Sericite-chalcocite mineral particle interactions and hetero-aggregation (sliming) mechanism in aqueous media

Attractive particle interactions which lead to hetero-aggregation or “sliming” of gangue and valuable mineral particles are encountered in a number of hydrometallurgical and flotation processes. Sliming leads to poor recovery of the valuable, hydrophobic minerals and high recovery of hydrophilic gangue particles in flotation concentrates. In the present work, the influence of interfacial chemistry and dispersion conditions on particle interactions which underpin the mechanism of hetero-aggregation between sericite and chalcocite particles was investigated in the pH range 5-9 at 23 °C. Hetero-aggregation is shown to occur under aqueous dispersion conditions where the fluid shear rate was high and the individual chalcocite and sericite particles were negatively charged, as shown by the electrokinetic potential data. Continuous flow particulate adsorption and rheological studies revealed that the hetero-aggregation behaviour was strongly pH and oxidative environment dependant. Sliming was greater at lower than higher pH and under air saturation than N₂ gas purge. The unexpected hetero-aggregation is ascribed to chalcocite (Cu(I)₂S) surface oxidation and dissolution which accentuated with decreasing pH and Cu (II) ion hydrolysis effect. Specific adsorption of Cu (II) complexes onto the mix minerals’ surfaces had a striking impact on the interfacial chemistry, reflecting significantly enhanced dispersion shear yield stress. The attractive particles’ interactions are believed to be due to a combination of Cu(II)-mediated mechanisms including: electrostatic-charge patch attraction, van der Waals attraction, adsorbed ion-particle bridging, surface nucleation and cementation. Sliming mitigation was demonstrated by the use of N₂ gas and higher pH, as evidenced by markedly lower sericite-chalcocite dispersion shear yield stress and greatly reduced particulate adsorption behaviour.     

Rheological properties of nanosized barium titanate prepared by HGRP for aqueous tape casting

The conditions for the preparation of stable nanosized barium titanate suspensions with high solids content for the production of aqueous tape casting are identified. The rheological behavior of colloidal barium titanate suspension with Ammonium polyacrylate (NH₄-PAA) as a dispersant to aid the powder dispersion has been investigated. Nanosized barium titanate powder was synthesized by a continuous high-gravity reactive preparation (HGRP) technique, and then annealed at 900 °C for 2 h. Measuring the zeta potential, the particle size distributions and ball-milling time, assessed the optimum conditions of the suspension with low viscosity and stability. An isoelectric point (IEP) at pH = 2.8 was found. Particle size distribution tests identified an optimum pH value about 10 and an optimum dispersant addition about 1.2 wt.% (based on the dry powder weight). As the ball-milling time was longer than 8 h, the amount NH₄-PAA adsorbed on the barium titanate reached to saturation. The maximum solid content attained during this work was 45 vol.% at pH of 10, with dispersant addition 1.2 wt.%. High green density value (up to ∼55.4% of the theoretical density) in BaTiO₃ sheet was achieved with a solid content 40 vol.%. After sintering at 1200 °C for 2 h a final density of 95% is reached.     

Effects of dispersant concentration and pH on properties of lead zirconate titanate aqueous suspension

Lead zirconate titanate (PZT) aqueous suspensions were prepared at 60 wt.% solids loading using a commercial ammonium polyacrylate (APA) dispersant. Effects of the dispersant concentration on rheological behavior, dispersion and stability of PZT aqueous suspensions were investigated by means of zeta potential, viscosity and sedimentation height measurements. The results showed that, under suitable conditions, APA dispersant promoted particle dispersion and stabilization in PZT aqueous suspensions. For 60 wt.% solids loading suspensions, the dispersant concentration yielding the lowest viscosity was 0.5 wt.% based on PZT powder dried weight basis. Effects of pH on particle dispersion in the suspensions prepared with APA were studied by laser light scattering technique and scanning electron microscopy. The results showed an improvement in particle dispersion for the alkaline condition, which led to relatively low viscosity and highly stable suspension. Possible particle stabilization mechanisms at various pHs were discussed based on dissociation of the dispersant in water, polymer conformation and adsorption behavior of the dispersant on the particle surface.     

Investigation of the role of interfacial chemistry on particle interactions, sedimentation and electroosmotic dewatering of model kaolinite dispersions

The influence of pulp chemistry on particle interactions and dewatering behaviour of colloidal kaolinite dispersions has been investigated under coagulation conditions. The dispersion shear yield stress, settling rate and consolidation showed strong dependence upon pH and ionic strength, indicating a maximum at ∼pH 3.2 which was established as the isoelectric point (iep) by particle zeta potential analysis. A “gel point” solid concentration at which the dispersion began to be significantly networked and gravity-driven consolidation of the pulp was completely suppressed, occurred at 13 vol.% (∼28 wt.%). The dewatering rates due to coagulation were significantly lower than those commonly achieved by polymeric flocculation, however the sediment consolidation was ∼25% higher when compared with flocculated pulps. Electroosmosis was found to be effective in consolidating pre-sedimented pulps to spadeable pastes (∼30 vol.%) at pH values away from the iep where zeta potential was higher and ionic strength low (10^− 3 M). This pulp consistency or markedly improved consolidation behavior is not achievable under coagulation and/or flocculation conditions.     

The cutting of multi-walled carbon nanotubes and their strong interfacial interaction with polyamide 6 in the solid state

The cutting of multi-walled carbon nanotubes (MWCNTs) using solid state shear milling (S³M) method and their strong interfacial interaction with polyamide 6 (PA6) in the solid state were studied. Transmission electron microscopy showed that after milling, the CNTs were greatly reduced in length, and disentangled, being straighter with open ends. Fourier transform infrared spectra and differential scanning calorimeter analysis indicated the existence of strong interfacial interactions between MWCNTs and PA6 of the pan-milled PA6/CNTs powder. It was further quantified by thermogravimetric analysis that about 30 wt.% of PA6 formed a strong combining force with CNTs after pan-milling. The mechanism of cutting CNTs and the reason for their strong interfacial interactions with PA6 in the solid state were discussed. A fine and homogeneous dispersion of CNTs throughout PA6 matrix was observed by scanning electron microscopy. The tensile properties of the composites prepared by the S³M method were significantly improved compared to those of pure PA6 and composites prepared by conventional melt mixing. Upon incorporation of only 1.5 wt.% MWCNTs, the tensile modulus of PA6 was enhanced from 2448 MPa to 4439 MPa, by about 80%, and the tensile strength was increased by about 23%.     

Preparation and rheology of biochar, lignite char and coal slurry fuels

Slurries of an Oil Mallee biomass char, a low rank coal char and sub-bituminous coals were prepared by mixing the finely milled solids with water and a range of additives including polyacrylic acid, charged copolymers D101 and D102, and sucrose. The resultant slurries were subjected to rheological characterizations including apparent viscosity and yield stress. The effect of the solid type, particle size distribution, and the additives on preparation of highly loaded slurries with the desired rheological behaviour were systematically examined in terms of apparent viscosity and yield stress. The additives D101 and D102 were found to be most effective in producing highly loaded suspensions with a low apparent viscosity and yield stress. Particle size distributions were manipulated to improve the solid loading. Suspensions produced by powder mixture containing equal weight precent of 30 min and 1 min milled powders gave a broad size distribution and is very effective in increasing the solid loading in slurries. The significant improvement in the solid loading was shown to be achieved by (i) increasing particle packing density via size distribution control and (ii) minimising the strength and number of the interactions between colloidal particles. The maximum solid loading of flowable (or relatively low apparent viscosity) slurries achieved with the Oil Mallee char is ∼40 wt.%, with the sub-bituminous coals 56-63 wt.%, and with the lignite char ∼65 wt.%. This study has shown that for low rank coals such as lignite, thermal and densification treatment is essential to achieve the solid loading of slurry fuel.     

A case study on suspended particles in a natural gas urban transmission and distribution network

Suspended particles in the natural gas transmission and distribution network of the city of Kerman, Iran were investigated. Particle concentration and size distribution were measured in different locations of the natural gas pipeline network. Particle samplings were carried out in two seasons: summer, when there is the lowest consumption, and winter, when there is the highest consumption of natural gas. Additional particle characterization was carried out by scanning electron microscopy coupled with energy dispersion X-ray (SEM/EDX) and X-ray diffraction(XRD) analyses. Particle concentration was found to be significantly higher in winter as compared to summer. The range of particle concentrations in summer was from 0.12 mg/Nm³ at the end of the pipeline to 4.7 mg/Nm³ at the network entrance, and from 0.30 mg/Nm³ to 22.1 mg/Nm³ in winter. Particle size distribution showed a higher frequency of smaller particles in winter than in summer. Larger particles were more likely to exist at the network entrance as compared to the exit. The average particle size ranged from 181 μm at the network end to 253 μm at the entrance in summer, and from 74 μm to 209 μm in winter. Particle characterization confirmed the presence of corrosion products in the suspended particles.     

Preparation of high solid loading,low viscosity ZrB2–SiC aqueous suspensions using PEI as dispersant

PEI was used as dispersant for ZrB₂ and SiC powders in water. The dispersion behavior of ZrB₂ and SiC in water was studied by zeta potential measurements, particle size distribution measurements and interparticle interaction calculations. Well-dispersed ZrB₂ and SiC aqueous suspensions were obtained using 0.6 wt% PEI at pH 6. The rheological behavior of ZrB₂–SiC aqueous suspensions was also investigated. Finally, a high solid loading (52 vol%), low viscosity (980 mPa s at 100 s⁻¹) ZrB₂–SiC aqueous suspension was successfully prepared.     

Morphological characteristics of mechanically processed ZnO and MnO2 powder mixtures

The evolution of structural-morphological characteristics of ZnO + 1 wt.% MnO₂ and ZnO + 10 wt.% MnO₂ samples during prolonged mechanical treatment (MT) was investigated by X-ray, AFM, LPS, SEM and IR methods. Changes in crystallite size due to mechanical activation were determined from X-ray measurements. These results were correlated with changes in particle size followed by SEM and AFM. Particle and aggregate size distributions were determined from LPS measurements. Interpretation of the evolution of IR-absorption peaks during mechanical activation using the theory of average dielectric constants enabled analysis of changes of particle and agglomerate shapes. All these results enabled establishment of a relationship between crystallites, particles, aggregates of particles, and superficial processes that occur during mechanical activation.     

Electrochemically instantaneous reduction of conducting polyaniline-coated latex particles dispersed in acidic solution

A cathodic voltammetric wave was observed in an aqueous suspension of mono-dispersed, spherical polyaniline-coated polystyrene particles, whereas no anodic wave was detected. This irreversibility was common to particles with eight different diameters ranging from 0.2 to 7.5 μm. Such irreversibility cannot be found at polyaniline-coated electrodes, and thus is a property of the dispersion of polyaniline latex. The reduction current was controlled by diffusion of dispersed particles. The reduction, being the conversion from the electrical conducting state to the resistive one, should begin at a point of contact between the conducting particle and the electrode in order to be propagated to the whole particle rapidly. In contrast, the oxidation proceeds slowly with the propagation of conducting zone, during which Brownian motion lets the particle detach from the electrode. The number of loaded aniline units per particle, determined by weight analysis, ranged from 6 × 10⁶ (Ø 0.2 μm) to 3 × 10¹¹ (Ø 7.5 μm) and was proportional to 2.9 powers of the particle diameter. The diffusion-controlled current of the cathodic wave was proportional to 2.4 powers of the diameter. The difference in these powers, 0.5, agreed with a theoretical estimation of the diffusion-controlled current, the diffusion coefficient for which was given by the Stokes-Einstein equation.     

Influence of support porosity and Pt content of Pt/carbon aerogel catalysts on metal dispersion and formation of self-assembled Pt-carbon hybrid nanostructures

Three carbon aerogels with different meso-macropore networks were used as supports for Pt catalysts using [Pt(NH₃)₄]Cl₂ as precursor salt. Results obtained showed mesopore volume and mean mesopore size to be important parameters that control Pt particle size and dispersion in catalysts containing 2 wt.% Pt. Once the most appropriate porosity to obtain the highest dispersion was determined, the metal content was increased to 20 wt.% Pt. However, the mean Pt particle size only increased from 1 to 2 nm, indicating the importance of an appropriate mesoporosity for obtaining a high dispersion at high metal loading. Mean Pt particle size was always slightly smaller by transmission electron microscopy than by H₂ chemisorption, because some Pt particles were not reduced during pre-treatment, as confirmed by X-ray photoelectron spectroscopy. Finally, transmission electron microscopy observations of catalysts with metal loading of 8-20 wt.% before pre-treatment showed the formation of self-assembled Pt-carbon hybrid nanorods and nanowires. To the best of our knowledge, this is the first observation of this phenomenon in Pt/C catalysts.     

Preparation of high solid loading titania suspension in gelcasting using modified boiling rice extract (MBRE) as binder

Preparation of high solid loading homogeneous titania suspension using modified boiling rice extract (BRE) as consolidator (network-former)/binder for gelcasting application has been investigated. To achieve in situ consolidation forming of TiO₂ ceramic, the gel network formed by swelling and gelatinization of the modified BRE (MBRE) with 2-hydroxyethylmethacrylate was studied. The dispersion behaviour of the titania powder and rheology of the suspension under the influence of binder content, dispersant (Darvan 821A) concentration and pH of the dispersing media have been discussed. The present process of gel casting deals with 50-80 weight% solid loading of titania particles with MBRE (2-10 weight% to that of total solid loading) in presence of dispersant (ammonium salt of polyacrylic acid [(C₄H₅O₂-NH₄⁺)n]). The influence of BRE concentration and solid particle loading on rheological properties of aqueous titania suspensions has been analyzed under steady and oscillatory shear conditions. Thermogravimetry (TG) and differential thermal analysis (DTA) on gelcast green body has been evaluated and analyzed. The characterization of green and sintered body has been done with respect to density, porosity and microstructure.     

Optimising the dewatering behaviour of clay tailings through interfacial chemistry, orthokinetic flocculation and controlled shear

The effect of shear on dewatering behaviour and particle interactions of Na-exchanged smectite and kaolinite clay dispersions has been investigated at pH 7.5, using hydrolysable Ca(II) and Mn(II) ions as coagulants and high molecular weight anionic and non-ionic polyacrylamide (PAM A and PAM N, respectively) and polyethylene oxide (PEO) flocculants. Metal ion addition enhanced the flocculation performance by dramatically reducing the magnitude of the particle zeta potential and, in the case of smectite pulp, suppressing osmotic swelling. Under optimum orthokinetic flocculation conditions of controlled agitation rate and duration, PAM A and PEO-based flocs settled faster than those of PAM N whilst kaolinite pulps produced higher sedimentation rates than smectite pulps. The settling rates are nearly an order of magnitude greater than those observed under standard flocculant-pulp mixing/flocculation methods of inversion and plunging. The difference in the flocculant behaviour is attributed to the more expanded conformation of PAM A and PEO polymer chains in contrast to PAM N, whilst the lower yield stresses, reflecting inter-particle bridging and floc network structure strength that are conducive to faster clarification, were displayed by kaolinite pulps. Following shear, similar consolidation enhancement of ≈ 5-7 wt.% solid for both pulps was achieved at an optimum agitation range of 100-200 rpm. This was accompanied by decreased yield stress in the case of PAM A-based pulps, indicating non-reversible disruption of polymer mediated particle and floc network structure. In contrast, the yield stresses of PAM N and PEO flocculated dispersions indicated similar and stronger particle interactions, respectively, upon consolidation following shear. The findings show clear links between effect of shear, interfacial chemistry and polymer structure on pulp particle interactions and dewaterability.     

Study of the dispersion behaviour of l-leucine containing microparticles synthesized with an aerosol flow reactor method

l-leucine containing particles having salbutamol sulphate or sodium chloride as a main component have been produced by an aerosol flow reactor method. In the method, aqueous solute droplets were transferred into a heated laminar flow reactor where droplet drying took place. The geometric number mean diameter (GNMD) of the produced particles varied between 0.50 and 1.01 μm. Amino acid l-leucine, due to its surface-active nature in water, formed the outer layer of the initial droplet and in the product composite salbutamol and NaCl powders. The morphology of the amorphous salbutamol particles changed from spherical to wrinkled and that of the crystalline NaCl particles from faceted to rounded but fractured due to incorporated l-leucine. These powders mixed with coarse lactose powder were tested in a novel deagglomeration apparatus where they experienced continuous turbulent flows with jet flow rates from 15 to 90 l/min intended to disperse powder agglomerates. In general, the incorporation of l-leucine improved dispersion efficiency as well as decreased dependence on dispersing flow rate of all the powders. The influence of l-leucine was observed particularly at low flow rates: The particle number concentration of the dispersed NaCl particles increased ∼ 19 times and that of the salbutamol particles ∼ 12 times with 20 wt.% of l-leucine at a flow rate of 15 l/min. Added l-leucine affected the dispersion of salbutamol particles more than that of NaCl particles due to different particle surface. Moreover, the salbutamol-l-leucine agglomerates were reduced to the primary particles at high flow rates. This was not observed for the NaCl-l-leucine agglomerates. Fine particle fractions (FPF, D ≤ 5 μm) of NaCl-l-leucine and salbutamol-l-leucine composite particles at a flow rate of 60 l/min increased, respectively, from 0.14 to 0.29 and 0.19 to 0.39 with increasing l-leucine content. Commercial micronized salbutamol powder gave an FPF of 0.15.     

Monodisperse carboxylated polystyrene particles: synthesis, electrokinetic and adsorptive properties

Effect of conditions of styrene dispersion polymerization initiated by 4,4′-azo-bis-(4-cyanopentanoic acid) in ethanol solutions of polyvinylpyrrolidone was investigated. Suitable methods ensuring the control of final particle size, surface structure, and surface concentration of carboxylic groups in the polymerization process are discussed. Particle ability to interact with protein was also studied. Monodisperse particles of diameters up to 4 μm had a complex surface layer containing polyvinylpyrrolidone-graft-polystyrene copolymers as well as carboxylic groups of the initiator. The effect of this surface structure on the isotherms of adsorption and chemisorption of bovine serum albumin was revealed. Electrophoretic mobility of the particles and their isoelectric point values before and after protein binding depending on pH and ionic strength were determined. These data depend on conditions of particle preparation as well as on protein coating values.     

Effect of mineral geology, mineral size and settling time on selective dispersion and separation process for recovering iron value from iron ultra fines

The dispersion study was conducted on ultra fines of iron ore collected from two locations, Joda and Noamundi slime ponds in the states of Orissa and Jharkhand (India) respectively using sodium hexametaphosphate as a dispersing agent. The Joda slime has been separated into two parts by using hydro cyclone to identify the effect of particle size on selective dispersion and separation of iron ore slimes. Among the gangue minerals present in the slimes, zeolite is more selective towards separation through dispersion process. Under a particular settling condition, a high grade concentrate containing 67.9 wt.% Fe, 1.2 wt.% Al₂O₃ and 1.25 wt.% SiO₂ with 58% iron recovery is achievable using off grade iron ultra fines containing 57.8 wt.% Fe, 7.55 wt.% Al₂O₃ and 7.15 wt.% SiO₂.The efficiency of selective dispersion process strongly depends on mineral geology and particle size. The iron ore ultra fines collected from the Joda slime pond have better selectivity towards dispersion resulting in better separation of gangue minerals in comparison to iron ore ultra fines collected from Noamundi slime pond. This can be attributed to better liberation of Joda slime and difference in nature of the mineral content in Noamundi slime.     

Bubble–particle collision and attachment probability on fine particles flotation

Particle size is an important parameter in flotation and has been the focus of flotation research for decades. The difficulty in floating fine particles is attributed to the low probability of bubble–particle collision. In this research, the influence of hydrodynamic parameters on collision probability of fine particles was investigated. Collision probability was obtained using Stokes, intermediate I and intermediate II and potential equations. Maximum collision probability was 5.65% obtained with impeller speed of 1100 rpm, air flow rate of 30 l/h and particle size of 50 μm. Also, attachment probability under Stokes flow, turbulent and potential flow conditions was calculated 100, 99.49 and 81.87% respectively. Maximum attachment probability was obtained with impeller speed of 700 rpm, contact angle of 90°, particle size of 20 μm and air flow rate of 15 l/h. Collision angles were obtained between 60.71° and 60.18° and attachment angles were obtained between 9.15° and 59.83°.     

Analysis of dilute solid–liquid suspensions in turbulent stirred tanks

In this work, dilute suspensions of solid particles in stirred tanks are investigated by Particle Image Velocimetry measurements, which were specifically designed to determine the effects of the dispersed phase on mean velocity and turbulence levels of the continuous phase and the local solid–liquid slip velocity. In order to determine the effect of particle size and concentration, glass particles of narrow size distribution were selected; the particle content was increased stepwise up the maximum of 0.2 vol.%. Overall, moderate dampening of liquid turbulent fluctuations was found with the smaller particles, while turbulence enhancement was observed with the bigger ones. Continuous phase turbulence was found to affect the local map of the particle settling velocity, which was also discussed on the basis of a force balance analysis. The reduction of particle settling velocity due to free stream turbulence under specific conditions is confirmed.     

Characterization of cast AlSi(Cu) alloys by scanning Kelvin probe force microscopy

The aluminum cast alloys undergo surface treatments involving micro-electrochemistry for improved hardness, wear and/or corrosion resistance. The susceptibility to local galvanic coupling for three different permanent mold cast aluminum alloys, i.e. 99.8 wt.%Al, Al-10 wt.%Si and Al-10 wt.%Si-3 wt.%Cu was investigated using the scanning Kelvin probe force microscopy (SKPFM) technique. All the particles detected in the alloys (i.e. Al-Fe, Al-Fe-Si, Si, Al₂Cu) revealed a positive Volta potential difference relative to the matrix. However, the nobility depended on particles composition. The highest Volta potential differences were determined in the Al-10 wt.%Si-3 wt.%Cu alloy between the copper containing intermetallics and the aluminum matrix whereas the lowest differences were detected for the Al-Fe particles in the 99.80 wt.%Al specimen. Further, the matrix of the 99.8 wt.%Al specimen showed Volta potential differences possibly due to compositional gradients following casting.     

Synthesis of Nd:YAG powders leading to transparent ceramics: The effect of MgO dopant

Neodymium doped yttrium aluminum garnet (Nd:YAG) ultrafine powders were synthesized by co-precipitation method using MgO as dopant. The addition of small amount of MgO can reduce the agglomeration and particle size of the produced Nd:YAG powders. The results show that pure phase YAG powders can be achieved by calcining of the precursors at 1000 °C for 2 h. The MgO doped Nd:YAG powders show better dispersion compared with the undoped powders. When the MgO content is 0.01 wt.%, well-dispersed Nd:YAG powders with spherical particles of 100 nm diameter were obtained. The transmission of the corresponding Nd:YAG ceramics is 82.6% at the wavelength of 1064 nm, which is comparable to Nd:YAG single crystals.