The conversion catalysts were inspected after the shutdown for major repairs; both the second and third stage catalysts showed varying degrees of blackening. In mild cases, the blackened layer was 20 centimeters thick, while in severe cases the entire catalyst stage was blackened. In the second and third stages, the color can sometimes turn blue, bluish-green; there’s no specific information available on this
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Request for an analysis regarding the color of the catalyst
The blackening is caused by vanadium loss in the earlier stages and deposition in later stages; it has already been encased, so it is definitely unusable. Both excessive temperature and poisoning can lead to vanadium loss. As for the blue-green one, there should be no major issues; it probably has a slight vanadium loss.
There are two scenarios for catalyst blackening: one is a purplish-black color, which may be due to lower temperatures on the inner surface of the converter; sulfuric acid condenses and evaporates there from time to time, resulting in a rough surface that turns black both on the outside and inside. Another reason is the dark black color, which is mainly caused by poor performance of the gas purifier in the period leading up to temperature drops, high moisture levels, lower temperatures in the latter part of the converter, and the condensation of sulfuric acid vapor in that latter section. Furthermore, when the second and third stages of the catalyst are permanently poisoned by arsenic, a black shell forms on the surface of the catalyst, causing it to turn black as well. In such cases, the catalyst usually needs to be replaced.
Catalyst blackening is likely due to arsenic encapsulation; we have conducted experiments on this
Blackening or bluing of the second and third stage catalysts generally indicates that the levels of arsenic and moisture in the flue gas are too high. In cases of arsenic poisoning, the only solution is to replace the catalyst; moisture and condensed acids can be addressed by raising the conversion temperature. The key is to properly control the purification and drying processes