Patentamt J JEuropâischesEuropean Patent Office (§) Publication number: 0 081 063 Office européen des brevets B 1 © EUROPEAN PATENT SPECIFICATION ® Dateof publication of patent spécification: 10.07.85 @ ht. Cl.4: C 01 B 15/08 ® Application number: 82109350.7 (22) Date offiling: 08.10.82 @) Method for producing sodium persulfate. (§) Priority: 10.11.81 US 320145 (7jj) Proprietor: FMC Corporation 2000 Market Street Philadelphia Pennsylvania 19103 (US) (§) Date of publication of application: 15.06.83 Bulletin 83/24 (7?) Inventor: McCarthy, MichaelJoseph 14 Lannigan Drive (§) Publication of the grant of the patent: Lawrenceville New Jersey 08648 (US) 10.07.85 Bulletin 85/28 Inventor: Schillaci, Phillip Vincent 226 Kaymar Drive Tonawanda New York (US) (§) Designated Contracting States: AT BE CH DE FR GB IT LI LU NL SE @) Representative: Barz, Peter, Dr. et al Patentanwalte Dr. V. Schmied-Kowarzik Dipl.- (68) References cited: Ing. G. Dannenberg Dr. P. Weinhold Dr. D. Gudel FR-A-1 493 723 Dipl.-lng. S. Schubert Dr. P. Barz US-A-2 899 272 Siegfriedstrasse 8 US-A-3 71 6 629 D-8000 Miinchen 40 (DE) US-A-3 954952 The file contains technical information submitted after the application was filed and not included in this specification Note: Within nine months from the publication of the mention of the grant of the European patent, any person may give notice to the European Patent Office of opposition to the European patent granted. Notice of opposition shall be filed in a written reasoned statement. It shall not be deemed to have been filed until the opposition fee has been paid. (Art. 99(1) European patent convention). Courier Press, Leamington Spa, England. reaction solution. This patent also discloses that sodium sulfate builds up in the recirculated This invention relates to a method for mother liquor and after 24-30 cycles can be separately crystallizing sodium persulfate and removed by cooling to precipitate sodium sulfate sodium sulfate decahydrate from a mother liquor decahydrate plus some sodium persulfate. If regenerated with ammonium persulfate and excess water builds up in the system, it is also sodium hydroxide or sodium carbonate. removed at this time by adding anhydrous Sodium peroxydisulfate (commonly called sodium sulfate to the system prior to cooling thus sodium persulfate) is a commercial chemical removing the water as the water of crystallization. particularly useful for etching printed circuits, for Zumbrunn partially controls the loss of active cleaning copper before plating or soldering, and oxygen due to hydrolysis by maintaining reaction for initiating polymerization reactions. Other com- temperatures at 15°C to 25°C. mercially available persulfate salts are Nevertheless, active oxygen is lost by crystal- ammonium persulfate and potassium persulfate; lizing with sodium sulfate and by reacting with however, ammonium persulfate introduces ammonia to produce sulfate and nitrogen gas. objectionable ammonium ions into solution, This loss of active oxygen and the periodic while the solubility of potassium persulfate is purification of the mother liquor add to the cost of much less than ammonium or sodium persulfate. the product by this process. Consequently, sodium persulfate is the preferred Hall et al in U.S. Patent No. 3,954,952 disclose a compound for some applications. process for the production of sodium or Sodium persulfate can be prepared by elec- potassium persulfate by continuously reacting a trolysis of a solution containing sodium and solution of the corresponding sodium or sulfate ions according to U.S. Patent No. potassium hydroxide with a solution of slurry of 4,127,456 and U.S. Patent No. 4,144,144, as well ammonium persulfate. The reaction conditions as by the reaction of sodium hydroxide with are maintained at 15°C to 45°C, at atmospheric ammonium persulfate as taught by U.S. Patent pressure and in the pH range of 11-13; the No. 2,899,272 and U.S. Patent No. 3,954,952. The ammonia concentration in the reaction medium is electrolytic processes yield a high quality product, kept below 6 weight percent at 25°C by air or inert but do not permit the flexibility of producing gas stripping to avoid the formation of explosive ammonium, sodium, and potassium persulfates mixtures with air. The product is recovered by utilizing the same electrolyte; processes based on spray drying or other conventional means. While the electrolytic production of ammonium per- the potassium persulfate produced by this sulfate optionally followed by metathesis to method is satisfactory, the sodium persulfate sodium and potassium persulfate and ammonia tends to cake badly because of the greater provide the desired flexibility of operation. solubility of sodium persulfate, which permits a However, none of these processes are completely greater buildup of by-product sodium sulfate satisfactory for the production of sodium from the side reaction with ammonia. The persulfate. process results in a fine product which must be The process disclosed by Flach et al in U.S. compacted and granulated to reduce its tendency Patent No. 2,899,272 reacts aqueous solutions of to cake which is caused by the sodium sulfate in ammonium persulfate and an alkali metal or the product. alkaline earth hydroxide or carbonate at tem- The present invention is a process for con- peratures between 0°C and 45°C. Ammonia is tinuously making sodium persulfate from an removed from the reaction zone by vacuum aqueous mother liquor which is recycled and stripping. The reaction solutions are held for over replenished by reacting solutions of ammonium 24 hours in a crystallizer to provide time for the persulfate and either sodium hydroxide or crystals to grow and the water and ammonia to be sodium carbonate. A slurry of sodium persulfate removed. This long residence time is a disadvan- crystals is formed by cooling the mother liquor. tage because it allows the persulfate to react with Sodium persulfate crystals are extracted from a ammonia to produce the corresponding alkali first part of the slurry while a second part is fed to sulfate. The sulfate levels build up in the a combined fines destruction loop and gas crystallizer mother liquor. Eventually, the alkali stripper which removes ammonia, as well as sulfate will precipitate with the persulfate or the water vapor from the mother liquor. A third part alkali sulfate must be removed from the mother of the mother liquor is diluted so that pure liquor, or if permitted, the liquor must be sodium sulfate decahydrate crystals are formed discarded. on further cooling and are removed to prevent a Zumbrunn, in French Patent No. 1,493,723, buildup of sodium sulfate in solution and teaches the isothermal reaction of solid sodium coprecipitation with the product sodium hydroxide or sodium carbonate with ammonium persulfate. persulfate in a saturated sodium persulfate In the drawing, Figure 1 is a ternary phase solution to produce crystalline sodium persulfate. diagram of the system: water, sodium persulfate, Preferably, the reaction is maintained without and sodium sulfate illustrating the phase relation- cooling at temperatures of 15°C to 25°C with the ships during the process of the present invention. ammonia and water formed by the process partially removed by passing air through the Figures 2A and 28 are flow diagrams of alter- native embodiments of the process of the present crystallization loop which is physically shown invention. operating as a subloop between lines 23 and 27 in Figure 1 includes two superimpsoed ternary Figure 2A or between line 21 and 24 in Figure 2B phase diagrams of the system sodium persulfate which comprises line 25 delivering mother liquor (point S), sodium sulfate (point R), and water and line 39 delivering dilution water to a cooling (point O). The compositions represented by crystallizer 6 operated at temperature T' thereby letters without primes and solid lines are for the crystallizing a slurry of sodium sulfate deca- system at 25°C and represents in the general hydrate crystals. The slurry is conducted by line discussions, the temperature of the sodium 30 to a filter or centrifuge 7 separating the slurry persulfate crystallizer, T; the compositions repre- into a low sulfate mother liquor which is returned sented by letters with primes and dashed lines are to the system by line 31 and the sodium sulfate for the system at 15°C and represents in the decahydrate crystals for disposal through line 38. general discussions, the temperature of the It is obvious that the source of mother liquor for sodium sulfate crystallizer T'. The compositions many of these loops could be wholly, or in part, within the areas OMKJ and O'M'K'J' are not in supplied by other mother liquor sources. For equilibrium with solid sodium persulfate or example, line 24 feeding the ammonium per- sodium sulfate phase at the temperature T or T' sulfate make-up tank 8 could be fed from line 27, respectively. Compositions on the lines MK and from the ammonia stripper 5. The particular M'K' are in equilibrium with sodium persulfate at arrangement of lines and tanks would, in practice, temperatures T and T' respectively. Similarly, the be dictated by the equipment and selected compositions KJ and K'J' are in equilibrium with operating conditions. For example, an a sodium sulfate crystal at the temperatures T and evaporation crystallizer with a fines destruction T', and the sodium sulfate crystal will be sodium loop would combine the functions of the primary sulfate decahydrate at the temperature T'. crystallizer loop and the combined fines Figures 2A and 2B represent two of many destruction and stripper loop. possible embodiments of the apparatus of the In operation, the persulfate values are replaced invention. Both show a cooling crystallizer 1 with in the recirculating mother liquor by adding a stirrer 10, although an evaporation crystallizer ammonium persulfate, preferably as a solid; the would be satisfactory.