Characterisation of Nickel Hydroxide Gel

Abstract

Precipitation of nickel and cobalt as a mixed hydroxide is an increasingly popular step used to produce an intermediate product in the processing of laterite ores. The main constituent of the precipitate is nickel at approximately 50% of the solid w/w. The industrial precipitation of the mixed hydroxide is carried out with seeding and at elevated temperatures (50°C) to ensure a filterable precipitate forms. Research was carried out last year at UQ investigating methods of improving the quality of the precipitate product. An observation was that at room temperature the mixed hydroxide precipitates as a voluminous green gel-like suspension. The aim of this thesis was to investigate the formation and properties of hydroxide “gels”, specifically focusing on nickel with mind to understanding the chemical, physical and thermodynamic properties of the “gel” and mixed hydroxide systems in general. This will be achieved by: • Collating the current knowledge regarding nickel hydroxide and gel-like precipitates and the factors affecting their formation. • Experimentally defining the physical, chemical and thermodynamic properties of the nickel hydroxide gel-like precipitates and investigating the factors affecting their formation. • Providing scoping and ground work for further research. Industrial mixed hydroxide precipitation is carried out in aqueous sulphate systems, so the scope of this investigation was limited to studying these simplified industry analogous systems. Literature pertaining to the nickel hydroxide “gel” is minimal, as previous research focuses on either the alpha- or beta-phase crystalline precipitate in relation to its use in batteries. The alpha-phase is an irregularly layered, highly hydrated precipitate which is believed to decompose into the well orientated and layered precipitate beta-phase over time as the impurities are slowly occluded by the crystal structure. This study proposes that the nickel hydroxide “gel” precipitated at room temperature forms an amorphous, highly hydrated gel as defined by Stokes and Frith (2008) when in solution and once filtered it dehydrates to form a vitreous or glassy solid with no long range atomic structure. Calculations on the energy of formation for the amorphous precipitate show that it is less stable that crystalline nickel hydroxide with its energy of formation at -443.2 ±2.0 kJ/mol as opposed to crystalline form at -453 kJ/mol. This indicates that the amorphous precipitate is a metastable phase with the crystalline form thermodynamically favourable but not achievable due to energy constrains. Equilibrium experiments showed that nickel hydroxide precipitates with between 10 and 25% less than the stoichiometrically required amount of hydroxide however, month long experiments found the precipitate began to absorb in more hydroxide over time. Therefore the system was not fixed at a chemical equilibrium and may have been morph rearranging into the crystalline phase over the experimental time frame. Other precipitation system components significantly affected the precipitation thermodynamics of the amorphous precipitate. When separate, cobalt hydroxide will form at a slightly higher pH to nickel hydroxide but when cobalt is included in the nickel hydroxide precipitation system, the cobalt is stabilised to the solid phase at much lower pH values. When the nickel and cobalt hydroxide precipitate was filtered, it formed a cake approximately 3 times larger than the combined volume of the individual nickel and cobalt hydroxide precipitates. Within the aqueous sulphate system the amorphous nickel hydroxide has a hydration factor of approximately 200-300 water molecules per nickel hydroxide molecule. Once it is filtered the hydration drops to approximately 25 water molecules per nickel hydroxide molecule. Over time the filtered precipitate dehydrated further. When the dry cake was submerged in distilled water it was observed to shatter and eject shards approximately 1mm diameter within the space of two minutes. Further research has also been proposed: • Longer term experiments are required to prove whether the amorphous precipitate was in fact decomposing into a more crystalline structure or simply shifting around within the bounds of the metastable phase. • XRD analysis is required to prove that the filtered precipitate is in a glassy state with no long range crystal structure. • Analysis on the gel inclusions is required to gain a greater understanding of the stability of different species within the gel and the attractive and repulsive forces during the formation of the gel.