Method for producing hydraulic binders from water treatment residues

Abstract

A method for making a high alumina hydraulic binder includes hydrating a source of aluminum ions with a source of calcium ions in the presence of water to form a mineral hydrate and then heating the mineral hydrate to form the high alumina hydraulic binder.

Description

Field of the Invention

[0001] The present invention relates to a method for producing a hydraulic binder from water treatment residues or other aluminum-rich waste residues, in particular to a method for producing a hydraulic binder containing calcium aluminate and / or calcium sulfoaluminate. Background of the Invention

[0002] Water treatment residuals (WTR or simply residuals) are waste products from the industrial purification of drinking water using coagulation with flocculants. They typically contain aluminum compounds such as aluminum sulfate (Al2(SO4)3·14H2O), often called alum, or aluminum ferric sulfate (ferrum). Coagulation removes particles suspended in untreated natural water. The chemical reactions involved include:

[0003] Al2(SO4)3·14H2O→2Al(OH)3(s)+6H + +3SO4 2- +8H2O

[0004] The solidified solids are separated from the treated water to form an aluminum-rich waste slurry, which is called WTR. WTR is generally composed of amorphous aluminum hydroxide, water, organic materials, residual flocculants and other minor components.

[0005] Currently there are only a few viable recycling options for this material, with most ending up in landfill. This waste is generated worldwide and the ability to recycle it would have global appeal.

[0006] Cement is a binder, a building substance that sets, hardens, and adheres to other materials to bind them together. Cement is rarely used on its own, but is used to bind sand and gravel (lumps) together. Cement mixed with fine aggregate produces mortar for masonry, or cement mixed with sand and gravel produces concrete. Cements used in construction are generally inorganic, usually based on lime or calcium silicate, and can be characterized as hydraulic or non-hydraulic, depending on the cement's ability to set in the presence of water (in the case of hydraulic binders).

[0007] High-alumina cements, such as calcium aluminate cement and calcium sulfoaluminate cement, are typically made from limestone and bauxite. Hydration of calcium aluminate cement generally produces calcium aluminate hydrate, while hydration of calcium sulfoaluminate cement generally produces ettringite, and specific physical properties (such as expansion or rapid reaction) can be obtained by adjusting the availability of calcium and sulfate ions. Calcium aluminate and calcium sulfoaluminate cements are typically industrially produced through high-temperature sintering processes (>1200°C), requiring significant energy resources and resulting in the production of large amounts of CO2.

[0008] The object of the present invention is to provide a process for producing high-alumina hydraulic binders, in particular calcium aluminate cement and / or calcium sulfoaluminate cement, at relatively low temperatures and using water treatment residues as raw materials, thereby providing a more environmentally friendly and less costly alternative to conventional high-alumina cement production processes. SUMMARY OF THE INVENTION

[0009] According to a first aspect of the present invention, there is provided a method for producing a high alumina hydraulic binder, comprising hydrating an aluminum ion source and a calcium ion source in the presence of water to form a mineral hydrate, and subsequently heating the mineral hydrate to form the high alumina hydraulic binder.

[0010] In one embodiment, a sulfate ion source may be mixed with the aluminum ion source and the calcium ion source prior to and / or during hydration.

[0011] The source of aluminum ions preferably comprises water treatment residues.

[0012] Water treatment residues can be formed, in particular, in the form of drinking water treatment residues, which can be purified more easily than other residues, such as wastewater treatment residues, etc., in particular because they are less contaminated. Since drinking water and wastewater have different compositions, they (drinking water / wastewater) are not directly comparable.

[0013] Organic material present in the water treatment residue may form the fuel energy source during the heating step and air or oxygen may be added to the mixture during the heating step to facilitate oxidation of the organic material.

[0014] The method may further comprise comminuting the hydrated mineral prior to the heating step.

[0015] Preferably, the hydrated mineral is heated to at least 350°C during the heating step to obtain the most efficient binder.

[0016] According to another aspect of the present invention, there is provided a process for producing calcium aluminate cement comprising the steps of mixing a source of calcium ions with a water treatment residue or other aluminum-rich waste residue in the presence of water in a mixing step to form a mineral hydrate, followed by heating the resulting mineral hydrate to a temperature of at least 350° C. in a heat treatment step to oxidize organic material present and at least partially dehydrate the hydrated mineral to form a final cement product.

[0017] According to yet another method for producing calcium sulfoaluminate cement, it comprises the steps of mixing a calcium ion source and a sulfate with a water treatment residue or other aluminum-rich waste residue in the presence of water in a mixing step to form a mineral hydrate, followed by heating the resulting mineral hydrate to a temperature of at least 350° C. in a thermal treatment step to oxidize the organic material present and at least partially dehydrate the hydrated mineral to form the final cement product.

[0018] The mineral hydrate may be comminuted prior to the heat treatment step.

[0019] The water treatment residue may be at least partially dewatered prior to said mixing step.

[0020] Air may be added to the material during the thermal treatment stage to promote oxidation of the organic material.

[0021] The source of calcium ions may comprise lime or hydrated lime.

[0022] Embodiments of the present invention

[0023] Methods for producing hydraulic binders from WTR or other aluminum-rich waste residues according to various embodiments of the present invention will now be described by way of example only.

[0024] The water treatment residue consists of water, organic material, and inorganic material. The inorganic material primarily consists of amorphous aluminum oxide and / or amorphous aluminum hydroxide (typically 75-80% amorphous aluminum oxide, with a 20-25% crystallinity, as measured, for example, by spectroscopic methods, DSC measurements, or X-ray diffraction experiments). The remaining inorganic components (primarily varying amounts of quartz and iron oxide) are essentially inert fillers. The amorphous aluminum oxide / aluminum hydroxide can be used in the hydration / dehydration process to form a cementitious mineral in the methods according to various embodiments of the present invention, as described below.

[0025] Calcium aluminate hydrate can be formed when WTR is mixed with a calcium ion source (eg, Portland cement, hydrated lime) in the presence of water.

[0026] When WTR is mixed with a calcium ion source (such as Portland cement, hydrated lime) and a sulfate ion source (such as calcium sulfate) in the presence of water, ettringite (Ca6[Al(OH)6]2(SO4)3.26H2O) can be formed.

[0027] The exact formulation of the precursor materials used to make the desired cementitious material will depend on the chemical composition of the water treatment residue used and the specific industrial application of the final product.

[0028] According to various embodiments of the present invention, a method for producing a hydraulic binder is provided that uses water treatment residues or other high aluminum content residues containing a reactive aluminum source (such as amorphous aluminum hydroxide, amorphous aluminum oxide, or aluminum chloride) as a raw material.

[0029] The term "amorphous" in mineralogy refers to a solid / material that lacks a crystalline form. In Northern Ireland, a typical chemical composition of water treatment residue (WTR) would indicate that 80-90% of the inorganic material is amorphous aluminum hydroxide. Another common composition is between 50-80% amorphous aluminum hydroxide. Other WTRs contain inorganic material with less than 50% amorphous aluminum hydroxide.

[0030] When manufacturing cement products, it is advantageous for the treated material to contain as much amorphous aluminum hydroxide as possible. This is because the crystalline form of aluminum hydroxide / aluminum oxide is essentially inert (chemically unreactive) in cementitious systems. However, the amorphous forms of aluminum hydroxide, oxyhydroxide, and oxide are chemically reactive in cementitious systems and, for example, will react with hydrated lime in the presence of water to form calcium aluminate hydrate. This reaction generates strength and leads to the formation of a hardened material. Other advantageous properties (rapid setting, hardening, and drying) may also be achieved. However, WTR containing lower amounts of amorphous aluminum hydroxide may still be useful, depending on the application.

[0031] In a first embodiment, the precursor materials preferably include a calcium ion source (e.g., lime, hydrated lime) and aluminum-rich WTR (or any other source of amorphous aluminum oxide / hydroxide (e.g., ECEM) or any other suitable aluminum-containing waste material containing a reactive or soluble aluminum ion source). A water source is also required. The water source can come from the water present in the WTR.

[0032] The method comprises the following steps:

[0033] Step 1: Blending / mixing precursor materials

[0034] The precursor material is mixed with water to form a paste / slurry. The WTR can be used as is. Alternatively, the WTR can be partially dehydrated and ground or otherwise comminuted to a particle size suitable for the application, for example, about 40 microns.

[0035] The composition and proportions of the precursor materials can be varied to produce a range of calcium aluminate hydrates (eg, hemicarbonate and katoite).

[0036] The choice of equipment for mixing will depend on the rheology of the mixture. In one embodiment, the mixing equipment is envisioned to be similar to a concrete mixer. The mixing equipment may be adapted to provide batches or to provide a continuous supply of mixed precursor materials.

[0037] Step 2: Hydrate

[0038] During the hydration period, hydrates are formed and the material can solidify. Preferably, the hydration is completed in less than 24 hours. Depending on the choice of precursor material, a range of hydrates are formed.

[0039] Step 3: Heat Treatment

[0040] In the heat treatment step, the hydrated material is heat treated, for example to a temperature of at least 350° C. Preferably, air is added during heating at a rate suitable to ensure that substantially all of the organic material in the WTR is oxidized:

[0041] The caloric value of the organic material in the WTR (~5-10 MJ / Kg) can be used as an energy source (full or partial) for the thermal treatment process.

[0042] Heat treatment temperatures may range from 350° C. to greater than 1200° C. It may be preferred that the heat treatment does not exceed 800° C. to maximize material reactivity.

[0043] Prior to heat treatment, the hydrated material may be partially dehydrated and ground or otherwise comminuted to a particle size suitable for the application, such as about 40 microns.

[0044] Heat treatment may occur in a suitable heating chamber such as a rotary calciner, flash calciner, or the like.

[0045] This heat treatment step produces a range of anhydrous calcium aluminate minerals. The type and amount of each mineral in the final product depends primarily on the precursor material ratios and composition of the WTR or other precursor materials used.

[0046] As the temperature reached in the heat treatment step increases, the calcium aluminate mineral becomes more crystalline.

[0047] Step 4: Final product: CA cement

[0048] The final product (consisting of a mixture of anhydrous calcium aluminate minerals) is cooled. After cooling, the final product may optionally be ground or otherwise comminuted to form a final cement product.

[0049] According to a second embodiment of the present invention, a method for producing calcium sulfoaluminate cement is provided, which utilizes water treatment residues or other high aluminum content residues as raw materials mixed with a calcium ion source and a sulfate source.

[0050] In one example, the precursor materials may include a calcium ion source (e.g., lime, hydrated lime), a sulfate (e.g., gypsum, anhydrite, hemihydrate), and aluminum-rich WTR (or a source of amorphous aluminum oxide / hydroxide (e.g., ECEM). As in the first embodiment, the water source may be water in the WTR.

[0051] Step 1: Blend WTR with calcium ion source and sulfate

[0052] Hydrated lime (or lime), sulfate (such as gypsum, anhydrite, hemihydrate) and aluminum-rich WTR are mixed with water to form a paste / slurry.

[0053] The WTR can be used as is. Alternatively, the WTR can be partially dehydrated and ground or otherwise comminuted to a suitable particle size.

[0054] Depending on the composition of the WTR, the addition of a sulfate source may be optional (ie, the sulfate source may already be in the WTR).

[0055] The composition and proportions of the precursor materials can be varied to produce a range of calcium sulfoaluminate hydrates (ettringite) and calcium aluminate hydrates (eg, hemicarbonate or katoite).

[0056] As with the first embodiment, the choice of mixing equipment used to mix the precursor materials depends on the rheology of the mixture. The mixing equipment can provide a continuous mixture or a batch mixture.

[0057] Step: Hydration

[0058] During hydration, hydrates form and the product solidifies. Preferably, hydration is complete in less than 24 hours. Depending on the composition and ratio of the precursor materials, a range of hydrates form. An example of this is the formation of the mineral ettringite.

[0059] like Figure 1 The table shown shows the XRD traces of a hydrated mixture containing WTR, hydrated lime and calcium sulfate produced according to this method.

[0060] Step 3: Heat Treatment

[0061] The hydrated material produced from the second step is heat treated (eg at a temperature of at least 350°C to substantially maximise dehydration of the hydrates). Preferably, air is added during heating at a rate suitable to ensure that substantially all organic material in the WTR is oxidised.

[0062] The caloric value of organic carbon from WTR (~5-10 MJ / Kg) can be used as a fuel source (full or partial) for the thermal treatment process.

[0063] The heat treatment temperature may range from 350° C. to greater than 1200° C. Preferably, the heat treatment does not exceed 800° C. in order to maximize the material reactivity, in particular to ensure that the amorphous residue Al 2 O 3 remains in its amorphous state and therefore remains reactive.

[0064] Prior to heat treatment, the hydrated material may be partially dehydrated and ground or otherwise comminuted to a suitable particle size, for example, to about 40 microns.

[0065] The heat treatment step may occur in a rotary calciner, flash calciner, or similar equipment.

[0066] The thermal treatment step can produce a range of calcium sulfoaluminate and calcium aluminate minerals (see Figure 2 ).

[0067] The type and amount of each mineral depends mainly on the composition and ratio of the precursor materials and in particular the composition of the WTR.

[0068] As the temperature reached in the heat treatment step increases, the calcium aluminate and calcium sulfoaluminate minerals become more crystalline.

[0069] Step 4: Final Product: CSA Cement

[0070] The final product (composed of calcium sulfoaluminate and calcium aluminate minerals) is cooled.After cooling, the final product may optionally be ground or otherwise comminuted to the desired particle size.

[0071] The cement produced may include a range of calcium aluminates and calcium sulfoaluminates, such as the mineral calcium sulfoaluminate (C4A3S-Ca4(AlO2)6SO4), monocalcium aluminate (CA-CaAl2O4), calcium aluminate (C12A7-Ca 12 Al 14 O 33 ) and tricalcium aluminate (C3A-Ca3(AlO3)2).

[0072] It is conceivable that the final hydraulic binder produced by the method according to the present invention could be used in the production of floor screeds. It has been found that the treated residue, in particular in the form of ettringite, exhibits shrinkage-compensating properties. Therefore, products produced from the treated residue could be direct competitors to Portland cement-based screeds.

[0073] The present invention provides a process for making high alumina hydraulic binders which can utilize waste materials and requires much lower energy input than prior art processes and thus provides a more environmentally friendly and lower cost alternative to conventional high alumina cement manufacturing processes.

[0074] The invention is not limited to the embodiments described herein but may be modified or varied without departing from the scope of the invention as defined in the appended claims.

Claims

1. A method for making a high alumina hydraulic binder comprising hydrating an aluminum ion source with a calcium ion source in the presence of water to form a mineral hydrate and subsequently heating the mineral hydrate to form the high alumina hydraulic binder.

2. The method according to claim 1, wherein Prior to and / or during hydration, a source of sulfate ions is mixed with the source of aluminum ions and the source of calcium ions.

3. The method according to claim 1 or 2, wherein: The aluminum ion source includes water treatment residues.

4. The method according to claim 3, wherein: Organic material present in the water treatment residue forms a fuel energy source during the heating step, and air or oxygen is added to the mixture during the heating step to facilitate oxidation of the organic material.

5. The method according to any one of the preceding claims, further comprising comminuting the hydrated mineral prior to the heating step.

6. The method according to claim 1, wherein: The hydrated mineral is heated to at least 350°C in the heating step.

7. A method for producing calcium aluminate cement, comprising the following steps: A source of calcium ions is mixed with water treatment residues or other aluminum-rich waste residues in the presence of water in a mixing step to form mineral hydrates, followed by heating the resulting mineral hydrates to a temperature of at least 350° C. in a thermal treatment step to oxidize the organic material present and at least partially dehydrate the hydrated minerals to form the final cement product.

8. A method for producing calcium sulfoaluminate cement, comprising the following steps: A source of calcium ions and a sulfate salt are mixed with water treatment residues or other aluminum-rich waste residues in the presence of water in a mixing step to form a mineral hydrate, followed by heating the resulting mineral hydrate to a temperature of at least 350° C. in a thermal treatment step to oxidize the organic material present and at least partially dehydrate the hydrated mineral to form the final cement product.

9. The method according to claim 7 or 8, wherein The mineral hydrate is comminuted prior to the heat treatment step.

10. The method according to any one of claims 7 to 9, wherein: The water treatment residue is at least partially dewatered prior to the mixing step.

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