Crushing product size distribution

Recently, crushing has been widely used in chemical industries. Although the crushed product size distribution in the operation has been expressed by various PSD functions, the most utilized PSD function is the log-normal PSD function. However, there is no physical background for applying the log-normal PSD function to the crushed product size distribution. Additionally, when the crushed product size distribution is expressed by various PSD functions, it becomes difficult to discuss the relationship between the parameters in the PSD and the operation condition. This fact is one of the obstacles in the development of particle technology. 

To examine whether the new PSD defined by Eq. (5.18) can sufficiently express the crushed product size distribution in the grinder. 

A fixed weight feed is fed in the ball mill and the ball mill is rotated at a fixed rotational speed. After the rotation has begun, the crushed product is removed at constant time interval, and the crushed product size distribution is measured by using a Tyler standard sieve. 

Crusher Product Sizes Table 20-10 relates product size to the discharge setting of the crusher in terms of the percent smaller than that size in the product. Size-distribution curves differ for various types of materials crushed, and a general set of curves is not vahd. 

Figure 5.6 Data of crushed product size probability density distribution and fitted PSD curve based on new PSD function.

These tertiary crushers employ smooth or toothed heavy-duty impact and abrasion-resistant steel-rimmed rolls. The rolls are mounted inline in a horizontal manner and turn toward each other at equal speeds to create a nip into which a friable feed material is introduced (Fig. 4). Heavy-duty compression springs with automatic reset are used to dampen crushing shock and to protect the crusher from tramp iron and oversize material. An adjustable screw that adjusts spring tension changes the crusher opening. A flywheel is used to even out pulses and economize on power consumption. These crushers have a theoretical maximum reduction ratio of 4 1 and will only crush materials to about 10 mesh. Roll crushers produce a controlled product size distribution without a lot of fines. The narrow particle size distribution is achieved by controlling a combination of variables including roll speed, gap measure, differential speed, feed rate, and roll surface. 

The granulation of powders by size enlargement, whereby agglomerates are first formed, then crushed into the desired particle size range, and finally classified to obtain the desired product size distribution, is even more difficult to analyze than the complex size reduction of solids. The reason for this is twofold. 

Roller Mills Solids crushed between rotating cylinder and wall of second cylinder. Variable gap. Narrow product size distribution but limited size reduction ratio. 

Sometimes a product that does not meet these requirements must be adjusted by adding a specially crushed fraction. No crushing device available will give any arbitrary size distribution, and so crushing with a small reduction ratio and recycle of oversize is practiced when necessary. 

Alternative terms for size reduction to describe the operations that subdivide solids mechanically are crushing and grinding. An ideal crusher or grinder should (i) have a large capacity (ii) require a small power input per unit of the product and (iii) yield a product of the size and/or the size distribution desired. 

The particle-size and size-distribution of solid materials produced in industrial processes are not usually those desired for subsequent use of these materials and, as a result comminution and recrystallization operations are carried out. Well known processes for particle size redistribution are crushing and grinding (which for some compounds are carried out at cryogenic temperatures), air micronization, sublimation, and recrystallization from solution. There are several practical problems associated with the above-mentioned processes. Some substances are unstable under conventional milling conditions, and in recrystallization processes the product is contaminated with solvent, and waste solvent streams are produced. Applying supercritical fluids may overcome the drawbacks of conventional processes. 

Figure 12.1. Normalized cumulative size distribution curves of comminuted products, (a) From various kinds of crushing equipment, (b) From rod and ball mills, (c) RRS plots of two curves Taggart, 1951).

Crushing is applied to large lumps of feed stock and grinding to smaller lumps, often the products of crushing, but the size distinction is not overly sharp. The process of size reduction results in a range of product sizes whose proper description is with the complete cumulative size distribution, but for convenience a characteristic diameter corresponding to 80% pass in the cumulative distribution curve is commonly quoted. 

Wood powder is a kind of upgraded fuel that is burned in large-scale combustion plants for heat production. However, it is possible to use wood powder for power generation as well. It is a biofuel made of sawdust, shavings and bark. The raw material is crushed, dried and milled to tine particles in order to obtain the best fuel properties. There are many different wood powder qualities dependant on different physical properties such as particle size distribution, particle shape and also moisture content. The powder is usually handled in a closed system from milling to storing in silos to avoid the risk of dust explosions. The raw materials and type of mill used determine the properties of the wood powder (Paulrud et al, 2002). 

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