Acid Mine Drainage (AMD) which occurs when sulfide minerals are exposed to water and oxygen with an excavation is one of the serious environmental problems in the world. A dry cover system is generally constructed in waste dump for the prevention of AMD in Indonesia by virtue of low cost and availability of waste rocks for a cover layer. However, the failure of the system caused by the lack of information related to the construction of cover system in mines, which leads to AMD, has been reported recently in Indonesia. In this study, the field investigation was conducted in pit and waste dump in open cast coal mine in Indonesia with the aim of obtaining the information on the construction of a cover layer and backfilling conditions of waste rocks in the waste dump. The rock samples taken in two areas of the mine were analyzed by geochemical analysis and sequential extraction with acids. The results indicated that Net Acid Producing Potential (NAPP) of the rocks in the waste dump down to 100 cm depth in both areas was from 10 to 30 kg H 2SO 4/ton, suggesting that Potentially Acid Forming (PAF) was backfilled in a cover layer. The backfill of PAF was contrary to the concept of cover system, which caused the failure of constructing a cover layer. The cause of the failure was likely attributed to the shortage of cover rocks which are classified as Non Acid Forming (NAF) or the failure of proper placement of them by an operational problem in the areas. Moreover, the results of the extraction with acids suggested that the form of iron and sulfur has to be taken into account to discuss the occurrence of AMD.
The negative impacts on the nature of coal development by open cast mining have been reported with expansion of coal production recently in Indonesia. Acid Mine Drainage (AMD), which is a serious environmental problem, is attributable to the contact of sulfide minerals with water and oxygen in excavation activities in mining. This problem affects the nature in the field [
The generation of acidic water proceeds with the dissolution of sulfide minerals such as pyrite (FeS2), followed by the reaction that Fe2+ changes into Fe3+ through oxidation by oxygen (Equations (1) and (2)) [
Fe3+ precipitates as ferric hydroxide (Fe(OH)3) by reacting with water (Equation (3)), leading to red-brown color in acidic water. Additionally, Fe3+ oxidizes sulfide minerals and Fe2+ forms (Equation (4)).
The chemical reaction continues until sulfide minerals are depleted, and produced H+ decreases the pH. Thus, it is important to prevent the contact of sulfide minerals with oxygen and water to prevent these reactions.
A dry cover system is a reliable method for prevention of exposure of sulfide minerals to oxygen and water in order to control AMD due to low cost and less works in comparison with the others [
In this system, waste rocks are classified into Potentially Acid Forming (PAF), Non Acid Forming (NAF), or uncertain materials on the basis of the geochemical properties of rocks by conducting several geochemical analysis [
In this study, field investigation and geochemical analysis with the rock samples taken in mines were performed with a focus on the amount of waste rocks and the geochemical properties of rocks in order to evaluate and obtain the information on the construction of a cover layer and backfilling conditions of waste rocks in waste dump in open cast coal mine in Indonesia.
A field survey was conducted with sampling of waste rocks in the coal mine in Indonesia with the objective of examining the conditions of construction of a cover layer and backfilling waste rocks in the waste dump. This mine is an open cast mine and one of the largest coal mines in Indonesia. The annual production has been around 10 million ton recently, and there is a trend toward an increase in the production. Most of the produced coals which consist of Sub-Bituminous and Lignite are used in domestic and exported to Asian counties. The average value of sulfur content in waste rocks is about 1%, leading to AMD with the expansion of mining area. Waste rocks are backfilled in the waste dump in accordance with cover system after the classification of them into PAF and NAF following the standard proposed in 1997 for the prevention of AMD in this mine [
The samples were dried at 50˚C for 48 h in a nitrogen atmosphere to supply to X-Ray Diffraction (XRD) and X-Ray Fluorescent (XRF). Major minerals in the samples were analyzed using wide angle goniometer RINT 2100 XRD under the following condition: radiation CuKα, operating voltage 40 kV, current 26 mA, divergence slid 1 deg, anti-scatter 1deg, receiving slit 0.3 mm, step scanning 0.050˚, scan speed 2.000˚/min, scan range 2.000˚ - 65.000˚.
They were analyzed by ABA test and NAG test to classify them into PAF or NAF by following the standard [
Sequential extraction with acids was conducted with the samples taken both in pit and waste dump in order to discuss the occurrence of AMD within the waste dump. Sulfide minerals, such as pyrite, form sulfate with the progress of AMD [
The amount of each type of rocks in pit was calculated as based on the results of classification of them into PAF, NAF, and uncertain in area I and II as shown in
The average amount of Fe and S at each stage of the extraction in pit in area I and II is shown in
Area | (mg/g) | HCl | HF | HNO3 |
---|---|---|---|---|
Area I | Fe | 30.1 | 8.6 | 4.4 |
S | 1.9 | 0.5 | 3.5 | |
Area II | Fe | 16.3 | 10.3 | 3.1 |
S | 0.8 | 0.8 | 2.1 |
extracted Fe and S by HNO3 is slightly larger in area I than that in area II in
The XRD patterns of the rocks in the waste dump from the surface down to 100 cm in area I and II are described in
NAPP at 20 cm intervals down to 100 cm depth in the waste dump in area I and II are presented in
coefficient 0.9659) nearly corresponding to that of pyrite (FeS2) where as a good correlation was not observed at HCl and HF stage. Thus, pyrite was leached by HNO3 and readily-soluble minerals were extracted by HCl in this study [
was 30.1 and 1.9 mg/g, respectively. In short, the amount of Fe and S which exist as readily-soluble minerals decreased after the backfill in the waste dump. Considering that it has taken more than a year since the waste dump was constructed in area I and II, these minerals dissolved inside of the waste dump by infiltration of precipitation into the waste dump and flowed in waste water. Furthermore, the amount of extracted Fe and S with HNO3 was larger at each depth in area I than that in area II. The difference of them between the areas significantly increased from the surface down to 20 cm depth, and the amount of them in area II was nearly zero. This may be attributed to the dissolution of sulfide minerals in the layer in area II inside of the waste dump for the results since there was not a large difference in the amount of sulfide minerals extracted by HNO3 between the areas in pit before backfilling in waste dump as discussed above. Sulfate is formed by the reaction of pyrite with oxygen and water and iron dissolves in water in the form of ferrous or ferric ion as AMD proceeds [
As demonstrated above, it was observed that there was not a great difference in acid potential in rocks in the waste dump until 40 cm depth in area I and II from the point of NAPP. However, acidic water can occur from the surface layer in area I due to the presence of sulfide minerals; on the other hand, it cannot occur for a long term from the layer until 20 cm depth in area II since the amount of sulfide minerals indicated nearly zero. AMD can, nevertheless, occur from the deeper layer with sulfide minerals when oxygen and water infiltrate in the waste dump in area II. Thus, the progress of AMD in the waste dump should be discussed by considering not only geochemical properties such as NAPP but also the form of Fe and S.
It became evident that about 40% of waste rocks in pit consisted of PAF in the areas in this study. Furthermore, the backfill of PAF in the surface layer until 100 cm depth in the waste dump in both areas was contrary to the concept of cover system in which a cover layer is constructed with NAF. The cause of the failure of constructing a cover layer was not clearly elucidated by only the results in this study, leastwise attributed to the shortage of NAF and/or the failure of proper placement of cover rocks by operational problems. The case that up to 80% of waste rocks consist of PAF and it may make it difficult to construct a cover layer was reported in Indonesia in the past research [
The main conclusions are summarized as follows: NAPP of waste rocks in the waste dump in area I and II was from 10 to 30 kg H2SO4/ton, suggesting that PAF rocks were backfilled in a cover layer. Additionally, the ratio of PAF in waste rocks in pit was 40% in the areas. The backfill of PAF in a cover layer in the areas in this mine was contrary to the concept of cover system in which a cover layer should be constructed with NAF. The cause of the failure of constructing a cover layer can be attributed to the shortage of NAF and/or the failure of proper placement of cover rocks by operational problems in this study. A larger amount of readily-soluble Fe and S in the waste dump in area I in comparison with that in area II, moreover, resulted in red-brown precipitation in the area. This suggested that the form of Fe and S has to be taken into account to discuss the occurrence of AMD over time. Furthermore, AMD progressed at different rates in the waste dump in area I and II, especially until 20 cm depth. This may be attributed to the difference in physical properties of a cover layer. Hence, the cause of the difference in the progress of AMD inside of the waste dump should be discussed from a point of the physical properties contributing to the performance of a cover layer.
The authors are grateful to the Mitsui Matsushima Co., Ltd. for financial support. The authors would like to ex-press their gratitude and appreciation to the mine for providing the rock samples and for kind assistance with fieldwork.
ShinjiMatsumoto,HidekiShimada,TakashiSasaoka,Ginting J.Kusuma,Rudy S.Gautama, (2016) Construction of Dry Cover System for Prevention of Acid Mine Drainage at Mine Waste Dump in Open Cast Coal Mines, Indonesia. Journal of Environmental Protection,07,160-169. doi: 10.4236/jep.2016.72014