Abstract:
A method of enriching the iron content of low grade iron ore bearing materials has been developed which produces a high iron ore concentrate suitable for processing into pig iron and steel. The process includes reducing the low grade iron ore bearing materials to a fine particulate form and treating a water slurry of this material by applying a combination of ultrasonic treatments in a plurality of high and low intensity magnetic separation operations to remove interfering materials and concentrate magnetic and paramagnetic iron bearing materials into a high grade ore stock.

Description:
CROSS-REFERENCED TO RELATED APPLICATIONS 
     Not applicable 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable 
     BACKGROUND OF THE INVENTION 
     I. Field of the Invention 
     The present invention relates generally to the processing of iron bearing ore materials and, particularly, to a process for enriching the iron ore content of low grade iron bearing feed materials such as are found in tailings piles and which heretofore have not been commercially usable. 
     II. Related Art 
     Throughout northeastern Minnesota and other iron mining regions of the world, there exists extensive stock piles of commercially unusable, low-grade iron ore that were rejected as tailings during the active ore removal mining phase because they lacked sufficient quantities of key mineral ores to justify further commercial processing. These significant volumes of low grade ores typically contain less than 34% iron and may contain high concentrations of unusable forms of iron and silica-bearing or clay materials which render these wastes or deposits as not fit for further processing into taconite pellets or high grade ore for producing pig iron. 
     Specifically, the material contained in these large, non-commercial ore deposits contains several mineral forms of iron ores, including magnetite (Fe 3 O 4 ), hematite (Fe 2 O 3 ), goethite (FeO.OH), siderite (FeCO 3 ) and limonite (FeO.OH.nH 2 O). All of these forms would be desirable as a concentrate, with the exception of limonite, as the high quantity of an attached water of hydration is an undesirable factor. Also present is a large amount of gangue material which includes several silts and clay materials, namely, chamosite, stilpnomalanene and kaolin. These small clay particles, also known as slimes, contain silica contaminates that are difficult to remove from the mix due to their strong adhesion properties. The clay particles are very small (&lt;5 microns) and have a propensity to coat particles of iron-bearing materials making the extraction and concentration of those materials very difficult. 
     It is known to use ultrasonic techniques to dislodge gangue particles from iron ores. Various techniques have been employed and an example of this is found in U.S. Pat. Pub. 2010/0264241 A1, which uses an ultrasonic crusher pipe system to separate gangue from ore in a water borne slurry. Magnetic separators have also been employed to enrich magnetic ore concentrations in a feed material, as shown in U.S. Pat. No. 5,868,255 to McGaa. Although such techniques have been employed with some degree of success, no practical process has heretofore been developed to economically enrich low grade ores. 
     It would present a distinct advantage if an overall complete process could be developed whereby non-commercial low-grade iron bearing materials of various compositions, presently considered waste material, could be processed into a concentrate containing a much higher percentage of iron that can be cost effectively converted into metallic iron and steel. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a method of enriching the iron content of low grade iron ore bearing materials has been developed which produces a high iron ore concentrate suitable for processing into pig iron and steel. The process includes reducing the low grade iron ore bearing materials to a fine particulate form and treating a water slurry of this particulate material employing a combination of ultrasonic treatments and a plurality of high and low intensity magnetic separation operations to remove interfering materials and concentrate magnetic and paramagnetic iron bearing materials into a high grade ore stock. 
     As used herein, the term “paramagnetic” refers to materials not normally magnetic themselves, but which may react and align when placed in a sufficiently strong magnetic field. These include hematite (Fe 2 O 3 ), goethite (FeO.OH) and siderite (FeCO 3 ) materials, which may be present in the feed material. 
     In a preferred embodiment, the process includes forming a water slurry of low grade iron bearing feedstock materials which have been reduced to a relatively small particle size by subjecting the low grade iron bearing material to crushing and ball mill grinding operations. A preferred particle size is at least −325 mesh and preferably −400 to −500 mesh. The slurry is subjected to a screening step to confirm particulate size and thereafter is subjected to an ultrasonic treatment that is sufficient to dislodge and separate gangue including clays and interfering materials from the iron containing particles. The ultrasonically treated material is then subjected to a plurality of relatively low, intensity magnetic separation steps concentrate the higher magnetic ore fraction (magnetite) with the slurry containing the separated gangue materials and the paramagnetic ore materials being removed for further treatment as a non-magnetic/paramagnetic tail fraction. 
     The non-magnetic/paramagnetic tail fraction is subjected to a further ultrasonic step to again separate interfering gangue materials from the ore containing particles. This material is concentrated in a thickener and separated from the overflow slurry water, the heavier iron containing materials remaining in the underflow or bottom fraction. The underflow material is then subjected to a plurality of relatively high field strength magnetic separation stages to separate out other desirable ore fractions. 
     The first relatively high magnetic separation stage has sufficient field strength to concentrate the hematite ore fraction and the ensuing stages are operated at a higher field strength to separate out siderite and other desirable ore fractions. The concentrated ore fractions are then subjected to further concentration filtering and drying stages where the magnetic and paramagnetic compound fractions can be combined and made available for use. 
     A wide variety of feed material compositions can be successfully processed and the final product is in the form of a loose processed material having a moisture content of from 0-10% and an iron content of from 54%-62% total iron and 7-9% silica. The concentrate may be further processed into briquettes, if desired, with various additives using a variety of binders. 
     The process water can be recycled, including cyclone separation and clarifying steps to separate the solid final tailings so that the process actually requires a minimum of makeup water. The solid tailings can be separately stored. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic flow diagram illustrating an embodiment of the process of the invention; 
         FIG. 2  is a schematic flow diagram illustrating tailings treatment and process water recovery. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description illustrates one or more specific embodiments in which the invention may be practiced. The description is intended to present the process by way of example and is not intended to limit the scope of the inventive concepts. 
     The present invention is directed to a comprehensive process for enriching low grade iron ore bearing materials that have heretofore been found to be unusable and have generally been disposed of in tailing stockpiles, tailing basins, or the like. The present process makes the use of these materials economically feasible for the production of iron and steel. As indicated, the low grade iron bearing materials may stem from a variety of sources and include various fractions of a wide variety of desirable iron compounds and interfering materials. The low grade material may also contain large amounts of undesirable or unusable forms of iron which are not easily processed into metal. Interfering materials or gangue may include fine particulate silica bearing or other clay materials, which tend to cling to the particulate iron compounds tenaciously. 
     The present process enriches the low grade iron bearing materials by concentrating desirable constituents including magnetite (Fe 3 O 4 ), hematite (Fe 2 O 3 ), goethite (FeO.OH) and possibly siderite (FeCO 3 ). Magnetite and hematite are the main desired ore compounds. 
     The low grade iron bearing material is the feed material or feedstock for the present process. In this regard, it will be appreciated that the relative amounts of the desirable constituents may vary widely between feed materials, particularly, the relative amounts of hematite (Fe 2 O 3 ) and magnetite (Fe 3 O 4 ) may vary widely. An important aspect of the present process is that it adapts successfully to a wide variety of feed material compositions. 
     In the process, low grade iron bearing materials are obtained, generally from discarded stockpiles, and fed into a conventional ore crushing mill, as shown at  10  in  FIG. 1 . This step is designed to crush the material to a size of ¾ inch (1.9 cm), or less, and preferably the material is reduced to a size of ¼ inch (0.64 cm), or less. 
     The crushed feed material is next fed into a commercially available ball mill at  12 , along with an amount of water at  14 , where it is further reduced to a size of about −300 to −500 mesh, and preferable to at least −400 mesh. Such ball mills are commercially available in various sizes and capacities and one such mill is a Vertimill® obtainable from Metso Corporation of Finland. Upon leaving the ball mill, the material may be mixed with additional water at  16  to form a slurry which is subjected to screening at  18  and  20  with the oversize particulates being recycled to the ball mill at  22  and  24 . The sizing screens are preferable vibrating screen devices, which are well known. Such screens are available in various capacities from Derrick Corporation of Buffalo, N.Y., for example. 
     Material passing the screens proceeds in streams  26  and  28  to undergo ultrasonic treatment at  30  as a slurry of ˜−400 mesh or less particulate matter in which the ore compound particles are covered with a layer of fine clay particles, or the like. The surface chemistry interactions of the particles creates a complex environment of electrically charged surfaces that cause fine particles of non-iron bearing materials to adhere to iron bearing particles in a manner that makes them difficult to separate using conventional physical separation techniques. The fine non-iron bearing or gangue materials represent a significant fraction of the low grade ore materials and are chiefly small clay particles (slimes) containing silica contaminates. The clay particles are by nature very small (&lt;5 microns) and need to be separated from the iron-bearing materials in order to allow the material to achieve the desired high iron concentration. 
     It has been found that the associated turbulence produced by the application of a sufficiently strong ultrasonic treatment can cause the adherence tendency to weaken and allow the materials to separate. The ultrasonic treatment at  30  causes the slurry to undergo such a highly turbulent phase produced by the ultrasonics, as will be explained. 
     In ultrasonic treatment, as is well known, ultrasonic waves are produced by applying an AC voltage to a crystal such as lead zirconate titanate which undergoes continuous shape changes sending pulsations that travel through the slurry; and, if generated with sufficient amplitude, the pulsations will produce bubbles that grow to a large resonant size and suddenly collapse causing high local pressure changes and a great deal of violent turbulence in the slurry. This type of ultrasonic treatment has been found to be very beneficial in separating silica and clay materials from the iron bearing compounds in the feed material. The intensity of the ultrasonic turbulence can be controlled as needed to accomplish the desired separation. 
     The material exiting the ultrasonic treatment stage  30  at  32  is a mixture of iron bearing compound fractions and separated particulates of clay and silica material and other tailing materials. This material generally contains both magnetic and paramagnetic iron ore fractions. 
     The slurry stream  32  is subjected to a first or rough low intensity wet magnetic separation at  34  using a conventional continuous wet magnetic separator that produces a magnetic field of about 800-1600 gauss. These devices are well known and available commercially in a range of capacities. 
     The rough magnetic separation further concentrates the magnetic fraction in the slurry at  36  and a separate tail fraction containing paramagnetic materials is diverted at  38 . Further magnetic separation is carried out in cleaner separators at  40  and  42  and additional makeup water may be added at  44  and  46 . In each of the cleaner magnetic operations, the tail or non-magnetic fraction is recirculated in line  48  to undergo further ultrasonic and rough separation where the paramagnetic and interfering materials are ultimately removed at  38 . 
     It will be appreciated that the magnetic separation sequence represented by  34 ,  40 ,  42  may be carried out by any desired number of separators which may be operated at any desired intensity level as needed to produce good separation. This may depend on the relative size of the magnetic fraction in a particular feed stock, which may vary widely. The separation generally involves relatively low intensity magnetic fields between about 800 gauss and 3000 gauss as the magnetic fraction will readily separate under these conditions. 
     The concentrated magnetic fraction at  50  may have additional water added as at  52 . This material is then removed to a container at  54  and concentrated and thickened and water decanted at  56 . Thereafter, it is filtered and the filter cake dried and stored at  58  for shipment separately or in combination with a paramagnetic fraction, as will be explained. The material at  58  is a loose processed material having a solids content of 90-95% and may be compressed into pellets or briquettes using well known binders. 
     The primary tail stream  38 , which includes the paramagnetic iron ore fraction, along with the interfering materials such as clays, undergoes further treatment in parallel with the magnetic fraction. As shown in the schematic flow diagram of  FIG. 1 , the tail stream  38  is subjected to a further ultrasonic treatment at  60  to again separate the silica and clay fine particulates from the ˜−400 mesh iron bearing materials. The outlet stream  62  proceeds to a separation step in the form of a thickener  64  which is essentially a clarifier where the heavier iron-bearing materials settle out. This allows a portion of the lighter non-iron bearing materials in the slurry including some silica-containing materials and clays to be removed in an overflow stream at  66 , which becomes part of the final or total tailing fraction at  88 . 
     The thickened or underflow stream leaving the thickener  64  at  70  is subjected to a further series of magnetic separation operations, as shown at  72  and  74  using a high gradient magnetic separator such as a SLon vertical ring pulsating high-gradient magnetic separator which utilizes the combination of magnetic force, pulsating fluid and gravity to continuously process fine, weakly magnetic or paramagnetic materials. While these separators are generally classified as high intensity magnetic separators, they can be operated over a range of field strengths. The device of  72  is operated at a relatively low field strength of about 1000-3000 gauss, which is sufficient to separate out the hematite fraction which is conducted at  76  to an intermediate container at  78 . The tailing stream  80  is conducted to the second high gradient magnetic separator  74 . The magnetic separator  74  is operated using a relatively high field strength of about 7500-12,500 gauss which is strong enough to accomplish the separation of the remaining desirable iron ore fraction which is generally chiefly siderite and goethite. 
     As with the separation of the magnetic constituents, the two stages of high gradient magnetic separators  72  and  74  represent as many stages as may be necessary to accomplish the desired separation. As with the magnetic fraction, the paramagnetic materials are thereafter concentrated and allowed to settle and the liquid fraction is decanted off at  82 . The concentrate is filtered and the filter cake is then allowed to dry at  84  and is in the form of a loose material having a solids content of 90%-95%, which can be processed into pellets or briquettes and/or thereafter be mixed with the magnetic material for further processing into steel. 
     The tailing fractions  66  and  86  are removed in line  88  and  90  as total tailings. The total tailing fraction is thereafter treated to clarify and separate the water for reuse in the process. 
     The tailings deposit and water recovery aspects of the process are illustrated in the schematic diagram of  FIG. 2  in which the supply and crushing operations are represented at  100  and the grinding circuit at  102 . The magnetite low intensity magnetic separation circuit, including the several stages, is represented by  104 . The tailings fraction from the magnetic separation operation  104  is seen at  106 . The paramagnetic high intensity magnetic separation operation circuit is shown at  108 . the processed magnetic and paramagnetic concentrate fractions are shown combined for concentration at  110 , filtering at  112  and storage at  114 . The combined tailings/overflow from the concentration operations is shown at  116 , which combines with tail portion  118  to form a total tailings stream at  120 . The total tailings fraction is subjected to a cyclone separation operation at  122  and the mainly water overflow stream is shown at  124  where it joins feed stream  126  which proceeds to a clarifier  128 . The tailings underflow of bottom discharge stream from the cyclone separator  122  at  130  and the clarifier at  132  are combined at  134  and fed into a tailings pressure filter at  136  where the solid filter cake is collected at  138  for transport to a tailings storage facility and the liquid containing fraction or filtrate material is sent to the clarifier at  140 . The clear water from the clarifier proceeds to  142  where it can be recirculated into the process at  144 . 
     It is important to note that it is the particular combination of ultrasonics and magnetics that enables the iron content of low grade, commercially unusable or deposits into commercially viable feedstocks for iron and steel making processes that contain 54%-62% iron. 
     Table I shows typical enrichment rates for Roast Taconite (magnetite) and hematite constituents and an average 50-50 mixture. 
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
               
               
                 Different Feed Sources for SMR Concentrate 
               
             
          
           
               
                   
                 Crude Feed 
                 Concentrate 
                 Wt. Recovery- 
                 Wt. Recovery- 
                 Total Weight 
               
               
                 Material Type 
                 Total Iron 
                 Total Iron 
                 Magnetite 
                 Hematite 
                 Recovery 
               
               
                   
               
             
          
           
               
                 Roast Taconite 
                 30.00% 
                 62.00% 
                 10.00% 
                 6.00% 
                 16.00% 
               
               
                 Roast Taconite 
                 31.00% 
                 62.00% 
                 10.00% 
                 7.00% 
                 17.00% 
               
               
                 Roast Taconite 
                 32.00% 
                 62.00% 
                 10.00% 
                 8.00% 
                 18.00% 
               
               
                 Roast Taconite 
                 33.00% 
                 63.00% 
                 10.00% 
                 9.00% 
                 19.00% 
               
               
                 Roast Taconite 
                 34.00% 
                 63.00% 
                 10.00% 
                 10.00% 
                 20.00% 
               
               
                 Average 
                 32.00% 
                 62.40% 
                 10.00% 
                 8.00% 
                 18.00% 
               
               
                 Hematite 
                 33.00% 
                 63.00% 
                 0.00% 
                 15.00% 
                 15.00% 
               
               
                 Hematite 
                 34.00% 
                 63.00% 
                 0.00% 
                 16.00% 
                 16.00% 
               
               
                 Hematite 
                 35.00% 
                 63.00% 
                 0.00% 
                 17.00% 
                 17.00% 
               
               
                 Hematite 
                 36.00% 
                 63.00% 
                 0.00% 
                 18.00% 
                 18.00% 
               
               
                 Hematite 
                 37.00% 
                 64.00% 
                 0.00% 
                 19.00% 
                 19.00% 
               
               
                 Hematite 
                 38.00% 
                 64.00% 
                 0.00% 
                 20.00% 
                 20.00% 
               
               
                 Average 
                 35.50% 
                 63.33% 
                 0.00% 
                 17.50% 
                 17.50% 
               
               
                 50% Roast Tac. + 50% Hematite 
                 33.75% 
                 62.87% 
                 5.00% 
                 12.75% 
                 17.75% 
               
               
                 Use 
                 Use 32% to 35% 
                 Use 60% to 63% 
                 Use 5% to 8% 
                 Use 10% to 13% 
                 Use 17% to 18% 
               
               
                   
               
             
          
         
       
     
     Samples of the enriched ore material in the form of nuggets have been successfully processed directly into metallic steel (about 1% carbon). 
     This invention has been described herein in considerable detail in order to comply with the patent statutes and to provide those skilled in the art with the information needed to apply the novel principles and to construct and use embodiments of the example as required. However, it is to be understood that the invention can be carried out by specifically different devices and that various modifications can be accomplished without departing from the scope of the invention itself.