Formation of needle-like lepidocrocite fine particles by oxidation of an aqueous suspension of ferrous hydroxide in a bubble column

Fine particles of needle-like lepidocrocite (gamma-FeOOH) were synthesized by the oxidation of aqueous suspensions of ferrous hydroxide using a bubble column with draft tube at a constant temperature ranging from 20 degrees C to 30 degrees C. The oxidation steps leading to green rust tan intermediat e) and lepidocrocite (final product), termed Step I and II, respectively, c ould be described apparently as first order, with respect to oxygen, and ze ro order, with respect to total ferrous species. When the concentration of oxygen in the feed stream was varied under a constant gas velocity, the mea n size based on the major axis of needle-like particle decreased from 0.60 to 0.35 mu m with increasing oxidation rate. When the gas velocity was vari ed under a constant oxygen concentration, the particle size was almost inde pendent of the oxidation rate and was equal to ca. 0.6 mu m. By the additio n of a small amount of sodium dihydrogenphosphate (NaH2PO4), the major axis could be reduced to 0.2 mu m with the minor axis and the oxidation rate al most unchanged.