Mercury waste treatment

Mercury waste treatment

For many years we have been dealing with all types of mercury waste: Sludge, solids, waste lamps, catalysts, building rubble and various other wastes.
Today one of our Business Development Engineers would like to share some of his experiences with you, to give you a short introduction to the wide field of mercury waste treatment:

What is mercury and why is it so special?

Mercury, quicksilver or simply Hg is a very special element. It has been fascinating people ever since it was first discovered in antiquity. Mercury is the only metal that is liquid at ambient temperature.

  • Melting point: – 38.8 ° C
  • Boiling point: 357.1 ° C (225 °C at 50 mbar)
  • Vapour pressure: 0.00163 mbar at 20 ° C
  • Density: 13.53 kg/l

Why is mercury a problem?

Mercury and most of its compounds are extremely toxic. One of the major risks is that it can be accumulated by fish and concentrated along the food chain (biomagnification). If humans consume these affected fish, fatal mercury poisoning can occur (Minimata disease). Another characteristic of mercury that makes it particularly troublesome is that it is not biodegradable and is therefore a persistent contaminant.

Where is mercury used?

Nowadays the use of mercury has been phased out for most applications due to its toxicity. However, in some countries it is still used for several applications, e.g. artisanal gold mining, industrial applications (chlorine-alkali industry), thermometers, fluorescent lamps, batteries, switches, tannery, pesticides, wood impregnation and many more. It is also generated as an unwanted by-product during oil and gas exploration and processing. Altogether this creates a large amount of highly problematic mercury contaminated waste streams.

So what can we do with all this waste?

The special properties of mercury make it difficult to deal with using traditional waste disposal techniques. Unlike other wastes, the two typical solutions of dumping on a landfill or incineration are usually not possible. Dumping of mercury waste in landfills is forbidden as its high density would quickly result in the waste seeping towards the groundwater – even in engineered landfills. Likewise, incineration cannot be used as the mercury would evaporate during incineration, and is difficult to remove from the huge off-gas stream of a normal waste-incineration plant (high emissions).
However, the special characteristics of Hg do offer one possibility to remove it from waste. The solution is evaporation. Due to its extremely low boiling point (compared to all other metals) it is possible to remove mercury completely by thermal desorption.
Several thermal treatment technologies have been used in the past with varying success:

  • Heated screw conveyors or continuous mixers
  • Vacuum retort
  • Indirectly heated, discontinuous vacuum mixer (VacuDry®)
  • Rotary kilns, indirectly heated
  • Rotary kilns, directly fired

In fact, so far only two methods have proven to be suitable for the treatment of mercury contaminated soil and mercury waste while remaining economically viable on an industrial scale.

  • Indirectly heated, discontinuous vacuum mixer (VacuDry®)
  • Rotary kilns directly fired

How does it work?

Indirectly heated, discontinuous vacuum mixer (VacuDry®)

At the core of the indirectly heated, discontinuous vacuum mixer is a vacuum evaporator chamber, which uses heat and controlled vacuum to evaporate contaminants with boiling points up to 450 °C (under atmospheric pressure). To heat up the evaporator, temperature resistant synthetic oil is circulated inside the evaporator´s heating jacket and its central shaft. The rotating shaft inside the still-standing cylindrical evaporator vessel ensures intensive mixing during the process. Material temperatures rise to 370 °C and pressure is reduced to 50 mbar(abs).
The evaporation process is divided into several phases: Water evaporation phase, oil evaporation phase (if oil is present) and mercury evaporation phase. The off-gas is led into a heat exchanger, which separates the vaporized compounds (water, oil, mercury) from the off-gas stream. The remaining vapour passes through a vacuum unit and is cleaned of residual contaminants by an activated carbon filter, before finally being discharged into the atmosphere. Water, hydrocarbons, mercury and the clean solids are then recovered as separate materials.

For more information please refer to VacuDry® 3D-Animation

Rotary kilns (directly fired)

The treatment of material inside a rotary kiln is a continuous process. The feedstock is dosed and continuously feed into the rotary drum e.g. by use of a screw conveyor. The rotational movement and slight incline of the rotary kiln allow the feedstock to pass through the kiln with ease. During this process the material heats up via the hot inner wall of the rotary kiln, or in the case of directly-fired rotary kilns, by the burner flame (burner mirror) and the hot exhaust gases. The mercury is evaporated together with the water.

At the end of the rotary kiln the solid material drops out and is cooled by cooling conveyors before it is finally discharged. The off-gas is lead to a chamber where dust, carried along with the vapours, is separated. Afterwards water and mercury contained in the off-gas are removed in a washer and collected as sludge together with the residual dust. The remaining off-gas is then subject to afterburning in a post-combustion chamber, where it is exposed in oxidizing conditions for 4 seconds at 850 °C. The goal of the post combustion is to avoid the formation of toxic substances (e.g. dioxins) which can be created during the incineration process. In the next step the off-gas is filtered and undergoes a number of additional treatment steps before passing through an activated carbon filter. Afterwards the cleaned off-gas is discharged.

Which technology for which application?

Due to its closed vacuum system design, the VacuDry® technology should always be the technology of choice for mercury waste. The only exception is waste containing mercury(I/II) chloride and mercury sulphide (HgS), in which case a rotary kiln is required. However, for the majority of cases a VacuDry® pre-treatment is still beneficial or even mandatory due to energy efficiency and the limitations of the rotary kiln regarding the water and oil content of feed material.

Comparison of characteristics
Table 1: comparison of the characteristics of VacuDry® and direct fired rotary kiln technology

Parameter VacuDry® Rotary kiln (directly fired)
Heating Thermal oil heated,
indirect heating
Direct fired
Treatment temperature Up to 370 °C 650 °C to 1,150 °C
Hg level after treatment < 1 ppm (elemental Hg) < 1 ppm
Off-gas stream 100 to 1,000 Nm³/h 5,000 to 25,000 Nm³/h
Safety Operation under vacuum, inert atmosphere 3 mbar differential pressure, oxidizing atmosphere
Approx. energy consumption per ton soil treated
(sandy; 15 % moisture)
~ 210 kWh/t ~ 700 kWh/t
Energy efficiency > 80 % ~ 30 %
Off-gas treatment Vapour filter
Two-step condensation unit
Activated carbon filter
Cyclone
Post-combustion
Gas-washer
E-filter
Activated carbon filter
Environmental licensing State of the art
Due to closed system, permitting process in sensitive areas is no problem.
State of the art
In sensitive areas, permitting process can be difficult.
Additional produced waste water, per ton material treated (additional to water contained in input material) None ~ 0.5 -1 ton waste water per ton material treated
(from off-gas cleaning)
Recovered mercury Pure Mercury Mercury sludge (Needs to be processed in additional treatment step)

Typical Applications
Table 2: Comparison of typical applications

Contaminants VacuDry® Rotary kiln (directly fired)
Elemental mercury yes yes
Methyl mercury yes yes
Mercury(I/II) chloride no yes
Mercury sulphide (HgS) no yes
Hydrocarbons no percentage limitation up to a maximum of 5 %
PAH, TPH, PCB, Dioxins, Furans, Organolead yes yes
High water content no limitation up to 25 %
High mercury content no limitation no limitation

 

This was a short introduction on mercury waste and how it can be dealt with. For more information please have a look at “Mercury wastes – Mercury recovery from waste and soil” or at our Mercury waste treatment centre.

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