The main reasons for the damage of graphite rotor in graphite materials are as follows:
1. High temperature oxidation: Graphite is mainly composed of carbon. Graphite is oxidized visibly in air at over 600 C. The product of carbon oxidation reaction is CO and CO2 gas, which can not protect the rotor. Generally speaking, the degassing tank can not be completely sealed, and most of the chambers are not fully protected by gas, so the oxidation of graphite rotor is inevitable. As a result of oxidation, the diameter of the rotor shaft decreases gradually until it is broken and discarded.
2. Aluminum scouring: Graphite rotor immersed in the liquid aluminium in the degassing tank, usually rotates at the speed of 200 r/min to 400 r/min and stirs the liquid aluminium strongly. The liquid aluminium will produce strong scouring and friction to the rotor. Especially, the contact area between liquid level and rotor rod becomes finer gradually, and eventually it will break and scrap.
3. The influence of transmission system: the transmission system of degassing device is unstable, especially the connection with the rotor is not centrifugal, the rotor sways during operation, on the one hand, the friction between the rotor and the sealing plug causes damage; on the other hand, the rolling of the liquid surface is increased, the oxidation and erosion of the rotor are intensified, and the loss of the rotor is accelerated.
How to prolong the service life of graphite rotor in graphite material is introduced in detail from the angle of operation.
1. Preheating before use: Before immersing the graphite rotor in liquid aluminium, preheating the graphite rotor at about 100mm above the liquid surface for 5 to 10 minutes to avoid the influence of quenching on the material quality; before immersing the rotor in liquid, the gas must be introduced into the rotor; after the rotor puts forward the liquid surface, the gas can be stopped to prevent the blockage of the air holes in the rotor nozzle.
2. Isolation of air: Nitrogen or argon is introduced into the purifying chamber to ensure positive pressure in the chamber and to isolate external air to prevent oxidation of graphite rotor.
3. Rotor immersion depth: The reasonable depth of graphite rotor immersed in aluminium melt can make the strengthening sleeve expose about 80 mm of aluminium liquid level and immerse below 60 mm of liquid level, which can effectively increase the anti-oxidation loss and erosion of the rotor.
4. Steady transmission system: Graphite rotor and transmission system are connected by connecting rod (pipe). Long-term deformation of connecting rod under high temperature or loosening of relevant parts of transmission equipment will affect the neutrality and smooth operation of the rotor, which will easily lead to breakage or knock of the graphite rotor. Argon or nitrogen should be pure: If the leakage of pipelines, joints and other components causes the argon or nitrogen gas into the aluminum melt to be impure, the upper part of the rotor will be seriously oxidized, and even the lower part of the rotor jet holes will be oxidized, which will greatly shorten the service life of the rotor.