The main steps in the processing of tea involves drying and rolling. Tea Powder may contain iron filings because of the method of processing it undergoes. 

Tea leaves are dried in a sieve fitted with a mesh and then cut using iron rollers. Iron machinery is used to crush the tea leaves and some very fine iron particles may get stuck to the tea grains, Magnets are used to remove these iron filings from powdered tea. It has been found that some particles may still remain or get powdered with the tea leaves. In order to ensure that tea is not contaminated with iron particles, FSSAI has approved a gravimetric estimation of iron particles using a magnet of strength ~ 1000 gauss for determination of iron filing in tea.

Read the complete method here.