Pyrometallurgical Recovery of Indium from Dental Metal Recycling Sludge

by Chlorination Treatment with Ammonium Chloride

Osamu Terakado*1, Takashi Saeki*2, Ryoji Irizato*3 and Masahiro Hirasawa

Department of Materials Science and Engineering, Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan

In the present paper we address the novel chlorination process for recovery of indium selectively from dental metal recycling sludge whichcontains considerably high amount of indium. The process is based on the utilization of ammonium chloride, NH4Cl, as chlorination reagent. Itwas found that indium could be successfully recovered from the sludge in the form of volatile indium chloride by heating the mixture of sludgeand NH4Cl at the temperature of 400�C under inert atmosphere. The influence of process parameters, such as composition of NH4Cl, wasinvestigated in order to achieve high process efficiency. [doi:10.2320/matertrans.M2010020]

(Received January 21, 2010; Accepted March 3, 2010; Published April 15, 2010)

Keywords: indium, recovery, chlorination, dental metal recycling sludge, ammonium chloride

1. Introduction

Indium is a trace element in some zinc, lead, copper, andtin ores, and produced mainly as by-product of zinc refiningprocess.1,2) The global demand of indium metal is steadilyincreasing with the increase in the production of flat paneldisplay, in which indium is used in transparent electricalconductor film as indium tin oxide, ITO. Recently, indium isalso utilized as an alloying element of lead-free electronicsolders. The establishment of the recovery process fromindium-containing waste is, therefore, of great importance.Various techniques have been proposed so far, mainly basedon the hydrometallurgical processes.3)

Among the wastes containing indium, dental metal recyclesludge is one of the attractive indium resources. The sludge isa by-product of precious metal recycling from dental alloyscraps and contains �0:08mass% of indium in some cases.In the present paper we report the novel pyrometallurgicalmethod of the indium recovery from the indium-rich sludgebased on a selective chlorination reaction.

Chlorination process plays an important role in theextractive metallurgy. Typical chlorination reagents are Cl2or HCl gas and metal salts such as NaCl and CaCl2. Forexample, Ohwa et al.4) reported the development of acommercial plant for the production of high-purity metalssuch as gallium, indium and bismuth by chlorination processusing Cl2 gas as chlorination reagent. Besides the goodeffectiveness and usefulness of the processes, chlorine andhydrogen chloride gases are very corrosive in nature, so thatspecial attention must be paid for the leakage of the reactor.As for the metal salts, the handling of reagent is quite easy,but one needs higher treatment temperature, usually over1000�C, than that in the case of chlorine gas in order toachieve sufficient reaction rate. Thus, exploring alternativechlorination reagents that can be used at lower temperaturewith convenient handling is intriguing to establish simplerand effective process. As an example, Jena et al.5) examined

the chlorination of vanadium pentoxide at the temperaturebetween 553 and 788K by carbon tetrachloride gas, thatshows, however, also corrosive property.

In the present study we have successfully appliedammonium chloride as chlorination reagent for the chlorina-tion of indium in the sludge. The process is based on thethermal decomposition of NH4Cl into NH3 and HCl gases,and the consequent chlorination of indium specie in thesludge. Indium chloride has high vapour pressure at�400�C,and it deposits at colder part of �280�C in the reactor.The treatment is especially profitable with respect to therelatively low treatment temperature (�400�C) as well assimpler handling of the reagent in comparison with thecases of conventional chlorination reagents. Various processparameters including the composition of the sludge–NH4Clmixture, reaction atmosphere, and deposition temperature arediscussed.

2. Experimental

2.1 MaterialsDental metal recycling sludge rich in indium was supplied

from Yamamoto Precious Metal Co., Ltd. A SEM image ofthe sludge is shown in Fig. 1. The morphology is quitecomplex: fibrous and spherical materials with different sizesare observed. For the analysis of the contents of metallicelements in the sludge, we have leached 600mg of the sludgewith 40mL of aqua regia, and the obtained solution has beenanalyzed with ICP spectrometer (SPS 1500VR, Seiko Instru-ments). The result of the quantitative analysis of somemetallic elements is summarized in Table 1. Obviously in thetable, the sludge consists of various compounds of metalelements. Typical phases of the sludge were characterizedby scanning electron microscopy with energy dispersivespectrometer, SEM-EDS, (JSM-6330F&JED-2140, JOEL) asshown in Fig. 2. The presence of SiO2, alkaline and alkalineearth oxides as well as the complex oxides of indium and tinwas observed.

2.2 Chlorination treatmentThe chlorination reaction was carried out in a quartz tube

equipped in a horizontal electric furnace. The size of the

*1Corresponding author, E-mail: teramon@numse.nagoya-u.ac.jp*2Graduate Student, Nagoya University. Present Address: Nagoya Railroad

Co., Ltd., Nagoya 450-8501, Japan*3Graduate Student, Nagoya University. Present Address: BOSCH Co.,

Ltd., Tokyo 150-8360, Japan

Materials Transactions, Vol. 51, No. 6 (2010) pp. 1136 to 1140#2010 The Japan Institute of Metals

quartz tube was inner diameter of 26mm, outer diameterof 30mm, and the length of 800mm. Samples for thechlorination reaction were prepared by mechanical mixing ofthe sludge and ammonium chloride powder (99.5%, Wakopure chemicals). The total weight of the mixture was 600mg.The powder mixture was pelletised with the load of ca.2:5� 108 Pa. Reactions were conducted under helium(99.9999%) or air gas flow with the flow rate of 100mL/min. After the temperature of high temperature zoneof the furnace was stabilised, sample mixture contained in amullite reaction boat was introduced quickly from the coldpart to the reaction zone inside the reaction tube. Typicalreaction temperature was 400�C and the reaction time was30min, if not specified in the present report. A water trapwas set in the off-gas line connected to the reaction tube inorder to collect the evolved gas.

After a reaction run the condensation of yellowish materialwas observed at the cold part of the quartz reaction tube.This material is hereafter denoted as reaction products.The product was carefully leached by pure water or dilutehydrochloric acid aqueous solution (ca. 0.6M). The solutionwas then filtered with mixed cellulose filter paper with theaverage pore size of 0.1 mm. The amount of metal ions in thefiltrate was analyzed by ICP spectrometer. The amount ofammonium and chloride ions were also measured for thewater-leached solution of the reaction products, the watertrap as well as the water-leached solution of the residueremaining in the reaction boat with an ion meter (IM-40,TOA-DKK).

3. Results and Discussion

3.1 Chlorination treatment with ammonium chlorideTable 2 shows the typical results of the ICP analysis for

the reaction products. They are presented in terms of metal

concentration of the water-leached solution of the reactionproducts, whereby the sample solution is prepared to the totalvolume of 100mL precisely. The deposition of reactionproducts occurs obviously at reaction temperature above280�C in the presence of ammonium chloride. The analytical

300 µ m

Fig. 1 Representative SEM image of the sludge employed for chlorination

reaction in the present study.

Table 1 Metal content of supplied sludge (in mg metal/g sludge, dry matter).

In Pd Pt Sn Cu Ni Co Ag Au Ti

78� 4 1:9� 0:2 0:5� 0:2 38� 2 22� 1 10� 1 2:0� 0:2 1:5� 0:3 1:4� 0:1 0:70� 0:03

10 µ m

Na

OCa

10 µ m

Sn

In O

Si

O

10 µ m

Fig. 2 Elemental mapping of representative phases of the sludge.

Pyrometallurgical Recovery of Indium from Dental Metal Recycling Sludge by Chlorination Treatment with Ammonium Chloride 1137

results of the amount of Cl� and NH4þ in the water-leached

solutions of reaction residue and reaction product as wellas in the water trap, together with the value of pH, aresummarized in Table 3. A reasonable mass balance has beenobserved for the ions in the present reaction. The results showthat the main product in the water trap is obviously ammonia,while the reaction residue contains mainly HCl. The reactionproduct is considered as indium chloride together withhydrogen chloride.

It is well known that ammonium chloride decomposes intoHCl and NH3 above 332

�C, and the latter defuses faster thanthe former decomposition product. The reaction mechanismis thus summarized briefly as follows: (1) decomposition ofNH4Cl, (2) evaporation of ammonia, (3) chlorination ofindium, and the consequent evaporation (with the simulta-neous HCl evaporation) and the deposition at the cold part ofthe reaction tube.

3.2 Influence of the concentration of ammonium chlo-ride

One of the most important process parameters forchlorination treatment is the partial pressure of chlorine aswell as that of oxygen. This can also affect the concentrationof impurities in the reaction product, most probably tinchloride. We have, therefore, studied the dependence ofcomposition of the ammonium chloride in order to examinethe influence of the partial pressures. Figure 3 shows theresult of the recovery ratio, i.e. 100 � (Recovered amountof metal)/(Initial amount of metal in the sludge sample), ofindium and tin as a function of concentration of ammoniumchloride in the initial mixture. The experiments wereperformed at 400�C for 30min under helium atmosphere.Clearly, the recovery ratio of indium increases withincreasing amount of the chlorination reagent. As for theinfluence of the reaction atmosphere, the recovery rateof indium decreases at higher oxygen partial pressure,as shown in Fig. 4 for the reaction under helium and airatmosphere.

An interesting observation is the different leachingbehaviour of tin and indium by pure water and HCl solution.While the leaching behaviour of indium does not change bydifferent leaching solutions, tin could not be detected by ICPmeasurement after leaching with pure water. This is due tothe formation of white colloidal precipitates during theleaching treatment; the precipitates are removed from theleached solution by filtration prior to ICP measurement.The compounds are water-insoluble tin oxide resulting fromthe hydrolysis of tin chloride in water solution of pH �3

(Table 3). On the other hand, the precipitation of tincompounds does not occur in the acid-leached solution.The result indicates the possibility of separation of indium

Table 2 Representative results of the recovery of metals at different reaction conditions (in mg/L�).

Composition (mass%)

(Sludge : NH4Cl)

Reaction temperature

(�C)In Pd Pt Sn Cu Ni Co Au Ti

2 : 3 400 152.7 n.d. 0.2 n.d. 1.3 n.d. 0.2 n.d. 0.1

2 : 3 280 0.8 n.d. 0.2 n.d. n.d. n.d. 0.2 n.d. 0.0

1 : 0 400 n.d. n.d. 0.2 n.d. n.d. n.d. 0.2 n.d. 0.0

�Metal concentration in aqueous solution where reaction products from sludge sample of 600mg were collected with pure water and the ICP sample was

prepared to the volume of 100mL.

Table 3 Results of Cl�, NH4þ and pH of the leached solutions of residue

and reaction product as well as water trap�.

Cl� NH4þ pH

Residue leached with pure water 39� 1 0:3� 0:0 4.4

Reaction product leached with pure water 61� 2 31� 2 2.8

Water trap 12� 1 55� 3 10

Total 112� 2 87� 1

�Results for ions are presented as (amount of ion)/(total amount the

corresponding ion in the initial sample) � 100.

0 4020 60 80 1000

20

40

60

80

100

In, Leaching with HCl soln.In, Leaching with H2OSn, Leaching with HCl soln.Sn, Leaching with H2O

NH4Cl (mass%)

Rec

over

y ra

te (

%)

Fig. 3 Recovery ratio of indium and tin as a function of NH4Cl

concentration in the mixture. The products are leached with either pure

water or dilute hydrochloric acid aqueous solution. The reaction is

conducted at 400�C for 30min.

0 20 40 60 80 1000

20

40

60

80

100

In, HeliumIn, Air

NH4Cl (mass%)

Rec

over

y ra

te (

%)

Fig. 4 Recovery ratio as a function of ammonium chloride composition

under helium and air reaction atmosphere (Reaction temperature at 400�C,

Reaction time = 30min). The products are collected by pure water.

1138 O. Terakado, T. Saeki, R. Irizato and M. Hirasawa

and tin by leaching of chlorination product with water andthe consequent filtration.

Thermodynamic aspects of the chlorination reaction ofthe sludge have been investigated with the software packageFactSage Ver.6.0 (Thermfact and GTT-Technologies).Figure 5 shows potential stability diagrams of the In-Cl-Oand Sn-Cl-O systems at 400�C. We have also evaluated theoxygen and chlorine partial pressure for sample mixture withcomposition of 50mass% of NH4Cl under inert atmosphere,assuming the coexistence of indium and tin solid oxides,namely In2O3 and SnO2, in the sludge. The calculation resultof the partial pressure is displayed as circle in the figure.As clearly seen in the figure, both indium and tin can bechlorinated under the equilibrium condition. The increasein oxygen partial pressure is, on the other hand, unfavour-able for the formation of indium and tin chloride. Thethermodynamic consideration can explain the experimentalobservation of the influence of the reaction atmospherereasonably. The potential phase stability diagram indicatedthat selective chlorination of indium was difficult, so thatother possibility of the selective recovery of indium shouldbe considered.

3.3 Possibility of the selective recovery of indium bycontrol of the condensation temperature

The influence of condensation temperature of the reactionproduct was examined. For this purpose we have collectedthe reaction products with HCl solution at different positionsalong the reaction tube; the average temperature of eachposition was measured prior to the reaction. Figure 6 showsthe recovery ratio of indium and tin and the correspondingtemperature along the reaction tube. The chlorinationreaction was carried out at a temperature of 400�C for thecomposition of ½sludge� : ½NH4Cl� ¼ 3 : 2 by weight. Theseparation of indium and tin inside the reaction tube isobvious in the figure. Indium chloride is mainly collected at270�C, while the tin chloride is collected at lower temper-ature (�250�C). Figure 7 shows the vapour pressure ofsome indium and tin chlorides as a function of temperature,that is calculated from thermochemical data.6) At temper-atures between 200 and 300�C, the vapour pressures of tinchlorides are higher than those of indium chloride. Thus,selective collection of indium is possible with the control ofthe deposition temperature.

With respect to the reaction mechanism, our preliminaryexperiment of the chemical analysis of tin products showedthat they deposited in oxidation state 4 even for the productformed in helium atmosphere. At the tin-rich depositiontemperature, i.e. �250�C, tetravalent tin chloride should

-50 -40 -30 -20 -10 0-50

-40

-30

-20

-10

0

-50 -40 -30 -20 -10 0-50

-40

-30

-20

-10

0

In (l)

InCl (l)

InCl3 (s)

In2O3 (s)log(

pC

l 2)

log(pO

2

)

Sn (l)

SnCl2 (l)

SnCl4 (g)

SnO2 (s)log(

pC

l 2)

log(pO

2

)

Fig. 5 Potential stability diagram of the In-Cl-O and Sn-Cl-O systems at

400�C. A circle inside the figure is the partial pressure of oxygen and

chlorine in the present experimental condition calculated with FactSage

software with assumption where indium and tin exists as pure solid oxides,

i.e. In2O3 and SnO2.

100 200 300 400-8

-6

-4

-2

0

2

4

6

InCl3

SnCl2

SnCl4

log(

P/P

a)

Temperature, T /°C

Fig. 7 Vapour pressure of some indium and tin chlorides as a function of

temperature that is calculated with thermochemical data.6)

14 16 18 20 220

20

40

60

80

100

Tem

pera

ture

, T/°

C

Distance from the center of the furnace, d /cm

Rec

over

y ra

te (

%)

InSn

220

240

260

280

Fig. 6 Recovery ratio and the corresponding average temperature at dep-

osition point (Reaction temperature = 400�C, Reaction time = 30min.)

Pyrometallurgical Recovery of Indium from Dental Metal Recycling Sludge by Chlorination Treatment with Ammonium Chloride 1139

evaporate, as indicated by the vapour pressure data shown inFig. 7. Further study is needed in order to clarify the reactionmechanism, since many factors are involved including theformation of ammonia, a possible reductant, that can play acomplex role in the reaction. Moreover, indium forms varietyof chloride compounds with complex valency and differentthermal properties which can affect the process performanceof the separation. The existence of tin compounds can alsoaffect the chlorination behaviour of indium.

3.4 Outlook of the processWe have experimentally shown the selective chlorination

of indium from the sludge by the reaction with ammoniumchloride at relatively low temperature (�400�C). The processis quite simple: satisfactory recovery of indium is possibleby heating of the simple mechanical mixture of sludge andNH4Cl powder. The handling of the chemicals is easy, andthe process does not require severe gas-tight reaction line incontrary with the processes using Cl2 or HCl gas. Further-more, in comparison with other salt type chlorinationreagents such as NaCl and KCl, the reaction temperature ismuch lower in the present system. A possible contaminationof tin can be eliminated by the leaching of chlorinationproduct with pure water as well as by a careful control ofcondensation temperature.

Moreover, there is another advantage of the presentprocess which results from the fact that both the decom-position products of ammonium chloride, i.e. acidic HCl andalkaline NH3, are employed in metal recovery. The formationof In(OH)3 is feasible by a treatment of reaction productswith ammonia aqueous solution, namely water trap in thepresent experimental setup. The hydroxide can be utilised byconventional metal recovery processes.

A disadvantage of the current process is that high amountof the chlorination reagent is required. More than 50mass%of ammonium chloride is needed to achieve the indiumrecovery ratio of 70%. The increase in the contact areabetween the sludge and the chlorination reagent can be a key

point for the improvement of the process efficiency.It is considered that the present simple method can be

applied not only for the sludge but also for the recovery ofindium from other kinds of indium-containing wastes.Further studies including ITO glass slide wastes are currentlyunder investigation.

4. Summary

In the present study we have shown that ammoniumchloride can be successfully applied as chlorination reagentfor the recovery of indium from dental metal recyclingsludge. Heat treatment of the mixture of sludge andchlorination reagent at 400�C results in a sufficient indiumrecovery. This simple process has potential application forthe recovery of indium from other waste. Further fundamen-tal researches such as reaction kinetics are required tooptimize the process parameters.

Acknowledgements

Part of the present work was supported by a grant-in-aidfor scientific research (K2130) from the Ministry of Environ-ment, Japan.

REFERENCES

1) A. M. Alfantazi and R. R. Moskalyk: Minerals Eng. 16 (2003) 687–694.

2) C. E. T. White and J. A. Slattery: Hydrometallurgy of Copper, Its

By-products, Rarer Metallurgical Processing Dallas Symposium, ed. by

L. A. Haas and D. R. Weir, (Society of Mining Engineers of the AIME,

New York, 1983) pp. 95–102.

3) A. P. Paiva: Sep. Sci. Technol. 36 (2001) 1395–1419.

4) H. Ohwa, A. Yukinobu, J. Nabeshima and M. Yasukawa: Extraction

Metallurgy ’89, (Institution of Mining and Metallurgy, London, 1989)

pp. 885–898.

5) P. K. Jena, E. A. Brocchi and J. Gonzalez: Metall. Mater. Trans. B 36

(2005) 195–199.

6) I. Barin: Thermochemical data of pure substances, (VCH, Weinheim,

1995).

1140 O. Terakado, T. Saeki, R. Irizato and M. Hirasawa