Process for producing a concentrated aqueous sodium hydroxide solution

Abstract

A method for producing a concentrated aqueous sodium hydroxide solution from a purge stream deriving from a sodium carbonate, or sesquicarbonate, or wegsheiderite crystallizer, or sodium bicarbonate crystallizer, said purge stream comprising sodium carbonate and/or bicarbonate, and at least 1% of sodium chloride or sodium sulfate and a soluble impurity from an ore deposit comprising at least one of the following elements: As, Ba, B, Ca, Co, K, Li, Mo, P, Pb, Se, Sn, Sr, Te, Tl, Ti, V, and W, to be purified, the method comprising: causticizing at least 50 mol. % of the sodium carbonate into a caustic solution and into a calcium carbonate mud with lime and water; separating the mud from the caustic solution; concentrating the caustic solution by removing part of the water to obtain a concentrated caustic solution comprising at least 25% NaOH, and a crystallized solid comprising sodium carbonate and sodium chloride and/or sulfate; and separating the crystallized solid from the concentrated caustic solution, said crystallized solid to be disposed of or to be further valorized.

Claims

1. A method for producing a concentrated aqueous sodium hydroxide solution from a purge stream derived from an anhydrous sodium carbonate crystallizer, or a sodium carbonate monohydrate crystallizer, or a sodium carbonate decahydrate crystallizer, or a sodium sesquicarbonate crystallizer, or a wegsheiderite crystallizer, or a sodium bicarbonate crystallizer, said purge stream comprising: sodium carbonate and/or sodium bicarbonate from a sodium carbonate or sodium bicarbonate ore deposit selected from the group consisting of: trona ore, nahcolite ore, and wegscheiderite ore, at least 1% by weight of a sodium salt selected from the group consisting of sodium chloride, sodium sulfate and mixtures thereof, and at least one soluble salt or at least one soluble impurity from said sodium carbonate or sodium bicarbonate ore deposit, said soluble salt or soluble impurity comprising at least one of the following elements selected from the group consisting of: As, Ba, B, Ca, Co, K, Li, Mo, P, Pb, Se, Sn, Sr, Te, Tl, Ti, V, and W, said concentrated aqueous sodium hydroxide solution comprising: at least 25% by weight of NaOH, said method comprising the following steps: f) adding lime to the purge stream, in presence of water, to causticize at least 50 mol. % of the sodium from sodium carbonate and/or sodium bicarbonate, into an aqueous sodium hydroxide solution and into a calcium carbonate mud; g) separating the calcium carbonate mud from the aqueous sodium hydroxide solution; h) concentrating the aqueous sodium hydroxide solution by removing part of the water to obtain: the concentrated aqueous sodium hydroxide solution comprising at least 25% NaOH, and to crystallize a solid comprising sodium carbonate and comprising sodium chloride and/or sulfate, i) separating the crystallized solid comprising sodium carbonate and sodium chloride and/or sulfate from the concentrated aqueous sodium hydroxide solution; j) valorizing the concentrated aqueous sodium hydroxide solution as a salable sodium hydroxide solution or recycling at least one part of the concentrated aqueous sodium hydroxide solution to the anhydrous sodium carbonate crystallizer, or to the sodium carbonate monohydrate crystallizer, or to the sodium carbonate decahydrate crystallizer, or to the sodium sesquicarbonate crystallizer, or to the sodium bicarbonate crystallizer, or to processes upstream of the anhydrous sodium carbonate crystallizer, or of the sodium carbonate monohydrate crystallizer, or of the sodium carbonate decahydrate crystallizer, or of the sodium sesquicarbonate crystallizer, or of the sodium bicarbonate crystallizer; and wherein the at least one soluble salt or the at least one soluble impurity comprising at least one element selected from the group consisting of: As, Ba, Be, Bi, B, Ca, Co, Cu, F, K, Li, Mo, P, Pb, Se, Sn, Sr, Te, Tl, Ti, V, and W, is at least partially removed at steps f) to i) from the purge stream.

2. The method of claim 1 wherein the at least one soluble salt or the at least one soluble impurity from the trona ore deposit or from the nahcolite ore deposit, comprises at least one element selected from the group consisting of: Ca, Cl, Cu, Pb, S, Se, Te, Tl that is at least partially removed at steps h) to i) from the purge stream.

3. The method of claim 1 wherein at least 66% of sodium chloride present in the purge stream is removed at steps f) to i) in the concentrated aqueous sodium hydroxide solution.

4. The method of claim 1 wherein at least 89% of sulfur expressed as sulfate present in the purge stream is removed at steps f) to i) in the concentrated aqueous sodium hydroxide solution.

5. The method of claim 1 wherein at least 74% of Mg (magnesium) element from the at least one soluble salt or the at least one soluble impurity from the trona ore deposit or the nahcolite ore deposit and present in the purge stream is removed from steps f) to i) in the concentrated aqueous sodium hydroxide solution.

6. The method of claim 1 wherein at least 92% of P (phosphorus) element from the at least one soluble salt or the at least one soluble impurity from the trona ore deposit or the nahcolite ore deposit and present in the purge stream is removed from steps f) to i) in the concentrated aqueous sodium hydroxide solution.

7. The method of claim 1 wherein the concentrated aqueous sodium hydroxide solution comprises at least 114 mg P and at most 460 mg P expressed as PO4/kg.

8. The method of claim 1 wherein the concentrated aqueous sodium hydroxide solution comprises at least 187 mg Si/kg and at most 1970 mg Si/kg.

9. The method of claim 1 wherein the concentrated aqueous sodium hydroxide solution comprises at least 3200 mg S and at most 6684 mg S expressed as SO4/kg.

10. The method of claim 1, wherein at step h) the weight ratio of sodium carbonate to the sum of the sodium chloride and/or sodium sulfate in crystallized solid is at most 2.

11. The method of claim 1, wherein at step f) the lime is hydrated lime, or a milk of lime comprising hydrated lime.

12. The method of claim 1, wherein, at step f), the lime used as quick lime directly in the causticization step or the lime used before hydration to constitute hydrated lime, comprises more than 90% calcium oxide (CaO).

13. The method of claim 1, wherein the lime is generated by recycle and calcination of the washed calcium carbonate mud.

14. The method of claim 1, wherein at step f) the lime is quick lime or hydrated lime, and the lime comprises magnesium oxide or magnesium hydroxide wherein the magnesium to calcium molar ratio is less than 0.2 mol/mol.

15. The method of claim 1, wherein the purge stream comprises Na2CO3 and/or NaHCO.sub.3 in a quantity of at least 7% TA (Total Alkalinity) expressed as equivalent Na2CO3.

16. The method according to claim 1, wherein the purge stream is a purge derived from a sodium carbonate decahydrate crystallizer, or from a sodium sesquicarbonate crystallizer.

17. The method according to claim 16, wherein the sodium carbonate decahydrate crystallizer, or the sodium sesquicarbonate crystallizer are crystallizers in which a purge from a sodium carbonate monohydrate crystallizer is treated in order to control the sodium chloride and/or the sodium sulfate of the sodium monohydrate crystallizer.

18. A method for producing a sodium carbonate salt or a sodium bicarbonate salt from a sodium carbonate solution or a sodium bicarbonate solution derived from a sodium carbonate ore or a sodium bicarbonate ore selected from the group consisting of: trona, nahcolite, and wegsheiderite ores, or from a sodium carbonate/bicarbonate lake water, or from a reclaimed solid, or from a mine water, said ores, waters or solid comprising sodium carbonate/bicarbonate, comprising the following steps: a) optionally pre-treating the sodium carbonate solution or the sodium bicarbonate solution in removing part of organics and/or changing the carbonate or bicarbonate molar ratio in order to obtain an optional pre-treated sodium carbonate solution or sodium bicarbonate solution; b) crystallizing from the sodium carbonate solution or the sodium bicarbonate solution, or from the optionally pre-treated sodium carbonate solution or sodium bicarbonate solution, a sodium carbonate salt or a sodium bicarbonate salt with one of the means selected from the group consisting of: evaporation crystallization, cooling evaporation, carbonation crystallization and combinations thereof, said crystallization step of the sodium carbonate salt or the sodium bicarbonate salt generating a mother liquor, said mother liquor comprising sodium carbonate or bicarbonate, sodium chloride or sodium sulfate, and water; c) separating the sodium carbonate salt or the sodium bicarbonate salt from the mother liquor; d) recycling part of the mother liquor back into one of the step a), or step b) and removing part of the mother liquor in order to generate a purge stream to control the sodium chloride and/or the sodium sulfate concentration in the mother liquor of the crystallization step b), e) producing the concentrated aqueous sodium hydroxide.

19. The method according to claim 18, wherein the sodium carbonate salt is sodium carbonate monohydrate, and wherein the carbonate solution or bicarbonate solution is a trona ore solution mining solution and/or a trona ore mine water and/or a reclaimed solid comprising sodium carbonate, said carbonate solution or bicarbonate solution comprising at least 10% total alkalinity expressed as sodium carbonate and comprising sodium chloride and/or sodium sulfate, wherein: step a) comprises: a wet calcination in one or several steps for partly decarbonating the carbonate solution or bicarbonate solution to a sodium bicarbonate content of less than 5 w % NaHCO.sub.3, and a water evaporation operation to increase the total alkalinity of the carbonate solution or bicarbonate solution exiting step a) to at least 20% expressed as sodium carbonate, and a caustic calcination for further partly decarbonating the carbonate solution or bicarbonate solution to a sodium bicarbonate content of the carbonate solution or bicarbonate solution exiting step a) to less than 4 w % NaHCO.sub.3, using at least partly the sodium hydroxide from the concentrated aqueous sodium hydroxide solution; step b) comprises crystallizing from the carbonate solution or bicarbonate solution exiting step a), a carbonate salt in the form of sodium carbonate monohydrate salt or a sodium carbonate anhydrous salt with one of the means selected from the group consisting of: evaporation crystallization, cooling evaporation; and step c) comprises separating the sodium carbonate, monohydrate or anhydrous, salt from the mother liquor of step b), and drying and/or calcining the sodium carbonate, monohydrate or anhydrous, salt into dried anhydrous sodium carbonate.

20. The method of claim 18, further comprising recovering water at step a) or at step b) or at step h) as condensates from evaporators and recycling the condensates to a calcined carbonate or bicarbonate ore leaching or to a carbonate or bicarbonate ore solution mining.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a flow diagram which schematically illustrates the method of the present invention.

(2) FIG. 2 is a flow diagram which schematically illustrates one embodiment of the present invention applied to the purge treatment of a sodium carbonate monohydrate crystallizer.

(3) The reference figures and letters quoted below refer to the attached drawings.

DEFINITIONS

(4) For purposes of the present description, certain terms are intended to have the following meanings.

(5) The term “purge” refers to a stream withdrawn from a part of a process to limit impurity concentration in this process.

(6) The expression “derived from” for instance “a purge stream derived from a sodium carbonate monohydrate crystallizer” refers to a stream withdrawn as such from said crystallizer, or to a stream that have been subjected to one or several chemical engineering operation downstream the said crystallizer (such as: centrifuging, crystallizing, filtering, evaporating, diluting, heating, cooling operations), or that has been mixed with one or more other stream(s), though keeping at least one chemical element withdrawn from said crystallizer.

(7) The term “impurity” refers to a compound different from the sodium carbonate and/or the sodium bicarbonate salt to be produced.

(8) The term “solubility” refers to the solubility of a compound in an aqueous solution.

(9) The expression “total carbonate” refers to the carbonate and bicarbonate content. It may be expressed as total carbonate equivalent content.

(10) The term “carbonating” refers to the action of increasing the amount of total carbonate (i.e. carbonate and bicarbonate) of a stream.

(11) The term “decarbonating” refers to the action of decreasing the amount of total carbonate (i.e. carbonate and bicarbonate) of a stream.

(12) The term “bicarbonating” refers to the action of increasing the amount of bicarbonate of a stream.

(13) The term “debicarbonating” refers to the action of decreasing the amount of bicarbonate of a stream.

(14) The expression “total alkalinity” of a stream refers to the alkalinity measured with hydrochloric acid 1N using methyl orange pH indicator down to orange color change (pH of about 3,1): total alkalinity comprises hydroxide ions (OFF), carbonate ions (CO.sub.3.sup.−), and bicarbonate ions (HCO.sub.3.sup.−), and it is expressed as equivalent Na.sub.2CO.sub.3 concentration.

(15) Sodium carbonate derivatives in present description refer to compounds selected from: light soda ash, dense soda ash, sodium carbonate monohydrate, sodium carbonate heptahydrate, sodium carbonate decahydrate, sodium bicarbonate, sodium sesquicarbonate, wegscheiderite.

(16) The term ‘comprising’ includes ‘consisting essentially of” and also “consisting of”.

(17) In addition, if the term “about” is used before a quantitative value, the present teachings also include the specific quantitative value itself, unless specifically stated otherwise. As used herein, the term “about” refers to a +−10% variation from the nominal value unless specifically stated otherwise.

(18) The sign ‘%’ refers to ‘weight %’ unless specifically stated otherwise.

DETAILED DESCRIPTION

(19) The present invention is described here after, in more detailed embodiments.

(20) Item 1. A method for treating a purge stream derived from an anhydrous sodium carbonate crystallizer, or a sodium carbonate monohydrate crystallizer, or a sodium carbonate decahydrate crystallizer, or a sodium sesquicarbonate crystallizer, or a wegsheiderite crystallizer, or a sodium bicarbonate crystallizer, said purge stream comprising sodium carbonate and/or sodium bicarbonate and at least 1% by weight of a sodium salt selected among sodium chloride, sodium sulfate and mixtures thereof,

(21) the method comprising the following steps:

(22) f) adding lime to the purge stream, in presence of water, so that to causticize at least 50 mol. % of the sodium from sodium carbonate and/or sodium bicarbonate, into an aqueous sodium hydroxide solution and into a calcium carbonate mud; g) separating the calcium carbonate mud from the aqueous sodium hydroxide solution; h) concentrating the aqueous sodium hydroxide solution by removing part of the water in order to obtain: a concentrated aqueous sodium hydroxide solution comprising at least 25% NaOH, and to crystallize a solid comprising sodium carbonate and comprising sodium chloride and/or sulfate, wherein lime added at step f) and water removed at step h) are controlled so that the weight ratio of sodium carbonate to the sum of the sodium chloride and/or sodium sulfate in crystallized solid is preferably at most 2, preferably at most 1.5, more preferably at most 1, even more preferably at most 0.6, and most preferred at most 0.4, i) separating the crystallized solid comprising sodium carbonate and sodium chloride and/or sulfate from the concentrated aqueous sodium hydroxide solution, said crystallized solid to be disposed of or to be further valorized, j) valorizing the concentrated aqueous sodium hydroxide solution as a salable sodium hydroxide solution or preferably in recycling at least one part of the concentrated aqueous sodium hydroxide solution to the anhydrous sodium carbonate crystallizer, or to the sodium carbonate monohydrate crystallizer, or to the sodium carbonate decahydrate crystallizer, or to the sodium sesquicarbonate crystallizer, or to the bicarbonate crystallizer, or to processes upstream of the anhydrous sodium carbonate crystallizer, or of the sodium carbonate monohydrate crystallizer, or of the sodium carbonate decahydrate crystallizer, or of the sodium sesquicarbonate crystallizer, or of the bicarbonate crystallizer.

(23) Item 2. The method of item 1, wherein at step f) at least 70 mol. % of the sodium from sodium carbonate and/or sodium bicarbonate is causticized.

(24) Item 3. The method of item 1 or 2, wherein at step f) at least 85 mol. % of the sodium from sodium carbonate and/or sodium bicarbonate is causticized.

(25) Item 4. The method of any items 1 to 3, wherein at step f) at least 90 mol. % of the sodium from sodium carbonate and/or sodium bicarbonate is causticized.

(26) Item 5. The method of any items 1 to 4, wherein at step h) the concentrated aqueous sodium hydroxide solution comprises at least 30% NaOH.

(27) Item 6. The method of any items 1 to 5, wherein at step h) the concentrated aqueous sodium hydroxide solution comprises at least 40% NaOH.

(28) Item 7. The method of any items 1 to 6, wherein at step h) the concentrated aqueous sodium hydroxide solution comprises at least 50% NaOH.

(29) Item 8. The method of any items 1 to 7, wherein at step h) the weight ratio of sodium carbonate to the sum of the sodium chloride and/or sodium sulfate in crystallized solid is at most 2.

(30) Item 9. The method of any items 1 to 8, wherein at step h) the weight ratio of sodium carbonate to the sum of the sodium chloride and/or sodium sulfate in crystallized solid is at most 1.5.

(31) Item 10. The method of any items 1 to 9, wherein at step h) the weight ratio of sodium carbonate to the sum of the sodium chloride and/or sodium sulfate in crystallized solid is at most 1.

(32) Item 11. The method of any items 1 to 10, wherein at step h) the weight ratio of sodium carbonate to the sum of the sodium chloride and/or sodium sulfate in crystallized solid is at most 0.6.

(33) Item 12. The method of any items 1 to 11, wherein at step h) the weight ratio of sodium carbonate to the sum of the sodium chloride and/or sodium sulfate in crystallized solid is at most 0.4.

(34) Item 13. The method of any items 1 to 12, wherein the purge stream comprises Na.sub.2CO.sub.3 and/or NaHCO.sub.3 in a quantity of at least 7% TA, advantageously at least 9% TA, preferably at least 10% TA, more preferably at least 13% TA (total alkalinity) expressed as equivalent Na.sub.2CO.sub.3.

(35) Item 14. The method of any items 1 to 13, wherein the purge stream comprises at most 33% TA, more advantageously at most 22% TA, preferably at most 19% TA, more preferably at most 18% TA, even more preferably at most 16% NaHCO.sub.3 TA (total alkalinity) expressed as equivalent Na.sub.2CO.sub.3.

(36) Item 15. The method of any items 1 to 14, wherein the purge stream comprises at most 33% Na.sub.2CO.sub.3 or at most 16% NaHCO.sub.3.

(37) Item 16. The method of any items 1 to 15, wherein the purge stream comprises at most 15% NaCl or at most 10% Na.sub.2SO.sub.4.

(38) Item 17. The method of any items 13 to 16, wherein the quantity of lime and of water present on step f) is controlled so that the aqueous sodium hydroxide solution comprises at least 6, preferably at least 8, more preferably at least 10% NaOH.

(39) Item 18. The method of any items 1 to 17, wherein the quantity of lime and of water present on step f) is controlled so that the aqueous sodium hydroxide solution comprises at most 14, preferably at most 13, more preferably at most 11% NaOH.

(40) Item 19. The method of any items 1 to 18, wherein the amount of water removed at step h) is controlled so that the concentrated aqueous sodium hydroxide solution comprises at most 7% NaCl and/or at most 2.5% Na.sub.2SO.sub.4.

(41) Item 20. The method of any items 1 to 19, wherein the purge stream is a purge derived from a decahydrate sodium carbonate crystallizer, or from a sodium sesquicarbonate crystallizer.

(42) Item 21. The method of item 20, wherein the decahydrate sodium carbonate crystallizer, or the sodium sesquicarbonate crystallizer are crystallizers wherein a purge from a sodium carbonate monohydrate crystallizer is treated in order to control the sodium chloride and/or the sodium sulfate of the sodium monohydrate crystallizer.

(43) Item 22. The method of any items 1 to 21, wherein when the purge stream comprises sodium chloride, the causticizing of purge stream and the concentrating of the aqueous sodium hydroxide solution, result in a crystallized solid comprising sodium carbonate and sodium chloride in a weight ratio of at most 1.5, preferably at most 1.2, more preferably at most 1.0, most preferred 0.7 ton Na.sub.2CO.sub.3 per ton of NaCl.

(44) Item 23. A method for producing a sodium carbonate salt or a sodium bicarbonate salt from a sodium carbonate/bicarbonate solution derived from a sodium carbonate/bicarbonate ore such as trona, nahcolite, and wegsheiderite ores, or from a sodium carbonate/bicarbonate lake water, or from a reclaimed solid, or from a mine water, said ores, waters or solid comprising sodium carbonate/bicarbonate,

(45) comprising the following steps:

(46) a) optionally pre-treating the sodium carbonate/bicarbonate solution in removing part of organics and/or changing the carbonate/bicarbonate molar ratio in order to obtain an optional pre-treated sodium carbonate/bicarbonate solution; b) crystallizing from the sodium carbonate/bicarbonate solution, or from the optionally pre-treated sodium carbonate/bicarbonate solution, a sodium carbonate salt or a sodium bicarbonate salt with one of the mean selected from the list of: evaporation crystallization, cooling evaporation, carbonation crystallization and combinations thereof, said crystallization step of the sodium carbonate salt or the sodium bicarbonate salt generating a mother liquor, said mother liquor comprising sodium carbonate or bicarbonate, sodium chloride or sodium sulfate, and water; c) separating the sodium carbonate salt or the sodium bicarbonate salt from the mother liquor; d) recycling part of the mother liquor back into one of the optional step a), or step b) and removing part of the mother liquor in order to generate a purge stream to control the sodium chloride and/or the sodium sulfate concentration in the mother liquor of the crystallization step b), e) treating the purge stream according to the method of any items 1 to 21.

(47) Item 24. The method of item 23, wherein the sodium carbonate salt is sodium carbonate monohydrate,

(48) and wherein the carbonate/bicarbonate solution is a trona ore solution mining solution and/or a trona ore mine water and/or a reclaimed solid comprising sodium carbonate,

(49) said carbonate/bicarbonate solution comprising at least 10, preferably at least 12% total alkalinity expressed as sodium carbonate and comprising sodium chloride and/or sodium sulfate,

(50) wherein:

(51) step a) comprises: a wet calcination in one or several steps for partly decarbonating the carbonate/bicarbonate solution to a sodium bicarbonate content of less than 5, preferably less than 4, more preferably less than 2.5 w % NaHCO.sub.3, and a water evaporation operation to increase the total alkalinity of the carbonate/bicarbonate solution exiting step a) to at least 20, preferably at least 25% expressed as sodium carbonate, and a caustic calcination for further partly decarbonating the carbonate/bicarbonate solution to a sodium bicarbonate content of the carbonate/bicarbonate solution exiting step a) to less than 4, more preferably less than 2.5 w % NaHCO.sub.3, using at least partly the sodium hydroxide from the concentrated aqueous sodium hydroxide solution; step b) comprises crystallizing from the carbonate/bicarbonate solution exiting step a), a carbonate salt in the form of sodium carbonate monohydrate salt or a sodium carbonate anhydrous salt with one of the mean selected from the list of: evaporation crystallization, cooling evaporation; step c) comprises separating the sodium carbonate (monohydrate or anhydrous) salt from the mother liquor of step b), and drying/calcining the sodium carbonate (monohydrate or anhydrous) salt into dried anhydrous sodium carbonate.

(52) Item 25. The method of Item 24, wherein: step e) comprises treating the purge stream to control the sodium chloride and/or the sodium sulfate concentration in the mother liquor of the crystallization step b), in three steps: first, optionally, lowering sodium bicarbonate concentration of the purge stream by adding sodium hydroxide, to obtain at most 2, preferably at most 1, more preferably at most 0.4, most preferred at most 0.1 w % of sodium bicarbonate, second, crystallizing at least 20, preferably at least 30, more preferably at least 50% of the sodium carbonate from the purge stream into a sodium carbonate decahydrate crystallization step by cooling and/or evaporating water, separating the sodium carbonate decahydrate crystals from second mother liquor, and said sodium carbonate decahydrate crystals being recovered to be further processed as to recover the corresponding sodium carbonate such as recycling it at step a) or b), third, treating the second mother liquor as a new purge stream with the method of any items 1 to 20.

(53) Item 26. The method of item 24, wherein: step e) comprises treating the purge stream to control the sodium chloride or the sodium sulfate concentration in the mother liquor of the crystallization step b), in three steps: first, optionally, partly carbonating the purge stream with carbon dioxide or with sodium bicarbonate addition to obtain 0.5 to 1.5 mole of sodium bicarbonate by mole of sodium carbonate, second, crystallizing by cooling or by water evaporation, or by carbonating, at least 20, preferably at least 30, more preferably at least 35% of the sodium carbonate from the purge stream into sodium sesquicarbonate, separating the sodium sesquicarbonate crystals from second mother liquor, and said sodium sesquicarbonate crystals are recovered to be further processed to recover the corresponding value of the sodium carbonate and sodium bicarbonate, third, treating the second mother liquor as a new purge stream with the method of items 1 to 20.

(54) Item 27. The method of any items 1 to 22 wherein the calcium carbonate mud is further used for flue gas mitigation, or for agricultural soil amendment, or after an optional carbonation as board or paper filler.

(55) Item 28. The method of any items 1 to 27, wherein the purge stream comprises at least one impurity of a soluble salt or at least one soluble impurity from ore deposits selected from: trona, nahcolite, or wegscheiderite ore, said soluble salt or soluble impurity comprising at least one element from: As, Ba, Be, Bi, B, Ca, Co, Cu, F, K, Li, Mg, Mo, P, Pb, Se, Si, Sn, Sr, Te, Tl, Ti, V, W, and wherein said soluble salt or soluble impurity is at least partially removed at step f) to i) from the purge stream.

(56) Item 29. The method of item 28, wherein the at least one impurity of a soluble salt or at least one soluble impurity from ore deposits selected from: trona, nahcolite, or wegscheiderite ore, comprises at least one element from: Ca, Cl—, Cu, Pb, S, Se, Te, Tl that is at least removed at step h) to i) from the purge stream.

(57) The method of present invention is efficient in treating variable concentrations of sodium chloride and/or sodium sulfate in the purge stream, such as high concentrations values encountered in pre-concentration of impurities in a decahydrate or a sesquicarbonate crystallizer pre-concentrating the purge from a sodium carbonate monohydrate crystallizer. Though too high concentrations of sodium chloride increase the density of suspension of the crystallized solid comprising sodium chloride and/or sulfate in the concentrated caustic soda solution. Therefore, preferably the purge stream comprises at most 15% NaCl or at most 10% Na.sub.2SO.sub.4.

(58) In the present invention the lime is quick lime or hydrated lime. The causticizing of the sodium carbonate and/or sodium bicarbonate with quick lime (Calcium oxide CaO) or hydrated lime (Calcium hydroxide Ca(OH).sub.2) relates in presence of water mainly to the same overall chemical reaction. Indeed quick lime reacts rapidly with water to form hydrated lime (Ca(OH).sub.2). Causticization of carbonate ions or bicarbonate ions with lime in presence of water generates an hydroxide ions solution, according to the following reactions:
Na.sub.2CO.sub.3+Ca(OH).sub.2.fwdarw.CaCO.sub.3+2NaOH  [1]
NaHCO.sub.3+Ca(OH).sub.2.fwdarw.CaCO.sub.3+NaOH+H.sub.2O  [2]

(59) Therefore during causticization reaction, sodium carbonate and bicarbonate concentrations decrease and the sodium hydroxide concentration increase.

(60) Moreover, part of sodium sulfate is also causticized by lime in presence of water according to the following reaction forming insoluble calcium sulfate and sodium hydroxide solution:
Na.sub.2SO.sub.4+Ca(OH).sub.2+2H.sub.2O.fwdarw.CaSO.sub.4.2H.sub.2O+2NaOH  [3]

(61) Though as calcium carbonate is less soluble than calcium sulfate, calcium sulfate in presence of sodium carbonate will partially dissolved and calcium will precipitated into calcium carbonate regenerating part of sodium sulfate.

(62) When quick lime (calcium oxide CaO) is put in contact with water, calcium oxide reacts to form hydrated lime (calcium hydroxide Ca(OH).sub.2). The quick lime (calcium oxide) reaction with water is strongly exothermic. When an excess of water is used for hydrating quick lime, the obtained hydrated lime forms generally finely divided solid particles in suspension in water. Calcium hydroxide is slightly soluble in water (about 0.185 parts in weight for 100 parts of water). The suspension of the divided solid particles in water is generally called milk of lime.

(63) In present invention, the lime (used directly in the causticization step when used as quick lime, or used before hydration to constitute the hydrated lime) should be of the highest possible quality, comprising preferably more than 90% CaO. The lime can be generated also on site by recycle and calcination of the washed CaCO.sub.3 mud.

(64) The quick lime or hydrated lime may comprise also magnesium oxide or magnesium hydroxide. Though because of the poor ability of magnesium hydroxide to be causticized with sodium carbonate or bicarbonate, and because of the poor filterability of the obtained calcium carbonate mud, comprising high amounts of magnesium hydroxide, it is preferred that the quick lime or hydrated lime comprises magnesium to calcium molar ratio of less than 0.2, more preferably less than 0.1 and most preferred less than 0.05 mol/mol.

(65) In present invention the causticizing step is preferably operated with hydrated lime, more preferably with a milk of lime comprising hydrated lime.

(66) In such embodiment using a milk of lime, the concentration of hydrated lime in the milk of lime, expressed in moles of Ca(OH).sub.2/liter of milk of lime is generally at least 0.5 mol/l, preferably at least 1.0 mol/l, more preferably at least 1.5 mol/l. The milk of lime comprises generally at most 7.0 mol/l, preferably at most 5.0 mol/l, more preferably at most 3.0 mol/l.

(67) The removal of water from the aqueous sodium hydroxide solution consumes energy. Therefore there is little economic interest to produce a too diluted solution of aqueous sodium hydroxide solution, in using for instance a diluted milk of lime or a purge stream with high content of water.

(68) Therefore it is advantageous in present invention that the water and lime amount be controlled at step f) so that the aqueous sodium hydroxide solution comprises at least 6, preferably at least 8, more preferably at least 10% NaOH.

(69) Also the causticizing (or caustifying) rate expressed as the molar ratio of equivalent hydroxide ion reported to the sum of: equivalent carbonate (2 mole equivalents per mole of carbonate) plus the equivalent of bicarbonate ion (1 mole equivalent per mole of bicarbonate ion) plus the equivalent of sodium hydroxide ion (1 mole equivalent per mole of hydroxide ion), is dependant from the sodium hydroxide concentration, because of the low solubility of calcium hydroxide. In present invention it is advantageous that the water and lime amount be controlled at step f) so that the aqueous sodium hydroxide solution comprises at most 14, preferably at most 13, more preferably at most 11% NaOH.

(70) Preferably in present invention, the purge stream is an aqueous solution. The total alkalinity (TA) of the purge stream is preferably around 16-17%. This enables to achieve caustification yields of about 85% and obtain a NaOH solution of about 10-11%. A higher total alkalinity results in higher final NaOH concentration, and therefore in lower energy consumption for water removal if done in an evaporation section, but lowers the caustification yield resulting in higher Na.sub.2CO.sub.3 losses in the crystallized solid during the concentration of the aqueous sodium hydroxide solution. A decrease in concentration (TA) of the purge stream will have the exact opposite effects. The economic optimum can be determined easily on each operating plant based on the respective costs of energy and feedstock (sodium carbonate/bicarbonate ore and lime).

(71) The causticization temperature should be kept preferably above 95° C. in order to increase the velocity of the reaction but most of all, in order to precipitate the CaCO.sub.3 in the form of calcite which is much easier to filtrate. A lower temperature will also favor the formation of pirsonite (CaCO.sub.3.Na.sub.2CO.sub.3.2H.sub.2O) leading to increased sodium losses with the caustification mud.

(72) The calcium carbonate mud (hereafter ‘the mud’) is separated from the aqueous sodium hydroxide solution through settlers, or other means, then eventually a filtration step if needed for disposal or handling of the mud. The mud, comprising mostly CaCO.sub.3, some unreacted CaO or Ca(OH).sub.2, precipitated impurities from the purge stream, and impregnating aqueous sodium hydroxide solution, can be disposed of in different ways.

(73) The most preferable when possible is to recycle the mud in a sodium carbonate trona leach system where it will be mixed with the insolubles from a sodium carbonate/bicarbonate ore such as trona ore. This has the double advantage of avoiding the investment for a separation unit (filters) and for the disposal of the causticization mud. It has also the advantage of allowing the reaction of any of the unreacted CaO with the bicarbonate present in the leach liquor.

(74) A second way is to separate the mud, filter it and wash it and then recycle it to a lime kiln, to produce the CaO for step 1 and eventually recover also pure CO.sub.2 for any process that needs this feedstock (bicarbonate) or for sequestration if calcining mud is operated with indirect heating. The economics of each option will be dictated by the price of energy, CO.sub.2, availability of cheap lime on the market.

(75) A third way is to separate the mud, filter it and wash it and sell the calcium carbonate (for example as dry sorbent for SO.sub.2 pollution control).

(76) A fourth way is to separate the mud, filter it and store it in mine voids separately from sodium carbonate/bicarbonate ore insolubles, or in solution mining cavities.

(77) A fifth but least preferred way is to separate the mud, filter it and store it on the surface (tailings pond).

(78) In present method, most of sodium chloride and/or sodium sulfate of the purge stream are removed in the purge stream treatment.

(79) The chloride ions are removed as sodium chloride solid precipitated during the concentration of the aqueous sodium hydroxide solution and subsequently separated from the concentrated aqueous sodium hydroxide solution.

(80) Along with the treatment of the purge stream comprising chlorides and sulfates ions, the present method has shown a surprising efficient synergy of causticization steps f) and g) combined with steps h) and i) to remove along with NaCl and Na2SO4 from the purge stream other soluble compounds such as: phosphates, silicates compounds removed with the calcium carbonate mud, and metallic ions either removed with the calcium carbonate mud or with the crystallized solid comprising sodium carbonate and sodium chloride and/or sulfate.

(81) In present invention, the concentrating of the aqueous sodium hydroxide solution may be done by any known technology in the art, such as an evaporation unit selected from: a mechanical vapor recompression evaporator, a falling film evaporator, and multiple effect evaporation unit, preferably with steam thermo-compressors. The aqueous sodium hydroxide solution will be evaporated in order to obtain a concentrated sodium hydroxide solution of generally from 30% up to 50% in weight of NaOH. The level of evaporation will depend on the economics that are dictated by the investment needed, in particular as evaporators made in nickel alloys recommended above 25% NaOH, the energy cost, the amount of impurities to be eliminated and the usage of the caustic solution. The higher the concentration of NaOH, the lower the solubility in NaCl, Na.sub.2SO.sub.4, and Na.sub.2CO.sub.3, and most of metal hydroxides in the resulting caustic solution, and higher the quantity of impurities removed (precipitation of the above mentioned salts). For instance NaCl solubility in a concentrated sodium hydroxide solution is given below in table 2.

(82) TABLE-US-00002 TABLE 2 NaCl solubility in concentrated aqueous sodium hydroxide solution NaOH aqueous g/kg 100 250 300 350 400 450 500 solution concentration NaCl solubility g/kg 148 71 49 32 22 17 12

(83) The sodium chloride remaining in the concentrated aqueous hydroxide solution can be controlled with the removal of water when concentrating the aqueous sodium hydroxide solution (from about 8% up to about 50 w % NaOH). In present invention it is advantageous to control water evaporation during sodium hydroxide concentration so that after removal of the crystallized solid comprising sodium carbonate and comprising sodium chloride and/or sulfate, the concentrated aqueous sodium hydroxide solution comprises at most 7, preferably at most 2, more preferably at most 1% NaCl and/or at most 2.5, preferably at most 1, more preferably at most 0.5% Na.sub.2SO.sub.4.

(84) In operating the present method comprising both a causticizing step and a concentration of the aqueous hydroxide solution, one can reduce sensitively the loss of sodium carbonate finally purged with the sodium chloride and/or sodium sulfate. In particular when the purge stream comprises sodium chloride, it is advantageous that the causticizing rate of sodium carbonate or bicarbonate from purge stream with lime, and the water removal when concentrating the aqueous sodium hydroxide solution, be controlled so that the crystallized solid comprising sodium carbonate and sodium chloride be in a weight ratio of at most 1.5, preferably at most 1.2, more preferably at most 1.0, most preferred 0.7 ton Na.sub.2CO.sub.3 per ton of NaCl.

(85) Indeed the more sodium carbonate is transformed into sodium hydroxide at step f) (that may be controlled for instance by lime addition, or causticization rate), the less sodium carbonate is present in the crystallized solid at step h) and i). Also the more water is withdrawn from the sodium hydroxide solution at step g), the more sodium chloride (and sulfate) is (are) removed as the crystallized solid at step i).

(86) Such a method is particularly interesting as when the purge stream is a purge from a decahydrate sodium carbonate (deca) crystallizer or from a sodium sesquicarbonate (sesqui) crystallizer, the deca or sesqui crystallizer being itself a pre-treatment of a purge from a sodium carbonate monohydrate crystallizer to pre-concentrate the sodium chloride and or sodium sulfate. In that case, the loss of sodium carbonate in final purge (ie the ‘crystallized solid’ of present method) is reduced of a factor 1.5 to 3.0 when reported per ton of purged NaCl, enabling the recovery of generally at least 60%, or at least 70%, and up to at least 90% of the sodium carbonate present in the purge of the sodium carbonate monohydrate crystallizer.

(87) Therefore, in an advantageous embodiment of present invention, the purge stream is a purge from a decahydrate sodium carbonate crystallizer, or from a sodium sesquicarbonate crystallizer. Even more advantageously the decahydrate sodium carbonate crystallizer or the sodium sesquicarbonate crystallizer can be crystallizers wherein a purge from a sodium carbonate monohydrate crystallizer is treated in order to control the sodium chloride and/or the sodium sulfate of the sodium monohydrate crystallizer. Indeed, the specific combination of a purge treatment from a monohydrate crystallizer, first in a decahydrate sodium carbonate crystallizer, or the sodium sesquicarbonate crystallizer, followed then with one of the methods described above of present invention, enables to produce a sodium carbonate monohydrate with low sodium chloride and/or sodium sulfate content, and then concentrating sodium chloride and/or sodium sulfate to constitute the purge stream of the present invention well-fitted for further steps of: causticization and removal of calcium carbonate mud, and then concentration of the previously obtained aqueous sodium hydroxide solution and separation of the crystallized solid comprising sodium carbonate and sodium chloride and/or sulfate from the concentrated aqueous sodium hydroxide solution. Though, when combining a purge treatment comprising a first step of sodium carbonate decahydrate crystallization and then the method of present invention, it is preferable to limit the sodium sulfate concentration in the decahydrate crystallizer solution to less than 10%, preferably less than 8% of Na.sub.2SO.sub.4 to avoid the crystallization of sodium sulfate decahydrate and to limit the formation of mixed salts of sodium carbonate and sodium sulfate.

(88) The present invention relates also to a method for producing a sodium carbonate salt or a sodium bicarbonate salt from a sodium carbonate/bicarbonate solution derived from a sodium carbonate/bicarbonate ore such as trona, nahcolite, and wegsheiderite ores, or from a sodium carbonate/bicarbonate lake water, or from a reclaimed solid, or from a mine water, said ores, waters or solid comprising sodium carbonate/bicarbonate, comprising the following steps: a) optionally pre-treating the sodium carbonate/bicarbonate solution in removing part of organics and/or changing the carbonate/bicarbonate molar ratio in order to obtain an optional pre-treated sodium carbonate/bicarbonate solution; b) crystallizing from the sodium carbonate/bicarbonate solution, or from the optionally pre-treated sodium carbonate/bicarbonate solution, a sodium carbonate salt or a sodium bicarbonate salt with one of the mean selected from the list of: evaporation crystallization, cooling evaporation, carbonation crystallization and combinations thereof, said crystallization step of the sodium carbonate salt or the sodium bicarbonate salt generating a mother liquor, said mother liquor comprising sodium carbonate or bicarbonate, sodium chloride or sodium sulfate, and water; c) separating the sodium carbonate salt or the sodium bicarbonate salt from the mother liquor; d) recycling part of the mother liquor back into one of the step a), or step b) and removing part of the mother liquor in order to generate a purge stream to control the sodium chloride and/or the sodium sulfate concentration in the mother liquor of the crystallization step b), e) treating the purge stream according to the method of claims 1 to 9.

(89) In an advantageous embodiment of the above method for producing a sodium carbonate salt or a sodium bicarbonate salt, the sodium carbonate salt is sodium carbonate monohydrate, and the carbonate/bicarbonate solution is a trona ore solution mining solution and/or a trona ore mine water and/or a reclaimed solid comprising sodium carbonate,

(90) said carbonate/bicarbonate solution comprising at least 10, preferably at least 12% total alkalinity expressed as sodium carbonate and comprising sodium chloride and/or sodium sulfate,

(91) wherein:

(92) step a) comprises: a wet calcination in one or several steps for partly decarbonating the carbonate/bicarbonate solution to a sodium bicarbonate content of less than 5, preferably less than 4, more preferably less than 2.5 w % NaHCO.sub.3, and a water evaporation operation to increase the total alkalinity of the carbonate/bicarbonate solution exiting step a) to at least 20, preferably at least 25% expressed as sodium carbonate, and a caustic calcination for further partly decarbonating the carbonate/bicarbonate solution to a sodium bicarbonate content of the carbonate/bicarbonate solution exiting step a) to less than 4, more preferably less than 2.5 w % NaHCO.sub.3, using at least partly the sodium hydroxide from the concentrated aqueous sodium hydroxide solution; step b) comprises crystallizing from the carbonate/bicarbonate solution exiting step a), a carbonate salt in the form of sodium carbonate monohydrate salt or a sodium carbonate anhydrous salt with one of the mean selected from the list of: evaporation crystallization, cooling evaporation; step c) comprises separating the sodium carbonate (monohydrate or anhydrous) salt from the mother liquor of step b), and drying/calcining the sodium carbonate (monohydrate or anhydrous) salt into dried anhydrous sodium carbonate.

(93) In a sub-embodiment of the advantageous embodiment of the above method for producing a sodium carbonate salt or a sodium bicarbonate salt, the step e) comprises treating the purge stream to control the sodium chloride and/or the sodium sulfate concentration in the mother liquor of the crystallization step b), in three steps: first, optionally, lowering sodium bicarbonate concentration of the purge stream by adding sodium hydroxide, to obtain at most 2, preferably at most 1, more preferably at most 0.4, most preferred at most 0.1 w % of sodium bicarbonate, second, removing at least 20, preferably at least 30, more preferably at least 50% of the sodium carbonate from the purge stream by a sodium carbonate decahydrate crystallization step, wherein the sodium carbonate decahydrate crystals are separated from second mother liquor, and said sodium carbonate decahydrate crystals are recovered to be further processed as to recover the corresponding sodium carbonate, third, treating the second mother liquor as a new purge stream with the method of claims 1 to 8.

(94) In a second sub-embodiment of the advantageous embodiment of the above method for producing a sodium carbonate salt or a sodium bicarbonate salt, the step e) comprises treating the purge stream to control the sodium chloride or the sodium sulfate concentration in the mother liquor of the crystallization step b), in three steps: first, optionally, partly carbonating the purge stream with carbon dioxide to obtain 0.5 to 1.5 mole of sodium bicarbonate by mole of sodium carbonate, second, removing at least 20, preferably at least 30, more preferably at least 35% of the sodium carbonate from the purge stream by a sodium sesquicarbonate crystallization step, wherein the sodium sesquicarbonate crystals are separated from second mother liquor, and said sodium sesquicarbonate crystals are recovered to be further processed to recover the corresponding value of the sodium carbonate and sodium bicarbonate, third, treating the second mother liquor as a new purge stream with the method of claims 1 to 8.

(95) The following examples are intended only to exemplify the invention and are not intended to limit the scope of the claimed invention.

EXAMPLES

Examples 1 and 2

(96) The purge from a natural soda ash crystallizer, a monohydrate crystallizer operating at 101° C., comprising 27.3-30.3% Na.sub.2CO.sub.3, 2-5% NaCl, 0.5% Na.sub.2SO.sub.4 and minor impurities such as silicates (1500 ppm Si), phosphates (165 ppm P), aluminates (79 ppm Al), and organics (2300 ppm COD), is treated in a first step in a sodium carbonate decahydrate crystallizer operated at about 15-20° C. in order to recover part of sodium carbonate of the purge (the ‘monohydrate purge’) as sodium carbonate decahydrate and a purge stream (a ‘decahydrate purge’) comprising 17% Na.sub.2CO.sub.3, 8% NaCl, and 1.1% Na.sub.2SO.sub.4.

(97) To this purge stream, lime comprising 95% CaO is added in a ratio of 9 t to 100 t of purge stream, with the addition also of 24 t of water, in a mixed reactor with a residence time of 1.5 hour and operated at 95° C., resulting into a suspension comprising an aqueous sodium hydroxide solution and a calcium carbonate mud.

(98) Analyses of the resulting suspension show that: 85% of sodium carbonate is causticized in the aqueous sodium hydroxide solution (the caustic solution) and the caustic solution comprises: 10% NaOH, 2.3% Na.sub.2CO.sub.3, 7.2% NaCl, and 0.2% Na.sub.2SO.sub.4.

(99) Analyses of the calcium carbonate mud and aqueous sodium hydroxide solution indicate that impurities such as silicates, and phosphates from the purge are extracted with the calcium carbonate mud and are sensitively decreased from a factor 1.5 to 10 in the caustic solution.

(100) The caustic solution is then concentrated in an multiple effect evaporator to reach: a concentrated aqueous sodium hydroxide solution (the concentrated caustic solution) of 30% NaOH for example 1, or 50% NaOH for example 2, and a crystallized solid comprising sodium carbonate, sodium chloride and sodium sulfate.

(101) The corresponding flow and mass balance are given for: example 1 with a concentrated caustic solution of 30% NaOH at table 3 example 2 with a concentrated caustic solution of 50% NaOH at table 4.

(102) On can see in each example that the sodium carbonate in crystallized solid is reduced down to respectively: for example 1: 0.64 t Na.sub.2CO.sub.3/t NaCl for example 2 and 0.57 t Na.sub.2CO.sub.3/t NaCl.

(103) Those figures illustrate that the method of present invention: eliminates 80% (example 1) and up to 90% (example 2) of the NaCl (Na.sub.2SO.sub.4 is similarly eliminated as NaCl with an even more important yield as sodium sulfate is partly precipitated at step f) plus at step h)) as a solid with a substantially reduced amount of Na.sub.2CO.sub.3 (loss) in the final purge constituted by the crystallized solid.

(104) Moreover a comparison to previous known methods of purge treatment wherein the purge of monohydrate crystallizer is treated: more than 80% to more than 90% of alkaline sodium (from sodium carbonate and bicarbonate of the purge) is recovered and may be recycled into the monohydrate crystallizer (or be valorized as caustic as salable liquor), comparatively the alkaline sodium recovery of purge treatment associating a sodium carbonate decahydrate crystallizer alone as described by US20050274678 is about 67%, comparatively the alkaline sodium recovery of purge treatment associating a sodium carbonate decahydrate crystallizer and a bicarbonate crystallizer as described by US2004057892 is about 70%, the loss of water is also sensitively decreased to less than 0.5 to 1 t water/t of sodium chloride purged. Comparatively, neither the decahydrate or the bicarbonate process can reduce the water losses per ton of NaCl purge which are about 9.5 t H.sub.2O/t NaCl.

(105) Therefore when the processed carbonate/bicarbonate ore, such as trona or nahcolite, or wegscheiderite, has an increased or fluctuating sodium chloride and/or sodium sulfate content, the method of present invention with a reduced amount of loss of sodium carbonate per ton of NaCl or per combined ton of NaCl+Na.sub.2SO.sub.4, remains particularly interesting compared to previous known methods.

Examples 3 to 6

(106) Four purge stream samples comprising different levels of impurities, said purge streams deriving from sodium carbonate monohydrate crystallizers fed with sodium carbonate aqueous solutions fed from different parts of trona deposits in Green River, Wyo. USA, were selected.

(107) The said monohydrate crystallizers operating conditions were similar to the ones described in ‘Natural Soda Ash’ from Donal E. Garrett, Van Nostrand Reinhold Editor, New York, 1992 at Chapter 8 ‘Monohydrate process’ pp 267-299.

(108) The selected four purge stream samples were analyzed and their content in main elements is listed in tables 5 to 8. The remaining of the purge stream composition is mainly water.

(109) The OH.sup.−, CO.sub.3.sup.2− et HCO.sub.3.sup.− were measured by potentiometry titration with chlorhydric acid (HCl) 1 mol/L (1 N), with determination of equivalence values by measuring derivative curve.

(110) Cl— was measured by potentiometry titration with AgNO.sub.3 0.2 mol/L (0.2N) with also determination of equivalence values by measuring derivative curve.

(111) All other elements were analyzed with standard method using inductively coupled plasma (ICP) atomic emission spectrometry, after dilution of the solution sample, acidification with HCl of the sample, filtration on 0.45 μm membrane, and the recovered solid was solubilized after alkaline fusion and redissolution in order to be complete on the related measured element.

(112) The purge stream samples were then causticized in a stirred laboratory reactor at 95° C. during 2 hours after addition of quick lime at stoichiometric ratio with an excess of 15% of lime, in regards of the sodium carbonate and bicarbonate contents of the purge streams, and in regard of equations [1] and [2] listed supra in present specification. Previous to lime addition, water was optionally added so that the final sodium hydroxide concentration of the solution be about 10+/−1% by weight. During causticization, nitrogen gas (N.sub.2) was continuously injected at a rate of about 50 L/h in the dome of the used 5 liters reactor, to avoid caustic solution carbonation with CO.sub.2 of ambient air.

(113) After 2 hours, where causticization of sodium carbonate/bicarbonate into sodium hydroxide and into a calcium carbonate mud took place, the reactor slurry was filtered with a stainless steel filter equipped with a Teflon membrane of 5 μm porosity to separate and obtain a sodium hydroxide solution (of about 10% NaOH concentration); and the calcium carbonate mud formed by the causticization of the purge stream was retained on the membrane of the filter.

(114) Analyzes of the obtained aqueous sodium hydroxide solutions are given in respective tables 5 to 8 (columns 3).

(115) The sodium hydroxide solutions were then concentrated: to about 30-39% NaOH concentrated solution (examples 3 & 4) by placing the content of the reactor at 95° C. and under a vacuum of 600 mbar, to about 43-46% NaOH concentrated solution (examples 5 & 6) by placing the content of the reactor at 115° C. and under a vacuum of 650 mbar,
and by removing part of the water of the sodium hydroxide solution and following the amount of obtained condensates to stop the water evaporation when the targeted value of water condensates was obtained for a given final NaOH concentration.

(116) The obtained suspensions, where part of: sodium chloride, sodium sulfate, and sodium carbonate were precipitated as solid during concentration of sodium hydroxide, were filtered to separate the concentrated NaOH solution that was recovered and analyzed, from the obtained precipitated solid. The used filter was heated at 95° C. and was equipped with a Teflon membrane of 5 μm porosity, and operated under pressure (at 1.5 bar), fed with nitrogen (N.sub.2) gas to avoid carbonation of caustic solution.

(117) The concentrated aqueous sodium hydroxide solutions thus obtained were analyzed in listed chemical elements and compared to the concentration of the solution in these elements before causticization and before caustic solution concentration. Analysis results are given on tables 5 to 8, along with the purification of impurities removal from purge solution expressed in weight %.

(118) For the causticization solution (aqueous sodium hydroxide solution or ‘Solution after steps f) to g)’), chlorides concentration (expressed as NaCl) was used as tracer to determine dilution/concentration factor with the original purge stream.

(119) For the concentration/evaporation solution (concentrated aqueous sodium hydroxide solution or ‘Solution after steps f) to i)’), hydroxide concentration (expressed as NaOH) was used as tracer to determine dilution/concentration factor compared to the aqueous sodium hydroxide solution or compared to the original purge stream solution.

(120) The analysis results (column 4 of the tables) show that the causticization step f) and associated filtering step g) of the calcium carbonate (plus calcium sulfate) mud enables an effective purification of the purge stream in following impurities:

(121) barium (Ba), beryllium (Be) bismuth (Bi), copper (Cu), magnesium (Mg), phosphor (P) expressed in phosphate, silica (Si), strontium (Sr), sulfur (S) expressed in sulfate, titanium (Ti), and to a lesser extent in arsenic (As), boron (B), potassium (K), lithium (Li), molybdenum (Mo).

(122) The analysis results (columns 6 of tables 5 to 8) show that the concentration step h) with water evaporation of the caustic solution and crystallization of part of the sodium carbonate, sodium chloride and sodium sulfate salts, into a crystallized solid, followed then by the separation step i) of said crystallized solid from the concentrated aqueous sodium hydroxide solution, enable an effective purification of the purge stream in following impurities: calcium (Ca), chloride (Cl.sup.−), copper (Cu), lead (Pb), sulfur (S) expressed as sulfate, selenium (Se), tellurium (Te), thallium (Tl) that are co-precipitated with the crystallized solid.

(123) Therefore the combination of the overall steps f) to i) (see columns 7 of tables 5 to 8) enable to decrease the amount of impurities comprising at least one of the following chemical elements:

(124) arsenic (As), barium (Ba), beryllium (Be), bismuth (Bi), boron (B), calcium (Ca), chloride (Cl.sup.−), copper (Cu), lead (Pb), lithium (Li), magnesium (Mg), molybdenium (Mo), nickel (Ni), phosphor (P) expressed as phosphate, potassium (K), selenium (Se), silica (Si), strontium (Sr), sulfur (S) expressed as sulfate, tellurium (Te), thallium (Tl), tin (Sn), titanium (Ti). Should the disclosure of any patents, patent applications, and publications which are incorporated herein by reference conflict with the description of the present application to the extent that it may render a term unclear, the present description shall take precedence.

(125) TABLE-US-00003 TABLE 3 Example 1 - flow and mass balance - Purge treatment with NaOH concentrated to 30%. 7 10 1 4 5 6 10% 8 9 Purge 11 12 Purge 2 3 CaCO.sub.3 10% 10% NaOH Steam Condensates NaCl 30% H.sub.2O stream Lime H.sub.2O Mud NaOH NaOH to EV to EV from EV out NaOH Evap Na.sub.2CO.sub.3 t/h 17 2.5 0.2 2.3 1.9 0.3 NaCl t/h 8 8 0.7 7.3 5.7 1.6 NaOH t/h 11 0.9 10.1 0 10.1 CaO t/h 8.1 0.4 CaCO.sub.3 t/h 13.7 H.sub.2O t/h 75.2 24 8.2 88.5 7.5 81 26.8 58.6 3.7 21.5 27.7 INERTS t/h 0.4 0.4 Total t/h 100.0 9.0 23.0 110.0 9.0 101.0 27.0 59.0 11.0 34.0 28.0 Na.sub.2CO.sub.3 % 17 2.3 2.3 2.3 17.1 1 NaCl % 8 7.2 7.2 7.2 50 4.9 NaOH % 10 10 10 30 CaO % 95 1.8 CaCO.sub.3 % 60.3 H.sub.2O % 75 100 36 80.5 80.5 80.5 100 100 32.9 64.1 100 INERTS % 5 1.9

(126) TABLE-US-00004 TABLE 4 Example 2 - flow and mass balance - Purge treatment with NaOH concentrated to 50% 7 10 1 4 5 6 10% 8 9 Purge 11 12 Purge 2 3 CaCO.sub.3 10% 10% NaOH Steam Condensates NaCl 50% H.sub.2O stream Lime H.sub.2O Mud NaOH NaOH to EV to EV from EV out NaOH Evap Na2CO3 t/h 17.0 2.6 0.2 2.4 2.4 0.0 NaCl t/h 8.0 8.0 0.7 7.3 7.0 0.3 NaOH t/h 10.8 0.9 9.9 0.0 9.9 CaO t/h 8.0 0.4 CaCO.sub.3 t/h 13.5 H.sub.2O t/h 74.9 22.6 8.1 87.0 7.5 79.5 31.5 68.9 4.6 9.6 32.5 INERTS t/h 0.4 0.4 Total t/h 100 8.4 22.6 22.4 108.4 9.3 99.1 31.5 68.9 14.0 19.8 32.5 Na.sub.2CO.sub.3 % 17.0 2.4 2.4 2.4 16.8 0.2 NaCl % 8.0 7.4 7.4 7.4 50.0 1.5 NaOH % 10.0 10.0 10.0 50.0 CaO % 95.0 1.8 CaCO.sub.3 % 60.3 H.sub.2O % 75.0 100.0 36.0 80.2 80.2 80.2 100.0 100.0 33.2 48.3 100.0 INERTS % 5 1.9

(127) TABLE-US-00005 TABLE 5 Example 3 - Solution analysis and percentages of impurities reductions in solutions: impurity impurity reduction reduction overall in solution in solution impurity Test after steps after steps reduction Example 3 Purge Solution f) to g) Solution h) to i) in solution Solutions stream after steps (based on after steps (based on after steps analysis solution f) to g) ΔNaCl) f) to i) ΔNaOH) f) to i) Concentration g/kg g/kg % g/kg % % NaOH 0 94 305 Na2CO3 163 48 28 82 NaHCO3 11 0 0 NaCl 44 49 53 66 66 mg/kg reported mg/kg reported Concentration mg/kg to 10% NaOH % to 30% NaOH % % Ba 2.1 <0.11 >95 <0.17 >95 Ca 2.3 90.0 19.3 93 93 Li 13 11 26 37.3 26 Mg 2.5 0.9 69 2.4 16 74 Na — 115624 263636 30 30 P as PO4 1237 75 95 319 95 Pb 0.6 <1.1 2.4 — S as SO4 15788 18101 6684 89 89 Si 545 329 46 187 82 91 Sr 0.10 <0.1 >7 0.17 <52 <56 Ti 0.02 <0.2 0.34 <52 <52 steps f) to g): causticization & solid separation steps h) to i): concentration − evaporation/separation steps f) to i): causticization/separation, +concentration − evaporation/separation.

(128) TABLE-US-00006 TABLE 6 Example 4 - Solution analysis and percentages of impurities reductions in solutions: impurity impurity reduction reduction overall in solution in solution impurity Test after steps after steps reduction Example 4 Purge Solution f) to g) Solution h) to i) in solution Solutions stream after steps (based on after steps (based on after steps analysis solution f) to g) ΔNaCl) f) to i) ΔNaOH) f) to i) Concentration g/kg g/kg % g/kg % % NaOH 0 88 392 Na2CO3 156 59 24 91 NaHCO3 13 0 0 NaCl 44 47 31 85 85 mg/kg reported Concentration mg/kg to 10% NaOH % mg/kg as it % % Ag 0.05 <0.02 >63 <0.04 >63 As 7.7 7.8 5 31.4 10 14 B 295 300 5 1160 13 17 Ba 3.3 0.03 99 0.04 70 99.7 Bi 1.6 <0.2 >89 <0.40 >89 Ca 2.3 8.6 5.9 85 85 Co 1.3 0.12 91 1.24 91 Cu 1.4 0.5 67 <0.20 67 K 2860 3065 12360 9 9 Li 12.3 11.2 15 47.4 5 19 Na 93000 95000 238000 44 44 P as PO4 1290 120 91 460 14 92 Pb 0.6 0.7 1.9 42 42 S as SO4 13500 14570 4300 93 93 Sb 1.5 1.4 14 14 Se 2.4 2.4 5 9.2 13 18 Si 650 565 19 1970 22 36 Sr 1.4 0.02 99 <0.02 >78 99.7 Te 1.6 1.4 19 6.2 2 20 Ti 0.19 0.05 75 75 Tl 1.6 1.6 5 0.5 93 94 W 0.56 0.54 10 10 steps f) to g): causticization & solid separation steps h) to i): concentration − evaporation/separation steps f) to i): causticization/separation, +concentration − evaporation/separation.

(129) TABLE-US-00007 TABLE 7 Example 5 - Solution analysis and percentages of impurities reductions in solutions: impurity impurity reduction reduction overall in solution in solution impurity Test after steps after steps reduction Example 5 Purge Solution f) to g) Solution h) to i) in solution Solutions stream after steps (based on after steps (based on after steps analysis solution f) to g) ΔNaCl) f) to i) ΔNaOH) f) to i) Concentration g/kg g/kg % g/kg % % NaOH 0 118 437 Na2CO3 174 39 14 91 NaHCO3 16 0 0 NaCl 29 32 23 80 80 mg/kg reported mg/kg reported Concentration mg/kg to 12% NaOH % to 44% NaOH % % As 4.9 4.7 12 12 B 163 141 22 22 Ba 5.9 0.11 98 0.06 86 100 Ca 2.2 17.6 21.7 66 66 K 1900 1368 35 35 Li 10 7.0 37 37 Mg 3.0 0.80 76 76 Mo 18 18.1 9 9 P en PO4 930 53 95 147 24 96 Pb 0.3 <0.6 1.3 45 45 S en SO4 15100 14200 3200 94 94 Sb 0.4 0.37 16 1.07 22 34 Se 0.5 0.8 1.5 48 48 Si 152 237 799 8 8 Sn <0.3 <0.6 <1.0 Sr 0.2 0.07 67 <0.04 >85 >95 Te <0.4 0.4 0.8 51 51 steps f) to g): causticization & solid separation steps h) to i): concentration − evaporation/separation steps f) to i): causticization/separation, +concentration − evaporation/separation.

(130) TABLE-US-00008 TABLE 8 Example 6 - Solution analysis and percentages of impurities reductions in solutions: % impurity % impurity reduction reduction % overall in solution in solution impurity Test after steps after steps reduction Example 6 Purge Solution f) to g) Solution h) to i) in solution Solutions stream after steps (based on after steps (based on after steps analysis solution f) to g) ΔNaCl) f) to i) ΔNaOH) f) to i) Concentration g/kg g/kg % g/kg % NaOH 0 111 460 Na2CO3 171 28 11 90 NaHCO3 17 0 0 NaCl 36 35 29 80 80 mg/kg reported Concentration mg/kg to 11% NaOH % mg/kg as it % % Al <0.3 8.2 31 9 9 As 6.2 5.4 10 18 23 31 B 176 169 2 560 20 22 Ba 7.6 0.13 98 <0.03 >94 99.9 Be 1.5 0.63 58 2.4 9 62 Bi 1.8 <0.8 54 <0.8 54 Ca 2.2 17 19 73 73 Cd <0.08 <0.08 <0.08 Co 1.6 0.12 92 0.52 92 Cu 1.5 1.18 19 <0.24 >95 >96 K 1929 1893 6187 21 21 Li 12 9.0 21 32 15 33 Mg 3.3 0.66 79 2.5 9 81 Mo 21 21 64 25 25 P en PO4 995 43 96 114 37 97 Pb <0.8 <0.81 1.2 63 63 S en SO4 20600 17800 3900 95 95 Sb 2.0 1.15 42 3.6 25 56 Se 2.7 2.5 4 1.1 90 90 Si 788 201 74 552 34 83 Sn 1.0 1.5 4.2 35 35 Sr 1.8 0.09 95 0.06 85 99 Te 2.1 2.0 0.7 92 92 Tl 2.2 1.8 15 0.6 93 94 V 5.3 5.6 21 11 11 steps f) to g): causticization & solid separation steps h) to i): concentration − evaporation/separation steps f) to i): causticization/separation, +concentration − evaporation/separation.