Heterocoagulation between SiO2 nanoparticles and Al2O3 submicronparticles; influence of the background electrolyte

The surface modification of alumina particles (d50 = 400 nm) by silica nanoparticles (d50 = 25 nm) has been studied according to the mass ratio between the two powders (0 < mSiO2/mAl2O3 < 4.5%) and to the background electrolyte (NaNO3, Ca(NO3)2, Na2SO4). Silica nanoparticles show a strong affinity for alumina surface in a large pH range because of electrostatic attractions and of the chemical reactivity between the surfaces. Its adsorption leads firstly to counterbalance the alumina surface charge and secondly to its reversal. Silica adsorption also occurs even bivalent ions or molecules such as catechol or Tiron influence electrostatic interactions. The electrokinetic properties of the suspensions permit to determine the pH range in which adsorption occurs. With a low amount of silica, the heterocoagulation phenomena leads to the rapid sedimentation of alumina agglomerates in which silica particles are well dispersed. This formulation can be useful for mineral separation.