TiO_2 plants are not the only sources of by-product gypsum.Much larger quantities are produced at phosphoric acid plants (about 80-100 million t.p.a.worldwide) and at flue-gas desulphurisation facilities (about 50 million t.p.a.worldwide).In addition,there are still a number of gypsum mines around the world,producing a substantial tonnage of natural gypsum.
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