United States Patent (1) 11 Patent Number: 4,574,075 Rickly et al. 45 Date of Patent: Mar. 4, 1986 54) PURIFICATION OF ALKALIMETAL Attorney, Agent, or Firm-Donald F. Clements; James NITRATES B. Haglind 75 Inventors: Jack D. Rickly, Dumont, N.J.; B. 57 ABSTRACT Timothy Pennington, Sulphur, La. A process is disclosed for removing heavy metal con 73) Assignee: Olin Corporation, Cheshire, Conn. taminants from impure alkali metal nitrates containing them. The process comprises mixing the impure nitrates (21) Appl. No.: 678,858 with sufficient water to form a concentrated aqueous (22) Filed: Dec. 6, 1984 solution of the impure nitrates, adjusting the pH of the 51) int.C.'.............................................. C01F 11/36 resulting solution to preferably within the range of 52 U.S. C. ..................................... 423/184; 423/198 between about 2 and about 4, adjusting the nitrite ion 58) Field of Search ................. 423/184, 198,55, 140, concentration to between about 0.07 molar and about 423/144, 395, 192, 194 1.0 molar, to effect reduction of the heavy metal con taminants, adjusting the pH of the reduced solution to (56) References Cited effect precipitation of heavy metal impurities, and sepa U.S. PATENT DOCUMENTS rating the solid impurities from the resulting purified aqueous solution of alkali metal nitrates. The resulting 4,517,164 5/1985 Fiorucci et al. .................... 423/84 purified solution of alkali metal nitrates may be heated OTHER PUBLICATIONS to evaporate water therefrom to produce purified mol J. W. Mellor, Inorganic and Theoretical Chemistry, vol. ten alkali metal nitrate suitable for use as a heat transfer VIII, pp. 476-478 (Mar. 1947), Longmans, Green and medium. If desired, the purified molten form may be Co. granulated and cooled to form discrete solid particles of alkali metal nitrates. Primary Examiner-John Doll Assistant Examiner-Wayne A. Langel 22 Claims, No Drawings 4,574,075 2 molar and about 1.0 molar to reduce heavy metal PURIFICATION OF ALKALI METAL NITRATES contaminants contained therein; d. adjusting the pH of the reduced concentrated aque This invention relates to the purification of alkali ous solution to a pH of at least about 6 to effect pre metal nitrates. More particularly, it is directed to the 5 cipitation of solid heavy metal contaminants within removal of heavy metal contaminants from a mixed said concentrated aqueous solution; and alkali metal nitrate heat transfer medium. e. separating said solid heavy metal contaminants from The energy crisis has generated the investigation of a the resulting purified solution of alkali metal nitrate. large number of alternate sources of power. One tech More in detail, any alkali metal nitrate containing nique presently under investigation is the use of solar O heavy metal impurities may be purified by the process energy to raise the temperature of a suitable heat trans of this invention. Typical examples of alklai metal ni fer medium and convey the heat transfer medium to a trates include sodium nitrate, potassium nitrate, and suitable steam generator or to a heating system for an lithium nitrate, or mixtures thereof. The process of this inhabited dwelling. One heat transfer medium under invention is particularly effective when treating a solar intensive investigation at this time is the eutectic mix 15 or industrial heat transfer medium comprised of an eu ture of sodium nitrate and potassium nitrate. This mix tectic mixture of sodium nitrate and potassium nitrate, ture usually contains about 60 percent by weight of wherein the sodium nitrate concentration is about 60 sodium nitrate and about 40 percent by weight of potas percent by weight and the potassium nitrate concentra sium nitrate, but may contain from about 30 to about 70 tion is about 40 percent by weight. However, the con percent by weight of sodium nitrate and between about 20 centration of sodium nitrate and potassium nitrate may 30 and about 70 percent by weight of potassium nitrate. each range from about 30 to about 70 percent by weight Mixtures of this type may have a relatively low melting of the mixture. point, i.e. about 400' F., and have a relatively low vis The process of this invention may be utilized in the cosity in the molten state. As a result, it can be easily purification of alkali metal nitrates in solid or molten transferred by pumping or otherwise within a closed 25 form. For example, sodium nitrate solids obtained by system. conventional mining and beneficiating techniques, Sodium nitrate and potassium nitrate frequently con which may contain heavy metal contaminants in the tain heavy metal contaminants as well as anionic con range of from about 0.03 to about 0.15 percent by taminants in the form of carbonates, oxides and sulfates. weight may be processed in accordance with the pro In addition, when the molten form of the mixed nitrate 30 cess of this invention. In addition, the process of this is circulated in the solar power system or industrial invention is useful in the purification of impure mixed process heat systems, small quantities of heavy metal nitrates produced synthetically, or impure mixed ni impurities may be dissolved from the materials of con trates obtained from solar heat systems, which may struction used as conduits, storage tanks and the like. In contain heavy metal impurities within the range from addition, local hot spots in the solar energy system may 35 about 0.03 to about 0.5 percent by weight. cause degradation of the alkali metal nitrates to form The impure alkali metal nitrates are dissolved in suffi oxides, oxygen and nitrogen. The oxides may react with cient water to form an aqueous solution having an alkali water and carbon dioxide to form hydroxides and car metal nitrate concentration within the range of from bonates. between about 20 to about 70 and preferably between 40 about 30 and about 50 percent by weight. Either solid, There is a need at the present time to provide a pro molten, or solution forms of the alkali metal nitrates cess for reducing the heavy metal contaminants found may be added to the water. in alkali metal nitrates, particularly those used in the Sufficient acid is added to the aqueous alkali metal preparation of solar energy heat transfer medium. nitrate solution to adjust the pH thereof to within the It is a primary object of this invention to provide an 45 range from about 2 to about 6 and preferably from improved process for purifying alkali metal nitrates. about 2 to about 4. Any suitable acidic substance which It is another object of this invention to provide a is nonreactive with the alkali metal nitrates may be process for reducing the concentration of heavy metal utilized for this purpose. It is preferred to utilize nitric impurities present in alkali metal nitrates. acid since it does not add foreign anions to the alkali Still another object of the invention is to provide a 50 metal nitrate being treated. However, if anionic impu process for purifying mixed alkali metal nitrates utilized rity is not a problem, then any other suitable acids such as a heat transfer medium in solar energy systems and as sulfuric acid, hydrochloric acid, phosphoric acid and industrial process heat systems. the like may be employed. It is preferred to employ a A further object of the invention is to provide a pro concentrated mineral acid when adjusting the pH in cess for purifying eutectic mixtures of sodium nitrate 55 order to minimize the amount of water that may ulti and potassium nitrate. mately have to be evaporated to obtain the desired These and other objects of the invention will be ap purified molten product. However, any suitable con parent from the following detailed description thereof. centration of acid may be utilized. It has now been discovered that the foregoing objects After the pH of the aqueous solution of alkali metal are accomplished in a process for removing heavy metal nitrate has been adjusted to the desired level, the con contaminants from impure alkali metal nitrates contain centration of nitrite anion in the aqueous solution is ing them, which comprises: adjusted to effect reaction of the nitrite ion with the a. admixing the impure alkali metal nitrates with suffi heavy metal impurities present. Sufficient nitrite ion is cient water to form an aqueous solution thereof; added to the aqueous solution to provide at least about b. adjusting the pH of the aqueous solution to preferably 65 6 times the stoichiometric amount, and preferably be within a range from between about 2 to about 4; tween about 8 and about 15 times the stoichiometric c. adjusting the concentration of nitrite ion in the aque amount necessary to reduce all of the heavy metal con ous solution to within the range between about 0.07 taminants to a reduced valent state. Sodium nitrite is 4,574,075 3 4 preferred for use as the nitrite ion forming agent, but potassium nitrate is a preferred ingredient for purifying other nitrite compounds such as potassium nitrite, lith in accordance with the process of this invention. ium nitrite, cesium nitrite, and the like, barium nitrite, When admixing the impure aqueous alkali metal ni strontium nitrite, calcium nitrite and mixtures thereof trate solution with acid, the nitrite ion compound and which form nitrite ions in aqueous solutions may also be basic material to effect the pH lowering, control of the employed. In addition, nitrite ions formed in situ, for nitrite ion concentration, and the pH raising, respec example, by electrolysis of aqueous solutions of alkali tively, suitable agitation means are employed. Fre metal nitrates, or by bubbling equimolar mixtures of NO quently, when the basic material is added to precipitate and NO2 through an aqueous alkali metal hydroxide or the heavy metal impurities, a porous floc-type precipi alkali metal carbonate solution may also be employed as 10 tate forms.