Combinability is about getting the raw materials to react, and how easily they do it.
If we were looking at potential raw materials for cement production, in addition to just looking at the analyses of the materials, we would need to know how well they combined with each other.
There are different approaches to this, but the basic idea is to do a series of trial mixes and assess how well they combine in a laboratory furnace.
First, we establish a fixed burning regime, eg: 30 minutes at the required temperature, intended to represent the time in the burning zone of the kiln.
For example, we might mix limestone and shale at an LSF of 0.94 and burn ten samples of this mix at different temperatures for 30 minutes – we’ll call this Mix A. When cool, we measure the free lime in each and draw a combinability curve for Mix A by plotting free lime against burning temperature.
We then repeat this for other mixes, eg: the same limestone and shale mixed to an LSF of 0.98; we’ll call this Mix B. For two mixes, A and B, we now have data relating burning temperatures and free lime (Figure 5.1). We would expect hotter burning to give lower free lime contents.
Figure 5.1 Examples of typical combinability curves for two mixes.
We then decide on a free lime level that we consider represents acceptable combination (eg: 1%, but it could be, say, 1.5% or 2%). We’ll use 1%.
The combinability temperature for each mix can be read from the plots at the intersection of the curve with the line CaO=1%. In Figure 5.1, the combinability temperatures, T(A) and T(B) for Mixes A and B are about 1430 ˚C and 1560 ˚C respectively.
Combinability depends on composition, specifically on the LSF, SR and AR.
Combinability also depends on particle fineness, so we would have to include a measure of fineness, such as the 90µm raw feed residue in the tests as well. The composition of the coarse residue is also important; a particle of silica 100µm across is more difficult to combine than a particle of limestone of the same size.
Combinability depends additionally on how well the raw materials react together;
a marl might be composed of small particles of calcium carbonate, silica and clay all together in one rock type in approximately the desired proportions to make cement. The small particles, already in close proximity with each other, will probably react more easily than a mix of the same overall composition containing a blend of three rock types say, chalk, silica and clay.
materials will combine, we need to do a large number of test burns. Assuming we have a suitable range of raw materials to control all three parameters (at least four materials) individual parameters can be changed to show how each parameter affects the combinability temperature for that mix.
First, we produce a series of mixes with varying LSF but the same SR and AR and fineness, we then burn each of these mixes at different temperatures to find the combinability temperature for that mix (ie: the temperature that gives us 1%
free lime). We then repeat, but varying the SR and then the AR. Finally, we repeat the whole exercise, this time using material of a different fineness.
(Fineness can mean the blended raw materials collectively, or we could test variations in the fineness of each component).
It will be clear that combinability experiments may involve doing tens or even hundreds of individual burns of test mixes and that this is not a trivial exercise.
Many hours of fun later, we would be able to produce standard plots of
combinability temperatures against the other variables. We would typically expect to find that:
Combinability temperatures increased with increasing LSF (Figure 5.2).
Combinability temperatures increased with increasing SR (Figure 5.3).
Combinability temperatures showed a minimum at about AR=1.4 (Figure 5.4).
The coarser the material, the more difficult it is to combine (Figure 5.5).
We could expect that the combinability temperature would increase with LSF; to produce a clinker in which the calcium silicates are all in the form of alite will require the constituents to be evenly mixed. Any heterogeneities will result in areas of excess lime or silica and these will be difficult to combine. With decreasing LSF mix heterogeneities become less important.
We would also expect the combinability temperature to increase with increasing SR because, as the SR increases, there will be less liquid to facilitate
combination.
The reason for a minimum in the combinability temperature at around AR=1.4 is less obvious. Alumina Ratios for ordinary Portland cement - normal grey cement - are in the range 1 to 4. However, at the lowest temperature at which liquid will form (1338 ˚C), the amount of liquid will be at a maximum at AR=1.38.
Since it is the liquid flux that mainly facilitates combination, mixes of AR ratio of 1.38 will be easiest to burn. Strictly, this applies to the CaO-SiO2-Al2O3-Fe2O3
system. Minor constituents can alter the optimum AR; magnesium oxide, for
example, increases it.
Combination is also affected by the viscosity of the liquid phase; a less viscous liquid will be more mobile and this will aid combination. Liquid viscosity increases with increasing AR and decreases with increasing temperature.
Finally, the coarseness of the reactants affect the rate of any chemical reaction, so it is no surprise that coarser material is harder to combine than fine material.
Figure 5.2 Example of a typical curve showing combinability temperature variation with LSF. With increasing LSF, it becomes progressively harder to achieve combination. Above 100% LSF, there will always be some uncombined free lime.
Figure 5.3 Example of a typical curve showing combinability temperature variation with SR. With increasing silica ratio, mixes become harder to burn, as there is less liquid present for ion transport, without which lime cannot combine with belite to form alite.
Figure 5.4 Example of a typical curve showing combinability temperature variation with AR at fixed LSF and SR.
Combinability temperatures are at a minimum at about AR=1.3-1.5.
Figure 5.5 Example of a typical curve showing combinability temperature variation for a fixed composition with 90µm sieve residue. Coarser material is harder to combine.
the previous section, suppose we want a high content of calcium silicates, with a high alite content. This means both a high LSF and a high SR. However, the curves in Figures 5.2 and 5.3 show that either will make the raw materials harder to combine, so in a mix with both a high LSF and a high SR, good combination might be difficult to achieve.
In practice, the main factor influencing the clinker composition will be the
compositions of the main raw materials to be used. For example, if a fairly pure limestone and a clay are to be used, the clay composition will largely determine the silica ratio and the alumina ratio, because the limestone won’t contain much silica and alumina. Much more control would be possible for the lime saturation factor, by adjusting the ratio in which the limestone and clay are mixed.
Of course, as we have seen, technically a cement works can have almost
complete control over clinker composition by blending raw materials of different composition to produce the desired result. Compositions are often a compromise between what is ideally wanted and what can be achieved in reality, given the composition of the available raw materials. Other important factors will include fuel and raw material grinding costs, and practical issues such as how finely the raw material mills can grind material at the required throughput.