The solubility of K3VO4 in the KOH–K3VO4–Ca(OH)2–H2O system and subsystems at (313.2 and 353.2) K were studied in a wide alkali concentration range, varying from 5 % to 45 % KOH. From the data, it is concluded that the K3VO4 solubility decreased obviously with the addition of Ca(OH)2 when the KOH concentration is low. Also, the calcification of K3VO4 can be achieved efficiently only in this KOH concentration range. The precipitate phase is mainly presented as Ca10V6O25, and the element K in the K3VO4 can be recovered as KOH. Besides, temperature is not the primary impact factor for the behavior of K3VO4 calcification.

The osmotic coefficients of the Na2CrO4−H2O system at 353.15 K were determined using isopiestic method ranging from 0.05 to 7.683 mol·kg−1. The experimental osmotic coefficients were excellently correlated with Pitzer equation to obtain the interaction parameters, the activity coefficients, and the solubility product for Na2CrO4. Furthermore, the solubility of Na2CrO4 in NaOH solutions at 353.15 K was predicted using binary parameters, and the predicted values agree with the experimental data, demonstrating the Pitzer equation is capable of optimizing the processes for the removal and recovery of sodium chromate in NaOH solutions at high temperature.

Equilibrium data for the NaOH−NaNO3−Na2CrO4−H2O quaternary system from (313 to 393) K were measured, and the phase diagram at 393 K was constructed, with the crystallization areas in the phase diagram discussed in detail. In addition, the solubility isotherms of Na2CrO4 and NaNO3 in NaOH solutions were plotted. The solubility of Na2CrO4 in NaOH solutions was compared with that in NaOH solutions saturated with NaNO3. On the basis of the phase diagram, a strategy for effective separation of Na2CrO4 and NaNO3 from the NaOH−NaNO3−Na2CrO4−H2O quaternary system has been proposed.

Equilibria data for the NaOH−Na2CrO4−Na2CO3−H2O quaternary system at (40, 60, and 80) °C were measured, and the phase diagrams at (80 and 40) °C were constructed. Furthermore, the crystallization areas in the phase diagrams are discussed in detail. In addition, the solubility of Na2CrO4 in NaOH solutions was compared with that in Na2CO3 saturated NaOH solutions, and the solubility of Na2CO3 in NaOH solutions was also compared with that in Na2CrO4 saturated NaOH solutions. On the basis of the phase diagrams, a strategy for effective separation of Na2CO3 from the NaOH−Na2CrO4−Na2CO3−H2O quaternary system has been proposed.

The full alkali concentration range phase diagrams of the Na2O−Al2O3−H2O system were constructed at (150 and 180) °C. The compositions of the clear liquids and wet solid phases were analyzed by inductively coupled plasma atomic emission spectrometry (ICP-AES), and the results show that, as the Na2O concentration increases, the Al2O3 solubility initially increases monotonically, to maximum values of 100 w (mass fraction) = 33.58 and 35.86 at (150 and 180) °C, respectively, and then decreases. At both temperatures, the solid phases were determined to be Al2O3·H2O, Na2O·Al2O3·2.5H2O, Na2O·Al2O3, and NaOH by X-ray diffraction coupled with Schreinemaker’s method. The phase diagrams indicate that, as the temperature increases, the Na2O·Al2O3·2.5H2O phase region shrinks, while that of the Na2O·Al2O3 phase expands.

To realize the objective of high alumina yield and a high molar Na2O/Al2O3 ratio in caustic solutions for Bayer plants, this paper presents the solubility of Al2O3 in methanol−water solvent mixtures with a fixed mass ratio of methanol to water of 1:1 at (30 and 60)°C and containing various Na2O concentrations. The Al2O3 solubility in the Na2O−Al2O3−H2O−CH3OH quaternary system increases with an increase of Na2O concentration, reaching maximum values of 1.70 % and 2.48 % by weight at (30 and 60) °C, respectively. Then, the solubility decreases with a further increase in the Na2O concentration. The equilibrium solids are aluminum hydroxide Al(OH)3 and mono-sodium aluminate hydrate Na2O·Al2O3·2.5H2O in the different alkali concentration regions. The data show that the solubility of Al2O3 in the Na2O−Al2O3−H2O−CH3OH quaternary system is much lower than that in the Na2O−Al2O3−H2O ternary system.

In a new vanadium production process, the effective separation of Na3VO4 and Na2CrO4 from the NaOH−NaNO3−Na3VO4−Na2CrO4−H2O system plays a very important role. In this regard, the dissolution behavior of Na3VO4 and Na2CrO4 in the NaOH−NaNO3−Na3VO4−Na2CrO4−H2O system was investigated by measuring solubility data, and finally the solubility diagram was constructed. From the data, a method of separating Na3VO4 via cooling crystallization and Na2CrO4 via evaporization in the concentrated alkaline solution is proposed.

The oxygen reduction reaction (ORR) on a polycrystalline Pt surface was studied using cyclic voltammetry techniques, and the influence of reaction media on the ORR is examined by comparing the ORR in NaOH and KOH solutions with concentration ranging from 0.5 to 14 M at 298 K. The results show that, in NaOH and KOH solutions, the ORR, a quasi-reversible diffusion-controlled reaction, is largely dependent on the electrolyte conditions, and KOH solutions are superior to NaOH solutions for the ORR process in both thermodynamic and kinetic consideration. As the alkaline concentration increases, the ORR performance frustrates, and the protonation of superoxide is suppressed; thus, the ORR shifts from a 2e reduction pathway to a 1e reduction pathway in both solutions.

Oxidation decomposition of a Vietnamese chromite ore in molten KOH under atmospheric condition was investigated. The decomposition and the further leaching processes were briefly described, together with a discussion on the related thermodynamics and kinetics. During decomposition, the silica in the mineral was first destroyed, followed by the chromite spinel and the activation energy of this process is 35.53 kJ/mol. In addition, the effects of reaction temperature, oxygen partial pressure, weight ratio of KOH-to-chromite ore, chromite particle size on chromium extraction, and behaviors of chromium, aluminum, and silicon were examined.

•Iron and chromium in chromite ore are both converted into valuable resource.•Pollution-free residue is obtained due to the formation of KFe3O5 particle.•The oxidative kinetics of chromite ore is systematically investigated.•The resistance of oxygen transmission during the reaction process is first studied.A new metallurgical process for extracting chromium from chromite ore is proposed to overcome the severe chromium (VI) pollution in chromate production industry. When the 60 wt.% KOH solution was used to decompose chromite ore, the chromium and iron ions in chromite ore were oxidized into soluble K2CrO4 crystal and insoluble KFe3O5 particle, respectively. The chromium levels in the new residue meet the emission standard of chromium-containing waste due to the KFe3O5 crystal's compact appearance, hence the pollution problem of chromite ore process residue is thoroughly resolved. The effect of the main variables on this clean metallurgical process was systematically investigated. Under the condition of leaching temperature 200 °C, oxygen partial pressure 2.0 MPa, and particle size 0.045–0.063 mm, the chromium extraction reached up to 99.4% after 5 h reaction. There is no product layer existing during the leaching process, and the reaction rate is controlled by the surface reaction. The resistances of oxygen transport during the leaching process were calculated step by step, and the surface chemical reaction was the main resistance factor that was consistent with the kinetics study.

An optimized process production of chromate by oxygen was proposed. The tests are executed by introducing NaNO3 to the decomposition process of chromite ore in molten NaOH to decrease the decomposition temperature. An interesting result is that the reaction temperature could decrease from above 500 °C to about 400 °C, and sodium nitrate has no consumption in the oxidation process, while the Cr yield is above 99%. Mechanism analysis shows that chromite ore is firstly oxidized by NaNO3, where NaNO2 is generated as a reduction product, and then is oxidized to NaNO3. NaNO3 plays the role of oxygen carrier, which can reduce the mass transfer resistance of oxygen. The kinetic study shows that the decomposition process was an internal diffusion controlled, with an apparent activation energy of 106 kJ/mol. Effects of liquid-to-ore ratio, chromite ore particle size, and reaction temperature on the decomposition of chromite ore were also studied.