POLYACRYLAMIDE FLOCCULANTS IN JUICE CLARIFliCATION

The importance of various factors relating to the preparation of polyacrylamide flocculant solutions was investigated. For optimum results, these solutions should be agitated for at least 2 hours before use while water of high ionic content should be avoided. The quantity of calcium phosphate precipitated during the clarification reaction and the calcium concentration in clear juice are shown to influence differently the behaviour of polyacrylamide flocculants of varying degrees of hydrolysis. Introduction The lime-defecation process for the clarification of raw sugar cane juice relies on the in situ precipitation of calcium phosphate. This is achieved by the addition of lime (Ca (0H)z) to raise the pH and thus precipitate the natural calcium and phosphate content of the juice. This process flocculates the particles in the raw.juice by binding them into the precipitate via the Ca2+ ions adsorbed at the particle-liquid interface'. The quality of the clear juice obtained by this method is thus dependent on the extent to which this flocculation process can scavenge the particles of the taw suspension. It has long been known 3 q 5 1 that the extent of defecation is dependent on the amount of phosphate present in the original juice. Inorganic phosphate is frequently added to juices deficient in this requirement in order to obtain clearer juice, faster mud settling and better mud filtrationg. Since the adjustment of juice pH with lime results in considerable addition of Ca *+ ions, the precipitation of the phosphate content of mixed juice is generally nearly complete. It has, however, been shown that addition of Ca *+ can improve the clarity of clear juice ) * I 3 . In order to improve the clarification system, polyacrylamide flocculants are added prior to the settling stage. These flocculants are copolymers of acrylamide and sodium acrylate (generally known as partially hydrolysed polyacrylamides) and possess molecular weights in excess of one million. The percentage of sodium acrylate groups is referred to as the degree of hydrolysis (DH). For their optimum use, care must be exercised in the preparation of the solution. The generally recognized principles of polymer solution preparation are a careful addition of the solid powder to the water so as to independently disperse the individual granules; gentle agitation of the solution for a sufficient period of time to ensure complete dissolution and equilibration; temperatures below 50°C; a two stage preparation in which a stock solution is prepared and diluted prior to use; and the avoidance of centrifugal pumps which can damage flocculant molecules4. The purpose of this investigation was to determine what magnitude of influence polymer solution preparation parameters have on clarification efficiency. The parameters considered were time of agitation of the stock solution, solvent ionic strength, flocculant solution concentration and solution pH. The second stage of this study concerned the influence of the amount of PzOs precipitate and the concentration of calcium in clear juice on the behaviour of polyacrylamide flocculants. Experimental The laboratory clarification unit and its operation have already been describedlO. Turbidities of clear juice were determined as the absorbance at 800nm in a lcm cell using a Zeiss PM4 spectrophotometer. Initial settling rates of the limed suspension Were found graphically from the initial linear section of the measured settling curves. Final mud volumes were determined by the extrapolation of a plot of mud volume against reciprocal time8. The minimum time required for flocculant solution prepara-, tioti was determined on Superfloc A 130, dissolved in distilled water at 0,05% and 0,5% concentration and agitated for periods between 15 min. and 6 hours before application. The flocculant dosage was 0,8 pprn on juice. All flocculant solutions were prepared by sprinkling the granular solid into the vortex of a stirrer. The effect of flocculant solution concentration on its efficiency in clarification was determined with Superfloc A130 (0,8 pprn on juice). Solutions were prepared in distilled water at various concentrations between 0,01% and 0,5%. The influence of dilution was investigated by diluting the higher concentrations immediately prior to application. The influence of the nature of the water used as solvent was studied with regard to pH and ionic concentration. The effect of the former was determined by adjusting the pH of flocculant solutions with either 0,lM HC 1 or 0,lM KOH. The flocculants investigated were Superfloc A1 10 (approx. 30% DH), Superfloc A 150 (Approx. 60% DH) and Talosep A3 (approx. 40% DH). Flocculants were prepared in distilled water as 0,05% solutions and applied at 3,3 pprn on juice. The effect of solvent ionic strength was studied by dissolving Talosep A3 (0,05% concentration a plied at 1,7 p m on juice) in water whose ionic strength ha f been adjustel with NaCI. Specific conductance determinations were used as a measure of ionic concentration. Samples of distilled water, factory condensate and factory tap water were also investigated. Juice for the above tests was obtained from IL. The effect of the amount of phosphate precipitated was studied at constant calcium concentration in clear juice (320 pprn). The PzOscontent of a JR mixed juice sample was adjusted with H3P04 and then the juice was clarified with Ca (0H)z using a liming pH of 7,5 (at 70°C). The clarification behaviour of the limed suspension was analysed without flocculant, with Superfloc A1 10, with Superfloc A130 (approx. 45O/0 DH) and with Superfloc A150. These flocculants !were prepared as 0,05% solutions in distilled water and dosed at 2 pprn on juice. The role played by the calcium concentration in clear juice was investigated at a PzOs concentration of 320 pprn in mixed juice. The calcium concentration in mixed juice was adjusted with calcium chloride and the pH raised to 7,5 with 2M KOH. Mixed juice samples were obtained from IL and TS. Results The relationship between initial settling rate and the time for which the solution is agitated prior to use is shown in Figure 1 for Superfloc A130. It is clear that for this polymer an equilibrium in solution is attained after 2 hours and that further agitation does not bring any improvement in the settling rate. This equilibration time is unaffected by polymer concentration. Although settling rates are affected by the period of agitation, a steady turbidity value and final mud volume had been attained even after only 15 minutes. Due to their high viscosity, flocculant solutions are generally prepared at concentrations of 0,5% or below. Low concentrations facilitate dosing and produce more efficient mixing with the limed juice. Figure 2 illustrates the effect that varying concentrations have on settling rates and shows that below a concentration of 0,05% a rapid decrease in flocculant efficiency occurs, whereas above this concentration a constant settling rate was observed. The variations in flocculant solution concentration appear to have no effect on Proceedings of The South African Sugar Technologists' Association June 1978 107 turbidity or final mud volume. The conformation of the flocculant polymer in solution and thus its efficiency in promoting clarificat~on is dependent on the pH and ionic strength of the solution. Once again, these parameters affect only the settling rates and do not produce a noticeable influence on turbidity or final mud volume. The magnitude of the effect of pH of the flocculant solution on the settling rate depends on the DH of the flocculant. It can be seen from Figure 3 that the optimum flocculant (Talosep A3) shows the greatest effect, increasing its activity as the pH of the solution is raised. The other two flocculants with DH above and Flocculant : Superfloc A130