**2. Mining wastes**

Mine operations may include 3 principle activities which are mining, mineral processing/dressing and metallurgical extraction/refining. All of these activities usually produce wastes that are unwanted and non-economic value. Solid mining wastes and other related wastes may be generated from each activity were summarized by Lakkopo (2002) as shown in Table 1.

Dusts, ashes and other atmospheric emissions may be routinely monitored by environmental scientists who have experienced in other industrial plants. Slag and waste water can also be tested before suitable processes of treatment and disposal will be designed by environmental engineers. Therefore, heterogeneous geological materials including overburden soils and rocks appear to be the most crucial solid wastes due to lack of geological knowledge of both environmental scientists and engineers. Geologist and mining engineer should share their opinion for environmental plans. However, top soils may have been utilized as construction materials during mining activities and reclamation at the mining end. Although, these top soils may contain some natural contaminants, particularly heavy metals in this case, they would have low impact to the environment. This is because they have been undertaken naturally erosion and weathering processes for several hundreds or thousands of years then transportation of contaminant have been taken place slowly ever since. Moreover, quantities of these top soils are usually much lower than waste rocks and tailings. Rocks usually have stable chemical forms of minerals but mining processes such as blasting, grinding and milling will reduce their sizes and increase surface of reaction. Consequently, chemical reactions would be activated rapidly leading to metal leach out form these rocks. On the other hand, tailings are the other solid waste left after ore and metal extractions which usually involve with chemical additives as well as alteration of the natural chemical bonding. This waste type should then be concerned for environmental monitoring plan.


Table 1. Summary of mining activities and their solid, gaseous and liquid wastes (modified after Lakkopo, 2002)

materials, mining designs and processes. Environmental protection should be carefully planned in order to eliminate and/or minimize any short- and long-term environmental impacts that may occur. Otherwise, serious problems may occur that may be very difficult

This chapter will review standard procedures for evaluation of AMD potential of rock waste and tailing generated from mining activity. Digestion techniques for analysis of heavy metals are also considered to give basic knowledge for environmental monitoring and impact assessment. Some cases studies in Thailand will be given for better understanding.

Mine operations may include 3 principle activities which are mining, mineral processing/dressing and metallurgical extraction/refining. All of these activities usually produce wastes that are unwanted and non-economic value. Solid mining wastes and other related wastes may be generated from each activity were summarized by Lakkopo (2002) as

Dusts, ashes and other atmospheric emissions may be routinely monitored by environmental scientists who have experienced in other industrial plants. Slag and waste water can also be tested before suitable processes of treatment and disposal will be designed by environmental engineers. Therefore, heterogeneous geological materials including overburden soils and rocks appear to be the most crucial solid wastes due to lack of geological knowledge of both environmental scientists and engineers. Geologist and mining engineer should share their opinion for environmental plans. However, top soils may have been utilized as construction materials during mining activities and reclamation at the mining end. Although, these top soils may contain some natural contaminants, particularly heavy metals in this case, they would have low impact to the environment. This is because they have been undertaken naturally erosion and weathering processes for several hundreds or thousands of years then transportation of contaminant have been taken place slowly ever since. Moreover, quantities of these top soils are usually much lower than waste rocks and tailings. Rocks usually have stable chemical forms of minerals but mining processes such as blasting, grinding and milling will reduce their sizes and increase surface of reaction. Consequently, chemical reactions would be activated rapidly leading to metal leach out form these rocks. On the other hand, tailings are the other solid waste left after ore and metal extractions which usually involve with chemical additives as well as alteration of the natural chemical bonding. This waste type should then be concerned for environmental

**Activities Mining Wastes**  Open pit and underground mining Waste rocks, overburden soils, mining water,

Table 1. Summary of mining activities and their solid, gaseous and liquid wastes (modified

emissions

atmospheric emissions

Tailing, sludge, mill water, atmospheric

Slag, roasted ores, flue dusts, ashes, leached ores, process water, atmospheric emission

to remediate and extremely cost enormously.

**2. Mining wastes** 

shown in Table 1.

monitoring plan.

fuel processing

electrometallurgy

after Lakkopo, 2002)

Mineral processing, coal washing, mineral

Pyrometallurgy, hydrometallurgy,

*Waste Rocks:* Large amount of waste rocks may have been removed from mining site, particularly for quarrying and excavation, to access to the ore body. These waste rocks are eventually remained in the site and surrounding areas after the mining end (see Fig. 1). Subsequently, they may become sources of environmental impacts. Although, mining design can reduce quantity of waste rocks; for example, mining excavation generates very less amount of waste rocks in comparison with open-pit mining. Geologic setting and ore formation are however the main factor for the mine planning; the open-pit mine may be economically more suitable in many cases. Besides, some waste rocks can be used for construction within the mining site; however, they must be tested prior to appropriate utilization. Otherwise, unexpected threats may occur.

Various types of waste rocks situated within ore deposits usually have different compositions that would be characterized for both mineralogical and geochemical constituents. Apart from heavy metals contained in these rocks, Acid Mine Drainage (AMD), a potential threat, may be activated and lasted for long period of time. AMD actually lowers pH of water; subsequently, the low pH drainage may flow over waste dumps including waste rocks and tailings and may in turn leach some heavy metals and contaminate surrounding area. Surface water and ground water would be crucial pathways of such contamination to ecosystem and food chain. However, most of these threats can be protected and prevented by good environmental management and monitoring plans.

*Tailings:* During the mineral processing (dressing), ore minerals and their host rocks have to be ground and milled prior to separation; besides, chemical additives may be added during the processes. Although, most of these chemicals are usually recovered and reused in the process, some of them may still remain in these tailings. Some chemical additives can be decomposed naturally within short period but many of them may be bound strongly and long lasted within the tailings. Moreover, these tailings may contain concentrate noneconomic minerals such as silicates, oxides, hydroxides, carbonates and sulfide that have never been collected throughout the dressing process. Therefore, these modified ingredients may partly be toxic and harm ecosystem. Tailings are similar to slurry, a mixture of finegrained sediment and water that have been disposed into tailing pond (see Fig. 2).

Fig. 1. Huge amount of rock waste generated from a gold mine in Thailand: left photo is waste dumping site; right photo shows placing process based on geochemical properties of each type of waste rock

Geochemical Application for Environmental Monitoring and Metal Mining Management 95

oxygen. Ferrous iron is released and sulfur is oxidized and changed to sulfate. This equation shown 2 moles of acidity generated from each mole of pyrite. The second equation is the conversion of ferrous iron to ferric iron. It consumes one mole of acidity. The third equation is a hydrolysis reaction which splits the water molecule; consequently, moles of acidity are generated as by-product. The fourth reaction is the oxidation of additional pyrite by ferric iron. The ferric irons generated in reaction steps 1 and 2 are cycle and propagation of the overall reaction. They take place very rapidly and continue until either the ferric iron or pyrite are depleted. In this reaction, iron is the main oxidizing agent instead of oxygen.

> <sup>222</sup> <sup>4</sup> 2 72 2 2 4 pyrite oxygen water ferrous iron sulfate acidity

2 3 2 2 4 442 ferrous iron oxygen acidity ferric iron water

2 3 4 12 4 ( ) 12 ferric iron Water ferrichydroxide acidity

3 2 2 22 4 14 8 15 2 16 pyrite ferric iron water ferrous iron sulfate acidity

Many procedures have been developed to assess the acid forming characteristics of mine waste materials. The most widely used methods are Acid-Base Accounting (ABA) test and

Characterization of rock types and geologic setting in the mine should be initially concerned prior to determination of capacity acid drainage generation of these rocks (Environment Australia, 1997). Acid-Base Accounting (ABA) is the most commonly-used static procedure that has been used for estimation/qualification of the acid generation potential of mine wastes (Furguson & Erickson, 1988). This procedure was developed at West Virginia University in late 1960s. ABA tests are designed to measure the balance between potentially acid-generating potential, particularly oxidation of sulfide materials and acid neutralizing potential in sample such as dissolution of alkaline, carbonates, displacement of exchangeable bases and weathering of silicate. The values arising from ABA are referred to the Maximum Potential Acidic (MPA) and the Acid Neutralizing Capacity (ANC), respectively. After MPA and NAC have been determined for a sample, both values are compared with set criteria. Two methods of combination commonly used are: 1) The difference in value between MPA and ANC or Net Acid Producing Potential (NAPP) where NAPP = MPA-ANC; 2) The ratio of ANC to MPA (ANC/MPA). NAPP is a theoretical calculation commonly used to indicate where a waste material has potential to produce acidic drainage. NAPP values represent balance between capacity of acid generation and capacity of acid neutralization. Unit of NAPP is also expressed as kg H2SO4/t in MPA and ANC. In addition, ANC/MPA ratio is also considered for assessment of acid generation from mine waste material. The main purpose of ANC/MPA ratio is to indicate relatively

the Net Acid Generation (NAG) test. These procedures are described below.

*FeS Fe H O Fe SO H* (4)

3

**3.1 Acid-base accounting** 

2 2

*Fe O H Fe H O* (2)

*Fe H O Fe OH H* (3)

*FeS O H O Fe SO H* (1)

Fig. 2. Tailing pond (left photo) for disposal of slurry-like waste produced from gold dressing and sample collection (right photo), a routine monitoring program which has to be carried out regularly

Due to tailings comprise both solid wastes mixing with water during the operation period of mine and they will become drying after the mining end, redox reaction would be taking place and may in turn change stability of some elements which can be leached out to the environment by accidence. Moreover, their property may also cause AMD. Therefore, routine monitoring plans for both water and tailing must be designed and continuously followed up. Monitoring data should be used for protection at the end and may be very useful for development of the mineral processing.
