build-ups

Buildups The compounds which cause build-ups in the smoke box/calciner and feed pipes will be different depending on the following factors;(a) The chemical balance of chlorides, alkalis and sulphur in the hot meal. (b) Oxygen content in the gases at the kiln inlet. (c) Burning zone and calciner temperature.

Chlorides: Alkalis (particularly potassium) preferentially react with chloride to form KCl and NaCl. Any left over alkalis then react with SO2 and oxygen to form alkali sulphates. Chlorides are far more volatile in burning zone than alkali sulphates and will substantially evaporate and recirculate to the back end of the kiln where they will be deposited on incoming meal. Some chloride will however escape with the clinker, and if this amount is equal to the amount of fresh chlorides entering the kiln with the raw meal, a balance will be maintained and few buildups will occur. However, if the chloride input changes or the burning zone temperature increases these factors will upset the balance and buildups will occur more frequently.

Sulphates and Oxygen: As mentioned before if akalis are in molar excess of chloride they will react with SO2 and oxygen to form alkali sulphates according to the following equation;K2O + 2 SO2 + O2 --> 2 K2SO4 If insufficient oxygen is present in the kiln inlet gases, the reaction above will not occur and very little alkalis or sulphur will exit the kiln in the clinker. Instead, K2O and SO2 levels in the back end of the kiln will increase rapidly causing the hotmeal to become sticky and heavily coat the walls and other surfaces of the smokebox/calciner/meal pipes, causing buildups. The solution is to have the residual alkalis and sulphur in molar balance AND to ensure at least 2-3% O2 at the kiln inlet. If sulphur is in large molar excess of alkalis, sulphate spurrite build-ups can form higher up in the preheater by reaction of C2S, CaO and SO3 ie;CaO.SiO2 + CaO + SO3 --> 3 CaO.SiO2.SO3

Sulphate spurrite is stable at temperatures between 900 -1200 degC and causes buildups which are very hard and difficult to remove.

Temperature: If the temperature in the burning zone is excessive, not only will more alkalis/sulphur and chloride be evaporated from the clinker charge, but the kiln inlet temperaure is likely to be too hot as well. This can make the hot meal even stickier (due to the low melting point of alkali/sulphate/chlorides eutectic mixtures (mp 500-600 degC) that occur in the hot meal. The result will be an increased tendency to form buildups.