Abstract:
A system and method directed to the economical recovery of valuable iron constituents from iron blast furnace and steel-making slag fines wherein the slag is obtained and subjected to a series of classification steps which progressively sort the slag fines by various physical characteristics, including magnetism, size, and density, into relatively iron-rich and relatively iron-poor classifications, resulting in the isolation of iron-rich commercial byproduct at one or more of the classification steps.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. provisional application Ser. No. 61/791,231 filed Mar. 15, 2013, the disclosure of which is hereby incorporated in its entirety by reference herein. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to recovering valuable constituents from the by-products of the iron and steel-making processes, and more particularly to recovering commercial quality iron product from iron-bearing slag fines. 
       BACKGROUND 
       [0003]    Slag, in various forms and constituent compositions, is a voluminous by-product of the iron and steel making process. In particular, kish slag or de-sulf slag is a by-product of iron making which includes a relatively low content of iron along with a relatively high content of non-ferrous materials, including sulfur and silicates. Though current attempts to reclaim iron-rich constituents from slag, including grinding, screening and magnetic sortation, have been employed to isolate and recover some of the iron-rich content, steel mills are left with constantly growing stockpiles of the remaining fine particle iron and steel slag. These ever-growing piles of “fines” must be disposed of economically and in an environmentally safe manner. 
         [0004]    Iron-bearing fines from various other similar manufacturing and reclamation sources, including iron blast furnace fines and steel fines, also include potentially valuable constituents for which to date an economical method of separating the valuable constituents from the aggregate has remained elusive. 
         [0005]    It is thus desirable to develop further processing methods and systems to re-claim as much of the valuable content of these slag fines for productive use, as well as reduce the disposal costs for the remaining material. 
       SUMMARY 
       [0006]    The disclosed system and method is directed to the cost-efficient recovery of valuable iron constituents from iron blast furnace and steel-making slag fines. The slag is obtained and subjected to a series of classification steps which progressively sort the slag fines by magnetic properties, size, and density into relatively iron-rich and relatively iron-poor classifications, resulting in the isolation of iron-rich commercial byproduct at the completion of one or more of the classification steps. The remaining, relatively iron-poor but sulfur and silicate rich residue may be used for other industrial purposes unrelated to steel or iron production, such as additives for cement aggregate or agricultural soil enhancement. 
         [0007]    In the disclosed method, the slag fines are sorted magnetically to isolate the relatively larger iron-bearing constituents from the non-magnetic materials. The resulting iron-bearing product is typically then sorted by size or by density in a series of separate classifying steps to progressively separate and isolate relatively iron-rich product from relatively iron-poor remainder. At the conclusion of each step, the relatively iron-rich sort resulting from that step may be isolated for sale or subjected to further processing to further classify the processed material by size or by density. 
         [0008]    An additional step of reducing the size of a batch of slag fines, such as by crushing or grinding the material, may be employed as desired to prepare the batch for any of the disclosed classifying steps. Various grinding mills or crushers may be employed for this purpose. 
         [0009]    The magnetic classification step(s) may include introduction of a batch of the slag fines to one of a variety of magnetic separators. 
         [0010]    The size classification step(s) may include the introduction of a batch of the slag fines onto a variety of dry-screen or wet-screen devices available for this purpose. 
         [0011]    The density classification step(s) may include introduction of a batch of the slag fines in slurry form into a hydraulic (or other non-compressible fluid) fluidized bed separator. 
         [0012]    This progressive method of magnetic, size, and density classification results in the separation and isolation of one or more separate commercial-quality iron-rich products from the progressively iron-poorer byproduct which then may be re-sold depending upon the nature of its non-ferrous constituencies. 
         [0013]    The disclosed system employs the above-described method in a fines processing plant which includes one or more magnetic sorting stations, one or more screening (i.e., size sorting) stations, and at least one fluidized bed for separation and classification of the fines by density/specific gravity into relatively heavier (i.e., metal-rich) and lighter (i.e., silicate-rich) constituencies. 
         [0014]    The frequency and order of the above classification steps, as well as the architecture of the system employed to implement this disclosed method, may be varied depending upon the characteristics of the particular slag fines being processed, as well as the targeted minimum iron content(s) for the recovered iron-rich product(s). Similarly, further processing and/or classification steps may be employed in addition to the disclosed steps to isolate and recover reusable product(s) from the remaining, relatively iron-poor fines. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a block diagram of the basic steps of one embodiment of the disclosed recovery method; 
           [0016]      FIG. 2  is a schematic of one embodiment of the disclosed recovery system; and 
           [0017]      FIG. 3  is a block diagram of the recovery method employed in the system disclosed in  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. 
         [0019]    Referring now to  FIG. 1 , in one embodiment, the disclosed recovery method  10  employs a series of processing and classifying steps which are progressively and, as required, iteratively employed to isolate relatively iron-rich products from slag fines. Iron and steel-making slag fines  12 , such as kish or de-sulf fines, are supplied as input in the process. In this disclosed embodiment, the slag fines are first input to a magnetic sorter  14  to separate the magnetic (i.e., iron-bearing) fines  16  from the non-magnetic fines  18 . 
         [0020]    Based upon various characteristics of the batch, including the iron content, particle size, and potentially the non-iron constituents, the material  16  resulting from the magnetic separation is then subjected to one or more additional classifying steps. In this disclosed embodiment the material  16  is then screened, at  20 , to separate the particles in the batch by size. It may be desirable, for example, and it is often the case, that the relatively larger-sized particle material  22  isolated as a result of this size classification may be sufficiently iron-rich (e. g., about 40-70% iron by weight) to be collected and sold without further processing. The relatively smaller-sized particle material  24  may then be provided as input to another processing or classifying step in order to further isolate and separate the relatively iron-rich and iron-poor constituents of the material. 
         [0021]    In this disclosed embodiment, material  24  may then be provided, at  26 , as an input batch to a fluidized bed separator where the material  24  particles are isolated and may be separated by density. The relatively light constituents of the input batch  24  (which typically include slag and other non-metallic particles)  28  are isolated and separated from the relatively heavy constituents  30  (typically including iron and other heavy metals). The heavy product  30  may, at this point, be sufficiently iron-rich (e. g., about 40-80% iron by weight) to be collected and sold without further processing. Alternatively, the heavy product  30  isolated from this density classification  26  may be supplied as an input batch for further processing, where it may be subjected to size or magnetic, or other separation processing to further isolate the relatively iron-rich constituents from the remaining material. The light product  28  may be disposed of, or may be reused as, for example an agricultural or cement product additive, depending upon the content of silicate, sulfur and other non-iron constituents of the product  28 . 
         [0022]    It will be appreciated that the particle size and constituents of the slag fines collected and used as input to the disclosed recovery system will vary from batch to batch. Similarly, the initial iron content (generally, by way of example, less than about 25% for steel slag and less than about 40% for kish slag), as well as the iron content of the relatively iron-rich batches developed at each station in the disclosed process, may vary. Thus the scope, order, type and number of classification operations may vary in order to obtain optimal iron-rich end products. In the disclosed system the typical raw material, kish fines, include iron content ranging from about 25-40% by weight, sulfur content ranging from about 0.9-3% by weight, and slag content ranging from about 60-30% by weight, as well as lesser amounts of metal (e.g. manganese, molybdenum) alloys. The characteristics of the resultant batches disclosed in the following description, also variable from batch to batch, reflect typical results using the aforementioned described raw material input. 
         [0023]      FIG. 2  illustrates one embodiment of the system which may be employed to implement the disclosed recovery methods. The system  50  may include a magnetic separator/conveyor  52  upon which an input batch of the raw material (e. g., kish slag fines) is placed. In this disclosed embodiment, a permanent head pulley is utilized for this purpose. However, other magnetic separators, including, for example, top pluck or cross belt magnetic separators, may be used as appropriate. 
         [0024]    As the input batch is conveyed, the magnetic portion of the batch remains on the conveyor, while the non-magnetic portion  54  of the batch is stockpiled for recovery as a potential non-ferrous by-product, or discarded. The magnetic portion of the batch is conveyed to a dry-screen station  56  where it is classified by size. The illustrated system employs a screen  56  suitable for separating batch particles of about 10 mm or greater in size from particles of less than about 10 mm. The relatively larger sized portion  58  is stockpiled. It has been found that this 10+ millimeter magnetic material has a high enough iron content (typically about 50-70%) to be sold as a product (known as “B Scrap”), typically to steel mills. Various commercially available dry screens may be employed for the size classification station  56 , depending upon the particular nature of the input batch and the desired size classification objectives. 
         [0025]    It should be appreciated that, while the system  50  is detected as a continuous, in-line process, various portions of the illustrated processing stations may be installed and operated at separate geographic locations. For example, in one embodiment the magnetic separating station  52  and the size screening station  56  are physically located at the site of the slag since, in this embodiment the source of the slag , a steel mill, retains the recovered B Scrap  58  at the site. Thus, for this embodiment, shipping costs (round-trip to the site of the remaining system stations) are avoided for the B Scrap portion of the process material. 
         [0026]    In this disclosed embodiment of the system  50 , the relatively smaller sized portion separated at screening station  56  is next conveyed as an input batch to a vertical shaft mill  60 , where the material is crushed, thereby breaking much of the slag away from the iron-bearing portion of the batch, as well as reducing the size of the batch particles. It will be appreciated that other commercially available mills, grinders, and/or crushers may be employed as an alternative to vertical shaft milling station  60 . 
         [0027]    The material output from the milling station  60  may next be provided as an input batch to a screening station  62  where the input batch particles are again classified by size. In the illustrated embodiment the screening station  62  employs a wet-screening process suitable for separating batch particles of about 1 mm or greater in size from particles of less than about 1 mm. It has been found that the relatively larger sized portion  64  is stockpiled. It has been found that this 1+ mm material often has a high enough iron content (typically about 60-80%) to be sold as a product. Again, as with the dry screen utilized in station  56 , various commercially available wet screens may be employed for size classification station  62 , depending upon the particular nature of the input batch and the desired size classification objectives. 
         [0028]    In the disclosed system  50 , the 1+ mm material separated at screening station  62  may optionally be provided as input to a magnetic separator/conveyor  67  where the non-magnetic content of this batch is separated, thereby further raising the iron content of the 1+ mm material  64 . 
         [0029]    The relatively smaller sized portion of the batch separated at screening station  62  may then be provided as an input batch to the controlled, hydraulic fluidized bed separator  66  where the material is then separated by density. At this stage, the input batch is in a slurry form as a result of the wet screening operation at screening station  62 . In the illustrated embodiment, the fluidized bed separator  66  includes one or more chambers capable of containing a fluidized bed comprising a non-compressible fluid, such as, for example, water. The slurry batch is introduced into the chamber(s). In the disclosed embodiment, water is supplied from the bottom of the chamber with a controlled, upwardly flowing current so that the input batch slurry and water form a fluidized bed having a very high turbidity, causing the relatively lower density constituents to migrate upward in a fluidized bed, while the relatively higher density constituents (e.g., iron) to settle in the receptacle. The water flow may be controlled to achieve the appropriate separation of the lighter density constituents from the heavier density constituents, and migration of the lighter density constituents from an upper outlet  68  while the heavier constituents exit from outlet  70 . 
         [0030]    In the disclosed embodiment of the system  50 , the density separator station  66  is controlled to separate the slurry batch into a portion that has a relatively lower density of about 2.30-2.70 g/cm 3 , and a portion that has a relatively higher density of about 5.0-6.0 g/cm 3 . The target densities may, of course, be varied based upon the types and densities of the different constituents present in the input batch, as well as the densities of those constituents targeted for isolation and recovery (e.g., iron). The fluidized bed density separation system may be controlled as described in U.S. Pat. No. 6,142,311, issued to Rolf Korber, for a “Process For Controlling A Sand And Gravel Sorting And Sizing Device,” the disclosure of which is hereby incorporated herein in its entirety. 
         [0031]    The relatively heavier portion developed at station  66  is collected at a de-watering screening station  72  where the still slurry batch portion is dried and moved by conveyor  74  to be stockpiled (at  76 ). Similarly, the relatively lighter portion developed at station  66  is collected at a de-watering screening station  78 , where this slurry is dried and moved by conveyor  80  to be stockpiled (at  82 ). Relatively small screens (typically less than about 0.8 mm openings, suitable for filtering out only the water and as little of the particulate as possible) may be employed at de-watering stations  72  and  78 . Various commercially available models of de-watering screens are available for use in stations  72  and  78 . 
         [0032]    The resulting relatively heavy product  76  typically includes an iron-rich (e.g., about 40-75% by weight) content, making it suitable for resale. This material is usable by iron and steel makers, and may as well be used for other applications, such as, for example, for making counterweights. The resulting relatively lighter dried product  82  is typically discarded. 
         [0033]    Process water collected from de-watering stations  72  and  78 , as well as directly from fluidized bed separator  66 , is collected in receiving tank  84 , and then pumped under high pressure into a hydro-cyclone  86 , where the remaining particulate is separated from the water by centrifugal force. The freshwater is then discharged from the hydro-cyclone  86  into a clean water tank  88  for reintroduction back into the system. The particulate recovered from the hydro-cyclone (not shown) may then be discharged onto a de-watering screen from which the water can be returned to clean water tank  88 , and the dried particulates thereafter discarded. 
         [0034]    It will be appreciated that cleaning and recycling the system water provides an energy-efficient, resource-efficient and cost efficient, closed-loop system. Other types of filtration systems, such as, for example, belt presses, filter presses, settling tanks, and flocculants, may, of course, be utilized to accomplish the same goal. 
         [0035]      FIG. 3  illustrates one of the methods that may be implemented with the system  50  shown in  FIG. 2 . In this illustrated method  100 , the slag fines may first be sorted magnetically, at  102 , to isolate the relatively larger iron-bearing constituents  104  from the non-magnetic materials  106 . 
         [0036]    The resulting iron-bearing product is typically then sorted, at  108 , by size, typically by dry-screening the material. The relatively larger (e.g., greater than about 10 mm) particles  110  separated during this dry-screening step comprise a relatively iron-rich (i.e., about 50-70% by weight) which may be isolated from further processing and resold to steel producers as B-scrap for use as input in the iron blast furnace. 
         [0037]    The remaining, relatively smaller and iron-poor fines  112  are then further processed, typically by milling or grinding, at  114 , to physically separate the slag portion from the iron portion of the material. The milling or grinding station also reduces the average particle size of the batch. 
         [0038]    The milled material is then again sorted by size, at  116 , typically by wet-screening. The relatively larger (e.g., greater than about 1 mm) particles  118  separated during this screening step have also been found to comprise another relatively iron-rich (i.e., about 60-80% by weight) product which may again be isolated from further processing for re-sale again, for example, to steel makers for use as input material in their sinter plant. 
         [0039]    The relatively smaller (e.g., less than about 1 mm) material  120  produced by the wet-screening step are then classified by density, at  122 . This resulting material, now a slurry, is introduced as feed material into a hydraulic fluidized-bed density separator which is controlled to separate the suspended particles by their differing densities. The relatively denser slurry (the “heavy product”)  124  is relatively iron-rich (i.e., about 40-75% by weight), and may have sufficient iron content to be resold again, for example, as input to an iron sinter plant. The lower density slurry (the “light product”)  126  comprises a greater proportion of slag material (and other low-density non-ferrous constituents). Each of the heavy product  124  and light product  126  slurries are dried, typically by discharging the slurries onto de-watering screens. The dried heavy and light products are then stockpiled for sale and/or disposal. 
         [0040]    It should be appreciated that, as previously described, the disclosed system and method may be employed to perform the various classifying processes in a variety of different sequences, depending upon the characteristics of the slag fines and the desired iron content recovery. Similarly, the portion of the system and method, utilizing certain selected, but not all of the disclosed, classifying methods where such alternatives are efficient and productive. For example, in one embodiment a method including only the steps described at  114 ,  116 , and  122  of  FIG. 3 , to effectively recover iron-rich product. 
         [0041]    It should similarly be appreciated that the system and method of the present invention may be modified to obtain recovered product of a variety of different ratios of iron/slag/minerals, where such products are indicated as useful in industries other than iron and/or steel-making, such as, for example, the cement industry, the agricultural industry, or the aggregate industry. 
         [0042]    While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the invention.