Your question is not very clear.  You talk about the concentrate mass increase when reporting to the pelletizing plant, then about the pelletizing reaction which occurs in the plant.

The process generally involves combining the concentrate with a binder which will have a minor increase in mass, followed by a sintering process to harden the pellets, which will drive off moisture and other volatiles reducing the mass.

The net result is a reduction mass but an increase in volume.  This is somewhat variable and depends on the moisture content of the concentrate and the binder.

As a general guide assume 95+% of the moisture is driven off.

SmartDog,

Ignore the moisture and work on a dry mass basis.

So if have we a magnetite concentrate, that reports to a pelletiting plant what is the impact on mass. Does it increase or decrease out of the pelletizer plant and what %?

Since you are not changing the amount of material, just how they are combined the only change will be the driving off of any volatile material.  If you are ignoring the water (the bulk of the volatile material) no change in mass will occur.

The volume will change so any measurement of the mass must consider the change in volume.  This means that weight per unit volume in is different from weight per unit volume out.  Also the mass per unit volume in will be different from the mass per unit volume out.  But net mass (ignoring volatiles) will be the same.

To prove this requires measuring the dry mass going in and then crushing and measuring the dried mass coming out.  Note this is NOT the same as measuring the dry weight in versus out as weight is NOT the same as mass.