Mo. Rev. Stat. § 290.020
It is hereby declared to be unlawful for any person, company or corporation engaged in carrying on any kind of mining, mechanical, chemical manufacturing or smelting business, to work their employees in any mill or mills, or plants, while engaged in crushing rocks and mine products, containing mineral or ores, or engaged in separating the minerals or ores from rock and such combination with which the mineral or ores are mixed, or reducing or roasting, or refining or smelting minerals or ores, from and after the time such rocks, or combination of rocks and mine products, or minerals or ores are taken out of the mines, at such labor or industry, for a period of time longer than eight hours in a day of twenty-four hours, without their consent, and it is hereby declared that eight hours shall constitute a day of employment, for all laborers, or employees, engaged in the kind of labor or industry aforesaid.
§ 290.020, RSMo