I doubt that there were even one oz. of iron in my scrap of 136 pounds total. I take everything apart and am very meticulous, no gaskets, screws, nails...not one bit. All aluminum. I should have taken the "irony" back home with me.
If that's the case change yards, tell it how it was to anybody that would run metals into them. It doesn't take long for folks to start driving past the gate of the place. Unless they're the big boy in town, and folks line up just because they like to rub shoulders with them, boot lickers.
In the case of yours if there was only like you said an once of iron it's still dirty Period. My brokers had a ZERO TOLERANCE for dirty (any foreign metal, material) in the alum. except for a little dirt, or greying of the metal. I would enforce this with a ZERO TOLERANCE with my staff. If they missed something and I got the metal returned, or a downgrade from the broker they heard about it, if they didn't comply it was good bye.
What we would do is separate the minor problem and if it was a case of a few minutes on the shear to clean it up then I would take 5-10 cents a pound to clean up the problem or you got it back.
I really had a hard time in the mid 90's when I opened up trying to stop the old scrap yard mentality of I screw you because I got screwed. I treated everybody the same that walked through the gates, educated the ones that would leave things unclean, unsorted, and with a little guidance they would make more money, I had less work to do, and that made me more money in the end.
Another thing is the scale issue are the scales checked on a regular basis? How do you know? Does the yard post the report? I had my electronic scale checked 4 times a year and I posted those up for every customer/ministry to see there was no BS going on with the weights. The small 5x5ft indoor scales just stand on each corner then in the middle all 5 should be the same weight. If not there's a load cell problem and I would just say thanks but no thanks and walk.
It's sometimes the case where one yard pays this crazy price for say copper, every yard works on a margin, if their price is too far out there then they're making it up screwing with the weights.
Long drawn out reply-but it just burns me arse to hear of companies that just screw a little guy over, and they get away with it.