Abstract:
The invention provides methods of using concentrated zinc chloride to protect zinc powders formed by electrowinning from oxidation during collection and removal from an electrowinning cell, during periods of prolonged storage and during melting. The invention provides a means of flushing zinc powders from all surfaces of an electrowinning cell and transporting and storing the zinc powders under a concentrated zinc chloride solution. The same concentrated solution can be used with ammonium chloride in a flux solution to melt the zinc powders prior to shaping the molten zinc into briquettes, pellets and the like.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims benefit under 35 USC §119(e) of U.S. Provisional Application No. 60/349,035, filed Jan. 17, 2002, which is incorporated herein in its entirety by this reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention is directed to recovering and melting zinc powder formed by electrodeposition from a zinc chloride solution while preventing the oxidation of the zinc powder during such procedures.  
         BACKGROUND OF THE INVENTION  
         [0003]    Zinc is one of the most widely used non-ferrous metals with annual world wide production of zinc exceeding nine million tons. Zinc is isolated from zinc-bearing ores and secondary sources such as steel scrap and zinc-bearing dusts. Chloride-based processes offer advantages over other technologies when secondary sources are treated to recover the contained zinc. U.S. Pat. Nos. 4,800,069 and 5,698,759 to Fray disclose methods of isolating zinc from sources such as furnace dust and galvanized materials using chlorine gas or a reactant that produces chlorine. One reason chloride-based methods have received little commercial development is the difficulty and cost associated with containment and collection of the chlorine gas evolved at the anode during electrolysis of chloride solutions.  
           [0004]    The problem of chlorine gas containment is largely overcome by using modem electrowinning cells, especially EMEW cells. Although zinc metal is plated out on the cathode in these cells, the metal is particularly difficult to remove from the inner surface of the cathode tube. Several means have been investigated to overcome this problem and remove the electroplated metal, including alteration of the electrowinning conditions to enable the zinc to be deposited in the form of metallic zinc powder. The zinc powder deposited at the cathode is easily removed by scraping or flushing with water or other liquids.  
           [0005]    The main problem with the electrolytic production of zinc powder is that the powder product is rapidly oxidized by contact with air or water. The oxidized powder, including powder that is only slightly oxidized, cannot be melted and cast into ingot form thereby decreasing the commercial value of this form of metallic zinc powder.  
           [0006]    Therefore, a method of preventing oxidation of the zinc powder formed in an electrowinning cell is needed. The method should prevent oxidation in the cell during the removal of the powder from the cathode and during subsequent processing operations including storage, dewatering, transfer between processing stages, pelletizing if performed, and during the melting operation.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention is directed to methods of preventing the oxidation of zinc powder formed in an electrowinning cell. In one aspect, the invention provides easy and efficient removal of electrolytic zinc powders from the cathode and interior surfaces of an electrowinning cell while simultaneously preventing oxidation of the zinc powder deposits comprising flushing said zinc metal from said electrowinning cell with a concentrated zinc chloride solution. Preferably, the electrowinning cell is an EMEW cell. The concentrated zinc chloride solution is typically greater than about 250 grams zinc chloride per liter and preferably, the concentration of said concentrated zinc chloride solution is greater than about 500 grams zinc chloride per liter.  
           [0008]    The method may be expanded to include the additional step of recycling the concentrated zinc chloride solution for further use in flushing an electrowinning cell. In this method, the recycling step may include adjusting the concentration of the concentrated zinc chloride solution.  
           [0009]    Another aspect of the present invention provides a method of recovering zinc metal powder formed in an electrowinning cell in the form of a pellet or similar desired shape by flushing the zinc metal from the electrowinning cell with a concentrated zinc chloride solution and then removing excess concentrated zinc chloride flush solution from the zinc powder which is then pressed into a desired shape. During this process, the zinc pellets or briquettes formed are protected from oxidation by residual concentrated zinc chloride solution that remains from previous operations performed on the powder or by concentrated zinc chloride solution added to the pressing operation used to form the pellets, briquettes or other desired shapes.  
           [0010]    Another aspect of the invention is a method of storing zinc metal including contacting the zinc powder with a concentrated zinc chloride solution for the duration of the storage. Typically, the concentration of the concentrated zinc chloride solution used in this method is greater than about 250 grams zinc chloride per liter and preferably, the concentration of the concentrated zinc chloride solution is greater than about 500 grams zinc chloride per liter. The zinc metal stored by this method may be in a form including powder, ingot, bar, pellet, briquette, brick and plate.  
           [0011]    Another aspect of the invention is a method of protecting zinc metal from oxidation comprising contacting the zinc metal with a concentrated zinc chloride solution. This method may include the additional step of removing excess concentrated zinc chloride solution from the zinc metal. The contacting step of this method may include adding concentrated zinc chloride solution to a mixture of zinc metal in water or other dilute solution. The contacting step may include dewatering the zinc metal and then contacting the dewatered zinc metal with a concentrated zinc chloride solution. This dewatering may be either complete or partial dewatering.  
           [0012]    Another aspect of the invention is a method of melting zinc powder by dewatering zinc powder previously in contact with a concentrated zinc chloride solution and then adding to the dewatered zinc powder a flux comprising ammonium chloride followed by heating the zinc powder and flux to a temperature between about 200° C. and about 700° C. Optionally, the flux of used in this method may include zinc chloride. Additionally, the zinc powder to be melted may first be dried in an oven in the presence of added the flux components zinc chloride and ammonium chloride. The method may also include the extra step of first placing the flux in a melting furnace and then adding the dewatered zinc powder to the flux before melting occurs. Alternatively, the dewatered zinc powder may be added to flux in contact with a molten metal phase. The melted zinc may first be pressed into a shape selected from the group consisting of pellets, briquettes, ingots, bars, plates and bricks to assist in the melting process. The method may include adding all or part of the flux to the zinc powder during the pressing process while the balance is added to the zinc pellets or other desired shapes in the melting furnace.  
         DETAILED DESCRIPTION  
         [0013]    Electrolytic zinc powder is readily oxidized by contact with air and water and by the dilute zinc chloride feed solution or other feed solution used in the electrowinning operation and also by the spent electrolyte resulting from electrowinning.  
           [0014]    Zinc deposits on the cathode may be in the form of very dense metallic plates, relatively loose and relatively coarse dendritic zinc particles, and zinc powder. Very often zinc powder may contain some dendrites and zinc dendrites may contain some powder. The present methodology applies mostly to zinc powder deposits and to dendritic deposits. For the purposes of this disclosure, reference to zinc powders includes dendritic zinc deposits and mixtures of zinc dendrites and zinc powders.  
           [0015]    During electrowinning, the zinc powders deposited on the cathode will not oxidize due to the electric charge on the cathode. The oxidation reaction begins immediately after the electric charge is removed from the electrowinning cell when water or oxygen molecules come into contact or remain in contact with the zinc powder. This occurs when the electrowinning process is stopped in order to remove the deposited zinc powders from the electrowinning cell. The powders are typically flushed from the cell and collected and stored for reuse in the refining process or for melting and casting into ingots.  
           [0016]    However, it has been found that the zinc powder is not oxidized by concentrated zinc chloride solutions. Zinc chloride is highly soluble in water. When zinc chloride dissolves water molecules arrange themselves around the soluble zinc chloride species and are said to become water of coordination. Without being bound by any single theory, it is believed that, as the concentration of the zinc chloride in solution increases, an increasing proportion of water molecules become bound as water of coordination and the number of uncoordinated water molecules available to react with or oxidize the zinc powder, strictly the activity of the water in the solution, decreases. In sufficiently concentrated zinc chloride solutions, the activity of water is too low to react with the metallic zinc powder and the powder is not oxidized.  
           [0017]    The solubility of zinc chloride is reported to be 4320 grams per liter of water at 25° C., but the practical effect of the coordination of water molecules in solution takes place at a lower concentration. Thus, the concentration of the zinc chloride solution in the different embodiments of the present invention will depend on economic consideration of the cost of the solution relative to the extent to which that solution is capable of preventing oxidation of the zinc powder produced in different situations. However, this zinc chloride concentration range is bound on the low end by the concentration at which the solution may begin to oxidize zinc powder. This is in part dependent upon the nature of the zinc powder being protected, the conditions under which the zinc powder is flushed and stored in the flush solution, and the length of time the flush solution will remain in contact with the zinc powder. The concentration of the zinc chloride in the flush solution is bound on the high end by the absolute solubility of the zinc chloride in solution. Generally, the zinc chloride solution is prepared containing greater than 250 grams of zinc chloride per liter of solution. Preferably the zinc chloride solution contains greater than 500 grams of zinc chloride per liter of solution, and more preferably, the zinc chloride solution contains greater than 1000 grams of zinc chloride per liter of solution.  
           [0018]    One embodiment of the present invention is a method of removing zinc powders from an electrowinning cell with a concentrated zinc chloride flush solution. This solution is used to flush zinc powder from the cathode and other interior surfaces of the electrowinning cell after the electric current is removed thereby preventing oxidation of the powder.  
           [0019]    Another embodiment of the present invention is the use of a concentrated zinc chloride solution to arrest oxidation of zinc powder that has been flushed from the electrowinning cell by means of the feed solution or other convenient solution lacking the ability to prevent oxidation. This can be achieved by rapidly settling the zinc powder followed by decanting the flush solution and quickly introducing a sufficient volume of concentrated zinc chloride solution. Alternatively, the dilute flush solution can be removed by filtering immediately on removal from the electrowinning cell followed by rapidly transferring the zinc powder into a concentrated zinc chloride solution. Alternatively the mixture of zinc powder and the flush solution may be directed into a sufficient volume of concentrated zinc chloride solution such that the diluting effect of the incoming flush solution does not reduce the concentration of the combined solutions below the lower bounds of the zinc concentration previously specified. In this embodiment of the invention it is important to minimize the time the zinc powder is in contact with the dilute flush solution to limit the extent of oxidation occurring before the powder gains the protection of the concentrated zinc chloride solution.  
           [0020]    Another embodiment of the present invention is a method of using a concentrated zinc chloride solution to prevent oxidation of zinc powders formed in an electrowinning cell during storage of the zinc powders prior to melting or other disposition. Zinc powders stored in a concentrated zinc chloride solution will remain essentially free of oxidation during storage for greater than 30 days.  
           [0021]    The specific gravity of the zinc chloride solution increases as the concentration increases. This is particularly advantageous as the higher the specific gravity of the flush solution, the greater will be the force applied to a zinc powder particle adhering to the cathode or other interior surfaces of the electrowinning cell. Thus, the use of a more concentrated zinc chloride solution will be more efficient in flushing the zinc powder from the cell while simultaneously preventing oxidation of the powder.  
           [0022]    Once the zinc powder has been flushed from the electrowinning cell, the flow of the zinc chloride feed solution is restarted and the current is again applied to resume electrowinning. Advantageously, the use of the concentrated zinc chloride flush solution will not contaminate or otherwise dilute the feed solution charged to the electrowinning cell. Thus, the concentrated zinc chloride solution can be used to flush the cell after which the flow offeed solution is resumed for continuation of the electrowinning process without any cleaning to remove the flush solution or prevent dilution of the fresh feed by the flush solution.  
           [0023]    The zinc powder removed from the cell in the concentrated zinc chloride flush solution or stored under a concentrated zinc chloride solution is recovered by using one or a combination of dewatering processes including thickening, filtering, centrifuging or any other process commonly known in the art. Residual concentrated zinc chloride solution retained in the dewatered zinc powder will continue to protect the metal from oxidation. The extent of dewatering is controlled to provide the appropriate amount of residual solution for the desired protection and the moisture content required for subsequent powder processing stages. Zinc chloride flush solution removed from the zinc powder in the dewatering process can then be recycled and used to flush subsequently formed zinc powders from the cell. During this recycling step, the concentration of the zinc chloride flush solution can be monitored and adjusted either by addition of water to dilute the solution or by concentrating the solution by any means known in the art. Typically, if it is necessary to concentrate the solution, solid zinc chloride can be added to raise the zinc concentration or the solution may be heated to evaporate excess water.  
           [0024]    The dewatered zinc powder can also be pressed into pellets or briquettes for convenient storage, sale or other uses, or to assist in melting. The zinc powder is pressed almost to the density of the metal, which further prevents oxidation of the metal. Residual zinc chloride solution remaining on the powder surfaces will protect the metal from oxidation during the pressing process and subsequent storage.  
           [0025]    In another embodiment of the present invention, flux is added to the dewatered zinc powder which is then melted. Typically, a melting temperature between about 200° C. and about 700° C. is used in the melting furnace. Preferably, the melting temperature is between about 400° C. and about 600° C. is used in the melting furnace. Preferably, the melting temperature is between about 450° C. and about 550° C. is used in the melting furnace.  
           [0026]    The flux is composed of ammonium chloride and zinc chloride and is used to help coalesce the metal particles by removing any metal oxidation from the particle surfaces. In the method described herein, the flux may initially be composed almost entirely of ammonium chloride. As solid ammonium chloride is added to the zinc metal powder, residual zinc chloride flush solution retained in the thickened zinc powder mixes with the ammonium chloride solution to form zinc chloride and ammonium chloride flux. The residual zinc chloride flush solution that is introduced to the furnace continues to protect the zinc powder from oxidation by water and air while the zinc powder undergoes initial heating and water is evaporated. At higher temperature the flux provides the protective effect. Excess zinc chloride is removed from the melting process in the form of dross and can be dissolved in water and recycled back to the flush solution.  
           [0027]    In one embodiment of this melting process, the zinc powder removed from the electrowinning cell is dewatered in a low temperature drying oven prior to being placed in the melting furnace. Preferably this is done in the presence of the zinc chloride and ammonium chloride flux components. In another embodiment of this melting process, the dewatered zinc powder is charged directly to the melting furnace into the flux layer above the molten phase. In another embodiment, the zinc powder can be added to the flux alone, prior to the formation of a molten phase.  
           [0028]    In another embodiment, the dewatered zinc powder is pelletized or briquetted under pressure. This increases the density of the zinc powder and facilitates the melting operation. The ammonium chloride flux is added to the melting furnace either by adding it to the zinc powder during the pelletizing process or by adding it directly to the furnace with the pellets. Although pressing minimizes the amount of solution retained in the powder, during heating the pellets may burst apart due to the release of entrapped steam. These explosions can be minimized by slowly preheating the pellets or by putting the pellets into the flux phase at less than 200° C.  
       
    
    
     EXAMPLES  
       [0029]    The following Examples are provided to illustrate embodiments of the present invention and are not intended to limit the scope of the present invention as set forth in the claims.  
       Example 1  
       [0030]    This example demonstrates the protection from oxidation afforded to zinc powder produced by electrowinning during collection, transfer and storage of the zinc powder for more than a month.  
         [0031]    Zinc powder was produced by electrowinning zinc chloride solution containing approximately 20 grams of Zn per liter of solution in an EMEW cell. Upon completion of the electrowinning cycle, the powder was removed from the cell by a flushing technique using the same zinc chloride solution that was used for the powder production. After that, zinc powder was immediately dewatered to approximately 80% solids by weight using filtration. The dewatered powder was then transferred to a beaker of concentrated zinc chloride solution containing approximately 500 grams of ZnCl 2  per liter of solution. After four weeks of storage, the zinc powder was dewatered and pelletized into a pellet with a density of 5.6 grams/cc. The pellet was placed in a crucible, covered with ammonium chloride (approximately 17 grams NH 4 Cl per 100 g of zinc) and then transferred into a furnace pre-heated to 500° C. After 30 minutes, the crucible was removed, cooled and the metallic zinc button was collected. The yield of metallic zinc was 91% of the original zinc powder weight. In a control test a pellet was pressed straight from the powder flushed from the electrowinning cell and melted without being stored in a concentrated zinc chloride solution. Melting of this pellet yielded less than 10% metallic zinc.  
       Example 2  
       [0032]    This example demonstrates the protection of zinc from oxidation during removal and transfer from an electrowinning cell and during storage thereafter.  
         [0033]    Zinc powder was produced by electrowinning zinc chloride solution containing approximately 20 grams of Zn per liter of solution in an EMEW cell. Upon completion of the electrowinning cycle, the powder was removed from the cell by flushing with a concentrated zinc chloride solution containing approximately 500 grams of ZnCl 2  per liter of solution. After two days of storage, the zinc powder was dewatered and pelletized into a pellet with a density of 6.3 grams/cc. Ammonium chloride was added into the pellet in an amount equaling approximately 20% of the zinc powder weight. The pellet was placed in a crucible, covered with ammonium chloride (approximately 14 grams NH 4 Cl per 100 g of zinc) and then transferred into a furnace pre-heated to 500° C. After 30 minutes the crucible was removed, cooled and the metallic zinc button was collected. The yield of metallic zinc was 97.5% of the original zinc powder weight.  
       Example 3  
       [0034]    This example demonstrates the protection of zinc from oxidation during removal and transfer from an electrowinning cell to storage and during formation of a molten flux by inclusion in the flux components.  
         [0035]    Zinc powder was produced by electrowinning a zinc chloride solution containing approximately 20 grams of Zn per liter of solution in an EMEW cell. Upon completion of the electrowinning cycle, the powder was removed from the cell by flushing with a concentrated zinc chloride solution containing approximately 500 grams of ZnCl 2  per liter of solution. After two days of storage, the zinc powder was dewatered to approximately 80% solids by weight, but was not pelletized. The powder was added to a mixture of 25% ammonium chloride and 75% zinc chloride flux, pre-heated in a crucible to approximately 200° C. to form a liquid phase. The temperature of the liquid was slowly increased to 450° C. over a period of two hours and the mixture of powder and flux was stirred gently during heating. Immediately after the molten zinc phase formed, the crucible was removed from the furnace. The yield of metallic zinc was approximately 75% of the original zinc powder weight.  
         [0036]    The foregoing examples of the present invention have been presented for purposes of illustration and description. Furthermore, these examples are not intended to limit the present invention to the form disclosed herein. Consequently, variations and modifications commensurate with the above teachings, and the skill or knowledge of the relevant art, are within the scope of the present invention. The embodiments described herein above are further intended to explain best modes known of practicing the present invention and to enable others skilled in the art to utilize the present invention in such, or other, embodiments and with the various modifications required by the particular applications or uses of the present invention. It is intended that the appended claims be construed to include alternative embodiments to the extent permitted by the prior art.