- Title
- Aqueous mineral carbonation via decarbonation
- Creator
- Oliver, Timothy Kenilworth
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2017
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- The main aim of this contribution is in demonstrating the process of decarbonation of aqueous solutions, or the controlled removal of carbon dioxide (CO₂) from aqueous solutions, as a process for aqueous mineral carbonation, specifically the production of magnesium (Mg) carbonates. Unlike more conventional mineral carbonation processes, which require the addition of alkaline agents, the engendering of supersaturation sufficient for precipitation is achieved through the inducement of a pH swing directly as a result of decarbonation. This pH change, dependent on the amount of CO₂ removed from solution, is calculable and measurable, and can thus be engineered both in terms of the degree of pH shift and the amount of Mg carbonate formed. Additionally, this thesis examines the sequestration of CO₂ using solutions augmented with heat activated serpentinite. The resultant Mg bicarbonate solutions can in turn be exploited for decarbonation and resultant mineral carbonation. The overall process, comprised of the sequestration of CO₂ using prepared solutions and the degassing of these solutions, can be operated under mild conditions.Experiments were conducted to examine the parameters affecting decarbonation processes. Mg sulfate (MgSO₄) and sodium bicarbonate (NaHCO₃) solutions were used in experimentation. Scenedesmus microalgae, a single celled green alga, was used for biological decarbonation experiments. Batch experiments using the Scenedesmus alga confirmed its role in accelerating decarbonation and consequently reducing the time needed to precipitate Mg carbonate. Research concentrated on decarbonation via degassing, using a variety of nitrogen (N₂) sparging devices, because of the rapidity, simplicity and controllability of the process. Batch experiments involved N₂ discharging into a solution-charged reactor and similar trends were observed for all experiments with CO₂ degassing rapidly producing Mg carbonates, with significantly accelerated rates of CO₂ degassing being achieved using smaller bubbles and higher temperatures. Nesquehonite (MgCO₃·3H₂O) was the precipitated mineral phase for both the 30°C and 50°C experiments. Degassing and precipitation experiments were also conducted with carbonic anhydrase (CA) as a supplement to reagents and these experiments showed greater rates of degassing and resultant precipitation. A mathematical model describing the kinetic response of the bulk solution to CO₂ degassing through N₂ sparging, and the resulting precipitation of Mg carbonate, was formulated and was found to closely follow that calculated through thermodynamic simulation. The model allowed the performance of batch, semibatch and continuous degassing reactors or crystallisers to be assessed. Extension of the kinetic model to semibatch and continuous mode of operation showed that rate of degassing was controlling to steady-state rate of precipitation. Experiments were also conducted on the dissolution of heat activated serpentinite in CO₂-saturated water in order to prepare solutions suitable for degassing. These dissolution experiments were conducted under mild conditions and it was found that extraction of the Mg (typically around 10% of total Mg in the activated mineral) and increase of the pH of the leachate occurred very quickly. A series of small scale isothermal dissolution experiments was also undertaken to understand the kinetics of the mineral dissolution process. Short-term system response was found to be consistent with rapid dissolution of the shallow external surface of the particle and an Avrami-Erofe’ev type model fitted data well. Mineral dissolution showed strong temperature dependency, and there was also clear evidence of both particle condition and pH limiting rate controls on continuing dissolution. The rate of CO₂ mass transfer to the solution was shown to be of prime importance in mineral dissolution response. The observed variation in pH was accounted for by coupling the apparent solid-state model with the Mg carbonate system kinetic model incorporating simplified representation of CO₂ mass transfer. The kinetic model of the system, particular to the experimental reacting system, was used to assess the performance of continuous mineral dissolution or leaching. The solutions derived from batch mineral dissolution experiments were used in subsequent small batch degassing and precipitation experiments. The trends exhibited in these experiments mirrored those of batch degassing and precipitation experiments using prepared reagent based solutions and also produced nesquehonite, although a small amount of an amorphous Mg silicate phase was precipitated. A process for aqueous mineral carbonation involving the dissolution of the thermally conditioned serpentinite and the degassing of the leachate derived from dissolution operated as separate but integrated process units on a continuous basis was presented. The process being operated at relatively low temperatures and pressures with recovery and recycling of both the solution and vented CO₂ gas streams being possible. It has application both to mineral carbonation of dilute and concentrated streams of CO₂.
- Subject
- mineral carbonation; decarbonation; aqueous solutions
- Identifier
- http://hdl.handle.net/1959.13/1333811
- Identifier
- uon:27155
- Rights
- Copyright 2017 Timothy Kenilworth Oliver
- Language
- eng
- Full Text
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