Magnesium treatment is one of the most important steps in ductile iron production. Magnesium reacts with sulfur and oxygen in molten iron and promotes the formation of spheroidal graphite. However, not all of the magnesium added during treatment is recovered in the molten iron. A significant portion may be lost through vaporization, oxidation, reaction with slag, or violent reaction during nodularization.
For foundries, improving magnesium recovery in ductile iron is not simply about adding more magnesium. Excessive magnesium addition can increase treatment costs, create stronger reactions, and may lead to casting defects. A better approach is to control the chemical composition of the base iron, treatment temperature, nodulizer addition, treatment method, and reaction conditions.
This article explains the main factors affecting magnesium recovery and practical ways to improve the recovery rate when using FeSiMg nodulizer.
What Is Magnesium Recovery in Ductile Iron?
Magnesium recovery refers to the percentage of the magnesium added during the nodularization treatment that remains effectively dissolved in the treated molten iron.
A simple calculation is:
Magnesium Recovery (%) = Recovered Mg ÷ Added Mg × 100
For example, if a foundry adds 10 kg of magnesium through a magnesium-containing nodulizer and approximately 1 kg remains effectively available in the treated iron, the apparent magnesium recovery is about 10%.
The actual recovery depends on the treatment process and how magnesium is measured. Therefore, foundries should use their own production data rather than relying on a single theoretical recovery value.
In ductile iron production, the objective is not to achieve the highest possible magnesium recovery at any cost. The real goal is to obtain stable residual magnesium with good graphite nodularity while minimizing magnesium consumption and treatment-related defects.
Why Is Magnesium Recovery Low?
Magnesium has a relatively low boiling point compared with the temperature of molten iron. When magnesium is introduced into hot molten iron, it can vaporize rapidly.
Several reactions can occur at the same time:
- Magnesium reacts with sulfur.
- Magnesium reacts with oxygen.
- Magnesium can vaporize from the molten iron.
- Magnesium can react with slag.
- Magnesium may be lost through the violent reaction at the beginning of treatment.
- Poor treatment conditions can increase magnesium loss.
This is why magnesium recovery is closely related to the nodularization process.
A foundry may use the same FeSiMg nodulizer but obtain different magnesium recovery rates because of differences in iron temperature, sulfur content, treatment ladle design, addition method, and reaction control.
1. Control the Sulfur Content of Base Iron
Sulfur is one of the first factors that should be considered when trying to improve magnesium recovery.
Magnesium has a strong affinity for sulfur. During nodularization, part of the added magnesium is consumed by desulfurization reactions before sufficient magnesium remains available to promote graphite spheroidization.
If the sulfur content of the base iron is too high, more magnesium may be consumed during treatment.
Therefore, maintaining a suitable and consistent sulfur level before magnesium treatment is important.
For many ductile iron processes, the base iron is desulfurized before nodularization when necessary. The exact target depends on the raw materials, furnace practice, iron grade, and treatment process.
Why low sulfur helps
A properly controlled sulfur level can:
- Reduce unnecessary magnesium consumption
- Improve magnesium recovery
- Reduce treatment reaction intensity
- Make residual magnesium easier to control
- Improve the consistency of graphite nodularity
However, sulfur should not simply be reduced as much as possible. The appropriate level should be determined according to the complete metallurgical process.
2. Optimize Treatment Temperature
Molten iron temperature has a direct influence on magnesium recovery.
When the treatment temperature is too high, the magnesium reaction can become more violent. Magnesium may vaporize quickly, increasing magnesium loss and producing more fumes and slag.
On the other hand, if the temperature is too low, the treatment reaction and subsequent pouring process may become difficult to control.
For this reason, foundries should establish a suitable nodularization treatment temperature based on their iron chemistry, treatment method, ladle size, and FeSiMg nodulizer specification.
In general, controlling the temperature rather than simply increasing it is the key.
Practical considerations
The foundry should monitor:
- Temperature before nodularization
- Temperature during treatment
- Temperature after treatment
- Temperature at pouring
- Time between treatment and pouring
This information can help identify whether magnesium losses are related to excessive temperature or excessive holding time.
3. Choose the Right FeSiMg Nodulizer
The selection of FeSiMg nodulizer has a major effect on magnesium recovery.
FeSiMg nodulizer is widely used to introduce magnesium into molten iron. Commercial grades can have different magnesium contents, particle sizes, rare earth levels, silicon contents, and reaction characteristics.
For example, foundries may use FeSiMg nodulizers with different magnesium ranges depending on their treatment process and target residual magnesium.
A suitable nodulizer should be selected according to:
- Base iron chemistry
- Sulfur content
- Treatment temperature
- Treatment method
- Desired residual magnesium
- Ladle capacity
- Treatment efficiency
- Required graphite nodularity
Higher magnesium content does not automatically mean better performance. A nodulizer with a suitable composition and controlled reaction can sometimes provide better process stability than simply increasing the Mg percentage.
4. Use an Appropriate Nodulization Method
The way FeSiMg nodulizer is added to molten iron has a strong influence on magnesium recovery.
Common treatment methods include:
- Ladle treatment
- Sandwich method
- Tundish cover treatment
- Plunging treatment
- Cored wire injection
The basic principle is to allow the magnesium-containing material to react with molten iron while limiting excessive magnesium vaporization.
Sandwich treatment
In the sandwich method, FeSiMg nodulizer is placed at the bottom of the treatment ladle and covered with steel scrap, iron filings, or another suitable covering material.
The covering layer delays the reaction between the nodulizer and molten iron.
This helps reduce the initial violent reaction and can improve magnesium utilization.
Cored wire treatment
In the cored wire process, FeSiMg or magnesium-containing alloy is enclosed in steel strip and injected into the molten iron.
The injection depth and feeding speed can be controlled, allowing the reaction to occur inside the molten metal rather than entirely at the surface.
For automated ductile iron production, cored wire treatment can provide good process repeatability when properly adjusted.
5. Control the Addition Rate
Adding FeSiMg too quickly can create a very strong reaction.
A rapid reaction may cause:
- Excessive magnesium vaporization
- Strong smoke and fumes
- Metal splashing
- Slag formation
- Lower magnesium recovery
- Unstable residual magnesium
Therefore, the addition rate should match the treatment equipment and molten iron quantity.
For cored wire treatment, injection speed and wire feeding length should be controlled according to the actual process.
The objective is to achieve a controlled magnesium reaction, rather than the fastest possible reaction.
6. Reduce the Time Between Treatment and Pouring
Magnesium does not remain stable in ductile iron indefinitely.
After nodularization, residual magnesium gradually decreases because of reactions with oxygen, sulfur, slag, and other elements.
This phenomenon is commonly referred to as magnesium fading.
The longer the treated iron remains at high temperature before pouring, the greater the possibility of residual magnesium loss.
Therefore, foundries should keep the treatment-to-pouring time as short and consistent as practical.
A well-controlled production process should monitor:
Treatment → Sampling → Inoculation → Pouring
rather than treating these operations as completely independent steps.
7. Improve Slag Control
Slag can also consume magnesium and reduce effective magnesium recovery.
During magnesium treatment, reaction products are generated and may form slag on the surface of the molten iron. If the slag is not properly controlled, it can interfere with the treatment process and contribute to magnesium loss.
Good slag management includes:
- Using clean charge materials
- Controlling furnace slag
- Avoiding excessive oxidation
- Removing unnecessary slag before treatment
- Keeping the treatment ladle clean
- Removing treatment slag efficiently after the reaction
A clean treatment environment helps make magnesium recovery more predictable.
8. Control the Oxygen Level
Magnesium reacts strongly with oxygen.
If the molten iron contains excessive oxygen or becomes exposed to air during treatment, part of the magnesium can be consumed by oxidation instead of contributing to graphite spheroidization.
This is another reason why treatment ladle design and operating practice matter.
Foundries should avoid unnecessary turbulence and excessive exposure of molten iron to air.
The treatment process should provide enough reaction time for magnesium to dissolve into the iron while limiting avoidable oxidation.
9. Use Inoculation Together With Nodularization
Magnesium treatment and inoculation have different metallurgical functions, but they work together in ductile iron production.
Magnesium is primarily used for graphite spheroidization, while inoculation promotes graphite nucleation and helps control the number and distribution of graphite nodules.
Common inoculants include:
- Ferrosilicon inoculant
- Silicon barium inoculant
- Silicon calcium inoculant
- Silicon strontium inoculant
Proper inoculation can improve graphite structure and reduce the tendency toward chill or carbide formation.
However, inoculation does not directly replace magnesium. A foundry should optimize magnesium treatment and inoculation separately but as part of the same process.
10. Monitor Residual Magnesium Instead of Only Magnesium Addition
One of the most useful ways to improve magnesium recovery is to measure the result of the treatment.
Simply recording how much FeSiMg nodulizer is added does not tell the foundry how much magnesium remains in the iron.
A better process-control system monitors:
- Initial sulfur content
- Initial iron temperature
- FeSiMg addition
- Nodulizer magnesium content
- Treatment temperature
- Treatment time
- Residual magnesium
- Graphite nodularity
- Nodule count
- Pouring temperature
By comparing these parameters over multiple heats, the foundry can determine the actual magnesium recovery under its own operating conditions.
This makes it possible to adjust the nodulizer addition more accurately instead of using an unnecessarily high addition rate.
Common Problems That Reduce Magnesium Recovery
Several production practices can lead to poor magnesium recovery.
| Problem | Possible Effect |
|---|---|
| High sulfur in base iron | More Mg consumed by desulfurization |
| Excessive treatment temperature | Higher Mg vaporization |
| Very rapid Mg reaction | Lower Mg utilization |
| Poor slag control | Increased Mg consumption |
| Excessive air exposure | More Mg oxidation |
| Long holding time | Magnesium fading |
| Incorrect nodulizer grade | Unstable treatment results |
| Improper particle size | Irregular reaction |
| Poor ladle design | Difficult reaction control |
| Inconsistent inoculation | Unstable graphite structure |
The important point is that low magnesium recovery usually has more than one cause. Changing only the FeSiMg addition rate may not solve the problem.
How to Calculate FeSiMg Nodulizer Addition
A simplified calculation can be used as a starting point:
FeSiMg Addition = Required Magnesium ÷ (Mg Content × Expected Recovery)
For example, suppose a foundry needs a certain amount of effective magnesium and uses an FeSiMg nodulizer containing 6% Mg.
If the expected magnesium recovery is 30%, the actual nodulizer requirement will be significantly higher than the theoretical amount calculated without considering recovery.
However, this formula is only a process estimate. Actual addition should be established through production trials and residual magnesium measurements.
The Mg recovery rate should be treated as a process parameter, not a fixed property of the nodulizer itself.
Best Practices for Higher Magnesium Recovery
For consistent ductile iron production, foundries can focus on the following points:
Before treatment
- Control sulfur and oxygen
- Maintain stable base iron chemistry
- Remove excessive slag
- Measure molten iron temperature
- Select the appropriate FeSiMg nodulizer
During treatment
- Use a suitable nodularization method
- Control nodulizer placement or injection depth
- Avoid excessively violent reactions
- Maintain appropriate treatment temperature
- Control addition or wire feeding speed
After treatment
- Remove slag properly
- Perform timely inoculation
- Minimize holding time
- Monitor residual magnesium
- Check graphite nodularity and nodule count
This approach is generally more effective than simply increasing the amount of magnesium alloy.
Conclusion
Improving magnesium recovery in ductile iron requires control of the entire nodularization process. Base iron sulfur content, treatment temperature, FeSiMg nodulizer composition, addition method, reaction intensity, slag condition, oxygen exposure, and treatment-to-pouring time can all influence the final residual magnesium level.
The best strategy is to achieve a stable and controlled magnesium recovery rate, rather than maximizing magnesium addition.
For foundries using FeSiMg nodulizer, process optimization should therefore start with actual production data. By measuring residual magnesium and graphite structure from different heats, manufacturers can gradually optimize nodulizer consumption, improve treatment consistency, and reduce unnecessary production costs.
For an experienced ferroalloy supplier such as Hongshun, FeSiMg nodulizer can be supplied in different Mg grades and particle sizes according to the customer’s ductile iron treatment process and technical requirements.
