The 72-hour dry-out curve after relining

The short version: dry-out removes water, not just temperature

In the 72 hours after relining, the dry-out has to pass three gates: free water leaves around 110 °C, chemically combined water leaves around 350 °C, and after 600 °C the lining finishes dehydration and heats through evenly. Keep the ramp rate within 15–25 °C/h throughout. Pushing past a hold point before the water is gone is the main cause of lining cracking. The ranges below are industry-typical; the actual curve must follow the dry-out schedule supplied by your refractory vendor.

A freshly cast lining is not dry. Part of the mixing water serves workability only and stays in the capillary pores as free water; the rest has reacted into the hydrate phases and is locked in as combined water. Dry-out lets both leave the lining at a controlled rate. Too fast, and steam pressure inside the lining exceeds the early-age tensile strength, causing spalling. Too slow, and you burn restart days you do not have. With a relining turnaround of 10–15 days for a rotary activation furnace and 15–20 days for a shaft furnace, dry-out is the last stretch of the job and the one you least want to repeat — and with the lining accounting for 8–15% of complete-plant investment, a failed dry-out costs far more than half a day of extra holding.

What each temperature band drives off

Band one removes free water: ramp from ambient to 110 °C and hold. At 100 °C and atmospheric pressure, water expands roughly 1,600 times in volume when it flashes to steam, so in this band the rate matters more than the temperature itself. Ramp too fast and the surface dries and skins over first, trapping steam inside the lining; the trapped pressure then blows spalling craters through the skin. Hold at 110 °C until the vents stop showing visible steam — typically 8–12 hours, and longer wherever the lining was cast wetter than specified.

Band two removes combined water: ramp from 110 °C to 350 °C and hold. The hydrate phases begin to decompose in this window, and the water they release still has to evaporate and escape, so the ramp stays gentle. A 6–10 hour hold at 350 °C is typical. This band is the one crews most often short-cut, because unlike the 110 °C hold there is little visible steam, even though the chemistry inside the lining is still actively changing.

Band three completes dehydration and equalizes heat: from 350 °C, past 600 °C, up to the lower edge of the activation working range — around 850 °C, depending on duty. By 600 °C the residual combined water is essentially gone; from that point the lining no longer releases water and only expands thermally, so the through-thickness temperature gradient becomes the dominant stress. Soak at the working temperature until the lining is hot all the way through before you consider feeding material.

Where linings actually crack

• Anchor roots: the heat-affected zone at the weld and the surrounding castable never reach the same temperature, so the first cracks appear there when the ramp runs fast — common in both rotary kiln linings and shaft furnace linings; see the notes on the rotary furnace lining and the shaft lining for anchor layout.
• Spalling craters under a skinned surface: the 110 °C band was ramped too fast or the vents were blocked, so the surface hardened first and trapped steam underneath.
• Around openings and feed ports: structurally weak sections that also heat up fastest locally — the classic spots for spalling and stone loss.
• Longitudinal cracks where expansion joints were undersized: during heat-through the lining expands as a whole, and a blocked joint shows up as a full-length crack.
• Cracks after an interrupted dry-out: a power or fuel cut mid-curve quenches the lining and leaves hidden micro-cracks. Treat the restart as a fresh ramp from ambient, never a continuation from the interruption point.