The change in granularity of iron grains in copper slag during coal-based deep reduction was identified using optical microscopy and the Image.' analysis software. The growth behavior of iron grains was investigated based on the Hillett dynamic model. The results indicate that the granularity and sphericity of iron grains are strongly affected by the reduction time and temperature during the deep reduction process. It is found that in isothermal condition, the growth rate of iron granularity increases with time exhibiting an S-shape characteristic. Meanwhile, in non-isothermal condition, the growth rate of iron granularity increases exponentially with temperature. When the reduction temperature is in the range of similar to 1423-1573 K and the reduction time was in the range of similar to 30-180 min, the grain growth kinetic parameters are calculated as follows: growth index n = 1.424 +/- 0.07855, apparent activation energy Q = 116.17 kJ.mol(-1), and pre-exponential factor as 20,839.38. (C) 2019 Published by Elsevier B.V.