Articles tagged with: Mine Engineering

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, instead assuming the reader is 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 curve.
However the pit optimization algorithm generates more data than shown in the NPV graph (see 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. It is quick and easy to examine more of the data than just the NPV.

In many 43-101 reports, limited optimization analysis is presented.  Perhaps the engineers did drill down deeper into the data and merely included the NPV graph for simplicity purposes. I have sometimes done this to avoid creating five pages of text on pit optimization alone. 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, I also prefer to examine the incremental economic impacts between the Revenue Factors.

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 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.

 

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Stories from 3000 Feet Down – Part 2

This is Part 2 of my story about working at a potash mine in Saskatchewan. If you haven’t read Part 1, here is the link to it “Stories from 3000 Feet Down – Part 1“.  It’s best to read it first.

Flooded Russian Potash Mine

As mentioned in Part 1, water seepage and potential leaks were always an operational concern. There were essentially two main fears; (i) that a borer would tunnel into an unforeseen collapse zone (i.e. an ancient sink hole) or (ii) that long term deformations in the mine would result in cracking of the overlying protective salt layer resulting in water inflow.
Any time wet spots were seen on the roof, the mining engineers or geologists were called out to take a look.
Sometimes the wet area was due to pockets of interstitial moisture. Other times it might have just been oil from a blown hydraulic hose on the borer. Wet spots all look the same when covered in dust.
Water ingress was such a concern in that an unmined pillar of 100 ft. would be left around all exploration core holes. These 3,000 ft. holes were supposedly plugged after drilling but you could never be certain of that. Furthermore, you couldn’t be certain of their location at potash depth. Hence, we didn’t want to mine anywhere close to them.
The process plant also had several water injection wells whereby excess water was injected into geological formations deep below the potash level. We left a 700 ft. pillar around these injection wells.
Carnallite is a magnesium salt that we encountered in some areas of the mine. It was weaker than the halite, resulting in more rapid room closure and deteriorating ground conditions. It was also hydroscopic and could absorb moisture out of the air. Sometimes we would water the underground travel ways to mitigate dust issues. With the resulting high humidity sometimes the carnallite areas might start to drip water. This resulted in another call to the engineers to come out and investigate.

Working on a production crew brings new learnings.

My period as a production foreman was great. When first assigned to this role, I was less than enthusiastic but when it was over 6 months later, I appreciated the opportunity. As a production foreman, we had a crew of 15 to 20 people responsible for mining 9,500 tons over a 12 hour shift. There was one foreman in the underground dispatch room and one face foreman travelling around supervising the borers and inspecting conveyors.
Ore grade control was a key responsibility of both the borer operator and mine foreman. The goal for the borer operator was to cut the roof even with the top of the high grade potash bed. The goal for the foreman was to ensure the operator was maintaining his goal. However, when looking at the potash face all the beds generally looked the same, especially when they are dusty.

Feel the glow

One key distinction was that the middle low grade bed had a higher percentage of insol clays. It was used as a marker bed. You would take your hardhat lamp and run it down against the wall. In the upper high grade potash bed, the cap lamp would have a halo, not unlike like the glowing salt rock lamps you see in stores. In the marker bed the halo would disappear, like putting a lamp against a rock wall.
Thus using the lamp you could identify the top of the marker bed, which was supposed to be kept 3.5 to 4 ft. off the floor. If borer operators were consistently more than 12 “ inches off optimal level, they could be written up (i.e. reprimanded). This was one of my tasks as foreman and I always hated doing it.
Lasers were used to guide the borer in a straight line but the marker bed was used for vertical control. Nowadays the use of machine mounted ore grade monitors is an option. By the way, if you’re looking at a potash project, be aware of the grade units being reported. A previous blog discusses this “Potash Ore Grades – Check the Units”.
The underground conveyor network consisted of 32” conveyors, feeding onto 42” conveyors, feeding on to 48”or 54” conveyors. Typically the length of the larger conveyors was in the range of a mile (5,000-6,000 ft.). The conveyors were all roof mounted, allowing for easy clean up underneath. Roof mounting was critical. Even some of the 600 hp conveyor drive stations were roof mounted, structure and all.

Conveyors were the reason for my one and only safety incident.

4-rotor side pass

4-rotor borer side pass

First a little background. The underground ore storage capacity at K1 was about 3,000 tons. If the underground bins were filled anytime during a shift, the entire conveyor system would automatically trip off; all 10 miles of it. If you happened to have a mile long conveyor shut off while it was loaded to the brim with ore, good luck starting it up again.
If this happened, there were two choices. You could bring in the operators from the borers to start shovelling off the conveyor until it was unloaded enough to re-start. This option went over like a lead balloon with the crew.
The other not so great option was to use a scooptram to lift the belt counterweight up to reduce the belt tension. This allows the drive pulleys to start spinning. Then by lowering the counterweight back down perhaps the drives can inch the belt along. The idea being that possibly the momentum of the moving mass of potash would keep it moving. The downside is that you could break the belt. There was nothing worse than having to tell the cross shift they have both a stalled conveyor and a broken belt. Have a good shift people.
At 4 a.m. working as night shift face foreman, I knew the underground bins were nearing full. Driving along I saw that the mainline conveyor was full and spilling over the sides. A worst case conveyor shutdown was imminent. I called the dispatch office to ask why he hasn’t been shutting down borers and conveyors but there was no answer.
After several unanswered calls I started to hustle back to dispatch. I remember driving alongside the conveyor thinking “don’t shut off, don’t shut off”. Just then a heavy duty service vehicle pulled out of a cross-cut and I plowed right into it. I wasn’t going very fast (~20 km), but when a Toyota Land Cruiser hits a steel plate truck, the Toyota is the loser. The front end crumpled, and I later was told that there was major frame damage.
I’m not sure why the other driver didn’t see my lights. I’m also certain his lights were not on. Nevertheless, I did get written up for this incident with a reprimand for my personnel file. The reprimander had become the reprimandee. I still insist that the incident wasn’t my fault.
The Land Cruiser laid around the shaft for a few days until it could be hoisted up to surface. Everyone coming on shift got to see it . Once it was hoisted up, it laid outside the headframe for a few more days for all the surface workers to see. Ultimately, this incident resulted in the underground miners giving me the nickname “Crash”.
This is where I’ll end the potash story.

Conclusion

Saskatchewan potash is a unique mining situation. It has its own history and mining method. The people are great and it’s a great place to learn the difference between engineering school and real life.

Mine Engineering Dream Team

By the way, I also learned that small town Saskatchewan loves their senior hockey league (20-30 year age range). The local towns compete all winter; sometimes combatively. It was Esterhazy vs. Langenburg vs. Whitewood vs. Rocanville vs. Stockholm vs. Moosomin.
One of the first questions asked in my job interview at Mosaic was if I played hockey. You see, the ideal candidate for the job would have been an ex-NHL player, preferably an enforcer, who is also a mining engineer. Gotta stack the local team.

 

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Stories from 3000 Feet Down – Part 1

Mosaic K1 Mine

Between 1985 and 1989 I worked at the Mosaic K1 potash mine in Saskatchewan, Canada.  I had roles that included mine engineer, chief mine engineer and production foreman.   This was relatively early in my work career so it represented an important learning period of time for me.  Each of these roles gave me a different perspective about a mining operation.   In this two part blog, I’ll share some stories relating to the uniqueness of potash mining.
The K1 mine is 15 kilometres outside the town of Esterhazy; about 2.5 hours east of Regina.  The mine is located at a depth of around 3,000 feet below surface, hence the title of this blog.  I will be referring to imperial units in this text because when the mine started production in the early 1960’s the maps and units were imperial and continued on through the 80’s.   At my previous job with Syncrude we used metric units, so it took some time to mentally recalculate back.
Geologically, the province wide Esterhazy Member was being mined, named after the town.  The layer was about 8 ft high.  In other regions of Saskatchewan they are mining a different potash layer (the Patience Lake Member) which is located a few feet higher up. These mines have slightly different mining conditions than we experienced in Esterhazy.  An example of their conditions is shown in the photo, where a parting layer can result in a roof collapse.
The Mosaic K1 and K2 mine shafts are interconnected at depth 6 miles.  The total expanse of the K1 and K2 underground workings probably extends laterally over 18 miles.  Back in the 80’s they said there were over 1,000 miles of tunnel underground and probably a lot more now since the mines are still operating.  There is now a new K3 shaft which allows access to more distant potash reserves.

4-rotor borer

Potash, also termed sylvanite, mainly consists of intermixed halite (rock salt) and sylvite (potash) crystals. There are also minor amounts of insolubles (clays) and sometimes carnallite (a magnesium salt).  Sylvite (KCl) is the pay dirt, salt is the gangue mineral.
The production rooms being mined were about 75 ft. wide and 5,000 ft. long.  That is quite a large span, not likely possible in many other rock types other than salt rock.  Overall, the mine would extract about 40-45% of the potash, leaving behind the rest in the form of large pillars to support the overlying rock mass.

2-rotor borer

The mines use continuous miners or borers. In the 80’s they were Marietta miners and now they use borers from Prairie Machine (read more here).   When I was there they were starting to transition from two-rotor to four rotor machines.   The small machines would carve out ~350 tph, and the large ones could do over ~700 tph.
All borers are directly connected to the massive underground conveyor network that moves the ore from the cutter head to the shaft in a continuous process.
Mining excavates an 8 ft. high room consisting of three layers of interest.  A 4.3 ft. layer of high grade ore, a 1.7 ft. layer of low grade ore, and a 2.5 ft. layer of moderate grade potash.  This adds up to 8.5 ft., so the miners try to take the best 8 feet.  The best head grade would be delivered if the borer cut exactly to the top of the high grade 4.3 ft. seam. One didn’t want to overcut into overlying salt or undercut and leave high grade behind in the roof.  I’ll have more on how grade control was done in Part 2.
The air temperature underground is about 25C, so it’s quite a pleasant short sleeve climate.  Around the cutting face, the electric motors, and cutter head, temps can reach into the high 30 degree range. Hot but better than freezing. The mine was dry with very low humidity.   Hence metal didn’t rust.  However, bring the metal to surface and it would rust in no time.
Potash is termed a plastic rock, in that it will deform slowly under stress.  Hard rock will build up stresses and erupt violently in a “rock burst”.  Potash will go with the flow and deform.  Pillars will compress vertically and expand horizontally.  Room heights can decrease over time as much as 6 inches over several weeks in higher stress areas.
Interestingly when the borer shut down and all was quiet, one could walk to the freshly cut face.   You could hear the potash walls gently crackling like rice krispies, as the potash expanded into the opening.  After all, a few moments earlier the potash was being laterally confined under a vertical stress of 3,000 psi. That confinement is suddenly gone.
Roof closure was always an issue.  In panels that were almost mined out, all the ground stress was being carried by the unmined ground.  When advancing into this ground, roof closure could be so rapid that once a borer finished a 4 to 6 week room, the miner operator would need to cut a few inches off  the roof to get back to the front of the room.  It took a couple of days to get the borer out, rather than during a regular shift.
In very high stress areas you could see the floor heaving up as pillars were compressing laterally. That’s when you really knew that mining panel was done and operations should relocate to a new area.
I recall inspecting some of the very old workings from 20 years past.  The rooms looked like new except the roof had compressed down to 5 ft. in height.   We’d have to drive leaning sideways out of the jeep because the steering wheel was literally scraping the roof.

Flooded Russian Potash Mine

The biggest fear in potash mining is water inflow and mine flooding. There can be a water bearing layer about 50-70 feet above the mining level.  A layer of salt rock separates the two.  If you have groundwater flowing through a dissolvable material, it’s no surprise that the pathway will keep getting larger and larger. That’s why any sign of water seepage is taken seriously and causes the engineering team to leap into action (more on that in Part 2).
At least one potash mine in Saskatchewan has flooded.  A couple of others have come close but have managed to deal with the water inflows.
Barren areas where the sylvite has been leached away and replaced with pure halite are called salt horses.   I never knew why and still don’t.
Exploration core holes could be a mile apart so salt horses would be encountered unexpectedly.  They could be 20 or 2,000 feet in size; you never knew which.   They were a nuisance and an economic penalty.  If you tried to tunnel through them, you would send a lot of uneconomic salt up the shaft to the mill (which they did not like). Conversely, pulling out and abandoning a room was a painful decision because it took that borer off line for a couple of days to relocate. It’s an unplanned outage, which is not good if another borer happens to be moving at the same time.
On occasion, a borer would abandon the room and move to the next room 50 feet away and could pass by the same area without seeing the salt horse.  Upon reflection, perhaps we should have persevered in the previous room and gone another 20 ft. and we might have been through it.  Often the mill manager hollering about the mill getting too much salt was a deciding factor in whether to abandon a room or keep cutting.
For a year or so,  I also worked as a production foreman.  It was an interesting role. How does a young mining engineer four years out of school tell  guys working underground for 20 years what they need to do?
This also leads to the time when I wrecked a Toyota Land Cruiser underground.   I’ll leave that account to Part 2.  I’ve rambled long enough for today.
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Two More Mining Productivity Innovations

The mining industry is always on the lookout for new innovations as it strives to keep up with other industries.  In that light, periodically I like to highlight new technologies that I become aware of.   I’m trying to help spread the word  about them, which in turn may assist them in their on-going growth and development.
In this article I want to briefly describe two hardware / software companies that are working on technologies related to mine equipment productivity.    In no particular order, the two companies are Loadscan and SedimentIQ.  SedimentIQ is more of a startup than Loadscan which has a longer operating track record.
These technology companies are both targeting the open pit and underground markets, looking to provide simpler and less costly productivity solutions. Their technologies may be well suited for small to mid tier mines that cannot afford or don’t require the comprehensive Minestar type fleet management systems.
For the record, I get no fee or commission for promoting these companies; I just like what they are doing.

Loadscan

Loadscan has been around for a few years, but I only became aware of it recently.  It is a technology that allows the rapid assessment of the load being carried in truck.  It does not rely on the use of load cells or weigh scales to measure the payload.
Instead Loadscan uses a laser scanner and proprietary software to three dimensionally map the surface of the truck payload and then calculate its volume.  The results will indicate how consistent and optimal truck loading is volumetrically.   One can then calculate the payload tonnage by applying a bulk density.
The Loadscan technology will assess whether trucks are being over or under loaded, whether the loads are off-centre, or whether there is excess carryback on the return trip.
Successive truck payloads can be tracked manually or with RFID tags.   A cloud based database and web based dashboard are used to store the data and summarize it. The output can include an image of each individual load.
What is interesting about this technology is that it is simple to install in an operation.  It does not require retrofitting of a truck.
Results are immediate.  Loadscan provided an example where a message readout board can let the shovel operator immediately know how well each truck was loaded, resulting in improved education and better performance efficiency.
One can also assess how much better shovel bucket factors are in well blasted rock versus in blocky rock.
The Loadscan system is already in use in several mines globally.  The vendors can provide more technical  data if you need it.
Their website is https://www.loadscan.com/

SedimentIQ

SedimentIQ is a vehicle tracking platform using a smartphone app.  Their technology makes use of a phone’s built-in GPS, Bluetooth, and accelerometer to track vehicle operation.  The phone’s sensor can measure vibrations produced by an operating truck or loader.
Vibration is a fingerprint of a vehicle’s activity.  Therefore using machine learning, the SedimentIQ app can produce an “activity score” that decides whether a machine is parked, idling, or performing productive work.  The phone is not connected to the machine diagnostics system, so its very easy to install, only needing a power source.
The system can be used on any vehicle you want tracked, including trucks, drills, loaders, graders, dozers, etc. The system has the capability to monitor equipment location and speed.
In an open pit environment it uses the phone’s GPS to monitor vehicle location. In an underground setting the phone reads inexpensive Bluetooth beacons mounted along the side walls to track location.
The app will identify delay and downtime based on equipment vibration levels.  The system currently requires no interaction with the operator, working in the background.  Hence it will not identify the cause of delay (i.e. blasting delay, breakdown, inter-equipment delay, etc).  I would expect that in the future they could add a feature for the operator to tag delay types on the touch screen.
The SedimentIQ software will aggregate the cycle time and delay information and upload it in real time to a cloud based database.  A web-based dashboard allows anyone with access to view the real time production data graphically or export it to Excel.
The SedimentIQ platform is less expensive than high end fleet management software.  Although it may not provide all the bells and whistles of the high end software, it may deliver just what you need to monitor productivity in your mining operation.  It relies on relatively inexpensive smart phones that are locked to the application.
I recall as a mining student doing time studies.  I rode the shift crew bus with pencil in hand, timing the travel  from the mine dry to the various shovels to measure start up times.  I recall sitting with a stopwatch timing shovel buckets and truck loading times.  Both of these tasks can be done for every shift, every truck by equipping the crew bus and mine trucks with the SedimentIQ tech.
The platform is currently being tested at a couple of trials mines and the founders are looking for more mines willing to adopt and further refine their technology.
The website is https://sedimentiq.com/.

Conclusion

Both of these innovative technologies can provide useful information to open pit and underground mine operations.  They are in the growth stage, looking for wider adoption.   Input from users, whether positive or negative, will assist them with on-going development and enhancements. Their websites obviously have more on what their technology offers, including presentations, white papers, and case studies.
It would be nice to meld these two technologies in some way to allow the SedimentIQ cycle times to also track payloads.
Check them out.  Try them out.  Tell them Ken sent you.

 

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O/P to U/G Cross-Over – Two Projects into One

Over the years I have been involved in numerous mining tradeoff studies. These could involve throughput rate selection, site selection, processing options, tailings disposal methods, and equipment sizing. These are all relatively straightforward analyses. However, in my view, one of the more technically interesting tradeoffs is the optimization of the open pit to underground crossover point.
The majority of mining projects tend to consist of either open pit only or underground only operations. However there are instances where the orebody is such that eventually the mine must transition from open pit to underground. Open pit stripping ratios can reach uneconomic levels hence the need for the change in direction.
The evaluation of the cross-over point is interesting because one is essentially trying to fit two different mining projects together.

Transitioning isn’t easy

There are several reasons why open pit and underground can be considered as two different projects within the same project.
There is a tug of war between conflicting factors that can pull the cross-over point in one direction or the other. The following discussion will describe some of these factors.
The operating cut-off grade in an open pit mine (e.g. ~0.5 g/t Au) will be lower than that for the underground mine (~2-3 g/t Au). Hence the mineable ore zone configuration and continuity can be different for each. The mined head grades will be different, as well as the dilution and ore loss assumptions. The ore that the process plant will see can differ significantly between the two.
When ore tonnes are reallocated from open pit to underground, one will normally see an increased head grade, increased mining cost, and possibly a reduction in total metal recovered. How much these factors change for the reallocated ore will impact on the economics of the overall project and the decision being made.
A process plant designed for an open pit project may be too large for the subsequent underground project. For example a “small” 5,000 tpd open pit mill may have difficulty being kept at capacity by an underground mine. Ideally one would like to have some satellite open pits to help keep the plant at capacity. If these satellite deposits don’t exist, then a restricted plant throughput can occur. Perhaps there is a large ore stockpile created during the open pit phase that can be used to supplement underground ore feed. When in a restricted ore situation, it is possible to reduce plant operating hours or campaign the underground ore but that normally doesn’t help the overall economics.
Some investors (and companies) will view underground mines as having riskier tonnes from the perspective of defining mineable zones, dilution control, operating cost, and potential ore abandonment due to ground control issues. These risks must be considered when deciding whether to shift ore tonnes from the open pit to underground.
An underground mine that uses a backfilling method will be able to dispose of some tailings underground. Conversely moving towards a larger open pit will require a larger tailings pond, larger waste dumps and overall larger footprint. This helps make the case for underground mining, particularly where surface area is restricted or local communities are anti-open pit.
Another issue is whether the open pit and underground mines should operate sequentially or concurrently. There will need to be some degree of production overlap during the underground ramp up period. However the duration of this overlap is a subject of discussion. There are some safety issues in trying to mine beneath an operating open pit. Underground mine access could either be part way down the open pit or require an entirely separate access away from the pit.
Concurrent open pit and underground operations may impact upon the ability to backfill the open pit with either waste rock or tailings. Underground mining operations beneath a backfilled open pit may be a concern with respect to safety of the workers and ore lost in crown pillars used to separate the workings.
Open pit and underground operations will require different skill sets from the perspective of supervision, technical, and operations. Underground mining can be a highly specialized skill while open pit mining is similar to earthworks construction where skilled labour is more readily available globally. Do local people want to learn underground mining skills? Do management teams have the capability and desire to manage both these mining approaches at the same time?
In some instances if the open pit is pushed deep, the amount of underground resource remaining beneath the pit is limited. This could make the economics of the capital investment for underground development look unfavorable, resulting in the possible loss of that ore. Perhaps had the open pit been kept shallower, the investment in underground infrastructure may have been justifiable, leading to more total life-of-mine ore recovery.
The timing of the cross-over will also create another significant capital investment period. By selecting a smaller this underground investment is seen earlier in the project life. This would recreate some of the financing and execution risks the project just went through. Conversely increasing the open pit size would delay the underground mine and defer this investment and its mining risk.

Conclusion

As you can see from the foregoing discussion, there are a multitude of factors playing off one another when examining the open pit to underground cross-over point. It can be like trying to mesh two different projects together.
The general consensus seems to be to push the underground mine as far off into the future as possible.  Maximize initial production based on the low risk open open pit before transitioning.
One way some groups will simplify the transition is to declare that the underground operation will be a block cave. That way they can maintain an open pit style low cutoff grade and high production rate. Unfortunately not many deposits are amenable to block caving.  Extensive geotechnical investigations are required to determine if block caving is even applicable.
Optimization studies in general are often not well documented in 43-101 Technical Reports. In most mining studies some tradeoffs will have been done (or should have been done).  There might be only brief mention of them in the 43-101 report. I don’t see a real problem with this since a Technical Report is to describe a project study, not provide all the technical data that went into it. The downside of not presenting these tradeoffs is that they cannot be scrutinized (without having data room access).
One of the features of any optimization study is that one never really knows if you got it wrong. Once the decision is made and the project moves forward, rarely will someone ever remember or question basic design decisions made years earlier. The project is now what it is.

 

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Are Engineers Too Pessimistic

Geological colleagues have often joked that engineers are a pessimistic lot; they are never technically satisfied. The engineers will fire back that geologists are an overly optimistic lot; every speck of mineralization makes them ecstatic. Together they make a great team since each cancels the other out.
In my opinion engineers are often pessimistic. This is mainly because they have been trained to be that way. Throughout my own engineering career I have been called upon many times to focus on the downsides, i.e. what can happen that we don’t want to happen.

It starts early and continues on

This pessimism training started early in my career while working as a geotechnical engineer. Geotechnical engineers were always looking at failure modes and the potential causes of failure when assessing factors of safety.
Slope failure could be due to the water table, excess pore pressures, seismic or blast vibrations, liquefaction, unknown weak layers, overly steepen slopes, or operating error. As part of our job we had to come up with our list of negatives and consider them all. The more pessimistic view you had, the better job you did.
This training continued through the other stages of a career. The focus on negatives continues in mine planning and costing.
For example, there are 8,760 hours in a year, but how many productive hours will each piece of equipment provide? There will delays due to weather conditions, planned maintenance, unplanned breakdowns, inter-equipment delays, operator efficiency, and other unforeseen events. The more pessimistic a view of equipment productivity, the larger the required fleet. Geotechnical engineers would call this the factor of safety.
In the more recent past, I have been involved in numerous due diligences. Some of these were done for major mining companies looking at acquisitions. Others were on behalf of JV partners, project financiers, and juniors looking at acquisitions.
When undertaking a due diligence, particularly for a major company or financier, we are not hired to tell them how great the project is. We are hired to look for fatal flaws, identify poorly based design assumptions or errors and omissions in the technical work. We are mainly looking for negatives or red flags.
Often we get asked to participate in a Risk Analysis or SWOT analysis (Strengths-Weaknesses-Opportunities-Threats) where we are tasked with identifying strengths and weaknesses in a project.
Typically at the end of these SWOT exercises, one will see many pages of project risks with few pages of opportunities.
The opportunities will usually consist of the following cliches (feel free to use them in your own risk session); metal prices may be higher than predicted; operating costs will be lower than estimated; dilution will be better than estimated; and grind size optimization will improve process recoveries.
The project’s risk list will be long and have a broad range. The longer the list of risks, the smarter the review team appears to be.

Investing isn’t easy

After decades of the training described above, it becomes a challenge for me to invest in junior miners. My skewed view of projects carries over into my investing approach, whereby I tend to see the negatives in a project fairly quickly. These may consist of overly optimistic design assumptions or key technical aspects not understood in sufficient depth.
Most 43-101 technical reports provide a lot of technical detail; however some of them will still leave me wanting more. Most times some red flags will appear when first reviewing these reports. Some of the red flags may be relatively inconsequential or can be mitigated. However the fact that they exist can create concern. I don’t know if management knows they exists or knows how they can mitigate them.
It has been my experience that digging in a data room or speaking with the engineering consultants can reveal issues not identifiable in a 43-101 report. Possibly some of these issues were mentioned or glossed over in the report, but you won’t understand the full extent of the issues until digging deeper.
43-101 reports generally tell you what was done, but not why it was done. The fact I cannot dig into the data room or speak with the technical experts is what has me on the fence. What facts might I be missing?
Statistics show that few deposits or advanced projects become real mines. However every advanced study will say that this will be an operating mine. Many projects have positive feasibility studies but these studies are still sitting on the shelf. Is the project owner a tough bargainer or do potential acquirers / financiers see something from their due diligence review that we are not aware of?   You don’t get to see these third party reviews unless you have access to the data room.
My hesitance in investing in some companies unfortunately can be penalizing. I may end up sitting on the sidelines while watching the rising stock price. Junior mining investors tend to be a positive bunch, when combined with good promotion can result in investors piling into a stock.
Possibly I would benefit by putting my negatives aside and instead ask whether anyone else sees these negatives. If they don’t, then it might be worth taking a chance, albeit making sure to bail out at the right time.
Often newsletter writers will recommend that you “Do your own due diligence”. Undertaking a deep dive in a company takes time. In addition I’m not sure one can even do a proper due diligence without accessing a data room or the consulting team. In my opinion speaking with the engineering consultants that did the study is the best way to figure things out. That’s one reason why “hostile” due diligences can be difficult, while “friendly” DD’s allow access to a lot more information.

Conclusion

Sometimes studies that I have been involved with have undergone third party due diligence. Most times one can predict ahead of time which issues will be raised in the review. One knows how their engineers are going to think and what they are going to highlight as concerns.
Most times the issue is something we couldn’t fully address given the level of study. We might have been forced to make best guess assumptions to move forward. The review engineers will have their opinions about what assumptions they would have used. Typically the common comment is that our assumption is too optimistic and their assumption would have been more conservative or realistic (in their view).
Ultimately if the roles were reversed and I were reviewing the project I may have had the same comments. After all, the third party reviewers aren’t being hired to say everything is perfect with a project.
The odd time one hears that our assumption was too pessimistic. You usually hear this comment when the reviewing consultant wants to do the next study for the client. They would be a much more optimistic and accommodating team.
To close off this rambling blog, the next time you feel that your engineers are too negative just remember that they are trained to be that way.  If you want more positivity, hang out with a geologist (or hire a new grad).

 

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Vertical Conveyors Give Mining a Lift

There are not many things that are novel to me after having worked in the mining industry for almost 40 years.  However recently I came across a mining technology that I had heard very little about.  It’s actually not something new, but it has never been mentioned as a materials handling option on any project that I am aware of.
That innovative technology is vertical conveying. Not long ago I read about a vertical conveyor being used at the Fresnillo underground mine, hoisting 200 tph up from a depth of 400 metres and had a capital cost of $12.7 million.
I was aware of steep angle conveyors being used in process plants.  However they tended to be of limited height and have idlers and hardware along their entire length. Vertical conveyors are different from that.
After doing a bit of research, I discovered that vertical conveyors have been used since the 1970’s.  Their application was mainly in civil projects; for example in subway construction where one must elevate rock from the excavation level up to street level.   The mining industry is taking vertical conveyors to the next level.
I have never personally worked with vertical conveyors.  Therefore I am providing this discussion based on vendor information.  My goal is to create awareness to readers so that they might consider its application for their own projects.

How vertical conveying works

The background information on vertical conveying was provided to me by FKC-Lake Shore, a construction contractor that installs these systems.  FKC itself does not fabricate the conveyor hardware.  A link to their website is here.
The head station and tail station assemblies are installed at the top and bottom of a shaft.  The conveyor belt simply hangs in the shaft between these two points.  There is no need for internal guides or hardware down the shaft.   The conveyor belting relies on embedded steel cables for tensile strength and pockets (or cells) to carry the material.
The Pocketlift conveyor system is based on the Flexowell technology.  This has been advanced for deep underground applications with a theoretical lift height of 700 metres in one stage.   The power transfer is achieved by two steel cord belts that are connected with rigid cross bars. The ore is fed into rubber pockets, which are bolted onto the cross bars.    The standard Pocketlift can reaches capacities up to 1,500 m3/h and lift heights up to 700 m, while new generations of the technology may achieve capacities up to 4,000 m3/h.
The FLEXOWELL®-conveyor system is capable of running both horizontally and vertically, or any angle in between.  These conveyors consist of FLEXOWELL®-conveyor belts comprised of 3 components: (i) Cross-rigid belt with steel cord reinforcement; (ii) Corrugated rubber sidewalls; (iii) transverse cleats to prevent material from sliding backwards.   They can handle lump sizes varying from powdery material up to 400 mm (16 inch). Material can be raised over 500 metres with reported capacities up to 6,000 tph.

 

The benefits of vertical conveying

Vendors have evaluated the use of vertical conveying against the use of a conventional vertical shaft hoisting.    They report the economic benefits for vertical conveying will be in both capital and operating costs.
Reduced initial capital cost due to:
  • Smaller shaft excavation diameter,
  • Reduced cost of structural supports vs a typical shaft headframe,
  • Structural supports are necessary only in the loading and unloading zones and no support structures in the shaft itself since the belt hangs free.
Lower operating costs due to:
  • Significantly reduced power consumption and peak power demand,
  • Lower overall maintenance costs,
  • No shaft inspections required,
  • The belt is replaced every 8 – 10 years.

Conclusion

I consider vertical conveying as another innovation in the mining industry. There may be significant energy and cost benefits associated with it when compared to conventional shaft hoisting or truck haulage up a decline.
With raise boring, one can develop relatively low cost shafts for the vertical conveyor.  Hardware installation would be required only at the top and bottom of the shaft, not inside it.
The vendors indicate the conveying system should be able to achieve heights of 700 metres.  This may facilitate the use of internal shafts (winzes) to hoist ore from even greater depths in an expanding underground mine. It may be worth a look at your mine.
As stated earlier, I have no personal experience with vertical conveying. Undoubtedly there may be some negative issues associated with the system that I am currently unaware of.
I would appreciate anyone sharing their experience with these conveyors either in a civil application or a mining environment.   I will gladly update this blog article with additional observations or comments.
Update: for those interested in open pit applications for high angle conveyors, here is a recent article.  This is a 37 degree angle 3,000 tph sandwich belt, which is different than the vertical conveyors discussed above.
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A Mining Career or a Travel Club

There have been some LinkedIn discussions about why the mining industry needs to attract more young people.  One of the selling points often mentioned is how mining gives a person the opportunity to experience the world.
Based on my own career, mining has definitely provided me with a chance to travel the world.  It will also help anyone overcome their fear of travel.   One will also learn that both international and domestic travel can be as equally rewarding.  There is nothing wrong with learning more about your own country.
The main purpose for my mining travel was due to either being a QP on a 43-101 study or visiting a site as a member of a due diligence team.   Other reasons have been to provide engineering support at a mine site or to meet with management teams for risk or strategy planning sessions.
Over the last year I haven’t traveled as much as in the past.  One reason is that not every QP working on a 43-101 report has to make a site visit. Fortunately, even when one doesn’t make a site visit, one still learns something about the local politics, legal system, infrastructure, and socio-economic situation in that country.
The map below shows places where I have been in my travels.  It also shows the locations of studies I was involved it. Mining really is a global business.   My map isn’t as cluttered as that of some geologists I know.  Exploration and resource geologists will visit many more destinations that an engineer will. After all, every project needs exploration drilling and a resource estimate, but not all projects advance to the engineering stage.

For those thinking of getting into mining, here is my list of pros and cons based on my own travel experience. Not everything is great about travel but some aspects of it can be fantastic.

What’s Good

  • I had the opportunity to visit many places for which there is a zero probability that I would have ever gone as a tourist.
  • Typically long duration long distance flights are in business class. You get lounge access and the perks associated with executive travel.   Less onerous short flights might be economy only, so be aware of your company policy.
  • All travel expenses, hotels, taxis, meals, etc. are paid for.  Just don’t get too exorbitant when wining and dining.  That’s the job of the senior person you are travelling with.
  • Upon arrival, often there will be a company representative to meet you at the airport.  They speak the local language and will take you where you need to go.  This saves you scrambling around an airport looking for a safe taxi to use.
  • You will get to meet local employees, go to dinner with them, travel around their country, and chat in the evenings. It’s a great way to learn about the people in the country you are visiting.
  • You will get to meet other technical people from around the world.  They might be expats working at a mine site or simply part of a multidisciplinary engineering team on the same visit.
  • You will be whisked away from tourist traps and thus have an opportunity to see the real countryside.
  • You will hit the ground running, get to visit mine sites, see some real live rocks, drill core, pit walls, equipment at work, and things happening.  You won’t get to see that while sitting in your downtown office.

What’s not so good

  • Unfortunately business trips are mostly of a very short duration since you’re not going there as a tourist.  You’re being paid for your time and expertise.

  • Mining trips are usually not to majors centers, so once arriving in the country you really haven’t arrived yet.  There might be more air flights or long pickup truck rides to get to the final destination. There can be a lot of waiting and the days can be long (and frustrating).   I remember on trip in northern Russia where four of us with luggage were jammed into a Volkswagen Rabbit in a snowstorm. That two hour trip took five hours, but we were just happy to make it back to the hotel.
  • Sometimes your accommodations will be less than stellar, i.e. one star hotels or tents.  So you’ll need to learn how to appreciate the charm of those places and not complain that it isn’t the Four Seasons Hotel.
  • Some travel locations can be potentially unsafe and require travelling security. That can lead to a bit of uneasiness.  I recall a trip to northern Mexico where we had two armed guards travelling with us.  I’m not sure if they were really needed and it was strangely more calming without them once they left.
  • Long east-west trips can leave you jet lagged and dog-tired. However the expectation is that at 7 am next morning you’re ready for breakfast and then head straight into the office.  You’re being paid to get to work, not to sleep.
  • The site visits will be focused on collecting or reviewing data and then immediately travelling back home to write your report.  Sightseeing opportunities can be limited other than what you will see during the course of your work. Sometimes you’ll get back home and think that you never really saw the place.

Conclusion

Business travel has always been one of the best parts of my mining career.   I can remember the details about a lot of the travel that I did.   Unfortunately the project details themselves will blur with those of other projects.
When I do travel now, it’s a nice change if just one flight gets you to your final destination.
During this Covid period, international travel is greatly restricted.  It will be interesting to see how soon things can return to normal, if they ever do.   To miss out on the travel aspect of a mining career would be a shame, unless the only travel you want to experience is sitting on public transit for a few hours each day.
By the way, my all time favorite place for a mining trip is…..Argentina.  It’s a long way from Toronto, but well worth it.

 

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Heap Leach or CIL or Maybe Both

Typically gold mines consist of either a heap leach (HL) operation or a CIL type plant. There are a few projects that operate (or are considering) concurrent heap leach and CIL operations. Ultimately the mineral resource distribution determines if it makes economic sense to have both.  This blog discusses this concept based on past experience.
A CIL operation has higher capital and operating costs than a heap leach. However that higher cost is offset by achieving improved gold recovery, perhaps 20-30% higher. At higher gold prices or head grades, the economic benefit from improved CIL recovery can exceed the additional cost incurred to achieve that recovery.

Some background

Several years ago I was VP Engineering for a Vancouver based junior miner (Oromin Expl) who had a gold project in Senegal. We were in the doldrums of Stage 3 of the Lassonde Curve (read this blog to learn what I mean) having completed our advanced studies. Our timeline was as follows.
Initially in August 2009 we completed a Pre-Feasibility Study for a standalone CIL operation. Subsequently in June 2010 we completed a Feasibility Study. The technical aspects of Stage 2 were done and we were entering Stage 3. Now what do we do? Build or wait for a sale?
The property’s next door neighbor was the Teranga Sabodala operation. It made sense for Teranga to acquire our project to increase their long term reserves. It also made sense for a third party to acquire both of us. The Feasibility Study also made the economic case to go it alone and build a mine.
While waiting for various third-party due diligences to be completed, the company continue to do exploration drilling. There were still a lot of untested showings on the property and geologists need to stay busy.
Two years later in 2013 we completed an update to the CIL Feasibility Study based on an updated resource model. Concurrently our geologists had identified seven lower grade deposits that were not considered in the Feasibility Study.
These deposits had gold grades in the range of 0.5 to 0.7 g/t compared to 2.0 g/t for the deposits in the CIL Feasibility Study. We therefore decided to also complete a Heap Leach PEA in 2013, looking solely on the lower grade deposits.
These HL deposits were 2-8 km from the proposed CIL plant so their ore could be shipped to the CIL plant if it made economic sense. Test work had indicated that heap leach recoveries could be in the range of 70% versus >90% with a CIL circuit. The gold price at that time was about $ 1,100/oz.
Ultimately our project was acquired by Teranga in the middle of 2013.

Where should the ore go?

With regards to the Heap Leach PEA, we did not wish to complicate the Feasibility Study by adding a new feed supply to that plant from mixed CIL/HL pits. The heap leach project was therefore considered as a separate satellite operation.
The assumption was that all of the low grade pit ore would go only to the heap leach facility. However, in the back of our minds we knew that perhaps higher grade portions of those deposits might warrant trucking to the CIL plant.
For internal purposes, we started to look at some destination trade-off analyses. We considered both hard (fresh rock) and soft ore (saprolite) separately. CIL operating costs associated with soft ore would be lower than for hard ore. Blasting wasn’t required and less grinding energy is needed. The CIL plant throughput rate could be 30-50% higher with soft ore than with hard ore, depending on the blend.
I have updated and simplified the trade-off analysis for this blog. Table 1 provides the costs and recoveries used herein, including increasing the gold price to $1500/oz.
The graph shows the profit per tonne for CIL versus HL processing methods for different head grades.
The cross-over point is the head grade where profit is better for CIL than Heap Leach. For soft ore, this cross-over point is 0.53 g/t. For hard ore, this cross over point is at 0.74 g/t.
The cross-over point will be contingent on the gold price used, so a series of sensitivity analyses were run.
The typical result, for hard ore, is shown in Table 2. As the gold price increases, the HL to CIL cross-over grade decreases.
These cross-over points described in Table 2 are relevant only for the costs shown in Table 1 and will be different for each project.

Conclusion

It may make sense for some deposits to have both CIL and heap leach facilities. However one should first examine the trade-off for the CIL versus HL to determine the cross-over points.
Then confirm the size of the heap leach tonnage below that cross-over point. Don’t automatically assume that all lower grade ore is optimal for the heap leach.
If some of the lower grade deposits are further away from the CIL plant, the extra haul distance costs will tend to raise their cross-over point. Hence each satellite pit would have its own unique cross-over criteria and should be examined individually.
Since Teranga complete the takeover in mid 2013, we were never able to pursue these trade-offs any further.
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Pre-Concentration – Maybe Good, Maybe Not

A while back I wrote a blog titled “Pre-Concentration – Savior or Not?”. That blog was touting the benefits of pre-concentration. More recently I attended a webinar where the presenter stated that the economics of pre-concentration may not necessarily be as good as we think they are.
My first thought was “this is blasphemy”. However upon further reflection I wondered if it’s true. To answer that question, I modified one of my old cashflow models from a Zn, Pb project using pre-concentration. I adjusted the model to enable running a trade-off, with and without pre-con by varying cost and recovery parameters.

Main input parameters

The trade-off model and some of the parameters are shown in the graphic below. The numbers used in the example are illustrative only, since I am mainly interested in seeing what factors have the greatest influence on the outcome.

The term “mass pull” is used to define the quantity of material that the pre-con plant pulls and sends to the grinding circuit. Unfortunately some metal may be lost with the pre-con rejects.  The main benefit of a pre-con plant is to allow the use of a smaller grinding/flotation circuit by scalping away waste. This will lower the grinding circuit capital cost, albeit slightly increase its unit operating cost.
Concentrate handling systems may not differ much between model options since roughly the same amount of final concentrate is (hopefully) generated.
Another one of the cost differences is tailings handling. The pre-con rejects likely must be trucked to a final disposal location while flotation tails can be pumped.  I assumed a low pumping cost, i.e to a nearby pit.
The pre-con plant doesn’t eliminate a tailings pond, but may make it smaller based on the mass pull factor. The most efficient pre-concentration plant from a tailings handling perspective is shown on the right.

The outcome

The findings of the trade-off surprised me a little bit.  There is an obvious link between pre-con mass pull and overall metal recovery. A high mass pull will increase metal recovery but also results in more tonnage sent to grinding. At some point a high mass pull will cause one to ask what’s the point of pre-con if you are still sending a high percentage of material to the grinding circuit.
The table below presents the NPV for different mass pull and recovery combinations. The column on the far right represents the NPV for the base case without any pre-con plant. The lower left corner of the table shows the recovery and mass pull combinations where the NPV exceeds the base case. The upper right are the combinations with a reduction in NPV value.
The width of this range surprised me showing that the value generated by pre-con isn’t automatic.  The NPV table shown is unique to the input assumptions I used and will be different for every project.

The economic analysis of pre-concentration does not include the possible benefits related to reduced water and energy consumption. These may be important factors for social license and permitting purposes, even if unsupported by the economics.  Here’s an article from ThermoFisher on this “How Bulk Ore Sorting Can Reduce Water and Energy Consumption in Mining Operations“.

Conclusion

The objective of this analysis isn’t to demonstrate the NPV of pre-concentration. The objective is to show that pre-concentration might or might not make sense depending on a project’s unique parameters. The following are some suggestions:
1. Every project should at least take a cursory look at pre-concentration to see if it is viable. This should be done on all projects, even if it’s only a cursory mineralogical assessment level.
2. Make certain to verify that all ore types in the deposit are amenable to the same pre-concentration circuit. This means one needs to have a good understanding of the ore types that will be encountered.
3. Anytime one is doing a study using pre-concentration, one should also examine the economics without it. This helps to understand the  economic drivers and the risks. You can then decide whether it is worth adding another operating circuit in the process flowsheet that has its own cost and performance risk. The more processing components added to a flow sheet, the more overall plant availability may be effected.
4. The head grade of the deposit also determines how economically risky pre-concentration might be. In higher grade ore bodies, the negative impact of any metal loss in pre-concentration may be offset by accepting higher cost for grinding (see chart on the right).
5. In my opinion, the best time to decide on pre-con would be at the PEA stage. Although the amount of testing data available may be limited, it may be sufficient to assess whether pre-con warrants further study.
6. Don’t fall in love with or over promote pre-concentration until you have run the economics. It can make it harder to retract the concept if the economics aren’t there.

 

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Climbing the Hill of Value With 1D Modelling

Recently I read some articles about the Hill of Value.  I’m not going into detail about it but the Hill of Value is a mine optimization approach that’s been around for a while.  Here is a link to an AusIMM article that describes it “The role of mine planning in high performance”.  For those interested, here is a another post about this subject “About the Hill of Value. Learning from Mistakes (II)“.
hill of value

(From AusIMM)

The basic premise is that an optimal mining project is based on a relationship between cut-off grade and production rate.  The standard breakeven or incremental cutoff grade we normally use may not be optimal for a project.
The image to the right (from the aforementioned AusIMM article) illustrates the peak in the NPV (i.e. the hill of value) on a vertical axis.
A project requires a considerable technical effort to properly evaluate the hill of value. Each iteration of a cutoff grade results in a new mine plan, new production schedule, and a new mining capex and opex estimate.
Each iteration of the plant throughput requires a different mine plan and plant size and the associated project capex and opex.   All of these iterations will generate a new cashflow model.
The effort to do that level of study thoroughly is quite significant.  Perhaps one day artificial intelligence will be able to generate these iterations quickly, but we are not at that stage yet.

Can we simplify it?

In previous blogs (here and here) I described a 1D cashflow model that I use to quickly evaluate projects.  The 1D approach does not rely on a production schedule, instead uses life-of-mine quantities and costs.  Given its simplicity, I was curious if the 1D model could be used to evaluate the hill of value.
I compiled some data to run several iterations for a hypothetical project, loosely based on a mining study I had on hand.  The critical inputs for such an analysis are the operating and capital cost ranges for different plant throughputs.
hill of valueI had a grade tonnage curve, including the tonnes of ore and waste, for a designed pit.  This data is shown graphically on the right.   Essentially the mineable reserve is 62 Mt @ 0.94 g/t Pd with a strip ratio of 0.6 at a breakeven cutoff grade of 0.35 g/t.   It’s a large tonnage, low strip ratio, and low grade deposit.  The total pit tonnage is 100 Mt of combined ore and waste.
I estimated capital costs and operating costs for different production rates using escalation factors such as the rule of 0.6 and the 20% fixed – 80% variable basis.   It would be best to complete proper cost estimations but that is beyond the scope of this analysis. Factoring is the main option when there are no other options.
The charts below show the cost inputs used in the model.   Obviously each project would have its own set of unique cost curves.
The 1D cashflow model was used to evaluate economics for a range of cutoff grades (from 0.20 g/t to 1.70 g/t) and production rates (12,000 tpd to 19,000 tpd).  The NPV sensitivity analysis was done using the Excel data table function.  This is one of my favorite and most useful Excel features.
A total of 225 cases were run (15 COG versus x 15 throughputs) for this example.

What are the results?

The results are shown below.  Interestingly the optimal plant size and cutoff grade varies depending on the economic objective selected.
The discounted NPV 5% analysis indicates an optimal plant with a high throughput (19,000 tpd ) using a low cutoff grade (0.40 g/t).  This would be expected due to the low grade nature of the orebody.  Economies of scale, low operating costs, high revenues, are desired.   Discounted models like revenue as quickly as possible; hence the high throughput rate.
The undiscounted NPV 0% analysis gave a different result.  Since the timing of revenue is less important, a smaller plant was optimal (12,000 tpd) albeit using a similar low cutoff grade near the breakeven cutoff.
If one targets a low cash cost as an economic objective, one gets a different optimal project.  This time a large plant with an elevated cutoff of 0.80 g/t was deemed optimal.
The Excel data table matrices for the three economic objectives are shown below.  The “hot spots” in each case are evident.

hill of value

hill of value

Conclusion

The Hill of Value is an interesting optimization concept to apply to a project.  In the example I have provided, the optimal project varies depending on what the financial objective is.  I don’t know if this would be the case with all projects, however I suspect so.
In this example, if one wants to be a low cash cost producer, one may have to sacrifice some NPV to do this.
If one wants to maximize discounted NPV, then a large plant with low opex would be the best alternative.
If one prefers a long mine life, say to take advantage of forecasted upticks in metal prices, then an undiscounted scenario might win out.
I would recommend that every project undergoes some sort of hill of value test, preferably with more engineering rigor. It helps you to  understand a projects strengths and weaknesses.  The simple 1D analysis can be used as a guide to help select what cases to look at more closely. Nobody wants to assess 225 alternatives in engineering detail.
In reality I don’t ever recall seeing a 43-101 report describing a project with the hill of value test. Let me know if you are aware of any, I’d be interested in sharing them.  Alternatively, if you have a project and would like me to test it on my simple hill of value let me know.
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Simple Financial Models Can Really Help

A few years ago I posted an article about how I use a simple (one-dimensional) financial model to help me take a very quick look at mining projects. The link to that blog is here. I use this simple 1D model with clients that are looking at potential acquisitions or joint venture opportunities at early stages. In many instances the problem is that there is only a resource estimate but no engineering study or production schedule available.

By referring to my model as a 1D model, I imply that I don’t use a mine production schedule across the page like a conventional cashflow model would.
The 1D model simply uses life-of-mine reserves, life-of-mine revenues, operating costs, and capital costs. It’s essentially all done in a single column.  The 1D model also incorporates a very rudimentary tax calculation to ballpark an after-tax NPV.
The 1D model does not calculate payback period or IRR but focuses solely on NPV. NPV, for me, is the driver of the enterprise value of a project or a company. A project with a $100M NPV has that value regardless of whether the IRR is 15% or 30%.

How accurate is a 1D model?

One of the questions I have been asked is how valid is the 1D approach compared to the standard 2D cashflow model. In order to examine that, I have randomly selected several recent 43-101 studies and plugged their reserve and cost parameters into the 1D model.
It takes about 10 minutes to find the relevant data in the technical report and insert the numbers. Interestingly it is typically easy to find the data in reports authored by certain consultants. In other reports one must dig deeper to get the data and sometimes even can’t find it.
The results of the comparison are show in the scatter plots. The bottom x-axis is the 43-101 report NPV and the y-axis is the 1D model result. The 1:1 correlation line is shown on the plots.
There is surprisingly good agreement on both the discounted and undiscounted cases. Even the before and after tax cases look reasonably close.
Where the 1D model can run into difficulty is when a project has a production expansion after a few years. The 1D model logic assumes a uniform annual production rate for the life of mine reserve.
Another thing that hampers the 1D model is when a project uses low grade stockpiling to boost head grades early in the mine life. The 1D model assumes a uniform life-of-mine production reserve grade profile.
Nevertheless even with these limitations, the NPV results are reasonably representative. Staged plant expansions and high grading are usually modifications to an NPV and generally do not make or break a project.

Conclusion

My view is that the 1D cashflow model is an indicative tool only. It is quick and simple to use. It allows me to evaluate projects and test the NPV sensitivity to metal prices, head grades, process recovery, operating costs, etc. These are sensitivities that might not be described in the financial section of the 43-101 report.
This exercise involved comparing data from existing 43-101 reports. Obviously if your are taking a look at an early stage opportunity, you will need to define your own capital and operating cost inputs.
I prefer using a conventional cashflow model approach (i.e. 2D) when I can. However when working with limited technical data, it’s likely not worth the effort to create a complex cashflow model. For me, the 1D model can work just fine. Build one for yourself, if you need convincing.
In an upcoming blog I will examine the hill of value optimization approach with respect to the 1D model.
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