This sequence represents the industry best practice for minimizing thermal stress and the risk of explosive spalling. A 24-hour ambient air cure allows the hydraulic bonds of the low-cement castable to develop initial strength. The subsequent slow, controlled heating ramp (50°F/h) through the critical temperature range (up to 600°F or 315°C) is essential. This controlled rate allows free and chemically bound water to be safely removed as low-pressure steam, preventing pressure buildup within the refractory's pore structure, which is the primary cause of cracking and spalling during dryout.

A. Firing directly to 1500°F without a controlled ramp-up induces extreme thermal shock and would almost certainly cause catastrophic failure of the lining due to rapid steam generation.

C. Accelerated drying with high-temperature forced air can cause surface sealing or "case hardening," trapping moisture within the lining and significantly increasing the risk of explosive spalling.

D. Using open flame burners with direct impingement on the refractory surface creates severe, localized thermal gradients, which is a well-documented cause of cracking and spalling.

1. API Technical Report 980

Monolithic Refractories: Installation and Dryout

2nd Edition

May 2018.

Section 6.1.1 (Curing): States

"For conventional castables

this curing period is typically a minimum of 24 hours..." This supports the initial step in the correct answer.

Section 6.2.2 (Heating Rate): Specifies

"The most critical part of any dryout is the initial heating phase where free water is removed... A typical heating rate in this range is 25 °F/hr to 50 °F/hr (14 °C/hr to 28 °C/hr)." This directly validates the heating ramp in the correct answer.

Section 6.2.3 (Burner Operation and Placement): Warns

"Direct flame impingement on the refractory lining shall be avoided as this can cause localized overheating

resulting in cracking or spalling of the refractory." This directly refutes option D.

2. Schacht

C. A. (2004). Refractories Handbook. Marcel Dekker

Inc.

Chapter 5

Section 5.4.2 (Drying and Firing): This chapter explains that the initial heating rate for monolithic refractories must be slow and controlled

particularly below 300°C (572°F)

to safely evaporate free water and water of hydration without generating destructive steam pressures. This principle supports the controlled ramp in option B and highlights the danger in options A and C.

3. VDI-Guideline 3470

Heat Treatment of Refractory Linings

Verein Deutscher Ingenieure

2012.

Section 5.2 (Heating-up): This guideline emphasizes that for hydraulically bonded refractories

the heating rate in the range up to 300°C should not exceed 20-30 K/h (36-54 °F/h) to prevent damage from steam pressure

aligning with the rate specified in option B.