Process of precipitating zirconium or hafnium from spent pickling solutions

A spent pickling solution containing a relatively small percentage of hydrofluoric acid and used for pickling zirconium or hafnium so as to be saturated with zirconium or hafnium fluoride, is treated by the addition thereto of sodium sulfate, Na.sub.2 SO.sub.4, to precipitate sodium zirconium or hafnium fluoride. The remaining solution is recycled for further pickling use, and may have fluoride concentration increased by the addition of calcium fluoride thereto resulting in the precipitation of calcium sulfate.

BACKGROUND OF THE INVENTION 
1. Field: 
The invention is in the field of chemical processing of a spent acid 
solution used for pickling a metal, in particular zirconium or hafnium. 
2. Description of the Prior Art: 
Zirconium and hafnium metals and alloys are normally conditioned, following 
production and before shipment to users, by a pickling procedure in a 
nitric acid bath containing a relatively small percentage of hydrofluoric 
acid. The spent pickle acid, saturated with zirconium or hafnium fluoride, 
is customarily sent to waste after being neutralized by the addition of 
lime. 
Proposals have been made heretofore for alleged commercially useful 
regeneration of the spent pickle liquor for reuse in the pickling circuit 
and, in some instances, for the recovery of useful by-products. 
Thus, in Megy et al., U.S. Pat. No. 4,105,469, the spent pickle liquor is 
regenerated by adding sodium fluoride (NaF), which, in the case of 
zirconium, precipitates sodium zirconium fluoride (Na.sub.2 ZrF.sub.6) out 
of the solution. After hydrogen fluoride (HF) and nitric acid (HNO.sub.3) 
are added to the residual solution to make up losses thereof, the 
regenerated solution is recycled for reuse in the pickling circuit. The 
precipitant by-product can be used in the making of zirconium-magnesium 
alloys or can be reduced to zirconium metal. 
To like effect is Fennemann et al., U.S. Pat. No. 4,330,342, which teaches 
precipitation of Na.sub.2 ZrF.sub.6 from a spent HF HNO.sub.3 pickle 
liquor by the addition of dissolved sodium hydroxide (NaOH) to such liquor 
after heating thereof, precipitation of the Na.sub.2 ZrF.sub.6 taking 
place after cooling of the so-treated liquor. 
Pansom, U.S. Pat. No. 4,738,747, teaches how such a spent pickle liquor 
resulting from the etching of zirconium metal or an alloy thereof can be 
regenerated for reuse in the etching circuit by the addition of 
appropriate amounts of hydrofluoric acid and a nitric acid following 
measurements and calculations indicative of the correct amounts, this 
being accomplished without the previous removal of dissolved zirconium 
from the spent solution. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, the normally waste pickle liquor 
is treated (for the recovery of a valuable commercial product and for the 
purification of the acid solution so that it can be recycled to the 
pickling tank) by the addition thereto of an effective amount of sodium 
sulfate, Na.sub.2 SO.sub.4. This results in the precipitation of sodium 
zirconium or hafnium fluoride. Such solution can be purified and increased 
in fluoride concentration by the addition thereto of an effective amount 
of calcium fluoride. The sulfate ions are precipitated as calcium sulfate 
(CaSO.sub.4). 
Advantages of the process are that it is possible to recycle the residual 
nitric acid pickle solution after adding make-up amounts of hydrofluoric 
and nitric acids, so that the need for neutralization and disposal as 
waste are eliminated, and the amount of hydrofluoric acid necessary to 
spike the nitric acid in the recycled acid solution is significantly 
reduced.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT 
As illustrated, a series of pickle tanks 10, here shown as four, are 
supplied with the usual zirconium or hafnium metal pickling acid (40% 
nitric acid and 3% hydrofluoric acid) from any suitable source of same. In 
this instance, zirconium metal or an alloy thereof is treated within these 
tanks in the usual manner well known in the art and the pickle solution, 
when spent, i.e., saturated with zirconium fluoride, is transferred to a 
storage tank 11, as by means of pumps 12, from where it is passed to a 
regeneration tank 13, as by means of a pump 14. Sodium sulfate, 
advantageously as a granulated solid, is introduced into tank 13 and mixed 
with the spent acid solution, as by means of a power mixer 15. 
Following precipitation of sodium hexafluoro zirconate in tank 13, both the 
precipitate and the residual acid solution are passed by a pump 16 into a 
series of filters 17 (here shown as two) from which the regenerated acid 
filtrate is passed to a recycle tank 18 and from there recirculated back 
into pickle tanks 10, as by means of a pump 19, following the introduction 
of calcium fluoride as a precipitant for calcium sulfate. 
Precipitation of calcium sulfate from the residual pickle acid solution by 
the addition of calcium fluoride increases the fluoride concentration of 
the nitric acid, which is advantageous. However, it may be found desirable 
to add make-up amounts of hydrogen fluoride and nitric acid as indicated 
before recycling to the pickle tanks. 
If there is a hafnium pickle circuit as well as a zirconium pickle circuit 
in the same plant, the spent solutions should be kept separate. 
EXAMPLE 
The process was carried out experimentally in the laboratory. A 25 
milliliter aliquot of the usual plant pickle acid was placed in a 100 ml. 
beaker containing a magnetic stir bar. As the solution was agitated, 0.500 
grams of Na.sub.2 SO.sub.4 was sprinkled over the surface. After ten 
minutes, the solution was filtered using No. 40 quantitative filter paper 
and the filtrate analyzed for metalic impurities. The results were 
compared with results of a similar analysis of the untreated acid. 
Using the analysis of the untreated acid, molar quantities of the zirconium 
and hafnium impurities were calculated. Based on this value, an equivalent 
quantity of Na.sub.2 SO.sub.4 was added to 50 ml of pickle acid as it was 
being agitated in a second 100 ml beaker. After ten minutes the solution 
was filtered, the filtrate and precipitate being retained for ICP 
analysis. Experimental date was as follows: 
______________________________________ 
TREATED 
25 ml. 50 ml. 
Pickle Acid 
Pickle Acid 
ANALYTE UNTREATED 0.5 g Na.sub.2 SO.sub.4 
0.300 g Na.sub.2 SO.sub.4 
______________________________________ 
Hf 200 ppm 16 ppm 110 ppm 
Zr 0.25% 100 ppm 0.13% 
Al 19 ppm 6 ppm 15 ppm 
Fe 137 ppm 36 ppm 30 ppm 
SO.sub.4 as S 
280 ppm 1.4% 0.4% 
______________________________________ 
Calculations were as follows: 
Zr: 2500 ug/ml (50 ml) = 1.25 .times. 10.sup.5 ug Zr 
##STR1## 
Hf = 200 ug/ml (50 ml) = 1 .times. 10.sup.4 ug Hf 
##STR2## 
Al = 19 ug/ml (50 ml) = 950 ug 
##STR3## 
Total metal inpurites=1.46.times.10.sup.3 u moles in 50 m. 
Need 1.46.times.10.sup.3 u moles Na.sub.2 SO.sub.4 for precipitation of 
metal impurities. 
These experimental results indicate that recovery of spent pickle acid is 
possible by the addition of sodium sulfate, which precipitates a majority 
of the metal impurities. When an excess of sodium sulfate was added, as 
illustrated by the data, 92% of the Hf, 96% of the Zr, and 70% of the Al 
were removed. Upon addition of only 49% of the required amount of sodium 
sulfate for total metal precipitation, 48% of the Zr, 45% of the Hf and 
21% of the Al was removed. The sulfur value was indicative of an 
incomplete reaction, which was not unexpected due to the limited reaction 
time (10 minutes). There should be minimal impact upon the reactivity of 
the pickle acid (Ka.sub.2 =1.2.times.10.sup.-2 for sulfuric acid) and, 
therefore, it can be utilized immediately upon filtration of the 
precipitate. 
Adding sodium sulfate to a warm solution would increase the solubility of 
the salt, and a subsequent cooling of the solution would promote 
precipitate formation. 
Whereas this invention is here illustrated and described with specific 
reference to an embodiment thereof presently contemplated as the best mode 
of carrying out such invention in actual practice, it is to be understood 
that various changes may be made in adapting the invention to different 
embodiments without departing from the broader inventive concepts 
disclosed herein and comprehended by the claims that follow.