Aqua-regia is a leaching process which is used commonly in the processing and analysis of ores containing gold. This process can be dangerous for chemists as aqua-regia is such a corrosive acid mixture. Robotics and automation provide a means to remove the possibility of accidents by removing chemists from hands on processing.

Argon?s robotic aqua-regia systems:

• Accept soil samples.
• Perform a sequence of chemical additions, weighing, heating, cooling, mixing & cleaning actions which comprise the aqua-regia leach.
• Output liquid samples ready for elemental analysis.
• Can be linked to analysis machines e.g. ICP-OES, ICP-MS etc.

Robotic systems such as this one have much greater sample processing rates in comparison to the old way using human processing. Robots are not limited to set working hours, they do not experience much downtime, they have the ability to work 24 hours a day 7 days a week.

Technical Info: Detailed description of Chemistry Underlying an Aqua-regia Leach

Aqua-regia is an extremely corrosive acid mixture and is used extensively for gold (Au) analysis. Aqua regia consists of a 3:1 mixture of concentrated hydrochloric acid (HCl) to concentrated nitric acid (HNO₃). Gold is an extremely unreactive metal and requires an extremely strong acid mixture such as aqua-regia to break its strong bonds; gold is sometimes known as a noble metal due to these properties.

The process of gold analysis using aqua-regia begins with the addition of the acid mixture to either the soil sample or the precious metal pellet produced from fire assaying. The aqua-regia is generally added sequentially, initially nitric acid is added to the sample, followed by the addition of a mixture of nitric and hydrochloric acids. Sometimes simply nitric is added first and hydrochloric second. Hydrochloric acid is never added first; if it was then a coating of Silver Chloride (AgCl) would form around the outside of the gold which would hinder the nitric acids ability to react with the gold. The chemical reactions which take place during gold dissolution are detailed below.

Reactions:

Au_{(s )} + 3NO₃^-_{(aq )} + 6H⁺_{(aq )} <-> Au³⁺_{(aq )} + 3NO_{2(g )} + 3H₂O_{(l )}

Au³⁺_{(aq )} + 4Cl^-_{(aq )} <-> AuCl₄^-_{(aq )}

It must be noted that neither acid independently would be able to affect the gold. When used in conjunction to one another however they obtain the ability to break the strong structure. Nitric acid is a strong oxidising agent and will produce gold ions (Au³⁺); the chloride ions (Cl^-) which are supplied by the concentrated hydrochloric acid form complex ions (AuCl₄^-) with the gold ions. Consequently to producing the complex ions it shifts the equilibrium position of the reaction to favour the oxidation of the gold which consequently allows for further complex ions to be produced.

Generally once the aqua-regia has been added it is allowed to sit for a while as the nitric acid produces gaseous NO₂, which has to bubble off. The mixture is then placed onto a heating mantle which accelerates the reaction by increasing the energy of the reacting particles as to create more particle collisions.

Once the gold is in solution it becomes a matter of making the solution up to a set volume and making the solution homogeneous. This is done so that the elemental proportions gathered from the detectors on the analysis instruments are able to be converted back to find out the elemental proportions of the gold in the initial soil sample.

Analysis Instrumentation:

The homogeneous solution is analysed for all elements of interest by high tech instrumentation. Some of the high tech instrumentation used for the analysis of gold after aqua-regia include Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES), Flame Atomic Absorption Spectroscopy (Flame-AAS), Inductively Coupled Plasma Mass Spectrometry (ICP-MS). These instruments have the ability to detect elements to a concentration of parts per million (ppm) or parts per billion (ppb) depending on sensitivity and the instrument.

Robotics in the Laboratory:

Most analytical laboratories perform these analyses by hand which exposes staff to harmful acids as well as repetitive boring work. However in recent years, developments in technology have arisen in the field of robotics and automation, and many labs are now converting their systems toward automation.

Laboratories around the globe are turning their sights to robotics and automation due to the ability to increase efficiency and reproducibility for sample analysis. Robots provide a means to deliver accurate, repeatable results with minimal time wastage as well as minimal materials wastage. Due to the ability of robots to be precisely programmed and their ability to follow complex algorithms, heating times and process times are able to be optimised according to requirements of specific samples.

Through the use of robots and simple automation, it is easy to perform many laboratory processes. Especially processes which mainly involve pick/place, dosing, weighing, heating, cooling, sample transfer and mixing actions, which covers most laboratory procedures.

For more information on how Argon Lab Systems will be suitable for your application contact us.