直硝法生产浓硝酸反应釜内筒及附件的腐蚀和结构的研究/study on corrosion and structure about the reacting ke
The process of concentrated nitric acid which produced using direct method started in the former Soviet Union. During the nearly 50 years development, inner vessel of the reacting kettle is still made by the high pure aluminum material, which service life is less than 1.5 years, which corrosion speed is very quick and which welded joint corroded frequently to perforation, so the degree of safty is lower. The predecessors have made some explorations in the manufacture aspect, but made less progress in the applying of material. The effect still was not very satisfactory. Since there is similarity between the C8 steel’s design conditions and the process conditions in the inner vessel of reacting kettle, it will have the great significance to develop the research results deeply.
This task is the applied research which according to the enterprise condition, it involves two research fields: Metal corrosion and selection; Pressure vessel structure analysis. Based on the above contents, we has established the C8 steel corrosion model. The writer took A group and B group as comparison, and each group has 17 short pipes which uniformly distributed in the reacting kettle’s inner vessel. The experiment has been for 3,672 hours according to the industrial condition. the following results are obtained :
1) Using a simple corrosion rate formula, the writer approximately figured out the corrosion rate in the reacting kettle’s inner vessel, drew the corrosion rate curve, and promulgated the C8 steel’s corrosion rule in the reacting kettle’s inner vessel.
2) Through the appearances of C8 steel pipes which have been corroded, we discovered that: both A group and B group corroded very seriously, but A group (melting condition) is better than B group (sensitized condition); the C8 steel pipe in the bottom of reacting kettle’s inner vessel is more serious than near top.
3) Through observing microstructure, we discovered that: parent metal is single-phase twin crystals austenite, and welded joint organization is austenite + δ ferrite. There is the ferrite pollution in the beveling process or the welding process, which creates the welded joint intercrystalline corrosion.
4) Through analysis of the corrosion product, we discovered that: the corrosion product mainly composed of SiO2, which confirmed that silicon is the main passivation element of the C8 steel.
The second aspect of the task is structure analysis about the reacting kettle’s inner vessel and outer vessel by using the finite element method. Under the design conditions, the writer calculated the displacement, and examined the outer vessel’s strength. We obtain the following results:
5) Both outer vessel’s and inner vessel’s maximal displacement are smaller than 1 mm, which tell us that the equipment deformation under the operating case is extremely slight.
6) After the analysis through the outer vessel’s dengerous section，the result is that the calculating stress intensity is smaller than the allowable stress intensity, so the section intensity is qualified.