RESUMEN
pH adjustment was considered a simple step in the hydrometallurgy process, but its complicated operation was ignored in the past. In some industrial applications, the leachate pH was slowly adjusted by a diluted alkaline solution, with the defects of doubling the leachate volume and causing droplet hydrolysis/coagulation. Up to date, promising routes have been developed for rapid pH adjustment, especially in sealed high-temperature/pressure vessels. New routes emerged in some redox/decomposition reactions of nitrate/urea and organics. Such reactions did not start and/or were slow at room temperature but started spontaneously at high temperatures to generate/consume free H+. This induced pH adjustment in a rapid and homogeneous way.
RESUMEN
Electroplating sludge was a hazardous waste comprised of heavy metals and other Fe/Al/Ca/Si impurities, and produced massively in surface treatment industry. In the past, it was commonly purified via hydrometallurgy, chlorination and reduction calcination routes, but also blended as additive in rotary kiln, to stabilize the heavy metals in geopolymer. Herein, an alternative strategy was developed to treat a real electroplating sludge for recycling magnetic Zn-rich spinel and stabilizing Zn in calcium metasilicate glass via a facile pyrometallurgy route with the blending of emulsion mud and coal ash. The sludge contained 35.6% Zn and 0.54% Cr and then was blended with 50% emulsion mud. After calcination at 1200 °C, the product was highly dispersed, whilst octahedral ZnAlFeO4 spinel with Zn content of 40.0% were formed and separated by using magnet, in accordance with the recycling efficiency of 51.2% Zn from the electroplating sludge. But after calcination at 1400 °C, the gypsum in emulsion mud was decomposed as CaO and accelerated the dissolution of Si-bearing substance as calcium metasilicate glass for covering ZnAlFeO4 spinel, resulting in the Zn leaching of 1568 mg/L. By adding 50% Si-rich coal ash in the calcination system, more calcium metasilicate glass were generated, and then the Zn concentration in the toxic leaching test was only 12.09 mg/L. During the calcination, Cr showed similar performance to Al/Fe and involved in the spinel formation. This provided a new route to recycle Zn from Zn-rich electroplating sludge and to solidify heavy metals via calcium metasilicate glass route.