PROCESS FOR SEPARATING A MIXTURE CONTAINING SALT

Eutectic freeze crystallization is a technique to separate salt from salt bearing aqueous solutions producing pure ice as by product. The output from the crystallizer is a salt slurry and an ice slurry.

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) separation and washing of the ice slurry.

The present invention provides a process for separating a mixture containing salt, into an ice slurry and a salt slurry, comprising the steps of feeding the mixture to a crystallizer having a first output for the ice slurry and a second output for the salt slurry, treating the salt slurry, so that a liquid part is recycled to the crystallizer, the salt remaining, and washing the ice slurry on the belt filter, whereby purified ice is outputted from the belt filter, as well a liquid which can be recycled to the crystallizer.

According to the present invention, the de-watered ice crystals (filter cake) are washed on the belt filter, increasing the wash efficiency, viz. using less water and/or producing more purified ice crystals.

Preferred applications for the process of the present invention relate to MgSO ₄ , CaCO ₃ , HNO ₃ among others. 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, producing more purified salt crystal .

The aqueous solution can be pre-concentrated in a conventional freeze crystallization, operating at a higher temperature than the eutectic temperature to produce ice crystals only. A conventional freeze crystallizer can operate at higher cooling 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 to a belt filter, which can either be the same belt filter receiving ice suspension from the eutectic freeze crystallizer or a separate belt filter.

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 a preferred embodiment of an apparatus to be used with the process according to the present invention;

Figure 3 a sectional view of the embodiment shown in Figure 2;

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

From a tank 10 (Figure 1) a solution comprising CaCO ₃ is fed to a buffer tank 12. From the buffer tank a solution is fed to a precooler 14 for cooling the solution. The precooled solution is fed from the precooler to a crystallizer 16 from which the ice slurry goes to an ice slurry buffer 18 and into a first gravitation separator 20 from which a salt slurry is fed back to the crystallizer 16.

An ice slurry is fed to an ice slurry buffer 22, from which the ice slurry is fed to a belt filter 24, having two stages 26 and 28 resp. In the first stage, liquid is sucked off and fed back to the buffer tank 12, while on the second stage 28 the slurry is washed with ice water from an ice water buffer 30, which is supplied with clean water from a tank 32 and the washing liquid being sucked off and fed back to the ice water buffer. The washed and cleaned ice is output into ice buffer 34. From crystallizer 16 a second output is output to a second gravitation separator 36, having an ice output which is fed back to the buffer tank 12 and a salt output being output to a belt filter 38, showing three stages 40, 42 and 44 resp. The output from the first stage being fed back to stage 12. At the second stage saturated soda is added from a buffer 46 from which second stage the output is drained to a spent wash buffer 48. The salt is dried in the third stage 44 by purging hot air indicated by arrow 50, so that pure salt is available at the end of the belt filter, which can be sold for commercial purposes.

A preferred embodiment 110 of the apparatus according to the present invention (Figure 2) comprises a lower hopper 112 which is 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, 125 for introducing a mixture or 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 thereof.

The top part 126 is provided with an discharge 127 for discharging 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 lower part 112. In the top part, the electric motor also drives a propeller 132 to press the ice

5 crystal through the discharge 127. 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 supportingO 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 cooling fluid for flowing in and out of helical line 143 which are provided in annular spaces between walls 140, 145 and 141, 144. When the arms5 136-139 are rotated, the scrapers scrape along walls 140, 145 and 141, 144 to scrape of salt and/or ice crystals, so that the solidified water or ice moves upwardly and the concentrated salt crystals slurry is discharged through discharge 109.

0 The above described apparatus can easily be used for recover of crystallized material including MgSO ₄ from aqueous solutions. Of course, it will also be possible to use it for salts, such as HNO ₃ or Ca(NO ₃ ) ₂ . The overall temperature of the solution can be e.g. within 2.0 ⁰ C, preferably 0.5 ⁰ C or 0.1 ⁰ C of the eutectic point of such medium, e.g. around -12°C for MgSO ₄ . The overall height of the apparatus can be a 1-10 metres.

The present invention is not limited to the above described preferred embodiment; the scope of the invention is

D to be defined by the annexed claims.