Abstract
Characteristics of refused materials are prerequisite information required to determine the strategic reclamation of extreme land degradation in post-tin mining areas. The objective of the study was to evaluate mineralogical, chemical, and physical properties and heavy metals of spoil and tailing as the basis for reclamation measures. Seven representative soil profiles were made and sampled to a depth of 130 cm for various soil analyses. Results showed that tin mining has drastically altered the soil texture from sandy clay loam under native conditions to loamy sand and sand under post-tin mining. Mineralogical constituents of refused materials were mainly mineral resistant to chemical weathering, consisting of predominant quartz with small amounts of tourmaline, opaque, zircon and garnet. Total X-ray fluoresce (XRF) elemental analysis showed extreme high SiO2 content (92-96%) associated mainly with quartz mineral, and extremely low oxides of Ca, Mg, P, K and S (< 0.2% altogether). This suggests all nutrients are severe problems for crops. Type of total heavy metals showed the Cr2O3 was high in sandy tailing (204 - 286 mg kg-1), while the SnO was low (0 -153 mg kg-1) and they were preserved in the structure of host minerals, thereby the health risk is negligible. Based on many serious constraints of soils, the strategic reclamation to recover soil productivity and ecological function was the building up soil organic matter, establishing “pot planting point” technique, complete fertilizer application, and selection of crops with an ability to fix N nutrient from the atmosphere, and adaptive to low soil nutrients.
KEYWORDS: Post-tin mining, soil characteristics, heavy metals, tailing, spoil, strategic reclamation
1 INTRODUCTION
Mining left the legacy of environmental problems attributed by disposal of refused materials. Global land use for mining between 1976 and 2000 is cumulatively about 3,700,000 ha or about 0.2% of earth’s land surface (Barney, 1980). Mines produce large amounts of refused-materials because the ore constituent of the total volume of the mined materials occurs only in a small fraction (Duhka and Adriano, 1997). For tin mining, the separation of tin ores, the heavy sand fraction (density > 4 g cm-3), from the clay, silt and light sand fractions leads to the dumping of large amounts of spoils and tailings, and their cumulative amounts increase with time. The disposal of tailings is a major environmental issue that has become more serious with the increased exploration for metals and the exploitation of lower grade mineral deposits (Ozkan and Ipekoglu, 2002).
Southeast Asian countries (Indonesia, Malaysia, Thailand and Myanmar) produced 9.6 million tonnes of tin, equivalent to 54% of the world’s tin production since 1800 (Schwartz et al., 1995). In 2017, Indonesia is the second larger world producer (50,000 metric tonnes) of tin after China (100,000 metric tonnes) with total world production of 290,000 metric tonnes (USGS, 2018). Further, tin world reserve is 4,800,000 metric tonnes in which China and Indonesian reserves are 1,100,000 and 800,000 metric tons, respectively (USGS, 2018). The major locations of tin producer in Indonesia are Bangka, Belitung and Singkep islands. Open pit mining of tin is destructive due to the complete removal of vegetation and topsoil. In addition, tin mining processes involving the dispersion and washing to separate heavy sand fraction of ores from refused materials could result huge piles of sandy tailing. The large idle bare land (abandoned tailings or waste piles) is massively occurred. The high economic return from tin mining activities are not reconciled with the existing environmental management.
In this study, tailing was referred to dumped materials left over after the process of separating the valuable heavy sand fraction ores from the uneconomic light sand fraction. Likewise, spoil was referred to an accumulation of displaced earthy material or other waste material removed during mining or excavation. Refused materials (e.g. spoils, tailings and slime) are mine’s byproducts accumulated from the excavation, washing, concentration or treatment of ground ores to extract valuable ores. The Bangka island has been exploited for tin since 1711 (Ko, 1986), resulting a widespread of huge spoil and tailing disposal as the extreme threat to the environment. Hence, characterization of spoil and tailings in terms of chemical, physical, mineralogical properties and heavy metal contents should be done to obtain reliable information as a basis for reclamation measures.
Generally, mining is considered to be one of the most significant sources of heavy metal contamination (Acosta et al., 2011; Dudka and Adriano, 1997; Fryer et al., 2006). Ores and mineral extraction have inflicted serious environmental damage, especially in the heavy metal pollution (Acosta et al., 2011; Komnitsas and Modis, 2006; Zhou et al., 2007). Metals of major interest in bioavailability studies, as listed by the U.S. Environmental Protection Agency (EPA), are Al, As, Be, Cd, Cr, Cu, Hg, Ni, Pb, Se, and Sb (McKinney and Rogers, 1992). Heavy metals such as Pb, Zn, Cd, Hg and Cr generally refer to metals and metalloids having densities greater than 5 g cm-3 (Oves et al., 2012). Li et al., (2014) reviewed the soil heavy metal pollution from mines in China and concluded that the mean concentrations of As, Cd, Cr, Cu, Ni, Pb, Zn, and Hg in soils are higher (varying from 0.4 to 36.5 times greater, depended on a heavy metal type) than the Grade II environmental quality standard for soils in China. The geochemical flux explained the lithological origin of potential toxic elements (As, Cd, Co, Cr, Cu, Fe, La, Mn, Ni, Pb, Sc, Th, V) (Pinto et al., 2017)
Mineralogical composition plays a crucial role in understanding native nutrient sources for crops and heavy metals both short and long-term period of time. However, mineralogical composition of refused materials has been ignored in post-tin mining areas, and more attention is given to chemical properties and heavy metals. Hence, we investigated mineral composition of refused materials deriving from different parent materials in post-tin mining areas and relating the minerals to soil chemistry, nutrients and heavy metals. The objective of the study was to evaluate mineralogical constituents, particle sizes, chemical and physical properties and heavy metal contents of refused materials in post-tin mining areas as the strategic basis for reclamation of soil productivity and ecological function.
2 MATERIALS AND METHODS
Study site and field samplings
The study areas locate in Bangka Island, Kepulauan Bangka Belitung province, Indonesia between X 626183 -Y 9757090 and X 632834 -Y 9752946 UTM grid, Zone 48 M with the elevation of 27 to 60 m above sea level. The areas were selected based on their lithological type that represents most of the post-tin mining areas in Kepulauan Bangka Belitung province. Lithologically, Bangka Island occurred on granite and sandstone parent rocks. The granitoid plutons of the Indonesian Tin Islands was Triassic age (193-251 Ma) (Aleva,1985; Ko, 1986). It was expected that the similarity in lithology of the areas would produce a similar property of tailings and spoils. The process of tin mining and deposition of refused material were mainly gravitational separation without any chemical treatment. The companies having tin mining land concession extracted the tin-host ores by dry-excavating of soils and the underlain deposit layers, mostly down 5 to 50 m. The companies used conventional bulldozers, loaders and haulers for the excavation of deposit ores. The separation of non-tin layer sediment such as white clay was directly dumped as a spoil, during the excavation. Hydraulic mining by spraying the ore deposits associated soils to form a slurry and pumped to the wet separation (concentrator) facility. The slurry was then passed through sequences of cyclones and separator in which clays, silts, and some very fine sands (collectively referred to as slimes) were separated gravitationally from the sand fractions. The heavy mineral sand of ores (particle density > 4.0 g/cm3) was further separated from the light sand fraction by cyclones water. The refused fractions consisting of light sand were dumped as tailings.
The characteristics of spoils (mostly a mixture of native soil, light sand and white clay) and sandy tailing piles of post-tin mining were assessed and evaluated as a basis for reclamation measures. Tailings and spoils deriving from sandstone and have been mined from 2000 to 2015 were represented by two sandy tailing profiles (TBB1 and TBB2), and a spoil profile (TBB3). The topsoil of TBB3 profile consisted of initial native soil with some tailing (0-30 cm), while the underlying layers consisted of a mixture of white clay and sandy tailing. For tailings resulting from granite materials and mining from 1960 to 1998 were represented by TBB6 and TBB7 profiles. The native soil profiles adjacent to post-tin mining areas were included for each type of parent materials (TBB4 for sandstone, and TBB5 for granite materials) as a comparison to spoils and tailings. We excavated a soil profile with the dimensions of 100 × 200 × 120–130 cm (width × length × depth) for morphological property observation and soil sampling. We collected a composite soil sample for each soil layer from all sides of the profile; the sample was thoroughly mixed prior to subsampling ~1 kg for mineralogy, chemical and heavy metal analyses.