FIELD OF THE INVENTION

The present invention relates to methods for removing lead from brass scrap comprising alloyed copper, zinc and lead.

BACKGROUND OF THE INVENTION

Brass is an alloy containing in its base form a 60/40 ratio of copper and zinc. In addition to this, other elements are added to improve the properties of the end- product. One example of such additives is lead (Pb), which is added to improve machinability of the brass. Lead is often added in concentrations of 1-4 wt%, typically around 2 wt%.

Due to its good corrosive resistant properties brass is widely used in plumbing applications. Various countries and organizations are working towards removing lead in brass to minimize human exposure to lead from drinking water, as brass is widely used in water taps. Currently, almost all circulating brass scrap contains lead, which causes difficulties during recycling. With improved methods for removing lead from brass scrap, it would be possible to increase the amount of brass that can be recycled and thereby achieve a greater resource efficiency.

Currently, there are three principal methods for removing lead from brass - dilution, vacuum distillation of zinc and lead, and intermetallic precipitation.

Diluting lead out of the stock of brass scrap demands an intensive use of virgin material and is time consuming.

Vacuum distillation can be used to remove zinc and lead from brass with high yield, but the method is energy demanding. Intermetallic precipitation, resulting in the formation of CaPb alloy precipitates, creates inclusions that cause to poor mechanical behavior during working of the recycled metal.

Accordingly, there is still a need for improved methods for removing lead from brass scrap, which can alleviate the deficiencies of the existing lead removal methods.

SUMMARY OF THE INVENTION

An object of the present disclosure is to provide a method for removing lead from brass scrap comprising alloyed copper, zinc and lead, which alleviates at least some of the deficiencies of existing lead removal methods.

Another object of the present disclosure is to provide a vacuum distillation method for removing lead from brass scrap comprising alloyed copper, zinc and lead, which can separate lead from a copper base of the brass with high yield.

Yet another object of the present disclosure is to provide a vacuum distillation method for removing lead from brass scrap comprising alloyed copper, zinc and lead, which offers a high degree of control over the process parameters.

The above objects as well as other objects that will become apparent to the skilled person in the light of the present disclosure are achieved by the various aspects of the invention as set out herein.

The present invention is based on the surprising realization that when subjecting brass scrap comprising alloyed copper, zinc and lead to heating under reduced pressure a significant portion of the Zn and Pb is evaporated when the brass, or the copper base of the brass, is still in the solid phase. Tests show that all, or substantially all, of the Zn content and up to 2/3 of the Pb content could be removed at temperatures where the brass was still in the solid phase. Also, the composition did not change further once the temperature had been increased above the melting point of the copper base of the brass. This, observation further supported the conclusion that most of the Zn and Pb can be evaporated while the brass is still in the solid phase.

The difference in condensation temperatures and pressures of Zn and Pb allow for separation of these two components during the condensation recovery process. With a condenser assembly comprising separate condensation chambers for pure or almost pure Zn and Pb (possibly together with some Zn) it is possible to obtain three product streams: the purified copper, one condensate consisting of pure or almost pure Zn, and one condensate consisting of a Pb-Zn mix. For example, the Zn may be collected in a primary condensation chamber and the Pb-Zn mix may be collected in a secondary condensation chamber.

The term reduced pressure as used herein generally refers to a pressure below normal atmospheric pressure (1013.25 mbar).

According to a first aspect of the disclosure, there is provided a method for removing lead from brass scrap, said method comprising:

- subjecting brass scrap comprising alloyed copper, zinc and lead to heating under reduced pressure at a temperature above the boiling point of lead at the reduced pressure but below the melting point of the copper base of the brass scrap at the reduced pressure, to evaporate lead and zinc, and

- recovering the evaporated lead and zinc by condensation.

In some embodiments, the recovery comprises recovering the evaporated zinc and lead separately by condensation.

In some embodiments, the recovery comprises recovering the evaporated zinc and lead by condensation and subsequently separating lead from zinc.

The inventors have found that a very low pressure is typically required to achieve evaporation of Pb from the solid brass scrap. The reduced pressure should preferably be kept well below 100 mbar, preferably well below 50 mbar. Typically, the reduced pressure should be kept below 25 mbar, preferably below 15 mbar, and more preferably below 10 mbar. In some embodiments, the reduced pressure is kept below 10 mbar. The lower limit of the reduced pressure is typically dictated by practical considerations. In some embodiments, the reduced pressure is above 0.01 mbar, 0.1 mbar, 1 mbar, or above 5 mbar. In some embodiments, the reduced pressure is kept above 0.01 but below 10 mbar, above 0.1 but below 10 mbar, or above 1 but below 10 mbar.

Keeping the reduced pressure during evaporation of zinc and lead may require further reducing the pressure, intermittently or continuously, during the evaporation process to compensate for the zinc and lead vapor generated. In some embodiments, the pressure is further reduced at least once to compensate for the zinc and lead vapor generated.

The temperature above the boiling point of lead at the reduced pressure but below the melting point of the copper base of the brass scrap at the reduced pressure will depend on the reduced pressure. Typically, the temperature will be below the melting point of pure copper of about 1085 °C. In some embodiments, the temperature is in the range of 800-1100 °C, preferably in the range of 850-1050 °C. In some embodiments, the temperature is in the range of 900-1100 °C, preferably in the range of 950-1050 °C. The temperature may vary within the specified ranges during the course of the process. For example, the temperature may be at a lower end of the range at the beginning of the process, and increase to a higher end of the range towards the end of the process. Even though the initial temperature may in some cases be slightly above the melting temperature of the brass starting material, the rapid evaporation of zinc, or zinc and lead, from the brass, will cause the melting point of the remaining alloy to increase to a point where the material will remain in solid form.

In some embodiments, the reduced pressure is kept below 10 mbar and the temperature is in the range of 800-1100 °C, preferably in the range of 850-1050 °C.

In some embodiments, the reduced pressure is kept below 10 mbar and the temperature is in the range of 900-1100 °C. The time for which the heating under reduced pressure is maintained may vary depending on range of parameters, such as pressure, temperature, chemical composition of the brass including concentration of zinc and lead in the brass, and the required degree of lead removal. In some embodiments, the heating under reduced pressure is maintained for a time of at least 0.5 hours, preferably at least 1 hour, more preferably at least 2 hours.

The heating under reduced pressure may also be performed sequentially in two or more steps with different temperatures and/or reduced pressures. Typically, a first step comprises evaporation of pure or almost pure Zn at a first pressure and temperature, and a second step comprises evaporation of Pb, or a mixture of Pb and Zn at a second pressure and temperature.

Thus, in some embodiments the method further comprises: a) subjecting brass scrap comprising alloyed copper, zinc and lead to a first heating under a first reduced pressure at a temperature above the boiling point of zinc at the first reduced pressure but below the boiling point of lead at the first reduced pressure and below the melting point of the copper base of the brass scrap at the first reduced pressure, to evaporate zinc, and b) recovering the evaporated zinc by condensation, c) subjecting the brass scrap to a second heating under a second reduced pressure at a temperature above the boiling point of lead at the second reduced pressure but below the melting point of the copper base of the brass scrap at the second reduced pressure to evaporate lead and zinc, and d) recovering the evaporated lead and zinc by condensation.

In some embodiments, step d) comprises recovering the evaporated zinc and lead separately by condensation. In some embodiments, step d) comprises recovering the evaporated zinc and lead by condensation and subsequently separating lead from zinc.

The second reduced pressure is preferably lower than the first reduced pressure.

In some embodiments, the first reduced pressure is above 10 mbar, preferably above 15 mbar, and more preferably above 25 mbar. In some embodiments, the first reduced pressure is kept above 10 mbar. In some embodiments, the first reduced pressure is below 50 mbar or below 100 mbar. In some embodiments, the first reduced pressure may be above 50 mbar or above 100 mbar. The first reduced pressure is below normal atmospheric pressure (1013.25 mbar).

The second reduced pressure is lower than the first reduce pressure. The reduced pressure should preferably be kept well below 100 mbar, preferably well below 50 mbar. Typically, the second reduced pressure should be kept below 25 mbar, preferably below 15 mbar, and more preferably below 10 mbar. In some embodiments, the second reduced pressure is kept below 10 mbar. The lower limit of the second reduced pressure is typically dictated by practical considerations. In some embodiments, the second reduced pressure is above 0.01 mbar, 0.1 mbar, 1 mbar, or above 5 mbar. In some embodiments, the second reduced pressure is kept above 0.01 but below 10 mbar, above 0.1 but below 10 mbar, or above 1 but below 10 mbar.

The temperature above the boiling point of zinc at the first reduced pressure but below the boiling point of lead at the first reduced pressure and below the melting point of the copper base of the brass scrap at the first reduced pressure will depend on the first reduced pressure. In some embodiments, the temperature of the first heating is in the range of 800-1100 °C, preferably in the range of 850-1050 °C. In some embodiments, the temperature of the first heating is in the range of 900-1100 °C, preferably in the range of 950-1050 °C. Even though the initial temperature may in some cases be slightly above the melting temperature of the brass starting material, the rapid evaporation of zinc, or zinc and lead, from the brass, will cause the melting point of the remaining alloy to increase to a point where the material will remain in solid form. The temperature above the boiling point of lead at the second reduced pressure but below the melting point of the copper base of the brass scrap at the second reduced pressure will depend on the second reduced pressure. In some embodiments, the temperature of the second heating is in the range of 800-1100 °C, preferably in the range of 850-1050 °C. In some embodiments, the temperature of the second heating is in the range of 900-1100 °C, preferably in the range of 950-1050 °C.

In some embodiments, the first and second temperature are in the same or overlapping ranges, while the first and second pressure are in non-overlapping ranges.

The time for which the heating under reduced pressure is maintained may vary depending on range of parameters, such as pressure, temperature, chemical composition of the brass including concentration of zinc and lead in the brass, and the required degree of zinc and lead removal.

In some embodiments, the first heating under the first reduced pressure is maintained for a time of at least 0.5 hours, preferably at least 1 hour, more preferably at least 2 hours.

In some embodiments, the second heating under the second reduced pressure is maintained for a time of at least 0.5 hours, preferably at least 1 hour, more preferably at least 2 hours.

In some embodiments, the heating under reduced pressure is performed in a vacuum furnace. The vacuum furnace may preferably be provided with one or more condensation chambers for collecting evaporated Zn and Pb. In some embodiments, the vacuum furnace comprises a primary condensation chamber configured to collect Zn and a secondary condensation chamber configured to collect a Pb-Zn mix.

In some embodiments, the copper base of the brass scrap remains in solid form throughout the lead removal procedure. This is advantageous since it allows for a more accurate control of the process conditions, e.g. pressure and temperature. The brass scrap preferably has a relatively well-defined elemental composition. This is advantageous since it allows for a more accurate control of the process conditions, e.g. pressure and temperature.

In some embodiments, the brass scrap comprises at least 50 wt%, preferably at least 55 wt%, of copper.

In some embodiments, the brass scrap comprises at least 5 wt%, preferably at least 10 wt%, of zinc.

In some embodiments, the brass scrap comprises at least 90 wt%, preferably at least 95 wt%, of copper and zinc combined.

In some embodiments, the brass scrap comprises at least 0.1 wt%, preferably at least 0.5 wt%, and more preferably at least 1 wt%, of lead.

In some embodiments, the brass scrap comprises 60-80 wt% copper, 20-40 wt% zinc, at least 90 wt% and copper and zinc combined, and 0.1-10 wt% lead.

In some embodiments, at least 10% of the Pb content of the brass scrap is removed by the method. In some embodiments, at least 25% of the Pb content of the brass scrap is removed by the method. In some embodiments, at least 50% of the Pb content of the brass scrap is removed by the method.

While the invention has been described herein with reference to various exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or feature to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.

EXAMPLES

A total of six tests were performed in a vacuum induction furnace. The lid of the furnace had been modified with a channel for conducting the evaporated off-gas to a condenser system for capturing evaporated Zn and Pb.

Experimental setup:

The tests were split into two batches of three tests each. Table 1 shows the parameters for each test. The brass was diluted using copper scrap in order to reduce the amount of Zn in the system so as not to overload the Zn condenser system.

Table 1. Test parameters for vacuum induction furnace tests for Pb removal

The experimental procedure for each of the performed tests is detailed in the following. PBV1 :

In this test the material was completely melted before lowering the pressure. Target temperature was 1200°C, and the N2 bottom gas feed was initially at 5 Nl/min. Once all material had melted, the pressure was reduced to approximately 100 mbar. Due to excessive boiling the gas feed was reduced to 2.8 Nl/min and the pressure was raised to 270 mbar. The holding time was 4 hours starting at the time when all material had melted. Samples were taken after 1 hour had passed, as well as just before tapping.

PBV2:

In this test the pressure was reduced to 5 mbar prior to initiating melting. N2 bottom gas feed was set to 2.8 Nl/min. About 50 min after test start, the material started to emit white smoke, most likely Zn(g). Once the material had melted, it immediately started boiling, forcing an increase of the pressure to 300 mbar. Samples were taken intermittently. The holding time was 4 hours starting at the time when all material had melted. Once this period had passed, the pressure was lowered to 150 mbar despite increased boiling, in order to improve the Pb removal. The melt was maintained at 150 mbar for 1 hour, followed by final sampling and tapping.

PBV3:

The aim of this test was to examine the Pb/Zn removal at high temperatures but prior to melting. The plan was to heat the brass sample to almost 1000°C (close to the melting point of the brass) but without melting it and hold there for 2 hours, then melt the material and hold for another 2 hours. In order to minimize boiling, no bottom gas feed was used in this test. The pressure was lowered to 5 mbar and the temperature was raised to just below the melting point, as indicated by optical inspection. This temperature was maintained for 2 hours, during which white smoke was emitted. Sampling was not possible since the material was in solid state. The test was terminated after the initial 2 hours due to short circuiting in the furnace. A final sample was taken from the cast brass.

PBV4:

This test was performed to investigate the Zn/Pb removal from brass in the solid phase observed in the earlier tests. The test used the same conditions of PBV3, i.e. holding the material just below melting point for 2 hours, then melting and tapping the material. A single sample was taken prior to tapping. PBV5:

This test was planned to be conducted like tests PBV3 and PBV4, but with a longer holding time for solid phase Zn/Pb removal. Unfortunately, after approximately 2.5 hours a significant portion of the material had melted. At this point, the temperature was raised with the goal to completely melt the material, similar to the initial plan for test PBV3. The test was terminated 30 min later, once again due to short circuiting. The off-gas channel temperature and pressure measurements were observed to be very similar to test PBV4, indicating these could be used to control the process. A single sample was taken prior to tapping.

PBV6:

This test attempted to complete the original plan for test PBV3, i.e. hold the temperature just below the brass melting point for 2 hours, followed by melting the material and holding for 2 more hours. The process was controlled using off-gas channel temperature and pressure measurements, along with knowledge gained during previous tests PBV4 and PBV5. The first sample was taken once the material had melted, and then sampling was conducted approximately every 40 minutes.

Chemical analysis:

The chemical analyses for the samples from testing are shown in Table 2. In all cases, the Cu content increased while both Pb and Zn decreased. It is also evident that it is possible to remove all Zn and a significant part of Pb in the solid phase. Additionally, only the tests where low enough pressure was achieved (PBV3-6) achieved complete separation of Zn and Cu.

The analysis of the condensate from tests PBV4-6 is shown in Table 3. The main component was Zn, as expected, with a few percent Pb as well as several other minor elements. Table 2. Chemical analysis of the Cu-alloy during and after vacuum processing

* 0 min = when the sample was melted

Table 3. Chemical analysis of the collected Zn in the condenser Mass balance and Pb-removal:

Table 4 shows the mass balance for Cu and Zn for all tests. The outgoing values of Zn does not include material forming coatings in the furnace, or material which condensed in the off-gas channel.

Table 4. Mass balance of vacuum induction furnace tests. All values are given in kg

The best Pb-removal degree was achieved in the PBV6 test. In this test the Pb removal degree was about 65%. The Pb-removal degree was calculated as follows:

PbiN = Weights ^* [Pb]i _N = 28.4 ^* 0.0068 = 0.192g

PbouT ⁼ WeightouT ^* [Pb]Brass OUT ⁼ 26.32 ^* 0.0026 = 0.068g

Pb removal = 100 ^* (Pbi _N - Pbou _T ) / Pb, _N = 100 ^* (0.192 - 0.068) / 0.192 = 65%