Process for treating a sodium carbonate purge |
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| 申请号 | EP13192428.4 | 申请日 | 2013-11-12 | 公开(公告)号 | EP2871156A1 | 公开(公告)日 | 2015-05-13 |
| 申请人 | Solvay SA; | 发明人 | VANDENDOREN, Alain; | ||||
| 摘要 | A method for treating a purge stream derived from a sodium carbonate, sesquicarbonate, wegsheiderite, or bicarbonate crystallizer, said purge stream comprising sodium carbonate and/or sodium bicarbonate and at least 1 % by weight of sodium chloride and/or sodium sulfate, the method comprising : - causticizing at least 50 mol. % of the sodium from sodium carbonate and/or sodium bicarbonate into a caustic solution and into a calcium carbonate mud with lime and water, - separating the calcium carbonate 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, - separating the crystallized solid from the concentrated caustic solution, said crystallized solid to be disposed of or to be further valorized, - recycling part of the concentrated caustic solution to the sodium carbonate, sesquicarbonate, wegsheiderite, or bicarbonate crystallizer. |
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| 权利要求 | |||||||
| 说明书全文 | The present invention relates to a method for treating a purge stream from a sodium carbonate crystallizer or from a sodium bicarbonate compound crystallizer, such method addressing the issue of reducing sodium alkaline loss, reducing water loss, and improving up-stream and down-stream purge treatments. The invention relates also to a process for producing a sodium carbonate salt or a sodium bicarbonate salt using such improved method. Sodium carbonate (Na2CO3), or soda ash, is one of the largest volume alkali commodities made world wide with a total production in 2011 of 53 million tons. Sodium carbonate finds major use in the glass, chemicals, detergents industries, and also in the sodium bicarbonate production industry. The main processes for sodium carbonate production are the Solvay ammonia synthetic process, the ammonium chloride process, and sodium carbonate or bicarbonate ore-based processes. Main exploited sodium carbonate or bicarbonate ores are mainly trona ore (Na2CO3.NaHCO3.2H2O), nahcolite ore (NaHCO3) or natron ore (sodium carbonate decahydrate : Na2CO3.10H2O). Most of those ore deposits may comprise also some Wegsheiderite (Na2CO3.3NaHCO3). Trona ore is a mineral that contains up to 99% sodium sesquicarbonate (Na2CO3.NaHCO3.2H2O). Trona-based soda ash is obtained from trona ore deposits in Green River (Wyoming), Turkey, China, and Kenya either by underground mechanical mining, or by solution mining, or by lake waters processing. The trona-based sodium carbonate from Wyoming comprised about 90 % of the total U.S. soda ash production. A typical analysis of the trona ore in Green River is as follows: Nahcolite ore is a mineral that contains up to 99% sodium bicarbonate (NaHCO3). Nahcolite-based soda ash has been produced from nahcolite ore deposits in Piceance Creek (Colorado). Sodium carbonate or bicarbonate ore deposits include beds of corresponding sodium carbonate or bicarbonate ores listed above, and include also halite (NaCl), and/or thenardite (Na2SO4), and/ or glauberite (Na2SO4.CaSO4) included within the sodium carbonate or bicarbonate ores beds or included in inter-beds layers. The quality of the ore varies depending on its particular location in the stratum and according geographic location. Aside sodium chloride, and/or sodium sulfate, the deposits contain other soluble impurities such as potassium chloride, potassium sulfate, alkaline metal borate, and alkaline metal phosphates, and slightly soluble minerals such as : alkali metal and alkali earth metal silicates, aluminates, titanates, vanadates, metallic compounds and salts. Sodium carbonate or bicarbonate ores comprise also organic impurities. Such impurities come from organic sediments that were captured during the formation of the deposits and that frequently have formed oil shales during geological aging. Those organic impurities may be present in the Sodium carbonate or bicarbonate ore and/or also present in inter-beds of the ore deposits. Other "insoluble" or very slightly water-soluble mineral impurities found in sodium carbonate or bicarbonate ores deposits or in the shales beds are generally mixtures of different minerals; the most frequent of which are calcite, dolomite, pirssonite, zeolite, feldspar, clay minerals, iron /aluminium silicates, and calcium sulfate. Two main techniques well known in the art are used to recover sodium carbonate or bicarbonate ores from ore deposits. The first technique is mechanical mining, also called conventional mining, such as a room and pillar panel operation or a longwall operation. The second technique is a solution mining recovering wherein the ore, such as trona or nahcolite, is dissolved with water and recovered as a solution. Among the several ways in which sodium carbonate can be recovered from sodium carbonate or bicarbonate ores that contains other salts and impurities, the most widely practiced is the so called "monohydrate process". In that process a mined ore, such as trona ore, is crushed, then calcined into crude sodium carbonate, then leached with water, the resulting water solution is purified and fed to a crystallizer where pure sodium carbonate monohydrate crystals are crystallized. The monohydrate crystals are separated from the mother liquor and then dried into anhydrous sodium carbonate. Most of the mother liquor is recycled into the crystallizer. However, the soluble impurities contained in the ore, tend to accumulate in the mother liquor in the crystallizer. To avoid build up of soluble impurities, the mother liquor must be purged. The purge liquor, which represents important quantities for industrial monohydrate plants, is commonly sent to an evaporative pond, also called tailings pond. The significant quantity of alkali which is contained in the purge liquor is consequently lost. Moreover, the stocking of large quantities of purge liquors in evaporative ponds raise environmental problems, because of the scarce availability of new areas for stocking, and induces also water loss that is detrimental in arid regions. Variants to produce sodium carbonate from a solution generated with sodium carbonate or bicarbonate ores, in particular from solution mining, are:
In those variants, the soluble impurities contained in the sodium carbonate or bicarbonate ore, tend to accumulate also into the monohydrate, or the sesqui or the sodium bicarbonate crystallizers. To avoid the build up of impurities, the mother liquors must also be purged, raising the same environmental problems in evaporative ponds as the monohydrate process. Sodium bicarbonate (NaHCO3), aside sodium carbonate, is another important alkali product with a wide range of applications including human food, animal feed, flue gas treatment, and chemical industries. The production of sodium bicarbonate is currently almost entirely made by the carbonation of solid or aqueous solutions of sodium carbonate with gaseous CO2 either produced in situ in the soda ash plants or purchased independently. Several technical alternatives have been proposed to reduce the purge volume from soda ash plants. However, there is still a need in the sodium carbonate and bicarbonate industry, to be able to further reduce : the purge volume and the loss of alkali in a simple way, without impairing operation conditions of the linked processes. In the natural soda ash processes, a high NaCl content, and/ or a high Na2SO4 content, in the crystallization section require substantial amounts of purge that represent losses in product (carbonate or bicarbonate) as well as storage space problems. In the past it was difficult to eliminate the chlorides or sulfate as there are the most soluble species present in the solutions that were handled. In the present invention, the purge stream from the process is caustified with lime (CaO) or hydrated lime (Ca(OH)2) to form a dilute solution of caustic soda, for instance 10 % NaOH solution, which still may contains 4.5 % Na2CO3 and the NaCl, and Na2SO4. This caustic solution is subsequently evaporated to about 30 % (up to 50 %) NaOH in which case, because of the order of the various solubilities, about 90 % of remaining Na2CO3 and about 80 % NaCl and at least part of Na2SO4 precipitates as solid and can be removed from the sodium hydroxide solution. This eliminates about 80 % (up to 90 % at 50 % NaOH concentration) of the NaCl and part of the Na2SO4 as a solid with a substantially reduced amount of Na2CO3 (loss). The NaOH solution can be recycled in the process (in particular in case of a solution mining based process) to calcine chemically the NaHCO3 into sodium carbonate, or can also be sold (in this case concentrating to 50 % NaOH is preferred as it is a commercial grade but it will also contain much less NaCl). Accordingly, the invention relates to 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, the method comprising :
A first advantage of the present invention is that it reduces considerably the amount of alkali loss to be stored in evaporative (or tailings) ponds. A second advantage of present invention is that it is efficient in reducing sodium carbonate loss in purge treatments of both main sodium impurities of sodium carbonate ores or sodium bicarbonate ores which are sodium sulfate and sodium chloride. A third advantage of the present invention is that it is also efficient to reduce a large spectrum of soluble impurities such as fluorides, silicates, phosphates, vanadates, titanates, and also potassium. A fourth advantage of the present invention linked to the second and third advantage is that it enables to exploit one or several sodium carbonate ores deposits such as trona deposit or sodium bicarbonate ores deposit such as nahcolite with different levels of soluble impurities while being able to treat the varying levels of the purges of a soda ash or a sodium bicarbonate plant with the same process described in the present invention. A fifth advantage of the present invention is that it enables to minimize the purge flow preparing the process for either dry-cooking the purge or for very small evaporative ponds or for reinjecting the purge into mined out cavities. A sixth advantage of the present invention is that it enables to reduce water consumption for soda ash production and/ or sodium bicarbonate production in recovering it as condensates from evaporators for either recycle it to a calcined carbonate or bicarbonate ore leaching or to a carbonate or bicarbonate ore solution mining, in particular to a calcined trona leaching or to a trona solution mining. A seventh advantage of the present invention is that it enables to improve the operation of a sodium carbonate decahydrate crystallizer treating in a first step the purge of a sodium monohydrate crystallizer as high levels of sodium chloride or sodium sulfate is detrimental to the good crystallization of sodium carbonate decahydrate. An eighth advantage of the present invention, is that it enables to improve also the operation of a sodium sesquicarbonate crystallizer treating in a first step the purge of a sodium monohydrate crystallizer as high levels of sodium chloride or sodium sulfate is also detrimental to the good crystallization of sodium carbonate decahydrate. A ninth advantage of the present invention, is that it enables to produce a technical caustic soda (NaOH) solution of constant low chloride and sulfate content from a sodium carbonate or bicarbonate ore deposit comprising variable content of chloride or sulfate salts such as halite (NaCl) or thenardite (Na2SO4). In this the synergy is particularly of interest rendering possible a smooth operation of 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, from content from a sodium carbonate or bicarbonate ore deposit comprising variable content of chloride or sulfate salts, along with the production of a technical caustic soda solution of constant chloride and sulfate content.
The reference figures and letters quoted below refer to the attached drawings. For purposes of the present description, certain terms are intended to have the following meanings. The term "purge" refers to a stream withdrawn from a part of a process to limit impurity concentration in this process. The term "impurity" refers to a compound different from the sodium carbonate and/or the sodium bicarbonate salt to be produced. The term "solubility" refers to the solubility of a compound in an aqueous solution. The expression "total carbonate" refers to the carbonate and bicarbonate content. It may be expressed as total carbonate equivalent content. The term "carbonating" refers to the action of increasing the amount of total carbonate (i.e. carbonate and bicarbonate) of a stream. The term "decarbonating" refers to the action of decreasing the amount of total carbonate (i.e. carbonate and bicarbonate) of a stream. The term "bicarbonating" refers to the action of increasing the amount of bicarbonate of a stream. The term "debicarbonating" refers to the action of decreasing the amount of bicarbonate of a stream. 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 (OH-), carbonate ions (CO3--), and bicarbonate ions (HCO3-), and it is expressed as equivalent Na2CO3 concentration. The term 'comprising' includes 'consisting essentially of" and also "consisting of". 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. The sign '%' refers to 'weight %' unless specifically stated otherwise. The invention relates to 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, the method comprising :
In present invention the purge stream comprises Na2CO3 and/or NaHCO3 generally in a quantity of at least 9 % total alkalinity expressed as equivalent weight Na2CO3. Advantageously the purge stream comprises at most 33 % Na2CO3 or at most 16 % NaHCO3. 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 % Na2SO4. 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)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)2). Causticization of carbonate ions or bicarbonate ions with lime in presence of water generates an hydroxide ions solution, according the following reaction : Na2CO3 + Ca(OH)2 → CaCO3 + 2 NaOH NaHCO3 + Ca(OH)2 → CaCO3 + NaOH + H2O When quick lime (calcium oxide CaO) is put in contact with water, calcium oxide reacts to form hydrated lime (calcium hydroxide Ca(OH)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. 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 CaCO3 mud. 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 caustified 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. In present invention the causticizing step is preferably operated with hydrated lime, more preferably with a milk of lime comprising hydrated lime. In such embodiment using a milk of lime, the concentration of hydrated lime in the milk of lime, expressed in moles of Ca(OH)2/ liter of milk of lime is generally at least 0.5 mol/ 1, preferably at least 1.0 mol/ 1, more preferably at least 1.5 mol/ 1. The milk of lime comprises generally at most 7.0 mol/ 1, preferably at most 5.0 mol/ 1, more preferably at most 3.0 mol/ 1. 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. Therefore it is advantageous in present invention that the aqueous sodium hydroxide solution comprises at least 6, preferably at least 8, more preferably at least 10 % NaOH. 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 aqueous sodium hydroxide solution comprises at most 14, preferably at most 13, more preferably at most 11 % NaOH. 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 Na2CO3 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). 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 CaCO3 in the form of calcite which is much easier to filtrate. A lower temperature will also favor the formation of pirsonite (CaCO3.Na2CO3.2 H2O) leading to increased sodium losses with the caustification mud. 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 CaCO3, some unreacted CaO or Ca(OH)2, precipitated impurities from the purge stream, and impregnating aqueous sodium hydroxide solution, can be disposed of in different ways. 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. 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 CO2 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, CO2, availability of cheap lime on the market. A third way is to separate the mud, filter it and wash it and sell the calcium carbonate (for example as dry sorbent for SO2 pollution control). 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. A fifth but least preferred way is to separate the mud, filter it and store it on the surface (tailings pond). In present method, most of sodium chloride and/or sodium sulfate of the purge stream are removed in the purge stream treatment. 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. Along with the treatment of the purge stream comprising chlorides and sulfates ions, the present method is also efficient to remove from the purge stream other soluble compounds such as :
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, Na2SO4, and Na2CO3, 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. TABLE 2 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 after removal of the crystallized solid comprising sodium carbonate and comprising sodium chloride and/or sulfate, it is advantageous that 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 % Na2SO4. 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, the causticizing of purge stream and the concentrating of the aqueous sodium hydroxide solution, may 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 Na2CO3 per ton of NaCl. 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. 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 Na2SO4 to avoid the crystallization of sodium sulfate decahydrate and to limit the formation of mixed salts of sodium carbonate and sodium sulfate. 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 :
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, 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, wherein :
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 :
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 :
The following examples are intended only to exemplify the invention and are not intended to limit the scope of the claimed invention. The purge from a natural soda ash crystallizer, a monohydrate crystallizer operating at 101°C, comprising 27.3-30.3 % Na2CO3, 2-5 % NaCl, 0.5 % Na2SO4 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 % Na2CO3, 8 % NaCl, and 1.1 % Na2SO4. 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. 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 % Na2CO3, 7.2 % NaCl, and 0.2 % Na2SO4. 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. The caustic solution is then concentrated in an multiple effect evaporator to reach :
The corresponding flow and mass balance are given for:
On can see in each example that the sodium carbonate in crystallized solid is reduced down to respectively :
Those figures illustrate that the method of present invention :
Moreover a comparison to previous known methods of purge treatment wherein the purge of monohydrate crystallizer is treated :
Therefore when the processed carbonate/ bicarbonate ore, such as trona or nahcolite, 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+Na2SO4, remains particularly interesting compared to previous known methods. 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. |
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