Mineral Phase Transformation Research Articles

Novel process for facile preparation of mesoporous silica-based materials with controllable pore structure from coal fly ash

Extraction of silica from fly ash to produce mesoporous silica materials is one of the most important utilization approaches. Mesoporous silica could not be synthesized on a large-scale by conventional sol-gel method. In this paper, facile preparation of mesoporous silica with controllable pore structure from fly ash by the template-free process via two steps of mineral phase transformation and selective acid etching was proposed. The influence of crystalline structure and acid etching degree on structure of as-synthesized mesoporous silica materials was revealed, as well as mechanism of crystalline structure transformation and pore structure formation. The results show that mullite and quartz could be transformed into acid-soluble kaliophilite when fly ash reacted with K2CO3 at temperature of 800–1100 °C. The hexagonal kaliophilite would be transformed into orthorhombic KAlSiO4–O1 phase when the temperature is controlled at 1100 °C. Mesoporous silica with specific surface area of 475.93 m2/g and 642.57 m2/g could be synthesized from activated fly ash with kaliophilite and KAlSiO4–O1 phase crystalline structure. By controlling the degree of acid etching, mesoporous silica materials with different pore structures can be obtained. This paper provides a cost-effective and large-scale process for the preparation of mesoporous silica materials with controllable pore structure from solid waste fly ash.

Magnetic separation for recovering iron resources from acid-leaching tailings of laterite nickel ore through mineral phase transformation

The acid-leaching tailings of laterite nickel ore is a typical hazardous waste in the laterite nickel ore industry. The current treatment involves stacking and landfills, which leads to considerable environmental pollution and resource wastage. The reduction of hazardous waste and the recovery of iron resources have environmental and economic value. In this study, a mineral phase transformation technology was proposed. The suitable transformation conditions were 700 ℃, 30 min, and 30 % CO, and optimal grinding and magnetic conditions were −37 μm content 92.42 % and 143.35 kA·m−1, respectively. Then the iron grade and recovery reached 52.00 % and 80.11 %, respectively. Meanwhile, hematite has converted into strongly magnetic magnetite with saturation magnetization increased from nearly 0 A·m2·kg−1 to 28.49 A·m2·kg−1. And particles transformed from dense to a porous honeycomb structure. Therefore, the mineral phase transformation achieves recovering iron resources during reducing hazardous waste. It proposed a new way of resource utilization of laterite nickel ore acid-leaching tailings which has positive environmental and resource benefits.

Thermal activation and mechanical properties of high alumina coal gangue as auxiliary cementitious admixture

In order to use high alumina coal gangue as auxiliary cementitious admixture via a simple and convenient thermal activation technique, the thermal transformation, mineral phase transformation and structure changes of coal gangue at calcining temperatures of 500–1000 °C were analyzed using x-ray diffraction (XRD), differential thermal Analysis (DTA), thermogravimetry (TG), infrared analysis (IR) and scanning electron microscope (SEM). The mechanical properties of cement mortar with 30% coal gangue auxiliary cementitious admixture were also measured to determine the optimal calcining temperature. As calcining temperature was increased, the coal gangue experienced the following transformations: carbon combustion, dehydroxylation, metakaolin transformation and mullite transformation. The cement mortar with coal gangue auxiliary cementitious admixture calcined at 700 °C presented the highest 28-d flexural and compressive strength, increasing by 8.27% and 11.85% respectively as compared with the benchmark cement mortar. The maximum dosage of coal gangue auxiliary cementitious admixture in cement mortar was further identified to be less than 30% by mechanical properties testing. The activity of high alumina coal gangues at different calcining temperatures was explained from the view points of hydration degree and products. The present investigation can provide a useful reference to utilize high alumina coal gangue as auxiliary cementitious admixture by means of a simple thermal activation at 700 °C.

Novel process for high value utilization of high-alumina fly ash: valuable metals recovery and mesoporous silica in situ preparation.

Extraction of valuable metals besides silica from high-alumina fly ash is one of the most important high-value utilization pathways. However, it is difficult to realize high-efficiency extraction due to the stable structure e.g. of quartz and mullite. In this paper, mineral phase transformation for valuable metal recovery and mesoporous silica in situ preparation from fly ash by a selective acid leaching method was proposed. The mineral phase transformation, dissolution behavior of each metal, and pore structure of fly ash derived mesoporous silica were systematically investigated. The results show that the co-activation of fly ash by Na2CO3-K2CO3 formed the phases of kalsilite and (Na, K)AlSiO4. During the acid leaching process, Al, Li, and Ga could be leached with the efficiencies of 86.17%, 89%, and 80% in the FK system. In the FN system, the efficiencies of Al, Li, and Ga are 92.38%, 95%, and 83%, respectively. The crystal plane (002) was destroyed for kaliophilite while all the crystal planes were destroyed for nepheline. With the increase of HCl solution concentration, the porous silica exhibited the same change order of pore shape. The pore structure of as-prepared porous silica was type IV and the hysteresis loop was type H3, and the specific surface areas could be 565.54, 448.02, and 746.76 m2 g-1, respectively. Finally, the leaching liquors can be used to produce crystal aluminum chloride, lithium carbonate and gallium. This paper might provide technical support for full recycling of high-value resources from fly ash.

Antimony(V) behavior during the Fe(II)-induced transformation of Sb(V)-bearing natural multicomponent secondary iron mineral under acidic conditions

Fe(II)-induced phase transformations of secondary iron minerals have attracted considerable attention due to their influence on antimony (Sb) mobility. However, Fe(II)-induced natural multicomponent secondary iron mineral (nmSIM) transformations and the corresponding repartitioning of Sb on nmSIM under acidic conditions upon Fe(II) exposure have not been systematically examined. Herein, we investigated the effect of Fe(II) on nmSIM mineralogy and Sb mobility in Sb(V)-bearing nmSIM at pH 3.8 and 5.6 at various Fe(II) concentrations over 15 d. The Sb(V)-bearing nmSIM phase transformation occurred under both strongly and weakly acidic conditions without Fe(II) exposure, while the presence of Fe(II) significantly intensified the transformation, and substantial amounts of intermediary minerals, including jarosite, ferrihydrite, lepidocrocite and fougerite, formed during the initial reaction stage, especially at pH 5.6. X-ray diffraction (XRD) analyses confirmed that goethite and hematite were the primary final-stage transformation products of Sb(V)-bearing nmSIM, regardless of Fe(II) exposure. Throughout the Sb(V)-bearing nmSIM transformation at pH 3.8, Sb release was inversely related to the Fe(II) concentration in the initial stage, and after maximum release was achieved, Sb was gradually repartitioned onto the nmSIM. No Sb repartitioning occurred in the absence of Fe(II) at pH 5.6, but the introduction of Fe(II) suppressed Sb release and improved Sb repartitioning on nmSIM. This transformation was conducive to Sb reimmobilization on Sb(V)-bearing nmSIM due to the structural incorporation of Sb into the highly crystalline goethite and hematite generated by the Sb(V)-bearing nmSIM transformation, and no reduction of Sb(V) occurred. These results imply that Fe(II) can trigger mineralogical changes in Sb(V)-bearing nmSIM and have important impacts on Sb partitioning under acidic conditions. These new insights are essential for assessing the mobility and availability of Sb in acid mine drainage areas.

Waste Corn Straw as a Green Reductant for Hematite Reduction Roasting: Phase Transformation, Microstructure Evolution and Process Mechanism

Mineral phase transformation (MPT) of hematite to magnetite by reduction roasting is a viable means of developing refractory iron ore resources. However, conventional coal-based reductants are prone to high carbon emissions and environmental pollution. Biomass, as a renewable green reductant, can make the MPT process more environmentally friendly while reducing the environmental impact associated with processing agricultural waste. This study systematically explored the feasibility of waste corn straw as a green reductant for hematite. Under the conditions of 8 min, 700 °C, a mass ratio of corn straw to hematite of 1:4, and a N2 flow rate of 300 mL/min, the best beneficiation indexes were achieved, with an iron grade of 69.82% and an iron recovery of 93.95%. During the MPT process, hematite was reduced under the action of corn straw, and the new magnetite particles were loose and porous, showing an acicular crystal structure. Meanwhile, the corn straw was converted into porous biochar.

Research on efficient utilization of high-phosphorus oolitic hematite for iron enrichment and dephosphorization by hydrogen mineral phase transformation

Research on efficient utilization of high-phosphorus oolitic hematite for iron enrichment and dephosphorization by hydrogen mineral phase transformation

Highly efficient and green separation of iron from complex low-grade polymetallic ore via hydrogen-based mineral phase transformation

The Bayan Obo low - grade polymetallic ore (BOLPO) is a complex polymetallic resource that has accumulated over decades. Conventional mineral processing struggles to meet specifications. In this study, a hydrogen - based mineral phase transformation (HMPT) technique was introduced. Using HMPT at 425 °C for 30 min, CO and H2 in a ratio of 1:3, 20% concentration, and a product grind size below 38 μm with 95% content, an iron concentrate achieved 65.56% grade and 93.68% recovery. x - ray diffraction, x - ray photoelectron spectroscopy and scanning electron microscopy with energy dispersive x - ray spectroscopy analyses showed selective phase transformation of iron to magnetite in BOLPO during HMPT, avoiding excessive wüstite formation, and no phase transformation of quartz, rare earths or fluorite was observed in the products. Vibrating sample magnetometer analysis showed improved magnetic properties, with saturation magnetization increasing from 7.25 A·m2·kg−1 to 47.28 A·m2·kg−1. HMPT selectively separates high - grade iron from low - grade polymetallic ores without compromising the subsequent recovery of rare earths and fluorite in non - magnetic components.

Insights into chloride sorption and phase transformation of the synthesized Ca-Al bimetallic oxides in flue gas desulfurization wastewater

Sorbents enriched with Ca3Al2O6 (C3A) and Ca12Al14O33 (C12A7) were successfully synthesized by a solid-state sintering method for Cl− removal from the flue gas desulfurization (FGD) wastewater. C3A and C12Al7-rich sorbents prepared at 1200 and 1100 °C achieved the highest Cl− removal of 83.3% and 73.1%, and the former had a higher aluminum utilization rate of 45.2%. Compositional analyses revealed that sorbents synthesized using α-Al2O3 had higher atomic density, higher ratio of absorbed oxygen to lattice oxygen, and higher AlO45−/AlO69− ratio than those using Al2O3, resulting in enhanced reactivity and superior Cl− removal performance. Higher calcination temperature also improved the crystallinity of bimetallic oxides. The sorption isotherms and thermodynamic studies confirmed that the sorption of Cl− on Ca-Al oxides was spontaneous and exothermic, and the maximum sorption capacities were 243.7 mg/g on C3A and 204.1 mg/g on C12Al7, respectively. Kinetics and characterizations suggested that the hydration-induced mineral phase transformation led to structural reconstruction, and the sorption mechanism involved a synergistic action of chemisorption (as the primary force) and physisorption. These results suggested that Ca-Al-based sorbents are highly promising alternatives for application in the zero liquid discharge process of FGD wastewater.

Characteristics and controlling factors of pore structure of shale in the 7th member of Yanchang Formation in Huachi area, Ordos Basin, China

To clarify the influence of organic matter and mineral composition on the pore structure of shale reservoirs in the 7th member of the Yanchang Formation (Chang 7 Member) in the Ordos Basin, we characterized the pore structure of Chang 7 shale reservoirs using argon ion polishing-field emission scanning electron microscopy (FE-SEM) and low-pressure nitrogen adsorption (LP-N2A). This characterization was combined with whole-rock mineral composition and organic geochemical experiments to analyze the main controlling factors of the pore structure of Chang 7 Member shale. The results reveal the presence of various pore types in shale, including organic matter pores, intergranular pores, intercrystalline pores, dissolved pores, and micro-cracks. The LP-N2A isotherms of shale consistently exhibit type Ⅱ isotherms with H3 and H4 hysteresis loop characteristics, indicating the relatively developed nature of mesopores and a pore morphology characterized by parallel lamellar and “ink bottle” shapes. The primary determinants of shale pore structure are identified as organic matter, clay minerals, quartz, feldspar, and pyrite. Among these factors, clay mineral phase transformation generates a substantial number of micropores and mesopores within the mineral crystal layers, serving as the main source of shale pores in the study area. Additionally, liquid hydrocarbons generated, solid bitumen, and euhedral pyrite fill inorganic mineral pores, thereby reducing the pore space of Chang 7 shale to a certain extent. These results provide a new cognition into understanding the pore structure characteristics and controlling factors of Chang 7 shale.

The Recovery of TiO2 from Ilmenite Ore by Ammonium Sulfate Roasting–Leaching Process

TiO2 production is a key part of Ti metallurgy and Ti recycling, and the process itself has turned out to be energy-consuming and material-consuming. New technologies are needed to utilize complex Ti ores, such as ilmenite, and reduce the carbon footprint of TiO2 extraction. Ammonium sulfate roasting has been revealed as an efficient way to carry out phase transformations of complex minerals. A low-temperature sulfation roasting approach was studied to chemically breaking down the crystal structure of ilmenite and generate metal soluble sulfates simultaneously. These roasted products were introduced to water leaching, then the residue of the water leaching was leached by diluted HCl acid, and the TiO2 product was enriched in the leaching residue. The effects of roasting temperature, roasting time, ilmenite-to-ammonium sulfate mass ratio, ilmenite particle size, and second-stage roasting on iron removal and titanium loss leaching efficiency were systematically studied. The results show that the optimum roasting conditions were a roasting temperature of 500 °C, a roasting time of 210 min, an ilmenite-to-(NH4)2SO4 mass ratio of 1:7, and an ilmenite particle size of below 43 µm. Under optimized conditions, the TiO2 grade in the obtained synthetic rutile reached 75.83 wt.%. Furthermore, the phase transformation and reaction mechanism during roasting are discussed and interpreted.

Dissolution and Passivation Mechanism of Chalcopyrite during Pressurized Water Leaching

In this study, chemical leaching, XRD, SEM, and XPS analyses were conducted to investigate the dissolution and passivation mechanisms of chalcopyrite under pressurized oxidative conditions in water. The chemical leaching results showed that the chalcopyrite could be dissolved by pressurized leaching without any acid addition, i.e., in an O2–H2O system, and the copper leaching rate reached 96.4% under the optimal conditions of 180 °C, 1.5 MPa, 900 rpm, and 90 min. The XRD, SEM, and XPS data suggested that a large proportion of the chalcopyrite dissolved in solution via the mineral phase transformation of CuFeS2→Cu2+ and CuFeS2→CuS→Cu2+, i.e., some of the chalcopyrite directly leached into solution as Cu2+, and some of it was first converted to CuS and then to Cu2+. The primary passivation layers during the chalcopyrite pressurized water leaching were hematite, pyrite, and covellite; however, none of them covered the un-leached mineral surface or inhibited chalcopyrite dissolution, as long as the agitation speed and leaching time were maintained over 700 rpm and 60 min, respectively. Finally, a model of chalcopyrite’s dissolution and passivation mechanism during pressurized water leaching was proposed.

A pilot study on recovery of iron from sulfur-bearing hematite ore using hydrogen-based mineral phase transformation followed by magnetic separation

Sulfur is a harmful impurity in iron ore, which seriously affects the subsequent smelting process and product quality. In this study, a novel process of hydrogen-based mineral phase transformation followed by magnetic separation was proposed to treat sulfur-bearing refractory iron ore. The results of pilot-scale experiments showed that in the oxidation heating stage, the sulfur element in the ore was oxidized to form SO2. During the reduction roasting stage, the hematite within iron ore powder was almost entirely transformed into magnetite at a reduction temperature of 520 °C using a gas combination of 45 % H2- 15 % CO- 40 % N2, which significantly enhanced the magnetism of the particles. At this time, iron minerals could be effectively recovered by a grinding and low-intensity magnetic separation process. Under the optimum roasting and magnetic separation conditions, the iron grade of magnetic concentrate reached 66.40 %, the iron recovery was 92.44 %, and the sulfur content decreased significantly from 0.547 % to 0.038 %. The hydrogen-based mineral phase transformation technology showed unique technical advantages and environmental potential in treating sulfur-bearing refractory iron ores.

Unveiling the crucial role of iron mineral phase transformation in antimony(V) elimination from natural water

Antimony (Sb) in natural water has long-term effects on both the ecological environment and human health. Iron mineral phase transformation (IMPT) is a prominent process for removing Sb(V) from natural water. However, the importance of IMPT in eliminating Sb remains uncertain. This study examined the various Sb–Fe binding mechanisms found in different IMPT pathways in natural water, shedding light on the underlying mechanisms. The study revealed that the presence of goethite (Goe), hematite (Hem), and magnetite (Mag) significantly affected the concentration of Sb(V) in natural water. Elevated pH levels facilitated higher Fe content in iron solids but impeded the process of removing Sb(V). To further our understanding, polluted natural water samples were collected from various locations surrounding Sb smelter sites. Results confirmed that converting ferrihydrite (Fhy) to Goe significantly reduced Sb levels (<5 μg/L) in natural water. The emergence of secondary iron phases resulted in greater electrostatic attraction and stabilized surface complexes, which was the most likely cause of the decline of Sb concentration in natural water. The comprehensive findings offer new insights into the factors governing IMPT as well as the Sb(V) behavior control.

Adsorption Characteristics of Oxyanions on Ferrihydrite and Mineral Phase Transformation

Adsorption Characteristics of Oxyanions on Ferrihydrite and Mineral Phase Transformation

Thermodynamic analysis and in situ powder X-ray diffraction investigation of the formation and transformation of mineral phases during pressure oxidation of pyrite

Acid oxidative pressure leaching is a cost-effective way for recovering gold from refractory gold ores such as gold-bearing pyrite, yet our knowledge of the dissolution, crystallization, and phase transformation during pressure leaching is still controversial. This is partially due to the lack of thermodynamic data and in situ studies of the leaching process. Here, we combine thermodynamic modelling and in situ powder X-ray diffraction (PXRD) to investigate mineralogical phase changes during oxidation of pyrite. The polyhedron method has been utilised to estimate the standard Gibbs free energy of formation (∆Gfo) and enthalpy of formation (∆Hfo) of 31 sulphate minerals from 25 °C to 300 °C. The calculated thermodynamic data for the sulphate minerals agree well with the literature, with an average relative error of 0.23% for both ∆Gfo and ∆Hfo demonstrating the reliability of the polyhedron method. Some new thermodynamic data for the mineral phases at elevated temperature was calculated and used to construct the Eh-pH diagrams for the Fe-S-H2O system at 225 °C, which were used to predict phase transformations and chemical reactions during oxidation of pyrite. Based on the analysis of thermodynamic diagrams, the formation of basic ferric sulphate (BFS: Fe(OH)SO4) is favoured at stronger acidic and higher solution redox potential conditions, while hematite and FeSO4∙H2O occur at low solution redox potential conditions. These predictions agreed with the results from laboratory-based in situ PXRD experiments, in which the mineralogical phase transformations were studied during acid pressure treatment of pyrite in a capillary. The influence of initial concentration of ferric ions on the change of mineral phase was revealed. The results confirmed the formation of FeSO4∙H2O and Fe(OH)SO4 phases during the heating (24–225 °C) and annealing stages. These phases may passivate pyrite surface and hence slow down further pyrite oxidation but were re-dissolved during the annealing and/or cooling stages, depending on the initial concentration of ferric ions.

Optimizing iron separation and recycling from iron tailings: A synergistic approach combining reduction roasting and alkaline leaching

Iron tailings, a type of refractory iron ores, pose a challenge for effective iron recovery due to the complex mineral combinations and liberation modes. In this study, we proposed a new approach to improve the quality of iron concentrates by combining alkaline leaching with magnetization roasting-magnetic separation, which promoted the leaching of silica and aluminum minerals from iron tailings. We investigated the leaching process conditions, phase transitions, and microstructure evolution of joint magnetization roasted iron tailings in the presence of an alkaline solution. Our research findings showed that with optimal leaching conditions, the iron concentrate from magnetic separation was improved from 59.97 % to 65.07 % in iron grade while maintaining an iron recovery of 92.05 %. The heat and mineral phase transformation during magnetization roasting destroyed the structure of iron tailings particles, resulting in the formation of pores and microcracks that weakened inter-mineral bonding. Additionally, alkaline leaching facilitated the extraction of Si and Al from the structure-damaged iron tailings particles, eliminating 83.75 % of Si minerals and 68.73 % of Al minerals. This study offers a promising solution for iron recovery from refractory iron ores, reduces environmental pollution from tailings, and could enhance current iron recovery technology.

Effects of Borax and Grinding Alkalinity on the Reduction–Magnetic Separation of Beach Placer

The effect of borax on the reduction characteristics of Indonesia beach placers was investigated. The effect of grinding alkalinity on the magnetic separation of the reduced sample was also studied in this paper. The mineral phase transformation, microstructures of reduction, and magnetic separation products were analyzed to reveal the enhanced separation mechanism of titanium and iron in beach placer. The borax could effectively improve the metallization rate and the growth of iron grains in a reduced sample. When 3% borax was added to the reduction process, the metallization rate of the reduced beach placer reached 95.64%, and the metal iron grains grew to about 50 μm. Adjusting the grinding alkalinity could prevent the metallic iron from being oxidized and promote the monomer dissociation between mineral particles. The iron powder concentrates with 94.07% total Fe and vanadium-rich titanium slag with 36.32% Ti were obtained by grinding magnetic separation as the grinding alkalinity pH was 13.

Investigation into the fusibility of biomass ashes and their mineral phase transformations at elevated temperatures by using the HT-XRD technique

In this paper, the fusibility of biomass ashes with varying chemical and mineral compositions were investigated and their mineral phase transformations were characterised in-situ by using the HT-XRD technique. Results showed that ash fusion temperatures (AFTs) of the analysed biomass increased in the order of walnut shell (WS) < wheat straw (WHS) < peanut shell (PES) < pine sawdust (PS). This is attributed partially to their chemical compositions, since WS and WHS ashes had high K2O contents (30.77%), but the PES and PS ashes were abundant in SiO2, Al2O3 and/or CaO. HT-XRD analysis showed that K-bearing minerals were firstly identified but then diminished in WS ash as temperature, confirming their role for slag formation. For WHS ash, the release of KCl and the formation of K silicates or aluminosilicate at different temperatures were confirmed, leading consequently to a decreased AFTs. In the PES ash, SiO2, MgO, CaAl2Si2O8, CaAl2SiO6 were identified, which also did not melt at the temperatures examined. Likewise, minerals in PS ash including CaO, Ca3SiO5 and Ca2SiO4 remain identified at elevated temperatures. Macro- and micro-morphological analysis shows that the WS and WHS ashes were prone to be sintered at temperatures as low as 650 °C, yet PES and PS ashes were less sintered even at 850 °C. These were confirmed by thermodynamic calculations when the phase and mineral transitions were considered. These mineral phases were also compared with those from conventional XRD, confirming the advantages of HT-XRD in determining mineral phase transformation and comprehending fusibility of the biomass ashes.

Formation and transformation of mineral phases in biomass ashes and evaluation of the feedstocks for application in high-temperature processes

Biomasses are known for their variety of ash systems, often accompanied by limiting effects such as fouling, sintering, slagging, agglomeration and corrosion, which affect thermochemical conversion processes (combustion, gasification, pyrolysis etc.). To explore those limiting effects, five different feedstocks from agricultural, woody, and processed biomasses are investigated after ashing in a temperature range between 200 and 815 °C. The ash composition and their specific distribution inside the particles are studied using X-ray fluorescence (XRF) and scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX). Distinct differences are detected in the ashes from various biomasses, indicating that the ash systems can be homogeneous (mainly authigenic origin) or heterogeneous (with significant contributions of detrital/technogenic origin). Using X-ray diffraction (XRD), the formation and transformation of mineral phases are monitored in the ashes produced at various temperatures. The data are then compared to the results of thermochemical calculations. The experimental findings agree almost completely with those calculated using the FactSage™ software package. In connection with the identified mineral phases, the release behavior of the main alkali species, potassium, is monitored as a function of the ashing temperature. Based on these results indicating the ashes' mineral phase composition and their observed heterogeneity, the high-temperature behavior of the ash fusion temperatures (AFT) and ash fusion interval (AFI) between deformation and flow temperature can be ascertained. Finally, based on their ashing behavior, the biomasses’ potential use in thermochemical conversion processes at high temperatures is evaluated.