A new method for producing calcium sulfoaluminate () clinkers is described. Sulfur is introduced from the gas phase as sulfur dioxide and oxygen and reacts with solids during clinkerisation. In this paper, the laboratory experiments are described and thermodynamic calculations are presented. The sulfur-containing phases ye'elimite and ternesite were stabilised together with belite to produce clinkers with various mineralogies. The influences of temperature and sulfur dioxide partial pressure were analysed and their effect on the formation of undesirable anhydrite and gehlenite was examined. The process by which a potentially hazardous waste material such as sulfur can be used as raw material, and possibly as fuel, to form cements, is shown to be successful.
At clinkering temperatures, typically >1250°C but lower than melting temperatures, which occur at >1300°C (Idrissi et al., 2010; Touzo et al., 2013), the loss of sulfur trioxide limits clinkering to a rather narrow window of temperatures and requires that the exhaust gas from the kiln is monitored and, if necessary, scrubbed to remove sulfur oxides (SOx). This paper shows that this loss need not be a problem, particularly on an industrial scale: the high pressures of sulfur oxides can be used to an advantage and vapour transport is shown to be an effective way of achieving reactions among the components of the raw meal. Kinetic studies show that the equilibrium between gas and solid components is achieved rapidly at ≈1300°C, even in the rapid flow rates achieved in commercial kilns.
Most experience of clinkering has been gained in laboratory experiments supplemented by pilot-plant ‘burns’. A somewhat different approach was taken in the work reported in this paper: experiments and thermodynamic calculations were combined to elucidate the clinkering process. This work demonstrated the importance of the vapour phase in clinkering and led to changes in kiln operation: the kiln was modified to work as a semi-sealed system in order to gain control of the kiln atmosphere. This was paralleled by using a small-capacity (10–100 g) laboratory kiln, permitting independent control of temperature and gas partial pressures of sulfur dioxide and oxygen and a total pressure of 1 bar (100 kPa).
Experiments were conducted in a tube furnace specifically modified to operate at 1 bar (100 kPa) total pressure but with controlled partial pressures of sulfur dioxide and oxygen (Galan et al., 2014); the furnace is shown in Figure 1. Pre-mixed gases, whose rates were monitored by means of precision mass flow controllers (Bronkhorst, NL), passed through the non-rotating furnace tube, maintaining the desired atmosphere during the experiment. The discharge end of the tube was connected to a scrubber that absorbs and neutralises unreacted sulfur oxides prior to gas discharge to the atmosphere. In this way, the exit gases comprised less than 1 ppm sulfur oxides. Temperature patterns (heating, idling and cooling) were programmed by means of the furnace control box.
The following two sets of raw materials were used in the experiments.
Set 1: laboratory grades of aluminium oxide (Al2O3) (Sigma-Aldrich 265 497, 10 μm, 99·7%), silicon dioxide (SiO2 or quartz) (Fluka 83 340, >230 mesh, >95%), calcium carbonate (CaCO3) (Sigma-Aldrich 795 445, >99%), iron oxide (Fe2O3) (Fisher Scientific I/1150/53, general purpose grade) and calcium sulfate (CaSO4) (Fisher C/2440/60, >95%).
Set 2: commercially available bauxite, clay and limestone. The oxide composition of the commercially available raw materials is shown in Table 1. The bauxite and clay were provided by Zhengzhou Haixu Abrasives Co. Ltd (China) and the limestone was provided by Samin (France).
|Silicon dioxide (SiO2): %||11·52||39·24||0·1|
|Aluminium oxide (Al2O3): %||69·32||38·18||0·00|
|Iron oxide (Fe2O3): %||1·21||5·98||0·009|
|Magnesium oxide (MgO): %||0·00||0·06||0·26|
|Calcium oxide (CaO): %||0·16||0·87||55·70|
|Sodium oxide (Na2O): %||0·00||0·00||0·00|
|Potassium oxide (K2O): %||0·455||0·624||0·00|
|Titanium dioxide (TiO2): %||3·409||1·773||0·00|
|Manganese oxide (MnO): %||0·000||0·001||0·00|
|Phosphorus pentoxide (P2O5): %||0·081||0·088||0·00|
|Loss on ignition: %||13·44||13·08||44·00|
In both cases the raw materials were weighed, mixed, placed in crucibles or boats of aluminous porcelain or platinum, and introduced into the furnace, which was ramped up to an isothermal level.
The variables evaluated in the experiments included sulfur dioxide partial pressure, peak clinkering temperature, proportioning of the raw materials and time. The oxygen partial pressure was kept sufficiently high to ensure oxidising conditions to (a) prevent the formation of undesirable sulfides and (b) ensure all sulfur dioxide was able to oxidise to sulfur trioxide if the equilibrium sought demanded the formation of solids containing sulfate. The minimum oxygen excess was targeted at 100%, resulting in weight ratios of sulfur dioxide:oxygen of at least 1:0·5 (or sulfur dioxide:air ratio of 1:2·5).
Approximately 25 compositions were tested and in all cases the atmospheric conditions were such that sulfur trioxide was transferred from vapour to solid to achieve the target mineralogy. This was achieved by providing an automatic check that the kinetics of transfer of sulfur species from gas to solid were rapid. The experiments evaluatedTable 2) and powder (Table 3) and from commercially available raw materials (Table 4) in powder form with a particle size of approximately 40 μm. In experiments 1–11 in Table 3, laboratory-grade calcium sulfate was also added to the raw mix.
In this set of experiments the sulfur dioxide plus oxygen flow was turned on when the furnace reached ≈600°C during ramp up (at 20°C/min) and turned off during the cooling cycle (at 20°C/min) below ≈600°C. To facilitate reactions between the gas and solid phases, a layer of the solid reactants several millimetres in thickness, ≈10 g in total, was placed in a 15 cm long ceramic boat in the middle of the hot zone of the tube (at constant temperature). The mix proportions used and the experimental conditions (sulfur dioxide:air ratios and temperature) are summarised in Table 3. The time allowed for reaction, ≈120 min, did not include ramping up and down times. The amount of calcium sulfate was, in all cases, sufficient to form the desired target compositions; the sulfur dioxide plus oxygen atmosphere was used to preclude sulfur losses from the solids and to keep an atmosphere with an excess of ‘sulfur trioxide’ at all times.
The products obtained were characterised by X-ray powder diffraction (XRD) using an Empyrean diffractometer (PANalytical) with strictly monochromatic CuKα1 radiation (λ = 0·154056 nm) at 45 kV and 40 mA. In order to determine the composition of the samples, they were analysed using the Rietveld methodology as implemented in the GSAS software package (Larson and Von Dreele, 2004). Final global optimised parameters included background coefficients, zero-shift error, cell parameters and peak shape parameters. Peak shapes were fitted using the pseudo-Voigt function (Thompson et al., 1987) with an asymmetry correction included (Finger et al., 1994). A March–Dollase ellipsoidal preferred orientation correction algorithm (Dollase, 1986) was used when the preferred orientation parameter needed refinement. The crystal structure descriptions for the different phases encountered were given by Cuesta et al. (2013) for orthorhombic ye'elimite, Cuesta et al. (2014) for cubic ye'elimite, Mumme et al. (1995) for β-belite, Colville and Geller (1971) for ferrite, Louisnathan (1971) for gehlenite, Irran et al. (1997) for ternesite, Hörkner and Müller-Buschbaum (1976) for calcium monoaluminate, Kirfel and Will (1980) for anhydrite and Sasaki et al. (1987) for perovskite.
As calculations suggested that the sulfur dioxide partial pressure was too high, another experiment was conducted in which the partial pressure of the sulfur dioxide component of the atmosphere was lowered: flow-rates of 0·1 g/min sulfur dioxide and 2·5 g/min air were used to give an air:sulfur dioxide ratio of 25:1. As shown in Figure 5, ternesite was successfully formed at 1075°C in the sulfur-containing atmosphere for the first time, in the presence of belite, anhydrite and unreacted lime. This temperature was chosen based on previous work by Pliego-Cuervo and Glasser (1978), who synthesised ternesite in sealed systems using belite and calcium sulfate as reactants. These experiments showed the combined influence of temperature and sulfur dioxide partial pressure on the stability of sulfur-containing phases. The field of stability of ternesite was subsequently mapped by Hanein et al. (2017), who quantitatively demonstrated the necessity of controlling the partial pressures of gas species if ternesite is the desired product.
At 1300°C, clinkers containing ye'elimite, belite and anhydrite were synthesised from mixes of calcium carbonate, silicon dioxide, aluminium oxide and iron oxide using mixes of the reactants in powder form and by pressing these same mixes in the form of 13 mm diameter pellets. The pellets were ≈2 mm thick. Three different mixes were used in order to obtain different proportions of the phases in the final product. The target compositions are given in Table 2. Figures 6 and 7 show the pellets corresponding to target compositions 1 and 2, respectively. In both cases the pellets were coherent and did not show cracking. However, the pellets with target composition 3, high in silica, crumbled during cooling and only powder could be retrieved.
|Experiment||Target mineralogy||Output mineralogy: wt%a||wRp: %b|
|1||60 wt% |
20 wt% C2S
20 wt% C6A2F
|O 20||α′ 1||23||26||1||—||—||—||5·28|
|C 28||β 2||—||—||—||—||—||—||—|
|2||40 wt% |
40 wt% C2S
20 wt% C6A2F
|O 15||α′ 2||18||21||1||—||—||—||4·99|
|C 23||β 19||—||—||—||—||—||—||—|
|3||20 wt% |
60 wt% C2S
20 wt% C6A2F
|O 2||β 25||—||1||28||4||20||1||5·48|
a The polymorphs of and C2S are shown: O and C stand for orthorhombic and cubic ye'elimite, respectively, and α′, β and γ are the three polymorphs of C2S. C represents calcium oxide, A represents aluminium oxide, represents sulfur trioxide, S represents sulfur dioxide and F represents iron oxide (Fe2O3)
b wRp is the weighted-profile R factor
|Experiment||Target mineralogy||Output mineralogy: wt%a||wRp: %|
|1||30 wt% |
60 wt% C2S
10 wt% C6A2F
|10||60 wt% |
30 wt% C2S
10 wt% C6A2F
Compositions with higher ye'elimite content (around 60%) could be achieved at temperatures as low as 1200°C (experiment 10 in Table 6); increasing the temperature to 1250°C led to an increase in both ye'elimite and belite contents (experiment 11 in Table 6).
The absence of ferrite in some clinkers could be attributed to the poorly crystalline ferrite, not ‘visible’ by XRD, the possible inclusion of some iron (probably not exceeding a few wt%) in ye'elimite (Touzo et al., 2013) and the limitations of XRD in detecting small amounts of phases.
|Experiment||Target mineralogy||Output mineralogy: wt%a||wRp: %|
|1||36 wt% |
32 wt% C2S
9 wt% C4AF
|2||O 8||β 4||48||25||—||1||9·30|
|C 13||α′ 2||—||—||—||—||—|
|3||O 20||β 11||29||4||2||11||6·79|
|C 17||α′ 7||—||—||—||—||—|
|4||O 30||β 23||22||—||1||0||5·63|
|C 16||α′ 8||—||—||—||—||—|
|5||34 wt% |
42 wt% C2S
11 wt% C4AF
|O 22||β 28||23||—||5||—||8·46|
|C 20||α′ 3||—||—||—||—|
|6||O 10||β 23||27||3||3||—||8·63|
|C 32||α′ 3||—||—||—||—||—|
|7||O 13||β 19||32||9||1||2||6·93|
|C 23||α′ 2||—||—||—||—||—|
|8||O 8||β 9||27||11||1||18||6·60|
|Composition||Model output: wt% using raw materials|
|C2S||C4AF||CT||CF (L)a||Magnesium oxide|
a L represents liquid state
Dilution of the sulfur dioxide from sulfur dioxide:air ratios of 1:25 (experiments 1 and 2 in Table 7) to 1:100 (experiments 3 and 4 in Table 7) led to a significant increase in ye'elimite and belite. In addition, the yields at 1300°C were notably higher than at 1250°C (experiments 1–4 in Table 7).
These experiments indirectly show the effect of the presence of impurities in the raw materials. Impurities affect the stability and formation of the phases, leading to different results and different effects of temperature and sulfur dioxide partial pressure. The mineralogical evolution with time can be observed from the results of experiments 5–7 in Table 7: at 1300°C and a sulfur dioxide:air ratio of 1:100, equilibrium seems to shift towards the formation of ye'elimite and belite with slow the disappearance of gehlenite and anhydrite.
According to the model predictions, the conditions used in experiments 2, 4 and 5–7 would lead to the target compositions. The reasons why these were not actually achieved are likely due to kinetic limitations, the ferrite not being ‘visible’ with XRD and possibly the cooling rate, which may have favoured the formation of anhydrite and gehlenite as opposed to ye'elimite and belite. It must also be noted that the model does not account for solid solutions (or liquid solutions) and therefore cannot predict the formation of entropy-stabilised phases such as ye'elimite with iron substitution or various aluminoferrite compositions.
The formation of ternesite was predicted in four of the compositions (2, 3, 8 and 9 in Table 8) but only detected experimentally in two (experiments 3 and 8 in Table 7). This can be understood by looking at the temperatures and partial pressures that were used. Comparing experiments 2 and 3 in Table 4, both were carried out at sulfur dioxide:air ratios of 1:100, but at different temperatures. Ternesite could be seen in the experiment at 1250°C, but a temperature of 1300°C appears to be too high for ternesite to stabilise. In experiments 8 and 9 in Table 4, both performed at 1275°C, the lower sulfur dioxide partial pressure (i.e. 1:100 sulfur dioxide:air ratio) favoured the formation and stabilisation of ternesite as opposed to the 1:50 ratio. Even though, under ideal conditions, ternesite would form in all cases, in reality too high temperatures and too high partial pressures make it more difficult for ternesite to be stabilised.
Sulfur species at elevated temperatures are dominated by sulfur dioxide and its partial pressure is fixed by temperature and by the sulfur content of the raw meal and fuel. As an approximation, the vapour pressure of sulfur dioxide in equilibrium with the clinker phases, for example anhydrite and ye'elimite, can be used to fix the minimum numerical value of the partial pressures necessary to stabilise these phases against evaporation. However, the actual pressure may significantly exceed that minimum, as for example is likely to occur in the course of combusting sulfur-rich fuels or when elemental sulfur is injected into the kiln to supply part of the thermal energy.
Under less secure control are the rates of sulfur transfer and their relation to the state or condition of the gas–solid surface available for exchange. These are functions of, among other factors, kiln size, gas flow rates and counter-current solid flow rates, as well as the total mass of transferrable components. As such, these factors are probably specific to specific equipment and cannot be readily calculated without process data.
By combining technical and thermodynamic limits, it is possible to control a kiln atmosphere simply by controlling the sulfur content of the fuel and raw mix and ensuring an excess of oxygen. The knowledge gained from these experiments and calculations (i.e. the influence of temperature and partial pressures of the gaseous components) partially informed the pilot-plant trials at Ibutec, Weimar, Germany, reported by Hanein et al. (2016a).
Another important aspect of this work – controlling the polymorphism and reactivity of belite – is still work in progress (Elhoweris et al., unpublished). In addition, hydration studies are needed to determine the properties of the resulting binders at all ages. The correlation of clinker mineralogy with cementing properties is under investigation (Jen et al., 2017).
The authors gratefully acknowledge the financial support provided by the Gulf Organisation for Research and Development (GORD), Qatar, through University of Aberdeen research grant number ENG016RGG11757.