Adsorption and depression effects of native starch, oxidized starch, and dextrin on graphite

Abstract

• Interactions of starch-based depressants with graphite characterised. • Oxidised starches with high ratio of –C=O to –COOH are effective depressants. • Hydrophobic interaction mechanism proposed based on negative effect of -COOH beyond a threshold • <15% surface coverage required for effective depression with oxidised starches. Carbon is a common gangue , often associated with base metal ores. The hydrophobicity of carbon causes interaction with air-bubbles during flotation , and results in carbon contamination of concentrate and grade reduction of base metals. Therefore, selective separation of carbon is important during froth flotation processing. Native and modified starch have been recommended to depress carbon during flotation, however neither the mechanism of interaction nor the efficiency of carbon depression by different forms of starch is currently clear. Using two sets of oxidized starches (native wheat starch oxidized by 2, 5 and 10% concentration of either H 2 O 2 or NaOCl at pH 4) and comparing them with dextrin and native wheat starch (NWS), the mechanism of depression of graphite (a carbon exemplar) was investigated through adsorption isotherm and micro-flotation experiments as a function of starch functional group content, molecular size distribution and surface coverage estimation. Efficiency of depression of graphite by oxidized starches was favoured by a high proportion of substituted -C = O compared to –COOH groups. Increasing proportion of –COOH beyond a threshold negatively impacted on depression of graphite. Dextrin had an intermediate efficiency to depress graphite whereas NWS was poor at low dosage range but with high dosage showed good depression. A low estimated surface coverage (<22%) by different starch polymers was enough to cause effective depression of graphite. The data are consistent with depression of graphite by starches being guided by a hydrophobic interaction mechanism. The results also demonstrate that controlled oxidation modification can be used to tailor-make starch for graphite (carbon) depression.