Production of lead monoxide from lead sulfate with acetic acid

An efficient and inexpensive method for producing lead monoxide from lead sulfate bearing materials such as recycled battery mud is provided comprising: PA1 (a) reacting said material with an ammonium carbonate solution to convert lead sulfate to lead carbonate; PA1 (b) decomposing the lead carbonate to form impure lead monoxide; PA1 (c) reacting the impure lead monoxide with acetic acid to form a lead acetate solution; PA1 (d) contacting the lead acetate solution with carbon dioxide to produce insoluble lead carbonate; and PA1 (e) decomposing the lead carbonate to form lead monoxide.

The starting material for the process of this invention is a lead bearing 
material, particularly a lead sulfate-bearing material such as battery 
mud. Such battery mud consists mainly of chemically reactive lead 
compounds such as lead sulfate, and varying amounts of lead dioxide, 
lead-antimony alloys and other complex lead bearing compounds. Such 
battery parts, namely grid metal, plastics, and battery mud fines by well 
known separatory methods known in the art, from the mud. 
According to this invention, the lead bearing material is slurried in water 
and then reacted with an ammonium carbonate solution wherein the lead 
sulfate contained therein is reacted and converted to insoluble lead 
carbonate and soluble ammonium sulfate according to the following 
reaction: 
EQU PbSO.sub.4 +(NH.sub.4).sub.2 CO.sub.3 .fwdarw.PbCO.sub.3 +(NH.sub.4).sub.2 
SO.sub.4 
Unreacted materials such as lead dioxide remain undissolved in admixture 
with the insoluble lead carbonate in the ammonium sulfate solution. 
Generally an aqueous solution of ammonium carbonate is employed containing 
from 1.5% to 12.5% ammonium carbonate and preferably about 6.5%. A lead 
battery mud containing from about 16-18 wt % of sulfate anion can be 
employed in the first step. Such mud is slurried with water to form a 
heterogeneous dispersion containing from about 10% to 60% by weight of mud 
and preferably about 35% by weight. The mud slurry and ammonium carbonate 
solution are then combined, at a mole ratio of ammonium carbonate to lead 
sulfate in the mud slurry of from 1:1 to 1.25 at temperatures of from 
25.degree. C. to 60.degree. C., and preferably 30.degree. C. Reaction time 
may vary from 1 minute to 60 minutes but generally all reactions are 
completed between 5 and 15 minutes. 
After the reaction is substantially completed, the ammonium sulfate 
solution is separated from the lead carbonate and other insoluble 
materials by conventional solid/liquid separation techniques. The isolated 
ammonium sulfate solution may then be crystallized to recover solid 
ammonium sulfate. 
Alternatively and more preferably, the ammonium carbonate desulfation of 
battery mud may be carried out in two stages wherein fresh ammonium 
carbonate solution is added to the second stage and the ammonium carbonate 
solution used in the first stage is the unexpected ammonium carbonate 
solution from the second stage. In this embodiment, virgin battery mud is 
slurried in the first or primary stage with recovered ammonium carbonate 
solution from the secondary desulfation stage. The slurry is thickened by 
removal of supernatant ammonium sulfate solution. The separated ammonium 
sulfate solution is then sent to an ammonium sulfate crystallizer for 
recovery of solid ammonium sulfate. The thickened slurry is then reacted 
with fresh ammonium carbonate in a secondary stage to convert 
substantially all lead sulfate in the battery mud to lead carbonate and to 
form additional ammonium sulfate. The lead carbonate and other insolubles 
in the form of a slurry is thickened and the thickened slurry is filtered 
and washed in a horizontal vacuum filter. The solution recovered from the 
thickener, containing both ammonium carbonate and ammonium sulfate is 
recycled to the first stage desulfation above. 
The lead carbonate and other insoluble material separated from the 
desulfation step are next calcined or heated at temperatures sufficient to 
decompose the lead carbonate to lead monoxide and carbon dioxide according 
to the following reaction: 
EQU PbCO.sub.3 .fwdarw.PbO+CO.sub.2 
Generally, the temperatures required to decompose the lead carbonate are 
from 400.degree. C. to 650.degree. C., preferably 600.degree. C. 
Preferably, the heating should be conducted in an inert atmosphere even 
though it may be conducted in a slightly oxidizing atmosphere. Heating is 
conducted for between about 15 and 90 minutes to convert substantially all 
the lead carbonate to lead monoxide. Most usually, however, all 
decomposition is completed within 60 minutes. The carbon dioxide evolved 
may be separated from the lead monoxide and reacted with ammonia to form 
ammonium carbonate which may in turn be used as the ammonium carbonate 
leach for desulfation of the battery mud. This will be discussed in more 
detail in connection with FIG. 1 depicting the continuous method of this 
invention. 
Any lead dioxide contained in the insoluble residue after ammonium 
carbonate treatment may also be decomposed along with the lead carbonate 
to form additional lead monoxide and oxygen according to the following: 
EQU 2PbO.sub.2 .fwdarw.2PbO+O.sub.2 
A portion of lead metal contained in the solids is also converted to lead 
monoxide. The lead monoxide product in admixture with undecomposed lead 
dioxide or lead carbonate and other insoluble materials is leached with an 
acetic acid solution (HAc). During this acetic leach step, the lead 
monoxide reacts with acetic acid to form soluble lead acetate and/or basic 
lead acetate. 
______________________________________ 
PbO + HAc .fwdarw. PbAc.sub.2 + H.sub.2 O 
(lead 
acetate) 
or 
2PbO + 2HAc .fwdarw. PbAc.sub.2 Pb(OH).sub.2 
(basic lead 
acetate) 
______________________________________ 
Any undecomposed lead carbonate is converted to soluble lead acetate by 
acetic acid during the reaction as follows: 
EQU PbCO.sub.3 +2HAc.fwdarw.PbAc.sub.2 +H.sub.2 O+CO.sub.2 
Alternatively to decompose and react lead dioxide in this step, a reducing 
agent such as hydrogen peroxide may be added with the acetic acid solution 
to form lead acetate as follows: 
EQU PbO.sub.2 +2HAc+H.sub.2 O.sub.2 .fwdarw.PbAc.sub.2 +2H.sub.2 O+O.sub.2 
Generally, a 0.1-15 wt %, preferably a 0.5-5 wt % solution of acetic acid 
is combined with the products of calcination as a water slurry in 
stoichiometric ratios of 1:0.5 to 1.5 acetate to reactive lead. While 
stoichiometric amounts of acetic acid to reactive lead may be employed, 
best results are achieved at mole ratios of 1:09 to 1.1 due to an 
unexpected increase in purity of the resultant lead acetate solution and 
precipitates. Such purity improvement ultimately results in increased lead 
monoxide purities in the final product as well. The concentration of 
reactive lead (lead carbonate, lead oxide and lead dioxide) in the 
calcined feed is not critical but for efficient operation the lead 
concentration should be from 50% to 99% by weight. The solution is 
contacted with the slurry at a wide range of temperatures. Increasing 
temperatures above about 15.degree. C. results in the concomitant increase 
in reaction rate with preferred conditions being atmospheric pressure at 
temperatures from about 50.degree. C. to the boiling point of the acetic 
acid solution. Reaction times between 5 and 60 minutes are necessary to 
complete the reaction. 
The lead acetate solution formed by the reactions between the acetic acid 
and lead monoxide, undecomposed lead carbonate, if any, and lead dioxide, 
if any, plus reducing agent are separated from the insoluble residue 
containing minor amounts basic lead acetate and lead gangue. The insoluble 
residue is then sent to a smelter to recover the lead values therefrom. 
It has also been discovered that the lead concentration of the acetic acid 
solution has a significant effect upon the purity of the lead acetate 
solution and precipitate. In this regard it is preferred to utilize a lead 
concentration of from about 1% to 9.5%. It is believed that an equilibrium 
reaction between the various soluble and insoluble basic lead acetate 
compounds exists in the acetic acid solution. It is desired to form 
PbOPbAc.sub.2, a soluble acetate. However, 2PbOPbAc.sub.2, an insoluble 
acetate is also formed. The insoluble acetate is lost in the gaugue during 
separation. When lead concentrations in the acetic acid solution on the 
order of about 8% are utilized, the gauge loss problem can be minimized to 
a large extent. 
It is somewhat difficult to separate the insoluble gauge materials under 
ambient temperature conditions. However, when the acetic acid solution is 
maintained at temperatures of about 50.degree. C. or greater, the gauge 
materials flocculate and thus separation is materially enhanced. 
A portion of the separated lead acetate solution may be used to prepare 
lead chemicals such as lead chromate, lead tungstate, lead molybdate, lead 
arsenate and the like by reaction with appropriate reagents. 
In order to prepare substantially pure lead monoxide from the lead acetate 
solution according to this invention, the lead acetate solution is next 
contacted with carbon dioxide gas under pressure to precipitate lead 
carbonate and acetic acid according to the following reaction: 
EQU PbAc.sub.2 +CO.sub.2 +H.sub.2 O.fwdarw.PbCO.sub.3 +2HAc 
EQU PbAc.sub.2 Pb(OH).sub.2 +2CO.sub.2 .fwdarw.2PbCO.sub.3 +2HAc 
Generally, the carbon dioxide is bubbled into the lead acetate solution at 
pressures of from atmospheric to 250 p.s.i.g. preferably from 20 to 100 
p.s.i.g. at temperatures of from about 5.degree. C. to 95.degree. C., 
preferably from 40.degree. C. to 60.degree. C. The carbon dioxide is added 
in an amount of from 0.5 to 1.2 moles per mole of lead in the lead acetate 
solution. Generally all conversion to lead carbonate is completed within 
60 minutes. 
The insoluble lead carbonate is next separated from the acetic acid 
solution and then calcined at temperatures of from 400.degree. to 
800.degree. C., preferably from 550.degree. to 650.degree. C., in an inert 
or slightly oxidizing atmosphere for from one minute to reaction 
completion, to produce substantially pure lead monoxide and carbon 
dioxide. The vapors from the calcination are partially condensed to yield 
an acetic acid solution which can be returned to the leach step. The 
non-condensed CO.sub.2 vapors can be compressed and recycled to the lead 
carbonate precipitation step in the continuous embodiment of this 
invention as well be shown in more detail below. The term substantially 
pure as used herein means the product contains less than 1% impurities 
such as primary iron oxide and antimony oxide and preferably less than 
0.1% impurities. 
FIG. 1 shows a process flow diagram of one continuous method according to 
this invention wherein by-products of various reactions such as carbon 
dioxide from lead carbonate calcination, acetic acid from the lead acetate 
carbonation, and carbon dioxide from the lead carbonate calcination are 
recycled or used to prepare reagents used in the process. 
A battery mud feed 11 containing lead sulfate, lead dioxide and other 
materials is reacted with ammonium carbonate solution 19. The products, 
comprising an ammonium sulfate solution and insoluble lead carbonate, as 
well as lead dioxide, are separated at 13. The separated ammonium sulfate 
solution is then crystallized to recoer solid ammonium sulfate at 14. The 
solid residues from the separation 13 are calcined at 15 to produce impure 
lead oxide, oxygen and carbon dioxide. The formed carbon dioxide is drawn 
off at 16 and combined with ammonia 17 and water 28 in tank 18 to form an 
ammonium carbonate solution. The ammonium carbonate solution is then added 
to the battery mud through line 19 at 12 to react with incoming battery 
mud. A carbon dioxide source 20 is used for start-up and make-up purposes 
to produce the ammonium carbonate solution. As previously mentioned, the 
ammonium carbonate desulfation may be carried out where fresh ammonium 
carbonate feed from the ammonia carbonation reaction at 18 is added to the 
ammonium carbonate reaction at 12. 
After calcination step 15, the impure lead monoxide and other impurities 
are subjected to an acetic acid leach at 21 to form a solution of lead 
acetate and an insoluble residue. The lead acetate solution is separated 
from the solids at 22. The solids, comprising a lead gangue is removed at 
23 for smelting. A portion of the lead acetate solution may be used at 
this point to prepare other lead chemicals as well known in the art. The 
lead acetate solution is treated with carbon dioxide at 24 to precipitate 
lead carbonate and form an acetic acid solution. After separation of the 
insoluble lead carbonate from the solution at 25, the lead carbonate is 
calcined by heating at 26 to form a pure PbO product at 27 and carbon 
dioxide. 
The acetic acid solution separated at 25 is then recycled to the acetic 
acid leach step via line 29 to leach incoming lead oxide at 21. Acetic 
acid solution 31, is used for make-up or start-up purposes. 
The carbon dioxide from the calcination 26 is withdrawn in line 32, 
compressed and recycled to carbonation step 24. Carbon dioxide source 34 
is used for start-up and make-up purposes for the carbonation. 
The invention will be more fully described with reference to the following 
Examples. All percentages given are by weight unless otherwise indicated. 
EXAMPLE I 
Battery mud containing 71% lead, 18% sulfate anion, 21% lead dioxide and 
minor amounts of antimony, iron and silica was fed to a desulfation 
reactor (primary reactor) with recycled ammonium carbonate from a second 
desulfation reactor and reacted to form a slurry containing about 30 to 
40% solids. The battery mud was then leached (reacted) at about 20.degree. 
to 30.degree. C. for 30 minutes to convert 70 to 75% of the lead sulfate 
content to lead carbonate. The resulting slurry was thickened to a 66% 
solids level by removal of a supernatant solution containing 17% ammonium 
sulfate. 
The solids slurry was then reacted with a fresh 6.5% ammonium carbonate 
solution in countercurrent fashion in a secondary desulfation reactor at a 
4 to 5:1 mole ratio of ammonium carbonate to lead sulfate to yield a lead 
carbonate slurry. The slurry was concentrated to a 66% solids level. The 
resulting slurry was filtered and washed in a horizontal vacuum filter to 
form a 77% solids cake. 
The filter cake was calcined at 550.degree. C. for about 1.5 hours in an 
inert or slightly oxidizing atmosphere to evaporate residual water and 
decompose the lead carbonate to lead monoxide and carbon dioxide. Fiber 
material associated with the battery mud was also decomposed along with 
lead dioxide to lead monoxide. 
The calcined, desulfated battery mud containing lead monoxide in admixture 
with other solid impurities was combined with a 3.5 to 4.0% solution of 
acetic acid to form a 14 to 15% solids slurry. The concentration of lead 
in the calcined feed is from about 75 to 90%. The so-formed mud was 
leached at 20.degree. to 30.degree. C. for 1 hour resulting in a 3 to 6% 
solids slurry which was concentrated to a 40 to 45% slurry in a thickener, 
and then filtered in a horizontal vacuum filter to form a 66% solids cake. 
The overflow from the thickener is an 8 to 9% lead acetate solution which 
was fed to a precipitation reactor. Gaseous carbon dioxide was bubbled 
into the solution at pressures of from 30 to 50 p.s.i.g. to precipitate 
lead carbonate. The slurry was then pressure filtered to a 90% solids lead 
carbonate cake. The filtrate, a 3.5 to 4% acetic acid solution, was 
recycled to the leach reactor. 
The lead carbonate cake was dried and decomposed to lead oxide and carbon 
dioxide at 600.degree. C. for 2 hours. The resulting substantially pure 
product had a total impurity concentration of around 1,100 ppm. 
EXAMPLE II 
The battery mud of example I was desulfated as described in Example I. 
The desulfated battery mud containing lead monoxide, lead oxide, lead 
dioxide, and lead carbonate in admixture with other solid impurities was 
combined with a 5.3 to 5.8% solution of acetic acid and a 30% hydrogen 
peroxide solution in a 1:1 stoichiometric ratio with the lead dioxide to 
form an 8 to 9% lead acetate solution. The mud was leached at 20.degree. 
to 30.degree. C. for 1 hour. The resulting 2 to 3% solids slurry is 
concentrated to a 40 to 45% solids slurry in a thickener which is filtered 
to a 66% solids cake. This cake or gangue is collected and is suitable for 
conventional smelting operations. The filtrate was returned to the 
thickener. 
The overflow from the thickener was an 8 to 9% Pb lead acetate solution 
which was fed to a precipitation reactor. Gaseous carbon dioxide is 
bubbled into the solution at pressures from 30 to 50 p.s.i.g. to 
precipitate lead carbonate. The slurry was then pressure filtered to a 90% 
solids lead carbonate cake. The filtrate, a 2.5 to 3.5% acetic acid 
solution, was concentrated and recycled to the leach reactor. 
The lead carbonate was then dried and decomposed to lead oxide and carbon 
dioxide at 600.degree. C. for 2 hours. The resulting substantially pure 
product had a total impurity concentration of around 630 ppm. 
The invention being thus described, it will be obvious that the same may be 
varied in many ways, such variations are not to be regarded as a departure 
from the spirit and scope of the invention, and all such modifications are 
intended to be within the scope of the following claims.