Cement Kiln Refractory Writeup.pdf

DEGREE OF OVERLAPPING STRESSES IN ROTARY CEMENT KILNS RELATED TO DIFFERENT KILN ZONE SECTION. 1. Upper Transition zone (Pre burning zone) Conditions: a. Unstable Coating b. Increased alkali, chlorine and Suppler attack c. Thermal Shock d. Redox Load e. Mechanical load due to Ovality in Tyre Area (1 x Diameter from center of the tyre – both sides)

2. Burning / Sintering Zone Conditions: a. Protection by stable coating b. Infiltration by liquid phase 3. Lower transition / Cooling / discharge Zone. Conditions: a. Unstable Coating b. High Thermal Shock c. High Abrasion by Clinker d. Alkali Attack e. Redox Load f. Mechanical Load due to ovality in tyre area (1 x Diameter from center of the tyre – both sides)

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REFRACTORY LIFE:The life of refractory in Rotary Kiln depends upon two major factors: 1) USER PART

Selection of refractory 2) PRODUCER PART

User’s Part: 1. Dry Storage 2. Installation of refractories 3. Service Conditions

Producer Part: 1.

Installation Drawings 2. Quality of Product 3. Quality of Raw Materials.

USER’S PART It is assumed that Dry Storage and Installation of Refractory has been done properly. Hence these topics are not described in detail here.

SERVICE CONDITIONS:1) Thermal Stress 2) Chemical Stress 3) Mechanical Stress

THERMAL STRESSES:1) Over Heating a. Very High Temperature than service temperature of Refractory b. Impingement of flame on the refractory Due to the effect of above, the brick samples removed from such sections often have smooth surface significantly sloping in one direction. Concavely eroded hot faces are also frequently observed. If the upper thermo chemical reaction limit of application of alumina brick is exceeded a chemical reaction between the kiln feed and the brick resulting in the formation of liquid reaction product which can be mechanically broken down (erosion/Scouring). To avoid this good coating to be obtained as early as possible. The limit for thermo chemical reaction for 85% Al2O3 brick is 1300 oC and for 70% Al2O3 bricks is around 1250 oC and for 50% Al2O3 bricks it is 1200 oC . Clinker litre weight should be taken as a guiding factor and to be maintained 1200+ 50 gm/litre. In no case it should go beyond 1300 gm/litre to avoid overheating .The flame impingement on the refractory must be avoided by adjustment of burner pipe.

2) Thermal Shock a. Running the Kiln in very upset conditions for longer duration.

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b. Frequent Stoppages. c. Frequent and Fast Heating and Cooling. d. In each and every rotation, there is a thermal shock due to change in temperature

3) Excessive

Thermal Load

Maximum Permissible Specific thermal load = 5.8 x 106 K cal / (M2 hr) Normal Specific Thermal Load = 3.5 to 5 x 106 K cal / (M2 hr)

Example:Production = 3000 TPD, 125 TPH, 12500 Kg / Hr Heat Consumption in Kiln Firing only = 800 K Cal / Kg of clinker Kiln Dia = 5.4M, 2.7M Radius Brick Thickness = 250 mm Cross Section =  (2.7 -0.25) 2 = 18.85 M 2 Thermal Load = 12500 Kg x 800 = 100 x 10 6 K Cal /Hr Specific Thermal Load = 100 x 10 6 K Cal /Hr 18.85 M 2 = 5.3 x 10 6 K Cal / (M2 Hr)

Chemical Stresses:1) Redox = (Reducing

Oxidizing)

Due to a) Carbon deposits in the joints of bricks and on the Kiln Shell b) Incomplete combustion of Coal c) High ash Contents in Coal d) High Residue of Coal / Pet Coke e) Substitute / Alternate Fuel f) Frequent change in Oxidizing and Reducing Atmosphere in the Kiln. Results in g) Redox causes Volume change between trivalent (Red) and bivalent (Green) Iron. Remedies h) Minimum 1.5 to 2.0 % Oxygen at Kiln Inlet is required for complete combustion. i) Coal Fineness to be improved to avoid Redox conditions.

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CLINKER MELT / LIQUID PHASE a) Normal Range = 22 to 28 % b) A/F Ratio = Al2O3 / Fe2O3 =1.38 Ideal Less than

Viscosity

1.38

Decreases

More penetration of liquid into the bricks pores

Bricks structure changes due to different thermal expansion (Thermal conductivity changes)

More Spalling of Bricks  A/ F = < 1.38 , 1338 oC Liquid Phase = 8.2 Al2O3 – 5.22 Fe2O3 + MgO + K2O + Na2O + SO3  A / F = > 1.38 , 1338 oC Liquid Phase = 6.1 Fe2O3 + MgO + K2O + Na2O + SO3  A / F = 0.64 , 1450 oC Liquid Phase = 3.0 Al2O3 + 2.25 Fe2O3 + MgO + K2O + Na2O + SO3

NB:- In the above formulas MgO %age should be taken 1.5 Max

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3) a)

ALKALI SALTS

Alkali Sulphate Ratio ( ASR ) Na2O / 62+K2O / 94 – Cl / 71

ASR = SO3 / 80

Should be 1(Balanced ratio of Alkali and Sulphate) When ASR is less than 1, surplus of Sulphate SO3/SO2 reacts with CaO to form CaSO4 anhydrided (Increase in volume by 275%). More Penetration in to the bricks Volume changes More Spalling

When ASR is more than 1, surplus of alkali reacts with MgCr2O4 (Magnesium Chromate) MgCr2O4 + Na , K = Na2 , K2 (CrO4) Alkali Cromate

Volume Changes Destroying the structure of Brick When ASR is = 1 , Balanced ratio Brick Lining free from alkali or sulphate

Alkali and Sulphate reacts with each other and forming neutral compound as Na2SO4 , K2 SO4 , NaCl,or KCl No reaction of Alkali / Sulphate with the Bricks Material

No attack on the Brick and no damage (5)

MECHANICAL STRESSES 1) 2) 3) 4) 5) 6)



Convex Spalling Concentric Stress Crack Spiralling Ovality and Deformation Formation of Grooves Pinch at Retainer Rings

Convex Spalling a) Axial pressure produced by dilatation exceeding brick resistance causing spalling. b) Convex spalling at joint/ corner in longitudinal direction of bricks. c) No proper expansion joint in magnesia bricks. d) Pre-attached cardboard missing. e) Use of bricks with pre attached cardboard for sufficient expansion.

 Concentric Stress Cracks a) Cracks in reaction zone of Steel Shim –lined bricks and Key bricks. b) Concentric stress cracks at 30-50 mm depth around the circumference. c) Steel Shims to be avoided. d) Mortar lining is recommended, e) Steel plates get oxidized and react with bricks forming a monolithic horizon of magnesia ferrite with volume increase.



Spiralling a) Lining displacements due to relative movement. b) Spiralling tilting and edging. Cold face wears from rubbing against the Kiln Shell. c) Due to Loose Installation 1. Increased shell ovality 2. Expansion and contraction due to frequent Kiln stoppages 3. Changing coating formation 4. Deformation in Kiln Shell d) Uniform Kiln Operation is needed. e) Repair of deformed Kiln Shell f) Ovality Control g) Proper installation (tight) with staggered joints. h) Fast cooling to be avoided.

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 OVALITY

:-

Maximum Permissible Ovality

 = D / 10 Example: - 4.75/10 = 0.475 % Maximum permissible Relative movement (Migration) between Kiln shell and Tyre ΔU = D/200 = 4750/200 = 23.75 mm Example: = 4750/200 = 23.75 mm Gap between Kiln Shell shoe and tyre S Theoretical = ΔU/  S Practical = ΔU/ 2.6

OVALITY:  = 0.0396 X S (Practical) + 0.0865 In case ΔU-exceeds D/200 a Shell test measurement is strongly recommended.

The Kiln Shell deforms under the brick weight, the material load weight and under its own weight. The circular cross section becomes elliptical and high compressive forces develop in the lining as the kiln runs. To improve shell stability , the riding rings or tyres are tightly adjusted to the shell with smallest possible clearance.

Even though preventive measures are taken, the shell becomes Oval and lining is Squeezed.  Kiln Shell Ovality in the tyre area, Squeezed and released during each rotation, Shear stressing the Brick heads.  Deep outbreaks of single brick, spalling here and there, in between completely perfect brick sections.  The effect of ovality stresses is up to 1 x Diameter of Kiln from the center of tyre ( Both sides)  Permanent monitoring of migration  Insertion of new spacers (shim)  Best is flexible lining with mortar.  Used tyre shoes increase the clearance leading to excessive ovality.  Limit for Ovality value maximum D/10 %

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FORMATION OF GROOVES : 1) Individual creeks having a redial (vertical) width of approximately 3 – 6 bricks (in circumference), extending several meters axially, not in the area of key bricks. It happens due to thermal shocks during heating up. Kiln is rotated intermittently. In these conditions, the kiln feed material protects the still relatively cold lining at the bottom of the ring for a relatively long time, whereas the exposed lining of the upper part is heated up by the flame. On further rotation of the kiln, the part of the lining previously protected by the feed material is now unprotected and exposed to the flame temperature. The thermal shock occurring amounts to several hundred Deg. C. This may exceed the brick strength and thus lead to spalling : a) Don’t keep the kiln stop in one position for longer durations. b) Installation of brick in deformed shell area should be done properly. 2) Parallel grooves 2-4 bricks wide of deep spalling appearing along the bricks axis. 3) Orientated premature wear channels with in sound lining. 4) Key line with excessive tightness. 5) Damage to Key bricks by Jack Hammer (micro cracks develop which are not visible) 6) More than one metal shim per joint to be avoided. 7) Correct insertion of Key Bricks. 8) Insert liners uniformly in the whole joints. 9) Use shim driver. 10) Use rubber edged hammer for keying.

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 PINCH AT RETAINING RING: 1) Collapse of brick rings against retaining rings and out let segments from pinch spalling. 2) Horizontal cuts and cracks of bricks at the upper edge of retaining rings and transpassing nose ring segments. 3) Thrust and Oscillation generates sheer cracks and grinding. 4) Flexing kiln shell out let / ovality accelerate the damage. 5) Stiff Kiln Shell is required. 6) Reduce Ovality as minimum as possible. 7) Use 300mm Long Bricks just after the retaining ring.

BRICKS FAILURE(COLLAPSE) : In Pre burning Zone coating does not exist and the failure / collapse of lining takes place due to the following main possible reasons. 1) Creeks forming in key section area. 2) Constriction due to localized cooling of exterior kiln shell during heavy rain. Individual creek having a radial (vertical) width of approximately 3-6 bricks (in the circumference) extending several meters axially (Horizontally) in which keying shims or keying bricks are observed. During the installation of bricks, if the key bricks are hammered or wedged in with such excessive tightness as to cause damage to the brick work where micro cracks develop which are not visible from out side. During running the kiln for sometimes this area becomes too weak & constriction of kiln shell takes place during heavy rain. So due to the combined effect of the above, bricks get crushed and finally collapsed.

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REMIDIES:a) Keying of the bricks should be done properly by a skilled person & strict supervision is required. b) Key bricks should not be cut by chisel & hammer. Only brick cutting machine should be used to cut the bricks to avoid any micro cracks in side the bricks. c) Key bricks should not be hammered excessively to avoid micro cracks during installation. d) Cutting of key bricks from standard size should be avoided by procuring different sizes of key bricks. e) MS shims of 2 mm thick should not be inserted near the key bricks. It should be inserted between the full bricks area to tighten the key bricks.

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