| Summary: | Research Doctorate - Doctor of Philosophy (PhD) Mineral carbonation represents a potentially suitable CO₂ abatement strategy for a country such as Australia, particularly the state of New South Wales, which is devoid of suitable geological sinks within 500 km from the cluster of coal-fired power stations but possesses ample feedstock, in the form of magnesium silicate deposits, to capture CO₂ over an estimated 300 years. This thesis examines the weak acid process, commonly known as the acetic acid route that basically encompasses the dissolution of wollastonite or serpentinite in a weak acid and subsequently carbonation with concomitant recycling of the acid. The initial phase of the study compares the efficiency of 0.1 M aqueous solutions of acetic, formic and DL-lactic acids as Ca²⁺ leaching agents from wollastonite (volume mean diameter = 17μm between 20 and 80 °C for 3 h. Metal ion analysis of the leachate, afforded on inductively coupled plasma-optical spectroscopy, revealed formic acid as being the most effective at 80 °C, in terms of extraction and rate of dissolution. Experimental calcium yields and solution pH values agree with thermodynamic predictions from OLI Analyzer Studio 3.0. Formic acid registered nearly complete extraction within 20 min, whereas acetic and DL-lactic acids achieved 60-70 % yield in the same lapse of time and temperature. The corresponding rates amount to 26(±7)x10 -5 , 14(±3)x10 -5 and 17(±4)x10 -5 mol m⁻² s⁻¹ for formic, acetic and DL-lactic acids, respectively. These results indicate an initial diffusion limitation in the silica layer formed around wollastonite particles for formic acid and a kinetic limitation for the other two acids. Scanning electron microscopic images provide further evidence for the effectiveness of formic acid on the basis of enhanced crazing in the silica layer. Additionally, the extractive process involving formic and acetic acids were modelled with ash/inert layer diffusion control and surface chemical reactions control standard shrinking core models, respectively, but the latter failed to fit experimental data. A modified version that incorporates a ‘variable activation energy’ term accounting for the occurrence of a complex network of reactions as well as varying conversion rates, successfully described dissolution in the weakly acidic media. An order of reaction of 0.45 was found to be the best fit for both cases pointing to mixed order reactions that comprise several elementary reactions. The next stage investigates the dissolution of partially and fully hydrated serpentinite specimens in 0.1 M formic acid at 80 °C Extraction was found to be dependent on particle size and degree of heat activation. Magnesium yields attained 42 % from the -25 μm forsterite-lizardite containing sample activated at 700 °C (29 % residual OH) and 66 % from the fully serpentinised antigorite mineral, which was pulverised to a particle size of -53 μm and conditioned at 720 °C (36 % residual OH). Heat treatment of lizardite and antigorite between 500 and 800 °C engenders the generation of amorphised material, forsterite and silica; enstatite forms from the amorphised material and silica at temperature exceeding 800 °C. Semi-quantitative X-ray diffraction, coupled with Fourier transform infrared spectroscopy, demonstrated that both forsterite and amorphous phases dissolve in the weak acid. However, the growth of amorphous silica formed either during forsterisation at elevated temperature or as a result of magnesium extraction, inhibits further dissolution. The complexity of carbonation in a weakly acidic medium and acid recycling constitute the focus of the last part of this study. The backbone of the acetic acid route consists of the assumption that acetic acid, with a pK a of 4.76 is weak enough to be displaced by carbonic acid (pK a1 6.4) and hence be recycled during carbonate precipitation. However, the thermodynamic framework OLI Studio Analyser 3.2 predicts that acetic acid remains stronger than carbonic acid over a temperature and pressure range of 25-200 °C and 10-200 bar, thus rendering recycling impractical. The pK a1 of carbonic varies, as function of temperature and pressure, between 6.2 and 7.2 while that of acetic acid reaches a maximum value of 5.5 from the initial 4.76. Even though dissolution yields can be maximised by using formic acid, finer particle sizes and heat activation, carbonation is unfeasible owing to the solubility of carbonates in acids. Inducing their precipitation through the addition of a base would impose additional financial burden on the process, unless chemicals are regenerated. The simulation software also shows that dissolution and carbonation of wollastonite in plain water is thermodynamically favourable at elevated, but achievable, temperatures (100 °C) and relatively low pressures (10-30 bar), which could compensate for pH swings.
|
|---|