Precipitation and characteristics of iron (III) oxyhydroxides from acid liquors

Evan John Jamieson

An important problem in the mineral processing industry is the removal of iron (III) from process and waste streams by precipitation as the metal hydroxide by elevation of pH. Caustic (NaOH),lime(CaO) and slaked lime (Ca(OH)2,), are cheap and effective reagents for this purpose, however iron (III) oxyhydroxides often form polymeric chains when precipitated at ambient temperature. This gelatinous material is slow to settle and difficult to filter, unlike the crystalline solids obtained at higher temperature. A systematic fundamental study was undertaken to assess the suitability of various characterisation techniques to the iron oxide and oxyhydroxide group of minerals. Techniques such as chemical dissolution methods, surface area, TEM and thermal gravimetry were found to be very useful for comparative purposes, but were not recommended as primary characterisation techniques. Mossbauer spectroscopy proved to be the most reliable method for deteimination of phase composition especially when used in conjunction with XRD. However cost and time for analysis prevent this technique being used extensively. It was found that XRD, settling rate and settled sediment volume were broadly applicable and able to identify variation between samples cheaply and efficiently. This study also methodically compared the settling rates, sediment volumes and iron oxide phases associated with using caustic solution, dry and slaked lime and dry and slaked magnesia. Pure sodium hydroxide and iron (III) chloride solutions were used to represent the simplest system Factors such as temperature, mixing, neutralisation rate, heterogeneous precipitation sulphate addition, the presence of divalent cations (Mg2') and strong iron (III) ligands (eg citrate, oxalate) were investigated with this system Generally the predominant iron phase precipitated was the poor order 0 or 2 XRD .line ferrihydrite. The temperature during precipitation was found to be the only critical parameter in changing product crystallinity and morphology. However, some other factors were able to produce small changes in agglomeration and settling rates, suggesting processes such as pulp recycle may prove beneficial. The iron (III) sulphate system induced a change in precipitates producing microcrystalline goethite, although settling rates were not improved. The use of slowly dissolving lime in the iron (III chloride system was unable to improve precipitate crystallinity or settling rate, however results suggest that lime and caustic can be used interchangeably. The lime / iron (III) sulphate system produced a co precipitate of ferrihydrite and gypsum of enhanced settling ability relative to the caustic system. The use of sparingly soluble magnesia failed to improve the precipitate of the iron (m) chloride system, however substantial improvement was found in the crystallinity and settling rate when using the iron (III) sulphate and mixed chloride / sulphate system, provided a range of conditions were met. This product resembled dense amorphous basic ferric sulphate (DABFS) and was identified as a highly ordered form of ferrihydrite by Mossbauer studies. It is suggested that this precipitate may form the basis for a new low temperature industrial precipitation process. To further investigate the iron precipitation process, iron (II) and mixed iron (II)/(III) chloride solutions were hydrolysed with the addition of sodium hydroxide, calcium oxide and magnesium oxide under well aerated conditions. Dense maghemite was produced with the caustic system at ambient temperature which settled faster than magnetite precipitated at 70 degrees C. The divalent cations Ca2+ and Mg2+ appear to hinder precipitation of crystalline maghemite at ambient temperature, forming the gelatinous poorly ordered ferrihydrite. This fundamental and systematic study of the precipitation process of iron oxides and oxyhydroxides has led to an improvement in the integral understanding of iron (III) hydrolysis. The investigation of characterisation techniques has also led to an enhanced knowledge of their interaction with these precipitates and hence their strengths and weaknesses.

Precipitation and characteristics of iron (III) oxyhydroxides from acid liquors

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