Unionized workers at Chile’s Codelco, the world’s largest copper producer, have returned to work following a 24-hour strike over the future of the state-owned company.

The more than 40,000 employees returned to their mines across the country Tuesday. They walked off the job Monday over modernization plans that union officials say will lead to job losses and possible modernization of some of the company’s plants. Codelco officials say the plants will not be privatized.

Union workers have threatened more protests if they are excluded from negotiations over the restructuring of the company.

The work stoppage was Codelco’s first in nearly 20 years. Codelco said it expected to lose $40 million from the lost production of about 4,900 tons of copper. Codelco produces 9 percent of the world’s copper, or about 1.7 million tons per year.

Their strike coincided with the 40th anniversary of Chile’s nationalization of the mining industry, a move by then-President Salvador Allende. The nationalization of the mines led to the creation of Codelco.