PROCESING OF INDIAN RED MUD FOR RECOVERY OF METALLIC VALUES

Abstract

Red mud is an inevitable by-product generated during the alumina production from bauxite ore in Bayer’s process. The red mud is considered as an industrial liability and usually dumped in the landfills. Red mud is caustic in nature and has fine particulate size. Scandium is the most valuable critical metal in red mud and correlation and characterization investigations inferred it to be associated with the anatase and hematite as a substitutional element of Fe and Ti ions. The metallurgical processes investigated for the recovery of valuable elements can be classified into acid leaching, reduction pre-treatment using carbon and hydrogen followed by acid leaching, and baking treatment using acid and alkali reagent followed by water and acid leaching. Stepwise acid leaching using hydrochloric partially dissolves Al (45%) and Si (55%), while oxalic acid leaching results in Fe (61%), Sc (50%) and Ga (48%) dissolution. The anatase was found unamenable to acid leaching with less than 15% Ti dissolution achieved during oxalic acid leaching. The mechanical activation improves Al (85%) and Si (90%) dissolution while Ti and Fe dissolution remains unaltered during hydrochloric acid leaching. The silica (85% purity), alumina (97% purity), and β-ferrous oxalate (99% purity) products were generated from the acid leach solutions. Iron values constituting 35-50 wt.% of red mud is recovered by reducing hematite to magnetite or ferrite by carbothermal/hydrogen reduction and magnetic separation. The reduction also helps in distortion of complex structured red mud, thereby liberating the phases. Microwave heating is integrated with carbothermal reduction and ~750 C is achieved in 10 min at 800 W power. The accelerated heating response of the red mud – carbon mixture during microwave irradiation resulted in formation of metallic iron (15 wt.%) with 78% reduction degree in 30 min. The carbon source acts as a thermal hot-spot and reductant during the microwave heating of the mixture. Microwave carbothermal reduction yields concentrate with 97% Fe recovery and hydrochloric acid leaching recovers 48% Al and generates refractory grade alumina product (>50% purity). In comparison, hydrogen reduction at 900 C achieves 97% iron metallization in 120 min. Magnetic separation was found inefficient in Fe enrichment from hydrogen reduced samples. Nevertheless, phase modification by reduction improves the dissolution of Ga (95%), Fe (92%), and Sc (54%) during sulfuric acid leaching. The final residue is enriched with Ti having TiO2 grade of 42%. Titanium values in red mud present as anatase was found unamenable to conventional leaching and reduction processes and Sc associated with anatase was unrecoverable resulting in partial Sc dissolution (<54%). The transformation of oxide phases to water-soluble metal sulfate salts by acid baking treatment using concentrated sulfuric acid efficiently Ti along with Sc, Fe, and Al values. Acid baking at 350 C in 1 h with 1 mL/g H2SO4 dissolves ~65% Ti, 50% Fe, and 55% Sc by water leaching in 1 h at 30 ºC. The microwave susceptible sulfuric acid rapidly heats the red mud – acid mixture during microwave irradiation and formation of micro-cracks and pores on the baked particles enhances the dissolution Al (~100%), Fe (74%), Ti (73%), and Sc (89%) at 600 W in 6 min, reducing the process duration by ~90%. The dosage of acid in the mixture and baking duration significantly effects the metal dissolution during leaching. The dissolved Ti ions are recovered as TiO2 product by thermal hydrolysis at pH of 1.5 in 3 h. The acid baking treatment significantly improves the metal dissolution from red mud. However, acid baking process results in evolution of corrosive SO3 gas due to decomposition of sulfuric acid. The gaseous evolution is tackled by replacing sulfuric acid with sodium hydroxide during the baking process which resulted in formation of acid/water amenable NaFeO2, Na2TiO3, NaAl(OH)4, NaAlSiO4 phases. The sodium aluminosilicate formation during alkali baking process reduces the Al dissolution during water leaching step. The formation of sodium metal oxide complexes significantly reduces the free energy change for the metal dissolution reactions with sulfuric acid, which was translated to improved metal dissolution during leaching process. Baking red mud with 75 wt.% NaOH at 600 C for 1 h results in 60% Al dissolution during water leaching and 82% Ti, 83% Fe, 97% Sc, and 64% Ga dissolution by 2M sulfuric acid at 80 C. The yield of the final residue is <10 wt.% and contains anatase phase. Alumina (85% purity) product is obtained by wet carbonation of alkaline solution and TiO2 product is obtained from acid solution by hydrolysis. The comparison between the acid leaching, reduction – leaching, and baking – leaching processes concluded that the baking treatment was the most economical and efficient for recovery of Ti, Fe, Al, Sc, and Ga values with minimal waste generation. The TiO2 product generates the maximum value and the dissolved Sc values in the solution is the second most valuable product in the red mud. The reduction process is best suited for the red mud containing higher iron concentration. The recovery of Sc and Ga from the leach solution by liquid-liquid extraction methods will be the scope of future work in the present study and recirculation of the by-products will help in achieving circular and sustainable processing of red mud.