Monday marked the second day of Copper 2016, a major industry event at the Kobe International Conference Center in Japan. Global demand has been climbing 2% a year, and the industry's hard times will be over in three years, said Diego Hernandez, former president of Chilean state copper company Codelco and former head of resource giant Antofagasta.

The audience was reassured by these comments but clearly remained shocked by the metal's recent upswing. A Trump win had been expected to send prices lower, but his promises of increased infrastructure investment sparked a rally. Three-month copper futures jumped on the London Metal Exchange last Wednesday, after Trump's victory became clear, and climbed past $6,000 a ton at one point Friday. They hovered around $5,660 in off-hours trading Monday evening.

"Investors started buying Oct. 25, anticipating an increase in Chinese construction demand," said Tatsufumi Okoshi, senior economist at Nomura Securities. Improving economic indicators raised expectations of a pickup in building and renovating such facilities as ports, airports and schools. The electrical wires and machinery used in construction require a great deal of copper. "Hopes of infrastructure investment in the U.S. and China led to a copper rebound," Okoshi said.

Yet not everyone in the industry is bullish. "I'm glad, but at the same time, we have to watch out for an influx of speculative money," JX Nippon Mining & Metals President Shigeru Oi said. The company is a unit of JX Holdings.

Many market watchers see covering of shorts by U.S. and European hedge funds and Chinese speculators as contributing to the rally. "It's unclear whether prices will stabilize," Sumitomo Metal Mining Chairman Nobumasa Kemori said.

Some also argue that the rebound has not gone far enough to justify resuming mine development. The average mine will not be profitable unless prices recover to $6,600, Hernandez said.

Cost-cutting is crucial for mining companies to ensure that they are not at the mercy of market fluctuations. JX Nippon Mining and Codelco have developed a process that uses bacteria to boost copper yields from ore by 30-50%, which the Japanese company will implement at its mines. Mitsubishi Materials has developed a process that examines aerial photographs shot by drones to detect topographical changes that could affect mining.

Copper prices topped $10,000 in 2011 but plunged after that as inventories swelled in China. Now that the supply-demand balance has improved there, the focus has shifted to the U.S. economy, which could be the next big factor to shake up the market.