WO2018065948A1

WO2018065948A1 - Process for desulphurising a lead- containing material in the form of pbso4

Info

Publication number: WO2018065948A1
Application number: PCT/IB2017/056177
Authority: WO
Inventors: Massimo Maccagni; Edoardo GUERRINI
Original assignee: Engitec Technologies S.P.A.
Priority date: 2016-10-07
Filing date: 2017-10-06
Publication date: 2018-04-12
Also published as: IT201600100862A1

Abstract

The present invention relates to a process for desulphurising a material containing PbSO₄, which comprises the following steps: (a) reacting said material in water with NaOH to produce an aqueous solution of Na₂SO₄ comprising a desulphurised material in the form of a dispersed solid containing lead oxide, lead hydroxide or mixtures thereof; (b) separating said desulphurised material from said aqueous solution of Na₂SO₄,- (c) subjecting said Na₂SO₄ aqueous solution coming from said step (b) to an electrolysis process in at least one electrolytic cell in order to form at least a H₂SO₄ aqueous solution, a NaOH aqueous solution and a desalinated aqueous solution comprising Na₂SO₄,- (d) recycling at least part of said NaOH aqueous solution and at least part of said desalinated aqueous solution comprising Na₂SO₄ to said step (a).

Description

PROCESS FOR DESULPHURISING A LEAD-CONTAINING MATERIAL IN THE FORM OF PBS0₄

The present invention relates to a process for desulphurising a material containing lead in the form of PbS0 . The process according to the present invention is particularly suitable for desulphurising the active mass based on lead (pastel) of exhausted lead-acid accumulators .

As is known, at the end of their useful life, lead- acid accumulators are subjected to recycling processes for recovering the materials forming the various components .

One of the most widely-used current recycling processes involves grinding the exhausted accumulators and subsequently separating the ground part into homogeneous fractions of materials.

The grinding of the accumulators is effected under wet conditions, after removing the exhausted electrolyte contained in the accumulators. The exhausted electrolyte essentially consists of an aqueous solution of sulfuric acid containing metal impurities .

In the most advanced form of the process, the material leaving the grinding step is subjected to hydrodynamic and hydrostatic separation treatment from which the following fractions of materials can be recovered :

(a) a metal fraction based on lead composed of the material forming the grids (electrodes) and the poles of the accumulators; this fraction is substantially composed of lead alloys (e.g. alloys with Sn and Ca) ;

(b) a fraction of polymeric materials deriving from the outer casings of the accumulators; this fraction is mainly composed of polypropylene;

(c) a fraction of polymeric materials deriving from the separators of the accumulators; this fraction is mainly composed of polyethylene and possibly a minor component of PVC;

(d) a lead pastel, i.e. the active mass of the accumulators on which, during use, the charging and discharging processes take place; the pastel is mainly composed of lead sulphate and lead oxides.

Examples of recovery processes of the components of exhausted lead-acid accumulators are described in US 1, 769, 116 and US 2006/0018819 Al .

The recovery of the lead present in the lead fraction, i.e. in the above fractions (a) and (d) , is mainly effected by means of pyrometallurgical processes, which involve melting the lead fraction under reducing conditions to obtain lead metal.

In some pyrometallurgical processes, the sulphur is removed from the pastel before this is fed, together with the metal fraction, to the melting furnace, in order to avoid the formation of sulphur oxides (mainly SO₂) , which would require onerous purification treatments of the gaseous effluents or the addition of reagents to the furnace charge which allow the sulphur to be fixed to the melting slag.

The desulphurisation of the pastel effected before melting the lead fraction, moreover, reduces fuel consumptions of the furnace and the quantity of slag produced in the melting process.

The desulphurisation process of the pastel can be carried out in various ways. In one embodiment, the pastel is reacted in water with an excess of an alkaline compound so as to solubilize the sulphur in the water, obtaining an aqueous solution containing sulphate ions. The most used alkaline compounds are NaOH, Na₂C0₃, NaHC0₃ and NH4HCO3.

When NaOH is used, the main reaction which takes place during the desulphurisation is the following:

PbS0₄ + 2 NaOH → Pb(OH)₂ + Na₂S0₄ (1) The solution of sodium sulphate obtained from the desulphurisation of the pastel is typically subjected to a purification treatment to eliminate the metal impurities present and subsequently to crystallization to obtain anhydrous sodium sulphate as final product of commercial value.

As the crystallization process, however, involves the evaporation of high volumes of water, it generates high energy consumptions and an onerous plant management .

Further disadvantages associated with the recovery of sulphur in the form of anhydrous sodium sulphate are also connected with the necessity of providing adequate storage spaces of this product in the plant and its relatively low commercial value due to the increasingly low request for this product on the market.

In an attempt to at least partially overcome the above-mentioned drawbacks, EP 0454257 Al proposes electrolytically treating the solution of sodium sulphate generated by the desulphurisation of the pastel with NaOH in a dual ion membrane electrolytic cell. The electrolysis generates an aqueous solution of NaOH and an aqueous solution of sulfuric acid. The solution of NaOH, after concentration by evaporation up to values of about 20% by weight, can be re-used in the desulphurisation process or for neutralizing the exhausted sulphuric acid of the accumulators. The solution of sulphuric acid, after concentration by evaporation up to values of about 35% by weight, can, on the other hand, be used as raw material for producing the electrolyte of new accumulators.

Although the process described in EP 0454257 Al entails reduced consumptions of NaOH, it has in any case the disadvantage of providing the concentration by evaporation of the electrolysis products. As already indicated, this process entails high energy consumptions and onerous plant investments. The evaporation of the water, moreover, makes it necessary to reintegrate it into the desulphurisation process, with a consequent increase in the overall consumptions of this reagent.

It is also noted that in processes in which a concentration step of aqueous solutions is provided for the recovery of the reagents, as in EP 0454257 Al, the solution used for the desulphurisation must be a solution having a relatively high concentration of alkaline compound (NaOH) , so as to limit the volume of water to be evaporated in the concentration step. The use of concentrated desulphurising solutions, however, has the disadvantage of generating a desulphurised pastel (so-called cake) containing considerable quantities of sodium sulphate dissolved in the imbibition water, which must be removed as much as possible through the subsequent washing with water of the desulphurised pastel. The sodium sulphate present on the desulphurised pastel, is in fact extremely undesirable due to the management problems of the melting slag which this can create when subjected to melting .

Furthermore, the electrolysis process described in EP 0454257 Al generates a waste solution (desalinated solution) containing Na₂SC> not converted into NaOH and H₂SO₄, which must be adequately disposed of.

A further drawback of the electrolytic process of EP 0454257 Al is connected with its implementation in batch mode. As the electrolysis proceeds, in fact, the concentration of sodium sulphate in the supply compartment of the cell progressively decreases, making the passage of the current less effective. This leads to a progressive increase in the cell voltage and therefore energy consumptions of the electrolysis process .

Considering the above state of the art, the Applicant has set the primary objective of providing a desulphurisation process of a material containing lead in the form of PbSC , such as lead pastel deriving from the recovery of lead-acid accumulators, in a simple and effective way, at least partially overcoming the drawbacks of the known art.

In particular, a first objective of the present invention is to provide a process for desulphurising a material containing PbSC , which results in a reduced consumption of energy and chemical reagents with respect to the processes of the known art.

A second objective of the present invention is to provide a process for desulphurising a material containing PbSC , which can be carried out in desulphurisation units having reduced dimensions and with a simpler management with respect to the plants used for implementing the desulphurisation processes of the known art .

A further objective of the present invention is to provide a process for desulphurising a material containing PbSC^, which can also be carried out in continuous mode, so as to make the desulphurisation process and also the recovery process of the components of the accumulators in which it is possibly inserted, more efficient.

The Applicant has now found that the above and further objectives, which will be better illustrated in the following description, can be achieved by means of a process for desulphurising a material containing PbSC , which uses NaOH as alkaline desulphurising agent, wherein the products generated by the electrolysis of the solution of sodium sulphate deriving from the desulphurisation are conveniently recirculated inside the desulphurisation process.

In particular, it has been observed that the contemporaneous recirculation to the desulphurisation step of the solution of NaOH and of the desalinated solution containing Na₂SC> generated by the electrolysis, not only reduces the consumptions of reagents, but also avoids the production of a process residue to be disposed of (i.e. desalinated solution) and significantly reduces the energy consumptions of the concentration step of the aqueous solutions.

In particular, according to the present invention, the NaOH obtained from the electrolysis is re-used as such in the desulphurisation process, without being subjected to any concentration step, thus significantly reducing the energy consumptions. The operations of concentration of the aqueous products are therefore limited, possibly, to the sole solution of H₂SO produced by the electrolysis thus making it suitable for subsequent uses, for example for the production of an electrolyte for new lead-acid accumulators. The process according to the present invention thus allows the use of very compact concentration and desulphurisation plants which are easier to manage. The process according to the present invention also allows to reduce the quantity of water that has to be reintegrated in the desulphurisation process, and thus the consumptions of this reagent.

Furthermore, as the recycling of the solution of NaOH generated by electrolysis implies a substantial reduction in the overall volumes of water to be evaporated, the desulphurisation step can be carried out using basic desulphurising solutions having a relatively low concentration, with the consequent advantage of reducing the concentration of sodium sulphate which remains imbibed in the desulphurised pastel, even up to 50% by weight less.

The Applicant has also surprisingly observed that by suitably recirculating the electrolytic solutions in the electrolysis cell, the electrolysis process, and possibly also the desulphurisation process, can be effected in continuous mode, further reducing the overall energy consumptions of the desulphurisation process.

According to a first aspect, the present invention therefore relates to a process for desulphurising a material containing PbSC , which comprises the following steps :

(a) reacting said material in water with NaOH to produce an aqueous solution of Na₂S0 comprising a desulphurised material in the form of a dispersed solid containing lead oxide, lead hydroxide or mixtures thereof;

(b) separating said desulphurised material from said aqueous solution of Na₂S0₄,-

(c) subjecting said aqueous solution of Na₂SC> coming from said step (b) to an electrolysis process in at least one electrolytic cell in order to form at least an aqueous solution of H₂SO₄, an aqueous solution of NaOH and a desalinated aqueous solution comprising Na₂S0₄;

(d) recycling at least one part of said NaOH aqueous solution and at least one part of said desalinated aqueous solution comprising Na₂S0₄ to said step (a) .

In a first preferred embodiment of the invention, the electrolysis process is a bipolar membrane electrodialysis process.

In a second preferred embodiment, the electrolysis process is carried out in at least one three- compartment membrane electrolytic cell.

In a third preferred embodiment, the electrolysis process is carried out in an electrolytic system comprising at least one first cationic permselective membrane electrolytic cell and at least one second anionic permselective membrane electrolytic cell, wherein an anode compartment of said first cationic permselective membrane electrolytic cell is fluidly connected to a cathode compartment of said second anionic permselective membrane electrolytic cell.

The electrolysis process is preferably carried out in continuous mode, for example keeping the concentrations of one or more electrolytic solutions substantially constant in the respective compartments of the electrolytic cell.

According to a second aspect, the present invention relates to a process for the recovery of one or more components of a lead-acid accumulator which comprises desulphurising a lead pastel of an exhausted lead-acid accumulator according to the above desulphurisation process .

For the purposes of the present description and annexed claims, the verb "comprise" and all the terms deriving therefrom also includes the meaning of the verb "consist" and the terms deriving therefrom.

The limits and numerical ranges expressed in the present description and enclosed claims also include the numerical value (s) mentioned. Furthermore, all the values and sub-ranges of a limit or numerical range should be considered as being specifically included as if they were explicitly mentioned. Further characteristics and advantages of the present invention will appear evident from the following detailed description of the invention in which reference is also made to the enclosed figures, in which:

figure 1 schematically represents a process for the recovery of the components of a lead-acid accumulator in which the desulphurisation process according to the present invention is integrated;

- figure 2 schematically represents an embodiment of the invention wherein the electrolysis is conducted in a three-compartment membrane electrolytic cell;

figure 3 schematically represents an embodiment of the invention wherein the electrolysis is conducted in an electrolytic system comprising two membrane cells connected with each other in series;

figure 4 schematically represents an embodiment of the invention wherein the electrolysis is conducted in a bipolar membrane electrodialysis cell (single unit cell);

figure 5 schematically represents an embodiment of the invention wherein the electrolysis is conducted in a bipolar membrane electrodialysis cell (double unit cell) .

With reference to figure 1, this describes the desulphurisation process according to the present invention integrated in a process for the recovery of materials forming the components of exhausted lead-acid accumulators .

In figure 1, the accumulators are fed, through line

1, to a grinding and separation unit MS of the various components into homogeneous fractions of material. The fractions of material separated in the unit MS include the lead pastel (line 2), the exhausted electrolyte (line 3), the fraction of polymeric materials (line 4) deriving from the separators of the accumulators, the fraction of polymeric materials 5 deriving from the outer casing of the accumulators (essentially polypropylene) and a metal fraction mainly deriving from the grids and poles of the accumulators (line 6) .

The lead pastel is fed through line 2, to a unit PD in which the desulphurisation reaction with NaOH takes place. The desulphurisation with NaOH can be effected according to methods known in the art. The pastel is preferably reacted with a stoichiometric excess of NaOH with respect to the sulphur content of the pastel, for example an excess of 5-30% of NaOH with respect to the stoichiometric quantity necessary for converting the sulphur present. The pastel to be desulphurised (line 2) can be reacted, for example, with a solution of caustic soda at 30-50%, possibly together with the exhausted electrolyte (line 3) . According to the present invention, the NaOH used in the desulphurisation is advantageously at least partly that produced by the subsequent electrolysis of the solution of Na₂S0 obtained at the end of the desulphurisation (line 7) . More preferably, the whole of the NaOH produced by the electrolysis step is used in the desulphurisation step.

The desulphurisation reaction is preferably carried out in a reactor at a temperature within the range of 20-80°C; preferably at a pressure within the range of 0.5-2 atm, more preferably at atmospheric pressure. The reaction is carried out for a time sufficient for obtaining a conversion of 95-99% by weight of PbSC^ into Na₂S0 , lead oxides and hydroxides.

At the end of the desulphurisation reaction, the reaction mixture is composed of a solution of Na₂S0 in which a desulphurised material comprising particles of lead oxide and/or lead hydroxide, is dispersed. The reaction mixture and the desulphurised material can also contain impurities of other elements, in particular metals, such as, for example, As, Sb, Sn, Na and Ca. The pH of the reaction mixture at the end of the desulphurisation is approximately within the range of 10-13.

The desulphurised material in solid dispersion containing lead oxides and/or hydroxides is then separated from the reaction mixture, for example by filtration in a filter-press, so as to obtain a desulphurised pastel (cake) (line 8) . The desulphurised pastel can be subsequently washed and then dried, before being fed, together with the metal fraction (line 6), to a furnace F of a pyrometallurgical process for the recovery of metallic lead (line 10) . The desulphurised cake can also contain sodium (in addition to the sodium sulphate present in the imbibition water) , deriving from the treatment with NaOH, for example in a quantity within the range of 0.1-3.0% by weight .

The solution of Na₂S0 substantially devoid of lead is fed, through line 11, to a purification and neutralisation unit NP . The NaOH which has not reacted during the desulphurisation is neutralised, in the unit NP, by the addition of sulphuric acid (lines 12, 13) . The sulphuric acid used is advantageously that generated in the subsequent electrolysis treatment of the purified and neutralised solution of Na₂S0 . The solution of Na₂SC> is preferably neutralised until a pH within the range of 8-10 is reached.

In the unit NP, the solution of Na₂SC>4 leaving the desulphurisation unit PD (line 11) can be optionally subjected to a purification treatment to eliminate the metal impurities. For this purpose, for example, sodium sulphide and a ferrous salt (e.g. iron (II) sulphate) can be added to the solution of Na₂SC> so as to precipitate the impurities contained in solution. The precipitate can then be separated from the solution of Na₂SC>4, for example by decanting, so as to obtain a clarified solution of Na₂S0₄ (line 14) which is subsequently fed to the electrolysis step in the unit ME .

An aqueous solution of NaOH (line 7), an aqueous solution of H₂SO₄ (line 12) and a desalinated solution containing Na₂SC>4 not converted by electrolysis (line 15) , are generated by the electrolysis process in the unit ME. According to the present invention, the solution of NaOH and the desalinated solution are sent, through the respective lines 7 and 15, to the desulphurisation unit PD.

The solution of H₂SO₄ which is possibly not sent to the unit NP through line 13, can be recovered (line 16) and destined for new uses, possibly after concentration . As, at the end of the desulphurisation reaction, part of the sodium introduced as NaOH typically remains entrapped in the desulphurised pastel, the portion of sodium missing can be reintegrated by adding further NaOH (make up), through line 17, to the desulphurisation unit PD.

For the purposes of the present invention, the membrane electrolysis process can be carried out with electrochemical systems and techniques known to skilled persons in the field. The density of current applied to the electrodes is preferably selected within the range of 100 - 5000 A/m². The electrolysis is preferably carried out at a temperature within the range of 20- 80°C, more preferably at room temperature (25°C) .

For the purposes of the present invention, however, it is particularly advantageous to use the electrolytic systems described hereunder.

According to a first preferred embodiment of the present invention, illustrated with reference to the enclosed figure 2, the membrane electrolysis process comprises the following steps:

(i) providing at least one electrolytic cell 201 comprising :

- at least one anode compartment 202 comprising at least one anode 203 immersed in an anolyte,

- at least one cathode compartment 204 comprising at least one cathode 205 immersed in a catholyte,

- at least one supply compartment 206 interposed between said anode compartment 202 and said cathode compartment 204;

said supply compartment 206 being separated from said anode compartment 202 by at least one anionic membrane AM;

said supply compartment being separated from said cathode compartment by at least one cationic membrane CM;

(ii) feeding the aqueous solution of Na₂SC> substantially devoid of lead 207, coming from the desulphurisation of the pastel, to said supply compartment 206 and applying a potential difference between said anode 203 and said cathode 205 so as to form an aqueous solution of H₂SO 208 in said anode compartment 202, an aqueous solution of NaOH 209 in said cathode compartment 204 and a desalinated aqueous solution 210 comprising Na₂S0₄ in said supply compartment 206, by means of electrolysis.

During the electrolysis, the potential difference applied to the electrodes induces the electrolysis of the water in the cell, with the production of H⁺ ions in the anode compartment 202 and OH- ions in the cathode compartment 204. The Na⁺ cations of the solution fed to the central compartment 206, under the thrust of the electric field, migrate towards the cathode compartment 204 passing through the cationic permselective membrane CM. In the cathode compartment 204, the Na⁺ ions combine with the OtT ions forming NaOH. Analogously, the S0₄ ^2~ anions of the solution of Na₂S0₄ 207 fed to the central compartment 206, under the thrust of the electric field, migrate towards the anode compartment 202 passing through the anionic permselective membrane AM.

In the cathode compartment, the S0₄ ^2~ ions combine with the H⁺ ions forming H₂SO₄. As the membranes AM and CM are ion-selective, the migration of the Na ions from the supply compartment 206 towards the anode compartment 202 and that of the S0₄ ^2~ ions towards the cathode compartment 204, is substantially inhibited.

During the electrolysis, due to the migration of the ions from the supply compartment 206 towards the adjacent compartments 202 and 204, the concentration of Na₂SC> in the solution 207 present in the supply compartment 206, is progressively reduced, forming a desalinated solution 210 leaving this compartment.

The electrolysis is accompanied by the formation of gaseous hydrogen 216 at the cathode and gaseous oxygen 217 at the anode.

The electrolysis process in the three-compartment membrane cell can be carried out either batchwise or in continuous mode. The electrolysis process is preferably carried out in continuous mode. In this case, in a preferred embodiment, the anolyte, the catholyte and the solution of sodium sulphate to be treated are recirculated in the respective anode compartment (circuit 208), cathode compartment (circuit 209) and supply compartment (circuit 210) .

The catholyte supplied to the cathode compartment is preferably an aqueous solution of NaOH.

The anolyte supplied to the anode compartment is preferably an aqueous solution of H₂SO₄.

Once the electrolysis process in continuous mode has reached regime, it can be advantageously controlled by maintaining the concentration of Na₂S0 in the supply compartment 206, the concentration of H₂SO₄ in the anode compartment 202 and the concentration of NaOH in the cathode compartment 204, substantially constant.

During the process in continuous mode, aliquots can be removed from each recirculation circuit, of the solutions circulating therein, to be used in accordance with the present invention, as previously illustrated. With reference to figure 2, for example, an aliquot of solution of H₂SO 211 can be removed from the anodic recirculation circuit 208, which can be destined for neutralising the solution of sodium sulphate leaving the desulphurisation, and/or which can be used as raw material for producing an electrolyte for new lead-acid accumulators. Analogously, an aliquot of solution of NaOH 212 is removed from the cathodic recirculation circuit 209, which can be used in the desulphurisation step. An aliquot of desalinated solution 213, instead, can be removed from the recirculation circuit of the supply compartment 210, which can be used in the desulphurisation step of the pastel.

In order to operate the electrolysis process keeping the concentration of the anolyte and/or catholyte constant, water can be fed, for example, to the respective anode and cathode compartments. The water is fed in such a quantity as to compensate the increase in concentration of the species in the anolyte and in the catholyte and also the quantity of water leaving the respective recirculation circuits as a result of the above withdrawals. With reference to figure 2, for example, the addition of water to the anodic recirculation circuit 208 and cathodic recirculation circuit 209 is effected through lines 214 and 215, respectively. Analogously, the continuous feeding of the sodium sulphate solution to be treated can be effected by providing a recirculation circuit 210 for the supply compartment 206, wherein the desalinated solution leaving this compartment is re-fed to the inlet of the same compartment. At operating regime of the electrolysis process, an aliquot of the desalinated solution can be withdrawn from the recirculation circuit 210, through line 213, and sent to the desulphurisation process together with the solution of NaOH leaving the cathode circuit 212. The water removed from the withdrawal 213 of the desalinated solution can be compensated by feeding, into the recirculation circuit 210, further aqueous solution of sodium sulphate 207 coming from the desulphurisation process of the pastel.

By conducting the electrolysis process in continuous mode and under substantially constant concentration conditions of electrolyte in the central supply compartment 206 (and possibly also in the anode compartment 202 and cathode compartment 204) an increase in the cell voltage is avoided, which instead, is observed with the progression of the electrolysis when operating in batch mode, with a consequent reduction in the energy consumption of the process.

According to a second preferred embodiment of the present invention, illustrated with reference to the enclosed figure 3, the electrolysis process comprises the following steps:

(i) providing at least one electrolytic system 300 comprising at least one first electrolytic cell 301 and at least one second electrolytic cell 302, wherein:

said first electrolytic cell 301 comprises:

- at least a first anode compartment 303 comprising at least a first anode 304 immersed in a first anolyte;

- at least a first cathode compartment 305 comprising at least a first cathode 306 immersed in a first catholyte;

said first anode compartment 303 being separated from said first cathode compartment 305 by at least one cationic membrane CM;

said second electrolytic cell 302 comprises:

at least a second anode compartment 307 comprising at least a second anode 308 immersed in a second anolyte;

- at least a second cathode compartment 309 comprising at least a second cathode 310 immersed in a second catholyte;

said second anode compartment 307 being separated from said second cathode compartment 309 by at least one anionic membrane AM;

said second cathode compartment 309 being fluidly connected to said first anode compartment 303 of said first electrolytic cell 301;

(ii) supplying the Na₂SC> aqueous solution substantially devoid of lead 311, coming from the desulphurisation, as anolyte in said first electrolytic cell 301 and applying a potential difference between said first anode 304 and said first cathode 306 so as to form, by electrolysis, said NaOH aqueous solution 312 in said first cathode compartment 305 and a partially desalinated aqueous solution 313 comprising a mixture of H₂SO and Na₂S0 in said first anode compartment 303;

(iii) supplying said partially desalinated aqueous solution 313 as catholyte in said second electrolytic cell 302 and applying a potential difference between said second anode 308 and said second cathode 310 so as to form, by electrolysis, said H₂SO₄ aqueous solution 314 in said second anode compartment 307 and said desalinated solution comprising Na₂S0 315 in said second cathode compartment 309.

During the electrolysis, in the first electrolytic cell 301, H⁺ ions are produced in the anode compartment 303 and OH^~ ions in the cathode compartment 305. Due to the electric field, the Na⁺ ions present in the anode compartment 303 migrate towards the cathode compartment 305, through the cationic membrane, where they form the solution of NaOH 312. The H⁺ ions present in the anode compartment, on the other hand, do not substantially migrate towards the cathode compartment, as the diffusion process of the cations through the cationic membrane mainly depends on the concentration of the cationic species present; in the anode compartment, the concentration of H⁺ ions is significantly lower than that of the Na⁺ ions. The H⁺ ions produced at the anode substantially remain in the first anolyte, which is therefore substantially formed by a mixture of H₂SO₄ and Na₂S0₄.

In the second electrolytic cell 302, the OH- ions produced at the cathode 310 combine with the H⁺ ions present in the partially desalinated solution 313 coming from the first anode compartment 303, forming ¾0. Due to the electric field, the SO₄ ^~ anions migrate towards the second anode compartment 307 passing through the anionic membrane AM. The concentration of sulphate ions therefore decreases in the second cathode compartment 309, producing a desalinated solution essentially formed by an aqueous solution of sodium sulphate .

Also in this case, the electrolysis in the two cells 301 and 302 is accompanied by the formation of gaseous hydrogen (lines 316 and 317) at the cathodes and gaseous oxygen (lines 318 and 319) at the anodes.

The catholyte is preferably an aqueous solution of NaOH. The anolyte is preferably an aqueous solution of H2SO4.

The electrolysis process can also be carried out in continuous mode with the electrolytic device represented in figure 3, as illustrated for the case of the three-compartment membrane cell, by suitably recirculating the electrolytic solutions present in the various compartments. The electrolytic solutions present in the first cathode compartment 303 and in the second anode compartment 307, for example, can be recirculated in the respective compartments 303 and 307 so as to form recirculation circuits 314 and 312 analogous to the anode recirculation circuit 208 and cathode recirculation circuit 209 of the cell of figure 2 previously described. The desalinated solution 315 leaving the second cathode compartment 309 can, on the other hand, be recirculated by feeding it to the head of the first anode compartment 303, thus forming a central recirculation circuit 313, 315 analogous to the recirculation circuit 210 of the supply compartment 206 of the cell of figure 2. The anode recirculation circuit 314 and cathode recirculation circuit 312 can be provided with withdrawal lines 320 and 321 for removing aliquots of the respective solutions and inlet lines 322 and 323 for supplying the water necessary for operating the electrolytic device, keeping the concentration of anolyte and catholyte constant. Withdrawal lines of the desalinated solution and inlet lines of the solution of Na₂S0 to be treated can be provided, on the other hand, on the central recirculation circuit.

The use of this electrolytic system allows the electrolysis to be conducted, keeping the anionic permselective membrane in contact with a catholyte having a relatively low pH, thus optimising the operating conditions of the membrane.

According to a third preferred embodiment of the present invention, illustrated with reference to the enclosed figure 4, the electrolysis process can be a bipolar membrane electrolysis process. In particular, the electrolysis can comprise the following steps:

(i) providing at least one electrolytic system 400 comprising in sequence at least one anode compartment 401, at least one unitary cell 402, at least one cathode compartment 403, and wherein:

- said anode compartment 401 comprises at least one anode 404 immersed in an anolyte,

- said cathode compartment 403 comprises at least one cathode 405 immersed in a catholyte,

- said unitary cell 402 comprises in sequence a bipolar membrane BM, an acid compartment 406, an anionic membrane AM, a supply compartment 407 and a cationic membrane CM;

(ii) supplying the aqueous solution of Na₂S0 408 to be treated, coming from the desulphurisation of the pastel, to said supply compartment 407 of said unitary cell 402 and applying a potential difference between the anode 404 and the cathode 405 so as to form an aqueous solution of H₂SO 409 in said acid compartment 406, an aqueous solution of NaOH 410 in said cathode compartment 403 and a desalinated solution 411 in said supply compartment 407.

The anolyte in the anode compartment 401 and the catholyte in the cathode compartment 403 are electrolytic solutions which are such as to allow the passage of current and closure of the circuit of the device. Said anolyte is preferably a solution of sulphuric acid. Said catholyte is preferably a solution of NaOH.

The electrolytic device 400 comprises a bipolar membrane (BM) of the type known in the art. A bipolar membrane is typically composed of a side permeable to anions and a side permeable to cations, separated by an interface containing a thin film of water. The side permeable to cations faces the cathode, whereas the side permeable to anions faces the anode.

During the electrolysis, in addition to the H⁺ ions and OH- ions produced at the anode 404 and at the cathode 405 respectively, further H⁺ ions and OH^~ ions are formed in the interface of the membrane BM, which, due to the electric field, migrate towards the electrodes having an opposite polarity. In particular, the H⁺ ions, attracted by the cathode, migrate from the bipolar membrane towards the compartment 406, in which they remain confined as they are not able to pass through the subsequent anionic membrane AM. In the supply compartment 407, the SC>4^2~ ions, attracted by the anode, pass through the anionic membrane AM reaching the acid compartment 406, where they remain confined as they are not able to pass through the bipolar membrane BM. In the acid compartment 406, the SC>4^2~ ions and H⁺ ions produced by the bipolar membrane form H₂SO₄. The Na⁺ ions present in the supply compartment 407, attracted by the cathode, migrate to the cathode compartment 403, passing through the cationic membrane CM. In the cathode compartment, the Na⁺ ions combine with the OH^~ ions produced at the cathode, forming NaOH.

The above-mentioned electrolytic system can advantageously operate in continuous mode, analogously to what is previously described for the other types of electrolytic systems. For this purpose, for example, the solution forming the catholyte can be recirculated in the cathode compartment 403 through the recirculation circuit 410, whereas the solution forming the anolyte is recirculated in the anode compartment 401 through the recirculation circuit 412. The desalinated solution is recirculated in the supply compartment 407 through the recirculation circuit 411, whereas the solution of H₂SO₄ produced during the electrolysis is recirculated in the acid compartment 406 through the recirculation circuit 409.

The cathode recirculation circuit 410 can be provided with a withdrawal line 413 for removing an aliquot of the solution of NaOH produced and with an inlet line 414 for supplying the water necessary for operating the electrolytic system, keeping the concentration of catholyte constant.

The anode recirculation circuit 412 can be provided with a withdrawal line 417 for removing an aliquot of the solution of H₂SO produced and with an inlet line 418 for supplying the water necessary for operating the electrolytic system, keeping the concentration of anolyte constant.

A withdrawal line 415 of the desalinated solution and an inlet line 408 of the Na₂SC> solution to be treated can, on the contrary, be provided on the recirculation circuit 411 of the supply compartment 407.

The bipolar membrane electrodialysis system described above has the advantage of guaranteeing greater protection of the anionic membranes from deterioration phenomena which may arise during the electrolysis compared to the electrolytic cells with anionic and cationic membranes of the type shown in figures 2 and 3. In the presence of impurities in the anolyte (chloride ions, for example) , in fact, the electrolysis can cause the evolution of corrosive oxidizing substances (gaseous chlorine, for example) capable of damaging the membrane adjacent to the anode. From this point of view, anionic permselective membranes are those most susceptible to deterioration. In a further preferred embodiment, the bipolar membrane electrodialysis system comprises at least two unitary cells connected to each other in series, as represented in figure 5.

The electrodialysis device 500 of figure 5 comprises in sequence: an anode compartment 501, a first bipolar membrane BM, an acid compartment 506, a first anionic membrane AM, a first supply compartment 507, a first base compartment 520, a second bipolar membrane BM, a second acid compartment 506, a second anionic membrane AM, a second supply compartment 507, a second base compartment 520 (coinciding with the cathode compartment) .

In the electrodialysis device of figure 5, the solution of Na₂SC> to be treated is fed contemporaneously to the supply compartments 507 of each unitary cell, producing a solution of H₂SO in each of the adjacent acid compartments 506, and a solution of NaOH in each of the adjacent base compartments 520.

During the electrolysis, the H⁺ cations (produced at the anode 501 and in the bipolar membranes BM) and Na⁺ (produced from the solution of sodium sulphate to be treated) migrate towards the cathode 505, possibly passing through the cationic membranes CM present on the path. The OH^~ anions (produced at the cathode 505 and in the bipolar membranes BM) and SC> ^2~ (produced by the solution of sodium sulphate to be treated) migrate towards the anode 501, possibly through the anionic membranes AM present on the path.

The solutions leaving the various compartments can be collected and subsequently used in the desulphurisation process according to the present invention . The treatment capacity of the bipolar membrane electrodialysis cells can be varied by connecting in series an adequate number of unitary cells in relation to the quantity of solution of sodium sulphate to be treated.

The bipolar membrane electrodialysis systems comprising a plurality of unitary cells can be operated in continuous analogously to those illustrated above for single unitary cell electrolytic systems, obtaining the same advantages.

An embodiment example of the present invention is provided hereunder for purely illustrative purposes, which should not be intended as limiting the protection scope defined by the enclosed claims.

EXAMPLE 1

11 kg of a wet lead pastel (humidity 9.37% by weight, sulphur 6.42% by weight) was subjected to desulphurisation with NaOH. For this purpose, the pastel was fed to a 20-litre reactor together with 1.92 kg of NaOH (an excess of 20% estimated) and about 13 litres of water. The reaction mixture was kept under stirring at 45°C for about 1 hour. The dispersion thus obtained was filtered so as to separate a desulphurised cake containing lead substantially in the form of hydroxides/oxides, which was subsequently washed with about 2 litres of water and then left to dry at a temperature of 100°C for 12 hours. A yield of the desulphurisation process was determined on the dry cake, equal to about 96.1% (weight percentage of sulphur removed with respect to the initial weight of sulphur present in the pastel subjected to treatment) . The desulphurised cake also contained about 1.8% by weight of Na .

An aqueous solution of Na₂S0₄ was also obtained from the filtration, to which 153.2 g of a solution of H₂SO (60% by weight) were added in order to neutralize the excess of NaOH, until a pH of about 8 was reached. After neutralization, the volume of the solution was 13.4 1 and the content of Na₂S0₄ was equal to about 203.4 g/l.

The solution of Na₂S0₄ was then fed to the central compartment of a three-compartment membrane cell equipped with two electrodes (each having dimensions of 250 x 250 mm) and two membranes: one cationic permselective membrane positioned in front of the cathode, and one anionic permselective membrane positioned in front of the anode. The cell was fed, at room temperature, with a current of 188 A (^s3, 000 A/m²) .

During the electrolysis, the catholyte (aqueous solution of NaOH) and the anolyte (aqueous solution of H2 S O4 ) were circulated in the respective compartments, adding water to each stream so as to operate with substantially constant concentrations of NaOH and H2 S O4 in the respective compartments.

The electrolysis process was started by circulating 3.12 1 of anolyte containing 95.3 g/l of H2 S O4 and 3.21 1 of the catholyte containing 204,6 g/l of NaOH.

The cell voltage which, at the beginning of the process was equal to about 4.1 V, increased during the process until it became stabilized at about 5.88 V (regime cell voltage) .

The solution of Na₂S0₄ coming from the desulphurisation, was circulated in the central supply compartment. During the electrolysis, the pH of this solution dropped from an initial value of about pH = 12.6 to a final value of pH = 8.2.

The electrolysis was carried out under regime conditions for about 340 minutes. At the end of the electrolysis, 10.36 1 of anolyte containing 104.5 g/l of H₂SO and 8.07 1 of catholyte containing 261.3 g/l of NaOH were recovered.

The cathode yield calculated was about 92.3%. The anode yield was about 40.6%. The overall power consumption estimated is equal to 4133 kWh/t of NaOH produced .

The desalinated solution recovered from the supply compartment had a volume of about 9.9 1 and a content of residual Na₂S0 equal to about 28.8 g/l. This means that about 86% of the Na₂S0 present in the solution coming from the desulphurisation was moved into the anode and cathode compartments.

On the basis of the experimental data, at the end of the process, the following products were recovered: (a) 83.9% by weight of sodium introduced as NaOH; (b) 0.87 kg of H₂S0₄. Net of the fraction of H₂S0₄ used for the neutralization of the sodium sulphate solution leaving the desulphurisation, the electrolysis allowed the recovery of 0.78 kg of H₂SO₄ available for other uses .

The solution of NaOH and the desalinated solution can be joined and used as such for preparing a new desulphurising solution for desulphurising further lead pastel .

Claims

1. Process for desulphurising a PbSC^-containing material, comprising the steps of:

(a) reacting said material in water with NaOH to produce a Na₂S0 aqueous solution comprising a desulphurised material containing lead oxide, lead hydroxide or mixtures thereof, in form of dispersed solid;

(b) separating said desulphurised material from said Na₂SC> aqueous solution;

(c) subjecting said Na2 S C> aqueous solution coming from said step (b) to an electrolysis process in at least one electrolytic cell in order to form at least a H2 S O aqueous solution, a NaOH aqueous solution and a desalinated aqueous solution comprising Na₂S0₄,-

(d) recycling at least part of said NaOH aqueous solution and at least part of said desalinated aqueous solution comprising Na₂S0₄ to said step (a) .

2. Process according to claim 1, wherein said electrolysis process is carried out in a continuous mode .

3. Process according to claim 2, wherein said electrolysis process is carried out keeping the concentrations of one or more of said H2 S O4 aqueous solution, NaOH aqueous solution and desalinated aqueous solution substantially constant in said electrolytic cell .

4. Process according to claim 1, wherein said electrolysis process is a bipolar membrane electrodialysis process.

5. Process according to claim 1, wherein said electrolysis process is carried out in at least one three-compartment membrane electrolytic cell.

6. Process according to claim 1, wherein said electrolysis process is carried out in a device comprising at least one first cationic permselective membrane electrolytic cell and at least one second anionic permselective membrane electrolytic cell, wherein an anode compartment of said first cationic permselective membrane electrolytic cell is fluidly connected to a cathode compartment of said second anionic permselective membrane electrolytic cell.

7. Process according to claim 1, wherein said membrane electrolysis process comprises the steps of:

(i) providing at least one electrolytic cell comprising:

- at least one anode compartment (202) comprising at least one anode (203) immersed in an anolyte,

- at least one cathode compartment (204) comprising at least one cathode (205) immersed in a catholyte,

- at least one supply compartment (206) interposed between said anode compartment (202) and said cathode compartment (204);

said supply compartment (206) being separated from said anode compartment (202) by at least one anionic membrane (AM) ;

said supply compartment (206) being separated from said cathode compartment (202) by at least one cationic membrane (CM) ;

(ii) supplying said Na₂SC> aqueous solution substantially devoid of lead (207) to said supply compartment (206) and applying a potential difference between said anode (203) and said cathode (205) so as to form said H₂SO aqueous solution (208) in said anode compartment (202), said NaOH aqueous solution (209) in said cathode compartment (204) and said desalinated aqueous solution comprising Na₂SC> (210) in said supply compartment (206) by electrolysis.

8. Process according to claim 1, wherein said membrane electrolysis process comprises the steps of:

(i) providing at least one electrolytic system (300) comprising at least one first electrolytic cell (301) and at least one second electrolytic cell (302), wherein :

said first electrolytic cell (301) comprises:

at least one first anode compartment (303) comprising at least one first anode (304) immersed in a first anolyte;

at least one first cathode compartment (305) comprising at least one first cathode (306) immersed in a first catholyte;

said first anode compartment (303) being separated from said first cathode compartment (305) by at least one cationic membrane (CM) ;

said second electrolytic cell (302) comprises:

at least one second anode compartment (307) comprising at least one second anode (308) immersed in a second anolyte;

at least one second cathode compartment (309) comprising at least one second cathode (310) immersed in a second catholyte;

said second anode compartment (307) being separated from said second cathode compartment (309) by at least one anionic membrane (AM) ;

said second cathode compartment (309) being fluidly connected to said first anode compartment (303) of said first electrolytic cell (301);

(ii) supplying said Na₂SC> aqueous solution substantially devoid of lead (311) as an anolyte in said first electrolytic cell (301) and applying a potential difference between said first anode (304) and said first cathode (306) so as to form said NaOH aqueous solution (312) in said first cathode compartment (305) and a partially desalinated aqueous solution comprising a mixture of H₂SO and Na₂SC>4 (313) in said first anode compartment (303) by electrolysis;

(ii) supplying said partially desalinated aqueous solution (313) as a catholyte in said second electrolytic cell (302) and applying a potential difference between said second anode (308) and said second cathode (310) so as to form, by electrolysis, said H₂SO₄ aqueous solution (314) in said second anode compartment (307) and said desalinated solution (313) in said second cathode compartment (309) .

9. Process according to claim 1, wherein said membrane electrolysis process comprises the steps of:

(i) providing at least one electrolytic system (404) sequentially comprising at least one anode compartment (401), at least one unitary cell (402), at least one cathode compartment (403), and wherein:

- said anode compartment (401) comprises at least one anode (404) immersed in an anolyte,

- said cathode compartment (403) comprises at least one cathode (405) immersed in a catholyte, - said unitary cell (402) sequentially comprises a bipolar membrane (BM) , an acid compartment (406), an anionic membrane (AM), a supply compartment (407) and a cationic membrane (CM) ;

(ii) supplying said Na₂S0 aqueous solution (408) coming from said step (b) to said supply compartment (407) of said unitary cell (402) and applying a potential difference between said anode (404) and said cathode (405) so as to form said H₂SO aqueous solution (409) in said acid compartment (406), said NaOH aqueous solution (410) in said cathode compartment (403) and said desalinated solution (411) in said supply compartment (407) .

10. Process according to claim 9, wherein said electrolytic system comprises two or more unitary cells

(402) .

11. Process for recovering one or more lead-acid accumulator components, comprising desulphurising a lead pastel of an exhausted lead-acid accumulator according to claim 1.