When your zeolite is already ammonia exchanged and calcined it is not the "iron" in form as charge compensation Fe-cations you want to get rid of, but the ironoxide.

Iron tends to form oligomeric species or even FeOx-clusters and Fe2O3 Particles in zeolites. After thoroughly ion echange and calcination most of the remaining iron will be Fe2O3 in my opinion.

I am not sure if you can really remove these particles without changing the zeolites composition by leaching processes or destroying the structure because of the perhaps quite big ironoxides particles.  

A very mild leaching with HCl (no more than 0.1 N) is known to work. The conditions depend on the (varying) Fe level in your natural zeolite sample. A quick check in the litterature should give you conditions; however you will have to adjust them to the properties of your specific natural zeolite. Verify that no dealumination takes place by chemical analysis (a side reaction taking place under harsh conditions of T, time, concentration of acid...) and check crystallinity and pore volume. Keep also in mind that natural zeolites are made of large crystals and diffusion issues (evacuation of the Fe debris from the micropores) might cause problems such as inhomogeneous removal of Fe (and Al) across the crystals.

Good luck.

just try to wash with mild organic acid, pH around 2-3.. hope it will give the maximum removal

Dear Krishnan Muthukumar,

Thanks a lot, Can you please send me some procedure for trying. Thanks

Regards.

you can take simple acid like acetic acid and dilute to the early mention pH range ... but don't go above four pH

carboxylic acid may remove more of Fe compared to Fe. you can try with those.

I agree with Jean-Pierre Gilson. As well as I know, HCl could work for this situation 

If you are using clinoptilolite then Fe has been shown to be in the framework sites