In general I like this concept.
Still I have my doubts.
It is correct and to everyones advantage, that fair and unfair materials end up in a pool from which all the companies get their stuff.
But the amount of fair and unfair materials being sent into the pool depends on contracts between the companies that use the materials and the companies that refine the materials or even the mining companies themselves.
Fairphone has explained their model in a blogpost, that has been discussed in another thread here already:
So, this model is working for the fair sourcing companies only if the companies using the materials are contracting them or the companies refining their materials especially for the fair sourced stuff.
Just feeding stuff into the pool on one side and taking it out on the other side is not how it works, as I understand from the Fairphone blogspot.
But I am really not a trading guy and have no experience or real knowledge concerning this field of business. So I might have got it totally wrong and your concept could work.
Though my understanding is the same, when it comes to contracting ecological energy supply in Germany. You pay e.g. Lichtblick or Greenpeace engery for delivering the energy to the pool and then you take the amount of energy from the pool (net) that you need. But the money and the contract is with the supplier of ecological (fair) energy.