EE was fueling heaters on 3/12/17 in prep plant, EE was struck in the hard hat by rust scale from overhead beam. EE worked w/o restriction until seeking med attn. on 3/17/17 becoming RLT. Cause of the incident was lack of EE knowledge. PPE was not a factor, mining equipment/mining systems was not a factor, compliance w/ rules regs was not a factor, miner proficiency was a factor