Productive recycling of basic oxygen furnace sludge in integrated steel plant

Transcription

1 Journal of Scientific & Industrial Research Vol. 64, September 2005, pp Productive recycling of basic oxygen furnace sludge in integrated steel plant Raj Kumar Agrawal 1 and Piyush Kant Pandey 2, * 1 Steel Authority of India Limited, Bhilai Steel Plant, Bhilai Centre for Environmental Science & Engineering, Bhilai Institute of Technology, Durg Received 07 March 2005; accepted 20 June 2005 The study describes techniques of recycling the gas cleaning plant (GCP) sludge generated during basic oxygen furnace (BOF) steel making process. Two different experiments were conducted to gainfully utilize converter sludge in iron making and steel making. The experiments resulted in two different products namely lime sludge briquette (LSB) and dolomite sludge mix (DSM). While LSB is used as a coolant in steel making, DSM is used for sinter production. Each one is having cost effective advantages on one hand and substantially reduced dumping of waste on the other. Keywords: Recycling, Basic oxygen furnace, Sludge, Blast furnace, Pollution Introduction Steel is an exceptionally recyclable material because steel products and by-products are excellent in terms of the reduce, reuse and recycle principle. On proper refining, the recycled products compete favorably with natural materials, improve recycling and preserve non-renewable resources 1. Indian steel industry is yet to reach the high standards achieved by its counterparts in developed world in the arena of waste minimizations and recycling. However, the Bhilai Steel Plant (BSP), Bhilai has taken some very bold steps in this area and very encouraging results are being obtained. One such attempt was the use of mixed pond ash for manufacturing superior quality bricks 2. In continuation of the task, an attempt was made to recycle the sludge obtained from the gas cleaning plant of basic oxygen furnace (BOF) more popularly known as LD converter. This research has particularly targeted the slurry from BOF for it s recycling. In all major steel producing countries, this slurry, which contains zinc content too high to directly recycle, is put into a settling tank and the water is pressed out of the slurry. The remaining fraction is either stockpiled or landfilled. Steel companies are prospecting for the processes to economically retrieve the zinc and iron units 3. This paper describes a breakthrough achieved which portents great savings economically and environmentally. *Author for correspondence Tel: ; Fax: bitdurg@sancharnet.ac.in Experimental Details Process Description Integrated steel production relies principally on virgin ore as the iron source and the use of some iron scrap is being made. The iron ore, mainly hematite (Fe 2 O 3 ) in India, is either sintered or pelletized and is then reacted with coke, preheated air and auxiliary fuels in the blast furnace (BF) to yield molten pig iron. This hot metal is then transformed into steel in and further processed before casting and rolling. Steel is an alloy of iron (< 1% carbon). Integrated steel plants produce steel by refining iron ore in several steps and produce very high quality steel with wellcontrolled chemical compositions to meet all product quality requirements. In BSP, three LD converters (capacity, 130 ton each) are installed. Steel is made in LD converters by lancing pure oxygen, which converts the carbon present in pig iron into carbon mono-oxide. Oxygen is blown through lances into the converter charged with hot metal, iron scrap, ferroalloys, lime and iron ore. Iron ore is used as a coolant, which adjusts hot metal scrap ratio to large extent. High purity oxygen blown into the furnace removes carbon and silicon in the molten iron mainly by oxidation. The basic oxide furnace is fed with fluxes to remove siliceous impurities. Certain alloying materials may also be added to enhance the characteristics of the steel. During oxygen blowing process, a large amount of fumes and gases are generated, which contain fine particles of the charged materials and carbon monooxide (CO) gas.

2 AGRAWAL & PANDEY: RECYCLING OF BASIC OXYGEN FURNACE SLUDGE STEEL PLANT 703 Fig. 1 Schematic diagram of basic oxygen furnace (LD Converter) and gas cleaning plant (GCP) Such discharges may vitiate the already stressed atmosphere existing in steel cities 4,5. Hence, this polluted gas is cleaned before discharge to the atmosphere. The gas is quenched and cooled using water and cleaned of suspended solids and metals. Evaporation of water due to high temperature saturates the gases with moisture. The trickled down water along with coarse dust particles is collected in expansion chamber and drained out to hydraulic guard tank. Gas, which is partially cleaned and cooled, passes through a Venturi where it attains high velocity. This leaves the occluded water particles behind, which subsequently separates out in elbow separator by centrifugal action. The wet slurry is discharged into hydraulic guard tank. The outgoing gas then passes through a zigzag path in baffle separator that removes the remaining dust particles and moisture. In the process of cleaning, the buoyancy is an unavoidable trade-off. Therefore, the gases are sucked by an I.D. fan to counter the negative draft. A change over device finally directs the outgoing gases either to recovery side or to the atmosphere. A high content of CO in the gas makes it suitable for its subsequent use as a fuel gas. Hence, in the recovery part, the gas is stored in a gasholder. However, CO content is variable therefore, a portion of the gas in different stages of the furnace operation is exhausted to the atmosphere through flare stack after combustion (Fig. 1). Slurry water from saturator, expansion chamber, elbow and baffle separator is collected in hydraulic tank guard. It is then discharged to radial settling tank through launder. Radial settling tank deposits, henceforth called GCP Sludge, is dumped in the landfill. Large production of this sludge has made it an environmental nuisance for the steel industry. Materials and Methods GCP sludge contains primarily iron oxide and free lime. Although BOF sludge is not suitable revert for use in BF because of high zinc content in other countries, whereas in India scrap is largely free from zinc. GCP sludge, analyzed by X-Ray fluorescence method, showed presence of high Fe content, which on recycling produced two types of products. (i) Lime sludge briquette (LSB); and (ii) Dolomite sludge mix (DSM). The major difficulty encountered when recycling sludge to iron or steel making is the moisture removal and material handling. Lime Sludge Briquette (LSB) Chemically, GCP sludge was found rich in iron oxides and total Fe content. CO used for drying is produced as a by-product in coke manufacturing in the coke ovens. CO gas burners were installed in an in-house furnace specially designed for the elimination of water component. Converter sludge was dried at 200 o C in CO gas fired furnace for

3 704 J SCI IND RES VOL 64 SEPTEMBER 2005 appropriate time. The dried sludge was screened and the size fraction ( 5 mm), collected after demagnetization, was charged into mixer of briquetting machine. Lime, used as the binder in briquetting process, was charged through a separate line into the mixer, and proportion was adjusted to form briquettes of adequate handling strength in briquetting machine. These briquettes were used as a coolant and replacement of iron ore in converter during steel making. Dolomite Sludge Mix (DSM) In LSB manufacture, CO, a by-product from coke ovens, was used to dry sludge. In this experiment, moisture and handling problem were overcome by a unique process of dehydration. Sludge dehydration was achieved by mixing sludge with hot dolomite collected from dust chamber of rotary kiln during calcination/sintering of raw dolomite. Calcination of dolomite, an essential step during sinter making operation, is carried out in sintering plants. The process of calcination/sintering of dolomite is carried out in the rotary kiln where the pulverized coal is supplied with a jet of the compressed air. Thus, large amount of fines, which are carried away by hot flue gases, is unavoidable in this process. Theses fines are separated from the flue gases in the dust chamber (temp, 600 C). At the time of disposal, these fines are generally at C. BSP envisaged using this heat energy for dehydration of GCP sludge. These outgoing gases from rotary kiln comprise of flue gases, ash, and dolomite fines. The rate of generation of rotary kiln fines is more than 40 ton/d. Prior to this experiment, these fines were cooled and then sent to dump yard as landfills. GCP sludge from converter was sprayed on the dolomite fines obtained from the rotary kiln. To achieve the same, a recycling process was developed in the plant premises. BOF sludge was supplied in tippers to the dolomite plant. Blending of the hot dolomite (~400 C) with sludge was done layer by layer with the help of grab bucket. The sludge was blended with dolomite in a predetermined mix ratios using layering effect of sludge dolomite sludge and so on. The mix ratios are not divulged here in view of the commercial significance of this process. The process uses available heat from hot dolomite to dehydrate the sludge and it resulted in a huge savings in drying costs. The treated mix is then allowed to cool for approx h. The undersize mix was then transported in rail wagons to the ore handling and preparation plant where it was mixed with the iron ore fines. Care was taken that in the preparation of iron ore fines, the moisture should not exceed (7%) in so prepared mix. Thus, the sludge was converted to non-dusting, free flowing material suitable as iron ore fines to be used in sinter making machine for sinter production. These sinters were charged in BF for making iron. Results and Discussion The major issues in GCP sludge recycling effort are economy and probable effects on the quality of steel produced. Generation of GCP sludge is 8 kg/ton of crude steel productions. For producing 1.8 million ton steel per annum, BSP dumps about ton sludge every year at a rate of production of 40 ton/d. After implementation of this scheme, entire quantity is utilized and the generation rate has reduced to 7.4 kg/ton. Chemically, GCP sludge contains mainly iron (50%), calcium oxide (16%), and magnesium oxide (4-5%), besides alumina, silica, basic oxides and zinc. Operationally, the final moisture content of mix is critical for proper handling and charging. Too low moisture may cause dust generation and too high may cause plugging of hoppers and bins. Presence of moisture may lead to the following problems: (i) Explosion in converter vessel may take place because the presence of O 2 and high temperature generation of H 2 ; (ii) Disintegration of briquettes will take place due to hydration of lime; and (iii) Wet sludge/briquette does not flow smoothly through Vibro-feeders and can cause chute jamming and lump formation in the bunker. Experiments showed that sludge (moisture, 3-4%) is optimum to avoid the above problems. A moisture level (< 3-4%) causes excessive fugitive dust, and moisture level (> 7%) starts choking in the flow line and equipments. Two tests, Shatter and Tumbler, were carried out to find out the suitability of the briquettes for industrial applications. In Shatter test, 20 kg of briquettes was allowed to fall 5 times from a height (2 m) on a steel plate (40 mm thick). The percentage fraction ( 5 mm), which remained intact, was reported as Shatter Index, which for 5 samples was found as follows: sample I, 89.2; II, 89.4; III, 90.1; IV, 91.0; and V, For Tumbler test, 15 kg of briquette was subjected to tumbling in a drum (diam, 1000 mm; height, 500 mm). The drum was rotated at 25 rpm for 8 min. The percentage fraction (+6.3 mm) was reported as Tumbler Index, which for 3 samples was found as follows: sample I, 75.8; II, 79.2; and III,

4 AGRAWAL & PANDEY: RECYCLING OF BASIC OXYGEN FURNACE SLUDGE STEEL PLANT 705 Fig. 2 Composition of lime sludge briquette (LSB) Fig. 3.Chemistry of dolomite fines Fig. 4 Composition of dolomite sludge mix (DSM) Results of both indexes show a high degree of suitability for the respective applications. Major components of LSB are CaO and Fe (Fig. 2). Initially, LSB (1 ton) was fed to the converter. On satisfactory performance, the quantity was increased (2, 5, 10 and now 15 ton) of LSB, which is being supplied every day. Thus, BSP is able to consume entire sludge this way. The use of iron rich GCP sludge has reduced the iron ore addition in the BOF converter. The reduced oxygen consumption is another benefit; as the presence of oxides is high in the GCP sludge. Further, the converter sludge, having high percentage of lime, would reduce lime consumption also. The most significant engineering benefit was in the form of a good vessel coating, which was achieved in using LSB. A better coating of the BOF directly results into a longer life of the coating of the converter. Experiments using dolomite fines have provided even better results (Fig. 3). In this process, the sludge dehydration was achieved by mixing it with hot dolomite fines (~400 O C) to produce low moisture material suitable for charging in sinter machine as a replacement of iron ore fines. The presence of an appreciable percentage of CaO and MgO in DSM (Fig. 4) makes it a suitable feed for the BF. In blast furnace, DSM is used in the form of pellet, which is manufactured by supplying DSM through vibro-feeders. In this process, the chute jamming and lump formation in the bunker are the major problems. To avoid these problems, the optimal moisture content (5%) in the DSM was maintained. This resulted in a proper flow ability of the mix ensuring a smooth operation. Greater economy by the use of otherwise wasted heat and the chemical properties of the DSM have proved the advantages, which can be accrued by such researches. Fiscal calculation shows that a saving of more than $10 per ton of steel produced is easily attained by this method. Thus, this research has successfully invented the method of waste recycling suitable to the Indian conditions. Another major technological issue concerns the high cooling effect of the oxidized by-products of BOF. The dust, scales and sludge from iron- and steel making processes, which usually have high iron content, are in oxidized form virtually without exception. Iron oxide containing material can be used as a substitutive coolant during converter blowing process. Traditionally scrap and iron ore (usually pellets or sinter) are used as coolants. Charging iron oxide to converter may also speed up slag formation in the beginning of the process and decarburization reactions. One of the biggest problems concerning utilization of oxidized by-products in BOF-steel making is probably a high cooling effect. This decreases the scrap melting capacity of a converter, which may reduce total amount of steel produced. Adding a reductant with the by-

5 706 J SCI IND RES VOL 64 SEPTEMBER 2005 product charge could diminish this problem. It is also possible to prevent oxidation of dust in some cases, when the cooling effect decreases. Anyway, at least sinter, which is used as a secondary coolant, could be replaced by a by-product with a high iron oxide content. The cooling effect of sinter is close to cooling effects of several dust, scales and sludge. The amount of dust produced during converter process may be increased when by-products are added. Some elements are also enriched in the dust. These factors may increase scaffold formation on the inner surface of the flue gas duct. Using by-products as raw material may increase content of some elements in raw steel. However, this experiment has successfully countered the problems identified as above because the actual trials in both BOF and BF have shown no adverse effect on the product quality and quantity. In this study, there was no negative cooling effect in either of the furnace following the mix proportions devised. Both experiments have developed a unique technology of sludge recycling in BSP in an environment-friendly way. Owing to its simplicity and visible benefits, the DSM technology has already been adopted by BSP. Overall, the use of LSB or DSM is advantageous due to the following reasons: (1) Iron ore use has been eliminated partially; (2) Reduction in oxygen consumption is envisaged; (3) Reduction in lime consumption; (4) Good coating of converter vessel was seen; (5) Land filling is reduced; (6) This has helped in controlling final temperature by reducing desired tapping temperature; and (7) Longer life and thus less frequent repairs of the BOF lining. Conclusions Converter sludge briquette/mix (LSB, DSM) are low cost, in-house produced by-products, which have been found to be valuable recyclable material. Their use is an eco-friendly solution to the problem of landfill disposal of huge quantity of BOF sludge, which is becoming increasingly unaffordable. This study will result in a net return to any integrated steel plant by the recovery of iron, reduced landfill cost and substantial reduction in steel making cost. Through this work, a viable industrial process has been developed for treating steel making sludge to produce useful by-product having improved flow rate properties in a recycling system. LSB and DSM are environment friendly and portend substantial cost savings. Acknowledgments Authors sincerely acknowledge the support provided by the Management of Bhilai Steel Plant, Bhilai. RKA acknowledges the support of Bhilai Institute of Technology. References 1 Hiltunen A & Poylio E, Sustainable development as a basis for recycling in the steel industry, in Seminar on Economic Aspects of Clean Technologies, Energy and Waste Management in the Steel Industry (UNECE, Linz), Pandey P K & Agrawal R K, Utilisation of mixed pond ash in an integrated steel plant for making superior quality bricks, Bull Mater Sci, 25 (2002) USEPA, Profile of the Iron and Steel Industry (EPA/310-R ) (U S Environmental Protection Agency. Washington, D C) 1995, Pandey P K, Patel K S & Subrat P, Trace elemental composition of atmospheric particulate in central India, Sci Total Environ, 215 (1998) Pandey P K, Patel K S & Lenicek J, Polycyclic aromatic hydrocarbons: Need of assessment of health risks in India? Study of an urban industrial location in India, Env Monit Asses, 59 (1999)