Changes in physical structure during calcination of carbonate rocks

• Autorzy: Turek P. , Hycnar E. , Bajda T.
• Konferencja: XVth International Conference of Young Geologists Her'lany 2014 : Międzybrodzie Żywieckie, Poland, May, 8th-10th 2014
• Języki publikacji: EN

Abstrakty

EN

Limestone and other carbonate rocks are commonly used as sorbents for removing sulfur oxides from coal combustion flue gases. The process is based on chemical reaction between calcium oxide CaO and sulfur dioxide SO2, which results in formation of anhydrite CaSO4. CaO required for the reaction is produced by thermal dissociation of calcite CaCO3 which is the major constituent of each limestone. Calcite during calcination is decarbonized according to the reaction CaCO3 → CaO + CO2. The quality of such sorbent is usually being related to its total uptake of sulfur dioxide, which is frequently considered to depend mainly on chemical composition of the sorbent, especially CaCO3 content. However, there are premises that the rate of desulphurization of contaminated gases depends more on the structure of the products of limestone calcination than on CaCO3 content. Exceptionally significant is the surface area of CaO which results from its high porosity. The formation of such structure can be affected by several factors among which the structure of the raw material, its porosity and pore distribution along with crystallinity and the size of crystals are considered to be the most important issues. The current study demonstrates that the size of calcite crystals plays a key role in developing high porosity during limestone calcination in spite of calcium carbonate content in raw material. Four samples of different carbonate rocks were investigated: 1.crystalline limestone with well-developed calcite crystals of size 1-2 mm; 98.75 wt. % CaCO3; 2. micrite limestone with calcite crystals of size not exceeding 2 (im and 94.08 wt. % CaCO3; 3. limestone with calcite crystals of size up to 50 (μm dispersed in a micrite background; 94.75 wt. % CaCO3; 4. marble with crystals up to 1 mm; 96.08 wt. % CaCO3. The samples were ground to 0.125-0.250 mm particle size and calcined in the fixed bed at 850°C for 1 hour. The observation for differences in morphology of raw and calcined samples were conducted using Scanning Electron Microscopy (SEM). The particles and pore size distribution were characterized by BET analysis using low-temperature nitrogen adsorption technique. The content of CaCO3 in raw samples was calculated using total Ca content which was determined by titration with EDTA after digestion of samples with hot hydrochloric acid. SEM images revealed a regular system of cracks in the sample 1. The whole surface of calcite grains fractured and became very rough and porous. Sample 2, which was cryptocrystalline, did not show such behavior. Cracks and fractures did not occur, but under large magnification a slight porosity could be observed. Large crystals of the third sample cracked in a similar way to sample 1. Noteworthy is also the fact of creating little voids on the grain contact. Calcite crystals in sample 4 fractured along wide cracks into large fragments. Smaller clefts also occurred but they created irregular pattern, in contrast to sample 1. BET analysis confirmed foregoing SEM data. The calcination process increased the porosity and surface area of the investigated samples which was more evident with respect to more crystalline samples. Moreover, the pore distribution changed significantly. Calcination was followed by formation of the new porosity of samples, especially in mesoporous and macroporous range. Cracks in coarse calcite crystals enhance calcination by creating fast diffusion ways for CO2 escape. They could play similar role in sorption of SO2, hence the structure of raw sorbent becomes more significant issue with respect to flue gases desulphurization problem.

Słowa kluczowe

EN

limestone; chemical reaction; calcite

• Wydawca: Wydawnictwa AGH
• Czasopismo: Geology, Geophysics and Environment
• Rocznik: 2014
• Tom: Vol. 40, no. 1
• Strony: 133--134

Twórcy

autor

Turek P.

• AGH University of Science and Technology, Faculty of Geology, Geophysics and Environmental Protection, Department of Mineralogy, Petrography and Geochemistry; al. Mickiewicza 30, 30-059 Krakow, Poland

autor

Hycnar E.

• AGH University of Science and Technology, Faculty of Geology, Geophysics and Environmental Protection, Department of Mineralogy, Petrography and Geochemistry; al. Mickiewicza 30, 30-059 Krakow, Poland

autor

Bajda T.

• AGH University of Science and Technology, Faculty of Geology, Geophysics and Environmental Protection, Department of Mineralogy, Petrography and Geochemistry; al. Mickiewicza 30, 30-059 Krakow, Poland