Detailed introduction of graphite rotor for aluminum liquid degassing

Graphite rotor belongs to graphite material, which is mainly composed of carbon element. It has excellent electrical conductivity, thermal conductivity and chemical stability. It can resist corrosion from various acids, alkalis, salts and organic solvents. In particular, its excellent high temperature resistance and thermal shock resistance make graphite an extremely important key material in high temperature and ultra-high temperature environments. Therefore, graphite rotors are widely used in metallurgy, chemical industry, machinery, electronics, aviation, aerospace, and nuclear industries. Among them, it is most widely used in aluminum alloy casting industry, and its service life in aluminum processing industries such as aluminum plate, aluminum foil casting, and aluminum alloy wheel hub is as high as 2 months.

1. Working principle of graphite rotor

The high-speed rotating graphite rotor breaks the nitrogen (or argon) blown into the aluminum melt into a large number of dispersed bubbles and disperses them in the molten metal. The bubbles in the melt absorb hydrogen and oxide slag by using the gas partial pressure difference and surface adsorption principle, and are taken out of the melt surface as the bubbles rise, so that the melt is purified. Because the bubbles are small and dispersed, they are evenly mixed with the rotating melt, and then they rotate in a spiral shape and slowly float up. They have a long contact time with the melt and will not form a continuous straight rising airflow, thereby removing harmful hydrogen from the aluminum melt and significantly improving the purification effect.

The graphite rotor rotating nozzle is made of high-purity graphite. In addition to considering the dispersion of bubbles, the nozzle structure also uses the centrifugal force generated by stirring the aluminum alloy melt to make the melt enter the nozzle and mix evenly with the horizontally ejected gas to form a gas/liquid flow ejection, increase the contact area and contact time between the bubbles and the aluminum alloy liquid, and improve the degassing and purification effect.

The speed of the graphite rotor can be adjusted steplessly by the frequency converter, up to 700r/min. The specifications of the graphite rotor are D70mm~D205mm, and the specifications of the impeller are D85mm~D305 mm. The high-purity anti-oxidation graphite rotor has the characteristics of high strength, high temperature resistance, and aluminum flow corrosion resistance. During the purification and degassing process, nitrogen is introduced into the surface of the aluminum alloy liquid in the box for protection, so that the part of the graphite rotor exposed to the aluminum alloy liquid is in an inert gas, which prevents the rotor from high-temperature oxidation and prolongs the service life of the rotor.

The impeller is streamlined, which can reduce the resistance during rotation. The friction and scouring force generated between the impeller and the aluminum alloy liquid is relatively small, and the degassing rate is above 50%, which shortens the smelting time and reduces the production cost.

2. SiC coating solves the problem of graphite rotor oxidation resistance

However, graphite also has its own shortcomings. In an oxygen atmosphere, once the temperature exceeds 400°C, the graphite material will oxidize rapidly, and the higher the temperature, the faster the oxidation rate. Therefore, this greatly restricts the application range of graphite and the service life of graphite products.

At present, SiC coating technology is mainly used at home and abroad to solve the problem of graphite oxidation resistance. SiC has excellent high-temperature oxidation resistance, good physical and chemical compatibility with graphite, and has high strength, high hardness and corrosion resistance. High-performance SiC coating can greatly extend the service life of graphite rotors, thereby improving economic benefits.

3. The main reason for graphite rotor damage

High temperature oxidation: Graphite is mainly composed of carbon. Graphite will undergo visible oxidation under air conditions exceeding 600°C. The products of the carbon oxidation reaction are CO and CO2 gases, which do not protect the rotor. Generally speaking, the degassing box cannot be completely sealed, and most of the box cavities are not filled with protective gas for protection. The oxidation of the graphite rotor is inevitable. The result of oxidation is that the diameter of the rotor shaft gradually decreases until it breaks and is scrapped.

Aluminum liquid flushing: The graphite rotor is immersed in the aluminum liquid in the degassing box, generally rotating at a speed of 200r/min~400r/min, and the aluminum liquid is strongly stirred. The aluminum liquid will produce strong flushing and friction on the rotor, especially the contact part between the liquid surface and the rotor rod gradually becomes thinner, and finally breaks and is scrapped.

Influence of the transmission system: The transmission system of the degassing device is unstable, especially the non-concentric connection with the rotor. The rotor swings during operation, which causes damage to the rotor and the sealing plug by friction on the one hand; on the other hand, it intensifies the rolling of the liquid surface, aggravates the oxidation and flushing of the rotor, and accelerates the loss of the rotor.

4. Tips for using graphite rotors

4.1. Preheating before use: Before the graphite rotor is immersed in the aluminum liquid, it should be preheated for 5min~10min at about 100mm above the liquid surface to avoid the impact of rapid cooling on the material; gas must be introduced before the rotor is immersed in the liquid; the gas supply can be stopped only after the rotor leaves the liquid surface to prevent the rotor nozzle pores from being blocked.

4.2. Isolation of air: Nitrogen or argon is introduced into the purification room to ensure that the room is under positive pressure, isolate the outside air, and prevent the graphite rotor from oxidizing.

4.3. Rotor immersion depth: The graphite rotor is immersed in the aluminum melt to a reasonable depth, so that the reinforcement sleeve is exposed to about 80mm above the aluminum liquid surface and immersed below about 60mm below the liquid surface, which can effectively increase the time for the rotor to resist oxidation loss and erosion wear.

4.4. Stable transmission system: The graphite rotor and the transmission system are connected by connecting rods (tubes). If the connecting rod is deformed at high temperature for a long time, or the relevant parts of the transmission equipment are loose, it will affect the centering and operation stability of the rotor, and it is also easy to break or bruise the graphite rotor, causing damage.

4.5. The argon or nitrogen must be pure: If there is leakage in the pipeline, joints and other parts, the argon or nitrogen entering the aluminum melt will be impure, which will cause serious oxidation of the upper part of the rotor and even oxidize several rotor injection holes at the bottom, greatly shortening the service life of the rotor.