Product coal particle-size distribution

Figure 6. The effects of three chemical additives on product coal particle size distribution while milling at constant conditions, (O) No Additives (A) 2 Ibs/ton Calcium Hydroxide (D)...

Toxicology. Epidemiological evidence suggests that workers intimately exposed to the products of combustion or distillation of bituminous coal are at increased risk of cancer at many sites, including lungs, kidney, and skin. The chemical composition and particle size distribution of coal tar pitch volatiles (CTPV) from different sources are significant variables in determining toxicity. ... 

Ubhayakar et al. [14] studied rapid devolatilization of pulverized coal in hot combustion gases, varying the input gas temperature between 1525 and 1975°C. They used three particle size distributions for the same type of coal as received, the fraction which remained on a 200 mesh screen and that which passed through the screen. The residence time in the gasifier was 7-70 x 10 s. The tests were conducted at a pressure of 1 atm, heating rates up to 10 °C/s, and volatile product yield up to 68% of the original dry-ash-free coal. 

Figure 1. Particle size distributions of the low temperature ash products from three different 5 cm blocks of Pittsburgh seam coal (o) Coal A (a) Coal B and (O) Coal C.

The conclusion of that study indicated that product coal particles larger in diameter than the band of mineral matter microparticles seemed to contain the whole particle distribution of the small microparticles. That is, there appeared to be a relatively homogeneous distribution of particles throughout, limited in top size by the largest particle in the test. To explore that aspect in more depth, a raw coal was wet milled to smaller sizes. 

When a raw coal is wet ball-milled for a sufficient time to produce a slurry with a particle diameter mode in the range of 4 pm there results two forms of mineral matter That fractured away from the coal and that which is still enveloped in the coal particles. Figure 3 illustrates a typical particle size distribution for the separated product coal as compared to the separated free mineral matter (90 weight percent ash) from one milling test. The separated mineral matter is clearly smaller in diameter than the coal which is probably due to its more brittle properties. Note that in Figures 3 and 4 an integration of the curves will yield 100% of the mineral matter (or ash) under consideration rather than the ash content of the coal as was the case in Figures 1 and 2. 

Figure 2. Particle size distribution of the low temperature ash product of a 48 + 4 pm product coal after the free mineral matter was removed.

Figure 3. When the products of wet milling 250 pm x 0 Pittsburgh coal (C) are separated, two products evolve Product coal and separated mineral matter. The particle size distributions for each are illustrated.

A specific case is examined in Table II where the particle size distribution data from the low temperature ashing of a 5 cm cube of coal and 44-53 pm coal was related to two product coal samples milled under different conditions all of which originated from the same source coal (C). The first column in Table II provides the average particle diameter points (y) at which the data were observed... 

Figure 5. A hypothetical particle size distribution curve of low temperature ash and product coal.

Figure 8. Weight percent ash in product coal versus the mode of the particle size distribution of the product coal, (O) No additives (O ) 7.9 Ibs/ton Sodium Hydroxide (O) 36 Ibs/ton Ammonium Hydroxide ( ) 20 Ibs/ton Sodium Ligninsulfonate ...

Type III or comparative method a method where the sample to be analysed is compared to a set of calibration samples, using a detection system which has to be recognised to be sensitive not only to the content of elements or molecules to be analysed but also to differences of matrix [11], Ignoring any difference in the matrix will lead to errors. Calibration of such methods requires (Certified) Reference Materials ((C)RMs) with a known matrix composition similar to the matrix of the sample. Such methods are rapid and are often used in monitoring of manufacturing processes (e.g. WDXRF in the production of metals, alloys, coal, cement, powdered oxides, etc.) or for the determination of basic parameters (e.g. viscosity, particle size distribution etc.). 

FIGURE 17 Distribution of concentrating table products by particle size and specific gravity. , coal , middling o, refuse. 

In addition to its chanical properties, the efficient use of a coal also requires a knowledge of its physical properties, such as its density (which is dependent on a combination of rank and mineral matter content), hardness, and grindability (both related to coal composition and rank). Other properties include its abrasion index (derived mainly from coarse-grained quartz) and the particle size distribution. Float-sink testing may also be included with the analysis process. This involves separating the (crushed) coal into different density fractions as a basis for assessing its response to coal preparation processes. Float-sink techniques may also be used to provide a coal sample that represents the expected end product of a preparation plant, in order to assess the quality of the coal that will actually be sold or used rather than the in situ or ROM material represented by an untreated (raw) coal sample. 

Coke sampling is marginally less problematic because the product from a single source derives from coal or blend of coals that have been prepared to a specification for ash, moisture, particle size distribution, etc. The final coke produced will be relatively homogeneous in all properties, with the exception of size distribution. Standard methods are available for coke sampling that reflects the somewhat less rigorous requirements for this material. 

This effect is well illustrated by a series of experiments on the grinding of coal in a small mill, carried out by Heywood(1). The results are shown in Figure 2.1, in which the distribution of particle size in the product is shown as a function of the number of... 

A graded size distribution, where fine particles fill the interstices between coarse particles as shown schematically in Fig. 4, will minimize the amount of void space that must be filled by fluid, and so will reduce the quantity of liquid needed to produce a flowable slurry. The best size distributions for this purpose have proven to be multimodal distributions, made up of several fairly narrow size fractions. An example of such a size distribution is shown in Table 1. To achieve such a multimodal size distribution, coal slurry production facilities are designed to generate several coal streams, which are each ground and sized to the desired particle sizes and then combined to give the proper size distribution. 

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