CARBONATION OF CALCIUM SULFATE CONTAINING MATERIALS

Abstract

A method for sequestering carbon dioxide includes providing a starting material having calcium ions, sulfate and/or sulfite ions, and alkali metal ions with a molar ratio ? Alk:? SO3>2.00, reacting the starting material with carbon dioxide at a pressure in the range from ambient up to 3.5 bar overpressure and/or a temperature in the range from 10 to 145? C. to obtain a carbonated product comprising calcium carbonate and alkali sulfate. The obtained product is used as a set regulator or accelerator for cement and/or as a minor or main cement component.

Claims

1. A method for sequestering carbon dioxide comprising providing a starting material comprising calcium ions, sulfate and/or sulfite ions, and alkali metal ions with a molar ratio ? Alk:? SO.sub.3?2.00, reacting the starting material with carbon dioxide at ambient pressure or at up to 3.5 bar overpressure and/or at a temperature in the range from 10 to 145? C. either in an aqueous liquid at a water:solids weight ratio from 1 to 100 or in a gas-solid reactor at a relative humidity from 75 to 100%, and obtaining a carbonated product comprising finely precipitated calcium carbonate and alkali sulfate.

2. The method according to claim 1, wherein the starting material is reacted with carbon dioxide at ambient pressure or at an overpressure of up to 1 bar and/or at a temperature in the range from 20 to 90? C.

3. The method according to claim 1, wherein the starting material is provided by mixing a material comprising calcium sulfate and a material comprising alkali metal ions, wherein the material comprising calcium sulfate is selected from the group consisting of gypsum, anhydrite, hemihydrate, phosphogypsum, waste concrete, slags, red and brown muds, bottom ashes, and mixtures thereof to provide calcium sulfate and the material comprising alkali metal ions is selected from the group consisting of dusts collected during cement and clinker production, especially bypass dust, clinker kiln dust, electro-static precipitator dust; paper ash; wood ash; biomass ash; and mixtures thereof to provide alkali metal ions.

4. The method according to claim 1, wherein the molar ratio ? Alk:? SO.sub.3 in the starting material ranges from 2.00 to 5.00.

5. The method according to claim 1, wherein sulfite and/or sulfide present in the starting material is oxidized to sulfate prior to carbonation and/or sulfite and/or sulfide present in a material used to provide the starting material is oxidized to sulfate.

6. The method according to claim 1, wherein a particle size of the starting material and/or a material used to provide the starting material is optimized by grinding or co-grinding and/or classifying.

7. The method according to claim 1, wherein exhaust gas from a cement kiln, lime kiln, waste incineration, coal fired power plant, gas fired power plant or exhaust gases combined from two or more of them is used as the carbon dioxide.

8. The method according to claim 1, wherein the reaction time ranges from 1 to 100 minutes.

9. The method according to claim 1, wherein carbonation is carried out in aqueous suspension, by bubbling the carbon dioxide in gaseous form through the aqueous suspension.

10. The method according to claim 9, wherein a solid product comprising calcium carbonate is separated from the aqueous suspension by precipitation.

11. The method according to claim 1, wherein carbonation is carried out in a gas-solid reactor.

12. The method according to claim 11, wherein calcium carbonate is separated from alkali sulfate by suspending the carbonated product in water and precipitating the calcium carbonate.

13. The method according to claim 1, wherein the obtained carbonated product comprising calcium carbonate and alkali sulfate is used as set regulator or as cement component.

14. The method according to claim 13, wherein the starting material contains at least 15 wt.-% sulfate and/or sulfite calculated as calcium sulfate to provide the set regulator or the cement component being an activator.

15. The method according to claim 13, wherein the starting material contains from 0.1 to 25 wt.-% sulfate and/or sulfite calculated as calcium sulfate and/or the alkali sulfate is removed from the carbonated product for use as minor or main cement component.

16. The method according to claim 2, wherein the molar ratio ? Alk:? SO.sub.3 in the starting material ranges from 2.00 to 4.00.

17. The method according to claim 3, wherein the molar ratio ? Alk:? SO.sub.3 in the starting material ranges from 2.00 to 3.00.

18. The method according to claim 6, wherein the particle size is adjusted to range from 0.01 ?m to 200 ?m or from 0.01 ?m to 100 ?m or from 0.01 ?m to 50 ?m.

19. The method according to claim 11, wherein the gas-solid reactor is a mill and calcium carbonate is separated from alkali sulfate by suspending the carbonated product in water and precipitating the calcium carbonate.

20. The method according to claim 14, wherein the starting material contains at least 50 wt.-% sulfate and/or sulfite calculated as calcium sulfate.

Description

Example 1

[0068] Two samples of bypass dust and NaOH were used to provide starting materials. The bypass dusts, designated BPD_H and BPD_L, had the oxide composition shown in table 1 which was determined by X-ray fluorescence (XRF). LOI 750? C. designates the loss on ignition at 750? C. NaOH was technical grade material.

TABLE-US-00001 TABLE 1 BPD_H BPD_L LOI 750? C. [wt.-%] 0.7 6.14 SiO.sub.2 [wt.-%] 19.82 13.12 Al.sub.2O.sub.3 [wt.-%] 4.698 3.789 TiO.sub.2 [wt.-%] 0.277 0.221 MnO [wt.-%] 0.03 0.027 Fe.sub.2O.sub.3 [wt.-%] 1.455 2.193 CaO [wt.-%] 59.64 47.76 MgO [wt.-%] 0.888 1.24 K.sub.2O [wt.-%] 5.008 10.2 Na.sub.2O [wt.-%] 0.339 0.313 SO.sub.3 [wt.-%] 4.256 9.762 P.sub.2O.sub.5 [wt.-%] 0.176 0.095 PbO [wt.-%] 0.032 0.078 SrO [wt.-%] 0.208 0.212 ZnO [wt.-%] 0.025 0.113 Cl [wt.-%] 2.522 4.131

[0069] For the carbonation experiments, solutions with either 0.1 mol/l or 0.5 mol/l NaOH in water were prepared, designated 01NH and 05NH, respectively. 15 g bypass dust was added to 150 ml solution after equilibrating the liquid with a gas mixture containing 10 Vol.-% CO.sub.2 in N.sub.2 that bubbled through the liquid. The starting material from bypass dust and NaOH was carbonated 6 hours.

[0070] For the solution analysis, the suspension was filtered by pressure-filtration using a 0.2 ?m Nylon filter. The gained solution was further analyzed within 2 hours. Concentrations of the elements in the solution were measured using inductively coupled plasma optical emission spectroscopy/mass spectros-copy (ICP-MS Agilent 7800 and ICP-OES Agilent 5110) depending on the element concentration. Solid samples were tested by XRF, TG and FTIR techniques allowing assessment of the phase assemblage. Before the measurements, the solid samples were dried at 40? C. for 24 hours. Table 2 shows the amounts of elements measured in the solutions.

TABLE-US-00002 TABLE 2 BPD_H + BPD_H + BPD_L + BPD_L + 01NH 05NH 01NH 05NH Al-ICP-OES Solution 0.1 0.0 0.4 0.0 Ca-ICP-OES Solution 165.0 7.3 388.5 11.1 Fe-ICP-OES Solution 0.0 0.0 0.0 0.1 K-ICP-OES Solution 4317.5 4602.5 8114.5 8710.0 Na-ICP-OES Solution 2290.0 10720.5 2391.5 9820.0 Mg-ICP-OES Solution 162.5 45.6 108.5 12.6 S-ICP-OES Solution 1439.5 1730.0 2780.0 3772.5 Si-ICP-OES Solution 56.8 51.5 77.0 42.3

[0071] As can be seen from table 2, significant leaching of sulfate to the solution occurred, confirming that carbonation of calcium sulfate in mixture with alkali metal ions provides dissolved alkali sulfates.

[0072] Further, the solid products were analysed with FTIR and TG, the results are shown in FIG. 1 and FIG. 2. From FIG. 1 is readily apparent that bypass dust BPD_H could not be carbonated without added NaOH. For bypass dust BPD_L containing more alkali metal ions some carbonation was achieved. Optimizing the ratio of calcium sulfate to alkali metal ions increased the carbonation degree and amount of sulfate leached into the liquid. The TG data in FIG. 2 confirm this finding.

Example 2

[0073] Technical grade gypsum and NaOH, Na.sub.2SO.sub.4, or NaOH and Na.sub.2SO.sub.4 were used to provide starting materials. For the carbonation experiments, solutions with 0.1 mol/l, 0.25 mol/l, 0.5 mol/l or 1 mol/l NaOH, with 0.25 mol/l and 0.5 mol/l Na.sub.2SO.sub.4 as well as 0.25 mol/l NaOH and 0.25 mol/l Na.sub.2SO.sub.4 in water were prepared, designated 01NH, 025NH, 05NH, 1NH, 025NS, 05NS, and 025NSNH, respectively. Pure water was used as control. 15 g gypsum was added to 150 ml solution after equilibrating the liquid with a gas mixture containing 10 Vol.-% CO.sub.2in N.sub.2 that bubbled through the liquid. The starting material from gypsum and NaOH/Na.sub.2SO.sub.4 was carbonated 6 hours.

[0074] The solid products were examined with FTIR and TG. The results are shown in FIGS. 3 and 4. From FIGS. 3 and 4 it is apparent that for gypsum as calcium sulfate the used amount of alkali metal ions was still not optimal, but the molar ratio of 0.7:1.5 tested was able to convert almost all calcium sulfate into alkali sulfate in a short time and under moderate conditions.

Example 3

[0075] Hardened cement paste made from Portland cement CEM III 42.5 N according to EN 197-1 standard, containing Portland cement clinker, setting time regulator (calcium sulfate), granulated blast furnace slag and a small quantity of limestone, was used. A CEM III cement was used to better simulate industrial RCP, as in many countries (for example the northern part of Europe) where recycling of the concrete is advanced, the supplementary cementitious materials are frequently used for cement and concrete production.

[0076] The mineralogical composition of the cement used is given in Table 3.

TABLE-US-00003 TABLE 3 phase amount [wt.-%] alite 32.7 belite 6.1 ferrite 5.8 calcite 3.0 anhydrite 1.5 bassanite 1.2 slag 43.0 others 2.9

[0077] The cement paste had a w/b=0.4 and was hydrated sealed at 40? C. for 3 months to mimic well-hydrated cement in mature concrete. After prolonged hydration, the hydration degree of the clinker was about 85%, while the hydration degree of the slag was 50% based on Rietveld calculation and SEM image analysis respectively. The main hydrates included C-S-H phase, portlandite, ettringite, hemi- and monocarbonate, hydrotalcite and small amount of hydrogarnet. After hydration, the samples were dried at 105? C. and ground in a laboratory ball mill to a D.sub.90?100 ?m.

[0078] This material was used as model for recycled concrete fines and carbonated analogously to bypass dust in example 1 and gypsum in example 2. Five different starting solutions were used: [0079] H.sub.2O=water [0080] 01NS=0.1 M Na.sub.2SO.sub.4 solution [0081] 025NS=0.25 M Na.sub.2SO.sub.4 solution [0082] 05NS=0.5 M Na.sub.2SO.sub.4 solution [0083] 025NS025NH=0.25 M Na.sub.2SO.sub.4+0.25 M NaOH solution

[0084] The kind and amounts of phases found in the solid products are shown in table 4.

TABLE-US-00004 TABLE 4 gypsum + gypsum + gypsum + gypsum + H2O 025NS 05NS 025NSNH C$ [wt.-%] 0.8 4.8 5.5 0.0 Ht [wt.-%] 3.2 2.9 2.8 3.3 AlSi-gel [wt.-%] 19.9 19.6 21.0 21.2 CaCO.sub.3 [wt.-%] 59.2 55.6 54.0 58.2 C + S [wt.-%] 16.9 17.1 16.7 17.2

[0085] As can be seen from table 4, the content of gypsum is depending on the concentrations in the solution.

[0086] The solid products were examined with FTIR. The results are shown in FIGS. 5 for the starting material without additional alkali metal ions and in FIG. 6 for the starting material with 0.25 mol/l NaOH and 0.25 mol/l Na.sub.2SO.sub.4 added in the aqueous suspension for carbonation. As is apparent the alkali metal ions present in the hardened cement paste achieved some conversion of calcium sulfate to alkali sulfate only when total alkalis concentration in the solution is higher than the sulfate. However, carbonization was readily achieved with the addition of alkali metal ions in a molar concentration of 0.5 M NaOH having the ration of alkali/sulfate ions 0.75:0.25.

[0087] This proves the advantageous effects on carbon dioxide sequestration and the calcium carbonate product achieved with the present invention by carbonation in the presence of alkali metal ions.