Kinetic tests

North America Acid Potential (AP)

Neutralisation Potential (NP) Net Neutralisation Potential (NNP)

(NNP = NP - AP)

Potential ARD, NNP = negative (-ve) Standard units, kg (t) CaC03 ( I

For materials where the potential for acid generation i s uncertain, kinetic test work is performed to attempt to define acid generation characteristics. In kinetic test procedures, the acid generation (and metal mobilisation and transport) characteristics of a sample are measured with respect to time. Kinetic test procedures include humidity cells (Sobek et aI, 1 97 8 ; Sullivan and Sobek, 1 982; ASTM, 1 996; Price, 1 997; Soregaroli and Lawrenc, 1 997), column tests (Sturey et aI. , 1 982; Price, 1 997), soxhlet reactors (Sullivan and Sobek, 1 982), Shake Flasks (Bruyensteyn and Hackl, 1 984), field lysimeters and test plots (Eger and Lapakko, 1 98 1 ) and barrel tests (Kalin et al. 1 995). Miller and Jeffrey ( 1 995) suggested that the static net acid generation test (NAG) procedure could also be considered as a kinetic test as the data generated provide information on the kinetics of the reaction from the short-term simulation of the weathering process. A development stage procedure, "Minewall" is also being tested for

determining the kinetic AMD c haracteristics of in-situ roc k such as pitwalls and the rock surfaces of adits, stopes and other underground workings (MEND, 1 995).

Currently there has been little standardisation of kinetic testwork procedures with regard to sample preparation, cell or column design and operation or data reporting. Most kinetics tests involve weathering of samples under laboratory controlled conditions to simulate time dependent chemical changes in the mine waste and determine the potential to generate net acidity, the rates of sulphide oxidation and neutralisation and the quality of the leachate/drainage. The most commonly used laboratory scale kinetics test procedures are humidity cells and columns.

Humidity cells are typically l aboratory units m which samples are subj ected to accelerated weathering by cyclic permeation of dry and humid air followed by flushing with water (Caruccio et aI . , 1 990; ASTM, 1 996). The test usually determines if a given sample will generate acidity but not when the material will produce acidity since the cells undergo accelerated oxidation of sulphide minerals (Price, 1 997). The accelerated rate of oxidation processes will thus result in an accelerated rate of generation of oxidation products as dissolved metals and/or precipitated metal compounds. The humidity cell tests are not designed to provide leachates that are similar to the actual leachate produced in field conditions and therefore are not intended to simulate site­ specific leaching conditions (ASTM, 1 996; Guard, 1 997) . The humidity cell tests were ori gi nally developed for coal overburden in which the lag period is shorter and then run for only 1 0 weeks. For hard rock materials, the lag periods are longer and a 1 0 week period is not adequate to give accurate results (Miller and Jeffrey, 1 995).

Column testwork is generall y conducted to simulate the leaching effects of precipitation infiltration into, and drainage from, material exposed to the atmosphere. The aim is to monitor leachate quality with time by c yclic flushing with water to simulate seasonal variations at site. Unlike humidity cells, there is no standard testwork procedure and the operation can be highly site or material specific with regard to material particle size range, sample mass, water infiltration rate and degree of oxygenation (Sturey et aI. , 1 982; Mills, 1 995). The column is operated without aggressive flushing s o that oxidation product may accumulate at particle surfaces in addition to being removed in

leachate. This behaviour parallels field conditions and, as a result, leachate analyses from column testworks are a better indicator of expected water quality than leachate analyses from humidity cells-particularly if column infiltration rate is varied to simulate site conditions (Mill , 1 995; 1 996).

There is no standardisation of column testwork procedures, allowing considerable flexibility, which permits column operation to be highl y site or material specific with regard to material particle size range, sample mass, water infiltration rate and degree of oxygenation. Column tests also allow treatments to be tested and compared which is maj or advantage over humidity cells (Miller and Jeffrey, 1 995). Column tests are intended to simulate natural conditions and are simple to construct, operate and monitor. Various environmental factors can be assessed, as can the influence of various control measures such as cover systems. However, the kinetics of reaction may not be distinguishable from rate limiting transport phenomena and bacterial populations may differ from those found under field conditions. Column tests, on the other hand can become water saturated and interpretations of the results become difficult ( Lawrence,

1 99 1 ; Miller and Jeffrey, 1 995; Perkins et aI . , 1 995).

Kinetic tests generally only attempt to predict what will happen in the early stages of acid generation processes (Miller and Murray, 1 988; Sherlock et ai, 1 995) and the data interpretation and modelling is complex irrespective of the nature of the tests carried out. Irrespective of the different types of kinetic tests, the overall objective of the tests is to provide data on the rate of acid generation and acid neutralisation under laboratory controlled conditions. In most kinetic tests water is commonly added to a sample, the mixture is allowed to incubate for a certain period to allow acid-ba e reactions, and samples of leachate or extracts are collected and analysed. The tests are required to run for a long time to generate overall acid generation information. The major parameters measured in kinetic tests are trends in pH, sulphate, acidity or alkalinity and metals. The pH identifies the stage of the acid generating process, sulphate production relates to rates of sulphide oxidation, acidity/alkalinity give an indication of the rate of acid generation/neutralisation and metal levels evaluate metal solubility and leaching behaviour.