Ion Exchange Resins for Metal Plating and Surface Finishing

ION EXCHANGE RESINS

Introduction 5

Plating Bath Rejuvenation 6

Rinse Water Recycling 8

Waste Effluent Treatment 12

Copper Recovery 16

Precious Metal Recovery 16

Purification of Galvanizing Solutions 17

Summary Table of Ion Exchange Resins 18

Contents

3

Introduction

Ion exchange resins (IER) can be used in the metalplating and surface finishing industries in a num-ber of ways: rejuvenation of plating baths and

pickling solutions, recycling of rinse waters, as wellas treatment of waste effluents. Environmental regu-lations, economic constraints and the quality of themanufactured metal products influence the decisionto use ion exchange technology.

Environmental considerations refer to meetingthe discharge limits for the various metals imposedby government regulations. These vary from countryto country, but in general the limits for discharge ofmetal finishing effluents into the environment rangefrom 0.1 ppm for the more toxic metals up to 3 ppmfor the less toxic metals.

For an economic analysis, the costs of treatmentof the plating baths or the rinse waters is compared tothe cost of waste discharge. In some cases it is pos-sible to recover and recycle chemicals for their value.Polishing of the effluents is achieved with the use ofselective resins, usually of the iminodiacetic acidtype, which selectively remove heavy metals fromsolutions with high Na+ and Ca2+ backgrounds.

Figure 1 illustrates the overall process of a metalfinishing plant. The major sources of the wastes arethe spent plating baths and the rinse waters. In manyplating industries, different plating baths may beinvolved. For example, chromium, copper, nickelplating are frequently performed in the same shop.Also, zinc and copper cyanide solutions can be pres-ent in the same plant. The wastes from such platingplants are generally treated chemically.

5

Figure 1: Overall Process of a Metal Finishing Plant

PlatingBath

ChemicalTreatment

Effluent

Rinse

H O2

6

Chemical Treatment Effluent

DilutionTank

Water

PlatingBath

ConcentratedChromic Acid

Condenser

Amberlyst15WET

Regenerant

Evap

orat

or

Figure 2: Rejuvenation Process for Chromium Plating Bath

Plating Bath Rejuvenation

Chromium plating baths are the most commonexample of a plating bath rejuvenation. Atypical spent chromium plating bath contains

about 300 g/L of chromic acid and 10-25 g/L of Cu2+,Fe3+ and Cr3+.

The use of a standard cation exchange resin toremove the metal impurities in order to rejuvenatethe plating bath is not possible under these condi-tions. The H+ concentration is so high that the resinwill remain in the H+ form, as the equilibrium reac-tion will shift to the left:

RSO3- H+ + M2+ (RSO3

-)2 M2+ + H+

In addition, the high oxidative potential of such asolution may rapidly degrade the resin.

To utilize an IER process, the chromic acid con-centration should be reduced to approximately 100g/L by dilution. A highly crosslinked macroreticularresin such as AMBERLYST 15WET, which has highoxidative stability, may be suitable for this applica-tion. Under these conditions, the resin has an operat-ing capacity of about 0.7 eq/LR

(1) at 3 BV/h(2) flowrate and a regeneration level of 300 g HCl per liter ofresin using 10-15% HCl (see figure 2).

Chromic acid baths used for aluminium anodiz-ing are easier to process because they are lessconcentrated. They contain about 100 g

CrO3/L and 10 g Al2O3/L as impurities and thereforeno dilution is necessary. AMBERLYST 15WET canagain be used in this application. This resin can treatabout 4 BV of such a solution with less than 1 gAl2O3/L average leakage. The same flow rates andregenerant levels are used as for chromium plating.

Phosphoric acid solutions can also be rejuvenat-ed with IER. Phosphoric acid is used to passivate andphosphatate iron and steel. These solutions haveupper limits for iron content above which the solu-tion becomes exhausted.

A common practice is to remove part of this solu-tion and pass it over a strong acid cation (SAC) resinlike AMBERLYST 15WET whereby iron isexchanged for H+ and the purified phosphoric acidreturns to the process. Operating conditions are sim-ilar as with chromic acid purification describedabove. The resin can remove up to 35 g Fe2+ per literof resin. Regeneration is done with 2 BV of 10-15%HCl or 10-15% H2SO4. Another case is the purifica-tion of HCl pickling solutions, contaminated withFe2+ and Zn2+. At these chloride concentrations, Zn2+

forms an anionic complex, ZnCl42-. Iron (Fe2+) forms

such complexes but at very high HCl concentrations(> 9 N) while Fe3+ forms such complexes at about 1.5N HCl. If both Zn2+ and Fe2+ are to be removed, Fe2+

is first oxidized to Fe3+. The HCl is then purified bypassing through a strong base anion exchange resinin the chloride form whereby both ZnCl4

2- and FeCl4-

are exchanged for the Cl-. Regeneration is carried outwith water, thereby dropping the HCl concentrationinside the resin beads and “breaking” the anioniccomplexes allowing Zn2+ and Fe3+ to come off theresin.

Gel type strong base anion (SBA) resins such asAMBERLITE IRA402 Cl, AMBERJET 4200 Cl,AMBERJET 4400 Cl or AMBERLITE IRA400 Clcan be used. The operating capacity depends on boththe Fe3+ and HCl concentrations.

Generally the higher these concentrations, thehigher the operating capacity. Operating capacitiesup to 35 g Fe per liter of resin can be achieved. Flowrates range from about 2-5 BV/h and regeneration isachieved with 4-8 BV of water. Figure 3 illustratesthe water elution curves for Fe and Zn usingAMBERLITE IRA402 Cl resin.

(1) eq/LR: equivalents per liter of resin(2) BV: Bed Volume, volume of solution per volume of resin

Figure 3: Water Elution of AMBERLITE IRA402 Cl

Flow rate : 2.5 BV/h

7

Rinse Water Recycling

Due to the use of different plating baths in thesame plant, the rinse waters can be acidic(plating baths) or basic (cyanide solutions).

The treatment of the rinse waters with IER is per-formed on the combined rinse waters (caution shouldbe exercised to insure that the pH is not acidic caus-ing formation of HCN), or separately on the acidicand the basic waters.

The concentration of the metal impurities shouldrange at about 1-2 meq/L for this process to be eco-nomically feasible. Typically, only the final rinsewaters are recycled, as shown in Figure 4.

8

Rinse

ChemicalTreatment

Rinse

Spent Regenerants

Effluent

Regenerants

PlatingBath

H O2

IonExchange

Resins

Figure 4 : Rinse Water Recycling with IER

Figure 5 illustrates the recycling of chromic acidplating rinse waters. The composition of suchwaters is similar to that of the plating bath

example given previously, but at a lower concentra-tion. These waters contain around 20 to 100 ppm ofchromates with metal impurities (Cu2+, Fe3+ andCr3+) at 5 to 20 ppm levels. The IER system involvesa SAC and a weak base anion (WBA) exchanger. Thewaste regenerant from the WBA resin, which con-tains the recovered chromates, is passed through aSAC exchanger in the H+ form which converts thechromates to chromic acid. This is subsequently con-centrated and recycled back to the plating bath.

A macroreticular SAC resin such as AMBERLYST16WET or a gel type resin such as AMBERLITEIR120 H can be used. In some installations,AMBERLYST 16WET is preferred due to itsmacroreticular structure which gives it increasedoxidative stability compared to the gel type resins. Inaddition, better removal of cationic detergents isachieved with this resin.

9

RinseTank

ChromeBath

Filtration

To Chemical Treatment

Amberlyst16Wet

Amberlyst16Wet

AmberlystA21

HClRegen.

NaOHRegen.

HClRegen.

Figure 5 : Acidic Rinse Water Recycling with Chromic Acid Recovery

Some chromates (Cr6+) may be reduced on the WBAresin to Cr3+ which can then precipitate on the resinas hydroxides. For that reason, it is recommended toperiodically perform an acid clean-up of both theSAC and WBA resins with 15% HCl.

Similar considerations apply for recycling rinsewaters from the passivation of steel metals withphosphoric acid. A typical composition of suchwaters may be 50-100 ppm of PO4

3- with 5-30 ppmof metal impurities such as Fe2+, Mn2+ and Zn2+. Aweak base anion exchange resin such asAMBERLYST A21 can again be used under similaroperating conditions and performance, as above.

The basic or mixed rinse waters can contain thefollowing ionic impurities:

The flow rate during service cycle can be ashigh as 20 BV/h. The operating capacity is0.8 to 1 eq/LR when the resin is regenerated

with 100 to 120 g HCl per liter of resin using 6-10 %HCl in a co-current mode. If H2SO4 is used as regen-erant, a 150 g per liter of resin level should be usedas a 10% H2SO4 solution. Rinse requirements areabout 5 BV. The regeneration level chosen dependson the quality of the recycled water desired. Forremoval of the strong acids, a macroreticular weakbase anion resin such as AMBERLYST A21 is rec-ommended. A typical operating capacity for CrO4

2- isapproximately 1 eq/LR when regenerated with 2 to2.5 eq of NaOH/LR (4% NaOH solution). Rinsing isdone with 5 BV of decationized water. In general,with the above regeneration levels for the SAC andthe WBA resins, a conductivity of about 30 µS/cm isobtained after the WBA resin.

10

Cations Anions

Cr3+ CrO42-

Fe3+ Fe(II)(CN)64-

Pb2+ Fe(III)(CN)63-

Cu2+ Cu(I)(CN)43-

Ni2+ Ni(CN)42-

Zn2+ Zn(CN)42-

Cd2+ Cd(CN)42-

PO3-

SO42-

citrates, acetates, tartrates

The same SAC and WBA resins can be used in thissystem as in the preceding one.

Strong base anion resins such as AMBERLYSTA26 OH can be used for this application. In thehydroxide form, these resins will remove all weakacids that leak through the WBA resin, such as HCN.Typically an operating capacity of 0.3 to 0.4 eq/LRcan be achieved using a 100 g NaOH per liter of resinregeneration level (4% NaOH solution). Five BV ofdecationized water is used for rinsing. A conductivi-ty of about 5 µS/cm is obtained after the SBA resin.

Separate regeneration of the WBA and the SBAresins is recommended in order not to mix the CrO4

2-

with the CN- streams.

11

Filtration

RinseTank

To Chemical Treatment

HClRegen.

NaOHRegen.

AmberlystA26 OH

AmberlystA21

Amberlyst16Wet

Figure 6: Mixed or Alkaline Rinse Waters Recycling

(3) T.V. Arden and F. de Dardel, “Techniques de l’Ingénieur”

Figure 6 illustrates the IER system used to treatthese rinse waters. The cationic metal impurities areremoved by the SAC resin. The following selectivi-ties of standard gel-type SAC resin are generallyobserved (3) for a cation with respect to H+ :

H+ < Na+ < NH4+ < Mn2+ < K+ < Mg2+ < Fe2+ < Zn2+

< Co2+ < Cu2+ < Cd2+ < Ni2+ < Ca2+ < Sr2+ < Hg2+

<Ag+ < Pb2+ < Ba2+

The selectivities of the WBA resin for the anions areas follows:

F- < oxalate < Cl- < SCN- < NO2- < NO3

- < SO42-

< CrO42- < Ni(CN)4

2- < Cu(CN)43- < Zn(CN)6

4-

< Fe(CN)64- <OH-

Waste Effluent TreatmentFigure 7 illustrates a chemical treatment of galvanicrinse waters. Cr6+ (CrO4

2-) is reduced to Cr3+ whilethe cyanides, CN-, are oxidized to CO2 and N2. TheIER process is followed by a neutralization and pre-cipitation step where the metallic precipitate is fil-tered, pressed and disposed as solid wastes. Since thefinal effluent may still contain some low levels ofheavy metals, a polishing unit containing a selectiveIER such as AMBERLITE IRC748(4) resin is fre-quently included.

The filtrate after the decantation step containsabout 5 to 20 ppm of heavy metals, in addition tohigher concentrations of Na+ and Ca2+ salts, in theorder of several grams per liter, coming from the neu-tralizing chemicals.

The stream can be treated by a selective chelatingresin, such as AMBERLITE IRC748, usually in atwo-columns-in-series, merry-go-round configura-tion, which can bring the total concentration of met-als below 0.1 ppm. AMBERLITE IRC748 has amacroreticular styrene divinylbenzene matrix with

12

(4) This resin and its use in this type of process are coveredunder US Patent # 5,804,606, Rohm and Haas Company

Filtration

CyanideRinse

WatersNeutralization Tank

Decantation

ChromateRinse

Waters

AmberliteIRC748

Reducing agent HCl NaOH or

Ca(OH)2

Oxidizingagent

AmberliteIRC748

Figure 7: Waste Effluent Treatment with Selective Resins

iminodiacetic functional groups which can formstrong complexes with transition or other metals. Thestability of these complexes is in general in the sameorder as the complexes of the metals with EDTA. Forthis reason, if the metals are found in the solution ascomplexes with EDTA, they can not be removedeffectively by AMBERLITE IRC748. Similarly, met-als such as Zn2+, Ni2+ and Cu2+ complexed with CN-

will not be removed by AMBERLITE IRC748. How-ever, in the presence of other complexing agents suchas citric acid, AMBERLITE IRC748 efficientlyremoves metal impurities.

The relative selectivities of several metal ions ata pH of 4 are shown below:

Ca2+ < Mn2+ < Fe2+ < Co2+ < Cd2+ < Zn2+

< Ni2+ < Pb2+ < Cu2+ < Hg2+ < Fe3+

The affinity for H+ in the above sequence is situ-ated somewhere between Pb2+ and Cu2+. Conse-quently, for the metals with selectivities less thanCu2+, the resin should be in the salt form (for exam-ple in the Na+ form) otherwise a relatively high metalleakage may be experienced.

Because of the high preference ofAMBERLITE IRC748 for the metals, thisresin can remove metals from solutions even

in the presence of high concentrations of sodium orcalcium salts, with very low metal leakage (less than0.1 ppm with two columns in series). The loadingstep can be done at a flow rate of 20-30 BV/h whileregeneration can be performed at 90 g HCl per literof resin regeneration level using 5 to 10% HCl, or theequivalent quantity of H2SO4.

After the acid elution, the resin is converted backto the Na+ form. Depending on the NaOH quantityper liter of resin, the conversion to the Na+ form maybe total or only partial. The degree of conversion ofthe resin to the Na+ form as well as the influent com-position will affect the effluent pH. In applicationsinvolving metal removal, with solutions containingmainly Na+ salts, the resin is converted to about 50%in the Na+ form. In that case, the effluent pH initial-ly is slightly acidic and it becomes progressively neu-tral as the resin is loaded with the influent Na+ andCa2+.

With solutions containing mainly Ca2+ salts, theresin is converted more fully (70% or more) in theNa+ form. In that case, the effluent pH initially isalkaline and becomes progressively neutral as theresin is loaded with the influent hardness ions. Inorder to more uniformly distribute these 50 or 70%of the sites in the Na+ form, the resin should be con-ditioned under agitation (see Table below).

13

Regeneration Step

Direction Flow rate Time Quantity Level (BV/h) (minutes) (BV) (g/LR)

Rinse Downflow Water 3 60 3Acid regeneration Downflow 6 % HCl 3 30 1.5 90Rinse Downflow Water 3 60 3 Caustic regenerationa) 50% Na+ form 4 % NaOH - 0.7-0.8 28-32

Agitation 20Stand 20

b) 100% Na+ form 4 % NaOH 3 30-40 1.5-2 60-80Rinse Downflow Water 3 60 3

4

0

1

2

3

3.5

0 1000200 400 600 1200800 1400

2.5

1.5

0.5

Treated Volume (BV)

Leak

age

(mg/

L)

AMBERLITE IRC748 with 70% of the functionalgroups in the Na+ form, can treat about 770 BV to a0.5 ppm Zn2+ end-point at a flow rate of 20 BV/h.Figure 8 illustrates an example with an influent

solution containing 5 ppm of Zn2+, 5 ppm Ni2+

and 2 ppm Cu2+ in CaCl2 and NaCl back-ground (1 g/L as Ca2+ and 1 g/L as Na+) at a pH of 7.

14

Figure 8: AMBERLITE IRC748 - Typical Leakage Profile

1 g/L Ca2+, 1 g/L Na+, 5 ppm Zn2+, 5 ppm Ni2+, 2 ppm Cu2+

Flow rate 20 BV/h - pH 7

Zn2+

Ni2+

Cu2+

0

Treated Volume (BV)

Leak

age

(mg/

L)

0

1

2

3

4

4.5

3.5

2.5

1.5

0.5

1000200 400 600 1200800 1400 1600

Figure 9: AMBERLITE IRC748 - Effect of Flow Rate on Operating Capacity

Zn2+, 30 BV/h

Zn2+, 20 BV/h

Ni2+, 30 BV/h

Ni2+, 20 BV/h

15

AMBERLITE IRC748 resin has excellentoperating capacity for nickel (Ni2+), a metalwhich is more toxic than zinc (Zn2+) and for

which the tolerated limits in the effluents are lowerthan those of Zn2+.

Another very important feature of AMBERLITEIRC748 is fast kinetics, thus, an increase of the flowrate from 20 to 30 BV/h, results in only a smallchange in operating capacity, especially if the end ofthe cycle is fixed at a low metal concentration (Fig-ure 9). The operating conditions for the data in Fig-ure 9 were close to those of Figure 8.

Precious Metal Recovery

The high price of noble or precious metals suchas gold, platinum and silver has resulted in theuse of IER for recovery from various waste

solutions. Gold and silver as cyanide complexes arestrongly fixed on SBA exchangers, such asAMBERLITE IRA402 Cl, and are removedquantitatively. In fact, they are so strongly fixed thatthe resin is dried and incinerated in order to obtainthe precious metals. Since this resin can fix up to 150g Au per liter of resin, the value of the recovered goldrepresents many times the value of the resin and thisprocedure is now a common practice.

Silver can be removed from photographiceffluents as a thiosulfate complex, [Ag(S2O3)2]

3-. Itcan be removed by both SBA resins such asAMBERLITE IRA402 Cl as well as a high capacityacrylic WBA resin such as AMBERLITE IRA67.The elution of Ag from the resin is more efficientlydone with ammonium thiosulfate, using 4 BV of 2 M(NH4)2 S2O3. The resin therefore works in the S2O3 -[Ag(S2O3)2]

3- cycle. The operating capacity of theresin depends on several factors including the ratioof S2O3 / [Ag(S2O3)2]

3- in the influent. Even thoughinitially the operating capacity of AMBERLITEIRA402 Cl may be higher than that of AMBERLITEIRA67, the latter resin regenerates more easily.Operating capacities up to 30 g /LR can be obtained,depending on free S2O3 concentration.

Platinum metals can also be recovered as Cl-

complexes on AMBERLITE IRA402 Cl. Thus, Pd,Pt and Ir are removed by AMBERLITE IRA402 Cland recovered from the resin by incineration. Rh isonly fixed weakly and can be eluted with water.

Copper Recovery

Copper can be recovered from acidic solutionsand recycled back to the process by using twocolumns, in series, of AMBERLITE IRC748

with a third column in regeneration (merry-go-roundsystem). AMBERLITE IRC748 can operate in the H+

form with no prior conditioning required with NaOH.With two columns of AMBERLITE IRC748

operating in series at a flow rate of 20 BV/h, the leadcolumn can be run to exhaustion before leakage fromthe second column exceeds effluent criteria. Thethird freshly regenerated column can then be put inthe polishing position when the lead column is takenoff line for regeneration. This operating mode max-imizes the capacity of the resin while consistentlyinsuring that the effluent meets specifications.

Regeneration is performed with 1.5 BV of 10%H2SO4 at a flow rate of 2 BV/h. The first half (0.75BV) of the regenerant contains most of the copper atconcentrations around 40 g/L. It is possible to recov-er copper from this fraction and recycle the secondhalf of the spent regenerant for the next regeneration.

As an illustration, when treating a stream con-taining 1 g/L of Cu2+ (in the form of CuSO4) at a pHof 3, the lead column will have an operating capaci-ty of approximately 35 g Cu2+ per liter of resin in theH+ form when Cu2+ in the effluent from the lag col-umn reaches a level of 10 ppm. When regenerated asdescribed above, the first half (0.75 BV) of the spentregenerant will contain about 40-45 g Cu2+/L whilethe second half, containing only minor quantities ofCu2+, can be recycled.

16

0 5 10 15 20 25 30

Treated Solution (BV)

0

1600

1400

1200

1000

800

600

400

200

Fe C

onc.

(mg/

L)3+

Fe2+ is oxidized to Fe3+ using H2O2 and then thesolution is passed through AMBERLITE IRC748 inthe H+ form at a flow rate of 6.5 BV/h. It should bemade sure that there is no excess H2O2 in the solutionbefore passing through the IER in order to avoidresin oxidation. The average iron concentration in thetreated solution is decreased by about one half whilethe resin, even at these highly unfavourable condi-tions of high ionic background and low pH, removesabout 13 g Fe3+ per liter of resin. Regeneration was performed using 2 BV of 15%H2SO4.

Zn/Ni electrolytic solutions can also be treated,where Zn2+ and Ni2+ concentrations are around 50g/L each, Na2SO4 around 80 g/L, total iron around 1g/L and a pH at 1.5-1.9. Operating capacities of 8-10g Fe per liter of resin can be obtained under theseconditions.

Figure 10: Iron Removal from Galvanizing Solutionswith AMBERLITE IRC748 Resin

Purification of Galvanizing Solutions

The high metal selectivity of AMBERLITE IRC748especially for Fe3+, is used to purify galvanizingsolutions from Fe3+ impurities. Figure 10 illustratesthe Fe3+ leakage curve where AMBERLITE IRC748is used to remove iron impurities from a ZnSO4 gal-vanizing solution. The composition of this solutionwas:

Fe2+ 0.66 g/L

Fe3+ 0.92 g/L

Zn2+ 93.5 g/L

Ni2+ 0.22 g/L

Na2SO4 55.0 g/L

pH 1.72

Flow Rate = 6.5 BV/h

17

Summary Table of Rohm and HaasIon Exchange Resins

Product Name Type Ionic Functionality Applicationform

Amberlite® IR120 H Gel Hydrogen Strong acid Rinse water recycling

Amberlyst® 15WET MR Hydrogen Strong acid Plating bath rejuvenation

Amberlyst® 16WET MR Hydrogen Strong acid Rinse water recycling

Amberjet® 4200 Cl Gel Chloride Strong base Plating bath rejuvenation

Amberjet® 4400 Cl Gel Chloride Strong base Plating bath rejuvenation

Amberlite® IRA400Cl Gel, Type I Chloride Strong base HCl pickling solutions

Amberlite® IRA402Cl Gel, Type I Chloride Strong base HCl pickling solutions

Amberlyst® A26 OH MR Hydroxide Strong base Rinse water recycling

Amberlite® IRC748 MR Sodium Chelating Heavy metal removal

Amberlite® IRA67 Acrylic Free Base Weak base Silver removal

Amberlyst® A21 MR Free Base Weak base Rinse water recycling

18

Ion Exchange Resinsfor Metal Plating and Surface Finishing

Rohm and Haas Company5000 Richmond StreetPhiladelphia, PA 19137U.S.ATel: (1)-215-537-4060Fax: (1)-215-537-4157

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185, rue de BercyFrance

75579 Paris Cedex 12Tel: (33)-1 40 02 50 00Fax: (33)-1 43 45 28 19

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4-26, Motoazabu 3-ChomeMinato-ku, Tokyo 106-0046

JapanTel: (81)-3-5488-3100Fax: (81)-3-5488-3179

Rohm and Haas Company makes no warranties, either expressed or implied as to the accuracy of appropriateness of this data and expressly excludes any liability upon Rohm and Haas arising out of its use. We recommend that the prospective users determine for themselves the suitability of Rohm and Haas materials and suggestions for any use prior to their adoption. Suggestions for uses of our products or the inclusion of descriptive material from patents and the citation of specific patents in this publication shouldnot be understood as recommending the use of our products in violation of any patent or as permission or license to use any patents of the Rohm and Haas Company.

Material Safety Data Sheets (MSDS) outlining the hazards and handling methods of our products are available upon request.

AMBERJET, AMBERLITE, AMBERLYST are trademarks of Rohm and Haas Company, Philadelphia, USA.

Metal01 - December 1999

Web Site: http://www.rohmhaas.com/ionexchange