m 7 'LfU

r-9 r A FLUORIDE RESISTANT TITANIA CERAMIC ELECTRODE FOR METAL RECOVERY AND POLLUTION CONTROL

Steve Harnsberger Iren Romoda

Ebonex Technologies, Inc 5915 Hollis Street

Emeryville, CA 94608

Summary

Electrolytic metal recovery processes are playing a critical role in the growing waste minimization movement in the metal finiehing industries. In many effluents, free fluoride (HF) can be found as it is widely used in metal finishing processes. Free fluorides present particular difficulties for conventional anode materlals. The presence of free fluoride in effluents has restricted effective application of electrochemical treatment technologies in these media. A comparison of two conventional inert anodes. platinum clad niobium and iridium oxide coated titanium, with a new iridium coated titania ceramic electrode was undertaken to . demonstrate the effects of fluorides on these electrodes. The results indicate that the titania ceramic exhibits superior corrosion resistance and life to both previously available metal anode substrates opening up new opportunities for electrolytic technologies in fluoride environments.

Introduction

A . Anodes, Fluorides and Pollution Control

The EPA's recent restrictions on land disposal of F006 metal hydroxide sludge from metal finishing operations has lead to strong growth in the market for waste minimization technologies. Advanced treatment equipment, such as electrodialysis and electrowinning technologies, have become both commonplace and economical due to these restrictions and the rising costs and liability of sludge disposal.

Estimates of market growth in electrochemical treatment equipent in metal finishing range from 20% to 25% annually in these technologies as of 1989. The rapid expansion of electrochemical effluent processes results from the combination of the increasing availability of new or improved sophisticated process technologies and components, such as new high performance electrodes or membranes, combined with the growth in environmental awareness, concern and regulation.

A major problem in electrochemical metal recovery is fluoride

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containing electrolytes. Fluoride ion Is very aggressive and will attack most metals, even in low concentrations. Fluorides are used in etchant solutions just for this purpose. Many electroplating solutions contain fluoride as a catalyst to improve the plating process; the effect on anodes however Is deleterious. In a wide variety of electrolytes, one will find tree fluoride ion (HF). Table 1 summarizes some of the sources of fluorldes in metal effluents by industry, process and electrolyte.

Table 1

Source of Fluorides in Metal Finishing Effluents

Industry Process Electrolyte

Circuit Board Tin/Lead plating Fluoboric acid Manufacture Acid etch

Plastic etch Chromic HF regeneration

Aerospace Etching Nit r i c/HF Titanium/aluminum

Metal Finishing Pretreatment Sulfuric HF pickles

Post treatment HF Stainless steel pickles

Chrome Plating Chromic acid, HF 6r regeneration catalyst systems

PLATING of: Tin/Lead Lead Copper Nickel

Fluoborate based systems

Ion exchange Fluoboric acid regeneration

The most common electrode materials used as anodes in electrowinning and electrodialysis equipment include carbon, lead, stainless steel, precious metal coated titanium, and platinized niobium. In addition, a new corrosion resistant titanium oxide ceramic, coated with similar iridium oxide catalysts, has recently become available for use in these applications .

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B. Objectives and Definition of Program

The purpose of this study was to select and compare the relative performance of the most commonly used electrowinning anodes, platinum clad niobium and iridium oxide coated titanium, with the titania suboxides under controlled laboratory conditions in a synthetic, sulfuric acid based, fluoride containing solution.

C. Description of Electrode Materials in Test

The following three electrode materials w@re evaluated in the lab program:

1) Bare titanium, coated with iridium oxide. 2) Bare niobium (columbium), clad with platinum. 3) Bare titania suboxides, coated with irldium oxide.

Titanium and niobium have been selected for comparison as they represent the industry standard in terms of corrosion resistant, insoluble anode substrates for electrolytic metal recovery.

Table 2 compares the general properties of the three electrode materials in this test.

Table 2

Properties of Titanium, Niobium and Titania Suboxides

Property Titanium Niobium Titania

Density g/cc 4.54 8.57 4.3 (theoretical)

Porosity X - - 5-50

Resistivity microohm-cm 42.0 x 13.1 x <40

Linear Coefficient of Thermal Expansion (per 'C at 26'C) 8.5 x 7.1 x 10-6 6.3 x

Breakdown Potential 12-15 (volts)

>120 NA

Catalyst Coating Ir02 Pt Ir02

Application Method Thermal Clad Thermal

Coating Thickness 15 g/m2 250 microinch 15 g/m2

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1. Titanium Electrodes

Precious metal coated titanium, in this case thermally deposited iridium oxide, is used widely in mesh or sheet form as the anode in effluent treatment processes, especially electrolytic metal recovery. Titanium based electrodes have been reported for poor corrosion resistance In fluoride electrolytes due to substrate attack by free fluoride ions and have been reco mended for use in solutions with less than 1-2 mg/l free fluoride . A survey of recent manufacturers' literature shows that neither of the two prominent US suppllers of precious metal coated titanlum electrodes recommend its use generally in fluoride concentrations greater than 10 ppm.

The titanium electrode selected for comparison in this study was a standard titanium sheet, coated with a conventional commercially2available iridium oxide, oxygen evolving catalyst, with a 15g/m loading on both sides.

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2. Platinum Clad Niobium

Platinum clad niobium (Pt/Nb) is used as an alternative Insoluble anode by some equipment manufacturers for electrowinning in fluoride containing solutions; the general market reports Its improved performance over titanium. It is available as clad woven wire, sheet or expanded mesh with platinum cladding of 125 or 250 microinches on one or both sideg. The published data on its suitability in fluorides is limited . The electrode selected for this study was the premium version of Pt/Nb, a solid sheet of a 250 microinch platinum thickness, clad on both sides.

3. Titania Ceramic Electrodes: Description and Backqround

The ceramic material is based on the lower oxides of titania and is composed of a dafinitive group of crystalline structures known as "Magneli phases 'I of the general formula TinO where n is a whole number between 4 and 10. The relation&i$' between the electrical conductivity, corrosion resistance and substoichiometry of the suboxides is shown in Figure 1. The phase Ti O7 is the most conductive of lower oxides and in combination with Ti Og, the preferred stoichiometry. The chemical composition of th8 actual materials Ti509, TigOll and Ti7011, (Ti0 where n = 1.75-1.85).

A general review of the ceramic electrode technology for use in other electrochemical applications. such as electrochlorination, cathodic protection, electro r anic synthesis, and electrophoresis has been recently documented . The titania ceramic is manufactured using conventional and advanced ceramic process technology to produce a vitrified

in these tests is generally a mixture of T i q O 7 ,

n

s g

4

WEIGHT LOSS

0 4

0 0 4 .-

I I

I I I 0 0 0

d 4 4 4 - 4

n

OX F K -

W

X

-5-

titanium oxide bulk product in plate, tube or rod form, which Is subsequently reduced by firing at elevated temperature in a hydrogen atmosphere. The product therein converts from a white titanium dioxide to black suboxide during reduction, and is transformed from an Insulator into a material with a conductivity similar to graphite. A comparison of the resistivities of some commonly used materials Is presented in Table 3.

Table 3

Reslstlvltles of Electrode Materials

Material

A luminum Carbon (graphite) Copper Iron Lead Mercury Platinum Silver Ti tanium Zinc

Resistivity (Ohm-cm x

2.7 1375 (O'C) 1.7 9.7 20.6 98.4 (50'C) 10.6 1.6 42 -> 48 5.9

For comparative purposes, some relevant electrical resistivities of the titanium suboxides are:

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7

Ti02 1 x 10

Tiel. 9995 Tiel. 995 Tiel. 875 Tiel. 75 Tiel. 5

1 x 10

1.3 x 10

3.2 x 10

6

4

(Ti4O7) 630; (i.e. 630 x ohm-cm) 5 2.5 x 10

TI0 100

* Minimum resistivity for compositions in range TiOlS5 to T102.

The titania suboxides have an unusual combination of properties. These electrodes are chemically stable, can be reversed electro-

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chemically, are not readily oxidized back to titanha, and show no propensity to form hydrides as does titanium metal. Because they do not hydride, the titania suboxides may be used either as anodes or cathodes.

Table 4 shows that under current reversal, the titania suboxides, when coated with iridium oxide, will significantly outlast a conventional iridium oxide coated titanium.

Table 4

Comparative Life of Titanium and Titania Suboxide Electrodes Under Polarity Reversal

Elect rode Life Ti/Ir02 < 350 hours

Titania Suboxides with Ir02 > 7000 hours

Electrolyte 3% NaCl Reversal Cycle, every 4 hours Currenz density 100 ASF 15 g/m Ir02

The stability of the ceramic under current reversal is useful in electrochemical cells where buildup of mineral scale on the cathode is a problem, such as in sodium hypochlorite generators used in hard water. Current reversal can be used to dissolve the scale eliminating the need for acid cleaning of the cathodes.

In metal finishing applications, the stability of the ceramic under current reversal is significant as the ceramic will tolerate stray cathodic currents which are deleterious to titanium and it may be used either as an insoluble anode or as an insoluble cathode.

Applications for the ceramic electrode in metal finishing are broad outside of effluent treatment processes. With appropriate coatings and connections, the ceramic may be used in precious and base metal plating systems and other electrolytic treatment processes such as electropickling, anodizing, etc.

The titania suboxides are available in various grad-. Two of

comparison of the typical properties of these two ceramic grades is presented in Table 5.

these grades were placed in the fluoride testing program. A

Table 5

Typical Properties of Two Grades Titania Suboxide Electrodes

Bulk Density

Porosity

Resistivity

Chemical Composition

Flex Strength

Surface Area

~~ ~~

Unit EMM-39 EMA- 3 1

g/cc 3.9 3.1

46 "10 "25

mOhm-cm 30-40 5-10

Ti 6'1 lWTi7'13 Ti4O7-Ti5O9

MPa 35

m2/g 0.1

60

0.5

The titania ceramics tested were all standard materials which received no special treatment. The iridium oxide coatings were applied using conventional, unmodified catalyst coating techniques.

Electrode Evaluations: Testinq Program

The electrodes were tested for fluoride resistance in two general ways. Firstly, samples of the uncoated substrates, Nb. Ti and the two grades of ceramic were immersed in 15% sulfuric acid at ambient temperature to which fluorides had been added In the form of sodium fluoride to bring the free fluoride concentration to a 1 g/l level (1000 ppm). Samples were periodically removed, cleaned, dried and weighed. The percentage weight loss over time was calculated and plotted.

Secondly, samples of each of the electrodes with their precious metal catalyst coatings were polarized in the same solution at 100 amps per square foot (ASF) current density on a single face of the anode surface. Voltage, weight change and actual life were measured. Life of the electrode was considered terminated when the voltage climbed dramatically, typically in 1-2 days, and the electrode ceased to effectively pass current.

A. Immersion and Weiaht Loss

Frequently, electrode materials are left to soak in solution with

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no current applied either when a part is not plating, or when a metalwinning unit is shut down overnight or on weekends. In the unpolarized state, electrode substrates may be exposed to electrolytes as the catalyst coatings are often not impervious due to pinholes, cracks or damage, or the nature of the coating process itself. Thus the performance of electrodes unpolarized is an important measure of their usefulness particularly in that increased electrode wear may be experienced in the unpolarized state.

In Figure 2, the weight loss of uncoated ceramic (grade reference: EMM-39) versus uncoated titanium and niobium is shown. Titanium exhibits rapid weight loss as expected climbing to almost 40% in less than 1000 hours. Niobium shows an improvement over titanium but still has lost close to 13% of its weight after 3000 hours. The ceramic exhibits the best overall resistance to weight loss unpolarized, showing 30 times less weight loss over time than the niobium (at 3000 hours 0.4% weight loss).

A comparison of the impact on weight loss on titanium and the titania suboxides of increasing the fluoride concgntration in a nitric acid solution has been detailed by Hayfield in a nitric/HF etchant solution. It is shown in Figure 3 in terms of percentage weight loss over time. The data shows that the ceramic has superior resistance to attack by fluorides in concentrations which will completely dissolve titanium. At higher fluoride concentrations, the titania ceramic begins to show that it too will be subject to attack.

Differences can be seen in the relative weight loss rate of the ceramic in a comparison of the sulfuric versus nitric acid data when both contain 1000 ppm of free fluoride. The data indicates that the impact of the fluoride solution will depend not only on the fluoride concentration but on the acid or electrolyte in which the fluoride is contained.

B. Polarized Testing

While unpolarized weight loss is an important part of the measure of the comparative corrosion resistance of various electrode materials, the only true test of performance comes when current is applied to the catalyst coated substrate and the useful life assessed.

1. Ceramic Uncoated

The titania suboxides are capable of supporting a current for a period of time without any precious metal coating whatsoever while neither niobium nor titanium can be used in this manner. Therefore, although the ceramic is not typically used without a catalyst coating, it was decided to compare the performance of the two ceramic substrates without a coating to see the relative life

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FIGURE 2 UNPOLARIZED IMMERSION TEST

15% H2S04 w/ 0.1% HF 10

0

0) v)

- : - I O -10

g -20

w I 0, .-

be

-30 o Titania Suboxide Niobium

x Titanium -40

Time (1000 Hours)

1 I i I

v)

0 0 ln n

K In b

M 0 In

K In N

K 0

SSOl lH313M %

of the two types. Grade EMA-31 and EMM-39 were polarized at 20 ASF at ambient temperature in the sulfuric/fluoride solution. The ceramic grade EMM-39 showed its superiority in this solution lasting close to 1000 hours with the EMA-31 lasting approximately half the time.

2. Catalyst Coated Electrodes

In the next set of tests the three coated electrode materials, IrO /Ti, Pt/Nb, and IrO /Titania were polarized in the 15% H2S04, 1 g?’l free fluoride sol8tion at 100 ASF.

In order to accelerate the evaluation, 100 ASF current density was selected. Typical anode current densities run in commercially available electrolytic metal recovery equipment range from 10-50 ASF with most processes operating on average at or below 25 ASF.

3. Polarization Curves

As a baseline of performance, polarization curves were plotted for each of the coated electrode types and the uncoated ceramic in the above mentioned solution as shown in Figure 4 . The uncoated ceramic exhibits very high oxygen overvoltage characteristics, a fundamental property of the ceramic. The iridium coated titanium and the iridium coated ceramic have similar curves due to the fact that the measurements are looking at the characteristics of the iridium catalyst coatings which are nearly identical. The Pt/Nb shows higher oxide overvoltages as expected.

After the polarization curves, each of the precious coated electrode substrates was polarized and time until failure monitored. Each electrode failed in a similar manner with the voltage climbing rapidly in a day or two whereupon the electrode ceased to function.

The life of each of the electrodes is shown in Figure 5.

4 . Discussion

The iridium oxide coated titanium showed high voltage in the first day of operation and failed after 4 8 hours. Loss of iridium oxide coatings was observed visibly during the first day of polarization. White deposits were observed above the area of polarization.

The 250 microinch Pt/Nb, double clad electrode, operated for approximately 1500 hours before the voltage went up and failed. In testing reported by others, a Pt/Nb electrode would last approximately 6000 hours at 100 ASF in 16% H SO without fluoridesg platinum thickness . In comparison, it can be seen from this testing that the addition of the 1 g/l free fluoride to the H2S04 has caused

when coated with a 40 microinch double 2c18i

12

W 0 (n

(n 3 v, rY

9 W c3 e a -J 0 >

4.0 1 FIGURE 4

POLARIZATION CURVES (lox H,SO,)

1 UNCOATED

3.0

2.0

Id2 on

Commic

Monium

1 .o I I I I I I , , I t I I

1.0 10 0.1

CURRENT DENSIN (kA/m2 )

i I

a I LI Kl 6 0 0 7 W

0 00 (D F

n Z \ a 4

0 0 0 *

0 0 0 M

0 0 0 -

0

4

this 250 microinch double clad Pt/Nb, which had a coating five times thicker, to fail in less than one third the time.

The ceramic lasted close to 3500 hours before the voltage climbed rapidly and the electrode ceased to operate. A slight color change was observed from black to lighter gray where the ceramic had been exposed to the HF.

The use of iridium oxide catalyst coatings will significantly improve the performance and life of the titania suboxide because the oxidation of the substrate is impeded by the low overvoltage for oxygen evolution catalyzed by the precious metal oxide coating.

While not unaffected by the fluorides, tho ceramic shows the highest degree of resistance of the electrodes in these tests. Table 6 shows the life of the ceramic coated with iridium oxide exceeds 2000 hours at a much higher current density of 700 ASF in 10% sulfuric without fluorides.

Table 6

Accelerated Testing Ir02/Titania: Life in 10% H2SOq at 100 ASF

~ ~ ~

Ceramic Substrate Grade Life

EMU-39 > 2500+

EMA-31 > 2500+

+ Electrodes continue to run

Ir02 Coating Loading 15 g/m2

5. Fluoboric Acid

Because fluoboric acid is such a prevalent fluoride el$ctrolyte, subsequent samples of both ceramic grades with 15 g/m iridium oxide coatings were polarized at 100 ASF in 10% fluoboric acid at ambient temperature. While the testing is not yet complete, over 2500 hours of stable performance have been achieved on the ceramic, with the electrodes continuing to run.

Summary/Conclusions

The following conclusions can be drawn from the above:

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1) Iridium oxide coated titanium electrodes, as expected, are not suitable for use in fluoride environments, certainly not at the 1 g/l free fluoride level. Titanium is readily attacked unpolarized and incapable of supporting a consistent current for more than one day.

2 ) Uncoated niobium is attacked unpolarized in this H SO /HF solution and shows appreciable weight loss. Pt/Nb may be o6ergted in this fluoride environment for a period of time. The addition of free fluorides to sulfuric acid greatly increases the wear rate of the platinum and diminishes the life of this electrode by as much as 10-15 times.

3) The titania suboxide substrates exhibit a degree of chemical stability in the fluoride/sulfuric electrolyte when unpolarized, which is many times greater than that of niobium, the main commercial alternative electrode for use in electrolytic treatment processes in the fluoride environment.

In the polarized testing, the ceramic with the conventional iridium coatings lasted twice as long as the Pt/Nb under identical conditions. This type of performance would suggest it is technically competitive and superior to Pt/Nb in most fluoride solutions.

Field trials are in progress in actual fluoride solutions and are not yet complete. Data will be reported at a subsequent date.

4 ) Data for the performance of the three types of electrodes varies in different fluoride containing solutions. Therefore, careful analysis of electrode performance is required depending on both the type of acid, the concentration and level of free fluoride concentration.

5 ) The performance of the ceramic in fluoborio acid indicates that it is a technically feasible alternative anode for electrolytic metal recovery from fluoborate solutions.

6) The use of electrolytic treatment processes in fluoride solutions has been inhibited due to the lack of corrosion resistant anodes. The new titania ceramic shows technical feasibility for expanding the use of electrochemical technologies in waste minimization in fluoride solutions. New and improved catalyst coating!; developed specifically for the ceramic are certain to improve its performance.

7) Other properties of the t.itania suboxide electrodes, such as their stability under reverse currents, make them good candidate materials for use in plai i n g processes where improved, nonpolluting, insoluble e1ec:-odes are of interest due to the growth in process waste minimi-ation technologies.

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References

I ) us patent 4,422,917. 2) Hardee, K.L., Mitchell L.K.. Rudd, E.J., "Catalytic Anodes for

High-speed Electroplating and Electrogalvanizing," Plating and Surface Finishing, 76(4), 68, Apr. 1989.

3) Baboian, R., Camp, B.K., Skomorooski R.M., Zobbi R.Q., "Performance of Platinum Clad Tantalum Anodes in Electroplating," Plating, 11/73.

4) Magneli, A., Acta Cham. Scand. 13, 5, 1959 5) Clarke, R.L., Hayfield, P.C.S., "The Electrochemical

Characteristics and Uses of Magneli Phase Titanium Oxide Ceramic Electrodes," Spring Meeting of the Electrochemical Society, May 1989.

6) Ibid. 7) Ibid. 8 ) Baboian, R., "Clad Metal Anode for Electrowinning," 78th

National Meeting of AICHE, 8/20/74.

Acknowledqments

This paper is dedicated to Mr. P.C. Hayfield who has spent his career in the development of improved insoluble anode technologies and who is the inventor of the titania ceramics described.

The authors wish to exprees their thanks additionally to the following individuals who have contributed their talents and efforts to this paper:

M r . Neil Ormesher Mr. Richard Pardoe Mr. Jimmy Cheng Ms. Kim Bell

Imperial Chemicals Industries, PLC Ebonex Technologies, Inc. Ebonex Technologies, Inc. Ebonex Technologies, Inc.

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