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How to Extend the service life of refractory in cement rotary kiln

factors affect the service life of the refractories are mechanical stress , thermal stress , production operation conditions  & raw mix design. we will analyze them one by one to reach a method for extending the service life of the kiln Refractories.

Abstract :

dry kiln production has become the mainstream trend in Cement industry, the dry production technology involve calcination where the clinker calcination temperature reach 1450 ℃  , such high temperature require special kiln refractories.

Function of Refractories in cement kiln

• to avoid high temperature kiln shell deformation
• reducing heat loss during firing to avoid the effect of the clinker to the kiln directly abrasion
• prevent chemical attack

Factors affecting the life of refractories:

Many factors affect the life of refractory brick kiln but there are  three main factors: the influence of mechanical stress, thermal stress and production operations.

influence of mechanical stress on the service life of refractory bricks

Mechanical stresses are due to external forces causes deformation , internal forces generated in the interaction between portions of the body, against the action of such external factors, and try to return an  from a deformed position to a position before the deformation.

The main factors of the mechanical stress has the following two reasons:

(1) ovality. Since the kiln lining bricks, kiln feed and barrel combination of factors caused by self-weight deformation of the kiln shell, under gravity and thermal load is applied, the cylindrical body of circular cross-section becomes an ellipse. When the operation of the kiln, cause mechanical stress to the refractory bricks ellipse, and the greater the mechanical stress generated ovality are greater. Change the ellipticity of the shearing stress generated in the tangential direction of each ring brick, so that an annular causing flaking brick, Figure 2. In general, stripping uniform thickness, hard.

Offset axis (2) of the rotary kiln. Rotary kiln is a belt, roller, support rollers, which axes of each section should kiln and the circle center point connection in a straight line, but in the kiln shell mounting portion and the cutting cylinder after replacement, or kiln after a period of operation thereof and unstable kiln thermal system, thermal load and under load of action, the axis of the kiln can cause migration. Thereafter, the operation and then by a long period of wear, the belt and roller, an outer roller and an inner partial partial, load conditions of each fulcrum vary, especially when the fulcrum load is too large, is likely to cause the pad roller burning, belt and roller surface anomalies peeling or cracking, which in turn further aggravate the kiln axis offset; bricks would result of compression deformation can cause damage or loss. After the damaged bricks shades shape

Thermal Stress

It refers to a thermal stress when the temperature changes, the object due to external constraints and mutual constraint between the portions of the interior, so that it is not completely free of shrink and expansion stresses generated. High temperature thermal expansion, easy to make bricks compressive stress to produce axial expansion, exfoliation is refractory kiln, breakage of the important reasons. Magnesium or chrome spinel brick, for example, when the expansion rate of 1.6% L400 deg.] C can be calculated according to [3], which is the amount of expansion of the refractory length up to 198mm 3.17mm, such a large expansion, if not properly to stay good girth, too large or too small will make the bricks appear ballot, out of brick, peeling, seriously shorten the service life of refractory bricks. In the form of bricks cause damage due to thermal stress.

effect of production operations on Refractory life time

Affect the operation of the mechanism of production of refractory bricks life is complex and many factors, it is mainly analyzed from two aspects.

(1) brick damage caused by high temperature calcination. NSP precalciner kiln flame temperature can reach more than 1700 ℃, the other transition zone which, throat and high temperature firing zone, cooling zone, kiln hood, cooler and the nozzle outer part of the operating temperature is far higher conventional kiln at a temperature corresponding parts. Even with the transition zone of high quality refractory material, a rotary kiln, the firing zone and cooling zone of the furnace lining life is generally 0.5 to 1 year, or even only a short three to five months; kiln lining and the nozzle life is generally only 0.5 to 1 year, or even less; kiln hood and cooler throat kiln lining life of about 2 years. In the pre-production stage, generally kiln operation rate is typically only 70% to 75% or less, very few can reach 85% to 90%. If we add the preheater, calciner adverse operating conditions, the degree of decomposition of the material into the kiln is very unstable, makes various process kiln with frequent changes in position, will lead to instability in the kiln operation, kiln lining damage faster. For example, if the calcination temperature is too high, it can cause refractory damage to the kiln crater, see the below figure circled.

Injury brick (2) due to the faster speed of the kiln. Dry process kiln speed often up to 3 ~ 3.7 r / min, even up to 4 r / min or more, the line speed of the rotary kiln reaches 1m / s or more. At high speed, and large-diameter high-temperature dry process kiln, the kiln lining thermal stresses, mechanical stresses and synthesis damaging effects of chemical attack is much greater than conventional kilns. This requires dry process kiln, whether in a cold state or hot conditions, which kiln lining shall have sufficient strength and stability.

technical measures to extend the service life of refractories

ovality deformation control

Ellipse rate is an important indicator reflects the extent of rotary kiln shell oval deformation, using the following formula (Switzerland Holderbank company first proposed):

Where: W- ellipticity;

  Da – the effective internal diameter of the kiln, m;

  D – outside diameter of the cylinder, m;

  σ– cylinder test shows the maximum deviation of the measured meter, m.

  Swiss Holcim ellipticity value management company (formerly Holderbank) presented, generally as ellipticity management reference value, this value has an upper limit value and the lower limit value, and kiln diameter linear relationship, between the upper and lower portions It is over the range shown in Figure 6. The upper limit is the maximum limit allowed on bricks that can damage the large kiln load or deformation due to the gap caused by the value exceeds the upper limit will adversely affect the bricks, causing the bricks falling and other accidents, damage caused by refractory bricks. The lower limit is to avoid the kiln shell belt is sandwiched (clamped phenomenon of so-called belt) is insufficient clearance values.

  Practice shows that the wear rate of the brick kiln thermal ellipticity certain correlation (Swiss company Holcim (formerly Holderbank) made) by the approximate formula obtained their relationship.

  W = 1.22 × 0.002Dm + 1.5V

  W ‘= W / D × 100%

  Where: W ‘- ellipticity%;

  W– ellipticity;

  Dm– tire diameter;

  D– cylinder diameter;

  V– slip.

  Calculated: of the kiln (kiln operation from the start to the maintenance shutdown time) is 1 year, the kiln thermal allowable value of the ellipticity should be controlled at 0.42% or less, the kiln period of six months, to control the ellipticity at 0.78% or less . However, in the baking zone, because the increase will cause ovality crust unstable, causing abnormal melting damage loss, so the firing zone to be more closely ellipticity management.

  Oval-rate management is not only to stabilize the kiln operation and prolong the life of the brick is important, but also through the hot kiln ovality can be measured periodically to accurately grasp the mechanical condition, so that it can develop a more accurate kiln maintenance plan.

Rotary axis offset control

The actual production process, strict control of the kiln axis offset can not exceed 3mm, or correction must be processed. Practice has proved that, when the axis of the kiln at the hot accurate adjustment, bending and twisting of the cylinder are minimized, load distribution on each of the fulcrum is relatively appropriate, kiln bricks, kiln cylinder, belt underlay plate (base plate), a belt, a support roller, roller shaft, the pressure on the pad roller is small; decreased and the wear rate of the refractory bricks, prolonged service life, and can greatly improve the operation rate of the kiln. Therefore, it is necessary to strengthen the timely corrective work kiln axis and the hot line detection offset.

  The method of adjusting the axis of the kiln there are two: one is in a cold condition of the kiln, the kiln is obtained inside the respective pivot axis and gear, using a light beam, while adjusting the rotary kiln axis side roller adjustment in a straight line on; the other is in a rotary kiln at a hot operating state, the axis of rotation of each pivot is obtained from an external kiln and roller side while adjusting the rotary kiln axis adjustment in a straight line. Practice has proved that the latter is more effective.

  3.3 reasonably determine girth sized to reduce thermal stresses

  When the rotary kiln is a refractory brick girth masonry, is expandable to reduce the high temperature firebrick produce an axial compression stress damage to the refractory bricks and a gap is provided between each ring of refractory bricks. Girth dimension size should be based on linear expansion coefficient of the refractory brick, the use temperature determined by calculation. By such calculations, the amount of expansion is fired with brick 3.17 mm, the actual masonry, girth dimension is between 3.5 mm ~ 4.0mm; and plate can be attached to each ring sutural high combustion temperatures fibreboard (Japan resistance industry-specific material), kiln drying should be studied and the system operation, but also the thermal stability control system during normal production, to minimize any axial compressive force.

  Controlling the calcination temperature 3.4

  Calcination temperature influence on the service life of refractory bricks is extremely significant. Thus, only the reasonable control of the firing temperature, to maximize the life of the refractory bricks, kiln to improve efficiency and reduce the consumption of refractory material, in order to increase production, cost reduction. The maximum temperature must enter the process management process management, it can be tested in conjunction with a flame color ratio method, thermometer, crystalline phase of clinker analysis. Wherein: the general color flame control flame temperature 1500 ~ 1600 ℃; thermometer clinker falls mainly for controlling inlet temperature, typically 1320 ~ 1400 ℃; crystalline phase analysis of the clinker to control the birefringence of C3S, general control of 0.007 to 0.008.

  Management and control of the kiln speed 3.5

  In the course of the production operation, can not blindly pursue high speed increase clinker production, to be matching the amount of material, kiln speed and kiln calcination conditions, in order to achieve the best results. The crystalline phase while the clinker size, growth conditions to determine whether the control value suitable kiln speed.

Conclusion

  As the refractory cement variety of industrial applications is increasing focus on energy saving Today, cement production enterprises should learn more about the performance of refractories, the rational use of refractories, take a series of technical measures and control, maximizing refractory material life, the refractory consumption to a minimum level. Control and management of the technology by the above factors, the original service life of refractory  to extend down to 14 to 16 months, kiln operation rate of over 85% to 90%, clinker production, mass They were all improved.