Chapter 11: Phenomena in the Rotary Kiln

Due to physical and chemical fluctuations inside the rotary kiln (kiln, preheaters, and cooler), build-ups or phenomena may appear that interfere with the production process and restrict the normal flow of the material.

What build-ups or phenomena are we mean to?

We mean the rings, boulder (large clinker ball), snowmen, rhino horn, red river, deposits in preheaters and ID fan, etc.

These build-ups and phenomena can cause detrimental consequences.

We quote the most relevant:

• They prevent adequate combustion, increasing the specific consumption of thermal energy,
• They accelerate the wear of the refractory lining,
• It can cause structural damage to the kiln since they concentrate the load in a single zone, consequently increasing the consumption of electrical energy and maintenance costs,
• May cause obstruct the uniform advancement of the material through the kiln or a sudden surge of material,
• Unexpected kiln shutdown to remove build-up,
• Poor heat exchange between clinker and secondary air (red river).

That is, build-ups, formations, or phenomena inside the rotary kiln negatively affect operating costs, and the environment, in addition, to increasing the risk of accidents.

In this article, we will briefly discuss what rings, boulders, rhino horns, and snowmen are, and why these formations appear. We will also comment on the red river in the cooler, its causes, consequences, and the parameters that help prevent its appearance.

RINGS

The rings are build-ups that form around the entire inner circumference of the kiln, in the rotary or static sections and can be of different dimensions and thickness. Rings belong to the group of sintered solid build-ups.

The rings can be classified depending on formation chemistry or formation location the following:

• Formation chemistry: sulfur ring, spurrite ring and alkali ring,
• Formation location: intermediate ring, sinter ring and coal ash ring.

Formation chemistry:

Sulfur Ring. - 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. 

The kiln working in conditions of high sulfur and low value of sodium oxide is critical operating scenario to formation of sulfur rings. However, 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.

Spurrite Ring. - Carbonate or spurrite rings are formed through CO2 desorption into the freshly formed free lime, or even through belite recarbonation. These rings are hard, layered, and exhibit the same chemistry as regular clinker. 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. 

Alkali Ring. - The alkali ring occurs whenever the sulfur-to-alkali molar ratio is 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.

Formation location:

Intermediate Rings. - Intermediate rings are dense, hard and seldom fall off during kiln operation. They are elongated, being some 10-15 meters long and extending from 7 to 11 kiln diameters from the outlet. This ring is clinker-like in color 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 alite of the inner layers may decompose into belite and secondary free CaO, resulting from cooling down of the inner layers to a temperature lower than the stability temperature of the alite (1260°C). 

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

Sinter Rings. - These rings occur in the burning zone inlet, some 4-5 diameters from the kiln outlet. They are greyish-black in color, 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 alite of the inner layers may decompose into belite and secondary free CaO.

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 1250°C. 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. - In kilns fired 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. As the ring grows up and the temperature of the inner layers falls down the alite may decompose into belite and secondary free lime.

The bonding mechanism is the freezing of molten coal ash particles and perhaps to a slight extent, the freezing of the clinker liquid phase. The molten coal ash 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, one ach kiln rotation, a portion of the still very fine kiln charge adheres to it.

BOULDER (large clinker ball)

Generally, clinker ball/ boulder formation is related to low clinker SM with high AM or related to very thick coating.

Boulder formation mechanism: large clinker balls initiate and grow behind a thick coating or ring. Build up often fall and slide into the kiln as large slabs and coming to rest behind a ring (dam effect). The slab can roll around behind the ring forming a ball which can then grow larger due to accretion.

SNOWMEN

Snowmen: Is a formation of large build-up on cooler first grate (static grate) or kiln discharge wall where the clinker falls from the kiln. Snowmen may eventually grow to reach the kiln outlet (mouth), thereby blocking the discharge of clinker from the kiln. Snowmen causes poor clinker distribution thereby poor heat exchange between clinker and secondary air.

Snowmen formation mechanism may divide to two:

1). Freezes of the clinker liquid phase as the clinker passes through the first cooling zone in the rotary kiln or on falling down the chute into the grate cooler. The clinker dust particles 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 snowmen. The clinker dust particles, having a superficial 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 superficial liquid phase freezes immediately.

2). Occasionally large lumps of coating discharge from the kiln, these lumps of kiln coating act as "seeds" for the formation of snowmen. Snowmen form when fines fall from the kiln above, onto the top surfaces of these lumps on top of the clinker bed within the cooler. As layer after layer of the fines fuse onto the lump, snowmen "grow" upwardly into stalagmite-like structures.

RHINO HORN

Is a build-up on the top of kiln burner pipe.

Rhino horn formation mechanism: clinker dust carried back to the kiln with the secondary combustion air loses velocity around the burner and that causes clinker dust to settle and build up on the top of the burner. 

RED RIVER

Is a phenomenon in grate cooler; it is often a red-hot narrow stream of fine clinker with a higher temperature than the neighbored clinker and appears far down in the cooler. 

The fact that especially large diameter type kilns tend to discharge fine clinker on the kiln’s load side and coarse clinker on the opposite side can make it difficult to get good clinker distribution. In addition to the segregation, a clinker bed with unilateral or bilateral slope on static grate tends to slide fine clinker down.

Due to non-uniform clinker bed and high air resistance of fine clinker; red rivers often are inevitable.

A fine clinker has a higher resistance to the airflow than the coarse clinker, so the cooling air takes the path of least resistance, which intensifies the “red river” formation. It sees that the particle size has a great influence on air distribution, which can be described by pressure losses.

Parameters to prevent the formation of red rivers

Proper clinker bed distribution on cooler grates by optimizing parameters may help us to avoid this phenomenon. 

These are the necessary attention points to avoid the appearance of the red river in the grate cooler:

1). Increasing the clinker bed thickness by slowing grate speed improves the overall material distribution, the heat exchange and the positive effect on grate wear rates. Slowing down the movement of the fine clinker bed diverts more fine clinker to the coarse clinker side, thus increasing the overall bed resistance by pushing more air through the fine clinker bed. (Good results have been obtained, with clinker beds in the grate cooler up to 1 meter deep).  

2). The cooler airflow optimization, as too high amounts of air promote fluidization of the clinker. As the finer clinker particles are likely to be entrained by the locally intensified airflow, large amounts of dust cycles can form between the kiln and cooler. Dust particles might also be picked up from highly fluidized areas and concentrated in others, thereby intensifying any red rivers. It is recommended that maximum airflow not exceed approximately 140 Nm3/min per square meter of cooler grate area.

3). A successful way to improve clinker distribution is to narrow the grate area on the fine clinker side. By doing so, the clinker bed becomes narrower and often eliminates severe segregation of fine and coarse clinker. It is recommended that the cooler inlet grate width not exceed 2.5 m for kiln capacities up to 2,500 MTPD/Clinker.

4). Corner areas often have a low clinker load which results in heavy air channeling and bypassing the clinker load. Blanked-off air holes ensure that cooling air is diverted into the clinker load.

5). The secondary air temperature optimization as the too high secondary temperature tends to form snowman which causes poor clinker distribution at the cooler inlet where improvement should start.

CONCLUSION

There are different factors that can favor the formation of phenomena in the rotary kiln mentioned in this article. Of course, technicians and operators must follow the guidelines of manufacturers and cement companies.

It is the author's wish that this information be informative and serve as support to avoid operating disturbances and personal accidents.

I wait for you in the next article The Cement Newsletter...!!

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Bibliography

SHABANA, N (2013) - Cement Rotary Kiln

BRITO, C (2003) - Rings Build-ups Formation in Cement Kiln