All that glitters is gold

The authors: Dr. Jürgen Reinemuth Managing Director, Thaletec Joachim Prior Technical Director, Prior Engineering

Dispersion of gaseous phases in liquids in glass-lined reactors is a well-known and frequently used process in the chemical and pharmaceutical industries. Hydrogenations and chlorinations are two typical processes where gas needs to be mixed with liquids. In biotech applications, gassing is necessary to supply cells and bacteria with the required oxygen.

In cooperation with the Swiss firm of Prior Engineering Services, Thaletec has developed a glass-lined reactor with the SEGTEC system (surface effect gassing technology) for surface gassing, to enable fine gold to be precipitated from hydrochloric tetrachloro gold (III) acid by means of SO2 gas. This process, which is marketed under the name Aureomat, allows fine gold qualities of more than 99.99 % to be refined by selective precipitation. In contrast to other, conventional gold precipitation processes such as precipitation with sodium metabisulphite (Na2S2O5), the use of technically pure SO2 gas as a reducing agent leads to a zero emission process, i. e. no SO2 gas escapes during the regular process. Furthermore, additional ions (e. g. Na+) – which could have a problematic effect, for example, in the process chemistry downstream of the gold precipitation – are avoided. The closed-system design of the equipment increases metal safety during the process, also suggesting that SO2 gas consumption is close to the theoretical value. There are several other advantages, too:

In the past, SO2 gassing of tetrachloro gold (III) acid has been realised in a rudimentary way with exchangeable plastic tubes suspended loosely in an open reactor, mostly without an agitating element, or by means of technically far more complex pressure gassing systems.

The technical challenge linked to the gold precipitation process using SO2 gas is that the gold precipitates spontaneously as soon as SO2 gas encounters the tetrachloro gold (III) acid. This chemical process for regular pressure gassing systems cannot be avoided technically, but with appropriate system design and/or technical know-how it can be controlled. Potential obstruction of the dip pipe, the partial precipitation of gold in gas-carrying lines (condensates) and the formation of gold deposits (pipelines, membranes in gas circulation pumps, etc.) are logical process-related risks as a result of spontaneously precipitated gold particles.

The SEGTEC surface gassing system used in the Aureomat process is different, however. The whole of the gas circulation unit, including the special gas circulation pump and its preventative maintenance cycles, has been elimi-nated. The SO2 gas is dispensed into the reactor’s gas-filled compartment and drawn in from it across the liquid surface. All surfaces in contact with the media are made of abrasion-resistant Thaletec Abrisist glass while the gas circulation itself is effected exclusively within the Aureomat reactor, which remains closed throughout the process. The complex chemistry of the gold precipitation is controlled more reliably using greatly simplified apparatus.

In addition, SEGTEC surface gassing is not impeded by the precipitated, very heavy gold particles with a density of 19,320 kg/m³. Cold welding/pressing of the ductile fine gold particles, which occurs relatively easily with fine gold qualities of 99.99 %, is avoided by a controlled process and adequate agitating system specifications, that is to say by ensuring the intended formation of the fine gold sponge as fine-grained, sand-like and lump-free.

The SEGTEC technology is suitable for all technical gassing processes and gas-liquid reactions – either without or with increasing or decreasing solid concentrations. In practice, the glass-compatible design results in hardly any process limitations with regard to chemical resistance, surface finish, process pressure ranges or cleaning efficiency in pharmaceutical production plants (CIP – cleaning in place). Besides applications in the precious metals sector, SEGTEC also facilitates new, innovative process methods for a variety of industrial branches, e.g. fermenters or technically complex gassing processes in pharmaceutical and food manufacturing, hydrogenation at high pressures or other gas-induced precipitation and/or solvent reactions. In principle, it is possible to equip existing gassing processes with this novel technology at a reasonable cost. Providing a feasibility check is carried out first, this also applies to glass-lined agitating and gassing systems from third party manufacturers.

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