US3909189A

US3909189A - Process for conditioning sinter draft for electrostatic precipitation of particulate material therefrom

Info

Publication number: US3909189A
Application number: US303953A
Authority: US
Inventors: Thomas E Ban
Original assignee: Mcdowell Wellman Engineering Co
Current assignee: Davy McKee Corp; Mcdowell Wellman Engineering Co
Priority date: 1971-08-25
Filing date: 1972-11-06
Publication date: 1975-09-30
Anticipated expiration: 1992-09-30

Abstract

There is provided an improved traveling grate sintering process for treating solid mineral matter such as iron ore and utilizing a sinter draft system which recycles a part of the total draft, minimizes the exhaust draft effluent, and reduces the costs of draft treatment to meet environmental air quality standards. The system contemplated herein recycles relatively cool draft from the initial windboxes adjacent the traveling grate to the hood or hoods downstream of the path of grate travel to significantly decrease the concentration of hydrocarbons in the sinter exhaust gases and to produce a hotter, more humid exhaust for subsequent processing to remove pollutants, particularly with respect to the electrostatic treatment of the exhaust draft to remove solid particulate material. At the same time a sinter product comparable in quality to that produced by the basic downdraft sintering system is obtained without substantial change in rates of production.

Description

1 1 Sept. 30, 1975 United States Patent [19] Ban 1 1 PROCESS FOR CONDITIONING SINTER DRAFT FOR ELECTROSTATIC PRECIPITATION OF PARTICULATE MATERIAL THEREFROM [75] Inventor: Thomas E. Ban, South Euclid. Ohio [73] Assignee: McDowell-Wellman Engineering Company. Cleveland, Ohio [22] Filed: Nov. 6. 1972 [21] Appl. No.: 303,953

Related U.S. Application Data [63] Continuation-impart of Ser. No. 174,714. Aug. 25.

1971. abandoned.

[52] U.S. C1. 432/16; 75/5; 432/18; 432/72 [51] Int. C1. ..1F27B 15/12 [58] Field of Search 75/5: 432/16. 18. 72

[561 References Cited UNITED STATES PATENTS 1.836.176 12/1931 Klcncke 266/21 2.006.368 7/1935 Reid...... 75/5 X 2.861.881 11/1958 Phelps 75/5 X 3.043.677 7/1962 Monaghan 75/5 3.205.064 9/1965 Chang 75/5 3.257.195 6/1966 Schwarz 75/5 3.264.091 8/1966 Ban 75/4 X 3.311.465 3/1967 Ban et a1 75/5 Primary Examiner-John J. Camby Attorney. Agent. or FirmMcNenny, Farrington, Pearne & Gordon [57] ABSTRACT There is provided an improved traveling grate sintering process for treating solid mineral matter such as iron ore and utilizing a sinter draft system which recycles a part of the total draft, minimizes the exhaust draft effluent. and reduces the costs of draft treatment to meet environmental air quality standards. The system contemplated herein recycles relatively cool draft from the initial windboxes adjacent the traveling grate to the hood or hoods downstream of the path of grate travel to significantly decrease the concentration of hydrocarbons in the sinter exhaust gases and to produce a hotter. more humid exhaust for subsequent processing to remove pollutants. particularly with respect to the electrostatic treatment of the exhaust draft to remove solid particulate material. At the same time a sinter product comparable in quality to that produced by the basic downdraft sintering system is obtained without substantial change in rates of production.

10 Claims, 1 Drawing Figure COLD fETCI (ZE hard? US. Patent Sept. 30,1975

PROCESS FOR CONDITIONING SINTER DRAFT FOR ELECTROSTATIC PRECIPITATION OF PARTICULATE MATERIAL TI-IEREFROM RELATED APPLICATION This application is a continuation-in-part of my copending application Ser. No. 174,714 filed Aug. 25, 1971, now abandoned.

BACKGROUND OF THE INVENTION AND PRIOR ART The present invention relates to an improved sintering process. Generally speaking, the well-known process of sintering has heretofore comprised submitting a particulate metallic oxide, e.g. iron ore or iron oxide (as from basic oxygen furnace fume), in a burden on a moving grate to transversely moving hot oxidizing gases to fuse particles of the ore together to form an agglomerated mass. The agglomerated mass may then be used in a known manner in iron-producing equipment, e.g. a blast furnace. Frequently, the ores contained naturally or were made to contain a combustible material, e.g. coke or coal, and a flux, e.g. limestone and/or dolomite, and a burden of a granular mixture with or without added moisture deposited upon a traveling grate. In the initial stages of the movement of the burden along its predetermined path on the traveling grate, the burden was ignited by passing under gas torches or oil burners whereby ignition of combustibles and a flame front were established in the burden. Thereafter, by means of fans, ambient air was drawn downwardly through the burden as it moved along the path causing the flame front to move downwardly through the burden toward the grates. The temperature of the burden by this process was raised along the flame front to approximately 2500F. (lower or higher depending upon the ore), whereby the individual particles became fused into a solid foraminous mass. As the hot sinter was discharged from the end of the traveling grate, it was broken into large chunks. The residence time on the traveling grate was sufficient to cause the fusion to occur substantially through the depth of the burden. The gases which were exhausted from the lower side of the burden when downdraft sintering was employed were generally exhausted to the atmosphere. These gases in addition to containing entrained and fumed solid particulate material also contained unburned hydrocarbons, sulfur compounds, and oxides of carbon.

Recent efforts to control atmospheric pollution have necessitated reconsideration of the methods by which sinter exhaust gases are handled. To remove hydrocarbons and lower oxides of carbon, afterburner or incinerator means have been utilized to ignite the combustible components of the exhaust sinter gases and convert them to harmless gases, e.g. carbon dioxide and water vapor. However, afterburner and incinerator means typically employ hydrocarbon fuels which can result in an increase in the concentration of hydrocarbons in the exhaust gases. Entrained sulfur or lower sulfur oxides are also converted to higher oxides which may be removed by scrubbing means in a known manner. The large quantity of solid particulatematerial entrained in such exhaust sinter gases typically is passed through electrostatic means which effect precipitation of such particulate material in a known manner. Unfortunately, conventional sintering practice is such that the moisture content and temperature of the exhaust gases are not optimal for cleaning by electrostatic means. Thus, in order to improve the combustion and formation of sinter, particularly in the case of iron ore, the mixture of iron ore and carbonaceous material and optionally a fluxing material, e.g. limestone and/or dolomite, has usually been moistened with up to 6 to 14 percent by weight of water to form wet agglomerated masses of approximately the size of rice. This enables the use of finely divided carbonaceous material and facilitates the distribution thereof with respect to the iron oxide in a wet agglomerated mass. The use of water as a binder aid results, during sintering, in considerable moisture in the gases traversing the burden. If the burden is too deep for the draft system employed as the gases encounter the lower regions of the burden which are at a much lower temperature, the moisture picked up at or near the flame front is condensed by and saturates the lower regions of the burden. When the problem becomes acute, a condition known as sogging out occurs and the passage of the gases through the burden is greatly impeded. Hence it has been desired to severely limit the depth of the burden or to minimize the amount of moisture in the burden and in the gases traversing therethrough in order to approach the capacity of the apparatus. However, when the moisture content of the gases is low, the resistivity of the gases and particles is high and the effectiveness of electrostatic precipitation means is greatly reduced. Also, the temperature of the gases normally passing through the burden in conventional procedures and exiting from the lower side thereof is low (225 to 300F.). At both low moisture content and such relatively low temperature, the electrostatic precipitation characteristics are generally inferior.

Thus, conventional sintering of mineral oxidecontaining ores as currently practiced is confronted by a dilemma: If the moisture content of the gases is raised within the system to improve the operation of electrostatic precipitation equipment, sogging out of deep sinter beds is encountered. If moisture is added to the exhaust gases after sintering, dew point condensation is reached which causes fouling and corrosion of the apparatus. While the capacity can be cut by using thinner beds or burdens to raise the moisture content of the exhaust gases, such cuts tend to be too severe for economical operation and the moisture pick-up per unit volume of gas because of the lowered quantity of water available in thinner beds is still below optimum for electrostatic precipitation techniques. Also, the temperature of the exhaust sinter gas is below optimum.

It has now been found that relatively deep horizontally disposed burdens of sinterable particles, comprised ofa metal oxide, shale, or clay, optionally a flux, a combustible carbonaceous material, and water, on a traveling grate of either linear or circular confirguration, may be sintcred economically by the process of the present invention, which includes the steps of igniting the burden and passing oxidizing gases through the burden to generate a transversely moving heat front in the burden while moving the burden horizontally along a predetermined path or length of travel. In accordance with the present invention, sogging out of the burden is avoided, pollutants in the exhaust gases are minimized, and the exhaust gases are conditioned for more effective electrostatic precipitation of entrained particulate solids by collecting moisture-laden gases exiting through the burden in a first portion of the path and recycling at least a portion thereof through a burden in the subsequent portion of the path. The exhaust sinter gases exiting from such subsequent portion are now found to possess substantially reduced hydrocarbon concentrations and more nearly optimum characteristics for treatment in electrostatic precipitation means. While recycling through sinter burdens is not per se new (see the patents to Dwight US. Pat. Nos. 1,215,636 and 1,215,637 and to Klencke US. Pat. No. 1,836,176), the ores so processed were sulfide ores naturally containing combustible sulfurous compounds, the sintering operation produced primarily sulfur dioxide and sublimed sulfur, but little or no hydrocarbon pollutants in the exhaust drafts, and there was no utilization of moisture for draft treatment. Dry gases cannot properly be electrostatically treated because the resistivity is too high. The technology with respect to sintering, on the other hand, has indicated it to be contrary to good sintering practice to introduce water vapor into a hood in a sintering process (see Agglomeration by Knepper, Interscience Publishers, 1962, Pages 464 466). It is also shown that the sinter output is decreased with an increase of the water vapor in the sintering air or gases.

The present process is, as indicated above, characterized by taking moisture-laden gases from an initial portion of the path traveled by the burden and deliberately recycling these relatively cool gases, preferably, after first passing them through a primary cleaner, into a hood and through the moving sinter bed or burden at a succeeding portion of the path traveled by the burden. 1

Those gases exhausting from the succeeding portion of the bed. as has been noted above, contain reduced hydrocarbon levels, have a high humidity, i.e. from about 5 to percent water by volume, are at an increased temperature (4()0 600F.), and are dustladen. The gases can be passed directly to an electrostatic precipitator for precipitation of the dust particles, the increased moisture content and temperature of the gases making them more amenable to such treatment. Alternatively, the exhaust gases flowing from the bed can be passed through a dust-collecting device to remove the coarser particles, then through an afterburner to burn any remaining hydrocarbons and a cooler to adjust the temperature to that which is proper for electrostatic precipitation. When either of these methods of conditioning exhaust sinter gas has been practiced, both the moisture content or humidity of the gas and the temperature thereof are more nearly optimum for proper operation of electrostatic precipitation equipment. No decrease in the rate of sinter production is observed, but numerous advantages, particularly from a pollution standpoint, are achieved. As indicated, electrostatic precipitation of solid particulate material is much more efficient. Also, it is apparent that the gases are sufficicntly conditioned for initial charging to the electrostatic precipitator and final charging to the afterburner. The recycling of the gases and the concomitant repassage of hydrocarbon, carbon and carbon monoxide gases through the sinter bed achieves combustion of the hydrocarbon moieties and conversion of the carbon and carbon monoxide to harmless carbon dioxide and water, and raises the temperature above the usual 250F. average. Sulfur impurities are also oxidized and may be scrubbed from the exhaust gases by suitable scrubbing means. Thus, both the desired objectives of economical sinter production and pollution control are accommodated by the present process. Additionally, the volume of effluent gas is reduced by the amount recycled, aiding not only in respect of atmospheric pollution but also in cost of handling gases.

The present invention may be practiced while using the sintering machine only for sintering and terminating the operation when the flame front reaches the grates simultaneously with the discharge of the hot sinter from the machine. In such case the sinter may be subsequently cooled by a separate cooler machine using forced ambient air as the cooling medium. If the sintering machine is extended to provide a cooling zone therein beyond the sintering zone, this cooling zone does not give rise to entrained hydrocarbons within the gases because sintering is completed before the cooling zone, whereupon the charge becomes completely depleted of hydrocarbonaceous material. However, if recycle draft containing entrained hydrocarbons is used as the cooling medium and is directed through the hot sintered ore layer in the sinter zone, the hydrocarbons still present are reduced and provide an exhaust gas depleted of entrained hydrocarbons.

BRIEF STATEMENT OF THE INVENTION Briefly stated, therefore, the present invention is in a process for sintering a burden of particles disposed on a traveling grate machine, such particles comprising any mineral or ore, e.g. iron ore, iron oxide, shale, or clay, a combustible carbonaceous material, e.g. coal, coke, coke breeze, tar residues, lignite, etc., and water, and optionally a flux, by igniting the burden and passing oxidizing gas, e.g. ambient air, through the ignited burden, preferably at a rate of from 1 5 lbs. of gas to each pound of burden, to maintain combustion of the carbonaceous material and generate a transversely moving heat front in the burden while moving the burden along the traveling grate through a combustion zone. The improvement of the invention comprises collecting moisture-laden exhaust gases and entrained combustible matter (both gaseous and particulate solid material) from an initial portion of the length of the combustion zone, recycling at least a portion thereof through the burden in a subsequent portion of the combustion zone to incinerate the entrained combustible matter within the bed, and withdrawing exhaust gases from the bed along the subsequent portion of the com-. bustion zone, whereby the latter exhaust gases from the subsequent portion of the combustion zone have a substantially reduced content of combustible matter and a substantially increased moisture content and are, therefore, more amenable to economical further treatment for incinerating any residual combustible matter therein in an afterburner or the like and for removal of entrained particulate matter in an electrostatic precipitator or the like.

Reference may again be had to Agglomeration by Knepper, supra, and particularly to the paper contained therein entitled The Continuous Sintering Process-Research and Applications" by Ban, Czako, Thompson and Violetta (pp. 51 l to 536) for a discussion and understanding of the concept of the flame front in a sinter bed. The usual vertical propagation rate through the burden of the flame front is in the range of from about 0.25 inch to 2 inches per minute. Many parameters affect the rate including oxygen content of the gas, fuel distribution, gas flow rate, moisture content of both the bed and the gases, porosity of the bed, etc. 1

BRIEF DESCRIPTION OF THE DRAWINGS In the annexed drawing there is diagrammatically shown apparatus suitable for carrying out the improved process of the present invention. The drawing shows a traveling grate moving along a predetermined path in a direction from right to left and having 16 windboxes disposed in underlying relation to the grate, and a gas hood disposed above the grate adjacent the terminal portion into which cold wet recycle gases are introduced. By cold in reference to these gases is meant from 150 to 300F.

DETAILED DESCRIPTION OF THE PROCESS As indicated above, the burden treated in accordance with this process is formed of particles of a plurality of materials. The solid materials are provided in the form of granules generally having a particle size in the range of one-fourth inch to fines. Thus, for example, granular iron ore, shale, or clay is mixed with a granular combustible carbonaceous material, e.g. coal, coke, coke breeze, lignite, returns, optionally flux material, e.g. limestone, etc. The proportions of these solid materials are generally in the range of from 2 to percent by weight of coke or.coal or other such carbonaceous material on the dry basis. In order to properly agglomerate the carbonaceous material and the metal oxide-containing moiety, a liquid binder may be used. The liquid binder composition comprises mainly water although, for example, an asphalt emulsion may be used. The amount of moisture should be in the range of from 6 to 14 percent by weight.

The particles from which the burden is made may be that which result from combining the essential ingredients above mentioned and tumbling in a drum or by treating in a pelletizing apparatus such as that described in US. Pat. No. 3,169,269 dated Feb. 16, 1965. The particles are desirably shaped as kernels of rice and have approximately that size. This condition is known in the trade as rice-like texture.

A typical formulation on the dry basis for the moist or green burden material to be submitted to the sintering process of the process invention is as follows:

Iron ore 3271 Mill scale 1071 Flue dust 27 Flux 23% Coke breeze 5% Returns 28% The dry materials are blended with water in any suit able blending or pelletizing apparatus to a moisture content in the amount of from 6 to 9 percent by weight, e.g. 6 percent. Any composition including a metal oxide moiety, optionally a flux material, and a carbonaceous material and moisture for sintering, numerous examples of which are known to those skilled in the art,

' may be used in the practice of this invention, such composition not being critical to the process in any respect.

In one embodiment of the sintering process of the present invention the moving burden is ignited at the uppermost layers thereof to initiate burning of carbon or other combustible carbonaceous material contained therein. Gases containing a sufficient amount of oxygen, e.g. air, are thereafter drawn downwardly through the bed to perpetuate the burning and cause vertical propagation of the heat front or flame front downwardly toward the grates. Where the metallic oxide moiety comprises iron ore, the temperature within the bed in the heat front approaches 2500F. Where clay is the metallic oxide-containing moiety, the temperature reaches approximately 2200F. Under these conditions, the granular mineral material is partially fused to form a foraminous cake of agglomerated sintered material. The process is distinguished from metallizing or reducing reactions because it is carried out in the presence of an oxidizing draft as distinct from a reducing draft. As the flame front moves through the bed, hot gas immediately ahead thereof effects volatilization or fuming of nonferrous metal oxides and a volatilization, distillation, or pyrollization of organic materials from the bed which are picked up by the moving gas and entrained therein. In general, organics are not burned in the draft stream below the flame front because the temperature and the oxygen content of the gas at this point is insufficient for combustion. With reference to the drawing, these phenomena are occurring within the burden adjacent windboxes 1 through 15. If sintering terminates adjacent windbox 15, the last windbox 16 is desirably used for cooling.

Referring now more particularly to the diagrammatic and schematic representation in the annexed drawing, a moist burden of blended particulate material having a depth of from about 5 to about 15 inches (the latter being regulated by conventional gating means) is cast upon the traveling grate (also of conventional design) adjacent windbox 1 and proceeds from there toward the discharge end as indicated in the drawing. The longer the path, the more rapid the movement required for the burden. While passing over the windboxes l and 2, the burden is ignited in the upper layers by means of gas torches or burders (not shown). This causes preliminary drying of the surface of the particulate burden and elevates the temperature of the burden sufficiently that in the portion of the path traveled by the burden above windboxes 3 6 burning of the fuel moiety is establishcd. Air downwardly drawn through the burden by means of the recycle fan promotes such combustion. In the initial portion of the total horizontal path traversed by the burden and indicated by windboxes 1 6, approximately 50 to percent of the water in the burden is removed by the gases. The gases drawn downwardly through the burden by means of the recycle fan at a rate producing a substantially violent draft, the rate being approximately from 0.5 2.5 pounds of gas per pound of burden treated. The rate of flow of the gases through the burden is in the range of from 300 standard cubic feet per minute per square foot of grate area (SCFM). Because of this relatively high flow rate, fine particles of aggregate and decomposition products thereof are mechanically entrained in the gas and pass through the bed. An initial dust collector 21 in series with the manifold connecting windboxes l 6 aids in elimination of some of the coarser particles, which particles may comprise a portion of returns" in subsequent green burden formulations. The temperature of the gases at this point is about 200F.a relatively cold recycle gas. These gases are propelled by means of a recycle fan 22 to a subsequent portion of the path traveled by the burden wherein the major portion of sintering actually occurs. By the time the burden enters the region above windbox 7, the flame front has been established due to the initiation of the combustion in the upper layers of the burden. It continues its downward propagation through the bed as the bed proceeds from right to left as shown in the drawing.

The cold recycle gas contains a substantial quantity of moisture and also contains unburned hydrocarbons, carbon monoxide, and dust particles. The recycle fan introduces such cold recycle gas into a hood 23 spanning windboxes 7 through in the embodiment shown. Under the influence of a sinter fan 24 in series with a header 26 joining windboxes 7 15 inclusive in the illustrated example the cold recycle gas is drawn downwardly through the sinter bed again at the rate which is on the order of from 0.5 2.5 pounds of exhaust gas per pound of solids treated.

The gases introduced into the hood 23 above the sinter burden for passage through the burden are very moist. Contrary to the previous experience, sogging out is not experienced. Perhaps this condition is alleviated by large gas flow. Hydrocarbons contained in the cold recycle are burned along with unburned carbonaceous material in the burden. Particulate material becomes entrained in the burden due to the fused condition of the burden at progressively deeper locations within the body of the burden as the burden moves from right to left. The balance of the moisture in the burden, i.e. approximately 50 to 10 percent, is also removed in this region.

As indicated above, the sinter exhaust draft is drawn downwardly by means of the sinter fan 24 and optionally, though preferably, through a dust collector 27 as also indicated in the drawing. These gases are of substantially reduced hydrocarbon content and increased temperature, and are oxygen-depleted. They may be conducted directly to an electrostatic precipitator or, alternatively, are introduced into an afterburner, as shown in the drawing, to further condition the gas for discharge to the atmosphere. The afterburner oxidizes carbon monoxide, burns any residual hydrocarbons or entrained carbonaceous material, and oxidizes any lower oxides of sulfur or entrained elemental sulfur. Natural gas and primary air may be introduced into the afterburner to aid combustion. The temperature of the gas entering the electrostatic precipitator or the afterburner is much higher, i.e. on the order of 400 to 400F. instead of the usual 250F. average obtained in the conventional process. Thus, where passage through an afterburner is contemplated, the amount of fuel and temperature elevation (up to about l500F.) required to effect afterburning is reduced.

Upon leaving the afterburner, the gas enters a gas cooler which may be in the form of a scrubbing tower whereby water-soluble components which would otherwise be discharged into the atmosphere as pollutants are efficiently removed. Alternatively, and preferbly, the cooling apparatus may involve heat transfer from the gas to the water through a barrier to maintain the gas and the water entirely separate. In this way watersoluble salts, for example, do not become cooling water pollutants, but are carried along in the gas stream for later removal in the electrostatic precipitator. By means of the cooler, the excessively high temperature of the gas exhausting from the afterburner is lowered to an optimum point for particulate solid removal in an electrostatic precipitator, i.e. to about 400 to 500F. instead of the usual 250F. average in conventional processes. The moisture content or humidity of the gases at this point is approximately 30 percent to 40 percent volume. The elevated temperature and the higher moisture content are conditions of the gas most favorable to successful electrostatic precipitation of entrained solid particulate material. The resistivity of the gas at these conditions is optimum. Thus, the gas is finally passed through an electrostatic precipitator diagrammatically illustrated beyond the cooling apparatus for exhaust therethrough to the atmosphere. The electrostatic precipitation apparatus is of conventional design and its operation is well known to those skilled in the art. An afterburner is not-required if the exhaust draft is sufficiently conditioned for electrostatic precipitation without an afterburner. Also, it is apparent that the exhaust draft may initially.;be charged to the electrostatic precipitator and finally to an afterburner.

In order to further cool the burden just prior to discharge of the sinter cake, primary air may be' drawn downwardly through the burden by means of a primary air fan 28 cooperating with windbox 16. Conventional dust-collecting means 29 may be desirably inserted in this line. This primary air, now w armed by passage through the hot burden and containing a higher percentage of oxygen thant he sinter exhaust, may be introduced in the afterburner along with natural gas if necessary to effect combustion of combustible components, solids, or gases in the sinter exhaust stream.

Also as a modification, a longer cooling zone can be extended beyond windbox l6 and some or all'of the draft, still containing some hydrocarbons, from windboxes 7 through 15 can be routed through this longer cooling zone as indicated by thedotted line 31 inthe drawing thereby thoroughly burning the hydrocarbons therein by contact with .hot sinter before this draft is finally exhausted to the electrostatic. precipitator as hydrocarbon-depleted gas, as indicated by dotted line 32 in the drawing- In this .case, the afterburner, cooler and electrostatic precipitator may be employed in series in the same manner previously described, as indicated by the solid'line 33in the drawing, or the afterburner may be bypassed asindicat'ed by the dotted line 34 in the drawing, or both the afterburner and cooled may be bypassed, as indicated by the line 32 in the drawing, as circumstances may warrant.

In a typical example for the system of FIGURE 1, the cold recycle amounts to 81,500 ACFM (actual cubic feet per minute) at a temperature of 230F. It analyzes Water 7 .771 Oxygen 15.8% Carbon dioxide 4.1% Carbon monoxide 0.8%

The sinter exhaust amounts to 136,000 ACFM at a temperature of 530F. The sinter exhaust analyzes as follows:

Water 14.3%

Oxygen 12.6% Carbon dioxide l4.l7r

Carbon monoxide 1.28%

The afterburner exhaust amounts to 318,000 ACFM at a temperatureof 1'400F. These gases analyze:

A typical exhaust from the cooler amounts to 213,000 ACFM at a temperature of 450F. This gas analyzes:

Water 36.671 Oxygen 7 .671 Carbon dioxide 6.671 Carbon monoxide 0.0%

The primary air in atypical example has a volume of 38,800 ACFM and exhausts at a temperature of 690F. These gases analyze as:

Water 0.0%

Oxygen 20.3%

Carbon dioxide 3.5%

Carbon monoxide By recycling sinter gases according to this invention, the total throughput of gas relative to the prior sintering process is about halved and the temperature of the exhaust gas nearly doubled, i.e. up to as much as 550F. Thus, since the initial moisture in the burden is entrained in approximately half the draft volume, the water content of the draft reaching the electrostatic precipitator according to this invention is virtually doubled. The effect a of this procedure is, as indicated above, to decrease the resistivity of the gas/dust particle medium and render the exhaust gas more favorably conditioned for treatment by electrostatic procedures.

This process accomplishes still another very desirable form of exhaust gas treatment. Current regulations are trending toward the direction of prohibiting the exhausting of any hydrocarbon or carbon-containing gaseous material, e.g. carbon monoxide, from a sinter process. In order to meet these requirements, it has been suggested that conventional sintering processes include an incinerator or afterburner piece of equipment to enable elevation of the temperature of the conventional sinter exhaust from the usual 250F. to as high as l300 or 1400F. This must be coupled with the introduction of air to effect combustion of these combustible ingreclients. In the present process, however, since the gases have been recycled, approximately 50 90 percent of the combustible materials will undergo incineration within the body of the sinter bed, i.e. in the regions spanned by windboxes 7 in the drawing. Therefore, there is a greatly reduced volume of these moieties to be incinerated by some form of aftertreatment. Moreover, the gases will exhaust at a temperature considerably higher than the usual 250F. average in prior processes, as well as in lesser volume, so that where an afterburner is employed the cost of fuel to raise the gases to incineration temperatures will be considerably less, and the amount of oxygen-containing gas required to effect combustion will also be greatly reduced, which further conserves fuel and other costs.

The terminal windbox 16 or windboxes are for the most part performing primarily the function of cooling the sinter material, and combustion is practically completed ahead of this portion of the path which the bur den traverses. Accordingly, the exhaust gases from this portion of the path largely consist of heated air which can serve as primary air of combustion in an afterburner as previously described. This further conserves heat and effects economies in the system. Also, as previously described, the terminal windboxes can be used for further incinerating exhaust gases by passing them through the cooling zone to effect combustion of the residual entrained hydrocarbons.

One of the truly surprising discoveries attendant the process of the present invention is that the recycle gases rich in hydrocarbons passing through the flame front, where the total time of passage is measured in milliseconds achieves substantially complete combustion of the burnable or combustible content of the gases within the body of the burden. Were these hydrocarbons to be treated in afterburner equipment, such equipment would have to be designed to a retention time of approximately 1 to 5 seconds in order to achieve complete combustion. It is believed that the presence of freshly formed metallic oxides in the sinter bed catalyzes the oxidation of the hydrocarbons in the gas stream. I

The significantly reduced concentration of hydrocarbons in the sinter exhaust gases attendant applicants process as compared to conventional sintering processes is demonstrated by the following: A sinter burden was prepared having the following composition on a natural basis, i.e. including water: classifier iron ore, 65 percent; manganese ore, 1 percent; limestone, 7 percent; dolomite, 7 percent; flue dust and sludge, 5 percent; coke breeze, 5 percent; and scale, 10 percent. The burden was sintered on a conventional Dwight- Lloyd sintering machine, having eight windboxes for providing draft.

In a first run no recycling was performed and exhaust gases from all eight windboxes were delivered to a main exhaust duct and the composite gas analyzed. The concentration of hydrocarbons in the analyzed gas was such that about 13.2 lbs. of hydrocarbons were discharged per hour.

In a second run the exhaust gases from the first three windboxes adjacent the feed end of the sintering machine were collected and recycled downwardly through the burden passing over the next four windboxes. The other operating conditions of both runs were essentially the same. However, the concentration of hydrocarbons in the composite gas exhausting from those four windboxes and the last windbox (to which gas was not recycled) was such that only about 3.0 lbs. of hydrocarbons were discharged per hour. This represents a decrease in hydrocarbon discharge of about 77 percent, achieved with asingle recycling phase. It is clear that these phases may be multiplied to recycle more frequently from one portion of the bed to a subsequent portion or portions as a multiphase system.

In the previous discussion, reference has been had generally to electrostatic precipitators. While the process may be practiced with both of the known types of electrostatic precipitators, i.e. the wet and dry types,

the process is most beneficial in conjunction with the dry-type electrostatic precipitators. Here, hot humid draft having in excess of about 10 percent humidity and a temperature in excess of about 400F. is produced by the process, which conditions are most favorable for dry electrostatic precipitation. The particulate material entrained in the draft comprises not only particles of the ore and sinter material which have been mechanically entrained, but also particles of volatile metal and particles of metal salts, usually water soluble. These metal salts are preferably precipitated by the dry electrostatic precipitator systems. In the latter, accumulations of particulate material upon the charged plates are removed by vibratory means. The high humidity and high temperature of the gas being treated aids in the conductivity of build-up of particulate material, this improving the operation of the precipitator. However, because of the high temperature, the particles are recovered in relatively dry form and may be disposed of without contamination of aqueous washing media such as are employed in wet electrostatic precipitation. The dry material recovered from the electrostatic precipitator may be sold, recycled or buried for disposal purposes.

Electrostatic precipitation, as is well known, contemplates passing particle laden gas through a highly charged electrostatic field between metallic conductors. The particles move toward a plate of opposite charge and are discharged on the surface thereof. As

the layer of discharged particles builds up, efficiency begins to decline and the particles must be removed periodically. This may be done ,by vibrators which shake the plates to dislodge accumulated particles, or by water washing. The former is the dry process, and the latter, the wet process as above mentioned. It has been found that with burdens containing high concentrations of flux materials to 30 percent) sharp, heavy blows are more efficient in cleaning the electrodes and plates than high frequency rapping.

What is claimed is:

1. In a process for sintering a bed of particulate mineral material mixed with combustible carbonaceous material and water while moving the bed on a traveling grate and while burning the carbonaceous material in the bed to supply heat thereto, wherein carbonaceous material in the bed adjacent one surface thereof is ignited during a first portion of its travel and combustion thereof progressively through the bed is thereafter maintained during its continued travel through an elongate combustion zone by passing oxidizing gases into the bed over the length of said combustion zone and exhausting from the bed the gaseous products of combustion of said carbonaceous material together with entrained combustible matter derived from the bed that includes a substantial amount of hydrocarbons, the exhaust gases from an initial portion of the length of said combustion zone also entraining and removing a major portion of the initial water content of the bed; the improvement comprising collecting the exhaust gases and entrained matter exhausted from the bed along said initial portion of said combustion zone and recycling the same into the bed along a subsequent portion of said combustion zone to incinerate the entrained combustible matter within the bed while withdrawing exhaust gases from the bed along said subsequent portion of the combustion zone, whereby the latter exhaust gases from the subsequent portion of said combustion zone have a substantially reduced content of combustible matter and a substantially increased moisture content.

2. An improved sintering process in accordance with claim 1 in which the gas flow through the bed over the length of said combustion zone is downward.

3. An improved sintering process in accordance with claim 1 in which the oxidizing gases and recycled gases are passed downwardly into the bed from the upper side thereof.

4. An improved sintering process in accordance with claim 1 in which said latter exhaust gases are passed through an afterburner to incinerate and further reduce their content of combustible matter.

5. An improved sintering process in accordance with claim 1 in which said latter exhaust gases are cooled to condition them for electrostatic precipitation of particulate matter entrained therein.

6. An improved sintering process in accordance with claim 1 in which said latter exhaust gases are passed through an afterburner to incinerate and further reduce their content of combustible matter and then through a cooler to condition them for electrostatic precipitation of particulate matter entrained therein.

7. An improved sintering process in accordance with claim 6 in which air is passed through the bed along a terminal portion of its travel to cool the sintered bed and preheat the air, and a portion of the thus preheated air is introduced into said afterburner'to support the combustion of combustible matter therein.

8. An improved sintering process in accordance with claim 6 in which air is passed through the bed along a terminal portion of its path of travel to cool the bed and preheat the air, and the preheated air is introduced into said afterburner to support the combustion of combustible matter therein.

9. An improved sintering process in accordance with claim 1 wherein the gas flow through the bed over the length of said combustion zone is downwardly there.- through from the upper surface thereof, said latter exhaust gases are passed through an afterburner to incinerate and further reduce their content of combustible matter, and gas exiting from the afterburner is cooled to further condition it for electrostatic precipitation of particulate matter entrained therein.

10. An improved sintering process in accordance with claim 9 in which air is passed through the bed along a terminal portion of its path of travel to cool the bed and preheat the air, and the preheated air is introduced into said afterburner to support the combustion of combustible matter therein.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENTNO. 9,189

DATED I September 50, 1975 INVENTOR(S) 1 Thomas E, Ban

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown betow:

Column 3, Line 1, after "through" change "a" to the Column 5, Line 48, after of the change "process" to present Column 6, Line 50, change "latter" to depth Column 7', Line #7, change tOOF. to 600%. ---7 Line 56 change "preferbly" to preferably Column 8, Line 5, after "percent" insert by Line 51, the phrase 'Istill containing some hydrocarbons,"

should appear after "15" in line 52;

Line 45, change "cooled" to cooler Column 9, Line 29, after half the insert former Column 11, Line 8, change "this" to thus i Signed and Scaled this twenty-third Day Of December 1975 [SEAL] A ttest:

RUTH C. MASON Arresting Officer C. MARSHALL DANN Commissioner ofParents and Trademarks

Claims

1. IN A PROCESS FOR SINTERING A BED OF PARTICULATE MINERAL MATERIAL MIXED WITH COMBUSTIBLE CARBONACEOUS MATERIAL AND WATER WHILE MOVING THE BED ON A TRAVELING GRATE AND WHILE BURNING THE CARBONACEOUS METERIAL IN THE BED TO SUPPLY HEAT THERETO, WHEREIN CARBONACEOUS MEATERIAL IN THE BED ADJACENT ONE SURFACE THEREOF IS IGNITED DURING A FIRST PORTION OF ITS TRAV EL AND COMUSTION THEREOF PROGRESSIVLY THROUGH THE BED IS AND COMBUSTION THEREOF PROGRESSIVELY THROUGH THE BED IS THEREAFTER MAINTAINED DURING ITS CONTINUED TRAVEL THROUGH AN ELONGATE COMBUSTION ZONE BY PASSING OXIDIZING GASSES INTO THE BED OVER THE LENGTH OF SAID CONBUSTION ZONE AND EXHAUSTING FROM THE BED THE GASSEOUS PRODUCTS OF COMBUSTION OF SAID CARBONACEOUS MATERIAL TOGETHER WITH ENTRAINED COMBUSTIBLE MATTER DERIVED FROM THE BED THAT INCLUDES A SUBSTANTIAL AMOUNT OF HYDROCARBONS, THE EXHAUST GASSES FROM AN INITIAL PORTION OF THE LENGTH OF SAID COMBUSTION ZONE ALSO ENTRAINING AND REMOVING A MAJOR PORTION OF THE INITIAL WATER CONTENT OF THE BED: THE IMPROVEMENT COMPRISING COLLECTING THE EXHAUST GASSES AND ENTRAINED MATTER EXHAUSTED FROM THE BED ALONG SAID INITIAL PORTION OF SAID COMBUSTION ZONE AND RECYCLING THE SAME INTO THE BED ALONG A SUBSEQUENT PORTION OF SAID COMBUSTION ZONE TO INCINERATE THE ENTRAINED COMBUSTIBLE MATTER WITHIN THE BED WHILE WITHDRAWING EXHAUST GASSES FROM THE BED ALONG SAID SUBSEQUENT PORTION OF THE COMUSTION ZONE, WHEREBY THE LATTER EXHAUST GASSES FROM THE SUBSEQUENT PORTION OF SAID COMBUSTION ZONE HAVE A SUBSTANTIALLY REDUCED CONTENT OF COMBUSTIBLE MATTER AND A SUBSTANTIALLY INCREASED MOISTURE CONTENT.

7. An improved sintering process in accordance with claim 6 in which air is passed through the bed along a terminal portion of its travel to cool the sintered bed and preheat the air, and a portion of the thus preheated air is introduced into said afterburner to support the combustion of combustible matter therein.

9. An improved sintering process in accordance with claim 1 wherein the gas flow through the bed over the length of said combustion zone is downwardly therethrough from the upper surface thereof, said latter exhaust gases are passed through an afterburner to incinerate and further reduce their content of combustible matter, and gas exiting from the afterburner is cooled to further condition it for electrostatic precipitation of particulate matter entrained therein.