Note: Descriptions are shown in the official language in which they were submitted.
~0-52A
METHOD FOR RECOVE~ING PLATINUM
IN A ~ITRIC ACID PLANT
Nitric acid is produced commercially by passing
ammonia and air over an oxidation catalyst which is
usually a gauze woven from platinum-rhodlum alloy wire.
Typically, the temperature of gas leaving the gauze
ranges from about 810Co to about 960C., most often
above 850C. As ammonia is oxidized, platinum is slowly
lost from the gauze, possibly in the form of the more
volatile oxides. Rhodium is also lost, but this is not
so severe a problem. The rate of loss depends upon the
t~pe of pl~nt. Typically, for each ton of ammonia
converted, a high pressure plant will lose more than one
gram of platinum, while lower pressure plants will lose
less. Even though the ra-te of catalyst loss is slow when
expressed in terms of weight, the cost is usually quite
substantial. In many operations, the cost of platinum
lost during production has been said to be the seconc
largest expense of t~e operation, exceeded only by the
cost of ammonia feedstock.
Many approaches have been tried to recovr~r sorne o~
the platinum and rhodium. Filters of various m~erials
have been placed downstream of the catalyst gauze to
mechanically catch and retain solid particles of platinum
and rhodium. ~ater~ it was discovered that various
palladium alloys had the a~ility to withdraw platinum-
containing vapor from the gas streamO The mechanism of
~.
this withdrawal has been a subject of some controversy,
but i-t has been theorized that, in the course of the
reaction, platinum oxide in the gas phase may revert to
platinum, which either returns to the catalys-t gauze or
is carried away by the stream to possibly alloy with
palladium and catalyze formation of volatile palladiurn
compounds. (See Holtzmann, Chemie- _genieur-Technik,
vol. 40, No. 24:1229-37, 1968.) A variety of alloying
elements have been selected, mainly for their ability to
improve the mechanical properties of palladium. Typical
commercial palladium alloys have contained about 80~
palladium and 20~ yold by weight. The recovery alloys
are usually employed in the form of multiple sheets of
woven gauze but knitted meshes or other foraminous
elements can also be used. The recovery gauze is usually
placed as close as possible to the catalyst gauze, often
withili a few millimeters, usually no more than 10 mm.
Since the catalyst gauze in a nitric acid plant is
changed regularly on a schedule of from about every 35 to
every 270 days, depending on plant design, as a practical
matter, the recovery gauze is usually replaced when the
catalyst gauze is changed, although it is possible to
replace it less frequently. This technology, which is
currently widely applied~ is described in more detail in
U.S. Patent 3,434,820; Platinum Metals Review~ Vol. 13
[No. ]]: Pages 2~8 (Jan., 1969); British Patent
1,082,105; and in Chemie- Ingenieur~Technik, Vol. 40, No.
.
24: 1229-37 (1968~. As applied, the recovery efficiency
of each sheet of recovery gauze obtained using this
technology has ranged ~rom about 10% to 60~, primarily
depending upon the type of plant which is usually speci-
fied in terms of the nitrogen loading of the plant.
Using the method of the present invention, it is
possible to increase the recovery efficiency of each
sheet of recovery gauze by several percent resulting in
the annual recovery of more than a hundred additional
~s~s~
--3--
troy ounces of plati.num in a medium pressure plant or to
obtain equivalent recoveries with fewer sheets o~ gauze.
These improvements can make a very significant di~ference
in the economic viability of a nitric acid plan-t. This
savings would be less for lower pressure plants, but for
higher pressure plants, the savings become even more
substantial. These savings are now possible because it
has been discovered that it is possible to estimate the
efficiency of platinum recovery of high pa].laclium content
yauzes based on the model that the process is mass
transfer limited, that is, the rate of withdrawal of
platinum from -the stream of gas coming from the catalyst
gauze is determined or limited by the rate at which the
platinum species diffuses through the gas to the surface
of the recovery gauze, the rate at which platinum at the
wire surface can be trapped or retained or "alloyed" with
the palladium in the gauze being much greater than the
rate at which the platinùm species can diffuse to the
wire surface from the gas stream. On this basis, it is
possible to rationally design and op-timize the configura-
-tion of the gauze to obtain improved efficiency without
incurring excessive pressure drop.
Using gauzes designed according to the present
inven-tion, it is possi.ble to increase the efficiency of
each sheet in the gauze pack by several percent, depen-
ding on the nitrogen loading of the plant. The method of
the present invention is especially desirable for use in
high pressure plants, since not only is more pla-tinum
lost per ton of ammonia converted, but also the number of
tons of ammonia processed is much greater than in lower
pressure plants. Further, prior art single gauze re-
covery efficiencies for high pressure plants have been
distressfully low, as recovering platinum in these plants
is extremely difficult. Thus, any improvement is par-
ticularly significant for the economics of these plants.
~ .3~
Recovery gau~es, accorcliny to -the present invenkion,
are designed and fabr.icated by the process comprising the
steps of
(1) measuring the flow ra-te, concli-tions
and composition of the gas- eous stream to be
treated with the yauze, -then
(2~ determining the physical properties
of the s-tream, either by measurement or
calculation;
(3) determining the mass velocity (G),
dynamic viscosity (~), and Schmidt No. (Sc)
for the process stream in which the gauze is
to be employed;
(4) estimating the recovery efficiency
of a selected gauze sheet based on the
assumption that platinum recovery is essen-
tially mass transfer limited; and
(53 fabricating and providing a gauze
which will provide a single sheet average
recovery efficiency over the catalyst cycle
within the range of this invention.
For example, recovery efficiencies can be estimated
for gauzes until an optimum configuration is determined,
which will have an average recovery efficiency exceeding
that given in column 2 of Table Io
Brief Description of Drawings
Figure 1 is a chart showing the predicted instan-
taneous recovery efEiciency of various recovery gauzes in
a nitric acid plant, having a nitrogen loading of 15 tons
of nitrogen in ammonia per square meter of catalyst gauze
per day, operating at a catalyst temperature of 900C.
and an ammonia concentration in the feed of 10 m/o (mole
percent).
3~
Figures 2 and 3 are charts which are analagous ~o
Figure 1, except that the corresponding nitroyen loadings
are 57 and 100 respectively.
Figure 4 is a plot o~ the recovery function " ~ " as
a function of wire diameter for a variety of mesh numbers
for a linen weave gauze.
Figure 5 is a chart showing anticipated platinum
loss as a function of nitrogen loading for a typical
nitric acid plant.
Fiyure 6 is a graph showing the average recovery
efficiencies obtainable with the present inven-tion over
the catalyst cycle as a function oE nitrogen loading for
a typical plant.
Figure 7 is a comparison of predicted recovery
efficiencies with a number of experimentally determined
points.
Figure 8 is an isometric view of a typical linen
weave gauze.
Figure 9 is a cross-sectional view of the catalyst
and recovery gauze package in a nitric acid reactor.
Figure 10 is a schematic of the nitric acid reactor.
In many cases, high efficiencies can be obtained by
using a gauze configuration in which the initial product
of the mesh (in wires per unit length) and wire diameter
exceeds at least about 0.2 for gauzes containing a major
proportion of palladium and a minor proportion of nickel.
Preferably, the initial product of the mesh and the wire
diame-ter will be in the range of from about 0.2 to about
0.9. For the lower swelling alloys, such as palladium-
gold, the initial product of the mesh and wire diameter
should be at least 0.3 and preferably in the range of
from about 0.3 to about 0.9, more preferably from about
0 35 to about 0~9. For 95% Pd:5~ Ni gauzes, it is
preferred that the mesh N be in the range of from about
10 to about 80, dw is in the range of from abou-t 0.003 to
about 0.090 inches, and their respective values are such
that the initial product of ~ and dw is greater than at
least about 0 2.
--6--
In practice, for purposes of the present invention
under the conditions encountered in most col~nercial
plants, the initial instantaneous recovery ef~iciencies
(~ ), that is the percentaye of platinum in the stream
-that is recovered by a single gauxe having high palladium
content, may be estimated by use of the quasi-empirica]
formula
e~p ~- 2C __ . adw _
L 2/3 Rem ~1-mJ
wherein "~ " is the volumetric void Eraction of the gauze
which is less than 0~76, preferably less -than 0.685, but
greater than 0; "Sc" is the Schmidt number for the
diffusion of oxidized platinum in the effluent from -the
catalyst gauze, which is usually between .8 and 1.0; "Re"
is the Reynolds number based on the wire diameter and
average velocity of the process stream just upstream of
the recovery gauze, iOe., Re = G dw, where "G" is the
mass velocity of the gaseous stream fed to the catalyst;
" ~ " is the dynamic viscosity of the effluent from the
catalyst gauze; typically, the Reynolds number will be
between 10 and 200, most often from 20 to 50; i'C" is the
appropriate mass transfer correlation coefficient for the
geometry of the trial gauze which usually falls within
the range of from about 0.4 to 1; "a" is the specific
bul}c surface area of the gauze; that is, the -total
surface area of one square inch of gauze divided by its
superficial ~olume, "a" usually has a value within the
range of from about 53 to 640 reciprocal inches; "m" is
the appropriate mass transfer correlation exponent for
the configuration of the gauze usually having a value of
from about 0.6-0.8; and "d " is the diameter of the wires
in the gauze. Most of the wires used in gauzes according
to the present invention r will have diameters varying
between 0.0015 and 0.02 inches.
For a square linen weave gauze, such as those most
often encountered in practice~ the following approxi-
mations are useful:
A = ~ [ 1 + N dw ] N
where "N" is the mesh or number of wires per inch, and
~ 7rNdw (1 ~ N dw )~
Methods of determining the appropriate mass transfercorrelation coeffi.cient "C" and mass transfer correlation
exponent "m'l are well~known to those skilled in the art.
A notable summary of the literature pertai.niny to the
usual configurations is found in "Esti.mat _ of Platinum_ _ _
Catalyst Requirement for Ammonia Oxidation" by ~oberts
and Gillespie in Advances in Chemistry Series, Number
133, Chemical Reaction Engineering II, 197~ pp. 600-611.
For more unusual configurations, these constants may be
determined experimentally. For the common stacked screen
gauzes, suitable correlations may be found in Satterfield
and Cortez, Ind. Eng. Chem. Fundamentals (19701 9, 613
and Shah, Ph.D. Thesis, University of Birmingham, England
(1970). For the purposes of this invention, equation 1
will work adequately with values for "C" of .94 and for
"m" of .7 for the screens, reactors and flow conditions
described in this application, if the values of Schmidt
number and viscosity given below are used, even though
the diEfusing species may not necessarily be platinum
oxide.
To expedite design of the recovery gauze for a
particular plant, efficiency vs. wire diarneter graphs
similar to Figures 1, 2 and 3 can be constructed using
formula 1.
As a practical matter, the properties of the gas
streams vary only by small amounts over the temperature
ranges encountered in practice of from about 810 to
about 960, so that properties at 900C. can be used with
only slight error. Similarly, the concentration of the
feed to the catalyst is normally regulated to between
10.0 to 10.5 m/o (mole percent) ammonia and 90.0 to 89.5
m/o air, so the composition of the reaction products from
the catalyst gauze remains cons-tant, so -that physical
properties in that range can be used. In these ranyes,
the Schmidt No. is about .9 - .95 for diffusion of
platinum oxide vapors in air and the dynamic viscosity of
the gas is about 42 x 10-5 poise.
Accordingly, the efficiency ~ is determined
primarily by the mesh "N", and wire diameter "d ", for a
given nitrogen loading "h", where the nitroyen loadiny
"L" is the number oE short tons of nitroyen (in ammonia)
passed through each square meter of the catalyst gauze
per day. I'hus efficiency can be plotted as a function of
wire diameter for a variety of mesh sizes. Further, void
fraction can be shown parametrically on the same graph,
so that efficiency and the void fraction can be deter-
mined simultaneously for each given combination of wire
diameter and mesh. For a yiven void fraction and number
of gauze sheets, the pressure drop through the gauze can
be estimated using known correlations. To obtain high
recovery efficiency without excessive pressure drop
across the yauze, it is preferred that the volumetric
void fraction (~) be between about 0.76 and about 0.5.
Volumetric void fractions from about 0.5 down to about
0.3 can provide even better recovery efficiencies, but
care must be exercised to properly support the recovery
gauze so that it is not damaged or displaced by the force
o~ the stream of gas passing through it. In many applica-
tions, volumetric void fractions between about 0.685 and
about 0.5 will provide an excellent combination of
especially high recovery efficiency with acceptable
pressure drop. Void fractions of about 0~3 and lower can
be used to provide extremely high recovery efficiencies,
but many existing plants would require modification of
the gauze supports to withstand and properly distribute
the resulting force of the stream on the gauze. In some
circumstances, the cost of power due to pressure drop may
also be of some significanceO However, in practice, it
is normally sufficient to limit consideration -to volu-
metric void fractions above about 0.3 and pre~erably .inthe range of from about 0.5 to about ~.76. The most
preferred range of void frac-tions is from about 0.5 to
about 0.685.
The method of fabricating yauzes accordiny to the
present invention is easily accomplishèd by plotting at
least a portion oE the appropriate effici.ency vs. wire
diameter graph for the conditions, such as temperature,
pressure and nitrogen loading of the plant under con~
sideration. Then the catalyst cycle length line can be
plotted on this graph using the followiny procedure, such
that if a mesh and wire diameter combination near the
catalyst cycle length line is chosen, the average re-
covery efficiency of the gauze over the catalyst cycle
( ~ ) will be within the range of this invention.
The catalyst cycle length line is plotted by deter-
mining which gauzes will yield efficiencies ( ) within
the range of this invention by first consulting Figure 6
and drawing a horizontal line corresponding to the
minimum efficiency determined from Figure b across the
appropriate efficiency vs. wire diameter graph, such as
Figures 1-3. Then the appropriate recovery gauze cycle
lengths "T" for a variety of mesh sizes and wire dia-
meters above this horizontal line are determined using
the formula
~ _ W
1.25~ bL
wherein "W" is the weight of each square meter of the
recovery gauze sheet and "b" is the amount of platinum
lost per ton of ammonia processed. In accordance with
the model of the present invention, the rate of platinum
recovery is approximately constant at least until the
recovery gauze cycle length has been reached, but de-
creases rapidly therea~ter. "W" in general is ~ Pw ~.
" ~ " for a single linen weave gauze can be determined
from Figure 4. For gauzes of a weave other than linen
weave, the weight may be calculated in a similar fashion
.35~
from first principles or i~ necessary may be de~ermined
empirically. If no better da-ta is available ~rom the
plant his-tory or the history of a similar plant, "b" rnay
be es-timated from Figure 5, presen-ting loss oE platinum
per ton of nitrogen processed as a function of nitrogen
loading on -the catalyst gauze. Finally, -the catalyst
cycle length line is drawn connecting the poin-ts where
the recovery gauze cycle length "T " coincides with the
planned catalys-t cycle length of -the plant, "Tp". Then a
gauze giving an acceptable efficiency and pressurc drop
is chosen near this line. Preferably, to minimize
interest costs, the minimum weight gauze which will both
yield an efficiency within the range of this inven-tion
and match the planned catalyst cycle length of the plant
should be chosen. It is preferred that the gauze sheets
used have a weight of less than 2.0S Troy ounces per
square foot or more preferably less than 1.9 Troy ounces
per square foot.
Provided that the recovery gauze cycle length of
preceding gauzes has not been exceeded, the recovery
gauze cycle leng-th of the nth gauze is determined by
using the formula
Wn
Tn n-l
1.25~n bL [ ~ i)]
where " ~ i" is the recovery efficiency of the ith re-
covery gauze sheet and Wn is the weigh-t of the nth gauze.
~s a practical matter, gauzes can be added until costs of
lost palladium, interest for the cost of the gauze,
fabrication and installation over the operating and
recovery cycle are not justified by the weight of the
platinum recovered. Normally, from about .3 to about .5
grams of palladium will be los-t from the recovery gauze
for each gram of platinum recovered. In many cases, it
will be advantageous to use gauzes of relatively coarse
mesh and large diameter wires in the initial layers of
~ 3
the recovery gauze, and to use finer mesh, -thinner wires,
or both, in the succeeding gauzes, even -though the
efficiency of the initial gauzes may not be as high as
could be obtained. By appropriately choosing the mesh
and wire diameter for each gauze, it is possible to
obtain recovery gauze cycle lengths which are close to
the planned catalyst cycle length for each gauze in the
pack. This result can be obtained since the efficiency
of the downstream gauzes can be made greater than the
efficiency of the upstream gauzes.
If it is desired to design recovery packs so that
there are approximately equal recovery gauze cycle
lengths obtained for each sheet in the pack, -the first
sheet of the pack should be designed as described pre-
vious~y, so that it will have an average recovery effi-
ciency over the catalyst cycle within the range of this
invention; i.e., greater than 1 - exp (- 3.45/L-7).
Preferably, the recovery gauze cycle length for this
first gauze sheet will be in the range of from about
nine-tenths to eleven-tenths of the planned catalyst
gauze cycle length for the plant. The geometric con-
figuration of each succeeding gauze sheet may then ~e
chosen, so that the following relationship is approxi-
mately satisfied for each gauze sheet:
a (dn)l m_ Sc / (G)mln
where an, dn and ~n are the specific bulk surface area,
wire diameter and void fraction, xespectively, for the
nth sheet in the gauze; ~1 is the average recovery
efficiency of the first gauze; ~i and~i are the respec-
tive recovery efficiencies and recovery functions for the
i gauze sheet; Sc, G, C, m and ~ are as defined
previously, while n is the number of the gauze sheet
'31.~L~3r3~r:~
-12-
being designed in the pack. For instance, for the second
gauze sheet in the pack, the rela-tionship should be
approximately satisfied with n = 2, the thir~ with n = 3,
and so on. Greatly improved results can be obtained by
insuring that at least one gauze (preferably at least
two) in the pac]c has an average recovery efficiency
exceeding 1 - exp (-3.~5/L-7) and that at leas-t one, but
preferably at :Least two, gauze sheets have a recovery
gauze cycle length of from about nine- tenths to about
eleven-tenths of the planned catalyst gauze cycle lenyth.
Using the method of the present inven-tion for a
given plant, it is possible to obtain average single
sheet recovery efficiencies over -the catalyst cycle ( ~ )
which are greater than the values given in column 2 of
Table I. Using preferred configurations, it is possible
to obtain average efficiencies greater than those given
in column 3. Figure 6 is a graph illustrating the
average recovery efficiencies obtainable over the cata-
lyst cycle length with the gauzes of the present inven-
tion as a function of nitrogen loading, as compared to
efEiciencies reported in the prior art.
- In practice, recovery gauzes almost always contain a
ma]or proportion of palladium or gold and minor additions
of other alloyiny elements which improve mechanical
properties. By major proportion of palladium, it is
meant that the recovery gauze contains at least about 70%
palladium by weight. Preferably, the recovery gauzes
will contain at least about 80% palladium and more
preferably 90~. The most preferred recovery gauzes
contain at least about 95% palladium by weight. Perhaps
the most widely used alloy has been an alloy containing
80% palladium and 20% gold. While this alloy has found
wide use, alternatives have been sought, since inclusion
of gold greatly increases the cost of the gauze. Other
alloying elements for palladium include other platinum
group metals, nickel, manganese, chromium, carbon, boron,
and the like. Particularly useful palladium alloys
~S~.S~i
-13-
include palladium/gold, palla~ium/platinum, palla-
dium/nickel, palladium/copper, palladium/ruthenium, and
palladium/silver. Alternatively, gauzes containing a
major proportion of gold and a minor proportion of a
platinum group metal have been suggested, since it has
been reported that yold does not volatilize to the same
extent as palladium. The ability of these gold-rich
alloys to withdraw platinum seems -to be somewhat less
than the ability of palladium-rich alloys. In the same
fashion as the palladium-rich alloys, the mechanical
properties of the gold-rich alloys may be improved by
adding me-tals which have a greater affinity for platinum
than for oxygen, such as tantalum, niobium, and the like.
Other suitahle alloying elements include -titanium,
zirconium, chromium, nickel, manganese, and the like.
TABLE I
Plant Loading Efficiency
Tons of Nitrogen of Gauzes of Efficiency
As Ammonia Per the Present of Preferred
m2 Per Day Invention Gauzes
10-15 49 52
15-20 41 44
20-25 35 37
25-30 31 33
30-35 28 30
35-40 25 27
40~45 23 24
45-55 21 22
55-65 19 20
65-75 17 18
75-85 16 16
85-100 15 15
100 ~ 13 14
-14-
~ or purposes of the present invention, the preferred
alloys are palladium/gold and palladium/nickel alloys,
particularly alloys containing at least about ~0% palla-
dium. 95~ palladium and 5% nîckel is a par-ticularly
advantageous alloy for the practice of the present
invention, since it is relatively inexpens:ive, is easily
fabricated and upon exposu:re to the hot pla-tinum-
containing effluent, the wires swell ancl may double in
diameter before they are to be removed. In some cases,
the diameter o:E the wires in the gauze may more than
double, reaching approximately 2~ times their initial
diameker. When properly allowed for, this swelling can
be particularly advan-tageous, as the efficiency of the
gauze increases as the wires swell. For example, in a
plant having a nitrogen loading of 57 tons per square
meter per day, a 36 mesh by .0068 in. wire diameter gauze
with an initial efficiency of about 11% could provide an
efficiency of about 14% after the wires swell to .012
in., and over about 18~ if the wires reach 2~ ti~es their
initial diameter, Thus, a gauze which provided an instan-
taneous efficiency which was initially outside -the range
of -the present invention, can swell to provide an average
ef:Eiciency in the range of the present invention provid-
ing a much higher efficiency than would have been pre-
dicted based on its initial configuration.
Thus, when nickel/palladium gauæes are used, a gauze
mcly be selected such that its recovery efficiency based
on its initial configuration is less than
1 - exp (-3.~5/L-7), but upon swelling, these gauzes
provide an average recovery efficiency over the catalyst
cycle in excess of that given in Table IA.
-15-
_BLE IA
Eficlenc~ Load~
-
48 10~15
15~20
3~ 20-25
25-30
27 30-35
2~ 35-40
22 ~0-~5
~5-55
18 55-65
16 65-75
75-85
14 85-100
12 100+
In the case of 95% Pd:5% Ni, the average recovery
efficiencies over the catalyst cycle ( ~ ) correlate best
when recovery is predicted based upon the geometric mean
of the initial and swelled diameters, but adequate
correlation for the 80% Pd:20% Au gauzes can be obtained
if recovery is predicted based upon initial diameter,
since the effect of swelling seems to be somewhat less
pronounced. I:E it is desired to account for the effect
of swelling in a palladium-gold alloy gauze, the
geometric mean wire diamete.r may be estimated by
multiplying the initial diameter by 1.1. Often for 95%
Pd:5% Ni, the geometric mean diameter can be estimated
satisfactorily by multiplying the initial diameter by a
factor in the range of from about 1~4 to 1.6, depending
on the location of the gauze in the recovery pack with
the higher end of the range being used for the first or
second layers in the pack and the lower end for the fifth
and sixth layers~ See Operating Example 11 for more
details. Thus, Equation 1 can also be used to estimate
3~
-16-
average efficiencies if geometric mean wire diameters are
used and the recovery gauze cycle lenyth is not exceeded.
Specific Embodimen-ts
As illustrated in Figure 8, the recovery gauzes of
the present invention may be employed in the form of
screens 10 having wires 20 and openings 30. As ex-
plained, the combination of the diameter of wires 20 and
the mesh or number of wires per lineal inch determining
the mass transfer parameters (MTP) of the screen accord-
ing to the formula
r~Tp = a dw
-
Then, the number of mass transfer units (MTU)
represented by a single gauze may be determined from the
relationship
MTU 2C MTP.
Sc
As shown in Figure 9, prior to a typical run, a
gauze ensemble 20 is placed into reaction chamber 40
(Figure 10) of a combustion vessel 42. This ensemble 20
includes recovery gauze pack 21 and catalyst pack 25
placed adjacent to one another. Catalyst pack 25 con-
tains individual sheets 24 of catalyst in the form of
net-tings or screens stacked one atop the other. In
Figure 9, the cata]yst pack is depicted with seven sheets
of catalyst, but it is to be understood that the precise
number of sheets is not critical and they may be
increased or decreased as needed to effect an essentially
complete conversion of ammonia to nitrogen oxides. One
such catalyst consists of 90% platinum/ 5% rhodium/5%
palladium, but other platinum- containing catalysts may
also be employed with good results. Recovery gauze pack
21 contains two sheets of recovery gauze 22 sandwiched
between separator screens 23. The recovery gauze packs
must be of sufficient mechanical strength to withstand
--17--
the force of the process stream at high reaction tempera-
tures while simultaneously enduring the corrosive effec-ts
of -the residual ammonia, oxygen and ni-trogerl oxide
products which are formed during the process.
Design ~ e~e I
._
A recovery gauze is to be designed for a nitric acid
plant operating at 900C., 10% NH3 and a loading of 15
U.S. tons of nitrogen in ammonia per square meter per
day. The plant operates on a cycle length of 130 days,
a-t a pressure of 100 p.s.i.g. To begin, a diagram
(Figure 1) is prepared of the single sheet efficiency of
a recovery gauze as a function of mesh size and wire
diameter. Figure 6 is then consulted, and it is deter-
mined that an efficiency in excess of 40% should be
obtainable. It can be seen from Figure 1 that a 50 mesh
gauze with wires .0095 in. in diameter would provide a
suitable instantaneous efficiency (~). Therefore, to
allow for swelling, a 50 mesh gauze with wires .006 in
diameter is prepared from 95% Pd:5% Ni. Upon use in the
reactor, the gauze swells by a factor of about 2.5 to a
wire diameter of about .015 in., providing an efficiency
(based on the geometric average wire diameter of .0095)
in excess of 40%. From Figure 5, it can be estimated
that such a plant can be expected to lose about .8 to .9
grams of platinum for each ton of nitrogen converted.
Thus, about 12.75 grams of platinum per day are presen-ted
to each square meter of gauze which weighs about 916
g/m2. Upon operation, the first gauze sheet can be
expected to remove over 40% of this for a recovery of
about 5.1 grams of Pt per day per square meter of gauze,
or about 665 g/m2 over the cataylst cycle. The recovery
gauze cycle length coincides closely with the planned
cycle length of the plant, so this gauze may be used
without a heavier, but less efficient gauze upstream of
it. About .3 to .4 grams of palladium can be expected to
~5~5~
~e lost for each gxam of Pt recovered, Three screens are
used to achieve an average recovery efficiency of 78%. A
successive finer and lighter screen may about be used
downstream -to recover a portion of the residual platinum,
if so desired.
Design ~xample II
A gauze is to be designed for a plan-t similar to
that in Design ~xample I, except that the loading :is 57
tons/m2-day, and the cycle length is 60 clays. According
to Figure 5, a plant of this type can be expected to lose
between about 1.4 and 1.6 grams of Pt per ton of ammonia
converted. Figure 6 shows that an efficiency of more
than 17% can be obtained. It can be seen from Figure 2
that this can be obtained with a 60 mesh screen having a
wire diameter of .006 in. ~n 80% Pd:20% Au screen having
these dimensions is selected. Upon operation, about 85
grams of Pt are presented to each square meter of the
screen and about 14.5 grams are collected each day. Six
screens are used to provide an overall avera~e recovery
efficiency of 67%.
~esign ~xam~le III
A gauze is to be designed for a plant having a
loading of 100 tons of nitrogen in ammonia per square
meter per day, and a cycle length of 60 days. According
-to Figure 5, a plant of this size can be expected to lose
between about 1.7 and 1.9 grams of platinum for each ton
of nitrogen converted, while an efficiency in excess of
12% can be obtained. However, if an 80% Pd:20% Au gauæe
having a mesh of 80 and a wire diameter of .005 in. is
used, even though an efficiency of over 15% is obtained,
the recovery gauze cycle length is shorter than the
catalyst gauze cycle length. Therefore, coarser, heavier
gauzes should be inserted upstream of the finer, lighter
-19-
recovery gauzes after the catalyst gauze. Since an 80
mesh by 0~005 in. wire diame-ter gauze of 80% Pd:20% Au
has a recovery gauze cycle length of 60 days with a
p]atinum recovery of 9~8 g/m2, -the number of grams of Pt
presented to each square meter of the first gauze must be
decreased from about 180 grams to about 105. Thus,
coarse gauzes of 50 mesh by .OOg5 wire diame-ter should be
followed by 4 fine gauzes oE 68 mesh by .006 wire
diameter to achieve an overall recovery of 67%.
Design Example IV
A recovery gauze system is -to be designed for a
nitric acid plant operating at 4.5 atmospheres pressure
and a nitrogen loading of 13.2 tons of nitrogen in
ammonia per square meter per day over a catalyst gauze
cycle l.ength of 150 days. The catalyst loss rate is
known to be 0.144 g. of Pt and Rh per ton of nitrogen.
The production rate oE the plant is 330 tons of HNO3 per
day, and the effective area of the reactor is 5.8 square
meters.
If two standard 80 mesh by .0031 in wire diameter
recovery gauzes of 80% Pd:20% Au are used, the predicted
recovery gauze cycle length is only about 130
days, resulting in an average recovery efficiency of the
plant cycle length of approximatley 46~ per gauze or a
total of 71% for both gauzes.
By following the procedure of Design Example I, it
can be seen that if two 50 mesh by .0064 in wire diameter
recovery gauzes of 95% Pd: 5% Ni are used instead of the
standard gauzes, the predicted recovery gauze cycle
length for the first gauze slightly exceeds 150 aays for
an average recovery efficiency over the cycle length of
72% per gauze for a total of 92%.
Thus, each square meter of the improved gauzes of
the present invention recover over 370 additional grams
of platinum over each cycle length.
S~;
-20-
In the followiny Operatiny Examples, a yaseous
stream of ai.r contai.ning about 10% N~I3 by volume was fed
to the reactor at a rate of 680 s-tandard cubic feet per
hour.
Prior to beyinning a run, the feed gas was preheated
to a temperature within the range of from about
290-310C.; duriny the run the gauze exit -temperature was
maintained at a relatively constant 930C. In Operating
Examples 1~6, infra, the run was conducted over an
appro~imately 1~6 hour period, and in Operating Examples
7 and 8, the runs were maintained for approximately 292
and 483 hours, respectively; however, it will he
appreciated that, in practice, the reaction period may be
varied over a wide range. In Operating Examples 9, 10,
ll and 12, the experiments were conducted in operating
nitric acid plants.
This invention will now be illustrated by making
reference to examples which specifically ~escribe the
gauzes of this invention and their use in recovery
processes; however, these examples are illustrative only
and this invention should not be construed as being
limited to these examples. In these examples, all
proportions for the metals comprising the oxidation
catalyst and recovery gauzes are in weight percent,
unless otherwise stated. All tonnages in this applica-
tion are in U.S. ( i.e.l short) tons.
Comparative Operating Example l
A recovery gauze pac~ consisting of two 80% Pd-19.4%
Au:.6% Ru, 80 mesh by .0039 inch wire diameter gauze
sheets was placed between three separator screens, as
shown in Figure 9, and this ensemble was placed into a
reaction chamber below 10 sheets of 90Pt/5Rh/5Pd oxida-
tion catalyst having a weight of 4.6769 g. The recovery
gauzes had a mesh (N) of 80 and wire diameter (dw~ of
0.0031 inches. The surface area of each recovery screen
-21-
(bulk surface ~rea of the wires per unit volume of
screen) was 263 in. 1.
Feed gas consisting of ammonia and air was forced
through the oxidation catalyst and recovery gauze pack as
a mixed gas stream under a pressure of 100 p.s.i.g. for a
loading of 57 -tons of nitrogen per square rneter per day.
The yield of nitrogen oxides (NO 3 was about 95%.
The average recovery efficiency (~*) for -the Pd/Au
recovery gauze pack was determined from assay da-ta by
measuring the Pt gain of each recovery gauze and thc Pt
loss of the ammonia oxidation catalyst ( 1.e., the
oxidation yauze pack) as follows:
* = Pt_Gain Per RecoverY Gauze
Pt Loss of Oxida- Totai Pt Gain For
tion Gauæe Pack - Preceding Gauzes
Following the run, the catalyst weighed 4.3951 g.,
and subse~uent assay data showed a Pt loss of 0.4203 g.
in the oxidation catalyst. By comparison, the first
layer of the recovery gauze pack weighed 0.5965 g., with
a Pt gain of 0.0853 g. Based on this data, the average
Pt pick-up efficiency (~') of the first recovery gauze
was found to be 20.3~. In calculating the Pt pick-up
e~ficiency of the second recovery gauze, the weight of Pt
gained by the first recovery gauze must be taken into
account. The second layer of the recovery gauze pack
weighed 0.57~7 g. with a Pt gain of 0.0592 g., and the Pt
pick-up efficiency of the second recovery gauze was found
to be 17.67~. The average Pt recovery efficiency was
found to be 19.00%.
Similar comparative studies on similar recovery
gauze screens of varying mesh (N) and wire diameter ~d )
were conducted, using the procedure described in Example
1 to further confirm the applicability of the present
model. In each study, the gauze screens and catalysts
employed were weighed prior to use and immediately
thereafter. Gauze assays were conducted for individual
35~S~
-22-
gau~e sheets to obtain the average Pt recovery effi.ciency
(~). Table II summarizes the geometry of the recovery
gauzes employed in Operating Æxamples 1 6. Based upon
these geometries and the flow conditions, the
dimensionless mass transfer unit for a single screen was
calculated from
MTU = 2C ~ adw
SC 2/3 Remgl~m -
Predicted average recovery efflciency (~ ) of a sinyle
sheet could then be estimated using the formula
~ exp (-MTU)~ In all runs, the conversion of
nitrogen oxides was in the range of 95-98.9~.
The Pt loss for the catalyst of Operating Examples
1-6, the Pt gains for the respective recovery screens,
and their recovery efficiencies are set forth in Table
III. These results are represented in Figure 7, illus-
trating the correlation of the present invention between
recovery efficiency ~) and MTU.
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Operating Example 7
~ recovery gauze pack, desiyned according to the
principles of the present invention, consisting of two
recovery gauze sheets (95Pd/5Ni) was placed between -three
separator screens, as depicted in Fiyure 9, and this
ensemble was placed into a reaction chamber of the -type
shown in Figure 8 below a 90Pt/5~h/5Pd oxidation ca-talyst
(15 sheets). The recovery gauzes were 60 mesh and had
wire diameters (dw) of 0.006 inches. The ammonia
oxidation catalyst weighed 7.1066 g. The separator
screens were in the form of a wire mesh gauze constructed
from a ferrous alloy.
The ammonia and air feed was forced through the
reaction chamber over a 292 hour period as a mixed gas
stream under a pressure of 100 p.s.i.g. for a nitrogen
loading of 57 tons/m2-day~
The average Pt pick-up efficiency (~) for the Pd/Ni
recovery gauzes was determined by measuring the Pt gain
of each recovery gauze and the Pt loss for the ammonia
oxidation catalyst from assay data.
Following the run, the catalyst weighed 6.1783 g.,
and the subsequent assay data showed a Pt loss of 1.0393
g. in the oxidation catalyst. The first layer of the
recovery gauze pack weighed 1.0110 g., and recovered
0.2709 g. of platinum, based on the gauze assay data for
an average platinum pick-up efficiency (~ of 26.07%.
The second layer of the recovery gauze pack weighed
0.9560 g. and recovered of 0.1998 g. of platinum, based
on the gauze assay data for a platinum pick-up efficiency
of 26.0%. The average platinum recovery efficiency was
found to be 26.04%, which is an extremely significant
improvement ~ver known getters operated under similar
reaction conditions.
The gauze screens and catalyst employed in each
study were measured and weighed prior to use and immedi-
ately thereafter. Assays were conducted on the catalyst
and individual gauze sheets in the manner described in
~3s~
-26-
the preceding paragraph to determine the average plat.inurn
recovery efficiency (~).
The configurations of the recovery gauzes employed
in this study are set forth in Table VI:
:~
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o o
o l
s~
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O
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O 0~
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L. ~
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t`~ ~ L' 0 ~1
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t~ ~( ~; Z
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a
P~ U~
~ - \
~1~85~
-28-
On the basis of this study, the geometric average
mass transfer unit for the recovery gauzes was calculated
at 0.260.
Operating _ ample 8
The procedure of Operating Example 7 was repeated,
except that the recovery gauze was operated over a period
of 483 hours. The results of this study, inclusive of
platinum 105s, platinum gain for the recovery gauzes, and
their recovery efflciencies are set for-th in rrable V.
These results are also represent:ed in E'igure 7.
.,
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~30-
The improvement in Pt recovery efficiency ~ for the
95Pd/5Ni recovery gauzes of Operating Examples 7 and 8 is
illustrated by Table VI. The beneficial effects
attributab]e to the use of palladium/nickel and the high
average mass transfer units for the Pd/Ni gauzes of this
invention makes -thern particularly suitable for
platinum/rhodium metal recove.ry. The data in Table VI
demonstrates the advantages of the Pd/N1 recovery gauzes
of this inventi.on and the improvement in platinum
recovery efficiency for the 95Pd/5Ni recovery gauzes of
Examples ~ ard 8, when compared against an ~OPd/19.4Au/
0.6Ru recovery gauze of Examples 3 ard 4 having a similar
initial MTU.
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-32-
Comparative Operatin~ Example 9
A recovery gauze pack consisting oE five 80~Pd:
20~ Au, 24 mesh by 0.008 inch diameter wire gauze sheets
were placed between six separator screens in an
arran~ement similar to that shown in Figure 9. This
ensemble was placed immediately downs-tream of a platinum
alloy ammonia oxidation catalyst pack (90Pt/5Rh/5Pd) in a
nitric acid plant having a nitrogen loading of 78 tons
nitrogen ~calculated as ammonia) per square meter of the
effective cross-sec-tional area of the recovery gauze per
day ( i.e., 78T(N)/m2/d3. The plant was operated for 77
days, during which the oxidation ca-talyst lost 205 troy
ounces in weight, of which 92% of 188 troy ounces were
estimated to be platinum. At the end of the 77 day
operating cycle, the recovery gauze ensemble was removed,
weighed and assayed -to determine the amount of pla-tinum
recovered. Platinum recovery was found to be 42 troy
ounces or approximately 22% of the estimated lost
platinum.
The mass transfer unit ~MTU) for a single gauze in
the recovery pack was calculated to be 0.05, based on its
mesh ~24), wire diameter (0.008 inches) and nitrogen
loading (78T(N)/m2/d). The total calculated p]atinum
recovery or the five sheets was 24~, a figure which
compares favorably with the observed xecovery of 22%.
To illustrate the effectiveness of this system, a
recovery gauze pack was constructed by placing five gauze
sheets (manufactured from an alloy of 80Pd/19.4 Au/0.6Ru,
having a mesh of 36 and a wire diameter of 0.0071 inches)
individually between six separator screens. The recovery
gauze paclc thus constructed was placed into a reactor
with a nitrogen loading of 78T~N)/m2/d. In this
operation, lhe single gauze mass transfer unit (MTU) was
calculated at 0.082, and it was predicted that five
sheets of recovery gauze would recover about 34O of the
platinum lost from the oxidation gauze catalyst.
-33-
The recovery gauze pack was installed in the plant
immediately downstream of the oxidation yauze pack and
the plant was operated for 78 days, during which -the
o~idation gauze lost 213 troy ounces in weight, of which
92~ or 196 troy ounces was estima-ted to be pla-tinum. A-t
the end o~ the 78 day cycle, the recovery gauze pack was
removed and the quanti-ty of platinum recovered was found
to be 35%, based on the recovery gauze pack weight and
platinum assay. This figure compares favorably with the
predicted recovery of 34%. These data are represen-ted on
Figure 7.
Comparative Operating Example 10
A recovery gauze pack consisting of six 36 mesh and
0.0071 inch diameter wire recovery gauze sheets were
individually placed between seven separator screens. The
recovery gauze sheets were manufac-tured from an alloy
composed of 80 weight percent palladium, 19.4 weight
percent gold and 0.6 weight percent ruthenium. The
recovery gauze was placed immediately downstream of a
platinum alloy ammonia oxidation catalyst pack
(9OPt/lORh~ in a nitric acid plant having a nitrogen
loading of 65 tons (in ammonia) per square meter of
reactor cross-sectional area per day, ( i.e.,
65T(N!/m2/d). The plant was operated for 61 days, during
which the catalyst pack lost 137 troy ounces in weight,
of which 92% or 126 troy ounces were estimated to be
platinum. Based on the wire size and mesh of the
recovery gauze sheet and the nitrogen loading for the
particular plant, the mass transfer unit (MTU) of a
single gauze was found to be 0.093, and the predicted
total pack recovery for platinum was calculated at 43%.
This predicted recovery figure ~43%) compared favorably
with the actual or observed platinum recovery of 52%.
This result is shown on Figure 7.
~ 3
-34-
Operatiny Exam
A platinum recovery gauze pack consisting of 95~
Pd/5~ Ni were individually placed between seven separator
screens. This pack contained six sheets of platinum
xecovery gauze, the first three having a mesh of 45 and a
wire diameter of 0.0083 inches and the last three having
a mesh of 60 and a wire diameter of 0.005 inches. This
pack was placed immediately downstream of a 90P-t/5Rh/5Pd
alloy ammonia oxidation catalyst gauze pack in a nitric
acid plant having a nitrogen throughput of 38 tons (in
ammonia) per square meter effective gauze cross-sectional
area per day ( i.e./ 38T(N)/m2/d). The furthest upstream
of the platinum recovery gauzes was gauze sheet
1, followed by gauze sheets 2, 3, 4, 5 and 6, that is,
gauze sheet 6 was lGcated the furthest downstream of all
of the gauzes. The plant was operated continuously for
71 days during which the ammonia oxidation catalyst pack
lost 443 troy ounces in weight, of which 408 troy ounces
(9~) were estimated to be platinum.
At the end of the 71st day in the operating cycle,
the platinum recovery gauze was removed from the plant
and disassembled for inspection. During operation, the
recovery gauze wires increased in size over
their original diameter, and this increase significantly
affected their mass transfer unit values. The wire
swelling factor (S) for each gauze sheet was determined
according to the following equation:
S = (Average Final Wire Diameter) (Initial Wire Diameter)
(Initial Wire Diameter)
and the results of these determinations are set forth in
Table VII.
95~
-35-
TABLE VII
Average Swelling
Gauze Sheet Fac-tor ~S)
1 1.45
2 1.30
3 1.08
4 1.08
0.90
6 0.90
On the basis of the initial wire diameter, mesh size
and nitrogen throughput, the total platinum recovery for
the gauze pack could be predicted to be 69.8%. On the
basis of the final wire diameters for each platinum
recovery gauze with identical mesh and nitrogen loading
parameters, the total platinum recovery could be
predicted to be 83.7%. The recovery, properly based on
geometric mean of the final and initial diame-ters of the
wires in the recovery gauze, is 76.4%.
An assay of the platinum recovery gauze pack showed
an ac-tual -total pack recovery of 306 troy ounces of
platinum (75.0%). The observed recovery of 75.0%
compares favorably with -the predicted recovery of 76.4.
A summary of the parameters for the recovery gauze
packs of Operating Examples 9-11 and their respective
recoveries of platinum metal are set forth in Table VIII,
infr_.
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.
~3S~
Operating ~xam~e 12
A recovery gauze pack consisting of two yauze sheets
(80 Pd/19.~ All/0.4 Ru) were placed ~e-tween three
separator screens, and this ensemble was placed in-to a
first reaction chamber below a 90 Pt/5 Rh/5 Pd oxidation
catalyst (10 sheets). The Pd/Au gauzes have a mesh of 36
wires per linear inch (N) and a wire diamcter (dw) of
0.0071 inches (N x dw: 0~2~6)~ The oxidation catalyst
weighed 4.6963 g., and the recovery gauze pack weiyhed
5.1737 g. prior to the run. The surface area "a" of each
recovery screen, that is, the surface area of the wires
per unit volume of screen packing, was 117 in2/in3 and
0.555 ft~ per Troy ounce of recovery gauze. The
separator screens were in the form of wide mesh gauze
constructed from a ferrous alloy.
In a second chamber, located immediately behind the
first chamber, there was placed a second gauze pack
consisting of two recovery screens (80% Pd:20~ Au),
sandwiched between three separator screens.
The two chambers were prehea-ted to 300C. and
ammonia and air were channeled therethrough as a mixed
gas stream under a pressure of 100 p.s.i.g. at a to-tal
flow of 6~0 SCFH. Ammonia constituted 10~ of the gaseous
mixture representing a throughput of 57 tons nitrogen per
square meter per day, that is, 57 t(N2)m2d. During this
run, the first chamber was maintained at a temperature of
930C., and the second chamber was maintained at 890C.
The test was run over a 146 hour period, and the yield of
nitrogen oxides (NOX) was 98.4~.
The weight recovery efficiency (~') for the Pd/Au
gauzes was determined by measuring the weight gain of
each recovery gauze pack and the weight loss for -the
ammonia oxidation catalyst. The difference in weight was
then converted to weight recovery efficiency ( ~ ')
according to the following equation-
~ R)l/n
3~ i~
-3~-
where n and ~' are as defined hereinabove, and R is the
weigh-t of precious metal recovered by the recovery gauze
pack divided by the weight of precious metal in the
stream presented to the pack.
Following the run, -the catalys-t weighed ~.3973 g., a
loss of 0.2989 g. from its starting weiyht. By
comparison, the recovery gauæe pack in -the first chambe~r
weighed 5.2350 g., a ga.in of 0.0623 g. Based on this
data, the pick-up efficiency ( ~') of the recovery gauze
pack in the first chamber was found to be 1~.6~. In
calculating the weight pick-up efficiency of the recovery
gauze pack in the second chamber, the weight recovery
efficiency of the first chamber must be taken into
account.
Comparative studies were conducted by repeating -this
procedure on recovery gauze screens of varying mesh (N)
and wire diameter (dw). In each study, the gauze screen
and catalyst employed were weighed prior to use and
immediately thereafter, and the the weigh-t changes were
conver-ted to weight recovery efficiency
( ~l). Both runs were conducted over an identical trial
period. The configurations of the catalyst and recovery
gauzes employed in Examples 12-17 are set forth in Table
IX. Both experiments afforded yields of nitrogen oxides
in the range of 96-98.9%.
9~3
~:5 ~COL~')N
u~ ~a~ ~u~ a~
~7 ~
o o o o o o
N
Ou- oa~ oa~ r~
~u~ D U~
Ou~~a~ a~u~ ~
E~ o ~ o o o o
r~
~ o
a) ."
~ s~r~r~ o ~ r~ r~
s~ , ~ ~ u~~o ~ ~ r~
O ~~IN
U~ .~1
-~
. r~
O t~l IH
~J Na~~ ~ r~u~
O~ ~ ~ o
X o a)
I--1a~ Q) ~N ~1~i ~N ~r ~`1
a~ P~ ~1 0
~1 U~
~1 u~ E~
~ _,
s~ U~
~ ~ ~ ~ a~a~ ~ o
a) ~ ~,r~ ~ ~ ~ r~ co
o o o o o o
O O O O O O
~1 ~ . . . .
.,~ _o o o o o o
~_
u~ z; ~ o o o o ~r
a)--~co u.~o .n
X
.~
1~ X ~ ~ ~ In ~ r~
O 1:':1--I ~ ~ ~ ~ ,~
~1~591~i6
- ~o -
The weight loss for the catalyst of Operating
Examples 12-17 and the weiyht gains for the respective
recovery screens and their recovery efficiencies are set
forth in Table X:
u
~ -
I ~ n I ~ In I ~ ~ I Lr ~r I a~ ~D I O
~l
I ~r ~ I ~ ~ I ~ ~ I 1` ~ I ~D O~ I O
` r-l ~ ~ ~ ~ ~ ~ ~
i 3 O r~~ ~ O ~ O r~ O ~rl co co
<I) c~ r--l r-l ~1 ~ u )~ 00 11~1 ~ ~r Ll~ t~l 1~ ~1 0 ~D (~
Z a~ 9 w ~D ~ ~ ~D U')(r) ~ ~D 11-\ 1~ ~ Il~ ~9 Ul
I 1~ O O ~ O O ~) O O~1 O O N O O ~ O O
5:~ ... ... ... ... ... ...
~ 000 000 000 000 000 000
rl U~ I + + I + + I + + I ~ I + ~ I + +
a) :~, ~
~:--
u~
~ ~ ~ ~ o ~ ~ r ~ o~ ~D ~r ~ ~ ~ u~
P~ a~ r~ CO 1~ ~ O ~ ~ ~ ~D ~ In o
U~ ~ ~-- ~ N L~ LO ~O r-l ~ ~D 1~ a~ ~ CC) I` t~ 1~ ~
N (~I~) ~ ~ ~ 1-- l . . . . . .
U ... .... - . .
I
4~ U~
O ~ ~_ o ~
. ~ ct) ~ t~ o ~u~ o r~ I` In O~1 ~ 1~-) U~ ~ ~ ~ O
~1 S ~ ~ n ~ OD O ~ Oa~ ~ In 5
W~ ~ a~ ~ r~ oo ~ ~co e:r ~7 cn o ~1` 1` 1~ a) u~
~1~,~ 3 ~ ~ ~~ . :~
O O ... ... ... . ~ r O ~-1
E~~ z; ~ In ~ O
h
~ a)
~n
o ~- ~
~ ~ ~ (Y~ ~ C)
o ~ C~ X X o X X o X X o X X o X X o ~C X
~ tn
.,~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ a
a ~ r~ O
~ ~ ~ ~ P~ P~ ~ ~
~ r~
~ ~ ~ ~:
~ ~1 a)
--Q~ ~ U~
>1 ~ ~ ~ ~ ~ ~ >1
~; ~ ~ E~ X a)
.
~ X ~ ~ ~r In ~ r~
o ~ ~-~ ~ ~ ~ ~ ~ *
-
-42-
This da-ta confirms the hiyh order of recovery (~')
attributed to -the recovery gauzes of this invention.
Furthermore, it demonstrates that the gauzes of Operating
Examples 15 & 16, having values of t~le product of mesh
and wire size above 0.3, provide excellent recovery
efficiency.
Assays were conducted to compare platinum recovery
efficiency (~ against weight recovery efficiency (~
Also, these assays confirm that the gauzes recover both
platinum and rhodium. See in this regard Table XL, where
the results of these studies are set forth lnclusive of
platinum and rhodium recovery expressed as a ratio.
~ABLE XI
TypeN x dw ~' (%) ~(~) Pt/Rh Recovery
3 .195 11.1 16.5 ----
2 .248 13.2 10.9 46.3
.312 15.9 17,5 39.9
On the basis of these studies, it was determined
that palladi.um/gold recovery gauzes having a mesh size
(N) in the range of from about 50-80 and a wire diameter
(dw) in the range of from about 0.003 to 0.018 inches,
exhibit particularly suitable precious metal recovery
properties provided they possess an N x dw of at least
about 0.3.
It was also found that the recovery gau~es in the
first reaction chamber and the recovery yauzes of the
same material and configuration in the second reaction
chamber exhibited no significant differnece in weight
recovery efficiency ¦~ '). Moreover, it appears from
this data that significantly improved results are ob-
tained using recovery gauzes of the type described in
Operating Examples 15-16, which possess the required
~43-
values of N x d O Significantly, -these p~rticular
recovery gauzes exhibit an initial N x d parame~er of at
least 0.3.
The foregoing data shows that the e~ficiency of an
gO% palladium and 20~ gold recovery gauze in ammonia
oxidation process is significantly improved by construc-
ting said gauze to an initial N x d parameter o~ at
least about 0.3~ A preferred embocliment of this inven-
tion comprises a recovery gauze ensemble comprised of
several such recovery screens sandwiched between several
separator screens.
The following example illustrates the improvemen-t in
weight recovery efficiency which can be realized with
Pd/Ni recovery gauzes.
OPERATING EXAMPLE 18
A recovery gauze pack consisting of two recovery
gauze sheets (Type Ni-B: 95% Pd/5% Ni) was placed between
separator screens and this ensemble was placed into a
first reaction chamber below a 90% Pt/5% Rh/5% Pd oxida-
tion catalyst (15 sheets~. The recovery gauzes contained
60 wires per linear inch (N) and had a wire diameter (dw)
of 0.006 inches (N x dw = 0.36). The ammonia oxidation
ca-talyst weighed 7.107 g., and the recovery gauze pack
weighed 5.164 y. prior to the run. The separator screens
were in -the form of a wide mesh gauze constructed from a
ferrous alloy.
In a second chamber located immediately downstream
from the first chamber, there was placed a second gauze
pack also consisting of two recovery screens (Type
Ni-A: 95% Pd/5% Ni~ sandwiched between three separator
screens. The recovery gauzes contained 45 wires per
linear inch (N) and had a wire diameter (dw) of 0.006
inches (N x dw = 0.27). The recovery gauze pack weighed
4.666 g. prior to the run.
The two chambers were preheated to 300C. and
ammonia and air were channeled therethrough as a mixed
Y,~
-4~-
gas stream under a pressure of 100 p.s.i.g. at a total
flow of 680 SCF~I. During -the operation, the first
chamber was maintained at a temperature of 930C. and the
second chamber was maintained at 890C. Ammonia
constituted 10% of the gaseous mixture, representing a
throughput of 57 tons nitrogen per square meter per day,
that is, 57 t(N2)/m2d.
The weight recovery efficiency (~') for the Pd/Ni
recovery gauzes in each reactor chamber was determined by
measuring the weight yain of each recovery yauze pack and
the weight loss for the ammonia oxidation catalyst.
These measurements were then converted to weiyht pick-up
efficiency (~ ') as per the equation: ~' = 1 - (l - R) /n
wherein n, ~' and R are as previously defined. Fol-
lowing -the run, the catalyst weighed 6.178 g., a loss of
0.929 g. from its original weight. By comparison, the
recovery gauze in the first chamber weighed 5.452 g., a
gain of 0.288 g. The recovery gauze in the second
chamber weighed 4.826 g., a gain of 0.160 g. On the
basis of this data, the weight recovery efficiency ( ~')
of the second chamber recovery gauze was 13.4%.
The used ammonia oxidation catalyst pack and the two
recovery gauze assemblies were assayed to determine their
actual platinum recovery efficiency (~) as well as
confirm that the Pd/Ni alloy recovery gauze recovered Rh
to the same exten-t as the Pd/~u alloy recovery gauzes.
The results of these assays are shown iII Table XII where
platinum and rhodium recoveries are expressed as a ratio.
~3~
-45-
TABLE XII
_
Effici.encies Recovery
Chamber Type N dw(in) Nxdw ~ ) of Pt/Rh
2 Ni-A 45 0.006 0~27 13.4 10.8 27.0
1 Ni-B 60 0.006 0.36 16.9 26.0 50.7
Operating Example 18 illustrates that both nickel-
containing alloys and gold-containing alloys are effec-
tive in recovering platinum and rhodium lost from ammonia
oxidation catalysts, and that the characteristicallv
improved recovery eEficiency associated with a high
N x dw product applies equally to -the gold and
non-gold-containing alloy recovery yauzes of this
invention.
