Application of Nanotechnology in Mining Processes

1 Water Pollution Monitoring and Remediation Initiatives Research Group, School of Chemical and Minerals Engineering, North-West University, Potchefstroom, South Africa

2 Solid-State Research Group, Department of Chemistry, School of Science and Technology, Cape Breton University, Sydney, Canada

3 Faculty of Engineering and the Built Environment, Chemical Engineering Department, Cape Peninsula University of Technology, Cape Town, South Africa

4 Institute for Nanotechnology and Water Sustainability (iNanoWS), College of Science Engineering and Technology (CSET), University of South Africa, Florida Science Campus, Johannesburg, South Africa

5 Department of Electrical and Mining Engineering, College of Science Engineering and Technology (CSET), University of South Africa, Florida Science Campus, Johannesburg, South Africa

Abstract

Mining supplies key resources necessary for technological advancement to ameliorate challenges imposed by the increase in the human population worldwide. One of the legacies of mining resources is the formation and discharge of acid mine drainage (AMD) during and even after active mining. It is a major environmental concern because it enhances the dissolution and increases the dispersion of contaminants, mostly toxic metals, in the environment. Many countries have now adopted or promulgated legislation that requires mining operators to treat and manage the formation of AMD, costing them a fortune from their profits. AMD can be an alternative source of valuable rare earth elements (REE), but the currently available extraction methods of REE from AMD are inefficient and costly, exceeding by many folds their conventional extraction from ores. Thus, there has been a growing effort to develop a novel and cost-effective method to recover REEs from AMD, in which extraction using polymeric nanomaterials, like Poly(amidoamine) (PAMAM) dendrimers, are growing in prominence. PAMAM dendrimers nanoparticles have high adsorption capacity, contributing highly to metal recovery from most wastewater. However, their application in the recovery of REEs from AMD is hampered by the low pH of the AMD, which protonates the amine functional groups forming cationic charges on the surfaces of the dendrimer nanoparticles. Therefore, designing these materials to adsorb metal ions in an acidic solution is paramount for treating AMD. This chapter discusses designing a cost-effective method for the recovery of REEs from AMD after alkaline treatment, using surface-functionalized magnetic PAMAM dendrimer nanoparticles. The environmental effect and shortcomings of AMD remediation methods will be highlighted as a background motivation in developing this procedure.

Keywords: Acid rock drainage, dendrimers, magnetic iron oxides nanoparticle, potentially toxic elements, rare earth element

The global human population has risen considerably since the industrial revolution and currently stands at above 7.7 billion worldwide, beyond the carrying capacity of the earth [1]. The rapid population growth is imposing tremendous challenges such as the easy spread of disease outbreaks, food scarcity, shortage of infrastructures, and insufficient communication networks, which require resources and technological advancement to ameliorate these predicaments. All resources required for this technological advancement come from mother earth and can be obtained only through two means; if they cannot be harvested (from farming), they should be mined. The mining industry plays a vital role in supplying these key resources but lacks the potential to obtain mineral resources without compromising environmental integrity [2]. Nevertheless, mining cannot be easily halted due to the growing need for mineral resources to support technological advancement required to artificially sustain the ever-increasing human population, which is beyond the earth’s carrying capacity. The mineral resources, including metals, are essential components in the advancement of several technologies, like the production of medications and vaccines, fertilizers for agricultural application to ensure food security, and the manufacture of building materials for construction of mega-infrastructures to improve road networks. They are also used as vital components to manufacture computers and cell phones for better communication networks. In addition, mining is a vital economic activity for many nations, bringing in much-needed foreign exchange earnings and employment [3]. Despite the benefits mentioned above, the legacy of mining activities includes major environmental pollution and heaps or piles of municipal solid waste (MSW) from mining [4]. For example, most valuable minerals like gold (Au), copper (Cu), sulfur (S), zinc (Zn), silver (Ag) or lead (Pb) occurs in sulfidic ore bodies ( Table 1.1) with more than one type of mineralization [5]. Once these valuable metals have been extracted from their sulfidic ores, vast volumes of mine water and leftover mining solid wastes and tailings are generated, which contain most of the sulfide mineral, like pyrite. The exposure of the pyrite-containing waste to oxygen and water leads to an acid-generating material, producing acid mine drainage (AMD). The discharge of AMD is attributed to most of the contamination being transported from mining sites to the receiving environments, affecting environmental water quality [6–8] ( Figure 1.1). Due to its well-known and publicized ecological impacts, many countries have adopted stringent regulations, such as Section 402 of the Clean Water Act of the Republic of South Africa, enabling mining operators to treat mine water discharging AMD [9–11].

On the one hand, treatment of AMD as required by environmental legislation has serious financial implications for the mining operators, but on the other hand, generation of AMD in former mining sites occurs long after active mining when the responsible mining companies no longer exist. Thus, treatment and remediation of AMD are an economic and financial burden to the mining company if the operation is still active, and to the community and their governments in closed or abandoned mines. Mine water discharging AMD is known to contain a consortium of dissolved elements, including precious metals and rare-earth elements (REE) [12]. Thus, AMD can be a valuable resource, especially of REE and precious metals that can generate more income and compensate for the treatment and remediation expenses of mine water and contaminated mining sites [13]. Furthermore, AMD can be regarded as an initial and natural process of hydrometallurgy of REE. However, extraction of REE from AMD is a big challenge due to the need for highly selective extraction methods, targeting only REE and leaving other metal ions dissolved in AMD to produce the required purity. Thus, recovery of REE from AMD is usually more expensive, resulting in AMD not being attractive as a source of REE. Nevertheless, using polymeric nanomaterials as hydrometallurgical extraction agents is promising to be a cost-effective and efficient method of extracting REE from AMD. One of the agent groups with high potential is Poly(amidoamine) (PAMAM) dendrimers. Consequently, this chapter discusses the application of PAMAM dendrimers as an extraction agent of REE, evaluating their performance potential and the development of methods in which they are applied.

Table 1.1 Different types of sulfide-bearing ore bodies, % content in ore body, uses, world deposit and origin [14, 15].