Note: Descriptions are shown in the official language in which they were submitted.
2U46~08
This invention relates to a process for the
regeneration of used foundry sand whose original molding
sand contains as molding material inorganic bonding agents
such as bentonite (= "inorganic used sand") and/or organic
bonding agents such as, for example, phenolic and/or furan
resins (= "organic used sand").
In inorganically bonded molding sands the bonding
agent bentonite or a similar material is fixed in shell-
form at the surface of the sand grains by oolithization,
depending on the degree of heat treatment during casting,
while molding sands with organic bonding agents suffer
thermal decomposition during casting and thus acquire at
the grain surface firmly adhering residues of carbon-rich
derivatives from the organic bonding agents. In addition,
the used sand is further contaminated because of additional
additives, such as the formation of lustrous carbons from
the additives.
While used (foundry) sand was formerly at least
predominantly simply disposed of, acute scarcity of
available deposit sites and also cost considerations today
have led to a demand for the regeneration, i.e. cleaning,
of used sand, so that it may be re-used. However, this
requires that the above-mentioned bonding agent shells and
other contaminations of the used sand be separated from the
quartz bodies and then eliminated.
A known method of regenerating used foundry sand
which consists at least predominately of organic used sand
is to treat it thermally by annealing at a temperature of
ca. 800 degrees C and/or possibly pneumatically. In this
process the bonding agent shells and other contaminations
of the used sand are generally so thoroughly removed that
the used sand can be reused as molding sand. On the other
hand, this thermal cleaning process does not give rise to
satisfactory results with inorganic used sands.
The thermo-mechanical process known from German
Patent No. 31 03 030 has useful results only if the
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proportion of inorganic sand is relatively small.
Consequently, this process does not solve the above
problem, since used foundry sand generally consists of more
than two thirds of inorganic used sand.
In view of these conditions, German Patent No. 38
15 877 has proposed (e.g. according to German Patent No. 31
03 030) that the thermo-mechanically pre-treated or treated
regenerate be subsequently treated ultrasonically in water
(as a coupling layer). In this way the bonding agent
shells, which are sintered onto the sand grains, are caused
to split off and the degree of oolithization is lowered to
normal values, while simultaneously the basic pH value of
the regenerate is adjusted to a near neutral value.
Quite apart from the complex and costly equipment
needed for implementing the above process, as well as the
high operating costs, further costs arise because the
cleaned sand still has to be dried, which makes this multi-
step process very uneconomical. In addition, it should be
noted that, due to a relatively high wear factor for the
sand grains, only a relatively low regenerate yield of
sufficient quality can be achieved.
For the regeneration of used inorganic sand
processes have been developed in which it was attempted to
remove the bonding agent shells by a wet process from the
surface of the sand grains. This was accomplished by means
of mechanical stirring in a watery sand suspension
resulting in intensive friction among the sand grains.
This process step, which is also called attrition, is
generally repeated several times and possibly affected or
intensified by creating special sand-water-mix conditions
(see e.g. U.S. Patent No. 27 83 511 or German OLS 30 19
096).
One disadvantage of this wet process is the fact
that hydrocarbon compounds of organic used sand portions,
as well as lustrous carbons and their generating agents,
cannot be sufficiently removed. These materials act
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practically as lubricants (as, e.g. argillaceous
components) and prevent the desired abrasion, so that
sufficiently satisfactory regenerating results cannot be
obtained even with multi-step attrition.
In order to solve this problem, Australian Patent
No. 387 921 suggested conducting a sludge removal (by means
of classifiers) after each attrition step, so that the
"lubricants" are thus removed from the circulating water
and the desired abrasion is intensified. However, even if
a satisfactory result could be obtained from a technical
point of view with three-, four- or manifold attrition,
this result would be considerably burdened with the
attendant costs. In addition, in this process an adherent
structure of finely distributed, porous silicic acid
remains on the grain surface, which necessitates increased
amounts of bonding agents when the cleaned used sand is
reused as new foundry sand.
It is an object of the present invention, while
avoiding the above-mentioned and other disadvantages, to
create a process for the cleaning of used foundry sand
which is satisfactory both from a technical and an
economical point of view, in which process (as compared
with the state of the art) a relatively low technical and
financial effort produces a regenerated material which is
extensively and generally usable and which contains
relatively few harmful components, and can be reused
without problems in place of new sand for mold and core
production in foundries.
Accordingly, the present invention provides a
process for the regeneration of used organic or inorganic
foundry sand formed from original molding sand which
contained, as the molding material, inorganic binders in
the case of inorganic used sand or organic binders in the
case of organic used sand, said binders being fixed in a
shell form at the surface of the sand grains of said used
sand, said process comprising the steps of:
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2 ~ 8
a) impregnating the used sand to be reconditioned
with water to such an extent to substantially fill the
pores of the grain shells of the sand with water;
b) providing a heated fluidized bed consisting of
sand;
c) maintaining said fluidized bed at a steady
temperature of about 750 to 950~C;
d) feeding said used sand impregnated with water
into the heated fluidized bed; and
e) discharging the sand from said fluidized bed.
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Thus, the used sand to-be-cleaned is (to begin
with) soaked in water and then placed in its wet condition
into the heated fluidized bed, where the shock-heating of
the used sand causes a spontaneous evaporation of the water
located in the pores thereof. The resulting considerable
increase in volume causes the shells (at least) of the
(inorganic) bonding agent to split off or at least to be
loosened to an extent that they can be directly separated
from the quartz grains in the fluidized bed. the
traditional drying of wet-regenerated used sand by hot air
at temperatures of ca. 150-300 degrees C in various devices
does not achieve such a splitting-off or loosening effect,
as is proven by the above-mentioned bonding agent
requirements.
The heat removal by water evaporation in the
immediate proximity of the quartz grains also prevents
possible changes of crystal modification and/or grain
decomposition, due to the shock-like heating of the quartz
grains. In addition, the sintering onto the grain surface
of mullite, which has been created from bentonite due to
the effect of heat, is prevented or at least inhibited.
Furthermore, it is not necessary in the process
of the invention that the used sand, which may consist of
any mixture of organically and inorganically bonded used
sands, must during wetting or soaking pass through an
attrition step which is otherwise required for wet
regeneration. The simplified operation and the less costly
equipment result in considerable economic advantages.
Consequently, it is not at all necessary in the
process of the invention (even though it may be practical
in certain cases) to assure a basic separation of organic
and inorganic used sand in the preparatory stages of the
cleaning, while it is indeed practical to clean the organic
and inorganic used sand essentially separately from one
another if they are not already separate, as this is often
the case.
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Furthermore, it is advantageous that practically
no harmful residues occur, that in accordance with the
above explanations it is not necessary to maintain a
certain proportion of different used sands for cleaning,
and that quite clearly no separate process branches are
required and have to be adjusted to one another, which, as
is well known, can generally cause considerable
difficulties especially in a hot process.
As a rule it has proved to be highly practical
to soak the used sand so thoroughly with water (while the
water may be given additives for lowering surface tension)
that the pores located between the grain shells are at
least essentially filled with water in order to optimize
the above-mentioned effect and, in the final analysis, to
optimize the entire used sand cleaning process and the
quality of the regenerate.
In a preferred embodiment of the invention the
soaking of the used sand is carried out with circulated
water in order to eliminate harmful components, especially
alkalis. This operating method is practical in cases where
the used sand contains, as a result of the use of alkali-
silicates or phenolates as bonding components, significant
amounts of such ions. If these were allowed to remain in
the sand regenerate, the resulting high basicity would
significantly limit reusability.
If necessary, the water used for soaking the used
sand may be kept at a pH value between 2 and 5 by
continuous addition of acid and is kept in circulation
until a maximum salt concentration (to be determined in
each case) has been reached. Furthermore, the exchange
with fresh water may be carried out continuously. Before
each entry into the soaking device the circulating water
passes through a settling tank and, if necessary, a filter,
where the solids absorbed by the water from the sand in the
form of sludge can be separated and the sludge can be
returned to the fluidized bed. The shock-like heating of
20~6508
these solids, which are continuously soaked with water,
also prevents a sintering onto the sand grains, and their
organic components burn out.
In another embodiment of this process variant,
carbon dioxide is used as an anionic component of the
decomposition of the alkali-silicates instead of mineral or
carboxylic acid. Here the circulating water acquires a
basic pH value; however, the alkalis are removed from the
used sand just as well, namely in the form of carbonates.
The alkaline solution which still adheres to the used sand
is rinsed from the grains by spraying on the drainage
screen with the fresh water required as replacement. The
resulting soda or potash solution can be evaporated with
exhaust or waste heat, crystallized and the alkali can thus
be recovered therefrom in a reusable form.
However, the process is extremely satisfactory
not only from a technical, but also from an economical
point of view, especially since the heated fluidized bed
itself can consist of sand (even of already cleaned sand,
if necessary).
In order to have available a fluidized bed with
sufficient heating capacity to obtain the desired results,
the ratio of the fluidized bed sand quantity to the amount
of wet used sand which is added per minute should be in the
range of about 50 to 100.
In accordance with the invention the fluidized
bed is most preferably heated from the top, preferably with
high-speed burners, for which gaseous fuels have proven to
be especially practical as additional fuels (beyond fuel
already contained in the used sand).
In order to obtain the above-explained effects,
the fluidized bed is preferably maintained at a stable
temperature in the range of about 750 to 950 degrees C.
According to a further preferred embodiment of
the invention, the temperature in the solid material, on
the one hand, and in the gas chamber above the fluidized
-- 20~6508
bed, on the other, can be variably adjusted, as can the
dwell time of the used sand in the fluidized bed, i.e.
depending on its composition.
In this manner complete burn-out of organic
substances even in flying dust, and thus is
decontamination, is achieved.
In order to intensify and/or accelerate the
wetting of the used sand it can be advantageous if the used
sand which is to be wetted and cleaned is wetted in a
vacuum.
It is also quite practical as a rule if the
propelling speed or velocity of the fluidized bed is
adjustable over a wide range, so that the operating
conditions can be optimally suited to the respective
requirements of the used sand.
Furthermore, it can be practical to separate sand
in a separator or similar device downstream of the
fluidized bed, in which case it may be further practical to
return this separated sand partially to the fluidized bed.
The used sand (regenerate) cleaned in this manner
can be subjected to subsequent mechanical cleaning if
desired.
A preferred embodiment of the invention will now
be described with reference to the accompanying drawing
which illustrates diagrammatically an apparatus for
carrying out the process for regenerating used foundry
sand.
The drawing shows as a diagrammatic
representation a bin 1, in which the used sand (which is
routinely a mixture of inorganic and organic used sand) is
collected. The used sand to-be-cleaned is moved from the
bin 1 into used sand pre-treatment equipment 2, consisting
of for example means for magnetic separation of Fe
particles, a lump crusher and a sifter, and is then moved
into an intermediate bin 3.
2o~6so8
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From the intermediate bin 3 the pre-treated used
sand is filled into a mixer 5 by means of a dosing device
4. Into this mixer 5 are added dosed water from a dosing
device 6 and materials for the reduction of surface
tension. The water is heated to a maximum of 95 degrees C
with exhaust or waste heat.
The wet sand is rotated in a conditioner 7 until
complete wetting of the pores of the grain shells is
attained and then reaches a fluidized bed oven 8 in dosed
form.
As a result of the extremely fast heat transfer
(and the good mixing in the fluidized bed) the wet used
sand is heated abruptly to an operating temperature of 800
to 850 degrees C, while the above-described formation of
steam splits the fire-clay shells off the (quartz) sand
grains, or at least loosens them considerably, and harmful
organic materials burn off completely. The separated
shells are trapped in a separator 9.
The oven exhaust gas is cooled in a heat
exchanger 11 by preheating the fluidizing air and then
passes through a filter 10.
To cool the oven discharge the latter first
passes through a fluidizing bed cooler 12, in which the
water needed for soaking is pre-heated to 70 to 95 degrees
C and the perceptible heat of the sand is recovered in the
form of steam or hot water.
A concluding treatment of the resulting
regenerate occurs in a pneumatic cleaning device 13 and by
fractionation into (at least) two grain sizes ("coarse" and
"fine"). The cleaned used sand is universally reusable,
while the granulating of the new sand which is to be formed
from the cleaned used sand (if necessary by adding certain
amounts of new sand) can be achieved by dosing from the
various fractionations in order to obtain used sand with
pre-determined average granulation or a certain grain bond.
After cooling and pneumatic cleaning of the
2Q~650~
treated sand, regenerate and filter dust are obtained. The
filter dust is free of harmful substances and may be
disposed of or used as construction material or additive
without any problem.
