SE1951540A1

SE1951540A1 - Method for removing lead from brass

Info

Publication number: SE1951540A1
Application number: SE1951540A
Authority: SE
Inventors: Guozhu Ye; Johan Stenman; Marcel Magnusson
Original assignee: Nordic Brass Gusum Ab
Priority date: 2019-12-20
Filing date: 2019-12-20
Publication date: 2021-06-21
Also published as: WO2021122974A1; SE543879C2; CN115151664A; EP4077749A1

Abstract

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.

Description

METHOD FOR REMOVING LEAD FROM BRASS 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. lnaddition 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 improvemachinability 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 plumbingapplications. Various countries and organizations are working towards removing leadin brass to minimize human exposure to lead from drinking water, as brass is widelyused in water taps. Currently, almost all circulating brass scrap contains lead, whichcauses difficulties during recycling. With improved methods for removing lead frombrass 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. lntermetallic precipitation, resulting in the formation of CaPb alloy precipitates,creates inclusions that cause to poor mechanical behavior during working of therecycled metal.

Accordingly, there is still a need for improved methods for removing lead from brass scrap, which can a||eviate 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 frombrass scrap comprising alloyed copper, zinc and lead, which alleviates at least someof the deficiencies of existing lead removal methods.

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

Yet another object of the present disclosure is to provide a vacuum distillationmethod 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 skilledperson 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 subjectingbrass scrap comprising alloyed copper, zinc and lead to heating under reducedpressure a significant portion of the Zn and Pb is evaporated when the brass, or thecopper base of the brass, is still in the solid phase. Tests show that all, orsubstantially all, of the Zn content and up to 2/3 of the Pb content could be removedat temperatures where the brass was still in the solid phase. Also, the compositiondid not change further once the temperature had been increased above the meltingpoint 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 forseparation of these two components during the condensation recovery process. Witha condenser assembly comprising separate condensation chambers for pure oralmost pure Zn and Pb (possibly together with some Zn) it is possible to obtain threeproduct streams: the purified copper, one condensate consisting of pure or almostpure Zn, and one condensate consisting of a Pb-Zn mix. For example, the Zn may becollected in a primary condensation chamber and the Pb-Zn mix may be collected ina secondary condensation chamber.

The term reduced pressure as used herein generally refers to a pressure belownormal atmospheric pressure (101325 mbar).

According to a first aspect of the disclosure, there is provided a method for removinglead from brass scrap, said method comprising: - subjecting brass scrap comprising alloyed copper, zinc and lead to heating underreduced pressure at a temperature above the boiling point of lead at the reducedpressure 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. ln some embodiments, the recovery comprises recovering the evaporated zinc andlead separately by condensation. ln some embodiments, the recovery comprises recovering the evaporated zinc andlead by condensation and subsequently separating lead from zinc.

The inventors have found that a very low pressure is typically required to achieveevaporation of Pb from the solid brass scrap. Typically, the reduced pressure should be kept below 25 mbar, preferably below 15 mbar, and more preferably below 10 mbar. ln some embodiments, the reduced pressure is kept below 10 mbar.

The temperature above the boiling point of lead at the reduced pressure but belowthe melting point of the copper base of the brass scrap at the reduced pressure willdepend on the reduce pressure. ln some embodiments, the temperature is in therange of 900-1 100 °C, preferably in the range of 950-1050 °C. ln some embodiments, the reduced pressure is kept below 10 mbar and the temperature is in the range of 900-1 100 °C.

The time for which the heating under reduced pressure is maintained may varydepending on range of parameters, such as pressure, temperature, chemicalcomposition of the brass including concentration of zinc and lead in the brass, andthe required degree of lead removal. ln some embodiments, the heating underreduced pressure is maintained for a time of at least 0.5 hours, preferably at least 1hour, more preferably at least 2 hours.

The heating under reduced pressure may also be performed sequentially in two ormore steps with different temperatures and/or reduced pressures. Typically, a firststep comprises evaporation of pure or almost pure Zn at a first pressure andtemperature, and a second step comprises evaporation of Pb, or a mixture of Pb andZn 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 thefirst reduced pressure but below the boiling point of lead at the first reduced pressureand 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 pressureat a temperature above the boiling point of lead at the second reduced pressure butbelow the melting point of the copper base of the brass scrap at the second reducedpressure to evaporate lead and zinc, and d) recovering the evaporated lead and zinc by condensation. ln some embodiments, step d) comprises recovering the evaporated zinc and leadseparately by condensation. ln some embodiments, step d) comprises recovering the evaporated zinc and lead bycondensation and subsequently separating lead from zinc.

The second reduced pressure is preferably lower than the first reduced pressure. ln some embodiments, the first reduced pressure is above 10 mbar, preferably above15 mbar, and more preferably above 25 mbar. ln some embodiments, the firstreduced pressure is kept above 10 mbar. ln some embodiments, the first reducedpressure is below 50 mbar or below 100 mbar. ln some embodiments, the firstreduced pressure may be above 50 mbar or above 100 mbar.

Typically, the second reduced pressure should be kept below 25 mbar, preferablybelow 15 mbar, and more preferably below 10 mbar. ln some embodiments, thesecond reduced pressure is kept below 10 mbar.

The temperature above the boiling point of zinc at the first reduced pressure butbelow the boiling point of lead at the first reduced pressure and below the meltingpoint of the copper base of the brass scrap at the first reduced pressure will dependon the first reduced pressure. ln some embodiments, the temperature of the firstheating is in the range of 900-1 100 °C, preferably in the range of 950-1050 °C.

The temperature above the boiling point of lead at the second reduced pressure butbelow the melting point of the copper base of the brass scrap at the second reducedpressure will depend on the second reduced pressure. ln some embodiments, the temperature of the second heating is in the range of 900-1 100 °C, preferably in therange of 950-1050 °C.

The time for which the heating under reduced pressure is maintained may varydepending on range of parameters, such as pressure, temperature, chemicalcomposition of the brass including concentration of zinc and lead in the brass, andthe required degree of zinc and lead removal. ln some embodiments, the first heating under the first reduced pressure ismaintained for a time of at least 0.5 hours, preferably at least 1 hour, more preferablyat least 2 hours. ln some embodiments, the second heating under the second reduced pressure ismaintained for a time of at least 0.5 hours, preferably at least 1 hour, more preferablyat least 2 hours. ln some embodiments, the heating under reduced pressure is performed in a vacuumfurnace. The vacuum furnace may preferably be provided with one or morecondensation chambers for collecting evaporated Zn and Pb. ln some embodiments,the vacuum furnace comprises a primary condensation chamber configured to collectZn and a secondary condensation chamber configured to collect a Pb-Zn mix. ln some embodiments, the copper base of the brass scrap remains in solid formthroughout the lead removal procedure. This is advantageous since it allows for amore accurate control of the process conditions, e.g. pressure and temperature.

The brass scrap preferably has a relatively well-defined elemental composition. Thisis advantageous since it allows for a more accurate control of the process conditions,e.g. pressure and temperature. ln some embodiments, the brass scrap comprises at least 50 wt%, preferably at least55 wt%, of copper. ln some embodiments, the brass scrap comprises at least 5 wt%, preferably at least wt%, of zinc. ln some embodiments, the brass scrap comprises at least 90 wt%, preferably at least95 wt%, of copper and zinc combined. ln some embodiments, the brass scrap comprises at least 0.1 wt%, preferably atleast 0.5 wt%, and more preferably at least 1 wt%, of lead. ln 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.

While the invention has been described herein with reference to various exemplaryembodiments, it will be understood by those skilled in the art that various changesmay be made and equivalents may be substituted for elements thereof withoutdeparting from the scope of the invention. ln addition, many modifications may bemade to adapt a particular situation or feature to the teachings of the inventionwithout departing from the essential scope thereof. Therefore, it is intended that theinvention not be limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the invention will include allembodiments falling within the scope of the appended claims.

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

Experimental setup: The tests were split into two batches of three tests each. Table 1 shows theparameters for each test. The brass was diluted using copper scrap in order toreduce the amount of Zn in the system so as not to overload the Zn condensersystem.

Table 1. Test parameters for vacuum induction furnace tests for Pb removal Test Temperature Pressure Brass Cu scrap Holding Gas flow(°C) (mbar) (kg) (kg) time (h) (NI/min) PBV1 1200 100-270 6.4 22 4 2.8PBV2 1200 5-300 6.4 35 4+1 2.8PBV3 ~1000 8 6.4 22 2 0 PBV4 ~1000 / 1100 7.5-25 6.4 22 2 0 PBV5 ~1000 / 1200 9-240 6.4 22 2.5 0 PBV6 ~1000 / 1200 9 6.4 22 2+2 0 The experimental procedure for each of the performed tests is detailed in the following.

PBV1:ln this test the material was completely melted before lowering the pressure. Targettemperature was 1200°C, and the Ng 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 NI/min and the pressure wasraised to 270 mbar. The holding time was 4 hours starting at the time when allmaterial had melted. Samples were taken after 1 hour had passed, as well as justbefore tapping.

PBV2: ln this test the pressure was reduced to 5 mbar prior to initiating melting. Ng bottomgas feed was set to 2.8 NI/min. About 50 min after test start, the material started toemit white smoke, most likely Zn(g). Once the material had melted, it immediatelystarted boiling, forcing an increase of the pressure to 300 mbar. Samples were takenintermittently. The holding time was 4 hours starting at the time when all material hadmelted. Once this period had passed, the pressure was lowered to 150 mbar despiteincreased 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 priorto melting. The plan was to heat the brass sample to almost 1000°C (close to themelting point of the brass) but without melting it and hold there for 2 hours, then meltthe material and hold for another 2 hours. ln order to minimize boiling, no bottom gasfeed was used in this test. The pressure was lowered to 5 mbar and the temperaturewas raised to just below the melting point, as indicated by optical inspection. Thistemperature was maintained for 2 hours, during which white smoke was emitted.Sampling was not possible since the material was in solid state. The test wasterminated after the initial 2 hours due to short circuiting in the furnace. A finalsample was taken from the cast brass.

PBV4: This test was performed to investigate the Zn/Pb removal from brass in the solidphase 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 thematerial. A single sample was taken prior to tapping.

PBV5: This test was planned to be conducted like tests PBV3 and PBV4, but with a longerholding time for solid phase Zn/Pb removal. Unfortunately, after approximately 2.5hours a significant portion of the material had melted. At this point, the temperaturewas raised with the goal to completely melt the material, similar to the initial plan fortest 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 bevery 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 thetemperature just below the brass melting point for 2 hours, followed by melting thematerial and holding for 2 more hours. The process was controlled using off-gaschannel temperature and pressure measurements, along with knowledge gainedduring 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. ln allcases, the Cu content increased while both Pb and Zn decreased. lt is also evidentthat 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 maincomponent was Zn, as expected, with a few percent Pb as well as several other minor elements. 11 Table 2. Chemical analysis of the Cu-alloy during and after vacuum processing sample Pb (%) cu (%) zn (%)iniiiai mix 0.58 90.52 8.80(PBv1 8 PBv8-5) PBv1-1* 0.58 90.58 7.5PBv1-2 0.71 98.2 4.7iniiiai mix (PBv2) 0.45 98.51 5.08PBv2-1* 0.14 95.45 1.12PBv2-2 (50 min) 0.29 95.50 1PBv2-8 (145 min) 0.28 95.84 0.91PBv2-4 (210 min) 0.29 95.70 0.84PBv2-5 (270 min) 0.25 95.54 0.81PBv8-1* 0.48 99.09 0PBv4-1* 0.42 99.48 0PBv5-1* 0.89 99.41 0PBv5-1* (0 min) 0.27 99.59 0PBv5-2 (40 min) 0.25 99.59 0PBv5-8 (80 min) 0.24 99.59 0PBv5-4 (120 min) 0.25 99.59 0 * 0 min = when the sample was melted Table 3. Chemical analysis of the collected Zn in the condenser Sample Zn Pb PBV4-Zn 97.87 1.41PBV5-Zn 96.35 1 .94PBV6-Zn 98.97 0.93 Mass balance and Pb-removal: 12 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 PBV1 PBV2 PBV3 PBV4 PBV5 PBV6 Cu IN 25.7 38.7 25.7 25.7 28.8 28.1Zn IN 2.5 2.5 2.5 2.5 2.5 2.5 Total IN 28.2 41.2 28.2 28.2 31.3 30.6Cu OUT 25.5 38.1 26.3 25.0 27.8 26.2Zn OUT 1.4 0.6 1.3 0.9 0.6 2.0 Total OUT 26.9 38.6 27.6 26.1 28.5 28.3 The best Pb-removal degree was achieved in the PBV6 test. ln this test the Pb removal degree was about 65%. The Pb-removal degree was calculated as follows: PbiN = Weightiw * [Pb]|N = 28.4 * 0.0068 = 0.192gPbouT = WeightouT * [Pbbrass ouT = 26.32 * 0.0026 = 0.068g Pb removal = 100 * (PbiN - PDOUT) / Pull = 100 * (0192 - 0.008) / 0.192 = 05%

Claims

1. A method for removing lead from brass scrap, said method comprising: - subjecting brass scrap comprising alloyed copper, zinc and lead to heating underreduced pressure at a temperature above the boiling point of lead at the reducedpressure but below the melting point of the copper base of the brass scrap at thereduced pressure, to evaporate lead and zinc, and - recovering the evaporated lead and zinc by condensation.

2. The method according to claim 1, wherein the recovery comprises recovering the evaporated zinc and lead separately by condensation.

3. The method according to claim 1, wherein the recovery comprises recovering theevaporated zinc and lead by condensation and subsequently separating lead from zinc.

4. The method according to any one of the preceding claims, wherein the reducedpressure is kept below 10 mbar.

5. The method according to any one of the preceding claims, wherein thetemperature is in the range of 900-1 100 °C, preferably in the range of 950-1050 °C.

6. The method according to any one of the preceding claims, wherein the heatingunder reduced pressure is maintained for a time of at least 1 hour, preferably at least2 hours.

7. The method according to claim 1, said method comprising: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 14 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 pressureat a temperature above the boiling point of lead at the second reduced pressure butbelow the melting point of the copper base of the brass scrap at the second reducedpressure to evaporate lead and zinc, and d) recovering the evaporated lead and zinc by condensation.

8. The method according to claim 7, wherein step d) comprises recovering the evaporated zinc and lead separately by condensation.

9. The method according to claim 7, wherein step d) comprises recovering theevaporated zinc and lead by condensation and subsequently separating lead from zinc.

10. The method according to any one of claims 7-9, wherein the first reducedpressure is kept above 10 mbar.

11. The method according to any one of claims 7-10, wherein the second reducedpressure is kept below 10 mbar.

12. The method according to any one of claims 7-11, wherein the temperature ofthe first heating is in the range of 900-1 100 °C, preferably in the range of 950-1050°C.

13. The method according to any one of claims 7-12, wherein the temperature ofthe second heating is in the range of 900-1 100 °C, preferably in the range of 950-1050 °C.

14. The method according to any one of claims 7-13, wherein the first heatingunder the first reduced pressure is maintained for a time of at least 1 hour, preferably at least 2 hours.

15. The method according to any one of claims 7-14, wherein the second heatingunder the second reduced pressure is maintained for a time of at least 1 hour, preferably at least 2 hours.

16. The method according to any one of the preceding claims, wherein the heating under reduced pressure is performed in a vacuum furnace.

17. The method according to any one of the preceding claims, wherein the copper base of the brass scrap remains in solid form throughout the lead removal procedure.

18. The method according to any one of the preceding claims, wherein the brass scrap comprises at least 50 wt%, preferably at least 55 wt%, of copper.

19. The method according to any one of the preceding claims, wherein the brass scrap comprises at least 5 wt%, preferably at least 10 wt%, of zinc.

20. The method according to any one of the preceding claims, wherein the brassscrap comprises at least 90 wt%, preferably at least 95 wt%, of copper and zinc combined.

21. The method according to any one of the preceding claims, wherein the brassscrap comprises at least 0.1 wt%, preferably at least 0.5 wt%, and more preferably at least 1 wt%, of lead.

22. The method according to any one of the preceding claims, wherein the brassscrap comprises 60-80 wt% copper, 20-40 wt% zinc, at least 90 wt% and copper and zinc combined, and 0.1-10 wt% lead.