formation des anneaux four rotatif

RING AND SNOW BALL FORMATION IN THE KILN

Sinter Ring at the junction of burning zone and transition zone

Spurrite and Sulfo-spurrite ring In calcining zone

Reduction in kiln Diameter due to Ring formation

Key reasons for ring formation

Raw mix Chemistry Variation in chemistry

Un stable kiln and calciner operation Flame &temperature Combustion

Hard Burning Due to the presence of Calcite and Quartzite

Fuel characteristics Volatile recycling

All reasons are inter-dependent and flame and combustion plays a very major role

Selection of refractories

The formation of a ring is a dynamic process in which the factors responsible for formation outweigh the forces of destruction. The forces considered to cause deposit formation can be grouped as follows: • Melting or freezing due to addition or removal of heat. • Interlocking of aggregates built up of finer particles hold together by surface forces. • Interlocking of long fibrous particles.

SINTER RING In general, chemical composition of the sinter ring material is that of the clinker with no appreciable concentration of volatile elements. From a mineralogical viewpoint, the normal clinker minerals like alite, belite and free CaO are observed. The bonding mechanism responsible for sinter ring formation is the freezing of the clinker. The rings, depending on their origin, may be dense or porous, and may develop slowly or just in a few hours. The first symptoms of ring formation are: • A higher negative pressure before the exhaust fan. • Incomplete fuel combustion as indicated by "CO" analyzer. • Intermittent charge retention. • Abnormal temperature gradients in the kiln shell. • Higher amperages in the kiln motor. The reasons for ring formation, well agreed between many plant operators, may be classified in three groups: • Process. • Operation. • Maintenance.

The next group deals with operation troubles that can lead to ring formation. These are: • Discontinuous feed. • Material segregation. • Flame pattern and position. • Improper combustion. • Successive kiln upsets.

Material segregation is not an uncommon situation in rotary kiln. For instance, the lime content we have in the meal, loss on ignition free, never coincides with the lime content in the clinker. The lighter lime particles are easily carried by the gases, while silica, iron oxide and alumina particles follow down the kiln into the burning zone. Depending on the degree of segregation, excess liquid may form in the burning zone, and even outside, if the melting point of the glass is lowered by segregation. The addition of sand in the raw materials modifies significantly the burnability because it shows lower reactivity and grindability. The grinding of quartz, during assemblage grinding, causes quartz concentration in the coarse fractions of raw mix and consequently material segregation in the kiln charge.

In the maintenance group we find: • Defective burner. • Defective instrumentation. • False air admission

Calcium Langebeinite K2Ca2(SO4)3

>1250

______

Porosity of the kiln brick Porosity of the brick may help to reduce ring formation but at the cost of heat loss. It can be reduced but it cannot be avoided.

Closed pores Open pores

Silica infiltrated surface or vitrified surface on brick can avoid infiltration of low melting ,low Viscous components .This avoids destruction of refracories but spurrite rings cannot be avoided fully. Properly balanced chemistry , well tuned flame with hot and oxdised conditions and properly operated kiln and Calciner can only elimiinate ring formation or reduce the aggravation.

Strong intense flame plays a major role In kiln coatng stablity

Uniform coating Preferably 5 or 6 % Of the diameter

What is the different between sulphur, spurrite, and alkali rings and how can we distinguish between these three? Formation of type of ring is based upon the kiln system atmosphere.If sulfur is excess in raw material and fuel then sulphate ring is formed due to formation of spurrite and sulphospurrite mineral.Sulphate ring generally formed in calcination zone and it is very hard in nature.Alkali ring is formed when alkali is present in excess than the sulphur in raw material and fuel.When high alkali concentration in raw material then Arcanite(K2SO4),Aphthitalite[K3Na(SO4)2] , and Thenardite[Na2SO4] mineral formed.You may analyse the coating sample by XRD to know the cause of coating formation and type of ring.

Sulphur Rings: Sulphur-induced rings are formed when the molar sulfur to alkali ratio in the system is more than 1.2. In such cases, there is a considerable amount of free SO3 circulating in the kiln. At a certain concentration level in the kiln gas, sulfation of the free lime occurs with anhydrite formation (CaSO4). If the kiln is burning under slightly reducing conditions, more volatile and lower melting sulfur salts may form, therefore increasing the severity of the problem. The salts, in molten state, coat the traveling clinker dust, forcing it to stick to the kiln wall in the form of rings. Sometimes the chemical analysis of such rings does not indicate high sulfur concentrations, proving that even a small amount of free sulfur is sufficient to cause rings. The severity of the problem increases with the dust concentration in the kiln gas. Dustier kilns have a tendency to form more rings than cleaner kilns.

Spurrite Rings: Carbonate or spurrite rings are formed through CO2desorption into the freshly formed free lime, or even through belite recarbonation. These rings are hard, layered, and exhibit the same chemistry as regular clinker. X-ray diffraction however, clearly indicates the presence of spurrite, a mineral with composition C2S.CaCO3. Spurrite is a form of carbonated belite. When the carbonate in the spurrite is replaced with sulfur the new mineral is called sulfated spurrite. Spurrite rings form whenever the partial pressure of CO2 above the bed of material is high enough to invert the calcining reaction. In coarsely ground, silica-rich raw mixes, the free lime does not have sufficient time in the calcining zone to react with silica, therefore increasing the chances for spurrite deposits.

Alkali rings: The third type of ring occurs whenever the sulfur-to-alkali molal ratio is( encrustration index) less than 0.83, usually in kilns with heavy chlorine loads. In such cases, low-melting potassium salts provide the binder for clinker dust travelling up the kiln. Through a "freeze-and-thaw" mechanism, these rings can assume massive proportions. Alkali rings are far less common than other types because sulfur and carbonates usually are in excess relative to potassium. In lime recovery kilns, for example, where alkali is always in excess of sulfur, severe ringing and balling sometimes occur.

-Statement by an experienced process man

Snow ball formation Long and lazy flame reduce the calcining zone length and increase the temperature. It increases Volatility of sulfur and alkalies ,increase the recirculation, and increase the low melting sufites .The rawmeal soaked in this temperature, in this low melting alkali and calcium sufites. gets balled up .As it reaches the burning zone it steals some more liquid , increases its size and many times blocks the flame causing deflection and impinges causing damage the refractory. .

around 1200 deg c

Spurrite ring in kiln calcining zone

Sulfo Spurrite -SEM

Chemical Analysis of spurrite ring layer by layer -- study by RHI,Vienna

Sulfo Spurrite ring -SEM

This is the result of bad flame which I witnessed In my own plant in my short tenure. 72 hours stoppage , loss of 11000 tons of clinker or 11 million Indian rupees as net cash loss. More over it is an unsafe activity

Do you need such condition and stoppage of kiln ?.

Snow ball formation as a result of ring formation and bad flame. Low gas retention time in calciner further aggravate the situation which we directly witnessed it. ILC is less friendly than SLC as far as gas retention and combustion efficiency is concerned but NOX emission is higher in SLC than that of in ILC where denoxification is achieved.

Bad flame activate the volatile recirculation esp sulfur and alkalies

Influence of flame length on volatile recycle and thermal stability Short and intense flame ,low back end temperature ,low volatile recycle

Favorable temperature profile

More absorption of sulfur and formation of more thermally stable sulfates of alkali and calcium

1500 degc

1200

1000 Long flame ,high back end temperature ,high volatile recycle Favorable area for spurrite ring formation Unfavorable Temperature profile, flat

1500

1200 Less sulfur volatile absorption and formation of more thermally unstable stable sulfites of alkali and calcium

1000

(- Reference from PCA)

Reference from Blue circle literature ( Holcim)

The burner can be more inclined ,shape the flame closer to the ring which reduces the Thickness. The jack and knuckle ball arrangement in the telescopic piece allow this up and down motion Which only Nova flam has.

Fives Pillard’s novaflam got the ability to slice the ring and avoid 3 days Stoppage.

The burner can be more inclined ,shaped the flame closer to the ring which reduces the Thickness. The jack and knuckle ball arrangement in the telescopic piece allow this up and down motion Which only Nova flam has..Caution –extra care must be taken When High alumina bricks are used which is inferior in its thermal properties than magnesite Bricks.

Pillard ‘s Novaflam got the ability to slice the ring and avoid 3 days Stoppages.

Shark teeth

Shark teeth forms when ash in coal is high and high dust circulation from cooler to kiln. This makes the clinker more sticky and many occasions lead to snow man formation on Static cooler and rhino horn on burner. Chemistry ,if it is not controlled can lead to dusty Clinker and shark teeth at the kiln tip.By pushing the burner in and adjusting flame gives great relief to this problem.Cooler air distribution also reduces this problem. Shark teeth lead segregation in the clinker discharge and leads to red river in the cooler .

By simple to and fro movement of swirler tip and by alteration of primary air Pressure at burner inlet , can change length and shape of the flame. Novaflam is user friendly ,simple and very versatile burner.

Do you need a old time master burner who does jugglery now and then to tune the flame by adjusting two or three valves for axial,radial and stabilising air like a magician on the Stage. Nova flam says “NO”. Novaflam has simple operation unlike DdD complicated operation with other burners. More user friendly than any other burner.

Make the kiln smile with good flame or you will weep

Novaflam makes you to smile

Thank you for your kind attention PRADEEP KUMAR Technical advisor - commissioning Fives combustion pvt ltd Baroda