US2739800A - Controls for pelletizing furnace - Google Patents

Description

c. w. slsco 2,739,800

CONTROLS FOR PELLETIZING FURNACE Filed June 13, 1952 J I ffm/f March 27, 1956 IN VEN TOR BY MIIUWAJe, M

a ATTORNEYS United States Patent coN'rRoLs FOR PELLETIZING FURNACE Carl W. Sisco, Toledo, Ohio, assignor to Erie Mining Company, Hibbing, Minn., a corporation of Minnesota Application June 13, 1952, Serial No. 293,264 2 Claims. (ci. 263-19) This invention relates to the art of indurating pelletsor equivalent small fluent bodies-of ore lines or the like in a shaft-type induration furnace, and is concerned with a method of and apparatus for correcting irregularities in the temperature obtaining at the stockline of the charge. The invention is particularly applicable in connection with the induration of pellets which contain within themselves an exothermically oxidizable component, in which case the maximum temperature obtaining in the charge column occurs in a normally horizontal and relatively thin zone or layer-hereinafter referred to as the heatline-high up in the column usually only some six to eighteen inches beneath the stockline.

lt heretofore had been appreciated that all of the countercurrent stream of heat-carrying gas should exit from the top of the charge column at a controlled, relatively low, temperature, and that local hot spots at the stockline should be avoided because the same represented areas wherein usable heat was being wasted into the exit gas and pellets were being improperly indurated. It had been thought that the heatline should be parallel with the stockline-i. e., should at all places in the cross-sectional areas of the column be at the same controlled depth beneath the stockline-and it further had been thought desirable if not absolutely necessary to maintain a level stockline in order to favor such parallel conditions. Copending application Serial No. 237,209, tiled July 17, 1951, in the names of Fred D. De Vaney and Donald Beggs, describes apparatus adapted for use in maintaining a level stockline. While the measure consisting in maintaining a level stockline does avoid one cause of heatline irregularity, namely, the formation of shorter and longer paths for the countercurrently moving stream of gases, it has been found that maintenance of a level stockline is not, in and of itself, sufficient to prevent heatline irregularities.

In the carrying out of the above-mentioned induration process the very desirable levelness of the heatline may be adversely affected by the happening of any one or more of a number of conditions, including: irregular descent of the charge column; channeling of the heated gases forced countercurrently through the column, by reason of localized congregations of pellets providing larger-or otherwise more readily traversable-interstices than obtain in other parts of the column; non-uniformity in the amounts of exothermically oxidizable component included in the pellets; non-uniformity in rate of oxidation, and other variables. In consequence, the heatline instead of being a zone or plane parallel with the stockline, i. e., the top of the column, becomes non-parallel with the stockline, e. g., irregular in contour, with the result that in some areas it approaches unduly closely to or even into the stockline while in other areas it lies at a desired depth beneath the stockline or ata depth unduly removed from the stockline. Such irregularity results in loss of heat and explosive decrepitation of pelletsin those areas where the heatline is unduly close to the stockline-and/or in the caking together of pelletsin 2,73%@ Patented Mar. 27, 1956 those areas wherel the heatline lies too far beneath the stockline. It has been found that such undesirable conditions may obtain beneath a stockline which per se is desirably level.

According to the concept of the present invention, the problem of insuring uniform thermal treatment of the green pellets being fed to the stockline of the column of the induration furnace is not met by invariant levelness of the stockline, or even by invariant parallelism between stockline and heatline, but rather by locating (across the stockline) any areas of abnormally high and/ or low temperature and counteracting such localized areas of abnormally high and/or low temperature by appropriate increase or decrease in the rate of addition of "green pellets to such localized areas whereby to reduce the stockline temperature (and, hence, the exit gas temperature) at such areas, to a value within a predetermined normal range. In consequence, local hot spots in the stockline (and exit gas) are promptly eliminated, tending to produce a uniform stockline temperature and uniform heating of the green pellets.

From the apparatus standpoint the present invention comprises, in combination, a shaft-type induration furnace to the top of which green pellets are fed and from the bottom of which indurated pellets are withdrawn, means for introducing a heat-exchange gas into the furnace at an elevated temperature, means for maintaining a predetermined average exit gas temperature, a translatable feeding device for laying down a layer of green pellets onto the stockline of a column of pellets occupying the furnace, and means responsive to localized deviations in exit gas temperature from the average for regulating the localized rate of feeding, whereby to minimize such deviations.

lThe invention will now be described in greater particularity and with reference to the accompanying drawing in which the single ligure is a schematic representation of an apparatus organization adapted for carrying out the concept of the present invention.

In the drawing there is indicated at l a shaft-type indurating furnace generally, the same being provided with a discharge pipe 2 for withdrawing pellets from the bottom of the furnace and with a translatable feeder mechanism 3 for laying down a layer of green pellets over the stockline 4 of a column of pellets occupying the furnace. Feeder mechanism 3 may be that described and claimed in U. S. Patent No. 2,538,556, de Coriolis and Campbell. A stockline control means (not shown), such as that described and claimed in my copending application Serial No. 6,743, filed February 6, 1948, now Patent No. 2,646,- 900, issued July 28, 1953, may be and preferably is used to maintain a normal average elevation of stockline 4 by regulating the relative average rate of withdrawal of indurated pellets to average rate of feed of green pellets, but forms no part of the present invention. Air under pressure from a blower or other suitable source (not shown) is fed through air conduit 5 into the interior of the lower part of the'furnace and to air inlet louver 6; air conduit 5 is provided with a motor-operated air Vcontrol valve 7. Air entering the furnace at louver 6 llows generally upwardly, a major portion by-passing the middle section of the furnace through combustion chamber 8.. ln this latter the air-preheated by passage through the lower sectionv of the pellets column-is heated to a desired elevated temperature by combustion of a fuel fed thereinto by fuel burner 9. The resulting heated gases reenter the upper section of the pellets column and flow generally upwardly through the latter, exiting at stockline 4 into a hood 10 and stack il.

In order to maintain the proper heat balance conditions in the upper or heating section of the furnace, the flow of air into the furnace is so regulated as to main- 3 tain a fixed predetermined average temperature of the exit gases leaving stoekline v4, asndescriled and claimed in application Serial No. 2,315, filed January 14, 1948, new Patent'2i67695, .issued Apr'lZ-Q, 119.54, in the 11me-S .of Fred D. De Valley and Donald Bessa TFO this end,

.a thermocouple 12 in .hood 10 serves yto aetuate air .con-

trol valve 7 by means of a temperature c orrtrol Iinstrument 13 and valve motor 14.

Feeder mechanism 3 is arranged to ,traverse Igenerally kthe .entire area of the -stoekline by Aany suitable means, .e. g., that describedandclaimedin Patent No. 2,538,556,

de Coriolis and Campbell. A temperature responsive means, e. g., a thermocouple, 15 is carried by feeder 3 in lits traverse, thermocouple 15 being so positioned with ,respect to the feeder yand the stoekline as lto :measure the-temperature of the exit `gas close `to the stoekline and at the area aboutfto be, or being, fed by feeder 3. Thermocouple 15 actuates aconventional temperature respon- .sive instrument 16 yhaving a lowcontact 17 `and a high contact 1S, each of which contacts is arranged to open as .the temperature increases past the setting o f the contact and to close when the temperature drops: below the setting of the contact.

Contacts 17 and18 aresuitably connected through electrical -relays to resistors `19 and 20 in the power circuit ,of -a variable speed motor 21 actuatingthe feeder traverse mechanism. When neither contact is open, the localized exit ygas temperature is unduly low (signifying that the apertaining area of the heatline is unduly low in the column) and motor 21 runs at high s peed. With motor .21,running at high speed, feedermechanism 3,is caused to travel vat a rate of speed in excessof its normal rate, with the result that athinner than normal layer of green vpellets is deposited at the stoekline area -under consideration.

Low contact 17 isset at a temperature somewhat below, preferably about 50 F. below, that ofjthe average exit gas temperature being controlled independently from thermocouple 12. High contact 1S similarly is set to operate at a temperature say 50 F..above that of the average exit gas temperature.

.range being controlled bythermocouple 1 2, Alow contact 17 is opened, motorv 21 runs atnormal lspeed,the rate of travel yof feeder mechanism 3 is normal and a-layer, of normal thickness, of greenvpellets is deposited. When thermocouple 15 detects anarea where the exit gas temperature is unduly high, -say 50 F. or more above the average exit gas temperature, both contacts 17 and 18 are opened, motor 21 'runs atlowspeed, the rate of travel of feeder mechanism 3 is decreased below normal, and a thicker than normal layer of green pellets isdeposited over the abnormally hot area. In this fashionthe cold Vareas, which usually (butnot Anecessarily always) are the high areas, receiveV a lightfeed, thus encouraging the gas current to work its way up through the affected area and thus bring the localized exit gas temperatureback to-normal. Conversely, the hot areas (frequently the low areas) receive a heavyI feed thereby tending to depress minutes (e. g., every four minutes), the above correction for heatline irregularity is being substantially constantly applied.

According to the foregoing description the deposition of a localized light, normal or heavyfeed layer is effected by fast, normal or slow traversel speed of the feeder mechanism over .the stoekline. This is the preferred mode of operation because the actual rate of .feed being put onto the stoekline in any given time interval need not be influenced by the speed of traverse.r However, the inventive concept applies as well .to other modes of feeding,-such for exampleV as a belt feeder having a fixed rate of traverse and a variable belt loading. Generally speaking, the invention is applicable with any feeding device whichtraverses the vstoekline in any fashion and is capable of feeding layens of varying thickness in response yto localizeddepartures,from no rrnalto exit gas temperature.

1. In an lapparatus for 4indurating small fluent bodies composed yof fnelydivided mineralsolids said apparatus including ashaft-type induration furnace adapted to be occupied `by a column of such uent bodies and to receive layers of green lfluentibodies at the top of such column and to discharge -indurated fluent bodies at the bottom thereof, means fory introducing a heat-exchange gas into the furnace'at "an .elevated temperature, means for maintaining a zpredetermined Aaverage exit gas temperature, anda variable-speed translatable feeding device for laying down alayer of green fluent bodies,

in a recurring pattern, onto the stoekline of a column of fluent bodies occupyingithe' furnace, the improvement which consists.inprovidingsaidfeeding device `with means responsive -tolocalize deviations in exit gas temperature for regulatingthe localized rate v'of translation of said feeding .devicle ),rllerebyfto .minimize Vsuch deviations.

'2. An improved apparatus for indurating pellets and otherV-srnalliluent bodicsgf devided mineral solids comprising, Iin cornbination, a shaft-type induration furnaceito the Vtopof Agreenpellets are fed and from the bottomofwhichjndurated ,pellets are withdrawn, a variable-speed Ltranslatable feeding device ymovable over the .mouth of.theifurnaceforlaying downa layer of green pellets,.in,arecurringpatterm onto the s tocldineof a column .of pellets .occupying the furnace, means for introducing Va heat-exchange gas into the ,furnace at an elevated temperatureand'for `forcing the vsame yupwardly through a column ofpellets occupying the furnace, means for maintaining a y predetermined average exit gas temperature, and means carried by said translatable feeding device and responsive tolocalizeddeviations in enit gas Atemperature from theaverage exit gas temperature for Iregulating the localized Yrate ,of translation of said feeding devicewhereby to deposiathicker than normal layer of green pellets on a localiliedlarea of such stockline above which the localizeduexitgas temperature exceeds the average exitgas temperature.

Referencescited in the le of this patent UNITED STATES PATENTS 2,533,142 V-Royster A Dec. 5, 1950 2,417,049 'Bailey et-al .Q-- Mar. ll, 1947 2,616,678 'Grossman Nov. 4, 1952 2,627,399 ADe`Vaney Feb. 3, 1953

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