INDUSTRIAL A N D ENGINEERING CHEMISTRY
290
Determinant Factors in Boiler Water Treatment
Since the solubility curves of the desirable and undesirable solid phases are divergent, it follows that the concentration of conditioning chemical which must be maintained in the boiler water is a function both of the unfavorable radical concentration and of the pressure a t which the boiler operates. Further, the point of introduction of the chemical, whether into the feed water or the boiler water, is a matter of indifference. To illustrate, consider two boilers operating a t 100 and 200 pounds gage pressure, respectively, and with raw water drawn from practically any lake or river in the country. Let the conditioning be done with soda ash. Note-In case of a raw water containing soda ash, as No. 1, Table I V , i t may be necessary, in order t o maintain the upper carbonate limit as desired, t o use a magnesium or calcium salt to decrease the carbonate concentration.
I n this case sulfate is the unfavorable radical to be considered. The minimum carbonate concentrations which may be maintained in the two boilers are as follows (p. p. m.) : Concentration of
so4
CONCBNTRATION OF COa
Boiler a t 100 pounds gage Boiler a t 200 pounds gage 500 22 72 1000 46 143 2000 88 285 Note-In view of the variation of the solubility product ratio with in-
creasing concentrations, t h e carbonate concentration, a s obtained directly from the equation, suffices for t h e upper limit at the higher sulfate concentrations.
The relatively high concentration of carbonate radical required a t the higher pressure naturally leads to the question-what is the limiting pressure a t which the use of soda ash is feasible, in view of its ready hydrolysis and decomposition under boiler conditions? Data on its decomposition a t different pressures have been presented elsewhere;13 a t 320 pounds gage it is impossible to keep enough carbonate in the boiler to prevent the precipitation of calcium sulfate13 and the high hydroxyl engendered may lead to the precipitation of calcium hydroxide, for which dS/dT is negative. The amount of carbon dioxide introduced into the steam by carbonate decomposition, although of almost negligible percentage, becomes another factor for consideration at the higher pressures because of its relation to corrosion in condensers and other points of condensation. Two methods of control may be suggested: (a) to maintain a low sulfate concentration by blow-down, or by the use of a barium salt; (6) to substitute a stable radical, such as phosphate, for the carbonate. It has been found satisfactory in practice, for boilers operating a t 150 pounds gage, to establish a suitable sulfate concentration (2000 to 2500 p. p. m. SO,) and maintain it by blow-down, thus making more uniform the inflow of soda ash. When the sulfate concentration of the feed water is high, the operating pressure a t which the substitution of phosphate for carbonate becomes desirable is probably not far removed from 200 pounds gage.
Vol. 17, No. 3
sing on to the boiler contains some carbonate crystals as nuclei. Although they are probably sufficient to lessen the formation of scale, they do not prevent it. TWOmeans of minimizing this formation of scale from a bicarbonate water suggest themselves. One method is the use of a d e a e r a t ~ r . ~ ?By the division of the water into a fine spray and the maintenance of a very low partial pressure of carbon dioxide in contact with it, partial decomposition of bicarbonate is much more quickly accomplished than otherwise would be the case, because of the large surface exposed. A second method is to introduce into the water leaving the feed water heater the hydroxyl and the finely divided calcium carbonate crystals in the effluent of a filter attached to a boiler for the purpose of maintaining a low content of suspended matter in the boiler water. The alkaline effluent water immediately removes any bicarbonate radical and provides nuclei for uniform crystal formation throughout the water in place of on the irregularites of the metal surface. Simple methods suffice for the removal of carbonate scale from feed lines. Cold water, saturated with carbon dioxide under pressure, disintegrates it.28 A somewhat higher hydrogen-ion concentration, under rigid supervision of a competent chemist who would realize the possibility of injury to the feed lines through overtreatment, would be more economical both in time and cost of material. In either method the water used in the treatment should be kept out of the boiler. Other Factors in Water Treatment
In addition to maintaining conditions in the boiler water so that hard adherent scale shall not form on the evaporating surfaces, any complete system of boiler water treatment must provide for the inseparably associated factors of sludge formation and wet steam, and must minimize corrosion. The prevention of scale formation is distinctly a chemical problem, and has been considered in this article. For minimizing corrosion both chemical and mechanical means should be employed;29 control of the other two factors is mechanical, and has been discussed e1~ewhere.l~ Acknowledgment
I n addition to those mentioned throughout the paper, the writer wishes to express his especial appreciation to J. M. Hopwood, president of the Hagan Corporation, and to A. C. Fieldner, superintendent and supervising chemist of the Pittsburgh Experiment Station, Bureau of Mines. n McDermet, Mcch. Eng., 42, 273 (1920); Jackson and McDermet, THISJOURNAL, 16, 959 (1923). Cross and Irvin, Power, 66, 422 (1922); Jones, Ibid., 60, 578 (1924);
private communication from W. P. Chandler, fuel engineer, Carnegie Steel Co., Duquesne (Pa.) Works. Hall, Meck. Eng., 46, 810 (1924).
Prevention of Soft Carbonate Scale
Production and Imports of Pyrites Continue Downward Trend in 1924
Water that has been given a correct treatment with lime and soda ash and sufficient time for complete precipitation, or with a zeolite exchange process, should cause little trouble a t this point in the boiler, as deposition of calcium carbonate will be a function of change in solubility with temperature only. If errors enter into the treatment, however, the conditions are most favorable for formation of carbonate scale, inasmuch as such waters are very free from suspended materials which may serve as nuclei in crystal formation, and thus more readily deposit crystals of calcium carbonate on the irregularities of the metal surface. I n the case of no outside treatment or treatment with soda ash alone, there is a gradual decomposition of bicarbonate radical and consequent precipitation of calcium carbonate. Some of this occurs in the feed water heater, and hence the water pas-
The production of pyrites in the United States, as indicated by figures received from the Geological Survey, dropped from 181,628 long tons, valued a t $661,000 in 1923 to 160,096 long tons, valued a t $645,262 in 1924 and, with the exception of 1921, when only 157,118 long tons were produced, was the smallest recorded since 1897. The figures for 1924 show a decrease of 12 per cent in quantity but of only 2 per cent in value as compared with 1923. The average value per ton was thus higher in 1924 than in 1923, being $4.03 as compared with $3.64. Although the production of pyrites dropped 12 per cent in 1924, the quantity of pyrites sold and consumed by producing companies dropped only 6 per cent. The imports of pyrites containing more than 25 per cent of sulfur in 1924 amounted to 243,237 long tons, valued at $582,794, according to the Bureau of Foreign and Domestic Commerce. Of this amount 242,786 long tons, or 99.8 per cent, were imported from Spain. The imports records for 1924 and 1923 indicate a decrease of about 50 per cent in the average value per ton of pyrites imported.
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