EP0156721B1 - Process for producing fine and ultrafine zinc powders by electrolysis in basic media - Google Patents

Description

The present invention relates to a process for the production of fine and ultrafine zinc powders by electrolysis in basic medium.

The electrolysis of zinc in a caustic medium has been studied for several decades but its industrial applications have been rare.

• 1. surtension de l'hydrogène en milieu caustique plus faible qu'en milieu acide;
• 2. production d'un dépôt de zinc très fragile, supprimant les coûts élevés de grattage des électrodes nécessaires en milieu acide;
• 3. problèmes de corrosion beaucoup moindre qu'en milieu acide, permettant un investissement plus léger.

This electrolysis has some significant advantages:

• 1. hydrogen overvoltage in caustic medium weaker than in acid medium;
• 2. production of a very fragile zinc deposit, eliminating the high costs of scraping the electrodes required in an acid medium;
• 3. much less corrosion problems than in an acid environment, allowing a lighter investment.

• - grande réactivité conduisant à une oxydation plus facile et donc, à un rendement de fusion moindre (85,90% contre 97,99% par les procédés habituels).

The zinc deposit which is obtained in an alkaline medium is not very coherent, has very little adhesion and provides zinc powders, hence, for the usual uses of zinc, the need to melt and ingot the powders produced. With this in mind, the production of zinc in fine particles can be considered as a disadvantage:

• - high reactivity leading to easier oxidation and therefore to a lower melting yield (85.90% against 97.99% by the usual methods).

This phenomenon probably explains the little success of electrolysis in a caustic medium. However, the existence of a large market for zinc powders and dusts, in the chemistry of catalysts, fillers in general and more particularly for the anti-corrosion paint industry, has guided the study of this type of electrolysis in recent decades.

The zinc powders currently sold are obtained thermally, by melting and vaporization under vacuum of zinc ingots. They provide a fairly wide range of production, from coarse zinc powder to fine and ultra-fine zinc dust.

• - aspect: poudre gris bleuté mat;
• - masse volumique 6,9 à 7,1 g/cm³;
• - masse volumique apparente 2 à 2,5 g/cm³;
• - surface spécifique 1 100 à 3 300 cm²/g;
• - forme: sphérique pour les poussières, déchiquetée et irrégulière pour les poudres;
• - d₉₀ = 6 à 17,5 µm;
• - d₅o = 5 à 20 µm selon les variétés;
• - refus à 50 µm inférieur à 1 %;
• - composition chimique:
  • - Zn total: 98,5%,
  • - Zn métal: 92 à 96%'
  • - Pb = 0,15%.

The powders and dusts obtained by thermal process have the following characteristics:

• - aspect: matt bluish gray powder;
• - density 6.9 to 7.1 g / cm ³ ;
• - apparent density 2 to 2.5 g / cm ³ ;
• - specific surface area 1,100 to 3,300 cm ² / g;
• - shape: spherical for dust, jagged and irregular for powders;
• - d ₉₀ = 6 to 17.5 µm;
• - d ₅ o = 5 to 20 µm depending on the varieties;
• - refusal at 50 µm less than 1%;
• - chemical composition:
  • - Total Zn: 98.5%,
  • - Zn metal: 92 to 96% '
  • - Pb = 0.15%.

• - aspect: poudre gris bleuté à éclat métallique;
• - masse volumique: 6,9 à 7,1 g/cm³;
• - masse volumique apparente: 0,9 à 1,5 g/cm³;
• - surface spécifique: 3 000 4 500 cm²/g;
• - forme: dendritique ou en aiguille;
• - d₉₀ : 160 à 325 µm;
• - dso : 60 à 90 µm;
• - refus à 50 µm: 60 à 70%;
• - composition chimique:
  • - Zn total: 99 à 99,7% - type Z 4-9;
  • - Zn métal: 90 à 97%;
  • - Pb: 0,005%.

On the other hand, the powders obtained by alkaline electrolysis have the characteristics:

• - appearance: bluish gray powder with metallic luster;
• - density: 6.9 to 7.1 g / cm ³ ;
• - apparent density: 0.9 to 1.5 g / cm ³ ;
• - specific surface: 3,000 4,500 cm ² / g;
• - shape: dendritic or needle;
• - d ₉₀ : 160 to 325 µm;
• - dso: 60 to 90 µm;
• - refusal at 50 µm: 60 to 70%;
• - chemical composition:
  • - Total Zn: 99 to 99.7% - type Z 4-9;
  • - Zn metal: 90 to 97%;
  • - Pb: 0.005%.

• sphérique pour les poudres par voie thermique et planaires pour les poudres obtenues par électrolyse alcaline.

The essential differences therefore lie in the higher specific surface for alkaline powders, their much coarser particle size and their very different forms, namely:

• spherical for powders thermally and planar for powders obtained by alkaline electrolysis.

Thus the powders obtained by alkaline electrolysis must be ground to approach the particle size standards of the powders obtained by thermal process. However, grinding does not make it possible to obtain powders having a d ₅₀ = 20 μm (to be compared with d _5o = 7.5 μm on average for the powders obtained by the thermal route).

Hence the research which has been carried out, intended to reduce the d ₅₀ of the powders obtained by electrolysis in an alkaline medium.

• - tension aux bornes
  • de la cellule U = 2,8 à 3,5 V
• - densité de courant J = 750 à 900 A/m²
• - distance anode cathode d = 2,0 à 3,8 cm
• - concentration en soude
  • caustique C_NaOH = 240 g/I
• - concentration en zinc C_zn = 5 à 45 g/I
• - température : ambiante 19 à 22°C.

The tests were carried out on the basis of the following parameters:

• - terminal voltage
  • of cell U = 2.8 to 3.5 V
• - current density J = 750 to 900 A / m ²
• - cathode anode distance d = 2.0 to 3.8 cm
• - sodium hydroxide concentration
  • caustic C _NaOH = 240 g / I
• - zinc concentration C _zn = 5 to 45 g / I
• - temperature: ambient 19 to 22 ° C.

• 1 - la nucléation: formation des ébauches de cristaux;
• 2 - la croissance du cristal.

Plating follows two processes:

• 1 - nucleation: formation of crystal blanks;
• 2 - the growth of the crystal.

The size of the grains obtained essentially depends on the speed ratio of these two processes. If the nucleation rate is higher than the growth rate of the crystal, the deposit will be fine and vice versa. The crystals will grow quickly and will therefore be coarse, if the metal-ions arriving at the solid-liquid interface are numerous. On the other hand, the nucleation centers will be abundant and the crystals will grow slowly and will therefore be fine, if the metal ions arriving at the solid-liquid interface are rare.

Hence, for the most part, the concentration of ions

• 1 - densité de courant;
• 2 - concentration de l'électrolyte;
• 3 - température;
• 4 - agitation du bain;
• 5 - présence de colloïdes;
• 6 - présence de complexes métalliques.

metal near the interface, therefore in the NERNST layer, will condition the particle size of the deposit. On these theoretical bases, previous studies have concerned the control of the diffusion process and therefore the following parameters:

• 1 - current density;
• 2 - concentration of the electrolyte;
• 3 - temperature;
• 4 - agitation of the bath;
• 5 - presence of colloids;
• 6 - presence of metal complexes.

All these techniques are interested in the electrolyte and faradyque current, but none studies the electrode.

However, electrolysis involves relationships between the current, the electrolyte and the electrodes. In work on alkaline electrolysis, the study of the cathode has always been seen from the angle of corrosion and it focused on the search for a material resistant to corrosion.

The essential conclusions of this study are that all materials more electropositive than zinc and not soluble in caustic solutions can be used to constitute the cathode. Ansi, the metals used were stainless steel, magnesium or alloys rich in magnesium (elektron) and incidentally zinc.

At the start of alkaline electrolysis, a very thin film of zinc covers the surface of the cathode; subsequently the zinc is deposited in the form of needles or dendrites.

The mechanical resistance and the appearance of this film is variable according to the metal constituting the cathode and according to the appearance of the surface of this cathode.

On all metals, except magnesium, this film is very adherent and the recovery of zinc, by a light scraping, respects it. However, on certain cathodes, blisters and blisters appear, corresponding to the detachment of this sub-layer. This phenomenon is very detrimental to particle size, because it produces plaques varying in size from millimeter to centimeter. Detailed observation shows that in these areas, very fine irregularities in the surface of the cathode are visible - streaks, pits, etc. - probably prior to the use of metal as an electrode.

After numerous tests relating to the influence of the surface state of the cathode on the particle size of the zinc powder obtained, it has been discovered, according to the invention, that a fine or ultrafine powder can be obtained thanks to special treatment of the cathode used for electrolysis.

For this purpose, according to the invention, this process for the production of fine and ultrafine zinc powders by electrolysis in basic medium is characterized in that a cathode coated with a film of varnish or silicone obtained from an aerosol.

According to an additional characteristic of the invention, the bath can be agitated, during electrolysis, by irradiating it by means of ultrasound preferably having an emission frequency of 40 kHz.

According to an additional characteristic of the invention, the initial concentration of zinc in the bath is preferably limited to around 30 g / l.

The process according to the invention makes it possible to obtain, by alkaline electrolysis, fine and ultra fine powders and this directly, without requiring additional grinding.

• la figure 1 est un diagramme illustrant l'influence de l'état de surface de la cathode sur la granulométrie de la poudre de zinc obtenue;
• la figure 2 est un diagramme illustrant l'influence de la concentration initiale en zinc sur la granulométrie de la poudre de zinc obténue, à la fois dans le cas d'une agitation du bain au moyen d'ultra-sons et sans cette agitation;
• la figure 3 est un diagramme illustrant l'influence de la concentration initiale en zinc sur le rendement faradyque.

Various embodiments of the present invention will be described below, by way of nonlimiting example, with reference to the appended drawing in which:

• FIG. 1 is a diagram illustrating the influence of the surface condition of the cathode on the particle size of the zinc powder obtained;
• FIG. 2 is a diagram illustrating the influence of the initial zinc concentration on the particle size of the zinc powder obtained, both in the case of agitation of the bath by means of ultrasound and without this agitation;
• FIG. 3 is a diagram illustrating the influence of the initial zinc concentration on the faradyque yield.

• 1 - une électrode de zinc corrodée par la soude caustique;
• 2 - une électrode de zinc non corrodée, mais ayant subi de nombreux grattages lors de la récupération de la mousse de zinc;
• 3 - une électrode en acier inoxydable, neuve et non corrodée mais striée à l'abrasif dans deux directions perpendiculaires;
• 4 - une électrode en acier inoxydable neuve, sans préparation particulière;
• 5 - une électrode en acier inoxydable polie à l'abrasif très fin.

To study the influence of the state of the cathode on the particle size of the zinc powder obtained, electrolysis tests were carried out with five different types of electrode, namely:

• 1 - a zinc electrode corroded by caustic soda;
• 2 - an uncorroded zinc electrode, but having undergone numerous scrapings during the recovery of the zinc foam;
• 3 - a stainless steel electrode, new and not corroded but striated with abrasive in two perpendicular directions;
• 4 - a new stainless steel electrode, without any special preparation;
• 5 - a very fine abrasive polished stainless steel electrode.

The use of these electrodes under conditions of current density, electrolyte concentration and identical temperatures leads to an immediate observation, namely that a significant variation in the particle size of the zinc powders is obtained, from a test to the other. The results of these tests appear from curves 1 to 5 of FIG. 1 corresponding respectively to the use of the five aforementioned electrodes. In this FIG. 1, the particle size D in microns (mesh) of the zinc powder obtained is indicated on the abscissa and the cumulative passer d in%, is indicated on the ordinate.

It is easy to see that the d _s α varies from 150 to 37 microns for tests 1 to 5.

• 1 - une très mauvaise adhésion et une épaisseur importante de la sous-couche pour l'essai 1;
• 2 - une très forte adhésion et une épaisseur importante de la sous-couche pour l'essai 3;
• 3 - une adhésion normale et des boursouflures de la sous-couche pour l'essai 2;
• 4 - une adhésion faible et l'existence de boursouflures de la sous-couche pour l'essai 5.

In addition, it should be noted:

• 1 - very poor adhesion and a significant thickness of the sub-layer for test 1;
• 2 - a very strong adhesion and a significant thickness of the sub-layer for test 3;
• 3 - normal adhesion and blistering of the underlay for test 2;
• 4 - a weak adhesion and the existence of blisters of the underlay for test 5.

The first conclusion of these tests leads to the observation that the surface condition has a lot of importance on the particle size and on the formation of the undercoat but that even fine polishing does not completely eliminate the detachments of the film of zinc.

Even very fine irregularities in the surface of the electrode have a significant influence on the formation of the undercoat.

A very fine polish becoming necessary, we tested, according to the invention, the use of an electrode coated with a film of varnish or silicone by aerosol.

• 1 - sous-couche très uniforme, très fine et très adhérente;
• 2 - d_5o = 12 µm; d₉₀ = 18 µm.

The results obtained are spectacular and they are illustrated by curve 6 in FIG. 1:

• 1 - very uniform, very thin and very adherent undercoat;
• 2 - d _5o = 12 µm; d ₉₀ = 18 µm.

The use of varnish or silicone deposited by aerosol on the electrode requires some precautions.

First of all, the varnish or silicon used must have chemical resistance to attack by caustic solutions.

Furthermore, the deposition of varnish or silicone must be as regular as possible, which supposes an appropriate solvent dosage. Indeed, a too viscous varnish, during its projection, forms granules which solidify without spreading and recreate by the same, by attenuating, a heterogeneity of surface. Likewise, a varnishing with a brush leads to poor results. It therefore appears essential that the solvent is sufficiently dosed to allow spreading by gravity flow of the varnish or of silicone. This condition being fulfilled, the surface can be considered very regular.

The excess of diluent is also not advantageous, the low viscosity then leading to flow along the electrode before drying and poor distribution of the varnish. The phenomena of blisters and blisters then reappear.

It is obvious that the cathodes thus treated must be free of dust and traces of fatty substance, or of any other element, preventing a close link between metal and varnish.

The process according to the invention can possibly be improved by playing on the control of other parameters, namely the zinc electrolysis concentration and the agitation of the bath.

At constant current density, constant temperature and agitation of the bath, the solid lines of FIG. 2 show the influence of the initial zinc concentration Ci (between 10 and 45 g / 1), indicated on the ordinate, on the d ₅₀ and the d ₉₀ of the powder obtained, in the absence of any agitation of the bath, the particle size D in microns of the powder being plotted on the ordinate.

These two curves demonstrate the existence of an abrupt change in behavior (threshold phenomenon) between 30 and 33 g / I for d ₉₀ = f (Ci) and between 35 and 40 g / I for d ₅₀ = f (Ci) . This phenomenon is related to the nucleation speed / crystal growth speed ratio.

In the slightly decreasing part of the curves, the electrolysis is controlled by the diffusion of the metal ions in the NERNST layer and the nucleation rate is higher than the growth rate of the crystals, and conversely in the strongly decreasing part.

It should be noted that the influence of Ci is stronger on dso than on d _5o beyond 30 g / I. Below this, the two curves are parallel and this corresponds to a reduction in the size of all the grains formed (translation of the grain size curve). This translation is of low amplitude. Thus, the industrial control of the grain size is facilitated. It can therefore be seen, from the above, that a large variation in the zinc concentration Ci of the electrolyte causes a small variation in d _50, provided that Ci remains less than 30 g / I. Only the upper limit of Ci is to be regulated industrially.

Agitation of the electrolyte allows homogenization of the concentrations in the bath; this agitation naturally exists by thermal convection and gravitational sedimentation.

The methods usually used reside in the pressure injection of the electrolyte in well chosen places or in a mechanical agitation.

These methods turn out to have little effectiveness. Indeed, the double layer near the electrodes is very little affected by this agitation, hence the search for an effective method of agitation, influencing the double layer.

A possibility is offered by the creation, within the liquid of high intensity cavitations by the irradiation with ultrasound.

The search for effective cavitation in a fairly viscous medium such as caustic soda at room temperature leads to choosing an emission frequency of 40 kHz. Thus tests were carried out at constant current density, constant temperature, initial zinc concentration variable from 10 to 42 g / I and irradiation with ultrasound at 40 kHz and an effective power of about 10 W / 1.

• 1 - diminution du d_5o de 6 à 2 µm par rapport aux essais sans irradiation avec des ultra-sons;
  • - diminution du d₉₀ de 12 à 5 µm par rapport aux essais sans irradiation avec des ultra-sons;
• 2 - resserrement net du fuseau granulométrique;
  • dgo - d₅o = 5 µm
  • d₉₀ - d₁₀ = 10 à 12 µm;
• 3 - le phénomène de seuil observé sans irradiation aux ultra-sons disparaît.

The results are given by the broken lines in Figure 2 and they call for three remarks:

• 1 - reduction of d ₅₀ from 6 to 2 μm compared to tests without irradiation with ultrasound;
  • - decrease in d ₉₀ from 12 to 5 µm compared to tests without irradiation with ultrasound;
• 2 - net tightening of the granulometric zone;
  • dgo - d ₅ o = 5 µm
  • d ₉₀ - d ₁₀ = 10 to 12 µm;
• 3 - the threshold phenomenon observed without ultrasound irradiation disappears.

• 1 - réduction de l'épaisseur de la double couche;
• 2 - désorption facilitée des gaz formés à l'électrode;
• 3 - création de centres de nucléation;
• 4 - rupture mécanique des gros cristaux.

The influence of ultrasound on the fineness of the grain during electrolysis is a phenomenon corresponding to several effects:

• 1 - reduction of the thickness of the double layer;
• 2 - facilitated desorption of the gases formed at the electrode;
• 3 - creation of nucleation centers;
• 4 - mechanical failure of large crystals.

The reduction in the thickness of the double layer seems doubtful because d ₅₀ and d ₉₀ = f (Ci) show that electrolysis is always controlled by diffusion.

The varnishing of the electrode according to the invention obviously introduces a faradyc loss of efficiency by adding an additional ohmic resistance to the cell. This loss is in fact very low (1 to 2%) of the order of the total uncertainty on the measurements, and it is obvious that the efficiency brought to the particle size control is without common measure with this reduction in yield. In addition, the very regular character of deposit (no warts or buttons creating short circuits) that the varnishing induced, makes it possible to reduce the anode-cathode distance and to largely compensate for this resistance resistance.

• 1 - de l'électrolyse du solvant: dégagement d'hydrogène à la cathode;
• 2 - de l'électrolyse des impuretés de la solution: chlore par exemple;
• 3 - de la corrosion par l'électrolyte du zinc électro- déposé - très faible si la solution est correctement purifiée et donc si le zinc est pur.

The faradyc yield depends on:

• 1 - electrolysis of the solvent: evolution of hydrogen at the cathode;
• 2 - electrolysis of impurities in the solution: chlorine for example;
• 3 - corrosion by the electrolyte of electro-deposited zinc - very weak if the solution is properly purified and therefore if the zinc is pure.

• - concentration en soude = 240 g/I
• - concentration en chlorure = 5 g/I à 10 g/1
• - concentration en zinc > 10 g/I
• - électrodes non corrodées et non siliconées;

le rendement faradyque moyen est de 82% et peu variable.Thus, with the following average parameters:

• - sodium hydroxide concentration = 240 g / I
• - chloride concentration = 5 g / I to 10 g / 1
• - zinc concentration> 10 g / I
• - non-corroded and non-silicone electrodes;

the average faradyque yield is 82% and hardly variable.

If the zinc Ci concentration of the electrolyte is less than 10 g / I, then the faradyque yield decreases to reach 30% at 0.5 g / 1 as indicated by the curve in Figure 3.

Under the same conditions with varnished cathodes the average faradyque yield is 83%.

It should be noted that the total polarization of the cell increases, in connection with the increase in chemical resistance (at anode-cathode distance equal).

With irradiation with ultrasound, the polarization of the cell decreases. This result was expected and perhaps related to the reduction in the thickness of the double layer of NERNST. Under these conditions - silicone cathode and ultrasound irradiation - the average faradyc yield is 88%.

From the above description it can be seen that the combined actions on the surface state of the cathode by varnishing or deposition of silicone, the irradiation by ultrasound and the concentration of the incoming electrolyte result in the production of ultra fine zinc powder never obtained by conventional alkaline electrolysis, the modification of the solid-liquid interface of the cathode being the paramount parameter of the process according to the invention.

Claims (4)

1. Process for production of fine and ultra-fine powders of zinc by electrolysis in a basic medium, characterized in that a cathode coated with a film of varnish or of silicon obtained from an aerosol is used.

2. Process according to claim 1, characterized in that the bath is stirred during electrolysis by radiating it by means of ultra-sounds.

3. Process according to claim 2, characterized in that the electrolysis bath is radiated with ultra-sounds having an emission frequency of the order of 40 kHz.

4. Process according to any of the preceding claims, characterized in that the initial concentration of zinc of the bath is limited to about 30 g/l.

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