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Патент USA US2404215

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Patented July 16, 1946
2,404,215
UNITED STATES PATENT OFFICE.l
sLUaRY FILTRATION
Robert Morris Cavanaugh, Woodbury, N. JY., as
signor to E. I. du Pont de Nemours & Company,
Wilmington, Del., a corporation of Delaware
Application October 2, 1943, Serial No. 504,711
1
6 Claims.
'
(ci. 21o-62)
2
This invention relates to a new and improved
tity of cake isA obtained. At this point suction
process of filtration, and more particularly to the
is discontinued and Water is sprayed into the top
filtration of slurries having particles of diverse
of the filter. Suction is applied again; `causing
the water to displace the acid in the interstices
The essential element in filtration is the filter
between the grains `of said cake andl leaving a
ing layer which brings about the isolation of the
small layer-of water thereon. This displacement
solid constituents from the liquid in a mixture.
wash may be repeated until the cake has been
The rate of filtration for any given area of said
reduced to the desired acidity- More Water in
layer depends, for the most part, on the extent
troduced into the top of the `ñlter and the .cake
to which the interstices thereof are kept free 10 is broken up by passing Water or water and air
from interfering matter. Forexample, in íilter
up through the bottom of the screen.. Mechani
ing a material having particles of different size
cal agitation is started and the cyclotrimethylene
there is a tendency for the smaller particles to
trinitramine is re-slurried and discharged for
size.
-
Y
-
settle and choke the filter. This renders por
tions of the filter impervious and thus materially
reduces the efticiency thereof. Due to this serious
drawback, conventional methods of filtering ma-terials of the type under discussion have ren
dered the operation tremendously slow, diiiicult
and inadequate.
An object of this invention is a process for
separating solid material of various sizes from
another medium.
A further object is a process
further processing.
'
c
In order to disclose _my invention ~more fully,
, Ythe following detailed description of one embodi
20
ment thereof is given in connection with the _ac
companying drawing, forming a part v0f this Spec
ification and, serving as a convenient means ,of
describing the principles `of my vnovel and im
proved process, with the understanding, however,
that the invention _is not to be .Confined to any
strict conformity with the showing of the draw
for separating particles of non-uniform size from
ing but may be changed or modified, 'provided
a liquid. A further object is a process for the 25 such alterations _mark no material departure from
isolation of the solid constituents from the liquid
the essential features of the invention, as ex
in a mixture, said .solid matter comprising par
presse'd in the appended patent claims.
ticles of at least two sizes. A still further object
Figure 1 is a cross section view of a filter which
is to provide such a process by means of which
may be employed in practicing this invention.
filtration may be carried out quickly and Without 30 Figure 2 shows the vtop .of the container and vthe
difliculty. Another object is a process for the
filter element broken away' in parts. .
filtration of slurries having different calibre
Referring in greater detail to the drawing, vthe
grains, said process being characterized by a ma
terial increase in the rate of filtration per unit
area of filtering surface without sacrifice of other
features. Other objects and advantages will ap
pear from the description hereinafter given.
In general, the foregoing disadvantages are
obviated and the objects obtained according to
this invention by introducing a liquid having
solid particles of different size therein, such as,
for example, an acid slurry of cyclotrimethylene
trinitramine, into a filtering vessel. Mechanical
and air agitation is applied, the air being intro-`
` two primary parts of the filter I comprise an
upper container A and a lower ïcontainer B,
lianged 2 at their point of union and ñtted with
a ö-'mesh screen 3, a 50G-mesh Screen 4, Vfollowed
by _another v6-rnesh screen 5, said Yscreens being
suitably reinforced on both sides >by means of
gratings 6 and 1. The screens lextend beyond
the inside diameter of the filter and 'between ¿the
iianges a sufficient distance to allow proper ,grip
ping therebetween. Gaskets surround 'the layers
of screen ‘between the flanges in order to prevent
pinching of the screen and seepage of Lslurry
duced from beneath the ñlter screens and serving 45 from the filter. Within the filter the „agitator
both to agitate the slurry and to maintain maxi
8 of the >fiat paddle type having a 45"i updraßft
mum screen porosity by forcing most of the small
pitch and located above and adjacentto'the up
grains, which otherwise would settle in the filter
per lgrating 6. Baiiìes '9 Within the iilteräpi‘QmOlîe
meshes and obstruct the passage of liquid -there
.turbulence
agitation. A spray system lll,
through, toward the surface of the slurry. As v50 including aduring
pipe having holes therein bored vat
soon as the slurry has been agitated sufliciently,
45° angles, is provided >for the introduction of
it is allowed to settle. Suction is applied and
water supplied by conduit Il, the flow lof water ,
the acid is drawn off, leaving a slight layer of acid
therethrough being regulated by Ameans of 'yaljve
on the resulting cake for safety purposes. This
procedure may be repeated until the desired quan 55 i2. `»Inlet I3 is for theintroductiongof theslurry
to be filtered, the `slurry iiow being Voo,iltrolllrßti 'by
A
4
- 3
tained. The spent acid thus recovered varies in
strength from 45 to '15% depending on the start
ïmeans of valve I4. In the top of the fllteris a
door or manhole I5. Near the bottom and above
the upper grating 6 is the outlet I6 and valve I1
for discharging the refined slurry. Beneath the
lower grating 1 is a vent I8 and valve I9 for4 per
`mitting air to pass Vinto the lower compartment
ing acid strength and the particular process
used previous to the iiltration stage.
The cake of crystalline material wet with spent
acid, some of which has been left in the cake
as a safety measure, remains on the ñlter.
ì of the ñlter toïaid in exiting the filtrate there
To
remove this, water is passed through the cake.
" from. ¿There -is also a fume Ventilating duct 20,
Thus valves 28, 29 and 30 are opened causing
water and» air to pass through screen 35 into the
33y II and the spray
cating withthe ñlter are appropriately valvedV n iilter by way of conduits
particles which may pass
system
Ill.
Hence,
solid
, service lines 2|, 22 and 23, through which'waten'.r '
through' the» main'screen because of leakage
ï air and suction, respectively, are applied. . LineV
l which is connected to the acid'recovery system,
i in the top of the ñlter.
Beneath and communi
_therethrough are sent back to the filter, thereby
ï 23 has a cleanup filter 34 provided with a A50G-f
. minimizingA waste and the hazard involved in
‘ mesh screen 35. `This line- is connected also to
l inlet'II by means of conduit 33._ Thetcleanup 1 y reconcentrating contaminated acid. When the
proper> arrn'iuntv of water has been introduced,
l ñlter is serviced from above by an air conduit e
theseY »valves are closed. Valves 25 and 21 are
3| and water conduit 32. The purpose of'this v
opened and the wash water ñltered through the
l ñlter is to prevent loss in the event of a break in,
ì or passage of solid particles through the main 20 cake. Valve I9 is opened when one inch of wash
water‘remains on the cake and I9, 25 and 21
are closed when airpasses into the receiver tank.
)T_he' process'> comprising a preferred embodi
This receiver is vented after turning 01TV the
à‘ment _of this invention may be described as fol
vacuum jet and the ñltrate from the first wash
Y .y An‘acidslurry of cyclotrirnethylenetrinitramine,- es water pumped, along with the spent acid Yfrom
the previous filtration, to the acid storage for
l commonly _known as cyclonite, obtained from
‘
screens.
`
nitration, the grains of said solid material being
reconcentration.
,
acid is displaced by water may be 'performed as
through conduit I3, the now therethrough being `
y I regulated by means of valve I4. After the iiltery
’
The portion pf the process Vin which the cake Y
, _of varoiusvsizes, is introduced into the filter I
eo
has_been_ñlled,_ valve I4 is closed, compressed air
i vvalve 24- is opened and rotation of the mechani
g cal agitator 8 is started with a suitable _driving
many times asdesirable; however, with the ma- `
terial under discussion it usually is carried out
three times. The second andthird displacement
washes are accomplished in the same manner as
Ythe first, except the wash water is‘introduced,
t means.r The opening of the latter valve causes4
air to' passl up through the bottom of the screens 35 with valves 25 and 21 open, into the ñlter
through conduit II by opening valve I2. In the
into the -slurry during the rotation of agitator 8.
second wash, valve I2 is opened also when one '
>`'I‘he'p‘ur-pose of this air current is twoffold, the
chief', one being to remove the small grains of
cyclonite Afrom the screens, a secondary purpose
'being' to 'assist in agitatingthe slurry. The op
3 L_timumvaircurrent is` one otjustsufûcient force
inch of liquid remains on the cake.
,
Ordinarily the volume of water used for each
Wash is-approximately the volume lof the cake.
lioV >There
is an important»,difference‘between the
¿
acid filtration and the wash water ñltrations in
. to, keepthe` small particles,l in suspension and
that'in the latter operations the cake is Washed
by displacement of the acid with water rather
than
reslurrying Vwith water and reñltering.
115
rate >of ñltr'a'tion„other things being equal, is
Thus the Washing is accomplished with the cake
' made possible. In the absence of such. an air
undisturbed so that substantially all the par
current, practically the opposite result would be
ticles thereof remain in the position in which
obtained. That is, the small grains would settle
they voriginally settled. The eiiluent from the
in the'screen'meshes and impede the passage of
awayiromïthe screen. In this manner, maxi
"mum >screen 'porosityand therefore a maximumV
liquid therethrough, even though the ñltration
50 first wash may be transferred to the weak acid
storagewhere it awaits concentration so it can
were carried out under a high suction, thereby
beused again in the process; Although the ef
ñuent from the succeeding washes ordinarily is
too weak in acid to warrant concentrating, it is
chanical agitation are discontinued and the cy
cloniteY is allowed to settle. During this settling 55 also reused in another portion of the process.
As is well known, washing a ñlter cake, which
operation, the larger particles fall rapidly inthe
seriously lreducing the efficiency of the iìlter.v
Following sufficient agitation, both air and me
is Yobtained from a slurry from which it is de
sired to recover the filtrate without unnecessary
dilution, by filtering Water through it is more ad
acid mediumI and tend to form a porous layer
fnear the’ñlter screens. The spent acid next is
separated from the cyclonite byY vacuum filtra
other conditions being equal, than Va
tion l'through the screens 3, 4 and 5, the acid 60 vantageous,
“repulp” wash in that: (l) it requires less time,
flowing into the base of the filter and on through
(2) permits recovery 'of considerable nitrate at
outlet 23 and auxiliary screen 35 into a receiver
high strength by making'a cut at the desired
Vtank from which it is ,taken to the acid area
point, and (3) requires less washing to reach a
for reconcentration and reuse in the process. To .
given crystal acidity. . _
65
facilitate this separation, suction ofy approxi
When the washing is complete, the cake is re- Y
mately 30 inches of mercury is applied beneath
slurried with water and conveyed 4to storage
the screens by starting the steam jet on said re
through conduit I6 for further processing. Th'us
ceiver and opening valves 25 and 21. Filtration " water and airare introduced into the ñlter by
vis continued until an >acid layer of about one inch. ._
remains on'the cake, after which -valve I9 is
n opened. Y' Valves I9, 25 and 21 are closed when air
opening valves I2, 24 and 2,6, agitator 8 being
'To started
when the cake is suinciently loose.v These
‘passes into -the -receiver'tank, this being indi
„c'a'ted‘ by a loss> of ‘suction Whichis shown on a
gauge(> 'The' steps described above may be ref-,I>
valves are closed when'the slurry is of proper
solids content, after which valve lI1 is opened.
Agitation is discontinued When theslurry level
"peated until fthe desired- amount of cake is ob-` .75., drops to the Vagitator' blade, valve I1 being closed
2,404,215
when the filter is drained. vSuction, applied to
the screen in removing the remaining charge, is
cut off when’air reaches the receiver tank. Nor
mally, a satisfactory slurry is formed when the
mixture comprises approximately 3.5 lbs. water
per lb. of cyclonite.
My invention has increased the rate of filtra
tion of cyclonite from 290 lbs. per hour according
tothe commonly used method of the prior art to
470 lbs. per hour. The increase in rate of filtra
tion of other materials realized by this invention
over conventional methods are comparable to that
realized in the filtration of cyclonite.
'
thus formed with water, passing air up through
the filter and thereby maintaining the particles _
in suspension, discontinuing the passing of air
to allow the large particles >to settle, passing the
wash liquid through the filter and removing _the
soïid from said vessel.
,
l
Y ‘
2. The process of separating diverse size ,solid
particles from Va liquid which comprises introduc
.ing the liquid containing the solid into-a vessel
having a filter across the bottom thereof, passing
air up through the> filter therein, thereby forcing _
the particles into suspension, stopping the up
ward flow of air allowing the large particles to
. settle by gravity upon said filter, aiding the flow
While the invention has been described with
reference to cyclonite, for the sake of concrete
of liquid through the filter by means of a vac- nes's, it is to be understood that the process is
uum,
and subsequently removing the solid from»
by no means restricted thereto, and that many
said filter,
modifications can be made without departing
3.
The
process
of
separating
multi-size solid
from the scope of the present invention.. For
from a liquid which comprises intro
example, the invention in its preferred applica 20 particles
ducing the liquid containing the solid into a ves
tion may be employed for the filtration and treat
sel
having a filter across 'the bottom thereofand
ment of materials having particles of at least two
-agitating same, passing air up through the filter
sizes, such as tetryl, ethylenedinitramine, penta
therein, thereby forcing the particles into sus
erythritol tetranitrate, sand and the like. Th'e
pension,
stopping the upward flow of air allowing
invention is applicable to substantially all ma
the large particles to settle by gravity upon said
terials, however when they are produced in such
filter, causing the liquid to fiow through the filter,
manner as to cause formation of different size
removing substantially all of the recoverable liq
crystals. Further, such conditions as filtration
uid remaining in the interstices between the par
temperature, volume of wash water, ratio of wa
ticles by water displacement, reslurring the solid
ter to refined solid in the reslurry, amount of
with water >and removing the slurry from said
suction applied, and the apparatus in general may 30 vessel.
be varied widely. For instance, it has been found
4. The process of separating different size par
according to this invention th'at the temperature
ticles of cyclonite from a spent acid solution
of filtration may be varied over a range of about
which comprises introducing the cyclonite solu
20° to 60° F. without any material effect on the
advance made by said’ invention. Nor is the in
vention limited to a process in which suction is
tion into a vessel having a filter across the
bottom thereof and agitating same, passing air
up through the filter therein, thereby maintain
applied to facilitate filtration, although the use
ing the particles in suspension, stopping the up- '
of suction is preferred. Moreover, my invention
ward
flow of air allowing the large particles to
is applicable not only to acid slurries but also 40
settle by gravity upon said filter, causing the acid
to liquid slurries in general. In the latter case,
to fiow through the filter by the application of
recovery of the filtrate may not be desired, in
suction
thereunder, removing substantially all of
which event dilution of th'e filtrate would be im
the recoverable acid remaining on the cake thus
material. Therefore, instead of washing the cake
formed and in the interstices between the par
by displacement, it may be accomplished by a “re
ticles thereof by at least one water displacement
pulp” or reslurry. Likewise, in dealing with cer- 45'
‘
treatment, -causing the acid and water to pass
tain materials it may not be necessary to use the
through said filter, forming a water slurry of’ the
clean-up filter. Obviously these and other con
refined cyclonite and removing same from said
ditions will depend on the material being proc
vessel.
essed. Nor is the liquid which' is used to Wash 50
5. The process of separating particles of cy
the cake limited to Water, since others that will . clonite
from a spent acid solution which com
accomplish the desired purification may be em
ployed.
f
prises introducing the said cyclonite solution into
a vessel having a filter across the bottom thereof
Although hereinbefore I have made an effort
to render my invention more readily understood 55 and agitating same, passing air up through the
filter therein, thereby maintaining the particles
by describing it in detail and citing specific ap
in suspension, stopping the' upward fiow of air
plications thereof, it will be appreciated that said
allowing the particles to settle upon said filter,
specific disclosure and the discussion of the theo
causing the acid to flow through the filter and
ries or principles underlying my process are for
aiding said fiow therethrough by applying suction
the purpose of clarity only and are not to be
construed as a limitation on the present inven 60 thereunder, providing a container under the f'll
ter, withdrawing the acid from the container _be
tion as more broadly made known.
neath said filter and assisting said withdrawal
I intend to be limited only by the appended
patent claims.
by allowing air to pass into said container through
a vent therein, removing substantially all of the
I claim:
65 recoverable acid remaining on the solid thus
l. The process of separating solid particles of
formed and in the interstices between the par
unlike size from a liquid which comprises intro
ticles thereof by at least one water displacement
ducing the liquid containing said solid into a
Wash, causing the acid and water to pass through
vessel having a filter across the bottom thereof,
passing air up through the filter therein, thereby 70 said filter into said container, promoting the pas
sage of acid and water through the filter by means
forcing said particles into suspension, stopping
of suction, withdrawing the acid and water from
the upward flow of air causing the large particles
said container and aiding said removal by allow
to settle by gravity upon said filter, causing the
ing air to pass into said container through a vent
liquid to flow through the filter by the applica
tion of suction thereunder, reslurrying the cake 75 therein, forming a slurry of the refined cyclonite
and removing same from said vessel, said process
8
jbeing carriedv out 'in‘such lmanner' that at least
Íthe iirstñltrate ispa'ssed through a clean-up ñlter
ïand the liquid for the first displacement Wash
l subsequently
is
»passed„ - together` with- air,
`through said clean-up filter beforeA introduction
`>into the ñlter vessel, said :Wash liquid being
lpassed 'through the clean-up lñlter in the oppo
î site direction tolthat in which said filtrate passed.
6. The process of separating solid particles of
5 Various rsize from a liquid which comprises'intro
ducing `the liquid containing said solid into a
3 Vessel having a ñlter acrossvthe bottom thereof
y and agitating same, passing air up4 through the
ñlter therein, -thereby keeping saidparticles in
1 suspension, stopping’` the upward 110W of air al- ;
` lowing the large particles to settle desirably upon
said ñlter by gravity, causing the liquid îto flow
ï through the i'ilterï'and aiding saidflow there
through by applying suction thereunder, >provid
intosaid, container through a Vent therein, re
moving substantially all of the recoverable liquid
Vremaining on the solid thus formed and in the
interstices between lthe particles thereof by at
-least one liquid displacement treatment, causing
the liquid to pass through said filter into said
container, promoting Y the passageV of
liquidV
through `>the filter lby applying suction thereto,
removing the liquid from said container and aid
ing said removal by Aallowing air to passv there-V
into >through a vent therein, forming a slurry of '
’ the'reñnedsolidland removing same from said
Vessel, said process beingk carried out in such va
manner that the ñrst nitrate is passed through a
lclean-11p filterV andthe liquid for the ñrst displace
passed, together with
air, through said clean-up ñlter before introduc
tion into the vñlter vessel, said wash liquid being
, ment lWash _subsequently is
passed
through the clean-upñlter in the oppo- Y _
site directionto that in which said ñltrate passed.
î ingl a container under said; filter, >withdrawing the {L_o .
g liquid from the container beneath said filterv and'
Q assisting said Withdrawal by allowing air to passA _
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