Stabilising liquid mercury in the mercury sulphide system
Since 1991, the company DELA GmbH has been successfully using vacuum technology to treat waste containing mercury thanks to its high level of operational security and the low emissions. The company DELA GmbH has developed a future-oriented stabilisation procedure for the environmentally compatible pre-treatment and disposal of waste that falls under the Mercury Guideline of the European Union which comes into effect in March 2011. It describes the chemical transformation of mercury to mercury sulphide with the objective of significantly reducing or completely eliminating the risk potential of the primary substance.
The process principle of the DELA stabilisation procedure is the reaction of mercury and sulphur in a closed temperature-controlled vacuum mixer. The proven vacuum technology stands out thanks to its high operational security and the fact that it works almost emission-free. Also, the production of non-desirable secondary products, such as mercury oxide and sulphur oxide, is avoided in the vacuum atmosphere during the stabilisation procedure. The primary objective of the stabilisation procedure is the complete transformation of liquid mercury into mercury sulphide.
Tested quality
We arrange for several external chemical institutes to check that the produced mercury sulphide complies with our quality criteria:
- Crystalline structure
- Thermodynamic and pressure stability
- Good eluate behaviour
- Free of elementary mercury
Dela started operating a system for stabilising liquid mercury on an industrial scale at its site in Essen in February 2010.
Brief profile of the system
| Special aspect | Produces a final product that, thanks to its properties (very low solubility product 10-54 mol²/l²), is considered to be non-toxic and not hazardous to water to allow safe disposal |
| Throughput | Approx. 800 kg mercury per batch |
| Process | Hg stabilisation via the reaction of mercury with sulphur in a closed temperature-controlled vacuum mixer |
| Delivery/collection | Steel, plastic kegs |
| Input | Liquid mercury from the treatment of industrial waste (e.g. catalysts or electrolyte water) |
| Output | Mercury sulphide powder |
| Optional | Production of pellets |
Procedure
Mercury and sulphur are weighed in a stoichiometric ratio and then placed into the designated hoppers. Once filled, the containers are tightly closed and rendered inert with nitrogen to prevent the creation of an explosive atmosphere inside the vacuum mixer. The sulphur powder is placed straight into the mixer whilst the mercury is dosed via a needle valve. Taking into consideration various influencing factors, the continuous mixing of mercury and sulphur in a vacuum (< 0.05 bar) means that the input materials are completed transformed into mercury sulphide.
Several external institutes conduct regular chemical analyses of the final product with regard to the crystallinity, thermal properties and eluate behaviour to ensure adequate product quality.
To produce the mercury sulphide as pellets, additives can be placed in a third hopper and then fed into the process. The production of mercury sulphide in pellet form has proved particularly beneficial because it significantly reduces dust emissions.
