Method for recovering nitric acid and purifying silver nitrate electrolyte

#The present invention relates to processes for the regeneration of an acid-based electrolyte through a hydrolysis process in order to remove undesirable metal-based compounds from such electrolyte. More particularly, it relates to a process for regenerating nitric acid based electrolytes such as the silver nitrate and nitric acid based electrolyte as used in certain prior art processes, including the electrolytic refining of silver using such electrolyte. The process of the current invention further comprises the hydrolytic removal of undesired metal-based compounds from the silver nitrate and nitric acid based electrolyte. Further, the metal nitrates other than silver nitrate which are in the nitric acid solution may be substantially purified into insoluble compounds and separated from the solution by filtration for processing or use.

#Electrolytic silver refining is normally carried out according to the Moebius or Balbach-Thum processes. The common contaminating metals, in particular copper, are an impediment to the refining process. These contaminating metals are dissolved and concentrated in the electrolyte. In typical silver nitrate baths used for electrolytic silver refining, a copper content of about 60 g/L is considered to be the maximum permissible value. In excess of this value, cathodic deposition of copper may occur whereby Cu salts are trapped in the refined silver so that the purity requirements are no longer met. In situations where there is an excess of copper, entrapment of metal salts also occurs. Therefore, the used electrolyte must be restored to being “fresh” electrolyte having a lower Cu content from time to time.

#In the past, silver refining has been performed by electrolyte recycling. The electrolyte recycling, i.e., removal of contaminating metals was typically accomplished by solvent extraction techniques. Swiss Patent No 614,238 describes a method employing solvent extraction for the removal of copper from silver nitrate electrolytes. The extractants used in this patent are commonly chelating agents such as phenoximes or quinolines, and remove substantially all of the copper, and associated metals such as nickel, from the silver nitrate electrolyte.

#The description which follows, and the embodiments described therein are provided by way of illustration of examples of particular embodiments of principles and aspects of the present invention. These examples are provided for the purposes of explanation and not of limitation of those principles of the invention. Wherever ranges of values are referenced within this specification, sub-ranges therein are intended to be included within the scope of the invention unless otherwise indicated. Where characteristics are attributed to one or another variant of the invention, unless otherwise indicated, such characteristics are intended to apply to all other variants of the invention where such characteristics are appropriate or compatible with such other variants.

#The process will now be exemplified by reference to the following examples.

#500 mL of spent silver refinery electrolyte, analyzing 81.1 g/L Ag, 66.4 g/L Cu and 4.78 g/L Zn was heated up to 160° C., driving off unbound water and free nitric acid. Cold water was then injected into the concentrated electrolyte at the rate of 1 mL/minute for a period of 2.5 hours while further nitric acid was evolved and separated from the solution and while the temperature was maintained at 160° C. Nitric acid distilled from the solution at this stage reaching a maximum concentration of 205 g/L. At the end of the injection period, heating was stopped, and water was slowly added to the solution to return the volume to its original value. This back-dilution occurred at a rate sufficient to ensure that the liquid phase did not freeze. The insoluble salts were then separated from the electrolyte by filtration. The concentration of the final solution was 77.1 g/L Ag, 24.5 g/L Cu and 4.3 g/L Zn. The recovered solids analyzed 0.07% Ag, 50.7% Cu and 0.64% Zn, representing the removal of 60.4% of the copper and 10.0% of the zinc in the original solution. X-Ray Diffraction analysis of the solids showed them to be basic copper nitrate.