Rotary degassing of molten aluminum typically takes 5–15 minutes for a standard holding-furnace batch, while published batch guidance spans 8–15 minutes depending on melt volume and starting hydrogen concentration [S3][S5].
These figures are starting points rather than volume-independent recipes: furnace capacity, melt temperature, alloy, rotor design, and required cleanliness determine the treatment time and purge-gas consumption [S1][S2].
Degassing Time Does Not Scale Directly With Melt Volume
The available 5–15 minute rotary range describes a standard holding-furnace batch, but a larger melt does not automatically require a proportional increase in treatment time [S3]. The controlling variables are bubble distribution, hydrogen loading, melt temperature, alloy-specific solubility, and the selected endpoint.
Published batch guidance places treatment at 8–15 minutes, explicitly linking cycle time to melt volume and initial hydrogen concentration [S5]. In practice, the furnace should be divided into a known working volume, and the rotor should be positioned to cover the active bath rather than treated as a small point treatment.
Rotary equipment performance also depends on head design. Fine bubbles improve hydrogen removal and can shorten treatment and reduce purge-gas use, while larger bubbles provide lower process efficiency even when the rotor is operating [S2].
Compare Rotary, Ultrasonic, and Inline Treatment
Rotary treatment is established for general foundry use because a spinning graphite rotor shears argon or nitrogen into fine bubbles that rise through the melt, collect hydrogen, and carry it to the surface [S2][S5]. It suits batch treatment in holding furnaces, transfer ladles, and crucibles, although each configuration requires its own operating routine [S2][S3].
Ultrasonic treatment can achieve comparable short-cycle results under specific laboratory conditions. A reported test on 10 kg melts found that 180 seconds of ultrasonic treatment produced a final bifilm index of about 42 mm, versus about 38 mm after 20 minutes of rotary treatment [S4]. At shorter settings, 90 seconds of ultrasonic treatment and 5 minutes of rotary treatment each produced an index of approximately 80 mm [S4].
Inline porous-plug systems and lance-fed argon or nitrogen injection offer continuous or localized alternatives, but the available material does not provide a directly equivalent treatment time for the same melt volume [S3]. Rotary and ultrasonic figures also do not establish a universal production specification because the 10 kg ultrasonic comparison did not measure hydrogen directly, rotor-tip amplitude was not reported, and the bifilm index combines gas-assisted porosity with oxide-related defects [S4].
Use Melt Volume, Temperature, and Alloy to Set the Cycle
Start with a documented working volume rather than furnace nameplate capacity, then record the initial hydrogen level or reduced-pressure-test result, alloy, melt temperature, rotor type, and treatment result. Rotary batches commonly fall within 8–15 minutes, but the published range itself states that melt volume and starting hydrogen concentration affect cycle time [S5].
Temperature changes the gas-removal burden. Above 760°C, the example cited for pure aluminum shows diminishing removal efficiency as hydrogen falls, while hydrogen solubility increases exponentially with temperature; an 111°C rise can approximately double solubility under the cited conditions [S2]. A hotter melt therefore needs a properly verified purge-gas allowance rather than a shorter timer.
Alloy choice changes the required effort. The cited casting-alloy correction factors show that difficult-to-degas alloys can take much longer than pure aluminum, with aluminum 535 cited as requiring four times the treatment time of pure aluminum [S2]. These relationships make a one-size-fits-all timer unsuitable for mixed-alloy foundries.
Match the Process to the Production Scenario
Use a rotary degassing unit for batch treatment where hydrogen must be reduced before pouring, especially in holding furnaces, transfer ladles, and crucible furnaces. Rotary impeller treatment is the most established route in the cited foundry guidance and offers established bubble-generation practice [S2][S5].
Use ultrasonic treatment only where the equipment, melt volume, and production cycle have been validated together. The reported 90-second ultrasonic result matched 5 minutes of rotary treatment on the tested 10 kg charge, but the study also showed that duration increased effectiveness and that the equipment comparison did not address production throughput, energy use, maintenance, or total operating cost [S4].
Use inline porous-plug treatment when continuous metal flow permits controlled gas introduction. Inline systems can fit the melt-delivery arrangement, but their treatment requirement cannot be translated directly from a rotary batch timer without line-specific hydrogen and residence-time data [S3]. Lance injection is a localized alternative, while tablet or flux treatment addresses selected inclusions rather than serving as a direct equivalent to rotary hydrogen removal [S3][S5].
Control Gas Consumption, Rotor Condition, and Melt Quality
Purge-gas demand rises sharply as the melt approaches low hydrogen levels. The cited example above 760°C gives a gas-removal ratio of 200, meaning 200 L of inert gas is required to remove 1 L of hydrogen under those illustrated conditions [S2]. This makes a fixed volume, fixed time combination useful only when the initial hydrogen level, melt volume, and endpoint are controlled.
Rotor condition must also be verified. An FDU study evaluated three graphite rotor geometries at 0%, 25%, 50%, 75%, and 100% wear. The compared rotors maintained equivalent degassing efficiency across the evaluated service life, while melt chemistry remained within the prescribed quality under EN 1706 [S1].
Do not accept a timer alone as evidence of treatment. Record reduced-pressure-test results, density or bifilm index where used, and mechanical-property results when they are part of the acceptance plan. A 2026 comparison linked increasing bifilm index with decreasing yield strength and greater mechanical-property scatter, so averages and consistency both matter [S4].
Known Limits and Failure Modes
Four principal failure modes can masquerade as insufficient degassing time: hydrogen entering with wet charge, damp tools or linings, or prolonged exposure to a humid atmosphere; poor rotary-head design that creates large bubbles; alloy and temperature effects that increase the required treatment; and worn or poorly maintained equipment [S2][S3][S5].
Treatment time is not a direct proxy for gas volume removed. The studies and guides cited here provide ranges for specific equipment, melts, and test conditions, not a universal conversion from furnace tonnes to minutes. Direct hydrogen measurement is also distinct from reduced-pressure testing, while a bifilm index reflects coupled gas and oxide behavior rather than hydrogen alone [S4].
For high-integrity aluminum die casting, validate the treatment cycle against the component acceptance criteria and the actual charge condition. A rotor that achieves a laboratory index in 180 seconds may still be unsuitable if it cannot process the production furnace volume uniformly.
Track three signals before changing the recipe: the initial hydrogen or reduced-pressure-test result, the post-treatment result, and the measured treatment duration for a fixed working volume. If the melt volume changes, reassess rotor positioning, gas flow, and treatment time rather than extending the old cycle mechanically.