Fines of hematite and magnetite were studied in the fluidized bed using a pure hydrogen atmosphere in the temperature range of 768 to 888 K. Hematite pellets were studied based on Thermogravimetric Analysis (TGA) experiments using hydrogen atmospheres containing 0–15 % pH2O, in the temperature range of 873 to 1173 K. Thermocouples in contact with the pellet’s surface and embedded in the pellet’s center recorded the thermal gradient in the pellet during reduction. The fluidized bed and the TGA experiments used an alternative method to start the reaction. The furnace was pre-heated with a reactive atmosphere. After this, the reaction was initiated by introducing the sample to the hot furnace, to eliminate the effect of gas dilution during gas switching. After the experiments, Scanning Electron Microscopy (SEM)analyses were employed to study the reduction microstructures. Both types of fines showed similar reduction rates. Fines and pellets showed high initial reduction rates, which increased with increasing temperatures. The reduction rate in the last reduction stage was low for both fines and pellets. An increasing pH2O content in the atmosphere lowered the reaction rate, and theeffect decreased with increased temperature. A difference between the pellet's surface and center temperatures was observed during reduction. The pellet'smacro-pore structure was seen to be unaltered by changes in temperature or atmosphere. However, at 873 K, the iron product microstructure was found to be highly porous. Furthermore, increasing temperatures caused dense iron to form. In addition, when porous iron or iron oxides were observed, increasing pH2O contentsincreased the pore diameter but decreased the pore amount. Pellet properties with varied pellet compositions were also investigated using Cold Crushing Strength (CCS), reduction in a TGA setup, and melting experiments. The composition was not found to influence the mechanical or reduction properties but significantly affected the phosphorus refining during melting.The results showed that a mixed reaction rate control occurred during the early reduction stage for both the fines and the pellets. The temperature differences observed during this reduction stage resulted from a combined effect of heat transfer and an endothermic chemical reaction. The impact of water in the atmosphere influenced the reaction rate through the backward reaction and mass transfer. At 873 K, the retarding effect is mainly caused by the backwardreaction. The results show the late stage of reduction to be primarily diffusioncontrolled. In addition, it should be possible to alter the pellet composition while maintaining pellet properties to increase the usefulness of the pellet.

Reduktion av hematit- och magnetitpulver studerades i ren vätgas i en fluidbädd, i temperaturintervallet 768–888 K. Hematitpellets studerades i en Termogravimetrisk Analysutrustning (TGA), i temperaturintervallet 873–1173 K med en atmosfär av vätgas och 0–15 % pH2O. Även termoelement borrades in i kärnan på pellets innan reduktion. Tillsammans med ett termoelement i kontakt med pelletens yta kunde temperaturgradienten i pelleten mätas under reduktionen. Ett alternativt sätt att starta reduktionen i både fluidbädden och TGA-utrustningen användes, där provet introducerades till en varm ugn och en stabil reaktiv atmosfär från ett icke-reagerande tillstånd vid låg temperatur. I den traditionella metoden värms provet upp till den experimentella temperaturen i inert gas, varefter gasen byts ut till en reaktiv gas för att starta reaktionen. Metodförändringen gjordes för att undvika osäkerheter som annars kan introduceras av utspädningssteget i den tidigare metoden. Analys med Svep Elektron Mikroskopi (SEM) genomfördes för att följa reduktionen. Båda typerna av malmpulver reducerade med en liknande hastighet. Reduktionen av både pulver och pellets var snabb i början, och hastigheten ökade med en ökande temperatur. Reduktionshastigheten under den sista delen av reduktionen var långsam, och där hade temperaturen inte stor inverkan. För pellets visade ett ökande pH2O värde på sjunkande reaktionshastighet, men effekten avtog med en ökande temperatur. Under reduktionen uppmättes en temperaturskillnad mellan pelletens yta och centrum vid samtliga temperaturer. Ingen skillnad i pelletens makroporositet kunde ses med en ändrad temperatur. Däremot skiljde sig mikrostrukturen i kornen åt. Vid 873 K bildade järnet en porös struktur, men vid en högre temperatur var det bildade järnet solitt. Med ökande pH2O värden vid 873 K sågs pordiametern i järnet öka men antalet porer minska. Porös järnoxid observerades under reduktionen vid samtliga temperaturer och pH2O värden, och ökande temperaturer eller pH2O värden sågs öka pordiametern och minska porantalet. Pelletsegenskaper utvärderades med avseende på sammansättning, genom att tre pelletsammansättningar testades genom ett Cold Crushing Strength (CCS) test, TGA- och smältexperiment. Sammansättningen visade inte på någon förändring med avseende på mekaniska egenskaper eller reduktionsegenskaper, men dess fosforreningsegenskaper varierade med sammansättningen. Resultaten visar på att reduktionen styrs av flera parallella mekanismer, både vid pulver- och pelletreduktion. Skillnaden mellan den uppmätta temperaturen i provet och den experimentella temperaturen visar en inverkan av både värmetransport och den endotermiska reaktionen. Effekten som pH2O ses ha, visar på inverkan från både diffusion och bakåtreaktionen. Resultaten visar också att den sista delen av reduktionen i huvudsak är styrd av diffusion. Det kunde också visas att pelletens sammansättning kan förändras för att förbättra pelletens användbarhet, utan att påverka dess mekaniska egenskaper eller reduktionsegenskaper.

A novel setup of fluidized bed was developed to study the reduction of iron oxides by pure hydrogen. The setup enabled the introduction of powder directly to pure hydrogen atmosphere at experimental temperature. The arrangement was to minimize the experimental uncertainties due to gas switching. Hematite and magnetite powders were studied in the temperature range of 768-888 K. Reduction rates of the two powders were found to be similar. For both powders, the reduction rates were very high before the O/Fe ratio reached 0.5. Thereafter, the reduction was sluggish. SEM analyses revealed that the later stages of reaction was controlled by diffusion through the product layer on bigger particles, irrespective of the type of oxide powder. The results have indicated that both hematite and magnetite powder could be employed in fluidized bed. The results have also suggested that process optimization is essential regarding the sluggish reaction below 0.5 O/Fe ratio.

In this study, the spreading and infiltration behavior of slag in contact with different grades of graphite was investigated. The wetting and infiltration of slag into graphite were found to be highly material dependent. Temperature and silica content of the slag also have a major influence on how slag spreads and infiltrates: The higher the temperature and silica content, the greater the slag infiltration, and the faster the rate of spreading. Reactions that generate gaseous products occurred during spreading of slag on graphite was evidenced by the observation of bubble formation. Silicon infiltrated into the graphite substrates much deeper than the slag phase, indicating gas-phase transport of silicon-bearing vapor species. Complete wetting of the interface and reduction of silica in the slag near the interface may lead to passivation by formation of a solid, CaO-rich layer.

In this study, the spreading and infiltration behavior of liquid slag in contact with different grades of graphite was investigated. The wetting and infiltration of slag into graphite were found to be highly material dependent. The reduction of silica by carbon is a characteristic of the system, and it generates gaseous products as evidenced by the observation of bubble formation. The higher the temperature and silica activity of the slag is, the greater the slag infiltration and the faster the rate of spreading. Silicon infiltrated into the graphite substrates much deeper than the oxide phases, indicating gas-phase transport of SiO(g) into the graphite pores. Fundamentally, in this system where the liquid and substrate are reacting, the driving force for spreading is the movement of the system toward a lower total Gibbs energy. Reduction of silica in the slag near the interface may eventually lead to the formation of a solid, CaO-rich layer, slowing down or stopping the reduction reaction.