Beneficiation of Iron Ores
Beneficiation of Iron Ores
Iron ore is the fourth most common element in the crust of the Earth. Iron (Fe) is necessary for the production of iron and steel and hence an essential material for the economic development of a nation. Majority of the iron ore resources around the world are composed of metamorphosed banded iron formations (BIF) in which iron is normally found in the form of oxides, hydroxides, and to a lesser extent carbonates.
Iron ore is a mineral which is used after extraction and processing for the production of iron and steel. In fact, it is the basic raw material for iron and steel industry. The chemical composition of iron ores normally has a wide range in chemical composition especially for Fe content and associated gangue minerals. Major Fe minerals associated with the majority of the ore deposits are hematite, goethite, siderite, limonite, and magnetite. The main contaminants in iron ores are silica (SiO2), alumina (Al2O3), sulphur (S), and phosphorous (P). The typical silica and alumina bearing minerals present in iron ores are quartz, kaolinite, gibbsite, diaspore, and corundum. Of these it is frequently observed that quartz is the main silica bearing mineral and kaolinite and gibbsite are the two-main alumina bearing minerals.
The majority of the world production of iron ore consists of haematite and magnetite ores. The chemical formula for pure hematite ore is Fe2O3 which contains 70 % of Fe. The chemical formula for pure magnetite ore is Fe3O4 which contains 72.41 % of Fe. The ore which is extracted at the mine is called ‘run of mine’ (ROM) ore.
Ores are normally associated with unwanted gangue material. Grade of iron ore is normally determined by the total Fe content in the ore. High grade iron ore after dry or wet sizing at the ore mine is that ore which contains higher than 62 % of Fe. This ore is normally known as ‘natural ore’ or ‘direct shipping ore’ (DSO). This ore can be directly used in the production of iron and steel. All other ores need beneficiation and certain processing before they are used in the production of iron and steel.
Iron ore can be classified in different ways. The most important classification is done with the iron content. In several cases, ore with a total Fe content of 62 % or higher, is regarded as high grade, and ore with a lower iron content is regarded as low grade. Iron ore is also classified as per its size and the processing method. Lump ore has the size of around 10 millimetres (mm) to 40 mm. Ore fines have the size of around 0.15 mm to 10 mm. Pellet feeds have the size of less than around 0.15 mm. Concentrate is the ore which has been enriched by the beneficiation process. The hematite ore is normally classified under the categories namely (i) massive ore, (ii) laminated (soft and hard) ore, (iii) lateritic ore, and (iv) blue dust / powdery ore.
Iron ore with Fe content of 65 % and above is desirable to achieve better productivity either through blast furnace or direct reduction processes for the production of hot metal (liquid iron) or direct reduced iron (sponge iron) respectively. Iron ore with low Fe content and high alumina / silica ratio cannot be used directly in the ore reduction processes. The alumina to silica ratio of higher than 1 causes serious problems during the reduction of ore.
Low grade iron ore cannot be used as such for the production of iron and steel and need to be upgraded for reducing its gangue content and simultaneously increasing its Fe content. This is carried out by the processes of beneficiation of the iron ore. Beneficiation of the ore is the process where ore is reduced in size and gangue is separated from the ore. Since each of the iron ore deposit has a unique mineralogy, the beneficiation process is specific to each deposit. Separation of certain minerals can be efficiently achieved by taking advantage of the physical, electrical, and magnetic properties.
Iron ore extraction is mainly performed through open pit mining operations, resulting in considerable generation of tailings. The iron ore production system normally involves two stages namely (i) mining activity, and (ii) processing activity. Of these, the processing activity ensures that an adequate iron grade and chemistry is achieved prior to its shipment. Processing includes crushing, classification, milling and concentration aiming at increasing the Fe content while reducing the quantity of gangue minerals. Each deposit of the ore has its own unique characteristics with respect to Fe and gangue bearing minerals, and hence it needs a different concentration technique.
ROM ore (also known as crude ore) which is mined in the natural state, seldom occurs in a pure state and needs some form of processing. Concentrations of as little as 30 % of Fe can be of commercial interest, provided other factors such as gangue content, the size of the deposit, and accessibility are favourable
The beneficiation of low-grade iron ores has got increased attention in the recent years because of the depletion of high-grade iron ore reserves and growth in the production of steel around the world. There are several issues associated with the beneficiation of low-grade iron ores. These include the mineralogical and textural complexity, high percentage of silica or /and alumina, and soft / hard nature of the ore, and the variability of the ore in the ore deposit etc. Detailed initial characterization of the iron ore is needed before developing a suitable ore beneficiation flow-sheet. The purpose of or characterization is to develop a cost-effective flow-sheet to beneficiate the low-grade iron ore to produce a product which meets the requirements of the ore reduction processes. Fig 1 shows a typical flow-sheet of ore beneficiation plant.
Fig 1 Typical flow-sheet of ore beneficiation plant
Several methods / techniques such as washing, jigging, magnetic separation, gravity separation, and flotation etc. are used to improve the Fe content of the iron ore and to reduce its gangue content. These techniques are used in different combinations for the beneficiation of iron ores. For beneficiation of a particular iron ore, the emphasis is normally to develop a cost-effective flow sheet incorporating necessary crushing, grinding, screening, and beneficiating techniques which are necessary for the upgrading of the iron ore.
Processing of iron ore for its beneficiation normally depend on the size and the nature of impurities present in the ore body. Depending upon the origin and mineralogical characteristics of the ore, different beneficiation methods are being adopted for iron ore ranging from simple crushing, screening, and separating various size fractions of the ore to complex concentration processes which beneficiate or upgrade the quality of the iron ore products.
Processing of the ore is done by physical processes, which remove impurities by difference in particle density, or size gravity, or size separation. Processing can be wet or dry. Further ore handling, washing, and screening operations are mechanized in the crushing and washing plants.
The processed ore is sent for stacking and stockpiling. The processed ore is stockpiled and blended to meet product quality requirements. Stackers are normally used for stockpiling so that bulk goods can later be reclaimed by reclaimers for loading onto a dumper truck or railway wagons for transporting to another stock-pile in the steel plant or port for ship-loading. Iron ores are transported from the mine site to the steel plant or port for export normally either by railway wagons or by dumper trucks. The ore which is despatched from the mine site after dry or wet processing is known as direct shipping ore. Fig 2 describes the typical iron ore beneficiation process.
Fig 2 Typical iron ore beneficiation process
Methods / techniques for iron ore processing
Scrubbing – Scrubbing is the process by which clays, slimes and any potential oxidization present in or on the ore are removed typically by using water. The conditioning of the ore surface is done by the scrubbing for further beneficiation. This process is primitive in nature and is widely used in lumpy iron ore processing to dislodge and remove friable and soft lateritic materials, fine materials, and limonitic clay particles adhering to the ore. Wet scrubbing is also useful in hard and porous ores, which invariably have cavity / pores filled with clayey material which need substantial removal. Crushing and grinding are preformed after the scrubbing of the ore.
Crushing, grinding, and screening – The purpose of crushing, grinding, and regrinding is to reduce the ore to a size small enough to liberate and recover the valuable minerals. The crushing, grinding, and screening systems of the iron ore beneficiation plant are to be designed taking into account the requirements of the downstream beneficiation processes.
Capital investment and operation costs of grinding equipment are high. Hence economics play an important part in planning for the degree of crushing and grinding needed to prepare ore for beneficiation. Other factors considered for the determination of the degree of crushing and grinding includes the value concentration of the ore, its mineralogy, hardness, and moisture content.
The first process which the ROM ore undergoes after it leaves the mine site is the sizing of the ore and separation of different fractions. In crushing and washing plants, the processing of the ROM ore is carried out. ROM ore undergoes crushing and dry screening. Sometimes grinding (also called milling) of the ore as well as wet screening is done by introducing high pressure water jets over screens, fines are classified and ultra-fines are deslimed by hydro-cyclone. This method of ore preparation uses the technique of mechanical separation of the grains of ore minerals from the gangue minerals, to produce an enriched ore containing majority of the ore minerals and a tailing (discard) containing the bulk of the gangue minerals.
However, Iron ore from different sources have their own peculiar mineralogical characteristics and need selection of proper crushing, grinding, and screening techniques for getting the ore product of desired quality. The choice of the technique depends on the nature of the gangue present and its association with the ore structure.
Crushing is the process of reducing the size of crude ore into coarse particles (typically coarser than 5 mm). Depending on the ore characteristics, the crushing of ROM ore can be single stage (primary) crushing, double stage (secondary) crushing, triple stage (tertiary) crushing, or even the stage of quaternary crushing. Jaw, gyratory, cone, and roll crushers are used for ore crushing. The efficiency of crushing depends on the efficiency of upstream processes (rock fragmentation because of blasting or digging in the extraction process) and, in turn, has a considerable effect on down-stream processes (grinding or separations).
In some cases, the primary crushing is located in the mine, while the secondary crushing or tertiary crushing are located at the steel plant. The primary crushing stage reduces the very large size of ROM ore to around 150 mm and further down in subsequent crushing stages to the size of calibrated iron ore (-40 mm to +10 mm), CLO, as the final product.
The crusher product is fed to the grinding (milling) operation for further size reduction when subsequent processing of ore is needed. Semi autogenous grinding and autogenous grinding circuits are used for grinding the ore. Both rod mills and ball mills are used for this purpose. Closed circuit grinding minimizes over grinding of very friable ores. The more is the recirculation load, the less is the over grinding of particles.
Screening is an important step for dry beneficiation of iron ore. Crushing and screening is typically the first step of the ore beneficiation processes. In the majority of the ores, valuable minerals are normally inter-grown with gangue minerals, so the minerals need to be separated in order to be liberated. This screening is a necessary step prior to their separation into ore product and waste mineral. Secondary crushing and screening can result in further classification and grading of iron ore. The fines fraction is normally of lower grade compared with lump ore.
The iron ore processing can be (i) dry processing, (ii) dry cum wet processing, and (iii) wet processing. Dry processing (Fig 3a) is done to meet the size requirements and involve multi-stage crushing and screening to meet the size requirements needed by different iron smelting processes. In dry-cum-wet process (Fig 3b), fines fraction (-10 mm) generated after dry processing is further processed in mechanical classifiers, and hydro-cyclones etc. to get -10 mm to + 0.15 mm size product which constitutes the feed material for the sintering process. The classifier / hydro-cyclone overflow i.e., -0.15 mm (100 mesh) size product constitutes the slime and dumped into the tailing pond. Fig 3 shows dry and dry-cum wet processing of ores.
Fig 3 Types of processing ores
The wet processing (Fig 4) is normally practiced for low / medium grade (60 % Fe to 63 % Fe) hematite iron ore. The wet process consists of multi-stage crushing followed by different stages of washing in the form of scrubbing and / or screening, and classification etc., but the advantage is only partial removal of adhered alumina and free silica in the CLO size fraction of -40 mm to +10 mm. The classifier underflow (-10 mm to +0.15 mm) is directly used for sinter making, while classifier overflow (100 mesh) is dumped in the tailing pond. This washing process marginally improves the handling properties of the ore because of the removal of the clayey material. Fig 4 shows the wet processing of ore.
Fig 4 Wet processing of ore
Gravity separation – Gravity separation is the proven and accepted technique of concentrating several minerals and has been used as a primary form of mineral concentration for decades. Because of its high efficiency and low cost, gravity separation is always the first consideration in a flow-sheet development and always features in any flow-sheet where there are sufficient differences between the specific gravity of the valuable material and gangue material.
This technique is used where iron bearing minerals are free from associated gangue materials. The specific gravity of iron bearing minerals is normally higher than the specific gravity of gangue materials. Effectiveness efficiency of the gravity separation depends largely on to proper crushing and sizing of the ore so as to ensure a proper size feed to the gravity separation equipment and also removal of slime from the equipment. A large variety of equipments / processes functioning on gravity separation principle are available. Some of them are described below.
Dense media separation process is also known as heavy media separation. The process is used for coarse ores (size range 3 mm to 50 mm). Ground ferro-silicon of -300 mesh size is used as suspension to create a parting specific gravity of 3 to 3.2 which is sufficient for the gangue material to float and get separated. The suspension material is recovered by using low intensity magnetic separator (LIMS). Feed for the dense media separation is to be hard and compact with non-porous gangue material.
In certain cases, dense media separation followed by complex gravity circuits in conjunction with multi gravity separation and / or other separation techniques, is needed to provide effective low-cost solutions for allowing the optimal economic recovery of complex iron ores.
Heavy media cyclone process is used for iron ore fines with size range of 0.2 mm to 6 mm. The cyclone type separator utilizes centrifugal as well as gravitational forces to make separation between ore and gangue material. Ground ferro-silicon of -325 mesh size is used as a media in cyclone.
Jigging is the process of sorting different materials in the ore in a fluid by stratification, based upon the movement of a bed of particles, which are intermittently fluidized by the pulsation of the fluid in a vertical plane. The stratification causes particles to be arranged in layers with increasing density from the top to the bottom. This particle arrangement is developed by several continuously, varying forces acting on the particles, and is more related to particle density than most other gravity concentrating methods.
Jigging of iron ores for its beneficiation is being practiced since several decades. The reasons for choosing jigging for the iron ore beneficiation over other processes include (i) relatively easy separation, (ii) beneficial trade-off between operating cost and reduced yield relative to dense medium processes, (iii) ability to treat ores needing cut specific gravities higher than 4, and (iv) physical characteristics of the ore which make heavy medium separation unsuitable (e.g., unacceptable media loss in macroscopic pores). Use of air-pulsed jigs for the beneficiation of iron ores is quite popular. This is since air-pulsed jigs are capable of generating the large pulse amplitudes needed to fluidize a deep bed of heavy ore, particularly lump iron ore.
In the jigging process, the particles are introduced to the jig bed (normally a screen) where they are thrust upward by a pulsating water column or body, resulting in the particles being suspended within the water. As the pulse dissipates, the water level returns to its lower starting position and the particles once again settle on the jig bed. As the particles are exposed to gravitational energy whilst in suspension within the water, those with a higher density settle faster than those with a lower density, resulting in a concentration of material with higher density at the bottom, on the jig bed. The particles are now concentrated according to density and can be extracted from the jig bed separately. In case of the beneficiation of the iron ore, the denser material is the desired enriched ore and the rest is needed to be discarded as floats (or tailings). The principle of the jigging process is shown in Fig 5.
Fig 5 Principle of jigging process
Spirals are the most practical equipment to use for gravity separation when high throughput is needed. Spiral concentrators are flowing film separation devices. General operation is a continuous gravitational laminar flow down on an inclined surface. The mechanism of separation involves primary and secondary flow patterns. The primary flow is essentially the slurry flowing down the spiral trough under the force of gravity. The secondary flow pattern is radial across the trough. Here the upper-most fluid layers comprising higher density particles move away from the centre while the lower-most concentrate layers of higher density particles move towards the centre. Fig 6 shows schematic diagrams of a spiral.
Fig 6 Schematic diagrams of a spiral
Spirals need addition of water at different points down the spiral to assist washing of the iron ore, i.e., transporting away the light gangue from the dense ore. The quantity of wash water and its distribution down the spiral trough can be adjusted to meet the operating requirements. Point control minimizes the total water requirements by efficiently directing water into the flowing pulp at the most effective angle. Feed size applicability is in the range of 0.3 mm to 1 mm. Spirals are normally operated at a pulp density containing 25 % to 30 % of solids.
Shaking tables have wide range of application in gravity treatment of iron ores. These tables are normally used in cleaning and scavenging circuits. Feed size applicability is in the range of 0.3 mm to 1 mm.
Multi gravity separator (MGS) is under development stage and are designed to treat fines and ultra-fine particles of iron ore. The laboratory / pilot plant scale C-900 MGS consists basically of a slightly tapered open-ended drum measuring 600 mm long with a diameter of 500 mm which rotates in a clockwise direction and is shaken sinusoidal in an axial direction. MGS is useful in processing of valuables from slimes and tails. Other gravity separation equipment, are Falcon and Knelson centrifugal separators, and hydro-sizers.
Cyclone – The cyclone, also referred to as simply ‘hydro-cyclone’, is a classifying equipment which utilizes centrifugal force to accelerate the settling rate of slurry particles and separate particles according to size, shape and specific gravity.
The Cyclone is a widely used classifier for in the ore processing. It is installed in close circuit between the grinding and conditioning paths for flotation in the complex flow-sheet of ore processing. It consists of a cylindrical section at the top connected to a feed chamber for continuous inflow of slurry, which is then expelled through an overflow pipe. The unit continues downward as a conical vessel and opens at its apex to the underflow of coarse material. Fig 7 shows schematic diagram of a cyclone.
Fig 7 Schematic diagram of a cyclone
Cyclones used for concentration of iron ores are of several types. These include hydro-cyclone, stub cyclone, and heavy media cyclone. Cyclones are cost effective and simple in their construction. The main parts of a cyclone consist of cyclone diameter, the inlet nozzle at the point of entry into the feed chamber, vortex finder, cylindrical section and cone section. There have been a proper geometrical relationship between the cyclone diameter, inlet area, vortex finder, apex orifice, and sufficient length providing retention time to properly classify particles.
The feed is pumped under pressure through the tangential entry which imparts a spinning motion to the slurry. The separation mechanism works on this centrifugal force to accelerate the settling of particles.
As the feed slurry enters the chamber, a rotation of the slurry inside of the cyclone begins, causing centrifugal forces to accelerate the movement of the particles towards the outer wall. The particles migrate downward in a spiral pattern through the cylindrical section and into the conical section. At this point the smaller mass particles migrate toward the centre and spiral upward and out through the vortex finder, discharging through the overflow pipe. This product, which contains the finer particles and the majority of the water, is termed as the overflow and is to be discharged at or near atmospheric pressure. The higher mass particles remain in a downward spiral path along the walls of the conical section and gradually exit through the apex orifice. This product is termed the underflow and also is to be discharged at or near atmospheric pressure.
The velocity of slurry increases as it follows in a downward centrifugal path from the inlet area to the narrow apex end. The larger and denser particles migrate nearest to the wall of the cone. The finer / lighter particles migrate toward the centre axis of the cone, reverse their axial direction, and follow a smaller diameter rotating path back toward the top. The oversized discharge fractions return to the mill for regrinding, while the undersized fractions move to the conditioning tank for flotation. Hydro-cyclones perform at higher capacities relative to their size and can separate at finer sizes than other screening and classification equipment.
Magnetic separation – Magnetic separation technologies are used to take the advantage of the difference in the magnetic properties for separating iron ore from the non-magnetic associated gangue materials. It can be conducted in either a dry or wet environment, although wet systems are more common. It is used in several flow-sheets.
Magnetic separation is typically used in the beneficiation of high-grade iron ores where the dominant iron minerals are ferro and para-magnetic. Wet and dry low-intensity magnetic separation (LIMS) techniques are used to process ores with strong magnetic properties such as magnetite while wet high-intensity magnetic separation is used to separate the Fe-bearing minerals with weak magnetic properties such as hematite from gangue minerals. Iron ores such as goethite and limonite are normally found in tailings and does not separate very well by either technique.
A full range of magnetic separators is available, from low intensity drum separators to high gradient / high intensity separators, and for either wet or dry feeds. Separation is achieved by exploiting differences in the magnetic susceptibilities of the component minerals. There are five basic types of separators designed for exploiting differences in the magnetic properties from the simplest low intensity unit for separating magnetite to high intensity / gradient units for removing minor impurities. These are (i) wet and dry, low intensity magnetic separation (LIMS), (ii) high gradient magnetic separation (HGMS), (iii) wet high intensity magnetic separation (WHIMS), (iv) roll magnetic separators for processing weak magnetic ores, and (v) induction roll magnetic separation (IRMS) for concentrating dry ores.
Magnetic separation operations can also be categorized as either low or high intensity. Low intensity separators use magnetic fields between 1,000-gauss to 3,000-gauss. Low intensity techniques are normally used on magnetite ore as an inexpensive and effective separation method. High intensity separators employ fields as strong as 20,000-gauss. This method is used to separate weakly magnetic iron ores such as hematite, from non-magnetic or less magnetic gangue materials. Other factors important in determining which type of magnetic separator system is to be used include particle size and the solids content of the ore slurry feed.
Typically, magnetic separation involves three stages of separation namely (i) cobbling, (ii) cleaning / roughing, and (iii) finishing. Each stage can use several drums in a series to improve separation efficiency. Each successive stage works on finer particles as a result of the removal of over-sized particles in earlier separations. Cobblers work on larger particles and reject substantial percent of feed as tails. Fig 8 gives principle and working ranges of magnetic separators.
Fig 8 Principle and working ranges of magnetic separators
Flotation process – The principle of the flotation process is based on the differences in the chemical properties of the mineral surface (hydrophilic, flowability). The flotation agent is used to selectively attach useful minerals or useless impurities to the bubbles for the purpose of separation. This method is also called chemical beneficiation.
A standard flotation circuit starts by separating the scrubbed ore into a coarse portion (e.g., +20 mesh), and a fine portion (e.g., -20 mesh). For designing optimum flotation circuits, it is necessary to understand the processing needs, and the technical and mineralogical characteristics of the ore. Successful flotation involves proper liberation, adding the proper reagents to induce selected minerals to become hydrophobic (water repelling) or hydrophilic (water attracting). Aeration (bubbles) is added through spargers at the bottom of the flotation cell. The bubbles attract and then float the hydrophilic minerals, leaving the hydrophobic component in the underflow as tailings. The flotation circuit then concentrates and separates the desired minerals. Fig 9 shows the schematics of flotation process.
Fig 9 Schematics of flotation process
Flotation is used to reduce the content of impurities in low-grade iron ores. Iron ores can be concentrated either by direct anionic flotation of iron oxides or reverse cationic flotation of silica, however reverse cationic flotation remains the most popular flotation route used in the beneficiation of the iron ore. The use of flotation is limited by the cost of reagents, the presence of silica and alumina-rich slimes and the presence of carbonate minerals. Moreover, flotation needs waste water treatment and the use of downstream dewatering for dry final applications.
The use of flotation for the concentration of iron ore also involves desliming since floating in the presence of fines results in decreased efficiency and high reagent costs. Desliming is particularly critical for the removal of alumina as the separation of gibbsite from hematite or goethite by any surface-active agents is quite difficult. Majority of the alumina bearing minerals occurs in the finer size range (less than 20 micrometres) allowing for its removal through desliming. Overall, a high concentration of fines (less than 20 micrometres) and alumina increases the needed cationic collector dose and decreases selectivity dramatically. Hence, desliming increases flotation efficiency, but results in a large volume of tailings and in the loss of iron to the tailings stream.
Flotation process uses a technique where particles of one mineral or group of minerals are made to adhere preferentially to air bubbles in the presence of a chemical reagent. This is achieved by using chemical reagents which preferentially react with the desired mineral. Several factors are important to the success of flotation activities. These include uniformity of particle size, use of reagent compatible with the mineral, and water conditions which do not interfere with the attachment of the reagents to the mineral or the air bubble.
Today flotation is primarily used to upgrade concentrates resulting from magnetic separation. Flotation, to be used all alone as a beneficiation method, is used rarely.
Chemical reagents used are mainly of three main groups namely (i) collectors/amines, (ii) frothers, and (iii) anti-foams. Reagents can be added in a number of forms which include solid, immiscible liquid emulsion, and solution in water. The concentration of reagents needs to be closely controlled during conditioning since adding more reagent than needed retards the reaction and reduce efficiency. Factors which affect conditioning include thorough mixing and dispersal of reagents through the slurry, repeated contact between the reagents and all of the relevant ore particles, and time for the development of contacts with the reagents and the ore particles to produce the desired reactions.
Dry processing of iron ore – It presents an opportunity to eliminate costs and wet tailings generation associated with flotation and wet magnetic separation circuits. STET (ST equipment and technology) has evaluated several iron ore tailings and run of mine ore samples at bench scale (pre-feasibility scale). Considerable movement of iron and silicates has observed. The results of this study have demonstrated that low-grade iron ore fines can be upgraded by means of STET tribo-electrostatic belt separator.
The STET dry electrostatic separation process offers several advantages over traditional wet processing methods, including the ability to recover fine and ultra-fine iron which otherwise is lost to tailings if processing is done with existing technology. In addition, the technology needs (i) no water consumption, which results in the elimination of pumping, thickening and drying, as well as any costs and risks associated with water treatment and disposal, (ii) no wet tailings disposal, hence avoiding the long-term risk of storing wet tailings in a tailing pond, and (iii) no chemical additions needed, which hence negates the ongoing expense of reagents and simplifies compliance to statutory regulations.
In the tribo-electrostatic belt separator (Fig 7) material is fed into the thin gap 9 mm to 15 mm between two parallel planar electrodes. The particles are tribo-electrically charged by inter-particle contact. For example, in the case of an iron sample comprising mainly hematite and quartz mineral particles, the positively charged (hematite) and the negatively charged (quartz) 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 counter current flow of the separating particles and continual tribo-electric charging by particle-particle collisions provides for a multi-stage separation and results in excellent purity and recovery in a single-pass unit. The belt allows for processing on fine and ultra-fine particles including particles smaller than 20 micrometres, by providing a method to continuously clean the surface of the electrodes and remove the fine particles, which otherwise adhere to the surface of the electrodes. The high belt speed also enables throughputs up to 40 tons per hour on a single separator by continuously conveying material out of the separator. By controlling different process parameters, the device allows for optimization of mineral grade and recovery. Fig 10 shows schematic diagram of tribo-electrostatic belt separator.
Fig 10 Schematic diagram of tribo-electrostatic belt separator
The separator design is relatively simple. The belt and associated rollers are the only moving parts. The electrodes are stationary and composed of a highly durable material. The belt is a consumable part which needs infrequent but periodic replacement, a process which is able to be completed by a single operator in only 45 minutes. The separator electrode length is around 6 meters and the width 1.25 meters for full size commercial units. The power consumption is less than 2 kWh per ton of material processed with the majority of the power consumed by two motors driving the belt.
The process is entirely dry, needs no additional materials and produces no waste water or air emissions. For iron ore beneficiation, the separator provides a technology to reduce water usage, extend reserve life and / or recover and reprocess tailings. The compactness of the system allows for flexibility in installation designs. The tribo-electrostatic belt separation technology is robust.
The STET dry electrostatic separation process offers several advantages over traditional wet processing methods, such as magnetics or flotation. There is no water consumption. The elimination of water also eliminates pumping, thickening and drying, as well as any costs and risks associated with water treatment and disposal.
There is no wet tailings disposal. The high-profile failures of tailings dams have long term risk of storing wet tailings. By necessity, iron ore processing operations produce tailings of some sort, but the STET electrostatic separator tailings are free of water and chemicals. This allows for easier beneficial re-use of the tailings. Tailings which do need to be stored can be mixed with a small volume of water for dust control.
Since, no chemical addition is needed as in the case of flotation chemicals, there is no ongoing operating expense for mineral processing operations. Further, there is suitable for processing fine powders. Desliming is not needed depending on ore mineralogy and grade. Moreover, there is lower investment cost (CAPEX) and lower operating cost (OPEX). Also, the process has lower environmental impact because of the elimination of water treatment