 Some molybdenum ores contain 0.001% to 0.2% rhenium.Rhenium(VII) oxide and perrhenic acid readily dissolve in water; they are leached from flue dusts and gasses and extracted by precipitating with potassium or ammonium chloride as the perrhenate salts, and purified by recrystallization.Total world production is between 40 and 50 tons/year; the main producers are in Chile, the United States, Peru, and Poland.