US3088723A - Air draft for pelletizing furnace - Google Patents

Description

May 7, 1963 K. M. HALEY ETAL AIR DRAFT FOR PELLETIZING FURNACE 2 Sheets-Sheet 1 Filed March 21. 1961 May 7, 1963 K. M. HALEY ETAL 3,083,723

AIR DRAFT FOR PELLETIZING FURNACE Filed March 21, 1961 2 Sheets-Sheet 2 HTI'QA'NE'YS United rates Patent 3,088,723 AIR DRAFT FOR PELLETIZING FURNACE Kenneth M. Haley, Bernard L. Arntz, and Richard L.

Reed, Silver Bay, Minn., assignors to Reserve Mining Company, Silver Bay, Minn, a corporation of Minnesota Filed Mar. 21, 1961, Ser. No. 97,256 3 Claims. (Cl. 2fi621) The invention relates to a novel and improved process and apparatus for use in a general method of heat-hardening or indurating nodules, or a gglomerates of heat susceptible material, such as pellets formed from the pulverulent products of iron ore benefication processes. As the amount of practically available iron ores of high iron con: centration is gradually being depleted it becomes more and more evident that processes must be developed for beneficiating the lower grade ores. One such ore is taconite, an extremely hard rock which can be reduced to usable form by a series of crushing and separating steps not necessary to set out in detail here. For a proper understanding of the advantages of the invention it is believed desirable to give herein a brief rsum of one successful method of pelletizing beneficiated taconite ore.

The beneficiated ore end product is in the form of a fine powder containing between sixty and seventy percent of iron which in one exemplification is in the form of a magnetic oxide having the formula Fe O but may be hematite having the formula Fe O This powder, or comminuted particles, is too fine for direct charging into a blast furnace since it would block the interstices of the blast furnace charge and prevent proper passage of the blast. It has been determined that the beneficiated ore is most efiiciently fed to the blast furnace in the form of small rounded agglomerated entities from about onequarter inch to one and one-half inches in diameter. These entities variously known in the art by such terms as balls, nodules, or glomerules, will herein be identified by the convenient and also frequently used term pellets.

A preferred method of pellet manufacture comprises moistening the pulverulent concentrate and rolling it into pellets in a large drum which is rotating on an axis somewhat inclined to the horizontal. The concentrate is charged into one end of the drum and rolls along the inner peripheral wall of the drum as it rotates. Nuclei form and increase in size as the growing particles progress towards the discharge end of the drum. The balling effect is analogous to the increase in size of a rolled snow ball on a snow slope. By various expedients not necessary to discuss here the pellet-forming procedure is controlled to yield pellets within the desired size range.

The beneficiated ore concentrate contains a certain amount of moisture, preferably around ten percent, which increases the tacky character of the powder so as to achieve improved balling properties. It may also contain a minor amount of some bonding material. The pellets, after formation, are subjected to rather vigorous handling. They must therefore be treated to increase their strength, hardness, and impact resistance, because in the progression from the pelletizing plant to the blast furnace they are transferred from conveyors to bins, thence perhaps to storage yards, thence to other conveying means, thence to ore-carrying boats, thence to railroad cars, etc. It is accordingly necessary that the soft moist pellets be given an induration treatment to prevent them from being substantially completely broken up by handling before they even reach the point of ultimate use.

This induration treatment has involved heating to an elevated temperature but preferably just below the range at which incipient fusion takes place. With beneficiated taconite a good operating upper limit is about 2400 F. If this temperature is materially exceeded incipient fusion ice and mutual adhesion occurs between the pellets, producing sintered lumps or clusters which are undesirable to the pelletizing operation and very destructive to grate bars and pallets.

Heating is supplied, for the most part externally. In one preferred system, the pellets are disposed in a layer or bed on the endless type of conveyor, consisting of a series of cars, moving in end-to-end contact, and having a gaspermeable floor consisting of slightly spaced grate bars supported on pallets. The layer may be from a thickness of six inches to twelve inches and upwards, and the conveyor is moved slowly through a series of zones comprising, in succession, a drying zone, a preheat, ignition, and reaction or furnace zone, a burning and recuperation zone, and a cooling zone. Preheating, ignition, and reaction can be accomplished by external fuel combustion, such as by burning gas or oil for heating the air which passes through the pellet bed. in a preferred method the pellets may be coated with a thin layer of a carbonaceous fuel, such as powdered coal. The pellet bed, as it moves through the preheating ignition, and reaction zone, is subjected to a penetrating current of heated air which supplies oxygen to the powdered coal coating and augments the effect of the heated air in raising the pellets to the desired reaction temperature.

As above noted, the iron oxide constituent of taooniteis originally substantially completely in the form of magnetite, namely Fe O During the induration process a certain major proportion is changed to ferric oxide, name ly, Fe O which is a non-magnetic type. The general reaction is exothermic, considerable heat is evolved, and a substantial saving is achieved in the quantity of oil, gas, or coal, which must be added in the reaction and recuperation stages.

An object of the present invention is to provide an improved process and apparatus for the induration of iron ore concentrate pellets of materially improved quality.

A further object of the invention is to provide a pellet indurating process and apparatus using a horizontal endless grate on which induration can be satisfactorily accomplished with a substantial decrease in B.t.u. input per unit weight of treated pellets.

A further object of the invention is the provision of a pellet indurating process in which the building temperature, mainly originating in the indurating furnace unit, has been substantially reduced to a more bearable working level.

A further object of the invention is to provide a method and means for cooling the lower strand of the endless pallet conveyor, thereby increasing pallet and grate bar life.

A further object of the invention reduces the amount of fines normally escaping to the plant atmosphere and settling elsewhere in the furnace room.

A further object of the invention is to promote faster and more thorough burning of the ignited pellets thereby increasing the efficiency of the process and improving the quality of the product.

As a consequence of the achievement of the various objects detailed hereinabove, a resulting object is the increase in pellet tonnage output per unit of time, with a decrease in fuel consumption rate.

Other objects and advantages will be apparent from a study of the following description of one embodiment of the invention, in conjunction with the accompanying drawings, in which:

FIG. 1 is a side elevational view of a pelletizing furnace embodying our invention.

FIG. 2. is a sectional view, somewhat enlarged, taken on the line 2-2 of FIG. 1.

Referring now to the drawings there is shown in partially schematic form a continuous grate pelletizing furnace including a series of small cars 20 advancing from right to left on the upper'strand or flight 21 of a continuous track, and from left to right on the lower flight 22 returning at a lower level. At its ends the track curves at 23 and 24 to connect the upper and lower flights. The manner in which the carsare supported and driven need not be specifically described or shown here, the system being generally similar to the arrangement used in sintering furnaces, well known in the art. The floor of each car is composed of a side-by-side assembly of grate bars supported on pallet frames there being slots between the grate bars to-provide air current passageways for supporting the reaction whereby the induration process progresses, The slots are large enough to pass an ample flow of air, but narrow enough to prevent any appreciable loss of pellets through the slots.

The cars are loaded with green pellets when arriving at level position on the top flight of the track, at the right end of FIG. 1, and they dump the indurated pellets by gravity at the left end of FIG. 1 as the cars tip over to upside-down position for thereturn trip on the lower track flight.

Immediately beneath the upper flight of the track throughout its full length is an end to end succession of wind boxes 25 (FIG. 2), the function of said windboxes being to guide air streams either upwardly through the pellet bed, or downwardly therethrough as the particular zone demands.

As indicated by the brackets in FIG. 1 the indurating machine may be divided into four zones, although their boundaries are not necessarily sharply defined. 'These zones are, proceeding from right to left in the direction of pellet travel, a drying zone A, a preheating, ignition, and reaction furnace zone B, a burning and recuperation zone C, and a cooling zone D. The designations indicate quite well the nature of the operation occurring in the respective zones. In the drying zone the moist pelf lets are dried by an updraft of heated air drawn from subsequent zonesthrough pipes '26, 27, 28, 29 etc. by means of pumps 30 31. 33 and thence by individual pipes 34, 35, etc. to respective windboxes, and thence through the pellet bed and saturated air-out stack. These brief characterizations are furnished as a general aid to an understanding of the background of the-invention, which will later be fully described, and a more specific disclosure is not believed nece'ssary/ In zone B a draft of air heated to between 2.100 F. and 2400 F. is drawn downwardly upon and through the dried pellet bed by means of pump 36, pipe 37, header pipes 38, and individual pipes 39 etc. (FIG. 1), each pipe going to an individual windbox beneath the moving pallet'cars. 'ThermaI reactions are thereby induced'in the top layer of the bed of pellets with an indurating effect which progresses downwardly through the bed as it advances into and through zone C, the reactions being substantially completed by the time the bed reaches the end of this zone C which we have previously termed the-burning and recuperation zone. Heated air is drawn downwardly through this burning and recuperation zone by means of pump 40, pipe 41, header 42, individual pipes 43 and windboxes disposed under the pallet car conveyor. As will soon appear, the inventive concepts are embodied in the novel manner and means whereby the air is heated and routed preparatory to its passage through the pellet bed in the burning and recuperation zone.

During burning and recuperation the pellets are indurated to a hardness adequate to stand up under subsequent handling and in zone D they are cooled by a draft of air by means of pump 44 header 45, pipes 46 The air is fed to headers 32,.

4 as well as surrounding structure, we show the loaded pallet car 20 supported on wheels 47 on tracks 21. Immediately below the car body ;;is the stationary windbox 25, in flow communication through a fitting or banjo 48, to a vertical pipe or downcomer 43 and thence to the manifold 42... Concentrate dust entrained in the air duct system can fallout into dust box 49, and is then dumped onto a conveyor 50 by which it may ultimately be returned to the pelletizing process. The conveyor and the air supply system are supported on frame members or piers 53 which of course rest on some suitable rigid foundation 54. Air from the manifold 42 is drawn downwardly by means of fan 40, shown in FIG. 1.

Heated air is supplied to the burning and recuperation zone by means of the suction just mentioned. Floor plates 55 are carried on piers 53. A domed housing 56 rests on the floor plates 5'5 and extends the full length of the machine including and subsequent to section (3/ As a consequence the suction in windboxes 25 must be satisfied by air drawn upwardly as shown by arrows. E. In its travel upwardly to the interior of housing 56. the air enters under depending side wall plates 57 and moves upwardly past the upper ends of downcomer ducts 43 the walls of which are at a temperature of about 300. F. so that the air begins to be heated. It .then passes the banjo fittings 48. which are also at about 300 'F., and then past windboxes. 25 the walls of which are at about 400 F. Some of the air .stream also travels past and through the lower flight 22. of the conveyor in its upside-down position, thereby picking' up another temperature increment, since the lower flight pallets may have a temperature of around 1000 F. The air next passes along the outside surfiace of the pallet side walls'60, which are at a temperature of about 1500 F. in the burning and recuperation zone, and finally down through the 'pellet bed. The air stream is therefore thoroughly preheated when it penetrates the bed in the recuperation zone. A sling system 62 is provided for easy removal of housing portions for service or inspection.

A number of advantages result from this method of routing the .air stream. The housing over the recuperation section reduces the heat in the building proper, which formerly was given off both by radiation and convection from the bed of pellets and also from the side plates and pallets as the'conveyor issues from the furnace or reaction zone. The average room temperature in a particular pelletizing plant has been reduced by more than ten de-; grees Fah'renheit'which is a material-factor in contributing; to workmens co'rnfont. The subst-a'ntizal increase in the temperature of the air supplied to the recuperation zone: (at least 300 F. higher than heretofore) has improved. the efliciency of a single furnace to' the point where its: average daily output tonnage has increased by eighty eight (88) tons. The service life of pallets'and grate bars: is materially extended because of the cooling effect of the; air flow on the lower flight of the conveyor.

In the appended claims the term windbox designates: any air intake means disposed below the path of movement of the upper conveyor-flight whereby suction can be: applied to each pellet-loaded car to draw 'an air stream through the pellet bed.

What is claimed is:

1. In a pelletindurating furnace of thetype described wherein a perforate endless conveyor carries 'a bed of' pellets through an ignition and reaction zone and then intoand through a burning and recuperation zone, means at said burning and recuperation zone directing a stream of air down through said bed including a Windbox assembly fixed immediately below the path of movement of said conveyor, means for creating suction in the windbox assembly including downward-1y directed duct means in communication at its upper end with said windbox and at its lower end with a suction pump whereby heated air is drawn through the burning pellet bed and through the said duct means and pump, and housing means for enclosing a space laterally around and above said conveyor in said burning and recuperation zone, said housing opening downwardly around said w-indbox and air duct means, whereby incoming air is caused to flow upwardly past the outer surface of said heated air duct means and windbox assembly into said housing, and thence downwardly through said pellet bed and then through said air duct means as aforesaid.

2. In a pellet indurating furnace of the type described wherein a perforate endless convey-or carries a bed of pellets through "an ignition and reaction zone and then into and through a burning and recuperation zone, means at said burning and recuperation zone for creating suction immediately below said conveyor whereby a stream of air is caused to move through the bed of pellets in a downward direction through said suction creating means, housing means for enclosing the space immediately above and laterally around said conveyor in said burning and recuperation zones, said housing means opening downwardly around said conveyor whereby said air stream moving through said bed must be fed from air moving upwardly into said housing from below said conveyor.

3. In a pellet indurating furnace of the type described wherein a perforate endless conveyor carries a bed of pellets on side-walled pallets through an ignition and reaction zone and then into and through a burning and recuperation zone, means at said burning and recuperation zone directing a stream of air down through said bed including a Windbox assembly fixed immediately below the path of movement of said conveyor, means for creating suction in the windbox assembly including downwardly directed air duct means in communication at its upper end with said windbox and at its lower end with a suction pump and with said reaction zone whereby reaction-heated air is drawn through the burning pellet bed and through the said duct means and pump, means for causing said incoming air to pass along the hot side walls of said pallets before it passes through said pellet bed including housing means enclosing a space laterally around and above said conveyor in said burning and recuperation zone, and additional means for causing said incoming air to pass upwardly along the outer surface of said heated air duct means, and said windbox assembly whereby to materially increase the temperature of .the incoming air stream.

References Cited in the file of this patent UNITED STATES PATENTS 1,856,548 Greene t May 3, 1932 2,254,323 Shallock Sept. 2, 1941 2,750,274 Lellep June 12, 1956

Claims (1)

1. 3. IN A PELLET INDURATING FURNACE OF THE TYPE DESCRIBED WHEREIN A PERFORATE ENDLESS CONVEYOR CARRIES A BED OF PELLETS ON SIDE-WALLED PALLETS THROUGH AN IGNITION AND REACTION ZONE AND THEN INTO AND THROUGH A BURNING AND RECUPERATION ZONE, MEANS AT SAID BURNING AND RECUPERATION ZONE DIRECTING A STREAM OF AIR DOWN THROUGH SAID BED INCLUDING A WINDBOX ASSEMBLY FIXED IMMEDIATELY BELOW THE PATH OF MOVEMENT OF SAID CONVEYOR, MEANS FOR CREATING SUCTION IN THE WINDBOX ASSEMBLY INCLUDING DOWNWARDLY DIRECTED AIR DUCT MEANS IN COMMUNICATION AT ITS UPPER END WITH SAID WINDBOX AND AT ITS LOWER END WITH A SUCTION PUMP AND WITH SAID REACTION ZONE WHEREBY REACTION-HEATED AIR IS DRAWN THROUGH THE BURNING PELLET BED AND THROUGH THE SAID DUCT MEANS AND PUMP, MEANS FOR CAUSING SAID INCOMING AIR TO PASS ALONG THE HOT SIDE WALLS OF SAID PALLETS BEFORE IT PASSES THROUGH SAID PELLET BED INCLUDING HOUSING MEANS ENCLOSING A SPACE LATERALLY AROUND AND ABOVE SAID CONVEYOR IN SAID BURNING AND RECUPERATION ZONE, AND ADDITIONAL MEANS FOR CAUSING SAID INCOMING AIR TO PASS UPWARDLY ALONG THE OUTER SURFACE OF SAID HEATED AIR DUCT MEANS, AND SAID WINDBOX ASSEMBLY WHEREBY TO MATERIALLY INCREASE THE TEMPERATURE OF THE IMCOMING AIR STREAM.

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