EP0331231A1 - Process for regenerating pickling solutions containing ZrF4 - Google Patents

Abstract

For the regeneration of ZrF4-containing pickling solutions by the addition of Na compounds, a process is proposed in which a batch of the pickling solution to be regenerated is analysed and heated, after which the amount of Na required for the regeneration is specifically determined and is mixed in with thorough stirring, and finally the mixture is cooled before the precipitated NaZrF compounds are separated off and a regenerated pickling solution is obtained. <IMAGE>

Description

The invention relates to a method for regenerating pickling solutions containing ZrF₄ 5 to 60 g / l HF 150 to 400 g / l HNO₃ 0.06 to 1.0 g / l so what 5 to 50 g / l Zr by adding Na compounds, precipitation of NaZrF compounds, separating the NaZrF compounds and supplementing HF, HNO₃ and possibly H₂O.

Such a method has become known from US Pat. No. 4,105,469, in which the used pickling solution is to be regenerated to a Zr content of 3 to 7 g / l by adding a sufficient amount of NaF. The precipitation of Na₂ZrF₆ is aimed at because NaZrF₅ is said to be gel-like and difficult to filter.

In European patent 0 035 804, the method known from the US patent is described as being relatively complex and difficult to handle. The relatively expensive precipitant NaF had to be added in crystalline form in order not to dilute the pickling solution too much. In addition, the process is very sensitive to under- and overdosing of the precipitant. If the residual NaF content was too high as a result of overdosing, there would be a risk that Na₂ZrF₆ would already precipitate out in the pickling bath, while gel-like NaZrF₅ x H₂O would be formed when underdosing, which is difficult to filter. It is therefore proposed to increase the used pickling solution to over 40 ° heat, add dissolved NaOH as precipitant, to separate the precipitated Na₂ZrF₆ by filtering after cooling to below 20 ° C. The addition of NaOH should be such that the completely regenerated pickling solution has a residual Na content of 1 to 3 g / l, preferably 1.5 to 2.5 g / l and a residual Zr content of 1.5 to 2, 5 g / l.

However, it has been found that the latter method is associated with very considerable difficulties in practice. This is mainly due to the fact that the regenerated pickling solution has a strong tendency towards oversaturation, which leads to uncontrolled precipitation and hard salt deposits in the entire system, especially in continuous operation. The process had to be interrupted after only a short period of operation because flow paths were blocked by deposits and had to be cleaned. An economical regeneration of the pickling solution could not be achieved.

It is therefore the task of modifying or developing the known method in such a way that the regeneration is not disturbed by uncontrolled precipitations as a result of oversaturation and that the regeneration can be carried out more economically overall.

• a) eine Charge der zu regenerierenden Beizlösung auf ihre Anfangsgehalte an HF_A, HNO_3A, Na_A und Zr_A analysiert und auf 35 bis 75°C erwärmt wird,
• b) die in der zu regenerierenden Beizlösung angestrebten Endgehalte Na_E und Zr_E vorgegeben und/oder berechnet werden und daß
• c) aus
  Na_Z = 0,3 (Zr_A-Zr_E) + (Na_E-Na_A)
  die erforderliche Menge an Na-Verbindung ermittelt wird, ferner
• d) der zu regenerierenden Beizlösung mindestens so viel HF zugegeben wird, daß die danach insgesamt vorhandene HF-Menge ausreicht, die zuzugebende Na-Verbindungs-Menge in NaF umzuwandeln, danach
• e) in die zu regenerierende Beizlösung die Menge Na-Verbindung unter intensivem Rühren eingemischt und schließlich
• f) die ausgefällten NaZrF-Verbindungen in an sich bekannter Weise abgetrennt und eine regenerierte Beizlösung erhalten wird.
  Index A: zu regenerierende Beizlösung
  Index E: regenerierte Beizlösung
  Index Z: Zugabe

To solve this problem, it is proposed to operate the aforementioned method in such a way that

• a) a batch of the pickling solution to be regenerated is analyzed for its initial contents of HF _A , HNO _3A , Na _A and Zr _A and heated to 35 to 75 ° C,
• b) the desired final contents Na _E and Zr _E in the pickling solution to be regenerated are specified and / or calculated, and that
• c) off
  Na _Z = 0.3 (Zr _A -Zr _E ) + (Na _E -Na _A )
  the required amount of Na compound is determined, further
• d) at least as much HF is added to the pickling solution to be regenerated that the total amount of HF present afterwards is sufficient to convert the amount of Na compound to be added into NaF, then
• e) the amount of Na compound is mixed into the pickling solution to be regenerated with vigorous stirring and finally
• f) the precipitated NaZrF compounds are separated off in a manner known per se and a regenerated pickling solution is obtained.
  Index A: pickling solution to be regenerated
  Index E: regenerated pickling solution
  Index Z: addition

Advantageous developments of the method are described in subclaims 2 to 24.

The method according to the invention is based on various insights gained in the investigation of incidents of the previously known method.

First of all, it was found that reliable information about the solubility of Zr and Na in the Pickling solution can only be obtained after the sample solutions have stood for a relatively long time, because stable equilibria in these solution systems are very slow. A satisfactory regeneration can therefore only be achieved if sufficient time is available, which is only possible with batch-wise regeneration and per se speaks against the continuous mode of operation which is considered advantageous in the prior art.

Another finding was the already mentioned strong tendency of the regenerated pickling solution to oversaturation. It often contains significantly more NaZrF compounds than the actual solvency, with the known effect in the case of supersaturations that small, often unidentifiable, disturbances can cause the unstable solution equilibrium to shift to a more stable phase, with precipitations that cannot be controlled in time and place, with all disadvantageous Consequences occur.

Finally, it was recognized that the formation of NaZrF₅ compounds does not necessarily lead to a poorly filterable gel and that one can control whether the predominantly NaZrF₅ or Na₂ZrF überwiegend compounds are formed by adjusting the temperature and the Na: Zr molar ratio. As the temperature rises, NaZrF₅ compounds predominantly precipitate out under the same conditions, while in the lower temperature range almost exclusively NA₂ZrF₆ compounds fail.

That being said, these two connections are not the only ones that form. There are probably a number of very complex compounds with a Na: Zr molar ratio between 1 and 2, the Na-rich Compound Na₂ZrF₆ with a molar ratio of 2 and the low-Na compound NaZrF₅ with a molar ratio of 1 form only partial complexes of the system. For example, the connection Na₇Zr₆F₃₁ was demonstrated.

In addition, the various NaZrF compounds are subject to constant conversion or reorientation, which can explain the inertia observed during precipitation or the tendency to oversaturation.

Through extensive studies with waiting times of several days until the steady state and addition of NaZrF salt to avoid oversaturation, it was determined how much Zr in a regenerated pickling solution with residual Na and acid contents of 5 mol / l HNO₃ and 1 mol / l HF can be solved during pickling. In addition to the residual Na content, the pickling temperature was varied. The following mathematical functions were determined from the found values by means of the compensation calculation Zr₂₀ = 4.72 x Na⁻⁰ʼ⁷⁴⁴ Zr₃₀ = 5.45 x Na⁻⁰ʼ⁷²⁰ Zr₄₀ = 6.08 x Na⁻⁰ʼ⁷²⁷ Zr₅₀ = 7.05 x Na⁻⁰ʼ⁷⁴⁶

These functions are shown as curves in FIG. 1. It can be seen that - in a first approximation - the product of the Na residue and Zr solubility gives a temperature-dependent constant, i.e. that the Zr solubility in all cases is greater the lower the residual Na content.

This dependency determined for the pickling process also applies at least qualitatively to the regeneration process. But while pickling at a very high level Zr solubility is interested, the residual Zr content in the regenerated pickling solution should be as low as possible in order to obtain a pickling solution with a high Zr absorption capacity. However, this would mean that the Na content in the regenerated pickling solution must be set relatively high in order to achieve the lowest possible initial Zr contents for the pickling. Precisely these considerations were the basis of the process according to European patent 0035 804, in which residual Na contents of 1 to 3 g / l and residual Zr contents of 1 to 2.5 g / l were aimed for. A further reduction in the residual Zr content can only be achieved with higher Na values, which is readily achievable by correspondingly higher addition of the regenerating agent, but on the other hand leads to an inadmissible Na enrichment in the regenerated pickling solution and the pickling side is negative influenced.

It follows that it is not possible to optimize the pickling and regeneration processes at the same time. Rather, one is forced to compromise and to find an acceptable setting for the residual Na and Zr contents in the regenerated pickling solution for both process sides.

Surprisingly, however, it has been shown that conditions can be considerably more favorable if the regeneration is carried out according to the invention. 2, in which the Zr content is again inversely proportional to the Na content, but assumes much lower values than were found for Zr saturation in the pickling process ( see Fig. 1).

It was found that when Na was added in an amount of up to 1.5 mol / molZr according to the invention The Na: Zr molar ratio in the precipitated salt is always approx. 1.2, the Zr precipitation rate and the residual Na content increasing in this range with increasing molar ratio. An accumulation of the residual Na content in the regenerated pickling solution can therefore be avoided if the Na addition is dimensioned such that the molar ratio - based on the amount of Zr to be precipitated - is always 1.2.

A salt with the empirical formula Na _1.2 ZrF _5.2 is formed in two stages by the process according to the invention.

The addition of Na compounds at a higher temperature and, if appropriate, the formation of NaF in the pickling solution to be regenerated initially precipitates predominantly lower-Na compounds with a total molar ratio Na: Zr less than 1.2. If the temperature is then lowered, the tendency to precipitate Na-rich compounds increases, but obviously no new crystals are formed predominantly, but the Na-poorer compounds are partially converted to Na-rich compounds by addition of NaF. For this second phase of the precipitation process, the initially poorly formed Na-poor compounds serve as precipitants for the NaF still present, whereby the Na residual content can be controlled by appropriate dosing of the Na compound addition and finally a salt with the Na: Zr - Total molar ratio of about 1.2 is obtained. This can also be calculated as a mixture of 20 mol% Na₂ZrF₆ + 80 mol% NaZrF rech.

However, the lower Na content in the regenerated pickling solution also means that the Zr content is much higher than they were previously thought to be optimal. Ultimately, however, the initial Zr content is less important than the dissolving capacity of the regenerated pickling solution, i.e. the difference between the Zr content after regeneration (Zr _E ) and the Zr content after exhaustion regeneration (ZR _A ).

Since the latter is higher, the lower the Na content, the process according to the invention is advantageously carried out with significantly lower Na but with a correspondingly higher Zr level than that provided for example in European Patent 0 035 804.

In Fig. 3 the Zr dissolving capacity, i.e. the difference between the values of FIGS. 1 and 2 is shown as a function of the Na content and the pickling temperature. It can be seen that in the preferred range from 0.1 to 0.6 g / l Na dissolving capacities from 4 to 30 g / l can be set, the regenerated pickling solution in any case containing less than 10 g / l Zr. In this way, the disadvantages of the known methods described at the outset can be overcome, i.e. on the one hand prevent uncontrolled precipitation during the pickling cycle and on the other hand work with a Na: Zr ratio that is only approx. 20% above the theoretical minimum of 1, which cannot be achieved for practical reasons.

During testing in the factory, it was found that the Zr solubilities in the regenerated pickling solution determined in the laboratory could be exceeded considerably (up to approx. 20 g / l Zr, depending on the temperature and composition of the pickling solution), since these supersaturations are relatively stable and can only be broken down under intensive precipitation conditions (stirring, salt charge). Ie that the reloadability of a regenerated pickling solution with Zr can be up to 20 g / l Zr higher than is obtained by calculating the difference from the values in FIGS. 1 and 2 and in accordance with FIG. 3. The Zr contents that can be achieved in the respective operating systems can be determined by regular analytical analysis of the pickling solution. A decreasing Na content indicates the beginning of NaZrF salt precipitation. The associated Zr content represents the limit that should not be exceeded.

In the process according to the invention, the Zr content belonging to the desired final Na content is determined using the equation
Zr _E = B - 3.8 ln Na _E
determined, where B is between 0 and 5.

This allows for the fact that conditions can rarely be set as precisely in operation as in laboratory and pilot plant trials. So vary. B. the HNO₃- and HF contents, fluctuations can occur in the dosing accuracy of the Na-containing precipitant, the stirring conditions and cooling ratios can not be transferred identically to large plants and salt residues not as completely removed as in the laboratory. These and other influences mean that the Na and Zr contents in the regenerated pickling solution can deviate from the values in FIG. 2. This deviation can be -1 to +4 g / l Zr.

To compensate for such deviations, the range from 0 to 5 is provided for B, and the specific value for a specific system or a specific procedure can be determined empirically. The theoretical curve can be measured by appropriate measurements Fig. 2 can be replaced by one tailored to the specific case.

The following exemplary embodiments serve to further explain the method according to the invention:
1. The first batch of used pickling solution was regenerated in a new system. This was naturally Na-free, was about 22 m³ and contained 39 g / l Zr, 250 g / l HNO₃ and 5 g / l HF. It was strengthened with concentrated nitric acid, whereby 26.5 m³ of solution with 317 g / l HNO₃, 4 g / l HF and 32 g / l Zr were obtained. This solution should be regenerated so that the Na content in the regenerated pickling solution is 0.1 g / l.
For a Na _E content of 0.1 g / l, a Zr _E content of 9.75 g / l results according to the specification of claim 2. This means that all values for determining the Na requirement are available, which in this case is 6.78 g / l.
When using 45% sodium blueberry as a precipitant, 694 kg of sodium hydroxide solution are required to regenerate the entire batch of 26.5 m³. The required RF requirements were also determined as a control. This is 6.78 x (20/23) = 5.9 g / l. Since the used pickling solution contains only 4 g / l HF, it had to be strengthened, at least by (5.9 - 4) x 26.5 = 50.4 kg. For operational reasons, the fortification was increased to 8 g / l HF.
The pickling solution prepared in this way was heated to 55 ° C. before the calculated amount of sodium hydroxide solution was added with vigorous stirring, the Temperature rose to 67 ° C. The mixture was then cooled to 35 ° C. with stirring and pumping over a heat exchanger and then filtered. About 2.1 t of moist salt or 1.4 t of dry salt with a Na: Zr molar ratio of 1.18 and 26.2 m³ of filtrate of the following composition were obtained: 10th g / l Zr 2nd g / l HF 0.2 g / l N / A 309 g / l HNO₃ The Zr content is about 3 g / l higher than in FIG. 2. The filtrate was supplemented with HF in accordance with the pickling requirements and was then available again as a regenerated pickling solution.
2. By using the regenerated pickling solution prepared in Example 1 in the pickling shop, which was sharpened according to the requirements with HF and HNO₃, 29 m³ of pickling solution with 325 g / l HNO₃, 12 g / l HF, 0.2 g were finally obtained / l Na and 28 g / l Zr. In order to determine the farm-specific values for establishing a curve corresponding to FIG. 2, a significantly higher Na _E content of 0.6 g / l was specified for the second precipitation. Using the regulations according to claim 1, a requirement of 885 kg of 45% sodium hydroxide solution was determined. It was not necessary to add HF before the precipitation. After treatment as in Example 1, a filtrate with 0.9 g / l Na and 4 g / l Zr was obtained. Compared to FIG. 2, the Zr content is accordingly about 2.5 g / l higher.
3. A regeneration comparable to Examples 1 and 2, with a Na _E content of 0.4 g / l resulted in a regrind with 0.6 g / l Na and 5 g / l Zr. The Zr content was accordingly about 2 g / l higher than in FIG.
4. Using the results from Examples 1 to 3, the following equation can be used for the existing system:
Zr _E = 3.5 - 3.8 lnNa _E (g / l).
On this basis, several batches were regenerated as in Examples 1 to 3, Na _{E in} each case being set at 0.4 g / l. Well reproducible results were obtained in all cases, namely
Na _E = 0.4 g / l (± 0.1 g / l)
Zr _E = 7 g / l (± 1 g / l)
Molar ratio in the precipitated salt:
Na / Zr = 1.2 (± 0.04)
The precipitated salts were easy to filter and dewater, so that only little adhering acid was carried away with the salt.
5. There were 27.5 m³ pickling solution with 0.15 g / l Na, 31 g / l Zr, 316 g / l HNO₃ and 9 g / l HF to regenerate. An Na _E content of 0.1 g / l was aimed for. For the precipitation, a requirement of 593 kg of 45% sodium hydroxide solution was calculated and added according to the plant-specific equation from Example 4. Before the precipitation, neither HF nor HNO₃ were added.
The temperature of the used pickling solution was 38 ° C before the precipitation and rose to 49 ° C during the precipitation. The mixture was then cooled to 19 ° C. with stirring and then filtered. About 1.8 t of moist or 1.3 t of dry salt with a Na / Zr molar ratio of 1.20 and about 27.2 m³ of filtrate of the following composition were obtained: 12 g / l Zr 4th g / l HF 0.11 g / l N / A 307 g / l HNO₃
6. In accordance with Example 4, used pickling solutions, each with 0.4 g / l Na (± 0.05 g / l) and 20 to 25 g / l Zr, were regenerated. The following precipitants were used instead of 45% sodium hydroxide solution:
15% sodium hydroxide solution
30% sodium hydroxide solution
45% aqueous NaNO₃ solution
fixed NaF
solid Na₂SO₄ x 10 H₂O.
In all cases, comparable results were obtained with Example 4.
7. In a further case according to Example 4, the precipitation was carried out at 30 ° C., the temperature in the pickling solution rising to 38 ° C. A slimy salt was precipitated, which was difficult to filter and to drain. Heating the suspension to 60 ° C. and then cooling did not result in any improvement either. The removal of the salt by filtration required about 10 times the time and the filter cake obtained contained significantly more adhering moisture (about 60% instead of 35% before). By raising the drop temperature to over 35 ° C, the aforementioned disadvantages could be avoided when regenerating the next batch.
8. In a further case, the pickling solution to be regenerated was heated to 75 ° C. before the precipitation, so that a temperature increase to approximately 85 ° C. resulted when sodium hydroxide solution was used as the precipitant. After the solution had been cooled to 39 ° C., positive results were achieved as in Example 4.
9. The filtrate from Example 5 was used after replenishing HF and HNO₃ as a regenerated pickling solution for pickling and was then used as a pickling solution of 29.0 m³ and the following composition: 41 g / l Zr 8th g / l HF 0.1 g / l N / A 316 g / l HNO₃ The Na content should again be 0.1 g / l after regeneration. According to the system-specific equation, a Na requirement of 8.6 g / l was calculated. The resulting HF requirement was 7.5 g / l and was contained in the pickling solution to be regenerated, so that no addition was necessary.
964 kg of sodium hydroxide solution (45% strength) were added. After the precipitation, about 3.2 t of moist or 2.1 t of dry salt with a Na / Zr molar ratio of 1.19 and about 28.4 m³ of filtrate were obtained, which had the following composition: 11.5 g / l Zr 1 g / l HF 0.1 g / l N / A 308 g / l HNO₃ This filtrate is available as a regenerated pickling solution after addition of HF and HNO₃ without other measures.

The examples show that with the quantification of the regenerating agents according to the invention and using the other measures according to the invention, a regenerated pickling solution can be obtained which, with regard to the residual Na content and the Zr dissolving capacity dependent thereon, is aimed specifically at desired values and thus also on the respective requirements of the Pickling processes can be set.

• Figur 1 zeigt die Zr-Löslichkeit in einer Beizlösung mit 5 Mol/l HNO₃ und 1 Mol/l HF beim Lösungsgleichgewicht.
• Figur 2 zeigt den Zr-Restgehalt der regenerierten Beizlösung nach der Salzabtrennung bei 20°C in Abhängigkeit vom Na-Gehalt (Laborversuche).
• Figur 3 zeigt die Zr-Lösekapazität der regenerierten Beizlösung in Abhängigkeit vom Na-Gehalt und von der Temperatur.

The disadvantages of known methods described at the outset are avoided, in particular the uncontrolled precipitation due to excess Na residual contents. Finally, the process is also very economical because only a little more precipitant is required than for the stoichiometric Na / Zr molar ratio of 1, which is not technically feasible.

• Figure 1 shows the Zr solubility in a pickling solution with 5 mol / l HNO₃ and 1 mol / l HF at solution equilibrium.
• FIG. 2 shows the residual Zr content of the regenerated pickling solution after the salt separation at 20 ° C. as a function of the Na content (laboratory tests).
• FIG. 3 shows the Zr dissolving capacity of the regenerated pickling solution as a function of the Na content and the temperature.

Claims (24)

1. Method for the regeneration of pickling solutions containing ZrFlösungen 5 to 60 g / l HF 150 to 400 g / l HNO₃ 0.06 to 1.0 g / l so what 5 to 50 g / l Zr by adding Na compounds, precipitation of NaZrF compounds, separating the NaZrF compounds and supplementing HF, HNO₃ and possibly H₂O, characterized in that a) a batch of the pickling solution to be regenerated is analyzed for its initial contents of HF _A , HNO _3A , Na _A and Zr _A and heated to 35 to 75 ° C, b) the desired final contents Na _E and Zr _E in the pickling solution to be regenerated are specified and / or calculated, and that c) off
Na _Z = 0.3 (Zr _A -Zr _E ) + (Na _E -Na _A )
the required amount of Na compound is determined, further d) at least as much HF is added to the pickling solution to be regenerated that the total amount of HF present afterwards is sufficient to convert the amount of Na compound to be added into NaF, then e) the amount of Na compound is mixed into the pickling solution to be regenerated with vigorous stirring and finally f) the precipitated NaZrF compounds are separated off in a manner known per se and a regenerated pickling solution is obtained.
Index A: pickling solution to be regenerated
Index E: regenerated pickling solution
Index Z: addition 2. The method according to claim 1, characterized in that the Na _E content is predetermined and the associated Zr _E final content
Zr _E = 1 - 3.8 ln Na _E
is calculated. 3. The method according to claim 1, characterized in that the Na _E content predetermined and the associated Zr _E content from
Zr _E = B - 3.8 ln Na _E
is calculated, where B is a value between 0 and 5 to be determined when the method is started up. 4. The method according to any one of claims 1 to 3, characterized in that a pickling solution with 5 to 40 g / l HF is regenerated. 5. The method according to any one of claims 1 to 3, characterized in that a pickling solution is regenerated with 0.06 to 0.8 g / l Na. 6. The method according to any one of claims 1 to 5, characterized in that the pickling solution to be regenerated is preheated to 35 to 65 ° C. 7. The method according to any one of claims 1 to 5, characterized in that the pickling solution to be regenerated is preheated to 45 to 55 ° C. 8. The method according to any one of claims 1 to 7, characterized in that the amount of pickling solution removed with the precipitated NaZrF compounds is 1 to 5% of the batch amount. 9. The method according to any one of claims 1 to 8, characterized in that the regenerated pickling solution is supplemented qualitatively and quantitatively to a full batch by adding HNO₃, HF and H₂O. 10. The method according to any one of claims 1 to 9, characterized in that the Na _E content in the regenerated pickling solution is predetermined at 0.06 to 0.9 g / l. 11. The method according to claim 10, characterized in that the Na _E content is predetermined at 0.08 to 0.8 g / l. 12. The method according to claim 10, characterized in that the Na _E content is predetermined at 0.2 to 0.6 g / l. 13. The method according to any one of claims 1 to 11, characterized in that NaOH is used as Na compound. 14. The method according to any one of claims 1 to 13, characterized in that an aqueous solution with 10 to 50% NaOH is used as the regeneration agent. 15. The method according to any one of claims 1 to 14, characterized in that the mixture is cooled to below 40 ° C with stirring. 16. The method according to claim 15, characterized in that the mixture is cooled to below 30 ° C. 17. The method according to any one of claims 1 to 15, characterized in that the mixture is cooled to 15 to 30 ° C. 18. The method according to any one of claims 1 to 17, characterized in that the separation of the precipitated NaZrF compounds is carried out in such a way that the stirring of the mixture is stopped, the precipitated NaZrF compounds can settle and the clear regenerated after a sufficient waiting time Pumped pickling solution from the sedimented NaZrF compounds. 19. The method according to claim 18, characterized in that the regenerated pickling solution is pumped off from the sedimented NaZrF compounds after 2 to 72 hours. 20. The method according to any one of claims 1 to 19, characterized in that instead of NaOH others, not fluorine-containing Na compounds can be used as regeneration agents. 21. The method according to any one of claims 1 to 20, but with the proviso that initially no HF is added (claim 1, feature d) and that NaF is used instead of NaOH as a regeneration agent. 22. The method according to any one of claims 1 to 21, characterized in that HNO₃ before the precipitation and HF - as far as necessary beyond the measure according to claim 1 d) - and optionally H₂O is added after the separation. 23. The method according to any one of claims 1 to 22, characterized in that the precipitation is accelerated by adding precipitated salts from previous batches. 24. The method according to claim 23, characterized in that the salt addition is 10 to 20 g / l.

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