Sorption of gold and silver on carbon adsorbents from thiocyanate solutions

Sorption recovery of thiocyanate gold and silver complexes on different carbon adsorbents has been studied using model solutions with concentrations 0.08-0.82 mmol/l and 0.16-1.06 mmol/l for gold and silver, respectively. The potassium thiocyanate concentration in these solutions was 0.25 mol/l and the pH of the contacting solution was ∼2. The degree of recovery exceeded 90% for gold and 80% for silver. The separate step-by-step desorption of thiocyanate gold and silver complexes was carried out by varying the initial concentration of thiocarbamide (desorption agent). The degree of recovery of noble metals can be increased up to 95% using basic thiocarbamide solutions (in 0.1-0.2 M NaOH) at the higher temperature of the process (up to 150 °C).     

An extremely rapid, convenient and mild coal desulfurization new process: Sodium borohydride reduction

The present work describes the desulfurization of coal using mildly reductive method. Both a Yanzhou and a Yanshan coal (referred to as YZ and YS coal, respectively), were treated in an aqueous media employing sodium borohydride (NaBH₄) as reducing agent, which is a well known hydrogen storage. Reaction variables investigated include concentration of reductant, time, pH of initial media, temperature, stirring rate and particle size. The calorific values and ignition temperatures of the coal samples before and after treatment were determined. Results show that the total sulfur removal improved with the increase in the concentration of NaBH₄, shaking rate and temperature and with the decrease in the particle size. Meanwhile, decreasing the particle size from − 250 to − 109 μm increased the organic sulfur removal by more than six times for either of the coal samples. Considering economic rationality and operational convenience, the desulfurization conditions determined were 1.6 mM of NaBH₄ concentration, − 109 μm of particle size, neutral pH of initial media, 1 min of treated time, 100 rpm of shaking rate, 30 °C of temperature. This led to 23.8% and 59.0% reduction in the pyritic, 70.4% and 100% reduction in the sulfate, and 11.0% and 15.0% reduction in the organic sulfur, giving 31.3% and 40.8% reduction in the total sulfur for the YZ coal and the YS coal, respectively. Moreover, this resulted in the increase in the calorific values by 3.4-6.9% and the decrease in the ignition temperatures by 2-21 °C for the coal samples. The desulfurization method described here is extremely rapid, convenient, inexpensive and mild, and therefore, has considerable technological interest.     

Synthesis of carbon nanotubes over gold nanoparticle supported catalysts

The synthesis of carbon nanotubes (CNTs) through the catalytic decomposition of acetylene was carried out over gold nanoparticles supported on SiO₂-Al₂O₃. Monodispersed gold nanoparticles with 1.3-1.8 nm in diameter were prepared by the liquid-phase reduction method with dodecanethiol as protective agent. The carbon products formed after acetylene decomposition consist of multi-walled carbon nanotubes with layered graphene sheets, carbon nanofilaments (CNFs), and carbon nanoparticles encapsulating gold particles. The observed CNTs have outer diameters of 13-25 nm under 850 °C. The influence of several reaction parameters, such as kind of carriers, reaction temperature, gas flow rate, was investigated to search for optimum reaction conditions. The CNTs were observed at a relatively low temperature (550 °C). The silica-alumina carrier showed higher activity for the formation of CNTs than others used in the screening test. With increasing temperature, the CNTs showed cured structures having thick diameters and inside compartments. When Au content on the support was over 5 wt.%, the gold nanoparticles coagulated to form large ones >20 nm in diameter and became encapsulated with graphene layers after decomposition of acetylene.     

In situ electrochemical generation of gold nanostructured screen-printed carbon electrodes. Application to the detection of lead underpotential deposition

In the present work, an electrochemical method for the reproducible and stable generation of gold nanostructures on the surface of screen-printed carbon electrodes was developed. This technique is based on the application of a constant current over an appropriate time interval. Gold nanostructured screen-printed carbon electrodes were characterized using both SEM and electrochemical methods. The mean diameter and the dispersion of gold nanoparticles that were generated electrochemically depended on the gold concentration, the time deposition and the current intensity. Smaller diameters and better distribution of nanoparticles were obtained when a shift of potential to −0.70 V occurred during the gold electrodeposition process. Moreover, the underpotential deposition (UPD) of lead on these nanostructured surfaces was studied, as was their behavior as array electrodes. The best results, using UPD combined with square wave stripping voltammetry, were obtained for gold nanostructured surfaces with a mean diameter of 78 ± 24 nm and a density of 4.4 × 10⁷ nanoparticles/mm². These gold nanostructured screen-printed carbon electrodes were obtained by applying a current intensity of −100 μA for 300 s using a gold concentration of 0.5 mM. The reproducibility and limit of detection obtained using these nanostructured electrodic surfaces were 2.4% (in terms of RSD) and 0.8 ng/mL, respectively.     

Numerical simulation of microparticles penetration and gas dynamics in an axi-symmetric supersonic nozzle for genetic vaccination

This paper describes two phase (solid particles/gas) flow in a supersonic nozzle that is part of a device for micromolecular vaccine/drug delivery. It accelerates micro solid particles to high speeds sufficient to penetrate the viable epidermis layer to achieve the pharmaceutical effect. Helium is used as the driving gas for the solid particles because of its high compressibility factor. A numerical parametric study was performed for gas pressures ranging between 3 and 6 MPa and gold particles of diameters 1.8 μm and 5 μm. The computed results show that uniform particle velocity was achieved at standoff distance of 2 exit diameters (D_e) downstream of the device exit with particles concentrated on the supersonic core jet. Increasing the helium pressure from 3 to 6 MPa caused an increase in the particle velocity of 24% for particles with a diameter of 1.8 μm and 7% for particles of diameter 5 μm at the standoff distance. Furthermore increased gas pressure has adverse effect on particles concentration. As the inlet pressure increases, the particles are concentrated more at the core of the nozzle. Semi-empirical particle penetration calculation confirms the numerical results that the 5 μm particles penetration distance is 45-135 μm and the 1.8 μm diameter penetration is 35-95 μm beneath the skin. Comparison of different geometries has been done in order to understand each section function and to gain optimum performance.     

Polymerization induced phase separation in poly(ether imide)-modified epoxy resin cured with imidazole

This paper studies the phase separation in poly(ether imide) (PEI) modified epoxy resin using imidazole (C₁₁Z-CNS) as epoxy hardener to control its morphology. The sponge-like phase structures were founded at higher PEI concentration (10-25 phr), while homogeneous structures are formed at low PEI concentration (5 phr). The effects of PEI concentration on curing kinetics and phase structures were studied by differential scanning calorimeters (DSC) and scanning electron microscopy (SEM). It is shown that although the addition of PEI does not change the curing mechanism, the separated morphology becomes finer at high PEI concentration. The curing rate and conversion decrease with the increase of the content of PEI. The chain growth polymerization of these systems caused an early gelation (conversion <10%) and early freezing of morphologies. The evolution of phase separation in the early stage was monitored by synchrotron radiation small angle X-ray scattering (SR-SAXS) and transmission electronic microscopy (TEM). It is suggested that the formation of sponge-like phase structure could be attributed to the strong viscoelastic effects in the early stage of phase separation.     

Crosslinkable functional moiety for the formation of highly crosslinked stable microspheres in the precipitation polymerization

The highly crosslinked stable spherical microspheres were successfully synthesized using styrene and three crosslinking agents having different number of crosslinkable functional moiety in comonomer using the precipitation polymerization. The crosslinking agents are ethylene glycol dimethacrylate (EGDMA), trimethylolpropane trimethacrylate (TMPTMA) and pentaerythritol tetraacrylate (PETRA). The maximum and minimum concentrations for forming the stable spherical particles were ranging at 20-90 mol% for EGDMA, 15-80 mol% for TMPTMA, and 5-40 mol% for PETRA, respectively. The number-average diameter of stable poly(S-co-EGDMA), poly(S-co-TMPTMA), and poly(S-co-PETRA) particles varied 4.1-3.06, 3.94-3.03 and 2.77-1.66 μm, respectively. Since the prepared microspheres are highly crosslinked, no glass transition temperature was observed. The TGA onset point of the thermal degradation temperature increased with the concentration of crosslinking agent and the number of crosslinkable functional moiety, which is EGDMA < TMPTMA < PETRA. As a result, the minimum and maximum concentrations for the formation of stable spherical particles of poly(S-co-EGDMA), poly(S-co-TMPTMA), and poly(S-co-PETRA), the particle size and its distribution, CV, yield and the TGA onset point are significantly affected by the number of the crosslinkable functional moiety. Thus, the number of the crosslinkable functional moiety and the different reactivity as well as the different copolymerization parameters of styrene with (meth)acrylates would influence the composition as well as the rate of formation of stable microspheres.     

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.     

Spherulite morphology and crystallization behavior of poly(trimethylene terephthalate)/poly(ether imide) blends

The spherulite morphology and crystallization behavior of poly(trimethylene terephthalate) (PTT)/poly(ether imide) (PEI) blends were investigated with optical microscopy (OM), small-angle light scattering (SALS), and small-angle X-ray scattering (SAXS). Thermal analysis showed that PTT and PEI were miscible in the melt over the entire composition range. The addition of PEI depressed the overall crystallization rate of PTT and affected the texture of spherulites but did not alter the mechanism of crystal growth. When a 50/50 blend was melt-crystallized at 180 °C, the highly birefringent spherulite appeared at the early stage of crystallization (t < 20 min). After longer times, the spherulite of a second form was developed, which exhibited lower birefringence. The SALS results suggested that the observed birefringence change along the radial direction of the spherulite was mainly due to an increase in the orientation fluctuation of the growing crystals as the radius of spherulite increased. The lamellar morphological parameters were evaluated by a one-dimensional correlation function analysis. The amorphous layer thickness showed little dependence on the PEI concentration, indicating that the noncrystallizable PEI component resided primarily in the interfibrillar regions of the growing spherulites.     

Reaction rate constant of methane clathrate formation

Particle size distribution measurements were performed during the growth stage of methane hydrate formation in a semi-batch stirred tank crystallizer. Experiments were carried out at temperatures between 275.1 and 279.2 K and pressures ranging from 3873 to 5593 kPa. The reaction rate constant of methane hydrate formation was determined using the model of Bergeron and Servio (AIChE J 2008;54:2964). The experimental reaction rate constant was found to increase with temperature, following an Arrhenius-type relationship, from 8.3 × 10⁻⁸ m/s to 6.15 × 10⁻⁷ m/s over the 4° range investigated, resulting in an activation energy of 323 kJ/mol. An increase in pressure of approximately 600 kPa did not have any effect on the reaction rate constant. Population balances, based on the measured critical nuclei diameter and that predicted by homogeneous nucleation theory, were also used for comparison purposes. The initial number of hydrate particles was calculated using the mole fraction of methane in the bulk liquid phase and compared to that predicted by an energy balance.     

Dithiocarbamate-protected ruthenium nanoparticles: Synthesis, spectroscopy, electrochemistry and STM studies

Stable ruthenium nanoparticles were synthesized in a biphasic system with a protecting monolayer of dithiocarbamate derivatives. The core size of the resulting Ru particles was found to vary with the initial ligand-metal feed ratio. UV-vis spectroscopic measurements showed a Mie scattering profile, with no obvious surface-plasmon resonance. The size and crystal structures of the particles were characterized by transmission electron microscopic (TEM) measurements. A significant fraction of the nanoparticles was found within the size range of 2-4 nm in diameter and of spherical shape from the TEM measurements. Clear lattice fringes could be observed in high-resolution TEM images with the fringe spacing consistent with the Ru(1 0 1) lattice planes. Electrochemical studies of Ru particles with different core size exhibited the solution-phase quantized charging of the particle double layers, analogous to those reported for gold and other transition-metal particles. The potential spacing between adjacent quantized charging peaks was found to vary with the particle core size, corresponding to the variation of the particle molecular capacitance. These charge-transfer properties were very consistent with the STM measurements of isolated nanoparticles which exhibit clear Coulomb blockade and staircase features.     

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.     

Kinetic theory based computation of PSRI riser: Part II—Computation of mass transfer coefficient with chemical reaction

The design of circulating fluidized bed systems requires the knowledge of mass transfer coefficients or Sherwood numbers. A literature review shows that these parameters in fluidized beds differ up to seven orders of magnitude.To understand the phenomena, a kinetic theory based computation was used to simulate the PSRI challenge problem I data for flow of FCC particles in a riser, with an addition of an ozone decomposition reaction. The mass transfer coefficients and the Sherwood numbers were computed using the concept of additive resistances. The Sherwood number is of the order of 4 × 10⁻³ and the mass transfer coefficient is of the order of 2 × 10⁻³ m/s, in agreement with the measured data for fluidization of small particles and the estimated values from the particle cluster diameter in part one of this paper. The Sherwood number is high near the inlet section, then decreases to a constant value with the height of the riser. The Sherwood number also varies slightly with the reaction rate constant. The conventionally computed Sherwood number measures the radial distribution of concentration caused by the fluidized bed hydrodynamics, not the diffusional resistance between the bulk and the particle surface concentration. Hence, the extremely low literature Sherwood numbers for fluidization of fine particles do not necessarily imply very poor mass transfer.     

Unsteady radiative heat transfer within a suspension of ZnO particles undergoing thermal dissociation

An emitting, absorbing, and anisotropically scattering plain medium containing a suspension of ZnO particles is considered, in which the particles are directly exposed to high-flux irradiation and undergo shrinkage during their endothermic dissociation into Zn(g) and O₂ at above 2100 K. The unsteady energy equation that links the rate of radiative heat transfer to the rate of the chemical reaction is formulated and solved numerically by the finite volume technique and the explicit Euler time-integration scheme. The path-length Monte Carlo method is applied for modeling the radiative transfer within the suspension using the absorption/scattering coefficients and the scattering phase function obtained from the Mie theory. It is found that the particle suspension can be heated rapidly from its initial 300 K to over 1800 K in less than 0.1 s, resulting in a more uniform temperature profile as the reaction progresses, particles shrink, and the suspension becomes optically thinner. The chemical conversion increases with decreasing initial particle diameter and volume fraction due to the efficient radiative absorption.     

Comparison of experimental pressure gradient and experimental relationships for the low density spherical capsule train with slurry flow relationships

In the experimental part of this study, pressure drops that occur in the flow of a low density spherical capsule train conveyed by water in a horizontal pipe were found to be 5-30% of the capsule transport concentrations. The developed experimental relations were compared with well-known relations used for slurry flow. Experimental variables (i.e. bulk velocity, diameter of particle or capsule, diameter of pipe, concentration of particle, density of particle, etc.) were applied to the pressure gradient expressions developed for the slurry flow (asymmetric suspension flow), so that the pressure gradients calculated for a concentration by 5% and 10% were compared with the experimental findings and with the developed mathematical model. It was observed that the pressure gradient expressions of the slurry flow did not simulate the experimental results of the capsule flow. However, a comparison of the empirical expression developed for the pressure drops of the spherical capsule train flow in region (2.5 × 10⁴ < Re < 1.5 × 10⁵) with the experimental findings revealed an average deviation of 3.37%.     

Particle size dependence of the charging of Au nanoparticles immobilized on a modified ITO electrode

Citrate-stabilized gold nanoparticles (Au-NPs) of a nearly spherical shape with four different diameters (3.7, 11.0, 21.7, and 40.8 nm) were immobilized on a 4-aminobutylsiloxane monolayer-modified indium-tin oxide (ITO) electrode. From the results of coulometric measurements using potential step sequences, the number of electrons per particle to be transferred to attain a new equilibrium state after applying a potential step was found to increase in proportion to the square of the diameter. The double layer integral capacitance of the Au-NP surface per unit area in the potential range from −0.4 to 0.6 V (Ag/AgCl/sat’d KCl) is ca. 70 μF cm⁻², being independent of the particle size. The differential capacitance of the Au-NP surface is a function of the potential with a maximum at 0.32 V, while the function is again independent of the particle size. The kinetics of the charging was discussed using the analysis of the potential step transient current. The potential-dependent shift of the plasmon absorption band obtained by constant-potential and potential-modulated transmission-absorption spectroscopic measurements revealed that a smaller Au-NP exhibits a greater blue-shift of the plasmon band when applying more negative potentials, being in line with the Mie-Drude theory.     

Particle formation and growth in ab initio emulsifier-free emulsion polymerisation under monomer-starved conditions

Particle formation and growth in the monomer-starved emulsifier-free emulsion polymerisation of monomers with different water solubility including methyl acrylate (MA), methyl methacrylate (MMA), and vinyl acetate (VA) were studied. The rate of formation of precursor particles, via homogenous nucleation, is proportional to the monomer concentration in the water phase. One may think that the maximum number of particles will be obtained when the water phase is saturated with the monomer. The number of PMA particles showed a maximum when the water phase was starved with the monomer. The number of PVA particles did not show any sensitivity to the monomer concentration in the water phase. More unexpectedly the final number of PMMA particles showed a minimum when the water phase was just saturated with the monomer. The minimum in the final number of PMMA particles was correlated with the enhanced rate of particle growth due to the gel effect. Under monomer-starved conditions, the number of particles produced was in the order of water solubility of the monomers; MA > VA > MMA. A reverse order was produced under monomer-saturated conditions as particle coagulation became progressively more important for some of the monomers.     

Solids concentration simulation of different size particles in a cyclone separator

To deepen our knowledge of the flow in cyclones, the solids concentrations of different size particles in a scroll cyclone separator were numerically simulated by using the Lagrange approach on the platform of commercial CFD software package, FLUENT 6.1. The numerical calculations visualize that there exists a spiral dust strand near the cyclone wall and a dust ring beneath the cyclone top plate. There are two regions in the radial solids concentration distribution, with which the solids concentration is low in the inner region (r/R(dimensionless radial position) ≤ 0.75) and increases greatly in the outer region (r/R > 0.75). Large particles generally have higher concentration in the wall region and small particles have higher concentration in inner vortex region. The axial distribution of the solids concentration in the inner vortex region (r/R ≤ 0.3) shows that serious fine particle re-entrainment exists within the height of 0.5 D (cyclone diameter) above the dust discharge port. We study the effect of solids particle on the gas flow field by two-way couple. The concepts of back-mixing rate, first escaping rate and second escaping rate are proposed for quantifying the local flow phenomena.     

Synthesis of carboxylic acid functionalized nanoparticles by reversible addition-fragmentation chain transfer (RAFT) miniemulsion polymerization of styrene

In this study, an addition-fragmentation chain transfer agent bearing carboxylic acid, 4-toluic acid dithiobenzoate (TADB), was used to synthesize carboxylic acid functionalized PS nanospheres via the miniemulsion polymerization. In addition, non-functionalized RAFT agent, benzyl dithiobenzoate (BDB), was also used to compare the surface properties of the PS nanoparticles. For the TADB system, the rate of polymerization was approximately two-fold faster than the BDB system, while the molecular weights and PDI of PS remain intact.With increasing the molar ratio of [TADB]/[AIBN] from 0 to 3.0, the average particle diameter is substantially increased from 90 to 126 nm. The absolute value of zeta potential and conductivity also correspondingly increase from 49.1 mV and 3.47 mS/cm to 53.9 mV and 4.21 mS/cm, respectively. The results indicate that the surface of PS nanospheres could be functionalized by means of a carboxylic acid group on the RAFT agent and the stability of the PS miniemulsion latex could be significantly improved.     

Experimental study and simulation of vibrated fluidized bed drying

The paper addresses numerical simulation for the case of convective drying of seeds (fine-grained materials) in a vibrated fluidized bed, analyzing agreement between the numerical results and the results of corresponding experimental investigation. In the simulation model of unsteady simultaneous one-dimensional heat and mass transfer between gas phase and dried material during drying process it is assumed that the gas-solid interface is at thermodynamic equilibrium, while the drying rate (evaporated moisture flux) of the specific product is calculated by applying the concept of a “drying coefficient”. Mixing of the particles in the case of vibrated fluidized bed is taken into account by means of the diffusion term in the differential equations, using an effective particle diffusion coefficient. Model validation was done on the basis of the experimental data obtained with narrow fraction of poppy seeds characterized by mean equivalent particle diameter (d_S,d = 0.75 mm), re-wetted with required (calculated) amount of water up to the initial moisture content (X₀ = 0.54) for all experiments. Comparison of the drying kinetics, both experimental and numerical, has shown that higher gas (drying agent) temperatures, as well as velocities (flow-rates), induce faster drying. This effect is more pronounced for deeper beds, because of the larger amount of wet material to be dried using the same drying agent capacity. Bed temperature differences along the bed height, being significant inside the packed bed, are almost negligible in the vibrated fluidized bed, for the same drying conditions, due to mixing of particles. Residence time is shorter in the case of a vibrated fluidized bed drying compared to a packed bed drying.