SUSTAINABLE CALCIUM HYDROXIDE PRODUCTION FOR GREEN CEMENT

Abstract

A method of making a composition of matter comprising calcium hydroxide. The method includes the steps of contacting a calcium-containing molecule with an aqueous solution of a water-soluble salt having ammonium cation and a counter-anion, under conditions effective to yield a compound containing calcium and the counter-anion; and reacting the compound comprising calcium and the counter-anion with ammonia and water under conditions to yield calcium hydroxide.

Claims

1. A method of making a composition of matter comprising calcium hydroxide, the method comprising: (a) contacting a material comprising calcium-containing molecules with an aqueous solution comprising a water-soluble salt comprising ammonium cation and a counter-anion, for a time, and at a temperature, pH, and pressure effective to yield a compound comprising calcium and the counter-anion; and (b) reacting at least a portion of the compound comprising calcium and the counter-anion with ammonia and water for a time, and at a temperature, pH, and pressure effective to yield calcium hydroxide.

2. The method of claim 1, wherein the water-soluble salt comprising ammonium cation and a counter-anion is selected from the group consisting of ammonium halide, ammonium acetate, ammonium phosphate, ammonium oxalate, and ammonium lactate.

3. The method of claim 1, wherein the water-soluble salt comprising ammonium cation and a counter-anion is ammonium chloride or ammonium acetate.

4. The method of claim 1, wherein step (b) yields calcium hydroxide and ammonium halide and further comprising: (c) recycling at least a portion of the ammonium halide formed in step (b) and using it as the water-soluble salt comprising ammonium cation and a counter-anion.

5. A method of making a composition of matter comprising calcium hydroxide, the method comprising: (a) contacting a material comprising calcium-containing molecules with an aqueous solution comprising a water-soluble salt comprising ammonium cation and a counter-anion, for a time, and at a temperature, pH, and pressure effective to yield a compound comprising calcium and the counter-anion; (b) reacting at least a portion of the compound comprising calcium and the counter-anion with ammonia and water for a time, and at a temperature, pH, and pressure effective to yield calcium hydroxide; and (c) recycling at least a portion of the ammonium halide formed in step (b) and using it as the water-soluble salt comprising ammonium cation and a counter-anion.

6. The method of claim 5, wherein the water-soluble salt comprising ammonium cation and a counter-anion is selected from the group consisting of ammonium halide, ammonium acetate, ammonium phosphate, ammonium oxalate, and ammonium lactate.

7. The method of claim 5, wherein the water-soluble salt comprising ammonium cation and a counter-anion is ammonium chloride or ammonium acetate.

8. A method of making a composition of matter comprising calcium hydroxide, the method comprising: (a) contacting a material comprising calcium-containing molecules with an aqueous solution comprising ammonium chloride, for a time, and at a temperature, pH, and pressure effective to yield a compound comprising calcium chloride; and (b) reacting at least a portion of the calcium chloride of step (a) with ammonia and water for a time, and at a temperature, pH, and pressure effective to yield calcium hydroxide and ammonium chloride.

9. The method of claim 8, further comprising: (c) recycling at least a portion of the ammonium chloride formed in step (b) and using it as the ammonium chloride in step (a).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] FIG. 1 is a stylized schematic diagram showing the overall method disclosed herein.

[0015] FIG. 2 shows the chemical reactions for the mineral dissolution and hydrolysis steps described here, along with the overall combination of the two reactions (which yields Ca.sup.2+ ions). Lastly is shown the ammonium absorption and precipitation reactions, in which the Ca.sup.2+ ions are reacted with Cl.sup.−, NH.sub.4.sup.+, and OH.sup.− ions to yield Ca(OH.sub.2), which precipitates from solution.

[0016] FIG. 3 is a formal schematic diagram showing an exemplary apparatus that can be used to practice the method disclosed and claimed herein.

DETAILED DESCRIPTION

[0017] Numerical ranges as used herein are intended to include every number and subset of numbers contained within that range, whether specifically disclosed or not. Further, these numerical ranges should be construed as providing support for a claim directed to any number or subset of numbers in that range. For example, a disclosure of from 1 to 10 should be construed as supporting a range of from 2 to 8, from 3 to 7, from 1 to 9, from 3.6 to 4.6, from 3.5 to 9.9, and so forth.

[0018] All references to singular characteristics or limitations of the present invention shall include the corresponding plural characteristic or limitation, and vice-versa, unless otherwise specified or clearly implied to the contrary by the context in which the reference is made. The indefinite articles “a” and “an” mean one or more.

[0019] All combinations of method or process steps as used herein can be performed in any order, unless otherwise specified or clearly implied to the contrary by the context in which the referenced combination is made.

[0020] The method disclosed herein can comprise, consist of, or consist essentially of the essential elements and limitations of the method described herein, as well as any additional or optional steps or limitations described herein or otherwise useful in chemical engineering.

[0021] The term “contacting” refers to the act of touching, making contact, or of bringing to immediate or close proximity, including at the molecular level, for example, to bring about a chemical reaction, or a physical change, e.g., in a solution or in a reaction mixture.

[0022] An “effective amount” refers to an amount of a chemical or reagent effective to facilitate a chemical reaction between two or more reaction components, and/or to bring about a recited effect. Thus, an “effective amount” generally means an amount that provides the desired effect.

[0023] Referring now to FIG. 1, the figure is a stylized schematic diagram of the novel method to make calcium hydroxide (Ca(OH).sub.2) as disclosed and claimed herein. Ca(OH).sub.2 is a critical ingredient in cement. (Ca(OH).sub.2 is also referred to as “slaked lime.”) It plays a central role in the hydration reactions that drive the curing of wet cements and concretes comprising the cements. It also plays a role in the final physical-mechanical characteristics of the cured cement/concrete.

[0024] As shown in FIG. 1, the raw material for the present method is calcium-containing waste streams, such as recycled concrete, bottom ash, fly ash, and the like. Typically, these materials are landfilled or stored in large “ash ponds.” An “ash pond,” also called a coal ash basin or surface impoundment, is an engineered storage structure used at fossil fuel power stations to hold bottom ash and fly ash. (“Bottom ash” is part of the non-combustible residue of combustion in coal-fired power plants, boilers, furnaces, and incinerators. Bottom ash is the heavier, non-combustible ash (“clinkers”) that form inside the combustion chamber and fall to the bottom of the chamber due to gravity. The lighter portion of the ash that escapes up the chimney is “fly ash.” In modern, coal-burning facilities, the lion's share of the fly ash is isolated using scrubbers and impounded in an ash pond along with the bottom ash. The ash pond is used as a landfill to prevent the release of the ash into the atmosphere. While certainly preferred to unrestricted release of the ash into the environment, ash ponds themselves are significant environmental hazards.

[0025] The method produces calcium hydroxide through an aqueous leaching-precipitation cycle aided by ammonia. The overall process is described by the following two reactions:

2NH.sub.4Cl+H.sub.2O+CaSiO.sub.2.fwdarw.CaCl.sub.2+SiO.sub.2↓+2H.sub.2O+2NH.sub.3↑  Reaction 1:

CaCl.sub.2+2NH.sub.3+2H.sub.2O.fwdarw.Ca(OH.sub.2)↓+2NH.sub.4Cl  Reaction 2:

[0026] In Reaction 1, calcium ions are extracted from calcium-bearing minerals using an aqueous solution comprising a water-soluble ammonium salt, preferably an ammonium halide salt, and most preferably ammonium chloride, which is shown as the exemplary ammonium salt in Reactions 1 and 2. Reaction 1 produces calcium chloride solution (as shown in Reaction 1) or a calcium salt comprising the anion from the water-soluble ammonium salt used, along with leached mineral residue, and ammonia gas. In Reaction 2, the calcium chloride solution and ammonia gas from the first step are collected and reacted to yield calcium hydroxide, which precipitates from the aqueous solution. The second reaction utilizes the low and inverse solubility of calcium hydroxide to induce precipitation at elevated temperature and mild pressurization.

[0027] Regarding the inverse (or retrograde) solubility of calcium hydroxide, the solubility of calcium hydroxide at 70° C. is about half of its value at 25° C. This counter-intuitive phenomenon arises because the dissolution of calcium hydroxide in water is exothermic process and follows Le Chatelier's principle. Thus, at lower temperatures, the elimination of the heat liberated through the process of dissolution increases the equilibrium constant of dissolution of calcium hydroxide.

[0028] Thus, it is preferred that Reaction 2 be conducted at a pressure above atmospheric pressure and a temperature ranging from roughly 25° C. to the boiling point of the reaction solution at the pressure chosen, and more preferably from about 70° C. to the boiling point of the reaction solution at the pressure chosen. A preferred pressure range is from roughly 2 bar to about 10 bar.

[0029] The calcium hydroxide precipitate is then separated from the reaction solution by conventional means. This can be done continuously or batchwise, as is known in the industry.

[0030] After the calcium hydroxide precipitates are separated, the ammonium chloride solution is recycled for use in the leaching step shown in Reaction 1.

[0031] For a feedstock, the method can use crystalline, amorphous, or hydrated phases of calcium silicates/aluminate/aluminosilicates. Such materials are abundant in a wide range of industrial waste streams, including crushed concrete, coal ashes, steel and iron slags, etc.

[0032] The sustainable calcium hydroxide produced from this process can replace limestone as the calcium source, offering a realistic pathway to reducing the carbon footprint of the existing cement industry by more than 50%. Furthermore, when combined with concrete recycling and/or carbonation-based cementation technologies, it can transform cement production into a carbon-negative industry. The technology enables a pathway for direct capture of atmospheric carbon as precipitated calcium carbonate, a valuable co-product, and stable form of bound carbon.

[0033] On this score, the present method is a distinct improvement over conventional methods to reduce the carbon emission of cement/concrete production. Conventional methods, such as those noted above, rely on blending cement with supplementary cementitious materials such as coal fly ash or other fillers. However, the typical replacement ratio is limited to 15-30%, and it provides little benefit to cement producers.

[0034] Reactions 1 and 2 can be broken down further as shown in FIG. 2. Reaction 1 can be parsed out as three separate reactions, representing mineral dissolution and hydrolysis:

2CaO.SiO.sub.2+4H.sup.+.fwdarw.2Ca.sup.+2+SiO.sub.2↓  1.a:

4NH.sub.4Cl.fwdarw.4NH.sub.4.sup.++4Cl.sup.−  1.b

4NH.sub.4.sup.+.fwdarw.4NH.sub.3↑+4H.sup.+  1.c:

The overall Reaction 1, excluding the water molecules, is thus:

CaO.SiO.sub.2+4NH.sub.4Cl.fwdarw.2Ca.sup.+2+2Ca.sup.+2+4Cl.sup.−+SiO.sub.2↓+4NH.sub.3↑  2

[0035] Similarly, Reaction 2 can be broken down as follows, representing ammonia absorption and precipitation of calcium hydroxide:

NH.sub.3(g)+H.sub.2O.Math.NH.sub.4.sup.++OH.sup.−(at increased pressure)  2.a:

Ca+.sup.2+2Cl—+2NH.sub.4.sup.++2OH.sup.−.fwdarw.Ca(OH.sub.2)↓+2NH.sub.4.sup.++2 Cl.sup.−  2.b:

[0036] FIG. 3 shows an exemplary schematic implementation of the method disclosed and claimed herein. The various apparatus and conduit shown in FIG. 3 is conventional and will not be described in any detail. Starting at the upper left corner of FIG. 3, the incoming powdered feedstock is fed into the process via a solid sample feeder 10. The feedstock, along with ammonium chloride 12 (which can be virgin or recycled from the process) are fed into a jacketed, temperature-controlled, stirred dissolution reactor 14. Reaction 1 takes place within reactor 14. The reactants may be reacted at room temperature up to the boiling point of solution (roughly 100° C.) and for a time sufficient to dissolve at least a portion of any calcium compound present in the feedstock.

[0037] The reaction solution is then transferred to separator/settling tank 16, which is dimensioned and configured to separate any precipitates from the reaction solution (principally SiO.sub.2). The calcium-rich supernatant is optionally cooled (if necessary) at process cooler 18 and pumped via pump 20 into reactor 22. Reactor 22 is operationally connected to a back-pressure regulator 24. As noted in FIG. 3, the preferred pressure for the reaction in reactor 22 is about 2 bar. Reaction 2 takes place in reactor 22.

[0038] The contents of reactor 22 are then transferred to separator/filtration unit 26 to recover the precipitated calcium hydroxide.

[0039] Ammonia recovered in separator 26 is recycled back into the process via condenser 28 and a separator 30. The separator 30 is dimensioned and configured to separate ammonia from the process water. The water is purged from the apparatus and send for treatment. The ammonia is sent to an ammonia make-up unit 32. Unit 32 is dimensioned and configured to mix the recycled ammonia with fresh ammonia and re-introduced into reactor 22 after optionally being passed through mixer 34 and chiller 36.

[0040] Additionally, NH.sub.4Cl present in the effluent from separator 26 is likewise recycled as shown at conduit 38 and used to dissolve the incoming feedstock. Make-up NH.sub.4Cl may also be added at 12.

[0041] The apparatus shown in FIG. 3 is exemplary only. Other equally suitable means for implementing the method will be apparent to chemical engineers of ordinary skill in the art.