Metal Leaching and Migration Processes

A series of reactions occurs along the path as the low pH water migrates from the source to the receiving environment. The resulting quality of the water is determined by the following factors:

• • Nature of the sulphides
• • Availability and type of soluble constituents
• • Nature of alkaline reactants
• • Physical properties of the waste

Some examples of the quality of ARD are given in Table 6.3.

Several naturally occurring physical, chemical, and biological properties of mine waste affect metal solubility and contaminant migration. The mobilization of metals is mainly controlled by chemical factors, while the processes that occur along the migration route are controlled by physical and chemical factors.

Physical properties that influence metal solubility include waste particle size and shape, and temperature and pressure of pore gases. However, chemical factors are more predominant than physical properties in the metal mobilization process. Physical properties are important in the migration rate of ARD and in the reactions that occur along the migration path. Important characteristics include:

• • Climatic conditions
• • Waste permeability
• • Availability of pore water
• • Pore water pressure
• • Movement mechanism, whether by stream flow or diffusion

These factors control the rate of movement of contaminant fronts, the amount of dilution, and the degree of mixing that occurs as the ARD moves from the source to the environment. The physical properties of the subsurface are different than those of waste, so a number of contaminant fronts

TABLE 6.3

Examples of Acid Rock Drainage Quality

^a Units are mg/L except pH.

Source: From Draft Acid Rock Drainage Technical Guide, Vol. 1 (Vancouver, Canada: BiTech Publishers, 1989). With permission.

may develop, all moving at different rates. Surface water yields tend to occur before groundwater yields in hard-rock waste dumps because of lower retardation and the resulting rapid migration through the waste rock. The quality of the yields, whether surface or groundwater, is a function of the dilution and buffering reactions that occur en route.

The solubility of the metals is generally determined by the pH of the leachate. Other chemical factors include the specific metal being dissolved, Eh, adsorption characteristics, and the chemical composition of the leachate. As the ARD moves away from the sulphide source through the waste material, more acid-generating material may be encountered, causing a reduction in pH. The drainage over alkaline material may also cause complete or partial neutralization.

With the gradual lowering of the pH level the dissolved metal load generally increases. However, a combination of chemical conditions could cause increasing mobilization of metals even at neutral or alkaline conditions. During the neutralization process of the drainage, precipitation of many of the soluble metals may occur, and the resultant drainage will contain the residual metals.²

An interesting phenomenon that has been observed in copper and massive sulphide ores is elevated zinc loadings in neutral drainage. Dissolved copper precipitates out as the pH is raised; the zinc remains at relatively high concentrations until the pH is raised to values above 9.5. As the ARD front moves through the waste or subsurface strata, the chemical composition of the front undergoes continuous change.³⁴

Biological activities along the route may influence metal dissolution. Metal leaching occurs where iron-oxidizing bacteria are present with iron and metal sulphides. Biological species can also attenuate the mobility of metals by absorption and precipitation.

Prediction of Acid Drainage

The prediction process for determining potential acid generation from metal mining operations includes;

• • Comparison with similar and neighbouring mines
• • A systematic sampling programme to collect representative samples
• • Static tests on the samples
• • Kinetic tests using anticipated on-site conditions using potentially acid-generating samples
• • Modelling

In the exploration stage, samples of the ore and waste rock should be collected for acid-base accounting analyses. These results early in the mine planning stages would indicate whether acid drainage might be a concern.

The geological units of ore and waste rocks should be identified based on lithology, mineralogy, and continuity of units. A comparison should be made with neighbouring mines and similar geological and paleoenvironmental areas for obtaining an initial indication of potential acid generation. A sampling programme for each geologic unit should be implemented. These samples should be subjected to acid tests to determine the potential for net acid production.

If the static test results give an uncertain indication of acid generation, then kinetic tests could be of value in determining acid production potential. If the potential for net acidity is identified for any geological unit, the mine plan should be revised. Mathematical models should be utilized along with kinetic tests to predict acid generation over a longer time period.

A simple approach to assess acid-generation potential involves geological comparison with nearby mines. This approach assumes that all factors influencing the acid-generation process are identical for the mines. This is rarely the case in vein deposits as the host rocks, alterations, and mineralogies are often dissimilar. On a larger scale, comparison over a wider geographical region is likely to be unreliable as the nongeological factors that affect acid generation, like climate and physiography, will vary.

A basic approach for the assessment of acid-generation potential is to compare paleoenviron- mental and geological characteristics. In this process, it is necessary to classify both deposits from the standpoint of acid generation. Some of the existing mineral deposit classification models can be useful in the prediction of ARD.

The geological factors controlling the generation of ARD include:

• • Oxidation state of minerals
• • Sulphide mineral compositions
• • Texture and crystal development in sulphides
• • Presence of acid-consuming minerals
• • Presence of rock structures that increase the permeability

Any available database to refine a geological classification in terms of ARD potential should also be helpful.

In the prediction process, a reliable sampling programme is the initial step. Beginning a sampling programme is complicated because static tests on samples from a defined geologic unit may indicate significant variability in acid-generation potential. This variability may indicate that the geologic unit actually consists of two or more units from the standpoint of acid generation. The sampling plan should be revised to define the additional units. Such an iterative sampling programme may be necessary to clearly define acid-generating units.

The sampling programme should be directly based on the mine plan. The samples should be taken from different areas in the mine plan. This approach will help predict the timing of acid generation as mining progresses.

The design of a sampling programme is initiated at the exploration stage when geological units are identified. In the next stage, a more detailed sampling programme is required to define more reliably the potential for net acid generation. The sampling programme should also respond to any change in the mine plan. A minimum number of samples to characterize each geological unit in terms of its potential to generate net acidity will be needed.

The potential sources of samples are outlined in Table 6.4.

TABLE 6.4

Potential Sources of Samples for Acid-Generation Prediction

Source: From Draft Acid Rock Drainage Technical Guide, Vol. 1 (Vancouver, Canada: BiTech Publishers, 1989). With permission.

For large-scale kinetic tests such as on-site rock piles, large amounts of a specific unit are required. For proposed mines such large volumes of specific units may not be available.