Patent Number: 053708270
Section: description

DETAILED DESCRIPTION OF THE INVENTION The present invention is directed to methods for treating various solutions which are contaminated with soluble and insoluble inorganic (including radioactive) species. While this process will be described primarily for removal of metals like uranium, thorium, lead, mercury, copper, cesium, barium, cadmium, and mixtures thereof, it is also suitable for removal of radium, arsenic, boron, chromium, silver, selenium, beryllium, nickel, antimony, molybdenum, vanadium, zinc, thallium, strontium, cobalt, plutonium, and the like. As used herein, the term "process stream" includes all forms of solutions in which contaminates may be found, such as, for example, groundwater, drinking water, soil washing extractants, leachates, effluents, etc. It also specifically includes uranium/thorium-containing waste streams produced by nuclear fuel handling/manufacturing facilities, and by mining facilities. The term "silicate" as used herein refers to the salt of silicic acid, or any compound that contains silicon, oxygen, and one or more metals, and may contain hydrogen. Silicates further include any of a group of minerals whose crystal lattice contains SiO.sub.4 tetrahedra, either isolated or joined through one or more oxygen atoms. The term "age" as used herein means to cause or allow to stand for a certain period of time (with or without stirring), or until certain transformations have taken place, namely, until the contaminant-containing silica matrix has gelled, polymerized and/or precipitated. The term "gel" as used in the present invention includes the formation of any jelly-like colloid, solution or suspension. For purposes of the invention described herein, "precipitates" include coprecipitates, and vice versa, and both terms include any substance precipitated from solution. The method of the invention begins by treating the contaminated process stream with precipitants, specifically silicate and ammonium hydroxide. In solution, the silicate becomes amorphous silica which has a large surface area and a high reactivity. It is believed that the amorphous silica reacts with the metal contaminants in solution by providing adsorption or exchange sites which capture the metal contaminants, thereby forming amorphous silica precipitates. The process stream may be treated with any suitable silicate known to the those skilled in the art, including, for example, sodium silicate, potassium silicate, tetraethylorthosilicate, tetramethylorthosilicate, or a mixture thereof. Preferably the stream is treated with sodium silicate having the formula: EQU Na.sub.2 O*nSiO.sub.2, where n=2 to 3.5 As the value of n increases, the silicate level increases, thereby increasing the likelihood that precipitation will occur. If n is outside of the these ranges, precipitation will not occur as efficiently, that is, more reactant will be required to accomplish the job or too much waste will be generated. Preferably, the sodium (or potassium) silicate is added as a liquid which comprises from about 1% to about 50% sodium (or potassium) silicate by weight, more preferably from about 2% to about 35%, even more preferably from about 2% to about 10%, and most preferably from about 4% to about 8%. The amount of silicate to be added is determined by the condition of the stream to be treated. Preferably, the amount of silicate added should be from about 0.5 to about 250 g/L of stream to be treated, more preferably from about 1 to about 100 g/L, and most preferably from about 5 to about 25 g/L. As shown in Table 2, increasing the amount of silicate (for example, from 5 to 15 g/L of solution to be treated) reduces the aging time required for successful gelation, polymerization and/or precipitation, as well as the filtration time. TABLE 2 ______________________________________ Effect of Silicate Concentration on Aging Time Required for Successful Filtration Filtration Silicate Aging Time, Filtration Level, g/L hr Time Contamination ______________________________________ 5 30 minutes 24 sec Highly Contaminated 5 180 minutes &gt;2 hours Low (&lt;1 ppm) 5 300 minutes 23 minutes Low (&lt;1 ppm) 10 75 minutes 60 minutes Low (&lt;1 ppm) 10 120 minutes 26 minutes Low (&lt;1 ppm) 10 180 minutes 15 minutes Low (&lt;1 ppm) 15 75 minutes 10 minutes Low (&lt;1 ppm) ______________________________________ Although treatment with silicates will significantly reduce the solubility of the contaminants in the stream, it generally will not be adequate to precipitate the contaminants to a degree which will permit collection and removal of contaminants to environmentally acceptable levels. Due to the nature of the precipitate (which tends to be a slimy or sludge-like), it may be difficult to collect the precipitate and separate it from the solution. In this regard, the addition of ammonium hydroxide solution and hydrochloric acid enhances contaminant removal and aids in the separation and collection of precipitate from the cleansed solution. The ammonium hydroxide solution promotes precipitation because the solubility of many metal hydroxides is relatively low. The silicate, which has a high surface area, then acts as a scavenger for the precipitated metal hydroxide contaminants. Preferably the stream is treated with ammonium hydroxide solution or ammonia gas comprising from about 1% to about 30% ammonium hydroxide by weight, even more preferably from about 10% to about 30% by weight, and most preferably from about 20% to about 30%. If ammonia gas is used, it can be sprayed directly into the solution. In order to ensure precipitation of substantially all of the contaminant, the ammonium hydroxide solution should be added in an amount of from about 0.001 to about 100 g/L of stream to be treated, preferably from about 0.01 to about 10 g/L, and most preferably from about 0.1 to about 1 g/L. The silicate and ammonium hydroxide precipitants may be added sequentially and in any order, or they may be added concurrently. However, the pH of the process stream should not be lowered with addition of acid until after the precipitants have been added. In the next step the stream is treated with any suitable acid known to those skilled in the art. Upon addition of acid to the stream, the contaminant-containing silica matrix will begin to gel, polymerize and/or precipitate. Generally, upon the addition of acid, clear liquid will begin to cloud and/or thicken, and solid particles will begin to form. Eventually the particles may get large enough in size to settle out of solution. For the reasons indicated above, it is important to obtain and maintain the pH of process stream in this step. Thus, the pH of the stream should be continuously monitored as the acid is slowly added. Preferably, acid should be added in a drop-wise fashion in an amount sufficient to lower the pH to between about 6 and about 8.5, more preferably to between about 7 to about 8, and most preferably to between about 7 to about 7.5. Mineral acids are most suitable for use in this step. Mineral acids selected from the group consisting of hydrochloric acid, nitric acid, sulfuric acid, phosphoric, or a mixture thereof, are preferred. Hydrochloric acid is even more preferred. In addition, acetic acid, formic acid, or other suitable organic acids known to those of skill in the art also may be used alone or in combination with the above mentioned mineral acids. In the next step of the process, the mixture is given sufficient time for the desired gelling, polymerization, and/or precipitation reactions to occur. Allowing for sufficient aging time is essential in order to remove the desired amounts of contamination from the solution and to produce a waste-containing "solid" which readily filters at an acceptable filtration rate. "Solid" as used herein and for purposes of the present invention includes any filterable precipitate, gel or polymer. Preferably, the stream is aged for about 5 minutes to about 15 hours, more preferably for about 30 minutes to about 10 hours, and most preferably for about 1 to 5 hours. If adequate time is not permitted for aging, the filtration rate is likely to be extremely slow and the filtrate will contain excessive and undesirable levels of contaminant. The gelled, polymerized and/or precipitated solids are easily handled and separated from the clean solution using any suitable technique known to those of ordinary skill in the art, including flocculation, settling and/or filtration techniques. Filtration techniques can be used to separate the waste-containing solid from the filtrate without substantial plugging or clogging of the filtration device, and at relatively rapid rates, i.e., within minutes. Suitable filtration techniques include but are not limited to vacuum filtration, filter press, or filter membranes. Preferably, each of the above described steps are undertaken sequentially and in the order set forth above; namely, the precipitating agents are added first, followed by addition of mineral acid, followed by the aging step, followed by separation of the clean stream from the contaminant-containing precipitate. In one preferred embodiment, addition of sodium silicate is followed by treatment with ammonium hydroxide solution, which is followed by treatment with hydrochloric acid, which is followed by aging and separation. In another embodiment, the ammonium hydroxide solution is added prior to treatment with sodium silicate. In yet another embodiment, the precipitants are added simultaneously. Although the above described method may be used in-situ and/or as a continuous process, it is intended to be used off-site and above ground in any suitable batch process wherein the entire process is carried out in one mixing tank. Although the method of the invention is ideal for treating uranium/thorium contaminated effluents, it also is suitable for treating extracting solutions used in various soil washing processes, such as those described in U.S. Pat. No. 5,128,068, which issued on Jul. 7, 1992, from U.S. patent application Ser. No. 529,092, filed May 25, 1990; U.S. patent application Ser. No. 648,673, filed Jan. 31, 1991, U.S. Pat. No. 5,268,128, which issued on Dec. 7, 1993, from U.S. Pat. No. 5,045,240, issued on Sep. 3, 1991, from U.S. patent application Ser. No. 345,852, filed May 1, 1989; and U.S. patent application Ser. No. 722,458, filed Jun. 27, 1991, in the name of Grant, et al., the disclosures of which are incorporated herein in their entirety. With the method of the present invention, it is possible to lower the amount of inorganic contamination to environmentally acceptable levels as set forth in the Federal Primary Drinking Water Standard (40 C.F.R., Part 141). The ability to accomplish solution decontamination using the methods of the invention, and in particular the novel combination of process steps, is demonstrated in the following example. EXAMPLE 1 A sample of water (approximately 400 g) contaminated with about 20 milligrams of copper was successfully treated according to the method of the invention as follows. Approximately 1 g of ammonium hydroxide (10 weight percent) was added to the water sample with stirring. Then approximately 13.6 g of sodium silicate (6 weight percent) was added to the solution with stirring. Next, concentrated hydrochloric acid (about 38 weight percent) was added dropwise to the solution while monitoring the pH. The pH of the stream dropped to between about 7.1 and 7.2 after the addition of about 3.5 g of acid. No additional acid was added. The solution was then mixed for an additional 5 minutes and then allowed to remain undisturbed for about 3 hours. The precipitated solids were easily separated from the solution by vacuum filtration. The resultant solution contained less than 0.2 milligrams of copper per liter of solution. From the above, it can be seen that the invention provides a simple, yet highly effective method for treating solutions contaminated with inorganic and radioactive species. The process utilizes a novel combination of steps which maximize contaminant removal, minimize waste volume, and produce a manageable waste stream. In addition, the method of the invention results in a precipitate which is readily treated and separated from the cleansed solution. The invention having now been fully described, it should be understood that it may be embodied in other specific forms or variations without departing from its spirit or essential characteristics. Accordingly, the embodiments described above are to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein.