Rings & Buildups Mechanisms

13
CEMENT & BUILDING MATERIALS NO. 11 - March 2003 52 Magnesita S.A I)INTRODUCTION Profiting the available resources and equipments of Magnesita Research Centre and Technical Assistance Department, Magnesita has participated on elimination of rings and build-ups at some cement kilns. This technical article does a general revision of this matter and introduces a methodology to face this problem. MAGNESITA remains at cement plants disposal for the conjoint study of this subject. The rings and build-ups are accumulations of solid materials in the rotary or static sections of the kiln. Ring and build-up formation has a detrimental influence on cement manufacture, by decreasing or stopping clinker production and by increasing the production costs. SOME DETRIMENTAL CONSEQUENCES OF RINGS AND BUILD-UPS FORMATION, ARE: The material and gas flow through the kiln isrestricted, reducing the kiln output. Ring formation impairs a proper combustion, and incomplete combustion may lead to increased specific fuel consumption, faster basic brick wear and unsafe kiln operating conditions. From time to time the build-ups break loose and choke the cyclones or the clinker cooler. This can even damage the clinker cooler. The uneven fall of deposits from exhaust gas fan rotor causes high bearing vibration, requiring shut down before mechanical damage leads to potential bearing failure. This is very costly as it reduces over-all kiln availability and increases maintenance costs. When a ring breaks loose, a sudden surge of material moves into the burning zone, leading the kiln into a serious upset condition, as the ring acts as a dam obstructing the uniform advancement of the material through the kiln. Build-ups in the cyclones increase the production costs by: - work force for cleaning it by using compressed air or high pressure water, - periodic use of compressed air. The kiln stoppage and cooling down in order to reach the ring zone and remove it. It abridges the refractory lining duration, especially in the basic zone. II) RINGS AND BUILD-UPS LOCATION CLASSIFICATION The accumulations of solid materials may be classified with regard to the type of material a t i s e n g a M s o l r a C e d o t i r B a t i s e n g a M SA a t i s e n g a M a t i s e n g a M a M a t i s e n g BY : chem. Eng. Carlos Brito, Brazil 1. 2. 3. 4. 5. 6. 7. 8. Cement Technology

Transcript of Rings & Buildups Mechanisms

Page 1: Rings & Buildups Mechanisms

CEMENT & BUILDING MATERIALS NO. 11 - March 2003

52

Mag

nesit

a S.

A

I)INTRODUCTION

Profi ting the available resources and equipments of Magnesita Research Centre and Technical Assistance Department, Magnesita has participated on elimination of rings and build-ups at some cement kilns. This technical article does a general revision of this matter and introduces a methodology to face this problem.

MAGNESITA remains at cement plants disposal for the conjoint study of this subject.

The rings and build-ups are accumulations of solid materials in the rotary or static sections of the kiln.

Ring and build-up formation has a detrimental infl uence on cement manufacture, by decreasing or stopping clinker production and by increasing the production costs. SOME DETRIMENTAL CONSEQUENCES OF

RINGS AND BUILD-UPS FORMATION, ARE:

The material and gas fl ow through the kiln isrestricted, reducing the kiln output.

Ring formation impairs a proper combustion, and incomplete combustion may lead to increased specifi c fuel consumption, faster basic brick wear and unsafe kiln operating conditions.

From time to time the build-ups break loose and choke the cyclones or the clinker cooler. This can even damage the clinker cooler.

The uneven fall of deposits from exhaust gas fan rotor causes high bearing vibration, requiring shut down before mechanical damage leads to potential bearing failure. This is very costly as it reduces over-all kiln availability and increases maintenance costs.

When a ring breaks loose, a sudden surge of material moves into the burning zone, leading the kiln into a serious upset condition, as the ring acts as a dam obstructing the uniform advancement of the material through the kiln.

Build-ups in the cyclones increase the production costs by:

- work force for cleaning it by using compressed air or high pressure water,

- periodic use of compressed air.

The kiln stoppage and cooling down in order to reach the ring zone and remove it.

It abridges the refractory lining duration, especially in the basic zone. II) RINGS AND BUILD-UPS LOCATION

CLASSIFICATIONThe accumulations of solid materials may be classifi ed with regard to the type of material

atisengaM

solraC ed otirB atisengaM SA

atisengaMatisengaM

aM atiseng

BY : chem. Eng. Carlos Brito, Brazil

1.

2.

3.

4.

5.

6.

7.

8.

Cement Technology

Page 2: Rings & Buildups Mechanisms

CEMENT & BUILDING MATERIALS NO. 11 - March 2003

53

Mag

nesit

a S.

Afrom which they are formed:UNSINTEREDSINTERED.

UNSITERED-Material build-ups within the I.D. fan-Accumulations of solid materials in the cyclones and grate preheaters

-Slurry rings-Rings in the calcining zone

SINTERED -intermediate rings -Sinter rings -Coal ahs rings -Clinker rings -Snowman formation within the clinker grate cooler -Clinker balls.

The location of build-ups and rings can be seen in Table I. III) BONDING FORCES OF RINGS AND

BUILD-UPS

The formation of accumulations of solid materials, rings and build-ups, from solid particles suspended in the gas stream and from solid particles which slide over the refractory lining, is only known in a qualitative way. Little quantitative knowledge is available on the formation of these deposits.The formation of a deposit of solid particles is always a dynamic process in which the factors responsible for formation outweigh the forces of degradation. After the transport of solid material to the area of deposition, a defi nite force is required to make it adhere to the wall as:

1. Turbulence within the gas stream,2. Centrifugal forces whenever the gas stream changes direction,3. A mechanical pressure.

The fi rst two types of foreces are predominant for preheater build-ups, whereas the mechanical pressure play a decisive role in ring formation.

According to RUMPF, the forces considered to cause deposit of solid materials can be grouped as follows:1. Melting or freezing, due to addition or removal of heat,2. Interlocking of very fi ne solid particles held together by surface forces,3. Interlocking of long, fi brous solid particles,4. Melting or softening of particles surface by friction or collision,5. Electrostatic attraction of solid particles.

The fi rst three mechanisms are the ones encountered in cement kiln operation. In general, the fi ner the powder, the greater the tendency towards agglomeration and in many cases the absence of solid particles smaller than a critical size, e.g. 5 mm, ensures freedom from deposit. IV) CHARACTERISTICS OF RINGS AND

BUILD-UPS

Tables II, III, IV, V and VI contain a list of typical properties of deposits encountered in many cement kilns.

EXHAUST GAS FAN DEPOSIT

Deposit of solid particles on the blades of exhaust gas fan, when dust-laden exhaust gas passes through the fan, causes vibration when the rotating fan is out of balance. Vibration reduces the bearing life time.

CHARACTERISTICS

The deposits are composed of the fi nest raw meal particles, they are usually red-brown, hard and brittle. Their structure is compact, layered, with a porosity smaller than 8%. Their chemical and mineralogical composition is that of the raw meal and the clay particles are deposited parallel to the blade surfaces. Due to their long stay in the system, these deposits are enriched in the volatile components, S03, K20 and Na20. The S03 is usually present as anhydrite, CaS04.

Cement Technology

Page 3: Rings & Buildups Mechanisms

CEMENT & BUILDING MATERIALS NO. 11 - March 2003

54

Mag

nesit

a S.

ATYPICAL VALUES:LSF 20 - 100%SR 1,0 - 1,5S03 4,5 - 6,0 %K20 + Na20 2,0 - 3,0 % BINDING MECHANISMThe temperature is so low that liquid phase involvement is insignifi cant. However, molten salts infl uence can sometimes be noticed.The dust particles, because of the high fan rotation, strike the blade surfaces at high velocity and are compacted. As the texture of the blade surfaces possesses undulations in the order of 0,5 to 20 mm, the very fi ne dust particles, mainly clay particles, can be mechanically “locked-on”. The development of the deposit follows an identical mechanism.

CYCLONE PREHEATER DEPOSITS

CHARACTERISTICSThese deposits form on the roofs, walls, outlet, riser pipes and transition chamber of cyclone preheaters, and vary in appearance and homogeneity. In general, they have a light colour, brown, indicating that the particles had not been heated higher than 1200?C. In some cases, darker colours of harder burnt material can be observed. The deposits range from a dense, compact structure, to a porous one, depending on their place of deposition and composition.Their chemical composition is usually characterized by a concentration of the volatile elements, in the following range:S03: 1 - 35% K20: 1 - 30%Cl: 1 - 25% Na20: 0 - 2%

It is very scarce to fi nd deposits in the cyclone preheater with no appreciable increase in concentration of the volatile elements.

Their mineralogical composition differs from that of the raw meal in that the clays are decomposed and intermediate minerals are formed. Minerals only containing the volatile elements can also be found. The most frequent found minerals are the following:

- FROM RAW MEAL:

CALCITE, CaC03 QUARTZ, Si02- INTERMEDIATE MINERALS: FREE LIME, Ca0 free PERICLASE, Mg0 BELITE, C2S MAYENITE, C12A7- MINERALS WITH LOW MELTING POINT: SYLVITE, KCl ARCANITE, K2S04 LANGBEINITE, 2CaS04.K2S04 HALITE, NaCl-MINERALS FORMED WITHOUT MELT INVOLVEMENT: CARBONATESPURRITE , 2C2S.CaC03

SULFATE SPURRITE, 2C2S.CaS04 ANHYDRITE, CaS04

FORMATION MECHANISMThe low melting point K20, Na20, S03 and Cl based compounds and the low melting point eutetics formed by these volatiles are the binding of these deposits. These compounds are molten in the kiln gas stream and are deposited on the walls of the cyclone preheater, or fi rst on dust particles which then themselves are deposited out of the gas stream in the accumulation area. Their cooling, on contact with the wall or with increasing thickness of the deposit results in an appreciable strengthning of the deposit texture.A chemical reaction with gaseous S03 can take place, resulting in the formation of lath shaped sulfate spurrite and carbonate spurrite which strengthen the deposit texture.

CALCINING RINGS IN LONG KILNS

CHARACTERISTICS AND FORMATIONThese rings are, in their properties and mechanism of formation, very similar to those of cyclone preheater deposits, the reason been that both deposits occur in the same temperature zone.Calcining rings are less troublesome than cyclone preheater build-ups because, due to their weight, kiln rotation, thermal fl uctuations and the mechanical action of the material stream, they fall off periodically. Because of the short duration of stay in the rotary kiln the texture of the deposits remains weak.

Cement Technology

Page 4: Rings & Buildups Mechanisms

CEMENT & BUILDING MATERIALS NO. 11 - March 2003

55

Mag

nesit

a S.

AINTERMEDIATE RINGS IN PREHEATER KILNS

CHARACTERISTICSThe intermediate rings are dense, hard and seldom fall off during kiln operation. They are elongated, being some 10 - 15 metres long and extending from 7 to 11 kiln diameters from the outlet. This ring is clinker-like in colour indicating it being composed of well burnt material. They have a layered structure, according the curvature of the kiln shell. Their chemical composition is very similar to that of clinker. No increase in concentration of S03 or alkalis takes place, and often the ring shows lower volatile element values than for clinker. The minerals found in intermediate rings are the clinker minerals, alite, belite, aluminate, ferrite and free Ca0. The alite of the inner layers may decompose into belite and secondary free Ca0, resulting from cooling down of the inner layers to a temperature lower than the stability temperature of the alite - 1260o C FORMATION MECHANISM

The mechanism of bonding is the freezing of the alumino-ferrite melt. The smallest clinker particles of 150-450 mm are carried back by the gas stream, fall down and are deposited on the rotary kiln refractory lining, in a zone where temperatures of below 1250oC exist. The clinker dust particles freeze in place, and because the kiln charge is still fi ne, it does not possess suffi cient abrasive action to remove the growing ring.

SINTER RINGS

CHARACTERISTICS

These rings occur in the burning zone inlet, some 4 - 5 diameters from the kiln outlet. They are greyish-black in colour, hard and formed by small clinker nodules and clinker dust. Because of the presence of large pores and voids, no layered structure is formed. Their chemical composition is that of the clinker with no concentration of volatile elements. The minerals found in sinter rings are the clinker minerals, alite, belite, aluminate, ferrite and free lime. The alite of the inner layers may decompose into belite and secondary free Ca0, similar to the case of intermediate rings.

BONDING MECHANISM

The bonding is created by the freezing of the clinker liquid phase. This phenomenon occurs especially in the burning zone inlet,

where the liquid phase is just starting to form, at approximately 1250oC. Due to the rotation of the kiln, the material freezes with each kiln rotation and deposit of clinker particles having less than 1 mm diameter may reach a large thickness.

COAL ASH RINGS

CHARACTERISTICS

In kilns fi red with a high ash content coal, rings can form at 7 - 8,5 diameters from the kiln outlet. They are dense, with a layered structure and sometimes glassy in appearance and built up from particles some 150 - 250 mm in size. They are rather less dense and have larger pores and voids than intermediate rings. Their chemical and mineralogical composition is very similar to that of clinker. The minerals are the clinker minerals, alite, belite, aluminate, ferrite and free lime. As the ring grows up and the temperature of the inner layers falls down the alite may decompose into belite and secondary free lime and the B - belite may convert into - belite. No enrichment of the volatile phases is observed. The belite content is higher than that of the clinker, because of the enrichment in coal ash, a silica rich material.

FORMATION MECHANISM

The bonding mechanism is the freezing of molten coal ahs particles and perhaps to a slight extent, the frezzing of the clinker liquid phase. The molten coal ahs droplets adhere to the kiln refractory lining in a zone where the temperature is high enough so that they are still partially sticky. When this layer passes under the kiln charge, on

Cement Technology

Page 5: Rings & Buildups Mechanisms

CEMENT & BUILDING MATERIALS NO. 11 - March 2003

56

Mag

nesit

a S.

Aeach kiln rotation, a portion of the still very fi ne kiln charge adheres to it.The ring is not formed from clinker dust carried back by the gas stream as the alite crystals are very small, smaller than those found on clinker particles.

CLINKER RINGS AND SNOWMAN FORMATION

These deposits are formed from small clinker nodules and clinker dust particles, and have a high porosity containing many voids. Their chemical and mineralogical composition is identical to clinker, but in some cases an enrichment up to 3,5% K20 and 3,0% S03 has been noticed.

The mechanism of bonding is the freezing of the clinker liquid phase, as the clinker passes through the fi rst cooling zone in the rotary kiln or on folling down the chute into the grate cooler. The clinker dust particules carried back by the secondary air stream from the clinker bed grate into the interior of the rotary kiln also play an important role on formation of these deposits. The clinker dust particles, having a superfi cial liquid phase layer, strike against the chute wall and the refractory lining at discharge side of the rotary kiln, lose their kinetic energy and the superfi cal liquid phase freezes immediately. V) FEED MOVEMENT THROUGH THE KILNRegarding the protection of the refractory lining, the movement of the material through the kiln can has the following consequences:

CIRCULATION

WEAR BY CORROSION, ABRASION AND EROSION

ACCUMULATION

The circulation would be completely free in case the refractory lining is absolutely inert for the material. However, there is always a certain degree of afi nity of the refractory lining to the material, resulting in corrosion or accumulation. Whenever in a certain lining zone, the corrosion is faster than the accumulation process, the lining is weared and the metallic casing loses its protection. A red spot is a sign of worn-out refractory lining.

The feed movement in the kiln wears-out the working face of the lining by abrasion or/and erosion.

Whenever the wear is compensated by accumulation of solid

materials, the circulation of the material remains easy. Oppositely, whenever the accumulations grow up the material circulation becomes hard. The accumulation of solid materials thicker than 500 mm in the rotary kiln is named a “ring”. VI) IRREGULARITIES AT KILN RUNNING BY DEPOSIT FORMATION

Rings and build-ups can lead to the following detrimental consequences:- The material circulation decreases, decreasing the kiln output.- The clinker quality suffers by unstable kiln running.

- The kiln thermal effi ciency diminishes by modifi cation of the thermal profi le within the kiln; the combustion rate slows down, the fl ame lengthens and its temperature decreases.

- The material dust content of the exhaust gas stream increases, lowering the I. D. fan capacity and the effi ciency of the dedusting system.

- A faster refractory lining wear occurs by intense thermal variations and by abrasion of the material retained upwards the ring.

VII) MAIN PROBABLE REASONS FOR DEPOSIT

FORMATION

There are many reasons for rings and build-ups development, which renders their identifi cation a diffi cult and time-consuming process. This phenomenon is yet more complex by the following points:

more than one factor can participate on ring or build-up formation, sometimes, the consequences and the causes mix.

For a question of simplicity of exposure, the probable reasons for

Cement Technology

Page 6: Rings & Buildups Mechanisms

CEMENT & BUILDING MATERIALS NO. 11 - March 2003

57

Mag

nesit

a S.

Arings and build-ups formation are sorted by their report with:

*the kiln *the combustion *the raw meal *other infl uencing factors.

CAUSES RELATED TO THE KILN

KILN DIAMETERA small kiln diametre is more prone to ring formation than a big one.

KILN ROTATING SPEEDThe tendency to ring formation decreases as the kiln runs at a faster speed: the faster material circulation decreases the chance of its deposit in a certain area. The kiln rotating speed should also remain as constant as possible. Whenever decreasing the kiln speed, the raw meal feeding rate should also be diminished accordingly, in order to have a constant kiln fi lling degree, avoiding the accumulation of material in a area favorable to ring development.

KILN ACCESSORY EQUIPMENTSThe inward or outward equipments of the kiln - raw meal drying and preheating systems and clinker cooling systems - developed to increase its thermal effi ciency, can make easy the rings and build-ups formation by disturbing the material and the gas streams and by formation of a favorable thermal profi le.Dedusting and preheating systems can also make easy the buil-ups formation by exhaust gas enrichment in volatile elements. To oppose this infl uence, the dust recovered by the electrostatic precipitator can be partially removed from the material stream, by its volatile enrichment.Sometimes even a by-pass is requested to deviate a portion of the exhaust gas stream.

KILN DEFORMATIONSThe most favorable kiln deformations to build-ups and rings formation are the ones that favour false air entrance - “cold air”- into the system. These deformations, located at cleanning oppenings of the cyclone preheater and at inlet and outlet joints of the rotary kiln, should be eliminated as soon as possible.

CAUSES RELATED TO THE COMBUSTION

FUEL CHEMICAL COMPOSITIONCoal ash can cause ring formation. The ash particles fall-down

at the beginning of the burning zone, increasing the amount of melt which makes easy the ring formation.The fuel S03 content can also make easier the formation of build-ups in the cyclone preheater, by unbalancing the ratio alcalis/sulfates + chlorides of the system [raw meal + fuel].

FUEL HEATING VALUEA fuel of high heating value decreases the tendency to ring formation especially whenever fi ring coal, as a high heating value coal has a low ash content.

COAL FINENESS AND LIQUID FUEL ATOMIZATIONThe fi ring time for pulverized coal is infl uenced by its fi neness: the fi ner the coal the faster its fi ring. A coarse coal fi res slowly and irregularly.The ash is the skeleton of the coal particles and preserves its original shape after fi ring. The biggest and heaviest ash particles carried back by the exhaust gas stream fall-down at a high temperature zone of the rotary kiln, while the smallest fall-down upwards, at a lower temperature kiln zone.Insuffi cient fuel oil atomization can lead to ring formation, as small fuel oil droplets are necessary to promote easy combination with the oxygen.The fuel oil water content and the humidity of pulverized coals, beyond an upper limit, act on combustion rate and can infl uence ring formation. EXCESS OF COMBUSTION AIRThe thermal profi le within the kiln is infl uenced by the amount of combustion air. This profi le can be irregular with zones of lower temperatures than expected. The material which has already started its melting, freezes and develops the ring. Excess of combustion air is an important factor to avoid ring formation.

Cement Technology

Page 7: Rings & Buildups Mechanisms

CEMENT & BUILDING MATERIALS NO. 11 - March 2003

58

Mag

nesit

a S.

A-AIR/FUEL MIXINGThe mixing of air and fuel within the rotary kiln is irregular, incomplete and slow. There is a lack of air in the vicinity of the burner tip. The central hot gas stream has an initial rate faster than the peripheral secondary air stream. This difference diminishes as the two gas streams get forward, in direction of the transition chamber. The external stream supplies oxygen for combustion in the central stream. The simultaneous presence of oxygen and carbon monoxide in transition chamber exhaust gas was already noticed in some kilns.

The type of atomizer nozzle is very important in order to ensure a turbulent motion, necessary for an adequate mixing air/fuel.Mechanical condition of the atomizer nozzle and of the primary air pipe nozzle is also very important to ensure an even mixing.The outlet joint of the rotary kiln should avoid false air entrance -

“cold air” - into the kiln, for a faster combustion.

CAUSES RELATED TO THE RAW MEAL

RAW MEAL CHEMICAL COMPOSITION

The raw meal favours the ring formation whenever melt formation starts at low temperature. In the following table we can remark the initial temperature for formation of melt of some systems.SYSTEM MELT FORMATION TEMPERATURESi02 + Ca0 2.065oCSi02 + Ca0 + Al203 1.455oCSi02 + Ca0 + Al203 + Fe20 1.338oCSi02 + Ca0 + Al203 + Fe20 + Mg0 1.300oCSi02 + Ca0 + Al203 + Fe20 + Mg0 + Na20 1.280oC

The liquid phase of raw meal starts at about 1250oC. The above table renders clear the action of Al203, Fe203, Mg0 and alkalies. The amount of liquid phase is infl uenced by the relative proportion of the different compounds. Whenever the potential proportion of C4AF is small, the amount of melt formed at low temperature increases with Fe203 addition.

The lime saturation factor should be aware of Ca0 losses through the exhaust gas draft and of coal ash absorption by the clinker, whenever using coal as a fuel.

According to practice, raw meals of low lime saturation factor favours the formation of rings. Concerning the formation of build-ups in the cyclone preheater, it is very important to have a balanced

ratio alkakis / sulfates + chlorides for the system [raw meal + fuel].

RAW MEAL HOMOGENEITY

Raw meals are not homogeneous mixtures, as we can notice by chemical analysis of every granulometric fraction of the raw meal. In this manner, the raw meal grinding should be made in such a way as to decrease the chemical composition differences of the granulometric fractions.The kiln raw feed should also have a chemical composition as constant as possible. The standard deviation of chemical analysis, during 24 hours of kiln running, shoul be smaller than:

SLSF < 1% or SCaC03 < 0,2%SMS < 0,1

The chemical or physical raw melt heterogeneity causes variations on fi rst melt formation temperature, which makes easy ring formation.

SEGREGATION WITHIN THE KILN

The smallest and lightest material particles, e.g., the Ca0 particles, are preferably carried back by exhaust gas draft. The dust draft is especially intense in the calcining zone. The lime content of the kiln charge decreases and it becomes easier to melt, increasing the tendency to ring formation. To counterbalance this segregation the raw feed lime saturation factor should be bigger than the theoretical value. The exhaust gas draft should also be adjusted in such a way as to ensure a complete combustion, but avoiding excessive draft.

Cement Technology

Page 8: Rings & Buildups Mechanisms

CEMENT & BUILDING MATERIALS NO. 11 - March 2003

59

Mag

nesit

a S.

A

OTHER CAUSES

CLASS OF REFRACTORY LININGFor decreasing tendency to build-up formation in cyclone preheaters, their lining is made of silicon carbide based low cement castables. For decreasing tendency to ring formation, the corresponding zone can be lining of low coating ability refractory bricks, e. g., magnesia-spinel bricks or direct bonded magnesia-chrome bricks.MAGNESITA technical staff is available to assist the user on selection of the most suitable refractory to line every kiln zone.MAN INFLUENCEThe kiln operator can favour the formation of rings and buld-ups by faulty kiln operating procedures, which increase thermal variations in the kiln system.STABLE KILN RUNNINGA stable kiln running and a small number of stoppages helps to prevent build-ups and rings formation.FLAME TEMPERATUREA too high fl ame temperature has, besides the well known hurtful consequences on refractory lining duration, a strong infl uence on sulfates and alkalies volatility. In this manner, a very high fl ame temperature favours the build-ups formation in cyclone preheaters.HIGH WATERHigh water can infl uence the ring formation especially in wet kilns, by increasing the salt content of the water used for slurry grinding.RAINNING INFLUENCERainning can increase the heterogeneity of the raw meal by diffi culties on raw materials transport and grinding especially for clay, by its bigger hygroscopy than limestone. This clay hygroscopy can increase the segregation within the rotary kiln. VIII) METHODS OF DEPOSIT ELIMINATION

MECHANICAL METHODS

* PERCUSSIONLong metallic bars are used to cut the thick coating in a defi nite site. Usually the ring falls down during turning of the kiln by the barring motor. This method is unsafe for the workers and abridges the service life of the lining.

* IMPACTThis method uses industrial cannons and has the following steps:The industrial cannon is assembled at kiln platformThe kiln is stopped

· The fuel fl ow is interrupted to suppress the fl ame

· The ring is cut at a defi nite site by shells impact

· The kiln is turned by using the barring motor.

· Usually the ring falls down, exception for very long rings.

· The number of shells used to cut the ring is infl uenced by the ring characteristics.

· For build-ups removal, compressed air or high pression water pipes are used, with a simultaneous action of percussion, impact and thermal shock.

* EXPLOSIONThis method is scarcely used for danger of explosive handling for workers and equipment. CHEMICAL METHODS

RAW MEAL CHEMICAL MODIFICATIONThis is an effi cient method during the fi rst stages of ring formation.Increasing the raw meal lime saturation factor can prevent the growth of the ring which can breaks loose during kiln rotation or during a kiln stoppage.

THERMAL METHODSCOOLING METHODThis method acts by:î formation of cracks which decrease the ring stability or/and by î formation of - C2S, from b-C2S, with volume increase of about 10%.Whenever the ring is just starting its process formation, we can envisage its cooling without stopping the kiln. Proper procedures are to push back the burner and to shorten the fl ame by changing the adjustments of the primary air fl ow, the waste gas draft and the burner nozzle. Whether the

Cement Technology

Page 9: Rings & Buildups Mechanisms

CEMENT & BUILDING MATERIALS NO. 11 - March 2003

60

Mag

nesit

a S.

Aring doesn’t fall down, the kiln is stopped and the ring is cooled down using compressed air. This procedure diminishes the refractory lining duration.

A WORD OF CAUTIONUSE OF WATER IS NOT RECOMMENDED BY DANGEROF STEAM EXPLOSIONS.

HEATING-UP TO MELTINGProcedures for ring heating-up are infl uenced by its location: burning zone inlet or outlet.

-RING LOCATED AT BURNING ZONE INLETThe burner pipe penetration into the rotary kiln is increased and the fl ame is lengthened. The burner pipe position to the rotary kiln axis is also modifi ed in order to have a direct impingement of the fl ame against the ring. To counter act the ring development, the following procedures are also recommended: to decrease the kiln feed, to increase the kiln rotation speed, to ensure oxidizing fl ame conditions, and to increase the raw meal lime saturation factor.

-RING LOCATED AT BURNING ZONE OUTLETIn this case, the proper procedures are to shorten the fl ame and to push back the burner pipe.

THERMAL SHOCKThe kiln is stopped during a short period, smaller than thirty minutes.During this period the fl ame is moved toward the upper portion of the ring. The uneven ring heating-up causes cracks formation which can break it. IX) CONCLUSION

Many factors can infl uence the build-ups and the rings formation. In order to avoid their repeated formation, the following methodology is recommended:

-Compile the kiln operating conditions during the deposit formation period.-Compare these conditions with the ones corresponding to a normal kiln running.-Have some samples of the accumulation material and provide for their chemical, mineralogical and texture analysis.

-Compare these analysis with raw meal and clinker analysis.-According to this comparison change a raw meal or a clinker production variable.-Check the infl uence of this modifi cation on deposit formation tendency.

X) REFERENCES

1) BLODA, Alberto Virella. Formacion y destruccion de anillos en los hornos rotativos para cemento portland. Instituto técnico de la construccion y del cemento. Tipografi a artistica, Alamed 12, Madrid.

2) HOLDERBANK. Second Cement Seminar on Process Technology. 1989, Ch. 11, pp 103-123.

3) MAGNESITA S. A. Internal Reports of the Research Centre and Technical Assistance Department.

Cement Technology

Page 10: Rings & Buildups Mechanisms

CEMENT & BUILDING MATERIALS NO. 11 - March 2003

61

Mag

nesit

a S.

A Cement Technology

Page 11: Rings & Buildups Mechanisms

CEMENT & BUILDING MATERIALS NO. 11 - March 2003

62

Mag

nesit

a S.

A Cement Technology

Page 12: Rings & Buildups Mechanisms

CEMENT & BUILDING MATERIALS NO. 11 - March 2003

63

Mag

nesit

a S.

A Cement Technology

Page 13: Rings & Buildups Mechanisms

CEMENT & BUILDING MATERIALS NO. 11 - March 2003

64

Mag

nesit

a S.

A Cement Technology