LD-Converter Slag Disposal Facilities
(Steel Making Department Pakistan Steel).
The LD-Converter (BOF) charge may consist of 10–25% of steel scrap and 75–90% of molten Pig iron. In order to remove the unwanted chemical elements of the melt, the LD-Converter is charged with fluxing agents, such as high calcium burnt lime (CaO), dolomitic lime (MgCa(CO3)2) and fluorspar (CaF2), during the oxygen blowing cycles.
Carbon dissolved in the molten Pig iron is oxidized to form carbon monoxide, causing the temperature to rise to 1660–1710°C .The scrap is thereby melted, and the carbon content of the molten iron is lowered, transforming it into raw steel. The impurities combine with the burnt lime or dolomite forming liquid slag and reducing the amount of undesirable substances in the melt. Resulting slag is a major by-product of the steelmaking process floats on top of the molten steel because of lower densities of slag constituents.
The main chemical constituents of the LD Converter (B.O.F) slag are CaO, FeO, and SiO2. During the conversion of molten Pig iron into steel, a percentage of the iron (Fe) in the hot metal cannot be recovered into the steel produced. Depending on the operating practice of LD Converter, the chemical composition of resulting slag ranges as follows,
CaO -------- 45 – 55 wt.%
MgO -------- 0.4 -- 4 wt. %
SiO2 -------- 14 – 20 wt.%
MnO -------- 7 -- 14 wt.%
P2O5 -------- 1-- 2 wt.%
Al2O3 -------- 0.5 – 2 wt. %
FeO -------- 5 -- 11 wt.%
FeO/ Fe2O3 (Total) may range 20 –30 wt %
X-ray diffraction testing of LD- Converter slag samples have revealed that the above chemical constituents are mostly present in the form of mineral phases as merwinite (3CaO·MgO·2SiO), olivine (2MgO·2FeO·SiO -C S (2CaO),·SiO), (4CaO·AlO·FeO), C F (2CaO·FeO ), CaO (free lime), MgO, FeO and C S (3CaO·SiO), and the RO phase (a solid solution of CaO-FeO-MnO-MgO).
Large quantities of calcined lime and dolomite are used during the process of conversion from Pig iron to steel and hence, the CaO content of LD Converter slag is typically very high (CaO >45%) in order to keep the basicity ratio higher (3 - 3.2). Although most of the lime exists in bound crystalline form with other constituents in slag but the un-dissolved or free lime content can be as high as 12wt%.
The slag samples are concurrently taken with steel samples at the turndown of every heat of LD-Converter subjected to regular auditing by Quality Assurance Department and the analysis are regularly communicated to Steel production manager so that corrective measures may be taken timely if required.
The higher the percentage of impurities in the Pig iron, the higher the amount of slag generated. The amount of slag generated per tonne of steel varies from plant to plant, because of the difference in pig iron analysis, type and quality of lime charged and the grades of steel produced. Normally an average of 150 - 200 kg of slag is generated per tonne of steel produced.
After tapping each heat, Converter’s slag is discharged into slag pot (mounted on a slag car positioned underneath the converter ) through Converter’s mouth by rotating the vessel to the charging side.
Above is the view of LD-Converter floor (Pakistan Steel) at + 8 meter height and ground floor beneath the LD-Converter, from where the electrically driven slag car is operated (travels on conventional rails) to transport the Converter’s slag to Slag dump yard of Steel Making Department.
Also the slag resulting as by-product of deoxidation/alloying of steel in steel ladles/ Ladle refining, is transported from continuous casting section via general slag car to slag dumping yard. Presently three slag cars are employed to fulfill the production requirement of 24 - 27 heats per day such that one for disposal of Continuous Casting slag whereas other two for transportation of slag from Steel Making Converters aisle.
To meet the future requirement of expansion of Converter shop to 2.2 million tonne, a provision of installation of third LD Converter and fourth slag car for disposal of slag was left in the Soviet design.
Slag is dumped from the slag pot by an overhead crane ( 30 tonne capacity ) into slag pit of Slag dumping yard, located at the far end of LD Converter charging aisle in the confines of Steel Making Department Pakistan Steel.
Generally an efficient and economical design of the slag handling system is to utilize, one slag pot per heat and turn-around time for one slag pot after cyclic process of slag dumping, safety inspection, lime spray coating, putting down a protective layer of dry slag at the bottom of slag pot and dispatching ready slag pot for next heat, should be as short as fifteen to twenty minutes in order to match the pace of peak production cycle.
Slag dumping yard of steel making department Pakistan Steel is comprised of four slag pits which remain in cyclic operation during peak production time such that one pit undergoes slag pouring/dumping operation, second under water spraying/cooling, third under slag removal/ disposal and fourth standby/ ready.
A slag pit is crammed after dumping slag of 50 - 60 heats, then it is stopped and slag is allowed to cool by water sprayers/ air. Subsequently the cooling slag is dragged and muddled, to allow further solidification of underneath layers of hot slag to turn into crystalline material and to get the slag pit completely dried.
Pouring of molten slag ( at 16000C ) on some left over excessive water in the slag pit may cause severe explosion and heavy slag splash out which can endanger the life of overhead crane operator and the people working around. It takes about 48 hours to get ready a slag pit to start next dumping operation. The method of water or air cooling employed effects the quality of slag obtained.
Disposal of Converter’s slag from slag yard Steel Making Department.
All above earth moving machinery used in slag handling at steelmaking department is deployed and managed by Process Auto Transport ( PAT) Department of Pakistan Steel.
The crystallized slag is loaded through excavating shovels and trucked away via haul trucks to slag granulation and metal reclamation plant.
At metal reclamation plant, metallics are recovered from the slag by magnetic separation. After removing the larger pieces of metal in the primary magnetic separator, the slag is put through a classifier, then crusher, after which the fine metallics are removed in secondary magnetic separator. The metallics are returned for recycling in steel making and metallic- free slag is sized and processed to make it salable in market.
Since large quantities of lime is used in LD-steelmaking and most of the CaO exists in bound crystalline form with other constituents in slag but LD slag can also contain pockets of free lime (CaO). This free lime can absorb water to form hydrated lime Ca(OH) which expands and swell, making the LD slag volumetrically unstable overtime. So a better understanding of the properties of steel slag is required for large volumes of this material to be utilized in a technically sound manner in civil engineering applications, mainly for road construction.
Industrially developed countries have found various ways of utilizing it. The UK, US and New Zealand use slag as aggregates in road construction, while Canada uses it as railway ballast. In China, a type of composite cement consisting of a mixture of LD slag and Granulated Blast Furnace Slag (GBFS) is used.