Attrition definition

In order to evaluate the extent of attrition and its impact on the particle size distribution, there is a need of a qualitative and quantitative characterization. This, however, is not as simple as it may seem at first. There are many different properties, parameters and effects that manifest themselves and could be measured. In addition, as will be shown, the choice of the assessment procedure is strongly connected with the definition of attrition which, on its part, depends on the degradation mechanism that is considered to be relevant to the process. Hence there are a lot of procedures and indices to characterize the process of particle attrition. Section 3 deals with those which are relevant to fluidized beds and pneumatic conveying lines. 

Special attention has to be paid to a definition of attrition rates in the case of continuous processes where fresh solid material is continuously added. This is particularly the case in heterogeneously catalyzed fluidized bed processes where fresh make-up catalyst must be added to compensate for attrition losses. The fresh catalyst may contain elutriable fines which add to the measurable elutriation rate thus leading to an apparently higher attrition rate. 

It should be noted here that the above definition of the attrition rate considers the bed material as a whole. More insights into the influence of elements of the fluidized bed apparatus, e.g., of the cyclone or of the gas distributor may be obtained from the observation of the change in the particle size distribution as has been demonstrated by Zenz and Kelleher (1980) and Lin et al. (1980). 

Both Rajdistr andR are introduced here as fines production rates with the dimension kg/s, which is in contrast to the definition of the overall attrition rate defined in Eqs. (2) and (3). 

However, the present results already allow a conclusion to be drawn with respect to the solids mass flux mc loss which is lost from the cyclone as a result of attrition inside the cyclone. From Eq. (23) it follows withw = -0.5 and with the definition of Ra c... 

However, in most cases of poisoning, fouling, sintering, solid-state transformation, volatilization and attrition, the loss of activity is irreversible and definite, and in the best scenario only a fraction of the initial activity can be restored. 

Particle behavior is a function of particle size, density, surface area, and shape. These interact in a complex manner to give the total particle behavior pattern [28], The shape of a particle is probably the most difficult characteristic to be determined because there is such diversity in relation to particle shape. However, particle shape is a fundamental factor in powder characterization that will influence important properties such as bulk density, permeability, flowability, coatablility, particle packing arrangements, attrition, and cohesion [33-36], Consequently it is pertinent to the successful manipulation of pharmaceutical powders that an accurate definition of particle shape is obtained prior to powder processing. 

The term tooth wear is commonly used to describe the loss of tooth hard tissue due to non-carious causes [1], This encompasses a variety of both chemical and mechanical causes of both intrinsic and extrinsic origin. The term tooth wear is preferred over some of the more precise definitions of individual hard tissue loss mechanisms, because it acknowledges the fact that wear is usually a multifactorial process one mechanism may dominate, but the overall wear is commonly due to the interaction between two or more wear mechanisms. In dentistry, the terms erosion, abrasion, attrition and abfraction are widely used to describe particular mechanisms of hard tissue loss. 

The mechanisms of tooth wear fall into two distinct types those of chemical origin (e.g. erosion) and those of physical origin (e.g. abrasion, attrition). In any individual, both chemical and physical insults to the tooth hard tissue will be present in some form or other, so tooth wear is the combined effect of these insults. Despite the clear definition of a number of distinct tooth wear mechanisms, it is uncommon to find a single wear mechanism present in the... 

For other powder properties, such as bulk density, particle attrition, or dustiness, some standardized methods [160] exist that may serve as a guide to establishing a test method but may not be applicable in every case. For example, the dustiness of a material is a matter of definition, depending on the initial particle size of the tested product. If the determination of dustiness is combined with testing the attrition stability of the particles, the stress mechanism also has to be considered. Consequently, the methods for determining dustiness and particle attrition are mostly very specific to the product and the stress mechanism that is to be simulated [161]. In many other cases (e.g., solubility, moisture, and temperature sensitivity), no general standardized tests are available, and different approaches are nsed to describe the powder properties (see examples in Refs 110 and 162). 

Solids may be reduced in size by a number of methods Compression or crushing is generally used for reduction of hard solids to coarse sizes. Impact gives coarse, medium, or fine sizes. Attrition or rubbing yields fine products. Cutting is used to give definite sizes. 

This definition, however, includes effects of initial breakage and of initial fines content and will therefore normally not give an unambiguous assessment of attrition. 

Even though most of the attrition tests presented in the literature deal with bubble-induced attrition, the respective attrition mechanisms are not quite clear yet. There are various theoretical and empirical approaches that can in accordance with Eq. P) be summarized in the following definition of a bubble-induced steady-state attrition rate ... 

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