Abstract:
A method for purifying an aqueous potassium hydroxide solution having rich silicon impurities has been disclosed in the invention, which is particularly related to a method that utilizes a low-carbon alcohol (such as ethanol) for extracting said aqueous potassium hydroxide solution, and includes the steps of mixing a low-carbon alcohol with an aqueous potassium hydroxide solution having rich silicon impurities; allowing the resulting mixture therefrom to divide into an aqueous phase layer and a low-carbon alcohol phase layer that contains the aqueous potassium hydroxide solution with reduced silicon impurities, and subjecting the low-carbon alcohol phase layer to a separation process for removing the low-carbon alcohol, thereby resulting in an aqueous potassium hydroxide solution having reduced silicon impurities.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method for purifying an aqueous potassium hydroxide solution having rich silicon impurities, and more particularly to a method that utilizes a low-carbon alcohol for extracting said aqueous potassium hydroxide solution, and allows an aqueous phase layer and a low-carbon alcohol phase layer that contains the aqueous potassium hydroxide solution with reduced silicon impurities to form. 
     DESCRIPTION OF PRIOR ART 
     Nowadays, the chemical mechanical polishing (CMP) process has become vital to the semiconductor industry because of its extensive application in the manufacturing process of silicon wafers. A critical component in the CMP process is the potassium hydroxide solution used for facilitating the overall process, which could result in reduced efficiency of chemical polishing when the concentration of silicon impurities thereof increases from continuous accumulation. Therefore, the potassium hydroxide solution has to be replaced when the silicon impurities thereof accumulates to a certain concentration. However, when the consumption of the potassium hydroxide solution is put into perspective, the potassium hydroxide solution that has been consumed due to actual applications only occupies a small proportion of the overall potassium hydroxide solution, while a major proportion of the potassium hydroxide solution is discarded as wastes simply because the concentration of silicon impurities has become excessively high that it reduces the efficiency of the CMP process. The wastes resulted from the CMP process take up a fairly high proportion in the overall wastes released from a semiconductor factory, and is difficult to process. As a result, it is plausible to suggest that costs and resources can be saved significantly by removing the silicon impurities from the potassium hydroxide solution that would be otherwise treated as wastes from the CMP process, which consequently purifies the potassium hydroxide solution to allow for further uses. 
     However, few studies about the purification of potassium hydroxide wastes are available at the moment, and the lack of efficient and cost-effective methods for the aforesaid goal remains the biggest challenge so far. For example, in U.S. Pat. No. 6,747,065 B1; methods of ion exchange, ultrafiltration, and electrodialysis are used to obtain a purified potassium hydroxide solution. But the methods are complicated and require high costs to carry out. 
     After continuous researching, the inventors of the present invention has discovered that an aqueous potassium hydroxide solution having rich silicon impurities may be divided into layers and purified by using a low-carbon alcohol (especially ethanol). This discovery contradicts with the widely known phenomenon that ethanol and water can be dissolved into each other at any proportions without dividing into layers. After repeated experimentation, a simple and cost-effective manufacturing process using the aforesaid discovery has been developed for actual industrial applications. 
     SUMMARY OF THE INVENTION 
     A primary objective of the invention is to provide a method for purifying an aqueous potassium hydroxide solution having rich silicon impurities. 
     Another objective of the invention is to provide a method for extracting potassium hydroxide from an aqueous potassium hydroxide solution having rich silicon impurities by using a low-carbon alcohol. 
     A further objective of the invention is to provide a method for reducing a concentration of silicon impurities in an aqueous potassium hydroxide solution having rich silicon impurities by using a low-carbon alcohol. 
     The invention discloses a method for purifying an aqueous potassium hydroxide solution having rich silicon impurities, comprising the following steps: 
     mixing a low-carbon alcohol as an extractant with an aqueous potassium hydroxide solution having rich silicon impurities so as to extract said aqueous potassium hydroxide solution, subsequently allowing the resulting mixture to form an aqueous phase layer as a lower layer, and a low-carbon alcohol phase layer containing the aqueous potassium hydroxide solution having reduced silicon impurities as an upper layer; 
     recovering the low-carbon alcohol phase layer; and 
     removing the low-carbon alcohol from said upper low-carbon alcohol phase layer, so as to obtain a purified aqueous potassium hydroxide solution. 
     In the aforesaid method for purifying an aqueous potassium hydroxide solution having rich silicon impurities, the methods used for removing the low-carbon alcohol may be any of the previously known techniques used to remove a low-carbon alcohol from a solution made up of the low-carbon alcohol and water; including: evaporation, distillation, adsorption, reverse osmosis using a membrane, liquid extraction, supercritical fluid extraction, and perstraction. The examples of the above-mentioned methods have been described in Taiwan Patent No. TW-057382. Among them the preferred methods are evaporation and distillation. 
     The aforesaid low-carbon alcohol refers to a carbon alcohol having a carbon number from two to four or a mixture thereof; such as ethanol, n-propanol, isopropanol, n-butanol, or a mixture thereof. In addition, the low-carbon alcohol is more preferably ethanol, a mixture made of ethanol and n-propanol, or a mixture made of ethanol and isopropanol, and is most preferably ethanol. 
     Moreover, a V/Wt ratio (volume-weight ratio of ml to g) between the aforesaid ethanol and the aqueous potassium hydroxide solution having rich silicon impurities should not be lower than 0.5, or no lower than 1.0, or even lower than 2.0. 
     In the aforesaid method for purifying an aqueous potassium hydroxide solution having rich silicon impurities, pure ethanol or an aqueous ethanol solution may be use, wherein the lower the weight percentage of ethanol in the aqueous ethanol solution used in the purification process, the worse the efficiency of extraction. Generally, the concentration of ethanol is preferably not lower than 50 wt %; is more preferably between 70 wt %-95 wt %, and is most preferably 95 wt %. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The aforesaid objectives of the present invention will be more clearly understood when considered in conjunction with the accompanying embodiments. 
     EXAMPLES 1-7 
     Respectively, 19.9 g (Example 1), 29.4 g (Example 2), 35.4 g (Example 3), 39.6 g (Example 4), 60.4 g (Example 5), 79.2 g (Example 6), or 100 g (Example 7) of 95 wt % ethanol aqueous solution were added into 50 g of an aqueous potassium hydroxide solution having rich silicon impurities and mixed thoroughly. In other words, a V/Wt (ml/g) ratio between the ethanol and the aqueous potassium hydroxide solution having rich silicon impurities is 0.5, 0.75, 0.9, 1.0, 1.5, 2.0, and 2.5, respectively; as shown in Table 1. The aforesaid aqueous potassium hydroxide solution has a concentration of silicon impurities of 2040 ppm, as well as a concentration of potassium hydroxide of 52.24 wt %. After thoroughly mixing the ethanol solution and the aqueous potassium hydroxide solution having rich silicon impurities together, the resulting mixture was left at room temperature without stirring, so that an upper layer and a lower layer were formed. The upper layer is an ethanol phase layer that includes the aqueous potassium hydroxide solution having reduced silicon impurities, while the lower layer is an aqueous phase layer. Subsequently, the upper ethanol phase layer was recovered from the layered mixture, and ethanol was removed therefrom by distillation to obtain an aqueous potassium hydroxide solution having reduced silicon impurities. The concentrations of potassium hydroxide and silicon impurities in the potassium hydroxide solution after the removal of ethanol are respectively listed in Table 1 below. 
     
       
         
               
             
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 An aqueous potassium hydroxide solution of 50 g, 52.24 wt %, and has a 
               
               
                 silicon concentration of 2040 ppm, has the listed silicon concentration after 
               
               
                 purification by using an ethanol solution. 
               
             
          
           
               
                   
                   
                 Aqueous KOH solution 
               
               
                   
                 The upper layer (after 
                 having a concentration 
               
               
                   
                 removing ethanol) 
                 adjusted to 45 wt % 
               
             
          
           
               
                   
                 Ethanol/KOH 
                 Ethanol 
                   
                 KOH 
                 Si 
                   
                 KOH 
                 Si 
               
               
                   
                 solution 
                 extracting 
                 Weight 
                 conc. 
                 conc. 
                 Weight 
                 conc. 
                 conc. 
               
               
                 Ex. 
                 (V/Wt) 
                 solution (g) 
                 (g) 
                 (wt %) 
                 (ppm) 
                 (g) 
                 (wt %) 
                 (ppm) 
               
               
                   
               
             
          
           
               
                 1 
                 0.5 
                 19.9 
                 11.7 
                 70.0 
                 297 
                 18.2 
                 45 
                 191 
               
               
                 2 
                 0.75 
                 29.4 
                 20.8 
                 58.3 
                 357 
                 26.6 
                 45 
                 276 
               
               
                 3 
                 0.9 
                 35.4 
                 23.4 
                 60.7 
                 379 
                 31.6 
                 45 
                 281 
               
               
                 4 
                 1.0 
                 39.6 
                 25.0 
                 64.0 
                 535 
                 35.6 
                 45 
                 376 
               
               
                 5 
                 1.5 
                 60.4 
                 37.2 
                 55.4 
                 701 
                 45.8 
                 45 
                 569 
               
               
                 6 
                 2.0 
                 79.2 
                 44.2 
                 54.8 
                 879 
                 53.8 
                 45 
                 723 
               
               
                 7 
                 2.5 
                 100.0 
                 46.6 
                 54.2 
                 890 
                 56.1 
                 45 
                 739 
               
               
                   
               
             
          
         
       
     
     The results from Examples 1-7 indicate that after purification, the concentration of silicon impurities in the aqueous potassium hydroxide solution is reduced significantly. Moreover, the volume of purified aqueous potassium hydroxide solution increases as the volume of ethanol extracting solution increases. 
     EXAMPLES 8-13 
     Respectively, 20.4 g (Example 8), 30.0 g (Example 9), 40.2 g (Example 10), 60.6 g (Example 11), 79.8 g (Example 12), 84.8 g (Example 13) of 95 wt % ethanol aqueous solution were added into 50 g of an aqueous potassium hydroxide solution having rich silicon impurities and mixed thoroughly. In other words, a V/Wt (ml/g) ratio between the ethanol and the aqueous potassium hydroxide solution having rich silicon impurities is 0.5, 0.75, 1.0, 1.5, 2.0, and 2.5, respectively; as shown in Table 2. The aforesaid aqueous potassium hydroxide solution has a concentration of silicon impurities of 1.85 wt %, as well as a concentration of potassium hydroxide of 39 wt %. After thoroughly mixing together the ethanol solution and the aqueous potassium hydroxide solution having rich silicon impurities, the resulting mixture was left at room temperature without stirring, so that an upper layer and a lower layer were formed. The upper layer is an ethanol phase layer that includes the aqueous potassium hydroxide solution having reduced silicon impurities, while the lower layer is an aqueous phase layer. Subsequently, the upper ethanol phase layer was recovered from the layered mixture, and ethanol was removed therefrom by distillation to obtain an aqueous potassium hydroxide solution having reduced silicon impurities. The concentrations of potassium hydroxide and silicon impurities in the potassium hydroxide solution after the removal of ethanol are respectively listed in Table 2 below. 
     
       
         
               
             
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 An aqueous potassium hydroxide solution of 50 g, 39 wt %, and has a 
               
               
                 silicon concentration of 1.85 wt %, has the listed silicon concentration after 
               
               
                 purification by using an ethanol solution. 
               
             
          
           
               
                   
                   
                 Aqueous KOH solution 
               
               
                   
                 The upper layer (after 
                 having a concentration 
               
               
                   
                 removing ethanol) 
                 adjusted to 45 wt % 
               
             
          
           
               
                   
                 Ethanol/KOH 
                 Ethanol 
                   
                 KOH 
                 Si 
                   
                 KOH 
                 Si 
               
               
                   
                 solution 
                 extracting 
                 Weight 
                 conc. 
                 conc. 
                 Weight 
                 conc. 
                 conc. 
               
               
                 Ex. 
                 (V/Wt) 
                 solution (g) 
                 (g) 
                 (wt %) 
                 (ppm) 
                 (g) 
                 (wt %) 
                 (ppm) 
               
               
                   
               
             
          
           
               
                 8 
                 0.5 
                 20.4 
                 12 
                 38.3 
                 3020 
                 10.2 
                 45 
                 3553 
               
               
                 9 
                 0.75 
                 30.0 
                 16.8 
                 35.9 
                 3184 
                 13.4 
                 45 
                 3992 
               
               
                 10 
                 1.0 
                 40.2 
                 21.8 
                 35.2 
                 2234 
                 17.1 
                 45 
                 2848 
               
               
                 11 
                 1.5 
                 60.6 
                 29.2 
                 32.5 
                 2269 
                 21.1 
                 45 
                 3140 
               
               
                 12 
                 2.0 
                 79.8 
                 33.4 
                 29.5 
                 2016 
                 21.9 
                 45 
                 3075 
               
               
                 13 
                 2.5 
                 84.8 
                 35 
                 25.0 
                 1734 
                 19.4 
                 45 
                 3128 
               
               
                   
               
             
          
         
       
     
     The outcome for Examples 8-13 shows that the concentration of silicon impurities in the aqueous potassium hydroxide solution is lowered significantly after being purified by using ethanol. As the volume of ethanol used for extraction is raised, the volume of purified aqueous potassium hydroxide solution is elevated as well. Furthermore, the V/Wt ratio between the ethanol and the aqueous potassium hydroxide solution is preferably no less than 0.5, wherein the V/Wt ratio between the ethanol and the aqueous potassium hydroxide solution is more preferably no less than 1.0. 
     In addition, if the V/Wt ratio between the ethanol and the aqueous potassium hydroxide solution is lowered to between 0.3 and 0.5, the efficiency of extraction would be reduced. The less the V/W ratio is, the worse the efficiency of extraction. 
     Moreover, if the ethanol aqueous solution being mixed into the aqueous potassium hydroxide solution having rich silicon impurities has a weight percentage less than 50 wt %, the aqueous potassium hydroxide solution cannot be divided into two different layers effectively, which prevents the extraction of purified aqueous potassium hydroxide solution from occurring successfully. Therefore, the concentration of the ethanol aqueous solution used for extraction is preferably no less than 50 wt %. 
     EXAMPLES 14-17 
     Respectively, 78 g of a mixed solution of 95 wt % ethanol and 5 wt % isopropanol (IPA, Example 14), or 78 g of a mixed solution of 90 wt % ethanol and 10 wt % isopropanol (Example 15), or 78 g of a mixed solution of 85 wt % ethanol and 15 wt % isopropanol (Example 16), or 78 g of a mixed solution of 80 wt % ethanol and 20 wt % isopropanol (Example 17), as an extracting solution of low-carbon alcohols, were added into 50 g of an aqueous potassium hydroxide solution having rich silicon impurities and mixed thoroughly. The aforesaid aqueous potassium hydroxide solution has a concentration of silicon impurities of 1.85 wt %, as well as a concentration of potassium hydroxide of 39 wt %. After thoroughly mixing the extracting solution with the aqueous potassium hydroxide solution having rich silicon impurities, the resulting mixture was left at room temperature without stirring, so that an upper layer and a lower layer were formed. The upper layer is a low-carbon alcohol phase layer that includes the aqueous potassium hydroxide solution having reduced silicon impurities, while the lower layer is an aqueous phase layer. The upper layer was recovered from the layer mixture, and the low-carbon alcohols were removed therefrom by distillation to obtain an aqueous potassium hydroxide solution having reduced silicon impurities. The concentrations of potassium hydroxide and silicon impurities in the potassium hydroxide solution after the removal of the low-carbon alcohols are respectively listed in Table 3 below. 
     
       
         
               
             
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 An aqueous potassium hydroxide solution of 50 g, 39 wt %, and has a 
               
               
                 silicon concentration of 1.85 wt %, has the listed silicon concentration after 
               
               
                 purification by using a mixed solvent made of ethanol and isopropanol. 
               
             
          
           
               
                   
                   
                 Aqueous KOH solution 
               
               
                   
                 The upper layer (after 
                 having a concentration 
               
               
                   
                 removing the alcohols) 
                 adjusted to 45 wt % 
               
             
          
           
               
                   
                 Extracting solution 
                 Weight 
                 KOH conc. 
                 Si conc. 
                 Weight 
                 KOH conc. 
                 Si conc. 
               
               
                 Ex. 
                 78 g 
                 (g) 
                 (wt %) 
                 (ppm) 
                 (g) 
                 (wt %) 
                 (ppm) 
               
               
                   
               
             
          
           
               
                 14 
                 95 wt % ethanol + 
                 31.4 
                 44.2 
                 1638 
                 30.8 
                 45 
                 1609 
               
               
                   
                 5 wt % IPA 
               
               
                 15 
                 90 wt % ethanol + 
                 26.8 
                 47.7 
                 1822 
                 28.4 
                 45 
                 1931 
               
               
                   
                 10 wt % IPA 
               
               
                 16 
                 85 wt % ethanol + 
                 26.6 
                 48.8 
                 1738 
                 28.8 
                 45 
                 1884 
               
               
                   
                 15 wt % IPA 
               
               
                 17 
                 80 wt % ethanol + 
                 26.2 
                 48.5 
                 1690 
                 28.2 
                 45 
                 1821 
               
               
                   
                 20 wt % IPA 
               
               
                   
               
             
          
         
       
     
     The results of Examples 14-17 show that the concentration of silicon impurities in the aqueous potassium hydroxide solution is lowered significantly after being purified by using a mixed solution of ethanol and isopropanol. 
     EXAMPLES 18-21 
     Respectively, add 100 g of 95 wt % ethanol with 5 wt % n-propanol (Example 18), or 90 wt % ethanol with 10 wt % n-propanol (Example 19), or 85 wt % ethanol with 15 wt % n-propanol (Example 20), or 80 wt % ethanol with 20 wt % n-propanol (Example 21), as an extracting solution of low-carbon alcohols, were added into 50 g of an aqueous potassium hydroxide solution having rich silicon impurities and mixed thoroughly. The aforesaid aqueous potassium hydroxide solution has a concentration of silicon impurities of 1.85 wt %, as well as a concentration of potassium hydroxide of 39 wt %. After thoroughly mixing the extracting solution with the aqueous potassium hydroxide solution having rich silicon impurities, the resulting mixture was left at room temperature without stirring, so that an upper layer and a lower layer were formed. The upper layer is a low-carbon alcohol phase layer that includes the aqueous potassium hydroxide solution having reduced silicon impurities, whereas the lower layer is an aqueous phase layer. The upper layer was recovered from the layered mixture, and the alcohols were removed therefrom by distillation to obtain an aqueous potassium hydroxide solution having reduced silicon impurities. The concentrations of potassium hydroxide and silicon impurities in the potassium hydroxide solution after the removal of alcohols are respectively listed in Table 4 below. 
     
       
         
               
             
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 An aqueous potassium hydroxide solution of 50 g, 39 wt %, and has a 
               
               
                 silicon concentration of 1.85 wt %, has the listed silicon concentration after 
               
               
                 purification by using a mixed solvent made of ethanol and n-propanol. 
               
             
          
           
               
                   
                   
                 Aqueous KOH solution 
               
               
                   
                 The upper layer (after 
                 having a concentration 
               
               
                   
                 removing the alcohols) 
                 adjusted to 45 wt % 
               
             
          
           
               
                   
                 Extracting solution 
                 Weight 
                 KOH conc. 
                 Si conc. 
                 Weight 
                 KOH conc. 
                 Si conc. 
               
               
                 Ex. 
                 100 g 
                 (g) 
                 (wt %) 
                 (ppm) 
                 (g) 
                 (wt %) 
                 (ppm) 
               
               
                   
               
             
          
           
               
                 18 
                 95 wt % ethanol + 
                 34.2 
                 46.7 
                 3053 
                 35.5 
                 45 
                 2941 
               
               
                   
                 5 wt % n-propanol 
               
               
                 19 
                 90 wt % ethanol + 
                 32.6 
                 45.1 
                 2758 
                 32.7 
                 45 
                 2750 
               
               
                   
                 10 wt % n-propanol 
               
               
                 20 
                 85 wt % ethanol + 
                 31.6 
                 44.6 
                 2345 
                 31.3 
                 45 
                 2367 
               
               
                   
                 15 wt % n-propanol 
               
               
                 21 
                 80 wt % ethanol + 
                 30.8 
                 45.4 
                 2060 
                 31.1 
                 45 
                 2040 
               
               
                   
                 20 wt % n-propanol 
               
               
                   
               
             
          
         
       
     
     The outcome from Examples 18-21 indicates that in comparison with the untreated aqueous potassium hydroxide solution, the aqueous potassium hydroxide solution purified by using a mixed solution of ethanol and n-propanol has a significantly lower concentration of silicon impurities. In addition, the concentration of purified potassium hydroxide increases slightly when the proportion of ethanol is raised. 
     Although particular embodiments of the invention have been described in details for purposes of illustration, it will be understood by one of ordinary skill in the art that numerous variations will be possible to the disclosed embodiments without going beyond the scope of the invention as disclosed in the claims.