It is important that all the catalyst is removed from the product for several reasons.
Any catalyst that is removed will reduce the quantity of catalyst remaining for further reactions. Replacing this is a cost that should be minimised since some commercial catalysts, such as platinum, can be extremely expensive.
Catalyst that remains in the product may lead to additional, unwanted chemical reactions if the product is to be used as a feedstock to a subsequent reaction.
Particular care needs to be taken if the product is to be used in the food, cosmetics or pharmaceuticals industries.
Catalytic material can be removed in a number of different ways depending on its properties.
The simplest way is to ensure that the catalyst is never released in the first place. For many applications the catalyst is bound to a solid substrate and so significant quantities cannot enter the liquid or gas phase mixture of reactants and products.
If the products of the reaction are gases then catalytic material can be removed by fine filters.
At 6.5 g cm-3 spongy nickel is very dense. It is relatively easy to remove large quantities of suspended catalyst using either sedimentation or centrifuging. This technique can also be used for platinum catalysts.
Nickel is a ferromagnetic material and so even small particles can be removed very effectively with the use of powerful electromagnets. Platinum on the other hand is a paramagnetic material and so experiences only a very weak attraction when subjected to a magnetic field.