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Starch modification of the flocculation and flotation of apatite Correa de Araujo, Armando


Although the technical literature contains abundant references on applications of starch in mineral processing, the majority is not concerned with phosphate mineral systems. Nevertheless, the interaction between starch and apatite surfaces is relevant to both selective flocculation and flotation of phosphate ores. The main objective of this thesis is to investigate in detail such interaction in order to provide a more clear understanding on the behaviour of apatite/starch systems. Considerable research effort was dedicated to a thorough characterization of the starch samples used, especially in those aspects most pertinent to the application of starches as flocculants and depressants. Presence of ionic impurities in the starch samples tested was identified by infrared spectroscopy and microelectrophoresis. These impurities (proteins, carboxylic groups and, possibly, phosphate esters)were found to play an important role in the mechanisms governing the interaction of starch macromolecules and mineral surfaces. In a first stage of this research, the interaction between aqueous solutions of starches (and starch fractions - amylose and amylopectin) with calcium ionic species and surfactants (flotation collectors) was investigated. Depression of solution electrical conductivity, experienced in Ca-starch systems, was indicative of chemical reactions taking place (complexation). For surfactants, evidence for their interaction with starch fractions was obtained by UV-Vis. spectroscopy. The spectra of starch/surfactant solutions in the presence of iodine were altered indicating the substitution of iodine species by surfactant molecules at the helical sites of starch macromolecules. The next step involved the study of the adsorption of starch onto both apatite and silica mineral surfaces. Preliminary tests pointed out that a much stronger interaction took place in the case of apatite. Starch adsorption isotherms obtained for fluorapatite and quartz confirmed the preferential adsorption of starch onto the phosphate mineral surface. Both amylose and amylopectin were strongly adsorbed on fluorapatite but the latter polymer displayed the largest extent of interaction on a weight per area basis. The shape of the adsorption isotherms for the two starch fractions on fluorapatite also corroborates the idea of a stronger interaction by amylopectin. In turn, whole starches displayed adsorption isotherms resembling more closely that obtained for amylopectin. Adsorption of starches on fluorapatite was increased considerably in the presence of Ca ionic species. In the absence of externally added Ca ionic species, the amount of Ca released by the mineral surface was dependent upon the amount of starch adsorbed. These two phenomena can be interpreted as indicating the importance of Ca sites and presence of Ca species for the adsorption of starches, hence justifying the preferential adsorption displayed for apatite. Adsorption of starch on quartz surfaces was also enhanced in the presence of Ca ionic species, once more confirming the important role played by calcium on the adsorption of starches. Flocculation studies were also conducted with fluorapatite, kaolinite and quartz suspensions in the presence of different starches. Under the conditions tested, all starches samples failed to promote aggregation of the two non Ca-bearing minerals. In turn, fluorapatite suspensions were flocculated rather strongly by all starches. Maximum flocculation of fluorapatite was achieved at partial polymer coverage conditions. With one exception (amylose), increasing the concentration of the polymers above an optimum level, generated partial re-stabilization of the suspensions, probably via a steric effect. All starches depressed both anionic and cationic flotation of fluorapatite. Amylose was the least effective depressant among all starches, especially for the cationic flotation system. The depressant action was a function of pH and of the relative amounts of polymer and surfactant. Alkaline pH favoured depression, whereas as the collector level was increased, the depressant action was diminished and eventually eliminated. The experimental evidence gathered in the present research supports a chemical mechanism for the interaction between starch and apatite surfaces. Calcium plays a dominant role, and its importance for the adsorption of starches onto mineral surfaces is most probably related to the formation of complexes between starch impurity-related ionic groups and Ca ionic species. Hydrogen bonding and to lesser extent electrostatic forces are also important for the overall interaction between starch and apatite surfaces. The larger extent of interaction for the amylopectin fraction(highest molecular wt.) as compared to that of amylase (lowest molecular wt. fraction) gives support for the accessory role of hydrogen bonding.

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