Deutsche Gold- Und Silber-scheideanstalt Vormals Roessler

Degussa Gmbh

Karl Achenbach

Gunter Turk

Silicon dioxide containing waste flue dust from the production of silicon metal and silicon alloys is converted to crystalline zeolite molecular sieves of type A by (1) dissolving the flue dust in alkali metal hydroxide solution with the formation of alkali metal silicate solution having a module (SiO.sub.2 :Me.sub.2.sup.I O) in the range of 2.0:1 to 3.5:1 at a temperature of 60.degree.-100.degree. C.; (2) purifying the thus obtained alkali metal silicate solution from organic constituents by treatment with activated carbon and/or an oxidizing agent and separating the non-decomposable residue from the solution; and, (3) reacting alkali metal aluminate solution present at room temperature with the diluted, purified alkali metal silicate solution with a module (SiO.sub.2 :Na.sub.2 O) of 2.0:1 to 3.5:1 obtained by steps (1) and (2) and subsequently crystallizing the reaction mixture ((2.0-4.4)Na.sub.2 O:Al.sub.2 O.sub.3 ; (1.3-1.8)SiO.sub.2 :(90-120)H.sub.2 O) at a temperature of 90.degree.-95.degree. C. for a period of 6-12 hours, subsequently filtering, washing to pH of about 10-11 and drying to recover a crystalline, zeolitic molecular sieve of type A having a degree of whiteness according to Berger (A. Berger, Die Farbe 8 Nr. 4/6 (1959) pages 187-202) at 460.mu. in the range of 90-95%.

Process for converting silicon dioxide containing waste flue dust to

Silicon dioxide containing waste flue dust from the producton of silicon metal and silicon alloys is converted to crystalline zeolite molecular sieves of Type Y by (1) dissolving the flue dust in alkali metal hydroxide solution with the formation of alkali metal silicate solution having the module (SiO.sub.2 :Me.sup.I.sub.2 O) in the range of 4.0:1 to 5.0:1 at a temperature of 60.degree.-110.degree. C.; (2) purifying the thus obtained alkali metal silicate solution from organic constituents by treatment with activated carbon and/or an oxidizing agent and separating the non-decomposable residue from the solution; and (3) reacting alkali metal aluminate solution present at room temperature with the diluted, purified alkali metal silicate solution with a module (SiO.sub.2 :Me.sup.I.sub.2 O) of 4.0:1 to 5.0:1 obtained by steps (1) and (2) and subsequently crystallizing the reaction mixture (SiO.sub.2 :Al.sub.2 O.sub.3 =4-7:1, Me.sup.I.sub.2 O:SiO.sub.2 =0.4-0.7, H.sub.2 O:Me.sup.I.sub.2 O=30-50:1) at a temperature of 75.degree.-100.degree. C. with stirring for a period of 8-48 hours, subsequently filtering to remove the product from the mother liquor, washing and drying to recover a zeolite molecular sieve of Type Y with a Faujasite structure having a degree of whiteness according to Berger (A. Berger, Die Farbe, 8 Nr. 4/6 (1959) 187-202) at 460.mu. in the range of 90-95%.

There is provided a process for treating air-borne dust containing silicon dioxide in order to form a silicic acid or a silicate. The air-borne dust is dissolved in an alkali metal hydroxide solution to form an alkali metal silicate solution. The alkali metal silicate solution is purified by treating it with activated charcoal and/or oxidation agents. Non-decomposable substances remaining after the purification step are removed from the alkali metal silicate solution. The alkali metal silicate solution is then reacted with acids and/or salts of aluminum, magnesium and calcium. The resulting product is then filtered, washed and dried, and then comminuted to form a finely divided, amorphous, substantially pure, precipitated, silicic acid or silicate having a whiteness according to Berger at a wave length of 460 m.mu. of about 90 to about 95% and a specific surface area of about 30 to about 800 m.sup.2 /g BET. The resulting products are useful as reinforcing fillers in the rubber and plastics industries, as well as additives in the paint, varnish, lacquer, paper, cosmetic, pharmaceutical, feed and pesticide industries.

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Frankfurt-am-Main, Germany

Karl Achenbach