Page 's to remove dissolved salts from water. Its main use was

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1 A CASE STUDY OF AN ELECTRODIALYSIS REVERSAL SYSTEM FOR THE RECOVERY OF NICKEL SALTS Thomas J. Susa and Richard A. Tata Ionics, Inc. Watertown, MA Introduction In recent years, environmental regulations have gotten more stringent, the associated costs of wastewater treatment and sludge disposal have increased, and metals prices have fluctuated wildly. These concerns have caused more and more platers to investigate the practicality of recovering or reclaiming metals from plating rinse waters. Reverse osmosis, evaporation, ion exchange, and electrolytic methods have been used for this purpose with varying degrees of success. Now the technology of electrodialysis reversal (EDR) has provided a reliable, economic means of recovering plating salts from plating rinse waters. Electrodialysis (ED) was developed in the late 1940's and early 1950's to remove dissolved salts from water. Its main use was for the purification of water for drinking or industrial use from brackish water sources in desert areas. The toughest problem for ED was the decrease in performance caused by membrane foulants in the solutions to be treated. This problem limited the potential applications for ED. In the 1970's a simple variation on the basic ED process proved to be a major step in minimizing the membrane fouling problem. This process variation is known as electrodialysis reversal (EDR). In the standard ED process a direct electrical current is always passed through the membranes in one direction only. In the EDR process the electrical current is periodically reversed. This reversal of current removes foulants previously driven onto the membrane, thus reducing or eliminating the need for chemical or mechanical cleaning, and greatly increasing the membrane life. Over the intervening years, the use of ED has grown to include many industrial applications involving the removal of ionic species from aqueous solutions, from producing electrolyte balanced infant formula, to concentrating salt water for use in solar ponds. For over thirty months an EDR system has been operating at a large industrial plating facility, recovering nickel salts from plating rinse waters and concentrating them for direct return to the plating baths. In this time the system has paid for itself and is producing a positive cash flow for the company. The ED and EDR Processes Both ED and electroplating utilize the basic principles of ion motion in an applied electric field. In electroplating the electric field is used to move metal ions from the anode through the plating bath to the cathode, the workpiece. In ED and EDR the electric field is used to move ions from one solution through an ion selective membrane to another solution. The ion selective Page 1

2 membranes are the heart of the process. The cation selective membrane consists of a polymeric matrix containing chemically bound negative charges and only permits the passage of positively charged ions, cations. The anion selective membrane likewise contains chemically bound positive charges and only permits passage of negatively charged ions, anions. A cation membrane, a flow path spacer, an anion membrane, and a second flow path spacer make up the basic ED unit called a cell pair. When direct electric current is passed through a cell pair, cations and anions move from one flowpath through their respective membranes into the next flowpath, thus reducing the total ion concentration in one stream and increasing the total ion concentration in the other stream. The cell pairs are "stacked" one on top of the next, and placed between a pair of electrodes to form an ED membrane stack. Up to 600 ED cell pairs can be arranged in a single ED membrane stack, and the stacks can be coupled in series or parallel to scale up the process. The largest ED plants produce millions of gallons per day of fresh water. EDR processes have relatively low operating costs. The major operating costs are usually for electric power and membrane replacement. Labor costs are minimal, as the EDR systems are automated and require little operator attention. A Case Stud In 1985 the'owners of Automatic Platinq Co., Inc. in Bridgeport, Connecticut, a large job shop plating facility, realized that a considerable amount of nickel was being lost to the wastewater treatment system, that could possibly be reclaimed. The owners investigated a number of technologies that could be used to reclaim the nickel in the rinse waters, including reverse osmosis, evaporation, ion exchange, and electrolytic systems. They decided that it would be advantageous if the metal could be reclaimed in the form of plating chemicals, rather than in the form of nickel metal; that eliminated the electrolytic recovery system. They realized that reverse osmosis or evaporation could be used, but were concerned that their brighteners and any contaminants in the rinse waters would also be concentrated in the reclaimed solution. They found that ion exchange or ED would be able to reclaim the plating salts without concentrating up the brighteners or contaminants. After carefully considering the economics of these two recovery systems, they chose the ED system. After extensive pilot plant testing, the owners ordered an EDR metal recovery system in November, The EDR system was installed and started up in March, The complete EDR system, as installed, consists of a feed water holding tank; an activated carbon bed filter; cartridge filters; a hydraulic skid containing pumps, valves, flowmeters, etc.; a power supply and control panel; and 480 cell pair EDR membrane stack. The system occupies a floor area of 14 by 20 feet, including maintenance clearance. The EDR system was designed to recover at least 80 % of the nickel from the rinse waters of up to three automated barrel rack Page 2

3 plating lines. The plating baths used on these lines are of the Watts nickel type, consisting of nickel sulfate, nickel chloride, boric acid, and brighteners. The total nickel concentration in these baths averages 85,000 ppm. The feed stream for the EDR system is taken from the first rinse tank of a three tank rinse system in which the rinse water flow is countercurrent to the work flow. (See Figure 1.) The nickel concentration in this stream has ranged from 700 to 7,000 ppm, with an average nickel concentration of 3,600 ppm. The flow volume from each plating line averages 2.25 GPM for a total of 4.5 GPM from two lines. This stream is fed directly from the first rinse tanks to the feed water holding tank. From the holding tank, this stream is pumped through 10 micron filter cartridges to remove particulates, and then through the carbon bed filter to remove any organics that might act as membrane foulants. This stream is then recycled to the demineralizing side of the EDR stack and back to the holding tank. A level control system in the holding tank determines the ratio between the recycle rate and the waste stream flow rate. During initial startups, when the system has been drained, the feed stream is also used to fill the concentrate recycle loop through the EDR stack. During normal operation the concentrate stream is continuously recycled, and the only volume increase comes from the water associated with the ions passing through the membranes. As the concentrate stream volume increases, it overflows to a product holding tank. The product is then pumped from the holding tank to the plating baths to replenish the plating chemicals as needed. The waste stream nickel concentration has ranged from 70 to 700 ppm, with an average nickel concentration of 350 ppm. The total flow of the waste stream is controlled by the feed water holding tank level control and averages 4.2 GPM, as only 0.3 GPM of water is transferred with the nickel salts and the boric acid. As shown, the EDR system reduces the nickel load on the wastewater treatment system by a factor of ten. The product stream nickel concentration has ranged from 22,000 to 44,000 ppm, with an average concentration of 35,000 ppm. The flow volume averages 0.3 GPM. During the first 30 months of operation the nickel recovery rate has ranged from 83 to 93 %, with an average of 86 %. The average electric power consumption of the EDR system has been 0.89 KWH per pound of nickel recovered. The system availability has averaged over 90 %. The EDR system is also equipped with a cleaning loop, to allow chemical cleaning of the membranes in the stack. During 30 months of operation, no chemical cleaning of the membranes has been required, so the cleaning loop remains unused. From the results obtained during the first 30 months of operation, an economic assessment was calculated and is presented in Table 1. The results are based on third quarter 1988 costs. The membrane replacement costs are based on an assumed membrane life of 5 years. The membrane replacement costs had been based Page 3

4 on an assumed membrane life of only two years when the system was designed, but after thirty months of operation, the EDR stack still contains 80 % of the original membranes. Membranes have been removed from the stack for analysis every three to six months, and so far the analyses have not shown any appreciable deterioration of membrane performance. From this data the membrane life for this application is estimated to be 5 to 7 years. The annual savings from recovery of plating chemicals is based on prices of nickel sulfate at $1.49 per lb., nickel chloride at $1.85 per lb., and boric acid at $0.47 per lb.. The annual savings from water treatment chemicals is based on 50 % sodium hydroxide solution at $1.35 per gallon. Conclusion EDR has shown itself to be a practical, reliable, and economically attractive method for the recovery of plating chemicals from the rinse waters of Watts nickel plating lines, The membrane life has proven to be much longer than originally estimated, and the system performance has not deteriorated over thirty months of operation in a commercial plating facility. Page 4

5 NiSOIJH& *DRAGIN Punw BATH REPLENISHMENT Ibl- RINSE WATER > 135 gavhr EVAPORATlON 2-g gclvhr *FINAL RINSE *DRAGOUT TOW Niekd = 40.5 lbldry 3600 ppm Ni 180 ppm Ni *DENOTES WORK FLOW PUnNG SYSTEM EFFLUENT (TO WASTE TREATMENT) 135 galhr NiS0,6H20 = Iblday NiC12.6W20 = 60.3 lblday HsBO3 = 22 Ib/day TotalNickd = 40.5lWday (a) I IONtCS PREFILlRAnON 135 gavhr 1 EDR RECYCLE L I I IONiCS EDR ELECTRODIALYSIS t - METAL CONCENTRATE RECYCLE EDR FEED RECOVERY 200 gavhr SYSTEM ppm NI ( 3 c PLATING SYSTEM EFFLUENT (TO WASTE TREATMENT) 126 galmr Total Nidcd = 2.8 lw- NCI, 6H20 = oz/gal H3WJ = 361 Or/gal Total Nidcd = Ib/day - *DENOTES WORK FLOW

6 TABLE 1 Economics of Electrodialysis for Nickel Salt Recovery from Plating Rinse Water at Automatic Plating Co., Inc. Items 1. Installed Cost 0 Equipment o Installation, labor & materials o Total 2. Annual Operating Cost o Labor, 100 $lo/hour o 2-1/2% of investment o Raw Materials Filter Cartridges Membrane, Spacfr, and Electrode Replacement o Electricity ($O.O11/KWH) o Total 3. Annual Fixed Cost o Depreciation, 10% of investment o Tax and Insurance, 1% of investment o Total Fixed Cost 4. Total Cost of Operation 5. Annual Savings o Plating Chemicals o Sludge Disposal Cost2 o Water Treatment Chemicals o Total 6. Net Savings (annual savings-(operating + fixed cost) 7. Net Savings After Tax 48% Tax Bracket 8. liverage KGi (%j (net savings after tax/total investment) 9. Cash Flow from Investment (net savings after tax + depreciation) 10.Payback Period = Total Investment/Cash Flow Assuming 5 year membrane life 2@ 95% solids 100,000 1,500 01,500 1,000 2,500 1,000 4, w 10, ,600 6, 080 2, , ,860 70, , yrs.