SOURCES OF HYDROGEN IN VACUUM CAISSON EQUIPMENT

Transcription

1 SOURCES OF HYDROGEN IN VACUUM CAISSON EQUIPMENT Martin Korbáš a, Milan Raclavský b a) VÍTKOVICE HEAVY MACHINERY, a.s., Ruská 2887/11, 76 2 Ostrava, ČR.martin.korbas@vitkovice.cz b) ECOFER s.r.o., Kaštanová 182, Třinec, Dolní Líštná, ČR.milan.raclavsky@ecofer.cz Abstract The article summarises the knowledge of hydrogen behaviour in metals, and the basis of kinetics of hydrogen reduction. It discusses models of hydrogen reduction according to Bannenberg and Kleimt, which are based on the equation of first order. The models are compared with the final results of hydrogen content experiments, for caisson equipment, and for vacuum equipment RH. The results prove complete agreement between the experiments and the model, confirming the expected mechanism of reduction of hydrogen content. The paper mainy focuses on analysing the composition of process gasses on the Integrated System of Secondary Metallurgy equipment (ISSM). The analyses performed show the deviations from the theoretical composition of process gasses. The article explains the causes of these deviations, the importance of the quality of ingredients for the composition of process gasses, for the quality of steel produced, and for the safety of the equipment. 1. SOURCES OF HYDROGEN The occurrence of hydrogen in the elemental form under normal conditions is considerably limited. Hydrogen is present in the air in very limited quantities. Composition of dry air, according to [1] is shown the following table: Gas Concentration (%) Nitrogen 78,8 Oxygen 2,95 Argon,93 Carbon dioxide,3 Neon,18 Helium,54 Methane,2 Krypton,11 Hydrogen,5 Xenon,87 Ozone,1 Iodine,35 Radon,6 The main source of hydrogen on the surface of the Earth is liquid water, moisture (gaseous water in the atmosphere) and water bound in hydroxides, the example of which is rust, which is a product of corrosion of iron based metals. Rust forms hydrated iron oxide, which is generated by action of moist environment and oxygen. Another example is hydrated lime. In the manufacture of steel other sources of hydrogen include reaction of iron and possibly other elements with hydrocarbons. In the primary production of iron, hydrogen is continuously dissolved in the melt iron and increasing temperature results in the increasing of its solubility. During metallurgical processes, hydrogen is

2 continuously removed from the melt, especially by carbon reaction. The final contents of hydrogen at the tapping from the main metallurgical units are the following: Electric arc furnace 4-8 ppm LD converter 2-4 ppm Bottom blown converter 6-12 ppm Other hydrogen enters the process during tapping. This occurs through the bare stream of steel, which is in contact with the surrounding atmosphere, deoxidation, alloying elements and slag-forming ingredients. The importance of individual inputs is not covered in literature. Their importance can be studied most from the vacuum processing on a device equipped with a process gas analyzer. There is a framework study of Electro-Nite Company [2] showing that the addition of materials to the melt increases hydrogen content in the melt. The authors unfortunately do not specify the amount and composition of additives. Fig. 1: Hydrogen content in steel before and after administration of additives [2] 2. DEHYDROGENATION Favourable conditions for reducing the hydrogen content of molten steel spilled into the reaction ladle can be created in vacuum caisson. Creating a vacuum in the area above the melt level at concurrent injection of an inert gas through the bottom of the reaction ladle creates a large free surface of molten steel without slag. Slag is a partially emulsified by molten steel, partially driven off to the side of reaction ladle. Free surface, continuously renewed by mixing gas, eliminates hydrogen according to Sieverts Law. Mixing gas bubbles passing through the melt multiply the effect of hydrogen elimination. Increased effect also causes the metal to spatter, which occurs during bursting of bubbles of the mixing gas upon reaching the level of molten metal. In parallel, hydrogen is eliminated from the slag, which thus ceases to be a source of secondary hydrogen after vacuum extraction. Thermodynamic equation of hydrogen removal reaction has the form 1): [H] =,5{H 2 } G = ,5T 1) and the corresponding equilibrium constant and its temperature dependence is determined by relations 2): K H p p 19 H 2 H 2 log K H 2, 423 ah f H * H T rovn 2) The rate of hydrogen elimination is then described by the differential equation 3):

3 d H d A V eff tav * k H * H 3) and its form after integration can be written as follows 4): 4) hence the necessary time required for hydrogen elimination 5): 5) Time dependence of the current content of hydrogen in the metal is given by equation 6): 6) Where [H] is the hydrogen content,k H is the equilibrium constant of hydrogen, p {H2} partial pressure, a [H], ƒ [H] activity, activity coefficient of hydrogen, A eff area through which the hydrogen, V tav volume of melt, k H rate constant,ƭ dhy dehydration rate of hydrogen. In the given volume of processed melt the hydrogen elimination rate is mainly influenced by areas between the bubbles of inert gas (Ar), in which the hydrogen is transferred, and the melt, respectively. melt surface area through which hydrogen passes into the gaseous phase (A eff ). The hydrogen atoms reach this interphase area by diffusion (diffusion coefficient of hydrogen is present in the reaction rate constant of elimination of hydrogen D H = 3,5 * 1-7 {m 2 /s} at 16 C and desorption reaction of origination of hydrogen gas according to the reaction (1) takes place on the inter-phase area (A eff ). Reducing the time of hydrogen elimination to a minimum is therefore a matter of achieving the greatest possible inter-phase reaction area at the given volume of metal and prevention of the flow of hydrogen from secondary sources, mainly from additives. So far a brief theoretical mathematical description of the thermodynamics and kinetics of reaction of hydrogen elimination. Based on similar equations Bannenberg [3] bysed a denitrification and dehydrogenation model in the caisson equipment. The results of this model are also listed in the most current book on metallurgy [4]. Bannenberg calculated the final hydrogen content after vacuum treatment and compared the results with measurement. The comparison is shown in figure 2.

4 Fig. 2: Bannenberg s model - comparison of the hydrogen content measured and computed after vacuum treatment on equipment developed for degassing. The figure shows and Bannenberg explicitly states [3] that the addition of lime has no effect on dehydrogenation of melt. This finding may have two explanations: Lime was dry Influence of lime added at tapping does not differ from lime added in vacuum o Lime releases hydrogen from moisture into the metal in only a very limited manner o Humidity of the lime in the vacuum treatment process is released only from the surface of the slag. 3. PROCESS GAS COMPOSITION, THEORY AND REALITY Bannenberg s model or any model created according to the above equations clearly leads to a gradual reduction in the evolution of hydrogen into process gases. With a reasonable assumption that the flow of other gases is roughly constant, the concentration of hydrogen in process gas should be dropping exponentially, (flow rate of argon, nitrogen flow to protect the camera and aspiration of air through leakages is roughly constant). The exponential progress of dehydrogenation can be disrupted by the addition of materials with dissolved hydrogen or moisture. The ISSM device is equipped with continuous flue gas analyzer. This analyzer is allowed to closely monitor the process gas composition, see the recording of the composition of process gas in heat 29473, Figure 3. Figure 3 also shows the moments and the amounts of added additives.

5 Složení (%) celková hmotnost (kg) nebo (dkg) Průtok (l/min) , Brno, Czech Republic, EU kg Al 219 kg CaO 59 kg Al 319 kg CaO kg Al kg FeMnaff 952 kg SiMn 232 kg FeMn Ch. ohřev :15 13:18 13:21 13:24 13:27 13:3 13:33 13:36 13:39 13:42 13:45 13:48 H2 (%) H2O (%) suma H2 (dkg) suma H2O (kg) V H2 (l/min) V H2O (l/min) Composition in (%), total weight (kg) Flow rate (l/min) Fig. 3: Progress of the composition of process gases, the volume of removed hydrogen and water vapour amount and the time to add ingredients The figure 3 clearly shows the progress of concentrations is not consistent with the theory of exponential decrease. It is further obvious from the figure that the addition of aluminium or lime leads to violent ejections of hydrogen and water vapour. The exponential progress of hydrogen content is apparent from the chart only between 1:17 p.m. to 1:22 p.m. In the time of 5 minutes more than 5% of the initial hydrogen concentration of 7 ppm is eliminated according to Bannenberg. As there is a continuous record of flow of argon and argon concentration, it is possible to calculate the flow of other gases based on dilution equation. It is also possible to integrate these flows and calculate the mass removed hydrogen and other constituents. Record of the hydrogen concentration is shown in Figure 4. Figure 4 is supplemented by the integral of the mass flow of hydrogen and water in process gases.

6 4,76 5,298 5,68 6,36 7,2 8,18 8,984 9,38 Složení (%) 13,419 Celková hmotnost (kg) nebo (dkg) , Brno, Czech Republic, EU 6 59 kg Al 219 kg CaO 59 kg Al 319 kg CaO kg Al 24,32 32 kg FeMnaff 952 kg SiMn 232 kg FeMn , Ch. ohřev 5 13:15 13:18 13:21 13:24 13:27 13:3 13:33 13:36 13:39 13:42 13:45 13:48 H2 (%) suma H2 (dkg) suma H2O (kg) Composition in (%) Total weight (kg) Fig. 4: Progress of the composition of process gases, the volume of removed hydrogen and water vapour amount and the time of addition of additives Heat H2 dkg delta H2 kg Type m (kg) % H2 % H2O ,76 5,298,592 Al 59,1,9 5,68 6,36,68 Al 84,8,73 7,2 8,18,116 CaO 219,5,42 8,984,966 Al 59,16 1,47 9,38 13,419,4381 CaO 319,14 1,24 Average,8713 sum 74,12 1,6 H2O (kg) Delta H2O (kg) m (kg) 16,57 24,32 7, ,47297 Total humidity (%) 2,11 The summary of the performed analyzes implies that: hydrogen is released at the beginning of the process - hydrogen from steel and primary slag, water is concurrently released from the slag, especially at the beginning of the process and then continuously, hydrogen is released after the addition of material and this addition induces significant hydrogen emanations that are manifested by sharp hydrogen peaks.

7 In general, the source of hydrogen in metallurgy is moisture. The balance implies that moisture additions in the above melt was about 2.1%. The main source is CaO and aluminium granulated into water 4. CONCLUSION The ISSM device is subject to extreme demands, which are often mutually exclusive. The technology supplied corresponds to the current and Know-How and in many ways is substantially enriched the knowledge of metallurgical processes. State-of-the-art technologies require superior materials and used raw materials, in particular strict compliance with the moisture in the additives. Problems with ISSM device were mainly due to technical solution of the post-combustion chamber, which was not designed for extremely high hydrogen emanations, while adding additives. The article clearly shows that the progress of concentrations is not consistent with the theory of exponential decrease that exponential progress of hydrogen content is disturbed at the moment of addition of aluminum or lime. LITERATURE [1] KVAPIL B., ŠTĚPÁNEK M., et al.: Československá encyklopedie (Little Encyclopedia of Czechoslovakia), part 6, page 68, Academia, [2] GLITSCHER W.: 15 year HYDRIS a revolutionary method to control hydrogen has become mature, Interoffice Memorandum, [3] BANNENBERG N., BERGMANN B., GAYE H.: Combined decrease of sulphur, nitrogen, hydrogen and total oxygen in only one secondary steelmaking operation. Steel research, 1992, number 1, pp [4] FRUEHAN R. J.:The Making, Shaping and Treating of Steel, 11 th Edition, Pittsburgh, 1998, pp