Aplicacions en expansió en sec triboelèctric separació de Minerals

Expanding Applications in Dry Triboelectric

Separation of Minerals

James D. Bittner, Kyle P. Flynn, i Frank J. Hrach

ST equips & Tecnologia LLC, Needham Massachusetts 02494 EUA

Tel: +1‐781‐972‐2300, email: jbittner@titanamerica.com

ABSTRACT

ST equips & Tecnologia, LLC (STET) has developed a processing system based on triboelectrostatic belt separation that provides the mineral processing industry a means to beneficiate fine materials with an entirely dry technology. In contrast to other electrostatic separation processes that are typically limited to particles greater than 75μm in size, El separador de cinturons triboelèctric és ideal per a la separació de molt fines (<1μm) a un gruix moderat (300μm) partícules amb un rendiment molt alt. The high efficiency multi‐stage separation through internal charging/recharging and recycle results in far superior separations that can be achieved with a conventional single‐stage free‐ fall triboelectrostatic separator. The triboelectric belt separator technology has been used to separate a wide range of materials including mixtures of glassy aluminosilicates/carbon, Calcita / quars, talc/magnesita, i barita/quars. An economic comparison of using the triboelectrostatic belt separation versus conventional flotation for barite / separació de quars il·lustra els avantatges del tractament sec de minerals.

Paraules clau: Minerals, dry separation, baritat, triboelectrostatic charging, Separador de cinturons, cendra volant

INTRODUCCIÓ

La manca d'accés a l'aigua dolça s'està convertint en un factor important que afecta la viabilitat dels projectes miners a tot el món. Segons Hubert Fleming, exdirector global de Hatch Water, "De tots els projectes miners del món que s'han aturat o alentit durant l'últim any, Ha estat, en gairebé 100% dels casos, resultat de l'aigua, either directly or indirectly” Blin (2013). Dry mineral processing methods offer a solution to this looming problem.

Wet separation methods such as froth flotation require the addition of chemical reagents that must be handled safely and disposed of in an environmentally responsible manner. Inevitably it is not possible to operate with 100% water recycle, requiring disposal of at least of portion of the process water, likely containing trace amounts of chemical reagents.

Els mètodes secs com la separació electrostàtica eliminaran la necessitat d'aigua dolça, i ofereixen el potencial de reduir costos. One of the most promising new developments in dry mineral separations is the triboelectrostatic belt separator. Aquesta tecnologia ha ampliat el rang de mida de partícules a partícules més fines que les tecnologies convencionals de separació electrostàtica., en el rang on només la flotació ha tingut èxit en el passat.

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Figura 2. Detall de la zona de separació

El procés és totalment sec, No requereix materials addicionals i no produeix aigües residuals ni emissions d'aire.. En el cas de carboni procedent de separacions de cendres volants, Els materials recuperats consisteixen en cendres volants reduïdes en contingut de carboni a nivells adequats per al seu ús com a barreja pozzolanica en formigó, i una elevada fracció de carboni que es pot cremar a la planta generadora d'electricitat. La utilització d'ambdós fluxos de productes proporciona un 100% Solució als problemes d'eliminació de cendres volants.

The triboelectrostatic belt separator is relatively compact. Una màquina destinada a processar 40 tones per hora és aproximadament 9.1 metres (30 M) llarg, 1.7 metres (5.5 peus.) ample i 3.2 metres (10.5 peus.) alta. L'equilibri necessari de la planta consisteix en sistemes per transportar material sec des de i cap al separador. La compacitat del sistema permet flexibilitat en els dissenys d'instal·lació.

Figura 3. Commercial triboelectrostatic belt separator

Comparison to other electrostatic separation processes

The triboelectrostatic belt 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

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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 required particle 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 (Figura 4) illustrates the fundamental features of this type of separator.

Figura 4. Drum electrostatic separator “Elder (2003)”

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 steady stream of gas to assist the conveying of the particles through the electrode zone.

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Figura 5. “Free fall” triboelectrostatic separator

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 be removed by some method. Particles of less than 75 μm cannot be effectively separated.

Another limitation 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 re‐charging of particles. Per tant, multi‐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 triboelectrostatic belt separator is ideally suited for separation of very fine (<1 μm) a un gruix moderat (300μm) materials with very high throughputs. The triboelectric particle charging is effective for a wide range of materials and only requires particle – particle contact. Del buit, alt camp elèctric, counter current flow, vigorous particle‐particle agitation and self‐cleaning action of the belt on the electrodes are the critical features of the 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 the conventional techniques.

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APPLICATIONS OF TRIBOELECTROSTATIC BELT SEPARATION

Cendra volant

The triboelectrostatic belt separation technology was first applied industrially to the processing of coal combustion fly ash in 1995. For the fly ash application, the technology has been effective in separating carbon particles from the incomplete combustion of coal, de les partícules minerals d'aluminosilicat vívides a la cendra de la mosca. The technology has been instrumental in enabling recycle of the mineral‐rich flyash as a cement replacement in concrete production. Des que 1995, 19 triboelectrostatic belt separators have been operating in the USA, Canadà, Regne Unit, and Poland, processing over 1,000,000 tonnes of fly ash annually. The technology is now also in Asia with the first separator installed in South Korea this year. La història industrial de la separació de cendres de mosca apareix a la taula 1.

Taula 3. Examples, mineral separations using triboelectrostatic belt separation

Use of the triboelectrostatic belt separator has been demonstrated to effectively beneficiate many mineral mixtures. Since the separator can process materials with particle sizes from about 300 μm to less than 1 μm, and the triboelectrostatic separation is effective for both insulating and conductive materials, the technology greatly extends the range of applicable material over conventional electrostatic separators. Since the triboelectrostatic process is entirely dry, use of it eliminates the need for material drying and liquid waste handling from flotation processes.

COST OF TRIBOELECTROSTATIC BELT SEPARATION

Comparison to Conventional Flotation for Barite

A comparative cost study was commissioned by STET and conducted by Soutex Inc. Soutex is a Quebec Canada based engineering company with extensive experience in both wet flotation and electrostatic separation process evaluation and design. The study compared the capital and operating costs of triboelectrostatic belt separation process to conventional froth flotation for the beneficiation of a low‐grade barite ore. Both technologies upgrade the barite by removal of low density solids, mainly quartz, to produce an American Petroleum Institute (Api) drilling grade barite with SG greater than 4.2 g/ml. Flotation results were based on pilot plant studies conducted by the Indian National Mettalurgical Laboratory “NML (2004)”. Triboelectrostatic belt separation results were based on pilot plant studies using similar feed ores. The comparative economic study included flowsheet development, material and energy balances, major equipment sizing and quotation for both flotation and triboelectrostatic belt separation processes. The basis for both flowsheets is the same, processament 200,000 t/y of barite feed with SG 3.78 to produce 148,000 t/y of drilling grade barite product with SG 4.21 g/ml. The flotation process estimate did not include any costs for process water, or water treatment.

Flowsheets were generated by Soutex for the barite flotation process (Figura 6), and triboelectrostatic belt separation process (Figura 7).

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Figura 6 Barite flotation process flowsheet

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Figura 7 Barite triboelectrostatic belt separation process flowsheet

Theses flowsheets do not include a raw ore crushing system, which is common to both technologies. Feed grinding for the flotation case is accomplished using a wet pulp ball mill with cyclone classifier. Feed grinding for the triboelectrostatic belt separation case is accomplished using a dry, vertical roller mill with integral dynamic classifier.

The triboelectrostatic belt separation flowsheet is simpler than flotation. Triboelectostatic belt separation is achieved in a single stage without the addition of any chemical reagents, compared to three‐stage flotation with oleic acid used as a collector for barite and sodium silicate as a depressant for the silica gangue. A flocculant is also added as a reagent for thickening in the barite flotation case. No dewatering and drying equipment is required for triboelectrostatic belt separation, compared to thickeners, filter presses, and rotary dryers required for the barite flotation process.

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REFERENCES

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