A Method For Producing An Activated Nesquehonite

Abstract

A method for producing an activated nesquehonite includes activating one or more nesquehonites by heating. The one or more nesquehonites may be formed by the reaction of carbon dioxide with aqueous magnesium ions at elevated pH, and may include barringtonite, nesquehonite, dypingite, hydromagnesite, and/or artinite and/or lansfordite. The activated nesquehonite may be useful in a building material, and have advantageous cementitious properties.

Claims

1. A method for producing an activated nesquehonite, which method comprises activating one or more nesquehonites by heating.

2. A method according to claim 1 wherein the one or more nesquehonites are activated by heating to a temperature of from 40 to 300 C., or from 75 to 400 C.

3. A method according to claim 2 wherein the one or more nesquehonites are activated by heating to a temperature of from 50 to 100 C., from 120 to 200 C., or from 150 to 250 C.

4. A method according to claim 1 which further comprises the steps of forming the one or more nesquehonites by the reaction of carbon dioxide with aqueous magnesium ions at elevated pH.

5. A method according to claim 4 wherein the carbon dioxide is reacted with alkali to form carbonate and/or bicarbonate anions at elevated pH, and the carbonate and/or bicarbonate anions are subsequently reacted with aqueous magnesium ions to form the one or more nesquehonites.

6. A method according to claim 4 wherein the carbon dioxide, or carbonate and/or bicarbonate containing aqueous solution, is reacted with the aqueous magnesium ions at a pH of 8 to 12, preferably 9 to 12, for example 10 to 11, measured at 25 C.

7. A method according to claim 5 wherein the carbon dioxide is reacted with the alkali at 1 to 3 equivalent moles of alkali per litre.

8. A method according to claim 5 wherein the equivalent moles of alkali per mole of captured carbon dioxide is between 1 and 2.

9. A method according to claim 5 wherein the carbonate and/or bicarbonate containing aqueous solution resulting from the reaction of the carbon dioxide with alkali reacts with the magnesium ions precipitating the one or more nesquehonites at a temperature of 10 to 80 C., preferably 20 to 70 C., more preferably 20 to 60 C.

10. A method according to claim 4 wherein the alkaline material for use in elevating the pH of the aqueous solution comprises Cement Kiln Dust (CKD).

11. A method according to claim 10 wherein a condensate is added to the CKD from the kiln gas phase.

12. A method according to claim 4 wherein the source of aqueous magnesium ions comprises reject water from a desalination plant, or formation waters.

13. A method according to claim 4 wherein the magnesium ions are present in the aqueous solution in an amount of 2 to 5 g/L.

14. A method according to claim 4 for capture and utilization/sequestering of carbon dioxide.

15. A method according to claim 1 wherein the one or more nesquehonites is selected from the nesquehonite-lansfordite family MgCO.sub.3.nH.sub.2O, the hydromagnesite-dypingite family, Mg.sub.5(CO.sub.3).sub.4(OH).sub.2.nH.sub.2O, and/or the artinite family, Mg.sub.2(CO.sub.3)(OH).sub.2.3H.sub.2O, and any mixture thereof

16. A method according to claim 15 wherein the one or more nesquehonites is selected from barringtonite, nesquehonite, and/or lansfordite, MgCO.sub.3.2H.sub.2O, MgCO.sub.3.3H.sub.2O and MgCO.sub.3.5H.sub.2O respectively, dypingite, hydromagnesite, and/or artinite, 4MgCO.sub.3.Mg(OH).sub.2.5H.sub.2O, 4MgCO.sub.3.Mg(OH).sub.2.4H.sub.2O and/or Mg.sub.2CO.sub.3(OH).sub.2.3H.sub.2O respectively, and any mixture thereof.

17. A method according to claim 15 for producing nesquehonite.

18. A material comprising one or more nesquehonites produced by a method according to claim 1.

19. A material according to claim 18 which is a cement, mortar or render, a board product, concrete, a construction block, a sound insulator and/or a thermal insulator, a fire retardant, and/or is suitable for use in a pharmaceutical product.

20. A material according to claim 18 which comprises a mouldable cementitious material comprising one or more rehydrated nesquehonites.

21. A material according to claim 20 which comprises a moulded product formed from one or more activated nesquehonites, either alone or with another material, by forming the one or more activated nesquehonites into a paste, filling a mould with the paste and curing in a wet atmosphere.

22. A material according to claim 21 wherein the paste is cured over a period of up to 36 hours, preferably up to 24 hours, at room temperature before de-moulding.

Description

[0034] The present invention will now be described in detail by way of an Example, with reference to the accompanying drawings in which:

[0035] FIG. 1 is a micrograph showing needles of Nesquehonite (NQ) MgCO.sub.3.3H.sub.2O obtained from the reaction of a magnesium-containing brine and a carbon dioxide-rich solution at 25 C.; and

[0036] FIG. 2 is a micrograph showing the microstructure of a fragment of a cube prepared with thermally activated Nesquehonite (NQ) MgCO.sub.3.3H.sub.2O.

EXAMPLE

Synthesis Study

[0037] Experiments were performed to study the effects of varying the temperature and reaction time on the phase of product formed from a preferred method of the present invention, including the yields of magnesium and consequently carbon dioxide in the product, the results of which are set out below in Table 2.

[0038] Thus, 100 ml of a 1M MgCl.sub.2 solution was added to 1L of a 0.1M Na.sub.2CO.sub.3 solution brought to the target temperature. After filtration, the solid was dried over silica gel, ground, and scanned by XRD and SEM.

[0039] The yield of the reaction for Mg was calculated. The mass of Mg in the filtrate was calculated from Atomic Absorption Spectroscopy (AAS) measurements: the lower the Mg concentration in the filtrate, the higher the yield and so the higher the amount of Mg being precipitated. The uncertainty of the yield values is +/5% due to uncertainties in amounts recovered and in analyses. For example, variations in the total volume as well as the amount of water incorporated in the solid products have been neglected: for 0.1 mol of NQ precipitated, the volume of water incorporated is the order of 5 mL, whereas for 0.02 mol of HM precipitated, the volume of water incorporated is the order of 2 mL (results for an initial total volume of approximately 1.1L).

[0040] The yield of the reaction for CO.sub.2 can be calculated from the Mg yield (N.B. DG.star-solid. is short-hand notation for a dypingite-like phase, i.e. a phase similar to hydromagnesite but with more than 4 moles of crystallisation water per formula unit, DG being the specific case where there are 5 moles of crystallisation water).

TABLE-US-00002 TABLE 2 Main phase in Mg yield CO.sub.2 yield T ( C.) t (h) product (mass %) (mass %) 25 1 NQ 59 59 2 NQ 82 82 4 NQ 91 91 24 NQ 86 86 35 1 NQ 68 68 2 NQ 77 77 4 NQ + DG* 77 76-77 24 DG* 73 58 45 1 NQ + DG* 77 76-77 2 NQ + DG* 82 81-82 4 NQ + DG* 82 75-78 24 DG* 86 69 55 1 NQ + DG* 77 76-77 2 DG* 73 58 4 DG* 82 66 24 HM + DG* 82 66 65 1 HM + DG 86 69 2 HM + DG 86 69 4 HM + DG* 86 69 24 HM 95 76

[0041] Thus, to summarise the data in Table 2, at the lowest temperature of 25 C. NQ was the predominant phase for all reaction times from 1 to 24 hours, and remained the dominant phase at 35 C. for shorter reaction times of 1 to 2 hours. From 4 to 24 hour reaction times at 35 C. through increasing temperature to 55 C. the predominant phase contained DG.star-solid., either with NQ, with HM (at 24 hours reaction time at 55 C.) or alone, and at 65 C. the predominant phase contained HM, with DG at shorter reaction times of from 1 to 2 hours, with DG.star-solid. at 4 hour reaction time, and alone at 24 hour reaction time.

[0042] FIG. 1 shows a scanning electron micrograph of NQ crystals formed at 25 C. and subsequently air dried. The magnification scale is X200, and the total length of the line formed by connecting dots is 150 microns (0.15 mm).

[0043] The crystals grow with characteristic needle to long prismatic shapes with clean surfaces and often with terminating pinacoids. The terminations show that the crystals are solid. Individual crystals do not generally cohere or join, with the result that dry powders and powder compacts have strong preferred orientation and a high proportion of interstitial space.

[0044] FIG. 2 shows a hydrated self-cemented cube formed from the NQ shown in FIG. 1, once activated. The image is of a fracture surface and the magnification relative to FIG. 1 is increased by approximately ten-fold.

[0045] To make the cube, NQ (FIG. 1) was activated by heating to 100 C. in air. Crystalline reflections characteristic of NQ disappear during heating but morphological relicts of the original crystals persist even after rehydration as hollow needles (arrowed in FIG. 2). On rewetting and curing at 20 C., NQ is regenerated and cemented by formation of intermediate products; the latter are fine-grained and, at this magnification, have a poor morphology. Several types of pore space are thus created in the cemented product, which has low bulk density, in this example ca 700 kg/m.sup.3. The presence of small air voids comprising a significant proportion of the whole volume contributes to the low thermal conductivity.

[0046] It will be understood that the embodiments illustrated above describe the invention only for the purposes of illustration. In practice the invention may be applied in different embodiments and applications.