The impact of mining solutions/liquors on geosynthetics

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Abstract

Mine owners and operators are presented today with a diverse range of geosynthetic products which all appear to provide similar benefits. Key factors in selecting geosynthetics for use in the mining industry include construction and operational durability issues such as slope stability, puncture resistance and resistance to weathering; but also their chemical resistance when they come into contact with the extreme liquors present on many mining operations and processes. The long-term performance of the geosynthetic depends largely on the type of polymer used in the manufacture, or in the case of geosynthetic clay liners (GCLs), also on the mineralogy and chemical make of the bentonite present in the GCL. This paper provides a guide to the characteristics of the leachates/liquors likely to be generated for a given mining process and the likely effect it will have on the performance of a given geosynthetic.

Introduction

The use of geosynthetics in various mining operations is now widespread. While they have not been accepted as readily as in the general construction market, growth in the mining industry is occurring as operators begin to understand the advantages associated with the use of these materials. For many of the applications such as load support and retaining structures the design and application is easily transferred to the mining field, albeit normally with far higher loading criteria. Examples of these types of applications are reported in Bouazza et al., 1995, Lupo and Morrison, 2007, and Ozcelik (2008).

When it comes to containment and mine site remediation however, it is not a simple matter of transferring the technology from the tried and tested geosynthetics applications common to waste containment facilities such as landfills, where the information is widely available and well established (Bouazza and Van Impe, 1998, Rowe, 1998, Bouazza, 2002, Bouazza et al., 2002, Bouazza et al., 2006a, Bouazza et al., 2007, Bouazza et al., 2008a, Jeon et al., 2007, Rowe et al., 2004, Rowe et al., 2009, Rowe, 2005, Touze-Foltz et al., 2008, Guyonnet et al., 2009) to the mining industry. This is primarily due to the extreme ranges in leachate properties generated from the various ore extraction processes and the harsh environment to which geosynthetic materials are therefore exposed. Nevertheless, the rapid growth seen in the past decade in mining exploration and operation has led to a sharp increase in the use of a wide range of geosynthetic materials by the mining industry for all type of applications. Smith (2008) reported that from 1987 to 2008 more than 60 square kilometers of geomembrane liners were installed in leach pads alone. In addition to geomembranes which are extensively used in evaporation ponds, heap leaching and disposal of tailings (Breitenbach and Smith, 2006, Thiel and Smith, 2004), other major applications include geosynthetic clay liners (Aubertin et al., 2000, Kim and Benson, 2004, Lange et al., 2007, Benson et al., 2010, Bouazza and Rahman, 2007, Benson et al., 2008), geosynthetic capillary breaks in cover systems (Park and Fleming, 2006, Bouazza et al., 2006c), geotextile tubes and electrokinetic geosynthetics to dewater tailings and minewater sludges (Newman et al., 2004, Fourie et al., 2007), and filtration and drainage (Gilbert and James, 2004, Lupo and Morrison, 2007, Majdi et al., 2007).

This paper will concentrate on the containment and remediation aspects of geosynthetics in mining due to the paucity of information available on this particular topic. It will focus in particular on the following mining operations: Coal (Table 1), Uranium (Table 2, Table 3), Aluminium (Table 4), Copper (Table 5, Table 6), Gold (Table 7, Table 8), Nickel (Table 9), Tin (Table 10) and Iron (Table 11). It should be stressed that the information provided in these tables is largely based on available literature.

Each mining process creates different leachates, each of which could possibly affect the long-term performance of the various polymers which make up the geosynthetic material. The aim of this paper is to define each mining process, the characteristic leachate/liquor associated with the mining process and what effect it might have on the generic polymer of the geosynthetic, as well as on the clay component of GCLs. It should be noted that while a generic polymer or mineral type may be better suited than others to a particular application, the chemical constituents may vary within a given polymer or clay type i.e. for example not all HDPE's will perform exactly the same way, which could have a marked effect on relative long-term performance of two HDPE's when compared to each other.

Section snippets

Geomembranes and geotextiles

Geomembranes have become critical components in mining facilities where their performance in containment of process solutions has been proven. These include base liners for heap leaching pads, liners to concrete basins and tank liners and in some cases control and mitigation of acid mine drainage (AMD) from tailings or waste rock dumps. Geotextiles are typically used, not as widely as geomembranes, in separation and filtration systems for waste rock dumps or tailings or for erosion control.

Heap

Coal

Coal is extracted from coal seams, the method of extraction i.e. open pit or underground, depends on the depth below ground level, geology and environmental factors. The majority of the world's coal reserves are recoverable by underground mining. Currently, almost two-thirds of worldwide hard coal production comes from underground mines. In Australia however, this proportion is significantly lower. Major Coal producing countries include China, USA, India, Australia and South Africa. Two types

Uranium

Traces of uranium occur almost everywhere on Earth, the ore is mostly present at relatively low concentrations. Most uranium mining is very volume-intensive, and thus tends to be undertaken as open pit mining. It is also undertaken in only a small number of countries worldwide, as the resource is relatively rare. Ores with as little as 0.1% uranium are mined, crushed in mills and processed by chemical methods including leaching and solvent extraction. Table 2, Table 3 show the effects that

Aluminium (bauxite)

Bauxite is the naturally occurring form of aluminium ore and is the third most abundant element in the Earth's crust. It is typically mined in open-pits and normally processed into alumina near the mining operation. Major bauxite producing countries include Australia, Guinea, Brazil, Jamaica, and the former Soviet Union. On a worldwide average, 4–5 tonnes of bauxite are needed to produce two tonnes of alumina, from which one tonne of aluminium can be produced. Table 4 shows the effects that

Copper

Sulfide and oxide ores are mined and processed to form copper. Copper ores are extracted (leached) with sulfuric acid, usually using the heap leach process. The ore rich solution is then transferred to tanks containing scrap iron and agitating the solution to precipitate the copper. Major copper producing countries include USA, Chile, Peru and Canada. Table 5, Table 6 show the effects that various leachates from copper processing might have on geosynthetics.

Gold

Gold is mined either using underground hard rock mining techniques which extract the ore through shafts or tunnels or from open cut mines. Once the ore is mined the gold is extracted by means of cyanide leaching. Major gold producing countries include South Africa, the United States and Australia. Table 7, Table 8 show the effects that various leachates from gold processing might have on geosynthetics.

Nickel

Nickel mining has moved from traditional underground methods to large volume open pit operations. A sulfuric acid heap leaching process is used to recover the nickel from the nickel laterites. Major nickel producing countries include Russia, Canada, Australia, Indonesia and New Caledonia. Table 9 shows the effects that various leachates from nickel processing might have on geosynthetics. Table 10 is related to waste rock piles, stockpiles and the heap leach process.

Tin

Approximately 80% of the world's tin deposits occur as unconsolidated secondary or placer deposits in river beds and valleys or on the sea floor and as such are mined using open pit mining methods. The tin is removed from the ore by acid leaching. Major tin producing countries include Brazil China, Bolivia and Indonesia. Table 10 shows the effects that various leachates from tin processing might have on geosynthetics.

Iron

Most economic sources of iron ore consist of iron oxide minerals, the primary form which is used in industry being hematite. The mining involves removal of large quantities amounts of ore and waste in open cut operations. The ore is then ground to a suitable level and stockpiled, to allow transport and processing. Major iron producing countries include China, Brazil, Australia, and India. Table 11 shows the effects that various leachates from tin processing might have on geosynthetics.

Summary and conclusions

This paper has shown that a number of factors should be considered when incorporating geosynthetics into a modern mining operation, as contact with many of the leachates has the potential to reduce the performance of the material. One should not, however, lose sight of the fact that the natural materials they are replacing/complementing or augmenting, such as compacted clay liners, will be affected in similar ways. Therefore, rigorous analysis of each component of the design should be carried

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