Print This Page

Iron Making in Vizag Steel

R N Bhattacharya, AGM (Oprm) N S Rao, DGM (Oprn), RINL

31 Jan' 2005

RINL operates two giant iron-making facilities at Visakhapatnam Steel Plant - Godavari and Krishna having 3200 M3 useful volume each. These are the first of its kind in India with a host of novel features like conveyor charging, bell less top, circular cast house with four tap holes and slag granulation plant at cast house. The potential energy of BF gas is utilized to generate power in gas expansion turbine. Godavari (BF#1) and Krishna (BF#2) are in midway of second campaign at this time. These two campaigns are earmarked by number of successful operational events particularly green field commissioning of BF# 1, low Si metal quality supplemented by high productivity with improved cast house management, better tuyering, quality control of raw materials, improved distribution and oxygen enrichment etc., Green field commissioning of Godavari (BF# 1) was done successfully in March 1990 with four pig casting machines to handle the total hot metal for few months as Steel Melting Shop started functioning little later in August 1990. The furnace achieved its rated capacity as per plan within a year of commissioning. Subsequent to stabilization of BF#I, 'Krishna' BF#2 was commissioned in March 1992. After commissioning of BF#2 both the furnaces had to run at lower regime due to the difficulty in hot metal utilization as SMS was in its initial period of stabilization. This had led to pouring of huge quantities of hot metal in PCM for example one million tonnes of hot metal (50% of total production) was poured in PCM in the year 1993 94 alone. Hot metal production varied in the range of 6000 tonnes/day to 7500 tonnes/day till 1993-94. As SMS started picking up during the year 1994 - 95, blast furnaces faced problems with poor availability of cast house, inferior quality of tap hole mass etc., while boosting up the hot metal production level with high regime and low Si operation. And furnaces could not be operated beyond 1.8 kg/ cm2 top pressure. Identification of bottlenecks and finding solution Key problematic areas were identified as Cast house availability Hearth drainage Taphole mass quality Trial with castables in more vulnerable areas of runner system were taken up on experimental basis from 1994 - 95 onwards. Full-scale usage of castables in runner system from the year 1995 - 96 had improved the cast house availability. The main runner profile and method of preparation was also modified to achieve more life in the runner and thus improvement in cast house management. The drill bit diameter was reduced for better drainage by controlled longer tapping. Bauxite based taphole mass was developed and stabilized by in-house trial to have smooth tapping at higher top pressure. With the above technological initiatives furnace could be operated at higher blast regime and achieved 94 per cent capacity for two successive years 1995-96 and 1996-97. Low silicon hot metal in the range of 0.40 - 0.45 were produced on constant basis. Period of set back During the year 1997-98 and 1998-99 the production suffered due to technological problem of BF# 1 in 1997-98 (top throat segments were severely deformed. and the furnace was not accepting wind because of this problem) and non availability of metallurgical coke in the second half of the year 1998-99. Due to the coke shortage furnaces were throttled and operated almost in staggered manner during this period. Capital repair Due to frequent stoppages of the furnace during the coke shortage period a number of cooling elements were damaged and the furnace health started deteriorating. With concentrated efforts, the year 1999-00 had recorded growth with 2.94 MT of hot metal against low production of 2.5 MT of hot metal during the year 1998-99. As the furnaces health was alarming it was decided to take up Cat - 2 capital repair of the furnaces in staggered manner. 2nd Campaign Fresh technological initiatives were made after the capital repairs to boost up productivity and minimizing fuel rate. Major thrust areas were chosen as Optimization raw chemical quality materials (Physical and chemical quality) Uniform tuyering (Uniform blast regime) Hearth drainage A special drive on optimization of raw materials, was taken up for improving the furnace running. The following was done in different stages. Attention on coke screening in coke oven end and in BF stock house. Stabilization of AbCh/ Si02 ratio in sinter and iron ore around 0.45 to control bosh slag quality and thereby improving the driving rate of furnace Optimization of CSR and CRI of coke in the range of 64 to 65 and 23 to 24 respectively. This was to get proper strength of coke and the required burning efficiency of coke in front of tuyeres. Better control on weighment of charge materials through PLC for bottom charging Finer control on burden distribution with the help of PLC for top charging. Uniform tyering as the main motto was adopted in second campaign. BF-1 and BF-2 was provided with dia of 130mm and 140 mm respectively with an aim t achieve tuyere velocity beyond 210-215 m/sec which was the requirement for such bigger furnaces. Blanking philosophy was totally avoided. RAFT was maintained stable in the level of 1950 - 2000°C to keep smooth furnace movement. With the addition of radiation pyrometer for on line hot metal temperature 20 measurements, RAFT was controlled in better way to achieve 1460 - 1480°C hot metal temperature in addition to the conventional way of maintaining Silicon level in metal to keep the furnace chemically thermally balanced. A total conceptual change was brought in tapping management front. Instead of no of tapping monitoring which had been the general approach during the first campaign, the tapping duration and the gap in between tapping became the prime monitoring parameter for improving hearth drainage. A target was fixed to have 68 - 70 per cent tapping duration of total running time of the furnace (with the closing to opening gap 40 - 45 minutes max.). Consistent quality of tap hole mass and increased arm length of drilling machine had helped in better tapping practice and thus tapping duration of about 70 per cent could be achieved. As a result of the effective attention on key areas as detailed above, the year 2000-01, 2001 - 02 had witnessed a growth of 8 to 10 per cent respectively, despite the loss due to capital repair in BF#l (00 - 01) & BF#2 (01 - 02). VSP at its peaks Relentless efforts were made to drive the furnaces at faster rate after the installation of oxygen enrichment system in June/July 2002 in cold blast line and the furnaces were tuned at 1.6 per cent of oxygen with the adjustment of RAFT at 2000 - 2050° C. By the above action the year 2002-03 could produce 3.94 MT (13% growth) and the hot metal production crossed 4 MT land mark in 2003-04. Present scenario The year 2004 - 05 had started with a bad note due to the crisis in availability of coal, coke at international market. The production was throttled to a level of 10,000 tonnes/day (earlier level of 11,500 tonnes/ day) by controlling oxygen enrichment and blast regime. As the crisis became acute in April (1st week onwards) and the production had to be restricted to 9200 tonnes/ day, a technological decision was taken to reduce tuyere area by blanking and putting copper sleeve (110 dia for BF#1 and 120 for BF#2) in uniform manner so as to achieve tuyere velocity beyond 200 m/ sec. at restricted blast regime such that the problem of loosing cooling elements which was experienced in 1998-99 do not get repeated. In May '04, coke availability in the international market improved in a smaller way whereas availability of coal was poor. And strategically furnaces operation was controlled as per requirement of steel melting shop, as hot metal pouring in pig casting machine was not technically viable with the use of very costly external coke. Subsequent to improvement in raw material availability improved towards the end of Aug '04, the furnaces were allowed to function in full regime. -Though BF#2 started picking up gradually, BF# 1 had some hick-ups while picking up as there were some uneven erosion in top segment areas (non cooled portion) which was gunned just one year back. The furnace started behaving erratic with uneven stock rod movement, the matter became worse with the poor quality of raw materials. The quality suffered particularly in coke front from CRI and in sinter and 1/O from Al2O3/ SiO2 point of view. As the bosh slag basicity became high coupled with high Al2O3, the furnace wind acceptance became very poor. After a lot of deliberation different actions were initiated Persuading NMDC to supply I/O fines of controlled alumina Procurement calibrated lump ore to minimise micro fines which was coming with iron ore lumps Selection of proper coal blend so as to get CRI around 23. Fine tuning of burden distribution in BF#l for improving the movement. (Allowing more peripheral movement) Re-gunning the upper stack area in Oct '04 The furnaces started responding from Oct '04 onwards with finer quality control in raw materials from source to stock houses of furnaces. The actions like high screening time, micro blending of coke in stock houses, using sinter of different size etc, showed better result. Further initiatives like control of Al2O3/SiO2 ratio around 0.40 with the availability of silicious ore from NMDC and maintaining constant coke quality having CRI of 23, frequent good inputs like CLO and coke from NINL (as good as VSP COKE), were taken to improve the production in November (The gunning of throat area of BF# 1 was completed in October). Conclusion With many challenges around BF department of VSP could excel in tough times by exploring new technology and adopting them at appropriate time in phased manner. While furnaces are being operated at the peak level, hot metal quality requirement is becoming more stringent day by day with the fast changing market, and with the dearth of required quality raw materials, further enhancement and sustaining the growth is an uphill task. But, with the strict technological discipline, with the adoption of new technologies and highly motivated and educated work force the set objective of 3.95 mt hot metal production for the year 2004 - 05 will be met.