Aqua Regia takes a Back Seat to a new Organic Process for Dissolving Gold and other Precious Metals

Solution of aqua regia in a laboratory
Photo by Thejohnier

Aqua Regia one of the few chemicals capable of dissolving gold and platinum group metals (PGMs) that was invented about 800 AD by the Arabic alchemist Gebar that was described in western literature in the 13 century by Paul of Taranto sometimes described as the “Pseudo Gebar.”  For centuries Aqua Regia reigned supreme until the development of organic solvents for gold were developed at the laboratory of Wei Lin and collogues at the Georgia Institute of Technology in the early 21^st century. 

Aqua Regia is composed of one part of fuming nitric acid and three parts of hydrochloric acid that was used with a solution of tin chloride for the reaction forming the Purple of Cassius the only definitive test for gold until the invention of the Atomic Absorption Spectrometer in the 20^th century.  Aqua Regia was also used in the gold refining process, but the same chemicals also dissolved PGMs making it difficult to remove PGMs from gold.  Lin’s new chemistry is capable of selectively removing the errant PGMs from gold by varying the constituents of the solution.

The new solution is based on the reaction of thionl chloride (SOCl₂) and the organic solvent pyrazine.  Further experiments disclosed that by substituting other organic solvents such as N-N dimethylformamide (DMF), imidazole and pyrazine.  The gold can be recovered by calcining the resulting mixture.

Gold dust precipitated from aqua regia.
Photo by Greenhorn 1

A mixture of SOCl₂ and DMS dissolves gold but doesn’t affect the PGMs making it possible to obtain 99.999 caret pure gold that is bullion grade.  By changing the recipe you can dissolve the PGMs without dissolving the gold.  This part of the process will make it easier to recover PGMs from catalytic converters then previous methods, and fuel cells in the future.

Even though the process can’t efficiently compete with conventional aqua regia at the present time Lin feels it will have uses beyond just dissolving gold and PGMs in the use of making nanostructures of the noble metals as well as recycling nano-coatings,  

Researches are ongoing with this technology, and it is expected that over the following years that advances in the technology will occur as well as new uses for the technology.  One of the major stumbling blocks that has to be overcome is the effects on the environment that is presented by this new technology.