The significance of the results coming from greenhouse gas (GHG) emissions to both the atmosphere and our lives has already been urged and is nowadays well-known. Because of the continuous increase of CO2
concentration in the atmosphere since the industrial revolution, various techniques are proposed. Carbon capture and utilization (CCU) is considered as the most promising technique in order to use the product in cement, transforming it into insoluble carbonate (mainly calcite and magnesite), that is able to remain stable in a geological timeframe [1
In order to accelerate mineral carbonation, some pretreatment processes are required (microwave heating, grinding, sieving, separation, thermal decomposition, and chemical treatment). The main goal of pre-treatment processes is to increase the carbonation rate and improve the process kinetics. Typical pre-treatment methods are particle size reduction, magnetic separation and thermal treatment. Particle size reduction incorporates various grinding methods for an increase of specific surface area. In magnetic separation, undesired ferrous particles are separated from the rest of the feedstock. Thermal treatment is necessary for hydrated minerals, such as serpentine that contains H2
O molecules in the mineral structure. Pre-treatment is usually required in direct carbonation processes [2
]. However, one must consider the balance between increase of reaction rate and additional energy costs, possible CO2
-production related to energy supply and the influence on the beneficial utilization of the final products. The major problem of pre-treatment is its high energy input, i.e., thermal treatment should be avoided due to the high energy demand and CO2
-emissions (depending on the energy source). Overall, the most potentially economical pre-treatment proved to be size reduction [3
]. Although there are large resources, it is not a feasible feedstock material because of its crystallographic stability and thus the necessary step of thermal pre-treatment.
Industrially produced by-products containing alkaline metals are also feasible for mineral carbonation, such as numerous types of slags, scraps, red gypsum, combustion residues, fly ashes and other forms of metal oxide materials such as red mud [4
]. Unlike natural feasible materials, industrial by-products usually do not require mining and pretreatment processes for utilization because they already have high alkaline metal contents which are sufficient for mineral carbonation [5
]. Because of high availability in minerals and secondary materials among all of the possible materials selections, calcium oxide and magnesium oxide are the most favorable options, as shown in Equations (1) and (2) [6
Although the carbonation process is an exothermic reaction, it requires an additional heat for better dissolution of carbon dioxide in water in order to form magnesium carbonate. The benefits of exothermic mineral carbonation may result in a positive net energy balance, which improves the net efficiency of a combined cycle power plant [8
]. Calculating energy balances of the process is essential for the determination of the process’ profitability which might be influenced by high energy costs. Furthermore, the overall reduction of carbon dioxide emissions has to be considered due to further emissions in the mineral carbonation process resulting from, e.g., transportation, grinding and processing of by-products. Furthermore, the potential use of the formed products should also be taken into consideration.
There are abundant calcium and magnesium rich minerals available in the earth’s crust. Although MgO and CaO are the most abundant alkali and alkaline metal oxides, they cannot be found as binary oxides in nature. Usually, they exist as hydroxides or silicate minerals. In a mineral carbonation process, these can also be used as feedstock to form carbonates that are chemically stable in a geological timeframe. Silicate minerals usually are richer in alkaline metal content such as magnesium, sodium, and calcium. Common silicate minerals suitable for carbonation are forsterite (Mg2
), antigorite (Mg3
) and wollastonite (CaSiO3
) and their overall reaction conversions are given in Equations (3) to (5).
The aforementioned mineral carbonation using slags as reactant also have the chance to be profitable if they are built closely to the steel production site in order to reduce production costs [9
]. Furthermore, heat integration with the steel production plant can reduce overall energy costs significantly [9
]. Carbonation of different types of slags are widely studied, one study by Georgakopoulos [4
] suggests that blended hydraulic cement BHC slag has the highest conversion rate which is 68.3%.
Red gypsum usually exists in the form of calcium sulfate dihydrate (CaSO4
O). Typically, red gypsum has a purity of 95% and has large resources in Malaysia [10
]. Due to the large calcium content its carbonation is also a promising CCS option, i.e., one ton of red gypsum can stably bind 0.26 tons of gaseous CO2
. A big advantage of red gypsum as carbonation resource is no mining cost is required since it exists in the form of fine powder which favors the carbonation reaction.
Generally, the reaction path for the indirect carbonation of forsterite in an aqueous solution can be described by Equations (6) to (12) which is also illustrated in Figure 1
]. For simplicity, olivine consists only of Mg2
, namely forsterite. First, gaseous carbon dioxide dissolves in the aqueous solution at a certain mass transfer rate as in Equation (6). Simultaneously, forsterite is dissolved in the aqueous solution (Equation (10)). In the aqueous solution, all species are assumed to be at equilibrium: Aqueous CO2
dissociates into bicarbonate, which further dissociates into carbonate (Equations (7) and (8)). Self-ionization of water is given by Equation (9). Aqueous silicic acid then precipitates as amorphous silica, which is a by-product, and lastly magnesium ions and carbonate form magnesite (Equations (11) and (12)).
The particular process is characterized by several equilibrium and non-equilibrium reactions. The determination of process parameters such as temperature, pressure and pH for maximum overall conversion rates is elementary. Direct CO2
sequestration at high pressure with olivine as a feedstock has already been performed in numerous studies at different temperatures and pressures with or without the use of additives such as carboxylic acid, and sodium hydroxide [11
]. It is reported that optimal reaction conditions are in the temperature range of 150–185 °C and in the pressure range of 135–150 bar [10
]. Additives are reported to have a positive influence on carbonation rate. Optimal addition of additives are reported by Bearat et al. [13
] in studies about the mechanism that limits aqueous olivine carbonation reactivity under the optimum sequestration reaction conditions observed as follows: 1 M NaCl + 0.64 M NaHCO3
, at 185 °C and P (CO2
) about 135 bar. A reaction limiting silica-rich passivating layer forms on the feedstocks grains, slowing down carbonate formation and raising process costs. Eikeland [14
] reported that NaCl does not have significant influence on carbonation conversion. The presented results show a conversion rate of more than 90% using a NaHCO3
concentration of 0.5 M, without adding of NaCl. Ideally, the solid phases exist as pure phases without growing together. In reality, different observations are made on the behavior of solid phases. Daval et al. [15
] reported about high influence of amorphous silica layer formation on the dissolution rate of olivine at 90 °C and elevated pressure of carbon dioxide. This passivating layer may either built up from non-stoichiometric dissolution, precipitation of amorphous silica on forsterite particles or a combination of both. In contrast to that, Oelkers et al. [16
] and Hänchen [17
] observed stoichiometric dissolution and no build-up of a passivating layer except during start-up of experiments. Additionally, magnesite may precipitate on undissolved forsterite particles leading to a surface area reduction and therefore a reduction on forsterite dissolution rate, which was reported by Turri et al. [18
]. Besides this undesired intermixing of solids, they observed pure particles of magnesite to be predominant in the smallest particle class, amorphous silica particles to be mainly present in the intermediate particle class and unreacted olivine particles to be predominant in the largest particle class. This knowledge may be of value for subsequent separation of products.
sequestration with olivine as a feedstock was performed in a rocking batch autoclave at 175 °C and 100 bars in an aqueous solution and a CO2
-rich gas phase from 0.5 to 12 h. Turri showed maintainable recovery of separate fractions of silica, carbonates and unreacted olivine. Characterization of the recovered solids revealed that carbonates predominate in particle size range below 40 µm. The larger, residue fraction of final product after carbonation consisted mainly of unreacted olivine, while silica is more present in the form of very fine particles. An addition of sodium hydrogen carbonate at 0.64 M, oxalic acid at 0.5 M and ascorbic acid at 0.01 M was successfully applied in order to obtain maximal carbonation. The positive influence of the above-mentioned additives on the carbonation efficiency was reported by Olajire [19
]. They studied the technology of CO2
sequestration by mineral carbonation with current status and future prospects, but the positive influence of additives was not explained in detail.
Formation of submicron magnesite during reaction of natural forsterite in H2
O-saturated supercritical CO2
was studied between 35 and 80 °C and at pressure of 90 bars [20
]. The magnesite particles formed under below-mentioned conditions exhibited an extremely uniform submicron grain-size and nearly identical rhombohedral morphologies at all temperatures. Then an evidence for carbonate surface complexation during forsterite carbonation in wet supercritical carbon dioxide was also considered. The effect of Fe on the measured rates of olivine carbonation and its role in the formation of Si-rich surface layers, which can significantly inhibit olivine dissolution and limit the extent of the carbonation reaction was considered by Saldi et al. [21
]. A series of batch and flow-through reactor experiments was conducted in pure water at 90 and 150 °C and under a CO2
partial pressure of 100 and 200 bar, using both a natural sample of Fe-bearing olivine and a synthetic sample of pure forsterite. Experimental results show that Fe plays an ambivalent role in the carbonation.
The preparation of a magnesium hydroxy carbonate from magnesium hydroxide and carbon dioxide includes the formation of a magnesium hydroxide slurry and sparging CO2
gas through it. Various experimental conditions are evaluated in order to obtain the conditions that result in the formation of the magnesium hydroxy carbonate [22
Our paper deals with the formation of magnesium carbonate using an Italian olivine (35.57 wt% MgO) and a synthetic reference material, mainly consisting of magnesia (97.56 wt% MgO) with special attention on the influence of the additives and different solid/liquid ratio. After carbonation, the settled solid fraction contained mainly carbonation products, which was studied by structure and composition analysis (X-Ray Diffraction XRD) and reactivity (Thermogravimetric analysis TGA and Differential Scanning Calorimetry DSC). The water solution was analyzed by Inductively Coupled Plasma Optical Emission Spectroscopy ICP-OES in order to determine the concentration of nickel, iron, magnesium and cobalt.