What happened at the French Catholic orphanage and cathedral in Tianjin in 1870?

The event you are referring to is known as the Tianjin Massacre, which occurred on June 21, 1870.

Prior to the violence, rumors spread throughout the city alleging that the French Sisters of Charity at the local orphanage were kidnapping and mutilating Chinese children. These unfounded accusations fueled intense xenophobic sentiment and hostility toward the French presence in Tianjin.1

The situation escalated when the French consul, Henri Fontanier, fired into a crowd of local representatives. While he missed the district magistrate, he killed the magistrate's servant. In the ensuing chaos, a mob killed and mutilated the consul and approximately 20 other individuals, most of whom were French. The incident nearly led to international warfare, ending only after the Chinese government executed 16 people and sent an official mission to apologize to France under pressure from Western powers.1