You could still use the steel pot if you filtered out the rust from the Sodium Chlorate/ide solution before you extracted the Chlorate.
Iron Oxide is insoluble in water. Sodium Chlorate is very soluble in water. After you boil the bleach solution down, you can dilute it again with water to make sure all of the NaClO3 is dissolved (if any was precipitating out before). Filter the Fe2O3 out, and then proceed with the KClO3 extraction. Potassium Chlorate’s solubility will decrease dramatically at low temperatures, so adding a couple of extra hundred mL to the solution won’t rob you of that much KClO3 from your yield. After filtration, you could even re-boil the diluted solution down again, but this time you shouldn’t get any Fe2O3 formation since there shouldn’t be any bleach left (in theory).