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Description  |
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FIELD OF THE INVENTION
The present invention relates to a method of and an apparatus for
regenerating foundry sand, i.e. sand used in the preparation of sand
casting molds for metallurgical purposes and the production of metal
workpieces. More particularly, the invention relates to the regeneration
of foundry sands of this type utilizing an abrasion process. The invention
also relates to the treatment of new sand for incorporation into a foundry
sand.
BACKGROUND OF THE INVENTION
Rising costs and legal requirements with respect to the disposal of foundry
sand in land fills and the like have increased the need for regeneration
and reuse of foundry sands. A variety of regenerating processes are known
and regeneration apparatus is commercially available and currently in use.
The regeneration of bentonite-containing mixed sands has posed particular
problems because of the very large quantity of contaminants which tend to
be present in the depleted foundry sand. At the present time,
thermo-mechanical regeneration is favored as is described, for example, in
EP 0 343 272 A1. In this thermo-mechanical process, the sand is initially
annealed in a thermal treatment stage at temperatures from 500.degree. to
900.degree. C. and, after an appropriate cooling, is fed batchwise into a
friction or grinding machine in which the dead-burned binder residue which
has not been volatized by the annealing process is abraded from the grains
of sand by rotating transverse arms. Compressed air blown through the sand
filling in the machine from time to time carries away the dust which is
liberated by the abrasion process.
Thermal regeneration treatments, however, have not been fully acceptable
heretofore because of the cost and time required.
The high capital cost, operating and maintenance costs of earlier apparatus
for the regeneration of foundry sand have made many of the earlier systems
unsuitable or unacceptable for small and medium-size foundries and have
resulted in the need to transport foundry sand from the foundry or to
return the foundry sand after treatment to the foundry with expensive
transport processes.
Old sand may have heat values in the form of carbon containing components
and some bentonite as valuable constituents which are dead burned and
lost. Sand grains can fracture as a result of the temperature variations
and can be transformed to waste. The amount of waste may thereby be
increased beyond that which is desirable or acceptable and the grain
spectrum or particle size distribution in the foundry sand may be varied
in an impermissible manner. Environmental problems arise from the
production of heat and carbon dioxide by the foundry in the regeneration
of foundry sand. Restrictions in the amounts of heat and CO.sub.2 which
can be liberated by a particular foundry can limit foundry furnace
operations in other respects.
In more recent publications, therefore, regeneration processes have been
described in which the dead burning of the entire bulk of the old sand can
be avoided (see DE 41 06 736 A1, DE 41 06 737 A1, DE 41 21 765 A1 and EP 0
465 778 A2).
When, however, the thermal regeneration step is eliminated, the
requirements for a mechanical regenerating machine are significantly
higher than with earlier machines since the mechanical operation must
satisfy the full need for cleaning the sand grains.
Investigation into commercially available machines has shown that these
machines have a variety of drawbacks and disadvantages which limit the
quality of regeneration or only allow satisfactory regeneration after long
processing times.
Impingement-type cleaning operations in which the sand grains are entrained
in high velocity air jets and the stream is directed against impingement
baffles, consume relatively too much compressed air and generate large
amounts of residues because of the rupture of the sand grains.
Rotary drums with plural drives and strippers are expensive when they must
be designed to accommodate hot depleted foundry sand and to tolerate the
high degree of wear and the tendency to breakdown which thus results.
With grinding machines which have commonly been used after a thermal
treatment stage, the removal of dust is generally effected either by
transverse air which can entrain the dust only over the batch in the
vessel, or by means of compressed air. When compressed air, however, is
fed to the grinding machine of EP 0 343 272 A1 by a multiplicity of
nozzles at the upper machine bottom, there is formed in the region of the
transverse arms, a fluidized bed. With sand which contains active clay,
there is insufficient abrasion which deterimentally affects the
suitability of the regenerated product for the production of foundry
cores.
To overcome this problem, machine operating times are increased or the
amounts of materials passed through the machine are reduced.
Furthermore, the abrasion does not always affect the entire charge in the
machine and it is found that at the bottom and side wall corners of the
machine there are dead zones where untreated sand can accumulate, the
untreated sand serving to contaminate the regenerated product.
OBJECTS OF THE INVENTION
It is therefore an object of the present invention to provide a simple
process for regenerating foundry sand and especially depleted or old sand
which still contains active bentonite, utilizing an apparatus which can be
operated at low cost and while avoiding the need for annealing or
otherwise thermally treating the sand.
Another object of the invention is to provide a method of and an apparatus
for the regeneration of a foundry sand which involves low capital cost,
low operating cost and low maintenance cost and is particularly suitable
for small foundries.
It is also an object of the invention, therefore, to provide an apparatus
which occupies a relatively small area, which is flexible in the sense
that it can be used for a wide variety of sand cleaning purposes and under
a wide variety of conditions and which can be used in a simple and
uncomplicated way for most required sand preparations.
It is also an important object of the invention to provide a process for
the purposes described which avoids fragmenting the sand grains by
impingement action and which nevertheless insures a high degree of
grain-to-grain friction so that even hard contaminants baked upon the sand
grains can be rubbed off and the grains themselves ground and
advantageously rounded.
According to another object of the invention, the grain-to-grain friction
in a regenerating charge should be clearly increased over prior systems so
that contaminants can be removed as completely as possible and the
duration of treatment of a particular charge can be reduced.
SUMMARY OF THE INVENTION
These objects and others which will become apparent hereinafter are
attained, in accordance with the invention, in a process in which a
horizontally rotating grinding or abrading tool is provided in an upright
grinding machine for the regeneration of foundry sand and a compressed air
source is provided.
According to the invention, the air is admitted only outside of the orbit
of the vanes of the tool through the bottom and/or the side wall in the
outer region of the sand filling and, in cooperation with the rotating
vanes, an upward fluidized column of the sand is generated in the
peripheral region of the chamber inducing the sand to move upwardly. The
dust-laden sand is removed above the filling in the chamber and then no
longer fluidized dense sand passes downwardly in a central stream.
More particularly, the method of the invention comprises the steps of:
(a) circulating depleted foundry sand in an upright dry abrading chamber by
causing the foundry sand to descend onto a horizontal rotating abrading
tool, displacing the foundry sand outwardly with the tool and inducing the
foundry sand to rise in the chamber outwardly of an orbit of the tool to a
level of foundry sand in the chamber;
(b) admitting air to the chamber at a bottom of the chamber and outwardly
of the orbit to fluidize a rising column of the foundry sand outwardly of
the orbit;
(c) removing a dust-laden air above the level of the foundry sand in the
chamber; and
(d) enabling nonfluidized foundry sand at the level to pass inwardly to a
downward central stream of the foundry sand descending to the horizontal
abrading tool.
In apparatus terms, the invention comprises:
means forming an upright chamber having a peripheral wall and a bottom;
at least one horizontally rotating abrading tool in the chamber located
close to the bottom for circulating depleted foundry sand by causing the
foundry sand to descend onto the tool, displacing the foundry sand
outwardly and inducing the foundry sand to rise in the chamber outwardly
of an orbit of the tool to a level of foundry sand in the chamber;
means for removing a dust-laden air above the level of the foundry sand in
the chamber; and
means for admitting air to the chamber outside the orbit, close to the
bottom and at the wall for fluidizing a rising column of the foundry air
in the chamber outwardly of the orbit whereby nonfluidized foundry sand at
the level passes inwardly from the column to a central downward stream of
the foundry sand descending to the horizontal abrading tool.
The admission of air can be effected with the injection of air as
compressed air or by drawing air under suction into the chamber.
According to a feature of the invention, the air is admitted to the chamber
through a plurality of locations horizontally and spaced apart from one
another at locations proximal to the bottom and to the peripheral wall of
the chamber. The upwardly moving fluidized column and the downwardly
moving stream can be separated by a partition in the chamber which is open
upwardly and downwardly.
According to a feature of the invention, the speed of the abrading tool
and/or the pressure, volume, inlet locations and inlet directions for the
air can be varied during the course of regeneration and can be matched to
the progress of cleaning of the sand grains and removal of dust.
The progress of cleaning can be monitored by monitoring the current or
power draw of the motor driving the tool.
The monitoring output signal can be used to control other operating
parameters in the processing of the batch, for example, the feed of the
material to the apparatus, the air flow, etc.
Air can be admitted, in addition, at other levels of the wall, either by
being blown in or being sucked into the chamber. The dust laden discharged
air can be led through a funnel-shaped sand catcher to the discharge duct.
The discharge of the dust-laden air can be effected by applying suction to
the duct and/or the blowing of transverse air into the chamber below the
sand catcher. The transverse air may also be used in the absence of
suction to displace the dust-laden air from the chamber and to control the
outflow of the dust-laden air.
The unburned valuable materials, such as heat values and active bentonite
in which the waste air is clearly enriched at least initially in the
treatment of the sand can be collected separately for reuse.
During a precleaning phase of the process, the abrading rotor need not be
driven at all or can be driven only slowly.
Advantageously, the abrading rotor can have at least two arcuate vanes and
can be driven by a speed-regulatable motor.
The admission of the air into the chamber can be effected at a corner
region between the bottom and the wall, preferably from at least two
groups of segment shaped individual chambers. The wall itself can be
surrounded by one or more air chambers having feed lines for the air which
can be controllable independently from one another. The inlets can be self
cleaning annular gaps, spaced apart ring segments or air slits and, where
air is admitted into the chamber, it can be admitted through sand-tight
inserts of porous sintered metal or sand filters.
According to a further aspect of the invention the apparatus is utilized to
prepare new or fresh sand by a pregrinding operation.
Since the air is only caused to pass upwardly through the periphery of the
sand filling in the container, to fluidify the sand in the annular
peripheral region externally of the orbit of the abrading rotor, the
fluidization only is effected along the peripheral wall. The compressed
air, therefore, tends to form bubbles in this column which entrain the
dust abraded from the sand grains, while lifting the sand grains
themselves, and thoroughly clean the surfaces of the sand grains and
entrain that dust out of the filling. The sand itself passes inwardly to
the descending stream and returns to the abrading tool. The sand particles
which may have been carried by the air out of the filling are collected in
the funnel, as a result of the decrease in velocity above the constructed
aperture of the funnel and can cascade back onto the surface of the
filling to pass downwardly in the stream to the abrading rotor. As a
consequence, the apparatus has a vertical circulation effect. The binder
residues adherent to the sand grains are not only abraded therefrom by the
abrading rotor or grinding rotor, but are also released by the
grain-to-grain friction within the moving mass of sand. This type of
frictional removal of the residues can be increased by injecting sharp
compressed air jets into the peripheral region of the filling and is
especially effective when electrostatically adherent plastic or synthetic
resin particles must be released.
Since an excessively high degree of fluidization in the region neighboring
the wall can significantly decrease the effectiveness of the rotor, it is
important to maintain the rising fluidized column wholly externally of the
orbit of the rotor.
This can be insured by admitting the compressed air from spaced apart
inlets at the junction between the bottom and the peripheral wall. In this
case, column like zones of fluidized sand are formed directly adjacent
these inlet locations and extend vertically, being separated from one
another by columns of dense sand.
As a consequence, the formation of a coherent tubular fluidized bed along
the machine wall can be avoided. Because intervening columns of more dense
sand or sand with a greater degree of packing can form between discrete
vertical columns at each inlet, the grinding machine need not have
additional baffles in the cylindrical chamber to break up a fluidized bed
or column which may propogate inwardly and may influence the effect of the
rotor. It is important, as will be apparent from what has been stated
previously, that the central stream of more dense descending sand be
permitted to be set into rotation by the rotor like a core in a bearing.
The vertical circulation effect can be reinforced when the upwardly and
downwardly moving streams are separated by an upwardly and downwardly open
annular partition.
The partition can open at its upper end above the filling or the "level"
mentioned previously and terminate at its lower end just above the
abrading rotor.
The number and size of the bubbles which are formed in the sand can be
varied by controlling the air introduced not only at the lowest point in
the chamber but partly also through the peripheral wall above the rotor.
The lateral inlets at upper levels may be angularly offset from one
another.
During the course of the regeneration treatment, the composition changes as
do various characteristics or parameters such as the flow properties or
rheology of the sand so that alterations of various operating parameters
is desirable. At the commencement of the treatment there is a temperature
equalization in the sand filling which is especially apparent between
newly added sand and sand which may have been retained in the chamber and
is most noticeable with depleted sand which has not been thoroughly mixed.
Initially as well, active bentonite which may be adherent to the sand
grains is, dried, rubbed off, and, together with carbon particles, is
carried away in the removed air which is enriched in these materials.
These valuable components can be separately collected, e.g. from a gas
cleaning cyclone or other gas cleaning installation for reuse in the
preparation of foundry sand.
The change in the flow conditions of the sand filling also changes the
current draw of the drive motor and can be monitored to indicate the
degree of regeneration or the progress thereof. The current draw can be
converted into a signal for varying the cross section of a compressed air
line and thus the supply of compressed air and/or for controlling the
speed of the rotor so that the abrading intensity can be varied to the
desired degree.
The control process can be automatic. With the aid of such a signal,
moreover, the completion of regeneration can be signalled for a specific
charge.
Indeed, the machine operating or residence time for a particular
regenerated product depends upon the composition of the depleted sand and
can be monitored through the drive motor as described as the composition
changes with mold sand circulation.
Many depleted sands contain relatively large amounts of dust, namely,
carbon dust and bentonite. It is advantageous to collect these materials
at the beginning of the abrading process and for that purpose I may
provide a precleaning phase in which the abrading rotor is at a standstill
or is rotated so slowly that the sand filling is mixed without significant
abrasion.
The discharge of dust can be effected or controlled by injecting into the
chamber above the sand filling, radially or tangentially, transverse air
from which the fine grain sand can be deposited at the latest in the
funnel-shaped collector by cyclonic action. This separated sand returns to
the sand circulation to maintain the grain size spectrum.
BRIEF DESCRIPTION OF THE DRAWING
The above and other objects, features and advantages of my invention will
become more readily apparent from the following description, reference
being made to the accompanying highly diagrammatic drawing in which:
FIG. 1 is a vertical section in highly diagrammatic form showing a
cylindrical foundry sand grinding and regenerating machine;
FIG. 2 is a section taken along the line II--II of FIG. 1; and
FIG. 3 is a section similar to FIG. 1 through a machine in accordance with
another embodiment of the invention.
SPECIFIC DESCRIPTION
The machine shown in FIGS. 1 and 2 has a cylindrical container 1 provided
with a cover 1a on which a drive motor 12 is mounted. The cylindrical
container 1 also has a bottom 9 and a peripheral wall 1b.
The lower portion of the container 1 has a jacket 2 which is formed with
the bottom 9 and defines with the peripheral wall 1b, air chambers 3, 3a
and 4 which have separate inlet lines or ducts 5, 5a and 6 for compressed
air with respective valves, only one of which has been represented at 5'
in FIG. 1.
The motor 12 is provided with a circuit 50 which monitors the current drawn
by the motor 12 and can feed a signal, for example, to the valve 5' or to
a speed controller 12a of the motor to regulate an operating parameter of
the machine as described.
Fluidizing and dust removing air is admitted to the interior of the housing
1 through an annular gap 7 between the peripheral wall 1b and the bottom 9
and through air inlet slits or nozzles 8 at higher locations along the
peripheral wall through the respective chambers 3, 3a and 4. The inlets 4a
from the chamber 4 are directed transversely across the top of the filling
below the sand catcher 20 to form the transverse jets which have been
described. The inlets 8 can be provided with sintered metal sand filters
preventing passage of sand through those inlets in the chamber 3a.
The cover 1a is provided with an outlet duct 23 through which discharged
air with entrained dust can be removed and valuable constituents can be
recovered from this air, e.g. in a cyclone downstream of the apparatus and
not shown. As is apparent, the motor 12, which is centrally mounted on the
cover 1a, is a speed-controllable motor which has a slightly downwardly
tapered shaft 13 at the free end of which a horizontal abrading or
grinding rotor 14 is mounted slightly above a bottom plate 24 of the
bottom. The bottom plate 24 is removable from the bottom to allow the
regenerated sand to be removed.
The rotor 14 can also be arranged on an eccentric and can be driven from
below, as desired, requiring only the shifting of the discharge opening.
As can be seen from FIG. 2, the rotor 14 can have two slightly curved
blades.
The sand is introduced into the chamber through a closable filling fitting
16 which is provided above the inclined surface of the funnel shaped sand
collector 20 and introduces sand into the chamber to the level 17.
FIG. 2 shows that the air chamber adjacent the bottom 9 is subdivided by
partition 25 into two groups of segment-like individual chambers 26, 27.
Such a subdivision also can be effected for the air chamber 3a shown in
FIG. 1 above the chamber 3.
The individual segmental air chambers are alternatingly supplied with
compressed air so that respective rising columns of fluidized sand will be
formed in the effective regions 29 in the direction of injection of the
air through the respective slits 7 as represented by the arrows 28.
In the outer region 10 beyond the orbit of the rotor 14, therefore, there
are zones 30 which are little affected by the air and thus have a
relatively dense packing of the sand and form braking and support bodies
of the sand which oppose the rotation of the sand by the rotor 14. As a
result, the rising columns 29 frictionally interact with the relatively
stationary columns between them and, moreover, the penetration of the
fluidized beds inwardly beyond the orbit of the rotor is minimized or
excluded. The action of the rotor, thereby remains effective.
In operation, the rising columns represented by the zones of arrows 28 of
fluidized sand whose dust is entrained with the air, pass to the top or
level 17, where the dense sand moves inwardly and the air is accelerated
through the narrower orifice 20a of the funnel.
The dust-entraining air is discharged at 23 and the heavier particles
separate out in the funnel and pass again into the sand filling below the
funnel. The path of the dust entraining air is represented by the arrow
22.
Along the center of the sand filling in the region 11, there is a
descending stream of sand represented by the arrows 19, the sand
contacting the rotor 14 and being subjected to abrasion thereby as the
sand is then centrifugally directed outwardly (arrows 15) to be entrained
anew upwardly in the columns.
To prevent untreated sand and dust or residues from collecting at the
corners of the zones in which air is not introduced through the slit 7,
the chambers 26 and 27 alternate in function, one receiving air while the
other is inactive and vice versa.
Any dead zones are thereby blown out upon alternation of the compressed air
flows to the chambers 26 and 27.
FIG. 3 shows another embodiment in which the bottom is formed with an
annular nozzle-forming gap 31 converging in the direction of the changer
or with a plurality of individual nozzles.
The descending stream is separated from the ascending column by an annular
partition 34 mounted with ribs 35 on the peripheral wall of the chamber.
In this embodiment, compressed air from the annular chamber 32 is injected
in sharply defined jets 33 into the sand to effect an additional friction
action in the outer region of the sand filling. The high velocity jets
promote complete removal of residues from the sand which is abraded by the
rotor 14 in the manner described and maintains the vertical vent of the
sand as described.
In this embodiment as well, valuable components can be recovered from the
entraining air.
In an early stage, before abrasive action is commenced by the rotor, the
rotor can be rotated slowly while the air jets remove most of the valuable
dust like materials in the precleaning operation. The circulation is
represented in this embodiment by the arrows 15, 33, 21, 19. The mixing of
the upwardly and downwardly moving streams of sand is prevented or limited
by the partition 34 and undesired rotation thereof is prevented by the
ribs 35.
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