The present patent application relates to a method for processing mercury-containing sludge and a system for processing mercury-containing sludge.

Sludge from, among other things, drilling activities for the purpose of for instance oil and natural gas extraction or some decontaminating activities usually also contains mercury (Hg) in addition to hydrocarbons such as (mineral) oil. The (mineral) oil also comprises aromatics such as benzene, toluene and xylene. This sludge occurs as‘exempt’, i.e. without radioactive contaminants, and contaminated with so-called Naturally Occurring Radioactive Material (NORM). In addition, both forms of the sludge occur as a variant with a rather high quantity of oil, which results in a substance with a so-called low flash point. Sludge (or dredged material, mud, alluvium and so on) is generally a mixture or suspension of solid particles, for instance earth or soil, in water.

Such sludge is usually treated by means of distillation in batch processes. These processes comprise of filling the furnace with sludge, hermetically sealing the lid of the furnace, applying a vacuum to the furnace, increasing the temperature in the furnace from room temperature to 650°C, keeping the furnace at this temperature and at a vacuum for about 20 hours so as to allow the mercury to evaporate and be captured. The furnace must then be cooled from 650°C to 100°C, which can take a long time, for instance 20 hours. If the mercury has not been removed to sufficient extent, the process is repeated.

In the above stated process the mercury is recovered from the waste as elemental and liquid mercury, in which form the mercury is highly toxic and polluting, due to which the mercury must be further processed and stored in a safe manner. The present patent application has for its object, among others, to provide an improved process for processing mercury-containing sludge.

US 6,399,849 B1 describes a process of treating mercury containing waste in a reaction vessel which includes stabilizing the waste with sulphur polymer cement (SPC). A first step of the process (step (a)) is the chemical stabilization of the mercury containing waste. This includes combining the waste with sulphur polymer cement. Thereafter, the resulting mixture is heated to temperatures in the range of 20 to 80 °C. In a second encapsulation step (step (b)) the chemically stabilized mixture resulting from step (a) is heated to form a molten product, which molten product is then cast as a monolithic final waste form. In this step (b), the stabilized mixture obtained in step (a) is heated to 120 to 150 °C.

According to a first aspect, a method for processing mercury-containing sludge is provided, wherein the mercury-containing sludge comprises water and a solid fraction, the method comprising of:

- adding sulphur to the mercury-containing sludge so that a mixture is formed;

- agitating the mixture; - heating the mixture to a temperature in the range of 90 to 360 °C to at least partially evaporate the water, so that the mixture is at least partially dried;

- allowing at least some of the mercury in the mixture to react with the sulphur to form mercury sulphide in the mixture;

- feeding out the evaporated water; and

- feeding out the mercury sulphide-containing mixture.

This process has the advantage that the mercury is stabilized in mercury sulphide and is thereby suitable for storage or for disposal. This process can particularly be carried out continuously, wherein the reaction into mercury sulphide, which is exothermic, is carried out in controlled manner because the mercury is surrounded by the sludge, whereby the temperature is easier to control. The evaporated water further has a low mercury content. The mercury sulphide- containing mixture which is fed out is generally a dry, granular mixture.

The heating is done such that water evaporates but the mercury does not, or hardly so. In other words, the temperature is chosen such that water does evaporate, and mercury does not, or hardly so. The waste is hereby dried and the quantity of remaining waste decreases. At least 80% of the water present in the sludge is preferably evaporated, more preferably at least 95%, most preferably at least 99%. This results in an end product of dried and stabilized sludge having a small volume, whereby less storage or disposal is required.

Agitation can for instance be mixing, such as by stirring or shaking. Agitating of the mixture preferably comprises of stirring the mixture.

The heating is preferably done simultaneously with the agitating. This provides for an accelerated heating of the mixture so that water can evaporate more quickly, and this further provides for the breaking up of mercury droplets in the sludge so that an increased surface area is created for reacting with the sulphur.

It is preferred to perform the step of allowing reaction simultaneously with the agitating and optionally the heating. In practice the reaction of sulphur with mercury will begin after the sulphurous substance is added to the mercury-containing sludge. The agitating is therefore in practice almost always done during the reaction of the sulphur and the mercury. When only little mercury is present in the sludge, it could be possible that the agitating and the heating do not take place until after the reaction has already been completed.

In an embodiment the method comprises of feeding in the mercury-containing sludge at an inlet of a reactor, wherein the reactor further comprises a first outlet and a second outlet for outfeed of respectively the evaporated water and the mercury sulphide-containing mixture. In this way the water can be separated from the mercury sulphide -containing mixture in a practical manner and optionally be further processed separately. The reactor can be any suitable means in which the mixture can be agitated and heated. The reactor is preferably configured to simultaneously bring about a continuous movement of the mixture from the inlet to the second outlet. The process can hereby be carried out continuously. In other words, the mercury-containing sludge is preferably fed into the reactor continuously and the mercury sulphide -containing mixture is fed out continuously.

In an embodiment the heating is done using a heating medium and by supplying the heating medium to the reactor such that the mixture is heated, wherein the reactor is configured such that the heating medium does not come into contact with the mixture. This can for instance be done by a jacket on an outer side of the reactor. The reactor can also be configured with agitating members such as paddles, wherein such members are hollow and the heating medium, such as water, steam or thermal oil, can flow therethrough.

The process is particularly advantageous when the mercury-containing sludge further comprises hydrocarbons. The method then preferably comprises of evaporating at least some of the hydrocarbons, feeding out the evaporated hydrocarbons together with the evaporated water and separating the mixture of evaporated water and evaporated hydrocarbons into a water fraction and a hydrocarbon fraction. The separated hydrocarbons have been found to contain very small quantities of mercury.

The hydrocarbon fraction is preferably used to heat the mixture of the sludge and the sulphur. The reactor can for this purpose be configured with burners. It is however preferred to apply a thermal oil system, wherein the oil is heated by means of a thermal oil boiler and the heated oil is used to indirectly heat the mixture in the reactor. This way of carrying out the process has the advantage that energy is gained since heat is recovered from the hydrocarbons which are present in the original sludge.

The mixture is heated to a temperature in the range of 90-360°C. In such ranges the water and possibly the hydrocarbons evaporate, while the mercury does not evaporate. Later on in the process the temperature may rise, also due to the exothermic reaction of the mercury with the sulphur. In this case a large part of the mercury has however already been stabilized into mercury sulphide, which makes the chance that the mercury evaporates increasingly smaller. When the hydrocarbons comprise a relatively large amount of hydrocarbons with long chains (for instance Ci5 and higher), a higher temperature will be applied, for instance 300-400 °C, preferably 300-360 °C, and/or preferably after most of the water has evaporated.

It is preferred for the process to be carried out at at least half an atmosphere of ambient pressure, preferably approximately at atmospheric pressure.

In an embodiment at least the steps of agitating, heating and allowing reaction are performed under an inert atmosphere. This prevents a reaction between mercury and oxygen at increased temperature, wherein mercury oxide (HgO) is formed. The presence of HgO in the final product is undesirable because HgO has a higher solubility in water than HgS, which is insoluble in water. The sulphur can here react with oxygen of air, and combust. This is prevented by use of the inert atmosphere. Nitrogen is preferably used as inert gas for obtaining the inert atmosphere.

Alternatively, use is not made of an inert atmosphere. This is because, when the reactor is filled with sludge or is being continuously replenished and is closed, relatively little air is present. The remainder of oxygen will be discharged quickly with the evaporation of the water and the oil fraction, and the danger of combustion of sulphur is also small.

Allowing reaction preferably comprises of allowing at least 95% of the mercury to react with the sulphur. This process achieves particularly that more than 99% of the mercury, i.e.

substantially all the mercury, reacts with the sulphur. This can for instance be achieved inter alia by applying an excess of sulphur. This can further be achieved by applying sufficient agitation.

In an embodiment the sulphur is (elemental) sulphur, wherein the sulphur is preferably added in a mass ratio of sulphur to mercury of at least 1 to 5. Such an excess is sufficient to allow at least 95% of the mercury to react, although at least 99% or even substantially 100% can be achieved.

According to a second aspect, a system is provided for processing mercury-containing sludge, wherein the mercury-containing sludge comprises water and a solid fraction, the system comprising a reactor which comprises an inlet, a reaction volume, a first outlet, a second outlet and agitating members; wherein the reactor is configured to:

- receive the mercury-containing sludge or a mixture of a sulphur and the mercury- containing sludge at the inlet;

- agitate the mixture in the reaction volume by means of the agitating members;

- heat the mixture to a temperature in the range of 90-360 °C in the reaction volume to at least partially evaporate the water in order to at least partially dry the mixture;

- allow at least some of the mercury of the mixture to react with the sulphur in the reaction volume to form mercury sulphide in the mixture;

- feed out the evaporated water via the first outlet; and

- feed out the mercury sulphide -containing mixture via the second outlet.

As will be apparent, such a system is particularly suitable for performing the method according to the first aspect, and the stated advantages also apply to this system.

The mercury-containing sludge and the sulphur can preferably be added at the inlet and the mixing will preferably be performed by the reactor.

In an embodiment the reaction volume extends from the inlet to the second outlet, wherein the reactor is configured to move the mixture from the inlet, via the reaction volume, to the outlet part. The agitating members are preferably configured to move the mixture toward the outlet. This forward movement is preferably applied continuously, so that the processing of the sludge can be carried out continuously. The reactor is preferably a mixing dryer. The mixing dryer more preferably comprises a plurality of paddles, wherein the paddles are configured to agitate the mixture and move it to the outlet part. These paddles are an embodiment of the agitating members. Such an embodiment is also referred to as paddle dryer.

The mixing dryer preferably comprises two shafts rotatable in opposite directions, wherein the paddles are arranged on each of the shafts. This embodiment of the mixing dryer provides for a mixing and agitation which bring the sulphur and the mercury into contact such that the conversion into mercury sulphide can be completed in a shorter time.

In an embodiment the mixing dryer is configured to rotate the paddles at a rotational speed of at least 40 revolutions per minute.

In an embodiment the reactor has a heating medium inlet for infeed of a heating medium and a heating medium outlet for outfeed of the heating medium, wherein the reactor is configured to heat the mixture using the heating medium and to keep the heating medium separate from the mixture.

Particularly when the mercury-containing sludge further comprises hydrocarbons, the reactor is preferably configured to evaporate at least some of the hydrocarbons and to feed out the hydrocarbons together with the water via the first outlet, wherein the system further comprises a separating device for separating the mixture of evaporated water and evaporated hydrocarbons which was fed out into a water fraction and a hydrocarbon fraction.

In an embodiment the system further comprises a heating system configured to heat the mixture in the reaction volume, wherein the heating system is configured to use combustion heat from the separated hydrocarbon fraction.

The system is preferably configured to heat the mixture to a temperature in the range of 70-400°C, preferably 90-360°C.

In an embodiment the system is configured as a mobile system which can be placed close to a source of mercury-containing sludge (such as close to a bore well or a storage silo) or close to a storage location for storage of the mercury sulphide-containing dried mixture.

Further advantages, features and details of the present invention will be elucidated with reference to the following description of figures relating to a preferred embodiment thereof, in which:

Fig. 1 is a schematic perspective view of a preferred embodiment of a system according to the second aspect;

Fig. 2 is a process diagram of a preferred embodiment of a method according to the first aspect. The method as described above comprises of bringing the mercury-containing sludge into contact with sulphur. The reaction of Hg with S is a reaction of a liquid (Hg) and a solid substance (S) and results in a solid substance (HgS).

The mercury is distributed throughout the sludge in the form of liquid globules or droplets. These globules are generally visible to the naked eye. The aqueous phase of the sludge may contain a very small amount of mercury; the solubility of mercury in water is 0.06 mg/1.

Sulphur is insoluble in water. The Hg dissolved in the water will not be able to react with S, or hardly so, in the aqueous phase. When the water evaporates, dissolved Hg will be precipitate, possibly directly in the form of HgS.

The HgS to be formed is a fine black powder (metacinnabar). In the case of very intensive mixing and higher temperatures red cinnabar is obtained. It is however not necessary to form red cinnabar. The difference is the crystalline form of the HgS: metacinnabar is cubic, cinnabar is hexagonal. Both shapes are stable and are naturally occurring. The solubility of HgS in water is very low to zero, at most 0.2 pg/l. This is at least a factor of 3000 lower than the solubility of the metallic mercury.

The reaction of Hg with S is exothermic, which means that heat of about 250 kJ/kg of Hg is released during the reaction. In order to initiate the reaction thermal energy and mechanical energy are applied by stirring/mixing (agitation).

It is likely that the sulphur reacts with the mercury at the surface of the Hg globules and the formed HgS stays on the surface of the globules and thus affects the reaction progress of Hg with S. By mixing the mixture of sludge and sulphur the HgS is at least partially removed from the surface. It is further noted that the chemical reaction of Hg with S accelerates when a large part, i.e. at least 70%, of the moisture has evaporated from the sludge.

Fig. 1 shows an embodiment of a trailer 1 within which is situated the system for processing the mercury-containing sludge. Trailer 1 is an embodiment of the system set up for mobile use. Stationary embodiments are of course also provided by the present aspects. The mobile embodiment can further also be configured in for instance a container. The mercury-containing sludge is situated in the supply container 2, which is connected via connection 3 to the processing system of trailer 1.

Referring to the process diagram of Fig. 2, mercury-containing sludge is supplied by suctioning the sludge from tanks or drums with a closed pump system. The sludge then preferably first passes a screen and/or filter (not shown) in order to remove the coarse parts. The filter and/or screen preferably takes a double form, wherein one is operative and the other can be cleaned. The pumped sludge is collected in a supply container 2. From supply container 2, dryer 10 is fed with the sludge via a screw conveyor or a sludge pump (not shown). The dryer 10 is an example of the reactor. The sludge can optionally be diluted with lye or an electrolyte in supply container 2, so that solid constituents in the sludge settle and a water layer results, which can be drawn off and stored separately. This drawn-off water fraction is stored separately because it may contain contaminants such as lye and fine, floating solid constituents and possibly low concentrations of

Hg.

The drying and precipitation of mercury sulphide takes place in the continuous mixing dryer 10. Sludge 2 and sulphur 4 are introduced into mixing dryer 10 via inlet 12 thereof. Both the mixing of the sludge with the sulphur 4 and the drying can take place in the same process in mixing dryer 10. The sulphur, an example of a sulphurous substance, is preferably supplied in bags 5 (for instance of 25 kg) and stored as such.

The paddles 18 of dryer 10 provide for continuous replenishment of the contact surfaces, whereby a good heat transfer for the drying process is achieved. Paddles 18 here also provide for mixing of the product, whereby the sludge dries more uniformly and contact between the sulphur and the mercury is increased.

Paddles 18 are formed such that the mixture of sludge 2 and sulphur 4 is moved gradually from inlet 12, via elongate intermediate part 13, to outlet 16 of dryer 10.

The mixture in mixing dryer 10 is heated indirectly with thermal oil 22 via walls 20 and shafts 19, to which paddles 18 are attached.

The sulphur 4 necessary for the stabilization is fed in at inlet 12 of dryer 10 with a dosing screw 24 which is driven by electric motor 25.

The drying and reaction process takes place in the dryer in three stages:

1) Heating of sludge and sulphur to about 100°C. As the temperature rises, increasingly more water evaporates, as do the lighter hydrocarbons such as the aromatics benzene and toluene.

2) Drying with increased speed begins, and a large part of the supplied heat is used to evaporate the water (and the lighter oil fraction) from the sludge, the product temperature will then not rise, or hardly so.

3) When most of the water has evaporated, the temperature of the mixture will rise

further to an expected final temperature of about 160°C. This temperature can also be higher, for instance 360°C, depending on the composition of the sludge that was fed in, for instance when more hydrocarbons with longer chains (for instance higher alkanes which are solid around room temperature and atmospheric pressure, or for instance C ₁₅ or longer) are present. In this temperature range the residual moisture will evaporate and the evaporation of the oil fraction will intensify. In this range the non-bound sulphur will become liquid (sulphur melts at around 115°C). The released reaction heat will be able to heat the product by 10 to 15°C, and thus has a limited effect on the overall heat balance.

At the prevailing final temperature the dried sludge containing stabilized mercury sulphide is discharged via second outlet 16 from dryer 10 with a screw 26.

The water and hydrocarbon vapours are exhausted from dryer 10 via first outlet 14 and here pass an indirectly cooled condenser 30. In this case the condenser 30 is air-cooled by fan 32, although this condenser 30 can for instance also be water-cooled. The water and a large part of the hydrocarbons condensate here. The condensed water 40 is discharged to a storage (not shown).

The uncondensed vapours 38, particularly benzene, toluene, ethyl benzene and xylene (BTEX), are carried to the afterburner (not shown), where they are combusted to form C0 ₂ and H ₂ 0. Alternatively, the BTEX can still be condensed and recovered.

The condensate 33 is collected in a collection tank 34 in which water 40 and hydrocarbons 42 are separated.

Discharge screw 26 of dryer 10 discharges the hot, stabilized sludge into containers 46.

The stabilized sludge is stored temporarily in these containers 46 in order to cool. After cooling, the containers 46 can be discharged, for instance in drums or in other packaging units, for permanent storage.

The condensed hydrocarbons 42 are stored in a supply container. These condensed (liquid) hydrocarbons can be used as fuel for the thermal oil system (TOS) which is used to provide the heated thermal oil 22 that heats the mixture in dryer 10. Alternatively, these hydrocarbons can be processed elsewhere.

The water fraction 40 from condenser 30 is stored in a tank.

The TOS is embodied with a burner which can combust domestic fuel oil (HBO). The HBO is the basic fuel, which can be alternated with the condensed hydrocarbon fraction.

The uncondensed hydrocarbons are light components and can be combusted with a burner (not shown) of the TOS in simple manner. The burner is for this purpose mounted on a pre combustion chamber, where the combustion takes place at at least 750°C. The hot combustion gases heat the thermal oil in a thermal oil boiler (not shown).

The air used to ventilate supply container 2 is utilized wholly or partially as combustion air for the TOS.

A number of variables affect the course of the process in mixing dryer 10.

The rotational speed of paddles 18 affects the mixing of sulphur and sludge, the diameter of the Hg globules, the removal of the HgS crystals from the surface of the Hg globules, and the heat transfer between the walls (and the shafts) and the mixture. The chemical reaction takes place on the surface of the Hg globules. The greater the surface area per kg of Hg, the faster the reaction will take place. Higher rotational speeds provide for a higher impulse on the Hg globules and will break them open, whereby smaller globules are formed and these globules have a greater surface area. The use of paddle dryer 10 has the advantage that this is achieved at a relatively low rotational speed and higher rotational speeds are not necessary. A rotational speed of at most 60 rpm (revolutions per minute), for instance 40 rpm, has been found to suffice.

The temperature further influences the reaction between the mercury and the sulphur. The reaction will speed up at higher temperature. The amount of time spent on average by the sludge in dryer 10 is determined by the temperature of the solid substances in the dryer. In addition, the amount of water and mercury in the sludge 2 that was fed in also determines the amount of time in dryer 10. The average time spent by the mixture in dryer 10 is for instance 60 minutes.

In order to be able to bond all the Hg an excess of sulphur is used. Based on experience, a mass ratio of 5 (Hg) to 1 (S) suffices. This is an excess of 25%. Higher ratios have not been found to provide any further advantage.

Further advantages and features of the method and the system for processing mercury- containing sludge follow below.

The system is displaceable and the equipment and control system can be set up and transported in sea containers.

The whole process takes place as far as possible in a closed system.

A mechanical dewatering step (centrifuge, decanter) can optionally be incorporated in the pretreatment of the sludge. Dewatering can here take place after settling of the solid constituents, optionally after the addition of lye or an electrolyte.

The system allows the control of the system to be such that it can run automatically from a supply container of sludge to the discharge of products (dried sludge, water, hydrocarbons).

The installation is preferably configured to have a processing capacity of 100 kg/h at an average composition of the Hg sludge of for instance about 10% mercury.

The stabilized and dried sludge is preferably stored directly in containers suitable for permanent storage.

Water and oil vapours from the dryer 10 are condensed with a condenser via indirect cooling, after which the water and hydrocarbon fractions are separated. This provides an increased safety of the system owing to a reduced presence of flammable vapours.

The water fraction is suitable, if necessary after an additional purifying step, to be discharged into the sewer or surface water.

An example of obtained dried and stabilized sludge comprised about 7% Hg. About 0.7 litres of condensation was captured. About 70% by volume of the clear liquid consisted of water, and the remaining 30% by volume of hydrocarbons (mineral oil). The Hg contents in the condensation were low: 1 pg/l in the aqueous phase and < 1 mg/kg (below the detection limit) in the hydrocarbons. Inspection of the dried and stabilized sludge showed no traces of elemental/metallic mercury, which indicates that almost all the mercury has been converted into mercury sulphide.

A further advantage of obtained dried and stabilized sludge, which was produced on the basis of about 250 kg of exempt sludge, is as follows. In this case the sludge had a relatively high moisture content and a high content of paraffin and/or wax. Although a thick, pasty mass of solid substance and paraffin/wax remained at temperatures of up to 160°C, it exited dryer 10 and the mass solidified after cooling. A further example of performing the present method with such sludge with a relatively high paraffin and/or wax content at a higher temperature resulted in a granular and dry final product. At a final processing temperature of 300°C only 0.8% by volume of residual moisture was present in the dried and stabilized sludge after an average residence time of 45 minutes in the dryer.

The disclosure further comprises the following embodiments

1. Method for processing mercury-containing sludge, wherein the mercury-containing sludge comprises water and a solid fraction, the method comprising of:

- adding a sulphurous substance to the mercury-containing sludge so that a mixture is formed;

- agitating the mixture;

- heating the mixture to at least partially evaporate the water, so that the mixture is at least partially dried;

- allowing at least some of the mercury in the mixture to react with the sulphurous substance to form mercury sulphide in the mixture;

- feeding out the evaporated water; and

- feeding out the mercury sulphide-containing mixture.

2. Method according to embodiment 1, wherein the heating is done simultaneously with the agitating.

3. Method according to embodiment 1 or 2, wherein allowing reaction is done

simultaneously with the agitating.

4. Method according to any one of the foregoing embodiments, further comprising of feeding in the mercury-containing sludge at an inlet of a reactor, wherein the reactor further comprises a first outlet and a second outlet for outfeed of respectively the evaporated water and the mercury sulphide-containing mixture.

5. Method according to embodiment 4, wherein the mercury-containing sludge is fed into the reactor continuously and the mercury sulphide -containing mixture is fed out continuously.

6. Method according to embodiment 4 or 5, wherein the heating is done using a heating medium and by supplying the heating medium to the reactor such that the mixture is heated, wherein the reactor is configured such that the heating medium does not come into contact with the mixture.

7. Method according to any one of the foregoing embodiments, wherein the mercury- containing sludge further comprises hydrocarbons, the method further comprising of evaporating at least some of the hydrocarbons, feeding out the evaporated hydrocarbons together with the evaporated water and separating the mixture of evaporated water and evaporated hydrocarbons into a water fraction and a hydrocarbon fraction.

8. Method according to embodiment 7, wherein the hydrocarbon fraction is used to heat the mixture.

9. Method according to any one of the foregoing embodiments, wherein the mixture is heated to a temperature in the range of 70-400°C, preferably 90-360°C.

10. Method according to embodiment 9, wherein agitating of the mixture comprises of stirring the mixture.

11. Method according to any one of the foregoing embodiments, wherein allowing reaction comprises of allowing at least 99% of the mercury to react with the sulphurous substance.

12. Method according to any one of the foregoing embodiments, wherein at least the steps of agitating, heating and allowing reaction are performed under an inert atmosphere.

13. Method according to any one of the foregoing embodiments, wherein the sulphurous substance is sulphur, wherein the sulphur is preferably added in a mass ratio of sulphur to mercury of at least 1 to 5.

14. System for processing mercury-containing sludge, wherein the mercury-containing sludge comprises water and a solid fraction, the system comprising:

- a reactor which comprises an inlet, a reaction volume, a first outlet and a second outlet; wherein the reactor is configured to:

- receive the mercury-containing sludge at the inlet;

- add a sulphurous substance to the mercury-containing sludge to form a mixture;

- agitate the mixture in the reaction volume;

- heat the mixture in the reaction volume to at least partially evaporate the water in order to at least partially dry the mixture;

- allow at least some of the mercury of the mixture to react with the sulphurous substance to form mercury sulphide in the mixture;

- feed out the evaporated water via the first outlet; and

- feed out the mercury sulphide -containing mixture via the second outlet.

15. System according to embodiment 14, wherein the reaction volume extends from the inlet to the second outlet, wherein the reactor is configured to move the mixture from the inlet, via the reaction volume, to the outlet part. 16. System according to embodiment 15, wherein the reactor is a mixing dryer.

17. System according to embodiment 16, wherein the mixing dryer comprises a plurality of paddles, wherein the paddles are configured to agitate the mixture and move it to the outlet part.

18. System according to embodiment 17, wherein the mixing dryer comprises two shafts rotatable in opposite directions, wherein the paddles are arranged on each of the shafts.

19. System according to embodiment 17 or 18, wherein the mixing dryer is configured to rotate the paddles at a rotational speed of at least 40 revolutions per minute.

20. System according to any one of the embodiments 14-19, wherein the reactor comprises a heating medium inlet for infeed of a heating medium and a heating medium outlet for outfeed of the heating medium, wherein the reactor is configured to heat the mixture using the heating medium and to keep the heating medium separate from the mixture.

21. System according to any one of the embodiments 14-20, wherein the mercury- containing sludge further comprises hydrocarbons, wherein the reactor is configured to evaporate at least some of the hydrocarbons and to feed out the hydrocarbons together with the water via the first outlet, wherein the system further comprises a separating device for separating the mixture of evaporated water and evaporated hydrocarbons which was fed out into a water fraction and a hydrocarbon fraction.

22. System according to embodiment 21, comprising a heating system configured to heat the mixture in the reaction volume, wherein the heating system is configured to use combustion heat from the separated hydrocarbon fraction.

23. System according to any one of the embodiments 14-22, wherein system is configured to heat the mixture to a temperature in the range of 70-400°C, preferably 90-360°C.

24. System according to any one of the embodiments 14-23, wherein the system is configured as a mobile system which can be placed close to a source of mercury-containing sludge or close to a storage location for storage of the mercury sulphide-containing dried mixture.

25. System according to any one of the embodiments 14-24, wherein the system is configured to perform the method according to any one of the embodiments 1-13.

The present aspects and embodiments are not limited to the above described preferred embodiments thereof; the rights sought are defined by the following claims, within the scope of which many modifications can be envisaged.