Note: Descriptions are shown in the official language in which they were submitted.
1C150209
The present invention is directed to the
reclaiming of clean sand.
Sand is an indispensible raw material for the
production process of the foundry. It occurs abundantly in
nature, but frequently the particular sands required
hy the foundry must be transported at great distances,
making this basic manufactured material an expensive one.
Accordingly, an economical process for the reclaiming of
used sand will alleviate the transportation expense and
to some degree storage problems associated with bringing
in new quantities of sand having a precise size and char-
acter distribution; in addition natural raw materials
will be conserved.
Early methods of reclaiming sand specifically
avoided using molding sand laden with resins or other
difficult to remove impurities. Only green sand and/or
dry sand was put through a reclamation process which
included calcining followed by washing which did in fact
remove a substantial portion of any impurities. This
process, however, required that the resulting sludge
from the washing operation be conveniently discarded in
rivers or settling ponds. With the advent of stricter
envieonmental protection regulations~ the ability to
dispose of a colloidal clay effluent, such as the sludge,
was virtually eliminated or at least made economically
prohibitive.
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Under the older production methods ~f molding and
curing sand, criteria for judging successful use of the re-
claimed sand was markedly less stringent than today. Today,
the requirements for accepting reclaimed sand demand that
there be almost no metallics, an absence of organic impurities,
absence of lime, and most importantly inert friable material
must be removed to provide the reclaimed sand with an acid
demand of 15 or less. These requirements are almost
impossible to meet by the present state of the prior art
and still meet environmental protection agency regulations.
Typical of current prior art methods, the process
includes crushing the dirty sand charge to size it to a
uniform mesh character, adding water and/or other suspension
agents to form a wet mud, scrubbing said mud and subject-
ing the mud to a vacuum filter to remove fines. The wet
sand is ther. subjected to a calcining operation, sprayed
with water following the calcining operation, and allowed
to cool. The high solid colloidal clay effluent from the
scrubbing and filtering operation had to be dumped in con-
venient areas such as rivers, etc. A conceptual modificationof the above process has been suggested; the sand charge
is fluidized during calcining by injection of gases. The
particulate body of sand is virtually suspended allowing
a vacuum system to remove certain particles based upon a
variation in density. Unfortunately, the fluid bed attrition
approach is highly inefficient and has not been successful
~rom a technical standpoint.
More current investigative work by the industry
has used a fatty acid for mixing resin impregnated or dirty
sand to form an aggregate, the fatty acid being oleic acid
or an equivalent. A frothing agent is then added on a
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continuous basis; t~.e mixture frot~s and boils over the
sand container
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carrying impurities in the frothing medium. The cost of
this process is very high and efficiency is not attractive,
but more importantly, the waste disposal problem of the
frothing mixture still remains.
In accordance with one aspect of the present
invention, there is provided a method of upgrading sand,
comprising: (a) calcining a charge of sand to be up-
grade~ at a temperature-pressure condition for a period
of time effective to convert any clay on or in the sand
charge to a friable material and to convert any dolomitic
lime in the sand charge from a carbonate to an oxide, and
(b) subjecting the calcined charge to severe agitation
so as to burst or split any shell-like coating appearing
on any sand of the charge whereby clean sand may be
separated from the charge to render a statistical average
acid demand of ]5 or less.
In accordance with a second aspect of the
present invention, there is provided a method of reclaiming
clean sand from a sand mixture impregnated with resin
and containing dolomitic lime and clay or improving
unused sand, the clean or improved sand being particularly
charac~erized by a low level of inert friable material
rendering an acid demand of 15 or less, the method com-
prising: (a) sizing the grains of a charge of sand
coated wi~h resin, clay or a mixture thereof to AFS 40 to
130, (b) calcining the sized charge at a temperature to
substa~tially convert all clay to a friable material and
convert dolomitic lime from a carbonate to an Qxide, (c)
subjecting the grains of the calcined sized charge to
severe agitation so as to burst or split any shell-like
coatings appearing on the calcined grains, the coating
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being typically composed of inert friable material,
cooling the calcined grains of the charge to a temper-
ature of 200F either before or after agitation, and
(d) collecting and separating the burst or split shell-
like coatings from the grains, the latter constituting
the clean sand.
In accordance with a third aspect of the
present invention, there is provided a method of preparing
sand molds or cores from sand previously impregnated with
resin, comprising: (a) sizing the grains of a charge of
resin impregnated sand to AFS 40 to 130, (b) separating
metallic elements from the sand charge either before or
subsequent to the sizing step, (c) calcining the sized
sand charge at a temperature in the range of 1200 to
1500F for about one hour, (d) immediately following
calcination, dry scrubbing the sand charge to produce a
reclaimed sand product characterized by being free of
injurious coatings of in-combustible oxides or inert
. friable material, (e) mixing the reclaimed sand product
. 20 with a resin curing agent and a catalyst, thereafter
forming the sand mixture into molds or core bodies, and
(f) either with or without the use of heat, curing
the bodies, each body having a cure rate such that at
the end of five seconds, the cured body will have a
tensile strength of at least 90 psi, and at the end of
30 seconds each cure body will have a tensile strength
of at least 300 psi~
In accordance with a fourth aspect of the
present invention, there is provided a reclaimed sand
product, comprising: (a) a collection of sand grains
sized in the range of AFS 43 to 53 and substantially
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distributed as follows:
20 mesh 0.1 max.
30 mesh 2.0 max.
40 mesh 3.0 to 15.0
50 mesh 26.Q to 40.0
70 mesh 35.0 to 53.0
100 mesh 5.0 to 15.0
140 mesh 4.0 max
200-270 mesh 0.3 max.
(b) the collection of sand grains having a shiny clear
appearance when magnified, a pH of 7.5 to 9.0, a clay ~-
content no greater than 0.1, an acid demand no greater
than 10, and a lime content rendering no appreciable
effervecence when a 10 gram sample of the sand is placed
in 30 milliliters of 1-1 HCl.
Two principal characteristics provided in the
reclaimed sand product of this invention are: (a) a
highly desirable and much lower acid demand capability,
and (b) an ability to form densified sand bodies, cured or
uncured, at higher tensile strengths not attainable by the
prior art.
Acid demand is defined herein as the amount of a
limited quantity of test acid consumed by a defined quantity
of sand composition (the acid is principally or preferQn-
tially reacted with the impurity content of the
composition). The procedure followed in determining
an acid demand according to that required of this invention
is as follows: -~
(1) Take a 50 gram sample of a sand composition
to be tested and add to it 15 cc of water and 50 cc of a
1/10 normal hydrochloric acid solution.
(2) Stir the acid, water and sand mixture by
an automatic stirrer for approximately 7 minutes, such
stirring being carried out by magnetic means.
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t3) Allow the stirred mixture to set for 1
hour.
(4) Filter the mixture so as to separate the
sand from the water and acid; wash the sand residue with
distilled water.
(5~ Take the separated sand or filtered sand
and back titrate to 7 pH utilizing a 1/10 normal sodium
hydroxide solution.
(6) Substract the amount of sodium hydroxide
utilized from the known amount (50 cc) of 1/10 normal
! hydrochloric acid utilized, the dividend of such sub-
traction rendering an acid demand number.
According to this invention, the acid demand
must be 15 or less (preferably 10 or less) indicating
that the content of alkaline material, such as clay or
other impurities, in the
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reclaimed sand product will react with less than 15 cc of
acid. The acid will be absorbed at such a low level that
the later utility of the reclaimed sand in hot box core
processes and other stringent uses will be enhanced.
Heretofore, processing techniques for reclaimed
sand have resulted in an acid demand for the reclalmed
product typically about 30. When the alkaline material
occu~red as a carbonate, the high acid demand was not harm-
ful in subsequent core and molding operations, since it did
not affect the cure rate of the reclaimed product when
mixed with resins. However, when spent sand is calcined
to remove organic impurities, much of the alkaline material
will convert to an oxide and this is extremely harmful to
subsequent core and molding operations if simultaneously
the acid demand is allowed to remain typically at about 30.
With the alkallne material in the oxide form, the acid
demand must be reduced to 15 or less and preferably about
10 or less to provide the first of the principal character-
istics.
Under the earliest methods, only 15 - 75% of the
spent or dirty sand was reclaimable. This was helped some-
what by the shell molding process which introduced a new
molding techniqùe which was non-aqueous. With the shell
molding type sand, all organic impurities could be burned
off and the resulting sand looked very good under visual
inspection. However, when dried in a hot box core process,
it was not capable of meeting required cure strength re-
quirements because of the presence of the oxide alkaline
content and thus did not provide the second of the principal
characteristics herein.
Calcining is an integral and important step in
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any reciaiming process because it permits a continuous type
operation rendering efficient and economical cost factors.
The problem is how to make a process, utilizing a calcining
step, successful in the sense that the ultimate product
will work in a hot box core process as well as o-ther demand-
ing molding operations. To these ends, the present in-
vention is set forth.
A preferred method sequence is as follows:
(l) A charge of spent or dirty foundry sand is
sized to AFS 43-53 utilizing a screening sequence, the
sized distribution should preferably be about as follows:
20 mesh .1 max.
30 mesh 2.0 max.
40 mesh 3.0 - 15.0
50 mesh 26.0 - 40.0
70 mesh 35.0 - 53.0
- lO0 mesh 5.0 - 15.0
140 mesh 4.0 max.
200 - 270 mesh .3 max.
(2) Either before or after the sizlng step, the
spent sand charge is subjected to a magnetic separator
whereby metallic eIements may be removed and thereby
separated from the charge.
`~ (3~ The sized and magnetically processed charge
is subjected to a calcining operation whereby the collection
of sand grains is heated to a temperature range of 1300 -
1500F and for a period of time about one hour. A con-
ventional calcining apparatus may be utilized and may pre- ~-
ferably be of the two~stage type in which the collection of
sand gralns are indexed upwardly in temperature and sub-
jected to differential temperature zones for different
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periods of time. The two-stage ~peration permits spent
gases from calcination to be used to preheat the sand grains
prior to entry into the calciner. The off-gases from
calcination may be conveyed to a continuous rotary kiln
acting as a preheater.
Another apparatus that can be used for this
purpose has a vertical steel cylinder lined with fire
brick and has a number of stationary brick hearths attached
to~a center drive shaft with a number of arms,`two for
each hearth, which carry rabble teeth that plow the sand
through the furnace. A number of oil or gas burners make
up the combustion system. The movement of the sand through
the furnace is a slow, rolling, tumbling, cascading flow
back and forth across the plurality of hearths, during
which time the grains are exposed to intense heat which
ignites and burns the carbonaceous materials from the grains,
leaving the sand virtually carbon free upon its exit from --
` the furnace.
Calcinlng is important because it converts the
resin constituent of the spent sand to a gas whereby so
that it may be removed. The binder resin referred to is
typically a phenolic resin either of the one or two step
type and may be in one of several different forms: dry
powder, lump, flake, granular or liquid. Carbonaceous
materials, clays and/or filler materials will form a glaze
on eac~ sand grain as a result of calcination. Washing will
not remove this glaze; it has been earlier determined that
if the reclaimed sand was washed or utilized in the glazed
condition in a subsequent molding operation, the glaze
would have a detrimental effect on rebinding the sand
with oils or new resins and the washing would only pose sn
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1~50209
environmental disposal problem.
In the event the spent sand was previously used
in a shell molding operation, then certain foreign material
must be removed from the sand prior to reclamation, such as
shot. All spent sand may contain iron fragments such as
gates, risers and fins. The collecting system usually has
screens and magnetic separators in abundance to separate
out shot or other metallic material; manual labor may
also be utilized to ca~ry out the separation.
(4) Sand leaving the last of the hearths of the
calcining furnace is approximately at 1500F and preferably
is sent to a sand cooler. The cooler is generally com-
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prised of steel cylinder filled with a high number of
steel tubes about which cooling air is circulated. The
sand descends slowly through the cooler tubes, the rate
being determined by the length of time the discharge gate~
at the bottom of the coolers are opened as well as the
interval between the gate openings. It is in this slow
descent, in conjunction with the large volume of cooling
20 air that reduces the sand temperature to less than 200F. ~ -
In the calcining operation, all of the clay content of
the spent sand is converted to a friable material which
encases each grain of sand and all the dolomitic lime is
converted to an oxide,
(5) The most important step of the process is
the subjection of the calcined sized charge to severe and/-
or violent dry agitation which must ~e greater in scrubbing
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action than that afforded by attrition systems and much
greater than that provided by simple gentle agitation
system. A rugged mulling machine must be utilized to achieve
the kind of severe agitation contemplated by the present
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invention, although a hurricane like blower system may be
envisioned to accomplish same. The charge should be sub-
jected to such severe scrubbing in a dry condition for a
period of time, such as from 15 - 25 minutes, whereby all
of the shell-like glaze coatings on each of the sand grains
are burst or split to expose and release the encased sand
grain, leaving it in a shiny condition.
(6~ Means should be utilized to separate the
burst shell-like coatings from the released sand grains so
that the clean sand grains may constitute the sole con-
stituent of the reclaimed sand product.
(7) The clean reclaimed sand may then be further
subjected to a washing operation, if desired, but it is
not necessary since the product in its present condition
will achieve an acid demand of 15 or less according to
this invention, and preferably 10 or less.
(8) The clean reclaimed sand is then recycled
into a core making or molding process by mixture with
certain resins or suitable compounds.
This process also achieves utility with respect
to new or raw sand, as opposed to reclaimed sand. Availability
and cost factors cause raw sand supplies to typically have
an acid demand of about 30. The raw or new product can be
improved in physical characteristics to promote better and
more eficient casting. Furthermore, continuous recycling
of raw or reclaimable sand will progressively upgrade the
product proportionate to the number of recycles; it is
possible that acid demands of about 3 can eventually be
achieved.
The reclaimed sand product resulting from the
method of this invention, has achieved a new status in that
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it is a new type of product having character~stics neverbefore achieved. The product will have a pH of between 7.5
and 10.0, an acid demand of 15 or less, and visual or
microscopic inspection of the sand grains will indicate
that they are clean, shiny, and free of any glazed coating
and other impurities. The product has a lime content such
that there will be no appreciable effervescence when a 10
gram sample is placed in a 30 milliliter solution of
1-1 hydrochloric acid. The clay content of the product
is .1 or less as determined by the American Foundry Society
sand test method. It is desired that the clay content be
substantially 0, but a tolerance up to .1 is provided.
The moisture content of the sand product is .25% or less
and has a calcium oxide and magnesium oxide content of
.2 ~ax. The grain size of the product should be preferably
about AFS 43-53.
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