Articles tagged with: Mine Engineering

Don’t Cut Corners, Cut Cross-Sections Instead

Exploration cross-sectionThis article is about the benefit of preparing (cutting) more geological cross-sections and the value they bring.
Geological sections are one of the easiest ways to explain the character of an orebody. They have an inherent simplicity yet provide more information than any other mining related graphic.
Some sections can be simple cartoon-like images while others can be technically complicated, presenting detailed geological data.
Cartoon-stylized sections are typically used to describe the general nature of the orebody. The detailed sections can present technical data such as drill hole traces, color coded assays intervals, ore block grades, ore zone interpretations, mineral classifications, etc.
Sections provide a level of clarity to everyone, including to those new to the mining industry as well as those with decades of experience.
This article briefly describes what story I (as an engineer) am looking for in sections. Geologists may have a different view on what they conclude when reviewing geological sections.
I will describe the three types of geological sections that one can cut and what each may be describing. The three types are: (1) longitudinal (long) sections; (2) cross-sections; (3) bench (level) plans. Each plays a different role in helping to understand the orebody and mining environment.
There is also another way to share simple geological images via3D PDF files. I will provide an example later.

Longitudinal (Long) Sections

Geological long section examplesLong sections are aligned along the long axis of the deposit. They can be vertically oriented, although sometimes they may be tilted to follow the dip angle of an ore zone.
Long sections are typically shown for narrow structure style deposits (e.g. gold veins) and are typically less relevant for bulk deposits (e.g. porphyry).
The information garnered from long sections includes:
  • The lateral extent of the mineralized structure, which can be in hundred of metres or even kilometers. This provides a sense for how large the entire system is. Sometimes these sections may show geophysics, drilling to defend the basis for the regional interpretation.
  • Long sections will often highlight the drill hole pierce points to illustrate how well the mineralized zone is drilled off. Is the ore zone defined with a good drill density or are there only widely spaced holes? As well, long sections can show how deep ore zone has been defined by drilling. On some projects, a few widely spaced deep holes, although insufficient for resource estimation purposes, may confirm that the ore zone extends to great depth. This bodes well for potential development in that a long life deposit may exist.
  • Sometimes the long section drill intercept pierce points can be contoured on grade, thickness, or grade-thickness. This information provides a sense for the uniformity (or variability) of the ore zone. It also shows the elevations of the higher grade zones, if the deposit is more likely an open pit mine, an underground mine, or a combination of both.

Cross-Sections

Geological pit sectionCross-sections are generally the most popular geological sections seen in presentations. These are vertical slices aligned perpendicular to the strike of the orebody. They can show the ore zone interpretation, drill holes traces, assays, rock types, and/or color-coded resource block grades.
As an engineer, my greatest interest is in seeing the resource blocks, color coded by grade. Sometimes open pit shells may be included on the section to define the potential mining volume. The engineering information garnered from block model cross-sections includes:
  • Where are the higher-grade areas located; at depth or near surface?
  • If a pit shell profile is included, what will the relative strip ratio look like? Are the ore zones relatively narrow compared to the size of the pit?
  • How will the topography impact on the pit shape? In mountainous terrain, will a push-back on pit wall result in the need to climb up a hillside and create a very high pit slope? This can result in high stripping ratios or difficult mining conditions.
  • Does the ore zone extend deeper and if one wants to push the pit a bit deeper, is there a high incremental strip ratio to do this? Does one need to strip a lot of waste to gain a bit more ore?
  • Are the widths of the mineable ore zones narrow or wide, or are there multiple ore zones separated by internal waste zones? This may indicate if lower-cost bulk mining is possible, or if higher cost selective mining is required to minimize waste dilution.
  • How difficult will it be to maintain grade control? For example, narrow veins being mined using a 10 metre bench height and 7 metre blast pattern will have difficulty in defining the ore /waste contacts.
  • Cross-sections that show the ore blocks color coded by classification (Measured, Indicated, Inferred), illustrate where the less reliable (Inferred) resources are located and how much relative tonnage may be in the more certain Measured and Indicated categories.
Geological cross-section exampleWhen looking at cross-sections, it is always important to look at multiple cross-sections across the orebody. Too often in reports one may be presented with the widest and juiciest ore zone, as if that was typical for the entire orebody.  It likely is not typical.
Stepping away from that one section to look at others is important. Possibly the character of the ore zones changes and hence its important to cut multiple sections along the orebody.

Bench (Level) Plans

Mining Bench PlansBench plans (or level plans) are horizontal slices across the ore body at various elevations. In these sections one is looking down on the orebody from above.
Level plans are typically less common to see in presentations, although they are very useful. The level plans may show geological detail, rock types, ore zone interpretations, ore block grades, and underground workings.
The bench plan represents what the open pit mining crews would see as they are working along a bench in the pit. The information garnered from bench plans that include the block model grades includes:
  • Where are the higher-grade areas found on a level? Are these higher grade areas continuous or do they consist of higher grade pockets scattered amongst lower grade blocks?
  • Do the ore zones swell or pinch out on a bench? A vertical cross-section may give a false sense the ore zones are uniform. The bench plan gives an indication on how complicated mining, grade control, and dilution control might be for operators.
  • Do the ore zones on a bench level extend out beyond the pit walls and is there potential to expand the pit to capture that ore?
  • On a given bench what will the strip ratio be? Are the ore zones small compared to the total area of the bench?
As recommended with cross-sections, when looking at bench plans, one should try to look at multiple elevations.  The mineability of the ore zones may change as one moves vertically upwards or downwards through a deposit.

Never mind cross-sections – give me 3D

While geological sections are great, another way to present the orebody is with 3D PDF files to allow users to view the deposit in three-dimensions. Web platforms like VRIFY are great, but I have been told they sometimes can be slow to use.
Mining 3D PDF file3D PDF files can be created by some of the geological software packages. They can export specific data of interest; for example topography, ore zone wireframes, underground workings, and block model information. These 3D files allows anyone to rotate an image, zoom in as needed and turn layers off and on.
You can also create your own simplistic cross-sections through the pdf menus (see image).
A simple example of such a 3D PDF file can be downloaded at this link (3D DPF File Example). It only includes two pit designs and some ore blocks to keep it simple.
The nice thing about these PDF files is that one doesn’t need a standalone viewer program (e.g. Leapfrog viewer) to view them. They are also not huge in size. As far as I know 3D PDF files only work with Adobe Reader, which most everyone already has.  It would be good if companies made such 3D PDF files downloadable along with their corporate PowerPoint presentations.

Conclusion

Exploration cross-section exampleThe different types of geological sections all provide useful information. Don’t focus only on cross-sections, and don’t focus only on one typical section.  Create more sections at different orientations to help everyone understand better.
In 2019 I wrote an article describing the lack of geological cross-sections in many 43-101 technical reports. The link to that article is her “43-101 Reports – What Sections Are Missing?
Geological sections are some of the first items I look for in a report. Sometimes they can be hidden away in the appendices at the back of the report. If they are available, take the time to actually study them since they can explain more than you realize.
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Grade-Tonnage Curves – Worthy of a Good Look

Most of us have seen the typical “grade-tonnage” table or graph, showing ore tonnes and grade at varying cutoff grades. It is usually part of every 43-101 technical report in Section 14.  We may glance at it quickly and then move on to more exciting chapters. Section 14 (Mineral Resources) can be a very complex chapter to read with statistics, geostatistics, and mathematical formulae.  However the grade-tonnage curve aspect isn’t complicated at all.
The next time you see the grade-tonnage relationship, I suggest taking a few seconds to study it a bit further.   There might be some interesting things in there.

Typical Grade-Tonnage Information

Typically, one will see grade-tonnage data in 43-101 Technical Reports towards the back of Section 14 "Mineral Resources".  The information is normally presented in either of two ways; (i) a grade-tonnage table or (ii) a grade tonnage graph.  Examples of each are shown below.  The grade tonnage graph typically has the cutoff grade along the bottom x-axis and the two separate y-axes  representing the ore tonnes above cutoff and the average ore grade above cutoff.
typical grade tonnage table
typical grade tonnage curve
Rarely do you see both the table and curve in the report, although ideally one would want to see both.  Given the option, I would prefer to see the graph more than the table of numbers.  The trend of the grade-tonnage information is just as important as the values, maybe even a bit more important.  Unfortunately, a data table by itself doesn’t illustrate trends very well.

Useful Grade-Tonnage Curve Information

mining grade tonnage curveWhen I am undertaking a due diligence review or working on a study, very early on I like to have a look at the grade-tonnage information.  This could be for the entire deposit resource, within a resource constraining shell, or in the pit design.
The grade-tonnage information gives an understanding of how future economics or technical issues may impact on the mineable tonnage.
An example of a typical grade-tonnage curve is shown here.
The cutoff grade along the x-axis will be impacted by changes in metal price or operating cost. The cutoff grade will increase if metal prices decrease or if operating costs increase.
The question is how sensitive is the mineable tonnage to these economic factors. The slope of the tonnage and grade curves will help answer this question.
In the example shown, the tonnage curve (blue dots) is fairly linear, meaning the ore tonnage steadily decreases with increasing cut-off grade.  That is expected and is reasonable.
mining grade-tonnage curveHowever, if the tonnage curve profile resembled the light blue line in this image, with a concave shape, the ore tonnage is decreasing rapidly with increasing cutoff grade.   This is generally not a favorable situation.
It indicates that a significant portion of the tonnage has a grade close to the cutoff grade.  If that’s the situation, the calculation of the cutoff and the inputs used to generate it are important and worthy of scrutiny.  Are they reasonable?  Over the long term, is the cutoff grade more likely to increase or decrease?
The same logic can be used with the ore grade curve in the graph.  As  shown in this example, the ore grade increases steadily as the cutoff is raised.  This is because lower grade ore is being shifted from ore to waste, and hence the remaining ore has better quality.  If the cutoff is raised from 0.4 g/t to 0.5 g/t, then some material with a grade of about 0.45 g/t is moved from ore to waste.
I also like to compare the ratio of the average grade to the cutoff grade.  Its nice to see a ratio of 4:1 to 5:1 to ensure the overall average grade isn’t close to the cutoff.  In this example, the cutoff grade is 0.5 g/t and the average grade is 4.5 g/t, a ratio of 9:1.
The tonnage curve and grade curve provide information on the nature of the mineral resource. Study them both.

Reporting Waste Within a Shell

One complaint I have about reporting mineral resources inside a resource constraining shell is the lack of strip ratio information. This applies whether disclosing a single mineral resource estimate or variable grade-tonnage data.
In my view, the strip ratio is even more important to be aware of when looking at grade tonnage data.
The strip ratio within a shell will climb as an increasing cutoff grade results in a decreasing ore tonnage.  Sometimes the strip ratio will increase exponentially. The corresponding amount of waste remaining in that pit shell increases, hence the ratio of the two (i.e. strip ratio) can escalate rapidly.
mining strip ratio curveRegarding mineral resources, one should be required to disclose the waste tonnage and strip ratio when reporting resources inside a constraining shell. The constraining shell and cutoff grade are both based on defined economic factors such as unit mining costs, processing cost, process recoveries, and metal prices.  With respect to the mining cost component, the strip ratio is a key aspect of the total mining cost, yet it normally isn’t disclosed.
Its common to see mention that the mining cost is (say) $2.50/t, but if the strip ratio is 10:1, that equates to an effective mining cost of $27.50 per tonne of ore.   That’s an important cost to know, especially if one is pushing a pit shell deep to maximum the mineral resource tonnage.
Each mineral deposit resource model can behave differently.  Hence, in my view, the waste tonnage should be included when reporting mineral resource tonnages (or presenting grade-tonnage data) within a constraining shell.  This waste tonnage or strip ratio can be in the footnotes to the mineral resource summary table.

Spider Diagram Downsides

In 43-101 technical reports, the financial Chapter 22 normally presents the project sensitivities expressed in a spider diagram or a table format.
In a previous blog post I had discussed the flaws in the spider diagram approach.  That article link is at “Cashflow Sensitivity Analyses – Be Careful”.  The grade-tonnage curve helps explain why that is.
In the spider diagrams, we typically see sensitivities related to +/- 20% on metal prices and operating costs.    If either of these factors change, then in reality the cutoff grade would change.
If the metal price decreases by -20%, or the operating cost climbs by +20%, the cutoff grade must increase.  This adjustment is normally not made in the sensitivity analysis because it requires a lot of re-work.
Elevating the cutoff grade would shift the pit ore tonnage towards the right on the grade-tonnage curve, showing a decrease in mineable tonnes.   However, in the spider diagram logic, the assumption is that production schedule in the cashflow model is unchanged and simply the metal prices or operating costs are adjusted.  Therefore, the spider diagram can be a misleading representation of the downside risk, showing a more positive situation than in reality.

Conclusion

The grade-tonnage information is always presented in technical reports. It examines the sensitivity of the orebody size to changes in cutoff grade. The next time you see grade-tonnage data, don’t skip over it.  Take a minute to study it further to see what can be learned.
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Mining Under Lakes – Part 2: Design Issues

This is Part 2 of a blog post related to open pit mining within bodies of water. Part 1 can be found at this link “Mining Under Lakes – Part 1“, which provides a few examples where this has been done successfully. Part 2  focuses on some of the social and technical issues the need to be considered when faced with the challenge of open pit mining within a water body.
dike construction in waterThe primary question to be answered is whether one can mine safely and economically without creating significant impacts on the environment.
The answer to this question will depend on the project location and the design of the water retaining structure.
I have worked on several projects where dike structures were built. I have also undertaken due diligence reviews of projects where dikes would be required. Most recently I have participated in some scoping level studies where mining within a lake or very close to a river were part of the plan.
In some instances, the entire orebody is located in the lakebed. In others, the orebody is mainly on land but extends out into the water. Each situation will be unique. In northern Canada, given the number of lakes present, it would be surprising if a new mining project isn’t close to a river or lake somewhere.

Dike concepts consider many factors

Different mining projects may use different styles of dikes, depending on their site conditions. Some dikes may incorporate sheet piling walls, slurry cutoff walls, low permeability fill cores, or soil grouting. There are multiple options available, and one must choose the one best suited for the site.
The following is list of some of the key factors and issues that should be examined.

ESG Issues

One’s primary focus should be on whether building a dike would be socially and environmentally acceptable. If it is not, then there is no point in undertaking detailed geotechnical site investigations and engineering design. One must have the “social license” to proceed down this path.
Water Body Importance: Is there a public use of the water body? It could be a fresh water source for consumption, used for agricultural or fishery purposes, or used as a navigable waterway, etc. Would the presence of the dike impact on any of these uses? Does the water body have any historical or traditional significance that would prevent mining within it?
Lake Turbidity: Dike construction will need to be done through the water column. Works such as dredging or dumping rock fill will create sediment plumes that can extend far beyond the dike. Is the area particularly sensitive to such turbidity disturbances, is there water current flow to carry away sediments?
At Diavik, a floating sediment curtain surrounding the dike construction area was largely able to contain the sediment plume in the lake.
Regional Flow Regime: Will the dike be affecting the regional surface water flow patterns? If the dike is blocking a lake outflow point, can the natural flow regime be maintained during both wet and dry periods?

Location Issues

If there are no ESG issues preventing the use of a dike, the next item to address is the ideal location for it.
Water depth: normally as the dike moves further away from land, both the water depth and dike length will increase. The water depth at the deepest points along the dike are a concern due to the hydraulic head differential created once the interior water pool is pumped out. The seepage barrier must be able to withstand that pressure differential, without leaking or eroding. A low height dike in shallow water may be able to use a simpler seepage cutoff system than a dike in deep water.
Islands: Are there any islands located along the dike path that can be used to shorten the construction length and reduce the fill volumes? Is there a dike alignment path that can follow shallower water zones?
Diavik open pit dikesPit wall setback: Given the size and depth of the open pit, how far must the dike be from the pit crest? Its nice to have 200 metre setback distance, but that may push the dike out into deeper water.
If the dike is too close to the pit, then pit slope failures or stress relaxation may result in fracture opening and increase the risk of seepage flows or catastrophic flooding. The pit wall rock mass quality will be the key determining factor in the setback distance.
Maximizing ore recovery: If the ore zone extends further out into the lake, maximizing ore recovery may require using a steep pit wall along the outer sections of the pit. This may require positioning haulroads with switchbacks along other sides of the pit rather than using a conventional spiral ramp layout.
At Diavik (see image), the A154 north open pit wall was pushed to about 60 metres of the dike to access as much of the A154N kimberlite ore as possible. Haulroads were kept to the south side of the pit.
It may be possible to recover even more ore by pushing out the dike even further. However, this may result in a larger and costlier dike or even require a different style of dike. There will be a tradeoff between how much additional ore is recovered versus the additional cost to achieve that. There will be a happy medium between what makes both technical sense and economic sense.

Design Issues

Once the approximate location of the dike has been identified, the next step is to examine the design of the dike itself. Most of the issues to be considered relate to the geotechnical site conditions.
Lakebed foundation sediments: What does the lakebed consist of with respect to soft sediments? Soft sediments can cause dike settlement and cracking, or mud-waving of fill material.
Will the soft sediments need to be dredged prior to construction, and if so, where do you dispose of this dredge slurry, and what impact will dredging have on the lake turbidity?
Lakebed foundation gravels: Are there any foundation gravel layers that can act as seepage conduits beneath the dike? If so, will these need to be sub-excavated, or grouted, or cut off with some type of barrier wall?  Sonic drilling, rather than core drilling, is a better way to identify the presence of open gravel beds.
Upper bedrock fracturing: Is the upper bedrock highly fractured, thereby creating leakage paths? If so, then rock grouting may be required all along the dike path to seal off these fractures.
Major faults: Are there any major faults or regional structures that could connect the open pit with the lake, acting as a source of large water inflow?. At Diavik, we attempted to characterize such structures with geotechnical drilling before construction. Upon review, I understand there was one such structure not identified, which did result in higher pit inflows until it was eventually grouted off.
Water level fluctuations: In a lake or river one may see seasonal water level fluctuations as well as storm event fluctuations. The height of the dike above the maximum water level (i.e. freeboard) must be considered when sizing the dike.
Ice scouring: In a lake or river that freezes over, ice loads can be an important consideration. During spring breakup as the ice melts, large sheets of ice can be pushed around and may scour or damage the crest of the dike. The dike must be robust enough to withstand these forces.
Construction materials available on site: Is there an abundance of competent rock for dike fill? Is there any low permeability glacial till or clay that can be used in dike construction? If these materials are available on site, the dike design may be able to incorporate them. If such materials are not available, then a alternate dike design may be more appropriate, albeit at a cost.

Conclusion

Each mine site is different, and that is what makes mining into water bodies a unique challenge. However many mine operators have done this successfully using various approaches to tackle the challenge.
Even at the exploration stage, while you are still core drilling the orebody through the ice, you can start to collect some of this information to help figure it all out.
The bottom line is that while mining into a water body is not a preferred situation, it doesn’t mean the project is dead in the water. It will add capital cost and environmental permitting complexity, but there are proven ways to address it.
On the opposite side, I have also seen situations where a dike solution was not feasible, so ultimately there are no guarantees that engineers can successfully address every situation. Lets hope your project isn’t one of them.
There could be a 3rd part to this post that discusses issues associated with underground mining beneath bodies of water; however that is not my area of expertise.  I would be more than happy to collaborate on a article with someone willing to share their knowledge and experience on that subject.
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Mining Under Lakes – Part 1: Examples

Mining Under Lakes
Springpole Project

Springpole Project

I recently saw an investor presentation from First Mining Gold about their Springpole Project. The situation is that their open pit is located within a lake and will require the construction of a couple of small cofferdams to isolate the pit area from the lake. The concept is shown in this image.
Over the last couple of years I have been involved in a few early-stage studies for mining projects in which nearby bodies of water play a role in the design.    In Canada’s north there are thousands of lakes and rivers, so its not surprising to find mines next to them.
That got me thinking about how many other mines are in the same situation, i.e. projects that may be located very close to, or within, a lake, river, or ocean. Hence I have compiled a short list of a few such mines.
I have been directly involved with some of those in the list, while others are only known to me with limited detail. Some mines I had never heard of before, but their names were provided to me by some Twitter colleagues.
My observation is that building a mine within, or adjacent to, a body of water is nothing new and this has been done multiple times successfully.
Some of these projects may refer to the dams as “dikes”, “cofferdams”, “sea walls” but I assume they are all providing roughly the same function of holding back water for the life of the project.  They are not viewed as permanent dams.
This is Part 1 of a two-part blog post. Part 1 provides some examples of projects where water bodies were involved in the design. Part 2 provides a discussion on specific geotechnical and hydrogeological issues that would normally have to be examined with such projects.

Some Lake Mining Examples

The following are some examples of operating mines involving lakes. I have captured a few Google Earth images, unfortunately some have only low resolution vintage satellite imagery.

Diavik Diamond Mines, NWT

This is a project I was working on with in 1997 to 2000 while it was still at the design and permitting stage.  My role focused on pit hydrogeology and geotechnical as well as mine planning.
The project would require the construction of three dikes in sequence to mine four lakebed kimberlite pipes.
The three dikes were named after the associated kimberlite pipe being mined inside it; A154, A418, and A21.
The first dikes were built in 2002 and the last dike (A21) was completed in 2018.
The total dike length for the three dikes is about 6.2 km.
For those interested in learning a bit more about Diavik, I have posted an earlier article about the open pit hydrogeology there, linked to at " Hydrogeology At Diavik – Its Complicated".
Diavik mines

Gahcho Kue, NWT

This is a DeBeers diamond project was built in 2016 and required the construction of several small dikes to allow access for open pit mining.    The photos show the pre-mining situation and the site as it is today.   One can see the role the lake would play in the site layout and the need for multiple small dikes.
Gahcho Kue diamond mine

Meadowbank, NWT

This is an Agnico-Eagle gold mining operation built in 2010 that required a cofferdam to be built around one of their open pits (see image).
The total dike length is about 2000 metres.   I don't know much more about it than that unfortunately.
Meadowbank Mine

Cowal Gold Mine, Australia

Yes, a lake in Australia ! This is a former Barrick operation, now owned by Evolution Mining, and is another example where the mine is located within the shoreline of a lake (Lake Cowal).    I don't know much about this, the name was kindly provided to me by a colleague.
The total dike length appears to be about 3000 metres.
Cowal Gold Mine

Rabbit Lake Sask

The historical Rabbit Lake uranium mining operation required the construction of cofferdams around a few of their open pits.  They are now reclaimed and flooded.
Rabbit Lake uranium

St Ives Gold Mine, Australia

This is a unique situation in that several pits are located within an ephemeral (intermittent) salt lake and dikes were required to prevent pit flooding during wet season.
St Ives gold mine

Some River Mining Examples

The following are some examples of operating mines involving rivers.  Rivers provide a somewhat different design challenge since they have flowing water, who's volume and velocity may change seasonally.    Constrictions in the river created by the dike itself may increase the flow velocity and erosion potential.

Gorevsky Mine, Siberia

This lead-zinc operation has an orebody that extends into the Angara River.
This mine has built a fairly large cofferdam into the river, and is currently mining a large pit within it.  The total cofferdam length appears to be about 4000 metres.
It would be interesting to see how close the pit will get to the cofferdam.   We'll check back in a few years.
Gorevsky Mine

BHP Suriname Bauxite Mine

This is a project I was involved with several years ago.  The bauxite deposit extends beneath the Suriname River and the goal was to mine as much ore as possible.
Given the flow rates in the river, especially during the wet season, it would be difficult to maintain a cofferdam out into the river.
The shoreline overburden consisted of sands and soft clays, so the decision as made to construct a sheet piling wall along the river bank to protect the pit from river erosion.   This was mined out successfully and eventually reclaimed.
Suriname Bauxite Mine

McArthur River, Australia

In situations where the river (creek) is small enough and the topography allows, one can divert the entire river around the mine.
There are several examples of this in Canada and elsewhere.  Here’s the McArthur River lead-zinc mine in Australia, where they channeled their small river around the open pit.
McArthur river diversion

An Ocean Example

There are some examples of mining near the ocean. These operations may need to deal with large storm water level surges and large tidal fluctuations.   The Island Copper Mine on northern Vancouver Island is an example where they mined close to the shoreline but not actually into the ocean (as far as I am aware).

Cockatoo Island Mine, Australia

This interesting iron ore mine has an ore zone that dips 60 degrees, is 35 metres wide, with a strike length of more than one kilometre.
A sea wall was constructed to prevent any tidal water from entering the open pit that was to be mined, with reportedly high tidal fluctuations there.
Cockatoo iron mine

Conclusion

As one can see, the idea of mining into a body of water is nothing new.   Its not a preferred situation, but it can be done economically and safely.   The technical challenges are straightforward, and engineers have dealt with them before.  However there also are instances where the design could not economically address the water issue, and thus played a role in the mine not getting built.
If you know of other mines not listed above that have successfully dealt with a water body, please let me know and I can update this blog post.
This concludes Part 1.  Part 2 can be read at this link " Mining Under Lakes – Part 2: Design Issues" discusses some of the concerns that engineers need to consider when building a mine in these situations.

Pantai Remis tin mine

Finally, the worst-case scenario is shown in this grainy video of a tin mine (Pantai Remis Mine) pit slope failure.  It seems they mined too close to the ocean.  Watch to the end, its hard to believe. Its looks like something out of a Hollywood disaster movie.

 

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Steeper Pit Slopes Can Save Money

We likely have all heard the statement that increasing pit wall angles will result in significant cost savings to the mining operation.
What is the potential cost saving?
The steeper wall angles reduce waste stripping volumes, which also provide other less obvious benefits.
I was recently in a situation where we undertook some comparative open pit designs using both 45 and 50 degree inter-ramp angles (“IRA”). I would like to share some of those results and discuss where all the benefits may lay.

Comparative Pit Designs

In this project, four separate open pits were designed with 45 and 50 degree IRA’s in an area with hilly topography. Some of the pits had high walls that extended up the valley hillsides. Its not hard to envision that waste stripping reductions would be seen along those areas with steepened walls.
The results of applying the increased  inter-ramp angle to each of the four pits is shown in the Bar Chart. Note that the waste reduction is not necessarily the same for each pit.  It depends on the specific topography around each pit.
However, on average, there was an overall 15% reduction in waste tonnage.
The Table shown below presents the cumulative tonnage for all four pits. The 50 degree wall results in a waste decrease of 25.4 million tonnes (15%), with a strip ratio reduction from 5.8:1 to 5.0:1.
There is also a very minor decrease in ore tonnage. This is because the 50 degree slopes did lose some ore behind the walls that is being recovered by the 45 degree slope.
In both scenarios the project life would be about 10 years at an assumed ore processing rate of 3 Mtpa.

4 Positive Impacts of Steeper Walls

In general one can typically see four positive outcomes from adopting steeper pit walls. They are as follows:
1. Cost Savings: The waste tonnage reduction over the 10 year life would be about 25.4 million tonnes. At a mining cost of $2.00/tonne, this equates to $50.8 million tonnes spent less on stripping. This could move the project NPV from marginal to profitable, since most waste is normally stripped towards the front part of the mining schedule with less discounting.
The next time you are looking at the NPV from an open pit project, take a quick look to see if the pit slope assumptions are conservative or optimistic. That decision can play a significant role in the final NPV.
2. Equipment Fleet Size: Over the 10 year life, the average annual mining rate would range from 20.5 Mtpa (45 deg) to 18.1 Mtpa (50 deg). On a daily basis, the average would range from 56,100 tpd (45 deg) versus 49,700 tpd (50 deg). While this mining rate reduction is not likely sufficient to eliminate a loader, it could result in the elimination of a truck or two.   This would have some capital cost saving.
3. Waste Dump Size: The 15% reduction in the waste tonnage means external waste dumps could be 15% smaller. This may not have a huge impact but could be of interest if waste storage sites are limited on the property. It could have a more significant impact if local closure regulations require open pit backfilling.
4. Pit Crest Location: The steeper wall angles result in a shift in the final pit crest location. The Image shows the impact that the 5 degree steepening had on the crest location for one of the pits in this scenario.
Although in this project the crest location wasn’t critical, there are situations where rivers, lakes, roads, mine facilities, or public infrastructure are close to the pit.  A steeper wall could improve ore recovery at depth while maintaining the same buffer setback distance.

Conclusion

Steeper pit walls can have multiple benefits at an open pit mining operation. However, these benefits can all be negated if the rock mass cannot tolerate those steeper walls. Pit wall failures could be minor or they could have major impacts. There are the obvious worker safety issues, as well as equipment damage and production curtailment concerns with slope failures.  Public perception of the mining operation also comes into play with dangerously unstable slopes.
Steepening of the pit walls is great in theory, but always ensure that geotechnical engineers have confirmed it is reasonable.
It is relatively easy to justify spending additional time and money on proper geotechnical investigations and geotechnical monitoring given the potential slope steepening benefits.
When designing pits, there is some value in looking at alternate designs with varying slope angles to help the team understand if there are potential gains and how large they might be.
In closing, I previously wrote a related blog post about how pit walls are configured to ensure safe catch bench widths and decisions as to whether one should use single, double, or triple benching. That earlier post can be read at this link. Pit Wall Angles and Bench Widths – How Do They Relate?
Feel free to share your personal experiences if you are aware of other benefits (or even downsides) to steeper pit walls that I did not mention.
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Games People Play

Are you a board game player?  Personally I am not.  However I understand there is a huge board game community out there. These game enthusiasts meet in small coffee shops and attend large gaming conventions. I read that 35% of Americans say they play board games several times a month. Think about how many board games you may have laying around in your own place, even if you’re not a hard core gamer.
Recently an avid board game player sent me an email asking if I was aware that there several mining related games. That was a surprise to me. Who would create a board game about mining and for what demographic market?
Curiosity took over and I had to check out the links sent to me on the Board Game Geek website. Here’s a few of the games and what they do.   Now that Germany may be moving back into coal, the first three games may come back into fashion.  The 4th game listed is a bit of a head scratcher.

A few of the mining board games

Game 1: Haspelknecht: The Story of Early Coal Mining (2015)
This game is part of a coal-mining game trilogy created by Thomas Spitzer in Germany. The players take the role of farmers with opportunities to exploit the presence of coal in the Ruhr region of Germany. During the game, players acquire knowledge about coal, extend their farms, and dig deeper in the ground to extract more coal.
Players must select the correct tasks while being mindful of quickly accumulating pit water, for it can stall efforts and prevent extraction of coal.  The game info link is here.
Game 2: The Ruhr: A Story of Coal Trade (2017)
In the second game of Spitzer’s trilogy, you are still in the Ruhr region in the 18th century, at the beginning of the industrial revolution. The Ruhr river presented a transportation route from the coal mines. However, the Ruhr was filled with obstacles and large dams, making it incredibly difficult to navigate.
The players transport and sell coal to cities and factories along the Ruhr river in the 18th and 19th centuries. In the beginning, players have access only to low value coal but can gain access to high value coal. The players also build warehouses, locks, and export coal to neighboring countries in the pursuit of the most points.
The info link is here.
Game 3: Schichtwechsel: Die Förderung liegt in deiner Hand (2021)
This game may still be in German text only. Players are the administrator of a coal mine, and experience competition while living through a piece of Ruhr Valley history.
They bring coal and overburden from underground to the surface, let the miner go through a “shift at the colliery”, produce coke, or build the typical colliery settlements.
The info link is  here.
Game 4: The Cost (2020)
This game takes on a more negative view of the mining industry. It is described as “A bold take on the economics in the brutal industry that is asbestos.” The game players assume the role of a global asbestos company.
Players make their fortune in mining, refining, and shipping. Whoever ends the game with the most money wins. The last part of the description is the gem “When players mine or refine asbestos, they must choose to either maximize profits for short-term gains or sacrifice their hard-won money to minimize deaths, thus sustaining the industry.” That’s every mining executive’s dilemma; profits or deaths.   The info link is here.
Some of these game boards look more complicated than the actual industry. To find other games you can go to the Board Game Geek website and search for different themes. Most mining games listed there are not realistic but are more about dwarves mining gems or they just have an activity called “mining”.   Here’s one called Copper Country.

Free Excel Mining Game

In 1983 my brother, at the age of 10, got his Commodore 64 computer and was eagerly learning to program in BASIC. He was always looking for ideas on what he could write programs about. I had graduated from McGill in Mining Engineering a few years earlier, so I suggested he write a simple computer game about mining as his project.
I provided him with the logic and in no time he had it written and functioning. That game is long gone, likely at the bottom of a landfill stored inside the chips of his Commodore 64. Some 40 years later, my brother is still coding as a software development manager. I guess I managed to convince him the mining industry wasn’t a career path.
Over the last few months I decided to learn VBA (Visual Basic for Applications). VBA is a programming language the works with Microsoft Office products, mainly Excel.
I always enjoyed programming. In university we wrote FORTRAN programs using stacks of punch cards to feed the machine the code. I had also learned the BASIC language, from my brother’s VIC 20 and Commodore computers.
A good way to learn something is to watch a few pf the many tutorial videos on YouTube. An even better way to learn VBA is by taking on an actual coding project from scratch. So, what worked 40 years ago, would work again. Rather than write something useful, I decided to re-write the mining game from 1983, albeit enhanced with the Excel application capability and more years of personal mining experience.
This coding process would force me to learn how to write code, figure out logic, create loops, if-then statements, and handle debugging. Already knowing Excel makes the entire process easier.  Combining Excel functionality with VBA delivers capabilities that would have been difficult to do in BASIC alone.  Note: It appears that BASIC is no longer in use, having been replaced by Python as the preferred programming language.

Download it.. if curious

If you are curious about the capability of VBA, the Excel mining game can be downloaded. A descriptive overview of the game is included in the PDF file at this link.

Junior Mining CEO game screenshot

The very simple game is called Junior Mining CEO. The object is to find gold, raise the share price, and not go bankrupt given the pitfalls that often befall the mining industry. The input parameters have a lot of optionality, although I have protected the macro code itself for this edition. You can borrow money and issue equity to fund your mining activity.
The Excel file can be downloaded at this link. The was written using Excel 365 but it may also work on older versions of Excel.
You will first need to save the game to your computer to run the macros. Since there are macros, many computers will disable such Excel files because they can contain viruses. You may need to toggle the file Properties in File Explorer to unblock the file to allow the macros to run.
Is there a junior mining corporate sponsorship opportunity here? Sure. For a small fee, I will add your company logo to the game and pre-set all the input parameters so that everyone is a big winner all the time.

Conclusion

As mentioned in a blog from a few months ago, “ A Junior EIT Mining Story” some gamification of mining may help introduce and educated people on the industry. Augmented reality (AR) and Virtual reality (VR) are both technologies that can be used to help reach out to the younger generations (I’m not talking about investor outreach).
How about a new board game that does to mining what Monopoly did to real estate investing? Look at real estate prices today, no doubt being influenced by everything we learnt playing Monopoly as kids.
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Clays and Mining – Friends or Foes?

Overburden is a generalized termed used to describe unconsolidated material encountered at a mine.  It can consist of gravels, sands, silts, and clays and combinations of each. Usually overburden is not given much focus in many mining studies. Very often, the overburden as a unit, is not adequately characterized.
This blog will explain why proper characterization can be an important issue,  particularly the clay component.
I also want to share some personal mining experiences with clays, all types of clays.  There is more to them than meets the eye; a fact often not apparent to those involved only in hard rock mining.
Clays have unique geotechnical properties that can make for challenging situations and require special consideration in project design.   Many simply view clay as a sticky cohesive material – no big deal. So let’s examine this a bit further. I tried to avoid geotechnical lingo where possible, since this blog isn’t being written for geotechnical engineers.
There are several types of clays, or clay-like materials that can be encountered in mining. Here are the ones that I have been lucky (or unlucky) enough to have dealt with over the years.
  • Normally consolidated clays
  • Over-consolidated clays
  • Sensitive (or quick) clays
  • Swelling clays
  • Saprolite clays
  • Kimberlite clays (muds)

What are the challenges?

Each of the clays listed above can be found in different locations, have unique properties, will behave differently, and can create specific mining challenges.   Clays can also cause problems in process plant circuits, but that is a subject outside my area of expertise.

Normally consolidated clays

These are the clays most people are familiar with, i.e. a sedimentary deposit of very fine particles that have settled in a calm body of water.   Normally consolidated clays are generally not a problem, other than having a high moisture content.  As such, they can be very sticky in loader buckets, truck boxes, and when feeding crushers.
When wet, they can become sloppy and difficult to handle efficiently.  They can creep and run when placed into waste dumps.  For these reasons, engineers must be aware if a large proportion of the overburden will consist of clays so they can avoid surprises.

Over-consolidated clays

These clays have undergone greater vertical compression in their history than in their current condition.  For example, perhaps they were once pre-loaded and compressed by a mile of glacial ice sheet during an ice age, which has subsequently melted.
Clays in general consist of very fine plate like particles, as shown in this sketch.   In over-consolidated clays, these particles have been flattened and tightly compressed as in the right image.   The result is that the clay may be dense, have a good cross bedding shear strength, but very low shear strength along the plates.  This characteristic is analogous to the lubricating properties of graphite, which is facilitated by sliding along graphite plates.
My experience in working with over-consolidated clays was at the Fort McMurray oil sands mining operations.  In that region the Clearwater clays formed part of the overburden sequence above the oil sands.  Stripping these clays with trucks and shovels was not exceptionally challenging.  They had low moisture content and were stiff.   The challenge really came when needing to build on top of them, for example building a waste dump or tailings dam.
The cross-bedding shear strength was good, with peak friction angles exceeding 25 degrees.  However after any creep or deformation, the peak shear strength was gone and the residual friction angle would now control stability.   The residual friction angles could drop as low as 6 degrees (very weak) and, upon surcharging the clay could maintain high internal pore pressures.   Due to these factors it was not uncommon to see tailings dams or waste dumps with 15:1 (H:V) downstream slopes.  This compares to the 3:1 slopes one may normally see at hard rock mine sites.
Building a 15:1 dam or dump is much less volume efficient than building a 3:1 embankment.  It also doesn’t take much instability to cause an embankment to creep along a foundation with only a 6-degree friction angle.  Hence the over-consolidated clays presented a unique engineering challenge when working in the oil sands.

Sensitive (quick) clays

Referring to the clay particle sketch shown above, quick clays represent a card house structure (on the left image).  These clays were often deposited in a quiet marine environment, where electrical charges prevented the clays from settling uniformly.  Instead, the clay particles tend to stack up like a house of cards.  The large void spaces are filled with water, whereby moisture contents can exceed 100% by weight.
When these clays are disturbed by vibration or movement, the house of cards structure collapses.  Combined with the excess void water, these clays will flow…. and flow a lot.   This video shows a slope failure in quick clays in Norway.  Try to stop that failure once it has initiated.
My experience with sensitive clays was at the former BHP bauxite mining operations along the northern coast of Suriname.   There were Demerara clay channels up to 20m thick over top of many of their open pits.   The bucketwheel excavators used for waste stripping would trigger the quick clay slope failures, sometimes resulting in the crawler tracks being buried and unfortunately also causing some worker fatalities.
I recall walking up towards a bucketwheel digging face as the machine quietly churned away.   About 70 metres from the machine, we would see cracks quietly opening all around us as the ground mass was starting to initiate its flow towards the machine.   Most times the bucketwheel could just sit there and dig.  Instead of the machine having to advance toward the face, the face would advance towards the machine.
To address the safety issue, eventually mine-wide grids of cone penetration tests were used to define the Demerara clay channels.  Dredges were then brought in to remove these channels before allowing the bucketwheels to strip the remaining sands and normally consolidated clays.

Swelling clays

In some locations, mines may contain swelling clays.  The issue with these clays is that they can absorb water rapidly, swell by 30%, and become extremely soft to operate on.  If they form part of the ore zone and find their way to the tailings pond, one may find they don’t want to settle out in the pond. Water clarification and clean water recycle to the plant can become an operational issue.   Mineralogy tests will indicate if one has swelling clays (smectites, montmorillonites, bentonite).  The swelling clays do have a functional use however, discussed later.

Kimberlite clays (muds)

The formation of the diamond deposits in northern Canada often involved the explosive eruption of kimberlite pipes under bodies of water. The lakebed muds and expelled kimberlite by the eruption would collapse back into the crater, resulting in a mix of mud and kimberlite (yellow zones in the image).   This muddy kimberlite could be soft, weak, and difficult to mine with underground methods.
Normally as one descends deeper into the kimberlite pipe,  the harder primary kimberlite dominates over the muddy material.   An upside is that the muddy kimberlite can be scrubbed fairly easily during processing, with the very fine clay particles being washed away.

Clays can’t be all bad?

Encountering clays at a mine site can’t be always negative?  There must be some benefits that clays can provide?   Well there are a few positive aspects.

Saprolite clays 

At many tropical mining operations (west African gold projects for example) the upper bedrock has undergone weathering, resulting in the fresh rock being decomposed into saprolite.  This clay-rich material can exceed 50 metres in thickness, can be fairly soft and diggable without blasting.   This is an obvious mining cost benefit.
As well, grinding circuits can easily deal with saprolite.  For example, if a 1000 tpd grind circuit is designed for the underlying deeper bedrock, it may be able to push through 1400 tpd of saprolite.   This would yield a 40% increase in mill throughput for little added cost.  This will boost early gold production.  However as the blend of saprolite to fresh rock declines over the years as the pit deepens, the plant throughput will decrease to the original design capacity.
One concern with saprolite sometimes is its sticky nature.   A truck load of saprolite ore dumped on a crusher grizzly may just sit there.  Possible some prodding or water flushing may be required to get it moving.  Nevertheless, this is normally an easily resolvable operating issue.

Clay core dams

One of the ways miners build water retention or tailings dams is to use mined waste rock.   The issue with this is that a dam built solely with waste rock will leak like a sieve, which can lead to piping failure.  One solution is to build an internal clay core in the center the dam to act as a seepage barrier.   Having on-site access to good quality clean clay fill is a benefit when such dams are required.   If the clay fill isn’t available at site, then more complex synthetic liners or internal seepage control measure must be instituted.
Compacted clay fill can also be used as a pond liner material for water retention ponds.
One can also purchase rolls of geosynthetic clay liners (GCL), whereby a thin layer of dried swelling clay is encapsulated between two sheets of geotextile.  Once the liner is laid out and re-hydrated with water, the clays swell and will act as an impervious liner.   The installation approach is somewhat simpler than for HDPE liners and such liners can be self-healing if punctured.  A downside is that the transport weight of these GCL liners can be significant.
See, there are some positives with having clay at site.

Conclusion

All clays are not the same.  The mining of clays can create unique challenges for mining engineers and operating personnel.   Whenever I see study happily mention that their open pit mine waste consists of “free digging overburden”, I say to myself “Be careful what you wish for”.
One must ensure that the overburden is properly characterized, even though it may be viewed as an unimportant or uninteresting material.  Determine whether it consists of gravels, sands, silts, tills, or clays, or combinations thereof.   It can make a big difference in how it is mined, disposed of, and whether it can have any secondary uses on site.  In many studies that I have reviewed, the overburden tends to be forgotten and does not get the technical respect it deserves.
Please feel free to share any thoughts on your experience in working with clays.
We had to build mine haul roads across large swamps underlain by soft clays.   One option was to use geo-textiles or swamp vegetation to assist us.  Another option was to place the sand fill hydraulically.  You can read how we did this at the following blog post “Using Pumped Sand to Build Mine Roads“.
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A Junior EIT Mining Story

We know the mining industry is having trouble attracting talent in all sorts of disciplines, including operations, technical, and supervision. Industry people have no shortage of ideas (right or wrong) on how this issue can be addressed in the future. Myself…I don’t really have any good suggestions.
Not long ago I was speaking with a 2020 graduate mining engineer (EIT = Engineer in Training). During our conversation, I was curious to know what attracted him to the industry and if he had any advice on how to reach out to others in his age group. I asked if he was willing to share his thoughts on my blog site. After all, who better to hear from than a recent graduate. He said “yes”, so for your interest, here is his story and his thoughts. (I decided to leave his name out of this article although he was not insisting on anonymity.)

So here’s the story (in his words)

Mining has been a part of my life for as long as I can remember. Being born in Sudbury, many of my family members have been, or are currently involved, in mining through a variety of occupations, including my father who I idolized. However, I never knew my true interest in the industry until my 11th-grade technology class. I had a teacher who was passionate about the mining industry, and he created a project that involved developing a very basic mine design.
Once I started the project, I realized that I enjoyed the design work, as it requires problem-solving which constantly stimulates the mind. After the conclusion of this project, I started doing my own research to expand my knowledge and realized that the financial side of mining had great interest to me as well. This led me at age 16 to start investing in the mining sector, which I continue to this day.
With this developed interest, and my family’s mining history, the decision to study Mining Engineering at Laurentian University was an easy decision. It allowed me to support my hometown and will allow me, given my career ambitions, to put this small school on the international map.
Before my first year of university, I had a summer job tramming at Macassa Mine in Kirkland Lake Ontario, which has been in production since 1933. My mentality was to get the boots on the ground and get the job done, whatever it took (with proper safety precautions of course). Using rail systems, dumping ore cars manually, jackleg drilling, etc. gave me the perspective that mining was archaic, mining was rough, and mining was only about the ounces.
Therefore, when I started the Mining Engineering program at Laurentian University in 2016, I already had a (somewhat negative) preconceived notion of the mining industry, but as my short career progressed, my opinions morphed into something different, something more positive.
Now that I have graduated and been employed for a couple of years, my perspective has changed. However, I feel that the perspective of the general population has not. People within mining have a (positive) bias and realize its importance in our everyday lives. This is showcased in the famous saying “if it is not grown, it is mined” and I believe that to get the industry to progress at an even faster rate, we need to get everyone on board.
It cannot be an industry that just takes from the Earth, it needs to be seen as one that values sustainability, supplies the world with required goods, and creates jobs with high employee satisfaction. Although this has started with companies taking more of an interest in stakeholder value and employee job satisfaction, based on my limited years in the industry, there is still lots of room for growth.
To change the negative view around mining, I believe the main focal point should be electric equipment and the ability for remote operation/work. With all this newly developed technology at our fingertips, I know that future operations will be safer and more sustainable, which should be better portrayed.
The battery-electric equipment will surely increase employee satisfaction since I know firsthand that one of the worst feelings as a worker is to have a scoop operating in a heading that is already 25+ degrees. It will also create more sustainability since the industry can transition from being reliant almost solely on fossil fuels.
In addition, I believe that remote equipment operation is not being used to its full capacity or explained to the younger generation. Right now, there has been equipment running in Canada that was operated in Australia. What is stopping mines from having equipment operators all over the world in urban office spaces or out of their own homes? I believe that for a company to visit a high school, or even a trades school, to sell the idea that you can operate a massive piece of equipment from the creature comforts of home, almost like a real-life video game, would be quite compelling to this audience.
Even creating a mining simulation video game where you can run through a story of being a manager, excavator/scoop operator, truck driver, etc. would get the thought of mining brought into the coming generations at a younger age. This would increase the talent pool from the more typical operator because more and more youth are getting skilled at remote operation through video games due to their increased screen time.
Not only equipment operators, but technical staff could be made fully, or partially, remote. When I describe my job to my (non-mining) peers, many are interested since mining is a fast-paced, stimulating, and rewarding industry.
But as soon as I describe the remote nature of the work, many young professionals, or high school students, get turned away. Therefore, showing teenagers, through school presentations or workshops, that a technical career in mining can lead you down so many avenues (scheduling, ventilation design, drill & blast, etc.) would pique their interest, but I believe adding the ability to work remote, with some occasional travel, would drive the point home.
InternPeople get comfortable and people are afraid to leave home, so selling a career that allows for boundless flexibility in job tasks and constant stimulation while living wherever you desire could allow a shrinkage in the current technical gap.
Overall, the mining awareness and outreach (approach) is still old school. Getting to youth and teenagers through various media streams could be the key to getting engagement from not only the current mining towns but larger urban centres as well.
I mentioned a mining simulation video game previously, but what is stopping us there. Many of my peers, and youth younger than myself, are entering the professions of doctors, lawyers, finance, or criminal investigators.
I might be wrong, but, intriguingly, these are industries are the base professions glorified on TV. Why not develop more TV shows based on mining? I know that there would be some population interest considering many people ask me if the gold we mine underground looks like what comes out of the pans in Yukon Gold Rush.
So do I think the mining industry is archaic…. not anymore.
Do I think the mining industry is rough… somewhat.
Do I think it is only about the ounces…., yes, since a mine will not run any other way.
However, I believe that there has been more importance placed on employee and stakeholder satisfaction. So, with more time, and more engagement from the public of all ages, I think this industry can have a bright future ahead.
END

Conclusion

Firstly, I would like to thank this engineer for taking time to write out his well formed thoughts, and for allowing me to share them.
Many of the mining people I meet are following along in family footsteps. No surprise there. However, the industry cannot rely on that farm system alone. It should be reaching out to broader society, although that may require some out-of-the-box thinking. People’s attitudes and personalities are different today than they were 20 years ago.  Many different doors are being held open as career options.
The discussion above has some interesting ideas from a person who would be the target of outreach efforts. It likely will take more effort than simply telling people “Hey, your iPhone uses metal, therefore mining is good, and you should work in mining”.
This guest blog post is an industry perspective from the view of a young engineer.  If you interested in the perspective from a seasoned engineer who has worked around the globe, check out the book that is described in this blog post “Life as an Engineer – Read All About It“.
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Hard Rock Continuous Cutting – Reducing Drill & Blast

Several years ago I worked in the Saskatchewan potash industry where I grew my appreciation for continuous mining systems. Some of the key benefits were the high productivity per man-hour and the safe operating conditions.  On a 12 hour nightshift, with a crew of 16 people we could mine over 9,000 tonnes of ore.  Productivity is  likely even higher now with the larger machines.   Therefore, since that time, I have always kept an eye out for when similar technology can be applied in hard rock mining.
One of the research areas we are seeing these days is the development of continuous cutting technology for hard rock mine development.  The idea is to replace the conventional drill & blast approach with something more efficient and safer.  No need to deal with explosives, noxious gases, shatter the wall rock, or have personnel scale their way under loose conditions.
Recently I was contacted by someone associated with Robbins asking if I was aware of their MDM5000 mine development technology.  I wasn’t aware of it, but I wondered if there finally is a light at the end of the tunnel.

4-rotor miner

In Saskatchewan potash the entire mining operation relies on track-mounted continuous miner technology.   The miners are connected directly to the shaft area using a network of conveyors, up to 10 km worth of conveyor.
The potash miners are able to undertake both development work and production mining whilst connected to conveyor.
From time to time they will rely on shuttle cars, scooptrams, or grasshopper conveyors for small development tasks. A roadheader may also be available for localized ground stabilization.
All of this mechanical cutting is done in potash (sylvinite rock), considered a soft rock with a compressive strength of 20-40 MPa. For comparison, hard rock can have compressive strengths exceeding 250 MPa.

Two approaches in hard rock

Hard rock piloting trials are underway at a few operations, using different vendors with different equipment.  These trials include companies like Komatsu, Robbins, Sandvik, and Epiroc.  Each are testing their own equipment and cutting technologies.
The hard rock cutting approaches generally fall into two camps.

Roadheader style

There are the track mounted roadheader style cutters, typically with a movable arm used to shape the excavation.  Any excavation shape is possible.

Tunnel Borer Style

Then there are the tunnel borer styles, where the machine propels itself with hydraulic shoes and the opening shape is based on the machine configuration. Normally a circular shaped opening is the result.
The roadheader style cutter is normally restricted to softer rock (< 50 MPa) while the tunnel borer is capable of much harder rock (200-250 MPa).

Robbins MDM5000

One system that peaked my interest is the Robbins MDM5000 because it can both cut hard rock and create a rectangular opening. Speaking with the vendor, this unit uses shoes to propel itself while cutting a rectangular shaped opening about 5m x 4.5m in size (see image).  A rectangular shape is preferred to the circular opening whereby the floor invert must be backfilled to provide a level operating surface.

MDM5000 opening shape

The MDM5000 configuration and advance rate allows the installation of ground support and utilities behind the advancing face.   Water sprays and dust collectors help to maintain visibility and air quality at the working face.
The Robbins unit is best suited for long straight drives although reportedly it can turn curves with 450-m radius.  Tighter turns may be feasibility in the future by tweaking the machine design.   Interestingly driving a drift uphill is easier than driving downhill due to the more efficient cuttings removal capability.
The MDM5000 unit can be linked to a Robbins conveyor system, which includes a head drive and an extensible belt storage unit that can feed out the conveyor belt as the machine advances forward.  This operation is similar to that used in the Saskatchewan potash industry.

Robbins MDM5000

A Robbins machine has been in operation at the Fresnillo mine for several years with favorable results (check out the link here).
One nice thing about disc cutters is that they can accommodate variable rock types (softer and harder) while road headers can be hindered by hard rock zones.  Roadheaders require a bit more consistency in rock quality.
Continuous cutting systems, such as the MDM5000, can be combined with vertical conveying technology, leading to safe and rapid development (>200m per month) in the right situation.

Conclusion

No doubt that we will eventually see more application of hard rock continuous cutting technology in the right situations.  The Stillwater Mine in Montana has been using a Robbins tunnel borer for years for development tasks.
No matter how well these new systems perform, there will still be some limitations.  This means the conventional drill and blast development will always be around.  However, keep your eyes on this mining technology sector as improvements in cutter head design and equipment mobility continue to evolve.
Coincidentally International Mining (Nov-Dec 2021) recently published an in-depth article on the various systems being looked at.  The link is here.

 

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Pit Optimization – More Than Just a “NPV vs RF” Graph

In this blog I wish to discuss some personal approaches used for interpreting pit optimization data. I’m not going to detail the basics of pit optimization, assuming the reader is already familiar with it .
Often in 43-101 technical reports, when it comes to pit optimization, one is presented with the basic “NPV vs Revenue Factor (RF)” curve.  That’s it.
Revenue Factor represents the percent of the base case metal price(s) used to optimize for the pit. For example, if the base case gold price is $1600/oz (100% RF), then the 80% RF is $1280/oz.
The pit shell used for pit design is often selected based on the NPV vs RF curve, with a brief explanation of why the specific shell was selected. Typically it’s the 100% RF shell or something near the top of the NPV curve.
However the pit optimization algorithm generates more data than just shown in the NPV graph.  An example of that data is shown in the table below. For each Revenue Factor increment, the data for ore and waste tonnes is typically provided, along with strip ratio, NPV, Profit, Mining cost, Processing, and Total Cost at a minimum.
Luckily it is quick and easy to examine more of the data than just the NPV curve.

In many 43-101 reports, limited optimization analysis is presented.  Perhaps the engineers did drill down deeper into the data and only included the NPV graph in the report for simplicity purposes. I have sometimes done this to avoid creating five pages of text on pit optimization alone, which few may have interest in. However, in due diligence data rooms I have also seen many optimization summary files with very limited interpretation of the optimization data.
Pit optimization is a approximation process, as I outlined in a prior post titled “Pit Optimization–How I View It”. It is just a guide for pit design. One must not view it as a final and definitive answer to what is the best pit over the life of mine since optimization looks far into the future based on current information, .
The pit optimization analysis does yield a fair bit of information about the ore body configuration, the vertical grade distribution, and addresses how all of that impacts on the pit size. Therefore I normally examine a few other plots that help shed light on the economics of the orebody. Each orebody is different and can behave differently in optimization. While pit averages are useful, it is crucial to examine the incremental economic impacts between the Revenue Factor shells.

What Else Can We Look At?

The following charts illustrate the types of information that can be examined with the optimization data. Some of these relate to ore and waste tonnage. Some relate to mining costs. Incremental strip ratios, especially in high grade deposits, can be such that open pit mining costs (per tonne of ore) approach or exceed the costs of underground mining. Other charts relate to incremental NPV or Profit per tonne per Revenue Factor.  (Apologies if the chart layout below appears odd…responsive web pages can behave oddly on different devices).

Conclusion

It’s always a good idea to drill down deeper into the optimization output data, even if you don’t intend to present that analysis in a final report. It will help develop an understanding of the nature of the orebody.
It shows how changes in certain parameters can impact on a pit size and whether those impacts are significant or insignificant. It shows if economics are becoming very marginal at depth. You have the data, so use it.
This discussion presents my views about optimization and what things I tend to look at.   I’m always learning so feel free to share ways that you use your optimization analysis to help in your pit design decision making process.
As referred to earlier, there is a lot of uncertainty in the input parameters used in open pit optimization.  These might include costs, recoveries, slope angles and other factors.  If you would like to read more, the link to that post is here.  “Pit Optimization–How I View It”.
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