Antimony trioxide is made when antimony is made. Stibnite is heated with air to make antimony trioxide. It is separated from arsenic by the boiling of the arsenic trioxide before the antimony trioxide boils. Antimony trioxide can also be made by a two step process. Stibnite is burned in air with calcium chloride to make calcium sulfate and antimony trichloride, which is reacted with water to make antimony trioxide.