Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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1V~ETAL RECOVERY FROM SKIMMED SLAG
The present invention relates to a process for recovery of metal from smelting
waste, and more particularly for recovery of metals from skimmed slag.
BACKGROUND CIF~THE INVENTION
The smelting; of metal sulphides for the recovery of metals has been practised
for many years and is the major process for producing valuable metals, such as
copper
and nickel. During smelting and converting a matte is formed, which contains
the
majority of copper and nickel. A slag is formed containing iron and other
elements,
which is immiscible with the matte and is of a lower specific gravity.
The slag floats on the surface of the matte, and is separated from the matte
in
furnaces and conven:ers by a process referred to as "skimming". Skimming
relies on
the differences in specific gravity between the matte which contains the
valuable
metals and the slag which contains mainly the waste products.
Once skimmed from the matte, slag is conventionally transferred into ladles or
pots for transportation to a waste disposal site. Unfortunately, during the
skimming
process, some of the matte is entrained with the slag, resulting in the loss
of valuable
metals when the skimmed slag is discarded.
Conventional solutions have attempted to recover the valuable metals from
skimmed slag. Such solutions have included the use of fuming processes
utilizing
oxygen, fuel and red.uctant in order to obtain metal enriched fiunes from slag
(e.g. US
4,820,340; US 5,588, 436; and US 4,252,563). In US 4,009,024 and US 3,891,428
there is disclosed the; recovery of metals from slag that comprises the
injection of fuel
and air into the molten slag to reduce the slag, followed by metal recovery.
Typical
metal recovery involves the use of settling units which allow the entrained
matte to
separate followed by a second slag skimming, or grinding and flotation of the
slag to
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recover the sulphide metals into a flotation concentrate. GB 1,435,371
discloses a
method for separating metal-containing slag using a wind tunnel, with
separation
arising as a result of differences in specific gravity. Alternatively,
attempts have been
made to reduce the amount of valuable metal entrained in the slag prior to
skimming,
for example by addition of reductants to the molten metal or slag which reduce
dissolved metal oxides, thereby increasing their specific gravity. This allow
the
reduced metals to selparate into the matte layer. Inconveniently, these
conventional
approaches may require large capital expenditures, specialized equipment, and
increased operating <;osts. The cost of milling and flotation of all of the
slag generated
from the smelting process may exceed the value of the metal recovered using
conventional recovery methodology.
There is a need within this art for a cost effective and simple method for the
recovery of metals from the slag produced during the steel making process. It
is an
object of the invention to provide a simple and efficient process for the
recovery of
copper, nickel, cobalt or other metals of interest from skimmed smelting slag.
SUMMARY OF THE INVENTION
The present invention relates to a process for recovery of metal from smelting
waste, and more particularly for recovery of metals from skimmed slag.
According to the invention, there is provided a process for recovery of metal
from skimmed slag, the process comprising the steps of: storing skimmed slag
in a
container for a period of time sufficient for at least a portion of the metal
to
precipitate or separate from the slag and freeze to the walls of the vessel,
removing
molten slag from the; container while leaving a slag skull within the
container,
cooling the slag skull, and recovering metal from the slag skull. This
invention also
comprises this method wherein the period of time sufficient for at least a
portion of
the metal to precipitate from the slag is from about 15 min. to about 12
hours.
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Furthermore, the above method is suitable for the recovery of metals selected
from
the group consisting of copper, nickel, cobalt, and combinations thereof.
Metal recovered from a slag skull would otherwise have been discarded as a
waste product of the: smelting process. Advantageously, the process allows for
greater recovery of :metal and reduces the waste as a result of smelting.
Furthermore, the step of storing the skimmed slag, as defined in the above
method,
may take place during transport of the slag to a slag disposal site.
Additionally, the invention provides a process for recovery of metal from a
slag skull comprising; breaking a slag skull into pieces, grinding the pieces
of slag
skull into fragments, and separating the fragments to obtain fragments
concentrated
in metal. The separated fragments may then be further processed by smelting.
Conventional. approaches to the recovery of metals from slag may require
large capital expenditures, specialized equipment, and increased operating
costs. The
present invention is directed to providing a cost effective and efficient
method for the
recovery of metals from metal-enriched skulls, and the recovery of the metal
from
these skulls.
DESCRIPTION OIL PREFERRED EMBODIMENTS
The present invention relates to a process for recovery of metals from
skimmed smelting slag. The metals recovered according to the invention include
any
metal typically found in the matte portion upon smelting. The metals recovered
generally have a greater specific gravity than slag, and thus settle out of
molten slag.
Examples of metals recovered from slag include, but are not limited to copper,
nickel
and cobalt.
A layer of srr~elting slag is skimmed off of the matte. During the skimming
process, a small amount of the matte typically becomes entrained in the slag.
The
matte content of skimmed slag is typically low between 0.25 and 0.35% by
weight,
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but it can be much higher under upset or abnormal operation, up to, for
example,
about 20% by weigr~t.
Once skimmed, molten slag and any matte content entrained therein is
deposited into a container, such as a ladle or a pot and may be transported to
a slag
disposal site. Due to the higher specific gravity of matte relative to slag,
the matte
entrained in the slag precipitates to the bottom of the container while the
slag is in
liquid form. As a result, a layer comprised of slag enriched with matte forms
in the
bottom of the container.
Any metals entrained within the slag are allowed to settle for a time
sufficient
to permit at least a portion of the metal to separate from the slag and
freeze. For
example, this period of time may comprises from about 15 minutes up to about
12
hours or more, depending on the size of the pot, and the slag composition.
Longer
periods of time may be employed for larger pots, where slag may remain in a
molten
state for a longer period. The slag cools in such a way that the molten liquid
first
solidifies near the base and wall of the pot while the inner content of the
container
remains in molten state due to heat retention. The metals may settle our of
the slag en
route to a slag disposal site, or while remaining stationary for a period of
time. In this
way, the bottom layer within the pot, which comprises slag enriched with matte
and
metals, solidifies ag~unst the bottom of the container.
Once the desired amount of solidification of the slag has been achieved,
either
by design or through the time required to transport the slag to a dump site,
the
remaining molten slag is removed from the container. This portion of slag may
be
disposed of as waste or may be poured into a secondary container, to allow
further
settling of metals contained therein. The solidified layer containing matte-
enriched
slag remains encrusted to the bottom of the container. The solidified layers
of
material that encrust the pot are called "skulls".
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The emptied container, having the layer formed along the bottom thereof, may
then be re-filled with freshly skimmed molten slag and the cycle repeated.
Upon
repetition of the above-described process, layers of combined matte-enriched
slag
build up along the swface of the container, gradually decreasing the available
volume
of the container. T'he skull is removed from the container either when the
available
volume of the contaainer is inadequate for further waste disposal of molten
slag, or
when the size of the skull is optimum for metal recovery as determined by
testwork.
Factors affecting this optimum include, container geometry, waiting time,
travel time
to disposal facilities, outside temperatures and operating temperature of the
smelting
unit.
The degree crf recovery of metals may depend on the degree of oxidation of the
metals in the slag (see for example US 4,588,436 and US 4,009,024). For
example,
the higher the degree of oxidation, the less recovery of the metal of
interest. Thus,
metals in the molten slag may be reduced from the oxidized form prior to
settling in
the pot, thereby increasing recovery of the metals.
To remove the solidified layers, the container is allowed to cool to a
partially
frozen state, allowing the combined layers to solidify. The skull is removed
from the
container and cooled to a frozen state, preferably to ambient temperature. The
skull
contains layers of rr~atte-enriched slag, and is thus more concentrated, than
is slag, in
valuable metals.
The skull m.ay represent up to about 2 to about 5% of the total mass of the
slag
produced, depending on operating conditions, but the skull comprises about 3
to 6
times more valuable metal than slag. The skull contains a high concentration
of the
metal of interest, thus avoiding the need to process all of the slag
generated.
After cooling, the skull is broken up into pieces, for example, using a
pneumatic or hydraulic hammer into fragments that can be manually sorted,
however,
the use of machine-assisted sorting may also take place. Skull pieces
containing high
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concentrations of the metal of interest are then selected by any suitable
process. Such
processes include, but are not limited to, visual inspection, the use of metal
detectors,
specific gravity, or weight. The pieces of skull identified and sorted
according to the
concentration of the metal of interest contained therein, may be re-processed
by
smelting and further refining.
According to an embodiment of the invention, once the skull has been broken
up, those pieces containing a layer rich in the metal to be recovered may then
be
identified and selectively separated from the pieces of broken skull. The
metal of
interest may then be recovered by milling the pieces of skull into smaller
fragments,
followed with further grinding to achieve appropriate sized fragments
containing the
metal of interest for further processing, for example, by flotation separation
or gravity
separation using a conventional shaking table.
The ground fragments may be returned to smelter or may be further separated
on the basis of specific gravity, using any acceptable method of separation
such as
flotation separation. Metals of interest are generally of a greater specific
gravity than
slag, and will thus settle out from the remaining slag content of the skull.
By using the process as described herein, a significant portion of the metal
present in slag, and up to about 80% of the metal present within the skull may
be
obtained. The concentration of the recovered metals within the product have
been up
to 45% Cu, Ni, and Co are typically of about 15 to about 25%. The weight ratio
of
materials processed to product is variable but is typically from about 20:1 to
about
30:1
Examples arc: provided below which describe particular embodiments of the
invention, but are not to be construed as limiting.
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Example 1
Slag is skimmed from a furnace at about 1100 degrees Celsius into steel ladles
or pots
which hold between 50 and 100 tons. These are transported to a slag dump and
during the transfer (betwf:en l and 2 hours) heat is lost through the walls of
the vessel
and the slag begins to freeze. During this time any entrained matte which
contains the
valuable metals, settles to the bottom of the vessel and preferentially
freezes. The slag
is poured off at the dump site leaving behind in the vessel a skull, that is,
a skin of
slag enriched with matte. The quantity of solidified material varies with many
factors
as discussed earlier. In a typical operation the skull can represent between
about 1 and
3 tons per cycle. After a number of cycles the skull is removed from the
vessel,
allowed to cool, broken into pieces and the high grade metal bearing layers
recovered.
Recovery includes father feeding directly to a smelter or further
concentration by
grinding and flotation or upgraded using conventional gravity separation
techniques
such as a shaking table. Recovery of the valuable metals from the entrained
matte that
is skimmed with the' slag carp be up to 80% with this process.
Various modifications rnay be made without departing from the invention. It is
understood that the invention has been disclosed herein in connection with
certain
examples and embodiments. However, such changes, modifications or equivalents
as can be used by those skilled in the art are intended to be included.
Accordingly,
the disclosure is to be construed as exemplary, rather than limiting, and such
changes within'~he principles of the invention as are obvious to one skilled
in the art
are intended to be included within the scope of the claims.
