小中大铁精矿粉 is not easy to do with 压片法; because Fe has different forms Fe3O4, Fe2O3, Fe(OH)3, Fe etc. if the proportion between those forms are not firmed stable, the background will be too much different. So 压片法 is not linear perfectly.
For example , 40% Fe3O4 and 80% Fe3O4 , for sure the impurity has big difference.