V . t • . !>* > - » • • « *Vk ft> . i • o «^' C v ♦' r . >. 0* »• VL** "*b j* v . & °- X/ . » • J* <+. ••• .* o 4-""^ V V . « * o "^ ^ Jp ^ I* . t • I V °y> * • » ...' ,G V <> *'T^ ./\ » ^ • <> *'...' ,& »♦ . N «5°- .4" H o^ •♦. .*% ^•0* 5° o , ' ^ v o > **F ?• J\. ''$£&; ^ : '0 > ... v^\^ ^^^v v-^y v^v \*^* 4 / ^ -•fife' *.^ :dbt'. ^** SuMtir. V>* *!dfe\ ♦* .*♦ >Wa % W Sgffl-. \>S S» \S #fc \S :gS£* \S .-M- r.- ^ -.fw: #'%. -,u^.- ^'*, -..^^.- ^»*^ -• IC 8931 Bureau of Mines Information Circular/1983 Economic Evaluation of a Method To Regenerate Waste Chromic Acid-Sulfuric Acid Etchants By Deborah A. Spotts UNITED STATES DEPARTMENT OF THE INTERIOR faj&Msii*, ^ox^- y*>^) y Information Circular 8931 Economic Evaluation of a Method To Regenerate Waste Chromic Acid-Sulfuric Acid Etchants By Deborah A. Spotts UNITED STATES DEPARTMENT OF THE INTERIOR James G. Watt, Secretary BUREAU OF MINES Robert C. Horton, Director # \ fc This publication has been cataloged as follows: -\\\l^ ■ a V> X\0> Spotts, Deborah A Economic evaluation of a nic hod to re generate waste chromic add- sulfuric acid etchants. (Information circular / Bureau of Mines ; 8931) Bibliography: p. 7. Supt. of Docs, no.: I 28.27:8931. 1. Etching reagents— Recyc ing— Economic as uce ts. 2. Chromic acid. 3. Sulphuric acid. I. T itle. II. Series: Information circular (United States. Bureau of Mines) ; 8931. TN295.U4 [TU899.M43] 622s [671.71 83-600071 CONTENTS Page : Abstract 1 Introduction 2 Operation of electrolytic cell 2 Economics 3 Capital costs 3 Operating costs 4 Economic evaluation 4 References 7 ILLUSTRATIONS 1. Daily direct operating cost 6 Daily sludge treatment and disposal savings 6 3 . Daily sodium dichromate savings 6 TABLES 1. Estimated fixed capital cost 3 2. Estimated annual operating cost 4 ^ UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT ft foot mo month gal gallon pet percent kW'h kilowatt-hour yr year lb pound ECONOMIC EVALUATION OF A METHOD TO REGENERATE WASTE CHROMIC ACID-SULFURIC ACID ETCHANTS By Deborah A. Spotts ] ABSTRACT Researchers at the Bureau of Mines have developed a technique for re- generating chromic acid-sulfuric acid etching solutions used in metal surface treatment operations. The technique utilizes a diaphragm cell equipped with a cation-selective membrane to oxidize Cr 3+ to Cr 6+ at the anode and to remove copper, the major metallic contaminant, at the cathode. Normally, spent etchant is discarded after approximately 3 days of use. Using the electrolytic cell, the etchant can be used for a year without replacement. From data obtained from industrial-scale cells, the installation of a regeneration cell with a 1,000-gal catholyte- holding tank has been estimated to save at least $240 per day. The payback period for the investment is estimated to be about 10 mo or less. Because the magnitude of these cost savings will vary at different locations, several graphs are presented to aid in calculating payback for a specific site. Using these graphs and the capital costs pre- sented in this study, the payback period can be determined for install- ing a regeneration cell with a 500- or a 1,000-gal catholyte-holding tank in an existing surface treatment plant. 1 Chemist, Avondale Research Center, Bureau of Mines, Avondale, Md. INTRODUCTION In 1980, there was no domestic mine production of chromium in the United States; however, the United States con- tinued to be a major chromium consumer (1) .2 To aid conservation of the metal, researchers at the Bureau of Mines de- vised a method to regenerate chromic acid-sulfuric acid etching solutions, thereby saving the chromium lost when the solution is dumped. These solutions currently are used for brass and printed circuit board etching and other surface treatments . For these solutions to be effective, the chromium must be in the hexavalent state. As the solutions are used, the Cr 6+ is reduced to Cr 3+ ; the dissolved solids content, and therefore the specific gravity, of the solution increase. During plant operation, sodium dichromate is added to the solution to replace the reduced chro- mium and losses due to drag out. These losses increase as the specific gravity increases. Sodium dichromate additions can extend the life of the etchant some- what, but when the solution no longer performs properly, despite additions, it is discarded to a waste operation. Dur- ing treatment all the metals including chromium are precipitated as waste solids (usually hydroxides) for disposal in a landfill. The method devised by the Bureau of Mines, and subsequently demonstrated in a brass etching plant, uses a diaphragm cell equipped with a cation-selective membrane to oxidize the Cr 3+ to Cr 6+ at the anode and remove copper, the major metallic contaminant, at the cathode ^2-4). This increases the solution life, reduces waste disposal costs significant- ly, and lowers chromium losses to drag out. Bureau of Mines researchers, after per- forming bench-scale and small pilot plant studies, designed and operated a process research unit at Gould, Inc., Valve and Fittings Division, Niles, 111.3 (now Im- perial Clevite, Inc.) for a 17-day peri- od. After evaluating the performance of the Bureau's cell, personnel at Gould, Inc., contacted Scientific Control Labo- ratories, Inc., Chicago, 111., and re- quested that a larger cell be constructed and installed for continuous use. This cell has been in operation for over a year without a need for etchant disposal. This operation has been used as an addi- tional source of data for this study. OPERATION OF ELECTROLYTIC CELL The electrolytic cell described herein is designed to continuously regenerate and purify chromic acid etching solu- tions. It is assumed that the cell is added to an existing surface treatment plant that uses chromic acid etching solutions. The electrolytic cell is located adja- cent to an etching tank containing a chromic acid-sulfuric acid etchant. This cell consists of one to five anode chambers and cathodes in a single tank containing a sulfuric acid catho- lyte. Each anode chamber consists of a ^underlined numbers in parentheses re- fer to items in the list of references at the end of this report. lead-antimony electrode surrounded by a cation-selective Nafion membrane. This membrane allows the positively charged ions to pass but inhibits the passage of negatively charged ions. Test results indicated that this membrane is not de- graded by dichromate ions. The cathodes are sheets of copper metal, and the cath- olyte in the tank is a 10- to 20-pct sul- furic acid solution. The concentration of sulfuric acid in the catholyte must be within 5 pet of that in the spent etchant to minimize the transfer of water across the membrane. ^Reference to specific companies, trade names, or manufacturers does not imply endorsement by the Bureau of Mines. Spent etchant is pumped from the etch- ing tank through a cartridge filter to remove any metal particulates. The solu- tion then is metered through manifolds into the anode chambers. The surrounding cation-selective membrane does not allow the etchant to mix with the catholyte. In the anode compartment, the following reaction occurs: 2Cr 3+ + 7H 2 -»- Cr 2 7 2 - + 14H + + 6e - . The complementary reaction at the cathode is 3Cu 2+ + 6e" * 3Cu. (These reactions are simplified versions of more complex reactions. The Cr 3+ and Cu 2+ probably exist as complex sulfate species.) The Cu 2+ ions migrate from the etchant through the membrane into the catholyte. Regenerated etchant is re- turned to the etching tank. Copper deposits loosely on the cathodes and, in one modification, slides down the cathodes into plastic buckets attached to the cathode assemblies. To remove the copper, the cathodes are lifted from the cell with a hoist approximately once a week, and the buckets are dumped manual- ly. After washing, the cathode product is 99 pet copper. In addition to the cell, some accessory equipment is required for operation, in- cluding a hoist for lifting the cathodes and piping between the etching tank and the electrolytic cell. Ductwork and a blower are added for ventilation of the cell, and electrical connections are needed for the cell and the blower. ECONOMICS The cost estimate in this evaluation is based on data from both the Bureau of Mines process research unit and the in- dustrial etchant regeneration operation. CAPITAL COSTS The capital costs presented in this evaluation are based on manufacturer's cost quotations and capacity-cost data. The electrolytic cell cost quotations were supplied by Scientific Control Labo- ratories, Inc. These costs include the rectifier, all required busing, the transfer pump, the cartridge filter, and the exhaust hood as well as the tank, electrodes, and membranes. Costs for cells with 500- and 1,000-gal catholyte- holding tanks are presented in table 1. The 1,000-gal catholyte-holding tank con- tains twice the anode area and therefore has twice the capacity of a 500-gal tank. The accuracy of a plant-addition esti- mate is very difficult to determine be- cause of the many variables that must be considered for each individual plant. In this study, it has been assumed that clear space is available for the cell and that the piping and electrical lines will be less than 200 ft long. If it is nec- essary to clear space for the cell or add additional electric service, the capital costs would be increased. Because the installed cell cost represents about 90 pet of the capital cost, the estimated costs in this study should be satis- factory to show the value of using the cell. The capital costs presented in table 1 include installation labor. It has been assumed that the cell will be deliv- ered to the plant site, installed, and TABLE 1. - Estimated fixed capital cost Electrolytic cell.... Hoist Blower Ductwork Electrical Piping Total 1,000-gal cell $65,700 3,400 1,000 100 1,800 200 72,200 500-gal cell $42,000 3,400 1,000 100 1,100 200 47,800 Basis: Second quarter 1982 cost. guaranteed to be operational; therefore, no additional startup costs are included in the estimates. Additional working capital has not been included since it has been assumed to be minimal. OPERATING COSTS The estimated operating costs (table 2) are based on an average of 350 days of operation per year over the life of the equipment. The regeneration cell and the etching tank with which it is associated will operate intermittently. Electrical requirements and chromium savings data are therefore based on operational data averaged over a long period of time. The operating costs are divided into direct and fixed costs. Direct costs include utilities and plant maintenance. It has been assumed that no additional direct labor will be required to operate the electrolytic cell. Occasional checks are required to determine if the cell is functioning properly, and copper removal is neces- sary. Since these activities require a minimal amount of time, they can be per- formed by existing plant employees. Payroll overhead for maintenance per- sonnel includes vacation, sick leave, social security, and fringe benefits. Fixed costs include the cost of taxes (excluding income taxes), insurance, and depreciation. Depreciation is based on a straight-line 20-yr period. ECONOMIC EVALUATION Etching solution life can be signifi- cantly lengthened by using the electro- lytic regeneration technology described herein. Normally, spent etchant is dis- carded after approximately 3 days of use. With the use of the electrolytic cell, the etchant can be used for a year with- out replacement. This results in signif- icant cost savings in several areas, as follows : Better product quality control . — Per- formance of the regenerated " etching solution remains constant and is superior to that of the untreated etchant prior to dumping. This will result in a product with a more consistent quality and less of f -specif ication product. Reduced waste solution treatment and disposal costs . — Chemical and labor costs for waste solution treatment and costs for sludge haulage to a landfill will de- crease because only the drag out from the cell will need to be treated. As capital investments have already been made at TABLE 2. - Estimated annual operating cost 1,000-gal cell 500-gal cell Direct cost: Utilities: $5,500 $2,800 5,500 2,800 Plant maintenance : 600 600 300 400 1,200 200 700 100 6,900 700 700 3,600 3,600 Fixed cost: 500 500 2 , 400 11,900 7,000 existing plants for waste treatment facilities, no credit was considered for the reduced need for waste treatment. Reduced sodium dichromate consump- tion . — Since the etchant will last a year or more, the only sodium dichromate losses will be those due to drag out. Reduced drag out losses . — Because the etchant is continuously regenerated, the specific gravity will remain fairly con- stant. Without treatment, the specific gravity of the etchant increases. Drag out losses increase with increased spe- cific gravity; thus, the use of the cell will lower these losses. Copper byproduct recovery . — Copper met- al recovered can be sold as a secondary copper product. Only a small quantity of copper is recovered, so this will repre- sent a minor cost savings. An example of cost savings calculations is presented in the following discussion. From process research unit data, it has been determined that the electrical con- sumption is 3.6 kW*h per pound of sodium dichromate regenerated, and each pound of sodium dichromate regenerated reduces the amount of sludge generated in a waste treatment plant by 9.4 gal (5). A plant installing a cell with a 1,000-gal catholyte-holding tank can reduce sodium dichromate consumption by 100 lb per day and consequently reduce the waste sludge generated by 940 gal per day. This pro- vides a significant cost savings. At costs of 20^ per gallon for sludge treat- ment and disposal and 68^ per pound of sodium dichromate saved, this is a total cost savings of $256 per day. However, to determine the net cost sav- ings, the cost of operating the cell must be considered. If the electric power cost is 4.5)6 per kilowatt-hour, the oper- ating cost for the example plant operat- ing 350 days per year will be $20 per day (not including taxes and depreciation). Thus, the net savings will be $236 per day. A convenient method to measure the eco- nomic value of this investment is to de- termine the payback period. Payback pe- riod is defined as the time required to recover the original investment through cost savings. Most company managements consider 2- to 3-yr payback periods sat- isfactory for new plants and less than 2 yr satisfactory for plant modifica- tions. A formula for calculating payback period is capital cost of the cell and related equipment m yearly net cost savings + depreciation For a 1,000-gal cell, the capital cost and depreciation, as found in tables 1 and 2, are $72,200 and $3,600, respec- tively. The daily cost savings are mul- tiplied by the number of operating days per year to obtain the yearly net cost savings. Therefore, for the example plant operating 350 days per year, the payback period is 72,200 (350)(236) + 3,600 = 0.84 yr (approximately 10 mo). With such a short payback period, in- stalling a regeneration cell with a 1,000-gal catholyte-holding tank in the example plant would be considered a prof- itable investment. Costs for sludge treatment and dispo- sal, sodium dichromate, and electric power will vary for each individual plant considering installing a regeneration system. For this reason, graphs have been provided to show how variances in these costs affect the cost savings. Figure 1 shows the effect of the elec- tric power cost on the operating cost for both a 500- and a 1,000-gal cell. Fig- ure 2 illustrates the effect of sludge treatment and disposal costs on the cost savings for both cells, and figure 3 shows the effect of the cost of sodium dichromate on the cost savings for both cells. Using this information, the capi- tal cost (table 1), and the depreciation (table 2), the payback period for in- stalling either a 500- or 1,000-gal cell can be calculated for a specific plant. The cost savings presented in this evaluation do not include savings due to the better product quality control because data are not available to mea- sure this quantity. In addition, no credit has been included for the copper metal recovered from the cell, which can be sold as secondary copper product. In- clusion of this credit will decrease the payback period slightly. 1,500 >- to XI o 1 ,000 500 500-gal cell 20 kO 60 80 100 120 1**0 SLUDGE TREATMENT AND DISPOSAL COST, cents per gal Ion FIGURE 2. - Daily sludge treatment and dis- posal savings. 30 h- 25 co O o >- to o -a 20 ■z. — 1_ 1- 0) < a. 15 UJ if) O- L. O TO ' — 10 1— — o O uj -a DC — 1 1 1 1 1 1 y — y/ — ^n,000-gal cell ^^-^^500-gal cell 1 1 1 1 1 l >- -a a. O ■a C3 100 i i — i — i — r 500-gal eel 1 1 I I I l 50 55 60 65 70 75 80 ELECTRIC POWER COST, cents per kilowatt-hour FIGURE 1. - Daily direct operating cost. SODIUM DICHROMATE COST, cents per pound FIGURE 3. - Daily sodium dichromate savings, REFERENCES 1. Peterson, E. C. Chromium. BuMines Minerals Yearbook 1980, v. 1, p. 190. 2. George, L. C, D. M. Soboroff, and A. A. Cochran. Regeneration of Waste Chromic Acid Etching Solutions in an Industrial-Scale Research Unit. Proc. 3d Conf . on Advanced Pollution Control for the Metal Finishing Industry (Kissimmee, Fla., Apr. 14-16, 1980). EPA-600/2-81- 028, February 1981, pp. 33-36. 3. Soboroff, D. M. , J. D. Troyer, and A. A. Cochran. Regeneration of Waste Metallurgical Process Liquor. U.S. Pat. 4,337,129, June 29, 1982. 4. Soboroff, D. M. , J. D. Troyer, and A. A. Cochran. Regeneration and Recy- cling of Waste Chromic Acid-Sulfuric Acid Etchants. BuMines RI 8377, 1979, 13 pp. 5. Horter, G. L. , and L. C. George. Demonstration of Technology To Recycle Chromic Acid Etchants at Gould, Inc. Pres. at 4th Recycling World Congress and Exposition, New Orleans, La., Apr. 5-7, 1982, 13 pp.; available from G. L. Hor- ter, Bureau of Mines Rolla Research Cen- ter, Rolla, Mo. 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