Physical Separation Barriers for Water and Oxygen
The most prevalent and conventional approach to limiting the entry of either oxygen or water, or both to sulphide-bearing waste, is to use physical barriers that consist of wet or dry covers (Kuyucak, 2002; Sahoo et al., 2013; Skousen et al., 2019). Argunhan-Atalay and Yazicigil (2018) also emphasise that barriers, particularly, dry cover barriers, retard the movement of water or oxygen into areas containing acid-producing rocks. It is noted, however, that though the exclusion of oxygen from waste materials is a very effective means of preventing sulphide oxidation, it is equally difficult to achieve than excluding water (Kuyucak, 2002). The sections that follow these opening remarks discuss in detail the dry and water covers.
188.8.131.52 Dry Covers
Pozo-Antonio et al. (2014) argue that dry covers have a number of roles:
- (1) stabilisation of mining waste so as to prevent erosion by wind and water,
- (2) improvement of aesthetic appearance and (3) prevention and/or inhibition of the release of AMD pollutants. The prevention and/or inhibition of the release of AMD pollutants is the focus of this section. The dry covers designed to inhibit AMD generation primarily comprise of a sealing layer with low hydraulic conductivity (Figure 6.1) which restricts the supply of
oxygen and limits the percolation of water into the sulphide waste materials, thereby reducing the rate of sulphide oxidation (Kleinmann, 1990; Hallberg et al., 2005; Sahoo et al., 2013; Pozo-Antonio et al., 2014; Skousen et al., 2019). Indeed, the dry covers also referred to as oxygen barriers slow down the movement of water or oxygen into areas containing acid-producing rocks (Argunhan-Atalay and Yazicigil, 2018; Skousen et al. 2019). Furthermore, Vila et al. (2008) acknowledge that the properties of dry covers, which include low permeability and increased moisture content, enable the covers to be used as a barrier to oxygen diffusion.
There is a large number of dry cover designs, but most importantly, the cover should be stable and provide long-term protection (Pozo-Antonio et al., 2014). Fine-grained soil is one of the materials that has been used as effective dry covers (Sahoo et al., 2013). However, according to Vila et al. (2008), the dry covers are usually made out of clay materials. It is also noted that soil covers vary depending on climate, types and volume of waste material, size and geometry of the waste storage facility, available cover materials in the field, etc, (Argunhan-Atalay and Yazicigil, 2018), and thus the choice of appropriate soil covers is important. For several years, some studies indicated that soil covers decreased the generation of acid remarkably compared to uncovered waste materials (Payant et al., 1995). Other studies that used soil cover on the surface of waste rock found that, over time, the effectiveness of the soil cover reduced (Harries and Ritchie, 1985; Yanful and Orlandea, 2000; Wang et al., 2006). Some of the shortcomings of soil covers that lead to their ineffectiveness include sidewall passage of oxygen and water (Yanful and Orlandea, 2000), precipitate infiltration (Wang et al., 2006) and high maintenance cost (Yanful, 1993; Yanful et al., 1999). Most importantly, Swanson and O'Kane (1999) state that "soil cover design is climate specific and a common misconception in soil cover design is the use of compacted clay covers". According to Swanson et al. (1997), low-permeability barriers such as clay are not necessarily the most effective covers in dry climates. This is due to the high potential for drying and cracking, which results in water bypassing the soil matrix (Morris et al., 1992; Daniel and Wu, 1993). Therefore, the ultimate result is a failed cover system (Swanson and O'Kane, 1999).
For decades, researchers have embarked on finding other solutions to minimizing and/or eliminating the generation of AMD. For example, synthetic materials such as plastic liners and polyethylene have been studied and subsequently used to control AMD generation (Sahoo et al., 2013). In a study by Caruccio and Geidel (1983), polyvinyl chloride (PVC) covers were used to completely cover a waste site and the study found that there was a substantial decrease in total acid loads. However, the use of synthetic plastic or polymer liners has a number of limitations such as (1) being too expensive for covering a large volume, (2) being susceptible to cracking and (3) repair costs being exorbitant (Sahoo et al., 2013).
On the other hand, organic materials have been found to be a good replacement for clay as long as the layer is thick enough (Pozo-Antonio et al., 2014).
Several studies have actually indicated that organic carbon-rich materials are able to remove oxygen from sulphide waste materials and thus limit AMD generation (Tremblay, 1994; Peppas et al., 2000; Sahoo et al., 2013; Park et al., 2019). Ideally, the organic covering has proven effective in preventing oxygen from reaching the tailings (Tremblay, 1994). Examples of organic carbon-rich waste materials that can be used as dry covers include wood waste, wood chips, sawdust, municipal sludge, sewage sludge, composted municipal wastes, manure, peat, paper mill sludge and vegetation (Backes et al., 1987; Sahoo et al., 2013; Park et al., 2019). As can be seen from reaction 6.3, the carbon-rich materials consume oxygen that subsequently maintains very low dissolved oxygen within the waste material thus suppressing the oxidation of sulphide waste materials and limit AMD generation (Sahoo et al., 2013; Park et al., 2019).
Sahoo et al. (2013) also argue that organic waste provides a pH buffer that neutralises acids. In addition, INAP (2014) states that when organic materials are mixed with sulphide-bearing wastes the organic materials consume oxygen and promote metal reduction in an anoxic environment by naturally occurring bacteria. Bacteria can reduce available sulphate and create insoluble metal sulphide precipitates in the presence of suitable organic substrates. Some studies have indicated that the complexation of free Fe (III) by soluble microbial growth products (SMPs) that are produced by the microorganisms growing in waste rock in the presence of organic matter can reduce the effectiveness of ferric iron as an oxidant (Pandey et al., 2011; Sahoo et al., 2013).
Although the organic carbon-rich waste materials have a number of advantages in terms of cost and are effective in a short term, they also possess some downsides (Sahoo et al., 2013; Park et al., 2019). For example, the materials may contain organic acids that may infiltrate the waste areas and leach out toxic metals such as arsenic, copper, nickel and zinc (Pond et al., 2005; Sahoo et al., 2013). Similarly, organic cover may induce the reductive dissolution of secondary minerals like Fe (Ill)-oxyhydroxides, leading to the release of toxic elements (e.g., arsenic, cadmium, copper, lead and selenium) previously adsorbed onto or co-precipitated with these phases (Ribet et al., 1995; Park et al., 2019).
A significant number of materials have also been tested as dry covers and were found to reduce the generation of AMD because the materials were able to maintain the high degree of water saturation in the overlying/covering layers (Ribet et al., 1995; Bellaloui et al., 1999; Peppas et al., 2000; Demers et al., 2008; Park et al., 2019). These materials included non-reactive fine mine residue and natural till, low-sulphide tailings, desulphurised tailings, silty materials, alkaline waste, and industrial or municipal wastes such as fly ash, bottom ash, cement kiln dust, red mud bauxite, paper mill waste, pulp/paper residue and organic wastes (Ribet et al., 1995; Bellaloui et al., 1999; Peppas et al., 2000; Bussiere et al., 2004; Demers et al., 2008; Park et al., 2019). Demers et al. (2017) also evaluated the possibility of using AMD neutralisation sludge as an alternative for natural soil used as covering materials. The study results, after about almost one and half years, showed that a cover layer composed of sludge-soil mixture (25%—75% by weight) over either waste rock or tailings effectively suppressed AMD generation.
Many other dry covers have been tested and applied, and these include impervious membranes, dry seals, hydraulic mine seals and grout curtains/ walls. Some of these have already been discussed in Section 6.2.1.
184.108.40.206 Water Covers
Research has shown that the disposal of sulphide-bearing materials under a water cover such as lakes, oceans, fjord and ponds (Fraser and Robertson, 1994; Skousen et al., 2019) is a suitable technique for excluding oxygen from sulphides and thus limits the generation of AMD (Kuyucak, 2002; Sahoo et al., 2013; Park et al., 2019; Skousen et al., 2019). In fact, water covers are an economical alternative to dry covers because oxygen has a very low solubility in water and its diffusion rate in water is almost four orders of magnitude less than in air (Kuyucak, 2002). Park et al. (2019) also argue that water is widely used as an oxygen barrier because of oxygen's slow diffusion in water (1.90 x 10~4 m2/s) compared to its diffusion in the air (1.98 x 10~5 m2/s) as illustrated in Figure 6.2. Therefore, by having less amount of oxygen under water, highly anoxic conditions are created thus inhibiting the oxidation of sulphidebearing wastes (Sahoo et al., 2013). In addition, Kleinmann and Crerar (1979)
Sub-aqueous disposal. (From Park et al., 2019.) state that "microbial catalysis, in association with other mechanisms such as metal hydroxide precipitation and development of sediment barriers between sulphide-bearing wastes and overlying waters also inhibit oxidation".
Water covers have been extensively utilised for decades to reduce the rate of oxygen contact with the sulphide-bearing wastes since it is comparatively less expensive than other options (Sahoo et al., 2013). However, the technique has a number of drawbacks. This is in addition to the practice of disposing the sulphide-bearing wastes into lakes and ocean having been banned by most countries (Skousen et al., 2019). For example, the water cover technique is not applicable in arid and semi-arid regions where annual evaporation exceeds precipitation because the drying out of water cover exposes the sulphide-bearing waste to atmospheric conditions, thereby generating AMD (Lottermoser, 2003; Park et al., 2019). In other words, the technique is limited to sites that can be flooded, or where the water table may be permanently altered to cover sulphide-bearing wastes (Sahoo et al., 2013). Research has also found that water cover is not suitable at sites where the influx of oxygen-containing water occurs, or where mines are only partially flooded (Johnson and Hallberg, 2005). It is also observed that despite water covers minimizing acid generation, there could be a slow release of some metals thus increasing the metal concentration that may increase the recommended water standards (Aube et al., 1995; Kuyucak, 2002). Most importantly, the actual flooding of mine wastes requires a rigorous engineering design and proper maintenance in order to eliminate or minimise the risk of any failure in the embankment that is definitely costly when it occurs (Sahoo et al., 2013).