Every year, the cement industry loses millions of dollars in unexpected kiln shutdowns caused by mid-kiln rings. Money also is lost when the kiln is slowed down or stopped for a few hours to enable operators to shoot several rounds of ammunition in an attempt to destroy rings. Kiln slowdowns also can damage the refractory lining.

If the mid-kiln rings are periodic, as opposed to progressive, they induce kiln upsets by retaining and releasing large volumes of underburned or overburned raw mix to the burning zone. Clinker properties, such as setting time, strength, and free lime, become quite erratic under such burning conditions.

Most articles dealing with mid-kiln rings describe how they are formed, their chemical and mineral composition, or their texture and hardness. Few writers, however, propose cost-effective ways to eliminate rings, the goal of this article.

How mid-kiln rings are formed Mid-kiln, or calcining zone, rings can be classified in three major groups: sulphur rings, spurrite rings, and alkali rings.

Sulphur-induced rings are formed when the molal 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. Th e 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.

Carbonate, or spurrite, rings are formed through CO2 resorption 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.

The third type of ring occurs whenever the sulfur-to-alkali molal 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. In lime recovery kilns, for example, where alkali is always in excess of sulfur, severe ringing and balling sometimes occur.

How to solve the ring problem Since the conditions that lead to ring formation can rarely be changed, the solution to the problem is to destabilize the ring as it forms. To do that, a silicon-carbide tumbling system has been developed that is welded directly to the kiln shell. The material of choice was Hoganas' 562 Denscast Sicto, for its proven non-wetting characteristics.

The tumblers are precast and prefired, before being welded to the kiln shell 90 degrees apart along the circumference. The space between tumblers is lined with regular high-alumina brick or semi-insulating brick.

The tumbling and mixing action changes the thermal and chemical profile of the kiln load, therefore inhibiting sulfation and carbonation. Even in cases where the reactions occur, the tumbling action mechanically removes the buildup in an autogenous type of action.

Photo 1 (page 9) shows a close-up view of a tumbler in the kiln, at the intersection point with the brick. The white deposit has the same composition of the rings. The darker area shows the contact points of the buildup material with the silicon-carbide, after the coating was removed with the fingertip. The non-adhering properties of the silicon-carbide tumbler became obvious during this inspection.

Photo 2 (page 9) shows massive ring formation adjacent and outside the tumbler area in the kiln. This ring measured 2 ft (600 mm) thick from the lining, and extended through 10 ft (3 meters) of kiln above and below the tumbler area.

Photo 3 (page 9) presents a better view of massive ring formation above and below the tumbler area. This photo, taken from the tumbler area after one year in service, is dramatic evidence of ring elimination through the use of silicon-carbide tumblers.

Additional benefits of tumblers Properly designed tumblers can provide several additional benefits to cement, lime, lime sludge, and mineral processing kilns in general. To mention a few:

* Better heat transfer from gas to solids, wall to solids, and within the bed of the material itself;

* Elimination of "kidney beans" in lime kilns;

* Prevention of mud-ball formation in lime sludge kilns;

* Better mixing of load components with different densities;

* Increase in material residence time in the kiln; and

* Fuel savings.

Through careful tumbler design, all of these benefits can be accomplished without any increase in dust loss or circulation.