Friability

Friability is of interest primarily because friable coals yield smaller proportions of the coarse sizes that may (depending on use) be more desirable, and there may also be an increased amount of surface in the friable coals. This surface allows more rapid oxidation hence, conditions are more favorable for spontaneous ignition, loss in coking quality in coking coals, and other changes that accompany oxidation. These economic aspects of the friability of coal have provided the incentive toward development of laboratory friability tests. 

The method employs a cylindrical porcelain jar mill fitted with three lifters that assist in tumbling the coal. A sample (usually 1000 g) of sized coal is tumbled in the mill for a specified time at a specified number of revolutions per minute. The coal is then removed and screened and the friability is reported as the percentage reduction in the average particle size during the test. For example, if the average particle size of the tumbled coal was 75% that of the original sample, the friability would be 25%. As with several other tests, the test parameters can be adjusted to suit the purpose of the investigation, but the precise parameters must be reported with the data. 

Attempts have been made to correlate the friability of coal with rank (Table 8.4). Thus, lignite saturated with moisture was found to be the least friable, and friability increased with coal rank to a maximum for coals of the low-volatile bituminous coal. The friability of anthracites is comparable with that of 

Considerable information is available on the properties of coal that relate to its hardness, snch as friability and grindability, but little is known of the hardness of coal as an intrinsic property. 

The scratch hardness of coal can be determined by measuring the load on a pyramidal steel point required to make a scratch 100 pm in width on the polished surface of a specimen. The scratch hardness of anthracite is approximately six times that of a soft coal whereas pyrite is almost 20 times as hard as a soft coal (Table 9.8). Similar data were noted for anthracite and cannel coal but durain, the reputedly hard component of coal, was found to be only slightly harder than vitrain and cannel coal. 

Although the resistance of coal to abrasion may have little apparent commercial significance, the abrasiveness of coal is, on the other hand, a factor of considerable importance. Thus, the wear of grinding elements due to the abrasive action of coal results in maintenance charges that constitute one of the major items in the cost of grinding coal for use as pulverized fuel. Moreover, as coals vary widely in abrasiveness, this factor must be considered when coals are selected for pulverized fuel plants. A standardized, simple laboratory method of evaluating the abrasiveness of coal would assist, like the standard grindability test now available, in the selection of coals suitable for use in pulverized form. 

Actually, the abrasiveness of coal may be determined more by the nature of its associated impurities than by the nature of the coal substance. For example, pyrite is 20 times harder than coal, and the individual grains of sandstone, another common impurity in coal, also are hard and abrasive. 

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It may be easier to fit the parameters by forcing them to follow specified functional forms. In earhest attempts it was assumed that the forms should be normahzable (have the same shape whatever the size being broken). With complex ores containing minerals of different friability, the grinding functions S and B exhibit complex behavior near the grain size (Choi et al., Paiiiculate and Multiphase Processes Conference Proceedings, 1, 903-916.) B is not normalizable with respecl to feed size and S does not follow a simple power law. 

Material characteristics, both chemical and physical, should be considered, especially flowabihty. Abrasiveness, friability, and lump size are also important. Chemical effects (e.g., the effect of oil on rubber or of acids on metal) may dictate the structural materials out of which conveyor components are fabricated. Moisture or oxidation effects from exposure to the atmosphere may be harmful to the material being conveyed and require total enclosure of the conveyor or even an artificial atmosphere. Obviously, certain types of conveyors lend themselves to such special requirements better than others. 

Size stability refers to the abihty of coal to withstand breakage during handhng and shipping. It is determined by twice dropping a 23-kg (50-lb) sample of coal from a height of 1.8 m (6 ft) onto a steel plate. From the size distribution before and after the test, the size stabdity is reported as a percentage factor (see ASTM D 440). The friability test... 

Absolute Density Bulk Density Size Distribution Friability... 

Britischgummi, n. British gum, dextrin, Brockchen, n. small bit, small piece, crumb, brockelig, a. brittle, friable, crumbly, fragile. Brdckeligkeit, /. brittleness, friability, brbckeln, v.t. crumble. 

The adverse reactions seen with penicillamine include pruritus, rash, anorexia, nausea, vomiting, epigastric pain, bone marrow depression, proteinuria, hematuria, increased skin friability, and tinnitus. Penicillamine is capable of causing severe toxic reactions. 

Administration of penicillamine has been associated with many adverse reactions, some of which are potentially serious and even fatal. The nurse carefully evaluates any complaint or comment made by the patient and reports it to the primary health care provider. Increased skin friability may occur, which may result in easy breakdown of the skin at pressure sites, such as the hips, elbows, and shoulders. If the patient is unable to ambulate the nurse changes the patient s position and inspects pressure sites for skin breakdown every 2 hours. 

Initial Situation An experimental granulation technique is to be evaluated a sample of tablets of the hrst trial run is sent to the analytical laboratory for the standard batch analysis prescribed for this kind of product, including content uniformity (homogeneity of the drug substance on a tablet-to-tablet basis, see USP Section (905)" ), tablet dissolution, friability (abrassion resistance), hardness, and weight. The last two tests require little time and were therefore done first. (Note Hardness data is either given in [kg-force] or [N], with 1 kg = 9.81 Newton). 

The pattern of inflammation in UC is continuous and confluent throughout the affected areas of the GI tract. The inflammation is also superficial and does not typically extend below the submucosal layer of the GI tract (Fig. 16-2). Ulceration or erosion of the GI mucosa may be present and varies with disease severity. The formation of crypt abscesses within the mucosal layers of the GI tract is characteristic of UC and may help to distinguish it from CD. Severe inflammation may also result in areas of hypertrophied GI mucosa, which may manifest as pseudopolyps within the colon.12 The inflammatory response may progress in severity, leading to mucosal friability and significant GI bleeding. 

Purulent drainage from the ulcer is indicative of infection. When pus and inflammatory symptoms are not present, the clinician must be astute to more subtle findings. These include delayed healing, increase in lesion size, prolonged exudate production, malodor, and tissue friability. Abnormal granulation tissue also may be present, as evidenced by color change (from... 

Ulceration friability of the lesion with bleeding or oozing. This is a danger signal. 

More specifically a certain weight of tablets, W0, is subjected to a well-defined level of agitation in a fixed-geometry, closed container for a specific time. They are then reweighed, W. The measure of abrasion resistance or friability, B, is usually expressed as a percentage loss in weight ... 

It might be advantageous to relate friability to unit time or number of falls. 

The Roche Friabilator is one of the most common methods used to test for resistance to abrasion [147], In this case, a minimum of 6 g (often 20 tablets) of dedusted and weighed tablets are placed in a 12 in. high drum, which is then rotated for 100 revolutions. A... 

This optimization method, which represents the mathematical techniques, is an extension of the classic method and was the first, to our knowledge, to be applied to a pharmaceutical formulation and processing problem. Fonner et al. [15] chose to apply this method to a tablet formulation and to consider two independent variables. The active ingredient, phenylpropanolamine HC1, was kept at a constant level, and the levels of disintegrant (corn starch) and lubricant (stearic acid) were selected as the independent variables, X and Xj. The dependent variables include tablet hardness, friability, volume, in vitro release rate, and urinary excretion rate in human subjects. 

Slightly different constraints are used to illustrate the mathematical technique. In this example, the constrained optimization problem is to locate levels of stearic acid (X ) and starch (X2) that minimize the time of in vitro release (y2) such that the average tablet volume (jy) did not exceed 9.422 cm2 and the average friability (y3) did not exceed 2.72%. 

Fig. 7 Optimum in vitro t50% release rate as a function of restrictions on tablet friability. (From Ref. 15.)...

Fig. 13 Contour plots for (a) disintegration time (b) tablet hardness (c) dissolution response (%) (d) tablet friability as a function of disintegrant level and compressional force. Dashed lines on ordinate denote limits of experimental range (—1.547 to + 1.547 eu see text for details).

The process conditions will influence the particle degradation by generating the stress on the individual particles on the one hand and by affecting the material properties and consequently the particle friability on the other. 

Data from attrition tests are usually presented as simple numbers called friability or attrition indices. Most of these indices are used as measures in quality control by subjecting the materials to a standard procedure. By comparing the test results with those of known materials, it is possible to give a relative characterization of the tested materials. Examples are given in Sec. 4.3. 

The large number of experiments that are termed attrition tests can be divided into two major fields of application, namely tests of material friability and experiments to study attrition phenomena. They will be separately discussed in the following subsections. The various test devices will be discussed afterwards. 

In friability tests the material s susceptibility to attrition is evaluated. But it is not as simple as it may seem at first to select the suitable test procedure. In this context Pell (1990) gave a simple thought experiment to illustrate the difficulties If we took a batch of rubber stoppers and a batch of diamonds, and rubbed them on abrasive paper, we would conclude that the diamonds were more attrition resistant. If we instead struck the particles with a hammer we would conclude that the rubber were more attrition resistant. So, different test methods can rank materials differently with respect to their attritability. This effect was for example observed by Knight and Bridgwater (1985). They subjected spray-dried powders to a compression test, a shear test and a test in a spiral classifier. They found that each test gave a different ranking of the materials. Obviously, there is no... 

Friability tests can be used for various purposes. They are widely used in quality control. Here, samples of produced material are subjected to a more or less arbitrary but well defined stress. The attrition extent is assessed by comparison with a standard value and a decision is reached whether the material meets the standard. Moreover, friability tests are often used for comparison of different materials to select the most attrition-resistant one. This is a usual procedure in the case of catalyst development. For example, Contractor et al. (1989) tested anew developed fluidized bed VPO-catalyst in a submerged-jet attrition test (described below). Furthermore, the specific attrition rate of a material in a certain process can be roughly estimated by friability tests. In this case the stress must be similar to that occurring in the process and the obtained degradation extent must be compared with those of other materials from which the process attrition rate is known. 

Results that are obtained by different friability tests are usually not comparable. All these tests give only a relative attrition resistance and numerical results are only useful in connection with detailed information about the test considered. 

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