How the physical forms of starch affect filterability at a carbonatation refinery. Part II: simulated carbonatation filtration

Authors: Cole, Marsha; Eggleston, Gillian; RATHKE, THOMAS - Imperial Sugar Co; NAIKI, JUNE - American Sugar Refining, Inc; Triplett, Alexa; St Cyr, Eldwin

Submitted to: International Sugar Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/18/2016
Publication Date: 9/1/2016
Citation: Cole, M.R., Eggleston, G., Rathke, T., Naiki, J., Triplett, A., St Cyr, E. 2016. How the physical forms of starch affect filterability at a carbonatation refinery. Part II: simulated carbonatation filtration. International Sugar Journal. 118:650-658.

Interpretive Summary: Concentrations of starch, an impurity extracted with sugarcane juice, in U.S. raw sugars have generally increased in the last decade. Current measures of raw sugar quality, starch, and filterability do not predict the effects sugar impurities have on filtration performance at the refinery since raw sugar is very complex. Total starch concentration and physical starch form not only directly affected filtration but also the chemical reactions before it too. Both starch forms initiated the production of small calcium carbonate crystals which limits the efficiency of refinery filtration. Quality traits of raw sugar, whether soluble or insoluble, affect carbonatation reactions and press filtration at the refinery both individually and collectively and it does so with varying degrees of complexity.

Technical Abstract: Although starch is known to detrimentally impact carbonatation filtration at sugarcane refineries, the physical form(s) of starch responsible is unknown. Thus, a study was undertaken to elucidate the effects of the physical starch forms, i.e., soluble, swollen, and insoluble, and their concentrations on carbonation clarification reactions using simulated melt liquors and six international raw A sugars. Simulated carbonatation clarification reactions and press filtration on melt liquors with known concentrations and ratios of soluble and insoluble starch as well as the international raw sugars were investigated. Both soluble and insoluble starch forms had a major impact on press filtration by adversely affecting the nucleation and induction periods of carbonatation that precede calcium carbonate (CaCO3) precipitation. Similar trends were found between these starch forms and their effects on nucleation time, although the impact was more dramatic with soluble starch. Soluble starch prolonged carbonatation induction whereas insoluble starch remained marginally unchanged. Reaction rates were dramatically faster when soluble starch was abundant in simulated melt liquors and slower with increasing concentrations of insoluble starch, and both starch forms correlated strongly with the total reaction time for the carbonatation reaction. Two size populations of CaCO3 fines (<5 µm) were implicated in filtration: <3 and 5-8 µm, and exhibited strong correlations mostly with insoluble starch concentration, filtrate turbidity, and filtration flow rates. In general, the international raw sugars showed variable responses to simulated carbonatation refining with reaction rates 10,000x faster than the simulated melt liquors containing soluble and/or insoluble starches. Mud yields ranged from 17-80% and were attributed mostly to all raw sugar impurities rather than only starch content. Comparison of the filterability indices determined with the Domino method showed moderately strong correlations with nucleation time (R=-0.693) and reaction rate (R=0.812). Although this filterability method did not account for the many soluble raw sugar impurities that influenced their filterability, it did implicate possible refinery performance with particular emphasis on nucleation time and overall reaction rate. Overall, CaCO3 inhibition in sugar refining is influenced significantly by all quality traits of raw sugar as they affected each process individually and collectively and it did so with varying degrees of complexity.