EP0652032A1 - Process for steadying of waste by crystallization - Google Patents

Abstract

Process for deactivating residues from burning of industrial or household waste or of metal hydroxide sludge, enabling the toxic metals present in this waste to be immobilized in the long term by stabilization. The process includes especially the stages of heating of a mixture including the waste to be treated and one more nucleating agents containing especially ferrous compounds. The heating is carried out in such conditions that it makes it possible to obtain a mixture in liquid phase, also called molten mixture or melt, in which at least 5 % of the oxides Fe2O3 are converted into oxides FeO. The process also includes the controlled cooling of the melt until a solid material is obtained in which the heavy metals present in the treated waste are included within crystal structures, and the recovery of the solid material is obtained. The stage of heating may comprise a stage of stabilization of the mixture at a temperature permitting the complete liquefaction of the mixture to be treated.

Description

The present invention relates to a waste inerting process, making it possible to immobilize the toxic metals contained in this waste in the long term by stabilization.

The term "waste" in the context of the present application means residues from the incineration of industrial or domestic waste or slurries of metal hydroxides.

Among the wastes likely to be treated in accordance with the process of the invention, mention is therefore made, by way of examples, of slag (or bottom ash) originating from the incineration of industrial or domestic waste, metal hydroxide sludge.

The usual incineration of domestic and industrial waste generates products which are suddenly cooled during the hydraulic quenching experienced at the outlet of the oven, form a polyphase material containing crystals of iron oxides and aluminosilicates embedded in a weak glassy matrix by extreme thermal variations. The toxic heavy metals, shared between the different structures, are then easily mobilized by external agents, natural or provoked, and make the slag potentially dangerous with respect to the environment.

The glasses produced during incineration have often been considered as stable products capable of immobilizing heavy metals and this in a lasting manner. It is important to emphasize that these slags, only "pro parte" glassy and evolving over time, represent a deferred danger. In fact, the aluminosilicate baths produced in industrial incinerators undergo very damaging hydraulic quenching. During this brutal cooling of a polyphase material, generally consisting of spinels embedded in a vitreous material, differential contractions appear due to the significant difference between the coefficients of isobaric thermal expansion of the spinels and the vitreous phase. Contracting more on cooling, the glass fractures and / or accumulates stresses, the relaxation of which is the source of micro-fracturing on the storage site. Thermodynamically unstable, glass is also mechanically unstable.

As the mass of metals extracted by the leaching of the vitreous phase is proportional to the surfaces coming into contact with the solution, it is conceivable that these slag (also called bottom ash), can happen to be percolated to the core in durations of a few years, durations very brief compared to the geological unit of time: the million years.

In order to ensure both legal compliance and the long-term safety of these clinkers, one solution consists in immobilizing these harmful elements in crystalline buildings, stable over the geological time scale.

The crystalline phase behaves differently from the behavior of the glassy phase upon leaching. Indeed, crystal is a much more stable organization of matter than glass of the same composition. Also the destruction of one or more mineralized species crystallized from rocks by combined dissolution, oxidation and hydrolysis is excessively slow and takes place on the scale of a million years. The harmful metals they contain, released at the same rate, are therefore practically harmless to our environment.

Vitrification has also been proposed. A first route involves making a glass at high temperature (4000 to 5000 ° C), using a plasma torch, modifying very little the chemical composition of the original product. A second way requires the synthesis of an alkali borosilicate glass, insoluble, in which the waste is dispersed. This process considerably changes the content of the initial product.

No other solution for treating slag from the incineration of industrial or household waste seems to have been proposed. The stabilization treatment of bottom ash by recrystallization modifies very little the chemistry of the waste and in particular of the slag which it contains, and brings only very little additional material allowing the transformation of a waste into a material close to a material natural.

To provide a solution to the problem of storage of harmful industrial or domestic waste, the inventors of the present application therefore propose to transform this waste into a material having the properties of natural rocks as regards their crystal structure.

For this purpose, a process has been developed which consists, according to the structures and compositions of the waste, of directing the migration of the metals which they contain, from the vitreous material towards the growing crystallized phases.

• le chauffage d'un mélange comprenant les déchets à traiter et un ou plusieurs agents de nucléation contenant notamment des composés ferreux, ledit chauffage étant réalisé dans des conditions telles qu'il permet l'obtention d'un mélange en phase liquide encore appelé mélange de fusion ou "melt", dont au moins 5% des oxydes Fe₂O₃ sont pour l'essentiel transformés en oxydes FeO ;
• le refroidissement contrôlé du "melt" jusqu'à l'obtention d'un matériau solide dans lequel les métaux lourds contenus dans les déchets traités sont stabilisés au sein de structures cristallines contenant des cristaux de la famille des spinelles ;
• la récupération du matériau solide obtenu.

The method according to the invention for the treatment of waste is according to a first embodiment, characterized in that it comprises the following steps:

• heating a mixture comprising the waste to be treated and one or more nucleating agents containing in particular ferrous compounds, said heating being carried out under conditions such that it makes it possible to obtain a mixture in liquid phase also called mixture of fusion or "melt", at least 5% of the Fe₂O₃ oxides are essentially transformed into FeO oxides;
• controlled cooling of the melt until a solid material is obtained in which the metals heavy contained in the treated waste are stabilized within crystal structures containing crystals of the spinel family;
• recovery of the solid material obtained.

The purpose of the process thus characterized is to allow, firstly, the complete liquefaction of the waste to be treated and, secondly, the inclusion, during controlled cooling, of the metals contained in the waste, within crystalline structures. developing as the melting mixture solidifies during its cooling. In other words, controlling the gradual decrease in temperature during cooling of the "melt" makes it possible to control the germination process of the crystals formed including heavy metals contained in the waste. These crystal structures contain crystals of the spinel family.

Preferably, 10% to 80%, or even more, of the Fe₂O₃ oxides are transformed into FeO oxides.

Advantageously, the treatment process of the invention makes it possible to obtain between approximately 20% and approximately 50% of spinels, or even more, expressed by weight of the mixture to be heated.

The relative proportions of the waste to be treated and the nucleating agents by weight relative to the weight of the mixture to be heated are determined with respect to the chemical composition of the waste to be treated, whether it is slag or hydroxide sludge.

To this end, the composition of waste oxides of the SiO₂, CaO, Fe₂O₃, Al₂O₃, TiO₂, MgO type is mainly taken into account. The invention may likewise be applied to waste which differs from the previous ones but whose chemical analysis reveals a relationship with the slag or the sludge of metal hydroxides for the main elements.

The treatment process of the invention having to favor the formation of spinels (A ^II B ^III X₄) and not the unique formation of magnetite (FeO (L) + Fe₂O₃ (L)), it is necessary to favor the reduction of the oxides Fe₂O₃ in particular by providing nucleating agents capable of reacting preferentially with these Fe₂O₃ oxides rather than with oxides such as CaO. These nucleating agents are, for example, iron filings or any composition containing iron in proportion to the weight of the mixture to be heated during the heat treatment, varying from approximately 0.5% to approximately 15%.

When the waste to be treated is hydroxide sludge, the nucleating agents are advantageously slag.

Preferably, inerting of the waste is achieved by complete crystallization of the material in the form of a structure including metals contained in the waste. A crystallization allowing the immobilization of 80 to 100%, preferably 90 to 100% of the metals (expressed as a percentage of the weight) contained in the waste already appears satisfactory.

The process described has the advantage of leading to the immobilization of metals in a stable manner over time on a geological scale (inerting), thus ensuring the harmlessness of the metals contained in the waste.

The term “controlled cooling of the melting mixture” or “melt” is used to mean cooling, the reduction of which is controlled at least until the solidification of the melting mixture is complete. In other words, it is a progressive cooling, the progression of which is determined to be sufficiently slow.

Full solidification of the melting mixture is normally achieved at a temperature between about 1050 ° C and about 1250 ° C, preferably between about 1100 and about 1210 ° C.

It marks the end of crystallization and is considered to have immobilized at least 80% of the metals, expressed as a percentage of the weight of the treated waste.

Preferably, the cooling will be carried out in such a way that the temperature decrease is of the order of approximately 1 ° C. to 10 ° C. per minute at least until crystallization or even until the complete solidification of the melting mixture. . Generally, until this solidification is obtained, the cooling must be as slow as possible.

Different known cooling techniques can be used. We could for example carry out this cooling in air.

Another possibility is to use the heat capacity of the melting mixture which on contact with a surface having a lower temperature will undergo a temperature decrease. In this hypothesis, it is possible, by knowing the thermal conductivity of the mixture, to determine the cooling rate of this mixture and to choose the temperature of the surface in contact with which the cooling must take place, in order to obtain the desired slow cooling.

The heating step referred to above in the treatment process of the invention must be carried out so as to obtain a completely liquefied mixture. To do this, use may be made of a temperature stabilization step when it has reached its highest level, in order to ensure this complete liquefaction. The stabilization time varies according to the highest temperature of the heating stage; it can, for example, last approximately 1 hour or be adapted according to the quantity of waste treated.

According to a first embodiment of the method of the invention, the waste to be treated is slag resulting from the prior incineration of industrial or domestic waste.

The average composition of the slag is given in the examples which follow on the basis of samples of industrial waste. In general, slag mainly contains noncombustible mineral matter combined in multiple oxidized forms. In particular, slag generally contains a relatively large proportion of iron oxides.

These iron oxides are for example in the form of magnetite which belongs to the family of spinels, and the process of the invention has in particular for object to lead to the formation of additional spinels, different from magnetite.

Thus during the heating carried out within the framework of the process of the invention, the magnetite as well as the other solid elements of the slag, are melted and their progressive cooling leads then in particular allows the substitution of the iron atoms contained in the magnetites, by atoms of metal contained in the waste. The same substitution process can take place with other spinels formed from the slag during the treatment according to the invention.

This phenomenon can be favored by the addition to the slag from waste, nucleating agents, in particular ferrous compounds, for example iron filings.

By way of example, the slag to be treated can represent from 100% to 85% estimated by weight, of the reaction mixture subjected to the heating step, the nucleating agents representing in this case from 0% to about 15% by weight of the reaction mixture.

According to another embodiment of the invention, the treated waste is metal hydroxide sludge.

The composition of the metal hydroxide sludges is also illustrated in the examples which follow.

In the case of the treatment of metal hydroxide sludges, the method of the invention is advantageously implemented by replacing all or part of the ferrous compounds constituting the nucleating agents, by slag. In this case, there can be a reaction mixture comprising, expressed as a percentage relative to the weight of the waste to be treated, from 70% to 30% of sludge and from 30% to 70% of slag.

Optionally, a fraction of the slag acting as nucleating agents promoting the formation of spinels can be replaced by ferrous compounds as described above. Thus, the proportion of ferrous compounds can be around 5% by weight of the mixture to be heated.

• a) la liquéfaction de la magnétite contenue dans les scories et la libération des oxydes Fe₂O₃, et
• b) la réduction de l'oxyde Fe₂O₃ formé, en oxyde FeO par addition d'agents tels que le fer.

The treatment method according to the invention is according to a preferred embodiment, characterized in that the heating step is carried out under conditions such that it allows

• a) the liquefaction of the magnetite contained in the slag and the release of the oxides Fe₂O₃, and
• b) the reduction of the Fe₂O₃ oxide formed, into FeO oxide by addition of agents such as iron.

Advantageously, the temperature of the mixture to be treated by heating is brought to about at least 1450 ° C, preferably up to about 1500 ° C. This temperature can be adjusted as a function of the quantity of waste to be treated and of its initial composition.

In a preferred embodiment of the invention, the ferrous compounds used as nucleating agents are iron.

In another embodiment of the invention, these ferrous compounds are iron filings, preferably in a proportion greater than 0.5% by weight of the mixture to be treated by heating and so particularly preferred, in a proportion of about 1% by weight of the mixture to be treated.

By way of example, the waste to be treated according to the process of the invention mainly contains the following oxides in a proportion given as a percentage relative to the total weight of the waste: slag sludge If 26.25 to 45.65 1.4 - 2.7 Al₂O₃ 7.43 - 17.98 1 - 3 Fe₂O₃ 14.53 - 34.92 3 - 15 MnO 0.54 - 1.88 0.05 - 0.2 MgO 1.64 - 2.86 0.5 - 1 CaO 6.41 - 10.16 20 - 32 Na₂O 2.66 - 10.82 1.5 - 4 K₂O 0.56 - 1.61 0.2 - 0.3 TiO₂ 3.66 - 7.69 0.01 - 0.1 WHERE 0.96 - 2.54 5 - 18 CO₂ 0.33 - 4.32 5 - 11 S total 0.03 - 2.53 4 - 14

The implementation of the process of the invention can be facilitated when the waste to be treated, whether slag or sludge, is previously mixed in a homogeneous manner with the nucleating agents and in particular with the ferrous compounds.

In the case of slag, homogenization can be carried out from slag having a particle size less than 10 mm or less, preferably less than 2 mm.

Quite interestingly, the homogenization could be improved by a porphyrization step of the mixture to be treated, for example so as to obtain a particle size of less than 10 μm.

Figure 1 :

Structure d'un spinelle direct de formule AB₂O₄

Figure 2 :

Diagramme d'équilibre des feldspath plagioclases

Figure 3 :

Diagramme de stabilité de Fe - Fe₂O₃en fonction de log pO₂

Other advantages and characteristics of the invention appear in the figures and in the examples which follow.

Figure 1 :

Structure of a direct spinel of formula AB₂O₄

Figure 2:

Plagioclase feldspar balance diagram

Figure 3:

Stability diagram of Fe - Fe₂O₃ as a function of log pO₂

EXAMPLES I. Slag treatment I.1. Composition of bottom ash A. Chemical composition

Slag results from the incineration of organic waste. It mainly contains incombustible mineral matter combined in multiple oxidized forms. Depending on the incinerated waste, the contents of oxides and chemical elements vary in the proportions given in Table 1 below.

Due to the limit values of the contents of certain heavy metals, this study will only be concerned with transition metals which, in the event that they are completely dissolved, should be the subject of a specific treatment: Cr, Cu, Ni, Pb and Zn.

B. Structural composition

If glass is the phase mainly present, 2/3 of the volume, magnetite (Fe ^II O-Fe ^III ₂O₃) is the most widespread crystallized form and at the most advanced stage of development (size = 10 to 20 µm) . It coexists with other important mineral species such as hematite (Fe₂O₃), lepidocrocite (FeO.OH), quartz (SiO₂) and calcium feldspar CaAl₂Si₂O₈ or sodium NaAlSi₃O₈.

• Le chrome et le nickel sont localisés dans le réseau de Fe₃O₄ en substitution soit de Fe^II et/ou de Fe^III.
• Le zinc et le cuivre se partagent entre le verre et la magnétite en proportion inégales et variables selon les échantillons.
• Le plomb est non détecté dans les spinelles. Son absence s'explique pour des raisons stériques.
• Les autres métaux sont difficilement décelables par suite de leur faible teneur et ne représentent de plus qu'un risque modéré.

The advantage of the presence of magnetite, a ferric spinel, is that it allows substitutions within its crystal lattice. The transition metals can replace iron in its various degrees of oxidation (II and III) without any modification of the structure and its stability. This is why studies of the distribution of elements in bottom ash have given us information on their distribution coefficients:

• Chromium and nickel are located in the network of Fe₃O le in substitution for either Fe ^II and / or Fe ^III .
• Zinc and copper are shared between glass and magnetite in uneven and variable proportions depending on the samples.
• Lead is undetected in spinels. His absence is explained for steric reasons.
• The other metals are difficult to detect due to their low content and therefore only represent a moderate risk.

I.2. Heat treatment A. Purpose

The basic concept consists in reducing the unstable glass fraction and sequestering the polluting metals by inserting them in crystalline networks in the presence or to synthesize. It is about recreating a real natural material from a waste by modifying very little its chemistry

II.a) Instability of the glassy slag phase

Vitrification techniques are already used in the stabilization of waste, especially radioactive. These are very high quality special glasses that are not found in clinkers from industrial incinerators. In fact, observations under the microscope revealed an intense micro-fracturing of the glass, which affected the crystallized phases very little. Its origin is to be found in the development of clinker. On leaving the incineration furnaces, the molten material undergoes hydraulic quenching. The stresses developed by this thermal shock are sufficient to develop intense fracturing, thereby weakening the material. Furthermore, the glass results in an alumino-siliceous gel caused by the departure of transition metals in a complexing acid solution. This gel, as opposed to glass, has the mechanical property of expanding or contracting in the ambient phase, occupying the volume offered to it. During leaching, there is extraction of atoms from the glassy phase which induces a reduction in the specific volume. This is observable by a discoloration of this phase, linked to the departure of transition metals. This is why the acid attack causes intense corrosion of the glass solubilizing certain metals which were initially attached to it: iron, copper, lead and very little nickel and chromium.

II.b) Stability of the crystallized slag phase

The phenomenon described above is not detected in the crystallized phases which retain their chemistry after leaching. No attack pattern could be detected on the crystallized phases, under the same leaching conditions as for glass, concluding that these crystalline buildings have notable stability even by very sensitive optical methods such as differential interference contrast.

B) PRINCIPLE

Incineration residues from industrial waste are generally rich in iron. When the waste temperature rises in the incinerator, iron, thermodynamically unstable under the partial oxygen pressure prevailing at temperatures below 1500 ° C, is oxidized to magnetite Fe₃O₄ (Fe ^II Fe ^III O₄) which crystallizes in the system cubic.

• (1) A ^II = Mg, (Fe) [Cr, Mn, Ca, Co, Ni, Cu, Zn, Cd, (Hg), Sn]
• (2) B ^III = Al [ Ga, In, Ti, V, Cr, Mn, Fe, Co, Ni, Rh ]

This substance belongs to the vast family of spinels whose general structural formula is:

A ^II B₂ ^III X₄ X = O, S, ...


with

• (1) A ^II = Mg, (Fe) [Cr, Mn, Ca, Co, Ni, Cu, Zn, Cd, (Hg), Sn]
• (2) B ^III = Al [Ga, In, Ti, V, Cr, Mn, Fe, Co, Ni, Rh]

Figure 1 illustrates the normal spinel structure.

The slag from the incinerator is generally rich in magnetite. This mineral effectively traps the elements in square brackets of line (1) in place of Fe ^II and those of line (2) in place of Fe ^III . These harmful metals are then permanently immobilized in the magnetite network.

The most immediate idea is therefore to increase the magnetite content of the slag in order to accentuate this trapping phenomenon. It must therefore be melted and cooled very slowly. Tests in this direction have shown that the magnetite content of the slag can be increased dramatically. But the proportion of glass remains important; this is due to the fact that certain elementary or associated oxides (polymers) of the alumino-silicate bath or "melt" are enriched in the residual bath, at the end of the crystallization of the magnetite, and notably increase the viscosity thereof. These include Al₂O₃, MgO and TiO₂. The crystallization is therefore slowed down or even inhibited; nucleation and diffusion having become inoperative.

The originality of the process proposed in the present application consists precisely in preventing the magnetite from forming by forcing the alumino-silicate bath to crystallize spinels rich in Al₂O₃, MgO and TiO₂ so as to prevent the increase in the viscosity of the “melt”. residual "which would prevent its total crystallization in reasonable durations. The spinels thus formed continue to receive the harmful heavy metals between the hooks of lines (1) and (2). Pb²⁺ has a much greater ionic radius (1.29 Å) than that of transition metals - between 0.4 and 0.8 Å. It cannot therefore be welcomed by these spinels. It is inserted into the much larger crystallochemical sites that the structure of plagioclase feldspar spares. These are intermediate of the poles of the solid solution NaAlSi₃O₈ (albite) - CaAl₂Si₂O₈ (anorthite) that represents Figure 2.

By melting a slag consisting of glass and magnetite crystals, a liquid is produced consisting of elementary oxide molecules such as MgO, CaO, Al2O3, SiO₂, FeO, Fe₂O₃ and TiO₂. These molecules will associate in varying degrees to form aggregates whose stability depends on the importance of the permanent dipole moments of the different constituents.

Pure magnetite basically melts at 1590 ° C under a total pressure of 1 atmosphere according to:

Magnetite = FeO (L) + Fe₂O₃ (L)

et $$\mathbf{\text{Hématite <--> Fe₂O₃(L)}}$$

apparaissent respectivement à 1369 °C et 1565 °C. Le bain silicaté dissout donc du monoxyde et du sesquioxyde de fer.
Réduire Fe₂O₃ en FeO au moyen d'un additif approprié a pour effet :

• d'accroitre la fluidité du "melt" par augmentation de l'activité en FeO du système,
• d'empêcher de formation ultérieure de magnétite par destruction d'un constituant indispensable (Fe₂O₃) à sa formation .

The elementary oxides FeO and Fe₂O₃ are liquid at the melting point of pure magnetite since the phase changes $$\mathbf{\text{Wüstite <--> FeO (L)}}$$

and $$\mathbf{\text{Hematite <--> Fe₂O₃ (L)}}$$

appear at 1369 ° C and 1565 ° C respectively. The silicate bath therefore dissolves monoxide and iron sesquioxide.
Reducing Fe₂O₃ to FeO using an appropriate additive has the effect of:

• to increase the fluidity of the "melt" by increasing the activity of FeO in the system,
• to prevent subsequent formation of magnetite by destruction of an essential component (Fe₂O₃) in its formation.

For this purpose, Fe metal (Fe °) is incorporated which under the prevailing conditions (T = 1550 ° C) is unstable and reacts with Fe₂O₃: $$\mathbf{\text{Fe ° (L) + Fe₂O₃ (L) -> 3 FeO (L)}}$$

The stability diagram of Fe - Fe₂O₃ as a function of the oxygen pressure is given in Figure 3.

In the "melt", the major spinel-forming oxides capable of hosting the harmful "spinellisable" metals are now: Al₂O₃, TiO₂ FeO and MgO.

the cooling of the "melt" leads, taking into account the Al₂O₃ / TiO₂ ratio, to the crystallization of hercynite (FIG. 4), which traps the transition metals from the very fluid bath, then from TiFe₂O₄ ulvöspinelle as soon as the temperature of the eutectic is reached (T _eu = 1355 ° C, Cheney and Muan, 1972). At this stage of cooling most of the harmful metals have been immobilized in the spinels.

The total crystallization of the bath occurs between 1205 ° C, (melting temperature of pure fayalite: Fe₂SiO₄) and 1118 ° C (melting temperature of sodium plagioclase NaAlSi₃O₈) It is a close association of needle-like crystals of fayalite _Sol.Sol and _fibrous plagioclases (60% anorthite) enriched in Pb.

Indeed, from these analytical results, we are faced with an alternative:

or optimize the growth of magnetite in the glassy gangue

The magnetite network has the possibility of receiving more than 15% of elements in substitution of Fe ^II and Fe ^III , and thus plays the role of a "crystalline sponge"; it makes sense to continue the growth of germs already present. This is why if the slag is heated beyond its melting temperature with the addition of ferrous compounds and by slow cooling of the molten phase, it feeds and thereby considerably increases the size of the crystals (100 μm). . The residual glass is considerably reduced and purified of these metals.

or completely recrystallize the slag by the synthesis of new structural patterns.

When the slag / addition mixture is brought to higher temperatures (1500 ° C.), a fully crystallized material is obtained after cooling. At this stage we reach the formation of new minerals (complex spinels, feldspars and olivine) which give it the harmlessness of a natural rock, the metals then being all strongly immobilized.

C. Handling

To proceed to the elementary substitutions of metals from the vitreous phase towards the crystallized phases, it is necessary to remelt then to cool the slag by controlling the processes of germination and recrystallization. This is why the structural studies and the contents define the following operating conditions.

a) Preparation of samples

The tests carried out in the laboratory are carried out using raw slag, sampled at the outlet of the incineration furnaces. After 24 hours of drying at 70 ° C, the clinker is crushed and then dry ground to reach a particle size <2 mm. After quartering a 1 kg sample, add 1% by mass of iron filings of extra pure quality MERK article 3819 (see Table 2).

Homogenization is done in a closed container and stirred manually. Even if a relationship between the maximum acceptable particle size and the efficiency of the process has not yet been demonstrated, we believe that grinding to 2 mm produces enough fines to have a correct homogenization of solids; ie a random spatial distribution of clinker and iron filings. The porphyrization of the whole after mixing (grinding <10mm) would undoubtedly improve the homogeneity. But the tests performed on the 2 particle sizes lead to the same results. ELEMENTS CONTENTS (%) Fe 99.5 insol. (H2S04) 0.1 Pb 0.002 Cu 0.002 Mn 0.002 Zn 0.002 Ace 0.0002 NOT 0.001 S 0.002 TOTAL 99.6112 Table 2: Composition of MERCK iron filings (art 3819)

b) Heating conditions

They are inspired by the manufacture of vitroceramics. These are prepared by recrystallization of all or part of an amorphous material. This elaboration first requires the synthesis of a high temperature glass (1500 ° C-1600 ° C). The liquid obtained is maintained at 1500 ° C for one hour. Cooling is then programmed and the temperature drops from 1 to 5 ° C / min.

c) Procedure

Laboratory experiments are carried out using the slag / filings mixture. 10 grams of the formulation described above are placed in a sintered alumina crucible (99.7% pure) sintered 20 mm in diameter and 30 mm in height. The crucible is then introduced into the center of a horizontal tube furnace CARBOLITE STF 16/75. There are 2 heat shields inside through the 2 orifices of the alumina cylindrical tube making it possible to best limit the heat losses in its center. Finally, the tubing is closed using 2 stainless steel plugs.

The regulator / programmer is used to set the heating and cooling profiles. The time required to reach 1500 ° C from room temperature depends on the maximum heating power that the oven can develop. It increases with the aging of the heating resistors. It takes approximately 4 hours and 30 minutes to complete. The cooling time varies according to the programmed speed.

III. Results IV. Thermal behavior IV.1.1. Differential thermal analyzes

• -1- Entre 800°C à 970 °C, le flux est exothermique. Il traduit l'énergie dégagée par une réaction dans l'état solide : oxydation de la magnétite en hématite.
• -2- Entre 1020°C et 1350°C, le flux mesuré est endothermique. Il correspond à l'énergie à apporter pour permettre la fusion de la scorie.

The differential thermal analysis made it possible to identify the reaction behavior of the slag / filings mixture during its heating. Two reaction domains, exo- and endothermic flux seats, are detectable within distinct temperature ranges:

• -1- Between 800 ° C to 970 ° C, the flow is exothermic. It translates the energy released by a reaction in the solid state: oxidation of magnetite to hematite.
• -2- Between 1020 ° C and 1350 ° C, the measured flux is endothermic. It corresponds to the energy to bring to allow the fusion of the slag.

The same analysis was carried out on the raw slag and shows little difference with the values cited above. Namely, an interval of 780 ° C to 990 ° C for the first flow, an interval of 1020 ° C to 1350 ° C for the second peak.

IV.1.2. Warm-up enthalpies

The heating enthalpies, per gram of product, were determined using a calorimeter from room temperature to the temperatures before and after the fusion. They are shown in Table 3. TEMPERATURES ENERGY (Joules / g) ENERGY (KWh / t) RAW SCORE Q before merger from 23 ° C to 1010 ° C 564 156.67 Q after merger from 24 ° C to 1390 ° C 1468 407.78 Q of fusion from 1010 ° C to 1390 ° C 900 250 SCORIE + 1% addition Q before merger from 23 ° C to 1010 ° C 500 138.89 Q after merger from 24 ° C to 1390 ° C 1334 370.56 Q of fusion from 1010 ° C to 1390 ° C 834 231.67 Table 3: Slag heating enthalpies as a function of temperature

These enthalpy measures allow us to appreciate the amounts of energy to provide to transform the slag and especially the energy gain brought by iron filings when incorporated into the environment.

V.2. TESTS

Heat treatments were carried out on 4 types of slag with different chemical compositions, STL 2, STL 3, STL 6 and STL 8, mixed with 1% iron filings (MERCK article 3819). For each clinker class, a slag / filings sample is brought, according to the procedure described, at 1500 ° C, at 1400 ° C and at 1300 ° C, in addition, for the STL 8/1% iron filings mixture.

X-ray diffraction analysis before and after heat treatment confirms that beyond 1500 ° C, the magnetite has disappeared to make way for the hercynite FeAl₂O₄ and the ulvite (or ulvöspinelle) Fe₂TiO₄.

Finally, to verify the stability of the synthesized products, the NFX 31.210 standardized leaching test is carried out on the raw samples and on the recast products. This experimental standard applies a method allowing the obtaining, under special conditions, of the solubilized fraction of a waste sample under aqueous conditions suitable for analytical characteristics. It applies to waste taken in the solid or plastic state, and remaining in this state, regardless of the degree of division of the material. The results of the characterization of the leachate make it possible, by comparison with the acceptance thresholds for landfill, to assess the level of stability of the waste and its final destination, in particular, its landfill. This can involve potential risks which mainly reside in the entrainment by water, of polluting elements contained in the waste.

The test is performed on the waste crushed to 4 mm. 100 g of residue are removed by quartering, which is placed in a 2 liter bottle and which is brought into contact with 1 liter of leaching solution. It is prepared from demineralized water, saturated with CO₂ gas which is stripped by an air bubbling. The bottle is shaken for 16 hours using a tray shaker allowing the linear reciprocating movement. At the end of the operation, the residual material and the solution are separated. The leachate is then available for carrying out analyzes. The residual material will be subjected to 2 other successive leaching operations which will lead to the production of leachate which will also be analyzed.

V.2.1. Tests on the STL 2 sample * Chemical composition of the raw sample

Majeurs (%)   99,95
Traces(ppm)   29391
Composition (%)   102,89

Major (%) 99.95
Traces (ppm) 29391
Composition (%) 102.89

* Structural characterization of the raw sample

• 20 % de quartz SiO₂
• 20 % de magnétite Fe₃O₄
• 10 % d'augite (Mg, Fe, Ca).(Si, Al)O₃
• Σ de feldspath (Na, Ca).(Al, Si)₃O₈

The raw STL 2 sample is mainly composed of:

• 20% SiO₂ quartz
• 20% Fe₃O₄ magnetite
• 10% augite (Mg, Fe, Ca). (Si, Al) WHERE
• Σ of feldspar (Na, Ca). (Al, Si) ₃O₈

• 17 à 18 % de Si   - 6 à 6,5 % de Al
• 11 à 12 % de Fe   - 3 à 3,5 % de Na
• 11 à 11,5 % de Ca   - 2 à 2,5% de Ti
• de l'ordre de 1% pour K, Mg,Mn et P

dontThe rest, about 50%, is made up of the amorphous phase. The chemical composition of this glassy phase could be estimated using point spectroscopic analysis (Castaing electron microprobe), namely:

• 17 to 18% of Si - 6 to 6.5% of Al
• 11 to 12% Fe - 3 to 3.5% Na
• 11 to 11.5% Ca - 2 to 2.5% Ti
• around 1% for K, Mg, Mn and P

whose

* Structural characterization of the remelted sample

• la hercynite Fe Al₂O₄
• l'ulvite, ou l'ulvöspinelle, Fe₂TiO₄
• un plagioclase

à 1400 °C : Le matériau n'est pas entièrement cristallisé. il reste encore approximativement 40 % de phase vitreuse.

• augmentation du pourcentage de magnétite Fe₃O₄
• apparition de cristaux d'ilménite FeTiO₃
• la fayalite Fe₂SiO₄
• feldspath (Na, Ca).(Al, Si)₃O₈

at 1500 ° C : It is fully crystallized and is composed of 3 phases

• hercynite Fe Al₂O₄
• ulvite, or ulvöspinelle, Fe₂TiO₄
• a plagioclase

at 1400 ° C: The material is not fully crystallized. approximately 40% of the glass phase remains.

• increase in the percentage of magnetite Fe₃O₄
• appearance of FeTiO₃ ilmenite crystals
• fayalite Fe₂SiO₄
• feldspar (Na, Ca). (Al, Si) ₃O₈

* Leaching tests

• Le cuivre et le nickel mobilisables pour l'échantillon brut se trouvent stabilisés dans les produits refondus surtout à 1500 °C où ils ne sont mêmes pas détectés dans les lixiviats.
• L' arsenic n'est plus détectable pour les produits refondus
• Les concentrations en chlorures et en sulfates chutent aussi brutalement pour devenir nulles ou pratiquement nulles dans STL2-1500°C.

If we compare the results of the leaching tests of the remelted products at 1500 ° C, at 1400 ° C with those of the raw sample, we notice:

• The copper and nickel that can be mobilized for the raw sample are stabilized in the remelted products, especially at 1500 ° C where they are not even detected in the leachate.
• Arsenic is no longer detectable for recast products
• The concentrations of chlorides and sulphates also drop suddenly to become zero or practically zero in STL2-1500 ° C.

V.2.2. Tests on the STL 3 sample * Chemical composition of the raw sample STL 3 SAMPLE

P.F 1010°C   -3,03
Majeurs   94,79
Traces(ppm)   35981
Composition (%)   98,39

Mp 1010 ° C -3.03
Major 94.79
Traces (ppm) 35981
Composition (%) 98.39

* Structural characterization of the raw sample

• 35 % de magnétite Fe₃O₄
• 13 % d'augite (Mg, Fe, Ca).(Si, Al)O₃

The raw STL 3 sample is mainly composed of:

• 35% Fe₃O₄ magnetite
• 13% augite (Mg, Fe, Ca). (Si, Al) WHERE

• 21 à 22 % de Si   - 5 à 5,5 % de Al
• 7,5 à 8 % de Fe   - 4 à 5 % de Na
• 8,5 à 9 % de Ca   - 2 à 2,5 % de Ti
• de l'ordre de 0,5 % à 1% pour K, Mg , Mn et P

dontThe rest, about 52%, consists of the amorphous phase. The chemical composition of this glassy phase could be estimated using point spectroscopic analysis (Castaing electron microprobe), namely:

• 21 to 22% of Si - 5 to 5.5% of Al
• 7.5 to 8% Fe - 4 to 5% Na
• 8.5 to 9% Ca - 2 to 2.5% Ti
• in the range of 0.5% to 1% for K, Mg, Mn and P

whose

* Structural characterization of the remelted sample

• la hercynite Fe Al₂O₄
• l'ulvite, ou l'ulvöspinelle, Fe₂TiO₄
• la fayalite Fe₂SiO₄
• un plagioclase

à 1400 °C : Le matériau n'est pas entièrement cristallisé. il reste encore approximativement 40 à 50 % de phase vitreuse.

• augmentation du pourcentage de magnétite Fe₃O₄
• apparition de cristaux d'ilménite FeTiO₃ et de pseudo-brookite Fe₂TiO₅
• un plagioclase à 50% d'anorthite

at 1500 ° C : It is fully crystallized and is composed of 4 phases

• hercynite Fe Al₂O₄
• ulvite, or ulvöspinelle, Fe₂TiO₄
• fayalite Fe₂SiO₄
• a plagioclase

at 1400 ° C: The material is not fully crystallized. approximately 40 to 50% of the glass phase remains.

• increase in the percentage of magnetite Fe₃O₄
• appearance of FeTiO₃ ilmenite crystals and Fe₂TiO₅ pseudo-brookite
• a plagioclase with 50% anorthite

* Leaching tests

• Le cuivre, le nickel et le zinc mobilisables pour l'échantillon brut se trouvent stabilisés dans les produits refondus surtout à 1500 °C où ils ne sont mêmes pas détectés dans les lixiviats.
• L'arsenic n'est plus détectable dans les lixiviats des produits refondus
• les fractions solubles des produits refondus sont de 4 à 20 fois plus faibles que pour l'échantillon brut.
• Les concentrations en chlorures et en sulfates chutent aussi brutalement pour devenir nulles ou pratiquement nulles dans STL3-1500°C.

Par ailleurs

• le plomb est légèrement déstabilisé dans STL3-1400 °C, mais il ne se solubilise que faiblement.
• On a diminué la solubilité du cuivre plus fortement dans STL3-1500°C que dans STL3-1400°C

If we compare the results of the leaching tests of the remelted products at 1500 ° C, at 1400 ° C with those of the raw sample, we notice:

• The copper, nickel and zinc that can be mobilized for the raw sample are stabilized in the remelted products, especially at 1500 ° C where they are not even detected in the leachate.
• Arsenic is no longer detectable in the leachate from remelted products
• the soluble fractions of the remelted products are 4 to 20 times lower than for the raw sample.
• The concentrations of chlorides and sulphates also drop suddenly to become zero or practically zero in STL3-1500 ° C.

otherwise

• the lead is slightly destabilized in STL3-1400 ° C, but it solubilizes only slightly.
• The solubility of copper was decreased more strongly in STL3-1500 ° C than in STL3-1400 ° C

* Chemical composition of the raw sample

STL 6 SAMPLE Major elements Trace elements Oxides Content (%) Elements Content (ppm) SiO2 45.65 Ba 12000 Al2O3 10.41 Be 2.29 Fe2O3 14.53 Co 1449 MnO 0.54 Cr 6716 MgO 2.86 Cu 971 CaO 6.98 Ga 39 Na2O 7.41 Nb 49 K2O 1.61 Or 1957 TiO2 3.66 Rb 51 P2O5 0.96 Sc 12.8 CO2 0.50 Sr 386 S total 1.1 Th 12 V 181 Y 16 Zn 2577 Zr 208 Pb 558 CD 1 Sn 24 Ace 8.6 Hg 0.05
Mp 1010 ° C 1.47
Major (%) 97.68
Traces (ppm) 27219
Composition (%) 100.40

* Structural characterization of the raw sample

• 11 % de quartz SiO₂
• 17 % de magnétite Fe₃O₄
• 12 % d'augite (Mg, Fe, Ca).(Gi, Al)O₃
• 3 % d'hématite Fe₂O₃
• e de Soufre

The raw STL 6 sample is mainly composed of:

• 11% SiO₂ quartz
• 17% Fe₃O₄ magnetite
• 12% augite (Mg, Fe, Ca). (Gi, Al) WHERE
• 3% hematite Fe₂O₃
• e of Sulfur

• 23 à 24 % de Si   - 5,5 à 8 % de Al
• 7 à 8 % de Fe   - 2 à 2,5 % de Na
• 6 à 6,5 % de Ca   - 3 à 3,5% de Ti
• de l'ordre de 1 à 2 % pour K, Mg,Nn et P

dontThe rest, about 57%, is made up of the amorphous phase. The chemical composition of this phase glassy could be estimated using point spectroscopic analysis (Castaing electronic microprobe) either:

• 23 to 24% of Si - 5.5 to 8% of Al
• 7 to 8% Fe - 2 to 2.5% Na
• 6 to 6.5% Ca - 3 to 3.5% Ti
• 1 to 2% for K, Mg, Nn and P

whose

* Structural characterization of the remelted sample

• la hercynite Fe Al₂O₄
• l'ulvite, ou l'ulvöspinelle, Fe₂TiO₄
• un plagioclase

à 1400 °C : Le matériau n'est pas entièrement cristallisé. il reste encore approximativement 30 % de phase vitreuse.

• augmentation du pourcentage de magnétite Fe₃O₄
• apparition de cristaux d'ilménite
• la fayalite Fe _Z SiO₄
• feldspath (Na, Ca).(Al, Si)₃O₈

at 1500 ° C : It is fully crystallized and is composed of 3 phases

• hercynite Fe Al₂O₄
• ulvite, or ulvöspinelle, Fe₂TiO₄
• a plagioclase

at 1400 ° C: The material is not fully crystallized. approximately 30% of the glass phase remains.

• increase in the percentage of magnetite Fe₃O₄
• appearance of ilmenite crystals
• fayalite Fe _Z SiO₄
• feldspar (Na, Ca). (Al, Si) ₃O₈

* Leaching tests

• L'arsenic, le chrome, le cuivre, le nickel, le plomb et le zinc mobilisables pour l'échantillon brut se trouvent stabilisés dans les produits refondus surtout à 1500 °C où ils ne sont mêmes pas détectés dans les lixiviats.
• les fractions solubles des produits refondus sont de 1 à 8 fois plus faibles que pour l'échantillon brut.
• Les concentrations en chlorures et en sulfates chutent aussi brutalement pour devenir nulles ou pratiquement nulles dans STL6-1500°C.

Par ailleurs
   - le plomb est légèrement insolubilisé dans STL6-1400 °C alors qu'il l'est complètement pour STL6-1400 °C. Ce phénomène est observable aussi pour le cuivre et pour le plomb.If we compare the results of the leaching tests of the remelted products at 1500 ° C, at 1400 ° C with those of the raw sample, we notice:

• Arsenic, chromium, copper, nickel, lead and zinc which can be mobilized for the raw sample are stabilized in the remelted products, especially at 1500 ° C where they are not even detected in the leachate.
• the soluble fractions of the remelted products are from 1 to 8 times lower than for the raw sample.
• The concentrations of chlorides and sulphates also drop suddenly to become zero or practically zero in STL6-1500 ° C.

otherwise
- the lead is slightly insolubilized in STL6-1400 ° C while it is completely insoluble in STL6-1400 ° C. This phenomenon can also be observed for copper and for lead.

V.2.4. Tests on the STL 8 sample * Chemical composition of the raw sample

P.F. 1010°C   -1,11
Majeurs(%)   94,31
Traces(ppm)   46544
Composition (%)   98,96

Mp 1010 ° C -1.11
Major (%) 94.31
Traces (ppm) 46544
Composition (%) 98.96

* Structural characterization of the raw sample

The raw STL 8 sample is mainly composed of:
- 40% Fe₃O₄ magnetite

• 17 à 17,5 % de Si   - 6 à 6,5 % de Al
• 13,5 à 14 % de Fe   - 2,5 à 3 % de Na
• 8 à 8,5 % de Ca   - 4,5 à 5% de Ti
• de l'ordre de 1% pour Mg et P
• de l'ordre de 0,5 % pour K et Mn

The rest, about 60%, is made up of the amorphous phase. The chemical composition of this glassy phase could be estimated using point spectroscopic analysis (Castaing electron microprobe), namely:

• 17 to 17.5% of Si - 6 to 6.5% of Al
• 13.5 to 14% Fe - 2.5 to 3% Na
• 8 to 8.5% Ca - 4.5 to 5% Ti
• around 1% for Mg and P
• around 0.5% for K and Mn

* Structural characterization of the remelted sample

• la hercynite Fe Al₂O₄
• l'ulvite, ou l'ulvöspinelle, Fe₂TiO₄
• la fayalite Fe₂SiO₄
• un plagioclase

à 1400 °C : Le matériau n'est pas entièrement cristallisé. il reste encore approximativement 30 % de phase vitreuse.

• augmentation du pourcentage de magnétite Fe₃O₄
• apparition de cristaux d'ilménite FeTiO₃ et de pseudo-brookite Fe₂TiO₅
• un plagioclase à 50% d'anorthite

at 1500 ° C : It is fully crystallized and is composed of 4 phases

• hercynite Fe Al₂O₄
• ulvite, or ulvöspinelle, Fe₂TiO₄
• fayalite Fe₂SiO₄
• a plagioclase

at 1400 ° C: The material is not fully crystallized. approximately 30% of the glass phase remains.

• increase in the percentage of magnetite Fe₃O₄
• appearance of FeTiO₃ ilmenite crystals and Fe₂TiO₅ pseudo-brookite
• a plagioclase with 50% anorthite

* Leaching tests

The leaching tests were carried out for this sample on 3 recast products at 1550 ° C, 1400 ° C and 1300 ° C. A comparison of the leachate analyzes with those of the raw sample does not prove that there is an increase in the inertia of the raw slag after treatment. Indeed, the raw sample is initially stable. However, this fact must be hung up with the structure of this slag, consisting essentially of magnetite.

However, the solubility of chlorides and sulfates is reduced in proportion to the increase in temperature.

II Heat treatment of metal hydroxide sludge

The thermal treatment of metal hydroxide sludge must make it possible to sequester the polluting metals from the waste by inserting them into crystalline networks in the presence or to be synthesized.

The goal is to achieve, as in the case of slag, the composition and structure closest to a stable natural rock.

The process involves melting the mixture of sludge and appropriate mineral additions to produce, during slow cooling, an artificial rock. During this crystallization, the transition metals migrate from the vitreous material towards the phases during crystallization.

The principle is to use the elements already present in the waste and to make the additional additions to orient the complete crystallization

An originality of the process applied to sludge resides in the additives which could be the slag from incineration.

II.1 Characterization of sludge and mixture sludge / slag A. Chemical composition: 1. Sludge

These are hydrophilic mineral sludges, made up of aqueous suspensions of metal hydroxides themselves formed by precipitation of metals in solution with lime and then with sodium sulfide (Process for the depollution of industrial effluents loaded with metals). The basic composition of the five samples of hydroxide sludge collected daily on the site is shown in Table II.1 below:

The five samples show a certain consistency for the contents of the major elements, mainly calcium and phosphorus. Some variations should be noted in terms of sodium, iron, sulfur ...

All samples contain high levels of chromium (1.5 to 4%), zinc (0.6 to 4%), copper (0.1 to 1%), nickel (0.4 to 0, 8%), lead (from 830 to 1500 ppm), cadmium (from 22 to 202 ppm) and tin (from 0.1 to 3.7%).

2. Slag

Slag results from the incineration of industrial waste. They mainly contain incombustible mineral matter combined in multiple oxidized forms varying according to the incinerated waste.

ST8 slag was used; they are the richest in iron. Their quantometric analysis is indicated in Table II.2 in comparison with the sludge T2 and T5 used for the tests described here.

Slag contains more silicon, aluminum, iron and titanium than sludge, and significantly less phosphorus and calcium. The trace metal contents vary considerably from one sample to another. The ST8 sample is rich in chromium, copper and zinc.

3. ST8 Sludge / Slag Mix

Mixing was carried out with T2 and T5 sludge and increasing proportions of ST8 slag (10 to 50%).

Table II.3 shows the composition of these mixtures.

B. Structural composition of metal hydroxide sludge

The structure of the dried, crushed raw hydroxide sludge has a crystallized phase consisting of approximately 12% apatite Ca₅ (PO₄) ₃ (F, Cl, OH), 35% bassanite (CaSO₄, _1/2 H₂O ), the remainder being 53% consisting of an amorphous matrix.

For our tests, the sludge is dried in an oven at 105 ° C.

II.3: Results and discussion A. Reminder of the latest results

• amélioration globale de la tenue mécanique et de la stabilité du déchet au delà de 1000°C grâce à une cristallisation totale,
• diminution de la perte au feu,
• diminution de la fraction soluble (disparition quasi totale des chlorures et des sulfates à partir de 1380°C,
• Les résultats des tests de lixiviation selon la norme X31-210 sont rappelés dans les tableaux récapitulatifs II.4 et II.5, à savoir :
• diminution du COT
• concentrations en Zn, Ni, Cd dans les lixiviats inférieures aux seuils règlementaires définis dans l'arrêté du 30 mars 93 pour des essais effectués au delà de 1000°C.

The first conclusions concerning the heat treatment of sludge without additions are as follows:

• overall improvement in mechanical strength and stability of the waste beyond 1000 ° C thanks to total crystallization,
• decrease in loss on ignition,
• decrease in the soluble fraction (almost complete disappearance of chlorides and sulfates from 1380 ° C,
• The results of the leaching tests according to the X31-210 standard are recalled in summary tables II.4 and II.5, namely:
• decrease in TOC
• Zn, Ni, Cd concentrations in the leachate below the regulatory thresholds defined in the decree of 30 March 93 for tests carried out above 1000 ° C.

• libération de chrome VI proportionnellement à l'élévation de température,
• formation de ferrialuminates de calcium (ferrites).

On the other hand, the problems encountered were as follows:

• release of chromium VI in proportion to the rise in temperature,
• formation of calcium ferrialuminates (ferrites).

The addition of iron filings (1 to 5%) and ST8 slag (1 to 5%) seemed to contribute to the trapping of chromium in stable crystal lattices (Cf Table II.5).

These observations made it possible to foresee other experiments in order to favor the formation of spinels compared to ferrites.

Since the addition of slag is clearly more advantageous than iron filings in terms of economic feasibility, the process has been optimized by varying the contents of ST8 slag from 10 to 50% in the mixture. Table II.4: Effect of temperature on stabilization of sludge T2 and T5 Leaching test (afnor X31-210 standard) T2 T2-1000 ° C T2-1200 ° C T2-1380 ° C TOC (ppm) 1495 86 162 <63 Metals (mg / Kg DM) Zinc 17.1 552 <1.2 <1.2 Nickel 13.7-24.1 435 3-7 <6 Cr VI <0.8 0.3-0.5 2678 5976 Chlorides 11958 11920 1070 473 Sulphates 141580 73700 75350 626 T5 T5-1000 ° C T5-1200 ° C T5-1380 ° C TOC (ppm) 2724 35.5 129 <60 Metals (mg / Kg DM) Zinc 39.6 <1.2 <1.2 <1.2 Nickel 28-34 38 <6 <6 Cr VI <0.9 <0.3 2618 2536 Chlorides 8514 9940 872 147 Sulphates 96377 47900 58841 540 Effect of iron or ST8 slag additions on the stabilization of T2 and T5 sludge at 1380 ° C Leaching test (afnor X31-210 standard) T2-1380 ° C T2-1380 ° C-1% Fe T2-1380 ° C-5% Fe T2-1380 ° C-1% ST8 T2-1380 ° C-5% ST8 TOC (ppm) <63 168 174 133 166 Trace metals (mg / Kg DM) Zinc <1.2 <1.2 <1.2 <1.2 <1.2 Nickel <6 <6 <6 <6 <6 Cr VI 5976 322 192 765 1204 Chlorides 473 1030 890 620 530 Sulphates 626 83-108 <79 486 627 T5-1380 ° C T5-1380 ° C-1% Fe T5-1380 ° C-5% Fe T5-1380 ° C-1% ST8 T5-1380 ° C-5% ST8 TOC (ppm) <60 132 136 142 138 Trace metals (mg / Kg DM) Zinc <1.2 <1.2 <1.2 <1.2 <1.2 Nickel <6 <6 <6 <6 <6 Cr VI 2536 215 229 207 596 Chlorides 147 17 10-21 24 27 Sulphates 540 159-185 <79 418 150
Detection limits: Total Cr <0.200 mg / l, CrVI <0.010 mg / l, Ni <0.200 mg / l, Cd <0.040 mg / l, Pb <0.500 mg / l, Zn <0.040 mg / l, As <0.005 mg / l, Hg <0.0005mg / l, free CN <0.100mg / l.

The minimum slag concentration threshold to be used to prevent the solubilization of chromium VI is 30%.

It is still necessary to increase this quantity of slag up to 50% to favor the formation of spinels compared to ferrites.

B. Thermal behavior of samples 1. Loss of mass of sludge after heat treatment

The loss of mass of the treated samples is recorded in Tables II.6 and II.7 Table II.6: Evolution of the sludge mass loss as a function of the treatment temperature Weight loss in% 1000 ° C 1200 ° C T2 18.1 21.3 T5 18.2 19.3 Evolution of the sludge mass loss as a function of the ST8 slag concentration Weight loss in% 5% ST8 slag 30% ST8 slag 50% ST8 slag T2 / ST8 29.6 21.9 15.2 T5 / ST8 - 21.3 14.7

• Plus on augmente la température, plus la volatilisation d'éléments est importante,
• L'ajout de scories diminue significativement la perte de masse des échantillons.

We observe that:

• The higher the temperature, the higher the volatilization of elements,
• The addition of slag significantly reduces the loss of mass of the samples.

2. Differential thermal (ATD) and thermogravimetric (ATG) analysis

The differential thermal analysis made it possible to identify the reaction behavior of the two sludge samples T2 and T5 as well as the slag ST8. In the case of sludge, important "events" only come off at low temperatures (<500 ° C). At high temperature, there are two peaks which can possibly be attributed to partial fusions around 1100 ° C and around 1400 ° C. The two samples T2 and T5 behave almost identically.

In the case of slag, the start of the melting zone is around 1000 ° C and the end of melting around 1300 ° C.

In view of the ATDs, it would seem that the effective thermal zone for treating raw sludge or mixtures of sludge and slag is in the range 1200 ° C-1400 ° C. The problem is different if you make other types of additions.

Thermogravimetric analysis at 1500 ° C shows a mass loss of 30 to 40% for the 2 samples of raw sludge tested. In terms of slag, the loss of mass is low, of the order of 2%. Two areas of rapid mass loss should be noted from 20 ° C to 300 ° C and from 1200 ° C to 1500 ° C.

3. Multi-element analysis of the treated residues

In order to be able to quantify the elements which have volatilized, a multi-element analysis of the residues resulting from the heat treatment at different temperatures was carried out by ICP-MS spectrometry.

For the analysis of raw sludge treated alone, the results were as follows:

* At 1000 ° C

The results are illustrated in Table II.8. In terms of major elements, we see the total departure of CO₂, a drop in the K₂O, S and Cl contents.

With regard to heavy metals, mercury, lead, cadmium are almost completely volatilized. The tin and zinc contents decrease slightly.

* At 1200 ° C

Table II.9 confirms the total disappearance of CO₂, of around 50% of S and Cl (only for T2). There is also a decrease in Na₂O, K₂O (T2), and F. As for trace metals, with the total volatilization of Pb, Hg, and Cd, there is a decrease in the concentration of Cr, Cu (T2), Ni, Zn, Sn (T2), and As (T2). Variations should be noted in the two samples processed.

The first results at 1380 ° C relate only to the mixture of sludge with 30% slag (see Table II.10).

* At 1380 ° C

There is essentially a greater departure of Na₂O, K₂O and Cl. As regards the trace metals, Cd and Hg are volatilized. On the other hand, less Pb is released into the atmosphere than at 1200 ° C, which could be explained by its trapping. It is the same for Cr, Ni, Zn and As which are perhaps also partly integrated in crystalline phases.

C. Leaching tests

By comparison with the regulatory thresholds defined in the decree of March 30, 93 relating to hydroxide sludge and bottom ash from the incineration of special waste as well as the thresholds recommended for recovery in Public Works of bottom ash from the incineration of household waste, the results obtained on the sludge-slag mixtures (30 and 50%) heat treated at 1380 ° C are summarized in Table II.11.

• diminution du COT
• concentrations en métaux dans les lixiviats inférieures aux seuils, même relatifs à la valorisation en travaux publics
• et surtout, diminution considérable de la solubilité du chrome VI par rapport aux essais à 1380°C sur les boues brutes (de 2678-5976 ppm à 4,3-9,4 ppm).

In conclusion, it can be confirmed that the leachate has the following characteristics compared to the untreated controls:

• decrease in TOC
• metal concentrations in leachate below thresholds, even relating to recovery in public works
• and above all, considerable decrease in the solubility of chromium VI compared to the tests at 1380 ° C on raw sludge (from 2678-5976 ppm to 4.3-9.4 ppm).

A minimum of 30% slag in the sludge / slag mixture to be treated is required to insolubilize the chromium.

The best results are obtained with 50% slag added to the sludge. Table II.11: Effect of adding 30% and 50% slag on the stabilization of T2 and T5 sludge at 1380 ° C T2 / 30% sc. 1380 ° C T5 / 30% sc. 1380 ° C T2 / 50% sc. 1380 ° C T5 / 50% sc. 1380 ° C Standard (March 30, 94) TP mg / Kg MS standard TOC (mg / Kg) 184 161 26-66 22-62 <1500 - Trace metals (mg / Kg DM) Cadmium <0.15 <0.15 <1.2 <1.2 <25 <1 Total chrome 9.4 4.3 <6 <6 <50 - Chrome VI - - 0.7 <0.3 <5 <1 Nickel <1.5 <1.5 <6 <6 <6 <50 Lead <0.60 0.25-0.6 <15 <15 <50 <15 Zinc <1.5 <1.5 <1.2 <1.2 <250 - Arsenic <0.30 <0.30 <0.02 <0.02 Mercury <0.03 <0.03 <0.04 <0.06 <5 <0.2 CN free - - <0.3 <0.3 <5 - Chlorides - - 30 35 - - Sulphates - - 55 122 - <70,000
Detection limits: Total Cr <0.200 mg / l, CrVI <0.010 mg / l, Ni <0.200 mg / l, Cd <0.040 mg / l, Pb <0.500 mg / l, Zn <0.040 mg / l, As <0.005 mg / l, Hg <0.0005mg / l, free CN <0.100mg / l.

D. Structural study 1. Mineralogical analysis by light microscopy and by scanning electron microscopy (SEM).

The melting-crystallization cycle in the case of adding 50% slag leads to the development of spinels, apatite crystals and calcium ferri-aluminates.

Apatite, the ideal composition of which is Ca₅ (PO4) ₃ (F) but where the Ca ions are replaced to varying degrees by cations and PO₄, F by the anions SO₄, SiO₄ and CrO₄, are generally little colored under the microscope photonics, unlike fusion tests on raw sludge.

The sludge / slag mixtures (50% / 50%) lead to a phase with crystallochemical structure of spinel having the highest reflectance. The variations in the reflectance correlated with the chemical data by electron microprobe make it possible to establish that the core of the spinel crystal is enriched in chromium unlike the rims richer in Titanium.

By this observation process, the ferrites are also well highlighted: they are honey yellow skeletal crystals with very characteristic shapes.

In fact, fluoro apatite for the most part, and / or plagioclases (byttownite variety) fill the intersertal voids of spinel crystals exclusively.

Claims (18)

Waste treatment method, characterized in that it comprises the following stages: the heating of a mixture comprising the waste to be treated and one or more nucleating agents containing in particular ferrous compounds, said heating being carried out under conditions such that it makes it possible to obtain a mixture in the liquid phase also called mixture melting or "melt", of which at least 5% of the Fe₂O₃ oxides are transformed into FeO oxides; - controlled cooling of the "melt" until a solid material is obtained in which the heavy metals contained in the treated waste are stabilized within crystalline structures containing crystals of the spinel family; - recovery of the solid material obtained. Treatment method according to claim 1, characterized in that the heating step comprises a phase of stabilization of the mixture at a temperature allowing the complete liquefaction of the mixture to be treated. Treatment method according to claim 1 or claim 2, characterized in that the treated waste is slag resulting from the incineration of industrial or domestic waste. Treatment process according to any one of Claims 1 to 3, characterized in that the nucleating agents are ferrous compounds such as iron filings. Treatment process according to any one of claims, characterized in that the waste to be treated is sludge of metal hydroxides and in that the mixture to be heated comprises the above-mentioned sludges of metallic hydroxide and, as agents nucleation, slag resulting from the incineration of industrial or household waste. Treatment process according to claim 5 characterized in that the mixture to be heated comprises between approximately 70% and 30% of hydroxide sludge and between 30% and 50% slag by weight of the mixture. Treatment process according to any one of Claims 1 to 6, characterized in that approximately 20% to 50% of spinels are recovered at the end of the treatment. Treatment process according to any one of Claims 5 to 7, characterized in that the slag is replaced by a ferrous compound up to 5% by weight of the mixture. Treatment method according to any one of claims 1 to 8, characterized in that the heating step is carried out under conditions such that it allows a) the liquefaction of the magnetite contained in the slag and the release of the oxides Fe₂O₃, and b) the reduction of the Fe₂O₃ oxide formed, into FeO oxide by addition of agents such as iron. Treatment process according to any one of Claims 1 to 9, characterized in that the temperature of the mixture to be treated by heating is brought to about at least 1450 ° C, preferably up to about 1500 ° C. Treatment method according to any one of claims 1 to 10, characterized in that the ferrous compound is iron. Treatment process according to any one of Claims 1 to 11, characterized in that the ferrous compound is iron filings in a proportion greater than 0.5% by weight of the mixture to be treated by heating, preferably 1% by weight of the mixture to be treated. Process according to any one of Claims 1 to 12, characterized in that the agents nucleation represent from about 0.5% to about 15% in percentage expressed by weight of the mixture subjected to the heating step. Treatment process according to any one of Claims 1 to 13, characterized in that the waste to be treated is slag mainly containing the following oxides in a proportion given as a percentage relative to the total weight of the waste: If 26.25 to 45.65 Al₂O₃ 7.43 - 17.98 Fe₂O₃ 14.53 - 34.92 MnO 0.54 - 1.88 MgO 1.64 - 2.86 CaO 6.41 - 10.16 Na₂O 2.66 - 10.82 K₂O 0.56 - 1.61 TiO₂ 3.66 - 7.69 WHERE 0.96 - 2.54 CO₂ 0.33 - 4.32 S total 0.03 - 2.53 Treatment process according to any one of Claims 1 to 13, characterized in that the waste to be treated is slurry of metal hydroxides mainly containing the following oxides in a proportion given as a percentage relative to the total weight of the waste:

Treatment method according to any one of claims 1 to 14, characterized in that the heating step is carried out on slag having a particle size less than 10 mm, preferably less than 2 mm. Treatment process according to any one of Claims 1 to 16, characterized in that the slag is homogenized with the nucleating agents, in particular with the ferrous compound (s). Treatment process according to any one of claims 16 or 17, characterized in that the mixture of slag and ferrous compound (s) is porphyrized so as to obtain a particle size less than 10 µm.

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