Calcined coke is a high-purity carbon material used for aluminum and steel production. It is produced by heating green petroleum coke (GPC) to drive off moisture and volatile matter.
GPC with different VM levels produce anode-grade CPC with different porosity and bulk density. Aluminum smelters typically blend different sources of GPC to achieve consistent anode-grade CPC.
The most common method of reducing oxygen during steelmaking is using calcined coke. This process reduces oxygen levels to prevent oxidation and slag formation. Oxygen reduction also improves metallurgical properties, such as strength and hardness.
GPC is produced by calcining petroleum coke in a rotary hearth or shaft kiln. The resulting calcined coke is called fuel grade petroleum coke (FGPC).
In order to produce the high quality carbons required by aluminum production, FGPC must be reduced to anode grade. To do this, the parent material is washed and pre-treated with nitric acid to remove impurities and volatiles.
The low sulphur content of calcined coke makes it an excellent choice for smelters. It has a lower carbon and sulfur level than coal, and it is a much cleaner fuel for the steelmaking process. In addition, blending calcined coke into the smelter feedstock can help increase anode density, which can lead to higher productivity and efficiency in the blast furnace.
Many of the energy-intensive industrial processes that use petroleum coke emit carbon dioxide, a greenhouse gas that contributes to global warming. Reducing the amount of coke used in steelmaking can lower these emissions.
The high carbon content of calcined coke increases the level of carburization when it is burned as an energy source in the iron reduction furnace. This reduces the amount of supplemental fuel needed to achieve an equivalent reduction in emissions.
All GPC has residual volatile matter (VM) when it is cut from the drum, and the VM can vary from 9-14%. Typically, the higher the VM, the harder the sponge coke and the lower its Hardgrove Grindability Index (HGI). Independent or merchant calciners usually blend different quality sponge cokes together to produce CPC with an HGI of around 68.
Variations in the porosity and bulk density of different cokes can result in substantial pitch level and anode density variations in smelter preparation. Blending the cokes on a consistent basis minimizes these differences and improves anode quality.
As a fuel, calcined petroleum coke provides a stable, cost-effective source of energy for industrial processes. Its high carbon content and low ash content makes it an excellent replacement for other fossil fuels. Increasing construction spending in developed and emerging economies worldwide is expected to drive the global market for calcined coke during the forecast period.
Most calcination takes place in shaft kilns, which are steel cylinders lined with high-temperature refractory brick. They have a slow rotational movement and a residence time of 24-48 hours, depending on the size of the kiln.
A rotary kiln is also used for calcination, but its rotational speed can’t be controlled as accurately. It requires much more manpower to operate, but it allows for the kiln to be sized for a higher capacity than shaft kilns.
Some smelters use sophisticated blending systems, concentrating different types of coke in different parts of the aggregate recipe. This allows them to control the anode density in a more consistent way. Higher VBD and lower reactivity coke are positioned in coarser fractions of the recipe, while finer fractions contain higher S and lower VBD cokes.
A major advantage of using calcined coke is that it reduces the amount of moisture in steel. This is important because excessive moisture will cause slag formation and reduce metal production. Moisture in steel also affects its ductility and strength.
The use of calcined coke also lowers the need for limestone in the ironmaking process, which reduces transport emissions and ash generation. In addition, it decreases energy consumption because of the lower heat requirement.
Green petroleum coke (GPC) is a solid carbon fuel with high residual volatile matter levels and trace metal impurities such as sulfur and nickel. It can be produced as blocky sponge coke from delayed or fluid bed cokers or in a shot size form from shaft calcination. GPC used for anode production is typically calcined by independent or merchant calciners which blend different quality cokes. This allows the smelter to achieve consistency in anode characteristics for the lowest cost.
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