kiln Refractories & lining – bricks refractories lining calculator sheet

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Kiln lining

The operational time of rotary kilns depends prirnar- ily on the quality of the kiln lining. To insure long- time kiln operation one willing珍makes allowance for a.higher price for good quality kiln refractory. Unscheduled kiln downtime caused by poor quality kiln lining, results in costly production losses.

The function of the kiln’s refractory lining is:

1. To protect the kiln shell from the influence of the flame and of the hot kiln feed;
2. Reduction of heat losses caused by radiation and convection of the rotary kiln shell.

The kiln lining also takes over the heat energy from the kiln gases, transmitting it to the kiln feed. Since the temperature of the kiln gases is considerably higher than that of the kiln feed, the kiln lining is exposed to periodical heat shocks during the kiln’s rotation. Due to the kiln’s rotation, the kiln lining is also exposed to mechanical stresses. Also the abra- sive action of the kiln feed on the lining is a wear fac– tor which should not be neglected.

In the burning zone of the rotary kiln, where the material temperature amounts to 1400一1500″ C, and where gas temperatures are several hundred degrees higher, there is一besides the above mentioned influ-ence factors一an additional element which acts upon the kiln’s lining, viz, the chemical attack caused by the gases and the kiln feed.

The ever increasing specific throughput of the rotary kilns, and the increase in heat capacity require higher quality kiln lining [282工 The content of liquid phase in the clinker is decisive for the reaction between kiln feed and kiln lining in the burning zone.

The demands made on the kiln’s refractory depend on the following properties [275]:

1. Mechanical strength
2. Refractoriness
3. Resistance to temperature changes (thermal shock)
4. Resistance to chemical attack
5. Thermal expansion or stability of volume
6. Thermal conductivity
7. Resistance to abrasion
8. Porosity

Refractoriness

Mechanical strength

Mechanical strength (also called cold crushing strength) of kiln lining bricks must comp厅with the followinc recouirernents:

Refractory bricks with high A1203-content:

350一500 kg/cm2 cold crushing strength; dolomite and magnesite bricks 500一700 kg/cm2 cold crushing strength.

The cold crushing strength of refractory bricks can be easily tested; this strength is a critirion for the behav- ior of the brick at the operational temperature in the rotary kiln. The high compressive stress of the refrac- tory brick exposed to the operational temperature in the kiln, requires a high cold crushing strength.

In connection with this, reference is made to the sta- bility of the rotary kiln’s shell, and its deformation during kiln operation. The largest deformation of the rotary kiln, i.e. the deviation from the circular shape of the shell occurs near the riding rings. Continuous measurements during kiln operation with the shell- test apparatus showed that radial deformation or ovali诊，i. e. the difference between the horizontal and the vertical diameter amounts to 0.3 0/0 during the revolution of the kiln older kilns showed differences up to 0.6一0.7%.This cyclic pressure and unburden- ing, if it is not absorbed by the kiln’s lining, can lead to its destruction [276, 277].

Testing of the cold crushing strength is performed according to ASTM-Standard C-133.

Resistance to temperature changes (thermal shock)

During one kiln revolution which normally takes less than one minute, the kiln lining is once exposed to the hot kiln gases, and the other time it is covered by the kiln feed. The periodic temperature fluctuations of the lining’s surface can amount up to 400 degrees. The ability to resist these constant temperature changes is denoted as resistance to thermal shock.

Resistance to thermal shock is tested by heating the sample brick during 40 minutes up to 950 00, and sub- sequently cooling it with cold water (3 minutes), up to a weight loss of 50 0/ (German Standard DIN 1068). High alumina bricks should resist 15一25, magnesite bricks 5一15, and chrommagnesite bricks up to 20 test cycles. Generally, basic bricks are tested with corn- pressed air.

The resistance to thermal shock is also a criterion for heating up the rotary kiln’s burning zone magnesite bricks. It is the recommendation can magnesite manufacturers [278] that should be heated up to 1100″C within 4一5 hours at a uniform rate. From that point on, raw mix should be fed into the kiln. After aimroximatelv two hours iaepenaing on tile length 01 tile lain), tile raw mix will enter the burning zone, which in the meantime was heated up to the operational temperature; other man- ufacturers recommend a heating time of 12一16 hours. Sudden heatings are not as detrimental to the magnesite and dolomite lining as sudden coolings; this should be particularly taken into consideration during kiln operation.

In connection with this, special attention should be paid to the sensitivity of the magnesite bricks to the influence of water and water vapor, especially in the temperature range between 60一295″ C. In this tern- perature range, the free magnesium oxide of the mag- nesite bricks shows a special disposition to combine with water or water vapor, forming magnesium hydroxide. This process is connected with an increase in volume, resulting in the breakdown of the magnesite brick’s structure. This phenemenon can occur when the combustion water of the flame, espe– cially of a natural gas flame condenses on the cold refractory lining. The tendency of the magnesium oxide to combine with water vapor decreases with increasing temperature and discontinues at 295″ C. In practical operation when starting the rotary kiln, it is the intention, to pass the critical temperature range of 60一295 O C as quickly as possible, to prevent hydration of the magnesium oxide.

Resistance to chemical attack

The chemical character of the kiln feed with a CaO- content of about 65 0/ is strong basic. In the high tem- perature of the burning zone, the kiln feed develops a strong chemical attack against the lining of the burn- ing zone. Besides, the kiln feed consists in the burn- ing zone by about 20一25 0/0 of liquid phase, which makes it even more apt to attack the refractory uin- ing. Almost all refractory material in the burning zone is subject to chemical attack by the kiln feed, except magnesite and dolomite; since these materials themselves have basic properties, they are immune to the clinker’s corrosive attack. This applies especially to dolomite, because of its high CaO-content.

The aggressiveness of the clinker depends also upon the value of the silica- and alumina ratio; with an increasing value of these ratios, also the aggressive- ness of the clinker increases. With the following val– ues of the mentioned ratios, the clinker shows no aggressiveness:

When firing coal, relative珍fast melting ashes con- tact the kiln lining with which they enter into reac- tion. Also combustion gases, especially carbon mon- oxide, can contribute to the destruction of the kiln lining.

thermal expansion or stability of volume

Despite the circumstance that the expansion coeffi- cient of the kiln shell is higher than that of the kiln lining, the linear thermal expansion of the kiln shell is lower than that of the lining. The reason for this is the kiln’s shell temperature, which under normal con- ditions does not exceed 280一365 00, whereas the mean temperature of the kiln lining, amounts to about 800一900 00. In the burning zone, the inside surface of the kiln’s lining can reach temperatures of up to 1350一1400 00. This causes high compressive stresses, especially on the free surface of the lining, which sometimes results in spalling of the brick’s upper layer. In the United States, the resistance against spalling is tested according to ASTM-Stan– dard 0-122 (Spalling Resistance Test).

Magnesite is characterized by the highest thermal expansion, which at 1400 00 is 2%.The linear ther- mal expansion of special magnesite bricks is 1 / at 1000 00, and 1.5 0/ at 1400 00. Refractory bricks with 70 0/o Al203 content show a linear thermal expansion of 1.0一1.2%.

On the example of a 3 m diameter rotary kiln with magnesite lining in the burning zone, it can be seen that the lining along the circumference in relation to  the kiln shell, will expand more by the following amount (assumed kiln shell temperature 365″ C, ambient temperature 20″ C, expansion coefficient of steel 0.000012/degree C, linear expansion of magne- site brick 2%):

3 m 0 x 3.14 x [0.02一（345 x 0.0000 12)]~0.149 m or 149 mm. When lining the kiln, this length is compen- sated by expansion joints, mostly by the use of asbes- tos lamellas or cardboard spacers.

The linear expansion of the burning zone lining (12 m long) will amount:

l2m x [0.02一（345 x 0.000012)]~0.190 m or 190 mm. This length also must be compensated by expansion joints.

In the United States, the thermal expansion of refrac- tory is tested according to ASTM-Standard C-l13 (Permanent linear change in heating).

In connection with the above named spalling effect, it should be mentioned that in the last several years magnesite bricks were supplied with sheet metal plates (socalled steelkiad plating). These plates manufactured from easily oxidizable steel, are mechani-cally attached to the side wall of the magnesite brick. At temperatures above 1000″ C the plates oxidize and combine with the bricks to a solid block, thus making the kiln lining resistant to spalling; besides, the provision increases the stability of the lining in the rotary kiln. The oxidation of the steel plates and the process of reaction with the magnesite, cause the formation of magnesioferrite, MgFe2O4, which corn- bines the magnesite bricks to a monolithic mass. As is well known, magnesioferrite, also called magnoferrite [279], usually constitutes the brown coloring material in magnesite bricks [2801. Also dolomite bricks are supplied with steel kiad plating.

As is known in practice, magnesite bricks are high- priced and represent twice the price of dolomite, and three times the price of high alumina refractory. Therefore magnesite bricks are only economically applicable if their operational lifetime will be two or three times longer than that of dolomite or high alu- mina brick respectively. In some countries however, the price of high alumina bricks is the same as that of magnesite bricks. Therefore when saving on opera- tional costs, it is advisable to limit the length of the magnesite lined burning zone to the necessary limits. When calculating the quantity of magnesite bricks for kiln lining, one can ascertain an almost uniform determination of the burning zone length. In the European cement industry the following rules pre- vail: the length of the burning zone 1n rc with up to 4 m diameter, equals four times ing zone diameter, measured on the shell. kilns with diameters over 4 m, the length of the burn- g zone is determined as five times the kiln diameter 81]. The cement industry of the Soviet Union deter- mined the length of the burning zone in all kilns equal to five burning zone diameters, measured on bricks [283.

Thermal conductivity

The thermal conductivity of refractory is denotedwith A. (thermal conductivity coefficient) and is expressed as kcal/m . h . “C.The thermal conductiv­ity of the kiln lining determines the heat losses through the kiln shell. With most refractory material the thermal conductivity increases with increasing temperature; on the contrary, with magnesite and dolomite, this ratio is reversed. The thermal conduc­tivity coefficientfor high alumina refractory at 20°C, A. = 1.00kcal/m . h . “C,and at 1000″C.A. = 1.30.On the other hand, for magnesite material at 20° C, A. = 5.00and at 1000″C, A. = 3.00kcal/m . h . °C; this results in high heat losses,which however, are widely
reduced by the coating covering the lining. To a large extent, the thermal conductivity depends on the porosity of the refractory. An insulating firebrick with a bulk density of 0.8g/cm-, A. = 0.40kcal/m . h .

“C,which is very low.The thermal conductivity coef­ficient of the coating is A. = 1.5kcal/m . h . °C. The kiln shell’s steel has a thermal conductivity coeffi­cient of A. = 40. A high thermal conductivity of the kiln’s lining results in overheating of the kiln shell, and thus in extreme heat losses. On the other hand, with a low thermal conductivity it is difficult to get a protective coating on the refractory lining. The dia­gram in Fig. 23.1.showsheat loses by radiation in per­cent of the applied fuel. depending on the mean tem­perature of the kiln shell in the area of the burning zone, whereby for calculation purposes the length of the burning zone was assumedto be 12m[283].

In addition to the thermal conductivity, the thickness of the lining is also of importance. In the UnitedStates, 150mmthick refractory is used for kiln diame­ters up to 3.65m. Rotary kilns with diameters above3.65m are lined with 230mm and 250mm thick refractory. In Germany the following thickness of refractory is recommended[284]:

Here it should be mentioned that the refractory thickness does not always depend on the kiln’s dia- meter. With good coating conditions in the burning zone, the refractory bricks are often selected one or even two grades thinner.

In cases where the rotary kilns are not located inside a building, the motion of the surrounding air influ- ences the heat losses of the kiln’s shell by radiation. The following tabulations enumerate the losses at ambient still air, as well as with air which is in motion. At ambient still air temperature of 21″ C, the heat losses by radiation per square meter of kiln shell and hour are [285]:

On the one hand a thicker kiln lining decreases theuseful kiln volume and thus the throughput of therotary kiln; on the other hand however, a thicker kilnlining improves the heat economy of the kiln in reducing the kiln shell’s heat losses by radiation. In the United States, the thermal conductivity is tested according to ASTM-Standard C-201 (Heat flow through refractory).

Resistance’to abrasion

The kiln feed which during the kiln’s revolution slides on the kiln lining, causes abrasion of the refrac– tory. The resistance to abrasion depends on the mechanical strength of the refractory. The resistance to abrasion is measured in cm3 per cm2 of the exposed surface. Magnesite and dolomite bricks as well as high alumina refractory (70一80 0/0 Al203), should have an abrasion loss of not more than 0.2 cm3/cm2. Refractory with an abrasion loss of more than 0.25 cm3/cm2 should not be used in rotary kilns.

In the burning zone, the resistance to abrasion is mostly exploited only during kiln start-up operation. After formation of coating, the lining is protected against abrasion. Despite high mechanical strength and resistance to abrasion, a magnesite lining would not withstand the stress in the burning zone without protective coating.

On fireclay bricks coating is formed一if at all一by reciprocal action between the basic liquid phase of the clinker e mnd the acidic fireclay at high tempera- tures. In the case of basic magnesite and dolomite bricks, which show neutral behavior against the basic liquid phase of the clinker, the surface of the lining must first become soft by adequate heating; then the clinker’s liquid phase combines mechanically rela- tive珍easily with the lining’s softened surface, thus creating coating.

The coating drops off periodically, followed by a new formation of coating. The coating’s drop off is thought to be a result of chemical reactions, enumerated schematically as follows [2861:

The FeO acts as liquid phase, and the deposited hite causes expansion.

The expansion can also be explained as the resulf of formation of cementite, Fe3C, which causes an increase in volume by 14.8 0/0.

Or, also the following reaction may be possible:

The coating’s drop off also causes occasional spalling of the upper layer of the refractory lining, which may result in gradual destruction.

The abrasion results in the wear of the refractory material. The following wear rates apply for rotary kiln lining:

For the total kiln arrangement, the following addi- tions apply: the hot and cold kiln hood, preheater, and clinker cooler. For this equipment 20 O/ should be added to the above figures. Based on a 6-year sta- tistic of kiln lining wear, the German Portland Cement Association ascertained a wear rate for the rotary cylinder itself of 0.9 kg/ton of clinker. The costs for material, installation and repair of the refractory lining participate with 2 0/ in the cement’s manufacturing cost.

Porosity

The refractory’s porosity is distinguished between open pore space (relative or open porosity and true porosity. The true porosity is the total of both open and closed pores and is ordinarily expressed as the percentage of the total volume of the refractory brick:

The increase in weight of a refractory brick in boiling water serves as the basis to calculate the apparent (relative) porosity. Magnesite and dolomite bricks show a relative porosity of about 15一21 0/o. relative porosity of fireclay bricks is about 18一25 0/. In the United States, the relative porosity is ascertained according to ASTM-Standard C-20.

In practical rotary kiln operation the closed pore vol– ume is less important and is not determined. On the other hand a high volume of open pores promotes the permeability to kiln gases as well as the sediinenta- tion of condensed gas components in the pores; This may result in the destruction of the refractory. This is true especially for alkali laden kiln gases. In connec- tion with this it should be mentioned that fireclay brick with 50 0/ alumina is less alkali-sensitive than high alumina brick [287].

Installing refractory lining in rotary kilns

The installation of the kiln lining is basically per- formed in three ways:

1. Brick lining with mortar
2. Dry lining of bricks
3. The adhesive method; this method can be applied when dry lining as well as when using mortar.
4. Installation of the kiln lining on the basis of mor tar, employing special mortars for fireclay as well as for magnesite bricks, is mostly applied in Europe. After lining of refractory bricks in the lower semi-cir cle of the rotary kiln, the freshly mortar bonded

bricks, lined up to the horizontal center line of the kiln are kept in position by using screw jacks to press the bricks against the kiln shell. Then the rotary kiln is turned twice, each time by about 90 0, to line the other semi-circle,as it is shown in Fig.23.2.

When using mortar, the installation of the kiln lining is mostly made in the way of longitudinal bonding (see Fig. 23.3.)

In the United States and Europe, the dry installa- tion method of the rotary kiln is ordinari污in use; this method has the advantage that during installa- tion of the lining, turning of the rotary kiln is not necessary. With the dry installation method, the refractory bricks are bonded in parallel rings along the kiln’s circumference, as it is shown in Fig. 23.4.

For the dry installation procedure a steel jack screw assembly as shown in Fig.23.5.is in use [288].

Dry lining of the refractory bricks in the rotary kiln is installed in parallel rings. After installing the lower semicircle, the jack screw assembly is positioned on these bricks, whereupon the lining of the upper semi-circle is accomplished.The particular bricks in the upper semicircle of the lining are held in place by screws until positioning of the key brick.

c. The adhesive method – In large diameter rotary kilns, handling jack screw assembly is difficult, also keeping the bricks in position by using screw jacks is no longer operationally reliable. In addition, this arrangement can result in deformation of the kiln
shell. Therefore a method has been worked out, to line large diameter rotary kilns without the use of scaffolds. This is the so-called adhesive method according to which refractory bricks adhere to the kiln shell with the help of glue with high adhesive strength. The applied glue is a modified synthetic resin (Epichlorhydrin and Diphenylpropane), whose adhesiveness exceeds the refractory’s tensile strength. The bricks are lined in longitudinal bonding. The lining is performed with stripes of glued (epox­ied) bricks, and with loose bricks laid in between and
held in place by epoxied bricks. Only about 25- 30 % of the bricks along the kiln’s circumference are epox­ied (see Fig.23.6.)

When heating the rotary kiln, the synthetic resin loses its adhesiveness between 100 and 400°C; thusthe thermal expansion of the lining is not impaired by the epoxied briks.

Examples for rotary kiln lining

In the United States, a 4.85 x 75 m preheater kiln is lined with refractory as follows; it is measured from the kiln’s discharge end. The lining is throughout 9″(230 mm)thick.
3 m- Fireclay, 70 % Al203 with high resistanceto abrasion
25 m- Magnesite brick, 80 % MgO,possiblychromemagnesite or also dolomite
21 m- Fireclay, 70 % Al203
26m- Fireclay, 50 % Al203
75 m- Kiln length
The following shows the lining of a wet process rotary kiln, size 3.40 x 3.95 x 129 m. the 18 ill long chain section is lined at a length of 15 m with 100 mm high castable refractory.

The ‘brick thickness is 150 mm.

The count starts from the discharge end of the kiln:

3 m- Fireclay, 70 % Al203 with high resistance to abrasion

18 m- Magnesite brick, 80 % MgO,possibly
chromemagnesite or also dolomite
15 m- Fireclay, 70 % Al203
35 m- Fireclay, 60 % Al203
35 m- Fireclay, 50 % Al203
15 m- Castable refractory
8m- Blank zone

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