Low-temperature Stage Research Articles

Petrogenesis of Jian forsterite jade solely composed of end-member forsterite (Fo 99.8): Constrained by trace element and oxygen isotope

The Jian forsterite jade, named for its enrichment of end-member forsterite, is a new type of jade discovered in Ji'an County (Jilin Province, Northeast China). The formation mechanism of this rare end-member forsterite remains unclear. In this study, we present the mineral chemistry and oxygen isotope analysis of end-member forsterite to elucidate the formation mechanism of the Jian forsterite jade. The Jian forsterite jade can be divided into three main types: yellowish-green, dark-green, and black. The Jian forsterite jade consists mainly of forsterite, serpentine, as well as minor brucite, spinel, phlogopite, dolomite, calcite, magnesite, and magnetite. The Fo% content (100 × Mg/[Mg + Fe], mol%) of forsterite varies from 96.69 to 99.82. All tested forsterite samples show low Ni, Mn, Cr, and Ca contents and very high B contents. The δ18O values of forsterite range from 13.79 ‰ to 18.57 ‰, which are significantly higher than those from the mantle and are similar to those from skarn, marble, and calc-silicate rocks. The oxygen isotope values of forsterite, coupled with a distinctive trace element signature, suggest that the genesis of end-member forsterite results from a siliceous fluid-mediated decarbonation of Mg-rich dolomite. The genesis of the Jian forsterite jade can be outlined in two main metamorphic stages: (1) high-temperature prograde stage related to the interaction of the gneiss-derived SiO2-rich fluids with overlying dolomite. High temperature leads to the formation of forsterite, spinel, and calcite mineral assemblage. (2) low-temperature retrograde stage characterized by the infiltration of hydrous fluids. Hydrous silicate minerals, mostly serpentine, phlogopite, and talc, are formed during this stage. Our work highlights and confirms the effectiveness of the trace element geochemistry of forsterite in identifying magnesian skarns, and classifies the forsterite in the Jian forsterite jade as an extreme example of this occurrence.

Multiscale thermal behavioral characterization of spontaneous combustion of pre-oxidized coal with different air exposure time

To comprehensively investigate the multi-scale characteristics of coal spontaneous combustion, in situ infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC) were utilized. The samples were pre-oxidized at 70 °C for 2, 4, 6, and 8 h (the raw coal and above samples were named R0, R2, R4, R6, and R8), then the secondary oxidation experiments were carried out. The results showed that the contents of –OH, –C=O, and –C=C- increased with the pre-oxidation time. The aromatic structure of pre-oxidized coal is more susceptible to oxygen erosion during secondary oxidation. The migration and transformation of –OH, –C=O are the underlying causes of water evaporation and gas desorption during low-temperature oxidation. The spontaneous combustion behaviors in stage I and stage II have different reaction mechanisms and kinetic characteristics. In light of Flynn-Wall-Ozawa (FWO) and Kissinger-Akahira-Sunose (KAS), the apparent activation energy of stage I is R0>R2>R4>R6>R8, and stage II is R6>R8>R2>R4>R0. Long pre-oxidation time in stage I will inhibit coal spontaneous combustion, while the opposite is true in stage II. By the Pearson correlation analysis, –OH is the key structure affecting the low-temperature endothermic stage of coal, revealing the correlation between the multi-scale thermal behavior of pre-oxidized coal spontaneous combustion.

Mechanism research on hydrogen production from catalytic pyrolysis of waste tire rubber

Waste tires are currently disposed of through various approaches such as landfill and incineration, which not only causes environmental pollution, but also lead to the wastage of resources. As an important type of solid waste, tire rubber has a high carbon-to-hydrogen ratio, which makes it a good material for hydrogen production from pyrolysis. In this study, molecular dynamics simulations combined with experimental methods are used to explore the mechanism of hydrogen production from the catalytic pyrolysis (CP) of waste tire rubber (WTR) over Ni, ZSM-5 and Ni/ZSM-5. The simulation results show that the three catalysts potentially promote the release of hydrogen radicals from monomers and facilitate attack by H· radicals to generate hydrogen. Compared with ZSM-5 and Ni, the Ni/ZSM-5 catalyst exerts better catalytic effects, leading to higher contents of produced H2 and hydrocarbons. In the low-temperature stage, the long chain cracks into monomer compounds, which mainly include isoprene, styrene, and 1,3-butadiene. Meanwhile, in the high-temperature stage, free radicals attack monomers to generate small molecules. The main gas products from the catalytic pyrolysis of waste tire rubber are hydrogen, methane, and ethylene. The use of all three catalysts leads to a low final catalytic pyrolysis temperature, high catalytic pyrolysis rate, and excellent catalytic effect. This work aims to provide a deeper understanding of the reaction thermodynamics and kinetics for the formation mechanisms of gaseous products, primarily hydrogen.

TitaniQ temperatures and textural analysis as a record of the deformation history in a major continental shear zone system, Borborema Province, Brazil

We combine quartz textural analysis with Titanium-in-quartz (TitaniQ) geothermometry to investigate the late stages of deformation in mylonitic rocks from the Sucuru dike swarm in the Borborema Province. The wide range in TitaniQ temperatures (>340 to 740 °C) closely related to grain sizes are attributed to inefficient Ti resetting during bulging (BLG) and subgrain rotation (SGR) recrystallization. The highest temperature values (>500–740 °C) occur in the core of quartz ribbons and porphyroclasts. They are interpreted as the record of early-stage plastic deformation, likely during the intrusion of magmatic bodies, and are progressively overprinted during a lower-temperature stage (>340–500 °C) associated with intense dynamic recrystallization and production of a large amount of fine-grained matrix. The dominantly weak crystallographic preferred orientations (CPOs) can be attributed to a large variety in crystallographic orientations of parent grains, which control to a large extent the CPO evolution of daughter grains, and to activation of multiple slip systems in the <a > direction (basal, rhomb and prism) due to synkinematic temperature decrease. The CPOs are consistent with strain localized by operation of dislocation creep-accommodated grain boundary sliding (DisGBS) during the low-temperature stage. DisGBS seems to be less important in coarse pure quartz domains, where moderate to strong CPOs indicate the dominance of dislocation creep. Several evidence of intracrystalline deformation in quartz ribbons and porphyroclasts point to dominant activation of dislocation creep at the early high-temperature stage of deformation, with activation of prism <c>. The dike orientations, geometry and evidence of solid-state deformation in their margins, fit well with progressive E-W dextral shearing, where they intrude and crystallize in a NW-SE direction and rotate clockwise during shearing. This implies that the dextral shear regime that dominated the Borborema province from ca. 600 Ma was still in effect into the Cambrian some 60–70 million years later.

Boosting Electrical Response toward Trace Volatile Organic Compounds Molecules via Pulsed Temperature Modulation of Pt Anchored WO3 Chemiresistor.

Insufficient limit of detection (LoD) toward volatile organic compounds (VOCs) hinders the promising applications of metal oxide chemiresistors in emerging air quality monitoring and/or breath analysis. There is an inherent limitation of widely adopted strategies of creating sensitive chemiresistors then operating at the optimized temperature via a continuous heating (CH) mode. Herein, a strategy combining Pt single atoms anchoring (chemical sensitization) with pulsed temperature modulation (PTM, physical sensitization) is proposed. Apart from generating abundant surface asymmetric oxygen vacancy (Pt-VO -W) active sites at pulsed high temperature (HT) stage, inward diffusion of trace target VOCs across the sensing layer at pulsed low temperature stage (driven by PTM induced concentration gradient), can greatly enhance the charge interaction probability between the generated surface active species and the surrounding VOCs, and thus offers a novel avenue on addressing the bottleneck issue of low LoD by PTM. Triggered by HT of 300°C, the responses of Pt anchored WO3 chemiresistor to 1 ppm trimethylamine (TMA) and xylene can be drastically boosted from 1.9(CH) to 6541.5 (PTM) and 1.5 (CH) to 1001.1 (PTM), respectively. And ultra-low theoretic LoD of 0.78 ppt (TMA) and 0.18 ppt (xylene) are successfully achieved, respectively.

Coupling effect of high temperature and thermophilic bacteria indirectly accelerates the humification process of municipal sludge in hyperthermophilic composting

Due to its high treatment efficiency and environmental friendliness when used to treat organic waste, hyperthermophilic composting (HC) has attracted extensive attention in recent years. However, it is still unclear how HC accelerates the degradation and humification process of organic matter. This study investigated the effects of high temperature and thermophilic bacteria on the degradation and humification of organic matter during HC. In the process of HC and conventional composting (CC), total organic carbon content decreased by 30.76 mg/g and 18.50 mg/g, while humic acid carbon content increased by 2.26 mg/g and 1.41 mg/g, respectively. These results showed that HC accelerated the humification process by enhancing the degradation of organic matter and the production of humic acid compared to CC. Redundancy analysis revealed that the rapid humification process of HC was related to high temperature and thermophilic bacteria. Thermophilic bacteria accelerated the degradation of organic matter in the high temperature stage, and the degradation products provided sufficient substances for the humification reaction in the low temperature stage, thereby accelerating the humification reaction. In conclusion, the coupling effect of high temperature and thermophilic bacteria indirectly accelerated the humification process. This work contributes to understanding the humification mechanism of HC.

Preparation and characterization of energetic composite films with mutual reactions based on B/PVDF mosaic structure

In this study, to solve the boron oxide (B2O3) formed on the boron (B) surface, which reduces the reactivity and energy release. The energetic composite films with mutual reactions of B as fuel and polyvinylidene fluoride (PVDF) as oxidant are designed, based on the mosaic structure. Then, samples with different contents of B are successfully prepared by the novel synergistic emulsification and transient freezing methods. The morphology and composition results show that B is embedded in PVDF films, which makes the energetic composite films have excellent hydrophobicity and oxidation resistance. Interesting, the thermal analysis results show that in the low-temperature stage, the addition of B advances the initial decomposition temperature of PVDF; in the high-temperature stage, the gas product hydrogen fluoride (HF) of PVDF decomposition has a pre-ignition reaction (PIR) with B2O3, which destroys the shell of B2O3 and accelerates the oxidation of internal active B. And the heat release of 50 % B content is the highest (3845.4 J/g). Furthermore, the ignition and combustion results show that all samples can be ignited smoothly and easily. But the combustion performance is greatly affected by the content of B. Finally, the reaction mechanism of samples is preliminarily obtained by molecular model through the characterization of combustion products. These results show that composite films with mutual reactions based on B/PVDF mosaic structure have an excellent prospect for application in thermite with fluoropolymer.

Spontaneous combustion characteristics and mechanism of water-immersed and air-dried brown coal

ABSTRACT To reveal the influential laws and mechanism of water immersion on the spontaneous combustion traits of lignite in the low-temperature oxidation stage. Raw coal (HL), water-immersed for 30 days (HL-S30), 90 days (HL-S90) and 150 days (HL-S150), were taken as the research objects. The pore evolution characteristics and functional group migration law of coal were explored by Scanning Electron Microscopy (SEM), Low-temperature Nitrogen Adsorption Test (LNAT) and Fourier Transform Infrared Spectroscopy (FTIR). The low-temperature oxidation procedure was simulated by Programmed Heating Test (PHT), and the gas release characteristics, exothermicity and ultimate spontaneous combustion parameters of the relic coal were compared. These findings demonstrated that the discriminant indexes of spontaneous combustion tendency of four specimens change periodically with temperature, but generally the spontaneous combustion intensity of HL-S30 and HL-S150 is greater than HL, and HL-S90 is the least. The micro reasons for increasing oxidation activity of HL-S30 and HL-S150 are as follows: the number of micropores of water-immersed coal is almost 3 ~ 4 times that of HL, and the pores expanded with immersion which helps to enhance the adsorption of oxygen; the content of broken aromatic hydrocarbon -C = C- reached 50%, the total amount of hydroxy -OH decreased to 2/3 of HL, while the content of carboxyl -COOH and ether-oxygen bond -C-O- of active group doubled which are beneficial to accelerate the process of free radical chain reaction, and promote the gas release and the temperature of residual coal to reach the ignition point. The research findings enrich the basic theory of spontaneous combustion traits of water-impregnated coal, which help reduce the fire accidents and hidden dangers in actual production.

Effect of the Iron Component on Microcrystalline Structure Evolution of Hydrochloric Acid-Demineralized Lignite during the Pyrolysis Process.

Hydrochloric acid-demineralized Shengli lignite (SL+) and iron-added lignite (SL+-Fe) were thermally degraded using a fixed-bed device to better understand the effect of the iron component on the microcrystalline structure transformation properties of lignite during the pyrolysis process. The primary gaseous products (CO2, CO, H2, and CH4) were detected by pyrolysis–gas chromatography. X-ray diffraction and Raman spectra were adopted to analyze the microcrystalline structure of lignite and chars. The results indicated that the iron component had a catalysis effect on the pyrolysis of SL+ below 602.6 °C. The pyrolysis gases released in the order of CO2, CO, H2, and CH4, and the addition of the iron component did not change the sequences. The iron component promoted the generation of CO2, CO, and H2 in the low-temperature stage. During the high-temperature stage, the iron component inhibited the formation of CO and H2. The formation of CH4 was inhibited by the iron component throughout the pyrolysis process. The evolution characteristics of −OH, C=O, C=C, and C–H functional groups were not significantly affected, and the fracture of aliphatic functional groups and C–O functional groups was inhibited by the iron component during the pyrolysis process. The iron component restricted the spatial regular arrangement tendency of aromatic rings and facilitated the decrease in the small-sized aromatic ring but inhibited the formation of large aromatic rings (≥6 rings) and the content decrease in side chains during the pyrolysis process. Notably, the effects of the iron component on the formation of gaseous products were associated with the microstructure evolution of lignite.

Analysis of a high-performance polygeneration system for efficient waste heat recovery of an organic flash cycle: Multi-criteria analysis and optimization

The waste heat from a geothermal-driven organic flash cycle has an outstanding potential to be recuperated. The present research is an effort to recover the thermal losses of a geothermal-based organic flash cycle to a feasible extent by employing a configuration that has not been studied in the literature. In this respect, the energy losses in the high-temperature and low-temperature throttling stages of the organic flash cycle are recuperated in a screw expander and an LiCl-H2O absorption chiller, correspondingly. Moreover, the heat loss in the condenser of the organic flash cycle is recovered in a Seebeck power generator, and the electricity produced in the expander and Seebeck generator is transmitted to a proton exchange membrane electrolyzer to generate hydrogen. In addition, a heat exchanger is embedded to produce hot water utilizing the residual energy of the geothermal water before reinjection. The proposed system is analyzed by the engineering equation solver from energy, exergy, and exergoeconomic standpoints. The exergoeconomic analysis is performed by employing the specific exergy costing method, and multi-objective optimization is carried out utilizing MATLAB software. The optimized thermo-economic performance of the system reveals that the heat loss recovery enhances the useful exergy of the system by producing 178 kW of exergy rate for cooling, heating, and hydrogen. Thus, the exergy efficiency of the polygeneration system is obtained as 30.3%, which is 10.5% points higher than the case of electricity generation by the turbine of the flash cycle. Above all, the low unit cost of polygeneration and payback period, respectively equal to 7.3 $GJ-1 and 1.4 years, make the designed system superior when is compared with similar geothermal-based polygeneration systems.

Evaluating the dynamic characteristics and energetic performance of a paddy multistage counter-flow dryer

Paddy drying is a spontaneous dehydration process following the principles of high product quality, high drying efficiency, low energy consumption, and environmental protection. However, its complex properties hinder the development of optimal control during drying. In this study, the real-time paddy drying state of a paddy multistage counter-flow dryer is simulated utilising numerical simulation technology. Results indicate the simulation results agreed well with the experimental data with mean relative deviation of less than 15%. Moreover, the steady and transient characteristics of present dryer are analysed under multiple disturbances. In addition, the effects of ambient and ventilation parameters on productivity, specific energy consumption, heat loss, and exergy loss characteristics are investigated in detail. Based on the simulation and analysis, a uniform design experiment is performed to determine the optimal ventilation parameters of paddy with initial moisture content of 0.238 g water g −1 wet matter when ambient temperature and relative humidity are 20 °C and 50%, respectively. Drying air temperatures of 60 °C and 40 °C, and flow velocity of 1600 m h −1 and 1150 m h −1 , respectively, in the high- and low-temperature drying stages are found to be the optimal ventilation parameters. • Captured the steady and transient characteristics of paddy multistage counter-flow dryer. • Analysed the energetic performance under different ambient and ventilation parameters. • Proposed an optimal ventilation strategy considering productivity and energy consumption.

Reaction mechanism of syngas produced via pyrolysis of enteromorpha polysaccharides

The mechanism of gas-phase products from pyrolysis of polysaccharides from Enteromorpha prolifera (PE) is researched. The sulfated polysaccharides are the most important polysaccharides in Enteromorpha prolifera. The molecular dynamics (MD) method is used to simulate the pyrolysis process of sulfated polysaccharides of Enteromorpha prolifera. Combined with the results of the pyrolysis simulations and density functional theory (DFT), rhamnose, glucuronic acid, glucose, and xylose are used as model compounds to estimate and calculate the reaction paths of hydrogen and carbon monoxide. In addition, fixed-bed pyrolysis experiments are performed with the above four monomers. The pyrolysis simulation results show that the pyrolysis process of sulfated polysaccharides is approximately divided into three stages: low-temperature, medium-temperature, and high-temperature stages. Gas-phase products are mainly produced in the high-temperature stage, and the main components of syngas are hydrogen, methane, and carbon monoxide. The simulation and experimental results show that pyrolysis of xylose is the most likely to generate hydrogen, followed by glucose, rhamnose, and glucuronic acid. The process of producing carbon monoxide from monomers goes through reactions such as dehydration, isomerization, C-C bond breakage, and decarbonylation. Moreover, the results of fixed-bed experiments indicate that when the four monomers are pyrolyzed individually, the highest proportions of hydrogen, carbon monoxide, and methane in the gas-phase products are obtained from the pyrolysis of xylose, glucuronic acid, and rhamnose. When the four monomers are mixed and then pyrolyzed, the proportion of hydrogen and carbon monoxide in the obtained gas-phase products decreases, and the proportion of carbon dioxide increases.

Improved creep forming efficiency and retained performance via a novel two-stage creep aging process of Al–Zn–Mg–Cu alloys

A novel two-stage creep aging forming (CAF) process with an elevated temperature followed by peak-aging temperature was developed to notably improve the creep forming efficiency and to achieve a favorable combination of strength and corrosion resistance in an Al–Zn–Mg–Cu alloy. Compared with traditional single-stage creep aging, the proposed two-stage creep aging (TSCA) halved the formation time while doubling the creep deformation without sacrificing strength. This specific aging procedure can manipulate precipitation evolution, thus effectively affecting the deformation and performance of the studied alloy. The creep responses, properties, microstructure evolution and their relationship of 7B50 aluminum alloy subjected to TSCA were investigated and discussed through creep and property tests, transmission electron microscopy, and differential scanning calorimetry. Both precipitate dissolution and high temperatures were beneficial for the exceptional creep response in the high-temperature creep aging process, whereas the re-precipitation of new Guinier–Preston (GP) zones in the low-temperature stage could practically counteract the strength loss at elevated temperatures. Moreover, the corrosion resistance was persistently enhanced throughout the process owing to coarse and discrete grain boundary precipitates. These results are of significance in the development of CAF technology for Al–Zn–Mg–Cu alloy components.

Fractional condensation of pyrolysis oil from fast pyrolysis of food waste digestate for enrichment of high value-added nitrogen-containing components

Fast pyrolysis over CaO coupled with fractional condensation was proposed to facilitate the efficient utilization of food waste digestate (FD) and upgrading py-oil. The three-stage fractional condensation temperature was optimized and the effect of CaO catalytic pyrolysis as well as the fractional condensation behavior for enrichment of high value-added nitrogen-containing components (NCCs) were evaluated. The results indicated that fractional condensation temperature was set to 0 °C, 60 °C and 120 °C, which were suitable for enriching NCCs, water and phenols, respectively. In addition, the application of CaO inhibited the production of water during the pyrolysis process of FD and increased the lower heating values of py-oil. Fast pyrolysis over CaO coupled with fractional condensation decreased the content of oxygen-containing compounds (OCCs) and promoted the enrichment of NCCs (71.14 %) in the low-temperature condensation stage, especially the pyrroles (29.03 %). In addition, the formation path of pyrroles and the effect of CaO on the process evolution were proposed.

Systematic variations in trace element composition of pyrites from the 26°S hydrothermal field, Mid-Atlantic Ridge

Variations in trace element composition of pyrites in inactive chimneys from the SMAR 26°S hydrothermal field provide important insights into the genesis and evolution of volcanic-hosted hydrothermal mineralization. Petrographic observations of zoned chimney samples integrated with in-situ LA-ICP-MS analysis of morphologically different pyrites allowed the assessment of fluid evolution during the chimney growth. Pyrite varieties precipitated with increasing temperatures from the outermost chimney wall (zone A) through the intermediate zone (zone B) to the inner zone (zone C). At the late mature stage, pyrite precipitates as interstice pore fillings. The distribution of trace elements in pyrites across the chimney indicates a strong dependence on time, temperature, and associated sulfide minerals. Variations in the trace elements composition of the different pyrite varieties indicate that the hydrothermal system evolved from low-temperature low-chloride liquid-dominated fluids (enriched in Zn, Cd, Tl, Ag, Pb, Mn, Mo, and V) to higher temperature, vapor-dominated fluids (Cu, Au, Te, and Bi), and then to high-temperature fluids (Co and Se). In the waning stage of the hydrothermal system, circulating hot fluids in auxiliary conduits were depleted in most trace elements. The systematic variations in trace element compositions of hydrothermal fluids induced by changes in physicochemical conditions are properly governed by the law of normal distribution of trace elements. The LA-ICP-MS time-depth analysis indicate that Cu, Zn, Cd, Tl, Ag, Te, and Bi are related to micro-/nano-inclusions, whereas Co, Se, and Mo may occur as substitutions in pyrite lattice. The As, Pb, Au, and Sb show both modes of occurrences as substitutions and as inclusions. Adsorbed films on pyrites control the distribution of V and Mo. The incorporation processes of trace elements in pyrites change with fluid evolution during the chimney growth: trapping of micro-/nano-inclusions and surface adsorption is seen more frequently at the low-temperature stage, and lattice substitution mainly at the elevated temperature stages.

Performance analysis of a cascade room temperature magnetic refrigerator for improving temperature span

A two-stage cascade magnetic refrigerator (CMR) dedicated to improving the temperature span is developed in this paper. The magnetic field of the CMR is provided by two identical C-shaped permanent magnets with an average magnetic field intensity of 1.41 T. Gd is first filled in the active magnetic regenerators (AMRs) of both the high-temperature stage (HTS) and low-temperature stage (LTS), called Gd-Gd MCM configuration. Then, the AMRs of the HTS and LTS are packed with Gd and Gd0.965Er0.035, respectively, termed Gd-GdEr MCM configuration. The performance of the CMR with two MCM configurations and the single-stage magnetic refrigerator (SMR) using Gd as the MCM are tested and compared at the same hot end temperature of ∼293.1 K. The results show that the maximum no-load temperature spans of 4.2, 5.7, and 6.9 K are obtained by the SMR, the CMR with Gd-Gd MCM configuration, and the CMR with Gd-GdEr MCM configuration, respectively. Compared to the SMR, the maximum no-load temperature spans of the CMR with Gd-Gd and Gd-GdEr MCM configurations are increased by 35.7% and 64.3%, respectively. This indicates that a promising approach for increasing the temperature span is to employ the CMR and fill its HTS and LTS with the MCMs of high and low Curie temperatures, respectively. Moreover, the SCPs of the CMR with Gd-Gd and Gd-GdEr MCM configurations are 6.4 and 10.2 W kg−1, respectively, over a 5.0 K temperature span. This study may provide new ideas for the design of magnetic refrigerators with large temperature spans.

Advanced two-stage cascade configurations for energy-efficient –80 °C refrigeration

In response to the COVID-19 pandemic, some vaccines have been developed requiring ultralow-temperature refrigeration, and the number of these freezers has been increased worldwide. Ultralow-temperature refrigeration operates with a significant temperature lift and, hence, a massive decrease in energy performance. Therefore, cascade cycles based on two vapor compression single-stage cycles are traditionally used for these temperatures. This paper proposes the combination of six different cycles (single-stage with and without internal heat exchanger, vapor injection, liquid injection, and parallel compression with and without economizer) in two-stage cascades to analyze the operational and energy performance in ultralow-temperature freezers. All this leads to 42 different configurations in which the intermediate cascade temperature is optimized to maximize the coefficient of performance. Ultra-low global warming potential natural refrigerants such as R-290 (propane) and R-170 (ethane) for the cascade high- and low-temperature stage have been considered. From the thermodynamic analysis, it can be concluded that liquid and vapor injection cascade configurations are the most energy-efficient. More specifically, those containing a vapor injection in the low-temperature stage (0.89 coefficient of performance, 40 % higher than traditional configurations). Then, using an internal heat exchanger for such low temperatures is unnecessary in terms of energy performance. The optimum intermediate cascade temperature varies significantly among cycles, from −37 °C to 2 °C, substantially impacting energy performance. Parallel compression configuration improves energy performance over single-stage cycles, but not as much as multi-stage (between 20 % and 30 % lower coefficient of performance). For most of low-temperature cycles, the high-temperature stage can be based on a single-stage cycle while keeping the maximum coefficient of performance.

The Miwah high sulphidation epithermal Au–Ag deposit, Aceh, Indonesia: Dynamics of hydrothermal alteration and mineralisation interpreted from principal component analysis of lithogeochemical data

The principal component analysis (PCA) of whole-rock major and trace element data has unravelled some of the dominant geochemical processes and identified the most relevant component as an exploration vector for many types of magmatic-hydrothermal ore deposits. An important exception is the high-sulphidation (HS) epithermal class of deposits. Here, the results of a PCA of different segments of the Miwah HS epithermal Au–Ag deposit in Indonesia show that the geochemical signature of the deposit varies from a tight group of Au, Ag, Cu and As in PC1 for the predominantly hypogene main zone to a moderately correlated assemblage of Au, As, Cu and Sb in PC1 for the variably oxidised rocks along the southern periphery to an association of Au, Ni, Cr, Co and Mo in PC1 for the overlying rocks in the north. PC2 in all zones represents the largest variability of Pb and Zn and has moderate contributions from Au and As in the periphery and the highest Au contribution in the north. By contrast, PC3 in the main and peripheral zones, shows a high variability of Cu. Representatives of auriferous vuggy-massive silica rocks from the periphery are in equilibrium with hydrothermal fluids with δ18O values of +6.18, +7.48 and +9.28‰, whereas oxidised, silicified rocks from the analogous northern zone are in equilibrium with hydrothermal fluids with δ18O values of +8.0 and +8.6‰. The results of the above PCA, complemented by centred-log-ratio bivariate diagrams of grade-scaled variables, mineralogical relationships and oxygen isotopes, are interpreted to indicate that PC1 in the main zone is consistent with the deposition of Au and related metals from the condensation of magmatic gas mixtures during the early-stage silicification of protoliths. By contrast, PC2 is related to late-stage Pb- and Zn-rich fluids that migrated away from the main conduits. A hybrid process, intermediary between PC1 and PC2, leached Cr, Ni and Co from the adjacent ultramafic rocks and superimposed the metals on the Au + Cu mineralised rocks. Processes related to PC3 and other smaller PC occurred at later and lower temperature stages in which steam-heated oxidation led to the local destruction of sulphide and arsenide phases and the resulting acid fluids decoupled Au-Ag alloys and enargite to produce separate Au- and Ag-based minerals and covellite. The PCA of individual alteration types shows metal associations that mimic those of the main-stage mineralisation, thereby providing a useful exploration tool. The ability of PCA to separate Au and the associated metals from the Cr + Ni + Co assemblage can be used to identify areas that have been affected by hyperacid fluids originating from epithermal systems; hence, such areas become exploration targets not only in the vicinity of Miwah but also throughout Sumatra or elsewhere.

Pyrolysis Kinetic Analysis of Sequential Extract Residues from Hefeng Subbituminous Coal Based on the Coats-Redfern Method.

Sequential extract residues (Ri, i = 1, 2, 3, 4, and 5) were obtained from Hefeng acid-washing coal (HFAC) by petroleum ether, carbon disulfide, methanol, acetone, and isometric carbon disulfide/acetone mixture, sequentially. Pyrolysis behavior of the residues was determined using thermogravimetry analysis. The Coats–Redfern method with different reaction orders was used to analyze the pyrolysis kinetic of each sample, and the kinetic parameters, including correlation coefficient (R2), activation energy (E), and pre-exponential factor (A), were calculated. Results showed that the weight loss of extract residues was higher than HFAC, and pyrolysis behavior varies greatly for residues, which is related to the unstable structure after extraction. From conversion–temperature (α–T) curves, the pyrolysis process was divided into three stages: low-temperature stage (150–350 °C), medium-temperature stage (350–550 °C), and high-temperature stage (550–950 °C). The medium-temperature stage made great contribution to the process of pyrolysis, which was dominated by depolymerization and decomposition reaction. The relationship between kinetic parameters and reaction order showed that the swelling effect is an important reason for the discrepancy of E for each sample in the process of pyrolysis.

Activation energy of phase transformations at high-temperature synthesis of tungsten carbide by electrothermal explosion under pressure

The effect of activation energy on phase transformations (transitions) in the W–C system during the synthesis induced by an external heat source was investigated by electrothermal explosion (ETE) under pressure. The ETE technology combines self-propagating high-temperature synthesis (SHS) with additional sample heating by Joule heat – electric current passing through the synthesized mixture, and it makes it possible to determine the chemical reaction rate that is highly susceptible to external impacts such as pressure, concentration, sample shape, any film present on combustion products, etc. The chemical reaction rate, i.e. external source current, may be controlled by changing the activation energy. The study was conducted in the following conditions: temperature Т = 293÷3700 K; carbon concentration of 49.8–50.2 at.%; quasi-static compression at P = 96 MPa; external source voltage and current density V = 10 V, I = 20 МА/m2, respectively; samples 8 mm in diameter weighing 6 g. The Т–τ thermogram of the W–C system was used to determine the following parameters: four stages of the synthesis process, temperatures of special points of phase transformations, temperature boundaries of phases and process activation energy. Thermograms of intermediate states are presented as isothermal plateaus of phase transformations. The analysis of experimental results and the physical representation of the process make it possible to assert that temperature plateau parameters are the effective value of activation energy for synthesis mode maintenance. Each of the 4 W–C mixture synthesis stages is described. Pre-explosion stage I – sample heating in the temperature range of Т = 293÷563К, endothermic reaction, effective activation energy for synthesis mode maintenance Q = 2.96 kJ, and taking into account 1-mole mass Еа = 111.6 kJ/mol. Low-temperature (563–1190 К) stage II – ignition, Q = 5.46 kJ, Еа = 109.2 kJ/mol. High-temperature stage (III) in the range of Т = 1190÷2695К, order–disorder transformation, Q = 14.25 kJ, Еа = 424 kJ/mol. Finally, Stage IV occurs in the range of Т = 2695÷3695К, Q = 14.31 kJ, Еа = 143.2 kJ/mol. It was shown that the limiting stage with the highest activation energy is the melting process.