Jürgen Tomas

Impact crushing of concrete for liberation and recycling

Abstract A test rig (large-scale pneumatic cannon) has been designed, constructed and tested. This apparatus allows the adjustment of intensive stressing conditions, e.g., impact and double impact, single and multiple stressing, velocity and energy. The accelerating tube diameter limit was given by the condition of a representative feed sample (model concrete spheres 150 mm sample diameter limited by an upper particle size of aggregate do=16 mm). The crushed fragments are described as subcollectives (truncated logarithmic normal distributions) of a multimodal distribution function. For impact stressing, liberation grades between 0.3 and 0.6 (0% adherent hardened cement paste) as well as more than 0.9 (0–50% adherent hardened cement paste) were obtained.

Expression behaviour of fine particle suspensions and the consolidated cake strength

Abstract The high-level expression of suspensions up to 50 bar is concerned with the recovery of finest particles, e.g. submicron particles. A so-called Press-Shear-Cell was designed, constructed and tested to measure the most important filtration process parameters. In a first filtration stage, a compressible filter cake builds up from a fine particle suspension. The following cake consolidation is applied to reduce the liquid amount in the small pores of the particle packing. To describe the two-stage expression process of fine sludge, a new dynamic model is introduced. In contrast to the common Tiller–Shirato model, which assumes constant average material properties of particle pressure ps, packing density es and permeability k inside the cake during constant pressure expression, the new dynamic expression model considers the time and local variation of these material properties. Furthermore, the wall friction resistance is taken into account with the wall friction angle ϕW and the lateral pressure ratio λ of the cake. The pressure-dependent constitutive functions of kaolin and titanium dioxide filter cakes are shown. Yield loci in a water saturated particle packing were in situ measured with comparatively high normal loads of 100–1000 kPa.

Toxicity of polymeric nanoparticles in vivo and in vitro

Polybutylcyanoacrylate nanoparticles (PBCA NPs) are candidates for a drug delivery system, which can cross the blood–brain barrier (BBB). Because little is known about their toxicity, we exposed cells to PBCA NPs in vitro and in vivo and monitored their life and death assays. PBCA NPs were fabricated with different surfactants according to the mini-emulsion technique. Viabilities of HeLa and HEK293 cells after NP incubation were quantified by analysing cellular metabolic activity (MTT-test). We then repetitively injected i.v. rhodamine-labelled PBCA NP variations into rats and monitored the survival and morphology of retrogradely labelled neurons by in vivo confocal neuroimaging (ICON) for five weeks. To test for carrier-efficacy and safety, PBCA NPs loaded with Kyotorphin were injected in rats, and a hot plate test was used to quantify analgesic effects. In vitro, we found dose-dependent cell death which was, however, only detectable at very high doses and mainly seen in the cultures incubated with NPs fabricated with the tensids SDS and Tween. However, the in vivo experiments did not show any NP-induced neuronal death, even with particles which were toxic at high dose in vitro, i.e. NPs with Tween and SDS. The increased pain threshold at the hot plate test demonstrated that PBCA NPs are able to cross the BBB and thus comprise a useful tool for drug delivery into the central nervous system (CNS). Our findings showing that different nanoparticle formulations are non-toxic have important implications for the value of NP engineering approaches in medicine.

Measurement of the degree of internal friction of two native silica packing materials

The degree of internal friction of two different packing materials for liquid chromatography (Kromasil NP10 and DuPont Pro Sil) has been measured using conventional methods of the mechanics of continuous solids applied in the study of the storage, handling and processing of powders. The former material was found to have a lower angle of internal friction and to flow more freely than the latter one. Possible relationships between these properties and the qualities of the column beds obtained are discussed.

Flow Properties of Cohesive Powders Tested by a Press Shear Cell

The most important design parameters for roller presses can be referred to flow and compression characteristics of bulk materials. Usually the flow properties are measured in the low stress range 1–50 kPa at the shear rate of about 1 mm/min. But this does not fit the stress regimes in the roller press. Therefore, the compression and flow behavior of the powder have to be investigated at higher pressures, shear rates, and shear displacements. These properties of bulk materials in the so-called medium pressure range 50–1000 kPa can be analyzed using a press shear cell. Tests were implemented with limestone, bentonite, and microcrystalline cellulose at average 23°C powder bed temperature using shear rates from 0.00042 to 0.042 m/s and a more realistic preshear displacement from 0.1 to 2 m for practical applications in powder compaction. Physical observation based compression functions were developed for the low and medium pressure range, which include simple equations for the compression rate and specific compression work.

Compression, permeation and flow behavior of wet nanoparticle cakes, in situ tested with a press-shear cell

A new test apparatus for in situ simulation of the expression operation and rheological testing of compressed cake was developed to combine the compression-permeability cell with a high-performance ring shear tester. The yield loci for water-saturated titania (dS = 200 nm) are measured. These flow parameters can be used to determine in situ the shear strength and compressive strength of a consolidated cake after the pressure filtration steps. The permeability of the compressed cake is characterized by the conventional filter cake resistance versus particle pressure and, for comparison, by an averaged pore size.

Liberation and Separation of Valuables from Building Material Waste

In general a recycling process starting with development, planning, production, utilization and disposal of a product, e.g., of a building, during its lifetime may be regarded as a system of material cycles. The main object is to minimize the input of raw materials and the output of waste for every stage of a building. These material cycles may be designed by reprocessing the building material waste to valuables of high quality. To approach this aim a joint research project dealt with the liberation and separation of aggregates (d = 2–16 mm) from concrete waste. The liberation is accomplished by a pneumatic cannon to simulate the stressing in impact crushers. With this cannon spherical concrete samples (d = 150 mm) are comminuted. The comminution is evaluated by the liberation grade. Fully liberated, partially liberated and non-liberated particles are distinguished. The experiments show an unexpectedly high liberation grade of up to 95% with a comparably small specific energy consumption of about 1 kWh/t. For separating both fully liberated and partially liberated aggregate, a pilot scale zigzag apparatus consisting of a zigzag channel, an air cyclone, a blower, a filter and a feeding system was used. If the diameter of the particles is kept nearly constant (e.g., by sieving) a multi-stage density separation in a turbulent cross-flow channel can be carried out. The separation performance may be evaluated by means of the separation function or grade efficiency curve, respectively. For the partially liberated aggregate a comparably good grade sharpness of about 0.74–0.84 is obtained for a 15-stage zigzag apparatus with different particle size fractions and particle contents in air.

Oblique impact simulations of high strength agglomerates

Different type of particle compounds like concrete particles can be considered as a model material of high strength agglomerates. It is necessary to investigate and understand the fracture behaviour of these agglomerates in order to avoid breakage during storage, handling and transportation. The aim of the research is to examine the comminution behaviour of high strength agglomerates during oblique impact loadings. A two dimensional finite element analysis has been carried out to understand stress pattern distributions before crack initiation. Then a two dimensional discrete element method has been performed to study the fragmentation behaviour of the agglomerates. Concrete particles of B35 strength category have been chosen to represent the high strength agglomerates. The analysis is done with oblique impact loadings at different velocities from 7.7 m/s to 180 m/s. The stressing conditions comprise low flow rate transportation and handling to high speed impacts during fall down in bunker, stock piles, ship loading or stressing in crushers and mill operations. Particle size distributions and new surface generation have also been evaluated in the paper. It is shown that at higher velocities, particle size distributions are identical to each other regardless of the impact angle. Increasing impact velocity does not necessarily produce more new surfaces after a certain velocity limit.

Effect of Applied Vibration on Silo Hopper Design

The vibration-promoted flow of cohesive particulate solids was investigated for limestone and titania powder, each with an average particle size of about 1 µm. The flow behavior of fine cohesive powders can be improved by the application of harmonic vibrations, as reflected in the flow properties. Based on the work of Roberts (1978, 1984), a shear testing technique has been established that allows the measurement of the effect of vibrations on the flow properties of particulate solids. The experimental results show that the vibration application leads to a significant reduction of the shear strength. The unconfined yield strength decreases with increasing vibration velocity, whereas the angle of internal friction is nearly independent of applied vibration. Also, the unconfined yield strength and the wall friction angle \varphi_{\rm W} can be reduced by applying mechanical vibration. Based on the lab-scale shear tests, the hopper half angle and the unconfined yield strength were estimated. For the given example, the conical outlet width is reduced from 0.88 m to about 0.4 m, and the maximum hopper angle increases from 5° up to approximately 20° due to the vibration application. powder vibration flow behavior shear cell

Simulation of Adhesive–Dissipative Behavior of a Microparticle Under the Oblique Impact

The adhesive–dissipative behavior of a microparticle under the oblique impact is investigated numerically and the new discrete element method (DEM)-compatible interaction model is elaborated. The modeling approach is based on the Derjaguin–Muller–Toporov model of normal interaction for the adhesive elastic contact. Adhesion hysteresis is specified by the loss of the kinetic energy governed by the fixed amount of the adhesion work, required to separate two adhesive contacting surfaces. This effect is captured in the new interaction model by adding an additional dissipative force component to normal contact during unloading and detachment. The essential feature of this approach, differing from that of the viscous damping model, is that, according to the proposed method, the amount of the dissipated energy is not influenced by the actual initial velocity during the entire contact. The influence of adhesion on slip friction is reflected by considering the adhesive normal force components in the Coulombs law of friction. The contribution of the adhesion-related dissipation is illustrated by a comparison of the behavior of the attractive–dissipative and attractive–non-dissipative models. The oblique impact of a microparticle on the plane surface at the intermediate impact angle is also investigated numerically. The link between adhesion and friction is supported by the numerical results.