Selective molecular weight adsorption from polydisperse polysaccharide depressants

• Low molecular weight fraction of a polydisperse CMC or guar adsorbs preferentially.
• Low molecular weight fraction is not displaced by larger fraction over time.
• Adsorption energy, not conformational entropy may be the driving force.

Guar and carboxymethyl cellulose (CMC) polysaccharides play an important role in the mineral processing industry as gangue depressants in the flotation process. As with all polymers, they are polydisperse molecules, exhibiting a range of molecular weights about a mean. Thus, the aim of this study was to determine whether there is competitive adsorption between chain lengths of different molecular weights and to determine which chain lengths adsorb preferentially. Gel permeation chromatography (GPC) was used to assess the molecular weight distribution of guar and CMC depressants of low and high molecular weights, before and after adsorption onto a talc substrate. The depressant solutions were allowed to react for increasing periods of time and the change in the molecular weight of the adsorbed fraction over time was assessed. Equilibrium adsorption isotherms and cryogenic scanning electron microscopy were utilised to complement the GPC data.It was found that the shorter chain lengths of the high molecular weight depressants adsorbed preferentially. The kinetics of adsorption were rapid and the molecular weight of the adsorbed fraction did not change substantially thereafter. This was contrary to most of the literature which suggests that the shorter chain lengths will diffuse rapidly to the mineral surface and adsorb, but will be replaced by the longer chain lengths over time due to favourable changes in entropy. An explanation is offered in terms of a greater dependence on adsorption energy in the case of this system. The low molecular weight depressants adsorbed more evenly across the entire molecular weight range, but still showed some preferential adsorption of the shorter chains. The implications for the flotation industry are discussed.

Cryo-SEM images of (a) CMC and (b) guar depressant on a talc surface.Figure optionsDownload as PowerPoint slide