The HIsarna is a coal-based smelting reduction process for ironmaking to drastically reduce CO2 emission and is one of the most promising alternative ironmaking processes under development in the world. The furnace consists of two inter-connected reactors: i) a smelting cyclone, ii) a smelting reduction vessel. The smelting cyclone is expected to provide about 20 % pre-reduction degree through thermal decomposition and reduction. However, as a new technology still at its development stage, the iron ore behaviour in the smelting cyclone is not well studied. Three HIsarna pilot plant campaigns have been conducted successfully during the years of 2011-2013 at the Tata Steel site in IJmuiden, the Netherlands. The laboratory study has been carried out in the same period at Delft University of Technology as part of the research programme of the Matrials innovation institute M2i, to focus on the thermal decomposition and reduction kinetics of individual particles in the smelting cyclone of the HIsarna, forming the main part of this thesis. In this study, a special experimental set-up has been designed and commissioned: the high temperature drop tube furnace (HTDF). The HTDF was used to investigate the reduction mechanism at high temperature of individual ore particles without agglomeration. The reaction gas is a mixture of CO, CO2, H2 and N2, and the composition is controlled with mass flow controllers. The gases are firstly guided into a gas mixing station and form a flow of gas mixture. And then the gas mixture is split up into two flows. A small flow of the gas mixture enters the syringe pump feeder as particle carrier gas. The other flow is preheated to 773 K through a pre-heat furnace before entering the electrically heated furnace. The particle feed rate is controlled by a syringe pump feeder. The individual iron ore particles from the syringe pump feeder pass through a water-cooled injection probe before entering the hot zone. The iron ore reduction takes place during the flying time of the particle in the hot zone. The off-gas and the partially reduced particles are received by a water-cooled sampling probe. Finally, the iron ore particles are collected by a sample collector. The study of the thermal decomposition behaviour of hematite ore started with a theoretical evaluation. The experimental study has been conducted with the TGA-DSC analyser and in a high temperature horizontal furnace at a steady state. The individual particle thermal decomposition behaviour has been investigated in the HDTF under different conditions. The theoretical evaluation shows that, in the inert gas environment, the lowest start thermal decomposition temperature of Fe2O3 could be room temperature when the partial pressure of oxygen is close to zero bar. The thermal decomposition of Fe2O3 can be accelerated when the temperature is higher than 1473 K. The thermal decomposition of pure Fe3O4 can also take place at room temperature when the partial pressure of oxygen is close to zero bar and the thermal decomposition can be accelerated when the temperature is higher than 1673 K. These results have been confirmed by the following experimental studies. From the TGA-DSC analysis, it was found that a sharp weight loss in a short time appears on the TG curve. However, the weight loss is quite small during the time elapsed after the sharp weight loss stage. The experimental study in the horizontal furnace provided accurate results of the thermal decomposition degree of hematite ore at different temperatures and holding times by chemical titration. The thermal decomposition degree of iron ore increases with the increase of temperature. At the same temperature, the thermal decomposition degree of iron ore increases slowly with the increase of holding time (1 h, 2 h and 3 h). In addition, no significant difference was observed between the results at 1673 K and 1773 K. Furthermore, whether the sharp weight loss stage occurs and how much of the thermal decomposition degree could be achieved in the smelting cyclone has been further tested with the HTDF. The effects of different inert gases: N2, Ar, CO2 (CO2 is the primary gas in the pilot plant besides CO), temperature, residence time, and particle size on the final thermal decomposition degree were studied. It was found that the sharp weight loss stage observed in the TGA-DSC experiments can be mostly achieved in the HTDF in the CO2 gas rather than N2 and Ar. For example, at 1750 K in CO2 gas, the thermal decomposition degree of iron ore in the HTDF is around 10.8 % which is slightly lower than the value of 12.6 % obtained in the horizontal furnace. This is because the fine iron ore particles could be heated up faster in CO2 gas than in N2 and Ar gas due to the strong radiation properties (emission and absorption) of CO2 gas. Temperature plays an important role in determining the iron ore thermal decomposition which dramatically goes up with the increase of temperature. However, there is no significant influence of particle size and residence time on the thermal decomposition degree observed, when the particle diameter is smaller than 250 µm. It indicates that the thermal decomposition of iron ore quickly takes place in the first 210 ms. Based on the results of thermal decomposition of iron ore particles, the individual particle reduction mechanism of hematite ore in the smelting cyclone has been investigated with the HTDF. Under the studied experimental conditions, the maximum reduction degree of iron ore particle is in the range of 23-30 %. Before reaching the reduction equilibrium state, the reduction degree of iron ore goes up with the increase of residence time and temperature, and decreases with the increase of Post Combustion Ratio (CO2 + H2O)/(CO + CO2 + H2+ H2O). The gas-solid particle reaction takes place in all the studied residence times at 1550 K and 1600 K, and at 1650 K in the residence times of 210-970 ms. The 100 % gas-molten particle reduction takes place at 1700 K in the residence times of 700-2020 ms and in all the studied residence times at 1750 K. The reduction degree of iron ore in the first 210 ms is the combined result of reduction and thermal decomposition. During the reduction process, quantities of micro pores were formed due to the different crystal structures of hematite, magnetite and w?stite. The micro pores could accelerate the reduction process. The reaction mechanism of iron ore reduction at high temperature has been revealed through the kinetic analysis. The kinetic model was determined by microscopic examination. The unreacted shrinking core model was applied to both gas-solid particle reaction and gas-molten particle reaction. The rate controlling step of gas-solid particle reduction was obtained by the model-fitting method and confirmed by the model-free method, and the rate controlling step of gas-molten particle reduction and mixed reduction was obtained by the model-fitting method. The reaction rate controlling step was found to be the chemical control for gas-solid particle reduction, mixed reduction and gas-molten particle reduction from a macrokinetics point of view. Through further study, the rate controlling step of gas-solid particle reduction was found to be mass transport of cations and electrons inside the matrix along the interfaces from a microkinetics point of view. The most typical operating conditions of the smelting cyclone of the HIsarna process have been studied in this study and the fundamental understanding of the reduction kinetics of the individual iron ore particle is presented in this thesis. At the same time, new questions about the reactions and the reactor systems have been raised like the reduction mechanism of bigger particle size, reduction behaviour of ‘particle swarms’, the particles’ collision behaviour and so on. The further laboratory study seems significant for future development of the HIsarna process.