Influence of suspension stability on wet grinding for production of mineral nanoparticles

Grinding behavior of nanoparticles in an attritor mill and the minimum achievable particle size are strongly influenced by the suspension stability. In the present work, suspension stability (i.e. (-potential) of nanoparticles was studied by measuring pH as a function of grinding time in the wet milling process. It was found that after a certain time in an attritor mill, there is no further size reduction and the average product particle size increases monotonically. One of the reasons is that the production of submicron particles leads to more particle-particle interactions and consequently pH of the suspension decreases with grinding time. Usually pH value is related to suspension stability and it can be enhanced by addition of NaOH solution. The maximum negative (-potential of -51.2 mV was obtained at pH of 12 for silica. The higher the (-potential with the same polarity, higher will be the electrostatic repulsion between the particles. Hence, the maximum electrostatic repulsion force was maintained by the adjustment of pH value in wet milling. The experiments were conducted at different pH conditions which were maintained constant throughout the experiments and nanosized particles were obtained consequently.     

DEM prediction of particle flows in grinding processes

Particle size reduction is a critical unit process in many industries including mineral processing, cement, food processing, pigments and industrial minerals and pharmaceuticals. The aim is to take large feed material and as efficiently as possible reduce the size of the particles to a target size range. Over time, a very large range of equipment has been developed to perform this for many materials and in many different conditions. Discrete element method (DEM) modelling is a computational tool that can allow detailed exploration of the particle flow and breakage processes within comminution equipment and can assist in developing a clearer and more comprehensive understanding of the detailed processes occurring within. In this paper, we examine the particle and energy flows in representative examples of the equipment used in many grinding processes. We study a 36′ semi‐autogenous mill used in primary grinding for mineral processing, a ball mill used for cement clinker grinding, a grinding table also used for cement grinding, a tower mill used for fine grinding both in mineral processing and for industrial minerals and finally for an Isamill, which is used for ultra‐fine grinding in mineral processing. Copyright © 2008 John Wiley & Sons, Ltd.     

Bifurcations and equilibria in the extended N-body ring problem

We consider the motion of an infinitesimal particle under the gravitational field of (n+1) bodies in ring configuration, that consist of n primaries of equal mass m placed at the vertices of a regular polygon, plus another primary of mass m₀=βm located at the geometric center of the polygon. We analyze the phase flow, determine the equilibria of the system, their linear stability and the bifurcations depending on the mass of the central primary (parameter β).This study is extended to the case when the central body is an ellipsoid or a radiation source. In this case, the topology of the problem is modified.     

Rheological and mechanical properties of silica colloids: from Newtonian liquid to brittle behaviour

Rheological and mechanical properties of aqueous mono-disperse silica suspensions (Ludox? HS40) are investigated as a function of particle volume fraction (ϕ _p ranging from 0.22 to 0.51) and water content, using shear rate tests, oscillatory methods, indentation and an ultrasonic technique. As the samples are progressively dried, four regimes are identified; they are related to the increasing particle content and the existence and behaviour of the electrical double layer (EDL) around each particle. For 0.22 ≤ ϕ _p ≤ 0.30), the suspensions are stable due to the strong electrostatic repulsion between particles and show Newtonian behaviour (I). As water is removed, the solution pH decreases and the ionic strength increases. The EDL thickness therefore slowly decreases, and screening of the electrostatic repulsion increases. For 0.31 ≤ ϕ _p ≤ 0.35, the suspensions become turbid and exhibit viscoelastic (VE) shear thinning behaviour (II), as they progressively flocculate. For 0.35 ≤ ϕ _p ≤ 0.47, the suspensions turn transparent again and paste-like, with VE shear thinning behaviour and high elastic modulus (III). At higher particle concentration, the suspensions undergo a glass transition and behave as an elastic brittle solid (IV, ϕ _p = 0.51).     

Rheological properties and dispersion stability of magnetorheological (MR) suspensions

In the present article, the rheological responses and dispersion stability of magnetorheological (MR) fluids were investigated experimentally. Suspensions of magnetite and carbonyl iron particles were prepared as model MR fluids. Under an external magnetic field (H ₀) and a steady shear flow, the yield stress depends upon H ₀ ^3/2. The Yield stress depended on the volume fraction of the particle (φ) linearly only at low concentration and increased faster at high fraction. Rheological behavior of MR fluids subjected to a small-strain oscillatory shear flow was investigated as a function of the strain amplitude, frequency, and the external magnetic field. In order to improve the stability of MR fluid, ferromagnetic Co-γ-Fe₂O₃ and CrO₂ particles were added as the stabilizing and thickening agent in the carbonyl iron suspension. Such needle-like particles seem to play a role in the steric repulsion between the relatively large carbonyl iron particles, resulting in improved stability against rapid sedimentation of dense iron particles. Furthermore, the additive-containing MR suspensions exhibited larger yield stress, especially at higher magnetic field strength. Received: 4 April 2000 Accepted: 6 November 2000     

Pozzolanic reactivity of silico-aluminous fly ash

For some years it has been possible to control the particle size of fly ashes, by-products of thermal power stations. Incorporating these very fine particles （obtained by grinding and/or pneumatic selection） improves the physical-mechanical characteristics of mortars and concretes. In this study, we measured the lime consumption of the various fractions （granulometric and densimetric） and identified by X-ray diffraction the neoformed phases by the pozzolanic reaction, to show that it is not sufficient to simply define the pozzolanicity of products based on lime consumption since it does not take into account the nature of the phases formed. The size of the particles used in the test samples also has a determining effect on the quantity of lime consumed. Before comparing results, it is necessary to ensure that the size of the particles （of the global ash and its constituents） be the same. Two distinct neoformed ohases appear： CSH in the largest granular fractions （d〉 40 μm） and C3AH6 in the smaller fractions.     

Synthesis of Fe3O4 nanoparticles by wet milling iron powder in a planetary ball mill

Fe3O4 nanoparticles with sizes ranging from 30 to 80 nm were synthesized by wet milling iron powders in a planetary ball mill. The phase composition and the morphologies of the as-synthesized products were measured by X-ray diffraction （XRD）, scanning electron microscopy （SEM） and transmission electron microscopy （TEM）. Nanosized Fe3O4 particles were prepared by wet milling metallic iron powder （-200 mesh, 99%） in a planetary ball mill equipped with stainless steel vials using iron balls under distilled water with a ball-to-powder mass ratio of 50：1 and at a rotation speed of 300 rpm. The use of the iron balls in this method played a key role in Fe3O4 formation. The present technique is simple and the process is easy to carry out.     

Role of particle stiffness and inter-particle sliding friction in milling of particles

Discrete element method （DEM） has been used to investigate the effects of particle elastic modulus and coefficient of inter-particle sliding friction on milling of mineral particles. An autogeneous mill of 600 mm diameter and 320 mm length with 14,500 particles has been selected for the simulation. Various mill performance parameters, for example, particle trajectories, collision frequency, collision energy and mill power have been evaluated to understand the effects of particle elastic modulus and inter-particle sliding friction during milling of particles. For the given model, it has been concluded that at high energy range, as the elastic modulus and particle sliding friction increase the energy dissipated among the particles increases. The collision frequency increases with the increase in elastic modulus, however, this trend is not clearly observed with increasing inter-particle sliding friction. The power draw of the mill increases with the increase in fraction of mill critical speed.     

Comportement rheologique de sols d'acide polysilicique. II. Etude de la gélification

Résumé L'analyse du comportement rhéologique de sols d'acide polysilicique en solution aqueuse de concentration supérieure à 7 g 1^–1 de silice en fonction du pH entre 6 et 11 et de la température entre 25 et 45 °C a permis de situer la gélification dans un domaine de pH compris entre 8 et 10. Ce processus prend une intensité maximale à 30°C. Le comportement rhéofluidifiant à bas gradients de cisaillement et antithixotrope donne naissance sur les rhéogrammes à un point d'inversion à pH = 9. L'ionisation des groupes silanols à la surface des particules colloïdales permet la réorganisation du milieu après cisaillement. Un état d'ionisation trop élevé est responsable d'un effet electro-répulsif à pH = 10. L'influence de la concentration et de la température sur ces processus est discutée.

---

The rheological behavior of polysilicic acid in aqueous solutions at higher concentrations than 7 g 1^–1 of silice is studied for pH between 6 and 11 with temperature varying from 25° to 45°C. This study indicates that gelification takes place between pH 8 and 10. The process of gel formation attains the highest intensity at 30°C. The shear-thinning behavior at low shear rate gives rise to an inversion point at pH = 9 in shear stress—shear rate curves. The state of ionization at the surface of colloïdal silice particles after shear involve shear thickening observed at pH = 9. A high ionization degree causes an electrostatic repulsion at pH = 10. The concentration and temperature effects upon the organization of network are discussed.

     

Rheology in dispersion testing and mill base formulation optimization

Conclusions Rheology offers only slight advantages in terms of mill base formulation, because of the comparatively low shear rates available on existing viscometers and the considerable changes in mill base rheology which occur during the milling cycle. These changes can however be utilised as a means of dispersion testing and satisfactory and reproducible milling isotherms can be obtained for a wide variety of dispersion and size reduction operations. It would seem possible to characterise grinding and dispersion by changes in mill base rheology in that grinding shows an increase in non-Newtonian characteristics whereas dispersion shows a decrease in these characteristics. Care must be taken to ensure that controlled pretreatment of the samples is adopted be ore satisfactory results can be ensured, but extensive storage periods tend to magnify differences between samples.Presented at the Joint Meeting of the British Society of Rheology and Research Association of British Paint, Colour and Varnish Manufacturers at Teddington, April 29, 1964.     

Sedimentation acceleration of remanent iron oxide by magnetic flocculation

Sedimentation based processes are widely used in industry to separate particles from a liquid phase. Since the advent of the ＂Nanoworld＂ the demand for effective separation technologies has rapidly risen, calling for the development of new separation concepts, one of which lies in hybrid separation using the superposition of a magnetic field for magnetic particles. Possible product portfolio of such separation consists of pigment production, nanomagnetics production for electronics and bio separation, A promising step in that direction is magnetic field enhanced cake filtration, which has by now progressed from batch to continuous ooeration. In sedimentation processes in a mass force field the settling behaviour of particles strongly depends on physico-chemical properties, concentration and size distribution of the particles. By adjusting the pH, the interparticle forces, in particular the electrostatic repulsion, can be manipulated. For remanent magnetic particles such as magnetite, pre-treatment in a magnetic field could lead to a change of interparticle interactions. By magnetizing the particles apart from van der Waals attraction and electrostatic repulsion, an additional potential is induced, the magnetic attraction, which could easily dominate the other potentials and result in agglomeration in the primary minimum. By sedimentation analysis, a wide spectrum of parameters like pH, magnetic field strength and concentration have been investigated. The results show a strong increase of sedimentation velocity by magnetic flocculation of the raw suspension. This leads to a rise in throughput due to the acceleration of sedimentation kinetics by imparting a non-chemical interaction to the physico-chemical properties in the feed stream of the separation apparatus.     

Filtration in a Porous Granular Medium: 2. Application of Bubble Model to 1-D Column Experiments

This paper describes the formulation of a quasi-1-D network model, referred to as the ‘bubble model’, and its application for simulating particle transport and filtration through a granular filter bed. The model comprises a series of homogeneous sites linked through bundles of cylindrical bonds that represent flow pathways through distributions of pores and pore throats. This model incorporates pore scale processes of particle sieving and infiltration are based on numerical simulations described in a companion paper. The modeling of infiltration is further refined based on detailed experimental observations and measurements of the filtration of a dilute suspension of acrylic particles through a column of glass beads reported by Yoon et al. (2005 Water Resour. Res., to appear). Their data distinguish (a) between the collection of particles on grain surfaces and at grain-to-grain contact points, and (b) between particles that are fully entrapped and those that are hindered (temporarily collected) and can later become detached. These effects are represented by two parameters that characterize the probability of attachment and are linked to the surface roughness of the grains; one that describes the minimum particle size that can be fully entrapped, and one that describes the detachment rate. These parameters can be readily calibrated from conventional measurements of effluent concentration and effluent particle size distribution. Detailed comparisons with the data reported by Yoon et al. show that the proposed bubble model is able to achieve reliable predictions of the spatial distribution of particles within the filter bed following phases of particle injection and washing.     

Application of DEM modified with enlarged particle model to simulation of bead motion in a bead mill

We applied the discrete element method (DEM) of simulation modified by an enlarged particle model to simulate bead motion in a large bead mill. The stainless-steel bead mill has inner diameter of 102 mm and mill length of 198 mm. The bead diameter and filling ratio were fixed respectively at 0.5 mm and 85%. The agitator rotational speed was changed from 1863 to 3261 rpm. The bead motion was monitored experimentally using a high-speed video camera through a transparent mill body. For the simulation, enlarged particle sizes were set as 3–6 mm in diameter. With the DEM modified by the enlarged particle model, the motion of enlarged particles in a mill was simulated. The velocity data of the simulated enlarged particles were compared with those obtained in the experiment. The simulated velocity of the enlarged particles depends on the virtual frictional coefficient in the DEM model. The optimized value of the virtual frictional coefficient can be determined by considering the accumulated mean value. Results show that the velocity of the enlarged particles simulated increases with an increase in the optimum virtual frictional coefficient, but the simulated velocity agrees well with that determined experimentally by optimizing the virtual frictional coefficient in the simulation. The computing time in the simulation decreases with increased particle size.     

Experiments on the local heat transfer characteristics of a circulating fluidized bed

The role of particle diameter in the heat transfer of a gas–solid suspension to the walls of a circulating fluidized bed was studied for particles of uniform size. This work reports and analyzes new experimental results for the local bed to wall heat transfer coefficient, not including the radiation component, in a long active heat transfer surface length laboratory bed, which extend previous findings and clear up some divergences. The research included determining the effects of extension and location of the heat transfer surface, circulating solids mass flux and average suspension density. An experimental set-up was built, with a 72.5 mm internal diameter riser, 6.0 m high, composed of six double pipe heat exchangers, 0.93 m high, located one above the other. Five narrow sized diameter quartz sand particles − 179, 230, 385, 460 and 545 μm − were tested. Temperature was kept approximately constant at 423 K and the superficial gas velocity at 10.5 m/s. The major influence of suspension density on the wall heat transfer was confirmed, and contrary to other authors, a significant effect of particle size was found, which becomes more relevant for smaller particles and increasing suspension density. It was observed that the extension of the heat transfer surface area did not influence the heat transfer coefficient for lengths greater than 0.93 m.The heat transfer surface location did not show any effect, except for the exchanger at the botton of the riser. A simple correlation was proposed to calculate the heat transfer coefficient as a function of particle diameter and suspension density.     

Manipulation of particles using dielectrophoresis

A numerical scheme based on the distributed Lagrange multiplier method (DLM) is used to study the motion of particles of electrorheological suspensions subjected to non-uniform electric fields. At small Reynolds number, the time taken by the particles to collect at the minimums or maximums of the electric field is primarily determined by a parameter defined to be the ratio of the dielectrophoretic and viscous forces. Simulations show that in non-uniform electric fields the collection time is also influenced by a parameter defined by the ratio of the electrostatic particle–particle interaction and dielectrophoretic forces. The collection time decreases as this parameter decreases because when this parameter is less than one, particles move to the regions of high or low electric field regions individually. However, when this parameter is greater than one, particles regroup into chains which then move toward the electric field maximums or minimums without breaking. It is also shown that when the real part of the Clausius–Mosotti factor (β) is negative the positions of the local minimums of the electric field, and thus also the locations where particles collect, can be modified by changing the electric potential boundary conditions.     

Irbesartan drug formulated as nanocomposite particles for the enhancement of the dissolution rate

Irbesartan (IBS), an angiotensin II receptor antagonist, is a poorly water-soluble drug. To enhance the dissolution rate, IBS nanocomposite particles were produced via an anti-solvent precipitation combined with a spray drying process. Four pharmaceutically acceptable excipients, including three different polymers and one charged surfactant, were evaluated as stabilizers to control the particle size and to prevent the agglomeration of particles. The experiment results indicated that polyvinylpyrrolidone (PVP) combined with sodium dodecyl sulfate (SDS) significantly decreased the particle size and enhanced the stability of drug nanoparticles. As a result, we finally obtained stable IBS nanoparticles with an average size of approximately 55 nm. In the dissolution test, the IBS nanocomposite particles showed a significantly enhanced dissolution rate and 100% of the drug dissolved within 20 min. In contrast, the physical mixture with the same recipe as the IBS nanocomposite particles and the raw IBS reached only 8% and 40% of drug dissolved in 20 min, respectively, and both of them did not dissolve completely, even after 120 min.     

Dynamics of Brownian particles in a turbulent channel flow

Turbulent channel flows with suspended particles are investigated by means of numerical simulations. The fluid velocity is computed by large eddy simulation. Motion of small graphite particles with diameter of 0.01–10 m, corresponding to the Schmidt number, Sc, of 2.87 × 10²–6.22 × 10⁶ and the particle relaxation time in wall unit, _p⁺, of 9.79 × 10^–5–4.51, is computed by Lagrangian particle tracking. Relation between the particle relaxation time and the computed deposition velocity is found to be in good agreement with an empirical relation. The statistics of the particle motion in the vicinity of the wall are studied. Clear differences are found in dynamical behavior of particles with different sizes. Medium size particles show a strong dependence on the structure of the fluid flow, while small and large particles are considerably less sensitive.     

Effects of deformed volume, volume fraction and particle size on the deformation behaviour of W/Cu composites

Tungsten/copper (W/Cu) particle reinforced composites were used to investigate the scaling effects on the deformation and fracture behaviour. The effects of the volume fraction and the particle size of the reinforcement (tungsten particles) were studied. W/Cu-80/20, 70/30 and 60/40 wt.% each with tungsten particle size of 10 μm and 30 μm were tested under compression and shear loading. Cylindrical compression specimens with different volumes (D_S = H) were investigated with strain rates between 0.001 s⁻¹ and about 5750 s⁻¹ at temperatures from 20 °C to 800 °C. Axis-symmetric hat-shaped shear specimens with different shear zone widths were examined at different strain rates as well. A clear dependence of the flow stress on the deformed volume and the particle size was found under compression and shear loading. Metallographic investigation was carried out to show a relation between the deformation of the tungsten particles and the global deformation of the specimens. The size of the deformed zone under either compression or shear loading has shown a clear size effect on the fracture of the hat-shaped specimens.The quasi-static flow curves were described with the material law from Swift. The parameters of the material law were presented as a function of the temperature and the specimen size. The mechanical behaviour of the composite materials were numerically computed for an idealized axis-symmetric hat-shaped specimen to verify the determined material law.     

Electrical double layer and rheological properties of yttria-stabilized zirconia suspensions in solutions of high molecular weight polyacrylic acid polymers

This work deals with the effect of the adsorption of two high molecular weight polyacrylic acid polymers (Carbopol) on the interfacial properties, and the rheology of aqueous zirconia suspensions. Since the Carbopol-covered particles can be thought of as soft colloids, Ohshimas theory was used to gain information on the surface potential and the charge density of the polymer layer (Ohshima H (1995) Electrophoretic mobility of soft particles. Colloids Surf A Physicochem Eng Aspects 103:249–255). The effect of the pH of the solution on the double layer characteristics is related to the different conformations of the adsorbed molecules provoked by the dissociation of the acrylic groups present in polymer molecules. The electrokinetic properties of the suspensions are studied for different pH and Carbopol concentrations in solution in order to investigate the possible stabilization of the suspensions by electrostatic repulsion between the particles. The rheological behavior of the suspensions was investigated in steady-state and dynamic conditions, and the corresponding yield stress and storage modulus were obtained in absence and presence of polymer in solution. The competition between bridging flocculation provoked by polymer adsorption and electrosteric stabilization determines the rheological properties of the suspensions. In the pH range investigated, bridging flocculation predominates at the neutral pH because of the graft of the uncoiled polymer to more than one particle, while at the extreme pH values (pH 3, pH 9) steric or electrosteric stabilization seems to be the predominant mechanism that explains the rheological results. These facts were confirmed by estimating the zirconia particle (or aggregate) diameter in the liquid medium by means of light scattering measurements.