These salts are usually either violet or green solids that are soluble in water. Additionally, ill-defined but commercially important “basic chromium sulfates” are known. The ammonia remains in the solution.Reddish-brown crystals (anhydrous), purple crystals (hydrated) Ultimately, the copper is all removed from the complex and precipitated as copper sulfide. Copper sulfide, for example, is a very insoluble compound and the presences of soluble sulfide precipitates the copper as it dissociates from the ammonical complex. The sulfide solubility chart below demonstrates the solubility of the metal sulfide compounds. The most economical method is to add soluble sulfide ions and break the ammonical complex by precipitating the metallic sulfide compounds. The addition of soluble ferrous ion as either ferrous sulfate or ferrous chloride will co-precipitate the metallic ion with the iron hydroxide.However, the cost is prohibitive when compared to other methods.
Eliminating the ammonia destroys the complex. The ammonia ion may be destroyed by oxidation with chlorine or ozone.There are several methods conventionally used to destroy the ammonical complex and precipitate the metallic ion. The ammonical metal complexes remain vary soluble at the higher pH values prohibiting the precipitation of the respective metal hydroxide. Certain metal ions, primarily copper, zinc and cadmium readily form metallic complexes with ammonia.A hydroxide precipitation curve is attached demonstrating the relationship Most heavy metal ions readily precipitate by raising the pH of solution, forming the respective metal hydroxide compound.The sulfide solubility is several orders of magnitude lower than the comparable hydroxide. Attached is the heavy metal sulfide solubility curves.Chromium does not form insoluble sulfide precipitates and must be precipitated as the hydroxide at 7.0 – 8.0. If nickel is present it must be precipitated with sulfide as the metallic sulfide ion. If chromium must be precipitated to a level less than 0.5 mg/l the pH must be operated at 7.0-8.0.A pH value of 9 – 9.5 will usually precipitate both ions to their required level. The effluent limitations for chromium and nickel are both 2.4 mg/l to discharge to a city sewer in the U.S.The net is a metallic ion concentration lower than would be predicted from the solubility curve. Ferric hydroxide and/or aluminum hydroxide precipitate and tend to form co-precipitate with nickel and chromium. Even when not added they are present from other metal processing solutions such as the pickling bath. Metallic coagulant such as ferric chloride or aluminum sulfate are generally used to accelerate the coagulation and precipitation of the heavy metals. The theoretical solubility usually does not exist in practice.It is common to utilize a pH of 9.0 – 9.5 to precipitate both metals. If both chromium and nickel are present a pH value that precipitates both ions must be chosen.Chromium reaches its least theoretical chromium solubility of 0.08 at pH of 7.5. Several metals such as chromium and zinc are amphoteric, being soluble at both alkaline and acid conditions.At a pH of 8.0 nickel has a solubility of 70 mg/l and at a pH of 10.2 the solubility is 0.1 mg/l. Nickel has a similar curve but it occurs at 3 pH points high.If copper is reviewed, it is seen that at a pH of 6 copper has a solubility of 20 mg/l and at a pH of 8.0, the solubility is 0.05 mg/l.
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