1. Field of the Invention
The present invention relates to a crystallizer for crystallizing solute contained in a solution to be treated.
2. Description of the Related Art
It is generally accepted that known crystallizers are of three types.
Firstly, there are what are called forced-circulation crystallizers, which comprise a chamber intended to contain the solution to be treated and a circuit external to this chamber for conveying said solution to be circulated, through the action of a circulating pump, from an outlet located at the bottom of said chamber to an inlet located at the top of the latter, passing through a heat exchanger in order to create a state of supersaturation within the solution. This type of crystallizer gives crystals of moderate size (usually 250-400 μm) with quite a wide crystal size distribution.
Secondly, there are what are called Oslo crystallizers, which comprise a chamber intended to hold the solution to be treated and in which this solution is made to circulate from the top downward through a central downflow tube, before rising back up in the chamber around this tube, and being extracted therefrom, in a relatively clear form, i.e. with a low content of crystals, by a circuit external to the chamber, which brings this solution back to the top of the chamber after passing through a heat exchanger in order to create a state of supersaturation. The crystals obtained with this type of crystallizer are relatively coarse (usually 1.5-2 mm) with a moderate crystal size distribution.
Thirdly, there are what are called DTB (draft tube baffle) crystallizers, in which the solution to be treated is made to circulate, under the effect of a stirrer creating an ascending movement, from the bottom up through a central tube and then from the top down around this tube, before reaching a calm zone where the solution has a low content of crystals and from where said solution is extracted, before being reheated in order to dissolve the finest crystals (or fines) and then reintroduced into the base of the crystallizer. The crystals obtained are relatively coarse (usually 1-2 mm) because the fines have been removed, but with a relatively broad size distribution because of the mechanical destruction of the crystals by the stirrer. Furthermore, the use of this stirrer has various drawbacks such as: difficulty of installation; sealing problems; necessary monitoring, in order to detect any blockage.
As will have been understood, the existing crystallizers described above have design and implementation difficulties and/or inadequacies in terms of crystal size and crystal size distribution.