Method and system for drying salts, in particular hydrated salts

Technical Field

The present invention relates to a method and system for drying salts and in particular hydrated salts in which the amount of wastewater is reduced by the introduction of a dehumidifying system.

Background Art

When drying salts in particular hydrated salt it is a challenge not to dry too much such as to remove crystal water. There exist inorganic and organic hydrates. An organic hydrate is created when a water molecule is added to a carbonyl group of an aldehyde or ketone. In organic hydrates, the water molecules have chemically reacted with the compound and bonded to it and are thus less vulnerable to becoming anhydrous. The water molecules in inorganic hydrates are only loosely bonded to the compound, and there is no chemical reaction involved. The water molecule(s) can be removed from the compound relatively easy, such as through heating. An inorganic hydrate that has lost its water molecules is known as "anhydrous." Inorganic hydrates are the most common type of hydrates. Drying inorganic salt hydrates such that the crystal water is not removed is a challenge.

DE 197 19 483 relates to methods for drying of materials with least possible energy requirement, the materials to be dried are not further described. Methods for specifically drying salt hydrates are described in the prior art such as US 10,215,492 and US 10,914,519 where wet material, of particularly iron sulphate heptahydrate, is dried in a fluid bed dryer. The wet gas leaving the dryer is fully recirculated to the dryer where the wet gas is dried through a condensing step, where the moisture content of the drying gas is set using the temperature of the condenser, and, when operating in a closed cycle system, water is selectively removed from the system using a surface or wash condenser. Thus, the water evaporated from the product corresponds to the condensed water which is removed from the process. Hence, drying is a process that consumes large amounts of resources both in terms of wastewater generation from the drying process, and resources in the form of supplying energy for heating and cooling.

It is an object of the present invention to provide a method and system that solve one or more of the above problems, where drying is performed at a lower energy consumption and preferably with reduced wastewater generation, and more preferably while still being able to operate at a specific moisture content which is critical for drying hydrated salts.

Summary of the Invention

One or more of the above is solved by a method and system according to the invention where in a first aspect a method is provided for drying a moist salt, said method comprising the steps of: i) feeding a moist salt, and optionally semi dried salt from a filter, to a dryer, such as a fluid bed dryer; ii) drying the moist salt, and optionally the semi dried salt, with a drying gas having a predefined moisture content thereby providing a dried salt and a moist exhaust gas; iii) feeding the moist exhaust gas, in which moist reduced salt may be entrained, to an optional filter to provide a moist gas and optionally the semi dried salt; iv) feeding a portion of the moist gas from the optional filter to a dehumidifying section, in said dehumidifying section, the portion of the moist gas subjected to the steps of a. feeding the portion of the moist gas through a filter capable of removing particles in the moist gas, preferably a mechanical air filter, such as a high efficiency particulate air filter (HEPA filter); b. optionally feeding the portion of the moist gas to a pre-treat- ment air cooler or condenser to provide a water reduced gas slipstream and water; c. feeding the moist gas or the optionally water reduced gas slipstream to an adsorption dehumidifier to provide a dehumidified gas slip stream, said dehumidified gas slip stream exits the dehumidifying system; v) mixing the dehumidified gas slipstream with the moist gas to provide a mixed airflow; and vi) detecting the humidity of the mixed airflow and wherein if the humidity of the mixed airflow is different from the predefined moisture content of the drying gas of step ii), the flowrate of the moist gas and the portion of the moist gas are adjusted.

By introducing a dehumidifying section that provides clean water and combining this with a moisture control feedback mechanism it is possible to carefully control the amount of regeneration air necessary for obtaining the desired humidity of the final drying air. Having a carefully adjusted humidity of the dying air is specifically important when drying hydrated salts, since it is critical that the crystal water does not evaporate.

Such carefully adjusted humidity comes with a large energy input and generation of wastewater, which naturally affects large scale productions most. The solution of the present invention was shown to provide the required humidity at a much lower energy consumption as compared to the prior art as can be seen in the examples illustrating the effect of the invention. Furthermore, very little wastewater is produced since the condensed water is clean and can be utilized in upstream processes such as the crystallization process without any substantial treatment. Only clean water vented with the regeneration gas is not immediately reused.

In preferred embodiments the salts are hydrated salts. Most preferred salts are metal salts, and even more preferred hydrated lithium salts such as lithium hydroxide monohydrate.

In further embodiments, step iv) b. of feeding the portion of the moist gas to a pre-treatment air cooler or condenser to provide a water reduced gas slipstream and water, is mandatory. In this way water is produced which may be reused, furthermore in embodiments where the pre-treatment is cooling, the thus water reduced slip stream is conditioned for optimal removal of water in the adsorption dehumidifier, which is more efficient at lower temperatures. The adsorption dehumidifier is suitably a desiccant, for example using montmorillonite clay, calcium oxide, calcium sulfate, activated carbon, superabsorbent polymers, silica gel, zeolites, molecular sieves or activated alumina. The adsorption dehumidifier suitably operates at temperatures around 15 - 30 °C, typically around 22 - 27 °C depending on the cooling water.

According to the invention the moisture content of the drying gas is adjusted to be in accordance with the predefined range such that when the humidity of the mixed airflow is higher than the predefined moisture content, the flowrate of the moist gas is decreased, and when the humidity of the mixed airflow is lower than the predefined moisture content, the flowrate of the moist gas is increased.

The predefined moisture content range depends on the specific salt to be dried and the skilled person will know how to find specific values. As such in a presently preferred embodiment when the salt is a hydrated lithium salt, the predefined moisture content is in the range of 40 to 60 g water/kg dry gas.

The dryer operates at a predefined pressure range. Thus, suitably a dryer air pressure sensor is attached outside the dryer, an exhaust gas fan is located on a discharge side of the filter and a bleed valve is positioned on an exit side of the exhaust gas fan. The sensor senses the air pressure within the dryer and if it is outside of the predefined pressure range, a signal is sent to the exhaust gas fan to change the speed thereof, thereby increasing or decreasing the removal rate of the moist gas from the dryer in order to return the air pressure within the dryer to the predetermined pressure range. The pressure is regulated by regulating the bleed valve. Suitably the predefined pressure is in the range of -5 mbar (i.e. , a slight under pressure) to 50 mbar, preferably a slight overpressure, more preferred 5 mbar to 30 mbar and even more preferred 10 to 20 mbar.

According to the invention the moisture content of the drying gas is controlled by splitting the moist exhaust gas and treating a portion of the moist gas in the dehumidifying section. In particular embodiments, the portion of the moist gas fed to the dehumidifying system is in the range of 10 to 40 %. This range has proven to provide an optimal effect on the overall process in simulation, balancing drying efficiency, i.e. taking out enough moisture from the mixed airflow, and energy costs for driving the dehumidifying section.

In some embodiments, in which the dried salt is a hydrate, if the humidity of the dehumidified gas slip stream is different from a second predefined moisture content, the flowrate of feeding the regeneration air to the adsorption dehumidifier is adjusted, such that the second predefined moisture content lies within a predefined range. It is presently preferred that the range is 5 - 15 g/kg, more preferably 10 g/kg. In a further variation, the second predefined moisture content may be expressed as a ratio of the moisture content of the moist gas, and a presently preferred value is around 1/6 of the moisture content of the moist gas.

As is evidenced in the examples the method of the invention can be used for reducing wastewater of such a process, hence in a particular embodiment the method of the invention is a method for reducing wastewater and in another aspect the invention provides the use of a method as detailed for reduction of wastewater. Since the water generated in the process is pure high- quality water it can readily be reused for other purposes or upstream in a crystallization process.

Also provided is a system for drying a moist salt into a dried, preferably hydrated salt, said system comprising a drying section and a dehumidifying section; in which the drying section comprises at least a dryer, a filter, and at least one humidity controller, said dryer having a feed material inlet, a product outlet, a drying gas inlet and an exhaust gas outlet and said filter having at least an exhaust gas inlet, a moist gas outlet and a semi dried salt outlet; wherein the dehumidifying section has an inlet and an outlet and wherein the dehumidifying section comprises a filter capable of removing particles, a adsorption dehumidifier, a dehumidifier regeneration system and a humidity controller, and wherein the outlet of the dehumidifying section is connected to the drying gas inlet of the dryer of the drying section, preferably via a heating unit.

In a preferred embodiment the dehumidifying section further comprises a pre-treatment cooler or condenser, where the inlet of the pre-treatment cooler or condenser is connected to the outlet of the filter capable of removing particles and the inlet of the adsorption dehumidifier and further wherein the pre-treatment air cooler or condenser has a water outlet. In more preferred embodiments the pre-treatment cooler or condenser is an air cooler.

The invention also relates to a dehumidifying section as a stand-alone unit that may be retrofitted to an existing drying plant.

Brief Description of Drawings

In the following description embodiments of the invention will be described with reference to the schematic drawings, in which:

Fig. 1 shows an overview of an embodiment of a system according to the invention.

Fig. 2 shows an overview of another embodiment of a system according to the invention.

Fig. 3 is an expanded view of a dehumidifying system according to the invention.

Fig. 4 is a view corresponding to Fig. 2, of an embodiment of an alternative aspect of a method and system according to the invention.

Detailed description

According to the invention salts to be dried include both organic and inorganic salts, preferred are hydrated organic and inorganic salts. Preferred hydrated salts according to the invention are metal salts and more preferred are hydrated inorganic salts. Examples of such hydrate salts according to the invention include: CuSO4-5H2O, Copper(ll)sulfate pentahydrate, CoCl2*6H2O, Cobalt(ll) chloride hexahydrate; BeSO FhO, Beryllium sulfate tetrahydrate; K ₂ CO ₃ -1.5H ₂ O, potassium carbonate sesquihydrate; CaSO4-0.5H2O, Calcium sulfate hemihydrate; Epsom Salts: Magnesium sulfate heptahydrate, MgSO4-7H2O; Washing Soda: Sodium carbonate decahydrate, Na2CO3-10H2O; Borax: Sodium tetraborate decahydrate, Na2B40y10H20; Lithium hydroxide monohydrate, LiOH-lFhO. Most preferred is lithium hydroxide monohydrate. Lithium hydroxide monohydrate is a high margin high volume salt that requires accuracy in its production. Hence, there is a big need for cost effective, yet accurate, processes for providing this hydrate.

According to the invention the dryer can be but is not limited to fluid bed dryers (with or without built-in heat exchanger), rotary dryers (drum dryers), flash dryers, or fluidized bed spray granulators. Preferred is a fluid bed dryer.

The method and system according to the invention preferably constitute a substantially closed system. By a closed system is meant a system where the process streams are confined in the process. The system is open to the surroundings via the feed of starting material and withdrawal of final product. Furthermore, there is provided for bleed-off and purging of carbon dioxide free, and optionally oxygen free, gas from and into the system. Moreover, condensed water and steam leaves the system in the dehumidifying system.

According to the invention a hydrate salt is to be understood as a salt where the water of crystallization or crystal water is still part of the salt crystal. Hence any reference to hydrate, salt with crystal water, salt containing water of crystallization have the same meaning.

Carbon dioxide and oxygen in the drying gas are undesired for many salts as they can react and produce undesired by-products, therefore these gases, and in particular carbon dioxide, are preferably removed from the system and drying gas. Carbon dioxide gas can react with lithium hydroxide to produce lithium carbonate, which is an undesired by-product.

In the following, embodiments of the invention will be described in more detail.

Referring now to figure 1 the inventive method and system invention are illustrated in the broadest sense of the invention. The method and system will be described jointly such that details referring to the method aspect may be applied to the system aspect and vice versa. The system comprises a drying section 1 and a dehumidifying section 2. The sections are connected such that a feed line for a portion of moist gas g9 is an inlet from the drying section 1 to the dehumidifying section 2, and a feed line for a dehumidified gas slip stream, g11 , is an outlet from the dehumidifying section into the drying section.

The system is closed and is only open to the surroundings in the following manner. The drying section 1 also has a feed line for moist salt, f1 , and an outlet for dried salt, f2. The dehumidifying section 2 has a feed inlet for regeneration air, g12, and an outlet for moist regeneration air, g13, and a water outlet, 114. If need be and as illustrated, a bleed off valve 16 and a purge valve 17 may be present in the system to accommodate flows which also lead away from and to the system, respectively. This is to regulate pressure if needed.

With reference to figure 2 an embodiment of the method and system will be described in more detail. Elements described for figure 1 are the same. The feed of moist salts f1 , i.e., salts with surface water and water of crystallization, is fed to a dryer 11 of the drying section 1 .

In addition, a flow of semi dried salt f3 that has been entrained in moist exhaust gas g7 may be fed to a top of the dryer 11 from a filter 12, in which the moist salts may be treated and/or mixed with a dried product. In some embodiments the semi dried salt from the filter may also be fed to a top end of a mixer (not shown) and mixed with the moist salts and fed to the dryer.

In the dryer inlet drying gas g5, g6, which has an initial moisture load, i.e., humidity, circulates around the moist salts and evaporates surface water and/or water of crystallization to provide the final product f2. In preferred embodiments the surface water evaporates while keeping the water of crystallization intact, i.e. in case of hydrates.

The moist exhaust gas g7 contains the moisture from the initial drying gas g5, g6 and moisture which has been evaporated from the moist salts in the dryer 11 .

In operation the moisture of the initial drying gas g5, g6 is moisture built up from drying the moist salts in the dryer 11 . Hence, as will be understood in a start-up phase, and ending phase of a batch operation, a moist gas will have to be provided from an external source, but once the method and system is in operation, the moisture comes from the process itself, i.e. from evaporating moist from the feed f1 .

A dryer air pressure sensor 10 is attached outside the dryer 11 . When the sensor 10 senses that the air pressure within the dryer 11 is outside of a predetermined pressure range, a signal is sent to change the speed of an exhaust gas fan 16a located for example on a discharge side of the filter 12 thereby increasing or decreasing the removal rate of the moist exhaust gas g7 from the dryer 11 in order to return the air pressure within the dryer to the predetermined pressure range.

For this regulation the bleed valve 16 is positioned in the moist exhaust gas flow g7 on an exit side of the exhaust gas fan 16a to allow the release of some of the moist exhaust gas as required depending on the predetermined pressure. The predetermined pressure in the dryer is at a slight overpressure, such as - 5 mbar to 50 mbar, preferably a slight overpressure, more preferred 5 mbar to 30 mbar and even more preferred 10 to 20 mbar.

The remaining moist gas g8 i.e. , the portion of the exhaust gas which is not bled off from the bleed valve 16, circulates within the drying method and system. If there is a need for supply of gas to the circulating system, a purge gas g15 without carbon dioxide (and/or oxygen) is added to the system. A purge gas may be added at any position of the system or method. In the embodiment shown it is positioned immediately upstream from the bleeding off of the moist gas g8 and fed to the filter 12. The purge gas could in principle comprise any gas that has been treated so as not to contain carbon dioxide, oxygen, or other constituents.

Referring now also to figure 3, a portion of the circulating moist gas, g9, is directed to the dehumidifying system 2. The portion g9 is fed through a filter capable of removing particles from a gas stream, here illustrated as a HEPA filter 21 . The filtering is followed by an optional dehumidifier element 22, which may be a condenser or an air cooler or other means of taking out water.

In the embodiment shown the dehumidifier element 22 is present in the form of an air cooler, which is a preferred element, because it conditions the gas for an efficient water adsorption in the next step. Following the HEPA filter 21 and the dehumidifier element 22 including the air cooler, a water reduced gas slipstream g10 passes directly through an adsorption dehumidifier 23 to remove remaining water and to provide the dehumidified gas slip stream g11 , and then feed the dehumidified gas slip stream g11 back to the drying section 2 to mix with the moist gas g8 to provide a mixed airflow g4.

Before being sent back to the drying system, a first humidity controller 24 is positioned in a return flow path of the dehumidified gas slip stream g11 , and the first humidity controller 24 detects whether the humidity of the dehumidified gas slip stream g11 is according to a predefined moisture content before the dehumidified gas slip stream is mixed with the moist gas g8 and to provide the mixed air flow g4. By the term “humidity controller” is meant a unit capable of sensing, monitoring and adjusting a humidity level in a given setting. The humidity controller may further comprise control units and transitory memories for storing values and instructions and is capable to communicate with other units to carry out means for assisting in adjustment of the humidity level.

If the moisture content of the dehumidified gas slip stream is determined to be outside a predetermined humidity range of the drying gas, a signal is sent to the adsorption dehumidifier 23, and a controller 25 for a heater 26 of regeneration air g12 ensures adjustment of the regeneration air g12 fed to the adsorption dehumidifier 23 to change the output and thereby adjust the humidity level in the moist regeneration air g13 leaving the adsorption dehumidifier 23. The adsorption dehumidifier 23 is regenerated by supplying hot air to the adsorption dehumidifier.

The dehumidified gas slip stream g11 from the adsorption dehumidifier 23 leaves the dehumidifying system 2 and is mixed with the moist gas g8 to form the mixed air flow g4. The mixed air flow g4 is fed to heating unit of the drying section 1 to provide the drying gas to the dryer 11 as will be described in more detail below.

As the mixed air flow g4 returns to the dryer 11 , a second humidity controller 15 is positioned to measure the humidity in the mixed air flow g4 and, if the humidity is outside of the predefined moisture content of the drying gas g5, g6, the second humidity controller 15 sends a signal to a valve 15a located on the moist gas g8 flow path. The valve 15a may be positioned at any location but is suitably positioned immediately after directing a portion of the moist gas g9 to the dehumidifying system 2 for ease of operation. The valve controls the flowrate of the moist gas g8 flow rate.

In an embodiment of the method and system according to the invention and as shown, the mixed air flow g4 is fed to a heating unit comprising one or more heaters, here first and second heaters 13, 14, to provide the drying gas g5, g6 to the dryer 11. After the mixed air flow g4 is passed through an inlet drying gas fan 18, the flow is split into a first inlet drying gas portion g5 that passes through the first heater 13 and then into an air inlet (not shown in detail) of the dryer 11 , and a slipstream inlet drying gas portion g6 that may bypass the second heater (not shown) 14 can be separately added directly to the dryer 11 in controlled amounts.

After leaving the heater 13 the first inlet drying gas portion g5 passes an inlet drying gas flow controller 13a, which measures the flow rate of the first inlet drying gas portion g5. If the flow rate of the first inlet drying gas portion g5 is measured by the inlet gas flow controller 13a to be outside of a predetermined flow rate range, the inlet gas flow controller 13a sends a control signal to the inlet drying gas fan 18 to either speed up or slow down to return the flow rate of the first inlet drying gas portion g5 to the predetermined flow rate range.

The slipstream inlet drying gas portion g6 passes through a slipstream inlet drying gas portion flow control valve 14b and a slipstream inlet drying gas portion flow controller 14a, which measures the flow rate of the slipstream inlet drying gas portion g6. If the flow rate of the slipstream inlet drying gas portion g6 is measured to be outside of a predetermined flow rate range, the slipstream inlet drying gas portion flow controller 14a sends a control signal to the slipstream inlet drying gas portion control valve 14b to either increase or decrease the flow rate of the slipstream inlet drying gas portion g6 to the predetermined flow rate range.

The dehumidifying section 2 of the system shown in figure 2 and shown as an expanded view in figure 3 may be a standalone unit that can be retrofitted to existing drying plants in order to improve energy consumption and improve the wastewater.

All equipment used in the system and method according to the invention are standard units well known to the skilled practitioner. Examples

The effect of the method and operation of the system will now be illustrated by way of the following non limiting examples. Reference is made to an embodiment as illustrated in figure 2.

Example 1 - calculation of process runs

Three different runs were made using a GEA proprietary custom made simulation program, but simulations could similarly be made with commercially available software like Chemcad and Aspen. Case 1 and 2 had the same initial moisture content of the moist salt but different flow rates whereas run 3 had a lower moisture content but similar flow rate as case 1 .

The end product which in the example is LiOH-hydrate had a moisture content of 0.1 %. In case 1 the portion of the moist gas fed to the dehumidifier system, 2, was 4115/13959, i.e. around 30%. In case 2 the portion was around 15% and in case 3 around 23%. The pressure of the system was in the cases in the range of 10-20 mbar. The drying gas temperature was around 120 to 130 °C, and the product feed temperatures around 70 to 90 °C and the product temperature around 50 - 60 °C.

According to the invention and as can be seen from the cases below no water is fed to the drying gas, only gas is supplied to the system. Hence, the desired moisture content of the drying gas, which is needed in particular when drying hydrated salts, is maintained by using water from the moist salt feed.

It could also be seen that up to 80% and around 75 % of the water fed to the system was taken out as pure water in stream 14, meaning that only around 25% was lost in the form of steam. The water generated was very clean and thus it may be discharged as is or used elsewhere such as in a production facility for example in the upstream crystallization process. Thus, for example in case 1 , the evaporated water from the product was 207 kg/h. Water taken out in stream 14 is 159 kg/h, corresponding to 77%. Similarly, in case 2 it was 74% and in case 3, 75%. Table 1

Table 2 Table 3

Example 2 - energy consumption The energy consumption was calculated for the embodiment of case

1 in example 1 compared to prior art method using a scrubber condenser. As can be seen from the above example there is substantial duty savings in both heating and cooling. In the embodiment shown the optional pretreatment air cooling was included, without this the difference in energy would be even more significant. Thus, the method and system for operating the method is obtained without compromising the quality of the end product, in terms of preserving the water of crystallization.

In an alternative aspect of the invention, a method and a system for drying a moist salt are provided in which the dehumidifying section comprises the steps of i) feeding a moist salt, and optionally semi dried salt from a separation device, to a dryer, such as a fluid bed dryer; ii) drying the moist salt, and optionally semi dried salt, with a drying gas having a predefined moisture content thereby providing a dried salt and a moist exhaust gas; iii) feeding the moist exhaust gas, in which moist reduced salt may be entrained, to an optional separation device to provide a moist gas and optionally the semi dried salt; iv) feeding a portion of the moist gas from the optional separation device to a dehumidifying section, said dehumidifying section comprises the steps of a. feeding the portion of the moist gas through a filter capable of removing particles in the moist gas, preferably a mechanical air filter, such as a high efficiency particulate air filter (HEPA filter); b. optionally feeding the portion of the moist gas from the filter to a pretreatment air cooler or condenser to provide a water reduced gas slipstream and water; c. providing a dehumidified gas slipstream from an external source; v) mixing the dehumidified gas slipstream with the moist gas or optionally water reduced gas slipstream, to provide a mixed airflow, and vi) detecting the humidity of the mixed airflow and wherein if the humidity of the mixed airflow is different from the predefined moisture con-tent of the drying gas of step ii), the flowrate of the moist gas and the portion of the moist gas are adjusted.

Referring now to figure 4, an embodiment of the method and system of the alternative aspect is shown and will be described as follows:

A first step comprises i) feeding a moist salt f 1 , and optionally semi dried salt f3 from a separation device 12’, to a dryer, such as a fluid bed dryer 11 . The fluid bed dryer may be provided with a distribution device (not shown), such as a conical hat.

A second step comprises ii) drying the moist salt with a drying gas g5, g6 having a predefined moisture content thereby providing a dried salt f2 and a moist exhaust gas g7; iii) feeding the moist exhaust gas g7, in which moist reduced salt may be entrained, to an optional separation device 12’ to provide a moist gas g8 and optionally the semi dried salt f3; iv) feeding a portion of the moist gas g9 from the optional separation device 12’ to a dehumidifying section 2, said dehumidifying section 2 comprises the steps of a. feeding the portion of the moist gas g9 through a filter capable of removing particles in the moist gas, preferably a mechanical air filter 21 , such as a high efficiency particulate air filter (HEPA filter); b. optionally feeding the portion of the moist gas g9 from the filter 21 to a pre-treatment air cooler or condenser 22 to provide a water reduced gas slipstream g10 and water 114; c. providing a dehumidified gas slipstream g11 ’ from an external source 30; v) mixing the dehumidified gas slipstream g11 ’ from an external source with the water reduced gas slip stream g10 to provide a combined dehumidified gas slip strem g11 ”, vi) mixing the combined dehumidified gas slipstream g11 ” with the moist gas g8 to provide a mixed airflow g4, and vi) detecting the humidity of the mixed airflow g4 and wherein if the humidity of the mixed airflow g4 is different from the predefined moisture content of the drying gas g5 of step ii), the flowrate of the moist gas g8 and the portion of the moist gas g9 are adjusted.

By the alternative configuration, it is thus possible to omit, or bypass, the adsorption dehumidier 23 of the first embodiment. The gas stream g11 ’ from the external source may comprise a stream of treated gas. As in the above definition, a treated gas could in principle comprise any gas in which the contents of carbon dioxide, oxygen etc. has been reduced or removed.

In the alternative aspect, it is also possible to configure the loop through a purge valve, corresponding to purge valve 17 of the first embodiment.