Workers to strike at Chile's Escondida copper mine next week -union

(Adds detail on mine, background)

SANTIAGO, Sept 15 (Reuters) - The union at the world's largest copper mine, Chile's Escondida, has called a strike for Sept. 22 and 24, aiming to paralyze activity to win improved working conditions, the union said on Monday.

In a surprise announcement on Monday evening, the Sindicato No. 1 union said it would call on its 2,800 members to stage two 24-hour strikes at the mine, which is controlled by global miner BHP Billiton Plc (NYSE: BBL - news) .

The union, which represents the vast majority of workers at the mine, said the stoppage would affect mining and port operations.

Escondida, in northern Chile's copper belt, produced 1.19 million tonnes of copper last year, about 20 percent of the output from Chile, the world's top copper producer.

The union carried out a similar 24-hour stoppage over pay and conditions last year, without causing any long-term impact on copper production. However, it stunned the copper market in 2011 by staging a two-week strike that sent the mine's output tumbling.

Escondida "systematically infringed on labor norms," the union said in a statement on Monday, citing overtime, holidays, hygiene and safety issues. It said on its blog that it held a series of meetings with company representatives on Thursday.

The stoppage was a "warning," the union said. If the problems were not resolved, the action would be extended, it added.

BHP declined to comment.

BHP owns a 57.5 stake in Escondida. Rio Tinto Ltd (Xetra: 855018 - news) holds a 30 percent stake, and Jeco Co Ltd has 10 percent.

(Reporting by Felipe Iturrieta and Rosalba O'Brien; Writing by Sarah Marsh and Rosalba O'Brien; Editing by David Gregorio, Cynthia Osterman and Steve Orlofsky)