3.4 Burning in a kiln – formation of cement clinker
The next step in the process is to heat the blended mixture of raw ingredients (the raw mix) to convert it into a granular material called cement clinker. This requires maximum temperatures that are high enough to partially melt the raw mix. Because the raw ingredients are not completely melted, the mix must be agitated to ensure that the clinker forms with a uniform composition. This is accomplished by using a long cylindrical kiln that slopes downward and rotates slowly (see Figure 3-1).
Figure 3-1: A rotary kiln for cement manufacture. The inset shows a view inside the hot kiln. (Image courtesy of the Portland Cement Association).
To heat the kiln, a mixture of fuel and air is injected into the kiln and burned at the bottom end (see Figure 3-2). The hot gases travel up the kiln to the top, through a dust collector, and out a smokestack. A variety of fuels can be used, including pulverized coal or coke, natural gas, lignite, and fuel oil. These fuels create varying types and amounts of ash, which tend to have compositions similar to some of the aluminosilicate ingredients in the raw mix. Since the ash combines with the raw mix inside the kiln, this must be taken into account in order to correctly predict the cement compassion. There is also an increasing trend to use waste products as part of the fuel, for example old tires. In the best-case scenario, this saves money on fuel, reduces CO2 emissions, and provides a safe method of disposal.
The burning process
This description refers to a standard dry-process kiln as illustrated in Figure 3-2. Such a kiln is typically about 180 m long and 6 m in diameter, has a downward slope of 3-4%, and rotates at 1-2 revolutions per minute.
(After Mindess and Young, Fig. 3.2)
Figure 3-2: Diagram of the reactions occurring in a typical dry process cement kiln without a preheater.
The raw mix enters at the upper end of the kiln and slowly works its way downward to the hottest area at the bottom over a period of 60-90 minutes, undergoing several different reactions as the temperature increases. It is important that the mix move slowly enough to allow each reaction to be completed at the appropriate temperature. Because the initial reactions are endothermic (energy absorbing), it is difficult to heat the mix up to a higher temperature until a given reaction is complete. The general reaction zones (see Figure 3-2) are as follows:
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 (see Figure 3-3). 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%).
Figure 3-3: A nodule in the clinkering zone.
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.
Figure 3-4: Nodules of portland cement clinker after cooling (Photograph courtesy of the Portland Cement Association).
Suspension preheaters and calciners
The chemical reactions that occur in the dehydration and calcination zones are endothermic, meaning that a continuous input of energy to each of the particles of the raw mix is required to complete the reaction. When the raw mix is piled up inside a standard rotary kiln, the rate of reaction is limited by the rate at which heat can be transferred into a large mass of particles. To make this process more efficient, suspension preheaters are used in modern cement plants to replace the cooler upper end of the rotary kiln (see Figure 3-2). Raw mix is fed in at the top, while hot gas from the kiln heater enters at the bottom. As the hot gas moves upward it creates circulating “cyclones” that separate the mix particles as they settle down from above. This greatly increases the rate of heating, allowing individual particles of raw mix to be dehydrated and partially calcined within a period of less than a minute.
Alternatively, some of the fuel can be burned directly within the preheater to provide even more heating to the suspended particles. The area of the preheater where fuel is burned is called a precalciner. With a precalciner, the particles are nearly completely calcined as they enter the rotary kiln. Preheaters and precalciners save on fuel and increase the rate at which the mix can be moved through the rotary kiln.
Figure 3-5: Suspension preheater.
Grinding and the addition of gypsum
Once the nodules of cement clinker have cooled, they are ground back into a fine powder in a large grinding mill. At the same time, a small amount of calcium sulfate such as gypsum (calcium sulfate dihydrate) is blended into the cement. The calcium sulfate is added to control the rate of early reaction of the cement, as will be discussed in Section 5.3. At this point the manufacturing process is complete and the cement is ready to be bagged or transported in bulk away from the plant. However, the cement is normally stored in large silos at the cement plant for a while so that various batches of cement can be blended together to even out small variations in composition that occur over time. Cement manufacturers go to considerable lengths to maintain consistent behavior in their cements over time, with the most important parameters being the time to set, the early strength development, and the workability at a given water content.
Cement kiln dust
As the hot kiln gas moves through the kiln, it carries with it the smallest particles of the raw mix as well as volatilized inorganic substances such as alkalis (sodium and potassium) and chlorides. As the gas cools, the volatiles condense back round the small particles, and the resulting powder is called cement kiln dust (CKD). In the old days, the CKD was simply vented out of the smokestack, after which it would continuously settle out of the air to create a thin coating of grey dust on the surrounding countryside. This is no longer allowed. In fact, environmental restrictions even prevent CKD from being buried in landfills because of the tendency for the alkalis and chlorides to leach into groundwater. In modern cement plants, the CKD is removed in the suspension preheater and by and electrostatic precipitators located near the base of the smokestack.
Approximately 30 million tons of CKD are produced worldwide each year , and finding a way to use or dispose of this material has become a rather urgent problem for the cement industry. Some CKD can be returned to the kiln as raw mix, but this is limited by the tendency for the recycled CKD to increase the alkali and chloride contents of the cement clinker to unacceptable levels. High alkali contents can promote deleterious reactions with aggregate particles that damage the concrete, while chloride ions corrode reinforcing steel. Up to 15% of the total input of raw kilnstock can be lost as CKD.
Because CKD contains a high proportion of soluble alkali chlorides and sulfates, an obvious use is as an activator for blended cements, pozzolans, and hydaulic slags. At present, the drawbacks of CKD as a cement additive seem to outweigh the advantages, although research in this area is ongoing. This is expanded a bit in the following optional section.
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