Thermal Stress Cracking in Calcined Materials: The Role of Kiln Loading and Bed Depth
Thermal stress cracking in calcined materials is usually blamed on fast cooling, but the damage often starts much earlier, inside the kiln. How full the kiln is and how deep the bed of solids sits both decide how evenly each particle gets heated. When heating is uneven, particles leave the hot zone with temperature and shrinkage differences built into them, and these later show up as cracks, fines and weak lumps.
This article explains how kiln loading and bed depth affect thermal cracking in minerals such as limestone, dolomite, magnesite and kaolin, and which checks help process teams keep the product intact.
What thermal stress cracking means in calcined solids
Calcination heats a solid to the point where it loses a volatile part, such as carbon dioxide from limestone or chemically bound water from clay, without melting it. As the volatile part leaves, the solid shrinks, becomes more porous or changes its crystal structure. Each of these changes alters the volume of the particle.
Cracking begins when one part of a particle tries to change volume more than the part next to it. The stress that builds up depends mainly on three things: the temperature difference across the particle, how stiff the material is, and how much it expands or contracts with temperature. Most mineral solids handle squeezing well but fail when they are pulled apart, so cracks usually start at the surface where tension is highest. Many of these cracks are microscopic, yet they lower the strength of the lump and make it easier to break during handling.
Residual thermal stress is the stress that stays in a particle after it has cooled to room temperature. It stays locked in because different zones of the particle shrank by different amounts, or because one zone changed phase earlier than another. A lump can look sound and still carry this stress until a drop, a load or moisture pickup releases it.
How kiln loading changes heat flow through the bed
The number of solids in the kiln cross section is known as fill degree, or kiln loading. It is usually in the range of 8 percent to 15 percent in rotary kilns and depends on the material and the kiln design. Heat reaches the bed from the hot gas, the flame and the refractory wall. The majority of it falls on the exposed top surface of the bed and under the bed on the wall. From there, it flows toward the inside by the close contact of particles which is slow due to the presence of gas between the particles and the poor thermal conductivity of this gas.
The thicker the bed the more surface is exposed when the load is increased. The heat input has to be shared with more material, meaning that each one of the material particles receives less heat input. The residence time of the particle in the near surface area will also become shorter during each revolution of the kiln. This translates to a broader range of particle temperatures at a given position in the kiln, and it is that spread that is the source of cracking problems. Some of the particles are completely calcined and others have an uncalcined core.
Such light loading presents another problem. Particles warm rapidly, the outside of each particle is subjected to a rapid increase in temperature from flame radiation and the outside of the particle expands long before the inside. Furthermore, light loads are likely to result in higher dust carryover which, in turn, means that fines will be lost before they can be identified as a quality problem. Neither side is right. The most important thing is the filling degree that remains constant throughout the shift.
Bed depth and temperature differences inside the material
Bed depth is the depth of the bed material from the bed surface to the kiln shell. There are two parts to the bed in a rotating kiln. The very thin top layer rolls and cascades, and thus, the particles are repeated in contact with the hot gas. The lower part moves along with the wall like a solid block until it is lifted into the active layer.
For a deep bed, a higher percentage of particles will be retained in the lower portion of the bed that flows slowly. These particles arrive at the active layer late, with a cooling time longer than the time it takes for the gas to be much hotter. The rapid temperature increase is called thermal shock. There is another complication within the lump. When the outside is hot and the inside is still breaking down, the gas will be forced to emerge through a hard-shell covering, such as a carbonate rock. Pressure can be created causing the particle to split. This is frequently referred to as “thermal decrepitation” and is one of the major causes of fines during calcination of limestone and dolomite.
Loading is not the only parameter that determines the bed depth. A slower rotation speed and a flatter kiln slope will both cause material to spend more time in the kiln, and will also make the bed deeper. In surging fed conditions the depth of the bed changes constantly and hence the heat treatment of the particles varies significantly within a batch.
This also depends on the particle size. The rotation of beds causes the separation of the beds by size, smaller particles will be in the center and the bigger lumps will be on the outside. The particles that will have the longest heat path will often be the ones that will be in the worst position.
The cooling stage and calcination cooling rate
Solids proceed into a cooler after the hot zone. The additional stress induced at this stage will be determined by the calcination cooling rate. Large lumps, materials with low thermal conductivity and materials with high thermal expansion are affected most.
Cooler is continuous of kiln loading. A thick and rough layer on a grate cooler allows air to flow through channels which causes some lumps to get a very cold surface whereas others cool slowly. The existing stress on the lump is then further pushed beyond the limit due to the presence of the kiln. If the product is quenched using water or moist air, another problem may arise, for example hydration cracking in lime, and should be separated from the thermal cracking problem when diagnosing a fault.
Signs of thermal cracking in a finished product
Thermal cracking is often silent but some indications are readily apparent. Screening may reveal more fines than would be indicated by the size of the feed. Lumps can break when being conveyed or pressed lightly together. If the broken lump is noticeably different colour at the core, then some uneven calcination has occurred. Reactivity can be batch-to-batch. If a larger piece of the rock is available, then hairline cracks can sometimes be seen with a hand lens.
Calcined material testing for cracking and residual stress
Thermal cracking is not a single test, but rather a combination of a few simple tests is used in most plants. A tumble or drop test is followed by a sieve analysis that demonstrates the reduction in sample size due to breakage as compared to the initial size distribution. A spread of values is beneficial if it is made by crushing individual lumps and recording the spread of the values of strength; a large spread will often indicate that the heat treatment has not been even. The loss on ignition is calculated for the outer shell and core separately and this can be used to determine if the core has been completely calcined. The measurements of porosity and surface area provide insight into the degree to which the structure has opened up and is able to react. To study or troubleshoot, the sectioned sample, when viewed through an optical or an electron microscope, shows the microcracks directly. The results are much more useful if they are taken at two locations. Samples taken at the kiln discharge and after cooler will indicate if cracking occurs on the material when it leaves the kiln, or will crack during the cooling cycle. The fill level, rotational speed and feed rate at the time of sampling should be recorded for each sample, allowing the results to be correlated to the operating conditions.
Reducing thermal stress cracking in calcined materials
Industrial calcination process optimisation for cracking usually comes down to a few steady habits.
Keep the feed rate and feed size consistent. Screening out very fine and very coarse fractions before the kiln narrows the range of heat paths inside the bed. Adjust fill degree in small steps and judge each change by product strength and core loss on ignition rather than by throughput alone. Choose rotation speed and slope together, so the bed depth stays the same from one shift to the next.
Look at the temperature profile along the kiln. A gentle rise through the preheating zone is easier on the material than a sharp jump near the flame. Where possible, use bed temperature probes or shell scanners so changes in bed behaviour are noticed early.
At the cooler, spread the material evenly, keep air distribution balanced and avoid sudden quenching unless the material is known to tolerate it. A slower first stage of cooling gives the core time to follow the surface.
Bringing it together
The heat received by each particle and heat distribution will depend on the amount of heat that is introduced and the depth of the bed. Then, on top of that, cooling put additional stress. If these are all constant, the particles exiting the process will have minimal temperature changes and low residual stresses, with little risk of cracking. If one of them backs out of alignment, it’s difficult to know at first until fines and weak lumps are seen downstream.
Frequently asked questions
Is thermal stress cracking the same as thermal shock in calcined materials?
They are closely related but not identical. Thermal shock is a sudden temperature change that causes cracking quickly. Thermal stress cracking is the wider term and includes slow build up of stress from uneven heating, uneven shrinkage and uneven cooling. Thermal shock is one way thermal stress shows up.
Can a kiln be overloaded even when the product is fully calcined on average?
Yes. An average value can hide a wide spread. In an overloaded kiln some particles may be fully calcined while others at the bed core are not, and the overheated ones may carry the most stress. This is why checking the core and the outer shell of lumps separately is more informative than one bulk result.
How can I tell whether cracking started in the kiln or in the cooler?
Compare samples from the kiln discharge and from the cooler outlet. If lumps already show internal cracks, core colour differences or high breakdown in a tumble test at the kiln discharge, the cause is mostly on the kiln side. If they are sound at discharge but produce more fines after cooling, the cooling stage is the more likely source.
Does particle size change how bed depth affects cracking?
It does. Larger lumps take longer to heat through and have a bigger temperature difference between the surface and the core, so they crack more easily. Size separation in the rotating bed can also place fine particles in the core and large lumps toward the outside, which changes how each size is heated. Narrowing the feed size range helps reduce this effect.
Is some microcracking acceptable in calcined products?
In some uses it is. Fine cracks can increase surface area and reactivity, which certain applications actually want. The concern is uncontrolled cracking that creates fines, weak lumps or batch to batch variation. The right level depends on how the product will be used, so it should be judged against the specification for that use.
