Flocculation behavior of clayey dolomites in borax solutions

Flocculation behavior of two clay rock samples with different dolomite and clay mineral (smectite) contents was examined using a non-conventional, medium-anionic polyacrylamide-based flocculant in borax solutions. The samples were representative of two major types of gangue material discarded from the borax processing plant at Kirka/Turkey as borax-saturated tailings stream which present serious disposal problems from technical and environmental points of view. The effect of the dissolved borax concentration by itself, and the effects of Ca2 + and Mg2 + ions at a constant borax concentration of 3% w/v were investigated with dispersions flocculated at the buffered pH 9.4. Flocculation efficiency was mainly assessed by the supernatant turbidity. Increasing the borax concentration decreased the supernatant turbidity. The effect was attributed to the reduced negative electric field around the dolomite and clay mineral particles due to the high ionic strength and the adsorption of sodium cations on the surfaces of the clay mineral particles, facilitating the hydrogen bonding of the amide groups of the flocculant. Ca2 + ions were essential for improving the floc formation and obtaining clear supernatants, but Mg2 + ions were detrimental to floc formation and turbidity removal. Positively-charged adsorbed calcium-hydroxo species on the particles should have promoted stronger salt linkages with the negative pendant groups of the flocculant. The adverse effect of Mg2 + was attributed to its higher hydration energy and the likely precipitation of Mg(OH)2 on particle surfaces. FTIR studies provided some supporting evidence for the suggested mechanisms of flocculant adsorption.