Browsing by Subject "SELECTIVE RECOVERY"

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  • Heliövaara, Eeva; Liljeqvist, Henri; Muuronen, Mikko; Eronen, Aleksi; Moslova, Karina; Repo, Timo (2021)
    Development of new, environmentally benign dissolution methods for metallic gold is driven by needs in the circular economy. Gold is widely used in consumer electronics, but sustainable and selective dissolution methods for Au are scarce. Herein, we describe a quantitative dissolution of gold in organic solution under mild conditions by using hydrogen peroxide as an oxidant. In the dissolution reaction, two thiol ligands, pyridine-4-thiol and 2-mercaptobenzimidazole, work in a cooperative manner. The mechanistic investigations suggest that two pyridine-4-thiol molecules form a complex with Au-0 that can be oxidized, whereas the role of inexpensive 2-mercaptobenzimidazole is to stabilize the formed Au-I species through a ligand exchange process. Under optimized conditions, the reaction proceeds vigorously and gold dissolves quantitatively in two hours. The demonstrated ligand-exchange mechanism with two thiols allows to drastically reduce the thiol consumption and may lead to even more effective gold dissolution methods in the future.
  • Räisänen, Minna; Heliövaara, Eeva; Al-Qaisi, Feda'a; Muuronen, Mikko; Eronen, Aleksi; Liljeqvist, Henri; Nieger, Martin; Kemell, Marianna; Moslova, Karina; Hämäläinen, Jani; Lagerblom, Kalle; Repo, Timo (2018)
    Dissolution of elemental gold in organic solutions is a contemporary approach to lower the environmental burden associated with gold recycling. Herein, we describe fundamental studies on a highly efficient method for the dissolution of elemental Au that is based on DMF solutions containing pyridine-4-thiol (4-PSH) as a reactive ligand and hydrogen peroxide as an oxidant. Dissolution of Au proceeds through several elementary steps: isomerization of 4-PSH to pyridine-4-thione (4-PS), coordination with Au-0, and then oxidation of the Au-0 thione species to Au-I simultaneously with oxidation of free pyridine thione to elemental sulfur and further to sulfuric acid. The final dissolution product is a Au-I complex bearing two 4-PS ligands and SO42- as a counterion. The ligand is crucial as it assists the oxidation process and stabilizes and solubilizes the formed Au cations.
  • Xu, Junhua; Koivula, Risto Tapio; Zhang, Wenzhong; Wiikinkoski, Elmo Werneri; Hietala, Sami; Harjula, Risto Olavi (2018)
    The purpose of this study was to investigate the separation of Co, Nd and Dy from a ternary Co-Nd-Dy solution using amorphous zirconium phosphate (am-ZrP). Am-ZrP was synthesized by a precipitation method at room temperature and subsequently characterized by Fourier transform-infrared spectrometry, thermogravimetry, scanning electron microscopy, X-ray diffraction, solid-state 31P magic angle spinning nuclear magnetic resonance spectrometry and sodium hydroxide titration (with and without background salt). The ion exchange kinetics of am-ZrP that were determined in ternary 1 mM equimolar solutions at equilibrium pH 2.5. The effect of pH on the adsorption was studied in ternary 1 mM equimolar solutions and the uptakes of the metals increased with increasing pH until approximately pH 3.5. The adsorption isotherms of Co, Nd and Dy were tested in a series of ternary equimolar solution, the total uptake amounted to 4.13 meq/g at pH ~ 3.0. The preference of am-ZrP for these metals occurred in decreasing order Dy > Nd ≫ Co. The separation of Co, Nd and Dy from their 1 mM equimolar ternary mixture was investigated on an am-ZrP column. Effects of loading (7.8%, 62% and 100%) on the separation were compared by measuring the corresponding HNO3 elution fractions. It was found that with a lower metal loading of 7.8%, three clear elution bands were obtained. Am-ZrP exhibited selective separation properties towards the ternary Co-Nd-Dy system, which contribute to the future scale-up studies for the recycling of NdFeB magnets.
  • Zhang, Wenzhong; Avdibegovic, Dzenita; Koivula, Risto; Hatanpää, Timo; Hietala, Sami; Regadio, Mercedes; Binnemans, Koen; Harjula, Risto (2017)
    The separation of rare-earth elements (REEs) is usually carried out by a multi-stage solvent extraction process utilising organophosphorus extractants. Inspired by the structure of the solvating extractant tri-n-butyl phosphate (TBP), new sorbents were designed by covalently attaching short n-alkyl chains (ethyl, n-propyl and n-butyl) to titanium(IV) phosphate functionalised mesoporous MCM-41 silica by a layer-by-layer grafting route. Mesoporous MCM-41 silica served as a versatile porous support and the grafted titanium(IV) derivatives provided enhanced acid stability and solvating extraction capability. Various characterisation methods including solid-state C-13, Si-29 and P-31 magic-angle spinning (MAS) nuclear magnetic resonance (NMR) and simultaneous thermogravimetry and differential scanning calorimetry-mass spectroscopy (TG/DSC-MS) were used to confirm the ligand attachment. The hybrid materials showed a better uptake of rare-earth ions from nitrate feed solution than the unmodified inorganic material. The optimal separation factor (SF) obtained for scandium-lanthanum separation is in excess of 100 000 at pH 2.1. The SFs calculated for dysprosium-neodymium are approximately 3, which is comparable to that of TBP in a typical solvent extraction setup. This study provides a new strategy to design inorganic-organic hybrid sorbents based on the structure of organophosphorus extractants via metal(IV)-O-P bonds.