Ca-bearing Minerals Research Articles

Aqueous carbonation of steel slags: A comparative study on mechanisms

This study investigated the aqueous carbonation mechanisms of three typical steel slags: ladle metallurgy furnace (LMF) slag containing high Al content, electric arc furnace (EAF) slag featuring high Si content and relatively low Al content, and ladle-arc fusion (LAF) slag with medium-Al content. It was found that the carbonation kinetics of the three slags were similar and followed the surface coverage model within the first 6 h of carbonation. Initially, the carbonation process was primarily governed by the reaction product precipitation. After 3 h of carbonation, the process was dominated by mineral dissolution, controlled by the uncovered reactive sites. The carbonation-reactive Ca-bearing minerals in the slags, including silicates (CRSis) and aluminates (CRAls), sequestered CO2 to form calcite during carbonation, accompanied by the formation of silica gel and alumina gel, respectively. CRSi, mainly larnite, was present in EAF and LAF slags, showing high reactivity, whereas mayenite (i.e., C12A7), a CRAl mineral present across all slags, exhibited high reactivity in LMF slag but lower reactivity in EAF and LAF slags. Furthermore, AFm phases and katoite (i.e., C3AH6) were detected in LMF slag as CRAls along with mayenite, and their carbonation reactivity decreased in the order of AFm>mayenite>katoite. As a result, low-Al steel slag tends to have higher carbonation reactivity, as manifested by the high carbonation degree of EAF slag throughout the reaction period, notably achieving a 40 % carbonation degree within 30 min and 71 % after 24 h under the studied conditions.

A retarded foam acid fluid system for low-temperature dolomite geothermal reservoir stimulation and its action mechanisms

Acidizing improves geothermal reservoir productivity by dissolving rocks to increase formation permeability, which is achieved by injecting acid fluids into reservoir formations. However, conventional acid fluids either can not achieve effective acidizing or cause geothermal reservoir damages. Foam acid fluids, which have been applied for oil-gas reservoir stimulation, can realize effective acidizing by retarding acid-rock reactions, and can protect reservoirs. Here, a foam acid fluid system is proposed, aiming at achieving efficient acidizing and reservoir protection in low-temperature dolomite geothermal reservoirs. Formic acid (FA) and acetic acid (HAc) are used to replace a portion of hydrochloride (HCl) acid. Alkyl glycine surfactant AGS-12 is added for foaming, while xanthan gum and nano silica particles are used to stabilize foams. Corrosion inhibitor and ferric ion stabilizer are added to inhibit acid corrosion of steel and ferric ion precipitations. The acidizing effect was evaluated through foam stability, dissolution performance, corrosion inhibition performance, and compatibility. Microscopic observation, field emission scanning electron microscope, and numerical simulation were applied to reveal the mechanisms of temporal evolution of foams and acid dissolution behaviors. Results show that the foam acid fluid with 5 wt% FA, 5 wt% HAc and 10 wt% HCl acid has the best foam stability and retardation effect. Compared to non-foam acid fluid, the decrease degree of the cuttings mass change rate in foam acid fluids exceeds 50 %, and foaming primarily retards acid-rock reactions. Simulation results is consistent with the experiment results, demonstrating that the cuttings dissolve less in FA and HAc. Calcium (Ca) content on the cuttings surfaces after acid-rock reactions in foam acid fluids with FA and HAc is less than that after acid-rock reactions in foam acid fluid with sole HCl acid, indicating that FA and HAc inhibit Ca-bearing mineral precipitations, as formate and acetate ions can chelate with Ca2+. The replacement of HCl acid by FA and HAc contributes to a low corrosivity and good compatibility of the foam acid fluids due to a reduced hydrogen ions resulted from partial dissociation of weak acids. This study shows the feasibility of foam acid fluids during geothermal reservoir stimulation with eliminated reservoir damages.

Laboratory experiments of carbon mineralization potential of the main terrestrial basalt reservoirs in China

Against the background of realizing the goal of “carbon peaking and carbon neutrality”, using basaltic rocks for carbon mineralization is one of the most promising approaches to reduce the rise in atmospheric CO2 concentrations. This study conducted a series of experiments to assess carbon mineralization in nine basalt samples from the main terrestrial basalt reservoirs in China within CO2-H2O/brine-rock systems at low temperatures (≤35 °C). The results indicate that the secondary carbonates formed in the CO2-H2O/brine-basalt system are predominantly calcite rather than Mg-carbonate minerals at low temperatures (≤35 °C). Hence, at low temperatures (≤35 °C), basalt rich in Ca-bearing minerals promotes the formation of stable carbonate minerals more effectively than basalt containing Mg-bearing minerals. Furthermore, under conditions of low temperatures (≤35 °C) and pressures of approximately 3 MPa, the results suggest that alkaline olivine basalt, with a higher content of Ca-minerals and typical alkaline minerals (nepheline and Na-sanidine), exhibits the highest pH value and the highest amount of calcite. Hence, the alkaline minerals, nepheline and Na-sanidine, serve as pH buffers to increase the pH and promote the precipitation of calcite within CO2-H2O– basalt systems at low temperatures (≤35 °C). Among the nine evaluated basalts, basalt from the Shandong Linqu-Changle volcanic basin exhibits the highest rate of carbon mineralization at low temperatures (≤35 °C). Hence, Cenozoic alkaline olivine basalt from Shandong Linqu-Changle volcanic basin is one of the most promising basalt reservoirs in China for future in- situ carbonation. As for ex- situ carbonation, compared with olivine, diopside or Ca-plagioclase may be more appropriate for increasing ocean negative emissions.

Tiebas Castle’ tuiles vernissées: Characterization, raw clay provenance, and manufacturing technology

Tiebas Castle was built between 1254 and 1264 as a royal residence of the kings of Navarre. The Castle was decorated with architectural luxuries imported from the French court. Some decorative elements of this French style are the polychrome roof tiles, called tuiles vernissées; and the glazed floor tiles, called carreaux de pavement. Both are the unique that have been found in the Iberian Peninsula. Elemental and mineralogical analysis allowed us to distinguish two different types of pastes that were used for both tuiles vernissées and carreaux de pavement: yellowish and reddish. The reddish paste was composed mainly of quartz, and to a lesser extent of haematite and illite. The yellow pastes were very rich in calcite and other calcium-bearing minerals (gehlenite, anorthite, diopside or wollastonite) and poorer in quartz and haematite. The different colour tones of the yellow paste samples allowed them to be classified into five subgroups (YP-1, YP-2, YP-3, YP-4, and YP-5). This classification turned out to coincide with a somewhat different mineral composition. The study of the mineral phases newly formed (gehlenite, anorthite, diopside and wollastonite) or destroyed (illite) during firing allowed us to estimate the maximum firing temperature of each of the subgroups. The temperature ranges for each subgroup were as follows: 750–800 °C (YP-5), 850–900 °C (YP-4), 900–925 °C (YP-3), 925–950 °C (YP-2), and 950–1000 °C (YP-1). The study of its possible raw materials allowed us to identify that the yellow pastes from tuiles vernissées and carreaux de pavement were prepared from a mixture of two clays. One of them was the decalcification clay (A15 clay) with which they also made the reddish pastes. The other component of the mixture was the marl from Castle hill. The proportion that the artisans used of both raw materials was 1:2 (twice as much marl as decalcification clay).

Fate of Ca2+ and metal impurities of a typical coal fly ash in amino acid mediated CO2 mineralization and simultaneous CaCO3 recovery

CO2 mineralization is a promising technology to achieve a waste-to-resource chain by integrating CO2 sequestration and alkaline solid waste utilization. Amino acids as a multifunctional reagent was proposed to avoid the extensive consumption of acid and alkaline reagents in traditional processes. However, the detailed interaction between Ca-bearing minerals and amino acids are still not fully understood, as well as migration pathways of metal impurities. To fill these gaps, this paper investigated the leaching and carbonation behaviors of Ca2+ and metal impurities of a typical coal fly ash (CFA) at various reagent types, reagent concentrations and leaching temperatures, thereby exploring the fate of these metal impurities. Results showed that the leaching and carbonation of Ca2+ was influenced by various aspects such as initial pH, reagent pKa, complexation, and pH buffering. The CaCO3 products obtained from glycine (Gly) and alanine (Ala) were vaterite due to the stabilizing effect of these amino acids on vaterite preventing it from converting to calcite. The optimal reaction conditions were 1.0 M Gly and leaching temperature of 35 °C with 26.63 % leaching efficiency, 56.08 % carbonation efficiency, and 42.6 g/kg vaterite yield. In addition, Ala induced an unexpected high leaching efficiency of Cd (75.79 %), Pb (76.08 %), and Zn (41.62 %), respectively, indicating promising mitigation of the environment risk of leached residue and valuable metal recovery from CFA. Although Gly displayed limited metal leaching efficiency at 1.0 M and 25 °C, increasing leaching temperature induced a remarkable enhancement of Cd and Pb leaching, which were 88.54 % and near 100 % at 35 °C, indicating promising mitigation of the environment risk of leached CFA. However, the high binding affinity of amino acids to impurity metals led to the low purity of CaCO3 product. This study derives the migration pathways of various metal ions during the mineralization and utilization of CFA, making it of great significance for the resource management and utilization of solid wastes.

Amino acid assisted Ca2+ leaching-CO2 mineralization: Thermodynamic equilibrium analysis and experimental investigation

An amino acid-mediated Ca2+ leaching-CO2 mineralization process was proposed to address the challenges associated with slow kinetics and extensive consumption of exogenous reagents in CO2 mineralization. While the underlying reaction mechanism between amino acid and alkaline minerals/metals remains unclear, this study explored the interactions between amino acid-species and Ca-bearing minerals/Ca2+ by investigating the evolution of active compounds from the perspectives of thermodynamic equilibrium analysis and experimental study. Thermodynamic equilibrium analysis showed that the active Ca-bearing minerals of coal fly ash (CFA) varied in the systems of ammonium salt, acid, and amino acid, e.g., the active phases in glycine were portlandite (Ca(OH)2), lime (CaO), and gehlenite (2CaO*Al2O3*SiO2). High Ca2+ concentrations of 0.26 M, 0.14 M, and 0.1 M can be achieved after 3 min of leaching assisted by amino acids of Glycine (Gly), L-Alanine (Ala), and L-Arginine (Arg), respectively, which implied the kinetics and thermodynamical feasibility. This phenomenon was demonstrated to relate to the fact that Gly and Ala had minimal pH buffering capacity in the early leaching stage. The deprotonated forms of Gly and Ala with a higher affinity with Ca2+ increased from 0.03 % and 0.04 % to 33.9 % and 12.88 %, respectively, thereby promoting Ca2+ leaching. The Ca2+ utilization efficiencies from CFA were 26.45 %, 15.0 %, and 13.3 % for Gly, Ala, and Arg, respectively. Gly achieved the best CaCO3 yield of 114.8 g/kg, due to the combined effects of Gly’s buffering capacity and strong chelation effect for Ca2+, which drove forward the dissolution of the active phase and precipitation of CaCO3.

Understanding impacts of introducing CO2 in N2 and operation conditions on physicochemical property and fusion behaviour of solid products during thermal decomposition of corn stalks

The total annual production of corn stalk (CS), a byproduct of corn processing, in China was as high as 3.1 × 108 tons, the utilization of which had great potential. The objective of this study was to investigate the characterization of the resulting solids during thermal conversion of CS under different atmospheres and to deepen the understanding of the effects of introducing CO2 in N2. The impacts of temperature, atmosphere, and particle residence time on the solid yield, ash characteristics were elucidated. It was found that the solid yields decreased with increasing temperature. For a given temperature below 600 °C, the solid yields in 20% CO2/80% N2 were higher than those in N2. But when the temperature was above 600 °C, the opposite trend was observed. Increasing residence time substantially improved carbon conversion. Compared with the solids formed below 600 °C, the 800 °C solids contained more agglomerates, which were composed of K-bearing and Ca-bearing minerals. The ash fusion temperature (AFT) increased with rising pyrolysis/gasification temperature. After substituting N2 with CO2 in the pyrolysis agent, the AFT became higher. Under the current conditions, ash deposition could occur even if the operating temperature was lower than the softening temperature (ST) due to the formation of low melting eutectics.

Enhancement of H2 and light oil production and CO2 emission mitigation during co-pyrolysis of oily sludge and incineration fly ash

The proper treatment and utilization of oily sludge (OS) and incineration fly ash (IFA) remains a significant challenge due to their hazardous nature. To attain effective recovery of petroleum hydrocarbons and synergistic disposal, this study investigated the co-pyrolysis of OS and IFA, resulting in successful energy recovery, CO2 mitigation, and heavy metal immobilization. Results revealed that the peak ratio of light oil to heavy oil fractions reached 179.42% with 20 wt% IFA addition, accompanied by the highest aromatic hydrocarbons selectivity of 30.72% and the lowest coke yield of 106.13 mg/g OS under the optimal temperature of 600 °C. In-depth analysis indicated that IFA inhibited the poly-condensation of macromolecular PAHs and promoted their cracking into light aromatic hydrocarbons. The addition of 50 wt% IFA significantly increased H2 yield (21.02 L/kg OS to 60.95 L/kg OS) and facilitated CO2 sequestration due to its higher content of Ca-bearing minerals. Moreover, high IFA ratios promoted the reduction of Fe species in OS to a low-valence state. Heavy metals in co-pyrolysis char were well immobilized into stable fractions with lower environmental risks. This work highlights the potential of co-pyrolysis as a viable approach for simultaneous disposal of multiple hazardous wastes and offers new insights for their utilization.

Experimental determination of the reactivity of basalts as a function of their degree of alteration

The element release rates from naturally altered basalts were measured in batch experiments at 27 °C, and in mixed-flow experiments at 25 °C in reactive fluids initially consisting of pH 3 or 4 aqueous solutions. The basalts reacted in this study consist of 1) surface weathered basalts ranging in age up to 13 million years, and 2) hydrothermally altered basalts from zeolite to actinolite alteration zones (temperatures of ∼50 to 300 °C). The in situ pH in the batch experiments increased over time with the final pH of the fluids ranging from to 4 to 8. Most experiments exhibited a preferential release of Ca and Mg compared to Si from the dissolving basalts, both initially and over the long-term. This behavior is attributed to the combination of an initial rapid cation-proton exchange reaction and the relatively fast dissolution rates of Ca-bearing minerals. The BET surface area normalized Si release rates for all of the altered basalts are within one order of magnitude of each other, and from one to two orders of magnitude lower than corresponding rates from fresh basaltic glass and fresh crystalline basalts. This is interpreted to stem from 1) the fast removal of reactive basaltic glass and olivine from fresh basalts during the natural alteration of the rocks, 2) the similarity of the dissolution rates of the remaining albitic plagioclase, pyroxenes, epidote, and zeolites, and 3) the high surface areas of some less reactive secondary minerals such as clays. Overall, the results suggest that altered basalts will be effective in consuming CO2 during both natural and engineered processes, though at a somewhat slower rate than unaltered basalts.

Large seasonal and spatial variability in δ44/40Ca values in the Godavari River, India: Implications for basalt weathering, groundwater interaction, and carbonate precipitation

We present stable calcium isotopic compositions (δ44/40Ca) of the Godavari River, the largest river in peninsular India, from the upper 750 km of its course, where it primarily drains the Deccan basalts. In addition to river water samples collected during the pre-monsoon and monsoon seasons, we also present δ44/40Ca values of groundwater, bedrock Deccan basalts, suspended and bedload sediments, and carbonate nodules from the Godavari Basin as well as zeolites and hydrothermal calcite from several locations within the Deccan Traps. The δ44/40Ca values (reported relative to NIST SRM 915a) of the dissolved load of the Godavari River main channel display large variability during both the pre-monsoon (0.73–1.64 ‰) and the monsoon (0.56–1.28 ‰) seasons. In the upper reaches (within 160 km of the source), the river water samples during both seasons show δ44/40Ca values (0.56–0.89 ‰) and Sr/Ca (mmol/mol) ratios (1.47–2.86), which are in the same range as that of the bedrock Deccan basalts (0.53–0.78 ‰ and 1.1–5.19 mmol/mol, respectively), suggesting control of water chemistry by basalt weathering. In contrast, in the middle reaches of the river (beyond 160 km from the source), the riverine δ44/40Ca values (0.99–1.64 ‰) are much higher than δ44/40Ca of bedrock basalt, but similar to that of groundwater in the middle reaches. We conclude that groundwater discharge to the river during the low-flow period has a significant control on the observed high δ44/40Ca values of the Godavari River. Additionally, the positive correlation between δ44/40Ca values versus Sr/Ca ratios and weak positive correlation of δ44/40Ca values versus saturation index of calcite suggests that carbonate precipitation from the river water impacts the δ44/40Ca values of the Godavari River in the middle reaches; this conclusion is consistent with the low δ44/40Ca of acetic-acid leached carbonate nodules (0.52–0.56‰) collected from the Godavari riverbed.The δ44/40Ca values of zeolites exhibit a large range from −0.72‰ (heulandite) to 1.67‰ (apophyllite) while the δ44/40Ca values of leached hydrothermal calcite samples range from 0.75‰ to 0.92‰. These values indicate that weathering of these Ca-bearing minerals has limited contribution to the calcium isotopic composition of the Godavari River, which contrasts with observations from relatively smaller rivers draining basalt in Iceland. The δ44/40Ca values of the dissolved load of the basalt-draining Godavari River is significantly higher than the estimated global riverine input to the oceans (0.88‰); given the high weatherability of basalts, the high δ44/40Ca values of basalt-draining rivers suggests that the estimate of the δ44/40Ca value of the average continental flux to the oceans may have been underestimated. The results of this study also demonstrate distinct differences in geochemical and Ca isotopic compositions of basalt-draining rivers from Iceland and India, which reflects fundamental differences in the weathering process in very different climatic regions.

Effects of co-combustion Fe-rich mediocre ash coal on slag deposition of Ca-rich Zhundong coal in a 0.4 MW pilot-scale facility

A type of coal with low calorific value and high ash content, locally known as mediocre ash coal (MAC), has been discovered in the interlayer of stratified coals in Jiang Jun Temple area Zhundong district, Xinjiang, China. In order to investigate slag deposition on the convective heating surface during co-combustion of MAC and Wucaiwan Ca-rich Zhundong Coal (WCW), a 0.4 MW pilot-scale facility, with tube panels #1 and #2 located in horizontal flue gas ducts, was constructed. The tube panels were designed to simulate the superheater and reheater of the Π-type boiler. FactSage was utilized to simulate the material balance, whereas the classic models were employed to calculate the slag viscosity. Combustion tests were conducted while maintaining flue gas temperatures (FGT) of 1050∼1100 °C and 900∼950 °C at tube panels #1 and #2, respectively, which were consistent with those observed during sole combustion of WCW. The slags collected from tube panels were analyzed by X-ray fluorescence, X-ray diffraction and Scanning Electron Microscopy coupled with Energy Dispersive X-ray Detector. Furthermore, Computer-Controlled Scanning Electron Microscopy analysis was used to obtain a quantitative characterization of the symbiotic relationship among mineral elements. The study of co-combustion MAC led to the following conclusions: The liquid phase ratio of slag decreased by 44% compared to sole combustion of WCW at 1100 °C. The viscosity was higher than that of sole combustion of WCW from 900 to 1650 °C. Compared to the sole combustion of WCW, the average S, Ca, and Na contents in the slag obtained on the two tube panels decreased by more than 85%, 40%, and 43.5% respectively. Sulfate formation in the slag on the tube panel was significantly reduced at medium to low FGT, and no Na2SO4 and CaSO4 compounds could be detected in the slag. The interaction between Fe-bearing minerals and Si- and Al-bearing minerals during combustion was evidently stronger than that between Ca-bearing minerals and Si- and Al-bearing minerals during combustion. The interaction of Fe with Ca/Na during medium to high FGTs led to a decrease in Si/ Al consumption, thereby increasing their availability for reacting with Ca/Na at medium to low FGTs. As a result, the formation of Na2SO4 and CaSO4 was inhibited.

Seasonal and Spatial Distribution of Phosphorus Fractions in Surface Sediments of the Southern Caspian Sea

This study assesses seasonal fluctuations, spatial distribution and fractions of phosphorus (P) in the surface sediment layer of the southern Caspian Sea. Seasonal fluctuations were insignificant (p-value > 0.05) relative to the mean total P (TP) concentrations. Still, the highest levels were in autumn samples (1555 mg kg−1), followed by winter (1405 mg kg−1), spring (1378 mg kg−1), and summer (1130 mg kg−1). These minor temporal fluctuations in P concentrations are explained by seasonal differences in runoff amount and intensity of rivers discharging into the Caspian sea and thereby their sediment loading and physicochemical characteristics. The large riverine influx has led to TP contamination hotspots in the river deltas of Anzali wetland, Babolrood, and Sefidrud, where high loadings of suspended particles are discharged into the sea. The spatial distribution of TP is thus site-specific and uneven. The main P fraction was calcium-bound P (CaP), reflecting the phosphate (PO43−) strong affinity for, and association with, Ca-bearing minerals. Only a minor fraction of P was determined as loosely bound P (LP). The fraction of the mud size particles was the main explanatory factor for the spatial distribution of overall low levels of non-residual (or bioavailable) P forms (i.e., LP and iron- and aluminum- bound P: FeP and AlP, respectively) during spring and summer, while the sand fraction had strongest explanatory value for the distribution of residual (non-bioavailable) P form (CaP) during autumn and winter. This study demonstrates that P bioavailability in sediments is mainly governed by the physicochemical characteristics of the sediment material, which again is steered by seawater chemistry. A low content of bioavailable P fractions could therefore be explained by the relatively low content of fine-grained (< 63 µm, i.e., mud) particles in sediments of the southern Caspian Sea.

Leaching of heavy metal(loid)s from historical Pb–Zn mining tailing in abandoned tailing deposit: Up-flow column and batch tests

This study aims to investigate the water-leaching characteristics of heavy metal(loid)s (HMs) from historical Pb–Zn mine tailing of an abandoned tailing deposit in eastern China. Up-flow column and batch leaching tests were conducted at different liquid-to-solid (L/S) ratios to estimate the releases of HMs and investigate the controlling mechanisms. Calcite and silicate were the dominant minerals in the tailing and the HMs contents followed the order of Zn (2371 mg/kg) > Pb (2061 mg/kg) > Cu (109 mg/kg) > Cr (47.8 mg/kg) > As (15.9 mg/kg) > Cd (5.1 mg/kg). Moreover, considerable fractions of Pb, Zn, and Cd existed in the acid-soluble forms (41–47%). Column and batch leaching tests consistently showed that limited quantities (<0.002%) of HMs could be leached from this historical tailing. In particular, variations in column conditions (e.g., length, flow rate, and initial saturation) significantly affected the release fluxes from the columns but had a relatively limited effect on the leaching mechanisms. The estimated results of HM release suggested that the leaching process was predominantly solubility-controlled and the dissolution of Ca-bearing minerals (e.g., calcite) primarily controlled the release of HMs. The studied tailing had a limited impact on the quality of the surrounding aquatic environments because the water-leaching concentrations of HMs were generally lower than the Chinese standards for drinking water. Only for Pb, the leaching results in column tests were significantly lower than those in batch tests; whereas the results in column tests for other HMs were comparable to those in batch tests to a certain extent. Based on the column test results, the amounts of HMs potentially released from the abandoned tailing deposit (height, 10 m; footprint area, 30,000 m2; tailing dry density, 1.9 × 103 kg/m3) followed a decreasing order of Zn (4.2 × 105 kg) > Cu (2.3 × 104 kg) > Pb (1.4 × 104 kg) > Cr (2.3 × 104 kg) > Cd (1.6 × 103 kg) > As (1.2 × 103 kg) over the 75-year assessment period (corresponding to an L/S ratio of 10 L/kg with an annual precipitation of 1500 mm).

Preliminary Assessment of Anomalously High Background Radioactivity in Makinit Hot Spring, El Nido, Philippines

Hot springs in the Philippines are found mostly near volcanic regions and are the least places with possibly high background radiation. However, Makinit hot spring in El Nido, Palawan exists not only on the tectonically stable northern Palawan Microcontinental Block but also records anomalously high natural radioactivity. To quantify and determine the nature of radioactivity, this study conducted a preliminary radiological and geochemical study in the vicinity of this hot spring. The measured average specific activities of uranium and thorium were 1528.62 and 920.96 Bq/kg, and these exceed world median values on terrestrial radionuclides in the soil of 35 and 30 Bq/kg by 44 and 31 times higher, respectively. The average values of all seven calculated radiological risk parameters – namely, dose rate with an amount of 13.77 mSv/y, radium equivalent of 2862.10 Bq/kg, external and internal hazard indices of 7.73 and 11.86, respectively, absorbed gamma dose rate of 1271.42 nGy/h, annual effective dose equivalent of 1.56 mSv/y, and excess lifetime cancer risk of 5.46 x 10–3 – go beyond reference limits set by the UNSCEAR. The preliminary assessment of spring water physicochemical properties showed that the water temperature is greater than 55 °C and the pH averages around 6.62, indicating weak acid-neutral hot waters and a low to moderate temperature geothermal system. The sediments surrounding the Makinit hot spring were analyzed to be travertine, composed only of calcite, suggesting that the uranium and thorium ions are potentially bonded or are associated with the Ca-bearing mineral. This study can serve as a baseline for further research regarding the source of radioactivity and geothermal heat source and as a consideration for the development of the Makinit hot spring area.

Evaluation of the carbon sequestration potential of steel slag in China based on theoretical and experimental labile Ca

Mineral carbonation with steel slag reduces greenhouse gas emissions and steel slag stockpiles. The mineral carbonation potential of steel slag was investigated by comparing fresh and weathered steel slag. Brownmillerite was discovered to be the most abundant inactive Ca-bearing mineral in steel slag, and the theoretical labile Ca was calculated using this information. The experimental labile Ca was studied using flow-through and batch experiments at ambient and CO2-rich conditions; the theoretical and experimental labile Ca agree well and are both at around 80%, indicating high mineral carbonation reactivity. The experimental results are also well supported by PHREEQC modeling, charge balance, and mass balance. We discovered that each ton of steel slag can capture approximately 250 kg of CO2. Furthermore, 450 Mt CO2 could be directly captured via steel slag CO2 mineralization. We offer dependable experimental and analytical methods for evaluating the mineral carbonation potential of steel slag.

The factors controlling equilibrium inter-mineral Ca isotope fractionation: Insights from first-principles calculations

Equilibrium isotope fractionation factors are crucial to quantitatively interpreting Ca isotope data of natural samples. Recent studies have revealed significant equilibrium Ca isotope fractionation between minerals, but the controlling factors remain poorly understood. Using density functional theory, this study calculates the reduced partition function ratios (RPFRs) among amphiboles (richterite and tremolite), sorosilicates (akermanite and gehlenite), K-bearing carbonates (butschliite), Na-bearing diopside (jadeite), and other Ca-bearing minerals (fluorapatite, anhydrite, CaTiO3 perovskite, and fluorite) at 0 GPa. The RPFRs of diopside and anorthite over a pressure range from 0 to 5 GPa are calculated to investigate the pressure effect. The effect of force constant, bond length, coordination number, anion type, solid solution composition, and pressure on RPFRs are discussed by compiling the literature data. The RPFRs show good correlations with the force constant and bond length but have no clear correlation with the coordination number. This suggests that the bond length is more reliable than the coordination number for roughly predicting the signs and magnitudes of inter-mineral isotope fractionation. Na-bearing clinopyroxene (e.g., jadeite) has similar RPFR with Na-free clinopyroxene, suggesting that the jadeite effect should be insignificant in natural samples. The RPFR of anorthite is lower than that of diopside at low pressure, and the two minerals both show a positive correlation between RPFR and pressure. Notably, the heavy Ca isotope enrichment between these two minerals can be reversed at high pressure (>3 GPa at 1000 K) because anorthite’s RPFR changes more sharply with increasing pressure than that of diopside. However, such a reverse may not occur in Earth's modern crust due to the stability of anorthite at lower pressures. Combining the theoretical predictions of amphibole and plagioclase and natural sample observation on granitoids, we infer that the RPFR of granitic magma may be lower than that of basaltic magma.

Application and uncertainty of a geochemical speciation model for predicting oxyanion leaching from coal fly ash under different controlling mechanisms

Three primary mechanisms (adsorption to iron oxides or analogous surfaces, co-precipitation with Ca, and substitution in ettringite) controlling oxyanion retention in coal fly ashes (CFAs) were identified by differentiating the leaching behavior of As, B, Cr, Mo, Se, and V from 30 CFAs. Fidelity evaluation of geochemical speciation modeling focused on six reference CFAs representing a range of CFA compositions, whereby different leaching-controlling mechanisms of oxyanions were systematically considered. For three reference CFAs with low Ca and S content, calibration of adsorption reactions for the diffuse double-layer model for hydrous ferric oxides improved the simultaneous prediction of oxyanion leaching, which reduced uncertainties in Se and V predictions caused by nonideal adsorption surfaces and competitive adsorption effects. For two reference CFAs with intermediate Ca content, the solubility constants for Ca-arsenates from literature and postulated phases of B, Cr, Se, and V were used to describe co-precipitation of oxyanions with Ca-bearing minerals under alkaline conditions. For the reference CFA with high Ca and S content, an ettringite solid solution was used to capture the simultaneous retention of all oxyanions at pH> 9.5. Overall, the simultaneous leaching predictions of oxyanions from a wide range of CFAs were improved by calibration of adsorption reactions and controlling solid phases.

Reactions and transformations of mineral matters during entrained flow coal gasification using oxygen-enriched air

Clarification of the reactions and transformations mechanisms of mineral matters during entrained flow gasification using oxygen-enriched air is necessary for the reliability of the gasifier. To explore the impacts of different gasification temperatures (1000 and 1200 °C) and oxygen concentrations (21, 50, 100 vol % O2 in N2) on the mineral behaviors, the gasified chars obtained from a pilot-scale gasifier were characterized by the means of ICP-OES, XRD, PSD, SEM-EDX, and AFT methods. Results indicated that gasification with oxygen-enriched air not only promoted the retention of inorganic elements in the solid phase but also changed the mineral types and contents. The main components of gasified chars were SiO2 and Ca-bearing minerals, and the increase in gasification temperature and oxygen concentration would promote the transformation of CaCO3 and CaSO4 to CaO, CaS, and Ca2Al2SiO7. With increasing oxygen concentration under 1000 °C gasification, the average particle size of gasified chars was getting smaller and the high-melting mineral Al6Si2O13 was likely to form, leading to the increase in the fusion temperature. However, when the gasification temperature rose to 1200 °C, the re-carbonation of CaCO3 was largely promoted with the increase of oxygen concentration due to the more abundant CO2 in the reaction atmosphere. Meanwhile, the morphologies of apparent agglomerate and sintering began to appear in the 1200 °C gasified chars, leading to a significant increase in particle size. It is deduced that the enrichment of Ca and Fe in the 1200 °C gasified chars would accelerate the formation of low-temperature eutectic mixtures, thus reducing the ash fusion temperatures of 1200 °C samples. Overall, the ash fusion temperatures of the gasified chars are closely linked to mineral composition, particle size distribution, and agglomeration behavior during the gasification process.

Separation of alumina and silica from metakaolinite by reduction roasting−alkaline leaching process: Effect of CaSO4 and CaO

Limestone (CaCO3), which could promote sulfur fixation, was added to coal gangue during roasting in a circulating fluidized bed (CFB) boiler. CaO and CaSO4 were the main Ca-bearing minerals while metakaolinite was the major Al-bearing mineral in CFB slag. The effect of CaSO4 and CaO on the separation of alumina and silica from metakaolinite by reduction roasting−alkaline leaching process was studied. Results showed that metakaolinite was completely converted into hercynite and silica solid solutions (i.e., quartz and cristobalite solid solutions) by reduction roasting with hematite. More than 95% of silica in the reduced specimen was removed by alkaline leaching. The addition of CaSO4 and CaO remarkably decreased the separation efficiency of alumina and silica in metakaolinite, which could be attributed to the formation of Si-bearing minerals: (1) Fayalite and anorthite were formed during the reduction roasting process; (2) Fayalite was stable while anorthite was converted into sodalite and wollastonite during the alkaline leaching process. This study demonstrates that sulfur in coal gangue should be fixed by treating the exhaust gas instead of controlling the combustion process of CFB to achieve the comprehensive recovery of silica and alumina from the CFB slag.

Silicate weathering contributed to arsenic enrichment in geotherm-affected groundwater in Pliocene aquifers of the Guide basin, China

High arsenic (As) groundwater in reduced shallow Holocene and Pleistocene aquifers has been intensively investigated, but the occurrence and the genesis mechanisms of high As groundwater in deep Pliocene aquifers affected by geothermal activity still remain unclear. To address these issues, geochemical characteristics of groundwater and aquifer sediments in both middle-Pliocene aquifers and Quaternary aquifers of the Guide basin were investigated to clarify groundwater-sediments interaction and the causes of As enrichment in groundwater from middle-Pliocene aquifer. Higher As and Na+ concentrations were observed in groundwater from middle-Pliocene aquifer (GPA) than those in groundwater from Quaternary aquifer (GQA), while GPA had lower Ca2+ concentrations than GQA. Results showed that middle-Pliocene aquifer sediments had low contents of carbonate minerals, and water-soluble Ca-bearing minerals relative to Quaternary aquifer sediments, which explain higher concentration of Ca2+ in GQA than in GPA. Na+ from weathering of silicates (i.e. (Na+)*), being calculated based on mass balance, accounted for high proportion of dissolved Na+ (up to 68%) in GPA. Weathering of silicates was related to As accumulation in GPA, which was proved by a positive correlation between As and (Na+)* in GPA, and high proportion of As bound to unweathered silicates (up to 65.7%) in middle-Pliocene aquifer sediments. The weathering of silicate minerals directly released As bound to silicates into GPA, and indirectly led to As desorption from solid surfaces by increasing pH, HCO3– and CO32–. Both (Na+)* and As in groundwater increased with the increasing groundwater temperature, showing that high temperature was conducive to weathering of silicates and As enrichment. This paper establishes a bridge between high groundwater temperature and high As concentration with weathering of silicates in aquifers.