable, namely, between pH 7.2 to8.5, and also pH 10.2 to 10.6, indicating differentoptimum iso-electric points of colloid flocculation.


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Row sheet ojClariJicatlon b the CarbonatationProcess at Natal Estates Mount E d ~ e c o m be, Natal.


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We have not been able to confirm that preliminaryor progressive liming to an optimum pH confers theadvantages claimed by the beet technologist to thesubsequentlyprepared carbonated juices.jn the wayof filtration rates and lime economy.'

44The quality of our refined sugar is nevertheless achallenge to the hitherto accepted dogma of darkcolour formation with high-temperature alkaline'cane juiceclarification. 'We propose to read, at a 'later date, a contributionOur inadequate tank space for time of' contact to the study of loss of sugar in molasses as revealedbefore gassing and the absence of a reliable automatic by the percentage composition of the non-sugars ofpH recorder,may be responsible for this unsatisfac- various factory molasses for a number ofyears. Datatory finding, which will be further investigated. on pH curves, gas absorption, speed of settling andfiltration, reducing sugars destruction, elimination ofContrary to the experience of java," we have de- ash, gum, wax and, organic non-sugars, have beenrived no material benefit in lime economy by using studied in great detail and will also form the subjectde Haari's modification of liming and gassing. of short-papers.

We have adopted the simultaneous liming and I t is sufficient to state that, with the high eliminagassing on account of the absolute failure met with, tion of non-sugars during clarification, the averageat the earliest attempts with heavy liming in one rise in purity from raw juice to clarified juice is 5.06operation, followed by gassing, when the excessive, degrees for the past 25 years, with an optimum riseformation of the gelatinous and frothyhydro-sucre- in excess of 6 degrees for two complete seasons.carbonate of lime increased the time of preparation ' Very seldom has the weight of molasses exceededof one tank to two hours. 3 per cent. cane, and for the past twelve seasons thisfigure has been reduced to 2.5 per cent. cane.We rely entirely on the progressive physical andchemical action of incremental doses of lime car- With a viewto economyinlabour, cloth and equipbonated in the juice at low alkalinity in order to ment, and to reduce sugar losses,we have been theobtain a high non-sugar elimination and high filtra- pioneers in South Africa in discarding the plate andtion rates. 12 to 14 per cent. of 20 Baume lime- frame presses, which have been replaced by rotarycream on volume of juice is our normal practice for vacuum self-dischargingMaussfilters working in consafe work at thefil ters.: junctionwith settler-thickeners of our own design.

In this system the muddy juice from the first car-The volume of lime neutralised per unit of time, bonatation tanks, limed and carbonated to a pH ofi.e. efficiency of gas absorption, is of capital im- 9.5 to 10.0, is settled in a continuous sett ler workingportance. This has been found to be retarded by too on the upward mud filtration principle, where 60 to'low a pH at gassing, although not to the same 65 per cent. of clear juice is withdrawn and thedisastrous extent as in t h case ofexcessivealkalinity. bottoms continuously delivered in the form of aOur efforts have concentraledon the design of thickened sludge containing 25 to 30 per cent. filtercarbonatation tanks, where the gas is split up into cake to the Mauss rotary vacuum filters.

bubbles by a combination of distribution pipes and Vacuum filtration with only 15to 17inches suctionperforated baffles, together with lime admission at postulates for continuity of operations muddy juicesvarious points in view of a homogeneous reaction, of extremely good filtrability, and consequently this,not easily obtainable in tanks with 10 feet of juice. method of handling our large filter cake productionhas influenced our technique of carbonatation.Much as wewould like to preserve our already low, reducing sugars, we have not been able to maintain I t is,surprising to find that the total effective fil-consistently a high throughputby working our liming tering area of our battery of 15 filters is only 675and gassing at temperatures under 55C., and in view sq. ft., out of which 10filters or 450 sq. ft. are someof the definite improvement in settling and filtering times sufficient to take care of 18 tons of filter cakerate the temperature of the raw juice going to car- ' per hour, washed to a sucrose content of less thanbonatation is kept near 70C. 1 per cent., equivalent to 4.5 sq. ft. of filtering areaper ton of cane per hour-a removal of 53lbs. of wetThe subsequent exposure of reducing sugars to cake per sq. ft. per hour. At maximum efficiencyalkaline reactions during settling does" not help these machines may remove 80 lbs. of wet cake (50reducing sugar preservation. per cent. moisture) per sq. ft. per hour.On the score of exhaustion of final molasses, our The mixture of decanted juice from the settler andsystem is open to criticism, with its formation of of filtered juice from the Mauss, both at a pH ofharmful reducing sugar decomposition products, No . !).5to10.0; is'subjected to a secondcarbonatation byfroth fermentation is, however, noticeable in our last a continuous gassing system where the alkaline reacmassecuites, which are dropped somewhat hot from tion is brought down to a pH 8.2 to 8.4 (faint pinkthe pan, namely, 70-75C. colour to phenolphthalein solution).


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45In the first ten years of its adoption up to 1929,188,367 tons of refined sugar was manufactured; andfrom 1930up to the war period this figure increasedto 304,752 tons; whilst during the six years of warup to the last season 249,540 tons of refined sugarwere produced.Summing up the production of refined, Government grade and cargo sugars, from its inception, wearrive at the respectable total of 1,075,999 tons ofcommercialproduct having beenmanufactured fromcane juice treated by the carbonatation process inSouth Africa. Our national economy has been thericher by the use of 482,933 tons of limestone and

5 2 , 0 5 ~ tons of coke obtained from our soil.Well can we say, after the silver jubilee of its life,

that the carbonatation process has merited its rightof citizenship as a South African industry, and justlydeservesthe praises ofHonig' as "one ofthe cheapest,cleanest and most reliable processes in the sugarindustry; ensuring standard quality of the sugar."References.

1 Baerts, F. (1934): Le prechaulage Dedek-Vasatko. RapportsIII. Congres International Tech., Paris.2 Campbell, W. A. (1920): A New Method of Manufac tu re inNatal . Jour. of Ind ., S.A., Oct. 954.3 Deerr, Noel (1933): A Review of Java Factory Practice.I .S.J. 8.4 Farnell, R. G. W. (1925): Invest igat ion of the Colloids of

Cane Juice and Syrup in Natal and Mauritius, I.S.J. 254.5 Geerligs, P. (1915): Practical White Sugar Manufacture. 69.6 Maxwell, F. (1923-4): The Case for the Carbonatation Processin the Manufacture of White Sugars. I .S.J. 572.7 Lyle, Oliver (1941): Technology for Sugar Refinery Workers.254. .8 Spencer and Meade (1945): Manual for Cane Sugar Manufacturers. 119.

THE PRESIDENT, in opening .the paper for dis:"cussion, said that the Natal Estates had been thepioneers in introducing the carbonation process inSouth Africa more than twenty-five years ago, butit had been a pioneering move, the initiative ofwhich had never been followed by the rest of theindustry. The reason was probably that we knewtoo little of what was being done by Natal Estatesin their unique process, and we were therefore verygrateful to Mr. Rault for presenting this paper to ~ s Mr. RAULT said that during the last fewyears theyhad SO many visitors to the factory who enquiredabout the process, but no satisfactory answers couldbe given during .the course of conversation. Theyhad also been honoured by a visit ofthe leading technologists of the industry last season. There was

therefore a great deal of interest taken in the carbonatation process and although not much was saidin the past, the process was by no means a secret;in fact it was the standard process in beet factories


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46in other countries, but they had had to adapt it toSouth African' conditions, and certain deviationsfrom standard practice overseas had to be made.He had often been struck by the differences intechnique as outlined in text books and as followedin actual practice. That , then, was th e reason forwriting this paper. After agreeing to submit a paperon the carbonatation process he wondered whetherto deal with one aspect of it or to give a generaland necessarily scanty review of the whole. He haddecided on the latter and he had therefore tried togive their 25 years' experience in carbonatation inone short paper. He might. however, deal withcertain .interesting aspects of the process at a laterdate.

Mr. VIGER said he noticed in the flow sheet a gasabsorption tower on top of the first carbonatat iontank. He wanted to know if the pre-heated acidraw juice from the heaters went through this towerbefore entering the carbonatation tank. I f this wasthe. case, the acid hot juice would not absorb anycarbon dioxide as it had not been previously limed.He was aware that the gas from the kilns containedabout 30 to 40 per cent. carbon dioxide, could Mr.Rault tell him what the carbon dioxide content ofthe gas escaping from the first carbonatation tankwas?Mr. BOOTH endorsed and emphasised Mr. Rault's

statement that juice filtration rates were the essenceof successful juice treatment and the advantagesderived from proper juice handling were of greaterand more immediate economic importance than thestriving after optimum conditions laid downin theobscure theories on the exhaustability of final molasses. He thought filtration had been a neglectedsubject in our sugar factories. The Oliver type of .filter was a strainer, not a true filter, and should beregarded as such even where sugars of 96 and 98pol were produced.THE PRESIDENT wanted Mr. Rault to tell the

conference what the fertilizer value of carbonatation filter cake was as compared with sulphitationcake.Mr. WHEELER asked for the quantity 'of carbonatat ion cake per ton of cane. He thought it was muchgreater than that produced by the sulphitation factories. More impurities were removed from the rawjuice and the purity of clarified juice was thereforeincreased considerably. He was not sure, however,what the effect of this elimination of impurities wason the purity of the final molasses.Mr. RAULT, in reply to these questions raised,

said that Mi. Viger was quite correct in his readingof the flowsheet which indicated that the raw heatedjuice from the mills was pumped up a tower whereit cascaded through a countercurrent to the waste

gas from the carbonatation tanks, . This hot rawjuice was unlimed and still of apH of about 5.6 andconsequently the carbon dioxide absorption waspractically nil. .The tower, however, served a veryuseful purpose. I t was a prolongation of all the shortexhaust gas pipes of the battery of carbonatationtanks which delivered into one main pipe connectedto the bottom of this tower. Any alkaline juicecarried up the waste gas pipes in the form of a frothmet the incoming cascading raw acid juice and wasthereby broken up and returned to the tank in theprocess of filling. In spite of maintaining a low pHduring the first carbonatation there was still occasionalfoaming and this tower was an excellentsave-all.The average live gas from the lime kiln contained

30 to 35 per cent. carbon dioxide and the exhaustgases going up to the tower still had 12 to 18 percent. carbon dioxide. In fact it was as rich in carbon dioxide as the live gas from the sulphur burnerswas in sulphur dioxide content and richer in carbondioxide than the average gas from the bagassechimney stack. When compared with the efficiencyof gas absorption in the sulphitation factories theircarbonatation gas system seems very wasteful. I tshould be remembered that a lime kiln produced notonly the 44 parts of carbon dioxide for every 56parts of quicklime strictly required for neutralisation of the lime produced, but also an additional 30par ts carbon dioxide from the coke' burned. Andthis had to be wasted-it was a surplus-unless additional lime from an outside source was used inthe juice. A 60 to 70 per cent. gas absorption wastherefore the maximum required.With reference to a query from Mr. Dymond, 'thespeaker said that Tate and Lyle's "mild carbonatation" referred to in his paper consisted in the carbonating of barely half a per cent. lime on melt andconsequently it was possible to gas this small amountof lime by means of weak concentration of carbondioxide present in the flue gases on lines similar tothose tried at Darnall on raw juice where from 2 to3 grams of lime per litre of juice had to be neutralised. At the Natal Estates' the flue gases could notpossibly be used for treating the heavy amount oflime required for their process as the gas handlingplant would be disproportionally cumbersome. Theirexperience was that a carbon dioxide content ofless than 28 per cent. in the kiln gas invariablymeant an insuffiicent liming and a slowing down oftheir filtration rate.In the 1942 Proceedings some details as regardsthe comparative values of carbonatation and sulphita tion filter cakes were given. As could be expected, carbonatation filter cake was higher in limecontent. The phosphate, nitrogen and organic matter in the carbonatation cake was lower than that

'j


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47of the sulphitation cakes because it was dilutedmore with lime. The cake was neverthelessvaluableagriculturally and was used in the fields and usuallyapplied to the lands with irrigation water. .

,The wet filter cake from the factory varied from10 to 12 per cent. on cane. Larger quantities ofimpurities were eliminated in the carbonatation process of clarification than in sulphitation. Mineralimpurities as well as non-sugars were eliminatedand there was a definite drop in ash content fromraw to .clarified juice. The ash composition of clari-

fied juice in the carbonatation process was differentto that of sulphitation process. Invert sugar wasunfortunately also reduced. In the raw juice thereducing sugar ratio was about 3 per cent. and itfell to 2 per cent. in clarified juice and consequentlythe purity of their molasses was somewhat higheras a. result of an unfavourable reducing sugar ashratio, but the smaller quantity ofmolassesproduced,resulting from the higher non-sugar elimination partly compensates for the higher sucrose content ofmolasses.

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1946_Rault_Working With the Carbon at at Ion