Abstract:
A device for discharging dust from a dry dust collector of a blast furnace includes a dust discharge vent located downstream of a dust discharge opening of the dry dust collector. A fully enclosed dust conveying system is located downstream of the dust discharge valve and provides mechanized transport of the dust discharged through the discharge valve. A control system is utilized to control the opening of the dust discharge valve in relation to the residual conveying capacity of the dust conveying system.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a device for discharging dust from a dry dust collector of a blast furnace. 
     BACKGROUND OF THE INVENTION 
     The object of a dry dust collector of a blast furnace (as e.g. a dust-catcher, a cyclone or a filter) is to remove in a dry state as much as possible of the dust with which the blast furnace gas is heavily laden, before the blast furnace gas passes into succeeding wet-cleaning units. Such dry dust collectors generally have a cone bottom in which the blast furnace dust accumulates and from where it must periodically be evacuated. 
     Known devices for discharging the dust from the cone bottom of a dry dust collector in a blast furnace plant comprise a fully enclosed mechanical conveyor, normally a conveyor including propelled paddles for pushing the dust through a closed duct. The conveyor collects the blast furnace dust at a dust discharge opening in the cone bottom of the dry dust collector to drop it e.g. into a railroad car. Water is injected into the conveyor so as to slightly moisten the blast furnace dust, thus preventing the dust from blowing around too much as it drops out of the conveyor. 
     At least one isolating valve is mounted between the conveyor and the dust discharge opening. This isolating valve is used to interrupt dust discharge from the dry dust collector if a new rail road car has to be placed under the outlet of the conveyor or if the dust level in the cone bottom of the dry dust collector has fallen below a certain level. Maintaining a minimum dust level in the cone bottom of the dry dust collector indeed warrants that pressurised gas does not blow through the conveyor when the isolating valves are open. 
     It is also known to provide a lock with a gastight outlet and inlet valve between the screw conveyor and the dust discharge opening. The gastight dust discharge valve of the look is closed when dust is discharged from the cone bottom of the dry dust collector into the lock. The gastight inlet valve of the lock is closed when dust is discharged from the lock Into the screw conveyor. 
     Known devices for discharging blast furnace dust comprise isolating valves that have either a plate shaped or a convex cone shaped closing body mounted on an arm that is articulated laterally of the dust discharge opening, so that the closing body can be pivoted around a horizontal axis between a closed position and an open position, wherein it is located completely outside of the dust flow. These valve types are used because they are rather insensitive to clogging and abrasion. 
     A major problem with known devices for discharging blast furnace dust is that the discharge flow rate of the dust from the cone bottom of the dry dust collector or from the lock is not at all constant This is inter alia due to the fact that the dust can be very fluid at one moment, but can also bake together one moment later. It follows that the operating conditions of the conveyor downstream of the discharge valve are subjected to violent changes. At one moment the conveyor may be running idle and just a moment later there may be a complete clogging in the conveyor. 
     It will be noted in this context that the standard solution for warranting a more or less uniform discharging of dustlike materials from a hopper is a cellular wheel. This solution is however not at all suited for discharging blast furnace dust from a dry dust collector because the blast furnace dust easily clogs the wheel cells and is also too abrasive for the wheel. Indeed, after a relatively short operating time, the lock edges of the cellular wheel are heavily worn out, so that blast furnace dust simply runs through the stopped cellular wheel. 
     JP 59-185711 discloses a method for automatically controlling the discharging of blast furnace dust from a dry dust collector. The discharging device disclosed in this document comprises three discharge valves in series, which are successively closed by a microcomputer. A mechanical conveying system, which comprises a screw conveyor and an enclosed belt conveyor in series, is located downstream of the dust discharge valves for providing mechanized transport of the dust discharged through the discharge valves. An oxygen sensor measures the oxygen content in the belt Conveyor. When the sensor detects a sharp decrease in the oxygen content, the discharge valves are closed by the microcomputer. According to JP 59-185711 a sharp decrease in the oxygen content is an indication of a blast furnace gas break-through and an automatic closing of the discharge valves in case of such a gas break-through helps to prevent environmental pollution by black smoke. It will be noted that JP 59-185711 does not concern itself with an improvement of the operating conditions of the mechanical conveying system. 
     The technical problem underlying the present invention is to provide a simple device for discharging blast furnace dust from a dry dust collector with improved discharging conditions. 
     SUMMARY OF THE INVENTION 
     The present invention concerns a device for discharging dust from a dry dust collector of a blast furnace comprising a dust discharge valve located downstream of a dust discharge opening of the dry dust collector and a fully enclosed dust conveying system located downstream of the dust discharge valve. In accordance with an important aspect of the present invention this device comprises a control system that is designed so as to control the opening of the dust discharge valve in function of the residual conveying capacity of the dust conveying system. In other words, the control system closes the discharge valve before the conveying system risks to clog and opens the discharge valve if the conveying load drops. It follows that the average conveying capacity, i.e. average discharge capacity, is substantially increased, whereas the risk of clogging is nevertheless reduced. As a result, breakdowns of the conveying system are less frequent and less overhauling operations are necessary. It will further be appreciated that in a conveying system that is more equally loaded, a gas break through is less probable. Consequently, with a device in accordance with the invention, it is generally not necessary to provide a lock with a gastight outlet and inlet valve between the dust discharge opening and the conveyor. 
     The conveying system of the device may comprise a fully enclosed mechanical conveyor, wherein the control system is designed so as to control the opening of the dust discharge valve in function of the power absorbed by the mechanical conveyor. 
     In a preferred embodiment of the device, the fully enclosed conveying system comprises a pneumatic conveying system, wherein the control system is designed so as to control the opening of the dust discharge valve in function of the pressure in the pneumatic conveying system. This device allows to efficiently discharge the blast furnace dust from the dry dust collector in a completely closed circuit. A preferred embodiment of such a pneumatic conveying system for the blast furnace dust comprises: a dust storage hopper located near the blast furnace; a pneumatic conveying conduit connected between the dust discharge valve the dust storage hopper, for transporting the discharged dust from the dry dust collector to the dust storage hopper; a fluidizing hopper connected to the dust storage hopper; and injection means for injecting the fluidized dust into the blast furnace. 
     It will be appreciated that the present invention also provides a dust discharge valve that is excellently suited for providing the required control function in the discharging device. This dust discharge valve includes: a housing with an inlet opening bounded by a concave annular surface that lies on a first fictive cylinder having a horizontal central axis; and a closing body mounted in the housing so that it can be pivoted around the horizontal central axis between a closed position and an open position. The closing body of this valve has a convex cylindrical closing surface that lies on a second fictive cylinder, which has a diameter that is slightly smaller than the diameter of the first fictive cylinder. In order to prevent that this valve is damaged or blocked by bigger nodules of agglomerated dust during the closing operation, its closing body is advantageously provided with a cutting edge that sweeps across the inlet opening when the closing body is pivoted between the closed position and the open position. Such a cutting edge is capable of shearing even very hard nodules of agglomerated dust when the valve closes. It will be noted that the cutting edge is advantageously given a concave form, so that shearing takes place mainly during the end phase of the closing movement. If the concave cutting edge is moreover substantially symmetric with regard to a center plane of the closing body, it is warranted that the closing body is more or less symmetrically charged during the shearing operation. 
     In a preferred embodiment of the discharge valve, the closing body includes a cylindrical closing plate with two lateral flanges, each of the flanges supporting a lateral journal. The housing of this valve includes a vertical dust passage channel below its inlet opening and a bearing located on either side of the dust passage channel for supporting each of the two journals of the closing body, so that the latter can be pivoted around the aforementioned horizontal central axis. It will be appreciated that the vertical dust passage channel is completely free when the closing body is in its open position. In other words, no component of the valve is located in the highly abrasive dust stream in the vertical dust passage. 
     In a preferred embodiment the closing includes a cylindrical closing plate with two lateral flanges, wherein the closing plate and the lateral flanges are made in one piece of metal carbide. Secured to each of these metal carbide flanges is a mating steel flange that supports a steel journal. It will be appreciated that this design makes it possible to obtain a metal carbide closing body with two perfectly aligned journals. 
     The housing of the discharge valve includes advantageously an inflatable seal, which is mounted around the concave annular surface so that it is pressed against the closing surface in the closed position when it is inflated, but spaced therefrom when it is deflated. This embodiment of the discharge valve warrants a gastight closure. It will be appreciated that the sealing element has a considerable service life, because it is exposed neither to contact with the dust stream, nor to contact with the pivoting closing surface. 
     The housing of the valve may furthermore include an outlet opening bounded by a ring made of metal carbide, wherein the ring is mounted in a ground plate below the closing body, so as to define a retaining shoulder for retaining a dust cushion on the ground plate around the outlet opening. This embodiment of the discharge valve distinguishes itself by an excellent wear resistance of the valve housing at the level of its outlet opening. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
     FIG.  1 : is a diagrammatic view of a first embodiment of the device for discharging dust from a dry dust collector of a blast furnace; 
     FIG.  2 : is a diagrammatic view of a second embodiment of the device for discharging dust from a dry dust collector of a blast furnace; 
     FIG.  3 : is a top view of a discharge valve to be used in the device of FIG. 1 or FIG. 2; 
     FIG.  4 : is a section along the line  4 - 4 ′ in FIG. 3; and 
     FIG.  5 : is a simplified three-dimensional view of a closing body to be used in a discharge valve of FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Reference number  10  in FIGS. 1 &amp; 2 identifies a dust-catcher of a blast furnace plant. This dust-catcher is a dry dust collector whose object is to remove as much as possible of the flue dust blown from the blast furnace gas, before the latter is wet-cleaned in succeeding wet-cleaning units. It is indeed easier and more economical to handle the blast furnace dust in a dry state than in a wet state. 
     Reference number  12  in FIGS. 1 &amp; 2 identifies a single downcomer coming from the top of the blast furnace and carrying the blast furnace gas, which is heavily loaded with flue dust, to the top of the dust-catcher  10 . In the latter, about 60 to 75% of the dust load of the blast furnace gas drops out of the gas stream and accumulates in a cone bottom  14  of the dust-catcher  10 . This cone bottom  14  is provided with a dust discharge opening  16 , which is equipped with a dust discharge valve  18 . It will be noted that the gas pressure in the dust catcher is only slightly smaller than the gas pressure in the blast furnace. 
     In the embodiment of FIG. 1, a conduit  20  connects the outlet of the discharge valve  18  to an inlet port of a fully enclosed mechanical conveyor  22 , which is known per se. This conveyor includes at least one electrical motor  24  for propelling a set of paddles  26 , which push the dust through a closed duct  28 . At its outlet opening the conveyor  22  drops the dust into a recipient  29 , which may be e.g. a railroad car. The electrical power absorbed by the mechanical conveyor  22  is a function of its instantaneous conveying load. This electrical power is continuously measured and used as controlled value  30  in a control system  32 , which produces a control signal  34  for the for the discharge valve  18 . If the electrical power absorbed by the mechanical conveyor  22  exceeds a pre-set value, the discharge valve  18  receives a control signal to close. If the electrical power absorbed by the mechanical conveyor  22  falls below said pre-set value, the discharge valve  18  receives a control signal to open. It follows that the mechanical conveyor  22  is more equally loaded, whereby gas break through and clogging become less probable. With the cone bottom  14  are associated a low level probe  36  and a high level probe  38 . A high level signal from the high level probe  38  is used to start the discharging operation. A low level signal from the low level probe  36  is used to close the valve  18  and stop the discharging operation. 
     In the embodiment of FIG. 2, the outlet of the discharge valve  18  is connected via pneumatic injector  40  to a pneumatic conveying conduit  42 . The latter is used to transport the dust into a dust storage hopper  44  located near the blast furnace. Blast furnace gas from the dust catcher  10  or an auxiliary gas, preferably an inert gas as for example nitrogen, may be used as transporting gas in the pneumatic conveying conduit  42 . The pressure in the pneumatic conveying conduit  42  is continuously measured by a probe  31  and used as controlled value  30 ′ in a control system  32 ′, which produces a control signal  34 ′ for the discharge valve  18 . If the pressure in the pneumatic conveying conduit  42  exceeds a pre-set value, which is an indication that the dust stream is getting denser, the discharge valve  18  receives a control signal to close and less dust is injected into the conduit  42 . If the pressure in the pneumatic conveying conduit  42  falls below a pre-set value, which is an indication that the dust stream is getting less dense, the discharge valve  18  receives a control signal to open and more dust is injected into the conveying conduit  42 . Just as for the device of FIG. 1, a high level signal from the high level probe  38  is used to start the discharging operation, and a low level signal from the low level probe  36  is used to close the valve  18  and stop the discharging operation. 
     It will be appreciated that the device of FIG. 2 is a completely closed system. From the storage hopper  44  the dust is discharged into a fluidising hopper  46 , which is located nearer to the blast furnace. This fluidising hopper  46  is connected vian injector  48  to a pneumatic distribution system  50 , which is used to inject the blast furnace dust with the hot-blast  52  through blast connections  54  back into the blast furnace. 
     A preferred embodiment of the discharge valve  18  will now be described with reference to FIGS. 3 to  5 . This discharge valve  18  includes a housing  60  with a dust passage channel  62  passing vertically across. At the inlet side the vertical dust passage channel is formed by an exchangeable wear resistant inlet tube  64  that is removably secured in the housing  60 . This inlet tube  64  has a lower edge  66  that is slightly protruding into an internal valve chamber  68 , wherein it defines an inlet opening  69  for the dust. This inlet opening  69  is bounded by a concave annular front surface  70  of the lower edge  66 , which lies on a first fictive cylinder having a horizontal central axis  72 . In other words, the concave annular front surface  70  bounding the inlet opening  69  is formed by the intersection of the lower edge  66  of the vertical inlet tube  64  with a first fictive horizontal cylinder having the horizontal axis  72  as central axis. A closing body  74  is mounted in the internal valve chamber  68 , so that it can be pivoted around the horizontal central axis  72 . This closing body  74  has a convex cylindrical closing surface  76  that lies on a second fictive cylinder that is coaxial to said first fictive cylinder but has a diameter that is slightly smaller than the latter. It follows that the closing body  74  can be pivoted around the horizontal central axis  72  from the completely open position shown in FIG. 5, wherein the closing body  74  is arranged laterally of the vertical dust passage channel  62 , by an angle of about 90° in the direction of the arrow  78  into a closed position, wherein the closing body  74  is arranged centrally under the inlet opening  69 . In this closed position the closing body  74  closes the inlet tube  64  with the exception of a small air gap subsisting between the concave annular front surface  70  of the inlet tube  64  and the opposite cylindrical closing surface  76 . 
     It will be noted that the cylindrical closing surface  76  is greater than the cylindrical surface delimited by the outer contour line of the concave annular front surface  70 , so that an outer ring surface of the cylindrical closing surface  76  surrounds the concave annular front surface  70  when the closing body  74  is in its completely closed position. This outer ring surface is used as a contact surface for an inflatable seal ring  80  that is mounted in a ring channel in the housing  60 , which surrounds the concave annular front surface  70 . When the closing body  74  is immobilised in its closed position, the seal ring  80  is inflated through a gas passage  82  and thereby firmly pressed against said outer ring surface of the cylindrical closing surface  76  to provide a gastight sealing of the small air gap subsisting between the concave annular front surface  70  of the inlet tube  64  and the opposite convex cylindrical closing surface  76 . When the closing body  74  has to be pivoted, the seal ring  80  is first deflated. It will be appreciated that in its deflated state, the seal ring  80  lies completely within its ring channel, so that it cannot come into contact with the moving cylindrical closing surface  76 . 
     The housing  60  includes an outlet opening  84  of the dust passage channel  62 , which is bounded by a ring  86  made of metal carbide. This ring  86  is mounted in a ground plate  88  below the closing body  74 , so as to define a small retaining shoulder  89  for retaining a dust cushion on the ground plate  88  around outlet opening  84 . This dust cushion helps to protect the rim of the outlet opening  84  against abrasion by the dust flow. A lateral inspection opening  90 , which is normally closed in a gastight manner by a plate (not shown in FIG.  4 ), gives access to the valve chamber  68 . 
     As can be seen on FIG.  3  and FIG. 5, the closing body  74  has a concave cutting edge  92  that is symmetric with regard to the centre plane of the closing body  74 . This cutting edge  92  sweeps across the inlet opening  69  when the closing body  74  is pivoted between its closed position and its open position. It is capable of shearing even very hard nodules of agglomerated dust when the valve closes. As can be seen on FIG. 3, in which the closing body  74  is shown in a position very close to its completely closed position, the special shape of the cutting edge  92  warrants that hard nodules of agglomerated dust are pushed towards the centre plane of the closing body  74 . Thus it is warranted that the closing body  74  is subject to a more or less symmetric stress situation during the shearing operation. 
     Further features of the closing body  74  will now be described with reference to FIG.  5 . The preferred closing body  74  shown on FIG. 5 includes a cylindrical closing plate  100  with two lateral flanges  102 ,  104  (the flange  104 , which is not seen in FIG. 5, is symmetrical to the flange  102 ), wherein the closing plate  100  and the lateral flanges  102 ,  104  are made in one piece of metal carbide. To each of the metal carbide flanges  102 ,  104  is secured a mating flange  102 ′,  104 ′, which supports a lateral journal  106 ,  108 , wherein the mating flanges  102 ′,  104 ′ and the journals  106 ,  108  are made of steel. It will be appreciated that this design makes it possible to obtain a metal carbide closing body with two perfectly aligned journals  106 ,  108 . The latter are used to support the closing body  74  in two gastight bearings (not shown) that are located on either side of the dust passage channel  62 , so that the closing body  74  can be pivoted around the aforementioned horizontal central axis  72 . Thus, a very resistant closing body  74  is achieved, which is moreover capable of withstanding high shear forces during the closing operation. 
     Coming back to FIG. 3, it remains to be noted that the two journals  106 ,  108  axially traverse their gastight bearings. Each journal  106 ,  108  is provided outside the housing  60  with a crank arm  110 ,  112 . The latter are actuated by two hydraulic cylinders  114 ,  116 , which are equipped with a continuous positioning device, so that the valve  18  can be used for a continuous control of dust discharge, as described above.