Calcinated coke

Petroleum coke is an excellent fuel, and that is its main use, especially for the coke from fluid coking". There are some other markets that have to do with calcined coke electrodes for aluminum production or for all other electrolytic cells, carbons for electro-mechanical equipment, graphite, and pigments. 

Coals mesophase pitch coal chars coal tar pitch carbon mesocarbon microbeads, carbon fibers semi-coke, calcined coke activated carbons premium cokes, carbon fibers, binder and matrix... 

In recent years researchers at West Virginia University have developed coal-derived pitches on a laboratory scale in quantities sufficient to make 1 kg samples of calcined coke for fashioning graphite test specimens. The pitches were derived by uhlizmg solvent extraction with N-methyl pyrrohdone (NMP). This solvent is able to isolate coal-based pitches m high yield and with low mineral matter content [13]. It is this work that will form the basis of the discussion for the later part of this chapter. 

Raw coal NMP-extract Residue Green coke Calcined coke... 

The green cokes were calcined by placing a weighed amount of green coke into an alumina tube. The tube was fitted with end caps to allow for a constant purge of nitrogen. The alumina tube was then inserted into a high-temperature furnace and the temperature raised to about 1000°C for a period between 30 and 60 minutes. The furnace was turned off, cooled to room temperature, and the product recovered to determine the calcined coke yield. 

Polarized light photomicrographs were taken of the green and calcined cokes, as well as their corresponding test graphites. The untreated extract cokes are characterized by very small amsotropic domains on the order of 3 microns or less. This type of optical structure is believed to be highly desirable for nuclear graphite applications. 

Table 20. Yield of calcined coke for WVU test graphites...

A key factor in the suitabihty of cokes for graphite production is their isotropy as determined by the coefficient of thermal expansion. After the calcined coke was manufactured into graphite, the axial CTE values of the graphite test bars were determined using a capacitance bridge method over a temperature range of 25 to 100°C. The results are summarized in Table 24. Also included in the table are bulk density measurement of calcined cokes and the resistivity values of their graphites. 

Table 24. Some properties of WVU coal-derived calcined cokes and their graphites ...

Coals coal chars semi-coke, calcined coke activated carbons... 

Table 21 reports the ash content and ash composition (determined by inductively coupled plasma-atomic emission spectroscopy, ICP-AES) for all of the calcined cokes used to fabricate the test graphites. It can be seen that the amount of ash and its make-up are variable, but are within the range observed for petroleum-based calcined cokes. Although the ash contents in all of the calcined cokes appear rather high, these materials may still be acceptable because many of the metallic species are driven off during graphitization. This aspect is addressed in the next section. 

Table 21. Ash content and composition of calcined cokes used to make the WVU graphites... 

In order to assess the loss of inorganic contaminants during graphitization, the ash composition of most of the graphites was analyzed by ICP-AES. The total ash contents of the WVU graphites are compared to those for the precursor calcined cokes in Table 22. Also included are data for H-451 and VNEA, which are the current qualified nuclear-grade graphites. 

Table 22. Ash contents of calcined cokes and their processed graphites (ppm)...

Dust control material is applied to calcined coke to help maintain a dnst-free environment. It adds weight to the coke and can have a negative effect on the quality of carbon and graphite artifacts made from the treated coke. Hence, a maximum amount may be specified. 

The coke for synthetic graphite is also calcined before further processing. It is calcined until a sample of —35 + 100 mesh material shows an electrical resistivity of less than 0.05 ohm-inch. The same type of calciners are used that are used to calcine coke for the aluminum industry (29). The calcined coke retains a major portion of the sulfur present in the green petroleum coke (1). Low-sulfur, green coke is preferred. 

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