1969_Matic_Aspects of Flotation Clarification

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Proceedings of The S outh Africa n Sugar Technologists' Association - June 1969ASPECTS FLOTATION CLARIFICATION

MIXED JUICEBy M. MATIC

Sugar Milling Research InstituteAbstractTh e influence of several param eters on removal ofvarious impurities during vacuum flotation clarifi-cation of mixed juice has been investigated, andpossible ways of dealing with the secondary preci-pitate obtained in this clarification process have beenexamined.Dispersed a~ir flotation clarification is describedand the experience gained with this separation

technique is recorded.IntroductionThe vacuurn flotation process of juice clarificationwas introduced to South Africa in 1966 by Rabe l.The efficiency of this process for starch removal wasimmediately recognised but there were, at the-sametime, several aspects of the process, both operationaland fundamental, which required further investiga-tion.A literature survey revealed that a clarificationprocess using phosphoric acid and lime, with theremoval of phosp hate precipitate b y air flotation,was patented by Williamson in 1918'. Instead ofvacuum, heat was used in this process for release ofdissolved air. 'The Williamson clarifier and particu-larly various modifications of it introduced byJacobs" and IBulkley-Dunton and Sveen Peterson4became firmly established in sugar refineries fortreatment of raw sugar melt. A thorough study ofvarious parameters influencing performance of oneof these c1arifil:rs has been made by Saran ins .In raw suga r mills the ap plication of a flotationtechnique to the treatm ent of cloudy filtrate obtaine dfrom rotary vacuum filters was described by Higgin-botham" and Foster et a17 studied the application ofthis technique to syrup. A preliminary report bySloane8 appears to be the only publication dealingwith the application of flotation to the clarificationof mixed juice.The additional information required in connection

with the Rabe process was therefore not available.In an attempt to fill this gap the S.M.R.I. investi-gated several aspects of the flotation process and apar t of this work, t o various stages of which E. J.Buchanan, A. Jullienne, R. M. Morris and A. J.MacRitchie, particularly contributed, is summarisedin this paper.Influence of pH , flocculant level and freshness of

juice on removal of impuritiesNormally, the o perating conditions durin g vacuumclarification are as follows: mixed juice is heatedto 60C and limed to pH 8.1 or slightly higher. Thejuice is then treated with mono-calcium phosphateor phosphoric acids to reduce pH to 7.4 and, afterthe addition of a coagulant, introduced into thevacuum clarifier8.It was of some interest to investigate other pHlevels at which precipitate could be floated and theinfluence of these pH levels on removal of variousimpurities.A series of experiments were carried out withjuices obtained from several mills. In each run onelitre aliquots of juice were heated to 6 0 C and th erequired amount of calcium saccharate (about 20 mgCaO per rnl) was added in order to obtain thedesired pH . After the addition of 10 ppm of floc-culant (0.05% solution of Nc 1273 - Dow Chemical)the juice was transferred to a conical vacuum flaskand the precipitate floated at 15" vacuum. Fromthe heating stage to flocculant addition the juice wasstirred at constant speed. For comparison purposesa portion of th e same juice was defecated a t pH 7.5.The clear juice thus obtained was analysed forstarch, P,O,, SiO,, protein and Ca + Mg using stan-dard S.M.R.I. methods. Although absolute valuesvaried with the origin of t he juice, the trend wasalways the same and a set of typical results isreproduced in Table 1.TABLE 1' Influence of pH on removal of impurities by vacuum flotation -

I I

Pol %juice . . . . . .. . . . . . . .ef. BrixPurity . . . . . . .S ta rc h p p m j ~ x . . . . .P,O, mg/l . . . . . . . .. . . . . . . .i02 mg/lProtein % on BX . . . .Ca + Mg mg/l . . . .

Mixed I Defecation I Vacuum flotation at pHJuice I p H 7 . 5 1 1 8 1 9 1 1 0 1 1 1


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200 Proceedings o f The S outh Afr ican Sugar Technologists' A ssociation - une 1969There appears to be a gradual improvement inremoval of impurities by vacuum flotation as thepH increases from 7 to 11. As far as starch, phos-phate, silica and to a certain extent protein are con-cerned, the improvement is marked when approxi-

mately p H 9 is reached. However, there is also asharp increase in Ca + Mg content of the clarifiedjuice under those conditions. Furthermore, a muchmore voluminous precipitate is produced at thesehigh pH's and this may be of considerable import-ance in both subsequent mud treatments and theexpected sucrose losses in filter cake.An advantage of vacuum flotation at any pHover simple defecation as far a s starch and to a lesserdegree sjlica removal is concerned is clearly evidentfrom Table 1. The removal of other impuritiesapp ears to be of a similar order.The influence of flocculant level on the efficiencyof vacuum clarification a t pH 9 was assessed bymeasuring the turbidity of the clear floated juice.It was previously established" that a significantcorrelation exists between starch content and tur-bidity of the juice measured a t 900 mp.It is apparent from Table 2 that as the amount offlocculant decreased the turbidity of the juiceincreased. In this experiment, however, the juiceclarity was very good even at the 3 ppm level. Inthe absence of flocculant no, or only partial, flotationoccurred. There were exceptions to this, particularlywhen flotation at high pH was carried out, and itappears that this behaviour was a consequence ofjuice quality. For example, juice obtained fromN:Co.376 cane variety usually floated easily whilstthat obtained from Co.331 was often very difficultto float.

TABLE 2Effect of flocculant level on juice turbidity

The effect of freshness of juice on efficiency offlotation was investigated in a series of tests per-formed over six hours on two batches of the samejuice, one of which contained a p reservative,Hg C1, (T able 3). Th e turbidity of the clear juiceafter flotation of the preserved juice decreased withtime, whereas that of the unpreserved juice increased.The latter could possibly be explained by the factthat a partial hydrolysis of starch present in thejuice occurred and, as only insoluble starch isremoved by flotation, the amount of "soluble"starch"' in the clear juice, and therefore juice tur-bidity, increased. No explanation could be offeredfor the decrease of turbidity found in preservedjuice. Secondary precipitation

Flocculant (Nc. 1273)pprn on juice20753

A formation of small amounts of precipitate inthe clear juice obtained by vacuum flotation on

Turbidity at 900 mp2 cm cell0.0530.0670.0680.074

TABLE 3Effect of juice freshness on turbidity ofclear juice after flotation

standing, and particularly after heating, was noticedin early laboratory experiments. The full implica-tions of this secondary precipitation was, however,realised only after the full scale start of th e Ra beprocess at Darnall, where after a few days ofoperation heavy scale deposited in heaters andevaporators.

Th e analysis of this scale (Table 4) revealedthat two-thirds of the heater scale and half ofthe evaporator scale was made up of organicmaterial, most of which was protein. The maininorganic constituents were calcium and phosphate.Clearly, a temperature higher than that used invacuum flotation was required for precipitation ofprotein contained in the juice. The clear juiceobtained after flotation had therefore to be heatedto boiling and then settled in a conventional way.

Timehr0135

TABLE 4Analysis of scale ex Darnall

Turbidity at 900 mp

Normally four zones can be distinguished inlaboratory settling tests with defecation muds whichare usually performed in a tall cylinder immersed in aconstant temperature bath: (a) clear liquid on thetop followed by (b) a zone of uniform solidconcentration, (c) a zone of increasing solids con-centration and finally (d) a zone of compaction. Incontrast with this, the secondary precipitate fromthe flotation process gave only zones (b) and (d),i.e. one of a uniform concentration of precipitateand another of the concentrated precipitate. Thelatter gradually increased from zero to a maximumof 4% after. 18 minutes (see Figure 1) and there-after began to compact. However, even after onehour a small amount of very fine precipitateremained floating in the juice. It appears, therefore,that different settling mechanisms are operative dur-ing settling of defecation muds and secondary pre-cipitate.

Juice preservedwith Hg C10.1380.0740.0480.024

Nopreservative0.1110.1130.1560.175

Evaporator scale% dry sample -55.50.64. 612.110.821.3Loss on ignitionSiO,RzO,Cat+pzo,Protein

Heater scale% dry sample78.30.84.05.37.034.7


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Proceedings of The South African Sugar Technologists' Association - une 1969100 Q FIG. 1 SETTLIN G OF DEFECATION MUDS AN D SECONDARY PRECIPITATE

TIM E M INIt was sugge:jted at the time'' that both evaporatorscaling and the ash content of sugar m anufacturedby the Rabe process could be reduced by flotationat high pH's and subsequent carbonatation of theclear juice ~bt~ained.In order to test this suggestion juices from severalfactories and juices expressed in the S.M.R.I. millfrom N:Co.376 and a P.O.J. cane variety were usedin a series of experiments. T he following clarifica-tion techniques were used always on aliquots ofthe same batch of juice:(a) defecation by liming to pH 7.5, heating to

boiling and settling the precipitate;(b) Ra be vacuum flotation at 60 C after limingto pH 81.4 and phosphating to pH 7.4. Theclear juice was heated to boiling and settled;(c) flotation at 6 0C after liming to pH 10 or 11;(d) clear juices obtained in (c) were carbonatedusing 30% CO,/air mixture to pH 8.5,heated to 75C and filtered;(e) carbonatation according to de Haan. Th e juicewas limed to a p H of 10.5 at 55 "C, gassing. using a 30% CO,/air mixture was started andthe pH held constant for 15 minutes bycontinu ous add itio n of milk of lime. Th e juicewas filtered and the filtrate carbonatated top H 8.5, heated to 70 C and filtered.The clear juices obtained by these various tech-niques were analysed and some typical results are

presented in Table 5. Except for defecation, starchremoval in all cases was very good but residualcalcium even after carbonatation was high. The levelof remaining impurities was comparable.In order to establish the manner in which theresidual Ca was retained in the clear juice a furtherseries of experiments was carried out and the resultsindicated that the formation of organic acids atthe high alkalinity levels used was mainly resoonsible.for this. In carbo natation , total alkalinity is of greaterimportance than pHt2 and when de Haan carbona-tation was carried out, maintaining the alkalinity at

250 mg CaO/litre, an amount of calcium and mag-nesium in clear juice as low as 217 ppm wasobtained. However, when the same technique wasapplied to juice floated at pH 9.8 (this would cor-respond with an alkalinity of about 250mg CaO/l)the level of Ca + Mg obtained in clear juice was ofthe order of 500 mg /l. T his is at the best only aslight improvement on the results obtained by defe-cation or normal Rabe clarification and, in view ofthis, it was concluded that no real advantage wouldbe obtained if the suggested modification wasadopted.Dispersed air flotation

The temperature at which the Rabe flotation pro-cess can be op erated is limited by t he "boiling pointof juice at the particula r v acuum used. A t theserelatively low temperatures only partial clarification


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202 ' Proceedings of T h e South Af rica n Slrgor Technologists' Association - une 1969TABLE 5Analyses of clear juices obtained by various clarification techniques

Clarification Purity Starch ProteintechniqueDefecation pH 7 . 5 .. . .Rabe vacuum flotation . .Flotation at pH 10 . . . .Flotation at pH 11 . . . .Flotation pH 10 + carbon-atation . . . . . . .Flotation pH 11 $ carbon-atation .. . . . .de Hahn caibonatation:1st filtrate . . . . . .2nd filtrate . . . . . .Mixed juice . . . . . .

171421 .51 .6NilNil

11

252

of juice is achieved as evidenced by the forma tion ofsecondary precipitate. Furthermore, the amount ofair available for formation of bubbles which effectthe flotation is limited by the solubility of air a tthe particular temperature. A clarification techniquewhich could combine the efficiency of high tempera-ture phospho-defecation with low retention timesachieved by vacuum flotation would have manyadvantages. It would eliminate the subsiders andwith them the exposure of clarified juice to hightemperature for prolonged periods. As a result thealways present danger of inversion of sucrose andof destruction of reducing sugars would be con-siderably reduced and the undesirable formation ofcolour substantially decreased.

Possible ways to achieve this were thereforeinvestigated at the S.M.R.I. It was obvious from thebeginning that the fragile floc produced on phosphodefecation could not be treated in the same harshway as solid particles in mineral flotation. Recently,however, Baarson and Ray13 described a new flota-tion technique in which several metal hydroxides,which are gelatinous and flocculant materials, werefloated by means of dispersed air introduced at lowrates in order to prevent breaking of the floc andits redispersion. Various parameters influencing this"precipitate flotation" technique were studied by ,Rubin et all4# 5.Preliminary experiments with cane juice werecarried out using sintered glass funnels as flotationcells. Defecated juice was poured into the funneland air under pressure introduced through the stem.By forcing air through the sintered glass disc finedispertion of air bubbles was obtained. After abouton e minute of sparging precipitate was completelyfloated and clarified juice of exceptional clarity wasobtained. When the clarification and flotation werecarried out at 60C starch removal was comparableto that achieved by the Rabe process. Analysis ofsugar boiled i n the laboratory from clarified juiceobtained in this way showed a starch content of 70ppm and a gum content of 635 ppm. The behaviourof the syrup during boiling in the laboratory panwas quite normal. Flotation of the juice defecatedin the normal way and heated to boiling was also

and type of floc are of utmost importance in thisprocess. Fo r example, only if sintered glass of poro-sity 2 was used was the floc floated. Similarly, onlythe floc obtained after the addition of flocculant wasalways amenable to flotation. The point of additionof flocculant was also of importance. When theflocculant was added after aeration of the juice morerapid rise of the floc and better flotation wasachieved. This was attributed to entrapment ofbubbles inside the floc when the latter coagulated inthe aerated medium. In the absence of flocculant,the flotation was erratic and appeared to depend onthe type and quality of juice processed.A photomicrograph study of the bubble-particleattachment indicated that bubbles playing an activepart in the flotation process have a mean diameterof 82 microns and a standard deviation of 1.9. Sizedistributions were determined using a Patterson andCawood eyepiece graticule. Photographs of theaerated and floated floc indicated that bubbles wereattached to the floc by a combination of flocculationand collision. Bubbles were found to b e fairly evenlydistributed inside and on the surface of the flocparticles. It was noted that surface attachmentoccurred frequently in small cavities in the irregularsurface of the floc (Figure 2).

completely satisfactory. FIGURE 2. Photomicrograph of bubbles affecting flotationIt was soon realised, however, that bubble size of cane juice floc.


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Proceedings of The South African Sugar Technologists' Association - une 1969 203For further experiments a continuous laboratorymodel of 2lIm in capacity was designed and built.The appara tus consisted of two compartments(Figure 3) into the first of which (A) t he limed juicewas introduced above a sparger (B). A layer ofscum forms on the surface of the juice and is con-

tinuously scraped by baffles of an endless belt (C)into ovedlow (D). The clear juice is removed fromthe bottom of the second compartment (E). Diffi-culties were experienced with this model because ofturbulence which could not be completely preventedin the compa.rtment (E). A part of the floated flocwas carried downwards and redispersed, finallyfinding its waiy into the clear juice.

were considered. It was concluded that difficultywould be experienced with the limited availabilityof suitably fine spargers. The fine dispersion ofair by surface tension depression was examinedusing a wide range of frothing agents. The most suit-able additives were found to be iso-amyl alcoholand ethyl alcohol. However, even the most economi-cal additive, ethanol, at a rate of 2m l/l would betoo expensive for commercial application.As an alternative method of a ir dispersion, thejuice was introduced into a clarifier by means of acentrifugal pump with an air bleed on the suctionpipe. Although this was fairly successful the opera-tion was somewhat unstable. Slight improvement was

, FIG. 3 CONTINUOUS DISPERSED AIR CLARIFIER (MARK I!

JUICEI NLE T* MUDS

JLT \-* CLEAR JUICEIn an attempt to overcome this difficulty a newcontinuous model was built (Figure 4). The spargingcolumn used In batch flotation experiments was fittedwith a glass cylinder at the top in order to providea compartment for collection of clear juice. Twoinlets, one foir the juice and the other for flocculant,

were drilled in the Perspex column. A gutter wasplaced around the top of the glass cylinder for flocremoval. A ring-shaped juice outlet was placed atthe bottom of the clear juice compartment. Thelevel inside the clarifier was controlled by means ofa level controller connected to the clear juice out-let. Th e mud formed a t the top of the clarifier wasremoved by ovedlowing into the gutter.An improv~ement n performance, compared to theprevious motlel, was achieved, but again the tur-bulence at the top of th e column, caused mainly bya considerabl'e number of rising large air bubbles,affected the floated floc layer. Consequently someof the floc sank and was carried away in the clearjuice.At this stage the practical implications of intro-ducing air by sparging in a commercial scale unit

achieved by the addition of an air injector. However,by introducing a positive displacement pump aheadof the centrifugal pum p a very steady aeration wasobtzined.A third continuous model, designed for a juicerate of one-third gal/min was built (Figure 5). Thiswas an adaptation of the clarifier used for water pol-lution studies by Stander et all6.The juice and floc-culant were introduced at the top of tube A, thelatter in order to compensate for the shearing actionof the pu mp on the floc. Th e velocity down tube Awas 4 ft/min which was sufficient to carry the flocdownwards but allowed all bigger bubbles intro-duced with the juice to escape up the tube A. Theupward velocity in B and downward velocity in Cwere both 15 ftlhr. T he residence time of juice inthis clarifier was 15 min.The experiments with this model were conductedat Melville using the juice flowing to the factoryRabe clarifier. The juice was used as such or afterit had been heated to 9 5 C in a n electrically heatedtank before it was fed to the laboratory clarifier. Inboth cases clear juice of a quality comparable to


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FIG. 4.CONTINUOUS D I S P E R S E D AIR C L A R I F I E R ( M A R K 2)

Proceedings of The South African Sugar Technologists' Association - une 1969

GUTTER (lo"dl

M U D OUTLET 1 % ' ' $1

GLASSCYLINDER (8" # I 12" high

rO L U M N 2" d - C ' H I G HI =I=- J U I C E I N L E T I$" #)

I 1 A IR COMPRESSOR CONNECTION

the factory clarified juice was obtained in paralleltests. However, a small amount of very fine precipi-tate always remained in the clear juice of the labora-tory clarifier. Insuff icient mixing of juice . and floc-culant at the top of tube A, the detrimental effectof the centrifugal pump on floc, and turbulence atthe top of the clarifier may be the reasons for theimperfect clarification and improvements are pos-sible. Nevertheless the fact that apparently anadditional settling of the precipitate would still berequired at this stage in order to obtain perfectlyclarified juice defeats, at present, the whole purposeof high temperature flotation clarification.

F I G . 5.C O N T I N U O U SD I S P E R S E D A I R CLARIFIERM A R K 31

I JUICE INLETMUD COLLECTOR

JTO LEVELCONTROLLERReferences

1 . Annon. S.A. Sugar Journal, 1966, 50, 628.2. Meade, G . P., Sugar Journal, 1952, 15, 33.3. Greven, J. P., Ind. Eng. Chem., 1942, 34, 633.4. S~en cer-M eade: Cane Sugar Handbook. Wilev. New.~ o r k , th Ed., 335.5. Saranin. P. A.. Int. Sugar Journal. 1966. 68. 37.6. ~ i ~ ~ i n b o t h a m , ', sugar ~ournal , '1955: 1'8, 28.7. Foster, D. H., Sockhill, B. D. and Wright, P. G.,Technical Report No. 70, Sugar Research Institute,Mackay.8. Rabe, A. E., PAC.S. Afr. Sugar Technologists' Assocn.,1967, 41 , 42. .9 . MacRitchie, A. J. , and Morris, R. M., Int. SugarJournal, 1968, 70, 362.10. Prince, P. A., Proc. S. Afr. Sugar Technologists' Assocn.1968, 42, 53.1 1 . A. van Hengel. private communication.12. Douwes ~ekke r , -K., Archief voor de Suikerindustrie

1931. 41, 1100.13. ~ a a r s o n , . E. and Ray, C. L., Am. Inst. Mining, Metaland Petrol Eng. Symposium, 1963.14. Rubin, A. J., Johnson, J. D. and Lamb, J. C., Jnd.Eng. Chem., Process Design and Development, 1966,5. 368., - - .15. Rubin, A. J. and Johnson, J. D., Anal. Chem., 1967,39 , 298.16. Von Vuuren, L. R. J., Stander, G. J., Henzen, M. R.,Van Blerk, S. A. V. and Hamman, P. I., WaterResearch, 1968, 2, 177.


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Proceedings of The South African Sugar Technologists' Association - une 1969 205Discussion

Mr. Boyes:: There appear to be two objectivesfor research. One is to develop, as described here,a flotation unit to work quickly and effectively inremoving mud . Th e other is a sequestering agent toprevent build up of calcium on the replacement ofmagnesia.This is aimed at the ideal, namely, to removephysical im purities, from the juice.A t present for the starch problem th e enzyme pro-cess seems the cheapest and most effective but it isa degradation process, not a removal process, andprobably carries down sugar in final molasses.Mr. van Hlengel: I have suggested that the car-bonatation process should be added to the flotationprocess, not only to reduce the calcium and mag-nesium content of the clear juice, but also to givecrystal-clear juice and to cut out the use of P,O,,which is expensive. A d isadv antag e would be the ,introduction 'of filtration.The de Halan process is described on page 3 ofthis paper where juice is heated to 55C. Nowthis temperature is critical and yet in the flotation -process the tests during carbonatation have beendone at 60C with poor results as regards calciumand magnesium, up to 500 ppm.Mr. Rault mentioned the large quantity ofmolasses when Natal Estates was operating thecarbonatation process but I must point out thatit was carried1 out at a temperature of 70C.The de Haan process requires a retention time ofabout 15 mircutes for application of gas and lime.The Rabe tank requires 6 minutes. I thereforecannot see why the 217 ppm calcium and mag-nesium claimed for the de Haan process at 55C infifteen minutes a t a dangerously 'high level of p Hcannot be repeated at 9.8 pH for 250 ppm at a

retention time of 6 minutes. T he only reason I canthink of is the temperature of 60C.Mr. Jennings: Has the idea been tried of spargingCO, at the same time as air?Dr. Matic: No, this has not been tried but it isan interesting suggestion.Mr. Alexander: These tests were apparently dis-continued because there was always a certain amountof material in the clear juice that would not floatproperly and I would like to know if any analyseswere done to determine why this would not floatlike the rest of the precipitate.Dr. Matic: The material was not analysed but wewere of the op inion that the centrifugal pump wasdamaging the floc.I think t he last type of clarifier could be improvedby introducing air in such a way as to prevent someof the turbulence that is still occurring and by amor e efficient mixing of the flocculant th at is adde d.I think mechanical alteration of the equipment

is more important than the chemical aspect in orderto get clear juice.Mr. Buchanan: I do not think that there is achemical or physico-chemical reason for the carryover of part of the suspended matter. It is purelya mechanical limitation. Th e juice ca nnot be sloweddown sufficiently to avoid disrupting the bubble solidcontact.Mr. Alexander: I presume batch tests were doneon the ordinary juice and the juice preserved withmercuric chloride, and yet the differences inturbidity are attributed to starch.Dr. Matic: Yes, because th e juices were completelyclear in respect of suspended matter and starch iscolloidally dispersed. In static tests the juice isabsolutely clear but, as soon as the continuousprocess is attempted, we cannot get rid of suspendedmatter.

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1969_Matic_Aspects of Flotation Clarification