Utilization of Gaseous Carbon Dioxide and Industrial Ca-rich Waste for Calcium Carbonate Precipitation: A Review

: Technologies for the management of various types of waste and the production of useful products from them are currently widely studied. Both carbon dioxide and calcium-rich waste from various production processes are problematic wastes that can be used to produce calcium carbonate. Therefore, the purpose of this paper is to provide an overview about the state of the development of processes that use these two wastes to obtain a valuable CaCO 3 powder. The paper reviews the current research on the use of post-distillation liquid from the Solvay process, steelmaking slag, concrete, cement, and gypsum waste as well as some others industrial Ca-rich waste streams in the calcium carbonate precipitation process via carbonation route. This work is an attempt to collect the available information on the possibility of influencing the characteristics of the obtained calcium carbonate. It also indicates the possible limitations and implementation problems of the proposed technologies.


Introduction
Carbon dioxide is the main anthropogenic greenhouse gas [1]. The rapid economic growth resulted in an increase in energy demand and, consequently, an increase in the consumption of fuels, in particular fossil fuels such as oil, coal, and natural gas [2,3]. During their combustion, large amounts of CO2 are emitted into the atmosphere, which has an adverse effect on the environment and is the cause of global warming. There is a strong pressure on the EU countries to make their economies more energy-efficient and zero-emission. However, despite this fact, the projected global coal production continues to increase by around 3%, and the global carbon-related CO2 emissions from the energy sector will increase by 0.1% annually between 2015 and 2040 [4]. It is not surprising that numerous studies are focused on the management of waste gaseous carbon dioxide [5,6]. Carbon capture and storage (CCS) and carbon capture and utilization (CCU) technologies are the most popular processes to reduce CO2 emissions [7][8][9] and play an important role in meeting the global targets specified at COP25 [10]. In the case of both technologies, three main CO2 capture systems depending on the type of combustion process can be distinguished, post-combustion, precombustion, and oxyfuel combustion [6,8,9]. The choice of capture technology depends on the type of plant, i.e., the composition of the exhaust gas [11,12]. Post-combustion technology is the simplest to implement and is mainly based on chemical absorption [9,13,14]. Thus, this option is usually used as a modification to existing power plants. However, due to the low CO2 content in flue gases (4% in the case of natural gas combustion, 7-14% in the case of coal combustion), it is relatively expensive

Post-Distillation Liquid from Solvay Process
The Solvay method used for the production of sodium carbonate (soda ash) is one of the processes in which waste is a serious environmental problem [36]. The main and most problematic waste generated during this process is post-distillation liquid. It contains mainly water (956 kg/m 3 of waste stream), calcium chloride (112 kg/m 3 ), and unconverted sodium chloride (56 kg/m 3 ), but also calcium carbonate (10 kg/m 3 ), calcium hydroxide (7 kg/m 3 ), silicon oxide (3 kg/m 3 ), calcium sulfate (1 kg/m 3 ) and ammonia (0.1-0.01 kg/m 3 ) [37,38]. The post-distillation liquid flows out the stripping columns and is directed to the ground settlements called "white seas" or directly to natural water reservoirs [37,39]. However, such activities contribute to the strong salinity of nearby groundwater with calcium chloride, as well as the pollution of natural water reservoirs and soil [39].
Due to the fact that Ca-rich waste from the Solvay process is liquid, it can be used directly as the reaction medium without carrying out a calcium ion extraction step. In this section, two possible paths of post-distillation liquid management for CaCO3 precipitation are discussed: using (i) liquidliquid method with aqueous Na2CO3 solution and (ii) carbonation, in which gaseous waste CO2 can also be applied.

Precipitation Using Aqueous Carbonate Solution
In most studies, using a post-distillation liquid from the Solvay process containing calcium chloride, sodium carbonate is applied to precipitate CaCO3 as a source of carbonate ions. In this case, the occurring reaction is described by Equation (3). The sodium chloride solution resulting from this reaction, after filtering the calcium carbonate particles, can be recycled and reused in the soda ash manufacturing process [40].
In the research conducted by Kasikowski et al. [37], precipitation of calcium carbonate using calcium ions present in the post-distillation liquid and carbonate ions derived from an aqueous soda ash solution has been proposed. Soda from the Solvay process that does not meet the standards for dense or light soda ash is a substrate for preparing an aqueous Na2CO3 solution. Calcium carbonate in calcite form (chalk) obtained by such precipitation is used in the pharmaceutical and cosmetics industry because it is characterized by very high quality, especially low content of ferrous or ferric ions and sodium chloride. In addition, by using well-chosen conditions of the precipitation process, it is possible to obtain pure semi-brine above the precipitated CaCO3. After filtering off calcium carbonate, the semi-brine can be recycled and reused in the production of soda ash or evaporated salts after saturation. For example, about 4000 tons of calcium carbonate is produced annually in the Janikowo Soda Factory (Janikowo, Poland) using this method, and this amount depends on the demand. This technological solution results in a profit of $560,000 per year [37]. In another research, Białowicz et al. [41] used a post-distillation liquid from the Janikowo Soda Factory and sodium bicarbonate solution for the precipitation process of calcium carbonate in the presence of urea. CaCO3 powder was obtained with very low packing density. Moreover, both the increase in reaction time and the increase in urea concentration resulted in an increase in bulk and packing densities [41].
Somani et al. [40] studied the effect of process temperature and sodium carbonate concentration on the crystallization of individual CaCO3 polymorphs. A model post-distillation liquid with a composition of 1.25 M CaCl2 and 0.85 M NaCl was used. Only calcite and aragonite are formed in such a system, but no crystallization of vaterite has been observed. In addition, the increase in sodium carbonate concentration resulted in an increase in the calcite content of the precipitated calcium carbonate. However, the use of diluted Na2CO3 solution caused the appearance of aragonite. The formed CaCO3 particles had a rhombohedral and needle-like shape [40]. However, this work did not contain information on the efficiency of the precipitation reaction, as it is not known how many calcium ions present in the initial reaction mixture were consumed to precipitate CaCO3. A diagram showing the calcium carbonate precipitation process by the liquid-liquid method using post-distillation liquid from the Solvay process and an aqueous solution of sodium carbonate is presented in Figure 1a.
Another method proposed by Trypuć et al. [42] is based on CaCO3 precipitation using a postdistillation liquid containing calcium ions and post-filtration solution as a source of carbonate and ammonium ions. Figure 1b shows a diagram of the CaCO3 precipitation process carried out by this method. Post-filtration solution, which is the waste generated after filtering the NaHCO3 precipitate in the Solvay process, consists of CO3 2− (0.69 mol/dm 3 ), NH4 + (4.962 mol/dm 3 ), Cl − (4.389 mol/dm 3 ) and SO4 2− (0.012 mol/dm 3 ). It was found that increasing the time of the post-filtration liquid dosage into post-distillation liquid causes the increase of both packing and bulk densities. Moreover, the formation of agglomerates and only a small amount of single calcium carbonate crystals was observed. In this method, the post CaCO3 filtration liquid contains NH4Cl, (NH4)2CO3, and unreacted NaCl. NH4Cl and NaCl can be recovered from this solution using evaporation and fractional crystallization processes. Valuable solid products are obtained in this way. For example, the recovered sodium chloride can be recycled to the Solvay process for brine preparation or used in other industries. Thanks to this it is possible to fully use of Cl − ions, which in a conventional process are directed as waste to natural water reservoirs. In addition, the proposed precipitation of calcium carbonate at temperatures not exceeding 50 °C results in obtaining chalk with the desired physicochemical properties due to easy control of process conditions [42]. Gao et al. [43] also proposed a method for utilizing both post-distillation liquid and post-filtration solution to precipitate calcium carbonate in the presence of selected surfactants (CTAB, SDBS and Brij 35). In this process, spherical ultrafine precipitated calcium carbonate was obtained. The results show that the presence of selected surfactants had a significant effect on the mean particle size. Without the addition of surfactants, the average particle size was about 5.5 μm, while the presence of these additives results in the formation of fine particles with a size of about 1.5 μm. Furthermore, the authors suggest that the use of such technology allows soda factories to obtain benefits such as the sale of CaCO3, but also release from fees for environmental pollution [43]. Mikhailova et al. [44] proposed the calcium carbonate precipitation method with the use of postdistillation liquid and excess of mother solution from Solvay process. The authors determined the optimal precipitation conditions leading to the obtaining of calcium carbonate with characteristics qualifying it for commercial applications. It has also been estimated that the application of the proposed process enables the production of 5000 tons of CaCO3 per year, taking into account the production of liquid waste from the Solvay process in Ukraine [44]. Table 1 summarizes the process parameters of the selected calcium carbonate precipitation technologies using a post-distillation liquid and an aqueous sodium carbonate solution (liquid-liquid method). All presented methods were conducted under atmospheric pressure, while the temperature was in the range of 5 to 95 °C. The use of higher temperatures for liquid-liquid processes is due to the fact that the reaction (Equation (3)) is endothermic. In this case, heat is absorbed from the surroundings, so that elevated temperatures favor this reaction.

Precipitation Using Flue Gas
Technology that allows the use of flue gases to precipitate calcium carbonate from postdistillation liquid has also been proposed [45]. It can be a variation of the wet-lime method, in which the absorption of combustion gases arising during combustion occurs in the overflow of distiller waste [37]. In the Solvay process, an excess of lime milk is added during the regeneration of ammonia from the filtration liquid. As a result, the distillation waste is strongly alkaline, which promotes the absorption of acid flue gas. However, it should be noted that several products are obtained, depending on the reacting gas component. ↑-increase, ↓-decrease.
The absorption of carbon dioxide results in the formation of calcium carbonate, while the absorption of sulfur oxides causes the precipitation of calcium sulfate and sulfite. The reactions that occur during the absorption of the flue gas are presented below (Equations (4)-(9)): It is also possible to simultaneously use waste gases containing carbon dioxide and post-distillation liquid from the Solvay process. However, of the individual components of distillation waste, only Ca(OH)2 can react with CO2 according to Equation (4). Calcium chloride, although its concentration in the liquid is still high, does not react with carbon dioxide. This is because CaCl2 is the salt of a strong acid while carbonic acid is a weak acid. Therefore, to create conditions conducive to CO2 absorption and the formation of carbonate ions in the solution, an absorption promoter, such as amines or ammonia, should be added to facilitate the CO2 transfer from the gas phase into the liquid phase [37,46].
Barzagli et al. [22] proposed a semi-continuous technique for capturing carbon dioxide from a gas mixture applying a dilute aqueous Na2CO3 solution and CaCl2 aqueous solution obtained as a by-product in the Solvay process. Firstly, sodium carbonate solution is saturated with CO2 and then such mixture can react with calcium chloride solution. Using this method, 80% absorption efficiency is obtained and high-quality calcium carbonate is precipitated. This technique may allow to minimize the energy demand due to the fact that the process is carried out at room conditions. By using this method, the main disadvantages of processes using aqueous solutions of alkanolamines, ammonia, or alkali metal carbonates are avoided. There is no energy-consuming regeneration of the absorbent, or costs associated with the evaporative loss of ammonia and oxidative and thermal degradation of amines [22]. The scheme of this method is presented in Figure 2.
In another study, a method for CO2 sequestration from flue gases using a model post-distillation liquid from Solvay process in the presence of ammonia has been proposed [47]. The scheme of this method is shown in Figure 3. As a result, calcium carbonate precipitates mainly in the form of vaterite and is a valuable product that can be used, among others, for paper production or as an adsorbent for heavy metal ions removal [48]. An increase in ammonia concentration favored CO2 and Ca 2+ consumption. About 80% of calcium ions from the post-distillation liquid and about 83% of carbon dioxide from a mixed air-CO2 gas stream were consumed under the most favorable conditions of this onestage precipitation process [47]. After filtering off the CaCO3 particles, a waste solution containing ammonium chloride is formed. However, it can be used to recover metal ions or as a leaching agent [49,50]. The NH4Cl solution can also be applied in the carbon capture and storage process [51].
Li et al. [52] proposed a new technology for processing post-distillation liquid and waste gaseous carbon dioxide. Calcium chloride is converted into calcium carbonate and hydrogen chloride based on the reactive extraction and crystallization process. Isoamyl alcohol and commercial tertiary amine N235 were used as the extraction system. The conversion rate of calcium chloride under optimal process conditions is 75%.  During this method, the overall reaction described by Equation (10) occurs. The addition of a tertiary amine results in a constant shift of equilibrium in Equation (10) in the right direction: An important issue to consider in processes using liquid waste are the additional compounds and ions present in them. In the case of post-distillation liquid from the Solvay process, they are, among others, Na + , K + , Mg 2+ , Al 3+ cations, and SO4 2− , NO3 − anions. The influence of these ions on CaCO3 precipitated by carbonation was studied by Dong et al. [53]. It turns out that Na + , K + , and NO3 − have a very weak effect on calcium carbonate, and therefore small amounts of these ions can remain in solution without consequences and no removal is necessary. However, Mg 2+ and SO4 2− can adsorb on the crystal surface at the early crystallization stage and inhibit the transformation of vaterite into calcite [54]. Magnesium ions also promote the growth of aragonite and inhibit calcite precipitation [55]. Therefore, the content of these ions should be reduced to 1% before using the liquid as a precipitation medium. In addition, a significant amount of very small particles with a diameter less than 1 μm are due to the presence of SO4 2− . Accordingly, these anions must be removed before industrial production [53]. Table 2 summarizes the process parameters of the selected calcium carbonate precipitation technologies using a post-distillation liquid and gaseous carbon dioxide (carbonation process). When analyzing the process parameters of all presented methods, it is worth noting that in each case the process pressure is atmospheric, while in most cases the temperature is moderate, which corresponds to the conditions most favorable to exothermic CO2 absorption.

Steelmaking Slag
Steel production is one of the largest industries in the world where wastes are a serious problem [56,57]. Most of the steel is produced by an integrated technological path, in which limestone or burnt lime is applied to remove impurities from molten iron and crude steel. As a result of this treatment, steelmaking slags, which mainly consist of calcium silicates and calcium oxide, are formed. Their specific composition depends on the production process. Steelmaking slags are used, among others, as lime in agriculture, in road construction as a substitute for gravel [58], or in cement and concretes as an additional cementing material [59]. However, it is not always possible to use them economically due to stringent environmental regulations. In such situation, they are problematic postproduction waste. Therefore, alternative methods of utilizing steelmaking slag are sought [60]. One of the proposed approaches is the use of this waste for the production of calcium carbonate by carbonation route (according to Equations (11) and (12)) due to the high content of calcium oxide. The resulting CaCO3 could replace the limestone used for purification in the steel industry. However, if the manufactured product were of sufficiently high quality, then it could be sold and used in other industries [61]: Potentially, calcium silicate (wollastonite) is the largest source of calcium ions in steelmaking slag. According to Kojima et al. [62], the rate of direct CO2 reaction with CaSiO3 is too slow to be practically used. Thus, to use calcium from CaSiO3, as well as to prevent unwanted pollution, the separation of calcium from the slag must first be carried out. It results in an accelerated reaction rate between Ca 2+ and gaseous CO2. One of the methods reported in the literature is the dissolution of calcium silicates with acetic acid according to Equation (13). A solution of calcium acetate and solid silicon oxide are formed. Gaseous CO2 is then introduced into the solution to precipitate calcium carbonate by carbonation according to Equation (14). The acetic acid added in the first stage is regenerated and can be reused [63]. Furthermore, flue gases from concentrated emission sources can be a source of gaseous CO2, which allows for its sequestration and removal: Eloneva et al. [61] investigated CaCO3 precipitation process using gaseous CO2 and waste steel converter slag, the diagram of which is presented in Figure 4. In this research, a modification of the method proposed by Kakizawa et al. [63] was used because acetic acid produced simultaneously with calcium carbonate prevents the precipitation of most calcium from the solution in the case of low acetic acid concentrations (30% or less). Therefore, the addition of sodium hydroxide was used to prevent the formation of acetic acid and favors the formation of CaCO3. This method is described in Equations (15)- (17). When a higher concentration of acetic acid is used, a greater efficiency in leaching of calcium ions is obtained, but at the same time the dissolution of other undesirable impurities (like silicon and iron) is increased. 86% of Ca 2+ ions present in the solution were consumed to precipitate calcium carbonate, while the obtained product had a purity of 99.5-99.8%. In addition, CO2 concentration has no effect on either the quality or the amount of CaCO3 precipitated. This is very beneficial because flue gases can be used directly in this process without prior CO2 separation and purification. The lowest CO2 concentration (10% vol.) resulted in longer precipitation time, but this effect can be eliminated by raising the temperature or increasing the flow rate. This is the solution to the problem of low concentration of carbon dioxide in the exhaust gas. It has been estimated that using the proposed method, a maximum of 42,000 t per year of pure CaCO3 can be obtained from the annual production of waste steel converter slag from the Raahe Works factory (Raahe, Finland). Although this process is also suitable for other steel plants, the necessary consumption of NaOH and CH3COOH results in too high process costs, so its implementation cannot be economical. This problem can be solved by replacing acetic acid with a salt solution and sodium hydroxide with a certain alkaline liquid waste stream, which would reduce the cost of the process [61]:  In another method of using steel converter slag for the production of CaCO3 by carbonization, calcium ions are extracted from this waste with an aqueous solution of ammonium chloride, ammonium nitrate and ammonium acetate [58,64,65] using different slag to liquid ratios. To prevent the escape of ammonia, Ca 2+ extraction is conducted in a closed reactor. The carbonization process is carried out in an open reactor at room temperature and atmospheric pressure, which reduces energy demand and costs [64]. The resulting calcium carbonate has high quality. The main problem of the proposed method is the formation of waste residual slag. Finding a method for recycling this waste can generate additional process costs [66]. It has been estimated that from 4.7 ton steel converter slag it is possible to produce 2.3 tons of CaCO3 with simultaneous production of 3.4 tons of residual slag. The analysis was made on the example of Raahe Works, which produces about 200 kt steel slag per year. Assuming the consumption of all waste slag, 42,000 tons of CO2 can be bound in the form of about 96,000 tons of calcium carbonate per year, which will generate an income of around 10 million euro per year [64]. However, it should be noted here that these estimates do not take into account other costs of running the process. Notwithstanding, this solution has great economic potential. Studies using an aqueous solution of ammonium chloride as the extraction agent for Ca 2+ ions have also been carried out [51,67,68]. Currently, it is the most commonly used method, because it turned out to give the highest Ca 2+ selectivity [69]. Table 3 summarizes the reaction conditions for calcium carbonate precipitation depending on the type of slag and the extraction agent and presents the characteristics of the obtained product. Atmospheric pressure was used in most of the research, while the tested temperature range was 20 to 80 °C. Due to the fact that CO2 absorption is an exothermic process, the increased temperature does not have a positive effect on it, although it has an influence on the polymorphism of the obtained CaCO3. Moreover, the use of elevated temperature generates additional costs of the process. As the literature shows, carbonation processes using steelmaking slag as a source of calcium ions and gas streams containing CO2 allow to obtain highly pure calcium carbonate, which can be used as a substrate in other industries. However, there are many economic issues to consider as to whether the proposed methods are worth implementing.

Concrete Wastes
Concrete as the main building material in the world, is manufactured in an amount of 4000 Mt per year (2017) and its market increased by 2.5% annually, while producing approximately 7% of global carbon dioxide emissions [71,72]. For example, the production of 1 kg of Portland cement generates more than 0.8 kg of CO2. Waste concrete from demolished buildings and other constructions is a serious problem, because their quantity is constantly increasing. Therefore, possible technologies for its utilization or reuse are sought [73]. There is a lot of research regarding the use of waste concrete for the production of PCC [74]. The potential amount of calcium that can be recovered from waste concrete depends on the content of this element in cement. For example, Portland cement contains 46.53% wt. of calcium [75], whereas concrete consists of 37% wt. coarse aggregate, 33% sand, 19% cement, and 11% water. Therefore, 88 kg of calcium can be recovered from a ton of Portland cement-based concrete [76]. Commonly used processes for the chemical recovery of calcium from waste concrete include three energy-intensive stages: crushing, milling, and leaching [74]. For the production of PCC from waste concrete, numerous studies have been conducted on various leaching agents. Water [77][78][79], acetic acid [80], ammonium salts [69,81], and nitric acid [82] have already been used. These solvents showed a relatively low calcium leaching efficiency of less than 60%, although higher pressure (about 10 bar) was used to improve the water leach. In addition, when producing CaCO3, it is recommended to use an even lower yield (around 30%) to avoid contamination of the final product. An alternative solution is to use a stronger acid, e.g., hydrochloric acid [83,84], which maximizes the leaching efficiency. However, carbonization does not occur in a solution with a low pH, so when using an acid as an extraction agent, a base, such as ammonia, must be added to the resulting Ca-rich solution [85,86].
Jo et al. [87] examined the impact of the type of extraction agent on the waste concrete carbonation process. In this research, aqueous solutions of ammonium chloride, acetic acid, hydrochloric acid, and pure deionized water were used, and the processes were carried out at room temperature and atmospheric pressure. The results indicate that the type of extraction agent has no effect on calcium carbonate precipitation, because in each case the resulting precipitate consisted mainly of CaCO3 in the form of a mixture of rhombohedral calcite and spherical vaterite. However, the weight fractions of individual polymorphs were not determined. However, when NH4Cl solution was used, the highest extraction and carbonization efficiency were obtained.
Van der Zee and Zeman [74] proposed a method of producing precipitated CaCO3 from waste concrete, where HCl was used as the leaching agent. Precipitation of calcium carbonate was carried out in a separate reactor by mixing the calcium-rich solution obtained by leaching and aqueous sodium carbonate solution. Na2CO3 solution can be produced by introducing any gas stream containing carbon dioxide into an aqueous NaOH solution. The overall reaction is shown in Equation (18) and the diagram of this process is shown in Figure 5. The HCl and NaOH used in this process were regenerated by membrane bipolar electrodialysis, which is an additional step requiring energy input. The authors suggest that reducing the costs of this stage can be achieved by using HCl concentration lower than 0.5 M: Another important problem in the concrete industry is the formation of concrete sludge (waste cement) when fresh concrete is used to manufacture construction or concrete elements [88]. Concrete sludge is a fine fraction recovered during the recycling of waste concrete as a coarse aggregate [76] and has a composition very similar to fresh concrete diluted with water, because it is calcium-rich and strongly alkaline. An example composition of concrete sludge is water (77.1% wt.), CaO (17.2% wt.), SiO2 (3.2% wt.), Fe2O3 (1.9% wt.) and Al2O3 (0.6% wt.) [89]. Usually, the utilization of this waste consists in its reuse as an additive to road materials or its storage in landfills [89]. However, the first solution is an expensive process because it requires the separation of solids from liquids, resulting in a strongly alkaline liquid that must be neutralized with acid. On the other hand, the storage of concrete sludge in landfills is environmentally harmful. Therefore, alternative methods are sought to dispose of problematic cement waste [89].
One of the solutions is the use of concrete sludge as a raw material in the carbonation process, which results in the precipitation of calcium carbonate. Cement hydration products, such as calcium hydroxide, calcium silicate hydrate, and calcium aluminate hydrate, react with the gaseous carbon dioxide introduced and dissolved in the aqueous solution [90,91]. Occurring reactions are described in Equations (19)-(21): Iizuka et al. [92] conducted research on bench-scale installation for the concrete sludge recycling process using gaseous CO2. Industrial concrete sludge and flue gas from oil combustion boilers were applied. Authors proposed a two-stage recycling method for concrete sludge, the diagram of which is presented in Figure 6. The first step is the extraction of calcium ions, which involves diluting the concrete sludge with water (hydration) and extracting Ca 2+ ions into the water phase assisted by mixing (Equation (22)). In another work, Ca 2+ extraction from waste concrete was carried out by carbonic acid solution prepared using pressurized carbon dioxide [93,94]. The remaining precipitate is then separated from the extract. Gaseous carbon dioxide is introduced into the resulting calcium-rich solution, thereby precipitating calcium carbonate. Importantly, the proposed method can directly use fumes from the combustion of fossil fuels without purifying or compressing them. The solution formed after filtering the CaCO3 particles is neutralized with carbonic acid. In addition, the resulting calcium carbonate can be used in many industries. The feasibility of the proposed technology was also assessed. However, commercialization of this process has not been shown to be economically viable [92]: The same research team designed and built a pilot installation for recycling concrete sludge and CO2 waste to precipitate calcium carbonate [89]. Concrete sludge from the production of piles and poles, boiler gas, and groundwater were used. The process had the same two stages as for bench scale [92]. Using this technology, it is possible to obtain high-quality CaCO3 with a purity above 97%, as well as to reduce CO2 emissions from waste gases. Importantly, this process is energetically beneficial because it is carried out at standard temperatures and atmospheric pressure, and moreover there is no need to purify the exhaust gas or increase its pressure. Table 4 summarizes the process parameters of the selected calcium carbonate precipitation technologies using concrete waste or concrete sludge and gaseous carbon dioxide (carbonation process). All listed methods were carried out under atmospheric pressure. In the case of temperature, most of the processes were carried out at room temperature, while the influence of elevated temperature (30, 50, 70 °C) on the precipitation process was also investigated. When CO2 absorption is accompanied by an exothermic reaction (Equation (22)), the lower process temperature favors such a transformation.

Gypsum Wastes
There are three types of gypsum waste, phosphogypsum (PG), desulfurization gypsum (DG), and red gypsum (RG). PG is produced during phosphoric acid production, DG is generated in power plants through a reaction between sulfur gas and lime or limestone sorbents in the desulphurization unit, while RG is the waste from processing ilmenite ore to obtain TiO2 particles. The waste PG and DG consist mainly of CaO, SO3, and water, as well as minor impurities such as Fe2O3, SiO2, and Al2O3 [95]. Due to the high calcium content, gypsum wastes have great potential to be used for CO2 sequestration and calcium carbonate production. The mineral carbonation of gypsum obtained during flue gas desulfurization (DG) proceeds according to the reaction in Equation (23). According to the reaction stoichiometry, in such a process for every 1 ton of produced ammonium sulphate, 0.9 tons of CaCO3 is generated and 1.6 tons of gypsum and 0.4 tons of CO2 are consumed [96]: Song et al. [97] proposed the direct carbonation of DG process, which was carried out in ambient conditions using an aqueous ammonia solution. A diagram of this process is shown in Figure 7. DG is characterized by high carbonation reactivity at room temperature and atmospheric pressure [97]. The polymorphic form of CaCO3 obtained during direct and indirect carbonation was also compared. In the case of the direct process, the formation of a mixture of rhombohedral calcite and spherical vaterite was observed, with the vaterite content being dependent on the carbonation time. The longer the time, the greater the amount of vaterite in the resulting product. However, during the two-stage (indirect) process, the precipitated calcium carbonate was mainly in the form of calcite. Moreover, what is important, in the proposed direct carbonation process, obtained calcium carbonate was practically free of impurities, which allows for further use of such a product [97]. In another work, Song et al. [98] focused on the possibility of controlling the polymorphic form and morphology of calcium carbonate obtained during direct carbonation of DG using an aqueous ammonia solution. The controlling factor was the addition of ethanol to the reaction mixture under stoichiometric conditions and with an excess of ammonia. During both processes investigated by Song et al. [97,98], a pure and valuable product from industrial DG is produced. However, the problem is to obtain a small amount of CaCO3 crystals. Additionally, these research are focused mainly on the polymorphic form and morphology of calcium carbonate and do not contain any discussion about the efficiency of CO2 sequestration.  Table 5 summarizes the process parameters of the selected calcium carbonate precipitation technologies using waste gypsum and gaseous carbon dioxide (carbonation process). All presented processes were carried out under atmospheric pressure and at moderate temperatures (20-40 °C). Such conditions favor both CO2 absorption and the exothermic reaction (Equation (23)).

Other Ca-Rich Waste
There are several other examples of alkaline liquid waste in the literature that can be used as Ca-rich raw material for the production of CaCO3 by the carbonization method. Paper sludge ash (PSA) from paper manufacturing has a very small particle size, high pH (>12) [100] as well as high calcium content of about 22% wt. [101,102]. Kim et al. [102] conducted research on CO2 storage by indirect carbonation with PSA at room temperature and atmospheric pressure. In this work, various types of solvents for the extraction of calcium ions from PSA were investigated: acetic acid, hydrochloric acid, ammonium acetate, ammonium chloride, sodium citrate and water. Gaseous CO2 was introduced into the obtained extract to precipitate calcium carbonate. To increase the CO2 storage efficiency, additional steps such as pH swing or changing the amount of supplied CO2 were used. The results indicate that the highest CO2 capture efficiencies were obtained using acetic acid (324 kgCO2/ton PSA) and hydrochloric acid (297 kgCO2/ton PSA) by treating the extract with sodium hydroxide. In the case of the ammonium salts, efficiency in the range of 168-175 kgCO2/ton PSA was obtained. The lowest efficiency was recorded for the processes with the use of sodium citrate and water, 136 and 29.1 kgCO2/ton PSA, respectively. In addition, sodium citrate and ammonium salts can be recovered and reused. This is an important factor that increases the economic feasibility of the proposed carbonation processes. According to Zevenhoven et al. [101], PSA is a very suitable material for carbonation as pure calcium carbonate (90-98.5% wt.) is obtained compared to steelmaking slag and wollastonite. However, the water consumption in the PSA process is relatively high, making wastewater treatment problematic.
Another raw material is oil shale ash (OAS) which, depending on combustion technology, contains 10-25% wt. of free lime [103]. Some of the shale oil ash is used in agriculture, road construction, and as raw material for the production of construction materials, while the majority of this waste is stored in piles located near the power plant [104]. Due to the high content of free lime and anhydrite, which form alkaline leachate with a pH of 12-13, the disposal of this type of waste is a large environmental problem [105]. Tamm et al. [106] investigated the feasibility of CaCO3 precipitation from shale ash using process modeling. According to the obtained results, such technology can be used in the processes of CO2 mineralization and waste stabilization and valorization. Uibu et al. [103] constructed and tested a continuously operating reactor for the precipitation of CaCO3 from oil shale ash-water suspensions with model gases containing CO2 on a laboratory scale. The obtained products contained 17-20% wt. bound carbon dioxide and 0.6-2% wt. unreacted lime. According to the authors, the use of this technology allows for the capture of about 1.2 million tons of CO2 from flue gases annually, estimated for the SC Narva Power Plant in Estonia (2007). However, further optimization of the process conditions is required to enable the production of PCC with a fine and uniform particle size [103].

Recommendations for Future Research
The state of the art of the presented methods using various calcium-rich wastes to capture CO2 indicates that additional research is needed to implement these technologies. Most of the presented studies lack clearly defined optimal process conditions. The use of concrete and cement waste is the most advanced. For these solid wastes, indirect two-stage CaCO3 precipitation methods have been proposed and tested on the bench and pilot scale [89,92,107]. The production of calcium carbonate is carried out at atmospheric pressure and at ambient temperature and there is no need to purify the exhaust gas.
Another significant problem is the generation of secondary waste, therefore the direction of their management should also be indicated. Only some studies suggest solutions to this problem [37,42,47,101,102].
Maintaining efficiency and energy demand for processing are important issues for future commercialization [25]. A useful approach to assessing a technology under development in terms of its energy efficiency is an exergy analysis [108][109][110]. Such attempts have been made for some CO2 absorption processes [111,112] and oxy-fuel technologies [113][114][115][116]. The exergy analysis is an effective tool to improve the efficiency of the installation, reduce waste heat, and indicate the possibility of heat integration. Therefore, it is reasonable to apply this method to validate and optimize design concepts prior to implementation in the next generation carbon capture technologies using Ca-rich waste.
A further limitation is the small amount of research into the scaling up the process. Pilot-scale studies for carbonization of waste concrete and cement have been published [89,107].
The consequence of the above problems is a limited number of techno-economic analysis (TEA) of the proposed methods. The available data show that the mineral carbonation CCU technology has an economic advantage in energy penalty, power plant construction, and operating cost over other proposed CCS and CCU technologies [117]. Recommendations for a techno-economic analysis of CCS and CCU processes have recently been published [118], and the application of this TEA guideline may improve the comparability of TEA studies.

Conclusions
Nowadays, a lot of attention is paid to the issue of the increasing amount of produced industrial waste, which is a serious environmental hazard. Numerous attempts are made to develop and implement technologies aimed at both disposal and reuse of this waste. Carbon dioxide emitted during the combustion of fuels or released during the manufacture of many products is indicated as one of the main greenhouse gas that contributes to rapid global warming. Another group of nuisance pollutants is calcium-rich wastes, such as distillation liquid from the Solvay process, metallurgical slag from steel production, cement industry waste, gypsum waste, ash from paper sludge and oil shale ash. The alkalinity of individual liquid waste and the liquid after leaching of solid waste allows them to be used to capture carbon dioxide from flue gas. Therefore, an interesting idea for the management of CO2 and Ca-rich waste is to develop a technology for producing calcium carbonate via carbonation route. Such technologies help reduce harmful emissions while producing a valuable product that can be sold and used in many industries, such as paper, rubber, pharmaceuticals, and many others. Therefore, an important criterion in the evaluation of these technologies is the possibility of producing CaCO3 meeting numerous criteria, such as high purity, adequate humidity, particle size, and shape, or polymorphic form. In addition, for the technology of calcium carbonate precipitation from waste liquids and CO2 to be successfully implemented in production, this process must be characterized by low electricity demand. In general, the technology must be economically viable, so that the benefit from the sale of precipitated CaCO3 should be more profitable higher than the cost of its production. Therefore, most of the technologies proposed and described in this work are carried out at room temperature and atmospheric pressure, so as not to generate additional costs associated with heating or maintaining the desired pressure of the reaction system. However, there are many other obstacles, such as the necessary reagent regeneration processes, which are expensive, or the generation of other waste streams for which further utilization processes should be implemented. An important issue is also the fact that most potential Ca-rich waste is in solid form, which requires the extraction of calcium ions into the solution. Therefore, the efficiency of the Ca ion leaching process is another important factor influencing the profitability of the designed process. In addition, the implementation of a given waste management technology may also be determined by political factors, when legal regulations of a given country force entrepreneurs to reduce emissions and pollutants.

Conflicts of Interest:
The authors declare no conflict of interest.