BACKGROUND OF THE INVENTION
Lead is often used as a shielding material in radiation evaluation
equipment in order to reduce the system background radiation. Lead, however, contains
small amounts of radioactive isotopes including lead-210, bismuth-210 and polonium-210.
In electronic devices, lead and lead alloys are often used in contacts
and solder pads. Integrated circuit memories can suffer from soft errors that can
destroy the data in a memory cell and are caused by the alpha particles emitted
from the decay daughters of Pb-210, particularly Po-210. Pb-210 has a half-life
of 22 years.
Po-210 is well-known as a source of alpha particle emission and it
is, therefore, of prime importance to use a lead that has a low alpha particle
emission, especially in the abovementioned applications. The emission is usually
measured as a count expressed in the number of alpha particles emitted per cm²
per hour (alpha count hereinafter). Commercially available lead has alpha counts
that may vary from as low as 0.25 to as high as 10 and, unless each batch of lead
is analyzed for its alpha count, there is no method for predicting which commercial
lead has a low count. There is no commercial process known whereby the Pb-210 can
be easily removed from commercial lead. Japanese Patent 59-64 791 describes producing
a low alpha lead, containing <50 ppb radio isotopes and an alpha count of ≦
0.5, by electrolyzing a sulfamic acid-lead electrolyte using a lead anode. In
spite of the fact that Pb-210 has a half-life of 22 years, even lead that is several
hundred years old, such as recovered from sunken ships or from church roofs in
Europe, has counts of 0.03 to 0.07. These alpha counts are much higher than the
level required for electronic devices and integrated circuits. The desired alpha
count in the electronics industry is 0.02 or less.
Zone refining, which is a successful method for removing substances
that emit alpha particles (alpha emitters hereinafter) from aluminum, does not
remove Pb-210 from lead. Although a temporary decrease in alpha count is obtained
when lead is zone refined with the initial removal of Bi-210 and Po-210, the count
increases again with time to its original level as secular equilibrium is regained,
indicating that Pb-210 is not removed.
SUMMARY OF THE INVENTION
The invention is based on the discovery that alpha emitters in lead
mineral-containing orebodies are associated with the host rock. Thus, we have found
that lead with a low alpha particle emission, i.e. low alpha lead, can be simply
produced by carefully selecting the orebody, recovering the lead mineral as a
concentrate and reducing the concentrate without the introduction of alpha emitters.
More particularly, we have found that by mining a lead deposit that
contains lead mineral in a coarsely-disseminated form, substantially free from impurities,
in a host rock with associated minerals that are relatively low in alpha emitters,
milling the mined ore and subjecting the ground ore to a gravity separation, the
alpha particle-emitting host rock or gangue and associated minerals are effectively
removed, and a lead concentrate is obtained that has a low alpha count. Subjecting
the concentrate to a suitable reduction operation without the addition of any
material that can introduce alpha emitters, yields lead metal that has an alpha
count of about 0.02 or less. Suitable reduction operations comprise the reductions
of sulfidic lead minerals with sodium carbonate in an oxidizing atmosphere or
in a nonoxidizing atmosphere, or with hydrogen, iron or charcoal, and the reduction
in a bath of molten lead chloride with the application of an electric current,
provided that these materials have a low alpha count. The reduction may also include
a prior conversion step to convert the concentrate into a form suitable for reduction.
The reduction, as herein described, is understood to include a prior conversion
as required. As desired, the lead recovered from a reduction may be subjected
to electro-refining to reduce its impurity content.
Accordingly, there is provided a method for the production of lead
with a low emission of alpha particles which comprises the steps of selecting an
orebody containing lead mineral in a coarsely-disseminated form substantially free
of impurities, and in a host rock together with associated minerals and relatively
low in alpha emitters; mining said orebody to produce mined ore; milling said mined
ore to form ground ore having particle sizes such that separation of lead mineral
from said host rock and associated minerals can be effected; forming a fluid suspension
of said ground ore; subjecting said suspension to gravity separation to remove
said host rock and associated minerals from said lead mineral; recovering said
lead mineral as a concentrate; subjecting said concentrate to a reduction; and
recovering lead having an alpha count of 0.02 alpha particle per cm² per hour
It is, therefore, an object of the present invention to provide a
method for producing low alpha lead. It is another object to provide an economical
method for producing large quantities of low alpha lead on a commercial scale.
These and other objects of the invention will become apparent from the following
Lead occurs mainly as galena but also in the form of carbonate, and
sulfate, as well as in other forms. The lead minerals usually occur in combination
with other minerals and impurities many of which are alpha emitters. The lead
minerals are present in host rocks, many of which are relatively high alpha emitters,
i.e., relatively high in uranium and thorium and, consequently, high in the Pb-210
isotope. Other host rocks, especially the carbonate-type host rocks that are usually
of a sedimentary type, are relatively low alpha emitters, i.e., relatively low
in uranium and thorium, and hence relatively low in Pb-210. Moreover, in many
deposits the lead mineral is present in a finely-disseminated form, that is closely
associated with impurities. Unless treated in a complex and expensive manner,
it is generally not possible to separate the lead mineral from such deposits into
a concentrate that can yield low alpha lead.
In order to produce lead with a low alpha count it is, therefore,
necessary to select deposits wherein the lead mineral is present in a coarsely-disseminated
form substantially free of impurities. Such deposits include the carbonate-type
orebodies at Polaris on Little Cornwallis Island and at Pine Point in the Northwest
Territories, and at Bixby, Missouri. These orebodies all contain galena as the
main lead mineral as well as some oxidized lead forms. The galena is present in
a coarsely-disseminated form substantially free of impurities in a host rock that
has an alpha count of less than about one alpha particle per cm² per hour.
It is pointed out that low alpha lead can be made directly by reducing
pure galena, which can be recovered such as by hand-picking from ore bodies. Such
a recovery is, however, not an economical method for producing low alpha lead on
a commercial scale.
After selecting an orebody with coarsely-disseminated lead mineral
substantially free of impurities in a host rock relatively low in alpha emitters,
i.e., preferably having an alpha count of less than about one, the ore is mined
in the usual well known manner to produce a mined ore. The mined ore is milled
to produce a ground ore. The milling is carried out to a degree sufficient to be
able to separate the lead mineral from the host rock and the associated minerals.
Depending on the ore, a coarse-milling is usually adequate for effecting a subsequent
separation of mineral from rock and the associated minerals. Milling of ore obtained
from the above-mentioned orebodies to particle sizes smaller than about 35 mesh
(Tyler Standard Screen Scale Sieves Series) is preferable. The milling is carried
out using a known method and known equipment.
The ground ore is formed into a fluid suspension suitable for separation
of the lead mineral from the host rock and associated minerals by gravity separation.
In one embodiment, the ground ore is mixed with water to form an aqueous suspension.
The suspension is then subjected to a gravity separation using known equipment
such as a spiral, a Wilfley or Deister Table or other suitable gravity separation
equipment. In a second embodiment, the ground ore is formed into a fluid suspension
using air as the medium to form a gaseous suspension and subjected to gravity
A gravity separation is more efficient when the particles in the
fluid suspension are substantially of the same size. Preferably, therefore, the
ground ore is subjected to a sizing operation, such as by screening or hydro-sizing,
prior to forming the fluid suspension, to form a fraction with a narrow range
of particle sizes of the ground ore. Preferably, such a fraction may have particle
sizes in the range of about minus 35 to plus 325 mesh. It is understood, however,
that other particle size ranges such as, for example, the minus 325 mesh fraction,
may be used to give the desired results. Preferably, the ground ore is separated
into a range of narrow particle size fractions, each fraction being formed into
a fluid suspension which is subjected to a gravity separation for the formation
of a lead mineral-containing concentrate separated from host rock and associate
minerals. For example, three particle size fractions may be formed by screening
or hydro-sizing, these fractions having particle sizes in the ranges of about minus
35 to plus 100 mesh, about 100 to plus 200 mesh, and about 200 to plus 325 mesh,
The gravity separation of a fluid suspension of ground ore is effective
in separating the host rock that substantially contains the alpha emitters, especially
Pb-210, and the associated minerals, from the lead mineral-containing concentrate.
The lead concentrate is subjected to a suitable reduction operation
for the recovery of lead metal that has a low alpha count. Optionally, the concentrate
may be subjected to a washing or etching operation prior to reduction. The washing
or etching may be carried out to remove residual host rock and associated minerals,
and may be effected with organic chemicals or hydrochloric acid substantially free
of alpha emitters. It is understood that a suitable reduction may include a conversion
of the concentrate into a form that is reducible to lead with a low alpha count.
For example, such a conversion may be the conversion of lead sulfide into lead
oxide, lead chloride, lead carbonate or like lead compounds that can be subjected
to electrolytic reduction for the recovery of lead with a low alpha count.
The reduction process must be a simple reduction, because the more
complex processes used in large-scale commercial lead smelting operations routinely
require the use of additives and fluxes that generally are alpha emitters. The
commercially-used smelting processes are, therefore, not suitable for reducing
the lead concentrate, not even pure galena, to a low alpha lead.
Suitable reduction processes comprise reductions of the lead concentrate
with, for example, hydrogen, iron, or charcoal, and the electrolytic reduction
in a bath of molten lead chloride as electrolyte. These reductions are well-known.
The reducing agent or electrolyte must be a material that has no or a low alpha
count. When reducing a lead concentrate, it is also desirable to avoid the evolution
of noxious gases, such as hydrogen sulfide and sulfur dioxide. The preferred reduction
processes using a low alpha count reducing agent and without the evolution of noxious
gases are the processes of smelting lead sulfide (galena) concentrate with sodium
carbonate with and without the addition of an oxygen-bearing gas. In the reduction
of the concentrate with sodium carbonate with the addition of an oxygen-bearing
gas, sodium chloride is added as a fluxing agent to form a low melting point salt
phase. The sodium chloride and the sodium sulfate formed during smelting form
a low melting point salt phase at about 600°C. Both sodium carbonate and sodium
chloride must have no or a low alpha count. The oxygen-bearing gas is chosen from
the group consisting of oxygen, air and oxygen-enriched air. The smelting reaction
in the presence of oxygen takes place according to the following equation:
2PbS + 2Na&sub2;CO&sub3; +3O&sub2; + 2NaCl → 2Pb + 2(NaCl.Na&sub2;SO&sub4;)
Preferably, the lead sulfide concentrate is mixed with an excess
of sodium carbonate and sodium chloride, and is smelted in a suitable vessel, made
of a material with a low alpha count such as graphite, with the lancing of oxygen-bearing
gas. The molten lead is easily separated from the molten salt, and lead metal is
recovered as low alpha lead with an alpha count of about 0.02 alpha particle per
cm² per hour or less.
The smelting reaction with sodium carbonate in the absence of oxygen,
i.e. reduction without the addition of an oxygen-bearing gas, takes place according
to the following equation:
4PbS + 4Na&sub2;CO&sub3; → 3Na&sub2;S + Na&sub2;SO&sub4; + 4CO&sub2; + 4Pb°
The reaction occurs with the evolution of a considerable amount of
carbon dioxide. In order to control the reaction, the charge mixture, which is
a well mixed blend of appropriate amounts of lead sulfide concentrate and sodium
carbonate, is continuously fed at a low and steady rate into a bath of hot reacted
material. The reacted material, i.e. sodium sulfide, sodium sulfate and lead, is
contained in a suitable vessel made of a material with a low alpha count, e.g.,
graphite. By only partly filling the vessel, thus leaving considerable freeboard,
the reaction is further controlled. The feed mixture preferably contains an excess
of sodium carbonate, e.g., 10 to 15% excess. If desired, the feed mixture may
also contain an amount of sodium chloride, which will tend to lower the temperature
of the reacted material, i.e. the matte. The reaction commences at a temperature
of about 850°C and may be carried out at temperatures as high as 1300°C. Preferably,
the temperature is maintained at about 1050°C. At this temperature the steady
input of new feed charge causes a rapid reaction with manageable evolution of carbon
The molten lead collects in the bottom of the vessel and is recovered
therefrom as a low alpha count lead with an alpha count of about 0.02 alpha particle
per cm² per hour or less. Optionally, the molten lead recovered from the smelting
vessel may be further purified by first treating with a small amount of sodium
hydroxide and then with a small amount of an oxygen-bearing gas to reduce the sulfur
and sodium sulfide contents.
The gases from the smelting vessel consist mostly of carbon dioxide
and small amounts of PbS, PbO, SO&sub2;, Na&sub2;SO&sub4; and, if used, NaCl. The
off gases are conventionally treated using a baghouse or scrubber. The salt phase,
or matte, from the smelting vessel is removed from the process. If desired the
matte may be quenched in and leached with water while being agitated and subsequently
settled. The solids may be separated from solution, dried and returned to the smelting
vessel. Sodium sulfide in the solution may be substantially oxidized by bubbling
an oxygen-bearing gas through the solution, followed by the addition of a small
amount of hydrogen peroxide.
As an alternative to a smelting reduction, the lead concentrate is
reduced electrolytically in a bath of molten lead chloride with the evolution of
elemental sulfur. This process is disclosed in US Patent 2,092,451, hereby included
by reference. The process according to the patent comprises separating lead and
sulfur from lead sulfide-containing material in fused lead chloride, the fused
chloride containing 1-10% lead sulfide. A current is applied at a current density
between about 5000 and 10000 A/m² to bipolar electrodes with a voltage drop of
1.2 to 1.4 V over each gap. The sulfur is evolved at the anode and is collected
and condensed. The lead is evolved at the cathode and is removed in molten state
from the cell.
This process may be successfully used for the preparation of a lead
with a low alpha count, provided the materials of the cell and electrodes as well
as the fused lead chloride electrolyte have no or a low alpha count. Preferably,
the cell and the electrodes are made of graphite, and the lead chloride is prepared
by chlorination of lead, lead sulfide or lead sulfide concentrate with a low alpha
count. In a preferred embodiment, the cell is a cylindrical graphite vessel acting
as cathode, and has a single hollow cylindrical anode open at its top and bottom
positioned centrally in the vessel some distance above the bottom of the vessel.
A suitable cover closes the cell and the anode. A mixing device is centrally located
at the lower end of the anode, the shaft of the mixer protruding through the cover.
The anode is provided with a number of spaced slots at its lower extremity to
improve mixing and with a number of openings at its upper end to allow circulation
of electrolyte, as well as to provide passage of evolved sulfur vapor. The cell
cover is provided with a passage for the feeding of concentrate into the anode
and for the syphoning of molten lead from the bottom of the cell. An opening is
provided in the cover for the removal of sulfur vapour. The cell, cover and passages
are well-insulated to reduce heat loss.
The process is preferably operated at a temperature maintained in
the range of about 500 to 600°C, using a concentration of lead sulfide in the lead
chloride in the range of about 2.5 to 25%, preferably 10% by weight, maintaining
a spacing between anode and vessel wall of about 5 cm, and using a current density
in the range of about 6000 to 9000, preferably about 7000 A/m². Lead sulfide concentrate
is continuously added at a rate to maintain the desired concentration in the electrolyte.
Molten lead is periodically syphoned from the cell. The electrolyte is skimmed
and bled at suitable intervals to remove impurities, and electrolyte is added as
required to maintain the desired level in the cell. The electrolyte is agitated
at a suitable rate to circulate the cell contents. The lead recovered from the
process is low alpha lead with an alpha count of about 0.02 particle per cm² per
It is noted that the alpha count of lead produced according to the
process of the invention remains substantially constant with time.
If desired, the low alpha lead recovered from the reduction processes
may be further purified by electro-refining. The electro-refining of lead in a
hydrofluosilicic acid or sulphamic acid electrolyte is well known, and may be carried
out according to either the well-known Betts Process or the bipolar process, provided
that substantially no alpha emitters are present or introduced. As in the reduction
processes, the electrolyte, as well as the lead cathode, in case of the Betts
Process, must have no or a low alpha count. In the electro-refining of low alpha
lead, the lead from a reduction process, as described, is made into anodes that
are immersed in the electrolyte and are refined under standard, well-known conditions.
Refined, low alpha count lead with a reduced impurity content is recovered from
the electro-refining process.
The invention will now be illustrated by means of the following non-limitative
This example illustrates the method of the invention.
Coarsely-disseminated lead mineral substantially free of impurities
was selected from the carbonate-type galena ore body at Pine Point, N.W.T. The
ore body was mined and the ore was coarse crushed to smaller than one inch, fine-crushed
to smaller than 3/8 inch using jaw crushers, ground in a pulverizer, and screened
to minus 35 mesh. The alpha count of a sample of screened ore was 0.24 alpha particle
per cm² per hour. The screened ore was made into a fluid suspension by the addition
of water and subjected to a gravity separation using a Deister table, model RH15SSD.
Two hundred and twenty eight kg of lead concentrate containing 84% lead was separated.
The alpha count of a sample of the concentrate was 0.02. This concentrate was
again subjected to gravity separation yielding a second concentrate containing
86% lead with an alpha count of less than 0.01. A portion of the lead concentrate
was mixed with an above stoichiometric amount of sodium carbonate and with sodium
chloride, these salts having an alpha count of 0.03. The mixture was smelted with
air sparging in a graphite crucible (low alpha count) for six hours at a temperature
in the range of 800 to 1000°C. Eighty two kg of lead metal, which separated readily
from the slag, was recovered. The grade of the lead metal was 99.99%. The alpha
count of the recovered metal was less that 0.01. Upon monitoring the count over
a period of time, it was determined that the alpha count remained essentially constant.
The results show that low alpha lead can be produced from lead mineral
that is coarsely-disseminated in a host rock substantially free of impurities and
relatively low in alpha emitters by subjecting crushed ore in a fluid suspension
to a gravity separation, and smelting the resulting concentrate with a reducing
agent with no or a low alpha count. The results also show that alpha emitters are
associated with the host rock.
Galena ore was hand-picked from the Polaris, Pine Point and Bixby
ore bodies. The galena was coarsely-disseminated in a carbonate-type host rock
and was substantially pure.
The hand-picked galena, which was substantially free of host rock
and impurities, each had alpha counts of less than 0.01. Nine hundred grams of
hand-picked galena from each ore body was mixed with 600 g of sodium carbonate
and 300 g of sodium chloride and smelted in a graphite crucible for two hours
at 950°C. Lead metal was recovered from each smelting with an 80% recovery, and
was determined to have an alpha count of less than 0.01 in each case. The alpha
counts of the lead recovered from each smelting did not increase with time.
The results show that pure galena has a low alpha count and that
the alpha count does not increase when the galena is smelted according to the method
of the invention.
This example illustrates that low alpha lead can not be produced
by conventional, commercially-used processes, even when the lead mineral is present
in a coarsely-disseminated form in a low alpha count host rock.
A lead concentrate was produced by crushing, grinding and froth flotation
of ore obtained from the Pine Point mine. The alpha count of the lead concentrate
was 0.428. This concentrate was subjected to conventional, commercial smelting
with the addition of lime-rock, silica and coke. A sample of lead metal recovered
from this smelting had an alpha count of 0.06. The alpha count increased, however,
with time to a value of 0.17 after twelve months.
Nine hundred grams of the same lead concentrate with an alpha count
of 0.428 was smelted as in Example 2. The lead recovered from this smelting had
an alpha count of 0.05. The count was also found to increase with time.
The results show that the usual commercial processes used for concentrating
lead mineral do not yield a lead concentrate that has even a relatively low alpha
count. Furthermore, the results show that neither commercial-type smelting nor
smelting with agents that have no or a low alpha count of a froth flotation concentrate
yield low alpha lead with an alpha count that remains constant with time.
This example illustrates the preferred reduction of lead sulfide
concentrate using sodium carbonate without the addition of oxygen-bearing gas.
A lead concentrate was prepared from Pine Point ore by crushing, grinding and
gravity separation as described in Example 1. The concentrate contained 82% lead
and had an alpha count of 0.02 alpha particle per cm² per hour. 2500 g of the lead
concentrate was mixed with 1450 g Na&sub2;CO&sub3;, i.e., 30% excess over stoichiometric,
and 725 g NaCl. 3140 g of the mixture was heated by induction in a graphite crucible
to a temperature of 1050°C. The reaction was continued for one hour and 1220 g
of lead were subsequently recovered. The recovery was 89%, the grade of lead metal
was 99.99%, and the alpha count of the recovered lead metal was less than 0.01.
The alpha count did not increase with time.
The electrolytic cell as described is used for the electrolysis of
lead concentrate that was prepared from Pine Point ore by grinding and gravity
separation as described in Example 1 and contained 82% lead with an alpha count
The graphite cell has an inside diameter of 40 cm and a height of
60 cm. The cell is filled with an amount of molten lead chloride prepared by the
chlorination of low alpha count lead (alpha count less than 0.01). A graphite
anode with a diameter of 30 cm and a height of 45 cm is immersed in the bath such
that the agitator circulates melt through the openings at the upper end of the
anode while leaving space for the passage of evolved sulfur vapor. The space between
the anode and the cell wall is 5 cm and that between the anode and the cell bottom
is 10 cm.
A non-alternating potential difference is applied between anode and
cell wall to give a direct current flow at a density of 0.7 A/cm² of anode surface.
Lead sulfide concentrate is continuously added through the cover into the anode
at a rate of 12.5 kg/h. The temperature is 525°C. The electrolyte is agitated and
the concentration of lead sulfide in the electrolyte is maintained at about 10%
by matching the feed rate to the current flow. Lead is formed at a rate of 10
kg/h and is periodically withdrawn from the bottom of the cell. Sulfur vapor exits
from the top of the cell. The withdrawn lead has an alpha count of 0.02 or less.
Molten lead from the reduction process of Example 1 was poured into
anodes and subjected to electro-refining according to the Betts Process. A sample
of the lead had a total impurity content of 568 ppm, as determined by spark-source
emmission spectroscopy, and had an alpha count of 0.014. Both the lead fluosilicate-fluosilicic
acid electrolyte and the lead cathodes were made from low alpha count lead. The
lead anodes were immersed in 1.5 L electrolyte, and a current of 3 A was applied
between cathode and anodes. The cell potential drop was 1.2 V. Electrolysis was
continued for 90 h, after which 950 g of lead was recovered. The recovered lead
had a total impurity content of 68 ppm and an alpha count of less than 0.01.
It is understood that modifications may be made in the process of
the invention without departing from the scope of the appended claims.