PRODUCTION OF ALKALI METAL CARBONATES AND/OR BICARBONATES FROM ALKALI METAL SULPHATES

Abstract

The invention provides a method of producing a carbonate or a bicarbonate of an alkali metal, in solid form, from a sulphate of the alkali metal. The method includes, in a first reaction step, reacting, in aqueous medium, a sulphate of an alkali metal with one or more alkaline earth metal sulphides, thus forming an aqueous solution of one or more sulphides of the alkali metal and one or more sulphates of the alkaline earth metal in solid form. The method also includes, in a second reaction step, in the aqueous solution of one or more sulphides of the alkali metal, reacting the one or more sulphides of the alkali metal with carbon dioxide (CO.sub.2) in gaseous form, thus forming an aqueous solution of a bicarbonate of the alkali metal and gaseous hydrogen sulphide. The method further includes, in a recovery step, recovering a carbonate or the bicarbonate of the alkali metal, in solid form, from the aqueous solution of the bicarbonate of the alkali metal.

Claims

1. A method of producing a carbonate or a bicarbonate of an alkali metal, in solid form, from a sulphate of the alkali metal, the method including: in a first reaction step, reacting, in aqueous medium, a sulphate of an alkali metal with one or more alkaline earth metal sulphides, thus forming an aqueous solution of one or more sulphides of the alkali metal and one or more sulphates of the alkaline earth metal in solid form; in a second reaction step, in the aqueous solution of one or more sulphides of the alkali metal, reacting the one or more sulphides of the alkali metal with carbon dioxide (CO.sub.2) in gaseous form, thus forming an aqueous solution of a bicarbonate of the alkali metal and gaseous hydrogen sulphide; in a recovery step, recovering a carbonate or the bicarbonate of the alkali metal, in solid form, from the aqueous solution of the bicarbonate of the alkali metal; and in an alkaline earth metal sulphide regeneration step, regenerating one or more alkaline earth metal sulphides by subjecting one or more sulphates of the alkaline earth metal, formed in the first reaction step, to carbothermal reduction and thus obtaining one or more regenerated alkaline earth sulphides.

2. The method according to claim 1, wherein the alkali metal is selected from sodium and potassium; the alkaline earth metal is selected from calcium and barium; and the one or more alkaline earth metal sulphides is/are selected from one or a combination of a sulphide and a hydrosulphide of the alkaline earth metal.

3. The method according to claim 1, wherein the one or more alkaline earth metal sulphides include/s a hydrosulphide of the alkaline earth metal and the method includes producing the hydrosulphide of the alkaline earth metal as an aqueous solution thereof by reacting a sulphide of the alkaline earth metal in water with gaseous hydrogen sulphide produced in the second reaction step.

4. The method according to claim 1, which includes, in a first separation step ahead of the second reaction step, separating the one or more sulphates of the alkaline earth metal, produced in the first reaction step in solid form, from the aqueous solution of one or more sulphides of the alkali metal.

5. The method according to claim 1, which includes, in a residual alkaline earth metal recovery step, recovering residual alkaline earth metal that is dissolved in the aqueous solution of the one or more sulphides of the alkali metal by reacting such residual alkaline earth metal with one or more of CO.sub.2 in gaseous form, alkali metal carbonate, and alkali metal bicarbonate, thus producing a precipitate comprising a carbonate of the alkaline earth metal in solid form.

6. The method according to claim 5, wherein the residual alkaline earth metal recovery step is performed ahead of the second reaction step and the method includes, in a second separation step, separating the precipitate comprising the carbonate of the alkaline earth metal in solid form from the solution of the one or more sulphides of the alkali metal, or the residual alkaline earth metal recovery step is combined with the second reaction step and the method includes, in a second separation step, separating the precipitate comprising the carbonate of the alkaline earth metal in solid form from the aqueous solution of the bicarbonate of the alkali metal.

7. The method according to claim 1, wherein the alkaline earth metal is calcium and the alkaline earth metal sulphide that is regenerated in the regeneration step is calcium sulphide, and the method includes reacting regenerated calcium sulphide, in water, with gaseous hydrogen sulphide produced in the second reaction step, thus producing an aqueous solution of regenerated calcium hydrosulphide.

8. The method according to claim 1, wherein the alkaline earth metal is barium and alkaline earth metal sulphide that is regenerated in the regeneration step is barium sulphide, and the method includes dissolving the regenerated barium sulphide in water, thus producing an aqueous solution of regenerated barium sulphide.

9. The method according to claim 6, wherein the alkali earth metal is calcium and the carbonate of the alkaline earth metal, separated form from the aqueous solution of the bicarbonate of the alkali metal in the second separation step, is calcium carbonate, and the method includes regenerating calcium hydrosulphide by subjecting the calcium carbonate to thermal treatment, thus producing calcium oxide (CaO); and reacting the CaO with the hydrogen sulphide formed in the second reaction step, to produce regenerated Ca(HS).sub.2.

10. The method according to claim 6, wherein the alkali metal is barium and the carbonate of the alkaline earth metal, separated form from the aqueous solution of the bicarbonate of the alkali metal in the second separation step, is barium carbonate, and the method includes reacting the barium carbonate with alkali metal sulphate ahead of the first reaction step, thus producing insoluble barium sulphate and recovering, in solid form, the residual alkaline earth metal that was dissolved in the aqueous solution of the one or more sulphides of the alkali metal.

11. The method according to claim 1, which includes using the regenerated alkaline earth metal sulphide or regenerated calcium sulphide or regenerated calcium hydrosulphide or regenerated barium sulphide as the alkaline earth metal sulphide in performing the first reaction step.

12. The method according to claim 1, wherein the recovery step includes subjecting the aqueous solution of the bicarbonate of the alkali metal to one of evaporative crystallisation, cooling crystallisation, and eutectic freeze crystallisation to recover the carbonate or bicarbonate of the alkali metal in solid form from the aqueous solution of the bicarbonate of the alkali metal.

13. The method according to claim 12, wherein evaporative crystallisation is performed and the bicarbonate of the alkali metal in the aqueous solution of the bicarbonate of the alkali metal is sodium bicarbonate, and sodium carbonate is recovered in solid form; or the bicarbonate of the alkali metal in the aqueous solution of the bicarbonate of the alkali metal is potassium bicarbonate, and potassium bicarbonate is recovered in solid form.

14. The method according to claim 12, wherein the bicarbonate of the alkali metal is recovered in solid form and the method includes subjecting the bicarbonate of the alkali metal in solid form to heat treatment, thus producing a carbonate of the alkali metal in solid form.

15. An alkali metal carbonate and/or an alkali metal bicarbonate in solid form produced according to the method of the invention.

Description

[0070] THE INVENTION WILL NOW BE DESCRIBED IN MORE DETAIL and by way of example only, with reference to the accompanying drawings, in which

[0071] FIG. 1 shows one embodiment of a process for performing the method of the invention; and

[0072] FIG. 2 shows another embodiment of a process for performing the method of the invention.

[0073] Referring to FIG. 1 of the drawings, reference numeral 10 generally indicates one embodiment of a process for performing the method of the invention, to produce sodium carbonate (Na.sub.2CO.sub.3) from sodium sulphate (Na.sub.2SO.sub.4) by reaction of the sodium sulphate with calcium hydrosulphide (Ca(HS).sub.2).

[0074] In the process 10, the alkaline earth metal sulphide used according to the method of the invention, is therefore Ca(HS).sub.2.

[0075] The process 10 includes a Na.sub.2SO.sub.4 dissolution stage 12.

[0076] In the dissolution stage 12, solid Na.sub.2SO.sub.4 is dissolved in water. Thus, an aqueous solution of Na.sub.2SO.sub.4 is produced.

[0077] The process 10 further includes a gypsum precipitation stage 14 (first reaction stage) in which the first reaction step of the method of the invention is performed.

[0078] In the gypsum precipitation stage 14, the aqueous solution of Na.sub.2SO.sub.4 from the dissolution stage 12 is mixed with approximately a stoichiometric amount of Ca(HS).sub.2 in aqueous solution. In accordance with the invention, there may have been a prior step of forming Ca(HS).sub.2 in aqueous solution by dissolving CaS in water and reacting with H.sub.2S, thus mixing an aqueous solution of an approximately stoichiometric amount of Ca(HS).sub.2 with the aqueous solution of Na.sub.2SO.sub.4 in the gypsum precipitation stage.

[0079] Mixing of the two solutions results in Na.sub.2SO.sub.4 reacting with Ca(HS).sub.2, thus forming an aqueous solution of sodium hydrosulphide (NaHS) (hereinafter referenced as the aqueous solution of NaHS) and precipitated gypsum (CaSO.sub.4.Math.2H.sub.2O). This is in accordance with reaction equation 3:

Na.sub.2SO.sub.4(aq) 30 Ca(HS).sub.2(aq)+2H.sub.2O=2NaHS(aq)+CaSO.sub.4.Math.2H.sub.2O(s) . . . Eq. 3

[0080] The process 10 also includes a first separation stage 16.

[0081] In the first separation stage 16, gypsum is separated from the aqueous solution of NaHS. Such separation may be effected by way of known solid liquid separation techniques, including filtration, centrifuging or sedimentation. In the case of the present example, filtration is used.

[0082] Whereas gypsum has a finite solubility, the aqueous solution of NaHS obtained after separation of gypsum, also contains some dissolved calcium cations. Such cations are usually undesired in final products.

[0083] The process 10 therefore includes a residual calcium recovery stage 18.

[0084] In the recovery stage 18, calcium is removed from the aqueous solution of NaHS (the filtrate from the first separation stage 16), by the addition of a sufficient amount of carbonate to precipitate calcium carbonate (CaCO.sub.3) from the solution. Calcium carbonate has a much lower solubility in water than gypsum, i.e. about 100 times lower.

[0085] In the present example, calcium carbonate precipitation is effected by adding CO.sub.2 to the solution of NaHS, in the residual calcium recovery stage 18. As an alternative, it is also possible to add sodium carbonate (Na.sub.2CO.sub.3) or sodium bicarbonate (NaHCO.sub.3) instead of CO.sub.2.

[0086] The process 10 further includes a second separation stage 20.

[0087] In the second separation stage 20, precipitated CaCO.sub.3 is separated from the aqueous solution of NaHS from the residual calcium recovery stage 16, thus producing a NaHS solution filtrate.

[0088] The NaHS solution filtrate obtained from the second separation stage 20 contains mostly HS.sup.anions, but it may also contain some carbonate, bicarbonate, sulphide and sulphate anions.

[0089] The process further includes a H.sub.2S stripping stage 22 (second reaction stage), in which the second reaction step of the method of the invention is performed.

[0090] In the stripping stage 22, H.sub.2S is stripped from the NaHS filtrate with CO.sub.2.

[0091] It is noted that since CO.sub.2 is a stronger acid than H.sub.2S, all the S.sup.2and HS.sup.anions in the solution are converted to molecular H.sub.2S, which is stripped from the solution. This is in accordance with reaction equation 4:

NaHS(aq)+CO.sub.2+H.sub.2O=NaHCO.sub.3(aq)+H.sub.2S(g) . . . Eq. 4

[0092] After stripping of H.sub.2S from the solution, a solution comprising mostly dissolved sodium bicarbonate (NaHCO.sub.3) is obtained (hereinafter referenced as the NaHCO.sub.3 solution), but there may also be some sulphate and carbonate ions present in the solution.

[0093] In accordance with the invention, the residual calcium recovery stage 18 and the stripping stage 22 may be combined, in which case the second separation stage 20 would follow such a combined recovery-and-stripping stage 18, 22.

[0094] The process 10 also includes a NaHCO.sub.3 concentration stage 24 (alkaline earth metal bicarbonate or carbonate recovery stage).

[0095] In the concentration stage 24, the NaHCO.sub.3 solution is concentrated to crystallize NaHCO.sub.3 from the solution, thus producing NaHCO.sub.3 crystals and a residual liquor. In accordance with the invention, cooling or eutectic freeze crystallisation may be used for this purpose. As an alternative, evaporative crystallisation may be used, to produce Na.sub.2CO.sub.3 crystals.

[0096] The process 10 further includes a third separation stage 26.

[0097] In the third separation stage 26, NaHCO.sub.3 crystals are separated from the liquor. Of course, if Na.sub.2CO.sub.3 was precipitated (crystallised) instead, then the third separation stage would be for the separation of Na.sub.2CO.sub.3 crystals from the liquor.

[0098] The liquor is a saturated aqueous solution of NaHCO.sub.3 that also contains some dissolved Na.sub.2SO.sub.4.

[0099] The process 10 includes a liquor recycle line 28.

[0100] Along the recycle line 28, the liquor is recycled to the dissolution stage 12, thus minimising losses of sodium species across the process 10 in performing the method of the invention.

[0101] The process 10 also includes a drying and calcination stage 30.

[0102] In the drying and calcination stage 30, NaHCO.sub.3 crystals separated from the liquor are dried and then calcined, to produce Na.sub.2CO.sub.3 as a desired product of the process 10.

[0103] The process 10 further includes a H.sub.2S conversion stage 32.

[0104] In the conversion stage 32, some of the H.sub.2S from the stripping stage 22 is converted to sulphur using conventional technology, such as the Claus process, or it is combusted and converted to sulphuric acid, both of which are further desired products of the process 10.

[0105] The process 10 further provides for production of an aqueous solution of Ca(HS).sub.2 that is used in the gypsum precipitation stage 14.

[0106] More specifically, the process 10 includes a gypsum reduction stage 34 in which gypsum from the first separation stage 16 is carbothermally reduced by reacting it with carbon, typically originating from coal, at an elevated temperature (800 to 1200 C.).

[0107] A solid product is thus produced, comprising mostly CaS mixed with ash originating from the coal. This is in accordance with reaction equation 5:

CaSO.sub.4+2C=CaS+2CO.sub.2. . . Eq. 5

[0108] CO.sub.2 produced in the reduction stage 34 may be recovered and cleaned by scrubbing in a scrubbing stage 35 and used in the residual calcium recovery stage 18 and in the stripping stage 22.

[0109] The process further includes a Ca(HS).sub.2 dissolution stage 36.

[0110] In the dissolution stage 36, CaS in the mixture of CaS and ash from the gypsum reduction stage 34 is dissolved in water by reacting it in water with H.sub.2S recycled from the stripping stage 22, to form Ca(HS).sub.2 which has a high solubility in water in the presence of H.sub.2S that is a weak acid. This is in accordance with reaction equation 6:

CaS+H.sub.2S=Ca(HS).sub.2(aq) . . . Eq. 6

[0111] The process 10 also includes a fourth separation stage 38.

[0112] In the fourth separation stage 38, following the dissolution of CaS to form an aqueous solution of Ca(HS).sub.2, the undissolved impurities such as ash from the coal and excess carbon, is separated from the aqueous solution. This is typically done by filtration as indicated in the fourth separation step. Naturally, other means to separate solids from liquid can also be employed.

[0113] The filtered Ca(HS).sub.2 solution is then fed to the gypsum precipitation stage 14 to react with Na.sub.2SO.sub.4 to produce the aqueous solution of NaHS and precipitated gypsum as described before.

[0114] As also mentioned before, precipitated gypsum that is separated from the aqueous solution of NaHS in the first filtration stage 16 is reduced in the gypsum reduction stage 34. This gypsum includes the CaCO.sub.3 that is separated from the aqueous NaHS solution in the second separation stage 20. In the gypsum reduction stage 34, the CaCO.sub.3 is converted to CaO. The CaO formed in this step also reacts with recycled H.sub.2S in the Ca(HS).sub.2 dissolution stage 36 to dissolve in the water as Ca(HS).sub.2.

[0115] Thus, it will be appreciated that the calcium sulphate used in the process is recycled and two useful products from the process are Na.sub.2CO.sub.3 and sulphur, either as elemental sulphur or sulphuric acid.

[0116] Referring now to FIG. 2, reference numeral 100 generally shows an alternative process for performing another embodiment of the method of the invention. The process 100 is a variation of the process 10. The process 100 is therefore described below with reference to the same process stages as the process 10, referenced with suffix A.

[0117] In the process 100, an aqueous solution of Na.sub.2SO.sub.4 is reacted with CaS in aqueous suspension, rather than with Ca(HS).sub.2 in aqueous solution. The compound of calcium and sulphur used according to the method of the invention, is therefore CaS.

[0118] As in the process 10, the first step of the process is to dissolve Na.sub.2SO.sub.4 in water in a sodium sulphate dissolution stage 12A.

[0119] The aqueous solution of sodium sulphate thus obtained is then, in a gypsum precipitation stage 14A, mixed with an approximately stoichiometric quantity of CaS.

[0120] The CaS is suspended in the sodium sulphate solution, thus reacting with the Na.sub.2SO.sub.4 to form an aqueous solution of Na.sub.2S that also contains some NaHS and also some residual dissolved CaSO.sub.4 (hereinafter simply referenced as the solution of Na.sub.2S). Gypsum and a relatively small quantity of Ca(OH).sub.2 precipitate.

[0121] Following the precipitation of gypsum, the precipitated gypsum, the co-precipitated Ca(OH).sub.2 and insoluble compounds in the CaS, such as ash, are separated from the solution of Na.sub.2S in a first separation stage 16A. Standard solid liquid separation techniques can be used such as filtration, centrifuging or sedimentation. In the case of this example, filtration is used.

[0122] Whereas gypsum has a finite solubility, the solution of Na.sub.2S from the first separation stage 16A also contains some dissolved calcium cations which may be undesirable in the final product. Therefore, the process 100 also includes a residual calcium recovery stage 18A.

[0123] More specifically, in the residual calcium recovery stage 18A, calcium is removed from the solution of Na.sub.2S by the addition of a sufficient amount of carbonate, to precipitate CaCO.sub.3 as in the previous example. As in the process 100, this is also achieved by adding CO.sub.2 to the solution. It is also possible to add sodium carbonate (Na.sub.2CO.sub.3) or sodium bicarbonate (NaHCO.sub.3), instead of CO.sub.2.

[0124] The precipitated CaCO.sub.3 is then separated from the solution of Na.sub.2S in a second separation stage 20A.

[0125] Although the Na.sub.2S solution obtained as a filtrate from the second separation stage 20A contains mostly S.sup.2 anions, it may also contain some carbonate, bicarbonate, bisulphide and sulphate anions.

[0126] In a H.sub.2S stripping stage 22A, H.sub.2S is stripped from the solution of Na.sub.2S from the second separation stage 20 using CO.sub.2. Since CO.sub.2 is a stronger acid than H.sub.2S, all the S.sup.2 and HS.sup. anions in the solution are converted to molecular H.sub.2S which is stripped from the solution.

[0127] In accordance with the invention, the recovery stage 18A and the stripping stage 22A may be combined, in which case the second separation stage 20A would follow such a combined recovery-and-stripping stage 18A, 22A.

[0128] After stripping of H.sub.2S from the solution, the solution comprises mostly dissolved NaHCO.sub.3 (hereinafter referenced as the NaHCO.sub.3 solution), but there is also some sulphate, carbonate and bicarbonate ions present in the solution.

[0129] The NaHCO.sub.3 solution is then concentrated in a NaHCO.sub.3 concentration stage 24A to crystallize NaHCO.sub.3 from the NaHCO.sub.3 solution, thus producing NaHCO.sub.3 crystals and a residual liquor. As in the case of the process 10, this would be effected by cooling or eutectic freeze crystallisation, and if Na.sub.2CO.sub.3 is required, evaporative crystallisation may be used instead.

[0130] The NaHCO.sub.3 (or Na.sub.2CO.sub.3 if evaporative crystallisation was employed) crystals are separated from the liquor in a third separation stage 26A.

[0131] In a drying and calcination stage 30A, NaHCO.sub.3 crystals separated from the liquor are dried and then calcined, to produce Na.sub.2CO.sub.3 as a desired product of the process 100.

[0132] In a H.sub.2S conversion stage 32A, the H.sub.2S from the stripping stage 22A is converted to sulphur using conventional technology, such as the Claus process, or it is combusted and converted to sulphuric acid, both of which are further desired products of the process 100.

[0133] The separated liquor is a saturated aqueous solution of NaHCO.sub.3 that also contains some dissolved Na.sub.2SO.sub.4. The liquor is recycled, along recycle line 28A, to the dissolution stage 12A, thus minimising losses of sodium species across the process 100 in performing the method of the invention.

[0134] The process 100 further provides for the production of the CaS that is used in the gypsum precipitation stage 14A.

[0135] More specifically, gypsum is carbothermally reduced in a gypsum reduction stage 34A by reacting it with carbon, typically originating from coal, at an elevated temperature (800 to 1200 C.).

[0136] A solid product is thus produced, comprising mostly CaS mixed with ash originating from the coal. CO.sub.2 is also produced.

[0137] CO.sub.2 produced in the reduction stage 34A may be recovered and cleaned by scrubbing in a scrubbing stage 35A, and used in the gypsum precipitation stage 14A and in the stripping stage 22A.

[0138] The precipitated gypsum and Ca(OH).sub.2 that are separated from the aqueous Na.sub.2S solution in the first filtration stage 16A may be recycled to the gypsum reduction stage 34A. However, some of this may have to be purged in order to prevent the accumulation of ash in the circuit.

[0139] Thus, it will be appreciated that, as in the case of the first example, the calcium sulphate used in the process is recycled and two useful products from the process are Na.sub.2CO.sub.3 and sulphur, either as elemental sulphur or sulphuric acid.

[0140] In accordance with the invention, the alkaline earth metal in either the process 10 or the process 100 may be barium instead of calcium. In such a case, the fundamental chemistry of the method of the invention and, therefore, the method and corresponding process steps remains virtually unchanged.

[0141] There are, however, certain advantages to barium being the alkaline earth metal over calcium. These advantages apply to and thus characterise the invention both as characterised in the examples above and as characterised in the summary of the invention.

[0142] One advantage is that barium sulphate, which would be formed when reacting the alkali metal sulphate with barium sulphide or barium hydrosulphide, has a lower solubility in water compared to gypsum.

[0143] As a result, virtually no sulphate remains in the solution of the one or more sulphides of the alkali metal, if a stoichiometric excess of barium sulphide or hydrosulphide is used. Using a slight excess of BaS would cause the barium sulphate to precipitate, and allow for its removal in the first separation stage. Thus, the recycle line 28 shown in FIG. 1 or recycle line 28A shown in FIG. 2 may be omitted.

[0144] Another such advantage is that significantly less energy is required to reduce barium sulphate than that which is required to reduce calcium sulphate, as represented by reaction equations 7 and 8 below:

BaSO.sub.4, 25 C.+2.sub.25 C.=BaS.sub.1000 C.2CO.sub.2(g).sub.1000 CH=359 kJ/mol . . . Eq. 7

CaSO.sub.4.Math.2H.sub.2O.sub.25 C.+2C.sub.25 C.=CaS.sub.1000 C.+2CO.sub.2(g).sub.1000 C.H=503 kJ/mol . . . Eq. 8

[0145] A further advantage is that the amount of gas that is released in the process is also lower in the case of barium being the alkali earth metal than in the case of calcium being the alkali earth metal.