The Electrochemical Recovery of Nickel from Plating Residues

P. Ramachandran, K.V. Venkateswaran and S. Visvanathan

In viewor1he increusing demhnd for nicul Qnd its compounds, the rwroery of nickel VCllues from any smmthlry source ;s of great importana, pi2rliculilrly in countries such 05 Indio where there is nodames/ic sourceof the metal. ~ /lui/dups (or nodilles) thAt accumulate during the nickel plating pro­cesses are one secondary 50urce of metal. To take advantage of this resource, an electro­chemical approach has been attempted to recouer the nickel from plating wastes ill the fann of nickel suI/aft. Theapproach involves selective QtJodic dissolution of the /lickLl nodules in Q sulfuric acid medium and crystafliznt ion of the resultant 'Iickel sulfate. Addity, nickel concentration in the ti«tr(}o lytealuJ thux/ent ofdissolution of the nodules have been examined with the intent of avoiding the loss of nickel at the CIlthodeand reducing copper contamination in the elec­trolyte.

34

INTRODUCTION

Very few countries produce primary nickel by exploiting sulfide, or lateritic, ore bodies. Many countries are totally dependent on the import of nickel and its com­pounds; this dependence is motivating a drive to exploit secondary sources of the metal. For example, secondary nickel can be obtained from a number of sources, including spent nickel oxidecatal yst, the bleed solution of a copperelectrorefining cell, sludge produced during the pickling of stainless steel, and scrap metal.

The buildups, or nodules, generated in the nickel plating process are another source. During plating, these nodules usually grow at the contact points where the articles to be plated are fixed in jigs. Accumulated in tonnage quantities. these nodules can conveniently be processed for the production of nickel or its pure salts. Further, these plating wastes are relatively dean compared to other secondary sources.

TREATMENT METHODS

In the chemical method ' that is commonly used for preparing nickel sulfate, nickel shot and powder are treated with concentrated sulfuric acid, which creates problems related to high-temperature handling, control of fumes and intake of impurities. Hence, the starting material should be of high quality.

The published infonnationu regarding the electrochemical preparation of nickel sulfate is scarce. In the literature, nickel in the form of a solid or powder and nickel­copper alloy or scrap material is used as the starting material. The sulfuric acid used is in the concentration range of 1 D-4U%. 1 t is necess.1ry that free acidity be no lower than 0.2-1 %. Hence, the amount of sulfuric acid used in the eledrolyte is limited. to slightly more than that stoichiometrically required to convert all nickel to nickel sulfate. At the crystallization stage, which is the last step in the process, the density of nickel sulfate electrolyte is around 1.5.

In recent work, nickel plating nodules have been investigated as a source for the electrochemical recovery of nickel in the form of nickel sulfate, which can be used lor captive consumption. Factors like current density, addity, nickel concentration in the electrolyte and the extent of dissolution of nickel nodules were examined.

EXPERIMENTAL PROCEDURES

The nickel nodules obtained from an electroplating unit could be conveniently grouped into four fractions: small lumps, bunches of nodules, nickel-coated copper wires, and foils and dust. Small, discrete lumps were found to be advantageous because of their purity and because they are easily handled. The other major fraction of the material (by weight> was in the form of bunches with clusler-Iike formations of nodules having phosphor·bronzerods (covered with plaslicsleeves) al the center. The wire fraction was troublesome because il not only contained minimum nickel, but also contaminated the end product with its higher content of copper. Conveniently, this fraction is physically separated from the bulk. The portion in the form of dusty, glittering foil was found to be voluminous.

Theelectrolytic cell consisted of a one-liter glass beaker, with an electrolyte of2.0 M H?04' The nickel nodules (in 100-200 g lots) were treated anodically in a perforated. titanium box covered with a tightly fitting terelync cloth bag; two stainless steel strips served as the cathodes. Direct current of 0.5-2.0 A was applied using a regulated d.c. power supply.

As the duration of electrolysis increased, nickel concentration in the electrolyte increased with consequent depletion of free acid. Electrolysis was continued until the solution was distinctly acidic when tested with pH indicator paper. The nickel concentration was monitored by periodical analysis of nickel content in the electrolyte using a disodium salt of ethylenediaminetetraacetate (EDT A) in a complexometric titration method . After electrolysis, the nickel sulfate solution was filtered to remove residues. Subsequently, the nickel sulfate solution was evaporated and the crystallized nickel sulfate was collected.

Ouring electrolysis, anode potential was measured with resped to a saturated calomel electrode (SCE). The reported values are corrected to hydrogen scale. The amount of copper contained in the nickel sulfate crystals was determined by atomic absorption spectroscopy.

JOM • June 1991

ELECTRODE REACTIONS

The p reparation of nickel sulfate is essentially based on anodic d issolution of nickel scrap. An understanding of the electrode processes associated with nickel dissolution and deposition gives a better insight into the practical problems and conditions followed in the experiment.

The dissolution process essentially comprises three major steps:4 active dissolution, passivation and reactivation.

In the active dissolution step, the nickel goes into solution under anodic polar­ization, and the concentration of nickel ion gradually increases in the electrolyte.

Ni --+' NP- + 2e- (I)

In prolonged electrolysis or with high anode current densities, the anode becomes passive, leading to an increase in cell voltage. Passivation is due to the change in mechanism from nickel dissolution to the formation of nickel oxide.

Ni + Hp --+' NiO + 2H' + 2e- (2)

In the reactivation step, the hydrogen ion concentration (a.cid concentration in the electrolyte) plays an important role in inhibiting the passivation through a chemical route.

NiO + 2H' --+' Nil- + Hp (3)

Moreover, the addition of chloride ion, which is specifically adsorbed on theanode, facilita tes reactivation in the following manner:

NiO + 2CI- -+- Hp --+' NiCl~ + 20H- (4)

INFLUENCE OF COPPER

The major contaminant of the nickel nodules is a varying percentage of copper with a maximum of 10%. On prolonged ;modic polarization, it is likely that copper goes into solution when the material is enriched in copper.

(5)

This reaction has a high positive potential (Eo = +0.35V) compared to the nickel disso­lution reaction (Ec = -O.25V) represented by Equation 1, so nickel is preferentially d issolved, forming nickel sulfate. Under extreme conditions, copper goes into solu­tion, and its enrichment in the electrolyte leads to copper deposition at the cathode.

Cu2• + 2e- -)0 Cu (6)

The other possible cathodic reaction is deposition of nickel from the electrolyte.

Nil· + 2e- -)0 Ni (7)

This reaction should largely be suppressed, as it would result in a poor yield of nickel sulfate.

OPERATING CONDITIONS

The acidity of the electrolyte plays a critical role in facilitating smooth anodic dissolution of nickel nodules and avoiding nickel deposition at the cathode. If the acidity is too low, conductivity is impaired and frequent replenishing of free sulfuric acid is required. Moreover, low acidity promotes the loss of nickel value through the cathodic reactions, with attendant disadvantages. On the other hand, very high acid concentration restricts the solubility of nickel sulfate and causes associated problems such assail crystallization at the anode su rface, thereby inhibiting further dissolutio n. Therefore, the ideal sulfuric acid concentration has been determined to be 2 M.

Table I lists typical operating conditions. The voltage indicated is the average value. In the initial period of the dissolution p rocess, the cell voltage was below LO V. The voltage increased grad ually as the free acid in the electrolyte decreased in the course of the p rocess.

Copper was also found to go into solution in trace amounts, particularly in the later stages of the dissolution process. The copper intake in the electrolyte and subsequent copper contamination of the nickel sulfate crystals depended mainly on the extent to which the nickel nodules were d issolved. The copper content was in the permissible limit when the anodic dissolution was continued up to a stage where approximately 50% of the initial weight of nickel material taken was consumed, as shown in Table II. Beyond this stage, higher intakeof copper in the electrolyte was found, resul ting in the deposi tion of copper in the form of powder.

Nickel concentration in the electrolyte beyond certain levels was found to pose operating problems. The anode potential, a major component of the cell volt~ge, remained low up to a nickel concentration of 100 gil (Table Ill), and it gradually increased up toa nickel concentration of 120 gi l. Thisobservation is in agreement with those of nickel matte leaching.5 Beyond that, the anode potential increased markedly. Various measures such as agitation of the electrolyte, periodic cleaning of the anode samples and further addition of acid were found to give only temporary ad vantage

1991 June . JOM

Table I. Typical Operating Conditions Parameter

Weight of Nodules HzSO. Concentration Current Apparent Anodic Current

Density Volume of Solution Duration Dissolution Current

Efficiency Cell Voltage Nickel Sulfate Crystals

Collected

Value

100 8 2.0M 1.0 A

200A/m2 SOO ml 48h

93-97% 1.0V

2308

Table II. Copper Contamination at DIfferent Stages 01 DIssolution

Stage of D issolution

25% of Material Consu med 50% of Material Consumed 80% of Materia l Consumed

Cu in NiSO. (wt,,,, )

0.0005-0.001 0.0015 0.055

Ta ble 111. Influence of Nickel Corw:entrafion In the Electrolyte

on Anode Potential Nickel Content

(gil)

100 120 140 160

Anode Potentia l (V)

- 0.5 -0.6 2-< >6

35

ABOUT THE AUTHORS, ____ _

P. Ramachandran received his Ph.D. in electrochemistry from Madurai-Kamaraj Uni­versity in 1988. He is currenlly a scientist at the Central Electrochemical Research Insti­tu te (CECAl). Karaikudi, India.

K.V. Vankat.swaran received his M.Sc. in analytical and inorganic chemistry from Ma­dras University in Bangalore, India, in 1970. He is currently a scientist at CECAL

S. Visvanathan received his Ph.D. in elec­trochemistry from Banaras Hindu University in 1969. He is currently heading the electro­hydrometallurgy division at CECAL

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with respect to voltage and smooth functioning of the electrolysis. The possible reason, as reported in a similar investigation6 on the electrolytic dis­

solution of nickel shot, is the formation of a highly resistant film of nickel sulfate salt at the anode surface, leading to a high cell voltage.

CONCLUSIONS

The electrochemical approach, where nickel waste materials like plating nodules are dissolved anodically, offers a clean and efficient means of recovering nickel- Anode passivation leading to high cell voltage, contamination by copper from the starting material and loss of nickel value in the cathodic processes are some of the operating problems identified . Selecting the proper acidity, anode current density and extent of dissolution largely overcomes these difficulties and allows the selective dissolution of nickel. Recovering nickel in the form of nickel sulfate permits the captive use of this plating chemical by metal finishing industries.

The success of the process depends upon classification of the raw material, selective dissolution of nickel to the maximum extent, and separate treatment of residue either by a dry procedure to enrich with nickel or a wet method to remove the copper. However, the economics of the residue treatment process must be considered.

ACKNOWLEDGEMENTS

The authors express their sincere thanks to Prof. S.K. RDngarajan, director, Central Electrochemical Research Institute (CECRI), Karaiktldi, India, for his constant encouragemtnt and lwm ilzterest in this work. The SZipfXlrt of MIs. T.l. Cycles of India is gratefully acbuJlolrdgrd.

References 1.0.11. Antonsm. -Nid<eJCompounds," h<),</optdioofOotmicoITtdlnoIogy,lrd ed .. "1>1. IS,ed. K.OIItmer(New Y""k:Job,n Wiley .. s.on.. 1981). pp. 801-819. 2. G. Ilkchio-ra.i. "Nickel s...Jl'h.I!~.-!ta&n pol 419.784 (1947);CIotrtt. AM .. 43(19491, p. 4S68i. 3. CP. Suppo, -lnorganIcSoIt:o." Frmch pol. 1.000.628 (1952);0-. AM .• 51 (1957). p. 243<1. 4. V. Ebonbo.cIo. l(s.:h"'~"" ~rod K. Rj...".. "On . ... Ki~""'''''P'''''ntion"fl"",-Ct.boll and Nkkd," f.1trtmt!timiao Act •. 12 (1 967). pp. 927-93/1. 5. F. Habuhi. ~ ~~llurgy""Sulphldfos in "'1""""" SoIutions.- Mi...,qs Sri. EnS .• 3(3) (1 971). pp.l-U . fo. S. Kamachar.dran .r.d N.V. P'rlhalU'l~y. -m-otyti< DisooIutionof NicbI in s...Jphuric Add.-~ Ago'" Ind ... 15(4) (] %4). pp. 552- 555.

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