Triboelectric Belt Separator for Beneficiation of Fine Minerals (2023)

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Procedia Engineering 83 ( 2014 ) 122 – 129

Peer-review under responsibility of the Scientific Committee of SYMPHOS 2013doi: 10.1016/j.proeng.2014.09.021

ScienceDirect

“SYMPHOS 2013”, 2nd International Symposium on Innovation and Technology in the Phosphate

Industry

Triboelectric belt separator for beneficiation of fine minerals

J.D. Bittner

_{, F.J.Hrach}

_{, S.A.Gasiorowski}

_{*, L.A. Canellopoulus}

_{, H. Guicherd}

a_{Separation Technologies, LLC, Needham, Massachusetts, USA}b_{SeparationTechnologies, LLC, Athens, Greece}

Abstract

Separation Technologies, LLC (ST) has developed a processing system based on triboelectric charging and electrostatic separation that provides the mineral processing industry a means to beneficiate fine materials with an entirely dry technology. The environmentally friendly technology can eliminate wet processing and required drying of the final material. The process requires little if any pre-treatment of the material other than grinding and operates at high capacity – up to 40 tonnes per hour by a compact machine. Energy consumption is low, approximately 1 kWh/tonnes of material processed. Since the only potential emission of the process is dust, permitting is typically relatively easy. In contrast to the other available electrostatic separation processes that are typically limited to particles greater than 75 μm in size, the ST belt separator is ideally suited for separation of very fine (<1 μm) to moderately coarse (300 μm) materials with very high throughputs. The triboelectric particle charging is effective for a wide range of materials and only requires particle – particle contact. The small gap, high electric field, counter current flow, vigorous particle-particle agitation and self-cleaning action of the belt on the electrodes are the critical features of the ST separator. The high efficiency multi-stage separation through charging / recharging and internal recycle results in far superior separations and is effective on fine materials that cannot be separated at all by conventional electrostatic techniques. Since 1995, this triboelectric technology has been extensively used for the beneficiation of coal fly ash with eighteen separators in place and over 130 machine-years of operation at locations in North America and Europe. The technology has been also successfully applied to the beneficiation of a variety of minerals including calcium carbonates, talc, and potash. Recently, dry electrostatic separation has been successfully demonstrated for beneficiation of phosphate ores.

Peer-review under responsibility of the Scientific Committee of SYMPHOS 2013.

Keywords: electrostatic; separation; minerals; fine particles; calcium carbonate; talc ; potash

1. ST technology overview

The ST separator utilizes electrical charge differences between materials produced by surface contact or triboelectric charging. When two materials are in contact, material with a higher affinity for electrons gains electrons and thus charges negative, while material with lower electron affinity charges positive. This contact exchange of charge is universally observedfor all materials, at times causing electrostatic nuisances that are a problem in some industries. Electron affinity is dependent on the chemical composition of the particle surface and will result in substantial differential charging of materials in a mixtureof discrete particles of different composition.

* Corresponding author. Tel.: +1-603-523-7024.

E-mail address: sgasiorowski@titanamerica.com

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In the ST separator (Figures 1 and 2), material is fed into the thin gap 0.9 – 1.5 cm (0.35 -0.6 in.) between two parallel planar electrodes. The particles are triboelectrically charged by interparticle contact. For example, in the case of coal combustion fly ash, a mixture of carbon particles and mineral particles, the positively charged carbon and the negatively charged mineral are attracted to opposite electrodes. The particles are then swept up by a continuous moving open-mesh belt and conveyed in opposite directions. The belt moves the particles adjacent to each electrode toward opposite ends of the separator. The electric field need only move the particles a tiny fraction of a centimeter to move a particle from a left-movingto a right-moving stream. The counter current flow of the separating particles and continual triboelectric charging by carbon-mineral collisions provides for a multistage separation and results in excellent purity and recovery in a single-pass unit. Thehigh belt speed also enables very high throughputs, up to 40 tonnes per hour on a single separator. By controlling various process parameters, such as belt speed, feed point, electrode gap and feed rate, the ST process produces low carbon fly ash at carbon contents of 2 % ± 0.5% from feed fly ashes ranging in carbon from 4% to over 30%.

Figure 1: Schematic of ST triboelectric separator

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The separator design is relatively simple. The belt and associated rollers are the only moving parts. The electrodes are stationary and composed of an appropriately durable material. The belt is made of plastic material. The separator electrode length is approximately 6 meters (20 ft.) and the width 1.25 meters (4 ft.) for full size commercial units. The power consumption is about 1 kilowatt-hour per tonne of material processed with most of the power consumed by two motors driving the belt.

The process is entirely dry, requires no additional materials and produces no waste water or air emissions. In the case of carbon from fly ash separations, the recovered materials consist of fly ash reduced in carbon content to levels suitable for useas a pozzolanic admixture in concrete, and a high carbon fraction which can be burned at the electricity generating plant. Utilization of both product streams provides a 100% solution to fly ash disposal problems.

The ST separator is relatively compact. A machine designed to process 40 tonnes per hour is approximately 9.1 meters (30 ft) long, 1.7 meters (5.5 ft.) wide and 3.2 meters (10.5 ft.) high. The required balance of plant consists of systems to conveydry material to and from the separator. The compactness of the system allows for flexibility in installation designs.

Figure 3: Commercial ST separator

2. ST Belt separator versus other electrostatic separation processes

The ST separation technology greatly expands the range of materials that can be beneficiated by electrostatic processes. The most commonly used electrostatic processes rely on differences in the electrical conductivity of the materials to be separated. In these processes, the material must contact a grounded drum or plate typically after the material particles are negatively charged by an ionizing corona discharge. Conductive materials will lose their charge quickly and be thrown from the drum. The non-conductive material continues to be attracted to the drum since the charge will dissipate more slowly and will fall or be brushed from the drum after separation from the conducting material. These processes are limited in capacity due to the required contact of every particle to the drum or plate. The effectiveness of these contact charging processes are also limited to particles of about 100 μm or greater in size due to both the need to contact the grounded plate and the requiredparticle flow dynamics. Particles of different sizes will also have different flow dynamics due to inertial effects and will result in degraded separation. The following diagram (Figure 4) illustrates the fundamental features of this type of separator.

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Figure 4: Drum electrostatic separator (Elder and Yan, 2003 [1])

Triboelectrostatic separations are not limited to separation of conductive / non-conductive materials but depend on the well known phenomenon of charge transfer by frictional contact of materials with dissimilar surface chemistry. This phenomenon has been used in “free fall” separation processes for decades. Such a process is illustrated in Figure 5. Components of a mixture of particles first develop different charges by contact either with a metal surface, or by particle to particle contact in a fluidized bed feeding device. As the particles fall through the electric field in the electrode zone, each particle’s trajectory is deflected toward the electrode of opposite charge. After a certain distance, collection bins are employed to separate the streams. Typical installations require multiple separator stages with recycle of a middling fraction. Some devices use a steadystream of gas to assist the conveying of the particles through the electrode zone.

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This type of free fall separator also has limitations in the particle size of the material that can be processed. The flow within the electrode zone must be controlled to minimize turbulence to avoid “smearing” of the separation. The trajectory of fine particles are more effected by turbulence since the aerodynamic drag forces on fine particles are much larger than the gravitational and electrostatic forces. The very fine particles will also tend to collect on the electrode surfaces and must beremoved by some method. Particles of less than 75 μm cannot be effectively separated.

Another limitation of the free fall separator is that the particle loading within the electrode zone must be low to prevent space charge effects, which limit the processing rate. Passing material through the electrode zone inherently results in a single-stage separation, since there is no possibility for recharging of particles. Therefore, multisingle-stage systems are required for improving the degree of separation including re-charging of the material by subsequent contact with a charging device. The resulting equipment volume and complexity increases accordingly.

In contrast to the other available electrostatic separation processes, the ST belt separator is ideally suited for separation ofvery fine (<1 μm) to moderately coarse (300 μm) materials with very high throughputs. The triboelectric particle charging is effective for a wide range of materials and only requires particle – particle contact. The small gap, high electric field, countercurrent flow, vigorous particle-particle agitation and self-cleaning action of the belt on the electrodes are the critical featuresof the ST separator. The high efficiency multistage separation through charging / recharging and internal recycle results in farsuperior separations and is effective on fine materials that cannot be separated at all by the conventional techniques. 3. Applications of the technology

3.1 History of Separation Technologies, LLC

ST was founded in 1989 to develop commercial applications for a proprietary electrostatic separation process invented by David Whitlock, one of the company’s founders. By 1994, ST was focusing on processing fly ash that could be used in higher value concrete production rather than being placed in landfills. Installation of mandated NOx control equipment at coal-fired power plants which increased the carbon as measured by loss on ignition (LOI) content of previously marketable fly ash created this opportunity to apply the ST separation process to fly ash beneficiation.

ST began operation of the first commercial fly ash processing plant to control the LOI content of fly ash in 1995. In 2002 ST was acquired by the Titan Cement Company S.A. Titan Cement Company, based in Greece, has operations in Europe, the Eastern Mediterranean, and North America and turnover of more than €1.5 billion. Titan America operations include cement plants, ready-mix concrete plants, concrete block plants, quarries, import and rail terminals, as well as fly ash production facilities.

Controlled low LOI fly ash is produced with ST’s technology at twelve power stations throughout the U.S., Canada, the U.K. and Poland. The processed fly ash is marketed under the ProAsh® brand. ProAsh® fly ash has been approved for use by over twenty state highway authorities, as well as many other specification agencies. ProAsh® has also been certified under Canadian Standards Association and EN 450:2005 quality standards in Europe. ST ash processing facilities are listed in Table 1.

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Table 1: ST Commercial Operations

Utility / power station Location Start of commercial operations

Facility details Progress Energy – Roxboro Station North Carolina USA Sept. 1997 2 Separators Constellation Power Source Generation -

Brandon Shores Station

Maryland USA April 1999 2 Separators. ScotAsh (Lafarge / Scottish Power Joint

Venture) - Longannet Station

Scotland UK Oct. 2002 1 Separator Jacksonville Electric Authority - St. John’s

River Power Park, FL

Florida USA May 2003 2 SeparatorsCoal/Petcoke blends South Mississippi Electric Power Authority

R.D. Morrow Station

Mississippi USA Jan. 2005 1 Separator New Brunswick Power Company

Belledune Station

New Brunswick, Canada April 2005 1 Separator Coal/Petcoke Blends RWE npower Didcot Station England UK August 2005 1 Separator

PPL Brunner Island Station Pennsylvania USA December 2006 2 Separators

Tampa Electric Co. Big Bend Station Florida USA April 2008 3 Separators, double pass RWE npower Aberthaw Station (Lafarge

Cement UK)

Wales UK September 2008 1 Separator EDF Energy West Burton Station (Lafarge

Cement UK, Cemex)

England UK October 2008 1 Separator ZGP (Lafarge Cement Poland / Ciech

Janikosoda JV))

Poland June 2010 1 Separator

Korea South-East Power Yeongheung Unit 5 & 6

South Korea To be commissioned 2013 1 Separator

3.2 Mineral applications

Electrostatic separations have been extensively used for beneficiation for a large range of minerals. [2] While most applications utilize differences in natural or induced conductivity of materials with the corona-drum type separators, triboelectric charging behavior with free-fall separators is also used at industrial scales. [3] A sample of applications of electrostatic processing reported in the literature is listed in Table 2. While this is not an exhaustive listing of applications, this table illustrates the potential range of applications for electrostatic processing of minerals.

Use of the ST triboelectric separator has been demonstrated to effectively beneficiate many mineral mixtures . Since the ST separator can process materials with particle sizes from about 500 μm to less than 1 μm, and the triboelectric separation is effective for both insulating and conductive materials, the technology greatly extends the range of applicable material overconventional electrostatic separators. Since the ST process is entirely dry, use of it eliminates the need for material drying and liquid waste handling from flotation processes.

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Table 2: Reported Electrostatic Separation of Minerals

Mineral Separation Reference ST Experience

Potassium Ore - Halite Manouchehri, Hanumantha & Foressberg [2] Searls [4]

Berthon & Bichara [5] Brands, Beier & Stahl [6]

Talc - Magnesite Fraas [7] Fraas [8]

Lindley & Rowson [9]

*Limestone - quartz Fraas [7]

Lindley & Rowson [9] *

Brucite - quartz Fraas [7] _*

Iron oxide – silica Manouchehri, Hanumantha & Foressberg [2] Brands, Beier & Stahl [6]

Fraas [7] Inculet [10]

Phosphate – calcite - silica Fraas [7] Feasby [11] Stencel & Jiang [[12]

Mica - Feldspar - quartz Manouchehri, Hanumantha & Foressberg [2]

Manouchehri, Hanumantha & Foressberg [3] *Wollastonite - quartz Manouchehri, Hanumantha & Foressberg [3] * Fluorite – silica Fraas [15]

Boron minerals Lindley & Rowson [9]

Celik & Yasar [16] *

Barites – Silicates Fraas [8] *

Zircon – Rutile Elder & Yan [1]

Manouchehri, Hanumantha & Foressberg [2] Brands, Beier & Stahl [6]

Fraas [7]

Venter, Vermaak, & Bruwer {14} Silver and gold slags Manouchehri, Hanumantha & Foressberg [3]

Carbon – Silica Fraas [7] *

Beryl – quartz Fraas [8]

Fluorite – Barite - Calcite Manouchehri, Hanumantha & Foressberg [3]

ST has conducted extensive pilot plant and field testing of many challenging material separations in the minerals industry. Examples of separations achievable with ST’s technology are shown in Table 3.

Table 3: Examples, ST Mineral Separations Mineral Separated

materials

Feed composition Recovered product composition

Mass yield product Mineral recovery Calcium Carbonate CaCO3 – SiO290.5% CaCO3 / 9.5% SiO2 99.1% CaCO3 / 0.9% SiO282 % 89% CaCO3 Recovery

Talc Talc/ Magnesite 58% talc / 42% Magnesite 95% talc/5% Magnesite

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4. Summary

The ST triboelectric separator provides the mineral processing industry a means to beneficiate fine materials with an entirely dry technology. The environmentally friendly process can eliminate wet processing and required drying of the final material. The ST process requires little, if any, pre-treatment of the material other than grinding and operates at high capacity – up to 40 tonnes per hour by a compact machine Energy consumption is low, approximately 1 kWh/tonnes of material processed. Since the only potential emission of the process is dust, permitting is typically relatively easy.

References

[1] Elder, J. , & Yan, E., 2003. “eForce.- Newest generation of electrostatic separator for the minerals sands industry.” Heavy Minerals Conference, Johannesburg, South African Institute of Mining and Metallurgy.

[2]Manouchehri, H.R., Hanumantha Roa, K., Foressberg, K.S.E., 2000 Review of Electrical Separation Methods, Part 1: Fundamental aspects, Minerals & Metallurgical Processing, Vol 17, No. 1 pp 23.

[3]Manouchehri, H.R., Hanumantha Roa, K., Foressberg, K.S.E., 2000 Review of Electrical Separation Methods, Part 2: Practical Considerations,Minerals & Metallurgical Processing, Vol 17, No. 1 p. 139.

[4]Searls, James P., 1985 Potash, in “Mineral Facts and Problems” United States Bureau of Mines, Washington DC. [5]Berthon, R., Bichara, M. , 1975 “Electrostatic Separation of Potash Ore.” United States Patent # 3,885,673. [6]Brands, L., Beier, Peter-M., Stahl, I., 2005. Electrostatic Separation, Wiley-VCH verlag, GmbH & Co. [7]Fraas, F., 1962. Electrostatic separation of Granular Materials, US Bureau of Mines, Bulletin 603, 1962 [8]Fraas, F. 1964. “Pretreatment of minerals for electrostatic separation.” US Patent 3,137,648.

[9]Lindley, K.S., and Rowson, N.A., 1997. Feed preparation factors affecting the efficiency of electrostatic separation, Magnetic and Electrical Separation, Vol 8 p, 161.

[10]Inculet, Ion I., 1994. Electrostatic Mineral Separation, Electrostatics and Electrostatic Applications Series, Research Studies Press, Ltd, John Wiley & Sons, Inc.

[11]Feasby, D.G., 1966. Free-Fall Electrostatic Separation of Phosphate and Calcite Particles, Minerals Research Laboratory, Labs Nos. 1869, 1890, 1985, 3021, and 3038, book 212, Progress Report, 1966.

[12] Stencel, J. M., Jiang, X., 2003. Pneumatic Transport, Triboelectric Beneficiation for the Florida Phosphate Industry, Florida Institute of Phosphate Research, Publication No. 02-149-201, 2003.

[13]Manouchehri, H.R., Hanumantha Roa, K., Foressberg, K.S.E. 2002. Triboelectric Charge, Electrophysical properties and Electrical Beneficiation Potential of Chemically Treated Feldspar, Quartz, and Wollastonite, Magnetic and Electrical Separation, Vol 11, No 1-2 p. 9.

[14]Venter, J.A., Vermaak, M.K.G., Bruwer, J.G., 2007. “Influence of surface effects on the electrostatic separation of zircon and rutile.” The 6th International Heavy Minerals Conference, The Southern African Institute of Mining and Metallurgy.

[15]Fraas, F., 1947. Notes on Drying for Electrostatic Separation of Particles, AIME Tec. Pub 2257 November, 1947

[16]Celik, M.S. and Yasar, E., 1995. Effects of Temperature and Impurities on Electrostatic Separation of Boron Materials, Minerals Engineering, Vol. 8, No. 7, p. 829.