Titanium Extraction from Spent Selective Catalytic Reduction Catalysts in a NaOH Molten-Salt System: Thermodynamic, Expe
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RODUCTION
TITANIUM dioxide (TiO2), a nontoxic inorganic material, has been widely applied as a photocatalyst in the self-cleaning and photodegradation of organic contaminants, as a pigment in paints and cosmetics, and as a filler in paper and plastic.[1,2] The sulfate process and chloride processes are the two main industrial routes used to produce TiO2.[3] In China, 95 pct of TiO2 producers use the sulfate process.[1,4] In the sulfate process for producing TiO2,[5] the main feedstock is ilmenite, which is first leached with sulfuric acid and forms tetravalent titanium ions, which are subsequently subjected to thermal hydrolysis in a highly acidic solution with the F value [m(effective H2SO4)/ m(TiO2)] around 1.9.[6] Finally, the as-obtained tetravalent titanium hydrous oxides are calcined at a high temperature to obtain TiO2 powder. However, the depletion of ilmenite inevitably raises environmental concerns. For example, ~ 8 tons of ~ 25 wt pct sulfate acid waste and ~ 4 tons of iron sulfates are generated per ton of TiO2 produced.[7] Thus, to protect the environment and preserve domestic titanium resources, a cleaner and more efficient approach for producing TiO2 using high-grade and secondary titanium resources is urgently needed.
QIJUN ZHANG, YUFENG WU, and TIEYONG ZUO are with the Institute of Circular Economy, Beijing University of Technology, Beijing 100124, P.R. China. Contact email: [email protected] Manuscript submitted July 16, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS B
Spent selective catalytic reduction (SCR) catalysts, which are derived from coal-fired power plants, contain up to 80 wt pct TiO2.[7,8] Several previous studies have focused on the recovery of titanium resources from spent SCR.[9–12] For example, Li et al. developed an oxidative ammonium bicarbonate leaching method to selectively leach vanadium, tungsten, arsenic, potassium, and sodium from spent SCR and to maintain the crystal structure of the anatase TiO2 in the spent SCR after treatment.[10] Chen et al. developed a Na2CO3 roasting method to treat spent SCR.[12] The spent SCR was first calcined at 700 C for 3.5 hours, with a mass ratio of Na2CO3 to SCR of 3.8:1, followed by water leaching at 80 C for 2 hours; then, the obtained filter residue was leached with 5 wt pct H2SO4. The recovery efficiency and purity of TiO2 reached 92.15 and 96.28 pct, respectively. NaOH molten-salt roasting is considered as a simple, low-pollution, efficient, and low-cost method for treating hardly decomposed minerals and waste.[13–18] For example, Chen et al. proposed a NaOH molten-salt roasting process for recovering titanium from rich titanium–vanadium slag.[13] Moreover, the titanium conversion yield reached 96.3 pct when roasted at 500 C for 60 minutes with a NaOH-to-slag mass ratio of 1:1. Liu et al. prepared a rutile TiO2 white pigment with good pigment properties from low-grade titanium slag based on the NaOH molten salt pretreatment process.[17] Using this NaOH molten salt pretreatment process, with a temperature of 550 C and a roasting time of 90 m
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