The Utilization of CO₂, Alkaline Solid Waste, and Desalination Reject Brine in Soda Ash Production

Abstract

Conventional Solvay process, which utilizes CO₂ to synthesize sodium carbonate, has been well-known for more than a century, but the only purpose of this process is to produce soda ash. Annually, significant amounts of CO₂ are converted into soda ash using Solvay process; however, this conventional process does not sustain CO₂ and waste cycle. Increasing the awareness of global warming and climate change linking to CO₂ emission has pressured this industry to move for a more eco-friendly process that should exploit its potential for CO₂ utilization and sequestration and thus contribute to both CO₂ emission and waste management. In conventional Solvay process, to produce one mole of Na₂CO₃, at least one mole of CO₂ might be emitted to atmosphere due to the use of CaO. Therefore, a number of studies have been done to explore novel processes or to modify the Solvay process in order for it to engage more in CO₂ mitigation. This chapter introduces the most up-to-date modified Solvay process and novel pathways to produce soda ash and baking soda in the consideration of waste and CO₂ utilization. Simultaneous waste and CO₂ utilization offers a great opportunity for shifting to a green production, not only soda ash industry but others where their exhausted CO₂, alkaline solid wastes, or reject brine can be utilized. However, there are challenges which require further research and technological development initiatives for the idea to be industrially implemented.

9.1 Potential Contribution of Inorganic Waste in CO₂ Emission Mitigation

9.1.1 Carbon Utilization with Alkaline Solid Waste

Industrial wastes such as slag or fly ash may have composition similar to cement; therefore, they have great potential to use as active additives to replace a certain fraction of cement in concrete [1,2,3]. Recent study indicated that the strength of cement mortar can be significantly enhanced by adding carbonated slag or fly ash [4, 5]. These wastes contain significant amount of CaO and MgO which has been extensively investigated for CO₂ sequestration [6]. The most effective CO₂ sequestration for those wastes is gas–liquid method in which slag is dispersed in water and then CO₂ gas is introduced into the slurry. Divalent ions such as Ca²⁺ leached from slag react with dissolved CO₂ according to Eqs. 9.1–9.3 to form calcium carbonate [7,8,9]. The CaCO₃ reacts with Ca₃Al₂O₆ (C₃A) phase in concrete as shown in Eq. 9.4 that results in the formation of calcium carboaluminate hydrate (C₃A · CaCO₃ · 11H, where H stands for H₂O) and (C₃A · 0.5CaCO₃ · 0.5Ca(OH)₂ · 11.5H). The formation of calcium carboaluminate hydrate could contribute to enhance the mechanical strength of concrete [4]. With an estimated 400 million tonnes of iron and steel slag (for every tonne of steel produced, two tonnes of CO₂ emission, 400 kg of blast and basic oxygen furnace slag, and 200 kg of electric arc furnace slag are generated) [10], 750 million tonnes of fly ash from coal-fired power plant [11], and huge quantity of alkaline waste from other industries’ discharges annually, this alternative could contribute to reduce a great amount of CO₂ emission.

$$ {\text{H}}_{2} {\text{O}} + {\text{CO}}_{2} \to {\text{H}}_{2} {\text{CO}}_{3} \to 2{\text{H}}^{ + } + {\text{CO}}_{3}^{2 + } $$

(9.1)

$$ {\text{CaO}} + {\text{H}}_{2} {\text{O}} \to {\text{Ca}}^{2 + } + 2{\text{OH}}^{ - } $$

(9.2)

$$ {\text{Ca}}^{2 + } + {\text{CO}}_{3}^{2 - } \to {\text{CaCO}}_{3} $$

(9.3)

$$ \begin{aligned} & 2{\text{C}}_{3} {\text{A}} + 1.5{\text{ CaCO}}_{3} + 0.5{\text{Ca}}\left( {\text{OH}} \right)_{2} + 22.5{\text{H}} \to {\text{C}}_{3} {\text{A}} \cdot {\text{CaCO}}_{3} \cdot 11{\text{H}} \\ & \quad \quad + {\text{C}}_{3} {\text{A}} \cdot 0.5{\text{CaCO}}_{3} \cdot 0.5{\text{Ca}}\left( {\text{OH}} \right)_{2} \cdot 11.5{\text{H}} \\ \end{aligned} $$

(9.4)

Global phosphate industry, mainly fertilizer producers, discharges approximately over 200 million tonnes of phosphogypsum annually (4.5–5.5 tonnes for one tonne of P₂O₅ produced), and about 85% of this is disposed without any treatment [12, 13]. This waste is dumped in large stockpile causing serious environmental impact. Phosphogypsum has high content of calcium sulfate, about 24–34 wt% as CaO and 48–58 wt% as SO₃. This make it high potential for CO₂ utilization and sequestration [14,15,16,17,18,19,20]. However, not only sequestration, this solid waste could be converted to marketable (NH₄)₂SO₄ fertilizer [21]. CO₂ utilization and (NH₄)₂SO₄ fertilizer production is based on the conventional Merseburg process in which CaSO₄ reacts with (NH₄)₂CO₃ to form CaCO₃ and (NH₄)₂SO₄ as Eq. 9.5 [22, 23].

$$ {\text{CaSO}}_{4} \cdot 2{\text{H}}_{2} {\text{O}} + \left( {{\text{NH}}_{4} } \right)_{2} {\text{CO}}_{3} \to \left( {{\text{NH}}_{4} } \right)_{2} {\text{SO}}_{4} + {\text{CaCO}}_{3} + 2{\text{H}}_{2} {\text{O}} $$

(9.5)

$$ {\text{CaSO}}_{4} \cdot 2{\text{H}}_{2} {\text{O}} + 2{\text{NH}}_{4} {\text{OH}} + {\text{CO}}_{2} \to \left( {{\text{NH}}_{4} } \right)_{2} {\text{SO}}_{4} + {\text{CaCO}}_{3} + 3{\text{H}}_{2} {\text{O}} $$

(9.6)

Conventionally, (NH₄)₂CO₃ that previously produced by bubbling CO₂ into ammonium solution is added into the slurry of gypsum or phosphogypsum. Recent studies reveal that bubbling CO₂ or flue gas directly into ammonium-saturated phosphogypsum solution helps reduce reaction process from two steps to one step which could result in saving expense and lower production cost [8, 24, 25]. This modified Merseburg process is chemically described in Eq. 9.6. Accordingly, for every tonne of (NH₄)₂SO₄ produced, 0.9 tonnes of CaCO₃ is generated which can permanently sequester 0.4 tonnes of CO₂, and 1.6 tonnes of waste gypsum is consumed [25]. With over 200 million tonnes of phosphogypsum from phosphate industry together with significant amount gypsum from flue gas desulfurization process in power plant [24], its reveals the high potential of gypsum waste and CO₂ utilization to produce marketable products such as CaCO₃ and (NH₄)₂SO₄.

9.1.2 CO₂ Utilization with Aqueous Inorganic Waste

Beside the solid alkaline wastes that contain mainly Ca²⁺ and Mg²⁺ with the large capacity of CO₂ sequestration, there are also aqueous solution wastes that have high concentration of Na⁺, Mg²⁺, and Ca²⁺ such as reject brine from desalination plant [26, 27], saline brine from CO₂ storage sites [28] or textile industry [29]. High demand on clean water due to the world population growth and scarcity of freshwater has driven the increase in the deployment of desalination plants worldwide. Desalination has become an important source of drinking water production with the capacity of estimated >86 million m³/day from >18,000 plants in 2015 [28]. Over 15,000 desalination plants were installed in 2009 in which reverse osmosis technology accounted for 80% and global market for reverse osmosis alone is predicted to reach $8.1 billion by 2018 [30,31,32]. Desalination is the most viable way that can produce enough clean water to meet the high demand. However, this method faces some issues that are high energy consumption and reject brine management and disposal. According to Dawoud and Mulla, every 1 m³ clean water produced results in 2 m³ of reject brine [33]. Advanced technologies may enhance the water recovery and reduce the reject flow, but the reject volume still remains very large. Reject brine disposal is a costly process which accounts for 3–33% of the total cost of desalination depending on the selection of disposal method [34]. The major constituents in reject brine comprise of calcium, magnesium, sodium cations, sulfate and chloride anions, and sometime antiscalants. Their concentrations are dependent on the water sources, desalination technologies, and water recovery ratio or concentration factor [27, 34]. Seawater desalination plant recovery is typically limited to 40–65%, and its total dissolved solid concentration usually ranges from 65,000 to 85,000 mg/l [27]. Reject brine disposal either inland or into the sea may result in negative environmental impact due to locally increasing in temperature and salinity; hence, a proper management and treatment procedure is required for the disposal [31, 35]. A number of methods such as solar evaporation, membrane distillation, electrodialysis, forward osmosis, zero liquid discharge, salt recovery have been proposed to reduce and eliminate the brine disposal in which the methods that combine brine treatment with the recovery of valuable component have drawn significant interest [31, 32, 35,36,37]. Recently, several studies have considered the recovery of salt and minerals from rejected brine in the context of CO₂ capture and utilization [38,39,40,41,42]. Simultaneous CO₂ and brine utilization is based on the reaction between CO₂ and major inorganic cations prevailed in the rejected brine. Generally, CO₂ utilization with brine can be divided into two methods: direct precipitation of earth alkaline cations of Ca²⁺ and Mg²⁺ by CO₂ gas to form CaCO₃ and MgCO₃ [43,44,45,46] and indirect precipitation of Na⁺ to form sodium bicarbonate [41]. In the former method, less soluble carbonates removed from water can be value-added products; nevertheless, with the low Ca²⁺ and Mg²⁺ concentrations of several hundred to several thousand ppm, this method is potentially used for brine pre-treatment other than the recovery of CaCO₃ for marketing [45], whereas, the latter method makes up a core technology for the industrial production of soda ash (Na₂CO₃) [47]. Soda ash is an important raw material used in many industries such as glass, chemicals, soaps and detergents, metallurgy, water treatment, pulp, and paper. Producing soda ash from rejected brine and CO₂ is an opportunity for at the same time utilizing two waste streams in one product. By producing soda ash, CO₂ can be sequestered in the way that generates the added value for CO₂ captured. In the following section, the CO₂ utilization in the most updated sodium bicarbonate (NaHCO₃) and sodium carbonate production process will be introduced; especially, its relevance to CO₂ mitigation strategy and brine and solid waste treatment will be considered.

9.1.3 Conventional Solvay Process and Its Potential for CO₂ Utilization

Solvay process developed in 1861 by Ernest Solvay is the most commercially accepted process for soda ash production due to its low investment and maintenance costs compared to others. This process produces soda ash from brine (NaCl) and limestone (CaCO₃) with the overall reaction given in Eq. 9.7 [48]. In fact, it is more complicated and goes through many different steps that are schematically described in Fig. 9.1. Brine after purification is ammoniated and carbonated by passing the CO₂ gas (Eq. 9.8). The carbonation leads to the precipitation of bicarbonate (NaHCO₃) that is subsequently filtered and calcined to produce soda ash (Eq. 9.9). Filtrate mainly consisting of NH₄Cl and residual NaCl is mixed with CaO in the distillation process to regenerate NH₃ that is returned to ammoniation process (Eq. 9.10). CaO reacts with NH₄Cl and NaCl in the filtrate to form CaCl₂ as a major chemical species in distiller waste (Eqs. 9.10 and 9.11). Limestone is burned to supply CaO for NH₃ regeneration and CO₂ for carbonation process (Eq. 9.12). The conventional Solvay process faces a significant problem of waste management [48,49,50]. To ease its impact on the environment, the waste utilization is usually added into the traditional Solvay process in which CaCl₂ can be converted to CaCO₃, calcium phosphate, magnesium phosphate, or gypsum that are commercializable by-products [50,51,52,53,54,55]. The Solvay process itself does not substantially contribute to reduce CO₂ emission due to the burning fossil fuel for CaO and CO₂. This process consumes a significant amount of CO₂ because of the high demand on soda ash and therefore has high potential for CO₂ utilization and sequestration, but it can become reality only when the whole process is revised with the replacement of CaO and CO₂ sources. These chemicals are abundant in many industrial wastes such as fly ash, slag, and flue gas. Thus, the exploration of those wastes as major sources for CaO and CO₂ instead of burning CaCO₃ should be considered. This strategy does not only reduce CO₂ emission but also mitigate other wastes to be discharged to the environment. In the following parts of this chapter, the modified Solvay method and related methods to produce NaHCO₃ and Na₂CO₃ in the consideration of waste and CO₂ utilization will be introduced.

Fig. 9.1

Schematic diagram of a traditional Solvay process

$$ 2{\text{NaCl}} + {\text{CaCO}}_{3} \to {\text{Na}}_{2} {\text{CO}}_{3} + {\text{CaCl}}_{2} $$

(9.7)

$$ {\text{NaCl}} + {\text{CO}}_{2} + {\text{NH}}_{3} + {\text{H}}_{2} {\text{O}} \to {\text{NaHCO}}_{3} + {\text{NH}}_{4} {\text{Cl}} $$

(9.8)

$$ 2{\text{NaHCO}}_{3} \to {\text{Na}}_{2} {\text{CO}}_{3} + {\text{H}}_{2} {\text{O}} + {\text{CO}}_{2} $$

(9.9)

$$ 2{\text{NH}}_{4} {\text{Cl}} + {\text{CaO}} \to {\text{CaCl}}_{2} + 2{\text{NH}}_{3} + {\text{H}}_{2} {\text{O}} $$

(9.10)

$$ 2{\text{NaCl}} + {\text{CaO}} + {\text{H}}_{2} {\text{O}} \to {\text{CaCl}}_{2} + 2{\text{NaOH}} $$

(9.11)

$$ {\text{CaCO}}_{3} \to {\text{CaO}} + {\text{CO}}_{2} $$

(9.12)

9.2 Modified Solvay Process and Solid Waste Utilization

9.2.1 Solid Waste Utilization in a Modified Solvay Process

Looking at the Solvay process as shown in Fig. 9.1, we can find a number of precursors or process steps where the traditional sources can be partially or even completely replaced by wastes. First of all, let us analyze the CO₂ cycle in the process; every mole of Na₂CO₃ produced requires one mole of CO₂ from burned CaCO₃ fed into system. This process looks like a carbon neutral one emission because CO₂ in a close loop, however, there is a hidden event, i.e., CaCO₃ burning process that combusts fossil fuel and generate significant amount of CO₂ [56]. The emission from fossil fuel combustion in lime production may vary with the type of kiln and fuel, but according to Shan et al., an average of 56.55 million tonnes CO₂ emitted for the production of 207.5 million tonnes lime in China in 2012 [56]. This means that every tonne of CaO produced generates 0.27 tonnes CO₂ that are not used in soda ash production. Steinhauser indicated that the production of one tonne of soda ash emits from 0.2 to 0.4 tonnes of CO₂ [48]. This makes the traditional Solvay process not attractive in the term of CO₂ emission. CO₂ is however very abundant in flue gas from power plant or industry that burns fossil fuel as energy source. Flue gas from coal-fired power plant, for example, contains approximately 15 vol.% of CO₂ that can be a viable CO₂ supply for soda ash industry. The issue now lies in the CaO source because burning CaCO₃ does not only provide CO₂ but also provide CaO. Stop burning CaCO₃ means the alternatives for CaO source must be taken into account. Fortunately, as discussed in earlier section, a variety of alkaline solid wastes such as ash and slag which have high content of CaO are available from coal-fired power plants and other industries that burn coal for energy as well. Depending on industry, technology, and type of coal, chemical composition and alkaline contents of solid wastes can be varied. In the industrial process, e.g., steelmaking illustrated in Fig. 9.2, slag can be collected from different process steps with different quantities and qualities. Table 9.1 shows the chemical composition of major alkaline wastes that are available from the literatures [6, 7, 57, 58]. The slag collected from blast furnace (BF slag) contains about 38–42 wt% CaO, while that collected from basic oxygen furnace (BOF slag) has 46.3–55.9 wt% CaO. As can be seen in this table, there are a number of wastes that have high CaO content with maximum above 50 wt% such as municipal solid waste (MW) incinerator bottom ash (53 wt%), fly ash class C (54.8 wt%), blended hydraulic slag cement (56.9 wt%), air pollution control residue (60 wt%), argon oxygen decarburization (AOD) slag from steel industry (60.7 wt%), and paper sludge incineration ash (69 wt%). These two waste streams, flue gas and alkaline solid waste, could be perfect alternatives for CO₂ and CaO sources and positive contribution to CO₂ emission and solid waste mitigation.

Fig. 9.2

Reprinted with permission from [59], copyright 2008 American Chemical Society

Flowchart of a typical integrated steelmaking process.

Table 9.1 Chemical composition of different alkaline solid wastes

In the Solvay process, thermal energy consumption accounts for 30% of production cost and considerable fraction of it comes from firing CaCO₃ for CaO [63]. The use of solid waste instead of CaO does not only reduce CO₂ emission but also directly contributes to lower the cost of product thanks to the reduction in energy consumption and low cost of solid waste. In order to improve the input parameters such as energy and material cost and diminish the amount of waste in conventional process, Carvalho Pinto et al. proposed a modified Solvay process as shown in Fig. 9.3 where lime used in the regeneration of ammonia is partially replaced by steel slag milk [63]. The steel slag having 40.1 wt% CaO was utilized in the laboratory investigation. The effect of various conditions such as temperature, time, NH₄Cl concentration, and slag/NH₄Cl ratio on the extraction of Ca²⁺ ions from slag was investigated. The maximum Ca²⁺ extraction efficiency reaches 5 wt% at the optimum condition of NH₄Cl 1.5 mol/L at 90 °C for 60 min with a solid-to-liquid ratio of 2 wt%. The Ca²⁺ leaching is however significantly affected by the NaHCO₃ concentration remained in the filtrate from the Solvay process after solid NaHCO₃ is separated. For a better Ca²⁺ leaching, it requires a thermal treatment to decompose NaHCO₃ prior to the addition of alkaline waste [54, 63]. By thermal treatment, the Ca²⁺ concentration is improved from 192 to 1520 mg/L, which is close to leaching efficiency of pure NH₄Cl (1840 mg/L). NH₃ recovery also varies with reaction conditions and is possible to reach a maximum of 40 wt%. The study at bench scale indicated that the maximum NH₃ recovery of 40 wt% can be reached only when multi-recovery stages is applied.

Fig. 9.3

Reprinted with permission from [63], copyright 2015 Springer Nature publisher

Conventional Solvay process and its modified one by partial replacement of CaO with steel slag.

It is easily understandable that the success of alkaline solid wastes as alternatives to CaO depends on the amount of Ca²⁺ ions that can be extracted as active components. The extraction is controlled by several factors including the type of solid waste, CaO content of waste, the nature of calcium in solid waste, extracting solution, and temperature [7, 59,60,61,62, 65, 68,69,70,71, 73]. CaO content in solid waste is a primary parameter to evaluate the potential of a waste utilization. The CaO content greatly varies with the type of solid waste. As shown in Table 9.1, CaO contents in different solid wastes are variable, the higher CaO wastes are usually expected to have more amount of Ca²⁺ extracted, but it also depends on which type of crystalline phase existing in the wastes. In a research to extract Ca²⁺ from steel slag containing 40.1 and 30.5 wt% CaO using NH₄Cl solution and the filtrate of the Solvay process conducted by de Carvalho Pinto et al., the maximum Ca²⁺ extraction reached only 22 wt%, much lower compared to many others as shown in Table 9.2 [54, 63]. Even though only 22 wt% calcium was extracted, 84.3 wt% of the NH₃ inlet can be recovered, demonstrating a great potential of solid waste use in a modified Solvay process. Therefore, the key to the success of this process now lies on calcium extraction method. In the following section, the essential parameters for effective calcium extraction using the NH₄Cl solution or the filtrate from the Solvay process will be intensively discussed based on the available literatures.

Table 9.2 Calcium extraction by NH₄Cl solution in the literature

9.2.2 Calcium Extraction by Extracting Agent

The number of studies has been conducted to extract the calcium from solid waste for a purpose of CO₂ sequestration and calcium carbonate production [6, 7, 58, 61, 65, 67, 70, 73, 74]. The calcium extraction efficiency is used as a major parameter to evaluate the effectiveness of an extraction system. The extraction efficiency is defined as the percentage of Ca extracted by extracting solution to the total calcium in the solid waste used in the test as expressed in Eq. 9.13. Pure water has very poor calcium extraction because it can dissolve only part of free CaO in solid waste. In a research conducted by Said et al. [61], only 0.81 wt% of calcium in steel-converted slag was extracted by pure water. However, in another research, He et al. can extract more than 10 wt% of calcium from fly ash using pure water even though the fly ash contains less CaO, 30.47 wt% (Table 9.2) [62]. The difference in calcium extraction efficiency is probably due to the different contents of free calcium oxide or the waste microstructure that prevents the migration of Ca²⁺ into water phase. The calcium extraction efficiency offered by water is usually far below the requirement for an application, and thus, it needs a more effective method. Various extracting agents such as acids (HCl, HNO₃, CH₃COOH, and CH₃CH₂COOH) [59, 74,75,76] and ammonium salts (NH₄Cl, NH₄NO₃, CH₃COONH₄, and (NH₄)₂SO₄) [59,60,61,62, 71] have been used to improve the Ca²⁺ extraction. Recent works have indicated that the addition of these extracting agents helps enhance the Ca²⁺ extraction significantly thanks to their possibility to selectively dissolve some calcium compounds in solid waste, e.g., the dissolution of calcium silicate and calcium ferrite phases given in Eqs. 9.14 and 9.15 [65, 66, 68, 70, 72].

$$ {\text{Ca}}\;{\text{extraction}}\;{\text{efficiency}}\left( \% \right) = \frac{{m_{Ca} \left( g \right)}}{{m_{t} \left( g \right) \times \frac{{C_{CaO} \left( \% \right)}}{100} \times \frac{{M_{Ca} \left( {\frac{g}{mol}} \right)}}{{M_{CaO} \left( {\frac{g}{mol}} \right)}}}} \times 100 $$

(9.13)

where m_Ca is the mass of Ca extracted by the extracting solution (g), m_t is the mass of solid waste used (g), C_CaO is the CaO content in solid waste (wt%), M_Ca is molecular weight of calcium (g/mol), and M_CaO is molecular weight of CaO (g/mol).

$$ 2{\text{CaO}}_{{ ( {\text{S)}}}} \cdot {\text{SiO}}_{{2 ( {\text{s)}}}} + 4{\text{NH}}_{4} {\text{Cl}}_{{ ( {\text{aq)}}}} \leftrightarrow 2{\text{CaCl}}_{{ 2 ( {\text{aq)}}}} + {\text{SiO}}_{{2 ( {\text{s)}}}} + 4{\text{NH}}_{{3 ( {\text{g)}}}} + 2{\text{H}}_{2} {\text{O}}_{{ ( {\text{l)}}}} $$

(9.14)

$$ {\text{Ca}}_{2} {\text{Fe}}_{2} {\text{O}}_{{5 ( {\text{s)}}}} + \, 2{\text{NH}}_{4} {\text{Cl}}_{{ ( {\text{aq)}}}} \leftrightarrow 2{\text{Ca}}^{2 + }_{{ ( {\text{aq)}}}} + 2{\text{Cl}}^{ - }_{{ ( {\text{aq)}}}} + 2{\text{NH}}_{{ 3 ( {\text{g)}}}} + {\text{ Fe}}_{2} {\text{O}}_{{3 ( {\text{s)}}}} + {\text{ H}}_{2} {\text{O}}_{{ ( {\text{l)}}}} $$

(9.15)

The calcium extraction from fly ash by water and ammonium salts at different temperatures was reported by He et al. [62] (Fig. 9.4). It is evident that the calcium extraction improves significantly by adding those extracting agents. Temperature also contributes to the improvement, but it should be controlled in a moderate range. The higher temperatures do not significantly improve the extraction but incur the water vaporization that causes the loss of solvent, difficult process control, and energy intensiveness. In another research, Lee et al. showed the effect of ammonium salts on two types of slag, granulated slag and air-cooled slag [71]. They again confirm the extraction efficiency caused by the type of both extracting solvent and solid waste. NH₄NO₃, NH₄Cl, and CH₃COONH₄ have a comparable extraction efficiency, approximately 49–52 wt% for granulated slag and 11–17 wt% for air-cooled slag (Fig. 9.5). (NH₄)₂SO₄ has worse extraction among four ammonium salts due to the formation of less soluble CaSO₄ on the outer layer of slag granules that prevents the dissolution of Ca²⁺ into liquid phase. Using 2 mol/L of NH₄Cl, NH₄NO₃, and CH₃COONH₄, Said et al. can extract from 45 to 54% calcium from BOF slag that has the particle size of 74–125 µm. Similar results were observed by Sanni et al. in which acids that form soluble salts with calcium such as CH₃COOH, HNO₃, and CH₃CH₂COOH possess the highest extraction efficiency (85–100%) followed by ammonium salts (NH₄NO₃, NH₄Cl, and CH₃COONH₄) with the efficiency ranging from 50 to 80%, while (NH₄)₂SO₄, (NH₄)₂HPO₄, and NH₄H₂PO₄ showed a poor extraction [67]. For cement waste, 1 mol/L solution of NH₄NO₃ and CH₃COONH₄ has the extraction efficiency of ~69% that is better than NH₄Cl and (NH₄)SO₄ with only 32 and 4%, respectively [60]. Generally, ammonium salts of NH₄Cl, NH₄NO₃, and CH₃COONH₄ were recognized as the best solution for extracting calcium from solid waste thanks to their high extraction efficiency and NH₃ recyclability.

Fig. 9.4

Reprinted with permission from [62], copyright 2013 American Chemical Society

Influence of temperature and the type of extraction solution on Ca²⁺ extraction efficiency (ammonium salt concentration: 1 mol/L, solid-to-liquid ratio: 50 g/L, mixing rate: 500 rpm, time: 60 min).

Fig. 9.5

Reprinted with permission from [71], copyright 2016 Elsevier

Calcium extraction efficiency of (a) granulated slag, and b air-cooled slag when different ammonium salts are used. Solvent concentration (mol/L), reaction temperature (°C), reaction time (min), and solid-to-liquid ratio are 2 mol/L, 30 °C, 60 min, and 10 for the condition 9.1 and 1 mol/L, 30 °C, 30 min, and 10 for the condition 9.2, respectively. Further detail on the condition 9.1 and 9.2 can be found in Table 9.2. Abbreviations for the solvents: AN: NH₄NO₃, AC: NH₄Cl, AA: CH₃COONH₄, and AS: (NH₄)₂SO₄.

Variety of agents have proved to be effective for calcium extraction, however, to apply in the Solvay process, and only NH₄Cl is a suitable candidate because it is the main component in the filtrate that will react with CaO for NH₃ recovery. In a typical Solvay process, NH₄Cl concentration in the filtrate is around 3.6–4.9 mol/L, but it could not directly apply for calcium extraction because of the presence of HCO₃⁺ in the filtrate that reacts with Ca²⁺ to form insoluble CaCO₃ [54, 55, 63]. The CaCO₃ may form the less soluble carbonated shell around slag particles that prevents the dissolution of Ca²⁺ ions [7, 9, 58]. Thermal treatment can decompose the HCO₃⁺ to release CO₂, but also remove NH₄⁺ as NH₃ in the gas phase; this causes the decrease in the NH₄Cl concentration. The reduction in NH₄Cl concentration may influence the extraction capacity of the filtrate. Recent researches have indicated that NH₄Cl concentration plays a very important role in the calcium extraction [60,61,62,63, 65, 67, 70, 72]. Table 9.2 shows the calcium extraction from various solid wastes using NH₄Cl in the literatures. In this table, numerous parameters including NH₄Cl concentration, solid-to-liquid ratio, particle size, temperature, and extraction time and their influence on the extraction efficiency, calcium selectivity, and calcium concentration in leachate are introduced. A good agreement among published works is that the extraction efficiency improved with the increase in NH₄Cl concentration; however, its concentration above 1 mol/L would be preferable. A rapid increase in the extraction efficiency was observed at the NH₄Cl ranging from 0 to 1 mol/L. Above 1 mol/L, the increase is not significant (Fig. 9.6a). The decrease in NH₄⁺ concentration due to thermal treatment of the filtrate has been verified by de Carvalho Pinto et al. [54, 63]. By boiling the filtrate containing NH₄Cl 4.2 mol/L for 30 min, they can recover 16.7% NH₃ which causes the NH₄Cl concentration decrease to 3.5 mol/L [63]. In another study, the reduction on the NH₄Cl concentration from 3.6 to 2.8 mol/L was found by boiling the filtrate [54]. These suggest that a fraction of NH₃ can be recovered by thermal treatment, but it does not influence on the following extraction activity because the NH₄Cl concentration is remained in preferable range. By boiling the filtrate for 30 min prior to the addition of slag, de Carvalho Pinto et al. can enhance the calcium extracted into filtrate solution from 192 to 1520 mg/L [63] confirming the effectiveness of the thermal treatment.

Fig. 9.6

Reprinted with permission from [63], Springer Nature publisher

Effect of NH₄Cl concentration (a) and solid-to-liquid ratio (b) on the calcium extraction efficiency.

9.2.3 Type of Solid Waste

Types of solid waste and the crystalline phases existing in solid waste are the most influent factors that decide the success of extraction using NH₄Cl. CaO content and its compounds in waste vary with the waste sources and directly affect the extraction efficiency. The calcium extraction from different wastes under various conditions is provided in Table 9.2. In general, BOF slag has a high extraction efficiency. Most of BOF slag samples show the extraction efficiency above 50 wt%, while only few samples of desulphurization lag and BF slag attain 50 wt% calcium extraction. Investigating on the calcium extraction from different slags, Sanni et al. recognized that free lime in BOF and desulphurization slags contributes to high extraction with about 70 and 50%, respectively, while all calcium in BF and ladle slags exists in silicate form that explains for a poor extraction with a efficiency below 15% [67]. According to Lee et al., air-cooled slag contains higher CaO content (32.9 wt%) compared to granular slag (22.5 wt%) but the extraction efficiency is much lower, 11–17 wt% compared to 49–52 wt%, respectively [71]. The differences are attributed to the crystallinity of these two slags caused by collecting methods. Granular slag is generated by rapid cooling using water produced about 50% amorphous phase and 50% crystalline phases including mainly wustite (15.23%), Ca₂Fe₂O₅ (13.32%), beta-Ca₂SiO₄ (13.10%), and magnetite (10.96%), whereas air-cooled slag is obtained by slow cooling under an ambient atmosphere containing mostly crystalline phases, which account for the poor extraction from this slag. To demonstrate the high extraction contributed by free CaO in solid waste, the comparison between MgO + CaO mixture and fly ashes was made by Hosseini et al. [69]. Two fly ashes collected from electrostatic precipitator of coal power plant that are Hazelwood fly ash (32.4 wt% CaO) containing mainly CaSO₄, CaO, calcium ferrite (CaFe₂O₄), and silicate and Yallourn fly ash (9.4 wt% CaO) containing mostly magnesia ferrite, quartz, and amorphous CaO were tested. As shown in Fig. 9.7, above 80 wt% of calcium in MgO + CaO mixture can be extracted, while only 32 and 37 wt% of calcium was leached from Hazelwood and Yallourn fly ash in the first cycle using NH₄Cl, respectively.

Fig. 9.7

Reprinted with permission of [69], copyright 2014 American Chemical Society

Calcium extraction efficiency for five leaching-carbonation cycles at optimum conditions (80 °C and 30 min) using MgO + CaO mixture, Hazelwood and Yallourn fly ash.

9.2.4 Particle Size and Microwave Treatment

Particle size distribution is another crucial characteristic that influences on the calcium extraction. Data on the effect of size on the calcium extraction can be found in Table 9.2. Investigating the calcium leaching from various slags with size <150 and 150–500 µm by Hall et al. indicated an impressive increase in extraction efficiency by size reduction with a maximum 50% higher in small size samples [68]. By reducing the size of slag from 250–500 to 0–125 µm, Said et al. can improve almost 50% of the extraction efficiency of a steel converter slag which contains the majority of CaSiO₄, Ca₂Fe₂O₅, Fe_0.925O, and Fe₃O₄ without free CaO (Fig. 9.8) [61]. Further extraction improvement can be executed by the application of microwave [72], ball milling, and heat treatment [70]. Tong et al. found that the calcium leaching can be improved about 10% by the combination of NH₄Cl extracting agent with microwave treatment on electric arc furnace slag [72]. The research indicated that Ca₂SiO₃, Ca₂SiO₄, and Ca₃SiO₅ can be dissolved by NH₄Cl solution generating leaching Ca²⁺ and SiO₂. Lee et al. showed a calcium extraction improvement by combining NH₄Cl extracting agent with ball milling [70]. The improvement is attributed to the particle size reduction that decreased from <10 to 1–5 µm by ball milling.

Fig. 9.8

Reprinted with permission from [61], copyright 2012 Elsevier

Effect of particle size on calcium extraction from steel converter slag in NH₄Cl solution 1 mol/L (solid-to-liquid ratio: 20 g/l).

9.2.5 Solid-to-Liquid Ratio

Solid-to-liquid ratio (g/L) for extraction is another key parameter. Most of studies agree that lower solid-to-liquid ratio offers better extraction efficiency (Table 9.2) [61,62,63, 65]. The relation was clarified by de Carvalho Pinto et al. as shown in Fig. 9.6b; the extraction efficiency drastically declines as the slag concentration increases [63]. This is in congruence with the observation by Said et al. where the calcium extraction from BOF slag by ammonium salts reached a maximum (73%) at the lowest solid concentration (5 g/L) and the lowest extraction (6%) by the highest solid concentration (100 g/L) [61]. The analogous effect was also observed when calcium was extracted from fly ash [62]. The reason for this reduction in the extraction efficiency is most likely due to the high viscosity causing the low mobility of solution that prevents proper mixing and consequently reducing calcium extraction. It is also believed that the higher buffer effect of the NH₄⁺/NH₃ couple is attained at pH = 9.25. At this pH, NH₄⁺ and NH₃ concentrations are equal and their interconversion occurs that prevents the further Ca²⁺ extraction [63]. In contrast to the extraction efficiency, the Ca⁺ concentration in the liquid increased a long with the increase in solid-to-liquid ratio; this relation is illustrated by He et al. in Fig. 9.9. In a practical application, an optimum solid-to-liquid ratio must be determined to balance the extraction efficiency, Ca²⁺ concentration, and water consumption for efficient production.

Fig. 9.9

Reprinted with permission from [62], copyright 2013 American Chemical Society

Effect of solid-to-liquid ratio on calcium extraction efficiency (a) and Ca²⁺ concentration in leachate (b) at 25 °C for 60 min with the NH₄Cl concentration of 1 mol/L and mixing rate 500 rpm.

9.2.6 Extraction Selectivity

Since solid waste has a diverse chemical composition, the extraction may simultaneously leach various metal ions into solution. Selectivity of a metal is a fraction of its concentration to total metal concentration in the leachate. A good solid waste should generate high selectivity for Ca over others. Impurity leached into the filtrate does not impact on the quality of resulting NaHCO₃ or Na₂CO₃ product, but it may affect the NH₃ recovery efficiency and relate to the strategy for later waste management. High selectivity of unreactive species toward NH₄Cl or metal ions that form stable complexes with NH₃ will result in low NH₃ recovery. In general, ammonium salt solution shows a good selectivity for calcium against undesired species (Table 9.2) [54, 60, 65, 68, 70]. Relatively small quantity of undesired metals, e.g., Mn, Fe, Ni, Al, Si are detected in the leachate. A study by Jo et al. on the calcium extraction from cement waste using ammonium salts showed that most of ammonium salts have a good selectivity for calcium, which is greater at higher salt concentration, reached above 98% at the salt concentration of 1.0 mol/L (Fig. 9.10) [60]. Other studies on steel slag by Hall et al. and Lee et al. show that the calcium extraction can attain more than 95% selectivity [68, 70]. Sun et al. found only 75.7% Ca selectivity when extracting it with NH₄Cl 2 mol/L from oxy furnace steel slag; however, total selectivity for two alkaline metals of Ca and Mg reached 96% [64]. Since magnesium could contribute well for NH₃ recovery in the Solvay process, the high selectivity for these two elements is acceptable.

Fig. 9.10

Reprinted with permission from [60], copyright 2014 Elsevier

Selectivity of major metal ions in different ammonium salts at the concentration of a 0.5 mol/L and b 1.0 mol/L.

9.3 Modified Solvay Process with Amines and Reject Brine Utilization

Finding a way to prevent or mitigate sea disposal of concentrated brine wastes has huge environmental benefits. The idea to combine CO₂ utilization with waste brine treatment derives from the unique opportunity provided by the availability of vast amounts of brine wastes from desalination plant. Due to its ability to treat two environmental wastes, CO₂ utilization with brine is a promising technology for solving the problem of desalination brine wastes [36]. Over the years, different ideas have been proposed to accomplish this kind of process, most of which are based on the Solvay process first developed by Ernst Solvay in 1861 [77]. Recently, methods involving the use of CaO and layered double hydroxides have also been shown to give promising results with regard to this process. The goal of the following sections is to provide a more detailed description of these processes and their performances with respect CO₂ utilization and reject brine treatment.

The conventional Solvay process involves reacting CO₂ with ammonia/brine solution to produce NaHCO₃ and ammonium chloride (Fig. 9.1). The NaHCO₃ is precipitated and separated, while the ammonium chloride solution (filtrate) is reacted with lime to produce ammonia, which is recycled, and CaCl₂ as a waste product. While this process scheme can be applied directly for CO₂ utilization and brine treatment as demonstrated by El-Naas [78, 79], it is not very appropriate for the processes where sodium removal is the main goal due to its low sodium conversion. This, in addition to the volatility of ammonia at the temperatures at which CO₂ capture processes operate, poses a problem for the application of Solvay process for CO₂ utilization and brine treatment [80, 81]. Also, Solvay process applies CaO for the regeneration of ammonia. This is counterproductive for the CO₂ capture process due to the fact the production of CaO is CO₂ intensive. For every two moles of CO₂ from power plants captured, one mole of CO₂ from calcination of limestone is released [82].

Modifications of the Solvay process for CO₂ capture and utilization processes have been mostly focused on the replacement of volatile ammonia with alcohol amines which are stable at the temperature range of CO₂ capture processes. Alcohol amines also tend to give better performance results (Na conversion/removal) than ammonia with dilute CO₂ streams such as flues gas as shown in Table 9.3.

Table 9.3 Na conversion in brine treatment with the Solvay process

Huang et al. used a sterically hindered alcohol amine and methyl aminoethanol (MAE), in the place of ammonia, and reported a CO₂ capacity of 0.9 mol/mol MAE and a NaHCO₃ yield of 14.3 g NaHCO₃/mole MAE (~17.14 g/kg solution) at MAE and NaCl concentration of 1.2 and 3.4 M, respectively [82]. In similar vein, Dindi et al. investigated the use of different classes of alcohol amines as replacement for ammonia and found that secondary or sterically hindered amines such as 2-amino, 2-propanol-amine gave the best performance [83, 86]. The reaction of amines with CO₂ could follow either a carbamate or the bicarbonate route depending on the class of the amine. Primary and secondary amines react with CO₂ to form carbamates following the zwitterion mechanism Eq. 9.16, while tertiary and sterically hindered amines react with CO₂ to form bicarbonates according to Eq. 9.17 [87].

$$ 2{\text{R}}_{1} {\text{R}}_{2} {\text{NH}} + {\text{CO}}_{{2 ( {\text{aq)}}}} \leftrightarrow {\text{R}}_{1} {\text{R}}_{2} {\text{NCOO}}^{ - } + {\text{R}}_{1} {\text{R}}_{2} {\text{NH}}_{2}^{ + } $$

(9.16)

$$ {\text{R}}_{1} {\text{R}}_{2} {\text{R}}_{3} {\text{N}} + {\text{CO}}_{{2 ( {\text{aq)}}}} + {\text{H}}_{2} {\text{O}} \leftrightarrow {\text{R}}_{1} {\text{R}}_{2} {\text{R}}_{3} {\text{NH}}^{ + } \cdot {\text{HCO}}_{3}^{ - } $$

(9.17)

$$ {\text{R}}_{1} {\text{R}}_{2} {\text{NCOO}}^{ - } + {\text{R}}_{1} {\text{R}}_{2} {\text{NH}}_{2}^{ + } + {\text{H}}_{2} {\text{O}} \to {\text{R}}_{1} {\text{R}}_{2} {\text{NH}}_{2}^{ + } \cdot {\text{HCO}}_{3}^{ - } + {\text{R}}_{1} {\text{R}}_{2} {\text{NH}} $$

(9.18)

$$ {\text{R}}_{1} {\text{R}}_{2} {\text{NH}}_{2}^{ + } \cdot {\text{HCO}}_{3}^{ - } + {\text{NaCl}} \to {\text{NaHCO}}_{3} + {\text{R}}_{1} {\text{R}}_{2} {\text{NH}}_{2}^{ + } \cdot {\text{Cl}}^{ - } $$

(9.19)

$$ {\text{R}}_{1} {\text{R}}_{2} {\text{R}}_{3} {\text{NH}}^{ + } \cdot {\text{HCO}}_{3}^{ - } + {\text{NaCl}} \to {\text{NaHCO}}_{3} + {\text{R}}_{1} {\text{R}}_{2} {\text{R}}_{3} {\text{NH}}^{ + } \cdot {\text{Cl}}^{ - } $$

(9.20)

Primary or secondary amines react with CO₂ to form carbamate (Eq. 9.16) accordingly, two molecules of amine are needed to capture one mole of CO₂. For this reason, the CO₂ absorption capacity of such amines cannot exceed 0.5 mol CO₂ per mole of amine theoretically. Meanwhile, the tertiary or sterically hindered amines which follow the bicarbonate pathway have the potential of reaching a capacity of 1 mol CO₂ per mol amine as shown in Eq. 9.17, although they may be prevented from reaching this capacity due to kinetic limitations of the bicarbonate pathway. Sometimes, the capacity of primary or secondary amines exceeds the theoretical limit due to the carbamate hydrolysis (Eq. 9.18) which releases an amine to react with more CO₂, thereby increasing the CO₂ absorption capacity beyond the theoretical limit. This effect is even more pronounced in the presence of brine.

The CO₂ absorption capacity of amines increases as a result of precipitation of NaHCO₃. Na in the brine reacts with ${\text{HCO}}_{3}^{ - }$ generated in Eq. 9.17 (tertiary amines) and Eq. 9.18 (primary/secondary amines), thereby shifting the equilibrium to the right and causing more CO₂ to be absorbed and more NaHCO₃ to be precipitated in Eqs. 9.19 and 9.20 [88]. The process continues until the solution pH reduces below 9 in which case NaHCO₃ cannot be precipitated and equilibrium is reached or until all the amine is consumed to form an amine chloride. This explains why amines give better CO₂ absorption results in the presence of NaCl [82, 83].

Although amines give better results during the carbonation stage, a major drawback of the use of amines in the Solvay process is the difficulty of regenerating the free amine from the amine chloride by-product produced during the carbonation step. Although Huang proposed a method for regenerating ammonia from NH₄Cl using activated carbon as a means of avoiding the use of CaO for ammonia regeneration, no method was proposed for regenerating the MAE chloride [82]. The activated carbon was used to remove HCl from a solution of NH₄Cl with the liberation of ammonia according to Eq. 9.21.

$$ {\text{NH}}_{4} {\text{Cl}} + {\text{AC}} \leftrightarrow {\text{NH}}_{3} + {\text{AC}} \cdot {\text{HCl}} $$

(9.21)

The HCl-loaded activated carbon was then regenerated with water and used repeatedly to produce about 0.0012 mol HCl per cycle. In addition, the ultra-high lime with aluminum (UHLA) method was proposed by Dindi et al. to recover AMP from the AMP-HCl formed during the carbonation step. However, the efficiency of this process was limited by increasing amine concentration and by the presence CO₂ in the solution [83]. No other method for regenerating amines from amine chlorides has been reported in the literature. Hence, this constitutes a major obstacle in the application of the amine-based Solvay process for brine treatment and CO₂ capture and utilization

Recently, El-Naas proposed a process in which the CaO was used to replace NH₃ in the Solvay process. The CaO is dispersed in brine to increase its pH and then applied for CO₂ capture. The higher pH facilitates absorption of CO₂ into the brine and subsequently the precipitation of NaHCO₃. The system was effective for CO₂ capture and Na removal with an optimum temperature of 10 °C and CaO concentration of 20 g/L. Beyond this concentration, Na removal was reduced due to competitive formation of CaCO₃. The modified process had a maximum CO₂ capture capacity of 1.44 mol/L and sodium removal efficiency of 35%, while the conventional process had a maximum CO₂ capacity of 1.14 mol/L and a 29% sodium removal efficiency. The modified process is also potentially less costly than the conventional process due to the elimination of energy-intensive ammonia regeneration, fewer unit operations, and lower costs of CaO relative to the cost of ammonia [89].

9.4 Layered Double Hydroxide Process

Due to the shortcomings of the modified Solvay processes for the simultaneous treatment of desalination brines and CO₂ utilization, a new, layered double hydroxide (LDH)-based process has been recently proposed which has the potential to perform better than modified Solvay processes [90]. LDHs, also known as hydrotalcite, are clay materials consisting of alternating layers of mixed metal hydroxides and anion-bearing interlayers, similar to structure of the hydrotalcite mineral as shown in Fig. 9.11 [46]. The mixed metal hydroxides layer are made up of positively charged di or trivalent cations, while the anion layer contains negatively charged balancing anions which combine to give the formula $\left[ {{\text{M}}_{{1{-}x}}^{2 + } {\text{M}}_{x}^{3 + } \left( {\text{OH}} \right)_{2} } \right]_{x} + \left( {{\text{A}}^{-} } \right){\text{ x}}/{\text{n}} \cdot {\text{mH}}_{ 2} {\text{O}}$. M²⁺ represents divalent cations like Mg, Fe, Co, Cu, Ni, or Zn, M³⁺ represents trivalent cations like Al, Cr, Ga, Mn or Fe, and A⁻ is the interlayer anion such as ${\text{CO}}_{3}^{ - 2}$, ${\text{HCO}}_{3}^{ - }$, ${\text{SO}}_{4}^{ - 2}$, Cl⁻, or OH⁻. The value of x is equal to the molar ratio of M²⁺/(M²⁺ + M³⁺) and is generally in the range 0.2–0.33.

Fig. 9.11

Reprinted with permission from [90], copyright 2018 Elsevier

Hydrotalcite structure.

Carbonate LDHs, in which a ${\text{CO}}_{3}^{2 - }$ ion exists within the layers, are the most common LDH forms due to the strong affinity of the LDHs for the ${\text{CO}}_{3}^{2 - }$ ion. While LDHs can exchange their interlayer ions with other ions present in a solution, this is not always possible depending on the relative affinity of the anion to the LDH. However, when calcined, they release the interlayer anion and OH groups to produce mixed metal oxides, which can reconvert to the original LDH structure when being dispersed in an aqueous solution of the appropriate anion, making them excellent materials for the sorption anions. This property of LDHs has been harnessed for the removal of different kinds of contaminants from wastewater like iodide [91], silicate [92], chlorides [93], heavy metals [94], and rare earth elements [95].

In the case of reject brine treatment, the objective is to reduce chloride and sodium concentrations to levels that are safe for disposal. LDHs can be used to remove chloride from brine as shown in several studies by Kameda and co-workers [93, 96, 97]. Given that the removal of chlorides from brines by Mg–Al–O also results in the increase in OH⁻ ions in the solution leading to a rise in solution pH, this increased alkalinity can then be employed for CO₂ absorption and precipitation of NaHCO₃ which ultimately leads to sodium removal from the brine. In this way, CO₂ capture/utilization and reject brine treatment are combined in one single process as further described in the following section.

9.4.1 Process Description

The process consists of three major stages which are the chloride removal, carbonation, and regeneration stages as shown in Fig. 9.12. In the chloride removal stage, Mg–Al mixed metal oxide, derived from Mg–Al hydrotalcite, is dispersed in the brine to remove the chloride ions via an ion exchange process. Chloride moves from the liquid phase to the solid phase, while the solution is enriched in OH⁻ ions due to the solvation of the mixed metal oxide. When the slurry is separated, the brine solution becomes more alkaline while mixed metal oxide is transformed to a chloride-form hydrotalcite, Cl⁻–HT, according to Eq. 9.22. The resulting alkaline brine solution, a mixture of NaOH and NaCl, is then sent to the carbonation stage for the precipitation of NaHCO₃. Depending on the initial concentration of the brine and the extent of chloride removal, a concentration step may be required to bring this solution to a concentration high enough to precipitate NaHCO3.

Fig. 9.12

Schematic diagram of the proposed process

In the carbonation stage, the CO₂ captured from flue gas reacts with the alkaline solution to precipitate NaHCO₃, removing the Na⁺ from the brine in the process as described in Eqs. 9.23–9.28. The liquid effluent from this stage has reduced salinity and can therefore be disposed as treated brine, while the NaHCO₃ can be dried and packaged for sale. To ensure efficiency of the process, the spent Cl–HT must be reused. This is done thermally in the regeneration stage.

Chloride removal step:

$$ {\text{Mg}}_{0.8} {\text{Al}}_{0.2} {\text{O}}_{1.10} + 0.2{\text{NaCl}} + 1.1{\text{H}}_{2} {\text{O}} \to {\text{Mg}}_{0.8} {\text{Al}}_{0.2} \left( {\text{OH}} \right)_{2} {\text{Cl}}_{0.2} + 0.2{\text{NaOH}} $$

(9.22)

Carbonation step:

$$ {\text{CO}}_{{2 ( {\text{g)}}}} \leftrightarrow {\text{ CO}}_{2} \left( {\text{aq}} \right) $$

(9.23)

$$ {\text{CO}}_{{2 ( {\text{aq)}}}} + {\text{OH}}^{ - } \leftrightarrow {\text{HCO}}_{3}^{ - } \left( {\text{aq}} \right) $$

(9.24)

$$ {\text{HCO}}_{3}^{ - } + {\text{OH}}^{ - } \leftrightarrow {\text{H}}_{2} {\text{O}}\left( l \right) + {\text{CO}}_{3}^{2 - } \left( {\text{aq}} \right) $$

(9.25)

At high pH,

$$ {\text{Na}}^{ + } \left( {\text{aq}} \right) + {\text{CO}}_{3}^{2 - } \left( {\text{aq}} \right) \to {\text{Na}}_{2} {\text{CO}}_{3} \left( {\text{aq}} \right) $$

(9.26)

At low pH when OH⁻ has been depleted by reaction 9.24,

$$ {\text{Na}}^{ + } \left( {\text{aq}} \right) + {\text{HCO}}_{3}^{ - } \left( {\text{aq}} \right) \to {\text{NaHCO}}_{3} \left( {\text{aq}} \right) $$

(9.27)

$$ {\text{NaHCO}}_{3} \left( {\text{aq}} \right) \to {\text{NaHCO}}_{3} \left( s \right) \downarrow $$

(9.28)

During the regeneration, the Cl–HT is calcined at above 450 °C to produce the original Mg–Al mixed metal oxide (Mg–Al–O) by releasing Cl, OH, and CO₂ present in the Cl–HT structure. The mixed metal oxide is then returned to the chloride removal stage and the process loop continues, while the released chloride, on the other hand, provides opportunity for the production of Cl-based products which brings additional value to the process. Overall, the process results in a more extensive treatment of the brine because of the removal of both Na and Cl ions while creating more potential for revenue through the sale of CO₂- and Cl⁻-based chemical products.

9.4.2 Process Evaluation and Performance

A detailed assessment of the proposed process has been presented in [90]. The process was evaluated based on its ability to accomplish the two process objectives of brine treatment and CO₂ utilization. The performance indices for the evaluation of brine treatment were expressed as overall chloride and sodium removal efficiencies, and for CO₂ capture and utilization, as CO₂ absorption capacity and yield of NaHCO₃, respectively. Firstly, hydrotalcite and mixed metal oxide were synthesized; thereafter, cyclic tests were carried out to determine the performance indices. Conversion between hydrotalcite and the mixed metal oxide in a process cycle can be demonstrated by XRD investigation (Fig. 9.13).

Fig. 9.13

Reprinted with permission from [90], copyright 2018 Elsevier

XRD spectra and of the synthesized CO₃–HT and Mg–Al–O.

The Mg–Al–O was found to be effective for the removal chloride from the brine across the six cycles tested as shown in Fig. 9.14. Chloride removal efficiencies varied from 90 to 58%, from the first cycle to the sixth cycle, with an average of 70% across six cycles. The extent of sodium removal from the brine, due to precipitation of NaHCO₃, varied from 26 to 17% from the first cycle to the sixth cycle, with an average of 20% across six cycles, while an average NaHCO₃ yield of 44 g/kg carbonated solution was also obtained over six cycles of the process. The characterization of the produced NaHCO₃ confirmed that a pure NaHCO₃ product can be obtained from the process as presented in Fig. 9.15. Based on these results, the proposed process was found to be feasible for the simultaneous treatment of the reject brine and CO₂ utilization.

Fig. 9.14

Reprinted with permission from [90], copyright 2018 Elsevier

Initial and final concentrations of Cl⁻ (a) and Na⁺ (b) in brine.

Fig. 9.15

Reprinted with permission from [90], copyright 2018 Elsevier

SEM images at different magnifications (a, b), XRD spectra (c), and EDS spectra (d) of precipitated NaHCO₃.

The proposed process concept is an important contribution to research efforts on brine treatment and CO₂ utilization. It promises to be more effective than earlier processes because it is able to remove both Na and Cl ions from brine, whereas other proposed processes remove only Na ions. Also, in many regions where reject brines are generated in large volumes, desalination plants are usually integrated with power plants; hence, there will be little logistical constraint to the implementation of this process and the potential cost savings from the combined treatment of two industrial wastes as well as the revenue from the sale of two chemical products might justify the investment in this technology. Nonetheless, a detailed techno-economic study is required to determine the economic viability of the proposed process concept.

9.5 Conclusion

Current progress has been demonstrated promising outcomes from solid waste, reject brine, and CO₂ utilization for greener soda ash production. No doubt that the waste and CO₂ utilization in an integrated process has huge potential on the mitigation of CO₂ emission and waste management. Instead of burning CaCO₃ for CO₂ and CaO in the conventional Solvay process, those agents can be extracted from the waste streams of power plant or steel plant in a modified Solvay process. The process can further integrate with reject brine treatment from desalination plant to create process that completely consumes wastes as raw materials. This process is suitable to combine with other industries such as power plant or steel plant so that it can at the same exploit both waste material and waste heat for a marketable product production. The revenue received may be partially offset the cost of waste treatment and CO₂ capture that will encourage the effort toward waste and CO₂ emission mitigation.

The calcium extraction from solid waste is influenced by multiple parameters including CaO content and its crystalline phase, particle size distribution, extracting agent concentration (NH₄Cl), and solid-to-liquid ratio. CaO content, particularly free CaO, and the size of solid waste has the most decisive role for an efficient extraction. While the NH₄Cl concentration in the filtrate of the Solvay process is usually in the range of optimum concentration for extraction (>1 mol/L), the solid-to-liquid ratio becomes a very important parameter for balancing Ca²⁺ concentration in the filtrate and extraction efficiency. Recent initiative reveals that steel slag can partly replace CaO in the Solvay process to recover 40% NH₃ [63]; however, it could be significantly improved with a more suitable solid waste (high CaO content with low crystallinity and small particle size).

The modified Solvay process by replacing NH₃ with alcohol amine may render high soda ash recovery, but it faces the critical problem of amine recyclability. The layered double hydroxide process using Al₂O₃–MgO mixture to adsorb Cl⁻ ions generating NaOH solution that can capture CO₂ for soda ash production is novel and promising route, but it is still in the research stage.

To accelerate the waste and CO₂ utilization in the soda ash production, several challenges must be solved in future research, i.e., (1) to recognize the waste sources that are most suitable for NH₄Cl extraction and NH₃ recovery, (2) to find calcium extraction assistant method to effectively boost the extraction, (3) to revise the extractor, e.g., using multiple stage or combine mixing with microwave, (4) to look for an efficient way to recycle alcohol amine in modified Solvay process, (5) to prevent the formation of spinel phase during calcination of the layered double hydroxide for the long recyclability of Al₂O₃–MgO system, and finally, an economic evaluation for the proposed process is necessary.