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Sodium release characteristics and revegetation of fine bauxite refining residue (red mud)

Wong, Jonathan Woon-Chung (1990) Sodium release characteristics and revegetation of fine bauxite refining residue (red mud). PhD thesis, Murdoch University.

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The revegetation of bauxite refining residue, red mud, produced from alumina refining is extremely difficult because of the high amount of Na in the residue especially in the desilication product (DSP). The high Na content imparts to red mud highly saline and sodic characteristics. The present study was to develop means to revegetate red mud through the control of Na release and the utilization of acidic and organic ameliorants together with tolerant plant species.

Neutralization of red mud with HCl showed that the DSP, which was found to be a zeolite mineral akin to nosean, decomposed at pH between 5.4 and 7.0 and with a maximum Na release of 98.6 meq/100 g occurring at pH 5.4. At pH >7, the release of Na was due to the cation exchange between Na+ in red mud with H+ , and Ca2+ from dissolution of calcite. At pH between 5.4 to 7.0, Na release from the DSP was due to acid hydrolysis.

The cation exchange capacity (CEC) obtained with monovalent cations was higher than with divalent cations. About 83 meq Na /100 g red mud could be displaced by K+ with repeated washings. Divalent cations could only displace about one third of the Na. The external CEC of red mud was estimated to be 15.4 meq/100 g while the internal CEC of red mud was 49.6 meq/100 g. A selectivity sequence for red mud was established: K > Li > NH4 > Ba ≥ Ca> Mg at a cation fraction on red mud ≥ 0.2.

Four first-order reactions could be used to describe the release of Na from Na-K and Na-Ca exchanges in red mud with a total release of 61 and 31 meq/100 g red mud respectively. Sodium located in the external sites was released in about 6 h and 3 days for K and Ca exchange respectively. The final rates of Na desorption from the DSP were low for both exchanges. This indicates that the sodicity of red mud may increase with time.

A glasshouse pot trial was used to investigate the effectiveness of copperas (FeSO4) and gypsum (CaSO4) for red mud reclamation. Copperas was effective in reducing the EC and Na content of red mud, but gypsum resulted in lower pH and higher Ca content at application rates of 5% and 8% w/w). Seed germination and dry weight yield of Agropyron and Cynodon were enhanced with 8% gypsum or 5% and 8% copperas amendments, but the performance of Agropyron was found to be better than that of Cynodon. Addition of sewage sludge to 5% copperas and 8% gypsum-amended red mud caused an additional reduction of the pH, EC and Na content of red mud. Gypsum-sewage sludge amendment was more effective in reducing pH and ESP, increasing Mg and P04 content, and resulted in lower soil Al content. Yields of Agropyron obtained in red mud receiving 8% gypsum and 16% sewage sludge were significantly higher than in copperas and sewage sludge-amended red mud. A minimum leaching of 84 mm was required to reduce the pH, EC, Na and Ca content of gypsum-sewage sludge-amended red mud, and a supplementation of N, P, and K fertilizer was recommended for the growth of Agropyron in red mud amended with gypsum and sewage sludge.

In field trials, the same rate of gypsum and sewage sludge amendment also reduced the pH, alkalinity, and ESP of red mud, but was less effective than the same rate in the glasshouse trial. Dry weight yield and plant covering percentage of Agropyron in plots receiving 38.5 t/ha (8%) gypsum and 77 t/ha (16%) sewage sludge were significantly increased but the yield was only about half that of plots having a surface placement of sandy soil. Tissue analysis of Agropyron grown in gypsum and sewage sludge-amended red mud indicates N and Mg levels were marginal, while Mn appeared to be insufficient.

Varying the gypsum to soil volume ratio (G:V) by amending with 38.5 t/ha gypsum and 77 t/ha sewage sludge to different depths of 5, 10, and 20 cm had no significant effect on the pH, EC, and ESP of red mud. However, increasing the application rate to 77 t/ha gypsum and 154 t/ha sewage sludge but maintaining the same G:V ratio by incorporating to a depth of 20 cm was effective in reducing the pH, alkalinity, and ESP of red mud. A higher amount of Ca and Mg was found in soil extending to a greater depth. This significantly enhanced the dry weight yield and plant covering percentage of Agropyron.

The increased yield following gypsum and sewage sludge amendment was attributed to improved soil physical properties. Addition of sewage sludge significantly reduced the bulk density, particle density and increased the total porosity. Both sewage sludge and gypsum contributed to increases in air-filled porosity and saturated hydraulic conductivity. The improved soil structure and hydraulic property were found to be significantly correlated with the dry weight yield.

The present study indicates that following the removal of Na in the external exchange sites, the use of gypsum and sewage sludge was effective in red mud reclamation. The slow released Na with time can be counteracted by the addition of an excess amount of gypsum, which will require periodic soil monitoring. This management strategy indicates direct revegetation is feasible and no heavy earth moving techniques are required.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Biological and Environmental Sciences
Supervisor(s): Ho, Goen
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