EP0497675B1 - Process for manufacturing a fine dendritic cadmium powder and powder obtained by this process - Google Patents

Description

The present invention relates to a process for preparing a dendritic cadmium powder, as well as a powder obtained by the process.

Within the general framework of the development of nickel / cadmium accumulators, there is a constant tendency to seek an improvement in performance. Thus the electrode structures are particularly studied in order to be able to contain a charge of active material as high as possible (Number of amperes.hourest hour) and as available as possible (maximum intensity).

A particular aspect of this research consisted in developing new techniques for manufacturing electrodes which use smaller quantities of these materials.

Thus, in particular in the field of portable accumulators, the negative electrode structure, usually made of sintered nickel, has been replaced by a so-called PBT structure in which a mixture of cadmium oxide and metallic powder is coated on a strip. In this known technique, the role of the conductive powder is to distribute the current of electrons homogeneously in the volume of the active mass of cadmium hydroxide.

Conventionally, a certain number of types of metallic powders are used, either cadmium or nickel. Often spherical or spheroid, these known powders are added in significant proportions, typically of the order of 20% by weight, to achieve the resistivity required for the electrode.

Japanese patent application published under No. 55-76569 on 9.6.80 teaches the preparation of such a powder and its incorporation into the electrode in such proportions.

The present invention is based on the observation that, with a powder urn which deviates from the spherical shape (in other words with a form factor which is much greater than 1), and in particular with a dendritic powder, the electrical performance is greatly improved, and in particular the volume energy is increased. This means that, to obtain the same performance as with an electrode of the prior art, a dendritic powder is added to the paste of the electrode in a quantity substantially lower than a spherical powder, resulting in a gain of weight.

Document FR-A-2 194 792 teaches a process for the preparation of a porous electrode from a cadmium powder of acicular or dendritic nature. This powder, obtained by deposition on an electrode then dry scraping, is then compressed to form the electrode, and is not intended to be used as a current distribution agent as mentioned above. More specifically, the operating conditions described in this document are such that the powder does not have the required fineness. In addition, the principle of electrolysis described in this patent oblique to work with small amounts of electricity, with extremely frequent scraping.

The present invention thus aims to propose an electrolytic process for obtaining a dendritic cadmium powder, which makes it possible to obtain, by appropriate control of simple parameters, a powder of quality and of particularly suitable characteristics, in particular in terms of fineness, incorporation into a negative nickel / cadmium accumulator electrode. It also proposes a process that can be implemented with large amounts of electricity, to obtain before powder reduction an electrode thickness of up to several centimeters without suffering from homogeneity defects.

To this end, it relates to a process for obtaining a fine powder of dendritic cadmium having the features of claim 1.

The invention also relates to a dendritic cadmium powder as defined in claim 8.

An advantage of the method according to the invention resides in that it offers the possibility of constituting a metallic matrix whose physical characteristics can be chosen without having to undergo the strong constraint of the particle size distribution. Thus, the inventors were able to determine the conditions for obtaining, by electrolytic deposition, a dendritic structure or else a sponge structure. The transition from one form of structure to another follows from the mode of crystallization. Thus the dendritic structure evolves towards the sponge when the section of the crystals oriented in the field decreases and the two-dimensional germination continues on the pre-existing dendrites.

It thus creates, by entanglement, polymorphic dendrites constituting a true porous structure which is characterized by a very low volume density of cadmium (of the order of 0.1 kg / dm ³ ). It has also been observed that the conditions for generating the sponge can be adjusted to obtain a high faradaic yield, typically greater than 70%, by making the particularly energy-efficient process.

The constitution of the sponge by deposition on the cathode does not pose any particular starting difficulty. Substrates can be used, for example stainless steel or titanium. It does not matter whether the surface is virgin or whether there remains a cadmium residue from the previous operation.

• une bonne homogénéité de structure sur toute l'épaisseur;
• un développement en épaisseur également homogène (absence de protubérances ou de creux sur la surface libre);
• une bonne adhérence de l'éponge sur son substrat, permettant d'extraire l'électrode de la cellule d'électrolyse dans risque de séparation ou de chûte de l'éponge.

Thanks to the process according to the invention, it is possible to produce cadmium sponges of very large thicknesses (typically 3 to 6 cm) which are characterized by:

• good structural uniformity over the entire thickness;
• an equally uniform development in thickness (absence of protuberances or hollows on the free surface);
• good adhesion of the sponge to its substrate, making it possible to extract the electrode from the electrolysis cell at risk of separation or of falling of the sponge.

The separation of the sponge from its substrate (stripping) is carried out by light mechanical means of conventional type.

The separated sponge is then washed in order to recover the electrolyte which still permeates it. Here again, the particular structure of the sponge allows very effective washing with a very small amount of water.

Once washed, the sponge proves to be perfectly chemically stable, either with respect to dissolution by acid attack or by air oxidation.

The second operation of the process consists in performing a shredding of the sponge. It is carried out in tank disintegration devices provided with specific stirring mobiles, operating continuously or discontinuously. In order to promote a complete release of particles of the sponge, it is preferable here to work with a pulp rate that does not exceed 200 g of dry matter per liter. As will be seen below, the peripheral speed of the stirring mobile is an important factor in obtaining an appropriate particle size.

After the disintegration operation, a sieving operation intended to remove coarse particles, and preferably particles larger than 125 μm, is advantageously provided, but optionally.

Another important characteristic of the present invention resides in that the disintegration stage releases particles whose size and solidity are determined essentially by the operating conditions of the electrolytic stage of constitution of the sponge, and are only very weakly influenced by a too long residence time of the material in the disintegration apparatus and by the choice of the geometry of the stirring mobiles. Several types of shredding mobiles have been tested, with or without counter-blades, without significantly modifying the morphology and the particle size of the powder. In addition, no overgrinding of the powder was observed.

It was also found that the particle size of the powder obtained, evaluated by the rejection rate at the sieving step, was only influenced by the peripheral speed of the grinding mobiles.

The good mechanical strength of the particles constituting the pulp allows storage in the decanted state without modifying the particle size distribution. In addition, the pulp obtained at the end of the disintegration can be pumped, for example by a centrifugal vortex pump, without undergoing any particle size alteration.

As will be seen in detail below, the morphology of the powder obtained is characteristic of the process according to the invention. The particles have the form of ferns made up of a central column from which leave, with an angle of the order of 60 °, secondary ferns. The overall shape is generally acicular, a shape well suited to the intended application.

We will now describe in more detail the concrete implementation of the method according to the invention.

The electrolysis cell can be supplied either with a pure cadmium solution or with metallic cadmium of appropriate purity.

In the case of a cadmium solution, a concentrated solution is preferably chosen. The associated anion is advantageously sulfate. The acidity of the solution can vary for example between 5 and 80 g / l of sulfuric acid. In order to obtain a cadmium powder of purity compatible with the intended application, the total content of metallic impurities in the solution, expressed relative to the cadmium, must be less than 100 g / t.

In the case where the electrolysis cell is powered by cadmium metal, it can take any suitable form, preferably with a purity of 99.99% or better. One can use an anode cast or supplied with metal balls or rods. The tests made it possible to note that, whatever the type of supply, the anodic reaction in no way limits the process of obtaining the cadmium sponge at the cathode.

The electrolyte is composed of cadmium sulfate and sulfuric acid. The acid content is conditioned by the search for a good ionic conductivity of the electrolyte. This content is advantageously between 5 and 100 g / l, a value close to 50 g / l being particularly advantageous because it gives a very good conductivity while limiting the acid corrosion of the sponge.

où

• J est la densité de courant exprimée en A/m²;
• (CD) est la concentration en cadmium exprimée en kg/m³.

The choice of cadmium concentration is closely linked to the choice of cathodic current density. Tests carried out by the Applicant have made it possible to discover that, in the range of current densities ranging from approximately 700 to approximately 1500 A / m ² , it is advantageous for the cadmium concentration to respond to the following relationship: $$\text{100 Am / kg ≤ J / (CD) ≤ 200 Am / kg}$$

or

• J is the current density expressed in A / m ² ;
• (CD) is the cadmium concentration expressed in kg / m ³ .

Thus, for a current density between 900 and 1200 A / m ² , the cadmium concentration is preferably between 4 and 15 g / l, more preferably between 7 and 11 g / l.

The operating temperature is preferably maintained in a range between 20 and 35 ° C, more preferably between 25 and 30 ° C.

As indicated, the cathode substrate is preferably stainless steel or titanium. It was found that good adhesion of the sponge was obtained with a surface roughness corresponding to the raw rolling state.

The circulation of the electrolyte is ensured either naturally when the oxygen is released, for a cell with insoluble anodes, or in a provoked way. The choice of circulation type has practically no influence on the morphology of the sponge.

The duration of electrolysis between two debates is preferably between 4 and 8 hours. Under the optimized current density and cadmium concentration conditions as mentioned above, a duration of the order of 6 hours is particularly suitable.

The concrete design of the electrolysis cells is of the conventional type, and will not be described in detail. It is possible, for example, to use cells of the type used in the zinc or copper industry.

In the case of an electrolysis process with soluble anodes, it is observed that the composition of the electrolyte does not remain stable. Indeed, the reaction at the cathode, where the protons are reduced and hydrogen is generated, constitutes a parasitic reaction which causes a decrease in the acidity of the medium, with which is associated an increase in the cadmium concentration.

According to a particular aspect of the present invention, in order not to have to purge the cadmium system and to add acid, provision is made to combine the electrolysis process with soluble anodes with an electrolysis process with insoluble anodes, working on the same electrolyte. By simple adjustment of the cathodic surface of the process operating with insoluble anodes, so that this surface constitutes a determined percentage of the total cathodic surface, a percentage equal to the cathodic faradic yield of hydrogen release, the excess of aforementioned anodic dissolution. The acidity consumed by the above-mentioned parasitic reaction is also generated on the insoluble anodes. The system is therefore globally balanced and can operate under stable conditions practically without the need for addition or purging, which guarantees constant quality for the sponge formed, and therefore for the powder.

After electrolysis then deburring and washing of the sponge as indicated above, the sponge is subjected to the disintegration operation. The shredding action is carried out by an agitating mobile having no significant pumping or shearing function. We are mainly looking for a shock effect on the peripheral parts of the mobile which have a small active surface.

As mentioned above, the essential parameter is the peripheral speed of the mobile. It is preferably situated between 20 and 50 m / s for diameters of stirring mobiles varying between 83 and 380 mm. For speeds lower than this range, a rapid increase in the rate of particles refused to sieving is observed. Concretely, it was found that, for a mobile with a diameter of 380 mm, a peripheral speed of 30 m / s was sufficient to reach a rejection rate at sieving at 125 µm of less than 0.5%.

The residence time of the sponges in the disintegration apparatus is for example between 3 and 5 minutes. However, it has been found that an excess of residence time of 100 to 200% over these durations has no consequence on the particle size distribution.

The pulp rate is fixed at a value compatible both with the productivity requirements of the process and with the requirement of conservation of the particle size distribution. Concretely, an amount of dry matter per liter of pulping solution between 50 and 200 g / l is suitable. Beyond the upper limit, the particle size distribution becomes coarser.

After the disintegration operation, the pulp is sieved, as indicated above, for example using a vibrating sieve, to remove particles of a size greater than a determined size limit, preferably about 125 µm. The pulp is then decanted and conditioned. An oxidation rate of less than 1% per month has been observed under wet storage conditions.

$${\text{d}}_{\text{90}}\text{≈ 37 µm}$$

$${\text{d}}_{\text{10}}\text{≈ 7 µm}$$

Illustrated on the microscope views of Figures 1 and 2 the appearance of the cadmium powder obtained. The magnifications used were 200 and 800 respectively. A dendritic powder is observed, the particles of which are in the form of ferns characterized by a central core, with a cross section of between approximately 4 and approximately 20 μm ² , on which secondary dendrites have developed which are oriented obliquely to the direction of the core, with an average inclination of approximately 60 °. The specific area of the powder, measured according to the BET method, is between 1 and 3 m ² / g. The average diameter, determined by laser particle size, is approximately 20 μm, a typical particle size distribution being as follows: $${\text{d}}_{\text{98}}\text{≈ 64 µm}$$

$${\text{d}}_{\text{90}}\text{≈ 37 µm}$$

$${\text{d}}_{\text{10}}\text{≈ 7 µm}$$

Furthermore, the process of the invention guarantees a very high titer in metallic cadmium compared to the total cadmium. Thus, it can be seen that, despite a very high specific surface, the final product is very little oxidized.

• Cadmium total ≥ 99%
• Cadmium métal ≥ 95%
• Zn ≤ 50 g/t
• Pb ≤ 30 g/t
• Ni ≤ 10 g/t
• Cu ≤ 10 g/t
• Fe ≤ 30 g/t
• SO₄= ≤ 50 g/t

A typical composition is as follows:

• Cadmium total ≥ 99%
• Cadmium metal ≥ 95%
• Zn ≤ 50 g / t
• Pb ≤ 30 g / t
• Ni ≤ 10 g / t
• Cu ≤ 10 g / t
• Fe ≤ 30 g / t
• SO ₄ = ≤ 50 g / t

Claims (12)

1. Process for obtaining a fine powder of dendritic cadmium, characterised in that it comprises the following steps:

   (a) electrolytic production of cadmium metal on an electrode, with a current density between 700 and 1500 A/m², a cadmium concentration in the electrolyte specified by the relationship: $$\text{100 A.m/kg ≤ J/(CD) ≤ 200 A.m/kg}$$

   

   where J is the current density expressed in A/m² and (CD) is the cadmium concentration expressed in kg/m³, and with an electrolytic solution composed of cadmium sulphate and sulphuric acid, the sulphuric acid content being between 5 and 100 g/l, in order to form a sponge consisting of tangled polymorphic dendrites,

   (b) removal and washing of the sponge,

   (c) disintegration of the sponge in a pulpy medium in order to release the dendrites and obtain a dendritic powder of particle size essentially less than a specified limit.
2. Process according to Claim 1, characterised in that step (a) is carried out at a temperature between 20 and 35°C, preferably between 25 and 30°C.
3. Process according to either of Claims 1 and 2, characterised in that step (c) is effected in a disintegration tank with an amount of pulp between 50 and 200 g of cadmium per litre of pulping solution and with a motory agitation device whose active surface exposed to the pulp is low and whose peripheral velocity is between 20 and 50 m/s.
4. Process according to Claim 3, characterised in that step (c) is carried out over a period of more than three minutes.
5. Process according to one of Claims 1 to 4, characterised in that step (a) is carried out by supplying an electrolysis apparatus with cadmium metal and in that the apparatus comprises at least one cell operating with soluble anodes and at least one cell operating with insoluble anodes, the cathodic surface of the cell or cells operating with insoluble anodes being chosen in such a manner as to constitute a percentage of the total cathodic surface equal to the cathodic coulombic efficiency for hydrogen evolution.
6. Process according to one of Claims 1 to 5, characterised in that step (c) is followed by a step (d) for sieving the pulp in order to eliminate the solid particles of a size greater than a specified limiting size.
7. Process according to Claim 6, characterised in that step (d) eliminates the particles of a size greater than approximately 125 µm.
8. Dendritic cadmium powder for producing an electrode of a nickel/cadmium accumulator, characterised in that its particles have the shape of ferns comprising a central stem from which secondary dendrites branch off obliquely, in that the cross-section of the central stem is between approximately 4 and approximately 20 µm², and in that it has a specific surface area, measured according to the BET method, between approximately 1 and approximately 3 m²/g.
9. Powder according to Claim 8, characterised in that it has a particle size less than 125 µm.
10. Powder according to Claim 9, characterised in that it has a d₉₀ of the order of 35 µm and a d₁₀ of the order of 7 µm.
11. Powder according to one of Claims 8 to 10, characterised in that it comprises at least 95% of cadmium metal.
12. Powder according to one of Claims 8 to 11, characterised in that the average angle of the secondary dendrites in relation to the central stem is of the order of 60°.

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