In many of the past mining studies that I have worked, stockpiling strategies were discussed and eventually implemented. Sometimes study team members were surprised at the size of the stockpiles that were generated by the production plan. It became apparent that not all members of the team were clear on the purpose of the stockpiling strategy or else they had preconceived ideas on the rationale. To them stockpiling may have seemed to be a good idea until they saw it in action.
In this blog I won’t go into all the costs and environmental issues associated with stockpile operation but will focus simply on the reasons for stockpiling and why stockpiles may get large or numerous .
In my experience there are four main reasons why stockpiling might be done at an operation. They are:
1. Campaigning: For metallurgical reasons, there may be certain ore type(s) that can cause process difficulties if mixed in with other ores. Therefore the problematic ore(s) might be stockpiled until sufficient inventory is built up until it makes sense to process that ore (i.e. campaign) through the mill. Such stockpiles will only grow as large as the operator allows them to, before processing the material and eliminating the stockpile. Be aware that if the mine operations are still delivering different ore types to the crusher area, then those ores may need to be stockpiled during the campaigning period. More different ore types may mean more stockpiles.
2. Grade Maximization: This stockpiling approach is used in situations where the mine delivers more ore than is required by the plant, thereby allowing the best material to be processed directly and the lower grade material to be stockpiled for a future date. Possibly one or more low grade stockpiles may be used, for example a low grade and a medium-low grade stockpile. Such stockpiles may not be processed for years, possibly remaining in place until the mine is depleted or until the mined head grades are lower than those in the stockpile. Such stockpiles can grow to enormous size if accumulated over many years.
3. Surge Control: stockpiling may be used in cases where the mine may have a fluctuating ore delivery rate and on some days excess ore is produced while other days there is underproduction. The stockpile is simply used to make up the difference and provide a steady primary crusher feed rate. These stockpiles are also available as short term emergency feed if for some reason the mine is shut down (e.g. extreme weather). In general such stockpiles may be relatively small in size since they are mainly used for operational surge control.
4. Blending: blending stockpiles may be used where a processing plant needs a certain quality of feed material with respect to head grade or contaminant ratios (silica, iron, etc.). Blending stockpiles enables the operator to ensure the plant feed quality to be consistent and uniform. Such stockpiles may not be large individually; however there could be several of them depending on the orebody character.
There may be other stockpiling strategies beyond the four listed above but those four capture the bulk of the situations.
Using today’s automated production scheduling software, one can test multiple stockpiling strategies by applying different cutoff grades or using multiple grade stockpiles. The scheduling software will have algorithms to determine whether one should be adding to the stockpile or drawing from it. It will track the grades in the stockpile and sometimes be able to model stockpile balances assuming reclaim by average grade, or first in-first out (FIFO), or last in-first out (LIFO).
Stockpiling in most cases will provide some potential benefits to an operation and the project economics. Even if metallurgical blending or campaigning is not required, one should always test the production schedule and project economics with a few grade stockpiling scenarios. Unfortunately these are not simple to undertake when using a manual scheduling approach and so are another reason to move towards automated scheduling software. Also make sure everyone on the team understands the rationale for the stockpiling strategy and what the stockpiles might ultimately look like. They might be surprised.
Can pre-concentration become a key savior for the mining industry and help in lowering metal production costs?
Pre-concentration is a way to reduce the quantity of ore requiring higher cost downstream processing, i.e. grinding in particular. By using a low cost method to pre-concentrate mineral-bearing particles into a smaller volume, one can attain significant cost savings in overall energy consumption and operational expenses. A previous blog “Remote Sensing of Ore Grades” discussed a new pre-concentration method under development.
Pre-concentration is nothing new and has been around for many years but is generally limited in the techniques available. Hence many ore types are not amenable to it..unfortunately. The main methods being used are:
Ore sorting can be done using automated optical, electrical, or magnetic susceptibility sensors to separate ore particles from waste particles. The different sensor types can include colour recognition, near infrared radiation sensors, x-ray fluorescence, x-ray transmission, radiometric, or electromagnetic sensing. The sensors can determine if a particle contains valuable mineral or waste, thereby sending a signal to activate air jets to deflect material into ore and waste bins.
Density or specific gravity difference is another property that some pre-concentration methods can rely on. Gravity based systems such as dense media separation (DMS), jigs, or centrifugal concentrators are currently in production use.
Another simple pre-concentration method used is scrubbing, whereby simply washing away fines may remove some deleterious materials prior to final processing.
Pre-concentration can provide several benefits to an operation:
-If done underground or at remote mine site, the net ore hoisting and ore transport costs can be reduced.
-If the pre-concentration rejects can be used as mine backfill, this can reduce backfilling costs.
-Processing of higher grade pre-concentrated mill feed can reduce total energy costs and ultimately reduce the cash cost of metal produced.
-Grinding costs can be reduced if waste particles are harder than the ore particles and they can be removed beforehand.
-Minimizing waste through the process plant will reduce the quantity of tailings that must be disposed of.
-Lowering operating costs may potentially allow lowering of the cutoff grade and increasing mineral reserves.
-Higher head grades would increase metal production without needing an increase in plant throughput.
Not all ore types are amenable to pre-concentration and therefore a rigorous testing program is required. In most cases the pre-con method would be relatively obvious to the metallurgical engineer but testing is still required to measure performance. Testing is required to determine the amount of waste rejection that can be achieved without incurring significant ore loss during the process. Generally one can produce a higher quality final product if one is willing to reject more ore with the waste, so it becomes a trade-off of recovery versus total processing cost.
Fine particles from the primary and secondary crushing stages might require bypassing the pre-con circuit. If this bypassed material is sent for downstream processing, one may need to examine crushing systems that minimize fines generation to avoid too much material bypassing the pre-con circuit.
One must also decide if the pre-con system should reject waste particles from the material stream or reject ore particles from the stream since the overall recovery and product quality will be impacted depending on which approach is used.
My bottom line is that the mining industry is continually looking for ways to improve costs and pre-concentration may be a great way to do this. Every process plant design should at least take an initial look at it to see if is feasible for their ore type. While the existing pre-concentration methods have their limitations, future technologies may bring in new ways to pre-concentrate and so this is probably an area where research dollars would be well spent.
The mining industry must continually find ways to improve and modernize. The most likely avenue for improvement will be using new technologies as they become available. One of the new players on the scene is a start-up company called “MineSense Technologies Ltd.” They are a British Columbia company looking to improve ore extraction and recovery processes based on the sensing and sorting of low-grade ore (pre-concentration in other words). They hope their pre-concentration methods will improve mine economics by reducing the consumption of energy, water, and reagents.
It’s not entirely clear to me how developed their technology is, but MineSense is relying on a combination of ground-penetrating sensors with other sensor technology in order to measure and report the grade of ore in real time. Existing ore sorting technologies seem to focus on distinguishing mineralized material from gangue, but MineSense seems to be targeting using actual ore grades as the defining factor. They hope to be able to eventually integrate their technology into equipment such as shovels, scooptrams, conveyors, feeders, and transfer chutes.
More specifically their proprietary technology is based on High Frequency Electromagnetic Spectrometry and High Speed X-Ray Fluorescence sensors. Reportedly these can deliver better sensitivity and operate at high speeds. They plan to develop two distinct product lines; shovel-based systems; and conveyor belt-based systems.
Their ShovelSense system would be a real-time mineral telemetry and decision system and used for measurement of ore quality while material is being scooped into the dipper, then reporting the ore quality and type to the grade control/ore routing system, and then enabling real-time online ore/waste dispatch decisions. Additional features may include tramp metal and missing tooth detection.
Their belt conveyor systems (SortOre and BeltSense) will use high-speed multi-channel sensing to characterize conveyed ore and waste in real time, allowing grades and tonnages to be reported and allowing ore to be diverted to correct destinations based on the sensor responses. MineSense say that pilot units are operating at 20 tph and systems of up to 2000 tph are in the development stages. Ore sorting has been around for a long time, with companies like Tomra (www.tomra.com), but possibly the MineSense technical approach will be different.
My bottom line is that we should all keep an eye on the continued development of this technology, especially as MineSense completes larger field trials. Hopefully they will readily share the results with us since it will be critical for industry players to see more actual case study performance data on their website. I recognize that developing new technology will have its successes and failures. Setbacks should not necessarily be viewed as fatal flaws since it takes time to work out all the kinks. Hopefully after being able to fine tune their technology they can advance to their next stage which will be to convince the mining industry to adopt it.
P.S. Unfortunately it appears MineSense don’t have a newsletter sign-up form on their website to help us in following their progress.
I have often wondered if any of the junior gold producers has ever tried to model itself after a gold Exchange Traded Fund (“ETF”). This hybrid-model may be a way for companies to provide shareholders a way to partly leverage themselves to physical gold rather than leveraging solely to the performance of a mine. Let me explain further.
Consider two identical junior mining companies starting up a new mine. Each of their two projects is identical; 2 million gold ounces in the reserves with annual production of 200,000 ounces resulting in a 10 year mine life. On an annual basis, let’s assume their annual operating costs and debt repayments can be paid for by the revenue from selling 180,000 ounces of production. This would leave 20,000 gold ounces as “profit”. The question is what to do with those 20,000 ounces?
Company A sells their entire gold production each year. At $1200/oz, the 20,000 oz gold “profit” would yield $24 million. Income taxes would be paid on this and the remaining cash can be spent or saved. Companies may decide to spend more on head offices costs by adding more people, or they may spend some on exploration, or they could spend on an acquisition to grow the company. There are plenty of ways to use this extra money but returning it to shareholders as a dividend isn’t typically one of them. Now let’s jump forward several years; 8 years for example. Company A may have been successful on grassroots exploration and added to reserves but historically exploration odds are working against them. If they actually saved a portion of the annual profit in the bank, say $10M of the $24M, after 8 years they may have $80M in cash reserves.
Company B only sells 180,000 ounces of gold each year and puts the remaining 20,000 ounces into inventory in a vault. Their annual profit-loss statement shows breakeven status since their gold sales only cover their financial commitments and nothing more. In this scenario, after 8 years Company B would have 160,000 gold ounces in their vault, valued at $192 million at $1200 gold price.
If you’re an investor looking at both these companies in the latter stages of their mine life, which one would you rather invest in? Company A has 400,000 ounces remaining in mineral reserves and say $80M cash in the bank. Company B also has 400,000 ounces of mineral reserves and $192 million worth of gold in the vault. If I’m a gold bull investor and foresee a $1700/oz gold price, then to me Company B might theoretically have $272M in the vault. If I’m a super gold bug, then their inventory could be worth a lot more..theoretically.
I assume that the enterprise value of Company A would be based on its remaining reserves at some $/oz factor plus its cash in the bank. Company B could be valued the same way plus its gold inventory. So for me Company B may be a much better investment than Company A in the latter stages of its mine life. In fact Company B could still persist as an entity after the mine has shutdown simply as a “fund” that holds physical gold. If I am a gold investor, then Company B as an investment asset might be of more interest to me.
My bottom line is that it appears that most of the time companies sell their entire annual gold production to try to show profit on the annual income statement. Possibly this is to put some cash in the bank and to show “earning per share” to the analysts. My question is why not inventory the extra gold and wait for prices to rise if the company doesn’t really need the money or doesn’t want to gamble it on exploration or acquisitions? This concept wouldn’t be a model for all junior miners but might be suitable for a few companies to target certain gold investors. They could provide an alternative junior mining investment especially interesting in the final years of a mine life. Who wants to buy shares in a company who’s mine is nearly depleted? I might if they hold a lot of gold.
When metals prices are high, we are generally taught that we should lower the cutoff grade. Our cutoff grade versus metal price spreadsheet tells us this is the correct thing do. Our grade-tonnage curve reaffirms this since we will now get more ounces of gold in the mineral reserve. But is lowering the cutoff grade really the right thing to do?
Books have been written on the subject of cutoff grades where readers can get all kinds of detailed logic and calculations using Greek symbols (F = δV* − dV*/dT). Here is one well known book by Ken Lane that sells for $998 on Amazon the last time I checked. You can also download a 38-page abridged sample of this book at THIS LINK and the full version is available for $150 at the COMET Strategy web site.
Recently we have seen higher production cash costs at operating mines when commodity prices are high. Why is this? It may be due to higher operating costs inputs caused by increasing labour rates or supplies costs. It also may be partly due to the lowering of cutoff grades, thereby lowering the milled head grade, which then requires more tonnes to be milled to produce the same quantity of metal.
A mining construction manager once said to me that he never understood us mining guys who lower the cutoff grade when gold prices increase. His rationale was that, since the plant throughput rate is fixed, when gold prices are high you suddenly decide to lower the head grade and produce fewer ounces of gold and at a higher cash cost. His point was that we should be doing the opposite; when prices are high you should produce more ounces of gold, not fewer. In essence, in times when supply is low (or demand is high) may not be the right time to further cut back on supply by lowering head grades.
Now this is the point where the grade-tonnage curve comes into play. Certainly one can lower the cutoff grade, lower the head grade and produce fewer ounces now with the upside being extending the mine life. By doing this a company is able to report more ounces in their mineral reserve and the overall snapshot of the company looks better if it is being valued on reserve ounces.
The problem with this is that there is no assurance that metal prices will remain where they are or that the new lower cutoff grade will remain where it is. If the metal prices dip back down next year, the cutoff grade will be increased and the mineral reserve is back to where it was. All that was really done was accept a year of lower metal production for no real benefit. Such a trade-off essentially contrasts a short term vision (i.e. annual production) against a long term vision (i.e. mineral reserves).
My bottom line is that there is no simple answer on what to do with the cutoff grades, hence the need to write books about it. Different companies have different corporate objectives and each mining project will be unique with regards to the impacts of cutoff grade adjustments on their orebody. I would like to caution that one should be careful when taking your cutoff grade spreadsheet, plugging in new metal prices, and then running off to the mine operations department with the result. You need to fully understand the long term and short term impacts of that decision.
Have worked in the mining industry for over the last 30 years it is always interesting to see the herd mentality that exists and how we all can easily get caught up in fads. All it takes is a short term spike in a commodity price or a big discovery somewhere and then off we go running in that direction. It doesn’t matter the rationale driving the event, all we know is that we need to be there and our investors want to be there too.
Based on my experience, the fads that grab us can include specific commodities, locations, or technologies. The industry is very flexible in that regard. I’ll give a few examples below and you probably have more examples from your own experience.
It seems that as soon as there is a price spike or positive market narrative, any commodity can take a life of its own. The following are a few examples and when you think about them ask how many actually came into production or successful production.
Potash – a few years ago potash prices spiked and potash properties were all the fad no matter where they were located around the globe, be it Canada, Russia, Ethiopia, Thailand, Brazil, etc.
Lithium / Graphite – as soon as green technology started to be promoted in the news, miners couldn’t run fast enough to pick up the lithium properties, same idea hold for the graphite and vanadium and rare earth categories.
Uranium – years ago uranium prices spiked and Ur properties were hot everywhere.
Nickel; a spike in nickel caused a surge in nickel properties being it sulphide nickel, laterite nickel, or other forms.
Iron Ore – in conjunction with the Chinese construction boom, iron ore properties were hot around the globe, in high cost or low cost jurisdictions, it didn’t matter where the property was.
Diamonds – in conjunction with the first diamond discoveries in Canada, quickly diamond properties because hot, whether in the Canada or around the globe. If you couldn’t get a property in Canada’s boom area, anywhere else was fine.
China in general – whereby every base metal project was thought of as either a potential supplier to China or a potential acquisition for Chinese companies. As long as it could meet Chinese investor interest it was good.
We have all seen the staking rushes that occur when a world class prospect is discovered. I’m sure we can all recall getting the large claim maps (as shown below) with their multicolored graphics showing the patchwork of acquisitions around a discovery. PDAC was great for distributing these and they were well done and interesting to study.
Picking up properties in hot areas became the fad and share prices would move upwards regardless of whether there was any favorable geology on the property. Who recalls the following?
Voisey Bay; with a mad staking rush around there, with nothing else really paying off in the long run.
Saskatchewan; and the potash staking rush where almost every inch of the potash band was staked with only a couple of companies eventually moving forward and only one going into production.
Indonesia; during Bre-X people could get properties in Indonesia fast enough.
NWT; where the diamond property staking rush was crazing in the mid 1990’s.
Even mining or processing technology could get caught up in somewhat of a wave and become a fad for further study, a rationale often driven by suppliers or consultants. Who can recall…
Paste Tailings; with numerous conferences and consultants promoting thickened or paste tailings technology as the panacea leading to numerous studies related to thickening, pumping, and disposal.
Block Caving; whereby in order to deliver high tonnages at low cost, bulk underground mining was being promoted. Everyone wanted their underground project to be a low cost caving style operation.
High Pressure Grinding Rolls (HPGR); where process consultants would tout HPGR as the new replacement for conventional grinding mills. I’m not sure this technology has taken the industry by storm as they were hoping.
IPCC; whereby inpit crushing and conveying was being promoted in many articles and global conferences as the solution to operating cost pressures. I think implementation of IPCC technology isn’t as simple as envisioned.
Dot.com; in the early 2000’s many junior miners left exploration behind and transitioned to the dot.com boom, a fad with generally poor results.
Medical marijuana; seems to be the hopeful target for some junior miners today. Unfortunately there is only so much marijuana you can sell.
Pre-concentration; this seems to be a growing technology fad that may be gaining momentum, with a few consultants pushing for it to be studied more. This isn’t new technology and will have its benefits but a big stumbling block is how many deposits are actually suitable for its application.
My bottom line is that over the years it has been interesting to watch the mining industry react to events. Sometimes it seems like we’re passengers on a boat running from one side to other side and then back again. Unfortunately that doesn’t necessarily make for smooth sailing and can result in upset stomachs.
What’s the next fad? I don’t know but if you could predict it we can probably make a lot of money.
Just as a reminder for all QP’s doing financial analysis for PEA’s, don’t forget that one needs to present the financial results on an after-tax basis. Every once in a while we still see a PEA technical report issued only with pre-tax financials. That report is likely to get red- flagged by the securities regulators and the company will then have to amend their press release and technical report in order to show the after tax results. No harm done other than some red faces.
When doing a tax analysis in your model, where can you find regional tax information? For those of you that prepare financial models or are simply looking at mining projects in different jurisdictions, PWC has a very useful tax-related website. The weblink was sent to me by one of my industry colleagues and I thought it would be good to share this.
The PWC micro-site provides a host of tax and royalty information for selected countries. The page is located at http://www.pwc.com/gx/en/industries/energy-utilities-mining/mining/tax.html
On the site they have tax information for specific countries and you can either view the information on your computer screen or download a PDF version. Below is a screen capture from the PWC website.
The PWC tax and financial information includes topics such as:
Corporate tax rates
Excess profits taxes
Mineral taxes for different commodities
Rates of permissible amortization
VAT and other regulated payments
Fiscal stability agreements
Social contribution requirements
PWC has a great web site and hopefully they will keep the information up to date since changes in the laws are occurring constantly. It would be nice to see them add more countries to their 22 country database but it’s already good as it is. Check it out.
One of the first things we look at when examining a resource estimate is how much of the resource is classified as Measured / Indicated (“M&I”) versus the tonnage classified as Inferred. It’s important to understand the uncertainty in the estimate and to a large degree the Inferred proportion gives us that. At the same time I think we tend to focus less on the split between the Measured and Indicated tonnages.
We are all aware of the study limitations imposed by Inferred resources. They are speculative in nature and hence cannot be used in the economic models for feasibility and pre-feasibility studies. However Inferred resource can be used for production planing in Preliminary Economic Assessments (“PEA”).
Inferred resources are also so speculative that one cannot add them to the Measure and Indicated tonnages in a resource statement, although that is what just about everyone does when looking at a project. I don’t think I fully understand the concerns with a resource statement if it included a row that adds M&I tonnage with Inferred tonnes as long as everything is open and transparent. When a PEA production schedule is presented, the three resource classifications are combined into a single tonnage number but in the resource statement itself the M&I&I cannot be totaled. A bit contradictory I feel.
With regards to the M&I tonnage, it appears to me that companies are most interested in what part of their resource meets the M&I threshold but are not as interested in how the tonnage is split between Measured and Indicated. It seems that M&I are largely being treated the same. Since both Measured and Indicated resources can be used in the feasibility economic analysis, does it matter if the split is 100% Measured (Proven) or 100% Indicated (Probable)? The NI 43-101 and CIM guidelines provide definitions for Measured and Indicated resource but do not specify any different treatment like they do for the Inferred resources.