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Lucas Rojas Mendoza, ST equips & Tecnologia, EUA
lrojasmendoza@steqtech.com
Frank Hrach, ST equips & Tecnologia, EUA
Kyle Flynn, ST equips & Tecnologia, EUA
Abhishek Gupta, ST equips & Tecnologia, EUA
ST equips & Tecnologia LLC (STET) ha desenvolupat una novel·la basada en la separació de cinturó funda Tribó-electrostàtica que proporciona la indústria càrnia minerals un mitjà per materials nobles beneficiate amb una tecnologia totalment sec i l'eficiència energètica sistema de processament. In contrast to other electrostatic separation processes that are typically limited to particles >75μm en grandària, el separador de cinturó triboelèctric STET és adequat per separació de molt fina (<1µm) a un gruix moderat (500µm) Partícules, amb un rendiment molt alt. The STET tribo-electrostatic technology has been used to process and commercially separate a wide range of industrial minerals and other dry granular powders. Aquí, bench-scale results are presented on the beneficiation of low-grade Fe ore fines using STET belt separation process. Bench-scale testing demonstrated the capability of the STET technology to simultaneously recover Fe and reject SiO2 from itabirite ore with a D50 of 60µm and ultrafine Fe ore tailings with a D50 of 20µm. The STET technology is presented as an alternative to beneficiate Fe ore fines that could not be successfully treated via traditional flowsheet circuits due to their granulometry and mineralogy.
Mineral de ferro és l'element més comú en quart lloc en l'escorça de la terra [1]. Ferro és essencial per a la fabricació d'acer i per tant un material essencial per al desenvolupament econòmic global [1-2]. Ferro és àmpliament utilitzat en la construcció i la fabricació de vehicles [3]. La majoria de recursos de mineral de ferro es componen de formacions de ferro bandat transformat (BIF) en el qual ferro es troba comunament en forma d'òxids, hidròxids i en menor mesura carbonats [4-5]. Un tipus particular de les formacions amb més alt contingut de carbonat de ferro són itabirites dolomític que són un producte de la dolomitization i metamorfisme dels dipòsits BIF [6]. Els dipòsits de mineral de ferro més gran del món es pot trobar a Austràlia, Xina, Canadà, Ucraïna, L'Índia i Brasil [5].
La composició química de minerals de ferro té un ventall aparent composició química especialment per a continguts de Fe i minerals associades gangue [1]. Minerals de ferro més importants associats amb la majoria dels minerals de ferro són Hematites Veure, goethite, web i magnetita [1,5]. Els principals contaminants en minerals de ferro són SiO2 i Al2O3 [1,5,7]. El típic sílice i alúmina tenint minerals presents en els minerals de ferro són Quars, caolinita, gibbsite, diaspore i Corindó. D'aquests sovint s'observa que el quars és el sílice mitjana tenint minerals i caolinita i gibbsite són les dues principals alúmina tenint minerals [7].
Extracció de mineral de ferro es realitza principalment mitjançant les operacions mineres de pou obert, resultant en la generació de residus importants [2]. El sistema de producció de mineral de ferro en general implica tres etapes: mineria, processament i activitats de pellet. D'aquests, processament assegura que un grau adequat de ferro i química s'aconsegueix abans de l'escenari pelletizing. Tramitació inclou aixafament, classificació, mòlta i concentració per a augmentar el contingut de ferro mentre que la reducció de la quantitat de minerals de gangue [1-2]. Cada dipòsit mineral té unes característiques úniques pel que fa al ferro i gangue tenint minerals, i per tant requereix una tècnica diferent concentració [7].
La separació magnètica s'utilitza típicament en la beneficiació de minerals de ferro d'alt grau on els minerals de ferro dominants són ferro i paramagnètic [1,5]. Seques i humides baixa intensitat separació magnètica (LIMS) tècniques s'utilitzen per processar els minerals amb propietats magnètiques forts com magnetita mentre humida alta intensitat separació magnètica s'utilitza per separar tenint Fe minerals amb propietats magnètiques febles com Hematites partir de gangue minerals. Minerals de ferro tal goethite i web trobem comunament en runams i no se molt bé amb qualsevol tècnica [1,5]. Mètodes magnètiques presenten reptes pel que fa a les seves capacitats baixes i pel que fa a la necessitat de mineral de ferro de ser susceptible de camps magnètics [5].
Flotació, D'altra banda, s'utilitza per reduir el contingut d'impureses en minerals de ferro de baixa qualitat [1-2,5]. Minerals de ferro pot ser concentrada ja sigui per la directa anionic flotació d'òxids de ferro o inversa cationic flotació de sílice, però inversa cationic flotació segueix sent la ruta de flotació més popular utilitzat en la indústria del ferro [5,7]. L'ús de flotació seu limitat pel cost de reactius, la presència de sílice i alúmina ric llots de depuradora i la presència de minerals de carbonat [7-8]. D'altra banda, flotació requereix tractament d'aigües residuals i l'ús de dewatering aigües avall per a aplicacions de finals secs [1].
L'ús de flotació per a la concentració de ferro implica també desliming com surant en presència de resultats multes en disminució de l'eficiència i costos d'alt reactiu [5,7]. Desliming és especialment crític per a la retirada d'alúmina com la separació de gibbsite de l'hematita o goethite per qualsevol agents tensoactives és molt difícil [7]. Majoria d'alúmina tenint minerals que es produeix en el rang de mida més fines (<20Um) permetent la seva eliminació a través de. Global, una alta concentració de multes (<20Um) i l'alúmina augmenta la dosi de col·lector cationic requerida i disminueix dramàticament la selectivitat [5,7].
D'altra banda, the presence of carbonate minerals – such as in dolomitic itabirites- can also deteriorate flotation selectivity between iron minerals and quartz as iron ores containing carbonates such as dolomite do not float very selectively. Dissolved carbonates species adsorb on the quartz surfaces harming the selectivity of flotation [8]. Flotation can be reasonably effective in upgrading low-grade iron ores, but it is strongly dependent on the ore mineralogy [1-3,5]. Flotation of iron ores containing high alumina content will be possible via desliming at the expense of the overall iron recovery [7], while flotation of iron ores containing carbonate minerals will be challenging and possibly not feasible [8].
Modern processing circuits of Fe-bearing minerals may include both flotation and magnetic concentration steps [1,5]. For instance, magnetic concentration can be used on the fines stream from the desliming stage prior to flotation and on the flotation rejects. The incorporation of low and high intensity magnetic concentrators allows for an increase in the overall iron recovery in the processing circuit by recovering a fraction of the ferro and paramagnetic iron minerals such as magnetite and hematite [1]. Goethite is typically the main component of many iron plant reject streams due to its weak magnetic properties [9]. In the absence of further downstream processing for the reject streams from magnetic concentration and flotation, the fine rejects will end up disposed in a tailings dam [2]. Tailings disposal and processing have become crucial for environmental preservation and recovery of iron valuables, respectivament, and therefore the processing of iron ore tailings in the mining industry has grown in importance [10].
Clearly, the processing of tailings from traditional iron beneficiation circuits and the processing of dolomitic itabirite is challenging via traditional desliming-flotation-magnetic concentration flowsheets due to their mineralogy and granulometry, and therefore alternative beneficiation technologies such as tribo-electrostatic separation which is less restrictive in terms of the ore mineralogy and that allows for the processing of fines may be of interest.
La separació tribo-electrostàtica utilitza diferències de càrrega elèctrica entre materials produïts pel contacte superficial o la càrrega triboelèctrica.. De manera simplista, Quan dos materials estan en contacte, the material with a higher affinity for electron gains electrons thus charges negative, mentre que el material amb menor afinitat electrònica carrega positiu. In principle, low-grade iron ore fines and dolomitic itabirites that are not processable by means of conventional flotation and/or magnetic separation could be upgraded by exploiting the differential charging property of their minerals [11].
Here we present STET tribo-electrostatic belt separation as a possible beneficiation route to concentrate ultrafine iron ore tailings and to beneficiate dolomitic itabirite mineral. The STET process provides the mineral processing industry with a unique water-free capability to process dry feed. The environmentally friendly process can eliminate the need for wet processing, downstream waste water treatment and required drying of final material. A més a més, The STET process requires little pre-treatment of the mineral and operates at high capacity – up to 40 tones per hour. Energy consumption is less than 2 kilowatt-hours per ton of material processed.
Material
Dos minerals de ferro mica fina van ser utilitzats en aquesta sèrie de proves. El primer mineral consistia en una mostra de residus de mineral de Fe ultrafines amb un D50 de 20 µm i la segona Mostra d'una mostra de mineral de ferro itabirite amb un D50 de 60 µm. Ambdues mostres presenten reptes durant la seva beneficiation i no es pot processar eficaçment a través de circuits de la tradicional concentració desliming flotació-magnètics per la seva granulometria i mineralogia. Ambdues mostres van ser obtingudes de les operacions mineres al Brasil.
La primera Mostra obtenia un circuit desliming flotació-magnètics concentració existent. La mostra es va recollir d'una presa runams, llavors seques, homogeneïtzada i ple. La segona mostra prové d'una formació de ferro itabirite al Brasil. La Mostra va ser aixafat i ordenats per la mida i la fracció fina obtinguda a la fase de classificació més tard es va sotmetre a diverses etapes de desliming fins a un D98 de 150 es va aconseguir µm. La Mostra va ser assecada llavors, homogeneïtzada i ple.
Distribucions de mida de partícula (PSD) van ser determinats utilitzant un analyzer de mida de partícula de difracció de làser, Mastersizer de un Malvern 3000 E. Ambdues mostres també es van caracteritzar per pèrdua-en-ignició(LOI), XRF i DRX. La pèrdua en ignició (LOI) estava decidit posant 4 grams de Mostra en un 1000 Forn º c per a 60 minuts i informar la LOI en una base tan rebudes. L'anàlisi de composició química es va completar amb una longitud d'ona dispersiva de raigs x de fluorescència (WD-XRF) instrument i les principals fases cristal·lines van ser investigats per la tècnica DRX.
La composició química i LOI per a la Mostra de runams (Runams), i per a la Mostra de formació de ferro itabirite (Itabirite), es mostra en la taula 1 i distribucions de mida de partícula per ambdues mostres es mostren en la figura 1. Per a la Mostra de residus les principals fases recuperables Fe són goethite i Hematites Veure, i el mineral de gangue principal és de quars (Figa 4). Per a la Mostra de itabirite les principals fases recuperables Fe són Hematites Veure, i minerals gangue principals són quars i dolomita (Figa 4).
Taula 1. Resultat d'anàlisi química d'elements principals en runams i mostres de Itabirite.
| Mostra | Grade (WT) | |||||||
|---|---|---|---|---|---|---|---|---|
| Fe | SiO2 | Al2O3 | MnO | MgO | CaO | LOI** | Others | |
| Runams | 30.3 | 47.4 | 4.3 | 1.0 | * | * | 3.4 | 13.4 |
| Itabirite | 47.6 | 23.0 | 0.7 | 0.2 | 1.5 | 2.2 | 4.0 | 21.0 |
Particle Size Distributions
Mètodes
Una sèrie d'experiments van ser dissenyats per investigar l'efecte de diferents paràmetres en moviment de ferro en ambdues mostres de ferro utilitzant tecnologia de separador STET privatiu cinturó funda Tribó-electrostàtica. Experiments es van fer mitjançant un separador de Banc-escala cinturó funda Tribó-electrostàtica, més enllà es referia com "benchtop separador". Escala-Banc de proves és la primera fase d'un procés d'implantació de tecnologia de tres fases (Veure taula 2) avaluació de Banc-escala incloent, escala pilot de proves i implementació comercial-escala. El separador de la banqueta s'utilitza per al cribratge d'evidència de càrrega tribo-electrostàtica i per determinar si un material és un bon candidat per al benefici electrostàtic.. Les principals diferències entre cada equip es presenten a la taula 2. Mentre que l'equip utilitzat dins de cada fase difereix en la mida, El principi de funcionament és fonamentalment el mateix..
Taula 2. Procés d'implementació trifàsic mitjançant tecnologia de separador de cinturó tribo-electrostàtic STET
| Fase | S'utilitza per: | Elèctrode Dimensions (W x L) cm | Type of Process/ |
|---|---|---|---|
| Bench Scale Evaluation | Qualitative Evaluation | 5*250 | Fornada |
| Escala pilot Proves | Quantitative Evaluation | 15*610 | Fornada |
| Commercial Scale Implementation | Commercial Production | 107 *610 | Continu |
STET Operation Principle
El principi d'operació del separador depèn de la càrrega electrostàtica tribu. En el separador de cinturó funda Tribó-electrostàtica (Figures 2 i 3), material s'alimenta en l'estreta escletxa 0.9 - 1.5 cm entre dos elèctrodes planars paral·lel. Les partícules triboelectrically paguen per contacte interparticle. El mineral carregat positivament(s) i el mineral carregat negativament(s) són atrets a davant elèctrodes. Dins el separador partícules són escombrats per un cinturó de xarxa oberta en moviment continu i transmès en direccions oposades. The belt is made of plastic material and moves the particles adjacent to each electrode toward opposite ends of the separator. The counter current flow of the separating particles and continual triboelectric charging by particle-particle collisions provides for a multistage separation and results in excellent purity and recovery in a single-pass unit. The triboelectric belt separator technology has been used to separate a wide range of materials including mixtures of glassy aluminosilicates/carbon (cendra volant), Calcita / quars, talc/magnesita, i barita/quars.
Global, the separator design is relatively simple with the belt and associated rollers as the only moving parts. Els elèctrodes són estacionaris i formada per un material durable adequadament. La longitud de l'elèctrode del separador és aproximadament 6 metres (20 peus.) i l'amplada 1.25 metres (4 peus.) per a unitats comercials de mida completa. The high belt speed enables very high throughputs, fins a 40 tons per hour for full size commercial units. El consum d'energia és menor que 2 kilowatt-hours per ton of material processed with most of the power consumed by two motors driving the belt.
Esquema del separador de cinturons triboelèctrics
Detall de la zona de separació
Com es pot veure a la taula 2, la principal diferència entre el separador de la part superior del banc i els separadors a escala pilot i a escala comercial és que la longitud del separador de la part superior del banc és aproximadament 0.4 temps la longitud de les unitats a escala pilot i a escala comercial. Com que l'eficiència del separador és una funció de la longitud de l'elèctrode, Les proves a escala de banc no es poden utilitzar com a substitut de les proves a escala pilot. Les proves a escala pilot són necessàries per determinar l'abast de la separació que el procés STET pot aconseguir, i per determinar si el procés STET pot complir els objectius de producte sota les taxes d'alimentació donades. En canvi, el separador de la taula de banc s'utilitza per descartar materials candidats que és poc probable que demostrin una separació significativa a nivell pilot.. Els resultats obtinguts a escala de banc no estaran optimitzats, i la separació observada és menor del que s'observaria en un separador STET de mida comercial.
Les proves a la planta pilot són necessàries abans del desplegament a escala comercial, No obstant això, es fomenta la prova a escala de banc com a primera fase del procés d'implementació de qualsevol material donat. D'altra banda, en els casos en què la disponibilitat material sigui limitada, el separador de sobres banc proporciona una eina útil per al cribratge de possibles projectes d'èxit (i.e., projectes en què es poden complir els objectius de qualitat del client i de la indústria mitjançant la tecnologia STET).
Proves a escala de bancs
Standard process trials were performed around the specific goal to increase Fe concentration and to reduce the concentration of gangue minerals. Different variables were explored to maximize iron movement and to determine the direction of movement of different minerals. The direction of movement observed during benchtop testing is indicative of the direction of movement at the pilot plant and commercial scale.
The variables investigated included relative humidity (RH), temperatura, electrode polarity, belt speed and applied voltage. D'aquests, RH and temperature alone can have a large effect on differential tribo-charging and therefore on separation results. Aquí, optimum RH and temperature conditions were determined before investigating the effect of the remaining variables. Two polarity levels were explored: Jo) top electrode polarity positive and ii) top electrode polarity negative. For the STET separator, under a given polarity arrangement and under optimum RH and temperature conditions, belt speed is the primary control handle for optimizing product grade and mass recovery. Testing on the bench separator helps shed light on the effect of certain operational variables on tribo-electrostatic charging for a given mineral sample, and therefore obtained results and trends may be used, to certain degree, to narrow down the number of variables and experiments to be performed at the pilot plant scale. Taula 3 lists the range of separation conditions used as part of phase 1 evaluation process for the tailings and itabirite samples.
Taula 3 lists the range of separation conditions
| Parameter | Units | Range of Values | |
|---|---|---|---|
| Runams | Itabirite | ||
| Top Electrode Polarity | - | Positive- Negative | Positive- Negative |
| Electrode Voltage | -kV/+kV | 4-5 | 4-5 |
| Feed Relative Humidity (RH) | % | 1-30.7 | 2-39.6 |
| Feed Temperature | °F (° C) | 71-90 (21.7-32.2) | 70-87 (21.1-30.6) |
| Belt Speed | Fps (m/s) | 10-45 (3.0-13.7) | 10-45 (3.0-13.7) |
| Electrode Gap | Inches (mm) | 0.400 (10.2 mm) | 0.400 (10.2 mm) |
Les proves es van dur a terme al separador de la banqueta en condicions de lot, with feed samples of 1.5 lbs. per test. A flush run using 1 lb. of material was introduced in between tests to ensure that any possible carryover effect from the previous condition was not considered. Before testing was started material was homogenized and sample bags containing both run and flush material were prepared. At the beginning of each experiment the temperature and relative humidity (RH) was measured using a Vaisala HM41 hand-held Humidity and Temperature probe. The range of temperature and RH across all experiments was 70-90 °F (21.1-32.2 (° C) i 1-39.6%, respectivament. To test a lower RH and/or higher temperature, feed and flush samples were kept in a drying oven at 100 °C for times between 30-60 minutes. En canvi,, higher RH values were attained by adding small amounts of waters to the material, followed by homogenization. After RH and temperature was measured on each feed sample, the next step was to set electrode polarity, belt speed and voltage to the desired level. Gap values were kept constant at 0.4 polzades (10.2 mm) during the testing campaigns for the tailings and itabirite samples.
Abans de cada prova, a small feed sub-sample containing approximately 20g was collected (designat com a "Feed"). En establir totes les variables d'operació, el material es va introduir al separador de la banqueta utilitzant un alimentador vibratori elèctric a través del centre del separador de la banqueta.. Es van recollir mostres al final de cada experiment i els pesos de l'extrem del producte 1 (Es tracta de "E1") Final del producte 2 (Es tracta de "E2") es van determinar utilitzant una escala de recompte legal per al comerç. Following each test, small sub-samples containing approximately 20 g of E1 and E2 were also collected. Mass yields to E1 and E2 are described by:
OnYE1 i YE2 are the mass yields to E1 and E2, respectivament; and are the sample weights collected to the separator products E1 and E2, respectivament. For both samples, Fe concentration was increased to product E2.
Per a cada conjunt de sub-mostres (i.e., Alimentació, E1 i E2) LOI and main oxides composition by XRF was determined. Fe2 O3 contents were determined from the values. For the tailings sample LOI will directly relate to the content of goethite in the sample as the functional hydroxyl groups in goethite will oxidize into H2 Og [10]. Contrari, for the itabirite sample LOI will directly relate to the contain of carbonates in the sample, as calcium and magnesium carbonates will decompose into their main oxides resulting in the release of CO2g and sub sequential sample loss weight. XRF beads were prepared by mixing 0.6 grams of mineral sample with 5.4 grams of lithium tetraborate, which was selected due to the chemical composition of both tailings and itabirite samples. XRF analysis were normalized for LOI.
Finalment, Fe recovery EFe to product (E2) i SiO2 rejection QSi were calculated. EFe is the percentage of Fe recovered in the concentrate to that of the original feed sample and Qsio2 is the percentage of removed from the original feed sample. EFe i Qsi are described by:
On CJo,(feed,E1,E2) is the normalized concentration percentage for the sub-sample’s i component (eg., Fe, sio2)
Mostres Mineralogia
The XRD pattern showing major mineral phases for the tailings and itabirite samples are shown in Fig 4. For the tailings sample the main Fe recoverable phases are goethite, hematite and magnetite, i el mineral de gangue principal és de quars (Figa 4). For the itabirite sample the main Fe recoverable phases are hematite and magnetite and the main gangue minerals are quartz and dolomite. Magnetite appears in trace concentrations in both samples. Pure hematite, goethite, and magnetite contain 69.94%, 62.85%, 72.36% Fe, respectivament.
D patterns. A – Tailings sample, B – Itabirite sample
Experiments a escala de banc
A series of test runs were performed on each mineral sample aimed at maximizing Fe and decreasing SiO2 content. Species concentrating to E1 will be indicative of a negative charging behavior while species concentration to E2 to a positive charging behavior. Higher belt speeds were favorable to the processing of the tailings sample; No obstant això, the effect of this variable alone was found to be less significant for the itabirite sample.
Average results for the tailings and itabirite samples are presented in Fig 5, which were calculated from 6 i 4 experiments, respectivament. Figa 5 presents average mass yield and chemistry for feed and products E1 and E2. A més a més, each plot presents the improvement or decrease in concentration (E2- Alimentació) for each sample component e.g., Fe, SiO2 Positive values are associated to an increase in concentration to E2, while negative values are associated to a decrease in concentration to E2.
Fig.5. Average mass yields and chemistry for Feed, E1 and E2 products. Error bars represent 95% confidence intervals.
For the tailings sample Fe content was increased from 29.89% a 53.75%, on average, at a mass yield YE2 – or global mass recovery – de 23.30%. This corresponds to Fe recovery ( and silica rejection (QE2 ) values of 44.17% i 95.44%, respectivament. The LOI content was increased from 3.66% a 5.62% which indicates that the increase in Fe content is related to an increase in goethite content (Figa 5).
For the itabirite sample Fe content was increased from 47.68% a 57.62%, on average, at a mass yield YE2 -de 65.0%. This corresponds to Fe recovery EFe( and silica rejection (Qsio2) values of 82.95% i 86.53%, respectivament. The LOI, MgO and CaO contents were increased from 4.06% a 5.72%, 1.46 a 1.87% and from 2.21 a 3.16%, respectivament, which indicates that dolomite is moving in the same direction as Fe-bearing minerals (Figa 5).
For both samples,AL2 O3 , MnO and P seem to be charging in the same direction as Fe-bearing minerals (Figa 5). While it is desired to decrease the concentration of these three species, the combined concentration of SiO2, AL2 , O3 , YE2 MnO and P is decreasing for both samples, and therefore the total effect achieved using the benchtop separator is an enhancement in the product Fe grade and a decrease in the contaminants concentration.
Global, benchtop testing demonstrated evidence of effective charging and separation of iron and silica particles. The promising laboratory scale results suggest that pilot scale tests including first and second passes should be performed.
Discussió
The experimental data suggests that the STET separator resulted in an important increase in Fe content while simultaneously reducing SiO2 content.
Having demonstrated that triboelectrostatic separation can result in a significant increase in Fe content, a discussion on the significance of the results, on the maximum achievable Fe contents and on the feed requirements of the technology is needed.
To start, it is important to discuss the apparent charging behavior of mineral species in both samples. For the tailings sample the main components were Fe oxides and quartz and experimental results demonstrated that Fe oxides concentrated to E2 while quartz concentrated to E1. De manera simplista, it could be said that Fe oxide particles acquired a positive charge and that quartz particles acquired a negative charge. This behavior is consistent with the triboelectrostatic nature of both minerals as shown by Ferguson (2010) [12]. Taula 4 shows the apparent triboelectric series for selected minerals based on inductive separation, and it shows that quartz is located at the bottom of the charging series while goethite, magnetite and hematite are located higher up in the series. Minerals at the top of the series will tend to charge positive, while minerals at the bottom will tend to acquire a negative charge.
On the other hand, for the itabirite sample the main components were hematite, quartz and dolomite and experimental results indicated that Fe oxides and dolomite concentrated to E2 while quartz concentrated to E1. This indicates that hematite particles and dolomite acquired a positive charge while quartz particles acquired a negative charge. Com es pot veure a la taula 4, carbonates are located at the top of the tribo-electrostatic series, which indicates that carbonate particles tend to acquire a positive charge, and in consequence to be concentrated to E2. Both dolomite and hematite were concentrated in the same direction, indicating that the overall effect for hematite particles in the presence of quartz and dolomite was to acquire a positive charge.
The direction of movement of the mineralogical species in each sample is of paramount interest, as it will determine the maximum achievable Fe grade that can be obtained by means of a single pass using the tribo-electrostatic belt separator technology.
For the tailings and itabirite samples the maximum achievable Fe content will be determined by three factors: Jo) The amount of Fe in Fe-bearing minerals; ii) the minimum quartz (SiO2 ) content that can be achieved and; iii) The number of contaminants moving in the same direction as Fe-bearing minerals. For the tailings sample the main contaminants moving in the same direction of Fe-bearing minerals are Al2 O3 MnO bearing minerals, while for the itabirite sample the main contaminants are CaO MgO Al2 O3 bearing minerals.
| Mineral Name | Charge acquired (apparent) |
|---|---|
| Apatite | +++++++ |
| Carbonates | ++++ |
| Monazite | ++++ |
| Titanomagnetite | . |
| Ilmenite | . |
| Rutile | . |
| Leucoxene | . |
| Magnetite/hematite | . |
| Spinels | . |
| Garnet | . |
| Staurolite | - |
| Altered ilmenite | - |
| Goethite | - |
| Zircon | -- |
| Epidote | -- |
| Tremolite | -- |
| Hydrous silicates | -- |
| Aluminosilicates | -- |
| Tourmaline | -- |
| Actinolite | -- |
| Pyroxene | --- |
| Titanite | ---- |
| Feldspat | ---- |
| Quartz | ------- |
Taula 4. Apparent triboelectric series for selected minerals based on inductive separation. Modified from D.N Ferguson (2010) [12].
For the tailings sample, the Fe content was measured at 29.89%. XRD data indicates that the predominant phase is goethite, followed by hematite, and therefore the maximum achievable Fe content if a clean separation was possible would be between 62.85% i 69.94% (which are the Fe contents of pure goethite and hematite, respectivament). Ara, a clean separation is not possible as Al2, O3 MnO and P-bearing minerals are moving in the same direction as the Fe-bearing minerals, and therefore any increase in Fe content will also result in an increase of these contaminants. Then, to increase the Fe content, the amount of quartz to E2 will need to be significantly decreased to the point it offsets the movement of , MnO and P to product (E2). As shown in Table 4, quartz has a strong tendency to acquire a negative charge, and therefore in the absence of other minerals having an apparent negative charging behavior it will be possible to considerably decrease its content to product (E2) by means of a first pass using the triboelectrostatic belt separator technology.
For instance, if we assume that all the Fe content in the tailings sample is associated to goethite (FeO(Oh)), and that the only gangue oxides are SiO2, Al2O3 i MnO, then Fe content to product would be given by:
Fe(%)=(100-SiO2 – (Al2 O3 + MnO*0.6285
On, 0.6285 is the percentage of Fe in pure goethite. Eq.4 depicts the competing mechanism that takes place to concentrate Fe as AL2O3 + MnO increases while SiO2 decreases.
For the itabirite sample the Fe content was measured at 47.68%. XRD data indicates that the predominant phase is hematite and therefore the maximum achievable Fe content if a clean separation was possible would be close to 69.94% (which is the Fe content of pure hematite). As it was discussed for the tailings sample a clean separation won’t be possible as CaO, MgO, Al2 O3 bearing minerals are moving in the same direction as hematite, and therefore to increase Fe content SiO2 content must be reduced. Assuming that the entirety of the Fe content in this sample is associated to hematite (Fe2O3) and that the only oxides contained in gangue minerals are SiO2, CaO, MgO, Al2O3 i MnO; then Fe content in the product would be given by:
Fe(%)=(100-SiO2-CaO+MgO+Al2O3+MnO+LOI*0.6994
On, 0.6994 is the percentage of Fe in pure hematite. It must be noticed that Eq.5 includes LOI, while Eq.4 does not. For the itabirite sample, the LOI is associated to the presence of carbonates while for the tailings sample it is associated to Fe-bearing minerals.
Evidently, for both tailings and itabirite samples it is possible to significantly increase the Fe content by reducing the content of SiO2; No obstant això, as shown in Eq.4 and Eq.5, the maximum achievable Fe content will be limited by the direction of movement and the concentration of oxides associated to gangue minerals.
In principle, the concentration of Fe in both samples could be further increased by means of a second pass on the STET separator in which CaO,MgO Al2 O3 i MnObearing minerals could be separated from Fe-bearing minerals. Such separation would be possible if most of quartz in the sample was removed during a first pass. In the absence of quartz, some of the remaining gangue minerals should in theory charge in the opposite direction of goethite, hematite and magnetite, which would result in increased Fe content. For instance, for the itabirite sample and based in the location of dolomite and hematite in the triboelectrostatic series (Veure taula 4), dolomite/hematite separation should be possible as dolomite has a strong tendency to charge positive in relation to hematite.
Having discussed on the maximum achievable Fe contents a discussion on the feed requirements for the technology is needed. The STET tribo-electrostatic belt separator requires the feed material to be dry and finely ground. Very small amounts of moisture can have a large effect on differential tribo-charging and therefore the feed moisture should be decreased to <0.5 wt.% (en anglès). A més a més, the feed material should be ground sufficiently fine to liberate gangue materials and should be at least 100% passing mesh 30 (600 Um). At least for the tailings sample, the material would have to be dewatered followed by a thermal drying stage, while for the itabirite sample grinding coupled with, or follow by, thermal drying would be necessary prior to beneficiation with the STET separator.
The tailings sample was obtained from an existing desliming-flotation-magnetic concentration circuit and collected directly from a tailings dam. Typical paste moistures from tailings should be around 20-30% and therefore the tailings would need to be dried by means of liquid-solid separation (dewatering) followed by thermal drying and deagglomeration. The use of mechanical dewatering prior to drying is encouraged as mechanical methods have relative low energy consumption per unit of liquid removed in comparison to thermal methods. About 9.05 Btu are required per pound of water eliminated by means of filtration while thermal drying, D'altra banda, requires around 1800 Btu per pound of water evaporated [13]. The costs associated with the processing of iron tailings will ultimately depend on the minimum achievable moisture during dewatering and on the energetic costs associated with drying.
The itabirite sample was obtained directly from an itabirite iron formation and therefore to process this sample the material would need to undergo crushing and milling followed by thermal drying and deagglomeration. One possible option is the use of hot air swept roller mills, in which dual grinding and drying could be achieved in a single step. The costs associated with the processing of itabirite ore will depend on the feed moisture, feed granulometry and on the energetic costs associated to milling and drying.
For both samples deagglomeration is necessary after the material have been dried to ensure particles are liberated from one another. Deagglomeration can be performed in conjunction to the thermal drying stage, allowing for efficient heat transfer and energy savings.
The bench-scale results presented here demonstrates strong evidence of charging and separation of Fe-bearing minerals from quartz using triboelectrostatic belt separation.
For the tailings sample Fe content was increased from 29.89% a 53.75%, on average, at a mass yield of 23.30%, which corresponds to Fe recovery and silica rejection values of 44.17% i 95.44%, respectivament. For the itabirite sample Fe content was increased from 47.68 % a 57.62%, on average, at a mass yield of 65.0%, which corresponds to Fe recovery and silica rejection values of 82.95% i 86.53%, respectivament. These results were completed on a separator that is smaller and less efficient than the STET commercial separator.
Experimental findings indicate that for both tailings and itabirite samples the maximum achievable Fe content will depend on the minimum achievable quartz content. A més a més, achieving higher Fe grades may be possible by means of a second pass on the STET belt separator.
Els resultats d'aquest estudi van demostrar que les multes de mineral de ferro de baix grau es poden actualitzar mitjançant el separador de cinturó tribo-electrostàtic STET. Further work at the pilot plant scale is recommended to determine the iron concentrate grade and recovery that can be achieved. Based on experience, la recuperació i/o qualificació del producte millorarà significativament en el processament a escala pilot, en comparació amb el dispositiu de prova a escala de banc utilitzat durant aquests assajos de mineral de ferro. The STET tribo-electrostatic separation process may offer significant advantages over conventional processing methods for iron ore fines.
