The rotor is the core functional component of the rotary jet degassing equipment, and its structural design and material properties directly determine the refinement effect of bubbles, the shape of the melt flow field, and the final degassing efficiency. The rotor structure of sunken equipment is mainly divided into two categories: pure rotor centrifugal shear type and stator rotor composite guide type; The material forms two mainstream systems: graphite and silicon nitride ceramics.

1. Structural types and performance differences of rotors

Pure rotor nozzles, also known as centrifugal shear rotors, have a rotating body with blades or grooves. Inert gas is introduced from the central air duct of the rotor and sprayed out from the bottom or side air holes of the rotor; By relying on the centrifugal and shear forces generated by the high-speed rotation of the rotor, the continuous airflow is torn into small bubbles, while driving the melt to make circular flow. Common structures include impeller type, porous type, turbine type, etc., which are the most widely used basic configurations in the industry.

Its advantages lie in:

• simple structure, low manufacturing difficulty, and low procurement and replacement costs;
• The rotor has strong stirring force, fast melt circulation speed, and good adaptability to high viscosity and high inclusion melts;
• The flow channel is open and not easily blocked by floating slag, which is friendly to the production line for melting dirty and high slag waste, and has a long cleaning and maintenance cycle.

The limitations are reflected in three aspects:

• firstly, the uniformity of bubble size is generally average, with some large bubbles that float up quickly and have a short gas-liquid contact time, which limits the further improvement of degassing efficiency;
• The second issue is that the melt mainly flows in a horizontal circular flow, with short paths for bubbles to rise and some bubbles quickly escaping from the liquid surface, resulting in insufficient release of mass transfer potential;
• Thirdly, strong stirring can easily disturb the liquid surface, suck up air and surface floating slag, which may actually exacerbate the oxidation and inclusion pollution of the melt.

The stator rotor composite structure adds a fixed stator guide cover around the rotating rotor, and the stator has regularly arranged guide holes or guide blades. The gas-liquid mixture ejected by the rotor first undergoes secondary shear and flow guidance by the stator, and the gas is further dispersed into more uniform microbubbles. At the same time, the stator guides the melt to form an axial upward and downward circulation flow, significantly prolonging the residence time of bubbles in the melt and increasing the gas-liquid contact area.

The core advantage of this structure is high purification efficiency:

• the refinement effect of bubbles is better, the size is smaller and the distribution is more uniform, the gas-liquid specific surface area is larger, the mass transfer efficiency is significantly improved, and the degassing efficiency is 15% to 25% higher than that of pure rotor type under the same working conditions;
• The axial circulation flow field significantly extends the upward path of bubbles, prolongs the gas-liquid contact time, and fully utilizes the purification potential of inert gases;
• At the same time, the surface flow of the melt is smoother, reducing the risk of secondary oxidation and slag entrapment caused by liquid level disturbance, and the amount of floating slag produced is lower.

Its disadvantages are:

• the double-layer structure of stator and rotor is complex, high machining accuracy is required, and the manufacturing cost is significantly higher than that of pure rotor;
• The gap between the stator and rotor is narrow, making it easy to be blocked by floating slag and oxide slag. Regular shutdown and cleaning are required, and maintenance frequency is higher;
• The high requirement for melt cleanliness and high slag waste can easily cause stator slagging failure, making it unsuitable for recycled aluminum production lines that mainly use heavily polluted waste aluminum as raw material.

2. Differences in rotor material types and performance

The rotor works in high-temperature aluminum melt at 700-750 ℃ for a long time, and is subjected to aluminum liquid erosion, thermal shock, and chemical corrosion. The material properties directly determine the service life and operational stability of the rotor. At present, the mainstream rotor materials in industry are divided into two categories: graphite and silicon nitride ceramic.

Graphite rotors are generally processed and formed using high-strength and high-density isostatic pressed graphite, and the surface is usually treated with anti-oxidation impregnation, making it the largest rotor material currently in stock. Its advantages are very prominent:

• excellent processing performance, capable of processing complex blades and flow channel structures, suitable for various rotor design schemes;
• Low cost, with a unit price of only 1/5~1/3 of silicon nitride rotors, and low operating costs for consumables;
• Low density and light weight, low requirements for the load of the drive motor, strong adaptability for equipment modification;
• Good thermal shock resistance, less likely to explode under alternating cold and hot working conditions, and high fault tolerance.
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The disadvantages are also obvious:

• the high-temperature oxidation resistance is limited, and graphite above 700 ℃ will continue to oxidize and lose oxygen in the air. The service life of ordinary graphite rotors is usually only 1-3 months, and frequent replacement will affect production continuity;
• The detachment of graphite particles can enter the melt and form carbon inclusions, posing a pollution risk to high-end clean aluminum alloys;
• The overall strength is relatively low, and if there is collision or uneven slagging during high-speed rotation, it is easy to cause shaft breakage and blade fracture faults.

Silicon nitride (Si ∝ N ₄) rotor is a structural ceramic based on silicon nitride, which is prepared by reaction sintering or hot pressing sintering process. It has excellent high-temperature mechanical properties and aluminum liquid corrosion resistance, and is the preferred rotor material for high-end clean aluminum alloy production. Its advantages are concentrated in long service life and high cleanliness:

• its ability to withstand high temperatures and aluminum corrosion is much better than graphite, and it hardly reacts with aluminum at high temperatures. Its normal service life can reach 6-18 months, greatly reducing replacement frequency and improving continuous production efficiency;
• The material is pure and does not introduce carbon or metal impurities into the melt, fully meeting the purity requirements of high-end clean aluminum alloys;
• High temperature strength, wear-resistant and erosion resistant, able to withstand higher rotational speeds, more stable bubble refinement effect, and minimal long-term performance degradation.
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The main shortcomings lie in cost and brittleness:

• high manufacturing costs, complex raw materials and sintering processes, and large initial procurement investment;
• Ceramic materials have high brittleness, weak thermal shock resistance, and are prone to cracking or even fragmentation due to rapid cooling, heating, and mechanical impact. They require strict installation and operation standards;
• The difficulty of forming complex structures is high, and the degree of freedom in rotor design is lower than that of graphite materials;
• Once a fracture occurs and cannot be repaired, the overall cost of scrapping is relatively high.