US20150315676A1

US20150315676A1 - Process for the treatment of solid alkaline residue comprising calcium, heavy metals and sulphate

Info

Publication number: US20150315676A1
Application number: US14/650,136
Authority: US
Inventors: Jaap STEKETEE
Original assignee: TAUW BV
Current assignee: TAUW BV
Priority date: 2012-12-06
Filing date: 2013-12-03
Publication date: 2015-11-05
Also published as: EP2928624A2; DK2928624T3; CA2893965A1; EP2928624B1; CN104968446A; SG11201504405VA; CN104968446B; HK1215800A1; WO2014088413A3; WO2014088413A2

Abstract

The invention relates to a process for the treatment of solid alkaline residue comprising calcium, heavy metals and sulphate, the process comprising: a)reacting the solid alkaline residue with carbon dioxide by contacting the solid alkaline residue with a carbon dioxide containing gas to obtain carbonated solid material; and c)washing the carbonated solid material with an aqueous stream to obtain washed solid material and wash water comprising calcium and sulphate and precipitating at least part of the calcium in the wash water as calcium carbonate.

Description

FIELD OF THE INVENTION

The invention relates to a process for the treatment of solid alkaline residue comprising calcium, heavy metals and sulphate, in particular incineration residue.

BACKGROUND OF THE INVENTION

Solid alkaline residues produced in many high temperature industrial processes, such as for example steel production, domestic waste incineration and other incineration processes and combustion processes, may find application in the field of civil-engineering, for example as building material, as foundation or embankment material in road construction or as aggregate in concrete.
Bottom ash obtained in domestic waste incineration for example, is the coarse residue that is left on the grate of the waste incinerator. It typically contains glass, ceramics, minerals, metals and some unburned material. Not all minerals obtained after the incineration process are thermodynamically stable and therefore, the material is susceptible to ageing and weathering by atmospheric water, oxygen and carbon dioxide. As long as metastable minerals and mineral phases are present in the bottom ash, the material will undergo chemical and physical changes, resulting in for example leaching of heavy metals such as antimony, lead, copper, molybdenum and zinc and of salts such as chlorides and sulphates. Leaching of heavy metals to the environment is highly undesirable. National authorities have therefore set limits for such leaching. In order to meet the requirements for cationic metals, i.e. metals that exist mainly as cations in aqueous solutions, incineration residue is sometimes subjected to an accelerated weathering treatment, wherein the material is contacted with carbon dioxide in order to achieve so-called carbonation of the material. Carbonation is a chemical reaction in which calcium hydroxide reacts with carbon dioxide and forms insoluble calcium carbonate:
Ca(OH)₂+CO₂→CaCO₃+H₂O
In WO 94/12444 for example is disclosed a process for carbonation of slags from waste incineration plants (WIP slags), wherein incineration residue is treated with a carbon dioxide-containing gas in order to decrease leaching of heavy metals like copper and lead. The leaching of antimony, however, increases as a result of the carbonation treatment.
In ES 2137071 is disclosed a process for the stabilisation of in particular zinc and lead in solid alkaline waste, by reacting the solid alkaline waste with carbon dioxide at a low pressure and for a short residence time.
Known carbonation processes, however, result in an increased leaching of anionic metals (i.e. metals that exist mainly as anions in aqueous solutions) such as for example antimony, chromium (as chromates) and vanadium (as vanadates). Leaching of salts such as chlorides and nitrates is not reduced by a carbonation process. Leaching of sulphates strongly increases due to the conversion of ettringite into gympsum that is induced by carbonation. Carbonated material typically exceeds the levels permitted by most national norms for leaching of sulphates and chlorides.
It has been proposed in the art to wash carbonated material with water to reduce leaching of chlorides and sulphates. A problem with water washing, however, is that a large amount of washing water is necessary in view of the low solubility and slow release of sulphate.
Therefore, there is a need in the art for a process for the treatment of solid alkaline residue, such as incineration residue, that not only results in a reduced leaching of cationic metals from the material, but also in a reduced leaching of sulphate and preferably also reduced leaching of anionic metals such as antimony.

SUMMARY OF THE INVENTION

It has now been found that leaching of salts, in particular the leaching of sulphates, from a solid alkaline residue, such as for example steel slag or residue of an incineration or combustion process, can be reduced by first subjected such solid alkaline residue to a carbonation step by reacting the solid residue with carbon dioxide to obtain a carbonated solid material, and then washing the carbonated material with an aqueous stream under such conditions that part of the washed out calcium is precipitated. By precipitating washed out calcium, the amount of sulphate that dissolves in the wash water and is therewith removed from the solid material, importantly increases.
Accordingly, the present invention relates to a process for the treatment of solid alkaline residue comprising calcium, heavy metals and sulphate, the process comprising:
a) reacting the solid alkaline residue with carbon dioxide by contacting the solid alkaline residue with a carbon dioxide containing gas to obtain carbonated solid material; and
c) washing the carbonated solid material with an aqueous stream to obtain washed solid material and wash water comprising calcium and sulphate and precipitating at least part of the calcium in the wash water as calcium carbonate.
An important advantage of the process according to the invention over washing without calcium precipitation is that sulphate is dissolved quicker and more completely and therewith a solid material with less leachable sulphate is obtained. It will therefore be possible to use the treated material in civil engineering applications without exceeding the limits for allowable sulphate leaching to the environment. Moreover, less water will be needed for washing the same or even a higher amount of sulphate from the solid material.
In a preferred embodiment, the carbonated material obtained in step a) is mineralised before being washed in step c). In this mineralisation step b), the carbonated solid material is contacted with an oxygen-containing gas for a period in the range of from 1 week to 12 months, preferably at a mineralisation temperature in the range of from 20 to 90° C. and preferably under conditions that the moisture content of the material is kept between 5 and 20 wt %. In this mineralisation step, anionic metals, in particular antimony, are fixated in the material resulting in less leachable anionic metals. Moreover, mineralisation typically results in a further increase in the amount of leachable sulphate, so that more sulphate can be removed from the material in washing step c).
It is an advantage of the process according to the invention that the preferred mineralisation temperature can be achieved without making use of an external heat source. This is achieved by making use of the heat generated by the exothermic carbonation and mineralisation reactions. By for example carefully choosing the dimensions of any depot or reservoir wherein the solid residue to be treated is provided, and carefully choosing the flows of the carbon dioxide containing gas and the oxygen-containing gas used in steps a) and c), the mineralisation temperature can be controlled.

DETAILED DESCRIPTION OF THE INVENTION

In the process according to the invention, solid alkaline residue is treated in order to decrease the amount of leachable components by first carbonating the material by contacting the solid alkaline residue with a carbon dioxide containing gas to obtain carbonated solid material (carbonation step a)) and then washing the carbonated solid material with an aqueous stream to obtain washed solid material and wash water comprising calcium and sulphate and precipitating at least part of the calcium in the wash water as calcium carbonate (washing step c)). Preferably, the carbonated solid material obtained in step a) is mineralised in a mineralisation step b) prior to being subjected to washing step c). Reference herein to decreasing the amount of leachable components is to fixation of leachable components or to removal of leachable components.
The solid alkaline residue may be any solid alkaline residue from an industrial process involving very high temperatures, incineration or combustion. Preferably, the solid alkaline residue is selected from the group consisting of steel work dust, steel slag, residues from an incineration process, and residues from a combustion process. More preferably, the solid alkaline residue is bottom ash from an incineration process, even more preferably bottom ash from incineration of domestic waste or comparable waste.
Such solid alkaline residues typically are an assemblage of metastable minerals and mineral phases that include single or mixed oxides of calcium, magnesium, and iron and silicates, and further chlorides, sulphates, and heavy metals, such as zinc, lead, molybdenum, antimony, cadmium and copper.
In step a), the solid alkaline residue is carbonated by contacting the residue with a carbon dioxide-containing gas.
In carbonation step a), cationic heavy metals such as for example copper, lead, zinc and cadmium are fixated in the solid material and sulphate and antimony are released from sulphate-containing minerals such as for example ettringite or from calcium-containing minerals such as romeite. As a result, leaching of cationic heavy metals will decrease and sulphate can be removed in a subsequent washing step to yield a final material showing decreased sulphate leaching.
The carbon dioxide-containing gas may be any suitable carbon dioxide containing gas, for example a mixture of air and carbon dioxide, or a flue gas from a combustion process. Preferably, the carbon dioxide-containing gas is a mixture of air and carbon dioxide, more preferably a mixture of air and carbon dioxide comprising in the range of from 4 to 16 vol % carbon dioxide based on the total volume of the mixture. In case flue gas is available at the location at which the carbonation will take place, it may be advantageous to use the available flue gas.
Carbonation of solid alkaline residue such as incineration residue is well-known in the art. Any suitable carbonation conditions known in the art may be used.
The gas may be contacted with the solid residue in any suitable manner, typically by leading a flow of the carbon dioxide-containing gas through a depot of solid alkaline residue, for example by injecting the gas into the depot. Such depot may be an open-air depot of solid residue or a depot in a container, such as for example a silo or a composting tunnel. Preferably, the flow of carbon dioxide containing gas through the solid material is in the range of from 0.1 to 10 m³gas per ton solid material per hour, more preferably in the range of from 1 to 5 m³gas per ton solid material per hour.
Carbonation is preferably carried out until conversion of any calcium or magnesium hydroxide or reactive calcium or magnesium silicate into its corresponding carbonate is complete or substantially complete. The degree of conversion can suitably be monitored by determining the pH of the solid residue. This can for example be done by measuring the pH in a slurry of the solid residue in water. Preferably, carbonation is carried out until the pH of the solid residue has decreased to a value in the range of from 8.0 to 8.5. Typically, in case of carbonation in large depots with a height of e.g. five metres or more, a carbonation time in the range of from 3 to 10 days, usually 4 to 7 days, will be required to achieve such pH decrease. It will be appreciated that the required time will inter alia depend on the height of the depot, the temperature, the pressure, the concentration of carbon dioxide in the carbon dioxide-containing gas, and the flow velocity of the gas through the solid residue.
Step a) may be carried out at any suitable temperature, preferably at a temperature in the range of from 20 to 90° C., more preferably of from 25 to 60° C. Any suitable pressure may be applied. Preferably, step a) is carried out at atmospheric pressure.
The carbonated solid material obtained in step a) is preferably subjected to a mineralisation step b), prior to being washed in washing step c), to obtain mineralised carbonated solid material. In mineralisation step b), the carbonated solid material is contacted with a molecular-oxygen containing gas, preferably air. Preferably, the solid material is kept at a mineralisation temperature in the range of from 20 to 90° C. during the contacting in step b), more preferably in the range of from 40 to 60° C. Preferably, the solid material has a moisture content in the range of from 5 to 20 w %, more preferably of from 8 to 15 wt %, during the contacting with oxygen-containing gas.
It has been found that if the temperature of the material is kept in the preferred range of from 20 to 90° C., more preferably of from 40 to 60° C., and the moisture content of the material in the range of from 5 and 20 wt %, preferably of from 8 to 15 wt %, antimony can be fixated to an extent that antimony leaching from the material can be kept below the maximum level allowed. A further effect of the mineralisation step is further release of sulphate from sulphate-containing minerals.
The oxygen-containing gas may be contacted with the carbonated solid material in any suitable manner, typically by leading a flow of the oxygen-containing gas through a depot of carbonated solid residue, for example by injecting the gas into the depot. Such depot may be an open-air depot of solid residue or a depot in a container, such as for example a silo or a composting tunnel. Preferably, the flow of oxygen-containing gas through the solid material is in the range of from 0.01 to 10 m³gas per ton solid material per hour, more preferably in the range of from 0.1 to 1 m³gas per ton solid material per hour. Suitably, carbonation step a) and mineralisation step b) are carried out by first leading a mixture of carbon dioxide containing gas and air through a depot of alkaline solid residue and then, after sufficient carbonation has taken place, continue with only leading air through the depot.
Mineralisation will be carried out until sufficient mineralisation of anionic metals, in particular antimony, has taken place. Reference herein to sufficient mineralisation is to so much mineralisation that antimony leaching of the mineralised material does not exceed the applicable norms. The extent of mineralisation can be determined by performing a leaching test, e.g. a column test or other leaching test known in the art or required by the local authorities. Typically, the mineralisation time needed for sufficient fixation of antimony and other metals will be in the range of from 1 week to 12 months, preferably in the range of from 2 weeks to 12 months, more preferably of from 6 weeks to 6 months. Although a relatively slow process step, it is a significant acceleration compared to the speed of mineralisation by natural weathering processes that occur if the material is stored in the open air without further treatment.
Preferably, the temperature of the carbonated solid material is kept within the preferred mineralisation temperature range without using an external heat source. This can be done my making use of the heat generated during the exothermic carbonation and mineralisation reactions that take place in steps a) and b). By carefully choosing the dimensions of the depot of solid material through which the gas is leaded and carefully choosing the flow of gas through the solid material (in terms of volume of gas per weight per time unit), the heat generated is sufficiently retained in the material in order to achieve the desired temperature.
It has further been found that a moisture content of the solid material that is contacted with the oxygen-containing gas in mineralisation step b) below 20 wt % is preferred, more preferably below 15 wt %, even more preferably below 12 wt %. It has been found that such relatively low moisture content results in an increased fixation of antimony. Preferably, the moisture content is not lower than 5 wt %, more preferably not lower than 8 wt %. A moisture content in the range of from 8 to 12 wt % is particularly preferred. If step b) is carried out at the preferred elevated mineralisation temperature (elevated compared to ambient temperature) the desired moisture content can be achieved by carefully choosing the dimensions of the depot of solid material through which the gas is leaded and carefully choosing the flow of gas through the solid material, so that any moisture evaporated due to the elevated mineralisation temperature can be discharged from the depot with the gas stream through the depot.
In step c) the carbonated solid material, preferably after a mineralisation step b), is washed with an aqueous stream to obtain washed solid material and wash water comprising calcium and sulphate. At least part of the calcium in the wash water is precipitated as calcium carbonate. Washing with an aqueous stream may be carried out in any suitable way, preferably by spraying the aqueous stream over a depot of the solid material and draining wash water from the bottom of the depot.
Calcium may be precipitated in the wash water before or after separation of the wash water from the washed solid material. If calcium is precipitated after separation of the wash water from the washed solid material, a calcium-depleted wash water is obtained that is recycled to wash step c) to form part of the aqueous stream with which the carbonated solid material is washed.
Preferably, at least part of the calcium is precipitated in the wash water by adding a carbonate salt to the aqueous stream used for washing the solid material or by adding a carbonate salt or a carbonate-forming compound such as for example carbon dioxide to the wash water after separation of the wash water from the washed solid material.
The carbonate salt may be any carbonate salt that is soluble in water under the conditions applied in washing step c), typically ambient conditions. The carbonate salt preferably is sodium carbonate, potassium carbonate or magnesium carbonate. In one preferred embodiment of the invention, sodium carbonate is added to the aqueous stream used for washing the carbonated solid material. As a result, the wash water will comprises carbonate and at least part of the washed out calcium will precipitate as calcium carbonate. The resulting lower concentration of dissolved calcium in the wash water increases the solubility of sulphate, which normally is present as gypsum, and therewith, more sulphate can be removed from the carbonated solid material and a lower volume of aqueous stream is needed for removing sulphate.
Alternatively, the carbonated solid material may be washed with water as aqueous stream under simultaneous contacting of the carbonated solid material with a carbon dioxide containing gas, for example pure carbon dioxide or a mixture of air and carbon dioxide, preferably comprising 4-8 vol % carbon dioxide. Calcium carbonate will then precipitate, typically on the solid material. As a result of the presence of carbon dioxide, the pH of the water that is contacting the solid material may decrease to a value below 7. If this is the case, the pH of the water is adjusted by adding an alkaline compound to the aqueous stream so that the pH of the water that is contacting the solid has a value in the range of from 7 to 8. In this embodiment (adding a carbon dioxide containing gas to the solid material), wash water separated from the solid material may be recycled to the solid material to form part of the aqueous stream with which the carbonated solid material is washed.
Instead of adding a carbonate salt to the aqueous stream that is contacted with the solid material or of adding carbon dioxide to the solid material during washing, a carbonate salt or a carbonate-forming compound such as carbon dioxide may be added to the wash water after it has been separated from the washed solid material in order to precipitate calcium from the separated wash water and obtain calcium-depleted wash water. The calcium-depleted wash water thus obtained is then at least partly recycled to the carbonated solid material to be washed to form part of the aqueous steam used for washing. As a result of the recycling, the concentration of sulphate in the aqueous stream and wash water will increase. After the sulphate concentration in the wash water has achieved a predetermined level, for example 1 wt %, wash water will be withdrawn from the process in order to remove sulphates from the process.
In a preferred embodiment of the invention, a carbon dioxide containing gas is added to the wash water separated from the washed solid material as a carbonate-forming compound. In order to achieve precipitation of calcium carbonate, the pH of the separated wash water needs to be in the range of from 7 to 8. If necessary, the pH of the separated wash water to which carbon dioxide is added is adjusted to a value in the range of from 7 to 8, preferably in the range of from 7.3 to 7.7, by adding an alkaline compound, for example sodium hydroxide, to the separated wash water.
The aqueous stream preferably is water. The amount of carbonate salt or of carbonate-forming compound added to the aqueous stream or to the separated wash water is preferably in the range of from 0.8 to 1.2 mole carbonate or carbonate equivalents per mole of sulphate to be removed from the solid material.
As a result of washing step c), the amount of leachable anionic metals in the solid material, in particular antimony, may have increased. Therefore, it might be desirable to subject the washed solid material obtained in step c) to a mineralisation step d). Step d) may suitably be carried out in the same way as hereinabove described for step b). Preferably, the washed solid material is kept at a temperature in the range of from 20 to 90° C. during the contacting in step d), more preferably of from 40 to 60° C. Preferably, the washed solid material is kept at a moisture content in the range of from 5 to 20 wt %, more preferably of from 8 to 15 wt %, even more preferably of from 8 to 12 wt % during the contacting in step d). In step d), however, an external heat source might be required to achieve the preferred mineralisation temperature and/or moisture content, since no use can be made of the heat generated in a preceding exothermic carbonation step.
The invention will be further illustrated by means of the following non-limiting examples.

EXAMPLES

Example 1

Effect of Wash Step

Experiment 1—Washing with Water (Not According to the Invention)
Bottom ash from a domestic waste incinerator was washed with water by upwardly flowing a stream of 0.18 m³of water per ton bottom ash per day through a container with bottom ash that was open at the top. The flow velocity was such that wash water was allowed to flow away from the top of the solid material without entraining solid material. The washing was carried out until 2.5 m³of water per ton bottom ash had flown through the bottom ash.
The concentration of sulphate in the wash water separated from the bottom ash was determined by means of inductive coupled plasma emission spectroscopy (ICP-ES) analysis at several points in time during the washing step and the cumulative amount of sulphate washed from the solid material (mg sulphate per kg dry bottom ash) at such point of time was calculated.
Experiment 2—Washing with a Solution of Sodium Carbonate in Water (According to the Invention)
Bottom ash from a domestic waste incinerator was washed with a solution of sodium carbonate in water (9.54 grams sodium carbonate per litre water) by upwardly flowing a stream of 0.18 m³of sodium carbonate solution per ton bottom ash per day through a container with bottom ash that was open at the top. The flow velocity was such that wash water was allowed to flow away from the top of the solid material without entraining solid material. The washing was carried out until 1.0 m³of solution per ton bottom ash had flown through the bottom ash. The washing was then continued with water until 2.5 m³of aqueous stream (total of sodium carbonate solution and water) per ton bottom ash had flown through the bottom ash.
The concentration of sulphate in the wash water separated from the bottom ash was determined by ICP-ES analysis at several points in time during the washing step and the cumulative amount of sulphate washed from the solid material (mg sulphate per kg dry bottom ash) at such point of time was calculated.
Experiment 3—Washing with Recycled Calcium-Depleted Wash Water (According to the Invention)
Bottom ash from a domestic waste incinerator was washed by flowing a stream of 0.18 m³of water per ton bottom ash per day through a container with bottom ash that was open at the top. The flow velocity was such that wash water was allowed to flow away from the top of the solid material without entraining solid material. In a separate reactor, the wash water that was separated from the bottom ash was contacted with a flow of pure carbon dioxide gas with a velocity of 0.15 litres carbon dioxide per litre wash water per hour, whilst the pH of the wash water was maintained at a value of 7.5 by addition of sodium hydroxide to the separate reactor. The wash water was then separated from the precipitated calcium carbonate and recycled to the bottom ash to serve as the water stream with which the bottom ash was washed. This washing with recycled calcium-depleted wash water was carried out until 1.0 m³of solution per ton bottom ash had flown through the solid material. The washing was then continued with water until 2.5 m³of aqueous stream per ton bottom ash had flown through the solid material.
As in experiments 1 and 2, the concentration of sulphate in the wash water separated from the bottom ash was determined and the cumulative amount of sulphate washed from the solid material calculated.
In FIG. 1, is shown the amount of sulphate (Y-axis; in mg sulphate per kilogram dry solid material) washed from the bottom ash as a function of the quantity of aqueous stream flown through the bottom ash (X-as; in cubic metres aqueous stream per ton of solid material) for experiments 1-3.
FIG. 1 shows that if the washing step is carried out under conditions that calcium precipitates in the wash water, a much lower volume of aqueous stream is needed for the washing in order to remove the same amount of sulphate.

Example 2

Effect of Mineralisation Step

Experiment a: carbonation step (1 week), followed by washing step, followed by post mineralisation step (8 weeks) as described hereinbelow.
Experiment b: carbonation step (1 week), followed by mineralisation step (2 weeks), followed by washing step, followed by post mineralisation step (8 weeks) as described hereinbelow.

Carbonation Step

During one week a stream of 5 vol % carbon dioxide in air was led with a flow rate in the range of from 0.9 to 1.4 m3 per ton bottom ash per hour through bottom ash of a domestic waste incinerator that was kept in a container which was open at the top. The bottom ash was kept at a temperature of 40° C. and a moisture content of 15 wt % (based on the dry weight of the bottom ash)

Mineralisation Step

During two weeks a stream of air was led with a flow rate in the range of from 0.9 to 1.4 m³per ton bottom ash per hour through the carbonated material obtained in the carbonation step. The bottom ash was kept at a temperature of 40° C. and a moisture content of 15 wt % (based on the dry weight of the bottom ash).

Washing Step

The carbonated or mineralised bottom ash was washed with water until a liquid/solid ratio of 8 cubic metres water per ton bottom ash was reached.

Post Mineralisation Step

During eight weeks, the washed solid material (50 tons) was exposed to the open air.
The copper, antimony and sulphate emissions from the solid material was determined after each treating step by first leaching copper, antimony and sulphate from the material by means of a batch leaching test with water (liquid/solid ratio of 20 litres water per kg solid material) and then determining the amount of leached copper, antimony and sulphate by means of ICP-ES analysis. In the Table is shown the results of these measurements.

TABLE

Emissions of Cu, Sb and sulphate after several process
steps. All emissions in mg per kg dry solid material

Experiment a

Experiment b

	Cu	Sb	SO₄	Cu	Sb	SO₄

Start	2.9	0.98	6,500	2.5	1.13	6,000
Carbonated	1.9	1.44	9,500	n.m.	n.m.	n.m.
Mineralised	n.a.	n.a	n.a.	1.2	0.80	8,000
Washed	1.0	0.86	2,600	0.87	0.61	1,670
Post mineralised	0.8	0.65	2,450	0.56	0.45	1,160

n.m.: not measured; n.a.: not applicable.

It can be seen from the results that a mineralisation step between the carbonation and washing step results in a significant reduced amount of antimony in the carbonated and mineralised material compared to the material that is only carbonated. The final emissions (in the post mineralized material) are clearly lower in Experiment b than in Experiment a. Further, the wash water obtained in Experiment b is less polluted than the wash water obtained in Experiment a and therefore easier to treat.

Claims

1-15. (canceled)

16. A process for the treatment of solid alkaline residue comprising calcium, heavy metals and sulphate, the process comprising:

(a) reacting the solid alkaline residue with carbon dioxide by contacting the solid alkaline residue with a carbon dioxide containing gas to obtain carbonated solid material;

(b) washing the carbonated solid material with an aqueous stream to obtain washed solid material and wash water comprising calcium and sulphate and

(c) precipitating at least part of the calcium in the wash water as calcium carbonate.

17. The process according to claim 16, wherein at least part of the calcium is precipitated in the wash water by adding a carbonate salt to the aqueous stream.

18. The process according to claim 16, wherein at least part of the calcium is precipitated in the wash water by washing the carbonated solid material with an aqueous stream while flowing a stream of carbon dioxide-containing gas through the carbonated solid material.

19. The process according to claim 16, wherein precipitating comprises:

(i) washing the carbonated solid material with an aqueous stream comprising recycled wash water or water and recycled wash water;

(ii) separating wash water from the carbonated solid material to obtain the washed solid material and the wash water comprising calcium and sulphate;

(iii) supplying the wash water to a separate container;

(iv) adding a carbonate salt or a carbon dioxide-containing gas to the wash water in the separate container to precipitate part of the calcium and to obtain calcium-depleted wash water, wherein, in case a carbon dioxide-containing gas is added to the wash water, the pH of the wash water is adjusted to a value in the range of from 7 to 8 by adding an alkaline compound to the wash water; and

(v) recycling the calcium-depleted wash water to the carbonated solid material as part of the aqueous stream.

20. The process according to claim 16 further comprising:

contacting, for a time period in the range of from 1 week to 12 months, the carbonated solid material obtained in step (a) with a molecular-oxygen containing gas, to obtain mineralised carbonated solid material,

wherein in step (c) the carbonated solid material is the mineralised carbonated solid material obtained in step (b).

21. The process according to claim 20, wherein the molecular-oxygen containing gas is air.

22. The process according to claim 20, wherein the solid material is kept at a mineralisation temperature in the range of from 20 to 90° C. during the contacting in step (b).

23. The process according to claim 22, wherein the mineralisation temperature is in the range of from 40 to 60° C.

24. The process according to claim 22, wherein no external heat source is used to achieve the mineralisation temperature.

25. The process according to claim 20, wherein the solid material is kept at a moisture content in the range of from 5 to 20 wt % during the contacting in step (b).

26. The process according to claim 25, wherein the moisture content of the solid material is kept in the range of from 8 and 12 wt % during the contacting in step (b).

27. The process according to claim 16, further comprising:

contacting the washed solid material obtained in step c) with an oxygen-containing gas, for a time period in the range of from 1 week to 12 months.

28. The process according to claim 27, wherein the washed solid material is kept at a temperature in the range of from 20 to 90° C. during the contacting.

29. The process according to claim 27, wherein the washed solid material is kept at a moisture content in the range of from 5 to 20 wt % during the contacting.

30. The process according to claim 16, wherein the solid, alkaline residue is bottom ash from an incineration process.