Tadahisa Nishimura

Removal of Selenium(VI) from Aqueous Solution with Polyamine‐type Weakly Basic Ion Exchange Resin

Abstract The adsorption of selenium(VI) on the polyamine‐type weakly basic ion exchange resin (Eporasu K‐6) was studied by batch and column methods, for developing a process to remove selenium(VI) from waste water. Selenate ion (SeO4 2−) was very strongly adsorbed on the resin over a wide pH range 3 to 12. The adsorption of SeO4 2− on the resin followed a Langmuir type isotherm, and the saturation capacity and Langmuir constant were 1.7 mmol/g and 1.2×104 cm3/mmol, respectively. As a rise in concentration of sulfate ion (SO4 2−) in solution significantly reduced an adsorption ability for SeO4 2− · SeO4 2− in waste water must be removed by precipitating first as CaSO4 · 2H2O and then as very insoluble BaSO4 prior to use of the resin for removing SeO4 2−. Selenium(VI) adsorbed can easily be eluted from the resin with small volumes of 1 M HCl solution and the resin can be used repeatedly. The adsorption technique with the resin may be used for the final removal of selenium(VI) below the level of the limit of the industrial waste water regulation (0.1 mg/L) in the combination process based on the barium selenate precipitation technique.

Oxidative precipitation of arsenic(III) with manganese(II) and iron(II) in dilute acidic solution by ozone

Abstract Oxidative precipitation using ozone has been examined for removal of arsenic with manganese from dilute acidic sulfate solution under the following conditions: the initial concentrations of arsenic(III) and arsenic(V)=1–100 mg/L, the initial mole ratio of Mn/As=10–100, pH=0.4–5.0 and temperature=15–80 °C. The oxidation–reduction potential (ORP) of the system was continuously measured to monitor the process of oxidation reactions. The O 3 –O 2 gas mixture (ozone partial pressure=0.010–0.031 atm, feed rate=500 mL/min) was found to produce an adequately high potential to oxidize arsenic(III) and then manganese(II). By formation of manganese(III) arsenate and uptake of arsenic by manganese dioxide formed by ozone oxidation, arsenic entities are removed to a sufficiently low concentration to meet the regulation against arsenic contaminant level for industrial waste water in Japan (0.1 mg/L) in the pH range 1.0–3.0 at 25 °C, where coprecipitation with ferric hydroxide is not very effective to remove arsenic. Furthermore, coexistence of an appropriate amount of iron(II) promotes significantly removal of arsenic with manganese at pH below 3 by ozonation.

Comparison between purification processes for zinc leach solutions with arsenic and antimony trioxides

More than 75% of zinc metal in the world is produced by the roast-leach-electrowinning process. In the roast-leach process, very pure zinc sulphate solution must be prepared for zinc electrowinning, because the zinc ion is less noble than the hydrogen ion. Therefore, the purification process is one of the most important unit processes in zinc hydrometallurgy. In general, the cobalt ion is focused on as a typical impurity in zinc electrowinning and several purification processes for cobalt removal have been adopted such as the beta-naphthol process, and the arsenic trioxide or antimony trioxide process. Recently, modernized plants are using the arsenic trioxide or antimony trioxide process, but there are very few published papers on the chemistry of these purification processes. In this paper, firstly the potential-pH diagrams for the systems MAsH2O and MSbH2O are established for conditions which are close to the operating conditions, and the chemistry of the purification process is deduced from the potential-pH diagrams. Secondly, the phenomena influenced by the chemistry are confirmed experimentally and then a comparison between arsenic and antimony trioxide purification is made based on the experimental results.

Chemistry of the M (M=Fe, Ca, Ba)-Se-H2O Systems at 25 °C

The chemistry of the M (M=Fe, Ca, Ba)-Se-H2O systems at 25 °C is reviewed based on our previous papers. In this paper, the phase equilibria in the Fe(III)-Se(IV)-H2O, Ca-Se(IV,VI)-H2O and Ba-Se(IV,VI)-H2O systems at 25 °C are discussed. Then, the three-stage process for removal of selenium from industrial waste water [Se(IV,VI) < 1,500 mg/L] containing sulfuric acid was introduced. This seems to be a promising process for selenium removal from acidic sulfate waste water containing high concentration levels of selenium to below 0.1 mg/L.

Separation of cobalt and nickel by ozone oxidation

The utilization of ozone for the separation of cobalt from nickel sulfate was investigated by determining the oxidation rate for Co(II) and Ni(II) ions under various ozonation conditions at 60°C. The oxidation reaction was observed to follow a first order rate with respect to the ozone partial pressure of the O3-O2 mixture gas and to be promoted considerably by vigorous agitation. The oxidation rates were virtually constant down to a fairly low concentration of the oxidizable ions. Nickel ion was found to be oxidized more easily at lower pH in the mixed sulfate solutions than in solutions of a single sulfate. At pH 2.5–5.0, ozone oxidation seems to be effective to separate cobalt ions selectively from nickel sulfate solutions, due to the extremely slow oxidation of the nickel ion in comparison with cobalt.

Changes in the Characteristics of Manganese Dioxide Produced by Ozone Oxidation during Heat Treatment.

The manganese dioxide prepared by ozone oxidation in acidified solutions of MnSO4 at different temperatures (OMD) was characterized by thermal analysis (DTA and TGA). The changes detected in the thermal analysis were further investigated by X-ray diffraction and oxidation capacity measurement through oxalate-reduction for the OMD samples heat-treated at various temperatures. The discharge behavior was also determined for the heat-treated OMD samples.The ozone oxidation conditions, temperature and sulfuric acid concentration, remarkably affect the changes appearing in thermal analysis due to the crystal transformation of MnO2 from γ-or α-type to β-type and the first thermal decomposition of MnO2 to Mn2O3 as well as due to the liberation of the water in a temperature range below 200°C.The oxidation capacity of OMD sample, expressed in MnO2 equivalent, was observed to decrease in duration of heat treatment, more pronouncedly for OMD precipitated at higher acid concentration and low temperature.Discharge characteristics of OMD were significantly changed by heating for 2 hours at temperatures lower than 200°C. The heat treatment at around 100°C improved the discharge behavior of OMD prepared at 60 to 80°C and acid concentration up to 1.0 M, the discharge capacity being comparable to or better than that of EMD. The OMD samples heated to above 150°C showed poor discharge behavior.It is suggested that the crystal lattice space and activity as an oxide of OMD are changed in close corelation with water liberation during heating at fairly low temperatures.