BELO HORIZONTE, Brazil—In November 2015, an earthen wall holding back mining waste collapsed, sending a giant wave of mud speeding toward the town of Mariana in southeast Brazil. Among 19 people killed was 5-year-old Emanuelly Fernandes, whose mangled body was found 40 miles down the valley.

Vale SA, which co-owned the mine, vowed to improve safety at dozens of other dams. When new chief executive Fabio Schvartsman took over in May 2017, he coined a company motto: “Mariana, Never Again!”

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