2.695 3.

Article

Removal of Copper, Nickel, and

Iron from Lead–Tin Composite by
Segregation of Intermetallic Silicon
Phases

Stefan Jessen, Danilo C. Curtolo and Bernd Friedrich

Special Issue
10th Anniversary of Metals: Metallurgy and Metal Technology
Edited by
Prof. Dr. Hugo F. Lopez, Prof. Dr. Bernd Friedrich and Prof. Dr. Christof Sommitsch

https://doi.org/10.3390/met11010081
 metals
Article
Removal of Copper, Nickel, and Iron from Lead–Tin Composite
by Segregation of Intermetallic Silicon Phases
Stefan Jessen 1 , Danilo C. Curtolo 2 and Bernd Friedrich 2, *,†

1 BSB Recycling GmbH, IME Institute for Process Metallurgy and Metal Recycling, Emser Straße 11,
56338 Braubach, Germany; sjessen.bsb@berzelius.de
2 IME Institute for Process Metallurgy and Metal Recycling, RWTH Aachen University, Intzestr. 3,
52056 Aachen, Germany; DCurtolo@metallurgie.rwth-aachen.de
* Correspondence: bfriedrich@metallurgie.rwth-aachen.de; Tel.: +49-0241-80-95851
† This author is the Principal Investigator.

Abstract: In the reduction stage of the secondary copper production process, copper, nickel, lead,
and tin are collected in a “black copper”, while zinc is volatilized and precipitated as ZnO in the
flue dust. The slag coming from this reduction stage is low in valuable metals and is disposed. In
the conversion stage, lead and tin are oxidized and incorporated in the slag phase; in accordance
with the oxygen potential, this “converter slag” also contains higher contents of copper and nickel.
This slag is then reduced in two stages. From the first stage, a copper–nickel metal is returned to the
converter stage, and in the second stage, a crude PbSn composite with copper contents of around 10%
and nickel contents of around 2.5%, as well as a further usable slag, is obtained from the “secondary”
slag. Iron is used as a reducing agent, so that the metal obtained can contain iron of up to over 10%,
depending on the reduction duration. The motivation of this investigation is to provide a method for
the subsequent refining of the raw PbSn composite, with the aim of obtain a saleable PbSn composite
as well as returning the copper and nickel contents quantitatively to the main copper route or to
sell them as an alloy. Therefore, the present work aims to investigate the refining of the raw PbSn
composite by the separation of the copper, nickel, and iron via the segregation of intermetallic phases.



For that, a series of experiments were performed on the formation and subsequent segregation of


intermetallic phases, by introduction of an additional element to the system. The results indicated
Citation: Jessen, S.; C. Curtolo, D.;
sharper separation of PbSn composite and copper, nickel, and iron due to the higher thermodynamic
Friedrich, B. Removal of Copper, Nickel,
stability of these phases (selectivity).
and Iron from Lead–Tin Composite
by Segregation of Intermetallic Silicon
Keywords: lead–tin composite refining; intermetallic segregation
Phases. Metals 2021, 11, 81. https://
doi.org/10.3390/met11010081

Received: 27 November 2020

Accepted: 24 December 2020 1. Introduction
Published: 2 January 2021 During the reduction stage conducted in the secondary copper production process,
the metals copper, nickel, lead, and tin are collected in the black copper. Zinc, on the other
Publisher’s Note: MDPI stays neu- hand, is volatilized and later precipitates as ZnO in the flue dust. The slag formed during
tral with regard to jurisdictional claims this reduction stage is disposed, as it contains low amounts of valuable metals [1].
in published maps and institutional Later on, the lead and tin are oxidized and incorporated in the slag phase during
affiliations. the conversion stage. In accordance with the oxygen potential, this formed slag phase
also contains higher amounts of copper and nickel. Being a valuable source of material,
the formed slag is then reduced via two stages. From the first stage, a copper–nickel
Copyright: c 2021 by the authors. Li-
metal is returned to the converter. In the second reduction stage, a crude PbSn composite
censee MDPI, Basel, Switzerland. This (containing 10% Cu and ∼2.5% Ni) and a further usage slag are obtained. Moreover, as iron
article is an open access article distributed is used as reducing agent, up to 10% iron can be present in the obtained metal. The obtained
under the terms and conditions of the raw PbSn composite is then refined with the aim of obtained a saleable material as well as
Creative Commons Attribution (CC BY) to return the copper and nickel back into the process chain or sell it as an alloy [1].
license (https://creativecommons.org/ Previous investigations in the field of metal separation has shown that minority
licenses/by/4.0/). components can be removed from lead–tin–copper melts by adding metals. The added

Metals 2021, 11, 81. https://dx.doi.org/10.3390/met11010081 https://www.mdpi.com/journal/metals

Metals 2021, 11, 81 2 of 16

metal is supposed to form intermetallic phases with the minority components, which are
insoluble in the base metal and can be separated by segregation [1–6].
A metalloid that has a great affinity to the metals of the iron group (e.g., Fe, Ni, and
Co) is silicon. It forms intermetallic phases with these metals, which should be separable
by segregation if the base melt does not form any compounds with the silicon [7]. Here,
the low density of silicon (2.33 g/cm3 ), and thus of the intermetallic phase formed with
silicon, is helpful.
In the processing of the second-stage converter slag, so-called “Glanzmetall”, which con-
sists of lead (10%), antimony (25%), tin (30%), and copper (35%), the copper content could
be reduced by 95 to 98% of the initial content by performing a one- or two-step addition of
silicon, followed by phase separation via segregation. By increasing the number of treatment
steps, more than 99% of the copper could be separated. Already in the first treatment step,
95% of the copper could be converted into a silicide phase containing low amounts of lead,
tin, and antimony [2,8].
To remove cobalt and nickel from lead–tin-based alloys by using a “solution metal”,
the effectiveness of silicon as a purification reagent was tested. Lead–tin alloys containing
iron, nickel, and cobalt with different Pb/Sn ratios were treated with silicon at 1250 ◦ C.
In addition to iron, nickel and cobalt were also converted into an iron–silicon phase,
which separated from the remaining lead–tin phase. The distribution coefficients, deter-
mined by the equation L X = %X( Fe−Si) /%X(Sn− Pb) , show that the increasing tin content of
the melt has a decreasing effect on the distribution coefficients (see Table 1) [9].

Table 1. Results of the investigations of Yazawa and others [9,10].

Removal of Cobalt and Nickel from a Lead-Tin Melt by Adding Silicon at 1250 ◦ C
Pb-Sn Ratio of Distribution Coefficient
Separated Fe-Si Phase
the Initial Melt Si Phase/Pb Phase
Pb [%] Sn [%] Fe [%] Si [%] Co [%] Ni [%] LCo L Ni LFe
75 25 71.4 18.1 4.5 5.06 1670 80 3330
50 50 72.1 17.4 4.96 4.53 430 24 770
25 75 72.7 17.4 5.60 2.36 95 15 630

This investigation also dealt with the influence of different silicon amounts added
to a lead–tin–nickel melt (see Table 2). Through the reaction at 1250 ◦ C, a Ni–Si phase
and a lead or lead–tin phase were formed. It is clear that the distribution coefficient for
nickel increases strongly with increasing silicon addition. The investigation also shows the
significant decrease of the distribution coefficient with increasing tin content of the melt.

Table 2. Results of the investigations according to Yazawa et al. [9,10].

Effect of Silicon Addition When Removing Nickel from a Lead or Lead-Tin Melt at 1250 ◦C
Ni-Si-Phase Distribution Coefficient
Pb-Sn-Phase
Ni [%] Si [%] L Ni
87.1 12.9 63
100% Pb 69.8 30.2 2700
50.0 50.0 100000
87.0 13.0 12
50% Pb–50% Sn 69.7 30.3 400
49.4 50.6 1180

For the thermodynamic consideration of the formation of intermetallic phases of

copper, nickel and iron with silicon and of copper and nickel with tin, most of the data
Metals 2021, 11, 81 3 of 16

available are only related to their respective binary systems. For these systems, the melt is
considered to be homogeneous and values for the free partial enthalpy, free enthalpy, and
enthalpy are determined [10–12]. The values allow the calculation of the activity for silicon
and tin in the respective melt, the estimation of the possibility of certain reactions taking
place and the estimation of the resulting heat effects. The negative free enthalpy values
show a clear tendency towards the formation of compounds for nickel, iron, and copper
with silicon, and a much weaker tendency for copper with tin.
The free enthalpy values suggest the sequence nickel, iron, and copper when removed
from PbSn composite by the formation of intermetallic phases with silicon. Only then does
the formation/separation of a Cu–Sn phase occur. The negative enthalpy values for Ni–Si
and Fe–Si clearly indicate an exothermic reaction process, which—although weaker—still
occurs with Cu–Si.
The Copper-Nickel-Silicon Three-Component System
A review of the published information on the three-component system copper–nickel–
silicon [13] shows that ternary intermetallic phases were not found in all cases. From the
few available data, it can be concluded that the nickel silicides have a solubility for copper
and the copper silicides for nickel. The sections Cu-Ni2 Si, Cu-Ni3 Si, Cu-NiSi, Cu3 Si-Ni3 Si,
Cu3 Si-NiSi are not quasibinary, as at least one true intermetallic phase (Cu12 Ni2 Si5 ) is
formed. The resulting liquidus projection results is shown in Figure 1 [13,14].

Figure 1. Liquidus projection in the Cu–Ni–Si three-substance system [13].

As in the present work a fixed copper–nickel ratio (82.1/17.9) is used in the PbSn
composite, the following explanations are limited to the line from the given copper—
nickel ratio to the silicon corner (Figure 2). Within the scope of this investigation, silicon
ratios between 17.5 wt.% and 40 wt.% based on the copper and nickel quantities have
been selected.
If one calculates the composition in atomic percent for the minimum and maximum
silicon content, the results can be drawn as the points 1 and 4 in the ternary state diagram
from Figure 2.
Metals 2021, 11, 81 4 of 16

Figure 2. Section of the ternary system Cu–Ni–Si with the working line (points 1 to 4 of the composi-
tion for the cooling curves).

At points 1 and 2, a primary crystallization of Cu12 Ni2 Si5 is expected, while at points 3
and 4, a primary crystallization of NiSi2 and Si occurs, respectively. In the following, the
expected cooling curves for the maximum and minimum silicon turnover are shown in a
highly simplified form (Figure 3, points 1 and 4). Points 2 and 3 show compositions for
which a significant change in the cooling curve and thus in the microstructure is to be
expected. The resulting phases are Si, Cu12 Ni2 Si5 , NiSi2 , and Cu3 Si.

Figure 3. Qualitative cooling curves of points 1 to 4 from the ternary system Cu–Ni–Si.

2. Materials and Methods

2.1. Pilot Plant
The first experimental step consists of the melting and the silicon treatment, followed
by a second step for the separation of the phases by segregation. Both processes were
conducted under normal air atmosphere.
A cold wall vacuum induction furnace was used for the melting process and for the
subsequent silicon treatment. As the silicon has a significantly lower density than the
prepared PbSn composite melt, it could not be properly introduced into the melt even by
inductive stirring. Therefore, the silicon was introduced by mechanically mixing it into
the melt.
For the segregation process, an insulated resistance-heated muffle furnace (Nabertherm
chamber furnace) was used to set a reproducible and sufficiently slow cooling rate. After the
Metals 2021, 11, 81 5 of 16

silicon had been added and dissolved, the melt was poured into a second preheated clay–
graphite crucible with a graphite insert (electrode quality graphite, max impurity of 200 ppm
ash content), due to the volume expansion of silicon (for pure silicon about 8% volume
increase) or of silicon-containing alloys occurring during solidification. The graphite insert
was arranged in such a way that the solidifying silicides floating on the melt were retained
and could be removed with the help of a graphite rod (Figure 4). The remaining PbSn
composite melt could then be poured off without any problems.

Figure 4. Segregation insert for the separation of silicide product and PbSn composite melt.

2.2. Experimental Plan and Methodology

With regard to the silicon input, attention was paid to ensure that an intermetallic
phase could form. The temperature for the end of the segregation should be as high as
possible to allow further processing of the metal without or with minimum additional
heating. The Table 3 shows the experimental plan.

Table 3. Experimental plan.

Series Si-Content [%] Temperature End of Segregation [◦ C] Cooling Rate

I 0 650 ∼5 ◦ C/min
II 17.5–40 650 ∼5 ◦ C/min
III 25 600, 650, 750 ∼5 ◦ C/min
∼5 ◦ C/min
IV 25 650 ∼5 ◦ C/min with ∼30 min holding at 800 ◦ C
∼10 ◦ C/min
25 (9/16) 650 ∼5 ◦ C/min
V
25 (9/16) 900/650 ∼5 ◦ C/min
VI 17.5–37.5 FeSi 650 ∼5 ◦ C/min

The tests were conducted with 8 kg batches, each with a lead/tin ratio of 69/31,
a copper content of 11 wt.%, and a nickel content of 2.5 wt.%. The silicon or FeSi batch was
based on the desired formation of the intermetallic phases. The silicon carriers used (1 to
5 mm in size) were preheated to 1100 to 1200 ◦ C in an air atmosphere before feeding in
order to minimize the temperature shock during the addition.
If one assumes that copper and nickel are completely removed, then, according to
the above-mentioned ternary system, NiSi2 , Cu12 Ni2 Si5 (with the exception of 17.5 wt.%
Si addition) are formed by the addition of the investigated silicon additives. In theory,
the silicide product formed has a composition of 67.5 weight percent copper, 15.3 weight
percent nickel, and 17.2 weight percent silicon. During this investigation, trials were carried
Metals 2021, 11, 81 6 of 16

out both with a shortage of silicon (silicon addition 17.5% and 20%) and with significant
silicon surpluses (25% to 40%).

2.3. Experimental Procedure

Single-Stage Process Control
After silicon was added, a spontaneous temperature increase to well over 1300 ◦ C was
recorded after a short time without changes in the power input.
After the silicon particles had dissolved in the melt under stirring (both inductive
and mechanical), the formed dross was removed and the melt was poured for segregation
into a crucible equipped with the graphite insert, preheated to 1200 ◦ C in a muffle furnace.
After the metal was poured, a graphite rod (φ 3 cm) was immersed in the melt.
After segregation, the graphite rod was pulled out of the melt. The melt level dropped
due to the missing volume displacement and an improved separation of PbSn composite
and silicide product was achieved. The silicides solidified in a temperature range between
850 and 750 ◦ C. The melt was then further cooled depending on the experimental design.
An increase in volume was noticeable in the resulting silicide products. Once the specified
temperature for the segregation end had been reached, the graphite insert with the silicide
product inside was removed and the melt was then poured into an ingot mold.

Two-Stage Process Control

The silicon addition in the first stage was chosen so that silicon was only offered for
the formation of a NiSi2 phase. After the segregation of the resulting silicide product, the
PbSn composite was again superheated to 1300 ◦ C. The amount of silicon added in the
second stage was then calculated from the difference to the total amount of silicon required.
The first segregation stage was stopped at 900 ◦ C because the solidification interval of
Ni-Si2 is already between 980 and 990 ◦ C. In the second stage, segregation was continued
to 650 ◦ C, as the phases formed should be completely eliminated.

3. Results and Discussions

To determine the silicon turnover XSi , the quotient of added silicon and the analyzed copper
and nickel contents in the PbSn composite is calculated according to Equation (1). All chemical
analyses were conducted by ICP-OES, model Spectro ICP-OES Spectro Ciros Vision.

mSi
Si −/FeSi − addition
XSi = (1)
mCu Ni
MZ + m MZ

For the evaluation of the results, the proportion of silicon that is actually available
for the segregation of copper and nickel is referred to in the following as “available”
silicon. The “available” silicon is calculated from the added silicon (silicon addition) minus
all product materials not included in the segregation process, such as dross or residues
remaining in the crucible.
The percentage of “available” silicon is calculated by the quotient of the silicon avail-
able (analytically detectable) in the PbSn composite at the start of segregation and the
copper and nickel contents analytically detectable at the start of segregation. As a rule,
this value is lower than the value for the silicon conversion due to losses caused by
dross formation.
The calculation of the “available” silicon for the tests with FeSi addition is calculated
analogously, taking into account the silicon bound to iron. For this purpose, the assumption
is made that all the iron remains intermetallically bound as FeSi and passes through
the process quasi inert without reaction. This means that a part of the silicon becomes
“ineffective” in addition to the above-mentioned losses.

3.1. Final Copper and Nickel Contents in PbSn-Composite

Figure 5 shows that nickel is always almost completely removed, whereby the residual
Metals 2021, 11, 81 7 of 16

contents are independent of the silicon addition or the “available” silicon. For the final
copper contents achieved, on the other hand, there is a decreasing tendency with increasing
“available” silicon quantities.

Figure 5. Final copper and nickel contents in PbSn composite as a function of the “available” silicon.

The lowest final copper contents were achieved by eliminating copper and nickel
without prior addition of silicon. However, unacceptably high losses of lead and tin were
recorded in this process (see Figure 11). The addition of FeSi provides results comparable
to those of technical silicon. The cooling rate of the melt has no significant or reproducible
influence on the final copper and nickel contents.
The final copper content in the PbSn composite decreases slightly as casting tempera-
tures falls (Figure 6), as further copper-containing phase fractions are separated from the
PbSn composite melt in the temperature range from 700 to 600 ◦ C.

Figure 6. Final copper and nickel contents in PbSn composite as a function of casting temperature.

3.2. Quantity of Segregation and Silicide Product

Figure 7 shows that three, sometimes even four products, were produced, but that
significant quantities of copper always remained in the PbSn composite.
In the tests without precipitation reagent, the amount of the PbSn composite product is
significantly lower than in the tests with silicon addition, because the segregation product
Metals 2021, 11, 81 8 of 16

fills the complete melt volume as a porous “cake” with a high PbSn composite content.
The other proportions are dross, sample material, or metallic residues, which will not be
considered in detail in the further evaluation.

Figure 7. Quantity distribution for the products depending on the “available” silicon (cooling
rate ∼5 ◦ C/min, temperature segregation 650 ◦ C).

The casting temperature has no significant influence on the phase distribution. At

700 ◦ C, the phases are almost completely separated from each other and can be recovered
separately. There is also no dependence of the quantity distribution on the cooling rate.
Even in the tests with FeSi addition, a clear dependence of the mass distribution on
the “available” silicon is apparent. The proportion of the poured PbSn composite and the
silicide product increases. At the same time, the proportion of the undesirable “further”
segregation product is decreasing (Figure 8).

3.3. Degree of Copper and Nickel Separation

The following approach (Equation (2)) is used to determine the degree of removal
for copper and nickel. The composition and quantity of raw PbSn composite (RMZ)
and refined PbSn composite (MZ) are only subject to a minor error. If the yield of the
removed copper and nickel contents is calculated via the silicide or segregation product
analyses, large inaccuracies result due to the inhomogeneous segregation product structure
(only conceivable via complete homogenization/sampling).
RMZ − m MZ
mCu,Ni
Cu,Ni Cu,Ni
ηRemoval = RMZ
· 100% (2)
mCu,Ni

Figure 9 shows that by increasing the silicon or FeSi rate, the degree of removal can
be increased from about 55% copper to up to 85% copper. More than 90% copper can be
removed by silicon-free segregation, but as mentioned above, with massive lead–tin losses
(Figure 11). Nickel is completely removed in all cases.
Metals 2021, 11, 81 9 of 16

Figure 8. Quantity distribution of the products of the experiments with FeSi addition as a function of
the “available” silicon (cooling rate ∼5 ◦ C/min, temperature segregation 650 ◦ C).

Figure 9. Percentage of copper and nickel separation in relation to the entry as a function of the
“available” silicon.

Figure 10 illustrates that the values of Si effectiveness (calculated by Equation (3)), i.e.,
the copper and nickel contents removed from the PbSn composite per kg of “available”
silicon, decrease significantly with increasing silicon addition or “available” silicon.

kg (Cu + Ni )removed
ηSi = (3)
kg Si available
Metals 2021, 11, 81 10 of 16

Figure 10. Cu and Ni mass removed from the PbSn composite per kg of “available” Si as a
function of the “available” Si.

This leads to the conclusion that with a higher silicon supply, there is either the
formation of phases with low copper and/or nickel contents, or that the higher silicon
contents or silicon remains “unused” in the segregation product. On the other hand,
this disadvantage is offset by lower final copper contents in the PbSn composite.
The consideration of the degree of copper and nickel removal for the other test
parameters (cooling rate and casting temperature) showed no dependencies.

3.4. Lead and Tin Losses in the Segregation Products

Figure 11 shows the losses of lead and tin in the segregation or silicide product in
relation to the flow. Lead losses are constantly below 1.5%, while tin losses increase from
about 2 to 5% with increasing silicon addition. At over 20%, lead losses are significantly
higher by a factor of 10 to 15, and tin losses of over 50% are significantly higher when
segregating without silicon addition, and do not allow this method for an economical
refining of the raw PbSn composite.

Figure 11. Lead and tin losses in the silicide product depending on the “available” silicon (cooling
rate 5 ◦ C/min, temperature segregation 650 ◦ C).
Metals 2021, 11, 81 11 of 16

The specific lead and tin losses in the silicide product per kg of “available” silicon
show a decreasing tendency for the tests with technical silicon, whereas no tendency is
visible for the tests with FeSi addition in the investigated range (Figure 12).

Figure 12. Specific lead and tin losses in the silicide product per kg of “available” silicon as a function
of “available” silicon.

3.5. Experiments with Two-Stage Process Control

The temperature of 900 ◦ C selected for casting the first PbSn composite melt allowed
only a silicide product with high nickel content to be segregated in the first stage. As ex-
pected, the copper contents in the silicide product are significantly lower than in the
single-stage tests. In the first stage, only about 46% of the nickel and 4.5% of the copper are
converted into a silicide product, although silicon addition was designed for 100% nickel
removal as NiSi2 . The yield of silicon in the silicide product is about 23%, as a large part (of
the comparatively small amount) of silicon is oxidized. Lead and tin losses are extremely
low at 0.34% and 0.63%, respectively. This first stage still has potential for optimization.
The second stage resulted in a PbSn composite quality comparable to that of single-
stage process control. The remaining nickel was completely removed and the final copper
content was 3.85%. The result was a copper-rich (67% Cu) silicide product with a nickel
content of about 12% and a silicon content of about 14%. Thus, it allows partial pre-
separation of nickel from the overall process without major copper losses. However, the
two-stage process requires the PbSn composite to be overheated twice to 1300 ◦ C with a
corresponding duration for the silicon solution.

3.6. Metallographic Examination on the Phase Formation in the Silicide Product

A sample of the silicide product from one of the trials was cut out over its entire height,
polished, and then examined by scanning electron microscopy (GeminiSEM 500, from
company Carl Zeiss AG, Oberkochen - Germany). Table 4 shows the average composition
of the silicide product, which was determined in advance by wet chemical analysis.

Table 4. Result of the wet chemical analysis of the silicide product.

Element Pb Sn Cu Ni Si
[%] 1.21 8.80 51.10 21.00 19.10

In the SEM, several samples from the upper edge (phase boundary silicide product–
atmosphere) to the lower edge (phase boundary silicide product–PbSn composite) were
scanned and the occurring phases were examined for their composition. The following
Metals 2021, 11, 81 12 of 16

drawing (Figure 13) shows the locations where the samples were taken from the silicide
product and the position of the selected images.

Figure 13. Drawing of the point where the sample was taken from the silicide product to conduct the
SEM analysis shown in Figure 14.

Figure 14 shows the SEM images of each analyzed section. Further analysis were made
in the regions within the marked square for the sections a, b, and f. These are illustrated in
Figure 15.

Figure 14. SEM images of the silicide product (subfigure (a) is from the phase boundary between
silicide product and atmosphere, and subfigure (f) is from the phase boundary between silicide
product and PbSn composite. Subfigures from (b–e) refers to the silicide product. The rectangles
indicates the location of the partial sections seen in Figure 15.
Metals 2021, 11, 81 13 of 16

Figures 14 and 15 of the silicide product show that predominantly coarse, primarily
solidified crystals are visible. These are surrounded by a residual phase which subsequently
solidified. In addition, other phases (white areas), which are also enclosed by residual
melt, can be seen in isolated cases. Those are partially attached to the primarily solidified
crystals. The regions of the silicide product at the phase boundaries to the atmosphere and
the PbSn composite are characterized by areas of fine crystalline dendrites in the matrix.

Figure 15. Enlarged sections of Figure 14a,b,f. TF = partial area, as seen in Table 5.

In the following Tables 5 and 6, the micro-analyses are listed in groups according
to their chemical composition and solidification morphology. The area percentages were
calculated based on the density and length dimensions of each group.

Table 5. Results of the phases composition within a silicide phase.

[wt.%]
Group Picture/Nr. Area Percentage [%]
Pb Sn Cu Ni Si
f /1 - - 9.33 50.89 39.78
e /1 - - 7.15 52.08 40.76
1 b /1 - - 6.59 52.66 40.76 ∼45.8
b /2 - - 8.64 50.30 41.06
Average - - 7.93 51.48 40.59
f /4 - 1.50 86.02 2.64 9.85
b /1 - - 88.20 1.62 10.18
2 a /2 - 2.17 85.55 2.65 9.63
a /TF - - 84.02 5.85 10.13
∼49.5
Average - 0.92 85.95 3.19 9.95
f /3 - - 73.09 14.52 12.38
3 b /TF - 1.16 73.46 13.69 11.69
Average - 0.58 73.28 14.11 12.04
b /2 - 98.82 1.18 - -
4 a /4 - 98.49 1.51 - - ∼4.4
Average - 98.66 1.35 - -
f /5 - 61.25 37.88 0.87 -
5 a /3 - 61.27 38.73 - - <0.1
Average - 61.26 38.31 0.44 -
6 a /1 - - - 70.62 29.38 <0.1
7 f /2 3.48 94.15 2.37 - - <0.1

Looking at the stoichiometric compositions (Table 6), the solidification mechanism

and the different compounds can be interpreted. In this case, Group 1 (as the analytical
Metals 2021, 11, 81 14 of 16

mean) is most likely to represent NiSi2 or NiSi in which copper is incorporated. Group 2
would result in Cu3 Si with low nickel contents. Group 3 would therefore represent approx-
imately Cu12 Ni2 Si5 . The other groups play only a minor role in terms of quantity, but are
indispensable for the interpretation of the processes.

Table 6. Stoichiometric composition of the phases (according to the average values).

Group Pb Sn Cu Ni Si Known Phase

1 - - 1 7 12 NiSi2
2 - - 25 1 7 Cu3 Si
3 - - 5 1 2 Cu12 Ni2 Si5
4 - 39 1 - - Sn
5 - 1 1 - - SnCu (h-Phase)
6 - - - 1 1 NiSi
7 1 47 2 - - Sn

The SEM investigations on a silicide product (silicon addition 30%), including the
knowledge from the ternary state diagram copper–nickel–silicon, reveal the following
solidification mechanisms.
• Primarily solidification of NiSi2 or NiSi (dark large crystals) occurs. Within NiSi2
copper is obviously integrated.
S ⇒ NiSi2 ∨ NiSi
• After nickel depletion, solidification of Cu3 Si or Cu12 Ni2 Si5 (light gray phase between
the dark large crystals) occurs.
S ⇒ Cu3 Si ∨ Cu12 Ni2 Si5
• The light gray phase is not homogeneous and shows white precipitates. These consist of
tin or CuSn. It is to be assumed that they will react accordingly via secondary reactions:
3Cu3 Sn + 2Si ⇒ 3CuSn + Cu3 Si
3CuSn + Si ⇒ Cu3 Si + 3Sn
• Only in the contact area of PbSn composite–silicides small amounts of lead are detected
in addition to tin and copper. These are PbSn composite adhesions.

4. Conclusions
The removal of copper and nickel from PbSn composite is achieved by simple seg-
regation without the addition of a precipitating reagent up to 650 ◦ C, whereby nickel is
removed completely and copper down to 1.7%. However, the lead and tin losses in the
resulting segregation product are very high, as, in this case, higher-melting copper–tin,
nickel–tin, or copper–nickel–tin phases are formed, as it can be expected from the phase
diagrams of the binary systems. Lower final copper contents can be achieved by continuing
the segregation to temperatures below 650 ◦ C, as copper–tin phases continue to crystallize.
Phase separation is poor with this type of classical segregation due to the low density
difference of the phases formed.
If silicon is added to the melt as a precipitating reagent, comparable final copper
and nickel contents can be achieved as in the experiments without a precipitating reagent.
Here too, nickel is completely removed in all experiments. For this form of segregation
of intermetallic phases, comparable results are obtained with technical silicon or with
FeSi75 (see Table 7). Iron did not show any influence on the refining effect of the silicon.
It can therefore be assumed that the proportion of iron bound to silicon passes through the
process quasi-“inert”. This assumption is supported by iron contents below the detection
limit in the cast PbSn composite. By using the silicon or ferro-silicon, very good separation
Metals 2021, 11, 81 15 of 16

could be achieved compared to the experiments without precipitation reagent due to the
large difference in density of the phases forming.

Table 7. Average composition of crude PbSn composite, silicide product, and PbSn composite.

Pb [%] Sn [%] Cu [%] Ni [%] Fe [%] Si [%]

Raw PbSn
63–68 34–36 ∼11 ∼2.5 <0.01 <0.01
composite
PbSn composite 62–66 30–33 3.5–6 <0.01 <0.01 <0.01
Silicide product <2 6–7 50–60 20–25 <7 15–18

Important for a sufficient copper removal is the amount of silicon actually “available”
during segregation, which cannot be equal to the added amount during the tests due to
oxidation. In the range between 8 and 20%, “available” silicon (based on the initial copper
and nickel content), nickel, and iron can be completely removed and final copper contents
of about 5% can be achieved, with higher additions even below 4%. Only 60–70% of copper
can be removed from the PbSn composite, as it is strongly bound to tin. With a higher
silicon supply, higher quality nickel–silicon compounds are formed, so that a smaller
proportion of the silicon additive is available for copper removal. An increase in silicon
addition above 30% is hardly economically viable, as with the larger volume, increasing
lead and tin losses in the silicide product can be expected. The effectiveness of the silicon
(kg [Cu + Ni]removed /kg Siavailable ) also decreases with increasing silicon turnover.
Neither the cooling rate nor the temperature of the segregating end has any influence
on the refining effect. It can be concluded from this that complete phase separation between
the PbSn composite and the silicide product has already taken place at 700 ◦ C. Likewise,
phase separation is not hindered during the tests at the maximum investigated cooling rate.
Temporarily holding the temperature at about 800 ◦ C did not improve the phase separation.
The development of a segregation crucible insert for the separation of the silicide product
forming and the PbSn composite has proved to be effective.
Metallographic investigations of the silicide product have shown that nickel–silicon
phases precipitate first. Only then are copper–silicon or copper–nickel–silicon phases
formed. Cu–Sn phases are formed in secondary reactions.
It could be shown that the removal of copper, nickel, and iron is possible by segregation
of intermetallic phases in a technically interesting, simple process. The obtained PbSn
composite can be processed in a post-refining process to a saleable alloy. The silicide
product can be returned to the copper recovery route.

Author Contributions: B.F. is the principal investigator. S.J. conceived and designed the experiments.
S.J. analyzed the data. D.C.C., S.J., and B.F. wrote and edited the manuscript. All authors have read
and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Data sharing not applicable.
Acknowledgments: The authors thank Ole Krüger for his assistance in editing this manuscript.
Conflicts of Interest: The authors declare no conflict of interest.

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