Technical Field [0001 ] Present invention relates to a method and apparatus for neutralizing and stabilizing fly ash. More specific, present invention relates to a method and apparatus for neutralizing and stabilizing alkaline fly ash to achieve improved leaching properties.

Background Art [0002] Processing and deposit of waste are governed by complex governmental regulations, such as the EU’s waste regulations (https://ec.europa.eu/environment/waste/landfill/index.htm).

[0003] For processing and deposit of inorganic waste, such as for example fly ash from incineration plant, the waste is categorized in different categories according to the present regulations:

Category 1 : Hazardous waste Category 2: Non-hazardous waste Category 3: Inert waste. [0004] For the categories mentioned above, it will be an increasing content of environmentally hazardous compounds from inert to hazardous waste. Consequently, it is the concentration of environmental hazardous compounds which defines the category if the waste. [0005] When the waste has been categorized based on the content of environmental hazardous compounds, it is a second requirement in the waste regulations regarding the amount of the environmental hazardous compounds that can leach out into water from the waste. [0006] Table 1 : limit values for leaching of different compounds from environmental hazardous waste, non-hazardous waste and inert waste, respectively are as follows ¹ : [0007] There are strict regulations regarding the imperviousness of landfills and how these should be constructed, and the general requirement is that the leaking of water from the deposit shall not exceed 10 ^-9 (m ³ /s)/m ² .

[0008] Further, the waste regulations allow landfilling of hazardous waste in a waste disposal site for non-hazardous waste if the leaching requirements for

¹ Waste regulations appendix 1, paragraphs 2.2.1, 2.3.1 and 2.4.1 stable, non-reactive hazardous waste are fulfilled.

[0009] There are considerable different processes involved in developing and operating a waste deposit for hazardous waste and a deposit for non-hazardous waste, both regarding economy and technically.

[0010] W02005040437A teaches a device and process for neutralizing fly ash, wherein a first process step, the fly ash is washed with water, and this mixture is mixed with hydrochloric acid to e pH < 4 and stirred for 30-60 minutes at 50 °C. Thereafter, this mixture is led to a neutralization step where a neutralizing agent, for example calcium hydroxide, until a pH between 3,5 and 5 is achieved. This mixture is then sent to a separation step, for example a filter press. The filtrate contains heavy metals such as Zn, Pb and Cu, and the residue contains Si, Al, Fe and similar. The filtrate is sent to yet another neutralization step where a neutralizing agent such as slaked lime is added, until a pH between 5 and 9 is achieved at room temperature to 80°C and heavy metals in the liquid are precipitated, while Ca-ions remain in the liquid. This liquid is neutralized again and the filtrate is mixed with sulfuric acid for precipitation of gypsum. This publication describes en multistage process with gradually increasing pH and where the metals are removed in multiple successive separation steps.

[0011 ] US6319482B1 teaches a process for the removal of heavy metals, especially lead, from fly ash. The purpose of this process is to provide a method for treating fly ash residues so as to provide or recovery of several by-product streams, including commercial grade CaCl2 liquor, lead in a form suitable for recycling and a calcium and silicate enriched non-hazardous residue which can be marketed as adjunct to concrete, ceramic bricks or asphalt products, or disposed of as non-hazardous waste material. The purpose of this process is also to remove unwanted metals such as Pb and obtain a non-hazardous residue enriched in calcium and silicate.

[0012] JP2000354843 describes a process for removal of heavy metals from a fly ash/water mixture by adding acid and where it is obtained a filter cake containing and a liquid phase containing heavy metals. The purpose of this process is thereby also to separate heavy metals from the fly ash/water mixture. Fly ash

[0013] Fly ash is a waste fraction form incineration of municipal waste in a waste incineration plant. This is the residual product of cleaning of the flue gas from the incineration (electro filter, textile filter). Fly ash is highly alkaline with a high content of heavy metals, and is therefore categorized as hazardous waste. The composition of the fly ash depends on several factors, for example the composition of the incinerated waste, incineration technology and flue gas cleaning. The fly ash has to be pretreated and stabilized before it can be landfilled. Fly ash is highly alkaline with a pH in the range of 11 -12,5. Mineralogical analysis (X-ray diffraction, XRD) of fly ash samples from the applicant’s present plant, exhibit high levels of calcium hydroxides, calcium sulphates (gypsum, anhydride) and carbonates (calcite), in addition to oxides/hydroxides. Carbonates and oxides give the ash a high acid neutralization capacity (ANC) which can be utilized in neutralizing waste acid. [0014] The reactions between fly ash and sulfuric acid are assumed to be as follow:

Rx 1 : Ca(OH) + H2SO4 CaS0 ₄ + H2O

[0015] Rx. 2: CaCOa + H2SO4 CaS0 ₄ + CO2 + H2O [0016] Normally, the fly ash has a low content of organic material (total organic carbon, TOC < 1%). The measured values of TOC are mainly due to elementary carbon (soot particles and residues of active carbon added in the smoke gas cleaning step). The ash can contain traces of PAH and chlorinated organic compounds (for example dioxins, chlorobenzene). These compounds have a low solubility in water and will bind well to the soot particles. Soot residues and active carbon are stable and not degradable compounds.

[0017] Fly ash also contains considerable amounts of heavy metals and these contribute to defining the fly ash as hazardous waste. Figure 2 shows average composition of fly ashes NOAFI is receiving

[0018] Fig. 2. Average content (XRF analysis) of elements in fly ashes NOAH is receiving.

[0019]

Average content fly ash

1000000

100000 iOQOO I

[0020] In an exemplary embodiment of a known process, fly ash is mixed with waste acid from the production of titanium. Ilmenite (FeTiOa) is extracted with sulfuric acid (H2SO4) and the residue from this process comprises diluted sulfuric acid (2-23 % by weight, 3,6 M and pH<0). The waste acid has a high content of iron (Fe(ll)) of approx. 4 %. In general, the content of pollutants is characterized as low. The exception is chromium with an amount of about 280 mg/I. Fig. 3 shows the typical content of pollutants in the waste acid. Fig. 3. Pollutants in waste acid.

[0021 ] These and other advantages are obtained with a method for the treatment of fly ash from incineration processes, said method comprises the following steps:

(a) providing a supply of fly ash and water and mix the fly ash with water to provide a fly ash/water slurry;

(b) leading the fly ash/water slurry to a first reaction vessel; (c) supplying acid to the fly ash/water slurry in the first reaction vessel and mix the ash/water slurry with the acid;

(d) leading the acid/water/fly ash slurry from the first reaction vessel to a second reaction vessel;

(e) supplying additional acid and additional fly ash/water slurry to the second reaction vessel and mix the content of the second reaction vessel;

(f) repeat steps (d) and (e) in one or more subsequent reaction vessels and gradually increase the pH in each of the subsequent reaction vessels;

(g) leading the mixture from the last one of the subsequent reaction vessels to a dewatering device, (h) washing the mixture from step (g) with water;

(i) remove water from the mixture of step (h) to obtain a substantially solid phase and a liquid phase,

(j) deposit the substantially solid phase from step (i) and lead the liquid phase from step (g) to a subsequent water treatment process. [0022] In a preferred embodiment, the fly ash is fly ash from incineration of municipal waste.

[0023] The acid is preferably chosen from the group consisting of sulfuric acid (H2SO4), hydrochloric acid (HCI), phosphoric acid (H3PO4 ) and CO2(g)0r mixture of such acids.

[0024] Preferably, the method is performed at a temperature range of 35-50 °C.

[0025] The invention also relates to an apparatus for treatment of fly ash, said apparatus comprising:

- a supply of dry fly ash;

- a supply of water;

- a mixer connected to the source of fly ash and the source of water;

- a conduit connecting the mixer to a homogenization unit, for homogenization of the fly ash/water mixture from the mixer;

- a conduit connecting the homogenization unit with a plurality of successive reaction vessels;

- a source of acid connected to the first reaction vessel;

- a conduit connecting the last reaction vessel with a dewatering device; and

- obtain a dewatered phase and a saline water phase.

[0026] The apparatus further comprises a source of moist fly ash, and a conduit leading the moist fly ash and water to a ball mill, and a conduit leading the fly ash/water mixture to the homogenization unit.

[0027] The fly ash is preferably fly ash form incineration of domestic waste.

[0028] The acid is preferably chosen from the group consisting of sulfuric acid (H2SO4), hydrochloric acid (HCI), phosphoric acid (H3PO4 ) and C02(g)or mixture of such acids.

Brief description of drawings

[0029] Figure 1 depicts a preferred embodiment of a method and an apparatus according to the invention. Detailed description of the invention

[0030] Fig. 1 depicts an embodiment of a method and a device for neutralizing and stabilizing fly ash with acid according to present invention. The device comprises a supply of dry fly ash (1 ), water supply (2) and a supply of moist fly ash (3). The fly ash is preferably fly ash from incineration of domestic waste. The dry fly ash (6) is mixed with water (5) in a mixer (4) to provide a fly ash/water slurry (7). The moist dry ash (8), which contains lumps, is mixed with water (9) and is led to a ball mill (10) and is milled together with water. The water/fly ash mixture (7) from the mixer (4) and the water/fly ash mixture (11) from the ball mill (10) are both led to a homogenization vessel (12) to mix the streams (7, 11) and equalize possible quality differences. A part of the homogenized slurry of water/fly ash (13) is then led to a first reaction vessel (14) where it is mixed with acid (15) from an acid supply (16). The acid (15) is preferably sulfuric acid, but other acids such as hydrochloric acid, phosphoric acid, CO2 are conceivable within the scope of present invention. The pH in the first reaction vessel (14) is typically approx. 4. The acid/fly ash/water mixture from the first reaction vessel (14) is then led to one or more additional reaction vessels (17-20) where additional water/fly ash slurry (13) from the homogenization vessel (12) is added to each reaction vessel (17-20). In these reaction vessels (17-20), the pH of the acid/water/fly ash slurry is gradually increased in a controlled way. In the exemplary embodiment shown in fig. 1 , there are a total of 5 reaction vessels (14, 17-20), however, the number of reaction vessels might differ from the number shown in fig. 1. In the reaction vessels (14, 17-20) the reaction between acid and water/fly ash slurry will produce a gypsum slurry according to the following formula:

Rx 1 : Ca(OH) + H2SO4 CaS0 ₄ + H2O

Rx 2: CaC03 + H2SO4 ^® CaSC + CO2 + H2O [0031] It is of utmost importance that the reaction time/retention period in each reaction vessel (14, 17-20) is sufficient to obtain a gypsum slurry whose pH will not change after depositing of the final product. Typical retention time is 30 minutes in reaction vessels (14, 17-19), while the retention time in the final reaction vessel (20) is preferably longer (1-2 hours). pH in the reaction vessels are typically 2 in reaction vessel (14), 6-9 in reaction vessels (17) and 9-11 in reaction vessel (18). Typical temperature is 35-50 °C in all the reaction tanks. The retention time in each of the reaction vessels (17-20) might be different, since the reaction speed will decrease with increasing pH. It is therefore important that the final reaction vessel (20) has the longest retention time, since the reaction between sulfuric acid and the fly ash will be at the lowest at a pH in the range of 9-11. The high pH in the slurry from the final reaction vessel (20) is crucial to obtain an end product with the required leaking characteristics.

[0032] The reaction between acid and fly ash can also be carried out starting with fly ash slurry in the first reaction vessel (14) and where acid is added in reaction vessels (17-20) until the wanted pH is achieved. Further, other acids than sulfuric acid can be applied in the treatment of fly ash, such as CO2, HCI and H3PO4.

[0033] The gypsum slurry from the last reaction vessel (20) has a pH of approx. 9,5 - 11 , and has a specific gravity of approx. 1 ,35 kg/I and a content of dry matter of approx. 35 weight-%.

[0034] From the final reaction tank (20), the gypsum slurry (21 ) is led to a dewatering device, preferably a filter press (22) or similar dewatering equipment such as a vacuum bed filter, centrifugal separator etc.

[0035] In order to remove chlorides from the gypsum slurry, the slurry in the dewatering device (22) is washed with water. In a preferred embodiment, the amount of wash water supplied to the dewatering device (22) is approximately equal to the dry matter content of the gypsum slurry. After the washing step, the gypsum slurry is pressed to a filter cake (23) which subsequently is being blown with air to remove surplus water. Tests have shown that this process will provide a filter cake which satisfy the leaching criteria for chlorides for a stable, non-reactive hazardous waste less than 15000 mg/kg. The dry matter (DS) of the filter cake is typically 60-70 w-%. Typical bulk density of the filter cake is 1 ,0 kg/I, but under compression, the bulk density can easily be increased to 1 ,6 kg/I. [0036] Part of the water from the dewatering device (22) (typically the washing water) can be used to slurry the fly ash in vessels (4) and (12). Heavy metals from the surplus water are removed in a water treatment system and the surplus water is either led to sea or used as a raw material for salt recovery of chloride salts.

Examples

[0037] In Table 2, leaching results from a filter cake obtained according to present invention are presented based on a one stage batch-leaching test at a liquid to solid ratio of 10 l/kg (NS-EN 12457-2). Limit values for acceptance to deposit stable non-reactive hazardous water on a landfill for non-hazardous waste is included for comparison. All leaching values are well within the limits for landfilling of stable, non-reactive hazardous waste on a landfill for non- hazardous waste.

[0038] Table 2. Leaching values on filter cakes using H2SO4, HCI and CO2 as acids and limit values for landfilling of stable, non-reactive hazardous waste on landfill for non-hazardous waste (NA - not analyzed).

[0039] The process to stabilize heavy metals is not limited to using sulfuric acid. Alternative acids such as HCI, H3PO4 and C02(g) can also be applied to reduce the leaching of metals from the fly ash. Metal leaching data from stabilized fly ash using HCI and CO2 are also shown. As shown, all leaching data for the metals are well within the limits for stable, non-reactive hazardous waste. For H3PO4, these results will be even further improved as is well known that most metal phosphates are insoluble at neutral to slightly alkaline pH range.

[0040] For HCI and H3PO4, similar conditions and process equipment as shown in Figure 1 can be applied. Simplified stochiometric reactions for the two acids with fly ash are as follow:

[0041] Rx 3: Ca(OH)2 + 2 HCI CaCI ₂ + H ₂ 0

Rx. 4: 3 Ca(OH)2 + 2 H3PO4 Ca ₃ (P04)2 +6 H2O

[0042] When using CO2 as acid to stabilize fly ash, either pure CO2 gas or CO2 rich flue gas can be applied. The leaching data shown in Figure 2 are based on flue gas containing approximately 16 vol-% CO2. The reaction between C02 and fly ash is:

[0043]

[0044] Rx 5: Ca(OH) + CO2 CaCOa + H2O [0045] [0046] In the present description, two different methods have been applied to stabilize fly ash by using CO2. In the first example, flue gas is reacted with fly ash slurry. To increase the reaction rate between CO2 and fly ash, a special designed gas inlet system is applied to achieve a large gas surface area.

Similar process equipment as shown in Figure 1 can be applied, with the exception that the CO2 flue gas is introduced as a gas into the reaction tank and not as liquid.

[0047] For the moist fly ash, ash from stock (3) are reacted with CO2 in a special designed reactor system. After the reaction is carried out, the carbonated fly ash can be slurryfied with water to remove soluble salts and subsequently dewatered and washed in the press filter (13) to produce a filter cake.

[0048] Ecotoxicological testing of the leaching water from the filter cake have been carried out using Artemia salina and Daphnia magna as test organisms. The leaching procedure was performed using batch-leaching test at a liquid to solid ratio of 10 l/kg (BS EN 12457-3). Artemia salina and Daphnia magna were exposed to the leaching water to measure the acute toxicity and the genotoxicity using Genotox profile®. Artemia salina has higher tolerance towards saline water to that of Daphnia magna. Artemia saline and Daphnia magna are used to mimic salt water and freshwater recipients, respectively.

[0049] The acute toxicity was measured by using the standard method OECD test 202 for Daphnia magna. For Artemia salina there does not exists any established method to measure acute toxicity, so the same method applied for the Daphnia magna (OECD test 202 was applied. Artemia salina and Daphnia magna were exposed to leaching water from the filter cake at the following L/S ratio 10-10000. For Artemia saline there was not revealed any acute toxicity after exposure of the leaching water at any of the measurements. For the Daphnia magna EC5 (5 % reduction of the population) was observed at L/S 10, thus ECsowas not observed for any of the samples.

[0050] The genotoxicity studies (Genotox profile®) showed only minor influence on two genes related to heavy metals such as Zn, Cu, Cd and Cr. The conclusion based on the results from the acute toxicity and the genotoxicity studies is the filter cakes where the salt content is reduced can be regarded as non-hazardous waste.

[0051] REFERENCE NUMERALS

1 dry fly ash supply

2 water supply 3 moist fly ash supply

4 mixer

5 dry fly ash

6 water

7 fly ash/water slurry 8 moist dry ash

9 water

10 ball mill

11 water/fly ash mixture

12 homogenization vessel 13 homogenized fly ash/water slurry

14 first reaction vessel

15 acid

16 acid supply

17 second reaction vessel 18-19 reaction vessels

20 final reaction vessel

21 gypsum slurry

22 dewatering device

23 substantially dry phase, filter cake 24 saline water