U.S. patent application number 13/388012 was filed with the patent office on 2012-07-26 for reclaiming of lead in form of high purity lead compound from recovered electrode paste slime of dismissed lead batteries and/or of lead minerals.
This patent application is currently assigned to Millbrook Lead Recycling Technologies Limited. Invention is credited to Federica Martini.
Application Number | 20120186397 13/388012 |
Document ID | / |
Family ID | 41800621 |
Filed Date | 2012-07-26 |
United States Patent
Application |
20120186397 |
Kind Code |
A1 |
Martini; Federica |
July 26, 2012 |
RECLAIMING OF LEAD IN FORM OF HIGH PURITY LEAD COMPOUND FROM
RECOVERED ELECTRODE PASTE SLIME OF DISMISSED LEAD BATTERIES AND/OR
OF LEAD MINERALS
Abstract
An all-wet process for reclaiming the lead content of impure
electrode paste or slime from discarded lead batteries and/or lead
minerals, in form of high purity lead compound, comprises a)
suspending the impure lead containing material in a lead sulphate
dissolving aqueous solution of a salt belonging to the group
composed of the acetates of sodium, potassium and ammonium; b)
adding to the suspension sulphuric acid in an amount sufficient to
convert all lead oxides to lead sulphate soluble in the acetate
salt solution and slowly adding to the suspension either hydrogen
peroxide or a sulphite or bubbling sulphurous anhydride through it,
in a measure adapted to reduce any lead dioxide to lead oxide
converted eventually to soluble lead sulphate by the sulphuric
acid; c) separating a limpid acetate salt solution containing
dissolved lead sulphate from a solid phase residue including all
undissolved compounds and impurities; d) adding to the separated
solution of lead sulphate either carbonate or hydroxide of the same
cation of the acetate salt of the lead sulphate dissolving solution
for precipitating highly pure lead carbonate/oxycarbonate or lead
oxide or hydroxide, respectively, while forming sulphate of the
cation, soluble in the acetate salt solution; and e) separating the
precipitated high purity lead compound from the acetate salt
solution now containing also sulphate of the same cation of the
acetate salt. The acetate salt solution containing also sulphate of
the same cation of the acetate salt separated from the precipitated
compound of lead is re-cycled to step a) and the content of
sulphate of the same cation in the solution is maintained below
saturation limit by continuously or periodically cooling at least a
portion of the solution separated from the precipitated lead
compound to cause selective crystallization of sulphate salt of the
same cation of the acetate salt and removing it as a by-product.
Optionally, the separated solid phase comprising insoluble
compounds of lead and/or undissolved concretions of lead compounds
is treated in hot concentrated hydroxide of the same cation of the
selected acetate salt and converting these compounds of lead and/or
undissolved concretions of lead compounds to soluble plumbites, and
the separated lead containing alkaline liquor may be added to the
limpid acetate solution for precipitating all reclaimable lead in
form of high purity lead oxide or hydroxide.
Inventors: |
Martini; Federica; (Spinea,
IT) |
Assignee: |
Millbrook Lead Recycling
Technologies Limited
Dublin 2
IE
|
Family ID: |
41800621 |
Appl. No.: |
13/388012 |
Filed: |
July 30, 2009 |
PCT Filed: |
July 30, 2009 |
PCT NO: |
PCT/IT2009/000344 |
371 Date: |
April 16, 2012 |
Current U.S.
Class: |
75/739 ;
266/101 |
Current CPC
Class: |
H01M 10/54 20130101;
C22B 3/08 20130101; Y02P 10/234 20151101; C22B 13/045 20130101;
Y02P 10/20 20151101; Y02W 30/84 20150501; C22B 7/007 20130101 |
Class at
Publication: |
75/739 ;
266/101 |
International
Class: |
C22B 13/00 20060101
C22B013/00; C22B 3/02 20060101 C22B003/02; C22B 3/08 20060101
C22B003/08 |
Claims
1. A process for reclaiming the lead content of impure electrode
paste or slime from discarded lead batteries and/or lead minerals,
in form of high purity lead compound, comprising the steps of a)
suspending the impure lead containing material in a lead sulphate
dissolving aqueous solution of a salt belonging to the group
composed of the acetates of sodium, potassium and ammonium; b)
adding to the suspension sulphuric acid in an amount sufficient to
convert all lead oxides to lead sulphate soluble in the acetate
salt solution and slowly adding to the suspension either hydrogen
peroxide or a sulphite or bubbling sulphurous anhydride through it,
in a measure adapted to reduce any lead dioxide to lead oxide
converted eventually to soluble lead sulphate by the sulphuric
acid; c) separating a limpid acetate salt solution containing
dissolved lead sulphate from a solid phase residue including all
undissolved compounds and impurities; d) adding to the separated
solution of lead sulphate either carbonate or hydroxide of the same
cation of the acetate salt of the lead sulphate dissolving solution
for precipitating highly pure lead carbonate/oxycarbonate or lead
oxide or hydroxide, respectively, while forming sulphate of the
cation, soluble in the acetate salt solution; e) separating the
precipitated high purity lead compound from the acetate salt
solution now containing also sulphate of the same cation of the
acetate salt.
2. The process of claim 1, wherein said acetate salt solution
containing also sulphate of the same cation of the acetate salt
separated from the precipitated compound of lead is re-cycled to
step a) and the increasing content of sulphate of the same cation
in the solution is maintained below saturation limit by
continuously or periodically cooling at least a portion of the
solution separated from the precipitated lead compound to cause
selective crystallization of sulphate salt of the same cation of
the acetate salt and removing it as a by-product.
3. The process of claim 2, wherein prior to cooling for selectively
precipitating sulphate salt of the same cation of the acetate salt,
the solution is contacted with a chelating resin adapted to
sequester any residual lead ions from the solution for selectively
precipitating substantially lead-free sulphate salt of the same
cation of the acetate salt, upon cooling.
4. The process of claim 1, wherein the selective dissolution of the
lead sulphate is carried out by suspending the impure material in
an aqueous solution of sodium acetate having a concentration
comprised between 10 g and 120 g of salt in 100 g of water at a
temperature comprised between 20.degree. C. and boiling point, for
a stirring time comprised between 5 and 180 minutes.
5. The process of claim 1, wherein selective dissolution of the
lead sulphate is carried out by suspending the impure material in
an aqueous solution of ammonium acetate having a concentration
comprised between 20 g and 120 g of salt in 100 g of water at a
temperature comprised between 20.degree. C. and boiling point, for
a stirring time comprised between 5 and 180 minutes.
6. The process of claim 1, wherein selective dissolution of the
lead sulphate is carried out by suspending the impure material in
an aqueous solution of potassium acetate having a concentration
comprised between 20 g and 120 g of salt in 100 g of water at a
temperature comprised between 20.degree. C. and boiling point, for
a stirring time comprised between 5 and 180 minutes.
7. The process of claim 1, further comprising suspending said
separated solid phase residue including all undissolved compounds
and impurities in hot concentrated hydroxide of the same cation of
the selected acetate salt for converting and dissolve compounds of
lead or concretions thereof that could not be dissolved in the
acetate salt solution in form of plumbite salt of the hydroxide
cation, separating a lead plumbite containing alkaline liquor from
a solid phase of impurities contained in the starting material and
adding the separated lead containing liquor to the separated
acetate solution for precipitating all reclaimable lead as highly
pure lead oxide or hydroxide.
8. A plant for reclaiming the lead content of impure electrode
paste or slime from discarded lead batteries and/or lead minerals,
in form of high purity lead compound, comprising: a) a first
reactor (RAC (1)) having stirring and heating means for suspending
the impure material in a lead sulphate dissolving aqueous solution
of a salt belonging to the group composed of the acetates of
sodium, potassium and ammonium, means for controlled addition of
sulphuric acid and means for controlled addition of a reagent
belonging to the group composed of hydrogen peroxide, sodium
sulphite and sulphurous anhydride; b) a first solid-liquid
separator (F1) for separating a solid phase constituted by
insoluble compounds of lead and/or undissolved concretions of lead
compounds and impurities from a limpid acetate salt and lead
sulphate solution; c) a second reactor (RAC (2), RAC (3), RAC (4))
adapted to hold the limpid aqueous solution of acetate salt and
lead sulphate solution, having stirring and heating means and means
for adding to the solution either a carbonate or a hydroxide of the
same cation of said acetate salt, for precipitating insoluble
carbonate/oxycarbonate of lead or lead oxide or hydroxide,
respectively, and forming sulphate of the same cation of the
acetate salt, soluble in the aqueous solution; d) a second
solid-liquid separator (F(2), F(3), F(4)) for separating the solid
phase constituted by said precipitated compound of lead from an
aqueous solution of acetate salt now containing also sulphate of
the same cation of the acetate salt; e) means for re-cycling the
separated solution to said first reactor (RAC (1)); f) a third
reactor (RAC (5)) having stirring means and means for controlled
cooling and/or heating of a continuously or periodically treated
portion of said recycling solution for selectively crystallizing
sulphate of the same cation of the acetate salt contained in the
solution; g) a third solid-liquid separator (F(5)) for separating
said crystallized sulphate of the same cation of the acetate salt
of said treated portion of recycling solution that is then recycled
to said first reactor (RAC (1)).
9. The plant of claim 8, further comprising a column filled with
chelating resin, through which said continuously or periodically
treated portion of recycling solution is passed for sequestering
residual lead ions from the solution, before introducing it in said
fourth reactor (RAC (5)) for selectively crystallizing sulphate of
the same cation of the acetate salt contained in the solution.
10. The plant of claim 9, further comprising means for periodically
stripping sequestered lead ions from said chelating resin by
circulating acetic acid through the column and means for
introducing the circulated amount of acetic acid to said first
reactor (RAC (1)).
11. The plant of claim 8, further comprising a fourth reactor (RAC
(2bis)) having stirring and heating means for suspending said
separated solid phase comprising insoluble compounds of lead and/or
undissolved concretions of lead compounds in hot concentrated
hydroxide of the same cation of the selected acetate salt and
converting said compounds of lead and/or undissolved concretions of
lead compounds to soluble plumbites; a fourth solid-liquid
separator (F (2bis)) for separating lead containing alkaline liquor
from a solid phase of impurities; and means for introducing the
lead containing alkaline liquor into said second reactor (RAC (2),
RAC (3)) for precipitating all reclaimable lead in form of high
purity lead oxide or hydroxide.
12. The plant of claim 8, further comprising an oven in which the
separated lead compound, if in form of lead carbonate and/or
oxycarbonate or hydroxide, is decomposed to lead oxide and carbon
dioxide or water, respectively.
Description
FIELD OF THE INVENTION
[0001] This disclosure relates to techniques for reclaiming high
purity lead compound from impure mixtures, like recovered electrode
paste slime of dismissed batteries or lead minerals.
BACKGROUND OF THE INVENTION
[0002] Departing from the long established processes for reclaiming
valuable lead content from recovered electrode paste of dismissed
lead batteries or lead minerals based on calcination, prior PCT
patent application No. PCT/IT2008/000022, in the name of the
present applicant, the content of which is herein incorporated by
express reference, disclosed a novel process almost completely
based on wet treatment of the recovered electrode paste or lead
minerals capable of producing highly purified lead carbonate at
high yield with much reduced burden of disposing of noxious
residues and efficient use of reagents. Nevertheless, in order to
use the reclaimed lead for making new battery paste, the lead
carbonate has to be decomposed to lead oxide by heating the
carbonate to about 400-450.degree. C. This is expensive in terms of
energy required.
[0003] According to the process disclosed in the above-mentioned
prior PCT patent application, the starting material in the form of
electrode paste slime or finely ground and eventually pre-treated
lead mineral, is leached using an acid different from sulphuric,
adding hydrogen peroxide or other reducing agent or lead dioxide
that may be present in the starting material and sulphuric acid for
converting all lead compounds to insoluble lead sulphate which is
separated together with other insoluble substances and is
thereafter selectively dissolved in an aqueous solution of a
solubilizing compound. To the separated clear solution of lead
sulphate is added a carbonate salt for precipitating
carbonate/oxycarbonate of lead.
[0004] Besides the energy required for eventually converting the
carbonate to lead oxide, the acid leaching step of the impure
starting material implies certain costs and treatment plant
complexities.
OBJECTIVES AND SUMMARY OF THE INVENTION
[0005] With the aim of reducing the lead reclaiming cost in terms
of plant inventory of treatment vessels and related agitators,
heaters and/or coolers, filters, overall treatment plant
complexity, pumping and energy requirements, the applicant has
found that an outstandingly effective simplification of the all wet
lead reclaiming process flow is not only viable but even more
efficient.
[0006] The novel approach, unexpectedly found outstandingly
efficient consists in directly suspending the impure starting
material in a lead sulphate dissolving aqueous solution of an
acetate salt and adding thereto either hydrogen peroxide or a
sulphite or alternatively bubbling sulphurous anidride through it,
in a measure adapted to reduce any lead dioxide expected to be
present in the impure starting material to lead oxide, and
sulphuric acid in a measure adapted to convert all lead oxide to
lead sulphate that remains dissolved in the selected dissolving
salt solution.
[0007] A clear solution containing the dissolved lead sulphate may
then be separated from a solid phase residue that includes all
undissolved impurities contained in the impure starting
material.
[0008] Together with the solid phase of all insoluble substances,
separated from the lead sulphate dissolving acetate salt solution,
depending on the origin of the impure starting material to be
processed, there may be present certain compounds of lead such as
oxysulphates or other oxides that could not be dissolved in the
acetate salt solution. Even the lead of these compounds undissolved
by the acetate salt solution can eventually be reclaimed if
considered economically or for other reason desirable to do so.
This can be carried out by suspending the separated solid phase
consisting of impurities and insoluble compounds of lead in a
concentrated solution of hydroxide of the same cation of the
acetate salt for decomposing and convert these compounds to soluble
plumbites, that dissolve in the hot hydroxide solution which may
then be separated from the remaining insoluble impurities. The
hydroxide solution now containing the residual lead stripped from
the previously separated solid phase of impurities may be
introduced in the liquid acetate solution containing the lead
sulphate in the vessel in which hydroxide of the same cation of the
acetate salt is introduced for precipitating the lead contained as
lead sulphate in the liquid acetate solution in form of lead oxide
or hydroxide.
[0009] Precipitation of high purity lead compound from the clear
lead sulphate solution may then by effected either by adding to the
solution carbonate of the same cation of the acetate salt used for
selectively dissolved lead sulphate for precipitating insoluble
carbonate/oxycarbonate of lead, as contemplated in the process
disclosed in said prior PCT patent application No.
PCT/IT2008/000022, or, more preferably, according to an alternative
embodiment, instead of a carbonate salt, to the clear solution of
lead sulphate is added hydroxide of the same cation of the acetate
salt used for selectively dissolved lead sulphate for causing
precipitation of either oxide or hydroxide of lead, depending from
the temperature of the precipitation bath, thus eliminating even
the burden of eventually having to convert the reclaimed lead
carbonate to lead oxide by heating the carbonate in an oven.
[0010] The applicant has found that whether a carbonate salt or a
hydroxide is used for causing it, precipitation of all lead in the
solution as highly pure lead compound is practically complete.
Therefore, separation of a solid phase of the highly pure lead
compound from the acetate solution is carried out from the same
acetate solution in which the impure starting material had been
suspended.
[0011] Though the clear solution of acetate salt becomes
progressively enriched of sulphate of the same cation of the
acetate salt used for selectively dissolving lead sulphate, it may
be recycled to the suspension step of the impure starting material
for as long as the sodium sulphate concentration remains below
saturation. When the sulphate concentration in the clear acetate
salt solution approaches saturation, the solution may be cooled to
about 10.degree. C. for selectively crystallizing and precipitating
solid phase constituted by sulphate of the cation of the acetate
salt used, which is recovered by filtering. The clear acetate salt
solution freed of the sulphate salt may then be recycled to the
suspension bath of the impure starting material.
[0012] Preferably, before cooling it for selectively crystallizing
and precipitating solid phase constituted by sulphate of the cation
of the acetate salt used, the acetate solution is percolated
through a column filled with chelating resin for sequestering any
residual lead ions in the solution, before cooling the solution in
order to produce lead-free sulphate salt, as a by-product, of
broader market acceptance.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a flow sheet of the main steps of the process of
this disclosure for reclaiming high purity lead oxide from
dismissed battery electrode slime or lead minerals, alternatively
for producing pure lead carbonate/oxycarbonate or pure lead
oxide/hydroxide.
[0014] FIG. 2 is a simplified diagram of a plant for reclaiming
highly pure lead compound from dismissed battery electrode slime or
lead minerals, alternatively as pure lead carbonate/oxycarbonate or
pure lead oxide/hydroxide, according to the alternative process
flows of FIG. 1.
DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0015] Referring to the flow sheet of FIG. 1, according to a first
embodiment of the process of this invention, particularly suited
for treating raw electrode paste slime recovered from crushed
dismissed batteries, but usable, with eventual trivial adaptations,
also with finely ground lead minerals, for example galena commonly
roasted for converting lead sulphite to lead sulphate, anglesite
and lesser common minerals, the impure starting solid material is
suspended in an aqueous solution of a salt capable of dissolving
lead sulphate, such as for example an acetate salt of sodium,
ammonium, potassium, urea, mono-, di- or tri-ethanolamine.
[0016] To this suspension, sulphuric acid in an amount necessary to
convert all oxides present in the starting material to sulphate,
and a reducing agent chosen among hydrogen peroxide, a sulphite
salt and sulphurous anhydride, is gradually added or bubbled
through the suspension bath in an amount necessary to reduce any
lead dioxide that may be contained in the starting material (as
would be the case with slime from discarded lead batteries) to lead
oxide.
[0017] Therefore, in this first Step (1) of the process flow sheet
of FIG. 1, in the suspension bath of the starting material take
place the chemical reactions that are formally described herein
below in view of the fact that they take place simultaneously,
causing the conversion of all lead compounds to sulphate that
dissolves in the aqueous solution containing the specific
dissolving salt mentioned above.
[0018] The below described reactions make clearly recognizable the
existence of a synergical action that results in a sensible
increment of the speed of the chemical conversion process, in view
of the fact that dissolution of lead sulphate frees the lead oxide
to which (for the case of dismissed battery electrode paste slime)
it was originally intimately tied in the electrode paste, to form
compounds of the type 3PbOPbSO.sub.4 and 4PbOPbSO.sub.4, in
consequence of which the oxide quickly reacts, transforming itself
to sulphate, which in turn dissolves in the acetate salt solution
that, in the considered embodiment, is sodium acetate, while the
lead dioxide physically embedded in concretions of the
above-identified oxysulphates, become more easily reached by the
reacting species and therefore is more quickly reduced to lead
oxide.
[0019] The following reactions relate to an exemplary embodiment of
reclaiming high purity lead compound from electrode paste slime of
dismissed batteries in which lead sulphate is present as measure
component (close to about 60% by weight) in the impure starting
material and dissolved in the aqueous solution of sodium
acetate.
[0020] Reaction 1: Dissolution of Lead Sulphate
PbSO4 (insoluble)+4CH3COONa.fwdarw.soluble complex of PbSO4
[0021] Reaction 2: Dissolution of Lead Oxide
2CH3COONa+H2SO4.fwdarw.2CH3COOH+Na2SO4
PbO (insoluble)+2CH3COOH.fwdarw.Pb(CH3COOH)2 (soluble)+H2O
Pb(CH3COOH)2+Na2SO4.fwdarw.PbSO4 (insoluble)+2CH3COONa
PbSO4 (insoluble)+4CH3COONa.fwdarw.soluble complex of PbSO4
[0022] Reaction 3: Reduction of Lead Dioxide and Dissolution
2CH3COONa+H2SO4.fwdarw.2CH3COOH+Na2SO4
PbO2 (insoluble)+H2O2.fwdarw.PbO (insoluble)+H2O+1/2O2
PbO (insoluble)+2CH3COOH.fwdarw.Pb(CH3COOH)2 (soluble)+H2O
Pb(CH3COOH)2+Na2SO4.fwdarw.PbSO4 (insoluble)+2CH3COONa
PbSO4 (insoluble)+4 CH3COONa.fwdarw.soluble complex of PbSO4
[0023] (Optional) Reaction 4: Solubilization of Insoluble Lead
Compounds Possibly Present in the Impure Starting Material
Separated Together with Impurities for Reclaiming of Such a Minor
Amount of Lead
4PbO*PbSO4
(insoluble)+12NaOH.fwdarw.5Na2PbO2(soluble)+Na2SO4+6H2O
[0024] By way of example, in order to process electrode paste slime
recovered from crushed discarded lead batteries, an aqueous
solution of tri-hydrated sodium acetate dissolved in water in a
concentration comprised between 37.5 and 54.5% by weight can be
satisfactorily used. Sulphuric acid is added in an amount
corresponding to or just exceeding the stoichiometric requirement
for converting all lead oxides to lead sulphate as pre-evaluated
for the impure starting material to be processed. Preferably, after
having added the required amount sulphuric acid, to the suspension
bath is added hydrogen peroxide the amount of which may also be
pre-calculated in terms of the stoichiometric requirement for
reducing the lead dioxide contained in the starting material.
[0025] The amount of electrode paste that can be treated in a
certain volume of solution, depends on the solubilizing capacity of
the lead sulphate in the solution of the selected acetate salt and
of the added amount of sulphuric acid. The ability of dissolve lead
sulphate of the acetate salt solution depends from its salt
concentration. By way of example, one liter of aqueous solution
with a concentration of 37.5% by weight of sodium acetate is able
to dissolve 100 g of lead sulphate. By increasing the concentration
of the acetate salt, the amount of lead sulphate that can be
dissolved increases proportionally. The temperature at which the
above described reactions can be carried out in the suspension bath
may be comprised between about 10.degree. C. up to boiling point.
The suspension may be stirred with a pails or turbine mixer in
order to favor breaking down of lead compound aggregates.
[0026] The combination of the chosen operating conditions (type and
fineness of the starting solid material, type and concentration of
the lead sulphate dissolving salt solution, eventual lead dioxide
reducing agent addition, temperature, stirring mode) will influence
the time needed for completing this first step (1) of the all-wet
reclaiming process. The sulphuric acid used for the sulphation of
all the lead oxide in the solution should preferably have a high
concentration in order not to excessively dilute the lead sulphate
dissolving solution.
[0027] Once the reaction time that, depending on the combination of
the numerous parameters may generally range between 6 and 15
minutes, a limpid acetate solution containing lead sulphate may be
separated from the solid phase residues, for example by filtration.
All insoluble impurities and substances are therefore separated
from the solution (Step 2 of the flow sheet of FIG. 1).
[0028] The subsequent reaction conducted in the limpid acetate
solution containing substantially all the lead content of the
starting material in form of lead sulphate by adding to the
solution a hydroxide of the same cation of the selected acetate
salt (i.e. either sodium, potassium or ammonium), according to the
preferred alternative contemplated for the Step 3 of the flow sheet
of FIG. 1, at a temperature sufficiently high to ensure
precipitation of all the lead in solution in form of PbO (of yellow
aspect) instead of in form of lead hydroxide (of white aspect)
produces a selective precipitation of lead oxide because of its
much lower solubility than that of lead sulphate. Generally, the
critical temperature is in the vicinity of 70.degree. C., therefore
the precipitation may be carried out at about 72-73.degree. C.
(unless for some reason one should prefer to recover a highly
purified lead hydroxide, thermally convertible to lead oxide
eventually).
[0029] The other alternative contemplated for the Step 3 of the
flow chart of FIG. 1, consists in adding to the liquid acetate
solution containing substantially all the lead content of the
starting material in the form of lead sulphate, instead of a
hydroxide, a carbonate of the same cation of the selected acetate
salt (i.e. either sodium, potassium or ammonium), which produces a
selective precipitation of lead carbonate or a mixture of lead
carbonate and oxycarbonate, because of their much lower solubility
than that of lead sulphate. In this alternative embodiment,
precipitation may be conducted at any temperature comprised between
ambient temperature up to boiling point.
[0030] Once the reaction of Step 3 is complete, the precipitated
lead oxide or hydroxide or carbonate/oxycarbonate is separated by
filtration (Step 4 of the flow chart of FIG. 1) from the solution
while sulphate of the cation of the hydroxide or carbonate used for
precipitating the lead as insoluble oxide (or hydroxide) or
carbonate (and/or oxycarbonate) remains in the solution.
[0031] The limpid acetate solution, now containing also the
sulphate of the same cation of the acetate salt, can be integrally
recycled to the suspension bath of selective dissolution of the
lead sulphate (Step 1) of the process, for as long as the content
of sulphate remains below saturation (this limit depends primarily
on the type of dissolving salt solution of the lead sulphate and
processing conditions).
[0032] Of course, precipitation of excess sulphate salt together
with the lead oxide or hydroxide or carbonate/oxycarbonate must be
prevented. Therefore, excess sulphate salt must be eventually
eliminated from the solution, well before approaching the
saturation limit (Step 8 of the flow sheet of FIG. 1). 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) from that of the corresponding acetate salt for
selectively crystallizing the sulphate and separating it from the
acetate solution.
[0033] The concentration of the aqueous solubilizing salt solution
and the temperature at which lead sulphate dissolution in it is
carried out are not essential parameters because they simply
influence the time necessary for completing the reactions discussed
above and the quantity of lead sulphate that may be dissolved in
the solution. In practice, if the solubilizing solution, after
precipitation of the dissolved lead as oxide or hydroxide or
carbonate/oxycarbonate, is recycled back and therefore one's
operates with a recycled solution, more and more recycles may be
necessary to complete dissolution of a given quantity of lead
sulphate.
[0034] The novel approach of this disclosure has proved itself
outstandingly suitable to process electrode paste slimes where the
amounts of the three main lead compounds, namely lead sulphate,
lead oxide and lead dioxide, oscillate in the vicinity of a mean
value by a range of variability of about 2% by weight and this may
in practice impede to calculate exactly the quantity of sulphuric
acid solution for converting to sulphate all lead oxides present in
the impure starting material.
[0035] Nevertheless, if in conducting the novel process of this
disclosure sulphuric acid happens to be added in excess of the
stoichiometrically necessary amount, after having precipitated the
pure lead compound by adding a hydroxide or a carbonate of the same
cation of the selected acetate salt, an excess of sulphate of the
cation of the added compound forms compared to the amount strictly
relative to the precipitation of lead sulphate, because of the
presence in the solution of free sulphuric acid. Vice versa, if
sulphuric acid happens to be added in defect of the
stoichiometrically necessary amount, incomplete conversion of
oxides to sulphate occurs, thus a residual amount of oxide remains
undissolved in the acetate solution when separating the solid
impurities. Should this accidentally occur, the separated solid
phase may be simply reintroduced in the suspension bath to be
eventually converted by introducing an excess of sulphuric
acid.
[0036] Continuously or intermittently, whenever the sulphate
concentration in the clear acetate salt solution approaches
saturation, the solution is preferably percolated through a column
filled with chelating resin for sequestering any residual lead ions
in the solution (Step 5 of the flow sheet of FIG. 1), before
cooling the solution to about 10.degree. C. for precipitating a
crystalline solid phase (Step 6 of the flow sheet of FIG. 1),
constituted by sulphate of the cation of the acetate salt used,
which is recovered by filtering (Step 7 of the flow sheet of FIG.
1). The clear acetate salt solution freed of the sulphate salt may
then be recycled to the suspension bath of the impure starting
material while the lead-free sulphate salt constitutes a marketable
by-product.
[0037] 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
[0038] 80 g of recovered dried electrode paste having a lead
content, expressed as metal equivalent, of 72% was treated under
stirring with 1000 ml aqueous solution of tri-hydrated sodium
acetate at 37.5% by weight, with the addition of g 12.2 of
concentrated sulphuric acid at 94-96% by weight, at the temperature
of 83.degree. C. Successively, hydrogen peroxide at 32% by weight
was slowly added to the suspension (dropwise for about 10 minutes)
until no further clarification of the suspension was observed.
[0039] The hot suspension was then filtered and the separated solid
phase was constituted by insoluble lead compounds and lead compound
concretions, electrode grid fragments and various additives used
for making the electrode paste such as carbon black, barium
sulphate, fibers, etc. and impurities such as sand, plastic
materials, etc. The amount of this dark grey solid phase was about
4-12% by weight of the solid mass of the dry electrode paste.
[0040] The filtered limpid solution containing lead sulphate was
stirred at 83.degree. C. adding thereto sodium hydroxide until
reaching a practically complete precipitation of the lead in the
form of lead oxide. The suspension was thereafter filtered
separating the precipitate from the solubilizing solution of sodium
acetate now enriched of sodium sulphate that was recycled to the
stirred lead sulphate dissolution vessel for as long as the content
of sodium sulphate in the solution remained below saturation.
[0041] When the content of sodium sulphate in the sodium acetate
solution became close to the saturation limit, the solution was
percolated through a column filled with chelating resins, for
example of the commercial type denominated Chelex-100 or Dowex A-1,
though any other equivalent resin may be used. The resin filler
sequestered almost completely the surprisingly small quantity of
lead ions residually present in the sulphate solution.
[0042] Subsequently the purified sulphate solution (practically
lead-free) was slowly cooled to 10.degree. C., under slow stirring,
for precipitating a crystalline solid phase constituted by sodium
sulphate that was then recovered by filtering the suspension, while
the clear solution was recycled to the dissolution vessel of the
lead sulphate.
[0043] The filtered lead oxide accurately rinsed with de-ionized
water was dried at 160.degree. C. for as long as reaching constancy
of weight.
[0044] The separated dark grey solid phase was suspended in sodium
hydroxide at 40% by weight, at 50.degree. C. for 15 minutes. The
separated limpid liquid phase was introduced into the limpid
acetate solution containing also the lead sulphate, as part of the
required amount of sodium hydroxide for precipitating all lead in
solution as lead oxide or hydroxide (according to a preferred
embodiment) in consideration of the fact that also the lead present
in the solution as sodium plumbite converts itself to lead oxide
(or hydroxide).
[0045] At the end of the tests, the following mass balance was
recorded.
[0046] In 80 g amount of recovered electrode paste used in an
experiment, there were 4 g of insoluble substances of dark grey
color containing metallic lead and extraneous substances such as
sand, carbon black, barium sulphate and other substances in minor
amounts.
[0047] The calculated maximum quantity of recoverable lead oxide
was of 62.05 g while the quantity of lead oxide effectively
recovered was of 62.03 g for a recovery yield of 99.96%.
[0048] Chemical analysis of the recovered solid product confirmed
that it was constituted exclusively by PbO at 99.99% purity, while
the sodium sulphate that was eventually recovered had a purity of
about 99.90%.
[0049] The following table summarizes relevant conditions,
peculiarities and results of other four exemplary embodiments of
the process of Example 1, described in detail above, but with the
indicated alternative conditions and the results that were
obtained, always using as starting material electrode paste of the
same lot recovered from crushed dismissed batteries.
TABLE-US-00001 Lead sulphate Reaction time Obtained dissolving and
Precipitating purified lead Yield n. solution temperature compound
compound % 2 1000 ml 10 minutes NaOH Pb(OH)2 99.94 Sodium
65.degree. C. acetate, @37.5% 3 1000 ml 8 minutes NaOH PbO 99.95
Sodium 90.degree. C. acetate, @40.0% 4 1000 ml 12 minutes Na2CO3
PbCO3 99.91 Sodium 45.degree. C. acetate, @42.5%
[0050] The lead oxide (whether directly produced by the all-wet
process or obtained by heating lead carbonate/oxycarbonate produced
by the all-wet process) is perfectly suitable for preparing
electrode pastes for new batteries.
[0051] The practice of the process of this invention using as
starting material a mineral or a mixture of minerals of lead may be
substantially similar to the above described embodiments, an
essential pre-step being that of converting as much as possible any
different salt of lead present in the mineral to either lead
sulphate or to lead oxide. For example, in case of galena, the most
common lead mineral, the mineral should be heated in air, according
to common roasting techniques, until oxidizing the lead sulphite to
sulphate. The other common mineral anglesite does not need any
prior treatment being itself already constituted by lead sulphate.
Of course the mineral(s) should be finely ground for facilitating
their processing.
[0052] FIG. 2 is a schematic diagram of possible embodiments for an
industrial plant for reclaiming variable lead in form of high
purity compounds from recovered electrode paste of dismissed lead
batteries and/or finally ground led minerals, eventually pretreated
for converting as much of the lead compounds to lead sulphate.
[0053] The scheme of FIG. 2 provides a multi-embodiment
illustration of the discussed processing alternatives (though
sodium is indicated as the exemplary selected cation of both the
acetate salt and of the alternatively added compounds for
precipitating the desired pure compound of lead, according to the
various alternatives).
[0054] In practice, the plant requires essentially three stirred
and temperature-controlled reactors. To a first reactor RAC (1) in
which the impure material is suspended in an aqueous acetate salt
solution, is associated a first solid-liquid separator F(1) for
separating the lead sulphate containing solution from the solid
phase constituted by insoluble impurities of the impure starting
material.
[0055] To a second reactor for precipitating the desired lead
compound of high purity, that in the multiple alternative scheme of
FIG. 2 is anyone of the reactors RAC (2), RAC (3) and RAC (4), is
associated a second solid-liquid separator, that is the related one
F(2), F(3) and F(4).
[0056] The third and last reactor RAC (5) and associated third and
last solid-liquid separator F(5) are required for at least
periodically (or more preferably continuously) treating the
recycling acetate salt solution and to recycle it to the first
sulphate dissolving reactor RAC (1). The treatment consists of
selectively crystallizing by virtue of the significantly different
solubilities of the acetate salt and of the sulphate of the same
cation of the acetate salt, introduced in the second reactor for
precipitating the desired lead compound, and removing it from the
system. This step must be performed (continuously or
intermittently) in order to prevent saturating the recycling
acetate salt solution with the sulphate of the same cation, which
if let to occur would cause co-precipitation of this salt together
with the lead sulphate (making vain the purification process).
[0057] According to the preferred embodiment, in order to ensure
that the "by-product" sulphate salt (e.g. sodium sulphate) be
substantially lead-free and thus economically marketable, the plant
may include an exchange resin column C(1) filled with an
appropriate chelating resin, through which the solution, when
directed to the selective sulphate crystallization reactor RAC (5)
(whether continuously or periodically) passes, for sequestering
residual lead ions that may be present in the solution.
[0058] Of course, the chelating resin filler will gradually loose
its activity and periodically a stripping of sequestered lead ions
must be carried out by circulating through the column (C1) acetic
acid for a certain period of time. The lead ridden stripping
solution of acetic acid used for this periodical re-activation of
the exchange resin, now containing lead acetate, may be "disposed
of" by simply introducing it into the first reactor RAC (1), as
shown by the relative line.
[0059] The multi-embodiment plant diagram of FIG. 2, illustrates
also the optional reactor RAC (2bis) in which, if desirable, in
consideration of the composition of the impure starting material to
be processed, residual amount of lead that may remain associated
with the separated solid phase of impurities, in form of compounds
or concretions that could not be dissolved during the treatment of
the impure material in the first reactor RAC (1). The separated
solid phase is suspended in hot concentrated solution of hydroxide
of the same cation of the selected acetate salt for dissolving also
these compounds of lead or concretions thereof. The associated
liquid-solid separator F (2bis) permits to separate all non-lead
impurities from a liquor of hydroxide containing sodium plumbite
dissolved in it which may be conveniently used as part of hydroxide
addition in a second reactor RAC (2) or RAC (3) that may be used
for precipitating all the lead in the solution as lead oxide or
lead hydroxide, according to a preferred embodiment.
* * * * *