Patent Number: 052231810
Section: description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a process for reducing the volume of thorium bearing radioactive waste for disposal from radioactive contaminated sites, thereby significantly reducing the cost for radioactive burial. The present process also allows for the recovery of valuable magnesium compounds for resale. The process is also economical to run on large volumes of material, using reagents that can easily be brought to the site for processing and can be recycled, and does not result in further disposal problems for the reagents or by-products from the process. Specifically, the present process extracts magnesium from magnesium slag. However, for the present invention the magnesium slag contains radioactive thorium (.sup.232 Th and .sup.230 Th) and radioactive Th daughters. The term "radioactive Th daughters" means .sup.232 Th or .sup.230 Th daughters. These radioactive Th daughters include, as .sup.232 Th daughters, actinium-228 (.sup.228 Ac), bismuth-212 (.sup.212 Bi), lead-212 (.sup.212 Pb), polonium-212 (.sup.212 Po), polonium-216 (.sup.216 Po), radium-224 (.sup.224 Ra), radium-228 (.sup.228 Ra), radon-220 (.sup.220 Rn), thallium-208 (.sup.208 Tl), and thorium-228 (.sup.228 Th) and, as .sup.230 Th daughters, astatine-218 (.sup.218 At), bismuth-210 (.sup.210 Bi), bismuth-214 (.sup.214 Bi), lead-210 (.sup.210 Pb), lead-214 (.sup.214 Pb), mercury-206 (.sup.206 Hg), polonium-210 (.sup.210 Po), polonium-214 (.sup.214 Po), polonium-218 (.sup.218 Po), radium-226 (.sup.226 Ra), radon-222 (.sup.222 Rn), thallium-206 (.sup.206 Tl), and thallium-210 (.sup.210 Tl). When the term "average .sup.232 Th daughters" is used, it is defined as ##EQU1## where [.sup.228 Ac] means the concentration of .sup.228 Ac measured using a germanium [Ge] gamma detector, and similarly for all the other indicated isotopes. Also [.sup.208 Tl] is divided by 0.35 to account for its branching ratio. Only the above indicated five isotopes given in the formula are measured for the radioactive Th daughters. The other isotopes will behave similarly and decay over time to one of the measurable isotopes. Typically, the non-radioactive components of the magnesium slag include as the major component, hydromagnesite [4 MgCO.sub.3.Mg(OH).sub.2.4 H.sub.2 O], and as minor components BaMg(CO.sub.3).sub.2 and Mg.sub.6 Al.sub.2 CO.sub.3 (OH).sub.16.4H.sub.2 O and others. Thus the starting material used in the present process termed "magnesium slag" includes both the radioactive and non-radioactive components. The magnesium slag is typically a heterogeneous mixture of the components. The process operates under pressure and uses carbon dioxide and water as the reagents. The basic processing technology is well known and was first used in the mid 1800's for separating magnesium from calcined dolomite. This process is often referred to as the Pattinson process (British Patent 9102, issued Sep. 24, 1841). Several modifications of the Pattinson process have been reported throughout the years. The selectivity in the present process needed to achieve a radioactive material volume and weight reduction is exceedingly high, as minute quantities of thorium and its daughters can cause the extracted magnesium material (for example, MgO, MgCl.sub.2, Mg metal, particularly MgCO.sub.3) to be radioactive and thus prevent its sale and pose a further disposal problem. There are ten radioactive .sup.232 Th daughters. Five of these daughters can be analyzed by gamma spectroscopy. The value obtained from the gamma spectroscopy measurement gives an estimate of the .sup.232 Th activity since the daughters for the slag that was utilized were found to be in equilibrium with the parent .sup.232 Th. Surprisingly, the Pattinson process as modified by the present invention does not result in the dissolution of radioactive thorium. The process of the present invention is highly efficient in that it uses reduced quantities of water, produces an effluent that is below regulatory concern, permits the recovery of magnesium for sale, selectively concentrates the radioactive thorium and its daughters such that the radioactivity is separated from the magnesium, and reduces the volume and weight of radioactive solids for disposal as radioactive waste. The process may be run as either a batch or continuous process. The reagents employed, water and carbon dioxide, are easily brought to a site and can be recycled in the process. To more clearly indicate the process of the present invention, the following reaction scheme is provided. In the Reaction Scheme, FIG. 1, Step I involves the digging of the crude magnesium slag (Mg Slag, crude) from the site location and separating the debris (for example, parts of trees or brush, large waste items such as tires), and grinding the crude magnesium slag to provide the refined magnesium slag. Step II of the reaction adds water to the refined magnesium slag to give a magnesium slag slurry. The ratio of the refined magnesium slag to water is such that it permits adequate mixing of the slurry (e.g. stirring). The ratio of water to magnesium slag is preferably at least about 1:1, more preferably from about 1:1 to about 10:1, most preferably about 1:1 to about 5:1, and especially preferred at about 3:1. The magnesium slag slurry is then reacted with carbon dioxide (CO.sub.2). CO.sub.2 can be introduced by sparging at atmospheric pressure (approximately 14.7 psi). However, higher yields of magnesium, as Mg(HCO.sub.3).sub.2, can be extracted if the reaction is carried out in a vessel pressurized with CO.sub.2 gas. Pressures of CO.sub.2 can be as high as 1,000 psig (about 7,000 kPa). Alternatively, the refined magnesium slag material may be heated to liberate CO.sub.2 prior to contacting the slag with water and CO.sub.2. The time of the reaction is not critical but must be sufficient so that some of the magnesium forms magnesium bicarbonate, usually from about 1 minute to about 24 hours, preferably from about 5 minutes to about 4 hours. The temperature of the reaction is not critical but appears to be most commercially suitable if it is from about -10.degree. to about 70.degree. C., with from about 4.degree. to about 35.degree. C. preferred. In Step IV the CO.sub.2 magnesium slag slurry is filtered to separate the radioactive solids (Th solids) from the Mg(HCO.sub.3).sub.2 liquor. The solids contain the radioactive .sup.232 Th and .sup.230 Th with Th daughters and processed slag. The liquor contains the soluble components, including Mg(HCO.sub.3).sub.2. The radioactive solids, which are now of a reduced volume can be treated by several processes. In Step V the radioactive solids may be disposed of in a radioactive burial site. Alternatively, the radioactive solids may be compacted by conventional means in Step VI to further reduce their volume for disposal in a radioactive burial site. Alternatively, the solids may be heated and/or compacted to further reduce their volume. The radioactive solids may be recycled in Step VII. The liquor containing the Mg(HCO.sub.3).sub.2 is radioactively below regulatory concern (i.e. the extracted magnesium is essentially void of radioactivity) and may be disposed of in Step VIII in any acceptable way. Alternatively, the liquor may be treated in Step IX by removing the CO.sub.2 by conventional methods such as by reducing the pressure, agitating, aerating, heating or combinations of these methods (CO.sub.2 may be recycled in Step XI). The resulting liquor can then be filtered to obtain MgCO.sub.3, which may be sold or converted to other products such as MgO, MgCl.sub.2, and MgSO.sub.4 in Step X. The water may optionally be recycled in Step XII. To ensure that the MgCO.sub.3 is not radioactive in Step IX, it is desirable that barium sulfate be added to precipitate the .sup.232 Th/.sup.230 Th daughters with the radioactive solids. BaSO.sub.4 may be added preferably at Step II or III, or following Step IV (the filtration step). If the addition takes place after Step IV an additional filtration step is required to remove the BaSO.sub.4 /Th-daughter coprecipitate. Alternatively, BaSO.sub.4 can be formed in situ by adding BaCl.sub.2 and Na.sub.2 SO.sub.4 to the Mg(HCO.sub.3).sub.2 liquor following Step IV. This addition must be done following the CO.sub.2 removal (by conventional methods) or after the solution has been acidified with HCl or H.sub.2 SO.sub.4. Using this scheme also requires an additional filtration step to remove the BaSO.sub.4 /Th-daughter coprecipitate. The present process utilizes carbon dioxide (CO.sub.2) and water to react with Th containing magnesium slag. The magnesium slag is placed under CO.sub.2 pressure. The CO.sub.2 reacts with the water insoluble magnesium compounds present in the slag to form Mg(HCO.sub.3).sub.2, which is soluble in the carbonated water. The Mg(HCO.sub.3).sub.2 liquor is separated from the remaining solids by filtration and the excess CO.sub.2 removed to precipitate MgCO.sub.3. With repeated extractions (preferably from 2 to 20 times), volume and weight reductions of the radioactive material for disposal as radioactive waste of at least 50%, preferably from about 50 to 90%, by weight and volume can be attained. No Th is extracted into the Mg(HCO.sub.3).sub.2 liquor, however very small amounts of the .sup.232 Th daughters are extracted. These .sup.232 Th daughters precipitate with the MgCO.sub.3 causing the resulting MgCO.sub.3 to contain radioactive isotopes and be considered radioactive waste. A small amount of barium sulfate (BaSO.sub.4) is added to cause coprecipitation with any solubilized .sup.232 Th daughters which are then removed by filtration. In the presence of excess sulfate, solubilized barium is converted into BaSO.sub.4, which is non-leachable. When the process is completed the concentration of radioactivity has increased by at least about 200%, preferably from at least about 200% to about 1,000%, from that present in the magnesium slag. The invention will be further clarified by a consideration of the following examples, which are intended to be purely exemplary of the present invention. EXAMPLE 1 A 240 G sample (approximately 325 mL) of dried, ground, radioactive magnesium slag, where the average thorium-232 (.sup.232 Th) daughter activity of the slag was 259 pCi/G (7,000 Bq/G) by gamma analysis using a germanium (Ge) detector, was added to a two liter Parr bomb reactor, followed by the addition of 1,200 mL of deionized water. The reactor was charged with 145 psig (1,100 kPa) of carbon dioxide, placed in an ice/water bath and stirred for two hours. The slurry was removed and the solids separated by vacuum filtration. To the filtrate [liquor, Mg(HCO.sub.3).sub.2 ] was added HCl (18% by weight) until the pH was between 0 and 1. Metal analysis and radioactivity measurements were performed on both the remaining solids and liquor. The remaining solids following extraction and separation were dried and weighed. The dried solids were then re-extracted by returning them to the reactor, adding 1,200 mL of deionized water, and recharging the reactor with CO.sub.2. The process was repeated ten more times (a total of 12 extractions). Following 12 extractions, the final weight of the remaining solids was 43.7 G. A final weight reduction of 83% of the slag was realized. The extraction was highly selective for magnesium. The total amount of magnesium recovered after 12 extractions was 44.4 G (calculated on a magnesium metal basis). The remaining slag increased in radioactivity by approximately 6 times (600%). Metal analysis (by atomic emission spectroscopy using an inductively coupled plasma, "ICP"), isotopic Th (by alpha analysis), and gamma analysis for Th-daughters all demonstrated the absence of Th in the Mg(HCO.sub.3).sub.2 liquor. A small amount, about 0.6 pCi/mL (16 Bq/mL) of the .sup.232 Th daughters were extracted into the liquor. EXAMPLE 2 Dried, radioactive magnesium slag [249 G, 259 pCi/G (7,000 Bq/G)] was treated as in Example 1 for one extraction. After filtering the remaining solids, the Mg(HCO.sub.3).sub.2 liquor contained 0.6 pCi/mL (16 Bq/mL) average .sup.232 Th daughters. The liquor was aerated to remove the CO.sub.2 and precipitate MgCO.sub.3. The MgCO.sub.3 precipitate was analyzed for radioactivity and found to have 20 pCi/G (541 Bq/G) average .sup.232 Th daughters. EXAMPLE 3 The acidified Mg(HCO.sub.3).sub.2 liquor from Example 1, extraction 3, was analyzed for radioactivity by gamma analysis. The average .sup.232 Th daughter activity was 0.44 pCi/mL (12 Bq/mL). Approximately ten drops of concentrated H.sub.2 SO.sub.4 was added to the liquor, followed by 0.5 G of BaCl.sub.2.2H.sub.2 O in 10.5 G of deionized water. The liquor was then re-filtered and the filtrate analyzed for radioactivity. The average .sup.232 Th daughters activity was 0.035 pCi/mL (0.9 Bq/G). Upon reanalysis of the filtrate three days later, no detectable levels of .sup.232 Th daughters were found. EXAMPLE A COMPARATIVE Dried, radioactive magnesium slag, 251 G, was treated as in Example 1 for one extraction. However, after filtering the liquor was not acidified. The liquor was then treated with 0.5 G of BaCl.sub.2.10H.sub.2 O and 0.8 G of Na.sub.2 SO.sub.4. The liquor was filtered again and aerated to remove CO.sub.2. The white MgCO.sub.3 precipitate was analyzed for radioactivity and found to have 62 pCi/G (1676 Bq/G) average of .sup.232 Th daughters. The liquor following both precipitations was found to have 0.01 pCi/mL (0.3 Bq/mL) average of .sup.232 Th daughters. EXAMPLE 4 Dried, radioactive refined magnesium slag [253 G, 259 pCi/G (7,000 Bq/G)] was treated as in Example 1 for one extraction. After filtering the remaining solids, 10 mL of a BaSO.sub.4 suspension (prepared from 0.225 G of BaCl.sub.2.2H.sub.2 O, 12 G of K.sub.2 SO.sub.4, 6 mL (1.1 G) of H.sub.2 SO.sub.4 and 100 mL of water) was added to the Mg(HCO.sub.3).sub.2 filtrate. The liquor was then filtered to remove the BaSO.sub.4 coprecipitated with the .sup.232 Th daughters. The filtrate was analyzed for radioactivity and the average .sup.232 Th daughter activity was 0.145 pCi/G (3.9 Bq/G). The MgCO.sub.3 precipitate activity was approximately 4.4 pCi/G (119 Bq/G). Three days later no .sup.232 Th daughters were found in the MgCO.sub.3 precipitate. EXAMPLE 5 RECYCLE OF BaSO.sub.4 Dried, radioactive magnesium slag, 274 G, was treated as in Example 1 for one extraction. In addition, 10 mL of a BaSO.sub.4 suspension (prepared as in Example 4) was added. The remaining solids were filtered and Mg(HCO.sub.3).sub.2 liquor aerated to remove the CO.sub.2 and precipitate MgCO.sub.3. The MgCO.sub.3 was analyzed for radioactivity and found to have 1.8 pCi/G (49 Bq/G) average of .sup.232 Th daughters. The remaining slag solids were then recycled as in Example 1 for one extraction without adding more BaSO.sub.4. The precipitated MgCO.sub.3 was analyzed for radioactivity and found to have 4.4 pCi/G (119 Bq/G) average of .sup.232 Th daughters. After a third extraction (by the procedure of Example 1), the precipitated MgCO.sub.3 was found to have 14 pCi/G (378 Bq/G) average of .sup.232 Th daughters. EXAMPLE 6 NON-RADIOACTIVE MAGNESIUM SLAG A. CO.sub.2 Process A 240 G sample (325 mL) of dried, ground, non-radioactive crude magnesium slag was extracted 10 times in the manner described for Example 1. A metal analysis by ICP was performed on the acidified Mg(HCO.sub.3).sub.2 liquors. The remaining solids following extraction and separation were dried and weighed. Following 10 extractions the remaining solids weighed 54.1 G. A final weight reduction of 77.5% of the slag was realized. The process was highly selective for magnesium and the total amount of magnesium recovered after ten extractions was 46 G (calculated on a magnesium metal basis). B. Compaction Process A 49 G portion of the remaining dried slag material after the ten extractions was mixed with 20 mL of deionized water and then compacted to 41 mL using a Harvard miniature compactor [40 lb. (1.8 kg) spring]. Thus the 54 G sample could be compacted to 45 mL. Combining both the extraction process and the compaction method permitted an 86% volume reduction. Other embodiments of the invention will be apparent to those skilled in the art from a consideration of this specification or practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with the true scope and spirit of the invention being indicated by the following claims.