by Craig J. Brown, P. Eng. ECO-TEC LIMITED Toronto, Canada ABS TRA CT

Transcription

1 RECOVERY SYSTEMS FOR THE LIGHT METAIS FINISHE~-RYIE H by Craig J. Brown, P. Eng. ECO-TEC LIMITED Toronto, Canada ABS TRA CT A trend towards chemical recovery is emerging in light metals finishing, Utilizing cation and anion exchange as well as acid retardation principles, a new technique called Reciprocating Flow Ion &change has been applied to a wide variety of recovery applications in this field. Basic system design is discussed as well as ways in which such recovery systems have helped stabilize and improve finishing operations, abate pollution and reduce costs. 1. INTRODUCTION Although recovery of spent process baths and rinsewaters has been widely practised in the electroplating industry over the past several years, the light metals industry has not practised chemical and water recovery to the same extent. This could be due to the lower value of the chemicals employed in finishing light metals or the lack of suitable techniques by which these chemicals can be effectively and economically recovered, A trend towards recovery is now emerging in this industry, both in new and existing plants. There are a number of factors which have contributed to this development Increased water and chemical costs. Because of the relatively large tonnage of chemicals consumed in light metals finishing, a small increase in chemical costs can be translated into significant savings through recovery. Commercialization of a number of new recovery processes, some of which were originally developed for the electroplating industry. Increased environmental pressures. Recovery systems can save money and at the same time help abate pollution or improve the operation of a conventional waste treatment system. Recognition of the fact that proper utilization of a recovery system can frequently stabilize and/or improve the finishing process _itself, from both a cost and quality point or view.

2 " % Page..' 2 One of the unit operations that has been extensively utilized for chemical recovery in the electroplating industry is ion exchange. The purpose of this paper is to discuss some ways that ion exchange can be utilized for chemical recovery in light metals finishing. 2. WATER AND CHEMICAL RECOVERY BY ION EXCHANGE Until fairly recently the use of ion exchange for pollution abatement and recovery in the metal finishing industry has been limited. This was primarily due to the limitations of the conventional fixed bed technology that has evolved over the past twenty or thirty years in the water treatment industry. Generally speaking, this technology is not well suited to the treatment and/or production of the fairly concentrated solutions encountered in metal finishing. The advent of continuous counter-current ion exchange systems improved the technical feasibility of the ion exchange process somewhat but the high capital cost and complexity of these systems has severely limited their applications. A patented l0 ion exchange technique called Reciprocating Flow Ion Exchange or "RECOFLO" has overcome many of these limitations and expanded the range of application of the ion exchange process in this industry. 3. RECOFLQ ION EXCHANGE While detailed discussion of RECOFLQ fundamentals is beyond the scope of this paper, it can be simply characterized by a number of features that distinctly set it apart from conventional ion exchange technology. These include; 1. Fine resins 2. Short beds 3. lbw exchanger loadings 4. Countercurrent regeneration and rinsing Liquid stratification in the bed High flow rate 7. Short cycles These features combine to Improve the feasibility of the ion exchange process in the following ways; 1. smaller equipment 2. lower capital costs 3. improved regeneration and uptake efficiencies 4. ability to treat and produce concentrated solutions with minimum intermixing or dilution 5. short offstream times for regeneration eliminate the need for standby equipment.

3 *.. Page 3 As a result it has been possible to utilize ion exchange for a number of chemical recovery applications which were previously not feasible by conventional ion exchange. The RECOFIQ technique has been utilized in a number of chemical recovery applications in the electroplating industry since RANGE OF APPLICATION Cation and anion exchange can be utilized for chemical recovery in a number of ways. A lesser known process called "acid retardation" is also finding wide application. These processes can be utilized by themselves, or in combination. 4.1 Cation Exchanqe Cation exchange is applied in two main areas: 1. Removal of metallic cations from dilute rinsewa ters. Regeneration of the cation resin with sulphuric or hydrochloric acid usually yields a more concentrated solution of the metal which can often be recycled to the process bath. 2. Removal of metallic cation impurities from concentrated process baths. Application of this sort are restricted to baths which are not too acidic, since uptake of metals onto the resin is poor under highly acidic conditions. The use of cation exchange for removal of aluminum impurities from process baths has been limited in the past. This is because of the nature of the aqueous aluminum ion. An excess of acid representing several times the stoichiometric or theoretical dosage is normally required in the regeneration of strong acid cation resins. This excess acid must normally be neutralized prior to recycle or discharge of the spent regenerant solution. This problem is particularly acute for aluminum since the resin dramatically prefers the trivalent aluminum cation over the monovalvent hydrogen cation. The low equivalent weight of aluminum (i. e. 8.9) results in very low resin capacities for aluminum. Moreover, the high resin preference for aluminum further reduces resin working capacities so that frequent regeneration of the resin is necessary. RECOFK) techniques have proven beneficial in overcoming these difficulties to a very large degree allowing the use of low exchanger loadings - and relatively low regeneration dosages.

4 Page 4 A number of RECOFIL)'%lecationizers"have been used for removal of A13+, Cr3+, Fe3+, and Cu3+ impurities from hard chrome plating baths with excellent results. Experienced gained in this area has proven applicable in light metal work. 4.2 Anion Exchanqe The main application of anion exchange has been removal of anions from dilute rinsewaters. Regeneration of the anion resin is normally accomplished with sodium hydroxide to yield a concentrated sodium salt solution of the original anion. This recovered salt can be subsequently converted to the acid form by cation exchange if desired. 4.3 Cation/Anion Exchanqe By combining cation and anion exchangers it is possible to completely deionize an aqueous solution. Apart from de-ionization of tap water, the main application of this process is purification of rinse waters to allow recycle of the water Acid Retardation Some time agdit was observed that certain polymeric resins would 'sorb' (ie. absorb/adsorb) strong acids such as sulphuric, hydrochloric, and nitric but would not sorb metallic salts of these acids. Desorption of the acid is readily achieved with water. By utilizing RECOFLO techniques it is possible to elute a purified acid product from the resin with no dilution of the acid strength. This process,known as acid retardation& not as well known as cation and anion exchange and was in fact not commercially exploited to a significant extent until RECOFIQ techniques were applied to the process in The process has been packaged into a standard, skid mounted piece of equipment called an Acid Purification Unit or "APU". There are two basic steps in the APU process (see figure 1). In the "upstroke" acid feed solution containing a metallic salt impurity is metered into the bottom of the resin bed. The free acid is sorbed by the resin and the metalllc salt byproduct solution flows from the top of the bed. In the "downstroke"

5 UPSTROKE Metallic salt (bv tirodti( f l COMf'RtSSED AIR I 1 RESIN BED JI Spent acid (leedl Spent acla (feed) i RESIN BED water I I v Purified acid (product) FIGURE I - APU Process PROCESS BATH... feed product water APU byproducl L rigure 2 - APU Bath Purification

6 a w L Page 6 -I K iz T I'

7 Page 7 water de-sorbs the acid from-the resin producing a purified acid product solution from the bottom of the bed. The APU has seen application in two main areas: 1. Purification of strongly acidic process baths (see figure 2); 2. Recovery of excess acid from cation exchange regenerant solutions (see section 4.5); The fact that only water is consumed in the process has made recovery of a number of relatively inexpensive acids feasible. 4.5 Cation Exchanqe/Acid Retardation The APU provides an ideal means of recovering the excess acid from the cation exchange regenerant solution. The purified acid product from the APU can be re-fortified with a small amount of fresh concentrated acid and re-used for regeneration of the cation resin. The net result is that the regeneration can be accomplished with a dosage only marginally above the theoretical or stoichiometric amount. With the aid of an APU, cation exchange can be applied in areas which were previously not feasible. Figure 4 shows the basic layout of a typical DCU/APU system. Contaminated process solution flows to the DCU and the purified process solution flows back to the bath. The DCU is periodically regenerated with acid and washed with water. The dilute washings flow to waste and the concentrated portion of the spent regenerant flows to the APU. The acid in the spent regenerant solution is retained by the APU and the impurities are rejected in the APU byproduct stream. Water elutes the acid from the APU which,after re-fortification with a small amount of fresh acid,can be reused by the DCU. The APU can be set up to remove 90% of the metallic impurities and recover about 90% of the free acid fed to it. 5. SULPHURIC ACID ANODIZING Most anodizing is done!xi sdphuric acid elez?rc!ytes. In all of these baths dissolved aluminum tends to build up in proportion to the ampere hours of anodizing carried out. It has been estimated that, under normal anodizing conditions, 15 grams of aluminum are dissolved per square meter of work processed per hour of anodizing time a. Eventually the aluminum reaches a level where it impairs the process and. quality of the film. At this point all or part of the bath must be discarded and replaced with fresh solution. -

8 Page 8 Utilizing the acid retardation principle, the APU can be used to continuously remove aluminum sulphate impurities from the bath! By this means bath life can be extended indefinitely. In March 1977, the first APU (see figure 3) was started up to purify a sulphuric acid anodizing bath. Since that time a large number of similar units have been installed around the world. Users of these units have reported a number of benefits associated with use of the APU including the following; 1. removal of phosphate and miscellaneous metallic impurities as well as aluminum. 2. savings in labour formerly required to decant and makeup fresh baths. 3. reduced alkali requirements for waste treatment. 4. reduced sulphuric acid purchases. 5. more uniform film thicknesses and reduced seal times. 6. lower operating voltage with corresponding electrical energy savings. 7. reduced cooling requirements. Sulphuric acid anodizing baths containing other minor components such as boric acid, glycerine, glycolic acid etc. can also be treated by this method. Figure 3 shows an APU that will remove 1.8 kg/h of aluminum while holding the sulphuric acid anodizing bath at about A1 = 10 g/l. Typical operating data are shown in table 1. TABLE I TYPICAL APU RESULTS - HQSOA Y ANODIZING RECOVERY Feed (bath) Product Waste Water LL. L

9 Page 9 6. INTEGRAL COIQUR ANODIZING A var ety of organic acids are used in the,.itegral colour anodizing of a!uminum including sulphosalicylic, sulphophthallic, boric etc. These baths are more sensitive to dissolved aluminum impurity buildup than conventional sulphuric acid baths and the aluminum content has to be maintained at a much lower level and within much closer limits. The higher replacement valve of these baths necessitates some means of impurity removal. Rates of aluminum buildup in these baths havebeen estimated as 27 grams per square meter of work per hour of anodizing time (at 2.58 amp/dm2)9. The lower free acidity of these electrolytes allows the use of cation exchange for aluminum removal and indeed conventional cation exchange has already been used extensively for this application. The large amount of downtime for frequent regenerations and the high regeneration dosages normally required allow considerable room for improvement however and the APU/DCU system as shown in figure 4 can be shown to process significant advantages in these respects. 7. PHOSPHORIC ACID ANODIZING Falling somewhere between sulphuric acid and the organic acids in terms of acid strength, phosphoric acid anodizing baths can be purified by either an APU or DCU. Taking into consideration the relatively high value of phosphoric acid and the role of the phosphate ion as a pollutant, the best approach appears to be the DCU/APU type system as shown in figure 4. Solution is fed directly from the bath to the DCU. Some dilution of the phosphoric acid may occur during treatment by the DCU. In this case a simple atmospheric evaporator that operates directly for the bath may be necessary to remove this excess water (see figure 13). 8. NITRIC ACID DE-SMUTTING The APU has been used in the electroplating industry for removal of metallic impurities from nitric acid rack stripping baths. Nitric acid baths, 20-50% w/w in concentration, are frequently used for de-smutting aluminum. The actual amount of metal dissolved in the acid is so bw that only rarely does the bath have to be dumped because of metallic contamination. Usually the problem is one of maintaining the strength of. acid in the bath.

10 .- Page 10 The bath is constantly being depleted in acid strength due to dragout of acid and drag-in of water. Because these baths operate at room temperature and there are no evaporation losses, the only way to maintain bath strength is by decanting part of the bath and topping it up with concentrated acid. This is undesirable from several points of view the toxic nitrate ion cannot be removed by simple neutralization alkali requirements for neutralization are high handling nitric acid is hazardous The APU has been successfully used in the electroplating industry for purification of nitric acid rack stripping baths? No major modifications were required for this application. The operating conditions of an APU can be adjusted so that the product is sllghtly higher in nitric acid concentration and lower in flow than the feed solution from the bath. The APU can then be used to remove water from the bath to allow makeup of fresh concentrated acid. Assuming one wants to maintain an acid strength of 35% w/w HN03 (50% vol.) and the dragout/ drag-in rate is 0.12 litre/m2 (3 US gav1000 ft2) then one must remove approximately 0.06 litres of water from the bath for each square meter of work process ed. An APU has recently been installed to remove 25 I/h of water from a nitric acid desmut bath. It was found that special attention must be paid to filtration of fine suspended solids in the bath solution for successful operation of system. 9. NITRIC AND HYDROCHLORIC ACID ELECTROCHEMICAL ETCHING Dilute nitric acid and hydrochloric acid solutions, often with additions of various organic acids and/or small amounts of other inorganic acidsare used for electrochemically etching aluminum foils. These baths of course become contaminated with dissolved aluminum and must be frequently dumped. Because these baths are often fairly dilute they may lend themselves to purification by cation exchange better than acid retardation. It is necessary to utilize aii ATU in conjunction with the decationizer to improve chemical regeneration efficiency sufficiently to make recovery truly viable. A large system of this type was recently delivered to a manufacturer of lithographic plates for removal of aluminum from a hydrochloric acid based solution.

11 .... Page 11 I Concentrated phosphoric acid based baths usually with additions of nitric, sulphuric, acetic and/or chromic acid are used to polish aluminum. After brightening, the adhering bright dip solution, now contaminated w-ith aluminum, must be immediately rinsed from the metal with water. Typically 5-10 grams of aluminum are etched for each square meter of work processed. Due to the high acid concentration (ie % H3P04) and high viscosity, dragout losses are appreciable, typically I/m2 compared to about 0.12 t/m 2 for a "water-like" process solution. The rinsewater can be reconcentrated to bath strength by evaporation but the aluminum contamination must be first removed before recycling to avoid fouling the bath. Most previous purification systems based upon cation exchange or acid retardation have failed, mainly because the cost of treating the final waste stream and disposing of the resulting sludge generated by the system was excessive. The DCU/APU system has greatly alleviated this problem. A typical phosphoric acid recovery system is shown in figure 5. Concentrated rinsewater flows to the DCU where it is purified and then to the evaporator where it is concentrated to bath strength and recycled. The DCU is regenerated in conjunction with the APU as discussed in section 4.5. f /- i T-ASm 1 i c/ sulphate waste byproduct produced by the system can be neutralized in the plant waste treatment system or concentrated by evaporation and sold as liquid alum. Table 2 shows projected results when this approach is applied to recovery of dragout from a typical phosphoric/nitric bright dip opera tion. Figure 6 and 7 shows a Decationizer and Acid Purification Unit respectively that will be employed in conjunction w-ith an evaporator in a Japanese anodizing plant for recovery of a phosphoric/sulphuric acid bright dip solution. The large recovery potential from processes of this type make recovery an extremely attractive proposition. When total operating costs of 4.3C/lb are subtracted from a value of 20C per Ib. for 80% phcsphoric acid, ar! installation consuming 500 tons per yearsave up to $140,000 per year based upon a typical recovery efficiency of 9 0%. -\

12 . - PHOSPHORIC ACID RECOVERY SYSTEM WATER n I WATER- APU Of Figure 5 WASTE

13 .... * i - w 0

14 . I 1, Page 14 TABLE 2 - TYPICAL RESULTS - PHOSPHORIC/NITRIC BRIGHT DIP RECOVERY lb/lb 80% H,POd - Slb. 80% Hypo4 66O Be sulphuric acid $ lime $ steam 5.65 $ sludge disposal 1.11 $ ~- TOTAL OPERATING COSTS $ CHROMATE PROCESSES Chromate, either as chromic acid or the potassium (or sodium) salt is used in several aluminum finishing processes including chromic acid anodizing, conversion coating, de-smutting and dichromate sealing (see table 3). To avoid pollution hazards there has been a recent trend away from chrome based processes even though these baths are often better understood, easier to control, superior in performance and less expensive to operate than the chrome-free baths. The availability of viable chrome recovery systems suggest that a re-evaluation of any moves in this direction should be considered. Recovery of hexavalent chrome from plant wastewater not only reduces costs but make waste treatment easier and less costly thereby tipping the scale back in favour of the chromate process. bllution results from chromate processes in two ways: 1. dragout to rinsewater; and 2. process bath dumps due to contamination with aluminum and/ or trivalent chromium. Since 1970 ion exchange units using RECOFIQ techniques have been used in over fifty chrome plating installations for recovery of purified chromic acid from rinsewaters and purification of hard chrome plating baths. This technology has been recently successfully applied to other chrome processes with excellent results. r- Ano d i z i nq I -/-- TABLE 3 - CHROME BASED PROCESS FORMULATIONS Cr03 = g/l De- smuttins i Conversion Coa tinq Cr03 = 6-20 g/l H3PO4 = g/l F- = 2-6 g/l Sea linq KZCr207 = g/L Na2C03 = 4-18 g/l i

15 Page Rins ewa ters The cation/anion exchange process is used for chromate recovery from rinsewaters, Actually, for chromic acid recovery a three bed cation/anion/cation process is often employed (see figure 8). Rinsewater is first pumped through a cation "pretreater" bed which removes cationic metallic impurities such as aluminum and trivalent chrome and then through an anion bed which removes the chromate ion. The resulting deionized water is recycled back to the rinse tanks. The pretreater is regenerated with acid, discharging the contaminants to waste. Regeneration of the anion bed with sodium hydroxide yields sodium chromate, which when passed through the other cation bed is converted to chromic acid at a concentration of 7-10% which can be recycled back to the process bath. Figure 9 shows such a three bed chrome recovery unit. In cases such as the conversion coating and de-smutting baths shown in table 3, the chromate constitutes a minor portion of the bath's chemical inventory. For these processes it may not be desirable or economical to recover all of the constituents since the chromate not only represents the largest value but also the major pollution hazard. It is possible to selectively recover the chromate ion as a purified concentrate and allow the remaining components to flow to waste. In this manner both capital and operating costs can be reduced. This is possible since the dichromate ion is vastly preferred on the anion exchange resin to sulphate, phosphate, fluoride, nitrate or most of the other anions that may be present Process Baths Chromic acid anodizing and conversion coating baths must be periodically dumped because of aluminum and/or trivalent chromium contamination. A Decationizer or DCU/APU type system can be used to continuously remave contaminants from the bath and extend bath life indefinitely. It has been found that the presence of fluoride ion in conversion coating baths hinders the uptake of aluminum and trivalent chrome impurities on the cation resin. As a result, low exchanger loadings and high regenerant dosages must be employed. This problem is handled quite effectively by the methods discussed here.

16 FIGURE 8: CHROME RINSE RECOVERY Page 16 STEP 1 ONSTREAM m REGENERATION t PRODUCT CATION * ANION WASTE r-l PRETREATER L-3 i H20 H2 WASTE mz L WASH - WATER

17 Figure 9 - Chromic Acid Recovery Units Page 17

18 Page ELECTROLYTIC COLQURING A prime area where the possibility of reclaiming valuable metallic salts presents itself is in electrolytic colouring operations. Electrolytes based upon nickel, copper, cobalt or tin, frequently in conjunction with boric acid, magnesium sulphate, aluminum sulphate or various organic acids are used to electrolytically impart colour to the anodized surface. Recovery of these metals fiom rinsewater is advantagebus for two reasons. First of all, salts of cobalt and tin and to a lesser extent nickel and copper represent significant value. Secondly, plants discharging to a sanitary sewer are frequently allowed to discharge aluminum hydroxide but the presence of hydroxides of these heavier metals would preclude this practice and in any case make ultimate sludge disposal m3re difficult. Numerous systems recovering copper and nickel sulphate from electroplating rin~ewaters~have demonstrated that cation exchange systems can effectively recover metallic salts from rinsewaters. Only minor modifications were required for this application. Careful sulphuric acid regeneration of a cation resin loaded with these metals yields a concentrated metallic sulphate solution at a ph of approximately In the case of baths buffered to higher ph's, a modified APU (or "de-acidification unit (DAU)") can be used to raise the ph of the recovered product to bath operating levels. Most of these baths operate near room temperature so that evaporation losses are negligible. Atmospheric evaporators that artificially increase bath evaporation at low operating temperatures must be used to allow recycle of the recovered metallic salt solution to the colouring bath. Figure 10 shows a typical recovery system layout for an electrolytic colouring operation Rinse flows from the first rinse tank through the decationizer (DCU) which removes the metals and discharges the purified rinsewater to waste. Deionized water is used to feed the rinses. Sulphuric acid regeneration of the DCU yields metai sulphate prociuc t which after de-acidifica tion is collected and periodically recycled back to the colour bath. Bath solution is continuously recirculated through the atmospheric evaporator to remove water from the bath. Figures 11, 12, and 13 show a decationizer, de-acidification unit and atmospheric evaporator used for recovery of a valuable metal salt from an electrolytic colouring system.

19 ELECTROLYTIC COLOUR BATH RECOVERY ATMOSPHERIC EVAPORATOR D.I. WATER H2S (D w cb Figure 10

20 Fiaure I? - l3p-acidificr7iinn TTni t.. Page 20 Figure 11 - Metal Salt Recovery Decationizer

21 Figure 13 - Atmospheric Evaporator Page 21

22 Page WATER RECYCLE Large quantities of water are consumed in rinsing work after each process bath. Most of the water can be easily de-ionized by catiodanion exchange and re-used. Indeed since de-ionized water is required for several rinses and makeup of many baths,the same unit can serve both purposes. Recovery of rinsewaters in this manner simplifies waste treatment because it is much easier and cheaper to treat a concentrated, low volume stream than a dilute, high volume stream. Reaction tanks, Teagent pumps and clarifiers are all reduced in size. Moreover the character of sludge precipitated from a concentrated wastewater is generally superior to that produced from a dilute solution. The nature of the rinsing process is such that typically only 10 per cent of the water is used to rinse 90 per cent of the chemicals from the work. Conversely 90 per cent of the water is consumed in actually rinsing only 10 per cent of the chemicals. Operating costs in ion exchange are mainly related to chemical loadings and only to hydraulic flows to a very minor degree. These two principles can be used to great advantage in designing a viable rins ewater recovery system. The system shown in figure 14 illustrates this point. Although only one process step is shown, several processes would of course be tied in together in a similar manner. Conductivity controllers (CC) mounted in rinse tanks R1 and R2 control additions of water to the tanks. The first rinse R1 is maintained at about one tenth of bath strength through additions of a very small amount of water equivalent to about ten times the rate of dragout. Overflow from R1 flows directly to the waste treatment unit (WTU). Desired final rinsewater purity in R2 is maintained through additions of large amounts of recovered water. The overflow fiom R2 flows to a deionizer or water recycle unit (MU) where it is purified and recycled. To facilitate monitoring and maintainance, the various conductivity controllers are best located in one central panel such as the one which is shown in Figure 15. (The other unit to the right is a central monitor panel indicating the status of the various recovery systems in the plant). Automatic regeneration of the WRU may occur once or twice each hour for a period of only about 5 minutes if RECOFLQ techniques are used, eliminating the need for a duplicate standby unit. Regeneration yields a low volume, concentrated liquid that flows to the waste treatment unit (WTU). -

23 RINSWVATER RECYCLE m ri=- V PROCESS BATH R2 1 I r 1 I WTU 'd W Q rd Figure 14

24 Figure 15 - Central Rinsewater Control Unit (left) Page 24

25 Page 2'5 14. CONCLUSION Technology for recovery of chemicals and water used in the finishing of light metals is now emerging and finding wide acceptance. In particular, a wide variety of recovery processes based upon ion exchange technology are now available. Recovery of even inexpensive process baths such as sulphuric acid anodizing can be shown to be worthwhile when side benefits such as pollution control and process stabilization and improvement are considered along with direct economic savings. The benefits of recovery of more expensive and/or toxic chemicals such as chromic acid are even more pronounced. Any existing or proposed new light metals finishing facility should seriously investigate chemical and water recovery as a means of abating pollution, reducing costs and improving their processes.

26 Page 26 REFERENCES R.F. Hunter, U.S. Patent 3,385,788 (1968) R.F. Hunter, U.S. Patent 3,386,914 (1968) M. J. Hatch, J.A. Dillon, Industrial and Enqineerinq Chemistry Process D e s i g i ; C. J. Brown, D. Davy, P. J. Simmons, "Purification of Sulfuric Acid Anodizinq Solutions, 'I Plating and Surface Finishing, January C. J. Brown, D. Davy, P. J. Simmons, "Recovery of Nitric Acid Surface Treatment Baths", Proceedings of the Annual Technical Conference of the American Electroplaters Society, Atlanta, Georgia, June R.F. Hunter, I.H. Spinner, P. J.' Simmons, K.S. Tan,'The Use of Reciprocatinq Flow Ion Exchanqe For the Recovery of Chromic Acid", Presented at the All-Day Symposium of Waste to Profit, sponsored by the Toronto Section of the C.S. Ch.E., November 8, C. J. Brown, D. Davy, P. J. Simmons, "Nickel Salt Recovery By Reciprocatinq Flow Ion Exchange", presented at the 62nd Annual Technical Conference of the American Electroplater's Society, June P.G. Sheasby, personal communication, November Aluminum Company of America, Duranodic 300 Outline of Equipment, July 1975.