INFLUENCE OF ALKALIS ON MECHANICAL PROPERTIES OF LUMPY IRON CARRIERS DURING REDUCTION

Transcription

1 INFLUENCE OF ALKALIS ON MECHANICAL PROPERTIES OF LUMPY IRON CARRIERS DURING REDUCTION Anton PICHLER 1, J.L. SCHENK 1, M.B. HANEL 1, H. MALI 2, F. HAUZENBERGER 3, C. THALER 4, H. STOCKER 5 1 Chair of Ferrous Metallurgy, Montanuniversitaet Leoben, Franz-Josef-Straße 18, Leoben, Austria 2 Chair of Geology and Econoic Geology, Montanuniversitaet Leoben, Austria 3 Sieens VAI Metals Technologies GbH, Linz, Austria 4 voestalpine Stahl GbH, Linz, Austria 5 voestalpine Stahl Donawitz GbH, Leoben, Austria Abstract: Alkaline etals like sodiu and potassiu are generally known as unwanted eleents in ironaking processes. Beside the negative effects on refractory and coke consuption, alkalis have a negative influence on the echanical stability of the feed. Previous research work has shown the correlation between alkali content and the destruction of iron ore pellets. To investigate the effects of sodiu and potassiu on all kinds of lupy iron carriers, reduction and tubling tests were executed under various experiental conditions. Lionitic and heatitic raw ores and pellets were treated with alkali bearing aqueous solutions to soak the particles. Investigations by SEM and icroprobe showed different alkali adsorption properties of ineralogical phases of the feed s. Elevated contents of sodiu and potassiu were detected in lionite, gangue and glass phases after soaking. The soaked s were reduced in a vertical retort furnace according to ISO standards and at conditions coparable to those of industrial processes. After tubling the reduced the grain size distribution was deterined. A significant effect of alkalis on disintegration, degradation and sticking behaviour of lupy iron carriers during reduction could be deonstrated. Additionally the different ipacts of sodiu and potassiu were quantified. Due to investigations on different types of lup ore and pellets, various effects of sodiu and potassiu on echanical properties were quantified. Keywords: ironaking, alkalis, reduction, sticking, degradation, lup ore, pellets; 1. INTRODUCTION Fro experience in industrial applications, we know that alkali etals as sodiu and potassiu are disadvantageous eleents in ironaking processes. The ain issues concern acroscopic effects like the foration of scaffolds by gaseous and liquid alkaline copounds or the general increase of fines due to increased values of alkali etals in the reactor [1]. However, these effects are ainly caused by icroscopic reactions. Previous research priarily focused on iron ore pellets and coke. Investigations on alkali-treated pellets show swelling during reduction [1,2]. Of course these changes in volue lead to degradation of the particle and to higher values of fines. In addition, tests with alkalitreated coke lead to higher aounts of dust in coparison with untreated [3]. It is also necessary to evaluate other input s for ironaking processes, e.g. different types of lup ore. A ethodology was developed to characterize lupy iron carriers and their behaviour under the influence of alkali etals, which is described in this paper. Variable input for ironaking like heatitic lup ore, lionitic lup ore and two brands of pellets were soaked with sodiu or potassiu. The separation between these two alkali etals was essential to obtain clear inforation about the specific effects; especially for industrial applications a clear partition of the different alkali etals is desirable. Reduction tests were perfored in both standardized and odified near industrial process conditions for non-soaked and soaked. Afterwards sieve analyses and tubling tests showed a process near picture of echanical load and its effects on particle strength. The paraeters for the experients and the results with lup ore and pellets are shown in the following.

2 2. EXPERIMENTAL SETUP The reduction of the lupy iron carrier is carried out in a vertical lab scale reduction retort. The experiental setup including the ain technical data is shown in Figure 1. The process gas for the reduction tests is supplied by a gas supply unit. For the required testing procedures specific reducing gas copositions are produced by ixing pure coponents like CO, CO2, H2 and N2. The core of the plant is a retort with 75 in diaeter, and is heated by an inductive heating syste. The reducing agent enters the bed fro the botto side. The teperature is easured by a therocouple in the fixed bed. The retort hangs on a weighing cell, which easures the reoval of oxygen of the iron carrier by ass loss. Based on this inforation the change in the reduction degree is calculated by the ratio of reoved oxygen during reduction to the total content of oxygen originally bound to iron. The process control syste of the plant allows for the tests to be executed with defined tie profiles for the saple teperature and reducing gas coposition. The ain test paraeters are listed in Table 1 [4,5]. The echanical properties are deterined in a tubling syste (see Figure 2). The reduced is then fed into the rotating chaber and dru for 3 inutes with a rotation speed of 3 rp. The separation of the different grain size fractions occurs by sieving, thereby providing the ass for the disintegration and the abrasion tendency calculations in Table 2. The initial ass prior to tubling is - 1 stands for the ass portions > 6.3 in g after tubling- 2 stands for the ass portions <.5 in g after tubling. Table 1: Testing paraeters for reduction tests including calculation and description of characteristic values [4] Saple portion 5 [g] Max. testing teperature 95 [ C] Size fraction of input [] O reoved Reduction degree RD 1 Otot Tie to achieve 8 % of reduction RD 8 [in] Figure 1: Plant layout of the testing equipent including gas supply syste [4] Table 2: Testing paraeters for echanical testing and description of characteristic values [4] Revolutions 9 [---] Rotation speed 3 [rp] Figure 2: Scheatic set up of the tubling equipent [4] Disintegration tendency 1 1

3 Abrasion tendency EXPERIMENTAL PROCEDURE OF SAMPLE PREPARION, REDUCTION AND TUMBLING In order to copare untreated with alkali-soaked, a representative ethodology is essential (descriptive flow sheet in Figure 3). The feeds are globally traded iron carriers, 2 lup ores and 2 pellet brands (refer to Table 4). They had been soaked in a 5 olar KOH and NaOH solution for 14 days resulting in alkali adsorption by the lupy. Finally the soaked was dried and sieved (according to ISO in order to obtain a hoogeneous bed feed with particles of in size). A subsaple was taken for cheical analysis. Two types of tests were operated, a standardized test according to ISO 4695 [6] and an industrial scale process condition test with a teperature and gas coposition profile siilar to blast furnace (BF) conditions. The industrial scale process conditions test was run at a specific gas ratio of CO and CO2 with addition of 3 % H2 (testing paraeters for both tests refer to Table 3). Both tests were stopped when a reduction degree of 8 % was reached. Tests with non-soaked as well as with soaked s were perfored. raw (Lup,sinter,pellets) drying Table 3: Testing paraeters of the standardized ISOtest and the industrial scale process conditions reduction test K/Na bearing aqueous solution SOAKING drying & sieving REDUCTION used aqueous solution cheical analysis test data Test ISO 4695 Phase Teperature [ C] abient to 95 C Gas coposition [vol.-%] Tie [in] 1 % N 2 6 I 95 C 4 % CO, 6 % N 2, RD 8 sieving TUMBLING sieving grain size distribution grain size distribution Figure 3: flow sheet for the evaluation of specific echanical indices of alkali treated iron carriers Industrial scale process conditions abient to 3 C I 3-5 C II 5-95 C III 95 C 1 % N % CO, 23 % CO 2, 3 % H 2, 5 % N 2 continuous fro phase I to phase III 42 % CO, 5 % CO 2, 3 % H 2, 5 % N RD 8 After the reduction tests the was sieved to deterine the grain size distribution before tubling. The final sieving took place after tubling and the disintegration and abrasion indices had been calculated with the respective fraction asses received. 4. PETROGRAPHY OF NON-REDUCED MERIAL Table 4: Cheical coposition of raw s Saple Specific surface area [²/g] Fe tot Fe 2+ Al 2O 3 CaO K 2O Na 2O MgO SiO 2 LOI B 2 B 4 Heatitic ore Lionitic ore Pellet brand Pellet brand

4 The analysis data of the raw saples in Table 4 shows the cheical analysis including the loss on ignition (LOI) and the specific surface area, which was deterined by the BET-ethod. The basicity (B2/B4) was calculated in accordance with the cheical analysis. The alkali adsorption of the soaked saples was analysed by cheical analysis and by icroprobe apping. In general the orphological structure of the saples did not change as a result of the treatent. The cheical analyses show an increase in the alkali eleents in the soaked saples, yet are different in agnitude due to the different types of the raw s used. The bar charts in Figure 4 show the K2O and Na2O contents in the lionitic and heatitic lup ore before and after soaking (grey and black bar respectively). The apping of the eleent K with the icroprobe proved the increase of alkalis where they were adsorbed ore readily by the gangue and lionite than by heatite. Figure 4: Cheical analysis and K-apping of the K-treated lup iron ores (top: lionitic lup ore, botto: heatitic lup ore) The pellets show a siilar behaviour as the heatitic ore. The alkalis were absorbed ainly by gangue and glassy phases. The increase of K2O was 1.9 % for pellet brand 1 and 1.1 % for pellet brand 2. The Na2O content of pellet brand 1 increased fro originally.5 % to 1.1 % by the soaking (.2 % to.94 % for pellet brand 2). Furtherore, a proportional correlation between olarity of the solution and the assiilation of K and Na was also detectable. 5. MECHANICAL PROPERTIES AFTER REDUCTION AND TUMBLING The initial grain size of all saples before the reduction tests was 1 to 12 illietres. Figure 5 shows the sieve analysis of the reduced saples after tubling. The lionitic ore was characterized by a strong affinity to sticking. The surface of the reduced shows a higher aount of coarse iron whiskers in soaked copared to the non-soaked ones. Conversely, the portion of fines doubled after tubling (Table 5). Sticking and abrasion effects were always higher in the ISO-tests than under near industrial BF-conditions. Heatitic ore was less affected in relation to lionitic orethe influence of alkalis on sticking phenoena was siilar for both lup brands. Macroscopically both

5 pellet brands were also affected by the addition of alkalis. Due to the grain size distribution Na caused higher degradation of pellet brand 1 whereas K induced higher disintegration rates of pellet brand 2. The different behaviours could be caused by different cheical copositions and different basicity of the gangue. Figure 6 shows wustite covered by etallic iron of non-soaked and soaked pellets. In the untreated saple fine porous etallic iron and iron shells were detected. In contrast to that finding the Na-treated saple is characterized by dense iron nuclei on the surface of the wustite grains, by the absence of finely porous etallic iron, by bigger pores and by fewer iron bridges between the grains, thus causing the echanical properties to deteriorate. Figure 5: Grain size distributions of lionitic lup ore (left) and the pellet brand 1 (right); both after tubling test of treated and untreated Figure 6: Micro-photos of untreated and Na-soaked reduced (ISO 4695 testing conditions) 6. RESULTS Alkalis show a significant influence on the abrasion, disintegration and foration of iron whiskers, thereby causing enhanced sticking behaviour during the reduction process. Microscopic investigations prove this acroscopic observation. Coarse etallic iron scraps and whiskers on the surface of the wustite grains were priarily found in reduced soaked. The growth of large iron crystals induces ore stress and a higher tendency towards crack foration, respectively. On the basis of the specific echanical indices of the investigated untreated and treated s (suarized in Table 5) the following conclusions can be drawn: The change in abrasion tendency of lionitic lup ore is ore pronounced under ISO-conditions. The indices under BF-conditions are siilar for both treated and untreated. Abrasion of heatitic

6 lups was hardly affected by alkali addition. The biggest influence on disintegration and abrasion was detected on lionitic ore reduced under ISO-conditions. All lup ores showed increased sticking tendency. Pellets coprise ore coplex correlations on the addition of alkalis. Not all treated s indicated significant changes in the echanical indices. However, Na-treated pellet brand 1 showed a very high degradation tendency during reduction under ISO- conditions. Under near BF- conditions both pellet types showed poorer echanical stability. Although soe degradation of the particles was also detected on untreated, the aount of fines increased significantly. Table 5: Coparison of specific echanical indices according to different reduction conditions and different aounts of alkalis in the Saple [values in %] non soaked Reduction test at ISO- conditions K-soaked Na-soaked Reduction test at industrial scale conditions (BF) non soaked K-soaked Na-soaked Heatitic ore Lionitic ore Pellet brand Pellet brand CONCLUSION Lupy iron carriers as lup ore and pellets are significantly affected by alkalis. An increase of sticking behaviour, of the aount of fines and the general degradation was deterined by reduction and tubling tests. Under the icroscope coarser etallic iron nuclei were observed in soaked s as opposed to in untreated ones. This confirs the action of alkalis on the iron foration echanis during reduction and, as a consequence, the following degradation of the product. As a atter of course, testing paraeters control the variables; at ISO- and BF- reduction conditions the abrasion and disintegration behaviours of the iron carriers were significantly different. The variation of the echanical indices between the two pellets brands is higher, which indicates the influence of the basicity on the effect of alkalis on disintegration. ACKNOWLEDGEMENT This work has been perfored within the K1-MET, eber of COMET Copetence Centers for Excellence Technologies, and is financially supported by voestalpine Stahl GbH, voestalpine Stahl Donawitz GbH, Sieens VAI Metals Technologies GbH, the BMVIT, BMWFJ, the provinces of Upper Austria, Styria and Tyrol, SFG and Tiroler Zukunftsstiftung. COMET is anaged by FFG (Austrian research prootion agency). LITERURE [1] ABRAHAM, K.P., STAFFANSSON, L.I., The Alkali Proble in the Blast Furnace, Scandinavian Journal of Metallurgy, 4, 1975, pp [2] ENDE, H., GREBE, K., THOMALLA, S., Alkalien als Ursache von Schwell- und Zerfallsvorgängen bei der Reduktion von Eisenerz, Stahl und Eisen, 9, 197, pp [3] GUPTA, S., FRENCH, D., SAKUROVS, R., GRIGORE, M., SUN, H., CHAM, T., Minerals and iron-aking reactions in blast furnaces, Progress in Energy and Cobustion Science, 34, 28, pp [4] HANEL, M. B., MALI, H., SCHENK, J.L., SKORIANZ, M., HAUZENBERGER, F., THALER, C.; STOCKER, H., Characterization of Different Lup Ore Brands According to Industrial Scale Process

7 Conditions by Means of Reducibility Testing and Morphological Investigation, Proceedings of AISTech213- Pittsburgh, USA, 213, [5] HANEL, M.B., MALI, H., SCHENK, J.L., SKORIANZ, M., HAUZENBERGER, F., THALER, C.; STOCKER, H., Characterization of burden for ironaking by eans of reduction tests with orphological characterization, Proceedings of the 14 th ISIJ-VDEh-Jernkontoret Joint Syposiu- Osaka, Japan, 213, [6] International Organization for Standardization: ISO 4695 Iron ores - Deterination of reducibility (1995)