Note: Descriptions are shown in the official language in which they were submitted.
~2~3~i8~i3
Title: Casting Metal and Reclaiming Foundry Sand.
This invention rels~tes generally to casting metal using shape-defining
parts such as moulds and/or cores made of organically bonded foundry sand
and more particularly relates to a method of reclaiming the organical1y
bonded foundry sand after it has been used for casting.
Conventionally, organically bon~ed foundry sand has been reclaimed by
burning off the organic binder components using a fuel fired (gas or oil)
heated system operating in the temperature range 800C to 1000C and in
which the used sand is agitated ciuring burning off of the organic binder
components, for example in a rotqry kiln.
In our prior specification GB 2,091,148A there is described a method of
reclaiming usecl organically bound foundry sand in which the used sand is
separated from castings and placed in a stationary container wherein the sand
is maintained without agitation whilst combustion supporting gas is provided
to the container and allowed to percolate through the sand by natural
convection. The sand is held in a temperature range of 250C to below
400C for a time sufficient to reclaim the sand. Whilst this method avoids
the need for plant capable of operating at high temperature and of agitating
the sand, such as a rotary kiln, it suffers from the shortcoming that for
certain applications the time taken for reclamation is inconveniently long and
involves the storage of large quantities of sand in large containers as well as
problems of conveying the sand and of control. The capital cost of the plant
is, therefore, high and its efficiency relativeiy low.
From one aspect the present invention provides a method of reclaiming
used foundry sand containing an organic binder comprising the steps of
separating said sand ~rom a casting9 thereafter supplying said sand to a fluid
bed at a feed region thereof without heating said sand in a fluid bed in the
presence of combustion supporting gas prior to supplying said sand to said
fluid bed, fluidising said sand in said fluid bed with combustion supporting gasintroduced into said sand at a plural;ty of locations so that the sand remains
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in said fluid bed in a fluidised state in an elevated treatment temperature
range to reclaim said sand, allowing the sand to pass whilst in the fluid bed
from the feed region to a discharge spaced from the feed region and removing
reclaimed sand from said fluid bed through the discharge and the fluidising
5 gas being devoid of hot products oF combustion suf~icient to maintain said
sand in said temperature range.
As a result we have found that satisfactory reclamation is achieved in a
time measured in minutes as opposed to the time measured in hours qs
disclosed in GB 2,091,148A, the need for storage of large quantities of sand
lû in large containers is avoided and conveying and control problems are also
avoided.
Preferably~ after separating said sand from the casting, said sand is
supplied to said fluid bed without fluidising said sand prior to entry into saidfluid bed. Preferably, after separating said sand from said casting, said sand
15 is not heated in the presence of combustion supporting gas prior to supplyingsaid sand to said fluid bed. This is further to avoid evolution of fumes prior
to heating of the sand at the treatment station since the sand is brought to
the treatment temperature range relatively quickly, e.g. in the order of ten
secs. If the used sand is brought relatively slowly to the treatment
2û temperature range in the presence of fluidising gas, particularly if the gas is
combustion supporting gas, then excessive fumes will be evolved.
Said sand may be supplied into said fluid bed in said treatment
temperature range by virtue of being heated in a metal casting process in
which said sand has been previously used, the metal casting temperature and
25 the metal-to-sand ratio being such that the sand is heated so as to be in the treatment temperature range.
Alternatively the sand may be initially at or substantially at ambient
temperature and said sand may be supplied into said fluid bed at or
substantially at ambient temperature and heat is applied to said sand in said
30 fluid bed to bring said sand to said treatment temperature range.
Further aiternatively, said sand may be supplied into said fluid bed at a
temperature between ambient temperature or substantially ambient
temperature and said treatment temperature range by virtue of being heated
in a metal casting process in which said sand has been previously used and
35 further heat is applied to said sand in said fluid bed to bring said sand to said
treatment temperature range.
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Where the method involves heating said sand in said fluid bed, said sand
may be introduced into a firsf fluid bed section wherein said sand is fluidised
by said combustion supportiny gas and heat is applied to said sand to heat said
sand to s~id treatment temperature range and wherein said sand is partially
5 reclaimed and the part;ally reclaimed sand passes into a further fluid bed
section wherein said sand is fluidised by said combustion supporting gas at a
temperature in said treatment temperature range to continue reclamation of
the sand.
The sand mqy remain in the further flvid bed section without the
lû application of heat or with the application of heat but a lesser amount of
heat than is applied in the first fluid bed section.
Said fluid bed or fluid bed section may be heated by means which avoid
combustion of fuel within said sand so that said combustion supporting gas is
available for combustisn of the binder.
Said sand may be heated by a plurality of heating elements immersed in
the sand, the heating elements may be electrical heating elements immersed
in the sand, or a combustion chamber and/or smoke tubes of a furnace in
which fuel is burned.
After treatment of the sand in the first fluid bed section or the first
2û and second fluid bed sections where two fluid bed sections are provided, thesand may be passed to a cooling fluid bed section wherein said sand is
fluidised and cooled to a desired temperature.
Said fluid bed sections may be disposed in a horizontal sequence~
The fluidised sand may flow from the feed region to the discharge.
The reclaimed sand may be removed from the fluid bed throuah the
discharge whilst supplying further used foundry sand to said feed region of
the fluid bed.
After separating said ~and from said casting, said sand may be reduced
to grain size or substantially to grain size.
Said sand may be selected from the group consisting of si3ica, zircon,
olivine or chrornite sand.
The binder muy be selected from the group consisting of a chemicolly
hardened resin binder or a thermo-setting resin or a ~ot box resin and may
be selected frorn the group consisting of phenolic, furane, isocyanate or
35 acrylic resin binder.
The treatment temperature may lie in the range 250C to ~00C, and
preferably lies in the range 300C to below 400C and more preferably lies in
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the range 400C to 530C, and still more preferably lies in the range 450C
to 500C.
According to another aspect of the invention, we provide a method of
rnaking a metal casting comprising the steps of making a mould using
5 reclaimed used foundry sand and ~n organic binder, hardening the mould,
casting molten metal into the mould to rnake a casting, separating said sand
from the casting, reclaiming said sand by a method according to the first
aspect of the invention and then using the thus reclaimed sand in a repetition
of the method.
The metal may be a non-ferrous metal selected from the group
consisting of aluminium, magnesium, copper and alloys based on said metals
and said reclaimed sand of which the mould is made may be predominently
reclaimed used zircon sand.
The mould may comprise in addition to a catalyst or hardening agent
for the resin binder;
5û% to 100% zircon sand, by total weight of sand;
0.4% to 1% resin binder, by total weight of sand;
0% to 5û% sand or sands other than zircon sand, by total weight of
sand.
2() Said sand of which the mould is made may comprise wholly or
substantially wholly zircon sand.
The metal may be melted in a melting vessel, transferred from the
melting vessel to a casting vessel and transferred upwardly from the casting
vessel into the mould.
The metal may be transferred from the melting vessel into the casting
vessel by flow of metal under gravity and the level of the top surface of the
metal as the metal leaves the melting vessel may be above the top surface of
the metal in the casting vessel by not more than a mGximum distance above
which excessive turbulence occurs; this maximum distance may lie within the
range 50mm to 200mm.
The use of zircon sand as a foundry moulding material is well known in
the production of iron and steel cs3stings. Zircon sand is used mainly because
vf its high refractoriness, i.e. it does not fracture nor melt under conditions
of thermal shock when casting ferrous materials. Usually such moulds qre
produced by the Croning (Shell) process~ which uses phenolic based thermo-
setting resins. Although the process has a reasonab1e reputation for the
accuracy of the resulting castings, the uccuracy is necessarily limited by the
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use of hot metal patterns, which are subject to thermal distortion and the
distortion of the thin shell moulds.
In ferrous foundries using the Shell Process, the expensive zircon sand is
re-claimed by a number of existing thermal reclamation systems, most of
which heat the sand to a temperature in the range 800C to 1,000C to burn
off the remaining resin prior to re-coating with fresh resin. The high cost of
such reclamation is usually recoverable in the relatively high price of such
ferrous shell-moulded castings.
Because aluminium, magnesium, copper and other metals having a
melting point below that of ~errous metals and alloys based thereon do not
require the refractoriness of zircon sand, moulds for these metals have
traditionally been made only in silica or other cheaper sands. Only
occasionally have cores been made in zircon sand.
The use of zircon sand and its reclamation in accordance with the
present invention provides a metal cqsting process having a number of
extremely important and unexpected benefits as follows
The accuracy of cored holes and wall thicknesses defined by cores, are
improved by a factor of up to twenty times. External features of castings
are typically five times more accurate than their silica sand cast
counterparts. This improvement in accuracy follows from the low expansion
combined with the high thermal capacity of zircon sand compared to silica.
It enables the accuracy to exceed the accuracy of all other casting methods
known to date including investment and pressure die casting. The high
thermal capacity increases free~ing rate and also improves mechanical
properties.
Sand expansion defects, such as scabs, rat-tails, finning, flash and the
like are eiiminated.
The levels of cddition of the resin binder are significantly lower than
those used for the Croning Shell Process. Because the patterns are used at or
near to room temperature, they retaîn their accuracy and so produce
accurate moulds and cores. Also, the moulds can be made of any convenient
thickness as a thick shell, or in the form of block moulds, or can be made in
steel bo~es or frames. In this way, also9 aCCUrQCy can be conserved
(compared and contrasted with a thin Croning Shell mould which is easily
distorted).
According to a third aspect of the invention we provide a casting made
using the method according to the second aspect of the invention.
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According to a further aspect of the invention, we provide an apparatus
for reclaiming used foundry sand containing an organic binder comprising
means for separating a mass of said sand from a casting, sand supply means
to supply the separated sand to a fluid bed, fluidising means to introduce
5 combustion supporting gas into said sand at a plurality of locations in said
fluid bed to fluidise the bed, means to maintain said sand in said fluid bed in
an elevated treatment temperature range to reclaim said sand and means for
removing the reclaimed sand from the fluid bed wherein said fluid bed
comprises a first fluid bed section into which said sand supply means supplies
lû said sand, in use, and having fluidising means to introduce combustion
supporting gas, which is devoid of hot products of cornbustion sufficient to
maintain said sand in said temperature range, into said sand at a plurality of
locations to fluidise the bed in the first fluid bed section and a heating meansto heat the bed in the firs`t fluid bed section to said treatm~nt temperature
15 range par~ially to reclaim said sand and sand passage means to pass partiallyreclaimed heated sand from the first fluid bed section to a further fluid bed
section having fluidising means to introduce combustion supporting gas, which
is devoid of hot products of combustion sufficient to maintain said sand in
said temperature range9 into the sand in the further fluid bed section at a
20 plurality of locations to Fluidise the bed therein and means to maintain the
bed in the further fluid bed section in said treatment temperature range
whereby reclamation of the sand is continued.
Preferably said sand supply means supplies said sand to said fluid bed
without fluidising said sand prior to entry of said sand into said fluidised bed.
25 Said sand supply means may supply sa;d sand to said fluid bed without heating said sand prior to entry of said sand into said fluid bed.
More generally, said sand supply means may restrict contact between
sGid sand and combustion supporting gas prior to introduction of said sand
into said fluid bed.
3~ Means may be provided whereby said sand remains in said further fluid
bed section in s~id treatment temperature range without application of heat
so as to continue rectamation utilising heat in said sand frorn the first fluid
bed section.
Alternatively, said further fluid bed section may be provided with
35 heating means whereby said sand is maintained in said further fluid bed
section in said treatment temperature ran~e so as to continue reclamation
partly by utilising heat in said sand from said first fluid bed section and with
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application of a lesser amount of heat than is applied in said first fluid bed
section.
Where the fluid bed or a fluid bed section has heating means, the
heating means may comprise a plura1ity of heating elements immersible in
5 the sand.
The heating elements may comprise electrical heating elements which
m~y be electrical conductors in direct contact with the sand and through
which electrical heating current at low voltage is passed, in use.
Alternatively, the heating elements may comprise a combustion
lû chamber and/or smoke tubes of a furnace in which fuel is burnedl in use.
Said fluid bed may comprise sand passage means to pass sand from the
further fluid bed section to a third, cooling, fluid bed section having fluidising
means to introduce combustion supporting gas, which is devoid of hot
` products of combustion sufficient to maintain said sand in said temperature
15 range, into the sand in the third fluid bed section at a plurality of locations to
fluidise the bed therein and means to cool the bed in the third fluid bed
section.
The fluid bed sections may be disposed in a horizontal sequence.
The fluidising means may comprise a plurality of discrete openings
2û through which said combustion supporting gas is directed to the sand, said
openings being provided with shield means to shield the openings from ingress
of sand.
Means may be provided to reduce the sand to grain size or substantially
to grain size after separating said sand from said casting.
The invention will be described in more detail by way of example with
reference to the accompanying drawings wherein:-
FIGURE I is a schematic illustration of a light alloy foundry casting
plant for carrying out a method embodying the invention;
FIGURE 2 is a diagrammatic cross-sectional view of the melting and
casting apparatus of the piant of Figure l;
FIGURE 3 is a diagrammatic pian view of a sand reclamation apparatus
of the plant of Figure l;
FIGURE 4 is a sect;on on the line 4~ of Figure 3;
FIGURE 5 is a graph showing rise in temperature of sand during
reclamation QS a function of the sand's original temperature and air flow
rate;
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FIGURE 6 ;s a set of Time Temperature Transformation (TTT) curves
for different extents of reclamation expressed as residual binder content as
measured by loss on ignition (Lol) at an air flow rate of 4001/min/m2;
FIGURE 7 is a similar set of TTT curves but for an air flow rate of 2000
1/min/m2; and
FIGURE 8 is a similar set of TTT curves but for an air flow rate of 4000
I/min/m2.
Referring now to Figure I, a foundry plant suitable for casting
aluminium~ magnesium, copper and alloys based on said metals or the like
lû comprises a moulding station 10 where moulds are made of organically
bonded foundry sand, in the present exnmple zircon sand, but which may be
made of other sands such as silica sand, olivine sand, chromate sand or a
mixture of such sands~ The sand is bonded with an organic binder such as a
chemicclly hardened resin, such as a g~s hardened resin, for example furane
resin with methyl ethyl ketone peroxide (MEKP) hardened with Sû2 or
isocyanate resin, h~rdened with amine gas or acrylic resin with MEKP
hardened with Sû2, or a liquid cataiyst hardened resin, for example furane
resin hardened with sulphonic acid, or a thermosetting resin for example a
thermosetting phenolic resin or hot box resin, or with any binder which can
be reclaimed by heating in the presence of a cornbustion supporting gas to
produce gaseous products of combustion/oxidation.
The mould may be made by ramming but, in the present example, are
blown using an automatic mould blowing machine. The mould may be used in
conventional mould boxes or a boxless process, as in the present example may
be used. In the present example, the mould comprises 98% zirconium silicate
sand; û.S5% furane resin, by weight of sand; 0.20% methyl ethyl ketone
peroxide, by weight of s~nd; 2% usual impurities such as oxides of transition
elements and bound with furane resin~ The resin was hardened using S02 gas
amount;ng to about 0.25% S02 equivalent by weight of sand. The sand has an
average grain size of 145mm.
If desired, the rnould may aomprise 50% to 100% zircon sand, by total
weight of sand; 0.4% to 1% resin binder, by total we;ght of sand; 0% to 50%
sand or sonds other than zircon sand, by to~aJ weigh~ of sand. Preferably
however, the sand comprises wholly or substantially wholly zircon sand.
The zircon sand rnay have an average particJe grain size Iying in the
range 50 ~o 500mm.
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A core or cores similarly made of organic resin bonded sand either the
same sand and binder system as the remainder of the mould or otherwise, is
positioned, as necessary, within the mould cavity at a mould assembly station
I l where the cope and drag halves of the mould are closed.
These moulds are transferred by a conveyor 12 fo a casting station 13
where malten metal, in the present example magnesium alloy, is cast through
ingates into the mould cavity and around the core or cores when present.
Details of the melting of the metal and casting are described below with
reference to Figure 2. After casting, the filled mould is transferred by a
conveyor 12 to a shakeout station 14 where the sand of the mould and core or
cores, ~hen present, is shaken out of the casting and the used sand is fed by a
conveyor 15 to a sand reclaiming plant 16 to be described hereinafter in more
detail with reference to Figures 3 to 8, where the sand is reclaimed and the
thus reclaimed sand is then supplied by a conveyor 17 to the moulding station
10 where binder is mixed with the reclaimed sanà and new cope and drag
parts of the mould made and transferred to the mould assembly station 11.
In the present example, the metal, which is a magnesium alloy, is
melted in a melting vessel 20 comprising a conventional lip action tilting type
fornace mounted for tilting movernent about a horizontal axis 21 coincident
with a pouring lip 22. The furnace is electrically heated by means of an
induction coil 23 and has a refractory lining 24 within an outer steel case 25
and an insulated lid 26. A ceramic launder 27, provided with an insulated lid
28 having electric radiant heating elements 29, extends from the lip 22 to a
casting vessel 3û. The casting vessel 30 comprises a holding furnace having a
lid 31 with further electric radiant heating elements 32 therein and has a
relatively large capacity, in the present example one ton.
The casting vessel is of generally rectangular configuration in plan view
but has a sloping half 33 (to maximise its area/volume) extending towards the
launder 27.
Interposed between the launder 27 and the filling spout 33 is a filter box
34 provided with a lid 35 having electric radiant heat elements 36. A weir 37
extends between side walls of the filter box 34 and has a bottom end 38
spaced above the bottom 39 of the filter box. A replaceable filter element
4û is positioned between the weir 37 and the downstream end wall 41 of the
filter box and is made of a suitable porous refrqctory material.
A pump 42 is positioned in relation to the casting vessel 30 so that an
inlet 43 of the pump wiil be immersed in molten metal wîthin the casting
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vessei and has q riser tube 44 which extends to a cqsting station so as to
permit uphill filling of a mould 45 thereat. The pump 42 has a stopper rod 46
to close the inlet 43 and an inert gas supply conduit 47. When it is desired to
pump metal from the casting vessel 30 into the mould 45, the stopper rod is
5 moved downwardly to close the inlet 43 and inert gas under pressure is
supplied through the conduit 47 to force metql within the pump body
upwardly through the riser tube 44 into the mould 45. The gas pressure is
maintained at qn appropriate level to ensure sqtisfactory filling of the mould
sufficiently long to ensure that the casting formed in the mould cavity is
IQ solidified but not so long as to cause solidifioation to extend down ingates.The gqs pressure is then released by venting the gas to atmospheric pressvre
and lifting the stopper rod 46 to open the inlet 43 thereby allowing the metal
within the pump 42 to attain the same level as the metal in the casting vessel
30.
The axis 21 about which the melting furnace is tilted is positioned so
that, in the present example, the top surfqce of the metql qs it leaves ~he
melting vessel is lûûmm qbove the minimum height to which it is intended
that the level of metql in the casting vessel 3û and hence in the Iqunder 27
should fall, so thqt the distance through which the metal falls freely is
2û limited to lOOmrn. Whilst a height of lûûmm is the distqnce in the present
example, if desired the distance may be such thqt during the pouring the level
of the ~op surfqce of the metql leqving the ~urnace is qt a maximum distance
of 2ûûmm qbove the qbove rnentioned minimum level. The above described
melting, transferring casting procedure provides a method which is capable of
high and continuous production cqpacity in which turbulence and its effects
ure substqntiqlly eliminqted qnd from which high quqlity castings are
consistently produced, this is because free fall of metql through the
atmosphere is minimised and pumping is performed in a quiescent manner.
Referring now to Figures 3 and 4, there is illustrated the reclaiming
3û apparatus 16 in which sand is reclaimed by being maintained in a treatment
ternperature range in the presence of a combustion supporting g~s.
The apparatus Iû comprises a series of connected fluid bed sections 51,
52 and 53, disposed in a horizont~l sequence, and a hopper 54 to which sand to
be reclaimed is fed.
The sand in the present example is fed from a shake or knock-out
station 14 of the magnesium alloy foundry casting plqnt at such q rate that
the sand is only slightly above ambient temperature. Alternatively the sqnd
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I I
may be fed in such a way, or stored in the hopper 54 for such a period, that it
is at ambient temperature on leaving the hopper 54.
However, the sqnd may be delivered, for example, from a ferrous
casting plant knock-out at such a rate and temperature and the reclaiming
5 apparatus 16 operated at such a rate that the sand leaving the hopper 54 is ata temperature in the treatment temperature range. Alternatively, operating
conditions may be such that the sand is at a temperature Iying between
ambient or substantially ambient temperature and the treatment temperature
range. The sand is reduced to grain size or substantially to grain size by the
I û shake-out or if necessary by an attrition unit, crushing unit or other means.
Sand to be reclaimed is fed from the hopper 54 to the first fluid bed
section 51 by means of a screw conveyor, air slide or other convenient
controlling device. The sand advantageously may be Fed into the fluid bed
section 51 helow the surface of the bed by ior example a screw conveyor.
The fluid bed section 51 contains a high density of electrical heaters 55
which heat the sand to a temperature Iying in the treatment temperature
range. The heaters 55 are preferably low voltage heaters operating at about
4û-5~ volts and comprising stainless steel strips through which low voltage
electric heating current is passed in direct contact with the sand. The
2û heaters 55 heat the sand as rapidly as possible so as to reduce smoke or other
fume emission and to gain as much energy from the burning resin as possible.
Sand is raised from slightly above ambient temperature to the treatment
temperature range very rapidly in a matter of a few seconds at most in the
present example.
Application of heat to the sand in the presence of combustion
supporting gas prior to heating in the fluid bed section 51 is avoided and
especially avoided are any steps involving feeding fluidising gas into contact
with the sand prior to the sand entering the fluid bed section 51. For
example there is no fluidisation of the sand at elevated temperture prior to
entry of the sand into the fluid bed section 51 thereby minimising evolution
of smoke and other fomes from the sand.
Qf course, when the sand is already at a temperature in the treatment
temperature range, no such heaters 55 are required and when the sand is
substantially above arnbient temperature, but below the treatment
temperature range, a lower density of heaters in the fluid bed 51 may be
3 provided. In such cases, it is preferred to take steps to minimise the time the
sand remains at said elevated temperature prior to entering the ~luid bed
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section 51 and to minimise contact of the hot sand with any high velocity gas
which will drive out smoke and other noxious pollutants from the hot sand.
The first fluid bed section 51 is separated from the further fluid bed
section 52 by means of a weir 56 and sand passes from the first to the second
sec~ion over the top of the weir. In the further fluid bed section 52 sand is
allowed to dwell in the treatment temperature range without further input of
heat, thus allowing combustion of the organic binder to proceed in the
fluidising air by utilising the heat in the sand from the first fluid bed section.
As to be discussed in more detail hereinafter, the cooling of the sand from
lû the fluidising air is measurable but not important in this section of theapparatus. If in any particular application supplementary heating is required
in this section, the requisite number of heating elements may be provided to
make up losses as neçessary. If desired, Q further fluid bed section or
sections may be provided at which the sand is maintained in said treatment
temperature range. The first and the or each further fluid bed section are
thermally insulated to prevent heat loss therefrom except that heat can
transfer between ~he first and further fluid bed sections via the weir 56.
The third fluidised bed section 53 is thermally insulated from the
previous sections by means of an insulating weir 57 and a baffle 58 and
contains cooling tubes 59 which are conveniently cooled by water but may be
cooled by other liquid or gas. The sand is cooled in this section to the desiredtempera1ure. For example, with an S02/furane resin binder system the sand
is cooled to a temperature in the range 30-35C which is an optimum
temperature for reuse with this sort of binder system.
In the present example, reclamation of the sand by combustion of the
binder is completed to the required standard in the further fluid bed section.
However, the method may be operated so that the reclamation is not so
completed in the further fluid bed section and reclamation ;s completed to
the required standarà in the cooling fluid bed section before the temperature
of the sand is reduced to below the treatment temperature range in which
reclamation occurs. In all cases however, reclamation is completed to the
required standard whilst the sand is fluidised. In the present example, the
sand is reclaimed to the extent that the residual binder content as measured
by "Loss on Ignition" lies in the range 0.05 to û.10% but the process may be
operated with more or less reclamation so Is)ng as adequate reclamation for
re-use of the sand is achieved and the term "reclaimed" is used herein to
refer to such an extent of reclamation. Typically, the minimum extent of
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reclamation for re-use is a residual binder content of 0.2% but we do not
want to be limited to this since, for example, some people vsing furane resin
binder accept a higher residual binder content whilst other people using
phenolic resin binder insist on a lower residual binder content than û.û5%.
Other binder systems require the sand to be cooled to other optimum
temperatures. For example, for the recoating of phenolic resins on shell
sands the temperatures may lie in the range 120~15ûC at the discharge from
the cooling or third section 53. In this latter case air cooling in at least some
of the cooling tubes may be desirable or the water cooling section reduced in
size for the number or water cooling tubes reduced.
The sand exits from the third fluidised bed section 53 via a discharge
tube 60.
Although in the present example the three fluid bed sections are
separated by weirs, if desired they may be separated by other means such as
separating plates having one or more apenings therein or may be completely
independent bed sections connected by ducts. Alternatively, the sections
may not be sepqrated by any physical barrier; each section being functionally
defined. For example the first section may comprise a part of a single fluid
bed having a high density of heater elements, the second section by a part of
the fluid bed having no or a lower density of heating elements than in the
first section and a cooling section, when provided, by a part of the bed which
may have cooling elements.
Hot dusty gas is exhausted via ducts 61, 62 and is passed through a dust
extraction system, not shown, prior to exhausting to atmosphere.
Fluidising gas, in the present example air, is provided to all three
sections through a plurality of discrete openings which are provided with
shield means to shield the openings from ingress of sand. The openings may
be provided for example in a plurality of tubes immersed in the sand and to
which fluidising gas is fed~ The fluidising gas may be provided by means of
30 sparge tubes or by means of a porous bottom to the sections for example a
gas permeable material, a foramirlous plate or mesh. It is preferred however
to introduce the gas through discreJe openings since this avoids problems
which can arise using porous material, a foraminous plate or mesh since these
can suffer from blockage of an area of the poroos material, plate or mesh
35 causing the bed to become non-fluidised above these regions leading to
locr lised overheoting.
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The above described apparatus is used to carry out a method of
rectaiming used foundry sand which comprises, in the present example,
feeding sand to be reclaimed into the hopper 54 and then from the hopper 54
into the first fluidised bed section 51.
The sand to be reclaimed is heated in the first fluidised bed section 51
to a treatment temperature range in which reclamation occurs. This range
may be 45ûC to 6ûOC. However, if desired the temperature may be
anywhere in the range 250C to 500C, preferably in the range 3ûûC to
below 4ûûC and more preferably in the range 400C to 500C. The sand in
the second fluid bed section 52 may be at the same temperature as in ~he
first fluid bed section or at a lower temperature but still within the above
mentioned treatment temperature range.
Since, in the present example~ the temperature of the sand does not
exceed 6ûOC the apparatus described with reference to Figures I and 2 may
be made, for example, of mitd stee1 as it does not have to withstand high
temperatures. Of course, the electrical heaters are required to be made of a
suitable high temperature material since these run at temperatures of up to
800C. The electrical heaters may comprise instead of the above described
low voltage heaters, conventional electric heaters operating at high voltage
alternating current, i.e. a heating element within a tube so as to be
electrically insulated from the sand. Electrical heating of the bed is useful
to achieve sirnple control and to ensure that all the available oxygen in the
air is available for burning the organic binder (and not the added fuel as is the
case for a fuel fired bed).
If desired other means of heating the bed may be used which avoid
corr~bustion of the fuel within the bed or other pressure in the bed of hot
products of combustion sufficient to maintain the sand in the treatment
temperature range. For example gas or oil or other fuel may be combusted in
a combustion chamber and the products of aombustion passed through smoke
;~ 30 tubes immersed in the fluidised bed. If desired the combustton chamber may
be immersed in the fluidised bed and may be with or without smoke tubes.
Experiments have been carried out to determine the range of
temperature and air flow rates over which the method embodying the
invention may be performed.
It has been found that if oir is used as the combustion supporting gas
and is forced through the sand to be reclaimed at a rate of at least 400
litres/min/metres2 (i/min/m23 and preferabty e~uat to, or in excess of, 2000
i3
` - 15 -
I/min/m2, then an adequate level of reclamation is achieved in a time scale
measured in minutes. For example, approximately 10 minutes at 400C and
three minutes at SOûC for furane polymer resin containing (Methyl Ethyl
Ketone Peroxide (MEKP) and hardened with sulphur dioxide gas.
Additionally, at a flow rate in the range approximately 2000-2400
I/min/m2 is found that most foundry sands start to fluidise and satisfactory
fluidisation is generally achieved over the range 25û0-8000 I/min/m .
Fluidisation can occur at lower flow rates depending on the grain size of the
sand. Thus the apparatus used is simple, efficient and compact being in the
form of a series of fluid beds through which the sand flows automatically
without requiring any conveyor means.
At a flow rate above about 3ûûO l/min/m2 cooling by the excess air
starts to be noticeable although does not become significantly deieterious
until the flow rate exceeds about 8ûOû l/min/m2.
Referring now to Figure 5, there are shown three curves for reclaiming
used sand at an original temperature of 5ûOC, 4ûOC and 3ûOC
respectively. The temperature rise in the sand to be reclaimed, as a result of
the exothermic reclaiming reaction, is plotted against the air flow rate in
litres per minute per square metre. The curves indicate that for all three
20 original temperatures an optimum performance is achieved in the region of
2000-3000 1/min/m2.
Referring now to Figures 6 to 8, there is shown the rates of reclamation
from sand having an original organic binder content of 0.75% as measured by
loss on ignition (Lol) for air flow rates 4ûO, 2ûO0 and 4000 1/min/m2
25 respectively.
Referring now to Figure 6, this shows the rates of reclamation at a
below optimum flow rate of air of only 400 1/min/m2. The time ternperature
transformation (TTT) curve for a residual organic binder content of û.2%
(measured by Lol) indicates a useful working level at which reclamation is
30 ade~uate for many processes, i.e. the sand is satisfactory for reuse. At thisflow rate a time of 6û rninutes is necessary to adequately reclaim sand at
400C. At the optimum rate of air flow ~2000 1/min/m2 shown in Figure S
the time required is thirteen minutes~ This time lengthens to eighteen
minutes as cooling becomes noticeable at ~OOû l/min/m2 as shown in Figure
35 8.
At 300C the times for the same residual binder content, i.e. 0.2% (Lol)
6;~
- 16
are thirty-three, thirteen and thirty-three hours respectively as can be seen
from Figures 6, 7 and 8.
In practice the temperature and flow rate for the practical application
of our method are chosen on the basis of TTT curves to achieve optimum
5 operating conditions for the application concerned.
If desired the sand may be reclaimed at a treatment station 16
comprising a single fluid bed at which the sand is fluidised at a temperature
in the treatment temperature range and heating elements may be provided at
only the entry end of the bed or throughout the length of the bed at a uniform
10 distribution or a non uniform distribution so as to cause the temperature in
the bed to decrease towards the exit end thereof. If desired, in any
particular application the sand may be fed from the treatment station at
elevated temperature without passing through a cooling fluidised bed section.
Compositions herein are expressed in % by weight.
IS The features disclosed in the foregoing description, or the foltowing
claims, or the accompanying drawings, expressed in their specific forms or in
terms of a means for performing the disclosed function, or a method or
process for attaining the disclosed result, or a class or group of substances orcompositions, as appropriate, may, separately or any combination of such
2û features, be utilised for realising the invention in diverse forms thereof.
