TOTEM Co Ltd.

Operating efficiency of Rotary Charging Unit

as Used on Blast Furnaces at the West Siberia Steel Works.

A.F.Avtsinov, B.M.Boranbayev, V.N.Vakoulin, V.V.Gaydouk, M.F.Maryasov

West Siberia Steel Works, Totem Co. Ltd.

3rd Mytischinskaya Str., 16, block 60, 6th floor, Moscow, 129626, Russia

Tel.: +7(495)631-62-09, Fax: +7(495)631-47-77

E-mail: totem@totem-engineering.com

Key words: Rotary Charging Unit, Blast Furnace

As of late, there has been a growing interest among ironmakers in Russia and abroad towards the performance of the Rotary Charging Unit (RCU), used on blast furnaces, which has lead to the necessity to analyse once again its efficiency at the West Siberia Steel Works, both at the time of it being commissioned and what it is nowadays. The experts from WSSW and Totem Co. Ltd have carried out such an analysis jointly. 

RCU is an up-to-date charging device of new generation, with its salient feature being its rotating distributor of burden, which is located inside the furnace and designed as a five vane rotating propeller. The general view of RCU is given in Figure 1. 

RCU consists of the following main parts: guiding funnels to guide stock as it goes from skips to the receiving funnel; upper bell, transfer hopper/ lock with a gas isolator, which receives stock before it is sent into the furnace; lower bell; angular gearbox for the rotor drive; vane rotor. The charging process is controlled automatically through a specific automatic control system called “Rotor”. 

Burden is distributed throughout the top span by varying speed and spinning direction of the rotor. Depending on the speed, the bulk of stock is lain and the burden ridge is formed at various distances from the furnace central line. As it happens the ridge can be shifted from one annular zone to another, smoothly without sharp changes in profile, which makes it possible to vary ore/coke ratio along the furnace radius in the same smooth way. Owing to 5 vanes, one revolution of the rotor would make 5 layers of stock in the top. One discharged portion of stock makes 30 to 40 layers, on the average. Such a practice ensures a high degree of circular distribution uniformity and averaging of burden in terms of chemistry and size as it is being discharged into the furnace. 

The first RCU, which had been fabricated by the Chief Mechanic Division of the WSSW, was installed on blast furnace No.2 (2000 m³) in June 1989, at the time of 3rd category capital repair. In the first RCU design, in addition to the above-said components, there was a provision of a revolving burden distributor. 

Prior to the erection of RCU a research had been carried out, using a RCU 1:25 scale model and full-size model, to look into the burden distribution pattern in the top. Another research was carried out with the help of RCU in the top before blow-in, again to check the distribution pattern. A comparison of the results, obtained from the physical models and the furnace before blow-in showed adequacy of the data generated by all tests for the distribution of burden along the top radius. This fact made it much simpler to plan later new promising modes of charging operations, on the basis of preliminary data, obtained from a small RCU physical model [1].

Fig.1. Blast Furnace with RCU

It was found out by research that the most flexible control of the desired distribution of burden along the top radius could be achieved by a separate discharge of ore-bearing material and coke, using, in most cases, the ring-wise pattern of charging. 

After blow-in and further blowing the modes of charging practice had been streamlined and specified, taking into account their impact upon the heat main indices (furnace productivity and specific coke rate). Also, main criteria for adjusting modes of charging practice were specified, like pressure drop along the furnace height, distribution of carbon dioxide along the top radius, utilisation of chemical and thermal energy of gas [2]. 

Table 1 shows a comparison of main operating indices of blast furnace No.2 during the base period (1^st half of 1989), when a typical two-bell apparatus was used, and trial periods after the installation of RCU, in August-December 1989, respectively.

Table1. Basic performance indices of BF No.2 (2000 m³) at WSSW, after replacing 2-bell apparatus with RCU 

Аs analysis of the furnace performance showed, thanks to RCU, in the trial periods it became possible to redistribute ore-coke ratio and gas flow respectively along the top radius. CO₂ content in gas in periphery went up from 14.0 to 17.5 - 19.4%, while in the centre it went down from 16.6 to 6.9 - 11.1%. A more uniform distribution of ore bearing material in the peripheral and central zones of the top and appearance of the central gas vent due to a more concentrated discharge of coke all these helped to make the gas flow more even throughout the whole span of the top, furnace run steadier and utilisation of flew gas better. The utilisation rate of CO went up from 45.1 to 45.47 - 45.73%, while the temperature of top gas went down by 7 - 13 ° C.

Table 2. Basic performance indices of blast furnace No.1 (3000 m3), WSSW, as a two-bell apparatus was replaced with RCU 

Having reduced the heat indices to similar conditions, the following results were obtained: furnace productivity in the trial periods had been increased by 7.6 - 10.4% (by 8.8% as average), the specific coke rate has been decreased, as compared with the base period, by 3.37 - 7.1% (by 5.28% as average). It should be also noted that during period No.2, on some dates, when smelting conditions happened to be quite favourable, the output of hot metal would go up to 5200 - 5500 t/day. 

In 1991, a charger with revolving burden distributor of improved design was erected on blast furnace No.1 (3000 m³ useful volume). The RCU on BF-1, which is still in operation, has two guiding funnels and upgraded rotating distributor of burden with a 34 m³ funnel which can accommodate two skips. The interbell space of 63 m³ can hold four skiploads. In practice, burden is charged in two-skip separate (one coke, one ore) portions, though sometimes in mixed halfportions [3]. 

It should be noted that as of late the rotating burden distributors of blast furnaces No.1 and No.2 have been put out of operation. As a long-lasting practice proved, their performance does not exert practically any effect upon the circular uniformity of distribution. This inference has been substantiated by trials on the RCU physical models. 

Table 2 shows the main indices of blast furnace No.1 performance in the base period (1^st quarter of 1991) when a typical two-bell apparatus was in use, and during trial periods (April, May and June, 1992) when RCU was mounted on the furnace. It should be noted that due to a slump in the steel products market, the run of the furnaces in 1991-1992 had been somehow put under restraint. 

As can be seen from Table 2, in the trial periods, when RCU was in use, the furnace productivity, as adjusted to similar smelting conditions, went up by 4.97- 8.1% (by 7.0% average), and an adjusted coke rate went down by 5.87-7.65% (by 6.8% average). Similarly to blast furnace No.2, a research had been carried out on BF No.1 with the aim of finding rational charging procedures, capable of additional loading of ore bearing material to the periphery of the top, stretching of the ridge in the central part of radius and unloading of stock from the central zone. As RCU was used, CO₂ content in the periphery increased from 15.4 to 16.2 - 19.2%, in the middle point of radius it decreased by 0.7 - 2.0% and in the centre decreased from 16.0 to 7.3 -12.5%. 

Thanks to RCU, the run of the furnace became steadier, the furnace became less susceptible to poor quality of coke and ore bearing material. 

The Chief Mechanic Division also fabricated RCU for BF No.1 at the WSSW. All in all, 4 RCUs had been fabricated: 2 for BF No.2 and 2 for BF No.1. 

The first RCU of BF No.2 was taken off during scheduled capital reline work after 15 months of performance, to inspect the condition of components and parts. The lining of vanes and other elements of the Unit and contact surfaces of the bells were found in a satisfactory condition. RCU-1 and RCU-2 on BF No.2 had been operated by turns for 11 years altogether, with the campaign being extended up to 23-25 months. And during that period there were no sign of a sizeable wear on the contact surfaces of bells and cups, lining of the receiving funnels, gas isolator and rotor. There has been no repair of the rotors during the period of their performance, except for replacement of some lining plates on vanes that happened to be slightly eroded. 

RCU-3 and RCU-4 on BF No.1 are also in a satisfactory condition after 9 years of service (also by turns). Some lining plates on the vanes have been also replaced.

 Nowadays all chargers on BF No.1 and BF No.2 are in a satisfactory condition, still in operation. For the sake of comparison, a two-bell apparatus with valves is being used on blast furnace No.3. Its service life is 1.5 years. When taken off, all members of this apparatus are to be reclaimed: old facing from big and small bells, big and small bells funnels, cups and valves is to be removed. After that the revolving burden distributor and valves mechanisms are reconditioned and bells and cups are hard-faced with expensive material and undergo grinding. Sometimes it is necessary to repair or replace big and small bells rods. Cost of the reconditioning of a typical two-bell apparatus (without valves), with allowance to its service life reduced to 1 year, is likely to be even higher.

 It would be interesting to look into the comparison of performance of two similar blast furnaces at the WSSW (BF No.1 and 3000m³ BF No.3), that are equipped with different chargers. On blast furnace No.1 a circular and radial distribution of burden is carried out with the help of RCU, whereby burden is laid at fixed radii in 9 annular zones of the top. On blast furnace No.3 the distribution of burden is controlled by movable throat armours in accordance with the preset charging cycle, with some separate charges like CC¯¯OO, when ore halfportions are loaded into the periphery and coke portions into the furnace central line zone. 

Table 3. Performance indices of blast furnaces No.1 and No.3, WSSW, in October-December 2000, as adjusted to similar smelting conditions 

The blast furnaces are in a satisfactory condition, practically similar ore bearing material is smelted in them. During the year 2000, bituminous lump coal from Bacha colliery (grade SSPK) was used as a substitute of coke in burden. The yearly average rate of it amounted to 8.8 kg/t of hot metal.

 The furnaces are fed with different grades of coke: for BF No.1 it is +35 mm wet quenched coke from quite worn-out batteries No.No.3, 5 and 6, for BF No.3 its is dry quenched +35 mm coke from a new battery No.7, of better quality.

 As was found out earlier, when BF No.1 was switched over to dry quenched coke from battery No.7, the furnace productivity would go up by 3.0%, and specific coke rate would go down by 3.7%.

 To compare the performance efficiency of the furnaces quite an extended period of 4 months was chosen (October - December, 2000), which was characterised by a high output, without failures, going haywire and long downtime. Table 3 shows basic techno- economics of BF No.3 and BF No.1 performance.

 As can be seen from this Table, the adjusted productivity of furnace No.1 with RCU is higher than that of blast furnace No.3 by 5.58% (relative), and the adjusted coke rate is lower by 4.36% (relative).

 A long-lasting experience of operation with various chargers shows that:

 -        In case of BF No.1 with RCU, it is possible, if required, to arrange a developed central gas flow with reduced CO₂ content in the furnace centre down to 0 and to make even ore/coke ratio in the central and peripheral zones of the top, with lowering the peripheral gas temperature down to 100‑150 °C. The variation of temperature of the peripheral gas in the furnace circumference from the mean value is 50‑80° C, which gives evidence of a high degree of burden circular distribution uniformity. The furnace features a considerable reserve in making its run steady and is less susceptible to a worse size distribution in burden. Sour pellets that are used in burden (as of late, their rate has been increased to 25-30%) are charged with the help of RCU into the top radius span and into the furnace central zone thus preventing them from contacting the stack lining. As a result, there have been no cases observed when fusible ferriferrous scull would be built up and then removed. This may help to increase the service life of stacks. 

-        On BF No.3 with a two-bell apparatus and movable throat armours, the burden distribution can be controlled when the stock line has been lowered to 2.1 - 2.3 meters. The variation of the peripheral gas temperature along the furnace circumference is much bigger and comes to 180-220 °C, from the mean value. Attempts to level up the circular distribution of stock with the help of combined actions by movable throat armours and revolving distributor, which operates in the automatic mode of control over gas flow, turned out to be not efficient enough. The furnace is operated with additionally loaded centre and is susceptible to a worsening quality of burden stock. The charged low-basicity pellets would go to the peripheral zone of the top, which results in the build-up of fusible ferriferrous scull on the stack walls. Scull peel-off and the circular non-uniformity of gas flow distribution would be conducive to disabilities in heating the hearth. This is corroborated by a greater variation of silicon in hot metal, as compared with BF No.1, in the period of the 4^th quarter of 2000.

The advantage of RCU is quite obvious and therefore a decision has been made at the WSSW to replace the existing charger on blast furnace No.3 with a RCU.

1. The experience of using RCU on 2 blast furnaces during 9-11 years proves high dependability of its members and components, a small scope of needed maintenance and repair. 

2. RCU helps to improve ironmaking indices. As was found out by research, thanks to RCU the productivity of BF No.2 has increased by 7.89% and specific coke rate decreased by 4.86%. On blast furnace No.1 its productivity has increased by 7.0% and coke rate decreased by 6.8%. 

3. As comparison of BF No.1 and BF No.3 performance in 2000 shows, in case of BF No.1 with RCU, as compared with BF No.3 with movable throat armours, the adjusted productivity has increased by 5.58% and adjusted coke rate decreased by 4.36%. 

4. In view of the obviously better cut of RCU above others, a decision has been made to replace the existing apparatus on BF No.3 with a RCU. The preparatory activities have been already started.

1. G.V.Abramin, B.M.Boranbayev, A.V.Koshelnikov, D.Y.Yankovky. Stahl No.3, 1999, p.p. 1 - 3.

 2. S.F.Bougayev et al, Chornaya Metallourgiya, NTI Bulletin 1992, No.8, p.p. 19-20

 3. S.F.Bougayev, A.A.Antonov, I.I.Basalayev Stahl, 1997, No.6, p.p. 19-22

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