goldbarb,
My thoughts only on what I understand of the question:
I understand you are recovering gold and refining the gold and precipitating it from solution.
But sending powders to refiners?
I would consider melting the powders to bars, where they can be core drilled and the drill samples tested by an assay.
I have not sold gold to refiners, or have had any dealings with them so I cannot comment.
Local buyers here will buy gold, one I deal with uses only nitric acid to test the gold he buys using a scratch test he is very good at it, but he can only determine a relative percentage of the gold content (karat),(he cannot test refined gold).
Other local buyers may use an acid test kit and the scratch test with different acids, but the result is the same they cannot test for pure gold and will buy the gold by the percent determined by their testing methods which can only determine if gold content is at least 91% or 22 karats the highest gold percentage that they can determine using these testing methods.
If I sell my refined gold to him I have to add copper to alloy the gold down to around 75%(18K gold) or 58% (14K gold) which he can test for.
I believe the more details you share about what you are doing, the more it will give others a chance to help answer your question.
Edit in order to correct my bad math skills, When it comes to numbers and words I am always getting some of them wrong or backward.
Can you give more details on the ore and recovery? when you stated "activated carbon loaded (?) with precious metals" what type of leach or extraction methods? What type of ore? Are you doing any pre-treatments on the ore?