Abstract—Neutralisation of acid-mine drainage (AMD) using limestone is cost effective, and good results can be obtained. However, this process has its limitations; it cannot be used for highly acidic water which consists of Fe(III). When Fe(III) reacts with CaCO3, it results in armoring. Armoring slows the reaction, and additional alkalinity can no longer be generated. Limestone is easily accessible, so this problem can be easily dealt with. Experiments were carried out to evaluate the effect of PVC pipe length on ferric and ferrous ions. It was found that the shorter the pipe length the more these dissolved metals precipitate. The effect of the pipe length on the hydrogen ions was also studied, and it was found that these two have an inverse relationship. Experimental data were further compared with the model prediction data to see if they behave in a similar fashion. The model was able to predict the behaviour of 1.5m and 2 m pipes in ferric and ferrous ion precipitation. Keywords—Acid mine drainage, neutralization, limestone, modeling. I. INTRODUCTION HE mining industry has played a very big role in the stabilization of the South African (SA). SA started exporting coal in the early 1970’s. The major consequence associated with the mining of coal is that of AMD (an unintended consequence facing the mining industry world- wide). Even though other mineral mining can cause AMD, coal mining remains the number one culprit. As the demand of coal increases, so is the problem of AMD, making it to be a growing environmental crisis [1], and coal mining remains the main culprit in the production of AMD [2]. AMD is mainly caused when iron sulphide is exposed to oxidation conditions. Abundant and cheap coal reserves will therefore almost certainly remain SA’s most important energy resource for at least the next 75 years [3]. Active and passive neutralisations of AMD using limestone are some of the treatment methods available for use. Active neutralisation is characterised by agitation, requires constant maintenance, and is relatively expensive, whereas passive neutralisation is simple, no agitation is required, no/less maintenance is required, and cheap to use [4]. For the purpose of this study, passive neutralisation of AMD will be conducted using limestone. Limestone has a neutralisation efficiency of 30%, so it requires large doses to be more efficient. The overall Reneiloe Seodigeng, Hilary Rutto and Tumisang Seodigeng are with the Department of Chemical Engineering, Vaal University of Technology, RSA (e- mail: renematlou@gmail.com, hilaryr@vut.ac.za, tumisangs@vut.ac.za). Malwandla Hanabe was with the Department of Chemical Engineering, Vaal University of Technology, RSA (e-mail: mahanabe@gmail.com). Haleden Chiririwa is with the Centre for Renewable Energy and Water, Vaal University of Technology, RSA (e-mail: harrychiririwa@yahoo.com). reaction of metal-acid wastewater neutralization process by limestone is given by (1): CaCO 3 + H 2 SO 4 → CaSO 4 + CO 2 + H 2 O (1) Limestone is widely used, economically friendly, easily accessible, has low maintenance costs, and requires minimum monitoring. Yet this process has limitations, it cannot be used for highly acidic water which consists of Fe (III) ions. When Fe (III) ions react with CaCO 3 , it produces impermeable metal hydroxide coating widely known as armoring [1]. The disadvantages of this reagent include slow rate of dissolution, and its effectiveness decreases with time. Previous studies of armor formation have been conducted by several researchers who investigated the effect of armoring limestones on acid mine neutralisation with limestones, and concluded that even after armoring, limestone was still partly effective in acid neutralization [5]. Few studies address the composition of armor coating on limestones in AMD neutralisation application, armor coating composed of metal oxyhydroxides and Ca sulfate (Gypsum). Rutto et al. described the gypSLIM process that can be used for processing gypsum into sulphur and CaCO3 [6]. The three main objectives of this work were to investigate the effect of pipe length on ferric and ferrous ion precipitation, to model the diffusion of H + ions, and to model the diffusion and precipitation of ferric and ferrous ions. II. EXPERIMENTAL A. Materials Limestone was obtained from Phalaborwa Mining Company (PMC). Raw AMD was obtained from the East Rand of Johannesburg. Barium diphenylamine sulphonate, potassium dichromate, 32% hydrochloric acid, 98% sulphuric acid, and starch indicator were bought from Merck South Africa. Reneiloe Seodigeng, Malwandla Hanabe, Haleden Chiririwa, Hilary Rutto, Tumisang Seodigeng Passive Neutralization of Acid Mine Drainage Using Locally Produced Limestone T World Academy of Science, Engineering and Technology International Journal of Bioengineering and Life Sciences Vol:11, No:1, 2017 5 International Scholarly and Scientific Research & Innovation 11(1) 2017 scholar.waset.org/1307-6892/10006153 International Science Index, Bioengineering and Life Sciences Vol:11, No:1, 2017 waset.org/Publication/10006153