Method for producing solar cell-grade silicon from rice hulls

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to the production of high purity silicon and in particular to the production of solar grade silicon from ordinary rice hulls.
Rice hulls are removed from the grain in the ordinary process of rich threshing.
The rice hulls are about 4 to 5 mm.
in length, about 1 to 2 mm.
in width, and about 0.
5 mm.
in thickness.
The untreated hull contains cellulose, lignin and siliceous material (referred to as ash subsequent to pyrolysis), the latter constituting greater than 95% SiO.
sub.
2.
The chaff of the rice stalk contains SiO.
sub.
2 and may accompany the hulls in subsequent processing or be separated by conventional sieve techniques.
In accordance with the invention, the rice hulls which hereinafter may or may not include the chaff, are subjected to a leaching step of bringing the hulls in contact with an acid such as 10-50% concentration of HCl, preferably of semiconductor grade purity.
Though HCl has demonstrated the most favorable results, other strong mineral acids such as sulfuric have been shown to be operative in the present invention.
The acid leaching is followed by a treatment of the hulls in distilled, deionized water to effect a thorough rinsing.
The process of leaching in acid and water reduces overall impurity levels of about 10,000 to 40,000 parts per million (ppm) for unprocessed hulls, to about 400 ppm.
Of particular importance is the significant reduction of calcium and magnesium which constitute mobile ions, detrimental to semiconductor performance.
Of similar importance is the substantial reduction of manganese and iron which would degrade carrier lifetimes in a semiconductor device.
Repeating the sequence particularly with different batches of acid each time may serve to further reduce the impurity content; however, the added processing has not been deemed of significant advantage.
Similarly, acid followed by caustic leaching has not proven substantially advantageous over the aforementioned procedure.
The leached hulls are dried by conventional means (i