Rising carbon dioxide emissions due to fossil fuel combustion has led to the urgent need to investigate and adopt different energy solutions that can mitigate this problem. Hydrogen has surfaced as a promising alternative in the pursuit for CO2-neutral energy systems. Microwave pyrolysis of methane has recently emerged as an innovative method to accomplish this goal. To enhance our understanding of this technique and its scalability, it is essential to explore the microwave characteristics of the carbon used and generated during this process. This work investigates the microwave properties of two carbon samples (seed carbon; SC and product carbon; PC) from microwave-driven pyrolysis of methane. The cavity perturbation technique was employed from room temperature to 1250°C for frequencies of 397, 912, 1429, 1948 and 2467 MHz. Thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and X-ray diffraction (XRD) analysis was also performed to elucidate the permittivity results. It was found that SC initially showed a decline in permittivity values up to 200°C which is attributed to the release of moisture from the sample. These results were corelated to TGA/DSC which showed 5% mass loss from 100-155°C. The permittivity gradually reached a peak after which it started to fall due to high conductivity. In the case of the PC, the permittivities exhibited undulations but the values remained consistent. Since this form of carbon is formed at elevated temperature, no loss in moisture was seen in TGA/DSC. These findings indicate that the microwaves can penetrate and heat both the samples uniformly across their entire volume, resulting in efficient heating. SC demonstrated higher permittivity magnitudes compared to PC, suggesting its better responsiveness to microwave fields. Nonetheless, the possibility of thermal runaway in SC renders it less favorable for applications involving microwave-driven pyrolysis. XRD analysis showed that the samples SC and PC demonstrated amorphous carbon structures, with PC showing indications of graphitization to some extent. Both SC and PC have the potential to serve as microwave heat carriers in the methane pyrolysis process. This suggests that utilizing the carbon produced can enable a self-sufficient process, eliminating the necessity for costly catalysts.

Nickel (Ni) is used to fabricate a variety of products, including various alloys- stainless steel being a prominent one, batteries, and catalysts. Global Ni demand is projected to increase by up to 350% by the year 2050 relative to 2017, while the high-grade Ni sulfide ore deposits have largely been depleted. The increasing demand for Ni from emerging markets such as electric vehicles and energy storage requires newer sources of nickel. In addition, the systemic exploration of nickel sulfide ores has led to resources that are more challenging to process due to the higher impurities. The processing complexity necessitates the development of new technologies and pathways to sustainably process globally abundant low-grade Ni sulphide ores. This work systematically reviews and compares various Ni resources in terms of their composition, occurrence, challenges, and opportunities. The conventional processing methods of low-grade ultramafic Ni ore are discussed along with emerging technologies. Key challenges remaining and future perspectives that tie together fundamental and applied research are also provided.

We present a carbonation-assisted flotation method for nickel (Ni) separation from low-grade ultramafic ores. The high process economics of conventional flotation of low-grade ultramafic ores makes Ni unrecoverable on an industrial scale. Based on the fundamentals of CO2 sequestration in minerals, we carbonated a low-grade serpentine (lizardite) ore specimen under extreme conditions (185 C-degrees, 15 MPa, and 24 h) and attained a conversion efficiency of serpentine to magnesite of 22% and 16% in the presence and absence of inorganic salts, respectively. Rhombohedral structures on carbonated particles detected using scanning electron microscopy suggested the formation of magnesite. We also found that Ni-Fe existed as sulfides, as confirmed by elemental mapping micrographs, and did not leach out as Ni2+ and Fe2+, as confirmed through carbonation supernatant analysis. Here, we introduced Denver cell froth flotation experiments after carbonation, which showed that the carbonated ore in the presence of inorganic salts had 29% higher Ni recovery and 0.15 wt% higher nickel grade than the uncarbonated ore. This study is the first to successfully show that the carbonation of serpentine containing nickel-bearing pentlandite leads to magnesite formation, does not lead to the dissolution of nickel sulphide, and enhances the nickel separation efficiency during froth flotation. This study paves the path forward for the integration of mineral carbonation in the processing of low-grade ultramafic nickel ores.

Environmentally unsustainable and toxic chemical flocculants and dispersants from nickel (Ni) processing contribute to industrial effluents that greatly impact biodiversity and aquatic life. Despite the industry's efforts to reduce its ecological footprint-primarily due to the lack of commercially available biodegradable, environmentally benign, and nontoxic reagents- these reagents continue to harm natural ecosystems. Chemicals used during the processing operations often target a specific unit operation that negatively affects downstream operations. Herein, we discover the concentration-dependent behavior of cellulose nanocrystals (CNCs) as a dispersant and flocculant, avoiding the use of various harmful chemicals in the nickel processing stages. Electrophoretic, optical brightfield microscopy, and quartz crystal microbalance-dissipation studies detected charge neutralization behavior by renewable and biodegradable CNCs, which can greatly benefit Ni processing operations. Microflotation experiments demonstrated that CNCs enhanced Ni recovery from 62 to 77 wt % and concentrate grade from 15 to 20 wt %. Settling and turbidity studies demonstrated the dual flocculant-dispersant behavior of CNCs resulting from the alignment of CNC fibers along the octahedral brucite basal and amphoteric edge plane of serpentine. CNCs can be used as dispersants during froth flotation to improve Ni beneficiation and as postprocessing flocculants for tailings management and dewatering, which is one of the major environmental and social concerns facing the mining and mineral processing industry. In addition, this study paves the way for CNCs to be used as flocculants and dispersants in a range of industries from healthcare to pharmaceuticals to semiconductor devices.

The depletion of high-grade nickel sulfide ores has led to an increased interest in low-grade ultramafic ores, which are widely available but challenging to process due to their high content of phyllosilicate serpentine. The anisotropic surface charge and shape of serpentine particles lead to attractive interactions, which increase the viscosity of ore slurry and associated operational and equipment costs. To overcome this challenge, we investigate the application of an environmentally benign reagent, sodium citrate, to modify the particle-particle interactions in serpentine aqueous suspensions. Our results show that at alkaline pH (10), sodium citrate selectively adsorbs on the brucite plane of serpentine, making the entire particle overall negatively charged, which weakens the interparticle attraction and reduces the suspension's shear viscosity, yield stress, and storage modulus. We confirmed these results using colloidal probe atomic force microscopy (AFM) technique, which showed a significant weakening of attraction between silica and brucite surfaces upon the addition of citrate. This work enhances the understanding of surface interaction mechanisms of phyllosilicate serpentine particles in aqueous media with the addition of sodium citrate and provides useful implications for the application of green reagents in mineral operations involving phyllosilicate gangues. Our findings suggest that sodium citrate has great potential as an effective and sustainable reagent in the processing of abundantly available low grade ultramafic nickel ores.

Study of comminution of carbonaceous solids during fluidized bed combustion process is essential for understanding the actual combustion rate, thermal efficiency, and particle size distribution. Comminution in a fluidized bed reactor involves a sequence of events that occur either separately or simultaneously, and with combustion process. These events are: primary fragmentation, which is attributed to stresses caused by devolatilization; secondary fragmentation, which is due to the burning up of the linkages inside the char particles; percolative fragmentation, which is a result of internal burning; and attrition, which occurs by collisions with other particles, or with the surface of the reactor. Several studies have demonstrated that due to the impact of particle comminution within the fluidized bed reactor, the carbon conversion efficiency is significantly affected. Furthermore, fine particles produced by comminution are elutriable, and they reduce the fuel residence time as well as thermal efficiency due to incomplete carbon conversion. Hence, there is a need for a complete understanding of particle comminution during practical combustion process in order to increase conversion efficiency. Considering the last review about this topic was written in 1991, the objective of this paper is to provide an updated overview of recent studies on comminution of carbon-based fuels in a fluidized bed reactor. The paper discusses different types of comminutions and their impacts on reactor efficiency, summarizes the experimental set ups used to investigate particle fragmentations, and presents the discussion on different factors of feed fuels that affect particle fragmentation. Literature experiments revealed that primary and secondary fragmentation greatly influence the fuel particle size and distribution, thus ignoring comminution phenomena will lead to errors while assessing actual fuel particle size in the reactor. It was found that fines generation and their postcombustion can cause increase in coarse char combustion rate. Carbon loss by elutriation is substantial in case of low reactive fuels only. Factors such as porosity, particle size, volatile content, and bed temperature heavily influence particle fragmentation. Fragmentation is also influenced by the initial particle size which in turn effects the particle size after de-volatilization and the size distribution throughout the reactor bed. The average size of the particles after de-volatilization and predictions of the size distributions were found to change depending on fuel type.

Aqueous suspensions of swelling clays display a nematic sol-gel transition at very low solid concentrations. The underlying microstructure of the gel has remained a point of contention since the time of Irving Langmuir and has been a major obstacle to fully realizing the potential of clays for practical applications. Here, we comprehensively probe the microstructure of a smectite clay suspension using ultra-small angle neutron/X-ray scattering and find that the nematic gel is structurally ordered and contains entities that are at least an order of magnitude larger than the individual particles. Complementary cryo-electron microscopy shows the presence of domains having particle-particle ordering responsible for nematic texture and regions of particle-particle aggregation responsible for gel-like behavior. We find that the smectic clay gels have a hybrid microstructure with co-existing repulsive nematic domains and attractive disordered domains.

Tailings storage is a prescient issue in mining, representing a visible and destructive liabi l i t y . Dewatering tailings to a paste-like consistency is a popular option to reduce tailings volumes and increase dam safety, but dewatering further to a bul k cake consistency has mostly been reserved for small operations in locations where seismicity or terrain prohibited the construction of dams. The recent high-profile failures of tailings dams such at Funda~o and Brumadinho facilities along with the push toward socially conscious investing have generated renewed and urgent interest in tailings dewatering, including stacked ( "filtered " or "dry-stack ") tailings. While geochemistry and civil enginee r i n g topics relating to tailings in general are both extensi v e l y studied, very little academic literature is available specifically on stacked tailings, with discussion mostly confined to conference articles or extrapolation about the behavior of dewatered tailings. Furthermore, the nomenclature around and definition of this method of tailings storage is inconsistent, which can make researching the topic unnecessarily tedious and confusing and, in the worst case, can open the door to intentional misrepresentation of a tailings storage facility's safety as this is a technology sti l l being scaled up. In this article, the term stacked tailings (ST) is introduced to describe what has been otherwise called filtered tailings or dry-stack(ed) tailings to remedy the inaccuracies with the two existing names and gather the existing literature in one place. Topics covered include the existing definitions of dewatered and stacked tailings; opportunities and barriers to adoption; and construction and general operation for such tailings storage facilities. These are supplemented by an overview of literature in geotechnical and geochemical topics relevant to tailings dewatering (ex. clays) and tailings storage facilities. Emerging dewatering technologies and innovations which might be useful in the scaling up of the technology are also discussed. The aim is to provide a thorough background on the topic for a general audience, outline gaps in the literature, and encourage further exploration of stacked tailings and tailings cake by furnishing a comprehensive list of sources. Stacked tailings are discussed in a rigorous way to make the topic a more approachable way to encourage standardization and adoption, scale up, and the development of associated technologies.

Non-renewable chemical reagents are commonly used as dispersants or flocculants of phyllosilicate clay particles in several industrial fields such as water/wastewater treatment, food production, papermaking, and mineral processing. However, environmentally benign reagents are highly desired due to the non-biodegradability and negative impacts of synthetic reagents on aquatic life. In this work, the dispersion and flocculation behavior of sustainable polymers (anionic and cationic biopolymers) sourced from proteins and polysaccharides were studied in serpentine phyllosilicate suspensions using the following bench-scale tests: zeta potential, microflotation, settling and turbidity, and isotherm adsorption using total organic carbon. The anionic polysaccharide-based biopolymer pectin acted as a switchable biopolymer for serpentine. That is, it could switch from being an efficient flocculant at pH 7 to an effective dispersant at pH 10.

Mining and mineral processing operations are responsible for 4 to 7 percent of greenhouse-gas emissions globally and are under a huge pressure to reduce their emissions. Decarbonization through CO2 capture and utilization is a lucrative option only when the captured CO2 can be utilized to bring down the costs such as in oil recovery from reservoirs where the injected CO2 increases the recovery. Unfortunately, in mining operations, the captured CO2 doesn’t have any utilization so far which is a primary reason of mining industry’s sluggish response for decarbonization commitments. Herein, we provide a first proof of CO2 capture and utilization in nickel mineral processing industry by using CO2 as a conditioning agent prior to froth flotation resulting in the conversion of monohydroxide complexes to carbonates which increases electrostatic repulsion between the valuable mineral and the gangue leading to enhanced froth flotation recovery. The inclusion of CO2 to the froth flotation improved the nickel recovery and grade by an extra 10% and 4%, respectively while permanently storing the CO2 at the same time. The study provides a strong motivation for decarbonatization in the industry through CO2 utilization.

The processing of ultramafic ores requires fine grinding, which can result in slime-coating of the nickel-bearing mineral, pentlandite, by the phyllosilicate gangue mineral, serpentine. This lowers nickel recovery and concentrate grade during froth flotation operations. The presence of divalent cations in process water exacer-bates the detrimental effects of phyllosilicates on nickel recovery by enhancing the positive surface charge of serpentine and causing charge reversal of pentlandite. To mitigate these issues, an environmentally benign, nontoxic, and biodegradable reagent, sodium citrate, was used in this study to chelate divalent cations on the pentlandite surface and desorb them from the mineral surface. The metal-citrate complexes adsorb onto the positively charged serpentine surface, reversing its charge. Sodium citrate was able to restore the surface charge and hydrophobicity of pentlandite, reverse the charge of serpentine, desorb the divalent cations from the mineral surface, and prevent MgO gangue recovery to the froth phase. Microflotation studies demonstrated that upon the addition of sodium citrate, nickel recovery increased from 74 to 96% and nickel grade was improved from 11 to 15.1%. The hydrophobicity of pentlandite was restored upon conditioning with citrate as evident by increase in water static contact angle from 50 degrees to 70 degrees , DLVO interactions showed that addition of citrate prevents slime coating of pentlandite by serpentine. The results demonstrated the depressive and dispersive action of sodium citrate.

Mitigation of colloid clay particles is critical during flotation and flocculation processes in mineral processing. Most organic and inorganic mitigation reagents have negative impacts on the environment and human health; therefore, biologically derived substances have been attracting attention as alternative reagents. Given the anisotropic nature of clay surfaces, it is imperative to understand reagent adsorption on the individual edge and basal plane surfaces of clays. Quartz crystal microbalance with dissipation (QCM-D) was used in this study to determine the adsorption characteristics of three biopolymers (the protein-based biopolymer, lysozyme, and protein and polysaccharide-based oligomers; protamine and pectin) on model surfaces of the anisotropic edge and basal planes of the clays (e.g., kaolinite and serpentine). SiO2 sensor representing the tetrahedral basal plane, Al2O3 and Mg(OH)2 representing the octahedral basal planes, AlSiO and MgSiO representing the edge faces of clays were used as model surfaces of clay minerals. For kaolinite, protamine adsorbed preferentially on the silica (SiO2) tetrahedral surface at pH 7 and on the alumina (Al2O3) surface at pH 10. Protamine adsorbed primarily on magnesium hydroxide (Mg(OH)2), representative of serpentine, at pH 7 and 10. Lysozyme adsorbed preferentially and irreversibly on the edge basal plane surfaces of both clays at pH 10, while it showed a higher affinity for octahedral surfaces (alumina and magnesium hydroxide) at pH 7. In contrast, pectin adsorbed strongly on the magnesium hydroxide, representative of the basal plane surface of serpentine. An adsorption study revealed that electrostatic attraction and/or hydrogen bonding mechanisms contributed to the adsorption of biopolymers on clay surfaces. This investigation provides a fundamental and practical understanding of biopolymer interactions with clay surfaces during selective flotation and flocculation.

Aqueous suspensions of geometrically anisometric (2D) sodium-montmorillonite (Na-Mt) particles display a sol-gel transition at very low solids concentrations. The underlying microstructure of the gel has remained a point of contention since the time of Irving Langmuir. An in-situ investigation encompassing length scales much larger than the individual particles is required to provide support for one of the two models proposed in the literature: 1) a percolated network governed by electrostatic attraction between platelets; and 2) a jammed suspension stabilized by repulsive electrostatic forces between particles. We settle this debate by comprehensively probing the microstructure of Na-Mt suspensions using ultra-small angle neutron/X-ray scattering and found that it is ordered and contains entities that are at least an order of magnitude larger than the individual particles. Complementary cryo-electron microscopy showed both the presence of domains having strong particle-particle ordering and regions of particle-particle aggregation. These data indicate 1) the presence of nematic domains, which refutes a purely attractive nature, and 2) assembly of particles, which refutes a purely repulsive nature. Na-Mt gels appear to have a hybrid microstructure with both attractive and repulsive domains.

A proof-of-concept for the carbonation-assisted processing of ultramafic nickel ores is presented. Carbonation converts serpentine, the primary gangue or undesirable mineral, to magnesite. It prevents slime coating of fine gangue minerals on pentlandite, the main nickel-bearing mineral, during froth flotation, and improves nickel recovery and concentrate grade. Additionally, CO2 is captured and stored in the form of solid carbonates, thus removing it from the atmosphere. Microflotation experiments demonstrated improved nickel recovery (61.2 to 87.4 wt%) and concentrate grade (20.6 to 24.7 wt%) in carbonated vs. uncarbonated systems. The mechanism behind the improved nickel flotation was investigated by zeta potential measurements, optical imaging microscopy, X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry. These analyses confirmed the absence of slime coating in the carbonated system under the flotation conditions tested. Finally, a preliminary techno-economic analysis was performed to evaluate the cost metrics of incorporating carbonation into nickel mineral processing.

HYPOTHESIS:Sodium-montmorillonite (Na-Mt) particles are geometrically anisometric that carry a pH dependent anisotropic surface charge. Therefore, it should be possible to manipulate the particle-particle interaction of colloidal range Na-Mt suspensions through pH changes which in turn should alter the soft glassy dynamics of Na-Mt suspensions. EXPERIMENTS:Rheological experiments were used to probe the impact of pH mediated colloidal particle-particle interaction on the physical aging, linear viscoelastic response, and yield stress behavior of Na-Mt suspension. FINDINGS:The temporal evolution of the storage modulus (G') was stronger in the acid regime (pH < 9.5) than the base (pH ≥ 9.5) pH regime. Horizontal shifting of the aging curves in the acid and base regimes led to aging time-H+ concentration and aging time-OH- concentration superposition. An aging time-Na-Mt concentration superposition was also observed in both pH regimes. The critical stress associated with the viscosity bifurcation behavior increased linearly with G' but with different slopes for acid and base regime. We propose that positively charged patches on the Na-Mt particle edge merge with the characteristic surface as a function of H+ ions in the system. This leads to a strongly associated microstructure at low pH and a relatively weak but associated microstructure at natural pH, hence confirming the hypothesis.

Oil recovery from carbonate reservoirs is low because of their complex nature which is characterized by the presence of high permeability streaks/fractures, broad pore size distribution and oil-wet/mixed wet state. The presence of high permeability streaks/fractures causes the injected fluid to bypass the low permeability pores that contain a high percentage of producible oil. One of the most effective and promising methods to control the fluid bypass is fluid diversion using chemical agents where a chemical agent is placed in the high permeability zone to block the path for the subsequent fluid, which ultimately diverts the injected fluids to the less permeable and oil-containing region. In this work, we investigate the potential use of cellulosic nanocrystals (CNC) for fluid diversion applications. Upon contact with brine, the nanosized CNC particles agglomerate to form micron size flocs showing a viscosity increment of an order at reservoir shear rates. In our core flooding experiments, the permeability of carbonate core decreased from 2827 mD to 127 mD when a CNC dispersion of 3000 ppm was injected. Injection of pre-equilibrated de-ionized water following CNC injection showed that the permeability reduction is permanent signifying the stability of CNC agglomerates in high temperature-high salinity environments. The blocking of high permeability zones due to the agglomeration of CNC particles results in fluid diversion. This improves the sweep efficiency and allows the injected fluid to come in contact with the oil. This is the first application of CNC in the area of fluid diversion for high temperature and high salinity fractured carbonate reservoirs which are present in the Middle East.

Schizophyllan is a natural polysaccharide that has shown great potential as enhanced oil recovery (EOR) polymer for high-temperature, high-salinity reservoirs. Nevertheless, the adsorption behavior of schizophyllan over carbonate minerals remains ambiguous element towards its EOR applications. Here, we investigate the adsorption of schizophyllan on different carbonate minerals. The effect of mineral type, salinity, and background ions on adsorption is analyzed. Our results indicate the adsorption capacity is higher on calcite and dolomite compared to silica and kaolin and the adsorption capacity decreases with salinity. Moreover, the adsorption kinetics follows pseudo-second order mechanism regardless of the mineral type. Adsorption over calcite is diminished in presence of water structure making ions and enhanced in presence of structure breaking ion and in presence of urea. Gel permeation chromatography results reveal the preferential adsorption of longer chains. The adsorption over carbonate minerals proceed via complex formation between polymer molecule and mineral surface.

In this work, we developed an extended surface complexation model (SCM) that successfully fits all tested zeta-potential data (63 in total) of synthetic calcite and three natural carbonates (Iceland spar, Indiana limestone, "SME" rock from a Middle East field) in brines with divalent ions in a wide range of ionic strengths (0.001-0.5 M). To develop this extended model, our previous reported SCM is first optimized by incorporating the zeta-potential of synthetic calcite in a wide range of ionic strength (0.001-0.5 M) along with previously published data for parameter refitting. The model is then applied to predict the surface charge of synthetic calcite in concentrated solutions up to 5 M NaCl to reveal the role of high salinity in calcite wettability. Eventually, the model is extended to fit the zeta-potential of natural carbonates by adding surface reactions for impurities such as silica and organic-based carboxylic acids. The coverage of the organic impurities is found to be essential for explaining why the zeta-potential of natural carbonates is more negative compared to that of synthetic calcite. Naphthenic acid (assumed to have one carboxylic group) and humic/fulvic acid (assumed to have six carboxylic groups) are tested in the model calculation as possible sources of surface impurities to demonstrate the effect of the number of carboxylic groups in the acid molecule. Finally, the effect of a humic acid pretreatment on the zeta-potential of synthetic calcite is investigated experimentally to verify the assumption that absorbed organic impurities on the calcite surface contribute significantly to a more negatively charged natural carbonate surface when compared to that of pure calcite surfaces.

Wettability alteration of carbonate rocks from an oil-wet state to a water wet or mixed-wet state during water flooding is known to enhance the recovery of oil from the reservoirs. Previously, the wettability of porous mediums has been extensively investigated using numerous macroscopic methods. Among those, the contact angle is a highly employed method for wettability measurements; however, this technique, because of its lower spatial resolution, fails to provide a complete chemical understanding of all the factors affecting the reservoir wettability. In an attempt to overcome this, we employed a multiscale approach involving macro-, micro-, and nanoscopic analytical techniques to investigate the wettability of calcite. Studies were performed by aging two different planes of freshly cleaved calcite in ambient atmosphere and with deionized (DI) water. Contact angle measurements and AFM force profiles were recorded at the macroscale and nanoscale, respectively. Wettability transition was observed from super hydrophilic to hydrophobic nature in ambient atmosphere and super hydrophilic to hydrophilic nature in DI water. When AFM studies were performed on samples aged in DI water there were always patches of water present, which were observed only at the nanoscale. These water patches affect the contact angle measurements and make the macroscopic wettability results inherently ambiguous. This work has shown that the contact angle measurements should not be taken as the absolute measurement of wettability.

The underlying mechanism of wettability alteration is vital for enhancement of oil recovery (EOR) by water flooding in carbonate reservoirs, such as calcite rock. Based on first-principles molecular dynamics simulations and core-flooding measurements, the authors find that proximal adsorption of ions in brine affects the wettability of calcite: Some ions primarily disturb the interfacial water structure, while others modify the effective surface charge, thus inhibiting and enhancing oil recovery respectively. This study provides needed insight into the physics of wetting at the atomic scale for EOR.