US3700564A - Continuous process of producing shaped metallurgical coke - Google Patents

Abstract

IMPROVED PROCESS OF PRODUCNG SHAPED COE ARTICLES MADE BY PELLETIZING COAL WITH OR WITHOUT ADMIXTURE THEREWITH OF COKE AND/OR MINERALS SUCH AS ORE, WHICH PROCESS CARBONIZES THE SHAPED ARTICLE IN A FIRST STAGE FOR ABOUT 2 TO 5 MINUTES WITH A CONCURRENT SOLID, PARTICULATE HEAT CARRIER AT ABOUT 500 TO 600* C.; A SECOND STAGE FOR ABOUT 30 TO 90 MINUTES WITH A COUNTER-CURRENT OR TRANSVERSE CURRENT

SOLID, PARTICULATE HEAT CARRIER HAVING AN INITIAL TEMPERATURE OF ABOUT 400 TO 500* C. INCREASING TO A FINAL TEMPERATURE OF 500 TO 600* C.; AND THEN IN A THIRD STAGE WITH A COUNTERCURRENT OR TRANSVERSE CURRENT SOLID, PARTICULATE HEAT CARRIER HAVING AN INITIAL TEMPERATURE OF ABOUT 550 TO 650* C. INCREASING TO A FINAL TEMPERATURE OF 850 TO 1000* C.

Description

Oct. 24, 1972 J. wlLLlBALD ErAL 3,700,564

CONTINUOUS PROCESS OF PRODUCING SHAPED METALLURGICAL COKE Filed Sept. 4, 1969 www .r w C1 N Hrw a T qmwT m n n In Tow United States Patent O U.S. Cl. 201-6 5 Claims ABSTRACT OF THE DISCLOSURE Improved process of producing shaped coke articles made by pelletizing coal with or without admixture therewith of coke and/or minerals such as ore, which process carbonizes the shaped article in a iirst stage for about 2 to 5 minutes with a concurrent solid, particulate heat carrier at about 500 to 600 C.; a second stage for about 30 to 90 minutes with a counter-current or transverse current solid, particulate heat carrier having an initial temperature of about 400 to 500 C. increasing to a final temperature of 500 to 600 1C.; and then in a third stage with a countercurrent or transverse current solid, particulate heat carrier having an initial temperature of about 550 to 650 C. increasing to a final temperature of 850 to 1000 C.

This invention relates to the production of shaped metallurgical coke from coals having dilferent caking and expansion properties. The present invention refers to a process of continuously producing shaped metallurgical coke by a carbonization of shaped articles which have been made by a pelletizing or briquetting operation from ground coal, with optional admixtures of ne coke or minerals, and a binder, which carbonization is effected with the aid of a dine-grained, solid heat carrier.

Shaped metallurgical coke of uniform particle size is increasingly preferred by the iron and steel industry because the use of preshaped or pelleted ore and of uniform pieces of sinter for charging blast furnaces is increasing and the efficiency of the blast furnaces is increased by the use of coke having a uniform particle size. For use in metallurgical furnaces, the shaped charge stock, whether ore or coke, must have a high wear resistance, resistance to deformation, and crushing strength.

To enable an operation of a blast furnace at its highest etliciency, it is desirable to use a shaped coke which has the same weight per piece as the preshaped ore. lExperience has shown that the most desirable weight of the shaped coke is about 30-50 grams per piece. The shaped particles or pieces should have high crushing and abrasive strengths and should have no protruding edges. They should have such a shape as to promote the formation of relatively large voids in the blast furnace charge. A shape which is particularly desirable from this aspect resembles a pillow or loaf of bread and has slightly rounded edges.

Various processes for producing shaped coke are known in which briquettes or pellets made from coal or a coalcoke mixture are mixed with hot, dine-grained heat carriers, such as sand or other fine-grained materials. This mixing results in heating of the briquettes or pellets. In other known processes, shaped pieces of coal or coke are embedded in sand and are carbonized by the action of hot gases to form a smokeless fuel. The shaped pieces to be carbonized are made from comminuted coal or from lowtemeprature coke or mixtures thereof. The coal and coke may consist of mixtures of components differing in origin. Fine-grained ore or mineral additives may be added to the carbonaceous material which is to be briquetted. The

3,700,564 Patented Oct. 24, 1972 ICC shaping is effected by a pelletizing or briquetting operation with an addition of pitch, bitumen, bentonite, sulfite solution or other suitable known binders.

When it is desired to produce a shaped coke article which burns without forming smoke and soot and may be used as a fuel in heat-generating furnaces, it is suicient to carry out a heat treatment with oxygen-containing gases at low temperatures up to about 350 under oxidation conditions. The resulting briquettes have large residual contents of volatile constituents .and Ia relatively low strength. For this reason they cannot be used for metallurgical purposes. When it is desired to produce shaped metallurgical coke, particularly blast furnace coke, a very high-temperature carbonization up to end temperatures between 800 and 1000 C. has been found to be essential.

A known process for the high-temperature carbonization of fuel briquettes with the aid of solid heat carriers utilizes a shaft furnace, in which highly heated sand and the shaped carbonaceous articles are jointly moved in `downwardly directed, concurrent streams. As the briquettes enter the furnace, they are immediately subjected to the highest treating temperatures. The rate of temperature rise at the surface of the briquettes is initially very high and decreases as the temperature equalization between the sand and the shaped carbonaceous articles and between the surface and core of the shaped articles proceed. This treatment is satisfactory only with a few fuels, particularly with low-gas fuels, and the shaped coke made therefrom has in most cases only a moderate wear resistance. When shaped articles are used which have been made from coals containing more volatile constituents, the shaped articles burst whereby producing small particle sized coke or a shaped coke which has much reduced mechanical strength properties as a result of internal stresses. For this reason, the above-mentioned process of carbonizing shaped articles has been carried out in two states with the shaped articles and the sand moved in countercurrent streams with respect to each case. The same disadvantages will arise when pellets of coal or of mixed coal and ore are charged in a rotary kiln which is heated with combustion gases and where the pellets are embedded in a bed which is at a temperature of at least about 500 C. and in which the pellets are heated to an end temperature of about 850 C. at a rate of about 30- 50 C. per minute.

U.S. Pat. 3,018,226 describes a process in which briquettes made from a mixture of caking bituminous coal and non-caking coal or coke and of pitch as a binder are carbonized by irst subjecting them to a heat shock so that the surfaces of the briquettes substantially immediately assume a temperature of between about 480 and 680 C., and the heating of the briquettes is continued so that the surface of the briquettes remains in that temperature range until the temperature has been equalized throughout the interior of the briquettes. The briquettes are subsequently heated to temperatures in excess of 760 C. until the content of volatile constituents in the shaped coke article has been reduced to below 2%. In this process, the briquettes may be heated with hot, inert gases or with fine-grained heat carriers and are preferably heated in the rst stage in a fluid-ized bed.

U.S. Pat. 3,018,227 describes a process of producing shaped coke articles in which briquettes made from a mixture of l0-35% by weight of a moderately caking bituminous coal, -45% by weight of coke produced in a fluidized bed, and 6-20% by weight of pitch, are rapidly heated to a temperature which is higher than the plastic range of the briquetting mixture but lower than 677 C. and are carbonized until the temperature has been equalized throughout each briquette, whereafter the briquettes are subjected to dry distillation at a higher temperature.

We have found that such a heat shock treatment cannot be generally applied to briquettes made of all materials but will be withstood only by moderately caking briquette mixtures. When briquette mixtures containing more highly caking coal are shocked (suddenly heated), they tend to form bubbles and foam in their interior to an increased degree and often burst. Even if they do not burst, they are often so damaged by heat cracks that they break and splinter easily in further use. If the briquette mixture is so lean that it will withstand the treatment involved in that procedure, e.g. as a result of an addition of non-caking, low-gas coke or coke, the wear resistance and the crushing strength of the resulting coke shaped article will be much reduced so that it can no longer be used in a blast furnace.

It is an object of this invention to provide a novel method of carbonizing carbonaceous shaped articles which is less subject to the disadvantages of prior art techniques.

It is another object of this invention to provide an improved coke article.

`Other and additional objects of this Iinvention will become apparent from a consideration of this entire specification including the drawing and claims hereof.

In accord with and fulfilling these objects, one aspect of this invention resides in the improved process of carbonizing pellets of the type referred to above by subjecting such to an initial sudden temperature rise for such a time that only the outermost layer of each pellet (briquette), which layer has a thickness of only a few millimeters, is rapidly heated above the plastic range of the coal mixture followed by subsequent resolidification. The carbonization of the inside of each briquette, however, must be carried out much 4slower and should suitably begin at a temperature which is lower than that which was used for the sudden heating of the surface layer. The completion of carbonization of each briquette is carried out at a low rate of temperature rise and the end temperature at which the temperatures of the skin and core of each shaped article are equalized is below 600 C. The briquettes which have thus been carbonized are subsequently subjected to dry distillation at temperatures up to 1000 C. The highly different temperatures necessary in each of the several stages of the treatment according to this invention are achieved through the use of fine-grained solid, particulate heat carriers.

The process according to the invention is characterized by treating shaped carbonaceous articles in a first stage for two to `five minutes with a solid, particulate heat carrier which is heated to SOO-600 C. and moves concurrently with the shaped articles; treating the shaped articles in a second stage for 30-90 minutes with a solid, particulate heat carrier which moves countercurrent or transverse current with respect to the shaped articles and has an initial temperature of 40C-500 C., increasing to SOO-600 C., and finally treating the shaped articles in a third stage with a solid, particulate heat carrier which moves in a countercurrent or transverse current to the shaped articles and which has an initial temperature of S50-650 C., increasing to S50-1000 C.

The heat carrier consists preferably of sand. The ratio of the weight of the heat carrier to that of the shaped articles is about 2:1 to 4:1 in the rst stage, about 3:1 to 6.1 in the second stage, and about 1:1 to 2:1 in the third stage.

According to the invention, the shaped articles consisting of briquettes or pellets are supercially carbonized in the rst stage lby being suddenly heated with heat carriers at a temperature of about 50G-600 C. s0 that the surfaces of the shaped articles are suddenly heated above the plastic range of the briquette mixture and then suddenly resolidiiied. The resolidilication should be restricted to a depth of 1 to 3 millimeters. In the second stage, the temperature of the heat carriers is reduced to about 450 C. The temperature of the heat carriers is then slowly increased at a rate of 0.5-2 C. per minute until a temperature between 500 and 600 C. has been reached so that this slow heating of the shaped articles for 30-90 minutes causes their core to be heated through the plastic temperature range of 40G-550 C. and to be resolidified. Temperatures above 600 C. will always be detrimental during the initial sudden heating and during the slow heating in the second stage. The reduction of the treating temperatures after the initial sudden heating of the shaped articles is important to ensure that there is sufficient time for the escape of the gases and vapors released inside each briquette. Otherwise, the treatment would promote the formation of large pores, bubbles or foam so that higher stresses would be induced in the shaped articles.

When the shaped articles have been resolidiiied, they are subjected to dry distillation in the third stage while being heated to a temperature of 800-1000 C. at a rate of 3-10 C. per minute. It is desirable to heat the shaped articles at a lower rate first and at a higher rate thereafter. During this treatment, the shaped articles move preferably countercurrent or transverse current with respect to fine-grained solid heat carriers, which are at a temperatue of about S50-650 C. at the beginning and at a temperature of about 850-1000 C. at the end of their action on the shaped articles.

The ratio of the weight of the heat carriers to that of the shaped articles is about 2:1 to 4:1 in the first stage and should be sufficient to ensure that the soft, plastic briquettes are completely embedded in the solid, particulate heat transfer medium. In the second stage, the ratio of the weight of the heat carrier to that of the shaped articles is in the range of about 3:1 to 6:1 so that a slow continuous temperature rise is carried out in this stage. A temporary plastic state of the shaped articles is accomplished in this range. In the third stage, in which a dry distillation is effected, a ratio of the weight of the heat carrier to that of the shaped articles of about 1:1 to 2:1 is sufiicient.

The temperature profile during the countercurrent treatment with the hot heat carrier moving countercurrent thereto will depend on the ratio of the mass of the heat carrier to that of the shaped articles. If the mass of the heat carrier is large so that the water equivalent is high, the temperature rise will be initially rapid and slower thereafter. The water equivalent is the product of the mass and specific heat of the heat carrier.

Conversely, where the mass of the heat carrier is small, so that the water equivalent is low, the temperature rise will be initially slow and faster thereafter.

The times and temperatures for and at which the treatment is carried out may be selected to influence the pore size and porosity of the carbonized product so as to obtain desired values of these parameters. A treatment for a longer time generally reduces the pore size and porosity whereas the use of a temperature at the upper limit so that the time of the treatment is reduced will result in larger pores and a higher porosity. The duration of the treatment and the upper temperature limit will be selected in each case dependant on the caking and expansion properties and the softening behavior of the coal-pitch mixture which is employed. These are determined experimentally for the individual coals and briquette mixtures used. It will be obvious that the carbonization of shaped articles having a higher weight per article will take more time than the carbonization of shaped articles having a lower weight per article.

The formation of proper pores is of great importance for the quality of a metallurgical coke. The porosity is suitably between 40% and 50% and the pores should not be too ne. Fine pores tend to become clogged more easily by soot or the like so that they become ineffective. Large pores do not present a sufficiently large reaction area. Whereas the disadvantage of the large pores is compensated to some extent by a higher porosity, larger pores result in a lower bulk density and in most cases in a lower Wear resistance. For this reason it is important to form medium size pores in the shaped article as far as possible. The porosity and pore size may also be influenced by the caking capacity of the coal mixture and by the proportion of admixed binder, such as tar pitch or petroleum bitumen. If the porosity is too large, it may be reduced by using a leaner coal mixture and/or by a pre-admixing of the binder. To increase the porosity, the carbonization time may be reduced or a coal mixture having a higher caking capacity may be used and/or the proportion of admixed binder may be increased. The caking capacity of the coal and the proportion of binder are suitably selected so that the shaped articles are subjected to temperatures in the range between about S50-600 C. for a time which is as short as possible, e.g., 30 minutes, so that the total time of the treatment is as short as possible and the carbonizing apparatus has a high throughput capacity.

In the process according to the invention, shaped articles having a higher caking and expansion capacity than those used before can be carbonized and processed into a high-grade shaped coke. As a result, the proportion of admixed non-caking coal or coke can be reduced, as is often desired. On the other hand, the resulting shaped cake has internal stress cracks to a smaller extent and has a higher abrasion resistance. This is of great advantage particularly when the coke is to be used in a blast furnace.

Where coals having excessively high caking and expansion capacities are used in the process according to the invention it is desirable to use a leaner mixture because otherwise too long treatment times would be required particularly in the second stage. This would increase the capital and operating costs of the plant and reduce the overall economy thereof. For this reason, such coals should be made lean but they need not be so lean for being carbonized by the process according to the invention as in the processes known before. To make the coal lean, a non-caking coal or a slightly caking coal or cokes of various kinds or origins may be admixed to form the material to be briquetted. In many cases, suitable coal is not available and it is often undesirable and less economical to produce in a separate -plant additionalcoke for use in making a lean mixture. In such cases it may be desirable in `the production of blast furnace coke to add fine ore and/or lime to the briquetting mixture before it is shaped. Fine ore and/or lime must be added to the blast furnace in any case so that it may" be economical to incorporate such in the coke. When the shaped articles are carbonized, fine ore and/ or lime will reduce the caking and expansion capacity to a larger extent than non-caking coal'or coke. The heating in three stages as taught by the invention is useful also with shaped articles which have moderate caking and expansion capacities and which could be carbonized by known processes. In these cases, the treatment can be carried out at temperatures at the upper limits of the ranges taught by the invention and the duration of the treatment may be reduced so that the Iresulting shaped coke has a higher wear resistance and a lower tendency toV splinter than the shaped coke articles which can be produced by the previously known processes.

The sudden heating of the shapedarticles in the first stage from a briquetting temperature of about 80 C. with pitch-bonded briquettes or from a similar temperature with dried pellets must not be carried out for an excessively long time. By this treatment, only an outermost layer having a thickness of 1-3 millimeters should be rapidly heated above the plastic range and resolidited. If deeper layers are also subjected to the rapid heating, larger pores and bubbles may be formed and/or the shaped articles may be splintered. For this reason, the sudden heating should be carried out for 2-5 minutes. A time at the lower limit will be used were the heat carrier is at a relatively high temperature and where the shape articles have a high tendency to cake and expand.

The fine-grained heat carrier consists preferably of sand having a particle size of 0.2-2 millimeters, preferably of 0.5-1.0 millimeter. Other known particulate heating media which may be used include sillimanite, corundum, coke or the like.

The process according to the invention is preferably carried out in a rotary kiln. During their suddent heating, the shaped articles are suitably moved concurrently with the heat carrier, which is charged together with the shaped articles to a rotary drum. For their slow second heating and mild treatment, the shaped pieces are desirably moved countercurrent to the heat carrier, suitably in a rotary kiln having internal fixtures, such as disclosed in German patent application M 75,578, P 15 83 472.7. The third heating stage for the dry distillation of the shaped articles can also be desirably performed in a rotary kiln in which the heat carrier is moved countercurrent to the shaped articles.

Whereas all three heating stages may be combined in a single rotary kiln, it is particularly desirable for relatively large plants to carry out the third heating stage for dry distillation in a separate rotary kiln or even to provide separate rotary kilns for each of the three stages.

To perform the heating in three stages in accordance with the invention, the shaped articles may be advanced on a horizontal or inclined grate and the particulate heat carriers may ow downwardly through a layer of said shaped articles in a transverse current. The temperatures and flow rates of the heat carriers may be selected so that the shaped articles can be subjected to carbonization and dry distillation in accordance with the invention in different portions of the overall length of the grate. A bed which consists of sand, coke or the like and which is continuously or periodically iluidized may also be used in the first heating stage.

The treatment in all three heating stages according to the invention or in one or two of said stages may be alternatively carried out with the aid of hot inert gases in a shaft furnace. This mode of operation requires a higher expenditure, cannot be supervised as easily and does not result in a shaped coke having as uniform good quality. For this reason, it is preferable to use fine-grained solid particulate heat carriers in the carbonizing treatment according to the invention.

The process will now be explained more fully with refence to an example and the ligure, which is a schematic view of the process and apparatus of this invention.

In the figure, a first rotary kiln 1 is used to treat briquettes in the first stage and a second rotary kiln 2 to treat the briquettes in the second and third stages. Briquettes, at a temperature of about C., are introduced at a rate of 1000 kilograms per hour into the first rotary kiln 1 through a conduit 3. Sand at a temperature of 550 C. is introduced at the same time at a rate of 3000 kilograms per hour through a conduit 4 into the first rotary kiln 1. These briquettes and sand are moved concurrently through the first rotary kiln, in which they reside for two minutes. During this treatment of the briquettes with a concurrently moving heat carrier, the surface of the briquettes is rapidly heated to a temperature which is similar to that of the sand. The briquettes are carbonized only at their surface whereas their core portions are heated only slowly. In each briquette, a skin having a thickness of 2-3 millimeters is hardened as a result of the carbonization so that the briquettes are well able to withstand the subsequent treatment.

The mixture of briquettes and sand is transferred by a chute 5 from the first rotary kiln 1 into the second rotary kiln 2. Immediately after the mixture has been received by the rotary kiln 2, the mixture is separated by screen fixtures 12. 'I'he briquettes are conveyed through the second rotary kiln 2 to the other end thereof whereas the sand which has been screened off leaves the second rotary kiln 2 immediately through a conduit 7. Sand at 900 C. is charged through the conduit 6 at a rate of 1250 kilolgrams per hour to the second rotary kiln 2 and is moved throughout the length of the second rotary kiln countercurrent to the briquettes. Sand at 500 C. is charged at a rate of 2500 kilograms per hour through a conduit 8 into the second rotary kiln and, by a centrally disposed lance tube 9, is blown into the second rotary kiln 2 approximately as far as to the middle thereof.

The briquettes charged to the second rotary kiln 2 are heated while moving countercurrent to the sand and in the first half of the rotary kiln are treated with a mixture of the sand from conduits 6 and 8. This mixture has an initial temperature of 500 C. The sand is cooled to 400 C. until it reaches that end of the rotary kiln where the briquettes are charged and leaves the second rotary kiln 2 also, through conduit 7. The briquettes are slowly and continuously heated by the sand and in the middle of the rotary kiln have a temperature of 520 C. at their surface and of 480 C. in their interior. While the coal is thus treated in a plastic state, it is subjected to a slow dry distillation during 40 minutes without the occurrence of undesired` phenomena due to expansion or cracking.

In the second half of the rotary kiln, the briquettes are carbonized while the carbonization of the briquettes is completed during an additional period of 60 minutes while the briquettes are moved countercurrent to the sand. During this time, the sand is cooled from its inlet temperature of 900 C. to 550 C. and in the middle of the rotary kiln is mixed with the sand which is charged through the lance. The resulting mixture is at a temperature of 500 C. 'Ihe dry distillation of the briquettes proceeds slowly and continuously. Coke at a temperature of 850 C. and at a rate of 700 kilograms per hour is discharged from the second rotary kiln through a conduit 10 and is subsequently cooled. Part of the sand which is discharged at 7 is heated to the temperature of 900 C. and is divided among conduits 3, 6 and 8, in which it is mixed with sand at a lower temperature. The vapors and gases which are thus evolved are vented through a conduit 11.

What is claimed is:

1. A process of producing shaped coke articles which comprises forming a shaped article comprising highly caking carbonizable carbonaceous material; contacting said shaped articles with a solid, particulate heat carrier for about 2 to 5 minutes in a iirst stage wherein said first stage heat carrier is at about 500 to 600 C. and moves concurrently with said shaped articles to raise to about 500 C. only up to about 3 mm. of said shaped articles, contacting the shaped articles emerging from said rst stage with a solid, countercurrently moving particulate heat carrier for about 30 to 90 minutes in a second stage wherein said second stage has an initial temperature of about 400 to 500 C. increasing to a final temperature of about 500 to 600 C., the temperature of said shaped articles upon entering said second stage falling sufiiciently to harden the surfaces of said shaped articles; and contacting the shaped articles emerging from said second stage with a solid, countercurrently moving particulate heat carrier in a third stage wherein said third stage has an initial temperature of about 550 C. to 650 C. increasing to about 850 to 1000 C.

2. The process claimed in claim 1 wherein said heat carriers are sand.

3. The process claimed in claim 1 wherein the Weight ratio of heat carrier to shaped article in said first stage is 2:1 to 4:1, in the second stage is 3:1 to 6:1 and in the third stage is 1:1 to 2: 1.

4. A shaped coke article produced by the process claimed in claim 1.

S. The process claimed in claim 1 wherein the contact with and heating of said shaped articles by said heat carrier is elected in a rotary kiln.

References Cited UNITED STATES PATENTS 3,018,226 1/1962I Batchelor et al. 201-5 3,018,227 `l/l962 Baum et al. 201-23 3,444,046 5/ 1969 Harlow 201-6 3,444,048 5/ 1969 Schmeling 201-12 FOREIGN PATENTS 503,199 5/1951 Belgium 201--12 X NORMAN YUDKOFF, Primary Examiner D. EDWARDS, Assistant Examiner U.S. Cl. X.R. 201-l2, 20, 32, 44

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