DAF–dissolved air flotation: Potential applications in the mining and mineral processing industry

Industrial application of microbubble generation methods for recovering fine particles through froth flotation: A review of the state-of-the-art and perspectives

The depletion of high-grade and coarse-grain ores has led to an increasing demand for the development of efficient separation technologies for low-grade and fine-grain ores. However, conventional froth flotation techniques are not adequate to efficiently recover fine and ultrafine particles (typically <10-15 μm) due to the low collision probability between these particles and the relatively large bubbles used in the process. The introduction of microbubbles has shown promise in enhancing particle recovery, making it a subject of significant interest. Thus, this review focuses on microbubble generation methods that have the potential to be scaled up for industrial applications, with a specific emphasis on their suitability for froth flotation. The methods are categorized based on their scalability: high-hydrodynamic cavitation, porous media/medium-dissolved air, electrolysis/low-microfluidics, and acoustic methods. The bubble generation mechanisms, characteristics, advantages and limitations of each method and its applications in froth flotation are discussed to provide suggestions for improvement. There is still no appropriate technology that can optimize bubble size distribution, production rate and cost together for industrial froth flotation application. Therefore, novel approaches of combining multiple methods are also explored to achieve the potential synergic effects. By addressing the limitations of current microbubble generation methods and proposing potential enhancements, this review aims to contribute to the development of efficient and cost-effective microbubble generation technologies for fine and ultrafine particles in the froth flotation industry.

Chemical and ecotoxicological indicators for evaluating the treatment of coal mining wastes

This paper consists of the evaluation in regards to the ecotoxicological effectiveness of a treatment applied to a coal mining waste. The treatment consisted of separating the particles based on gravimetric concentration in spirals, generating three fractions: heavy, intermediate, and light, with high, moderate, and low pyrite content, respectively. The intermediate fraction represents the larger disposal volume of the waste on soils. To evaluate the effectiveness of the treatment, metal determination and bioassays Eisenia andrei, Folsomia candida, Lactuca sativa, Daphnia similis, and Raphidocelis subcapitata were applied to the intermediary fraction. To evaluate the toxicity to aquatic organisms, elutriates were generated from the unprocessed waste and the intermediate fraction. The intermediate fraction showed a decrease of metal concentrations compared to the untreated waste. Metal concentrations in the intermediate fraction were below the Brazilian thresholds for soil quality. Avoidance bioassay with E. andrei and germination tests of L. sativa showed no significant effects. The bioassay with F. candida indicated a significant reduction in reproduction at the highest doses used (24% and 50%). Bioassays with D. similis and R. subcapitata revealed a reduction in toxicity of the intermediate fraction compared to the untreated waste. However, the toxicity levels of the intermediate fraction to aquatic organisms still require attention, especially in regards to pH that played a crucial role in the toxicity. Finally, the results suggest that the treatment performed on the coal waste was efficient, even though significant toxicity have still been detected in the treated waste and additional steps are still required for adequate final disposal.

Investigating the Corrosive Influence of Chloride Ions on Slag Recovery Machine Shells in Power Plants

An effective strategy for achieving cost-effective and environmentally friendly desulfurization wastewater in coal-fired power plants involves the incorporation of desulfurization wastewater into the slag water system. The objective of this study was to analyze the corrosion behavior of Q235-A slag-picker shell material upon the introduction of FGD wastewater into the slag water system. The dynamic weight loss method, electrochemical testing method and microscopic phase characterization were employed to investigate the impact of varying chloride ion concentrations (ranging from 1000 mg/L to 30,000 mg/L) of flue gas desulfurization wastewater (FGD wastewater) on the corrosion of Q235-A slag-picker shell material. The test results indicate that as the concentration of chloride ions increases, the corrosion rate increases from 1.1487 mm/a to 1.5590 mm/a when the concentration is less than 10,000 mg/L. However, when the concentration exceeds 10,000 mg/L, the corrosion rate decreases from 1.559 mm/a to 1.0393 mm/a. The corrosion rate is above 1 mm/a at all concentrations. As the Cl- concentration, the quality of the corrosion product film initially increases and then decreases. The primary components of the corrosion product are α- FeOOH, γ-FeOOH, β-FeOOH, Fe3O4 and γ-Fe2O3.

Oxidative Capacity of Oxygen Nanobubbles and Their Mechanism for the Catalytic Oxidation of Ferrous Ions with Copper as a Catalyst in Sulfuric Acid Medium

Nanobubble (NB) technology has demonstrated the potential to enhance or substitute for current treatment processes in various areas. However, research employing it as a novel advanced oxidation process has thus far been relatively limited. Herein, we focused on the oxidative capacity of oxygen NBs and investigated the feasibility of utilizing their enhanced oxidation of ferrous ions (Fe²⁺) in a sulfuric acid medium when using copper as a catalyst and their effect mechanism. It was demonstrated that oxygen NBs could collapse to produce hydroxyl radicals (·OH) in the absence of dynamic stimuli using electron spin resonance spectroscopy, and methylene blue was used as a molecular probe for ·OH to illustrate that NB stability, determined by their properties, is the critical factor affecting ·OH release. In subsequent Fe²⁺ oxidation experiments, it was discovered that both strong acidity and copper ions (Cu²⁺) contribute to accelerating the collapse of NBs to produce ·OH. While ·OH derived from the collapse of NBs acts on Fe²⁺, the molecular oxygen generated homologously with ·OH will further activate the catalytic oxidation of Fe²⁺ by interacting with Cu²⁺. With the synergistic effect of the above two oxidation-driven mechanisms, the oxidation rate of Fe²⁺ can be significantly increased up to 88% due to the exceptional properties of oxygen NBs, which facilitate the formation of an atmosphere with persistent oxygen supersaturation and the generation of oxidation radicals. This study provides significant insight into applying NBs as a prospective technology for enhanced oxidation processes.

Ultrafine Kaolinite Removal in Recycled Water from the Overflow of Thickener Using Electroflotation: A Novel Application of Saline Water Splitting in Mineral Processing

The presence of ultrafine clay particles that are difficult to remove by conventional filtration creates many operational problems in mining processing systems. In this work, the removal of clay suspensions has been investigated using an electroflotation (EF) process with titanium electrodes. The results show that EF is a viable and novel alternative for removing ultrafine particles of kaolinite-type clay present in sedimentation tank overflows with low salt concentrations (<0.1 mol/L) in copper mining facilities based on the saline water splitting concept. Maximum suspended solid removal values of 91.4 and 83.2% in NaCl and KCl solutions, respectively, were obtained under the experimental conditions of the constant applied potential of 20 V/SHE, salinity concentration of 0.1 mol/L, and electroflotation time of 10 and 20 min in NaCl and KCl solutions, respectively. Furthermore, the visual evidence of particle aggregation by flocculation during the experiments indicates a synergy between EF and electrocoagulation (EC) that enhances the removal of ultrafine particles of kaolinite.

Microbubbles in Food Technology

Microbubbles are largely unused in the food industry yet have promising capabilities as environmentally friendly cleaning and supporting agents within products and production lines due to their unique physical behaviors. Their small diameters increase their dispersion throughout liquid materials, promote reactivity because of their high specific surface area, enhance dissolution of gases into the surrounding liquid phase, and promote the generation of reactive chemical species. This article reviews techniques to generate microbubbles, their modes of action to enhance cleaning and disinfection, their contributions to functional and mechanical properties of food materials, and their use in supporting the growth of living organisms in hydroponics or bioreactors. The utility and diverse applications of microbubbles, combined with their low intrinsic ingredient cost, strongly encourage their increased adoption within the food industry in coming years.

Inline imaging reveals evolution of the size distribution and the concentration of microbubbles in dissolved air flotation

Dissolved air flotation (DAF) is an efficient process to remove impurities from fresh or salt water. As the removal is based on the agglomeration of impurities on the generated microbubbles, the size distribution and concentration of air bubbles are key parameters in dissolved air flotation. However, the development of microbubbles in the whole flotation process remains unexplored. In this study, we show that state-of-the-art inline microscopy enables the image acquisition of bubbles in DAF. Based on image analysis, thousands of microbubbles (10-200 µm) were analyzed within 6-12 min experiments. Consequently, bubble size distributions and bubble concentrations can be determined with moderate effort. Bubble size distributions were measured in a lab-scale DAF comprising a saturation unit, a decompression valve in/after which the bubbles are formed, and the actual flotation tank. The state of the microbubbles is not only determined at different positions within the tank but also in the supply pipe from the decompression valve to the tank. All bubble size distributions were unimodal and can be described well with Burr XII distributions. For fresh water, bubble size increased while bubble concentration decreased along the supply pipe between the decompression valve and the inlet of the flotation tank, indicating bubble coalescence. Compared to freshwater, saltwater inhibited this bubble coalescence in the pipe. Within the flotation tank, the bubble size did not change drastically for neither salt- nor freshwater. However, the bubble concentration decreased for both waters, which could be explained by dilution effects. Our results demonstrate that the developed inline method is a promising tool to study the evolution of microbubbles in flotation systems. Further, it might also be applied to investigate microbubbles in other processes such as fermentation, decomposition of organic compounds, and fouling mitigation in membranes.

Generating Lifetime-Enhanced Microbubbles by Decorating Shells with Silicon Quantum Nano-Dots Using a 3-Series T-Junction Microfluidic Device

Long-term stability of microbubbles is crucial to their effectiveness. Using a new microfluidic device connecting three T-junction channels of 100 μm in series, stable monodisperse SiQD-loaded bovine serum albumin (BSA) protein microbubbles down to 22.8 ± 1.4 μm in diameter were generated. Fluorescence microscopy confirmed the integration of SiQD on the microbubble surface, which retained the same morphology as those without SiQD. The microbubble diameter and stability in air were manipulated through appropriate selection of T-junction numbers, capillary diameter, liquid flow rate, and BSA and SiQD concentrations. A predictive computational model was developed from the experimental data, and the number of T-junctions was incorporated into this model as one of the variables. It was illustrated that the diameter of the monodisperse microbubbles generated can be tailored by combining up to three T-junctions in series, while the operating parameters were kept constant. Computational modeling of microbubble diameter and stability agreed with experimental data. The lifetime of microbubbles increased with increasing T-junction number and higher concentrations of BSA and SiQD. The present research sheds light on a potential new route employing SiQD and triple T-junctions to form stable, monodisperse, multi-layered, and well-characterized protein and quantum dot-loaded protein microbubbles with enhanced stability for the first time.

The Challenges and Prospects of Recovering Fine Copper Sulfides from Tailings Using Different Flotation Techniques: A Review

Flotation is a common mineral processing method used to upgrade copper sulfide ores; in this method, copper sulfide mineral particles are concentrated in froth, and associated gangue minerals are separated as tailings. However, a significant amount of copper is lost into tailings during the processing; therefore, tailings can be considered secondary resources or future deposits of copper. Particle–bubble collision efficiency and particle–bubble aggregate stability determines the recovery of target particles; this attachment efficiency plays a vital role in the selectivity process. The presence of fine particles in the flotation circuit is because of excessive grinding, which is to achieve a higher degree of liberation. Complex sulfide ores of markedly low grade further necessitate excessive grinding to achieve the maximum degree of liberation. In the flotation process, fine particles due to their small mass and momentum are unable to collide with rising bubbles, and their rate of flotation is very slow, further lowering the recovery of target minerals. This collision efficiency mainly depends on the particle–bubble size ratio and the concentration of particles present in the pulp. To overcome this problem and to maintain a favorable particle–bubble size ratio, different techniques have been employed by researchers to enhance particle–bubble collision efficiency either by increasing particle size or by decreasing bubble size. In this article, the mechanism of tailing loss is discussed in detail. In addition, flotation methods for fine particles recovery such as microbubble flotation, column flotation, nanobubble flotation, polymer flocculation, shear flocculation, oil agglomeration, and carrier flotation are reviewed, and their applications and limitations are discussed in detail.

Magnetic-Actuated Robot Enables High-Performance Underwater Bubble Maneuvering on Laser-Textured Biomimetic Slippery Surfaces

Controllable underwater gas bubble (UGB) transport on a surface is realized by geography-/stimuli-induced wettability gradient force (F_wet-grad). Unfortunately, the high-speed maneuvering of UGBs along free routes on planar surfaces remains challenging. Herein, a regime of magnetism-actuated robot (MAR) mounting on biomimetic laser-ablated lubricant-impregnated slippery surfaces (LA-LISS) is reported. Leveraging on LA-LISS, MAR-entrained UGBs can move along arbitrary directions through the loading of a tracing magnetic trigger. The underlying hydrodynamics is that MAR-entrained UGBs would be actuated slipping upon a giant magnetic-induced towing force (F_M//). Once the magnetism stimuli is discharged, F_M// vanishes immediately to immobilize the UGBs on LA-LISS. Thanks to the MAR's robust bubble affinity, a typical UGB (20 μL) on the optimized LA-LISS can be accelerated at 500 mm/s² and gain an ultrafast velocity of over 205 mm/s that far exceeds previously reported figures. Moreover, fundamental physics renders MAR antibuoyancy, steering locomotive UGBs on the inclined LA-LISS. Significantly, an MAR propelling UGBs to configure desirable patterns, realize on-demand coalescence, remedy the cutoff switch, as well as facilitate a programmable light-control-light optical shutter is successfully deployed. Compared with previous smart surfaces, the current multifunctional regime is more competent for harnessing UGBs featuring an unparalleled transport velocity independent of the feeble F_wet-grad.

Recent advancements in the removal/recovery of toxic metals from aquatic system using flotation techniques

The paramount cause of water scarcity is pollution, which is becoming a massive issue since the last century. Besides, it is evident that water pollution is the main cause of emerging contaminants that are left untreated from industries, can cause serious threats to humans and biota as well. One of the best ways in remediating pollutants and finding a way for generating useable water is to use this contaminated water after the necessary treatment. Heavy metals are of major concern in treatment because of their toxicity, non-biodegradability, carcinogenicity, and they can cause inevitable damages even at low concentrations. In this review article, available different flotation techniques are discussed to address this issue. Flotation tends to be one of the promising techniques that have shown a high scope because of its high produce, low sludge formation, and ease of operation. From the several pieces of literature, it can be inferred that the flotation process can be conducted in one step, and that does not need any expensive materials. Further, this paper deliberates the versatility of each process in disclosing its advantages, limitations, further scope of research and fills the loopholes in the process for better effectiveness. Overall, flotation is a highly probable as well as effective treatment technology to eradicate noxious pollutants present in wastewater and thus helps to compromise environmental and social sustainability.

Generation of Bulk Nanobubbles by Self-Developed Venturi-Type Circulation Hydrodynamic Cavitation Device

Bulk nanobubbles (BNBs) have attracted substantial interest from academia and industry owing to their peculiar properties and extensive potential applications. However, a scalable engineering method needs to be developed. Herein, we developed a nanobubble generator based on venturi-type recirculating hydrodynamic cavitation. The existence of nanobubbles produced by our generator was confirmed using physicochemical test methods, including the Tyndall effect, multiple freeze-thaw degassing experiments, and trace metal analysis. Subsequently, the effects of different operating parameters (circulation time and operating pressure) on bulk nanobubble production and properties, as well as their stability, were investigated. The results suggest that the characteristics of BNBs varied with the circulation time (5-20 min) and operating pressure (2-5 bar). However, all the particle size distribution of BNBs had a bimodal distribution with a mean diameter of 180-210 nm for the different circulation time and operating pressures. For example, by increasing the circulation time from 5 to 20 min, the peak value of size distribution decreased from 333/122 nm to 218/52 nm, and the average sample scattering signal count rate (Avg. Count Rate) increased from 133 to 303 Kcps. The evaluation of the stability of the BNBs formed for the circulation time of 15 min and the operating pressure of 3 bar showed that they could continue existence and stability in the suspension for 72 h. The study results might provide a valuable method for further investigation of industrial applications of venturi-type nanobubble generators.

From Microbubbles to Nanobubbles: Effect on Flotation

Attachment of particles and droplets to bubbles—the latter being of various fine sizes and created by different techniques (as described in detail)—forms the basis of flotation, a process which indeed was originated from mineral processing. Nevertheless, chemistry often plays a significant role in this area, in order for separation to be effective, as stressed. This (brief) review particularly discusses wastewater treatment applications and the effect of bubble size (from nano- to micro-) on the flotation process.

Oily water treatment in a multistage tower operated under a novel induced pre-saturation process in the presence of a biosurfactant as collector

The increase in water-oil separation efficiency as a function of biosurfactant in a novel process of a continuous induced pre-saturation tower (IPST) with stages was described. The pre-saturation of the effluent in a new IPST prior to its entrance in each stage enabled enhancing the effect of the biosurfactant on the flocculation of oil droplets due to the close contact with the air during the formation of microbubbles inside a centrifuge pump. This change of a conventional dissolved-air flotation device enabled each stage to serve as a final flocculation chamber and flotation separator. The initial flocculation step occurred nearly entirely within the centrifugation pump adapted for the generation of microbubbles. Experimental tests in a bench-scale prototype of an IPST enabled drafting two operation diagrams based on the absence and presence of the biosurfactant produced by the bacterium Pseudomonas cepacia CCT 6659. We used an effluent composed of water and semi-synthetic motor oil at 500 ± 13 mg L-1. The oil removal efficiency was estimated with the aid of Damköhler numbers applied under the analogy of considering the IPST to be a set of perfect-mixture tanks in series. To quantify the increase in efficiency achieved with the addition of the biosurfactant, we identified the kinetic laws corresponding to the addition and non-addition of the biosurfactant. The addition of the biosurfactant led to an increase in the oil removal rate in the IPST from 92.5 % to 97.0 %.

Removal of flocculated TiO2 nanoparticles by settling or dissolved air flotation

Engineered nanoparticles of TiO₂ (TiO₂-NPs) are used in the industry for a great number of applications. After their usage, the particles end up in aquatic environments, contaminating supply waters and watercourses. Bench-scale studies report removal of TiO₂-NPs (450 nm, the mean volumetric diameter) by flocculation followed by settling or by dissolved air flotation (4 bar saturation pressure and 30% recycling ratio). Floc formation was conducted after heterocoagulation with iron hydroxide (30-40 mg L^-1Fe³⁺⁾ and gelatinized corn starch (10-20 mgL^-1) as flocculant, at pH 7. Particle size distribution and zeta potential, removal efficiencies as a function of time and microphotography of flocs were analyzed. Mechanisms involve ferric hydroxide precipitation, heterocoagulation with the nanoparticles and flocculation of the loaded carrier precipitates with gelatinized starch. Best results showed removals between 95-100% of TiO₂-NPs, either by settling or flotation after 5 min. Clear treated waters with low turbidity < 3 nephelometric turbidity units (NTU) and TiO₂-NPs concentrations <1 mg L^-1 were obtained. A practical advantage in DAF was the higher solids content (1.9% w/w) of the sludge, when compared to settling (0.7% w/w). This would facilitate the sludge dewatering and disposal, but DAF has the disadvantage of the poor efficiency at high concentration of the nanoparticles of titanium oxide (>100 mg L^-1). Conversely, the removal by settling of the flocs increased at high dosages. It is believed that both processes are sustainable in terms of reagents and the removal efficiencies of TiO₂ nanoparticles from water.

Auto-Aspirated DAF Sparger Study on Flow Hydrodynamics, Bubble Generation and Aeration Efficiency

A novel auto-aspirated sparger is examined experimentally in a closed-loop reactor (CLR) at lab scale using particle image velocimetry, high-speed camera and oxygen mass transfer rate measurements. State-of-the-art 3D printing technology was utilized to develop the sparger design in stainless steel. An insignificant change in the bubble size distribution was observed along the aerated flow, proving the existence of a low coalescence rate in the constraint domain of the CLR pipeline. The studied sparger created macrobubbles evenly dispersed in space. In pure water, the produced bubble size distribution from 190 to 2500 μm is controlled by liquid flow rate. The bubble size dynamics exhibited a power-law function of water flow rate approaching a stable minimum bubble size, which was attributed to the ratio of the fast-growing energy of the bubble surface tension over the kinetic energy of the stream. Potentially, the stream energy can efficiently disperse higher gas flow rates. The oxygen transfer rate was rapid and depended on the water flow rate. The aeration efficiency below 0.4 kW/m3 was superior to the commonly used aerating apparatuses tested at lab scale. The efficient gas dissolution technology has potential in water treatment and carbon capture processes applications.

Agglomeration–Flotation of Finely Ground Chalcopyrite Using Emulsified Oil Stabilized by Emulsifiers: Implications for Porphyry Copper Ore Flotation

Flotation is the conventional method for processing porphyry copper deposits, one of the most economically important sources of copper (Cu) worldwide. The rapidly decreasing grade of this type of Cu ore in recent years, however, presents serious problems with fine particle recovery using conventional flotation circuits. This low recovery could be attributed to the low collision efficiency of fine particles and air bubbles during flotation. To improve collision efficiency and flotation recovery, agglomeration of finely ground chalcopyrite (CuFeS2) (D50 = 3.5 μm) using emulsified oil stabilized by emulsifiers was elucidated in this study. Specifically, the effects of various types of anionic (sodium dodecyl sulfate (SDS), potassium amyl xanthate (KAX)), cationic (dodecyl amine acetate (DAA)), and non-ionic (polysorbate 20 (Tween 20)) emulsifiers on emulsified oil stability and agglomeration–flotation efficiency were investigated. When emulsifiers were added, the average size of agglomerates increased, resulting in higher Cu recovery during flotation. This dramatic improvement in flotation efficiency could be attributed to the smaller oil droplet size in emulsified oil and their higher stability in the presence of emulsifiers. The utilization of emulsifiers during agglomeration–flotation not only lowered the required agitation strength for agglomeration but also shortened the agglomeration time, both of which made the process easier to incorporate in existing flotation circuits.

Agglomeration-Flotation of Finely Ground Chalcopyrite and Quartz: Effects of Agitation Strength during Agglomeration Using Emulsified Oil on Chalcopyrite

In flotation, the size of mineral particles is one of the most important parameters: when the size becomes fine, collision efficiency of the particles and air bubbles becomes low, causing low flotation recovery. To improve the collision efficiency and flotation kinetics, agglomeration using the emulsified oil of finely ground chalcopyrite (D50 = 3.5 μm) was carried out before flotation. In this study, the effects of agitation strength during agglomeration, kerosene dosage and potassium amyl xanthate (KAX) dosage on the flotation were investigated. Agglomeration using emulsified oil improved Cu recovery because the median diameter of agglomerate increased. With increasing agitation strength, KAX and kerosene dosages, Cu recovery was further increased. Agglomeration-flotation of a mixture containing chalcopyrite and quartz with 1:1 ratio (w/w, weight by weight) showed that Si recovery in froth was low and did not change with varying conditions (agitation strength, KAX and kerosene dosages); however, Cu recovery was significantly improved with increasing agitation strength, KAX and kerosene dosages, and thus the separation efficiency was improved.

Study on Coagulation Kinetics of Disk-like Particles under Simple Shear Flow

In this study, a theoretical study is carried out on the collision of two disk-like particles to understand the coagulation of disk-like particles suspended in liquid under a shear flow. The diameter of the particle is fixed at 2 μm while the length is varied so that the aspect ratio (length/diameter) varies from 0.1 to 0.4. The liquid viscosity is changed from 0.01 to 1 Pa s. The minimum Peclet number is 10, and the Brownian motion is considered to be negligible. Both hydrodynamic and van der Waals interactions are included in tracking the position and the orientation of each particle. The Hamaker constant is fixed at 1.06 × 10^-20 J. The boundary integral formulation is used to calculate the hydrodynamic interaction. To obtain the kinetic constant of coagulation, the time-independent orientation distribution function for a particle is obtained under the noninteracting condition. The kinetic constant of coagulation is obtained by considering the presence of collision between two particles initially separated by a long distance, the orientations of two particles, and the flux of the liquid flow. The result shows that the kinetic constant of coagulation is reduced to approximately 1/3 of the value for the noninteracting particles when the viscosity is 1 Pa·s. As collision modes, side-side, face-edge, side-edge, and edge-edge are considered. The edge-edge mode is frequently observed in the given range of the aspect ratio. The collision mode does not change much from the noninteracting case except for the side-side mode.

Removal of iron ore slimes from a highly turbid water by DAF

This paper addresses Dissolved Air Flotation (DAF) process variables, such as the flocculation parameters and the recycle water addition, as well as the pretreatment chemical variables (coagulation conditions), to determine the optimal values for the flotation of iron ore slimes found in a highly turbid water sample from the Gualaxo do Norte River, a tributary of the Doce River Basin in Minas Gerais, Brazil. This work was conducted using a flotatest batch laboratory-scale device to evaluate the effectiveness of DAF for cleaning the water polluted by the Samarco tailings dam leakage and determine the ability of DAF to reduce the water turbidity from 358 NTU to values below 100 NTU, aiming to comply with current legislation. The results showed that the four types of tested coagulants (PAC, ferric chloride, Tanfloc SG and Tanfloc SL) provided adequate conditions for coagulation, flocculation and flotation (in the range of 90-99.6% turbidity reduction). Although the process variables were optimized and low residual turbidity vales were achieved, results revealed that a portion of the flocs settled at the bottom of the flotatest columns, which indicated that the turbidity results represented removal caused by a combination of flotation and sedimentation processes simultaneously.

Bulk nanobubbles in the mineral and environmental areas: Updating research and applications

In the last decade, the research with bulk nanobubbles (ultrafine bubbles with a diameter <1 μm, according to ISO 20480-1:2017) has been rapidly increasing in the academic and industrial environments. Nowadays, there are many applications reported in the literature, with several patents, procedures, and techniques on nanobubbles generation and an evergrowing research and many applications. Yet, most of those publications reporting bulk nanobubbles generation devices, do not bring information on measurements of size distribution or bubbles concentration (if nanobubbles). Further, there is a problem of scale and many of these products are small bench discontinuous rigs difficult to scale up, which might serve small scale purposes, but are not able for treating high flow-rate wastewaters or minerals pulps at industrial scale. These nanometric bubbles present interesting and peculiar properties such as high surface area per volume unit, high stability and longevity, surface charge in water and the ability to aggregate hydrophobic particles. These findings demonstrate their high potential for applications in many technological areas, which occur not only as isolated bubbles but also jointly with micro (~ 1-100 μm diameter) and/or macrobubbles (~100 μm - 2 mm diameter). This paper reviews the evolution of basic research on nanobubbles, the challenges concerning generation and stability and their applications in the mineral (flotation) and environmental areas (treatment of water and wastewaters or remediation of contaminated environments). Herein, because the importance in engineering, as a whole, most of the studies are based on the nanobubbles generated by depressurisation/hydrodynamic cavitation of the air-saturated water in flow constrictors (venturi, needle valves). In the mineral area, they appear to be responsible for increasing the recovery and flotation kinetics of fine (<74 μm) and ultrafine (<13 μm) particles at lower frother and collector dosages. In the environmental area, nanobubbles have been reported to enhance the removal of a variety of pollutants (emulsified oil, colloidal solids, organic/inorganic precipitates, ions) by flotation associated with bigger bubbles. More, the application of isolated nanobubbles on the removal of residual pollutants, such as amine and oil (both as flocs) were reported. Also, the use of ozone and oxygen nanobubbles has been studied for the remediation/decontamination of soil and aquatic ecosystems and for the oxidation of emerging pollutants in water and wastewater treatment. The future of nanobubbles in flotation separation research is highly promising; operating costs of the different forms of nanobubbles generation and bench studies should be validated through pilot and real scale with the continuous injection of these bubbles.

The dynamics of rising oil-coated bubbles: experiments and simulations

Air bubbles rising through an aqueous medium have been studied extensively and are routinely used for the separation of particulates via froth flotation, a key step in many industrial processes. Oil-coated bubbles can be more effective for separating hydrophilic particles with low affinity for the air-water interface, but the rise dynamics of oil-coated bubbles has not yet been explored. In the present work, we report the first systematic study of the shape and rise trajectory of bubbles engulfed in a layer of oil. Results from direct observation of the coated bubbles with a high-speed camera are compared to computer simulations and confirm a pronounced effect of the oil coat on the bubble dynamics. We consistently find that the oil-coated bubbles display a more spherical shape and straighter trajectory, yet slower rise than uncoated bubbles of comparable size. These characteristics may provide practical benefits for flotation separations with oil-coated bubbles.

BS12-assisted flotation for the intensification of SNPs separation from CMP wastewater using a novel flotation column

In view of the extremely small size, high stable dispersion and intricate colloidal nature of silica nanoparticles (SNPs) in chemical mechanical polishing (CMP) wastewater, they might not only have hazards for environment and human health, but also cause low separation efficiency by classical water-treatment processes. Thus, it would be an important challenge to develop an efficient flotation technology for the separation SNPs. For this propose, this paper firstly presented the interaction between SNPs and dodecyl dimethyl betaine (ambient-friendly surfactant). Secondly, a novel flotation column was developed for strengthening interfacial adsorption by micro-bubbles and enhancing foam drainage by internal of regular-decagonal hollow frustum (RHF). One vital finding was that the mixture of micro-bubbles and macro-bubbles was conducive to improving the flotation performance. Under the suitable operating conditions, the enrichment ratio (E) and recovery percentage (R) of SNPs could reach 30.4±1.5 and 90.8±4.5%, respectively. The great E and R were obtained simultaneously, revealing a good participation of RHF in the flotation. Without a doubt, owing to the low chemical reagent addition and the high flotation performance, it was clear that our flotation has huge implications for the separation of nanoparticles from their wastewaters.

Separation of emulsified crude oil in saline water by flotation with micro- and nanobubbles generated by a multiphase pump

The flocculation-column flotation with hydraulic loading (HL, >10 m h^-1) was studied for the treatment of oil-in-water emulsions containing 70-400 mg L^-1 (turbidity = 70-226 NTU) of oil and salinity (30 and 100 g L^-1). A polyacrylamide (Dismulgan, 20 mg L^-1) flocculated the oil droplets, using two floc generator reactors, with rapid and slow mixing stages (head loss = 0.9 to 3.5 bar). Flotation was conducted in two cells (1.5 and 2.5 m) with microbubbles (MBs, 5-80 μm) and nanobubbles (NBs, 50-300 nm diameter, concentration of 10⁸ NBs mL^-1). Bubbles were formed using a centrifugal multiphase pump, with optimized parameters and a needle valve. The results showed higher efficiency with the taller column reducing the residual oil content to 4 mg L^-1 and turbidity to 7 NTU. At high HL (27.5 m h^-1), the residual oil concentrations were below the standard emission (29 mg L^-1), reaching 18 mg L^-1. The best results were obtained with high concentration of NBs (apart from the bigger bubbles). Mechanisms involved appear to be attachment and entrapment of the NBs onto and inside the flocs. Thus, the aggregates were readily captured, by bigger bubbles (mostly MBs) aiding shear withstanding. Advantages are the small footprint of the cells, low residence time and high processing rate.

Lamella dissolved air flotation treatment of fish farming effluents as a part of an integrated farming and effluent treatment concept

Nutrient emissions from fish farming can be reduced by a bag pen, i.e., a floating circular basin which serves simultaneously both as a fish cultivation tank and a swirl separation tank. Solid matter (excreta and uneaten feed) is collected at the bottom of the bag pen and pumped as an underflow to a dissolved air flotation (DAF) unit for nutrient removal. DAF equipped with lamella elements was studied in real conditions. Altogether 3000 rainbow trout females (2.0 kg each) were cultivated. Solid-water mixture was pumped from the bottom of the bag pen to an equalizing basin using a sequence of 2-min pumping followed by a 4-min pause. In some tests the influent was pumped directly and continuously from the bag pen to DAF. The influent quality changed substantially: average suspended solids (SS) and phosphorus (P) concentrations were 290 mg l⁻¹ ± 110 mg l⁻¹ and 3.2 mg l⁻¹ ± 1.2 mg l⁻¹, respectively. When the influent was fresh and P strongly associated with SS, DAF without precipitation chemicals produced up to 86% SS and 83% P removals. The influence of chemical doses was studied using 6.4-29.2 mg Fe l⁻¹ with hydraulic loadings (HLs) of 11.0-11.7 m h⁻¹. SS and P removal did not change substantially and the effluent concentration levelled at 30 mg SS l⁻¹ and 0.20-0.30 mg P l⁻¹, respectively. The lamella DAF, coupled with ferric precipitation, produced up to 90% P and 80% nitrogen reductions. HLs, excluding recycle water flow and lamella projection, up to 21 m h⁻¹ could be used.