Abstract:
A physical process for recovering iron from magnetic cementitious spherical particles generated from metallurgical by-products such as slag from a basic oxygen converter (B.O.F.S) by applying a thermal process assisted by Ar—He plasma, which allows the microstructural and morphological transformation of these compounds based on iron, resulting in the generation of magnetic phases, among others. The separation process is based on the classification of the particles according to their new magnetic properties. Thus, when applying a magnetic field, the particles are separated into two fractions: a highly magnetic one and with low cementitious properties, and another fraction with high cementitious properties and almost no magnetic properties. The highly magnetic fraction is mixed with an aqueous medium to form an agglomerate and facilitate its transportation to be reefed to an iron melting process. The fraction with low magnetic properties has potential applications as a densifier in non-conventional cement and concrete.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    This invention relates to a process for recovering iron, more particulary a process for recovering iron from magnetic cementitious spherical particles generated from metallurgical by-products. 
       BACKGROUND OF THE INVENTION 
       [0002]    For recovering iron from steel slags, several processes have been developed which typically include crushing of them, in one or more stages to bring them to a sinter plant, if any, or directly to the B.O.F. furnace to adjust CaO and MgO contents, and recovery of iron and magnesium. However, in some cases the high content of phosphorus limits the amount of slag that can be used in this process. Currently, this involves low throughput rates due to the reuse of only 25% of the produced slag [Deyong Wang, Maofa Jiang, Chengjun Liu, Yi Min, Yuyuan Cui, Jian Liu, and Yongcang Zhang. Enrichment of Fe-Containing Phases and Recovery of Iron and Its Oxides by Magnetic Separation from B.O.F. slags. Steel Research Int. 83 (2012) No. 2, 189-196]. In the present invention, development is for the use the B.O.F. slag (B.O.F.S.) as a whole, where the iron found in the slags is recovered, at high values of efficiency, and the remaining non-magnetic material can be used as cementitious/pozzolanic for generating new building systems and non-conventional cementation. 
         [0003]    Moreover, at present machinery like grinders and pulverizers has been developed for recovering metals from slag, because, as is known, slag may contain on average between 10 and 15% metals of interest, treatment of which involves recovery of values and process optimization. Normally, once the slag has been pulverized, it is easier to handle it for its separation and disposal. Starting from this process, separation steps have been added and the resulting product is then separated by gravity or by magnetic means [Deyong Wang, Maofa Jiang, Chengjun Liu, Yi Min, Yuyuan Cui, Jian Liu, and Yongcang Zhang. Enrichment of Fe-Containing Phases and Recovery of Iron and Its Oxides by Magnetic Separation from B.O.F. Slags. Steel Research Int. 83 (2012) No. 2, 189-196]. It is noteworthy that the crushing and grinding stages may consist of several sub-steps until reaching the desired size, and then the material is passed to a magnetic separator, wherein the magnetic portion has the highest metal content. These procedures can process from 2 to 20 T/h of slag depending on the number of steps and equipment used. The extraction of iron from slag has allowed reducing losses by 30%. [Y. A. Kabanov, O. A. Stolyarskii, E. N. Agapeev. Recovering and recycling scrap from steelmaking slags Containing dumps. Metallurgist, Vol. 50, Nos. 1-2, 2006]. However, and as was mentioned previously, the material without any magnetic iron is not used in any application since it has no properties of interest. It remains important to note that the portion of non-magnetic iron in this material is not recovered. In the present invention the loss of iron material is reduced to a minimum, because the non-magnetic phases (wustite) are transformed through plasma treatment to magnetic phases, increasing the efficiency of the recovery of iron. Additionally, there is no material loss and the B.O.F.S. is fully used, since the remaining non-magnetic material of the present invention may have a variety of applications, particularly such as a densifier in advanced and non-conventional cementitious systems. Moreover, D. Wang et al. [Deyong Wang, Maofa Jiang, Chengjun Liu, Yi Min, Yuyuan Cui, Jian Liu, and Yongcang Zhang. Enrichment of Fe-Containing Phases and Recovery of Iron and Its Oxides by Magnetic Separation from BOF Slags. steel research int. 83 (2012) No. 2, 189-196] developed a process for recovering metals from two types of B.O.F.S. re-melted at 1873 K. The liquid slags were cooled under different conditions such as: granulation in water, sprinkling, cooled in air and cooled in a furnace to study the influence of the cooling speed in the behavior of enrichment in iron of the components. Subsequently, magnetic separation was applied in humidity to evaluate the relationship between the percentage of recovery and the cooling conditions. It was found that both the throughput, as the iron content in the magnetic concentrate increase when using slow cooling rates. Therefore, slow cooling of industrial process slag is recommended. These special processes of re-melting and cooling are very expensive because of their energy requirements. The present invention transforms the B.O.F.S. material to a material with new crystalline phases, which provide magnetic, cementitious and pozzolanic properties. This is accomplished in one sole step, through the use of industrial equipment of Ar—He plasma, known and frequently used, and without any necessity for any special sophisticated cooling equipment/processes, thus using a collection system suitable for this new material, where simply the air occupying its interior achieves its cooling and conversion into a material with new phases of interest. Subsequently, a conventional process of dry magnetic separation is used to finally agglomerate the magnetic material with water to reinject it into the conventional melting processes of production of iron and/or steel, and the non-magnetic material can be packed and marketed for applications in the construction area. Finally, the material described in this application and basis of the present innovation was developed by Y. Perera et al. who generated a new magnetic cementitious material from the B.O.F.S. [Y. Perera and E. Reyes; “Production of spherical particles with magnetic and cementitious dual properties”; Patent Application No. MX/a/2011/013658 (IMPI); Folio: MX/E/2011/091426, date of application: Dec. 15, 2011]. However, the process does not mention the magnetic separation/recovery of iron from B.O.F.S., much less of the magnetic cementitious spherical particles, as presented herein, which gives high added value to the current steel processes, safely extracting from the B.O.F.S., high amounts of iron found in the highly magnetic fraction which may be separated and can be used as raw material to be reinjected into the production process of iron as into the blast furnace and basic oxygen furnaces. 
       SUMMARY OF THE INVENTION 
       [0004]    The present invention shows a new process resulting from the application MX/a/2011/013658, which includes the projection assisted by argon (Ar)/helium (He) plasma of different iron-rich powder precursors, which are common byproducts of various industrial metallurgical activities. Iron contained in this type of industrial byproducts is generally found as iron oxide, typically wustite (FeO), a non-magnetic phase, and also present in a minor proportion as a mixture of other iron oxides. Specifically, the present invention uses slag resulting from the steel process of the basic oxygen furnace (B.O.F.). The slag is microstructural and morphologically transformed, showing complete oxidation of the wustite phase and its total conversion to crystalline phases with magnetic properties. Subsequently, this new microstructure makes possible a magnetic separation of the iron-rich material. Moreover, this magnetic material is mixed with low quantities of phases which react with water and help particles agglomeration, which makes easier the transportation of the compacted material to any system of iron melting furnace such as the blast furnace or the basic oxygen furnace itself. Thus, the iron in the slag from the B.O.F. is recovered. 
         [0005]    Therefore, the object of the present invention is to provide a physical process for recovering iron from magnetic cementitious spherical particles generated from metallurgical by-products, the physical process includes the steps of separating by magnetic means the magnetic cementitious spherical particles to obtain a highly magnetic fraction and a highly cementitious fraction; agglomerating the highly magnetic fraction obtained in a) with an aqueous medium to obtain a magnetic agglomerate; and reinjecting the magnetic agglomerate to an iron melting process for the recovery of iron. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    The characteristic details of the invention are described in the following paragraphs together with the attached drawings, with the purpose of defining the invention, but without limiting its range. 
           [0007]      FIG. 1  shows the schematic outline of the process for recovering iron from magnetic cementitious spherical particles generated by the metallurgical by-products, specifically B.O.F.S. The section enclosed in dotted lines belongs to the application for protection of patent No. MX/a/2011/013658 (IMPI); Folio: MX/E/2011/091426, date of application: Dec. 15, 2011. The rest of the process shows the methodology of the present invention. 
           [0008]      FIG. 2  shows the X-ray diffraction patterns (XRD) of the samples of the present invention. The diffractogram of the bottom part belongs to the E.B.O.F. and the upper diffractogram shows the crystalline phases (mostly magnetic) of the material of high magnetic fraction. Likewise, some secondary crystalline phases can be observed that allow these particles to be agglomerated by the water. 
           [0009]      FIG. 3  shows the photomicrograph obtained by Field Emission Electronic Microscopy Scanning (FE-SEM) of the highly magnetic particles that are rich in iron. 
           [0010]      FIG. 4  shows the magnetization curves of the fraction of highly magnetic spheres and its precursor, the original B.O.F.S. 
           [0011]      FIG. 5  ( a ) shows the photomicrograph of Field Emission Scanning Electron Microscopy where the particle agglomerates system produced from the interaction between the highly magnetic fraction and water can be observed. ( b ) Shows the qualitative chemical analysis by X-rays energy dispersive spectroscopy (EDS, by its acronym in English) of the agglomerated material of  FIG. 5   a , observing here the Fe, Si, Ca and Mg as major elements of these agglomerates. ( c ) A photomicrograph of the Field Emission Scanning Electron Microscopy which details the surface of highly magnetic particles agglomerated with water, and the growth crystals is evidenced, that help bind these highly magnetic spherical particles. ( d ) Shows the qualitative chemical analysis by X-rays energy dispersive spectroscopy (EDS) of the crystals from  FIG. 5   c , generated during the agglomeration of highly magnetic spheres that are responsible for the binding of them, showing the Ca, Si, and Mg as major elements of these agglomerates. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]    The following description is only intended to represent the way of how the principles of the invention can be implemented in various embodiments. The embodiments described herein do not intend to be a comprehensive representation of the invention. The following embodiments are not to limit the invention to the precise form published in the following detailed description. 
         [0013]    The present invention takes as the starting material the one obtained under the process described in patent application MX/a/2011/013658 called “Production of spherical particles with magnetic and cementitious dual properties”, which is briefly described below: The production of the magnetic cementitious spherical particles starts with a suitable mineralogical treatment of the precursor material, B.O.F.S. in this case, which consists of the steps of: (a) crushing, (b) drying, (c) grinding and (d) sieving. Upon completion of these processes, the precursor reaches the desired particle size, preferably less than 150 μm, more specifically between 10 nm and 150 μm. Moreover, it is important to note that the B.O.F.S. in general show a chemical composition as reported in Table 1. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Weight % 
               
             
          
           
               
                 SiO 2   
                 Al 2 O 3   
                 CaO 
                 MgO 
                 Mn 
                 FeO 
                 Na 2 O 
                 K 2 O 
                 Zn 
                 C 
                 S 
                 TiO 2   
                 P 
               
               
                   
               
               
                 8-12 
                 1.5-3 
                 38-42 
                 6-9 
                 1-3 
                 20-25 
                 &lt;0.5 
                 &lt;0.5 
                 &lt;0.5 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;1 
               
               
                   
               
             
          
         
       
     
         [0014]    Subsequently, the B.O.F.S. particles are transformed to magnetic cementitious spherical particles through the conversion of their microstructure and morphology while being projected into the argon (Ar) and helium (He) plasma plume using an electric arc plasma industrial equipment of the brand Plasmadyne (Praxiair) SG-100, with direct current (DC), and using such ionization gases as Ar and He, which generate the plasma plume and whose combinations vary according to the degrees of ionization thereof. The magnetic cementitious spherical particles were produced with the optimum parameters, including: the proportion of gases such as Ar:He equal to 1:1 for a flow of mixture of these gases equal to 2 L/min and an electric current of 1000 A. 
         [0015]    Next, the physical process is described for the recovery of iron from magnetic cementitious spherical particles generated from metallurgical by-products, and the object of the present invention: 
       STAGE I: Magnetic Separation of the Magnetic Cementitious Spherical Particles to Obtain a Highly Magnetic Fraction and a Cementitious Fraction. 
       [0016]    The magnetic cementitious spherical particles have at least one magnetic phase, said phase is optionally magnetite and maghemite, the particles are subjected to at least one magnetic field, which is able to classify particles into a highly magnetic fraction and a cementitious fraction. The highly magnetic fraction is used to recover the iron that was originally found in the B.O.F.S, and the highly cementitious fraction is optionally used as a densifier material in non-conventional cementitious formulations. 
         [0000]    STAGE II: Agglomeration of the Highly Magnetic Fraction Obtained in a) with an Aqueous Medium to Obtain a Magnetic Agglomerate. 
         [0017]    The highly magnetic fraction obtained in a) is homogeneously mixed with an aqueous medium at a ratio of 1:4, media:fraction, the mixture is maintained at rest for 24 to 48 hours at controlled temperatures between 25 to 60° C. to form a magnetic agglomerate which maintains the entire highly magnetic fraction in a cohesive manner; the aqueous medium is water and optionally silica nanoparticles are added from 0.1 to 1% w/w with a particle size between 5 and 15 nm. The addition of silica enables a more compact and manageable magnetic agglomerate before it is being subject to the next step. 
       STAGE III: Reinjection of a Magnetic Agglomerate to an Iron Melting Process for the Recovery of Iron 
       [0018]    The magnetic agglomerate obtained in b) is reinjected to an iron melting process under conditions well known; the iron melting process is optionally iron blast furnace and basic oxygen converter. This stage allows to retrieve and use the iron in the magnetic agglomerate and reuse the precursor metallurgical by-products of the magnetic cementitious spherical particles. It is noteworthy that the cementitious elements of the magnetic aggregate are separated and incorporated into the slag of the iron melting process without modifying the industrial process. 
       EXAMPLES OF THE INVENTION 
       [0019]    The invention will now be described with respect to the following examples, which are solely for the purpose of representing the way of carrying out the implementation of the principles of the invention. The following examples are not intended to be a comprehensive representation of the invention, or try to limit the scope thereof. 
       Example 1 
     General process for the recovery of iron from magnetic cementitious spherical particles produced from the E.B.O.F. 
       [0020]    The process set forth in  FIG. 1 , shows a section enclosed within a dashed line box, this section corresponds to Patent Application No. MX/a/2011/013658. However, once the new material, cooled off (magnetic cementitious spherical particles), leaves the collector, it passes to a magnetic separation device capable of classify the material into two or more fractions according to the applied magnetic field. Once separation is achieved, basically in two fractions: (1) an iron-rich and highly magnetic one, and (2) another cementitious fraction with practically very little or almost no magnetic properties, said highly magnetic fraction is mixed with an aqueous solution, which may or may not contain silica nanoparticles to form a magnetic aggregate. Adding silica nanoparticles promotes the chemical reaction with products generated by the hydration of the highly magnetic spherical particles, producing a higher percentage of C—S—H gel, which allows these particles to agglomerate to form aggregates which are easily transportable to be re-injected at any iron and/or steel melting process furnace such as blast furnace or basic oxygen furnaces. 
       Example 2 
     Characterization by X-Ray Diffraction (XRD) of the Precursor Material (B.O.F.S.) and the Separated Fraction of Highly Magnetic Spherical Particles 
       [0021]    The XRD pattern of the original B.O.F.S, and the fraction of highly magnetic spherical particles, was obtained using an X-ray diffractometer, model Siemens 500, with Kα radiation of Cu (λ=1.5418 Å), at 35 kw and 25 mA. The diffractogram of  FIG. 2  indicates the presence of different original crystalline phases of the B.O.F.S. (bottom XRD), such as: Wustite (FeO), Magnetite (Fe 3 O 4 ), Brownmillerite (Ca 2 AlFeO 5 ), Larnite (CaSiO 4 ), Bixbyite (Mn 2 O 3 ), Diopside (CaMg(SiO 3 ) 2 ) and Merwinite (Ca 3 Mg(SiO 4 )). On the other hand, the upper difractrogram corresponds to the fraction of highly magnetic spherical particles, and can be noticed the presence of phases such as Magnetite (Fe 3 O 4 ), Maghemite (γ-Fe 2 O 3 ), Mervinite (Ca 3 Mg(SiO 4 )), Bixbyite (Mn 2 O 3 ), Diopside (CaMg(SiO 3 ) 2 ), Tricalcium Aluminate (3CaOAl 2 O 3 ), Dicalcium Silicate (2CaO.SiO 2 ) and Tricalcium Silicate (3CaO.SiO 2 ). The rearrangement of crystalline structures are due to the thermodynamic and kinetic effects to which the precursor particles are subject within the Ar—He plasma plume, which causes the original phases of the B.O.F.S. such as: the Wustite (FeO), the Larnite (CaSiO 4 ) and Brownmillerite (Ca 2 AlFeO 5 ) to disappear to make way for new crystalline structures that ultimately give new properties to the generated products (fraction of highly magnetic spherical particles). 
       Example 3 
     Morphology and Chemical Composition by Field Emission Scanning Electron Microscopy of the Separated Fraction of Highly Magnetic Spherical Particles 
       [0022]    Morphological analyses of the spherical particles that form the highly magnetic fraction were performed by Field Emission Scanning Electron Microscopy, using a JEOL model JSM-7401F equipment and its respective energy dispersive X-rays spectroscopy (EDS) equipment Noran-200, to determine the semiquantitative chemical composition was performed through the analysis by EDS.  FIG. 3  shows the photomicrograph of the Field Emission Scanning Electron Microscopy of the separated fraction of highly magnetic spherical particles, where the surface of the spheres, and their sphericality and roughness. Moreover, and as indicated in Table 2, all oxides present in the magnetic cementitious spherical particles remain very similar in proportion and amounts to those shown by the precursor used (B.O.F.S.) However, and about the sulfur content (reported as SO 3 ), the disappearance of this element is observed, which could be due to the separation of said content as SO 3  gas, disappearing completely from the composition of this new material. 
         [0023]    Moreover, it is clear that the highly magnetic fraction has the largest amount of iron (Fe). It is important to note that these data were generated using a particle size of precursor (B.O.F.S.) between 74-105 μm, so that when reducing these values of particles sizes, the recovery of iron (Fe) increases. Additionally, it is similarly appreciated that for this highly magnetic fraction other oxides present in its chemical composition are SiO 2 , Al 2 O 3 , CaO and MgO are reduced compared with the precursor material (B.O.F.S.) and also compared with the magnetic cementitious spherical particles. Finally, the highly cementitious and with low in magnetic properties fraction shows being the one with the lower amount of iron (Fe) and the higher amount of oxides: SiO 2 , Al 2 O 3 , CaO, and MgO, which is the reason why its cementitious properties increase. 
         [0000]    
       
         
               
             
               
               
             
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Chemical composition of the precursor (B.O.F.S.) of 
               
               
                 the magnetic cementitious spherical particles produced 
               
               
                 and of the separate fractions for a particle size 
               
               
                 of the injected precursor between 74-105 μm 
               
             
          
           
               
                 Chemical 
                 Sample 
               
             
          
           
               
                 composition 
                 Spherical 
                 Magnetic 
                 Cementitious 
               
             
          
           
               
                 (Weight %) 
                 E.B.O.F. 
                 particles 
                 Fraction 
                 Fraction 
               
               
                   
               
             
          
           
               
                 SiO 2   
                 11.87 
                 12.02 
                 8.20 
                 15.54 
               
               
                 Al 2 O 3   
                 2.76 
                 2.82 
                 2.38 
                 3.30 
               
               
                 CaO 
                 42.02 
                 41.80 
                 34.69 
                 47.92 
               
               
                 MgO 
                 8.83 
                 8.91 
                 6.63 
                 12.42 
               
               
                 Mn 2 O 3   
                 2.42 
                 2.40 
                 1.79 
                 1.96 
               
               
                 Fe 2 O 3   
                 28.83 
                 29.02 
                 44.27 
                 16.99 
               
               
                 Na 2 O 
                 0.40 
                 0.35 
                 0.49 
                 0.60 
               
               
                 K 2 O 
                 0.42 
                 0.38 
                 — 
                 0.24 
               
               
                 SO 3   
                 0.25 
                 — 
                 — 
                 — 
               
               
                 TiO 2   
                 0.93 
                 1.05 
                 0.85 
                 0.47 
               
               
                 P 2 O 5   
                 1.27 
                 1.25 
                 0.70 
                 0.56 
               
               
                   
               
             
          
         
       
     
       Example 4 
     Evaluation of the Magnetic Properties of Magnetic Cementitious Spherical Particles 
       [0024]    The magnetic properties of the highly magnetic separated fraction were measured using a vibrating sample magnetometer (MMV) (VSM—by its acronym in English), with a system for measuring magnetic properties (PPMS) of Quantum Design and a maximum magnetic field equal to 9T.  FIG. 4  shows the magnetization curves of the fraction of highly magnetic spherical particles and their E.B.O.F. precursor, showing that the spheres reach their maximum magnetization (30 emu/g to 10,000 Oe) at magnetic field values much lower than the maximum magnetic field used for the studies (10000 Oe), thus showing proof of the ferromagnetic behavior of these materials, this being described by magnetically soft materials. These properties make the magnetic separation possible, basis of the present invention. 
       Example 5 
     Agglomeration Tests with Water and its Characterization by Field Emission Scanning Electron Microscopy 
       [0025]    A test of agglomeration of the particles separated and identified as a highly magnetic fraction was performed, by Field Emission Scanning Electron Microscopy ( FIGS. 5   a  and  5   c ) and EDS ( FIGS. 5   b  and  5   d ) the reaction of the particles of the highly magnetic fraction when they come into contact with water was studied. The mixtures were cured for 21 days at ambient condition. The formula of the agglomeration mixture used a relation of water and particles of the highly magnetic fraction equal to 0.25 g of water per gram of spheres.  FIG. 5   a  shows the photomicrograph by Field Emission Scanning Electron Microscopy, detailing the bonding of highly magnetic spherical particles, generating an agglomerate material and demonstrating its capacity to agglutination upon contact with water. The EDS analysis shown in  FIG. 5   b  indicates as major parts of the general chemical composition: calcium (Ca), silicon (Si), iron (Fe), and magnesium (Mg). A detailed view of the generation of the secondary phase between the spherical particles is shown in  FIG. 5   c . This phase includes scales and/or flakes and in this area, where an EDS analysis was performed, which is shown in  FIG. 5   d , which indicates that the material growing on the surface of the highly magnetic particles is rich in the following elements, mainly: calcium (Ca), silicon (Si), and magnesium (Mg). The phases rich in these elements are responsible for the generation of hydrated phases of the type: C—S—H gel and/or C—S-M-H, which allows the agglutination of this new material. Finally, it is noted that the consolidated material is attracted by magnetic fields generated by conventional magnets. 
         [0026]    Although the invention was described with reference to specific embodiments, this description is not intended to be built in a limited sense. The different modifications of the embodiments published, as well as alternative embodiments of the invention will be apparent to persons knowledgeable in the state of the art when referring to the description of the invention. For this reason it is considered that the appended claims cover such modifications that fall within the scope of the invention, or their equivalents.