SCALE-UP EFFECT ON SULFIDE PROCESSING OF WASTE ELEMENTAL MERCURY USING A PLANETARY BALL MILL
The Minamata Convention on Mercury was adopted in 2013, which requires that elemental mercury collected from mercury waste should be managed under safe conditions. Elemental mercury is liquid at room temperature and has a high volatility, allowing it to move easily in the environment. It is preferable that elemental mercury be recovered by a distillation method that converts it to a more stable chemical form for long-term storage or permanent disposal. We have developed a stabilization method by producing mercury sulfide using a planetary ball mill. However, in order to apply the method to a real plant, the challenges on the scale-up effect should be confirmed. In this study, we conducted scale-up tests to confirm that mercury sulfide can be effectively produced using a larger sized planetary ball mill, and investigated options for improving the operating conditions for stabilizing mercury. Firstly, the mercury concentration in the headspace above the stabilized materials was measured to evaluate degree of the reaction between elemental mercury and sulfur.
In previous studies, the maximum capacity for sulfide processing was calculated as 120 g-Hg/h/pot using 250 mL pots with balls of 19.04 mm diameter in a laboratory-scale planetary mill. In this study using 2,400 mL pots with balls of 19.04 mm diameter, the maximum capacity (1152 g-Hg/h/pot) was equivalent to the value calculated by extrapolating the results from previous laboratory-scale experiments. Ball size was the most important parameter studied under various conditions. As the ball diameter increased, the stabilization progressed faster under stronger pulverization. When the ball diameter was 25 mm, the reaction time was shortened to 20 min. The powdered products obtained using the planetary mill were identified by X-ray diffraction analysis of cinnabar and metacinnaber, two types of mercury sulfide. Our study demonstrates that sulfide processing of mercury can be effectively conducted using a larger sized planetary ball mill.
THE ROLE OF MERCURY IN NUCLEAR FUSION TECHNOLOGY
Nuclear fusion, the merging of two hydrogen isotopes (deuterium and tritium) to helium under the release of huge amounts of energy, is considered as one important element to satisfy the world-wide growing demand of energy. This technology would be carbon-free, safe, almost unlimited in resources and does not produce long-living radioactive waste. In several countries in the world (EU, USA, Japan, China, Korea, India, Russia) large development programs have been launched to bring this technology to maturity. To realize fusion power plants, high technical challenges have to be tackled. Two of them are closely linked to the use of mercury.
One area of specific concern is the fusion fuel cycle which processes large amounts of the radioactive tritium fuel. Tritium is a weak -emitter and decays with a half-life of 12.3 years to form helium. As the tritium isotope is chemically reactive and easily implanted in any organic species via an isotope exchange reaction against the normal hydrogen, all such hydrogen-containing species (such as polymers, organic working fluids, oils, lubricants, hydrocarbons etc) are not allowed to be used in the processing machines. The best fully tritium-compatible alternative working fluid is mercury, as it does practically not interact with tritium.
As tritium itself is not a natural resource, it is produced in-situ via a nuclear reaction with lithium-6 that takes place in the so-called tritium breeding blankets attached to the first wall of the reactor. The lithium-6 has to be taken from natural lithium which is a lithium-6/lithium-7 mixture with 7.5% lithium-6. There are various technologies to enrich the lithium-6 content of natural lithium to the required level; the most promising approach is based on isotope exchange between lithium salt and lithium amalgam, exploiting the very high affinity of mercury to lithium-6.
This paper shortly presents the EU roadmap to a fusion power plant until the 2050s. It then introduces in the main missions and delineates the technical requirements for the fuel cycle and the breeding blankets. Here, issues of safe and reliable mercury handling with minimum releases are addressed. Finally, an optioneering exercise is conducted that clearly supports the use of mercury related technologies, and a near- and mid-term RD programme is outlined to bring these concepts to higher technical readiness levels. This paper is also thought to create awareness of these opportunities in the industrial mercury community.
MERCURY AS WORKING FLUID IN SPECIAL-PURPOSE VACUUM PUMPS
Since the very early days of vacuum technology, mercury is used in various applications: For the measurement of pressure, Evangelista Torricelli used a glass tube filled with mercury in 1643. The first high vacuum pump on the market, invented by Wolfgang Gaede in 1915, has been operated with mercury as working fluid and opened the door to scientific fields like accelerator physics and space applications that are unthinkable without high vacuum conditions.
Nowadays, the importance of mercury in vacuum technology has decreased: pressures are now measured using electronic gauges and for vacuum pumping, pumps are available on the market that are dry (i.e. no working fluid needed) and simple to operate (i.e. no heat-up times, no cool water and cold traps required). However, despite of all this new technologies, there is still a small but growing field where mercury as working fluid cannot be replaced: high vacuum pumping for very reactive and radioactive gases.
In the past, mid of the 20th century, mercury has been used extensively for vacuum pumping and tritium (a radioactive hydrogen isotope) processing. At that time, mercury was not handled with care and a lot of environmental contamination occurred, leading to a bad image of this element. In the 21st century, the development of a new source of energy is ongoing that needs a very large vacuum system where huge amounts of tritium have to be processed: nuclear fusion.
This paper presents the development of novel vacuum pumps for fusion applications at KIT, namely mercury ring pumps and linear mercury diffusion pumps. For both pump types, the requirements are defined and the development path is shown up. Special focus during the development is given to the minimization of the working fluid inventory and the trapping of mercury inside the pumps. As conclusion, future RD activities are outlined.