Method for preparing lightweight aggregate

Abstract

The present invention relates to a method for preparing a clay-based lightweight aggregate, which is suitable for use in civil engineering, particularly road applications and construction. The present invention relates to a method for preparing a lightweight aggregate, which comprises at least the following steps: a) granulating a clay-based mixture to obtain an aggregate, b) drying the obtained aggregate to obtain a dried aggregate, c) heat treating the dried aggregate, which comprises two successive substeps: i) a first heat treatment substep carried out in a reducing atmosphere at a temperature T1 between 900 and 1200°C, ii) a second heat treatment substep carried out in an oxidizing atmosphere at a temperature T2 between 1050 and 1300°C, and d) cooling the aggregate.

Description

[Technical field]

[0001] The object of the present invention is a method for preparing a lightweight clay-based aggregate, said aggregate being particularly suitable for use in public works, in particular road applications, and in construction. [Background technology]

[0002] Industrial waste, especially sludge generated in municipal or industrial wastewater treatment plants, contains organic matter, minerals including some fractions of clay fines, metals and toxic pollutants. This waste is in very large quantities and many methods are being directed towards its treatment and reclamation.

[0003] Thus, there is a continuing need to develop methods to recycle industrial waste while producing materials of interest for industrial or other uses.

[0004] Recycling of waste materials, particularly sludge and industrial by-products, means that the waste materials are considered inert, i.e. they do not decompose, burn, undergo physical or chemical reactions, or degrade other materials with which they come into contact in a manner that is harmful to the environment or human health.

[0005] European standard NF X30-402-2 specifies conformity tests that provide information on the leaching of fragmented waste and sludge under defined experimental conditions. This European standard concerns the compliance of waste characterization, especially the leaching of fragmented waste and sludge. The leachable fraction of many elements or compounds must therefore not exceed certain concentration thresholds set by current regulations, in particular the guide values ​​set by the Center for Studies on Risks, the Environment, Mobility and Urban Planning (CEREMA). As an indication, the permissible limits set in the Regulation on Inert Waste Storage Facilities (ISDI) (Annex II to the Order of December 2014) are given in Table 1 below, in mg / kg of dry matter.

[0006] [Table 1]

[0007] EP 1,571,135 describes a method for the preparation of expanded clay aggregates containing a fraction of material derived from organic sludge. The method comprises a first heat treatment step carried out at a temperature between 500 and 750 °C, allowing the removal of organic matter and the volume expansion of the aggregate, and a second heat treatment step carried out at a temperature between 900 and 1200 °C, which allows, on the one hand, the final expansion of the volume of the aggregate and, on the other hand, the obtaining of a definitive cohesion of the aggregate due to the eutectic effect between the respective mineral fractions of the clay and the sludge. However, this method is not entirely satisfactory in that it does not produce aggregates that comply with current regulations, in particular since the leachable fractions of some elements or compounds obtained from aggregates prepared according to this method exceed the concentration thresholds set in the standard NF X30-402-2.

[0008] There is therefore a need for a method for preparing lightweight aggregates from clay-based mixtures. By "clay" is meant clay minerals. By "clay-based mixtures" is meant clay products consisting mainly of waste materials from water treatment, waterway and harbor dredging, and municipal engineering and public works. There is a need for a method that allows overcoming the shortcomings of the prior art and that obtains aggregates that are acceptable, especially from a regulatory point of view. Summary of the Invention [Problem to be solved by the invention]

[0009] Therefore, a first object of the present invention is a method for preparing lightweight aggregates, comprising at least the following steps: a) granulating the clay-based mixture to obtain aggregate; b) drying the obtained aggregate to obtain a dried aggregate; c) heat treating the dried aggregate, i) a first heat treatment substep carried out in a reducing atmosphere at a temperature T1 of about 900 to 1200 ° C; ii) a second heat treatment substep carried out in an oxidizing atmosphere at a temperature T2 of about 1050-1300°C; A process including two consecutive sub-processes: d) Cooling the aggregate. [Means for solving the problem]

[0010] According to a particular embodiment of the invention, each of the heat treatment steps is carried out in a rotary furnace, and preferably the residence time of the product in the furnace is adjustable by controlling the rotation speed of the furnace and, consequently, the drive speed of the aggregate.

[0011] According to the method of the invention, the above two heat treatment sub-steps, carried out in this order, surprisingly make it possible to remove a very large part of the leachable fraction of many chemical elements or compounds, so as to obtain aggregates in which the concentrations of a very large part of these chemical elements and compounds in the aggregate are below the concentration thresholds set by regulations. In fact, the first heat treatment sub-step, carried out in a reducing atmosphere, makes it possible to extract the heavy metals contained in the matrix under the given temperature conditions, since they are first reduced and then extracted into the gas phase. Similarly, the decomposition of sulfates is only possible in the above temperature range under a reducing atmosphere. The second sub-step makes it possible to block some metal contaminants in the mineral matrix as insoluble compounds, such as spinel-type compounds.

[0012] The method according to the invention has the advantage of producing lightweight aggregates from materials containing metallic and organic contaminants. "Organic contaminants" and "metallic contaminants" respectively mean organic or metallic elements that are harmful to the environment or human health when present in amounts above the concentration thresholds set in the standard NF X30-402-2.

[0013] The process according to the invention therefore produces aggregates from which contaminants have been removed. Indeed, the implementation of the process according to the invention results in the destruction of organic matter, the thermal destruction of pathogenic agents such as bacteria and viruses, the extraction of heavy metals such as mercury, cadmium, lead and zinc, and the partial or total decomposition of sulfates and carbonates. The implementation of the process does not result in the entrapment of the abovementioned contaminants in the aggregates.

[0014] In particular, the aggregates prepared by the method of the invention have the great advantage, compared to non-thermally treated aggregates, of being decontaminated, i.e. recyclable as a raw material replacement according to current standards, with respect to at least one, and preferably several, of the following elements: - metallic arsenic (As), barium (Ba), cadmium (Cd), total chromium (Total Cr), copper (Cu), mercury (Hg), molybdenum (Mo), nickel (Ni), lead (Pb), antimony (Sb), selenium (Se), zinc (Zn), and / or - Chlorides, fluorides, sulfates, and / or - phenol index.

[0015] Extraction efficiency can reach 99% effectiveness depending on the contaminant.

[0016] The method according to the invention makes it possible to reduce the concentration of pollutants in the leachate by at least 95% compared to its initial concentration. In particular, the concentration is reduced as follows: Copper and copper compounds are 99.6% Nickel is 97.6% Zinc is 99.6% - 99.9% of organic compounds - Sulfates: 95.9%.

[0017] In particular, the aggregates prepared by the method of the invention are desulfated. The level of decontamination of the aggregates for the above elements can be demonstrated by leaching tests carried out according to standard NF X30-402-2 at L / S=10 L / Kg. The aggregates prepared by the method of the invention also have the great advantage, compared to non-thermally treated aggregates, of being decontaminated, i.e. practically reusable, for at least one, and preferably several, of the following elements: - Total organic carbon (TOC), - Benzene, Toluene, Ethylbenzene and Xylene (BTEX), - 7 polychlorinated biphenyl congeners (PCBs), - Hydrocarbons (HCT), and - Polycyclic aromatic hydrocarbons (PAHs).

[0018] The total of the various contaminants mentioned above can amount to 30% by weight of the clay-based mixture before treatment.

[0019] The level of decontamination of the aggregates, and therefore the effectiveness of the treatment obtained by the method of the invention, for the abovementioned elements can be demonstrated by leaching tests on raw waste, as described in standard NF-X-30-402-2.

[0020] The chemical and physical properties of the aggregates obtained by the method of the invention make it suitable for the preparation of materials intended for various applications, in particular in the fields of public works and construction.

[0021] The method according to the invention has the advantage that it allows the production of lightweight aggregates from clay-based mixtures containing 5% to 30%, 10% to 30%, or 20% to 30% by mass of contaminating elements.The method according to the invention further has the advantage that it allows the preparation of lightweight aggregates from clay-based mixtures containing a large proportion of industrial sludges and industrial by-products, thus allowing the recycling of large amounts of industrial waste.

[0022] The method according to the invention has the advantage of a potential energy saving. Indeed, according to a particular implementation of the method of the invention, the energy required to dry the aggregates can be recovered during the method, for example when the aggregates are cooled after the thermal treatment. Moreover, according to a particular embodiment of the method of the invention, the first thermal treatment substep, which is carried out in a reducing atmosphere, is autothermal. Indeed, in this embodiment, the required energy is provided by the decomposition and partial combustion of the organic compounds present in the clay-based mixture. The synthesis gas thus produced is pumped into a recovery boiler, which ensures the complete combustion of the gases and the energy recovery. The energy is recovered in the form of hot air or steam, depending on the needs of the process.

[0023] Furthermore, the temperature difference between the first and second heat treatment steps may be small, which helps limit the fossil energy consumption of the method compared to other processing methods.

[0024] In the method of the invention, the heat treatment step under reducing conditions also makes it possible to significantly reduce the volume of fumes treated compared to state-of-the-art methods, allowing the recycling of the fumes for other applications.

[0025] Furthermore, compared to prior art processes, the heat treatment step under oxidizing conditions requires less gas; the amount of natural gas required is approximately one-tenth of that required in conventional processes for producing aggregates from noble clays, which contributes to a reduced carbon dioxide footprint of the process.

[0026] Finally, the process according to the invention is carried out in stages, since the different reactors can be separated, so that each step can be optimized completely independently, whether in terms of temperature, atmosphere or residence time of the aggregate in the reactor.

[0027] By "lightweight aggregate" is meant aggregate having a density of less than 1, preferably between 0.6 and 0.95, more preferably about 0.8. The aggregates prepared by the method of the present invention are porous, hard and resistant.

[0028] The aggregates produced by the process of the present invention are recyclable in the construction industry, public works, landscaping and agriculture.

[0029] Other advantages and features of the method according to the invention will become apparent from a consideration of the detailed description of the invention and various embodiments thereof, which are given by way of illustration and do not limit the scope of the invention. When a range of values ​​is given, the lower and upper values ​​are included. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] The method according to the invention comprises a first step a) of granulating a clay-based mixture, said clay-based mixture consisting of a mixture of different materials, each material containing a certain proportion of clay and organic matter, said materials being homogeneously mixed by techniques known to those skilled in the art.

[0031] Materials that can be used in the method of the present invention include: - so-called "inert" clays, in particular from clay extraction sites, and / or clays present in sludges and clay materials originating from industrial by-products, preferably selected from clay deposits from dredging, fines from the washing of contaminated soils, filter cakes from liquid waste treatment after centrifugation or passing through filter presses, sludges from wastewater treatment plants (WWTP sludges), sludges from municipal wastewater treatment plants (UWWTP).

[0032] The clay-based mixtures usable in the process according to the invention in particular comprise 10-90% clay, expressed by weight relative to the weight of dry matter. In the clay-based mixtures usable in the process according to the invention, the clay is present, on the one hand, due to the presence of so-called "inert" clays and / or, on the other hand, due to the presence of at least one contaminant, which comprises clay. More specifically, the clay-based mixtures usable in the process according to the invention comprise 10-90% clay, expressed by weight relative to the weight of dry matter.

[0033] In step a) of the method according to the invention, the preparation of the clay material likely to be incorporated into the clay-based mixture may in particular comprise the preparation of a clay matrix in a plastic state by incorporating a sufficient amount of liquid to obtain a moisture content of 30% to 50%, preferably 40%. This liquid is preferably water, but can also be chosen from industrial water, wastewater or leachates. According to a particular embodiment of the invention, one or more additives in liquid or solid form may also be added to the clay material.

[0034] In the method according to the invention, additives which may be added are intended to accelerate some chemical reactions or to improve the mechanical properties of the lightweight aggregate. Such additives are, for example: - Barium carbonate (to neutralize sulfates), - Fixed carbon (allows the extraction of some heavy metals and enhances the reducing properties of the atmosphere) It is.

[0035] The organic matter incorporated in the clay-based mixture used in step a) of the method according to the invention is derived in particular from a material selected from sludge and industrial by-products, preferably from sludge from wastewater treatment plants (WWTP sludge) or from sludge from municipal wastewater treatment plants (UWWTP).

[0036] According to a more specific implementation of the method according to the invention, the clay-based mixture comprises: (i) 10-25%, preferably 20%, of an inert clay material; and (ii) at least one material obtained from sludge and / or industrial by-products, said material being: - Organic sludge, such as sewage treatment plant sludge, - dredging sediments, - filter cake, and - combinations of the above materials, is selected from The material has been previously cleaned of foreign objects such as stones, shells, wood chips, etc.

[0037] Even more specifically, in the implementation of the method according to the invention, a clay-based mixture is obtained by mixing 20% ​​inert clay, 40% clayey dredged soil and 40% WWTP sludge, said WWTP sludge containing up to 40% organic matter, i.e. between 1% and 40%, preferably between 20% and 30%. In this particular case, the organic matter content of such a clay-based mixture is of the order of 8-12%.

[0038] The clay-based mixtures usable in the process according to the invention preferably contain, expressed as a percentage of the total dry weight, between 5% and 40% by weight of organic dry matter, more particularly between 10% and 30% by weight of organic dry matter. By "organic dry matter" is meant carbon or nitrogen compounds which, when heated to high temperatures, give rise to off-gases, the release of which contributes to the particular porosity of the material obtained.

[0039] The organic material is also preferably free of any foreign matter, in particular stones, wood chips and plastic chips. The material is then preferably crushed and mixed to obtain a homogeneous mixture. One or more additives in liquid or solid form can also be added to the organic material.

[0040] In the method according to the invention, said organic material may optionally consist of sludges and industrial by-products of different origin, which are then mixed or combined together.

[0041] The preparation of a clay-based mixture usable in step a) of the method according to the invention comprises homogeneously mixing the materials. The moisture content of the clay-based mixture can be adapted by adding a suitable amount of liquid to obtain a moisture content of preferably 30% to 50%, preferably about 40%. In particular, the moisture content of the mixture can be adjusted by adding a suitable amount of water, industrial water or leachate. One or more additives in liquid or solid form can also be added to the clay-based mixture.

[0042] According to a particular embodiment of the method of the present invention, the method does not involve the addition of any additives.

[0043] According to a particular embodiment of the method of the present invention, step a) of granulating the clay-based mixture comprises grinding and shaping the clay-based mixture to obtain a homogeneous mixture. The mixing, grinding and shaping of the raw materials is preferably carried out in one and the same device. According to a preferred embodiment of the method of the present invention, the clay-based mixture comprises 10-25%, preferably about 20%, by weight relative to the total weight of dry matter, of inert clay.

[0044] Thus, according to a preferred embodiment of the method of the present invention, step a) of granulating the clay-based mixture comprises mixing the clay with at least one material as described above, grinding and shaping the mixture to obtain a homogeneous mixture.

[0045] Granulation may be carried out by any means known to those skilled in the art, in particular by extrusion or by passing over a pelletizing disc.

[0046] Then, in step b) of the process according to the invention, the aggregates are dried, preferably to a moisture content of less than 20%. Drying step b) can be carried out by any means known to the skilled person, preferably at low temperatures, i.e. at temperatures below 250° C., to avoid the release of organic elements and the bursting of the aggregates. The drying step reduces the moisture content of the aggregates and increases their hardness. The drying step can be carried out in a suitable dryer. The energy required for drying may be obtained from the calorific values ​​recovered in the cooling step d) after the thermal treatment of the aggregates, for example by a direct heat exchanger.

[0047] Thus, according to a particularly preferred embodiment of the method of the invention, said method further comprises recovering calories from cooling the aggregate in step d).

[0048] After the drying step b), the aggregate is subjected in step c) to a heat treatment according to the method of the invention in two successive sub-steps at appropriate temperatures and under defined conditions.

[0049] This heat treatment allows for the extraction of heavy metals, the production of syngas, the decomposition of sulfates and carbonates, the destruction of pathogens, the creation of porosity, and partial vitrification.

[0050] The first heat treatment substep, or pyrolysis, involves treating the aggregate in a reducing atmosphere at a temperature T1 of about 900-1200°C.

[0051] By "reducing atmosphere" is meant an atmosphere that is free of oxygen and that contains a gas selected from carbon monoxide, volatile hydrocarbons, hydrogen, or a combination of these gases. A reducing atmosphere can be obtained, for example, by substoichiometric combustion of the organic compounds present in the mixture with air.

[0052] The first heat treatment substep may be carried out by any suitable means known to the person skilled in the art. It is in particular carried out in a furnace, for example a rotary kiln, in which the atmosphere is reducing. It is preferably carried out under substoichiometric conditions. It results in the production of a synthesis gas rich in carbon monoxide (CO) and volatile hydrocarbons (CxHy) and free of oxygen.

[0053] During the first heat treatment substep, heavy metals such as mercury (Hg), cadmium (Cd), zinc (Zn), and lead (Pb) are partially or totally reduced and volatilized.

[0054] These metals are in gaseous form in the synthesis gas produced during the reaction. The main sulfates are decomposed and sulfur and chlorine compounds are extracted into the gas phase.

[0055] The conditions used in this first heat treatment sub-step optimize the amount of synthesis gas produced. The synthesis gas produced in this first sub-step can be directed, inter alia, to a boiler, an engine or another device. The synthesis gas from the heat treatment step in an oxidizing atmosphere can be used to start the reaction in the heat treatment step in a reducing atmosphere.

[0056] The temperature T1 of the first heat treatment sub-step is about 900 to 1200°C, preferably about 950 to 1200°C, more preferably about 1050 to 1150°C, and even more preferably about 1110 to 1150°C.

[0057] This temperature range makes it possible in particular to extract the sulfur compounds without oxidizing them to sulfates.

[0058] The exact temperature of the first heat treatment sub-step will depend on the composition of the clay-based mixture and any synthesis gas that may be injected into the enclosure in which the heat treatment takes place.

[0059] The duration of the first heat treatment sub-step is generally about 30 to 150 minutes, preferably about 60 to 120 minutes, and more preferably about 120 minutes.

[0060] The second heat treatment substep consists of combustion in an oxidizing atmosphere at a temperature T2 of approximately 1050-1300°C.

[0061] An "oxidizing atmosphere" refers to at least one oxidizing agent, preferably oxygen (O 2 An oxidizing atmosphere is obtained, for example, by the excess stoichiometric combustion of methane with air, resulting in an atmosphere with an oxygen percentage greater than 3%.

[0062] The second heat treatment substep can be carried out by any suitable means known to the person skilled in the art. The second heat treatment substep is in particular carried out in a rotary kiln.

[0063] The temperature T2 of the second heat treatment sub-step is about 1050 to 1300°C, preferably about 1050 to 1150°C, more preferably about 1110 to 1150°C, and even more preferably about 1125°C.

[0064] This second heat treatment substep completes the decomposition of the carbonates and the complete combustion of the organic compounds, and metals such as iron (Fe), nickel (Ni) and chromium (Cr) may further react with each other to form insoluble spinel compounds. The decomposition of the organic compounds, sulfates and carbonates results in a porous structure that produces a lightweight aggregate, i.e., an aggregate having a density preferably less than about 1. The high temperature of the reaction results in the ceramification and partial vitrification of the material, which imparts hardness and mechanical strength to the aggregate.

[0065] The exact temperature of the second heat treatment sub-step depends on the melting point of the aggregate, which in turn depends on the chemical composition of the clay-based mixture.

[0066] The duration of the second heat treatment sub-step is generally about 30 to 150 minutes, preferably about 60 to 120 minutes, and more preferably about 60 minutes.

[0067] According to a particular embodiment of the method of the invention, the temperature T1 is less than or equal to the temperature T2.

[0068] According to a more particular embodiment of the method of the present invention, T1 is equal to T2.

[0069] After the heat treatment of step c), aggregates are obtained whose density is generally about 0.6-1 and whose particle size is 1-15 mm.

[0070] According to a particular embodiment of the method of the invention, the combustion gases generated in the second heat treatment sub-step are injected into the enclosure in which the first heat treatment sub-step takes place.

[0071] In step d), the aggregates thus obtained can be cooled by any known and suitable means, in particular by introduction into a cooler, said cooler operating under ambient air. This step makes it possible to cool the aggregates to a temperature below about 100° C. This cooling step also allows the recovery of calories, since the transfer of calories from the aggregates to the ambient air warms this air from a temperature of about 15° C. to a temperature of about 250° C. The hot air thus produced can be recovered, in particular for use in a step of drying the aggregates prior to the heat treatment step c).

[0072] The implementation of the method of the invention thus results in lightweight aggregates characterized by a density of less than 1. Said aggregates are also characterized by their spherical appearance and by their hardness. Said aggregates are also characterized by no or low levels of release of organic and / or metallic pollutants, or at levels corresponding to parameters defined for recycling, for example in road technology or construction materials, when carrying out a leaching test according to standard NF X30-402-2, compared to aggregates taken before the thermal treatment in the two sub-steps of the method of the invention.

[0073] The lightweight aggregates obtainable by the method of the invention are of particular interest for the production of materials such as drainage materials, snow removal substrates, sand, insulating materials, insulating lightweight blocks, rooftop greening, etc. The lightweight aggregates obtainable by the method of the invention are of particular interest for the production of construction materials such as lightweight concrete.

[0074] An object of the invention is therefore also a material comprising aggregate obtainable by the method according to the invention.

[0075] A second object of the invention is the use of the aggregates obtained by the process of the invention in the construction, public works, gardening or agricultural industries.

[0076] More specifically, one object of the present invention is the use of the aggregate obtained by the method of the present invention for road applications, preferably selected from first road applications, second road applications and third road applications.

[0077] The first road use is the use up to 3 meters high as the subbase of a road or shoulder of a paved road structure, in particular as embankment, subbase, superbase and base (road) under the structure. The second road use is the use up to 6 meters in technical embankments related to road infrastructure or on shoulders, insofar as the use is within the road structure in question. The second road use also includes the use with a thickness of more than 3 meters and a height up to 6 meters in the subbase of a road or shoulder of a paved road structure. The third road use is not subject to any constraints regarding the mounting thickness. The third road use is in particular the use as the subbase or shoulder of a roadway, for example, for the construction of tracks at construction sites, forest roads or towpaths. EXAMPLES

[0078] Example 1: Preparation and characterization of pre-mixed products Three clay-based mixtures containing additional sludge were prepared in the following proportions: WWTP sludge: 40%; dredged sand, 20%; filter cake, 20%; inert clay, 20%.

[0079] A leaching analysis was performed for each product intended for incorporation into the blend, and Table 2 below summarizes the results of this analysis and the extrapolated values ​​for the blend.

[0080] [Table 2]

[0081] The results of the analysis of untreated waste leachate show that some of the guide values ​​set out in the standard are exceeded.

[0082] Example 2: Baking test The various components were first weighed out and then mixed in a mixer. The prepared clay-based mixture (weight about 30 kg) was mixed with H 2 The mixture was air dried to a moisture content of about 20%, expressed as % of O. Moisture measurements were performed in a Mettler-Toledo infrared dryer. 500 grams of the above base mixture was then ground to a fine powder in a blender type mixer. Unwanted elements such as stones, shells, wood chips, etc. were sieved out. The resulting product was mixed again in a mixer and any type of additive selected was added. With continued mixing, the product was hydrated to obtain a mass for producing aggregate. The resulting aggregate was then dried in an electric oven at 120°C for 24 hours. After drying, 500 grams of the sample was subjected to a two-stage heat treatment.

[0083] In the preliminary tests, the samples were subjected to two treatment steps successively: a first calcination at 700 °C for 20-30 minutes, followed by a heat treatment at 1075 °C for 60 minutes. After carrying out the heat treatment, the beads were cooled in air. The leaching tests were carried out according to the NF X30-402-2 (NF EN12457-2) standard. Chemical analyses were then carried out according to the recommendations stated in the said standard. The results obtained before and after the treatment were compared. The average results are shown in Table 3 below.

[0084] [Table 3]

[0085] The mean values ​​obtained in the analyses carried out after treatment exceed the thresholds permitted by ISDI and are therefore not recyclable. In particular, high concentrations of Mo, organic carbon and sulfate were found in the leachate.

[0086] Example 3: Samples treated according to the invention, first series of tests A clay-based mixture was prepared as described in Example 1. An aggregate was then prepared as described in Example 2. The resulting aggregate was then dried in an electric oven at 120°C for 24 hours. After drying, 500g samples were subjected to a two-step heat treatment. Controlled atmosphere testing was carried out in a tube furnace in a closed quartz tube. Pure gases (air, CO, CO) from Air Liquide were used if necessary. 2 , N 2 The atmosphere in the furnace was reconstituted with a mixture of 1,000 mL of 1 ...

[0087] The leaching tests were carried out according to the standard NF X30-402-2 (NF EN12457-2). Chemical analyses were then carried out according to the conditions referenced in the standard NF X30-402-2. The results obtained were compared with the permissible thresholds of the CEREMA Guide for the Reuse of Alternative Materials in Road Engineering, which defines three types of applications depending on the limit values ​​obtained for the leachate.

[0088] Road materials that can be used for road use of type 1, 2 or 3 are those whose composition includes alternative materials that comply with the limit values ​​for type 1, 2 or 3, respectively. Table 4 below shows the permissible values ​​for road use of type 1, 2 or 3, determined in a leaching test according to standard NF X30-0402-2. These values ​​are expressed in mg / kg of dry substance. The results of the values ​​obtained in the leaching test of untreated samples without additives or heat treatment are given in the column "Test 0" in Table 4 below.

[0089] [Table 4]

[0090] The above results show that the resulting mixture has many guide value exceedances (numbers in bold) and cannot be recycled as is.

[0091] Example 4: Samples treated according to the invention, first series of tests In this example, a base mixture was prepared in the form of an aggregate according to example 2, according to the procedure described in example 1, and heated as described in example 3, with the addition of additives in tests 4 and 6. This mixture was subjected to a heat treatment comprising a first sub-step carried out in a reducing atmosphere followed by a second sub-step carried out in an oxidizing atmosphere (tests 4 to 6).

[0092] Test 4 was performed on the untreated mix in a mixed (oxidizing / reducing) atmosphere. Test 5 was performed on the untreated mix in a mixed (oxidizing / reducing) atmosphere with 5% BaCO3 added. Test 6 was performed on the untreated mix in a mixed (oxidizing / reducing) atmosphere with 5% reducing agent (carbon) added. Table 5 below shows the mixes produced (with or without additives) and the sequence used during heat treatment.

[0093] [Table 5]

[0094] The results are shown in Table 6 below.

[0095] [Table 6]

[0096] Comparing the results to the untreated sample shows that metals, except for molybdenum, are not leached after treatment, regardless of the treatment method.

[0097] Total organic carbon dropped from a concentration of 9600 mg / kg to less than 20 mg / kg after treatment. The concentration of sulfates in the leachate, divided by 6, reached an average of 4600 mg / kg. However, this value is still too high for the purposes of the present study. The lowest sulfate concentration was obtained in test no. 5 (2400 mg / kg). This result was obtained for an untreated sample (without additives) with a thermal treatment sequence of 60 min at 1075 °C in a reducing atmosphere, followed by 30 min at the same temperature in an oxidizing atmosphere. Compared to tests 4 and 5, it can also be seen that the addition of additives to the base mixture did not have a positive effect on the quality of the treatment.

[0098] Example 4: Samples treated according to the invention, second series of tests In this example, the base mixture was first treated in a reducing atmosphere and then heat treated in an oxidizing atmosphere as described in Example 1. The temperatures and residence times varied depending on the test and are listed in Table 7 below.

[0099] [Table 7]

[0100] Table 8 below summarizes the analysis obtained on leaching of the products after treatment.

[0101] [Table 8]

[0102] The results obtained confirm that no metals, except molybdenum, are leached after treatment. Total organic carbon (TOC) was reduced from a concentration of 9600 mg / kg to less than 10 mg / kg after treatment. The concentration of sulfates in the leachate reached an average of 2300 mg / kg after treatment, divided by 12. In tests 11 and 12, the sulfate concentration was reduced to less than the threshold value of 1300 mg / kg, which is the current regulation for recycling into type 3 road material and is the strictest limit. These results were obtained with a residence time of 120 minutes in a reducing atmosphere and 60 minutes in an oxidizing atmosphere. The optimal temperature range is considered to be 1110-1125 °C.

[0103] The implementation of a protocol including a two-stage heat treatment, alternating a first stage heat treatment in a reducing atmosphere immediately followed by a heat treatment in an oxidizing atmosphere, has resulted in a significant improvement in efficiency compared to no treatment or, in particular, compared to state-of-the-art treatments such as those described in EP-A-1,571,135.

[0104] In particular, in the temperature range of 1110-1125°C, the sulfate conversion efficiency allowed sulfate levels in the leachate to be below the current threshold for aggregate recycling in road engineering.

[0105] This study shows that the addition of various reagents has no effect on the treatment quality. In particular, the addition of barium carbonate had no effect on the treatment of sulfate in the tests.

Claims

1. A method for preparing lightweight aggregate, comprising at least the following steps: a) granulating a clay-based mixture to obtain an aggregate; b) drying the obtained aggregate to obtain a dried aggregate; c) heat treating the dried aggregate, i) a first heat treatment substep carried out in a reducing atmosphere at a temperature T1 between 900 and 1200°C; ii) a second heat treatment substep carried out in an oxidizing atmosphere at a temperature T2 between 1050 and 1300°C; a step including two successive substeps of d) cooling the aggregate; A method for preparing lightweight aggregate, comprising:

2. The clay-based mixture comprises: (i) 10-25%, preferably 20%, of an inert clay material; and (ii) at least one material derived from sludge and / or industrial by-products, said material comprising: - organic sludge, such as sewage treatment plant sludge, - dredged soil, - filter cake, or - combinations of the above materials, is selected from The material has previously been cleaned of any foreign matter.

2. The method of claim 1.

3. 3. The method of claim 2, wherein step a) of granulating the clay-based mixture comprises mixing the clay with at least one material obtained from sludge and / or industrial by-products, and grinding and shaping the mixture to obtain a homogeneous mixture.

4. The method of claim 1 , wherein the clay-based mixture further comprises at least one additive.

5. 2. The method of claim 1, wherein the drying step b) is carried out at a temperature below 250°C.

6. 10. The method of claim 1, further comprising recovering calories from the cooling of the aggregate during step d).

7. 7. The method of claim 6, wherein the energy required for the drying step b) is obtained at least in part from the calories recovered during the cooling step d).

8. 2. A method according to claim 1, characterized in that the first heat treatment substep is carried out at a temperature T1 between 1050 and 1150°C, preferably between 1110 and 1150°C.

9. 9. A method according to claim 8, characterized in that the first heat treatment substep is carried out for a period of 30 to 150 minutes, preferably 120 minutes.

10. 2. The method of claim 1, wherein the first heat treatment substep is carried out in a rotary furnace in which the atmosphere is reducing.

11. 2. A method according to claim 1, characterized in that the second heat treatment substep consists of oxidative combustion at a temperature T2 between 1050 and 1150°C, preferably between 1110 and 1150°C, more preferably about 1125°C.

12. 12. A method according to claim 11, characterized in that the second heat treatment substep is carried out for a period of 30 to 150 minutes, preferably 60 minutes.

13. 2. A method according to claim 1, characterized in that said temperature T1 is less than or equal to said temperature T2, preferably T1 is equal to T2.

14. Use of aggregates obtained by the method according to any one of claims 1 to 13 in the construction, public works, landscaping or agricultural industries, in particular in road applications.