EP0494563B1 - Process for electrowinning of zinc - Google Patents

Description

The present invention relates to the electroextraction of zinc in an acid medium, in particular in a sulfuric medium.

In the electroextraction of zinc carried out in acid sulfate medium, the presence of small amounts of metallic impurities (Ge, Sb, Ni, Co, As, ...) induces difficulties on the electrocrystallization process of zinc: decrease in Faradaic yield of electrocrystallization, stimulation of hydrogen evolution and redissolution of the zinc deposit. Thus, for example, for Ni or Co concentrations greater than 5 mg / l, the yield decreases rapidly after a stable incubation period the duration of which depends on the concentration of the impurity. The elements Ge and Sb have a particularly harmful effect on the yield, even at very low concentrations (approximately 0.1 ppm) and practically without an incubation period. In general, the drop in yield caused by an impurity goes hand in hand with a depolarization of the zinc electrode, after incubation in the case of nickel or cobalt, but almost immediate in the case of germanium.

• le plomb (E.J. FRAZER, J. Electrochem. Soc., 135, 1988, p. 2465)
• la gomme arabique (M. MAJA et al, Oberflache-Surface, 24, 1983, p. 234)
• la glu (D.J. MACKINNON et al, J. Appl. Electrochem., 17, 1987, p. 1129)
• la réglisse "liquorice" (T.J. O'KEEFEE et al, J. Appl. Electrochem., 16, 1986, p. 913)
• le 2-butyne-1,4-diol (M. SIDER et al, J. Appl. Electrochem, 18, 1988, p. 54)
• un molybdate (M.M. JAKSIC, Surf. Coat. Technol., 28, 1986, p. 113)
• le chlorure de tétrabutyl ou tétraéthyl ammonium (D.J. MACKINNON et al, J. Appl. Electrochem., 9, 1979, p. 603)
• un mélange d'alcool éthoxyacétylénique (HOCH₂C ≡ CCH₂OCH₂CH₂OH), de chlorure de triéthyl benzyl ammonium et de polyéthylèneglycol (Chr. BOZHKOV et al, Proceedings of the 7th European Symposium on Corrosion Inhibitors, Ferrare, Suppl. n° 9, 1990, p. 1211).

Work to remedy these difficulties is based on the use of additives in the electrolyte. In particular, the following additives have been studied:

• lead (EJ FRAZER, J. Electrochem. Soc., 135, 1988, p. 2465)
• gum arabic (M. MAJA et al, Oberflache-Surface, 24, 1983, p. 234)
• glue (DJ MACKINNON et al, J. Appl. Electrochem., 17, 1987, p. 1129)
• liquorice liquorice (TJ O'KEEFEE et al, J. Appl. Electrochem., 16, 1986, p. 913)
• 2-butyne-1,4-diol (M. SIDER et al, J. Appl. Electrochem, 18, 1988, p. 54)
• a molybdate (MM JAKSIC, Surf. Coat. Technol., 28, 1986, p. 113)
• tetrabutyl chloride or tetraethyl ammonium (DJ MACKINNON et al, J. Appl. Electrochem., 9, 1979, p. 603)
• a mixture of ethoxyacetylene alcohol (HOCH₂C ≡ CCH₂OCH₂CH₂OH), triethyl benzyl ammonium chloride and polyethylene glycol (Chr. BOZHKOV et al, Proceedings of the 7th European Symposium on Corrosion Inhibitors, Ferrara, Suppl. No. 9, 1990, p. 1211).

Ethoxyacetylene alcohol, which must be present in a high concentration, is not a commercial product. It also has the disadvantage of being consumed during electrolysis.

It has now been found that the conditions for electrocrystallization of zinc in the presence of metallic impurities (in particular germanium) can be stabilized by using as surfactant a compound comprising a perfluoroalkyl group linked to a hydrophilic polyoxyethylene group, amine- oxide or betaine.

dans lesquelles R_F représente un radical perfluoroalkyle contenant de 4 à 20 atomes de carbone, m est un nombre allant de 6 à 18, n est égal à 0 ou 2, p est égal à 2 ou 3, q est égal à 1 ou 2, X représente un groupe CO ou SO₂, R représente un atome d'hydrogène ou un radical alkyle contenant de 1 à 4 atomes de carbone, et R' et R'', identiques ou différents, représentent chacun un groupe alkyle contenant de 1 à 4 atomes de carbone.The surfactant compound according to the invention can be chosen from the known compounds of formula:

${\text{R}}_{\text{F}}{\text{-CH₂CH₂O (CH₂CH₂O)}}_{\text{m}}\text{H (I)}$

in which R _F represents a perfluoroalkyl radical containing from 4 to 20 carbon atoms, m is a number ranging from 6 to 18, n is equal to 0 or 2, p is equal to 2 or 3, q is equal to 1 or 2 , X represents a CO or SO₂ group, R represents a hydrogen atom or an alkyl radical containing from 1 to 4 carbon atoms, and R 'and R'', identical or different, each represent an alkyl group containing from 1 to 4 carbon atoms.

A particularly preferred group of additives according to the invention consists of the compounds in which R _F contains from 6 to 10 carbon atoms, R is a hydrogen atom, R 'and R''methyl groups, X a SO₂ group, m a number ranging from 10 at 12, n is 2, p is 3 and q is 1.

The amount of fluorinated surfactant compound according to the invention to be added to the electrolyte can vary within wide limits depending on the nature and the concentration of metallic impurities present in the electrolyte. Without this affecting the course of the electroextraction process, this amount can generally range from 0.01 to 5 millimoles of fluorinated additive per liter of electrolyte; it is preferably between approximately 0.1 and 2 mmol / l.

For a given metallic impurity, there is generally an optimal concentration of fluorinated additive allowing the best yield to be obtained. This optimum concentration, which varies according to the additive under consideration and the concentration of the metallic impurity, can be easily determined by a person skilled in the art.

The following examples illustrate the invention without limiting it.

• densité de courant : 50 mA/cm²
• température : 36°C
• électrode verticale en aluminium
• sans agitation

   En suivant le potentiel de l'électrode au cours du temps, on constate que la période d'incubation (c'est-à-dire le temps de déstabilisation du système) est de 15 minutes.An electrolyte containing 120 g / l of H₂SO₄, 55 g / l of Zn²⁺ and 90 mg / l of nickel is used. Electrolysis is carried out under the following conditions:

• current density: 50 mA / cm²
• temperature: 36 ° C
• vertical aluminum electrode
• without agitation

By following the potential of the electrode over time, it can be seen that the incubation period (that is to say the time of destabilization of the system) is 15 minutes.

This time is greater than 48 hours when the test is repeated by adding 0.33 millimole / liter of the compound C₆F₁₃CH₂CH₂O (CH₂CH₂O) ₁₁H to the electrolyte.

In the presence of manganese (15.4 g / l) in the electrolyte, the incubation period drops to 4 hours, the manganese stimulating the evolution of hydrogen. This period goes back to 72 hours by adjusting the concentration of the electrolyte in compound C₆F₁₃CH₂CH₂O (CH₂CH₂O) ₁₁H to 2 millimoles / liter.

The electrolysis is carried out under the same conditions as in Example 1, with an electrolyte containing 120 g / l of H₂SO₄, 55 g / l of Zn²⁺ and variable concentrations of germanium.

In the absence of an additive, an almost immediate destabilization of the electrolysis conditions is observed with redissolution of the zinc deposit.

The addition of the compound C₆F₁₃CH₂CH₂O (CH₂CH₂O) ₁₁H makes it possible to stabilize the potential of the electrode for at least 8 hours. The faradaic yield of electrocrystallization then varies as a function of the concentrations of germanium and of polyfluorinated compound (see the following table). Concentration of the electrolyte in: Faradic yield (%) Germanium (mg / liter) C₆F₁₃C₂H₄O (C₂H₄O) ₁₁H (millimole / liter) 0.127 0 0 0.127 0.094 88.9 0.254 0.094 88.3 0.381 0.094 55.7 0.508 0.094 49.2 0.508 0.190 79.6 0.508 0.280 73.8 0.635 0.280 75.4 0.889 0.280 84.5 1.180 0.280 71.7 1.180 0.380 74.0 1.180 0.470 76.2 1.180 0.570 61.0 1,700 0.570 63.0 2,100 0.570 75.7 2,300 0.570 73.4

In the presence of the polyfluorinated compound, the optimal yield always corresponds to fine-grained zinc deposits, without the imprint left by the hydrogen bubbles.

The electrolysis is carried out under the same conditions as in Example 1, with an electrolyte containing 120 g / l of H₂SO₄, 55 g / l of Zn²⁺ and 1.18 mg / l of germanium.

   Le tableau suivant montre l'évolution du rendement faradique de l'électrocristallisation en fonction de la concentration en composé A ou B.

The addition of the compound (A) or (B) below makes it possible to stabilize the potential of the electrode for at least 8 hours.



(A) = C₆F₁₃C₂H₄SO₂NHC₃H₆NO (CH₃) ₂


$$\text{(B) = C₆F₁₃C₂H₄SO₂NHC₃H₆N⁺ (CH₃) ₂CH₂CO}\genfrac{}{}{0ex}{}{\text{-}}{\text{2}}$$

The following table shows the evolution of the faradic yield of electrocrystallization as a function of the concentration of compound A or B.

With these compounds A and a, good homogeneity of the yields is obtained. The zinc deposits are formed by sets of parallel lamellae and arranged perpendicular to the aluminum substrate.

An electrolyte containing 120 g / l of H₂SO₄, 55 g / l of Zn²⁺ and 4.16 (or 8.32) mg / l of nickel is used and the electrolysis is carried out under the same conditions as in the example. 1.

In the absence of surfactant, the potential is destabilized and the faradaic yield drops to zero before eight hours.

The potential of the electrode is stabilized for more than 8 hours by adding 0.094 millimole / liter of the compound C₆F₁₃CH₂CH₂O (CH₂CH₂O) ₁₁H to the electrolyte. The faradic yield is approximately 86%.

Claims (8)

1. Process for the electroextraction of zinc in an acidic medium, characterised in that there is added to the electrolyte a fluorinated surface-active compound selected among the compounds of formulae :
   
   
   
           R_F-CH₂CH₂O(CH₂CH₂O)_mH   (I)
   
   

   

   wherein R_F denotes a perfluoroalkyl radical containing from 4 to 20 carbon atoms, m is a number ranging from 6 to 18, n is equal to 0 or 2, p is equal to 2 or 3, q is equal to 1 or 2, X denotes a CO or SO₂ group, R denotes a hydrogen atom or an alkyl radical containing from 1 to 4 carbon atoms, and R' and R'', which may be identical or different, each represents an alkyl radical containing from 1 to 4 carbon atoms.
2. Process according to Claim 1, in which R_F contains from 6 to 10 carbon atoms, R is a hydrogen atom, R' and R'' are methyl groups, X is SO₂, m is a number ranging from 10 to 12, n is equal to 2, p is equal to 3, and q is equal to 1.
3. Process according to Claim 1, in which the compound C₆F₁₃CH₂CH₂O(CH₂CH₂O)₁₁H is employed as additive.
4. Process according to Claim 1, in which the compound C₆F₁₃CH₂CH₂SO₂NHC₃H₆NO(CH₃)₂ is employed as additive.
5. Process according to Claim 1, in which the compound C₆F₁₃C₂H₄SO₂NHC₃H₆N⁺(CH₃)₂CH₂CO $\genfrac{}{}{0ex}{}{\text{-}}{\text{2}}$

   

   is employed as additive.
6. Process according to one of Claims 1 to 5, in which the electrolyte contains from 0.01 to 5 millimoles of fluorinated additive per litre.
7. Process according to Claim 6, in which the electrolyte contains between 0.1 and 2 millimoles/litre.
8. Process according to one of Claims 1 to 7, in which the operation is carried out in a sulphuric acid medium.