Abstract
This research reports on an investigation of the performance of inorganic membranes for use in the percrystallisation of nickel sulphate hydrate. In this novel process, the separation of the solvent (water) and the crystallised solute (nickel sulphate hydrate) occurs continuously in a single-step, avoiding further downstream processing (crystal filtering and drying). The inorganic membranes were synthesised with sucrose solution followed by a post vacuum-assisted impregnation of the coated film on a α-alumina substrate and carbonisation under nitrogen atmosphere. The highest fluxes measured were 22 L m−2 h−1 and 1 kg m−2 h−1 (40 g L−1) for water and nickel respectively. Interestingly, the transport of solution through the membrane also affected the hydration state of the nickel sulphate, as well as the crystal type and shape. High water fluxes delivered pure nickel sulphate heptahydrate with elongated and laminar crystal particles (~200 μm). Lower water fluxes produced both heptahydrate and hexahydrate salts with approximately spherical particles (also ~200 μm). There a number of factors that influence the crystallisation reaction such as the rate of evaporation which affects water availability and the resultant temperature at the permeate side of the membrane. Finally, the activation energy for nickel sulphate crystallisation was estimated to be approximately 16 kJ mol−1 based on feed solution temperatures.
Highlights
Owing to its electrochemical and electric properties (Lei et al, 2014; Xiong et al, 2015; Dimaras, 1956; Lascelles et al, 2005), nickel has been gaining increased interest from the research community
Nickel sulphate is highly soluble in water and its solubility was shown to have a positive correlation with temperature in water over the range of 60–80 °C and a negative correlation with sulphuric acid concentration; the temperature-solubility correlation reverses when the sulphuric acid concentration is above about 25 wt% (Havlik et al, 1996)
As water evaporates from the wet thin film, it provides the ideal conditions for supersaturation, leading to nucleation and crystallisation of nickel sulphate hydrate
Summary
Owing to its electrochemical and electric properties (Lei et al, 2014; Xiong et al, 2015; Dimaras, 1956; Lascelles et al, 2005), nickel has been gaining increased interest from the research community. Madsen et al (Madsen et al, 2018) reported that the size of the resultant sodium chloride crystals were controlled by the conditions used to prepare the porous carbon membranes and operating process conditions. This work demonstrates for the first time the use of carbon membranes for the percrystallisation of nickel sulphate, where operation conditions affect the hydration state of the formed salt. This work shows that membrane percrystallisation is an effective way of crystallising the solute whilst concomitantly separating the solvent (water) from the target salt in a single-step process. The formed crystals were analysed by scanning electron microscopy, thermogravimetry and X-ray diffraction to determine the particle size, morphologies and the degree of hydration of the resultant salt
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