The age old question - how do we scale for the lab to the field? The answer is that there is no clear answer and it depends on your outcomes for doing the testing in the first place. ASTM D5744-07 Standard Test Method for Laboratory Weathering of Solid Materials Using a Humidity Cell specifically states: 1.6 this test method is not intended to provide leachates that are identical to the actual leachate produced from a solid material in the field or to produce leachates to be used as the sole basis of engineering design. 1.7 This test method is not intended to simulate site specific leaching conditions. It has not been demonstrated to simulate actual disposal site leaching conditions. Furthermore, the test is not designed to produce effluents that are in chemical equilibrium with the solid phase sample.

By the time you get to kinetic testing you should have an understanding of lithologies and the associated static geochemistry (total metals, leachable metals, ABA, mineralogy) that guide sample selection. Samples can either be a range of single lithology or a composite of likely conditions to be encountered in the waste pile, or as means for testing rock interaction and potential treatment techniques. All of these need to be considered during the planning process.

ASTM suggests running the tests for a minimum of 20 weeks, however, longer may be required. Again, the clearer you are going into the testing, the better you will be able to review the data and make appropriate decisions. Reviewing the data weekly will again help in evaluating when to terminate and termination needs to be based on data evaluation rather than a set time frame.

The GARD guide has more detailed information on answering your questions as well. The last suggestion I have is, depending on where you are, the humidity cell may not be the most appropriate kinetic test. AMIRA uses a Free Draining Leach Column which may be more appropriate for arid environments rather than the humidity cell.

You need to understand the ABA data first. Potentially acid forming (PAF) samples with no time lag to acid onset should present acid drainage characteristics quickly. Kinetic testing of non-acid forming (NAF) samples will confirm drainage quality (acidity/alkalinity and metals, noting that NAF samples can still produce highly metalliferous drainage). Samples that have a reasonable acid neutralisation capacity (ANC) and are PAF can have a time lag to acid onset. ABA data will indicate what samples are likely and then kinetic testing will confirm the time lag, which could be a number of years for samples with a high ANC. The AMIRA work (Roger Smart and Stuart Miller) has derived some time frames for the time lag to pH 4.0 in the column leach test versus the time lag to pH 4.0 in the kinetic NAG test, which will provide you with an indication of test duration for such samples.

Consider the conceptual model of the test:

•The sulphide(s) oxidize based on their intrinsic reaction rate given the exposure (i.e. specific surface area and the free-energy characteristics of the specific form of sulphide.

•The H2SO4 produced then undergoes acid-base reactions with the non-sulphide gangue of the specific sample, again characterized by some detailed mineralogy and surface chemistry.

These are all part of the fundamental chemistry of the sample under leach. The specific water-rock ratio (and experimental variable) used in the test then transport soluble reaction products.

To the extent that HCTs accelerate anything it is a result of the specific surface area being larger per unit mass than would be expected in waste-rock piles. [For tailing, there is no reason to expect a surface-area effect at all.] But even in waste rock, one must bear in mind that the PSD used in the test likely exists within the waste-rock pile, too, and may approximate the dominant surface- area fraction for reaction. That is, the very coarse fraction may represent a small fraction of the total surface area.

In short, to the extent that the HCT is being used to establish rates of solute release per unit surface area of representative mineralogies/lithologies, I don't believe that HCTs accelerate reaction rates at all. These are not accelerated-life test equivalent to flipping and on-off switch for light bulbs.

The 20-week value is utterly arbitrary: it may be longer than is needed for some purposes with some samples, it may be woefully inadequate with respect to other purposes or in other samples.

How to proceed? Firstly, one must be clear as to the purpose of the test, and that the HCT as you intend to conduct it is the proper test design. At least questions could be addressed with HCTs

• Are Static Predictions of ARD Status Confirmed?

•Will Active Engineering Management, Perhaps Including Water Treatment, Likely Be Required?

•What is the Expected Water Chemistry of Effluents That Could Be Generated, for Example after Closure?

Secondly, you need to be certain that the samples being tested are representative of the materials to which their results will be imputed. This is essential scientifically, but also to ensure your Principals that you are spending money wisely.

Thirdly, you need to understand the mineralogy of the samples you are testing and the static ABA behaviour that those materials exhibit.

With such information and some calculations, perhaps bounded by literature for parable deposits, you may be able to garner a notion of whether your question (s) can be answered quickly, or only over very long times. But, in the end, you have to be guided by the data themselves with respect to criteria you set to answer the question you are asking. I have recently presented data showing that for some samples 12 weeks would be entirely adequate, whereas other samples *in the same deposit* needed > 100 weeks to reach decision-making criteria for water quality or even engineering controls, and were still highly reaction after >250 weeks. 

You all have summed it up very well. There is no "magic" number as to how long the humidity cell tests should run. It is very much dependent upon the materials under test and their reactivity. We have run tests from 20 weeks to over 250 weeks (5 years) and they have still not gone through the full sulphide oxidation reaction. It gets back to what is your end goal (typically water quality objective) that you have to meet either through a license or internal company condition. Starting the test work early once you have identified the waste rock/tailing lithology's is important, so you have the time to understand the material characteristics and commence planning for waste management or closure options.

Below some links to literature that explaining purpose and interpretation of static and kinetic tests:

This one is old but concise:

http://mend-nedem.org/mend-report/draft-guidelines-and-recommended-methods-for-the-prediction-of-metal-leaching-and-acid-rock-drainage-at-minesites-in-british-columbia/

The last MEND report offers quite comprehensive and up to date coverage. You can get it from:

http://mend-nedem.org/wp-content/uploads/2013/01/1.20.1.pdf

If you wander around http://mend-nedem.org/category/uncategorized/ you will find these reports and much more literature on a wide array of environmental mine drainage problems.

The Mine Drainage Assessment Group has a book called Environmental Geochemistry of Minesite Drainage that presents loads of case studies and even has exercises. It is written as a text book and explains all the necessary concepts provided you have a basic grasp of geochemistry. It is not free but it is available at:

http://www.mdag.com/book.html

I hope this helps in finding the answer to your question.

Thank you all for your support, I will keep my mind to your ideas and continue deeply research HCT results and correlate with static test results.