D. Feng

Adsorption of heavy metals by biomaterials derived from the marine alga Ecklonia maxima

Abstract The adsorption of heavy metals onto biomaterial derived from the marine alga Ecklonia maxima was investigated via batch experiments. The adsorption equilibria of Cu, Pb and Cd could be represented by Langmuir isotherms and the capacity of fresh alga for Cu, Pb and Cd was approximately 85–94, 227–243 and 83.5 mg/g dry alga, respectively. The rate of adsorption onto the marine alga was high. The alga particle size played an important role in the adsorption behaviour. The coarse alga particles had a higher adsorption capacity and slower adsorption kinetics and could be regenerated without significant loss of capacity. In contrast, the fine alga particles had a lower adsorption capacity and faster adsorption kinetics and could not be regenerated without significant loss of capacity. Comparison with a commercial resin indicated that the activated biomass derived from E. maxima could be used as an efficient biosorbent for the treatment of waste waters containing heavy metals.

Leaching behaviour of sulphides in ammoniacal thiosulphate systems

Systematic studies have been conducted to understand the leaching behaviour and dissolution mechanisms when common gold-host sulphides such as chalcopyrite, pyrite, arsenopyrite and pyrrhotite are treated by oxidative ammoniacal thiosulphate leaching. Leach solution composition strongly influenced the sulphide leaching, and the presence of sulphides also enhanced the decomposition of thiosulphate. The XRD patterns of the binary mixtures before and after leaching indicated that the relative leaching rates of the sulphides in the ammoniacal thiosulphate system were in the order chalcopyrite>pyrrhotite>arsenopyrite>pyrite, which was in accordance with the observations made from the leaching tests. SEM analysis with the aid of EDAX indicated the formation of iron oxide at the chalcopyrite, pyrrhotite and pyrite surfaces and the formation of iron arsenate at the arsenopyrite surface after leaching. SEM analysis also demonstrated that the high-energy defect sites and crystal boundaries favoured the sulphide leaching. Raman spectroscopy indicated that haematite was formed during the leaching of chalcopyrite. Iron and arsenic concentrations in the leach solutions were very low due to the formation of iron oxide and iron arsenate during the leaching reactions. Pyrite enhanced chalcopyrite and sphalerite dissolution. Chalcopyrite and sphalerite also enhanced pyrite dissolution.

The role of heavy metal ions in gold dissolution in the ammoniacal thiosulphate system

Abstract A study has been conducted to understand how heavy metal ions affect the dissolution behaviour of gold in the ammoniacal thiosulphate system. The effect strongly depended on ion types and concentrations, and reagent concentrations. Zn had a slightly positive effect on gold dissolution at very low concentrations, and would retard it at high concentrations. Cd, Co, Cr and Ni retarded gold dissolution to different extents at all reagent concentrations, and the dissolution kinetics of gold decreased with the increase in the ion concentrations. The free thiosulphate concentration decreased in the presence of heavy metals, especially at higher concentrations. The Eh–pH diagrams of the Metal–NH3–S2O32− systems indicated that the predominant species for Cd, Co, Ni and Zn in the leaching region are Cd(NH3)42+, Co(NH3)2+, Ni(NH3)62+ and Zn(NH3)42+, respectively. On the other hand, Co(OH)2, NiO and ZnO may also be present in the solutions containing high concentrations of Co, Ni and Zn, respectively. These oxy- or hydroxyl-metal species may hinder gold dissolution. The Co3+/Co2+ couple can be used as an alternative to Cu2+/Cu+. At low lead concentrations, the predominant species for Pb in the leaching region is Pb(OH)+ in the Eh–pH diagram of the Pb–Au–NH3–S2O32− system. AuPb2 may also be present in the leaching region. However, PbO is possibly the predominant species at high Pb concentrations. Consequently, Pb accelerated gold dissolution at low concentrations, and retarded it at high concentrations. Pb retarded gold dissolution at low ammonia concentrations because PbO is the predominant species in such cases. The EDS result indicated the presence of Pb species at the surface of the gold plate leached in the 20 mg/L Pb system.

Preg-robbing phenomena in the thiosulphate leaching of gold ores

Abstract The ability of various sulphide and gangue minerals and gold ores to adsorb gold during thiosulphate leaching was examined. The effect of free thiosulphate concentration, metal species, gold thiosulphate concentration, ammonia concentration, particle size and cupric addition on preg-robbing was examined in detail, and likely preg-robbing mechanisms were proposed as well. It was found that all the gold ores were highly preg-robbing in thiosulphate deficient systems and that the preg-robbing process was fairly fast. The presence of free thiosulphate could greatly decrease or eliminate preg-robbing in both mineral and gold ore systems, depending on its concentration. Physical adsorption of gold was reversible in the presence of thiosulphate. Lead and zinc ions could reduce gold adsorption to some extent, and competitive adsorption of the gold thiosulphate with the lead and zinc thiosulphate species on the minerals or ores was expected. The gold elution tests demonstrated that preg-robbing was dominated by physical adsorption for the gangue minerals and was controlled by both chemical and physical adsorption for the sulphide minerals. The ability of an ore to adsorb the gold thiosulphate species from solution was found to be intimately associated with the free thiosulphate concentration. Surface topological studies by SEM coupled with EDS showed that gold was adsorbed at defect sites on the surfaces of the minerals.

Modelling of hydrocyclone performance based on spray profile analysis

Abstract Spray profile measurements can be used to calculate the underflow rate, and consequently, be related to hydrocyclone performance. The flow geometry of the spray discharge is found to arise from velocity patterns at the outlet orifice. Through a videographic example of an industrial hydrocyclone, it is shown that underflow profiles are typically parabolic, a feature which is indicative of the velocity at which the fluid exits. The inclusion of gravity in this model clearly highlights deficiencies in currently used models. Moreover, an intimate knowledge of factors affecting the profiles of the underflow of a cyclone is essential for the correct interpretation of videographic images. Subsequently, image data are used to estimate outlet velocities, which give an excellent insight into various fluid mechanical phenomena that are not appreciated by analysing basic operational variables. The exit velocities are used to calculate underflow rates, which are related to mass recovery in the underflow.

The dynamic behaviour of coarse particles in flotation froths: Part I: Model

Abstract The effects of various physical parameters on the rupture of bubble films in two-phase foams were investigated in order to develop a better understanding of the behaviour of coarse particles in the froth phase of a novel flotation cell. This novel flotation technique is based on the fact that coarse particles, if they are selectively rendered hydrophobic by conditioning, would act as bubble film breakers. If the feed was introduced onto the surface of the froth, such particles would settle through the froth under gravity to be recovered as an underflow (concentrate) product, while the hydrophilic gangue would be supported by the bubble films and be recovered as a float (tailings) product. The efficiency of this technique––reverse froth flotation––depends on the interaction between various characteristics of particles and the froth. In order to simulate the experimentally observed trends, and hence investigate the various mechanisms qualitatively, a fundamental model of these interactions was developed. Various particle properties were taken into account, including surface properties, shape, size and density. To account for the changing nature of the froth at different positions in the cell, the model predicts the trajectory of a particle over discrete time events. This was accomplished by calculating bubble flow streamlines and modelling the bubble size, thickness of bubble films, air hold-up and bubble velocity at any point on the streamline.

The use of bubble loads to interpret transport phenomena at the pulp–froth interface in a flotation column

A calculation procedure is proposed whereby the bubble load and interstitial concentration of di-erent species can be estimated at di-erent levels in the pulp phase of a pilot-scale %otation column. The model requires experimentally measured concentrations of species in the pulp (overall concentrations), the %ow rates of species in the concentrate and tailings, air hold-up, the rate of aeration, the wash water rate, as well as mineralogical liberation data as input parameters. Various transport mechanisms, such as the preferential rejection of particles from bubble wakes, settling of particles, entrainment of particles, the %ow of the bulk slurry and bubble transport, are incorporated in the calculation procedure. Calculated results can be used to investigate transport phenomena at the pulp–froth interface (PFI), to estimate pulp recoveries and froth recoveries, as well as to identify regions of high and low particle–bubble attachment in %otation columns. The %otation of chromite in a 15 cm diameter column serves as a case study to demonstrate the application of the calculation procedure. ? 2001 Elsevier Science Ltd. All rights reserved.

Ultrasonic elution of gold from activated carbon

Abstract The feasibility of using ultrasound to enhance the elution of gold from activated carbon was investigated. A laboratory-scale elution column immersed in an ultrasonic bath was used to investigate three eluant systems. The kinetic activity of the activated carbon was improved in the elution with sonication. Ultrasonic radiation improved the recoveries and kinetics of elution considerably. This could possibly be attributed to an increase in diffusive transport caused by acoustic vortex microstreaming and microjets, as well as the prevention of activated carbon fouling which can lead to the blockage of active sites. The rate of elution was significantly increased by the addition of ethanol to the aqueous eluant, especially in the presence of ultrasound. It could be attributed to the increased activity of smaller ions in preference to gold cyanide in the presence of ethanol. Ethanol, by capturing the primary radicals (H * and OH * ), generated secondary radicals (C 2 H 4 OH*) which could be responsible for the increased efficiency. The introduction of ultrasound into the conventional elution systems made it possible to elute gold cyanide from activated carbon under less severe conditions.

The dynamic behaviour of coarse particles in flotation froths Part III: Ore particles.

Abstract A froth separating device was tested in which heavy, hydrophobic particles break froth films and sink to the bottom, while light, hydrophilic particles are supported by the froth to form a top tailings product. Two potential industrial applications for the reverse froth flotation process were evaluated. As a coarse particle flotation technique for sulphide bearing ores, it was found that xanthate pre-conditioning of the ore results in concentrating the sulphur bearing particles to the concentrate. By increasing the xanthate addition, the relative mass recovery as well as sulphur recovery increases. This increase in mass and sulphur recovery is not linear. These results were promising in terms of finding a coarse particle flotation technique for the pre-concentration of sulphide bearing minerals. Preliminary tests indicated that the reverse froth flotation process is not suitable for the replacement of grease belts for fine (−3 mm) diamond recovery. The major concern is the high potential losses of valuable material to the tailings. These losses are mainly due to the fact that the major separation process in the reverse froth flotation cell is based on particle mass beyond a certain critical value. The surface properties of the particle account only for a secondary separation process. Nevertheless, this technique still offers the potential to be applied to coarse coal flotation and recycling waste separation within certain operating limits.

The dynamic behaviour of coarse particles in flotation froths Part II: Density tracer tests.

Abstract A novel flotation cell was used in which hydrophobic particles act as film breakers and sink through the froth as concentrate, while hydrophilic particles are supported by the upward flow of froth and are recovered as a top product tailings. Experimental results on density tracers showed that the behaviour of particles (within the size range tested) in the froth phase of the cell is primarily dependent on the mass of a particle. In general, the higher the mass, the steeper is the trajectory of the particle in the froth, i.e., an increase in particle mass results in an increased recovery to the concentrate. The contact angle on the particle surface has only a secondary influence on the overall particle trajectory, in that an increase in the equilibrium contact angle will result in an increased recovery. However, the particle contact angle has very little influence on the behaviour of large, high-density particles, as well as small, low-density particles. Particles will therefore only separate on the basis of contact angle as long as their mass is between an upper and lower critical value. Any particle with a mass greater than the critical value will fall through the froth irrespective of the contact angle. Similarly, the upward force component acting on a particle with mass less than the lower critical value will dominate the force balance. The particle will therefore remain supported by the froth, irrespective of the particle contact angle and bubble film rupture time. For particles within these mass limits, the effect of the contact angle increases with a decreased mass. It was further concluded that these mass limits are dependent on the operating conditions of the cell as well as the particle shape. The particle shape determines the mass to cross-sectional surface area ratio (M/A0). Where particles therefore have the same mass, the M/A0 ratio would govern the particle trajectory. The higher the M/A0 ratio, the more particles would be recovered to the concentrate, while a decrease in the M/A0 ratio would result in flatter particle trajectories in the froth, thereby increasing the probability of a particle reporting to the tailings. A mathematical model provides an understanding of the interrelationship between the various parameters.