The effect of molecular weight on the adsorption and efficacy of polysaccharide depressants

Long chain polysaccharide polymers, such as carboxymethyl cellulose (CMC) and guar gum, are commonly used in flotation to depress gangue minerals. These depressants are in widespread use in the flotation of platinum group minerals contained in the Merensky ore body where the dominant gangue minerals are talc, pyroxene and feldspar. The molecular weight and degree of substitution of these polysaccharides are known to be significant factors influencing the behaviour of such depressants. In the present study CMC and guar at different dosages and with different average molecular weights were used to investigate their effectiveness in depressing gangue minerals. The high molecular weight depressants ranged between 600,000 and 700,000 g/mol and the low molecular weights, 40,000-70,000 g/mol. The depressants were tested using microflotation, batch flotation and equilibrium adsorption studies. The investigation showed that, at starvation dosages (100 g/t) during batch flotation, the high molecular weight polymers did not depress naturally floating gangue (NFG), whereas the low molecular weight depressants did. At higher dosages (300 g/t), both high and low molecular weight polymers depressed all NFG, without depressing sulfide recovery in the pulp. The high molecular weight polymers were significant froth destabilisers, which suggested that they were acting as good slime cleaners. This, combined with their ineffective depression of NFG at low dosages, suggested that the high molecular weight polymers were selectively adsorbing to hydrophilic gangue (pyroxene and feldspar). Adsorption isotherm experiments gave the maximum adsorption densities attainable at equilibrium. These were compared to the adsorption densities of the polymers in the microflotation and batch flotation experiments. This information showed that very little polymer was required to destabilise bubble-particle interaction during microflotation tests. At conventional plant depressant dosages of 100-300 g/t, adsorption densities range from ∼20% to 50% of maximum.