Characterization of kiln feed limestone by dynamic heating rate thermogravimetry

    Characterization of kiln feed limestone by dynamic heating rate thermogravimetry Matias Eriksson PII: DOI: Reference:

S0301-7516(16)30001-1 doi: 10.1016/j.minpro.2016.01.001 MINPRO 2837

To appear in:

International Journal of Mineral Processing

Received date: Revised date: Accepted date:

31 March 2015 23 November 2015 4 January 2016

Please cite this article as: Eriksson, Matias, Characterization of kiln feed limestone by dynamic heating rate thermogravimetry, International Journal of Mineral Processing (2016), doi: 10.1016/j.minpro.2016.01.001

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ACCEPTED MANUSCRIPT Characterization of kiln feed limestone by dynamic heating rate thermogravimetry

Matias Eriksson

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Umeå University Thermochemical Energy Conversion Laboratory

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Department of Applied Physics and Electronics SE-901 87 Umeå, Sweden Tel: +46 (0)90-786 50 00

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[email protected]

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ACCEPTED MANUSCRIPT Abstract

Quicklime is a product rich in calcium oxide produced in industrial kilns. The process involves thermal decomposition

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of minerals with high content of calcium carbonate. The kiln feed properties vary with the geological formation from

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where the mineral is quarried or mined. Characterization of feed properties is necessary to achieve an optimized kiln

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production. In this work the decomposition of four different types of calcite ore were investigated by comparing conventional constant heating rate and dynamic heating rate thermogravimetric methods The conclusion of this work is that the conventional method always “overshoots” the calcination temperature when

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continuously heating during calcination compared to the dynamic rate method that resembles the kiln by holding temperatures constant during the calcination event. This justifies the used of the dynamic rate method. By a correct

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experimental parameter setup the dynamic rate method can be adapted for individual kilns and feed fractions, giving new additional value to the kiln operator and increasing the high value use of limestone deposits. This new method to

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characterize calcination properties of kiln feed materials can be utilized in normal kiln operations and when developing

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new mixes of different quality limestone. The results show differences when comparing the methods and different materials even though CaCO3 is present only as calcite. In addition, the dynamic rate method is faster than the

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conventional method. Besides quicklime production the method can also be applied in other industries calcining

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limestone, such as cement clinker production.

KEY WORDS: Limestone, lime kiln feed, dynamic rate thermogravimetry, thermal decomposition

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Introduction

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From an industrial perspective the thermal decomposition behavior of different minerals is of practical importance. Quicklime is an industrial product with a high concentration of calcium oxide (CaO) produced in industrial kilns by thermal decomposition of minerals rich in calcium carbonate (CaCO3). The thermal decomposition can be described by the following reaction: CaCO3 (s) → CaO (s) +CO2 (g)

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(1)

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1.1 Limestone and limestone based products Limestone is an abundant rock. The main component is CaCO3. Six different phases of CaCO3 have been reported; amorphous CaCO3, ikaite, monohydrocalcite, vaterite, aragonite and calcite (Zhang et al. 2012). All phases except calcite are unstable at ambient temperature and pressure (Jamieson 1953, Simmons and Bell 1963, Brecevic and Kralj 2007, Demichelis et al. 2013, Demichelis et al. 2014). Although aragonite can be found in natural limestone, the limestone of industrial interest consists mainly of calcite.

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Limestone is used in a wide range of applications and the quality requirements are high. Many limestones contain significant amounts of dolomite (CaMg(CO3)2) and the two fundamental types, high calcium limestone and dolomitic limestone are identified by their Mg content. There are several methods and systems for the further classification of limestone. In this work the chemical classification originally by Cox will be used (Cox et al. 1977, Harrison and Adlam 1985, Mitchell 2011), see Table 1. Limestone can also be classified for example according to grain type and grain size (Folk 1959) or by depositional texture (Dunham 1962). On average limestone is impure and estimated to contain about 77 wt.-% CaCO3 (Weast 1980). Industrial applications, with some exceptions, usually requires a significantly higher purity. Some limestone product quality data can be found in Table 2. Typical minor components are oxides of Al, Ba, Cr, Cu, K, Mn, Na, Ni, P, Pb, S, Sr, Ti, Zn and Zr (Mitchell 2009a). The main use of limestone is within construction. As illustrated by Table 3 the value of the limestone product varies with quality and use. Specific use of limestone per ton of product for some industrial applications can be seen in Table 4.

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The production of limestone through quarrying involves several steps; overburden removal, drilling, blasting, loading and hauling to the processing plant. The processing comprise of operations such as crushing, grinding, sizing, washing, sorting, and scalping before product storage. The whole production process requires extensive sampling and quality control (e.g. Oates 1998). Limestone properties can vary within a deposit, e.g. concerning concentrations of Fe, Si, S or Mg (e.g. Harrison and Adlam 1985). To increase high value utilization, e.g. quicklime production see Table 3, the mixing of different property limestone within a deposit is of interest. Limestone mixing can be made based on chemical properties but also for example on physical fraction or color. Quarry fines is usually of low value and increasing the utilization fines will have positive environmental and economic consequences (Mitchell 2009b).

1.2 Quicklime products and quicklime production Quicklime is used in many applications, as such or through the further processing by slaking and carbonation. The use can be divided into four main areas: construction, chemical and industrial, environmental, and metallurgical (e.g. Boynton 1980, Schorcht et al. 2013, Dowling et al. 2015). The most common production technologies for quicklime are shaft kilns and rotary kilns. The theoretical energy consumption according to reaction (1) is 3.2 GJ per ton CaO. The energy consumption for rotary kiln lime production is reported at 5.1 – 9.2 GJ per ton product. The shaft kiln is more sensitive to kiln feed and fuel properties but has lower energy consumption at 3.2 - 4.9 GJ per ton of product (Schorcht et al. 2013). The fraction of the limestone feed to the kilns range from <1 mm up to 350 mm depending on the kiln type. Kilns with limestone feed up to 3000 ton per day are in operation (Oates 1998). For 2013 the world production of lime products, mainly quicklime, has been estimated to 353 Mton (Miller 2014).

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The kiln process is highly dependent on quality parameters of the limestone feed (e.g. Watkinson and Brimacombe 1991, Cheng and Specht 2006). Full utilization of the quarried raw material, e.g. through optimized kiln feed fractions and mixes, together with adequate kiln operation for each feed mix is a positive sustainability measure for the lime operator. It allows for high product quality and high capacity utilization, with high energy efficiency and low waste generation. Typical quality control of lime kiln feed includes e.g. sieving, x-ray fluorescence and thermogravimetric analysis. Determining the variations in key parameters within a quarry or mine or between different deposits, is critical for kiln operations. Changes in the kiln feed need to be continuously addressed to avoid unwanted events such as subquality product or sintering of heavy blocks in kiln shafts. The kiln process needs to be adapted and optimized, e.g. high calcination temperature in the kiln increases kiln production but reduces quicklime reactivity (e.g. Moropoulou et al. 2001 and Commandré et al. 2007).

Materials and methods

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In this work the decomposition of four limestones intended for quicklime production have been investigated by dynamic heating rate thermogravimetry (DRTG) and conventional constant heating rate thermogravimetry (CRTG). The purpose of the work is to compare lime kiln feed properties determined by these two methods. The compared parameters are (i) the extrapolated onset temperature of the CaCO3 decomposition, (ii) maximum decomposition reaction rate, and (iii) temperature at maximum reaction rate. Since the time from limestone blasting, pretreatment and sampling to feeding into the kiln can be relatively short, a fast analytical method for quality control is preferred compared to a slower one. Therefore the experimental time is assessed as a secondary parameter.

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2.1 Calcite and calcination Calcite has five structures, calcite I-V. At ambient pressure, as in these experiments, CaCO3 is expected to be found as calcite I up to 800°C and as calcite IV above 800°C (Mirwald 1976, Kawano et al. 2009).

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The CaCO3 decomposition behavior varies with the properties of the specific mineral investigated and atmosphere conditions, e.g. product gas partial pressure. The partial pressure of CO2 in the atmosphere, pCO2, effects the decomposition, see Figure 1. At low pCO2 the full decomposition equilibrium occurs at a lower temperature. The pCO2 also influences the type of decomposition reaction, at low pCO 2 there is a gradual onset and at higher pCO2 it is a threshold reaction.

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The decomposition event evolves along a reaction zone. The reaction zone starts at the surface of the CaCO3 particle moving towards the center, described as a “shrinking core” (e.g. arc a-Labiano et al. 2002), a “contracting sphere” (Haines 1995) or an “unreacted core” (Ar and Doğu 2001). The CO2 gas migrates through the CaO layer and is released to the atmosphere. The temperature profile varies within the particle. The endothermic decomposition reaction decreases the temperature in the reaction zone. Heat is transferred from the surroundings to the reaction zone. The event can be seen in Figure 2, showing the uncalcined core of a piece of lump lime from shaft kiln operation. A SEM (Scanning Electron Microscopy) image of a lime surface can be seen in Figure 3 (Päärni 2008). The grains have a size up to 1 μm. A “spherical grain model” can be used to describe the porous structure emerged after calcination. Literature values for the initial CaO specific surface area after calcination are reported at a few m2/cm3, and particle size at 1-2 μm (Stendardo and Foscolo 2009, Gallucci et al. 2008). Several processes are involved in the decomposition; (i) heat transfer to the particle surface and through the CaO and CaCO3 layers to the reaction zone, (ii) mass transfer of CO2 released at the reaction zone through the CaO layer and away from the particle surface or (iii) the chemical reaction (Wang and Thomson 1995, Oates 1998, arc a-Labiano et al. 2002, Cheng and Specht 2006). What process is rate controlling at a given setup depends on factors like; heating rate, temperature, impurities, limestone crystalline structure, pCO2 and other properties of gas atmosphere. In laboratory conditions the experimental setup and sample preparation can influence which process is the actual rate-limiting process (e.g. Wang and Thomson 1995).

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ACCEPTED MANUSCRIPT 2.2 Decomposition of dolomite Dolomite, CaMg(CO3)2, is a common component in limestone and some of the samples in this investigation contain a dolomite phase. Dolomites are reported to decompose in a single step or via two discreet stages and some to “decompose in an intermediate manner” (Oates 1998). The decomposition varies with atmosphere conditions.

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CaMg(CO3)2 (s) → CaCO3 (s) + MgO (s) + CO2 (g)

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In a CO2 atmosphere dolomite decomposes in two steps, first to CaCO3 and MgO and secondly to CaO according to reaction (1). The first decomposition step occur in the temperature range 550-756°C and can be described through the following reaction (Engler et al. 1989): (2)

In air, at 700-750°C, the decomposition process proceed as follows:

2 CaMg(CO3)2 (s) → CaCO3 (s) + CaO (s) + 2 MgO (s) + 3 CO2(g)

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At 750°C the CaCO3 start to decompose while CaMg(CO3)2 still present. In the range 750-785°C, there is a simultaneous decomposition of CaCO3, according to reaction (1), and CaMg(CO3)2 according to reaction (4): (4)

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CaMg(CO3)2 (s) → CaO (s) + MgO (s) + 2 CO2 (g)

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2.3 Limestone samples The materials investigated are two very high purity high calcium limestone in single calcite phases (samples A and D), one dolomitic limestone (sample B), one high purity high calcium limestone with quartz and dolomite phases (sample C) and one dolomite mineral with only a small calcite phase (sample E).

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X-ray diffraction (XRD) was used to determine the crystalline content of the samples. Diffraction data were collected by continuous scans in the 2θ-intervall 10 - 80° with a Bruker d8 Advance instrument in θ−θ mode and Cu Kα radiation (40 kV and 40 mA), where the optical configuration consisted of a primary Go bel mirror and a Vantec-1 detector. The total data collection time lasted for at least 2.5 h by adding repeated scans. The initial phase identification was made using Bruker software and the PDF-2 database (ICDD 2004)). Subsequent semi-quantitative analysis of crystalline material was performed with Rietveld refinement using structures from Inorganic Crystal Structure Database (ICSD 1978). The XRD analysis can be seen in Table 5.

2.4 Thermogravimetry In conventional TG the sample temperature is changed in a predetermined scheme set by the user. In dynamic heating rate the temperature change varies in a user non-predetermined way (Gill et al. 1992, Ozawa 2000, TA Instruments 2001). The extrapolated onset temperature studied in this work is defined as the point of intersection of the tangent drawn at the point of greatest slope on the leading edge of the peak with the extrapolated base line (IUPAC 2002). The maximum reaction rate is determined by peak maximum of the first derivate weight curve. TA Instruments Q600 STD (CRTG) and TA Instruments Q500 HiRes™ T (DRTG) were used for the analysis. The Q600 has a horizontal furnace and balance design. It operates with dual beams up to 1500°C. The sample capacity is 200 mg and the heating rate from ambient to 1000°C can be set between 0.1°C/min and 100°C/min. The balance sensitivity is 0.1 μg. The Q500 has a vertical furnace and balance design. It operates with a dual hanging holder up to 1000°C. The sample capacity is 1000 mg and the heating rate from ambient to 1000°C can be set between 0.01°C/min and 100°C/min. The balance sensitivity is 0.1 μg. Both instruments are equipped with mass flow controllers for purge gas. No reaction gas was used. Since some samples contain a dolomite phase the decomposition reactions need to be established. The CaMg(CO3)2 decomposition was investigated by DRTG at different pCO2.

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In the lime kiln the feed material remains in almost isothermal conditions as long as the decomposition reaction proceeds, in this way the DRTG resembles the kiln. Therefore the influence of the DRTG experimental setup is of interest, i.e. does the DRTG allow for wide enough variation to simulate different kiln types, e.g. shaft kilns with long residence times and rotary kilns with shorter residence times? In addition to the DRTG-CRTG comparison a short evaluative series on the influence of the experimental parameters of heating rate and resolution setting of the DRTG was performed to investigate this. The experimental setup can be seen in Table 6.

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Comparative studies on conventional and dynamic rate thermogravimetry (also called high resolution TG) can be found in literature (Lever and Sutkowski 1993, Berbenni et al. 1999, Li and Huang 2000, Masson and Bundalo-Perc 2005, Brozek et al. 2007, Tobón et al. 2012). Some publications are available on dynamic rate thermogravimetric analysis of specific samples such as polymers, minerals, fillers, fuels and fuel additives (Bereznitski and Jaroniec 1996, Li 1999, Li and Huang 1999, Gonzalez et al. 2006, Li 2000, Wang et al. 2001, Zanier 2001, Frost et al. 2003, Frost and Weier 2004, Kök 2008, Tobón et al. 2012). No publications on dynamic rate thermogravimetric analysis of limestone or CaCO3 have been found.

Results and discussion

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The kinetics of thermal decomposition of CaCO3 and the interpretation of thermogravimetric data has been widely debated (e.g. Sharp and Wentworth 1969, Johnson and Gallagher 1972, Zsakó 1973, Elder and Ready 1986, Khinast et al. 1996, Brown 1997, Roduit 2000, Burnham 2000, Brown et al. 2000, Czarnecki and Šesták 2000, L’vov 2001, L’vov 2002, L’vov et al. 2002, Stanmore and Gilot 2005, Khawam and Flanagan 2006, Cheng and Specht 2006, Vyazovkin et al. 2011, Galwey 2012, Vyazovkin et al. 2014, L’vov 2015). A scientific consensus about the details of the decomposition kinetics is not yet achieved.

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3.1 Distinguishing the decomposition of calcite and dolomite To be able to distinguish the calcite and dolomite decomposition samples B and E were analyzed in different atmospheres. This is important for the comparison of the DRTG and CRTG of the lime kiln feed, in section 3.2. The experiment were performed by DRTG, experimental runs No. 13-16, in Table 6. The results can be seen in Figure 4 and Figure 5. Due to the rapid reaction at 900°C the dynamic rate TG decreases the temperature resulting in a nonlinear temperature signal. Therefore the onset temperatures are determined on a time axis.

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Figure 4 shows the decomposition of the dolomitic limestone. In pCO 2 = 0 the simultaneous decomposition of the calcite and dolomite phases according to reactions (1), (3) and (4) start at 704°C. Increasing pCO2 increases the onset temperature of the decomposition and allows for the two step decomposition of CaMg(CO 3)2 initially according to reaction (2), followed by the decomposition of CaCO 3 according to reaction (1). For the pure dolomite sample in Figure 5 the situation is similar. In summary, the results show that in high pCO2 a rapid first step of decomposition of dolomite to CaCO 3 and MgO starts at approximately 750°C followed by the threshold decomposition of CaCO3 at approximately 900°C, in accordance with reactions (2) and (1). In N2 atmosphere (pCO2 = 0) the overlapping decomposition of dolomite and calcite starts at approximately 700°C, in accordance with reactions (1), (3) and (4). When comparing the lime kiln feed in N2 the dolomite will decompose simultaneously with the calcite according to reactions (1), (3) and (4) and the decomposition can be viewed as a single event.

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Comparing constant heating rate and dynamic heating rate thermogravimetric analysis of lime kiln feed limestone The obtained temperature-time profiles of experimental runs No. 1-8 (see Table 6) can be seen in Figure 6. In Figures 710 results for the individual samples are compared. The results are summarized in Table 7.

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The difference between CRTG and DRTG is clearly shown in Figure 6. The temperature profile of the DRTG is not predetermined by the user, but a result of the experimental conditions and the sample properties. Figure 6 shows that while the CRTG heating rate remains constant all through the experiments, the heating rate of the DRTG decreases in the temperature range of 750-850°C. The DRTG result resemble kiln operations. When calcination is complete the feed temperature quickly rise causing sintering and reducing product reactivity. The different calcination properties will influence quicklime product quality. In this way DRTG can be used to characterize calcination properties of existing kiln feed for optimization of quarry and kiln operation. It can also be used for developing new kiln feed products by mixing, and when prospecting, evaluating or developing new deposits.

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In Figures 7-10, showing results for the individual samples, the sample weight (green) and 1st derivative of sample weight (blue) can be seen as a function of temperature. The weight loss starting at approximately 750°C corresponds to the decomposition reactions (1), (3) and (4). The end weight corresponds with the calcite and dolomite content of the samples. Since SiO2 undergoes no weight loss in this temperature range, sample C, having a quartz phase has the highest end weight, see Figure 9. The very high purity limestone, samples A and D, have the lowest end weight, see Figures 7 and 10. Low in impurities, the dolomitic limestone, sample B, also has a low end weight, see Figure 8. When the end weight is reached, and 40-44 % of the initial weight has been lost, no CaCO3 or CaMg(CO3)2 remain it the samples. The peak of the 1st derivative weight signal shows the maximum weight change, i.e. maximum rate of the decomposition reactions. When a function of temperature, as in Figures 7-10, the peak occurs at a lower temperature and with a higher maximum value for DRTG (solid line) as compared to the CRTG (dotted line).

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In Table 7 the results from the individual samples are summarized with respect to the selected methods and parameters. Table 7 shows that for all samples the CRTG onset is higher than the DRTG onset. For the individual samples the onset temperature varies at most by 10°C, sample A, compare Figure 7. The maximum reaction rates with respect to temperature [%/°C] and time [%/min] all occur at higher temperatures for CRTG (T ≈ 825°C) than for DRTG (T ≈ 784°C). For all samples the reaction rate, expressed as [%/°C], is higher for DRTG, on average 64% and at most 83% for sample A. If the reaction rate is expressed as [%/min], the situation is the opposite, then the DRTG reaction rates are lower for all samples.

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The results show that there are differences between the kiln feeds. The residence time of the lime kiln is easy to control. Comparing maximum decomposition rates in %/min and the temperature at which this occurs, as in Figure 11, illustrates the different calcination properties. The results can be utilized e.g. when defining kiln feed fractions, when mixing different kiln feed or when preparing to switch between kiln feed. For example mixing A and D will result in a partly over burned and a partly unburned product stream. Decomposition of kiln feed A starts at a slightly lower temperature, 748°C as compared to D at 751°C. During decomposition feed A reaches higher decomposition rates 7.3 %/min as compared to D at 6.5 %/min. Furthermore feed A reaches the maximum decomposition rate at a lower temperature 781°C as compared to D at 792°C. Applied, e.g. to rotary kiln production, the A and D fractions would be fully mixed and could not be separated to representable quicklime product samples. The rotary kiln is operated on residual CO2 content in the product stream. Kiln feed A will have a lower residual CO2 and a possibly a high degree of sintering and low reactivity, while kiln feed D will have a higher residual CO2 with less sintering and a higher reactivity. To compensate the different calcination properties the fraction of the kiln feeds can be adjusted, so that the faster feed, A, is feed as a coarser fraction, e.g. 20-40mm, and the slower feed, D, is feed as a finer fraction e.g. 1030mm. In the shaft kiln limestone feed is usually coarse, e.g. a 40-90 mm fraction, and the resident times are longer, e.g. 2030h. The residence time needs to be adjusted to the fraction and the properties of the feed. A feed with slower reaction rate, such as sample D, will need a smaller fraction or a longer residence time in the kiln to achieve the same residual CaCO3 in the end product as compared to a feed with a faster reaction rate, e.g. sample A. In a mixed feed the fractions can be adapted as for the rotary kiln case. The shaft kiln feed can be mixed prior to feeding or feed alternately to achieve a vertical layers of the different fractions. The results also show that kiln feed A and B have similar calcination properties under pCO2 = 0 conditions. As shown in Figure 4, kiln feed B containing dolomite is sensitive to kiln gas conditions and further investigations should focus on determining kiln feed properties at different pCO2 resembling different kiln types and conditions.

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3.3 The effects of HRTG experimental setup and adaption to specific processes To study the effect of experimental parameters of the dynamic rate method the heating rate and resolution setting were varied in an experimental series with sample A, Table 6 No. 1 and No. 9-12. The obtained temperature-time profiles for sample A at different experimental setup can be seen in Figure 12. The decomposition event profiles can be seen in Figure 13. The results can be summarized as seen in Table 8.

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For the Q500 the resolution setting can be set between -8.0 and + 8.0. With a negative setting the mode is constant reaction rate TG and with a positive setting dynamic rate TG. If resolution setting is zero the instrument operates as a conventional constant heating rate TG. Increasing the resolution setting increases the experimental resolution between consecutive reactions. The resolution setting influence the detected transition temperature. A high resolution setting lowers the reaction temperature at which a transition is detected, bringing it closer to the theoretical decomposition temperature. A high resolution setting also increases the experimental time. Increasing the resolution setting by a whole number, e.g. from 3.0 to 4.0, generally increases the experimental time by a factor of between 2 and 5. The effect is clearly seen in Figure 12 where the high resolution setting of run No. 10 increases the experimental time by almost 7 times as compared to experimental run No.1.

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Figure 13 shows the variations related to different experimental setups. The lowest onset temperature is achieved with the highest resolution setting, No. 10, and the highest onset temperature can be found at the lowest resolution setting, No. 9. As summarized in Table 8, the results show that increasing the heating rate at constant resolution setting increases the onset temperature while decreasing the experimental end time. On the other hand, increasing the resolution setting at constant heating rate lowers onset temperature while multiplying the time consumed to end of experiment. The results show that resolution setting has a more significant impact on the experimental result than the heating rate.

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By adjusting experimental settings, primarily the resolution setting, the calcination event can be adapted to specific kiln residence times. By standardizing and verifying the method for a specific kiln the results can be utilized in kiln process control, e.g. as an operating parameter in an automated control system on the kiln, or in the quarry for quality control and as a parameter for mixing. Adapted to the specific kiln the potential is achieving an improve quicklime quality, a more stable calcination process, a reduced kiln energy consumption and thereby reduced costs and environmental impact, e.g. lower CO2 emissions and waste. Increased use of quarried material for high value purposes will increase quarry profitability.

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The method can also be adapted to other industries calcining limestone, such as cement clinker or blast furnace processes.

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Conclusions

At pCO2 = 0 the decomposition of dolomite will proceed simultaneously with calcite and therefore the onset temperatures and reaction rates between the pure calcite kiln feeds and the kiln feeds with dolomite phases can be compared. The rate of thermal decomposition as defined by CRTG and DRTG varies between different materials although CaCO3 is present only as calcite. The conventional T always “overshoot” the calcination temperature while continuing heating during calcination. The DRTG resembles, or mimics, the kiln by holding temperatures constant, or near constant, during the calcination event. DRTG experimental resolution setting has a more significant impact on the experimental result than the heating rate. When DRTG experimental setup is properly adjusted for the individual kiln, this new thermogravimetric method provides the kiln and the quarry operator with valuable new information on kiln feed behavior.

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ACCEPTED MANUSCRIPT The new method to characterize calcination properties of kiln feed materials can be utilized in normal kiln operations and when developing new mixes of different quality limestone. Besides quicklime production the method can also be applied in other industries calcining limestone, such as cement clinker production. In addition, DRTG can provide additional value for the industrial operator by reducing the experimental time.

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Further work should focus on full scale application at a specific kiln and determining of kiln feed properties at different pCO2 resembling different kiln types and conditions.

Acknowledgements

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The author would like to acknowledge Rainer Backman at Umeå University for valuable discussions and Dan Boström at Umeå University for the XRD analysis and valuable discussions. Erik Viggh at Cementa Ab and Umeå University is acknowledged for proof reading of the manuscript.

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The Swedish Mineral Processing Research Association (MinFo) is acknowledged for project support. The Swedish Governmental Agency for Innovation Systems, Vinnova (No. 2014-04073 and 2015-02519), The National (Swedish) Strategic Research Program Bio4Energy, the Swedish Energy Agency (No. 2006-06679/30527-1) and Nordkalk Oy Ab are acknowledged for financial support. The funding sources have had no involvement regarding the content of this study.

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L'vov, B.V., 2015, Kinetic parameters of CaCO3 decomposition in vacuum, air and CO2 calculated theoretically by means of the thermochemical approach, Reaction Kinetics Mechanisms and Catalysis, Vol. 114(1), pp. 31-40, DOI: 10.1007/s11144-014-0767-2

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Masson, J-F., and Bundalo-Perc, S., 2005, Solventless fingerprinting of bituminous materials: A high-resolution thermogravimetric method, Thermochimica Acta, Vol. 436(1-2), pp. 35–42, DOI: 10.1016/j.tca.2005.06.025

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Mitchell, C., 2009b, Quarry Fines and Waste, British Geological Survey, originally published in Quarries & Mines 2009, http://nora.nerc.ac.uk/6290/1/Quarry_Fines_and_Waste.pdf (Accessed 2015-09-23) Mitchell, C., 2011, High purity limestone quest, Industrial Minerals, November issue, pp. 48-51

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Moropoulou, A., Bakolas, A., and Aggelakopoulou, E., 2001, The effects of limestone characteristics and calcination temperature to the reactivity of the quicklime, Cement and Concrete Research, Vol. 31(4), pp. 633-639, DOI: 10.1016/S0008-8846(00)00490-7

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Stanmore, B.R., and Gilot, P., 2005, Review—calcination and carbonation of limestone during thermal cycling for CO 2 sequestration, Fuel Processing Technology, Vol. 86(16), pp. 1707-1743, DOI: 10.1016/j.fuproc.2005.01.023

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Stendardo, S., and Foscolo, P.U, 2009, Carbon dioxide capture with dolomite: A model for gas–solid reaction within the grains of a particulate sorbent, Chemical Engineering Science, Vol. 64(1), pp. 2343-2352, DOI: 10.1016/j.ces.2009.02.009 TA Instruments, 2001, TGA Hi-ResTM Option User Reference Guide, 109 Lukens Drive, New Castle DE 19720

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Watkinson, A.P., and Brimacombe, J.K., 1991, Limestone calcination in a rotary kiln, Journal of Electronic Materials, Vol. 20(12), pp. 369-378, DOI: 10.1007/BF02816008 Weast R.C. editor, 1989, CRC Handbook of chemistry and physics, CRC Press Inc., Boca Raton, Florida, p. F-89, and p. D-64

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World Steel Association, 2012, Sustainable steel: at the core of a green economy, ISBN 978-2-930069-67-8, http://www.worldsteel.org/dms/internetDocumentList/bookshop/Sustainable-steel-at-the-core-of-a-greeneconomy/document/Sustainable-steel-at-the-core-of-a-green-economy.pdf (Accessed 2015-03-31) Vyazovkin, S., Burnham, A.K., Criado, J.M., Pérez-Maqueda, L.A., Popescu, C., and Sbirrazzuoli, N., 2011, ICTAC Kinetics Committee recommendations for performing kinetic computations on thermal analysis data, Thermochimica Acta, Vol. 520(1), pp. 1-19, DOI: 10.1016/j.tca.2011.03.034 Vyazovkin, S., Chrissafis, K., Di Lorenzo, M.L., Koga, N., Pijolat, M., Roduit, B., Sbirrazzuoli, N., and Suñol, J.J., 2014, ICTAC Kinetics Committee recommendations for collecting experimental thermal analysis data for kinetic computations, Thermochimica Acta, Vol. 590, pp. 1-23, DOI: 10.1016/j.tca.2014.05.036 Zanier, A., 2001, High-resolution TG for the characterization of diesel fuel additives, Journal of Thermal Analysis and Calorimetry, Vol. 64(1), pp. 377-384, DOI: 10.1023/A:1011586407704 Zhang, Z., Xie, Y., Xu, X., Pan, H., and Tang, R., 2012, Transformation of amorphous calcium carbonate into aragonite, Journal of Crystal Growth, Vol. 343(1), pp. 62-67, DOI: 10.1016/j.jcrysgro.2012.01.025 Zsakó, J., 1973, Kinetic analysis of thermogravimetric data, VI, Some problems of deriving kinetic parameters from TG curves, Journal of Thermal Analysis, Vol. 5(2-3), pp. 239-251, DOI: 10.1007/BF01950372

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Figure 1. Equilibrium temperature for CaCO3 decomposition as a function of pCO2. Figure 2. Split lump of quicklime.

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Figure 3. Quicklime surface (from Päärni 2008).

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Figure 4. Sample B, DRTG investigation of the thermal decomposition of calcite and dolomite, in high pCO 2 (solid line) and at pCO2 = 0 (dotted line), as a function of temperature. Onset temperatures of decomposition steps marked.

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Figure 5. Sample E, DRTG investigation of the thermal decomposition of dolomite and calcite, in high pCO 2 (solid line) and at pCO2 = 0 (dotted line), as a function of temperature. Onset temperatures of decomposition steps marked. Figure 6. Samples A-D, temperature-time profiles of experimental runs No. 1-8. DRTG No. 1-4 and CRTG No. 5-8.

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Figure 7. Sample A, comparison of DRTG (solid lines) and CRTG (dotted lines), weight (green lines) and 1 st derivative weight (blue lines) as a function of temperature. Experimental runs No.1 and 5.

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Figure 8. Sample B, comparison of DRTG (solid lines) and CRTG (dotted lines), weight (green lines) and 1 st derivative weight (blue lines) as a function of temperature. Experimental runs No.2 and 5. Figure 9. Sample C, comparison of DRTG (solid lines) and CRTG (dotted lines), weight (green lines) and 1 st derivative weight (blue lines) as a function of temperature. Experimental runs No.3 and 6.

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Figure 10. Sample D, comparison of DRTG (solid lines) and CRTG (dotted lines), weight (green lines) and 1 st derivative weight (blue lines) as a function of temperature. Experimental runs No.4 and 8.

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Figure 11. Samples A-D, maximum decomposition rates as a function of time and temperature at maximum decomposition rate, comparison of DRTG (black) and CRTG (green).

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Table 1. Classification of high calcium limestone. SiO2

Fe2O3

> 55.2 54.3 - 55.2 52.4 - 54.3 47.6 - 52.4 < 47.6

< 0.8 0.8 - 1.0 1.0 - 3.0 > 3.0 > 3.0

< 0.2 0.2 - 0.6 0.6 - 1.0 < 2.0 > 2.0

< 0.05 0.05 - 0.1 0.1 - 1.0 > 1.0 > 1.0

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MgO

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> 98.5 97.0 - 98.5 93.5 - 97.0 85.0 - 93.5 < 85.0

CaO

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Very high purity High purity Medium purity Low purity Impure

CaCO3

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[wt.-%]

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CaO

MgO

SiO2

Paint (n=100)

92 - 99.35

51.55 - 55.67

0.15 - 1.2

0.05 - 4.5

Paper (n=35)

96 - 99.35

53.79 - 55.76

0.15 - 1.2

0.05 - 0.4

Plastic (n=88)

92 - 99.35

51.55 - 55.67

0.15 - 1.2

0.05 - 4.5

Food & Pharmaceuticals (n=34)

97 - 99.5

54.35 - 55.75

0.24 - 0.42

0.1 - 0.12

Ceramic (n=14)

98.8 - 99.35

55.36 - 55.67

0.22 - 0.38

0.06 - 0.12

Rubber (n=51)

92 - 99.35

51.55 - 55.67

0.15 - 1.2

0.05 - 4.5

Adhesives & sealants (n=65)

92 - 99.35

51.55 - 55.67

0.15 - 1.2

0.05 - 4.5

Agriculture & animal feed (n=14)

92 - 99.35

51.55 - 55.67

0.22 - 0.96

0.06 - 4.5

0.3

0.1

96.4

(b

Metallurgy (n=1)

97.6

(b

> 90

54

54.7

> 50.4

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Functional filler(a (n=1)

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Table 2. Overview of commercial limestone products, product quality as regards to CaCO3, CaO, MgO and SiO2, n = number of products, based on Mitchell 2011, except a) Nordkalk 2015 and b) Carmeuse 2015

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Average value [USD/ton] 10 4 16 16 11 14 19 9 9

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Construction Cement Quicklime Agriculture Flue gas desulphurization Chemical and metallurgical Other Unspecified Total/Average

Volume [Mton] 254 57 13 11 6 2 5 422 769

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Reference Schorcht et al. 2013 Stoichiometric Schorcht et al. 2013 Remus et al. 2013, Oates 1998 World steel 2012, Remus et al. 2013 Ruth and Dell'Anno 1997 Ruth and Dell'Anno 1997, Scalet et al. 2013 Scalet et. al. 2013 EC 2013 EC 2013

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Limestone consumption [ton/tonproduct] 1.4 - 2.2 1.3 1.27 - 1.57 0 - 0.08 0.064 - 0.3 0.12 - 0.19 0.08 - 0.42 0.1 - 0.4 0.011 - 0.022 0.387

Product Quicklime Hydrated Lime Cement clinker Hot metal Crude steel Fiber glass/glass wool Glass Glass lamp bulbs Kraft pulp ADt Fine paper

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Table 5. XRD analysis of limestone and dolomite. Samples A-E.

99

E 0.5 99.5

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Sample C D 97 99 1 2 100 100 99 B 88 12

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Phase [wt.-%] Calcite, CaCO3 Dolomite, CaMg(CO3)2 Quartz, SiO2 Sum:

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Purge gas Sample [/min] 10ml N2 10ml N2 10ml N2 10ml N2 10ml N2 10ml N2 10ml N2 10ml N2 10ml N2 10ml N2 10ml N2 10ml N2

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Isothermal at 1000°C [min] 1 1 1 1 -

Purge gas Balance [/min] 25ml N2 25ml N2 25ml N2 25ml N2 100ml N2 100ml N2 100ml N2 100ml N2 25ml N2 25ml N2 25ml N2 25ml N2 25ml CO2 25ml N2 25ml CO2 25ml N2

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Method DRTG DRTG DRTG DRTG CRTG CRTG CRTG CRTG DRTG DRTG DRTG DRTG DRTG DRTG DRTG DRTG

Heating to 1000°C [°C/min] 50 50 50 50 20 20 20 20 50 50 80 20 50 50 50 50

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Sample A B C D A B C D A A A A B B E E

DRTG resolution setting 3 3 3 3 0.5 6 3 3 4 4 4 4

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No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Mass [mg] 21.6 26.9 22.9 24.6 22.4 27.1 22.5 23.0 20.3 21.9 25.3 28.2 18.2 18.0 6.4 18.6

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(1),(3),(4)

C

(1),(3),(4)

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(1)

1-4

DRTG

5-8

CRTG

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Average ± st.dev.

f ′(t) Rate at Tmax [%/min] 7.30 9.92 6.96 9.12 6.25 8.35 6.50 8.56 6.75 ± 0.41 8.99 ± 0.61

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Tmax [°C] 781 825 780 821 786 824 792 837 785 ±5 827 ±6

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f ′(T) Rate at Tmax [%/°C] 0.93 0.51 0.81 0.47 0.62 0.42 0.68 0.43 0.76 ± 0.12 0.46 ± 0.03

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(1)

Method DRTG CRTG DRTG CRTG DRTG CRTG DRTG CRTG

Tmax [°C] 780 818 777 816 785 822 795 835 784 ±7 823 ±7

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No. 1 5 2 6 3 7 4 8

Tonset [°C] 748 757 745 750 748 756 751 752 748 ±2 754 ±3

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Table 7. Summary of results for comparison of selected parameters for limestone samples A-D: DRTG = dynamic heating rate TG. CRTG = constant heating rate TG. Tonset = onset temperature of decomposition reaction [°C]. f ′ = first derivative of weight change signal with regards to temperature (T) or time (t). Tmax = temperature at peak maximum. Rate = rate of decomposition event with regards to temperature [%/°C] and time [%/min]. End = end of experiment.

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tend [min] 23 43 24 42 24 43 24 44 24 ± 0.5 43 ±1

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Tmax [°C] 781 847 648 770 794

f ′(t) Rate at Tmax [%/min] 7.30 18.9 0.4 4.9 8.6

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f ′(T) Rate at Tmax [%/°C] 0.93 0.46 2.85 0.84 0.94

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Tmax [°C] 780 846 647 770 793

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Tonset [°C] 748 776 640 733 762

tend [min] 23 18 157 47 16

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Heating rate [°C/min] 50 50 50 20 80

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No. 1 9 10 11 12

Resolution setting 3.0 0.5 6.0 3.0 3.0

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Dynamic rate thermogravimetry as a new method to characterize lime kiln feed The method can be adapted to specific kilns to improve quicklime production The method can be used to increase high value use of limestone deposits The method can be transferred to other industries, e.g. cement clinker production

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