Humidity cell testing
Henna Punkkinen, Markku Juvankoski, Tommi Kaartinen, Jutta Laine-Ylijoki, Elina Merta, Ulla-Maija Mroueh, Jarno Mäkinen, Emma Niemeläinen & Margareta Wahlström, VTT Technical Research Centre of Finland Ltd, P.O. Box 1000, FI-02044 VTT, Finland.
Introduction
A humidity cell test is probably the best known kinetic test method. It is a laboratory weathering procedure commonly used in the mining sector to estimate the long-term acid generation behaviour of sulphide bearing tailings and waste rocks (ASTM D5744 2007, Technical Committee CEN/TR 16363 2012). The humidity cell is intentionally operated to accelerate sulphide mineral oxidation and acid production, which will also result in an enhanced rate of generation of alkalinity, dissolved metals, precipitated metal compounds and other oxidation products (ASTM D5744 2007, Technical Committee CEN/TC 292 2012). Generally, results of the humidity cell testing can be used to determine if the test material will produce an acidic, alkaline, or neutral effluent; identify solutes which represent dissolved weathering products that are formed during a specific time period; and determine the mass and rate of solute release under controlled test conditions (ASTM D5744 2007). It must be kept in mind that the humidity cell test does not predict the actual moment when acid leachate will appear, or simulate the drainage quality in the field (European Commission 2009) as the formation of secondary minerals is obstructed in the test (Price 2009). However, in combination with geochemical modelling, humidity cell test results can be used to make cautious conclusions about the drainage chemistry (INAP 2009).
The humidity cell test method ASTM D5744 evaluated here is the only standardised kinetic test procedure currently existing. The studied method is modified from a laboratory weathering procedure for mining wastes originally developed by Caruccio (1968) and revised by Lawrence (1990) and Ferguson & Morin (1991) (ASTM D5744 2007). Standards are continuously rewised. The newest version of the method available at the moment, ASTM D5744-13e1, was published in 2013. However, the examination made here is based on the older version of the standard, ASTM D5744-07. The humidity cell test has also many other non-standardized variations, which are not discussed further in this chapter.
Description of the test method
In the test setup more than 1 kg of solid well characterised sample material, crushed to less than 6.35 mm, is placed in a column with a lid. The shape and size of the column depends on the particle size of the tested material (waste rock/tailings). During the test air is conveyed into the column to assure fully oxygenated conditions and samples are subjected to periodic leaching. The standard method consists of two alternative options for performing the test procedure. Both options comply with weekly cycles; in the first option dry air (<10 % relative humidity) and water saturated air (95 % relative humidity) are passed through column three days each, and on the seventh day sample is rinsed with specific volume of water. In the second option column is not subjected to dry-/humid-air cycles, but is stored between leaches for six days under controlled and relatively constant humidity and temperature. In this option oxygen is conducted to the sample material rather by diffusion of ambient air than by pumping. (ASTM D5744 2007)
After leaching, the leachate water is collected, measured and chemically characterised. Depending on the testing objectives, for example the following parameters can be measured on a specific frequency: pH, Eh, conductivity, alkalinity/acidity, sulphate, TDS (total dissolved solids), cation/anion concentrations, and metal and trace metal concentrations. (ASTM D5744 2007, Technical Committee CEN/TR 16363 2012) Figure 1 illustrates a common setup for humidity cell testing.
Figure 1. Humidity cell test setup (Technical Committee CEN/TR 16363 2012).
The test is continued until sulphate generation and metal leaching have been in a stabile level at least for five weeks (Price 1997). It is important to ensure that the tests are carried out until NP has been consumed and final pH levels attained. A minimum test duration is 20 weeks (ASTM D5744 2007), but it is not uncommon to have test durations up to a year or more (Technical Committee CEN/TR 16363 2012). A specific closedown procedure after the test is terminated enables better interpretation and post-test validation of the test results (Price 2009).
The current version of “International Kinetic Database (IKD©,TM)” created by Morin & Hutt contains results of over 500 humidity cells tests from 72 different mine sites around the world. Also several peer-reviewed articles focusing on humidity cell tests can be found on the internet.
Appropriate applications
The humidity cell test method ASTM D5744-07 is suitable for waste rocks, tailings and ores with a particle size less than 6.3 mm (ASTM D5744 2007). However, the method is not appropriate for fine-grained tailings having low hydraulic conductivity. For fine-grained tailings a diffusion of oxygen may become rate limiting for the oxidation process, as only a part of the reactive surface area is in contact with the humidity and can become oxidised. Also the low permeability of the fine-grained material may limit the leaching of reaction products. (Technical Committee CEN/TR 16363 2012)
Some advantages and disadvantages of humidity cell testing are presented in Table 1.
Table 1. Advantages and disadvantages of humidity cell testing (Lapakko 2003, ASTM D5744 2007, Price 2009, Technical Committee CEN/TR 16363 2012).
| Advantages | Disadvantages |
|---|---|
| Standardised method: consistent reproducible test conditions, enables the comparison of results | Long test duration |
| Bulk sulphide reaction rates, leachate water quality at regular intervals | Laboratory conditions may enhance or depress reaction rates: grinding and crushing of samples may cause unnatural and too forcible reactions. |
| Primary reaction rates can be measured because samples are leached once a week. However, as rinsing will only remove water soluble weathering products it should be checked that sulphate and base cations are in water soluble form. | Laboratory test does not simulate actual field circumstances/drainage chemistry, for example: Laboratory test is restricted to (usually more reactive) finer particles, material is more heterogeneous in the field, water percolation may be faster in the column than in the field, secondary mineral formation is obstructed in the test, constant operation temperature and regular air cycles/optimized oxidation do not simulate field conditions, testing conditions do not conserve heat generated by sulphide oxidation, some factors affecting sulphide oxidation in the field may be unknown or difficult to simulate in the laboratory etc. |
| Easy to handle | High costs |
| Both acid and neutralising reactions can be studied | Secondary mineral precipitates may accumulate even though the aim is to prevent secondary mineral formation. This can cause misinterpretation when predicting primary reaction rates. |
Performance
Multiple columns can be placed in parallel to enable testing of multiple splits of the same sample, or testing of different samples simultaneously (ASTM D5744 2007). Regular maintenance (rinsing, air cycle adjusting etc.), sampling, and monitoring for maintaining the desired test conditions are needed during the test. Possible aspects that may be monitored and adjusted before, during, and/or after the testing include: weight or volume of material, mineralogy and element specification, grain size, temperature, test duration, water flow rate and saturation, and air flow/oxygen supply (Technical Committee CEN/TR 16363 2012) and consumption, drainage chemistry, changes in colour etc. (Price 2009). Although no expensive equipment is needed for the test performance, the long duration of the test and the need for regular maintenance raise the costs of the testing. The costs also depend on the amount of the samples tested; testing just one sample is relatively more expensive than if multiple samples are tested simultaneously.
Pre-test and post-test characterisation of the sample material is important (Price 2009). The results of the humidity cell test should be evaluated together with these characterisation results, and possibly with the results of geochemical modelling, to be able to fully understand the processes taking place during the test period. However, there is always a relatively high uncertainty when the application of laboratory test results to represent field conditions and predictions of the long-term behaviour are made. That is why these kinds of applications should always be done cautiously, and the comprehension of differences between the natural setting and the test setup needs to be taken into consideration. (Technical Committee CEN/TR 16363 2012)
Design requirements
When selecting and designing the suitable kinetic test method, the objectives of the testing should be carefully considered. Also a proper characterisation of the sample material together with well-defined testing objectives will aid in choosing the optimal kinetic test method. If the method is appropriately chosen, it is possible to obtain the desired information to make interpretation and evaluation of the test data. (Technical Committee CEN/TR 16363 2012) It must be kept in mind that the use of standardised test can lead to incorrect conclusions if the test parameters are not suitable for the material tested.
Method maturity
The method ASTM D5744 was originally developed in 1996 and after that modified at least in 2001, 2007 and 2013. Repeatability and reproducibility of the test method is proved by intralaboratory and interlaboratory tests (Lapakko 2003). Interlaboratory tests have shown this method to be suitable for mine waste rocks. Test application for metallurgical-processing waste, such as for mill tailings, is outside the scope of the method. (ASTM D5744 2007)
References
ASTM D5744 2007. Standard Test Method for Laboratory Weathering of Solid Materials Using a Humidity Cell. ASTM D5744-07.
European Commission 2009. Reference Document on Best Available Techniques for Management of Tailings and Waste-Rock in Mining Activities, January 2009.
INAP 2009. The GARD Guide. The Global Acid Rock Drainage Guide. The International Network for Acid Prevention (INAP). http://www.gardguide.com/
Lapakko, K.A. 2003. Developments in Humidity-cell tests and Their Application. In: Jambor, J.L., Blowes, D.W. & Ritchie, A.I.M. (Eds.) Environmental Aspects of Mine Wastes. Mineralogical Association of Canada, Short Course Series 31.
Price, W.A. 1997. DRAFT Guidelines and Recommended Methods for the Prediction of Metal Leaching and Acid Rock Drainage at Minesites in British Columbia. British Columbia Ministry of Employment and Investment.
Price, W.A. 2009. Prediction Manual for Drainage Chemistry from Sulphidic Geologic Materials. MEND Report 1.20.1.
Technical Committee CEN/TC 292 2012. Characterization of waste – Overall guidance document for characterization of wastes from extractive industries. CEN/TR 16376:2012.
Technical Committee CEN/TR 16363 2012. Characterization of waste – Kinetic testing for assessing acid generation potential of sulfidic waste from extractive industries. Technical Report.
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