Abstract:
A method of separating and recycling shards and fines that occur in a briquetting operation using an non-reactive carrier gas to separate and entrain the hot shards and fines, and conduct them to a material inlet of the briquetting machine, while separating and recycling the carrier gas. Apparatus for carrying out the method is also disclosed.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 60/191,036, filed on Mar. 21, 2000. 
    
    
     FIELD OF INVENTION 
     This invention relates to an improved method for separating and recycling hot fines, and is especially useful in recycling hot fines produced in the hot briquetting of reduced iron. 
     BACKGROUND OF THE INVENTION 
     Hot briquetting of reduced iron consists of feeding the output material of a direct reduced iron (DRI) process, for example, from a direct reduction shaft furnace, into a press in which the material is formed into uniform, pillow-shaped briquettes. The material is substantially metallized with an iron content exceeding 85%. The material temperature at the egress of the furnace depends on the type of furnace used, but is greater than 600° C. in all cases. The material is not cooled before the briquetting operation. The briquettes usually are formed into sheets of connected briquettes. A breaking operation follows the briquetting press so that briquettes abutting and connected in the press are separated at the egress of the briquette process. “Fines” are particles of DRI that consist of shards of small (less than 12 mm) irregular shapes and other particulates that are created in the breaking operation. These fines must be removed from the egress material but are nonetheless valuable DRI material. Therefore, fines must be removed and recycled in any economically feasible method. 
     The current State of the Art for recovering fines is to pass hot direct reduced iron, DRI briquettes, shards and fines over a vibrating screen through which the small particles fall, then recycle the fines to a briquette feed screw via a bucket elevator. This method involves mechanical equipment and concomitant maintenance problems. For example, when the vibrating screen becomes hot it loses surface hardness, and therefore wears out quickly. Replacement of the screen is expensive and requires process interruption. The hot bucket elevators require excessive maintenance. These bucket elevators have many moving parts which are subject to undesired thermal expansion and creep. Ensuring that the system is hermetic and that only inert gases contact the hot DRI is also a problem with known fines separation and recycling systems. Hot DRI fines are subject to reoxidation and burning, including catastrophic burning or explosion upon exposure to oxygen. 
     Applicants are aware of the following prior art: 
     Biscofsheim et al., U.S. Pat. No. 3,630,353, teaches a method for removing fines or dust from broken sinter cake before delivery to a blast furnace. Biscofsheim works by blowing air upward through a perforated bottom wall of a rotating cooler and through the broken sinter thereon. The fines are carried by the air to an exhaust hood and then to a precipitator where the fines are collected and then reintroduced into the sintering machine. Air pressure and air velocity are changed according to the thickness of sinter material and the size of the fines to be collected. 
     Rohaus, U.S. Pat. No. 3,116,996, teaches a method for handling fines which remain after a briquetting operation without the use of a cyclone or a screen. Rohaus handles fines by blowing the briquettes with air while in a cooling shaft and reintroducing the fines into a kiln. Rohaus does not mention the reduced or oxidized status of the briquettes or fines, but seems more concerned with moisture content. The method utilizes the air, heated by the briquettes, as secondary combustion air within the kiln. The recycled fines are passed only through a final portion of the kiln since the fines are already substantially moisture free. Howell, U.S. Pat. No. 4,249,906, teaches a process for recovering and using fines of flux, such as limestone, used in metallurgical processes. Howell collects airborne fines with a baghouse or a similar piece of equipment and the fines are then transported to a first processing zone. The method of transportation is called ‘dense phase collection’, a method in which particulate matter is carried by a gas in a manner resembling a fluidized bed. The material is formed into briquettes. The briquettes, fines and larger pieces of flux are temporarily stored. Upon removal from storage the material is abrasively deflashed, then transported to a vibrating screen classifier. Appropriately sized pieces are delivered for processing. Other sizes, both too large and too small are recycled to the briquetting machine. 
     Metz, U.S. Pat. No. 4,274,863, teaches a method oftreating flue gas. Metz describes a single gas treatment station used to clean varying gases from the steel mill before discharge to the environment. The flue gas containing fines and other contaminants is introduced into a sintering plant. The flue gas is introduced with air and incorporated into the combustion reaction inside the sintering plant. Many of the pollutants in the flue gas are oxidized, and the dust in the flue gas is affixed to the sintering material. Pollutants may be selectively added to the sintering process in the location where materials are typically decomposed or removed. For instance, those gas streams containing large amounts of sulfuric components may be introduced into the upstream zone of the sintering bed. Metz also provides that crude dust may be removed before the gas and dust mixture enters the sintering plant. 
     Coyne, Jr., U.S. Pat. No. 4,917,723, teaches a method for capturing nonferrous materials used in metallurgical processes. The process begins by collecting plant waste including fines, sludge, dust, etc. The captured dust and fines are fed to a briquette press and then to a smelter. The process is repeated so that nonferrous constituent material is repeatedly captured, briquetted, smelted, volatilized, cooled, then captured again. This process continues until the dust has a sufficient percentage of nonferrous material to warrant removal and recovery. The goal of the process is to recycle iron containing material while recovering nonferrous material. This patent applies to direct reduced iron though it is not limited to DRI. 
     Batterham et al., U.S. Pat. No. 5,364,446, teaches a process for heating solid non-reducible particulate material by carrying it in a hot gas. This process may be used in the production of Portland type cement or may be used to heat carbonate materials for a metallurgical process. The detailed description in the patent uses an off gas to heat a carbonate in preparation for calcination. Problems with stickiness from molten or sticky particles in the off gas are solved by cooling particles slightly or establishing flow patterns to minimize the interaction between particles to be carried by the flue gas and particles already present in the flue gas. The goal of this invention is to heat carbonate material with flue gas with minimal interaction between the carbonate material and the particulate matter in the flue gas. 
     The present invention differs substantively from the teachings of the above references. The invented process requires that the briquette temperature at the egress of the briquette operation is not substantially lower than the temperature egress of the DRI furnace. Entraining DRI fines at more than 600° C. in air is inoperable since hot DRI readily, possibly explosively, reacts with oxygen. Note that the term “fines” in the present invention is defined as particulate materials, the size of which particles range from less than 0.1 mm dust to 12 mm shards of DRI material. 
     SUMMARY OF INVENTION 
     The invention is a method and apparatus for separating and recycling hot DRI fines produced in the hot briquetting of reduced iron. The method pneumatically separates fines from briquettes upon breaking of the briquettes, and pneumatically transports the fines to the material feed inlet of the briquetting machine. This method eliminates the need for a hot screen and for a bucket elevator, both of which require unacceptably high maintenance expense. The invented method utilizes a hermetic system to avoid problems with re-oxidation, such as combustion, or even explosion. It also eliminates the need for a separate dust collection system. 
     OBJECTS OF THE INVENTION 
     The principal object of this invention is to provide a mechanically simplified method of separating and recycling fines and shards that occur in the breaker chamber of a briquetting machine. 
     It is another object of provide a method of recovering waste fines from a briquetting machine when such machine is used for briquetting hot, substantially metallized, iron-bearing materials. 
     Another object of the invention is to provide an improved method and apparatus for recovering and recycling hot direct reduced iron fines which is reliable and requires low maintenance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects will become more readily apparent by referring to the following detailed description and the appended drawings, in which: 
     FIG. 1 is a schematic diagram of the apparatus for carrying out the method according to this invention. 
     FIG. 2 is vertical cross-section of the fines collection chamber of the invention. 
     FIG. 3 is an enlarged view of a portion of FIG.  1 . 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, the method of the invention may be embodied in the apparatus  10 , as shown. The gas used in the invented method is a gas that does not react with hot DRI, for example, an inert gas such as nitrogen or argon. Hot DRI is transferred by a conveying means  22 , such as a screw conveyor to entry port  23  of a briquetting machine  24 . Briquetting of hot DRI is accomplished by briquetting rolls  26 , a roller press, or other compressing means. The briquettes are formed into a sheet  40  of attached briquettes, then are separated into individual briquettes by contact with breakers  28  located below the briquetting rolls  26 . The act of breaking the briquettes apart creates fine particles, or “fines”, which are substantially reduced iron. The fines are separated from the larger particles and recycled. The fines separation and recycling apparatus consists of a chamber  34 , fines discharge nozzle  12 , a pneumatic fines conveying conduit  14 , a fines-gas separator  16 , a wet scrubber  18 , a compression blower  20 , and associated conduits. 
     The fines are drawn from the chamber  34  housing briquette breaker  28  and into the associated fines discharge nozzle  12  by means of a negative pressure created by the blower  20 . The fines are conveyed pneumatically through the pneumatic conveying conduit  14  and into the inlet  15  of the fines separator  16 . The fines separator, which may be a cyclone or a hot gas filter, separates the fines from the inert conveyor gas, and the fines drop to the bottom of the separator  16  to be returned to the fines inlet  30  of the briquetting machine. The gas pressure within the fines separator is isolated from the screw housing by means of a rotary valve  22 , double dump valve, trickle valve or other device which makes a seal. The gas is removed from the fines separator  16 , via conduit  17 , is cleaned in wet scrubber  18 , compressed by means of compressor/blower  20  and reintroduced to the briquetting machine breaker housing inlet nozzle  36 . A vent V, whith associated control valve  38  is provided downstream from the blower  20  to cause negative pressure to be maintained in the briquette machine  24 . This negative pressure prevents fugitive dust emissions. Non-reactive makeup gas from source  32  is added as needed to the system before gas re-enters the briquetting machine breaker chamber. 
     The non-reactive gas is preferably nitrogen or a principally nitrogen gas, such as reformer seal gas, or it may contain argon or other inert gas. In the event that some small amount of air enters the system, the oxygen therein will be reacted in short order with hot components, or even with reducing agents in the briquette materials, leaving principally nitrogen in the gas. 
     A grid  44  may be provided on the fines exit side of the breaker chamber  34  which prevents the entrance of large particles or briquettes into the gas discharge nozzle  12 . The briquettes fall downward by gravity for further handling, or into a briquette collector  46 . 
     The process allows the size of the fines collected and recycled to be controlled by adjusting the velocity of gas in the fines discharge nozzle  12  and/or by changing the nozzle configuration. Conveying gas velocities are typically in the range of, but are not limited to, from 7 to 25 m/s, and fines discharge temperatures from the briquetting machine are typically in the range of, but are not limited to, from 600° C. to 750° C. Heat loss from the invented system is minimized by insulation on the conveying conduits and the cyclone  16 . The gas cleaning efficiency of the fines separator is high, typically more than 99%. The percentage of fines recycled depends on the amount of conveying gas used. The pressure required at the blower egress depends on the nozzle, conveying conduits, fines separator, and scrubber configurations, as well as the solids to gas ratio in the conveying line  14 . In the event of a pressure drop in the system, particles may fall to the bottom of vertical conveying line  14  and into a receptacle  42 . 
     The invention solves the aforementioned problems by pneumatically separating the fines from the briquettes and conveying the fines back to a briquetting machine feed screw. The invention eliminates the need for a hot screen and a bucket elevator. The invented method has the added advantage of reducing the required height of a briquetting system in a greenfield hot briquetted iron (HBI) plant. Because the system is entirely closed, and inert gas is continually added, the problem of isolating the recycle mechanism from air associated with any presently known system is resolved. The only moving part involved with the gas is the blower  20 , which is isolated from the dust and heat because the gas/fines separator  16  and the wet scrubber  18  are positioned upstream from the blower. The invented system thus is much less expensive than the known systems. 
     The invented system is also useful with briquetting of other materials, including coal, coke, and other metallic and oxide materials. 
     SUMMARY OF THE ACHIEVEMENT OF THE OBJECTS OF THE INVENTION 
     It is readily apparent from the foregoing that we have invented an improved method and apparatus for separating and recycling fines and shards that occur in the breaker chamber of a briquetting machine when such machine is used for briquetting hot, metallized iron bearing materials and that we have solved the reliability and maintenance problems associated with known briquetting methods. 
     It is to be understood that the foregoing description and specific embodiments are merely illustrative of the best mode of the invention and the principles thereof, and that various modifications and additions may be made to the method or apparatus by those skilled in the art, without departing from the spirit and scope of this invention, which is therefore understood to be limited only by the scope of the appended claims.