The leading suppliers offer their outlook on the current state of gravity-separation technology
By Simon Walker, European Editor
With the possible exception of visual sorting—"this one's green, this one's not," or however the Bronze Age equivalent ran—being able to separate minerals using differences in their density is the most ancient mineral-processing technique known to man. It is, after all, emulating the natural processes that formed the alluvial deposits in which gold and other dense minerals such as cassiterite were first found and, because of its effectiveness, it still plays a valuable role in a variety of modern mineral-recovery circuits.
Panning and vanning, later upgraded by mechanization into shaking tables and jigs, formed the main means of recovering dense minerals for centuries, and it was only the advent of flotation for sulphide ores and cyanidation for gold that relegated gravity separation to a minor place in the processing hierarchy. By contrast, its use in coal washing—initially using jigs then with enhanced process capabilities through the introduction of dense-medium systems—began in the late 19th century and has remained a washery mainstay ever since. Likewise the use of spirals for iron ore beneficiation. The big innovations in the past 25 years have been the development of 'assisted gravity' equipment, such as bowls for gold recovery, and the transfer of jig technology to a range of other minerals. Here, the contrast is that the concentrate is formed from sink material whereas in coal washing, the sinks are waste.
As the Australian company, Gekko Systems, notes, in recent years flowsheet designers have been reassessing gravity-separation systems for cost and environmental reasons, since they do not use increasingly expensive chemical reagents. And, the company says, there are many situations where a significant proportion of the valuable minerals in run-of-mine ore can be recovered into a pre-concentrate, thereby cutting subsequent mineral-processing costs.
Jigging Applications Range Extends
One of the most widely used gravity-separation technologies, jigging, has extended its range of applications significantly over the past quarter-century. Originally used almost exclusively in coal washing, jigging is now a recognized concept for separating a much wider range of minerals.
The concept of the jig is based on synthesizing natural processes of stratification in which denser material is concentrated below the less-dense in a water bath—comparable to the formation of alluvials but in a controlled process that can handle large throughputs. And whereas the original jig designs used mechanically induced pulsing to assist in stratification, modern systems use air-pulsing to achieve the same effect, but with much greater control and efficiency.
E&MJ asked one of the major suppliers of jig technology for minerals separation, Delkor (now a Tenova Mining & Minerals company), to explain in more detail how jig applications have been extended in the recent past. The company's product manager for jig technology, Anup Dutta, confirmed how in earlier times jigs were only used for coal processing, but that in the past 20–30 years under-bed pulsed jigs have become used increasingly for separating hard and dense materials such as iron ore, diamonds, gold, manganese ore, barite and heavy-mineral sands. "Jigs have also been used very successfully to recover metals such as ferrochrome and ferro-manganese from slags, at size ranges varying from 0.5 to 30 mm at varying granulometry," he added.
Dutta went on to explain how Delkor is now taking the lead within Tenova Mining & Minerals in terms of its jigging technology. "Bateman developed its proprietary technology known as the Apic jig," he said, "which was a technologically advanced version of some of the older air-driven jigs. It successfully commissioned Apic jigs in coal and heavy minerals such as iron ore and manganese ore, along with ferrometal slag applications. Since Tenova acquired Bateman and Delkor in early 2012, its jig technology has been consolidated under Delkor, with its products now being marketed as Delkor Apic jigs."
Benefits claimed for Apic jigs include their ability to deal with both fine and coarse material effectively and efficiently, and that they can concentrate either lighter float or heavier sink material in a wide range of particle sizes. Delkor also points out that they can cope with a wide range of deslimed and non-deslimed feed and discharge rates, as well as continuously varying feed grades. Another significant advantage is that their operating costs are lower than competitive separation technologies, while there is no loss of heavy media since none is used.
"Compared to other separation technologies, and specifically to dense-media processes, jigging has the edge in terms of its energy usage," said Dutta. "Generally speaking, jigging has a specific power consumption of around 2.75–3.25 kWh/mt of raw material feed, while dense-media systems need 3.75–5 kWh/mt, depending on the design considerations and the technology applied in relation to the feed characteristics and the throughput."
From Coarse to Fine
Dutta also explained that the sizing and design of a jig plant is very sensitive to the nature and beneficiation characteristics of the raw material to be processed. "Although attempts have been made to standardize the design, we feel that some design changes or modifications will be needed for each kind of feed. This will optimize the efficiency for that particular material, or if the system needs to be flexible enough to handle material coming from a number of feed sources."
To illustrate this, Delkor now produces jigs in sizes ranging from 500 mm to 8 m wide, covering applications ranging from pilot plants to full industrial installations. The number of jigging chambers, from two to seven, is also dependent on the required separation efficiency and the number of products, with individual unit capacities varying from 10 to 1,000 mt/h for coal plants and from 10 to 350 mt/h for heavy-mineral applications. In terms of particle size-handling capabilities, the company's fines jigs operate in the range 0.5–20 mm, medium-grain jigs from 20 to 80 mm and coarse-grain jigs from 75 to 125–150mm.
Today's jigging technology has become much more sophisticated and controllable through the use of systems such as Delkor's Jig Scan PLC software. By controlling parameters such as the pulsation frequency, stroke and pattern, this in turn means that the separation cut point can be fine-tuned more effectively.
Optimizing the cut point depends on achieving proper settling of the sink material, Dutta told E&MJ, which requires uniform feed distribution as well as controlling the specific throughput relative to the jig width, and the jigging air quantity and pressure. It is also important to have accurate measurements of the settled bed height, with advances in instrumentation and ultrasonic sensors having improved this. In addition, he said, the sink-discharge mechanism plays an important role in obtaining better cut-point control.
One of the biggest changes in jig applications has been the transfer of the concept from essentially coarse feed material, in which the density differences between float and sink constituents are clearer, to its use for separating fines. As Delkor notes, for example, jigging iron-ore fines is very difficult, and factors such as fluctuating pulse frequencies, pulse dampening, the extraction methods and dead areas on the jigging surface—which can be neglected in the case of coarse iron-ore jigging—can have a significant effect on the jig performance in this situation.
Bateman installed two of its Apic jigs for handling iron-ore fines. The first, at Corumba in Brazil, was subsequently optimized with Delkor applying the changes made to the next installation at an Indian iron ore operation. In both cases, the systems have worked well, Dutta said.
Changes made included reducing the bed depth in the first two jig compartments, allowing the jig pulse to dilate all parts of the jig bed effectively. This improved the unit's performance and the product grade, but at the expense of lower recovery, so Delkor then cut the velocity of the hutch water over the end weir to thicken the reject layer and prevent scouring of the jig bed. Finally, the company optimized the control settings on the jig, and the height of the chamber levels, testwork having indicated that recovery could be improved by around 8% by running chamber levels high compared to low.
An Australian Viewpoint
E&MJ also sought the views of Gekko Systems' business development director, Sandy Gray, on the way in which the use of jig technology has extended in the recent past. "Gekko has seen projects in polymetallics, which have not been a traditional gravity-separation area, and with gold sulphides being more reliant on gravity separation. We believe this to be the future, especially in coarser separation," he said.
"For instance at Pirquitas [Silver Standard Resources' mine in Argentina], we did an operation for silver, tin and zinc pre-concentration ahead of the mill in which we upgraded the ore by 30%–40%," he added. "Applications in polymetallics are starting to increase, and certainly in all types of sulphide recoveries."
Gekko has developed its primary gravity-separation system, the Inline Pressure Jig (IPJ), over the last 17–20 years, Gray told E&MJ, noting it has become a reliable continuous separator that can recover minerals down to the 100 µm. "The uniqueness of the jig allows it to be used in applications where there is a broad range of sizes to treat, and it is extremely useful in grinding circuits," he explained. "The other focus is the recirculating loads and gangue rejection with this unit."
"The key target area is in gangue rejection," he said. "We are not only focusing on the recovery of the valuable minerals into a small concentrate. Much emphasis is being placed rejecting gangue materials before spending energy on them—discarding them early in the process cuts the costs of energy, time and chemicals used down-stream. Every ton of gangue removed from the system early allows a further fresh ton to be treated, which increases the metal value delivered to the downstream process for the same level of energy absorbed."
According to Gray, Gekko's jigging technology was developed predominantly for production in extremely low-grade, high-tonnage operations and for recovering gold. However, early research and development work highlighted the potential for high recovery of sulphides and other heavy materials, leading the company to develop its InLine Spinner to recover free gold from IPJ concentrates that are typically heavy with sulphides and free gold. Designed for heavy concentrate feeds, the spinner deliberately exerts relatively low 'g' forces on the feed and has different separation mechanisms compared to other bowl-type centrifugal concentrators, he said.
"Coarse liberation and gravity is, in our mind, the way of the future, because of its reduced energy consumption throughout the process, and the focus on primary pre-concentration and gangue rejection ahead of the milling system," Gray stated. "Gekko's Python technology allows for all the concepts of gravity, pre-concentration and gangue rejection into one package. Developing the Python flowsheet and modular plant allows for a low capital and operating cost system with pre-designed flexibility."
Designed as a compact, modular processing system, Python consists of a process train encompassing coarse and fine crushing, screening, gravity and/or flotation separation, concentrate handling and a tailings-disposal system. By pre-concentrating underground, the company says, transport volumes can be reduced and costs cut. In the most recent edition of its Footprint newsletter, Gekko describes an updated version of the Python that has been re-engineered to handle gold recovery from surface waste-rock dumps. Gold Fields has been a long-term user of its equipment, according to Gekko, with an underground Python system at its Kloof mine in South Africa, and plans to use a surface Python to recover gold from its KDC East waste-rock dumps.
Challenges and Solutions
Gray identified several challenges being faced when working with gravity separation technology. Critical to success, he said, is getting the testwork right. It is essential to ensure the samples are representative due to the generally low number of samples tested in most programs. The success of testwork to the application is dependent on the reliability of the sampling to provide an accurate representation of the orebody.
A second area of concern is the skill level at mine sites. Gray noted that there is an inherent need to ensure that the skill level and education of the operators is taken into account, and that there is a focus during the commissioning process to allow for adequate operator training time. This is critical to the success of the plant, he said, adding that there appears to be a strong correlation between gaps in training and the efficiency experienced at the mine. Commissioning time, post-commissioning and optimization are pivotal to a system's success, he went on, with Gekko now offering operating contracts to assist in the optimization of plant installation and production ramp-up.
Gekko says that in general with the IPJ, the fine-tuning of cut points has been managed through the development of new ragging material. While parameters traditionally used to control the cut point in a jig include the upflow of water and pulse rate of the unit, in the IPJ the internal ragging performs this function. Instead of using naturally occurring ragging materials, which are essentially uncontrollable, the company has developed ragging made from spherical, metal-filled polymer balls, and defines the separation cut-point by changing the density and size of the balls. As well as increasing the efficiency and reliability of the process, controlling the ragging density and consistency in this way enables it to predict outcomes accurately.
New control systems provide very accurate and repeatable control of all the IPJ's operating parameters, such as the number of cycles per minute, downstroke speed and length of stroke, and will have a significant impact on achieving repeatable results and making on-line changes to the unit, Gekko adds.
Asked about improvements that could be made to current technology that would not require significant investment to provide a noticeable return, Gray replied that there are two key areas, the first of which is using fine crushing to produce the right product to feed gravity-separation systems. The second is fine gravity separation and continuous units to handle high tonnage of fine material, he went on, pointing out that this is a large R&D area where investment is required. "We need to consider gravity separation as a primary technology rather than a secondary area to allow for significant return and improvements," he said.
FLSmidth Develops Knelson
In September 2011, FLSmidth bought Knelson, the Canadian company that had—through its concentrator designs—established itself as a world leader in gravity-separation technology, with over 3,000 installations in more than 70 countries. Since then, FLSmidth reports, the innovative spirit that made Knelson one of the most widely recognized and accepted brands in the minerals industry continues to thrive. Doug Corsan, FLSmidth Knelson's global product director, told E&MJ: "Over the past 18 months we've made tremendous strides in the on-going development of our application-based CONE*Logic™ concentrate cone-selection system. This enables us to work with our clients to assess their specific metallurgical needs and operating conditions, to tailor a concentrate cone solution that is precisely to their requirements.
"We began development of the CONE*Logic system in 2003, and over the past nine years we have worked closely with many of our clients to devise a method of cone selection that looks at five fundamental factors of operation. These include mineralogical test data, target particle size and shape, circuit water constraints, water quality and ore abrasivity," Corsan added.
As part of its development of the industry's first fully customized concentrate cone solution, the company has developed a new cone manufacturing platform called the Matrix Cone. During extensive field testing over the past three years, it says, the Matrix Cone has proved to provide superior metallurgical recovery, along with extended cone-cleaning intervals on average by a factor of five times, while at the same time reducing operating and maintenance costs.
Shaking Tables Still Play a Role
Shaking-table technology may appear archaic to many mineral processing engineers, yet as Chris Bailey, managing director of UK-based Holman-Wilfley Ltd., told E&MJ, the company is still finding new applications for its two well-known brands. "We have remained committed to the development of gravity tables, providing niche separating areas in the mineral processor's flowsheet—often still unmatched by alternative more recent technology developments," he said. "This is coupled with an expansive demand in minerals from developing-world markets such as China and India for specific products like tin metal and titanium pigments.
"Tables are often needed at the end of a circuit so, for example, we see our units being used for cleaning-up pre-concentrates from Knelson concentrators in gold-recovery plants."
Bailey went on to explain that Holman technology evolved strongly in the 1960s and 1970s in tin-processing flowsheets around the world, and that these machines are now well-proven for reliability and for the metallurgical advantage of 'spreading' the concentrate products over a wider discharge area—allowing finer control of product grades. This design also lends itself to the separation of difficult minerals that have close specific gravities, such as are found in alluvial mineral sands. In modern flowsheets, the gravity table provides the final concentrating stages of high-volume rougher treatment by gravity spirals.
The company's Wilfley product line is marketed to the recycling industry for the separation of metals in electrical and electronic scrap, Bailey told E&MJ. "These units have long been in use in Europe for copper-cable reprocessing, and remain highly respected and capable in newer, advanced plants for processing electrical and electronic waste.
"In some cases these units operate with feed sizes well outside recommended gravity-table use," he said, "for example minus-6 mm, but they still provide the necessary separation in a way that is unmatched by more accepted dry-processing routes."
Expanding Application Interest
As can be seen, interest in gravity-separation technology is by no means at a low ebb. Take, for example, the use of cones in heavy-mineral recovery, or spirals in coal and iron-ore processing. The Australian company, Mineral Technologies, reported that during 2010-2011 it supplied ArcelorMittal with the largest single spirals order in its history, used for iron-ore upgrading at the Mont Wright operation in Canada. The company has also supplied its Kelsey centrifugal jigs to customers in Australia, South Africa, Brazil, Peru, Bolivia, India and the U.S.A. for processing zircon, rutile, tin, tantalum, tungsten, gold and nickel.
Meanwhile, in 2011 German manufacturer allmineral won a contract to supply an 80-mt/h alljig® unit for Rio Tinto's low-grade iron-ore pilot plant in Western Australia, a unit that can handle both fines and lump material up to 32 mm as well as treating a wide range of particle-size ratios, the company says. It had earlier supplied two alljigs to Outokumpu's Tornio ferrochrome plant in Finland, where they are used for reprocessing smelter slag.
And MBE-CMT reports that over 300 of its Batac jigs are now in operation around the world, handling iron, tin and manganese ores and alloy slags, as well as coal. The main advantages of Batac jigs, the company said, are higher efficiency, better product quality, higher availability and greater throughput rates, with units offering throughput rates of between 30 and 1,000 mt/h while handling 1–150-mm feed sizes.
Gravity separation is clearly still a valuable component in the mineral-processing toolkit and, as new applications are identified within the non-coal sector, its niche position looks certain to expand further.
Air Classification Enhances Gravity-Separation Effects
In the most recent edition of its customer magazine Results, Metso Minerals reported on two applications where its air-classification technology has helped customers achieve product-quality results by using centrifugal force to enhance gravity effects. In the first, Arizona-based Salt River Materials Group uses Metso centrifugal air classifiers to produce fine fly-ash for use as a feed material in the local ready-mix concrete and other markets.
The raw fly-ash obtained from coal-fired power stations contains a range of particle sizes, while the company's customers need coarse particles removed. Salt River now has three plants in operation, with a combined capacity of around 1.45 million mt/y, using classifiers that are designed for separations in the range 20–100 µm (635–140 mesh). Metso notes that wear in the classifiers is minimal, since there are no moving parts, that adjustments to the cut point can be made by fine-tuning the airstreams, and that the concept is suitable for other industrial-mineral and fine-grinding applications.
In the second case study, Luck Stone Corp. uses Metso gravitational inertial classifiers to produce finely graded engineered sand from rock produced at its quarries in the eastern U.S. Faced with the disadvantages inherent in wet processing systems, the company evaluated a number of dry-processing routes before selecting this type of classifier. Metso explains that it now offers three classifier systems: gravitational classifiers, which produce 0.15–1.65 mm (12–100 mesh) separations and are suitable for coarse industrial minerals; centrifugal classifiers, which produce 0.02–0.15 mm (100–600 mesh) separations and are suitable for industrial mineral, mining, fly-ash and cement applications; and gravitational inertial classifiers, which produce 0.063–0.3 mm (50–230 mesh) separations and are used for precise engineered-sand applications. By using these, and Metso air classifiers, Luck Stone now produces engineered sand to meet both asphalt and concrete industry specifications.