Method for grinding silicon metalloid

A method for grinding silicon metalloid comprising grinding silicon metalloid in the presence of an effective amount of a grinding aid selected from the group consisting of carboxylic acids comprising at least 8 carbon atoms, alkali metal salts of carboxylic acids comprising at least 8 carbon atoms, and polydiorganosiloxanes.

BACKGROUND OF INVENTION 
The present invention is a method for grinding silicon metalloid comprising 
grinding silicon metalloid in the presence of an effective amount of a 
grinding aid selected from the group consisting of carboxylic acids 
comprising at least 8 carbon atoms, alkali metal salts of carboxylic acids 
comprising at least 8 carbon atoms, and polydiorganosiloxanes. 
Silicon metalloid (herein in the alternative referred to as "silicon") is a 
key raw material in organohalosilanes produced commercially in the 
fluidized-bed process often referred to as the Direct Process. In the 
Direct Process for producing alkylhalosilanes, an alkylhalide is reacted 
with particulate silicon in the presence of a catalyst comprising copper. 
It has long been recognized that in the Direct Process the size of the 
particulate silicon is important in determining the efficiency of the 
reaction of alkyl halides with silicon to form alkylhalosilanes. The 
efficiency of the reaction of alkyl halides with silicon is determined 
based on the amount of silicon charge that is converted to 
diakyldihalosilane and this conversion is in part dependent on the silicon 
particle size. 
Windle, U.S. Pat. No. 3,604,634, describes a method of grinding calcium 
carbonate minerals comprising the steps of (a) forming an aqueous 
suspension comprising at least 25 percent by weight of the calcium 
carbonate mineral, and (b) grinding the aqueous suspension of calcium 
carbonate mineral with a particulate grinding material such as silica 
sand, calcined clay, flint granules, ceramic and glass beads. 
Manfroy et al., U.S. Pat. Nos. 4,162,044 and 4,274,599, describe a process 
for grinding coal or ores containing metal values comprising carrying out 
the grinding in a liquid medium and with a grinding aid comprising an 
anionic polyelectrolyte derived from polyacrylic acid and dispersible in 
the liquid medium, the grinding aid being present in an amount effective 
to provide increased grinding efficiency. 
Klimpel et al., U.S. Pat. No. 5,131,600, describe a process for the wet 
grinding of silica or siliceous gangue-containing solids comprising 
carrying out the grinding operation in the presence of liquid medium and 
at least one alkanol amine dispersible in the liquid medium. 
Freeburne et al., U.S. Pat. No. 5,312,948, teach a process for the reaction 
of an alkyl halide with particulate silicon in a fluidized-bed process, 
where the particle size of the silicon is within a range of one micron to 
85 microns. 
The present invention provides a method for grinding silicon in the 
presence of an effective amount of a grinding aid selected from the group 
consisting of carboxylic acids comprising at least 8 carbon atoms, alkali 
metal salts of carboxylic acids comprising at least 8 carbon atoms and 
polydiorganosiloxanes to increase mill capacity, increase silicon grinding 
rate, increase silicon flowability, decrease silicon agglomeration and to 
narrow silicon particle size distribution. 
SUMMARY OF INVENTION 
The present invention is a method for grinding silicon comprising grinding 
silicon in the presence of an effective amount of a grinding aid selected 
from the group consisting of carboxylic acids comprising at least 8 carbon 
atoms, alkali metal salts of carboxylic acids comprising at least 8 carbon 
atoms, and polydiorganosiloxanes. 
DESCRIPTION OF INVENTION 
The present invention is a method for grinding silicon metalloid comprising 
grinding silicon metalloid in the presence of an effective amount of a 
grinding aid selected from the group consisting of carboxylic acids 
comprising at least 8 carbon atoms, alkali metal salts of carboxylic acids 
comprising at least 8 carbon atoms, and polydiorganosiloxanes. 
The silicon useful in the method may be chemical grade silicon metal often 
used in the direct synthesis of methylchlorosilanes which have an 
elemental composition of 0.100 to 0.280 Wt. % aluminum, 0 to 0.150 Wt. % 
calcium, 0.150 to 0.500 Wt. % iron and 0.015 to 0.050 Wt. % titanium. 
Chemical composition can enhance the reactivity and selectivity of the 
reaction to produce the alkylhalosilanes. Atomized silicon metal may also 
be employed in the method, where the chemical composition is 0.05-1% by 
weight Fe; 0.01-1% by weight Al; 0.0001-1% by weight Ca; 0-0.5% by weight 
Na 0-0.5% by weight Li; 0-0.5% by weight K; 0-0.5% by weight Mg; 0-0.5% by 
weight Sr; 0-0.5% by weight Ba; 0-0.5% by weight Be; and the remainder 
other impurities in small amounts. Also employable in the method is less 
expensive particulate water granulated silicon. 
The grinding may be conducted in a mill such as a ball mill where the 
milling is performed by means of a rotating chamber within which there are 
placed free-rolling grinding media such as steel, stainless steel or 
tungsten carbide balls, ceramic cylinders, or flint pebbles, plus the 
silicon to be ground. The rotating chamber causes the grinding media to 
engage in abrasive action, crushing and grinding the silicon to the 
desired particle size. Preferably, the silicon is ground in a ball mill 
with stainless steel balls as the grinding media, however the grinding 
media may be any material that is compatible with the ground silicon. The 
ground silicon may be further classified as to particle size distribution 
by means of, for example, screening or use of mechanical classifiers such 
as a rotating classifier. 
The grinding aid may be fed into the mill, for example, by using a metering 
pump. 
The method for grinding silicon may be conducted in a batch process, 
semi-batch process, or a continuous process. 
In the present method, it is desirable to grind the silicon to a particle 
size within a range of one micron to about 150 microns. Preferred, the 
silicon particle size is within a range of one to about 85 microns. Most 
preferred, the silicon particle size is within a range of two to about 50 
microns. It is preferred that the silicon have a particle size mass 
distribution characterized by a 10.sup.th percentile of 2.1 to 6 microns, 
a .sub.50.sup.th percentile of 10 to 25 microns, and a 90.sup.th 
percentile of 30 to 60 microns. Most preferred is when the particle size 
mass distribution of the silicon is characterized by a 10.sup.th 
percentile of 2.5 to 4.5 microns, a 50.sup.th percentile of 12 to 25 
microns, and a 90.sup.th percentile of 35 to 45 microns. 
The grinding aids used in the present method are selected from the group 
consisting of carboxylic acids comprising at least 8 carbon atoms, alkali 
metal salts of carboxylic acids comprising at least 8 carbon atoms, and 
polydiorganosiloxanes. An example of a suitable carboxylic acid are oleic 
acid, heptanoic acid, heptanedioic acid, 3-biphenylcarboxylic acid, 
naphthoic acid, octanedionic acid, nonanedioic acid and decanedioic acid. 
Alkali metal salts of carboxylic acids useful in the method contain at 
least 8 carbon atoms and include, for example sodium salts and potassium 
salts of carboxylic acids. Examples of alkali metal salts of carboxylic 
acids are potassium oleate, sodium oleate, potassium heptanoate, potassium 
heptanedioate, potassium 3-biphenylcarboxylate, potassium naphthoate, 
potassium octanedionioate, potassium nonanedioate, potassium decanedioate, 
sodium heptanoate, sodium heptanedioate, sodium 3-biphenylcarboxylate, 
sodium naphthoate, sodium octanedionioate, sodium nonanedioate, and sodium 
decanedioate. 
The polydiorganosiloxanes useful as grinding aids in the present method can 
have a viscosity in the range of from 0.65 to about 2000 mm.sup.2 /sec at 
25.degree. C. Examples of suitable polydiorganosiloxanes are 
octamethylcyclotetrasiloxane, decamethylpentasiloxane and 
polydiorganosiloxanes, such as, for example polyphenylmethylsiloxane, 
poly-3,3,3-trifluoropropylmethylsiloxane and trimethylsiloxane endblocked 
polydimethylsiloxane fluids having a viscosity in the range of from 0.65 
to about 2000 mm.sup.2 /sec at 25.degree. C. Preferably the 
polydiorganosiloxane fluids have a viscosity in the range of from 1.0 to 
about 1000 mm.sup.2 /sec at 25.degree. C., and most preferable the 
viscosity is from 10 to about 20 mm.sup.2 /sec at 25.degree. C. The 
polydiorganosiloxanes are not limited to trimethylsiloxane endblocked 
polydiorganosiloxane fluids, but may include phenylmethyl, alkylmethyl, 
methylvinyl, and hydroxy endblocked fluids. A specific example of a 
trimethyl endblocked polydimethylsiloxane fluid useful in the present 
method is DOW CORNING 10 centistoke 200 Fluid.TM. (DOW CORNING 
CORPORATION, Midland, Mich.). Also useful in the present method are metal 
containing polydiorganosiloxanes such as trimethylsiloxane endblocked 
polydimethylsiloxane fluids containing metals such as copper, iron, zinc, 
and tin. In a preferred method a trimethylsiloxane endblocked 
polydimethylsiloxane fluids containing titanium, zirconium or hafnium is 
used as the grinding aid. A specific example of a metal containing 
polydiorganosiloxane useful in the present method is SYLTHERM 800.TM. (DOW 
CORNING CORPORATION, Midland, Mich.). 
An effective amount of grinding aid is any amount that effects at least one 
of the following: increases mill capacity, increases silicon grinding 
rate, increases silicon flowability, decreases silicon agglomeration, or 
narrows silicon particle size distribution. Typically, an effective amount 
of grinding aid ranges from about 10 ppm to 1000 ppm. Preferably the 
amount of grinding aid ranges from about 100 ppm to 300 ppm. The optimum 
amount of grinding aid used may depend on such factors as the particular 
particle size distribution desired, mill type, and the grinding aid used. 
The maximum amount of grinding aid used is typically limited by economic 
constraints. 
In grinding aid studies, literature sources cite the angle of repose as a 
measure of flowability in grinding aid studies. Increased mill capacity, 
increased grinding rate, increased silicon flowability, decreased silicon 
agglomeration, and narrowed silicon particle size distribution can all be 
determined by measuring the angle of repose. Angle of repose can be 
measured by pouring the ground silicon through a funnel with a 0.95 cm 
drain hole positioned three inches above a flat surface until a pile 5.08 
cm to 7.62 cm high is formed. The angle of repose is calculated from the 
arc-tangent of twice the cone height divided by the pile diameter. In the 
present method amounts of the grinding aid which decrease angle of repose 
by 2 degrees to 3 degrees are considered as significant. Using grinding 
aids described herein, the angle of repose was reduced from a baseline 
average of 46 degrees to as low a 33 degrees. Flowability increases with 
the decrease in the angle of repose. Enhanced flowability can provide 
faster ground silicon removal from the production mill which leads to 
increase mill capacity, narrowed particle size distribution, and increased 
silicon grinding rate.

The following examples are provided to illustrate the present invention. 
These examples are not intended to limit the scope of the provided claims. 
EXAMPLE 1 
Evaluation of grinding silicon metal without a grinding aid. Silicon metal 
(800 g) was sieved to 4.times.6 mesh and charged into a cylindrical 
laboratory ball mill 25.4 cm in diameter and 18.4 cm in length containing 
135, 2.54 cm diameter stainless steel balls totaling 8253 g in weight to 
grind the silicon metal. The ball mill was sealed and purged with nitrogen 
for 2 minutes. The ball mill was rotated at 51 rpm for 20 minutes. The 
resulting powder was sieved to 100 mesh and analyzed for particle size 
distribution using a Sedigraph Model 5100 manufactured by Micromeritics, 
One Micromeritics Drive, Norcross, Ga., 30093-1877. The results of the 
particle size analysis were used to calculate the cumulative mass percent 
for geometric size intervals from 13 to 150 microns. Data for geometric 
size intervals from 13 to 150 microns and the angle of repose, are 
presented in Table 1. 
EXAMPLE 2 
Evaluation of grinding silicon with oleic acid as a grinding aid. 
The procedure as described in Example 1 was followed with the exception 
that 100 ppm of oleic acid was added to the ball mill. Data for geometric 
size intervals from 13 to 150 microns and angle of repose are presented in 
Table 1. 
EXAMPLE 3 
Evaluation of grinding silicon with SYLTHERM 800.TM. as a grinding aid. The 
procedure as described in Example 1 was followed with the exception that 
100 pm of SYLTHERM 800.TM. was added to the ball mill. Data for geometric 
size intervals from 13 to 150 microns and angle of repose are presented in 
Table 1. 
EXAMPLE 4 
Evaluation of grinding silicon with DOW CORNING 10 centistroke 200 
Fluid.TM. as a grinding aid. The procedure as described in Example 1 was 
followed with the exception that 100 ppm of DOW CORNING 10 centistoke 200 
Fluid.TM. was added to the ball mill. Data for geometric size intervals 
from 13 to 150 microns and angle of repose are presented in Table 1. 
EXAMPLE 5 
Evaluation of grinding silicon with potassium oleate as a grinding aid. The 
procedure as described in Example 1 was followed with the exception that 
100 ppm of potassium oleate was added to the ball mill. Data for geometric 
size intervals from 13 to 150 microns and angle of repose are presented in 
Table 1. 
TABLE I 
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Angle of Repose and Cumulative Mass Percent 
Silicon DOW 
Metal CORNING 
Geometric 
Without 10 
Size Grinding Oleic Syltherm centistoke 
Potassium 
(Microns) 
Aid Acid 800 .TM. 200 Fluid .TM. 
Oleate 
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Cumulative Mass Percent 
150 62% 68% 68% 66% 67% 
106 55% 59% 59% 55% 59% 
75 46% 49% 49% 48% 51% 
53 37% 39% 41% 38% 42% 
38 30% 31% 33% 32% 32% 
27 23% 23% 24% 22% 26% 
19 17% 15% 17% 17% 18% 
13 12% 10% 11% 11% 11% 
Angle of 
45.8 43.0 32.8 34.1 38.3 
Repose 
(Degrees) 
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