The National Labor Relations Board has ruled that a subsidiary of Overland Park-based Compass Minerals violated federal laws in contract negotiations with a United Steelworkers local that represents 100 workers.

The ruling involves North American Salt Co., which operates the Cote Blanche mine in Louisiana.

A three-member panel affirmed an earlier ruling that the company violated the federal labor laws when it implemented a final contract offer to union Local 14425 on March 31, 2010, without a “valid impasse in bargaining,” according to the labor board ruling. The contract expired the same day, the union said Wednesday.

The NLRB also determined that the local union members engaged in an unfair labor practice strike, which began April 7, 2010, and continued for about two months, when workers offered to return to work.

The labor board said in its ruling that the company violated the law, among other things, by “threatening to permanently replace strikers, failed to reinstate former strikers promptly after their offer to return to work,” and changed seniority procedures.

The labor panel ordered North American to cease and desist from these violations. It also ordered the company to negotiate in good faith with the union and restore the terms and conditions of employment that existed before March 31, 2010, until a new contract is reached or there is a valid impasse in the talks.

“It’s time for the company to obey the law and work with us to obtain a fair agreement that is good for both sides,” Mickey Breaux, a Steelworkers director, said in a statement.

North American said in a statement that it was disappointed by the labor board’s decision “as the company believes that it has acted correctly every step of the way.” The company said it plans to file an appeal and added that the mine continues to “operate safely and efficiently providing rock salt to keep America’s roadways safe.”

Compass Minerals is one of the largest providers of industrial and commercial salt.