The flocculation of cane sugar muds in Australian raw sugar factories

Crees, Owen Lloyd (1988) The flocculation of cane sugar muds in Australian raw sugar factories. PhD thesis, James Cook University.

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Abstract

The flocculation of cane muds in the factories of the Australian raw sugar industry has been investigated under factory operating conditions, in laboratory simulations of factory operations and with model particle systems.

The polymers employed as flocculants in Australian sugar mills are exclusively polyelectrolytes of the acrylamide/acrylic acid copolymer type. The effectiveness of a wide range of these polymers in flocculating muds has been found to depend primarily upon their molecular weights and on the relative amounts of the two comonomers. For a series of polymers of any given composition, the flocculating ability increases approximately linearly with molecular weight. No evidence has been found to indicate a limiting molecular weight although the practical problems associated with the large scale use of such large molecules increase as their size increases.

There was a very pronounced dependence on the copolymer composition, with a definite optimum composition for any given cane mud. The optimum composition was found to depend on the zeta potential of the mud particles, with a higher proportion of ionic monomer required as the zeta potential tended towards higher negative values. It has been shown that the adsorption of the flocculating polymer occurs under conditions where both the polymer and the particle carry negative charges. Adsorption occurs through the acrylate functional group of the polymer, probably by the formation of a cation bridge with another carboxylic acid group at the particle surface. Adsorption is favoured in the presence of calcium ions which are the predominant multivalentions in cane juice.

On the basis of the results presented here, a mechanism has been postulated for the flocculation of cane muds, consistent with the generally established principles of bridging flocculation but with some novel features. The optimum conditions for flocculation represent a compromise between the proportion of acrylate groups available for adsorption and the energy of adsorption of the individual ionic groups. When the product of these two parameters is less than the optimum value, polymer absorption is weak and reversible with the result that the aggregates formed are also weak and easily broken, even by the shear forces encountered during sedimentation. The settling rate is low and the supernatant turbidity high. When the product is higher than the optimum, adsorption is irreversible although some subsequent rearrangement of adsorbed molecules can occur. The growth rate of the aggregates is limited by the lack of mobility of the adsorbed polymer. The settling rate is low due to the smaller size of the aggregates but the supernatant turbidity remains low. The optimum value therefore represents the condition where adsorption is strong enough to resist the shear forces and to collect and hold all of the particles while still allowing the adsorbed polymer to retain its solution conformation and flexibility. The growth of the aggregates is not limited by either desorption or mobility of the polymer so the settling rate is maximized and the supernatant turbidity minimized.

Investigations of the solution properties and the monomer sequence distributions of commercial copolymers have shown that flocculation is favoured by a regular distribution of ionic groups along the chain. Random distributions, block copolymers and branching or crosslinking are all deleterious to flocculation.

A very important consequence of this research, and one of its main aims, has been its rapid and successful application to the industrial environment of all Australian sugar mills. Through this application and the cooperation of several of the world's major manufacturers of polyacrylamide flocculants, the industry has been able to effect substantial improvements in process operations and in product quality.

Item ID: 27714
Item Type: Thesis (PhD)
Keywords: cane mud; flocculation processes; optimum flocculation conditions; polyelectrolytes; polymers
Date Deposited: 26 Jun 2013 07:09
FoR Codes: 03 CHEMICAL SCIENCES > 0305 Organic Chemistry > 030599 Organic Chemistry not elsewhere classified @ 100%
SEO Codes: 97 EXPANDING KNOWLEDGE > 970103 Expanding Knowledge in the Chemical Sciences @ 100%
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