After the Mt Polley and Samarco tailings failures, there have been ongoing conversations about the benefits of filtered or dry stack tailings as the only way to eliminate the risk of catastrophic tailings failure. Mining companies would all like to see a similar risk reduction at their own project. However what mining companies don’t like is the capital and operating costs associated with dry stacking. The dry stack tailings processing cost and the transport cost are both costlier than for conventional tailings disposal and therefore would negatively impact on the overall value of the project. Obviously this reduction in value would get offset against an improved environmental risk and a better closure condition. So what’s a company to do?

 

Filtered tailings stack

Example of a Dry Stack Tailings

In my experience when designing a new mining project, all mining companies at one point in time complete a trade-off study for different tailings disposal methods and disposal sites. Contrary to some environmental narratives, companies really do wish to know how the different tailings options compare because they would adopt the dry stack approach if it was the most advantageous method. The mining companies are fully aware of the benefits but the dilemma the company runs into is the cost and being able to somehow justify the technology. Complicating their final decision, companies also have options for reducing their tailings risk even if using another tailings disposal method and so the final decision can get very complex.
Often proponents of the risk analysis approach will use a risk-weighting approach to assign an expected economic cost to their tailings plans. For example, if the cost of a failure is $200 million and the risk is 0.1%, then the Expected Value is $200,000. The problem is that this is a theoretical calculation on an assumed likelihood of failure but in reality either the dam will fail or it won’t. So failure remediation money will be spent or it won’t be spent, it won’t be partially spent.
The degree of acceptable tailings risk therefore becomes a subjective factor. While implementing a dry stack may reduce the risk of catastrophic failure to zero, implementing a $100,000 per year monitoring program on a conventional tailings pond will reduce its risk. Implementing a $500,000 per year monitoring program would reduce that risk even further. Installing in a water treatment plant to enable periodic water releases may further lower the tailings risk. The company can look at different mitigations to keep lowering their risk, although recognizing that none of the mitigations would necessarily bring the risk down to zero. Finally the companies could compare the various risk mitigation costs against the incremental dry stack costs in order to arrive at an optimal path forward.
So the question becomes how low does one need to reduce the tailings storage risk before it is acceptable to shareholders, regulators, and the public. I don’t think the answer is that one must lower the risk down to zero. There are not many things in today’s world that have zero risk. Driving a car, flying in a plane, shipping crude oil by ocean tanker, having a natural gas furnace in your house..none of these have zero risk yet we accept them as part of living in modern society.
Environmental groups are always discussing ways of forcing regulators and mining companies to take action against the risk of tailings failure. This is commendable, however they generally fail to provide any guidance on what level of risk would be acceptable to them or to the public. It seems to be impossible for these groups to define what an acceptable risk is or provide any ideas other than the standard “shut down all mining” solution.
We know that in the long run mining is here to stay so we all should work together towards solutions. The solutions need to be realistic in order to be taken seriously and for them to play a role in redefining tailings disposal in modern mining. Dry stack may not be the only solution and we should be open to ways of improving the other tailings disposal methods so that companies have more low risk options available to  them.
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4 thoughts on “46. Tailings Disposal Method Risk

  1. hardrockminer

    There is another factor inhibiting the use of dry stack and that’s volume. What is the largest tonne per day facility using dry stack? The ones I’ve seen are usually less than 10 ktpd. What would dry stack look like for 250 ktpd? Also, what about closure? Dry stack would require treatment forever. And lastly, submerged tails usually include PAG waste. If you go with dry stack you will also have to treat PAG waste forever.

    I’m not ready to give up on submerged tailings just yet. If risk is the issue then simply add a larger “shit happens” factor. Make it 2.0 or 3.0 or some number bigger than present, which is currently, I believe 1.5. What is the rationale behind 1.5 anyway?

    Mt Polley TSF failed for two major reasons. First, the foundation wasn’t adequately understood. And second, operating practices were not optimal. Either one of these on their own may not have resulted in a failure, but together they led to a catastrophe. Perhaps more geotechnical work and greater construction scrutiny would have led to a different result, but so would a higher FOS. If the cost of more geotech and more buttress is too great then projects may opt for dry stack.

    We should never abandon the goal of zero failures over the long term. To do so would mean just accepting a lower failure frequency. As you noted, statistics and probability are useless tools when considering a single failure. Each dam should be designed as if failure is not an option. No doubt that will increase costs, but having just experienced a rather significant failure at Mt Polley we know that another similar failure would result in a virtual industry shutdown. So failure is not an option.

  2. hardrockminer

    A very cost effective alternative to dry stacking (which is very expensive due to de-watering and conveying costs) is to remove the pyrite and store it in a submerged environment and then use the coarse NAG tailings sands to build the dam face. This can’t be done in all cases but it should be part of the tailings alternatives review during FS studies.

  3. Ken Kuchling Post author

    Good idea. Tailings treatment is something I have rarely see studied before, other than treatments such as thickening or cyanide destruction. Removal of bad constituents is often not thought about.

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