The influence of metal cations on the behaviour of carboxymethyl celluloses as talc depressants

Metal cations have been shown to depress the floatability of siliceous minerals indicating an interaction with the mineral surface. In this work, the effect of metal cations on the interaction of sodium carboxymethyl cellulose (CMC) depressants and talc has been studied. Equilibrium adsorption and microflotation studies were conducted on pure talc with a range of ions (Ca2+, Mg2+, K+) and ionic strengths, using CMCs of two different degrees of substitution (DS). In order to probe the effect of the ions on the polymer molecules themselves, intrinsic viscosity measurements were made where the ionic strength (IS), pH and type of ion were varied. The results of the adsorption studies showed that the divalent cations Ca2+ and Mg2+ were far more efficient at promoting adsorption on to the talc surface than the monovalent K+ ion. The higher DS CMC had a much more stringent requirement than the low DS CMC for the presence of high ionic strength electrolyte in order to achieve good adsorption. The microflotation studies complemented these results and showed that talc was more efficiently depressed by CMCs when Ca2+ and Mg2+ ions were present than when only K+ was present and that the high DS CMC was less efficient at the ionic strength used than the lower DS CMC. Intrinsic viscosity measurements of a CMC in different ionic strength solutions of K+ and Ca2+ ions showed that over a wide pH range, Ca2+ ions caused the CMC molecule to coil more than the K+ ions. This is in keeping with the observed increase in adsorption density on the talc surface with Ca2+ ions and subsequent improvement in the depressant activity. These results were complemented by those from size exclusion chromatography with triple detection where the light scattering data revealed that for a solution of CMC of DS 0.78 in 10−2 IS Ca(NO3)2 the radius of gyration was 28% smaller than in 10−2 IS KNO3. The adsorption studies show however, that at high ionic strength (10−1), when the CMCs are fully coiled, the enhancement of bonding due to Ca2+ and Mg2+ ions is still significantly greater than for K+ ions, which implies that there is also an interaction between the CMCs and the divalent ions on the talc surface which increases CMC adsorption and facilitates depression.