Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
21521'0
BACKGROUND OF THE INVENTION
This invention relates to the treatment of volcanic igneous rocks, commonly
called volcanic
cinders, to recover the gold, silver and platinum group metals therefrom.
A common volcanic ore is composed of silicates and oxides with major amounts
of iron,
magnesium and aluminum, with lesser, but significant, amounts of sodium,
boron, calcium, fluorine,
and phosphorus (from tenths of a percent to three percent of these elements).
The ore contains a
major amount of magnetite. At a grind of 43 percent minus 65 mesh plus 200
mesh and 57 percent
minus 200 mesh, 54.6 percent of the sample was strongly magnetic. Not all of
this magnetic
material is magnetite, however, since much of the magnetite is locked with non-
magnetics. The
phosphorus and fluorine are assumed to come from apatite, CasF(POQ),, a
mineral commonly
associated with magnetite in igneous rocks.
Volcanic cinders, as typified by the ore described above, are extremely
resistant to all of the
procedures normally used in the treatment of precious metal ores. A typical
smelting process is one
in which all of the materials charged to a smelting furnace are completely
fused or melted, resulting
in two or more liquid products which stratify into separate layers upon
standing, with the slag, the
CA 02152770 2001-12-11
liquid of lowest specific gravity, forming the top layer which is skimmed off.
In most smelting
opf;rations the slag is a waste product which serves as a vehicle to eliminate
substances which are not
desired in the valuable products recovered from the lower layers of molten
material. 'Che bottom
layer can be a metal, such as lead in lead smelting, iron in iron smelting, or
copper in some types of
copper smelting. An intermediate layer can be a matte (principally molten
sulfides), or a speiss
(molten arsenides and/or antimonides). It is well known that molten lead
serves as a solvent, or
collector, for gold, silver, and the platinum group metals in the smelting of
lead ores. Similarly,
liquid copper and liquid mattes (molten sulfides of copper and iron, or nickel
and iron) serve as
collectors for the precious metals (gold, silver, and the platinum group) in
copper and nickel
smelters.
SUMMARY
In the investigations leading to the invention, lead was first considered as a
possible molten
solvent or collector of the precious metals in the smelting of volcanic
cinders. Standard fire assaying
is a miniature, laboratory sized, lead smelting process in which the materials
charged to the
laboratory crucible are smelted to result in two liquid layers, a lead layer
which settles to the bottom
of the crucible and a lower specific gravity slag layer on top of the lead
layer which carries off the
undesirable wastes in the ore. The lead contains the precious metals which are
then recovered from
the lead to complete the assay.
A second procedure used in the investigation employed an iron alloy as the
collector. In this
z 0 procedure, four furnace charges consisted of twenty-five (25) pounds of
ore and twenty-one and one-
half (21.5) pounds of borax for flux were used. The ore contains several
percent carbonaceous
maeterial which occurs naturally, and this provides most of the reducing
agent. The material serves
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not only as a reducing agent, but also as an alloying element which allows the
melting point of iron
to be reduced. Just as in pig iron production, the second most abundant
element in the alloy after
iron is a few percent of silicon. The iron also contains two (2) to three (3)
percent phosphorus,
which together with the silicon and carbon reduces the melting point of the
resulting alloy to the
terr~perature range of the investigations ( 1000 ~ - 1260 ~ C) . The amount of
iron alloy produced was
1.75 pounds from 100 pounds of ore.
The iron alloy was then dissolved electrolytically in a twenty percent (20% )
sulfuric acid
solution. The iron alloy served as the anode and a stainless steel sheet was
used as the cathode. A
standard fire assay of the residue which remained after dissolving the iron
showed that gold was a
major constituent of the residue and that the iron alloy was a good collector
for gold, in the absence
of lead. This procedure avoided the toxicity and environmental problems
associated with lead.
A flux is a material which converts compounds that are infusible at a certain
temperature into
others which melt at that temperature. For example, silica (SiOZ) is fusible
only at a very high
temperature, but if sodium carbonate is added the mixture can be fused at a
much lower temperature.
A mixture of one mole of silica (SiO~) and one mole of sodium oxide (Na20)
will melt at about
10 20 C to form a slag of the composition Na2Si03, much lower than the melting
points of either
NazO or SiO,. The borax and sodium carbonate used in the experiments coverd by
this
investigation, when mixed with the ore, lowered the melting point of the
resulting mixture to
allow the use of 1000 - 1260 degrees centigrade for forming a molten fluid
slag. Copper was
added to help in the collection of gold since copper and gold show complete
mutual solubility in
each other.
The mixture of ore, copper, borax and sodium carbonate was heated to 1260
degrees
centigrade and held for 30 minutes. In the experiments covered by this
disclosure 1.25 pounds of
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copper, 25 pounds of ore, crushed to minus one-eighth inch, 16.7 pounds of
borax, 8.3 pounds of
sodium carbonate and one-quarter pound of lluorspar were charged to the
furnace crucible. At the
end. of the furnace melt, the slag was skimmed off and discarded and another
identical charge was
add'~ed to the crucible, after four charges and four slag skimmings the molten
metal remaining was
poured from the crucible into an anode mold where eight pounds of copper and
iron metal were formed
into an anode bar. After solidification, the bottom layer of the metal bar was
grey and had the
appearance of cast iron, while the top layer had the typical reddish
appearance of copper. However,
there was a considerable amount of alloying between iron and copper at the
interface and the copper
had enough iron in it to be magnetic. Each twenty-five (25) pounds of ore
melted yielded about two
(2) pounds of iron and copper metal. Electrolysis of the eight (8) pound metal
bar, approximately 5'
x 5' x 1", in 15-20 weight percent sulfuric acid at 3.5 volts and SO amps was
complete in
approximately sixty (60) hours.
The anode was held in a cloth bag in which the anode residue was collected.
For 8 pounds
of cropper-iron metal the dry weight of the anode residue was about 30 grams.
The residue was
washed, dried, and further smelted by well known techniques to produce a final
Dore' metal, which
is essentially an alloy of gold and silver, containing the platinum group
metals.
These iron bearing ores were first smelted in a highly reducing atmosphere
with the addition
of cropper to the furnace charge to produce a mixture of copper and iron
metal. These metals act as
a solvent, or collector, for gold, silver, and the platinum group metals. This
action is similar to the
2 0 smelting of copper ores in which a molten matte (a mixture of iron and
copper sulfides) collects
gold, silver, and the platinum group metals, and to the smelting of lead ores
in which molten lead
collects these precious metals. The iron and copper mixture is separated from
the slag, then leached
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in a mineral acid, either with or without electrolysis to dissolve the iron
and the copper. The
remaining insoluble residue is filtered to separate it from the liquid. The
solid residue contains gold,
some silver, and the platinum group metals. Most of the silver is dissolved in
the acid. The residue
contains iron, silicon, and a number of other elements, mostly metals. The
residue can be treated by
well known sweating methods to produce a precious metal bullion containing
gold, silver, and the
platinum group metals.
There are two unique features in this invention. A first unique feature is the
use of copper
and iron in combination as a collector for gold, silver, and the platinum
group metals. The practice
of iron smelting is well known. Also, the ability of copper to collect the
precious metals is well
known. Furthermore, the use of electrolytic acid dissolution to dissolve the
copper, followed by the
recovery of these metals from the anode residue is conventionally used in
copper smelting and
refining, and is thus well known. However, the combination of iron smelting
with the addition of
copper, followed by electrolytic acid dissolution of a mixture of copper and
iron metal, and recovery
of these precious metals from the acid residue is a new way to recover these
metals.
The process of the invention will be more specifically illustrated in the
following example:
EXAMPLE
The following specific example is illustrative, but not a limitation of the
practice of the
invention. Four furnace charges, each consisting of 25 pounds of ore crushed
to minus one-eighth
inch, 16.7 pounds of borax, $.3 pounds of soda ash (sodium carbonate), one-
quarter pound of
2 0 fluorspar, and 1.25 pounds of copper were thoroughly mixed, heated to 1260
degrees centigrade and
held for about one-half hour. The natural carbonaceous conte~tt of the ore
acted as a reducing agent.
The slag and metal were poured into a mold and allowed to coo(. The cold slag
was broken away
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from the copper and iron metal. The total weight of metal from the four
furnace runs was 7.5
pounds. The metal was then cast into an anode and dissolved electrolytically
in a 15-20 weight
percent sulfuric acid solution at 3.S volts D.C., and SO amps for 60 hours
until the metal node
wa:~ completely dissolved. The anode was enclosed in a cloth bag, so that at
the end of
dissolution the insoluble residue was collected in the bag.
The copper from the anode was deposited on two cathodes, while most of the
iron went into
solution. The residue was washed onto a paper filter, filtered, and washed
with water.. The dried
residue weighed 30 grams which amounted to 0.88 percent of the weight of the
anode. The residue
was smelted with a standard fire assay flux containing lead oxide, borax, and
soda ash. To this was
added 15 grams of silver. The lead button, weighing 117 grams, and containing
gold, silver, and
the platinum group metals was recovered from the melt after pouring and
cooling. The lead button
was cupelled in the standard fire assaying manner at 800-850 degrees
centigrade. In cupellation
molten lead oxide is formed which absorbs into the cupel, a cup made of bone
ash. The absorption
continues until all of the lead oxide is gone leaving only a bead containing
gold, silver, and the
platinum group metals on the surface of the cupel. The weight of the precious
metal bead was 16.66
grams. The bead was parted in 25 weight percent nitric acid which dissolved
the silver and some of
the platinum group metals, leaving the gold and some of the platinum group
metals undissolved.
The; gold bearing metal was then wrapped in sheet lead and recupelled at 800-
850 degrees centigrade
to give a final bead weighing 1185 milligrams. The yellow color of the bead
suggested that it was
2 0 principally gold. This calculates to be 0.76 ounce per ton of ore. These
figures indicate that an
avordupois ton of ore would yield 0.76 ounce of gold and platinum group metals
by standard
methods of recovery from the anode residues.
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