PROCESS FOR SEPARATING A MIXTURE CONTAINING SALT

Eutectic freeze crystallization is a technique for separating salt or other soluble substances from a solution producing ice and salt.

Known processes in this field are for instance described in US patent US-A-3, 541, 804; US-A-3 , 400, 548 ; US-A- 3,541,804; US-A-3 , 885 , 399; US-A-4 , 091, 635 ; US-A-2 , 354 , 633 ; US-A-5,537,832; and EP-B-1230194. It is an object of the present invention to improve upon these known processes by using a (belt) filter for the (further) treatment such as separation and/or washing of the ice slurry, or more specifically for filtering and washing the streams of ice and salt slurry to increase the purity thereof.

A process for separating a salt or any other soluble substance from a stream of an aqueous solution, into a stream of ice and a stream of solid salt crystals or other substance, comprises the following steps: feeding the solution into a crystallizer having a first outlet for a slurry of ice and a second outlet for a slurry of crystallized salt or other substance, treating the ice slurry from said crystallizer in a filter, whereby purified ice is discharged from the filter, treating the salt slurry in a filter, whereby purified salt or other substance is discharged from the filter.

An important advantage of this process is that very pure ice and salt is produced as a final result. In addition by adding heat in the final step of the belt filter for the salt slurry the content of water in the cake can be reduced. It is also possible to reduce the amount of water bound chemically in the salt crystals as crystal water

(de-crystallization) . By pre-concentrating the salt solution the overall efficiency of the process is increased.

Both salt and ice are produced in solid state. The process preferably comprises the following steps: a) The salt solution is fed into a crystallizer having an output for salt slurry - a mixture of water, dissolved salt and solid salt crystals - and an output for ice slurry - a mixture of water, dissolved salt and solid ice crystals. The crystallizer can be for example a eutectic freeze crystallizer working at or near the eutectic temperature of the salt solution or it can be a freeze crystallizer working at a temperature significantly above the eutectic temperature of the salt solution. The salt is separated from the solution, and because the salt crystals have a higher specific gravity than the slurry in the crystallizer, the salt crystals settle in the bottom part of the crystallizer, from where they are discharged as a slurry to a first belt filter. Water is also removed from the solution as (solid) ice crystals and because the ice has a lower specific gravity than the slurry in the crystallizer, the ice is moving upwardly to the top part of the crystallizer and it is then discharged to a second belt filter; b) Salt crystals and the salt solution are discharged to the first belt filter as slurry either directly from the crystallizer or from a storage tank. The treatment of the salt slurry on the first belt filter takes place in three different steps. In the first step the slurry is dewatered and the salt crystals form a cake on the belt filter due to the differential pressure over the filter cloth created by a vacuum pump. The major part of the liquid phase is removed in this step as filtrate. The second step includes washing of the salt crystals to obtain salt crystals almost free from

impurities. The washing is carried out by spraying clean cold water on the bed of salt crystals. Alternatively a saturated solution of the salt can be used. The washing can take place in several sequential steps in order to further reduce the content of impurities in the salt. The washing can also take place as counter current washing i.e. the used wash water from one washing step is fed as new washing water into the previous washing step. In the third step the cake of purified salt crystals is dried by sucking air through the cake. Step three may also include addition of heat to the filter cake in order to reduce the water content through evaporation or through reduction of the amount of chemically bound crystal water in the salt crystals. Said addition of heat may take place by sucking or blowing hot gas such as air through the filter cake. Heat can also be added by using micro waves or infrared radiation or other type of heat source. After the treatment of the crystals the cake of purified salt crystals is discharged from the belt by gravity or by using scrapers; and c) Ice crystals together with salt solution from the crystallizer are discharged to a second belt filter as a slurry either directly from the crystallizer or from a surge tank. The treatment of the ice slurry on the belt filter takes place in three steps. In the first step the slurry is dewatered and the ice crystals form a cake on the belt filter due to the differential pressure over the filter cloth created by a vacuum pump. Most of the liquid phase is removed in this step as filtrate. In the second step the cake of ice crystals is washed by spraying clean water on the cake. The washing can take place in several sequential steps in order to further reduce the content of impurities in the ice. The washing can also take place as counter current washing i.e. the used wash water from one washing step is fed as new

washing water into the previous washing step. In the third step as much liquid water as possible is removed from the cake of ice crystals by means of sucking air through the cake at the end of the belt prior to cake discharge by gravity or by scrapers .

Salts or substances for which the present invention can be used include - among others inorganic acids such as H ₂ SO ₄ , H ₃ PO ₄ , HNO ₃ , organic acids such as oxalic, citric, lactic, formic, tartaric acid, amino acids such as glutamic, threonine, aspartic acid or vitamins, antibiotics, proteins, enzymes, or any derivatives of the above or inorganic salts such as MgSO ₄ , Na ₂ CO ₃ , KNO ₃ , CuSO ₄ among others although the present invention is not limited to mentioned salts.

The aqueous solution of salt can be pre-concentrated in a conventional freeze concentration process, operating at a temperature significantly above the eutectic temperature to produce ice crystals only. Since only water is removed as ice from the solution, the salt concentration is increased. A freeze crystallizer can be operated at significantly higher heat transfer rates above the eutectic temperature and hence less cooling area is required and the overall costs are reduced. The pre-concentration can be done in one or a cascade of several crystallizers to obtain maximum efficiency of the process. The pre-concentrated salt solution is then fed into the eutectic freeze crystallizer.

Soluble substance may include - among others - acids, including mineral acids such as H ₂ SO ₄ , H ₃ PO ₄ , HN ₃ , organic acids, such as oxalate, citric, lactic, formic, tartaric, animo acids, such as glutamic, threonine, aspartic acids, or vitamines, anitibiotics, proteins, enzymes, any derivatives of the above .

According to a preferred embodiment of the present invention, the de-watered ice crystals (filter cake) are

washed on the belt filter, increasing the wash efficiency, and/or producing (more) purified ice crystals.

Preferred applications for the process of the present invention relate to MgSO ₄ , CaCO ₃ , HNO ₃ , N ₂ CO ₃ , KNO ₃ , HNO ₃ , CuSO ₄ among others although the present invention is not limited to these salt .

In the preferred embodiment also the salt slurry is treated on a belt filter, preferably in three stages, so that purified salt is obtained at the end of the belt filter. In the second stage, the de-watered salt crystals are washed with a saturated salt solution in cold water, producing more purified salt crystal.

The aqueous solution can be pre-concentrated in a conventional freeze crystallization process, operating at a higher temperature than the eutectic temperature to produce ice crystals only. A conventional freeze crystallizer can operate at higher heating transfer rates than in a eutectic freeze crystallizer, resulting in a use of less cooling surface area and reduced capital cost. In the pre- concentrated ice solution can be fed into eutectic freeze crystallizer.

Further details, characteristics and advantages will become clear when reading the following description of a preferred embodiment thereof; the figures showing: Figure 1, a flow diagram of a preferred embodiment of the process according to the present invention;

Figure 2, a side view of an apparatus to be used in the process according to the present invention;

Figure 3, a sectional view of the apparatus shown in Figure 2;

Figure 4, a sectional view of the apparatus shown in figures 2 and 3 ;

Figure 5, a more detailed sectional view of the apparatus shown in Figures 2 , 3 and 4 ; and

Figure 6, a diagram of an alternative embodiment for the washing in Figure 1. From a tank 10 (Figure 1) a salt solution, such as an aqueous solution of Na ₂ CO _3, is fed into a buffer tank 12. From the buffer tank 12, the solution is fed into a pre-cooler 14 for cooling the solution to a temperature close to the temperature where ice crystals start to form. The pre-cooled solution is fed into crystallizer 14a which pre-concentrates the salt solution removing water as ice, which is fed to the belt filter 24. The operating temperature of crystallizer 14a is significantly above the eutectic temperature of the solution allowing maximum heat transfer rate for the cooling surfaces in the crystallizer. The pre-concentration step is optional, if the salt concentration is high enough this step is not necessarily needed. For very dilute salt solutions a cascade of several freeze crystallizers, each lowering the temperature of the solution with a part of the total temperature drop between feed into tank 10 and the eutectic temperature, can be used to obtain a higher overall heat transfer efficiency. The pre-concentrated and pre-cooled solution enters the eutectic freeze crystallizer 16, which separates water and salt producing slurry of solid salt crystals and ice crystals. The operating temperature of the crystallizer 16 can be at or close to the eutectic temperature of the salt solution. From the top part of crystallizer 16, ice slurry is fed into a slurry buffer tank 18 and further into a first gravitation separator 20. Ice slurry is fed to an ice slurry buffer tank 22 and further to a vacuum belt filter 24, having three stages 26, 27 and 28.

In the first stage 26, ice slurry is dewatered to a cake of ice crystals with a lower content of water and the filtrate is fed back to buffer tank 12.

In the second stage 27 the cake of ice crystals is washed with external cold water. Alternatively the washing can be done by using counter current washing (CCW) (Figure 6) . In this case the filtrate from one stage 27c is used as wash water in the previous stage 27b. The filtrate from 27b is used as wash water for stage 27a. In the third step 28, the content of liquid phase in the cake of washed ice crystals are is further reduced by a flow of air through the cake.

After the third stage the cake of ice crystals is discharged from the belt filter. The ice can be used for cooling purposes such as for pre-cooling the stream of salt solution fed into the process by using a heat exchanger.

From the bottom part of crystallizer 16, a slurry of salt crystals is fed into a second gravity separator 36, having a liquid recycling line to buffer tank 12 and a discharge of salt slurry to a second vacuum belt filter 38, having four stages 40, 42 and 44.

In the first stage 40 the salt slurry is dewatered to a cake with significantly lower water content. The filtrate from this step is fed back to buffer tank 12. In the second stage 42, the cake of salt crystals are washed using external water, which is relatively cold-having a temperature of for example 4 or 5 ⁰ C. The wash water is sprayed on the cake using spray nozzles distributing the liquid evenly over the cake. Alternatively the washing can be done using cold saturated solution of the salt being separated in the crystallizer 16, in order to minimize the dissolution of solid salt during the washing. Alternatively the washing can be done with a saturated solution of a salt

solution from a buffer tank 46. The CCW principle described above can also be used in washing the cake of salt crystal in order to reduce the amount of wash water or wash solution. In the CCW any number of washing steps may be used whichever is optimal for the washing result and for the amount of wash water used.

In the third stage 44, the filter cake of washed salt crystals is dried by a flow of gas such as air passing through the cake. To make the drying more efficient hot gas can be used for the drying. After the drying with gas the cake still contains some liquid as free liquid and usually also water bound chemically as crystal water. The flow of gas through the cake is indicated by arrow 50 in Figure 1. In order to obtain drier cake of salt crystals an additional source of heat 50a such as a radiator of infrared heat or microwaves or other source of heat can be used. Water is removed with several different mechanisms of which the most important are: flow of water as liquid through the cake due to the differential pressure, evaporation of water and release of crystal water due to the elevated temperature (de-crystallisation) . For example in the case of MgSO ₄ the content of crystal water is 12 molecules of H ₂ O (MgSO ₄ x 12H ₂ O) at a temperature of -4 ⁰ C. If the temperature is increased above 0.4 ⁰ C, the amount of crystal water is reduced to 7(MgSO ₄ x 7H ₂ O)

After the stage 44, the cake of purified and dried (and de-crystallised) salt crystals is discharged from the belt filter for commercial use.

A preferred embodiment 110 of the apparatus according to the present invention (Figure 2) comprises a hopper 112 in the bottom part of the crystallizer, supported by a steel construction comprising legs 113, 114 and 115, as well as

intermediate modular parts 120, 122, 124 as well as a top part 126.

Each of the modules 120, 122, 124 is provided with an inlet 121, 123, and 125 for introducing the salt solution into the interior of the modules, of which interiors are in open connection with the interiors of the modules below and above. Each of the intermediate modules is also provided with a manhole 127 for inspection and reparation purposes.

The top part 126 is provided with a discharge 127a for discharging slurry of ice crystals. The top part 126 includes a variable speed electric motor 128 (see also Figure 3) , driving a central shaft 129 extending through the module to a scraper 130 in the bottom part 112. In the top part, the electric motor also drives a propeller 132 to press the slurry of ice crystals through the discharge 127a. A clutch mechanism 133 is provided for driving the propeller 132.

Also referring to Figures 4 and 5, each of the intermediate modules 120, 122, 124 is provided with arms 136, 137, 138, 139 coupled to the central shaft 129 for supporting scrapers 146 to scrape ice from two cylindrical walls 140, 145 and 141, 144. Each of the intermediate modules is provided with a connection 142 for pumping cooling fluid through a helical line 143 which are located in annular spaces between walls 140, 145 and 141, 144. When the arms 136-139 are rotated walls 140, 145 and 141, 144 are scraped by the scrapers to remove crystals of salt and/or ice from the surface of said walls. The ice crystals move upwards being discharged through the discharge 127a. The salt crystals settle to the bottom and they are discharged through the discharge 109.

The above described apparatus can easily be used for recovering salts such as MgSO ₄ from aqueous solutions such as waste streams. The operating temperature of the solution in

the crystallizer can be within 2 ⁰ C preferably 0.5 ⁰ C or 0.1 ⁰ C from the eutectic temperature of the solution, e.g. approx. -12 ⁰ C for MgSO ₄ . The overall height of the apparatus can be 1-10 metres.

The present invention is not limited to the above described embodiment; the scope of the invention is to be defined by the annexed claims .