Note: Descriptions are shown in the official language in which they were submitted.
l~Z9165
This invention is concerned with prills and their
manufacture, more especially the invention i5 concerned with an
apparatus and method for producing a sulphur containing prill.
This application is a divisional of Canadian Patent
Application S.N. 346,710, filed February 29, 1980.
Sulphur is widely employed as a fertilizer or soil
nutrient, since sulphur is essential to plant life and in many
areas the soil must be supplemented with sulphur. In alkaline
soils, addition of sulphur is desirable to increase soil water
intake and aeration, improve the physical condition of the
soil, eliminate harmful alkalinity and sodium problems and
increase the availability to the plant of elements in the
soil necessary for plant life.
Use of sulphur results in improvement in both yield
and quality of the crop.
Sulphur is most suitably employed in elemental form
and different processes have been proposed for providing
sulphur in a form for use as a fertilizer.
Canadian Patent 1,054,821, Bob L. Caldwell, issued
May 22, 1979, describes a method of producing granular sulphur-
bentonite mixture for use as a fertilizer in which a uniform
mixture of molten sulphur and bentonite is poured onto a wet
thermally conductive metal plate and is allowed to cool to a
thickness of 0.25 to 2 inches whereafter the solid is comminu-
ted to provide granules. In U.S. Patent 4,133,669,
Bob L. Caldwell et al, issued January 9, 1979, there is des-
cribed a method of producing a water degradable solid pellet of
bentonite and elemental sulphur in which droplets of a mixture
of molten sulphur are allowed to fall into a liquid fertilizer
solution to anneal the droplets into pellets. An aqueous
liquid fertilizer solution of low water content is used
because the sulphur-bentonite mixture does not pelletize in
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llZ9165
water but rather degrades.
These prior processe$ although producing acceptable
water-degradable fertilizer products have certain disadvan-
tages. The granular product produced in accordance with
Canadian Patent 1,054,821 comprises non-spherical particles of
irregular shape having a non-smooth surface. The granules thus
- have a greater volume when packed for transport than would cor-
responding spherical particles and thus the transportation
costs are higher than they would be for spherical particles.
The non-smooth surface of the irregular shaped granules is
friable such that the granular product has an undesirable
content of fine particles or dust which is herein referred to
as "fines". These fines are produced during the manufacture
reducing the efficiency of yield of granular product and during
subsequent storage, transportation and use of the granular
product, particularly when the product is mobile and individual
granules rub against each other. The fines are particularly
undesirable in that during processing and use of the granules,
clouds of dust may be generated in the air which is unpleasant
or workers and may be hazardous to health, under certain
circumstances there is also a danger of explosion.
In addition the irregular surfaces of the granular
product renders the product susceptable to degradation at the
surface on exposure to a humid environment, such that the
product cannot be stored for long periods and also some
degradation may occur during transport.
The pellets of U.S. Patent 4,133,669 are allegedly
dust-free, a nitrogen coating on the pellet retarding escape
of dust from the pellet, however, the process of manufacture
requires the use of a large volume of expensive nitrogen-
containing liquid fertilizer to anneal the molten droplets and
form the coating. A less costly aqueous annealing medium
- 2 -
1~2gl65
would be economically advantageous. In addition the pelletsformed are not spherical, because the molten droplets are first
formed in an alr space below a perforation plate, whereafter
the preformed droplets fall through the air under gravity into
the liquid fertilizer. This results in spheroidal rather than
spherical particles, the particles being flattened or oblate
spheroids, and such spheroidal particles occupy a larger volume
when packed and are thus more costly in storage and transport.
By means of the present invention it is possible to
obtain a substantially spherical particle having a hard, smooth
surface, for use as a fertilizer; in particular the particle
may be a sulphur particle or a water-degradable particle
containing sulphur and a swelling clay.
The invention particularly provides a novel apparatus
for producing such a particle.
Still further the invention provides a process for
producing such a particle particularly for use as a fertilizer.
There is disclosed herein a substantially spherical
storage stable sulphur-clay particle having a smooth, hard
surface, comprising sulphur and a swelling clay, said clay
being present in an amount effective to render said particle
water degradable.
There is also described a process for producing
water degradable, sulphur clay particles comprising: passing a
mixture comprising molten sulphur and a particulate swelling
clay into an aqueous solution of water soluble electrolyte
salt, maintaining said solution at a temperature effective to
cool said mixture below its solidification temperature,
allowing said mixture to solidify into sulphur clay particles
in said solution, withdrawing said particles from said
solution and drying said particles.
In accordance with one aspect of the invention there
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~I.Z~65
is provided an apparatus for forming solid, substantially
spherical particles from molten, solidifiable material compri-
sing a deep holding tank adapted to maintain a high head of
said molten material, level control means to maintain said
head at a constant static depth, a first plurality of orifices
in a floor of said holding tank, a shallow forming tray below
said holding tank to receive molten material from said holding
tank, adapted to maintain a low static head of said molten
material, a second plurality of orifices in a floor of said
forming tray, said orifices of said second plurality being
circular in cross-section, and a particle forming tank adapted
to maintain a forming medium below said second plurality of
orifices, including level control means to maintain the upper
surface of the forming medium at a constant distance below the
second plurality of orifices, said orifices of said second
plurality being of smaller cross-sectional area than the
orifices of said first plurality, and the total cross-sectional
area of orifices of said second plurality being greater than
that of orifices of said first plurality.
In another aspect of the invention there is provided
in a method of forming solid, substantially spherical particles
from a molten, solidifiable material, comprising feeding the
molten material in a plurality of continuous streams into a
forming medium effective to cool said molten material below
its solidification temperature to form solid, substantially
spherical particles, the improvement wherein said molten
material is allowed to fall from a constant high static head
through a first plurality of orifices to form a constant low
static head of said molten material above said forming medium,
and allowing said molten material to fall through a second
plurality of orifices from said constant low static head in
said plurality of continuous streams into said forming medium.
-- 4 --
1~2~65
In a particular embodiment of the invention the
apparatus includes a prilling head for producing a solid prill
from molten material comprising: a support frame, a deep
holding tank for molten material mounted in an upper part of
said from, said holding tank having a tank floor with a
plurality of feed orifices therethrough, a shallow distribu-
tion tray disposed below said tank floor in said frame, said
tray having a tray floor with a plurality of distribution
orifices therethrough, and a shallow forming tray disposed
below said distribution tray floor in said frame, said forming
tray having a forming tray floor with a plurality of circular
forming orifices adapted to discharge continuous molten
streams of the molten material.
Process of Producinq Particles
The solid particles are formed by cooling droplets
of moiten sulphur containing the sweliing-clay in an
aqueous solution of a water soluble electrolyte salt.
In an especially preferred embodiment a mixture
comprising molten sulphur and a particulate swelling clay
is allowed to flow under gravity through a plurality of
spaced apart circular prilling orifices to form a plurality
of liquid streams, which are generally circular in cross-
section such that the streams can be considered as being
generally cylindrical continuous streams.
The liquid streams enter the aqueous solution
of the electrolyte salt which causes the molten streams
to break up because of the surface tension of the liquid
sulphur, and form substantially spherical droplets which
cool and solidify, while falling through the aqueous
~olution, to substantially spherical particles or prills.
~Z9165
During free fall through the aqueous solution
or under minimal di~ruptive forces, surface tension causes
the liquid sulphur to assume its equilibrium shape, namely
a spherical droplet, having a lesser surface area per unit
volume than any other shape.
It is believed that the ions of the electrolyte
salt dissolved in the water forming the aqueous solution
may interact with the strongly dipolar water molecules
to reduce the water adsorption at the droplet surface.
In prilling the mixture,the disruptive forces
working against the surface tension which tendsto form
truly spherical droplets, are the impact force of the
stream against the surface of the solution and the drag
force exerted on the droplets during settling and solidify-
ing in the solution.
The impact force and the drag force are directly
proportional to the squared product of the velocity and
the diameter of the stream or droplet, and may
become excessively large with relatively small increases
in velocity and diameter, thereby disturbing the spherical
shape.
Moreover, since the surface tension force is
inversely proportional to the diameter of the stream or
droplet, smaller diameter or thinner streams at lower
velocities will minimize the disruptive forces and maximize
the effect of the surface tension forces favouring the
formation of spherical droplets and thus solid, spherical
particles or prills,
The temperature of the molten mixture and the
temperature of the aqueous solution are also significant
in the prill formation,
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~29~65
Since the surface tension of both liquids
decreases with increase in temperature, spherical droplet
and prill formation is favoured by employing the molten
mixture at lower rather than higher temperatures and by
employing the aqueous solution at higher rather than
lower temperatures.
Thus the temperature of the molten mixture and
of the aqueous solution, the orifice diameter and the
velocity of the streams must all be considered in producing
spherical particles.
The sulphur-clay mixture is suitably formed by
810wly adding the particulate clay to molten sulphur which
is suitably at a temperature of 250F to 270F. A temperature
of 250F and 260F iS especially preferred for prilling.
The mixture i9 continuously stirred, while maintaining the
temperature in the indicated range to form a uniform mixture.
Balling of the clay is avoided by stirring the mixture
slowly.
The aqueous solution is maintained at a temperature
~0 effective to cool the molten material below its solidifi-
cation temperature.
In addition it is appropriate to maintain the
solution in an essentially quiescent state, this is achieved
by maintaining the temperature below the boiling temperature
of the solution. If the solution boils or is close to
the boiling point, the internal movement of the solution
caused by bubbling disturbs the formation of spherical
droplets and prills.
In particular it is found that if the aqueous
BolutiOn i9 maintained at a temperature of 140F to 170F,
good prill formation is obtained.
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As indicated above the diameter of the streams
is significant in obtaining good prill formation. It is
found that circular prilling orifices having a diameter
of 1/16 to 3/32 inches results in good prill formation.
It is appropriate to have the streams spaced apart to avoid
coalescence between adjacent streams and between droplets of
adjacent streams, to this end it is appropriate to space
the orifices at least 3/4 inches apart.
The velocity of the streams is determined by the
head of the mixture retained over the prilling orifices
and the distance from the orifices to the surface of the
aqueous solution below.
The preferred apparatus of the invention maintains
a constant head of mixture of only small depth above the
orifices and the orifices are suitably spaced from ~ to
6 inches from the surface of the aqueous solution below.
The water soluble electrolyte salt in the aqueous
solution is suitably employed in a dissolved concentration
of about 5 to about 15%. The minimum dissolved concentration
for effective prill formation without water-degradation
"
appears to be about 5%. It is desirable not to employ
concentrations in excess of about 15%, higher concentrations
are unneceqsary, and have the disadvantage that they cause
excessive salt deposits on the prill surface, making the
prills sticky and difficult to dry.
The electrolyte salt should be soluble in water,
at least in an amount of 5%, by weight, at the solution
temperature employed,
The electrolyte salt is preferably a strong
electrolyte that is highly dissoclated into its ions even
at moderate dilutions and does not obey Ostwald's dilution law.
165
Especially preferred are inorganic salts which are strong
electrolytes, by way of example, there may be mentioned the
water soluble halides, sulphates, nitrates, carbonates,
phosphates and polyphosphates of potassium, sodium,calcium
and ammonium. As the halides there may be mentioned in
particular the chlorides.
It is appropriate to avoid the use of salts
which are hazardous as being toxic, corrosive or explosive
or otherwise hazardous to health even though they can be
successfully used to produce good prills. It is also
appropriate to avoid salts which have a tendency to foam
as this will disturb the desired quiescent state of the
solution, Finally it is appropriate to avoid salts that
are detrimental to plant life or the enviranment. Since
some salt may be entrained in the prills during their
formation and subsequently retained,~it is especially
appropriate to employ salts which are beneficial to plant
life or to the soil or in any event which are agronomically,
agriculturally and horticulturally acceptable.
As examples of salts which are preferably avoided
there may be mentioned potassium nitrate and ammonium
nitrate which are both explosive. Ammonium nitrate can,
however, be stabilized by urea and used as a salt in the
invention.
As examples of suitable electrolyte salts there
may be mentioned ammonium sulphate, potassium chloride,
potassium sulphate, potassium carbonate, potassium phosphate,
sodium chloride, calcium chloride and ammonium polyphos-
phate.
112~165
The electrolyte salt may comprise a mixture of
two or more salts so long as they do not react with each
other in a deleterious manner.
The particulate swelling clay is employed in
admixture with the molten sulphur in an amount to provide
a swelling clay content in the prills effective to render
the prills water-degradable.
The minimum effective content of the swelling
clay is about ~/O by weight of the prills. If the amount
of swelling clay in the admixture with the molten sulphur
is too high it becomes difficult to form the prills. Suit-
ably the upper limit for the swelling clay content of the
prills is about 12% by weight.
. Since some swelling clay is lost in the aqueous
solution during processing, the amount of swelling clay
added to the molten sulphur should be higher than the
desired swelling clay content of the prill. In general the
swelling clay content of the molten mixture should be 1
or 2% higher than the desired clay content of the prill
in particular for a prill containing, in weight %, 9~/O
sulphur and l~/o ~welling clay it is appropriate to employ
a molten mixture comprising, in weight %, 88 to 89% sulphur
and 11 to 12% swelling clay to a total of 100%.
The formed prills are removed from the aqueous
solution and dewatered, suitably by means of an inclined
tray-type conveyor which transports the prills from the
forming tank containing the solution, toa dryer, The
conveyor trays comprise mesh wire screens which allow free
water and fines to pass through and flow back into the
forming tank. Swelling clay fines and product fines are
suitably recovered from the aqueous solution through a
filtering system.
-- 10 _
The prills have a content of fines as prilled
of about 0.1 to about 0.8%~ by weight, on a -50 U.S. sieve.
The prills, before drying, typically have a surface
moisture content, in weight %, of about 3 to about 5% and
are dried to a surface moisture content of 0 to about 1.5%,
suitably in a rotary hot air dryer. Prills with a surface
moisture content up to about 1.5% can be stored without
caking or degradation. Some internal water is retained
during the drying. Depending on the narrowness of size
range desired in the product prills, the dry prills can be
screened to remove oversized and smaller particle sized
prills, whereafter the removed prills can be recycled to
the molten mixture.
Prills
The substantially spherical water-degradable
particles of the invention arqiin this~pecification,referred
to as "prills", which term is intended to exclude non-
spherical particles such as the irregular shaped granules
of the afore-mentioned Canadian Patent 1,054,821 and the
spheroidal particles of U.S. Patent 4,133,669.
The novel prill of the invention consists
essentially of elemental sulphur and a swelling clay and
may additionally include trace amounts of electrolyte salt
and water,
In particular the prills consist essentially of
about 88 to about 93% of sulphur and about 7 to about 12%
of swelling clay, and preferably contain about 8g to about
90/O of sulphur and about 9 to about l~/o of swelling clay.
The swelling clay is a clay mineral which adsorbs
or takes up water and swells. In use the prills adsorb
moisture from the environment, for example, the soil or
the atmosphere, and swell. The swelling brings about the
~Z9165
disintegration of the prills to provide particles of
sulphur in a finely divided form, in the soil.
Clay minerals are layer~lattice minerals and are
basically hydrous aluminium silicates or hydrous magnesium
silicates, however, not all clays are capable of inter-
layer swelling. The smectite group which is also known
as the montmorillohite group and which includes montmoril-
lonite, nontronite and beidellite, is especially notable
for its swelling capacity in water. A preferred swelling
clay for the prills of the invention is bentonite which
is composed largely of montmorillonite and beidellite.
In particular bentonite occurs in a sodium form,
sodium bentonite, and a calcium form, calcium bentonite.
Sodium bentonite has a high swelling capacity in water
and has strong adsorbing properties, more specifically sodium
bentonite may swell 12-fold when contacted with water,
Calcium bentonite, on the other hand, has a negligible
swelling capacity.
The bentonite employed in this 1nvention must
therefore include the sodium form. A mixture of the sodium
and calcium forms can be used, especially when both forms
are present in the source of the material, but it is the
sodium form that is active in the degradation of the prills
in use.
The swelling clay should be in a relatively
fine powder form, and it is found especially appropriate to
employ a clay having a particle size of +325 to ~200 mesh
(U.S. sieve).
The prills of the invention display excellent
storage stability, andexperimental samples of the prills have
been stored for up to a year under normal variation of ambient
atmospheric temperature and humidity, without showing any
deterioration.
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~Z9~65
The prills, however, degrade or disintegrate into
finely divided particles within four hours of being completely
immersed in pure distilled water.
The prills of the invention suitably have a prill
size of +16 to -4 mesh (U.S. sieve) and a loose bulk
density of about 70 to about 85 lb/ft3.
The prills have a hard,smooth surface and display
low friability. Prills of the invention were tested for
friability according to the sulphur prill testing procedure
S5-77 developed by the Sulphur Development Institute of
Canada (SUDIC), Calgary, Alberta, Canada, In this test
prills are tumbled in a 28 inch diameter cylinder rotated
at a speed of 31 + 1 RPM for a total of 40 revolutions.
The test measures prill degradation or the amount of fines
which would be generated by field handling and transportation.
The prills of the invention have a friability in terms of
the percent fines generated in this test of about 2 to
about 5% (-50 U.S. sieve size) and more particularly about
2,5 to 2.75%, specifically about 2.6%, with the overall
prill breakdown being about 3.5 to 4%, particularly about
3,~/0.
The low friability and stable storage characteristics
of the prills are also demonstrated by the fact that only
about 0.1 to about 0.5% fines are generated when the prills
are stockpiled to a height of 50 ft.
The prills of the invention are further characterized
by an angle of repose of about 25 to 35; the angle of
repose being the angle of maximum slope at which a pile of
the prills will stand, without sliding.
The novel prills typically have an internal
moisture content, in weight %, of about 0.02 to 0.1 and a
surface moisture, after drying of, in weight %, 0 to 1.5%.
_ 13_
1~2g~65
Apparatus
The prills of the invention may be produced in
an apparatus adapted from that described in Canadian Patent
863,221, Roy E. Campbell, issued February 9, 1971, for
producing sulphur prills, In particular the apparatus
described therein would require modification to provide a
mixing tank to maintain a supply of uniformly mixed molten
sulphur and particulate swelling clay.
Water-degradable particles can also be produced
employing the apparatus described in the afore-mentioned
U.S. Patent 4,133,669, employing an aqueous solution of
an electrolyte salt in accordance with this invention rather
than the nitrogen-containing liquid fertilizer. The particles
produced will not have the advantages of the novel prills
of the invention, which are substantially spherical, never-
theless thetspheroidal particles produced represent an accept-
able commercial product produced more economically than in
the system in the U.S. patent in view of the lower cost of
maintaining the aqueous solution of the invention as compared
with the nitrogen-containing liquid fertilizer.
However, the prior apparatus described in the
U.S. patent and the apparatus adapted from the apparatus
described in the Canadian patent are less preferred.
The apparatus of the Canadian patent includes a
holding tank to maintain a constant level o~ molten material
to be prilled, The apparatus is such that a fairly deep
static head of molten material is maintained. The depth
or level of the static head of molten material on the forming
orifices which discharge the molten stream under gravity
is a factor in successful prill formation as well as prill
size.
_ 14 _
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In order to form spherical particles it is
necessary that the streams be thin, that is of small dia-
meter, and that they have a low velocity. The deep static
head provided by the apparatus in the Canadian patent
dictates against these features. The velocity is a
function of both the orifice size and the height or depth
of the static head and the depth of static head required in
the apparatus of the Canadian patent, for a particular
orifice size to produce a particular prill size, results
in streams of relatively high velocity, thereby caus1ng
deviation in the prills formed, from the desired spherical
shape.
The apparatus of the invention is adapted to
maintain a low static head of molten material over the
prilling orifices and permits the formation of low velocity
streams even with orifices of relatively large diameter.
In particular in the apparatus of the invention
most of the molten material is maintained in a holding tank
and is fed under gravity to a distribution tray below it,
which in turn distributes the molten material under gravity
to a forming tray below it. Both the distribution tray
and the forming tray are dimensioned to maintain a much
lower static head of molten material than the holding tank.
In addition the forming tray is dimensioned to maintain a
lower static head of molten material than the distribution
tray.
The individual orifices in the floor of the
holding tank, distribution tray and forming tray
decrease in cross-sectional area from the holding tank to
the forming tray although the number of orifices and the
total area of the orifices increases from the holding tank
to the forming tray.
_ 15 -
~il2~65
In this way the molten material is delivered from
the holding tank to the forming tray in a stepwise manner,
the velocity of the molten material decreases from the
holding tank to the forming tray, and only a small static
head of molten material, determining the velocity of the
streams through the prilling orifices is maintained.
The invention is further illustrated in a preferred
embodiment by reference to the accompanying drawings in
which:
FIGURE 1 is a schematic representation of an
apparatus for producing prills in
accordance with the invention, and
FIGURE 2 is an isometric view of an improved
prilling head which forms part of the
apparatus of the invention.
With further reference to~the drawings a prill
forming apparatus 8 comprises a prilling head 10, a forming
tank 12, a dewatering tray conveyor 14, a rotary dryer 16,
a screen 17, and a storage container 15. The apparatus 8
further includes a mixing tank 18, a solution tank 20, a
cyclone separator 22 and a filter 24.
Mixing tank 18 includes a mixer 26 for mixing
particulate swelling clay and molten sulphur, a steam heater
28 to maintain the sulphur in a molten state, a swelling
clay inlet line 30, a molten sulphur inlet line 32 and a
recycle inlet line 34.
A molten mixture line 36 delivers molten material
from the mixing tank 18 to the prilling head 10.
A metering valve 38 in line 36 is controlled by a
level control 44 to maintain a constant head of the molten
mixture in the prilling head 10.
- 16 -
~2~165
Prilling head 10 includes mixers 40 to maintain
the molten mixture uniformly mixed and a steam heater 42 to
maintain the temperature of the molten mixture,
Forming tank 12 includes a steam heater 46 to
maintain the temperature of the aqueous electrolyte solution
therein.
Solution tank 20 includes a water inlet line 48
: and an electrolyte salt inlet line 50. A pump 52 delivers
solution from tank 20 through a solution line 54 to forming
tank 12. A valve 56 in solution line 54 is controlled by a
level control 58 to maintain a constant level of solution
in forming tank 12.
The dewatering conveyor 14 is inclined with its
lower portion submerged in the solution in forming tank 12~
Conveyor 14 comprises a plurality of trays 59 for collecting
formed prills and a return chute 57. The trays 59 suitably
comprise mesh wire screens.
The rotary dryer 16 comprises an air inlet 60,
an air filter 62, an air blower 64, a heater 66 and a rotary
drum 67.
A slurry line 68 from the bottom of forming tank
12 communicates with cyclone separator 22 through a slurry
pump 70.
A heavy particle line 72 from cyclone separator
22 communicates with screw conveyor 78.
A light particle line 74 from cyclone separator 22
delivers undi~solved fine particles and electrolyte salt
solution to filter 24. The undissolved fine particles
collected in filter 24 are delivered via a filter particle
line 76 to screw conveyor 78.
~2~i~S
Electrolyte salt solution is returned from filter
24 to forming tank 12 through a tank recycle line 85, A
first portion of the solution in line 85 is returned to
line 74 through filter recycle line 82 having a valve 84
therein.
A second portion of the solution in line 85 is
taken through cooler line 86 to a cooler 88 where it is
cooled, and from there to a cooled solution line 89,
Lines 85 and 89 meet at a valve 90 which is
controlled by a temperature control 92,
The swelling clay and fine particles in conveyor
78 are removed by a disposal line 80, Optionally line 80
may be connected to line 30.
The prills from dryer 16 are screened on screen
17 to provide prills of a desired size, which are then passed
to the storage container 15, Fines and oversized prills
are recycled through line 34 to mixing tank 18,
With reference to Figure 2 there is shown a
prilling head 10 for use in the apparatus 8 of Figure 1,
Prilling head 10 comprises a holding tank 100,
a distribution tray 102 and a forming tray 104 mounted in
a supporting frame 106,
Supporting frame 106 comprises a pair of U-shaped
side supports 108, each support 108 comprising a pair of
vertical legs 110 and 112 connected by a horizontal arm 114,
A pair of L-shaped supports 116 is mounted between
legs 110 and between legs 112 respectively,
A second pair of L-shaped supports 118 is similarly
mounted between legs 110 and between legs 112 respectively,
below L-shaped Rupports 116,
- 18 -
l~ Z9~65
Struts 120 extend between the supports 116, and
are secured to the bottom thereof. Struts 122 extend between
the supports 118 and are similarly secured to the bottom
thereof.
An H-shaped support 124 is disposed intermediate
~ide supports 108 and includes a pair of vertical legs 126
connected by an arm 128.
The legs 126 of support 124 are secured at an
inner surface to the upright outer surfaces of supports 116
and 118, with arm 128 above supports 116.
Holding tank 100 is mounted on arm 128 and is
secured to the upper ends of legs 110 and 112.
Holding tank 100 includes walls 130 and a floor
132; A plurality of circular orifices 134 is formed in
floor 132, in spaced apart rows.
Distribution tray 102 comprises walls 136 and
floor 138 and i8 slidably mounted on the upper horizontal
portions of supports 116. A pair of elongated feet 142
extend from the underside of floor 138 to support tray
102 on the struts 116.
A plurality of circular orifices 140 is formed,
in spaced apart rows, in floor 138.
Forming tray 104 comprises walls 144 and a floor
146, and is slidably mounted on the upper horizontal portions
of supports 118. A pair of elongated feet lS0 on the under-
side of floor 146 supports tray 104 on struts 122.
A plurality of circular orifices 148 is formed
in spaced apart rows in floor 146.
Side supports 108 mount the prilling head 10 across
opposed upper edges 152 of forming tank 112.
The number of orifices 140 is greater than the
nu~ber of orifices 134 and the number of orifices 148 is
greater than the number of orifices 140.
. ' - 19 -
~129~65
The diameter of orifices 140 is less than that
of orifices 134 and the diameter of orifices 148 is less
than that of orifice~ 140. The total cross-sectional area
of orifices 140 is greater than the total cross-sectional
area of orifices 134; and the total cross-sectional area
of orifices 148 is greater than the total cross-sectional
area of orifices 140.
Trays 102 and 104 are, as indicated above slidably
mounted in frame 106. In this way trays 102 and 104 can be
readily removed from frame 106 for cleaning.
The operation is described with further reference
to the drawings, swelling clay and molten sulphur are
delivered to mixing tank 18 through lines 30 and 32
respectively, where they are continuously stirred by mixer
26 to produce a uniform mixture, at a controlled temperature
maintained by heater 28.
The mixture is metered from tank 18 through line ,
36 and metering valve 38 to the holding tank 100 (Figure 2)
of prilling head 10. The metering of the mixture is
controlled by the level control 44 to maintain a constant
static head of the mixture in holding tank 100.
The mixture is continuously stirred in holding
tank 100 by mixers 40, the temperature being maintained by
heater 42.
With particular reference to Figure 2, the
mixture in holding tank 100 passes through orifices 134
into the distribution tray 102 and through the orifices
140 into forming tray 104.
Forming tray 104 provides a significantly smaller
static head of molten material than does holding tank 100.
Molten material pa~se~ through the circular orifices 148
- 20 -
1129~65
in the floor 146 of forming tray 104, in continuous streams,
and the continuous streams enter the aqueou~ electrolyte
salt solution in forming tank 12, the temperature of the
solution being maintained by heater 46.
The solution is maintained at a constant level
below the floor 146 of forming tray 104 by the level
~ control 58.
- As the streams of molten material are submerged
in the solution in the forming tank 12, they are broken up
into substantially spherical droplets which solidify in
the solution as they fall therethrough towards the bottom
of the tank 12. .
The bottom of the tank 12 is suitably sloped and
the formed prills are collected on the trays 59 of conveyor
14. As the trays 59 emerge from the tank 12 water is drained
from the trays 59 into return chute 57 and returned thereby
to the tank 12.
The dewatered prills which still contain residual
water are dried by means of hot air in rotary dryer 16 which
is of a conventional form.
The dried prills from dryer 16 can be stored as
such or screened by means of screen 17 to obtain a desired
prill size. Fines and oversized prills from screen 17 are
returned to mixer 18 via line 34.
Slurry accumulating in the bottom of tank 12 is
pumped by slurry pump 70 through slurry line 68 to the
cyclone separator 22, where the heavier particles are
separated and removed through line 72 to screw conveyor 78.
The lighter undissolved particles and salt solution from
cyclone 22 are passed through line 74 to filter 24, The
undissolved fine particles are collected in filter 24 and
are passed to the screw conveyor 78 through line 76. The
_ 21 -
~.~Z9165
salt solution from filter 24 is returned to forming tank
12 through recycle line 85.
A first portion of the salt solution in line 8S
is returned to line 74 for refiltering in filter 24 vla
line 82 and valve 84.
A second portion of salt solution in line 85 is
bled-off through a cooler line 86 to a cooler 88, the
resulting cooled solution being returned to line 85 through
line 89.
The relative proportions of solution in lines 85
and 89 which are fed into tank 12 are controlled by valve 90
in response to temperature control 92, so that the solution
in tank 12 is maintained at a desired temperature.
The particulate material in screw conveyor 78
is removed through line 80 and can be recycled to the
tank 18.
The prilling head 10, while maintaining a
constant high static head of material in holding tank 100
suitable for a continuous operation, delivers continuous
streams of the molten material at low velocity to the
forming tank 12. Thi~ is achieved by the step down of the
head of molten material in tank 100, by means of the dis-
tribution tray 102, so that a lower head is formed in
tray 102, and a still lower head in tray 104.
The low static head of molten material in tray
104 produces continuous streams of the molten material of
low velocity,
The velocity of a molten material flowing down-
wardly through an orifice is determined by the head of
molten material above the orifice. However, the orifice
dimension also affects the velocity, a critical orifice
dimension being reached, as the orifice dimension is
l~Z~3~65
decreased, at which point the orifice produces a drag in
the flow and retards the flow thereby reducing the velocity
of flow. The critical orifice dimension depends on the
viscosity of the molten material.
In order to produce spherical particles it is
necessary to produce continuous small diameter streams
of the molten material at low velocity.
- The prilling head 10 achieves this by a step-
wise reduction of the orifice size from the tank lO0,
through tray 102 to tray 104. At the same time the total
cross-sectional area of the orifices is increased, in a
stepwise manner, by increasing the number of orifices from
the tank lO0, through tray 102 to tray 104 to compensate
for the reduction in velocity of the molten material
brought about by the lo~er static heads in each of trays
102 and 104 and the smaller orifice sizes in each of trays
102 and 104, as compared with tank 100.
The ~tal orifice cross-sectional area in the dis-
tribution tray is about three times as large, and in the form-
ing tray about nine times as large, as that in the holdingtank, A constant liquid level needs to be maintained only in
the holding tank. By virtue of the increased total orifice
cross-sectional areas in the lower trays, constant but sub-
stantially lower liquid levels are established naturally in
the lower trays.
By means of prilling head lO it is possible to form
a low static head below the high static head. The high static
head typicall~ having a depth of 40 to 50 and preferably about
45 times the depth of the low static head,
~Z9165
In general it is found appropriate to limit the
depth of the low static head ~o not more than about 0,5
inches, and a low static head maintained at a depth in the
range of about 0.1 to about 0.5 inches has been found
especially useful.
It is found that at low static head depths
greater than about 0.5 inches the prills formed begin to
depart from the desired spherical shape.
EXAMPLE
In an example a prilling head 10 as shown in
Figure 2 was employed in the apparatus 8 of Figure 1.
The holding tank 100, distribution tray 102 and
forming tray 104 were of substantially square cross-
section, measuring 6 ft. by 6 ft.
The holding tank 100 had a depth of 2 ft. and
the floor 132 had 144 circular orifices 134, spaced 6 ins.
apaxt, in rows, each orifice 134 having a diameter of
5/32 ins.
- 24 -
l~lZ~165
The distribution tray 102 had a depth of 4 in~.
and the floor 138 had 1152 circular orifices 140, spaced
2 ins. apart, in rows, each orifice 140 having a diameter
of 3/32 ins.
The forming tray 104 had a depth of 2 ins. and
the floor 146 had 4700 circular orifices 148, spaced 1 in~
apart, in rows, each orifice 148 having a diameter of
5/64 ins.
A molten mixture comprising 90/O by weight of
molten sulphur and lG% by weight of a particulate bentonite
clay comprising a mixture of sodium bentonite and calcium
bentonite, having a particle size ranging from -200 to
+350 mesh (U.S. sieve), was maintained at a temperature
of about 255F in tank 100, at a constant static head of
5,5 inches.
The static head formed in tray 104 was about
1/8".
The molten mixture was delivered from tray 104
in 4700 continuous streams into a 10% aqueous solution of
potassium phosphate, at a temperature of 15S~F, in a tank
12. The upper surface of the solution was at a level
maintained 4 ins. below the orifices 148 of tray 104.
The prills formed in tank 12 were removed and
dewatered by conveyor 14 having trays 59 comprising 12
mesh (U.S. sieve) screens.
The prills were dried in rotary dryer 16; the
following prill size distribution was obtained:
- 25 -
~Z9~65
U.S. Wt.%
Sieve Size Retained
3/81' 0
No. 4 1.2
No. 5 28.4
. 6 43~5
~O. 7 21.5
No. 8 4.7
~O. 12 0.6
lOMinus No. 50 0.1
lOQ.00
In this run 98.7% of product was within the size
range of 4 to +12 mesh.
The prills were substantially spherical particles
having a hard, smooth surface and a friability of 2.6%,
with an overall particle breakdown of 3.8% (testing
procedure S5-77).
Each prill containeda pin hole extending from
the prill surface into the core, this being generally
characteristic of prills formed in l quid solutions.
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