Alkali Removal Research Articles

Optimized ceramic membrane-based method for efficient and acid-free rice protein preparation from alkaline extracts

Rice protein, stands out as a high-quality plant-based protein, suitable for dietary supplementation and food processing. Traditional methods, involving alkaline extraction and acid precipitation, are challenged by the extensive use of chemicals and the difficulty in desalination. This study proposed a novel separation method that utilizes ceramic membranes for direct alkali filtration, eliminating the formation of salts by acid neutralization. The membrane pore size and the operating parameters such as transmembrane pressure, cross-flow velocity, and pH were optimized to enable a high flux (>80 L m−2·h−1) and favorable protein rejection (>95 %) while permitting the removal of excess alkali. Additionally, the effects of non-protein substances, e.g., starch, present in the alkaline extract on the separation performance were explored. Subsequently, an optimized strategy for starch removal was proposed, involving the concentration of the alkali extract using an ultrafiltration membrane, followed by enzymatic hydrolysis of starch and diafiltration to eliminate the residuals of starch hydrolysates and alkali. Under stabilization of protein particles by carboxymethyl cellulose, rice protein with a uniform particle size of 25 nm and a purity of over 80 % was produced. The direct removal of alkali from alkali extracts of rice by ceramic membranes presents a promising strategy for producing rice protein with high purity and reduced aggregation degree, enabling better functionalities such as solubility than the conventional alkali-extraction and acid-precipitation method.

Catalytic pyrolysis of wheat straw based on dual catalyst CaO/ZSM-5 with acid washing and torrefaction pretreatment to enhance aromatic yield in bio-oils

In this paper, the catalytic pyrolysis integrating combined pretreatment (acid washing and torrefaction) and dual catalysts was adopted to improve the quality of bio-oil and the selectivity of aromatic hydrocarbons. Combined pretreatment is a highly effective method to improve the quality of biomass feedstock. It was capable of the removal of alkali and alkaline earth metals from wheat straw, with a K removal rate of 97.79 %. Under the combined pretreatment, the aromatic hydrocarbon content in bio-oil increased to 68.8 % using the ZSM-5 catalyst alone. Compared with ZSM-5, CaO could remove part of the oxygenated functional groups and had a better acid removal effect, but the aromatic hydrocarbon yield was low to 7.95 %. After combined pretreatment using simulated aqueous phase bio-oil for acid washing, catalytic pyrolysis using CaO/ZSM-5 dual catalysts greatly enhanced the quality of the bio-oil. The oxygenated compounds content was reduced to 17.58 %, and the total hydrocarbon yield was increased to 82.42 %, especially the aromatic hydrocarbons yield was increased to 79.91 %, of which the monocyclic aromatic hydrocarbons were as high as 68.38 %, and the benzene, toluene, and xylene content reaching 49.39 %. Thus, integrating dual catalysts (CaO/ZSM-5) with combined pretreatment can effectively increase the aromatic yield for producing high-quality bio-oil.

Integration of lactic acid biorefinery with treatment of red mud from alumina refinery: win–win paradigm for waste valorization

The cost of detoxification and neutralization poses certain challenges to the development of an economically viable lactic acid biorefinery with lignocellulosic biomass as feedstock. Herein, red mud, an alkaline waste, was explored as both a detoxifying agent and a neutralizer. Red mud treatment of lignocellulosic hydrolysate effectively removed the inhibitors generated in dilute acid pretreatment, improving the lactic acid productivity from 1.0 g/L·h−1 to 1.9 g/L·h−1 in later fermentation. In addition, red mud could replace CaCO3 as a neutralizer in lactic acid fermentation, which in turn enabled simultaneous bioleaching of valuable metals (Sc, Y, Nd, and Al) from red mud. The neutralization of alkali in red mud by acids retained in lignocellulosic hydrolysate and lactic acid produced from fermentation led to effective dealkalization, rendering a maximum alkali removal efficiency of 92.2 %. Overall, this study offered a win–win strategy for the valorization of both lignocellulosic biomass and red mud.

Experimental study on the dealkalization of red mud using the freeze-thaw and acid washing method

The strong alkalinity of red mud is a key reason that threatens the environment and restricts the secondary use of red mud. Freeze-thaw (FT)-acid washing (AW) is proposed to remove alkali to improve the dealkalization efficiency of red mud. FT-AW tests were performed using red mud with oxalic acid (OA) and citric acid (CA) to examine the effect of FT cycles and eluent concentration on the dealkalization efficiency. Results showed that the FT-AW method significantly increased the dealkalization efficiency under the synergy of FT and AW. After 5 times of FT-AW, the alkali removal efficiencies of red mud with OA and CA as eluents reached 97.5% and 95.5%, which were 27.3% and 18.9% higher than those of the control group, respectively. The effect of OA on dealkalization was more sensitive than that of CA. The results of SEM-EDS and XRD showed that FT cycles destroyed the crystal structure of red mud and released part of the bonded alkali. The FT-AW method provides a novel idea for future large-scale red mud dealkalization by using FT alternation in seasonally frozen soil areas.

Simultaneous dealkalization of red mud and recovery of valuable metals by a novel marine fungus tolerating high alkalinity and salinity

Red mud is a highly saline and alkaline solid waste generated in alumina refinery, which entails dealkalization for safe disposal and valorization. Herein, Trichocomaceae sp. K-2, a novel fungus isolated from marine sediment, was used for the simultaneous dealkalization of red mud and recovery of valuable metals. The fungus demonstrated robust growth at high salinity (45 g/L NaCl), good tolerance to high pH (up to 12), and capability of utilizing low-cost substrates derived from lignocellulosic biomass. Both structural and free alkali can be removed by Trichocomaceae sp. K-2 to achieve a significantly lower minimum pH (<4) in a considerably short timeframe (<16 days), and the alkali removal efficiency as high as 94% was attained. Moreover, concomitant leaching of valuable Nd, Sc and Y from red mud during dealkalization was feasible. Physical disintegration and chemical erosion by the secreted organic acids were responsible for the effective dealkalization, as well as leaching of valuable metals. This study sheds lights on how marine microorganisms can be harnessed to the green disposal of red mud, and provokes the exploration of more robust and versatile marine microbes for the sustainable and cost-effective management of other highly saline and alkaline solid waste.

Effects of various pretreatments on fast pyrolysis of biomass to bio-oil: A case study of levoglucosan

Levoglucosan (LG), a valuable chemical derived from carbohydrates, is challenging to obtain with high yield directly from raw biomass. In this study, pyrolysis of a lignocellulosic biomass to LG was studied with various pretreatment methods, including acid, alkali, and torrefaction pretreatments. The results indicated that the hydrochloric acid washing effectively promoted the production of LG when the concentration of HCl was lower than 0.5 mol/L, mainly caused by the improved crystallinity of the cellulose fraction and the removal of alkali and alkaline earth metals (AAEMs). When 4% of NaOH solution was employed for the alkali pretreatment, the relative content of LG significantly increased from 3.07 to 73.59 with the temperature ranging from 130 to 170 ℃, owing to the effective removal of lignin and hemicellulose fractions. Biomass torrefaction at temperature lower than 250 ℃ slightly promoted LG production, but it inhibited the formation of LG when the temperature reached 300 ℃.

Microstructures in the Baima layered intrusion, SW China: Constraints on the late-stage magmatic processes during solidification of a crystal mush

Magmatic microstructures in layered intrusions record key petrographic evidence for the late-stage magmatic processes that took place during solidification of a crystal mush. Systematic examination of the magmatic microstructures can reveal dynamic evolution of interstitial liquid in crystal mush, which however has been only conducted in a few studies on layered intrusions by far. In order to constrain the late-stage magmatic processes related to the Fe-Ti oxide-bearing layered intrusions, we investigated the magmatic microstructures of the Baima intrusion in the Panxi region (SW China). The intrusion is divided into three major units from the base upwards, including the lower zone (LZ), middle zone (MZ) and upper zone (UZ). Through detailed examination of a series of drill core samples, we found several different types of reactive microstructures, such as replacive symplectites, fish-hook pyroxene/olivine/amphibole and amphibole rim. Replacive symplectites include fine-grained, lamellar intergrowths of An-rich plagioclase + olivine/clinopyroxene/amphibole/phlogopite/biotite that are rooted to Fe-Ti oxides and replacing plagioclase primocrysts (Type 1) and intergrowths of An-rich plagioclase + orthopyroxene/olivine/amphibole that are rooted to olivine and replacing plagioclase primocrysts (Type 2). These microstructures are particularly abundant in the LZ except for the Type 2 symplectites, which are rare throughout the intrusion. An-rich plagioclase in the replacive symplectites and fish-hook textures have An contents up to 96, much higher than those of plagioclase primocrysts. Overall, fish-hook amphibole and amphibole rims contain 0.19−4.24 wt.% TiO2, higher than those of amphibole lamellae in the replacive symplectites that varying from 2.38 wt.% TiO2 to below the detection limit. These microstructures were developed during solidification of the intrusion due to the reaction between plagioclase primocrysts and ambient Fe-rich liquid, accompanying with addition of Fe, Ti and Ca and removal of alkalis and silica in an open system. The abundance of such reactive microstructures in the LZ are therefore attributed to silicate liquid immiscibility within a crystal mush, in which the segregation of the buoyant Si-rich component left behind the dense Fe-rich liquid in the lower part of magma chamber. The common presence of amphibole and phlogopite/biotite in the replacive symplectites indicates that continuous fractionation of the Fe-rich liquid leads to hydration of interstitial liquid during the late-stage solidification.

Analysis of Alkali in Bayer Red Mud: Content and Occurrence State in Different Structures

The application of large amounts of red mud in the field of building materials is one of the main ways to reuse this material, but the high alkali content of red mud limits its application. In this paper, the washable alkali, removable alkali, and lattice alkali contents of Bayer red mud were studied, and the occurrence states of potassium and sodium in red mud were studied using XRD, IR, XPS, and NMR. On this basis, the removal mechanism for potassium and sodium in red mud was analyzed. The results showed that the Na in the red mud was mainly deposited in the shelf silicon voids of hydroxy sodalite (Na8(AlSiO4)6(OH)2(H2O)2) in the form of Si-O-Na or Al-O-Na. K is deposited in the shelf silico-oxygen void of potassium feldspar (KAlSi3O8) in the form of Si-O-K or Al-O-K. The washable Na and K contents of the mud were 13.7% and 4.47%; the alkali removal agent CaO removed 83.1% and 50.8% of Na and K in the red mud; and the lattice alkali Na and K contents were 3.20% and 44.8%, respectively. In the process of red mud dealkalization, Ca2+ ions can enter the internal voids of the hydroxyl sodalite and potassium feldspar silica skeleton and then replace Al3+ in the Si-O skeleton and Na+ and K+ in the skeleton voids. The replacement reaction changes the silica tetrahedron network structure, resulting in the disintegration of the frame-like silica tetrahedron in the hydroxyl sodalite and potassium feldspar, forming an isolated, island-like silica tetrahedron in hydrated garnet.

Element distribution of grading in red mud at different temperatures based on TIMA and EDS analysis.

In this paper, element distribution patterns of red mud particles with different grading temperatures were explored based on TIMA and EDS, and alkali removal performance of different particle sizes under high temperature grading was compared. The results show that non-clay phases in the particles coagulate with the clay phases of different sodium contents during stacking process, thus forming a mixture phase containing clay phase and other impurities. The potential of grading utilization of red mud is displayed by process mineralogy studies. The elements and phases of different particle sizes of red mud cannot be effectively separated by grading at room temperature. Due to high-temperature grading, red mud is divided into three particle sizes, namely, a (above 100 μm), b (38-100 μm), and c (below 38 μm), with Na2O contents of 3.25%, 2.31%, and 8.13%, respectively, decreasing to 1.00%, 0.27%, and 2.99% after alkali removal. The different elements and phases of red mud can be effectively separated by high-temperature grading, which promotes the classification of different particle sizes and the comprehensive utilization of red mud.

Efficient removal of alkali and alkaline earth metals from biodiesel using Ion-exchange resin: Performance and mechanism

Raw materials, production process, and other factors cause several metal ions to exist in biodiesel. Furthermore, the existence of alkali and alkaline earth metals seriously affects the quality and performance of biodiesel. To improve fuel quality, a 732 ion-exchange resin was used to reduce the contents of alkali and alkaline earth metal ions in Jatropha biodiesel. The adsorption efficiency of the ion-exchange resin for alkali and alkaline earth metal ions in biodiesel was evaluated using ion chromatography, while the adsorption mechanism of ion change resin on alkali and alkaline earth metal ions in biodiesel was elucidated via FTIR and XPS. In addition, the effects of ion-exchange resin on several key physiochemical properties of biodiesel, including oxidation stability, corrosivity, heat value and kinematic viscosity, were also explored. The results showed that the total content of alkali and alkaline earth metal ions in biodiesel was reduced by 88.19%, and the K+, Na+, Mg2+, and Ca2+ contents were reduced to 0.11 mg/kg, 0.89 mg/kg, 0.53 mg/kg, and 0.78 mg/kg, respectively, after removing the ion-exchange resin. In the removal process, the hydroxyl group of sulfonic acid group on the ion-exchange resin is broken, and the substitution adsorption reaction between H+ and the alkali and alkaline earth metal ions in biodiesel is carried out. Moreover, the oxidation induction period of biodiesel was prolonged by 40.8% owing to the reduction in the content of alkali and alkaline earth metal ions, which significantly blocked the decomposition pathways of unstable oxidation intermediates in biodiesel. The degree of copper corrosion in biodiesel did not change significantly before and after ion-exchange resin adsorption during short-term corrosion (3 h); however, the copper corrosion grade decreased from 2c to 1b after ion-exchange resin adsorption during long term corrosion (240 h). Nevertheless, the heat value and kinematic viscosity of biodiesel were observed to be independent of the ionic metal content.

Red mud with enhanced dealkalization performance by supercritical water technology for efficient SO2 capture

The total de-alkalization treatment of industrial solid waste red mud (RM) has been a worldwide challenge. Removing the insoluble structural alkali fraction from RM is the key to enhancing the sustainable utilization of RM resources. In this paper, supercritical water (SCW) and leaching agents were used for the first time to de-alkalize the Bayer RM and to remove sulfur dioxide (SO2) from flue gas with the de-alkalized RM slurry. The results showed that the optimum alkali removal and Fe leaching rates of RM-CaO-SW slurry were 97.90 ± 0.88% and 82.70 ± 0.95%, respectively. Results confirmed that the SCW technique accelerated the disruption of (Al–O) and (Si–O) bonds and the structural disintegration of aluminosilicate minerals, facilitating the conversion of insoluble structural alkalis to soluble chemical alkalis. The exchangeable Ca2+ displaced Na+ in the remaining insoluble base, producing soluble sodium salts or alkalis. CaO consumed SiO2, which was tightly bound to Fe2O3 in RM, and released Fe2O3, which promoted Fe leaching. RM-SCW showed the best desulfurization performance, which maintained 88.99 ± 0.0020% at 450 min, followed by RM-CaO-SW (450 min, 60.75 ± 6.00%) and RM (180 min, 88.52% ± 0.00068). The neutralization of alkaline components, the redox of metal oxides, and the liquid-phase catalytic oxidation of Fe contributed to the excellent desulfurization performance of the RM-SCW slurry. A promising approach shown in this study is beneficial to RM waste use, SO2 pollution control, and sustainable growth of the aluminum industry.

Ultrasound-assisted alkali removal of proteins from wastewater generated during oil bodies extraction

In this study, an ultrasonic-assisted alkaline method was used to remove proteins from wastewater generated during oil-body extraction, and the effects of different ultrasonic power settings (0, 150, 300, and 450 W) on protein recovery were investigated. The recoveries of the ultrasonically treated samples were higher than those of the samples without ultrasonic treatment, and the protein recoveries increased with increasing power, with a protein recovery of 50.10 % ± 0.19 % when the ultrasonic power was 450 W. Amino acid analysis showed that the amino acids comprising the recovered samples were consistent, regardless of the ultrasonic power used, but significant differences in the contents of amino acids were observed. No significant changes were observed in the protein electrophoretic profile using dodecyl polyacrylamide gel, indicating that sonication did not change the primary structures of the recovered samples. Fourier transform infrared and fluorescence spectroscopy revealed that the molecular structures of the samples changed after sonication, and the fluorescence intensity increased gradually with increasing sonication power. The contents of α-helices and random coils obtained at an ultrasonic power of 450 W decreased to 13.44 % and 14.31 %, respectively, whereas the β-sheet content generally increased. The denaturation temperatures of the proteins were determined using differential scanning calorimetry, and ultrasound treatment reduced the denaturation temperatures of the samples, which was associated with the structural and conformational changes caused by their chemical bonding. The solubility of the recovered protein increased with increasing ultrasound power, and a high solubility was essential in good emulsification. The emulsification of the samples was improved well. In conclusion, ultrasound treatment changed the structure and thus improved the functional properties of the protein.

Staged characteristics of red mud dealkalization by CO2 and SO2

As a by-production of alumina industry, red mud has caused serious environmental issues such as water, soil, and air pollution. The disposal and comprehensive utilization of red mud are restricted by the alkalinity seriously. The dealkalization of red mud by acid neutralization has been widely investigated. Nevertheless, the involved issues concerning reaction mechanisms are not fully understood, which questions the feasibility of the practice. This study focuses on and compares the dealkalization behaviors of red mud by CO2 and SO2 to provide a full insight into the neutralization. The differences are analyzed by simulating state changes of structural alkali by Phreeqc. Results show that the removal of structural alkali is the point of improving the dealkalization efficiency of red mud since structural alkali is the main form of alkali in red mud, especially sodalite, accounting for 83.0% of the total alkali content. CO2 mainly removes soluble alkali and has a limited effect on structural alkali, thus exhibiting a low dealkalization efficiency, only at 18.4%, which is similar to the effect of water. SO2 removes both the soluble and structural alkalis resulting in 95.5% dealkalization efficiency. The saturation index simulated by Phreeqc shows that the dissolution of sodalite requires a pH environment between 7 and 4, where CO2 is too weak to reach, and the dealkalization of red mud can be divided into three stages based on the development of pH and temperature: the neutralization of solution alkali at a pH range from 10 to 7, the neutralization of structural alkali at a pH range from 7 to 4 and the continuous dissolution of the excess SO2 at a pH lower than 4.

Effect of removal of alkali and alkaline earth metals in cornstalk on slagging/fouling and co-combustion characteristics of cornstalk/coal blends for biomass applications

Co-combustion of cornstalk/coal blends is feasible for large-scale renewable energy commercialization. However, alkali and alkaline earth metals (AAEMs) in cornstalks can cause slagging/fouling problems. In this study, the ash micro-characteristics, slagging/fouling indexes, and combustion behavior of cornstalk, coal, cornstalk after pretreatment, and cornstalk/coal blends were investigated using scanning electron microscopy, X-ray diffraction, X-ray fluorescence, and thermogravimetric experiments. The results demonstrate that cornstalk ash is smoother than coal ash. There are few significant differences in the micromorphology and minerals between the coal and blends. Various chemicals were used to remove AAEMs, with the most to the least effective being (i) soaking in hydrochloric acid, (ii) soaking in ammonium acetate ≈ soaking in distilled water, and (iii) untreated. After soaking in hydrochloric acid, the mineral content containing potassium, calcium, and magnesium significantly decreased in cornstalk ash, except for sodium salt. All pretreatments can effectively lower the slagging/fouling indexes of cornstalk ash and blends. Pretreatment to remove AAEMs would restrain volatile release in cornstalk. The blend resists the volatile release of cornstalk but is helpful for the coke combustion of coal. AAEMs can lower some characteristic temperatures but do not increase the combustion rate and combustion characteristic parameters in the co-combustion of blends.

Experimental Study on Desulfurization and Removal of Alkali Behavior of BF Slag System in Low-Slag Ironmaking

The increased utilization of pellets in blast furnaces is one of the directions for low-carbon ironmaking. As a result, the low slag rate may affect the desulfurization of the hot metal and the removal of alkali in the blast furnace. Effective desulfurization and the removal of alkali in the low slag ironmaking process have become the focus of the steel industry. In this paper, the effects of slag quantity, temperature, reaction time and slag composition on the desulfurization and removal of alkali were studied using the slag-metal reaction method. It was found that the slag quantity had the same influence trend on the desulfurization and the removal of alkali. The greater the slag quantity, the more effective the desulfurization and the removal of alkali. The slag composition, temperature and reaction time had the opposite effect on the desulfurization and the removal of alkali. High temperature, long reaction time, high MgO concentration, high CaO/SiO2 ratio and low Al2O3 concentration increased the desulfurization of hot metal but reduced the removal rate of alkali from the blast furnace. Applications of the experimental results on high-proportion pellet blast furnace operation are discussed.

Efficient Removal of Alkali and Alkaline Earth Metals from Biodiesel Using Ion-Exchange Resin: Performance and Mechanism

Efficient Removal of Alkali and Alkaline Earth Metals from Biodiesel Using Ion-Exchange Resin: Performance and Mechanism

Dissociation and removal of alkali and alkaline earth metals from sewage sludge flocs during separate and assisted thermal hydrolysis

High levels of alkali and alkaline earth metals (AAEM, including K, Na, Ca, and Mg) in sludge needs to be removed in pretreatment process for alleviating adverse effects on subsequent disposal. Theoretically, the liquid environment provided by the pretreatment technology of thermal hydrolysis (TH) is the ideal condition for the dissolution of AAEM. Therefore, this work quantified AAEM removal efficiency of TH and carbonaceous skeleton (CSkel) assisted TH that we previously proposed for sludge dewatering. Then the mechanism of AAEM dissociating from sludge was explored through the new perspective of biological structure evolution and chemical species transformation. The results showed that all of the AAEM in raw sludge was trapped in extracellular polymer substances (EPS) and cells. Only the water-soluble K/Na in EPS could be released by TH to the supernatant, the residual K/Na in EPS was organically linked with humic matters that were generated through the degradation of proteins. Water/NH4Ac-soluble K/Na in cells still stayed inside with a more stable form of HCl-soluble after TH. Fortunately, with the assistance of CSkel, this part of K/Na could be leached out due to organic acids derived from hemicellulose decomposition. In such a case, the removal efficiency of K/Na was elevated to 55.5% and 72.5%, respectively. Unlike K/Na, nearly all the Ca/Mg in EPS were transferred to cell residuals during TH. They were combined with the bio-phosphorus in cell residuals as the form of HCl-soluble Ca/Mg-P precipitates, rather than carbonates, sulfates or other compounds. This precipitation reaction was also moderately suppressed in CSkel-assisted TH with low pH, then 7.7% and 34.1% of Ca/Mg were taken away by filtrate. This means that appropriately raising the reaction temperature and adding CSkel with high hemicellulose/cellulose contents can promote the removal of AAEM in sludge during TH process.

Effects of torrefaction on ash-related issues during biomass combustion and co-combustion with coal. Part 2: Ash fouling behavior

Torrefaction is a promising pretreatment technology that allows the utilization of biomass on large scales. This work provides new knowledge regarding the effects of torrefaction on ash fouling during biomass combustion and co-combustion by correlating them to element repartitioning due to torrefaction, which has little been evaluated in the literature. Two herbaceous biomasses (rice husk and corn stalk) and one woody biomass (sawdust) were torrefied on a fixed bed at 270℃ in N2. The raw and torrefied biomass were fired and co-fired with two coals on a drop-tube furnace at 1300℃ in air. The samples were characterized by various techniques such as X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectrometry, and ion chromatography. The results show that the fuel interactions during co-combustion inhibit the formation of inside fouling while the incorporation of alkalis in the aluminosilicates promotes the growth of outside deposits. Torrefaction reduces the formation of chlorides and sulfates during combustion directly and consequently inhibits the inside deposition due to the fractional removal of Cl, S, and alkalis in biomass. Besides, the less condensation of chlorides/sulfates on the surface of the tube and the coarse particles tend to reduce the formation and growth of the outside deposits. However, the elemental repartitioning during torrefaction leads to more retention of alkalis which can aggravate the formation of mixed aluminosilicates during combustion and favors the outside deposition. Both co-combustion and biomass torrefaction have the potential to decrease the ash fouling tendency during biomass combustion considering that they reduce the inside fouling deposits which are problematic in practice. Based on the ash composition of the fuels, a modified fouling index [soluble (Na2O + MgO + K2O + CaO)] * [SiO2/(SiO2 + Al2O3)] is proposed which shows a superior linear correlation with the inside fouling rates of the various fuels investigated, indicating its feasibility for prediction of the ash fouling tendency.

Hydrothermal preparation of lightweight calcium silicate powder from alumina-leaching residue of low-calcium sintering red mud

In order to realize the complete and high-value utilization of red mud, a hydrothermal process was proposed to prepare the lightweight calcium silicate powder from a sodium calcium silicate residue after alumina extraction. The synthesized products were characterized by XRD, XRF, SEM, EDS, and PSD, and the corresponding transition mechanism and physical properties were also studied. The main silicon- and aluminum-bearing phases in the clinker after the low-calcium sintering are Na2CaSiO4 and NaAlO2, and the former silicate keeps stable during the alumina leaching process under atmospheric pressure. In the hydrothermal process, Na2CaSiO4 in the alumina-leaching residue is converted into NaOH and Ca5Si6O16(OH)2·4H2O in the alkali solution, and the residual alumina is converted into Ca3Al2SiO8·4H2O which promotes the generation of Ca5Si6O16(OH)2·4H2O. Increasing the reaction temperature, duration and liquid-solid ratio of the hydrothermal process can facilitate the alkali removal efficiency in calcium silicate powder and decrease its bulk density, while the excessive hydrothermal temperature and duration reduce the alkali removal efficiency. The Na2O residue and bulk density of the calcium silicate powder are 0.42% and 0.30 g·cm−3 obtained at the optimal hydrothermal conditions. The flaky particles of Ca5Si6O16(OH)2·4H2O are aggregated into a porous cauliflower structure, which improves the absorption and insulation properties of calcium silicate powder.

Alkali Recovery of Bauxite Residue by Calcification

Bauxite residue (red mud) generated during alumina production is a highly alkaline solid waste. The red mud is mainly stored on land, but it can cause harm to the surrounding environment and human health. The transformation of red mud into soil is a feasible method for the large-scale disposal of red mud, but alkali removal is the key process that controls the transformation of red mud into soil. In this study, the calcification dealkalization of red mud with a small particle size was carried out below 100 °C. The results show that the sodium in red mud is predominately distributed in small particles, mainly because the lattice alkali and alkali present between the crystals are exposed to the surface of red mud particles by ball milling. The dealkalization process was controlled by the internal diffusion of the shrinking-core model (SCM), and the apparent activation energy was 23.55 kJ/mol. The dealkalization rate and the Na2O content of dealkalized red mud reached 92.44% and 0.61%, respectively. The dealkalization rate increased with increasing reaction time, reactant concentration, and leaching temperature, and this result was consistent with the results of the kinetic study. In addition, calcification enhances the flocculation of particles, so the filtration performance of red mud improved.