METHOD FOR REMOVING HEAVY METALS FROM AQUEOUS SOLUTIONS BY COPRECIPITATION

Abstract: A method for removing heavy metal ions from an aqueous solution to yield a less contaminated aqueous affluent. The method comprises coprecipitating the heavy metal ions with a carrier precipitate which is formed in situ within the aqueous solution.

Description: METHOD FOR REMOVING HEAVY METALS FROM AQUEOUS SOLUTIONS BY COPRECIPITATION Technical Field This invention relates generally to the removal of heavy metals from aqueous solutions and, in particular, to the removal of heavy metals from aqueous solutions by the method of coprecipitation. As used herein, the term "heavy metals" refers to non-ferrous metals and metaloids (e.g., arsenic) which have an atomic number greater than that of calcium. Background Art There is increasing concern over the hazards posed by the rising levels of heavy metals within the world's water supplies. Most heavy metals are toxic to some degree to all life forms. Aqueous concentrations of as little as 0.05 ppm can have a deleterious effect on aquatic flora and fauna. In humans, toxic heavy metal poisoning can lead to severe nervous system disorders and can cause death. Even trace amounts of heavy metals within an organism's environment are potentially dangerous, because heavy metals do not decompose over time (as do most organic pollutants) and often accumulate within the organism throughout its lifetime. This accumulation effect is accentuated in succeeding species along each food chain. As a consequence of the increasing concern over aqueous heavy metal concentration levels, industry is being required to virtually eliminate heavy metals from its aqueous wastes. For many industries, however t OMPI WIPO this requirement is very difficult to fulfill. The metal finishing industries, for example, employ a variety of processes which generate large volumes of aqueous waste material. Many of these wastes contain high concentrations of heavy metals (often as high as 10 percent) , including zinc, nickel, copper, chromium, lead, cadmium, tin, gold, and silver. The combined quantity of these wastes generated daily is very large (over 3.785 billion liters in the United States) , and the number of plants employing metal finishing processes is also large (nearly 8,000 in the United States) . Numerous heavy metal removal methods have been proposed for the metal finishing industries, including dilution, evaporation, alkali-precipitation, absorption, dialysis, electrodialysis, reverse osmosis and ion exchange, but none has been found to be entirely satisfactory. By far the most common heavy metal removal method is alkali-precipitation. In this method, a sufficient quantity of base is added to the aqueous waste solution to precipitate the desired quantity of heavy metals as insoluble metal hydroxides. However, as governmental heavy metal regulations have become stricter, the alkali-precipitation method has become exceedingly costly, more difficult to use and, in some instances, inappropriate. Alkali-precipitation must be carried out at high pH (between about 9.0 and about 12.0) in order to reduce the soluble heavy metal concentrations to within acceptable limits. Additive chemical volumes can therefore be quite high. Large quantities of base are required to raise the waste solution pH to treatment conditions and to precipitate the requisite quantity of. heavy metals. Large quantities of acid are often required to reduce the pH of the resulting treated effluent, prior to its recycle or disposal. Additive chemical unit costs are also quite high because a costly base such as caustic soda must be employed. The most preferable base, aqueous ammonia (because it is less expensive and easier to handle than caustic soda) , is impractical in the alkali-precipitation method. At the high solution pH levels required by the alkali- precipitation method, aqueous ammonia forms soluble complexes with many heavy metal species (especially with copper, nickel, and zinc) thereby preventing their precipitation. Waste streams containing hexavalent chromium, a common contaminant in many metal finishing industry waste solutions, require costly pretreatment because the alkali-precipitation method is ineffective in precipitating hexavalent chromium. The pretreatment step entails reducing the hexavalent chromium to the trivalent state by reaction with a suitable reducing agent, such as sodium bisulfite, at pH levels below 3.0. After pretreatment, the trivalent chromium is precipitated from the solution as a hydroxide by raising the solution pH to above about 9.0. Waste streams containing organic and nitrog¬ enous complexing agents, also common contaminants in many metal finishing industry waste solutions, require a specialized and especially costly alkali-precipitation treatment. To counter the tendency of the complexing agents to solubilize heavy metals, large quantities of calcium hydroxide must be added to the waste solution. OMPI These large quantities of base necessarily raise the pH of the solution to very high levels, and make necessary the eventual use of large quantities of acid to neutral¬ ize the resulting effluent. The necessary use of calcium hydroxide also results in significantly in¬ creased operating costs because calcium hydroxide exists as a slurry at treatment conditions and is, therefore, very difficult to handle and control. Furthermore, having to use calcium hydroxide in such high concentrations results in large precipitate sludge disposal costs because abnormally large volumes of sludge are produced. This abnormal sludge production stems from (a) the fact that, in addition to the formation of heavy metal precipitates, calcium precipi- tat.es are formed as well, and (b) the fact that calcium precipitates tend to retain a large amount of water. There is, therefore, a need for a superior method for removing heavy metals from aqueous streams, especially from aqueous waste streams produced in the metal finishing industries. Thus, it is an object of this invention to provide a superior method for removing heavy metals -from aqueous waste streams. It is a further object of this invention to provide a superior method for removing heavy metals from aqueous waste streams without having to adjust the pH of such streams to pH values above 8.0. It is a still further object of this invention to provide a less costly method for removing heavy metals from aqueous waste streams. OMPI It is a still further object of this invention to provide a superior method for precipitating heavy metals from aqueous waste streams requiring less additive base. It is a still further object of this invention to provide a method for reconditioning a heavy metals- containing acid stream requiring less additive acid. It is a still further object of the invention to provide a superior method for removing chromium from an aqueous waste solution. It is a still further object of the invention to provide a superior method for substantially reducing ' the concentration of heavy metals within the aqueous waste streams of the metal finishing industries. These and other objects and advantages of the invention will become apparent to those skilled in the relevant art in view of the following description of the invention. Disclosure of Invention In accordance with the present invention, a superior method is provided for removing heavy metals from an aqueous solution. The method comprises co¬ precipitating the heavy metal ions with a carrier precipitate which is formed in situ within the aqueous solution. Use of the method of the invention results in reducing the concentrations of aqueous heavy metal ions to below their thermodynamic equilibrium concentrations. This extraordinary result affords the user the unique ability to substantially reduce high aqueous concentra¬ tions of heavy metals, often to within legally accept¬ able concentrations, while maintaining the aqueous solution at near neutral pH. Brief Description of the Drawing The present invention will be more readily understood by reference to the drawing which schemat¬ ically illustrates the preferred embodiment of the invention. Modes, Including Best Mode, for Carrying Out Invention ' The method of the invention can be used to remove dissolved heavy metals and/or iron from nearly any aqueous stream. The invention is particularly useful in removing the large concentrations of copper, nickel, zinc, gold, silver, cadmium, tin, chromium, and lead from pickling acid wastes and other acidic waste streams formed in the metal finishing industries. In the method of the invention, a selected carrier precip- itate is created within an aqueous waste solution which is contaminated with heavy metals and/or iron. The contaminants are thereby caused to coprecipitate with the carrier precipitate and are thus removed from the aqueous solution. "Coprecipitation" as used with respect to the invention described herein refers to the chemical phenomenon where, within an aqueous solution containing a cationic carrier precipitate precursor, an anionic _OMPj carrier precipitate precursor and one or more coprecip- itant precursors, the cationic and anionic carrier precipitate precursors are caused to chemically react and precipitate out of the aqueous solution as carrier precipitate particles; and, as the carrier precipitate particles are formed, coprecipitant precursors are removed from the aqueous solution by adsorption onto the surface of the carrier precipitate- particle and/or by occlusion within the interior of the carrier precip- itate particle. The term "occlusion" as used herein refers to the entrapment of foreign ions within a precipitate particle by physical encapsulation within the particle walls and by chemical bonding within the particle structure. It has been discovered that by the method of the invention a heavy metals-rich aqueous solution can be transformed into a metastable liquid-solid mixture wherein the liquid-phase heavy metal concentrations are substantially lower than their respective equilibrium concentrations. Although the invention is not limited to any particular theory of operation, the reduction of heavy metal concentrations below their equilibrium concentrations is believed to result, in part, from the large quantities of heavy metals which are occluded within the carrier precipitate structure. The proportion of the dissolved heavy metals which become occluded is typically greater than 10 weight percent, frequently greater than 50 weight percent and often greater than 80 weight percent. By segregating the liquid phase of the mixture from the solid phase, a stable aqueous effluent is produced which is substan¬ tially free of heavy metals. It has been further discovered that the method of the invention results in liquid-phase heavy metal concentrations which are so much lower than their respective equilibrium values that environmentally innocuous liquid effluents can be produced from heavy metals-concentrated solutions, even when the liquid phases are maintained at near neutral pH. In the preferred embodiment illustrated in the drawing, an acid waste stream containing heavy metal ions in concentrations frequently greater than 1,000 ppmw and often greater than 10,000 pp w is generated in industrial process 6. This stream is transferred from industrial process 6 via conduit 8. Carrier precipitate precursor cations from source 10 are added to the stream via conduit " 12 to raise the concentration of such cations sufficiently so that the molar ratio of such cations to heavy metal ions within the stream is preferably greater than 1:1, more pref¬ erably greater than 4:1 and most preferably greater than 8:1. In a preferred embodiment of the invention, wherein the carrier precipitate is amorphous oxyferric hydroxide (hereinafter referred to as "ferric hydroxide") , little or no addition from source 10 may be necessary since waste streams rich in heavy metals are commonly rich in dissolved iron as well. Carrier precipitate precursor anions are also added to the waste acid stream. Preferably, such anions are added in sufficient quantities to raise the concentration of such anions within the waste stream to above the stoichiometric concentration necessary to react with all solubilized carrier precipitate precursor cations, and more preferably above 110 percent of such stoichiometric concentration. The addition of such anions can be made by injection into conduit 8 (not shown) or by addition to vessels 14 and 16 (as described hereinafter) . In the preferred embodiment wherein the selected carried precipitate is ferric hydroxide, addition of such anions (hydroxyl ions) is made in two stages to allow for accurate pH control. Accordingly, as illustrated in the drawing, the waste acid stream from industrial process 6 is transferred via conduit 8 to mixing vessel 14. Base from source 18, preferably aqueous ammonia, is added to the acid waste solution within vessel 14 via conduit 20. Rapid mixing of the solution within vessel 14 is preferably assured by the use of mixing device 22. Sufficient base is added to the solution within vessel 14 to raise the solution pH to between about 5.5 and about 6.5. The partially neutralized waste solution is then transferred, via conduit 24, to precipitation vessel 16. Via conduit 26, additional base from source 18 is added to the waste solution within vessel 16 in sufficient quan- tities to raise the solution pH to between about 6.5 and about 8.0, preferably to between about 6.5 and about 7.5. Within vessel 16, the carrier precipitate precursor cations are caused to react with the carrier precipitate precursor anions and precipitate out of solution. As the carrier precipitate forms, substan¬ tial quantities of heavy metal ions coprecipitate with the carrier precipitate and are thereby removed from the solution. In the preferred embodiment wherein the carrier precipitate is ferric hydroxide, precipitation is triggered by the oxidation of ferrous ions to ferric ions. Accordingly, as illustrated in the drawing, an oxidizing agent from source 28, preferably air, is added to the acid waste stream via conduit 30. Suffi¬ cient oxidizing agent is added to rapidly oxidize essentially all of the dissolved ferrous ions to ferric ions. When air is the selected oxidizing agent, the rate of air addition is preferably sufficient to oxidize all of the ferrous ions and to air-saturate the solution. Dispersion device 32 and/or mixing device 34 can be used to assure rapid and thorough mixing of the waste solution, additive base and additive oxidant within vessel 16. The coprecipitant reaction is very rapid. Typically, more than 95 weight percent, and usually more than 99 weight percent, of the heavy metals are removed from the waste solution within about 8 minutes after the formation of the first 5 weight percent of the carrier precipitate. After this 8-minute period, the remaining solubilized heavy metals continue to be adsorbed onto the precipitate particles. However, because the solution-precipitate system is metastable, in some solutions the rate of this additional heavy metal adsorption may tend to be counterbalanced by the slow resolubilization of particular coprecipitated heavy metal species. Thus, the net value of additional contact between the precipitate particles and the supernate liquid (after the initial 8-minute period) varies from system to system. It follows that the ideal residence time of the aqueous solution within vessel 16 and separator 36 (described hereinafter) varies with each particular operating situation, and that the optimizing of such .residence time will, in each situation, require some routine adjustment. From vessel 16, aqueous effluent, now sub¬ stantially reduced in dissolved heavy metal content, is transferred together with the nascent precipitate to solids separator device 36 via conduit 38. Within separator 36 the effluent and precipitate are segregated into two streams. Separator 36 is comprised of a clarifier, filter, centrifuge, settling pond or other suitable liquid-solid separating equipment capable of segregating the precipitate particles from the aqueous effluent. Segregated precipitate is removed from separator 36 as a sludge and is transferred to a suitable disposal site (not shown) via conduit 40. The corresponding aqueous effluent, which typically contains less than 15 ppmw heavy metals and usually contains less than 5 ppmw heavy metals, can be recycled to industrial process* 67 via conduit 42. In those cases where it is desired that the recycled effluent be less basic than the solution within separator 36 (e.g., where the recycled effluent is to be used as an acid makeup solution) , acid from source 44 is added to the recycled effluent via conduit 46. Optionally, the treated effluent from separator 36 is discharged to a disposal site (not shown) via conduit 48. Preferably, the concentrations of heavy metals within the treated effluent are reduced to below the relevant legal limits so that non-recycled effluent can be discharged directly to a municipal sewer. Although the preceding description of the preferred embodiment assumes that the aqueous waste solution is an acid waste, it is understood that the invention is not limited to the treatment of such wastes. Furthermore, although the preceding descrip¬ tion of the preferred embodiment describes a continuous process, it is understood that the invention can also be practiced as a batch process. Preferably, the choices of carrier precipitate precursors and operating conditions are made so as to maximize the removal of heavy metal ions while minimiz¬ ing treatment costs. Towards that end, the choices are generally made so as to (1) produce a carrier precipi¬ tate structure which is conducive to heavy metal occlusion, (2) produce a carrier precipitate particle surface which is conducive to adsorption, (3) form the carrier precipitate as rapidly as possible, and (4) minimize extraneous reactions which interfere with heavy metal coprecipitation. . • The carrier precipitate cation and anion are generally chosen so that, when the carrier precipitate is forming, the developing precipitate is conducive to the occlusion of heavy metals. The carrier precipitate cations of choice are those which have approximately the same ionic diameter as most of the contaminant heavy metals. The similarity of ionic diameter makes it conducive for the forming carrier precipitate to accept heavy metal ions in substitution for carrier precipitate cations. When substituted heavy metal ions are similar in size to the cations, the precipitate structure is not unduly stressed by the heavy metal inclusion. Thus, preferably, the ionic diameter of the carrier precipitate precursor cation is between about 75 percent and about 125 percent of the ionic diameter of the most common heavy metal contaminant within the waste solution. The preferred carrier precipitate cations are metal ions, with the ions of aluminum, calcium, iron, and magnesium being more preferred. Most preferred are iron ions, because such ions closely approximate the size of most contaminant heavy metals and because it is common for large natural concentrations of iron ions to be dissolved within heavy metals-rich waste streams. The carrier precipitate anions of choice are those which form insoluble salts with the contaminant heavy metals as well as with the carrier precipitate cations. Such anions have a strong attraction for heavy metal ions, and the degree of heavy metal occlu¬ sion is directly proportional to the strength of the anion-heavy metal bonds. This proportionality stems from the fact that, before heavy * metal occlusion can occur, the heavy metal ions must first be drawn to and strongly held by the anions at the surface of the carrier precipitate. When the carrier precipitate cations are ferric ions, the preferred carrier precipi¬ tate anions are hydroxyl, phosphate and carbonate ions. When the carrier precipitate cations are calcium ions, the preferred anions are hydroxyl, phosphate, carbonate and sulfate ions. When the carrier precipitate cations are aluminum, the preferred anions are hydroxyl and phosphate ions. When the primary precipitate cations are magnesium, the preferred anions are hydoxyl, phosphate and carbonate ions. The preferred carrier precipitates are aluminum hydroxide, ferric hydroxide, calcium sulfate, iron phosphate and calcium phosphate, with ferric hydroxide being most preferred. The operating conditions are also generally controlled so as to produce a carrier precipitate particle surface which is conducive to the adsorption of heavy metal ions. As explained above, the carrier precipitate anion is chosen from among those anions which form strong bonds with the contaminant heavy metal ions. In addition, the concentration of carrier precipitate precursor anions in solution is maintained in most cases at levels sufficiently in excess of the concentration of the carrier precipitate precursor cations so as to ensure that the carrier precipitate particle surface is anionic. The anionic particle surface attracts the heavy metal ions, binds them (adsorption) and makes them available for incorporation within the precipitate structure (occlusion) . When hydroxyl ions are the chosen carrier precipitate anions, maintaining such anion excess is a matter- of pH control. Where ferric hydroxide is the chosen carrier precipitate, solution pH during coprecipitation is maintained above about 6.0 because solutions which are more acidic cause the ferric hydroxide precipitate surface to take on a cationic character. In general, the larger the carrier precipitate surface area, the more heavy metals are removed from solution. Thus, the carrier precipitate and the conditions of operation are preferably chosen so as to maximize the surface area of each unit mass of precipi¬ tate. The total mass quantity of produced carrier precipitate is thereafter controlled, where possible, to the minimum value sufficient to remove the requisite quantity of heavy metals. The carrier precipitate is generally formed as rapidly as possible because the removal of heavy metal ions by both the adsorption and occlusion mecha- __OMPI_ nis s is markedly greater at higher precipitation rates. Typically, about 95 percent of the carrier precipitate is formed within about 15 minutes, pref¬ erably within about 10 minutes, and more preferably within about 5 minutes. The adsorption of heavy metal ions is increased by an increase in the precipitation rate because adsorption is surface area-dependent. When the carrier precipitate is formed rapidly, it forms as a large number of small individual particles, each having a high surface area-to-mass ratio. By relative comparison, when the carrier precipitate is formed slowly, it forms as a small number of large particles, each having a low surface area-to-mass ratio. Thus, for a given mass of carrier precipitate precursors, the faster the precipitate is formed, the larger is the combined surface area of the resulting * precipitate particles. The occlusion of heavy metal ions is increased by an increase in precipitation rates because occlusion is adsorption-dependent and diffusion time-dependent. As alluded to above, heavy metal ions are more likely to be occluded within the carrier precipitate when they are first adsorbed at the carrier precipitate surface. Thus, the number of heavy metal ions occluded within the carrier precipitate is proportional to the number of heavy metal ions adsorbed onto the carrier precipitate surfaces during the growth of the carrier precipitate particles. The number of heavy metal ions which are occluded within the carrier precipitate lattice structure is inversely proportional to the relative ionic diffusion times available in the vicinity of the forming carrier precipitate. Heavy metal ions which initially bond with precipitate surface anions and which might otherwise be eventually incorporated as a part of the particle framework, tend to be displaced by competing carrier precipitate precursor cations which diffuse to the precipitate surface. Thus, it can be seen that, if the rate of carrier precipitate formation is relatively fast with respect to the rates of ionic diffusion near the forming particle surfaces, the localized diffusion times are relatively small and more heavy metals are occluded. The choices of carrier precipitate and operating procedures are therefore preferably made, in part, so as to maximize the rate at which the carrier precipitate is formed. In the preferred embodiment wherein the carrier precipitate is ferric hydroxide, ' the precipitation rate » depends on two reactions, .the oxidation of ferrous ions to ferric ions and the reaction of ferric ions with hydroxyl ions. The precipitation rate is almost solely controlled by the oxidation reaction since oxidation is much the slower of the two reactions. Thus, the basic strategy is to maximize the rate of oxidation. Since the oxidation reaction is accelerated as the solution pH is raised, the pH of the reaction medium is maintained as high as possible during the introduction of the oxidizing agent. The oxidation reaction is also accelerated by the presence of a suitable catalyst. Most soft Lewis bases can be employed as suitable catalysts, with iodine and oxygen-containing soft Lewis bases being preferred. The most preferred catalyst is ferric hydroxide since it is manufactured in situ by the method of the invention. In the continuous process embodiment of the invention illustrated in the drawing, no addition of catalyst is necessary because ferric hydroxide is continually being formed and is constantly present within precipitation vessel 16. In a ferric hydroxide batch process embodiment of the invention, however, it is preferred that a suitable catalyst, most preferably ferric hydroxide product, be mixed with the aqueous waste stream at the time the oxidizing agent is added. In addition to promoting rapid precipitation, the rapid oxidation of the ferrous ions may promote occlusion in another way. In aqueous solutions, ferrous ions tend to form soluble complexes with heavy metal and hydroxyl ions. If the ferrous ions of such, complexes are rapidly oxidized to ferric ions, these complexes tend to precipitate out of solution en masse, including the originally complexed heavy metal ions which, during precipitation, become occluded within the precipitate. The operating conditions are also preferably controlled to minimize extraneous reactions which interfere with the heavy metal coprecipitation. Thus, ' the concentration of superfluous ions is maintained as low as practical (for instance, by dilution of the waste stream) since such ions interact with carrier precipitate precursor and heavy metal ions, thereby impeding the coprecipitation reactions. Also, the pH of the aqueous solution is maintained at sufficiently low levels to. minimize the effects of complexing agents which solubilize heavy metal ions at high pH. For instance, certain nitrogenous compounds, including ammonia, will complex with several heavy metal ions. especially with copper, nickel, and zinc, at pH levels above about 8.0. Where such complexing agents are present in the aqueous solution and where "cσmplexable" heavy metal ions are also present, the pH of the aqueous solution is therefore maintained below about 8.0, preferably below about 7.5. Accordingly, in the embodiment of the invention illustrated in " the drawing (wherein aqueous ammonia is used as a base) , the pH of the waste solution in precipitation vessel 16 is preferably maintained between about 6.5 and about 7.5 in order to oxidize the ferrous ions as rapidly as possible but not form significant quantities of ammonia- heavy metal complexes. Since this pH operating range is relatively narrow, since the relationship between dissolved ammonia and solution pH is very sensitive within this operating range, and since the pH of the acid waste stream generated in industrial process 6 can fluctuate significantly, pH control is preferably accomplished in two steps. First, the pH of the acid waste stream is raised to pH levels between about 5.5 and about 6.5 within vessel 14. Second, the solution pH is carefully raised to operating levels (preferably between about 6.5 and about 7.5) within precipitation vessel 16. The method of the invention is unique in its effectiveness for removing substantial quantities of heavy metals from aqueous solutions at near neutral pH. The effective removal of heavy metals at near neutral pH is most advantageous to the industrial operator. It substantially reduces problems caused by the aforemen¬ tioned presence of heavy metal complexing agents, especially nitrogenous complexing agents, which are commonly found in aqueous waste streams. Accordingly, OMPI it allows the additive use of aqueous ammonia, a most cost-effective base. The ability to operate at near neutral pH also eliminates the need to add neutralizing acid to the treated effluent before disposal. Like- wise, it markedly reduces the consumption of fresh acid necessary to reacidify the treated effluent when the effluent is employed as a recycle acid. Finally, operating at near neutral pH produces a precipitate which settles faster than precipitates formed at higher pH levels. This last fact allows the operator to separate the treated effluent from the nascent precipi¬ tate particles with smaller and less expensive separat¬ ing equipment than would be required by other precipi¬ tation methods. The preferred embodiment of the invention employing ferric hydroxide as the carrier precipitate has the additional unique advantage over conventional hydroxide precipitation methods of requiring less additive base to precipitate a given quantity of iron and contaminant heavy metals. In conventional alkali- precipitation methods, base is consumed in the precipi¬ tation of individual iron ions, and additional base is consumed in the precipitation of individual heavy metal ions. In the embodiment of the invention illustrated in the drawing, base is consumed in the precipitation of individual ferric ions, but little additional base is required to precipitate the heavy metal ions. Furthermore, in the preferred embodiment of the inven- tion, a substantial proportion of the base required by the process is manufactured by the process itself. For every ferrous ion that is oxidized to a ferric ion, a hydroxyl ion is produced pursuant to the following reaction: 2 Fe ÷2 + h 0 2 + H 2 0 2 Fe +3 + 2 OH~ The method of the invention is also unique in its ability to remove chromium ions from an aqueous waste solution without having to first reduce the hexavalent chromium ions to trivalent ions. Typically in the preferred embodiment of the invention, more than about 95 percent of the hexavalent chromium is removed from the aqueous waste solution at the same time and by the same method as are other heavy metals. Thus, the method of the invention eliminates the need for seg¬ regating and separately treating hexavalent chromium- containing waste streams and saves the costs of acid, base, and reducing agent required by such treatment. Finally, the method of the invention is superior to conventional precipitation methods in that the method of the invention produces less precipitate sludge. The lower sludge production stems, in part, from the fact that the volume of sludge is smaller when several metals are coprecipitated than when those metals are precipitated separately. The difference in sludge production is even greater when the method of the invention and conventional precipitation methods are compared in the treatment of aqueous solutions containing significant quantities of heavy metal - complexing agents. As stated above, the conventional treatment of such aqueous solutions requires the use of large quantities of calcium hydroxide and results in the formation of sludge volumes which are even larger than normal. The invention can be further understood by considering the foregoing specific examples which are illustrative of specific modes of practicing the invention and are not intended as limiting the scope of the appended claims. Example 1 Two aqueous waste solution samples are obtained from a commercial electroplating process. The first sample is taken from a 40,878 liters/day waste water stream containing approximately 0.6 weight percent total dissolved solids. The second sample is taken from a 5,299 liters/day waste acid stream containing approximately 15 weight percent total dissolved solids. • • Twenty-three milliliters of the waste acid sample is mixed in a mechanically agitated beaker with 177 ml of the waste water sample to yield 200 ml of a combined waste solution sample. Immediately thereafter, 4.5 ml of a 28 weight percent aqueous ammonia solution is rapidly added to the beaker. Thereupon, 25 ml of an aqueous solution containing approximately 4 weight percent of a ferric hydroxide-heavy metal precipitate is added to the beaker. Immediately thereafter, air is commenced to flow through a sintered glass tube at the bottom of the beaker so as to cause air bubbles to rise through the solution. A precipitate is observed to appear within the solution, and the solution pH is observed to begin dropping. Aqueous ammonia is periodically added to the solution over about the next 30 minutes in order to maintain the solution pH between about 7.0 and about 7.5. After about 30 minutes, the solution pH is observed to stabilize. The flow of air is ceased but the solution is agitated for an additional 30 minutes. Thereafter, a pipette is used to extract a sample of the solution-precipitate mixture. The precipitate particles are removed by filtering the sample through #41 (coarse) filter paper. The resulting filtrate is clear and colorless. The filtrate is analyzed for heavy metals content, and compared to the heavy metal content of the original combined waste solution sample. The results are presented in Table 1. TABLE 1 Solu¬ Metals Concentrations, ppmw tion Sample Cd Cr Cu Fe Ni Pb Zn EH Untreated combined waste solution 0.6 2.3 5 4,850 3.5 106 953 3.5 Treated filtrate after treatment <0.1 <0.1 1.4 <0.1 0.2 <0.5 2.8 7.3 Example 2 A 50 ml sulfuric acid waste sample from a commercial electroplating process is diluted with distilled water to 200 ml. The waste solution is neutralized by the addition of 17.5 ml of a 28 weight percent aqueous ammonia solution, whereby the solution pH is observed to be 7.7. The slurry is added to a mechanically, agitated beaker containing 22 ml of an aqueous ferric hydroxide slurry. Air is commenced to flow through a sintered glass tube at the bottom of the beaker so as to cause air bubbles to rise through the solution. Aqueous ammonia is periodically added to the solution so as to maintain the solution pH between 7.0 and 7.5. About 15 minutes after neutralization, the pH of the solution is stabilized at about 7.35 and a precipitate is observed within the solution. Air dispersion is terminated but mechanical agitation is continued. A sample of the solution-precipitate mixture is extracted with a pipette and filtered through coarse filter paper. About 25 minutes after neutralization, a second sample is similarly extracted and filtered. The filtrate from both samples is clear and colorless. The filtrate from both samples is analyzed for heavy metals content and the results are compared to the heavy metals content of the original acid waste sample. The comparison is summarized in Table 2. TABLE 2 Metals Concentration, ppmw c Sample Cd Cr Cu Fe Ni Zn PPm £H Untreated acid waste 0.75 8.75 2.0 3,625 8.5 132 66,250 ' N.T. Treated filtrate 15 minutes after neutralization <.01 <.01 0.2 <.01 1.7 2.1 N.T. 7.35 I to tf-*. I Treated filtrate 25 minutes after neutralization <.01 <.01 0.2 <.01 1.7 2.1 N.T. 7.35 N.T. denotes that the parameter was not tested. Example 3 A 200 ml volume of a heavy metals-containing acid solution is prepared. The solution is placed in a mechanically agitated beaker and neutralized to a pH between about 7.0 and about 7.5. Immediately there¬ after, 0.142 gr of ferric oxide is added to the solution and air is commenced to bubble through the solution. Additional base is periodically added to maintain the solution pH between 7.0 and 7.5. About 7 minutes after neutralization, the solution pH is observed to have stabilized and precipitate particles are visible within the solution. A sample of the solution-precipitate mixture is extracted and filtered with coarse filter paper. The filtrate, which is clear and colorless, is analyzed .for heavy metals content. The solution-precipitate mixture is agitated for an additional 15 minutes (a total of 22 minutes after neutralization) without addition of base or air. A second sample is extracted and filtered through coarse filter paper. The filtrate, which is clear and colorless, is analyzed for heavy metals content. The solution-precipitate mixture is agitated for an additional 7..5 hours (a total of 18 hours after neutralization) . A third sample is extracted and filtered, and the clear, colorless filtrate is analyzed for heavy metals content. The results of the filtrate heavy metals analyses are compared to the heavy metals content of the original acid solution. A summary of this compar¬ ison is set forth in Table 3. OMPI TABLE 3 Metals Concentration, ppmw Solution Sample Cd Cr Cu Fe Ni Zn Untreated acid Solution 102 74 96 5,684 96 101 N.T. Treated filtrate 7 min. after neutralization <.01 0.2 <.01 3.4 <0.1 7.5 I M I Treated filtrate 22 min. after neutralization <.01 09 <.01 7.3 0.6 7.5 Treated filtrate 18 hr. after neutralization 9.3 <.01 09 <.01 6.7 0.8 6.9 Example 4 A sample of an acid waste solution from a commercial galvanizing process is placed in a mechan¬ ically agitated beaker, neutralized with aqueous ammonia and oxidized with bubbled air while maintaining the solution pH between about 7.0 and about 8.2. When the solution pH stabilizes, addition of aqueous ammonia and air is discontinued except as noted below. A solution-precipitate sample is extracted with a pipette 7 minutes after neutralization. The sample is filtered and analyzed for heavy metals. A second sample is also extracted (60 minutes after neutralization) , filtered, and analyzed for heavy metals. Additional aqueous ammonia is thereupon added to the solution and a third sample is extracted (73 minutes after neutralization) . This sample is also filtered and analyzed for heavy metals. Two additional samples are similarly extracted, filtered, and analyzed for heavy metals. The results of all analyses are summarized in Table 4. C TABLE 4 Metals Concentration, ppmw Solution Sample Cd Cr Cu Fe Ni Pb n Untreated acid waste 6.25 256 72 54,400 206 5,125 354 N.T. Treated filtrate 60 min. after neutralization <.01 <.01 5.6 1.3 6.6 <0.5 1.2 7.0 I Treated filtrate 00 after base added, I 73 min. after neutralization <.01 <.01 7.2 <.01 3.2 <0.5 0.6 7.8 Treated filtrate 120 min. after neutralization <.01 <.01 5.4 <.01 1.5 0.6 0.6 7.6 Example 5 A 50 ml acid waste sample from a commercial electroplating process is added to a mechanically agitated beaker and diluted with distilled water to 200 ml. To this diluted solution is added 18 ml of a 28-weight-percent aqueous ammonia solution, whereby the solution pH is observed to be 7.0. An additional 10 weight percent iron is added to the solution in the form of ferric hydroxide particles. Immediately thereafter, air is bubbled through the solution. Aqueous ammonia is periodically added to maintain the solution pH between 7.0 and 7.5. After about 8 minutes, the solution pH is observed to have stabilized. A sample is extracted with a pipette, filtered, and analyzed for heavy metals. Air dispersion is halted but solution agita¬ tion is continued. The pH of the solution is raised to about 9.0. A second sample is immediately extracted, filtered, and analyzed for heavy metals. The solution is agitated for an additional 60 minutes during which time aqueous ammonia is period¬ ically added to maintain the pH at about 9.0. A third sample is extracted, filtered, and analyzed for heavy metals.. The results are summarized in Table 5. TABLE 5 Metals Concentration, ppmw Solution Sample Cd Cr Cu Fe Ni Pb Zn ES Untreated acid waste 0.75 8.75 3,625 8.5 132 132 N.T. Treated filtrate 8 min. after neutralization 0.04 <.01 0.2 0.3 0.7 <0.05 2.4 7.0 eo Treated filtrate o 10 min. after I neutralization 0.1 <.01 1.6 0.3 3.3 <0.05 30 9.0 Treated filtrate 70 min. after neutralization 0.1 <.01 1.6 0.3 2.6 <0.05 30 9.0 Industrial Applicability The method of the invention is suitable for removing substantial quantities of heavy metals from a wide variety of industrial processes. The method is especially suitable for the removal of heavy metals from aqueous wastes processes used in the metal- finishing industries. Such aqueous wastes are typically heavily contaminated with heavy metals, including such difficult to remove metals as copper, nickel, zinc, and chromium. By the method of the invention, a substantial proportion of such heavy metal contaminants, typically greater than 95 weight percent, can be removed from such aqueous wastes under conditions of near neutrality. Having now described the invention, I claim:

Claims: 1. A method for reducing the heavy metal content of an aqueous solution containing carrier precipitate precursors and heavy metal ions, comprising: (a) selectively causing said carrier precip¬ itate precursors to react within said aqueous solution so as to rapidly form by coprecipitation an amorphous precipitate which contains at least a substantial proportion of said heavy metal ions; and (b) separating said precipitate from said solution so as to form an effluent solution having a reduced concentration of heavy metal ions. 2. The method defined in claim 1 wherein, during step (a) , the pH of said aqueous solution and the ratio of said carrier precipitate precursor ions to said heavy metal ions- are controlled such that at least about 95 percent of said amorphous precipitate is formed within about 15 minutes. 3. The method defined in claim 2 wherein said carrier precipitate precursors comprise cationic and anionic carrier precipitate precursor ions and wherein said method further comprises the step of adding additional cationic carrier precipitate precursor ions to said aqueous solution so as to increase the molar ratio of said cationic precursor ions to said heavy metal ions in said solution during treatment in step (a) . 4. The method defined in claim 3 wherein the concentration of said anionic precursor ions is at least 110 molar percent of the stoichiometric concentration required to react said anionic precursor ions with all of said cationic precursor ions. 5. The method defined in claim 1 wherein said amorphous precipitate comprises ferric hydroxide, aluminum hydroxide, calcium sulfate, iron phosphate, calcium phosphate, or mixtures thereof. 6. The method defined in claim 1 wherein said heavy metal ions contained in said aqueous solution prior to step (a) comprise hexavalent chromium ions and wherein said method further comprises the step of, during step (a) , c »ontrolling the pH of said aqueous solution .and the ratio of said carrier precipitate precursor ions to said heavy metal ions such that said amorphous precipitate contains greater than 95 percent of said hexavalent chromium ions. 7. A method for removing heavy metal ions from an aqueous solution containing heavy metal ions and ferrous ions, comprising: (a) oxidizing at least a portion of said ferrous ions to ferric ions while controlling the pH of said solution so as to rapidly form an amorphous precipitate comprising ferric hydroxide and a substantial proportion of said heavy metal ions; and (b) separating said precipitate from said solution so as to form an effluent solution having a reduced concentration of heavy metal ions. 8. The method defined in claim 7 wherein during step (a) the pH of said aqueous solution is controlled above about 6.0 and the rate of oxidation of said ferrous ions to said .ferric ions is controlled such that about 95 percent of said amorphous precipitate is formed within about 15 minutes. 9. The method defined in claim 8 wherein the pH of said solution is controlled during step (a) between about 6.5 and about 8.0 by the addition of a base. 10. The method defined in claim 9 wherein the pH of said solution prior to treatment in step (a) is less than about 5.5 and wherein said method further comprises the step of, prior to step (-a) , adding a base to said solution so as to raise its pH to between about 5.5 and about 6.5 before treatment thereof in step (a) . 11. The method defined in claim 9 wherein said base is selected from the group consisting of caustic soda and calcium hydroxide and mixtures thereof, 12. The method defined in claim 9 wherein said base comprises ammonia. 13. The method defined in claim 12 wherein the heavy metal ions contained in said solution prior to treatment in step (a) comprise the ions of copper, nickel, zinc or mixtures thereof and wherein at least a substantial portion of said ions of copper, nickel, zinc or mixtures thereof are contained in said precipitate. 14. The method defined in claim 7 wherein during step (a) the pH of said aqueous solution is controlled above about 6.0, and the rate of oxidation of said ferrous ions to said ferric ions is controlled such that said amorphous precipitate contains more than about 95 weight percent of the heavy metal ions con¬ tained in said aqueous solution prior to treatment in step (a) . 15. The method defined in claim 14 wherein a substantial proportion of the heavy metal ions contained in said aqueous solution prior to treatment in step (a) are occluded within said amorphous precipitate. 16. The method defined in claim 15 wherein the concentration of heavy metal ions in the effluent solution from step (b) is less than about 15 ppmw. 17. The method defined in claim 15 wherein the concentration of heavy metal ions in the effluent solution from step (b) is less than about 5 ppmw. 18. The method defined in claim 15 wherein the concentration of heavy metal ions in said aqueous solution prior to treatment in step (a) is greater than about 1,000 ppmw. 19. The method defined in claim 7 wherein said heavy metals contained in said aqueous solution prior to treatment in step (a) comprise hexavalent chromium ions and wherein during step (a) the pH of said aqueous solution is controlled between about 6.5 and about 8.0 and the rate of oxidation of said ferrous ions to said ferric ions is controlled such that said amorphous precipitate contains greater than about 95 percent of said hexavalent chromium ions contained in said aqueous solution prior to treatment in step (a) . 20. The method defined in claim 7 further comprising the step of adding ferrous ions to said aqueous solution so as to increase the molar ratio of said ferrous ions to said heavy metal ions in said solution during treatment in step (a) . 21. The method defined in claim 7 wherein said method further comprises contacting said aqueous solution throughout step (a) with a catalytic amount of a ferric hydroxide catalyst so as to enhance the speed of oxidation of said ferrous ions to ferric ions. 22. The method defined in claim 21 wherein said ferric hydroxide catalyst comprises at least a portion of the amorphous precipitate formed during treatment of said solution in step (a) . 23. A method for removing heavy metal ions from an aqueous solution containing heavy metal ions and ferrous ions in a ratio of at least one mole of ferrous ions per mole of total heavy metal ions, comprising: (a) intimately contacting said solution with a molecular oxygen-containing gas and adding a base to control the pH of said aqueous solution between about 6.5 and about 8.0 so as to rapidly oxidize at least a substantial amount of said ferrous ions to ferric ions and to rapidly form an amorphous precipitate comprising ferric hydroxide and at least 95 weight percent of said heavy metal ions; and (b) separating said precipitate from said solution so as to form an effluent solution containing less than about 15 ppmw total heavy metal ions. 24. The method defined in claim 23 wherein said aqueous solution has a molar ratio of ferrous ions to total heavy metal ions less than about 1:1 prior to treatment in step (a) and wherein said method further comprises the step of adding ferrous ions to said aqueous solution so as to increase the molar ratio of said ferrous ions to said heavy metal ions in said solution during treatment in step (a) to above 1:1. , 25. The method defined in claim 23 wherein said base is selected from the group consisting of caustic soda and calcium hydroxide and mixtures thereof. 26. The method defined in claim 23 wherein said base comprises ammonia. 27. The method defined in claim 23 wherein the heavy metal ions contained in said aqueous solution prior to treatment in step (a) comprise ions of copper, nickel and zinc or mixtures thereof and wherein said precipitate contains more than about 95 weight percent of said ions of copper, nickel, zinc or mixtures thereof. 28. The method defined in claim 23 wherein said heavy metal ions contained in said aqueous solution prior to treatment in step (a) comprise hexavalent chromium ions and wherein said amorphous precipitate formed in step (a) contains greater than about 95 percent of said hexavalent chromium ions. 29. The method defined in claim 23 further comprising contacting said aqueous solution throughout step (a) with a catalytic amount of a ferric hydroxide catalyst comprising at least a portion of said amor¬ phous precipitate. 30. A method for removing heavy metal ions from an aqueous solution containing ferrous ions and heavy metal ions in a ratio of at least four moles of ferrous ions per mole of total heavy metal ions, comprising: (a) in a contacting zone wherein the mean residence time for said aqueous solution is less than about 15 minutes, contacting said ferrous ions with air in the presence of a catalytic amount of a ferric hydroxide catalyst and adding a base to control the pH of said aqueous solution in said contacting zone between about 6.5 and about 8.0 so as to rapidly oxidize at least a substantial amount of said ferrous ions to ferric ions and rapidly form an amorphous precipitate comprising* ferric hydroxide and at least 95 weight percent .of said heavy metal ions wherein said ferric hydroxide catalyst comprises at least a portion of said amorphous precipitate; (b) withdrawing from said contacting zone a slurry comprising said aqueous solution and said amorphous precipitate; and (c) separating said amorphous precipitate from said aqueous solution so as to form an effluent solution containing less than about 15 ppmw total heavy metal ions. fry 31. The method defined in claim 30 wherein said aqueous solution has a molar ratio of ferrous ions tσ total heavy metal ions less than about 4:1 prior to treatment in step (a) and wherein said method further comprises the step of adding ferrous ions to said aqueous solution so as to increase the molar ratio of said ferrous ions to said heavy metal ions in said aqueous solution during treatment in step (a) to above about 4:1. 32. The method defined in claim 30 wherein the concentration of heavy metal ions in the effluent solution from step (c) is less than about 5 ppmw. 33. The method defined in claim 30. wherein said base is selected from the group consisting of caustic soda and calcium hydroxide and mixtures thereof. 34. The method defined in claim 30 wherein said base comprises ammonia. 35. The method defined in claim 30 wherein the heavy metal ions contained in said aqueous solution prior to treatment in step (a) comprise ions of copper, nickel and zinc or mixtures thereof and wherein said precipitate contains more than about 95 weight percent of said ions of copper, nickel, zinc or mixtures thereof. OMPϊ 36. The method defined in claim 30 wherein said heavy metal ions contained in said aqueous solution prior to treatment in step (a) comprise hexavalent chromium ions and wherein said amorphous precipitate formed in step (a) contains greater than about 95 percent of said hexavalent chromium ions. 37. The method defined in claim 30 wherein the pH of said solution prior to treatment in step (a) is less than about 5.5 and wherein said method further comprises the step of adding a base to said aqueous solution so as to raise its pH to between about 5.5 and about 6.5 and thereafter introducing said aqueous solution into said contacting zone. 38. A method for removing contaminant substances from an aqueous solution having a pH of less than about 6.0 and containing contaminant substances, comprising adding ammonia to said solution so as to control the pH of said solution between about 6.0 and about 8.0 and adding cationic precipitate precursors which react with hydroxyl ions so as to rapidly form an amorphous precipitate which contains at least a substantial proportion of said contaminant substances, and, thereafter, separating said precipitate from said solution so as to form an effluent solution having a reduced contaminant substance content. 39. The method defined in claim 38 wherein said cationic precipitate precursors comprise ions of iron, aluminum or mixtures thereof. 40. The method defined in claim 39 wherein said cationic precipiate precursors comprise ferric ions. 41. The method defined in claim 38 wherein the pH of said aqueous solution is controlled to between about 6.5 and about 7.5. fURE, OMPI

Inventors: WALKER DOUGLAS T, WALKER, DOUGLAS, T.
Applicants: UNION OIL CO, UNION OIL COMPANY OF CALIFORNIA
Application year: 1984
Main classification: C02F1
IPC codes: C02F1, C02F9