EP1343734A2

EP1343734A2 - Method for inerting ash, artificial pozzolan obtained by said method

Info

Publication number: EP1343734A2
Application number: EP01985901A
Authority: EP
Inventors: Claude Criado; Fabrice Giraud; Jean-Emmanuel Aubert; Bernard Husson
Original assignee: Solvay SA
Current assignee: Solvay SA
Priority date: 2000-12-13
Filing date: 2001-12-12
Publication date: 2003-09-17
Also published as: CA2431252A1; FR2817858B1; FR2817858A1; US6962119B2; JP2004526555A; AU2002235790B2; US20040055519A1; AU3579002A; US20050188904A1; PL361803A1; WO2002048066A3; WO2002048066A2; JP4022469B2; CN1518528A; US7128006B2; BR0116128A; MXPA03005306A; CN1258492C

Abstract

The invention concerns a method for inerting ash derived from urban waste incineration which consists in successively treating (19) the ash with a water-soluble phosphate (20) in the presence of water, in conditions adjusted to crystallise the hydroxylapatite and/or the whitlockite and calcining (22) it. The invention also concerns an artificial pozzolan obtained by subjecting ash derived from urban waste incineration to said inerting method.

Description

Method for inerting an ash, artificial pozzolan obtained by said method

The invention relates to a method for inerting ash from the incineration of urban waste. Incinerators used for the destruction of urban waste (household waste and hospital waste) produce generally large volumes of ash, both in the form of bottom ash in incineration furnaces and of fly ash entrained in smoke. The mineralogical composition of these ashes hardly varies, whatever their origin, and one generally finds there, although in proportions which can vary from simple to double, or even more, chlorides of alkali metals (NaCl and KC1), of calcium (usually in the form of calcium carbonate, sulphate, hydroxide or hydroxychloride, in particular lime and anhydrite), quartz, vitrified aluminosilicates, heavy metals in the metallic or combined state (in particular tin, zinc, lead, cadmium, mercury, copper and chromium), chlorinated organic derivatives and unburnt. Among the unburnt, there is usually metallic aluminum.

International application WO 00/29095 describes a process for inerting residues from famines of incinerators, according to which these residues are dispersed with a hydraulic binder in water and then filtered.

The process has been improved by using, for the filtration step, a belt filter, which makes it possible to subject the filter cake to effective washing. This washing of the cake improves the mechanical properties and the resistance to leaching of the inert waste resulting from the process. European patent EP-883 585 [SOLVAY (Société Anonyme)] proposes a process for the inerting of ash containing heavy metals. According to this known process, the ashes are first subjected to washing and filtration to remove the water-soluble materials which they contain and the aqueous cake collected from the filtration is treated with phosphoric acid or an alkali metal phosphate. The phosphate residue thus obtained is then subjected to calcination, then added with a hydraulic binder and water to form a hydraulic mortar. At the end of this known process, an inorganic solid mass is obtained, which is substantially inert with respect to atmospheric agents and which respects the leaching toxicity standards, in particular the TCLP standard ("Toxicity Characteristic leaching Procedure", USA).

These known methods apply to all types of ash, in particular fly ash from the incineration of household or hospital waste. They involve a hydraulic binder.

In the case of ash from the incineration of urban waste, an improvement has now been found in these known processes, which makes it possible to dispense with the hydraulic mortar without harming the inert nature of the treated ash. Consequently, the invention relates to a process for the inerting of an ash from the incineration of urban waste, according to which the ash is successively subjected to a treatment with a water-soluble phosphate in the presence of water and to a calcination; according to the invention, the treatment with phosphate is carried out under controlled conditions to crystallize hydroxylapatite and / or whitlockite.

In the method according to the invention, the ash comprises a bottom ash from an urban waste incineration oven and / or a fly ash which has been separated from the smoke from such an incineration oven.

Urban waste means household waste and hospital waste. These wastes normally contain metals (including heavy metals and aluminum), calcium compounds (usually calcium hydroxide and calcium sulfate), sodium compounds (including sodium chloride) and compounds organic (including chlorinated organic compounds and plastic objects, especially polyvinyl chloride).

Heavy metals are understood to mean metals with a specific mass of at least 5 g / cm ^, as well as beryllium, arsenic, selenium and antimony, in accordance with the generally accepted definition (Heavy Metals in Wastewater and Sludge Treatment Processes; Vol. I, CRC Press, Inc; 1987; page 2).

The quantity of heavy metals in the ash subjected to the process according to the invention depends on the origin of the urban waste. It is usually between 0.5 and 15 parts by weight per 100 parts by weight of dry matter of the ash, more generally between 3.0 and 10 parts by weight per 100 parts by weight of the ash.

Aluminum is generally present in the ash in an amount of 2.0 to 8.0 parts by weight per 100 parts by weight of the ash, plus - __)

generally in an amount of 3.0 to 5.0 parts by weight per 100 parts by weight of the ash.

In the process according to the invention, the water-soluble phosphate reacts with the ash calcium to form calcium phosphate. The water-soluble phosphate is generally phosphoric acid or an alkali metal phosphate, for example sodium phosphate. Orthophosphoric acid is preferred.

According to the invention, the treatment with phosphate is carried out under conditions regulated so that the calcium phosphate which forms is in the state of hydroxylapatite and / or whitlockite crystals.

Hydroxylapatite is a mineral with the general formula Ca ₅ (PO ₄ ) ₃ (OH). Whitlockite is a mineral with the general formula Ca ₉ Fe _x Mgι. _x H (PO ₄ ), where x is a whole or fractional number between 0 and 1.

The hydroxylapatite and whitlockite crystals have the property of allowing a substitution of part of their calcium atoms by heavy metal atoms, by isomorphism. The amount of hydroxylapatite and / or whitlockite crystallized must therefore be sufficient for these minerals to absorb the heavy metals from the ash in their respective crystal lattices. The optimum amount of hydroxylapatite and / or whitlockite to be crystallized will therefore depend on the amount of heavy metals present in the ash and it must therefore be determined in each particular case by routine laboratory work. The amount of hydroxylapatite and / or whitlockite to be crystallized will itself condition the amount of water-soluble phosphate which must be added to the ash. Depending on the origin of the latter, the weight quantity of water-soluble phosphate used (expressed in the H ₃ PO state) may for example vary from 10 to 20% of the weight of dry matter of the ash. Furthermore, the formation of hydroxylapatite and whitlockite requires the presence of calcium in the ash. Depending on the amount of hydroxylapatite and / or whitlockite to be crystallized, the weight amount of calcium (expressed as CaO) can for example vary from 10 to 35% of the weight of dry matter of the ash. Alternatively, if necessary, additional calcium (metallic or combined) must be added to the ash before or during the addition of the water-soluble phosphate, to obtain the required amount of hydroxylapatite crystals and / or whitlockite. Advantageously, calcium carbonate is used, preferably avoiding basic calcium compounds, especially lime. In the process according to the invention, the water-soluble phosphate is added to the ash in the presence of water. The water must be present in an amount at least sufficient to crystallize the hydroxylapatite and / or the whitlockite. In practice, it is present in an amount greater than 10% (preferably at least equal to 25%) of the weight of dry matter of the ash. In practice, it is not beneficial for the amount of water used to exceed 100% of the weight of dry matter in the ash. Values of 30 to 75% are generally suitable.

In the process according to the invention, the calcination has a double function. On the one hand, it is used to destroy the organic compounds of the ash. On the other hand, it recrystallizes hydroxylapatite and / or whitlockite. The calcination is normally carried out at a temperature above 400 ° C, preferably at least equal to 600 ° C. There is no point in exceeding a temperature of 1000 ° C. Temperatures from 600 to 950 ° C are especially advantageous. The calcination can be carried out in an inert atmosphere (for example, under a nitrogen atmosphere). It is preferred to perform the calcination in the presence of air, so as to cause combustion of organic compounds, in particular ^{organo-halogenated.}

The recrystallization of hydroxylapatite and / or whitlockite has the advantageous result of reinforcing the insoluble character of these compounds in water.

In an advantageous embodiment of the process according to the invention, before treating the ash with phosphate, the latter is subjected to an aqueous alkaline wash, at a pH greater than 8.5, for example from 9 to 14, of preferably from 9.5 to 13. In this embodiment, the pH is measured on the aqueous solution collected from the wash. This embodiment of the process aims to remove the water-soluble compounds from the ash. It has the additional advantage that the ash collected from washing is in the optimum conditions to obtain the crystallization of hydroxylapatite and / or whitlockite during treatment with the water-soluble phosphate. In another particular embodiment of the method according to the invention, specially adapted to the case of an ash containing chromium in the hexavalent state, a reducing agent is added to the ash collected from the treatment with phosphate. In this embodiment of the method, the reducing agent has the function of reducing the hexavalent chromium to bring it to a state of lower valence. Iron metal (e.g. iron filings) and carbon (e.g. activated carbon) are preferred reducing agents. The agent the reducing agent is advantageously used in an amount by weight substantially between 0.3 and 1% of the weight of the ash.

The ash collected from the process according to the invention is in the form of a pulverulent or granular mineral mass, which is inert vis-à-vis the environment and atmospheric agents and which respects the standards of toxicity by leaching, particularly the TLCP standard defined above. This mineral mass has the remarkable and unexpected property of having pozzolanic power, which makes it suitable for the constitution of hydraulic binders. In the case where the ash subjected to the process according to the invention contains calcium sulphate, it may prove desirable to decompose it. It has in fact been observed that if, at the end of the calcination, the ash contains calcium sulphate, this alters the hydraulic properties of the ash.

To this end, in a particular embodiment of the process according to the invention, the ash is subjected, before or after calcination, to washing with a solution of sodium carbonate or sodium hydroxide (for example at pH 13 ), so as to dissolve the aluminum by oxidation thereof. Washing can be carried out at room temperature or at a higher temperature, for example from 40 to 75 ° C. In a particularly advantageous embodiment of the process according to the invention, the ash is subjected to washing with an aqueous solution of alkali metal carbonate at a pH greater than 10, in order to decompose the calcium sulfate. The washing is advantageously carried out at a pH of 12 to 13. In this embodiment of the invention, the pH is. measured on the aqueous solution collected from the wash. In the embodiment which has just been described, the washing of the ash with the sodium carbonate solution can be carried out on the ash downstream from the calcination. However, it is preferred, according to an advantageous variant of the invention, that the washing with the aqueous sodium carbonate solution is superimposed on the washing of the ash, upstream of the treatment with the water-soluble phosphate.

It has been observed that, in the case where the ash collected from the calcination contains metallic aluminum, this alters the pozzolanic properties of the ash, causing uncontrollable swelling of the mortars. To this end, in a preferred embodiment of the method according to the invention, the ash is subjected to oxidative heating to oxidize the metallic aluminum to alumina. For the implementation of this embodiment of the invention, the oxidative heating is advantageously superimposed on calcination, the latter then being carried out in an oxidizing atmosphere (for example in air), at a temperature above 800 ° C., preferably from 900 to 1000 ° C.

The ash collected at the end of the process according to the invention can be stored in a public landfill, without risk for the environment (in particular for ground and surface water). It can also be used as it is in civil engineering works, for example in ballasts or as a filler in hydrocarbon road surfaces. Because of its pozzolanic properties, the ash collected at the end of the process according to the invention finds a particularly advantageous use for the manufacture of hydraulic cements.

Consequently, the invention also relates to an artificial pozzolan, obtained by subjecting an ash from the incineration of urban waste to an inerting process according to the invention, as well as hydraulic binders comprising this artificial pozzolan. The invention is illustrated by the following description of the single figure of the appended drawing, which represents the diagram of an installation implementing a particular embodiment of the method according to the invention.

The installation shown in the figure comprises an oven 1 in which the incineration of urban waste is carried out 2. An oven 1 is collected on the one hand, a clinker 3 and, on the other hand, a smoke 4. The smoke 4 is charged with fly ash and it is also contaminated by toxic gaseous compounds, particularly hydrogen chloride and volatile heavy metals. It is first treated in a dust collector 5 (for example an electrostatic filter), where the fly ash 6 is separated therefrom. The dusted smoke 7 is then treated in a purification device 8, known per se, in order to extract the acid gas compounds, then it is discharged to the chimney 9.

The bottom ash 3 and the fly ash 6 are mainly composed of mineral compounds and chlorinated organic compounds, in particular dioxins and furans. The slag is processed in a crusher 10 and crushed slag 11 is then introduced with the fly ash 6 in a chamber ^"23. mixture is collected from the chamber 23 ash 24 which is passed into a washing chamber 12. In the washing chamber 12, the ash 24 is dispersed in a sufficient quantity of water 13 to dissolve substantially all of the water-soluble compounds which it contains, and sodium carbonate 14, intended to react, is also introduced into the chamber 12. with calcium sulphate from the ash to form insoluble calcium carbonate and passing sodium sulphate in solution. The pH in the washing medium of chamber 12 is established at around 12.5. An aqueous suspension 15 is collected from the washing chamber, which is immediately treated on a filter 16. Separated from the filter 16 is an aqueous solution 17 on the one hand and an aqueous cake 18 on the other hand. cake 18 contains most of the heavy metals as well as the dioxins and furans of the ash 24. It also contains calcium (mainly in the form of calcium hydroxide and carbonate) and is impregnated with water. According to the invention, it is introduced into a reaction chamber 19 where orthophosphoric acid 20, a make-up of water 25 and iron filings are added thereto. The phosphoric acid 20 reacts with the compounds calcium and with water to crystallize hydroxylapatite and / or whitlockite. The function of the iron filings 26 is to reduce hexavalent chromium. An ash 21 containing hydroxylapatite and / or whitlockite crystals is collected from the reaction chamber 19. It is transferred to a calcination chamber 22, where it is heated to a temperature of approximately 950 ° C., in the presence of air, for a time sufficient to decompose dioxins and furans, oxidize the metallic aluminum with alumina state and recrystallize hydroxylapatite and / or whitlockite. Is collected from the calcination chamber 22 a dry, pulverulent mass 27 which is inert and which has the properties of an artificial pozzolan. The artificial pozzolan 27 can advantageously be used for the manufacture of hydraulic binders.

The examples whose description follows will show the advantage of the invention.

In these examples, a fly ash from the incineration of household waste, the composition of which is given in Table 1 below, was treated (analysis carried out by X-ray fluorescence).

Table 1

Example 1

100 g of ash were removed and subjected to a washing operation. To this end, the ash was dispersed in an aqueous solution consisting of 1 l of demineralized water containing 50 g of sodium carbonate and the aqueous suspension thus obtained was subjected to moderate stirring for 2 hours. ambient temperature. The aqueous suspension was then filtered, the filter cake was dried and the dry cake was weighed: 78.2 g.

70 g of the dry cake were removed, which was subjected to the inerting process. To this end, 70 g of water and 7 g of phosphoric acid were added to it successively (which corresponds to 0.1 kg of phosphoric acid per kg of dry cake) and the resulting mixture was calcined at 750 °. C for 1 hour. The dry powder collected after the calcination was subjected to a standardized toxicity test by leaching. To carry out this, 30 g of the powder were removed and subjected to three successive leachings with 300 ml of demineralized water. The three leach solutions were collected, mixed, and the composition of the mixture of leach solutions thus obtained was analyzed. The result of the analysis is shown in Table 2 below.

Table 2

Example 2

The test of Example 1 was repeated, under the following conditions:

Washing solution: 1 1 of demineralized water, as it is, without sodium carbonate, Phosphoric acid: 4.5 g (which corresponds to 0.1 kg / kg of dry cake), Calcination: 750 ° C for 1 hour .

The results of the leach toxicity test are shown in Table 3 below. Table 3

Example 3

In order to highlight the pozzolanic character of the ash treated, the following comparative test was carried out. 75 g of “ARC 3” hydraulic road binder (78% slag, 11% quicklime and 5% clinker) were mixed with 25 g of ash treated according to the invention. The resulting mixture was mixed with water and subjected to setting and hardening. The mechanical properties of the mortar obtained were measured according to standard EN 196-1 and compared to the properties of a mortar obtained starting from 100% of hydraulic road binder "ARC 3".

The substitution of 25% of binder by treated ash does not reduce the mechanical properties of the mortar, which shows the pozzolanic character of the treated ash.

The preceding examples show the good ability of the process according to the invention to inert the heavy metals from the fly ash.

A comparison of the results of Example 1 with those of Example 2 further shows the advantage of incorporating sodium carbonate into the water used for washing fly ash, as regards the inerting of the toxic metals, in particular chromium (especially chromium VI), aluminum, molybdenum and strontium.

Claims

- EVENTS

1. A method for inerting an ash from the incineration of urban waste, according to which the ash is successively subjected to a treatment with a water-soluble phosphate in the presence of water and to a calcination, characterized in that performs the treatment with phosphate under controlled conditions to crystallize hydroxylapatite and / or whitlockite.

2. Method according to claim 1, characterized in that before the treatment with phosphate, the ash is subjected to an alkaline washing, at a pH greater than 8.5

3. Method according to claim 2, characterized in that the washing is carried out at a pH of 9.5 to 13.

4. Method according to any one of claims 1 to 3, characterized in that a reducing agent is added to the ash.

5. Method according to any one of claims 1 to 4, characterized in that, in the case where the ash contains calcium sulphate, it is subjected to washing with an aqueous solution of alkali metal carbonate at pH greater than 10.

6. Method according to claim 5, characterized in that the washing of the ash is carried out with the alkali metal carbonate solution at pH 12 to 13.

7. Method according to any one of claims 1 to 6, characterized in that in the case where the ash contains metallic aluminum, it is subjected to an oxidation treatment of said metallic aluminum. ι I

8. Method according to claim 7, characterized in that the oxidation treatment of metallic aluminum comprises oxidative heating of the ash.

9. Method according to claim 8, characterized in that to carry out the oxidative heating of the ash, calcination is carried out at a temperature of 900 to 1000 ° C.

10. Artificial pozzolan, obtained by subjecting an ash from the incineration of urban waste to an inerting process according to any one of claims 1 to 9.