METAL STRIPPING SYSTEM AND AN OPERATION PROCESS THEREFOR

Abstract: A metal stripping system and an operation process there­ for are disclosed. A main part of this system is a crystallization apparatus fo metal stripping comprising a mixing zone of or­ ganic solvent and fluoride series stripping solution, a settling zone for organic solvent, a settling zone for stripping solution, a separation zone for fluorinated metal complex crystal and a stripping solution cooling zone. In the operation of the system wherein the organic solvent containing extracted metal ions is contacted with the fluoride series stripping solution to deposit the fluorinated metal complex crystal and then the crystal is re­ covered as metal or metal oxide, the stripping solution is circu­ larly used by supplying NH₄HF₂ solution to the cooling zone, sullpying the stripping solution discharged from the apparatus and absorbed with a decomposition gas of the crystal to the mixing zone, and supplying a filtrate after the crystal separa­ tion to the settling zone for stripping solution.

Description: This invention relates to a metal stripping system wherein an organic solvent, containing extracted metal ions, is mixed and contacted with a fluoride series stripping solution (an aqueous solution containing one or more of HF, NH₄HF₂, NH₄F; see Japanese Patents laid open No. 57-42,545 (EP-A-0046973), No. 57-73,138, No. 57-73,141 and No. 57-85,943 (EP-A-0052354)). to deposit crystals of a fluorinated metal complex and then the organic solvent, stripping solution and deposited crystals are individually recovered, and to a process for operating such a system. Recently solvent extraction has become noted as a method for obtaining high-purity metal or metal oxide because it involves a simple purification step and requires small energy consumption. Metals which can, be extracted include Mg, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Cd, Nb, Zr, Ta, Mo, W, In and the like. In conventional solvent extraction methods, the stripping (back-extraction) of the extracted metal ion has experienced problems, which have been solved by the use of a fluoride series stripping solution Further, there has been proposed a crystallization apparatus of an inverted frustocone structure for use in the solvent extraction in Japanese Patent laid open No. 58-81,402 (EP-A-0079246). However, such a crystallization apparatus has been used only on a laboratory scale and has not yet been put to practical use on an industrial scale. The inventors of the present invention have made various studies in order to industrialise the crystallisation apparatus disclosed in Japanese Patent laid open No.58-81,402 (EP-A-0079246) and were the first to develop an industrial scale apparatus having a crystal production capability of 45 tons/month and being able to operate continuously for a long period of time. This work culminated in EP-A-0 157 562. However, when using such an apparatus, the nature and position of the chemicals fed to the circulating flow of the fluoride series stripping solution tend to give rise to troubles such as deposition of crystals inside the pipes or the pump, clogging of the pipes or pump and the like. In the comparable metal separation system shown in Fig.7 of the accompanying drawings, the above troubles are often produced during operation, as mentioned in the following comparative example, and this requires the operation of the system to be stopped. It is an object of the present invention to provide a metal stripping system in which the required chemicals are fed to the system at a position such that clogging of the system by crystals is minimised so that continued operation can be carried out for as long as possible. Accordingly a first aspect of the present invention provides a process of metal stripping which comprises (i) introducing, to a mixing zone, a fluoride series stripping solution and an organic solvent containing extracted metal ions so that the solvent and solution are contacted together and the metal ions form a fluorinated metal complex; (ii) allowing the mixed organic solvent and stripping solution to settle in settling zones; (iii) precipitating the fluorinated metal complex as crystals; (iv) cooling the resultant mixture of crystals and mother liquor in a cooling zone; and (v) recovering the crystals as a metal or a metal oxide, characterised in that the stripping solution is recycled and the concentration of NH₄HF₂ and HF in the stripping solution discharged from the settling zone is adjusted by (vi) supplying an aqueous solution of NH₄HF₂ to the stripping solution cooling zone; (vii) discharging stripping solution from the settling zone, contacting it with a gas produced by thermal decomposition of fluorinated metal complex crystals, and returning it to the mixing zone; and (iix) supplying the mother liquor, after the separation of the crystals therefrom, to the settling zone for the stripping solution. According to a second aspect of the present invention, there is provided a metal stripping system comprising (i) a crystallisation apparatus for metal stripping and comprising an upper portion and a lower portion of inverted frustoconical shape, the apparatus including (a) in the upper portion a mixing zone for contacting an organic solvent containing extracted metal ions with a fluoride series stripping solution so that the metal ions form a fluorinated metal complex and settling zones disposed around the mixing zone for receiving organic solvent and stripping solution respectively from the mixing zone, (b) a zone below the stripping solution settling zone for precipitating crystals of the fluorinated metal complex from the stripping solution, and (c) in the lower portion a stripping solution cooling zone below the crystal precipitating zone for growing the crystals; (ii) a tank connected so as to receive the crystals and mother liquor from the apparatus; and (iii) a liquid-solid separation device for receiving the crystals and mother liquor from the tank and separating the crystals from the mother liquor characterised in that the system includes: (iv) a device, connected so as to receive crystals from the liquid-solid separation device, for drying and thermally decomposing the crystals; (v) an absorbing device to receive and absorb the decomposition gas produced in the drying and decomposing device; (vi) a device for supplying an aqueous solution of NH₄HF₂ to the crystallisation apparatus; and (vii) a device for measuring the concentration of the stripping solution, wherein said decomposition gas absorbing device is connected so as to receive stripping solution from the stripping solution settling zone of the apparatus, contact it with the decomposition gas and return it to the mixing zone; said NH₄HF₂ solution supply device is connected to the apparatus so as to supply NH₄HF₂ solution to the stripping solution cooling zone in the lower portion of the apparatus; and said liquid-solid separation device is connected to the apparatus so as to return said mother liquor to the settling zone for the stripping solution in the apparatus. A diluted solution of an alkyl phosphoric acid or carboxylic acid extractant with n-paraffin can be used as the organic solvent for extracting the metal ions, while an aqueous solution containing one or more of NH₄HF₂, HF and NH₄F is used as the fluoride series stripping solution. Particularly, according to the invention, the NH₄HF₂ solution and the fluorine series decomposition product and, if necessary, ammonia are supplied to the crystallisation apparatus at proper positions, respectively, and the concentration of the fluoride series stripping solution is preferably adjusted on the basis of two items viz total HF content and total HH₄F content. In the case where the metal ions are Fe³⁺, the concentration of the stripping solution discharged from the settling zone should be adjusted so that the solution contains at least 85g/ℓ NH₄HF₂ (preferably from 85 to 115g/ℓ NH₄HF₂) and not more than 10g/ℓ HF. The solvent extraction method using the apparatus of Japanese Patent laid open No.58-81402 consists mainly of extraction and stripping (back-extraction) stages. At these two stages, the working temperature is usually adjusted to be relatively high, or to be not excessively low, in order to enhance the extraction and back-extraction efficiencies. If the working temperature is too low, the viscosity of the organic solvent increases and the handling thereof becomes somewhat difficult, and also the extraction or back-extraction efficiency and rate become lower. Particularly, as the stripping temperature in the metal stripping step becomes higher, the back-extraction rate of the metal ion increases and also the crystallisation conditions for the resulting fluoride compound become advantageous. On the other hand, if the working temperature for the organic solvent is too high, deterioration of the organic solvent becomes a problem. That is, the deterioration of the organic solvent results from a certain chemical change or the like and becomes apparent as a reduction of metal extraction rate or the like. Especially, when the organic solvent is continuously contacted with a strong acid or stripping solution, the deterioration is caused by hydrolysis or the like even at room temperature. Also, the degree of deterioration becomes more conspicuous as the contacting temperature rises. In this connection, Japanese Patent laid open No.55-18,512 merely discloses that the deterioration of organic solvent for the extraction of mineral and (consisting of a neutral phosphate ester extractant and an aromatic hydrocarbon diluent) is suppressed by restricting the working temperature to not more than 30 o C. In accordance with a particularly preferred embodiment of the present invention, the temperature is controlled so as to prevent deterioration of the organic solvent without impairing the efficiency of extraction. More particularly, the temperature of the organic solvent supplied to the apparatus is kept at 20-25 o C, the liquid temperature during the mixing of the organic solvent and the stripping solution is kept below 40 o C, and the temperature of the stripping solution in the cooling zone is kept at 15-20 o C. Particularly, the temperature of the organic solvent is adjusted to be 25-40 o C inside the mixing and settling zones of the crystallisation apparatus and 10-30 o C elsewhere inside the crystallisation apparatus, as well as in the pipes and tanks. For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, wherein: Fig. 1 is a flow diagram for a system for metal stripping by solvent extraction using a circulating fluoride series stripping solution; Fig. 2 is a schematic sectional view of an embodiment of a crystallisation apparatus for carrying out the metal stripping (back-extraction) step of the flow diagram of Fig. 1 as applied to iron stripping; Figs. 3 to 6 are graphs showing solubility curves of (NH₄)₃FeF₆ in fluoride series stripping solutions containing 75-150 g/ℓ of NH₄HF₂ and 0-20 g/ℓ of HF, respectively; Fig. 7 is a schematic view of a comparative apparatus known to the inventors for carrying out metal stripping in accordance with the flow diagram of Fig. 1; Figs. 8 and 9 are schematic views of apparatus according to the present invention for carrying out metal stripping in accordance with the flow diagram of Fig. 1; Fig. 10 is a graph showing the relationship between the liquid temperature during the mixing of the two phases and the Fe³⁺ extraction rate in the organic solvent after the mixing when carrying out iron stripping in accordance with the flow diagram of Fig. 1; Fig. 11 is a graph showing the relationship between the temperature of the organic solvent during the mixing and contacting with an aqueous solution of nitric-hydrofluoric acid and the Fe³⁺ extraction rate after the contacting when carrying out iron stripping in accordance with the flow diagram of Fig. 1; Figs. 12 to 14 are neutralization titration curves (differential type) for the fluoride series stripping solution by means of a potentiometer titration device, respectively, which show measurement results on stripping solutions containing 100 g/ℓ of NH₄HF₂ and a varied amount (0, 10 and 20 g/ℓ ) of HF when carrying out metal stripping in accordance with the flow diagram of Fig, 1; and Fig. 15 is an equiconductivity curve, as measured by an electromagnetic conductivity meter, of the fluoride series stripping solutions at 25 o C when carrying out metal stripping in accordance with the flow diagram of Fig. 1. The invention will be described in detail with reference to the drawings illustrating, by way of example, the case of Fe³⁺ extraction, but it is not intended as a limitation to the metal ion and systems shown in the illustrated embodiments. Fig. 1 is a flow diagram of the metal stripping system including the circulation step of the organic solvent and fluoride series stripping solution as used according to the invention. As shown in Fig. 1, an aqueous solution A containing metal ions is first mixed and contacted with an organic solvent S in a metal extraction step 1 whereby the metal ions are extracted from the aqueous solution A into the organic solvent S. The aqueous solution left after step 1 is a raffinate R, which is a recovered acid if A is a metal containing waste acid. When the metal ion in the aqueous solution A is Fe³⁺ and the organic solvent S contains an ion exchange type extractant such as alkyl phosphoric acid or the like (which is abbreviated as HR hereinafter), the metal extraction is expressed by the following reaction formulae: Fe³⁺ + 3HR → FeR₃ + 3H⁺ (1) FeF₂⁺ + 3HR → FeR₃ + H⁺ + 2HF (2) FeF₂⁺ + HR → FeF₂R + H⁺ (3) The reaction formulae (2) and (3) occur when using an iron-containing nitric-hydrofluoric acid waste liquid as the aqueous solution A, wherein Fe³⁺ in the solution A mainly takes the ionic form FeF₂⁺. The organic solvent S containing the extracted metal ion is charged from step 1 to a metal stripping (back-extraction) step 2 where the organic solvent S is mixed and contacted with a warmed fluoride series stripping solution D to convert the metal ion into a fluorinated metal complex crystal X. When the metal ion is Fe³⁺ and the stripping solution D is an aqueous solution consisting mainly of NH₄HF₂, the stripping (back-extraction) is expressed by the following reaction formulae: FeR₃ + 3NH₄HF₂ → 3HR + (NH₄)₃FeF₆ ↓ (4) FeF₂R + 3NH₄HF₂ → HR + (NH₄)₃FeF₆ ↓ + 2HF (5) FeR₃ + 3NH₄HF₂ + 3NH₄F → 3NH₄R + (NH₄)₃FeF₆ ↓ + 3HF (6) After the stripping of the iron, a part of the extractant is in an ammonia form (NH₄R) as shown in reaction formula (6). The stripping solution D containing the crystals X is discharged from step 2 and fed to a liquid-solid separation step 3 where the crystals X are separated from the stripping solution. After the separation of the crystals, the mother liquor is again circulated to the stripping step 2 for re-use. The separated crystals X are fired in a drying-decomposition step 4 to produce a metal M or a metal oxide O. The metallic iron or iron oxide is produced from the fluorinated iron complex crystal (NH₄)₃FeF₆ by the following reaction formulae: (NH₄)₃FeF₆ + 3/2H₂ → 3NH₄F + 3HF + Fe (7) (NH₄)₃FeF₆ + 3/4O₂ → 3NH₄F + 3/2F₂ + 1/2Fe₂O₃ (8) A fluorine series decomposition gas G comprising HH₄F, HF, F₂ and the like, which is generated from step 4, is returned to the stripping solution D for reuse. The organic solvent S, containing the extractant in ammonia form, is introduced from step 2 into a solvent conversion step 5 where it is contacted with an aqueous hydrochloric acid solution C to convert the ammonia form into a hydrogen form (HR) by the following reaction formula (9), which is returned into step 1 for re-use: NH₄R + HCl → HR + NH₄Cl (9) The aqueous hydrochloric acid solution C containing NH₄Cl is fed from step 5 to an ammonia recovering step 6 where it is subjected to neutralization and distillation to recover an ammonia water E according to the following reaction formula: NH₄Cl + NaOH → NH₄OH + NaCl (10) The recovered ammonia water E (ammonia liquor) is returned into the stripping solution D for re-use, if necessary. In connection with the concentration and composition of the fluoride series stripping solution D, the characteristic curve for the solubility of the fluorinated metal complex will be described taking the case of (NH₄)₃FeF₆ when the metal is Fe³⁺. Fig.2 shows an embodiment of a crystallisation apparatus 10 used in the iron stripping step 2. The organic solvent S containing extracted Fe³⁺ is introduced into a mixing zone 13 in the upper portion of the apparatus 10 through a temperature regulator 11, while the fluoride series stripping solution D containing NH₄HF₂ or the like is introduced into the mixing zone 13 through a heating device 12. In the mixing zone 13, an iron-fluorine complex (NH₄)₃FeF₆ is produced by the iron stripping reaction as shown in the formulae (4), (5) and (6). After the iron stripping, the organic solvent is discharged via an outlet 15 from a settling zone 14. Since the iron-fluorine complex has relatively small solubility in the stripping solution, it deposits as crystals during the continuation of the stripping. The stripping solution containing the resulting crystals descends downwards through a downpipe 16 and arrives at a stripping solution cooling zone (or a crystal growth zone) 17 in the frustoconical lower portion of the apparatus 10. This zone 17 is connected to a heat exchanger 18 and the like through pipes, whereby the warmed stripping solution is cooled to promote the growth of the crystals. If necessary, a forced circulation zone 19 is formed inside the zone 17 and connected to an external circulating pump so as to give fluidity to the stripping solution, whereby the adhesion of crystals to the inner wall of the apparatus and the accumulation and agglomeration of crystals at the bottom of the zone 17 can be prevented. The stripping solution is finally discharged from an outlet 21 through a crystal separation zone 20. On the other hand, the deposited crystals are properly discharged from an outlet 22 as a slurry, which is then subjected to liquid-solid separation. The solubility curve of (HH₄)₃FeF₆ in the fluoride series stripping solution when operating the apparatus of Fig. 2 is shown in Figs. 3 to 6, wherein the concentrations of NH₄HF₂ and HF in the stripping solution are varied over the ranges 75-150 g/ℓ and 0-20 g/ℓ, respectively. When Fe³⁺ is stripped at the stripping step 2 according to the formula (4), NH₄HF₂ in the stripping solution D is consumed causing a decrease in the NH₄HF₂ concentration. On the other hand, when the aqueous solution A containing the metal ion is an iron containing nitric-hydrofluoric acid waste liquid and Fe³⁺ is stripped according to formula (5), not only does the NH₄HF₂ concentration of the stripping solution D decrease, but also the HF concentration increases because F⁻ transported from the organic solvent to the stripping solution forms HF. In any case, as is apparent from the solubility curves, the solubility of (NH₄)₃FeF₆ increases with a decrease in NH₄HF₂ concentration in the stripping solution D. This tendency becomes particularly conspicuous as the HF concentration increases. Thus, the NH₄HF₂ concentration decreases with the progress of the metal stripping reaction, and when the metal ion is extracted as a fluoride ion, the HF concentration increases. Therefore, in order to circularly re-use the stripping solution, it is necessary to adjust the concentration and composition of the stripping solution. If the cooling of the stripping solution is caused outside the crystallization apparatus, this is likely to produce unfavorable phenomenon for the operation of the apparatus such as deposition of (NH₄)₃FeF₆. Further, the stripping solution is cooled by atmospheric temperature in the cold season such as winter or the like or is at a cooled state when inside a pipe connected to the cooling zone even in the summer season. Therefore, the composition and concentration of the stripping solution need to be adjusted. The chemicals and the like to be supplied for adjusting the concentration and composition of the circularly re-used fluoride series stripping solution D are as follows. In order to increase the NH₄HF₂ concentration, there are two methods. One of these is the addition of an aqueous solution of NH₄HF₂ (the concentration is usually 30-40%) J and the other of which is the absorption of fluorine series decomposition gas G produced according to formulae (7) and (8) by the stripping solution in a suitable manner. Since the composition of the decomposition gas G produced according to formulae (7) and (8) substantially corresponds to the composition of NH₄HF₂, i.e. NH₄F+HF, the absorption of the decomposition gas G has substantially the same effect as the addition of the aqueous NH₄HF₂ solution J. Further, in order to decrease the HF concentration, it is sufficient to add ammonia water E or NH₃ gas to neutralize HF. The addition of ammonia E also contributes to the increase of NH₄HF₂ concentration as shown by the following equation: HF + 1/2NH₃ → 1/2NH₄HF₂ (11) As mentioned above, the addition of the aqueous NH₄HF₂ solution J or the absorption of the fluorine series decomposition gas G increases the NH₄HF₂ concentration in the stripping solution D, while the addition of ammonia water or NH₃ gas E decreases the HF concentration and increases the NH₄HF₂ concentration in the stripping solution D. In any case, the supply of these chemicals acts to reduce the solubility of the fluorinated metal complex in the stripping solution. Therefore, when the chemicals and the like for the concentration adjustment are supplied to the stripping solution D having an increased concentration of fluorinated metal complex as the metal stripping reaction progresses, the solubility of the fluorinated metal complex rapidly reduces to immediately deposit the fluorinated metal complex crystals X. As a result, when the location at which the chemicals are supplied is inappropriate, undesirable deposition of the crystals is caused to produce the aforementioned troubles such as clogging of the pipes and the like. The supply location of the chemicals for the concentration adjustment will now be described with reference to the comparative metal stripping system as shown in Fig.7 as well as the metal stripping systems according to the present invention as shown in Figs.8 and 9. In these Figures, parts corresponding to parts of Fig.2 are denoted by like reference numerals. The aqueous NH₄HF₂ solution J (30-40%) is added to the lower portion of the crystallisation apparatus 10. As shown in Figs. 8 and 9, this supply position 73 corresponds to that region of the crystallisation apparatus 10 where the fluorinated metal complex crystals X gradually grow. Therefore, the addition of the solution J at the position 73 causes crystal growth as a result of the concentration gradient of the stripping solution D as well as the temperature gradient thereof, whereby the crystal growth is accelerated. On the contrary, undesirable crystal deposition occurs at positions other than the position 73. In the case of the system of Fig.7, for example, the solution J is supplied to an absorbing liquid tank 62 in an absorbing device 60 for decomposition gas, but in this case the crystals X are deposited in the absorbing liquid tank 62 which results in the adhesion and clogging of the crystals X inside the absorbing liquid pump 63 and the decomposition gas absorbing tower 61 or the pipes connected therebetween. If the crystal receiving tank 30 is used as the supply position of the solution J, the mother liquor discharged from the crystal receiving tank 30 is supplied to settling zone 24 for stripping solution located beneath the settling zone 14 for organic solvent in the crystallization apparatus 10, so that the effect obtained by adding the solution J to the tank 30 is inferior compared to the case where the solution J is added to the lower portion of the apparatus 10. Ammonia E may be added as ammonia water or NH₃ gas, if necessary. Preferably, ammonia liquor recovered from the ammonia recovering step 6 of Fig. 1 is used as the ammonia water. It is desirable that the ammonia is supplied to the lower portion of the crystallization apparatus 10 as shown in Fig. 8 or to the crystal receiving tank 30 as shown in Fig. 9. The reason why the ammonia E is supplied to the lower portion of the apparatus 10 is the same as in the case of adding the aqueous NH₄HF₂ solution J. On the other hand, when the ammonia E is supplied to the crystal receiving tank 30, the amount of HF in the stripping solution decreases as a result of neutralization, while the NH₄HF₂ increases by an amount corresponding to the decreased amount of HF so that crystals X are further deposited. Further, the mother liquor after the liquid-solid separation is suitable for use as a stripping solution to be supplied to the settling zone 24 of the apparatus 10 because the HF concentration is decreased. If the ammonia E is supplied at positions other than the above two positions, undesirable crystal deposition still occurs. For instance, the NH₃ gas E is supplied to the decomposition gas absorbing tower 61 in the absorbing device 60 for the decomposition gas in the comparative system of Fig.7, but in this case unfavourable deposition, adhesion and clogging of the crystals X are caused. A situation occurs which is similar to that occurring when the aqueous NH₄HF₂ solution J is added to the absorbing liquid tank 62. Further, the fluorine series decomposition gas G is absorbed into the stripping solution discharged from the outlet 21 of the crystallisation apparatus 10 at the decomposition gas absorbing tower 61 and is then supplied to the mixing zone 13 of the apparatus 10. Even in the above preferred method there is a possibility of the crystals X depositing in cold conditions due to the increase in the NH₄HF₂ concentration in the discharged stripping solution. This possibility can be avoided by maintaining the concentration of NH₄HF₂ and HF in the discharged stripping solution within certain ranges. According to Example 2 as mentioned later, the concentration of the stripping solution to ensure no deposition of (NH₄)₃FeF₆ crystals X at the outside of the crystallization apparatus under cold conditions is not less than 85 g/ℓ as NH₄HF₂ and not more than 10 g/ℓ as HF in the case where the waste acid is nitric-hydrofluoric acid. These concentration conditions correspond to a temperature gradient of not more than about 0.15 g/ℓ/°C at the low temperature region (not more than 20°C) of the solubility curve shown in Figs. 3 to 6. For instance, when using an organic solvent consisting of 30 V/V % of di-(2-ethylhexyl) phosphoric acid (hereinafter abbreviated as D2EHPA) and 70 V/V % of n-paraffin, the extracted Fe³⁺ concentration in the organic solvent is within a range of about 12-20 g/ℓ. Furthermore, when the flow ratio of organic solvent S (organic phase) to stripping solution D (aqueous phase) in the supply to the crystallization apparatus, which is hereinafter referred to as O/A, is varied within the range of 1/3 to 2/3, the decrease of the NH₄HF₂ concentration in the stripping solution by the back-extraction reaction is within the range of 12.2-40.7 g/ℓ, so that the varying range of the NH₄HF₂ concentration is about 30 g/ℓ. Since the concentration range of NH₄HF₂ in the stripping solution discharged from the crystallization apparatus should be from 85-115 g/ℓ for Fe³⁺ (see Example 2), it is sufficient to adjust the NH₄HF₂ concentration of the stripping solution contained in the crystallization apparatus within a range of 100±15 g/ℓ. In order to obtain a NH₄HF₂ concentration of more than 115 g/ℓ, the concentration to be supplied to the apparatus must be more than 130 g/ℓ, a concentration which is difficult to maintain economically. Fig. 10 shows the relationship between the liquid temperature during the mixing of the two phases in the back extraction step (step 2) and the Fe³⁺ extraction rate in the organic solvent as shown in the following Example 3. As is apparent from Fig. 10, the Fe³⁺ extraction rate after the mixing becomes lower as the liquid temperature during the mixing becomes higher, which tends to deteriorate the organic solvent. Assuming that practicable four-stage mixer-settlers are used for iron extraction and the Fe³⁺ extraction is performed at the same extraction rate in every stage, if the Fe³⁺ extraction rate at each stage is not less than 68%, the overall Fe³⁺ extraction rate is more than 99%. In this connection, the liquid temperature corresponding to a Fe³⁺ extraction rate of 68% is 40°C, so that it is desirable to adjust the liquid temperature during the mixing of the two phases to be not more than 40°C. Moreover, the temperature of the organic solvent to be supplied to the apparatus is adjusted so as to obtain the desired liquid temperature during the mixing. The temperature of the organic solvent supplied is preferably from 20-25°C considering that it needs to be within a range of 25 to 40°C, in the mixing zone and that the range over which it can be adjusted by warming or cooling is 20°C. Therefore, in this case the temperature of the stripping solution to be supplied to the apparatus in a flow ratio (O/A) of 1/3-2/3 is within a range of 45-53°C. Furthermore, the temperature of the stripping solution in the cooling zone is desirably held within a range of 15-20°C considering that the temperature adjusting range is within 20°C with respect to the atmospheric temperature (0-35°C). On the other hand, the back-extraction velocity and rate of Fe³⁺ extraction become higher as the liquid temperature in the mixing chamber becomes higher so that the liquid temperature is maintained near 40°C as mentioned above. Therefore, the temperature distribution in the crystallization apparatus is held substantially constant throughout the year and the composition and concentration of the stripping solution are adjusted as previously mentioned, so that stable crystal deposition occurs according to the illustrated solubility curves and the continuous service of the apparatus is easily performed for a long period of time. Fig. 11 shows the relationship between the temperature of the organic solvent during mixing with an aqueous solution of nitric-hydrofluoric acid in the metal extraction step (step 1) and the Fe³⁺ extraction rate in the organic solvent as shown in the following Example 4, from which it is apparent that when the organic solvent temperature is more than 35°C, the reduction of the Fe³⁺ extraction rate and hence the deterioration of the organic solvent becomes conspicuous. Therefore, it is desirable that the organic solvent temperature is not more than 30°C during the steps other than the metal stripping step (step 2). Furthermore, Example 5 shows the results of Fe³⁺ extraction from hydrochloric acid at low temperatures with the same organic solvent as in Examples 1 and 4. When the temperature is less than 10°C, the viscosity of the organic solvent containing extracted Fe³⁺ becomes higher, and a third phase is formed in the settler part of the Fe³⁺ extraction mixer-settler. Therefore, it is desirable that the organic solvent temperature is not less than 10°C, and the preferable temperature range is 10-30°C during the steps other than the metal stripping step (step 1.) As is apparent from the above, the working temperature of the organic solvent repeatedly used in the system should generally be within a range of 10-30°C bearing in mind the deterioration of the organic solvent at higher temperatures and the production of the third phase at lower temperatures but it is preferably in a range of 25-40°C in the mixing and settling zones of the crystallization apparatus itself bearing in mind the back-extraction efficiency and crystallization conditions. Preferably, the temperature of the organic solvent supplied to the crystallization apparatus is maintained at 20-25°C considering the atmospheric temperature. In this way the temperature condition inside the apparatus is substantially constant through the year whereby the operation of the metal stripping system can be stably performed. By setting the working temperature of the organic solvent for metal extraction in the above mentioned ranges, the deterioration of the organic solvent due to elevated temperature can be prevented without damaging the efficiency of the metal stripping system. According to the invention, the aqueous NH₄HF₂ solution, and optionally ammonia, are supplied to effective positions lying in a region extending from the lower portion of the crystallisation apparatus to the crystal receiving tank which region has no affect on the crystal deposition, while the fluorine series decomposition gas is absorbed into the discharged stripping solution for recycling to the mixing zone of the apparatus whereby the operation of the metal stripping step can continue smoothly as is apparent from the following Examples. The invention will now be described with respect to the metal stripping system. In Figs.7, 8 and 9 there are shown the fundamental structures of a metal stripping system operating in accordance with the flow diagram of Fig. 1. This system comprises the crystallisation apparatus 10 for metal stripping, the tank 30 for receiving the crystals, a liquid-solid separation device 40, a crystal drying and decomposition device 50, the absorbing device 60 for decomposition gas, a device 70 for supplying an aqueous solution of NH₄HF₂, a device 80 for supplying ammonia, and a device 90 for measuring the concentrations of the stripping solutions. As the crystallisation apparatus 10 for the metal stripping, use may be made of the apparatus disclosed in the previously mentioned Japanese Patent laid open No.58-81,402, but it is preferable to use a crystallisation apparatus for metal stripping of the inverted frustocone type provided at its lower portion with a forced circulation zone 19 for stripping solution in the manner shown in Fig.2, which is described in EP-A-0 157 562. As the liquid-solid separation apparatus 40, use may be made of the filter 42 shown in Figs. 8 and 9 or a centrifugal separator. As the crystal drying and decomposition device 50, an assembly 51 comprising a crystal drying kiln and a crystal decomposition kiln or various furnaces can be adopted. The decomposition gas absorbing device 60 comprises the decomposition gas absorbing tower 61, the absorbing liquid tank 62, the absorbing liquid pump 63, a stripping solution supply pump 64 and the like. This device 60 is connected to the stripping solution outlet 21 and to an inlet inside the mixing zone 13 in the crystallisation apparatus 10 through pipes and also the fluorine series decomposition gas G obtained from the crystal drying and decomposition kiln 51 is introduced into the device 60. The supply device 70 for aqueous NH₄HF₂ solution J comprises a tank 71, a pump 72, the outlet 73 and the like, wherein the outlet 73 is located at the lower portion of the crystallization apparatus 10 as shown in Figs. 8 and 9. The ammonia supply device 80 may be disposed according to the need, and comprises an ammonia water tank 81, a pump 82 and an outlet 85 as shown in Fig. 8 or an NH₃ gas holder 83, an NH₃ gas valve 84 and an outlet 85 as shown in Fig. 9. In this latter case, the outlet 85 opens into the lower portion of the crystallization apparatus 10 or into the crystal receiving tank 30. Since the deposition of the crystals X newly occurs near the outlet 73 for the aqueous NH₄HF₂ solution and the outlet 85 for ammonia, it is desirable that the stripping solution D is in a fluidized state at such areas. For this purpose, the forced circulation zone 19 is disposed in the lower portion of the crystallization apparatus 10 as previously mentioned, or a stirrer 31 is arranged in the crystal receiving tank 30. The systems of Figs. 8 and 9 are preferred embodiments of the invention and are not intended as limitations thereof. Next, the invention will be described with respect to the control of the concentration of the fluoride series stripping solution. As previously mentioned, the fluoride series stripping solution D for separating the fluorinated metal complex crystals X from the organic solvent S containing the extracted metal ions by contacting together during the metal stripping step 2 is an aqueous solution of NH₄HF₂ and HF. Since the composition of NH₄HF₂ is the same as NH₄F+HF, it is effective to control the concentration and composition of the stripping solution D in terms of the total HF (hereinafter abbreviated as T.HF) and the total NH₄F (hereinafter abbreviated as T.NH₄F). When the molar concentrations (mol/ℓ) of NH₄HF₂ and HF in the stripping solution D are a and b , respectively, there are established the following relationships: T.HF = a + b (12) T.NH₄F = a (13) The measurement of T.HF and T.NH₄F can be performed by a neutralization titration method using a potentiometric titration meter or by a combination of conductivity measurement using an electromagnetic conductivity meter with an analysis of the total fluorine (hereinafter abbreviated as T.F) using an ion electrode process as mentioned below. In Figs. 12 to 14 there are shown neutralization titration curves (differential type) of the stripping solution D (1 mℓ) with an aqueous solution of N/2 NaOH as measured by the potentiometric titration meter, , wherein Fig. 12 represents the case of a stripping solution having a (NH₄HF₂ concentration) = 1.75 mol/ℓ = 100 g/ℓ and b (HF concentration) = 0 mol/ℓ = 0 g/ℓ, Fig. 13 represents the case of a stripping solution having a = 1.75 mol/ℓ = 100 g/ℓ and b = 0.50 mol/ℓ = 10 g/ℓ, and Fig. 14 represents the case of a stripping solution having a = 1.75 mol/ℓ = 100 g/ℓ and b = 1.00 mol/ℓ = 20 g/ℓ. As seen from Figs. 12 to 14, two main peaks appear in each titration curve. When both the concentrations are calculated by assigning a region from the beginning of titration (0 ml) to the first peak and a region from the first peak to the second peak to T.HF and T.NH₄F, respectively, the calculated values are well coincident with the T.HF and T.NH₄F concentrations measured from a and b values in the equations (12) and (13). Therefore, the values of T.HF and T.NH₄F are measured from the neutralization titration curve, from which the values of a and b can be found by the following equations: a = T.NH₄F (14) b = T.HF - T.NH₄F (15) Fig. 15 shows equiconductivity curves based on values of the electric conductivity C in stripping solutions D having various values of a and b at 25°C as measured by the electromagnetic conductivity meter, wherein the abscissa is the T.HF value and the ordinate is the T.NH₄F value. The equiconductivity curves are slightly downward to the right. When the electric conductivity C is measured by the electromagnetic conductivity meter and the value of T.F is analyzed, for example, by an ion electrode process, the T.HF and T.NH₄F values of the stripping solution D can be measured as follows. For instance, when C = 160 mS/cm and T.F = 3.00 mol/ℓ, if normal lines are drawn from an intersection of the curve of C = 160 mS/cm with the line segment XY of T.F = 3.00 mol/ℓ to the abscissa and ordinate in Fig. 15, the values at the intersections with the co-ordinates correspond to the T.HF and T.NH₄F values of the stripping solution D. Strictly speaking, a fluorinated metal ammonium complex is dissolved in the stripping solution D. For example, when the metal ion is Fe³⁺, the stripping solution D is a solution of NH₄HF₂-HF-(NH₄)₃FeF₆. In general, however, the fluorinated metal ammonium complex has relatively small solubility in the stripping solution D, so that the dissolved amount of the fluorinated metal ammonium complex hardly influences the analysis and the measurement. Therefore, the stripping solution D can be considered as NH₄HF₂-HF solution in order to determine the T.HF and T.NH₄F values. As mentioned above, the concentration and composition of the fluoride series stripping solution can be controlled by measuring the values of T.HF and T.NH₄F according to any one of the aforementioned two methods. The neutralization titration method can directly measure the values of T.HF and T.NH₄F, so that it is very convenient but its measurement is intermittent. On the other hand, the combination of conductivity measurement and total fluorine analysis can continuously control the concentration and composition of the stripping solution because the conductivity measurement is continuous even if the total fluorine analysis is performed intermittently. In the system according to the invention as shown in Figs.8 and 9, the concentration and composition of the stripping solution D is measured at three locations i.e. a measuring location 91 for the supplied stripping solution, a measuring location 92 for the discharged stripping solution and a measuring location 93 for the mother liquor after the separation of the crystals. At these measuring locations, the concentration and composition of the stripping solution are properly determined within the controlling ranges while measuring the T.HF and T.NH₄F values, whereby the required chemicals may be supplied to the crystallisation apparatus as appropriate. As previously mentioned in detail, the concentration of the fluoride series stripping solution is controlled in terms of T.HF and T.NH₄F to enable the supply of the necessary chemicals at proper positions in the crystallisation apparatus whereby the stable operation of the metal stripping system can be realised over a long period while preventing the occurrence of troubles such as the clogging of pipes and the like. The following Examples are given by way of illustration of the invention and are not intended as limitations thereof. Comparative Example With the comparative metal stripping system shown in Fig. 7, the separation of Fe³⁺ extracted in an organic solvent consisting of 30 V/V % of D2EHPA and 70 V/V % of n-paraffin was carried out at a rate of 45 tons/month as crystals of (NH₄)₃FeF₆. In the crystallization apparatus 10 for metal stripping, the liquid temperature inside the mixing chamber 13 was adjusted to 30-40°C, and the liquid temperature inside the cooling zone 17 was adjusted to 15-20°C. Then, in order to adjust the concentration and composition of the stripping solution to 85-115 g/ℓ of NH₄HF₂ and 0-10 g/ℓ of HF at the measuring location 92, a 30% aqueous solution of NH₄HF₂ was added from the outlet 73 to the absorbing liquid tank 62. In 24 hours after the beginning of the addition, (NH₄)FeF₆ crystals deposited in the absorbing liquid tank 62 adhered to the insides of the pump 63 and the decomposition gas absorbing tower 61 to cause clogging thereof, resulting in the cessation of operations. Further, when NH₃ gas E was added to the decomposition gas absorbing tower 61 apart from the NH₄HF₂ solution J, clogging with (NH₄)₃FeF₆ crystals still occurred inside the pump 63 and the tower 61. Example 1 With the system according to the invention shown in Fig. 9, iron separation was carried out under substantially the same conditions as in the Comparative Example. In order to maintain the concentration and composition of the stripping solution D at the measuring location 92 within the same range as in the Comparative Example, the 30% aqueous solution of NH₄HF₂ was adequately supplied to the forced circulation zone 19 of the stripping solution cooling zone 17 of the crystallisation apparatus 10. Even after 30 days of operation, there was observed no deposition and clogging of (NH₄)₃FeF₆ crystals due to the addition of the NH₄HF₂ solution J. Apart from the NH₄HF₂ solution J, ammonia E was properly added to the lower portion of the crystallisation apparatus 10 as ammonia water (Fig.8) or to the crystal receiving tank 30 as NH₃ gas (Fig.9). Even in this case, the deposition and clogging of (NH₄)₃FeF₆ crystals due to the addition of ammonia E was not observed after 30 days of operation. Example 2 The separation of Fe³⁺ from nitric-hydrofluoric acid waste liquid was carried out in the crystallisation apparatus 10 for metal stripping having a (NH₄)₃FeF₆ crystal production capability of 45 tons/month as shown in Fig.2. In this case, an organic solvent S consisting of 30 V/V% of D2EHPA and 70 V/V% of n-paraffin and containing 14.9-19.5 g/l of the extracted Fe³⁺ was charged into the mixing chamber 13 at a rate of 2 m³/hr, while the stripping solution D containing 65-131 g/l of NH₄HF₂ and 0-20 g/l of HF was charged into the mixing zone 13 at a rate of 3.5 m³/hr (O/A=1/1.75). The temperature of the organic solvent supplied was adjusted to 20-23°C, and the temperature of the stripping solution supplied was adjusted to 45-50°C. Also, the temperature of the stripping solution in the cooling zone 17 was adjusted to 15-20°C. As regards the concentration and composition of the stripping solution discharged from the apparatus 10, the NH₄HF₂ content was 54-126 g/ℓ and the HF content was 0-25 g/ℓ. The depositing state of crystals inside the pipes connected to the cooling zone is shown in the following table. Run No. Concentration of stripping solution discharged Deposition of crystal inside pipes NH₄HF₂ (g/ℓ) HF (g/ℓ) 1 54 3 presence 2 90 0 none 3 104 8 nearly none 4 115 12 presence 5 85 21 some presence 6 104 25 some presence 7 94 13 presence 8 82 15 presence 9 87 8 nearly none 10 80 7 presence 11 110 3 none 12 126 0 none As apparent from the above table, there is hardly any deposition of crystals inside the pipes when NH₄HF₂ is not less than 85 g/ℓ and HF is not more than 10 g/ℓ. Example 3 The same organic solvent S as used in Example 1 (about 20°C) was mixed and contacted with a stripping solution D containing 125 g/ℓ of NH₄HF₂ (20-50°C) at a rate (O/A) of 1/3 by a batch process for 1 hour. Then, the extractant in the organic solvent was treated with an aqueous solution of HCℓ to convert the NH + / 4 form into the H⁺ form. Subsequently, the organic solvent containing the H⁺ form extractant was contacted with nitric-hydrofluoric acid waste liquid containing 32.2 g/ℓ of Fe³⁺ and 45 g/ℓ of HF at a rate (O/A) of 1.5 and the Fe³⁺ extraction rate was measured. As seen from the results of Fig. 10, the Fe³⁺ extraction rate became lower as the temperature of the stripping solution increased. Example 4 After the organic solvent having the same composition as in Example 1 and containing no metal ion was mixed and contacted with an aqueous solution of nitric-hydrofluoric acid (180 g/ℓ NHO₃+40 g/ℓ HF) at 20-45°C and a rate (O/A) of 1.5 by a batch process for 24 hours, it was contacted with the same nitric-hydrofluoric acid waste liquid as in Example 1 at a rate (O/A) of 1.5 and the Fe³⁺ extraction rate was measured. As seen from the results of Fig. 11, the Fe³⁺ extraction rate lowers considerably as the temperature of the organic solvent during the mixing becomes higher. Example 5 The same organic solvent as in Example 4 and hydrochloric acid waste liquid containing completely oxidized Fe²⁺ (Fe³⁺=90.5 g/ℓ, HCℓ=64.8 g/ℓ) were charged into a one-stage mixer-settler having a size of 100 mm × 100 mm × 200 mm (height) at flow rates of 5 ℓ/hr and 1 ℓ/hr, respectively and were mixed by means of a stirrer revolving at 800 rpm to perform Fe³⁺ extraction. This procedure was repeated three times by adjusting the liquid temperature during the mixing to 6.5°C, 10.8°C and 14.3°C on average, respectively. The formation of the third phase in the settler part was observed in the case of the liquid temperature of 6.5°C but was hardly observed at the liquid temperatures of 10.8°C and 14.3°C.

Claims: A process of metal stripping which comprises (i) introducing, to a mixing zone (13), a fluoride series stripping solution (aqueous solution containing one or more of HF, NH₄HF₂, NH₄F) and an organic solvent containing extracted metal ions so that the organic solvent and fluoride series stripping solution are contacted together and the metal ions form a fluorinated metal complex; (ii) allowing the mixed organic solvent and fluoride series stripping solution to settle respectively in a settling zone (14), for organic solvent and a settling zone (24) for stripping solution (iii) precipitating the fluorinated metal complex as crystals; (iv) cooling the resultant mixture of crystals and mother liquor in a cooling zone (17); and (v) recovering the crystals as a metal or a metal oxide, characterised in that the fluoride series stripping solution is recycled and the concentration of NH₄HF₂ and HF in this stripping solution is adjusted by (vi) supplying an aqueous solution of NH₄HF₂ to the cooling zone (17); (vii) discharging stripping solution from the settling zone (24) for stripping solution, contacting it with a gas produced by thermal decomposition of fluorinated metal complex crystals, and returning it to the mixing zone (13); and (iix) supplying the mother liquor, after the separation of the crystals therefrom, to the settling zone (24) for the stripping solution. A process according to claim 1 wherein the metal ions are Fe³⁺ ions and the concentration of the stripping solution discharged from the settling zone (24) for stripping solution is adjusted so that the solution contains at least 85 g/ℓ NH₄HF₂ and not more than 10 g/ℓ HF. A process according to claim 2 wherein the concentration of the stripping solution discharged from the settling zone (24) for stripping solution is adjusted to be from 85 to 115 g/ℓ NH₄HF₂. A process according to claim 1, 2 or 3 wherein the concentration of said fluoride series stripping solution is adjusted in dependence on the total HF and total NH₄F contents. A process according to any preceding claim wherein ammonia is supplied to the mixture of crystals and mother liquor. A process according to any preceding claim wherein the temperature of the organic solvent is adjusted to be 25-40 o C in the mixing and settling zones and to be 10-30 o C elsewhere. A process according to claim 6 wherein the temperature of the stripping solution in the cooling zone is from 15 to 20 o . A metal stripping system comprising (i) a crystallisation apparatus (10) for metal stripping and comprising an upper portion and a lower portion of inverted frustoconical shape, the apparatus including (a) in the upper portion a mixing zone (13) for contacting an organic solvent containing extracted metal ions with a fluoride series stripping solution (aqueous solution containing one or more of HF, NH₄HF₂, NH₄F) so that the metal ions form a fluorinated metal complex and settling zones (14,24) disposed around the mixing zone (13) for receiving organic solvent and stripping solution respectively from the mixing zone (13), (b) a zone (20) below the stripping solution settling zone (24) for precipitating crystals of the fluorinated metal complex from the stripping solution, and (c) in the lower portion a cooling zone (17) below the crystal precipitating zone (20) for growing the crystals; (ii) a tank (30) connected so as to receive the crystals and mother liquor from the apparatus; and (iii) a liquid-solid separation device (40) for receiving the crystals and mother liquor from the tank (30) and separating the crystals from the mother liquor characterised in that the system includes: (iv) a device (50), connected so as to receive crystals from the liquid-solid separation device (40), for drying and thermally decomposing the crystals; (v) an absorbing device (60) to receive and absorb the decomposition gas produced in the drying and decomposing device (50); (vi) a device (70) for supplying an aqueous solution of NH₄HF₂ to the crystallisation apparatus (10); and (vii) a device (90) for measuring the concentration of HF, NH₄HF₂ and NH₄F in the stripping solution, wherein said decomposition gas absorbing device (60) is connected so as to receive stripping solution from the stripping solution settling zone of the apparatus (10), contact it with the decomposition gas and return it to the mixing zone (13); said NH₄HF₂ solution supply device (70) is connected to the apparatus (10) so as to supply NH₄HF₂ solution to the stripping solution cooling zone (17) in the lower portion of the apparatus (10); and said liquid-solid separation device (40) is connected to the apparatus (10) so as to return said mother liquor to the settling zone (24) for the stripping solution in the apparatus (10). A system according to claim 8, which further comprises a device (80) connected so as to supply ammonia to said stripping solution cooling zone (17) of the crystallisation apparatus (10), to said crystal receiving tank (30), or to the system at any location between said stripping solution cooling zone (17) of the crystallisation apparatus (10) and said crystal receiving tank (30).

Inventors: HOSHINO, MINORU C/O CHIBA WORKS, I, KANJI C/O RESEARCH LABORATORIES, NAKAZATO, YOSHIO C/O TOKYO HEAD OFFICE KAWASAKI, UCHINO, KAZUHIRO C/O RESEARCH LABORATORIES, WATANABE, TOSHIO C/O RESEARCH LABORATORIES, YAMAMOTO, AKIRA C/O RESEARCH LABORATORIES
Applicants: KAWASAKI STEEL CORPORATION
Application year: 1985
Main classification: B01D9
IPC codes: C01G1, C22B3, B01D9, C01B9, B01D11