Types of rings

Formation of type of ring is based upon the kiln system atmosphere.If sulfur is excess in raw material and fuel than 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.

u Sulphur 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. 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.

 

u Spurrite Rings: 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. 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.

 

u Alkali rings: 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.

Best Regard

Hi Nael, very good infos. Could you please explain what is the meaning of "if the kiln is burning under slightly REDUCING CONDITIONS"?