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ROBERT L. JOHNSON', MERK HOBSON, and JAMES H. WEBER Department of Chemistry and Chemical Engineering, University of Nebraska, Lincoln 8, Neb.
Continuous Dissolution of Copper rate of copper by nitric acid has been studied (7-4,6, 7) but degrees of agitation, size and shape of copper specimens, and size of equipment used have differed. Therefore flow of acid over the metal surface was not uniform, and the dissolution rates obtained cannot be reduced to a common basis for comparison. The work reported here indicates that dissolution rate of copper by nitric acid is not controlled entirely by chemical reaction rate and that it is affected by degree of agitation. Buben and FrankKamenetskil (5) have proposed that dissolution is a diffusion-controlled reaction and the rate of diffusion depends on the conditions of flow past the metallic surface. T o obtain more accurate information, continuous dissolution of copper bars, 19.1 mm. in diameter and 432 cm. long, was studied, using 1 to 5.5iv nitric acid and an acid mass velocity of 6 to 19 grams/sec./sq. cm. At acid concentrations of 4.3N and below, dissolution rate decreased with an increasing flow rate, but with 5.5N acid, it increased only slightly. The results using 1 N nitric acid were varied, but the dotted line in the graph shown is a reasonable average. This uncertainty for acid concentrations of 1 N 1 Present address, Dow Chemical Co., Midland, Mich. B * T C H - W I S E DISSOLUTION
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When copper i s dissolved in nitric acid, dissolution rate, although affected b y acid concentration, i s nearly independent of acid mass velocity
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and below has been observed previously (5, 7). When lit' acid was used, an interesting phenomenon occurred. Steady state conditions were apparently not reached for some time, and tu determine when these conditions were established, the copper content of the outgoing solution was measured at various times throughout the runs. The point where this became constant was used as the criterion of steady state operating conditions in the dissolver. The amount of copper in the outgoing acid stream increased steadily with time to a maximum, then decreased to a point where the copper content remained essentially constant. The final steady state conditions were approximately the same for all runs in which I N acid was used, but the initial rates of increase and maximum concentrations varied widely among the different runs. Although this was studied for 1 N acid only, the same phenomenon was ' observed with 3N acid. With acid concentrations higher than this, however, the unsteady state period is either absent or much shorter. For example, steady state conditions were reached within 5 minutes with 2 41 .%' acid as compared to an average of 30 minutes for 1N acid. To determine if wall effect was present,
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the results from six runs using a copper bar of 12.7-mm. diameter were compared with those using a 19.1-mm. bar. The dissolution rates per unit of area at the same mass velocities agreed within 47, when normality of the acid was 4.31%~ or higher. However, with 3N acid at a mass velocity of 0.3 gram/sec./sq. cm., the 12.7-mm. bar gave a dissolution rate 257, higher than that obtained with the larger bar, but at a mass velocity of 7.8, the rate was only one fourth as high. These data were checked and the differences are probably not caused by experimental error. Using 3 N acid, two copper bars of the same dimensions (19.1 mm. in diameter) gave essentially the same results, the differences being less than loyo. At low flow rates with 3N acid, the effect of flow rate on the rate of dissolution is quite marked-dissolution rate increased rapidly with decreasing flow rate. In comparing results obtained using acid more concentrated than 4;L' with those obtained using 3A' and low acid concentrations, it is obvious that the dissolution rate is dependent on concentration and, therefore, is not a diffusion-controlled reaction. During the experimental work, colorless gas bubbles formed on the surface of the copper when acids of 4iv or higher were used. This did not occur with acids of 3;L' or less. The rates of dissolution reported by Berg (2) are half those obtained in this work when a mass velocity of 16.5 grams/sec./sq. cm. was used. In his batch tests, turbulence was probably developed at the surface of the rotating metal sample, but in the work reported here, the liquid was in laminar flow. Possiblv at higher flow rates, the curve at 5.5L1r acid in the illustrated graph would start downivard, similar to that for 4 . 3 5 acid. Literature Cited (1) Berg, T. G. Owe, Z. anorg. allgem. Chem. 266, 118, 130 (1951). (2) Zbid., 269, 210 (1952). (3) Zbid., 273, 101 (1953). (4) Ibid., 2.Metallk. 44, 82 (1953). (5) Buben, N. Ya., Frank-KamenetskiY, D. A., J . Phvs. Chem. (U.S.S.R.) . 20.. 225 (1946). . (6) Centnerszwer. M.. Heller. W..' Metaux @ Corrosion' 14,'37 (1939). (7) Krasil'shchikov, '4.I.? Dedova, I. V., J . Gen. Chem. (U.S.S.R.) 16, 537 (1946). RECEIVED for review September 25, 1957 ACCEPTED March 6, 1958 I
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