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Thermal Decomposition of H ydromagnesite

Thermal Decomposition of H ydromagnesite. Rafael Snell- Feikema , Neil Mehta, and Dr. Thomas DeVore. Introduction. Hydromagnesite is a naturally occurring magnesium carbonate mineral with the chemical formula Mg 5 (CO 3 ) 4 (OH) 2 -4H 2 O.

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Thermal Decomposition of H ydromagnesite

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  1. Thermal Decomposition of Hydromagnesite Rafael Snell-Feikema, Neil Mehta, and Dr. Thomas DeVore

  2. Introduction • Hydromagnesite is a naturally occurring magnesium carbonate mineral with the chemical formula Mg5(CO3)4(OH)2-4H2O. ( W.B. White; Environmental Geology (1997) 30: 46-58 )

  3. Applications • Commercial applications include perfume retainers, fire retardants, rubber reenforcers and antacids. • Hydromagnesite is also an intermediate in the possible method of carbon sequestration starting with magnesium oxide† †V. Vágvölgyi; M. Hales; R.L. Frost; A. Locke; J. Kristóf; E. Horváth; J Therm Anal Calorim (2008) 94:523-528

  4. Samples studied • Acros “MgCO3” • Fisher “MgCO3” • Synthesized Mg5(CO3)4(OH)2 † • Dissolved 0.01 moles magnesium sulfate in 50 mL of water. • Dissolved 0.01 moles sodium hydrogen carbonate in 50 mL of water. • Heated to boiling and mixed. • Vacuum filtered the precipitate. • Air dried for twenty-four hours. †Z. Zhang; Y. Zheng; Y. Ni; Z. Liu; J. Chen; X. Liang; J. Phys. Chem. B 2006, 110, 12969-12973

  5. IRs

  6. IRs †N. Koga; Y. Yamane; J Therm Anal Calorim (2008) 93:963-971 ‡J. Lanas; J.I. Alvarez; ThermochimicaActa(2004) 421:123-132

  7. XRD - hydromagnesite

  8. Experimental TGA • Analysis was done using a MettlerToledo TGA /SDTG 851e • N2 flow rates: purge = 150 ml/ min; protect = 50 ml/ min • Capped and uncapped 70 ml alumina cells

  9. TGA/DTG – all samples uncapped

  10. Experimental EGA • EGA-FTIR was done on a Thermo Nicolet 6700

  11. EGA – Synthesized

  12. IRs – Fisher decomposition Temperature (K) 425 575 625 675 725 925

  13. Open cell mass loss • Step 1 matches in both cases and is fairly slight, low temperature – it’s surface drying • Accounting for this addition to the mass, we can approximate the other steps Mg5(CO3)4(OH)2-4H2O <=> Mg5(CO3)4(OH)2 +4H2O Mg5(CO3)4(OH)2 <=> 2Mg(CO3)+ 3MgO + 2CO2 + H2O (overlapping step #2) Mg(CO3) <=> MgO + CO2

  14. TGA/DTG – all samples capped

  15. SDTA – all samples capped

  16. TGA – effect of sample size

  17. TGA – effect of heating rate

  18. Kinetics Theory • DTG data can be used to find activation energy via the Kissinger equation† : • Where β is the heating rate and T is the temperature at the maximum reaction rate • Graphing ln(β/T2) vs 1000/T gives -E/R as the slope in kJ/mol †X.W. Liu; L. Feng; H.R. Li; P. Zhang; P. Wang ; J Therm Anal Calorim (2012) 107:407-412

  19. Kinetics - dehydroxylation

  20. Kinetics - decarbonation †X.W. Liu; L. Feng; H.R. Li; P. Zhang; P. Wang ; J Therm Anal Calorim (2012) 107:407-412

  21. Hypothesis • MgCO3 (s) <=> MgO(s) + CO2 (g) • As the pressure of CO2 rises, it drives the equilibrium to the left, causing the apparent decomposition to occur at a higher temperature. • Amorphous MgCO3 turns to crystalline MgCO3 at 808 K, which then decomposes rapidly, giving the observed “new” transition. • Fisher and Acros vary due to differing apparent densities • Fisher and our synthesized sample vary due to differing morphologies† †D. Bhattacharjya; T. Selvamani; I. Mukhopadhyay; J Therm Anal Calorim (2012) 107:439-445; “Thermal decomposition of hydromagnesite: Effect of morphology on the kinetic parameters”

  22. Acknowledgements • Dr. Reisner (X-ray diffraction lab) Research Corporation Departmental Development Grant #7957 NSF: MRI 0340245 (TGA-MS) NSF: DMR 0315345 (XRD) NSF: REU - 1062629

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