DAF–dissolved air flotation: Potential applications in the mining and mineral processing industry

Application of dissolved air flotation on separation of waste plastics ABS and PS

The aim of this research was to separate waste plastics acrylonitrile butadiene styrene (ABS) and polystyrene (PS) by dissolved air flotation in a self-designed dissolved air flotation apparatus. The effects of wetting agents, frother, conditioning time and flotation time on flotation behavior of waste plastics ABS (w-ABS) and PS (w-PS) were investigated and the optimized separation conditions were obtained. The results showed that when using 25 mgL(-1) tannic acid, 5 mgL(-1) terpineol, 15 min conditioning time and 15 min flotation time, mixtures of w-ABS and w-PS were separated successfully by dissolved air flotation in two stages, the results revealed that the purity and recovery rate of w-PS in the floated products were 90.12% and 97.45%, respectively, and the purity and recovery rate of w-ABS in the depressed products were 97.24% and 89.38%, respectively. Based on the studies of wetting mechanism of plastic flotation, it is found that the electrostatic force and hydrophobic attraction cannot be the main factor of the interaction between wetting agent molecules and plastic particles, which can be completed through water molecules as a mesophase, and a hydrogen bonding adsorption model with hydration shell as a mesophase was proposed.

Feasibility of electroflotation to separate solids and liquid in an activated sludge process

In this study, electroflotation (EF) has been applied as a secondary clarification in the activated sludge process to improve the efficiency of the solids-liquid separation, which is essential in maintaining effluent quality. The effects of sludge settleability were examined through a series of batch and semi-continuous experiments. The results of the batch experiments revealed that thickening efficiencies using EF were 2.6 to 9.2 times higher than those with gravity settling (GS). In addition, clarification efficiencies were not significantly influenced by sludge settling properties, as compared with GS as a control. In the semi-continuous EF experiments, the concentrations of solids in the float layer were maintained above 10 g L(-1) during flotation, regardless of variations in sludge settleability. Furthermore, the volumetric gas proportion in the float layer increased as the gas to solids (G/S) ratio rose. This allowed the float layer to be more stably suspended against gravity at the top of the reactor. Based on the results obtained from these batch and semi-continuous experiments, an anoxic/oxic (AO) reactor combined with EF clarifier remained in successful continuous operation for four months. In comparison with conventional AO processes using a GS clarifier, enhanced clarification and thickening efficiencies were achieved through the EF-AO system. In addition, higher mixed liquor suspended solids concentrations (averaging 5300 mg L(-1)) in the bioreactor (EF-AO) were maintained via the return of highly concentrated sludge (averaging 16,400 mg L(-1)) from the EF clarifier. These findings suggest that EF could be a promising and effective alternative for the solids-liquid separation of poorly settling sludge.

A review of the fundamental studies of the copper activation mechanisms for selective flotation of the sulfide minerals, sphalerite and pyrite

A review of the considerable, but often contradictory, literature examining the specific surface reactions associated with copper adsorption onto the common metal sulfide minerals sphalerite, (Zn,Fe)S, and pyrite (FeS(2)), and the effect of the co-location of the two minerals is presented. Copper "activation", involving the surface adsorption of copper species from solution onto mineral surfaces to activate the surface for hydrophobic collector attachment, is an important step in the flotation and separation of minerals in an ore. Due to the complexity of metal sulfide mineral containing systems this activation process and the emergence of activation products on the mineral surfaces are not fully understood for most sulfide minerals even after decades of research. Factors such as copper concentration, activation time, pH, surface charge, extent of pre-oxidation, water and surface contaminants, pulp potential and galvanic interactions are important factors affecting copper activation of sphalerite and pyrite. A high pH, the correct reagent concentration and activation time and a short time delay between reagent additions is favourable for separation of sphalerite from pyrite. Sufficient oxidation potential is also needed (through O(2) conditioning) to maintain effective galvanic interactions between sphalerite and pyrite. This ensures pyrite is sufficiently depressed while sphalerite floats. Good water quality with low concentrations of contaminant ions, such as Pb(2+)and Fe(2+), is also needed to limit inadvertent activation and flotation of pyrite into zinc concentrates. Selectivity can further be increased and reagent use minimised by opting for inert grinding and by carefully choosing selective pyrite depressants such as sulfoxy or cyanide reagents. Studies that approximate plant conditions are essential for the development of better separation techniques and methodologies. Improved experimental approaches and surface sensitive techniques with high spatial resolution are needed to precisely verify surface structures formed after copper activation. Sphalerite and pyrite surfaces are characterised by varying amounts of steps and defects, and this heterogeneity suggests co-existence of more than one copper-sulfide structure after activation.

Surfactants as bubble surface modifiers in the flotation of algae: dissolved air flotation that utilizes a chemically modified bubble surface

In this paper we present an investigation into the use of bubbles modified with surfactants in dissolved air flotation (DAF). Bubble modification was investigated by dosing surfactants of varying character into the saturator of a DAF unit in turn. The cell removal efficiency only improved when using a cationic surfactant where optimum removal of Microcystis aeruginosa cells was obtained when using 0.0022-0.004 mequiv L(-1) surfactant. However, the magnitude of the removal differed according to the hydrophobicity of the surfactant. Typically, the more efficiently the surfactant adsorbed at the bubble interface, the better the removal efficiency. When the dose to saturator ratio was kept constant and the recycle ratio varied, the removal efficiency improved with increasing recycle ratio, reaching a maximum removal efficiency of 87% for M. aeruginosa. This value was comparable with that predicted by a theoretical model. The bubble collection efficiency of a maximum of two cells per bubble was constant irrespective of the influent cell number or recycle ratio. Treatment of additional species in this way revealed a relationship between increasing size and both increasing removal efficiency and decreasing surfactant dose, which is supported by theoretical relationships.