Dust Reduction in Bauxite Red Mud Waste using Carbonation, Gypsum & Flocculation

Abstract

Producing alumina by the Bayer Process creates fine air borne red dust in de-watered dumps which devalues property and causes irritation to the human respiratory system. Samples of red mud waste (1) treated 10 years before at the 0–15 cm depth zone with 40 t ha⁻¹ of gypsum (2) from the subjacent 15–30 cm zone, were collected, crushed and passed through a 0.5 mm diameter sieve. Leaves from Acacia senensis (a legume) were finely chopped to <1 mm and thoroughly mixed with the sieved bauxite waste at 25- and 50 %, and the samples incubated for 6 weeks at ambient room temperatures, at 60 % soil water-holding capacity. To determine the fraction of potential dust, the treated samples were submerged in de-ionized water for several days until there was no change in discoloration (due to clay dislocation) of the water. The samples were removed from the water and the water evaporated and the residues dried and weighed. This submergence process was repeated. The first submergence produced 30- and 20 % w/w dust respectively from the 25- and 50 % phytogenic treatments, and 0 % dust from the non-phytogenic treatments. Submergence #2 released no clay (wet dust) particles from any sample. Finally, the same samples were air-dried, re-crushed and dry-sieved through a 100 µm sieve, because dust-size particles are <100 µm in diameter. Dry-sieving produced dust as follows: non-phytogenic-treated subjacent red-mud zone = 50 % w/w, non-phytogenic—0–15 cm depth = 30 % w/w, phytogenic—at 25 % = 15 % w/w, phytogenic at 50 % = 7 % w/w. In total, the dust-reducing capabilities of the treatments were: 50 % phytogenic >25 % phytogenic >0–15 cm depth non-phytogenic >15–30 cm-depth non-phytogenic. The 50 % phytogenic-treatment reduced potential particles of fugitive dust by 70 % over the untreated controls and 95 % over the crushed-only (subjacent red mud; no organics added) samples. The binding of clay particles into larger entities by organic gums produced agglomerates which (1) reduced the number of <100 µm particles and (2) hence, particles which would have resisted air movements. The initial increases in fine clay dislocation amongst phytogenic treatments was attributed to the increase of small negatively charged organic colloids from the decomposition process, which repelled similar-sized negatively charged clay particles. All in all, phyto-organics increased average particle size >100 µm by flocculation, thereby creating stable agglomerates which resisted disintegration and breakdown under simulated rain. This treatment promises the use of gypsum + phyto-organics for reducing the emanation of surface dust from red mud waste sites onto surrounding areas.