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Some electrochemical aspects of the Becher process

Marinovich, Yadran (1997) Some electrochemical aspects of the Becher process. PhD thesis, Murdoch University.

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      Abstract

      The Becher process is used to upgrade ilmenite (~60% TiO2) to synthetic rutile (92-95% TiO2). Reduced ilmenite (FeTiO2) is an intermediate mineral product and the industrial process whereby metallic iron is corroded from the TiO2 matrix (leaving synthetic rutile) using air and ammonium chloride solutions is termed aeration. This thesis is concerned with the electrochemistry of the corrosion reaction that is the heart of this aeration step.

      Synthetic rutile is almost exclusively used as a feedstock for the chloride process which produces pigment grade TiO2. The Becher process is but one a number of processes that produce feedstocks for the chloride process. Together with the Becher process, these alternative processes are briefly reviewed in Chapter 1 followed by a detailed review of the Becher aeration step. Also included in Chapter 1 is a brief review of the thermodynamics for the iron-water system and, following a literature review on ferrous-ammine chemistry, a new Eh-pH diagram for the iron-water-ammonia system is presented using recently published data.

      The investigation of the electrochemistry of anodic iron dissolution in Chapter 3 reveals some interesting results about the influence of aqueous NH4Cl. An air-formed film only a few monolayers thick (9-18 Å) passivates iron after surface preparation. Ammonium ion is responsible for the rapid removal of this film after immersion which then allows fast active dissolution to occur upon anodic polarisation. Without ammonium ion the air-formed film passivates the iron during polarisation. Ammonium ion also increases the active dissolution current density while chloride inhibits active dissolution. However, chloride is responsible for breaking down the air- and anodicallyformed films at relatively high anodic potentials. The activation energy for anodic iron dissolution in 0.2 M NH4Cl up to 150°C is reported.

      In Chapter 4 it is shown that oxygen is reduced on iron via a 4-electron mechanism in air-saturated 0.2 M NH4Cl and that reduction is diffusion controlled. On gold, every oxygen molecule requires between 0 and 4 electrons depending on the potential. The oxygen reduction current density is proportional to the oxygen partial pressure up to 300 kPa. At 300 kPa oxygen partial pressure, the current density nearly doubled between 80 and 130_C due to the combination of three different effects.

      In Chapter 5 it is shown that in air-saturated 0.2 M NH4Cl the air-formed film inhibits iron corrosion for the first 20 to 30 minutes but then is removed allowing iron to corrode quickly and uniformly. _-FeOOH forms continuously as a porous film adhering to the surface and begins to inhibit corrosion only after 3 hours immersion. Ammonium ion is necessary for the removal of the air-formed film while chloride alone results in the localised breakdown of the film leading to pitting corrosion. However, above pH 7.5, the air-formed film is stable even in the presence of ammonium ion and, in the presence of chloride, pitting corrosion also results. Even though iron corrosion is essentially oxygen diffusion controlled at ambient pressures, the corrosion rate is not proportional to the oxygen partial pressure due to the formation of a thick _-FeOOH film. Increasing the temperature to above 80°C increases the corrosion rate at high pressure largely due to solubilisation of the thick film.

      A novel application of the carbon paste electrode technique to the investigation of the electrochemistry of anodic iron dissolution from whole reduced ilmenite grains is described in Chapter 6. Anodic dissolution is potential dependent but strongly limited by an air-formed passive film. Ammonium ions do not assist in the removal of this film; rather, chloride ions were found to increase the anodic current density by assisting in the breakdown of the air-formed passive film. Also presented in this Chapter is some preliminary evidence linking the variability of the aeration time with the stability of the air-formed film: the less stable the air-formed film, the shorter the aeration time.

      Publication Type: Thesis (PhD)
      Murdoch Affiliation: School of Chemical and Mathematical Science
      Supervisor: Bailey, Stuart and Avraamides, Jim
      URI: http://researchrepository.murdoch.edu.au/id/eprint/9235
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