Evaluation of acid-base accounting methods and the prediction of acid-mine drainage in the Middelburg area

[摘要] English: South Africa has several coal mines of which promised economic wealth since the 1700, however the exploitation of this commodity has a negative effect to the environment. Mining operations are a source of Acid Mine Drainage (AMD) in South Africa therefore salt loading and acidic conditions are of concern. Although lab methods such as Acid-Base Accounting (ABA) are used to predict the impact and effects of AMD, it is still difficult to predict its rate. The importance of laboratory analysis is to give the best and worst case scenario of the area, provided that the samples are representative of the area investigated. This thesis is conducted to note the difference between South African and Australian Laboratory methods in determining the potential and existing acidity of the samples. Furthermore, AMD is predicted using ABA methods for the Middelburg area. South Africa uses similar ABA laboratory guidelines as USA and Europe while Australia uses its own guidelines that are modified from time to time. After the comparisons of the methods were evaluated, the Australia's versus the South African results showed a very good correlation. However the Ca percentage from the ICP results had a lower correlation compared to other cations in the Actual Acidity Method. The KCl and NaOH used in the Australian Actual Acidity Method either displays the Ca as double or half of the South African ICP results. However, both countries' Actual and Potential Acidity Methods are comparable. The Australia's Neutralising Potential Method is derived from lime analysis disciples and can be an overestimate of the Neutralising Capacity. Whether the methods and results of Neutralising Capacity can be compared or not, is inconclusive. Other ABA tests have to be done on the same samples to verify if personal, analytical or instrumental error was significant. To achieve another aim of this thesis, The IGS was approached by Middelburg mine services to update the 2006 groundwater model at Goedehoop, Hartebeesfontein, Bankfontein and Klipfontein Collieries. At a later stage, Acid-Base Accounting (ABA) methods were requested for AMD prediction. The aim of the project was to do groundwater assessment of the collieries and predict future impacts of AMD. The locality of the investigated area is in the Middelburg, which falls in the central Highveld of the Mpumalanga province in South Africa. Open-cast coal mining is in operation at Klipfontein, Hartebeesfontein and Goedehoop. Klipfontein is along south of the Vaalbankspruit while Hartebeesfontein and Goedehoop spreads north of the Spookspruit. The area is in the Karoo Supergroup which underlines two thirds of the Republic of South Africa with typical rocks like sandstone, mudstone and shale. About 4.7 % of the representative samples from the Middelburg Collieries showed a Net Neutralizing Potential in the Static test method whereas Kinetic tests results proved that only a handful of the samples had a low salt load and a limited buffering capacity. In conclusion, the representative samples showed the investigated Middelburg Collieries had high-medium risk of acid-generation upon oxidation. With the prediction of AMD done, control measures can be put in place. The primary objective of AMD treatment is to remove metals (i.e. sulphides, hydroxides and carbonates) and raise the pH to alkalinity. Passive or Active treatments could be used to remedy the area. The deteriorated water quality caused by the coal mining area could be remediated and released into the Upper Olifants river catchment.

[发布机构] University of the Free State