The Trump administration says that the tariffs on metals imports are aimed at boosting domestic industries and addressing "trade imbalances."

Copper traders were caught flat-footed after refined copper was exempted from Trump's new tariffs. The exemption triggered a 20% drop in copper futures on the COMEX exchange. Traders now face excess inventory, with COMEX stockpiles at a 21-year high.

The president stopped short of putting a 50 percent tariff on refined copper, as he had threatened earlier this year. The market reaction was swift anyway and prices tumbled.