A REACTOR AND METHOD FOR MAKING CALCIUM HYDROXIDE

Abstract

A method for making a calcium hydroxide solution for use in a carbonation reactor containing supercritical carbon dioxide to form calcium carbonate particles of a selected size.

Claims

1. A method for making a calcium hydroxide solution comprising the steps of: adding water and calcium oxide to a slaking reactor comprising stirring means at an upper end thereof, and one or more outlets; stirring the water and calcium oxide such that a concentration gradient of calcium ions can be formed within the reactor; selecting an outlet to collect the resultant calcium hydroxide solution depending on the concentration of calcium ions required.

2. The method according to claim 1 wherein the speed of the stirring means can be selectively controlled.

3. The method according to claim 1 wherein the two or more outlets are provided at different vertical positions of the slaking reactor.

4. The method according to claim 1 wherein the reactor can be provided with an outlet which can be slid vertically to allow selection of the solution with the desired calcium ion concentration.

5. The method according to claim 1 wherein the solution is sprayed into a carbonation reactor containing supercritical carbon dioxide to form a slurry of calcium carbonate.

6. The method according to claim 5 wherein the solution is stirred slowly to provide a ‘clear top’ solution at an upper outlet of the slaking reactor, such that when sprayed into the carbonation reactor: a. without dilution, calcium carbonate is formed with a particle size of around 2-10 μm; or b. with dilution by 10-30% with water, calcium carbonate is formed with a particle size of around 300-600 nm.

7. The method according to claim 5 wherein the solution is stirred slowly to provide a ‘milk of lime’ solution at a lower outlet, or stirred quickly to provide a substantially homogenous ‘milk of lime’ solution in the slaking reactor, such that when sprayed into the carbonation reactor, calcium carbonate is formed with a particle size of around 10-30 μm.

8. The method according to claim 5 wherein the solution is provided to the carbonation reactor continuously at a flow rate of 0.5-1.5 L/min.

9. The method according to claim 1 wherein the solution is sprayed using an injector nozzle provided at the top section of the reactor, having a working pressure of around 80-400 bar.

10. The method according to claim 1 wherein the injector nozzle extends around 30-40% into the reactor.

11. A system for sequestering carbon comprising: a slaking reactor comprising stirring means at an upper end thereof for forming a concentration gradient of calcium ions within the slaking reactor when water and calcium oxide are added thereto; a carbonation reactor for receiving a solution from an outlet of the slaking reactor; the carbonation reactor comprising means for introducing supercritical carbon dioxide thereinto and an injector nozzle for spraying the solution thereinto; and wherein the concentration of calcium ions at an outlet of the slaking reactor can be selectively controlled to determine the particle size of calcium carbonate produced by the carbonation reactor.

12. The system according to claim 11 wherein the carbonation reactor comprises an outlet with a back pressure regulator at the bottom of the reaction chamber; wherein the regulator is adjustable such that a slurry can continuously flow out of the reactor via the outlet while maintaining a predetermined height of slurry within the reactor.

13. A slaking reactor according to claim 1.

14. Calcium carbonate made according to claim 1.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0036] It will be convenient to further describe the present invention with respect to the accompanying drawings that illustrate possible arrangements of the invention. Other arrangements of the invention are possible, and consequently the particularity of the accompanying drawings is not to be understood as superseding the generality of the preceding description of the invention.

[0037] FIG. 1 is a schematic diagram of the slaking reactor according to an embodiment of the invention.

[0038] FIG. 2 is a schematic diagram of the carbonation reactor.

[0039] FIG. 3 is a chart indicating the particle size distribution from the carbonation reactor depending on the output from the slaking reactor (a) lower output or slurry feed; (b) clear top solution feed; (c) diluted clear top solution feed.

[0040] FIG. 4 illustrates an analysis of the slaking reaction under different conditions.

DETAILED DESCRIPTION

[0041] With regard to FIG. 1, a slaking reactor 5 is provided with stirring mean 7 at the upper end thereof. The slaking reactor has at least two outlets, for example an upper outlet 9 and a lower outlet 11. Water 13 and calcium oxide 15 is added to the slaking reactor, and then stirred such that the calcium oxide dissolves to form a calcium hydroxide solution.

[0042] Under slow stirring conditions, the calcium ions form a gradient, being low concentration at the top of the reactor and high concentration at the bottom. Thus at the upper outlet 8 a clear top solution 1 can be collected, whereas at the lower outlet a milk of lime solution 2 can be collected. The clear top solution may be mixed with water for form a diluted clear top solution 3. The different solutions of calcium hydroxide may then be provided to a carbonation reaction.

[0043] With reference to FIG. 2, supercritical carbon dioxide is pumped by a low compression ratio pump 4 into a carbonation reactor 6 in a supercritical condition under high pressure (e.g. 80 bar, 30° C.; although it will be appreciated that pressure down to around 10 bar would also be acceptable). The selected calcium hydroxide solution is directed to the top of the carbonation reactor 6 via pump 22, then injected in the form of atomised droplets via nozzle 28 into excess supercritical carbon dioxide 30, where it precipitates as calcium carbonate 32 almost instantaneously. The calcium carbonate falls to the bottom of the reactor 6 and forms a slurry 34 which builds up and prevents egress of carbon dioxide through the regulator 36. However, as the injection of calcium hydroxide increases the reactor pressure, the slurry is eventually forced out of the reactor 6 via the regulator 36, which can be adjusted to suit the pressure and slurry flow i.e. while maintaining a sufficient height of slurry to substantially prevent the carbon dioxide from escaping. For example, in a cylindrical reactor 10 m high and 2 m in diameter, a slurry height of around 1.5 m may be maintained to prevent escape of carbon dioxide through the regulator. The wet precipitate 38 can then be processed further without having to disrupt the continuous flow operation of the reactor.

[0044] With regard to FIGS. 3a-c, the size distribution of the calcium carbonate particles produced from the carbonation reactor is illustrated, depending on the proportions of the original mixture (g CaO/L H.sub.2O), the feed rate of the solution into the carbonation reactor (L per minute), and the solution selected. Retention time was reduced by increasing the pressure and using a nozzle which extended around 15 cm into the 42 cm reactor.

[0045] FIG. 3a indicates a particle size distribution of around 10-30 μm when the ‘milk of lime’ solution is injected into the carbonation reactor at a rate of around 0.5-1.2 L/min. The solution can be obtained from a lower outlet of the slaking reactor when a mixture of 7 g CaO per litre of water is stirred at low speeds, or alternatively at any outlet when the mixture is stirred at high speeds such that the solution within the slaking reactor is substantially homogenous. The resulting particles can be used in the polymer and sealant industries.

[0046] FIG. 3b illustrates a particle size distribution of around 2-10 μm when the ‘clear top’ solution is injected into the carbonation reactor at a rate of around 1-1.5 L/min. Particles of this size can be used in the food and cosmetics industries

[0047] FIG. 3c shows that when the ‘clear top’ solution is diluted with water by up to 50%, typically 10-30% and injected into the carbonation reactor at a rate of around 1.5 L/min, the calcium carbonate is formed with a particle size distribution of around 300-600 nm. This is because crystalline growth is suppressed due to the limitation of secondary growth particles. These particles have applications in medicine, paper filler, and rubber glove manufacturing.

[0048] Thus it will be appreciated that a single system can be easily controlled to produce calcium carbonate particles of different sizes, depending on the relevant industry's requirements.

[0049] With regard to FIG. 4, there is illustrated an analysis of the slaking reaction under different conditions, by examining the calcium carbonate produced by the carbonation reactor. Phenolphtalein indicator is added to the feedstock which is then spray injected into the carbonation reactor at 1 L/min. If the purple colour of the phenolphthalein turns colourless than the reaction is complete i.e. no excess Ca.sup.2+ in the product. In the plant economics, a low reactor working pressure (about 20 bar) is required with a high calcium oxide federate. The results show that a calcium oxide concentration of 7 g/L is recommended for a pressure of 20 bar.

[0050] It will be appreciated by persons skilled in the art that the present invention may also include further additional modifications made to the system which does not affect the overall functioning of the system.