Effective plant process control needs data—lots of data—and sensor technology has taken big strides toward supplying the required information quickly and accurately

By Russell A. Carter, Managing Editor

On the scale of risk potential for industrial projects, the design and startup of a mineral processing plant most likely ranks in the upper percentile range. Unlike, say, an auto assembly plant, mine plants can’t be cloned from one site to another. Each mineral deposit differs from seemingly similar deposits elsewhere, and it’s impossible to pin down and eliminate every variable encountered when trying to process natural materials such as ore, coal or industrial minerals. Plus, unlike construction mistakes that often can be fixed quickly and sometimes even relatively cheaply, a defective flowsheet usually can’t be corrected quite so easily.

Apart from design errors, it occasionally happens that once a new plant progresses beyond the commissioning stage—after leaks and other obvious mechanical problems are found and corrected—process failure occurs due to misunderstanding or misapplication of laboratory or pilot plant findings. However, a quick scan of recent mineral processing literature hints that operator miscues also might be a bigger problem than expected as a cause of sub-par process performance. Because of inexperience or unfamiliarity with the technology, plant personnel may not understand or have a complete picture of what’s happening inside the plant’s process systems.

In addition to improved operator training, another way to solve this problem is through greater reliance on process control systems that make essential data easily accessible to operators, allowing them to make better, faster decisions. At the ‘business end’ of these advanced systems is a new generation of sampling and sensor technology that has progressed far beyond the warning-and-reaction role these devices originally filled. Today, sophisticated non-intrusive systems can provide operators with real- or near real-time understanding of not just what’s gone wrong, but also of what’s happening inside a plant’s pipes, tanks, columns and thickeners that can be adjusted or corrected to avoid problems and improve plant performance. The proliferation of wireless technology in industrial applications also provides the opportunity to provide operators with additional data, with less delay, often from points in the plant that previously were unmonitored due to physical or cost restraints. Mobile communications devices and ‘apps’ add another dimension of data accessibility, allowing plant personnel to keep track of equipment and system status without being chained to the control room chair.

From a broader standpoint, cumulative advances in sampling and sensing technology now allow mineral producers to maintain a digital data stream of information from the exploration and mining stages, through grinding and flotation, and extending to tailings management, environmental monitoring, site safety and security matters as well. Here’s a recap of selected recent product developments in this technological sector.

Before Processing Begins
Colorado, USA-based ASD Inc., a supplier of analytical instrumentation for materials measurement, has introduced two mining industry-specific spectroscopic measurement devices—the TerraSpec Explorer and TerraSpec Examiner—for analysis of minerals in exploration and production.

The TerraSpec Explorer spectrometer is designed for rapid qualitative minerals analysis during exploration and deposit mapping. According to the company, this portable instrument provides immediate mineralogical results and can assist in analyzing a wide variety of deposit types by determining the key alteration minerals. With a spectral range of 350–2500 nm and 6-nm resolution, it has the accuracy needed to help geologists evaluate samples across a broad area, rapidly determine the mineralogy of prospective mining locations, identify alteration patterns and classify ore systems.

The TerraSpec Examiner, says ASD, is designed for use in the lab or field to perform quantitative materials analysis of ore in production. It offers 10-nm resolution, a broad spectral range of 350–2500 nm, and can be used for monitoring moisture, acid-consuming gangue minerals, clays and other materials influencing process outcome. When combined with application-specific calibrations from ASD’s SummitCAL Solutions, mining operations can build on existing sample analysis to analyze a much larger set of samples in a significantly reduced amount of time. This can result in tighter process control and significant cost savings in a variety of mining processes, including agglomeration, flotation, ore sorting and heap leach.

As this report was being prepared, an article appeared in the October issue of CSIRO’s Process magazine describing a new, state-of-the-art X-ray imaging detector smaller than a postage stamp as possibly being the key to a powerful new method of characterizing mineral ores.

The photon-counting device, called Medipix, can take high-resolution images of minerals, detailing the materials present in an ore sample. Unlike conventional X-ray films and cameras, the new technique measures the energy of individual incoming X-rays and adds color to traditional black-and-white snapshots.

The author of the CSIRO article, Alexandra Roginski, writes: “While X-rays have long been used to image mineral ores, with shades of grey depicting the density of different components, these images do not easily identify the individual minerals present. It is akin to seeing a foreign object on an X-ray of a human body—knowing it shouldn’t be there, but not being able to identify exactly what it is. In the Medipix process, an X-ray tube fires beams through grains of ore. A sensor on the other side measures how X-rays of different energies are attenuated by the objects in the beam.

“Medipix then images the grains of ore in detail, producing pictures in which colors correspond to the elements contained in the sample. The detector collects information at more than 65,000 points simultaneously and multiple images can be obtained and joined together to increase the field of view.”

Research scientist Dr. Josef Uher said the new technique could revolutionize micro-imaging, and the mining industry.

“In every single pixel of the detector, you gain information about what the X-ray spectrum looks like. If you analyze it properly, you can determine whether the materials in the sample are nickel, copper, zinc, gold or something else,” Uher said.

According to the CSIRO article, Medipix could provide near real-time imaging of ores for plant monitoring and control in the mining industry.

Fine-tuning Flotation
Further downstream in the processing sequence, Numcore Oy’s new impedance-tomography imaging technology opens up entirely new possibilities to control flotation processes, said Jukka Hakola, the Finnish company’s vice president of sales and marketing. “Problems emerge if froth cannot carry the load of mineral particles or the process otherwise becomes disturbed. The froth bed then disappears, and restarting the process wastes valuable time,” Hakola said. “If the solid matter content and bubble size in the froth bed changes, and solid matter starts to gather under the froth, this can cause the froth to become rigid or to collapse. When this situation can be predicted, the problem can be solved by changing the operating parameters.”

With Numcore’s measurement devices, the size and quantity of air bubbles and the solid matter content of the froth bed can be monitored by means of electric conductivity distribution. Real-time performance is a key functionality in this technology; the system can continuously provide the operator with current data on what is happening in the flotation cells.

“Controlling a mineral concentration process has largely been based on experience-derived know-how. Now that operators can ‘look’ inside the process, it is possible for them to maintain an optimal mix all the time,” said Hakola, who cautioned that measuring, by itself, does not create added value—rather, the results should be used to control the operation. Consequently, Numcore has, in co-operation with key customers, developed measurement technology to better serve the routine requirements of day-to-day plant operation.

Depending on the diameter of the pipeline or tank, Numcore’s measurement devices are configured as either a flow-through sensor or as a probe-type sensor for installation inside large pipelines or tanks. By supplying a weak alternating current to the electrodes of the sensor, it measures conductivity differences between the phases—for example, with liquids inside pipelines and tanks, as different substances show different conductivity values. The actual image is created by means of inverse calculation, however, and this is where Numcore’s core expertise lies.

“Our CoreApus flow-through sensor lends itself for analyzing material flows inside pipelines of less than 1 m in diameter. Because processing tanks are generally larger than this, we have developed the probe-type CoreHydra sensor which can analyze a larger area of the tank. With several sensors, one can analyze even a large tank or cell,” said Hakola.

According to the company, both sensor types can accurately measure interfaces between liquids and solids, but CoreApus is also applicable for imaging rapidly flowing liquids.

“The biggest difference compared to previous systems is that now you can analyze why and how something is happening in the process, and how you can prevent an event if necessary. We focused on making the system easy to use to enable real-time and on-site corrective measures rather than relying on, say, laboratory samples,” Hakola said.

Employing a different approach, Melbourne, Australia-based Hawk Measurement Systems, a provider of level, positioning and flow measurement technology, has developed an acoustic device for optimizing flotation cell performance. Hawk’s very-low-frequency Acoustic Wave Transmitter is non-intrusive and can penetrate the froth to measure pulp height, according to the company. The sensor is mounted above the froth and pulp level, reducing maintenance or mechanical problems, and the transmitter can be mounted at walkway height for easy serviceability. The transmitter can be supplied ready for connection to a typical two-wire loop power supply used for the displacement float transmitter that it replaces. Remote-mounted transmitters are also an option.

Looking Inside, with Radiometric Help…
Thermo Fisher Scientific designed its new Nitus Gamma Backscatter (GBS) gauge to measure level, density and wall buildup in large mining vessels of any size and pipes 24 in. or larger. Using non-contacting gamma backscatter technology, the sensor measures overflow clarity and flocculants, facilitates control of solids in paste tanks and monitors solids concentration in settling tanks and clarifiers. The sensor on the Nitus gauge features a highly stable, sensitive scintillation detector that allows the system to respond instantly to level or density changes. Additionally, the sensor requires only small amounts of energy, needing only a small gamma source to achieve precise measurements. In many applications, says the company, the amount of gamma energy needed for a typical level or density measurement can be reduced by a factor of 200, without giving up speed of measurement or precision. The small source also lowers capital costs and improves plant safety.

Ronan Engineering claims its X96SI/R radiometric transmitter is ideal for difficult continuous level measurement applications in harsh process environments such as mining. The entire system mounts externally to the vessel or pipe and employs gamma energy to provide reliable and accurate measurement when other measurement techniques are not an option. The X96SI/R, according to the supplier, is the first radiometric transmitter to be fully Ethernet capable, enabling configurations, software updates, and data logging to be completed easily through the user’s PC using a standard Web browser. The X96SI/R includes a patented optical coupling that allows the transmitter and detector electronics assembly to be easily mounted to any detector configuration. The transmitter can also be remotely mounted in the field or control room. The X96SI/R is available in explosion-proof, weather-proof or stainless steel housings. The system is backward-compatible, enabling easy upgrades of existing systems to newer transmitter technology.

According to the company, the complete system can be installed and maintained while the process is running, without downtime, vessel modifications or risk of accidental release.

…and Without
Germany-based process measurement instrument specialist Krohne Messtechnik GmbH recently reported its Australia group had successfully applied Krohne’s Optimass Coriolis mass flowmeter to a tailings thickener underflow line to provide a non-radiometric density solution.

With annual production of more than 10 million metric tons of coking coal, the unidentified customer needed reliable thickener underflow density measurements to enable its preparation plant to optimize tailings disposal schemes to recover water, minimize tailings impoundment areas and comply with regulations over the life of the facility.

According to Krohne, radiometric density measurement has traditionally been employed on thickener underflow lines as they provide a non-contact solution to the high solids, high abrasion environment present in minerals processing. However, the company maintains the high total cost and regulatory requirements of owning radiometric devices is becoming prohibitive. In addition to the initial purchase price, licensing, transportation of nuclear source, documentation and administrative controls, radiometric devices require ongoing training of on-site radiation safety officers who can perform calibration, wipe tests and ensure regulatory compliance. Add to this decommissioning and disposal costs.

Krohne says its straight-tube Optimass 7000 series Coriolis mass flowmeters have been successfully used on abrasive fluids such as ore slurries and sand/water mixtures for more than 15 years. In these applications, a single straight measuring tube provides advantages over other designs with tube geometries that incur erosion and premature failure of flow dividers and bends in the abrasive fluid stream.

Krohne Australia also recently installed the supply flow, level, density, pressure and temperature instrumentation for the carbon-in-leach gold and silver processing plant at the Morobe Mining Joint Venture Hidden Valley mine in Papua New Guinea. The plant treats 4.6 million mt/y of ore and produces an average of more than 250,000 oz/y gold and 4 million oz/y silver.

Solid Results
For measuring levels of solid materials in storage, Siemens’ new Sitrans LR560 78-GHz (W Band) microwave radar level transmitter combines level measuring with high accuracy in tough environments, offering intelligent signal processing for optimum performance. The LR560 has a measurement range of 100 m (328 ft) and emits a narrow 4° beam. In addition to the advantages offered by its narrow beam configuration, its short-wavelength emissions provide excellent signal reflection, even from solids with a steep angle of repose. The lens antenna is highly resistant to material build-up and a built-in purge system is included for cleaning exceptionally sticky material. The high-frequency operation makes aiming unnecessary, but an optional aiming flange helps direct the beam to an area of specific interest, such as the discharge area of a silo cone. A high-resolution graphical interface guides the user and expedites start-time to less than 1 minute for accurate readings without need of fine-tuning.