Method and apparatus for reclaiming foundry sand

A method and apparatus for reclaiming foundry sand from contaminated foundry sand wherein ferromagnetic contaminants are first removed in a magnetic separating zone, and the remaining contaminated sand is thermally treated at a temperature sufficiently high to cause embrittlement of clay constituents among the remaining contaminants, and finally the thermally treated material is comminuted, preferably in a counterflow impact jet mill to recover a reclaimed foundry sand substantially free from contaminants.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention is in the field of reclaiming foundry sand for reuse after 
contamination by metals, binders, and the like. The invention is concerned 
with treating the contaminated foundry sand sequentially in a magnetic 
separating zone, a thermal treatment zone, and a comminution zone from 
which the reclaimed sand particles are recovered. 
2. Description of the Prior Art 
Used foundry sand represents a mixture of more or less large agglomerates 
consisting of silica sand grains, dust, binding agents, and metallic 
particles which may remain from the casting process. The binding agents 
usually exist in loose form in admixture with the sand, but they may also 
be partially adhered to the silica sand grains. Binding materials such as 
bentonite clay, synthetic resins based on phenols or resols, water glass, 
and the like, are frequently present as binding agents. 
The substantial costs involved in the preparation of sand for foundry use 
as well as the transporting and disposition of used sand, particularly in 
view of the requirements of keeping ground water clean, make a reclaiming 
of used foundry sand economically desirable. The reuse of the old sand, 
however, must be accompanied by processing which is directed toward the 
removal of contaminants from the sand which would interfere with the use 
of the sand in a mold making process. 
For example, iron particles or other ferromagnetic particles tend to sinter 
together during the casting process with the silica sand grains and must 
therefore be removed completely along with fine grain components and 
binding agents. In addition, a cleaning device which is used to break up 
the sand agglomerates from the binding agent must be cleanly removed from 
the silica grains without, at the same time, causing excessive fracturing 
of the grains which would provide an undesired amount of fine grains. 
The grinding of silica sand generally involves a significant wear on the 
grinding tools, particularly impact pulverizers and vibration grinding 
mills so that the economic feasibility of such recovery methods is 
impaired. Selection of a suitable cleaning process is also rendered 
difficult by the fact that the wear during the grinding process also 
produces unavoidable iron fines which are harmful in the casting process, 
and which have to be removed by a further step. It would be possible to 
use vibration grinding mills which utilize iron-free linings as well as 
appropriate grinding bodies, but this involves relatively high capital 
costs which limit the economic feasibility of the reclaiming process. 
SUMMARY OF THE INVENTION 
The present invention provides a method for recovering foundry sand from 
contaminated foundry sand in an economical manner while avoiding the 
difficulties mentioned previously. The reclaiming of the foundry sand is 
achieved by passing the same in series through at least one magnetic 
separating zone, one thermal treatment zone, and one cleaning zone. The 
magnetic separating zone serves to separate any ferromagnetic components 
from the contaminated foundry sand, and the thermal treatment zone serves 
to provide for the combustion of carbon, and the chemical conversion of 
binding agent components. For example, in the use of bentonite clay, this 
material is converted by means of the thermal treatment into mullite by 
dehydration and the loss of structural water. The mullite is considerably 
more brittle and lends itself to a nearly complete stripping from the 
surface of the silica sand so that the product of the cleaning zone is a 
mixture of clean silica sand grains free of binding agent incrustations as 
well as binding agent residues from which the silica sand grains can be 
recovered through known methods. 
In a preferred embodiment of the invention, the contaminated foundary sand 
is cooled after passing through the thermal treatment zone and before 
introduction into the cleaning zone. The cooling serves the purpose of 
thermally relieving the succeeding cleaning process and the heat retained 
by the sand from the thermal treatment zone can be partially regained and 
reused in any desired manner. 
In a particularly preferred embodiment of the invention, the cleaning of 
the used foundry sand is carried out in the cleaning zone by means of a 
counterflow jet impact process utilizing air velocities preferably in the 
range from 30 m/sec to 50 m/sec. This method of cleaning results in very 
little wear on the cleaning aggregate, so that by selecting appropriate 
air velocities, the cleaning can be carried out in such a manner that the 
silica sand grains are cleansed of adhering binding agent residues but are 
not themselves fractured.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to the drawing, used foundry sand is delivered as indicated by 
the arrow 1 into a multi-chamber storage bin 2 and is transported 
therefrom by means of a weighing type conveyor 3 into a magnetic separator 
4. In the illustrated embodiment of the invention, the magnetic separator 
4 takes the form of a weak field drum magnetic separator. The magnetizable 
portions of the used sand are separated out into a container 6 through a 
discharge line 5. 
The non-magnetic portions of the sand are subsequently delivered from the 
magnetic separator 4 through a surge bin 7 to a furnace by means of a tube 
conveyor 8 for the thermal treatment of the remaining foundry sand. 
The furnace 9 is preferably a fluidized bed furnace consisting of a 
cylindrical, vertically disposed lined combustion chamber into the lower 
portion of which hot gases are introduced as illustrated by the arrows 10. 
The hot gases are received from a hot gas source (not illustrated) and are 
introduced into the fluidized bed furnace 9 in such an amount and at such 
a temperature that temperatures of at least 870.degree. C. exist in the 
combustion zone of the furnace 9. The conditions are such that a fluidized 
bed consisting of an incandescent bed of sand is formed. A very intensive 
admixture with hot gases occurs in this fluidized bed in which the used 
foundry sand remains for a period up to 45 minutes. During the thermal 
treatment, the carbon present burns off and the quartz present is 
transformed into tridymite which is a vitreous form of pure silica. This 
transformation occurs with about a 16% increase in volume at a temperature 
of 870.degree. C. 
The thermal treatment in the fluidized bed furnace 9 also serves to 
separate the combustible components of the mixture, including carbonaceous 
materials. Any bentonite component is dehydrated with the loss of 
structural water to form a phase of mullite which is considerably more 
brittle than the original bentonite. At the same time, a fine grain 
component is separated out and leaves the fluidized bed furnace 9 as shown 
by the arrow 11 together with the hot exhaust gases. This fine grain 
component is separated out in a dust filter 12 and as shown by the arrow 
13 is delivered to a mixer 14. An exhaust gas blower 15 delivers the 
dust-free exhaust gases to a chimney 16. 
The thermally treated sand is directed from the fluidized bed furnace 9 to 
a cooling stage consisting of a pair of coolers 17 and 18 connected in 
series. After the last cooling stage 18, a tube conveyor 19 delivers the 
cooled sand into an air jet mill, preferably a counterflow impact jet mill 
20. At the output of the cooler 17, the sand has a temperature of 
approximately 250.degree. C. while when leaving the cooler 18, it has a 
temperature of approximately 25.degree. to 30.degree. C. 
The grinding zone of the impact mill 20 consists of two injectors disposed 
opposite one another and operated with compressed air jets which provide 
the material to be cleaned at velocities of approximately 30 m/sec to 50 
m/sec. The compressed air jets emerging from the injectors and carrying 
the solids collide with one another frontally, and the sand particles are 
cleaned solely by the collision of the particles against one another. The 
counterflow impact mill 20 thus serves the purpose of stripping solid 
crusts consisting of residues of binding agent from the surfaces of the 
silica sand without the silica sand grains themselves being comminuted. 
The discharge product of the impact mill 20 is thus a mixture consisting 
of silica sand grains which represent the coarse grain component and the 
fractured crust particles which are the fine-grained component. These two 
components are separated in a sifter 21. The coarse grained fraction is 
recovered through a line 22 and recycled to the foundry. By changing the 
operating variables in the sifter operation, different sand qualities and 
different grain categories can be produced in a simple manner. 
The fine-grained fraction of the sifter 21 is delivered by means of a line 
23 into a dust filter 24 where the remaining exhaust gas is discharged by 
means of an exhaust gas blower 25 into the chimney 16. 
The fine-grained fraction separated out in the dust filter 24 as well as 
the fine-grained fraction from the dust filter 12 are subsequently 
combined in a mixer 14 by the addition of water or a foundry waste slurry 
through a line 26 into a storable, non-dust-producing waste product which 
is delivered by means of a line 27 into a container 28. 
The three fractions produced according to the method of the present 
invention consist of magnetizable components collected in a container 6, 
fine-grained dust components constituting a second waste product collected 
in a container 28 and the reusable foundry sand delivered through the 
discharge line 22. 
It should be evident that various modifications can be made to the 
described embodiments without departing from the scope of the present 
invention.