Indirect mineral carbonation of blast furnace slag with (NH4)2SO4 as a recyclable extractant
Graphic abstract
Recent researches have been done to understand the reaction of (NH4)2SO4 with blast furnace slag and the mineral carbonation process with extracted calcium and magnesium.
Introduction
Carbon dioxide capture (utilisation) and storage (CCS/CCUS) is recognised as one of the options for tackling the increase in the atmospheric concentration of CO2 for climate change mitigation [1]. Although a huge amount of CO2 can be stored through geological and oceanic sequestration, proper geological conditions, such as depleted oil and gas fields, and geographical conditions, e.g. close to deep sea for the CO2 sources, as well as the inevitable use of expensive monitoring equipment to ensure that CO2 does not escape into the atmosphere for a very long time are required. Mineral carbonation is a potential CO2 sequestration method in which calcium and magnesium oxides in various minerals, particularly silicate minerals, react with CO2 and form thermodynamically stable carbonates. Silicate rocks are abundant in nature to the extent that, in theory, the potential for CO2 storage by mineral carbonation is higher than those of other CO2 storage methods [2].
Mineral carbonation consists of direct and indirect methods. The former mimics the weathering process in nature but proceeds at a much faster rate which can be enhanced using a variety of methods, including activation pre-treatment of minerals [3], the use of a high temperature and pressure [4], [5], and high-gravity methods [6]. However, the products of direct mineral carbonation are generally mixtures, which are difficult to separate for further utilisation or for selling as valuable products. Indirect CO2 mineralisation ordinarily comprises two successive steps: (1) extraction of Ca and Mg from minerals with an acidic or weakly acidic additive and (2) carbonation of Ca- and Mg-rich solutions or solids with CO2 in a basic or weakly basic environment. This method is now receiving widespread attention because of its relatively mild reaction conditions, high carbonation conversions of Ca and Mg, and purer, thus more valuable, by-products.
The key issues of the indirect mineral carbonation method that need to be solved are the regeneration of the chemical additives for reuse with low energy consumption and the improvement of the economy of the process. Thus far, the extractions of calcium and magnesium from silicate minerals have been attempted using a variety of acidic or weakly acidic additives including acetic acid [7], sulfuric acid [8], [9], [10], nitric acid and hydrochloric acid [11], ammonium nitrate, ammonium chloride, and ammonium acetate [12], ammonium bisulfate [13], [14], [15], and ammonium sulfate [16], [17], [18]. The extracted Ca and Mg were then carbonated with the aid of sodium hydroxide [11], [19] and ammonia [20], [21]. However, the recycling of most of these acidic and basic chemicals is either difficult or involves high energy consumption. When ammonium sulfate (AS) is used, it will decompose into acidic ammonium bisulfate and basic NH3 at over 250 °C and be regenerated after the subsequent mineral carbonation [22]. Eqs. (1) and (2) show the total reactions for the extraction and CO2 mineralisation, respectively.
Therefore, there have been several studies which have involved the use of easily recyclable ammonium sulfate or ammonium bisulfate + NH3 as additives to treat various silicate minerals, including serpentine [15], [16], [17], olivine [14], amphibole [14] and pyroxene [14], steel slag [23] and concrete aggregate [23].
The iron and steel industry is one of the largest industrial sources of emissions of CO2 and solid wastes. The main solid wastes include blast furnace (BF) slag and steel slag emitted in the iron-making and steel-making processes, respectively. Currently, approximately 300–1000 kg of BF slag is discharged per tonne of iron produced depending on the grade of the iron ores and the process conditions employed [24]. BF slag is a Ca- and Mg-rich silicate mineral, with CaO, MgO, and Al2O3 contents of 34%–52%, 6%–10%, and 10%–14%, respectively [19], [25]. In 2015, the global output of iron was approximately 1.6 billion tonnes [26]. If all of the Ca and Mg elements in the BF slag are utilised to fix CO2, theoretically, approximately 200–680 million tonnes of CO2 can be safely sequestered annually. Although the amounts are quite small in comparison to the capacity of natural mineral resources, using BF slag to store CO2 on site could be an inexpensive way to significantly reduce CO2 emissions for individual iron and steel plants. Meanwhile, a high-added-value by-product, namely Al(OH)3, an Al-rich precipitate, and carbonation products could be obtained for further use, which may improve the economy of the process. Al(OH)3 is mainly used as a raw material for the production of electrolytic aluminium. The global output of electrolytic aluminium was approximately 57.7 million tonnes in 2015 [27], and its production consumed approximately 120 million tonnes of natural bauxite (main constituents were Al(OH)3 or AlOOH) (calculated as Al2O3). If worldwide aluminium resources contained in BF slag could be recovered effectively, nearly the same amount of bauxite would be saved, thus leading to more sustainable use of natural resources.
Based on the above analysis, a process for combined carbonation of Ca and Mg extracted from BF slag using recyclable AS and recovery of high-added-value Al(OH)3 was proposed, and its schematic flowsheet is shown in Fig. 1. The present study focuses on the process parameters and efficiency of the roasting extraction, mineral carbonation, and recovery of aluminium. In addition, a preliminary economic analysis is also presented.
Section snippets
Materials
The water-quenched BF slag used in this research was provided by Zenith Steel Group Company Limited (Changzhou, China). The density and the specific surface area of the milled BF slag (D50 = 50.43 µm) are 1.3 g/cm3 and 0.512 m2/g respectively. The chemical composition analysed using X-ray fluorescence (XRF) is shown in Table 1.
X-ray diffraction (XRD) analysis showed that the as-received water-quenched BF slag was amorphous and could be converted into the crystalline state after annealing at 800
Effects of roasting conditions on extraction of Mg, Al, and Ca
The extraction ratios of Ca, Mg, and Al at different roasting temperatures are shown in Fig. 3. Clearly, all of the extraction ratios of the three elements increased monotonically with increasing roasting temperature, and beyond 350 °C, the reactions had been complete. It is well known [31] that the reaction of (NH4)2SO4 with silicates can be divided into two steps: (1) decomposition of (NH4)2SO4 into NH3 and NH4HSO4 and (2) further digestion of the silicates by NH4HSO4. The decomposition
Preliminary economic assessment
Based on the above experimental results, a preliminary analysis of the further utilisation and market values of the obtained carbonation products and Al-rich by-product was conducted. Approximately 1000 kg of product consisting of 60 wt% CaCO3 and 33 wt% SiO2 could be produced through carbonation of the leaching residue from 1 tonne of BF slag. This product could be used as a raw material for cement production to replace 600 and 330 kg of natural limestone and silica, respectively. Based on the
Conclusions
In the present study, a novel, facile process was proposed; in this process, BF slag is roasted with recyclable AS for the extraction of calcium, magnesium, and aluminium at an AS-to-slag mass ratio of 3:1 and at 370 °C. The extraction ratios of the three elements approached 100%. The NH3 released in the roasting was used to capture CO2 from flue gases to obtain NH4HCO3 and (NH4)2CO3. Carbonation of the leaching residue of the roasted slag with NH4HCO3 can enable the stable fixation of CO2 with
Acknowledgments
The authors are grateful for the financial support of the National Key R&D Program of China (2016YFB0600904).
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