ORIGINAL PAPER Characterization of Settled Particles of the Red Mud Waste Exposed to Different Aqueous Environmental Conditions N. Gangadhara Reddy 1 • B. Hanumantha Rao 1 Received: 16 March 2017 / Accepted: 20 March 2018 / Published online: 27 March 2018 Ó Indian Geotechnical Society 2018 Abstract Red mud waste is generally transferred to the storage area in a slurry form with solids content varying from 15 to 40%. Due to arise of a variety of environmental and climatic conditions in the due course of time, settling behaviour of particles as well as properties of settled par- ticles changes. Thus, understanding of the settling beha- viour of waste particles becomes essential after disposing the waste into the pond. In addition, studies related to the settling behaviour of particles become important during the extraction of alumina from bauxite ore (i.e. in settlers). In the present study, efforts are made to investigate the set- tling behaviour of red mud waste particles when exposed to different aqueous environmental conditions. For testing purpose, a series of differential free swell tests were con- ducted considering aqueous environmental conditions such as tap water, distilled water, kerosene, NaCl solutions of 1.71 and 3.42 M, and HCl solutions with a pH range from 1 to 4, respectively. In addition, efforts are also devoted to characterizing the post-exposed sediment samples to ascertain possible changes in the mineralogical, morpho- logical, and elemental compositions of the particles. Results reveal that aqueous environment has a considerable effect on the settling behaviour of particles, while the effect of pH found to be trivial. The analyses of characterization studies disclose a marginal to considerable changes in elemental, phase compositions and morphological charac- teristics of the post exposed particles. Keywords Red mud waste Á Settling behaviour Á Aqueous environment Á Mineralogical–morphological–elemental compositions Á Dispersion and flocculation Introduction In Aluminium refinery, generally, synthetic flocculants are used for facilitating the red mud waste particles to settle in settlers and washers during washing of the mud. The pur- pose is to separate solids from liquid and recover as much as caustic soda before the mud is transferred to the pond. There are six stages of washing by which the concentration of caustic soda is reduced from 70 to 80 g/L at the initial stage to about 6–8 g/L in the last stage, at this instant it is pumped into the pond. One of the techniques of removal of the red mud is by flocculated gravitational separation. There are a variety of flocculants commercially available and have been used successfully for this purpose for some time. These flocculants, though, quite expensive, must be added to achieve the desired settling rate and sedimentation of the particles. The common way of determining the optimum dosage of a flocculent is by performing batch- settling tests in the laboratory scale [1]. The separated red mud waste, usually, is transferred to the storage area in slurry form in which total solids con- centration occupies 15–40% by volume [2]. The red mud waste largely consists of fine particles, which make it difficult to filter them out. Thus, the sufficient impound- ment area is required for allowing the red mud particles to settle. The slurry consists of chemicals like caustic soda and solid particles. Several factors such as surface forces, ionic strength, and electrical interactions between particles & B. Hanumantha Rao bhrao@iitbbs.ac.in N. Gangadhara Reddy gn11@iitbbs.ac.in 1 School of Infrastructure, IIT Bhubaneswar, Argul, Jatani, Odisha 752050, India 123 Indian Geotech J (September 2018) 48(3):405–419 https://doi.org/10.1007/s40098-018-0300-z