AUTHORS: Patrick Leyser and Christian CortinaPaul WURTH S.A.

Wet granulation of blast furnace slag has beenpractised for many years. Various systems are in

use and all are based on the quenching of molten slagwith water. The original process was the OCP granulationsystem (gravel layer filtering plant) – the starting point forfurther development of the granulation process and plantdesign. Improvements have become a must since theenvironmental demands imposed by local authorities invarious countries become ever more stringent.

INBA GRANULATION PLANT – INITIAL PLANT DESIGN At the beginning of the 1980s, SIDMAR and PaulWURTH (PW) developed the INBA granulation system.The initial plant layout was designed with a blowing boxin combination with a cold runner (see Figure 1). In theoriginal system the water-slag mixture flowed to thereceiving hopper and was then fed to the de-wateringunit via an internal feeding arrangement.

The de-watering unit (see Figure 2) consists of a de-watering drum (1) which has vanes (2) on its innercircumference to remove the sand out of the drum and onits outer circumference there are screens for de-watering(3). The slurry is fed by means of a feeding arrangementcomprising a distributor (4) and slow-down boxes. The aimof the distributor is to distribute the water-sand slurryevenly over the whole drum length while the slow-downboxes reduce the impact of the slurry flowing from thereceiving hopper via the distributor into the drum, andthus protect the screens.

GRANULATION PROCESSThe aim of wet granulation is the fast quenching ofmolten slag. During the quenching process the moltenslag converts into glassy sand with 97% of the particlesless than 3mm, and an average slag sand particle size ofless than 1mm. The impact point of the slag stream and

As the demand for granulated BF slag continues to grow and environmental constraints becomemore severe, improvements to slag granulation technology are required. New designs involving thecombination of a granulation basin, counter-current condensation tower and use of very fine watersprays result in improvements in granulation, sand product and emissions.

INBA® slag granulation system with environmental control of water and emissions

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r Fig.1 Cold runner design

r Fig.2 Drum cross-section

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the granulation water depends on the slag flow, itstemperature and the slope and shape of the hot runner(see Figure 3). The heat exchange between molten slagand granulation water has to take place very quickly. Thegranulation water jets break up the slag stream intomolten slag lamellae which decompose initially intofilaments and then into droplets. The best heat transferoccurs when the contact surface between the molten slagand the water is at maximum, ie, when slag has beenconverted into droplets and fully enclosed with water. Thesolidification time depends on the size of the slagdroplets, the temperature difference between the moltenslag and the granulation water, and the contactenvironment between the slag and the water. Dependingon the granulation water temperature around the slagdroplet, different heat transfer mechanisms take place:

` Heat removal only through steam release –applicable if the granulation water temperature is equal to water boiling temperature

` Heat removal through steam release and heattransfer into granulation water – applicable to most cases

` Heat removal without steam release but only throughheat transfer into granulation water – applicable if the granulation water is cold and allows animmediate condensation of the generated steam

In general, boiling temperature will not be reachedwhen granulating with cold water, except for local spotsdue to transient high slag flows. Heat removal withoutsteam release can take place if granulating with coldwater and where good turbulence between the slag andthe water allows optimum removal of heat. However, the most common situation is heat removal throughsteam release and heat transfer into granulation water(see Figure 4).

The granulation process can be performed with hot orcold granulation water, allowing for two different watercircuit layouts:

Hot water circuit (see Figure 5) An installationdesigned as a hot water granulation circuit does nothave a cooling tower. The granulation water, circulatedin a closed loop, heats up close to boiling temperatureand heat removal from the molten slag during hot watergranulation is mainly via steam release. Cold make-upwater is added to the system only to compensate forsteam and moisture losses. The average watertemperature in the circuit is approximately 90–95°C. Atthe impact point, where the granulation water comes incontact with the molten slag, water temperatures of~95°C and even higher are to be expected.

r Fig.3 Slag – water interaction

r Fig.4 Schematic of heat transfer

r Fig.5 INBA system with hot water granulation

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Cold water circuit (see Figure 6) The closed coldwater circuit is equipped with a cooling tower whosepurpose is to keep the process (granulation) water at aconstant cold temperature. Heat removal from the moltenslag in contact with cold granulation water takes place viaheat transfer into the water and partly through steamrelease. Heat transfer through steam release variesdepending on the granulation water temperature and theinstantaneous slag flow. At low slag flows the heattransfer of the molten slag will be mainly through transferinto the cold water, whereas at high slag flows steamrelease takes place. An installation with a cold watercircuit has a higher potential for a fast heat removalcompared to a hot water circuit.

PLANT LAYOUTSCold runner design The cold runner is installed as acontinuation of the hot runner, with a built-in blowingbox at the front end. The blowing box is fully embeddedin the cold runner which is installed below the hot runnerend spout. The cold runner serves the purpose of guidingthe water-slag/sand mixture to the receiving hopper andis equipped with a wear-resistant lining as the granulatedslag sand is very abrasive. The heat flux of the molten slagrequires some water spraying alongside of the cold runnerat the front end.

GGrraannuullaattiioonn wwiitthh ccoolldd rruunnnneerr llaayyoouutt The granulationprocess starts when the granulation water comes intocontact with the molten slag (see Figure 7). The slagflow breaks up into lamellas and filaments, then intodroplets. Only part of the slag is granulated on the waythrough the cold runner to the receiving hopper, but islikely to be completed after hitting the impact plateinside the receiving hopper and falling into thereceiving hopper. With this design, only part of thewater flow is directly used for the granulation process aspart is used to cool the wear protection plates alongsideof the cold runner front end.

Granulation basin design The granulation basinlocated below the hot runner spout end consists of awater basin which can vary in size depending on theplant layout (see Figure 8). The basin, filled with water toa defined level, permits water additional to the circuitwater to be available for granulation. Thus granulation,being sustained by the turbulent water bath, takes placemuch faster when compared to the cold runner layout.The layout allows the design of water circuits with lesswater flow, but nevertheless having more water availablefor granulation, without compromising on safety. Thebasin can easily be protected against wear, which in thecase of the cold runner, needs high maintenance. Thebasin layout has the potential to reduce the amount of a

r Fig.6 INBA system with cold water granulation

r Fig.7 Schematic of granulation process in cold runner

r Fig.8 Schematic of granulation process in granulation basin

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total installation below ground level.GGrraannuullaattiioonn wwiitthh ggrraannuullaattiioonn bbaassiinn llaayyoouutt The molten

slag falls from the hot runner and hits the granulationwater jets of the blowing box. The impact point of the slagstream and the granulation water jets is located justabove the water level in the granulation basin. Thegranulation water breaks up the slag stream and helps to

push the slag into the granulation basin below the waterlevel. The heat exchange between the slag droplets andthe water is now not only given by the water jets from theblowing box, but also from the water surrounding andenclosing each droplet in the water basin. The water jetshitting the water surface inside the granulation basincontribute to creating turbulent conditions in the basinand help to promote a faster cooling effect of the slagdroplets into sand particles. Although this design has areduced water to slag ratio, more water volume isavailable for granulation, ie, water volume in the basinand the water flow at the blowing box. The granulationprocess takes place faster and thus the solidification timeis reduced.

An overview of the particle size distribution of the sandquality generated via the different installation layouts isshown in Figure 9.

EMISSION OF SULPHUR COMPOUNDSBlast furnace slag has a sulphur content of around 1–2wt.%. The major sulphur compound is calciumsulphide (CaS) and during granulation gaseous sulphurcompounds are generated and emitted. These consistmainly of hydrogen sulphide (H2S) and sulphur dioxide(SO2) according to the following simplified reactionequations:

CaS + H2O p H2S + CaO (1)CaS + 3/2 O2 p SO2 + CaO (2)

These reactions occur mainly at temperatures above1,100°C and are illustrated in Figure 10. As long as theslag droplet is liquid, CaS is sufficiently available to feedthe slag/steam surface. The supply of sulphur to thecontact surface takes place through flow and diffusion,however, once the surface of the droplet hardens (skin),the transfer of sulphur takes place only throughdiffusion. Since the coefficient for solid diffusion ismuch less than for liquids (10-4 versus 10-8), furthersupply of sulphur from the liquid to the surface isstopped. Once a hard skin has been formed, onlysulphur contained in the skin reacts with the steam. Asthe steam is the product of H2O vapour and gaseous

Hot water system Cold water system Cold water system with steam condensation (new design*)

Stack Stack Conditioning tower De-watering drumH2S mg/Nm3 100–800 30–300 – 3SO2 mg/Nm3 50–200 50–500 – < 5(*) Within limits as defined by TA LUFT(H2S concentration <= 3mg/m3; SO2 concentration <= 350mg/m3)Data are average figures

r Fig.9 Particle size distribution

r Fig.10 Chemical reactions

r Table 1 Emissions comparison

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sulphur compounds (H2S, SO2) in contact with thesurrounding granulation water the sulphur compoundswill go into solution according to the relevant partialpressures. The prevailing conditions like watertemperature, pH value of water and solubility of H2Sand SO2 define the amount of sulphur compoundsreleased via the steam and emitted to the atmosphereor bound with CaO contained in the water.

A comparative table of emissions between hot watergranulation, cold water granulation with cold runnerdesign and cold granulation with granulation basinincluding steam condensation is shown in Table 1. Thedata are average figures from various installations.

STEAM CONDENSATION SYSTEM (see Figure 11)Emissions cause potential odour and corrosionproblems. The gaseous sulphur compounds emitteddepend strongly on the type of granulation system, slagflow rate, slag/water ratio and on the granulationwater temperature. Since the solubilities of H2S andSO2 decrease with rising water temperature, lowergaseous sulphur compound emissions are observed incold than hot water systems. As a result of theenvironmental requirement specified for differentcountries, the plant layout for a cold water granulationsystem with steam condensation has become thepreferred solution for integrated steel plants situated inurban areas. In order to further reduce the gaseoussulphur compound emissions, the generated steamreleased inside of the granulation basin is routed into acondensation tower.

CONDENSATION TOWER DESIGN – CO-CURRENT FLOW/COUNTER CURRENT FLOWThe initial design of the condensation tower is co-currentflow. The tower consists of an outer shell and an innercondensation chamber with by-pass openings (see Figure12). The steam flows up the annular opening between theouter shell and the inner condensation chamber. At thetop the steam is introduced into the condensationchamber by means of the draft generated by the spraynozzles injecting the condensation water.

The water jets from the blowing box, also acting asinjectors, introduce air into the condensation system andthe two phase condensation system (steam/water)converts into a three-phase system, having air as a third medium. Since air cannot be condensed, thecondensation system gets disturbed if excess air entersthe system (herein after called ‘false air’).

In the case of the co-current flow design, false air issucked into the annular opening, the velocity inside theannular opening increases, reaching figures of

~20–25m/s. This high velocity reduces the reaction timeof the condensation water in contact with the steam. Thisproblem called for a modified design where the entry offalse air can be reduced or even avoided to keep a two-

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r Fig.11 INBA System with cold water granulation and steam condensation

r Fig.12 Co-current condensation tower design

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phase condensation system and where the velocity of thesteam is reduced to have an adequate thermal exchangetime for proper steam condensation.

The co-current condensation system in conjunction witha granulation installation with cold runner design doesnot allow the installation of a sealing hood (internalhood) against the entry of false air. The granulation basinhowever allows the installation of a sealing hood in theform of a siphon. A water screen installed at the front endof the hood and the introduction of a small opening at itslower end allows the steam to enter the condensationchamber and further keeps the steam from escaping tothe cast-house floor.

The counter-current flow condensation design consistsof a condensation chamber (tower) of larger diameterthan for the co-current design (see Figure 13). The wateris sprayed on the up-coming steam counter-current to thesteam flow direction. Due to the lower steam ascendingvelocity and virtually no false air in the system (designwith granulation basin), the performance of thecondensation tower has been improved.

Spray nozzles The thermal exchange of steamcondensation has been further improved by thegeneration of very fine condensation water droplets thusincreasing the water reaction surface for the thermalexchange while condensing. This was achieved throughintroduction of spray nozzles which generate very finedroplets (ds ~660μm).

CONCLUSIONSThe first step to reduce emissions is associated with theimplementation of a cold-water circuit granulationsystem. The introduction of the granulation basin alongwith the counter-current flow condensation system hasbecome a must if severe environmental constraints areimposed by the local authorities. The granulation basinallows the introduction of a sealing hood as a barrier tofalse air, besides generating additional benefits such ashigher resistance to wear, reduced height design andreduced pipe investment due to less water flow in thecircuits. The implementation of counter-current flowcondensation permits better thermal exchange betweenthe water sprays and the ascending steam. Theintroduction of the spray nozzles generating very finesprays contribute to further improve the thermal heattransfer, and therefore the condensation tower heightcan be reduced. The introduction of both granulationbasin and counter-current flow condensation hasbrought customer and their environmental authoritiesto no longer consider the condensation tower as anemission source. MS

Patrick Leyser is Vice-President, Concept & GeneralEngineering, and Christian Cortina is Head ofGranulation Technologies, both at Paul WURTH S.A.,Luxembourg

CONTACT: laurence.kayl@paulwurth.com

r Fig.13 Counter-current condensation tower design

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