A TWO STAGES EXTRACTION METHOD FOR SYNTHESIZING PRECIPITATED CALCIUM CARBONATE

Abstract

Present invention relates to a multi-stage method for preparing high purity calcium carbonate precipitate from wastes and by-products containing high concentrations of calcium and silica. The calcium and silica rich material is introduced into a stirred reactor containing the extraction solution. The calcium rich solution, produced in the reactor, is separated from residual material and a gas containing carbon dioxide is passed into the said solution to precipitate calcium carbonate. The calcium carbonate precipitate is then separated from solution. The recovered solution and residual material, from previous extraction stage; is stirred in a reactor to further extract calcium from the residual material. After separating the solids from solution, carbon dioxide containing gas is introduced into solution to again precipitate calcium carbonate. Calcium carbonate precipitate is then separated from solution. The residual material produced from the process is characterized by high silica content.

Claims

1. A multistage method for producing precipitated calcium carbonate and silica concentrate from calcium and silica containing waste and by-products, characterized in that the method comprises following steps: Step 1: adding solid calcium and silica bearing material into the extraction solution in a continuously stirred extraction reactor; Step 2: separating the solid residual material from the calcium rich solution; Step 3: subjecting the calcium rich solution from step 2 to carbonation, in the carbonation reactor, to precipitate calcium carbonate, wherein the carbonation is induced by introducing carbon dioxide gas directly into the calcium containing aqueous solution in a closed reactor; Step 4: separating the calcium carbonate precipitate from solution of the previous step; Step 5: directing the residual material from step 2 and the recovered extraction solution from step 4 to an extraction reactor, where a second calcium extraction step takes place; Step 6: separating the solid residual material from calcium rich solution; Step 7: subjecting the calcium rich solution from step 6 to carbonation, in the carbonation reactor, to precipitate calcium carbonate, wherein the carbonation is induced by introducing carbon dioxide gas directly into the calcium containing aqueous solution in a closed reactor; Step 8: separating the calcium carbonate precipitate from solution of the previous step.

2. A multistage method according to claim 1, wherein the calcium extracting agent is an ammonium based aqueous solution, preferably ammonium chloride (NH.sub.4Cl), most preferably ammonium acetate (CH.sub.3COONH.sub.4).

3. A multistage method according to claims 1 and 2, wherein the solid calcium and silica bearing material in step 1 is an industrial waste or by-product.

4. A multistage method according to claim 3, wherein the solid calcium and silica bearing material is oil shale ash or coal ash or waste cement, having nominal grain size <1000 μm, more preferably <500 μm and most preferably <200 μm; where preferable materials are mechanically sieved or more preferably mechanically comminuted in a dry environment.

5. A multistage method according to claims 1-4, wherein in step 1, the temperature in the extraction reactor is within the range of 3° C.-100° C., preferably within the range of 15° C.-80° C., most preferably within the range of 25° C.-70° C.; the solid to liquid ratio in the extraction reactor is between the range of 1:1-1:20, preferably between 1:1-1:10, most preferably between 1:1-1:5; the molarity in the extraction reactor is preferably between the range 0.1-3 M, more preferably between the range 1.5-2 M; and at the end of the extraction stage, the resulting mixture has a pH range between 8-13.

6. A multistage method according to claims 1-5, wherein in steps 3 and 7 the pH of solution during carbonation is preferably >7.5, most preferably >8.

7. A multistage method according to claims 1-6, wherein in steps 3 and 7 the carbon dioxide is introduced into the calcium containing solution by bubbling or spraying at the temperature between 3° C. to 80° C., preferably between 15° C.-60° C., most preferably between 25° C.-45° C.

8. A multistage method according to claim 7, wherein carbon dioxide is present in an amount >5 vol % of gas, where the gas preferably originates from industrial waste gases, and where most preferably the carbon dioxide in the industrial waste gas is separated, purified and concentrated before use.

9. A multistage method according to claims 1-8, wherein the separation techniques in steps 2, 4, 6 and 8, for separation of solids from liquids, comprise one of, or a combination of some or all from the following methods: sedimentation, centrifugation, decanting, filtration, reverse osmosis.

10. A multistage method according to claim 1-9, wherein the precipitated calcium carbonate produced is having; a calcium carbonate content >95 w/w %, an average particle diameter between 0.05-10 μm; the iron-III-oxide (Fe.sub.2O.sub.3) concentration is <0.2 w/w %, the Hunter Whiteness Index is >85%.

11. A multistage method according to claim 1-10, wherein the method comprises further steps of washing and de-watering of the precipitate of calcium carbonate and the residual output material with water.

12. A multistage method according to claim 11, wherein the ammonium salts in the wash solution are concentrated by distillation or more preferably by membrane separation; and re-used in the extraction process.

13. A multistage method according to any one of the previous claims, wherein the ferromagnetic material in residual output material from Step 6 is isolated and processed using magnetic and/or chemical extraction methods.

14. A multistage method according to any one of the previous claims, wherein the reactor for second stage calcium extraction in step 5 and reactor for second stage carbonation in step 7 is different to the reactor for first stage calcium extraction in step 1 and the reactor for first stage carbonation in step 3, respectively.

15. A multistage method according to any of the previous claim, wherein a single carbonation reactor is present, where the calcium rich solution from step 6 is subjected to the carbonation reactor described in step 3, to precipitate calcium carbonate.

16. A multistage method according to any of the previous claims, wherein a number of modules of the present multistage method according to claim 1 is set-up in a serial arrangement, and where each module consists of the following steps: i) extraction of calcium, ii) solid/liquid separation, iii) carbonation and precipitation, iv) solid/liquid separation to receive the calcium carbonate precipitate.

17. A multistage method according to any of the previous claims wherein residual material left after completion of calcium extraction is a fine-grained powder with a silica oxide and magnesium oxide concentration >45 w/w % and >15% respectively.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] FIG. 1 depicts the first or principal embodiment of the present invention.

[0034] FIG. 2 depicts the second embodiment of the present invention, whereby a single carbonation reactor is present, in comparison, the principal embodiment has two carbonation reactors.

[0035] FIG. 3 depicts the third embodiment of the present invention, whereby a single extraction and a single carbonation reactor is present, in comparison, previous embodiments have two extraction reactors and one or two number carbonation reactors.

[0036] FIG. 4 depicts the fourth embodiment of the present invention, whereby several modules are set-up in a serial arrangement. Each module consists of the steps; i) extraction of calcium, ii) solid/liquid separation, iii) carbonation and precipitation, iv) solid/liquid separation to receive calcium carbonate precipitate.

DETAILED DESCRIPTION OF THE INVENTION

[0037] In order to achieve the aims, the present invention provides the following technical process: a multistage method for producing precipitated calcium carbonate and silica concentrate, from calcium and silica containing waste and by-products, using ammonium based aqueous solution as the calcium extracting agent.

[0038] The principal embodiment of the invention comprises the following steps (ref FIG. 1). [0039] Step 1. Add solid calcium and silica bearing material into the extraction solution (in reactor—R1). [0040] For the first calcium dissolution step; the preferred operational parameters are influenced by several factors. Under optimum operational conditions, maximum amount of calcium ions will leach out, from the calcium bearing material, by using minimum energy and least amount of extraction chemical. The conditions shall also, discourage leaching of elements, such as iron or manganese, which are known to have a detrimental effect on the end product quality. Furthermore, the selected conditions shall avoid jelling of silicates, which can complicate the operation. [0041] The extraction process is carried out using an ammonium-based aqueous solution. The chosen solutions can selectively extract calcium ions, from silicate minerals containing calcium, without dissolving contaminants (such as salts, iron and manganese), which may co-precipitate with the calcium carbonate in the subsequent precipitation stage. The said solution is either ammonium acetate (CH.sub.3COONH.sub.4), ammonium chloride (NH.sub.4Cl) or any other ammonium-based aqueous solution, but preferably ammonium chloride (NH.sub.4Cl) or most preferably ammonium acetate (CH.sub.3COONH.sub.4). [0042] The material, from which calcium is extracted, can be any alkaline, calcium bearing compound but preferably is a material deemed to be an industrial waste or by-product; and can be sourced either directly or indirectly from production, landfills, deposits and/or stockpiles. Examples of such materials are oil shale ash from energy or oil and gas generation, coal ash and waste cement. The most preferable material, from these examples, for the disclosed invention is oil shale ash from energy generation. The material shall be fine grained with a nominal grain size of; preferably <1000 μm, more preferably <500 μm and most preferably <200 μm. Materials with larger sized particles are preferably mechanically sieved or more preferably mechanically comminuted in a dry environment. [0043] Lower temperatures will improve the ultimate amount of calcium ions that are extracted from the material but will also, inevitably lower the dissolution rate. The temperature in the extraction reactor shall be within the range 3° C.-100° C. but preferably within the range 15° C.-80° C. The most preferable temperature range for dissolution is 25° C.-70° C. [0044] The solid to liquid ratio in the dissolution reactor is between the range 1:1-1:20, preferably between 1:1-1:10 but most preferably between 1:1-1:5. [0045] The molarity in the dissolution reactor is preferably between the range 0.1-3M but most preferably within the range 1.5-2M. [0046] At the end of the dissolution stage, the resulting mixture has a pH range between 8-13. [0047] The reactor used in this stage is a continuously stirred reactor. [0048] Step 2. Separate the solid residual material from the calcium rich solution. The selected separation method will depend on grain size, grain distribution and the density of residual material. The separation technique may involve one of, or a combination of some or all the methods: sedimentation, centrifugation, decanting, filtration, reverse osmosis. [0049] Residual material from this step is directed to a separate reactor (R2) for a second calcium extraction stage. The calcium rich solution is subjected to carbonation to precipitate high purity calcium carbonate (in reactor CR1). [0050] Step 3. The carbonation is induced by introducing carbon dioxide gas directly into the calcium containing aqueous solution in a closed reactor. The carbon dioxide will hydrolyze to form bi-carbonates that will dissociate to a carbonate and hydrogen ion. The carbonate ion will readily react with the calcium ions in solution to form calcium carbonate precipitate.

CO.sub.2(l)+H.sub.2O.Math.HCO.sub.3.sup.−+H.sup.+  Carbon dioxide gas hydration reaction

Ca.sup.2++CO.sub.3.sup.2−.Math.CaCO.sub.3↓

CaCO.sub.3+H.sup.+++Ca.sup.2++HCO.sub.3.sup.−

[0051] It is noted that, carbon dioxide dissolution in water is influenced by pH. Carbon dioxide dissolving in water will mainly form CO.sub.3.sup.2− at pH>9 and HCO.sub.3.sup.− at pH<9. Therefore, in theory, the maximum amount of calcium carbonate crystals will form around pH 9.sup.9. In the present invention, the solution into which carbon dioxide is introduced, is not water, hence the optimal pH level for maximum crystal formation is different. To avoid back dissolution of calcium carbonate crystals, the pH of the solution is preferably >7.5 but most preferably >8.

[0052] The carbon dioxide is introduced into the calcium containing solution either by bubbling or spraying. The temperature during carbonation is kept between 3° C.-80° C. but preferably between 15° C.-60° C.; and most preferably within the range 25° C.-45° C. Carbon dioxide utilized for carbonation can be either carbon dioxide or preferably carbon dioxide containing waste gases (carbon dioxide>5 vol %). More preferably the carbon dioxide gas originates from industries such as oil shale thermal plants but most preferably this gas is also separated, purified and concentrated before use. [0053] Step 4. Separate the calcium carbonate precipitate from solution. Selected separation method will depend on grain size, grain distribution and the density of precipitate. The separation technique may involve one of, or a combination of some or all the methods: sedimentation, centrifugation, decanting, filtration, reverse osmosis. [0054] The ammonium based aqueous solution is recovered and directed back to extraction reactor R1, for use in the first calcium extraction stage. [0055] Step 5. Residual material from step 2 is directed to the extraction reactor R2 where a second calcium extraction step takes place. The conditions (temperature, solution molarity, solid to liquid ratio etc.) in reactor R2 is kept within the same ranges as in R1. [0056] In reactor R2, the residual material may be subjected to >1 number extraction cycles. This is determined based on the concentration of calcium ions leaching from a specific extraction cycle, relative to the magnitude of aqueous solution make-up and total energy consumed.

[00001]$\frac{\left[{\mathrm{Ca}}^{+2}\right]}{\mathrm{Energy}\mathrm{spent}\times \mathrm{Extraction}\mathrm{solution}\mathrm{makeup}}$ [0057] If the calcium ion concentration is deemed low, relative to the resources utilized, then the residue batch is directed to the solid-liquid separation step, together with solution. [0058] Step 6. Separate the solid residual material from calcium rich solution. The separation technique may involve one of, or a combination of some or all the methods: sedimentation, centrifugation, decanting, filtration, reverse osmosis. [0059] Step 7. The calcium rich solution is subjected to carbonation in carbonation reactor-CR2 to precipitate calcium carbonate. The carbonation is induced by introducing carbon dioxide gas directly into the calcium containing aqueous solution in a closed reactor. The carbon dioxide is introduced into the calcium containing solution either by bubbling or spraying. The physical parameters, such as temperature, in reactor CR2 is kept within the same ranges as in CR1 (presented in Step 3). [0060] The carbon dioxide utilized for carbonation can be either carbon dioxide or preferably carbon dioxide containing waste gases (carbon dioxide >5 vol %). More preferably the waste gas originates from industries such as oil shale thermal plants but most preferably this gas is also separated, purified and concentrated before use. [0061] Step 8. Separate the calcium carbonate precipitate from solution. The separation technique may involve one of, or a combination of some or all the methods: sedimentation, centrifugation, decanting, filtration, reverse osmosis. The ammonium based aqueous solution is recovered and directed, to extraction reactor R2, for use in the calcium extraction. [0062] If desired, the calcium carbonate precipitate produced from the second carbonation stage can have different physical properties, in comparison to the precipitate from the first carbonation stage. This can be achieved by varying the operational parameters (temperature, carbonation duration, rate of carbonation, carbon dioxide bubble size etc.) in the carbonation reactor. This would indicate that the described technology can produce, precipitated calcium carbonate exhibiting different physical properties simultaneously. [0063] The precipitated calcium carbonate produced from both streams are characterized by; a fine-grained, white colored powder with a calcium carbonate content >95 w/w % and an average particle diameter 0.05-10 μm. The iron-III-oxide (Fe.sub.2O.sub.3) concentration is <0.2 w/w % and the Hunter Whiteness Index is >85%. [0064] The residual material from first (Step 2) and second (Step 6) stream are characterized by; a fine-grained, light grey colored powder. The silica oxide and magnesium oxide concentration from first stream is >40 w/w % and >10 w/w % respectively. In the second stream the silica oxide and magnesium oxide concentrations are >45 w/w % and >15 w/w % respectively.

[0065] As further improvement, to the present invention, the precipitate of calcium carbonate is washed with water and de-watered to obtain a dry precipitated calcium carbonate product. Similarly, the residual output material is washed with water and de-watered. The ammonium salts, in the washed solutions, are concentrated by distillation or more preferably by membrane separation; and re-used in the calcium extraction process. This will reduce the loss of ammonium solvents from the system.

[0066] As further improvement, to the present invention, the ferromagnetic material in residual output material (from Step 6) will be isolated using magnetic and/or chemical extraction methods and processed.

[0067] As further improvement, of the present invention, the residual material (from Step 6) can be utilized in cement and/or concrete production with no or minimal pre-treatment.

[0068] As further improvement, of the present invention, the residual output material (from Step 6) can be further processed, to utilize as a substitute for silica minerals in industry.

[0069] As further improvement, of the present invention, the residual output material (from Step 6) can be directly landfilled or backfilled as inert material.

[0070] In another embodiment of the present invention (second embodiment, FIG. 2); the setup has a single carbonation reactor in comparison to the principal embodiment that has two carbonation reactors. In this arrangement, the calcium rich solution from Step 6, is re-circulated back to the carbonation reactor CR1.

[0071] In this set-up a single output stream of calcium carbonate precipitate is present. Relative to the principal embodiment, this arrangement has a; lower operational cost (due to lower chemical and energy consumption) and lower capital cost (due to lesser piping and reactor numbers).

[0072] In another embodiment of the disclosed method (third embodiment, FIG. 3); a single calcium extraction stage is present. While this approach has some advantages, in comparison to other embodiments; such as lower energy usage, lower chemical loss and a leaner process; the setup will produce a lower grade silica stream (high calcium concentration) compared to a multi-stage extraction. The calcium carbonate yield per unit mass of input solid material, will also be lower. The advantages of the principal embodiment were previously listed in section: overview of the invention.

[0073] In another embodiment of the disclosed method (fourth embodiment, FIG. 4); several modules are set-up in a serial arrangement. Each module consists of the steps; i) extraction of calcium, ii) solid/liquid separation, iii) carbonation and precipitation, iv) solid/liquid separation to receive the calcium carbonate precipitate. In this arrangement, the initial input material for each module is the solid residue from the solid/liquid separation step (after calcium extraction) of previous module. This arrangement gives a high degree of control during the extraction and precipitation stage, if desired, to synthesize physically different or similar calcium carbonate crystals from each stream. If the operational parameters are same in all the calcium extraction steps, the calcium ions leaching during each extraction stage will be lower, relative to the previous stage. This variation in calcium concentration can influence the physical characteristics of calcium carbonate crystals formed during carbonation. By adopting, this embodiment, it is possible to adjust the operational parameters, to cater for the reducing calcium concentration in each subsequent leachate solution. The number of modules in the series will be determined based on a cost/benefit analysis. This is directly related to the amount of leachable calcium present, in the calcium bearing material, in relation to the operational conditions employed.

REFERENCES

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