Kiln Drying & Heating Up the Lining: Guide

Drying and heating up a refractory lining is one of the most delicate operations in cement plant maintenance, and also one of the most frequently underestimated. After a kiln shutdown for refractory installation, the new lining contains water: free water from the mortar, from castables and concretes mixed on site, from gunning mixes and from the brick moisture absorbed during storage. If the kiln is simply brought back to full temperature, the steam generated inside the lining cannot escape fast enough, pressure builds up within the pores and the lining can explode violently — destroying a freshly installed million-dollar refractory lining in minutes and endangering personnel. This article, based on the well-known plant engineering document “Drying and Heating Up of Refractory Linings”, explains exactly how much water a new lining holds, how that water is released, what the heating-up curves for bricks, castables and concretes look like, what auxiliary measures (drilled holes, ventilation, rotation, controlled burners) are applied, and how the plant executes a safe heat-up from cold shell to full production. The complete 73-page reference is part of the Complete Cement Technical Package, the 931-file cement engineering library from cementequipment.org.

The subject matters because a wrong heat-up cannot be reversed cheaply: the damage it causes (delamination, spalling, cracking, loosened bricks, shifted arches) appears only weeks later as hot spots on the kiln shell, and the cost of an unplanned shutdown to repair that damage typically exceeds the entire cost of the heat-up fuel several times over. Drying and heating up is therefore treated in the cement industry with the discipline of a process operation: it has its own procedure, its own instrumentation, its own fuel supply and its own control room log, and it is executed exactly according to the heating-up curves described below.

1. Why Drying Is Necessary: Water in New Refractory Linings

Every freshly installed refractory system contains water in two different forms. Free (mix) water is the water added at installation: the water of hydration of refractory mortars and castables, the water used to dampen bricks for tight joints, and the residual moisture of gunning or shotcreting mixes. Free water is physically held in the pores and capillaries of the refractory, and it must be driven off as vapour below about 200 °C. Chemically bound water is part of the hydration products of the cement bond in castables (calcium silicate hydrates, aluminate hydrates); it is released only when those hydrates decompose, which for most cement-bonded castables occurs between about 300 and 500 °C. Some raw materials in magnesia-based bricks and in certain insulating concretes carry additional bound water (for example, brucite-type phases and magnesium hydroxide), released at similar temperatures.

The amounts are far from negligible. A typical cement-bonded castable contains between 8 and 12 percent water by weight at installation, roughly split into about 6–8 percent free water and 2–5 percent chemically bound water depending on the cement content and the mix water demand. Since the preheater and kiln inlet castable linings of a large modern kiln (4000–10000 tpd) easily total 300–1000 tonnes of monolithic refractory, the total water that must be evaporated during a heat-up can reach tens of tonnes. In extreme cases, plants with very thick monolithics (for example 300 mm or more of castable in cyclone cones and duct bends) report drying campaigns where the evaporated water amounts, in total, to the equivalent of many full-truck deliveries. Every kilogram of that water must leave the lining as vapour through the surface, which is why the heating curve must be built around the physics of water transport and steam pressure build-up.

2. The Physics: Evaporation, Steam Pressure and Explosive Spalling

When heat is applied to a wet refractory, a drying front develops. Near the hot face the water evaporates and the vapour moves outward through the open pores; deeper inside, the still-wet material remains cold. If the heating rate is too fast, the surface layer dries and shrinks, the temperature front moves inward faster than the vapour can escape, and the steam pressure inside the lining rises. The danger point is when the local steam pressure exceeds the tensile strength of the refractory at that temperature: the lining then bursts in a sudden, violent spalling that can eject large fragments with the force of an explosion.

This explosive spalling is particularly feared for dense, low-porosity castables with high strength, because their fine pore structure offers high resistance to vapour flow. The risk is highest between about 100 and 350 °C, exactly the range where free water is still boiling off and where the first chemically bound water begins to be released, and it increases with lining thickness: for a 100 mm lining the pressure relief is comparatively easy, while for linings thicker than about 150 mm the vapour path is long and the temperature gradient steep, making controlled drying mandatory. The design measures that reduce the risk are (a) limiting the heating rate, (b) providing vent paths for the steam (drilled holes, special vents in the casing), and (c) improving the drying geometry by making the lining as thin as the process allows. The same physics explains why castable linings must never be brought directly to process temperature: the standard rule of all refractory suppliers and the international heat-up recommendations is to hold the lining at the boiling range until the surface temperature confirms that the free water has been driven out, and only then to continue the temperature rise.

3. The Castable Families and Their Drying Behaviour

Monolithic linings in cement plants are classified by their cement content, and each family has a characteristic drying difficulty. The established classification is RCC (regular castable, also called conventional cement castable, with 15–25 percent cement), MCC (medium-cement castable, 8–15 percent), LCC (low-cement castable, 3–8 percent) and the ultra-low and no-cement types (ULCC, NCC) at 1–3 percent or below, together with SC (self-flowing castables) that are placed without vibration. The families are compared in the table below:

Type	Cement content	Mix water	Drying behaviour	Typical use
RCC (regular castable)	15–25 %	10–14 %	Most water to remove; long dry-out	Cheap linings, low-duty zones
MCC (medium cement)	8–15 %	8–10 %	Moderate water, moderate dry-out	Preheater cyclones, ducts
LCC (low cement)	3–8 %	5–7 %	Less water, denser pore structure, spalling risk	Kiln inlet, riser ducts, burner pipe
ULCC / NCC	0–3 %	4–6 %	Least water; needs special placing, careful cure	High-wear, high-temperature zones
SC (self-flowing)	3–8 %	6–8 %	Dense, must dry slowly; no vibration needed	Complex shapes, dense areas

Two practical consequences follow. First, the drying curve of a castable installation must be selected according to the actual class and thickness installed, not according to a plant standard value: drying an LCC on the fast curve of an RCC is a classic cause of spalling. Second, the chemically bound water of the cement bond requires the holding range around 300–500 °C; skipping those holds, even after the surface looks dry, leaves hydrates that later decompose during production, producing steam deep in the lining where no surface observation can detect it.

4. Drying Measures: Vents, Holes, Stitching and Air Paths

Because steam must leave the lining through defined paths, good installation practice prepares those paths before the heat-up begins. The most important measures, treated in detail in the refractory heating-up literature, are the following:

• Vent holes drilled through the lining. For thick monolithic linings (more than about 150 mm), plants drill small holes (6–12 mm diameter) through the full lining thickness in a grid, typically every 300–600 mm in the zones most exposed to steam accumulation, so that vapour escapes directly to the outside or to the interior rather than pushing against the shell.
• Vents in the casing. Where the lining is cast against a closed steel plate, small openings or drillings in the plate are provided; some vendors supply special vented anchor patterns that guarantee an escape path.
• Stitching (venting grooves). On large flat areas and on precast blocks, grooves are cut into the hot face or the back face to create a continuous vapour channel network that drains steam toward the edges.
• Dry curing rooms for preassembled blocks. Large precast blocks and kiln inlet modules are dried at the manufacturer or in the plant yard for days or weeks before installation, reducing the water load the heat-up has to remove.
• Air circulation. During the first drying phase the kiln is turned slowly with the preheater dampers and tower doors adjusted so that warm, dry combustion gas sweeps all surfaces; stagnant pockets where steam can accumulate are avoided.

Every one of these measures must be decided during the refractory design and checked during installation, because none of them can be added economically once the kiln is back in service. The heat-up procedure then merely exploits the prepared vapour paths.

5. Heating-Up Curves for Brick Linings

Brick linings contain far less water than monolithics — only the mortar in the joints and the dampness of stored bricks — but they have their own heating constraints: thermal expansion, joint closure and the thermal shock sensitivity of magnesia-based bricks. The classic heating-up curve for a brick-lined rotary kiln after a lining change starts with a slow phase to remove joint moisture, typically not exceeding 50 °C per hour up to about 200 °C, followed by a holding period at or near the boiling range. The plant then raises the temperature at rates of the order of 50 to 75 °C per hour through the intermediate range, with additional holds at the temperature where expansion of the brick assembly must be accommodated, and reaches the final firing range only after the preheater and cooler linings have also come up.

Typical values used in the industry for a kiln brick lining are a total heating time of roughly 24 to 36 hours from cold shell to firing temperature, but the governing factors are the kiln size, the brick types and the previous temperature history of the kiln shell. A practical schedule often used for modern dry-process kilns after a full reline is: 12–16 hours from cold to 800 °C with holds, then a further 6–10 hours to normal operating shell profile while the kiln is slowly rotated and the refractory is allowed to settle under its own weight. The rule that no stage may be shortened applies with special force to the first 200 °C, because joint mortar contains the majority of the brick lining’s water.

6. Heating-Up Curves for Castable and Concrete Linings

For castable and concrete linings the drying curve is the central document of the heat-up procedure. The universally applied shape is a stepped ramp: a low heating rate from ambient to roughly 110–120 °C (rate 10–25 °C per hour), a long soaking period at that level (frequently 6–24 hours, the exact duration depending on lining thickness, with many suppliers specifying 1 hour of soak per 10 mm of thickness up to a cap) to drive out free water, a second ramp at modest rate to about 300–350 °C with another hold to release the first part of the chemically bound water, a third ramp to about 500 °C with a hold where the calcium aluminate hydrates decompose, and only then the normal rapid heating to operating temperature.

An example of a practical castable heat-up curve for a preheater tower with 150–250 mm thick LCC linings is:

Phase	Temperature range	Heating rate	Hold time
1. Ambient to boiling	20 → 110–120 °C	10–20 °C/h	6–24 h
2. Free water removal	110–120 °C hold	0	8–24 h (per supplier curve)
3. To hydrate release	120 → 300–350 °C	15–30 °C/h	4–12 h at 350 °C
4. To decomposition range	350 → 500 °C	20–40 °C/h	2–8 h at 500 °C
5. To operating level	500 °C → process temperature	50–100 °C/h	—

The total duration for a large monolithic installation is therefore commonly 3 to 7 days, and the plant must plan the heat-up as a round-the-clock operation with a shift log. Exceeding the ramp in phase 1 or 2 is the single most common cause of explosive spalling in castable linings, and no amount of later careful heating repairs that damage.

7. How the Water Amount Is Estimated and Checked

Before and during the heat-up, the plant estimates how much water must be removed, to size the heat-up fuel, the ventilation and the duration. The estimate uses the installed weights: for every tonne of castable, the supplier’s mix water specification (usually 8–12 percent) gives the water content; for mortared brick linings, the mortar consumption (roughly 20–40 kg of mortar per m² of lining surface) times its water content gives the joint moisture; and the added dampness of stored bricks is estimated at a fraction of a percent. The total is then divided by the evaporation capacity of the heat-up gas flow to give the theoretical minimum drying time, which is normally shorter than the curve’s soak times — an indication that the soaks are governed by transport kinetics, not by the energy balance.

During the heat-up the progress of drying is verified by direct observation and instrumentation rather than assumed: the plant watches for condensation on cold surfaces, checks the smell and colour of the exhaust for the first traces of steam, and — most reliably — monitors the shell temperature against the internal temperature. A steady, even rise of the shell surface temperature while the hot face is held constant indicates that water is still evaporating (the latent heat keeps the shell cool); when the shell temperature starts rising quickly toward the hot-face value, the water front has passed and the next ramp may begin. On large kilns, infrared scans of the shell during the heat-up identify wet zones, which remain visibly cooler than their surroundings, and those zones are given extra soak time. Some plants additionally install temporary thermocouples at the refractory/shell interface in critical zones, which is the most informative measurement of all.

8. Heat-Up Equipment: Burners, Fuel and Ventilation

The heat source for the heat-up depends on the plant configuration. For a kiln with a preheater, the heat-up is normally performed with the kiln’s own main burner at very low firing rates, complemented in many plants by an auxiliary heat-up burner or by preheated air from an external hot-gas generator connected to the kiln inlet or to the cooler. Where the kiln burner alone is used, the limitation is the minimum stable flame rate of that burner, which is often still too high for the first drying phase; auxiliary low-capacity burners mounted in the kiln hood or in the preheater riser, fired on light fuel oil or gas, give the fine control needed at the bottom of the drying curve. The fuel is chosen for controllability and clean combustion, because soot deposition on the cold refractory is a real hazard during the low-temperature phase.

Ventilation is the second half of the drying equation. The kiln induced-draft fan is set to pull a moderate negative pressure, the preheater access doors and inspection hatches are cracked open to create a slow, steady air sweep through the tower, and the kiln is rotated periodically (for example, 90 degrees every 30 to 60 minutes in the early phase, later continuously at low speed) so that no lining zone remains in a stagnant steam pocket and so that the load-bearing arches of the brick rings settle uniformly. The flue gas temperature is controlled at the preheater outlet and at the kiln inlet rather than at the burner, because the drying of the upper tower stages depends on the gas temperature available there, not on the flame temperature.

9. Monitoring, Control and Documentation of the Heat-Up

A heat-up is documented like a process campaign. The control room log records, at least every hour: the fuel flow, the hot-gas temperature at the burner, the kiln inlet and preheater outlet gas temperatures, the draft at key points, the kiln rotation mode, the shell temperatures around the kiln (measured by the existing shell scanning pyrometer), and the exhaust steam observations. Any deviation from the curve (a temperature that rises too fast after a ramp, a shell zone that suddenly heats up, a condensation drip in the tower) is an event to be investigated, not merely logged.

The key supervisory rule is that the curve’s holds are never skipped to save time: a hold that is cut short transfers the drying burden to the next phase, where the higher temperatures make steam pressure more dangerous, not less. It is equally important that the heat-up does not stop and restart abruptly; if the heat source must be interrupted, the kiln is allowed to cool slowly, because a thermal cycle is itself damaging to the lining. When the heat-up is complete, the confirmation is made by the surface and shell temperatures reaching their normal operating relationship, after which the kiln feed is introduced gradually over the next hours, in steps, exactly as the commissioning procedure prescribes, and the refractory then experiences the stresses of production loading for the first time.

10. Special Cases: Thick Linings, Preheater Towers and Kiln Cool-Down

Special attention is paid to thick monolithic linings (above about 150 mm), where the vapour path is long and the pressure gradient high. For these, the heat-up curve is flattened further: heating rates are reduced by half, and the boiling-range hold is extended, because the drying front must travel deeper before the next ramp can begin. Stitching and vent drilling are considered mandatory in this thickness range rather than optional, and the plant verifies the water release by the shell temperature behaviour described above.

Preheater towers present their own challenges because their castable linings at different elevations dry at different times: the lower stages reach 500 °C while the upper stages are still in the free-water phase. The control point for the tower is the top-stage outlet gas temperature, and the curve is followed against that temperature, with the understanding that the lower stages dry themselves faster and must be protected by the vents and by the flue gas moisture content, not by individual temperature control. The kiln cool-down after production deserves the same respect: magnesia brick is thermal-shock sensitive, so the plant cools the kiln slowly (typically with the main burner off, the draft reduced and the kiln rotated continuously until well below 500 °C), because the brick lining — still hot, expanded and full of stored strain — spalls far more easily during cooling than most people realize. A careless rapid cool-down can destroy the brick in the burning zone even though the kiln was never shut down for that purpose.

11. The Transition to Production: From Heat-Up Completion to Kiln Feed

Completing the heating curve does not mean the refractory can immediately receive full production load. The final phase of a proper heat-up procedure is the careful handover to process conditions, because the lining must adapt to three new load types at once: the mechanical load of material, the chemical load of the process gases and the thermal load of the burning zone. The standard handover sequence begins with a stabilization period at minimum firing, during which the kiln rotates continuously at low speed and the shell temperatures are scanned repeatedly to confirm that no wet or weak zone has developed; any suspicious zone is re-inspected before feed starts, because repairing a lining after the feed begins costs a full shutdown.

The kiln feed is then introduced in steps: first a small percentage of the normal feed rate for several hours, then progressive increases, with the clinker quality and the shell temperatures monitored at each level. During this ramp the free lime and the burning zone temperature profile must be held within the normal range, because an over-hot burning zone in the first hours stresses the freshly settled brick and the new coating must be allowed to build up gradually. The first coating forms on the new brick surface slowly, and the plant intentionally runs at moderate burning temperatures for the first 12 to 48 hours so that a stable, well-adhering coating develops rather than a loose crust that falls off with the first thermal fluctuation. In the preheater, the castable linings reach their equilibrium temperature profile during the same period, and the tower gas temperatures are kept within the curve’s final level until the upper stages have come fully into balance. Only when the kiln reaches full production at stable quality and the shell scanning shows a uniform, low shell temperature all around does the heat-up campaign formally end.

Plant experience repeatedly confirms two truths about this handover. First, the failures that appear in the first weeks after a heat-up are almost always traceable to the heat-up itself: zones that were heated too fast, holds that were shortened, or vent paths that were omitted. Second, a well-executed heat-up followed by a careful feed ramp gives the longest lining life, because the brick and castable joints, the anchors and the coatings have all been allowed to settle under controlled conditions. The heat-up document should therefore be archived with the lining campaign records, together with the supplier’s curve and the installation quality records, so that the next campaign starts from measured experience rather than from memory.

12. Common Heat-Up Mistakes and the Lessons of Plant Practice

The refractory failure statistics of the industry repeatedly identify the same heat-up mistakes, and an honest plant engineering department uses them as the checklist for its own procedure. The first and most common mistake is shortening the boiling-range hold because the schedule pressure of a shutdown is high; the damage appears later as delamination and hot spots, and the total cost (re-shutdown, bricking work, lost production) is always greater than the hours saved. The second mistake is heating by flame brightness rather than by measured gas temperature, which leads to far higher real temperatures than intended in the zones directly exposed to the flame, especially at the kiln inlet and in the riser duct; every heat-up must be governed by thermocouple readings at defined points, not by visual judgment of the flame. The third mistake is ignoring the moisture load of new precast modules, which arrive with high residual water and are often mistaken for dry material; they must be treated with the same curve as cast-in-place monolithics.

Further lessons from plant practice: ventilation must never be fully closed during the drying phase, because stagnant steam accelerates the spalling risk; the kiln rotation during the heat-up must follow the documented schedule, because a stationary kiln allows the brick rings to sag and the coating pattern to freeze in an asymmetric shape; and the exhaust steam observations must be recorded, because they are the only direct indication of where in the system drying is actually occurring. Finally, the plant must never mix two different heating-up philosophies in one campaign — for example, following the brick curve for the kiln shell while the tower follows a fast castable ramp and the kiln inlet follows yet another. The whole system heats as one unit: the tower linings, the riser, the kiln inlet monolithics, the brick shell and the cooler linings share one gas path and one temperature history, and the campaign procedure must define one consistent curve for the entire train, with the controlling temperatures selected at the locations that dry last.

13. Frequently Asked Questions

Q1. How much water is inside a newly installed kiln lining?

For castable and concrete zones, 8 to 12 percent of the installed weight is water (mix water plus hydrate water); for a large preheater with several hundred tonnes of monolithics this means tens of tonnes of water that must be evaporated. Brick linings hold much less, essentially only the joint mortar moisture and brick dampness.

Q2. Why can’t the kiln be heated up quickly to save downtime?

Fast heating traps steam inside the lining; when the steam pressure exceeds the refractory’s strength, the lining explodes (explosive spalling), destroying the refractory and endangering personnel. The drying curve is the physical limit, not a procedural formality.

Q3. At what temperatures must the heating curve be held?

The essential holds are around 110–120 °C for free water removal, around 300–350 °C for the first hydrate decomposition, and around 500 °C for the remaining chemically bound water of cement-bonded castables. Holding durations follow the supplier’s curve and the lining thickness.

Q4. What is stitching and vent drilling for?

They are deliberate vapour escape paths: vent holes (6–12 mm, on a 300–600 mm grid) drilled through thick monolithics and grooves cut on the surface create channels that let steam leave the lining without building pressure, reducing the explosive spalling risk.

Q5. How do I know that drying is complete before raising the temperature?

The classic indicators: the shell temperature begins to rise quickly at constant hot-face temperature (the water front has passed), steam ceases to condense on cold parts, and temporary interface thermocouples approach their final values. The curve’s scheduled holds should still be respected as a minimum.

Q6. Can the heat-up burner of an existing kiln handle the drying phase?

Often not: the minimum stable flame of the main burner is too hot for the first phase. Plants use auxiliary low-capacity heat-up burners (fired on gas or light oil) or an external hot-gas generator for fine temperature control at the bottom of the curve.

14. Final Summary

Drying and heating up of a refractory lining is a controlled process operation whose physics (water transport, steam pressure, hydrate decomposition) dictates an unchangeable sequence of ramps and holds. The water content of the installation must be known and estimated, the vapour escape paths (vent holes, stitching, casing vents, air circulation) must be prepared during installation, the heat-up curve must be matched to the lining class — brick or castable, RCC, MCC, LCC or SC — and the whole campaign must be monitored, logged and confirmed by shell temperature behaviour before the kiln goes into production. The complete 73-page reference document provides the detailed curves, the estimation methods and the step-by-step plant procedure that make a heat-up safe, repeatable and fully documented.

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