CRYSTALLIZATION APPARATUS FOR METAL STRIPPING

Abstract: A crystallization apparatus for metal tripping of an inverted frustocone type structure is disclosed, which comprises a zone of mixing and contacting an organic solvent containing ex­ tracted metal ions therein with a fluoride series stripping solu­ tion to separate (back-extract) the metal ions as a crystal of metal fluoride or fluorinated metal complex into the stripping solution, a zone of cooling the stripping solution containing the deposited crystal of fluoride or metal complex therein to per­ form crystal growth, and a zone of separating the grown crys­ tal. In this apparatus, the colling zone is arranged in a lower part of the apparatus and provided at its outside with a heat ex­ changer, and a downpipe extending downward from the mix­ ing zone is terminated above the cooling zone, and a forced cir­ culation zone for generating cyclonic stream of the stripping solution is disposed in the lowest portion of the cooling zone and provided with at least one blow-off nozzle located in a sub­ stantially tangential direction thereof.

Description: CRYSTALLIZATION APPARATUS FOR METAL STRIPPING This invention relates to a crystallization apparatus for metal stripping, wherein an organic solvent containing extracted metal ions is subjected to a stripping (back-extraction) by mixing and contacting with a fluoride series stripping solution to thereby separate the metal ions from the organic solvent as a metal fluoride or fluorinated metal complex crystal and then the organic solvent is separated from the stripping solution. Lately, solvent extraction is watched as a method for obtaining a high-pur+ty metal or metal oxide, which is evaluated to be simple in the purification step and small in the energy consumption. And also, metals to be extracted include Mg, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Cd, Nb, Zr, Ta, Mo, W, In and the like. In the solvent extraction method, the stripping (back-extraction) of the metal ions has hitherto come into problem, which was solved by the use of a fluoride series stripping solution (an aqueous solution containing one or more of HF, NH4HF2, NH4F; see Japanese Patent laid open No. 57-42,545, No. 57-73,138, No. 57-73,141 and No. 57-85,943). Further, there is proposed a crystallization apparatus for use in the solvent extraction. For example, the crystallization apparatus described in Japanese Patent laid open No. 58-81,402 is of an inverted frustocone type relating to an improvement of a Krystal Oslo type (or a classifying fluidized bed type) crystallizer for stabilizing crystallization conditions, whereby the grain size of the resulting crystal is made very uniform. Heretofore, such a crystallization apparatus has been used only in a laboratory scale and has not yet been put to practical use in an industrial scale. Now, the inventors have manufactured an industrial scale crystallization apparatus having a crystal production capacity of 45 tons/month as shown in Fig. 1 and actually done the crystallization of metal ions. As a result, it has been confirmed to produce phenomena interfering with the continuous service of the apparatus, such as adhesion of deposited crystal to the inner wall surface of the apparatus, accumulation and agglomeration of deposited crystal in the lower portion of the apparatus, incorporation of the organic solvent into the crystal slurry and the like as mentioned in the following Reference Example. It is, therefore, an object of the invention to solve the problems resulted from the structure of the conventional inverted frustocone type crystallization apparatus and to provide a crystallization apparatus for metal stripping capable of facilitating the continuous service in industrial scale with keeping the classification performance of the conventional apparatus. According to the invention, there is provided in a crystallization apparatus for metal stripping of an inverted frustocone type structure comprising a zone of mixing and contacting an organic solvent containing extracted metal ions therein with a fluoride series stripping solution to separate the metal ions as a crystal of metal fluoride or fluorinated metal complex into the stripping solution, a zone of cooling the stripping solution containing the deposited crystal of metal fluoride or fluorinated metal complex therein to perform crystal growth, and a zone of separating the grown crystal, the improvement wherein said stripping solution cooling zone is arranged in a lower part of the apparatus and provided at its outside with a heat exchanger, and a downpipe extending downward from said mixing zone is terminated above said stripping solution cooling zone, and a forced circulation zone for generating cyclonic stream of the stripping solution is disposed in the lowest portion of said stripping solution cooling zone and provided with at least one blow-off nozzle located in a substantially tangential direction thereof. In a preferred embodiment of the invention, an outlet of an organic solvent supply pipe is opened in the mixing zone at a position lower than an outlet of a stripping solution supply pipe but higher than a lower end of a downpipe extending from the mixing zone toward the cooling zone. For a better understanding of the invention, reference is taken to the accompanying drawings, in which: Fig. 1 is a schematically sectional view of the conventional crystallization apparatus; Fig. 2 is a schematically sectional view of an embodiment of the crystallization apparatus for metal stripping according to the invention; Fig. 3 is a sectional view taken along a line Ill-Ill of Fig. 2; Figs. 4 and 5 are schematically sectional views of another embodiments of the crystallization apparatus for metal stripping according to the invention, respectively; and Figs. 6 and 7 are schematically perspective views of the organic solvent supply pipe used in the invention, respectively. Like parts are designated by like numerals throughout the different figures of the drawings. The invention will now be described in detail with reference to the accompanying drawings but is not limited to the illustrated embodiments. Fig. 1 shows an embodiment of the conventional crystallization apparatus for metal stripping, while Figs. 2 and 3 show an embodiment of the crystallization apparatus for metal stripping according to the invention. As shown in Fig. 2, an organic solvent 2 containing extracted metal ions is introduced into a mixing zone or a mixing chamber 6 through a supply pipe 2' provided with a temperature regulator 3, while a fluoride series stripping solution 4 containing NH4 HF2 or the like is also introduced into the mixing chamber 6 through a supply pipe 4' provided with a heating device 5. In the mixing chamber 6, the organic solvent and the stripping solution are mixed by means of an agitator 7, whereby the metal ions contained in the organic phase 2 are separated as a crystal of metal fluoride or fluorinated metal ammonium salt into the aqueous phase (or the stripping solution) 4. The aqueous phase 4 having a specific gravity increased by the stripping of metal ions descends in a downpipe 8 extending downward from the mixing chamber 6, during which the organic solvent is separated from the aqueous phase and rises upward into the mixing chamber and the crystal of metal fluoride or fluorinated metal ammonium salt having a relatively small solubility is precipitated from the stripping solution. According to the invention, the lower end of the downpipe 8 is not extended to the vicinity of the lower end portion of the conventional apparatus 1 as shown in Fig. 1 but is terminated at a position higher than an outlet 11 for a cooled stripping solution corresponding to an upper limit of a stripping solution cooling zone 10 as mentioned later. Thus, it is considered to promote the separation and rise of the organic solvent from the stripping solution and the deposited crystal descending through the downpipe 8. The organic solvent overflows from the mixing chamber 6 into a settling zone 12 and is discharged from an outlet 14 for organic solvent through an overflow wall 13. On the other hand, the stripping solution and crystal are introduced from the downpipe 8 into a stripping solution cooling zone 10 (or a crystal growth zone). The cooling zone 10 is connected to a pump 16 and a heat exchanger 17 through outlet ll and inlet 15 for the cooled stripping solution, whereby the stripping solution fed at warmed state from the mixing chamber 6 is cooled to accelerate the crystal growth and also classify a part of the deposited crystal. Further, a forced circulation zone 18 is disposed in a lowest portion of the cooling zone 10 and connected to an outlet 19 for the stripping solution, a pump 20 and at least one blow-off nozzle 21 for circulating the stripping solution. In the illustrated embodiment, two blow-off nozzles 21 are arranged at the same plane in the substantially tangential direction of the forced circulation zone 18 as shown in Fig. 3, through which the cooled stripping solution is blown into the zone 18 to generate cyclonic stream of the stripping solution. Thus, the temperature and the concentration and composition of the stripping solution are held uniformly while giving fluidity to the crystal slurry, whereby the adhesion of crystal to the inner wall of the apparatus and the accumulation and agglomeration of crystal in the lower portion of the apparatus can be prevented. After the stripping solution 4 is introduced into the cooling zone 10 through the downpipe 8, it passes upward through a crystal separating zone 22 and finally is discharged from an outlet 24 for the stripping solution through an overflow wall 23. On the other hand, the deposited crystal is conveniently discharged as a slurry from a crystal outlet pipe 25 and then subjected to a liquid-solid separation. In Figs. 4 and 5 are shown preferable modified embodiments of the crystallization apparatus for metal stripping of Fig. 2, respectively. In the apparatus of Fig. 2, an outlet end 2" of the supply pipe 2' for organic solvent is located in the upper portion of the mixing chamber 6 at a level substantially equal to an outlet end 4" of the supply pipe 4' for stripping solution, so that the residence time of the organic solvent 4 inside the mixing chamber 6 is short and the deposition of crystal grains occurs in a relatively upper portion of the mixing chamber 6. As a result, there may be caused a problem that fine particles of the deposited crystal are again entrapped in the organic solvent and then flowed into the settling zone 12 to produce adhesion and accumulation. In order to solve the above problem, it is preferable that the outlet end 2" of the organic solvent supply pipe 2' is disposed in the mixing chamber 6 at a position lower than the outlet end 4" of the stripping solution supply pipe 4' but higher than the lower end of the downpipe 8 as shown in Figs. 4 and 5. Thus, the residence time of the organic solvent inside the mixing chamber 6 becomes longer as compared with the case of using the apparatus of Fig. 2 and also the deposition of crystal is produced in a relatively lower portion of the mixing chamber 6. Since the organic solvent left from the outlet end 2" flows upward and the stripping solution left from the outlet end 4" and the deposited crystal flow downward, the organic solvent countercurrently contacts with the stripping solution and the deposited crystal in the mixing chamber 6. As a result, the fine particles of the deposited crystal entrapped in the organic solvent are removed into the descending stripping solution, so that the entrapped amount of fine crystal particles into the organic solvent is considerably reduced and the adhesion and accumulation of such particles in the settling zone 12 can be prevented as shown in the following example. As mentioned above, the effect of preventing the adhesion and accumulation of the fine crystal particle is achieved by disposing the outlet end 2" of the organic solvent supply pipe 2' at a position lower than the outlet end 4" of the stripping solution supply pipe 4' but higher than the lower end of the downpipe 8 in the mixing chamber 6. Particularly, such an effect is most conspicuous in the apparatus wherein the outlet end 2" of the organic solvent supply pipe 2' is disposed at a position just above the lower end of the downpipe 8. Moreover, the form of the organic solvent supply pipe 2' may be properly modified within the scope of the invention. Although the object of the invention is achieved by rendering the organic solvent supply pipe 2' into a single nozzle form as shown in Figs. 2, 4 and 5, it can be achieved more effectively by enlarging the effective opening area of the supply pipe 2' so as to enhance the contact efficiency between the rising flow of the organic solvent and the descending flow of the stripping solution. The preferred modification of the organic solvent supply pipe 2' is shown in Figs. 6 and 7. According to the invention, the downpipe extending downward from the mixing zone is terminated above the stripping solution cooling zone as compared with the conventional apparatus wherein the lower end of the downpipe locates near the lower portion of the apparatus, and at the same time the forced circulation zone is arranged in the lowest portion of the stripping solution cooling zone, whereby the adhesion of the separated crystal to the inner wall of the apparatus and the accumulation and agglomeration of the crystal on the lower portion of the apparatus can be prevented. Further, the outlet end of the organic solvent supply pipe is disposed in the mixing zone at a position lower than the outlet end of the stripping solution supply pipe but higher than the lower end of the downpipe, whereby the entrapment of fine crystal particles into the organic solvent can be suppressed to prevent the adhesion and accumulation of the fine crystal particles in the settling zone for the organic solvent. In this way, the continuous service of the apparatus according to the invention can easily be accomplished. The following examples are given in illustration of the invention and are not intended as limitations thereof. Reference Example In the conventional apparatus shown in Fig. 1, the organic solvent (210C) consisting of 30 V/V /O of di-(2-ethylhexyl) phosphoric acid (D2EHPA) and 70 V/V /O of n-paraffin and containing 17.6 g/Q of extracted Fe3+ was charged into the mixing chamber 6 at a rate of 2 m3/hr, while the aqueous solution of 125 g/Q of NH4HF2 (460C) as a stripping solution was charged into the chamber 6 at a rate of 3.5 m3/hr, whereby a crystal of (NH4)3FeF8 was deposited. However, the fine particles of the resulting crystal were adhered to the inner wall of the apparatus and the inner and outer walls of the downpipe in the lower half region of the apparatus and further the accumulation and agglomeration of the crystal were produced in the lower portion of the apparatus, and consequently it was difficult to continue the operation after 3 days of the beginning. Furthermore, since the lower end of the downpipe 8 extended to the vicinity of the bottom of the apparatus, the droplet of the organic solvent left from the downpipe rised upward in the stripping solution cooling zone, a part of which was sucked from the outlet 11 for the stripping solution to be incorporated into the crystal slurry. As a result of the analysis, the content of P in the crystal was 2-3%. Example 1 In the crystallization apparatus for metal stripping according to the invention shown in Fig. 2, the organic solvent (230C) having the same composition as in Reference Example and containing 18.7 g/2 of extracted Fe3+ was charged at a rate of 2 m3/hr, while the stripping solution of 125 g/Q of NH4HF2 (450C) was charged at a rate of 3.5 m3/hr, whereby a crystal of (NH4)3FeF6 was deposited in the mixing chamber 6. In this case, the adhesion of the crystal to the inner wall of the apparatus and the accumulation and agglomeration of the crystal onto the lower portion of the apparatus were not substantially observed even in 24 days after the beginning of the operation. Furthermore, since the lower end of the downpipe 8 was located above the upper limit of the stripping solution cooling zone 10, the incorporation of the organic solvent into the crystal slurry was less. As a result of the analysis, the content of P in the crystal was 0.31-0.60%. Example 2 In the crystallization apparatus for metal stripping according to the invention shown in Fig. 4, the organic solvent consisting of 30 V/V% of di-(2ethylhexyl) phosphoric acid (D2EHPA) and 70 V/V/O of n-paraffin and containing 16.3-18.4 g/R of extracted Fe3+ at a temperature of 21-230C was charged into the mixing chamber 6 at a rate of 2 m3/hr. In the mixing chamber 6, the outlet 2" of the organic solvent supply pipe 2' was located just above the lower end of the downpipe 8 and the outlet 4" of the stripping solution supply pipe 4' was located above 185 cm from the outlet 2". The stripping solution consisting of 118-130 g/Q of NH4HF2 and 4-9 g/Q of HF at a temperature of 45-47"C was charged at a rate of 3.5 m3/hr to conduct the continuous service of the apparatus. As a result, the adhesion of the crystal to the settling zone 12 was not substantially observed even in 30 days after the beginning of the operation, so that the continuous service of the apparatus was performed without troubles.

Claims: Claims 1. In a crystallization apparatus for metal stripping of an inverted frustocone type structure comprising a zone of mixing and contacting an organic solvent containing extracted metal ions therein with a fluoride series stripping solution to separate the metal ions as a crystal of metal fluoride or fluorinated metal complex into the stripping solution, a zone of cooling the stripping solution containing the deposited crystal of metal fluoride or fluorinated metal complex therein to perform crystal growth, and a zone of separating the grown crystal, the improvement wherein said stripping solution cooling zone is. arranged in a lower part of the apparatus and provided at its outside with a heat exchanger, and a downpipe extending downward from said mixing zone is terminated above said stripping solution cooling zone, and a forced circulation zone for generating cyclonic stream of the stripping solution is disposed in the lowest portion of said stripping solution cooling zone and provided with at least one blow-off nozzle located in a substantially tangential direction thereof. 2. The apparatus according to claim 1, wherein an outlet of an organic solvent supply pipe is opened in said mixing zone at a position lower than an outlet of a stripping solution supply pipe but higher than a lower end of said downpipe.

Inventors: HOSHINO, MINORU C/O CHIBA WORKS, UCHINO, KAZUHIRO C/O RESEARCH LABORATORIES, WATANABE, TOSHIO C/O RESEARCH LABORATORIES
Applicants: KAWASAKI STEEL CORPORATION
Application year: 1985
Main classification: B01D9
IPC codes: B01D9