Patent Number: 053708270
Section: summary

BACKGROUND OF THE INVENTION This invention relates generally to precipitation methods for decontaminating various types of solutions which are contaminated with a variety of contaminants such as heavy metals and radioactive compounds, using a novel combination of treatment steps. More particularly, this invention relates to methods for remediating water contaminated with uranium, thorium, mercury and/or copper, using sodium silicate, ammonium hydroxide and hydrochloric acid, to precipitate the contaminants and ultimately separate them from solution. There is increasing concern over the hazards posed by the rising levels of inorganic contaminants within the world's water supplies due to accidental spills, leaks, mining practices and poor disposal practices. Most heavy metal contaminants are toxic to some degree to all life-forms. In humans, toxic heavy metal poisoning can lead to severe nervous system disorders and can cause death. It has been suggested that various inorganic contaminants in solution can be removed via precipitation solution using, for example, carbonates, hydroxides, sulfides, and/or silicates. Such techniques are described in Canter, L. W., and Knox, R. C., Ground Water Pollution Control, Lewis Publishers, Inc., 1985, pp. 110-120; and Willey, B. R., Finding Treatment Options for Inorganics, in WATER/Engineering & Management, October, 1987, pp. 28-31. In particular, the use of sodium silicate (also referred to as water glass or Na.sub.2 SiO.sub.3) to remove uranium and/or thorium from waste streams has been suggested. For example, in U.S. Pat. No. 4,501,691, issued in the name of Tanaka et al., on Feb. 26, 1985, there is described a process for treating radioactive liquid waste in which the liquid waste is treated with sodium silicate to form a uranium containing silica precipitate. The precipitate subsequently is treated in a step-wise fashion with acid to recover the uranium, and then alkali metal hydroxide solution to regenerate the water glass. Silicate precipitation processes for uranium and thorium recovery also are described in U.S. Pat. Nos. 4,349,513, issued Sep. 14, 1982, in the name of Ishiwata et al.; U.S. Pat. No. 5,077,020, issued Dec. 31, 1991, in the name of Lahoda et al; and U.S. Pat. No. 4,338,286, issued Jul. 6, 1982, in the name of Nakai et al. In Ishiwata, the liquid waste is treated with sodium silicate in the presence of aqueous fluorine and ammonia to make a uranium/thorium-containing silicate precipitate which is filtered out of the process stream find sent to a holding tank. In Lahoda et al., the contaminated stream is treated with sodium silicate in the presence of ammonia, fluoride, and nitrate in water. Nakai et al. generally describe a precipitation process for treating liquids contaminated with uranium/thorium wherein sodium silicate is added to the solution in the presence of ammonia water and chlorine to cause a contaminant-containing silica precipitate to form. There are significant disadvantages associated with the application of each of these methods. For example, carbonate systems, while relatively easy to operate, are difficult to control and often result in processing problems such as premature plugging of equipment. Sulfide systems are difficult to handle, complex to operate, and frequently produce a high waste volume and harmful residual levels of precipitating agent. Hydroxide systems are widely used to remove inorganics because they are the most reliable, and have the added advantages of ease in chemical handling and low volume of sludge. However, the resulting sludge often is gelatinous and difficult to dewater, making treatment, separation, and storage of the contaminated material difficult. In addition, pH must be precisely controlled. Otherwise, contaminant-containing precipitate can readily go back into solution. The above mentioned silicate precipitation methods suffer from at least two critical drawbacks. First, typically such methods produce a contaminant-containing, slime or sludge which is not readily treatable, separable, or easily stored. For example, the slime/sludge is not easily dewatered, making further treatment with filtration devices either impossible (due to plugging) or impractical (due to excessively slow filtration rates). Second, silicate precipitation generally is not effective on other inorganic contaminants; for example, silicates do not readily precipitate other heavy metals like mercury. Consequently, silicate precipitation methods are slow, inefficient, and ineffective in reducing the level of uranium, thorium and other heavy metals to environmentally acceptable levels. What is needed is a simplified, easy-to-operate method of treating large volumes of solutions containing heavy metals and radioactive contaminants, singly or in combination, which effectively segregates the contaminates from the clean solution and concentrates the contaminated material in a manageable, low volume, concentrated waste stream. There is a further need for a system that can effectively and economically remove metals from contaminated solutions, whereby the contaminated material is readily separable from the cleansed solution, especially by filtration methods. There is also a need for a process which can effectively remove various metal contaminants like uranium, thorium, mercury, and copper from solution. SUMMARY OF THE INVENTION These and other needs are satisfied by the invention which is characterized by treating process streams such as groundwater, drinking water, soil extracting solutions, leaching solutions, and the like, which are contaminated with various inorganic contaminates, either singly or in combination, with a unique combination of treatment steps. In the process of the invention, the contaminated process stream is treated with a unique combination of precipitating/gelling/polymerization agents comprising silicate, ammonium hydroxide, and acid. The acid is added to the process stream after the addition of silicate and ammonium hydroxide, in an amount sufficient to lower the pH of the stream to between about 5 to about 9.5. Next, the stream is allowed to age for a time sufficient to allow the contaminant-containing silica matrix to gel, polymerize and/or precipitate to a filterable "solid". The resulting solid is readily dewatered and can be quickly separated from the clean stream. In practicing the precipitation method of the invention, it is important that the precipitating agents are added in the proper sequence and at the required amounts, that the proper pH of the stream is maintained, and that the stream is permitted to age for the requisite length of time. It has been found that the controlled addition of the sequence of precipitating agents, the maintenance of pH, and proper aging of the stream minimizes the consumption of precipitating agent and the generation of waste volume, and results in a contaminated solid which is readily filterable in minutes, as opposed to hours or even days. Consequently, a smaller amount of precipitant is used, a manageable volume of waste is generated, and a lower disposable/clean-up cost is incurred. Moreover, the inventors experimentally have determined that it is the combination of sodium silicate with ammonium hydroxide which renders the present process more effective at removing a wider variety of contaminants. For example, as indicated in Table 1, the use of sodium silicate alone, sodium silicate with ammonium chloride, or sodium silicate with sodium hydroxide, was not effective at removing mercury from a contaminated stream to desirable levels. It was only when both sodium silicate and ammonium hydroxide precipitants were added that levels of +99% mercury removal were obtained. Similarly, +99% copper removal was achieved only through the combined use of sodium silicate and ammonium hydroxide precipitants. TABLE 1 ______________________________________ Effectiveness of Ammonium Hydroxide Addition on Heavy Metal Removal Using Sodium Silicate % Contaminant Additive* Contaminant Removed ______________________________________ None Mercury 0% 0.25 g/L Mercury +99% Ammonium Hydroxide 0.25 g/L Mercury 71.4% Ammonium Chloride 0.25 g/L Mercury 3.5% Sodium Hydroxide None Copper 99% 0.25 g/L Copper +99% Ammonium Hydroxide ______________________________________ *Added with 5 g/L sodium silicate to contaminated water stream The inventors also have found that after the addition of silicate and ammonium hydroxide, the pH of the stream must be adjusted to between about 5 to about 9.5 with mineral acid. As FIG. 1 indicates, contaminant removal is optimized at a pH above about 7, while less than about 10% of contaminant is removed at a pH below about 5. Accordingly, if the pH of the stream is permitted to go below about 5, unsatisfactory amounts of contaminants are likely to remain in the process stream. With respect to one preferred embodiment of the invention, the method for removing metals from a contaminated stream comprises the steps of: a. treating the process stream with sodium silicate in an amount of from about 5 to about 25 g/L of stream to be treated, and ammonium hydroxide in an amount of from about 0.1 to about 1 g/L of stream to be treated to precipitate said contaminants; PA1 b. adding a mineral acid to the process stream in an amount sufficient to lower the pH of the stream to between about 7 to about 7.5; PA1 c. allowing said stream to age for about 1 to about 5 hours; and PA1 d. separating the clean stream from the precipitate. Accordingly, it is an object of this invention to provide a precipitation method for the decontamination of solutions which produces a clean solution having environmentally acceptable levels of contamination, and readily manageable waste having a relatively low volume. It is a further object of this invention to provide a precipitation method for the decontamination of solutions wherein the contaminant containing waste is easy to handle, and simple to treat, separate and store. It is yet another object of this invention to provide a precipitation method which can be utilized to remove a variety of heavy metals.