Articles tagged with: Tailings

51. Pre-Concentration – Savior or Not?

Can pre-concentration become a key savior for the mining industry and help in lowering metal production costs?
Pre-concentration is a way to reduce the quantity of ore requiring higher cost downstream processing, i.e. grinding in particular. By using a low cost method to pre-concentrate mineral-bearing particles into a smaller volume, one can attain significant cost savings in overall energy consumption and operational expenses. A previous blog “Remote Sensing of Ore Grades” discussed a new pre-concentration method under development.
Pre-concentration is nothing new and has been around for many years but is generally limited in the techniques available. Hence many ore types are not amenable to it..unfortunately. The main methods being used are:
Ore sorting can be done using automated optical, electrical, or magnetic susceptibility sensors to separate ore particles from waste particles. The different sensor types can include colour recognition, near infrared radiation sensors, x-ray fluorescence, x-ray transmission, radiometric, or electromagnetic sensing. The sensors can determine if a particle contains valuable mineral or waste, thereby sending a signal to activate air jets to deflect material into ore and waste bins.
Density or specific gravity difference is another property that some pre-concentration methods can rely on. Gravity based systems such as dense media separation (DMS), jigs, or centrifugal concentrators are currently in production use.
Another simple pre-concentration method used is scrubbing, whereby simply washing away fines may remove some deleterious materials prior to final processing.
 Jig Plant 1
Pre-concentration can provide several benefits to an operation:
-If done underground or at remote mine site, the net ore hoisting and ore transport costs can be reduced.
-If the pre-concentration rejects can be used as mine backfill, this can reduce backfilling costs.
-Processing of higher grade pre-concentrated mill feed can reduce total energy costs and ultimately reduce the cash cost of metal produced.
-Grinding costs can be reduced if waste particles are harder than the ore particles and they can be removed beforehand.
-Minimizing waste through the process plant will reduce the quantity of tailings that must be disposed of.
-Lowering operating costs may potentially allow lowering of the cutoff grade and increasing mineral reserves.
-Higher head grades would increase metal production without needing an increase in plant throughput.
Not all ore types are amenable to pre-concentration and therefore a rigorous testing program is required. In most cases the pre-con method would be relatively obvious to the metallurgical engineer but testing is still required to measure performance. Testing is required to determine the amount of waste rejection that can be achieved without incurring significant ore loss during the process. Generally one can produce a higher quality final product if one is willing to reject more ore with the waste, so it becomes a trade-off of recovery versus total processing cost.
Fine particles from the primary and secondary crushing stages might require bypassing the pre-con circuit. If this bypassed material is sent for downstream processing, one may need to examine crushing systems that minimize fines generation to avoid too much material bypassing the pre-con circuit.
One must also decide if the pre-con system should reject waste particles from the material stream or reject ore particles from the stream since the overall recovery and product quality will be impacted depending on which approach is used.
My bottom line is that the mining industry is continually looking for ways to improve costs and pre-concentration may be a great way to do this. Every process plant design should at least take an initial look at it to see if is feasible for their ore type. While the existing pre-concentration methods have their limitations, future technologies may bring in new ways to pre-concentrate and so this is probably an area where research dollars would be well spent.
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50. Landslide Blog – If You Like Failures

For those of you with a geotechnical background or have a general interest in learning about rock slides and slope failures, there is an interesting website and blog for you to follow. The website is hosted by the American Geophysical Union the world’s largest organization of Earth and space scientists. The blogs on their site are written by AGU staff along with contributions from collaborators and guest bloggers. Their website screenshot is shown below.
Landslide Blog screenshot

Landslide Blog web page screenshot

The independent bloggers have editorial freedom in the topics they choose to cover and their opinions are those of their authors and do not necessarily represent the views of the American Geophysical Union. This provides for some leeway on the discussions and the perspectives the writers wish to take.
One specific area they cover well in their Landslide Blog are the various occurrences of rock falls and landslides from any location around the globe. They will present commentary, images, and even videos of slope movements as they happen. Often they will provide some technical opinion on what possibly caused the failure event to occur. The Landslide Blog has a semi-regular email newsletter that will keep you updated on new stories as they happen.
Landslide
The following links are a few examples of the type of discussion that they have on the website.
Here is a description of a small water dam failure in Greece.
Here is some video of the Samarco tailings runout in Brazil.
Here is some video of boulders raining down on some buses along the Karakorum Highway in Pakistan.
From time to time the Landslide Blog will examine mine slopes, tailings dams, and waste dump failures, however much of their information relates to natural earth or rock slopes along roads or in towns and cities. Some of their videos are quite fascinating, illustrating the forces behind some of earth’s natural erosion processes. Check it out for yourself.
My bottom line on all of this is that the less the mining industry is mentioned in the Landslide Blog, the better it is for all of us.
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46. Tailings Disposal Method Risk

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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36. Fluid Tailings – Time to Kick The Habit?

It’s been awhile since my last blog article.   Work and other commitments seem to get in the way.
Smoking… we constantly hear about the negative effects of it.  We all know of people that have died due to lung cancer or other smoking related causes.  However we probably also know of people that have smoked their entire lives yet lived into their eighties.  Regardless of that, there still is a push to get people to kick the smoking habit because it is better for them and their families.  Short term pain for long term gain.
Let’s compare all of that with the concept of fluid tailings storage.
Tailings…those of us in the mining industry constantly hear about the negative effects tailings storage.  We know of numerous mines that have had failures resulting in fatalities and catastrophic damage.  However we also know of many mines that have used fluid tailings their entire operating lives without any incidents.  Given the recent Samarco tailings failure, for me the question has now become whether the mining industry should kick the habit of fluid tailings storage despite no failure in many circumstances.
Quitting smoking requires real effort, some pain, a change in lifestyle, and an overall commitment to quit.   It isn’t easy but generally pays off in the long run when you speak with those who have already quit.  Similarly for the mining industry, moving away from fluid tailings storage requires real effort, some pain, a change in operating style, and an overall commitment to quit.  It wouldn’t be easy but might pay off in the long run by avoiding major tailings incidents, less negative press, and fewer environmental permitting issues.  No longer would the consultants and regulators be disputing factors of safety of 1.3 versus 1.5, when they could be discussing factors of safety of 5 or 10.
Just as quitting smoking brings relief to oneself and family, quitting fluid tailings storage may bring relief to stakeholders, shareholders, regulators, and mine management.  They can all sleep better at night knowing there isn’t a large mass of fluid being restrained simply by a rockfill dam at a factor of safety of 1.5.  Engineers say they can design dams that if built properly will be stable for the long term, and I tend to agree with this statement.  However that is no guarantee for all tailings dams, the proof of which is major incident we seem to see regularly.
My bottom line is that no one wants to sit downwind of a smoker and no one wants to live downstream of a tailings dam.  Perhaps it is time for the mining industry to kick the habit of using fluid tailings storage, regardless of cost and discomfort that it causes. Short term pain for long term gain.
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