FR2881360A1 - Solution crystallization comprises use of reactor with series of compartments for recycling solution and recuperation of mother liquor - Google Patents

Abstract

Crystallization comprises circulating by gravity and in controlled supersaturation conditions a liquid (L) obtained from a solution (S) and containing crystals formed in situ through a series of compartments (C1, C2, C3...Cn) in a crystallization reactor (1) at a rate sufficient to maintain the crystals formed in suspension. A proportion of the liquid (Bc) containing crystals of medium size is recycled through the compartments, and a mother liquor containing fines (LMf) is recuperated from at least the last of the compartments by an overflow system, treated to remove the fines from it and recycled. The crystallization of the solution can be carried out by evaporation or precipitation.

Description

The present invention relates generally to crystallization.

  More specifically, it relates to a crystallization process and a crystallization device, suitable for implementing said method.

  Said method and device of the invention constitute improvements to the method and crystallization device of the prior art. The improvements in question appear clearly in consideration of the description below. It can already be pointed out that, according to the invention, within an unsophisticated device, favorable conditions for the formation of crystals, inorganic or organic, having a particle size of interest (large crystals, with a small size dispersion ) can be obtained, insofar as, within said device, there is: - a progressive magnification of the crystals, - no consequent recirculation of slurry charged with crystals, resulting in a low attrition; and - a destruction of the fines, with advantageously a limited residence time of said fines.

  Crystallizers, which have been and / or are the most used, are mainly of three types. They are perfectly known to those skilled in the art, in themselves, with their more or less specific field of application, their advantages and disadvantages. They are described in the literature, they are mentioned in particular in the techniques of the engineer, treated Process Engineering, J 2788-4 and following. It is more precisely: - crystallizers forced circulation: the most basic, which include an external recirculation loop with pump and heat exchanger. In these crystallizers, a large flow of slurry is circulated, which involves broken crystals in the pump and problems with viscous products. In addition, it inevitably faces clogging problems, pipes of the external recirculation loop and especially the surfaces of the exchanger; crystallizers of the Oslo or Krystal type: less used today, replaced by the crystallizers DTB presented below. These are fluidized-bed crystallizers with external recirculation of a finely charged supernatant. The recirculation flow, very important, must ensure agitation within the device; - SWENSON's "Draft Tube Baffle" (DTB) evaporator crystallizers: these crystallizers comprise an internal stirrer and are arranged so that they contain at least two zones, a stirred zone and a calm zone (called the settling zone). A charged slurry flow is withdrawn from said at least one quiet zone and recycled via an external fines dissolution circuit. Crystallizers of this type are more particularly described in patent application FR 2 062 756.

  In general, various methods are known for improving the size of the crystals generated in a crystallizer. In the context of these methods, it is recommended in particular to ensure: - maintaining a low supersaturation; and / or - to reduce attrition by optimizing pumping and / or stirring conditions; and / or - optimizing the conditions of pH, temperature, viscosity, impurity content; and / or - to destroy the fines, to reduce their residence time in the crystallizer. In a fines dissolving circuit, as described in application FR 2 062 756, said fines are dissolved and the raw material is recoverable (capable of generating large and medium crystals).

  In such a context, the inventor proposes a novel method and crystallization device.

  In the description of said process and device of the invention which follows and in the appended claims, the term "crystallization" is to be taken in the broad sense. Thus, by "crystallization process" is meant any process for obtaining a crystallized product (from crystals) from a solution, any process capable of creating a supersaturation, that is to say to act physically or chemically on said solution so that the concentration of the solute in the solution exceeds the solubility. This can be done in different ways depending on the shape of the solubility curve, for example: by cooling, by exchange or by evaporation of a third body (direct cooling), by cooling, by evaporation under vacuum (by re-injecting or not into the device the evaporated and condensed solvent), - by isothermal evaporation, during which it is the increase of the concentration which creates the supersaturation, - by salting out, by adding a salt (" salting out ") or a third solvent, -by chemical reaction between two soluble bodies to create an insoluble product, by chemical reaction on a soluble body by varying the pH conditions; in which case we will speak more generally of precipitation.

  The crystallization by isothermal evaporation or by precipitation is very often carried out.

  According to the invention, the process for crystallizing a solution or process for obtaining crystals from said solution typically comprises: circulating, under controlled supersaturation conditions, a liquid obtained from said solution, filled with crystals formed in situ, successively n n compartments (n> 2) of a crystallization reactor connected in series; said liquid filled with crystals progressing from compartment to compartment with a flow rate sufficient to maintain the formed crystals in suspension; with - recycling a slurry loaded with medium sized crystals; said slurry preferably from the n-compartment to be recycled to the first of said n compartments; and recovering mother liquor charged with fines from at least the nth of said n compartments; all or part of the recovered mother liquor being treated to be freed of fines and, after its treatment, recycled into n currents in each of said n compartments.

  Characteristically, in the context of the process of the invention: one operates successively within n compartments of crystallization. At each of said n crystallization compartments, the supersaturation conditions can be perfectly controlled. For such a control, one can play on two recycling rates: that of slurry and that of mother liquor; - A destruction of fines with recycling mother liquor, free of said fines, in each of said n 5 compartments; - Nitrate is advantageously recycled to the first compartment, a slurry of crystals of medium size. Because of the subdivision of the reactor, the skilled person understands that the flow rate of such recycling is here much lower than that used (internally or externally) in the crystallizers of the prior art. We can schematically say that it is divided by n, by the number of compartments. The slurry of crystals of medium size is thus advantageously recycled in a single current from the nth to the first compartment. This advantageous embodiment is however not limiting.

  With such conditions of implementation, that is to say: with progressive magnification of the crystals in the n compartments; with the use of a dissolution of the fines (the elimination of the fines which can be further improved, at the level of the recovery of said fines, see below); with little recycling of crystals, said recycling does not concern or very little large crystals; it is understood that the method of the invention is particularly effective in reference to the particle size, the width of the particle size spectrum, crystals produced. It is particularly suitable for the production of large crystals. In any case, it offers the possibility of adjusting the particle size of the crystals produced.

  The process of the invention is carried out in a compartmented or multi-stage reactor, which therefore comprises at least two compartments or stages connected in series. It has been seen that the number of compartments or stages makes it possible to limit the rate of recycling of the slurry. It is therefore interesting to increase this number, however reasonably, of course. It is indicated, in no way limiting, that the reactor of the invention generally comprises from 2 to 20 compartments.

  In the compartmented reactor of the invention, the liquid loaded with crystals must progress from compartment to compartment, with a flow rate sufficient to maintain the crystals formed in suspension. It should be limited, advantageously to avoid crystal deposits in the compartments.

  For the circulation of this liquid loaded with crystals, for its progression within the reactor, it is possible to use any suitable means. It will be ensured that the means in question do not disturb, in a harmful way, the growth of the crystals. In the context of an advantageous embodiment of the method of the invention, there is provided a particular arrangement of the compartments so that the liquid flows by gravity. Said compartments are then arranged in cascade.

  The mother liquor finely loaded, intended to be treated and then recycled, comes, as indicated above, from at least the nth (the last) of the n compartments of the reactor. Advantageously, it comes from several of said n compartments; very advantageously it comes from each of the n compartments from the second. In fact, it is advantageous to reduce the residence time of the fines in the reactor, to collect them as soon as possible after their formation (to be able to dissolve them and thus optimize the formation of large crystals). Within compartments of the crystallization reactor, it is therefore appropriate to provide settling zones-clarification where mother liquor loaded fines can be taken. Advantageously, said mother liquor finely loaded is recovered from the nth compartment at least, by withdrawal by overflow.

  To rid the mother liquor loaded with fine recovered, intended to be recycled into the compartments, freed of fines, it can implement any type of solubilization treatment of said fines. The recycled stream can be heat treated, a solvent of said fines can be added to said stream. These two embodiments of the solubilization of fines are not exhaustive. They can also be conducted jointly.

  It has been stated in the preamble that crystallization can be achieved by various types of physical or chemical action on a solution.

  In the context of the process of the invention, in each of the n compartments, the crystallization is advantageously obtained, according to a first embodiment, by evaporation and, according to a second embodiment, by precipitation. Other modes of implementation are obviously not excluded.

  The process of the invention, which the skilled person has already grasped the interest, can in particular be used for the crystallization of inorganic salts, salts and organic acids and sugars. It may more particularly be used for the crystallization of: - inorganic salts such as (NH4) 2SO4r KCl NaClO3 NaCl Na2SO4 K2SO4 CaSO4r 2H2O Ca CO3.

  - Organic salts such as citrates, gluconates, oxalates glutamates, acetates - organic acids such as citric acid, gluconic acid, oxalic acid ...

  It is now proposed to describe, in general terms, a crystallization device of the invention, crystallization device that is suitable for carrying out the crystallization process of the invention as described above.

  Said device typically comprises a series-mounted n-cell crystallization reactor and two recycle loops, one suitable for recycling a slurry loaded with medium-sized crystals and the other suitable for subsequent treatment. mother liquor charged to fines and its recycling, treated, in n currents in each of said n compartments of the reactor.

  Said device comprises, more specifically, a crystallization reactor with n compartments connected in series (n> 2), associated with: means capable of maintaining supersaturation conditions within each of said n compartments; means for circulating a liquid loaded with crystals, from the first to the nth of said n compartments; the generated crystals remaining in suspension; and two recycling loops: + the first suitable for recycling a slurry loaded with medium-sized crystals; said first recycling loop comprising means for recovering slurry from at least one compartment of said n compartments, means for circulating said recovered slurry and means for recycling said recovered slurry circulated in at least one upstream compartment, it being understood that advantageously said recovery means are arranged to recover said slurry from the last of said n compartments and said recycling means for recycling said slurry in the first of said n compartments; + the second suitable for recycling mother liquor not loaded with crystals, in the form of n currents in each of said n compartments; said second loop comprising means for recovering mother liquor finely charged from at least the nth of said n compartments, means for circulating all or part of said recovered fines mother liquor, treatment means, of said charged mother liquor circulated to remove said fines and recycle means suitable for recycling treated mother liquor in the form of said n currents.

  The means capable of maintaining supersaturation conditions within each of the n compartments are per se conventional means, adapted to the type of crystallization in question, the type of crystallization process to be implemented. They generally include means for controlling the temperature and the pressure.

  Temperature is usually a decisive parameter, especially in a context of isothermal evaporation.

  We can, in this or that context, operate under a more or less pronounced vacuum.

  Said means capable of maintaining supersaturation conditions within the n compartments may also comprise means for adding a compound, a reagent or a third solvent, and / or modifying the pH. We are in a context of precipitation.

  Within each of said n compartments, said means capable of maintaining supersaturation conditions are obviously activated in view of the incoming and outgoing flows.

  The means for circulating the liquid loaded with crystals are obviously suitable for limiting or even preventing any deposit of crystals, any accumulation of crystals, within the reactor.

  They are advantageously such that they limit, very advantageously as they avoid, any attrition or destruction of said crystals within said reactor. In this spirit, we have seen above that the n compartments of said reactor are advantageously arranged in cascade, to allow circulation by gravity. It can thus be overcome by any intervention of pump or agitator type means likely to break the crystals they put into circulation.

  The recycling loop suitable for recycling medium-sized crystal-filled slurry is, as indicated, advantageously of a simple design. It is thus advantageously arranged to allow recovery of said slurry from the last reactor compartment followed by recycling thereof in the first compartment. Other constructions are not excluded from the scope of the invention but do not present, a priori, no particular interest. The means for circulating the slurry generally consist of a pump.

  With regard to the recycling loop suitable for the recycling of mother liquor treated in each of the n compartments of the reactor, it mainly comprises, as indicated: - means of recovery of mother liquor charged with fines, coming from at least the last n compartments. It advantageously comprises such means for recovering said mother liquor loaded with fines from several of said n compartments, very advantageously such means for recovering said mother liquor loaded with fines, from each of said n compartments from the second. We have seen the interest of recovering the fines as far upstream as possible as they are formed.

  The recovery means in question advantageously comprise a tropplein, disposed in a clarification-settling zone arranged within the compartment, from which mother liquor charged with fines is recovered. The compartments of the reactor are therefore advantageously arranged to present, in function, such clarification-settling zones. We can speak of "calm zones", less concerned by the circulation of the slurry loaded with crystals. To present such zones, the compartments of the reactor can themselves be compartmentalized into three sub-compartments: an inlet of the flow of slurry, an intermediate, an outlet of said flow, in which is delivered the mother liquor treated.

  The "calm zones" are thus created in the upper part of the intermediate subcompartment. Such an arrangement is perfectly illustrated in Figure 2 attached. Other arrangements and / or the intervention of additional means such as hydrocyclones, to operate a clarification, are quite possible; means for circulating all or part of the mother liquor loaded with recovered fines: such means generally consist of a pump; means for treating the mother liquor loaded with fines, intended to rid it of said fines, to re-dissolve said fines. Such means may in particular consist of heating means (in particular surface or mixing exchangers) and / or means suitable for the addition of a solvent; means suitable for recycling the mother liquor treated, in the form of n currents, into each of the n compartments.

  Those skilled in the art, on reading the foregoing, understood the arrangement of the main means of the crystallization device of the invention. Said means can be declined according to several variants, to adapt to the nature of the product to be crystallized.

  In general, it is also noted that the crystallization reactor of the invention may be horizontal or vertical, depending mainly on the nature of the product to be crystallized and the capacity of said reactor. The bottom of said reactor may be flat or inclined; in the latter case, it can be inclined with slope or counterpoise .... It can also provide the arrangement of heat exchange surfaces within at least one compartment of the reactor ...

  It is now proposed to illustrate the invention, in its method and apparatus aspects, with reference to the appended figures.

  Figure 1 is a block diagram of the method of the invention.

  Figure 2 shows schematically a device suitable for implementing said method.

  The same references have been used in both figures. There is shown: in 1, the crystallization reactor or crystallizer. Typically, said reactor is compartmentalized into n compartments C1, C2, ..., Cn; - In 2, the recycling loop of the slurry charged with medium-sized crystals Bc - in 3, the mother liquor recycling loop. The acronym LMf corresponds to mother liquor loaded in fines while the acronym LMf corresponds to treated mother liquor, freed from its fines.

  in L, the crystal-laden liquid which progresses within the reactor 1, from compartment to compartment.

  In FIG. 1, the supply of solution S in the loop 3 and the withdrawal of slurry charged with large crystals BC in the loop 2 are provided.

  Pumps P2 and P3 respectively provide circulation in said recycling loops 2 and 3.

  It is shown schematically, upstream of the pump P2, in the portion 2 'of the recycling loop 2, the circulation of slurry charged with medium and large crystals Bc + BC; downstream of said pump P2, in the part 2 "of the recycling loop 2, the circulation of slurry charged with Bc.

  With regard to the recycling loop 3, it comprises the recycling means 3 'upstream of the processing means 4 and the recycling means 3 "+ 3" a + 3 "b + ... + 3" n downstream of said processing means 4. The mother liquor loaded with fine LMf taken from the crystallizer 1, is circulated by the pump P3 and then treated at 4 to be rid of its fines. It is then, referenced LMf, recycled in n currents 3 "to 3" n in each of n n compartments C1 to Cn of the crystallizer 1. Said crystallizer 1 and its internal arrangement are better represented in FIG. 2.

  The structure of the crystallization reactor 1 comprises: - an upper dome on which is arranged a steam outlet V, capable of communicating the interior space overlooking the compartments C1 to Cn with a suitable vacuum source, - an intermediate section of fixed diameter lb within which are arranged said compartments C1 to Cn. Said compartments are arranged in cascade; - A frustoconical lower section 1c which ends with a leg ld.

  In the lower part of this structure, below the compartments C1 to Cn at the lower part of the intermediate section lb, the lower section 1c and the leg - the recovery is envisaged (via the passage 5) and the settling liquid L having circulated in said n compartments. Three zones are schematically represented: in the lower part, that of the slurry charged with large crystals BC. The leg 1d of the reactor 1 comprises means for withdrawing said slurry BC; partly median, that of the slurry charged with crystals of medium size Bc. Part of this slurry Bc is recycled via the recycling loop 2; - in the upper part, that of the liquor loaded in fine LMf. Part of this mother liquor LMf is recycled, with treatment, via the recycling loop 3. There is shown schematically at 4 heating means, capable of ensuring the dissolution of the mother liquor liquids LMf.

  In the device shown, the solution supply S is provided between the pump P3 and said heating means 4.

  The n compartments of the reactor 1 are arranged (in cascade) on a plate 8. Each of said n compartments, from the second, is itself compartmentalized into three sub-compartments. In the upper part of the second of said three sub-compartments ("calm zone" of decanting clarification), there is a device 6 which, by overflow, associated with a pipe 6 ', is suitable for removing mother liquor loaded in fine LMf. The plate 8 is arranged for the passage of the pipes 6 '.

  The mother liquor charged with fine LMf is added with the feed stream S and then treated with 4. The LMF + fines are removed, added with the said feed stream S LMf + S, and it is recycled in n stream: 3 "a in the first compartment C1, 3 "b, 3" c, 3 "n in the third subcompartment of each of the other compartments from the second C2.

  In view of the description above and the appended figures, above commented, the skilled person has understood the whole point of the invention.

  In a crystallizer of the invention: the (progressive) growth conditions of the crystals are optimized; attrition is minimized, mainly because of the low recirculation of the slurry; viscous products which are more viscous than those which can be treated in crystallizers of the DTB or Oslo type can be treated.

  With reference to the crystallizers of the prior art, it can further be added that, according to the invention: - large crystals can be obtained, as in crystallizers DTB or Oslo type; large crystals can be obtained with a small particle size dispersion, as in the Oslo-type crystallizers; - There is no problem of holding the supersaturation as is found in crystallizers type Oslo.

Claims (16)

  Process for the crystallization of a solution (S), characterized in that it comprises: circulating, under controlled supersaturation conditions, a liquid obtained from said solution (S), charged with crystals (L) formed in situ, successively the n (n 2) compartments (C1, C2, Cn) of a crystallization reactor (1) connected in series; said crystal-loaded liquid (L) advancing from compartment to compartment with a flow rate sufficient to maintain the formed crystals in suspension; with - recycling a slurry loaded with medium size crystals (Bc); said slurry (Bc) advantageously from the n-compartment (Cn) to be recycled to the first (Cl) of said n compartments (C1, C2, ... Cn); and recovering mother liquor loaded with fines (LMf) originating from at least the nth (Cn) of said n compartments (C1, C2, ... Cn); all or part of the recovered mother liquor (LMf) being treated to be free of fines and, after its treatment, recycled into n currents in each of said n compartments (C1, C2, ... Cn).   2. Method according to claim 1, characterized in that said liquid loaded with crystals (L) is circulated by gravity.   3. Method according to claim 1 or 2, characterized in that said liquor mother liquor loaded (LMf) is recovered from several of said n compartments (C1, C2 ... Cn); advantageously from each of said n compartments from the second (C2, ... Cn).   4. Method according to any one of claims 1 to 3, characterized in that said mother liquor loaded fines (LMf) is recovered by withdrawal by overflow.   5. Process according to any one of claims 1 to 4, characterized in that the treatment carried out on recovered charged mother liquor (LMf) consists in a solubilization operation of the fines by heat treatment and / or addition of a solvent.   6. Method according to any one of claims 1 to 5, characterized in that the crystallization is obtained by evaporation.   7. Method according to any one of claims 1 to 6, characterized in that the crystallization is obtained by precipitation.   8. Device for crystallization of a solution (S), characterized in that it comprises a crystallization reactor (1) with n (n> _2) compartments (C1, C2, ... Cn) connected in series, associated to: means capable of maintaining supersaturation conditions within each of said n compartments (C1, C2, ... Cn); means for circulating a liquid loaded with crystals (L), from the first (Cl) to the nth (Cn) of said n compartments (C1, C2, ... Cn); the generated crystals remaining in suspension; and two recycling loops (2, 3): + the first (2) suitable for recycling a slurry loaded with medium-sized crystals (Bc); said first recycling loop (2) comprising means (5) for recovering slurry (Bc) from at least one compartment (Cj) of said n compartments (C1, C2, ... Cn), setting means in circulation (P2) of said recovered slurry (Bc) and recycling means (2 '+ 2 ") of said recovered slurry (Bc) circulated in at least one upstream compartment (Ci, i <j), being understood that advantageously said recovery means (5) are arranged to recover said slurry (Bc) from the last (Cn, j = n) of said n compartments (C1, C2, ... Cn) and said recycling means (2 "+2") for recycling said slurry (Bc) into the first (C1, i = 1) of said n compartments (C1, C2, ... Cn); + the second (3) suitable for the recycling of mother liquor not loaded with crystals (LMf '), in the form of n currents in each of said n compartments (C1, C2, ... Cn); said second loop (3) comprising means (6 + 6 ') for recovering mother liquor loaded with fines (LMf) from at least the n + th (Cn) of said n compartments (C1, C2, ... Cn), means for circulating (P3) all or part of said recovered mother liquor fines (LMf), treatment means (4), said charged mother liquor (LMf) circulated, for the removing said fines and recycling means (3 '+ 3 "+ 3" a, 3 "b, ... 3" n) suitable for recycling the treated mother liquor (LMf') in the form of said n currents.   9. Device according to claim 8, characterized in that the n compartments of said reactor (1) are arranged in cascade to allow circulation of said liquid (L) by gravity.   10. Device according to claim 8 or 9, characterized in that said second recycling loop (3) comprises recovery means (6 + 6 ') mother liquor loaded fines (LMf) from several of said n compartments ( Cl, C2, ... Cn); advantageously recovery means (6 + 6 ') of the mother liquor loaded with fines (LMf) from each of said n compartments from the second (C2, ... Cn).   11. Device according to one of claims 8 to 10, characterized in that said recovery means (6 + 6 ') comprise an overflow disposed in a clarification-settling zone arranged within the compartment, from which mother liquor loaded with fines (LMf) is recovered.   12. Device according to any one of claims 8 to 11, characterized in that said processing means (4) of said second recycling loop (3) comprise heating means (4) and / or means for adding a solvent.   13. Device according to any one of claims 8 to 12, characterized in that said means adapted to maintain supersaturation conditions comprise means for controlling the temperature and the pressure within each n compartments (C1, C2 , ... Cn) of the reactor (1).   14. Device according to any one of claims 8 to 13, characterized in that said means capable of maintaining supersaturation conditions comprise means for adding a compound, reagent or third solvent, and / or modify the pH within each said n compartments (C1, C2, ... Cn) of the reactor (1).   15. Device according to any one of claims 8 to 14, characterized in that said crystallization reactor (1) is a horizontal or vertical reactor.   16. Device according to any one of claims 8 to 15, characterized in that said crystallization reactor (1) has a flat bottom or inclined, with slope or against slope.