Stirrer Tip Speed Research Articles

Characterization Data for the Establishment of Scale-Up and Process Transfer Strategies between Stainless Steel and Single-Use Bioreactors

The reliable transfer of bioprocesses from single-use bioreactors (SUBs) of different scales to conventional stainless steel stirred-tank bioreactors is of steadily growing interest. In this publication, a scale-up study for SUBs with volumes of 200 L and 2000 L and the transfer to an industrial-scale conventional stainless steel stirred-tank bioreactor with a volume of 15,000 L is presented. The scale-up and transfer are based on a comparison of mixing times and the modeling of volumetric mass transfer coefficients kLa, measured in all three reactors in aqueous PBS/Kolliphor solution. The mass transfer coefficients are compared with the widely used correlation of van’t Riet at constant stirrer tip speeds. It can be shown that a van’t Riet correlation enables a robust and reliable prediction of mass transfer coefficients on each scale for a wide range of stirrer tip speeds and aeration rates. The process transfer from single-use bioreactors to conventional stainless steel stirred-tank bioreactors is proven to be uncritical concerning mass transfer performance. This provides higher flexibility with respect to bioreactor equipment considered for specific processes.

Effect of operating conditions on the particle size distribution and specific energy input of fine grinding in a stirred mill: Modeling cumulative undersize distribution

Simulation of the fine or ultra-fine grinding process by Population Balance Model (PBM) has its restrictions because some coarse particles maybe disappear or the breakage rate exhibits complex non-first-order breakage kinetics during the grinding process. In this study, the influence of grinding media diameter, stirrer tip speed, media fulling rate, and powder filling rate on the uniformity of particle size distribution and specific energy input were studied in a stirred mill. The cumulative undersize distribution of ground products was simulated by the Rosin-Rammler-Benne (RRB) function, from which the parameter k and characteristic particle xe of the RRB function were obtained. The time-dependence of the parameter k was investigated. And, the Walker model for describing the reduction in characteristic particle xe with specific energy input was examined. Based on these results, combining the Walker model with the RRB function to simulate the cumulative undersize distribution of ground products. Thereby, a new cumulative undersize distribution of the prediction model was developed, which was successfully validated by the experimental data against predicted data.

Comparison of open and closed circuit mode using a dry horizontal stirred media mill with special regard to the powder flowability and residence time distribution

A dry operated, horizontal stirred media mill was used for the fine grinding of limestone and compared in open and closed circuit modes. It was found that the choice of grinding aids as well as the stirrer tip speed are crucial for the mill operation, especially in terms of the grinding efficiency and the transport behavior through the mill. Independent of the process mode, both a too low and an excessive additive-induced flow behavior led to unfavorable stress conditions inside the mill, and thus, inefficient grinding. In the open circuit, especially high flowabilities turned out to be crucial, as thereby the stress conditions are negatively affected as a consequence of the material transport in combination with the overlapping impact of a mill- internal deflector wheel. Different important aspects were seen for the closed circuit grinding; especially when aiming for high finenesses or using high tip speeds, it is essential to ensure both a minimum flowability and a sufficient particle stabilization to ensure a fast removal of fine particles from the mill. High flowabilities in turn are not as critical during closed circuit grinding, but are accompanied by high material recirculations.

Effect of grinding media properties and stirrer tip speed on the grinding efficiency of a stirred mill

Owing to their high energy efficiency, stirred mills are commonly used in the fine or ultrafine grinding of low-grade and fine disseminated ores to enhance liberation. In this study, batch wet grinding studies were conducted in a stirred mill to evaluate the effect of the grinding media density, grinding media diameter, and stirrer tip speed on the ground products and specific energy input. The population balance model was used to simulate the particle size distribution of the ground products. The results obtained indicated that the breakage rates S1 and S2 were highly dependent on the operating parameters, while the breakage distribution value b21 was independent of the grinding media density and stirrer tip speed. Additionally, it was observed that the yield of the desired size (−45 + 10 μm) of ground products increased with grinding media diameter. With the increase of grinding media density and stirrer tip speed, the time efficiency of the grinding process was improved. Moreover, the attainable region method via a contour plot was used to produce the maximum amount of the desired size with lower energy. Furthermore, the results revealed that the grinding process became energy inefficient when the grinding time was close to the turning point.

Impact of ball filling rate and stirrer tip speed on milling iron ore by wet stirred mill: Analysis and prediction of the particle size distribution

The wet grinding of iron ore powder was investigated using a stirred media mill. In this study, the fitting degree of four cumulative undersize distribution functions was compared and the parameters of the four functions were discussed. It was concluded that the particle size distribution (PSD) of grinding products followed well with the Rosin Rammler Benne (RRB) function. In addition, operational parameters such as ball filling rates, stirrer tip speeds and grinding time have been investigated in terms of the width of PSD and specific energy input. The results indicated that slight differences in the width of PSD could be observed at different ball filling rates and stirrer tip speeds. It was also found that increasing the ball filling rate and stirrer tip speed could improve energy efficiency. Furthermore, an equation was established to predict PSD based on the characteristic particle size of the RRB function and size-energy model, which provided a sound fit to the grinding products under various grinding conditions

Scalable production of glass flakes via compression in the liquid phase

A comprehensive study on controlled shape formation with high yield of three commercially relevant SiO2-based amorphous glasses in a stirred media mill is presented. Stressing under well-controlled conditions leads to micron-sized amorphous glass flakes with high aspect ratios. This unique result is quite contrary to the fundamental observation that comminution processes generally result in irregular shaped particles. The influences of glass composition, processing time, stirrer tip speed and grinding media size on the obtained products have been investigated. The size and shape of the obtained glass flakes have been characterized by scanning electron microscopy and atomic force microscopy. The glass flakes exhibit thicknesses as low as 155 nm while the lateral dimensions are well within the micrometer range. Our study shows that particle size reduction occurs within the first hour of grinding. Afterwards plastic deformation of the fragments, which can be accompanied by densification of the glass network, leads to the formation and further thinning of the glass flakes. To demonstrate the quality and applicability of the glass flakes interference pigments were realized by a TiO2 coating via an aqueous titration process. The presented approach offers a simple, convenient and fully scalable top-down method to produce flake-like particles from various silica glasses. The obtained flakes are suitable substrates for further modifications and applications and the process can be transferred to other materials and glasses with tailor-made chemical compositions.

Impact of stress conditions on stirred media milling of a two component mixture

Grinding of a material mixture is common in a variety of industries, even though, impacts on the multicomponent grinding result are not fully understood yet. Within this study, the wet fine grinding of a model system consisting of limestone quartz mixtures was investigated using a stirred media mill. The influence of the relative component grindability, as well as the component mixing ratio and the stress energy, which was adjusted by different stirrer tip speeds and grinding media sizes, were examined. The knowledge of the breakage behavior of the individual components within the mixture is the basis to understand the process. Hence, a new particle sample preparation method was developed; enabling the measurement of particle sizes separately for each component in the mixture. The results were interpreted according to the energy split factor approach, which was first conceived for ore grinding but also seemingly works for fine grinding purposes. Thus, the dependency of the component particle size was discussed in relation to the total as well as to the component specific energy. A significant effect of the mixing ratio and the adjusted stress energy on the single component grinding efficiency in mixture could be elucidated.

Influence of the Carbon Black Dispersing Process on the Microstructure and Performance of Li‐Ion Battery Cathodes

The influence of the dispersion process and the carbon black (CB) particle size on the resulting structure and, hence, on the properties of lithium‐ion battery cathodes is investigated. N‐methyl‐2‐pyrrolidone‐based cathode slurries with 95.5 wt% (NCM622) and a high mass content (82.5 wt%) are processed in a planetary mixer PMH10 from NETZSCH with varying high‐speed stirrer tip speeds. Particle size analyses are carried out to measure the CB particle size at different time steps of the dispersion process. The resulting cathodes are characterized to determine mechanical and electrical properties. The microstructure of chosen electrodes is reconstructed and quantified by focused ion beam/scanning electron microscopy tomography and correlated with experimental data. In addition, discrete element method simulations are used for a deeper understanding of the dispersion process and breakage of CB aggregates. Correlations between process, structure, and properties of lithium‐ion battery cathodes are revealed.

Mill, material, and process parameters – A mechanistic model for the set-up of wet-stirred media milling processes

Stirred media milling is frequently used to generate nanoparticles for industrial applications such as paints, inks, and food or for the life sciences. Each product suspension has different requirements and therefore different material and formulation parameters. The first attempts to set up a new process are experimental in nature, especially the determination of a suitable composition. To adapt a process to the production scale, more experimental work is often needed to determine suitable operation parameters with regard to energy consumption, throughput, and investment cost. The energy consumption is influenced by operation parameters such as the size of the grinding media or the stirrer tip speed, whereas the investment costs are influenced by the mill geometry and size and the type of grinding media used. Therefore, it is challenging to transfer or scale up processes because lab-scale mills are smaller and may have different geometries than production-scale mills. Moreover, it is well known that the lab-scale operation parameters cannot be easily adapted to the production scale. In this study, the stress model developed by Kwade was improved by introducing parameters corresponding to the mill and the material in addition to the process parameters. Using this model, the optimum operating conditions for stirred media milling processes can be determined with a reduced amount of experimental work, even for geometrically unequal mills.

Grinding Kinetics of Lotus Leaf Powder in a Stirred Media Mill

Abstract. Stirred media mills are invoked as high energy utilization equipment to prepare superfine particles with a narrow particle size distribution. In this study, the effects of the stirrer tip speed and the grinding media diameter on the particle size distribution of lotus leaf powder (LLP) were investigated. The stress intensity and stress number, which were determined by the stirrer tip speed and grinding media diameter, were applied to estimate the wet grinding kinetics of LLP. The stress conditions with the minimum specific energy consumption were a grinding media diameter of 1.4 mm and stirrer tip speed of 8.4 m s-1. Additionally, FT-IR measurements of the samples confirmed that the chemical structures of the active ingredients were not damaged with reduction in particle size, which indicated the mildness of the grinding process.HighlightsA stirred media mill was initially applied for preparing superfine lotus leaf powder.The stress conditions with the minimum specific energy consumption were determined.The active ingredients were not denatured during the grinding process. Keywords: Grinding kinetics, Lotus leaf, Stirred media mill, Stress intensity.

Breakage, temperature dependency and contamination of Lactose during ball milling in ethanol

Wet stirred media milling is an established method for the production of drug nanoparticles which are increasing the bioavailability of poorly water-soluble active pharmaceutical ingredients (APIs). While the processability of various organic materials has been proved in aqueous suspensions, this work focuses on the milling of the organic substance Lactose in ethanol. The aim of this study was the comparison of these results with the former studies in stirred media mills with organic materials in water as well as the analysation of recrystallization processes induced by increasing suspension temperature. Regarding the milling performance different grinding media materials as well as sizes, stirrer tip speeds and the solids contents of the suspension were varied. Furthermore the amount of generated product contamination was measured and a correlation to the process parameters and the used grinding beads was analysed. A convincing valuation of the milling results is only possible if there are no agglomeration effects in the suspensions. Therefore, at the beginning a stabiliser screening with different ionic and non-ionic additives was performed in a planetary ball mill. This showed an effect of the process temperature on the recrystallization behaviour of the particles and, thus, the final product fineness and particle shapes what was furthermore confirmed by milling the Lactose under controlled conditions in a stirred media mill.

Effect of operational parameters and stress energies on stirred media milling of talc

Talc powder with a size of around 10μm is used in many industrial applications such as in paints and coatings. A special feature of talc is its exceptional particle shape: platy with a three-layer structure. The properties for some applications are improved when talc platelets are dispersed to individual platelets in a wet stirred media mill. This study examined the effects of the operational parameters of a stirred media mill on the particle size and width of the particle size distribution (PSD) of talc. The aim was to find optimal operational parameters of a stirred media mill in order to minimize specific energy consumption and to reduce particle size. The particle size reduction of talc platelets was slow, due to the delamination step in the beginning of the grinding process. After delamination, the individual platelets were broken down, which was regarded as a stronger decrease in particle size as a function of specific energy. The PSD broadened as grinding proceeded, due to the platy shape of talc platelets. The smallest particle size with the lowest specific energy was obtained when the stress energy of the grinding media was around 100μJ. The operational parameters at this point were the stirrer tip speed of 10m/s and yttria-stabilized zirconium oxide beads with a size of 559μm.

Formation of Nanoemulsions by Stirred Media Milling

Emulsions are widely used in the pharmaceutical and food industry. In recent years an increasing use of emulsion-based delivery systems for bioactive substances involving small particle sizes is notable. Apart from spontaneous formation of microemulsions, in general energy has to be introduced into the system, i.e. the formation process of nanoemulsions (thermodynamically metastable systems) is not spontaneous and the droplet-breakup process needs application of external energy input. Within this account we show that highly stable nanoemulsions can be produced by an alternative and scalable process applying stirred media milling. Alkane (hexadecane, dodecane)-Tween85-water-systems have been used as model emulsion systems to demonstrate the feasibility of the approach. The influence of fundamental process parameters such as emulsion formulation (c.f. oil mass fraction and oil-to-emulsifier mass ratio), stressing conditions (grinding bead size and stirrer tip speed), process time and especially temperature on the emulsion droplet size and size distribution has been studied in detail. Low oil-to-emulsifier ratios are most appropriate with respect to obtaining small mean droplet sizes and narrow droplet size distributions. It is shown that the smallest mean droplet sizes (at constant oil-to-emulsifier ratio) are obtained at the lowest oil mass concentrations. Product emulsions of Sauter diameters x1,2 below 25nm have been obtained in the hexadecane-Tween85-water system using ZrO2 grinding beads of 100μm and 400μm in diameter, respectively. Interestingly, working at high stress intensities is not appropriate: higher stress intensities lead to a more intense mixing that increases the coalescence rate. Remarkably, lowering the process temperature below the solidification temperature of the oil phase, i.e. ‘grinding’ of the solid oil, is advantageous as compared to emulsification of the liquid oil: Coalescence and ripening can be significantly reduced. Smallest product droplets with x1,2 < 10nm are obtained in the hexadecane-Tween85-water system after a short processing time of clearly below 1 hour. The feasibility of the process proposed to produce nanoemulsions using oil phases loaded with the hydrophobic organic compounds pyrene and trans-stilbene is demonstrated.

Attritor-milling of poly(amide imide) suspensions

The milling behavior of poly(amide imide), which serves as a prototypical hydrophilic high-performance polymer with a high glass transition temperature, was investigated. Various milling conditions (milling times up to 7h, stirrer tip speeds of 3.4–4.9m/s, and mass concentrations of 5–20%) were tested, and particle sizes as low as d50,3∼3μm were obtained. The milling was performed at 11°C in an attritor. Differential scanning calorimetry and thermogravimetric analysis were performed before and after milling to investigate the effect of milling on the glass transition temperature and on the decomposition behavior of the polymer. The suspension obtained after milling was observed to be stable without the addition of stabilizers or the adjustment of the pH value, and no negative effect of milling on the polymer properties was observed. The attritor technique proved to be an adequate and efficient milling tool for the production of micrometer-sized high-performance polymer suspensions.

Formation of nanoemulsions in stirred media mills

Alkane–Tween85–water-systems have been used as model emulsion systems to demonstrate the feasibility of emulsification in stirred media mills. The influence of fundamental process parameters, i.e. emulsion formulation (oil mass fraction and oil-to-emulsifier mass ratio), stress conditions (grinding bead size and stirrer tip speed), process time and especially temperature on the emulsion droplet size and size distribution has been studied for this approach. By lowering the process temperature below the solidification temperature of the oil phase, coalescence and ripening can be significantly reduced. Smallest product droplets with x1,2<10nm thus are obtained even for nominal oil mass fractions up to 20wt% in the hexadecane–Tween85–water system. The feasibility of the process proposed to produce nanoscale emulsions using oil phases loaded with compounds such as pigments and other fillers is demonstrated.

Prediction of energy effective grinding conditions

Grinding processes in general are extremely energy intensive. In order to optimize the energy consumption the choice of the process parameters is important. The decision on the process parameters often depends on experiences or a certain number of experiments before starting a process.Here a model will be shown which enables the prediction of optimum process parameters for ceramic materials in wet stirred media milling. The model was built on the basis of the model of stress energy. The optimum stress intensity (optimum stress energy related to particle volume) is correlated with the compression strength, due to the assumption that pressure is dominant type of stress in stirred media mills. The term describing the strength of the particles is correlated with the material property parameters taking the grinding limit into account. The enhanced model enables the prediction of grinding media size and stirrer tip speed for certain desired particle finesses, if material parameters like Young’s moduli and density of product and grinding media as well as the compression strength are known. This model can be used for different mill types if different the stress energy distributions are taken into account.

Effect of operational parameters and stress energies on the particle size distribution of TiO2 pigment in stirred media milling

Titanium dioxide with a particle size of approximately 0.3 μm is widely used as a pigment because of its notably high refractive index and significant scattering properties. To achieve optimum scattering in the visible range (400–700 nm), it is important to obtain a narrow particle size distribution, between 0.2 μm and 0.4 μm. TiO2 particles are usually finely ground to the desired particle size range using a wet-stirred media mill. In the present case this fine grinding was performed in the multiple passage mode using a laboratory-scale stirred media mill. Optimum values for the operational parameters were determined, and the stress energies were varied to obtain different stress mechanisms, and thus different particle size distributions. The narrowest particle size distribution around the target of 0.3 μm was achieved with operational parameters resulting in the lowest stress energy and the lowest specific energy consumption. The operational parameters at this point included a 255 μm Zirconium oxide grinding media, a stirrer tip speed of 9.2 m/s, and a solid concentration of 52 wt.% and a filling ratio of 70 vol.%. Comminution was expected to largely follow a compressive stress mechanism when this optimal combination was employed.

Generation of wear during the production of drug nanosuspensions by wet media milling

Wet media milling is an established technique for the commercialized top-down production of nanoparticulate drug suspensions. These drug nanosuspensions can be transferred into the related drug products, like capsules, tablets and injectables. The generation of wear during stirred media milling of a drug compound was investigated for grinding media made from yttrium stabilized zirconia. Drug compound and drug nanosuspension were characterized initially by their mechanical and rheological properties. The generation of wear from grinding media has been investigated simultaneously with the reduction of drug particle size by evaluating several grinding media supplier and diameter as well as process parameters stirrer tip speed and specific energy input. Grinding media quality and process parameters were identified with strong impact on the amount of generated wear and on drug particle size distribution. Wear from grinding media characterized by elemental zirconium and yttrium could be significantly minimized by operating with the favored grinding media quality and with optimal stirrer tip speed and specific energy input. Wear debris, respectively wear particles from grinding media, were identified with respect to morphology and particle size. Finally, the overall contamination by raw materials and by wear during processing characterized by elemental iron, silicium, yttrium and zirconium as well as the mean size of contamination particles are presented for selected drug nanosuspensions.

Role of Milling Parameters and Particle Stabilization on Nanogrinding of Drug Substances of Similar Mechanical Properties

The feasibility and efficiency of wet-nanogrinding of three drug substances (miconazole, itraconazole, etravirine) with similar elastic and plastic properties proved to depend primarily on the adequate electrostatic and steric stabilization of the nanoparticles and the specific energy input. Particle stabilization was provided by sodium dodecyl sulfate (SDS) and hydroxypropylcellulose. The specific energy input was defined by the grinding time, grinding bead size, and stirrer tip speed. Miconazole and itraconazole exhibited similar milling behavior, whereas etravirine nanosuspensions revealed agglomerates and increasing viscosity with increasing specific energy input. Agglomeration and viscosity increase were successfully counteracted by increasing the SDS concentration in the nanosuspension from 0 to 0.125 %. Under the provision of proper particle stabilization, the three drug substances could be nanosized to a mean size of ∼ 130 nm, with 90 % of all particles being smaller than ∼ 250 nm.

The Effect of Grinding Media Performance on Milling a Water‐Based Color Pigment

AbstractThe effect of grinding media performance on milling and operational behavior is demonstrated for different selected conditions of a water‐based red pigment slurry milling. A variety of grinding media materials and bead sizes, along with two different stirrer tip speeds, are used in the grinding process to reduce the particle size in order to increase the color strength of the red pigment. To determine the optimum milling parameters regarding the particle size and the pigment color, the collected test data is used to calculate and evaluate the specific energy and stress intensity under different milling conditions.