Process for obtaining a powdered hydraulic binder from biomass ash

Abstract

The invention relates to a process for obtaining a powdered hydraulic binder comprising a step of hydrating biomass ash, said hydration step comprising bringing said biomass ash into contact with water in liquid or gaseous form.

Description

Title of the invention: Process for obtaining a powdered hydraulic binder from biomass ash

[0001] The invention relates to the field of construction materials. It relates more particularly to the obtaining of hydraulic binders as well as to the hydraulic binders obtained and the dry mortar compositions comprising said hydraulic binders.

[0002] Dry mortar compositions are powdered mixtures comprising a hydraulic binder and aggregates. After mixing ("stirring") with water, a paste (wet or fresh mortar) is obtained that can be shaped and then hardened to form a hardened mortar. Such mortars can have various applications: facade coatings, tile adhesives, flooring products such as screeds, jointing mortars, masonry mortars, and many others.

[0003] There is a need to reduce the carbon footprint of construction materials, and in particular mortar mixes. The hydraulic binder in these mixes is often cement, especially Portland cement, the production of which releases large quantities of CO2 into the atmosphere (between 800 and 1000 kg of CO2 per tonne of cement produced), partly due to limestone decarbonation reactions, and partly due to the high temperatures (around 1450°C) required to produce hydraulic phases. To address this problem, various solutions have been proposed, some of which involve at least partially substituting cement with industrial by-products, such as slag, fly ash, or calcined clays. More recently, the use of biomass ash has been proposed.

[0004] Biomass ash is ash formed by the combustion of biomass, used for example to produce electricity, steam and / or heat. Biomass can come, for example, from agricultural products, forestry products, household and municipal waste, the paper or pulp industry and / or crops grown for energy production.

[0005] However, the use of biomass ash is not without its drawbacks. In particular, the hydration of certain types of ash can lead to uncontrolled swelling, which prevents their use or at least significantly reduces the maximum quantity of ash that can be used in mortar compositions.

[0006] The present invention aims to solve this problem by proposing a process for treating biomass ash to obtain a hydraulic binder exhibiting reduced swelling, or even no swelling, during its hydration.

[0007] To this end, the invention relates to a process for obtaining a powdered hydraulic binder comprising a step of hydrating biomass ash, said hydration step comprising bringing said biomass ash into contact with water in liquid or gaseous form.

[0008] In this presentation, the starting product of the process will be called "biomass ash" and the final product, in powder form, resulting from the hydration of biomass ash, and where applicable additional steps, will be called "hydraulic binder".

[0009] In the present exposition, the following terms are synonymous: free lime, which is a phase of chemical formula CaO, is also called quicklime or calcium oxide; slaked lime is also called calcium hydroxide or Ca(OH)2; calcium carbonate is also called calcite or CaCO3.

[0010] The process according to the invention is particularly useful for biomass ash containing free lime (CaO). The free lime content in the biomass ash is preferably at least 5% by weight, even at least 10%, for example from 6 to 18% or from 7 to 15% by weight. As explained in more detail later in the text, the hydration step reduces the amount of free lime by transforming at least a portion of it, preferably at least 50%, and even at least 60% or at least 70%, or even at least 80% or 90%, and even all of it, into slaked lime and / or calcium carbonate.

[0011] The process is also particularly useful for biomass ash containing metallic aluminum (Al), which exhibits the greatest swelling. Metallic aluminum is notably present in the form of fine particles. Without being bound by any scientific theory, it would appear that the dissolution of aluminum during the hydration of the hydraulic binder could lead to the generation of hydrogen gas. The metallic aluminum content in biomass ash is typically between 0.1 and 5.0% by weight, particularly between 0.5 and 4.0%, or even between 1.0 and 3.0%.

[0012] The strongest swelling was observed when the biomass ash simultaneously contained free lime and metallic aluminum. It would appear, in fact, that the presence of free lime creates conditions favorable to the dissolution of metallic aluminum.

[0013] Biomass ash is preferably ash obtained by burning waste (or residues, or by-products) from the paper or pulp industry. For example, this waste may be produced during the production of paper or board from recycled paper or board. This waste may, for example, be recovered during the pulping stage, during which water and chemicals such as hydrogen peroxide, sodium hydroxide, and sodium silicate are added to the shredded paper in order to separate the cellulose fibers, and / or during screening stages. This type of biomass ash is likely to contain small amounts of metallic aluminum, for example from barrier layers of cardboard packaging or from labels, caps or lids used with cardboard packaging.

[0014] Combustion is carried out, for example, by fluidized bed combustion, in particular by bubbling fluidized bed combustion. Other combustion processes include fixed bed combustion and circulating fluidized bed combustion. The combustion temperature is preferably at least 700°C, or even at least 800°C. Regardless of the combustion process, the biomass ash used is preferably recovered from the flue gases (for example, by filtration), as opposed to ash collected at the bottom of the boiler, which frequently contains undesirable materials such as heavy metals.

[0015] Biomass ash preferably has a chemical composition by weight comprising 5-35%, preferably 9-20%, SiO2, 3-20%, preferably 5-15%, Al2O3, 0-5%, preferably 0.5-3%, Fe2O3, and 30-70%, in particular 40-65%, CaO. This is a chemical, not a mineralogical, composition: the quantity of calcium, expressed here as "CaO," does not imply the mineral forms in which the calcium is present. It is preferably partly present as free lime, as indicated above, but also, preferably, incorporated into hydraulic phases.

[0016] Biomass ash preferably comprises at least 10% by weight of hydraulic phases, more particularly at least 15%, or even at least 20%, relative to the total weight of biomass ash. This content is advantageously at least 25% or even at least 30%. It is preferably at most 60% and even at most 50%. The hydraulic phases are preferably selected from calcium silicates, calcium aluminates, free lime, and mixtures thereof. Calcium silicates and calcium aluminates preferably comprise one or more of the following phases: C2S, C3S, C3A, C4AF, C2A2S, CA, C12A7, and CA2. Biomass ash preferably comprises at least 10% by weight of C2S phases, at least 5% by weight of C2A2S phases and at least 1% by weight of C3A phases, relative to the total weight of biomass ash.The C3S phase content is preferably no more than 1% by weight, or even zero, as it prevents the formation of aluminate phases during hydration.

[0017] Biomass ash may also include amorphous phases, notably comprising CaO, Al₂O₃ and SiO₂, in amounts sufficient to dissolve these elements. The quantity of amorphous phase is, for example, 10 to 30% by weight.

[0018] Biomass ash may include additional reactive phases, such as carbonates (in particular calcite), sulfates (for example sulfate) of calcium) or aluminosilicates. The calcite content is, for example, 10 to 40% by weight, relative to the total weight of biomass ash.

[0019] In terms of particle size, the D50 (based on volume distribution) of biomass ash is preferably from 8 to 50 µm, in particular from 10 to 30 µm, for example, approximately 15 µm. The maximum size (Dmax) is preferably less than or equal to 200 µm. These dimensions are generally measured by laser particle size analysis.

[0020] In order to avoid unduly negatively impacting the reactivity of the resulting hydraulic binder, the process according to the invention preferably reduces the total weight content of hydraulic phases in biomass ash, other than free lime, by no more than 30%, in particular by no more than 20%, or even by no more than 10%, relative to its initial content. This percentage of relative reduction is calculated using the following formula: (Hi-Hf) / Hi, where Hi is the total initial weight content (before hydration) of hydraulic phases (excluding free lime) and Hf is the total final weight content (after hydration) of hydraulic phases (again, excluding free lime).

[0021] When biomass ash contains free lime, the process according to the invention preferably reduces the free lime content by weight by at least 50%, in particular by at least 60%, and even by at least 70%, or even by at least 80% or 90%, relative to its initial content. In other words, the ash is brought into contact with water in such a way that the weight content is reduced by the percentages indicated above. This percentage of relative reduction is calculated by the following formula: (Ci-Cf) / Ci, where Ci is the initial total weight content (before hydration) of free lime and Cf is the final total weight content (after hydration) of free lime.

[0022] It has been shown to be possible to selectively hydrate free lime without excessively hydrating the other hydraulic phases. Given the much stronger affinity of free lime for water, the latter will tend to react first with the free lime, before potentially hydrating the other hydraulic phases. This is also possible by manipulating the conditions of contact between the biomass ash and the water, in particular the amount of water added and / or the contact time. For example, a longer contact time is likely to reduce the content of hydraulic phases other than free lime more significantly. The same is true when the amount of water added is greater.

[0023] During the hydration step, the biomass ash is brought into contact with water in liquid or gaseous form. The hydration step can be a continuous or batch process. The hydration step can be followed, if necessary, by a drying step.

[0024] The amount of water with which the biomass ash is brought into contact is preferably adjusted according to the free lime content in the ash. The amount The water content put into contact is preferably 1 to 5%, in particular 2 to 4% by weight relative to the weight of biomass ash.

[0025] In a first embodiment, the biomass ash is brought into contact with water in liquid form. For example, liquid water can be sprayed onto a bed of biomass ash, optionally moving on a conveyor, for example a belt conveyor. Alternatively, liquid water can be added to the biomass ash in a mixer, for example a screw conveyor or a rotary drum.

[0026] In a second embodiment, biomass ash is brought into contact with water in gaseous form. The ash can, for example, be brought into contact with a stream of humid air or water vapor. During the contact, the ash can be conveyed. Alternatively, the contact can involve the formation of a fluidized bed. According to yet another alternative, water vapor can be injected into a mass of ash. For example, a mass of ash can be contained by a gas-permeable jacket while water vapor is injected from below into the ash mass. The injection can be carried out by a plurality of homogeneously distributed nozzles.

[0027] Preferably, the process includes, after or during the hydration step, a carbonation step by contacting the biomass ash with carbon dioxide. The carbonation step allows at least partial conversion of the slaked lime into calcium carbonate (or calcite).

[0028] The carbonation step, in combination with the hydration step, makes it possible to significantly reduce, or even eliminate, swelling, particularly in the case of ash containing both free lime and metallic aluminum. It would appear that the carbonation of the lime prevents the conditions under which the hydration of the binder can lead to the generation of hydrogen. It is possible that this neutralization of the free lime initially present in the ash slows down the dissolution of the metallic aluminum, with hydrogen generation then occurring later, at a point when the mortar has sufficiently hardened.

[0029] According to a first embodiment, the carbonation step is carried out simultaneously with the hydration step. For example, the ashes can be brought into contact with humid air, which therefore contains both water in gaseous form and carbon dioxide.

[0030] According to a second embodiment, the carbonation step is carried out after the hydration step. The carbonation step can be carried out by contact with air. Air storage may suffice if the storage conditions allow for sufficiently intimate contact, for example by using thin beds. Alternatively, air or any other gas containing carbon dioxide can be injected into the ash that has undergone the hydration step.

[0031] Preferably, the carbonation step reduces the weight content of slaked lime (or calcium hydroxide) by at least 30%, in particular by at least 40%, and even by at least 50% or 60%, or even by at least 70% or 80%, or at least 90%, compared to its initial content. This percentage of relative reduction is calculated by the following formula: (Ei-Ef) / Ei, where Ei is the initial total weight content (before hydration) of slaked lime and Ef is the final total weight content (after hydration) of slaked lime.

[0032] The invention also relates to a powdery hydraulic binder obtained, or capable of being obtained, by the process according to the invention.

[0033] The resulting powdered hydraulic binder preferably comprises at least 8% by weight of calcium silicate and / or calcium aluminate hydraulic phases, in particular hydraulic phases selected from C2S, C3A, and C2A2S. This content is preferably at least 10%, or even at least 15%, for example, between 20 and 50%, or between 25 and 40% by weight. The free lime content is preferably at most 5%, in particular at most 3% by weight.

[0034] The D50, based on the volume distribution of the hydraulic binder obtained, is preferably from 8 to 50 pm, in particular from 10 to 30 pm.

[0035] Another object of the invention is also a dry mortar composition comprising such a powdery hydraulic binder as well as aggregates.

[0036] The hydraulic binder according to the invention can be mixed with other hydraulic binders and / or pozzolanic materials, in particular selected from Portland cement, aluminous cements, sulfoaluminous cements, lime (hydraulic or aerial), calcium sulfate sources, slags (in particular crushed granulated blast furnace slags), fly ash, silica fume, calcined shale, natural or calcined pozzolans or calcined clays.

[0037] The aggregates may include fillers, which are finely ground inert mineral materials, generally of the calcareous or siliceous type. Preferably, the aggregates include sands, in particular siliceous sands, and / or fillers, in particular calcareous or dolomitic fillers. The aggregates may also include lightweight aggregates selected from perlite, vermiculite, expanded glass beads, expanded polystyrene beads, cenospheres, expanded silicates, aerogels, and mixtures thereof.

[0038] The dry mortar composition may also include one or more additives, selected from rheological agents, plasticizers or superplasticizers, water-retaining agents, air-entraining agents, thickening agents, biocidal preservatives, dispersing agents, pigments, Accelerators and / or retarders of setting or hardening, polymeric resins and antifoaming agents. The total content of additives and adjuvants preferably varies between 0.001 and 5% by weight relative to the total weight of the dry composition.

[0039] The dry mortar composition preferably comprises an expansion inhibitor, in particular selected from lithium salts (especially lithium nitrate and sodium sulfate), nitrates (especially sodium or potassium nitrate), and carboxylic acids (especially oxalic acid and citric acid). The total content of the expansion inhibitor is preferably between 0.5 and 3.0%, in particular between 1.0 and 2.5% by weight of hydraulic binder.

[0040] The mortar composition can in particular be used to obtain floor products, facade coatings, tile adhesives, jointing mortars, repair mortars, masonry mortars or even mortars for additive manufacturing.

[0041] Examples

[0042] The following examples illustrate the invention in a non-limiting manner.

[0043] The various examples use biomass ash from the combustion of waste from the paper and cardboard industry, the chemical composition of which includes as main elements 11% SiO2, 11% Al2O3, 1% Fe2O3, 57% CaO and 2% MgO. In terms of mineralogical composition, the ash contains 50% by weight of calcite, 6% free lime, 5% Ca(OH)2, 19% larnite (C2S), 6% gehlenite (C2A2S), 2% C3A, 2% metallic aluminum and 19% amorphous phase. The median diameter D50 was 14 pm.

[0044] In a comparative example, a mortar-adhesive composition comprising these biomass ashes was mixed with water, and the resulting wet mortar was poured into a container. The mortar-adhesive comprised 20% by weight of biomass ash, 3% aluminous slag, 0.9% Portland cement CEM I 42.5, 0.4% hemihydrate, 0.5% sodium sulfate, 0.1% lime, a cellulose ether, and 48.7% sand. Significant swelling was observed.

[0045] In a first series of examples, a 5 mm thick ash bed was maintained at a temperature of 20°C and a relative humidity of 70%. After approximately 2 hours, about half of the free lime had been transformed into slaked lime, without any significant change in the content of other hydraulic phases. After 24 hours, almost all of the free lime had been transformed into slaked lime, while the amount of the C2S phase was reduced by only 20%.

[0046] In a second series of examples, a 5 mm thick bed of ash was placed in a closed chamber at 20°C and 100% relative humidity. The treatment enabled the transformation of 45% of the free lime into slaked lime after 4 hours, and 60% after 24 hours, without significant variation in the contents of the C2S and C3A phases.

[0047] In a third series of examples, a 5 mm thick ash bed was maintained at a temperature of 20°C and a relative humidity of 70%, under an atmosphere containing 10% carbon dioxide. After 6 hours of treatment, the free lime content was reduced by 80%, without any change in the C3A phase content, but with a change of approximately 30% in the C2S phase content. With ash treated in this way, no swelling was observed in the case of a mortar-adhesive of the same composition as that of the comparative example.

[0048] In a fourth series of examples, biomass ash was mixed with liquid water at a ratio of 2.7% by weight, then spread in a thin layer and exposed to air for 24 hours. With the ash treated in this way, no swelling was observed in the case of a mortar-adhesive of the same composition as that of the comparative example. In this case, thermogravimetric analysis showed that mixing with water followed by exposure to air converted a large portion of the free lime and slaked lime into calcite. Conversely, in the absence of exposure to air (storage in a closed container after the hydration step), moderate swelling was observed. In this case, the hydrated, but not carbonated, ash contained large quantities of slaked lime.

Claims

Demands

1. A process for obtaining a powdered hydraulic binder comprising a step of hydrating biomass ash, said hydration step comprising bringing said biomass ash into contact with water in liquid or gaseous form.

2. A process according to claim 1, wherein the biomass ash comprises free lime (CaO).

3. A method according to any one of the preceding claims, wherein the biomass ash comprises metallic aluminum.

4. A process according to any one of the preceding claims, wherein the biomass ash has a chemical composition by weight comprising 5-35% SiO2, 3-20% Al2O3, 0-5% Fe2O3 and 30-70% CaO.

5. A process according to any one of the preceding claims, wherein the amount of water brought into contact is 1 to 5% by weight, relative to the weight of biomass ash.

6. A process according to any one of the preceding claims, wherein the process reduces the total weight content of hydraulic phases of biomass ash, other than free lime, by not more than 30%, in particular by not more than 10%, relative to its initial content.

7. A process according to any one of claims 2 to 6, wherein the process reduces the free lime content by weight by at least 50%, relative to its initial content.

8. A process according to any one of the preceding claims, comprising, after or during the hydration step, a carbonation step by contacting the biomass ash with carbon dioxide.

9. A process according to any one of the preceding claims, wherein the resulting powdered hydraulic binder comprises at least 8% by weight of calcium silicate and / or calcium aluminate hydraulic phases, in particular hydraulic phases selected from C2S, C3A and C2A2S.

10. Powdered hydraulic binder obtained by the process according to any one of the preceding claims.

11. Dry mortar composition comprising a powdery hydraulic binder according to the preceding claim and aggregates.