Attrition process

Unfortunately, the basic physical mechanisms that control the attrition process are still poorly understood. As a consequence, particular test methods are used to evaluate the degradation tendency of the materials or to predict the rate of attrition for a given process. There are a lot of procedures using widely different devices and operations. Some of them observe the degradation of only one individual particle, whereas others treat a considerable amount of material. The particles are subjected to stress systems which range from well-defined ones like impact or compression, to those which are similar to the more or less randomized stresses occurring in natural processes. Section 4 attempts to summarize the huge variety of attrition tests in a systematic way. 

Fig. 4. General concept and experimental devices to determine the material function for attrition processes.

The results of the attrition experiments are compared to various material properties in order to identify those relevant for attrition. The long term goal is to develop a physical model of the attrition process which makes it possible to predict the amount of attrition that will be encountered in a pneumatic conveying installation by knowledge of the process and material functions. 

For the attrition experiments approximately 25 g of polymer particles were used. After three, six and nine consecutive impacts with a velocity of 40 m/s, the attrition rate A was determined as the relative loss of mass (see Eq. (1)). This procedure was necessary since the attrition rates for some polymer classes were not measurable with satisfying accuracy for lower impact numbers. Since the attrition process is highly statistical, each experiment was repeated three times. This also holds for the other experimental setups. In the following diagrams, the median values of these three repeated experiments are plotted with the standard deviations as error bars. For all experiments, the attrition rate A was calculated according to Eq. (1), where M denotes the initial particle mass. Ma is the mass of particles at the initial size after the attrition experiment. 

This is due to the fact that the respective measurement procedures for their determination do not take into account the dynamic nature of the attrition process. Therefore, in the present study dynamic mechanical analysis (DMA) was employed. 

The results presented show that three levels have to be distinguished when investigating attrition processes. The first one is the stress mode as derived from the process function which is essential to know if the attrition process is to be simulated successfully in a simple experimental setup. The second point is the material reaction to this stress mode, i.e. the material function which varies depending on material properties like storage and loss modulus as measured by DMA. Finally, the microscopic attrition mechanisms (see [18] for impact and [19,20] for sliding friction) describing the formation of attrition on a microscopic scale constitute the bottom level. 

Nevertheless, the presented results indicate that if it is carefully distinguished between the process function (stress mode), the material function (material specific attrition mechanisms) and microscopic attrition mechanisms (attrition formation on the microscale) in the analysis of attrition processes, significant progress in the understanding of the complex phenomenon of attrition can be made. 

The attrition process is described with a surface-proportional attrition coefficient. 

Since separation of stereisomers requires that this type of crystallization be all-growth, some means for creation of new seed particles must be provided for any crystallizer type. These particles can be added from an external source or can be created internally by attrition. An internal attrition process would be preferred to minimize the number of operations. 

Several hydrodynamic models of secondary nucleation in agitated crystallizers were applied to experimental data obtained from a 6-L agitated batch crystallizer using potassium sulphate by Shamlou, Jones and Djamarani (1990). They concluded that the secondary nuclei were produced by an attrition process with a turbulent fluid-induced mechanism with critical eddies in the... 

The equation of state. Eg. (10), can also be employed for determining the surface energy of fillers. Examples for the fillers investigated are an N330 carbon black and an attrited N330 carbon black. The attrition process of the carbon black used consists of two parts. We first heat the carbon black in a vacuum oven at 150 C for 24 hours. We then ball-milled the carbon black at 25 -225 C for 48 hours. The amount of carbon black put into the ball mill container was about one third of the volume of the container. 

As it is obvious from the above example, the individual modes do usually not occur separately. They are rather combined in varying proportions, which makes a description of the entire attrition process rather complicated. The only attrition mode that can occur separately is abrasion, since it has the lowest threshold energy. The other attrition modes, the threshold energy of which is much higher, are at least combined with abrasion. In general, the extent of abrasion in relation to fragmentation depends on various factors that will be discussed below. 

The assessment on the basis of ehanges in the particle size distribution considers the left-hand side of Eq. (5), i.e., the absolute change in mass Am,- is measured. But the individual contributions that led to these changes, i.e., the terms on the right-hand side of Eq. (5), are not considered by this type of assessment. It may be possible to take the contributions of feed and system loss by means of measurements into account, but it is not possible at all to unravel the internal transfer masses rrif j and between the size intervals just by observing the changes in the particle size distribution. For this reason, it is not possible to extrapolate the description to a different duration of the attrition process or even to transfer it to other initial particle size distributions. For this purpose a description via particle population balances is needed. 

Figure 7.27 shows the evolution of particle size distribution for a trial with 4 wt% of binder. The hold up material grows with time. Moreover, a fraction of very small particles appears after approximately 2.5 h. This peak is caused by breakage or attrition processes producing internal nuclei. 

Flow patterns of high-shear impellers, such as the bar turbine, Chemshear, and sawtooth impeller, are similar to those of radial flow impellers. The major difference is in lower pumping at higher shear. Backswept turbine and spring impeller also have similar radial flow patterns. It is important to understand the flow patterns around the impeller blades, where dispersion and attrition processes occur. Changing the blade geometry changes these flow patterns and alters the shear. 

The catch can also contain an error due to breakage and attrition, processes which are very difficult to trace. 

Several potential mechanisms exist for the attrition process with the breaking energy of particles originating from either bulk circulation e.g. Ottens and de Jong (1973), Nienow and Conti (1978), Conti and Nienow (1980), Kuboi etal. (1984), Laufhiitte and Mersmann (1987), PloC and Mersmann (1989) or the turbulent motion of the fluid e.g. Evans etal. (1974), Glasgow and Luecke (1980), Jagannathan etal. (1980), or both e.g. Synowiec etal. (1993). 

If it is assumed that the crystal-crystal attrition process is a function of ... 

Pumping does not affect the average x-ray cry.stallite size of slurry oxides [52]. Also, oxides which have been pumped as slurries, dried, and then calcined, show relatively little crystallite growth. From these considerations it would seem that the crystallite size as measured by x-ray diffraction line broadening represents the ultimate limit of the attrition process due to pumping. 

A great deal of interest has been shown in the concept of powder mixing polychloroprene. Certain grades are commercially available in powder form, being produced from the standard chip by an attrition process and maintained in that form with the aid of 5% of precipitated silica as... 

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