Mine Wastes

This book focuses on “problematic” solid wastes and waste waters produced and disposed of at modern mine sites. They are problematic because they contain hazardous substances (e.g. heavy metals, metalloids, radioactivity, acids, process chemicals), and require monitoring, treatment, and secure disposal. However, not all mine wastes are problematic wastes and require monitoring or even treatment. Many mine wastes do not contain or release contaminants, are “inert” or “benign”, and pose no environmental threat. In fact, some waste rocks, soils or sediments can be used for landform reconstruction, others are valuable resources for road and dam construction, and a few are suitable substrates for vegetation covers and similar rehabilitation measures upon mine closure. Such materials cannot be referred to as wastes by definition as they represent valuable by-products of mining operations.

Sulfide minerals are common minor constituents of the Earth’s crust. In some geological environments, sulfides constitute a major proportion of rocks. In particular, metallic ore deposits (Cu, Pb, Zn, Au, Ni, U, Fe), phosphate ores, coal seams, oil shales, and mineral sands may contain abundant sulfides. Mining of these resources can expose the sulfides to an oxygenated environment. In fact, large volumes of sulfide minerals can be exposed in: tailings dams; waste rock dumps; coal spoil heaps; heap leach piles; run-of-mill and low-grade ore stockpiles; waste repository embankments; open pit floors and faces; underground workings; haul roads; road cuts; quarries; and other rock excavations. When the sulfides are exposed to the atmosphere or oxygenated ground water, the sulfides will oxidize to produce an acid water laden with sulfate, heavy metals and metalloids. The mineral pyrite (FeS₂) tends to be the most common sulfide mineral present. The weathering of this mineral at mine sites causes the largest, and most testing, environmental problem facing the industry today — acid mine drainage AMD).

Water is needed at mine sites for dust suppression, mineral processing, coal washing, and hydrometallurgical extraction. For these applications, water is mined from surface water bodies and ground water aquifers, or it is a by-product of the mine dewatering process. Open pits and underground mining operations commonly extend below the regional water table and require dewatering during mining. In particular, mines intersecting significant ground water aquifers, or those located in wet climates, may have to pump more than 100 000 liters per minute to prevent underground workings from flooding. At some stage of the mining operation, water is unwanted and has no value to the operation. In fact, unwanted or used water needs to be disposed of constantly during mining, mineral processing, and metallurgical extraction.

Mineral processing of hard rock metal ores (e.g. Au, Cu, Pb, Zn, U) and industrial mineral deposits (e.g. phosphate, bauxite) involves size reduction and separation of the individual minerals. In the first stage of mineral processing, blocks of hard rock ore up to a meter across are reduced to only a few millimeters or even microns in diameter. This is achieved by first crushing and then grinding and milling the ore (Fig. 1.2). Crushing is a dry process; grinding involves the abrasion of the particles that are generally suspended in water. The aim of the size reduction is to break down the ore so that the ore minerals are liberated from gangue phases. In the second stage of mineral processing, the ore minerals are separated from the gangue minerals. This stage may include several methods which use the different gravimetric, magnetic, electrical or surface properties of ore and gangue phases (Fig. 1.2). Coal differs from hard rock ore and industrial mineral deposits as it does not pass through a mill. Instead, the coal is washed, and coal washeries produce fine-grained slurries that are discarded as wastes in suitable repositories. Consequently, the end products of ore or industrial mineral processing and coal washing are the same: (a) a concentrate of the sought-after commodity; and (b) a quantity of residue wastes known as “tailings”. Tailings typically are produced in the form of a particulate suspension, that is, a fine-grained sediment-water slurry. The tailings dominantly consist of the ground-up gangue from which most of the valuable mineral(s) or coal has been removed.

Cyanide is a general term which refers to a group of chemicals whereby carbon and nitrogen combine to form compounds (CN^-). Cyanide leaching is currently the dominant process used by the minerals industry to extract gold (and silver) from geological ores. Gold extraction is accomplished through the selective dissolution of the gold by cyanide solutions, that is, the so-called “process of cyanidation”. This hydrometal-lurgical technique is so efficient that the mining of low-grade precious metal ores has become profitable, and modern gold mining operations extract a few grams of gold from a tonne of rock. As a result, exceptionally large quantities of cyanide-bearing wastes are produced for a very small quantity of gold. The wastes of the cyanidation process are referred to as cyanidation wastes. At modern gold mining operations, cyanidation wastes occur in the form of heap leach residues, tailings, and spent process waters. While the bulk of the cyanide used in the mining industry is applied to gold ores, cyanide is also added as a modifier in the flotation of base metal sulfide ores in order to separate base metal sulfides from pyrite. Therefore, some process circuits of base metal ores may generate cyanide-bearing tailings and spent process waters.

Uranium ores have the specific issue of radioactivity, and uranium mine wastes are invariably radioactive. This property differentiates uranium mine wastes from other mine waste types. For example, gold mine tailings contain cyanide, and the cyanide can be destroyed using natural, naturally enhanced or engineering techniques. Sulfidic wastes have the potential to oxidize, and oxidation of sulfidic wastes can be curtailed using covers. In contrast, the decay of radioactive isotopes and the associated release of radioactivity cannot be destroyed by chemical reactions, physical barriers or sophisticated engineering methods. Therefore, appropriate disposal and rehabilitation strategies of radioactive uranium mine wastes have to ensure that these wastes do not release radioactive substances into the environment and cause significant environmental harm.

Plants and agricultural crops require phosphorus, potassium, and nitrogen as macronutrients. In order to maintain agricultural crop yields, these elements must be added to replace those lost from the soil. In most cases, the nutrients are added to agricultural land as mineral fertilizers (UNEP/IFA 2001). Nitrogen fertilizers are generally produced from atmospheric nitrogen, water, and energy. In contrast, the production of phosphate and potassium fertilizers relies on the provision of phosphate rock and potash ores. The majority of mined phosphate rock and potash ore is processed to fertilizer; a minor proportion of the mined material is used for other purposes. World population growth and the necessity to provide adequate food supplies have resulted in the significant growth of phosphate and potash mining, and fertilizer consumption over the last 1oo years. This growth has also led to the ever increasing volume of phosphate and potash mine wastes.