INDUSTRIAL AND ENGINEERING CHEMISTRY
248
Vol. 20, No. 3
Disposal of Acid-Iron Wastes from a Steel Mill Stuart E. Coburn METCALF &
EDDY,ENGINEERS, BOSTON,MASS.
T
HE presence of acid-iron wastes is often a serious hand-
icap in the puiification of polluted streams or in the treatment of sewage. The discharge of these wastes into a stream or other body of water tends to kill the fish and to render the water turbid and discolored by the precipitation of iron hydroxide on oxidation of the ferrous sulfate. This oxidation also tends to deplete the dissolved oxygen in the waters of the stream. If the acid-iron wastes are discharged into a sewer system, they may seriously affect the treatment of the sewage. The disposal of such wastes is an important problem. I n a recent investigation conducted by the writer at a steel mill, the pickling wastes were being discharged into a concrete sewer of the city and had caused serious disintegration of the concrete. The city authorities had notified the concern that the wastes must be either kept out of the sewer or treated so as to render them harmless before discharge into the sewer. Source of Wastes These wastes consist of spent acid liquor from pickling steel, rinse waters from washing the metal after pickling, and spent lime solution from neutralizing the small amount of acid adhering to the metal after rinsing. Two processes of pickling are in use a t this plant. I n one process the metal is passed continuously through the acid bath, rinsing arrangement, and lime bath; and in the other process it is allowed to remain in the acid bath as long as necessary, after which it is transferred to the rinse bath and then to the lime bath. There are two continuous picklers. The acid-bath tanks have a working capacity of about 1000 gallons each. When the acid bath is first made up, it contains about 8 per cent by volume of sulfuric acid. The bath is tested each day and reenforced with additional acid as found necessary until the dissolved iron reaches about 0.5 pound per gallon. The percentage of acid required then becomes so high that the bath is dropped, the time in use varying from 2 days to 1 week. During the treatment of the metal the bath is kept a t a temperature of about 180' to 190" F. by live steam, and the steam condensate is used to maintain the volume of the bath. When the bath is dropped, it contains from 3 to 5 per cent by volume of unused acid. The rinse water for the continuous picklers is supplied from perforated pipe lines which cause the water to spray over the moving metal. The two lime tanks for the continuous picklers have a total capacity of about 500 gallons, It is usually necessary to "drop" the lime baths every day or at least every other day on account of the accumulation of dirt. I n the tub process used for products that are not suited for the continuous process, there are four acid-bath tanks having a total working capacity of about 1500 gallons. AS previously stated, the metal is allowed to remain in the acid bath as long as necessary, after which it is transferred to the rinse bath and then to the lime bath. In this process it is possible to work the free-acid content of the pickling liquor down to about 1 per cent by volume. Volume of Wastes The wastes from the acid baths of each of the continuous picklers amount to about 1000 gallons every time the bath
is dropped. With both baths dropped on the average of twice a week, this is equivalent to approximately 4000 gallons per week or an average of about 700 gallons per working day. The discharges from the acid baths of the tub process average about 300 gallons per day, making the total average volume of acid-bath wastes about 1000 gallons per working day. The volume of rinse-water wastes from the two processes is estimated a t 7500 gallons per working day. The volume of lime-bath wastes from the two processes averages about 550 gallons per working day. Composition of Wastes Samples of the different wastes were collected for observation and analysis. The samples were all practically clear, but contained more or less solid matter. A portion of each sample was allowed to stand for 24 hours in a 50-cc. graduated glass cylinder and the volume occupied by the sediment determined. The results for a typical set of samples from the continuous process are shown in Table I. T a b l e I-Observations SAMPLE Pickling liquor Rinse waters Lime wastes
of S e d i m e n t in S a m p l e s f r o m C o n t i n u o u s
Process SEDIMENT REMARKS Per cent by volume 4.4 I/a-inch green crystals at bottom of bottle, with dark brown precipitate above 0 4 Brown flocculent precipitate 4.0 Chocolate colored precipitate
The results of the analyses of these wastes are given in Table 11. Table 11-Analyses DETERMINATION
of Wastes f r o m C o n t i n u o u s Process ACID RINSE LIMB BATE WATERS BATH Per cenl b y weight
Sulfuric acid (HzSOd: 19.60 Total" 8.17 Free acidb Iron (Fe) : Total 5.250 5.180 Ferrous Ferric 0.070 33.21 Total solids Specific gravity 1.220 Hydrogen-ion concentration, pH index 1-
0.85
h'eutralo
0.50
0.242 0.162 0.080
1.483 1.010 1.4
...
0,0125 0 0007 0,0118 0.183 1.000 6.5
By phenolphthalein at boiling temperature. b Computed by deducting from total acidity the sulfuric acid equivalent of the ferrous iron. c B y methyl orange.
Possible Methods of Disposal of Wastes The analyses show that the lime-bath wastes may be discharged into the sewer without harmful effects. They contain considerable suspended matter, but this is light in weight and not enough in amount to cause objectionable deposits in the sewer. The rinse waters are distinctly corrosive and require treatment before discharge into the sewer. There is no question that the acid-bath wastes must either be taken out of the sewer or treated to remove their corrosive action before discharge into the sewer. The following methods of disposal were considered: (1) dilution with water, and discharge into the sewer; (2) neutralization with lime and discharge into the sewer; (3) treatment with lime in separate tanks and disposal of resulting sludge; (4) disposal by cesspool method: ( 5 ) recovery of ferrous sulfate.
March, 1928
INDUSTRIAL A S D EAVGINEERING CHEMISTRY Dilution
If the acid wastes were sufficiently diluted with water before discharge into the sewer, no harm would result. It would be necessary, however, t o mix the water thoroughly with the wastes prior to discharge. I n the laboratory all the samples of wastes received were mixed together in approximate proportion to the average volumes produced, and it was found that one volume of the mixture would require a t least 100 volumes of water to reduce the hydrogen-ion concentration to a p H index of about 4,which, although slightly acid, would have no serious effect upon the sewer. The volume of water required for such dilution would be nearly 1 million gallons per day, and under conditions existing a t this plant this would involve a large expenditure for water. It is also questionable whether the city authorities would approve of putting such a large volume of water into the sewer. Neutralization with Lime
Experiments were made to determine the amount of lime required to neutralize the acid wastes. The amounts of lime required were determined on two bases: first, to neutralize the acidity indicated by methyl orange, and second, to neutralize the acidity indicated by phenolphthalein. Methyl orange indicates the most aggressive part of the acidity including the free sulfuric acid, while phenolphthalein indicates the total acidity including the ferrous sulfate acidity. The tests indicated that an average of about 1600 pounds of lime per day would be required to neutralize the total acidity of the wastes and about 500 pounds of lime per day to neutralize the most aggressive portion of the acidity. The use of lime for neutralizing the acidity is attended by the precipitation of the iron in solution forming hydroxide of iron. Undiluted samples of the acid-bath wastes after lime treatment became thick sludges in the case of neutralizing the acidity indicated by methyl orange, and pasty masses in the case of neutralizing the acidity indicated by phenolphthalein. The lime treatment could not be properly controlled in practice without pumping the wastes to separate treatment tanks and utilizing rinse waters for dilution. Even then, the proportion of sludge produced would be so large that it would be likely to form troublesome deposits in the sewer. Treatment with Lime in Separate Tanks and Disposal of Sludge
All the wastes could be pumped to a suitable lime-treatment plant including storage tanks, the effluent discharged into the sewers, and the sludge disposed of by other means. The tanks would have to be large enough to hold 2 or 3 days’ storage of wastes in order to secure proper mixing before treatment. The sludge could be discharged into lagoons if suitable low areas could be found or be dewatered on beds of porous material like sand or by filter presses or vacuum filters and the dewatered sludge hauled away to a suitable dumping ground. It would be practically worthless. The amount of lime required and the percentages of sludge produced were as follows: T o ~YEUTRALIIE ACIDITYB Y :
Methyl orange Phenolphthalein
SLUDGEPRODUCED ON LImE Lbs./1000 gallons 100 192
2 Hours Per cent 45 66
STANDING: 24 Hours
Per cent 42 5s
IENDOF
249
Assuming that the sludge in practice would be concentrated to one-half of the above percentages after 24 hours’ standing, the sludge from neutralizing the methyl orange acidity would amount to about 20 per cent of the volume of wastes or, say, 1800 gallons per day. There would be large expense involved in dewatering and disposal of this sludge, in addition t o the cost of the lime treatment. No further consideration was given to this method because of the possibility of other much less expensive methods. Disposal by Cesspool Method
It would be difficult to dispose of all the wastes by the cesspool method even with the sandy, porous soil available. As previously stated, the lime-bath wastes can be discharged directly into the sewer without any treatment, and the rinse waters can be so discharged after the addition of moderate amounts of lime sufficient to neutralize the acidity indicated by methyl orange. The acid-bath wastes could then be discharged into leaching cesspools, the liquid passing away into the soil. A more effective method of disposal would be by means of subsurface drains laid in porous soil with open joints properly surrounded with graded gravel. However, these wastes contain considerable amounts of suspended matter which would tend to clog the soil. Furthermore, as the wastes passed into the soil and became partially neutralized and oxidized, hydroxide of iron would be precipitated, tending t o clog the soil and render the system inoperative. This method of disposal is also open to the objection that the acid waters passing through the soil might come in contact with iron pipes for gas, water, etc., and cause serious corrosion. Furthermore, if the acid waters should get into wells or surface water supplies they would impair the quality of the water and might cause serious trouble. For these reasons this method of disposal was not considered satisfactory for permanent use. Recovery of Sulfate of Iron
The acid-bath wastes contain large amounts of dissolved iron, as shown by the analysis. This iron is in the form of ferrous sulfate, which can be recovered and sold as copperas. I n the recovery process the wastes are discharged into leadlined tanks, the excess water is evaporated by steam coils, and the concentrated liquor, after sedimentation to remove suspended matters, is discharged into a lead-lined tank, where the copperas crystallizes out on cooling. The free acid may be neutralized by addition of scrap iron before evaporation, thus increasing the amount of copperas produced and rendering the liquor suitable to discharge into the sewer with the addition of relatively small amounts of lime as may be required to neutralize any methyl orange acidity remaining. If scrap iron.is not available, the mother liquor, after the copperas is crystallized out, will be strongly acid and can be returned to the acid tanks. The analyses indicate that over 12,000 pounds of copperas per week could be recovered from the acid-bath wastes without the addition of scrap iron to neutralize the free acid, and over 20,000 pounds per week if the free acid were so neutralized. While this recovery process may not be profitable a t the present market price of copperas, it will prove in the long run to be the most satisfactory and most economical method of disposal of the acid-bath wastes. SYMPOSIUM]
