«Mining Mineralogy Mining is all about the recovery of value metals or minerals from their non-value host rocks Many methods through the years have ...»
NIR Mineralogical Analysis in Mining
Freeport-McMoRan Copper and Gold
I started working in the mining industry as a mineralogist about 35 years ago. I found out quickly that
mining was all about the recovery of value metals from their non-value host rocks. Everything we did
was to decide where the value metals and minerals were and what could we do to recover them. I found
there were many ways used to define the value metals.
Hand lens, Microscope, X-Ray Diffraction, SEM, Chemical analyses Mining Mineralogy Mining is all about the recovery of value metals or minerals from their non-value host rocks Many methods through the years have been used to determine which metals are present in which rocks
- Hand Lens
- Chemical Analysis Some are very accurate some are not Some are expensive and some are not Some are fast and some are not All are effective to an extent The need is to find a method that is simple, fast, inexpensive and accurate Some of these are very accurate and expensive, some are not. Some are fast and some are slow. They are all effective to a degree, but everyone was looking for a method that was simple, fast, inexpensive and accurate.
In my first job I was challenged to identify the minerals present in various processing streams at the mine production plant. The plant operators would often have problems with the plant and could not figure out what was happening. After checking to make sure no operating parameters had been changed, we all agreed that the issue had to be that the mineralogy had changed from the “normal” to something else.
What had changed was then the question. That sent me back to my lab with samples from the plant. I started with my hand lens to see what I could determine. Because of the size of the grains, between 50 and 150 microns, the particles were too small to identify. I then tried the polarizing microscope. This required a lot more sample preparation time, but allowed very good identification of the minerals present.
However, it was difficult to estimate percentages of the minerals present in the samples because of the huge variety of textures and intergrowths of some very fine-grained minerals.
This is still the situation today. The problems with plant upsets still occur every day somewhere in our world-wide operations. People still call and want to know what happened to the ore. The difference today is that we have many more methods available to help answer the question. We still have hand lenses and microscopes. They still have the same limitations. Today we also have X-ray diffractometers
Generally when an ore deposit is finally found and completely outlined and tested, there are then several different ways to recover the value metals. Which of these methods is used is dependent on the mineralogy of the value metal. Simplistically, mining deals with breaking the rock, crushing the rock and
processing the rock through:
Gravity separation Concentration Smelting / Roasting Leaching You have to know exactly which value minerals are present in the ore in order to determine which of these processes can be used effectively to recover the value metal.
To make the processes work, it is necessary to know how much of each mineral is present in the ores.
Once you know how much of each mineral (value metal minerals as well as host rock or gangue or waste
as it is called in mining) is present in the rock, one can determine or predict issues about the ore such as:
4 These factors and others have an impact on what must be done in order to extract the value metal from the rock. For example, hard and soft minerals have a tremendous effect on drilling of the rock. The rock must be drilled and then loaded with explosives so that it can be broken into a size that can be loaded into trucks for transportation to the milling plant. If the rock has hard minerals present, it will require more time to drill the blast holes and cost more money for drill bits. It will take more explosives to break the rock.
For concentrator ores, before the hard rock is delivered to the plant, there must be more crushing to reduce the rock to a useable size to feed into the plant.
5 Once in the plant the value metals must be released or liberated from the host or gangue rock. This is done with even more grinding in large SAG mills and ball mills. All crushing and grinding requires the use of tremendous energy to reduce the rock to the correct size. The percentage of hard and soft minerals present can make a huge difference in the operation of a mine and plant.
Once the ore is ready to be processed, other factors must be considered in order to recover the metals such as which reagents are the metal minerals reactive to? Can the metal be leached from the mineral? Will acid do the leaching? Do other gangue minerals consume the acid? Are there gangue minerals that will inhibit the permeability of an ore leaching pile? Does the mineralogy require that the whole mineral be floated away from the gangue rock? Are the minerals heavy enough that they can be separated from the gangue by gravity or do they need to be melted in a smelter to be separated?
6 So how do we determine the mineralogy of ores and host rocks? XRD is the standard that is used most of the time. We also have automated mineral analyzers like QEMSCAN. Both work very well, but have limitations. They can be expensive to buy, especially the QEMSCAN, which is a scanning electron microscope that has been fitted with 4 fast detectors for collecting elemental spectra, which is then converted into mineralogy based on a defined Species Identification Program (SIP). Sample preparation can be extensive for both instruments. Computerized Rietveld Refinement programs for XRD can now make quantification of minerals very fast and accurate. XRD has a limitation of materials that must be crystalline to be identified and QEMSCAN identifies minerals based on percentages of elements present in minerals, which can vary substantially. There is a need for an instrument that is simple, fast, inexpensive and accurate. Enter infrared.
7 Infrared has been used for mineral identification for at least 35 years. It is especially known for its use in Satellite Imagery.
8 For many years infrared spectroscopy, especially the near range, has been used for the characterization of various clay minerals, which are complex hydrous aluminum silicates. These instruments work by using the detection of molecular vibrations and the percent absorption or reflection of infrared light frequencies by these molecular bonds. Thus infrared can be used to identify minerals that are amorphous or noncrystalline as well as crystalline.
-6- 6 Swelling clays have a high cation exchange effect, which can disrupt the crystal lattices of the mineral and make it appear as an amorphous mineral to XRD. The chemical composition of these clays can be almost exactly the same as that of the original feldspars from which the clays altered, so QEMSCAN identification can be problematic.
9 Swelling clay is potentially one of the most detrimental or helpful minerals present in the mining industry.
It is critical to know how much swelling clay is present in the ore being processed. When dealing with various ores, we need to determine if the presence of clay is going to be detrimental or helpful. For example if we need to do a lot of drilling, the presence of clay will make the drilling go much faster and thus cost much less. This will allow much more ore to be mined and moved faster.
If we need to construct an impermeable liner below a stock pile, then the more swelling clay present the better.
However, on the other side of the coin, if we are trying to get leach solutions to pass through a stockpile to extract the copper, swelling clay can plug the stockpile and completely stop solution flow.
In the concentrating process, the presence of any clay can make the froth bubbles that we form to collect the copper collapse and very little copper will be recovered.
NIR can identify the swelling clay without difficult preparation. The percentage of swelling clay present can be quantified by a wet chemical cation exchange capacity (CEC) test. A quantitative NIR model can be built using the wet CEC measurements of matrix specific samples with variable swelling clay percentages over the full range of clay values present in the ores. This model is then loaded into the Indico Pro software on the ASD NIR and used to predict the percentage of swelling clay in mine samples.
The swelling clay value is determined in a matter of seconds with the NIR and this value is then used to
Often a complete mineralogy of ore samples is not needed. It may only be necessary to know if the material is “hard or soft”. For example drilling, blasting, crushing, and grinding are dependent on “hard” versus “soft” minerals. NIR can be used to identify the individual “soft” minerals such as clays, micas, talc, and others that have vibrational modes in the near range. The quantification of these additional minerals is calibrated with XRD measurements and used as predictive models as was the CEC measurements for swelling clay. The “hard” minerals are measured indirectly using the “soft” minerals as measured on the NIR. This measurement is done each day on hundreds of blast hole samples.
13 With the information obtained daily from NIR analysis of blast holes, the ore control geologist and mining engineers can make many decisions about what to do with the material from various parts of each blast. For example, before material is loaded into a truck a decision must be made whether there is enough clay to cause permeability issues in a leach stockpile or is there too much clay to send the material to a concentrator. If there is too much clay, then the material must go to a different type of process that minimizes the effect of clay. If the clay content is ok, then how much “hard” mineral is present and how will that affect the crushing and grinding circuits. Will too much extra time be needed? If so does the material need to be blended with softer ore to preserve throughput? Will metal recovery be lowered from lack of correct grinding time?
- 10 - 10 After the decision is made to route the ore material to one of the processes, decisions need to be made from this same information that affect the various processes. For example, the size of the blasted material may dictate which crushing unit is used. Once material gets on a conveyor after the first stage of crushing, water is added to keep dust down and ready the ore for treatment. In the leaching process the amount of water added is critical in order to form agglomerate balls that will not break down during the leaching process. The amount of water that needs to be added to form these agglomerates is dependent on the percentage of absorptive and adsorptive minerals present.
15 In the concentration process, the retention time needed in the various types of grinding mills, be they SAG or Ball mills, will be dependant on the percentage of “hard” and “soft” minerals present. The concentration process is built on the mixing of oily reagents with the water/ore slurry and then the addition of air to form bubbles. The bubbles then float to the surface of the slurry. The value metal minerals that have been liberated during the grinding have a surface reaction with the reagent bubbles so that they stick to the bubbles and are brought to the surface with the bubbles. This separates the value metal from the non value gangue minerals. However some of the gangue minerals have effects on the bubble production and the surface adherence of the minerals to the bubbles. This can cause non value minerals to float with the value minerals and dilute the percentage of metal recovered. If we know what kind and how much of the detrimental mineral are present, we can make adjustments in the type of reagents added and/or the time and amount of flotation steps needed to clean the gangue minerals from the concentrated value minerals. Once the value minerals have been separated from the gangue minerals, the gangue minerals must be separated from the water so that it can be recycled. This requires the addition of flocculant reagents.
Currently the measurements we make with the NIR are on blast holes in the mine before the material is actually mined and crushed. There is almost always a lag time between the day of mining and the day of
- 11 - 11 processing of the material. Sometimes there is a physical sampling device on a crushed ore conveyor belt from which samples can be prepared for testing. Usually once the result is returned from these samples, the ore has already been processed.
We still need to have a method of measuring the material being processed just as it is entering the processing plant so that correct decisions can be made about grinding times and reagent addition. We are currently working with ASD on an instrument for use on a conveyor belt to help determine the minerals present in the process circuit in a real-time, at-the-process, situation. Hopefully this will help us to work smarter, not just harder, to produce the metals needed for our civilization.
In the End, Hopefully this will help us be better neighbors to the other inhabitants of our world.
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