P. Mavros

Effect of Axial Agitator Configuration (Up-Pumping, Down-Pumping, Reverse Rotation) on Flow Patterns Generated in Stirred Vessels

Single phase turbulent flow in a tank stirred with two different axial impellers, a pitched blade turbine (PBT) and a Mixel TT (MTT), has been studied using Laser Doppler Velocimetry. The effect of the agitator configuration, i.e. up-pumping, down-pumping and reverse rotation, on the turbulent flow field, as well as power, circulation and pumping numbers has been investigated. An agitation index for each configuration was also determined. In the down-pumping mode, the impellers induced one circulation loop and the upper part of the tank was poorly mixed. When up-pumping, two circulation loops are formed, the second in the upper vessel. The PBT pumping upwards was observed to have a lower flow number and to consume more power than down-pumping, however, the agitation index and circulation efficiencies were notably higher. The MTT has been shown to circulate liquid more efficiently in the up-pumping configuration than in the other two modes. Only small effects of the MTT configuration on the power number, flow number and pumping effectiveness have been observed.

Innovations in flotation technology

1. Fundamentals.- 1.1. The Study of Thin Aqueous Films as Models for Froths and Flotation.- 1.2. The Mechanisms of Collector Adsorption-Abstraction (Ionic and Non-Ionic Surfactants) on Heterogeneous Surfaces.- 1.3. Electrochemistry in the Plant.- 1.4. Flocculation of Fine Particles.- 1.5. Applied Mineralogy and Materials Balancing Procedures. Evaluations of Flotation Concentrators.- 1.6. Liberation by Size Reduction. Consequences and Improvements on Flotation Kinetics.- 1.7. Elements of Physically-Based Modelling of the Flotation Process.- 2 . Chemical Technology Aspects.- 2.1. Flotation Kinetics.- 2.2. Mixing and Hydrodynamics in Flotation Cells.- 2.3. Case Studies of Flotation Control.- 2.4. Mineral Surface Complexation in Flotation.- 2.5. Amide-Based Technologies in Flotation and Hydrometallurgy of Precious Metals.- 2.6. Determination of Floatability Data using the EmDee Microflot Method.- 2.7. Froth Flotation Kinetics - Theory and Model Experiments.- 2.8 Electrolytic Flotation.- 3. Mineral Processing.- 3.1. Features and Rotation of Complex Cu-Pb-Zn Sulphides.- 3.2. Interactions of Anionic Collectors in Flotation of Semi-Soluble Salt Minerals.- 3.3. Flotation of Salt-Type Minerals.- 3.4. Vibroacoustic Improvements of Froth Flotation.- 3.5. The Institute of Geology and Mineral Exploration (IGME) of Greece and its Research Activities on Flotation.- 3.6. The Influence of Multivalent Cations on the Floatability of Scheelite, Fluorite and Calcite.- 3.7. Non-Cyanide Technology for the Flotation of Lead-Zinc Ores.- 4. Water and Wastewater Treatment.- 4.1. Flotation in Water Treatment.- 4.2. Mechanism of Selective Ion Flotation.- 4.3. Flotation Techniques in Waste Water Treatment.- 4.4. Flotation of Pigments and Inks from Waste Paper.- 4.5. A Case Study of Flotation Applied to an Edible Oil Factory Wastewater.- 4.6. The Need for Advanced Treatment of Stormwater Run-off from Separate Sewer Systems.- List of Contributors.- List of Participants.

Recovery of Metals by Ion Flotation from Dilute Aqueous Solutions

Abstract Flotation, a process originating from the minerals industry, is finding its way as a separation process for dilute aqueous solutions, with particular interest in metal ions recovery. The present paper reviewes the several flotation techniques available in this area, with certain illustrative laboratory examples. Metals investigated are the following : chromium, copper, zinc, arsenic, lead, iron and germanium. The various parameters affecting the process are examined and attention is paid towards the selectivity of the process.

Iron-modified hydrotalcite-like materials as highly efficient phosphate sorbents

Highly efficient sorbents for phosphate removal from aqueous solutions based on the calcined forms of Fe(III)-substituted Layered Double Hydroxides (LDH) materials have been developed in this study. Hydrotalcite-like materials with Mg/M(3+) approximately 3 (where M=Al(3+), Fe(3+) or combined) have been synthesized following simple co-precipitation method and were subsequently calcined in air at 450 degrees C. Both as-synthesized and calcined materials were characterized by means of X-ray Diffraction (XRD), Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES), elemental (C) analysis, N(2) porosimetry, Scanning Electron Microscopy (SEM). All the materials were evaluated for the sorption of phosphates by batch equilibrium sorption experiments and kinetic measurements (effect of contact time). It was shown that chlorides or nitrates, being the charge-balancing anions in the LDH structure, are more easily exchanged by phosphates compared to carbonates. In the Fe(III)-modified LDHs, an increase of the Fe loading led to the decrease of the sorption efficiency. The maximum uptake of phosphates for both the Mg-Al LDH and Mg-Fe LDH samples containing mainly carbonates as charge-balancing anions was relatively low (ca.<or= 25mgP/g sorbent) while it was higher for the LDH samples containing mainly chlorides (approximately 80mgP/g). On the other hand, the maximum sorption capacity for the calcined Mg-Al LDHs and the calcined Fe(III)-substituted sorbents were very high, ca. approximately 250 and approximately 350mgP/g, respectively. The sorption data of both the as-synthesized and calcined LDHs was best fitted by the Freundlich model. Both the Mg-Al and Fe-substituted LDH sorbents were regenerated with mixed aqueous solution of NaCl and NaOH and were reused with a small loss of removal efficiency.

A study and modelling of liquid-phase mixing in a flotation column

Abstract The mixing process of the liquid phase in a gas-liquid flotation column and the effect of the flowrates on it have been investigated. The increase of the gas and liquid flowrates had opposite effects: the former enhanced the mixing process, whereas the latter caused the behaviour of the column to approach plug flow. A mixed zones-in-series model is proposed for interpreting the experimental results, which involves two parameters: the number of zones and the ratio of backwards to net liquid flow. In this way, the mixing was related to the backflow and the increase in mixing was due to an intense internal recirculation. The model was able to predict accurately the column behaviour in the experimental range investigated. Its major advantage lies in its structured description of the column contents.

Dissolved-Air Flotation of Metal Ions

Metal ions (copper, nickel, zinc, and ferric ions) were separated from dilute aqueous solutions by dissolved-air flotation. The ions were either precipitated as sulfides or floated (as ions) by xanthates. Copper and nickel were selectively separated; promising results were obtained with single, binary, and ternary mixtures. The effect of several parameters (solution pH, addition of chemical reagents at varying concentrations, and the presence of other ions) on the removal of ions was studied. The collectorless flotation of copper ions was also investigated.

Simulation of flow generated by an axial-flow impeller: Batch and continuous operation

It is important to extend and to validate computational flow models to simulate continuous operation of stirred vessels and to capture possible interaction of feed inlet/outlet with the flow generated by impellers. In the present work, we have developed and used a computational model to understand the flow generated by an axial flow impeller in a batch and a continuously operated baffled vessel. A multiple reference frames approach was used to simulate flow generated by the Mixel TT impeller in stirred vessel. The predicted velocity results show reasonably good agreement (qualitative as well as quantitative) with the experimental data. Characteristics of flow around blades of Mixel TT were studied using the computational model. The computational model was extended to simulate flow and mixing in a continuous operation. Simulations were carried out to understand the interaction of the jet emanating from the feed pipe and the flow generated by the impeller. Model predictions were compared with published experimental data, obtained by laser Doppler velocimetry. The differences and similarities between batch and continuous operation are highlighted. Mixing simulations were carried out to examine possible short-circuiting and non-ideal behaviour of the continuous operation of the stirred vessel. Influence of the impeller speed, feed rate and location of inlet/outlet on mixing and on the extent of non-ideality of flow was studied. The computational model and results discussed in this work will be useful for understanding the mixing process in continuous-flow stirred vessels.

Removal of metal ions from wastewaters. The case of nickel

Abstract The removal of nickel ions from aqueous solutions, using fly ash, a cheap industrial by‐product, was investigated. Its addition to the solution raised the pH, effectively removing the totality of the nickel ions. The removal may be attributed, depending upon the dispersion pH, partly to adsorption but mainly to precipitation. The fly ash performance was compared to that of other adsorptive materials (activated carbon and a synthetic zeolite). The synthetic zeolite was more efficient in removing the nickel ions, especially at low pH values, but is evidently a more costly material.

Colour removal from aqueous solutions. Part I. Flotation

Abstract The removal of colours from aqueous solutions and/or dispersions has been studied by dispersed‐air flotation, in a semi‐batch flotation column. Three colours were used for the experiments: Basic Yellow 28 (acidic), Direct Black 22 (basic) and Disperse Orange 30 (dispersion colour). All three were effectively removed by flotation within 15 to 30 min. Sodium lauryl sulfate and dodecylamine were found to be effective as collectors in the removal of colour, which was also found to be related to the pH of the solution and the amount of collector added to it, with high collector dosages causing the process to become pH‐independent.

Laser Doppler Velocimetry in Agitated Vessels: Effect of Continuous Liquid Stream On Flow Patterns

Liquid velocities were measured by laser Doppler velocimetry for a standard Rushton turbine and an axial-flow Mixel TT agitator, with a stream of liquid, having a volumetric flowrate such that the residence time was approximately 1/10 of the mixing time, being fed continuously in the agitated vessel. Measurements taken in the feeding-tube plane showed a major disruption of the flow pattern, especially of axial velocities, due to the continuous liquid stream. The incoming liquid jet combined with the stream discharged by the impeller in a complex 3-D way; however, the disruption of the flow gradually attenuated, and measurements taken in the plane rotated 90° clockwise indicated a flow pattern, with characteristics like velocity magnitudes, turbulence intensity and energy dissipation, similar to the batch case (with no through flow).