Direct reduction in rotary hearth furnaces (RHF) can be used for metallurgical dusts recycling considering its high efficiency in reducing iron oxides and separating heavy and alkali metal elements. However, there is still a lack of understanding on the relationship between in-pellet behaviour and in-furnace phenomena of a RHF. In our previous study (Wu et al., 2017), a model was developed and validated and typical in-furnace phenomena and in pellet behaviour were described. In connection with the previous study, this paper reports a numerical study of process design and optimization using the integrated model under industrial-scale RHF conditions for evaluating the sensitivity of main operating and controlling parameters of the RHFs on product quality in terms of iron metallization and Zn/Pb/K/Na removal. It is found that the final iron metallization rate is most affected by furnace atmosphere, and other product indexes are more sensitive to furnace temperature and hearth rotary speed. Several new designs are proposed and numerically tested in this study. The results show that the new designs, such as ladder furnace heights and adjustable obstruction, are useful to improve gas flow and pressure distribution inside the furnace, favourable for pellet direct reduction. The optimization of furnace temperature and rotary speed of furnace hearth also shows a good effect in product assurance and energy conservation, when facing on-site changes in raw material composition, production schedule and product quality target. The model and numerical results provide a cost-effective tool to solve technical problems in RHF industrial practice. (C) 2017 Elsevier B.V. All rights reserved.