Takehito Hiraki

Thermodynamic criteria for the removal of impurities from end-of-life magnesium alloys by evaporation and flux treatment

Abstract In this paper, the possibility of removing impurities during magnesium recycling with pyrometallurgical techniques has been evaluated by using a thermodynamic analysis. For 25 different elements that are likely to be contained in industrial magnesium alloys, the equilibrium distribution ratios between the metal, slag and gas phases in the magnesium remelting process were calculated assuming binary systems of magnesium and an impurity element. It was found that calcium, gadolinium, lithium, ytterbium and yttrium can be removed from the remelted end-of-life (EoL) magnesium products by oxidization. Calcium, cerium, gadolinium, lanthanum, lithium, plutonium, sodium, strontium and yttrium can be removed by chlorination with a salt flux. However, the other elements contained in magnesium alloy scrap are scarcely removed and this may contribute toward future contamination problems. The third technological option for the recycling of EoL magnesium products is magnesium recovery by a distillation process. Based on thermodynamic considerations, it is predicted that high-purity magnesium can be recovered through distillation because of its high vapor pressure, yet there is a limit on recoverability that depends on the equilibrium vapor pressure of the alloying elements and the large energy consumption. Therefore, the sustainable recycling of EoL magnesium products should be an important consideration in the design of advanced magnesium alloys or the development of new refining processes.

Thermodynamic Analysis for the Refining Ability of Salt Flux for Aluminum Recycling

The removability of impurities during the aluminum remelting process by oxidation was previously investigated by our research group. In the present work, alternative impurity removal with chlorination has been evaluated by thermodynamic analysis. For 43 different elements, equilibrium distribution ratios among metal, chloride flux and oxide slag phases in the aluminum remelting process were calculated by assuming the binary systems of aluminum and an impurity element. It was found that the removability of impurities isn’t significantly affected by process parameters such as chloride partial pressure, temperature and flux composition. It was shown that Ho, Dy, Li, La, Mg, Gd, Ce, Yb, Ca and Sr can be potentially eliminated into flux by chlorination from the remelted aluminum. Chlorination and oxidation are not effective to remove other impurities from the melting aluminum, due to the limited parameters which can be controlled during the remelting process. It follows that a proper management of aluminum scrap such as sorting based on the composition of the products is important for sustainable aluminum recycling.

Oxidation of Pure Solid CaS with Ar-O2 Gas Mixture

Synopsis Oxidation behavior of CaS in oxidizing atmosphere has been investigated for the regeneration process of desulfurization slag by oxidation. The temperature range of oxidation of CaS reagent with Ar-21%O2 gas was 973 to 1423 K. The weight gain was observed in the lower temperature range and reached at the maximum value at 1173 K while weight gain turned to decrease when the reaction temperature was higher than 1173 K. It was clarified that CaS was oxidized to CaSO4 under 1173 K or to CaO over 1373 K by the measurement of oxidation rate, XRD and thermodynamic analysis. The results strongly suggested that sulfur removal from the sulfur containing slag may be possible at temperatures of 1223 K or higher by the oxidation of with air.

Up-grading of natural ilmenite ore by combining oxidation and acid leaching

Rutile (TiO2) is heavily used in pigments and colorants, and the most abundant source of rutile is ilmenite. Upon oxidation of ilmenite, rutile can be formed with modest energy consumption; furthermore, after leaching, only a few byproducts are formed. Unfortunately, one drawback is the necessarily long oxidative process of typically used methods. In this study, we show that a fluidized bed reactor can be used to oxidize ilmenite ore to rapidly form rutile and pseudobrookite (Fe2TiO5) phases. Ilmenite was oxidized with 5vol% O2 in Ar at temperatures of 1173 K or 1223 K and subsequently leached using a diluted H2SO4 solution to dissolve the pseudobrookite phase. The effects of acid concentration, temperature, and cooling rate after oxidation were investigated. We show that the ilmenite was rapidly oxidized to form rutile and pseudobrookite phases at 1173 and 1223 K in a 5vol% O2/95vol% Ar environment within 40 min. The final maximum rutile yield was 84.2mol% after leaching in (1 + 1) H2SO4 solution at 393 K for 12 h.

Hydrolysis Rate of Aluminum Nitride in a Sodium Hydroxide Solution

This paper describes a kinetic analysis of the hydrolysis of aluminum nitride (AlN) powder under highly alkaline conditions, in which the temperature dependence of the hydrolysis rate was measured to derive the most suitable kinetic reaction model. The hydrolysis reaction, AlNC NaOHC 3H2O! NaAl(OH)4(aq)C NH3, AlNC OH C 3H2O ! Al(OH) 4 C NH3, was experimentally monitored by the aluminum ion concentration measured via Inductively Coupled Plasma–Optical Emission Spectrometry (ICP-OES) at different constant temperatures of 295, 300, 305, 310, 316, and 327 K. The reaction accelerated exponentially with solution temperature. The time required for the complete dissolution of the AlN samples was highly dependent on temperature; it was 1 ks at 327 K and 120 ks at 295 K. Based on a comparative analysis of various kinetic models, the reaction curves were most successfully simulated by a second-order homogeneous reaction with an activation energy of 69.2 kJ mol .

In Situ Observation of Dross Formation During Melting of Al–Mg Alloy

Al–Mg alloy is one of the most important industrial aluminum products used in can lids, panel, wheel, welding materials etc. It is known, however, that magnesium in the alloy enhances dross formation on the molten aluminum surface during melting process. Suppression of the dross formation is one of the most important tasks in aluminum industry. In the present work, in situ observation of dross formation during melting of Al–Mg alloy was conducted. By the air oxidation of molten Al–Mg alloy at 800 °C, horizontally projected area of dross phase formed on the surface of the melt and weight gain of the alloy were measured. It was observed that the projected area of dross phase drastically increased after an incubation period and then it changed a little. On the other hand, weight gain of the alloy continuously increased and it became faster with increasing magnesium content of the alloy. These findings help to clarify the dross formation mechanism.

DIRECT PRODUCTION OF PRESSURIZED HYDROGEN FROM WASTE ALUMINUM WITHOUT GAS COMPRESSOR

An innovative environment-friendly hydrolysis process for generating high-pressure hydrogen with recycling waste Al has been proposed and experimentally validated. The effect of the concentration of NaOH solution on H2 generation rate was mainly examined. In the experiments, distilled water and Al powder were placed in the pressure-resistance reactor made of Hastelloy, and was compressed to a desired constant water pressure using a liquid pump. The NaOH solution was supplied by liquid pump with different concentrations (from 1.0 to 5.0 mol/dm) at a constant flow rate into the reactor by replacing the distilled water and the rate of H2 generated was measured simultaneously. The liquid temperature in the reactor increased due to the exothermic reaction given by Al + OH + 3H2O = 1.5H2 + Al(OH)4 – + 415.6 kJ. Therefore, a high-pressure H2 was generated at room temperature by mixing waste Al and NaOH solution. As the H2 compressor used in this process consumes less energy than the conventional one, the generation of H2 having a pressure of almost 30 MPa was experimentally validated together with Al(OH)3—a useful by-product. The energy losses in the proposed system (150.9 MJ) is 55% less than that in the conventional system (337.7 MJ) in which the gas compressor and production of Al(OH)3 consume significantly more energy.