Optimization of recycling of white bronze chips.

Jenicek, Stepan ; Jirkova, Hana ; Kucerova, Ludmila 等

1. INTRODUCTION

The recycling of metals has become an important issue in our society in recent years. The reason for this interest in recycling is its strong impact on the economy and the environment (ChriasteF et al., 2000).

Waste recycling in the original sense means to return the waste to the process which created it, so that it can serve its original purpose. The term 'recycling' is, however, also getting a new meaning. It is also used to describe new usage of production or consumer waste, materials and energies as new sources of secondary raw materials. Recycling is achieved by special recycling technologies, which usually consist of a set of successive production processes, procedures and operations that change the waste into raw material (Kristofova et al., 2003).

The chips produced when manufacturing sliding elements were chosen as the input material for this research. The reason was that this form of waste causes big problems when recycled by remelting. The large surface to volume ratio of the chips results in heavy oxidation of the charge and the metal does not remelt, but tends to burn out instead. The remnants of cutting fluid on the surface of the chips present another problem in the recycling process.

2. EXPERIMENT

Research work was carried out on the ternary alloy of zinc, aluminum and cuprum ZnAl35Cu5, which is also known under the commercial name ALZEN. This alloy is usually called white bronze due to its excellent sliding properties (Balicky et al., 1953; Michna et al., 2005).

2.1 First part of experimental work

Six packets were formed and remelted in the first step of the experiment. The large surface to volume ratio causes heavy surface oxidation of the chips in the furnace, so the chips were compacted by forming into packets. The surface of the packets also underwent great oxidation, however the core of the packets remained relatively intact. These packets were produced either from pure chips or from chips with the addition of charcoal, which was supposed to reduce oxidation of the charge in the furnace. The packets formed with charcoal were firstly placed into the cold furnace. The chips after heating and melting burned out completely. The second charge was made of pure chips and also placed into the cold furnace. This method turned out to be the most profitable and the overall efficiency of the recycling process of ZnAl35Cu5 was increased from the original zero to 41 % (Luka et al., 2007). Other tests were made with the packets with or without charcoal placed into the melt prepared from other packets or from lump waste, however the efficiency of these methods was lower, never reaching 40% (Skalova et al., 2007).

2.2 Second part of experimental work

On the basis of the results obtained from the first phase, further experimental worked focused on the packets made from chips without charcoal. The influence of forming pressure and cleaning of the chips prior to forming were investigated in this step. Cleaning the chips was necessary because they were covered by cutting fluids, which had been expelled during the forming of the packets. This effect was stronger when higher forming pressure was applied. Four methods of cleaning were therefore tested. The chips were either only dried, or washed in alcohol, washed in lukewarm water with degreasing agent or in hot water with degreasing agent.

The first method consisted of rotation of unwashed chips in a centrifuge at 2000 rpm. The centrifuge cleaned most of the cutting fluid from the chips, which were subsequently dried in hot air. The second method involved three-step washing of chips in alcohol. The chips were placed in the centrifuge after each washing step to remove the dirt and they were dried in hot air at the end. Manipulation with large volumes of alcohol was technically complicated and therefore washing in lukewarm water with degreasing agent was used instead. The procedure was very similar to the one with alcohol, but the third washing was done in water without a degreasing agent to remove the degreasing agent from the surface of the chips. The last cleaning method consisted of washing the chips in hot water with degreasing agent. A higher degreasing effect was expected because of the higher degreasing ability of hot water.

The packets were prepared with forming pressures of 200 MPa, 300 MPa and 400 MPa. To achieve higher compression of chips, the packets were formed gradually in three steps. The experiment was carried out in an electric resistance furnace and three different methods of melting were designed.

The influence of forming pressure on the compaction of the chips was observed on macrographs of the ground packets. Plane fractions of empty space between the chips were evaluated using Lucia image analysis software. The highest compaction was determined for a forming pressure of 400 MPa (Fig 1).

The first melting strategy was designed on the basis of the results obtained in the first phase of the experiment. The charge was placed in the hot furnace at a temperature of 720[degrees]C. Six different melting processes were carried out in this way with different parameters of charge preparation.

[FIGURE 1 OMITTED]

Procedure: 1. remelting strategy: basic remelting process

2. remelting strategy: temperature of furnace 900[degrees]C

3. remelting strategy: refining salt

Preparation of chips:

PRV--washed in hot water with degreasing

PRJ--washed in lukewarm water with degreasing

PRL--washed in alcohol

NTS--unwashed, only dried

The second remelting strategy tested the influence of a higher remelting temperature on the efficiency of the recycling process. The temperature of the furnace was therefore increased to 900[degrees]C.

The third remelting strategy checked the effect of refining salt on the efficiency of the recycling process. The primary bath at a temperature of 900[degrees]C was prepared from a mixture of free chips and refining salt.

3. RESULTS

3.1 Unwashed chips

Unwashed chips were processed using three different methods E, F, G (Tab.1), the only difference being the forming pressure. The aim of the experiment was to determine the influence of the forming pressure on efficiency. Unwashed chips provided reference values, which could be compared with the results of the washed chips. Standard deviation of the efficiency of methods E, F, G suggests that forming pressures applied in the range of 200-400MPa do not change the efficiency of the recycling process. The average efficiency of unwashed chips was thus around 60%.

3.2 Chips washed in alcohol

The chips washed in alcohol (C, D, Tab1) attained an efficiency of only 56%, which is less than in the case of the unwashed chips.

3.3 Chips washed in lukewarm water with degreasing agent

The average efficiency of the charge A (Tab.1) was 73.5%. The dross had the form of a metallic substance (Fig.2), which might be further adapted.

Even higher efficiency was obtained in method H (Tab.1), where the chips were prepared in the same way as in case A; only the charge was remelted at a temperature of 900[degrees]C. The efficiency of method H reached 65% and it can probably be further increased by a shorter hold in the furnace.

3.4 Chips washed in hot water with degreasing agent

Chips washed in hot water with degreasing agent (B, Tab.1) achieved an efficiency of 67.6%, which is slightly less than in the case of chips washed in lukewarm water. The drop in efficiency was probably caused by the higher oxidation effect of washing in hot water.

3.5 Experiment with refining salt

Recycling method J (Tab.1) proved the assumption that salt would work as outstanding protection against oxidation. This was also confirmed by the 84% efficiency of the process. This was the highest efficiency achieved in all the recycling methods in this work. The disadvantage of this method was the long time required to melt the salt in the furnace and also the need for a special melting pot resistant to the leaking of the salt bath. This remelting process was not further optimized because of these practical limitations.

4. CONCLUSIONS

The aim of this experimental work was to find a new technique for recycling Alzen chips so that the highest efficiency of input material was achieved. Several different methods of chip compression were tested with the help of compression tools and different forming pressures. Several methods of chip preparation and subsequent remelting were also designed. The influence of cutting fluid on the efficiency of remelting and on the whole recycling process was also determined.

The highest efficiency was achieved in the method using refining salt, where free chips were poured into the hot salt bath. The efficiency of this process was over 80%, however this process turned out to have some limitations, such as the long melting time of the salt bath and the need for a special melting pot resistant to the leaking of salt.

It was also found that removing the cutting liquid from the chips prior to compacting has a positive influence on the compacting process and remelting. Washed chips were extruded from the compacting tool with lower force and the dross after remelting usually had a metallic form. When the cutting liquid was not removed from the chips the dross tended to take the form of ashes.

The chips washed in lukewarm water with degreasing agent, which were remelted after compaction into packets, achieved a relatively high efficiency of 73.5%. This experiment proved that lukewarm water with a common cleaning agent can be used instead of alcohol. This result contributes to a significant cost reduction of the recycling process and it also improves work safety.

5. ACKNOWLEDGEMENTS

This paper includes results obtained within the project 1M06032 Research Centre of Forming Technology.

6. REFERENCES

Balicky, S. (1953). Loziskove slitiny bez clnu, (Bearing alloys without tin) SNTL, Praha

Chriastel, L. (2000). Recyklacia odpadov, (Waste recycling) Vydavatelstvo Slovenskej technickej univerzity Bratislava, ISBN 80-227-1403-8, Bratislava

Kristofova, D. (2003). Recyklace nezeleznych kovu (The nonferrous metals recycling), VSB-TU Ostrava, ISBN 80-2480485-9, Ostrava

Luka J. (2007). Zvysovdnl efektivity procesu metalurgicke recyklace kovoveho odpadu, (Escalation effectiveness of the process metallurgical recycling waste metal) diploma thesis, UWB in Pilsen

Michna, S.; Lukac, I.; Ocenasek, V.; Heinz, S. & Miskufova, A. (2005). Encyklopedie hlinlku (The Encyclopaedia of aluminium), Aidin s. r. o., ISBN 80-89041-88-4, Presov

Skalova, L. & Luka, J. (2008). Optimization of Recycling Process of White Bronze Chips, 12th International Research/Expert Conference--TMT 2008, ISBN 995861741-2, Turkey, August 2008, Istanbul

Tab. 1. Recycling methods

 Forming
 Preparation pressure Remelting Efficiency
Variant of chips [Mpa] strategy [%]

A PRJ 200 1 73.5
B PRV 200 1 67.6
C PRL 200 1 55.7
D PRL 300 1 56.0
E NTS 200 1 58.0
F NTS 300 1 62.3
G NTS 400 1 59.0
H PRJ 200 2 65.5
I NTS 400 2 30.4
J NTS 0 3 84.3