DE3337568A1 - Production of electrolytic manganese dioxide for alkaline cells - Google Patents

Abstract

If titanium salts, in particular titanium sulphide, or titanium oxides are added to the electrolysis bath in the production of gamma -manganese dioxide from acidic manganese (II) salt solutions by direct-current electrolysis, titanium is incorporated in the anode product. The gamma structure is related to the rutile lattice and is stabilised if titanium is incorporated in amounts of up to a few per cent. As a cathode depolariser, the titanium makes the magnesium dioxide capable of a much improved rechargeability.

Description

Manufacture of electrolytic manganese dioxide

for alkaline cells The invention relates to a method of manufacture of manganese dioxide as a depolarizer for galvanic elements, in which using of electrodes made of titanium, carbon or a lead alloy of acidic manganese (II) salt solutions be subjected to direct current electrolysis and the anodically deposited Product is subjected to post-treatment such as washing, crushing, drying.

Details on the extraction of electrolyte brownstone according to the the current state of technology are for example the essay by A. Kozawa in Batteries, Vol. 1, Manganese Dioxide (ed. By K.V. Kordesch, Ch. 3, Marcel Dekker 1974) can be found. Usually manganese-containing ore, in which the raw material in the form of oxides (MnO2, Mn2O3, MnO) or as MnCO3 (rhodochrosite), with leached by hot sulfuric acid.

The manganese sulfate solution obtained contains an excess of Sulfuric acid still impurities in Fe2 +, Pb2 +, Ni2 +, Co2 + and SiO2. By adding of MnO2 and blowing air into the solution, Fe2 + is oxidized to Fe3 + and then through targeted adjustment of the pH value with a neutralizing agent (Ca (OH) 2 or CaCO3) the iron precipitated together with the other impurities.

The filtered, purified manganese sulfate solution is 0.5 to 1.2 molar in MnSO4 and 0.5 to 1.0 molar in H2SO4. It is now fed to the electrolytic cells and subjected to direct current electrolysis at temperatures between 88 and 980 ° C., the current density being around 0.7 to 1.2 A / dm2. These details correspond to preferred temperature and current density conditions. The reactions taking place at the electrodes are whence the overall reaction results. Titanium, carbon or lead alloys have proven useful as electrode materials, in particular for the anode. DE-AS 1 796 305 also discloses a platinized titanium electrode for use in an electrolytic bath containing hydrochloric acid and manganese dichloride. In this case, the anodic development of chlorine gas places special demands on the corrosion resistance of the electrode.

That which occurs in layers up to 30 mm thick during electrolysis Anode product (electrolyte brown stone) is removed after the anode has been pulled out of the bath, supported by mechanical shocks, removed from it and by various Post-treatments such as washing, neutralizing, re-filtering, drying and Chopping brought to the desired quality level.

Electrolyte brown stone has a MnO2 structure and is coated with ramsdellite, MnO2 x H2O, isotype. This structure is derived from the rutile lattice by certain Resettlement of the lattice sites (exchange of O2 ions for OH ions, replacement from Mn4 + through Mn3 +). It is responsible for the electrochemical activity of a brownstone In general, the more favorable the more its fine structure is from a definable, ordered one Crystal lattice deviates.

In terms of batteries, the electrolyte brownstone is the natural brownstone consider why the commercial application, especially for the powerful Alkali-manganese cells, almost entirely limited to the electrolytically obtained6 MnO2. Above all, alkaline brownstone cells are characterized by an, albeit limited, one Rechargeability from, provided that one ensures that the oxidation state of the 3-valent Manganese is not fallen below during the discharge. The main reason for this lies in the fact that the reduction of the electrochemically active db MnO2 bis for structurally identical s MnO (OH) takes place in a homogeneous phase. According to K. Kordesch et al, Electrochimica Acta, Vol. 26, No. 10, 1497-1500 (1981) was used in comparative tests of alkaline MnO2-Zn cells with an electrolyte brown stone, which is an optimal Selection from a quality list (International Common Samples of manganese dioxide) represents a number of cycles of 42 until the end-of-discharge voltage 1.0 V is reached, the depth of discharge being 33%. One slight increase the cyclability beyond that was according to DE-OS 30 26 065 only by mechanical The brownstone-graphite mass can be clamped in a cage-like arrangement, which prevents the volume expansion of the mass during discharge.

The invention is based on the object of an electrolyte brownstone to make available, which as a cathode compact or as one with an elastic Binder-bound cathode bodies less for loosening up during cycling and tends to lose line contact and with the better reversibility the cathode reaction up to an oxidation value of approx. MnO1.6 Depth of discharge can be ensured.

The object is achieved according to the invention in that the electrolysis bath titanium salts, in particular titanium sulfide or titanium oxide, are also added.

When studying the electrolytic production of manganese dioxide it had been shown that with the anode product a higher than 100% The number of cycles that could be achieved up to then with the best brownstone, No. 2 from the series of International Common Samples, was found when the - preferably sulfuric acid - electrolysis bath soluble titanium compounds added had been. According to the most stable valency level of titanium come here mainly titanium (IV) compounds in question. The soluble titanium (IV) compounds they all have a very strong tendency to hydrolytic cleavage. Hence, at least they will partial hydrolysis as titanyl compounds with the TiO2 + ion. When smoking Ti (SO4) 2 is initially formed from TiO2 with concentrated sulfuric acid Dilute the solution in TiO (SO4), a white water-soluble powder that passes over. Similar Titanium (IV) halides can be dissolved as oxide salts of the type TiOX2 (X = Cl, Br), if the acid concentration of the solution is sufficient, complete hydrolysis to prevent TiO2 hydrate.

Finally, double salts (sulfatotitanates) can be used in the electrolysis bath. of the type Me2 [Ti (SO4) #J are present.

According to the invention, the amount of titanium added to the bath should be measured in this way be that the electrolysis bath is at least saturated with titanium ions. It's like that assume that only a certain amount of titanium salt is dissolved and that the bath composition plays a decisive role in this. Undissolved titanium salt remains as a slurry return.

Example 1: For an electrolysis bath with a total volume of 1 liter, 2.5 g titanium dioxide taken up with 5 ml concentrated sulfuric acid and smoked, until the residue is just damp. He is then in the 9400 hot bath brought in. The complete composition of the finished bath is as follows Table 1 can be found.

Table 1 Material concentration gil molar H20 1000 55.5 (distilled) MnSO4 50 0.33 (56 g MnSO4 H20) H2504 49 0.5 (density 1.84) TiO2 2.5 0.03 The others Manufacturing parameters (electrolysis conditions and electrode design) are in the tables 2 and 3 compiled.

Table 2 Cell voltage 4 to 5 V direct current 0.4 A duration 24 h temperature 94au (367 K) crude yield MnO2 5.99 pure yield MnO2 ~ 5.5 g Tabel 3 electrodes material current density area anode graphite 0.01 A / cm2 40.65 cm2 cathode Graphite 0.02 A / cm2 20.4 cm2 The electrode plates in the above embodiment a rectangular format. The distance between them is 20 mm.

It should be noted here that in practice the manganese sulfate solution is already used comes from the leaching process in ore processing and in purified form, after being reheated to over 8000, fed to the electrolytic cells will.

The small difference between the raw yield and the pure yield is through unavoidable losses during post-treatments, especially during washes conditional. These are in technical baths that are used to prevent A layer of oil or paraffin is located due to evaporation losses, with particular thoroughness to undertake.

The chemical analysis of the embodiment described above The resulting electrolyte brown stone had a TiO2 content of 4.8% by weight.

Example 2 Under otherwise identical execution conditions as in the example 1, excluding the voltage of the electrolytic cell, which in this case is only 1.8 to Was 2.2 V, titanium disulfide was selected as the additive according to the invention.

Table 4 summarizes some numerical values from this experiment so far they differ with the corresponding information from Example 1, Table 4 used Material: 1.5 g / l = 0.013 Mobil TiS2 crude yield MnO2: 14.2 g Net yield MnO2: 13.9 g Electrode gap: 30 mm Ti content in MnO2: 0.8% by weight (= 1.34% by weight TiO2) In a cell test, the rechargeability of an according to the invention manufactured brown stones tested. The depolarizer mixture prepared for this purpose consisted of 79.5% by weight MnO2; 20.0% by weight of graphite and 0.5% by weight of carbon black. One on the crowd the pressure constantly exerted was 27 N / cm2.

The discharge current was 9.55 mA / cm2, the depth of discharge 35%.

FIG. 1 shows in part a) the discharge behavior of an MnO2-Zn cell with electrolyte brownstone produced according to the invention in the depolarizer compound, in part b) for comparison the discharge behavior of a similar one, however cell equipped with a known electrolyte brown stone.

The numbers indicate the respective cycle. The discharge time of the individual cycles (the charging curves are not shown for the sake of clarity) is 60 minutes.

An electrolyte brownstone became a known comparative brownstone according to International Common Sample No. 2 elected. This product is used in electrolytic cells produced with alloyed lead electrodes (Pb Sb alloys).

According to FIG. 1, the manganese dioxide produced according to the invention achieved more than 100 cycles until the end-of-discharge voltage 1.0 V is reached, at the well-known manganese dioxide, the end-of-discharge voltage is already undercut after 42 cycles. The advantage of the product according to the invention is the increase in cyclability clearly visible by over 100%.

A hypothesis that is by no means certain about the effect according to the invention the addition of titanium to the electrolysis bath is that TiO2, which crystallizes in the rutile lattice, serves as a nucleating agent for the rebuilt ar-MnO2 structure, which is the rutile is very related. There is also the possibility that the Titan acts in the role of a "dopant". On the other hand, the fact that that simple mixing of brownstone and titanium dioxide no improvement that brings rechargeability. There could also be no improvements in this direction brownstone samples produced with titanium electrodes can be detected are (International Common Sample No. 1). The titanium electrodes bring opposite the otherwise commonly used graphite or lead electrodes have certain process advantages such as but have a longer service life of the system and lower cell resistance no noticeable influence on the cycling ability of the anode product.

The absorption of titanium up to a few percent by the manganese dioxide under the conditions according to the invention is surprising because assumed must be that the added titanium compounds due to their tendency to hydrolysis are only partially dissolved at the given acid concentrations. So much for that If so, one must suspect that first at the cathode of the electrolytic Cell reduction to Ti (III) ions takes place, which is then converted to Ti (IV) ions at the anode be oxidized. As such, they are then built into the host lattice of the # MnD2. The titanium has a stabilizing effect on this and is possibly also electrochemical active.

Claims (3)

Claims 1. A process for the production of manganese dioxide as Depolarizer for galvanic elements, in which using electrodes made of titanium, carbon or a lead alloy of acidic manganese (Ifl salt solutions of a Direct current electrolysis are subjected and the anodically deposited product Subsequent treatments such as washing, crushing, drying, characterized in that that the electrolysis bath also includes titanium salts, in particular titanium sulfide or titanium oxide attached. 2. The method according to claim 1, characterized in that the electrolysis bath contains free sulfuric acid. 3. The method according to claim 1 or 2, characterized in that the The amount of titanium additives is such that the electrolysis bath of titanium ions is at least is saturated.