Assessment of the physical fertility of bauxite residue with different textural and structural conditions

Buchanan, Simon James (2006). Assessment of the physical fertility of bauxite residue with different textural and structural conditions MPhil Thesis, School of Land, Crop and Food Sciences, The University of Queensland.

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Author Buchanan, Simon James
Thesis Title Assessment of the physical fertility of bauxite residue with different textural and structural conditions
School, Centre or Institute School of Land, Crop and Food Sciences
Institution The University of Queensland
Publication date 2006
Thesis type MPhil Thesis
Supervisor Hwat So
Total pages 112
Collection year 2006
Language eng
Subjects L
Abstract/Summary Bauxite mining and alumina processing in Gove, northern Australia, produces residues that contain significant quantities of sodium hydroxide, sodium carbonate, and sodium aluminate, making the material highly alkaline, saline, and sodic. In an untreated condition these residues are incapable of supporting plant growth, and if left exposed and unstabilised they are highly susceptible to wind and water erosion, presenting a hazard to both man and the environment. Covering these wastes, preferably with vegetation that is native to the area, is therefore considered desirable. The current method of dry stacking bauxite residue necessitates separation into fine grained red mud (RM) and coarse grained residue sand (RS). Adverse chemical properties of RM inhibit plant growth, but moderately successful plant growth has been obtained in ameliorated (seawater neutralised/freshwater leached) RS, where the limitations are largely associated with water stress during the post-monsoon dry season. The water storage capacity of RS can be improved by increasing its fines content, but this can have a detrimental impact on other physical properties of the bauxite residue that are important for vegetative growth. This project aims to determine what effect increasing the fines content of RS has on the physical properties of bauxite residues, by assessing the suitability of various mixtures of RS and RM for supporting plant growth. Two distinctly different textured bauxite residues dominated by silt (RMsilt) and clay (RMclay) particles, were separated from RM by sedimentation and then ameliorated to reduce pH 1:5 to 8.8, electrical conductivity 1:5 < 412 µScm-1, and Na + < 28 cmolckg-1. The RMsilt and RMclay materials were then added to RS at different ratios to create a range of clay (RS/RMclay) dominated mixtures; silt (RS/RMsilt) dominated mixtures; plus an equivalently textured RM. The effect of texture and structure on plant available water capacity (PAWC), saturated hydraulic conductivity (Ksat), and penetration resistance (Pr) was assessed. Structure of puddled mixtures was altered using both crushing (< 2 mm) to simulate thorough tillage, and wetting and drying cycles for consolidation. The PAWC of bauxite residue was not significantly improved until fines content (silt + clay) was increased from 6 % in RS, to 39 % and 62 % in the RS/RMclay and RS/RMsilt mixtures respectively. Increasing the fines content of RS altered the void ratios of the mixtures, changing the pore size distribution from being dominated by large micro-pores in coarse mixtures to be dominated by smaller micro-pores in finer mixtures, which resulted in negligible change in PAWC of coarser textures. Crushing and consolidation increased water storage at potentials of -0.1 to -10 kPa, with negligible effect on PAWC and water storage at lower water potentials. The PAWC of puddled RM was not greater than that of RS, but in crushed RM the PAWC was twice that of RS. The Ksat of mixtures tended to decrease as fines content increased, but increased with level of disturbance. In puddled mixtures Ksat was lowest (mean 0.52 mmh-1) and whilst not shown to be significant, Ksat seemed to be closely associated to packing density and void characteristics of the mixtures. Subjecting puddled materials to successive weathering cycles caused the samples to expand and crack increasing the Ksat of over half the textures by more than an order of magnitude. Crushing increased Ksat of all samples as water movement was no longer restricted to smaller inter-particle voids, but instead capable of moving through larger cracks and around secondary particles and aggregates. Wetting and drying of these crushed samples caused consolidation of particles, reducing Ksat due to the increased tortuousity. A quantitative measure of Ksat after wetting and drying cycles was not able to be attained in these experiments due to insufficient sample size. In puddled mixtures the penetrability at field capacity and wilting point was affected by packing of particles, wherein Pr increased with decreasing void ratio. Increasing the fines content of RS by only a small amount (13-15 %) resulted in a more than 6 fold increase in Pr of puddled mixtures. The Pr of all puddled mixtures other than RS was much higher than the penetrability considered highly restrictive to root elongation (2.0 MPa). This was substantially lower (up to 96%) in the crushed treatments, regardless of texture, and considered favorable to root growth, which suggests that a vital criterion for successful vegetation growth in neutralised bauxite residues is the breaking up of the massive structure. This work concluded that through drying and crushing is a prerequisite for plant growth in bauxite residues and that improvements in soil water availability are only identifiable in the RS when the fines content is increased substantially. To achieve successful plant establishment and sustainable vegetation cover it is recommended that deep ripping and thorough cultivation of puddled bauxite residues be carried out initially prior to propagation in order to improve water infiltration, reduce erodability, and facilitate root elongation.

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Created: Fri, 21 Nov 2008, 14:54:00 EST