Disposal of Waste Liquors from Chromium Plating e.
I
C. HOOVER AND J. W. MASSELLI Wesleyan University, Middletown, Conn. When
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metallurgical wastes containing chromium are discharged to sewers or open streams, objectionable conditions are produced and a relatively valuable metal, of which we have only a limited national supply, is lost. In this investigation waste solutions from chromium plating are the special subject of study. Two methods for preventing harmful effects in streams are suggested. One uses scrap steel to reduce the chromium to the trivalent condition and also lime as a neutralization and precipitation agent; the other employs barium sulfide as a combined reducing and precipitating reagent and also lime for the completion of neutralization and precipitation. The most feasible recovery process appears to involve the use of sulfur dioxide as a reducing agent and soda ash for precipitation. The filtered precipitate is roasted to produce chromium trioxide in salable form.
HE increasing use of chromium as a surface coating of metals is one of the noticeable developments of the past decade, The industrial operations of plating and brightdipping involved in producing such coatings create a new type of liquid waste disposal problem. Factors involved in this problem are (a) the elimination of polluting effects and (b) the prevention of economic loss due to the relatively high cost and national scarcity of chromium compounds. Pollution effects that have been noted are the persistent yellow color produced in natural water courses and the interference with biological processes of stream and sewage purification. Preliminary results of investigations in this laboratory indicate that, under conditions which obtain in sludge digestion, chromium compounds are comparable in germicidal effect to copper compounds. The economic loss is illustrated by the observation that in one plant investigated more than 100 pounds of chromium were lost each working day. Dodge ( I ) , in cooperation with the State Water Commission of Connecticut, investigated the recovery of chromium and other metal compounds from bright-dipping wastes. He suggests six different processes involving reduction of hexavalent chromium with sulfur dioxide or brass chips, precipitation of zinc, chromium, and copper with lime or soda ash, and ignition or solution of the precipitate in acid to recover compounds of the metals. This investigation was concerned with wastes in which it was not feasible to segregate the chromium. A pilot plant was operated over a period of 18 months, and an economic study of the cost of operating a full-scale recovery plant was made. The results are of importance in indicating the limitations of processes involving recovery of by-products. The conclusions drawn were that “one or two processes offer promise of being able to recover enough of value in the form of by-products to pay all the direct operating costs”, but that “no treatment process now in sight will be able to recover sufficient values to pay the entire cost of treatmentJJ. Wittmann and Wohlfahrt (3) suggested the use of sodium bisulfite followed by soda ash for recovering chromium from waste waters of the galvanizing industry, but no details are given. McKee ( 2 ) devised and patented a continuous electrolytic process for regenerating chromic acid from strong chromate solutions used for oxidizing organic matter. This investigation has as a primary object the amelioration of the harmful effects of chromium wastes on natural waters and sewage purification and, secondarily, the chemistry of recovery processes. It was hoped that these two objects might be combined, but in a process designed to eliminate the effects of chromium on natural waters, the purity of the treated effluent is a more important factor than in a recovery mocess. While not strictly within the scope of this paper, any possible decrease in the normal loss of chromium plating solutions in the operation of a plating plant serves the same end as a treatment and recovery process and should be carefully considered by investigators and manufacturers. The most obvious method of decreasing the loss of electrolyte, due to
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mechanical carry-over as the plated articles and racks are withdrawn from the plating bath, is the use of standing rinse tanks in which the work is immersed momentarily by hand or mechanical means. Two such tanks, in series, will retain a large proportion of the electrolyte and render it available for make-up liquid used in preparing new electrolyte. Other suggestions include (a) thorough shaking and draining of the cleaned work before it is placed in the plating bath, and thus preventing the dilution of this bath and enabling more solution from the rinse tanks to be used in maintaining the volume of electrolyte, and (b) as thorough shaking and draining of work immediately after removal from the plating bath as can be secured without discoloring the work. This draining may be over a lead trough leading back to the plating bath. Careful chemical control of recovered solution and the discharge of a definite proportion are necessary to avoid contamination of electrolyte. The effect of these means of decreasing chromium losses is partly shown in Table 11. Plant E has decreased the chromium content of its waste from approximately 350 to 247 p. p. m. by draining and shaking; and plant M , using two standing rinse tanks, discharges only 102 p. p. m. The latter method thus appears to be the better.
Composition of Wastes The results of analyses of typical complete composite Liquid wastes from three plants where chromium-plating and, in the case of plant M , bright-dipping operations are carried on are shown in Table I. 131
INDUSTRIAL AND ENGINEERING CHEMISTRY
132 TABLE I. Plant
PH
bfETAL-FINISHING PL.4NT M Y 2.7 6.2 7 -
Total Cr Crvr cu Ni Zn Fe
so. CN
4.1 3 5 7.1 0.1 6.8 2.3 215 1.1
EFFLUENTS
Parts per million 18.1 16.6 2.0 0.14 2.1 3.1 65 0.8
E 3.0
87 76 32 2
0 2.3 271 12
Segregation of waste solutions containing chromium is feasible in certain plants. Such segregated wastes were made the basis of the major portion of the treatment studies carried out in this investigation. Wide variation in composition of such segregated wastes was observed. The range of composition in one plant and typical average daily compositions in two plants are shown in Table 11. Copper, nickel, and cyanide were seldom entirely absent and were apparently carried over from preliminary plating operations in hollow articles. Chemical and engineering analyses of plating room practice have led to suggestions for decreasing the loss of plating solutions and some improvement has been effected, but the daily average compositions shown in Table I1 are typical of the waste solutions it was necessary to treat. TABLE 11. PH
SEGREGATED WASTE CHROMIUM PLATING SOLUTIONS Typical Daily Av. R~~~~of c o m p n . , Plant E Plant E Plant il4 1.8-3.2 2.1 3.5
Total Cr CrVI
cu Ni
F0
SOL
CN
Mineral acid
--
Parts per million 247 87-643 242 75-636 3.6 0.2-34 Trace-6.4 1.1 Trace 0.0-80 343 84-769 0 .o-22.4 1.2 55-634 308.6
-
102 97 0.9 0.2 0.8 199 0.3 108
Treatment Studies The methods of attack can be classified under three headings: (a) reduction of hexavalent chromium followed by precipitation of hydrated chromic oxide, (b) direct chemical precipitation of hexavalent chromium, and ( c ) miscellaneous precipitation, coagulation, and electrolytic processes.
Vol. 33, No. 1
As reducing agents the following materials were investigated: sodium sulfide, calcium sulfide, barium sulfide, sulfur dioxide, sodium sulfite, sodium bisulfite, calcium bisulfite, zinc hydrosulfite, copperas (ferrous sulfate), zinc dust, iron filings, and scrap steel. The mutual reductions with hydrogen peroxide, sodium peroxide, and potassium permanganate were also tried. Following reduction, the precipitation and neutralization with lime, soda ash, barium carbonate, and hydroxide were studied. Direct precipitation of hexavalent chromium was tried with lead, barium, copper, and zinc compounds. The miscellaneous methods employed included combination effects, partial reduction to form chromicchromate complexes, use of organic reagents such as gelatin and alcohol, and electroreduction and oxidation. Preliminary chemical treatments were carried out in small glass cylinders; and as less feasible methods were eliminated, quantities treated were increased to 200-gallon lots. It is obvious that the concentration of chromium in the waste determines the amount of reagents required. But it was also found that certain reagents were not so effective in strong as in weak solutions. While a wide range of concentrations was studied, for quantitative tests two (namely, 100 and 250 p. p. m. of total chromium) were chosen as covering the probable variations to be met in the plants where waste required treatment. Results obtained with 100-p. p. m. solutions and certain processes, shown after preliminary tests t o be most promising, are summarized in Table 111. The pH of all effluents was between 6.8 and 7.2, except barium hydroxide. The chemical costs indicated in Table I11 are based on current published prices plus cost of delivery and, in some cases, on direct quotations delivered a t the location of projected treatment plants. These prices do not, of course, cover the total cost of treating a thousand gallons of waste; but since the equipment and labor required in any of the processes being compared are similar, a high chemical cost is sufficient reason for rejecting several of the reagents listed in Table 111. This would appear to include sulfur dioxide and sulfites, but the possibility of preparing sulfur dioxide from sulfur a t a treatment plant seems to justify further consideration of this reagent. There appears to be no market a t present for the byproducts of the action of soluble barium and lead salts on the type of waste chromium solutions produced in the plating industry. Barium hydroxide is an ideally effective reagent for purifying chromium wastes, but the presence of sulfate ion in the solutions treated increases the expense and gives mixed precipitates of chromates and sulfates. The same objection is raised to the purity of lead chromate produced by precipitation with lead acetate or nitrate.
TANKS AND STIRRING EQUIPMENT FIGURE 1. ZO,OOO-GALLON TREATMENT
Selected Treatment Processes Considering all factors involved, three disposal processes were selected for more detailed study. They were based on sodium sulfide, barium sulfide, and scrap iron. I n addition, one recovery process, based on the use of sulfur dioxide, was investigated. I n comparing the four processes considered feasible, the major factors studied were: cost; completeness of removal of objectionable substances; the adaptability of the process to both treatment and recovery, and also to the treatment of all plating-room waste solutions as well as segregated chromium wastes; time required for a cycle of treatment operations; and the disposal of the precipitated solids. Table IV compares the four selected treatment and recovery methods.
INDUSTRIAL AND ENGINEERING CHEMISTRY
January, 1941
TABLE111. TREATMENT OF 100-P. P. M. CHROMIUM WASTE SOLUTIONS H2SOil Lb./
Lime, Lb./ 1000 Gal. 6 3 5 5'
Chem, coat Cent&
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I n deeper vessels the volume decreases, and settling and dewatering characteristics improve; but in any case i t dewaters slowly on a vacuum filter and dries slowly on sand beds.
Barium Sulfide Commercial barium sulfide (barium black ash) of 75 to 80 .. 26 per cent purity is obtainable in a fineness of approximately 20 . 2. 28 100 mesh, but even with this degree of fineness, when added 2 .. 2 9 to a waste solution and thoroughly stirred, reaction is slow and 4 .. 2 16 12 . 3. 75 11 19 incomplete. It is found, however, that a 30 per cent solution 3 3 3 6 31.5 of the commercial material can be prepared by stirring the 2 86 7 .. 5 solid with water a t a temperature between 60' and 65' C. 8 .. ... 5 56 without serious loss of the hydrogen sulfide produced by hydrolysis. The solution must be TABLE IV. SUMMARY OF SELECTED TREATMENTS kept warm, however, or it will (250-p. p m. chromium waste solutions, original pH 2.5, effluent pH 6.2-7.2, aosts on basis of 40.000 gal. per 16-hour crystallize to a solid of day, plant capacity 100.000 gal. per 24 hours) Total hexahydrate. The reaction of TreatCoat, ment Vol. Dewqter- Cents/ barium sulfide with a solution ?,"d/, HzSOi Secondary Reagents Cycle % ing Time 1000 Reagent Gals. Copperas CaO Hour)s Sludge Mm.:Sec: Gal. Nature Effluentof of chromate, sulfate, and hydronium ions is interesting. If Disposal Methods 50 Unobjectionable the sulfide is added until the re4 10 4:5 4 3 1-3 3 NazS 4 6 2 : 15 54 Unobjectionable ductionis complete, and the p H 7 .. . 1-3 4 Bas 5 3-4 .. . 6 Scrap ateel 8 5:10 4o Un'bJeCtionable is raised to 7 with lime, the preSO? 5 ... ... 7 4 11 6:20 42 Objectionable cipitate consists of a mixture Recovery Method0 of barium chromate, sulfate, 5 Soda. rtah, 8 4 8 3:45 51 Objectionable son hydrated chromium oxide, and a By-product, 3 pounds CrzOt. some sulfur. If less than the theoretical amount of barium sulfide to form sulfur and triAll processes precipitated the chromium completely. No valent chromium is added to a chromate solution and the solution is neutralized, all of the chromium may be precipidifficulties due to the formation of undissociated chromium tated, but on standing a yellow solution of chromate ion recomplexes were experienced. The small amounts of copper, appears. This behavior may be caused by the formation of nickel, cadmium, and zinc, sometimes present in chromium unstable, insoluble chromium chromates which decompose on wastes, were largely removed with sulfide and scrap iron processes, and partly with sulfur dioxide. Cyanide was restanding, or by the slow oxidation of sulfide ion to sulfate and the dissolving of barium chromate to form more insoluble duced in amount by all processes, and the scrap iron and sulfur barium sulfate. When using this reagent, as was noted with dioxide methods could be operated to remove approximately sodium sulfide, it is usually simpler to reach the point of 50 per cent. The sodium sulfide and scrap iron processes complete reduction by adding ferrous sulfate solution after can readily be adapted to purifying the complete wastes from lime has been added, since this reagent will react with an exr the chromium plating of steel. A neutral effluent can be secess of either chromate ion or of barium and sulfide ions and cured in all processes. The scrap iron process and, with carewill also coagulate any colloidal heavy metal sulfides that ful chemical control, the sulfide processes gave a final effluent may be present. The precipitate obtained by adding lime to which was unobjectionable when diluted in sewerage systems a completely reduced chromate solution obtained with barium and in open streams. The sulfur dioxide process, however, produced an objectionable effluent as it was carried out, but sulfide settles rapidly, occupies approximately half of the supplementary treatment with ferric sulfate and air could volume of an equivalent amount of hydrated chromium oxide, be applied to remove residual sulfites. Special characterand dewaters readily to a firm cake. istics of the different processes are discussed separately.
Reagent SOa liquid NaHSOa NazSOs NarS Bas Copperas Iron scrap Zinc Pb(Ac)a Ba(OH)a.SHtO a Copperas.
Lb./ 1000 Gal. 4 4 4
1000
Gal.
pHEnd Reaction 2 5 2 2.5 8 5 3 2 .. 55 3.7 2 8 10
Sludge, %in 2 Hr. 16 14 11 16 8 22 15 17
1000
Gal.
Scrap Iron
Sodium Sulfide Sodium sulfide appears superficially to be a satisfactory reagent for the reduction of chromic solutions. The commer.cia1 hydrated salt is readily soluble and reacts in almost stoichiometric proportions with chromate wastes containing 150 p. p. m. of hexavalent chromium or less. The commercial salt contains some iron, and this, together with the small amount of copper present in chromium wastes, forms a persistent brown colloidal dispersion of sulfur and sulfides which makes the effluent objectionable. Ferrous sulfate, when added t o the treated wastes, combines with the sulfide ion, removes the odor of hydrogen sulfide, and forms a floc which clears the solution. However, if the concentration of the chromium is greater than 150 p. p. m., this procedure is not easy to control, and a small amount of a strong oxidizing agent, such as a soluble permanganate, is necessary to produce a clear effluent. The precipitated hydrous chromium oxide settles slowly and after several hours occupies a volume as great as 15 per cent of the solution treated in solutions 2 feet deep.
Finely divided iron, in the form of steel wool or powder, reacts slowly with typical waste chromic acid solutions and reduces the chromium to the trivalent form. Pure dilute chromic acids of the concentration of typical wastes react very slowly owing to weak acidity, and concentrated solutions do not react owing to the well-known passivating action of chromic acid on iron. Small increases in the sulfuric acid concentration of the waste or larger increases of sulfate ion bring about a marked improvement in the rate of the reaction. Cupric and cyanide ions apparently increase the rate of reaction, while nickel has little effect and ferric ion retards the reaction. However, this retarding effect is not appreciable in concentrations capable of being formed in waste solutions. Scrap iron in the form of waste sheet steel is found to react readily with typical waste plating solutions when sulfuric acid is present, Since most chromium plating is carried out in manufacturing plants using sheet steel stamping, trimming, or punching operations, it was considered practical to use sheet steel scrap as a means of reducing chromic acid wastes.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
I n laboratory tests a large glass percolator was filled with scrap sheet steel punchings of such size that 100 pounds occupied 1 cubic foot, and waste chromic acid solutions of varying concentration and acidity were passed through or were allowed to stand for varying times. I n a 250-p. p. m. waste chromic acid solution, to which sulfuric acid had been added to produce a p H of 2.2 (3 or 4 pounds of sulfuric acid per thousand gallons), complete reduction was secured in 4 hours. Chromate solutions can be applied continuously at the top and reduced solution withdrawn at the bottom when 6hour detention time in the tower is allowed. The copper present in the wastes is deposited on the surface of the iron and facilitates the reduction. Ultimately this copper could be recovered. Some hydrogen bubbles pass u p through the tower and aid in removing liberated hydrocyanic acid. After treatment with scrap steel the waste solution is neutralized with lime in an approDriate tank. and the trivalent chroc mium is precipitated along with the ferric and ferrous salts formed during reduction. The mixed precipitates of hydrated chromium and iron oxides settle more rapidly and occupy less volume than the same amount of chromium compound alone, but the mixture dewaters slowly on vacuum filters and can best be disposed of on sand beds or by lagooning.
Sulfur Dioxide
It was evident from preliminary tests that liquefied sulfur dioxide could not compete on a cost basis with other reagents. However, it seems feasible to prepare this convenient reducing agent by burning the amount of sulfur required for a treatment in a closed, cast-iron pot which can be heated from without to start the combustion. For large installations a commercial sulfur burner could be used. The gas can be withdrawn or blown from the burner by a corrosion-resistant liquid-sealed rotary pump and forced through diffusion tubes placed in the bottom of the treatment tank containing the chromium waste. After reduction is complete, air can be blown through the solution to discharge some of the excess sulfur dioxide and hydrocyanic acid, and lime slurry can be used to precipitate the trivalent chromium. Air from the same blower can be used to stir the precipitating mixture. After precipitation, the gas diffuser system is withdrawn and the precipitate allowed to settle for a t least two hours. By careful and slow operation the precipitated sludge can be dewatered on sand beds or with filter aids on a continuous vacuum filter. Recovery Process The most feasible process for the recovery of a salable chromium compound that can be suggested as a result of this investigation appears to be a modification of the disposal process just outlined. An excess of sulfur dioxide should be avoided, the gas should be washed before being passed into the reducing tank, and mechanical stirring should be used. A hydrated-chromium oxide of a high degree of purity can then be precipitated with soda ash or caustic soda. The precipitated hydrated chromium oxide can be washed Once bJ7 decantation in the precipitating tank. The precipitate obtained by the use of soda ash can be dewatered on a continuous vacuum filter. The cake is dried and roasted a t 500" C., and forms the dense green P-chromium trioxide which is reported to be reasonably satisfactory as chrome green pigment.
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Treatment Plant Features As a result of the laboratory and small-plant scale tests and a consideration of factors which have just been discussed, a treatment plant is being built to handle from 40,000 to 100,000 gallons of segregated chromium plating wastes each 24 hours. The chief features of this plant are two 20,000-gallon wooden treatment tanks with sloping bottoms and an adjustable siphon arm for decanting the purified effluent (Figure l), two solution tanks holding 200 and 400 gallons, a 2000-gallon collecting sump on the plant drain, distributing pumps, mechanical stirrers in each tank, and a filter press for dewatering the sludge (Figure 2). This equipment can be adapted to any of the four processes discussed above. The plant is to begin operations with barium sulfide on account of the better dewatering properties of the precipitate produced and the purity of the effluent. Meanwhile a market for by-product chromium oxide is being sought, and if found the sulfur dioxide process will be recommended. If recovery of chromium oxide does not seem to be feasible, it is recommended that scrap steel be collected from the metal stamping department of the adjoining factory and that the process based on the use of this material as a reducing agent be carried out. I n this case one of the 20,000gallon tanks uill be filled with the scrap steel and used as a reducing tower, while the other 20,000-gallon tank will be used for precipitation with lime. Aclmowledgment This investigation has been supported by the State Water Commission of Connecticut as a part of its industrial waste disposal program; engineering plans were prepared by W. R. Copeland. Personnel provided by the Federal Works Progress Administration assisted in chemical analyses. The Electrolux Company of Old Greenwich, Conn., cooperated in furnishing wastes used in this investigation and erected a treatment plant, shown in Figures 1 and 2. Literature Cited (1) Dodge, B. F., Biennial Rept. State Water Comm. Connecticut, 5 , 51 (1934); 6, 69 (1936). (2) McKee, R.H., a n d Leo, T., J. IN=.ENG. CHEM., 12, 16 (1920): MoKee, R. H., U. S. P a t e n t 1,408,618 ( M a r c h 7, 1922). (3) W i t t m a n n , a n d Wohlfahrt, R., Chem.-Ztg., 61,496 (1937).
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PRESENTED before the Division of Water, Sewage, and Sanitation Chemiitry et the 100th AIeeting of the American Chemioal Society Detroit. Mi&
