Abstract:
A method and an apparatus for removing fine-grain silicon material from coarse-grain ground silicon material are disclosed. In the method, ground silicon material is selected that exhibits a predominantly brown color in an aqueous suspension, indicating that a considerable fraction of the ground silicon material has a grain size of less than 0.25 μm, and the ground silicon material is supplied to a reaction vessel. An aqueous or water-containing solution of a base is added to the ground silicon material, causing an etching process which chemically removes a fine fraction with a grain size of less than approximately 1 μm. Acid or water is then added to terminate etching and cause rapid sedimentation of a suspension in form of a relatively coarse-grain solid, which can be removed for further processing. The solution formed above the relatively coarse-grain solid can also be withdrawn.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application claims the priority of German Patent Application, Serial No. 10 2008 055 833.8, filed Nov. 4, 2008, pursuant to 35 U.S.C. 119(a)-(d), the content of which is incorporated herein by reference in its entirety as if fully set forth herein. 
       BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to a method for removing fine-grain silicon material, in particular with a grain size of less than 1 μm, from coarse-grain ground silicon material with a grain size of less than 500 μm. The present invention further relates to an apparatus for removing fine-grain silicon material with the method. 
         [0003]    The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention. 
         [0004]    Comminution processes produce a ground material with an inhomogeneous distribution of grain sizes independent of the way energy is introduced and independent of the technical conversion and the employed apparatus. Fine-grain fractions not only represent a loss of ground material; they may also have serious consequences for configuring a technical process. For example, fine-grain fractions can accumulate inside fittings (e.g., valves, faucets, pumps, etc.) and thereby impair their functionality or cause failure. Fine-grain particles can also rapidly clog filters and strainers. 
         [0005]    Moreover, fine-grain materials in form of dust may represent a health risk and may cause an explosion. This includes silicon, which is very reactive in small grain sizes. 
         [0006]    Comminution processes can also cause significant contamination of the ground material. This is caused by mechanical abrasion on the material of the employed apparatus, for example a ball mill or a crusher caused by the process. Abrasion is expected to increase with increasing milling time and with the hardness of the milled material. 
         [0007]    When comminuting silicon, in particular the small grain sizes (smaller than 10 μm) and the smallest fractions (grain sizes in a range smaller than 0.25 μm) are undesirable, wherein the latter can be identified from their brown color. These can no longer be separated using dry screening processes, and are also challenging in other separation processes. 
         [0008]    Typically, mechanical separation processes result in a ground material fraction which includes material with grain sizes from 0 μm to a grain size representing the largest size that can still be separated (comminuted) mechanically. This upper limit is between 1 and 50 μm, depending on the complexity of the process. The grain size is predominantly between 20 and 50 μm with dry screening processes. If the desired grain size fraction is smaller or an additional separation step in a range of less than 1 μm is to be performed, in order to separate for the aforementioned reasons the fraction of the very fine material, then a serious problem arises. 
         [0009]    It would therefore be desirable and advantageous to address prior art shortcomings and to effectively remove in a relatively simple and cost-effective manner from a ground silicon material with grain sizes smaller than 500 μm a fine-grain silicon material (“fine-grain fraction”, “fine fraction” or “very fine fraction”), in particular material with grain sizes less than 1 μm, while allowing the use of less fine silicon material (“coarse-grain fraction”, “coarse fraction”) for further processing, without significant losses. 
       SUMMARY OF THE INVENTION 
       [0010]    According to one aspect of the present invention, a method for removing fine-grain silicon material with grain sizes of less than 1 μm from ground silicon material having grain sizes of less than 500 μm, includes the steps of selecting ground silicon material that exhibits a predominantly brown color in an aqueous suspension, indicating that a considerable fraction of the ground silicon material has a grain size of less than 0.25 μm, and supplying the ground silicon material to a reaction vessel. The method further includes adding an aqueous or water-containing solution of a base, for example, NaOH or KOH to the ground silicon material, causing an etching process which produces a significant volume of foam and chemically removes a fine fraction with a grain size of less than approximately 1 μm, and adding acid, for example HCl, or water to terminate etching and cause rapid sedimentation of a suspension in form of a relatively coarse-grain solid. The sedimented relatively coarse-grain solid is then provided for further processing, or a solution formed above the relatively coarse-grain solid is withdrawn, or both. 
         [0011]    According to yet another aspect of the invention, an apparatus for removing fine-grain silicon material with grain sizes of less than 1 μm from ground silicon material having grain sizes of less than 500 μm includes a reaction vessel constructed by interconnecting a storage vessel for the ground silicon material, a storage vessel for a base or a aqueous or water-containing solution of a base, and a storage vessel for an acid, a controller for controllably supplying the ground silicon material, the base or the solution of the base and the acid to the reaction vessel, a device for agitating contents of the reaction vessel, a unit for interrupting supply of acid to the reaction vessel either at a time predetermined by a timer or at a time determined by a measuring device, a first withdrawal device installed on the reaction vessel for removing a sedimented fraction of a relatively coarse-grain solid, and a second withdrawal device installed on the reaction vessel for removing a liquid residing above the sedimented fraction. 
         [0012]    The disclosed process operates by suspending and chemically dissolving the very fine fraction of the ground silicon material. Importantly, instead of acid, an aqueous or water-containing solution of a base, such as in particular NaOH and KOH, is used. 
         [0013]    Hydrogen is produced when the surface of silicon is etched. As a result of this etching process, the entire suspension begins to move and foam, with small gas bubbles being released continuously. This foaming effect is very strong, in particular when silicon particles with grain sizes of approximately 0.25 μm or less are present. Ground silicon material produced with technical grinding processes frequently results in particles where up to 70% of the particles have grain sizes less than 100 μm, with a significant fraction thereof again having grain sizes of less than 0.25 μm. Accordingly, the preferred application of such ground material likewise produces a strong foaming effect. Such ground material of very fine grain size is also produced as waste material when silicon is processed (in the form of single pieces, such as blocks (e.g., ingots) or of discs, in particular during cutting, sawing, polishing, drilling). Such ground material is also commercially available. 
         [0014]    The term “significant foaming” is to be understood here as an increase in volume of at least 10%. Under certain circumstances, this increase in volume can reach 100%. The level of the strongly foamed suspension will typically be at least 1-3 cm. 
         [0015]    The aforementioned suspension can be agitated and transported without adding mechanical complexity. 
         [0016]    It has been observed: the smallest silicon grains are completely dissolved due to their higher reactivity, whereas the larger grains are only slightly etched. If the reaction time is selected accordingly, then the fraction of the smallest grains is completely dissolved and hence removed from the ground material. In this way, a desired grain size distribution and a desired separation step can be implemented. In this way, coarser ground silicon material with a considerably reduced very fine grain fraction can be obtained for semiconductor production or solar cell manufacturing. 
         [0017]    Advantageously, the quantity of the added base may be selected so that etching by the base is terminated after the base is completely consumed. 
         [0018]    The reaction can be terminated by adding large quantities of water (dilution) or, more effectively, by adding acid up to the neutral point (neutralization) or into the acidic range. It has been observed that the remaining particles precipitate very quickly regardless of their size and that the solution above the sentiment is clear and free of smallest silicon particles, e.g., has grain sizes smaller than 1 μm. Importantly, the precipitated solid can be easily suspended by agitation, in order to be transported or to start another process. 
         [0019]    The process takes advantage of the following fact: in a suspension of the ground silicon material, grains larger than about 0.25 μm have a black color. Conversely, silicon material with a grain size equal to or less than about 0.25 μm has a brown color. According to a preferred embodiment, the end of the etching process is determined or initiated by determining the color, preferably by determining the transparency or clarity. 
         [0020]    Instead of interrupting the etching process by introducing acid from the acid storage vessel followed by neutralization, etching can also be interrupted by dilution through the introduction of water from a water storage vessel. 
         [0021]    For this purpose, a device for measuring a clarity of a suspension residing in the reaction vessel may be provided. When this device indicates “clarity” or “transparency” of the suspension, the etching process is concluded, and additional steps (e.g., withdrawing the sedimented fraction and/or the liquid above) may follow. 
         [0022]    Importantly, a safety device may be provided to prevent a level of a foamed suspension formed in the reaction vessel from rising above a maximally allowed level. 
         [0023]    As described above, hydrogen is produced during foaming. The hydrogen should be carefully vented from the reaction vessel through an exit port. 
         [0024]    According to another advantageous feature of the present invention, dedicated control valves may be provided for controlling the inflow of the ground silicon material, the base or the aqueous or water-containing solution of the base and the acid. 
         [0025]    It should also be mentioned that in the semiconductor industry, in particular in the manufacture of solar cells, silicon material with a small particle size (less than approximately 1 μm) is undesirable. Instead, material with greater particle size, i.e. material that does not include very fine grains, is used in photovoltaic applications. Such coarser material can be produced cost-effectively with the disclosed method and the described apparatus from conventional, grainy silicon material or from the aforedescribed waste silicon generated during silicon processing. This represents a significant advantage. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0026]    Other features and advantages of the present invention will be more readily apparent upon reading the following description of currently preferred exemplified embodiments of the invention with reference to the accompanying drawing, in which: 
           [0027]      FIGS. 1-5  show schematic illustrations of a content of a vessel, after undergoing different process steps according to the present invention; and 
           [0028]      FIG. 6  shows a schematic illustration of a device for removing very fine-grain silicon particles from a grainy ground silicon material. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0029]    Throughout all the Figures, same or corresponding elements may generally be indicated by same reference numerals. These depicted embodiments are to be understood as illustrative of the invention and not as limiting in any way. It should also be understood that the figures are not necessarily to scale and that the embodiments are sometimes illustrated by graphic symbols, phantom lines, diagrammatic representations and fragmentary views. In certain instances, details which are not necessary for an understanding of the present invention or which render other details difficult to perceive may have been omitted. 
         [0030]      FIGS. 1 to 5  illustrate schematically a principle illustration of various process steps of a method according to the invention with reference to an exemplary embodiment. The described approach was successful in the laboratory, but can also be implemented on a commercial scale. 
         [0031]    Turning now to the drawing, and in particular to  FIG. 1 , in a first step grainy ground silicon material  4  is introduced in a vessel  2 . 3 kg mechanically ground, e.g., commercially available silicon with a fraction having a grain size smaller than 50 μm was used. The employed vessel  2  has a diameter of about 20 cm. The ground silicon material  4  contains about 5 wt.-% of fine-grain silicon with grain sizes below 0.25 μm. It should be noted that the vessel  2  in  FIG. 1  contains only solid ground silicon material  4  with a “fine fraction” and a “coarse fraction.” The fine fraction is to be removed without causing a noticeable loss of the silicon coarse fraction. 
         [0032]    In a second step, the silicon coarse fraction  4  is mixed with about 2.3 l deionized water H 2 O and suspended by stirring. The suspension  6 , shown in  FIG. 2 , has a brown color produced by the fine fraction of the ground silicon material  4 , in particular by a fraction with a grain size of less than 0.25 μm. 
         [0033]    In a third step, a base, in particular about 0.2 l of a NaOH solution with 4.4 wt.-%, is added immediately thereafter, while the silicon is further suspended by stirring. This suspension is indicated with the reference symbol  8 . In this described exemplary embodiment the resulting base concentration was about 0.05 wt.-%. 
         [0034]    It should be mentioned that the second and third step can also be combined. 
         [0035]    This suspension  8  undergoes an immediate chemical reaction, as evidenced by heat generation, bubble formation and significant foaming, with foaming illustrated in  FIG. 4 . The foamed product, which now fills almost the entire vessel  2 , is indicated with the reference symbol  8   a . As can be clearly seen, the surface  9   a  is located significantly above the surface  9  of  FIG. 3 , with the height of this surface  9  being determined by the sum of the volumes of Si, H 2 O and NaOH. Depending on the base concentration and the dimensions of the vessel, the surface  9   a  can rise above the height of the surface  9  by, for example, 3-4 cm or even more, for example 20 cm. 
         [0036]    Advantageously, the foamed suspension  8   a  is constantly agitated during the foaming process to prevent excess foaming. However, other safety measures can also be implemented to guarantee that a maximum level  9   a  is not exceeded. 
         [0037]    As shown in  FIG. 4 , etching may occur during foaming, which is maintained for about one hour. The fine material, preferably also the silicon particles with a particle size of less than 0.25 μm, are then practically completely dissolved. Strong foaming is also important for the production of the desired coarse material which is substantially free of fine material, in particular of silicon material with a grain size of less than about 0.25 μm to 1 μm. Foaming causes the particles to swirl, which enhances the chemical attack on the individual particles. 
         [0038]    In a subsequent fourth step, the foamed solution  8   a  is neutralized by adding acid, in the present example particularly by adding of about 40 ml HCl with 36 wt.-%. This stops the etching process. Alternatively, large quantity of water H 2 O could also be added. The foam  8   a  diminishes, with the remaining coarser ground material  10  completely precipitating within several minutes, as shown in  FIG. 5 . A clear (not brown) liquid or solution  12  is formed above the boundary  11  of the precipitated (sedimented) coarse-grain ground material  10 . The previously existing fine-grain ground material, shown in  FIG. 1 , is dissolved in this clear solution  12 . The surface  14  of the liquid  10  is located somewhat below the surface  9   a  of the foamed material  8   a.    
         [0039]    The clear solution  12 , which contains almost no particulate matter, can now be separated and withdrawn from the vessel  2 . 
         [0040]    The precipitated coarse-grain silicon  10  can now be likewise removed from the vessel  2  and a) treated again with the aforementioned steps  1  to  4  or b) transferred for further processing, e.g., rinsing, water absorption, drying. As mentioned above, the coarse-grain silicon material  10  is important, for example, for semiconductor and solar cell manufacturing. 
         [0041]    The following observation was made in the aforementioned exemplary process: if a sample of precipitated ground material  10  is placed on a fine-mesh filter (pore diameter less than 1 μm) and water is added to this sample, then the rinse solution passing through the filter shows no brown coloration. The fine-mesh filter also does not become clogged. 
         [0042]    It has also been observed that renewed precipitation of this separated coarse-grain fraction  10  in water (according to  FIG. 2 ) or in an acidic medium even after many hours does no longer show a brown discoloration. Instead, the remaining silicon  10  precipitates with a clear boundary, so that a predominantly clear solution remains in the upper region. As indicated by the (non-brown) color, the fine fraction of silicon is—as desired—completely removed. 
         [0043]      FIG. 6  shows an apparatus  20  for removing very fine silicon particles from grainy ground silicon material  4 , using the process described above with reference to  FIGS. 1 to 5 . 
         [0044]    The core component of the apparatus  20  is a reaction vessel  22 , to which storage vessels  24 ,  26 ,  28  and  30  for dry ground silicon material Si, sodium or potassium base NaOH and KOH, respectively, water H 2 O or an acid, such as for example hydrochloric acid HCl, are connected. Controllable throttle or shutoff devices  32 ,  34 ,  36  and  38 , depicted here as throttle or shutoff valves, are located in the supply lines between the storage vessels  24 ,  26 ,  28 ,  30  and the reaction vessel  22 . These shutoff devices  32  to  38  can be operated either manually or electrically and are used to control the corresponding supply into the reaction vessel  22 . The two storage vessels  26 ,  28  can also be combined into a single storage container (not illustrated), which then contains a base diluted with water. 
         [0045]    The ground silicon material  4  contains once more a significant fraction of silicon dust with a particle size or grain size of about 0.25 μm or less. 
         [0046]    The individual shutoff devices  32  to  38  can be individually or commonly associated with a control device. Such control device  40  is illustrated on the shutoff device  38  for acid supplied from the storage vessel  30 . 
         [0047]    A device  41  is centrally provided for agitating the content of the reaction vessel  22 . In the present example, an agitator with a blade  42  is used, which is rotated by an electric motor  44 . In principle, a different device  41  can be used for agitation, for example a device operating based on an electric-inductive principle. 
         [0048]    An outlet  45  with a shutoff valve  46  is located on the upper end of the reaction vessel  22 . The hydrogen H 2  produced by the reaction  10  here be vented. 
         [0049]    The uppermost level of the level in the vessel  22  is indicated by the reference symbol  48 . This level  48  is located considerably higher, for example by of up to 100%, than the level which, as illustrated in  FIG. 4 , corresponds to the sum of the non-foamed volumes of Si, H 2 O, and NaOH. Safety measures have been adopted to ensure that this uppermost level  48  is not exceeded during foaming. The boundary of the sedimentation, which according to  FIG. 5  separates the coarse-grain silicon  10  from the clear liquid  12  above, is indicated with the reference symbol  50 . 
         [0050]    A withdrawal device  52  is arranged at the lower end of the storage container  22  for removing the coarse fraction  10  desired for further processing (which is used in the field of semiconductor technology, for example, for producing silicon wafers). The withdrawal device  52  is indicated as withdrawal line  54  with a shutoff valve  56  and a conveying device or pump  58 . It will be understood that any type of withdrawal device  52  can be employed for the coarse-grain material  10 . It should also be mentioned that the particles of the coarse fraction  10  rarely adhere to one another or stick to the bottom of the reaction vessel  22 , which would prevent them from being dissolved. The withdrawal device  52  conveys the coarse-grain ground material  10  as needed to a container and/or a rinse and/or drying device  60 . From there the material  10  can be transferred via a shutoff valve  62  to an (unillustrated) receiving vessel. 
         [0051]    Likewise, a withdrawal device  64  is provided for removing the clear liquid  12 , which is also illustrated as withdrawal lines  66  with shutoff valve  68  and conveying device or pump  70 . This withdrawal device  64  transfers the clear liquid  12  (which contains in solution the fine-grain ground silicon material) to a catch vessel  72  commensurate with the process requirements. The liquid  12  can then be removed via a drain or shutoff valve  74  or transferred for recycling. 
         [0052]    A device  76  for measuring the clarity of the upper part of the full suspension  8   a  (see  FIG. 4 ) after introduction of the acid from the storage container  30  may be added for improving the operation of the device  20 . The device  76  can be used to determine when the precipitation of the coarse-grain fraction  10  is concluded and when this fraction  10  and the liquid  12  residing above the boundary  50  can be removed. The device  76  in the present example includes a light source  78 , which projects light  80 , after the light  80  has passed a filter  82 , through the liquid  12  onto a light detector  84  slightly above the expected boundary  50 . A signal generated at the output terminals  85  of the light detector  84  is transmitted, for example, to the control device  40  if the clarity is adequate. The control device  40  then interrupts or reduces the flow of acid from the storage container  30 . 
         [0053]    Instead of this special device  76 , a device can also be used which indicates when a desired grain size distribution in the reaction vessel  22  is obtained. For example, a device for online monitoring of the average grain size can be used, which may, however, add complexity. 
         [0054]    If the parameters of the process are known, then a clock or an adjustable timer  86 , which after a predetermined time, for example of 30 or 60 minutes depending on the process, interrupts or reduces supply of acid via the control device  40 , can also be used for interrupting the supply of acid. Alternatively, H 2 O can also be introduced into the storage vessel  22  through control by the timer  86  to interrupt etching by the base NaOH or KOH (dilution). 
         [0055]    A temperature value T measured by a temperature sensor  88  or a pH-value measured by a pH transducer  90  can also be used to determine the time of the interruption or reduction. The temperature T and the pH value are indicative of the condition of the suspension. Supply of acid can then be interrupted, as desired or according to the setting, when the measured pH value indicates “neutral” or “acidic.” 
         [0056]    In the present embodiment, a safety device  92  is arranged in the upper part of the reaction vessel  72  which is used to prevent excessive foaming which would cause the predetermined uppermost level  48  to be exceeded. In the example illustrated in  FIG. 6 , the safety device  92  is implemented as a fill level indicator with a float  94  which interrupts an electrical contact path  96  as soon as the float  94  rises to the uppermost level  48  on the foamed suspension  8   a . This interruption operates electrically on the control device  40  via a control line. 
         [0057]    Advantageously, with the illustrated apparatus  20 , the undesired, very small particles can be effectively separated and cost-effectively removed from ground silicon material, so that the still quite small, but comparatively larger particles (e.g., greater than 1 μm) which are desired for processing (for example in the semiconductor industry, in particular as raw material for solar cell production), can be obtained for further processing without the presence of very fine silicon dust. The process- and health-related disadvantages mentioned at the beginning are hereby eliminated. 
         [0058]    In summary, the described process and apparatus for carrying out the process are designed particularly for chemical dissolution of very fine silicon particles with a grain size of 1 μm or less by
       using the aqueous or water-containing solution of the base (e.g., NaOH or KOH),   strong foam formation,   particularly using silicon material that forms a brown-colored suspension.       
 
         [0062]    While the invention has been illustrated and described in connection with currently preferred embodiments shown and described in detail, it is not intended to be limited to the details shown since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. The embodiments were chosen and described in order to best explain the principles of the invention and practical application to thereby enable a person skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated.