Question

List the main thermal zones in a cement kiln. Describe the process in each zone.

- Dehydration zone (up to 500°C)

- Decarbonation zone (between 500°C to 950°C)

- Solid-state reaction zone (between 950 °C to 1350 °C)

- Clinkering Zone (between 1350°C to 1500°C)

- Cooling zone

Answer 1

Dehydration zone (up to ~ 450˚C): This is simply the evaporation and removal of the free water. Even in the “dry process” there is some adsorbed moisture in the raw mix. Although the temperatures required to do this are not high, this requires significant time and energy. In the wet process, the dehydration zone would require up to half the length of the kiln, while the dry process requires a somewhat shorter distance.

Calcination zone (decarbonation zone )(450˚C – 900˚C): The term calcination refers to the process of decomposing a solid material so that one of its constituents is driven off as a gas. At about 600˚C the bound water is driven out of the clays, and by 900˚C the calcium carbonate is decomposed, releasing carbon dioxide. By the end of the calcination zone, the mix consists of oxides of the four main elements which are ready to undergo further reaction into cement minerals. Because calcination does not involve melting, the mix is still a free-flowing powder at this point

Dehydration zone (up to ~ 450˚C): This is simply the evaporation and removal of the free water. Even in the “dry process” there is some adsorbed moisture in the raw mix. Although the temperatures required to do this are not high, this requires significant time and energy. In the wet process, the dehydration zone would require up to half the length of the kiln, while the dry process requires a somewhat shorter distance.

Calcination zone (450˚C – 900˚C): The term calcination refers to the process of decomposing a solid material so that one of its constituents is driven off as a gas. At about 600˚C the bound water is driven out of the clays, and by 900˚C the calcium carbonate is decomposed, releasing carbon dioxide. By the end of the calcination zone, the mix consists of oxides of the four main elements which are ready to undergo further reaction into cement minerals. Because calcination does not involve melting, the mix is still a free-flowing powder at this point

Solid-state reaction zone (900˚ - 1300˚C): This zone slightly overlaps, and is sometimes included with, the calcination zone. As the temperature continues to increase above ~ 900˚C there is still no melting, but solid-state reactions begin to occur. CaO and reactive silica combine to form small crystals of C₂S (dicalcium silicate), one of the four main cement minerals. In addition, intermediate calcium aluminates and calcium ferrite compounds form. These play an important role in the clinkering process as fluxing agents, in that they melt at a relatively low temperature of ~ 1300˚C, allowing a significant increase in the rate of reaction. Without these fluxing agents, the formation of the calcium silicate cement minerals would be slow and difficult. In fact, the formation of fluxing agents is the primary reason that portland (calcium silicate) cements contain aluminum and iron at all. The final aluminum- and iron-containing cement minerals (C₃A and C₄AF) in a portland cement contribute little to the final properties.   As the mix passes through solid-state reaction zone it becomes “sticky” due to the tendency for adjacent particles to fuse together.

Clinkering zone (1300˚C – 1550˚C): This is the hottest zone where the formation of the most important cement mineral, C₃S (alite), occurs. The zone begins as soon as the intermediate calcium aluminate and ferrite phases melt. The presence of the melt phase causes the mix to agglomerate into relatively large nodules about the size of marbles consisting of many small solid particles bound together by a thin layer of liquid Inside the liquid phase, C₃S forms by reaction between C₂S crystals and CaO. Crystals of solid C₃S grow within the liquid, while crystals of belite formed earlier decrease in number but grow in size. The clinkering process is complete when all of silica is in the C₃S and C₂S crystals and the amount of free lime (CaO) is reduced to a minimal level (<1%).

Cooling zone: As the clinker moves past the bottom of the kiln the temperature drops rapidly and the liquid phase solidifies, forming the other two cement minerals C₃A (aluminate) and C₄AF (ferrite). In addition, alkalis (primarily K) and sulfate dissolved in the liquid combine to form K₂SO₄ and Na₂SO₄. The nodules formed in the clinkering zone are now hard, and the resulting product is called cement clinker. The rate of cooling from the maximum temperature down to about 1100˚C is important, with rapid cooling giving a more reactive cement. This occurs because in this temperature range the C₃S can decompose back into C₂S and CaO, among other reasons. It is thus typical to blow air or spray water onto the clinker to cool it more rapidly as it exits the kiln.