Method for producing crystalline sodium sulfate

Abstract

1. A process for producing crystalline sodium sulfate comprising the steps of: (a) combining an aqueous solution containing a sulfate of nickel and at least one metal selected from cobalt and manganese with an aqueous solution of sodium hydroxide or sodium carbonate in a stoichiometric ratio of about 1:2 or 1:1, respectively, optionally in the presence of ammonia or a salt of ammonia; (b) removing the precipitated nickel hydroxide or carbonate and at least one metal selected from cobalt and manganese by filtration; (c) removing ammonia from the filtrate of step (b) by stripping in a distillation column; (d) passing the remaining liquid phase through the membrane, thereby obtaining a permeate; and (e) removing water from the permeate of step (d) by evaporation. The method includes:

Description

[Technical field]

[0001] The present invention relates to a process for producing crystalline sodium sulfate, the process comprising the steps of: (a) combining an aqueous solution containing a sulfate of nickel and at least one metal selected from cobalt and manganese with an aqueous solution of sodium hydroxide or sodium carbonate in a stoichiometric ratio of about 1:2 or 1:1, respectively, in the presence of ammonia or a salt of ammonia; (b) removing the precipitated nickel hydroxide or carbonate and at least one metal selected from cobalt and manganese by filtration; (c) removing ammonia from the filtrate of step (b) by stripping in a distillation column; (d) passing the remaining liquid phase through the porous membrane, thereby obtaining a permeate; and (e) removing water from the permeate of step (d) by evaporation. The present invention relates to a method comprising the steps of: [Background technology]

[0002] Alkali metal sulfates, such as sodium sulfate, are useful products in many applications, for example, they function as builders in laundry detergents. However, for such sulfates to be suitable builders, they must meet several criteria. They must not contain significant impurities. They must be colorless and must not impart an unpleasant odor to the laundry detergent. Additionally, they must not be a cost factor in the laundry detergent.

[0003] Large quantities of sodium sulfate are obtained from mineral sources, for example from mirabilite, however other routes are also of interest.

[0004] CN112299638A1 describes a method for obtaining sodium sulfate from the filtrate of a ternary precursor in which the sulfate salt is precipitated with sodium hydroxide. The resulting sodium sulfate may still not be suitable for builder or other home care applications.

[0005] WO2019 / 117411 describes a method for processing the filtrate, in which ammonia, a complexing agent commonly used in precursor production, is recovered as ammonium sulfate.

[0006] US 6,001,246 discloses a method for producing sodium sulfate from wastewater, the sodium sulfate containing about 0.200% NaCl by weight. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] CN112299638A1 [Patent Document 2] WO2019 / 117411 [Patent Document 3] US6,001,246 Summary of the Invention [Problem to be solved by the invention]

[0008] The object of the present invention is to provide a process for the production of sodium sulfate from inexpensive raw materials, which meets the above requirements of purity, odorlessness and colorlessness and does not have the disadvantages of the prior art processes. A further object of the present invention is to provide a setup in which the respective process can be carried out. [Means for solving the problem]

[0009] Thus, a method as defined at the outset has been found, which is also referred to below as the method of the invention. The method of the invention comprises steps (a) to (e), which is also referred to below as step (a) or simply (a), step (b) or simply (b), etc. The method of the invention is explained in more detail below.

[0010] Step (a) involves combining an aqueous solution containing a sulfate salt of nickel and at least one metal selected from cobalt and manganese with an aqueous solution of sodium hydroxide or sodium carbonate in a stoichiometric ratio of about 1:2 or 1:1, respectively, in the presence of ammonia or a salt of ammonia.

[0011] The relative amounts of nickel and at least one metal selected from cobalt and manganese may be selected within wide limits. Nickel may be present in an amount of 25-97 mol % of the metals other than alkali metals. Cobalt may be present or absent and may be present in an amount of 1-33 mol %, preferably 2-20 mol %, of the metals other than alkali metals. Manganese may be present or absent and may be present in an amount of 1-75 mol %, preferably 2-68 mol %, of the metals other than alkali metals. Further metals which may be present in step (a) are magnesium or aluminium, for example up to 2 mol % of magnesium, or up to 10 mol % of aluminium. When aluminium is present in an amount of 3-10 mol %, it is preferred that cobalt is also present.

[0012] During step (a), precipitation takes place.

[0013] In one embodiment of the invention, at least one of aluminum and magnesium is included in the precipitation of step (a) by introducing at least one of MgSO4, Al2(SO4)3, NaAl(OH)4, NaAl(SO4)2, and KAl(SO4)2.

[0014] Sodium hydroxide can absorb carbon dioxide from the air, and frequently technical grade sodium hydroxide contains some sodium carbonate, e.g. up to 0.5% by weight, preferably 0.1-0.4% by weight. Application of such technical grade sodium hydroxide precipitates a mixed hydroxide / carbonate.

[0015] In one embodiment of the present invention, in step (a), the stoichiometric ratio of sodium hydroxide to the sum of nickel and other metals is about 2:1. The term "about" in this context refers to a deviation of the hydroxide content of ±2 mole % from the ratio that would correspond to the exact stoichiometric ratio. If aluminum is present, the amount of hydroxide may vary depending on the nature of the aluminum source.

[0016] In one embodiment of the invention, the stoichiometric ratio of sodium carbonate to the sum of nickel and other metals is about 1: 1. The term "about" in this context refers to a deviation of the carbonate content of ±2 mole % from the ratio that would correspond to the exact stoichiometric ratio.

[0017] In step (a) a complexing agent is used, and the economically most preferred complexing agent is ammonia, which may be used directly or as an ammonium salt, e.g., ammonium sulfate. On neutralization with hydroxide, ammonia is formed.

[0018] The molar ratio of ammonia to the metal other than an alkali metal is preferably in the range of 1:10 to 1:1, more preferably 1:5 to 1:2.

[0019] A mixed metal hydroxide or carbonate is precipitated by combining an aqueous solution containing nickel sulfate and at least one metal selected from cobalt and manganese with an aqueous solution of sodium hydroxide or sodium carbonate in the presence of ammonia or a salt of ammonia, the mixed metal hydroxide or carbonate being suspended (slurried) in a continuous phase containing sodium sulfate and traces of nickel and further transition metal salts or excess sodium hydroxide and ammonia.

[0020] In one embodiment of the present invention, step (a) is carried out at a temperature in the range of 15 to 80°C, preferably 40 to 70°C.

[0021] Step (a) is preferably carried out in conjunction with a mixing operation, such as stirring.

[0022] In step (a) a slurry of precipitated mixed metal hydroxide or carbonate of nickel and at least one metal selected from cobalt and manganese is obtained, said mixed metal hydroxide or carbonate may contain further metals such as magnesium or aluminium.

[0023] In one embodiment of the present invention, step (a) is carried out at a pH value ranging from 10 to 13, preferably from 11.1 to 12.5. The pH values ​​are measured at 23° C. and refer to the continuous phase.

[0024] In one embodiment of the present invention, the concentration of the mixed metal hydroxide or mixed metal carbonate can be increased by carrying out step (a) in a stirred vessel connected to a clarifier, such as a lamellar clarifier, through which the continuous phase can be removed from the stirred vessel. The continuous phase thus removed can be combined with the filtrate from step (b).

[0025] In step (b), the precipitated nickel hydroxide or carbonate and at least one metal selected from cobalt and manganese are removed by filtration, which may be carried out batchwise or continuously, for example using a filter, in particular a filter press or a belt filter, by suction filtration or by centrifugation, with a filter press being preferred.

[0026] In embodiments where centrifugation is used, a supernatant and a precipitate are obtained, the supernatant serving as the filtrate in the subsequent steps and the precipitate serving as the filter cake in the context of the present invention, and are not referred to separately.

[0027] Filtration produces a filter cake and a filtrate. The filter cake is preferably washed with water or an aqueous solution of sodium hydroxide or sodium carbonate according to step (a). After washing, the wash water may be combined with the filtrate from step (b).

[0028] In one embodiment of (b), the filter cake from step (b) is pre-dried by blowing a gas, such as nitrogen or air, over the filter cake. The water thus removed may be combined with the filtrate.

[0029] In one embodiment of step (b), the pH value of the filtrate is in the range of 10 to 13.5, preferably 11.1 to 12.5.

[0030] In step (c), ammonia is removed from the filtrate of step (b) by stripping in a distillation or fractionation column (both expressions are used interchangeably in the context of the present invention) without or, preferably, with wash water.

[0031] Steps (a) and (b) may be carried out continuously or batchwise, however the distillation or fractionation column of step (c) is preferably operated continuously and at steady state.

[0032] In step (c), ammonia together with water is the low boiling fraction, and the aqueous solution containing sodium sulfate is the high boiling fraction. Preferably, the ammonia concentration in the low boiling fraction, water, is in the range of zero to 10 ppm, preferably 1 to 5 ppm.

[0033] All data given in ppm refer to ppm by mass.

[0034] In one embodiment of the present invention, the distillation or fractionation column in step (c) has a theoretical plate count in the range of 10-50.

[0035] The distillation or fractionation column in step (c) may have plates or trays with bubble caps.

[0036] In one embodiment of the invention, the distillation column of step (c) is operated at a temperature in the range of 80 to 120°C.

[0037] In one embodiment of the present invention, said distillation column in step (c) is operated at a pressure ranging from 10 mbar to 1.5 bar, with atmospheric pressure being preferred.

[0038] The high boiling fraction typically contains small amounts of precipitates, such as nickel hydroxide or possibly nickel carbonate, and at least one of cobalt or manganese, depending on the metals used in step (a).

[0039] The low boiling (or "volatile") fraction of step (c) contains primarily ammonia, which is then dissolved in water (washed) and recycled to step (a).

[0040] The high boiling fraction or liquid phase of step (c) is then subjected to step (d).

[0041] Step (d) comprises passing the high boiling fraction from step (c) (hereinafter also referred to as the "residual liquid phase") through a porous membrane, thereby obtaining a permeate, said membrane being preferably nanoporous.

[0042] The porous membrane may be made of a polymer, such as: PES, PTFE, PVDF, PE, PP (each with an average pore size of 5 nm to 2 or 3 μm or less). In another embodiment, the porous membrane may be made of a ceramic material, such as Al2O3, TiO2, ZrO2, SiC or sintered metal. More preferred ceramic membranes are made of Al2O3.

[0043] The polymeric or ceramic membrane preferably has pores with an average diameter of 2 nanometers to 3 μm, preferably 20 to 400 nm and more preferably 50 to 100 nm. To increase the wear resistance of the ceramic membrane, a porous membrane can be used that includes two or three layers of membrane, for example 800 nm, then 400 nm, and finally 100 nm, or 400 nm, then 200 nm, and finally 100 nm, and more preferably 400 nm, 200 nm, and 50 nm.

[0044] In one embodiment of the present invention, step (d) is operated at a temperature in the range of 20-95°C, preferably 50-70°C.

[0045] In one embodiment of the present invention, step (d) is operated at a pressure in the range of from 2 to 6 bar (abs), preferably from 2 to 5 bar (abs), and the high boiling fraction from step (c) is pressurized through a porous membrane.

[0046] In another embodiment of the invention, step (d) is operated at a submerged pressure, for example below 250 mbar, and the high boiling fraction from step (c) is drawn through a porous membrane.

[0047] From step (d), a permeate is obtained, hereinafter also referred to as brine. The brine preferably contains less than 10 ppm in total, preferably less than 0.1 ppm, more preferably less than detection levels of metals selected from nickel, cobalt and manganese. The brine is transparent to the naked eye and colorless. The concentration of sodium sulfate is preferably in the range of 5-20% by weight.

[0048] In one embodiment of the invention, the method of the invention comprises an additional step (d') in which reverse osmosis is carried out. Said reverse osmosis may be carried out by passing the permeate from step (d) along a porous membrane with nanopores, for example with a pore width (pore width 2 nm or less), under a pressure of 120 bar or less. The temperature of the reverse osmosis step is preferably in the range of 20-35 C. By reverse osmosis, a permeate is obtained which is almost pure water. Furthermore, the retentate may contain up to 160 g / l of sodium sulfate (which corresponds to 15% by weight or less). Reverse osmosis serves to remove water from the filtrate and to increase the concentration of sodium sulfate.

[0049] In step (e), the sodium sulfate from the permeate of step (d) or, where applicable, from the retentate of step (d') is solidified by evaporative techniques, such as a spray tower or an evaporative crystallizer.

[0050] Evaporative crystallizers are known per se. In one embodiment of the present invention, the evaporative crystallization is carried out by one of the following methods: - Falling film evaporation, or - Forced circulation crystallization.

[0051] In one embodiment of the present invention, the Na2SO4 crystals thus obtained are removed by centrifugation or filtration, preferably by centrifugation. Drying may be performed in a fluidized bed.

[0052] Preferably, prior to step (e), the pH value from the filtrate of step (d) or, if applicable, from the retentate of step (d') is adjusted to 6.5-9.5, more preferably 6.8-8.0, with sulfuric acid prior to step (e). The closer the pH value is to 10, the less corrosion problems are faced in step (e). However, strong bases, e.g. sodium sulfate with a high NaOH content, are not preferred for use as builders.

[0053] In the spray tower, the brine is sprayed to form droplets from which the water evaporates. Sodium sulfate powder is formed, which can be recovered. By selecting the temperature and pressure, the size of the droplets and therefore the particle size of the resulting sodium sulfate can be controlled.

[0054] Atomization is carried out through one or more nozzles per spray tower. Suitable nozzles are, for example, high pressure rotary drum atomizers, rotary atomizers, three-fluid nozzles, single-fluid nozzles and two-fluid nozzles, with single-fluid nozzles and two-fluid nozzles being preferred. The first fluid is the brine from step (d) and the second fluid is compressed hot gas, also called hot gas inlet stream, compressed at a pressure of, for example, 1.1 to 7 bar. The hot gas inlet stream may have a temperature in the range of at least 125°C to 250°C, preferably 150°C to 250°C, even more preferably 160°C to 220°C.

[0055] In one embodiment of the present invention, a single effect spray dryer is used. In another embodiment, a multiple effect spray dryer may be used.

[0056] In one embodiment of step (e), the brine obtained from step (d) is introduced in the form of droplets. In one embodiment of the present invention, the droplets formed during spray drying have an average diameter in the range of 10 to 200 μm, preferably 15 to 150 μm.

[0057] In one embodiment of the invention, the off-gas leaving the spray tower has a temperature in the range of 40-140° C., preferably 80-110° C., but in any case lower than the hot gas stream. Preferably, the temperature of the off-gas leaving the drying vessel and the temperature of the solid product present in the drying vessel are the same.

[0058] In one embodiment of the present invention, the pressure in the spray tower in step (e) is atmospheric pressure ±100 mbar, preferably atmospheric pressure ±20 mbar, for example 1 mbar below atmospheric pressure.

[0059] In a preferred evaporative crystallizer, the body of the crystallization vessel performs evaporation and crystallization simultaneously. The process is driven by a heat exchanger in the pump loop where steam is used to heat the circulating solution. A fresh supply of brine enters this loop before passing through the heat exchanger. The heated suspension re-enters the crystallization body and releases its heat via evaporation. The resulting increase in concentration causes crystallization. The steam leaving the crystallizer body is preferably recompressed through an MVR fan ("mechanical vapor recompression") so that it can be used to further drive the crystallizer operation. With this particular embodiment, there is no need to consume fresh costly steam.

[0060] By carrying out the method of the present invention, sodium sulfate is obtained which is highly suitable as a builder for the production of laundry detergents and has a whiteness of more than 85% as measured by a standard reflectance measurement method (e.g., Ket C-100). Thus, the sodium sulfate obtained has a very good whiteness and no malodor can be detected. The chloride content is preferably 500 ppm or less, more preferably 50 ppm or less.

[0061] Further disclosed is a configuration for carrying out the method of the present invention. Such a configuration of the present invention comprises various parts: (A) at least one stirred tank reactor connected to: (B) a filter device; (C) a distillation column; (D) A container containing a ceramic membrane; and (E) A vessel for removing water by evaporation, for example an evaporative crystallizer (E1) as a tubular heat exchanger or a spray dryer ("spray tower") (E2).

[0062] The sections are connected such that the fluid moves sequentially between them, i.e. the fluid may move through sections (A), (B), (C), (D) and (E) in that order. The fluid may be water containing sodium sulfate. However, in section (E) solid sodium sulfate and water or concentrated brine are obtained.

[0063] The vessel (E) may be selected from an evaporative crystallizer and a spray tower.

[0064] The porous membrane of part (D) may be selected from polymeric and ceramic membranes.

[0065] Part (D) may comprise a unit containing two or more membranes through which the permeate passes.

[0066] The set-up may further comprise a part (D') with a membrane for reverse osmosis. EXAMPLES

[0067] The following setup was used: a 10 liter stirred tank reactor (A.1) equipped with baffles, an overflow system, a layered clarifier, and an agitator, connected to a filter press (B.1). The filtrate may be transferred to a fractionation column (C.1). The low boiling fraction is transferred to a first vessel equipped with a porous ceramic filter (D.1), a reverse osmosis unit (D'.1), and from there the retentate is transferred to a crystallizer (E.1) or a spray dryer (E.2).

[0068] Process (a.1): Synthesis of precursor TM-OH.1 A stirred tank reactor (A.1) was charged with deionized water and 49 g of ammonium sulfate per kg of water. The solution was warmed to 55° C. and the pH value was adjusted to 12 by adding aqueous sodium hydroxide solution.

[0069] The coprecipitation reaction was started by simultaneously feeding the aqueous transition metal sulfate solution and the aqueous sodium hydroxide solution at a total flow rate resulting in a flow ratio of 1.8 and a residence time of 8 hours. The transition metal solution contained Ni, Co and Mn in a molar ratio of 8.5:1.0:0.5, and the total transition metal concentration was 1.65 moles / kg. The aqueous sodium hydroxide solution was a 25% by weight sodium hydroxide solution and a 25% by weight ammonia solution in a mass ratio of 6. The pH value was kept at 12 by separately feeding the aqueous sodium hydroxide solution. Starting from the start of all feeds, the mother liquor was continuously removed. After 33 hours, all feed flows were stopped.

[0070] Step (b.1): The resulting suspension was filtered through a filter press (B.1), washed with 1 wt% NaOH aqueous solution, dried at 120°C in air and sieved to obtain a mixed transition metal (TM) oxyhydroxide precursor.

[0071] The filtrate of step (b.1) was combined with the mother liquor and wash water taken in step (a.1). The combined filtrate thus obtained had a pH value of 12.0.

[0072] Process (c.1): The combined filtrate from step (b.1) was transferred to a fractionating column (C.1). A distillation column equipped with a bubble cap and a tray with 15 theoretical plates served as the fractionating column (C.1). Step (c.1) was operated continuously. The low boiling fraction was an aqueous solution of 15% by weight ammonia, and the "high boiling fraction" was a slightly turbid aqueous solution of Na2SO4 with a pH value of 12.

[0073] Process (d.1) The slightly turbid solution from step (c.1) was cooled to 50°C and transferred to a 20 liter vessel (D.1) and passed through a porous ceramic Al2O3 membrane unit (three membranes with average pore sizes of 800 nm, then 400 nm and finally 100 nm). A pressure of 3 bar was applied. A clear, colorless permeate was obtained.

[0074] After step (d.1), sulfuric acid was added to the permeate from step (d.1) to adjust the pH value to 7.0.

[0075] Process (d'.1): The neutralized permeate from step (d.1) was cooled to 25° C. and subjected to reverse osmosis. Water was removed from the permeate of step (d'.1) at a pressure of 100 bar, thereby obtaining the permeate and retentate of step (d'.1). The retentate was then transferred to step (e.1).

[0076] Process (e.1) The neutralized, clear, colorless filtrate from step (d.1) was then transferred to a tubular heat exchanger (E.1) functioning as a falling film evaporator. By repeatedly passing the solution through the tubular heat exchanger, the solution was heated to 100-110°C and water was evaporated. The thus concentrated, but still crystal-free, solution entered the crystallizer unit. Here, the solution was pumped through another heat exchanger and superheated to 110-120°C. Upon re-entering the crystallizer, water was evaporated and Na2SO4 crystals were precipitated. The suspension was pumped out of the crystallizer and the crystals were separated from the suspension in a laboratory-scale belt filter. The whiteness of the suspension was excellent and there was no bad odor.

[0077] Process (e.2) The neutralized, colorless and transparent filtrate from step (d.1) was then transferred to a single-effect laboratory spray dryer through a spray nozzle. Air was used as gas for the nozzle. A mist was formed. The spray tower was operated in co-current mode with respect to the gas flow and the mist. The gas temperature was 200°C. The filtrate was introduced through the nozzle at a pressure of 2500 kPa. A white, odorless Na2SO4 powder with an average particle size of 10 μm was obtained.

[0078] The nickel content of the Na2SO4 was below the level of detection in each case.

Claims

1. 1. A process for producing crystalline sodium sulfate comprising the steps of: (a) combining an aqueous solution containing a sulfate of nickel and at least one metal selected from cobalt and manganese with an aqueous solution of sodium hydroxide or sodium carbonate in a stoichiometric ratio of about 1:2 or 1:1, respectively, in the presence of ammonia or a salt of ammonia; (b) removing the precipitated nickel hydroxide or carbonate and at least one metal selected from cobalt and manganese by filtration; (c) removing ammonia from the filtrate of step (b) by stripping in a distillation column; (d) passing the remaining liquid phase through the porous membrane, thereby obtaining a permeate; and (e) removing water from the permeate of step (d) by evaporation. The method includes:

2. 2. The process of claim 1, wherein the filtrate of step (c) has a pH value in the range of 10 to 13.

3. 3. The method of claim 1 or 2, wherein the evaporation method in step (e) is selected from spray drying and evaporative crystallization.

4. 3. The process according to claim 1 or 2, wherein the chloride content of the sulfate filtrate after step (d) is in the range of zero to 10 ppm by weight of the filtrate.

5. In step (a), MgSO ４ , Al ２ (SO ４ ) ３ , NaAl(OH) ４ , NaAl(SO ４ ) ２ , and KAl(SO ４ ) ２ 3. The method of claim 1, wherein at least one of aluminum and magnesium is included in the precipitation by introducing at least one of:

6. 3. The method according to claim 1 or 2, wherein in step (d) a pressure of 5 atm or less is used.

7. 3. The process according to claim 1 or 2, wherein reverse osmosis (d') is carried out between steps (d) and (e).

8. 8. The process according to claim 7, wherein the pH value of the permeate of step (d) or, if applicable, the pH value of the retentate of step (d') is adjusted to between 6.5 and 9.5 with sulfuric acid before step (e).

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