Fuel 114 (2013) 216–223

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Fuel

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Briquetting of carbon-containing wastes from steelmaking formetallurgical coke production

0016-2361/$ - see front matter � 2012 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.fuel.2012.04.018

⇑ Corresponding author.E-mail address: madiez@incar.csic.es (M.A. Diez).

M.A. Diez ⇑, R. Alvarez, J.L.G. CimadevillaInstituto Nacional del Carbón, INCAR-CSIC, Apartado 73, 33080 Oviedo, Spain

a r t i c l e i n f o a b s t r a c t

Article history:Received 15 December 2011Received in revised form 12 April 2012Accepted 16 April 2012Available online 4 May 2012

Keywords:CoalBituminous and oily wastesBriquettingCarbonizationMetallurgical coke

This work focuses on the manufacture of briquettes by using carbon-containing wastes from steelmakingas fillers and binders for use in coke ovens to produce metallurgical coke. Coal-tar sludges from the tardecanter of a by-products coking plant were employed individually as a binder or combined with otherwastes, such as oils from the steel rolling mills and deposits from the coke oven gas pipelines. Anotherobjective of this study was to use alternative low-cost fillers such as the coal generated after routinecleaning operations in the coal stockyards, so as to reduce the overall cost of briquette manufacture.Carbon briquettes with different formulations produced by a roll-press machine were tested in a semipi-lot movable wall oven by adding them to a coking blend at a ratio of 10 wt%. The quality of the cokes pro-duced was assessed by measuring of their reactivity towards CO2 and mechanical resistance before andafter gasification with CO2. In general, the coke quality parameters did not show any significant deteri-oration as a result of the addition of carbon briquettes when the amount and the nature of the binderand the particle size of the filler were optimized. Partial briquetting of the charge enabled cokes to beproduced according to the specific requirements of blast furnace.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Coal briquetting consists in applying pressure to small coal par-ticles with or without the addition of a binder to form compact oragglomerate-shaped fuels for domestic or industrial applications.This well tried and established technology was introduced byWilliam Easby who, in 1848, patented a method to convert coalfines into solid lumps [1,2]. He described the process as follows:‘‘the formation of small particles of any variety of coal into solid lumpsby pressure’’. This, in turn, makes possible to take advantage of twoprocesses associated to the coal and steel industries: briquettingand recycling. The latter is well understood by the description ofthe relevance of this technology: ‘‘the utility and advantage of thediscovery are that by this process an article of small value and almostworthless can be converted into a valuable article of fuel for steamers,forges, culinary and other purposes thus saving what is now lost’’.Nowadays, the in situ recycling of any waste generated in an indus-trial process is a key priority, with the added challenge of nearzero-waste generation or efficient disposal. It goes without sayingwith 160 years of history, the coal briquetting technology hasundergone many improvements due to scientific and engineeringadvances and that new applications in several industrial sectors

have emerged. Similarly, the partial briquetting of coal charges �up to 30 wt% of briquettes – for the production of metallurgicalcoke has been developed and industrially implemented especiallyin Japan [3–5], giving rise to beneficial effects such as an increasein bulk density in coke ovens, the possibility of increasing pooror non-coking coals in coking blends and an improvement in coldand hot coke properties.

The manufacture of coal briquettes for cokemaking is based onthe agglomeration of several types of coal (filler) with a binder (i.e.coal-tar and its derived oil fractions and pitches, petroleum pitch,asphalt, petroleum oils, etc.) by means of roll presses under a rel-atively low pressure. The literature about briquetting coal withbinders for coke ovens or other applications is very extensive.Whereas some authors have reviewed the basic concepts of coalbriquetting for several applications and the design and operationalconditions for each material [6–10], other have focused on a widevariety of fillers and binders [3–5,11–26]. Special attention has alsobeen given to the physical and chemical characteristics of the bin-der, to the optimum amount required to manufacture a briquettedestined for a specific application, to how binder should be distrib-uted in the carbon composite and to the particle size distribution ofthe filler to be used [18–30]. All of these considerations influencethe quality of the briquettes by determining not only their physicalproperties (density and mechanical strength) but also the chemicalinteractions between the components during carbonization[22,23,28].

M.A. Diez et al. / Fuel 114 (2013) 216–223 217

Among binder feedstocks, coal-tar pitch has been one of the ear-liest and most extensively used bituminous binders until now. Itsexcellent properties enable it to agglomerate fine coal particlesinto a coherent briquette by applying a soft thermal treatmentwith steam to allow the pitch to be fluid, the optimum temperaturefor this purpose being about 20–25 �C above the softening point ofthe pitch [8]. However, because of the high price of bituminousbinders it was no longer able to compete with coal; and the man-ufacture of briquettes for cokemaking progressively declined.

Now, however, stringent environmental regulations governingwaste disposal practices, the need to reduce fossil fuel consump-tion, the interest in the use of low-grade coals or renewable carbonsources and the rise in coking coal prices have led to a renewedinterest in coal briquetting as a useful technology for recyclinglow-value and carbon-containing wastes generated in steelmakingand for increasing the amount of non-coking coals in the blends.

In a typical integrated steel industry with coke plant, blast fur-nace, BOF converter, hot and cool rolling and processing lines, awide spectrum of carbon-containing wastes are generated. Someof the carbon-containing wastes such as tar sludge from the clean-ing of decanter and tanks, residual pitch from the benzol distilla-tion column in the by-products coking plant and lubricating oilsfrom different parts of the installation have all been successfullyused as minor components in coal blends [11,30–32] or as bindersin partial charge briquetting for metallurgical coke production[12,13,29].

Moreover several technical benefits have been derived from theuse of wastes generated in the steelmaking process in the partialbriquetting of the coal charge including a more homogenous distri-bution of bituminous additives in the charge, a more compactcharge due to an increase in bulk density and an improvement incoke quality. Furthermore, the use of a combination of several het-erogeneous carbon-containing wastes from steelmaking as binderscontributes to the protection of the environment because the pro-cess consumes the wastes generated in situ, converting them intovaluable sources of raw material. It also leads to a stabilization ofthe wastes making them safer to store before they are used in cokeovens, easier to handle when incorporated into the cokemakingprocess. The possibility of removing them in a single operation alsosolves the problem to dispose them in well-controlled landfills. Fi-nally, the use of the coal generated during routine cleaning opera-tions in the stockyards leads to a reduction in coking coalconsumption thereby contributing to environmental protection.

In addition to the considerations outlined above, the mainobjective of this work was to determine the effects of partial bri-quetting process on the carbonization behavior of the charge andon the quality of the resultant cokes in order to establish theviability of using several carbon-containing wastes (fillers andbinders) to manufacture briquettes by means of a cold compacta-tion process.

Fig. 1. Briquettes manufactured with filler B and binder Mxa at a relativeproportion 70:30 w/w.

2. Experimental section

2.1. Coal blend and wastes

The coal blend P was supplied by ArcelorMittal-Spain. This wasprepared by mixing several bituminous coals of different rank,thermoplastic properties and geographic origin to obtain a blendwith the following characteristics: ash, 7.6 wt% db, volatile matter,25.5 wt% db, sulfur, 0.72 wt% db, Gieseler maximum fluidity, 544ddpm. Proximate analyses of the coal blend and an evaluation ofGieseler fluidity were carried out using the appropriate ISO andASTM standard procedures, respectively [33].

Two wastes from different parts of an integrated steel installa-tion (Tu and Lo) were selected to be used in combination with

coal-tar sludges from the tar decanter of the by-products cokingplant (Mx). The waste Lo is an oily mixture from the steel rollingmills whereas the waste Tu comes from deposits from the cokeoven gas (COG) pipelines. Two tar sludge samples, Mxa and Mxb,differing in the solid particle content were used.

The quinoline insoluble content of the wastes was determinedfollowing the ASTM D2318 standard procedure for QI separationfrom tar and pitch, in which no filter aid (celite) was used.

The coal generated during routine cleaning operations in thecoal stockyards – B – with a particle size of <5 mm and a volatilematter content of 21 wt% db was used as a low-cost filler. Morethan 90% of the filler was <3 mm in size while the fraction of lessthan 1 mm constituted about 65%, this particle size distributionbeing similar to that of the coal blends destined to be carbonizedin the coke ovens, avoiding any special preparation.

2.2. Manufacture and characterization of briquettes

The carbon briquettes were produced in a Komarek B050 roll-press machine using different combinations of carbon-containingwastes from the steel industry. All of the briquettes produced werepillow-shaped with the following dimensions: 39 mm long, 19 mmwide and 10 mm thick. The materials to be processed were placedin the feed hopper, and then mixed and stirred at room tempera-ture in a mixer fitted to the briquetting machine. After flowingalong a screw the mixture was compacted by squeezing it betweentwo rollers revolving in opposite directions at the same speed. Thegap between the two rollers was set at 1.5 mm and a roll force of35 kN was used. It should also be noted that the dust recoveredduring the briquetting process can be recycled by returning it toa fresh batch of briquetting mixture in the mixer. As an example,Fig. 1 illustrates the briquettes manufactured with filler B and bin-der Mxa, being added at a rate of 30 wt%.

The physical properties of the briquettes (density and mechan-ical strength) and the thermal behavior were measured in order toassess the effectiveness of the agglomeration process. The densityof the briquettes was determined by water immersion at 20 �Cusing 100 g of briquettes.

The mechanical strength of 22–23 briquettes (120 g) was testedin an I-type rotating drum by means of mechanical rotations vary-ing between 20 and 1200 revolutions at a rotation rate of 20 rpm.The I-type drum is the same as that used to determine the cokestrength after reaction with carbon dioxide (CSR). After each setof rotations, the material was sieved into different size fractionsand these were weighed and then returned to the drum. The

218 M.A. Diez et al. / Fuel 114 (2013) 216–223

amount of unbroken briquettes and broken briquettes with a sizeof >10 mm (CS10) and the amount of fines with a size of <1 mm(CS1) were used as an indicator of the strength of the briquettes.The two indices, CS10 and CS1, are expressed as the percentageof the initial weight of the briquettes.

The coke yield of the briquettes was determined by carboniza-tion in a Carbolite horizontal tubular oven from room temperatureup to 1000 �C at a heating rate of 5 �C/min and a soaking period of15 min. Nitrogen at a flow rate of 300 ml/min was used to sweepaway the volatiles evolved during carbonization.

SEM and EDX analysis were performed on the carbonized bri-quettes at 1000 �C (B13Mx13Tu and B15Mx15Tu) by using a FEIQuanta FEG650 scanning electron microscope coupled with an en-ergy dispersive X-ray microanalyzer EDAX Genesis Apex 4. Sam-ples were scanned in backscattered electrons (BSE) mode to givecontrasted images and to discriminate between the differentphases (minerals and carbon matrix). Selected areas were sub-jected to EDX analysis. In addition, quantitative chemical analysisof total iron, metallic iron and iron (II) contents were performedby using the ISO standard methods developed for iron ores and re-lated materials. These chemical methods includes a titrimetricmethod using potassium dichromate after reduction of the triva-lent iron by tin (II) chloride for the determination of the total ironcontent (ISO 2597-2), a bromine-methanol titrimetric method forthe metallic iron content (ISO 5416) and hydrogen chloride reduc-tion and dichromate titration for iron (II) content (ISO 9035). Theiron (III) content was calculated by difference.

2.3. Semipilot carbonization tests

The amount of briquettes added to the industrial coal blend Pwas 10 wt%. All of the carbonization tests were carried out in anelectrically-heated movable wall oven of about 17 kg capacity withthe following dimensions: 790 mm height, 250 mm length and150 mm width. One of the two heating oven walls is movable to al-low the force exerted on the wall during carbonization to be mea-sured and the coke produced to be discharged. The oven was fittedwith a thermocouple and a load cell with a capacity to hold1 tonne. The data from the load cell, pressure transducer and ther-mocouple were stored in a personal computer using the dataacquisition system.

During the carbonization tests, the temperature of the wall waskept constant at 1010 �C. The coking time was nearly 3 h 30 min.The process was finished when the center of the chamber hadreached a temperature of 950 �C. After a soaking time of 15 minthe hot coke was pushed out of the oven and quenched with waterspray. The oven used has the capacity to produce enough coke tobe characterized by the standard procedures used by the steelindustry. As the bulk density of the charge varies as a function ofgrain size and moisture content, both of these parameters werekept as close as possible in each carbonization test. The size ofthe coal blend was 83 wt% less than 3 mm and the moisture con-tent about 5 wt%, giving a bulk density of 782 ± 6 kg/m3 db.

2.4. Coke characterization

The quality of the resultant cokes was evaluated in terms oftheir reactivity to CO2 (CRI) and the mechanical strength of the par-tially-gasified coke (CSR) by the Nippon Steel Corporation – NSC-method. The design and the characteristics of the experimental de-vices are described in the ASTM D5341 and ISO 18894 standardprocedures. To determine the CRI, 200 g of coke with a particle sizebetween 22.4 and 19 mm was exposed to the action of CO2 at aflow rate of 5 l/min h at 1100 �C for 2 h. The weight per cent ofthe initial coke mass lost during the reaction is defined as CRI.The mechanical degradation of the partially gasified coke (CSR)

was measured as the weight of coke remaining on a 9.5 mm sieveafter 600 revolutions at a rotation rate of 20 rpm in an I-type drum.According to ISO 18894 standard procedure, an abrasion index ofthe partially-gasified coke (A0.5) was calculated. This index reflectsthe amount of fines (<0.5 mm) resulting from tumbling the reac-tive coke and it is completed with A1 which is the amount offines <1 mm.

The cold mechanical strength of the coke was evaluated from asample of 10 kg with an initial size of >20 mm, employing a JISdrum and rotating it for 150 revolutions at a rotation rate of15 rpm (JIS K2151 standard procedure). Three indices were derivedfrom this test: DI150/25, DI150/15 and DI150/5 which are definedas the amount of coke with sizes >25 mm, >15 mm and <5 mm,respectively, after the mechanical treatment.

Total porosity of the cokes was calculated from the true heliumdensity measured on a AccuPyc1330T Micromeritics apparatus andthe apparent water density which was determined by waterdisplacement using a 300 g coke sample of the same particle sizeas that used to determine the coke reactivity towards CO2

(19–22.4 mm).

3. Results and discussion

3.1. Composition and characterization of the briquettes

Table 1 shows the composition of the briquettes prepared usingthe coal left behind after routine cleaning operations in the coalstockyards (B) as low-cost filler and bituminous or oily wastes asbinder.

Three types of wastes were used as binder in the manufactureof the different briquettes: tar sludges (Mxa and Mxb), depositsfrom the coke oven gas (COG) pipelines (Tu) and an oily wastefrom the rolling mills (Lo). Tar sludge from decanter serves as amatrix type binder which has two functions. On the one hand,the material soluble in organic solvents such as quinoline withgood binding properties, similar to commercial tar, acts as a bin-der; while, on the other hand, the insoluble fraction composed offine solid carbon particles [34] serves only as a filler [22] and con-tributes to reduce interparticulate friction during the briquettingprocess. Due to fluctuations in the composition of tar sludges ina by-product coking plant, two different samples (Mxa andMxb) were used, the main difference between them being theamount of waste soluble in quinoline. While Mxa contains about47 wt% of quinoline soluble, Mxb has a very low content of ap-prox. 15 wt%.

The waste Tu is a heterogeneous mixture of light tar compo-nents and solid particles. This waste is characterized by a high Sand N content, 15–20 wt% and 6–7 wt%, respectively, a quinolinesoluble fraction of nearly 20 wt% and a high amount of inorganicmatter (69.8 wt%), mainly composed of iron oxides.

The waste Lo is nearly 100% soluble in organic solvents and con-tains a complex mixture of petroleum-based and synthetic lubri-cating oils, which are composed of aliphatic hydrocarbons andpolyol esters, respectively.

Mechanical resistance, density and thermal behavior is depen-dent on the amount and type of binder in the briquette [15–18]and these properties provide useful information about the effec-tiveness of the agglomeration process. Therefore, the effective bin-der in the briquette should be reasonable associated to thequinoline soluble fraction of all the binders. Assuming no interac-tions between briquette components at room temperature, an indi-cator of the total effective binder content in the briquette shall bethe sum of the individual contribution of the quinoline solublefraction present in the binder added to the briquette. This indexis defined as follows:

Table 1Series of carbon briquettes prepared with different wastes.

Briquettes Filler Amount of filler (wt%) Binder/s Amount of each binder (wt%) Total effective binder (wt%)

B30Mxa B 70 Mxa 30 14.2

B13Mxa13Tu B 74 Mxa 13 10.1Tu 13

B13Mxa7Lo B 80 Mxa 13 13.1Lo 7

B30Mxb B 70 Mxb 30 4.4

B15Mxb15Tu B 70 Mxb 15 6.7Tu 15

M.A. Diez et al. / Fuel 114 (2013) 216–223 219

Total effective binder ¼Pn

i¼1QSi xi

100

where QSi and xi are the mass percentage of quinoline soluble mate-rial and the mass percentage of each binder in the briquette, respec-tively; i refers to each binder component.

It is important to point out that for cokemaking, the briquettesdo not need to be very strong, but they must have a high enoughresistance to perform the transportation, loading and dischargingoperations of an industrial plant with minimum size degradation.There are no internationally standard tests for measuring themechanical properties of coal briquettes (cohesion and abrasion).In this study, the I-type drum used for the determination of CSRby the NSC method was employed as a means to measure the resis-tance of the briquettes to broken and to the generation of finesproduced by the severity of the mechanical treatment applied.

Fig. 2 shows the variation in size degradation as the severity ofthe mechanical treatment increased from 20 to 1200 revolutions.The CS10 index represents the volumetric breakage of the bri-quette, while the CS1 index is a measurement of the abrasion resis-tance. The briquettes manufactured with 30 wt% coal-tar sludge

Fig. 2. Variation of mechanical strength of the briquettes as a function of theseverity of the mechanical treatment. CS10 and CS1: fraction of >10 mm and <1 mmin size, respectively.

Mxa exhibit the highest mechanical resistance, being able to with-stand mechanical treatment without any breakage and with aminimum generation of fines even in the most severe conditions(1200 revolutions). In such conditions, they only produce about8 wt% of fine particles below 1 mm. When Mxa was combined withother wastes (Lo and Tu), the briquettes display a high stabilizationindex at a low number of revolutions of 20 and 40 revolutions. Thesize degradation at this stage was mainly due to abrasion. After-wards, surface breakage became more noticeable and the volumet-ric breakage of the briquettes occurred as a result of the activationof flaws. After the first 150 revolutions, the size degradation of thebriquettes B13Mxa7Lo accounts to about 55%, 20 wt% breaking upto small pieces and 35 wt% into fine particles of less than 1 mm. Byextending the duration of the mechanical treatment, the disinte-gration of the briquettes increased, there being predominant sur-face breakage over the broken briquettes. At the end of thetreatment, abrasion represented about 65%. Between the two bind-ers (Tu and lubricant Lo), the latter proved to be less effective pre-venting crack propagation. Tu was a better matrix binder, whereasthe lubricating oils improved the briquetting operation by reducinginterparticulate friction and by serving as a separating agent be-tween the briquettes and the rollers.

However, the other waste, Mxb, produces weaker briquettessubject to decreasing cohesion and increasing abrasion, as theseverity of the mechanical treatment increases. During the first20–80 revolutions, the reduction in size of the briquettes consistsof a combination of volumetric and surface breakage. After the first20 revolutions, all the briquettes start to disintegrate. Abrasioncontinues to increase with treatment in the drum, because smallfragments provide more surfaces for abrasion. After 1200 revolu-tions, this accounts for about 85%.

An increase in the binder components is achieved by the com-bination of Mxb with Tu waste at the relative binder proportionof 1:1 w/w. Although the mechanical strength of the briquettes isimproved, the high resistance levels of the Mxa series are notattained.

The density of the briquettes made up of just Mxa or combinedwith oil and carbon deposits ranges from 1.28 to 1.22 g/cm3, forB30Mxa and B13Mxa7Lo, respectively. The use of a poor bindersuch as tar sludge Mxb clearly undermines not only the mechanicalstrength, but also the resistance of the briquettes to water. In sum-mary, the briquettes with Mxb as binder break after only a lownumber of revolutions, generate a high amount of fines and disin-tegrate when immersed in water. The series of the briquettes withMxb would not resist prolonged storage in open stockpiles and theoperations of loading and unloading during the briquetting opera-tions and before being charged into a coke oven without breakingand generating fine particles. One possible way to solve the defi-cient binder properties of the tar sludge with a high amount ofquinoline insoluble fraction (Mxb) would be to add a quantity oftar to obtain briquettes as efficient as those manufactured usingthe tar sludge Mxa.

Fig. 4. Coke yield at 1000 �C of the briquettes manufactured.

220 M.A. Diez et al. / Fuel 114 (2013) 216–223

Applying the effective binder index defined above to the bri-quettes could be very useful way to understand the differencesin the mechanical strength of the briquettes. Basically, the twosamples of tar sludge employed differ in the amount of materialwith good binding properties. Whereas the quinoline soluble frac-tion of Mxa contains about 47 wt%, the other waste Mxb only hasabout 15 wt%. This basic difference may explain the divergentmechanical behaviors of the two series of briquettes. When thetar sludge contains a relatively high amount of soluble fraction,the binder components are accommodated within the intersticesbetween the coal particles and the pore structure of the fillerand, consequently, the accessibility of the tar components to thecoal surface during carbonization will be enhanced.

In the briquettes made with Mxb, the unfilled interstices be-tween the coal particles, which are considered as potential flaws,may also give rise to structural flaws in the carbonized briquettesand, consequently, in the coke produced from the charge.

Although the number of briquettes is limited, a relationship be-tween the content of the effective binder index and the earlybreakage of the briquettes, reflected by the CS10 at 20 revolutionscan be clearly appreciated in Fig. 3. It can be deduced that: (i) if theamount of the binder is optimum serving to keep the coal particlestogether, breakage of the briquettes will be due to abrasion result-ing in finely divided particles of <1 mm in size; (ii) the first stage ofbreakage is the result of the activation of the potential flaws in thebriquettes due to a deficient amount of binder and then the twodegradation mechanisms will start to have an effect. From thisgraph, an optimum amount of effective binder can be set at morethan 10 wt%.

The thermal behavior of the briquettes was also evaluated. Thecoke yield is in agreement with the composition of the wastes(Fig. 4). Due to the higher amount of fine solid particles in Mxb,the briquettes containing this waste give a higher amount of cokeat 1000 �C. Both binders, Tu and Lo, clearly decrease the coke yield,especially in the case of the carbon deposits from the COG pipe-lines (Tu). In fact, not only is the coke yield modified, but alsothe coke profile. Whereas the briquettes made with the Mxa binderswell during thermal heating and yield a coherent coke, the bri-quettes with Mxb do not experience any swelling and retain theirinitial shape (Fig. 5). Thus, during co-carbonization with coal thesebriquettes may create local areas with a deficient agglomerationand localized fissures and cracks in the matrix of the resultant met-allurgical cokes.

3.2. Partial briquetting carbonization and coke quality

Using movable-wall ovens of different capacities is the mostwidely accepted way of obtaining information about the blending

Fig. 3. Variation of the mechanical strength index CS10 at 20 revolutions with theamount of effective binder in the briquettes.

potential of coals and additives to be used in industrial coke ovensin terms of the gas pressure generated during the process, cokepushing and prediction of industrial coke quality. In this study, asemipilot-scale movable wall oven of 17 kg capacity wasemployed.

All the briquettes were carbonized with the industrial coalblend P at an addition rate of 10 wt%. No relevant effect on the gen-eration of coking pressure was observed from partial briquetting(1.4 kPa for the blend P vs. 0.8–1.4 kPa for carbonizations with par-tial briquetting). The ability to predict and control the coking pres-sure in the coke oven is extremely important, especially in the caseof old coke batteries [33,34]. According to our results, none of theformulations used for briquetting were detrimental to cokemaking.

Coke is a heterogeneous carbon composite that it contains fis-sures, cracks and other weaknesses due the lack of cohesion be-tween the carbon components. During the transportation of cokefrom the coking plant and when it is charged into the blast furnacebefore the onset of any chemical reactions, coke suffers mechanicalstresses and its size decreases due to volumetric or coarse breakagealong the structural defects giving rise to smaller fragments and tosurface degradation (abrasion) that eventually yields fine particles.When the cold mechanical strength indices (DI150/25 and DI150/15) of the resultant cokes are compared, a similar volumetricbreakage along the fissures and internal cracks is observed in thecase of the cokes from the charges which contain Mxa (Table 2).In the case of the coke of >15 mm (DI150/15) this amounts toaround 77–78%, and between 45% and 50% for the coke fractionabove 25 mm (DI150/25). This indicates that the size of the cokesis only slightly diminished in the case of the intermediate fraction(25–15 mm) there being a slightly higher resistance to abrasion(DI150/5). However, as was predicted, the poor agglomerationachieved in the briquettes manufactured with the low quinoline-soluble sludge Mxb is reflected by the cold mechanical strengthof the cokes. Clearly, there is a marked change in the mechanicalresistance due to the propagation of fissures, a loss of cohesionand a higher surface breakage of the cokes to abrasion.

This negative effect of the binder Mxb is also evident in the cokeproduced from the charge which contains a mix prepared withthree types of briquettes: 31.9% B30Mxb; 36.4% B15Mxb15Tuand 31.7% B13Mxa7Lo.

On the other hand, by partial briquetting of the charge, cokeporosity is not affected and it accounts over 50% of the volume ofthe coke.

In the blast furnace, coke must support the weight of thedescending burden. At the same time it provides voids which en-able the hot iron to pass through, to reach the lower hearth andreducing gas to be distributed around the furnace. The coke alsoreacts with the CO2 produced from the reduction of iron ore to gen-erate the CO which is required for the reduction of iron ore in theupper part of the furnace. Consequently, the coke charged into the

Fig. 5. Profiles of a green briquette (as an example) and the carbonized briquettes at 1000 �C. (For interpretation of the references to color in this figure legend, the reader isreferred to the web version of this article.)

Table 2Characteristics of the cokes produced from the coal blend P and partial briquetting of the charge (briquette addition rate: 10 wt%).

Coke DI150/25 DI150/15 DI150/5 CRI (%) CSR (%) A1 (%) A0.5 (%) P (%)

P 50.8 77.8 16.5 30.9 56.2 41.3 38.6 52.2P + 10B30Mxa 47.7 76.7 15.7 31.6 55.6 40.7 37.6 51.4P + 10B13Mxa13Tu 46.5 77.2 15.3 33.7 52.6 42.7 39.6 51.7P + 10B13Mxa7Lo 45.2 78.0 15.5 31.6 56.8 42.9 40.0 51.4P + 10B30Mxb 32.2 62.9 22.7 31.4 55.4 42.0 39.0 52.2P + 10B15Mxb15Tu 35.3 69.3 20.8 32.3 53.0 43.4 39.0 51.6P + 10Mix 35.5 68.5 20.6 33.0 51.7 45.1 41.6 52.1

M.A. Diez et al. / Fuel 114 (2013) 216–223 221

blast furnace must be capable of generating CO by reacting withCO2 while simultaneously maintaining its physical strength afterreaction. Thus, a high degree of coke gasification (high CRI) resultsin a weakening of the partially-gasified coke (low CSR) which isdetrimental for the blast furnace operation. This coke behavior isof great importance for blast furnace operation, especially whencoke rates are very low which is a common occurrence when theinjection rates of auxiliary reducing agents are very high and inlarge blast furnaces [35,36].

The high-temperature properties of the cokes, CRI and CSR, re-main constant, whether Mxa is used, individually or combinedwith the oily waste (Lo). In contrast, the incorporation of the wasteleft behind in the COG pipelines (Tu) clearly produces a more

reactive coke (CRI) but a less resistant coke after the reaction withCO2 (CSR). Furthermore, the CRI and CSR indices are not greatly af-fected as the cold mechanical strength is, when Mxb is used as bin-der. Thus, the variation in the high-temperature properties seemsto be a consequence of the type of binder used in the manufactureof the briquettes and not the degree of agglomeration achieved.

The increase in reactivity towards CO2 by about three points isexpected to be due to the catalytic effects of the iron-bearing spe-cies present in the inorganic fraction of the Tu waste. It is wellknown that iron-containing minerals from coal are transformedinto other amorphous mineral phase containing metallic iron, ironoxides and pyrrhotite during the carbonization process and, conse-quently, accumulated into the resultant cokes [37,38]. The degree

Fig. 6. SEM image and EDX spectra of selected areas of the carbonized briquettes at 1000 �C, containing the waste Tu as a binder.

Fig. 7. Content of total iron, metallic iron and iron (II) and iron (III) oxides in thecarbonized briquettes at 1000 �C, containing the waste Tu as a binder.

222 M.A. Diez et al. / Fuel 114 (2013) 216–223

of transformation depends on the coal characteristics and the car-bonization conditions. These iron mineral phases have been widelyrecognized as catalysts of the Boudouard reaction of cokes andother carbon materials [39]. The cokes studied in this work wereproduced from the same coal blend with and without briquetteaddition and under similar coking conditions. As a consequence,the transformation of the mineral components present in thewaste Tu during carbonization should play an important role incoke reactivity. By this reason, the carbonized briquettesB15Mxa15Tu and B13Mxb13Tu have been analyzed by SEM-EDX.Our results are in agreement with those reported by Grigoreet al. [37,38] on metallurgical cokes. An amorphous aluminosilicatefrom the filler is the dominant phase in the carbonized briquetteswith iron being a major element. SEM-EDX analysis suggests a highconcentration of iron inclusions in other minerals or dispersed inthe carbon matrix. As an example, Fig. 6 shows EDX spectra of indi-vidual particles of varying sizes and the corresponding SEM image.The two spectra clearly reveal the presence of metallic iron andiron (III) oxide, the later is supported by a Fe/O atomic ratio of0.67. Although chemical analysis showed presence of Fe in themost areas with Fe/O atomic ratios > 0.67, EDX patterns did notindicate clearly the stoichiometric ratio of wustite. The titrimetricmethods bring more light on the iron forms such as metallic iron,iron (III) and iron (II) oxides in the carbonized briquettesB15Mxb15Tu (Fig. 7). The relative amount of the iron species in-creases as follows: Fe (III) < Fe (II) < metallic Fe. These data suggestthat during the carbonization of the Fe-rich briquette components,Fe2O3 can react with the reducing gases evolved (mainly hydrogen)from the decomposition of coal and carbon-containing wastes orwith the carbon of the components. Under such conditions, itundergoes direct reduction to wustite and metallic iron. The pres-ence of these iron forms allows explaining the adverse effect on theCO2 reactivity of the coke produced using this binder Tu in thebriquettes. This is why the quantity of Tu waste to be added to

the briquette should be controlled to maintain the parameters ofcoke quality in appropriate values.

4. Conclusions

Partial briquetting can be considered as a good recycling routefor carbon-containing wastes of different types, origin and compo-sition in an integrated steel factory. An accurate evaluation of theamount of tar sludge necessary to act as binder in the briquetteis essential to ensure that the cold mechanical strength of thecokes resulting from partial briquetting is adequate and to retainthe amount of breeze coke. No negative effect was produced onthe high-temperature properties of coke, reactivity to CO2 andstrength after reaction. Provided that the amount of effective bin-der used to make the briquettes is controlled, the partial briquet-ting of the coal charge will yield cokes with the desiredcharacteristics to be used in blast furnaces.

M.A. Diez et al. / Fuel 114 (2013) 216–223 223

Acknowledgements

The financial support provided by PCTI-Asturias-Spain throughresearch Project PEST08-07 is gratefully acknowledged. We arealso grateful to ArcelorMittal-Spain for its collaboration.

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