Potash-ammonia fertilizer K₂SO₄ · (NH₄)₂SO₄ is prepared from the uncalcined alunite treated with ammonia water. The residue containing a large amount of alumina treated by alkali solution may be regarded as a resource of alumina of high purity. The residue treated with ammonia contains 45∼50% alumina, 25∼30% silicate, 10∼15% moisture and iron, After extracting the alumina which was reacted into the soluble sodium aluminate, carbon dioxide is blown to precipitate aluminium hydroxide. The slurry is them filtered, to produce alumina.