Note: Descriptions are shown in the official language in which they were submitted.
Description
PROCESS FOR THE RECOVERY OF LEAD
AND SILVER CHLORIDES
.
Technical Field
The invention lies in the field of recovery of the
chlorides of lead a~d silver by selectively solubiliz-
ing the chlorides from other solid materials and the
final recovery of the metals from the s~lubili~ed
ehlorides.
Back~round Art
Prior Art Stat~emPnt
U. S. Patent 4,113,471 disc.lo~es a process for
brine leaching oxide ores to non~selectively soluhi-
lize non-ferr~us metal values as chlorides, at elPvated
temperatures and pressures with the addition of oxygen.
Leaching is required for a time of 1~4 to 12 hours.
There is no sel~ctive leaching of lead or silver
~hlorides from their sulfide ores.
U. S. Patents 4,135,993 and 4,173,623 teach brine
leaching lead chloride at temperatures of 80-120C to
selectively solubilize ~helead chlorideoutof a sulfidic
residue containing the s~lfides of copper~ iron and
~inc as well as elemental sulfur. The leachin~ is not
done under pressure. Leach times ~f 1~4 to 2 hours
are requir~d in both proc~s~e~.
Internati~nal publication WO 80/00852, class 2Z~
~i
-2-
13/00, published under the patent cooperation treaty
on May 1, 1980 (01.05.80), discloses the recovery of
lead from crystallized lead chloride by reduction with
hydrogen or a hydrogen containing compound accompanied
by condensing lead chloride volatilized during the
reduction process.
Disclosure of the Invention
. . _ ..
Lead and silver chlorides are selectively separ-
ated from other solid materials by leaching the
materials with a brine lea~h at elevated temperatures
and pressures to selectively solubilize the chlorides,
followed by a liquid~solids separation. A starting
material on which the process is particularly effect-
! ive is the sulfidic residue obtained by selectively
leaching a sulfidic lead ore with a cupric or ahalogenating leach to produce solid leacl chloride.
Temperatures ranging from the hoiling point of the
solution to 170C are used ~o selectively solubilize
substantially all of the lead and silver chlorides in
a rapid time which can be not in excess of about three
minutes. If elemental sulfur is present in the
starting material, i~ is agglomerated at elevated
temperatures for ea~e in separation from the solid
residue. Lead chloride is crystallized from solution
by flashing from the high temperature of ~he leach to
lower tempe~atures to produce a large crop of lead
chloride crystals per pass and elemental lead of high
purity recovered from the crystallized lead chloride
by hydrogen reduction, or otherwise~ Silverchloride
is redovered from the mother li~uor and processed
for recovery of silver.
Best Mode for Carrying Out the Invention
~ .
Although the process is not limited in its appli-
i
cation to any particular starting materials containing
lead and silver chlorides, it has been found effective
for selectively leaching lead and silver chlorides at a
rapid rate from residues o~tained ~y cupric chloride
leaching o~ pyritic or sulfidic lead ores in which lead
and silver chlorides are selectively leached at saturation
into solid products. Among other materials, these
residues contain the sulfides of copper, iron and zinc.
The process is equally effective for selectively leaching
the residues produced from halogen leach of pyritic ores
in the processes of U.S. Patents 4,135,993 and 4,173,623.
The operation of the invention will be illustrated by
its application for the recovery of lead from the residue
resulting from the cupric chloride leach of sulfidic ores.
Pressure Brine Leach
The solids from the cupric chloride leach reaction,
comprising lead chloride, silver chloride, elemental sulfur,
unreacted metal sulfides of other metals, and gangue, are
treated ~or the selecti~e separation of lead and silver
chlorides. The lead and silver chlorides are selectively
solubilized from the residue in the illustrative embodi-
ment by leaching with an aqueous brine solution having a
sodium chloride concentration of from about 200 gpl to
saturation. Suitable substitutes for sodium chloride
are the other alkali metal chlorides~ lithium and
potassium chlorides, as well as the alkaline earth
metal chlorides, calcium and magnesium chlorides.
When other solutes than sodiu~ chloride are used the
upper limit o~ the amount used will change, the
;3
I
the minimum amount of solute preferably being above about
200 gpl. A leach pH of about 0-7 is preferred. Use
of too high a pH will precipitate lead compounds.
Hydrochloric acid (hydrogen chloride,) may also be used
as one of the chlorides. The solute must be a chloride
which provides maximum chloride ion concentration to
the saturation point under the reaction conditions.
The solubilization of silver chloride may be enhanced
by the use of an oxidant in the leach, such as, sodium
~ chlorate or oxygen.
The brine leach is conducted at a temperature in
excess of the solution boiling temperature, which, of
course, requires a pressurized system. The tempera-
ture is maintained between about 100C to about 170C,
the system pressure being selected so as to accommo-
date the solution temperature while preventing solution
boiling. Pressures from about 30 to about 150 psig
are suitable to accomplish 'chis purpose. If elmental
sulfur is presentin the residue the agglomeration
temperature o-E sulfur'is used. This is about 130C to
140C. It was found that.-~hen the sulfur is agglomer-
ated during the leach and separated from the liquid
chlorides in this form with the other solids it can
be readily separated from the other solids by physical
methods, such as, wet screening.
Th~ brine leach, conduGted under the described
temperatures and pressures, accomplishes a rela~ively
high solubility of lead and ~ilver chlorides in a
relatively short period of time, while leaving the
elemental sulfur and unreacted metal sulfides in the
residue ~hase. Retention times of from about 30
seconds to abou-t 5 minutes are generally ade~uate
to dissolve lead chloride to solution concentrations
of at least about 130 grams per liter of lead. A
preferred leach time is not in excess of about three
~ 3
minutes. Increased lead concentrations as a result of
high temperature and pressure brine leach significantly
facilitate further separation processing.
The brine leach may be conducted at lower pressures,
05 including atmospheric pressure, and lower temperatures,
as in the prior art. However, pressures and temperatures
lower than those recited for the preferred range of the
brine leach will require more of the brine solution per
amount of lead and si.lver chlorides and a longer retention
time in order to solubilize the chlorides.
In a typical application~ washed tails or residue
as a filter cake from the above-referred-to cupric
leach of sulfidic ores was brine leached in an exter-
nally heated concentric double pipe pressure leach.
~rine containing 280 g/l of NaCl and PbC12 cake was
heated to 135C under 50 psig pressure with a one
minute retention time to dissolve PbC12 up to a con-
i centration of 145 g/l Pb. This procedure reduces the
size of the crystallizer used in subsequent PbC12
crystalli~ation and circui-t flows to a fraction of
that of an ambient pressure system with a corresponding
reduction in heating and cooling needs.
To explore the efectiveness of the brine pres-
sure leach at high temperatures in rapidly solubilizing
large amounts of lead chloride, the solubility system
PbC12-NaCl-H2O was extended to 144C at two brine con-
centrations of 240 and 320 g/l of NaC1 used for
leaching a lead sulfide ore residue fxom a cupric
leach, the composition of -the residue being typified
by ~he brine leach residues of ~xamples 1 and 2. The
results of the solubility tests reported in Table 1 below
show a decided nonlinear increase in the solubility of lead
chloride with increase in temperature above the boiling point
~ 33
~6-
of the solution, and particularly above 125 C.
TABLE 1
~iluted Diluted Pb
Temper- Sample to Sample Total Pb Solubil-
NaCl ature Volume Volume ~b in Sample ity
g/l C ml ml g~l g g/l
_ _ _ _
240 42 10.0 200 0.746 0.149 14.9
10.0 200 1.450.290 29.0
g4 1/10.0 200 2.g40.588 58.8
100 -87.5 795 9.417.4885.5
124 ~/176.01360 6.20~.43110.9
14~ ~79.01945 7.1313.87175.5
320 47.510.0 200 0.972 0.194 19.4
- 69.010.0 200 2.970.~9459.4
96 10.0 200 6.021.2041~0.4
100 ~ .5 1320 8.9611.8139.1
124 - 8.3 2060 6.6713.7165.5
135 2/9.~1625 11.017.9194.3
143 1/8.22200 ~.8219.4236.6
1/ Sample withdrawn from pressuried autoclave (150 psig
N2) using sample bomb.
2/ Repeat run to check data, new solutions.
Since a pipeline brine leach is contemplated in
the most feasible commercial application of the pro-
cess, a minimum leach time is required in the interest
of reducing equipment cost and processing time. Rate
of brine leaching tests at high temperatures were made
on a residue obtained by the above-referred-to cupric
leach of a lead sulfide ore. The brine leach con-
tained 250 g/l of NaCl and a pH of about 1.5 was used.
The leach temperature was 140C. The results recorded
in Table 2 below indicate that substantially all of
the PbC12 i5 leached in a time not in e~cess of about
three minutes.
;1 ' .
.,
~ 3
T~BLE 2
Rate of Brine Leachlng Tes-t
Feed: 50 g cu+2 leached residue of a high grade galena
containing sa .68 percent Pb and obtained by
leaching 200 g of a high grade ~alena in 1.14
liter of 90 g/1 Cu and 200 g/l NaCl at pH =
1 (HCl) for one hour at 60 C.
Brine Leach Solution: 1 liter 250 g/l NaCl, pH 1.5.
Procedure: Feed added to solution with continuous
stirring. Thief samples removed at designated
time and immediately vacuum filtered without
rinsing.
_ . . . ~.. __. ._. __ . - -
Leach
Time Pb
(min.)_ Vol. Wt % Pb Extraction (%)
1 PF120 ml 26.1 g/l 89
Residue 1.9 g 20.6%
3 PF 110 ml 28.6 g/l 93
Residue 1.3 g 18.7~
PF 625 ml 28.5 g/l 92
Residue 9.1 g 17.2~
__ ~ ~ .. ._ _ .. _ ._ _.. , . .. _ .. ..
Sulfur Agglo _ration
The agglomeration of sulfur is accomplished
during leaching by operating the brine leach within
the ~ulfur agglomeration ternperature range thereby
permitting the sulfur to be readily separated from
¦ the remainder of the residue follo~ing liquid-solids
separation. ~gglomeration tests were run on a brine
leach residue from cupric leach of lead sulfide ore as
referred to above containing elemental sulfur to see
if the sulfur could be coalesced to a size lar~e 1 Y!~
enough for a wet screen separation. The autoc~lve ili ~J
leach was made a~ 130C. The results recorded in
;i9~
--8--
Table 3 below shows that th2 plus 200 mesh fraction
contains about 90 percent of the free sulfur with
a grade of 82 percent, thus showing that the procedure
is feasible for sulfur separation.
05
TABLE 3
Elemental Sulfur Distribution in + 200~mesh Size
F ctions of S-Agglornerated Autoclave Leach Residue
Feed to We-t Screening
10 5.0 g, S - agylomerated product from Test
1151-107-1:
Conditions: Test 1151-99-1 leach residue
130C
pH 11.8 with KOH
2 Hours
Di~tribution
Size Weight S Weiyht S
Fraction g ~ % %
20 Plus 200 (beads) 1.67 81.833.9 89.5
Minus 200 (fines) 3.26 4.92 66.1 10.5
Total/overall 4.93 (31.0) 100.0 100.0
Liquids-Solids Separation
Following the brine leach, the preynant lead and
silver chloride solution is separated from the re-
maining residue for subsequen-t recovery therefrom of
lead and silver values. As high temperatures and
pressures are utilized during the leach, the liquid-
solid separation must be conducted under pressure in
order to prevent flash crystallization of the lead
chloride from the solution. One suitable technique
to accomplish the separation while avoiding flash
crystallization is to employ small diameter pres-
surized liquld cyclones in parallel, the hydro-
clones operating to permit pressure reduction
_9_
to atmospheric as the cyclone operation effe~ts a
liquid-solids separation. A pressure drop of about
40 psi across the cyclone system occurs. Hydroclone
techniques such as those discussed in ~he Hydroclone,
05 D. Bradley, Pergamon Press, Lrd. 1965 may be utilized
in this context. Another solids separation device,
such as, an insulated or jacketed pressure leaf filter
can be used to accomplish the same objective.
In operation, the pipeline dissolver discharyes
through a bank of 10 mm alumina cyclones to remove
solids at about a 4 5 micron cut point with a let down
from a 50 psig pipeline leach to atmospheric, the
pressure being utilized to remove the solids. Floc-
culant may be injected at the cyclone inlet to improve
clarity of the cyclone vortex flow. The apex flow,
containing unreacted s^~lfides and agglomerated sulfur,
flashes to atmosheric pressure and mixes with concen-
trate and mother liquor from the subsequent PbC12
crystallization to quench the hot slurry and solidify
beads of ayglomerated sulfur. The slurry is gravity-
fed to A wet screen or similar separation device to
make a separation of agglomerated sulfur beads from
other solids, principally, unreacted sulfide tails.
The fines are dewatered and final:ly filtered by con-
ventional filtration. Filtrate is recycled to theleach feed tank and tails cake is discharged to a
solids disposal area. Prior to reaching the leaching
tank the leach can be purified by a bleed stream in
which copper and lead values are recovered by iron
cementation and soda ash used at pH 9 to precipitate
Fe, Mg, and Zn to permit recycle of barren brine. The
residue from sulfur separation is disposed of or further
pxocessed for recovery of metal values if warranted.
33
--10--
Lead Chloride Crys-tallization
Lead chloride is crystallized from the liquid
phase resulting from the liquid-solids separation on
~he brine leach solution for subsequent recovery of
05 elemental lead by hydrogen reduction, or otherwise.
Two-stage crystallization may be used with the first-
stage at atmospheric pressure and the second stage at
about 50 mm Hg absolute to cool the feed to about
40C. A pregnant brine containing up to 145 g/l Pb
flashes typically from 135C to ambient temperature in
the first stage to produce a large crop of crystals per
pass. Surface condensers may be used for the second
stage, with contaminated lead chloride condensate being
recycled to process. Mother liquor overflow and
crystal withdrawal elution leg are specific design
requirements to elute minus 5-micron impurities
not removed in the cyclones. Alternatively, polish
filtration techniques could be used to separate minus
5-micron solids.
Crystallizer under-flow is removed through an
elution leg at 40-50 percent solids and advanced to
a washing centrifuge. A three percent moisture PbCl2
cake is conveyed to a surge hopper above the PbC12
reduction furnace.
Lead Recovery from Lead Chloride
The lead chloride is reduced to high purity lead
by hydrogen directly without further refining. The
remaining solubilized silver chloride is treated for
recovery of silver by cementa-tion or other means.
Other co~ventional methods may be used to recover
elemental lead from the ]ead chloride. Hydrogen
supplying compounds, such as, methane and propane
may ~e used as a source for hydrogen.
Since the reduction of PbCl2 is endothermic,
heat must be supplied to the reaction~represented by
~11--
the formula PbC12 -~ H2- -Pb + 2 HCl when an
e~cess of hydrogen over stoichiornetxlc is used.
As lead chloride is extremely corrosive, -the reactor
cannot be made of conventional reactor materials but
05 must be made of material which is substantially im-
pervious to the corrosive action o~ lead chloride,
such as, castable or refractory brick. The materials
of which the reactor walls must be m~de have such a
low heat conductivity that it is practically impossible
to heat the reactor contents with heat applied to the
outside of the walls. Accordingly, it was necessary
to devise a practical procedure for internally heating
the reactor contents to a temperature up to 900C at
least. Two alternative procedures were found to be
feasible.
In accordance with one procedure a furnace ~r
reactor made of refractory brick was used. Heat for
the endothermic reaction occurring in the reaction
chamber was supplied by fire tubes submerged in molten
lead in con-tact with lead chloride and the other
reactants in the reaction chamber. Means are pro-
vided for introducing reactants into the reaction
chamber and for continuously or intermittently tapping
pure lead from the furnace. Means are also provided
for condensing vaporized lead chloride and returning
the vaporized lead chloride to the reaction chamber.
Lead chlori.de does not react with molten lead and
having a lesser specific gravity floats on top of the
molten lead.
The second proc~dure comprises introducing into
the reaction chamber a partially uncombusted gas
mixture supplying hydrogen, and completing the com-
bustion with oxygen in an endothermic reaction which
supplies heat for the endothermic lead chloride re-
duction reaction. Heat balance calculations showed
-12-
tha-t sufficient heat can be brought into the lead
chloride reduction reactor to supply -the endothermic
heat of reaction and other heat requirements, includlng
that caused by heat loss, by using a reducing com-
S bustion gas or gas mixture, the term "gas" as usedherein and in the claims including both. An~ hydro-
carbon or mixture of hydrocarbons which supply hydro-
gen can be used. A mix-ture produced by a partial
combustion of hydrocarbons, such as, methane or pro-
pane, provides both the hydrogen and the heat neededfor the endothermic reduction of lead chloride. Con-
trary to what migh-t be expected, introduction into the
reaction area of large volumes of water vapor formed
in the partial combustion reaction and diluent gases
does not adversely affect the reduction reaction. The
above described procedure applies also to the recovery
of copper from cuprous chloride by reduction of hydro~
gen.
Illustrative gases and gas mixtuxes found suitable
2 2' 2 CO CO2-H2O, and H2-CO-N2 The gases
used may or may not be supplemented by hot reducing
combustion gas. Oxygen gas or alr may be used to
supply oxygen.
The PbC12 cake was metered to a brick-lined PbC12
reduction furnace as described above operating between
600-900C, preerably at about 800C. A reducing ga~
feed of 98 percent H2 from an on-site H2 plant was
used. An excess of 240 perceh-t of theoretical H2 was
fed based on lab tests in batch tube furnace runs.
This produces an exit gas consisting of 60 pexcent
HCl and 40 percent H2, by volume. Some volatilized
PbC12 leaves the reactor zone with the off~as but is
refluxed back to the furnace by either a molten lead
splash condenser or an air-cooled surface condenser.
Any additional heat requirements for the endothermic
-13-
reduction reaction and to bring reactants up to
temperature may be supplied by indirect firing of
submerged fire tubes in the molten lead in the reactor
as described above. High puri-ty lead is tapped con~
05 ~inuously or intermittently from the furnace into a
casting machine.
Off-gas is scrubbed in a packed tower or similar
scrubbing device using liquor from cupric leach, and
a large excess of dilutlon air to lower H2 content to
a saEe level and also simultaneously consume scrubbed
HCl which may be used to reoxidize the cuprous ion
to cupric. A water scrubber may also be used to
recover the HCl. Exit gas, free of HC1 and particulate
matter, is exhausted to atmosphere.
lS Up to over 99 percent of lead was obtained from
the starting material. Lead having a purity of ~g9.9
percent was consistently obtained by the process. The
recoveries of lead and silver shown in Tables 4 and 5,
as produced by Examples 1 and 2, are representative
of xecoveries obtained by the process. The lead
purity obtained in Example 2 is also typical.
-14-
Example 1
Two different 100 gram samples of a lead con-
centrate having a composition of 18 percent lead,
26.2 percent zinc, 0.54 percent copper, 5.1 troy ounces
of silver ore ton of concentrate, 0.029 percen-t
antimony and 14.4 percent iron were treated with 250
milliliters of a cupric chloride leach solution com-
prising about 50 grams oE copper per liter as cupric
chloride and 200 grams of sodium chloride per liter~
The pH of the leach solution was maintained at about 1
through the addition of hydrochloric acid. After 3
hours, a total of 4~08 and 4.80 grams of hydrochloric
acid were added to Sample 1 and Sample 2, respectively.
The cupric chloride leach of Sample 1 was conducted
at a temperature of 60C and the cupric chloride leach
of Sample 2 was conducted at a temperature of 80C.
The r~idu~ o:E the cupric chlc)r;~.~lo l~(~ch o F ~ h ~f
the samples was separately brine leached in a brine
solution containing about 250 grams of sodium chloride
per liter at a temperature of 80-85C and about one
a-tmosphere for one-half hour. Each brine leach slurry
was filtered while hot and the residue was washed
first with hot brine solution and then with water.
The analyses of the brine leach residue and the results
of this extraction are set forth in Table 4. The
negative extracted copper percentages are due to a
portion of the cupric chlori.de of the leach solution
being precipitated to copper sulfide.
(3;3
--15--
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-16-
Example 2
A 125 gram sample of a lead concentrate haviny a
composi-tion of 25.1 percent lead, 9.57 percent zinc,
0.36 percent copper and 16~3 perc~nt iron was treated
with 500 milliliters of a cupric chloride leach solu-
tion comprising about 50 grams of copper per liter as
cupric chloride, 200 grams of sodium chloride per liter
and sufficient hydrochloric acid to maintain a pH of
about 1. The cupric chloride leach was conducted at a
temperature of 60C for two hours. The residue of the
cupric chloride leach was subjected to a 900 milliliter
brine leach at a temperature of 80-90C and about
atmospheric pressure of one half hour. The brine
solution contained about 250 grams of sodium chloride
per liter. The analysis of the brine leach residue,
which weighed 83 grams, and the results of the ex-
traction are set forth in Table 5.
The extraction resulted in 19.0 grams of lead
chloride being produced. This lead chloride was
reduced to lead in an atmosphere of 175 cubic centi-
Tneters per minute of hydrogen, 75 cubic centimeters
per minute of carbon monoxide, 75 cubic centimeters
per minute oE carbon dioxide at a temperature of 800C
for 35 minutes. The lead metal was assayed by
emission spectroscopy. The lead metal was 99.98 per-
cent pure. It contained impurities of 0.01 percent
silicon, 0.005 pel~cent iron, 0.001 percent copper and
0.001 percent bismuth with no other elements being
detected.
~17-
Table 5
B _ne Leach Residue Assay ! %
Ag
Pb Cu Zn Fe (oz/ton) Sb
0.21 0.80513.9 22.8 2.0 0.33
Extractlon,~
.
Pb CuZn _ Fe Ag Sb
99.4 -~8~5 3.6 7.1 81.6 32
