Leaching with ionic liquids for recovery of reactive metals (TI, SC, REES)

Abstract

Bauxite residue, also known as red mud, is the major solid waste generated during primary alumina production with the Bayer process. For each ton of alumina produced, 1 - 1.5 tons of bauxite residue is generated [1–3], leading to over 150 million tons per year of bauxite residue generated globally [2,4]. Management of bauxite residue is the major issue for alumina industry because of its high volume and alkalinity. However, bauxite residue is a polymetallic matrix, containing valuable metals like scandium, titanium, iron, aluminum and rare earth a comprehensive strategy is needed for recovering metals from bauxite residue and utilize the left-over residue in applications like cementitious industry or building materials. This “zero-waste” approach could contribute in finding a way to solve major issues for the management of bauxite residue and furthermore could help to tackle the raw material dependency of Europe. In this perspective, the main aim of this PhD thesis was to study a process for recovering valuable metals from bauxite residue, with the main objective being recovering valuable metals, such as titanium and scandium. A conceptual flowsheet was presented and two main parts can be outlined. The first part involved the dissolution of metals from bauxite residue through an innovative ionometallurgical approach. In particular, the direct leaching of bauxite residue by using the Brønsted acidic ionic liquid 1-ethyl-3-methylimidazolium hydrogen sulfate for recovering scandium and titanium at high recovery yields was investigated. To optimize the process, parameters like stirring rate, time, temperature and pulp density were evaluated. Their optimized combination has shown high recovery yields of scandium, nearly 80 %, and titanium (90 %), almost total dissolution of iron, while aluminum and sodium were partially extracted in the range of 30 – 40 %. Silicon and rare earth element dissolutions were found to be negligible, whereas calcium was dissolved and reprecipitated as calcium sulfate anhydrate, consuming about the 2 wt.% of the ionic liquid. Moreover, the left-over solid residue was fully characterized, providing explanations for the destiny of rare earths that remain undissolved during the leaching process. The solid residue produced after dissolution can be further treated to extract rare earths or employed in cement industry or for building materials. The second part of the conceptual flowsheet involved solvent extraction process for extracting metals from the pregnant liquid solution. Preliminary tests with four major extractants, three organophosphorus acids (D2EHPA, Cyanex 272 and Ionquest 801) and a neutral extractant (Cyanex 923), were tested in a comparative manner to understand the extraction behavior directly from ionic liquid leachates. Phase separation time, organic to ionic liquid ratio and extractant concentration were studied as variable parameters and kinetic studies were performed to understand metals extraction behavior over time. From the experiments performed, the acidic extractant D2EHPA at 20 % v/v and 1:1 O:IL gave the best results in terms of extracting metals, as almost the total amount of iron, aluminum, titanium and scandium were recovered from the pregnant liquid solution after fifteen minutes. On the other hand, scandium selectivity was achieved using the neutral extractant Cyanex 923. In this perspective, a multi-stage solvent extraction process for selectively recovering metals from pregnant liquid solution was proposed. In the first two stages Cyanex 923 was employed for recovering almost the total amount of scandium and aluminum, while iron and titanium were moderately extracted. After stripping and purification, aluminum and scandium could be employed in Al-Sc alloys industry. The third stage involved the use of the acidic extractant Cyanex 272 for extracting iron and titanium that can be further stripped and purified. Finally, the resulting ionic liquid phase could be regenerated and further used again in leaching process.