RECOVERY OF LEAD IN FORM OF HIGH PURITY LEAD CARBONATES FROM SPENT LEAD BATTERIES INCL.

ELECTRODE PASTE

BACKGROUND OF THE INVENTION

Valuable high purity lead compound and eventually metallic lead is obtained from minerals as well as from electrode paste or slime recovered from dismissed lead batteries by pyrometallurgy according either to a very high temperature (1500 - 1700 ⁰ C) process with attendant generation of large quantities toxic fumes to be abated, or to a high temperature (1000 - 1100 ⁰ C) process, as often practiced in case the starting material is recovered electrode paste, that implies a significant by-production of toxic slags to be disposed of as dangerous substances in special dumps.

An attempt to improve these situations has been pursued through a carbonation technique of the raw (impure) electrode paste in plants for producing secondary lead.

This technique gives substantial advantages in terms of reduced impact on the environment but poor economical results and therefore has not encountered diffused acceptance in the industry. Many have endeavoured to improve the performance of this technique as testified by the numerous patent publications such as: U.S. Pat. No. 5,827,347, also published as WO9966105; U.S. Pat No. 5,840,262 also published as WO9858878; the European Patents No. 1619261, No. 1656463; the Italian Patent No. 01239001; the U.S. Pat. No. 4,336,236 and No. 1,738,081; the BP patents No. 239,257 and No. 272,053; the Belgian Patents No. 273,660 and No. 691,028 as well as the project "Cleanlead Gypsum" partially fmanciated by the European Union.

Lead is extracted from a lead mineral such as galena, through a high temperature (over 1500°C) pyro-metallurgical process that produces impure lead. All sulphur

present in the starting material is converted to SO ₂ such that generally to the pyro- metallurgical plant is associated a plant for producing sulphuric acid in order to avoid inertization of SO ₂ in the form of CaSO ₄ , which has an extremely low commercial value.

OBJECTIVES AND SUMMARY OF THE INVENTION

It has now been found and is the object of the claimed invention a wet process that achieves a practically complete conversion of the lead content of recovered electrode paste or slime from dismissed lead batteries, typically a grossly impure mixture of dioxide, sulphate and oxysulphate of lead that compose the electrode paste of a battery and/or of a lead mineral, to carbonate and/or oxycarbonate of lead of very high purity.

The obtained high purity lead carbonate mixture may be eventually converted through a relatively low temperature heat treatment in oven to lead oxide of extremely high purity, above 99.9%, which is perfectly suitable to be directly re- used for preparing electrode pastes of new batteries.

Of course, the mixture of highly pure carbonate and oxycarbonate of lead that is produced may even be employed as starting material for producing other useful lead compounds or for producing metallic lead by thermal reduction or electrolysis.

The basic operations contemplated by the novel process of this invention are all performed in aqueous suspension or solution at atmospheric pressure, with indoubtable advantages from the point of view of reduced energy consumption and reduced environmental impact. Moreover, only simple apparatuses of generally common use in the chemical industry, such as for example stirred reactors, dosing pumps, filters and crystallizers are needed.

The reagents required by the process are:

a) an acid different from sulphuric acid capable of dissolving lead oxide;

b) hydrogen peroxide or alternatively sulphurous anhydride or a sulphite to reduce lead dioxide if it is present in the starting material, as for example in the case of electrode paste or slime according to one of the commonly used techniques to form lead sulphate;

c) sulphuric acid for converting dissolved lead compounds to insoluble sulphates;

d) a water soluble compound capable of selectively dissolving lead sulphate in respect to other insoluble sulphates;

e) a water soluble carbonate salt for forming insoluble lead carbonate and a water soluble sulphate of the cation of the carbonate salt employed.

Besides producing lead carbonate and/or oxycarbonate, the process produces as by-product sulphate of the cation of the carbonate salt employed for precipitating lead carbonates. The sulphate salts that remain in solution may all be separately recovered at high purity and may be easily commercialized in a remunerative manner on the market.

According to a first preferred embodiment of the process of this invention, particularly suitable for treating raw recovered electrode paste or slime, the impure material is subjected to the following steps directly without any pyrometallurgical pre-treatment:

1) acid leaching of the impure material employing an acid different from sulphuric acid and capable of forming soluble salts of Pb(II) such as for example acetic acid, nitric acid, fluo-boric acid and fluo-silicic acid;

2) addition to the acid leach suspension of hydrogen peroxide or of sulphurous anhydride or of a sulphite for reducing the undissolved lead dioxide to soluble compounds of Pb(II) which dissolve in the acid leach solution or for forming insoluble lead sulphate;

3) addition of sulphuric acid to the acid leach suspension for reacting with dissolved Pb(II) contained in the acid leach solution forming insoluble lead sulphate;

4) separation of the solid phase of the sulphatated suspension constituted by lead sulphate and of undissolved impurities of the starting material, recovery of the clarified acid leach solution, reconstituted by the preceding sulphatation step and eventual recycling of the acid solution to step 1);

5) treatment of the separated solid phase with an aqueous solution containing compounds capable of selectively dissolving the sulphate, as for example sodium acetate, ammonium acetate, potassium acetate, acetates of urea, mono-, di-, tri-ethanol amine, methyl, ethyl, propyl amine and of other amines and hydrosoluble ammides, sodium tiosulphate;

6) separation of a clear solution from the undissolved solid residues now constituted by only the impurities present in the starting impure material;

7) addition to the clear solution containing lead sulphate dissolved in the solubilizing solution of a carbonate (for example carbonate of sodium or of potassium or of ammonium) for precipitating insoluble lead carbonate and/or oxycarbonate and forming correspondent soluble sulphate of the cation of the carbonate salt employed in the solubilizing solution;

8) separation of the lead carbonates from the solution;

9) controlled cooling or heating of the clear solution for selectively crystallizing and separating sulphate of the cation of the carbonate salt used in the solubilizing solution and eventual recycling of the clear solution to step 5).

The so recovered mixture of lead carbonates and the sulphate of the cation of the carbonate salt used have both a high level of purity. The lead carbonates separated from the solution may be commonly converted to lead oxide and CO ₂ by heating

in oven at a relatively low temperature without adding any reagent. The lead oxide that is obtained has a purity in excess of 99.9% (normally higher than 99.99%) and may be used directly for preparing new batteries.

In case the starting material is a lead mineral or a mixture of minerals, if the starting material contains only acid leachable compounds of Pb(II), the process remains substantially identical to the one illustrated above for the case of presence of insoluble lead dioxide in the starting material (e.g. electrode slime), eliminating only step 2); differently, if the minerals contains only lead sulphate, the process obviously may start from step 5).

When starting from minerals that generally include a far larger proportion of impurities than a raw recovered electrode slime, there will be a larger residue left from the solubilization of lead sulphate. However, such a solid phase residue may be processed for extracting other metals contained in the mineral from a now more concentrated source having already extracted the lead content.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a flow chart of the main steps of a process for the recovery of lead carbonates from battery electrode slime according to a preferred embodiment. Figure 2 is a simplified diagram of a plant for recovering lead according to a preferred embodiment.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

According to a first embodiment of the method of this invention, particularly suited for treating raw electrode slime recovered from crushed dismissed batteries, but usable with eventual adaptations with a finely ground lead minerals, an acid leaching of the impure starting material with an acid different from sulphuric acid and capable of leaching compounds of Pb(II) (e.g. lead oxide) is first carried out (step 1) and hydrogen peroxide or sulphurous anhydride or a sulphite is added to the acid leach suspension (step 2) in order to reduce suspended lead dioxide

(Pb(IV)) to lead oxide (Pb(II)) that dissolves in the acid leaching solution or to form soluble lead sulphate.

For example an aqueous solution of acetic acid with a concentration that may be comprised between about 3% to 85% by weight can be satisfactorily used with dropwise addition of hydrogen peroxide for leaching completely all the lead (II) oxide and lead (IV) oxide contained in an electrode slime maintained suspended in the acid leach solution at a temperature that may be comprised between about 10 ⁰ C up to boiling point, stirring the suspension with a pails or turbine mixer in order to favor disgregation of lead dioxide aggregates.

Alternatively, a leaching solution of nitric acid with a concentration that may be comprised between about 2% and 30% by weight, or of fluoboric acid or fluosilicic acid in a concentration that may generally be comprised between about 5% and 40% by weight may be used instead of acetic acid.

The combination of the chosen leaching conditions (type and concentration of the acid, eventual hydrogen peroxide addition, temperature and stirring mode will influence the time needed for completing these steps of dissolution and/or reduction-dissolution of all the lead (II) oxide and lead (IV) oxide contained in the starting material.

Contrarily to the long established approach of prior art processes, according to a fundamental aspect of the process of this invention, a practically complete sulphatation of all the lead causing its precipitation as insoluble lead sulphate that is then separated from a thus clarified solution together with all undissolved impurities present in the impure starting material is carried out (steps 3-4).

The sulphuric acid used for this step should preferably have a high concentration in order not to excessively dilute the acid leach solution. The sulphuric acid may be introduced in a stoichiometric amount or slightly above it on the basis of an estimated content of dissolved lead in the suspension.

Thereafter, a selective dissolution of lead sulphate in respect to the other undissolved impurities separated from the acid leach solution is performed by suspending the separated solid fraction in an aqueous solution of an acetate or other suitable salt (step 5).

With the method of this invention a clear solution in which is present lead sulphate as the only dissolved salt other than the salt employed for making the solubilizing solution is obtained.

The subsequent carbonation reaction of the clear solution containing lead sulphate, by adding to the solution a carbonate of sodium or of potassium or of ammonium as commonly used for such a reaction (step 7) causes the selective precipitation of lead carbonate and of lead oxycarbonate because these salts have a solubility that is much lower than that of lead sulphate.

Once the reaction is complete, the precipitated lead carbonates are separated by filtration from the solution while sulphate of the cation of the carbonate salt used for precipitating the lead generally as a mixture of lead carbonate and of lead oxycarbonate remains in the solution.

The clear solution containing the sulphate of the cation of the carbonate salt employed, can be integrally recycled to the step of selective dissolution of the lead sulphate (step 5) of the process, for as long as the content of sulphate of the cation of the carbonate salt used does not reach saturation (this limit depending on the type of solution being used for dissolving the lead sulphate). Precipitation of the sulphate salt together with the lead carbonates must be prevented. Therefore, excess sulphate salt must be eventually eliminated from the solution before approaching saturation. This may be easily done by exploiting the different solubilities at different temperatures of the sulphate salt (i.e. of sodium, potassium or ammonium sulphate).

The concentration of the aqueous solubilizing salt solution and the temperature at which dissolving of the lead sulphate in it is carried out are not essential

parameters because they simply influence the time necessary for completing the treatment and the quantity of lead sulphate that may be dissolved in the solution. In practice, the solubilizing solution, after precipitation of the dissolved lead as carbonate, is recycled back and therefore when operating with a recycled solution with a declining concentration of acetates, more and more recycles may be necessary to complete dissolution of a given quantity of lead sulphate.

The lead carbonate and/or oxycarbonate that are obtained can be thermally decomposed in a rotating oven at the temperature of 350°C thus obtaining extremely pure lead oxide (>99.9%) that is perfectly suitable for preparing electrode pastes for new batteries.

It has been found and is an optional accessory aspect of the preset invention that it is possible to make with the lead oxide obtained by the process of this invention an electrode paste that may be employed even days after having been prepared. As a matter of fact, production and storage of lead oxide in the form of a paste reduces safety problems for operators attributable to the pulverulent form of lead oxide. According to an embodiment, the lead oxide is transformed into a paste by mixing 90.8% by weight of lead oxide with 7.49% by weight of water and 1.7% by weight of pure glycerin.

Similar results are obtained by employing as humidifying polar substance instead of glycerin, ethylenic, propylenic or polyethylenic glycol having an average molecular weight comprised between 100 and 2000. The amount of humidifying polar substance to be added, may vary between about 1% and about 3% by weight, the pastosity of the dough that is obtained is substantially the same, the only difference being in the storability of the prepared paste that will be possible for longer and longer time the greater is the quantity of polar substance that is added. These polar substances do not alter the normal functioning of the battery, because they do not interfere with the electrodic processes.

The practice of the process of this invention using as starting material a mineral or a mixture of minerals of lead may be completely similar to the above described

process, an essential step being that of converting for as much as possible any different salt or lead present in the mineral to either lead sulphate or to lead oxide form. For example, in case of galena, certainly the most common lead mineral, the mineral must be heated in air, according to common techniques until oxidizing the lead sulphide to sulphate. The other common mineral anglesite does not need prior treatment being itself already constituted by lead sulphate. Of course the mineral(s) should be finely ground for facilitating the contemplated wet process steps.

Herein below several examples are reported solely for illustrating different possible embodiments of the process of this invention without in any manner meaning to exclude other possible embodiments.

Example 1

15g of recovered dried electrode paste having a lead content, expressed as metal equivalent, of 72% was treated under stirring for 10 minutes with 100 ml aqueous solution of acetic acid at 15% by weight, at the temperature of 50°C. After 10 minutes hydrogen peroxide at 32% by weight has been added dropwise until obtaining a no longer increasing clarification of the suspension.

The stirring of the suspension was maintained for further 35 minutes and thereafter sulphuric acid at 96% by weight was added to the clarified suspension. The hot suspension was then filtered and the recovered solid phase was rinsed with a small quantity of deionized water whilst the recovered limpid solution was found to be practically free of lead and was thereafter employed for acid leaching a fresh 15 grams amount of dried electrode paste at the same treatment conditions.

This sequence of steps was repeated 24 times and the repeatedly recycled acid leach solution did not show any impairment of its effectiveness.

The recovered solid phase was suspended in a solution composed of 6Og of sodium acetate and lOOg of water and kept under stirring for 30 minutes at the

temperature of 70 ⁰ C. Thereafter the suspension was filtered for separating a limpid solution containing lead sulphate in the form of a soluble complex and a solid phase of dark grey color that was recycled to the initial step of acetic acid leach.

The limpid solution containing lead sulphate was stirred at 36 ⁰ C adding thereto sodium carbonate until reaching a practically complete precipitation of the lead in the form of lead carbonate and lead oxycarbonate. The suspension was thereafter filtered separating the lead carbonates from the solubilizing solution of sodium acetate now enriched of sodium sulphate that was maintained at the temperature of 36°C and recycled to the step of dissolution of the lead sulphate as long as the content of sodium sulphate in solution remained below saturation.

At this point, the solution was slowly cooled to 15°C under slow stirring. A crystalline solid phase constituted by sodium sulphate was recovered by filtering the suspension while the clear solution was recycled to the dissolving step of the lead sulphate.

The filtered lead carbonate and oxycarbonate accurately rinsed with de-ionized water were dried at 110°C for as long as reaching constancy of weight.

At the end the following mass balance was recorded.

In each 15g amount of recovered electrode paste used in the experiment, were present 1.2 g of insoluble substances of dark grey color containing metallic lead and extraneous substances such as sand, carbon black, barium sulphate, etc.

The maximum quantity of recoverable lead carbonate was of 13.02g while the quantity of lead carbonate effectively recovered was of 12.89g for a recovery yield of 99%.

Chemical analysis of the solid product recovered was constituted exclusively by the double salt of formula 2PbCO ₃ PbO and by PbCO ₃ at 99.99% purity, while the sodium sulphate that was eventually recovered had a purity of about 99.90%.

The following table summarizes relevant conditions, peculiarity and results of the process of the above Example 1 described in detail together with relevant conditions peculiarity and results that were obtained, always using as starting material 15g of electrode paste of the same lot recovered from crushed dismissed batteries, according to nine different examples of practicing the method of this invention.

Example 11

5g of lead carbonate salts mixture obtained according to the process described in Example 1, were heated in oven up to 350 ⁰ C and maintained at this temperature for 60 minutes.

At the end of the treatment 4.17g of a pulverulent substance of yellow color were recovered. The yellow substance proved to be constituted by α lead oxide of purity above 99.99%.

Example 12

5g of lead carbonate salts mixture obtained according to the process described in Example 7, were heated in oven up to 45O ⁰ C and maintained at this temperature for 60 minutes.

At the end of this treatment 4.17g of a pulverulent substance having an orange color were recovered. The substance proved to be constituted by β lead oxide of purity greater than 99.99%.

To 3 g of lead oxide were added water and pure glycerin in order to obtain a mixture having the following composition by weight:

lead oxide 91%, water 7%, glycerin 2%.

The mixture was accurately mixed obtaining a paste with the consistency of a stucco. The paste was placed in a sealed container and after 12 hours its consistency was verified. The paste was found to practically retain the same consistency for as long as 72 hours from its preparation.

Example 13

1000 g of lead carbonate mixture obtained according to the processing choices of Example 2, was treated in an externally heated, rotating tubular oven.

The temperature inside the oven was maintained at 35O ⁰ C for one hour and during

the treatment a stream of 30 liters of pure anhydrous nitrogen was passed through the oven.

The gases exiting the oven were conveyed to a condenser and cooled down to the temperature of -8O ⁰ C in order to liquefy the carbon dioxide released during the thermal decomposition of the carbonates. At the end of the test, 16Og of extremely pure carbon dioxide were recovered, usable even for gassing beverages.