Note: Descriptions are shown in the official language in which they were submitted.
HALOGENATION OF LEAD SULFIDE
BACKGROUND OF THE INVENTION
In standard methnds of obtaining metallic lead from concentrates,
the standard procedure has been to treat the lead sulfide concentrates in
a blast furnace. However, the pyrometallurgical procedure possesses~many
disadvantages and drawbacks. Primary among these disadvantages is that
the process will result in some major pollution problems such as the gen-
eration of sulfur oxide gas along with substantial fuming. The fuming
carries with it possible carcinogenic compounds whictl will contain lead,
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cadmiull1, etc. ThereFore, it is necessary to provide improved and safer
methods for obtaining metals such as lead in metallic or elemental form
by methods which will not contribute to pollution of the air or will be
safer to operate. The aforementioned lead smelting techniques will consist
S of roast sintering the lead sulfide concentrate whereby a major portion ofthe sulfur will be removed followed by melting in a blast furnace to obtain
the metallic lead.
In an effort to alleviate the pollution problem~ it is necessary
to develop new processes for obtaining lead which will be competitive as
an alternative to the conventional smelting practices. Prior work in the
hydrometallurgical field resulted in developing a non-aqueous processing
route whereby lead sulfide concentrates are chlorinated at temperatures
above 300C. to produce lead chloride and volatilized sulfur. However,
chlorination at these elevated temperatures will promote formation of
vol~tile chlorides of contaminating elements such as iron, magnesium,
aluminum, silicon, and zinc, as well as elemental sulfur which may be
; present in the lead sulfide concentrate. ~ther hydrometallurgical processes
which have been developed include the use of ferric sulfate as a leach
agent. In this method, the lead sulfide is sulfated to form lead sulfate.
This step is then followed by carbonat;on of the lead sulfate to form lead
carbonate and thereafter the lead carbonate is subjected to dissolution in
hydrofluosilicic acid for electrolysis to metallic lead. Yet another hydro-
metallurgical method which is developed for the recovery of lead is based on
the use of an acidic ferric chloride medium. This method involves a leach-
ing step whereby the lead sulfide is converted to lead chloride and there-
after subjected to steps of solubilizing, crystal7ization and electrolysis.
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.,, I . . ,.. ~.. .
The prior ar-t which discloses other processes for converting a
lead sulfide to elementdl or metallic 7eacl is also exemplified by U.S.
Patent No. 1,491,653. This reference describes the use of chlorides of
sulfur and in particular sulfur monochloride to selectively chlorinate lead
sulFide in a complex lead-zinc sulFide ore at temperatures ranging from
50 to about 150DC. However, in this method of operation, the solids are
reacted in a solution of sulfur monochloride to form a slurry. A disadvan-
. tage which is present when utilizing such a system is that certain metal
sulfides are solubilized due to the dissolution of sulfur into the slurry,
th-is dissolution being due to the wide range of compositions of sulfur chlo-
rides. Such an action can lead to the dissolution of some metal sulfides
thereby rendering the process more complex in nature. In addition to this
type o~ operation, another metal recovery system utilizes a dry chlorination
of complex sulFides in a two-stage process. The first stage consists in a
countercurrent chlorination of the ore in a tube mill Witil chlorine gas, the
temperatures of this process usually being in a range oF from about 100~ to
about 150C. to insure chlorination oF from about ~0~ to abollt 70% of the
metals. The important step in this stage is the chlorination of iron which
serves as a source of chlorine in the second step. The second step of this
two-stage process consists in a chloridizing roast wherein the final chlo-
rination is accomplished to convert all metals present in the ore to chlorides.
Much of this reaction is done by the release of chlorine by the oxidation of
I the initially formed ferric chloride to ferric oxide and chlorine. Follow-
! ing this, the metal chlorides are then leached in water and brines in order
¦ 25 to solubilize the metals. However, the chloridizing roast to produce ferric
chloride will also produce sulfur and sulfur chlorides as well as oxides,
i the roast temperatureswhich are necessary to accomplish this being above
138C. and prrba~ly above 150~C.
, . . .
One example of the two step process is found in United States
Patent ~,011,146. In this reference a dry chlorination of a sulfide ore
is conducted in which the products which are formed during the aforesaid
drying chlorination step are contacted with an inert sweep gas such as
nitrogen to convert any sulFur chlorides which are formed to metal chlorides
and elemental sulfur. The patent teaches that the lead sulfide present in
the ore is converted by an exothermic reaction to lead chloride and, there-
fore, some cooling must be effected or in the alternative inert materials
must be added.
10As will hereinafter be set forth in greater detail, it has now
been discovered that a lead sulfide source may be subjected to halogenation
in the presence of an oxygen-containing gas which is substantially inert to
the environment to provide a fluidized bed for the halGgenat~on reaction.
By utilizing this fluidized bed, it w;ll be possible to effect a more thor-
i 15GUyh hdlogenation of the lead sulfide source with a concomitant increase
in the yield of the desired lead~
SPECIFICATION
This invention relates to an improvement in a hydrometallurgical
process for the recovery of metallic lead. More specifically, the invention
;s concerned with an improved process for the halogenation and particularly
chlorination of a lead sulfide concentrate wherein the halogenation is
effected in a fluidized bed reactor in either a batch or continuous type
of operation.
It is therefore an object of this invention to provide an improved
process for the halogenation of lead-containing sources.
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A further object of this invention is to provide an improvement
in the chlorination of a lead sulFide concentrate whereby a more efficient
mixing o-F gas-solids which is a requirement of the reaction is effected.
In one aspect an embodiment of this invention resides in a pro-
cess for the halogenation of a lead sulfide which comprises halogenating saidlead sulfide at an elevated temperature with a halogen gas in a dry atmo-
sphere, the improvement which comprises halogenating said lead sul-Fide in
a fluidi~ed bed with said halogen ~as, said fluidized bed beiny efFected
by the introduction of a gas consisting essentially oF a mixture of said
halogen gas and an oxygen-containing gas at a rate of from about 4 centi-
meters per second to about l2 centimeters per second to said lead sulfide
within said fluidized bed, wherein the oxygen in said oxygen-containing gas
is substantially inert to said halogenation within said fluidized bed.
A specific embodiment of this invention resides in the process
for the halogenation of a lead sulfide in which said lead sulfide is
ch10rinated at a temperature in the range oF from about 90 to about 120C.
with chlorine gas in a dry atmosphere, said halogenation being effected in
a fluidi2ed bed operation which is afForded by the introduction of a gas
consisting essentially of a mixture of said chlorine gas and air at a rate
of from about 4 centimeters per second to about l2 centimeters per second,
said chlorine gas being present in said mixture in a ratio of from about
O.Ol:l to about 0.2:l parts by volume of chlorine gas per part of said air.
Other objects and embodiments wil1 be found in the following
further detailed description of the present invention.
2~ As hereinbefore set forth the present invention is concerned
with an improvement in a process for the halogenation of a lead source such
as lead sulfide concentrates. When subjecting a lead source such as a lead
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sulfide concen-trate to a halogenation reaction, a certain degree of care
must be taken to control the temperature inasmuch as the halogenation reac-
tion is exothermic in nature. The exothermicity of t'ne reaction can be
readily controlled in a batch type operation, a usual method being by con-
trolling the rate of the addition of the halogen gas. However, when utiliz-
ing a fluidized reaction in order to insure a more effective or efficient
mixing of the gas and the concentrates or solids thereby permitting a higher
halogenation of the lead in the lead sulfide, it is difficult to maintàin
a controlled flow of the halogen gas without permitting the exothermic
nature of the reaction to take over. It has, therefore, now been discoYered
that by introducing a mixture of halogen gas and a gas which is inert or is
not involved in the halogenation reaction~ into the reaction zone, it is
possible to maintain the solid lead sulfide concentrate in a fluidized
state while controlling the temperature of the reaction within the desired
lS range.
In the preferred embodiment oF the invention the halogenation of
the lead source wlth a halogen gas such as chlorine gas, bromine gas, fluo-
rine gas is effected in a dry atmosphere at temperatures in the range of
from about ~0 to ab~ut 120C., this temperature range being necessary in order
to effect a selective halogenation of the lead in the concentrate while
avoiding the halogenation of the other materials which are present such as
iron, copper, cadmium, zinc, etc. The aforementioned mixture of halogen
gas and a gas which is inert to the reaction may be in a ratio of from about
0.01:1 to about 0.2:1 parts by volume of the halogen gas per part of inert
gas. The gas which may be employed in admixture with the halogen gas and
which is substantially inert to the halogenation within the fluidized bed
comprises an oxygen-containing gas in which the oxygen may be present as
fre~ oxygen or as bonded oxygen, examples of these gases comprising air,
carbon monoxide, carbon clioxide, etc. The preferred gas ~hich is substan-
tially inert and which is not involved in the halogenation reaction comprises
air due to its greater availability and inexpensive nature. As hereinbefore
set forth, the mixture of the halogen gas and the inert gas is charged to
the reaction apparatus which comprises a fluidized bed reactor at a rate
which is suFficient to maintain the solid concentrate in a fluidized state.
Therefore, the rate of addition of the mixture of gases must be carefully
controlled inasmuch as if the gas is admitted at too low a rate there ~ill
be no fluidity of the particles. Conversely, if the gas is charged at too
high a rate the result will be a carry-over of dust which is detrimental to
the reaction and to the apparatus involved thereto. ThereFore, it has been
found that the introduction oF the mixture of halogen gas and inert gas to the
reactor is effected a-t a rate in the range of from about ~.0 to about 12.0
centimeters/second, the preferred rate being in a range of from about 6.5
to about 9.0 cen-timeters/second.
By utilizing this mixture of halogen gas and inert gas, it is
possible tc utilize either a batch type reaction or-a continuous type reac-
tion for the halogenation of lead sulfides. When utilizing a batch type
operation, the solid lead value source such as lead sulfide concentrate may
be introduced-into the fluidized bed reactor in particulate form through
conventional means such as a screw conveyor, rotary valve, doubling pinch
valve or any similar air-lock systems. Thereafter the mixture of halogen
gas such as chlorine gas and inert gas such as air wherein the halogen gas
and inert gas are present in the mix-ture in a range hereinbefore set forth
may be introduced at a controlled rate which is sufflcient to maintain the
particulates in a fluidized state. The reactor is then heated to the desired
temperature, following which the temperature is maintained by controlling
the amount of halogen gas which is introduced into the fluidized mixture at
a rate which is sufficien~ to control the exothermici-ty of the reaction.
Upon completion of the desired halogenation reaction, which may range from
about 0.1 up to about 4 hours or more in duration, the halogenated lead
material may then be removed from the system in a similar manner and there-
after subjected to a series oF steps hereinafter set forth in greater detail
to recover the desired lead.
In addition to utilizing a batch type operation, it is also
contemplated within the scope of this invention that the halogenation of
the lead sulfide may also be effected in a continuous manner of operation.
When such a type of operation is employed, the lead sulFide concentrate is
continuously charged to a series of reactors by means similar to that here-
inbefore set forth, that is, a screw conveyor, rotary valve, etc., at a
1~ predete~ ined rate. At the same time, a mixture of ha10gen gas and inert
gas is also continuously charged to the reactors which are maintained within
the desired operating temperature range in such a manner so as to aFFord
a countercu~^rent flow of the yases and the solids. After passage through
the reactors for a time su,ficient to effect the desired halogenation of
the lead sulfide, the particles are then also continuously removed through
similar means and passed to the subsequent steps for recovery of lead. By
utilizing this type of reaction, it is possible to control the heat of the
reaction by either controlling the amount of halogen gas which is introduced
into the fluidization zone, controlling the amount of inert gas which is
introduced as a diluent or by controlling the temperature of the off-gas from
the reactors by passing it through a hea-t exchanger and recycling said off-
gas to be mixed with fresh make~up gas at the bottom of the bed
The lead source which has been subjected to -the halogenation,
and particularly chlorina-tion, s-tep may then be subjected to a series of
further s-teps in order to obtain the desired metallic lead. For example,
after halogenating the lead sulfide concentrate in a dry atmosphere, the
term "dry atmosphere" referring to atmospheres or lead-containing sources
which possess a water content not greater than about 0.5%, is then leached
by the addition o~ a brine solution at an elevated temperature usually in
the range of from about ~0 to about 120C. The leaching of the mixture
with said brine solution which usually comprises an aqueous sodium chloride
solution containing from about 20 to about 35% by weight of sodium chloride
is effected for a period of time which may range from about 0.25 up to about
2 hours or more in duration, the residence time which is required being that
which is sufficient to dissolve the lead halide. Upon completion of the
leaching step, the solution may then be filtered while maintaining the tem-
perature at an elevated range o-f from about 80 to about 120aC. whereby the
lead halide is maintained in a soluble forlrn The filtrate, which contains
the soluble lead halide, may then be passed to a crystallization zone wherein
~ the soluble lead halide is crystallized due to a drop in the temperature,
: the temperature of the crystallization zone being mainta;ned at a temperature
somewhat lower than the leach temperature by external cooling means such as
a heat exchanger. For example, typical crystallization temperatures would
: be between about 60C. and ambient (20-25C.).
The thus crystallized lead halide may then be recovered and dried
to ren-love any trace of water which may still be present, the drying being
effected, for example, by placing the lead halide in an oven and subjecting
said halide to a temperature of about 100C. in an atmosphere of air for a
period of time ranging from about 0.1 to about ~ hours or more. Following
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the clrying of the lead halide, it is then placed in an appropriate apparatus
and subjected to a temperature sufficient to melt said halide until it
assumes a molten ~orm. The temperature which is utilized to effec-t this
melt may range from abou-t 380C. which is sufficient to melt lead bromide up
to about 875C. ~hich is sufficient to melt lead fluoride. Following this,
the lead halide in molten form may then be admixed with the salt of a metal
selected from the group consisting of alkali metals and alkaline earth metals
such as lithium chloride, sodium chloride, potassium chloride, cesium chlo-
ride, magnesium chloride, calcium chloride, barium chloride, sodium bro-
mide, potassium bromide, rubidium bromide, beryllium bromide, calcium bro-
mide, sodium fluoride, potassium fluoride, etc., in a fused salt bath. If
so desired, the salt of the alkali metal or alkaline eartll metal will be
comparable in the halide content to the lead halide which is to undergo
electrolysis. The thus formed fused salts are then subjected in a fused
1~ salt bath to electrolysis utilizing a su-Fficient voltage to effect said
electrolysis whereby metallic lead is deposited as a liquid which can be
removed from the -fused salt and recovered.
The following examples are given for purposes of illustrating
the process of the present invention~ However, it is tu be understood
that these examples are given merely for purposes of illustration and that
the present invention is not necessarily ljmited thereto.
EXAMPLE I
As an illustration of the applicability of a mixture of halogen
gas and inert gas to effect a halogenation of a lead source~ a series of
experiments was performed on a lead sulfide concentrate. The apparatus
,~,
which was utilized in these experiments consisted of a Pyrex reaction
vessel with a coarse porosity frit at the bottom to disperse the fluidizing
.. . . ............ ........
gas. The reactor was provided with a -thermocouple placed in a thermowell
to measure the temperature of the reac-tion and a por-t for sampling the bed
solids which ~ias located opposite the thermowell adjacent to the top of
the reactor. In addition, the apparatus was also provided with an expansion
chamber, a miniature cyclone and a caustic scrubber which was used to clean
the off-gas from the reactor. The expansion chamber and cyclone were used to
remove particulates from the gas while the caustic solution in the scrubber
was used to remove residual chlorine and sulfur chlorides from the off-gas.
A sample consisting of 600 grams of the lead sulfide concentrate in particu-
lo late form was poured into the ~luidizing zone of the reactor. The leadsulfide was then fluidized with air and the fluidizing zone was heated
using a 1000 watt electric heat gun. When the temperature reached 95C.
a mixture of chlorine gas and air was charqed to the reactor. Although
most of the heat necessary to maintain the temperature was supplied by the
exothermic nature of the reaction, external heat was added occasionally to
maintain the temperature in the desired range, that is, 100-110C. Samples
of the bed of solids were obtained through the port at intervals during each
test and analyzed to determine the amount of lead sulfide which had been
converted to lead chloride. It was found that the -feed solids changed
color from black to yellowish-gray during the reaction, the extent of
color change being used to judge the length of each test. Upon completion
of the reaction, the chlorine flow was terminated while air Flow was main-
tained to cool the reactor. When the reaction vessel had cooled to room
temperature, the final product was recovered, weighed and analyzed. The
results of three tests using this method are set -forth in Table I below:
TABLE I
Tirne, C12/Air Gas Velocity, % Lead
Test l~o. Min. Vol. Ratio Cm/Sec. Converted
A 0-10 0.021 8.03 ---
10-20 0.033 8.12 4.8
20-65 0.042 8.20 22.4
65-95 0.054 8.29 40.1
95-125 0.065 8.36 56.6
125-155 0.075 8.~6 60.4
155-185 0.084 8.52 &~.2
B 0-10 0.021 7.27 ---
10-20 0.032 7.35 ---
20~60 0.041 7.~1 43.0
60-90 0.052 7.49 76.4
90-120 0.06~ 7.57 92.2
120-150 0.075 7.66 91.1
C 0-20 0.01~ 6.47 ---
20-30 0.032 6.59 ---
30-60 0.042 6.~5 ---
60-90 0.052 6.72 56.0
90-120 0.063 6.78 87.2
120-150 0.07~ 6.85 94.g
E~ L.~ II
To illustrate the fact thal; the process of the present inven-
tion may also be used in a continuous manner of operation, a second experiment
- was performed in which a continuous operation~was simulated. The feed for
this test was prepared by blending 315 grams of a fresh dry lead sulfide
concentrate with 315 grams of a lead sulfide concentrate that was 98% chlor-
inated. A head sample of the blend was submitted for assay and 60~ grams
of the blend were then used in the test. The test was run for a period of
2 hours using conditions similar in nature to those set forth in Example I
above. The exothermic heat of the reaction was less than that found in Example
I and constant heating was necessary to maintain the temperature of the
reaction within the desired range. The results of this test are set forth
in Table II below. These results indicate that the initial conversion rate
of lead was relatively high.
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TABLE II
Time, C12/Air Gas Velocity, % Lead
rest No. Min. Vol. Ratio Cm~Sec. Converted
D 0-25 0.065 7.62 ---
25-50 0.077 7.71 70.6
50-75 0.090 7.~0 72. 4
75-100 0.102 7.88 ---
100-120 0.113 7.69 79.4
EXAMPLE III
In this example a lead bearing source which contained a mixture of
lead sulFide, copper sulFide, iron sulfide and zinc sulfide was chlorinated
at a temperature of 100C. in a dry atmosphere which did not contain more
than about 0.5% of water. The chlorination was efFected by passing a mix-
ture of chlorine gas and air over the metal sulfide mixture at a rate of
100 cc/min. of chlorine and 800 cc/min. of air. Upon completion of the
chlorination step, the product was analyzed by means of atomic absorption
principles.
In contradistinction to this, a chlorination step was e-ffected
utilizing the method as set forth in U.S. Patent 1,491,653. A lead concen-
trate sample similar ;n nature to that set forth in the above paragraph
was~slurry leached in an agitated magnetically stirred apparatus, ~iltered
and washed with fresh sulFur monochloride according to the method set forth
in the patent. Thereafter, the product was a~ain analyzed by means oF
~ atomic absorption principles. The results of the two chlorination steps
:¦ 25 are set Forth in Table III below:
I TABLE III
i Sulfur Monochloride
¦ Metal Dry C12 Conversion According
~-l Sulfide Converslon To U.S. 1,491,65
I
Pb 90.4% 75%
Cu 23.7% 51~
Fe 12.7% 25%
Cd -- 32%
Zn 5.0% 29%
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It is readily apparent from a comparison of the chlorination
conversions of the two methods as set forth in the above table that the dry
chlorination step set for-th in the present application is considerably more
selective for lead than is the chlorination conversion of the U.S. patent.
In addition to this more selective conversion, another advantage which is
present in the instant application is that no slurry is required to be
treated or filtered as must be done in the method set forth in the U.S.
patent.
EX~lPLE IV
To illustrate the fact that in order to obtain a greater selec-
tivity of halogenation, and particularly chlorination, of lead in metal
bearing sources such as those ores containing lead sulfide, zinc sulfide,
ferrous sulfide, copper-iron sulfide, iron-arsenic sulfide, etc., can be
obtained at relatively low temperatures, Table IV below contains data ~hich
was set forth in two Bureau oF Mines Publications, the first being RI-5~g4
titled "Chloridizing The Sulfides OF Lead, Zinc And Copper", and the second,
Rl-6052 t1tled "Chloridization Of Certain M;neral Sulfides".
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T_BLE IV
_emp. (~C.)% Chlorination
A. Sphalerite (ZnS~ 124 11
250 20
400 3~
500 42
600 100
B. Pyrite (FeS2) 100 12
150 ~1
200 72
250 96
300 100
C. Chalcopyrite (CuFeS2) 100 7
200 ~2
300 100
D. Arsenopyrite (FeAs S) 150 0
200 ~0
250 66
300 gO
400 100
It is therefore shown in Tables III and IV that by utilizing a rela-
tively low temperature as taught in the process of the present invention,
that is, from about 90 to about 120C., in the chlorination of a mixture
o-F metal sulfides while employing gas as a chlorinating agent and in a dry
atmosphere containing less than about 0.5% water, it is possible to selec-
tively chlorinate lead sulfide to lead chloride while leaviny the other metal
sul^Fides relatively unaffected. This selective chlorination facilitates the
separation of the desired lead chloride from ilnpurities in subsequent steps.
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