Mechanisms of Attrition

Particle attrition can be caused by the relative motion of mechanical parts such as a blade in the flow of bulk particles by the impact of particles with a solid wall or with other 

For particle surface abrasions, a satisfactory empirical formulation was suggested by Gwyn (1969) as 

Assume that the rate of abrasion is proportional to the reduction in radius of a particle raised to an arbitrary power, i.e., 

Consider particles with an initial size distribution in the sieve range from a to aj and define an as the initial radius of a particle, which shrinks to a value a at strain e. Hence, we have 

As a result, the mass fraction attrited from other sizes to a, w, can be expressed by 

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The approach taken here is to employ standard materials characterization tests to measure the materials properties of the granulated product. With this information, the mechanism of attrition, i.e., breakage versus erosion, is determined. The rate of attrition can then be related, semi-empirically, to material properties of the formulation and the operating variables of the process, such as bed depth and fluidizing velocity. 

There is clearly a need to investigate the mechanism of attrition to relate it to the fracture properties of the solids, and to develop a realistic attrition index , similar to that used for abrasion in cyclones. Such an index would indicate the relative importance of operating conditions and design variables such as inlet velocity, feed solids concentration or cyclone diameter. This could then be used in scale-up to predict (or minimize) the effect of the shape, the particle size distribution or the hardness and strength of the feed solids, if known, may allow such predictions without any experimental tests. Generally, better understanding of attrition and its relation to abrasion may lead to better equipment design and operation. 

Particles in fluidized beds undergo collisions and frictional contacts with each other and with fixed surfaces, sometimes causing the particles to break [40, 41]. The most important mechanisms of attrition are impact attrition (also called fragmentation) and abrasion. Impacts can be especially energetic, and therefore likely to cause attrition, when particles are accelerated in distributor jets, feed jets, or cyclone entrances and then collide with fixed surfaces or stationary particles. 

Product diameter is small and bulk density is low in most cases, except prilling. Feed hquids must be pumpable and capable of atomization or dispersion. Attrition is usually high, requiring fines recycle or recoveiy. Given the importance of the droplet-size distribution, nozzle design and an understanding of the fluid mechanics of drop formation are critical. In addition, heat and mass-transfer rates during... 

The mechanisms of comminution are complex involving breakage along particle cracks and fissures etc., and depend on the hardness and structure of the feed particle. The Institution of Chemical Engineers (London) produced a major report on comminution (IChem, 1975), which was followed by reviews by Bemrose and Bridgwater (1987), Prior etal. (1990) and Jones (1997). These reviews included sections on both the fundamental and practical aspects of comminution and attrition in process equipment, test methods and an extensive list of references. 

The significance of this novel attempt lies in the inclusion of both the additional particle co-ordinate and in a mechanism of particle disruption by primary particle attrition in the population balance. This formulation permits prediction of secondary particle characteristics, e.g. specific surface area expressed as surface area per unit volume or mass of crystal solid (i.e. m /m or m /kg). It can also account for the formation of bimodal particle size distributions, as are observed in many precipitation processes, for which special forms of size-dependent aggregation kernels have been proposed previously. 

The debris resulting from the attrition or other breakdown mechanism of filtration or exchange media. Typically, fines are particles of under 50 mesh. 

Both, the mechanism and the extent of particle degradation depend not only on the process type but also on properties of the solid material, and to a large extent on the process conditions. Clift (1996) has stated that attrition is a triple-level problem, i.e., one is dealing with phenomena on three different length and time scales the processing equipment, the individual particles, and the sub-particle phenomenon such as fracture which leads to the formation of fines. The appearance of attrition can, therefore, differ very much between the various applications. For that reason, the following section deals with the various modes of attrition and the factors affecting them. 

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. 

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. 

Attrition cannot normally be directly investigated in the large-scale process. It is, for example, impossible to analyze the whole bulk of material and it is nearly impossible or at least very expensive to vary parameters in a running industrial process. For that reason, attrition has to be investigated in small-scale experiments. The results of these experiments require a model or at least an idea of the governing attrition mechanisms to be applied to the large-scale process. In principle, there are two different philosophies of attrition modeling. 

Experimental Techniques. Jet attrition cannot be investigated in isolation, because there is always some additional attrition of the bubbling bed. For that reason, many authors (e.g., Blinichev et al., 1968 Kutyavina et al., 1972 Arastoopour et al., 1983 Contractor et al., 1989) considered the overall attrition rate resulting from both attrition sources. In order to get direct insights into the mechanisms of jet attrition, it is necessary to separate the jet contribution from the measured overall attrition rate. This can be done in two different ways. 

The mechanical strength of the carriers produced in lab scale (Fig. 8) was quantified in terms of attrition loss, side crush strength and drop test strength. An example of the results is given in Table 3 for carriers D and E prepared with... 

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