Abstrict
Sand, bentonite clay and organics recovered as foundry waste from
a green sand mold foundry are reclaimed for reuse in making new
green sand molds and mold cores by a multi-step process involving
both hydraulic and mechanical separation steps.
Claims
We claim:
1. A process for recovering sand, bentonite clay and organic additives
from the foundry waste produced by a green sand foundry, the foundry
waste being formed from bag house dust and molding waste, the process
comprising: forming an aqueous slurry of the bag house dust, hydraulically
separating the slurry in a first hydraulic separation step into
an overflow stream comprising at least 60% of bentonite clay originally
in the bag house dust and an underflow stream comprising at least
60% of the sand in the bag house dust; hydraulically separating
the overflow stream in a second hydraulic separation step to produce
an effluent stream comprising water and less than about 5% sand
in the bag house dust; and reusing the sand in the underflow stream
and the bentonite clay and organic additives in the effluent stream
for making green sand molds.
2. A process for reusing sand, bentonite clay and organic additives
used in a green sand foundry in the manufacture of green sand molds
and mold cores, the foundry also producing molding waste formed
from sand coated with bond, the process comprising: mechanically
removing bond from the sand particles to produce a lighter fraction
and a heavier fraction, combining the lighter fraction with water
to produce a slurry, hydraulically separating the slurry in a first
hydraulic separation step into an aqueous overflow stream comprising
at least 60% of the bentonite clay orginally in the lighter fraction
and an underflow stream comprising at least 40% of the sand in the
lighter fraction, hydraulically separating the aqueous overflow
stream in a second hydraulic separation step to produce an effluent
stream comprising a maximum of about 5% sand and at least 60% of
the bentonite clay orginally in the lighter fraction, reusing the
sand in the underflow stream and the bentonite clay in the effluent
stream to make green sand molds, and reusing the heavier fraction
to make mold cores.
3. The process of claim 2, wherein the heavier fraction contains
about 30% to 90% of the sand in the molding waste.
4. The process of claim 2, wherein sand in the heavier fraction
has an AFS clay of less than about 0.5.
5. A process for reusing sand, bentonite clay and organic additives
used in a green sand foundry in the manufacture of green sand molds
and mold cores, the foundry also producing molding waste formed
from sand coated with bond and bag house dust containing sand and
bentonite clay, the process comprising: mechanically removing bond
from the sand particles of the molding waste to produce a lighter
fraction and a heavier fraction, combining the lighter fraction
and the bag house dust with water to produce a slurry, hydraulically
separating the slurry in a first hydraulic separation step into
an aqueous overflow stream comprising at least 60% of the bentonite
clay orginally in the slurry and an underflow stream comprising
at least 40% of the sand in the slurry, hydraulically separating
the aqueous overflow stream in a second hydraulic separation step
to produce an effluent stream comprising a maximum of about 5% sand
and at least about 60% of the bentonite clay originally contained
in the slurry, reusing the sand in the underflow stream and the
bentonite clay in the effluent stream to make green sand molds,
and reusing the heavier fraction to make mold cores.
6. The process of claim 5, wherein the sand in the underflow steam
is a coarse sand product characterized in that at least 80% of the
sand in the coarse sand product has a particle size of at least
about 60 microns.
7. The process of claim 6, wherein the sand in the overflow steam
is a fine sand product characterized in that at least 80% of the
sand in the fine sand product has a particle size of less than about
20 microns.
Description FIELD OF THE INVENTION
The present invention relates generally to the field of sand cast
molding. More specifically, the invention relates to a process and
apparatus for recovering molding media in a foundry, and the process
for using the recovered molding media in the foundry.
BACKGROUND OF THE INVENTION
Green sand casting is a well-known process for forming cast metal
articles. In this process, a casting mold for making castings, formed
from molding media that is primarily sand and bentonite clay, is
used in only one molding cycle for the production of one or multiple
castings. Once the casting solidifies in the mold, the mold is broken
down and the casting cycle is complete. A portion of the molding
media can be recycled for another casting process, however, much
of the molding media exits the foundry as foundry waste. In the
U.S. alone, foundry waste accumulates at a rate of approximately
6 to 10 million cubic yards per year. The large volume of foundry
waste coupled with the increasing cost of landfill acreage and transportation
is problematic.
In Green Sand Foundries a casting mold is made using a "green
sand mold" that defines the external body of the casting and
a "core" that is placed inside the green sand mold to
define the internal configuration of the casting. FIG. 1 is a process
flow diagram illustrating the well-known manner in which molding
media is used to form green sand molds and cores used in a casting
cycle within a green sand foundry. Prime (i.e. new) silica sand
of input stream 1 and the chemical binder of input stream 3 are
used to produce cores in core-forming step A. The core, which must
withstand high pressure during formation of the casting, is made
by coating the particles of sand with any one of a number of chemical
binders, such as for example a two-part urethane system, and which
are well known in the art. The sand/chemical binder mixture is pre-formed
according to the internal configuration of the casting to be made
and the chemical binder is then reacted to complete a high-tensile
core. Prime silica sand 2, bentonite clay 4 and organic additives
5 are used to produce green sand molds at mold-forming step B. The
green sand mold is made by press forming sand that is coated by
a mixture of bentonite and organic additives, generally known as
"bond." The addition of water of input stream 6 hydrates
the bond and causes the grains of sand to adhere to one another
and take shape. The green sand molds typically comprise by weight,
from about 86% to 90% sand, 8% to 10% bentonite clay, 2% to 4% organic
additives and 2% to 4% moisture.
After the core and green sand mold are formed the core is inserted
into the green sand mold and molten metal is poured into the green
sand mold to produce a casting at casting step C. After the molten
metal solidifies, the casting undergoes "shakeout" at
shakeout step D to break apart the green sand mold and the core
into small particles or clumps. During shake out the particles of
the core flow out of the solidified casting and become commingled
with the particles from the green sand mold. A portion of the materials
that once made up the green sand molds and core, represented by
output stream 7, are recycled to make green sand molds at mold-forming
step B for a subsequent casting cycle, and an excess portion of
the materials that once made up the green sand molds and core, represented
by output stream 8, exits the process as "molding waste."
The addition of prime sand 2 at mold-forming step B compensates
for the "fine" sand that is taken out of the process after
each casting cycle. Prime bentonite clay 4 and prime organic additives
5 compensate for the additional bond needed to coat the uncoated
prime sand and also the uncoated sand that once made up the cores.
The addition of prime bentonite clay and organic additives also
compensates for molding media loss due to high temperature exposure.
The excess molding media, that is, foundry waste which cannot be
reused for subsequent casting cycles, is generated at several locations
within the foundry. The composition and particle size distribution
of foundry waste can vary depending upon the areas of the foundry
in which it is collected, but foundry waste can be generally classified
in two broad categories, namely, "molding waste" and "bag
house dust". The term "molding waste" refers to the
excess molding media from broken down green sand molds and cores,
output stream 8, produced during shakeout. Another source of foundry
waste, represented by stream 9, is generated by defective cores
that never get used in the casting operation. Molding waste can
include materials present in both output streams 8 and 9, as well
as molding media which fall from the conveyor system at various
stages throughout the foundry. In many green sand foundries, the
molding waste typically contains by weight from about 80% to about
90% sand, from about 6% to about 10% bentonite clay and from about
1% to about 4% organic additives. Molding waste includes sand that
is coated with bond as well as individual particles of sand, bentonite
and organic additives.
Attempts have been made to reduce the accumulation of molding waste
by mechanically removing the bond from the sand so that the sand
is sufficiently clean to be reused in the production of cores. In
such processes the sand is recovered, but the bentonite clay, which
costs several times more than sand on a weight basis, and the organic
additives are discarded. Another disadvantage of mechanical reclamation
is that the cost of prime sand is sufficiently low in many geographic
areas that the capital investment for sand recovery is economically
unfeasible.
Another large source of foundry waste, stream 10, includes fine
particles of sand, bentonite clay, organic additives and debris
collected in the foundry's air evacuation system. Foundry waste
10 is commonly known in foundries as "bag house dust".
Bag house dust contains substantially more bentonite clay than does
molding waste. Bag house dust typically comprises from about 40%
to about 70% sand, from about 20% to about 50% bentonite clay and
from about 10% to about 30% organic additives.
In some cases, certain foundries have been able to recover bentonite
clay by introducing the bag house dust back into the water system
that is used for making green sand molds in the casting process.
In this manner, the bag house dust is mixed into the water system
treated according to the advanced oxidation process (AO technology)
and is placed into a settling tank. See, Advanced Oxidants Offer
Opportunities to Improve Mold Properties, Emissions; Modern Casting,
September, 2000, p. 40-43. Upon settling, water containing bentonite
clay is pulled from the top of the settling tank and reused in the
green sand molding lines. A disadvantage, however, is that the sludge
which settles out of the settling tank and is discarded contains
most of the sand in the bag house dust.
Accordingly, there is a need to reduce the amount of foundry waste
exiting a green sand foundry. There is also a need for a process
to recover sand that has sufficient quality to be used in the foundry
to make cores and green sand molds and which can yield quality castings
in a subsequent casting process. There is also a need for a process
to recover sand, bentonite clay and organic additives to decrease
the amount of prime materials that enter the foundry as raw material.
SUMMARY OF THE INVENTION
These and other needs are addressed by the present invention which
is based on the recognition that much of the sand and bentonite
clay contained in foundry waste derived from a typical green sand
foundry can be recovered for reuse in making new green molds by
a two-step hydraulic separation procedure which first recovers coarse
sand suitable for reuse in making new green sand molds from the
waste and thereafter separates out fine sand unsuitable for use
in making new green molds from the remainder of the waste to produce
an aqueous byproduct bentonite clay stream that can also be used
in making new green molds.
Thus in one embodiment of the invention, bag house dust, after
slurrying in water, is hydraulically separated to produce an underflow
output stream containing at least about 40% of the sand originally
contained in the bag house dust as well as an aqueous overflow stream
containing at least about 60% of the bentonite clay in the bag house
dust. In accordance with the present invention, it has been found
that the relatively coarse sand contained in the underflow has a
particle size distribution allowing it to be directly used for making
new green sand molds for a subsequent casting cycle. Accordingly,
this coarse sand product is recycled to the green mold preparation
station, after optional removal of water, for reuse in making additional
green sand molds. The aqueous overflow stream produced as a byproduct
of the first hydraulic separation step, if desired, can be subjected
to a second hydraulic separation step to remove most of its sand
content. This sand is too fine to be useful in making additional
green sand molds and is therefore discarded. However, the effluent
output stream produced as a result of this second separation step,
which contains at least about 50% of the bentonite clay originally
found in the bag house dust but very little sand, can also be directly
used for making new green sand molds and accordingly is also recycled
to the green sand molding station for this purpose.
In another embodiment of the invention, the molding waste produced
during operation of a typical green sand foundry is processed in
essentially the same way as described above. However, in this instance
the molding waste is first mechanically separated to produce a lighter
and a heavier fraction. The lighter fraction contains most of the
bentonite clay and organic components in the mold waste and therefore
can be processed in the same way as described above, by itself or
together with the bag house dust produced by the foundry, to recover
its useful sand and bentonite clay values for making still additional
green sand molds. The heavier fraction produced by mechanical separation
is composed predominantly of sand. In accordance with still another
feature of the invention, this reclaimed sand product can be made
to exhibit a particle size and particle size distribution approximating
that of prime sand by carrying out the mechanical separation process
in an appropriate manner. Therefore, this heavier sand fraction,
when appropriately made in accordance with the present invention,
can replace at least some of the prime sand used in making new mold
cores, thereby significantly reducing the foundry's total demand
for prime sand in its overall green sand molding process.
DESCRIPTION OF THE DRAWINGS
The present invention may be more readily understood by reference
to the following drawings wherein:
FIG. 1 is a schematic process flow diagram illustrating how the
molding media used to form green sand molds and associated mold
cores are received, used and discharged in a typical green sand
foundry; and
FIG. 2 is a schematic process flow diagram illustrating the present
invention; and
FIG. 3(a) is a photomicrograph of typical sample of prime silica
sand used to make mold cores in a green sand foundry; and
FIG. 3(b) is a photomicrograph of a reclaimed sand product produced
according to the invention herein.
DETAILED DESCRIPTION OF THE INVENTION
In accordance with one embodiment of the invention, sand, bentonite
clay and organic additives are recovered from the bag house dust
produced by a typical green sand foundry and reused to make additional
green sand molds. Silica sand is commonly used and green sand can
also include, for example, silica sand, lake sand (silica and calcium,
shell, etc.), chromite sand, zircon sand, olivine sand, nickel slag,
and carbon sand. Also, different types of bentonite clay are used
and can include calcium bentonite, sodium bentonite and sodium-activated
bentonite, for example. Organic additives used in green sand foundries,
include but are not limited to, cellulose, cereals, starch, causticized
lignites, sea coal, gilsonite, and anthracite, for example.
This process to recover sand, bentonite clay and organic additives
in a green sand foundry is illustrated in FIG. 2, which shows bag
house dust 10 and water 22 being fed into a slurry tank and mixed
at slurry step E to produce slurry 24. Although any amount of water
can be added in slurry step E, normally the amount of water added
will be at least about 10 times the amount of bag house dust on
a weight basis. More typically, the amount of water added will be
enough so that the weight ratio of water to bag house dust is between
about 12:1 and 40:1, more preferably between about 15:1 and 30:1.
Slurry 24 is then transferred to separation step F where it is
hydraulically separated to recover the coarser, heavier sand particles
therein for reuse in making additional green molds. By "hydraulically
separated" is meant that the slurry is subjected to a force
such as gravity or centrifugal force so that the heavier, coarser
particles separate out from the other components of the slurry--i.e.,
the water and lighter, finer particles.
Various methods and equipment can be utilized to hydraulically
separate particles of different sizes and densities from one another.
For example, fluid handling equipment which imparts centrifugal
force on the slurry to move the larger or denser particles apart
from the smaller, lighter particles can be used. Examples of such
fluid handling equipment include hydroclones and centrifuges. A
hydroclone has a stationary, vertical cylinder with a conical bottom
that imparts centrifugal force on slurry which enters at an inlet
near the top. The incoming slurry receives a rotating motion on
entrance to the cylinder, and the vortex so formed develops centrifugal
force which forces the heavier sand particles radially toward the
wall of the hydroclone and separates them from the fluid containing
the fine particles. The centrifugal force imparted on the slurry
increases the settling rate of the coarser sand and causes the sand
to settle to the bottom well ahead of the finer particles. An underflow
stream containing the coarser sand particles exits out the bottom
of the hydroclone, while an overflow stream containing the particles
not having separated out exits through an outlet located above the
outlet for the underflow. A commercially-available example of such
a unit is Hydroclone Unit 212 available from Swaco Inc. of Houston,
Tex.
Separation step F is carried out in accordance with the present
invention so that at least about 40% of the sand in slurry 24 is
recovered in underflow output stream 28, while at least about 60%
of the bentonite clay in slurry 24 is recovered in overflow stream
26. In accordance with the present invention it has been found that,
when operating in this manner, at least about 80% of the coarse
sand product recovered in underflow output stream 28 will normally
have a particle size of at least about 60 microns. This particle
size is appropriate for making new sand molds, and so underflow
output stream 28 can be recycled directly to mold-forming step B
for reuse of the sand therein in making additional green sand molds
by the foundry, if desired.
In the particular embodiment shown, underflow output stream 28
is de-watered at de-watering step H to remove most of the water
from the recovered coarse sand therein. Solids fraction output stream
34, which contains substantially all of the sand in underflow output
stream 28 and no more than about 10 wt. % water, more typically
no more than about 2 wt. % water, can be recycled directly or indirectly
to mold-forming step B for manufacture of additional green molds.
Alternatively, the sand of output stream 34 can be dried and used
as an additive for core-forming step A or another application inside
or outside the foundry.
Separation step H also produces liquid fraction 36, which normally
contains about 1 to 3 wt. % of the bentonite clay and about 8 to
15 wt. % of the organic additives in slurry stream 24. This stream
can also be directly recycled back to mold-forming step B.
Many different types of commercially available equipment can be
used for carrying out separation step H. Examples are desilter units,
mud cleaners, and shaker decks. A particular example of one such
commercially available pieces of equipment is Desiltering Unit Model
No. 202 available as from the Swaco Corporation of Houston, Tex.
As indicated above, separation step F is carried out so that at
least about 40% of the sand in slurry 24 is recovered in underflow
output stream 28, while at least about 60% of the bentonite clay
in slurry 24 is recovered in overflow stream 26. When operating
in this manner, about 60% or more of the organics originally contained
in slurry 24 will also be recovered in overflow stream 26. Preferably,
separation step F is operated so that about 50 to 80% of the sand
in slurry 24 is recovered in an underflow output stream 28, while
about 70 to 95% of the bentonite clay and 70 to 90% of the organics
originally contained in this slurry are recovered in overflow stream
26. In some instances, separation step F is operated so that about
60 to 80% of the sand in slurry 24 is recovered in an underflow
output stream 28, while about 80 to 95% of the bentonite clay and
75 to 85% of the organics originally contained in this slurry are
recovered in overflow stream 26.
As well appreciated by those skilled in the art, the degree of
separation achieved when operating commercially available hydraulic
separation equipment depends on the various operating variables
of the equipment used, including the degree of centrifugal or other
force exerted on the slurry, the flow rate at which the slurry is
introduced into the equipment, residence time and so forth. The
effects of these processing variables can easily be determined through
routine experimentation to achieve the degree of separation desired,
as indicated above.
Depending on the composition of bag house dust 10 as well as the
way first hydraulic separation step F is operated, aqueous overflow
stream 26, which is also produced in separation step F, may contain
a significant amount of sand having a particle size of about 20
microns or less. Since this particle size is too fine to be of interest
in making additional green sand molds, overflow stream 26 is processed
to remove this sand content as well as other debris that may be
present in this stream. This is shown in FIG. 2 as second hydraulic
separation step G.
In accordance with the present invention, second separation step
G is accomplished to remove substantially all of the sand in aqueous
overflow stream 26 and thereby produce effluent output stream 30
comprising a maximum of about 5%, preferably about 3%, and even
more preferably, about 1% of the sand originally contained in the
overflow stream 26. Effluent output steam 30 also contains much
of the bentonite clay and organic additives originally in overflow
stream 26, and it has been found in accordance with the present
invention that a significant amount of this retained bentonite clay
is "active" in the sense that it will exhibit some active
binding properties when dehydrated then rehydrated. Accordingly,
this recovered bentonite clay can be used as a source of active
bentonite for making additional green molds by recycling effluent
output stream 30 directly or indirectly to mold-forming step B,
rather than discharging this stream to waste.
As in step F, separation step G may be accomplished using well-known
hydraulic, gravitational or centrifugal separation units, such as
a hydroclone or a centrifuge, for example, for imparting a gravitational
and/or centrifugal force on aqueous overflow stream 26 to increase
the differential settling rates of the heavier, larger particles
from the lighter, finer particles to physically move the particles
apart so they can be withdrawn separately. It has been found that
substantially all of the fine sand particles can be removed from
the effluent which maintains most of the bentonite clay.
As previously indicated, the sand particles in overflow stream
26 are too fine to be of interest for making additional green sand
molds. For example, 80% or more of the sand in solids discharge
stream 32 normally has a particle size of about 20 microns or less.
Accordingly, solids discharge stream 32 is normally discharged to
waste. Surprisingly, it has also been found that these sand particles,
together with the organic materials and other debris that might
be present, coalesce in the form of colloidal agglomerates, probably
because of the residual bentonite clay present. It is believed that
the encapsulation of sand and organic materials by the bentonite,
reduces environmental hazards associated with disposing of this
material.
In summary, the inventive process as described above recovers about
40% or more of the sand, about 60 wt. % or more of the bentonite
clay and about 20 wt. % or more of the organic additives originally
contained in the foundry's bag house dust. Previous known methods
do not recover these materials at all, or if they do recover these
materials, they only recover some of them under limited conditions
incidental to the operation of advanced oxidation technology. AO
technology is not necessary in accordance with the present invention,
although it can also be used, if desired. In any event, the recovered
materials produced in accordance with the present invention can
be recycled in the foundry to make additional green sand molds,
thereby substantially reducing the amount of prime (make-up) sand,
bentonite clay and organics that must be added to keep the foundry
running and also substantially reducing the amount of waste produced.
In another embodiment of the present invention, the above separation
technique is used to recover sand, bentonite clay and organics from
the molding waste also produced by green sand foundries. This aspect
of the present invention is also illustrated in FIG. 2.
Molding waste 8 derived from shake out step D and/or molding waste
9 derived from core-forming step A (and/or molding waste formed
from unused or defective green sand molds from mold-forming step
B) initially undergoes drying, screening and demagnetizing at preparation
step I to produce dry molding waste product 52. The molding waste
may also be subjected to a preliminary crushing step, before or
after drying, if necessary.
Dry molding waste product 52 should have a moisture content of
10 wt. % or less, preferably 4 wt. % or less, 2 wt. % or less, or
even 0.5 wt. %. In addition, it should have a particle size such
that no more than 20 wt. % has a particle size exceeding 8 mesh
and preferably 10 mesh. Molding waste product 52 is also desirably
free substantially of iron and other metallic components capable
of magnetic separation, as such materials constitute contaminating
waste. Equipment for drying, screening and demagnetizing foundry
waste as accomplished in preparation step I is commercially available.
Also, molding waste 8/9 need not be dried, screened and demagnetized
as described above, if desired, as the techniques and advantages
of the invention will be realized whether or not such pretreatment
is done. However, the processing steps described below will work
more efficiently to produce better quality reclaimed materials if
the molding waste is dried, screened and demagnetized in this manner.
According to the second embodiment of the present invention, molding
waste product 52 is subjected to mechanical separation in separation
step J. By "mechanical separation" it is meant a separation
process in which the molding waste is subjected to significant mechanical
impact or abrasion to physically break apart agglomerates containing
multiple sand particles and/or to separate from these sand particles,
at least partially, the bentonite clay, carbonaceous additives and
other chemical binders that may be present on the surfaces of these
particles.
Numerous different types of commercially available equipment can
be used for carrying out mechanical separation step J of the present
invention. In some, the material to be processed is propelled against
a solid object, such as by the action of a jet of air or other gas.
In others, the material is ground upon itself. A mechanical separation
unit that causes molding waste to be blown via a gas and impinged
onto a stationary plate is the EvenFlo Pneumatic Reclaimer unit
available from Simpson Technologies of Aurora, Ill. A mechanical
separation unit that abrades particles of molding waste against
one another is Model NRR32S unit available from Sand Mold Systems,
Inc. of Newaygo, Mich. As well appreciated by those skilled in the
art, the extent of separation achieved by these machines depends
upon a variety of operating factions including retention time, velocity
of the particles, number of iterations in which the particles of
waste are processed, and so forth.
Mechanical separation process step J yields a lighter fraction
(residual stream 56 in FIG. 2) composed of sand, bentonite clay
and organic additives and a heavier fraction (output stream 58 in
FIG. 2) composed primarily of coarse sand. In prior art methods
of recovering sand from molding waste, the residual sand, bentonite
clay and organic additives are discarded. In accordance with the
present invention, however, it has been found, however, that residual
output stream 56 can be processed in the same way as discussed above
in connection with bag house dust 10 to also recover the sand, bentonite
clay and organic additives in this residual stream for making still
additional green sand molds.
In accordance with this aspect of the present invention, therefore,
residual output stream 56 is transferred to slurry step E where
it is made into a slurry and then subjected to first hydraulic separation
step F and second hydraulic separation step G to produce aqueous
overflow stream 26, underflow output stream 28, effluent output
stream 30, solids discharge stream 32, solids fraction output stream
34, and liquid fraction 36, in the same way as described above.
As in the case of processing bag house dust, it has been found in
accordance with this aspect of the present invention that it is
also possible to recover about 40% or more of the sand, about 60
wt. % or more of the bentonite clay and about 20 wt. % or more of
the organic additives originally contained in residual output stream
56 by carrying out the first and second hydraulic separation steps
in the manner described.
In an especially preferred embodiment of the invention, as illustrated
in FIG. 2, both residual output stream 56 as well as bag house dust
10 are formed into slurry 24 for further processing. By this approach,
both sources of foundry waste--bag house dust and molding waste--can
be processed simultaneously to recover the sand, bentonite clay
and organics therein for making additional green sand molds. Accordingly,
the amount of make-up sand, clay and organics need to operate the
foundry, and the overall waste produced by the foundry, can be reduced
even more.
In addition to residual output stream 56, mechanical separation
process step J also yields output stream 54 composed primarily of
coarser sand. Normally, this coarser sand product will be composed
of about 30% to 90%, preferably about 50% to 85%, and even more
preferably about 75% to 85% of the sand in molding waste 8/9. In
accordance with the present invention, it has been further found
that this coarse sand product can be made to approach prime silica
sand in terms of composition and particle size distribution by carrying
out mechanical separation process step J in an appropriate manner.
Therefore, in accordance with a particularly preferred embodiment
of the invention, the coarse sand product in output stream 54, after
washing and drying at finishing step K, is recovered for reuse in
making additional new mold cores by recycling this product directly
or indirectly to core-forming step A.
Two factors help determine if the reclaimed sand product in output
stream 54 can be used as a replacement for prime (new) silica sand
in making new mold cores. The first is the amount of residual bentonite
clay and organic additives remaining on the surface of sand particles
of this product and the second is the particle size of this product.
The bentonite clay and organic additives remaining on the surface
of sand particles recovered from separation step J may interfere
with the new chemical binder added to these recovered sand particles
in the manufacture of new cores. This, in turn, may detrimentally
affect the strength of the new cores and ultimately the quality
of the castings made from these cores. Accordingly, separation step
J should be accomplished to remove enough of the clay and organics
originally on the sand in output stream 54 so that the bond strength
of new cores made with this reclaimed sand will not be adversely
affected to any significant degree.
One way to determine if enough of the clay and organics have been
removed in mechanical separation step J is to determine the "AFS
clay measurement" of the recovered sand according to AFS Procedure
No. 110-87-S. As well known to those skilled in the art, this test
method is a standard of the American Foundry Society which measures
the amount of fine particulate matter, including material other
than clay, on the surfaces of sand grains. The AFS clay of prime
sand entering green sand foundries typically has an AFS clay of
about 0.3. In accordance with the present invention, the reclaimed
sand recovered from separation step F desirably has an AFS clay
value that is less than about 0.5, preferably, less than about 0.4,
and even more preferably, less than about 0.3.
Another method for determining if enough clay and organics have
been removed in separation step J is to test the bond strength of
a test core made from the reclaimed sand. In other words, a test
core containing all of the ingredients intended to be used to make
product cores, including the reclaimed sand to be tested, can be
tested to determine its tensile strength by AFS Procedure N. 317-87-S,
for example. If the tensile strength of the test core exceeds the
minimum acceptable tensile strength suitable for withstanding the
pressure to be encountered in the planned casting process, then
it follows that sufficient clay and organics were removed in separation
step J.
In an alternative to this approach, the test core can be made using
reclaimed sand only. In other words, no prime sand is used to make
the test core, only reclaimed sand. Achieving an acceptable tensile
strength in this instance indicates that the reclaimed sand recovered
from separation step J will not reduce bond strengths below an acceptable
level, even if no prime sand is used to make product cores. This,
in turn, suggests that product cores made with significant amounts
of prime sand, in addition to reclaimed sand of the present invention,
should be even stronger than minimum acceptable levels.
It is also desirable that the reclaimed sand in output stream 58
have a particle size distribution that is similar to the particle
size distribution of the prime sand that it will be used to replace.
Sand particles can break down if too much contact force is used
in separation step J, which in turn can lead to a reclaimed sand
product containing too many fine sand particles to be useful. Therefore,
care should be taken during separation step J to avoid contacting
conditions so severe that the reclaimed sand product in output stream
58 contains more than about 3 wt. % fines defined as the sum of
the weight retained on the 200 and 270 screens and pans.
As will be understood by those skilled in the art, neither of the
above factors (particle size and surface residuals) is an absolute
requirement for allowing the reclaimed sand recovered in output
stream 58 to be used as a replacement for prime sand in core forming
step A, at least to some degree. Rather, these factors are guides
which will help determine how mechanical separation step J should
be accomplished in particular instances.
In other words, even if the particle size and surface residuals
of the reclaimed sand do not meet the above standards, it still
may be possible to use this reclaimed sand as a substitute for at
least some of the prime sand in making new mold cores. On the other
hand, the more the reclaimed sand resembles prime sand in terms
of both surface residuals and particle size, the more likely it
is that greater amounts of this product can be used as a prime sand
replacement without adverse impact on the mold cores produced. Therefore,
in carrying out specific instances of the inventive process, surface
residuals and particle size can be used as handy guideposts to help
determine exactly how mechanical separation should be carried out.
In order to more fully and clearly describe the present invention
so that those skilled in the art may better understand how to practice
the present invention, the following examples are given. The following
examples are intended to illustrate the invention and should not
be construed as limiting the invention disclosed and claimed herein
in any manner.
EXAMPLE 1
1600 pounds of bag house dust obtained from a green sand foundry
producing approximately 350 molds per hour was processed using the
hydraulic separation scheme illustrated in FIG. 1. The bag house
dust, which contained 864 pounds of sand, 448 pounds of bentonite
clay and 288 pounds of organic additives, was mixed with 20,164
pounds of water to make a slurry (Slurry 24). The slurry was then
fed into a hydroclone, model unit 212 from Swaco, to separate the
sand from the bentonite and organic additives in a first hydraulic
separation step (Step F). An overflow stream (26) and an underflow
stream (28) were produced. The underflow stream contained 518 pounds
of sand (60% of the sand present in the bag house dust), 13 pounds
of bentonite clay (3%), 53 pounds of organic additives (18%), and
4757 pounds of water. 80% of the sand product in the underflow stream
had a particle size larger than 60 microns, indicating that this
sand product could be reused to make additional green sand molds.
The overflow stream contained 435 pounds of bentonite clay (97%
of bentonite clay present in the bag house dust), 235 pounds of
organic fillers (82%), 346 pounds of sand (40%) and 15,403 pounds
of water. This overflow stream was then put through a centrifuge
to further separate (Step G) the sand fines and debris from the
bentonite and organic additives. Separation in the centrifuge produced
an effluent stream which contained 348 pounds of bentonite clay
(78% present in the bag house dust), 105 pounds of organic fillers
(36%) and 15,100 pounds of water. The effluent stream also contained
less than 1% sand, indicating it could be reused as make up water
in forming new green molds. All of the bentonite in the bentonite
stream was found to be active bentonite based on the results of
methylene blue clay testing.
The solids discharge, which was in the form of wet, colloidal agglomerates,
contained 346 pounds of sand (40%), 130 pounds of organic additives
(45%), 87 pounds of bentonite clay (19%) and 303 pounds of water
(1% total water). 80% of the sand had a particle size less than
60 microns, indicating that it was too fine to be of interest in
making additional green sand molds or mold cores.
EXAMPLE 2
To show the ability of commercially available mechanical separation
equipment to convert standard molding waste into a reclaimed silica
sand product capable of replacing prime silica sand, the following
example was conducted.
Approximately 2000 pounds of molding waste produced by the above
green sand foundry and having a moisture content of 1.84% was subjected
to a multi-pass mechanical separation process using mechanical reclamation
equipment available from Sand Mold Systems, Inc. of Newaygo, Mich.
Waste sand was introduced at the top of the two-cell unit and came
into contact with a rotary drum. Waste sand spun on the drum and
was abraded against sand that was built up on the shelf. The bentonite,
organic additives and the binder that was removed from the sand
grain was collected through a dust collection system and the heavier
sand grains fell to the bottom of the unit and were classified.
Six passes were run through the two-cell unit.
The data in Table I below lists several measured characteristics
of 1) the molding waste being processed 2) the molding waste after
each of the six passes through the two-cell unit, and 3) prime sand
(control). Each sample was classified for sand grain size distribution
and several physical properties of the sand were measured. In addition,
photomicrographs at 40.times. magnification were also taken of the
prime sand raw material used by the foundry in the manufacture of
mold cores as well as the reclaimed silica sand produced in as described
above after the sixth pass through the two-cell unit.
The results of these physical measurements are reported in the
following table 1, while the photomicrograph of the prime sand is
shown in FIG. 3(a) and the photomicrograph of the reclaimed silica
sand is shown in FIG. 3(b).
TABLE I Control Physical Waste First Second Third Fourth Fifth
Sixth (Prime Data Sand Pass Pass Pass Pass Pass Pass Sand) Screen
20 Sieve 2.7 0.1 0.0 0.0 0.0 0.0 0.4 0.0 30 Sieve 1.3 0.2 0.2 0.3
0.2 0.2 0.3 0.4 40 Sieve 7.7 4.6 4.1 4.8 3.9 4.1 5.8 6.6 50 Sieve
14.4 11.7 10.1 11.5 11.3 10.6 13.3 13.3 70 Sieve 35.4 35.0 31.3
32.8 34.2 33.1 34.2 33.7 100 Sieve 28.8 36.8 39.2 38.2 39.5 40.6
37.7 37.6 140 Sieve 7.3 10.0 12.8 11.0 10.2 10.6 8.0 7.3 200 Sieve
1.6 1.4 2.1 1.4 0.7 0.7 0.4 1.0 270 Sieve 0.5 0.1 0.1 0.0 0.0 0.0
0.0 0.1 Pan 0.4 0.0 0.0 0.0 0.0 0.0 0.0 0.0 AFS GFN 58.1 61.6 64.4
62.3 61.7 62.1 59.2 59.5 Base Perm 97 87 98 106 110 115 98 85 Moisture
1.84 0.52 0.21 0.14 0.15 0.09 0.07 0.01 AFS Clay 10.64 4.68 1.99
1.30 1.02 0.74 0.46 0.15 MB Clay 11.50 5.60 2.10 1.40 1.30 0.80
0.30 -- LOI 3.76 1.77 0.86 0.78 0.65 0.53 0.43 0.08 pH 9.89 9.95
9.75 9.62 9.49 9.40 9.02 6.97
As can be seen from Table 1 and FIGS. 3(a) and 3(b), the mechanically
reclaimed sand resembles the prime sand in size and shape, and the
particle size distribution of the mechanically reclaimed sand listed
in Table I is nearly identical to the particle size distribution
of the prime sand that entered the foundry. This indicates that
this reclaimed sand can be readily used as a replacement for at
least some of the prime sand used to make new mold cores.
EXAMPLE 3
In order to show the suitability of the reclaimed sand obtained
in Example 2 for replacing some or all of the prime sand used to
make new mold cores, the tensile strengths several different tensile
briquettes were tested. The different tensile briquettes were made
using 1) prime silica sand 2) reclaimed sand recovered after the
sixth pass through the mechanical separation unit of Example 2,
and 3) an 80/20 blend of this reclaimed sand and a prime sand. A
phenolic/urethane resin in the amount of 1%, 1.3%, and 1.8% by weight
was also included in each briquette as a binder. All tensile briquettes
were made according to the following procedure:
Approximately 4,000 grams of (Bridgman 1L-5W washed and dried silica
sand (AFS #50) from Bridgman Corporation was placed in a stainless
mixing bowl. A small pocket was made in the sand and 28.1 grams
of the Part I of the chemical binder resin was poured into the pocket.
Part I of the binder resin was a phenolic resin commercially available
as Part I from Delta HA Corporation of Detroit, Mich. The binder
resin was covered lightly with sand and mixed on a Hobart N-5D mixer
at #1 speed for one minute. The bowl was checked for unmixed resin
at the sides and bottom of the bowl and them mixed for an additional
minute. A small pocket was again made in the mixed sand and 23.4
grams of Part II of the binder resin was poured in the pocket. Part
II of the binder resin is an isocyanate compound commercially available
as Part II from Delta HA Corporation of Detroit, Mich. The same
mixing procedure for the Part II resin was repeated as per the Part
I resin to obtain the sand mix. The sand mix was stored in a polyethylene
container until it was ready for use in making tensile briquettes.
Tensile briquettes were made by transferring the sand mix from
the polyethylene container to a 3-gong capacity metal core box that
meets AFS specifications with vents per industry design. A gassing
manifold was applied to the core blower, a modified Redford-Carver
HBT-1 core blower from Redford-Carver Foundry Products, Sherwood,
Oreg., and amine, catalyst, triethylamine (TEA) available from Ashland,
Chemical, Cleveland, Ohio, was blown into the core box for seven
seconds. The center briquette was removed from the core box and
was thereafter placed in a tensile testing machine.
The tensile strength of each core was taken 1 hour after the sand
and the chemical binder were mixed and formed into a core. Tensile
strengths measurements were taken according to the Thwing-Albert
operating manual. Table II lists the results obtained:
TABLE II Binder Tensile Concentration Strength (1 hr.) Sand System
(wt. %) (psi) Prime sand 1 210 Reclaimed sand 1 81 80% RS/20% prime
1 96 Prime sand 1.3 275 Reclaimed sand 1.3 115 80% RS/20% prime
1.3 141 Prime sand plus 2% glass 1.3 231 Prime sand 1.8 361 Reclaimed
sand 1.8 169 80% RS/20% prime 1.8 223 Prime sand and 2% Macor 1.8
167
As can be seen from this table, the tensile strengths of briquettes
made with the reclaimed sand of the present invention, although
not as high as those briquettes made with prime sand, are still
reasonably high. Moreover, the tensile strengths of briquettes made
with the reclaimed sand of the present invention can be significantly
enhanced by adding small amounts of prime sand thereto. This suggests
that product briquettes with the desired tensile strengths can be
easily designed through appropriate selection of the amount of reclaimed
sand of the present invention to included therein.
EXAMPLE 4
Sand that was mechanically reclaimed according to Example 2 was
mixed with 1.8% chemical binder and poured into a core mold to produce
a core. The core was then placed inside a green sand mold and run
through the casting process. The casting produced met quality standards
for dimensions and surface quality.
Although only a few embodiments of the present invention have been
described above, it should be appreciated that many modifications
can be made without departing from the spirit and scope of the invention.
All such modifications are intended to be included within the scope
of the present invention, which is to be limited only by the following
claims. |