Given that more stringent NOx emission limits are expected in the near future, several industrial processes are in need of NOx mitigation measures. The Grate-Kiln process, applied in the iron ore industry, is one such process. NOx formation is inherently high in the process, and due to the combustion conditions, several standard mitigation strategies are impractical. Alternative solutions are thus needed. The current paper aims at developing a model capable of describing the NO formation under conditions relevant in iron ore rotary kilns and to identify governing parameters that may be modified for mitigation purposes. The developed model uses detailed reaction modeling for the homogeneous combustion chemistry combined with simpler modeling with apparent kinetics for the heterogeneous chemistry. The main findings are that thermal NO is of low significance and that the NO formation during char combustion is the main contributor to the high NOx emissions. Attempting to control the partitioning between the volatile nitrogen and the char-bound nitrogen is suggested as a mitigation strategy, since the combustion of char is challenging to control compared to the combustion of volatiles.