A roundup of current technologies that use the force of gravity for heavy mineral recovery
RMS-Ross’s variable-pitch sluice can be adjusted to tailor the slope to the individual site conditions.

A previous article (E&MJ, November 2012, pp. 54–58) on the use of gravity separation systems ended with the comment: “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.” So, has this prediction come to pass, and what technology developments have there been in the past four years? E&MJ asked some of the world’s leading suppliers of this type of equipment for their views.

Far predating many of today’s mainstream mineral processing technologies, gravity separation was unquestionably the earliest method of mineral recovery devised. An understanding of the mechanisms behind the formation of alluvial gold and tin deposits led to the invention of sluicing and panning—techniques that still provide small-scale miners worldwide with their livelihoods, and which have since been developed as mainstream recovery methods for heavy minerals.

The principles behind the water jig were well-understood in medieval times; dry jigging is a more recent development. With increasing interest in recycling, the use of tables has extended from primary mineral production to recovering the valuable components in scrap, while spirals are a key component for upgrading commodities such as mineral sands, metallurgical coal and iron ore.

And, of course, the relatively recent development of various types of bowl concentrators has improved on the concepts behind simple panning by enhancing the gravity effect available through the use of centrifugal force.

20 Years of Tables

Having inherited the rights to the technologies from the original equipment manufacturers, U.K.-based Holman-Wilfley has been producing Holman and Wilfley shaking tables for nearly 20 years. The company’s managing director, Chris Bailey, told E&MJ that its Holman option provides a unique design layout and separation capability, and provides higher concentration and recovery of ultra-fine minerals compared to standard Wilfley designs.

New construction materials and engineering techniques, incorporating know-
ledge from the automotive industry, have contributed to the machine advancements, Bailey added, although the known principles of hindered settling have remained the focus in an ever more challenging minerals market. “These machines are associated closely with other gravity devices as part of flowsheet development, although no recently developed devices are capable of matching the enrichment ratios possible with gravity tables,” he said.

Applications include titanium-bearing heavy mineral sands, tantalum, tin, tungsten, chromite and gold, with the company currently closely associated with tin and tungsten projects in Spain, Australia and South America. Holman-Wilfley also recently supplied 42 of its Holman 8000DD table decks to titanium producer Tronox for its Namakwa Sands Wet Gravity Extension Project, some 350 kilometers (km) north of Cape Town on South Africa’s west coast. The units recover zircon from rutile and ilmenite-separation tailings, with the initial concentration using spirals and final concentration on the Holman tables.

Some of the shaking tables supplied by Holman-Wilfley to Tronox for its Namakwa mineral-sands project.
Extending Jig Capabilities

According to marketing manager, Luciano Devoto, Gekko Systems developed its Inline Pressure Jig (IPJ) over 15 years ago. While it was originally used to pull free gold from milling circuits and within alluvial gold circuits, over the past few years, the equipment has been modified to treat a wide range of heavy minerals, utilizing different jigging parameters, screen configurations and ragging densities. The unit can now be completely automated and integrated into mill-control systems, making it a more robust option for recovering heavy minerals, he said.

In addition to these technical advances, the IPJ’s range of application has been expanded. Gekko now uses it as a preconcentration device, which enables operators to recover high mass yields to concentrate. This is very suitable for high sulphide recovery and enables the unit to be employed in preconcentration and gangue rejection applications.

Advantages claimed for the IPJ include high mineral recovery and unit throughput, better security since the equipment is fully enclosed and pressurized, local control of the jig parameters, and low installation costs, with improved plant capacity and lower operating costs being delivered by rejecting low-value material ahead of further processing. Gekko also stated that the IPJ uses 20% less power and consumes 10% of the water, compared to traditional jigs.

IPJs are available in several sizes, with feeds up to 250 metric tons per hour (mt/h) for larger operations, as well as having a small operational footprint and being easy to ship. Flexible jig components and adjustable operation modes allow the IPJ to be set to specific ores and current applications, Gekko noted.

For the coal industry, the company now offers its IPJ-Coal Separator (IPJ-CS) used to remove heavy gangue and ash from lightweight coal material, with less impact on the environment compared to conventional coal plants. Thoroughly tested on a range of Australian coals, the IPJ-CS helps to increase saleable coal quality and quantity as well as improving waste recoveries, reducing energy and transport costs, and streamlining coal processing.

A cut-away through FLSmidth’s continuous variable discharge (CVD) bowl-type concentrator.
Enhancing Gravity

With capacities ranging from laboratory scale to 1,000 mt/h of solids, the Knelson batch concentrator from FL-
Smidth combines centrifugally enhanced gravitational forces with a fluidized bed process to recover precious metals. Project engineer Maher Sabouneh told E&MJ that these units can operate on feed densities of up to 75% solids while achieving best-in-class metallurgical performance.

Meanwhile, the Knelson continuous variable discharge (CVD) concentrator is designed for the recovery of high-density minerals where these exceed 0.5% of the total feed solids. The CVD concentrator uses similar principles of mineral separation and recovery to that of the batch Knelson concentrator, but allows the concentrate to be ejected from the fluidized bed continually, Sabouneh explained. It can achieve mass yields of up to 50% on minerals such as gold sulphides, tungsten and chromite.

Knelson batch concentrators can be installed in grinding and regrind circuits, treating either a portion or the entire circulating load. This leads to improved overall gold recovery, higher and faster gold payment, reduced stripping and downstream reagent costs, and improved security, he added. They can also be installed in flotation streams in open circuit to remove gold values.

Used for preconcentration ahead of leaching, Knelson continuous concentrators are installed in the leach feed to recover slow-leaching species, allowing aggressive post-treatment such as fine grinding and intensive leaching to be employed. Alternatively, both batch and continuous types can be used for tailings treatment, with batch concentrators better suited for scavenging of free precious metals while continuous concentrators are used to recover slow-floating or slow-leaching sulphides.

Redesigning Table Options

John Christophersen, international sales manager for Deister Concentrator, told E&MJ the company specializes in manufacturing shaking tables for gravity concentration. Recent advances have included the development of enhanced riffling systems for sand and slime mineral separation, and the introduction of molded HDPE launder systems for Deister’s Triple-Deck 999, Double-Deck 99 and Single-Deck 6 shaking tables.

With the enhanced riffling systems, particular emphasis has been placed on cleaner applications with the introduction of “take-off” riffles for handling greater quantities of concentrate without sacrificing capacity. The molded HDPE launders provide the operator with better vision to observe the separation on the middle and bottom decks of a triple-deck table, while they are easier to adjust because of their lighter weight when compared to steel launders. In addition, the slick interior surface reduces material backups and the use of “push water.”

The company claims that its modular Triple-Deck 999 suspended tables offer a number of advantages over conventional designs, and they have been shown to outperform virtually any other concentrating tables on the market today. With a rhomboidal shape, they are easy to operate with low maintenance requirements, Deister stated, and since they are fully suspended, they require less floor space than floor-mounted units. This provides an advantage for the operator in remote locations, and also where a concentrator has to be moved from site to site.

Christophersen said the company is now completing designs for four-deck, five-deck and six-deck suspended tables. As mining companies consider gravity concentration of finer minerals, floor space becomes a consideration, he pointed out, with these new designs allowing for increased capacity per unit of floor space. The largest Deister table installation is currently located in southeast Asia, where more than 100 Triple-Deck 999 tables are used to concentrate tungsten ore.

He also noted that the company is seeing greater use of its products as finishing tables for gold recovery, especially with the single-deck six and lab-size 15-S tables, which have shown the ability to recover greater quantities of gold than other tables and table-type devices.

The Falcon C 1000 bowl concentrator from Sepro Mineral Systems can handle up to 74 m3/h of slurry, generating forces of 50-300 G.
Spirals for Minerals and Coal

With its headquarters in Queensland, Australia, Mineral Technologies is one of the world’s leading suppliers of spirals for mineral processing applications. The company’s business development manager, Dale Henderson, explained that over the past 15 years, the company’s technology development programs have resulted in the introduction of several new spiral models.

For example, the FM1 fine mineral spiral was developed specifically to process fine mineral sand ore in the range of 30-150 µm (100–400 mesh), but has since been applied to a number of other fine mineral separation applications. In this size range, it is also an alternative to flotation where a specific gravity difference exists between the valuable and gangue minerals. Available in 6.5- and 8.5-turn configurations, with single, twin or triple starts, each FM1 can handle 600 kilograms (kg) to 2 mt/h of solids at a 30%-55% solids pulp density.

The MG12 two-stage rougher/scavenging spiral has 12 turns and features three auxiliary concentrate (slide) splitters spaced at three-turn intervals. A pair of ganged, pivoting splitters at the bottom provide for a fourth concentrate cut and facilitate the middling/tailing split. The model is available as a single-, twin- or triple-start assembly.

The company said the high separation efficiency afforded by the 12 turns of the MG12 means that, while its trough diameter is no greater than that of an MG4 or MG6.3 spiral, its performance is more resilient to increased loading. Hence it has a higher feed capacity. The high concentrate grades and recoveries achievable on an MG12, in a single pass, lead to concentrator plants that are both greatly simplified and capable of higher overall metallurgical performance, it added.

Other recent developments have included the HC33 spiral, used to treat streams containing mostly high specific gravity (SG) material, and which can be operated with the use of additional wash water if required. Meanwhile, the four- and eight-turn LC3 spiral for coal cleaning consistently achieves an SG cut-point down to 1.45 and produces Ep values of 0.1, lower than any other spiral currently in the market, Mineral Technologies claimed. These spirals enable coal plant operators to generate cleaner (low ash) coal or a higher yield of coal with the same ash.

Lastly, the company’s CS1 spiral has been developed specifically for the beneficiation of chromite. It is a short, three-turn spiral that is claimed to provide high separation performance in a compact and low-cost device.

Sepro Sells to Russia

Canadian company Sepro Mineral Systems offers three versions of its Falcon gravity concentrator: the C, SB and UF models. Designed for continuous duty, with unit capacities of up to 100 mt/h, Falcon C can produce mass yields as high as 40%, its primary objective being to maximize mineral recovery and reduce tonnage to downstream processes. It can also be used for scavenging or preconcentrating, since no water is added during processing.

The larger Falcon SB is a semibatch machine, with continuous feed and periodic rinse cycles. Generating 50 to 200 G, capacities are up to 400 mt/h, with a typical application being the recovery of liberated precious metals within a grinding circuit or from placer deposits.

Meanwhile, the high gravitational fields (up to 600 G) generated by the Falcon UF allows the recovery of ultra-fine particles as small as 3 µm. A common application is in scavenging ultra-fine particles from a plant reject slimes stream. All of the Falcon models have a small footprint, and feature fully automated, one-touch operation that provides the least amount of offline time and highest possible concentrate security, Sepro said.

Earlier this year, the company reported the sale of further SB750B gravity concentrators to Polyus Gold’s Natalka project in the far east of Russia. Polyus’s operating subsidiary, Rudnik Imeni Matrosova, commissioned its gravity-only gold-recovery plant in 2015, with the first stage achieving around 65% recovery from ore grading about 2 g/mt. The additional Falcon units are expected to increase overall recovery to 72%-75%.

An array of Mineral Technologies spirals in a mineral sands concentrator. The Queensland, Australia-based com-
pany recently completed the world’s largest such installation for the Grand Côte project in Senegal, where the floating treatment plant has a feed capacity of 7,000 mt/h.
Outotec’s Spirals

The HC8000 spiral from Outotec is a seven-turn, wear-resistant polyurethane/fiberglass unit. Each spiral start has up to six super concentrate collection splitters designed for easy adjustment and repeatable positioning. The product discharge box splitter has a similar design. All splitter handle indicators are readily visible so that the operators can see and adjust large numbers of handles accurately and quickly. Spirals are supplied complete with detachable feed and product discharge boxes, and the product box will collect product, middlings and tails from each spiral start simultaneously.

The company’s five- or seven-turn H9000 series spirals are available as either washwater or washwater-less designs. According to Outotec, the washwater model (H9000W) is suited for the treatment of high-grade feeds, like iron ore, which might benefit from supplementary washwater to remove gangue minerals. Each spiral turn on the H9000W is equipped with washwater cups, with a washwater distributor proportionately distributing water to each cup down the length of the column, regardless of changes in the flow rate.

Both spiral models are available in single-, double- or triple-start configurations.

Derocking to Increase Reserves

Company President Scott Plummer told E&MJ that RMS-Ross Corp.’s Derocker system allows rapid easy processing and recovery of the values concentrated in bouldered alluvials. The units can handle boulders up to 1.2 m (4 ft) in diameter, bringing previously unmineable ground into reserves.

The company has recently changed to using high-impact flat-pan spray nozzles and high-pressure sprays, which has given more effective scrubbing of values from coarse boulder and cobble while using much less process water. The underflow can then feed downstream secondary recovery circuits, such as advanced jig plants and centrifugal concentrators. The Derockers are equipped with remote control for safer operation, with full speed and stroke adjustability to suit the ore.

Plummer also explained that RMS-Ross’s modular circular jig plants can be shipped, set up and taken down easily, having high commonality of parts. They have been redesigned to better handle feed ores with unusually high concentrations of heavy mineral, he added. All slurry paths can be autoclaved with rubber for long life. The modular design allows for scaling of the operation, while the potential use of a range of different specific gravity jig ragging or jig bed media can also allow for more effective separation of the target minerals or metals. The company’s new secondary jig design allows for the same travel distance of feed material as on the larger 9 ft jig, but in a compact arrangement, he said.

RMS-Ross’s variable-pitch sluice box has also been designed to make life easier for placer miners. Since they sit on level platform areas, these sluice boxes no longer have to be moved and the ground graded and regraded to get the pitch of the box just right for the feed material. They can be dialed in even when in full operation, Plummer said, to be sure that the riffle vortex action is maximized for the volume of feed material and process water.

The Gekko gravity concentration plant at Castlemaine Goldfields’ operation, Ballarat, Australia. The plant handled 250,000 mt of ore grading 6.1 g/mt in the year to March 2016, producing 41,516 oz of gold.
Other Options

E&MJ asked each of the companies for their views on whether they envisage gravity separation becoming used more widely within mineral processing, perhaps in a preconcentrating role. “This only makes sense in so many applications,” said RMS-Ross’s Plummer. “The gravity separation process economics are clear. Any preconcentration of heavy minerals that can be completed with gravity separation before getting into secondary and tertiary concentration and separations of product streams should certainly be considered first in any potentially applicable ore situation. This can considerably decrease downstream processing costs. There has been quite a resurgence due to its simplicity, effectiveness and low environmental impact.”

Dev Saini, spokesman for Sepro, agreed. “Absolutely,” he said. “Sepro is well aware of the need to reduce the capital costs associated with new projects as well as reduce industry-wide energy consumption. We believe that effective preconcentration and early waste elimination is going to play a key role in mineral processing plants of the future, and we believe that gravity concentration is perfectly positioned to play a role in these developments, specifically for particles that are too fine to be economically sorted through sensor-based techniques.”

For Gekko, Devoto said, “We believe that preconcentration will play a very important role in the future of mineral processing. The ability to recover a high mass yield using a continuous gravity concentrator is very appealing, as it offers a very low-cost solution to upgrade what would normally be considered uneconomic ores.”

And with Mineral Technologies’ perspective, Henderson added, “Our spiral separators have been applied as preconcentrating devices for many years. We believe these applications will grow.”

And for the future?

“Leaving aside new applications in terms of other minerals, are there any technology improvements that you can foresee that could offer better separation than can currently be achieved?” asked E&MJ.

From Holman-Wilfley, Bailey suggested the continued development of operational control and instrumentation. Devoto looked at manufacturing aspects. “Most of our work to optimize the ‘separation’ is in adjusting the internal ragging density used within the IPJ,” he said. “Gekko has its own injection molding machine enabling us to produce any ragging size at any SG. This really helps in fine tuning the separation of the mineral no matter what size or density split we are trying to achieve.

“The other thing is to look closely at the liberation characteristics of the ore,” he added. “Often a slightly finer crush can make a big difference in the separation efficiency of the unit.”

Henderson of Mineral Technologies concentrated on design matters. “Our current research and development is focusing on a significant change to the design of spiral separators,” he said. “It is too early to detail these developments, but suffice it to say that the spirals we supply into gravity concentration plants in five years’ time will look nothing like the spirals that we see today.”

For Plummer at RMS-Ross, a key development will be the use of gravity separation to widen operators’ product options. “One of the exciting advances that we have been seeing with our RMS modular jig plant is in tailoring the concentrate multistage separation and analysis lab work. Where previously customers had one or two target minerals or metals, since these plants concentrate the vast majority of all the heavy mineral, rather than rejecting all but the heaviest, we have been able to work with specialized mineralogical labs in order to create up to seven high-grade saleable products from the one concentrate stream and actually multiply what they had projected as their revenue stream, as well as greatly reduce the residual concentrate tailings.”

And for Dev Saini at Sepro, the future is already here. “Sepro is about to commercially release a centrifugal concentrator for gold recovery that does not use any additional fluidization water,” he told E&MJ. “We believe this is a significant technology breakthrough that not only improves gold recovery, but also reduces the fresh water demand in the milling circuit—something that is crucial not only for managing the water balance in milling circuits, but also for reducing pumping requirements and eventually reducing the amount of water pushed to tailings impoundments.”

Clearly, interesting times remain ahead for gravity separation in all its forms.

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