Patent Publication Number: US-2003221972-A1

Title: Electrochemical process for preparing zinc metal and process for preparing zinc dithionite using electrochemically produced zinc metal

Description:
FIELD OF THE INVENTION  
       [0001] The present invention provides an electrochemical process for producing zinc metal and a process for preparing zinc dithionite using said electrochemically produced zinc metal.  
       BACKGROUND OF THE INVENTION  
       [0002] Zinc metal is generally produced by pyrometallurgical or electrochemical process. Zinc dust and powder are particulate forms of zinc. The terms dust and powder have been used indiscriminately to designate particulate zinc materials. The term zinc dust designates material produced by condensation of zinc vapor, whereas zinc powder indicates the product obtained by atomizing molten zinc. Zinc dusts are manufactured in various size ranges and a typical commercial dust has an average particle diameter between 4 and 8 μm. Usually, dusts are screened to be essentially free of particles coarser than 75 μm (200 mesh). The particle size distribution for commercial zinc powders typically range from 44 to 841 μm. (325-20 mesh).  
       [0003] In the chemical and metallurgical industries, zinc dust is used as a reducing agent, in the manufacture of hydrosulfite compounds for the textile and paper industries, and to enhance the physical properties of plastics and lubricants. Efforts have been made to prepare zinc powder by electrolyzing a solution of zinc based compounds in strong alkaline solutions.  
       [0004] Orszagh and V. Vass (Hungarian Journal of Industrial Chemistry, Vol. 13, pp. 287-299 (1985)) disclose an electrochemical method to regenerate zinc mud obtained during sodium dithionite production. The zinc mud obtained in sodium dithionite production, contains 50-60% water and 30-35% zinc, in the form of zinc hydroxide. The zinc mud was dissolved, the solution clarified, and electrolyzed, and the zinc powder obtained was used to reduce sulfur dioxide. According to Orszag and Vaas, their process requires a number of labor intensive operations such as lifting of the electrodes, removal of the zinc powder, drying, grinding and sizing, etc., that make this process economically unattractive. Removal of all insoluble material was carried out by using sedimentation and filtration. Solution of the zinc oxide in an aqueous NaOH solution was electrolyzed to prepare zinc particles. Zinc particles so produced were washed with water (20 to 60 ml water per gram of the zinc), dried, ground and sieved to give zinc particles with the desired particle size distribution. Furthermore, their process requires a divided electrolytical cell. This leads to a significant increase of operating, maintenance and capital expenses.  
       [0005] The process of Orsagh and Vass further requires high cell voltage at a lower current density. This increases capital as well as operational expenses. In summary, this process is economically unattractive as an alternative to the currently used zinc dust based technology. Because of these disadvantages, there is a strong need for an improved cost effective process to recycle zinc oxide byproduct to zinc dithionite. The present invention provides such a process that overcomes the above-mentioned disadvantages.  
       [0006] In order to reduce the required operating, maintenance and capital expenses, it is also highly desirable to develop an electrochemical process for producing zinc metal with sufficiently high reactivity of the zinc metal particles that can be used for the production of zinc dithionite. The present invention provides such a process.  
       [0007] U.S. patent application Ser. No. 09/776,518 (filed Feb. 2, 2001) discloses an electrochemical process for preparing zinc powder which involves: a) providing to an electrochemical cell a basic solution of zinc oxide or any other zinc compound that reacts with an aqueous base to produce zinc oxide, the basic solution prepared by dissolving the zinc oxide or the other zinc compound in an aqueous 2.5 to 10.0 M base solution; and b) passing current to the cell at a current density of about 10,000 to about 40,000 A/m 2  for a time period sufficient to electrochemically reduce the zinc oxide to zinc powder, wherein the electrochemical process has a current efficiency of at least 70% and is substantially free from electrode corrosion.  
       [0008] U.S. patent application Ser. No. 09/776,644 (filed Feb. 2, 2001) discloses a continuous electrochemical process for preparing zinc powder which involves: providing to an electrochemical cell a solution or suspension in an aqueous 1.25 Molar to 10.0 Molar base solution of zinc oxide or any other zinc compound that reacts with an aqueous base to produce zinc oxide, the solution or suspension containing at least 2 millimoles of solubilized zinc based species per 100 grams of electrolyte; and b) passing current to the cell at a current density of about 500 to 40,000 A/m 2 , for a time period sufficient to electrochemically reduce the solubilized zinc based species to zinc powder, while continuously or intermittently adding a sufficient amount of the zinc oxide or the other zinc compound to the cell to maintain the concentration of the solubilized zinc based species at a level of at least 2 millimoles per 100 grams of electrolyte and continuously or intermittently removing at least a portion of the zinc powder formed; wherein the electrolyte includes the aqueous base solution and the zinc oxide or the other zinc compound.  
       [0009] U.S. patent application Ser. No. 09/965,157 (filed Sep. 27, 2001), provides a process for preparing a solution of zinc oxide in an aqueous base, said process comprising diluting a more concentrated solution of zinc oxide in aqueous sodium or potassium hydroxide to produce a resulting dilute solution of zinc oxide having a concentration of zinc oxide that is higher than that obtained by dissolving solid zinc oxide in aqueous sodium or potassium hydroxide, wherein the concentration of the aqueous sodium or potassium hydroxide used for dissolving the solid zinc oxide is substantially the same as the concentration of the aqueous sodium or potassium hydroxide in the resulting dilute solution of zinc oxide, and wherein the concentration of the aqueous sodium hydroxide in the resulting dilute solution ranges from 5 wt % NaOH to about 35 wt % NaOH, and the concentration of the aqueous potassium hydroxide in the resulting dilute solution ranges from 10 wt % KOH to about 55 wt % KOH.  
       [0010] U.S. patent application Ser. No. 10/015,185 (filed Dec. 7, 2001) provides a low corrosion electrochemical process for preparing zinc metal which comprises electrochemically reducing an aqueous basic solution or slurry of zinc oxide or any other zinc compound that reacts with an aqueous base to produce zinc oxide, wherein the electrochemical process is carried out in an undivided electrochemical cell, and wherein air or nitrogen is bubbled in through the solution or slurry of zinc oxide or said other zinc compound during said electrochemical process.  
       [0011] U.S. patent application Ser. No. 10/109,443 (filed Mar. 28, 2002) provides a process for preparing zinc dithionite comprising reacting zinc metal with sulfur dioxide, wherein the zinc metal used in the reaction is produced by electrochemical reduction in an undivided electrochemical cell of an aqueous alkaline slurry or solution of zinc oxide or any other zinc compound that reacts with aqueous base to form zinc oxide.  
       SUMMARY OF THE INVENTION  
       [0012] The present invention provides a low corrosion electrochemical process for preparing zinc metal which comprises electrochemically reducing an aqueous alkaline slurry or solution of zinc oxide or any other compound that reacts with an aqueous base to produce zinc oxide, wherein the electrochemical process is carried out in an undivided electrochemical cell and wherein sodium bisulfide (NaSH) is added to the aqueous alkaline slurry or solution.  
       [0013] The present invention also provides a process for preparing zinc dithionite which comprises reacting zinc metal with sulfur dioxide, wherein the zinc metal is produced by the aforementioned electrochemical process.  
       DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0014] The process disclosed in U.S. patent application Ser. No. 10/109,443 (filed Mar. 28, 2002) however suffers from following disadvantages:  
       [0015] 1. To achieve higher current efficiency, one needs to use expensive electrodes. For example, magnesium cathode is used to achieve a higher current efficiency. Use of a less costly copper cathode gives relatively lower current efficiencies.  
       [0016] 2. Use of lower cost copper, nickel, stainless steel etc. electrodes leads to a relatively hard deposit on the cathode. This is especially true when, an electrolyte with a lower sodium hydroxide concentration is used. In contrast, a magnesium cathode (significantly more expensive than copper, nickel, or stainless steel based electrodes) leads to a relatively soft and spongy deposit. A softer deposit is preferred because of the ease with which it can be removed from the cathode.  
       [0017] 3. Electrochemically produced zinc was separated from the electrolyte and then washed extensively so that the aqueous slurry of zinc particles in water has a pH of 6.5 to 8.5. In order to meet this requirement, one uses a relatively large amount of water and generates a large amount of an alkaline aqueous waste.  
       [0018] 4. When stainless steel is used as the anode, anode corrosion leads to the formation of iron ions in the electrolyte. These ions get deposited on the zinc. When sodium dithionite is produced from this impure zinc, zinc oxide containing a higher amount of iron ions is produced. As the entire process is cyclical in nature, iron content in the electrolyte increases with time. This increased iron ion concentration in the electrolyte may lead to a lower current efficiency because of a parallel redox reaction of these iron ions.  
       [0019] In order to reduce the required operating, maintenance and capital expenses, it is highly desirable to develop a process that utilizes an undivided cell and lower cost electrodes for the production of zinc metal that leads to a relatively higher current efficiency for the formation of zinc dithionite, a relatively easily removable zinc deposit on the cathode, a significantly reduced anode corrosion and needs significantly reduced washing of the electrochemically produced zinc particles. The present invention affords such a process.  
       [0020] Reactivity of the zinc particles produced by electrolysis of zinc based species is of major importance because it determines the rate at which it will react with sulfur dioxide. When reacted with sulfur dioxide, less reactive zinc particles will lead to a lower conversion and a lower yield of zinc dithionite. It is generally believed that the reactivity of zinc particles depends in part on the surface area of the zinc particles. This in turn suggests that the nature of the zinc deposit on cathode may in part determine the reactivity of the zinc particles produced electrochemically. For example, use of magnesium based cathode leads to the formation of a spongy zinc deposit. In contrast, use of a copper cathode leads to a relatively hard zinc deposit. This is especially true when nickel is used as the anode. Spongy nature of the zinc deposit on the magnesium cathode suggests that the reactivity of the zinc particles prepared by using magnesium cathode will be higher than the reactivity of the zinc particles prepared by using copper cathode where the zinc deposit is relatively hard.  
       [0021] Magnesium cathodes are significantly more expensive than copper or stainless steel cathodes. In order to achieve significant reductions in capital expenses, industry would like to use lower cost copper, stainless steel, etc. cathodes. We have now unexpectedly learned that the addition of a catalytic amount of sodium bisulfide to the electrolyte before starting the electrolysis modifies the nature of the zinc deposit on these lower cost cathodes. Nature of the deposit is relatively loose and hence easier to remove mechanically. Furthermore, data in Table 1 below clearly shows that the reactivity of the zinc particles produced where sodium bisulfide is added to the electrolyte is significantly higher than the zinc particles produced without using NaSH. Furthermore, the use of sodium bisulfide in the electrolyte leads to a significant increase in the current efficiency for the formation of zinc dithionite.  
       [0022] Extensive washing of the zinc powder produced by electrolysis of zinc based species is thought to be of major importance because the residual metal hydroxides such as sodium or potassium hydroxides in the wet zinc particles is not desirable. This preference for the absence of sodium or potassium hydroxide in zinc particles is thought to arise from the fact that the zinc dithionite produced from these particles will react with the residual sodium or potassium hydroxide to give sodium or potassium dithionite. Sodium or potassium dithionite are less stable than zinc dithionite and undergoes decomposition much faster when the reaction mixture&#39;s pH is below 4.0. However, we unexpectedly found that the presence of a small amount of sodium or potassium hydroxide does not lower the yield or quality of the zinc dithionite significantly. Thus pH of the electrolyte can be as high as 12.5, preferably as high as 12, and even more preferably as high as 11. This less stringent requirement leads to a significant reduction in cycle time and a significant reduction in the alkaline waste generated during the washing of the electrochemically produced zinc particles.  
       [0023] Reducing anode corrosion is very important because this leads to an increased amount of metal ion impurities in the electrolyte with time. Presence of these metal ion based impurities may lead to a reduced current efficiency because part of the current may be used in reducing these metal ion impurities on the cathode. Furthermore, the presence of these metal ion impurities may effect the reaction rate of these zinc particles with sulfur dioxide. We have unexpectedly found that the use of sodium bisulfide (sodium hydrosulfide, NaSH) in the electrolyte helps in controlling the corrosion of stainless steel based anodes.  
       [0024] The present invention provides a low corrosion electrochemical process for preparing zinc metal which comprises electrochemically reducing an aqueous alkaline slurry or solution of zinc oxide or any other compound that reacts with an aqueous base to produce zinc oxide, wherein the electrochemical process is carried out in an undivided electrochemical cell and wherein sodium bisulfide (NaSH) is added to the electrolyte, i.e., to the aqueous alkaline slurry or solution.  
       [0025] The use of an undivided electrochemical cell is preferred because it requires lower capital and operational and costs.  
       [0026] The sodium bisulfide can be added during the electrochemical reduction process or prior to beginning the electrochemical reduction, with the latter method being preferred. The sodium bisulfide preferably is used in a catalytic amount. In one embodiment, the concentration of the sodium bisulfide (NaSH) in the electrolyte is at least 0.005 g/100 g of the electrolyte, and in one preferred embodiment has a concentration range of 0.01 to 0.1 g/100 g of the electrolyte.  
       [0027] In one embodiment, the electrolyte comprises solubilized zinc based species, the species comprising at least one member selected from the group consisting of ZnO 2   2− , HZnO 2   1− , Zn(OH) + , Zn(OH) 2 , and Zn 2+  and a catalytic amount of NaSH. Zinc oxide is known to dissolve by reacting with water to form a variety of species (which includes ionic and neutral species) depending upon pH. Thus a solution of zinc oxide in alkaline solution may contain species such as ZnO 2   2− , HZnO 2   1− , Zn (OH) 2 , Zn(OH) + , and Zn 2+ . Furthermore, these species may react with NaSH to form species such as Zn(SH) +  Therefore, solubilized zinc based species may comprise one or more of these species in the solution.  
       [0028] The aqueous base solutions employed in the process of the invention are prepared by combining water with a source of alkali metal or alkaline earth metal ions, such as lithium, sodium, and potassium, and a source of hydroxyl (OH −  ions). A single source may of course provide both types of ions. The various alkali or alkaline earth metal ions are preferably supplied from various compounds such as hydroxides and oxides. Preferred base solutions are sodium and potassium hydroxide solutions.  
       [0029] The anode of the undivided electrochemical cell of the present invention may be made from any conventional suitable material such as platinum, or iridium, either of which may be coated over an inert support such as niobium or titanium. The anode may also be made of nickel, or from conventional materials having good alkali corrosion resistance, e.g., lead or stainless steel. The cathode may be made from any conventional suitable lower cost materials having good alkali corrosion resistance, such as copper, nickel and stainless steel. Preferably, the anode in the present invention is formed of stainless steel or nickel and the cathode is formed of stainless steel, nickel or copper. In one embodiment, the cathode and the anode are copper and stainless steel respectively, and in one embodiment, copper and nickel respectively.  
       [0030] In one embodiment, the electrochemical reduction of the presently claimed process is conducted at a temperature of from 10° C. to 85° C., more preferably from 20° C. to 70° C.  
       [0031] The present invention also provides a process for preparing zinc dithionite which comprises reacting zinc metal with sulfur dioxide, wherein the zinc metal is produced by the aforementioned electrochemical process.  
       [0032] The reaction of zinc metal with sulfur dioxide to produce zinc dithionite is a conventional process well known to those of ordinary skill in the art. Preferably, the reaction is carried out at a temperature of 20° C. to about 70° C., and in one embodiment, 40° C. to 60° C. In one embodiment, the zinc metal used in the presently invention is the form of wet zinc metal particles. By electrolyzing a slurry of zinc oxide by-product (from the reaction of zinc dithionite and sodium hydroxide to form sodium dithionite), wet zinc particles can be prepared (particle size distribution depending on the composition of the electrolyte, cathode and anode used), which can be washed and used without further processing to prepare zinc dithionite. Washing does not have to be extensive to remove all sodium or potassium hydroxide. However, the resulting aqueous slurry of these zinc particles should have a pH lower than 13 and more preferably less than 12 and even more preferably less than 11.  
       [0033] Current efficiency (C.E.) for the presently claimed process for the formation of zinc dithionite (ZnS 2 O 4 ) can be defined as follows:  
       [0034] C. E. for the formation of ZnS 2 O 4 =[(Moles of ZnS 2 O 4 )/(Moles of electrons passed/2)]×100.  
       [0035] Current efficiency for the formation of zinc dithionite can also be obtained by multiplying current efficiency for the zinc formation with conversion efficiency of zinc to zinc dithionite. Current efficiency of zinc formation can be quite high (such as&gt;90%). If reactivity of the zinc formed is similar to the zinc dust currently used in manufacturing zinc dithionite, conversion efficiency of zinc to zinc dithionite is also quite high. This then suggests that the current efficiency for the zinc dithionite formation should also be quite high. In one embodiment, the current efficiency for the formation of zinc dithionite is at least 75%, and in one embodiment, at least 80%. Experimental current efficiencies for the zinc dithionite formation in an undivided cell varied from 76% to 88% depending on several variables such as the electrodes used, current density used (such as 500 to 20,000 Amps/m 2 , preferably 2500 to 7500 Amps/m 2 , and more preferably 4000 to 6000 Amps/m 2 ), concentration of sodium hydroxide used to dissolve zinc oxide byproduct, etc. This also depends on the amount and nature of impurities in the zinc oxide byproduct. These results clearly suggest that the presently claimed process (wherein a catalytic amount of NaSH in the electrolyte is used) is highly effective and advantageous for the preparation of zinc dithionite from the zinc oxide.  
       [0036] In one embodiment of the present invention, the reaction of zinc metal with sulfur dioxide to produce zinc dithionite is conducted in the presence of cadmium sulfate, and in one embodiment in the substantial absence of cadmium sulfate. While not wishing to be bound by theory, it is believed that the addition of cadmium sulfate has a positive effect on the production of zinc dithionite, in that the yield of zinc dithionite is slightly improved. However, the reaction can also be carried out in the substantial absence of cadmium sulfate (e.g., where cadmium sulfate is not added to the reaction mixture), with good results.  
       [0037] The following specific examples will provide detailed illustrations of the methods of producing and utilizing compositions of the present invention. These examples are not intended, however, to limit or restrict the scope of the invention in any way and should not be construed as providing conditions, parameters or values which must be utilized exclusively in order to practice the present invention. Unless otherwise specified, all parts and percents are by weight. 
     
    
    
     EXAMPLES  
     Example 1  
     [0038] General Procedure:  
     [0039] In these experiments, a resin Kettle (5 inch (12.7 cm) in diameter and 18 inch (45.7 cm high) was used as the cell. A solution or slurry of zinc oxide (from Fisher Chemical Co.) in the aqueous sodium hydroxide solution (3 to 3.5 liters) at 20 to 80° C. was charged into the resin kettle. NaSH was then added to the electrolyte as an aqueous solution. Zinc oxide was also added during the electrolysis to maintain higher concentration of zinc ions in the electrolyte. A thermometer, and desired cathodes and anodes were positioned in the cell using laboratory clamps. Mixing was achieved by using a mechanical stirrer. In some experiments bubbling nitrogen in addition to mechanical stirring was used. Parts of the cathode and anode surfaces were covered with Teflon or electrical tape to achieve the desired active cathode and anode surface areas. Electrolysis was carried out at a current density of about 5,000 Amps/m 2 . A portion of the zinc deposited on the cathode was mechanically removed periodically and collected on the bottom of the kettle. At the end of the experiment, zinc particles were separated from the electrolyte by decantation, and washed with water. Wet zinc (A grams) was charged into a one-liter resin kettle equipped with a thermometer, nitrogen inlet and outlet adapter, a gas disparager, mass flow meter to measure the amount of sulfur dioxide passed, a pH probe, and a mechanical stirrer. An amount (X ml of 0.18%) of an aqueous solution of cadmium sulfate was then charged into the resin kettle. The step of adding the cadmium sulfate can, however, be eliminated. The reaction mixture was then heated to 43° C. and sulfur dioxide gas was passed through the reaction mixture. The sulfur dioxide addition rate was controlled to maintain pH of the reaction mixture above 3.73 and temperature was maintained at 43 to 55° C. Initial addition rate of sulfur dioxide was 3.4 g/minute. Zinc dithionite solution was decanted to give a translucent solution of the zinc dithionite. It was then titrated to determine the concentration of zinc dithionite.  
     Example 2  
     [0040] The results of electrolysis of zinc oxide followed by the reaction with sulfur dioxide under various conditions are shown below in Table 1.  
                                               TABLE 1                               Initial                                       Conc.   NaSH 2         Moles                       NaOH   of ZnO 1     Soln.   Cathode/   of e- 1     Moles of   CE   Initial       Exp. No.   (Wt %)   (Wt %)   Wt % 2     Anode   passed   Zn 2 S 2 O 4     %   pH 4                                                                      7G-3 3     25   6.0   0.00   Cu/Ni   6.32   2.28   72   11.2       7G-5 3     25   6.0   0.001   Cu/Ni   6.32   2.69   70   11.1       7G-7   25   6.0   0.006   Cu/Ni   6.32   2.69   87   12.35       7G25   25   6.0   0.03   Cu/Ni   6.32   2.78   88   8.4       7G-4   25   6.0   0.04   Cu/Ni   6.32   2.69   85   11.78       7G-2   25   6.0   0.15   Cu/Ni   6.32   2.69   85   11.1       7G-21   25   6.0   0.00   Cu/SS   6.32   2.24   71   10.5       7G-16   25   6.0   0.06   Cu/SS   6.32   2.49   79   13.0       7G-20   25   6.0   0.06   Cu/SS   6.32   2.44   77   10.8       7G18   15   2.0   0.06   Cu/SS   6.32   2.40   76   12.5       7B35 5     —   —   —   —   —   2.68   85   7.00       7B35 5     —   —   —   —   —   2.63   83   6.56                                                          
 
     [0041] The data in Table 1 above clearly shows that the reactivity of the zinc particles produced (as measured by the moles of zinc dithionite produced from using said zinc metal when sodium bisulfide is added to the electrolyte is significantly higher than the zinc particles produced without using NaSH. Furthermore, the use of sodium bisulfide in the electrolyte leads to a significant increase in the current efficiency (CE) for the formation of zinc dithionite.  
     Example 3  
     [0042] In these experiments, a 4 L resin Kettle (4 inch in diameter and 18 inch high) is used as the cell. A solution of zinc oxide (283 g), and sodium hydroxide (586 g), in water (586 g) was diluted with water (2450 g) and the diluted solution was charged into the resin kettle. A thermometer, a magnesium cathode and a stainless steel anode are positioned in the cell using laboratory clamps. Mixing is achieved by using a mechanical stirrer. Parts of the cathode and anode surfaces are covered with Teflon tape to achieve the desired active cathode and anode surface areas. Electrolysis is carried out at a current density of 20,211 amps/m 2  and 600,000 coulombs were passed during the electrolysis. A portion of the zinc deposited on the cathode is removed periodically. At the end of the experiment, zinc particles are separated from the electrolyte by decantation. It was then washed with water, sulfuric acid (1%), and methanol. Wet zinc product was then dried to give a yield of dry zinc particles (206.3 g). Analysis of zinc particles showed that these particles contained 193 ppm of iron.  
     Example 4  
     [0043] In these experiments, a 4 L resin Kettle (4 inch in diameter and 18 inch high) is used as the cell. A solution of zinc oxide (271 g), and sodium hydroxide (560 g), in water (560 g) was diluted with water (2344 g) and the diluted solution was charged into the resin kettle. An aqueous solution of NaSH (1.6 g) in water (3.4 g) was then added to the reaction mixture. A thermometer, a magnesium cathode and a stainless steel anode are positioned in the cell using laboratory clamps. Mixing is achieved by using a mechanical stirrer. Parts of the cathode and anode surfaces are covered with Teflon tape to achieve the desired active cathode and anode surface areas. Electrolysis is carried out at a current density of 20,211 amps/m 2  and 600,000 coulombs were passed during the electrolysis. A portion of the zinc deposited on the cathode is removed periodically. At the end of the experiment, zinc particles are separated from the electrolyte by decantation. It was then washed with water, sulfuric acid (1%), and methanol. Wet zinc product was then dried to give a yield of dry zinc particles (209.7 g). Analysis showed that these particles contained 20 ppm of iron.  
     [0044] Each of the documents referred to above is incorporated herein by reference in its entirety, for all purposes. Except in the Examples, or where otherwise explicitly indicated, all numerical quantities in this description specifying amounts and concentrations of materials, reaction and process conditions (such as temperature, current density, current efficiency), and the like are to be understood to be modified by the word “about”.  
     [0045] It must be noted that as used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural reference unless the context clearly dictates otherwise. Unless defined otherwise all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this invention belongs.  
     [0046] While the invention has been explained in relation to its preferred embodiments, it is to be understood that various modifications thereof will become apparent to those skilled in the art upon reading the specification. Therefore, it is to be understood that the invention disclosed herein is intended to cover such modifications as fall within the scope of the appended claims.