Kupfer-Symposium

Investigations of Possibilities for Cleaning of for the Recycling of Cast Alloys

Stelter, M.; Morgenstern, G. (1)

For the production of large ship propellers mainly further addition of up the preparation of the dust before the aluminium bronze cast alloys are used. This ma- to 5 wt. % and re-melting, additionally re-melting terial is highly resistant against sea water and it . This particular under usage of new types of refining- also offers all other necessary physical properties. kind of is used salts to keep the homogeneity of the In a cooperative project, which was sponsored by e.g. for the of melted in the furnace for good the German Federal Ministry of Education and ship propellers. quality of the castings is presented. Research (BMBF), methods and procedures for The foundry indus- the selective removal of impurities from alumini- try uses the alloy um bronze have been developed. CuAl10Mn5FeNi to The technology for cleaning of the dust after produce the ship grinding have been improved and prepared for the propellers and other elements for marine implementation into the industry. The re-melt- use. One of the opera- ing processes of this kind of material have been tions is to grind the investigated and optimized. cast propellers with By increasing the usage of scrap for the alloying the polishing papers, of aluminium bronze in cast shops the expensive mechanical tools for use of primary or alloys can be reduced. example, to produce Fig. 1: Grinding of the propeller In the beginning thermo-dynamical calculations the expected surface were performed to pre-estimate which impurities (Fig.1). The dust after Aluminium bronze dust are capable for a removal by chemical/metal- the polishing is col- lurgical reactions. Both, selective oxidation and lected and sporadi- There were 3 different samples of selective chloride and nitride formation were cally reused within the aluminum bronze dust taken for examined experimentally. Other methods to refine the foundry. Due to analysis. All fractions were separately the high aluminum collected within the industry. The aluminium bronze are the evaporation of volatile concentration in the first step of the investigations was elements or the application of refining salts. alloy one of the prob- to analyze the chemical composition The investigations showed that refining salts and lems was the in house of all three samples with the Atomic the evaporation of volatile components gave the re-melting of the dust Absorption Spectrometer (AAS). The best effects for cleaning and recycling. Except of in the foundry. Before chemical composition was determined nearly all potential impurities of alumini- the project the mate- (Tab.1) by dissolving of 10 g of each um and scrap can be removed. rial was sent to a cop- sample in hot diluted nitric acid and per plant for recycling the indissoluble residues were sepa- ithin the project, support- to produce the pure copper. rated by membrane filtration PTFE ed by BMBF, procedures During the refining process, all ele- (1.2 µm). The residues were washed Wand methods for selective ments, except of copper, were reduced with hot water, dried and weighed. extraction of disturbing impurities and removed with the slag. More Therefore, the sum of analyzed ele- from copper casting alloys had to be than 20 wt-% of the sent material ments did not reach 100 %, regarding developed, particularly from alumin- was slagged - even such expensive silicon and aluminum in the residue. ium containing bronze [1]. Moreover, metals like . In opposite, Silicon as an alloying element can metallic waste products that are a the in house recycling gives the pos- also be found in the dust. However, side effect of the production of pro- sibility to save all elements of the it is also a main component of the pellers, e.g. dusts, are to be included alloy with lower waste level and costs ceramics for grinding and polishing. into the refining technology that had of the energy to protect the environ- Silicon is used there in the form of to be developed (scrap cleaning). ment. Costs of the transport can be indissoluble SiO2 which is removed Copper alloys with less than 15 wt.-% also reduced. There is the presen- almost as residue in the analyses. The aluminium are called aluminium tation of the closing the material same problem occurs with the alumi- bronze. Excellent mechanical and flow for recycling within the foundry num concentration, where aluminum physical properties are obtained by using the internal recycling ways of on the one hand is a main part of the

694 · 61. Jahrgang · 11/2007 Rohstoffe/Recycling

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<420 µm. Sample 2 is much finer Element Sample 1 Sample 2 Sample 3 in comparison to sample 1. The fine Cu 78,91 74,74 63,92 fraction <10 µm was determined to Al 8,27 7,558 7,06 1.4% and to the average grain-size Ni 4,58 4,335 3,96 to approx. 100 µm. Sample 3 is finer Fe 3,83 3,664 3,89 in comparison to sample 1, too. The Mn 0,63 0,613 0,41 average grain-size is approx. 120 µm Zn 0,095 0,109 0,003 and 90% of the material is smaller Pb 0,015 0,025 0,0001 than 350 µm. However, the size also Na 0,080 0,116 0,013 shows that the sample 3 is a mixture Si 0,31 0,61 0,67 of a gross and a fine dust. It indicates Sn 0,01 0,01 0,10 that this material is collected without previous separation of factions. Tab. 1: Chemical composition of the dust samples in wt-% Magnetic cleaning of the aluminium bronze dust

alloy, on the other hand it is used The analyses have shown that it is

as Al2O3 in the abrasive. Because of possible to use the dust as a valuable the higher concentrations of this ele- material if it can be separated from ment in the alloy the influence of the impurities like silicon. To analyze inpurities (i.e. form abrasive) on the possibilities for separation the suscep- final result is much smaller compared tibilities of the different materials like Fig. 2: Rotating drum magnetic to silicon. alloy and oxides were compared by separator The grain-size distribution in all with those of silicon and his possible three samples was carried out with compounds. The aluminum bronze cally cleaned. To find the optimal the Particle analyzer Sympatec Helos with 2.5% - 3.5% of iron (propeller parameters and to minimize the non- and a grain-size spectrum can be alloys) shows relatively high magnetic magnetic residue, the paramagnetic determined in the area from 0.5 to susceptibility. Therefore the possibil- sample from the first cleaning step 1200 µm. For the investigations the ity to separate both fractions in the was cleaned again with the same samples were supplied to the analyzer magnetic field was suggested. The equipment. Fig. 4 presents the results and there the grain-size distribu- magnetic separation should offer the of the experiment and the weights of tion was determined by a laser-opti- chance to clean the dust from silicon each fraction. The silicon concentra- cal system. In Tab. 2 the grain-size and silicon compounds without any tion in the dust after the first clea- distributions of 3 examined sam- chemical treatment and to melt it in ning is presented in the Tab. 4. ples are shown. However, during the original physical form. A Rotating By sieving all particles >1 mm were the investigations sometimes material Drum Magnetic Separator 9630\110 separated. This fraction was found to was blocking the analyzer and there- was chosen with changeable poles be 1 wt- % of the incoming sample fore disturbed the measurement. With (Fig. 2). The investigations have been 2. The sieved 99 wt- % was magne- these investigations no fine fraction realized for all 3 samples separately. tically cleaned. 79.9 wt- % could be <10 µm could be determined in sam- After the sieves have been cleaned transferred into the magnetic product. ple 1. Only 10% of the material had a approx. 2 kg of sample 1 was treated By the process the silicon concentra- grain-size <70 µm, the average grain- with the magnetic separator. In Fig. 3 tion was reduced from 0.07 wt-% to size was found to be approx. 200 µm. the investigation steps are given. 0.025 wt-%. After the second cleaning 90% of the material had a grain-size 82.17% of the material could be step of the paramagnetic fraction the separated as magnetic sample. The magnetic fraction of sample 2 has a unwanted non-magnetic sample was concentration of 0.1 wt-% silicon. Sample Particle size [µm] lower than 5 % of the incoming 10 % 50 % 90 % material. Tab. 3 presents the silicon concentration in all cleaned fractions. Sample 1 < 70 200 < 420 The concentration was reduced from Sample 2 < 30 100 < 320 0,18 wt- % to 0,03 wt- % in the mag- netic fraction. Sample 3 < 41 122 < 350 The fine fraction of the dust was for experimental purpose re-melted in Tab. 2 Fractions of aluminum bronze the casting shop. To check the re- Fig. 3: Schematic presentation of dust particles melting possibility of 360 kg of dust, the separation process for alumi- sample 2 was sieved and magneti- num bronze dust - sample 1

696 · 61. Jahrgang · 11/2007 Rohstoffe/Recycling

Sample Magnetic Para­ Non- Element Cu Al Ni Fe Mn Mg Pb Si Sn Zn magnetic magnetic Aluminium- - - - - 2% - 8% 32% - 28% 1 0.03 % Si 0.13 % Si 0.51 % Si bronze 2 0.02 % Si 0.18 % Si 0.42 % Si 3 0.03 % Si 0.32 % Si 0.68 % Si Tab. 5: Amount of additions in scraps‘ supply for aluminium alloys (period: from September 1999 until September 2001) Tab. 3: Silicon concentrations in all cleaned samples in three different products of the separation loped, particularly from aluminium tent of these elements in the casting containing bronze. process has to be examined. Starting Sample Magnetic Para­ Non- The Mecklenburger Metallguss GmbH from the alloys‘ elements and from magnetic magnetic already utilises at limited extent the impurities of the aluminium cas- 2 0.1 0.07 0.33 copper and aluminium scraps beside ting, we can break down the data into pure metals for production of propel- groups specified in table 6. Tab. 4: Silicon concentration after the lers. The scraps are characterized by In literature mostly the recovery of second cleaning of the paramagnetic chemical composition, metal content, copper from scraps from electronic fraction [wt-%] humidity and size of the particular equipment is discussed in addition to piece. Having made research of scraps‘ the concerning technology [2] und [3]. supply for production of aluminium The usage of refining-salts in order Scrap cleaning bronze, we received the results in to prevent oxidation of metals and tab. 5. to absorb impurities is known from Included in the project technologies Because more than 30 % of the usage gaining and production of aluminium and methods for selective extraction of the supplied scraps has been due to [4]. Refining procedures of copper of contaminating impurities from high silicon and concentrations, casting alloys are not included in the copper casting alloys had to be deve- the possibility of reducing the con- literature. To what extent selective

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· 61. Jahrgang · 11/2007 697 Kupfer-Symposium

vacuum refining of aluminium and Group Element Description copper alloys have been made. This Group 1 Copper, aluminium, Casting metals of aluminium bronze way, , lead and zinc concentration nickel, manganese, iron alloys could be reduced, however, not the Group 2 Oxides (primarily Composition is to be prevented by concentration of silicon. Introducing Al O ) smelting; already existing oxide has to 2 3 this vacuum technology into a cas- be removed ting shop could not be the economic Group 3 Silicon and lead Both kinds of impurities negatively influence the propeller alloys and are way because of high investment cost to be removed and nevertheless no reduction of the Group 4 Magnesium This metal decreases the refining silicon concentration in the melt. ability of the salts Hence this kind of technology has Group 5 Primarily gases, such Have a negative influence on quality not been investigated and research as H , N and other of propellers‘ casts and therefore also 2 2 has been concentred on following have to be removed refining procedures: Group 6 Zinc, tin Are accepted in the propellers‘ casts to limited concentrations gas treatment of the melt, salt refining of the melt. Tab. 6: Classifications in groups of substance concerning content in the aluminium bronze alloy refining procedures including the use single components of the aluminium of refining-salts can remove contami- bronze can be considered. nating impurities had to be explained In Table 7 the partial pressures of by thermo-dynamical investigations. metals and oxides are given as well as free energy of the oxide formation Thermo-dynamical of the metals included in the alumi- investigations nium bronze at 1200 °C. or air treatment of alumini- Fig. 4: Schematic presentation With the help of thermo-dynamical um bronze at first leads to the forma- of the separation process of the analyses it had to be estimated, which tion of alumina. Therefore refining aluminum bronze dust - sample 2 impurities in the aluminium bronze by oxidation is possible only for could be removed by chemical reac- aluminium free copper scraps. Experiments tions. By comparison of the figures Similar calculations have been made of Gibbs free energy of oxide forma- for selective formation of nitrides and Gas Refining tion it could be determined, which chlorides of the impurities and alloy- element can be refined by a selec- ing elements. By selective formation According to thermo-dynamical cal- tive oxidation. With help of thermo- of nitrides or chlorides as well as by culations with the HSC – Chemistry dynamical analyses of the systems evaporation, tin, lead and zinc should 5.1 program [5] the theoretical pos- copper – impurities and aluminium be removable from the liquid alumi- sibilities for removing of impurities – impurities it is generally possible nium melt and respectively from the from copper alloys with means of to receive information on the refi- aluminium bronze melt. miscellaneous reaction gases can be ning ability of these contaminations. From literature it is also known shown. Analogical the partial pressure of the that further examinations concerning According to Fig. 5 the formation

of Si3N4 should be possible by the reaction of silicon with ammonia D 0 Element Partial-pressure Reaction of the oxida­ G , [kJ/mol at simultaneous release of hydro- [atm] 1200°C tion O2] at 1200°C 1 5 gen. With increasing temperature Zn 1 * 10 2 Zn + O2 = 2 ZnO -3.9 * 10 -6 5 the stability of this compound is Sn 7 * 10 Sn + O2 = SnO2 -2.7 * 10 -2 5 increasing. Therefore this reaction Pb 2 * 10 2 Pb + O2 = 2 PbO -1.6 * 10 -5 5 should be examined in a tempera- Al 1 * 10 4/3 Al + O2 = 2/3 Al2O3 -7.8 * 10 ture range of 1300 °C to 1500 °C. 5 Si n. g. Si + O2 = SiO2 -6.5 * 10 Primarily, the nitrogen in ammonia -6 5 Cu 4 * 10 4 Cu + O2 = 2 Cu2O -1.2 * 10 can get in touch with the silicon in -7 5 Fe 1 * 10 2 Fe + O2 = 2 FeO -3.4 * 10 the melt and thereby form silicon Ni 2 * 10-10 2 Ni + O = 2 NiO -2.2 * 105 2 nitride (Si3N4). This process, like all heterogeneous reactions is strongly Tab. 7: Partial pressure and free energy of oxidation of selected metals affected by the convection in the melt. At high temperatures the for-

698 · 61. Jahrgang · 11/2007 Rohstoffe/Recycling

treatment the zinc concentration was reduced from 0.12 or 0.14 wt.-% to 0.001 wt.-%. The content of lead and tin in the melt could only be decreased less. On the other side, silicon concen- tration increases significantly from under 0.1% to 0.14% at 1300 °C and up to 0.16 % at 1500 °C. The reason for this increasing Si-concentration can be the reduction of silicon dioxide to silicon from the graphite crucible or/and the loss of zinc by evapora- Fig. 5: Schematic illustration of temperature dependency on calculated DG0 value in the temperature range 1000 °C – 1400 °C for reactions of Si and Al with ammonia in aluminium bronze mation of aluminium nitride is pos- diameter. To prevent the oxidation of sible as well. the melt, always 5% of refining-salts Gas refining was examined from were added to the melt in the crucible. 1300 °C to 1500 °C in a Tamman Refining time was 30 minutes. furnace. During refining the tempe- As can be seen from Fig. 6 concentra- rature continuously was controlled tions of the impurities Pb, Sn and Zn Fig. 6: Behaviour of impurities during with a thermocouple Ni–CrNi. Gas was can be decreased during refining. the refining of aluminium bronze with injected centric into the melt through The removal rates depend on the tem- nitrogen at 1300°C the graphite lance with a 3 mm open perature of the melt. After 30 min of

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into the melt after reduction with alu- Element before refining after 10 after 30 min. minium from the alloy. Due to strong [wt. %] min.[wt. %] [wt. %] reaction of chlorine with aluminium Zn 0.18 0.13 0.03 in the sense of oxidation and the reac-

Pb 0.02 0.02 0.02 tion with SiO2 from the crucible, this Sn 0.02 0.02 0.02 gas is not appropriate for the refining Si 0.11 0.11 0.11 of aluminium bronze. Mn 1.18 1.19 1.13 In general one may express, that com- Fe 4.67 4.64 4.69 plete removal of zinc is possible with Ni 4.4 4.4 4.56 every reaction gas by evaporation. Al 8.46 8.33 8.02 The fastest result is obtained at higher Cu 80.9 81.11 81.37 temperatures. Silicon contents can not Tab. 8: Results of the gas refining of aluminium bronze with nitro- be reduced by gas refining, but in gen and 10% of ammonia at 1300 °C contrary can be increased by reactions

of SiO2 from the crucible. The contents of trace elements such as Cr, As or P tion from the melt. The reduction of Therefore gas refining with ammonia are not reduced by gas refining. The

SiO2 from the grid of the crucible can does not result in the desired goal – a contents of major elements of the happen by the intensive convection significant reduction of impurity con- alloy such as Cu, Mn, Ni, Al, and Fe of the liquid melt by means of molten centrations of tin, lead and silicon. are kept constant on the same level at salts. SiO2 further is reduced by Al and Hence injection of chlorine to the alloy 1300 °C. At 1500 °C injection tempe- dissolved in the melt. liquid melt has been investigated as an rature the losses on aluminium from Having made the experiments with alternative gas refining possibility. The the melt increases further more. pure gases, one can assume that refi- influence of chlorine on the melt leads ning efficiency of nitrogen and argon to a direct reaction of aluminium with Refining of Aluminium Bronze on aluminium bronze melt is limited. chlorine with simultaneous formation with Molten Salts Hence further experiments have been of AlCl3. This compound separates conducted with addition of reactive from the metallic melt to the surface Goal of these investigations was to gases, e.g. chlorine or ammonia with forming a salt slag. On its way to the develop new, high effective salt com- concentrations of 10 % and 20 % in surface it can agglomerate with oxide positions, which react selectively with the gas mixture. particles and other contaminations impurities and protect the surface of

The thermal decomposition of NH3 in incorporating them into the slag. the alloy from oxidation. H and N atoms in the melt can lead The injection of chlorine gas (with a Fluoride containing salts promised to the reaction of these reactive atoms content of 10 vol. % and 20 vol. %) best results for the application with with components of the alloy. During has been made by means of gas carrier aluminium bronze, because they are the refining, significant loss on alumi- nitrogen and argon at 1300 °C. able to bind oxides such as e.g. Al2O3 nium occurs, even up to 10 % of the The results in Fig. 7 show a 400 % and thus they show an influence on initial content. Thus the concentration increase of the silicon concentration. the refining. A technically used salt of copper as main component of the This proves that SiO2 from the crucible must smelt rapidly during handling alloy increases significantly (Tab. 8). is attacked with chlorine and dissolved and must be chemical reactive, so that Same happens to the others alloying components and trace elements. Behaviour of main alloying compo- nents prove that the metals like copper or nickel do not react with ammonia. The relative increase of contents can be explained with losses on alumini- um from the alloy by oxidation. Gas handling of aluminium bronze with ammonia at 1300 °C also evaporates zinc. Tin and lead contents remain on the same level, because they do not form nitrides nor can be evaporated. The results of gas treatment of alumi- nium bronze with argon and ammonia show that only zinc has been removed Fig. 7: Behaviour of impurity elements during treatment of alu- with simultaneous loss of aluminium minium bronze with argon and 10 % of chlorine at 1300 °C from the liquid melt.

700 · 61. Jahrgang · 11/2007 Rohstoffe/Recycling

ze were developed with the goal to Refining- Silicon concentration Aluminium concen­ Copper con­ eliminate harmful impurities from the salt in the alloy [wt.%] tration in the alloy centr. in the cast alloys. [wt.%] slag [wt.%] Concluding the results of the experi- before after before after after refining refining refining refining refining mental investigations the most effec- Flux 6 0.108 0.092 8.90 9.04 12.23 tive refining procedure for the interes- Flux 7 0101 0.126 8.84 8.81 24.31 ting type of alloy can be formulated Flux 8 0.110 0.111 9.22 9.04 14.03 as: removing of elements that can Flux 9 0.157 0.129 7.94 8.37 8.99 easily be evaporated at high tem- Flux 10 0.116 0.113 8.70 8.76 2.48 peratures and additionally covering Flux 11 0.106 0.113 8.18 8.55 16.86 the melt as protection for oxidation with the refining-salt. Apart from Tab. 9: Aluminium- and silicon concentrations in the melt before silicon, all potentially impurities in and after salt refining, copper concentrations in slag after salt copper- and aluminium-scraps can be refining of aluminium bronze removed by using these methods for refining when secondary material is added to melts of aluminium bronze fluoride components can react with These results indicate that the new in a casting shop. the oxide impurities. The resulting salt developed refining-salts are able to By conducting laboratory investiga- mixture, that has significantly lower protect the melt of the alloy bet- tions with parallel industry examina- density than the melt, covers the sur- ter from oxidation rather than the tions together with Schäfer Chemische face of the melt and can be removed common salt mixtures known before. Fabrik GmbH refining-salts have been after the reaction as a slag. From the characteristics of refining- developed which show good refining Thus systematic investigations on the salt behaviour during the process one effects on aluminium bronze alloys. evaporating and decomposition beha- can say that evaporating of compo- These salts have good wet ability by viour of different fluorides lead to a nents was on a very low level. The smelting of metallic swarfs and can be few thermally stable compounds in swarf is melted successfully with eve- classified as non-toxic. the interesting system. ry salt mixture used in the tests. Oxide Due to the investigations it is pos- Using thermal stable fluoride com- layers were removed fast, so that the sible to utilise more aluminium- and pounds and corresponding salt mix- alloy obtained a homogeneous and copper-scraps for production of this tures the compositions marked as Flux compact structure after the process by type of bronze. However, one should 6 to Flux 11 were created [6]. solidification. In contrary concentra- pay attention to silicon content, The smelting investigations have been tions of copper in the slag are relative- because a refining of this element is made in an 8000 Hz middle frequent ly high after the refining. This reveals not possible in principle until now. laboratory inductive furnace. Prima- that copper rich swarf particles are rily 2 kg bronze alloy was smelted in left un-reacted in the slag. This could References a graphite crucible and a sample was be explained with a low convection in taken. Later on a mixture of swarf and the crucible during laboratory tests. [1] Abschlussbericht BMBF-Projekt: “Inno- vatives Schrottcleaning für Kupferlegie- refining-salt (5 wt. % of total mass of The investigated refining-salts lead rungen”, 2004. the melt and swarf) has been added. only to a small increase of the sili- [2] Bombach,H., Stelter, M.: “Recycling of electronic scrap materials”, Erstes Frei- The molten material was treated in the con content in the melt. They are berger Europa-Seminar, 1997 furnace for further 5 min at 1200 °C. less aggressive against the crucible [3] Waltritsch, S., Hagleitner, C.: „Proces- Thereafter a second sample was taken material, what is proved by the silicon sing of Electronic Scrap“, EMC 2003, V3, S. 1009-1024. for the analysis of the aluminium and concentrations in Table 9. Having [4] Krone, K.: „Aluminiumrecycling“, Ver- silicon content. The results of these made laboratory investigations, the einigung Deutscher Schmelzhütten e.V., Düsseldorf, 2000. experiments using the salt mixtures new developed salt mixtures where [5] HSC Chemistry 5.1: “Chemical Reaction Flux 6 – Flux 11 are shown in Tab. 9. tested in the industrial scale. It has and Equilibrium Software”, 10/2002; A. Simultaneously another sample was been revealed that by simple addition Roine, Outokumpu Resarch Oy. [6] Stelter, M., Vogel, W., Jablonski, K.: taken from the solidified refining-salt of the refining-salts to the alloy and „Raffination von Aluminiumbronze in order to determine the copper con- the following addition of swarf it is mit Raffinationspräparaten“ Metall 59 (2005), S. 272-276. centrations in the salt slag as a rate of possible to smelt the mixture with copper loss (Tab. 9). The new refining- minimal losses of the original alloy. (1) Prof. Dr.-Ing. Michael Stelter, salts can cover and wet the melt very Dr.-Ing. G. Morgenstern, Tech- well, so that losses on aluminium are Summary nische Universität Bergakademie minimized. The increase of the Al- Freiberg, Institute for Nonferrous content in the melt in some cases is Within this project supported by Metallurgy and Purest Materials, due to melted swarf that does not have BMBF procedures and methods for Leipziger Str. 34, 09596 Freiberg, really constant Al-content (Tab. 9). selective refining of aluminium bron- Germany

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