CA1111366A - Production of metallurgical coke from oxidized caking coal - Google Patents

Abstract

"PRODUCTION OF METALLURGICAL COKE FROM OXIDIZED CAKING COAL"
ABSTRACT OF THE DISCLOSURE
A three step process is provided for producing high strength metallurgical coke from high volatile caking coals. Firstly, the caking coal is oxidized by heating the coal in an oxygen-containing atmosphere for a period of time. Secondly, the oxidized caking coal is blended with fresh or relatively non-oxidized caking coal. Thirdly, the blend is carbonized to yield metallurgical grade coke.

Description

11~1366 BACKGROUND OF;THE INVENTION
The present ;nvention relates to a process for producing metallurgical coke from high volatile caking coal.
Metallurgical cokes are high quali.ty blast furnace or foundry cokes characterized by relatively high strength and stability. These cokes are routi`nely formed by a high temperature carbon;zation of bituminous coal blends. A typical blend is composed of high volati:le, hi:ghly caking oituminous coals, herelnafter referred to as "caki:ng coals", and medium and/or low volatile bi.tumi:nous coals, hereinafte.r referred to as "coki.ng coals".
Presently caking coals account for approximately 8a% of the world's known reserves of Carboniferous coals (see Fisher, A.D~, Metallurgical Coals for the Futurei Proc.~ of 23rd Canadian Conference on Coal, Ottawa, Canada, pp 45 - 68, September, 1971). Many coal-produci.ng countri.es are rich in their supply of caking coals, but possess an inadequate quantity of coking coals for the metallurgical blendsA
Heretofore, i.t has been necessary to blend caking and coking coals ;n order to obtain the high strength necessary in a ~etallurgical coke. At the present state of the art, caking coals alone, when carbonized, produce. a coke having an inadequate strength. It ;s not feasible to use coki.ng coals alone to manufacture a metallurgi.cal coke because of their scarc;ty, relative high costs, and particularly due to the high expansion pressure exerted on the walls of a coke oven d~ring carbonization.
Thus there is a need for a feasib.le process for produci.ng coke, usable as metallurgical coke, from cak;ng coals.
Previous workers have observed a mi.nute increase in the strength of metallurgical cokes manufactured from s.ome charges which have been pre-heated to temperatures of about 300C pri:or to carboni:zation. This increase in strength has b.een attr;buted to the removal of water duri:ng the pre-heating step (e.g. see Beck, K.B., Requi.réments of Coking Coals and How They are Affected by the Technical Development of Pig Iron and Blast Furnace Coke Production; 25th Canadian Conference on Coal, Victoria, B.C.,

- 2 -September, 1973). A problem associated with this pre-heating treatment is the difficulty in obtain;ng a fast and uni:form pre-heati`ng of s.uch large quantities of coal. Additionally there is a poss:i:bility of coal dust explosions. resulting from the necessity of charging the coke oven with the hot pre-heated coal. Finally there are di.fficulties involved i.n transporting the coal between the pre-heater and the coke oven. The~e problems~ and difficulties appear to have prevented pre-heating from being applied com-mercially.
SUMMARY OF THE INVENTION
10 In accordance with the pres::ent invention it has been di:scovered that a caking coal, when exposed at elevated temperatures to an oxygen containing atmosphere, can yield on carboni.zation a stronger coke.. It has also been found that there is an optimum range of oxi:dation levels of the.
caking coal which will yield on carbonizati:on a coke havi`ng the high strength necessary in a metallurgi::cal coke~
It is. difficult to uni:formly oxidize cak;ng coal to the optimum level previously menti:oned. To overcome th;:s difficulty, we have found that a first portion of caking coal, whi.ch.has been oxidi:zed but without ach;eving homogeneity of oxidation, may be blended with a second porti.on of non-oxidi.zed caking coal to produce a mixture which, when later carbonized, yields. a metallurgical-grade coke. Thi:s blending step i`s a preferred feature of the invention.
The invention thus provide~ a process for producing a metallurgical coke from oxidized caki:ng coals or from blends of oxidized and fresh caking coals, thereby eliminating or reducing the need for the scarce and expensive coking coal component. Since the average level of oxidation need only be at least in the opti:mal range of oxidation levels, the oxidation step does not require critical and elaborate control.
Broadly stated, the i.nventi:on i:s a process for producing a metallurgical coke from caking coal which comprises: arti:ficially oxidizing the caking coal by heating i:t at a temperature les:s than about ~1.11366 350C in an oxygen-containing atmosphere for a period of t;me sufficient to oxidize part of the caking coal to at least the optimum level of oxi:dation required to produce a metallurgical coke when the coal is subsequently carbonized; and carboni:zing the oxidized caking coal to produce metallurgical coke.
The invent;on further extends to two novel products. The f;rst product is a caking coal wh;ch has been artific;ally oxidized and which, when carb.onized, yields metallurgical coke. The second product is a cak;ng coal blend comprising a fi:rst part of caki:ng coal wh;ch has been artifici:ally ox;dized so that at lèast part of ;t is at that optimum level of ox;dat;on requ;red to y;eld metallurgi:cal-grade coke on subsequent carbonization; and a second part of untreated caking coali the ratio of oxidized coal to untreated coal being that ratio which will produce, on carbon;zation, a metallurgical coke.
DESCRIPTION OF THE DRAWINGS
-Figure 1 is a plot show;.ng the strength of oxidized caking coal samples which have been subsequently carbonized varying with chang;ng oxidiz;ng conditions:.
Figure 2 is a plot showing the strengths of cokes derived from blends of oxidized and untreated caking coal, which blends have been s.ubsequently carbonized.
Fi:gure 3 is a schematic of the set-up used to artificially oxidize a large charge of cak;ng coal.
Fi:gure 4 is a plot shnwing the di:latation and strengths of cokes derived from caking coal samples located at vari:ous depths throughout the oxidation set-up shown in Figure 3.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The process of the pre.s.ent invention is preferably a three step process. to produce a metallurgical coke from a charge of caking coal.
Before dealing with the.details. of the process, i:t i:s worth-while to clari.fy a number of terms used ;n context with the present invention.

~11366 A "metallurgical coke" refers. to a coke resulting from a carbonization at high temperatures and ;s characterized by a s.tability factor of at least 50 as determined by ASTM procedures: D3038-72 and - 4a -D294-64 (1972). Other properties such as high reactivity and low ash and sulphur content are also desirable in a metallurgical coke. For the pur-poses of this invention however a coke i:s considered to be. a metallurgi:cal coke if it satisfi.es the mechanical stabili.ty requirement.
"Caki:ng coals" i.nclude bituminous coals having a hi:gh amount of volatile material and an elemental analys:is typical of thos:e gi:ven in Table 1. Thes:e are coals which exhi:bit on carbonization the properties of high swelling, high dilatation and high. fluidity.
"Carbonization" refers to a hi.gh temperature ( _ 1000C) thermal 10 decomposition of coal to produce a metallurgical coke. This is distinct from "semi-carbonization" and "low temperature carbonization" which are processes to produce gas, char and tar at relatively low temperatures.
Turning to the process of the present invention, the preferred combination of three steps include oxidizing, blending and carbonizing;
a caking coal in order to produce a metallurgi:cal coke.
The oxidation step includes heating the caking coal at a temperature of less than about 350C, which is the decomposition temperature of most caking coals, in an oxygen containi.ng atmosphere for a time suf-ficient to oxidize a substantial portion of the caking coal to at least an optimum level of oxidation. (The term "artificially oxidizing" is used in describing this process i.n the claims to distinguish from the level of oxidation which occurs naturally when the coal is exposed to outdoor atmosphere.) This optimum level of oxidati.on is defined as that degree of oxygen treatment producing an oxi:di.zed caking coal which on carbonization yields a metallurgical coke.
An optimum level of oxidation may be better understood by referring to Figure 2. An optimum level of oxidation in the coal precursor is represented by the portion of the curves. coinci.ding with a coke strength i:ndex of at least 50.
Oxidation in context wi:th th.e present i:nvention refers. to the chemical reaction and/~or chemi:sorption of oxygen with the caki:ng coal - . .

~11366 during the pre-oxidation step.
It will be evident to those skilled in the art that to produce a quantity of optimally oxidized caking coal in accordance with the process of the present invention, a number of oxidizi:ng conditions can be used.
The oxidation time will be dependent on such parameters as the pre-heating temperature and the concentration of oxygen in the atmosphere used. These interdependent parameters, which can be determined by routine experimentation, need not be criti:cally monitored since, as will b.e demonstrated later, the average level of oxidation achieved in the caking coal preferably need only be at least at an optimum level of oxi.dation. Samples. oxidi:zed beyond this level may be produced and utili.zed in the process.
It may be possible to homogeneously oxidize a large quantity of caking coal to an optimum level of oxidation and subsequently carbonize the charge to produce a metallurgical coke. Although such cases fall within the scope of the present i:nventi.on, it i5 usually not feasible to proceed in this manner. Preferably the caki.ng coal is oxidized such.that the average level of oxi.dati:on is at least at an opti:mum level of oxi.dation.This oxidized caking coal is then blended with a quantity of untreated ~:
or relatively non-oxidized caking coal. The quantity of untreated caking coal required in the blend is that quantity whi:ch when blended with the oxidized caking coal yields on carbonizati.on a metallurgical coke. The ratios of oxidized and fresh caking coals needed are determined by testing the strengths of the cokes derived from a number of thes.e blends.
The above blend of caking coals is charged into a coki:ng oven and carbonized by a conventional procedure at a temperature of at least 1000C to yield a metallurgical coke. The coal blends may be hot or cold charged i.nto the oven wi:thout affecting th.e quality of the coke. It is preferable to cold charge the coal in order to avoid the possibility of explosion of the hot coal fines The operabili:ty of the process, together with the opti.mum materials, procedures and condîtions are established in the following examples.

In order to establish that the strength of a coke derived from a caking coal can be increased by an artifi:cial oxidation treatment, the following experi.ment was performed.
Three types of high:volati.le bituminous. caking coals (hvab.)~
the characteri.stics and thermal rheological properti.es of which are summarized in Tables 1 and 2, were heatèd at 180 + 3C i.n an atmo~phere of commerci:ally available nitrogen (containi.ng oxygen) in a reactor for varyi.ng periods of time. The flow rate of nitrogen through the reactor was adjusted to 10 ml/min.
Following the oxidation trea~ment the coal samples were each carb.onized at temperatures of at least lOQ0C. For th.e detai:ls of the carbonization procedure see Ignasiak, B., and Berkowi:tz, N., CIM Bulleti:n, 67, No. 747,72, 1974.
After carboni:zation the strength index of each sample was obtained by the method set forth i:n the abovementioned reference to Ignasiak et al. These F5 45 s.trength i:ndi.ces. have been correlated wi:th the ASTM stability factors in the same reference. For comparison purposes an F5 45 strength index of 50 is approxi`matel~ equal to an ASTM stability factor of 50. Since a metallurgi:cal coke i.s. ch.aracterized by an ASTM
stab;lity facto.r of at least 50, those cokes having an F5 45 strength - index greater than 50 would certainly be classi:fied as a metallurgical coke.
Table 3 summari.zes the strength indices of the preoxidi:zed carbonized coke samples. For compari:son purposes strengths a.re given for a coke derived from a caki:ng coal s.ample which had not been oxidized prior to carbonization. Strengths are also given far a coke derived from the original coal sample which had been pre-heated under the same con-ditions but i:n an atmosphere of oxygen-free nitrogen.

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o Stren~th Indices (F5 45) Oxidat;on No Oxygen Oxygen-free Coal Time' Pre-trea~ment Pre-treatment Pre-treatment hvab ~1 120 hrs. 5 52 5 hvab #2 280 hrs. 0 55 0 hvab #3 7Z hrs. 6 43 6 The results show firstly that the oxygen pre-treatment of caking coals greatly i~proves the strength of the produced coke. Furthermore the same treatment in an oxygen-free atmosphere does not improve the ' mechanical properties of the coke. Thus the improvement i'n the quality of the coke produced results from oxidati'on and not water removal as was previously thought to be the case (e.g. s-ee Beck, K.G., 25th Canadian Conference on Coal, ~i'ctoria, g.C., September, 1973).

The following example demonstrates that the operating con-ditions used in the oxygen pre-treatment step can be varied without significantly changing the mechanical stabilit~ of the resulting coke.
Three samples (~ 1009 each) of the hvab #3 caking coal characterized in Example 1 were spread in trays in thin layers (< 1/8 inch) and placed in an oven. Oxidation was carried out according to one of the following conditions:
A. oxidation in pure oxygen at 200~C, B. oxidation in air at 150C; or C. oxidation in air at 100C.

The oxidizing gas was continuously passed through the oven at a flow rate of about lOa mllmin. Subsamples: were periodically removed duri'ng the oxidation step and carbonized as per Example 1. The F5 45 . ~ ~ . .. .. . . .

~L~1~3~6 strength indices of the coke were Measured and are shown in Figure 1.
Referring to these results it can be seen that by the proper selection of the reaction conditions, that is the temperature of oxidation, time of oxidation and concentration of oxygen in the oxidizing gas, it is possible to reach an optimal level of oxidation in the caking coals.
These optimally oxidized coals will produce on carbonization a metallurgical coke. This optimal level of oxidation in a given oxygen containing atmos-phere at a given temperature corresponds to a time of oxidation which yields on carbonization a coke having a strength index of at least 50.
In a pure oxygen atmosphere at 200C this optimal level of oxidation is reached in a few seconds, whereas in air at lOO~C this level is reached after about 20 hours.
It should be noted that the results of both Examples 2 and

3 demonstrate that any concentration in the oxygen-containing gas, from a few ppm up to about 100% oxygen, is suitable for the pre-treatment step.
Temperatures ranging from 100C to 200C did not influence the quality of coke obtained from an optimally oxidized coal. Therefore any temperature from ambient temperatures up to a few degrees below the temperature of thermal decomposition of the caking coal (r~350C) appears suitable for the pre-oxidation step.
The time of oxidation is shown to be a function of the temperature and concentration of oxygen in the oxidizing atmosphere. It thus must be modified accordingly. By proper combination of the reaction parameters, the oxidation time can be shortened to a few seconds or if desired, extended to several days.
The toleration with respect to the above three operating parameters makes it clear that the oxidation step does not require specific and elaborate oxidizing equipment.

It will have been noted in the previous examples that as the treatment of the caking coals under the oxidative conditions is prolonged an oxidized coal results which on carbonization yields a progressively weaker coke, having strengths less than those required for a metallurgical coke. The following example demonstrates that a caking coal which has been oz;dized to past a level of optimal oxidation can be mixed with a quantity of fresh or untreated caking coal to yield a blend which on carbonization will yield a high strength metallurgical coke.
Samples of the hvab #3 caking coal characterized in Example 1 were oxidized to complete loss of agglutinating properties by the pro~
cedures set forth in Example 2; that is~ the oxidized coal on carbonization yielded a coke having a F5 45 strength index of zero. This oxidized coal was blended in various ratios with fresh hvab #3 coal and the mixtures carbonized. The strength indices of the product coke were determined and are illustrated in Figure 2.
It can be seen that by optimizing the ratios of the oxidized and fresh caking coals prime metallurgical coke may be produced.

In a large scale coking industry it would usually not be feasible to pre-oxidize large charges of caking coal to an optimal level of oxidation, since this would require a complex set-up under critical monitoring. The following example demonstrates that carbonization of a charge composed of portions of variously oxidized caking coal can also yield on carbonization a coke improved in strength.

A 500 lb. charge of the hvab #3 caking coal characterized in Example 1 ~as subjected to a mild oxidation pre-treatment in the hopper 1 depicted in Figure 3. A steady flow of commercial grade nitrogen (900 ml!min) was admitted through inlet 2 into the hopper. The temperature of the hopper 1, as controlled and measured by gas heated oven iLl:~1366 3 and thermocouples 4 respectively, was increased gradually to 90C
after two days, to 120C after three days, and to 170C after four days.
On completion of the oxidation step, samples of the oxidized coal from different locations in the hopper were cooled rapidly at -10C in a nitrogen atmos;phere and subjected to dilatometric analysis The strength indices ~ere estimated from these analysis. The results, as depicted in Figure 4, show that the oxidation of the charge ~as not uniform throughout the hopper.
Nevertheless, when, after identical oxidizing treatments, the entire charge was either immediately carbonized (hot charged) or cooled down in nitrogen then exposed for a few days to the atmosphere and sub-sequently carbonized (cold charged), an impressive improvement in coke strength was observed. The results are summarized in Table 4. The quality of the coke was not affected by hot or cold charging. Thus it would not only be acceptable, but even recommended to avoid hot charging, thereby eliminating the risk of explosions of hot desiccative coal fines.

Evaluations of Cokes Produced in a 500 p Moveable-Wall Coke Oven from Fresh and Oxidized #3 Coal ASTM
Stahility Hardness 2 Factor (%1 Factor (%)1Breeze 1. fresh coal 35.2 64.5 3.9 2. oxidized in a hopper for 96 hrs. (hot charging) 45.8 64.5 3~6 3. oxidized in a hopper for 96 hrs. (cold charging~ 47.1 64.3 3.5 Notes - 1 - Hardness Factor describes the cumulative percentage remaining on a 1/4 in. s;eve, as determined by ASTM D294-64, 1972 2 - Breeze describes the percentage of the coke bet~een the lim;ts of 1/2 i`n. and 1 in.
The coke produced subsequent to the oxidative pre-treatment, as compared to the coke produced from untreated caking coal, is characterized by an unchanged hardness factor and a lower breeze content. These results show that other qualities of the coke have not been adversely affected by the oxidative pre-treatment. In fact, the breeze factor can be improved by this process.

The results of Examples 3 and 4 demonstrate that, provided there is a sufficient quant;ty of caking coal at least at a level of optimal oxidation, it does not matter whether the charge consists of homogeneously oxi.dized coal or is prepared by blending components at different levels of oxidation. An improved strength coke results from either of these cases.
~hile the pres.ent invention has been described in terms of a number of illustrative embodiments, it should not be so limi:ted, s.i.nce many variations in the process will be apparent to persons-skilled in the related art without depart;ng from the true spi:rit and scope of the invention.
Similarly, while it has been proposed that the benefi:ts~ result-ing from the process can be explained by oxi:dation of the caki.ng coal, the present invention should not be res.tricted to any parti.cular explanation of the me.chanism responsible for the benefits. achieved through the us:e of the process.

Claims (4)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for producing a metallurgical coke from caking coal which comprises:
artificially oxidizing the caking coal by heating it at a temperature less than about 350°C in an oxygen-containing atmosphere for a period of time sufficient to oxidize part of the caking coal to at least the optimum level of oxidation required to produce a metallurgical coke when the coal is subsequently carbonized; and carbonizing the oxidized caking coal to produce metallurgical coke.

2. A caking coal which has been artificially oxidized and which when carbonized, yields metallurgical coke.

3. A process for producing a metallurgical coke from caking coal which comprises:
(a) artificially oxidizing a first part of caking coal by heating it at a temperature less than 350°C in an oxygen-containing atmos-phere to oxidize at least part of the coal;
(b) blending the product from step (a) with untreated caking coal; and (c) carbonizing the blend to yield metallurgical coke.

4. A caking coal blend comprising:
a first part of caking coal which has been artificially oxidized so that at least part of it is at that optimum level of oxidation required to yield metallurgical-grade coke on subsequent carbonization; and a second part of untreated caking coal;
the ratio of oxidized coal to untreated coal being that ratio which will produce, on carbonization, a metallurgical coke.