RECENT PROGRESS. INTERNAL HOT METAL DESULPHURIZATION BY MULTI-INJECTION AT THYSSENKRUPP STAHL AG s (TKS) STEELWORKS

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1 RECENT PROGRESS IN INTERNAL HOT ETAL DESULPHURIZATION BY ULTI-INJECTION AT THYSSENKRUPP STAHL AG s (TKS) STEELWORKS van den Boom, Heinz; Ender, Alfred*; Kwast, Hartmut *Conference Section Chairman & Speaker ThyssenKrupp Stahl AG, Duisburg, Germany page 1 of 11

2 Abstract Hot metal desulphurisation at TKS has a long tradition. Until the early 8ies the total amount of hot metal had been desulphurised only with carbide blends in torpedo ladles outside the steel plant. The nominal sulphur content was <. %. An even lower content could be achieved by additional treatment in the steel plant. The ever increasing requirements to steel properties and the growing demand for steel qualities and quantities with lowest sulphur contents of down to.1 %, forced TKS to initiate a chain of developments concerning the injection process for internal hot metal desulphurisation. The development started with the mono-injection of magnesium blends in the mid-8ies, followed by the co-injection of magnesium and carbide blends and culminated in the multi-injection of magnesium, carbide and lime since the early 9ies, nowadays a standard desulphurisation process in the Bruckhausen and Beeckerwerth steelworks. The development of e.g. controlled injection nozzles put TKS in a position to stop the utilisation of mechanically less stable blends for mono-injection and resulted in a measured dosing of single, mechanically stable desulphurisation agents in multi-injection. The present plant technique allows of well-aimed sequencing of desulphurisation agents according to a thermo chemical point of view. The process is optimised with respect to costs or time by metallurgical software in the process computer. Since the introduction of mono-injection in the mid-8ies, the quantity of magnesium equivalents, necessary for the desulphurisation, could be practically halved by means of multi-injection. Approx. half of the desulphurisation costs are caused by the desulphurisation agents, the other half is generated by the loss of hot metal, slag handling and deposition. Therefore the reduction of hot metal losses is one of the major objectives. Trials with fluoride containing carbide blends were intensified. Thus hot metal losses could be significantly reduced by more than %. page 2 of 11

3 INTRODUCTION Up to now the increasing requirements to the properties of steel caused by technological innovations necessitated a steadily decreasing sulphur content down to ppm. Subsequently, with improving steel properties the hot metal desulphurisation philosophy had to be changed consequently from the BF desulphurisation in the 6ies, via the external torpedo ladle process with carbide until the early 8ies to a steel plant internal injection process, that developed from mono- and co-injection finally into a multi-injection process using magnesium, carbide and lime as desulphurisation agents.. The development of the hot metal desulphurisation at Thyssen Stahl AG until 1989 is described in detail in a different paper (please see reference 1). The following article deals with the development of internal hot metal desulphurisation based on injection treatment at ThyssenKrupp Stahl AG since OXYGEN STEEL PLANT In the following the oxygen steel plant, especially the hot metal desulphurisation facility, is referred to as an example for the description of the development in injection technology. Lay out The oxygen steel plant (Figure 1) has a crude steel capacity of about 5. m t/y. 6 % are cast on a 2- strand continuous casting machine. The other 4 % are cast on a CSP-line that started operation in Steel grades As far as the grades for slab processing are concerned, the production mix for the slab caster is shown in Figure 2. Electrical steel like non-oriented and grain-oriented sheet grades amounts to a quarter. Low carbon steel is mainly used for tinplate. 5 % of the electrical steel is grain-oriented. The ULCgrades are for automotive and D&I-applications. HOT ETAL DESULPHURIZATION TKS first multi-injection station, built in 1991, was replaced in 1999 with new equipment in cooperation with KÜTTNER. By means of this replacement the capacity was increased to 4.5 m t/y of hot metal. The desulphurisation station comprises two separate co-injection facilities with two lances each. Silo storage The segregation-free desulphurisation agents allow a storage in big units. Thus the facility consists of three 1 m 3 silos (Figure 3). 6 t agnesium, t carbide and 7 t lime can be stored at the same time. The material transportation out of silo trucks is performed by conveying with nitrogen at a rate of 4 kg per minute with a pressure up to 4 bar. Each silo is equipped with a pressure control (max. 35 mbar) and a gas control as a security check for oxygen and acetylene generation. From the silos the agents are conveyed through a screening device (4 mm mesh size) into 1 m 3 transfer dispensers (Figure 4). Further on the materials are stored in pressure- and gas controlled stock bins. 5 m 3 of g-agent, m 3 of carbide and m 3 of lime are disposable to fill the two dispenser lines. The whole system is working fully automatically. Operating process control system The system consists of control units for the silo plant, the injection units and the hydraulic system which are connected to the industrial (in-house) Ethernet-network. (Figure 5). The gateway PC is the junction between the a. m. Ethernet-network and the TCP/IP network of Level 2 computing. The visualisation system consists of server and clients which run the WINCC-visualisation software. The process is time- and cost-optimised by a metallurgical model that is run on a separate PC. page 3 of 11

4 ulti-injection of desulphurisation agents The order and the quantity of the desulphurisation agents are optimised with regard to the thermochemical point of view. Desulphurisation facility Each of the two dispenser lines consists of three independent dispensers, one for lime with a capacity of 2 m 3, one for carbide (2 m 3 ) and one for magnesium (1 m 3 ), see Figure 6. The injection could be optionally performed with difference- or top pressure control. The process is slide gate nozzle operated with 5.8 bar top and 2.8 to 3.2 bar transport pressure and a transport gas rate of 8 l per minute. The injection rates range from 4 to 6 kg per minute for lime, 3 to 45 kg per minute for carbide and 7 to 12 kg per minute for magnesium. For each desulphurisation position exists one lance carriage with two alternately used T-hole-lances, one for temperature-measuring and one for sampling. The carriage is laser induced coupled. The lifetime of lance is about 65 heats. Temperature measurement and sampling are carried out before and after desulphurisation. The lance immersion depth is a function of the hot metal volume in the charging ladle. The depth varies between 3.3 and 3.7 m, depending on the amount of hot metal. Development and optimisation (material amount) Figure 7 shows the advantages of multi-injection as result of the minimisation of g equivalent amounts as function of the final sulphur content compared to mono- and co-injection (2). The first drop in g equivalent of about to 25 % compared to mono-injection, resulted from the introduction of co-injecting magnesium (8%) with carbide/lignite. To improve the magnesium-yield, the gas-generating and turbulence-causing diamide lime (CaCO 3 ) in the carbide blend was substituted by lignite to get rid of the oxidising conditions caused by CO 2. The final drop of about 55 to 6 % in total, could be achieved by means of optimising measures concerning the multi-injection: - continuously adjustable injection nozzles - development of concentrated mechanically stable and segregationfree desulphurisation agents - optimisation of plant techniques Figure 8 shows the achieved cost reduction since 198. In comparison with the torpedo ladle. desulphurisation in 198 over the years a total reduction of 73 % has been reached. Optimisation (treatment time and costs) The development of nozzles with a continuously adjustable cross section together with a sophisticated electronic controlling device made it possible to realise more complex injection schedules (3), which allowed for an optimisation of costs and, if necessary, of treatment time. Figure 9 shows the range of injection schedules with operational influences on costs and treatment time. Cost reductions of up to % became possible. A reduction of treatment time by about 15 % occurred, but the costs rose by about 6 %. aterial Efficiency During torpedo ladle desulphurisation with carbide/diamide lime the consumption of material had been determined with mathematical functions tuned separately for each desulphurisation blend. As far as mono-injection was concerned there had been no problem. g equivalents The introduction of co- and later multi-injection necessitated, due to varying material proportions and to make handling easier, a new mathematical description. Thus the efficiency of each desulphurisation agent had been compared with g, resulting in the so-called magnesium-equivalents (g*). Following a physicochemical and kinetic approach, the g equivalents (g*) can be calculated as a function of initial (S i ) and final (S f ) sulphur content. g* (kg/thm) = A 1 * log(s f /S i ) + A (1) A represents a factor for the g loss due to slag conditioning. A more complex function had been used for the description of temperature dependence and g*s solubility product (4) g* (kg/thm) = [A 1 + A 6 * T i] * ln(s i /S f ) (2) +[A 2 + A 7 * T i] * ln 2 (S i /S f ) +[A 3 + A 8 * T i] * (S i S f ) page 4 of 11

5 +[A 4 + A 9 * T i] * 1/S f + A 5. Technical carbide The g equivalent factor for technical carbide had been evaluated by the minimisation of the residual deviation on the base of equation (2). Figure shows the result using operational data from the co-injection of a % g/carbide blend with carbide/lignite (4). At the extreme the g equivalent factor yields to.15 kg g* per kg technical carbide. Lime In Figure 7 the g equivalent factor has been defined constant as.75 kg g* per kg technical carbide. A recent evaluation of coinjection trials with 89 % agnesium and lime shows influences of temperature, hot metal composition e.g. silicon content on lime efficiency (Figure 11). From the TKS hot metal conditions results on average a g equivalent factor of.4 kg g* per kg lime. Ability to achieve specifications ulti-injection proved to be a precise and excellent process. Its reproduction is one of the suppositions for cost effectiveness and efficiency. Therefore altogether a high ability to achieve the specifications in more than 8 % of the cases had been achieved in the fiscal year 1/2 (Figures 12 and 13). The hitting rate increased from 8 % to more than 9 % with rising sulphur target contents >.2 %. For extremely low concentrations of ppm about 6 % of the target extent is accomplished. DESLAGGING Besides the desulphurisation treatment itself the deslagging of the sulphur-enriched process slag is another important cost effective measure. On the one hand the BOF process is poor in desulphurisation, because sulphur can be carried back into the bath. On the other hand extreme deslagging causes high hot metal losses and thus increasing costs; because of the bad slag preparation conditions. Deslagging facility Deslagging devices are integrated into each desulphurisation station. During the development of multi-injection the completion of deslagging had been aided by inorganic additives and/or nitrogen bubbling with a submerged lance. The deslagging facility is depicted in Figure 14. Nitrogen lance A similar effect is realised by means of a submerged lance, bubbling with nitrogen. The lance is positioned on the opposite side of the ladle lip. 15 l per minute are enough to float the slag into the direction of the ladle lip. inimization of hot metal loss The desulphurisation agents cause half of the desulphurisation costs. The other half occurs due to in particular hot metal loss and slag handling. Therefore the reduction of the hot metal loss is one of the major objectives. Ladle design The design of the ladle lip has a tremendous influence on deslagging as far as the separation of the desulphurising slag from the remaining hot metal is concerned. Radial ladle lips provoke considerably hot metal losses during completion of total slag removal, especially for grades with lowest sulphur specifications. A special ladle lip has been constructed (Figure 15). During deslagging this ladle lip forms an inclined ramp on which the slag and hot metal is easily separated. The consequent adoption of the above mentioned deslagging principle led to a tremendous reduction of slag skulls, thus considerably improving the preparation process of the slag. Additives based on fluoride Beside the minimisation of the mechanical hot metal loss additional endeavours had been done by the reduction of the hot metal granules captured inside the slag. The addition of surface-tension-effecting components to the desulphurising agents had proven positive. Useful for this purpose are fluorspar (CaF 2 ) or cryolite (Na 3 AlF 6 ). The addition of cryolite to the carbide/lignite blend in multi-injection in oxygen steel plants led to a reduction of hot metal losses of about 3.6 kg/t hm (Figure 16). This reduction amounted to more than %. page 5 of 11

6 By means of these fluorides the desulphurisation efficiency is negligibly negatively effected. But the overall reduction in desulphurisation costs is considerable. SUARY By means of multi-injection, realised with modern injection technology and tuned by a metallurgical process computer model, steel making costs can be reduced significantly. The combination of metallurgical measures, especially the introduction of thermo chemically adopted injection schedules, led to a minimisation of desulphurisation agents quantities necessary to achieve the sulphur specifications with high precision. In addition ladle design and fluoride based additives in the carbide/lignite blend led to a considerable reduction in hot metal losses. As far as the recycling of steel plant residuals is concerned the preparation and screening of the hot metal ladle slag could be essentially improved. page 6 of 11

7 REFERENCES (1) H. v. d. Boom, A. Ender, W. Florin and D. Kirsch, Recent progress in hot metal desulphurization using calcium carbide and magnesium-based mixtures at Thyssen Stahl AG, in: Past to Future, 47 th Annu. World agnesium Conf., 199, Cannes, p. 83. (2) A. Ender, H. Kwast and H. v. d. Boom, Development of the multi-injection for the hm-desulphurization with g agent, carbide and lime in the oxygen steel plant # 1 (R&D final report), Report No. 1/3, Business unit hot rolled coil, ThyssenKrupp Stahl AG, January, 1. (3) A. Ender and H. v. d. Boom, Calculation of injection schedules for the operation of adjustable tuyeres for the hot metal desulphurization in open charging ladles at oxygen steel plant # 2, Report No. 92/, R&D Division, Thyssen Stahl AG, September, (4) A. Ender,. Tutte and H. v. d. Boom, Functions for the calculation of specific amounts of desulfurization agents for the hmdesulphurization in oxygen steel plant # 2, Report No. 88/, R&D Division, Thyssen Stahl AG, Juni, (5) A. Ender and H. v. d. Boom, Co-injection of AZ91 with lime and lime efficiency, Report No. 2/17, Business unit hot rolled coil, ThyssenKrupp Stahl AG, November, 2. page 7 of 11

8 Casting bay Furnace hall Finishing train Laminar cooling Coilers FIGURES Silo 1 Silo 2 Silo 3 Oxygen steel plant with thin slab mill Loading station Transfer bin P< 1 F> Transfer bin P< 2 F> Transfer bin P< 3 L< Pressure surge Silo 3 Lining hall Auto filling Auto filling Auto filling Casting bay Gas analysis O 2 C 2 H 2 Stock bin 1 lime Gas a. O 2 C 2 H 2 Stock bin 2 carbide Gas a. O 2 C 2 H 2 Stock bin 3 g agent L nf F> F> F> Ladle furnace RH-plant LTS-plant Dispenser Line Inset hall ixer hall De-S-plant Slag hall Vessel hall Scrap shop Storage hall Fig. 4 Desulphurisation agent conveyed through transfer bins to silo storage for the distribution into the dispenser lines Fig. 1 Layout of an oxygen steel plant Quality mix Quality group frequency, % Low carbon steel 57 Ultra low carbon steel CC slab caster, fiscal year 1/2 Electrical steel 25 C - Grades 2 Fig. 2 CC quality mix 16 Fig. 5 Oxygen steel plant - Network structure hm-desulphurisation line Filling device Filling device Filling device Dispenser line 1 Pressure control L> L> CaO CaC 2 g Pressure Pressure control control L nf L nf L nf N 2 L< L< L< L> N 2 LC1 Lance carriage Laser coupling LC2 Position 2 Lance bogie Position 1 top bottom Ladle Fig. 3 Desulphurisation agent transport and silo storage Fig. 6 Desulphurisation facility of an oxygen steel plant page 8 of 11

9 g equivalents, kg/t hm Standardized desulphurization costs, % Co - injection ono - injection Standardized desulphurization costs, % Residual deviation, kg g/t hm Cumulative frequency, %,9,8,7,6,5,4 H-desulphurization, development ono-injection: g 5% Co-injection: g 8% / Carbide ulti-injection: g 97% / Carbide / Lime S initial :,4 % Co-injection: g/carbide blend (% g) with Carbide/lignite,4,3,2,1 n = 135 inimum equivalent factor,15, Final sulphur content, -3 % Fig. 7 Optimisation of the specific demand on g equivalents,,,5,,15,,25,3 kg g equivalent/kg technical carbide Fig. Evaluation of the g equivalent factor for technical carbide H - desulphurization, development Price reduction desulphurization agents 27 Co-injection: g(89%) agent lime g 9 % (CaF2) + Carbide/lignite 41 g 97 % + Carbide/lignite 5 Optimization plant technics 53 g (45;6 %) blend + Carbide/lignite 57 g blend ( %) + Carbide/lignite 62 g % Carbide/lignite 8 % 71 2 hole T - lance 94 g 5 % Al 2 O 3 5 % 111 Carbide 75 % CaCO 3 25 % Fig. 8 Development of the desulphurisation costs Fig. 11 Influence of hot metal silicon content and temperature on lime efficiency g blend: v max Carbide : v max Optimization plant technics, 1992 g blend: v min Carbide : v max g blend: v max Carbide : v min Relative treatment time, % kg/ 4 min 3 g time Carbide g blend: v5 % Carbide : v min Fig. 9 Optimisation of desulphurisation time and costs (co-injection g blend/carbide) Fiscal year 1/2 ulti-injection g 97% / Carbide / Lime S final = S aim S aim n ppm Sulphur content, ppm Fig. 12 Hitting rate multi-injection (S aim < 6 ppm) page 9 of 11

10 Cumulative frequency, % Laser Deslagging loss, kg/t hm Fiscal year 1/2 3 Deslagging, optimization ulti-injection g 97% / Carbide / Lime S final = S aim S aim n ppm - 15 inimization of hm loss 3,6 kg/t hm 4 3 ulti-injection g 97% / Carbide / Lime 7/ / / / 25 26/ Carbide/lignite with % Cryolite Carbide/lignite with 2 % Cryolite Sulphur content, ppm Total amount of desulphurisation agents, kg/t hm Fig. 13 Hitting rate multi-injection (S aim > 6 ppm) Fig. 16 Optimisation of hot metal loss Deslagging position 1 coupled LC1 Secondary dedusting Deslagging device 1 Auto dedusting gate Deslagging machine 1 Suction hood top bottom Oiling finished L> L< locked free L> Water cooling Deslagging track Ladle tilting device Adjustment Deslagging agent Hydraulic standby operation Fig. 14 Deslagging facility Hot metal Slag Fig. 15 Optimisation of charging ladle lip for the reduction of hot metal loss page of 11

11 FIGURE CAPTIONS Fig. 1 Lay out of an oxygen steel plant Fig. 2 CC quality mix of an oxygen steel plant Fig. 3 Desulphurisation agent transport and silo storage Fig. 4 Desulphurisation agent conveying through transfer bins to silo storage for the distribution into the dispenser lines Fig. 5 Network structure hm-desulphurisation line Fig. 6 Desulphurisation facility of an oxygen steel plant Fig. 7 Optimisation of the specific demand on g equivalents Fig. 8 Development of the desulphurisation costs Fig. 9 Optimisation of desulphurisation time and costs (co-injection g blend/carbide) Fig. Evaluation of the g equivalent factor for technical carbide Fig. 11 Influence of hot metal silicon content and temperature on lime efficiency Fig. 12 Hitting rate multi-injection (S aim < 6 ppm) Fig. 13 Hitting rate multi-injection (S aim > 6 ppm) Fig. 14 Deslagging facility Fig. 15 Optimisation of charging ladle lip for the reduction of hot metal loss Fig. 16 Optimisation of hot metal loss page 11 of 11