By studying the leaching kinetics of indium from waste liquid crystal displays (LCD), it was pointed out, that recoveries of > 90 % are possible with 0.5 M HCl at 80 °C or 2 M HCl at 55 °C. The anion-exchange sorption of indium was found practically negligible from sulphate solutions, however, in chloride solutions of 2 mol/dm3 HCl (or NaCl) indium was moderately sorbed (log D = 0.83), and a further increase of the distribution coefficient (log D = 1.1) was found until 6 mol/dm3 Cl- ion concentration. Based on the determined activation energies, at low (< 1 M) HCl concentrations, the process is controlled by the formation of the complex species, while above, diffusion and transport processes are more important. The devised chromatographic separation in an anion-exchange column was able to eliminate virtually all the practical impurities. After loading the leachate of 5 mol/dm3 chloride ion concentration, the non-sorbed alkali, alkaline earth and some transition metals can be removed by rinsing the resin bed with a solution of 2 mol/dm3 chloride ion concentration. Indium can be eluted by reducing the chloride ion concentration in the influent to 0.25 mol/dm3. The still retained elements (Zn, Sn, Bi) can be removed with 1 M NaOH solution. The pure solution was used to electrowon indium metal, which was analysed by SEM-EDS and - after dissolution – with ICP – OES. The purity of the metallic indium product was found to be 99.9997 %.
The possibility and the main characteristics of cementation applied for indium recovery from aqueous chloride and sulphate solutions were considered theoretically and examined experimentally. Aluminium and zinc, in the forms of plates and powders were examined as reducing agents. It was found that indium can be cemented on Al or Zn surfaces best in chloride solutions. Zinc plate proved to be inefficient, however Zn powder performed well. In the case of aluminium, both the plate and the powder worked well, and virtually complete recoveries of indium could be achieved within 10–60 min - depending on the pH and temperature of the solutions. The produced sponge can be detached easily from the Al plate and efficiently melted under protective atmosphere, vacuum or salt flux. The purity of the produced sponges can be generally above 99.9%, and the cementation on Al plate produces the purest indium.
Nagy mennyiségű és nagy sótartalmú végsalak származik az alumíniumötvözetek hulladék alapú gyártásánál keletkező primer olvasztási salakok meleg (termo-mechanikus) feldolgozásából. Az oxidos mátrix mellett ebben viszonylag kevés (5 ~ 10 %) fém és jelentős mennyiségű kloridos alkotók (30 ~ 40 % NaCl és KCl), valamint egyéb vegyületeket fordulnak elő. A vízben oldható, illetve azzal reagáló anyagok miatt, ez a Magyarországon is évi több ezer tonnás nagyságrendben keletkező ipari maradványanyag veszélyesnek minősül és valós környezetvédelmi terhet jelent. A gazdaságos technológiát célzó laboratóriumi kísérletek szerint, a kloridos alkotók kioldása néhány percen belül megtörténik legalább 1:1 cm3/g (víz térfogat:minta tömeg) folyadékarányt biztosítva a szedimentációt megakadályozó intenzitású vízszintes rázás mellett. Ugyanakkor, az edény telítettségi szintjének is van kimutatható hatása. A viszonylag kis költségű megvalósítás a só visszajáratását és a kezelt végmaradvány egyéb célú felhasználását is lehetővé tenné.
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Electrodeposition of Zn from chloride media had been studied by potentiodynamic method to understand the characteristics of Zn recovery from spent pickling liquor (SPL). Various concentration, pH, agitation speed and impurities contamination were examined in the series of experiments. The influence of Zn concentration in the range of 30-150 g/dm 3 relevant to the real SPL were studied along with acidity level change (pH 1.5-5.5). It was found that the cathodic deposition started with the uniform patern followed by sponge-like deposit and as the concentration of the electrolyte near the cathode surface decreased, the dendritic deposition start to grow, especially at the edge of the cathodes. It was found that the effect of iron concentration on the polarization curves of zinc from SPL is complex. Initially it has a negative effect on the generated cathodic current because of the enhancement of hydrogen bubble formation. At a higher concentration range, however, iron deposits at a relatively higher rate. Further increased iron concentrations may make the composition of the Zn-Fe deposit dominantly in favor of iron, resulting in a hydrogen dominated cathodic mechanism. It is also characterised by the loss of the dendritic structure – attributed mostly of zinc deposition when hydrogen bubbles are not blocking the cathode. As the gas evolution becomes more characteristic, the deposit tends to become more powdery. Under such conditions the cathode becomes smoother and the active surface is reduced. Increasing the stirring speed, the powder was easily detached from the cathode surface. Iron, however can enhance the cathodic deposition process of zinc. With 30 – 60 g/dm 3 iron in the 90 g/dm 3 Zn solution, an increased iron concentration resulted in a significant increase in the mass of the deposited zinc. This may be interpreted by a beneficial effect of the increased hydrogen evolution enhancing the zinc ion transport to the electrode surface. However further increments of iron had a contrary effect, possibly by the locally increased – and generally detected – pH of the solution indirectly causing a superficial precipitation of hydroxides. Technically pure zinc could be deposited from only low-iron (< < 30 g/dm 3 ) zinc solutions (of 90 g/dm 3 Zn) at relatively vigorous stirring speeds. High iron concentration in the solution is definitely unsuitable for obtaining the aimed quality of cathode zinc.
The main characteristics of electrolytic reduction applied to indium recovery from aqueous chloride solutions were considered theoretically and examined experimentally. Also other media were considered for comparison. Potentiodynamic examinations pointed out that chloride electrolytes are superior to sulphuric and nitric acid-based ones for efficient indium deposition. The optimum conditions, allowing higher than 90% cathodic current efficiencies, were determined by galvanostatic experiments. In contrast, similar sulphate electrolytes could only yield lower than 50% efficiencies because of a stronger hydrogen evolution. The galvanostatic results confirmed the findings of the potentiodynamic study. The cathodic polarisation curves revealed the major characteristics of the deposition process, implying a reduction of the electroactive In 3+ ions. Coupled processes are the dynamic transformations of complex species and various rates of simultaneous hydrogen evolution. The structure and the elemental composition of the—relatively dense—cathodic deposits were examined by scanning electron microscopy and energy-dispersive X-ray analysis. Various levelling agents were also tested, and gelatine (up to a concentration of 1 g/dm 3 in the chloride electrolyte) was found efficient in grain refining. Graphical abstract
White LED bulbs – dominant in lighting technology – can become a significant secondary source for the recovery of valuable rare metals, if a proper extractive metallurgical technology is developed. We have found an efficient way to separate the “phosphors” (phosphor coated LED chips) from the metallic base shells in dilute sodium carbonate and calcium hydroxide solutions. The leaching reactions of the separated phosphors was studied thermodynamically and the process was examined experimentally. The contained base metals and rare earth elements could be efficiently leached in 1 M HCl acid in 1 h, only Ga and In showed inert behavior toward acid solubilization. The separation of the valuable components has been devised by standard hydrometallurgical techniques.
