Processing Pollution into Product David L. Dean Eastern Washington University, Cheney, WA 99004 New process development in the chemical industry results from the interplay of several forces. Whereas new products are usuallv develo~edin resnonse to consumer need. new processes are usually developed in response to producer need. One need that has arisen reneatedlv is what to do with the NO and NO2 (collectively known a; NOx) that is produced whenever nitric acid is used as an oxidizing went. This frequently happens when scrap metals are dissdved in nitric acid for purification. Let us examine one case where this problem aroie in the nuclear industry and see how a processwas developed to overcome it. Nuclear fuel ~ l a n t convert s uranium hexafluoride into fuel pellets of urani& dioxide which are subsequently loaded into zircaloy fuel rods. In spite of close process controls, 100%of the uranium that comes into a plant is not converted into acceptable fuel pellets the first time i t is processed. Scrap mat e d can arise at various points in the overall process. For instance, small amounts of fuel spill out of the pellet presses during rapid automated filling. In addition, some uranium dioxide dust is generated when the nearly completed fuel pellets are ground to exact specifications for loading into the rods. S c r a fuel ~ which accumulates in these wavs is contaminated with dirt and grease from the factory. ~ e f i r iet can he made into aualitv . . fuel el lets i t is necessarv to remocess i t in a manner roughly analogous to processing raw ore. Scrap fuel reprocessing consists of oxidation by nitric acid to water soluble uranyl nitrate, UOz(NO& removal of insoluble impurities, and selective precipitation of uranium as
ammonium diuranate, (NH4)zU20,. The oxidation step caused the problem by giving off noxious NOx gases as hyproducts. The original answer to the problem was simply tovent the fumes to the atmosphere where the NOx gases were dispersed and diluted. While this procedure reduced the hazard to workers in the plant, it did nothing to eliminate the NOx. One result of this short term answer and others like i t was t o disperse the NOx into the upper atmosphere where it was carried away from the originator but from which it was ultimately returned to earth somewhere in the form of acid rains. Another result was that occasionally a brown plume of pollution could he seen hanging over the plant. There was always the possibility of exceeding the Federal Standard set on NOx emissions. Just how toxic are NO and NOz? Where would a chemist turn to find out? Chemists need to he aware of chemicaltoxicity for their own protection, for the protection of their colleagues, and for the protection of the general public exposed to effluent waste gas and liquid streams from the plant. Many companies require information to he on file before chemists can take chemicals out of the lab and into the pilot plant. Toxicity data should be considered when selecting between alternatives in new process design. A study of ACS member deaths during the period 19481967 showed a cancer mortality rate 25% higher than that of professionals in other fields ( I ) . Clearly, chemicals present many health hazards which are much more subtle than the obvious ones from acids, bases, and oxidizing agents. I t must
Volume 59
Number 8 August 1982
639
be nresumed that those ACS members incurring premature cancer deaths did so our of ignorance rather than intent. One reason may have been lack of information, a problem that has been attadked vigorously in the last decade. Let us examine some of the recently published toxicology references that chemistswould find useful as desk references and one that should be available in libraries if more information is required. The handiest. inexnensive small reference is a nocket-size pamphlet published b y the American conference of Governmental Industrial Hveienists (ACGIH) entitled "TLVSe-Threshold Limit values for Chemical Substances in Workroom Air" (2). This booklet is published annually and updated with the latest information. The ACGIH distinguishes three types of TLVs: (1) TLV-TWA, the time weighted average TLV concentration t o which workers can be repeatedly exposed 40 hrlweek, (2) TLV-STEL, the short term exposure limit TLV indicating the maximal concentration to which workers can be exposed for a period of up to 15 min without sufferine adverse effects. and (3) TLV-C. the TLV concentration chat should not be exceeded even instantaneouslv. The onlv entrv for NO9 is a TLV-TWA of 5 ppm. This low value indicates that NO2 is very toxic. The TLV for NO is 25 ppm, indicating it to be somewhat less toxic than NOz. However, since it is readily oxidized to NO2 by oxygen in the air, i t actually presents a hazard similar to that of --. NU2. More detailed information can be found in the one volume "Handbook of Industrial Toxicology" (3).Here the basic TLV for NO2 is supplemented with a description of the gas, common occupational exposure situations, a toxicity rating of "extremely toxic," the route of entry into the body, the mode of action. siens and svmotoms. diagnostic tests. treatment. likely res&&t disability,-and pieventive measures. The major limitation of this reference is its scope. I t covers only chemicals already in widespread use. There is no entry for NO. Another reasonable size reference with information on chemical properties as well as toxicity data on a much wider range of chemicals is "The MerckIndex" (4).Theindication h e r e is t h a t NO2 is a n insidious gas. One hundred ppm is dangerous for even a slight exposure, and 200 ppm has been fatal. This reference indicates that NO is aconsiderable hazard due to its facile oxidation to NOz. The source containine toxicitv data on the lareest varietv l of ~ o x i ~c f f e c t s of chemicals is that of thk ~ e d e r a"Registry of Chemical Substances," which is published and updated annually (5). Often referred to simply as the "Toxic Substances List," i t indicates that for NOz, the lowest known lethal concentration for man is 200 ppm for 1min. For NO i t gives the TLV and the results of animal studies. Although entries are brief. the available information includes animal toxicity data, TLV, OSHA standard (normally the same as the TLV), and several references to both original articles and reviews. Human toxicity is not the only disadvantage of releasing NOx into the air. Due to the reaction of NOz with water in the air to form nitric acid, the rain that falls t o earth can be acidic and frequently is. Although sulfur oxides from power plants and the resultant sulfur acids are bigger contributon than nitrogen oxides, the effects of acid rains are becoming increasingly serious problems in the United States and Western Europe. Some of the effects of acid rains are erosion of buildings and statues, death and injury to plant life, and death and injury to fish and other aquatic life in streams and lakes. If there is a significant time interval before atmospheric NO2
640
Journal of Chemical Education
reacts with water to form nitric acid, it can feed into the photochemical smog cycle. Due to the adverse effects of NO2 on life in general, the EPA has set the ambient clean air standard at 0.05 .. nnm (6). . . In designing a process to remove NO, gases from the offeas stream. i t was noted that NO2 reacts readily with water t o form nitric acid. Preparation 3N02
+ H20
-
2HN03 +NO
(1)
for this step required an examination of the reaction converting NO into NOz. Either air or oxygen could conceivably be used as the oxidizing agent. While the thermodynamics are favorable for this reaction (AG = -8.3 kcal/mole a t 25OC), it is somewhat slow M-2 sec-I). Consequently, pure oxygen was (k = 7 X determined to be advantageous since it increased the reaction rate bv" annroximatelv a factor of five over that of air. Still there was a question of whether the rate was fast enough to be practical. Ultimately, it was determined that a continuous process utilizing a long residence time took the reaction essentially to completion at room temperature. The long residence time was achieved through the use of a large column relative to the volume of the N o.r. stream. The conversions are carried out in a single, twenty-foot column. The top five-foot section is used as an oxidation chamber where the NOx is converted into NO2 in the presence of excess oxvwn. The center ten-foot section is used for reaction (1). 7% excess oxygen reacts with by-product NOx as it forms convertine essentiallv all of the NO? to nitric acid bv the time it reachesthe bottom live-four section ut'thect,lum~ wherc the oroduct acid accumulates. Mixina is facilitnred in the centerportion of the column by the useof ceramic "Intelox" packing. The entire column is made of corrosion resistant 317 stainless steel. The off gas stream, which is mostly oxygen, is continuously monitored for NOz. The NO2 emission level has been successfullv held to less than 5 nnm. The nitric acid produce2 is recirculated throGh the column until its concentration reaches 3070, a t which point it is drawn off as product. This nitric acid is then used in the uranium reprocessing system in place of nitric acid that would otherwise have to be purchased from vendors. The svstem described here was installed about seven vears ago. currently, it costs the company less to process NOX pollution into nitric acid product than it would to buy an equal amount of nitric acid. I t demonstrates that pollution control can be an important driving force in new process development and that sometimes the new process can even contribute to reduced operating costs.
..
Literature cited (I) Li, F. P.,Fraumeni,J. F., Msnte1.N.. andMiller, R.W..J Not. Concorlnsf., 43,1159
(196%
(21 e c ~ ~ ~ ~ * - ~ htimavalues r r a h o ~ dfor chemieal subatanee~ m worhm ~ i r . " ~ m e r i e a n Conferenceof Governmental Industrial Hygienints, 1980edifion. 6500GlenwiayAue.. Bldg. D-5. Cincinnati,OH 45211. Seeslso"Poe*et Guide toChemical Haeards."U.S. i ~ " centerfor ise ease control DHEW PubliDepr of ~ ~ ~ ~l t dh ~, ~endt welfare, cation No. (NIOSH) 78-210.
(31 P h n k & , E R,,~~Handhkd~nd~i~lToxieol~?Chemi~alPuhlishbgCo.,Ine,
NW+york. 1 9 7 6 ,298. ~~~~ (4) Windhdz. M. (Editor), "The MerckInder?9th Ed., Merck andCo., Inc.. Rahwsy.NJ, 1976. p. 858. (5) .w+try of Toxic ~ f f f f t r chemical suhitaneea? ( ~ d i t ~ levis. ,: ST.,~ i ~ J.), h ~ d US oept.~ f ~ d t~ h d, ~ ~ tW i~ ~~~ ,nR W ~ Md I I ~ , M D , H ~vbiication~o. EW (NIOSHJ 78-976.1978 edition. end Rempds of the Fed.rsl code of ,,( u.s. National F P ~ P RP ~~ ~ U I ~ ~~~ iO t40. Ml ,~ e c t~O.II,IW~. i ~ ~
