Drill Sampling the Bulk Commodities – Iron Ore and Copper

Never has the old adage “moving dirt makes money” been more prevalent than in the world of bulk commodity mining such as iron ore and copper. Bonuses are paid throughout the various stages of the mining process based purely on this ill thought out notion and with very little emphasis placed on the quality of the ore that is being dug from the outset. The inefficiency of this approach is comparable to that of the manufacturing industry prior to the industrial revolution of the late 1700’s.


Of course, in reality moving millions of tonnes of dirt costs money …a lot of money. And the more you move the more equipment you need to move it. And the more equipment you need, the more people you need to operate it. And the more people you need, the greater the risk of injury. And the greater the risk, the higher the insurance premiums, and so it goes on and on and on… If this dirt is not of a saleable grade, or it is full of impurities that make metal processing difficult, it is money wasted along with reputation and profit.

iron ore stock piling

With commodity prices across the board at some of the lowest levels in recent history, many mines are now, as a result of this inefficiency, operating below cost. Surprisingly to some, the approach of the three major players in iron ore has been to increase production during this time. Some feel that this is a good thing as it is seen as riding the bad times and protecting jobs, while to others it is nothing more than a power play to squeeze the smaller players out of the game. One thing is for sure, oversupply, particularly inefficient oversupply, will drive prices even lower and for longer and, in the long run, this cannot be good for the industry.

“So, what is the answer?”

Some of the smaller players have not only already worked this massive inefficiency loss out for themselves, they have begun to act on it. And in doing so they have identified a way to combat all of the threats currently faced – inefficiency, cost, risk, price, profit and “squeeze”. And in doing so, they are securing a sustainable future.

The answer is actually quite simple: Accurately find out what is in the ground before it is dug up. Know where the high, medium & low grade boundaries are from the waste and only mine saleable grade ore.

“But, we already do that …don’t we?”

You would think so but the reality, and perhaps the biggest concern here, is that many key personnel within mining operations, including some senior production staff and even some geology managers, are not even aware that their decisions are often the cause of the inefficiency. And sometimes, even when they know, they feel bound from making change for fear of ridicule and reprisal. Others wrongly assume that the lack of correlation between sample results and mined grade is simply down to natural variation.

Suffice to say, when the people on the ground fail to feedback reality, the decision makers at the top – mining boards and the analysts that advise them – are left completely in the dark.

07-board-room (2)

And like a revolving door, share prices fluctuate, senior executives come & go, commodity prices rise & fall on supply & demand and the ever present peaks & troughs of mining cycles continue forever more… Very few decision makers understand that there is a proven and relatively simple way to avoid all of this. Something that turns the marginal into profit …and significant profit at that.

Grade control drilling, the sole function that dictates every mining decision thereafter, in bulk commodity mining is dominated by the flawed preconception that samples taken from blast holes are representative. Not so. Not even close. The problems with blast hole samples are serious and many. There are a vast number of issues that individually cause the samples to be unrepresentative. But when combined, the sampling error can be enormous and decisions based on these results can be catastrophic. Below are some of the issues that contribute to the extremely unrepresentative and biased nature of blast hole samples:-

– Poor & inconsistent recovery from the hole.

– Losses of the bulk sample to voids and broken ground especially during hole collaring making the drilled interval itself unrepresentative and delivering inconsistent sample volumes from each hole.

– Extensive fines loss in the hole and to atmosphere when exiting.

– Poor duplicate repeatability.

– Labour intensive and highly susceptible to human error.

– High risk of injury.

– Produces a vast number of unrepresentative samples to analyse. Some feel that this large volume of samples helps overcome the issue of poor representivity. That more and more bad data somehow overcomes the problem.


– Suffers from cross contamination if interval sampling is attempted due to hole cave in and hole wall erosion.

– Limited to vertically drilled holes and single large sampling intervals.

– Lab test work cannot keep up with production demand for blasting – mining continues blind.

– Mine planning is compromised as it is unprepared and cannot react in time often with no decisions relating to grade made at all.

– Politically unsound concept because it attempts to mix the functions of geology and production into one which:-

– Slows down production

– Adversely affects bonuses

– Meets with departmental pressure, resistance and ultimately poor decisions

Manual Blast Hole Sampling

The issues are further compounded by the sheer scale of bulk commodity mining and, ironically, this goes some way to cloak the issues too because sampling in this reactive way often makes it too late to change course if identified at all. As a result, waste dumps are full of saleable grade ore that go completely undetected.

Iron Ore

Many people believe that quality sampling in iron ore is not a particularly significant issue to spend a great deal of time or money on as Fe is generally everywhere within the resource and reasonably consistent in grade throughout. In addition, it is often felt that any grade fluctuations will be picked up in the subsequent belt sampling stations downstream during the ore blending process and that this acts as a safety net for any previous misclassification of ore. It has even been suggested to the author of this article by some, but by no means all, senior members of geology departments within one of the major iron ore mining companies that, in their view, quality sampling systems at the drilling stage have no place in iron ore, only in the precious metals industry. Indeed, another major iron ore mining company even posed the idea internally to scrap all sampling, though this was later rejected. The point that is consistently missed in iron ore mining largely for the reasons given above is that it is not the iron content that we are altogether interested in from a sampling perspective. It is the content of impurities within the ore such as silica, alumina, sulphur, phosphorous, etc that can adversely affect steel production in the steel mills. Their known content is therefore critical to ensure the mill can be set up correctly for it. It is a product quality issue for both the ore supplied and the steel produced from that ore. These impurities are in the form of trace elements within the mined ore. It is therefore as important to sample iron ore correctly at the drilling stage in the same way as it is for a gold mine to sample for gold at the drilling stage. In both cases, it is trace elements within the ore that we are looking for and to find them, you need a quality sample every time. Such a sample cannot and will never be provided from a blast hole sample for all of the reasons explained before. Furthermore, when you consider the scale of iron ore production, the billions of tonnes mined, it has a greater potential to destroy profit in real dollar terms than it does in precious metals such as gold …and in a depressed market this could be the difference between viability and closure.


Another massive contributor to the unrepresentative nature of blast hole samples in general is the inherent loss of fines from both the bulk sample (leaving the hole during drilling) and the bag sample post splitting. The nature of conventional open hole drilling of blast holes causes material exiting the hole to be lost to voids and cracks in the ground. These can be both naturally occurring voids and, more often, broken ground resulting from the previous bench blast above and particularly its sub-drilled component. In order to prevent the hole from collapsing during drilling as a consequence of broken ground, it is often necessary to inject water to stabilise the hole. In doing so however, this also contributes to fines loss within the hole as it forms a paste with the water and never exits the hole as a result. Once the hole is stabilised and the material exiting becomes dry, the fines (dust) leaving the hole is then lost to atmosphere. In addition, conventional drilling has no ability to dry a sample when drilling in wet ground so, in this instance, there are no fines in the sample at all. In fact, it is not unusual to see zero drill cuttings (coarse or fines) coming to the surface in certain ground conditions. Copper mining faces a significant problem here because the copper itself is generally found in the oxide fines. If we lose the fines, firstly from the bulk sample and then more in the splitting, the resulting sample that is sent to the lab is completely unrepresentative based on fines loss alone and underestimates true copper content as it is a severely diluted sample.


It should be noted that in ores where the target mineral is generally found in the coarse particulate, fines loss has the effect of concentrating the sample leading to overestimation of mineral content. Either way, by dilution or concentration, the results and decisions made from them thereafter carry great risk and can be catastrophic …and, once again, in a depressed market this could be the difference between viability and immediate closure.

“So, how should we do it?”

RC (Reverse Circulation) Drilling for Grade Control is becoming more and more widely used in open cut mining throughout the world because it overcomes all of the issues presented by blast hole sampling. However, it is as critical to understand that RC Drilling rigs need to be set up with the right equipment on board to provide representative samples. We call this set up, “Balanced RC Drilling” and it comprises of the following list of equipment as a minimum requirement:-

Progradex on Schramm 450GT

– Rig Compressor correctly sized for the size and depth of hole.

– Reputable “Face Sampling” RC Hammers are used – In parts of the Americas, the term “RC” is referred to as a conventional hammer (similar to those used to drill blast holes) with a “cross-over sub” positioned above the hammer to redirect the cuttings from the hole into the inner tube. This is 1980’s technology that should not be used as it bears the same issues associated with blast hole sampling. A Face Sampling RC Hammer cuts and extracts the cuttings immediately and directly from the face of the drill bit preventing water ingress and contamination.

– Correctly sized Drill Rods with low pressure drop, sealing tool joints that have no ability to lose the sealing o-rings into the sample are selected.

– A quality Blow-down valve is installed – This keeps samples dry at all times preventing contamination, even when drilling below the water table.

– A correctly sized Sampling System is installed that is capable of capturing and representatively splitting all of the bulk sample that enters it (i.e. provides 100% recovery of both coarse and fine particulate for sampling). There is no point in going to the trouble of keeping the drilling dry, bringing the coarse and fine material to the surface only to lose the fines component at the sampling system.

– The Sampling System is capable of automation to completely remove people from the rig during drilling – removing risk of injury and human error.


This is Balanced RC Grade Control.

Note: The only sampling system in the world that is capable of providing 100% recovery for sampling and can completely remove sampling operators from a drilling rig is the Progradex Sampling System.


Progradex Protects Your People – Progradex Protects Your Asset