DESULPHURIZATION OF PIG IRON USING CALCIUM CARBIDE BASED REAGENTS 141

IntroductionSA Calcium Carbide has been making calcium carbidedesulphurizer mixtures for 30 years and has extensiveexperience in the use and application of this material.

Calcium Carbide is produced at the Newcastle site in a 50MW submerged arc furnace using calcium oxide andcarbon and electricity, the reaction is CaO + 3C = CaC2+ CO (Figures 1 and 2).

Ladle designThe design of the ladle is important, especially the depth ofinjection in order to maximize the reaction time for thereagents. The surface area should be minimized to avoidheat loss and air exposure, and a hood is often used for thisand to prevent splashing from the ladle on to adjacentstructures during reagent injection.

The pouring lip design is important to assist in deslaggingand to reduce iron loss as is the freeboard of the ladle toavoid splashing losses. As can be seen, it is important todesign the desulphurization treatment ladle to meet theplant throughput rate especially to injection depth,circulation/mixing and possible splashing of hot metal.

Injection system and lancesThe design of the injection system is also important,pressure vessels with a steep sided cone, fitted with afluidization cone or nozzles at the bottom to assist with aneven continuous flow of reagent into the nitrogen carriergas stream. In treatment ladles normally a refractory coatedlance is lowered vertically into the molten iron to within500 mm of the ladle bottom to maximize the injectiondepth. Co-injection systems are available for injecting morethan one reagent simultaneously into the hot metal.

Different lances are available—original straight bottomdischarge, T- lance with the discharge from 2 holes on theside of the lance parallel to the ladle bottom, 4 hole lance

BRODRICK, C. Desulphurization of pig iron using calcium carbide based reagents. The 7th International Heavy Minerals Conference ‘What next’, TheSouthern African Institute of Mining and Metallurgy, 2009.

Desulphurization of pig iron using calcium carbide basedreagents

C. BRODRICKSA Calcium Carbide

Use of calcium carbide based reagents is a long established practice for the desulphurization ofpig iron/hot metal resulting in the removal of the sulphur from the molten metal as calciumsulphide into the slag floating on the molten metal surface.

At SA Calcium Carbide desulphurizer reagents are produced by blending calcium carbide withvarious other additives and milling the mix to a fine powder with a particle size of approximately95% smaller than 75 microns.

This paper discusses factors such as ladle design, injection system and lances, gas and solidflow rates, sulphur levels, reagents, temperature and slag affecting the desulphurization. Alsoincluded is discussion on iron losses and reduction of iron loss. The last section is allocated tosafety requirements for calcium carbide based reagent, dealing with spillage and fire as well astransportation, transfer, handling and storage.

Figure 2. Calcium carbide ingots being cooled, using naturalconvection, from 2200°C to 200°C

Figure 1. View of the SA calcium carbide production facility inNewcastle

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and dual pipe lances have all been tried. Their use isdependent on the individual customer’s system. Lance lifeis mostly a function of heat stresses and cooling, andshorter injection times increases lance life. Improvedrefractory designs with lower conductivity, increasing thethickness and addition of stainless steel fibres to therefractory covering on the lance have extended lance life.Cleaning of the lance outlets, especially on multi portlances, improves lance life by reducing clogging.1, 2

The ladle side plug injection system has been tried andwhile good desulphurization and efficiencies were obtained,the cost of the side plug was expensive. Renewing the sideplug was also time consuming together with the added riskof burn-through at the bottom of the ladle.

It is important to get a smooth constant flow of reagent toprevent lance clogging and achieve good desulphurizationefficiency. This is achieved by the control of the toppressure in the injection vessel, the fluidization cone gasflow and the main carrier gas flow. Bends in conveyingpipe work should be kept at a minimum and where requiredlong radius bends should be used. Computerized control of

the whole injection system from lowering and raising thelance to control of gas pressure and gas and reagent flowhas improved the desulphurization process. More consistentfinal sulphurs and improved reagent efficiencies areachieved.3 (Figure 3.)

Gas and solid flow rateThe position of calcium carbide particles relative to the gasbubbles or in the molten iron strongly influences thedesulphurization efficiency. Particles on the surface of gasbubbles are able to make contact with sulphur in iron whilethose others in the bubble will not, unless they are able toreach the interface. Calcium carbide particles which enterdirectly into the liquid iron travel more slowly to the Ironslag interface than the particles in the bubbles and have alonger residence time, increasing the reaction with thesulphur in the iron. Plume reactions depend heavily on theinjection conditions and the gas to solids ratio. The risingbubbles cause the agitation, circulation and mixing of theiron. (Figure 4.)

Figure 3. Flow diagram of a typical injection unit

Legend: 1 – Ladle; 2 – Ladle cover; 3 – Lance; 4 – Lance support unit; 5 – Transfer pipe; 6 – Desulphurizer storage and weighing

Figure 4. Flow dynamics in the nitrogen/calcium carbide plume

Transport gas

DESULPHURIZATION OF PIG IRON USING CALCIUM CARBIDE BASED REAGENTS 143

An increase in the gas flow increases the velocity of therising plume and reduces the residence time of the particlesin direct contact with the iron thus, decreasing thedesulphurizing efficiency of these particles. Increased gasflow gives more and larger bubbles which creates a greatersurface area for more particles to make contact with themolten iron. Increasing the solids flow rate increases thenumber of particles inside the bubbles and makes it difficultfor particles to reach the bubble surface. It can be clearlyseen that it is essential to optimize the gas to solids ratio toachieve the best desulphurization efficiency within timeconstraints and process requirements. The circulation of themolten iron is beneficial, as this enables sulphur in the ironto also contact and react with calcium carbide and limeparticles that have risen through the iron and are sited at theiron-slag interface.4, 5

In many of the original open ladle, top lance injectionsystems with a single bottom outlet lance the carrier gasflow was of the order of 600 N litres/minute and reagentflow of approximately 60 kg/minute giving a gas to solidsratio of 10:1. It is my opinion that this needs to beoptimized for each system. Higher gas velocities assist inpreventing lance clogging, while lower flow rates ofreagent have been found to result in better desulphurizationefficiencies. Rates of the order of 35 kg/minute for carbidebased reagents appear to be preferred where the availabletreatment time allows. The multi-port lances like the Tlances with two outlets allow for higher gas flows and, asthis splits the reagent flow, it can give better efficiencies.

Sulphur levelsThe Sulphur in the molten iron from a blast furnaceoperation or TiO2.FeO separation in the case of ilmenitetreatment is influenced by the sulphur level in the reductantused in the process. Desulphurizer reagent usage increasesas the initial sulphur level increases, but also increasesexponentially as the final sulphur level aimed for decreases.Obviously the initial and final sulphur levels then influencethe desulphurizer reagent injection time and overalltreatment time.

Reducing conditions in the molten iron are necessary foreffective desulphurization.

Initially on injecting calcium carbide based reagents anyoxygen activity in the iron e.g. metal oxides, has to bereduced.

Some basic slag containing calcium carbide and lime alsoneeds to be present to trap the calcium sulphide rising to thesurface of the molten iron. In the case of the ilmeniteprocess there is little slag so the incubation period should beshort.

ReagentsThe particle sizing of the desulphurizer reagents isimportant to increase the particle surface area of reagentavailable for contact with sulphur and reduce the thicknessof the calcium sulphide on the particles. This makesdiffusion of sulphur through this layer to the calciumcarbide easier. To achieve suitable particle sizing of lessthan 75 microns, at SA Calcium Carbide, the calcium

carbide and other additives are blended together in acomputer controlled batch weighing system and milled in alarge combination rod and ball mill. (Figure 5.)

Addition of dolomite to the calcium carbide basedreagents assists with making more bubble surface areaavailable so reagent particles can contact the iron. It alsoincreases the agitation and mixing and helps to dispersemore particles directly into the molten iron. Assuming 0.6 Nm³/minute of nitrogen carrier gas, the decompositionof the dolomite to release CO2 on contact with the hotmolten iron adds considerably to the agitation in thetreatment ladle.

Temperature and slag affectingdesulphurization

While desulphurization can still be achieved with irontemperatures of slightly below 1300°C, temperatures of1350°C give better desulphurization and reagentefficiencies. The higher temperatures also reduce the slagviscosity with a reduction of iron trapped in the slag. If theslag contains significant levels of metal oxide, the calciumcarbide can react with the oxygen in the oxide first,reducing the reagent efficiency and this should becompensated for.

Reduction of iron lossThe main sources of iron losses in the desulphurizationoperation are pig iron granules trapped in the slag on top ofthe molten iron, splashing during injection of reagent, lossduring deslagging operations and skulling on the ladlerefractory sides.

The Marangoni effect causes iron droplets to be propelledinto the slag where they cannot continue to coagulate andare caught in the rigid net shape desulphurization slaginstead of falling back into the molten iron. Granules arefinely distributed and cannot be removed due to a highdegree of intergrowth. The pig iron cools during thedesulphurization operation, causing some of the dissolvedcarbon to drop out. This together with the carbon releasedfrom the reaction of sulphur with the calcium carbide isincorporated into the slag increasing its viscosity. It is thenmore difficult for the iron droplets to leave the slag back

Figure 5. Combination rod and ball mill for producing variouscalcium carbide based desulphurizer mixtures

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into the pig iron. Formation of di-basic calcium silicatetogether with the carbon (graphite) increases the viscosityand gives the slag its crumbly appearance. Chemical fluxesare the most effective way of reducing iron loss via the slag.fluoride based fluxes such as fluorspar and cryolite are mostcommonly used while soda ash on its own or with thefluoride fluxes has been used. While these fluxes reduce theviscosity of the slag and thus the iron loss, a higherconsumption of desulphurizer reagent is required to achievethe same desulphurization level (1% flux can increasereagent required by up to 3%). Thus for any operation theamount of flux and the reduction in iron loss needs to betraded against the increase in desulphurization reagent.Addition of approximately 3% flux appears to becommonly used. Alkaline silicates such as nephylinesyenite are reported as having been successfully used withan improvement in desulphurization and reduction in ironloss.6

The turbulence of the injection is influenced by thecarrier gas flow and the rate of desulphurizer reagentinjection. Carrier gas and the volume of gas released bydecomposition of or volatilization of components of thereagent needs to be controlled to avoid splashing of molteniron from the treatment ladle. The free board chosen in theladle design is also important to limit loss due to splashing.

The deslagging operation, on completion of treatment ofthe pig iron, needs to be carefully controlled to avoidunnecessary losses. Design of the ladle pouring spout isimportant to prevent run out of the molten iron during theraking process of deslagging. Tilting of the ladle duringdeslagging should be adjusted at different stages to reduceloss of molten iron. Use of a porous plug with a nitrogenflow to push the slag forward towards the spout reduces therequired number of deslagging strokes and thus the loss ofiron. (Figure 6.)

The lower the temperature of the slag, the higher theviscosity and the higher the iron loss will be. Also the moreslag created during the injection of reagents in the treatmentof the iron and together with any initial slag, the higher theiron loss will be.

Safety requirementsThe calcium carbide on its own is not hazardous butreaction of the calcium carbide with moisture from anysource releases acetylene which presents a serious hazard offire and/or explosions. If contact of the calcium carbidebased desulphurizer with moisture is avoided the material issafe to handle. To reduce the risk, nitrogen with a dew pointof -40°C or lower is used in blanketing of material stored insilos and for conveying reagent. This blanket excludesmoisture and reduces oxygen levels below the lowerexplosive limit for acetylene (LEL is approximately 2%acetylene in presence of > 6% oxygen).

Handling and storage

Offloading areas and storage silos must obviously bedesigned to prevent contact of desulphurizer reagent withrain and other moisture sources. Storage silos should have aconstant nitrogen purge to prevent ingress of moist airthrough a bag filter due to atmospheric temperaturefluctuations. Gas analysers should be installed to monitoracetylene and oxygen levels in storage vessels. This thenenables nitrogen purging to be adjusted to keep theacetylene and oxygen levels at acceptable concentrations

(SA Calcium Carbide believe acceptable levels to be < 0.3% acetylene and < 2% oxygen). Storage silos andvessels should have a steep sided cone shape with afluidizing ring in the bottom to ensure free flow of materialout the bottom. Desulphurizer is like all fine powders whichcan compact on standing, especially if vibration is present.

Transportation and transfer

Transportation of desulphurizer must meet the laid downrequirements for the transportation of hazardous substancesby road. The drivers of road tankers must have a validHazchem drivers licence and should be trained in thespecific hazards of the material being transported. Thetanker must have Hazchem stickers, in this case for calciumcarbide, on both sides and the rear while the ‘horse’ musthave an orange warning diamond on the front. TheHazchem stickers must show the UN number (1402 forcalcium carbide), a blue diamond indicating ‘dangerouswhen wet’ and emergency assistance numbers. In the cabinof the vehicle there must be a TREM card available, givinginformation on hazards of the product and information onhandling of incidents. (Figures 7 and 8.)

Figure 6. Schematic of ladle deslagging

Figure 7. Example of a pressurized tanker for safe transportationof desulphurizer mixtures

DESULPHURIZATION OF PIG IRON USING CALCIUM CARBIDE BASED REAGENTS 145

Carbide based desulphurizer must be transported under anitrogen blanket with the tanker gas space pressurized to100 kpa pressure. Mounting of nitrogen cylinders as anemergency back up on the tanker is advisable. On arrival atthe customer site the gas space should be tested to ensurethe acetylene (< 0.3%) and oxygen (2%) are at acceptablelevels before proceeding with offloading of the material. Ifacetylene and /or oxygen levels are high or the pressure onthe tanker is low, purging with nitrogen should be carriedout to ensure safe gas levels for offloading.

Nitrogen should be used as the pressurizing and carriergas for transferring the desulphurizer from the tanker to theStorage silo. An emergency stop, on the nitrogen supply tothe tanker, is required in case of an incident duringoffloading to minimize any spillage. It is also advisable tofit a remotely operated valve on the Tanker outlet pipe toimmediately stop the flow of material. The valve must besituated before the connection to the offloading flexible andbe operated from the pneumatics of the tanker or somesuitable supply of air or nitrogen from the plant. (Figures 9and 10.)

Spillage and fireIn case of spillage, prevent contact of the material withmoisture as far as possible. If dry conditions exist and thereis no reaction of the calcium carbide with moisture thematerial can be recovered by sweeping up into dry metalcontainers.

Desulphurizer mixtures can, in contact with sufficientmoisture, generate enough heat to spontaneously ignite anyacetylene produced. In case of a fire, remove all ignitionsources, and move personnel upwind. Allow the fire to burnitself out and only use dry powder extinguishers whereequipment or buildings are on fire or there is an imminentdanger of catching fire. Beware of areas where the drypowder has been used on top of the desulphurizer as if thereis still moisture present, flames might suddenly erupt fromunder the powder layer. When the flames have stopped andthe material is possibly glowing red, if deemed safe, a longhandled flat metal rake could be used to separate glowingmaterial from dry unreacted material. Persons should betrained to deal with spillages and fire and the assistance ofthe supplier sought if required. Disposal of residues shouldbe undertaken with care with the assistance ofknowledgeable and trained personnel.

References1. SALINAS, A. DS-Lances for hot metal

desulphurization, State of the art and newdevelopments. HTMK, Almamet and Polysius. TheVIII. International Symposium for Desulphurization ofHot Metal and Steel, September 20–24, 2004, Nizhny,Russia, p. 33.

2. CHANDRA, S., PATAK, S., MATHEW, K.S.,KUMAR, R., MISHRA, A., SEN, S., andMUKHERJEE, T. Improvements in ExternalDesulphurization of Hot Metal at Tata Steel,Jameshepur India. Almamet GmbH: The VII.International Symposium for Desulphurization of HotMetal and Steel, September 26–27, 2002 in Anif/Austria, p. 34.

3. ECHELMEYER, A. and BECKUM. P.A.G. Modernhot metal desulphurization plants and their processengineering potential. HTMK, Almamet and Polysius.The VIII International Symposium forDesulphurization of Hot Metal and Steel, September20–24,2004, Nizhny, Russia, p. 21.

4. CHIANG, L.-K., IRONS, G.A., LU, W.-K., andCAMERON, I.A. Department of Materials Scienceand Engineering, McMaster University, Hamilton

Figure 8. Relevant Hazchem transportation data prominentlydisplayed as per regulations

Figure 9. Close-up view of offloading connections

Figure 10. Emergency STOP station

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Ontario, Canada—Kinetic Studies of Calcium CarbideHot Metal Desulphurization by Powder Injection.Transactions of the ISS, 1990. pp. 35–53.

5. CHANDRA, S., PATAK, S., MATHEW, K.S.,KUMAR, R. MISHRA, A., SEN, S., andMUKHERJEE, T. Improvements in ExternalDesulphurization of Hot Metal at Tata Steel,Jameshepur India. Almamet GmbH: The VII.

International Symposium for Desulphurization of HotMetal and Steel, September 26–27, 2002 in Anif/Austria, p. 34.

6. GITTERLE, W. Almamet GmbH—The use of flux inhot metal desulphurization. HTMK, Almamet andPolysius. The VIII International Symposium forDesulphurization of Hot Metal and Steel, September20–24,2004, Nizhny, Russia p. 2.

C. BrodrickSA Calcium Carbide

Born in Pietermaritzburg and graduated with a BSc Hons. At Natal University (PietermaritzburgCampus) in 1963.Worked in Bulawayo Zimbabwe for 14 years before returning to South Africa, in 1978, to theCalcium Carbide business at Ballengeich, Newcastle KZN. Held various positions during his stay,including Laboratory Manager, Production Manager and Technical Manager during the 30 years inthe carbide business. In June 2008 he retired and is currently employed on an ad hoc basis as aConsultant for SA Calcium Carbide.