**Abstract**

Acid mine drainage (AMD) is perhaps one of the most relevant challenges the mining industry has faced during the last few decades. This issue is particularly important in the scenario of mine closure where mining processes cease to be active, and the sustainability of the sites needs to be re-established. This chapter reviews the fundamentals behind the generation of AMD as well as a set of physicochemical phenomena (chemisorption, precipitation, neutralisation, etc.) usually considered by researchers to mitigate it. Mine closure conditions where human presence is seldom or frankly rare turn the wastewater treatment even more challenging as it cannot be intensive in the utilization of reagents, energy, or human resources. Therefore, from a practical standpoint, passive-like wastewater treatment strategies mimicking nature are preferred. Finally, insights with regards to the complexities behind the implementation of pilot plant and industrial wastewater treatment systems conformed by long-term reactive barriers and constructed wetlands are also revised.

**Keywords:** acid mine drainage, mine closure, heterogeneous reactions, reactive barriers, wetlands

## **1. Introduction**

During last few decades, there has been an increasing awareness among the scientific community about the impact of carrying out mining activities [1, 2]. Before implementing standard ore exploitation activities, potential contaminant species remain restrained inside the original rock, however, such situation changes once mining activities kickoff and valuable material along with other toxic species are mobilised throughout the atmosphere or other media such as surface- or groundwaters. Among the latter, Acid Mine Drainage (AMD) has arisen as one of the most relevant multidisciplinary challenges in the mining industry [3]. The AMD corresponds to an aqueous stream which appears spontaneously from the natural contact, and therefore the natural interaction, between the surface of the rocks (or mineral particles) exhibiting at their surface primarily metal sulphide structures, and water

either in the form of vapour or liquid in conjunction with other atmospheric gases (**Figure 1**) [4, 5].

Perhaps the major difference between AMD and other sorts of pollution is that the former is not directly produced by mining activities. Mining activities would inevitably produce, to some extent, solid wastes and then, the environment in contact with them would eventually trigger the generation of AMD. In other words, the misplacing of solid wastes coming from anthropogenic mining activities in nature itself spontaneously transforms it into a different system with increased toxicity. In this context, the appearance of AMD depends largely on the local atmospheric conditions. For instance, higher humidity or rainy weather will favour the generation of AMD compared to dry conditions [4].

From a historical standpoint, one of the first reports indicating the generation of AMD was published in 1895 by F.G. Holman who glanced at the presence of a waterflow coming out from a small mine site in Forbestown, Sierra Nevada, California [6]. During AMD formation several physical, chemical, and biological phenomena are triggered while solid wastes and environment interact and are commonly summarised by many authors as simple as "weathering" [7, 8]. Weathering, though, is a wide concept that encompasses all the characteristics related to the environment including climate and biosphere. This makes it a bit too general to fully predict the specifics of AMD (timespan to appear, chemical composition, etc.) and its instantaneous or longterm impact on the mine site surrounding areas. The locations where mine sites are placed present a variety of different climates like desertic, Mediterranean, or other. Therefore, when carrying out any study on AMD prediction, prevention, treatment or other, the ambient conditions used will be crucial to get proper results [9].

Before examining the fundamentals behind the generation of AMD, a brief analysis of where AMD might be generated from a mineral processing perspective will be presented. There are many situations where AMD may appear across the mineral processing line, especially when solid wastes appear. For instance, it is well known that base metals occurrence covers a wide range of mineral structures such as oxides, sulphides, and intermediate phases [10]. Although oxide minerals bearing ores may also produce AMD due to their susceptibility to undertake leaching and metal hydrolysis steps in aqueous aerated conditions, sulphide-bearing minerals are considered the major ones responsible for it. In that context, AMD is mainly associated with base

**Figure 1.** *Scheme of acid mine drainage (AMD) production.*

*Fundamentals and Practical Aspects of Acid Mine Drainage Treatment: An Overview from Mine… DOI: http://dx.doi.org/10.5772/intechopen.104507*

metals and coal beneficiation plants where sulphide minerals occur as valuable or gangue material [11]. Only for exemplification purposes, and given its worldwide relevancy, the copper sulphide pyrometallurgical beneficiation path will be discussed. The traditional copper sulphide line of process usually includes blasting, rock size reduction (crushing and milling) and mineral selective separation commonly froth flotation [12]. **Figure 2** presents a block diagram of that line of ore processing, identifying the most relevant scenarios where AMD may be anticipated to take place.

From a practical perspective, it is all about sulphide-bearing material in the form of particles with different diameters being piled up which generate a certain natural porosity that will determine the access of atmospheric gases (or material weathering) towards the interior of the porous material. Different particle size distributions can be observed across the process line characterised in the picture by the maximum particle size only. Another common and better way to estimate a mean diameter biased to larger diameter values of a set of rocks or particles is through the Sauter mean diameter (*d***32**) [14]. As expected, the Sauter mean diameters follow a similar trend to the maximum particle diameter [Eq. (1)].

$$d\_{32,\ \text{ROM}} \ge d\_{32,\text{publules}} \ge d\_{32,\text{tailings}}\tag{1}$$

where,

$$d\_{\mathfrak{W}} = \frac{\sum\_{i} d\_i^3}{\sum\_{i} d\_i^2} \tag{2}$$

Eq. (2) is normally preferred as it considers the whole population of rocks or particles which brings up the significance of a correct sampling procedure, another crucial aspect of AMD. The mean diameter of a population of rocks or particles is

### **Figure 2.**

*Possible ore processing situations where AMD may appear, and the maximum diameter of rocks/particles involved in each scenario. Particle diameter is referred from [13].*

studied with different techniques depending on the relevant particle sizes. For instance, after blasting operation optical methods are widely used while for tailing materials sieving processes are preferred [15]. Nevertheless, particle size is not the only key variable to look at. The content of the valuable and gangue species present in the solid waste is also a variable to take into consideration. For instance, copper grades of sulphide minerals bearing ores fed to a concentrator usually contain around 0.8% copper leaving final tailings with about 0.1% of the metal [16]. Low-grade copper sulphide ores fed to dump leach operations contain copper grades between 0.1 and 0.3% and the extraction may reach values of around 50% [17]. The requirements for ore sorting vary from one case to another but it is common to impose a cut-off grade of around 0.2% [15].
