Closed Recycling Process for Al Based Composite Materials

Closed Recycling Process for Al-Based Composite Materials

Hiroyuki Sano

, Shinichi Kato

, Tatsuya Motomura

and Toshiharu Fujisawa

1_{EcoTopia Science Institute, Nagoya University, Nagoya 464-8603, Japan}

2_{Department of Materials Science and Engineering, Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan}

Closed recycling process for low-grade scrap of Al-based composite materials was developed. Flux treatment with water-soluble halide is a key technique in this process. In the present work, flux treatment conditions were discussed from the viewpoints of separation and recovery of base material, reinforcement and flux. Optimum separation condition was obtained for the NaCl–KCl–KF flux treatment. The recovered aluminum alloy can be recycled for various usages. The recovered SiC particles cannot be reused as reinforcement because of the coexistence of contamination, therefore it is necessary to eliminate these contaminates or to find the other uses. The recovered flux is reusable with supplemental addition of fluoride.

(Received June 27, 2005; Accepted September 6, 2005; Published December 15, 2005)

Keywords: composite materials, recycling, aluminum alloy, potassium fluoride, flux treatment, recovery

1. Introduction

Composite materials demonstrate a more excellent char-acteristic in combining materials with various properties.1–3)

Therefore, they are being used in various fields. A particle reinforced Al-based composite material is one of the typical metal matrix composite materials.4) It is used for various components such as automotive brake disks, engine parts and so on, because of the lightweight, high thermal conductivity and high wear resistance.5)The application of this composite is expected to be expanded and, therefore, a large amount of by-products and waste will be generated from the manufac-turing process in the near future.

High-grade scrap can be re-melted and directly reused, while low-grade scrap cannot be re-melted because of the contamination of the aluminum melt by impurities such as oxides.6,7)_{Therefore, it is needed for the recycling of the}

low-grade scrap to develop the separation and recovery method of the base material and the reinforcement.8–10)_{However, it is}

not easily to separate the base material and the reinforcement, because the particle reinforced Al-based composite material is not produced by considering recyclability but by focusing on high performance. So an effective recycling technology for the particle reinforced Al-based composite material has not been established.

In the present work, scraping waste of the Al–SiC composite material, which is one of the typical particle reinforced composite materials, was selected as low-grade scrap. It was aimed to separate the scrap to the aluminum alloy, base material, and the SiC particles, reinforcement, by using a water-soluble flux treatment at elevated temperature, and to recover the both at reusable conditions.11) _The

conceptual flow of the proposed closed recycling process is shown in Fig. 1.

2. Experimental Method

Scraping waste generated in the manufacturing process of the automotive brake disks was used. It consists of Al– 8.5%Si alloy and 20 vol%SiC particles as reinforcement. Table 1 shows the water-soluble fluxes used in the present work and the experimental temperatures. In order to transfer the SiC particle to the flux, it is required that the wettability of SiC and flux is larger than that of SiC and aluminum alloy.

Al-Based Composite Material

(e.g. Al-SiC)

Base Material (e.g. Al Alloy)

Reinforcement (e.g. SiC) Water-Soluble

Flux

Flux Leaching in Water Water-Soluble

Flux Treatment Filtration and Drying

Residue Filtrate

Fig. 1 Conceptual flow of the proposed closed recycling process.

Table 1 Fluxes used in this research and experimental temperatures.

Water Soluble Flux Temp.,T/K

NaCl 1083

MgCl2 1023

CaCl2 1055

CaCl2–32 mass%NaCl 1023

CaCl2–30.4 mass%NaCl–5 mass%KF 1023

CaCl2–28.8 mass%NaCl–10 mass%KF 1023

NaCl–50 mass%KCl 1023 NaCl–47.5 mass%KCl–10 mass%KF 1023 NaCl–45 mass%KCl–10 mass%KF 1023

*1_{Graduate Student, Nagoya University. Present address: Taiho Kogyo}

Co., Ltd., Toyota 471-8502, Japan

*2_{Graduate Student, Nagoya University. Present address: Saganoseki}

Smelter & Refinery, Nippon Mining & Metals Co., Ltd., Oita 879-2201, Japan

However, it has no available data about wettability of SiC and flux.12,13) _{Thus, the flux was selected based on the}

following points, large solubility to water, low melting temperature, and low price. In case when the melting temperature of the flux is lower than 1023 K, experimental temperature was 1023 K, and in the other cases it was 10 K higher than the melting temperature of flux.

About 20 g of the flux was charged into a graphite crucible (I.D. 35 mm, L. 140 mm) and heated in an electric furnace at a desired temperature under an argon atmosphere. After the flux was melted, scraping waste of the Al–SiC composite material of the same volume as the flux was put into the flux melt and stirring by Al2O3 rod was continued for 3.6 ks

(250 rpm). Then, by settling for 3.6 ks, aluminum alloy and flux were separated by the specific gravity difference. The sample was poured from the crucible into a water-cooled copper mold to quench rapidly.

After the experiment, the sample was soaked in about 200 cm3 _{of deionized water at room temperature. Only the}

flux was dissolved in water, and the recovered aluminum

alloy was collected from the solution and the chemical composition of the alloy was analyzed by ICP-AES. Next, the water dissolving flux was filtrated to separate into the residues and the filtrate. The residues were dried and the mass was measured, then the carbon content was analyzed by the combustion coulometric titration method to calculate the amount of recovered SiC. Impurities other than SiC included in the residues were identified by the X-ray diffraction analysis and SEM-EDX. The flux was recovered from the filtrate by evaporating water. It was subjected to the X-ray diffraction analysis. The chemical compositions of flux before and after the treatment were analyzed by ICP-AES and the ion chromatography.

3. Results and Discussion

3.1 Recovery ratio of SiC

A SiC recovery ratio,RSiC, was defined by the following

equation:

RSiC=%

½Mass of SiC Transferred to the Flux

½Mass of SiC Initially Included in the Specimen100 ð1Þ

Figure 2 shows the SiC recovery ratio by each flux.

3.2 Treatment without fluoride

When the flux treatment was conducted by the flux that did not contain the fluoride (KF), the recovery ratio of SiC was less than 60%. In addition, the aluminum alloys were recovered only as granulated form [see Fig. 3(a)]. The surface of granules was completely covered by some depositions. The depositions were identified as SiC and Al2O3 by the X-ray diffraction analysis. It was considered

that the deposited SiC particles disturbed the coalescence of aluminum alloy. Thus, the recovery ratio of SiC was low and it was difficult to recycle the recovered aluminum alloy under these conditions.

3.3 Treatment with fluoride by the CaCl2–NaCl–KF flux

It is necessary to improve the wettability of the flux system in order to recover the lump of aluminum alloy and to improve the recovery ratio of SiC. Fluorides decrease the

interfacial tension between the aluminum alloy and the flux.14–16)Then, a new set of experiments was conducted by using fluoride-added flux. In consideration of the solubility to water, KF was selected as the additive fluoride.17)

When 5 or 10 mass% of KF was added to the CaCl2–NaCl

0 20 40 60 80 100

NaCl _MgCl2 CaCl

2

CaCl

2-32%NaCl

CaCl

2-30.4%NaCl-5%KF

CaCl

2-28.8%NaCl-10%KF

NaCl-50%KCl

NaCl-47.5%KCl-5% KF

NaCl-45%KCl -10%KF

Recovery Ratio of SiC,

RSiC

/ %

Fig. 2 Recovery ratios of SiC by various flux treatments.

10 mm

10 mm

(a)

(b)

10 mm

10 mm

Fig. 3 Appearance of recovered aluminum alloy (a) by the CaCl2–32%NaCl flux treatment, and (b) by the CaCl2–30.4%NaCl–5%KF flux

flux, the aluminum alloy was still recovered as granulated form the same as the treatment by the flux without KF [see Fig. 3(b)]. The recovery ratio of SiC was also not improved. The fluoride addition to the CaCl2–NaCl system flux was not

effective.

As the existence of CaF2in the flux was identified by the

X-ray diffraction analysis, it was considered that the follow-ing reaction took place durfollow-ing the flux treatment:

CaCl2ðlin fluxÞ þ2KFðlin fluxÞ

!CaF2ðlin fluxÞ þ2KClðlin fluxÞ ð2Þ

Therefore, the effect of the KF addition did not result in the improvement of the wettability.

3.4 Treatment with fluoride by the NaCl–KCl–KF flux

When 5 or 10 mass% of KF was added to the NaCl– 50%KCl flux, a lump of aluminum alloy was recovered (see Fig. 4). The recovery ratio of SiC was improved to almost

100% as shown in Fig. 2. In this flux treatment, it is considered that the SiC particles moved completely into the flux phase and the aluminum particles could coalesce into a large mass.

3.4.1 Recovered aluminum alloy

The recovered aluminum alloy did not have any deposi-tions on the surface [see Fig. 4(b)], and the SiC particle was not observed in the alloy (see Fig. 5). Table 2 shows the chemical composition of the aluminum alloy before and after the treatment with the NaCl–45%KCl–10%KF flux. The composition was not changed by the treatment. The flux did not contaminate the alloy because sodium and potassium were not detected. Therefore, it is considered that the recovered aluminum alloy is reusable to various usages.

3.4.2 Recovered flux

From the results of the ion chromatography and the X-ray diffraction analysis, it was found that the recovered flux did not contain the fluoride. The fluoride was consumed during the treatment. Therefore, the supplemental addition of fluoride is required before reusing the flux.

3.4.3 Recovered SiC particulate

Figure 6 shows the SEM photograph of the recovered SiC particles. Two kinds of materials adhered on the surface of the recovered SiC particles were observed, identified as Al2O3 and K2NaAlF6 by the X-ray diffraction analysis and

SEM-EDX. Al2O3was supposed to come from the oxide film

originally existed on the surface of the scraping waste and the oxidation of aluminum alloy during the treatment. It was considered that K2NaAlF6was produced by reacting the flux

with the aluminum alloy during the treatment:

(a)

(b)

10 mm

10 mm

10 mm

10 mm

Fig. 4 Appearance of recovered aluminum alloy (a) by the NaCl–50%KCl flux treatment, and (b) by the NaCl–45%KCl–10%KF flux treatment.

10

µ

m

10

µ

m

(a)

(b)

Fig. 5 Microscopic photograph of cross section of aluminum alloy (a) before and (b) after flux treatment.

Table 2 Chemical composition of the aluminum alloy before and after the flux treatment.

Concentration,Ci/(mass%)

Before After

Si 10.3 10.4

Mg 0.32 0.26

Fe 0.40 0.42

KFðlin fluxÞ þNaClðlin fluxÞ

!KClðl in fluxÞ þNaFðlin fluxÞ ð3Þ

4KFðl in fluxÞ þ2NaFðl in fluxÞ þAlðlÞ !K2NaAlF6ðsÞ þ2Kðg orlin AlÞ

þNaðg orl in AlÞ ð4Þ

K2NaAlF6 could be eliminated from SiC particles by

leaching in NaOH solution and its reaction was represented as follows:

K2NaAlF6þ4OH

!2Kþ_þNaþ_{þ6Fþ ½Al(OH)}

4 ð5Þ

However, an effective elimination method of Al2O3was not

found.

Because of the coexistence of Al2O3, the recovered SiC

particles cannot be reused as reinforcement. It is necessary to invent an appropriate elimination method of Al2O3or to find

the other uses.

3.4.4 Effect of the amount of KF addition

The effect of the amount of KF addition to the NaCl– 50%KCl flux on the form of recovered aluminum alloy and the recovery ratio of SiC was investigated. It was confirmed that at least 1 mass% of KF was required to recover the

aluminum alloy as a lump and to collect almost all of the SiC particles in the flux (see Fig. 7).

Figure 8 shows the effect of the amount of KF addition on K2NaAlF6content in recovered SiC particles. The amount of

K2NaAlF6 could be minimized with decreasing the amount

of KF addition.

3.4.5 Timing of the KF addition

From the above examinations, it was found that KF reacted with Al-alloy during the treatment. Then, minimization of the consumption of KF was considered. KF was initially mixed with the flux until now. Under the condition, most of KF had been consumed to the production of K2NaAlF6 during the

early melting and stirring stages.

As the next trial, KF was not mixed with the flux initially, but added just before the settling and separation. Figure 9 shows the effect of the timing of the KF addition on the appearance of the recovered Al-alloy. As mentioned before, when 0.5%KF was initially mixed with flux, Al-alloy was recovered as a granulated form [see Fig. 9(a)]. On the other hand, by adding KF later, the Al-alloy could be recovered as a lump even decreasing to 0.25%KF [see Fig. 9(b)]. There-fore, by adding KF later, the less amount of KF was enough to separate Al-alloy and SiC particle, and Al-alloy coalesces into a large mass. In addition, by decreasing the amount of

SiC particle

10

µ

m

K

2

NaAlF

6

Al

2

O

3

Fig. 6 SEM photograph of the surface of the recovered SiC particles.

10 mm

10 mm

10 mm

10 mm

(a)

(b)

Fig. 7 Appearance of recovered aluminum alloy (a) by the NaCl–49.75%KCl–0.5%KF flux treatment, and (b) by the NaCl–49.5%KCl– 1.0%KF flux treatment.

0

2

4

6

8

10

Amount of KF Addition, C

/ mass%

0

4

8

12

16

K

NaAlF

6

Content,

C

2

NaAlF

6

/ mass%

Fig. 8 Effect of the amount of KF addition to the flux on K2NaAlF6

KF addition, the production of K2NaAlF6was also decreased

as was discussed in the previous section.

4. Concluding Remarks

The aluminum alloy and the SiC particulate can be recovered separately from the scraping waste of the Al–SiC composite material by using the flux of the NaCl–KCl–KF system. Only small amount of KF is enough to achieve a good result (KF<1mass%). The recovered aluminum alloy can be recycled for various usages. Because of the coex-istence of Al2O3and K2NaAlF6, the recovered SiC particles

cannot be reused as reinforcement. It is necessary to eliminate the contamination or to find the other uses. The recovered flux is reusable with supplemental addition of fluoride.

Recycling process flow was proposed based on the present result and is shown in Fig. 10.

(1) NaCl–KCl flux is melted, then, the scrap of composite material is charged into the flux melt.

(2) Al-alloys exist as a granulated form in the flux melt. (3) Fluoride is added into the Flux.

(4) Then, Al-alloy and flux are separated into two phases. (5) Al-alloy is discharged from the furnace and recovered. (6) On the other hand, the flux is leached in water. In the flux phase, these compounds shown in the figure co-exist.

(7) Because these compounds are not dissolved in water, by filtrating this solution, NaCl and KCl are separated as aqueous solution.

(8) By evaporating the water, NaCl and KCl are recovered and reused.

0.25% KF

10 mm

10 mm

10 mm

0.5% KF

0.5% KF

10 mm

10 mm

10 mm

(a) Initially Mixed

(b) Adding Later

10 mm

10 mm

10 mm

Fig. 9 Appearance of recovered aluminum alloy after the treatment: (a) KF was initially mixed with the NaCl–50%KCl flux, and (b) KF was adding later into the flux.

(9) SiC, Al2O3 and K2NaAlF6 are leached in NaOH

solution.

(10) From the residue, SiC and Al2O3 are recovered, and

by evaporating the solution, KF and NaF are recov-ered. These fluorides can be reused as flux, because NaF has a almost equivalent ability to improve the wettability.

Acknowledgements

The authors are appreciated to AISIN TAKAOKA CO., LTD. for the financial support.

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