Particle Detachment Research Articles

Dry Reforming of Methane over 5%Ni/Ce1-xTixO2 Catalysts Obtained via Synthesis in Supercritical Isopropanol.

A series of 5%Ni/Ce1-xTixO2 catalysts was prepared with nickel impregnation of mixed Ce-Ti oxides obtained via synthesis in supercritical isopropanol. All oxides have a cubic fluorite phase structure. Ti is incorporated into the fluorite structure. Small amounts of impurities of TiO2 or mixed Ce-Ti oxides appear with Ti introduction. Supported Ni is presented as the NiO or NiTiO3 perovskite phase. Ti introduction increases total samples reducibility and results in stronger interaction of supported Ni with the oxide support. The fraction of rapidly replaced oxygen and the average tracer diffusion coefficient also increase. The number of metallic nickel sites decreased with increasing Ti content. All catalysts except Ni-CeTi0.45 demonstrate close activity in tests of dry reforming of methane. The lower activity of Ni-CeTi0.45 can be connected to Ni decoration with species of the oxide support. The incorporation of Ti prevents detachment of Ni particles from the surface and their sintering during dry reforming of methane.

Lattice Boltzmann simulation of cathode catalyst layer degradation on transport reaction process within a proton exchange membrane fuel cell

The degradation of the catalyst layer significantly affects the gas transport and electrochemical processes in the porous electrodes, thus affecting the performance of proton exchange membrane fuel cells. To reveal the catalyst layer degradation impact, the microscopic porous structure of the cathode catalyst layer was reconstructed by a random algorithm in this work. Consequently, Lattice Boltzmann method was used to study the oxygen transport and electrochemical reaction processes at the limiting current density condition with considering the degradation of platinum, carbon particles, and ionomers under uniform and exponential degradation rates, respectively. The results reveal that the degradation of platinum reduces the reaction sites in the catalyst layer, thus deteriorating the electrochemical kinetics and lowering the total reaction rate. On the contrary, the degradation of carbon and ionomer shows two diametrically opposed effects. On the one hand, the oxygen transport is improved due to carbon and ionomer degradation, especially for ionomer degradation, thereby accelerating the total reaction rate. On the other hand, the degradation of carbon and ionomer triggers the detachment of platinum particles, leading to a decrease in reaction rate. In the early stages of the multi-component simultaneous degradation process, the total reaction rate is prohibited by oxygen transport limitation inside the catalyst layer; as the degradation degree increases, the oxygen transport through the ionomer films is enhanced and the electrochemical kinetics becomes the rate determining factor, especially for exponential degradation rate. This study provides a comprehensive assessment of the oxygen transport and electrochemical reaction within the catalyst layers with respect to different degrees of catalyst layer degradation, which can guide the design of high-performance anti-degradation catalyst layers for the next generation of fuel cells.

Droplets impact against the random rough surface with a liquid film

Aero-engine fouling will affect the economy and safety of aircraft, and online washing is the main method to solve the problem of aero-engine fouling. Online washing is considered to be the removal of fouling particles by the impingement of droplets on a flowing liquid film. Here, we simulate a flowing liquid film of droplets impinging on a randomly rough surface to predict the removal of fouling particles. We considered the situation that the horizontal flow velocity of the liquid film is between 5 and 20 m/s and the thickness of the liquid film is 5–20 μm. An analysis of the droplet impact velocity field, radial velocity, and extension length shows that the vortex strength, radial velocity, and extension length all increase with increasing liquid film flow velocity, while increasing liquid film thickness inhibits the growth. Moreover, we constructed a fouling particle detachment model to evaluate the effects of different liquid film flow velocities and film thicknesses on particle removal. A particle removal plot shows the dominance of liquid film flow velocity stations. This study not only reveals the removal process of fouling particles on random rough surfaces but also provides insight into the optimization of spray cleaning process parameters.

Influence of Sand Grain Size and Clay Mineralogy on Suffusion of Sand-Clay Mixtures

Suffusion is a type of internal erosion caused by the detachment of fine particles. Additionally, continuous suffusion reduces the stability of geotechnical infrastructures. In this study, the impact of clay mineralogy and sand grain size on the suffusion of sand-clay mixtures was investigated using two-dimensional laboratory experiments on nine different sand-clay mixtures made of three types of sands (K3, K4, and K5) and clays (kaolinite, illite, and bentonite). The breakthrough curve for each sample was obtained for the top/middle/bottom outlets of the designed cell to measure and analyze the amount of detached clay. It was observed that as the sand grain size decreased, the amount of detached clay was more and clays with kaolinite exhibited greater suffusion than those with illite. Bentonite exhibited the lowest amount of detached clay owing to its high swelling potential. The influence of clay mineralogy and sand grain size on the suffusion has been discussed based on the obtained breakthrough curves.

Response of particle detachment and hydrological processes on windward slope in laboratory flume experiments at different wind velocities and rainfall intensities

Wind-driven rain (WDR) is a natural phenomenon that causes soil erosion, a decline in land productivity, water pollution, and other environmental problems. Owing to the influence of topography on atmospheric circulation, WDR occurs most frequently on windward slopes. Studying the physical mechanisms of soil erosion on windward slopes is challenging because of the complex co-occurrence of wind and rainfall. This study aimed to investigate the effects of rainfall intensity (30, 60, and 90 mm h−1), wind velocity (0, 2, 3, 5, 7, and 8 m s−1), and induced flow hydraulic characteristics on the processes of runoff occurrence, infiltration, and sediment detachment on windward slopes based on an indoor artificial simulated WDR soil flume experiments with three replicates. The results showed that both rainfall intensity and wind velocity exerted significant effects on runoff and sediment transport on the windward slopes (P < 0.05). The contribution rates of wind velocity on runoff occurrence time, steady infiltration rate, runoff rate, and sediment detachment rate were 20.53 %, 39.44 %, 3.36 %, and 6.24 %, respectively. When the rainfall intensity was constant, the steady infiltration rate on the windward slope increased linearly with the increasing wind velocity, whereas the runoff and sediment detachment rates decreased with increasing wind velocity. The decrease in hydraulic parameters of average flow velocity (v), stream power (Ω), unit stream power (U), and unit energy of cross-section (E) influenced by wind blowing on overland flow is the main reason for the decrease in the sediment detachment rate on the windward slope. Meanwhile, the increase in flow depth (h) and shear stress (τ) caused an increase in the infiltration rate. Because of the inclusion of Ω and E in the total work done and total energy consumption of overland flow, respectively, the relationship between Ω and E of the windward water flow with the sediment detachment rate showed an exponentially increasing curve for all treatments (R2 greater than 0.97). The hydraulic parameters Ω and E of the windward water flow are the optimal factors for constructing an erosion dynamic model under WDR conditions.

Treatment of Oil Production Data under Fines Migration and Productivity Decline

Fines migration is a common cause of permeability and, consequently, injectivity and productivity decline in subterranean reservoirs. Many practitioners implement prevention or remediation strategies to reduce the impact of fines migration on field productivity and injectivity. These efforts rely on careful modelling of the underlying physical processes. Existing works have demonstrated the ability to predict productivity decline by quantifying the extent of particle decline at different fluid velocities. Fluid flows in porous media often involve multiple phases, which has been shown in laboratory experiments to influence the extent of particle detachment. However, no theory has directly accounted for this in a particle detachment model. In this work, a new model for fine particle detachment, expressed through the critical retention function, is presented, explicitly accounting for the immobile fines trapped within the irreducible water phase. The new model utilises the pore size distribution to allow for the prediction of particle detachment at different velocities. Further, an analytical model is presented for fines migration during radial flow into a production well. The model accounts for single-phase production in the presence of irreducible water, which has been shown to affect the extent of fines migration significantly. Combining these two models allows for the revealing of the effects of connate water saturation on well impedance (skin factor growth) under fines migration. It is shown that the higher the connate water saturation, the less the effect of fines migration. The appropriateness of the model for analyzing production well data is verified by the successful matching of 10 field cases. The model presented in this study is an effective tool for predicting the rate of skin growth, its stabilization time and final value, as well as the areal distribution of strained particles, allowing for more intelligent well remediation design. Further, the findings of this study can help for a better understanding of the distribution of fines within porous media and how their detachment might be influenced by pore structure and the presence of a secondary immobile phase.

Current and friction transients during tribocorrosion of a biomedical titanium alloy sliding against zirconia

The tribocorrosion mechanisms of a Ti–Nb–Zr–Ta (TNZT) alloy rubbing against a zirconia ball in a Phosphate Buffered Solution (PBS) were analyzed by considering the mechanical and chemical interactions that lead to the material degradation. To understand the governing mechanisms, tribocorrosion tests were carried out in a ball on flat contact and reciprocating tribometer applying two normal loads (6.7 and 11.2 N) and two different anodic potentials (400 and 1500 mV). Worn surfaces were characterized by SEM and FIB cross sections as well as surface analysis. During the tribocorrosion tests, transients consisting in simultaneous abrupt current increase and a decrease in the COF were found at different instants of the tests. Subsurface cracking as well as third body build up were observed. These different phenomena and their temporal sequence could be described by initial sliding tribocorrosion, including metal particle removal and surface oxidation, followed by the build-up of a protective third body. After a certain number of sliding cycles subsurface cracking due to strain accumulation beyond a critical level lead to the detachment of a large particles that reactivated the initial tribocorrosion process. This mechanistic view could be substantiated by a model based on a quantitative analysis of the results.

Improving the wear resistance of aluminum by a nickel-filled anodized porous alumina layer

Anodizing is a widely used method for increasing the wear and corrosion resistance of aluminum alloys. However, it is known that the porous brittle alumina layer can easily crack during tribological stresses, which results in the detachment of large particles from the worn surface. Electroplating, on the other hand, can produce a compact ductile metallic coating with enough wear resistance, though a specific pretreatment like the zincate process is needed for an aluminum substrate. In this paper, a combination of the two methods, anodizing and electroplating, is applied on the surface of pure aluminum intending to improve the wear resistance. Anodizing was performed in oxalic acid by applying a constant current density of 2 A/dm2. The anodized samples were then immersed in phosphoric acid for up to 10 h to increase the diameter of the pores. Nickel electroplating was carried out on the anodized samples at a current density of 0.5 A/dm2 in a Watts solution. Cross-sectional studies show that there is an optimum pore widening time that establishes a good adhesion between nickel and anodic oxide layers, while the structure of the oxide film is largely preserved. Sliding wear tests were performed on the samples using a ball-on-disk reciprocating tribometer. The results show much higher wear life for all Ni/Al2O3 composite coatings as compared with the alumina layer. The maximum wear life was obtained for the sample etched for an optimum pore widening time of 4 h.

Fouling by inorganic-particle-containing cake layers can be reduced by microorganisms at low fluxes

A microbe-rich cake layer (i.e., a membrane biofilm) may maintain a stable hydraulic resistance at low fluxes because the microorganisms may decompose organic foulants and create cavities in the cake layer. However, these microorganisms cannot decompose the inorganic particles, which can increase cake resistance because of the synergistic fouling effect with organic foulants. To gain a better understanding of the effect of microorganisms on the microbe-rich cake layer containing inorganic particles, we investigated the change in hydraulic resistance, structure, and composition during the transition of cake layers from a microbe-poor state to a microbe-rich state, at the flux of 5, 10, and 30 L·m−2·h−1. And we used SiO2 particles (2.5 μm in diameter) and sodium alginate to simulate inorganic particles and organic foulants. The results showed that when the cake layer was transformed from a microbe-poor state to a microbe-rich state, the hydraulic resistance was reduced, the structure became looser, and inorganic particles were partially detached. These changes were more pronounced for the cake layer under an ultra-low flux of 5 L·m−2·h−1, with 62% and 47% reductions in the hydraulic resistance and inorganic particles, respectively. These results indicated that the fouling by inorganic particles could be reduced by the microorganisms at ultra-low fluxes. Cake composition analysis, forces balance analysis, and computational fluid dynamics simulation suggested that the reduced fouling could be derived from the reasons 1) the biodegradation of organic foulants could promote the detachment of inorganic particles, and 2) the microbial-induced cavities at ultra-low fluxes could reduce the synergistic fouling effect by inorganic particles and organic foulants.

Codependent failure mechanisms between cathode and anode in solid state lithium metal batteries: mediated by uneven ion flux

An in-depth understanding of the degradation mechanisms is a prerequisite for developing the next-generation all solid-state lithium metal battery (ASSLMB) technology. Herein, synchrotron X-ray computed tomography (SXCT) together with other probing tools and simulation method were employed to rediscover the decaying mechanisms of LiNi0.8Co0.1Mn0.1O2 (NCM)|Li6PS5Cl (LPSCl)|Li ASSLMB. It reveals that the detachment and isolation of NCM particles cause the current focusing on the remaining active regions of cathode. The extent of Li stripping and the likelihood of Li+ plating into LPSCl facing the active NCM particles becomes higher. Besides, the homogeneity of Li stripping/plating is improved by homogenizing the electrochemical reactions at the cathode side by LiZr2(PO4)3 (LZP) coating. These results suggest a codependent failure mechanism between cathode and anode that is mediated by uneven Li ion flux. This work contributes to a holistic understanding of the degradation mechanisms in ASSLMBs and opens new opportunities for their further optimization and development.

Model Characterizes Formation Damage Caused by Fines Breakage and Migration

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 208810, “Formation Damage Caused by Fines Breakage and Migration,” by Abolfazi Hashemi, Sara Borazjani, and Bryant Dang-Le, SPE, University of Adelaide, et al. The paper has not been peer reviewed. _ Fines migration is a major cause of formation damage; the detached clays migrate and impair well productivity. Two types of damaging clays are encountered in petroleum reservoirs: authigenic clays that grew on the grain surfaces during geological times, and detrital clays that have been broken off the grains because of local stresses. The aim of the work described in the complete paper was the development of a laboratory procedure to estimate formation damage by authigenic clays and the derivation of a mathematical model for core scale. Background Detrital particles are attached to a substrate by an electrostatic force and can be modeled using the DLVO theory. Most core samples, however, have some authigenic clay content (i.e., clay particles bonded to rock). To study formation damage caused by the detachment of authigenic clay, a new formulation should be presented. The formulation should be based on failure or breakage criteria of the contact bond between the particle and a substrate. To the best of the authors’ knowledge, the theory for detachment of a single authigenic particle is not available. The purpose of this study was the development of a laboratory-based mathematical model to estimate formation damage by authigenic clays. In the complete paper, the authors assumed that the bond between the authigenic clay and rock acts as a cantilever beam. The cohesive beam model (CBM) was applied to formulate the detachment of the particle because of the applied drag force. The stress state at the bond was governed by the Timoshenko beam theory. These equations are applicable for the elastic limit of both brittle (elastic) and ductile (elastic/plastic) materials, and only different failure criteria were applied. Kaolinite in porous media was assumed to be elastic/perfectly plastic. For defining failure for this type of material, yield criteria were applied. The von Mises yield criterion was used to define failure. For detachment of every particle, a specific drag force or a fluid velocity must be applied. Because a large number of particles with different random properties exist in porous media, by considering a distribution for each property, a distribution for detachment velocity can be obtained. By taking an integral from this distribution bounding to experimental velocities, the amount of detachment between two fluid velocities can be obtained. The same procedure can be followed for detrital particles and authigenic particles. Total detachment concentration is the sum of authigenic and detrital particles. The total detachment is matched with laboratory data. Having obtained the matching parameters, a maximum retention function curve can be plotted. This new maximum retention function takes into account the detachment of both authigenic and detrital particles. In the complete paper, the authors initially presented the assumptions of their models, then governing equations for detrital particle detachment and detachment of authigenic particles; finally, the formation damage formulation and solutions were provided, which are applicable for both detrital and authigenic particles.

Changes in soil surface properties under simulated rainfall and the effect of surface roughness on runoff, infiltration and soil loss

Soil erosion by water is a result of detachment of particles or small aggregates from the soil surface followed by transport of the detached material. One of the elements that affects surface runoff and soil erosion is the soil surface roughness (SSR). Prior research reports that increasing SSR reduces generation of runoff and soil loss. In addition to that, it is widely reported that across-slope oriented roughness is better at controlling soil and water losses. However, to date there have been few studies into the effect of both magnitude and orientation of SSR on runoff, infiltration and soil erosion at the sub process level (i.e. by raindrop splash and overland flow), occurring simultaneously. In this study, the effects of up-down-slope oriented SSR (Treatment A), across-slope oriented SSR (Treatment B) and random SSR (Treatment C) were compared, along with a smooth surface (Treatment D). A moderate slope gradient of 10 %, a simulated rainfall intensity of 90 mm hr−1 and storm durations of 15 or 30 min were considered. The SSR was measured using the chain method, before and after the rainfall event. Images of the soil surface were taken using a hand-held laser scanner to monitor the effect of rainfall on the surface morphology. The outcome of this study shows that rainfall erosivity increases the SSR of the initially smooth surface, but decreases that of the initially rough surface, particularly in the random SSR treatment, where the decrease in SSR was 64 % of the pre-rainfall condition. This was due to the effects of raindrop impacts and overland flow. The random SSR treatment generated significantly more runoff and soil loss, and less infiltration than all other treatments (p < 0.001), but for raindrop splash erosion, there was no significant difference between random SSR and the other treatments. Contrary to expectations, the across-slope oriented SSR did not always reduce runoff and soil erosion compared to the up-down-slope orientation. This can be explained by degradation of surface microtopography by rainfall and runoff, as confirmed by the post-rainfall SSR measurements.

Incorporation of ZnO/Ag-ZnO nanoparticles into a cotton fiber by the LbL method for Self-Cleaning applications

This paper reports the ZnO particles synthesis and characterization stabilized in poly-(diallyldimethylammonium chloride) polymer (ZnO-PDDA). By means of the TEM images, ZnO nanoparticles showed ellipsoid-shaped particles, with diameters ranging from 100–500 nm. The LbL method was utilized for the cotton fibers functionalization, using Ag and ZnO nanoparticles. The nano-hybrids incorporation through this method was carried out after a chemical modification in the cotton fabric surface. The functionalized cotton fibers characterization was performed and showed an excellent photocatalytic activity, degrading about 75 % of methylene blue (MB) in 8 h of sun exposure. The presence of one layer of Ag nanoparticles improved photocatalytic efficiency reasonably, completely degrading the MB dye in 4 h. Fabric washing tests showed that even after 15 consecutive-cycle washes, the particle detachment is minimal (about 0.1 ppm), so the LbL technique is a good alternative for the functionalization of new materials, including textile materials.

On the variation of the correction factor of surface velocity with the measurement vertical for shallow flows over rough beds

AbstractConsidering that water flow energy affects the detachment of soil particles, the transport and deposit of the detached particles, the flow velocity is a key variable governing the soil erosion processes at the hillslope scale. The simple dye‐tracer technique for measuring mean flow velocity can be applied in non‐controlled field applications for which some measurement difficulties (e.g. due to sediment transport, and shallow flows) can occur. The correction factor is usually obtained as the ratio between the mean velocity, deriving from measurements of flow discharge and water depth, and surface velocity. Alternatively, the possibility of using the velocity profile in a given vertical to determine a suitable correction factor has not yet been explored. In this article, the flow velocity measurements were carried out in five verticals having a different distance from the wall of a flume whose bed was covered by hemispheres with different concentrations (9%–64%) and organized in two arrangements (square, staggered). Fifteen runs, characterized by different Reynolds and Froude numbers, were performed and the correction factor αvv was calculated for each vertical. For the axial vertical αvv was independent of the arrangement, decreased with the hemisphere concentration, and increased with both the Froude and Reynolds numbers. Furthermore, αvv decreased from the bank to the flume axis as a result of the varying shape of the velocity profile. The analysis showed that the frequency distributions of the ratio between αvv and its mean value αm were overlaid and αm was used to represent the correction factor, accordingly. A relation of αm with the distance from the flume wall was detected. For the measured velocity profiles having a power shape, the analysis also demonstrated that the αvv values, obtained by fitting the power law to the measurements, were close to both those calculated as the ratio between the mean velocity along the vertical and the surface velocity and their averaged values across the hydraulic section. These three methods give comparable average values of the correction factor, in the range of 0.72–0.75, which are close to the commonly applied value of 0.8. Finally, this investigation demonstrated that a single survey of a power velocity profile allows for obtaining the correction factor accounting for different verticals across the entire hydraulic section.

Spatial migration and distribution of suspended particles in saturated sand layer under the impact of temperature

AbstractThe migration, deposition and detachment of suspended particles in saturated sands are used in many fields. Temperature is an important influencing factor in the transport and deposition process of particles in porous media, and it plays an important role in ground‐water heat pump (GWHP) projects. This article discusses the independent development of a three‐dimensional percolation sandbox test device and the design of a particle migration test based on the Reynolds number in the context of the backfilling process of GWHP systems as the experimental background. The effects of various temperatures (10, 25, and 40°C) on the movement of suspended particles in porous media were studied. The experimental results showed that as the temperature rose, more particles were deposited in the sand layer, the uneven flow of particles in the sand layer accelerated, and the particles spread out in the direction opposite to the non‐percolation direction with a wider distribution range in the sandbox. Increased temperature also promoted the percolation of smaller particles, causing the diffusion zone to terminate sooner and the deposition zone to last longer, resulting in the capture of more particles on the surface and in the pore channels of the sand layer.

Flexible canvas produced from uncured-natural rubber composites filled with high calcium oxide fly ash/cement hybrid filler

Concrete canvas (CC) is an attractive composite based on a combination of textile and cement, applying for the slope protection and ditch lining. However, there is no binder between both the materials and its high thickness causes a strong detachment of cement particles, and the complex designing of CC shapes is limited. Therefore, the possibility of incorporating natural rubber (NR) as a binder for extending the applications of cement-based composites has been considered. In the present work, NR composites with fly ash (FA) and cement (CM) are prepared through an internal mixer and a two-roll mill with a total filler concentration of 1000 phr. It can be found that, FA/CM hybrid filler is successfully adhered together through the NR matrix, and it promotes the flexible canvas composites with controlled thickness and provides complex shapes. The existing calcium oxide (CaO) present in the filler reacts with water molecules, and the NR composites become hardened and restrict the deformation and decomposition. The optimum ratio of FA/CM hybrid filler has been fixed at 300/700 phr, which has recognized as the product-like concrete canvas. Tensile properties, dynamic properties and thermal stability are improved upon the incorporation of hybrid fillers and its flexibility is found to be sustained on soaking with water. The structural changes are confirmed from the infrared spectrum which clearly explains the transformation of CaO to the CSH network structure in the NR composites. Un-crosslinked composites exhibited similar mechanical properties compared to the chemically crosslinked NR with sulfur curing system. Thus the composites can be used as the flexible canvas, although it has not been vulcanized. To clarify this statement, tensile properties, abrasion resistance and bending propagation of NR composites, NR vulcanizates and commercial concrete canvas are compared. The composites exhibited comparable properties with superior flexibility performance. Along with the waste utilization, the uses of FA, provides an opportunity of using FA/CM as the flexible canvas.

Asymmetrically coupled two lane totally asymmetric simple exclusion process with extended Langmuir kinetics

The proposed study is motivated by the transport phenomena where the particle hopping, attachment, and detachment dynamics rely on the occupancy state of the corresponding site on another lane. To depict this kind of scenario, we consider an open two-lane Totally Asymmetric Simple Exclusion Process with the presence of asymmetrically coupled and extended Langmuir kinetics regulations. We have analyzed the steady-state properties of the system by calculating characteristics including phase diagrams, density profiles, phase transitions, and finite size effect. The theoretical results are captured by utilizing the framework of a mean-field theory, and it is recognized that numerical findings match closely with the Monte Carlo simulation outcomes.

Role of hydrophobic fine particles in coarse particle flotation: An analysis of bubble-particle attachment and detachment

It is widely accepted that hydrophobic fine particles improve froth stability and thus increase the recovery of coarse particles in the froth phase. In fact, the presence of fine particles also enhances coarse particle recovery in the collection zone. It is not yet known what causes this, nor is it known how fine particles affect interactions between bubbles and coarse particles (collision, attachment, and detachment). In the present study, the role of hydrophobic fine particles in improving the recovery of coarse particles in the pulp phase was discussed from the perspective of bubble-particle interaction. Flotation tests of coarse particles with different fine particle contents were first carried out. The bubble collision-attachment test, bubble-particle aggregate stability test, and interaction force test were used to further explain the effects of fine particle coverage on bubble-particle interactions. The results show that the recovery of coarse particles increases as the fine particle content in the flotation feed increases. Although the presence of hydrophobic fine particles prevents the rupture of the liquid film and the formation of the contact line between bubble and coarse particle, it can also greatly increase contact angle hysteresis, increasing the maximum force required to pull coarse particle from the gas-liquid interface into the aqueous phase. Furthermore, the coverage of fine particles helps to improve the energy dissipation during the bubble collision and reduce the oscillation of the bubble-particle aggregates. As a result, the increase in coarse particle recovery in the presence of fine particles is due to the decrease in the detachment of the bubble-particle aggregate. The important findings of this study are expected to provide a basis for understanding the interaction between multiple particles and bubbles in actual flotation.

Investigation into the Gaseous SO2 Attack on Sandstone in the Yungang Grottoes

The Yungang Grottoes, with over 1500 years of history, have been subjected to air pollution since the last century. Field investigations have indicated that acid gases, particularly sulfur dioxide (SO2), have accumulated on the surface of the sculptures and caused various types of decay that reduce their artistic value. To shed new light on the gas–stone interaction process, artificially accelerated weathering was performed on local sandstone in the laboratory. In a specially developed test device, fresh specimens were exposed to gaseous SO2 under different relative humidity and temperature conditions. The physical, mineralogical, and chemical changes of Yungang sandstone were evaluated conjointly using destructive and non-destructive methods. The results show that after weathering, the luminosity of all specimens changed, with a slight alteration in hue toward yellow. The weight increased to various degrees during the aging cycles, which depended on both the accumulation of matter and the detachment of particles. Higher relative humidity and cyclic temperature fluctuations favored the dissolution of carbonates and the hydrolysis of feldspar in sandstone. The concentration of ions, especially dissolved Ca2+ and SO42−, increased considerably over time in the near-surface region of the specimens. A trace of newly formed gypsum was detected in some specimens at the end of the test. Knowing the synergistic impact of different climatic variables will make it possible to identify the mechanisms of the deterioration of sandstone in complex environments.

Simulation of Rainfall Erosivity Dynamics in Romania under Climate Change Scenarios

Soil erosion is triggered by rainfall through the detachment of soil particles and their transport downslope, playing a key role in soil erosion models. Together with the vegetation cover, rainfall is a temporal dynamic factor, inducing corresponding time variations of erosion rates. Under current climate change, rainfall is also changing its characteristics and our study aimed to reveal whether these changes will significantly affect rainfall erosivity in Romania, and implicitly the soil erosion. To achieve this purpose, we developed a statistical non-parametric model for predicting rainfall erosivity on the basis on the modified Fournier index and applied it to future precipitation evolution scenarios. The precipitation data were extracted from the CHESLA database for the Romanian territory for two climate change contrasting scenarios (RCP 4.5 and 8.5). Average predictions from five selected climate models were used in order to minimize prediction uncertainty. The results show that rainfall erosivity is likely to increase, at least during the 2041–2060 period, especially in the south-western, western and eastern part of the country, which may cause a corresponding increase in soil erosion rates, with an average of 1–2 t ha−1 yr−1. During the 2061–2080 period, rainfall erosivity is likely to decrease in central and eastern Romania.