In many steel plants, ladle slag build-up (ladle skulls) has quietly become a hidden cost driver: lower tapping weight, higher empty ladle weight, more frequent repairs, and a higher risk of ladle breakout.
This case study explains why ladle slag adhesion occurs in aluminium-killed steel and how optimised slag modification can effectively reduce build-up while cutting refractory and steelmaking costs.
The ladle working lining is made of high-alumina bricks with an Al₂O₃ content of about 88%. All steel grades are aluminium-killed. The average composition of the ladle top slag is shown in Table 1.
| CaO | Al2O3 | SiO2 | MgO |
|---|---|---|---|
| 37.67 | 38.34 | 13.11 | 3.87 |
From Table 1 it can be seen that:
The Al₂O₃ content in the slag is high
The MgO content is relatively low
According to the CaO–Al₂O₃–SiO₂ phase diagram with 5% MgO (Figure 1), the slag composition falls in the 1500–1600 °C liquidus region. The CaO saturation, expressed as CaO/Al₂O₃ ≈ 0.98, is also relatively low. In other words, the ladle top slag has a high melting point and low basicity.
During teeming, as the steel temperature gradually decreases, the top slag begins to solidify. With the progress of pouring, the steel level drops and the semi-solidified slag adheres to the high-alumina working lining, forming a slag layer on the ladle wall.
The high content of deoxidation product Al₂O₃ in the top slag promotes reaction with the high-alumina bricks and the formation of Al₂O₃-rich deposits. This leads to:
Reduction of the effective ladle volume
Part of the adhered slag falling back into the ladle in subsequent heats, deteriorating steel cleanliness
At the same time, because the MgO content in the slag is too low, the melting point remains high, which further promotes Al₂O₃ deposition and aggravates slag build-up.
Besides slag chemistry, several operational and lining-related factors also aggravate ladle slag adhesion, such as:
Excessive BOF converter slag carry-over
Insufficient ladle turnaround speed and long waiting time
Poor ladle thermal insulation or absence of an insulation layer
Inadequate preheating/baking of the ladle
Localised lining damage, spalling and delayed repair
These factors and corresponding descriptions are summarised in Table 2.
| Item | Reasons for aggravated slag accretion |
|---|---|
| Converter slag carry-over | Large amount of BOF converter slag carried over, resulting in high content of Al2O3 deoxidation products in the ladle slag and severe slag accretion. |
| Secondary refining | During LF treatment the ladle age is still low and slag accretion is relatively light, whereas after RH and CAS-OB treatment slag accretion becomes much more severe. |
| Steel grade | When producing aluminium-killed steels, a large amount of deoxidation products is generated, leading to serious slag accretion. |
| Ladle turnaround | The longer the ladle is kept waiting (i.e. the slower the ladle turnaround), the more easily slag accretion occurs. |
| Ladle cover | If no ladle cover is used, molten steel loses temperature rapidly, which easily promotes slag accretion. |
| Insulation layer | If the ladle lining has no insulation layer, the ladle loses heat rapidly, which readily causes slag accretion. |
| Refractory wear | When the ladle lining is damaged, steel/slag penetrates the damaged areas and easily causes slag accretion. |
| Minor ladle repair | After minor repair, the ladle has a poor thermal condition, which easily leads to slag accretion. |
As slag build-up grows thicker with ladle age, it causes:
Average tapping weight to drop (e.g. to around 178 t/heat)
Empty ladle weight to increase (e.g. from ~92 t to ~103 t)
More frequent minor repairs, higher refractory consumption and higher cost per ton of steel
Difficult and inaccurate residual lining thickness evaluation, increasing the risk of ladle breakout
Therefore, alleviating or eliminating slag adhesion on the ladle lining is essential for safe and stable operation.
From the above analysis, reducing the melting point of the ladle top slag is the key to controlling slag build-up.
From the CaO–Al₂O₃–SiO₂ phase diagram with 10% MgO (Figure 2), the target is to adjust the slag composition so that it falls in the 1400–1500 °C liquidus region. This requires:
MgO content: 8–12%
CaO saturation: CaO/Al₂O₃ ≥ 1.0
By increasing MgO and basicity at the same time, the slag becomes more fluid at ladle operating temperatures and less prone to sticking to the high-alumina lining.
To achieve the desired slag chemistry, a new slag modifier was developed:
Raw materials: screened fines of BOF flux quicklime and lightly burned dolomite
Mixing ratio: 2:1 (lime : dolomite)
Processing: mixed thoroughly and briquetted with a dedicated pressing machine to form composite balls
These composite slag-modifying balls are added into the ladle during tapping. The addition rate is dynamically adjusted according to the oxygen content of the tapped steel; the recommended addition ranges are given in Table 3.
| Tapped steel oxygen content, ppm | Addition of burnt lime + lightly calcined dolomite balls, kg |
|---|---|
| ≤ 650 | 450–550 |
| 650–800 | 550–650 |
| 800–1000 | 650–750 |
| > 1000 | 700–800 |
After applying the new modifier, the composition of the ladle top slag becomes more stable and falls into the designed range. A typical composition after modification is shown in Table 4.
| CaO | Al₂O₃ | SiO₂ | MgO |
|---|---|---|---|
| 43.32 | 26.38 | 9.38 | 9.82 |
From the CaO–Al₂O₃–SiO₂–MgO (10% MgO) phase diagram, the liquidus temperature of this slag system is about 1400–1500 °C, which is lower than the steel temperature in the ladle during pouring. As a result, the top slag remains sufficiently fluid and is much less likely to adhere to the ladle wall.
In parallel with slag modification, several operational and lining measures were implemented to tackle secondary causes of slag build-up (see Table 5), including:
Reducing converter slag carry-over into the ladle
Speeding up ladle turnaround to shorten waiting time
Strengthening ladle preheating and thermal insulation
Timely repair of eroded or spalled areas of the ladle lining to prevent steel/slag penetration
Improving the quality and thermal shock resistance of ladle refractories, and optimising repair practice to avoid over-frequent minor repairs
| Category | Specific measures |
|---|---|
| Steelmaking process | Improve converter tapping slag-blocking operation to reduce converter slag entering the ladle. Control the number of ladles in service. Reduce ladle waiting time. Strengthen ladle preheating/baking. |
| Ladle insulation | Use effective ladle insulating agents. Apply an insulating layer in the permanent lining. Improve brickwork quality and control joint thickness. |
| Refractory materials | Reduce thermal stress in the ladle lining, improve thermal-shock resistance and reduce cracking. Improve the corrosion resistance of RH snorkel magnesia gunning mixes, magnesia–alumina bricks, ladle magnesia gunning mixes and magnesia-carbon bricks, thereby reducing their wear/slag erosion. Extend the service life of the ladle bottom and seating bricks, and reduce the frequency of minor repairs. |
| Ladle repair | Repair obvious burnt and spalled areas of the ladle wall in time, to prevent penetration of slag and molten steel and avoid excessive slag/skull build-up. |
| Minor ladle repair | Strengthen ladle baking, and after minor repair try to use the ladle mainly for medium- and high-carbon steels. |
After implementing the above slag-modifying and operational measures, the plant achieved clear improvements:
Average tapping weight increased from about 178 t/heat to 186 t/heat
Empty ladle weight decreased from about 103 t to below 93 t
Slag build-up on the ladle working lining was significantly reduced
Frequency of minor repairs and refractory consumption per ton of steel decreased
The overall production rhythm became more stable, supporting consistent casting and refining operations
For ladles lined with high-alumina bricks, the main cause of slag build-up is the excessively high content of deoxidation product Al₂O₃ in the ladle top slag, which raises its melting point and promotes slag adhesion. Factors such as steelmaking practice, steel grade, ladle turnaround, ladle insulation and lining maintenance can further intensify slag build-up.
To effectively reduce ladle slag adhesion, the ladle top slag must be modified. Using composite slag-modifying balls made from quicklime and lightly burned dolomite fines, added into the ladle during tapping and adjusted dynamically according to tapped steel oxygen content, can significantly lower the top slag melting point. At the same time, it is important to:
Reduce BOF converter slag carry-over into the ladle
Speed up ladle turnaround and strengthen ladle insulation
Repair eroded and spalled ladles in a timely manner
Only by combining slag chemistry control with good ladle operation and maintenance can slag build-up on the lining be reliably alleviated.
Q: What is the main cause of ladle slag build-up in aluminium-killed steel?
A: The main cause is a high Al₂O₃ content in the ladle top slag from aluminium deoxidation, which raises the slag melting point and makes semi-solid slag adhere to the high-alumina ladle lining. Low MgO content and low basicity further increase the liquidus temperature and aggravate slag adhesion.
If you are facing similar ladle slag build-up or skull problems, our team at Firebird New Materials Co., Ltd. can support you with slag system evaluation and refractory optimisation for your specific steel grades and ladle practice.
For technical discussion or tailored refractory solutions, feel free to contact us at service@firebirdref.com.
