The Role of Operating Conditions in the Precipitation of Magnesium Hydroxide Hexagonal Platelets Using NaOH Solutions.

Magnesium hydroxide, Mg(OH)2, is an inorganic compound extensively employed in several industrial sectors. Nowadays, it is mostly produced from magnesium-rich minerals. Nevertheless, magnesium-rich solutions, such as natural and industrial brines, could prove to be a great treasure. In this work, synthetic magnesium chloride and sodium hydroxide (NaOH) solutions were used to recover Mg(OH)2 by reactive crystallization. A detailed experimental campaign was conducted aiming at producing grown Mg(OH)2 hexagonal platelets. Experiments were carried out in a stirred tank crystallizer operated in single- and double-feed configurations. In the single-feed configuration, globular and nanoflakes primary particles were obtained, as always reported in the literature when NaOH is used as a precipitant. However, these products are not complying with flame-retardant applications that require large hexagonal Mg(OH)2 platelets. This work suggests an effective precipitation strategy to favor crystal growth while, at the same time, limiting the nucleation mechanism. The double-feed configuration allowed the synthesis of grown Mg(OH)2 hexagonal platelets. The influence of reactant flow rates, reactant concentrations, and reaction temperature was analyzed. Scanning electron microscopy (SEM) pictures were also taken to investigate the morphology of Mg(OH)2 crystals. The proposed precipitation strategy paves the road to satisfy flame-retardant market requirements.

Energetic Valorisation of Saltworks Bitterns via Reverse Electrodialysis: A Laboratory Experimental Campaign.

Concentrated bitterns discharged from saltworks have extremely high salinity, often up to 300 g/L, thus their direct disposal not only has a harmful effect on the environment, but also generates a depletion of a potential resource of renewable energy. Here, reverse electrodialysis (RED), an emerging electrochemical membrane process, is proposed to capture and convert the salinity gradient power (SGP) intrinsically conveyed by these bitterns also aiming at the reduction of concentrated salty water disposal. A laboratory-scale RED unit has been adopted to study the SGP potential of such brines, testing ion exchange membranes from different suppliers and under different operating conditions. Membranes supplied by Fujifilm, Fumatech, and Suez were tested, and the results were compared. The unit was fed with synthetic hypersaline solution mimicking the concentration of natural bitterns (5 mol/L of NaCl) on one side, and with variable concentration of NaCl dilute solutions (0.01-0.1 mol/L) on the other. The influence of several operating parameters has also been assessed, including solutions flowrate and temperature. Increasing feed solutions' temperature and velocity has been found to lower the stack resistance, which enhances the output performance of the RED stack. The maximum obtained power density (corrected to account for the effect of electrodic compartments, which can be very relevant in five cell pairs laboratory stacks) reached around 10.5 W/m2cellpair, with FUJIFILM Type 10 membranes, temperature of 40 °C, and a fluid velocity of 3 cm s-1 (as empty channel, considering 270 µm thickness). Notably, the present study results confirm the large potential for SGP generation from hypersaline brines, thus providing useful guidance for the harvesting of SGP in seawater saltworks all around the world.

Analysis of Operational Parameters in Acid and Base Production Using an Electrodialysis with Bipolar Membranes Pilot Plant.

In agreement with the Water Framework Directive, Circular Economy and European Union (EU) Green Deal packages, the EU-funded WATER-MINING project aims to validate next-generation water resource solutions at the pre-commercial demonstration scale in order to provide water management and recovery of valuable materials from alternative sources. In the framework of the WATER-MINING project, desalination brines from the Lampedusa (Italy) seawater reverse osmosis (SWRO) plant will be used to produce freshwater and recover valuable salts by integrating different technologies. In particular, electrodialysis with bipolar membranes (EDBM) will be used to produce chemicals (NaOH and HCl). A novel EDBM pilot plant (6.4 m2, FuMa-Tech) has been installed and operated. The performance of EDBM for single pass under different flowrates (2-8 L·min-1) for acid, base and saline channels, and two current densities (200 and 400 A·m-2), has been analyzed in terms of specific energy consumption (SEC) and current efficiency (CE). Results showed that by increasing the flowrates, generation of HCl and NaOH slightly increased. For example, ΔOH- shifted from 0.76 to 0.79 mol·min-1 when the flowrate increased from 2 to 7.5 L·min-1 at 200 A·m-2. Moreover, SEC decreased (1.18-1.05 kWh·kg-1) while CE increased (87.0-93.4%), achieving minimum (1.02 kWh·kg-1) and maximum (99.4%) values, respectively, at 6 L·min-1.

Electrodialysis with Bipolar Membranes for the Sustainable Production of Chemicals from Seawater Brines at Pilot Plant Scale.

Environmental concerns regarding the disposal of seawater reverse osmosis brines require the development of new valorization strategies. Electrodialysis with bipolar membrane (EDBM) technology enables the production of acid and base from a salty waste stream. In this study, an EDBM pilot plant with a membrane area of 19.2 m2 was tested. This total membrane area results much larger (i.e., more than 16 times larger) than those reported in the literature so far for the production of HCl and NaOH aqueous solutions, starting from NaCl brines. The pilot unit was tested both in continuous and discontinuous operation modes, at different current densities (200-500 A m-2). Particularly, three different process configurations were evaluated, namely, closed-loop, feed and bleed, and fed-batch. At lower applied current density (200 A m-2), the closed-loop had a lower specific energy consumption (SEC) (1.4 kWh kg-1) and a higher current efficiency (CE) (80%). When the current density was increased (300-500 A m-2), the feed and bleed mode was more appropriate due to its low values of SEC (1.9-2.6 kWh kg-1) as well as high values of specific production (SP) (0.82-1.3 ton year-1 m-2) and current efficiency (63-67%). These results showed the effect of various process configurations on the performance of the EDBM, thereby guiding the selection of the most suitable process configuration when varying the operating conditions and representing a first important step toward the implementation of this technology at industrial scale.

Electrodialysis with Bipolar Membranes for the Generation of NaOH and HCl Solutions from Brines: An Inter-Laboratory Evaluation of Thin and Ultrathin Non-Woven Cloth-Based Ion-Exchange Membranes.

The SEArcularMINE project aims to recover critical raw materials (CRMs) from brines from saltworks, thus facing a CRM shortage within Europe. To promote a fully circular scheme, the project valorises concentrated brines using electrodialysis with bipolar membranes (EDBM) to generate the required amounts of reactants (i.e., acids and bases). Regarding the performances of new non-woven cloth ion-exchange membranes (Suez): (i) an ultra-thin non-woven polyester cloth and (ii) a thin polypropylene cloth acting as the support structures were assessed. Additionally, the anion layer includes a catalyst to promote the water dissociation reaction. The effect of current density (100, 200, and 300 A m-2) on the performance of two combinations of membranes in an inter-laboratory exercise using 2 M NaCl was evaluated. According to statistical analysis ANOVA, there was an agreement on the results obtained in both laboratories. NaOH/HCl solutions up to 0.8 M were generated working at 300 A m-2 using both combinations of membranes. Regarding the performance parameters, stack set-ups incorporating thin polypropylene membranes showed lower specific energy consumption (SEC) and higher specific productivity (SP) than ultra-thin polypropylene ones. Hence, for ultra-thin polypropylene membranes, SEC was reported to be between 2.18 and 1.69 kWh kg-1NaOH and SP between 974 and 314 kg m-2 y-1.

Feasibility of Producing Electricity, Hydrogen, and Chlorine via Reverse Electrodialysis.

Reverse electrodialysis (RED) is a technology to generate electricity from two streams with different salinities. While RED systems have been conventionally used for electricity generation, recent works explored combining RED for production of valuable gases. This work investigates the feasibility of producing hydrogen and chlorine in addition to electricity in an RED stack and identifies potential levers for improvement. A simplified one-dimensional model is adopted to assess the technical and economic feasibility of the process. We notice a strong disparity in typical current densities of RED fed with seawater and river water and that in typical water (or chlor-alkali) electrolysis. This can be partly mitigated by using brine and seawater as RED feeds. Considering such an RED system, we estimate a hydrogen production of 1.37 mol/(m2 h) and an electrical power density of 1.19 W/m2. Although this exceeds previously reported hydrogen production rates in combination with RED, the levelized costs of products are 1-2 orders of magnitude higher than the current market prices at the current state. The levelized costs of products are very sensitive to the membrane price and performance. Hence, going forward, manufacturing thinner and highly selective membranes is required to make the system competitive against the consolidated technologies.

Influence of Operational Strategies for the Recovery of Magnesium Hydroxide from Brines at a Pilot Scale.

The continuous depletion of minerals caused by land mining and the increase in their demand have pushed the development of novel sustainable technological processes for mineral recovery from unconventional sources. In this context, magnesium (Mg) has gained considerable attention for its peculiar properties and high relevance of its compounds, such as magnesium hydroxide, Mg(OH)2. In the present work, the influence of several operating conditions on the Mg(OH)2 precipitation process was thoroughly investigated by adopting a novel multiple feed-plug flow reactor. The influence of (i) initial Mg2+ concentrations in the feed stream; (ii) brine and alkaline flow rates; and (iii) the product recycling strategy (seeded crystallization) was considered. The results marked the possibility of improving sedimentation and filterability properties of Mg(OH)2 suspensions by adopting the recycling strategy to overcome industrial issues associated with the production of Mg(OH)2 suspensions using NaOH solutions.

Economic Benefits of Waste Pickling Solution Valorization.

An integrated hybrid membrane process, composed of a diffusion dialysis (DD), a membrane distillation (MD) and a reactive precipitation unit (CSTR), is proposed as a promising solution for the valorization and onsite recycling of pickling waste streams. An economic analysis was performed aiming to demonstrate the feasibility of the developed process with a NPV of about EUR 40,000 and a DPBP of 4 years. The investment and operating costs, as well as the avoided costs and the benefits for the company operating the plant, were analyzed with an extensive cost tracking exercise and through face-to-face contact with manufacturers and sector leaders. A mathematical model was implemented using the gPROMS modelling platform. It is able to simulate steady state operations and run optimization analysis of the process performance. The impact of key operating and design parameters, such as the set-point bath concentration and the DD and MD membrane areas, respectively, was investigated and the optimal arrangement was identified. Finally, operating variables and design parameters were optimized simultaneously in a nonlinear framework as a tradeoff between profitability and environmental impact. We show how the integration of new technologies into the traditional pickling industry could provide a significant benefit for the issues of process sustainability, which are currently pressing.

Performance Comparison of Alternative Hollow-Fiber Modules for Hemodialysis by Means of a CFD-Based Model.

Commercial hemodialyzers are hollow-fiber cylindrical modules with dimensions and inlet-outlet configurations dictated mostly by practice. However, alternative configurations are possible, and one may ask how they would behave in terms of performance. In principle, it would be possible to depart from the standard counter-flow design, while still keeping high clearance values, thanks to the increase in the shell-side Sherwood number (Sh) due to the cross-flow. To elucidate these aspects, a previously developed computational model was used in which blood and dialysate are treated as flowing through two interpenetrating porous media. Measured Darcy permeabilities and mass transfer coefficients derived from theoretical arguments and CFD simulations conducted at unit-cell scale were used. Blood and dialysate were alternately simulated via an iterative strategy, while appropriate source terms accounted for water and solute exchanges. Several module configurations sharing the same membrane area, but differing in overall geometry and inlet-outlet arrangement, were simulated, including a commercial unit. Although the shell-side Sherwood number increased in almost all the alternative configurations (from 14 to 25 in the best case), none of them outperformed in terms of clearance the commercial one, approaching the latter (257 vs. 255 mL/min) only in the best case. These findings confirmed the effectiveness of the established commercial module design for the currently available membrane properties.

Bipolar membrane reverse electrodialysis for the sustainable recovery of energy from pH gradients of industrial wastewater: Performance prediction by a validated process model.

The theoretical energy density extractable from acidic and alkaline solutions is higher than 20 kWh m-3 of single solution when mixing 1 M concentrated streams. Therefore, acidic and alkaline industrial wastewater have a huge potential for the recovery of energy. To this purpose, bipolar membrane reverse electrodialysis (BMRED) is an interesting, yet poorly studied technology for the conversion of the mixing entropy of solutions at different pH into electricity. Although it shows promising performance, only few works have been presented in the literature so far, and no comprehensive models have been developed yet. This work presents a mathematical multi-scale model based on a semi-empirical approach. The model was validated against experimental data and was applied over a variety of operating conditions, showing that it may represent an effective tool for the prediction of the BMRED performance. A sensitivity analysis was performed in two different scenarios, i.e. (i) a reference case and (ii) an improved case with high-performance membrane properties. A Net Power Density of ~15 W m-2 was predicted in the reference scenario with 1 M HCl and NaOH solutions, but it increased significantly by simulating high-performance membranes. A simulated scheme for an industrial application yielded an energy density of ~50 kWh m-3 (of acid solution) with an energy efficiency of ~80-90% in the improved scenario.

Electromembrane Processes: Experiments and Modelling.

This Special Issue of Membranes journal focuses on electromembrane processes and is motivated by the increasing interest of the scientific community towards their characterization by experiments and modelling for several applications [...].

A Novel Ionic Exchange Membrane Crystallizer to Recover Magnesium Hydroxide from Seawater and Industrial Brines.

A novel technology, the ion exchange membrane crystallizer (CrIEM), that combines reactive and membrane crystallization, was investigated in order to recover high purity magnesium hydroxide from multi-component artificial and natural solutions. In particular, in a CrIEM reactor, the presence of an anion exchange membrane (AEM), which separates two-compartment containing a saline solution and an alkaline solution, allows the passage of hydroxyl ions from the alkaline to the saline solution compartment, where crystallization of magnesium hydroxide occurs, yet avoiding a direct mixing between the solutions feeding the reactor. This enables the use of low-cost reactants (e.g., Ca(OH)2) without the risk of co-precipitation of by-products and contamination of the final crystals. An experimental campaign was carried out treating two types of feed solution, namely: (1) a waste industrial brine from the Boleslaw Smialy coal mine in Laziska Górne (Poland) and (2) Mediterranean seawater, collected from the North Sicilian coast (Italy). The CrIEM was tested in a feed and bleed modality in order to operate in a continuous mode. The Mg2+ concentration in the feed solutions ranges from 0.7 to 3.2 g/L. Magnesium recovery efficiencies from 89 up to 100% were reached, while magnesium hydroxide purity between 94% and 98.8% was obtained.

Electrodialysis Applications in Wastewater Treatment for Environmental Protection and Resources Recovery: A Systematic Review on Progress and Perspectives.

This paper presents a comprehensive review of studies on electrodialysis (ED) applications in wastewater treatment, outlining the current status and the future prospect. ED is a membrane process of separation under the action of an electric field, where ions are selectively transported across ion-exchange membranes. ED of both conventional or unconventional fashion has been tested to treat several waste or spent aqueous solutions, including effluents from various industrial processes, municipal wastewater or salt water treatment plants, and animal farms. Properties such as selectivity, high separation efficiency, and chemical-free treatment make ED methods adequate for desalination and other treatments with significant environmental benefits. ED technologies can be used in operations of concentration, dilution, desalination, regeneration, and valorisation to reclaim wastewater and recover water and/or other products, e.g., heavy metal ions, salts, acids/bases, nutrients, and organics, or electrical energy. Intense research activity has been directed towards developing enhanced or novel systems, showing that zero or minimal liquid discharge approaches can be techno-economically affordable and competitive. Despite few real plants having been installed, recent developments are opening new routes for the large-scale use of ED techniques in a plethora of treatment processes for wastewater.

Diffusion Dialysis for Separation of Hydrochloric Acid, Iron and Zinc Ions from Highly Concentrated Pickling Solutions.

Acid recovery from pickling waste solutions is an important step to enhance hot-dip-galvanizing industry process sustainability. Diffusion dialysis (DD) can be used to separate acids and heavy metals (e.g., iron and zinc) from pickling waters, promoting the circular use of such raw materials. In the present study, a laboratory scale unit operating in batch and a continuous large scale unit, both equipped with Fumasep anionic exchange membranes, were tested. Results obtained show that zinc and iron concentration affect the HCl recovery in opposite ways. Iron chlorides enhance acid recovery, while zinc chlorides considerably tend to diffuse through the membrane because of negatively charged chloro-complexes formation and slightly reduce the acid diffusion. A multi-components mathematical model, with a time-dependent and distributed-parameters architecture, was adopted enabling the prediction of operations with hydrochloric acid, zinc, and iron metals both in batch and in continuous dialyzers. As a result, a good comparison between model simulations and experiments was achieved in both configurations.

CFD Investigation of Spacer-Filled Channels for Membrane Distillation.

The membrane distillation (MD) process for water desalination is affected by temperature polarization, which reduces the driving force and the efficiency of the process. To counteract this phenomenon, spacer-filled channels are used, which enhance mixing and heat transfer but also cause higher pressure drops. Therefore, in the design of MD modules, the choice of the spacer is crucial for process efficiency. In the present work, different overlapped spacers are investigated by computational fluid dynamics (CFD) and results are compared with experiments carried out with thermochromic liquid crystals (TLC). Results are reported for different flow attack angles and for Reynolds numbers (Re) ranging from ~200 to ~800. A good qualitative agreement between simulations and experiments can be observed for the areal distribution of the normalized heat transfer coefficient. Trends of the average heat transfer coefficient are reported as functions of Re for the geometries investigated, thus providing the basis for CFD-based correlations to be used in higher-scale process models.

Membrane Deformation and Its Effects on Flow and Mass Transfer in the Electromembrane Processes.

In the membrane processes, a trans-membrane pressure (TMP) may arise due to design features or operating conditions. In most applications, stacks for electrodialysis (ED) or reverse electrodialysis (RED) operate at low TMP (<0.1 bar); however, large stacks with non-parallel flow patterns and/or asymmetric configurations can exhibit higher TMP values, causing membrane deformations and changes in fluid dynamics and transport phenomena. In this work, integrated mechanical and fluid dynamics simulations were performed to investigate the TMP effects on deformation, flow and mass transfer for a profiled membrane-fluid channel system with geometrical and mechanical features and fluid velocities representative of ED/RED conditions. First, a conservatively high value of TMP was assumed, and mechanical simulations were conducted to identify the geometry with the largest pitch to height ratio still able to bear this load without exhibiting a contact between opposite membranes. The selected geometry was then investigated under expansion and compression conditions in a TMP range encompassing most practical applications. Finally, friction and mass transfer coefficients in the deformed channel were predicted by computational fluid dynamics. Significant effects of membrane deformation were observed: friction and mass transfer coefficients increased in the compressed channel, while they decreased (though to a lesser extent) in the expanded channel.

Long-run operation of a reverse electrodialysis system fed with wastewaters.

The performance of a Reverse ElectroDialysis (RED) system fed by unconventional wastewater solutions for long operational periods is analysed for the first time. The experimental campaign was divided in a series of five independent long-runs which combined real wastewater solutions with artificial solutions for at least 10 days. The time evolution of electrical variables, gross power output and net power output, considering also pumping losses, was monitored: power density values obtained during the long-runs are comparable to those found in literature with artificial feed solutions of similar salinity. The increase in pressure drops and the development of membrane fouling were the main detrimental factors of system performance. Pressure drops increase was related to the physical obstruction of the feed channels defined by the spacers, while membrane fouling was related to the adsorption of foulants over the membrane surfaces. In order to manage channels partial clogging and fouling, different kinds of easily implemented in situ backwashings (i.e. neutral, acid, alkaline) were adopted, without the need for an abrupt interruption of the RED unit operation. The application of periodic ElectroDialysis (ED) pulses is also tested as fouling prevention strategy. The results collected suggest that RED can be used to produce electric power by unworthy wastewaters, but additional studies are still needed to characterize better membrane fouling and further improve system performance with these solutions.

Reverse electrodialysis performed at pilot plant scale: Evaluation of redox processes and simultaneous generation of electric energy and treatment of wastewater.

This paper describes the experimental campaign carried out with a reverse electrodialysis (RED) demonstration plant (Marsala, Italy) with the main aims of: (i) evaluating the effect of various operating parameters, including the redox processes, on the system performances; (ii) using the plant for the simultaneous generation of electric energy and treatment of wastewater. The prototype (44 × 44 cm2, 500 cell pairs) was tested using both real (brackish water and brine) and artificial solutions. Tests with two different electrode rinse solutions (with or without iron redox couples) were performed. In agreement with the data obtained in the laboratory, the presence of iron ions contributes positively to the power production. The effect of flow rates in the electrode and saline compartments, as well as aging of the electrode rinse solution was also investigated. The possibility to remove an organic pollutant (the azoic dye Acid Orange 7) from the electrode solution was tested, obtaining a very fast and total removal of the pollutant. This experimental campaign represents the first demonstration in a real environment of the abilities of a RED plant to treat wastewater, thus giving useful indications for the spreading of RED technology in the near future.