Determining which materials are acid generating and which are not is key to sound engineering design and cost mitigation. In my opinion spending the money up front for abundant characterization and good modeling will lead to better decisions, lower treatment costs, lower bonds, and stronger social license. Many waste rock management plans I have attempt to sequester the acid generating rock in the center of the pile where oxygen diffusion may be limited.

I recommend that you follow the INAP/NSERC/DDMI - sponsored research on waste rock that is being conducted at the Diavik Mine in Canada. The work is being done by a consortium of Canadian universities, including Waterloo, UBC and University of Alberta. This is, by a great length, the best instrumented, best monitored, and best modeled waste-rock pile in the world. After years of preparatory work, results are just now starting to be published - there were summaries I am sure at the just-completed ICARD meeting in Ottawa. Several of your issues about how engineering controls, such as covers, affect water-quality outcomes are directly evaluated. Perhaps even more importantly, the experimental design of the work will allow much greater fundamental understanding of the 3-phase (solid-liquid-gas), non-isothermal, nn-homogeneous environment, and therefore more complete and accurate understandings of the basic controls on oxidation and dissolution/precipitation reactions that move us well beyond simple considerations of a binary difference between systems that do and do not generate acidic effluents.

There is one site that really does demonstrate leading practice and has been operating for more than 20 years. This is the Martha Mine in New Zealand. There is another mine in NZ, the Golden Cross mine which closed (for geotechnical reasons) more than 10 years ago and which is also demonstrating leading practice. Another site, which I adopted the Waihi model and is going very well with respect to ARD control (average S grade 3.5%S and no ANC), is the Phu Kham mine in Loa PDR. I have not got Mark there as yet, but he will be next.

I`m impressed with the great results achieved with 3.5%S and no NP capacity. In addition, one of the challenges we are also facing in Brazil is what we call NMD (neutral mine drainage). This is because even with an elevated ratio of ANC/MPA (2, 3 or 4) some metals can leach higher than the tight Brazilian standards for class II waters. How to manage the best way it`s possible is still a good question to be answered. 

The recommendations of visiting Waihi and Golden Cross are very good ideas, and although I have not been to Phu Kham, I know a little about it. There is another site in SE Asia where Stuart and I have worked at which inadequate execution of the strategies pioneered by Stuart in NZ has led to at least incipient probelms. It's not enough to have a plan - it actually has to be executed, then monitored, and modified as necessary in light of the accumulating empirical data and the mine's demonstrated capacity to execute the original idea.

We agree with you that Neutral Drainage is an issue that needs more consideration than it often has had in the past. What we actually care about is water-quality outcomes, in ways that are protective of water resources. We know that AMD can have deleterious effects, but the fact that an effluent is neutral or even alkaline is no guarantee whatsoever that it will meet discharge requirements. 

With neutral mine drainage the options for passive treatment of the mine water becomes increasingly possible - it does depend on the loads etc and the type of metals, but in general the mine water is more amenable to physical (settling ponds), biochemical and vegeated system treatment - climate prevailing of course.

I understand that the project design of the waste rock dumps must contemplate the geochemical control these days. One of the issues here in Brazil related to NMD is that in some cases, the pH is 7 or 7.5 and the concentration of metals, specially Mn, is still slightly above the POTABLE standard, which sometimes is the standard that EPA requires. But of course my biggest concern is to prevent ARD along the project and operation phase to avoid massive liabilities (maybe even perpetuity) in the future. Lets keep the discussion up!

I successfuly removed manganese with a surface limestone rock filter (not to be confused with the anoxic limestone drain) which was poulated with algae - the micro environment of the pH was sufficient to ppt the Mn without over the top armouring from Fe (as that had been romoved using a wetland system). The last set of kinetic results I have reviewed showed that with increasing pH, the solubility of U(VI) increased - nothing new there, but it does show that the kinetic testing (humidity cell) is not just about the acid conditions but also alkaline.

I'm surprised at your Mn success with limestone and wonder whether the benefit will last. In AMD conditions the effectiveness of most limestone particles is quickly diminished as acid reacts with the outer surface of the CaCO3, forming a layer that retards or eliminates further limestone reaction (neutralization). Although ABA presumes that all alkalinity magically reacts acid at the same rate sulfide tailings leach the acids, this simply does not occur.

With limestone "treatment" as with many things, the devil is in the detail. While great attention is paid to CCE, less is directed to the relationship between limestone particle size, surface area and porosity. And yet, it is these traits that drive the extent of reaction in the neutralization process. Consider, for example, that calcium hydroxide particles have many times the surface area and porosity of a minus 200 mesh limestone particle. Reaction is immediate. Now think with me for a moment. How much of a ton of limestone you now use will pass a 200 mesh sieve? And how much of that ton will take up space without yielding reactive alkalinity.

ABA is wonderful to the extent regulators accept it and grant approvals -- if that is the hurdle you must clear. But long term solutions for Mn with calcium carbonate?

Thanks, your concerns are why the Mn-system formed part of a treatment train of passive systems and therefore by the time the mine water had reached the rock filter, the water was not acidic and therefore the 'passivation' you described did not happen. Armouring, which can also be a major issue, was reduced by the preceding treatment and removal of iron. It is believed that the micro environment between the algae and the rock maintains an elevated pH which encourgaes the Mn to precipitate. Removal rates were consistent with litertaure values (around 5g/m2/d) and that was pretty consistent for 5 years I believe. The influence of the physical charaecteristic of limestone explains reduced neutralisation efficiencey when compared to hydrated lime. When comparing limestone to hydrated lime, limestone is only 30-50% efficient compared to the lime which is probably around 90%, and you need less lime (per unit of acidity). So for 50 tonnes of acid you need around 41 tonnes of hydrated lime or 2 or 3 times the amount of limestone.

Regarding the rates of limestone reaction - and the short comings of ABA - that is why kinetic tetsing in the lab or field should be undertaken following the intitial ABA work.

Long term treatment of Mn with CaCO3 in a rock filter is proven technology - but it does depend on what you view as long term - no treatment system is finitie and walkaway, so if you want a panacea of building and let the thing run for decades unabted - then of course that will not work - but appropriate management through a deisgned system will.

The Mn treatment sounds like a “Mn aerobic constructed wetland,” which is a special type of aerobic wetland that is highly oxidizing and contains populations of appropriate microorganisms (green algae and cyanobacteria).

Manganese wetlands are typically shallow rock wetlands (rocks protrude slightly out of the water), colonized with an algal mat designed to locally raise pH and the redox potential. The rocks are typically limestone. However, it’s the microorganisms, not the limestone, that catalyze the precipitation reaction by locally raising the pH to between 8 and 9. An aeration spillway typically precedes the wetlands to ensure that the inflow water is highly oxidized.

These wetlands have problems in climates with cold winters, especially if macro-vegetation is allowed to grow in them. During the winter, the vegetation dies back. Then, in the following spring, the rotting vegetation uses up oxygen and releases previously precipitated Mn back into the water column.

(Also, you need to remove any Fe and Al and raise the pH to above neutral BEFORE the water flows into the Mn treatment cell.)

Yes the system is as you described and yes there are potential problems - all these factors need to be taken into account, when selecting the system, passive or otherwise. In most abandoned mine settings it is a balance between the area, climate, the setting and what is achievable given all of these things. So in my case we were asked to do what was possible to remove the metals and improve the overall environmental setting. This in itself was considerably better that letting the mine pollute the river systems with no treatment.

The rate of Mn removal in a passive treatment wetland system occurs in the range of 0.5 - 10g/m2/day and so if you have a 60 mg/l Mn in the discharge you can look at what flow rate is treatable, theoretically. Given the high rainfall, then there will almost certainly require balancing lagoons, diversion ditches etc which can be sized on a hydrological basis given the climate-but can also be used to mix with the treated water downstream. Treatability trials are the way to go from bench scale to pilot scale testing to see if the system is possible.

My experience in wetland systems is that any adsorption of the Mn by Fe and Al is limited. Often if you removal Fe and Al in an anaerobic system, such a redox will not precipitate or control Mn and so you still have the Mn to treat eventually so if there is any adsorption it is likely to be short term. That is usually why Mn is treated last in a treatment train. However if you are looking at an oxidising system for the removal of Mn at circum neutral pH then that will also remove iron (any design would need to account for armouring etc). A rule of thumb regarding Mn is that it precipitates with positive redox (and sunshine!) and is mobile with reducing redox.