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1、R. Perruchoud Page 1 of 7 ABSTRACT Modern smelter technologies require consistent anode behaviour ending with long anode cycles for low gross and net carbon consumptions. Therefore high density thermal shock resistant anodes using high performance forming machines have been developed in the last dec
2、ade to achieve this goal. From the raw material side the coke properties such as: pulverizing factor and grain stability bulk density and resiliency are becoming critical for the production of crack free anodes. These properties influence not only the fines preparation and pitching aspects but also
3、the extent of lamination of the green anodes after forming and the crack propagation resistance. These mechanical and physical properties are influenced by the type of resid oil and also by the coking and calcining conditions. The impacts of the coke macrostructure (isotropy) from the coking to the
4、calcining steps are reviewed. The blending aspects of green or calcined cokes with different macrostructures and sulfur content are also discussed taking into account the anode burning behaviour. This knowledge allows optimizations of the coke production from the refineries to their usage in the car
5、bon plant. INTRODUCTION Most of the new projects for calcining coke include blending facilities. Blending of cokes purchased in different refineries is made to meet the purity specifications, the sizing requirements and sometimes the bulk density and reactivity figures. As shown in Table 1, the scat
6、ter of the green cokes available as “anode grade” in the refineries is larger than the blends used as kiln feed by the calciners. Blending of green coke simply to meet the coke sulfur and vanadium specifications figures is a passive approach which is no longer satisfactory for the aluminium industry
7、 (1). With a better understanding of the performance of coke from the refiners to the smelter and a more efficient communication between suppliers and users it is possible to manufacture high density anodes that are thermal shock and oxidation resistant. To reach this goal it is important to integra
8、te the role of the coke macrostructure and sulfur content in the physical and mechanical carbon properties. COKE CHARACTERISTICS FROM THE REFINERS TO THE SMELTERS Raymond C. Perruchoud, Markus W. Meier and Werner K. Fischer R logically an isotropic coke is milled preferentially to small particles ha
9、ving the size of the coke mosaics. This means that the ball mill circuit produces a rather high amount of filter fines (5 to 10 times more!). Therefore the setting of the classifier is chosen to maintain, by coarser “fines” selection above 100 m in size, the Blaine number or the 325 Mesh target. GRA
10、IN STABILITY % 8070605090TEMPERATURE GRADIENT C/min401208060100VCM10%12%14%ISOTROPIC ANISOTROPIC FINE MOSAICS OPTICAL TEXTURE COARSE FLOW ROUND PORE SHAPE ELONGATED Fig. 7 Optical micrograph of cokes surfaces Scale10 mR. Perruchoud Page 5 of 7 The characteristics of this type of fines are so differe
11、nt from a classical rather anisotropic anode grade coke that major disturbances of all green carbon processes have been experienced in the industry. Filter fines are indeed badly wetted by pitches during mixing and usually the stability of filter fines addition to the dry aggregate is one of the les
12、s controlled steps in a typical paste plant. FREQUENCY DISTRIBUTION %/Decade 7550250 5002321258100AnisotropicIsotropicIsotropicPARTICLE SIZE m Fig. 9 Particle size distribution of fines of different cokes after milling For a constant final temperature and throughput, which is a situation normally pr
13、evailing in a given kiln, a bulk density specification of 0.80 kg/dm3 for the 2-1 mm fraction is normally achieved when the VCM is below 12% for a typical anode grade coke. Nevertheless this is no longer sufficient for an anisotropic coke as shown in Figure 10. Fig. 10 Bulk density of coke depends o
14、n VCM and on macrostructure The easiest way of getting information on the macrostructure of the coke was found to be the ratio of HGI/VCM which is about 4 for isotropic fuel grade coke and 12 for a needle green coke. If the feedstock and/or coking conditions are changed towards the production of iso
15、tropic coke, it is safer, in a first phase, to decrease the temperature of coking thereby increasing the VCM level as shown in Figure 11. In a second phase the VCM level can be lowered again when the smelter has learned to use such a material without cracking issues as discussed in (15). TypicalFig.
16、 11 Volatile matter threshold for a constant calcined coke bulk density level These two diagrams explain also why a lower level of VCM ( 0.82 kg/dm3 for the 1-2mm bulk density). Also, as already published by Reis (20), and confirmed here (Figure 15 and 16), the resiliency (6) of the high S coke would not be critical (4%) with the approach of optimally calcined coke blending and this with unchanged pulveri
