Hydrogen bubble flotation of silica

In this study the flotation recovery of silica using air, and molecular and electrolytically-generated hydrogen was investigated. For comparison of air and molecular hydrogen recoveries, a laboratory Denver, type D12, flotation machine was used. For both gases, pH of the suspension, gas flow rate, concentration of collector and frother, solids concentration, particle size and speed of impeller were kept constant. Almost identical recoveries were obtained for both gases, suggesting that gas composition played no significant role in silica flotation. Electroflotation experiments were carried out using 12.6 μm mean diameter silica particles. While fine particles had very poor recovery in the Denver cell, greater than 70% recoveries were achieved in the electroflotation cell. This was thought to be the result of the very small (less than 40 μm) bubbles generated by the electroflotation process. A population-balance model, incorporating the hydrogen generation process, supported the conclusion that increased recovery for electroflotation, for very fine silica particles at least, was attributed to the reduced bubble size and not by the composition of the gas.     

Influence of the hydrophobic force model on the capture of particles by bubbles: A computational study using Discrete Element Method

Discrete Element Method computer simulations have been carried out to analyse the influence of the hydrophobic force model on the capture of particles by a central bubble. Two hundred particles, with diameters ranging between 24 and 66 μm, were randomly positioned within a maximum distance from the surface of a bubble of 2 mm in diameter. Initial particle velocities were random in direction and value and they followed Gaussian distributions with standard deviations between 0.0 and 1.0 m/s. Three possible models, named A, B and C have been used in the simulations. The models correspond to different published relationships of the hydrophobic force with the distance between particle and bubble surfaces, d. Model A corresponds to a hydrophobic force that decays in the form 1/d; the hydrophobic force given by Model B uses a relationship in the form 1/d²; Model C predicts a force that decays in an exponential way in the form exp(?d/λ). These models have also been compared with a base case in which the hydrophobic force only acted when the particles were in contact with the bubble. Therefore, we could better discern between the influence of the initial particle velocities and the long range component of the hydrophobic force. The differences in the capture efficiency of the particles predicted by the three models were drastic. All particles were captured by the bubble in the cases simulated using Model A for any particle–bubble surface distance smaller than 1 mm. However, only 40% and 60% of the particles were captured even for particles located at distances of less than 50 μm from the bubble surface in the cases simulated using Models B and C (λ = 1 μm), respectively. In fact, the capture of particles seems to be more strongly influenced by how the hydrophobic force decays with interparticle distance in the range of tens of micrometres rather than by the differences between the models in the range of micrometres. Therefore, this work should aid in the future determination of a general hydrophobic force model through an experimental comparison of the kinetics of collision of particles against bubbles in flotation cells with the simulation results.     

Effects of monovalent anions and cations on drainage and lifetime of foam films at different interface approach speeds

Ions of inorganic salts are known to affect bubble coalescence via ion size, charge density and polarizability. In this paper, a systematic study of the effect of monovalent anions (F⁻, Cl⁻, Br⁻ and I⁻) and cations (Li⁺, Na⁺ and K⁺) on the lifetime of liquid films between two bubble surfaces is carried out by applying the thin film interferometry method. To mimic realistic conditions of bubble coalescence in a bubble column, drainage and stability of saline water films driven by different interface approach speeds (10–300 μm/s) using a nano-pump was investigated. The results show significant effects of interface approach speed on transient film thickness and radius, film stability and rupture, and lifetime of saline water films. The experiments also indicate that there is a critical approach speed of 35 μm/s for pure deionised water above which the water films instantly coalesce, i.e., no water film can be obtained. High interface approach speed creates corrugation on saline water film surfaces, which rapidly increases the rates of film radial expansion and drainage, and shortens the film lifetime. There is a critical salt concentration above which the saline water film lifetime abruptly increases. This critical concentration is independent of the interface approach speeds of 10–300 μm/s. Our experimental results show a decreasing trend of film lifetime with increasing the size of either the cation or anion (NaF > LiCl > NaCl > NaBr > NaI). The order of the critical concentrations is the opposite of the order of lifetimes. The experimental results highlight the ion-specific effect of salt ions on the water structure and hence the behavior of saline liquid films. These results are relevant to a number of chemical engineering processes taking place in saline water, including mineral separation by flotation using air bubbles in saline water.     

Screen channel liquid acquisition device outflow tests in liquid hydrogen

This paper presents experimental design and test results of the recently concluded 1-g inverted vertical outflow testing of two 325 × 2300 full scale liquid acquisition device (LAD) channels in liquid hydrogen (LH₂). One of the channels had a perforated plate and internal cooling from a thermodynamic vent system (TVS) to enhance performance. The LADs were mounted in a tank to simulate 1-g outflow over a wide range of LH₂ temperatures (20.3–24.2 K), pressures (100–350 kPa), and flow rates (0.010–0.055 kg/s). Results indicate that the breakdown point is dominated by liquid temperature, with a second order dependence on mass flow rate through the LAD. The best performance is always achieved in the coldest liquid states for both channels, consistent with bubble point theory. Higher flow rates cause the standard channel to break down relatively earlier than the TVS cooled channel. Both the internal TVS heat exchanger and subcooling the liquid in the propellant tank are shown to significantly improve LAD performance.     

EDU liquid acquisition device outflow tests in liquid hydrogen: Experiments and analytical modeling

This paper presents experiments and modeling of the most recent set of liquid acquisition device (LAD) vertical outflow tests conducted in liquid hydrogen. The Engineering Development Unit (EDU) was a relatively large tank (4.25 m³) used to mimic a storage tank for a cryogenic storage and transfer flight demonstration test. Six 1-g propellant tank outflow tests were conducted with a standard 325 × 2300 rectangular cross-section curved LAD channel conformal to the tank walls over a range of tank pressure (158–221 kPa), ullage temperature (22–39 K), and mass flow rate (0.0103–0.0187 kg/s) per arm. An analytical LAD channel solver, an exact solution to the Navier-Stokes equations, is used to model propellant outflow for the LAD channel. Results shows that the breakdown height of the LAD is dominated by liquid and ullage gas temperatures, with a secondary effect of flow rate. The best performance is always obtained by exposing the channel to cold pressurant gas and low flow rates, consistent with the cryogenic bubble point model. The model tracks the trends in the data and shows that the contribution of flow-through-screen pressure drop is minimized for bottom outflow in 1-g, versus the standard inverted outflow.     

Utilization of hydrogen in electroflotation of silica

In this study the hydrogen bubble electroflotation of 3–15 μm diameter silica particles was investigated. Experiments were conducted to determine the influence of current density, solids concentration, mechanical agitation and the presence of dissolved gases on the rate of hydrogen gas production. It was found that approximately 98% of the theoretical hydrogen production resulted in gas bubbles. There was a very small increase in the hydrogen bubble production rate with the introduction of mechanical agitation, while the opposite trend was observed for the degassed electrolyte solution. Hydrogen gas production was found to be largely independent of the concentration of the suspended solids. Batchwise flotation experiments were also undertaken to determine the influence of particle diameter and solids concentration on flotation recovery. The experimental results were inputted into a recovery model, based largely on the work of Koh and Schwarz [1], that was applied to gain insight into the factors that influence the fractional coverage of the bubble surface by the particles. From the analysis it was found that, for this study at least, flotation recovery was controlled by either the bubble–particle aggregate rise velocity being greater than zero or the bubble–particle aggregate projected area being less than that of just the bubble.     

Dissolution of coated microbubbles: The effect of nanoparticles and surfactant concentration

The ability of solid particles to stabilise emulsions is a well known phenomenon which has recently been demonstrated for the stabilisation of gas bubbles. In this paper, a new theoretical model is developed which describes how an adsorbed layer of solid nanoparticles modifies the interfacial tension and diffusivity of a gas bubble in a liquid and hence its stability. In agreement with experimental observations on microbubbles coated with 15 nm diameter spherical gold particles, the results of simulations with the model indicate that the particles substantially decrease the rate at which bubble dissolution occurs and enables them to maintain a stable radius once a critical particle concentration has been reached.     

Development of porous plug phase separator and superfluid film flow suppression system for the Soft X-ray Spectrometer onboard ASTRO-H

ASTRO-H is the sixth Japanese astronomy satellite scheduled for launch in 2014. The Soft X-ray Spectrometer instrument is onboard ASTRO-H. This is a 6 × 6 array of X-ray microcalorimeters with an energy resolution of <7 eV at 0.5–10 keV. Superfluid liquid helium is utilized as a part of the cooling system. To retain the liquid helium in the tank under zero-gravity, a porous plug phase separator made of sintered stainless is used. Since the vapor mass flow rate is only 29 μg/s, any additional superfluid film loss influences the lifetime of the liquid helium. Therefore, a film flow suppression system consisting of an orifice, a heat exchanger, and knife edge devices is adopted based on the design used for the X-ray Spectrometer onboard Suzaku. The film flow will be suppressed to <2 μg/s, sufficiently smaller than the vapor flow rate. In the present investigation, the design and ground experiments of a helium vent system composed of the porous plug and film flow suppression system are presented. The results show that the phase separation and the film flow suppression are satisfactorily achieved.     

Elimination of stagnant particles from a N-valve with side aeration in circulating fluidized bed

The existence of stagnant particle layer in the conventional non-mechanical valves limits their utilization in CFB with the feedstock of caking particles. A new N-valve consisted of a fluidized weir chamber with bottom aeration and a moving-bed angled standpipe with side aeration was developed to eliminate the stagnant particle layer and reach high solids circulation rate G_s in CFBs. The particle flow behavior and its control in the N-valve were studied experimentally. By combining the bottom aeration for weir chamber and the side aeration for angled standpipe the G_s over 270 kg/(m² s) was achieved, and the stagnant particle layer completely disappeared. The G_s increased with increasing the side aeration gas flow rate Q_sa, and this loosing gas flow was optimally injected from the bend between the downcomer and the angled standpipe. At a constant but enough high Q_sa, the increase in the bottom aeration gas flow rate Q_ba elevated G_s linearly.     

Thermal behaviour of helium in silicon carbide: Influence of microstructure

We have studied the microstructure dependence of He bubble formation in silicon carbide. Helium accumulation in SiC was performed by 500 keV ³He implantation at room temperature with a fluence of 5 × 10¹⁵ cm^?2. Depth concentration profiles have been investigated in 6H-SiC single crystals and α-SiC polycrystals by NRA spectrometry. Cross-sectional TEM samples have been imaged to study bubble formation. After annealing at 1300 °C, results clearly demonstrate an influence of grain boundaries on He retention yield in α-SiC polycrystals while helium is totally released from single crystals. Polycrystals also display the formation of intragranular overpressurized bubbles while no bubbles are observed in single crystals. Interpretations are proposed on the basis of the nature of He traps.     

Extremely high surface area of ordered mesoporous MCM-41 by atrane route

Silatrane synthesized from inexpensive oxide precursor, silica and TEA was used as the precursor for MCM-41 synthesis at low temperature because of its stability in aqueous solutions. Using cationic surfactant hexadecyltrimethyl ammonium bromide (CTAB) as a template, the resulting meso-structure mimics the liquid crystal phase. Varying the surfactant concentration, ion concentration and temperature of the system, changes the structure of the liquid crystal phase, resulting in different pore structures and surface area. After heat treatment, very high surface area mesoporous silica was obtained and characterized using XRD, BET and TEM. XRD and TEM results show a clear picture of hexagonal structure. The surface area is extraordinarily high, up to more than 2400 m² g⁻¹ at a pore volume of 1.29 cm³ g⁻¹. However, the pore volume is up to 1.72 cm³ g⁻¹ when the surface area is greater than 2100 m² g⁻¹.     

Optimization of gas tungsten arc welding process by response surface methodology

Gas tungsten arc welding is widely used for connecting of boiler parts made of A516-Gr70 carbon steel. In this study important process parameters namely current, welding speed and shielding gas flow rate were optimized using response surface methodology (RSM). The simultaneous effects of these parameters on tensile strength and hardness were also evaluated. Applying RSM, simultaneous effects of welding parameters on tensile strength and hardness were obtained through two separate equations. Moreover, optimized values of welding process parameters to achieve desired mechanical properties were evaluated. Desired tensile strength and hardness were achieved at optimum current of 130 A, welding speed of 9.4 cm/min and gas flow rate of 15.1 l/min.     

Temperature responsive polymers as multiple function reagents in mineral processing

A processing scheme which uses a single chemical that has multiple functions to achieve both efficient mineral flotation and solids dewatering is presented. Temperature sensitive polymers which display hydrophilic/hydrophobic transitions in response to changes in temperature such as poly (N-isopropyl acrylamide) (PNIPAM) have been found to be useful as such multiple function reagents. This polymer can cause the mineral particles’ surfaces to be hydrophilic at temperature below the critical solution temperature (CST = 32 °C) and hydrophobic at temperature above the CST. Therefore, both particle surface wettability and inter-particle interaction forces are reversibly controllable. When the surface is hydrophilic, particle dispersion is achieved by repulsive inter-particle forces whereas when the surface is hydrophobic, particle aggregation is induced by inter-particle hydrophobic attractive forces. In addition, the hydrophobic surface condition allows for the attachment of particles to bubbles. Flotation and solid settling tests have been conducted with silica and kaolinite suspensions treated with (PNIPAM). Both effective suspension dispersion or hydrophobic aggregation and flotation without any additional collector have been demonstrated. In solid/liquid separation, rapid settling was obtained with hydrophobic aggregation at temperature above the CST and further sediment consolidation (and water release) occurred at temperature below the CST. The approach has the potential to reduce the amount and types of reagents required for mineral processing.     

Influence of air bubble size on float–sink of spheres in a gas–solid fluidized bed

The float–sink of density adjusted spheres of different diameter (10–40 mm) in a gas–solid fluidized bed was investigated at various bed heights (50–200 mm). The maximum density of floating spheres (ρ_float) and the minimum density of sinking spheres (ρ_sink) were determined by the float–sink experiments. The fluidized bed density (ρ_fb) was measured using the height and cross section of the fluidized bed and total weight of the fluidized media. The diameter of air bubbles at the bed surface was measured at each bed height, and was normalized by the sphere diameter. It was found that the value of ρ_fb–ρ_float approaches zero as the normalized bubble diameter decreases from 4 to 0.5 regardless of the sphere diameter. The value of ρ_sink–ρ_fb for sphere diameter = 10 mm approaches zero as the normalized bubble diameter decreases from 4 to 1.5, whereas the value for sphere diameter = 20–40 mm rises from zero as the normalized bubble diameter decreases from 1.5 to 0.5. The float and sink of spheres basically tend to follow the fluidized bed density with decreasing the normalized bubble diameter. However, relatively larger spheres do not sink based on the density difference as the normalized bubble diameter decreases, which may be due to that the fluidized bed viscosity becomes larger as the normalized bed diameter decreases.     

Pressure drop of slush nitrogen flow in converging–diverging pipes and corrugated pipes

Cryogenic slush fluids such as slush hydrogen and slush nitrogen are solid–liquid, two-phase fluids. As a functional thermal fluid, there are high expectations for use of slush fluids in various applications such as fuels for spacecraft engines, clean-energy fuels to improve the efficiency of transportation and storage, and as refrigerants for high-temperature superconducting equipment. Experimental flow tests were performed using slush nitrogen to elucidate pressure-drop characteristics of converging–diverging (C–D) pipes and corrugated pipes. In experimental results regarding pressure drop in two different types of C–D Pipes, i.e., a long-throated pipe and a short-throated pipe, each having an inner diameter of 15 mm, pressure drop for slush nitrogen in the long-throated pipe at a flow velocity of over 1.3 m/s increased by a maximum of 50–60% as compared to that for liquid nitrogen, while the increase was about 4 times as compared to slush nitrogen in the short-throated pipe. At a flow velocity of over 1.5 m/s in the short-throated pipe, pressure drop reduction became apparent, and it was confirmed that the decrease in pressure drop compared to liquid nitrogen was a maximum of 40–50%. In the case of two different types of corrugated pipes with an inner diameter of either 12 mm or 15 mm, a pressure-drop reduction was confirmed at a flow velocity of over 2 m/s, and reached a maximum value of 37% at 30 wt.% compared to liquid nitrogen. The greater the solid fractions, the smaller the pipe friction factor became, and the pipe friction factor at the same solid fraction showed a constant value regardless of the Reynolds number. From the observation of the solid particles’ behavior using a high-speed video camera and the PIV method, the pressure-drop reduction mechanisms for both C–D and corrugated pipes were demonstrated.     

Operating characteristics of a single-stage Stirling cryocooler capable of providing 700 W cooling power at 77 K

High cooling capacity Stirling cryocooler generally has hundreds to thousands watts of cooling power at liquid nitrogen temperature. It is promising in boil-off gas (BOG) recondensation and high temperature superconducting (HTS) applications. A high cooling capacity Stirling cryocooler driven by a crank-rod mechanism was developed and studied systematically. The pressure and frequency characteristics of the cryocooler, the heat rejection from the ambient heat exchanger, and the cooling performance are studied under different charging pressure. Energy conversion and distribution in the cryocooler are analyzed theoretically. With an electric input power of 10.9 kW and a rotating speed of 1450 r/min of the motor, a cooling power of 700 W at 77 K and a relative Carnot efficiency of 18.2% of the cryocooler have been achieved in the present study, and the corresponding pressure ratio in the compression space reaches 2.46.     

DEM simulation of single bubble flotation: Implications for the hydrophobic force in particle–bubble interactions

A 3D Discrete Element Method simulation model for a single bubble was developed in order to investigate the capture of hydrophobic particles. The bubble was considered stationary at the centre of the working space. Particle–particle and particle–bubble contacts were simulated using a linear spring-dashpot model. Gravitational, buoyancy, drag and hydrophobic forces were taken into account. The hydrophobic force was estimated through a single exponential decay law which depends on a pre-exponential parameter K and a decay length λ. It was observed that when λ was less than 10 nm, the number of the particles that were collected was independent of the strength of the hydrophobic force. In contrast, for values of λ within the range of 10–500 nm, the capture efficiency increased significantly with the strength of the hydrophobic force and λ. We have also demonstrated how these two parameters affect the particle trajectory around the bubble and thus produce a significant difference in particle collection when the strength and range of the hydrophobic force were varied.     

High-speed synchrotron X-ray phase contrast imaging for analysis of low-Z composite microstructure

The study of high performance composites such as plastic-bonded explosives under extreme conditions often requires innovative experimental techniques. Here, static synchrotron X-ray phase-contrast imaging (PCI) of simulated explosive materials has been performed at high speed in an effort to determine feasibility of imaging material response to dynamic, high-strain rate events (10²–10⁷ s^?1). The microstructure of pristine materials, idealized composites and simulated explosive composites has been characterized with synchrotron PCI at the Advanced Photon Source. High spatial resolution (2 μm) of the microstructure was achieved with 5 μs exposures, and features such as interfaces, cracks, voids, and bubbles were clearly observed. The likelihood of obtaining sufficient phase information at even faster exposures (e.g., 0.2–0.5 μs) is shown to be high.     

Design and experimental investigation of a cryogenic system for environmental test applications

This paper is concerned with the design, development and performance testing of a cryogenic system for use in high cooling power instruments for ground-based environmental testing. The system provides a powerful tool for a combined environmental test that consists of high pressure and cryogenic temperatures. Typical cryogenic conditions are liquid hydrogen (LH2) and liquid oxygen (LO2), which are used in many fields. The cooling energy of liquid nitrogen (LN2) and liquid helium (LHe) is transferred to the specimen by a closed loop of helium cycle. In order to minimize the consumption of the LHe, the optimal design of heat recovery exchangers has been used in the system. The behavior of the system is discussed based on experimental data of temperature and pressure. The results show that the temperature range from room temperature to LN2 temperature can be achieved by using LN2, the pressurization process is stable and the high test pressure is maintained. Lower temperatures, below 77 K, can also be obtained with LHe cooling, the typical cooling time is 40 min from 90 K to 22 K. Stable temperatures of 22 K at the inlet of the specimen have been observed, and the system in this work can deliver to the load a cooling power of several hundred watts at a pressure of 0.58 MPa.     

Continuous-wave lasing at 1.06 μm in femtosecond laser written Nd:KGW waveguides

We report on the buried channel waveguide laser at 1065 nm in Nd:KGW waveguides fabricated by femtosecond laser writing with dual-line approach. A relatively high scanning speed of 0.5 mm/s enables acceptable propagation loss less than 2 dB/cm. The fluorescence emission spectra of Nd³⁺ ions measured shows that the fluorescence properties were well preserved in the waveguide region. A stable continuous wave laser at 1065 nm has been obtained at room temperature in the buried channel waveguides by optical pumping at 808 nm. A maximum output power of 33 mW and a slope efficiency of 52.3% were achieved in the Nd:KGW waveguide laser system.