Note: Descriptions are shown in the official language in which they were submitted.
G.136
PROCESS FOR THE RECLAMATION OF FOUNDRY SANDS
This invention concerns the reclamation of foundry
sands.
Foundry sand is used with suitable binding agents for
making moulds and cores for casting metal. Various
binders, both inorganic and organic, are used to bond the
foundry sand and, for many years, the sand after use was
discarded.
In an attempt to reduce-the running costs of foundries
and to reduce environmental pollution, various processes
have been suggested in an attempt to enable the foundry to
re-use sand, preferahly many times over. Problems have
always arisen in that the amount of binder to be removed
from the used foundry sand to enable it to be re-used is
quite critical and, hitherto, it has only been possible to
use again some reclaimed sand together with some fresh sand
if satisfactory binding is to be achieved and suitable
moulds and cores made.
A paper entitled "Trends in Sand Reclamation" was
given at the British Cast Iron Research Association
International Conference in 1979, by Mr. D.A. Taylor and in
this paper reference is made to water washing and
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attrition, which can be carried out by various means
including crusher units, vibrators, pneumatic impellers and
shot blasting. For organic binders incineration has also
been proposed. In relation to silicate bonded sands, it is
suggested that only intensive attrition and/or water
washing can remove sufficient sodium oxide to allow re-use
of the sand.
British Patent Specification No. 1,322,864 is also
concerned with the problem of sand reclamation and
discloses a process in which the used ~oundry sand is
introduced into a blast of fluid under pressure, whereby
the sand is projected against lateral walls of a
hermetically sealed casing to cause a matrix of the binding
agent to burst releasing the sand. The product of this
process is then subjected to granulometric selection to
remove the fines generated by the high impact.
British Patent Specification No. 1,505,904 discloses a
process in which coated sand particles are projected in a
current of gas against the interior surface of a rotating
drum.
British Patent Specification No. 2,019,274 discloses a
similar process in which, again, the coated sand is
projected against a solid target.
O~her reclamation methods have been disclosed in, for
example, British Patent Specification No. 1,593,008, in
which used foundry sand is suspended in a fluidised bed and
subjected to jets of compressed air whilst in the ~luidised
state. Japanese Patent Specification No. 51109941,
published on 3 April 1978, concerns a similar method in
which fluidised sand is subjected to abrasion by stirrer
blades rotated at high speed inside the fluidised bed of
sand.
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The essential problem to be overcome in the sand
reclamation process is an adequate balancing of two
objectives, namely the effective removal of a hard and
tenacious coating and the avoidance of significant particle
degradation of the clean sand particles to produce fines
which are not useful in the reclaimed sand. The present
invention provides a process which effectively balances
these two objectives.
Experiments have shown that it is acceptable to have
up to a total of 0.8% by weight of soda on sand, including
the fresh silicate binder, when reclaimed sand is reused in
in foundry core work, without having to add fresh
uncontaminated sand to the core mixture. This is
equivalent to the removal of 65-70% of the soda coated onto
the sand in the the normal carbon dioxide or ester silicate
processes.
If less soda is removed a proportion of fresh sand
needs to be included to ensure the production of
satisfactory cores and moulds. Such re~oval can be
obtained when organic binders are removed from fou~dry sand
by attrition or incineration, but no satisfactory method
has hitherto been proposed for the removal of silicate
binder to this extent, without causing significant
disintegration of the sand, so producing useless fines
which merely contaminate ~he system.
Experiments involving increased fluid energy input
into a fluidised bed system have proved unsatisfactory in
that the additional energy destabilises the fluidised bed
and the higher energy sand merely erodes the surface of the
3 container carrying the fluidised bed.
Accordingly, the present invention provides a process
for the reclamation of binder coated foundry sand, in which
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the coated foundry sand is fed to a fluid energy mill and
fluid ener~y is imparted to the sand to a sufficient extent
to substantially remove the binder coating without causing
significant disintegration of the sand particles, and the
5 reclaimed sand is separated from the binder debris.
In a preferred form of the process the specific energy
imparted to the sand is less than 1,300 kilojoules per
kilograMme of soated sand. In the equipment used for this
work, the optimal energy range was from 400 to 1,200
kilojoules per kilogramme of sand. This work was carried
out in a microniser having a diameter of 10 cm and
containing 6 1.6 mm diameter inlet nozzles in the periphery
of the chamber. The feed rate of the sand varied from
0.33 to 1.54 g per second and the optimum feed rate was of
the order of 1 g per second for this apparatus.
Using a commercial production size of fluid energy
mill - or microniser the sand feed rate can be increased in
relation to the size of the mill and air throughput. The
specific energy imparted to the sand should be of the same
order of magnitude as previously defined but applied at a
commercially useful sand feed rate.
Fluid energy or jet mills (alternatively known
as micronisers) suitable for use in the process of this
invention are available from various suppliers in the
market and are described in, for example,
Chemical Engineering Handbook, Ed. J.H. Perry, 4th ed.,
McGraw Hillr p.8-42 and 43.
The fluid energy imparted to the sand in the mill is
determined by calculating the energy expended by the
adiabatic expansion of the compressed fluid through the
nozzles, and is given by:
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E ~ x W x R x T [~ ~ ~ -1 ]
where E ~ Energy due to expansion (kw)
R = Universal Gas Constant (8.314 kj/k mol k~
T = Absolute Temperature (K~
W = molal flowrate (k mol/s)
Pl = Absolute air pressure applied to nozzles
(N/m ) 2
P0 = Absolute air pressure after expansion (N/m )
assumed to be atmospheric pressure
~ = ratio of specific heats (Cp/CR).
The specific energy input is obtained by dividing by
the feedrate of sand (kg/s), i.e~
Es = - (kj/kg)
Ms
It'will be appreciated that the imparted energy
figures set out above rela~e to a specific piece of
apparatus, but it is thought that by simple experiment the
work can readily be applied to other types of fluid energy
mill. To assist in this generalisation and to relate the
forces involved to the fluid input stream, Reynolds Number
calculations have been carried out.
Reynolds Number is the ratio of inertial force to the
viscou's force of the flowing fluid. It is a measure of
the air fluid velocity in the jet stream which determines
the drag force exerted on the individual particles and thus
determines their acceleration and their impact'velocity.
Reynolds Number can be ca'lculated on the basis of the
fluid mass flowrate through a single nozzle (Qf) of
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nozzle diameter, dn,
4 Qf
Re ~rdn/U /u = air viscosity
In general, it is preferred that the Reynolds Number
does not exceed 6.5 x 104 and is preferably in the range
4O0 x 104 to 6.4 x 104.
Using the 10 cm diameter microniser referred to
earlier, a sample of foundry sand which had previously been
used in a silicate bonded core was treated in accordance
with the invention and the results are set out below.
Sieve Analysis of Qriyinal Fresh Uncoated Sand.
: ~ 150 /um 3.0
150 - 180 /um 10.0
180 - 250 jum 43.8
: 15250 - 355 /um 30.8
355 - 425 /um 10.5
~ 425 /um 1.9
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EXAMP.l EXAMP.2 EXAMP.3 EXPT. 1
Sand Feedrate (g/s) 1.54 1.0 1.0 1.0
Injection Air
Pressure (KN/m ) 310 228 310 434
Grinding Air
Pressure (KN/m ) 379 241 379 517
Specific Energy Input,
Es (Kj/Kg) 434 421 1000 1783
Sieve Analysis
C150 /um 11.94 6.96 14.27 35O62
150~180 /um 9.78 11.75 llr 639~ 82
180-250 /um 41.92 47.68 42.93 32.71
250-355 /um 27.46 27.15 24.47 17.50
355-425 /um 7.95 5.82 5.97 4.00
~ 425 /um 0-95 0.64 0.73 0.34
% Na20 on sand 0.287 0.355 0.2750.178
~ 150 /um (original
sand plus silicate
binder 0.675%)
20 % Na20 Removal 64.7 53.3 66.7 82.8
Renolds Number 5.3X10 3.91X104 5.73X10 7-89X104
It can be seen from the Examples that in the preferred
range of specific energy and Reynolds number mode of
operation of the process of this invention, that more than
60% of the soda on used sand can be removed whilst fines
are less than 15%.
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If higher energies are used, although even more soda
is removed, unacceptably large quantities of fines are
produced - see Exp. 1.
As mentioned earlier, if sufficient soda is removed,
sand can be reused without any addition of fresh sand, so
giving a valuable benefit to the user.
