1. Leaching Process of Purifying Metallurgical silicon
By: Weimi Li, Karl Visnovec, Bosco Yu Supervisor: Professor Torstein. A Utigard
Department: Material Science and Engineering
Abstract
Leaching is one method that researchers use to purify metallurgical grade
silicon (MGS) for use in solar cells. Using copper to alloy out the impurities in
MGS, determining which acid can best dissolve away the Cu-Si phase
without dissolving the Si phase was done. Starting with a eutectic
composition of Cu-Si (15wt% Si, 85wt% Cu), the alloy was dissolved using a
variety of acids which include Sulphuric (H2SO4), Hydrochloric (HCl) and
Nitric (HNO3) acids. To determine the amount of the alloy dissolved by the
acids, inductively coupled plasma (ICP) analysis was used and concluded
that HNO3, followed by HCl were best at dissolving the alloy. Leaching was
then carried out on an alloy with 97wt% Si-3wt% Cu using a variation of acid
compositions. According to the ICP tests, HNO3 was able to leach out the
highest amount of copper from the particles.
Background
-Reason for Adding Cu into MGS:
Cu3Si phases were created to absorb
impurity (Fe in Figure). Cu3Si-rich
particles were removed in the HMS
process.
-Reason for controlling the Cu-Si alloy
size range to 38-500um:
The Cu-Si alloy particles in this range
have the highest amount of Si particles
with minimal Cu3Si inclusions [1]
Fig 1. Impurity Fe in the Cu3Si phase
Table 1. EDS Result on the area containing Fe
Procedure
The addition of 2g Si particles (97wt%Si-3wt%Cu)
in
150ml of acids:
A. 10%HNO3
B. 10%HNO3 with 0.3g Ammonium Persulfate
C. 7%HNO3 + 5%HCl
D. 7%HNO3 + 5%HCl with 0.3g Ammonium
Persulfate
An overhead stirrer with a graphite impeller was
used to agitate the solution. Fig.3
The samples were leached at room temperature
for
4 hrs
Fig 3. The Set of the Carbon Stirrer
Results
-The composition of the 2g Si particles before leaching: 97wt%Si and
3wt%Cu
A B C D
ICP Result of the
leaching solution
Cu(mg/L) 3.48 3.02 2.64 1.25
Si (mg/L) 0.43 0.37 0.37 0.08
Extraction Percentage
wt% Cu 41.17 37.79 33.02 15.69
wt% Si 0.16 0.14 0.14 0.03
Weight % after
leaching
wt% Cu 1.79 1.89 2.03 2.54
wt% Si 98.21 98.11 97.97 97.46
As in the table, solution A (10% HNO3) is the most effective leaching
acid for removing the Cu3Si phase from the Si particles.
Table 2. ICP Result
SEM Image Analysis
A B C D
Si 99.49 97.86 96.96 99.32
Cu 0.51 2.14 3.04 0.68
Table 3. EDX result of sample composition
The results from the EDX are mostly in agreement with the ICP results
with the exception of sample D, which could be from a misrepresented
overall test area.
Future Work
•Crushing Process: Optimizing and narrowing the particle size range
•Leaching Process: Increasing the leaching temperature and time
Reference
[1] Mitrasinovic, Aleksandar. "Refining of Silicon." PHD.Thesis. Department of Materials
Science and Engineering. University of Toronto.
100.00100.00Total
39.9556.38Cu
1012.4Fe
50.0631.22Si
atm%wt%Element
100.00100.00Total
39.9556.38Cu
1012.4Fe
50.0631.22Si
atm%wt%Element
-Most challenging part of this project is to leach out the Cu3Si inclusions
from the Si particles.
Fig. 2 Steps in the alloying route of Cu-Si, along with the focus of the
project (highlighted). [2]
Crushing & Screening
(CS)
Alloying, Melting & Slow Cooling
(AMSC)MGS
Leaching
Solar Grade Si (SGS)
Directional Solidification
(DS)
-EDX Result
EDX test
area
-ICP result
Conclusion
After many tests, the most effective acid for leaching Cu3Si is HNO3.
Fig 6 Comparison of the leaching edge using different acids
(b) 10%HNO3 Acid(a) 7%HNO3 + 5%HCl
The 7%HNO3 + 5%HCl leached sample has more partly reacted layers
than the 10%HNO3 sample, which may prevent the mixed acid from
attacking the inner Cu3Si phase.
Si phase
-Cu3Si phase
Si phase
-Cu3Si phase
Most of Cu3Si phases at original position were leached
out, thus HNO3 is effective to dissolve the Cu3Si phase.
(a) Before leaching
Fig 5. Cu3Si phase area before and after leaching
(b) After leaching
Cu3Si phase
Si phase
Cu3Si phase
Si phase
-Cu3Si phase before and after leaching
-Cu3Si phase before and after leaching
Acknowledgment
Specially thank to Professor Utigard and his metallurgy group. Also I
would like to thank NECER for the financial assistance.
Fig 4. Cu3Si inclusion after leaching by 10%HNO3
As shown in Fig.4, there is a very high possibility that the Cu3Si
phase is an inclusion, which makes it more difficult to leach out.
-Cu3Si inclusion
Cu3Si phase
Si phase
Si phase
Cu3Si phase