Under the premise of reaching the same typical calcination endpoint temperature of 1250–1350°C, the calcined coke produced by rapid heating and slow heating exhibits systematic differences in yield, density, electrical resistivity, reactivity, and microstructure. This is not simply a matter of “whether it is fully calcined” — rather, the thermal history of the heating process profoundly reshapes the intrinsic quality of the carbon material.
1. Yield: Slow heating is significantly superior
Experimental data clearly show that when slow heating is applied before 900°C, the yield is markedly higher than with rapid heating. The reason is that during slow heating, volatiles have sufficient time to escape gradually, so the interior of the coke particles does not fracture and scatter due to sudden gas expansion. Rapid heating, by contrast, causes volatiles to be released in large quantities over a short period, making the coke particles prone to crumbling and pulverization, thus reducing yield. Moreover, a turning point appears around 1150°C — the yields of all four petroleum cokes drop sharply after this temperature, but the loss under slow heating is consistently smaller. For raw materials with high moisture content and large amounts of fine coke, the yield disadvantage of rapid heating is even more pronounced.
2. True density and bulk density: Rapid heating is actually higher, but at a cost
Rapid heating (on the order of 10³ K/s) involves an extremely short heating time, so the coke undergoes less thermal treatment, retains more active sites, and its microstructure does not fully rearrange and densify — hence its higher reactivity. However, in terms of density indicators, the true density and bulk density obtained by rapid heating after 900°C are often higher — because the degree of structural ordering is “frozen” at a relatively high level. Although slow heating allows the structure to fully rearrange toward densification, the prolonged exposure to high temperature also causes partial carbon to be oxidized and burned off, so while the yield decreases, the density indicators are not necessarily better.
It is worth noting that appropriately reducing the heating rate in the high-temperature range (900–1300°C) actually helps improve true density and oxidation resistance. This shows that faster is not always better — the heating rhythm in the high-temperature stage is equally critical.
3. Electrical resistivity: Slow heating is more conducive to lowering resistivity
Electrical resistivity decreases as calcination temperature rises, dropping sharply in the 1050–1100°C range and then leveling off. Slow heating allows the carbon structure to fully graphitize and become more ordered, creating more complete conductive pathways, and thus yields lower resistivity. Rapid heating can also reduce resistivity, but because structural rearrangement is less thorough, the effect is not as good as with slow heating. Low-sulfur, low-ash raw materials combined with slow heating most easily produce low-resistivity calcined coke (powder resistivity can be controlled below 650 μΩ·m).
4. Reactivity: Rapid-heating coke is more “lively”
This is the most fundamental difference between the two. Rapid-heating coke retains a large number of active sites because it is not held at temperature long enough after heating, so both its CO₂ reactivity and air reactivity are higher. Experiments show that the reactivity ranking under different heating rates is: rapid-heating coke > rapid-heating coke with 3s hold > slow-heating coke. Especially for coal types with inherently good reactivity (such as SH coal), the effect of heating rate is even more dramatic — the difference in reactivity between rapid and slow heating can reach several times.
But this double-edged sword means: although rapid-heating coke has high reactivity, it is more prone to deactivation due to oxidation during storage and transport. Slow-heating coke has lower reactivity but better chemical stability, making it more suitable for long-term storage and downstream applications with strict reactivity requirements (such as prebaked anodes).
5. Microstructure and impurity behavior
Slow heating favors the formation of micropores — the longer residence time allows a large number of micropores with diameters <8 nm to develop. The specific surface area at low temperatures (e.g., 500°C) may actually be lower than with rapid heating, but above 800°C the growth in specific surface area under slow heating is more significant. Rapid heating, on the other hand, causes rapid growth in the pore diameter range below 2.5 nm, resulting in a pore structure that leans more toward larger pores.
The catalytic effects of impurity elements are also influenced by heating rate: Ca and Na have a strong catalytic effect on CO₂ reactivity, S can inhibit this catalysis, and V strongly catalyzes air reactivity. Under rapid heating, the catalytic effects of these impurities are more easily “activated,” leading to higher reactivity.
In one sentence: Slow heating sacrifices yield for quality — higher yield, lower resistivity, better stability; rapid heating preserves yield but produces higher reactivity and poorer stability. The optimal industrial strategy is often “slow first, then fast”: slow heating before 900°C to preserve yield, then appropriately increasing the rate after 900°C to improve density, selecting an endpoint temperature of 1200°C rather than 1300°C, to achieve the best balance among energy consumption, yield, and quality.
Post time: Jun-05-2026