Technical Field:

The present invention relates to a process for treating fly ash into a harmless form. Background and prior art:

Fly ash from power plants and incineration plants comprises hazardous waste which often is send to waste disposal sites such as special land fill due to their hazardous contents of heavy metals and persistent organic pollutants (POPs). In Norway, amounts of about 50 000 tons per year of fly ash is produced and send to the special landfill at Langoya. This landfill is expected to run out of capacity over the next decade. Thus, there is an urgent need for solutions that can eliminate or

significantly reduce the amount of hazardous waste that needs to be deposited at special landfills.

Several processes are known to convert fly ash into harmless forms such as glass. One of them is vitrification. Scientific research reports show the process for vitrifying fly ash require high temperature plasma ovens wherein POPs are degraded to oxides and heavy metals are vaporized. The massive glass obtained can be deposited the same way that other inert materials are deposited, and the glass meets the requirements of heavy metal leaching concentrations set by the Norwegian authorities. Due to the vitrification process being such an energy consuming process, the process is much more expensive than the costs related to depositing fly ash at special landfills such as stable mines, thus the method is not used.

Various processes are known for treatment of fly ash in view of their reuse and final disposal, such as fly ash treatment by solidification with Portland cement into stable compounds. The solidification with Portland cement does however present some disadvantages like protection against humidity to prevent breakdown and leaching of heavy metals.

From EP 0186224 Al it is known to convert fly ash into harmless form by mixing with molten silicate-containing material whereafter the mixture is solidified. The method involves preheating the fly ash particles to at least 600 °C and then mixing it with molten slag material followed by solidification and breaking. Noxious products such as heavy metals being present in the particles cannot be leached out from the broken solidified composition. The method requires a lot of energy when preheating the fly ash particles to at least 600 °C and even more energy is required to keep the slag in molten state during mixing.

It is thus an object of the present invention provide a method for converting fly ash into harmless products and thereby significantly reduce the hazardous waste accumulation at special landfills. It is further object of the present invention to provide a process that is cost efficient, reuses already existing energy sources and results in a glass product that captures heavy metals.

It is also an object of the present invention to provide a process wherein the final glass product can be used as building materials, i.e. glass tiles, rock wool insulation material and substitute for crushed rock.

Summary of the invention:

The present invention is set forth and characterized in the independent claim, while the dependent claims describe other characteristics of the invention. The process of the present invention provides an environmentally friendly path for handling heavy metals and POPs in fly ash.

In particular, the invention concerns a process for treating fly ash comprising the following steps:

a) forming an aqueous suspension by mixing fly ash with an aqueous solvent; b) filtrating the suspension;

c) drying the residue to a humidity level below 15 weight%;

d) forming a mixture by adding the dried residue to a molten slag from a

smelting plant, wherein the molten slag is having a temperature of at least 1400 °C, and

e) solidifying the mixture by cooling.

The composition of the fly ash produced depends on the combustion process producing the fly ash, but oxides like S1O2, AI2O3, Fe20 ₃ , T1O2 and CaO are usually present as the main components. Further, the fly ash also comprises salts, such as NaCl, CaCh and KC1, and traces of hazardous materials like heavy metals such as lead and zinc, and organic compounds such as furans and dioxins.

The process step a) involves dissolving salts like sodium chloride, calcium chloride and/or potassium chloride in the aqueous solvent, wherein the solvent is an aqueous solution selected from at least one of water, acid or base. The aqueous suspension is in a preferred embodiment of the invention formed by mixing fly ash with an aqueous solvent at a temperature of at least 15 °C. The salts are dissolved before the filtration step b) to pass through the filter in the filtration step and thereby substantially reduce the amount of salts in the residue. If salts are not removed in the filtration step, the final product will have undesired properties like being hygroscopic and moist. Thus, this step, providing a residue with a substantially reduced amount of salts, is particularly important to obtain a final product that is not hygroscopic and/or moist, since the residue is used in making the final product.

In the filtrating step, step b) of the process, an industrial filter press can be used. Analyzes of the filtrate from the filtration step shows that the filtrate merely comprises an aqueous solution of solvent and ions of sodium, calcium and potassium and chloride.

With respect to easy and environmentally acceptable disposal of the filtrate, water may advantageously be used as the aqueous solution in step a) of the process. The aqueous suspension of water and dissolved salts can be directly discharged after the filtration step b) into a recipient such as seawater, since the dissolved salt are already present in seawater.

If the aqueous solvent is an acid or base, the suspension will need to be pH- neutralized before it can be disposed into the environment such as seawater.

In another embodiment of the present invention, the filtrate may advantageously be subjected to a separation step involving removal of possible traces of heavy metals such as zinc and lead before the filtrate is disposed. The separation process may involve magnetic separation, chemical precipitation, ion exchange or membrane filtration. After removal, the heavy metals are disposed at a suitable waste disposal for hazardous waste.

Analyzes of the residue from the filtration step, step b) shows that the residue comprises materials of mainly oxides of S1O2, AI2O3, Fe20 ₃ , T1O2 and CaO. Further, the residue comprises traces of hazardous materials like heavy metals such as lead (as PbO) and zinc (as ZnO). The residue also comprises hazardous persistent organic pollutants (POPs) such as dioxins and furans.

In another embodiment, in step c) of the process, the residue may advantageously be dried at a temperature of at least 100 °C, and the humidity level of the residue is reduced to below 15 weight%, preferably below 10 weight%. The drying step c) may advantageously be obtained by a rotary drying drum wherein hot air passes through the dryer to obtain a temperature of at least 100 °C.

A municipal waste incineration plant produces waste heat which advantageously may be applied to provide the thermal energy required for the drying of the residue in step c) of the invention. Thus, in an advantageous example embodiment, the drying step is provided from the waste heat from the plant producing the fly ash, such as a municipal waste incineration plant. In this way the waste energy produced by the incineration plant is being reused in the drying of the residue, thus no extra energy is required to dry the residue, since the energy has already been produced in the plant. The residue must be dried before it is mixed with molten slag at above 1400°C.

Further, the dying of the residue provides a lighter product, more than 30 % lighter, which is advantageous for handling and transportation of the residue. In step d) the molten slag has a temperature of preferably at least 1400 °C, more preferably 1500 °C, and even more preferred at least 1600 °C to provide complete combustion of POPs such as furans and dioxins when the dried residue is mixed together with the hot slag. A complete combustion of POPs at these temperatures oxidizes the POPs to carbon dioxides in the presence of oxygen.

In another advantageous embodiment according to the invention, the mixture in step d) is formed inside a thermally isolated ladle to avoid significant heat losses from the ladle into the surroundings.

In another example embodiment according to the invention, the ladle comprises a lid.

In an advantageous embodiment a burner, such as an oxy fuel burner or a plasma burner is used to maintain the temperature in step d) of at least 1400 °C, more preferably at least 1500 °C, and even more preferred at least 1600 °C.

The molten slag may advantageously be stirred before, during and after the adding of the dried residue to provide a close to or complete homogeneous mixture of the slag and residue.

In an advantageous example embodiment, the stirring is obtained by injecting compressed fluid through a plug at the bottom of the ladle. The fluid may

advantageously be a gas such as air. In an advantageous example embodiment of the present invention, the mixture of fly ash residue and slag comprises at least 40 weight% slag, more preferred at least 50 weight% slag, and even more preferred at least 60 weight% slag in order to obtain a solidified final glass product in step e) wherein the heavy metals are captured within the final product. The term "filtrate" is used herein to describe a liquid phase, which has passed through a filter during filtration, while the term "residue" is used herein to describe the slam or solid phase which has not passed through the filter during filtration.

The term "captured" is used herein to describe that the heavy metals are trapped and unable to be leached out from the final glass product. In the following description, numerous specific details are introduced to provide a thorough understanding of the process. One skilled in the relevant art, however, will recognize that the process can be practiced without one or more of the specific details, or with other components, systems, etc. In other instances, well-known operations are not shown, or are not described in detail, to avoid obscuring aspects of the disclosed embodiments. Brief description of the drawings

Fig. 1 shows a schematic view of the process according to the invention. Detailed description of the invention

The method steps of the present invention are represented in fig. 1.

The process involves mixing fly ash with slag from a smelting plant to obtain a solid glass product.

The following example embodiment of the invention relates to fly ash produced by a municipal waste incineration plant.

Fly ash produced from a municipal incineration plant normally comprise more than 30 weight% chlorides represented as salts. Salts in the final product would make the product hygroscopic and moist. A hygroscopic and moist product would be a disadvantage since the final product is applicable as building material for a variety of applications. Further, it is important that the final product does not leach any heavy metals into the environment, thus the product obtained by the present invention comprises heavy metals which are captured inside the final product.

The first step of the process, step a) thus involves washing the fly ash with an aqueous solvent to dissolve salts. The aqueous solvent can be water, acid or base.

If the fly ash is washed with an acidic aqueous solvent, such as HC1, or a basic solvent, such as NaOH, there is a need to pH-neutralize the filtrate before it is discharged into the environment. Thus, the preferred aqueous solvent is water, which is a pH-neutral solvent, and which can be discharged into the environment without any neutralization step.

The second step of the process involves filtration of the washed product. The filtrate after passing through the filter comprises the dissolved salts, thus separating the salts from the residue. The residue obtained is thus close to or completely free from chloride-containing salts.

Analyzes of the compositions in the filtrate have shown that the filtrate may comprise traces of heavy metals, in particular lead and/or zinc. These heavy metals are not allowed to be released into the environment, as they categorized as are hazardous materials. Thus, before releasing the filtrate into the environment, the filtrate passes through a separation step for removal of any heavy metals. Many different processes are known for such removal of heavy metals, such as magnetic separation by nanoparticles (Li. Wang, Liang. & Zhang. 2016), chemical

precipitation, ion exchange or membrane filtration (Fu & Wang, 201 1). Magnetic separation by nanoparticles is a preferred method as the remaining filtrate after removal of the heavy metals can be released into for example seawater, since the filtrate comprises salts already represented in seawater.

The next step, step c of the process, involves drying the residue from the filtration step, step b). The drying step involves removal of the aqueous suspension which is remained in the residue after the filtration step. The drying step involves drying the residue at a temperature of at least 100 °C, until the humidity level of the residue is below 15 weight%, preferably below 10 weight% to make it mixable with molten slag and to obtain a light weight residue.

Preferably, the energy needed to dry the residue can be retrieved from the waste energy of an incineration plant, thus the energy from the plant is reused instead of lost.

A known rotary drying drum can be used to dry the residue. The residue can then be placed inside the drum and hot air is passed inside the drum to remove the water from the residue while the drum rotates. An alternative is to dry the residue by heating the walls of the drum with hot water, wherein the drum comprises double walls.

The drying process can be a continuous process.

The dried residue is in the next step, step d), mixed with molten slag from a smelting plant, wherein the molten slag preferably has a temperature of at least 1400 °C when removed from the smelting plant. This step may further involve placing the molten slag into for example a ladle comprising isolating means on the outside to provide thermal isolation of the ladle. The ladle may for example be isolated by isolating mats, which are covered with sheets of steel to keep them in place. Thereafter the dried residue of fly ash is added into the molten slag which comprises a temperature of at least 1400 °C. Preferably the molten slag is stirred before, during and after the residue is added thereto. Stirring can be performed by for example injecting compressed fluid such as air through a nozzle and into the bottom of the ladle comprising the molten slag.

The fact that the molten slag arrives directly form the smelting plant in molten state, reduces the need for energy significantly. There is no need for energy to bring the slag into molten state. During adding and mixing of the residue with slag, it can be useful to provide heat supply to the ladle in order to make sure that the slag does not congeal before the fly ash residue is completely mixed therein. The energy supply can be provided by for example an oxy fuel burner or plasma burner. An oxy fuel burner can for example be arranged vertically from the top of the ladle in through an opening in a lid placed on top of the ladle, thus providing heat supply into the ladle. The high temperature used in the mixing process, step d) allows the POPs to oxidize to oxides, such as CO2, which can be released into the atmosphere.

The final glass product is obtained by cooling the slag with for example water.

In order to obtain a product that can be utilized as building material for for example buildings and roads, the composition of the mixture fly ash residue and slag comprises at least 40 weight% slag. Such composition ensures that any leaching of heavy metals from the product is avoided.

REFERENCES

Fu, F., & Wang, Q. (201 1). Removal of heavy metal ions from wastewaters: A review. Journal of Environmental Management 92, 407-418.

Li, S., Wang, W., Liang, F., & Zhang, W.-x. (2016). Heavy metal removal using nanoscale zero-valent iron (nZVI): Theoryand application. Journal of Hazardous Materials.