United States Patent (19) 3,896,209 Fournier Et Al
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United States Patent (19) 3,896,209 Fournier et al. (45) July 22, 1975 54 REDUCTION OF HEXAVALENT 2,288,320 6/1942 Morey............................. 423,607 X CHROMIUM 3,819,05 6/1974 Henley et al.......................... 21 OF50 75 Inventors. Louis Byron Fournier; Raymond Alan Meyer, both of Wilmington, OTHER PUBLICATIONS Del. Baxendale, "Advances in Catalysis,' Vol. 4, pp. 73) Assignee: E. I. du Pont de Nemours & 75-80. Company, Wilmington, Dei. 2 2 Filed: Jan. 22, 1973 Primary Examiner-Herbert T. Carter 2 Appl. No. 325,623 57 ABSTRACT 52) U.S. Cl. .................... 423/55; 42.3/53; 42.3/607; Under acidic conditions the hexavalent chromium 21 O150; 2 O/53 present in aqueous solutions is chemically reduced to 5 Int. Cl........................ C01g 37/02; COlg 37/00 trivalent chromium with hydrogen peroxide. This pro 58) Field of Search .......... 423/53, 55, 607; 2 O750, cess is improved if the pH of the aqueous solution is 2O/53 maintained below 1.5 and the hydrogen peroxide is gradually added to the solution to keep the chromate 56) References Cited in excess as long as possible. UNITED STATES PATENTS 2, (), 187 3, 1938 Williams ......................... 423f607 X 9 Claims, No Drawings 3,896,209 2 REDUCTION OF HEXAVALENT CHROMUM In the field of pollution abatement, hydrogen perox ide is an attractive alternative to commonly used chem BACKGROUND OF THE INVENTION ical treatment methods since its reaction and decompo Hexavalent chromium is an extremely toxic material. sition products (oxygen and water) are nonpolluting. It can be found in many aqueous solutions, e.g., aque 5 Moreover, excess reagent at normal levels is not itself ous wastes such as the effluents from the electroplating detrimental to most environments and may, in fact, be industry such as plating and rinse baths, and also efflu beneficial. Additionally in the case of accidental spills ents from the phosphatizing, anodizing, tanning, pig or overtreatments hydrogen peroxide entering the envi ments, and chemical industries. It can also be found in ronment in an uncontrolled fashion does not long per the blowdown from cooling towers and boilers. () sist. The most commonly used method of treating such It is known in the art that hydrogen peroxide is a re aqueous solutions containing hexavalent chromium is ducing agent for hexavalent chromium in acid solution, by chemical reduction to trivalent chromium followed e.g., S. B. Brown, Peter Jones and A. Suggett, Chro by precipitation of the reduced chromium with alkali. mium - Hydrogen Peroxide Reactions, Adv. in Catalysis, The reducing agent is generally sulfur dioxide, a deriva 5 4, pp. 193-200. However this reaction is exceedingly tive such as a sulfite or a bisulfite, or ferrous sulfate. complex and it is not quantitative under most condi With SO as the reducing agent, for example as in tions. As a result it has not been readily adapted to pol large plating shops, expensive equipment is required to lution control processes. combine gaseous sulfur dioxide with the aqueous solu tion to be treated. There is an inherent danger and diffi SUMMARY OF THE INVENTION culty in handling and using this toxic and odorous gas. We have found that the reduction of hexavalent Furthermore, to insure rapid and complete reaction, chromium to trivalent chromium in aqueous solutions, the pH of the solution usually is adjusted to pH = e.g., aqueous wastes, can be conducted on a quantita 2.0-3.0 with sulfuric acid. Additional automated in tive basis if the pH of the aqueous solution is main strumentation is normally required for this purpose. 25 tained below 1.5, preferably below 1.0, and the hydro The approximate chemical usage during the SO, gen peroxide is gradually added with agitation. treatment should theoretically be about one pound of The pH of the reaction can be maintained in the de SO, per pound of chromium trioxide in the solution. sired range, by the addition of a mineral acid, e.g., hy However, in actual practice this required amount of 30 drochloric, sulfuric, nitric or the like. It is important to SO can be increased dramatically by species such as note that during the reduction the pH of the solution copper ion and oxygen dissolved in the solution to be increases, i.e., the reduction consumes hydrogen ions. treated. Just as in the chlorine treatment of waste wa Thus even if solution originally has a pH below 1.5, it ters where many materials present in solution can cause must be maintained at this low level during the course a high chlorine demand likewise, acqueous solutions, of the reaction either by adding sufficient acid at the e.g., aqueous wastes, can have a background 'sulfur start of the reaction or adding it during the course of dioxide demand' which may equal or exceed the quan the reaction. tity of sulfur dioxide required to treat dissolved hexava By "gradually added' it is meant that the hydrogen lent chromium. peroxide is added as slowly as is reasonably possible To avoid the hazards and difficulties associated with such that the chromate remains in excess as long as SO sodium bisulfite has been used as an alternate ma possible. The slower the hydrogen peroxide is added, terial for the reduction of hexavalent chromium. The the better. Thus it is preferred to add the hydrogen per bisulfite may be added as a solid or as a solution. The oxide dropwise. When the hydrogen peroxide is rapidly addition of acid, normally sulfuric, is required just as added, a high concentration of HO, will be formed in with SO to maintain a pH of about 2 to 3 to obtain the solution and side reactions will occur. In this event rapid and complete reduction of the hexavalent chro the overall reaction will be less efficient. mium. The anhydrous form of sodium bisulfite, NaS The agitation is desirable to avoid local excesses of O. or sodium metabisulfite, also may be used in this the hydrogen peroxide during the addition. reduction step. However, sulfur compounds share with SO a prob DESCRIPTION OF THE INVENTION lem arising in municipal sewer systems when the sulfate The hexavalent chromium ion is found in small con ions formed from both processes are discharged. centrations in a multiplicity of industrial solutions, but Through bacterial action in anaerobic sewers, sulfate the source of most such solutions is the rinse waters ion forms sulfide ion; this leads to severe odor problems from chromic acid baths, or the discarded baths them and also to crown corrosion, which is a primary cause 55 selves, as used by the metal finishing industries con of sewer cave-ins. Thus, both sulfur dioxide and bisul cerned with the cleaning of metals or with the coating fite ion indirectly lead to damage in sewerage systems. of them with chromium in electroplating and anodizing Furthermore, excess sulfite needed in the reduction re processes. action imposes an oxygen demand. Most of the baths employed in these industrial opera In recent years other methods of removing chromium tions are acid, usually being made up with (1) "chro from solutions have been advocated such as the addi mic acid, 'CrO3, and sulfuric acid or sodium sulfate or tion of barium ions, ion-exchange, reverse osmosis, di (2) sodium dichromate, NaCrO 2HO and sulfuric alysis, electrolytic reduction of hexa- to trivalent chro acid. Anions such as the sulfate ion, chloride ion, or flu mium, and the use of scrap iron to reduce chromium. oride ion, is generally present in an amount equivalent None of these processes has gained the popularity of 65 to about one-hundredth of the chromic acid content. chemical treatment methods based on the reducing The concentration of hexavalent chromium in these ability of sulfur compounds. aqueous solutions can range from 20 to 300,000 ppm. 3,896,209 3 4 The process of the invention is broadly applicahle to cess hydrogen peroxide will reoxidize some trivalent any aqueous solution containing hexavalent chromium, chromium to hexavalent chromium. Thus it is desirable and can be used with solutions at room temperature or to ensure that no excess peroxide is present in the aque solutions at elevated temperatures. It is particularly ous solution when the alkali is added if the small useful with aqueous wastes. Thus it can be used with amount of hexavalent chromium thus created will metal plating baths, which are at elevated temperature cause a problem. and contain larger amounts of hexavalent chromium or This can be accomplished simply by permitting the it can be used with metal plating rinses which are usu solution to stand for whatever time is required to de ally at room temperatures and contain up to 100 ppm compose the peroxide by whatever decomposition cat of hexavalent chromium. () alysts may be present in the solution. Ferric ion, for in The aqueous solution is first treated with sufficient stance, is such a catalyst and is almost invariably pres mineral acid to insure that the pH will be maintained ent in plating waste. below 1.5, preferably below 10, throughout the entire Alternatively the destruction of this excess peroxide reduction reaction. As set forth previously this requires can be accomplished by the addition of a reducing the addition of excess acid as the reaction itself causes agent such as SO, or a derivative, e.g. sodium bisulfite a rise in pH. In a less desirable embodiment, additional or metabisulfite or other sulfur compounds, e.g., thio acid can be added during the course of the reaction to sulfates, sulfides, or hydrosulfites. This method has the maintain the desired pH. If the pH rises above pH 1.5 advantage in that if any chromate is still present, it is during treatment, chromium reduction still occurs, but also destroyed.