Mass spectrometric study of the impurity profile in Zn during reduction-distillation of ZnO with activated and inactivated Al

Abstract

In this work, we report on the application of hyphenated gas source mass spectrometry to study and understand the mechanism of the reduction-distillation of ZnO using Al powder as reductant in its activated and inactivated form. The experiments revealed that the purity of the Zn metal produced were superior using activated Al with respect to inactivated Al, i.e., m5N8 (99.9998% metallic based) versus m5N3. The achieved purity levels of Zn and the absence of high volatile Cd, Mg, and Sb impurities in the gas phase and the material collected were explained with respect to the impurity elements free-energy values, vapor pressure data, and an observed scavenging effect of the Ta crucible, which was supported by the on-line observed mass spectrometric profiles of the residual gas. [1,2]. Several researchers have carried out work on the aluminothermic reduction of ZnO using activated and inactivated Al for the preparation of Al 2 O 3 . However, the reduction was seldom utilized for the purification of zinc [3][4][5]. The present study focuses on the behavior of impurities during the purification of Zn by reduction-distillation of zinc oxide with activated and inactivated Al powder in vacuum. The experiments were conducted at different temperatures and the evolving metal atoms were monitored in real-time to investigate the impurity element's oxide reduction ability and the impurity element's ability for co-distillation. All experiments were performed in a resistance heated vacuum reduction distillation system connected to a gas source mass spectrometer.The activation process of Al is discussed in detail in [6]. The mechanical treatment of Al with added carbon powder as surfactant in vibration milling results in an extremely high chemical activated Al with the removal of inhibiting alumina layer. The addition of graphite also allows one to conserve the powder state of the material without getting oxidized after mechanical milling. The Al powder without pretreatment inherently contains an alumina film at its surface, and is usually denoted as inactivated Al [1]. A smaller particle size presents, in general, a faster reaction rate in the reduction process because of the greater contact area between the reactants; however, in the case of Al, more than particle size, the surface coating matters. ExperimentalThe vacuum reduction distillation system consists of a cylindrically shaped recipient (V ϳ 55 L) on a base plate connected to the mass spectrometer [7]. The schematic of the set-up, originally developed by EMPA, Switzerland, is given in Supplemental Figure S1, which can be found in the electronic version of this article. A turbo pump (PFEIFFER TCP-121) backed by a membrane pump is connected via a vacuum valve. The vacuum achieved was ϳ2.0 ϫ 10 7 mbar measured using a Pirani and a cold cathode gauge. The two copper electrodes mounted side by side to carry the crucible assembly are connected to a high current/low voltage supply. The crucible used for the reduction is made out of tantalum (9.0 mm o.d. ϫ 7.0 mm i.d. ϫ 8 mm ...

In situ vacuum thermal mass spectroscopy for the selection of an environmentally friendly, energy saving catalytic additive and optimization of the process by a novel ignition mechanism

“…A detailed description of the equipment is given elsewhere. 2 For the investigation of the decomposition products of the catalytic additives, 100 mg of the formates of Sodium, Calcium or Zinc (98% Alfa Aesar) were used. The catalytic reduction experiments were carried out separately using y0.5 g of zinc oxide (99.9%, Alfa Aesar) mixed with y0.1 g carbon (Spektralkohlepulver, Germany) as reductant and y0.01 g of catalytic additive.…”