title: Iron nickel separation at high concentration solutions using ion exchange
Creators: name: Silva Jimenez, René Alberto
Publisher: name: Memorial University of Newfoundland
Year: 2021

description: Separation of iron from nickel and other base metals is an indispensable step of leach liquor purification in hydrometallurgy. Currently, the removal of Fe³⁺ is carried out by precipitation increasing pH of leach liquor to the limits of iron solubility. Depending on the process conditions Fe³⁺ precipitates into jarosite, hematite, goethite, or magnetite. Mechanical filtration processes separate the Fe-precipitates from nickel leach solutions. The disposal of precipitate requires strict measures of waste management to reduce the potential of pollution. Iron removal through precipitation is a resource intensive process. Therefore, the pursuit of alternative processes for iron removal signify an opportunity for overall process improvements. Ion exchange is a potential alternative for Fe removal. Hydrometallurgy currently uses ion exchange resin at industrial scale for removal and purification of non-ferrous metals. However, the industry remains reluctant in applying ion exchange for iron removal. Partly due to relatively simplicity of Fe precipitation processes and the lack of environmental regulation related to waste disposal. Previous research on Fe removal by ion exchange focuses on systems at low concentrations (>0.1 M) and research related to Fe removal from electrolytes at high metal concentration is scarce. At large, most of the work is unavailable. It is protected by intellectual properties and company secrecy. Although selectivity of resins is stablished at low concentration systems, theory suggest that selectivity trends and adsorption potential change as metal concentration increases. Therefore, there is a need for investigation to confirm resins technical feasibility at electrolytes of high metal concentration. In this work, the adsorption capabilities and selectivity for Fe³⁺ over Ni²⁺ were evaluated using 30 commercial ion exchange resins of cross-linked polystyrene with divinylbenzene body with 16 different functional groups. The selected resins were classified into strong cationic exchangers (i.e., sulfonic resins), weak cationic exchangers (i.e., carboxylic resins), chelating resins (i.e., N-methyl-glucamine, amidoxime, iminodiacetic, bis-picolylamine, phosphinic, phosphoric, thiol, thiourea and isothiouronium, aminophosphonic, and combination of sulfonic and phosphonic resins), and mixed bed resin (i.e., combination of sulfonic and quaternary ammonium groups). Additionally, resin containing strong anion exchange characteristics (i.e., quaternary ammonium resins) acted as control experiments while non-functionalized resin acted as blank experiments. The potential for Fe³⁺ adsorption and selectivity was evaluated in bimetallic (Fe:Ni) and polymetallic (Fe:Ni:Co:Cu) systems. The bimetallic systems contained equimolar concentration of Fe³⁺ and Ni²⁺ satisfying rough averages of common concentration found in Ni-bearing sulfide and laterite ore leach liquors. The polymetallic systems contained excess of Ni²⁺ to represent leach liquors obtained from Ni-bearing sulfide ores. Among the resins tested, resins containing aminophosphonic acid groups (up to 94% removal), a combination of phosphonic and sulfonic (93%), iminodiacetic acid (92%), methylglucamine (91%), carboxylic acid (78%), quaternary ammonium (62%), amidoxime (67%), phosphoric acid (92%), and phosphinic acid (84%) were the best performing resins obtaining satisfactory results for the selective adsorption of Fe³⁺ from bimetallic and polymetallic systems. The best performing resins underwent systematic studies of single factor optimization to elucidate the parameters that most affect the efficiency of Fe³⁺ separations. Metal concentrations above 20 g/L and solution pH between 1.0 and 2.3 were dominant solution factors affecting the adsorption trends. Adsorption selectivity changed in time. Adsorption of Fe³⁺ increased with time up to a maximum of 120 min. Thereafter, the adsorption of Fe³⁺ was negligible. In periods under 10 min, resins obtained significant co-adsorption of base metals. After that, the co-adsorption of base metal decreased suggesting a base metal displacement by Fe³⁺. Conversely, temperature had little effect on the metal adsorption trends suggesting that ion exchange do perform appropriately at temperatures ranging from 20°C to 90°C. Not all best performing resins sustained Fe³⁺ adsorption selectivity after modification of process variables. Among the best performers, the resins sustaining Fe³⁺ adsorption selectivity were resins with a combination of phosphonic and sulfonic groups, resins with aminophosphonic acid, with carboxylic acid, iminodiacetic acid, phosphoric acid, quaternary ammonium, and methylglucamine. These best performing resins underwent a series of elution studies in H₂SO₄ and HCl solutions of 0.2, 0.5, 1, 5, and 10% v/v. Resins with a combination of phosphonic and sulfonic group showed the best elution performance amidst the resins analyzed and underwent a systematic analysis to determine the best conditions to enhance metal elution. A combination of primary and secondary elution using solutions of H₂SO₄ 5% v/v and EDTA 5% w/w resulted in almost 100% elution of Fe³⁺ and Ni²⁺. The thermodynamic characteristics of the metal adsorption on ion exchange resins with a combination of phosphonic and sulfonic groups confirmed preference for Fe³⁺ over Ni²⁺. The calculation of heat of adsorption, entropy, and the degree of spontaneity of the adsorption reaction was difficult due to the high metal concentration in electrolyte. However, the results showed that the adsorption of Fe³⁺ in ion exchange resins was favourable with exothermic and exergonic characteristics and positive changes in entropy. Conversely, Ni²⁺ adsorption was not favourable having negative changes in entropy while the change in Gibbs energy was positive. These observations confirmed the low adsorption percentages of Ni²⁺ compared to that of Fe³⁺. None of the kinetic models employed described satisfactory the adsorption rate of Fe³⁺ as they failed to achieve correlations above 90%. This suggest that the adsorption rate of Fe³⁺ on resins with a combination of phosphonic and sulfonic groups may be governed by unknown transport phenomena not accounted on the kinetic models used. Overall, the results obtained suggest ion exchange can potentially be an alternative to iron precipitation as an iron removal process in nickel purification. However, economic studies are still required to determine the cost benefit of ion exchange resins against precipitation.