Do you have a source with more details? 250,000 tons of copper doesn't really sounds like much. Wikipedia says annual WW production is 18M tons, so the US dumped 1.4% of one's year production on the market. Sure, enough to depress prices, but for how long? It would seem like you'd need to keep dumping over a long period of time otherwise the price would dip for a few months, then recover.