NO20170616A1 - Method and system for total dry refining of iron oxide ore through a magnetic separation unit - Google Patents

Description

Specification of the Patent of Invention for: "PROCESS AND SYSTEM FOR

TOTALLY DRY ORE-DRESSING THROUGH A MAGNETIC SEPARATION UNIT"

BACKGROUND OF THE INVENTION

The present invention relates to a process and a system for totally dry dressing of iron ore of fine to superfine granulometry lower than 150 microns, in a totally dry pathway. Iron ores of fine and superfine granulometry are found at waste barrages and in dump ores from existing plants.

At the present cold ore-dressing plants, which exist at large mining companies, the original projects used to envisage the processing of ores håving high iron contents, for example, iron contents higher than 54% Fe, so that the whole material lower than this percentage is considered dump, which has been restricting more and more the dressing process due to the lack of ore with iron contents higher than 54%.

However, in the terms of the present invention said dump is considered useful for feeding a process and a system for totally dry dressing of iron-oxide ore through a magnetic separation unit.

In this regard, the present invention aims at simplifying the process of recovering the iron ores that are still contained in said dump, bringing about high metallurgical and mass recoveries. Thus, it is possible to obtain a commercially superior product, more precisely an iron-oxide ore concentrate with contents higher than 63% Fe, and with a simple adjustment to increase the final content of the concentrate, which may reach up to 67% Fe(T). Such a result represents a significant advance from the environmental point of view, if one considers the risk historically represented by the dump basins of the mining industry in Brazil and around the world.

The innovatory characteristics of the dry process of the present invention meet advantageously and simultaneously meets the demands of economic, environmental and strategic nature of the mining industry, since they enable improved recovery of dump that constitutes a high risk of environmental impact and transformation thereof into marketable products in a technically and economically feasible manner, as well as the possibility of making use of the low-content iron ores that are dumped by the existing plants at present.

In this dry process, no water is used, and the final residue is formed by a pile of waste, entirely dispensing with the need for a waste barrage. Besides, by means of the dry process of the present invention, the waste generated becomes now by-products for the pavers-and-blocks industries of building construction and also in the cement industry.

Said pavers and blocks are manufactured from sand and cement. In a number of states in Brazil, the source of sand consists in obtaining this raw material from dismounting altered-granite hills. However, in this process the altered granite is removed by mechanical dismounting or with water-jet, undergoing a degradation process, by sieving and hydrocycloning, wherein the sand fractioning is separated from the clay fraction. In order to deposit this clay and recover water, it is necessary to build a waste barrage. Another way to obtain this raw material consists in removing the sand fraction from dug places by the draining process and the sand fraction is separated from the clay fraction by a hydrocycloning process in which the clay returns to the dug place. At the end of this process, due to the removal of a large part of the substrate constituent, there will be formation of large lakes, usually without economical utilization for these liabilities. For instance, this is the case of the region of city of Seropédica - RJ - Brazil, with great liabilities consisting of the presence of large saturated lakes of mineral sediments, due to the extraction of sand, without condition of sustaining any form of life or of economical utilization.

DESCRIPTION OF THE PRIOR ART

In the beginning of the mining activities on an industrial scale, little was known about the techniques for disposal of dump. The low interest in this area was still due to the fact that the amount of waste produced was reasonably small and the environmental problems still were not part of the operational concerns of the industry.

In this regard, the dump was usually thrown into streams at random. However, with the growth of the mining sector, the growing social concern about the environmental issue, as well as the occurrence of accidents with dump retention barrages ever since the Seventies in various parts of the world, including Brazil, the mining companies faced the challenge of guaranteeing operation of the industrial units with minimization of environmental impacts and reduction of accident risks through more careful and optimized projects.

In general, three techniques are used for disposal of mining dump, namely: - by wet pathway in barrages;

- by dry pathway in dump piles, or

- by using past-fill technology.

The difference between the disposal by dry pathway and by wet pathway is that by wet pathway in barrages there is retention of liquids in conjunction with the solid material discarded. The high-density magnetic separation is traditionally adopted for continuous flows of material, usually by wet operation, a process that is internationally known as WHIMS - Wet High Intensity Magnetic Separation).

With regard to the disposal in the form of paste, this is an alternative to the conventional practices, which has advantages such as greater recovery and circulation of water, greater rest angles and smaller impact on the environment. However, this process is carried out at high costs for implantation and operation.

For instance, Brazilian patent application BR PI0803327-7 discloses a magnetic concentration process with low consumption of water and little generation of dump mud. The wet magnetic separation and the pouring of the magnetic waste may decrease the throwing of a large part of the magnetic waste into settling barrages. However, this process does not deal with the recovery of this dump. So there no effective decrease in the environmental risk inherent in the activity.

Another document, BR PI0103652-1, describes a process for recovering iron-oxide dump. This dump can be obtained directly from the recovery of fines from reduction processes of metallurgy, as well as from the deviation of return of fines from companies that supply iron ore to iron-and-steel companies. The material is charged into a feed silo and follows through chutes and conveyor belts to a rotary drying furnace. The dried material is discharged for stock without undergoing any sorting/concentration process, or is then led directly to the reduction furnaces by a conveyor-belt system.

Document BR 102012008340-0, also belonging to the same applicant, discloses a system and a process equally intended for the separation of fines and superfines, but which are capable of processing ores that are considered dump. Besides, the separation unit disclosed in this document is operationally inadequate for processing minerals with high magnetic susceptibility (such as magnetite - FeOFe203). In addition, the feed control at the silos of the separation unites of this system is made by varying the vibration intensity of the vibrating motor installed there, which not always results in an adequate flowability of the material in the separator. Finally, the system and the process disclosed in this document do not enable immediate disposal of the non-magnetic fraction separated.

With regard to the step of drying the dump is for subsequent separation, the prior art traditionally employs a rotary drum dryer. By this technique, the presence of fines in the drier results in the formation of an expressive amount (30 to 50%) of pellets inside the dryer (which obviously runs counter to the objective of recovering fines), leading to low efficiency of the equipment for coarse particles and even greater inefficiency for fine particles, since the particles are not released, thus preventing the separation between the iron-oxide minerals and the impurities.

Fluid-bed dryers are recommended for coarse particles that enable one to form fluid beds, and it is impossible to form fluid bed for fine particles.

Spray Dry, widely used at present in the ceramic industries in preparing masses for the manufacture of porcelain floor tiles. However, for drying on a Spray Dry it is necessary to form a pulp with 50% solids to promote spraying of particles to be injected against a current of hot air. In order to feed 500 ton/h, it is necessary to add a magnetic separation unit that exhibits satisfactory efficiency for materials that are traditionally unfeasible to process by magnetic separators by means of permanent-magnet rollers of high rare-earth intensity (such as iron-boron-neodymium) and of low ferrite-magnet intensity (such as iron-boron).

Such objectives are achieved in an absolutely effective manner by reducing the potentiality of environmental risk in implanting the system, by promoting the rational use of natural resources, by recovering the dump that may present environmental risk in the event of an accident at the barrages or at piles, and by friendly interaction with the surrounding.

In times of increasing environmental demands, the present invention constitutes a definitive response to the challenge of generating economical results in an environmentally friendly manner, chieflycharacterized by: - greater mass and metallurgic recovery of the iron; - recovery of iron-ore fines in fractions < 100 mesh (about 150 microns) without losses by due to drag in the during the wet magnetic separation; - dean combustion without residues; - non-existence of residues to the atmosphere; - more efficient separation of the iron with generation of deaner dump with lower iron contents; - logistic optimization with localized treatment; - preservation of springs and aquifers; - elimination of the risk of accidents with dump barrages; - decrease in the physical space intended for the implantation; - low consumption of energy; - modularity and flexibility of the system; - increase in the useful life of the mines, by virtue of the possibility of treating iron-oxide ores with much lower contents.

As already said, the singularity of the solution provided by the present invention lies in adopting a totally-dry pathway for processing minerals, which requires the introduction of a drying and disaggregating unit before feeding the finer fractions to a magnetic separator.

The pathway that constitutes the pillar of the present invention can be summarized as follows: the ore moisture is reduced by means of a dryer with mechanical stirring (by using natural gas to prevent contamination or burning biomass), the ore being sorted into various fractions by different clycloning stages and then separated magnetically into each of the sorted ranges, with the important differential of being a process developed in a totally dry manner.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic diagram showing the dressing of fines from dump barrage and/or of the dump; Figure 2 is an operational flowchart of the process for fines from dump barrage; Figure 3 is an operational flowchart of the process for utilization of the dump by the wet plants in operation at present; Figure 4 shows a rapid dryer with mechanical stirring/mechanical stirring system used in the process and system of the present invention; Figure 5 shows an arrangement of the set of cyclones; Figure 6 is a scheme of the magnetic separation unit according to the present invention; Figure 7 is a representation of a side section of the magnetic separation unit according to the present invention. Figure 8 to 12 are graphs representing the granulometric distribution of the different samples obtained in the example described in the text, according to an exemplifying embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Before beginning to describe the invention, it is well to point out that the magnitudes set forth herein are given by way of example, so that they should not be tåken as being limitative of the protection scope of the present invention. A person skilled in the art, in the face of the presently disclosed concept, will know how to determine the magnitudes suitable to the

concrete case, so as to achieve the objectives of the present invention.

In figure 1, reference numbers 1 to 8 represent steps and components such as they are traditionally employed in the prior art, so that they do not incorporate the innovations brought by the present invention.

In this regard, for dump basins, the removal of fines and superfines from iron oxide is started by extraction with a dredge 2 and pumping as far as the bank to let off the excess water and form a pile for charging the material. A number of piles are formed along the dump bank for the purpose of separating as much water as possible from the dump. Once the percentage of moisture of about 6 to 8% has been reached, the dump is collected by an excavator 3, and carried on a dump-truck 4, for transport to the silo or chute hopper 5.

For the dump, the process is started by extraction with an excavator 3, which feeds directly a dump-truck 4 for transport as far as the silo or chute hopper 5.

The discharge from the silo or chute hopper 5 is equipped with a belt-feeder, which feeds a screen 7 intended for carrying out preliminary separation.

The screen 7 may consist, for instance, of a shaking screen for removal of contaminating material. Thus, the material is led to a lung-pile 8.

The capacity of said lung-pile 8 can be defined according to the operational capacity of the plant.

Additionally, one may provide a mist curtain around the chute hopper, so as to prevent dust from escaping out of the chute hopper. In this regard, the belt-feeder may be completely enclosed, thus preventing possible losses of material and the consequent emission of dust to the atmosphere.

Below the lung-pile 8, one may provide a duct comprising a shaking feeder (not shown), which provides transfer of the ore to the belt-feeder.

From the belt-feeder of the lung-pile 8, the material is then led to the first of the three unitary operations that constitutes the present invention, relating to the process and to the system for dry recovery of iron-oxide fines

and superfines.

The first unitary operation consists of the dry operation/disaggregation of fine particles. The process of drying fine and superfine particles is usually considered a technically complex operation, since the presence of other contaminating minerals, such as clay minerals and iron hydroxide, in the rotary-drum drying process, tends to form pellets, which is an aggregate of different minerals, thus preventing the possibility of carrying out the magnetic separation process.

Thus, in order to solve the already-mentioned problem of drying/disaggregating fine particles, and to obtain particles 100% individualized for achieving maximum efficiency of the magnetic separation process, one proposes the use of a dryer 9 with mechanical stirring, as shown in figure 4.

The dryer 9 is composed by a heating chamber 10, which generates hot air (temperature around 850°C) introduced into the main body, within which two axles 9.1 are provided with a plurality of blades 9.2 that move the particulates both vertically and horizontally. These gases go through a labyrinth system 9.5, forcing the heated air to come into contact with the material. The vertical movement of particles, besides providing contact of particles with the hot air to increase the efficiency of the drying process, still facilitates the removal of fines by the system of capturing fines provided by the negative pressure exerted by the exhaust fan.

In the dryer 9, a further efficient step of disaggregating iron-oxide ore fines and of the non-magnetic fraction takes place, through movement of particles vertically, so that the dry material moves along the main body as far as the discharge point 9.3.

The dryer may be sized for a capacity of up to 600 t/h. For larger capacities suffice it to add drying modules. Based on the characteristics of the material to be dried, the dryer may have, for instance, capacity to dry, disaggregate and, at the same time, remove the fines in which the material to be fed to the lower dryer at 100 mesh (about 150 microns) can reach about 98% of the total.

The main characteristics of the dryer employed in the tests carried out are listed hereinafter. - It is equipped with two shafts driven by a duly sized electric motor. The shafts are equipped with a plurality of blades for different positions, namely: inclined in the direction of discharge, which causes the material to move forward, straight blades to impel the material upward and blades inclined in the feeding direction, which tend to retard or control the velocity of the material inside the dryer; - discharge of the fraction > 100 mesh of the dry product; - sluice valves at both the feed and the discharge of the fraction > 100 mesh, these sluice valves tend to prevent the entry of cold air into the system, as well as the exit of hot gas, keeping the performance at the temperature of the hot gases, that is to say, providing optimization of the thermal balance; - there are two safety valves for each dryer, for the event of an explosion taking place; - a hot air generator with ducts that interconnect the generator to the dryer coated with refractory materials; - valves for entry of cold air to make the balance of the temperatures measured with thermocouple and pyrometers; these temperatures can be indicated and controlled on a control panel; - a set of cyclones and interconnection ducts for exit of gases plus the product and endless screws with rotary valves; a support structure is provided for the cyclones; - a duct interconnecting the cyclones to sleeve filters 22, besides screws for exit of the products, exhaust fan and chimney; - an electric board for the system, capable of providing automation and measurement and control instruments; - in the drying process, the dryer 9 needs to operate with a depression for removal of water vapor obtained in the drying process. Therefore, the dryer should be coupled to an exhaustion system. In this process of removing water vapor, the fines smaller than 150 microns are also

dragged by the exhaust system. For this reason, the exhaust system is composed by different cycloning stages and a final collection system close to the sleeve filters 22, so as to prevent any emission of particulates to the atmosphere. For generation of heat, as already said, one uses natural gas and/or biomass, which together with the adequate control of the flow of air, in a correct air/fuel ratio provides dean and complete combustion, håving as discharge the gases after passage through the sleeve filters 22.

The process of removing the gases, water vapor and fines is carried out by a high-capacity exhaust fan arranged at the end of the circuit. Associated to the circuit of the exhaust system there is a component that integrates the dryer to the so-called unitary operation of the process of the present invention. In other words, it consists of a set of cyclones in series, duly sized with losses of adequate load to make the separation by different granulometry ranges.

Therefore, the second unitary step of this inventive process consists in providing a set of cyclones arranged in series, each of the cyclones being sized to separate a granulometry range, the granulometry ranges being defined according to the release granulometry of the iron-oxide ore with its associated interfering minerals. However, the number of cyclones may be determined as from one to six units, according to the granulometry range to be processed. The cyclones are usually pieces of equipment used for collecting fines with granulometry bigger than 10 microns, exactly to diminish the charge of fines in the sleeve filters 22. However, in order to collect particles at different granulometry ranges, it is necessary to re-size the cyclone to capture in accordance with the desired granulometries. In this regard, the cyclones can collect efficiently 100% of the particles bitter than 10 microns.

According to the embodiment given by way of example in figure 5, the sizing of the number of cyclones, arranged in series and according to the intended granulometric cuts, three cyclones are provided in series, which collect the following granulometry ranges: in the first cyclone 11, the granulometric range collected is smaller than 150 microns and bigger than 45 microns; in the second cyclone 14, the granulometry range collected is smaller than 45 microns and bigger than 22 microns; and in the third cyclone 17, the granulometry range collected is smaller than 22 microns and bigger than 10 microns.

Finally, as to the superfines particles smaller than 10 microns, they are sucked and withdrawn in a set of sleeve filters 22.

The products collected in each of the cyclones 11, 14 and 17 arranged in series are then sent to respective cooling columns 12, 15 and 18, which have the function of lowering the temperature that is between 70°C and 100°C to a temperature of about 40°C. This cooling is necessary to preserve the magnetic intensity of the rare earth magnets (iron-boron-neodymium).

The material collected in the first cyclone 11, which corresponds to the fraction smaller than 150 microns and bigger than 45 microns, is sent to the first cooling column 12 and then fed to the first magnetic separation unit 13. The material collected in the second cyclone 14, which corresponds to the fraction smaller than 45 microns and bigger than 22 microns, is sent to the second cooling column 15 and then fed to the second magnetic separation unit 16. The material collected in the third cyclone 17, which corresponds to the fraction smaller than 22 microns and bigger than 10 microns, is sent to the third cooling column 18 and then fed to the third magnetic separation unit 19.

Indeed, the magnetic separation comprises the next unitary step of the present invention.

In the magnetic separation step, the products of each cyclone 1, 14 and 17, which feed successively the cooling columns 12, 15 and 18, classified in different granulometry ranges, are sent to respective sets of magnetic separators 13, 16 and 19, arranged in cascade, and by be from two magnetic rollers to four magnetic rollers or more rollers according to the need. However, this configuration depends chiefly on the separability charactristics of the iron-oxide minerals present.

For each of the granulometric ranges separated in the different cyclones positioned in series, the respective separators of high-intensity rare-earth (iron-boron, neodymium) rollers are fed with the due adjustments of velocity, as well as the positioning of splits, which will be discussed later.

Figure 6 shows the magnetic separation scheme with three rollers in cascade. In the first magnetic separation unit 13, the material from the first cyclone 11 and from the first cooling column 12 feed a first magnetic roller, which may be of low and/or high intensity, generating a first non-magnetic fraction that should be discarded immediately, a first magnetic fraction, which consists of a final product with contents higher than 64% Fe (T), and a first mixed fraction that feeds a second high-intensity magnetic roller. In the same sequence, the second magnetic roller 16 generates a second non-magnetic fraction, which is also discarded, and a second magnetic fraction with contents higher than 64% Fe (T), besides a second mixed fraction, which will feed the third magnetic roller. The third magnetic roller 10, in turn, generates a third non-magnetic fraction that is equally discarded, a third magnetic fraction with contents higher than 64% Fe (T) and a third mixed fraction, which is discarded together with the third non-magnetic fraction.

And so on, the products from the second cyclone will feed a cooling column and, then, the second magnetic separation unit 16, in the same sequence, just as in the first magnetic separation unit, feeds the first magnetic roller, which may be of low and/or high intensity, generating a first non-magnetic fraction that should be discarded immediately, a first magnetic fraction, which consists of a final product with contents higher than 64% Fe (T), and a first mixed fraction, which feeds a second high-intensity magnetic roller. In the same sequence, the second magnetic roller generates a second non-magnetic fraction that will also be discarded, and a second magnetic fraction with contents higher than 64% Fe (T), besides a second mixed fraction that will feed the third magnetic roller. The third magnetic roller, in turn, generates a third non-magnetic fraction that is equally discarded, a third magnetic fraction with contents higher than 64% Fe (T) and a third mixed fraction that is discarded together with the non-magnetic fraction. The same thing will happen in the third magnetic separation unit 19.

Further in figure 6, one shows the magnetic separation scheme with three rollers in cascade, the first magnetic roller may be of low intensity or high intensity. Depending on the characteristics of the material to be separated, the use of the low-intensity magnetic roller may be preferred, considering the fact that the permanent magnets are made from iron-boron, with magnetic intensity ranging from 500 to 3000 Gauss, being intended for the separation of high magnetic susceptibility (such as magnetite - FeOFe203). On the other hand, in the case of high-intensity magnetic rollers, the permanent magnets are made from iron-boron-neodymium with magnetic intensities ranging from 7,500 to 13,000 G, indented for the separation of minerals håving low magnetic susceptibility (such as hematite and iron-limonite hydroxides.

In figure 7, which is a representation of a side section of the magnetic separation unit, one illustrates in detail all the elements of the magnetic separation unit in cascade, which in the exemplified case has three rollers overlapping each other. As already seen, each of the cyclones, with granulometries duly classified, feeds a respective set of magnetic separators 13, 16, and 19. According to figure 7, the set is composed by a receiving silo 30, wherein the feed to one set may be alternatively controlled by the vibration intensity of a reducing motor (not shown, as will be discussed later). However, preferably the silo 30 configured with inclination angles that provide better flowability of the material to the magnetic separator assembly.

Then, the material is discharged onto a polyester belt 34 coated with PU, the belt is tensioned by a first low-intensity magnetic roller of ferrite (iron-boron) magnets 32 and by a support roller 33.

The control of the magnetic separation is made by varying the velocity of the magnetic roller and by positioning the splits. In order to contain the dissipation of powder and lead to the magnetic roller 32, one positions an acrylic plate. The split 36 separates the non-magnetic fraction from the mixed fraction, and the split 37 separates the mixed fraction from the magnetic fraction. The first non-magnetic fraction is collected by the chute 38, the first mixed fraction is collected by the chute 39 and the first magnetic fraction is collected by the chute 40. The chute of the first mixed fraction feeds the silo 41 of the second high-intensity magnetic roller of rare earths (iron-boron-neodymium) 42. The second high-intensity magnetic roller of rare earths (iron-boron-neodymium) magnets 42, after the magnetic separation, generates a second non-magnetic fraction, which will be discarded through the chute 43, the second magnetic fraction is discarded in the chute 45 and a second mixed fraction in the chute 44, which feeds the third high-intensity magnetic roller 47 of rare earth (iron-boron-neodymium) magnets through the chute 44 by means of the silo 46. The third high-intensity magnetic roller 47 of rare earths (iron-boron-neodymium) 46, after the magnetic separation, generates a third non-magnetic fraction, which will be discarded through the chute 48, a third magnetic fraction which will be discarded in the chute 50 and a third mixed fraction through the chute 49 is discharged together with the other non-magnetic fractions. Item 51 in the three magnetic separation units comprises support rollers for the polyester belt coated with PU 34.

The low-intensity and high-intensity magnetic rollers are inclined, and the inclination may range from 5 to 55 degrees, with an ideal working range of 15 - 25 degrees, the inclination being defined as a function of the release granulometry of the iron oxide. This inclination, by the tests already carried out, increases the efficiency of separating the magnetic fraction from the non-magnetic fraction.

Other characteristics of this equipment are presented hereinafter: - the high-intensity magnetic roller of high-gradient permanent magnets is preferably made with superponent neodymium magnets, which are resistant to a temperature of up to 60°C and a steel dise of high magnetic permeability; - the actuation of the magnetic roller may be made by means of an AC 2.0 CV motor, with variable velocity and frequency inverter (not shown); - a system for tensioning and aligning the belt 50 is provided, so as to prevent the occurrence of problems due the short distance between rollers of small diameters of think belt. Thus, it is possible to replace the belt in a few minutes, without need for special tools. The guide systems used on each of the magnetic rollers enables tensioning and aligning the respective belts, thereby increasing their useful life; - a separation belt 34 of the type with polyester fabric coated with a PU (polyurethane) layer, with thickness ranging from 0.6 to 1.00 mm; - the silo-type alignment systems 30, 41 and 46 operate by a discharge system by gravity, wherein the inclination angle is duly designed so as to enable a homogeneous discharge throughout the silo, for which reason one can even dispense with the use of a vibration system; - alternatively, if one adopts a feeding system by silo with a vibration drive motor, the latter may exhibit a configuration of 2.0 hp, 220 VCA, three-phase and frequency inverter, for regulation of the feeding speed. It includes a storage silo; this type of feeder enables controlled and uniform feed; - a support structure constructed in carbon-steel profiles with respective finish painting, so as to provide an assemblycharacterized bya compact and easy-to-install unit. One may also provide an entirely dust-proof control panel (not shown), including measurement instruments, velocity controllers, frequency invertere, power voltage:

220 VCA, 60 Hz, three-phase.

Such an arrangement can be viewed in the magnetic separators illustrated schematically in figure 1 under reference number 13, 16 and 19.

EXAMPLE1

ANALYSIS OF DUMP SAMPLE

With a view to test the efficiency of the totally dry process and system for dressing iron-oxide ore through a magnetic separation unit, a sample collected in the dump basin was subjected to a characterization study and was processed in the pilot unit, simulating the same operation pathway adopted by the plant of the process of the present invention.

The sample of ore from the dump pile exhibited an extremely simple mineralogy, constituted essentially by minerals bearing iron and by a non-magnetic fraction. The iron-bearing minerals of the sample collected were constituted by hematite and iron oxides and hydroxides, as shown below. The non-magnetic fraction is composes essentially by silica and a small amount of clay in the form of kaolinite. The percentage of these minerals is shown in Table 1 below.

The sample of dump, after being subjected to the characterization assays, exhibits iron contents of 33.62% Fe(T), the result of which can be seen in Table 2 below.

The sample of dump was subjected to a granulometric analysis and exhibits the following granulometry shown in Table 3 below.

The sample of the second cyclone exhibits the following distribution: 80% (d80) of the mass is smaller than 55 microns, with a median (d50) of 29 microns and 20% (d20) of the weight is smaller than 12 microns. These values can be viewed in the graph in figure 8.

The sample of dump was subjected to the air classification step at a pilot plant, the system being composed by three cyclones arranged in series, sleeve filters and centrifugal exhaust fan. During the operation, one collected samples from each of the three cyclones and fro the sleeve filters and subjected them to a granulometric analysis, in which they exhibited the following results.

The granulometric distribution of the first cyclone is shown in Table 4.

The sample of the first cyclone exhibited the following distribution: 80% (d80) of mass smaller than 75 microns, with a median (d50) of 47 microns and 20% (d20) of the weight smaller than 25 microns. These values can be viewed in the graph in figure 9.

The granulometric distribution of the second cyclone is presented in Table 5 below.

The sample of the second cyclone exhibited the following distribution: 80% (d80) of mass smaller than 39 microns, with a median (d50) of 25 microns and 20% (d20) of the weight smaller than 13 microns. These values can be viewed in the graph in figure 10.

The granulometric distribution of the third cyclone is presented in Table 6 below.

The sample of the third cyclone exhibited the following distribution: 80% (d80) of mass smaller than 27 microns, with a median (d50) of 26 microns and 20% (d20) of the weight smaller than 8 microns. These values can be viewed in the graph in figure 11.

The granulometric distribution of the fines collected sleeve filters is presented in Table 7 below.

The sample of the sleeve filters exhibited the following distribution: 80% (d80) of mass smaller than 13 microns, with a median (d50) of 5 microns and 20% (d20) of the weight smaller than 1 microns. These values can be viewed in the graph in figure 12.

The chemical analysis of the products obtained in the three cyclones positioned in series plus the product from the sleeve filters exhibited the following results, shown in Table 8 below:

As can be seen, the first cyclone exhibits retention of 52.3% by weight, with contents of 34.67% Fe (T) and exhibits 53.96% of the iron contained. The second cyclone with 22.6% by weight, with contents of 33.23% Fe (T), which corresponds to 22.35% the iron contained. The third cyclone exhibits retention of 16.2% by mass, with contents of 31.64% iron, which represents 15.25% the iron contained. The sleeve filters had 8.9% by weight, with contents of 32.05% Fe (T), which represents 8.53% of non-recoverable iron.

All the products collected in each of the cyclones 11, 14 and 17 were classified in different granulometry ranges, according to the above-cited granulometric distributions. Each of the fractions was processed in a magnetic separation unit, which in this case corresponds to the magnetic separation units 13, 16, 19 of the pilot plant, being composed by two magnetic rollers overlapping each other. All the magnetic rollers are high-

intensity rollers of rare-earth (iron-boron-neodymium) magnets.

Each of the products obtained in the cyclones 11, 14, and 17 respectively for each of the magnetic separation units 13, 16 and 19, as shown in figure 7, was inserted into the first silo 30 of the first high-intensity magnetic roller 32,o generating the first non-magnetic fraction, which is discarded from the magnetic separation unit through the chute 37. In this separation, one guarantees the first magnetic fraction, which is also removed through the chute 38 and further generates a mixed fraction that will feed the second magnetic roller, the material will be collected and led to the second silo 39 through the collecting chute 43. The silo of the second magnetic roller 39 feeds the second high-intensity magnetic roller of rare-earth magnets 40, which in turn generates a second non-magnetic fraction that will be collected and removed fro the magnetic separation unit through the collecting chute 41, and a second magnetic fraction is also generated, which will be collected through the chute 42, besides a second mixed fraction, which is collected through the chute 44, which in incorporated in conjunction with the second non-magnetic fraction.

All the non-magnetic products from the first roller and from the second roller of high intensity and, therefore, derived from chutes 37, 41, besides the mixed fraction derived from the chute 44, are collected on a conveyor belt called non-magnetic.

All the magnetic products from the first roller and from the second roller of high intensity and, therefore, derived from the chutes 38 and 42, are collected on a conveyor belt called non-magnetic.

For each of the magnetic separation units, after processing on the first magnetic roller, all the first magnetic products, first non-magnetic products and first mixed products from the first roller were collected and subjected to the calculation of mass balance and chemical analysis.

For each of the magnetic separation units, the mixed fraction from the first magnetic roller fed the second high-intensity magnetic roller, wherein the products generated were collected as second magnetic products and second non-magnetic products, besides second mixed products and also subjected to the calculation of mass balance and chemical analysis.

The result of the mass balance and the chemical analysis of the first high-intensity magnetic rollers is shown in Table 9.

In the three cyclones 11, 14 and 17, arranged in series, which correspond to the magnetic fraction -150 and +10 microns, one achieved a recovery of 28.28% by mass, with contents of 65.41% Fe(T) and a metallurgical recovery of 55.02%. One achieved a generation of a non-magnetic fraction -150 and +10 microns, 29.92% by weight, with contents of 2.42% Fe(T), which corresponds to a metallurgical loss of 2.15%. Besides, a first mixed fraction -150 and +10 microns of 32.90% by mass, with contents of 35.10% Fe(T) was generated, further with a presence of 34.35% contained iron to be separated in the second high-intensity magnetic roller.

The result of mass balance and chemical analysis of the reprocessing of the mixed fractions from the second magnetic rollers is shown in Table 10.

TABLE 10 - RESULTS, RECOVERY OF THE SECOND HIGH-INTENSITY MAGNETIC ROLLERS

For the magnetic fraction -150 and +10 microns, from the three rollers of the magnetic separation units, one observes a recovery of 14.90% by mass, with contents of 64.20% Fe(T) and with a metallurgical recovery of 28.46%. One further observes a generation of a combined non-magnetic and mixed fraction -150 and +10 microns, with 18.00% by weight with contents of 11.03% Fe(T), which corresponds to a metallurgical loss of 5.90%.

The composition of the final metallurgical balance, composing the magnetic fractions from the first rollers plus the magnetic fractions from the second rollers, is shown ion Table 11 below.

Composing the magnetic products from the first magnetic rollers with the products from the second magnetic rollers, one observes a recovery in mass of 43.18% by weight, with contents of 64.99% Fe(T), which corresponds to a metallurgical recovery of

83.48% contained iron.

The composition of the final metallurgical balance, composing the non-magnetic fractions from the first rollers plus the non-magnetic fraction and of the mixed from the second rollers is shown in Table 12 below.

Therefore, 47.92% by masse is discarded, with contents of 5.65 Fe(T), which corresponds to 8.92% unrecovered iron.

In the sleeve filters, the granulometric fraction smaller than 10 microns, which in the process of the present invention does not enable its magnetic recovery, exhibits 8.9% by weight, with contents of 32.05% Fe(T) and 8.48% non-recovered iron, as shown in Table 13 below.

Other modifications within the spirit and concept of this invention and evident to a person skilled in the art, after considering this specification, will also be regarded as being within the scope of the invention, as defined in the accompanying claims.

Claims (16)

1. A system for dry dressing of iron-oxide-ore fines and superfines present in dump barrages and low-content dump ores, which comprises: - drying means; - a set of air classifiers by cycloning in granulometry ranges; and - a set of magnetic separators for the granulometry ranges from the air classifiers; characterized in that: - the drying means comprise a hot-air injection dryer (9) with mechanical stirring means and shafts provided with blades (9.2) for disaggregating and moving the material in the horizontal and vertical directions; - the set of air classifiers is operatively connected to the outlet of the drying means, being composed by at least one to five cyclones (11, 14,17) arranged in series, which carry out classification of the fine and superfine iron ores into pre-determined granulometry ranges, particularly according to the degree of difficulty of magnetic separability of the material, each of the cyclones (11, 14, 17) being connected to a column cooling unit (12, 15, 18), so as to reduce the temperature of the iron-oxide ore; - each of the magnetic separators (13, 16, 19) of the set of magnetic separators comprise two to four magnetic rollers (32, 42, 47) arranged in cascade, formed by rare-earth magnets of low and/or high magnetic intensity, the magnetic rollers being arranged at an inclination angle that may range from 5 to 55 degrees.

2. The system for dry dressing of iron-oxide-ore fines and superfines according to claim 1,characterized in thatthe drying means is connected to a heating chamber (10) to generate heat at a temperature around 850°C.

3. The system for dry dressing of iron-oxide-ore fines and superfines according to claim 1 or 2,characterized in thatthe outlet of the drying means is constituted by exhaust means for gases from the drying, for water vapor present inside the dryer and for fine and superfine particles of iron-oxide ore.

4. The system for dry dressing of iron-oxide-ore fines and superfines according to any one of claims 1 to 3,characterized in thatthe cooling unit is a water-cooling column unit that reduces the temperature of the material to about 40°C.

5. The system for dry dressing of iron-oxide-ore fines and superfines according to any one of claims 1 to 4,characterized in thatthe rare-earth magnets of low magnetic intensity employed on the magnetic rollers are formed from iron-boron and the magnets of high magnetic intensity are formed by iron-boron-neodymium.

6. The system for dry dressing of iron-oxide-ore fines and superfines according to any one of claims 1 to 5,characterized in thatthe first magnetic roller (32) is formed by a rare-earth magnet of low magnetic intensity and the other rollers are formed by rare-earth magnets of high magnetic intensity.

7. The system for dry dressing of iron-oxide-ore fines and superfines according to any one of claims 1 to 6,characterized in thateach of the magnetic rollers is provided with means for discarding a non-magnetic fraction, means for collecting a magnetic fraction, and means from transferring a mixed fraction to the next magnetic roller, wherein at the last magnetic roller the means for discarding the non-magnetic fraction is arranged to collect the mixed fraction as well.

8. The system for dry dressing of iron-oxide-ore fines and superfines according to any one of claims 1 to 7,characterized in thatthe inclination angle of the magnetic rollers ranges from 15 to 25 degrees.

9. The system for dry dressing of iron-oxide-ore fines and superfines according to any one of claims 1 to 8,characterized in thata set of sleeve filters (22) is provided, connected to the last cyclone at the set of air classifiers, for sucking and retaining the superfine particles that are smaller than the granulometry of the latter.

10. A process for dry dressing of iron-oxide-ore fines and superfines present in dump barrages and low-content ore dump,characterized bycomprising the steps of: (a) hot-air drying with mechanical stirring and disaggregating and moving the material in the horizontal and vertical directions; (b) air classification of the fine and superfine iron ores into pre-determined granulometry ranges, particularly according to the degree of difficulty of magnetic separability of the material; (c) cooling the iron-oxide ore; (d) magnetic separation by means of magnetic rollers arranged in cascade with rare-earth magnets of low and/or high magnetic intensity, inclined at an angle between 5 and 55 degrees.

11. The process for dry dressing of iron-oxide-ore fines and superfines according to claim 10,characterized in thatthe drying is carried out by generation of heat at a temperature of about 850°C.

12. The process for dry dressing of iron-oxide-ore fines and superfines according to claim 10 or 11,characterized in that, after the drying step, exhaustion of the gases from the drying and of the water vapors present inside the dryer and from the fine and superfine particles of iron-oxide ore is carried out.

13. The process for dry dressing of iron-oxide-ore fines and superfines according to any one of claims 10 to 12,characterized in thatthe cooling step is made by means of a water-cooling column that reduces the temperature of the material to about 40°C.

14. The process for dry dressing of iron-oxide-ore fines and superfines according to any one of claims 10 to 13,characterizedion that the magnetic separation step further includes, at each magnetic roller, the disposal of a non-magnetic fraction, collection of a magnetic fraction and transfer of a mixed fraction to the next magnetic rollers, as well as the disposal of the mixed fraction together with the non-magnetic fraction at the last magnetic roller.

15. The process for dry dressing of iron-oxide-ore fines and superfines according to any one of claims 10 to 14,characterized in thatthe separation at the magnetic rollers takes place with the latter at an inclination angle between 15 and 25 degrees.

16. The process for dry dressing of iron-oxide-ore fines and superfines according to any one of claims 10 to 15,characterized bycomprising the suction and retention of the superfine particles smaller than the granulometry of the last cyclone at the set of air classifiers by means of a set of sleeve filters (22).