Dr Jianhua Du

The flocculation and solid/liquid separation of four well-characterized kaolinites (2 well, 2 poorly crystallized) have been studied for comparison of surface structure (SEM), aggregate structure during flocculation (cryo-SEM), settling rate, and bed density (with raking). It is shown that major differences in these properties are largely due to crystallinity and consequent surface structure of the extensive (larger dimension "basal") face. Well-crystallized kaolinites, with higher Hinckley indices and lower aspect ratios, have relatively smooth, flat basal surfaces and thicker edge planes promoting both effective initial bridging flocculation (largely edge-edge) and structural rearrangement to face-face during the raking process. This results in faster settling rates and more compact bed structures. Poorly crystallized kaolinites, with low Hinckley indices and high aspect ratios, exhibit ragged, stepped structures of the extensive face with a high proportion of nanosized islands forming cascade-like steps (i.e., multiple edges) contributing up to 30% of the specific surface area and providing flocculant adsorption sites (hydroxyl groups) across this extensive face. This leads to bridging flocculation taking place on both edge and extensive ("basal") planes, producing low-density edge-face structures during flocculation which leads to slow settling rates and poor bed densities. In particular, the complex surface morphology of the poorly crystallized kaolinites resists the transformation of edge-face structures to dense face-face structures under shear force introduced by raking. This results in low sediment density for poorly crystallized kaolinites. The studies suggest that the main influence on settling rates and bed densities of kaolinites in mineral tailings is likely to be related to the crystallinity and surface morphology of the kaolinite. They also suggest that interpretation of kaolinite behavior based on models of a flat (001) basal plane and edge sites only at the particle boundaries is not likely to be adequate for many real, less-crystallized kaolinites. © 2010 American Chemical Society.

The critical role of dissolved gas nano-bubbles at solid surfaces in particle association, aggregation, adsorption and flotation has been recognised in the recent literature. The principles of mineral processing, fine particle separation, and water recovery depend upon changing the surface properties at the solid-liquid interface. It has been assumed that the solid surfaces are either in direct contact with the liquid or may have nano-bubbles attached only at hydrophobic surfaces. This paper shows that gaseous layers 50-100 nm thick can be attached surrounding high proportions of solid clay mineral surfaces restricting reagent access, producing buoyancy and aggregation. Ultrasonic treatment before flocculant addition effectively removes these gaseous layers as well as dispersed micro-bubbles. Re-aggregation after brief ultrasonication produces denser (less buoyant) flocs, demonstrated with cryo-SEM statistical analysis, giving more complete access of the flocculant to the aggregate surfaces. In the subsequent flocculant addition, the settling rates of the denser flocs can be increased up to 40%. If ultrasonic action is continued, the bridged flocs are disturbed with some redispersion of smaller flocs and individual platelets and consequent slower settling rates. © 2009 Elsevier Inc. All rights reserved.

The destabilization of kaolinite suspension by anionic flocculant addition occurs in three zones; free settling, hindered settling and compression which usually includes a final bed raking process in mineral processing practice. This paper reports changes in the kaolinite aggregate and floc structures in the different settling and raking zones by cryo-vitrification/cryo-SEM techniques with image analysis combining micro- and macro-flocs. Cryo-SEM images indicate that, even during free settling, fine clay particles are bridged predominantly in edge-edge (E-E) with some face-face (F-F) configurations forming single, small flocs and some chain structures. When these small flocs and chains settle into the hindered settling zone, the collision between flocs and chains results in "honeycomb" network structures formed with lateral chain-like extension. The settled bed consists of these honeycomb structures with both inter-aggregate and intra-aggregate trapped water and has relatively low bed density (e.g. < 12¿wt.% for a 2¿wt.% slurry). The effect of the raking process in dramatically improving thickener underflow solids has been extensively studied but the structural changes in flocs and aggregates in this process are less well defined. Raking the compression zone for 1¿h at 3¿rpm can release some of the trapped water in the "honeycomb" structure and the bed density for 2¿wt.% slurry improves dramatically to more than 36¿wt.%. Cryo-SEM illustrates the extensive restructuring of flocs from predominantly E-E to predominantly F-F in many areas. The STructural IMage ANalysis (STIMAN) software is used to combine a series of images at magnifications from 1000× to 8000×, including both macro- and micro-flocs. This structural analysis comparing the un-raked and raked bed samples gives increases in total particle area of 30% and in relative particle area of 6%. The relatively low energy rake action of the shear stress results in the disruption of the E-E chains and the honeycomb structure, partly releasing the trapped water and inducing some E-E to F-F aggregate restructuring are clearly illustrated in these results. © 2009 Elsevier B.V. All rights reserved.

IR spectra were used to analyse the azo dye solution decoloration action by two kinds of iron oxyhydroxides. It was discovered that: (1) Acid Red G and methyl orange are apt to form complex on the surface of iron oxyhydroxides > FeOH, especially Acid Red G. which possesses two -SO3 Na structures has a relatively high decoloration efficiency as a result of complexation reaction; (2) after 2 hours adsorption, the IR spectra of iron oxyhydroxides show characteristic wave numbers at 1033 and 1030 cm-1 which belong to -SO3Na, whereas the peaks at wave numbers between 1450 and 1400 cm-1, which belong to azo dye, disappear. These phenomena indicate that azo dye molecules are adsorbed on the surface of iron oxyhydroxides due to the negative -SO3Na structure, and at the moment when azo dye molecules are adsorbed on the surface of iron oxyhydroxides, the electron transfer occurs between the azo dye molecules and the iron oxyhydroxides surface's Fe3+ centre, which could lead to the rupture of azo bond. It can be infered that the decoloration of azo dye molecules is the co-effect of the selective chemical absorption and the oxidation-deoxidation effect on the surface of iron oxyhydroxides.