Studies on the rehabilitation of manganese tailings at Groote Eylandt, Northern Territory

The thesis briefly-describes the location where the work was conducted ie Groote Eylandt, Northern Territory, its climate, biophysical environment, mining operation, ore beneficiation process, tailings dams and rehabilitation research programmes. A review of tailings disposal methods used throughout the world, including land and marine based systems, alternative uses, and reprocessing are discussed. The potential for air and water pollution from tailings, including the local examples of Captains Flat and Rum Jungle are described. Examples of the various means of surface stabilization ie vegetative, physical, and chemical and a brief outline of tailings stabilization work conducted at most major metalliferous mining and ore processing operations in Australia is given. An overview of the major aspects of soil manganese chemistry is presented in Chapter 2. Subjects discussed include manganese mineralogy, heating, storage and drying effects, the influence of lime, organic matter, and compaction and the interactions between manganese and other ions, in particular Calcium, Iron, Phosphorus, Aluminium and Cobalt. The research work was conducted in the period 1987-83 and had the primary objective of "finding a long term, self sustaining vegetative cover to stabilize tailings and prevent them from becoming sources of air and water pollution, while enhancing the aesthetics of the area". Detailed chemical and physical characterization of tailings from a 2 hectare study area was conducted. The study area was located within a major group of earthen walled tailings dams, some of which were being actively used for tailings disposal. The analyses found that the black coloured tailings were variable in physical properties with textures ranging from sands to clays, with corresponding variations in bulk density and water holding capacity. No extreme pH, soluble salt, or exchangeable aluminium levels were recorded, although total nitrogen and available phosphorus levels were found to be low. A series of sequential extractions were used to determine water soluble, exchangeable, easily reducible and dithionite soluble manganese fractions. The results obtained indicated that the exchangeable, easily reducible and total manganese levels would be above the critical levels for plant growth to occur. Characterization of the temperature regime on the tailings dams during starrier was undertaken using mercury in glass thermometers over the summer months. Comparisons obtained between the tailings study area, undisturbed open forest and a backfilled area rehabilitated with pasture, indicated that the tailings were not hotter than the other sites studied. Although the results indicated that no adverse microclimate exists in the tailings in terms of temperature regime, measurements with thermocouple needles at all three sites to confirm this finding would be useful. Because some local plants were growing on the dams, a series of agronomic trials were initiated using nine tropical pasture grasses and five tropical legumes, without the addition of any soils. Vigorous growth of the grasses occurred and despite many different treatment combinations, no successful growth of legumes occurred. The main fertilizer response from the grasses was dependent upon phosphorus with the addition of nitrogen, and various trace elements in the absence of phosphorus not resulting in any significant growth. Analysis of the results led to the conclusion that the key factor in determining the growth of the grasses in the tailings was the level of water soluble manganese. Following the success of the agronomic trials, work was then directed towards utilizing local plant species in the tailings rehabilitation work. Botanical surveys identified G1 plant species comprising 19 families growing on the dams. These were found to be nearly twice the number of plants identified by Langkamp, Ashton and Balling (1981) as growing on a manganese ore outcrop on Groote Eylandt. Comparisons between the tailings and ore outcrop, suggest that physical impedence to plant establishment and growth on the ore outcrop, rather than the manganese levels is the reason for the many differences between the two plant communities. The use of seed traps and a surface roughness and phosphorus trial, showed that a large number of grass seeds were being blown across the dams. It was shown that if a rough surface was provided, together with a phosphate fertilizer, colonization of abandoned dams would occur at a much faster rate than currently occurs. To complete the study programme, a large trial using the seed of 26 local trees and shrubs and four provenences of Eucalyptus cemadulensis was conducted. Treatments applied to the seeds included combinations of phosphorus, mulch and Rhodes grass. The trial determined that there are some local species which do not grow and survive in the tailings, yet many other common local species had impressive germination, survival and growth rates. The application of surface mulch resulted in higher germination percentages and the application of the cover crop appeared to be beneficial to the young seedlings. Overall, the regeneration trial established that the tailings dams can be rehabilitated by similar methods to the forest regeneration programme for mined out and backfilled quarry areas and that the broad objective of the research programme was achieved