Process for recovery of silicon from a reaction mixture

A process is described for the recovery of silicon from a reaction mixture comprising silicon and an alkali metal fluoride.

BACKGROUND OF THE INVENTION 
The reaction of an alkali metal and silicon tetrafluoride to form elemental 
silicon has been described in U.S. Pat. No. 4,442,082 and in Final Report 
"Novel Duplex Vapor--Electrochemical Method for Silicon Solar Cells," 
March 1980, both of which are incorporated by reference. Generally the 
reaction may be carried out in a suitable reactor by directly contacting 
silicon tetrafluoride with an alkali metal such as sodium. In such a 
reaction, the reaction product consists essentially of a mixture of sodium 
fluoride and silicon. This mixture may be separated by aqueous leaching 
using acidic solutions to neutralize any unreacted sodium and to decrease 
the spontaneous oxidation of silicon that occurs rapidly in alkaline 
solutions. 
It is known that the presence of soluble fluoride increases the amount of 
silicon that is lost by oxidation. This phenomenon is discussed in J. 
Electrochem. Soc. Vol. 127, No. 8, pp 1848-1851 (1980) wherein data is 
given which shows that the rate of oxidation of silicon, as measured by 
the evolution of hydrogen, is accelerated by the fluoride ion. 
The applicant has discovered that the addition of an alkaline earth metal 
chloride to the leaching bath will substantially reduce the oxidation of 
the silicon and thus increases the yield of silicon by rapidly converting 
the soluble fluoride ion to a substantially completing insoluble form. It 
has been discovered that the use of this technique increases the rate of 
leaching and aids in the reduction of aluminum impurities in the silicon 
by coprecipition with the insoluble fluoride. The evolution of hydrogen 
gas is also suppressed which eliminates a safety hazard and avoids the 
formation of foam in the leaching stages that interferes with filtration. 
The cost of recovery of the silicon is also reduced because (1) the sodium 
fluoride in the reaction mixture dissolves more rapidly, (2) less silicon 
is oxidized, and (3) fewer leaching steps are required. Accordingly, it is 
a primary object of this invention to provide an improved method for the 
recovery of silicon from a reaction mixture comprising silicon and an 
alkali metal fluoride. 
It is also an object of this invention to provide an improved method for 
the recovery of silicon from a reaction mixture comprising silicon and 
alkali metal fluoride that reduces the loss of silicon caused by 
oxidation. 
These and other objects of the invention will become apparent from a review 
of the present specification. 
DETAILED DESCRIPTION OF THE INVENTION 
The invention comprises a process for the recovery of silicon from a 
reaction mixture that comprises silicon and an alkali metal fluoride. The 
process is based on the leaching of said reaction mixture with aqueous 
alkaline earth metal chloride. The aqueous leaching in the presence of an 
alkaline earth metal chloride such as calcium chloride, barium chloride or 
magnesium chloride results in the formation of a substantially insoluble 
alkaline earth metal fluoride that is easily separated from the elemental 
silicon. The separations may be carried out very efficiently because the 
insoluble alkaline earth metal fluorides do not adhere to or form cohesive 
masses that interfere with the separation of the silicon from said 
insoluble alkaline earth metal fluoride. 
The aqueous alkaline earth metal chloride leaching solution should be 
acidified with an acid. A sufficient amount of acid should be utilized to 
achieve a pH of less than 7.0 in the leaching solution. A preferred pH 
range is from 1 to 2. Suitable acids include hydrochloric, nitric, 
perchloric, hydroiodic and hydrobromic. The preferred acid is 
hydrochloric. Acids such as sulfuric and phosphoric should be avoided 
because the phosphate and sulfate ions may be carried over as insoluble 
alkali metal salts into the separated silicon. Generally, reagent grade or 
better acids should be used to avoid introducing any impurities into the 
silicon. 
A stoichiometric excess of the alkaline earth metal chloride, based on the 
amount of fluoride that is present, should preferably be utilized in order 
to provide that all of the soluble fluoride ion will be converted to 
insoluble fluoride ion. Generally a molar ratio of from 0.9:1 to 1.3:1 or 
more preferably from 1.05:1 to 1.1:1 of alkaline earth metal chloride to 
alkali metal fluoride may be utilized. The use of a large excess of 
alkaline earth metal chloride should be avoided because the level of 
impurities introduced will be directly proportional to the amount of the 
alkaline earth metal chloride that is utilized. 
The alkaline earth metal chloride is preferably added as a concentrated or 
dilute aqueous solution. Generally, from 5% by weight to a saturated 
solution may be utilized. The aqueous solution may be added all at once or 
in divided amounts. The preferred alkaline earth metal chloride is calcium 
chloride. Refined grades are preferred although commercial grades that are 
free of transition metals may be utilized.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
EXAMPLE 1 
To 5.0 g of a reaction product containing silicon and sodium fluoride 
obtained by reacting sodium and silcon tetrafluoride (14.3% theoretical 
weight of silicon) is added 150 ml of calcium chloride solution containing 
13.8 g of calcium chloride and 0.3 g of concentrated hydrochloric acid. 
After the solution is combined with the mixture of silicon and sodium 
fluoride, the amount of hydrogen generated in a closed system as described 
in J. Electrochemical Soc. Vol. 127 No. 8 pp 1848-1851 (1980), was 
determined as a basis for the estimation of the amount of silicon that was 
oxidized. After 5 hours, a total of 0.67% by weight of the silicon was 
oxidized. 
To compare the efficiency of the use of aqueous calcium chloride with the 
prior art use of aqueous sulfuric acid, leaching was carried out using 4 
ml; 2.5 ml; 2 ml; and 1 ml of concentrated sulfuric acid in 150 ml of 
water as a leaching agent. 
It was found that after 5 hours, 0.17%; 0.58%; 1.4% and 2.5% by weight, 
respectively, of the silicon was oxidized. 
This data shows that the rate of oxidation of the silicon is reduced as 
increasing amounts of acid are used. The greater efficiency of calcium 
chloride for this purpose is shown by the data which shows that five times 
more sulfuric acid than calcium chloride is required to achieve an 
equivalent decrease in the quantity of silicon lost through oxidation. 
EXAMPLE 2 
To 156 g of a reaction product which consists of silicon and sodium 
fluoride in chunks no larger than 1" in diameter is added a 95% by weight 
calcium chloride solution and 4 liters of water containing concentrated 
hydrochloric acid (1.0 ml) which is added to achieve a pH of about 1. 
After the reaction product is added, an additional 12 ml of concentrated 
hydrochloric acid is required to adjust the pH to about 1.2. No foaming is 
observed which indicates that no hydrogen is evolved and hence no 
oxidation is taking place. The slurry is stirred for about 2 hours to aid 
in dissolving the sodium fluoride. After the stirring is stopped, the 
silicon settles as a black layer. The mixture is centrifuged under 
conditions that separate solid silicon but maintain substantially all of 
the calcium fluoride as a suspended solid in the leaching liquid because 
of the different settling rates. The mixture is washed with water seven 
times and about 3.5% by weight of calcium fluoride (by arc emission 
spectrographic analysis) remains with the silicon. The residual calcium 
fluoride may be removed as slag when the silicon is melt consolidated in 
the preparation of a single crystal using Czochralski crystal growing 
technology.