Method for treating waste sodium sulfate obtained from industrial process, use of waste sodium sulfate obtained from industrial process and industrial processing plant

Abstract

Disclosed is a method for treating waste sodium sulfate obtained from an industrial process, the method comprising providing a solution of waste sodium sulfate, providing potassium hydroxide in solid form or in solution form having a potassium hydroxide concentration of 25 wt% or higher and mixing with the solution of waste sodium sulfate to obtain a reaction mixture to convert the waste sodium sulfate into potassium sulfate and sodium hydroxide, and recovering the formed potassium sulfate and sodium hydroxide. The invention further discloses application of the waste sodium sulfate and an industrial processing plant.

Description

Technical Field

[0001] The present disclosure relates to a method for treating waste sodium sulfate obtained from an industrial process and an industrial processing plant. The present disclosure also relates to the use of waste sodium sulfate obtained from an industrial process. The present disclosure also relates to a fertilizer product. Background Art

[0002] Sulfate discharge increases water salinity and can exacerbate eutrophication. Soluble sulfates such as sodium sulfate (Na2SO4) increase water salinity. Wastewater with a higher salt content has a higher density than lake water. Due to the density difference, the water tends to form two layers: salty water at the bottom and less salty water above. This phenomenon is called stratification, and it reduces natural water mixing with the bottom and surface layers.

[0003] Several industrial activities such as the metal refining industry and the pulping industry produce metal sulfates which are increasingly subject to strict limits on sulfate concentrations in effluents. For example, kraft pulping effluents contain sodium sulfate due to the use of NaOH and Na2S as cooking chemicals. Sulfate concentrations in the effluent are typically at the level of 54,000 kg / day. In old pulp mills, sulfate emissions in the effluent can be as high as 1500 mg / l. Sulfate has not yet been considered as an effluent parameter for pulp mills. Therefore, sulfate is rarely removed in biological wastewater treatment. The World Health Organization (WHO) guideline for the maximum sulfate content in drinking water is 250 mg / l.

[0004] An emerging and rapidly growing industrial sector is the production of lithium-ion battery precursors, which are often prepared by coprecipitation from sulfate-based metal solutions. In Finland, for example, the limit values ​​for wastewater sulfate concentrations and the required treatment methods are determined by the company's environmental permit. A common limit value for sulfate concentration in wastewater is 400 mg / l. Sulfate limits are set to reduce environmental pressures caused by increasing saltwater concentrations in natural water bodies, especially freshwater.

[0005] In the prior art, sodium sulfate is converted into potassium sulfate or sodium sulfate, or is landfilled, which are not practical solutions to the emerging waste sodium sulfate problem.

[0006] While industrial production is increasing, there is a need to avoid the generation of sulfate-based waste and salinization. This requires more efficient wastewater and process water treatment methods for sulfate removal and reuse of sulfate-containing wastewater and process water. Summary of the invention

[0007] A method has been discovered to utilize spent sodium sulfate from industrial sources and reconvert it into usable chemicals. The inventive method overcomes the disadvantages of the prior art and provides a zero-waste, low-temperature method that enables cost-effective implementation and processing. The method enables utilization of the products NaOH and K2SO4 obtained from sodium sulfate.

[0008] The present disclosure provides a method for treating spent sodium sulfate obtained from an industrial process, the method comprising:

[0009] - providing a solution of spent sodium sulfate,

[0010] - providing potassium hydroxide in the form of a solid or a solution having a potassium hydroxide concentration of 25 wt % or more and mixing with a solution of waste sodium sulfate to obtain a reaction mixture to convert the waste sodium sulfate into potassium sulfate and sodium hydroxide, and

[0011] - Recovery of the potassium sulphate and sodium hydroxide formed.

[0012] The present disclosure also provides the use of waste sodium sulfate obtained from an industrial process for preparing sodium hydroxide by the method.

[0013] The present disclosure also provides the use of waste sodium sulfate obtained from an industrial process for preparing potassium sulfate by the method, and potassium sulfate is used to prepare fertilizer products.

[0014] The present disclosure provides an industrial processing plant, which comprises

[0015] -Industrial processes using sodium hydroxide,

[0016] - a source of spent sodium sulfate solution,

[0017] - an apparatus arranged to perform the method, the apparatus comprising

[0018] - reactor,

[0019] - a mixing device,

[0020] - a heating device,

[0021] - wherein the mixing device and the heating device are electrically controllable,

[0022] - the spent sodium sulphate source is arranged to be conveyed and / or transported to the reactor,

[0023] - the sodium hydroxide obtained from the reactor is arranged to be conveyed and / or transported to an industrial process utilizing sodium hydroxide, and

[0024] - The potassium sulphate obtained is arranged to be recovered from the reactor.

[0025] Main embodiments are characterized in the independent claims. Various embodiments are disclosed in the dependent claims. The embodiments and examples disclosed herein are freely combinable with each other, unless explicitly stated otherwise.

[0026] The method of the present invention utilizes inexpensive chemicals and is easily implemented at any industrial location, so any applicable industrial process that provides spent sodium sulfate can be supplemented with the method of the present invention with low investment. For example, the reactor or system of the present invention can be implemented near a source of spent sodium sulfate at low cost and can be operated without interfering with existing facilities and processes.

[0027] All products obtained from the process of the invention can be utilized, so that the process improves current solutions in a completely zero-waste manner and enables the use of the obtained NaOH, for example as a cooking chemical or precipitant, and the obtained K2SO4, for example for fertilizers. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 An example of the method of the present invention is shown.

[0029] Figure 2 An example of a method for treating waste sodium sulfate is shown.

[0030] Figure 3 The determined XRD spectrum of the solid product obtained by the process of the present invention is shown. DETAILED DESCRIPTION

[0031] In this specification, unless otherwise specifically indicated, percentage values ​​are based on weight (weight / weight, by weight or weight %). If any numerical ranges are provided, these ranges also include the upper and lower limits. The open term "comprising" also includes the closed term "consisting of" as an option.

[0032] As discussed, large amounts of metal sulfates are formed each year in industrial activities. Sulfate recovery methods that reduce the sulfate concentration of process water or wastewater in a commercially viable and efficient manner have been extensively studied. So far, there are no cost-effective technical methods for treating alkaline sulfate waste streams. Acidic sulfate waste streams are usually precipitated with calcium and the formed gypsum sludge is recovered. In this case, a techno-economic solution is also missing.

[0033] Alkaline spent sodium sulfate solutions are formed as by-products in the chemical and pulping industries. One example is the precipitation of battery precursors from metal sulfate solutions using NaOH as a precipitant. For example, the concentrations of the main elements in spent sodium sulfate are described in Table 1. In addition to the elements mentioned in Table 1, the solution contains ammonium ions in a concentration of 4 g / l as a residue from chemical coprecipitation.

[0034] Table 1. Physicochemical characteristics of spent sodium sulfate solution (pH 12.5). The values ​​are expressed in mg / l and only values ​​above 1.0 mg / l are reported.

[0035]

[0036] The present disclosure relates to a method for utilizing (i.e., reusing) spent sodium sulfate by reacting it with potassium hydroxide and water to produce potassium sulfate and sodium hydroxide. Potassium sulfate is a chemical used for fertilizers and is currently produced by high temperature processing. NaOH can be recycled in reverse to be used, for example, as a precipitant in the chemical industry or as a cooking chemical in the pulping industry.

[0037] The present disclosure provides a method for treating spent sodium sulfate obtained from an industrial process. An example of the method is disclosed in Figure 1 The industrial process 10 can be any suitable industrial process that provides sodium sulfate (Na2SO4) in a suitable form that can be an effluent of the industrial process. Preferably, the spent sodium sulfate is alkaline spent sodium sulfate, wherein the pH of the waste liquid is in the alkaline range, such as a pH of 8 or greater, 9 or greater, 10 or greater, 11 or greater, or 12 or greater. The sodium sulfate should be concentrated enough to, for example, discharge wastewater containing small amounts of sodium sulfate, such as less than 50 g / l, less than 30 g / l, or less than 10 g / l of sodium sulfate.

[0038] The method comprises providing sodium sulfate in the form of a solution, providing a certain amount of potassium hydroxide and mixing the potassium hydroxide with the sodium sulfate solution to obtain a reaction mixture. The reaction occurs in the reaction mixture 12 and a mixture of end products is obtained. The formed potassium sulfate and sodium hydroxide are recovered from the mixture of end products. The method may include washing the obtained solid material containing potassium sulfate, which will reduce the sodium content.

[0039] Potassium hydroxide may be provided in a molar excess relative to sodium sulfate, such as by addition. However, for certain applications (other than battery applications), sufficient final concentrations of potassium sulfate and sodium hydroxide may also be obtained by using a stoichiometric or substoichiometric ratio of potassium hydroxide to sodium sulfate.

[0040] The method may include converting spent sodium sulfate into potassium sulfate and sodium hydroxide in a one-step reaction

[0041] Na2SO4 + 2 KOH → K2SO4 + 2 NaOH.

[0042] This equation can also be viewed as a general equation describing the overall course of the process of the present invention, which can be carried out as a one-step reaction or as a two-step reaction.

[0043] The method can also be carried out by a two-step reaction having a first step (1) and a subsequent step (2):

[0044] (1)2 Na2SO4 + 3 KOH → K3Na(SO4)2 + 3 NaOH,

[0045] (2)K3Na(SO4)2 + KOH → 2 K2SO4 + NaOH.

[0046] The method may include controlling the stoichiometry of the reactants in step (1) and / or step (2) to control the reaction. More particularly, the reaction of step (1) may be carried out to an extent where less than all of the reactants react, i.e., a partial reaction is carried out. This may be carried out by maintaining the amount of KOH in a substoichiometric ratio with sodium sulfate. In step (1), a precipitation mixture is obtained, which is represented in the reaction as K3Na(SO4)2. More particularly, the precipitation mixture mainly comprises K3Na(SO4)2, but may also comprise K2SO4. It may also be referred to as "impure potassium sulfate" or "impure K2SO4", such as in Figure 2 The precipitation mixture is provided to step (2).

[0047] In step (2), potassium hydroxide may be provided in excess molar amounts relative to sodium sulfate (superstoichiometric ratio). It was found that this significantly increases the amount of solid K2SO4 obtained from the reaction and correspondingly reduces the amount of SO4 residues in the obtained NaOH (i.e., the obtained liquid phase), so that NaOH can be used in a variety of applications or industrial processes as such, i.e., without further purification, including purity-sensitive applications and technical fields, such as the battery industry, etc.

[0048] In one embodiment, the molar excess comprises a molar ratio of potassium hydroxide to sodium sulfate, such as KOH to Na2SO4 and / or K3Na(SO4)2, of 4:1 or greater, preferably 5.1 or greater. In the case of a molar ratio of 5:1 or greater, such as 6:1 or greater, substantially all sulfate can be effectively precipitated from the sodium sulfate solution. The amount of sodium in the final solid precipitate is also low, thus making it suitable for use as a fertilizer.

[0049] Potassium sulfate crystallizes in the process and can be separated from the sodium hydroxide, while the sodium hydroxide remains dissolved in the solution. The solid potassium sulfate obtained is recovered and can, for example, be provided as a fertilizer product or used in the preparation of a fertilizer product 14, which can be carried out in the same process or in a separate process. Other products containing or based on potassium sulfate can also be prepared. The solid potassium sulfate obtained can be transported to another location for further processing, such as for the preparation of another product, for example to a fertilizer manufacturer for the preparation of a fertilizer product.

[0050] The sodium hydroxide obtained is reclaimed and can be reused as an industrial chemical, for example in the same industrial process 10 or in a process associated with the same industrial process 10. The industrial process is an industrial process utilizing sodium hydroxide, for example using sodium hydroxide as a process chemical and / or for other purposes in the process or associated processes. The industrial process can be carried out in the same facility or plant, such as a plant, a manufacturing plant or any other applicable processing site. However, sodium hydroxide can be provided to another purpose and / or industrial process, preferably wherein such purposes and / or process are located near the place where the method is carried out. The transportation or delivery of the sodium hydroxide obtained can be arranged in most industrial plants or other facilities, for example by providing a pipeline or other conveying means, or by arranging the delivery in a container, for example by using a special conveyor or other transport means, such as a vehicle and / or a transport chain.

[0051] Solid potassium sulfate and sodium hydroxide solution can be separated and recovered by using any suitable method and equipment for separating and recovering solids and liquids. The purity of the separated and recovered fractions can be analyzed by any suitable means. For example, the purity of potassium sulfate can be analyzed from a sample by using X-ray powder diffraction (XRD) and related instruments (such as an X-ray powder diffractometer or a general XRD system for R&D) to obtain an XRD spectrum or a spectrum, which can be used to evaluate the purity of potassium sulfate and / or the degree of conversion in the method, the success of the method, etc. The obtained potassium sulfate and sodium hydroxide can be identified by analyzing the fractions. The progress of the reaction, such as the degree of conversion during the reaction, can also be monitored and / or estimated by using other means, such as monitoring the absorbance and / or turbidity of the reaction mixture, detecting the formed precipitate, etc.

[0052] In one embodiment, the method comprises using the recovered sodium hydroxide in the industrial process and / or in a process related to the industrial process.

[0053] Therefore, a closed or substantially closed process can be provided, particularly with regard to sodium sulfate, which utilizes all or substantially all materials provided to the method, particularly sodium hydroxide, and the materials obtained from the method. This makes it possible to provide an industrial process that produces less waste or does not produce waste. Since sodium sulfate is considered to be a waste that is problematic in further use, and this material is mainly discarded, the inventive method can utilize this waste and obtain valuable raw materials for the process. Since all spent sodium sulfate can be utilized, there is no need to find a disposal site for waste, or to apply for any authorization to dispose of waste. This makes it possible to implement an industrial process and a factory with fewer environmental issues, fewer authorizations, fewer wastewater purification units or factories and to be implemented in a variety of locations.

[0054] In one embodiment, the method is a waste-free method, which includes utilizing all or substantially all reaction products and / or reagents, including spent sodium sulfate. Waste-free refers to a process for treating sodium sulfate in which no or substantially no waste is generated. For example, a small amount of waste comprising or consisting of impurities present in spent sodium sulfate may be obtained, which impurities may have been separated in the process. However, residual impurities are not problematic in all cases, and therefore it may not be necessary to remove these impurities from the final product.

[0055] Sodium sulfate can be provided in a reactor or similar container as a solution (such as an aqueous solution). The sodium sulfate solution should have a sufficiently high concentration, wherein the concentration of sodium sulfate is 50g / l or higher, such as 80g / l or higher, 100g / l or higher, preferably 120g / l or higher. In one embodiment, sodium sulfate is provided in the form of an aqueous solution with 100g / l or higher concentrations. The solution can have a sodium sulfate concentration in the range of 100g / l-450g / l or 120g / l-450g / l, such as 120g / l-400g / l or 140g / l-300g / l. It can be obtained directly from the corresponding industrial process that produces sodium sulfate, so before being provided to the inventive method, spent sodium sulfate may not have been processed, such as purified. However, spent sodium sulfate solution may be concentrated and / or concentrated in the inventive method. The method may include determining and / or obtaining the sodium sulfate concentration in the spent sodium sulfate solution. This can be used to determine the required amount of potassium hydroxide, the suitability of the waste liquor for the process of the invention, the need to concentrate the waste liquor and / or the need to adjust any other process parameters, and a corresponding decision can be made based on the determined and / or obtained concentration whether to perform the action and / or to what extent the action is performed. The method can include, for example, providing (such as conveying and / or transporting) waste sodium sulphate from an industrial process or a process step that generates sodium sulphate waste to a container. The waste sodium sulphate may contain small amounts of impurities, but in testing it was found that the impurities do not interfere with the process and, if desired, they can even be separated from the sodium sulphate.

[0056] The sodium sulfate concentration can be adjusted to obtain an optimal precipitation of potassium sulfate. A suitable sodium sulfate concentration can be obtained after concentration, or the original waste liquor can already have such a concentration, such as 2 mol / l or less, or 1.8 mol / l or less, for example 0.3 mol / l - 2.0 mol / l, 0.5 mol / l - 2.0 mol / l (71 g / l - 284 g / l) or 1 mol / l - 1.8 mol / l (142 g / l - 256 g / l). In the case of concentrations above 2,5 mol / l, sodium sulfate tends to precipitate, which interferes with the process of the invention. It was found that in most cases, a concentration in the range of 1.0 mol / l - 2.0 mol / l (142 g / l - 284 g / l) is optimal.

[0057] Potassium hydroxide can be provided as a solid form or as a solution such as an aqueous solution. The solution can be a concentrated solution, such as having a KOH concentration of 25% by weight or greater, such as 30% by weight or greater, for example in the range of 30-50% by weight, for example about 30% by weight. Preferably, potassium hydroxide is provided in a solid form, such as in the form of particles or powder. When dissolved in an aqueous solution, solid potassium hydroxide generates heat, which is beneficial to the process. Providing KOH in solid or concentrated form in addition enables the method to be implemented in a simple, safe and compact form. This has advantages in terms of water and solution management and the implementation of equipment and systems, as it does not require a large amount of KOH and can be more compact.

[0058] Potassium hydroxide is mixed with sodium sulfate. This can be done in a reactor, which can be equipped with one or more mixing devices, such as one or more mixers, which can include one or more mixing blades, agitators, flow effects and / or the like, and the flow effect of the liquid can also be used to obtain mixing. A reaction mixture is obtained. The reaction mixture and / or the contents of the reactor are heated, preferably by using one or more heating devices, such as one or more heaters, which are arranged to heat the contents of the reactor. Mixing and / or heating can be performed to obtain a homogeneous solution. The reaction mixture can be heated to or have a temperature below 100°C, such as 95°C or lower, or 90°C or lower. The temperature is 60°C or higher, such as 70°C or higher, or 80°C or higher. The temperature can be in the range of 60°C-90°C, such as 60°C-80°C, 70°C-90°C or 80°C-90°C. Alternatively, or in addition, the sodium sulfate solution can be provided and / or heated at an elevated temperature.

[0059] The solubility of the sodium and potassium salts differs significantly at temperatures of 60°C or above, as shown in Table 2.

[0060] Table 2. Solubility of key Na and K salts as a function of temperature

[0061]

[0062] Heating and / or mixing may continue for a period of time necessary to allow all or substantially all of the reagents to react, such as to allow complete or substantially complete (eg, 90% or more, or 95% or more) conversion of sodium sulfate to potassium sulfate.

[0063] The reaction mixture can be concentrated to remove water. Concentration can be carried out by evaporation occurring at an elevated temperature. In one embodiment, the method comprises concentrating the reaction mixture, preferably by evaporation. This can be concentrated before and / or after adding potassium hydroxide.

[0064] The solution is cooled or allowed to cool, such as to 50°C or less, for example to 40°C or less, preferably to room temperature, such as to 25°C or less, for example 20°C-22°C, to obtain a crystalline potassium sulfate and sodium hydroxide solution. During the cooling process, potassium sulfate crystallizes with high purity (> 90%) and the formed NaOH remains in the solution. The crystallized potassium sulfate forms a suspension, which is precipitated to obtain a solid potassium sulfate precipitate, which can be separated from the remaining NaOH solution.

[0065] The solution is preferably cooled or allowed to cool to a temperature of not less than 2°C, such as not less than 5°C, for example not less than 10°C. Advantageously, the cooling is above 1.8°C, which prevents solidification of the sodium sulfate. The temperature may be, for example, in the range of 2°C-50°C, such as 2°C-40°C or 2°C-25°C, preferably 5°C-50°C, such as 5°C-40°C or 5°C-25°C, for example 10°C-50°C, such as 10°C-40°C or 10°C-25°C, for example to about 30°C.

[0066] In one embodiment, the method comprises

[0067] - heating the reaction mixture to 60°C-90°C, preferably to obtain a homogeneous solution,

[0068] - Cool the solution to 50°C or below to obtain a crystalline potassium sulfate and sodium hydroxide solution.

[0069] The method may be carried out in an apparatus comprising the device disclosed herein, which may be controllable, such as electrically controllable. The apparatus may be an automated or semi-automated apparatus and may be part of a complex or system. The apparatus or system may be an apparatus or system for pulping and / or papermaking processes and / or industries or battery industries.

[0070] In one example, the device includes

[0071] - reactor,

[0072] - a mixing device,

[0073] - temperature control means, such as heating and / or cooling means, for example heating and optionally cooling means,

[0074] -Wherein the mixing device and the temperature control device are electrically controllable and are preferably operatively connected to a control device, such as one or more control units, which is arranged to perform the method steps disclosed herein, such as at least controlling the temperature in the reactor and / or controlling mixing. The control device can be operatively connected to one or more devices, equipment, actuators, etc. disclosed herein, so that the control device can controllably operate the device, and / or be connected to one or more sensors and other devices arranged to monitor the process, i.e., to obtain information from the process, such as from the reactor / reaction mixture. The device may include a cooling device, such as one or more coolers, for example, implemented by liquid flow in the reactor housing. The cooling device can be used to cool the reactor mixture or homogeneous solution to induce precipitation. The cooling device can be operatively connected to a control unit.

[0075] The device may also include one or more of the following

[0076] - an inlet for spent sodium sulphate solution,

[0077] - an inlet for potassium hydroxide,

[0078] - an outlet for the solid potassium sulfate obtained,

[0079] - an outlet for the sodium hydroxide solution obtained,

[0080] - one or more sensors arranged to monitor one or more properties of the reaction mixture and / or the reactor, such as temperature, pH, turbidity, absorbance, flow rate, level, conductivity, etc.,

[0081] - one or more pumps for delivering the solution, said pumps being operatively connected to the control unit,

[0082] - one or more additional containers disclosed herein and required connections, such as pipes,

[0083] - one or more valves for controlling the flow of solutions and / or solids, and / or

[0084] - one or more actuators, which are connected to one or more movable members for mixing, moving and / or otherwise controlling the process, and which actuators are operatively connected to a control unit. The valve may be an electrically controllable valve, which includes an actuator operatively connected to a control unit. The apparatus may include a container for potassium hydroxide, which is connected via a controllable valve or other control device, such as operatively connected to a control unit, so that the dosing of potassium hydroxide can be controlled. The container may include a funnel for allowing the flow of solid potassium hydroxide. The apparatus may include means for discharging the solid potassium sulfate obtained, such as at the bottom of the reactor, for example connected to one or more movable members for moving the solid potassium sulfate, and which means may be operatively connected to a control unit.

[0085] The apparatus may include an inlet for solid potassium hydroxide or an inlet for a concentrated potassium hydroxide solution, which may have a concentration as disclosed herein.

[0086] The apparatus of the present invention utilizes a mixing device such as a mixer to ensure immediate dissolution of solid KOH or concentrated potassium hydroxide solution, which is beneficial to avoid local concentration gradients. Even if heating or cooling is not necessarily required initially, for example due to heat generated by KOH dissolution or due to the need to reduce the temperature, the use of a temperature control device can simply control the reaction by keeping the temperature of the reaction mixture at an optimum during the process. The temperature control device may include one or more heating and / or cooling elements arranged to control the temperature of the reactor and / or the reaction mixture in the reactor.

[0087] The inlet for solid potassium hydroxide or the inlet for concentrated potassium hydroxide solution can be connected to a container for solid potassium hydroxide or concentrated potassium hydroxide solution. The use of concentrated potassium hydroxide enables the provision of a container with a relatively small volume, which enables the system and the apparatus to be implemented in a compact form.

[0088] The industrial process plant may be an industrial process plant for a pulp and / or paper making process and / or industry, or an industrial process plant for the battery industry.

[0089] The control unit may be an electronic control unit, which may be programmable, comprising one or more processors, memory and software, the software being configured to perform one or more operations when executed by a processor in the control unit to implement the method, such as adjusting the temperature of the reaction mixture by controlling a temperature control device such as a heating and / or cooling device, controlling a mixing device to obtain a desired mixture of the reaction mixture, monitoring the temperature and / or other characteristics of the reaction mixture with one or more sensors in the reactor, and the like. The control unit may be arranged (such as programmed) to monitor one or more characteristics from the device, the system and / or the reactor, such as changes over time, and to perform one or more control actions in the device or the system as feedback to the monitored characteristics to adjust the function of the device to perform the method of the present invention. Characteristics such as temperature, pH, turbidity, absorbance, conductivity, flow rate, liquid level, control of substance addition, mixing rate, etc. may be monitored using one or more sensors arranged to monitor the characteristics. For example, the temperature may be controlled within a predetermined range and / or increased and / or decreased in a controlled manner to perform the method.

[0090] The present disclosure provides an industrial processing plant, or a system or equipment arrangement in the industrial processing plant, or a similar processing site, which includes

[0091] -Industrial processes using sodium hydroxide,

[0092] - a spent sodium sulphate source, which may be an industrial process utilizing sodium hydroxide, a related industrial process and / or a separate industrial process,

[0093] - an apparatus arranged to perform the method, the apparatus comprising

[0094] - reactor,

[0095] - a mixing device,

[0096] - temperature control means, such as heating and / or cooling means,

[0097] wherein the mixing device and the heating device are electrically controllable and are preferably operatively connected to a control unit arranged to perform method steps, such as at least controlling the temperature in the reactor and / or controlling the mixing,

[0098] - the spent sodium sulphate source is arranged to be conveyed and / or transported to the reactor,

[0099] - the sodium hydroxide obtained from the reactor is arranged to be conveyed and / or transported to an industrial process utilizing sodium hydroxide, and

[0100] - The potassium sulphate obtained is arranged to be recovered from the reactor.

[0101] The inventive method and overall process can be implemented in different ways by different operators, and the whole production chain (such as actions performed by different operators) can be promoted by the method. The method can be performed by one operator, or can be performed by two or more operators. For example, the first operator can produce sodium sulfate waste. Such an operator can run the industrial process, which can be carried out in a plant, factory or other applicable production or processing site, and the industrial process can be an industrial process 10 utilizing sodium hydroxide. Sodium sulfate waste can be collected in a container, or can be directly provided from the process, such as via a pipeline or similar conveying device. The first operator can also provide sodium sulfate waste to the place where the present invention is reacted, which can be referred to as a use site. Another operator can also provide sodium sulfate waste, such as when sodium sulfate waste is collected in a container and transported to a use site. The present invention reaction 12 can be performed by the first or second operator, but can also be performed by another, third operator. The operator operates the reactor, doses potassium hydroxide, and reclaims the reaction product. The operator may also provide the reaction product for further use, and the operator may also perform one or both of the further uses, i.e., use the sodium hydroxide as an industrial chemical and / or provide potassium sulfate to the preparation 14 of fertilizer, or prepare fertilizer. However, one or two further operators may also perform these steps, and / or the first operator may perform the step of using the sodium hydroxide as an industrial chemical in the industrial process 10. The operator who may utilize the obtained potassium sulfate may be a fertilizer manufacturer.

[0102] An example of the method of the present invention for treating spent sodium sulfate is Figure 2 The method utilizes two precipitation steps and can provide a high purity final product. Figure 2 The method may include Figure 1 in step 12.

[0103] In step 20, sodium sulfate (Na2SO4) waste liquor is fed into a reactor, concentrated (30%) potassium hydroxide (KOH) in solid form is added as pellets, and the obtained solution is mixed and heated to 80°C to obtain a homogeneous solution. The solution is concentrated by evaporation at an increased temperature. The mixture is cooled to 50°C in step 22, where potassium sulfate (K2SO4) crystallizes and contains some residual sodium sulfate. This obtained impure potassium sulfate is dissolved in a minimum amount of water at 80°C for purification crystallization step 26 and crystallized by cooling to 50°C. Pure potassium sulfate is obtained from step 26 and can be recycled.

[0104] The remaining solution from steps 22 and 26 is crystallized in the second crystallization step 24 by cooling to 0°C-20°C. Regenerated NaOH, Na2SO4 and residual K2SO4 are obtained in step 30. NaOH can be recovered and reused, such as sent to an industrial process. The residual solution containing Na2SO4 and residual K2SO4 from step 30 is sent back to step 20. Alternatively, the residual solution containing Na2SO4 and residual K2SO4 from step 26 can be directly sent back to step 20 to avoid diluting the NaOH solution of the second crystallization step 24.

[0105] The method of the present invention can be applied to different industrial processes 10 that provide waste sodium sulfate.

[0106] In one embodiment, the industrial process 10 is a pulping process. A pulping process refers to a process or system arrangement that includes or involves pulping and / or cooking in a pulping process, including the generation of cooking chemicals. One such source of sodium sulfate is the removal of sodium sulfate from the recovery cycle due to excess sulfur. The traditional way to control the sodium and sulfur balance is to remove a portion of the recovery boiler fly ash, which mainly includes sodium sulfate and sodium carbonate. Sodium sulfate is conventionally dissolved in wastewater or used as a supplementary chemical. If sodium sulfate is dissolved in wastewater, it will increase the discharge of sulfates. On the other hand, if it is used as a supplementary chemical, it can increase the excess sulfur in the Na / S balance. Excess sulfur can lead to increased fly ash purging, which also increases the discharge of sulfates.

[0107] By means of the process according to the invention the sodium sulphate content of wastewater or other effluents of a pulping process or pulp mill can be reduced and allows better control of the process since there is no need to use sodium sulphate as a make-up chemical.

[0108] In one embodiment, the method comprises using the recovered sodium hydroxide as a cooking chemical in the pulping process, such as caustic soda, for example in white liquor and / or in bleaching.

[0109] Sodium hydroxide can be used in papermaking processes, such as paper recycling to separate ink from the recycled paper. In one embodiment, the industrial process 10 is a papermaking process.

[0110] In one embodiment, the industrial process 10 is a battery industrial process. In such cases, the method may include using the recovered sodium hydroxide as a precipitation agent, such as for precipitating battery precursors from a metal sulfate solution.

[0111] In certain processes in the battery industry, transition metal (M) sulfates, such as nickel sulfate, are treated with sodium hydroxide to precipitate transition metal hydroxides by the following reaction:

[0112] MSO4 + 2 NaOH → M(OH)2 + Na2SO4

[0113] Sodium sulfate is produced as a waste liquid in the process.

[0114] In one example, nickel hydroxide is prepared as a precursor for making LiNiO2 (LNO), which will be used as a cathode material in lithium-ion batteries. A spherical Ni(OH)2 precursor is synthesized using alkali hydroxide coprecipitation in an inert gas atmosphere. The reaction also uses NaOH to precipitate nickel hydroxide in the following reaction:

[0115] Metal-ammine complex formation i2+ + nNH3 → [Ni(NH3)n] 2+

[0116] Metal-hydroxide precipitation reaction [Ni(NH3)n] 2+ + 2OH - → Ni(OH)2 + nNH3

[0117] In one embodiment, the method includes using the recovered sodium hydroxide as a precipitation chemical and / or pH adjuster to treat the wastewater.

[0118] Sodium hydroxide is also used as an industrial cleaner, where it can be used to clean process equipment, storage tanks, etc., as it can dissolve grease, oil, fat, and protein-based depots. It is also used in the preparation of soaps and other detergents.

[0119] The sodium hydroxide obtained or a portion thereof may be used for other purposes other than the industrial process 10 in question, such as for or in the preparation of cements (e.g., in plasticizers), cleaning agents, water treatment agents, food treatment agents, esterification and / or transesterification agents, solvents for amphoteric metals and compounds, or agents for the manufacture of artificial textile fibers. The method may include separating and / or recovering a portion of the sodium hydroxide obtained and using it as or in the preparation of any agent disclosed herein, and / or for any use disclosed herein.

[0120] In one embodiment, the method includes providing crystalline potassium sulfate for the preparation of a fertilizer product, preferably prepared by combining with one or more materials acting as fertilizers, fillers and / or stabilizers. In one example, the fertilizer is an NPK fertilizer. NPK fertilizers contain nitrogen, phosphorus and potassium, and they can be manufactured by steam granulation, chemical granulation, compaction or blending. The potassium sulfate obtained by the present invention can be provided as a component for the preparation of such fertilizers or other types of fertilizers. The preparation of the fertilizer product may include providing potassium sulfate, providing one or more materials acting as fertilizers, fillers and / or stabilizers, mixing to obtain a mixture, and forming the mixture into a fertilizer product. The fertilizer product may be formed into particles, powders or any other applicable forms. In one embodiment, the method includes preparing a fertilizer product comprising crystalline potassium sulfate.

[0121] A fertilizer product comprising potassium sulfate obtained by the method disclosed herein is disclosed. The fertilizer product may be in the form of a dry powder or dry granules, which may have a moisture content of 20 wt % or less, such as 15 wt % or less or 10 wt % or less.

[0122] The present disclosure provides the use of waste sodium sulfate obtained from an industrial process in the process, comprising converting the waste sodium sulfate into potassium sulfate and sodium hydroxide by the following reaction:

[0123] Na2SO4 + 2 KOH → K2SO4 + 2 NaOH,

[0124] wherein potassium sulfate is recovered and provided as a fertilizer product or used in the preparation of the fertilizer product, and / or wherein sodium hydroxide is recovered and preferably used in the industrial process and / or in a process related to the industrial process.

[0125] A process comprising converting spent sodium sulfate into potassium sulfate and sodium hydroxide may comprise any of the methods disclosed herein and / or use any of the apparatus disclosed herein.

[0126] The present disclosure provides the use of spent sodium sulfate obtained from an industrial process for preparing sodium hydroxide by the method disclosed herein. As discussed, the sodium hydroxide is preferably used in the industrial process and / or in a process related to the industrial process.

[0127] The present disclosure provides the use of spent sodium sulfate obtained from an industrial process for preparing potassium sulfate by the method disclosed herein. Preferably, potassium sulfate is used to prepare a fertilizer product.

[0128] Example

[0129] Example 1

[0130] The method of the present invention is carried out in a one-step process using sodium sulfate and potassium hydroxide as follows:

[0131] Na2SO4 + 2 KOH → K2SO4 + 2 NaOH

[0132] Alkaline spent sodium sulfate solution (250 ml) obtained from a process for preparing metal hydroxides for batteries was mixed with excess KOH solution (21 g KOH pellets in water) by stirring and heated to a maximum of 80°C-90°C to form a completely homogeneous solution. Potassium sulfate was precipitated by cooling the reaction system to 50°C (evaporation of about 125 ml), resulting in the formation of almost pure potassium sulfate (white powder) and the formation of concentrated NaOH solution (supernatant). These were separated and recovered.

[0133] Example 2

[0134] The method of the present invention is carried out in a two-step process using sodium sulfate and potassium hydroxide as follows:

[0135] 2 Na2SO4 + 3 KOH → K3Na(SO4)2 + 3 NaOH

[0136] K3Na(SO4)2 + KOH → 2 K2SO4 + NaOH

[0137] Alkaline spent sodium sulfate solution (250 ml) obtained from a process for preparing metal hydroxides for batteries was mixed with excess KOH solution (21 g KOH pellets in water) by stirring and heated to a maximum of 80°C-90°C to form a completely homogeneous solution. Potassium sulfate was precipitated by cooling the reaction system to 50°C (evaporation of about 125 ml), resulting in the formation of almost pure potassium sulfate (white powder) and the formation of NaOH solution (supernatant). These were separated and recovered.

[0138] The X-ray powder diffraction (XRD) spectrum of the obtained crystalline potassium sulfate was determined using a PANalytical Powder XRD instrument and Figure 3 As can be seen from the spectrum, most of the peaks represent potassium sulfate and only trace impurities are present, mainly unreacted sodium sulfate and sodium potassium sulfate.

[0139] Example 3: One-step reaction

[0140] A 1M Na2SO4 solution was prepared by dissolving solid Na2SO4 in deionized water. Eight separate examples, 3.1-3.8, were then performed, wherein in each example, 200 ml of the prepared Na2SO4 solution was added to a conical flask and the initial temperature of the solution was measured. Then the amount of KOH pellets as described in Table 3 below was added to the flask in small batches and continuously stirred by a magnetic stirrer to form a completely homogeneous solution. After all the KOH was added, the temperature of the solution was recorded, then mixing was ended and the solution was cooled to 30°C, followed by filtration to separate the solid and liquid fractions obtained.

[0141] Analysis of the obtained solid and liquid fractions was performed as described subsequently, including X-ray diffraction (XRD) for the solid and inductively coupled plasma (ICP) for elemental composition analysis of the solid and liquid, and the results are given in Table 4.

[0142] Table 3 .

[0143]

[0144]

[0145] In Example 3.1, where the molar ratio of KOH to Na2SO4 was 1:1, no solid material was obtained.

[0146] In Example 3.2, in which the molar ratio of KOH to Na2SO4 was 1.5:1, a small amount of solid was produced which was insufficient for ICP analysis.

[0147] In Examples 3.2 and 3.3, the main solid phase obtained was NaK3(SO4)2.

[0148] In Examples 3.6, 3.7 and 3.8 with a large excess of KOH, almost all sulfate could be precipitated from the sodium sulfate solution. The main solid phase obtained was K2SO4.

[0149] Using a large excess (4:1) of KOH produces primarily K2(SO4) as a solid material with some impurities.

[0150] Washing of the solid material obtained (not performed in this example) will reduce the sodium content.

[0151] Example 4: Two-step reaction

[0152] First, in Example 4.1, a 1M Na2SO4 solution was prepared by dissolving solid Na2SO4 in deionized water. Then 1 liter of the prepared Na2SO4 solution was added to a conical flask and mixed with 112.21 g of KOH pellets by a magnetic stirrer to form a completely homogenous solution. After all the KOH was added, a solution temperature of 33.2°C was recorded, and then the mixing was ended and the solution was cooled to 25°C, followed by filtration to separate the solid and liquid fractions obtained.

[0153] Analysis of the obtained solid and liquid fractions was performed as described subsequently, including X-ray diffraction (XRD) for solids and inductively coupled plasma (ICP) for elemental composition analysis of solids and liquids, and the results are given in Table 6. As reported in Table 6, the major phase of the obtained solid material of Example 4.1 was NaK3(SO4)2.

[0154] In addition, four separate examples, 4.2-4.5, were performed, wherein in each example, 4.0 g of the obtained solid material (i.e., the precipitation mixture containing NaK3(SO4)2) and 12 ml of deionized water were added and mixed in a conical flask, and then the amount of KOH pellets as described in Table 5 below was added to the flask in small batches and continuously stirred by a magnetic stirrer to form a completely homogeneous solution. After all the KOH was added, the temperature of the solution was recorded, and then the mixing was terminated and the solution was cooled to 30°C, followed by filtration to separate the obtained solid and liquid fractions.

[0155] Analysis of the obtained solid and liquid fractions was performed as described subsequently, including X-ray diffraction (XRD) for the solid and inductively coupled plasma (ICP) for elemental composition analysis of the solid and liquid, and the results are given in Table 6.

[0156]

[0157]

[0158] According to the XRD and ICP analysis in Example 4.1, the solid material obtained was NaK3(SO4)2.

[0159] In Example 4.2, the solid material obtained was mainly K2SO4 with some NaK3(SO4)2.

[0160] Washing of the solid material obtained (not performed in this example) will reduce the sodium content.

[0161] Example 5: Effect of Na2SO4 concentration

[0162] First, in Example 5.1, three different Na2SO4 solutions with concentrations of 0.5M, 1M, and 1.5M were prepared by dissolving solid Na2SO4 in deionized water. Then, three groups of examples were performed and analyzed in the same manner as in Example 5.1 by using each of the three Na2SO4 solutions with four different KOH:Na2SO4 molar ratios of 2:1, 4:1, 6:1, and 8:1 (Examples 5.2-5.12). Table 7 shows the concentrations of the Na2SO4 solutions, the molar ratios of KOH:Na2SO4, and the amounts of KOH pellets used in Examples 5.1-5.12, and the analysis results are given in Table 8, respectively.

[0163]

[0164]

[0165] Examples 5.1-5.12 show that when the concentration of the Na2(SO4) solution increases, the amount of solid fraction obtained at the same KOH:Na2SO4 molar ratio increases.

[0166] Examples 5.1-5.12 show that when the amount of KOH increases, the sulfate content in the liquid fraction decreases.

[0167] Washing of the solid material obtained (not performed in this example) will reduce the sodium content.

[0168] X-ray diffraction analysis of Examples 3-5 .

[0169] Using the X-ray diffractometer EMPYREAN from PANalytical ® The XRD spectra of the samples were measured under the conditions described in Table 9 below.

[0170] Table 9. XRD measurement conditions

[0171]

[0172] Elemental composition analysis of Examples 3-5

[0173] The contents of K, Na, and S in the obtained solid fraction and liquid fraction samples were measured by inductively coupled plasma-optical emission spectroscopy (ICP-OES) using a Thermo iCAP 6000 series ICP-OES instrument.

[0174] For solid samples, the sample for measurement is prepared as follows: in a 100ml volumetric flask, 0.25 grams of the powder sample of each embodiment is dissolved in deionized (DI) water. Fill the volumetric flask with deionized (DI) water until the 100mL mark, and then homogenize completely. Take out 1ml of the solution by a pipette and transfer it to a 100ml volumetric flask for a second dilution. When constant volume is made with deionized water (DI), add an appropriate amount of concentrated nitric acid by a pipette to achieve a 5% HNO3 solution, and then homogenize. Finally, the solution is used for ICP-OES measurement. The content of K, Na and S is expressed as %.

[0175] For liquid samples, the measurement sample is prepared as follows: 1 ml of the liquid sample is taken out by pipette and transferred to a 100 ml volumetric flask. Fill the volumetric flask with deionized (DI) water until the 100 ml mark and then homogenize thoroughly. Take out an appropriate amount of solution by pipette and transfer it to a 100 ml or 250 ml volumetric flask for a second dilution. When the volume is made up with deionized water (DI), add an appropriate amount of concentrated nitric acid by pipette to achieve a 5% HNO3 solution and then homogenize. Finally, the solution is used for ICP-OES measurement. The contents of K, Na and S are expressed as g / l.

Claims

1. A method for treating spent sodium sulfate obtained from an industrial process (10), the method comprising - providing a solution of spent sodium sulfate, - providing potassium hydroxide in a solid form or in a solution form having a potassium hydroxide concentration of 25 wt % or more and mixing with the solution of the waste sodium sulfate to obtain a reaction mixture (12) to convert the waste sodium sulfate into potassium sulfate and sodium hydroxide, and - Recovery of the potassium sulfate formed and of the sodium hydroxide.

2. The method according to claim 1, wherein the spent sodium sulfate is converted into potassium sulfate by the following reaction: Na2SO4 + 2 KOH → K2SO4 + 2 NaOH.

3. The method according to claim 1 or 2, wherein the waste sodium sulfate is converted into potassium sulfate by the following two-step reaction: (1)2 Na2SO4 + 3 KOH → K3Na(SO4)2 + 3 NaOH, (2)K3Na(SO4)2 + KOH → 2 K2SO4 + NaOH.

4. A process according to any one of the preceding claims, wherein the potassium hydroxide is provided in a molar excess relative to the sodium sulfate.

5. The method according to claim 3, wherein in step (2) the potassium hydroxide is provided in a molar excess relative to the sodium sulfate.

6. The method of claim 4 or 5, wherein the excess molar amount comprises a molar ratio of the potassium hydroxide to the sodium sulfate of 4:1 or greater.

7. A process according to any one of the preceding claims, wherein the potassium hydroxide is provided in solid form.

8. A method according to any one of claims 1 to 6, wherein the potassium hydroxide is provided in the form of a solution having a potassium hydroxide concentration of 30 wt% or more, such as in the range of 30 wt% to 50 wt%.

9. The method according to any one of the preceding claims, wherein the solution of spent sodium sulphate is a solution of alkaline spent sodium sulphate.

10. A method according to any one of the preceding claims, comprising - heating the reaction mixture to 60°C-90°C, - Cooling the solution to 50°C or below to obtain a crystalline potassium sulfate and sodium hydroxide solution.

11. A method according to any one of the preceding claims, wherein the sodium sulphate is provided in the form of an aqueous solution having a concentration of 100 g / l or higher, such as in the range of 120 g / l-450 g / l or 140 g / l-300 g / l.

12. A process according to any one of the preceding claims, comprising concentrating the reaction mixture, such as before and / or after adding the potassium hydroxide, preferably by evaporation.

13. The method according to any of the preceding claims, comprising using the recovered sodium hydroxide in the industrial process (10) and / or in a process related to the industrial process (10).

14. The method according to any of the preceding claims, wherein the industrial process (10) is a pulp and / or paper making process.

15. A method according to claim 14, comprising using recovered sodium hydroxide as a cooking chemical in the pulping process.

16. The method according to any one of claims 1-13, wherein the industrial process (10) is a battery industrial process, preferably wherein the method comprises using recycled sodium hydroxide as a precipitating agent for precipitating battery precursors from metal sulfate solutions.

17. A method according to any one of the preceding claims, comprising using the recovered sodium hydroxide as a precipitation chemical and / or pH adjuster for treating wastewater.

18. The process according to any one of the preceding claims, wherein the process is a waste-free process comprising utilisation of all reaction products.

19. A method according to any one of the preceding claims, comprising providing the crystalline potassium sulphate for use in preparing a fertilizer product (14), preferably by combining with one or more substances acting as fertilizers, fillers and / or stabilizers.

20. Use of spent sodium sulphate obtained from an industrial process (10) for preparing sodium hydroxide by a process according to any one of claims 1 to 19, the sodium hydroxide preferably being used in the industrial process (10) and / or in a process related to the industrial process (10).

21. Use of spent sodium sulphate obtained from an industrial process (10) for the preparation of potassium sulphate for the preparation of a fertilizer product (14) by a process according to any one of claims 1 to 19.

22. An industrial processing plant, comprising - industrial processes using sodium hydroxide (10), - a source of spent sodium sulfate solution, - a device arranged to perform the method according to any one of claims 1 to 19, the device comprising - reactor, - a mixing device, - temperature control means, such as heating and / or cooling means, - wherein the mixing device and the temperature control device are electrically controllable and are preferably operatively connected to a control unit, the control unit being arranged to perform method steps, such as at least controlling the temperature in the reactor and / or controlling the mixing, - the spent sodium sulphate source is arranged to be conveyed and / or transported to the reactor, - the sodium hydroxide obtained from said reactor is arranged to be conveyed and / or transported to said industrial process (10) utilizing sodium hydroxide, and - The potassium sulphate obtained is arranged to be recovered from the reactor.

23. An industrial process plant for the battery industry according to claim 22, wherein the equipment comprises an inlet for solid potassium hydroxide or an inlet for a concentrated potassium hydroxide solution.

24. An industrial process plant for the battery industry according to claim 22 or 23, wherein the industrial process plant is an industrial process plant for pulp and / or paper making processes.

25. The industrial process plant for the battery industry according to claim 22 or 23, wherein the industrial process plant is an industrial process plant for the battery industry.

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