Patent Application
20080041731
The present invention is a process for the addition of iron (III) nitrate to a high sodium chloride aqueous solution (electrolyte) in an amount commensurate to the amount of metal hydrolysates obtained in the electrochemical polishing of aluminum, to precipitate the hydrolysates to a surprisingly effective degree. The process is used to regenerate electrolytes in which the sodium chloride (or other sodium salt) concentration in aqueous solution is, for example, greater than 50%, preferably ranging between 50-60%. The high salt concentration of the electrolyte is not only an important factor for obtaining a high quality surface finish in the electrochemical polishing of aluminum, but also itself contributes to the new and unexpectedly improved precipitation of unwanted hydrolysates overall. In theory, without intention of being bound by the theory, the flocculation of the hydrolysates in the presence of the high salt content is enhanced as contrasted with relatively lower salt contents, possibly because the ready availability of sodium ions creates a complex with the hydrolysates to enhance precipitation of the hydrolysates.
In the prior art process for regenerating electrolytes as described in U.S. Pat. No. 4,737,250, the metal hydrolysates are separated from a sodium nitrate electrolyte by adding iron (III) nitrate followed by a known separation method. It is believed that about 10-25% of the finely dispersed hydrolysates do not bind to the iron (III) nitrate and, therefore, these contaminants remain in the electrolyte after the separation process. Even minor amounts of metal hydrolysates remaining in the electrolyte can adversely effect the reflectiveness and brightness of a surface in electrochemical polishing applications, however, for example, when minor amounts of the aluminum hydrolysates (e.g., about 1.0 wt. %) remain in the electrolyte during electrochemical polishing of aluminum surfaces, a white cloudy film develops on the surface, thereby substantially reducing the polished appearance of the aluminum surface.
The addition of iron (III) nitrate to an electrolyte mixture containing high sodium chloride concentrations produces a substantially contaminant-free electrolyte when separated (e.g., filtration, sedimentation or centrifugation). The added iron (III) nitrate undergoes hydrolysis and the ionic products thus formed in turn complex the metal hydrolysate impurities to form coarsely flocculent particles. The hydrolysis reaction, in theory, also causes the sodium ions to create a complex with the remaining hydrolysates that did not bind with the iron (III) nitrate, thus enhancing precipitation of the remaining hydrolysates. The coarsely flocculent particles and the sodium complexes are then easily separated from the electrolyte using known separation methods including filtration, sedimentation and centrifugation. Although the salt concentration of the electrolyte is slightly decreased as a result of flocculation and separation, the resulting electrolyte is substantially contaminant-free, and a minor amount of restorative salt may be added. The regenerated electrolyte can then be added back into an electrolyte reservoir in an electrochemical polishing process without affecting the quality of the polished surface.
The following example is illustrative.
In the electrochemical polishing of aluminum parts such as automobile wheel rims, an electrolyte containing about 60% sodium chloride is used. After the polishing process, at least one-half pound of aluminum hydrolysates are produced per rim, thus forming a sludge or gelatinous mixture containing the electrolyte (aqueous sodium chloride) and finely dispersed aluminum hydrolysates. This electrolyte mixture may have at least 5.0% by weight of solids (hydrolysates) based upon the total weight of the mixture. Iron (III) nitrate is added to the electrolyte mixture in an amount ranging from 10 to 100 mg of iron (III) nitrate per liter of electrolyte mixture. The mixture reacts thus forming both coarsely flocculent particles and sodium ion complexes with the hydrolysates. Centrifugation of the mixture results in a substantially contaminant-free electrolyte. Even with a slight decrease in salt concentration, restorative salts do not have to be added until the salt concentration of regenerated electrolyte falls below 50%.
By measuring the electric power, which is a measure of the removal of metal, it is possible to meter in the iron (III) nitrate which is to be added to the electrolyte in an electropolishing circulation system in a controlled manner. The addition of the iron (III) nitrate is expediently effective in the electrolyte reservoir or in a separate circulation system connected thereto, in which the hydrolysates are likewise removed.
Although the present invention has been described with references to specific details of certain embodiments thereof, it is not intended that such details should be regarded as limitations upon the scope of the invention except in so far as they are included in the claims.
