Concentrated Alkaline Solution Research Articles

Aspects of the Mechanism of Nucleation and Intergrowth of Gibbsite Crystals on Sodium Oxalate Surfaces in Concentrated Alkaline Solutions

To mimic the process of gibbsite nucleation on sodium oxalate crystals and their intergrowth in alumina Bayer process, such a coprecipitation process has been broken down into three distinct stages and studied by in situ AFM and ex situ SEM under static conditions. In the first stage, a solution saturated in aluminate and supersaturated in oxalate has been used for investigating the oxalate growth on its {110} face, which is controlled by a layer-by-layer mechanism. In the second stage, in a solution saturated in oxalate and supersaturated in aluminate, gibbsite secondary nucleation was preferred along macrosteps of sodium oxalate {110} faces and appeared as two-dimensional (2D) islands with pisolitic shapes. There does not appear to be a preferred epitaxial relationship at the heterointerface between gibbsite crystals and sodium oxalate substrates, indicating that the magnitude of the surface free energy for nucleation is greater than that associated with strain energy caused by lattice mismatch. In the third stage following gibbsite secondary nucleation, in a solution saturated in aluminate and supersaturated in oxalate, the inclusion of gibbsite crystallites into the bulk of sodium oxalate crystals through step growth on {110} faces of sodium oxalate was observed.

Solubility Equilibrium of the NaOH–H2O–Na2CrO4–Na2SiO3–NaAlO2 Multicomponent Systems Involved in the Liquid-Phase Oxidation of Chromite

Chromium salts are a group of important chemical raw materials with many applications. While the conventional manufacturing process, i.e. calcium roasting of chromite, has many disadvantages; the novel processes of liquid-phase oxidation, whose main reaction is the oxidation of chromite by oxygen (air) in highly concentrated alkaline solutions, are promising to achieve clean manufacturing. For the oxidation reaction of chromite in NaOH solutions, a significant amount of multicomponent systems consisting NaOH, Na2CrO4, Na2SiO3, and NaAlO2 are generated. It is a prerequisite to clarify the solubility equilibrium of the multicomponent systems for exploring highly efficient separation methods of Na2CrO4 with Na2SiO3 and NaAlO2. In this paper, Na2CrO4 solubility in the ternary system of NaOH–H2O–Na2CrO4 was first measured using the equilibrium analysis method and varying the NaOH concentrations from 100 to 800 g·L–1 and the temperatures from 353.15 to 403.15 K. Aferwards, the solubility of Na2CrO4, NaAlO2, and...

Chemical stability of basalt fiber in alkaline solution

The chemical stability of basalt fiber in alkaline solution is investigated by the means of weight retention and tensile strength retention. It has been observed that the basalt fiber immersed in weak alkaline solution is very stable whereas the basalt fiber immersed in strong alkaline solution shows poor weight retention due to the high dissociation constant. On the other hand, the tensile strength of basalt fiber in alkaline solutions is drastically decreased regardless of alkaline solution concentrations. Also basalt fiber in saturated Ca(OH)2 solution, which is similar to the alkaline environment of cement hydration, shows very low weight loss and high stability even after 3 months of immersion compared to glass fiber in the same condition.

Experimental investigation of neutralization dialysis in three-compartment membrane stack

This work is concerned with the feasibility of surface water desalination by the neutralization dialysis (ND) process and the difficulties related to it. The pH and the conductivity of model saline solution have been measured during different ND operations, using a three-compartment membrane stack of 64 cm2 active area for each ion-exchange membrane (IEM). The saline solution concentration is maintained at 0.02 mol L−1. The influence of three parameters has been investigated, i.e. the flow rate (from 35 to 100 mL min−1), the alkaline solution concentration, and the acidic solution concentration (from 0.02 to 0.1 mol L−1). We have shown that diffusion boundary layers have a great influence on the ion-exchange kinetics through the IEMs. A leakage of HCl solution through the cation-exchange membrane has been proved. Its influence is more pronounced when the acidic and alkaline solution concentrations are low. These leakages modify the shapes of the pH vs. time and conductivity vs. time curves, and decelerate the desalination efficiency. After 2–3 h of desalination process, the conductivity reached the value of 0.25 mS cm−1, as recommended by the Organisation Mondiale de la Santé (OMS) for a drinking water. The desalination process was achieved within a reasonable duration in all the experiments.

Experimental Study of Geopolymer Solidification Kinetics

Geopolymers are formed from silica and alumina oxides mixed with alkali hydroxide or alkali silicate. This paper presents the findings on the study of the solidification of fly ash geopolymer through setting time by varying alkaline activators, their concentrations and geopolymer curing temperature. This work focuses on the transformation of geopolymer from liquid paste to solid through Avrami’s Kinetic Theory. From the experimental results, alkaline activation with sodium silicate produced shortest time for geopolymer solidification as compared to KOH and NaOH. The increase in concentrations of alkaline solution and curing temperatures were found to reduce setting time for geopolymer’s solidification. From Avrami theory perspective, the growth forms of geopolymer in the geopolymerization process exhibit two and three dimensional structure with the presence of secondary nucleation.

A Sustainable Approach for the Geopolymerization of Natural Iron-Rich Aluminosilicate Materials

Two iron-rich clayey materials (L1 and L2, with the main difference being the level of iron accumulation) have been studied for their suitability as solid precursors for inorganic polymer composites. L1, with the lower iron content, was calcined at 700°C for 4 h and used as replacement, in the range of 15–35 wt%, for both raw laterites in the formulations of geopolymeric composites. The different mixtures were activated with a highly concentrated alkaline solution containing sodium hydroxide and sodium silicate. River sand with semi-crystalline structure was added to form semi-dry pastes which were pressed to appropriate shape. X-ray diffraction, Infrared spectroscopy, Scanning Electron Microscopy and Mercury Intrusion Porosimetry results demonstrated the effectiveness of the calcined fraction of L1 to act as nucleation sites and extend the geopolymerization to the matrix composites. A highly compact matrix with low porosity and good stability in water, together with a strength comparable to that of standard concretes was obtained allowing for conclusions to be made on the quality of laterites as promising solid precursor for sustainable, environmentally-friendly, and cost-efficient structural materials.

Light Weight Geopolymer Paste Made with Sidoarjo Mud (Lusi)

This paper presents the efforts of how to use the solid form of Sidoarjo mud as a base material of lightweight-geopolymer paste. The mud, which is called Lusi was mixed with a class F fly ash. Some experimental results have shown that mixing mud with fly ash and alkali was potential to become a binder in concrete, instead of Portland cement. Alkaline solutions used in the mixture were NaOH of 12 M and 14 M and Na2SiO3combined in the ratio of 1:2.5 by weight. Aluminum powder was applied as a foaming agent. It showed that the strength of paste made with mixing Sidoarjo mud and fly ash was influenced by mud content. The effect of chemical admixture and curing temperature was observed as well. A steam curing method at 60°C was applied to the paste for three and six hours. Test results showed that the compressive strength of the paste varied with the concentration of alkaline solution, mud content, admixture and curing temperature. The maximum strength of the lightweight paste at 21 days was 2.8 MPa with the density of 722 kg/m3.

A novel method to produce dry geopolymer cement powder

Geopolymer cement is the result of reaction of two materials containing aluminosilicate and concentrated alkaline solution to produce an inorganic polymer binder. The alkali solutions are corrosive and often viscous solutions which are not user friendly, and would be difficult to use for bulk production. This work aims to produce one-mix geopolymer mixed water that could be an alternative to Portland cement by blending with dry activator. Sodium hydroxide (SH) was dissolved in water and added to calcium carbonate (CC) then dried at 80°C for 8h followed by pulverization to a fixed particle size to produce the dry activator consisting of calcium hydroxide (CH), sodium carbonate (SC) and pirssonite (P). This increases their commercial availability. The dry activator was blended with granulated blast-furnace slag (GBFS) to produce geopolymer cement powder and by addition of water; the geopolymerization process is started. The effect of W/C and SH/CC ratio on the physico-mechanical properties of slag pastes was studied. The results showed that the optimum percent of activator and CC content is 4% SH and 5% CC, by the weight of slag, which give the highest physico-mechanical properties of GBFS. The characterization of the activated slag pastes was carried out using TGA, DTG, IR spectroscopy and SEM techniques.

Preparation and electrochemical performances of nanoporous/cracked cobalt oxide layer for supercapacitors

Nanoporous/cracked structures of cobalt oxide (Co3O4) electrodes were successfully fabricated by electroplating of zinc–cobalt onto previously formed TiO2 nanotubes by anodizing of titanium, leaching of zinc in a concentrated alkaline solution and followed by drying and annealing at 400 °C. The structure and morphology of the obtained Co3O4 electrodes were characterized by X-ray diffraction, EDX analysis and scanning electron microscopy. The results showed that the obtained Co3O4 electrodes were composed of the nanoporous/cracked structures with an average pore size of about 100 nm. The electrochemical capacitive behaviors of the nanoporous Co3O4 electrodes were investigated by cyclic voltammetry, galvanostatic charge–discharge studies and electrochemical impedance spectroscopy in 1 M NaOH solution. The electrochemical data demonstrated that the electrodes display good capacitive behavior with a specific capacitance of 430 F g−1 at a current density of 1.0 A g−1 and specific capacitance retention of ca. 80 % after 10 days of being used in electrochemical experiments, indicating to be promising electroactive materials for supercapacitors. Furthermore, in comparison with electrodes prepared by simple cathodic deposition of cobalt onto TiO2 nanotubes(without dealloying procedure), the impedance studies showed improved performances likely due to nanoporous/cracked structures of electrodes fabricated by dealloying of zinc, which provide fast ion and electron transfer routes and large reaction surface area with the ensued fast reaction kinetics.

The Effect of Variation of Molarity of Alkali Activator and Fine Aggregate Content on the Compressive Strength of the Fly Ash: Palm Oil Fuel Ash Based Geopolymer Mortar

The effect of molarity of alkali activator, manufactured sand (M-sand), and quarry dust (QD) on the compressive strength of palm oil fuel ash (POFA) and fly ash (FA) based geopolymer mortar was investigated and reported. The variable investigated includes the quantities of replacement levels of M-sand, QD, and conventional mining sand (N-sand) in two concentrated alkaline solutions; the contents of alkaline solution, water, POFA/FA ratio, and curing condition remained constant. The results show that an average of 76% of the 28-day compressive strength was found at the age of 3 days. The rate of strength development from 3 to 7 days was found between 12 and 16% and it was found much less beyond this period. The addition of 100% M-sand and QD shows insignificant strength reduction compared to mixtures with 100% N-sand. The particle angularity and texture of fine aggregates played a significant role in the strength development due to the filling and packing ability. The rough texture and surface of QD enables stronger bond between the paste and the fine aggregate. The concentration of alkaline solution increased the reaction rate and thus enhanced the development of early age strength. The use of M-sand and QD in the development of geopolymer concrete is recommended as the strength variation between these waste materials and conventional sand is not high.

Conical nanopores fabricated via a pressure-biased chemical etch

Controlling the size and shape of nanopores in polymer membranes can significantly impact transport of molecular or ionic species through these membranes. Here we describe a facile method to controllably form conical nanopores in ion-tracked polycarbonate membranes. Commercial polycarbonate ion-tracked membranes were placed between a concentrated alkaline solution and an acidic solution. By varying the height of the acidic solution, the hydrostatic pressure was controlled, regulating the acid flux through the nanopores. The resulting asymmetric etching of the membrane produced conical pores with controllable aspect ratios. Scanning electron microscopy of both the pores and nickel nanostructures electrolessly templated in the pores confirms their conical shape. This safe, straightforward approach obviates the need to use large voltages, currents, and/or plasma etching equipment traditionally employed to create conical nanopores.

Geopolymer ferrocement panels under flexural loading

AbstractEfforts are needed to develop innovative and environmental friendly materials in order to reduce greenhouse gas emissions. An experimental investigation on the flexural behavior of thin cementless composite panels reinforced with welded rectangular wire mesh and chicken mesh with varying number of mesh layers as well as varying concentration of alkaline solution is presented. A total of 30 panels have been tested under flexural loading. The size of the panel is 1000 mm (length)×200 mm (width)×25/35 mm (thickness). The parameters studied in this investigation include varying the concentration of NaOH (8, 10, 12, 14 m) and thickness of composite panels. In this work, cement is replaced by geopolymer mix to bind the ferrocement skeletal framework and its flexural behavior is studied. It is concluded that the first crack and ultimate loads increase with the increase in the thickness of the element and the concentration of alkaline solution. From the studies, it is observed that the load-carrying capacities, energy absorption, and deformation at ultimate load are high in the case of geopolymer ferrocement element. Further, it is observed that there is reduction in crack width, and increase in number of cracks in the case of geopolymer ferrocement indicates delay in crack growth.

Hydrogen diffusivity, kinetics of H2O/H2 charge transfer and corrosion properties of LaNi5-powder, composite electrodes in 6 M KOH solution

Meticulous analysis of galvanostatic charge/discharge dependencies of the LaNi5-based, powder composite electrode, in terms of determination of characteristic kinetic parameters of hydrogen storage electrode materials working in concentrated alkaline solution has been carried out. A special attention has been paid to the precise determination of charge and discharge times. The cathodic curves reveal their stepwise nature which allows to receive information of hydride material corrosion phenomena and determine the real time of atomic hydrogen absorption. The graphical way of determining of reduction times, based on differential cathodic curves is proposed. The knowledge of hydrogen absorption and desorption times allows to determine hydrogen diffusivity within the tested material with acceptable accuracy. The effect of external pressure (0.5–4 bar) on hydrogen absorption ability of LaNi5-based material is also discussed. The exchange current density of H2O/H2 system distinctly increases with external pressure, at the same time, kind of gas atmosphere (Ar or H2) scarcely affects the exchange current on the LaNi5 electrode. The hydrogen capacity increases when the charge/discharge rate decreases. The reduction times of oxide phases formed during electrode discharge can be a measure of material corrosion rate. It is shown that the rate of LaNi5 corrosion process strongly increases with the electrode cycling.

Solubility investigations in the quaternary NaOH–Na3VO4–Na2CrO4–H2O system at 40 and 80 °C

In a new vanadium production process using concentrated alkaline solution as reaction media, the effective separation of Na3VO4 and Na2CrO4 plays an important role. The mutual salt solubility data in the NaOH–Na3VO4–Na2CrO4–H2O system at 40 and 80°C therefore were measured and compared with ternary NaOH–Na3VO4–H2O and NaOH–Na2CrO4–H2O subsystems, respectively. With the crystallization areas discussed in detail, a strategy for effective separation of Na3VO4 and Na2CrO4 from the NaOH–Na3VO4–Na2CrO4–H2O quaternary system has been proposed.

Effect of Bi Addition on the Electrochemical Behavior of Zinc Electrodes in Concentrated Alkaline Solutions

The electrochemical behavior of zinc electrode with bismuth addition in 35%KOH solutions has been investigated systematically by electrochemical methods including linear polarization, potentiostatic polarization, potential-time measurements at a constant current density, combining the observations of scanning electron microscopy (SEM). Linear polarization results showed that the open circuit potential shifted positively with increasing bismuth content, which is explained based on the gassing data and change in the exchange current of the zinc electrode. Addition of bismuth increased the exchange current of zinc reaction and caused an increase in the measured open circuit potential. Galvanostatic results showed that the addition of bismuth shortened the passivation time. Scanning electron microscopy images showed that the addition of bismuth aggravated the corrosion of zinc electrode which may be responsible for the increased tendency to passivation at high current densities. It has been found that at low current densities the reaction kinetics may be increased by addition of Bi, which is general agreement with the discharging test of actual alkaline batteries.

Effect of Alkaline Pretreatment on Cellulosic Structural Changes of Paulownia

Four different concentrations of alkaline solution for pretreatment of Paulownia biomass were comparatively studied. The extracted celluloses were characterized using Fourier Transform Infrared spectra (FTIR) for changes of the chemical group, as well as X-ray Diffraction (XRD), Scanning Electron Microscope (SEM). The four samples were untreated biomass, treated with 2%, 4% and 6%peroxide alkaline, and thena comparison between alkaline and dilute acid pretreatment on those indexes were conducted. FTIR spectra results suggested that NaOH could mainly work on lignin of the raw material while the dilute acid work on hemicellulose. The CrI of acid pretreatment and alkaline pretreatment increased to 0.5467 and 0.6395 respectively comparing with the value of 0.4347 of raw material. The XRD results suggested that both of them could act on amorphous region firstly. The SEM micrographs showed the visual structure changes of this progress. The results of enzyme hydrolysis showed 93.3% and 90.6% of the potential glucose and xylose were recovered in the pretreated biomass at 6% NaOH that proved the effects of the NaOH on cellulose structural.

Enzymatic hydrolysis of wood with alkaline treatment

Pulverized samples of wood, cedar and eucalyptus were treated with 5 N NaOH solutions at 25–150 °C. Hemicellulose and lignin content in the samples decreased with increasing treatment temperatures, while the recovery of glucose was maintained at nearly 90 %. X-ray diffraction analysis showed that the content of the original cellulose I structure in the samples decreased with increasing temperature, and most of the cellulose in the sample treated at 150 °C was converted to cellulose II by mercerization. Enzymatic hydrolysis of the alkaline-treated samples was carried out at 37 °C using solutions comprising a mixture of cellulase and β-glucosidase. The samples treated at higher temperatures showed better enzymatic degradability. Treatment with an alkaline solution of lower concentration (1 N NaOH) at 150 °C was also used. Despite significant quantities of hemicellulose and lignin being removed, mercerization was not induced. The enzymatic degradability was much lower than that of the sample treated with a 5 N NaOH solution at 150 °C. Thus, treatment with concentrated alkaline solution at high temperature led to not only the removal of hemicellulose and lignin, but also to modification of the cellulose structure, which resulted in high efficiency of enzymatic saccharification of the wood samples.

Effect of Indium Addition on the Electrochemical Behavior of Zinc Electrodes in Concentrated Alkaline Solutions

The electrochemical behavior of zinc electrode with indium addition in 35%KOH(or saturated with ZnO) solutions has been investigated systematically by electrochemical methods including linear polarization, potentiostatic polarization, potentiodynamic anodic polarization, potential-time measurements at a constant current density, combining the observations of scanning electron microscopy (SEM). It is indicated that the indium addition makes the corrosion potential of Zn shifted positively and its corrosion current increased. Galvanostatic results showed that the addition of indium shortened the passivation time, indicating indium is an active element to the electrochemical performance of zinc alloy electrode. The potentiostatic polarization curves of the pure zinc plate and zinc-indium alloy electrodes in a 35%KOH solution saturated with ZnO indicated that the addition of indium improved the cathodic polarization of alloy electrodes and the deposition overpotential,mean while it depressed the deposition morphology of zinc on the electrode and accelerated the dendrite growth. Scanning electron microscopy images showed that the addition of indium aggravated the corrosion of zinc electrode which may be responsible for the increased tendency to passivation at high current densities. It has been found that at low current densities the reaction kinetics may be increased by indium addition , which is agreement with the discharging test of actual alkaline batteries.

Electrocatalytic oxidation of methanol on Ru deposited NiZn catalyst at graphite in alkaline medium

The methanol oxidation on C/NiZn-Ru electrode in a 1.00M KOH and 1.00M KOH+1.00M methanol solutions at different scan rates and temperatures was studied by the cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and chronoamperometry (CA) techniques. The graphite electrode is coated in a nickel–zinc bath by electrodeposition for use as anode materials for methanol oxidation in alkaline solutions. It is etched in a concentrated alkaline solution to produce a porous and electrocatalytic surface suitable for use in the methanol oxidation (C/NiZn). Ruthenium was electrodeposited on a C/NiZn electrode. The surface morphologies and compositions of electrodes were determined by energy dispersive X-ray (EDX) and scanning electron microscopy (SEM). Kinetic parameters of oxidation such as the anodic electron transfer coefficient (αa), cathodic electron transfer coefficient (αc), charge transfer rate constant (ks) and activation energy values were calculated. The effect of methanol concentration on methanol oxidation were also investigated. It was found that the ruthenium electrodeposited electrode showed higher catalytic activity and stability toward methanol oxidation than the C/NiZn electrode.

Calculation of bulk etch rate’s semi-empirical equation for polymer track membranes in stationary and dynamic modes

One of the most urgent and extremely social problems in environmental safeties area in Kazakhstan is providing the population of all regions of the country with quality drinking water. Development of filter elements based on nuclear track-etch membranes may be considered as one of best solutions this problem. The values of bulk etch rate and activation energy were calculated in view the effect of temperature, alkaline solution concentration as well as stirring effect. The semi-empirical equation of the bulk etch rate for PET track membranes was calculated. As a result of theoretical and experimental studies a semi-empirical equation of the bulk etch rate V B =3.4∙10 12 ∙C 2.07 ∙exp(-0.825/kT) for 12 microns PET film, irradiated by ions 84 Kr 15+ (energy of 1.75 MeV/nucleon) at the heavy ion accelerator DC-60 in Astana branch of the INP NNC RK, was obtained.