Mines today do not have the luxury any more to be able to just mine the good or stable quality areas. Permits might not be extended or the decline in material prices put pressure from the commercial side into increase the recovery rate. More autonomous recovery methods, less human interaction and a move towards Industry 4.0, demands increased automation, improved communication and monitoring, along with self-diagnosis and new levels of analysis. This enables faster decision-making procedures in a close-loop control and will result in smoother processes that will free up workers for other tasks. Conventional ‘offline’ methods like sampling and laboratory analysis are too slow for that decision-making process.
Conventional online analysis technologies and their limitations
There is a range of online analysis technologies available for the analysis of the mineral content in the mined material: Radioactive type analyzers, X-Ray analyzers, Laser-based systems. They are discussed and compared in numerous articles (references?).
The main drawback of the probably most commonly used prompt gamma neutron activation analyzers (PGNAA) is that it works with a radioactive source and that safety measures and permits are required. Supply and import of new sources is getting more and more difficult and starts to be banned in an increasing number of countries. On an analytical side, the technology is limited to an elemental analysis of inorganic components in the material and in most cases rather sensitive to belt load variations.
X-ray and laser-based systems have less requirements on an operational permit side. They are, same as the PGNAA, elemental analyzers for inorganic constituents. A main analytical difference is that the analysis is on the surface. Consequently, the surface needs to be representative and for the X-ray also preferably with a small particle size. In both technologies, only a very small area on the surface of the conveyor belt is analysed.
All of the above-mentioned technologies need a separate moisture analyzer as they cannot analyze moisture directly. As the analysis of the ore needs to be moisture corrected, the error of the moisture meter is a basic error in all of the above analysis technologies.
Looking at the limitations of the technologies above, the ideal analyzer would be free of any kind of radioactivity or permit requirements, so it can be installed and operated long term in any country and remote area. It should be able to directly analyze moisture, organic and inorganic components of the ore, possibly an online mineralogical analysis as the process control in most cases not only depends on an elemental analysis of the inorganic constituents, but also on moisture, organic, total carbon and mineralogy.
Belt load variations (PGNAA and PFTNA) and the small analysis spot (X-Ray, LIBS) also add to the range of errors, that should be avoided using a different technology.
Near Infra-Red Technology for Online Analysis of Mineral Ores
A still new technology in the market for the online analysis of minerals is based on a near infra-red (NIR) source: a simple halogen light bulb. NIR means electromagnetic radiation in a wavelength range of 700 nm to 2500 nm, emitted by a light bulb or just plain sun light.
The Near Infra-Red source gives the technology its name: NIR. In NIR applications, light emitted from the halogen bulb shines on the material that needs to be analyzed. The material absorbs part of the light, more exact part of the wavelength spectra. Each molecule of the material absorbs the light in its specific wavelength. What is not absorbed is reflected. The reflection is detected by a lens of a spectrometer. This spectrometer analyses the absorption and returns the corresponding spectra.
Figure 1. Schematic of the measurement process of the NIR online analyzer technology
One important aspect of the NIR technology is that it is a surface analysis. The light in the NIR application does not shine through the material. The technology analyses what it can see on the top. The consequence is that for an accurate analysis
- material needs statistical homogeneous: this is the case as the material goes through a crusher and falls (from a statistical point of view) in random order on the belt.
- a very high measurement frequency is required to ensure that all material is measured: the analyzers takes a measurement every 428 ms and averages these results over one minute
Under these conditions, especially avoiding installation locations of non-linear layering, the NIR Crossbelt analyzer has proven to work very accurate in cement and iron ore applications.
One other important aspect of the NIR technologies is that NIR can directly analyze moisture, organic and inorganic constituents and is sensitive to mineralogical changes in the raw material. The energy in the near-infrared range is absorbed by the material (minerals, organic, moisture) on the belt, the minerals and their molecular structure. Every mineral has its own unique signature in the NIR spectra as you can see in the graphs below:
Limestone vs. Dolomite
Hematite vs. Magnetite
Benefits of using NIR in Online Belt Mineral Analysis
Some of the benefits of a NIR based Crossbelt Online Analyzer are already indicated above:
- it’s a technology completely free of any kind of harmful radiation and no permits are required
- light bulbs as sources can be replaced easily by the plant personnel
- it’s a technology that can directly analyze moisture, organic and mineral content online, without interfering with the materials
A look at the actual analyzer setup will show additional advantages:
NIR CBA installation NIR Airslide installation
- light weight installation (<150 kgs), over belt, no conveyor belt requirements
- installation as Crossbelt (for coarse rock, <150mm) or Airslide (for pulverized material)
- whilst being a surface analysis the measurement spot covers the belt width
- simple maintenance and low OPEX (sources are halogen light bulbs)
- no recalibration required
Still, as for every analytical equipment there are preconditions required to obtain an ideal result:
- Representative Surface:
NIR along with LIBS and X-Ray technologies are surface analysis technologies. NIR though covers a much larger analysis area then the other technologies. The positive part of a surface analysis is that variations in belt load do not influence the measurement accuracy. What is required for the NIR technology though is a representative surface – locations with a non-linear layering should be avoided.
Situations where you can analyze the material after a crusher or a mill are usually ideal and give a representative surface:
Figure 3. The homogenization of raw material inside the crusher
Figure 3 shows a truck bringing the different raw materials. In the crusher these materials are crushed and mixed, opening many rock faces. These are measured afterwards by the NIR analyzer.
- Changes in mineralogy:
The NIR analyzer will detect changes in mineralogy and can be used to return online mineralogical information. At the same time this means that the analyzer needs to be trained (calibrated) to understand (read) the mineralogy and composition on the belt. This can be covered during the calibration procedure or new material can be added to the calibration at a later point in time. Situations where the mineralogy and sources of the material are undefined and constantly changing need to be closely evaluated
Fields where NIR can be deployed in Online Mineral Analysis
Application of the NIR technology is typically in the mine environment to have a very early idea about the composition of the ore. Targets can be either to sort or to blend the material. More autonomous mining equipment will make this online analysis technology even more important to sore the earth from the ore body.
Figure 4. General setup of an online analyzer for pile blending
A second application is in the process control in the processing plant. Here sometimes there are specific process parameters, which may not necessarily be the prime ore that is processed, but e.g. total carbon and sulfur for a roaster process or main ash minerals for slag formation. The goal is always to come to a closed loop control cycle so the system can react in time to adjust e.g. additives or process parameters like temperature, air intake, pressure, etc.
Applications of NIR Technology
The NIR technology can be applied in the following industries: (maybe as a table?)
- Bauxit: raw materials blending or sorting to stockpile (Crossbelt) – based on mineralogical analysis, process control at the refinery (Crossbelt)
- Cement: raw material blending to stockpile (Crossbelt), process control for stable raw meal (Airslide after the raw mill), process control at the cement mill for SO3 control in the finished cement and reduction of the clinker factor (Airslide after the finish mill)
- Coal: raw materials blending or sorting to stockpile (Crossbelt), grade control for export (Crossbelt), coal mix blend control after stockpiles (Crossbelt), process control for combustion to adjust air intake or flux additives (Crossbelt), fly ash analysis for ash composition and carbon in ash (Airslide or Crossbelt), also for analysis of Coke
- Gold: raw material blending to stockpile (Crossbelt), process control for roaster application (Airslide)
- Gypsum: raw materials blending or sorting to stockpile (Crossbelt), process control (Crossbelt or Airslide)
- Iron Ore: in the mine for raw materials blending or sorting to stockpile (Crossbelt), grad control for export / shipping (Crossbelt), incoming quality control (Crossbelt), process control in sinter and pelletizing applications (Crossbelt)
- Lime: control of remaining CaCO3 (Crossbelt)
- Manganese: raw materials blending or sorting to stockpile (Crossbelt), process control at the smelter (Crossbelt or Airslide)
- Nickel Copper: raw materials blending or sorting to stockpile (Crossbelt), process control at the smelter (Crossbelt or Airslide)
- Phosphate: raw materials blending or sorting to stockpile (Crossbelt), grade control for shipping (Crossbelt), grade control during processing (Crossbelt or Airslide)
- Potash: raw materials blending or sorting to stockpile (Crossbelt), grade control for shipping (Crossbelt),
- Refuse Derived Fuel (RDF / Waste): Quality control in RDF production (Crossbelt) and Process Control in RDF firing
- Zinc: raw materials blending or sorting to stockpile (Crossbelt), process control (Crossbelt or Airslide)
SpectraFlow NIR Online Analyzer Installations
The NIR mineral online analyzer technology was developed by ABB since 2007. SpectraFlow was founded in 2013 as a management buy-out and the key people joined the new company. 32 analyzers were commissioned by the SpectraFlow team since then.
In the cement industry the main driver for online analysis is to feed a stable raw meal to the kiln. As the quarry products are mostly very heterogeneous, it is important to balance the main components CaO, SiO2, Fe2O3 and Al2O3, while not exceeding undesired constituents like MgO, Cl, Alkalies etc.
Picture x. Cement process flow diagram with applications for SpectraFlow online analyzers
Consequently, the first installation point is after the crusher to control the chemistry on the stockpile and avoid too large variations or being completely off target. In this application, SpectraFlow currently has 12 analyzers running in that installation location in Europe, Middle East and Northern Africa.
The next step in the cement process is at the raw mill, where the raw materials are ground, and additives are added to obtain a chemically stable material. This is a prime example for the SpectraFlow analyzers as the analyzer is installed behind the raw mill on top of the airslide: it enables a very accurate analysis, including filter dust and allows a direct comparison with the XRF offline analysis. 16 analyzers are installed and running from China to Brazil. The typical result is that with the control of the the SpectraFlow airslide analyzer, the standard deviation in the LSF is cut in half.
The third possible installation point for a SpectraFlow analyzer is behind the finish cement mill to control SO3 and Chlorine and optimize the clinker factor (= CO2 reduction). Currently 2 projects are under execution.
The next main application for SpectraFlow is in the iron ore sinter plant.
Besides the control of the incoming raw material the analysis of the raw material mix feeding to the sinter drum is essential:
- the analysis of the carbon content will trigger the coke dosing to get an even temperature in the kiln
- moisture content is controlled and if necessary adjusted for a good formation in the sinter drum
- basicity is analyzed to adjust the lime addition
In this application SpectraFlow has 4 analyzers installed. Three more analyzers are installed for moisture and carbon control after the sinter drum and one more analyzer for the returns after the sinter kiln for carbon control and sinter quality control.
Other applications which are currently under commissioning are
- an airslide analyzer in a gold mine for the control of a roaster with sulfur addition (analysis of Total Carbon, Arsenic, Gold, CO3 and Sulfur). The goal is to dose additional sulfur per requirement and control the temperature in the roaster.
- A crossbelt installation at a bauxite Alumina bearing minerals can be in the form of a hydrate (e.g. Gibbsite, Boehmite, …) or as a silica bearing mineral (e.g. Kaolinite). This installation requires a mineralogical analysis as the silicates e.g. from quartz / sand in the ore need to be distinguished from the silicates in the kaolinite. Also, not the total alumina can usually be processed, and the analysis of the available alumina is of interest.
Next installations will be about
- nickel – copper ore to optimize the feed and the parameters at the furnace
- in a coal mine for coal blending and a constant feed to the power plant
All existing installations prove the stability and robustness of the equipment. The analyzer is available 24/7 and the upkeep can be done by rather unskilled labor. The sources (light bulbs) last between 9 to 15 months, which makes the operation and OPEX of the analyzer extremely efficient.
The flexibility in the applications, the robustness and that no radioactive sources are required make the SpectraFlow NIR Online Analyzer technology more and more popular throughout the world.