Silver nitrate produced by a continuous evaporative crystallization process

Silver nitrate crystals comprising a substantially non-platelet crystal morphology are disclosed. The non-platelet silver nitrate crystals have an aspect ratio in the range of from about 1:2:3 to about 1:1:1. FIGS. 4 and 5.

CROSS-REFERENCE TO RELATED APPLICATIONS 
This application is related to U.S. Application Ser. Nos. 848,446 and 
848,490, filed on Mar. 9, 1992. 
FIELD OF INVENTION 
The present invention relates to silver nitrate crystals produced in a 
continuous evaporative crystallization process and apparatus. In 
particular, the present invention relates to an improved form of silver 
nitrate crystals. 
BACKGROUND OF THE INVENTION 
Silver nitrate crystals are produced by crystallizing silver nitrate in a 
slurry in a crystallizer. Slurry containing crystals is then withdrawn 
from the crystallizer and introduced to a separator to separate silver 
nitrate crystals from the slurry. Silver nitrate crystals prepared by 
prior art processes have a platelet-type of crystal morphology that can 
render the crystals difficult to separate and dry. The separation and 
drying of such crystals can be time-consuming and costly, and crystals can 
tend to break and not separate out, further increasing process costs and 
inefficiency. 
It is the objective of the present invention to provide an industrially 
feasible and economically practical process that solves the above problem. 
SUMMARY OF THE INVENTION 
This invention provides silver nitrate crystals having a substantially 
non-platelet morphology as illustrated by FIGS. 4 AND 5. Typically, the 
crystals of the invention are non-platelet, as signified by the aspect 
ratio which is typically in the range of from about 1:2:3 to about 1:1:1, 
the latter defining a cubic structure. The crystals typically have a mean 
particle size in the range of about 200 .mu.m to about 600 .mu.m, and a 
size distribution of from about 70 .mu.m to about 1000 .mu.m. 
The silver nitrate crystals of the invention are prepared rapidly by 
evaporative crystallization. In other words, the residence time of 
material in the crystallizer is short, e.g. on the order of about 1 to 3 
hours. The crystals can therefore be prepared efficiently and at a 
reasonable cost. The crystals are easily separated and dryed, which 
further adds to the speed and efficiency of their preparation. The 
crystals also exhibit good resistance to breakage during separation, which 
further improves the efficiency of their preparation since more crystals 
are effectively separated and therefore less are recycled to the 
crystallizer. 
These and other aspects, objects, features and advantages of the present 
invention will be more clearly understood and appreciated from a review of 
the following detailed description of the preferred embodiments and 
appended claims, and by reference to the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The preparation of the aqueous silver nitrate solution used as a starting 
material in the invention is well known and is described, for example, in 
U.S. Pat. No. 5,000,928. The equation for the reaction of silver with 
nitric acid can be expressed as follows: 
EQU 4Ag+6HNO.sub.3 .fwdarw.4AgNO.sub.3 +NO+NO.sub.2 +3H.sub.2 O 
After preparation, the silver nitrate solution can be purified prior to use 
as a starting material in the invention as is discussed further below. 
Purification of silver nitrate solutions is described, for example, in 
U.S. Pat. Nos. 1,039,325, 2,581,519, 5,000,928, and Jap. Kokai No. 
52[1977]-60294. 
Referring now to FIG. 1, silver nitrate solution is introduced as the feed 
solution to evaporative crystallizer 10 via line 12 and flow control valve 
14, the level of solution in crystallizer 10 being shown schematically as 
the dotted line. For efficient operation of the crystallizer, the 
concentration of silver nitrate in the silver nitrate feed solution should 
be in the range of from about 60 to about 90 percent by weight. The 
temperature of the feed solution entering the crystallizer should be in 
the range of from about 20.degree. C. to about 85.degree. C. The flow rate 
of feed solution to the crystallizer and/or the slurry discharge from the 
crystallizer can be established based on the desired production rate. 
Crystal growth is dependent on mean residence time in the crystallizer and 
other factors that are further discussed below. 
The evaporative crystallizer employed in the invention should be a 
well-mixed evaporative crystallizer, such as is described below, in 
Perry's Chemical Engineer's Handbook, 6th Ed., Section 19, Liquid-Solid 
Systems, "Crystallization Equipment", Miller et al, pp. 19-35 to 19-40 
(hereinafter "Perry's"), incorporated herein by reference, and in Chemical 
Engineer's Handbook, 4th Ed., Section 17, "Crystallization", Perry et al, 
pp. 17-7 to 17-23 (hereinafter "Crystallization"). In the preferred 
embodiment shown in FIG. 1, crystallizer 10 is a draft tube evaporative 
crystallizer comprising draft tube 16. Means for introducing the silver 
nitrate feed solution to crystallizer 10 can comprise any convenient 
system, e.g. a continuous silver nitrate feed solution production facility 
having an output pipeline serving as the input pipeline to crystallizer 
10. Bladed agitator 20 is means for agitating the solution and the slurry 
along the flow path indicated by the direction arrows to promote the 
formation of a well-mixed slurry in a crystallization zone in and around 
draft tube 16. The solution level in the crystallizer should be maintained 
above draft tube 16 but by not more than about 2 inches (5 cm) to 6 inches 
(15 cm) to avoid leaving a poor slurry crystallization zone thereabove due 
to insufficient agitation and flow. At such a solution level, the 
crystallization zone effectively is the entire solution volume shown in 
crystallizer 10. Silver nitrate crystals typically form and grow while 
suspended in the crystallization zone. The well-mixed slurry promotes 
uniform silver nitrate crystal growth to achieve a desirable non-platelet 
crystal morphology as discussed below. 
Agitator 20, for example a bladed agitator, is turned by shaft 22 which in 
turn is driven by motor 24. The silver nitrate slurry is dense, and 
agitator 20, shaft 22, and motor 24 should be sized accordingly, based on 
factors such as desired agitation rate and maximum expected loading. One 
skilled in the art can readily select the appropriately sized and powered 
agitator system for the particular crystallizer system design. 
A crystallizer level controller system comprises level detector 26 that 
measures the level of solution in crystallizer 10 and provides a signal to 
level controller 28. Controller 28 provides an output signal to control 
valve 14, thereby automatically regulating the flow rate of feed solution 
to the crystallizer to maintain a desired level in the crystallizer. 
Means for heating and concentrating the solution in the crystallizer is 
steam jacket 30 to which steam, e.g. low pressure steam at about 3 psig 
(122 KPa absolute) to 15 psig (205 KPa absolute), is supplied via line 32 
and flow control valve 34. The heating evaporates water from the 
supersaturated solution, causing silver nitrate to crystallize and 
precipitate. Crystallizer 10 can also be heated by any other convenient 
heat transfer means such as are well known in the art. A preferred slurry 
temperature is in the range of from about 100.degree. F. (38.degree. C.) 
to about 140.degree. F. (60.degree. C.). At higher slurry temperatures, 
crystal growth can become too rapid and undesirable amounts of fines can 
form. 
Preferably, a partial vacuum is provided and maintained in the vapor space 
in the crystallizer. The vacuum, for example, can be in the range of from 
about 23 mm Hg absolute to about 200 mm Hg absolute, with a preferred 
level of about 112 mm Hg absolute. The solution in the crystallizer is 
heated while vacuum is drawn. Means for maintaining a partial vacuum can 
comprise a cooling condenser having a vacuum source such as a vacuum pump 
to which the process gases, primarily water vapor, from the crystallizer 
vapor space can be exhausted. The heating under vacuum creates a 
concentrated, supersaturated slurry in which silver nitrate crystals are 
formed and grow 
Crystallization conditions such as slurry density and mean residence time 
are controlled in order to obtain a desired crystal habit and size. 
Crystal formation and its mechanisms are further described in 
"Crystallization" at pp. 17-11 to 17-15, incorporated herein by reference. 
Means for measuring the slurry density and for controlling the means for 
heating to maintain the slurry density within a desired range comprises 
differential pressure transmitter 36 and controller 38 that controls valve 
34. Slurry density is measured by differential pressure transmitter 36 
with diaphragm seals, which transmitter is a standard well-known device 
such as the Model 3051C manufactured by Rosemount, Inc. The concentration 
of solids in the slurry is related to slurry density by the equation 
Concentration of Solids=Slurry Density--Mother Liquor Density. 
Slurry density is proportional to the pressure differential across 
differential pressure transmitter 36. Transmitter 36 provides a signal 
representative of solids content to controller 38. Controller 38 then 
provides a control signal to control valve 34 to adjust steam flow and 
thereby control the heating rate in the crystallizer. The slurry density 
is thus maintained at a desired level by controlling the heating of the 
slurry to thereby control evaporation from the slurry. In a preferred 
embodiment, slurry density is maintained in the range of from about 2.7 
g/cc to about 3.2 g/cc, corresponding to about 18 to 45 weight percent 
silver nitrate, respectively. Alternatively, slurry density can be 
monitored manually and the crystallizer heating can be controlled 
manually, although automatic control as described is preferred. 
The term "mean residence time" can be defined as the average time a unit of 
material, e.g. silver nitrate, remains in the crystallizer after 
introduction. The mean particle size and size distribution of crystals in 
the slurry is primarily determined by the mean residence time and the 
agitating regime in the crystallizer. For steady state conditions, mean 
residence time can be defined by the equation 
EQU Mean Residence Time=Vol.sub.s /v.sub.ws 
where Vol.sub.s is the volume of slurry in the crystallizer and v.sub.ws is 
the volumetric rate of withdrawing slurry from the crystallizer, with 
other parameters, e.g. slurry density and feed solution plus recycled 
solution flow to the crystallizer, being substantially constant. The 
smaller the mean particle size of the crystals, the more difficult it is 
to separate the crystals from the withdrawn portion of the slurry. 
Therefore, it is desirable to obtain crystals in the slurry having a mean 
particle size and size distribution capable of cost-effective separation, 
a factor which can influence the selection of minimum residence time. The 
upper limit on residence time is mainly determinable in regard to process 
efficiency, because crystal breakage tends to limit further process gains 
in crystal growth and size for long residence times in the apparatus of 
the invention. One skilled in the art can readily select the residence 
time for a particular system design and separator and for a desired 
product characteristic and output. For the preferred embodiment of the 
invention described herein for the stated operating parameters, e.g. feed 
concentration, feed temperature, and vacuum, a preferred residence time is 
in the range of from about 60 minutes to about 180 minutes. 
FIG. 2 shows the smaller mean particle size and size distribution of silver 
nitrate crystals formed by the process of the invention as compared to 
that formed by cooling crystallization. The size distribution and mean 
particle size of the crystals in the slurry are dependent on 
crystallization process conditions and on the crystallizer apparatus 
employed. Typically, the silver nitrate crystals of the invention have a 
size distribution in the range of from about 70 mm up to about 1000 mm and 
a mean particle size of from about 200 mm to about 600 mm. 
It has also been surprisingly found that, with the appropriate selection of 
residence time for a given set of process parameters, a form of silver 
nitrate crystal can be obtained that can be separated from the slurry and 
dried with less difficulty than with other known processes, as discussed 
further in the Examples. FIG. 3 shows silver nitrate crystals formed by a 
different process and apparatus. As shown in FIGS. 4 and 5, the process 
and apparatus of the invention produce silver nitrate crystals having more 
uniform growth along each axis than the silver nitrate crystals of FIG. 3. 
Prior art processes produce silver nitrate crystals having irregular 
crystal sizes and morphologies, and such crystals have a tendency to form 
agglomerates of platelet-like crystal structures. In comparison, the 
silver nitrate crystals of the invention have a more regular size and 
morphology and are more cubic than prior art crystals. The terms "more 
cubic" or "cubical" are meant to include orthorhombic crystals, that is, 
crystals having a polyhedron structure having x, y, and z axes, and each 
such pair of axes having about a 90 degree angle therebetween. 
Crystallization process parameters such as flow rates and residence time 
can be established to produce a slurry containing the cubic silver nitrate 
crystals of the invention. A preferred cubic silver nitrate crystal of the 
invention has an aspect ratio of between about 1:2:3 to 1:1:1, where 
aspect ratio is defined as the ratio of the x, y, and z orthorhombic 
crystal axes. 
As discussed in the Examples below, the cubic silver nitrate of the 
invention is more easily deliquored, i.e. separated from the slurry, than 
irregular or platelet silver nitrate crystals, and therefore the process 
of the invention requires less time and can employ smaller, simpler 
apparatus to produce a given quantity of usable product than other 
processes. "Crystallization", pp. 17-7 to 17-8, incorporated herein by 
reference, further describes and defines the various crystallographic 
systems. 
Means for withdrawing slurry comprising silver nitrate crystals from 
crystallizer 10, and for providing slurry to a separator for separation of 
crystals, is provided by variable speed pump 70 and line 72. Slurry flow 
rate is measured by flow monitor 74 that provides a representative signal 
to controller 76 for controlling the speed of pump 70 and thereby 
controlling the withdrawal of slurry from crystallizer 10 at a selected 
rate. At steady state conditions in the continuous process of the 
invention, the rate of feed and makeup solution to the crystallizer should 
be about equal to the rate of slurry withdrawal and exhaust of vapors to 
the condenser. 
Crystallization of the silver nitrate and separation of the crystals as 
described herein produces silver nitrate product having a very substantial 
improvement in purity compared to the feed solution. Impurities are 
concentrated in the solution remaining in the crystallizer and in the 
liquor remaining from the separation step, producing a very pure silver 
nitrate crystalline product. The silver nitrate crystal product can 
typically exhibit an increase in purity in the range of about 30 to 100 
times the purity of the feed solution as calculated by weight percent of 
the impurities present in the product and the feed solution. The pH of the 
crystal product is typically in the range of from about 3.5 to about 5.5, 
depending on the moisture and acid content in the product. Generally, pH 
increases as the moisture content decreases because the acid content in 
the product also decreases, and therefore the product also has a higher 
purity. 
The slurry withdrawn from crystallizer 10 is introduced to a separator, 
centrifuge 78, to separate silver nitrate crystals from the slurry. 
Centrifuge 78 separates silver nitrate crystals from the slurry, leaving a 
residue liquor that is recycled to crystallizer 10 via dissolver 82. FIG. 
6 further illustrates details of centrifuge 78. Centrifuge 78 is a rotary 
screen centrifuge, e.g. a Model GTLII manufactured by Heinkel Corp., with 
screen 84 shown in phantom. Other separators useful in the invention 
include continuous pusher centrifuges such as the Kraus-Maffei SZ30 and 
SB250 series units. The rotary screen centrifuge is a preferred separator, 
and particularly preferred is one having a high ratio of rated capacity to 
product throughput. The screen hole size can be readily selected depending 
on the size of the crystals desired to be separated from the slurry. In 
the well-known manner, the slurry is introduced to the intake of 
centrifuge 78 and is rotationally accelerated against screen 84, 
separating the slurry into a liquor and fines component which passes 
through screen 84 and a solids component comprising silver nitrate 
crystals which are stopped by screen 84 and discharged therefrom. 
The next step in the process of the invention is drying the separated 
crystals. Control of moisture content is important in the preparation of 
silver nitrate. It is desirable to provide a consistent product for the 
eventual end-use, such as the preparation of photographic silver halide 
emulsions. At higher moisture levels, crystals can stick to surfaces, 
resulting in poor flowability from the separator and difficulty in 
handling. Some moisture content may be desirable, however, in order to 
prepare product having a desired pH value, and product pH can be dependent 
on moisture content as discussed in the Examples below. A preferred 
moisture content in the separated, uncompacted silver nitrate crystals is 
in the range of from about 0.001 percent to about 0.2 percent by weight, 
and 0.05 percent by weight is particularly preferred. 
Surprisingly, it has been discovered that the invention provides a drier, 
more consistent product than do prior art processes and apparatus. Unlike 
the prior art, the invention does not require large, costly dryers. In the 
embodiment of the invention described herein, rotary screen centrifuge 78 
is also a dryer, as it provides a drying function in addition to its 
above-described separating function. The inherent drying capability of 
centrifuge 78 can be supplemented by the introduction of an airstream to 
provide further drying of the crystals, as shown in FIG. 6. Line 86 is 
means for providing an airstream to the discharge end of centrifuge 78 to 
provide further drying of the crystals. The flow rate of air through line 
86 to centrifuge 78 can be set so as to sufficiently dry the crystals a 
desired amount for a specified mass flow rate of separated, dried 
crystals. The moisture content of the separated crystal product depends on 
factors such as the speed of the centrifuge, the crystal habit, air 
addition rate, slurry flow rate, slurry temperature, and the hole size of 
the centrifuge screen. One skilled in the art can readily select the 
operating parameters to obtain a desired product moisture level in the 
practice of the invention. 
Extent of drying is determined by the ratio of the drying air flow rate to 
the crystal product flow rate. A preferred ratio of drying air flow rate 
to the dried crystal flow rate is in the range of from about 0.75 CFH of 
air/lb/hr of crystals (0.047 m.sup.3 /hr of air/kg/min of crystals) to 
about 1.5 CFH of air/lb/hr of crystals (0.047 m.sup.3 /hr of air/kg/min of 
crystals), to obtain product moisture contents of from about 0.1 percent 
to about 0.001 percent by weight, respectively. 
Separation and drying can be carried out on a slurry at any convenient 
slurry temperature, and is particularly good at elevated slurry 
temperatures, e.g. in the range of from about 45.degree. C. to about 
55.degree. C. At lower slurry temperatures, the moisture content in the 
crystal product can increase. At higher slurry temperatures, the slurry 
can be more difficult to handle because the pipelines have an increased 
tendency to plug up. The appropriate separator can be selected based on 
factors such as slurry and crystal characteristics and the like. For 
example, the rotary screen hole size can be selected based on the mean 
particle size and size distribution of the crystals in the slurry to be 
being separated. In a preferred embodiment, the aqueous slurry comprises 
silver nitrate crystals having a mean particle size in the range of from 
about 200 mm to about 600 mm, and the screen has holes sized to separate 
and dry crystals having a size of about 50 mm and up. 
Silver nitrate crystals can accumulate over time on screen 84, which can 
adversely affect the rotational stability of centrifuge 78 and cause 
unacceptable vibration levels. Accordingly, means for cleaning screen 84 
is provided, comprising a controlled, intermittent introduction of hot 
water to centrifuge 78. The hot water should have a sufficient flow rate 
for a sufficient time to substantially flush and clean screen 84. For 
example, a flow rate of about 5 gallons per minute (19 L/min), for a time 
of about 10 seconds, at a water temperature of about 50.degree. C. can 
provide adequate cleaning without substantial interruption of the 
continuous separation and drying of the silver nitrate crystals. During 
continuous operations, a preferred time interval between cleanings is 
about 1.5 hours. A standard vibration sensor 86 is mounted on centrifuge 
78 to monitor the vibration level. 
The invention also includes a novel way of treating the silver nitrate 
crystal product to facilitate subsequent use and handling of the product. 
As discussed above, the silver nitrate crystals produced in the process of 
the invention have a substantially cubic morphology compared to the 
complex aggregate or platelet-type structure of crystals produced by other 
processes. The crystal product can also have a mean particle size and size 
distribution smaller than that produced by prior art processes. As 
described above, these characteristics lend some advantages to the crystal 
product of the invention. An accompanying disadvantage, however, is that 
the smaller crystals may have a greater tendency to fuse, creating large 
chunks of material that are difficult to handle and use. 
Accordingly, in another aspect of the invention, after separation and 
drying, the silver nitrate crystal product is compacted to form a 
plurality of small, flowable, compacted silver nitrate bodies. Each 
compacted silver nitrate body has a compacted surface and density 
sufficient to allow the body to freely disengage from surface contact with 
other similar bodies so that the body is freely flowable despite the 
contact with the other such bodies. Referring now to FIGS. 6 and 7, in 
order to form the compacts, the separated, dried crystals drop from screen 
84 into solids discharge chamber 88. Chamber 88 should be polished, and 
have a discharge diameter of about 4 inches (102 mm) or larger and a cone 
angle of about 40 degrees to substantially prevent or minimize bridging 
and clogging of the crystals therein. The crystals discharge from chamber 
88 into feed tube 90. Feed tube 90 is seamless and without surface 
projections to avoid bridging of the crystals and blockage of crystal flow 
in feed tube 90. Chamber 88 has first end 92 engaged with the discharge 
end of centrifuge 78 and second end 94 engaged with first end 96 of feed 
tube 90. Second end 98 of feed tube 90 is engaged with compactor 100. 
The invention encompasses any well-known compactor, such as a tablet press, 
pellet press, rotary pan sphereinizer, or a roll compactor. The rotary pan 
sphereinizer produces compacts having a high moisture content and 
therefore increased drying of the product is necessary. The pellet press 
uses lubricating materials that may contaminate the product and can be 
inconvenient or impractical for a continuous process producing a large 
quantity of a highly purified product. A roll compactor is preferred, 
since it does not have the aforementioned problems, it is capable of 
providing a continuous, large throughput of uncontaminated product, and is 
economical to use. Roll compactors are described in Roll Pressing, W. 
Pietsch, Powder Advisory Centre, P.O. Box 78, London NW11 OPG, England 
(2nd Ed. 1987) (hereinafter "Pietsch"), incorporated herein by reference. 
Roll compactors comprise one of two typical designs, cantilevered and mill 
shaft. The mill shaft design can accommodate higher compaction pressures 
and roll forces and have a higher capacity than a comparably sized 
cantilevered roll compactor, and is preferred for compacting the dense 
silver nitrate crystals of the invention. A representative roll compactor 
useful in the invention is the Model 4B4LX10 Chilsonator, manufactured by 
The Fitzpatrick Company. 
Compactor 100, as shown in FIGS. 7 and 8, is a roll compactor. Compactor 
100 comprises fixed, drive roll 102 rotatably mounted within housing 104, 
and movable, idler roll 106 rotatably mounted within housing 104 and 
slidably mounted on opposing walls 108 and 110 of housing 104 by bearing 
blocks 112 and 114, respectively. Rolls 102 and 106 are mounted such that 
their axes of rotation are substantially parallel during compaction. 
Movable roll 106 is an idler roll that is gear-driven by a standard gear 
mechanism (not illustrated) driven by roll 102. Roll 102 is associated 
with standard drive means (not illustrated). The actuator of hydraulic 
piston 116 is connected to plate 112 to provide a force to movable roll 
106 during compacting. Machined spacer plate 118 establishes a minimum gap 
between the edges of rolls 102 and 106 in order to ensure and maintain a 
desired minimum flow of material through the nip and past the rolls while 
permitting a sufficient flow rate of material therethrough to minimize 
webbing between individual compacts. It is important to minimize webbing 
to minimize the amount of fines in the compacted product. Because roll 106 
is slidably mounted on housing 104, the roll gap can be adjusted and roll 
106 has some freedom of movement during compaction. The roll gap thus 
affects the percentage of fines in the compacted product. The roll gap 
should be small when using a roller design having a pocketed surface to 
limit webbing around the compact to an acceptable amount. Increased 
webbing can lead to increased fines, which can cause additional exposure 
to workers handling the compacted silver nitrate product. A preferred 
minimum spacer setting and minimum roll gap is about 0.002 inches (0.05 
mm). 
The size and density of a compact of silver nitrate depends on factors such 
as the type of compactor used and the compacting force employed. The 
density of uncompacted silver nitrate crystals can typically be about 2.25 
g/cc to about 2.30 g/cc. The density of a coherent, hard compact of silver 
nitrate is generally in the range of from about 3.5 g/cc to about the 
theoretical density of silver nitrate, which is 4.35 g/cc. 
Surprisingly, it has been found that the process and apparatus of the 
invention provide a coherent, hard compact that can be prepared from 
uncompacted silver nitrate crystals that are either wet or dry, and 
without necessitating the addition to the uncompacted powder of a binder, 
e.g. polyvinyl alcohol. The fact that a binder is unnecessary is a 
significant aspect of the invention, because a binder can contaminate the 
silver nitrate and make it impracticable to use in applications requiring 
high-purity silver nitrate. Examples requiring the use of highly-pure 
silver nitrate include catalysis, and the preparation of photographic 
emulsions. 
FIG. 8 illustrates a compacted body, or "compact", of silver nitrate such 
as is formed by compactor 100. The size of the compact is a function of 
the roll surface cavity design as illustrated in FIG. 9. The density of a 
compact is a function of the roll force, which is proportional to the 
compaction pressure. Compaction pressures in the range of from about 500 
psi (3,500 KPa) to about 80,000 psi (559,000 KPa) form a usable silver 
nitrate compact of the invention, although the integrity of the compact is 
better at a higher compaction pressure and less fines may be generated. 
Material fed into the roll gap is subject to a compaction force 
proportionate to the roll force, although a precise compaction pressure is 
difficult to measure. In the described embodiment, using a roll compactor 
having about a 4 inch (102 mm) wide compacting zone between two 10 inch 
(25.4 cm) diameter rolls, a preferred roll force is in the range of from 
about 16,000 pounds (7,270 kg) to about 40,000 pounds (18,200 kg) to 
produce good compaction pressures. 
The separated, dried silver nitrate crystals are typically fairly dense. As 
the roll force is increased there is a point at which the crystals 
approach maximum compaction. "Compaction ratio" is the ratio of the 
density of the silver nitrate compact to the density of the uncompacted 
silver nitrate crystals. Typically, the compaction ratio for silver 
nitrate has a maximum value of about 2 and is approached in the described 
embodiment at a roll force of about 32,000 pounds (14,545 kg). The density 
of a compact of silver nitrate is typically in the range of from about 3.5 
g/cc to about 4.35 g/cc for a roll force of from about 16,000 pounds 
(7,270 kg) to about 40,000 pounds (18,200 kg), respectively. 
Roll speed can be selected so as to optimize product throughput while 
maintaining compact coherency. The maximum speed should therefore not 
exceed that at which a starved feed condition occurs. A minimum speed 
should be established such that a sufficient throughput is achieved and 
such that product bridging can be substantially avoided. "Bridging" is the 
fusing together of crystals. One skilled in the art can readily select and 
maintain appropriate minimum and maximum operating roll speeds, and the 
roll speed can also be controlled automatically as is further described 
herein. 
It is desirable to maintain a substantially continuous feed of crystals to 
compactor 100 in carrying out the continuous process of the invention, but 
if the level of silver nitrate crystals is above feed tube 90, bridging of 
the crystals can occur between centrifuge 78 and compactor 100, slowing 
the flow of crystals. Accordingly, there is provided an optical level 
monitor comprising light source 120 and detector 122 which are each 
positioned such that feed tube 90 is positioned therebetween. Feed tube 90 
is fabricated of clear teflon or other translucent material to allow 
detector 122 to monitor the level of crystals in feed tube 90. Detector 
122 provides an output signal to controller 124, and controller 124 
computes a correction value and provides a control signal to adjust the 
speed of compactor 100, thereby maintaining the desired level of crystals 
in feed tube 90. One skilled in the art can determine the relative 
positions of light source 120, feed tube 90, and detector 122 and the 
appropriate instrument and control settings. For example, light source 120 
should be positioned sufficiently close to feed tube 90 for detector 122 
to accurately monitor light transmitted through feed tube 90 during 
operation, e.g. as dust collects on the inner surfaces of feed tube 90. 
Although compacting the silver nitrate crystals helps to minimize the 
amount of fines present in the product, fines are not reduced to zero. 
Fines can be further removed from the compacted product prior to storing 
the product by contacting the compacted product with a motive airstream to 
sweep fines from the product. In a preferred embodiment further described 
below, the airstream containing fines is also employed to dry separated 
silver nitrate crystals in the centrifuge and fines are recovered for 
compacting. 
The feeding of crystals to the compactor in the continuous process of the 
invention should achieve a uniform and continuous crystal feed. Feeding 
can be either gravity feeding or force feeding, both of which are well 
known in the art. In the embodiment of the invention using a centrifuge 
separator/dryer, with the compactor positioned under the centrifuge as 
shown in FIG. 7, gravity feed of crystals should generally suffice. 
Compacted bodies of silver nitrate are discharged from compactor 100 to 
line 126. Means for passing air through the compacted bodies to remove 
fines comprises line 127 by which air is introduced to line 126 to contact 
and pass through the compacts. Line 128 is means for recycling air 
containing fines via line 86 to centrifuge 78. The compacted product can 
be discharged directly into storage container 130, which can be maintained 
at a positive pressure to prevent contamination of the stored silver 
nitrate product by providing a filtered air supply as shown to storage 
container 130. FIG. 8 illustrates a typical compact prepared by the 
invention. The compact is preferably spherically or elliptically shaped as 
shown, presenting fewer contact points for crystals to fuse and allowing 
the compacts to separate easily such as when poured from a storage 
container. 
The process for preparing the compacted silver nitrate crystal product is 
continuous. Accordingly, it is important to substantially match product 
flow rates in the subsystems that comprise the overall system and 
apparatus, e.g. the crystallizer, the separator, and the compactor. One 
skilled in the art can readily select the appropriate such flow rates and 
can select the specific component sizes to achieve the desired system 
stability. Means for adjusting subsystem product flows to compensate for 
such changes or transients in flow rates can be provided. For example, the 
speed of the compactor is adjusted as described above to maintain a 
desired level of crystal feed to the compactor. Similarly, slurry 
withdrawal rate can be adjusted by varying the speed of pump 70 to adjust 
slurry flow to the separator. 
It is preferred that the process of the invention is carried out at a 
system pressure below ambient air pressure in order to minimize leakage of 
material from the system to the environment to limit exposure to workers 
and others. 
The invention is further illustrated by the following Examples. 
EXAMPLE 1 
Silver nitrate crystals were prepared by evaporative crystallization and by 
cooling crystallization to compare crystal growth rates and estimate 
comparative residence times. Tests were run with both types of 
crystallizers on a feed silver nitrate solution containing impurities, and 
also with the evaporative crystallizer on a feed silver nitrate solution 
substantially without impurities. Each test was conducted using a 4 liter 
crystallizer as follows: 
Vacuum Cooling Crystallizer 
A feed solution of silver nitrate containing impurities was introduced to a 
cooling crystallizer. The residence time in the crystallizer was 1.5 
hours, and the crystal growth rate was calculated to be 0.2444 mm/hr. 
Evaporative Crystallizer 
A feed solution of silver nitrate, which in a first run contained 
impurities and in a second run was substantially free of impurities, was 
introduced to an evaporative crystallizer. The residence time in the first 
run was 1.5 hours, and the crystal growth rate was calculated to be 0.3474 
mm/hr. The residence time in the second run was 30 minutes, and the 
crystal growth rate was calculated to be 1.2360 mm/hr. The results 
demonstrate that the evaporative crystallizer provides improved silver 
nitrate crystal growth rate, especially for a silver nitrate feed solution 
containing a low level of impurities. 
EXAMPLE 2 
A feed solution comprising 79 percent by weight of silver nitrate and about 
0.02 percent by weight of impurities was introduced to a 3 liter, 
jacketed, draft tube, evaporative crystallizer having an A310 impeller 
agitator that was run at a speed of 800 rpm. The solution was heated to 
50.degree. C. The solution was then cooled at a rate of 25.degree. C. for 
2 hours (which is therefore the `mean residence time` for the test) to 
form a slurry comprising silver nitrate crystals. The slurry was then 
removed and immediately filtered to collect the silver nitrate crystals. 
FIG. 3 is a photomicrograph of the silver nitrate crystals. The crystals 
exhibited a platelet-type of crystal habit not readily dewaterable by 
separation techniques such as centrifugation. 
EXAMPLE 3 
Tests were conducted using the apparatus as illustrated in FIG. 1, which 
was scaled up from the crystallizer apparatus used in Example 1. A silver 
nitrate feed solution with a density of 2.7 g AgNO.sub.3 /cc solution and 
a temperature of 74.degree. C. was introduced to a 125 gallon (473 liter), 
draft tube, agitated, evaporative crystallizer at a flow rate of 4 gallons 
per minute. The solution was agitated, and vacuum was maintained in the 
crystallizer of from between 61 mm Hg absolute and 112 mm Hg absolute, 
corresponding to solution temperatures of between 45.degree. C. and 
65.degree. C. The residence time varied from about 1.5 to 3.5 hours. 
FIGS. 4 and 5 are electron micrographs of the crystals formed at a 1.5 hour 
residence time. Surprisingly, the silver nitrate crystals formed in the 
scaled-up apparatus exhibit more even growth along the crystal axes, that 
is, they are regularly shaped cubical crystals and less platelet in shape 
than the silver nitrate crystals formed in the 3 liter evaporative 
crystallizer in Example 2 at the same residence time of 1.5 hours. The 
cubical crystals have a mean particle size in the range of from about 200 
to about 600 microns, depending on process parameters such as residence 
time and crystallizer slurry homogeneity. 
The cubical crystals of FIGS. 4 and 5 were found to filter out, and 
therefore are deliquored, more easily than the platelet-like crystals of 
FIG. 3. Representative distributions of particle size are shown in FIG. 2 
in which the mean particle size is indicated for each curve. 
EXAMPLE 4 
Silver nitrate crystals prepared in the apparatus and process of the 
invention, as illustrated by FIGS. 1 and 6, were dried in a rotary screen 
centrifuge, both without air supplied to the centrifuge, and with drying 
air supplied to the centrifuge at various flow rates. The results are 
shown in Table 1 below and in FIGS. 10-12. "N/A" means that data was not 
taken or was not available. 
TABLE 1 
__________________________________________________________________________ 
Dried Silver 
Air Flow/Product Rate 
Air Flow Rate 
Moisture Slurry 
Nitrate Product 
in CFH or air/lb/hr of 
in CFH Content 
Slurry Pump 
Denisty 
Rate in lb/hr 
crystals (m.sup.3 /hr of 
Run 
(m.sup.3 /hr) 
(Wgt %) 
Speed % 
(g/cc) 
(kg/hr) air/kg/min of crystals) 
__________________________________________________________________________ 
1 0 (0) 0.3000 
8.9 3.06 N/A N/A 
2 300 (8.5) 
0.0130 
7.0 N/A 281 (128) 
1.0 (0.06) 
3 500 (14) 
0.0800 
7.0 N/A 281 (128) 
1.7 (0.10) 
4 500 (14) 
0.0140 
8.5 N/A 342 (155) 
1.7 (0.09) 
5 540 (15.3) 
0.0470 
10.5 2.960 
135 (61) 
N/A 
6 500 (14) 
0.0045 
9.0 3.000 
182 (83) 
2.7 (0.16) 
7 500 (14) 
0.0035 
9.2 2.990 
212 (96) 
2.3 (0.14) 
8 500 (14) 
0.0016 
12.4 3.400 
316 (144) 
1.5 (0.09) 
9 387 (11) 
0.0035 
10.0 3.070 
285 (130) 
1.3 (0.08) 
10 583 (16.5) 
0.3300 
8.1 3.030 
285 (130) 
2.0 (0.12) 
11 583 (16.5) 
0.0011 
8.5 3.040 
285 (130) 
2.0 (0.12) 
12 387 (11) 
0.2100 
22.7 2.970 
667 (303) 
2.0 (0.03) 
13 420 (11.9) 
0.50 23.1 2.870 
292 (133) 
1.44 (0.08) 
__________________________________________________________________________ 
The product pH in runs 11, 12, and 13 was 5.5, 4.9, and 4.9, respectively, 
and the results are shown in FIG. 11. 
As can be seen from Table 1 and the figures, good percentage decreases in 
product moisture are exhibited until the drying air flow rate is about 1 
CFH/(lb/hr) (see FIG. 10), at which point increases in flow rate do not 
provide a high percentage of drying of the product. FIG. 11 shows that the 
product pH increases as air flow rate is increased. Thus, as the moisture 
content in the product decreases, the pH increases, due to removal of 
residual acidic liquors in the crystal product during the air-drying 
process. Variations in the results occurred and can be explained as 
follows: 
Run 3--the product exhibited a higher than expected moisture content, which 
was probably attributable to residual water blown into the product from 
the air lines upon start-up of the drying air. 
Run 5--moisture content in the product was higher than expected, probably 
due to the relatively low slurry density. 
Run 10--the moisture content in the product was higher than expected, 
probably attributable to residual water blown into the product from the 
air lines upon start-up of the drying air. 
EXAMPLE 5 
Compaction tests were carried out to determine the feasibility of 
compacting silver nitrate to form coherent, flowable compacted bodies. The 
compactor was a hydraulic press employing a cylindrical ram die with an 
inside diameter of 1.127 inches (28.6 mm) and a cross-sectional area of 
0.9975 square inches (6.43 cm.sup.2). 
Each test was conducted with a die loading of about 10 grams of silver 
nitrate powder. The uncompacted powder density was about 2.28 g/cc and had 
a moisture content of between 0.15 and 0.25 weight percent. The ram was 
inserted and spun gently to level out the surface of the powder. 
Measurements of the die length prior to compression were used to determine 
the uncompressed bulk density of the powder. The die was placed in the 
press and placed under the selected load for about 10 seconds. The 
resulting wafer was ejected from the bottom of the die and weighed and 
measured to determine the resulting wafer density. The results are shown 
in FIG. 13 as a graph of wafer density versus compression pressure. 
Compression pressures as low as 1000 psi (6,900 KPa) formed coherent wafers 
that were hard and brittle. At compression pressures of 20,000 psi 
(138,000 KPa) and greater, the wafers did not readily eject from the die. 
At pressures of 40,000 psi (276,000 KPa) and greater, a blow with a mallet 
was required to eject the wafers. At such pressures, the wafers formed 
were harder than those formed at pressures under 20,000 psi (138,000 KPa). 
At high compression forces, the density of the wafers approached the 
theoretical silver nitrate density of about 4.35 g/cc. 
Two samples of wet silver nitrate crystals prepared in a pusher centrifuge 
were also compaction-tested. Compression pressures of 5,000 psi (39,500 
KPa) and 20,000 psi (138,000 KPa) were employed. It was observed that at 
about 5,000 psi (34,500 KPa) several drops of liquid were forced out of 
the die, so that the crystals were being further dewatered in the press. 
In the test at 20,000 psi (138,000 KPa), both fluid and a paste were 
extruded from the die. In both tests, coherent, hard wafers were produced. 
The results show that silver nitrate powder, in either wet or dry form, can 
be pressed into coherent compacts without necessitating the addition of a 
binder. 
Operation of the present invention is believed to be apparent from the 
foregoing description and drawings, but a few words will be added for 
emphasis. 
The process and apparatus of the invention provide an economical, expedient 
way to prepare silver nitrate. The compacted silver nitrate product of the 
invention has superior flowability characteristics and is less prone to 
fuse than non-compacted silver nitrate. The apparatus and process of the 
invention are particularly suited for processing a substantially cubic 
crystal form of silver nitrate, which is easier to deliquor or separate 
from a slurry but has an increased tendency to fuse compared to silver 
nitrate having a platelet crystal morphology. 
While the invention has been described with particular reference to a 
preferred embodiment, it will be understood by those skilled in the art 
that various changes may be made and equivalents may be substituted for 
elements of the preferred embodiment without departing from invention. In 
addition, many modifications may be made to adapt a particular situation 
and material to a teaching of the invention without departing from the 
essential teachings of the present invention. For example, the invention 
is also considered to encompass a precompactor for initially compacting 
the silver nitrate crystal output of the separator and dryer prior to 
feeding the material to the compactor. 
As is evident from the foregoing description, certain aspects of the 
invention are not limited to the particular details of the examples 
illustrated, and it is therefore contemplated that other modifications and 
applications will occur to those skilled in the art. It is accordingly 
intended that the claims shall cover all such modifications and 
applications as do not depart from the true spirit and scope of the 
invention.