A LABORATORY EVALUATION OF THREE METHODS FOR THE CLARIFICATION

OF B-MOLASSES

L. M. S. A. Julienne and D. M. Frankland Sugar Milling Research Institute, Durban, South Africa

I , ABSTRACT ~

The clarification of B-molasses by three different methods namely phos- po-flotation, froth-flotation and sulphitation was investigated under labora- , tory conditions. The effect of the treatments on turbidity, suspended matter content and viscosity as well as on the chemical characteristics of ~ the molasses is discussed. I

INTRODUCTION 1

For many years the sugar industry has been interested in supplementing the conventional mixed juice clarification with a syrup or molasses clarificqtion. In the lafe fifties extensive factory trials were carried out in Australia and Mauritius with the solid-bowl centrifuging of B-molasses. At the 1962 ISSCT Congress a symposium on B-molasses clarification was held at which the Australian technologists " reported on their experiences with the solid bowl centrifu- ge. However, due to a combination of mechanical problems and excessive sugar losses, centrifuging was subsequently abandoned.

In recent years, Tate and Lyle Ltd have developed a phospho-flotation pro- cess for syrup and molasses clarification. In the 1975-1976 season a full scale syrup flotation trial >was run at the Noodsberg factory in South Africa which yielded better quality A-sugar and less viscous final molasses. There was, howe- ver, no conclusive evidence of any improvement in the boiling house recovery.

For part of the 1981-1982 season Tatesand Lyle Ltd, ran a full scale B-molasses clarification trial at this same factory using the phospho-flotation process. This has renewed the industry's interest in finding ways and means of improving the notoriously highly viscous South African C-massecuites.

It is with this in mind that the laboratory investigations into B-molasses clarifi- cation by phospho-flotation, froth flotation and sulphitation were undertaken.

EXPERIMENTAL PROCEDURE

The three methods which were investigated for clarifying B-molasses are sepa- rately described below. The &raw material used for the investigations was factory B-molasses. The bulk of the tests were done with molasses from the Noodsberg (NB) and Sezela (SZ) factories. Some sulphitation tests also included molasses from the Umfolozi (UF) factory.

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Phospho- Flotation

1. The B-molasses was diluted to brixes ranging from 40" to 60". 2. The diluted molasses was heated to 85OC.

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3. Two different levels of P205 were added as dilute phosphoric acid, to the hot molasses (250 and 500 ppm P205 on molasses brix).

4. Milk of lime was added to bring the p H of the molasses back to its original level (+ 6,4 usually).

5. A 500 ml portion of the flocculated molasses was aerated by mixing in a Moulinex mixer type 241 at 1 500 rpm for one second.

6. Different flocculants (Talofloc A 3 and A5 and Separan AP 273) at various dosages (15,25 and 50 ppm on brix) were gently mixed into the aerated molasses.

7. The molasses was then transferred to a one litre separating funnel placed in 'a hot water bath at 85°C and the scum flotation was allowed to proceed for 20 min.

8. The clear molasses was drained from the top scum and both portions were weighed.

9. The clarified molasses was kept for analysis. 10. The scum was de-sweetened by mixing with hot water at 85°C (ratio of water

to scum being 3 : l by weight), mixing 1 ppm of flocculant and floating in a one littre separating funnel at 85°C for 20 min.

11. The sweet water was drained from the scum and kept for analysis.

Froth- Flotation

The procedure used during the investigations was as recommended by Fabcon International, Inc.

1 . The B-molasses was diluted to brixes ranging from 40 to 60". 2. The diluted molasses was heated to 90°C for 3 min. 3. The hot molasses was aerated for 15 s in a Moulinex mixer type 241. 4. Different flocculants (Zuclar 1000 ST, Talofloc A 137) at various dosages

(25-75 pprn on brix) were added to the aerated molasses and gently mixed for one minute.

5. The molasses was transferred to a one litre separating funnel and the flotation allowed to take place at 90°C for 20 min.

6. The clarified molasses was drained from the top scum and the two portions were weighed separately. The clear molasses was kept for analysis.

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1 Sulphitation

1 1. The raw B-molasses was diluted to biixes varying between 45' and 60"Bx. 2. The diluted molasses was heated to 90°C. 3'. Varying amounts of lime as a slurry of A. R.

CLI(OH)~ (1% to 5% CaO on brix) were continously added to the stirred molasses over a 20 min period and sul ph ur dioxide gas (ex SO2 cylinder 99,9% pure) was simultaneously bubbled in at a controlled rate just sufficient to maintain the pH at its original value of .+ of 6,4.

4. The flocculated ,molasses was gently stirred at 90°C for 30 minutes. 5. A 500 ml portion was filtered at 90°C in a water jacketed pressure filter

(filtering area 30 cm2) using a filter cloth ex U F sugar refinery (NY 19 nylon cloth. supplied by Filtaflow Ltd). During the filtration the pressure were gradually increased to +I00 kPa gauge as the filter cake formes.

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. . 7. The rate of filtration and the applied pressure 'were recorded at regular

intervals. 7. The filtered molasses was kept for analysis.' 8. The calcium sulphite cake was weighed, crushed and mixed with hot water

i (ratio of water to cake was 1: l ) and filtered in the presure filter at + 50 kPa i gauge.

9. The de-sweetened cake and wash water were kept for analysis. ~ CHEMICAL ANALYSIS

The chemical methods used are as described in the Laboratory Manual for South African Sugar Factories (1977) except for suspended matter and turbidity which are described in the Appendix. The viscosity determinations were carried out with a Brookfield Model HBT viscometer on the molasses reconcentrated to 70-76" brix.

The list of analytical determinations done during the investigations is a follows:

Raw and Clarified Molasses

Pol, brix, reducing sugars, sulphated ash, gums, turbidity, viscosity and suspen- ded matter content.

Sweet Water

As above no turbidity, viscosity or suspended matter.

Sulphitation Cake ,

Pol and moisture.

Composition of Suspended Matter in B-Molasses

The suspended matter was obtained by centrifuging undiluted B-molasses in a Beckman 5-21 preparative centrifuge at 20 000 rpm for 2 h at room temperatu- re. The supernatant molasses was discarded and the suspendea matter was was- hed with cold water and centrifuged for 20 min. This latter procedure was repeated twice. The washed suspended matter was analysed for moisture, ash, Si02, Al2O3, CaO, MgO, K20, Na20, P205 and SO3.

i / RESULTS

Quantity and Composition of Suspended Matter in B-Molasses

The a-mount qf .suspended matter in some South African B-molasses sampled d;ring the 1981-1982 season is given in Table 1.

The results are expressed as percent suspended matter on brix in molasses.

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Table 1 . Amount of Suspended Matter in B-Molasses

Factory Month Susp. Matter % brix

NB sept . -bct . 1981 0,8 - 1,0 IL Dec. 1981 2,3 SZ Dec. 1981 2 3 GH Dec. 1981 2,1 D L Dec. 1981 J 2,3

The analysis of the suspended matter from 4 different factory molasses samples gave the results shown in Table 2.

Table 2. Chemical Composition of Suspended Matter in Factory B-Molasses ' Factory EM AK SZ UK

% loss at 105'C Ash % dry sample SiO, % ash Al2O3 % ash Fe203 % ash CaO % ash MgO % ash K,O % ash Na20 % ash P20, % ash SO1 % ash

' All the ash constituents are expressed as a percentage of total ash found.

Phospho-Flotation

1) Scum Formation -

The amount of scum formed in the phospho-flotation process under different conditions of molasses brix and flocculant addition is shown in Fig. 1.

2) Suspended Matter and Turbidity

The turbidity of B-molasses following different treatments is shown in Fig. 2. During the investigation it was found that a good correlation existed between

the turbidity of molasses and its suspended matter content. The plot is shown in Fig. 3 the relationship being:

For NB molasses:

Turbidity = 8 269 + 10 975 Susp. matter % brix (n = 12; r = 0,91)

d I 1

0.1 0 2 0 3 0.4 0 5 0 6 0 7 0.8 0.9 1.0 1.1 1 2 1.3

Suspended matter f%Bx)

Figure 3. Regression Curves for turbidity v/s suspended matter content for B-molasses.

For SZ molasses:

Turbidity = 3 914 + 11 283 Susp. matter % brix. (n = 9; r = 0,91).

Using the above equations and the turbidity results obtained during the investi- gation it is calculated that the maximum % suspended matter removal achieved at NB and S Z with 50" brix molasses was about 80%.

It was also found that the clarification was slightly better at 500 ppm P2O5 on brix compared with 250 ppm P2O5.

3) Viscosity '

The viscosity curves of the raw and clarified B-molasses are shown in Fig. 4.

4) Chemical Arfalysig Y

Table 3 shows the results of the chemical analysis carried out on the raw and clarified B-molasses.

- SZ Raw

- - - N B f Y X Clarified ,

e 1 - . d

70 71 f 2 f 3 r I " I

7 4 7 5 76 77

o Bx t

Figure 4. Viscosity curves for raw and clarified B-molasses at 22,8"C.

Table 3. Chemical Analysis of B-Molasses (NB average of 6 tests; SZ average of 7 tests)

Red. Sug. Red. Sug. Ash to Gums Purity ash brix brix PPm on

ratio ratlo ratlo brix

NB Raw molasses 42,4 1,OS 17.0 16.2 22 600 Treated molasses 42.8 1,09 16.9 15,6 23 600 Sweet water 40,O 0,75 - 13.0 17,4 20 700

sz Raw molasses 45 ,O 1,15 17.7 15.4 30 700 Treated molasses 45,4 1.24 18.1 14.6 28 900 Sweet water 41.8 1.10 17.6 16.0 27 200

Froth-Flotation

The investigations were carried out with NB and SZ B-molasses but only the SZ results are given since they are representative of the general trend.

1) Scum formation

This is shown in Fig. 5 for molasses at 45 and 55" Bx.

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5 + @

# w

b

' 2 5 5 0 , , - 7 5

ppm flocculant

Figure 5. Scum % B-molasses in froth flotation.

2) Turbidity

The turbidity of the B-molasses following treatment under different condi- tions of brix and flocculant additions is shown in Fig. 6.

3) Chemical Analysis

The results of the analysis of the raw and clarified molasses is given in Table 4.

Sulphitation

During this investigation it was found that a minimum addition of 3% CaO (NB) and 4 % CaO (SZ, UF) on brix in molasses was required to prevent the complete and rapid blinding of the filter cloth by a slimy deposit on its surface. It

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ppm flocculant

Figure 6 . Turbidity ok clarified B-molasses by froth-flotation. -

was also found that at a brix exceeding 50" the filtration rates would become excessively slow.

The results given here are therefore for the following treatments

NB-50" Bx molasses with 3% CaO on brix. SZ, UF-50" Bx molasses with 4% CaO on brix.

Table 4. Analysis of Raw and Clarified B-Molasses (SZ Average of 6 Tests)

Red. Sug. Red. Sug Ash to Gums Purity ash to brix brix PPm on

Ratio Ratio Ratio brix

Raw molasses 48,O 1,05 16,2 15,5 30 900 Cla. molasses 48,l 1,11 16,3 14,7 29 000

1) Filtration Characteristics

The main features of the filtration tests are given in table 5 .

Table 5. Filtration of Sulphitated R-Molasses

Average Maximum Pressure Filtrat~on Time

Factory @Pa)

Filtration Rate (mln) (1 . h-' . m-*)

2) Turbidity

Table 6 shows thq turbidity of the molasses before and after treatment.

Table 6. Turb~dity of Raw and Clarif~ed B-Molasses

Turbidity Molasses Factory % Removal

Raw Treated

NB 18 900 6 300 66 SZ 26 100 4 100 84 UF 26 900 5 100 81

3) Chemical Analysis

The results of the chemical analysis for the NB and S Z raw and treated B-molasses and sweet water (ex calcium sulphite cake washing) are given in Table 7.

Viscosity

Figure 7 shows the viscosity curves of the S Z raw and treated B-molasses.

Figure 7. V ~ s c o s ~ t y curves for raw and clarif~ed B-molasses at 20,s (sulphltation).

5 ) Filter Cake

T h e washed filter cake gave the analytical results shown in Table 8.

Table 7. Chemical Analysis of NB and S Z Raw and Treated B-Molasses and Sweet Water

Red. Sug. Red. Sug. Ash to Gums Purity to Ash to brix brix PPm on

Ratio Ratio Ratio brix

NB Raw mol. 47,O 0,90 13,9 15,5 20 750 Treated mol. 46,8 0,97 13,8 14,4 21 600 Sweet water 42,7 0,59 13,O 15,7 15 850

S Z Raw mol. 50,7 0,79 12,4 15,7 28 850 Treated mol. 50,4 0,95 12,5 13,3 30 300 Sweet water 48,5 0,72 12,7 17,5 22 000

Table 8. Sulphitation Cake Analysis

Weight of Pol in Cake - Pol % Factory Cake % brix Moisture % % brix in

Cake in Molasses Molasses

Factory Trials

During part of the 1981-1982 season a full scale B-molasses clarification trial by phospho-flotation was run at the NB factory. The B-molasses was clarified at f 50" Bx and the average results are as follows:

Suspended matter removal -55% Reduction in viscosity -28%

There was, however, no conclusive evidence of an impro"ement in the C-massecuite characteristics and final molasses exhaustion in the limited time during which the tests were run.

In December 1981 a 3 day B-molasses sulphitation trial was run in the UF raw sugar refinery.

Unfortunately the refinery equipment did not prove entirely satisfactory in handling B-molasses and the trials had to be stopped before any C-massecuite could be boiled, from the clarified molasses. In spite of te unsteady operation whiqh prevailed during the trials and the fact that the'flocculation was not entirely satisfactory, the treatment showed some marked improvements in the B-molasses characteristics. There were 52% and 48% reductions in turbidity and viscosity respectively.

It appears that South African B-molasses contains between one and three percent suspended matter on brix (see Table 1). This supended matter is, roughly speaking, made up of 75% organic mat t s -and 25% ash (see Tabk 2). 'The main ash consiiltuents are potassium and silica and this is an indication that the bulk of

the suspended matter does not originate from floc carry-over in juice clarification (the main constituents would then have been calcium and phosphate).

This laboratory investigation shows veri conclusively that all three treatments can remove fairly large percentages of suspendqd matter, and substantially improve the B-mdlasses characteristics. This is evidenced by the important reductions in the turbidity and viscosity of the treated B-molasses.

The three treatments differ in their clarification efficiency which is also influen- ced by other parameters such as brix and origin of the molasses, type and quantity of flocculant, and chemical addition.

On the basis of turbidity removal which, as shown earlier bears a direct relationship to suspended matter removal (see Fig. 3), sulphitation is the most efficient of the three processes followed by phospho-flotation which is better than froth flotation. At optimum conditions the three processes yielded the following turbidity removal:

Sulphitation: 80%. Phospho-flotation: 60%. Froth-flotation: 40%.

The main objective in clarifying B-molasses is to effect a reduction in its viscosity and in this connection it is important to note that the turbidity removal is accompanied by a drop in viscosity, as shawn in Fig. 4 and 7, which is about 25% for phospho-flotation and 50% for sulphitation in the brix range 72" to 76". Difficulties were encountered in obtaining viscosity curves at the 90°B level (where it really matters since the C-massecuite boilings take place at that level) but there were indications that the reduction in viscosity would be even more important.

All three processes have one important common drawback in that efficient clarification can only be achieved in diluted B-molasses. It appears that 55"Bx would be a maximum above which the floc in the flotation methods would not rise fast enough and the filtration rate in sulphitation would become exceedingly low. The adoption of B-molasses clarification in the industry would consequently entail the installation of additional evaporation equipment on the pan floor.

The amount of scum formed in the two flotation processes was rather high although froth flotation produced only about half the scum of phospho-flota- tion. The brix of molasses, quantity of flocculant and the suspended matter content of the molasses have an influence on the scum formation (see Figs. 1 and 5). The high scum quantity is disadvantageous in the sense that it would result in important quantities of B-molasses being re-cycled. It is, however, felt that part of the reason for the high scum formation could be the wall effect of the small diameter laboratory flotation chamber and that on an industrial scale lower quanti- ties could be expected.

On the analytical side, clarification by all three methods does not seem to effect any obvious change in the B-molasses purity (see Tables 3, 4 and 7). There is also no gum removal. The reducing sugar to ash ratio of the clarified molasses is higher than in the raw molasses. This is certainly due to the removal of the inorganic suspended matter and would not affect the molasses exhaustion which is only dependent on soluble ash. The reducing sugar to ash ratio does, however, show a marked drop in the sweet water of all three treatments which is an indication that part of the inorganic matter removed during the clarification goes into solution when the scumlcake is washed with hot water, This of course could present a problem on an industrial scale in that it could lead to a build-up of ash in the B-molasses (the sweet water being used for diluting the B-molasses before

clarification) andlor increase the soluble ash content of the final molasses (if part ' of the dissolved ash did not crystallise out) with a detrimetal effect on its exhaus- tion.

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CONCLUSIONS

The laboratory investigations show that different chemical methods are availa- ble for' the clarification of B-molasses. The results of the factory trials at NB and U F are an indication that these processes could be effectively usted on an industrial scale. The reduction in B-molasses viscosity which could be expec- ted, at 25 percent or more, would certainly have a beneficial effect on the C-massecuite processing, although at this stage no concrete factory results exist to confirm it.

Because of the difficulties in obtaining any conclusive results on the benefical effects of clarified B-molasses on a factory scale, a series of laboratory boiling down tests in presently being carried out at the Sugar Milling Research Institute using raw and clarified molasses to obtain an indication of the potential gains in exhaustion.

I A ACKNOWLEDGEMENTS

The authors would like to thank the Process staff of NB and U F for their valuable assitance, Messrs. G . Ghetty and C. Rungasamy for conscienciously carrying out the laboratory investigations and the staff of the Analytical Services Division of the SMRI for the analytical work.

APPENDIX. DETERMINATION O F SUSPENDED SOLIDS IN B-MOLASSES

Apparatus Erlenmeyer flask (250 cm3) and rubber stopper. I Analytical balance. High speed centrifuge (30 000 rpm). Vacuum oven (65" C). Desiccator. ' Procedure

1. Weigh 50 g of molasses plus 50 g of distilled water into the Erlenmeyer flask, stopper and shake to dissolve the sample.

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2. Weigh a clean dry centrifuge tube on the balance and record the mass (MI). 3. Stir the sample well and decant ca. 50 g of sample into the centrifuge tube.

Weigh the tube plus sample (M2). 4. Centrifuge for 30 min at 30 000 rpm and at 15' C. 5. Decant the supernatant and add 10 cm3 of distilled water to the precipitate in the

tube. 6. Shake for 5 min to disperse and wash the precipitate. 7 . Centrifuge for 30 min at 30 000 rpm and at 15" C. 8. Repeat steps 5 to 7.

, 9. Dry the centrifuge tube plus precipitate in the vacuum oven for 18 h, then cool in

I the desiccator for 1 h and weigh (M3).

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Calculation

Suspended solids % B-molasses = M 3 - " 1 ~ 2 x - loo or suspended M2 - M1 1

solids % brix in B-molasses = 100 100 M3 - - M I x - 2 X x - M2 - MI 1 B x

Turbidity of B-Molasses

Apparatus Spectrophotometer or colorimeter refractometer.

Procedure 1. Prepare 200 g of approximately 5" Bx molasses by dilution with cold distilled

water. 2. Measure the absorbance of the diluted molasses at 720 nm using distilled

water as reference in a 10 mm cell. 3. Read the exact brix of the + 5" Bx molasses.

Calculation

Turbidity x O.D. x 1000 1 X g of sucrose per ml at exact brix

REFERENCES

1. Foster, D. H. (1962): An Evaluation of Sludge Removal From A- and B-Molasses. Proc. Int . Soc. Sugar Cane Technol. l l t h Congress, 847-855.

2. Davis, C. W., Clarke, W. B. (1962): Removal of Sludge From B-Molasses by Centrifugal Separa- tion. Proc. Int. Soc. Sugar Cane Technol., l l t h Congress, pp. 856-864.

3. Bennett, M. C. (1976): Flocculation Technology in Sugar Manufacture Rev. Agric. Sucr. Maurice, vol. 55 (1, 2), pp. 275-283.

EVALUATION EN LABORATOIRE DES TROIS METHODES POUR LA CLARIFICATION

DES EGOUTS B

L. M. S. A. Julienne et D. M. Frankland Institut de recherches sur la fabrication du sucre, Durban, Afrique du Sud

L'exposC fait part d'une experience rCaliske en laboratoire sur la clarifi- cation des &gouts B suivant trois mkthodes diffkrentes: phospho-flottation, flottation au moussage, sulfitation. L'auteur explique l'effet de ces traite- ments sur la turbiditC, la teneur en matieres en suspension, la viscositd, et les caractkristiques chimiques des kgouts.

EVALUACI~N EN EL LABORATORIO DE TRES METODOS DE CLARIFICACI~N DE MIELES

DE SEGUNDA

L. M. S. A. Jullienne y D. M. Frankland Instituto de Investigaciones de Molienda Azucarera, Durban, Sudhfrica

RESUMEN

La clarificaci6n de las mieles de segunda, en condiciones de laboratorio, fue investigada mediante tres mktodos diferentes: por fosfo-flotacidn, flota- ci6n por espuma y sulfitaci6n. Se discute el efecto de 10s tratamientos sobre la turbidez, el contenido de materias en suspensi6n y la viscosidad, asi como sobre las caracteristicas quimicas de las mieles. ~