In order to support the growth of the global aluminum industry, calcined petroleum is used for the anode production. There is currently no commercially viable alternative to calcined petroleum coke for the production of carbon anodes in aluminum smelting. The sponge like structure of petcoke allows for binding materials to penetrate through coke particles into a solid carbon blocks that conduct electricity into aluminum smelting. The capacity performance of the anode depends on both the internal structure and the treatment temperature of the coke.
Calcined Petcoke is a by-product of the calcination procedure that takes place within the coker unit in a crude oil refinery. The high temperature of calcination is used to remove excess hydrocarbons as well as moisture from the petcoke. It also transforms the crystalline structure. The resultant coke has a high ash content and is hard. It is capable of being ground to a fine powder or granular size and then transported in bags, trucks, railcars, barges, or ships for distribution and use.
Kilns of either the rotary type or shaft type are used for coke calcination. Both types of kilns use a refractory fired with alumina or silica as the heat-transfer material. The refractory material is heated up to about 1200-1350degC. The high calcination temperature allows the coke be processed into an anode or recarburizer grade product.
Most smelters blend different quality GPCs together to create the anode-grade calcined petcoke (CPC) that they need to meet their specification requirements for sulfur (S), vanadium (V), nickel (Ni), and other trace metals. The blending can be done in a variety of ways, but most commonly through a weigh belt blending system. More smelters use more sophisticated blending techniques, such as concentrating cokes with higher S/CO2 or lower VM reactivity in coarse fractions, to maximize the anode density. They also use cokes with highest VBD for ball mill feed.
Aluminum smelters demand higher bulk densities from their anode-grade CPC to ensure that the anodes are capable of conducting electricity through the smelting pot over a long period of time. This is especially important for large-scale operations, such as those producing a million tons or more of aluminum per year. The higher CPC bulk density allows the anode to stay in the smelting cell for longer, giving the operator more opportunities to make aluminum before removing and recycling the anode butt.
The smelter requires a high CPC granularity to maintain electrical conductivity. This granularity depends on the smelter’s choice in calciner type, kiln design, GPC grading & processing technology.
The CPC anode grade granularity also depends on the vibratory bulk density (VBD). A rotary Kiln produces CPCs with higher VBDs and more granularity than shaft-type calciners due to a slower initial heating up of the coal in the kiln. Consequently, the granularity of a rotary-kiln produced anode-grade CPC is more consistent than that of a shaft-type calciner.
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