Note: Descriptions are shown in the official language in which they were submitted.
~L3~3~
"HYDROMETALLURGIC PROCESS FOR RECOVERING IN THE METAL
FORM THE LEAD CONTAINED IN THE PASTE OF THE EXHAUSTED
BATTERIES"
According to the various methods of desulphurization
of the paste, by treating the sludge of the exhausted
batteries with solutions of carbonate of alkali metals or
of ammonium~ a solution is obtained, into ~hich
practically all of the sulphur passes in the form of
soluble sulphate (of alkali metal or of ammonium), as
~ell as a mixture of insoluble lead compounds which,
after having been separated from the solution, and
th~roughly washed with water, is practically free from
sulphur.
Such a mixture can be easily reduced with coal at a
relatively low temperature (800-900 C), without
substantial emissions of sulphur dioxide, and without the
: product;on of lead mattes~
The pyrometallurgic reduction of this product is
thus the presently most used method~ also because of the
advantages oF the h;gh productivity and of the long life
of the reduction ovens and of the high yields of lead
~xtraction.
The pyrometallurgical processings of the lead-
containing materials ;nvolve however particular
precautions in the carriage and in the charging of the
furnaces, and the possibility of emission, by the same
furnaces~ of fumes and volatile dusts~
From here the need arises for the refiner to have
available large filtering plants, and a wide and
expensive system for the monitoring, as to the pollutant
lead, of the working premises and of the surroundings of
~ 3 ~
2.
the factories. For this reason, very important are the
processes for the recovery of lead of the paste in the
metal form, which may constitute a valuable alternative
to the pyrometallurgical processings, such as, above all,
those processes which operate according to a completely
hydrometallurgical route, such as the extraction processes by
electrolytic way, or electrowinning.
The desulphurized paste has on the average the
following composition:
- Lead carbonate (PbC03) 50 - 52%
- Lead sulphate (PbS04) 2.5 - 4%
- Lead peroxide (PbO2) 14 - 16X
- Lead oxide (PbO) 11 - 12%
- Metal lead (Pb3 5 - 7%
It is known that the practical problem of the
hydrometallurgical processing of the paste of the old
batteries ;s that of rendering soluble in the solution
selected for the electrolysis all of the components of
; the mixture, in paticular PbO2, which is conta;ned in it
in large percentages. In fact, the residual of the acidic
~; solubilization of the mixture must not constitute an
intolerable loss of lead.
~ On the other hand, owing to the well-known
;~ insolubility of PbO2 in the normal acids suitable to the
electrolytic process, without a preliminary treatment for
PbO2 reduction, the residue of the acidic solubili~ation
of the mixture is very rich in lead, and must be sent
back to the pyrometallurgical recovery.
In accordance with the prior art, the solubilization
of all of the components of the paste in the various
electrolyt~s has been accomplished by various ways.
~' .
~ 3 ~ 3 ~
In chronological order~ the following patents may be
cited:
; - C~E. Tucker in U.S.Pat. 1,148,062 suggests a heating of
the battery sludge, to convert PbO2 into soluble PbO
and Pb20.
- W.C. Smith in U.S.Pat. 1,752~356, ~o solubilize PbO2 in
view of the treatment with caustic alkali, submits
the paste to a heating in a reducing environment
~PbO is formed).
- J.H. Calbeck in U.S.Pat. 1,911,604 carries out the
Leaching of the battery paste by a sodium acetate
solution. Lead oxide and sulphate are dissolved,
whilst PbO2 is normally insoluble in that
electrolyte.
~ 15 ~ut, ;n the presence of metal Pb and in the said
; elertrolyte, a local couple is established, so that
PbO2 and an equivalent amount of metal lead are
dissolved.
- A.F. 6~umann in U.S. Pat. 4,107,007 leaches the paste
with a concentrated solution of alkaLi metal
hydroxide~ to which molasses,or raw sugar, or the
like, has been added.
In such a way, Pb oxide and sulphate are dissolved,
and fed to the electrolysisu The behaviour of PbO2
is not detailed~
R.D. Prengaman in U.S.Pat. 4,229,271 proposes two
; methods of eliminating PbO2 from the paste, and
rendering it wholly soluble in the customary acids
for the electrowinning process:
a) a drying at 100 C of the paste, follo~ed by a
roasting under a reduGing atmosphere at
i
'
; temperatures comprised within the range of from
290 to 375C;
b) a trea~ment of the aqueous suspension of the
paste ~ith sulphur dioxide, or with sulphite or
bisulphite of alkali metal or of ammonium~
- U.Ducati in U.S.Pat. 4,460~442 reacts the paste of the
battery, at 100-120 C, in the presence of a strongly
alkaline solution, to obtain a red lead precipitate,
which has the property of completely dissolving ;n
the hot concentrated solutions of fluoboric acid or
of fluosilicic acid.
-A.Y. Lee and E.R. Cole of Bureau of Mines suggest in
R~I. 8857 two ways for reducing the PbO2 contained
in the paste:
a) by the addition of Pb powder during the leaching
-~ with fluosilicic acid of the paste already
desulphurized with ammonlum carbonate;
b~ by the addition of ammonium bisulphite during the
treatment of desulphurization with ammonium
carbonate.
The suggested methods of thermal reduction of PbO2
under a reduc;ng atmosphere have the disadvantage of
add;ng two steps to the working cycle: the drying and the
- reducing roasting. These operation steps require a strict
control of the operat;ng ronditions, and they must be
furthermore carried out on a un;t tfurnace or roaster)
euipped with an adequate dust exhauster. Moreover~ e~en
~ if the temperatures are low, moving dry material is
: pollution-causinga
The method of reduction during the high-temperature
dissolving, by means of the add-ition of lead powder,
3 ~
;nvolves the transfer of a portion of produced lead, to
convert ;t into powder; as a consequence, this is an
expensive method.
The method of reduction with sulphur dioxide,
sulphite or bisulphite before the carbonating involves a
certain cost, due to the reactant, but, what's more
important, it causes the consumption of carbona~e for the
desulphurizing to increase by about 25%; the
correspond;ng expense is not negligible; to date, the
expense for the carbonate is the highest item of
individual expenditure of the whole process.
Purpose of the present invention is to solve the
above reminded problems concerning the processes of
hydrometallurgical treatment by electrow;nning of the
desulphurized paste of the exhausted batteries~
For such a purpose to be accomplished, the invention
proposes a hydrometallurg;cal process for recovering, by
extraction by electrolytic way, in pure metal form, the
lead of the paste, or desulphurized paste, of the
exhausted batteries~ characterized in that it comprises
the steps of:
a) leach;ng said paste with an aqueous solution of an
ac;d selected from those suitable to the electrolytic
~; extraction;
b) treating with hydrogen peroxide the leached paste as
~: of (a), with the concom;tant carrying out of the
following reactions:
~1) Pb + H22 ~~~> Pb + 2 t 2H
(2) Pb ~ Pb4+ --~ 2Pb2+
up to the quantitative reduction of all of the lead
dioxide contained in the sa;d paste,
c) separation of the solid residue from the so-obtained
dissolved lead-containing solution, which can be
directly sent to the electrolytic lead extraction.
The process of the invention, as substantially
5defined above, is described in greater detail.
The starting material of the process is the paste
already separated from the other components of the
exhausted batteries, and which has already undergone the
desulphurizing process in accordance with the known art.
10This material is leached with the aclueous solution
of an acid suitable to the subsequent eletrolytic
extraction ~preferably, either fluoboric or fluosilicic
acid), to which such an amount of hydrogen peroxide is
added, as to obtain the greatest solubilization of all of
~; 15the lead compounds present, and~ more specifically, of
PbO2 and of metal Pb.
Dur;ng the acidic treatment of the desulphurized
paste, the hydrogen peroxide causes the reaction:
Pb4 + H22 ~~~ Pb + 2 + 2H ~1)
20so that all of the PbO2 is reduced to PbO and dissolved
by the present acid.
; The cons;dera~le amount of 2 which is liberated
during reaction ~1) physicaLly actlvates the particLes of
metal Pb present in the paste, so that the reaction:
25Pb ~ Pb4+ _ ~ 2Pb2~ t2)
occurs more easily, and all of the metal Pb having the
right granulometry is d;ssolved, allowing also the
hydrogen peroxide, corresponding to the Pb4 ~hich is
reduced, to be saved.
30After the acidic leaching with the addition of
hydrogen peroxide, of the desulphurized paste only the
.
3~
organic substances (thin scraps of separators and
ebonite, f;bres and so forth), and Pb sulphate not
conuerted in the desulphurizing step remain undissolved.
The reacted liquid/solid mixture is separated by
filtration~ The solid residue from the filtration
contains not more than 5% of the lead present in the
starting material and, after a roasting to destroy the
organic substances, it can be returned to the
desulphuri2ing step.
The filtered solution contains all of the Pb in
ionic form and does not normally require any
purifications, because it is the same Pb powder,
physically activated by the oxygen being evolved in
reaction (11, ~hich carries out the displacement action
~also named as "cementation") of the impurities~ small
amounts of which may haue been dissolued during the
acidic leaching.
The self-purified solution can be then directly fed
to the electrolytic extraction of lead, which is carried
out in customary cells with insoluble graphite anodes and
with cathodes made from thin lead sheets. `
~y operating under suitable conditions, a cathodic
; deposit of excellent quality and purity can be obtained,
- ~ the deposition of PbO2 at the anode being prevented to
the highest extent~
The small amounts of PbO2, which are sometimes
unavoidably formed, must be returned to the step of
acidic leaching with hydrogen peroxide. The cathodes,
smelted into pigs, are marketed as electrolytic lead. The
solution coming off the electrolysis returns back to the
acid;c leaching of the paste.
8.
To the purpose of better understanding the
characteristics and the advantages of the invention, a
non limitative example of practical embodiment thereof is
described in the following.
E X _ m e '
An amount of 980 9 of well washed desuLphurized
paste, containing, as referred to the dry matter: Pb,
70.5%; Sb, 0.68%; S, 0.18%; has been leached with 5 l of
exhausted electrolyte from the electrowinning,
containing:
49 g/l of Pb
139 g/l of free HBF4
After a half an hour stirring at 50 C, hydrogen peroxide
~in the form of a solution at 35%) has been added in the
amount of 4.6% of the processed paste, and has been
allowed to react over a further half an hour.
By filtration, the following substances have been s_
parated:
80 g of insoluble residue having the composition:
Pb 34.2X
Sb 2.4%
S 2.2%
~ 5 l of lead-carrying electrolyte~ containing;
; 181.5 g/l of Pb
~+~
0.6 g/l of Sb
25.1 g/l of free HBF4
The leaching ~ith fluoboric acid and hydrogen
peroxide has extracted 96.01% of the total Pb contained
in the paste. All of the sulphur has remained in the
residue.
By submitting the 5 l;tres o~f lead-carrying
~ 3 ~
electrolyte to electrowinning on a cell with graphite
anodes (previously lined with PbO2) and cathodes of thin
sheet of eLectrolytic lead, 630 9 of cathodic lead,as
well as 9 g of PbO2 a~ the anode, have been produced.
The cell has been operating over 24 hrs at 7 A and
2.5 V.
The cathodic current density has been of 280 A/m .
The curren~ efficiency has been of 97%.
The smelted cathodic lead had the following
composition:
Sb c 0.001;
As < 0.001;
Sn < 0.001;
8; < 0.~0~;
Cu = 0.0003;
Ag < 0~0005.
The exhausted electrolyte, to be returned back to
~he leaching of new paste, contained:
53 g/l of Pb
20 `` \ 131 g/l of free HBF4.
ln the attached drawing, a block diagram is
reported, to exemplifying purposes, of the process of the
` invention.
In general, it can be observed that according to the
invention the use of hydrogen peroxide offers the
follow;ng advantages~ as compared with the other reducing
agents as proposed by the known art for the reduction of
PbO2:
~; - It does not show any toxicity.
- It does not supply foreign ions to the solution which
has to be sent to the electrolysis, and which must be
.
1 0 .
continuously recycled.
- It is a reactant easily availabLe from the market, at a
not ~oo high price~
- it can be metered with precision, avoiding the wastes,
because the end of PbO2 reduction is signalled by a
~ sharp turning of the colour of the suspension from a
: brown-red to a brown-grey colour~ The change in colour
can be easily detected visually.
- With a consumption of 4-5% of H202 (as a solution at
35%), an yield not lower than 95~ is reached in the
extraction of Pb.
~ ' .
,
~:`
