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.
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.
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.
Au(s ) + 3NO3-(aq
) + 6H+(aq ) <-> Au3+(aq ) + 3NO2(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.
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
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.