Union employees at Asarco's copper mines and other facilities will rally at the company's headquarters in Tucson Tuesday afternoon to demand a "fair contract," according to a press release from the United Steelworkers, one of eight unions representing hundreds of thousands of mine workers. 

The previous labor agreement expired at the end of June 2013, but production and maintenance employees at five Asarco locations in the country, including the ones in Arizona, continued to work under existing terms and conditions while current negations continue, the release says. 

The union negotiating committee is supposed to meet with Asarco again on Dec. 14, even though "management has announced its unilateral intention to implement the company's 'last, best and final' contract proposal on Tuesday instead of resolving the outstanding issues in good faith at the bargaining table as the unions have request."

From the press release:

The union considers the move illegal and will follow up with Region 28 of the National Labor Relations Board, which already plans to prosecute Asarco for unilaterally changing working conditions at the facilities on multiple occasions without first negotiating over the changes with union representatives as required by law.

The NLRB has scheduled a hearing for Jan. 12, 2016, when an administrative law judge will review evidence against Asarco and listen to testimony in the case.

Union workers are gathering at 3 p.m. tomorrow, joining employees protesting at four other Asarco locations in the U.S.

The company, which is under the umbrella of Grupo México—Mexico's largest mining corporation—owns three copper mines in Arizona.