The purification of metallurgically pure silicon having a silicon content of more than 95% by weight is accomplished by leaching with an acidic solution which substantially does not attack silicon. A mechanical treatment leading to continuous particle size reduction of the granulated silicon to be purified is combined with the chemical purification step

Pfeiffer, W.

Rath, H. J.

Sirtl, E.

English

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PROCESS FOR PURIFICATION OF SILICON
H. Rath, E. Sirtl,
and W.-Pfeiffgr
Translation of "Verfahren;zum Reinigen von Silicium",
German Patent Office; Offenlegungsschrift de 2722783, App. Date
May 20, 1977; November 30, 1978, pp. 1-7.
	 -
(NASA-TA- 7655 7) PROCESS F08 PURIFICATION OF	 N81-27221
SILICON (National Aeronautics and Space
Administration) 11 p KC A02/11F 1101 CSCL 07D	 Unclas
G3/25 26812
y
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
WASHINGTON, DC 20546
	 FEBRUARY 1981
L- -7-
STANUARD TITLE PAGE
1.	 Report No. a. Government Accession No. 3. Recipient's Catalog No.	 y
NASA TM-7655_ _7
S. Report Dote
February 1981__
1.	 Title and Subtitles
6. Perforating Organination CedePROCESS FOR PURIFICATION OF SILICON
7.
	 Author(s) S. Performing Organisation Resort Me.
H. Rath, E. Sirtl, and W. Pfeiffer
	
10. work Unit No.
Contract or Grant No.
ASW-31989. Performing Organization Name and Address
Type of Resort and Period CoveredSCITRA23
P.O. BOX 5456
SANTA BARBARA CA 93 ranslation
t14*.
12,	 Sponsoring Agency Name and Address
Spnsering Agency Code
15,	 Supplementary Notes
i
Mslat g	 en oof"Verfahren zum Reinigen von Silicium," Gfirlrman Patent
Of	 ce, VVii	 gungsschrift de 2722783, App. Date May 20, 1977;•
vember 30, 1978, pp. 1-7.
r
16.	 Abstract
17. Key words (Selected by Author(s)) 11. Distribution Statement
Unclassified - Unlimited
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19.	 Security Zlessif. (of this few) 20.	 Seeurity Clessif. (ef this pep) 21. No. of Peps 22. Price
Unclassified Unclassified 7
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NASA-NO
Int. Cl. 3	 B 01 17/18
Federal Republic of Ger+aany
German Patent Office
PATENT 27 22 783
File No. P 27 t22 783.4
Filing date:	 20 May 1977
Patent date:	 30 November 1978
Union Priority:
Title:
	
PROCESS FOR PURIFICATION OF SILICON
Applicant: Wacker-Chemitronic Company for Basic-Electronic
Materials, Inc.,	 8263 Burghausen
Inventor:
	
Rath, Heinz-Jbrg, graduate chemist; Dr.; 8263
Burghausen.
Sirtl, Erhard, graduate chemist; Dr.
Pfeiffer, Waldemar, 8261 -Markt.
2722783
	 •^:^ i•^i ='''i: l
i
e.
2722783
WACKBR-CKWTRONIC '	 Munich, 9 May 77
L-Pat/Dr.Ffwe
Company for Basic Electronic
Materials, Inc.
%.-Ch 7706
PROCESS FOR PURIFICATION OF SILICON
Patent Claims
1. Process for purification of metallurgLeally,pure silicon
with a silicon content of more than 95% by weight by leaching
silicon is an acidic solut .nr. w1dch is substantially not
corrosive, characterized by the fact that a treatment of
the silicon granulation to be purified, leading to a
continuous reduction in particle size, is superimposed
on.the previously known chemical purification step.
2. Process as in Claim 1, characterized by the fact that the
silicon granulation is ground during the action of the
aqueous acid solution.
3. Process as in Claims 1 and 2, characterized by the fact
that grinding elements of polycrystalline silicon are used.
4. Process as in one or more of Claims 1 to 3, characterized
by the fact that a rotating ball mill is used.
5. Process as in Claim 4, characterized by the fact that the
rate of rotation of the ball mill is 5 to 15 revolutions
per minute.
6. Process as in one or more of Claims 1 to 5, characterized
by the fact that the mechanochemical purification of the
silicon granulation is performed at a pressure of 1 to 1.8
bar.	 .
3
7, Process as In one or am of Claim I to 6 9
 characterized
by the fact that the NMVIWW	 Ical purification of the
silicon granulation is performed at a temperature of
20 to 80•C.
Process as in one or more of Claims I to 7, characterized
by the fact that an oxidant which does not dope silicon is
added to the aqueous acid solution.
9. Process as In Claim 89 characterized by the fact that air
is injected as the oxidant.
10. Process as In one or more of Claim 1 to 9, characterized
by the fact that the aqueous acid contains 4 to 7 moles of
acid per liter.
11. Process as in Claim 11 0
 characterized by the fact that
hydrochloric acid in used as the aqueous acid.
...
- . 12-i,-Process as in Claim 10 and 11, characterized by the fact
that the aqueous acid is composed of a mixturd-of hydrochloric
acid and hydrofluoric acid.
13. Process an in one or more of Claims I to 12,' characterized
by the fact that the silicon granulation is mixed with from
0,8 to 1.2 times its weight of acid.
The object of the invention is a process for purification
of metallurgically pure silicon having a silicon content of more
than 95% by weight by leaching with an acidic solution which
substantially does not attack silicon.
In energy considerations of recent years, generation of
electricity by*direct conversion of solar energy into electrical
4
energy has become ingreasifigly important. But, in order to
make silicon solar calls, for instance, economically competitive,
it is essential to make a substantial reduction in the cost of
the raw material of required purity, which exists on the earth
in essentially inexhaustible quantity. Gas phase purification,
such as is standard today in production of silicon for the
electronic industry, is immediately ruled out because such a
process is too energy-intensive and, therefore, too expensive
by a considerable factor. In addition, the purity requirements
for silicon to be used in solar cells are not as great as chose
for 'wafer silicon' to be used in production of expensive
components.
It is metallurgically well known, to be sure, that pure
silicon can be purified with_zorrotve acids or acid combinations;
but with the previously known processes, one either gets material
with inadgquate purity for manufacture into solar cells, or
extremely long processing times are needed (see, for instance,
L. P. Hunt et al., "Purification of Metallurgical Grade Silicon
to Solar Grade Quality", Solar Energy, Proc. Intern. Symp., 1976,
and Swiss Patent 567 435).
The invention is, then, based on the problem of purifying
metallurgically pure silicon having a silicon content greater
than 95% by weight, in a short time, enough that it can be
used for manufacture of solar cells, for example.
This problem was solved by combining a mechanical treatment
leading to continuous particle size reduction of the silicon
granulation to be purified with the chemical purification step,
which was itself well known.
This mechanical treatment is preferably performed by grinding,
as in ball mills or tube mills, which are best coated internally
with polytetrafluoroethylene, polypropylene, or other suitable
plastic which is resistant to acid and which does not liberate
•r .
impurities which dose silicon. .
5
The grinding elements may be, for instance, spherical or
cylindrical bodies of ceramic, or, preferably, silicon
carbide or.polycrystall'.ne silicon.
The size reduction of the silicon granulatim-used during
the action•of the aqueous acids can also, within the meaning
of the invention, be attained, for example, by counter-rotating
pairs of rolls of steel coated with the above-mentioned plastics.
The silicon to be purified is conveniently used as a
granulation with a particle size distribution in the range of
0.1 to 2 mm. -It is usually advantageous to use a granulation
which has already been pretreated in a known manner by action
of aqueous acid.
In the preferred ball or tube mills, the silicon granulation
is mixed with 0.5 to 2 times, and preferably 0.8 to 1.2 times
its weight of aqueous acid solution. The aqueous acid solution
may contain 3 - 10 mole, and preferably 4 - 7 mole pure acid
per liter. Suitable acids are, as examples, sulfuric acid
or hydro-halogen adds such as hydrochloric acid and hydro-
fluoric acid in particular, which can be used alone or mixed.
During the mechenochemical purification by the silicon
granulation, it is desirable to keep the mill at a pressure
of about 1 - 5 bar, preferably 1 - 1.8 bar, and at a temperature
of 10 - 90% preferably 20 - 80°C. The speed of mill rotation
is 0.5 to 50 revolutions per minute, and preferably 5 - 15
revolutions per minute.
The purifying action can be increased sometimes by addition
of oxidants, such as hydrogen peroxide, ammonium persulfate,
or other oxidizing materials which do not cause dosing of the
silicon granulation being purified, and which can remain,
decomposed or undecomposed, in the solution or are drawn off
as gas. Such chemical oxidants should be at most 2 mole percent
6
based on the silicon content, and preferably at 0.8 - 1.3 mole-%.
Air has proved to be the preferr*d_ oxidant. It is blown in
at a rate of 0.2 to 5.-standard cubic meter per hour per unit
volume of mill, and preferably at 0.4 - 0.8 standard cubic
mater per hour per unit volume of mill. The advantageous
effect of the air is particularly that it causes added mixing
of the reaction material. Also, of course, it is cheaper than
the chemical oxidants listed above.
The invention is explained by means of the figure, showing
a tube mill such as can be used to carry out the process of the
invention:
A cylindrical steel drum 1 coated with polypropylene, for
example, is surrounded by a heating jacket 2 through which
a heating fluid can be pumped by means of the pipe connections
3 and 4. These are connected through the axle with the
input and exhaust lines 5 and 6. It is filled through the
opening 7 with the silicon granulation to be purified, 8,
with the grinding elements, 9, and with the acid solution, 10,
so that the drum 1 is almost half full. The electric motor 11
working through the gear drive 12 rotates the drum about its
longitudinal axis. The and of drum 1 opposite the gear drive, 12;
has a coaxial tube through the axle, and air can be blown into
f the drum through the inner tube 13, with air or exhaust gas
exiting again through the outer tube 14. The drum 1 with
bearings 15 and 16 stands on the supports, 17 and 18, which
are solidly anchored in the base, 19. The time of treatment for
the raw silicon charge placed in the drum depends on the 14-vel
of purification. Through the steady rotation of the drum, the
crude silicon used is ground by the grind'.g elements, made of
polycrystalline silicon, for instance ., so new grain faces are
continuously exposed. The impurities, preferentially excluded
into the grain faces, can be treated through reaction with the
corrosive acid mixture.
7
r
After termination of the mechanochemical treatment, the entire
charge is cooled and dumped. The silicon is ,filtered off.
The grinding elements, much larger than the largest particles of
the .treated silicon granulation, are also removed. The silicon,
purified in this manner, is further processed after drying.
In the further processing it may be melted and cast into plates
or rods or other shapes, or it may be used as the raw material
for for-subsequent segregation processes, such as zone purification
or crucible pulling processes. Even withouL these further
purifications, the silicon can be used directly wherever its
electrical properties are not important, as in alloys, for
instance.
Example 1:
A tetrafluoroethylene jar insert with a volume of about 250 cm3
and with a cover is fitted into the stainless steel jar of a
bell mill. The grinding elements are three cylindrical
polycrystalline silicon pieces with a diameter of 30 mm and
a length of about 35 mm, with ground edges. Then silicon,
granulation with a particle size between 0.2 and 1.5 mm is added.
The silicon granulation has already been pre-purified by leaching
with hydrochloric acid and contains impurity levels of 400 ppm
iron and 250 ppm aluminum by weight, along with the usual
impurities, boron, phosphorus, titanium, chromium, vanadium,
manganese, nickel, copper, calcium and magnesium. This silicon
granulation is then mixed with 100 ml 18% aqueous hydrochloric
acid and 5 ml 40% aqueous hydrofluoric acid. The mill is
rotated at about 30 revolutions per minute. The temperature
of the s¢lt solution is 20-25°C. After 5 hours the mill is
turned off and the silicon separated. After drying, the
iron and aluminum contents were determined by analysis: the
iron content had diminished to 20 ppm and the aluminum content
to 50 ppm by weight.
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8
Example 2:
A polypropylene cylinder with a volume of 2.12 m3 was fitted into
a steel drum with an inside diameter of 100 cm and a length of
270 cm. The polypropylene cylinder was then filled with 500 kg
granulated crude silicon with a mean particle size of 0.2 - 1.5 mm,
500 kg 18% aqueous hydrochloric acid, and 25 k3 40% aqueous
hydrofluoric acid, as well as about 500 grinding elements of
polycrystalline silicon cylinders of various dimensions, with
diameters between 18 and 75 mm, and with lengths between 20 and
50 mm. Then the drum was rotated at 12 revolutions per minute.
The temperature in the-dr mi was adjusted with the heating jacket
to about 80°C. Air was blown into the drum through the axial
air inlet at 1 m3 per hour. After a treatment period of
6 hours the process was stopped. The silicon was separated
and dried, and the contents of the major impurities, iron
and aluminum, were again determined. The iron content had
dropped from the previous 400 ppm by weight to about 10 ppm,
and the a)- minum content had dropped from 250 ppm to less than
20 ppm.
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Process for purification of silicon

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