Metal detector coil SHIELDING?
The graphite type shields that line the apertures of industrial metal detectors are only required when the material being passed (in which you are searching for metal contaminants) is partially electrically conductive and causes what is known as "product effect". If the material is not conductive (but has a significant electric permittivity) then the graphite sheet is also required.

Industrial metal detectors in the food and pharmaceutical industry do make use of extensive product-effect algorithms to ignore the "signal" produced by "clean" product but the graphite sheets are still needed to homogenize the electric fields in the aperture. The effect that a poorly distributed electric field in the aperture has is difficult to eradicate with just algorithms.

The electric fields arise because the central coil generating flux is driven from a voltage source and that voltage source creates the electric fields in the aperture. These need to be "smoothed-out" in order to prevent effects that an algorithm cannot adequately deal with.