PROCEDURE OF PREPARATION OF LIGHT GRANULATIONS

Abstract

The present invention relates to a process for preparing lightweight clay-based aggregates, said aggregate being suitable for use in public works, particularly road construction, and building construction. The invention thus has as its first object a process for preparing lightweight aggregates comprising at least the following steps: a) a step of granulating a clay-based mixture to obtain aggregates, b) a step of drying the aggregates obtained to obtain dried aggregates, c) a step of heat treatment of the dried aggregates, said step comprising two successive substeps: i) a first substep of heat treatment carried out under a reducing atmosphere at a temperature T1 between approximately 900 and 1200°C, ii) a second substep of heat treatment carried out under an oxidizing atmosphere at a temperature T2 between approximately 1050 and 1300°C, and d) a step of cooling the aggregates.

Description

Title of the invention: METHOD FOR PREPARING LIGHTWEIGHT AGGREGATES

[0001] The present invention relates to a process for preparing lightweight clay-based aggregates, said aggregate being suitable for use in particular in public works, especially road uses, and construction.

[0002] Industrial waste, such as sludge produced by urban or industrial wastewater treatment plants, contains organic matter, minerals containing a more or less significant fraction of clay fines, metals, and possibly toxic pollutants. This waste represents a very substantial volume, and numerous processes exist for its treatment and recovery.

[0003] There is therefore a constant need to develop processes which allow for the recovery of industrial waste, while producing materials of interest for industry or for any other use.

[0004] The recovery of waste, in particular sludge and industrial by-products, implies that these are considered inert, that is to say that they do not decompose, do not burn, do not produce a physical or chemical reaction and do not deteriorate other materials in contact in a way that is detrimental to the environment or to human health.

[0005] European Standard NF X 30-402-2 specifies a conformity test providing information on the leaching of fragmented waste and sludge under defined experimental conditions. This European Standard relates to waste characterization, and in particular conformity for leaching of fragmented waste and sludge. Thus, the leachable fractions of a number of elements or chemical compounds must not exceed certain concentration thresholds set by current regulations, in particular the guideline values ​​defined by CEREMA (Center for Studies and Expertise on Risks, the Environment, Mobility and Planning). For guidance purposes, the permissible limits as defined in the regulations for inert waste storage facilities (ISDI) (Annex II of the December 2014 decree) are indicated in mg / kg of dry matter in Table 1 below:

[0006] [Tables 1] Parameters ISDI Guideline Values ​​LEACHING TESTS (Standard NF X 30-402-2) - LS = 10 L / kg 10 Arsenic (As) 0.5 Barium (Ba) 20 Cadmium (Cd) 0.04 Total Chromium (Cr total) 0.5 Copper (Cu) 2 Mercury (Hg) 0.01 Molybdenum (Mo) 0.5 Nickel (Ni) 0.4 Lead (Pb) 0.5 Antimony (Sb) 0.06 Selenium (Se) 0.1 Zinc (Zn) 4 Chlorides (3) 800 Fluorides 10 Sulfates (3) 1000 Phenols Index 1 Total Organic Carbon (TOC) on wastewater (1) 500 Soluble Fraction (3) (SF) 4000 TESTS ON RAW WASTE Total Organic Carbon (TOC) 30,000 Benzene, Toluene, Ethylbenzene, and Xylenes (BTEX) 6 Polychlorinated Biphenyls 7 Congeners (PCBs) 1 Total Hydrocarbons (THCs) 500 Polycyclic Aromatic Hydrocarbons (PAHs) 50 Sequestration (in %) 30

[0007] Patent application EP-A1-1 571 135 describes a process for manufacturing expanded clay aggregates comprising a fraction of materials derived from organic sludge. This process comprises a first heat treatment stage carried out at a temperature of 500 to 750°C, enabling the removal of organic matter and the expansion of the aggregate in volume, and a second heat treatment stage carried out at a temperature of 900 to 1200°C, said second stage enabling, on the one hand, the final expansion of the aggregate's volume, and on the other hand, the acquisition of definitive cohesion of the aggregate through a eutectic effect between the respective mineral fractions of clay and sludge. This process, however, is not entirely satisfactory insofar as it does not produce aggregates conforming to the standard. regulations in force, in particular because the leachable fractions of a number of elements or chemical compounds obtained from aggregates prepared according to said process exceed the concentration thresholds set by Standard NF X 30-402-2.

[0008] There is therefore a need for a process for preparing lightweight aggregates from a clay-based mixture which overcomes the drawbacks of the prior art process, and in particular leads to aggregates which are acceptable from a regulatory point of view.

[0009] The invention thus has as its primary object a process for preparing lightweight aggregates comprising at least the following steps: a) a step of granulating a clay-based mixture to obtain aggregates, b) a step of drying the aggregates obtained to obtain dried aggregates, c) a step of heat treatment of the dried aggregates, said step comprising two successive sub-steps: i) a first sub-stage of heat treatment carried out under a reducing atmosphere at a temperature Tl between approximately 900 and 1200°C, ii) a second sub-stage of heat treatment carried out under an oxidizing atmosphere at a temperature T2 between approximately 1050 and 1300°C, and d) a stage of cooling the aggregates.

[0010] According to the process of the present invention, carrying out the two heat treatment substeps, in this order, surprisingly eliminates a very large portion of the leachable fractions of a number of elements or chemical compounds, resulting in aggregates in which the concentration of a very large majority of these elements and chemical compounds is below the concentration thresholds set by regulations. Thus, the process according to the invention produces a decontaminated aggregate. Indeed, implementing the process according to the invention leads to the destruction of organic matter, the destruction of pathogens, the extraction of heavy metals, and the partial or total decomposition of sulfates and carbonates. Implementing this process does not lead to the entrapment of pollutants in the aggregates.

[0011] In particular, aggregates prepared using the process according to the invention have the significant advantage of being decontaminated, compared to unheat-treated aggregates, for at least one, and preferably several, of the following elements: - the metals 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 - the phenol index.

[0012] In particular, the aggregates prepared by the process according to the invention are decontaminated of sulfates. The level of decontamination of the aggregates for these elements can be demonstrated by means of leaching tests carried out according to standard NF X 30-402-2, with L / S = 10 L / Kg.

[0013] The aggregates prepared using the process according to the invention also have the great advantage of being depolluted, compared to unheat-treated aggregates, for at least one, and preferably several of the following elements: - Total Organic Carbon (TOC), - benzene, toluene, ethylbenzene and xylenes (BTEX), - polychlorinated biphenyls 7 congeners (PCBs), - hydrocarbons (HCT), and - polycyclic aromatic hydrocarbons (PAHs).

[0014] The level of depollution of aggregates for these elements can be demonstrated by means of leaching tests on raw waste, as described in standard NF-X-30-402-2.

[0015] The chemical and physical characteristics of an aggregate obtained by the process according to the invention make this process suitable for the preparation of a material intended for various uses, particularly in the field of public works and construction.

[0016] The process according to the invention also has the advantage of allowing the recovery of a large volume of industrial waste, because it allows the preparation of light aggregates from a clay-based mixture containing a large proportion of industrial sludge and by-products.

[0017] The process according to the invention is energy-efficient, as the energy required for drying the aggregates can be recovered during the process, for example, during the cooling of the aggregates after their heat treatment. Furthermore, the temperature difference between the first and second heat treatment stages can be small, which helps to limit the energy consumption of the process.

[0018] In a process according to the invention, the heat treatment step under reducing conditions also makes it possible to considerably reduce the volumes of fumes to be treated, compared with state-of-the-art processes, and allows their valorization towards other applications.

[0019] Moreover, in view of prior art processes, the heat treatment step under oxidizing conditions requires little gas, the need for natural gas being divided by about 10 compared to a conventional process of producing aggregates from noble clay, which contributes to a reduction in the carbon footprint of the process.

[0020] Finally, the process according to the invention is staged, the different reactors being able to be separated, which makes it possible to optimize each of the steps.

[0021] By "light aggregates" we mean aggregates having a density less than 1, preferably between 0.6 and 0.95, more preferably about 0.8. The aggregates prepared by a process according to the invention are porous, hard and resistant.

[0022] The aggregate produced by a process according to the invention is valuable in the construction industry, public works, landscaping and agriculture.

[0023] Other advantages and features of a method according to the invention will become apparent upon examination of the detailed description of the invention and various embodiments, which are given by way of illustration and not to limit the scope of the invention. Where ranges of values ​​are indicated, these include the values ​​of the lower and upper bounds.

[0024] The process according to the invention comprises a first step a) of granulating a clay-based mixture. Said clay-based mixture consists of a mixture of different materials, each material containing a greater or lesser fraction of clay and organic matter, said materials being homogeneously mixed according to techniques known to a person skilled in the art

[0025] A material usable in the process according to the invention may include: - so-called "noble" clay, in particular from clay quarries, and / or - clay present in a clay material from sludge and industrial by-products, said clay material being chosen preferably from: a clay sediment from dredging, fines from washing polluted land, a filter cake from liquid waste, sludge from wastewater treatment plants (WWTP sludge), sludge from urban wastewater treatment plants (WWTP).

[0026] A clay-based mixture usable in a process according to the invention comprises, in particular, between 10 and 90% clay, expressed by weight relative to the weight of dry matter. In a clay-based mixture usable in a process according to the invention, the clay is present due, on the one hand, to the presence of so-called "noble" clay and / or, on the other hand, to the presence of at least one contaminated material containing clay. More particularly, a clay-based mixture usable in a process according to the invention comprises between 10 and 90% clay expressed by weight relative to the weight of dry matter.

[0027] The preparation of a clay material suitable for incorporation into a clay-based mixture during step a) of the process according to the invention may, in particular, include the preparation of a clay matrix in a plastic state by incorporating liquid, in sufficient quantity to obtain a moisture content of between 30% and 50%, preferably 40%. This liquid is preferably water, but may also be chosen from industrial water, wastewater, or leachate. According to particular embodiments of the invention, one or more additives, in liquid or solid form, may also be added to the clay material.

[0028] The additives that may be added during the process according to the invention are intended to facilitate certain chemical reactions. These include, for example: - barium carbonate, which neutralizes sulfates, - fixed carbon, which allows the extraction of certain heavy metals and reinforces the reducing character of the atmosphere.

[0029] An organic material incorporated into a clay-based mixture used in step a) of the process according to the invention is notably derived from a material selected from industrial sludge and by-products. Preferably, this material is selected from sludge from wastewater treatment plants (WWTP sludge) or from urban wastewater treatment plants (EWTP).

[0030] According to a more particular embodiment of a process according to the invention, said clay-based mixture comprises (i) between 10 and 25%, and preferably 20%, of high-quality clay material and (ii) at least one material obtained from industrial sludge and by-products, said material being selected from: - organic sludge, such as sewage sludge, - a dredging sediment, - a filter cake, and - a combination of said materials, said material having been previously freed of any foreign matter.

[0031] More particularly, in one embodiment of a process according to the invention, said clay-based mixture is obtained by mixing 20% ​​high-quality clay, 40% clayey dredged sediments, and 40% sewage sludge. Said sewage sludge comprises 30% organic matter. In this particular case, the organic matter content of such a clay-based mixture is approximately 12%.

[0032] A clay-based mixture usable in a process according to the invention preferably comprises between 5 and 40% by weight of organic dry matter, expressed as a percentage of the total dry weight, more particularly between 10% and 30% by weight of organic dry matter. "Organic dry matter" means carbonaceous or nitrogenous compounds which, when heated to a high temperature, lead to the release of gases, the emission of which contributes to the specific porosity of the resulting material.

[0033] Said organic material is also preferably freed from any foreign matter, in particular stones, pieces of wood and plastic. It is then preferably ground and mixed to obtain a homogeneous mixture. One or more additives, in liquid or solid form, may also be added to said organic material.

[0034] In a process according to the invention, said organic material may optionally be consisting of industrial sludge and by-products from various sources, which are then mixed or combined together.

[0035] The preparation of a clay-based mixture usable in step a) of the process according to the invention comprises the homogenous mixing of the materials. The moisture content of the clay-based mixture can be adjusted by adding an appropriate amount of liquid, preferably to obtain a water content between 30% and 50%, and preferably around 40%. The moisture content of the mixture can, in particular, be adjusted by adding an appropriate amount of water, industrial water, wastewater, or leachate. One or more additives, in liquid or solid form, can also be added to said clay-based mixture.

[0036] According to a particular embodiment of the process according to the invention, it does not include the addition of an additive.

[0037] According to a particular aspect of the process according to the invention, step a) of granulating the clay-based mixture comprises grinding and shaping said clay-based mixture to obtain a homogeneous mixture. Said mixing, grinding, and shaping of the raw materials are preferably carried out in a single piece of equipment.

[0038] According to a preferred embodiment of the process of the invention, said clay-based mixture comprises 10 and 25%, and preferably about 20%, of noble clay, expressed by weight relative to the total weight of dry matter.

[0039] Thus, according to a preferred embodiment of the process according to the invention, step a) of granulating the clay-based mixture comprises mixing the clay and said at least one material, grinding and shaping said mixture, to obtain a homogeneous mixture.

[0040] Granulation can be carried out by any means known to a person skilled in the art, in particular by extrusion or by passing over a pelletizing disc.

[0041] In step b) of the process according to the present invention, the aggregate is then dried, preferably to a moisture content of less than 20% kg / kg dry matter. Step b) of drying can be carried out by any means known to those skilled in the art, preferably at a low temperature, i.e., below 250°C, to avoid the release of organic matter and the cracking of the aggregate. The drying step reduces the moisture content of the aggregate and increases its hardness. It can be carried out in a suitable dryer. The energy required for drying can be obtained from the heat recovered during step d) of cooling after the thermal treatment of the aggregate, for example, by means of a direct heat exchanger.Thus, according to a particular and preferred embodiment of the process according to the invention, said process further includes a step of recovering heat from the cooling of the aggregates during step d).

[0042] After step b) of drying, the aggregate is subjected, in step c), to a treatment thermal in two successive sub-steps, at appropriate temperatures and under defined conditions, according to the process which is the subject of the invention.

[0043] This heat treatment allows the extraction of heavy metals, the production of synthesis gas, the decomposition of sulfates and carbonates, the destruction of pathogens, the creation of porosity and gritting.

[0044] The first sub-step of heat treatment, or pyrolysis, consists of subjecting the aggregate to a temperature Tl of between approximately 900 and 1200°C, under a reducing atmosphere.

[0045] By "reducing atmosphere" is meant an atmosphere devoid of oxygen and comprising a gas chosen from carbon monoxide, volatile hydrocarbons, hydrogen, or a combination of these gases. A reducing atmosphere is obtained for example by the substoichiometric combustion, with air, of the organic compounds present in the mixture.

[0046] The first heat treatment substep can be carried out by any suitable means known to those skilled in the art. In particular, it is carried out in a furnace such as, for example, a rotary furnace in which the atmosphere is reducing. This step is preferably carried out under substoichiometric conditions. This step allows the production of a synthesis gas rich in carbon monoxide (CO), volatile hydrocarbons (CxHy), and devoid of oxygen.

[0047] During this first substep of heat treatment, heavy metals such as mercury (Hg), cadmium (Cd), zinc (Zn), and lead (Pb) are reduced and volatilize, partially or completely. These metals are found in the gaseous state in the synthesis gas produced during the reaction. The main sulfates are decomposed, and sulfur compounds, as well as chlorinated compounds, are extracted in the gas phase.

[0048] The conditions used during this first heat treatment substep minimize the quantities of syngas produced. The syngas produced during this first substep can be directed, in particular, to a boiler, a heat engine, or another device. It is possible to use the syngas from the heat treatment step in an oxidizing atmosphere to initiate the reaction in the heat treatment step in a reduced atmosphere.

[0049] The temperature Tl of the first sub-step of heat treatment is between approximately 900 and 1200°C, preferably between approximately 950 and 1200°C, more preferably between approximately 1050 and 1150°C, and even more preferably between approximately 1110 and 1150°C. This temperature range makes it possible, in particular, to extract sulfur compounds without oxidizing them into sulfates.

[0050] The exact temperature of the first sub-stage of heat treatment depends on the clay-based mixture and the composition of the synthesis gas possibly injected into the enclosure in which said heat treatment takes place.

[0051] The duration of the first heat treatment substep is generally between 30 and 150 minutes approximately, preferably between 60 and 120 minutes approximately, more preferably it is 120 minutes approximately.

[0052] The second sub-stage of heat treatment consists of combustion under an oxidizing atmosphere at a temperature T2 between approximately 1050 and 1300 °C.

[0053] By "oxidizing atmosphere" is meant an atmosphere comprising at least one oxidizing agent, preferably chosen from oxygen (O2). An oxidizing atmosphere is obtained, for example, by the combustion of methane with air, leading to an atmosphere in which the proportion of oxygen is greater than 3%.

[0054] The second sub-step of heat treatment can be carried out by any suitable means known to a person skilled in the art. In particular, it is carried out in a rotary oven.

[0055] The temperature T2 of the second heat treatment substep is between approximately 1050 and 1300°C, preferably between approximately 1050 and 1150°C, more preferably between approximately 1110 and 1150°C, and even more preferably around 1125°C. This second heat treatment substep completes the decomposition of carbonates and the complete combustion of organic compounds. Metals such as iron (Fe), nickel (Ni), and chromium (Cr) can also react with each other to form insoluble spinel compounds. The decomposition of organic compounds, sulfates, and carbonates leads to a porous structure, resulting in a lightweight aggregate, preferably with a density less than approximately 1. The high reaction temperature leads to the ceramization and grit formation of the materials, which gives the aggregates their hardness and mechanical strength.

[0056] The exact temperature of the second sub-stage of heat treatment depends on the melting temperature of the aggregates, which depends on the chemical composition of the clay-based mixture.

[0057] The duration of the second heat treatment sub-step is generally between 30 and 150 minutes approximately, preferably between 60 and 120 minutes approximately, more preferably it is about 60 minutes.

[0058] According to a particular embodiment of the process according to the invention, the temperature T1 is less than or equal to the temperature T2. According to a more particular embodiment of the process according to the invention, T1 is equal to T2.

[0059] After the heat treatment of step c), an aggregate is obtained whose density is generally between about 0.6 and 1, and whose particle size is between 1 and 15 mm.

[0060] According to a particular embodiment of a process according to the invention, the combustion gases generated during the second sub-step of heat treatment are injected into the enclosure in which the first sub-stage of heat treatment takes place.

[0061] In step d), the aggregates thus obtained can be cooled by any known and suitable means, in particular by introducing them into a cooler. Said cooler operates in ambient air. This step cools the aggregates to a temperature below approximately 100°C. This cooling step also recovers heat, as the transfer of heat from the aggregates to the ambient air leads to a heating of this air, from a temperature of approximately 15°C to a temperature of approximately 250°C. The hot air thus produced can then be recovered, in particular for use in the drying step of the aggregates before step c) of heat treatment.

[0062] The implementation of the process according to the invention thus leads to a lightweight aggregate characterized by a density of less than 1. Said aggregate is also characterized by its spherical appearance and its hardness. Said aggregate is also characterized in that, compared with an aggregate taken before the two-sub-step heat treatment of the process according to the invention, when subjected to a leaching test according to standard NF X 30-402-2, it is characterized by the absence or low level of release of organic and / or metallic pollutants, or the release of organic and / or metallic pollutants at a level compatible with the parameters defined for use in road construction or building materials, for example.

[0063] By "organic pollutants" and "metallic pollutants" we mean respectively organic or metallic elements which are harmful to the environment or to human health when they are present in quantities exceeding the concentration thresholds set by Standard NF X 30-402-2.

[0064] A lightweight aggregate obtained by the process according to the invention is particularly useful for manufacturing materials such as: a drainage material, a snow removal substrate, sand, an insulating material, a lightweight insulating concrete block, and a green roof. A lightweight aggregate obtained by the process according to the invention is also particularly useful for manufacturing construction materials such as lightweight concrete.

[0065] The invention therefore also relates to a material comprising an aggregate obtained by a process according to the invention.

[0066] The invention has as its second object the use of an aggregate that can be obtained by the process according to the invention in the construction, public works, landscaping or agricultural industries.

[0067] More particularly, the invention relates to the use of an aggregate obtained by the process according to the invention for road use, preferably chosen from: a type 1 road use, type 2 road use and type 3 road use.

[0068] Type 1 road uses are those with a maximum height of three meters in the sub-base of paved roadways or shoulders. These include embankments under structures, subgrade layers, foundation layers, base courses, and binder courses. Type 2 road uses are those with a maximum height of six meters in technical embankments connected to the road infrastructure or in shoulders, provided they are within covered road structures. They also include uses with a thickness greater than three meters and a maximum height of six meters in the sub-base of paved roadways or shoulders. Type 3 road uses are not subject to any restrictions on their thickness. These include, for example, uses as a sub-layer for road surfaces or shoulders, for the construction of construction tracks, forest roads or towpaths. Examples

[0069] Example 1: Preparation and characterization of products before mixing

[0070] A clay-based mixture, further comprising three types of mud, was prepared, with the following respective proportions: STEP sludge: 40%; dredging sediments, 20%; filter cake, 20%; noble clay, 20%.

[0071] An analysis of the leaching of each of the products intended to be incorporated into the mixture was carried out; Table 2 below is a summary of the results of these analyses, and an extrapolation of the values ​​of the mixture. Guide values ​​Filtration cake Sediments STEP sludge Noble clay Mixture ISD parameters Leaching tests standard NFX 30-402-2; LS = 10 L / Kg. Results expressed in mg / kg: Arsenic (AS) 0.5 0.2 0.2 0.2 0.52; Barium (Ba) 20 0.29 0.15 3.35 0.87 1.61; Cadmium (Cd) 0.04 0.002 0.002 0.011 0.002 0.01; Total Chromium (Cr) 0.5 0.1 0.1 0.1 0.1 0.1; Copper (Cu) 2 0.7 0.2 50.3 0.2 20.34; Mercury (Hg) 0.01 0.001 0.001 0.003 0.001 0.01; Molybdenum (Mo) 0.5 0.156 0.055 1.95 0.045 0.84 Nickel (Ni) 0.4 0.1 0.1 4.48 0.1 1.86 Lead (Pb) 0.5 0.1 0.1 1.32 0.1 0.59 Antimony (Sb) 0.06 0.1 0.01 0.026 0.022 0.04 Selenium (Se) 0.1 0.01 0.02 0.85 0.031 0.36 Zinc (Zn) 4 0.2 O ÎQ 8.39 n ? 3.48 Chlorides (3) 800 14870 1430 226 1605.7 Fluorides 10 5 7.12 5 8.82 6.19 Sulfates (3) 1000 14900 3040 2160 1080.0 4668 Phenol index 1 0.5 0.5 5.72 0.5 2.59 Total Organic Carbon (TOC) on waste 500 10 130 62000 50 24860 Soluble Fraction (3) (SF) 4000 23500 14400 159009 2040 71588 Raw Waste Tests Total Organic Carbon (TOC) 30000 44600 18600 263000 24000 122840 Benzene, Toluene, Ethylbenzene and Xylenes (BTEX) 6 0.05 0.05 0.07 0.05 0.06 Polychlorinated Biphenyls (PCBs) 1 0.121 0.01 0.01 0.01 0.04 Hydrocarbons (CW to C4G) (HCT) 500 1300 220 1000 45 713 Polycyclic aromatic hydrocarbons (PAHs) 50 21 0.99 0.61 0.05 4.66 .

[0073] It can be noted that the results of the analyses of the leachates of the raw waste exceed the guide values ​​defined in the standard.

[0074] Example 2: Cooking tests

[0075] Each of the different components was first weighed, then the different components were mixed in a mixer / blender. The clay-based mixture The prepared mixture, weighing approximately 30 kg, was then air-dried to a moisture content of around 20%, expressed as % H2O on a dry matter basis. Moisture measurements were taken using a Mettler-Toledo infrared desiccator. 500 grams of the base mixture were then ground in a blender to a fine powder. Undesirable elements such as pebbles, shells, and wood fragments were removed by sieving. The resulting product was remixed in the blender, with any selected additives added. While mixing continued, the product was hydrated to form a paste suitable for making aggregates. The resulting aggregates were then dried in an electric oven at 120°C for 24 hours. Once dry, a 500 g sample underwent a two-stage heat treatment.

[0076] In a preliminary test, the sample underwent two successive treatment steps: a first baking at 700°C for 20-30 minutes, then a heat treatment for 60 minutes at 1075°C. Once the heat treatment was completed, the beads were air-cooled.

[0077] Leaching tests were carried out according to standard NF X 30-402-2 (NF EN 12457-2). Chemical analyses were then performed according to the recommendations cited in said standard. The results obtained before and after treatment were compared; the average results are presented in Table 3 below. Guideline values ​​Before treatment After treatment ISDI parameters Average Average Leaching tests standard NF X 30-402-2; LS = 10 L / Kg, results expressed in mg / kg Arsenic (AS) 0.5 0.36 0.05 Barium (Ba) 20 0.85 1.52 Cadmium (Cd) 0.04 0.01 0.01 Total chromium (total Cr) 0.5 0.1 0.85 Copper (Cu) 2 87.5 0.08 Mercury (Hg) 0.01 0.000 0.000 Molybdenum (Mo) 0.5 1.61 15.17 Nickel (Ni) 0.4 6.17 0.05 Lead (Pb) 0.5 0.46 0.09 Antimony (Sb) 0.06 0.36 0.01 Selenium (Se) 0.1 0.45 0.18 Zinc (Zn) 4 5.90 0.03 Chlorides (3) 800 1770 96.83 Fluorides 10 5 1 Sulfates (3) 1000 12450 14433 Phenol index 1 0.6 041 Total organic carbon (TOC) on eluate 500 14500 1800 Soluble fraction (3) (SF) 4000 62400 24833

[0079] The average values ​​obtained from the post-treatment analyses exceed the permissible thresholds for ISDI and therefore cannot be used for commercial purposes. In particular, the leachates show a high concentration of Mo, organic carbon, and sulfates.

[0080] Example 3: Sample treated according to the invention, first series of tests

[0081] A clay-based mixture as described in Example 1 was prepared. Aggregates were then prepared as described in Example 2. The resulting aggregates were subsequently dried in an electric oven at 120°C for 24 hours. Once dry, a 500 g sample underwent a two-stage heat treatment. The controlled atmosphere tests were carried out in a tube furnace within a sealed quartz tube. Where necessary, the atmospheres in the furnaces were reconstituted from a mixture of pure gases (Air, CO, CO2, N2) produced by Air Liquide. The injection of the different gases was adjusted and controlled by previously calibrated microvolumetric flow meters. After the heat treatment, the granules were air-cooled.

[0082] The leaching tests were carried out in accordance with standard NF X 30-402-2 (NF EN 12457-2). Chemical analyses were then carried out according to the conditions referenced in standard NF X 30-402-2. The results obtained were compared to the acceptability thresholds for the valorization of alternative materials in road construction of the CEREMA Guide, which defines 3 types of use according to the limit values ​​obtained on leachates.

[0083] Road materials suitable for use in road applications of type 1, 2, or 3 are those for which the alternative materials used in their composition meet the limit values ​​for these applications, respectively type 1, 2, or 3. Table 4 below lists the acceptable values ​​for road applications of type 1, type 2, or type 3, determined during leaching tests according to standard NF X 30-0402-2. These values ​​are expressed in mg / kg of dry matter. The results of the values ​​obtained from the leaching tests of the raw sample, without additives or heat treatment, are presented in Table 4 below, in the "Test 0" column.

[0084] [Tables4] Type 1 Type 2 Type 3 Test 0 Arsenic (AS) 0.6 0.6 0.6 0.5 Barium (8a) 36 25 25 0.5 Cadmium (Cd) 0.05 0.05 0.05 0.012 Total Chromium (Cr) 4 2 0.6 0.4 Copper (Cu) 10 5 3 8.3 Mercury (Hg) 0.01 0.01 0.01 0.0004 Molybdenum (Mo) 5.6 cO 0.6 0.91 Nickel (Ni) 0.5 0.5 0.5 2.1 Lead (Pb) 0.6 0.6 0.6 0.05 Antimony (Sb) 0.6 0.3 0.08 0.5 Selenium (Se) 0.5 0.4 0.1 0.05 Zinc (Zn) 5 5 5 6 Chlorides (3) 10000 5000 1000 260 Fluorides 60 30 13 1 Sulfates (3) 10000 5000 1300 27000 Phenol index 2 Total organic carbon (TOC) on eluate 500 500 500 9600 Soluble fraction (3) 67000

[0085] These results show that the mixture obtained exhibits numerous overshoots (figures shown in bold) compared to the guide values ​​and cannot be valued as is.

[0086] Example 4: Sample treated according to the invention, first series of tests

[0087] In this example, the basic mixture, prepared according to the operating procedure described in Example 1, prepared in the form of granules according to Example 2 and heat-treated as indicated in Example 3, with the addition of an additive in tests 4 and 6. This mixture was then 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).

[0088] Test 4 was carried out on the crude mixture in a mixed (oxidizing / reducing) atmosphere. Test 5 was carried out on the crude mixture to which 5% BaCO3 had been added in a mixed (oxidizing / reducing) atmosphere. Test 6 was carried out on the crude mixture to which 5% reducing agent (carbon) had been added, in a mixed (oxidizing / reducing) atmosphere. Table 5 below describes the mixtures produced (with or without additives) and the sequences used during the heat treatment.

[0089] [Tables5] Parameters Test 4 Test 5 Test 6 Additive 5% BaCOS Non 5% BaCO3 Atmosphere Red / ox Red / ox Red / ox Reducing atmosphere Temperature (°C) 1100 1125 1110 Residence time (min) 120 120 120 Oxidizing atmosphere Temperature (°C) 1125 1110 Residence time (min) 30 30 30

[0090] The results are presented in the following table 6. Type 1 Type 2 Type 3 Test 4 Test 5 Test 6 Arsenic (As) 0.6 0.6 0.6 0.05 0.05 0.05 Barium (Ba) 36 25 25 3.3 1.8 1 2 Cadmium (Cd) 0.05 0.05 0.05 0.001 0.002 0.002 Total Chromium (Cr) 4 2 0.6 0.26 CJ O 0.43 Copper (Cu) 10 5 3 0.46 0.04 0.12 Mercury (Hg) 0.01 0.01 0.01 0.01 0.0003 0.0003 0.0003 Molybdenum (Mo) 5.6 2.8 0.6 1.4 1.6 2.1 Nickel (Ni) 0.5 0.5 0.5 0.05 0.05 0.05 Lead (Pb) 0.6 0.6 0.6 0.05 0.05 0.05 Antimony (Sb) 0.6 0.3 0.08 0.05 0.05 0.05 Selenium (Se) 0.5 0.4 0.1 0.05 0.05 0.05 Zinc (Zn) 5 5 5 0.05 0.05 0.05 Chlorides (3) 10000 5000 1000 18 16 14 Fluorides 60 30 13 1 1 1 Sulfates (3) 10000 5000 1300 5200 2400 3700 Phenol index 0.1 0.1 0.1 TOC on eluate 500 500 500 36 10 20 Soluble fraction (3) 22000 12000 13000

[0092] Comparison of the results with the raw sample demonstrates that, regardless of the treatment method, the metals, with the exception of Molybdenum, are no longer leachable after treatment.

[0093] Total organic carbon decreased from a concentration of 9600 mg / kg to less than 20 mg / kg after treatment. The sulfate concentration in the leachate was reduced sixfold to an average of 4600 mg / kg. However, this value remains too high for the objectives of this study. The lowest sulfate concentration was obtained in Test No. 5 (2400 mg / kg). This result was obtained on a raw sample (without additives) with a heat treatment sequence of 60 minutes at 1075°C under a reducing atmosphere followed by 30 minutes of baking under an oxidizing atmosphere at the same temperature. We can also observe that, compared to Tests 4 and 5, the addition of additives to the base mixture had no positive influence on the quality of the treatment.

[0094] Example 4: Sample treated according to the invention, second series of tests

[0095] In this example, the basic mixture, as described in Example 1, was first subjected to treatment under a reducing atmosphere, followed by heat treatment under an oxidizing atmosphere. The temperatures and residence times varied depending on the test and are described in Table 7 below.

[0096] [Tables7] Parameters Test 7 Test 8 Test 9 Test 10 Test 11 Test 12 Additive No No No No No No Atmosphere Red / ox Red / ox Red / ox Red / ox Red / ox Red / ox Reducing Atmosphere Temperature (GC) 1075 1075 1100 1100 1125 1110 Residence Time (min) 120 120 120 120 120 120 Oxidizing Atmosphere Temperature (CC) 1075 1075 1100 1100 1125 1110 Residence Time (min) 60 120 60 60 60 60

[0097] Table 8 below is a summary of the analyses obtained on the leaching of products after treatment.

[0098] [Tables8] Type t Type 2 Type 3 Essay 7 Essay 8 Essay 9 Essay 19 Essay EssaySc3i 12 Arsenic (As) Cadmium (Cd) 8.05 8.05 0.85 0.002 8.002 0.01 0.002 8.01 G ÜG Chromium iôlai (G? total) 4 2 0.6 8.4 0.44 0.02 0.51 0.23 8.20 8 0.83 0.03 0.03 0.83 0.02 6.03 Mercury 8.01 8.64 0.81 0.8803 8.3003 a.8oœ 0.0003 0.3083 9.0083 Moiybdèn 3.2 810, 6 , 8) 0.98 2..8 6.98 1.2 Nickel (Ni) 0.5 0.5 0.5 0.85 8.05 8.95 0.85 0.05 Lead (Pb) 8.6 0.6 0.6 0.65 0.51 Anfi 0.95 (5b) 8.6 0.3 8.86 8.05 0.æ 0.05 0.85 8.05 0.S5 Selenium (Se) 0.5 0.4 0.1 0.05 0.05 0.95 9.85 8.05 OnZt 0.09 8.32 8.1 0.02 8.02 Chtorores (3) iûœ» 50æ 1880 17 '16 830 548 510 Fluorides 60 38 13 j! 1 1 1 1' 1 Sulfates (31 40008 5000 1300 3800 3300 1600 2880 1106 1308 index phenols. 3.1 0.1 8.1 0.1 0.1 OT 0f 80 w æ C 10 10 w 10 Soluble fraction (3) 14000 13000 BIOS 12808 7106 6708

[0099] The results obtained confirm that the metals, with the exception of molybdenum, are no longer leachable after treatment. Total organic carbon (TOC) decreased from a concentration of 9600 mg / kg to less than 10 mg / kg after treatment. The sulfate concentration in the leachates was reduced by a factor of 12, reaching an average of 2300 mg / kg after treatment. In tests 11 and 12, the sulfate concentration fell below the threshold of 1300 mg / kg, which is the current regulatory limit for use as type 3 road construction material, the most stringent limit. These results were obtained with residence times of 120 minutes in a reducing atmosphere and 60 minutes in an oxidizing atmosphere, respectively. The optimal temperature range appears to be between 1110 and 1125°C.

[0100] The implementation of a protocol comprising a two-phase heat treatment, with an alternating first phase of heat treatment under a reducing atmosphere, immediately followed by heat treatment under an oxidizing atmosphere, greatly improves yields compared to no treatment or compared to treatments described in the state of the art, such as described in EP 1 571 135.

[0101] In particular, in a temperature range between 1110 and 1125°C, the sulfate conversion efficiency made it possible to achieve sulfate levels in the leachates below the thresholds currently in force for the valorization of aggregates in road construction.

[0102] This study demonstrates the ineffectiveness of adding different reagents on the treatment qualities. In particular, the addition of barium carbonate had no effect on the treatment of sulfates in the tests.

Claims

Demands

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

2. A process according to claim 1, characterized in that said clay-based mixture comprises (i) between 10 and 25%, and preferably 20%, of noble clay material and (ii) at least one material obtained from industrial sludge and by-products, said material being selected from: - an organic sludge, such as sewage sludge, - a dredged sediment, - a liquid waste filter cake, and - a combination of said materials, said material being previously freed of any foreign matter.

3. A process according to claim 2, characterized in that step a) of granulating the clay-based mixture comprises mixing the clay and said at least one material, grinding and shaping said mixture, to obtain a homogeneous mixture.

4. A process according to any one of the preceding claims, characterized in that the clay-based mixture further comprises at least one additive selected from barium carbonate and fixed carbon.

5. A method according to any one of the preceding claims, characterized in that step b) of drying is carried out at a temperature below 250°C.

6. A method according to any one of the preceding claims, characterized in that it further comprises a step of recovering the calories from the cooling of the aggregates during step d).

7. A process according to claim 6, characterized in that the energy required for step b) of drying comes at least in part from the calories recovered during step d) of cooling.

8. A method according to any one of the preceding claims, characterized in that said first sub-step of heat treatment is carried out at a temperature Tl between 1050 and 1150°C, preferably between 1110 and 1150°C.

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

10. A method according to any one of the preceding claims, characterized in that said first heat treatment substep is carried out in a rotating furnace in which the atmosphere is reducing.

11. A process according to any one of the preceding claims, characterized in that said second sub-step of heat treatment consists of an oxidizing combustion at a temperature T2 between 1050 and 1150°C, preferably between 1110 and 1150°C, more preferably at about 1125°C.

12. A method according to the preceding claim, characterized in that said second sub-step of heat treatment is carried out for a period of between 30 and 150 minutes, preferably for 60 minutes.

13. A method according to any one of the preceding claims, characterized in that the temperature Tl is less than or equal to the temperature T2, preferably Tl is equal to T2.