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Thermal Decomposition of H ydromagnesite

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Thermal Decomposition of Hydromagnesite

Rafael Snell-Feikema, Neil Mehta, and Dr. Thomas DeVore

- 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 )

- 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

- 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

†N. Koga; Y. Yamane; J Therm Anal Calorim (2008) 93:963-971

‡J. Lanas; J.I. Alvarez; ThermochimicaActa(2004) 421:123-132

- 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

- EGA-FTIR was done on a Thermo Nicolet 6700

Temperature (K)

425

575

625

675

725

925

- 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

- 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

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

- 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”

- Dr. Reisner (X-ray diffraction lab)

Research Corporation Departmental Development Grant #7957

NSF: MRI 0340245 (TGA-MS)

NSF: DMR 0315345 (XRD)

NSF: REU - 1062629