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Despite the abundant industrial use of various wet mills a detailed understanding of the mechanisms involved is far from being complete. During comminution stress distributions are applied to the individual particles of a processed suspension. In fact, the type of stressing and the acting stress energies are distributed. The distributions of acting stress energy and number of stress events per product particle are governed by the used mill and its operational conditions. Mill functions link the stress energy and stress number distributions to the operating parameters and are so far unknown. We introduce a new methodology to directly characterize the effective stress energy and effective stress number distributions within stirred media mills: Mechanically fully characterized spherical particles are used as particulate probes. The underlying finite element model for the energetic behavior of the probes during uniaxial compression is supported by manipulation experiments on the single particle level. The deformation behavior of individual particles and the respective changes of the particles' shapes are used to determine the stressing history of the particles and hence the stress energy and stress number distributions in the mill. Experimental limitations and prospects of the proposed methodology are outlined in detail. The new approach is comprehensively discussed for a lab-scale stirred media mill operated at exemplary process conditions. The transfer to other mills or unit operations, however, is straightforward. [ABSTRACT FROM AUTHOR]

Stöber Fink Bohn (SFB) silica is a well-known colloidal system: monodisperse spherical particles are readily obtained by this wet chemical synthesis route. Despite the frequent application of SFB silica, its mechanical properties are not comprehensively understood. Within this account, size dependent mechanical properties of single SFB particles are systematically investigated using in situ SEM and TEM techniques: particle properties are addressed in the size range of 1.5 μm down to 200 nm. Both, untreated particles and heat treated particles densified at 1000 °C are investigated. Structural characterization by vibrational spectroscopy, by solid state nuclear magnetic resonance spectroscopy, by scanning electron microscopy and by focused ion beam machining allows correlating mechanical properties and the internal particle structure. For untreated particles comprised of a weakly cross-linked silica network a high degree of plasticity associated with low Young's moduli and hardness values is found. At large strains only cracking without full fracture of the particles occurs, whereby the crack path is clearly linked to the synthesis conditions: for larger particles obtained from a multistep growth process cracks propagate along weak intraparticle interfaces and are deflected around an inner core structure. After the thermal treatment cross-linking of the silica network is significantly enhanced and Young's moduli comparable to vitreous silica accompanied by increased hardness values are found. Only the smallest particles exhibit cracking; fracture into two or more pieces is distinguished for particles above 500 nm. It is found by SEM imaging that the relative occurrence of cracking into several pieces is increasing with size — a behaviour associated with the brittle to ductile transition of silica. By testing a large quantity of particles, statistical significance is ensured. [ABSTRACT FROM AUTHOR]

We report on the development and characterization of a novel in situ manipulation device to perform stressing experiments on the submicron scale inside a high resolution field emission scanning electron microscope. The instrument comprises two main assembly groups: an upper part for positioning and moving a mounted probe and a force sensor as well as a specimen support as lower part. The upper part consists of a closed loop tripod piezoelectric scanner mounted on a self-locking coarse positioning stage. Two interlocked steel springs and a linear variable differential transformer measuring the springs' deflections compose the lower part of the instrument. This arrangement acts as force-sensor and sample support. In comparison to already well-established concepts a wide measuring range is covered by adjusting the spring constant between 30 N/m and 50000 N/m. Moreover, the new device offers striking advantages with respect to force calibration and sample deformation measurements. Force calibration is performed using the eigenfrequency of the force detection system directly inside the SEM. Deformation data are obtained with high accuracy by simultaneously recording displacements above and below the specimen. The detrimental apparatus compliance is determined, and the influence on measured data subsequently minimized: an easy to validate two-springs-in-series model is used for data correction. A force resolution in normal direction of 100 nN accompanied by a sample deformation resolution of 5 nm can be achieved with the instrument using an appropriate load cell stiffness. The capabilities and versatility of this instrument are exemplified by compression experiments performed on submicron amorphous silica particles. [ABSTRACT FROM AUTHOR]

Milling processes in stirred media mills can be essentially described by the collision bead frequency distribution (stress number) and the transferred stress energy distribution. Although general dependencies on the mills' operating parameters have been extensively studied, the underlying distributions are widely unknown. Recently, we established an experimental protocol to determine the aforesaid distributions by using well-characterized spherical metal particle probes. To this end, the plastic deformation of the particle probes which is caused by stressing in the apparatus is accessed by electron microscopy and is related to single particle compression testing described via a finite element model. Within this account we focus on the influences of grinding media size and fluid viscosity on the stressing conditions in a small-scale horizontal stirred media mill which is operated in open-circuit mode. We show that larger milling beads lead to higher stress energies, whereas an increase of viscosity leads to lower transferred stressing energies. Surprisingly, however, the overall amount of stressed particle probes decreases with the milling bead size. At the same time the fraction of the probes which has been stressed shows many contacts. We relate this effect to the slurry flow through the agitated packed milling bead bed. The effect of viscosity is attributed to squeezing flows which form as a result of approaching milling beads. [ABSTRACT FROM AUTHOR]

Graphical abstract Single particle probes are used to characterize stress intensity and stress number distributions for different grinding bead sizes and fluid viscosities in a stirred media mill which is operated in open circuit mode. Highlights • Assessing stressing conditions in a stirred media mill by single particle probes. • Single particle compression and finite element modelling. • Influence of grinding media size and fluid viscosity. • Unravelling capturing efficiency and stress energy. • Deconvolution of bead dampening and particle capturing efficiency. Abstract Milling processes in stirred media mills can be essentially described by the collision bead frequency distribution (stress number) and the transferred stress energy distribution. Although general dependencies on the mills’ operating parameters have been extensively studied, the underlying distributions are widely unknown. Recently, we established an experimental protocol to determine the aforesaid distributions by using well-characterized spherical metal particle probes. To this end, the plastic deformation of the particle probes which is caused by stressing in the apparatus is accessed by electron microscopy and is related to single particle compression testing described via a finite element model. Within this account we focus on the influences of grinding media size and fluid viscosity on the stressing conditions in a small-scale horizontal stirred media mill which is operated in open-circuit mode. We show that larger milling beads lead to higher stress energies, whereas an increase of viscosity leads to lower transferred stressing energies. Surprisingly, however, the overall amount of stressed particle probes decreases with the milling bead size. At the same time the fraction of the probes which has been stressed shows many contacts. We relate this effect to the slurry flow through the agitated packed milling bead bed. The effect of viscosity is attributed to squeezing flows which form as a result of approaching milling beads. [ABSTRACT FROM AUTHOR]