Reclamation of metal values from TiO2 production waste residues.

A novel process for recovery of niobium, vanadium and scandium from TiO2 residues in three stages consisting of acid leaching, selective precipitation and oxidative alkali leaching has been developed using a batch leach test rig. While technologies for recovering these valuable metals from various minerals as co-products or by-products are commercially available, they are not currently used for recovering them in combination. An integrated process for reclamation of the metals could increase the total values recovered while reducing the objectives to their subsequent disposal. Three batches of TiO2 filter cake were obtained from Huntsman Pigments and Additives between 2012 and 2014, and were analysed using ICP-OES, TGA, XRD, SEM-EDX and XRF. The samples contained different amounts of unreacted ore and coke, together with hydrated oxides of a wide spectrum of metals such as vanadium, scandium, iron and niobium. Initially, Pourbaix diagrams were utilised for mapping out when the metals would precipitate or be in solution depending on their concentration, solution pH and electrochemical potential. The information was compared with established metal recovery processes to determine the recovery route. This study showed that the metals could be separated by selectively precipitating and filtering niobium from acidic media at pH ≈ 1 followed by controlled co-precipitation of vanadium and iron at pH ≈ 2 and scandium at pH ≈ 4. It was also determined that further concentration enhancement could be achieved by oxidative alkali leaching after the selective precipitation steps. Following establishment of the metal recovery route, experiments were carried out to sequentially study the effects of temperature, lixiviant concentration, reaction time, pH, stirring speed, solid-liquid ratio and oxidant concentration on dissolution and precipitation of the metals. The chloride process of TiO2 production is operated under reducing conditions therefore most hydroxides in the filter cake are in a reduced state. NaNO3 was used as an oxidant for improving metal recovery and selectivity during Na2CO3 leaching. Vanadium purification by salt roasting was investigated and the conventional NH4VO3 precipitation route was utilised for vanadium precipitation from pregnant solutions. Recoveries of over 80 % were achieved for vanadium and niobium, with grades of 99.5 % and 16 % respectively. Zirconium, titanium and thorium were the key impurities in the niobium-rich precipitates. Nearly all the scandium was recovered with a grade of at least 50 % in an alkali solution where rare earths were the main impurities. Adding NaNO3 to the alkali leaching systems significantly improved the extraction of vanadium and scandium, while niobium’s selective precipitation at pH 1 was catalysed by sulphate ions. The random pore model governs the dissolution processes, with all acid leaching kinetics being controlled by pore diffusion and alkali by surface reaction. The developed process opens up opportunities for recovery of other metals, particularly titanium, iron and REE and can be employed for recovery of the metals from filter cake already landfilled.