CA1185954A - Foraminate element for the recovery of platinum metal and method of using same - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Improved recovery of platinum lost from the catalyst gauze in a nitric acid plant is obtained by using palla-dium containing recovery gauzes configured to enhance the mass transfer characteristics of the system.

Description

5~
~1-57A

~iETHOD FOR RECOVE~ING PLATINUM
IN A NIT~IC ACID PLAI~T

Nitric acid is produced commercially by passing ammonia and air over an oxidation catalyst which is usually a gauze woven from platinum-rhodium alloy wire.
Typically, the temperature of gas leaving the gauze ranges from about 810~C. 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 type of plant. 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 thouyh the rate of catalyst loss is slow when expressed in terms of weight, the cos-t is usually quite suhstanti~JO In many operations, the cost of platinum lost ~uring production has been said to be the secon~
l~r~est. expense of the operation, exceeded only by the c~st G~ ammonia feedstoc~.
Many approaches have been tried to recover some of the platinum and rhodium. Filters of various materials have been placed downstream of the catalyst gauze to mechanically catch and retain solid particles of platinum and rhodium. Later, it was discovered that various palladium alloys had the ability to withdraw platinum-containing ~apor from the gas stream. The mechanism of this withdrawal has been a subject of some controversy, but it 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 possib]y alloy wi-th palladi.um and catalyze formation of volatile pal.ladium compounds. (See Holtzmann, Chemie ~Lnleur-I'echnik, vol. 40, No. 24:1229~37, 1968.) ~ variety of alloying elements have been selected, mainly for their ability -to improve the mechanical properties of palladium. Typi.cal commercial palladium alloys have contained about 80%
palladium and 20~ gold by weight. The recovery alloys are usually employed ln the form of multiple sheets of woven gauze but knitted meshes or other foraminous elements can al.so 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 ~auze is usually replaced when the catalyst g~uze 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. 1]: Pages 2-8 (Jan., 1969); British Patent 1,082,105; and in Chemie- In~enieur-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 irom 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 troy ounces of platlnum in a medium pressure plant or -to obtain equivalent recoveries wi-th fewer sheets of yauze.
These improvements can make a very siynificant difference in the economic viability of a nitric acid plant. This savings would be less for lower pressure plants, but for higher pressure plan-ts, the savinys become even more substantial. These savinys are now possible because i-t has been discovered that it is possible to es-timate the efficiency of platinum recovery of hiyh pal:ladiurn conten-t gauzes based on the model that -the process is mass transfer limited, that ls, the rate of withdrawal of platinum from the stream of gas cominy from the catalys-t 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 yauze beiny much yreater than the rate at which the platinum species can diffuse to the wire surface from the yas stream. On this basis, it is possible -to rationally design and optimize the configura-tion of the gauze to obtain improved efficiency without incurring e~cessive pressure drop.
tJsing yauzes designed according to the present invention, it is possible 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 platinum lost per ton of ammonia converted, but also the number of tons of ammonia processed is much greater than in lower pressure plants. Fur~her, prior art sinyle 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.

Recovery gauzes, according -to -the present invention, are designed and fabricated by the process compris:ing the steps of (1) measuring the flow rate, conditions and composition of the gas- eous stream -to be treated with the gauze, then

(2) de-termining the physical properties of t~le stream, either by measurement or calculation;
(3t 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 (5) 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 I.
Brief Description of Drawings Figure 1 is a chart showing the predicted instan-taneous recovery efficiency 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 ~0 m/o tmole percent)O

? i~

Figures 2 and 3 are charts which are anaklgous to Figure 1, except that -the corresponding nitrogen loadings are 57 and 100 respectively.
Figure 4 is a plot of the recovery function " ~ " as a function of wire diame~er for a varie-ty 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.
Figure 6 is a graph showing the average recovery efficiencies obtainable with the present invention over the catalyst cycle as a function of nitrogen loading for a typical plant.
Figure 7 is a comparison of predicted recovery efficiencies wi-th 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 diameter 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 about 0.003 to about 0.090 inches, and their respective values are such that the initial product of N and dw is greater than at least about 0.2.

In practice, fo.r purposes of the present invention under the conditions encountered .in most commercial plants, the ini-tial. instantaneous recovery efficiencies (~ ), that is the percentage of platinum in the stream that is recovered by a single gauze having hiyh palladium content, may be e~stimated by use of -the quasi-empirical.
formula exp ~ 2C . a w _ _ L 2 / 3 Re ~ ~
wherein "~ " is -the volumetric void fraction 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 gauæe~ i.e., 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; "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 bulk surface area of the gauæe; that is, the total surface area of one square inch of gauze divided by its superficial volume, "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-Oo 8; and "d " is the diameter of the wires in the gauze. Most of the wires used in gauzes according to the present invention, 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-.7_mations are useful:
a = rr[ 1 + N2dW ]~N
where "N" is the mesh or number of wires per i.nch, and ~ NdW ~ N dw )~
Methods of determining the appropriate mass transfer correlation coefficient "C" and mass transfer correlation exponent "m" are well-known to those ski.lled in the art.
A notable summary of the literature pertain.inc~ -to the usual configurations is found in "Estimation of P1atinum Catalyst Requirement for ~mmoni.a Oxidation" by Roberts and Gillespie in Advances in Chemistry Series, Number 133, Chemical Reaction Engineering II, 1974 pp. 600-611.
For more unusual configurati.ons, 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 (1970) 9, 613 and Shah, Ph.D. Thesis, University of Birmingham, England (1970). For the purposes of this inv~ntion, 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 diffusing species may not necessarily be platinum o~ide.
To expedite design of the recovery gauze for a particular plant, efficiency vs. wire diameter 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 900~C~ can be used with only slight errorO Similarly, the concen-tration of the feed to the catalyst is normally regulated to be-tween 10.0 to 10.5 /o (mole percent) ammonia and 90.0 to 89.5 m/o air, so the composition of the reaction products from the catalys-t gauze remains cons-tant, so that physical properties in tha-t range can be used. In -these ranges, the Schmidt No. is about .9 .95 for diffusion of platinum oxide vapors in air and the dynamic viscosity of the gas is about ~2 x 10-5 poise.
Accordingly, the e~ficiency ~ i5 determined primarily by the mesh "N", and wire diameter 7~ ~I / for a given nitrogen loading "L", where the nitroyen loadiny "L" is the number of short tons of nitrogen (irl ammonia~
passed through each square meter of -the ca-talyst gauze per day. Thus efficiency can be plot-ted as a function of wire diameter for a variety oE 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 combina-tion of wire diameter and mesh. For a given 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 gauæe, it is pre~erred that the volumetric void fraction (~) be between about 0O76 and about 0.5~
Volumetric void fractions from about 0.5 down -to about 0.3 can provide even better recovery efficiencies, bu-t care mus-t be exercised to properly support the recovery yauze so that it is not damaged or displaced by the force oE 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 modlfication of the gauze supports to withstand and properly distribute the resulting force of the str~am on the gauze~ In some circumstances, the cost of power due to pressure drop ~a~
also be of some significance. However, in practice, it is normally sufficient to limit consideration to volu-,.

metric void frac-tions above about 0.3 and preferably in the range of from about 0.5 -to about 0.76. The most preferred range of void fractions is from about 0.5 to about 0.68~.
The method of fabrica-ting gauzes according to the present invention is easily accomplished by plot-ting at least a portion of the appropriate effi.ciency vs. wire diameter graph for the condition.s, such as temperature, pressure and nitrogen loading of -the plant under con-sideration. Then the catalyst cycle length line can be plotte~ on this graph using the followi.ng proce~ure, such that if a mesh and wire diameter combination near the catalyst cycle length line is chosen, the averaye 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 gauz-s 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 ~ across the appropriate efficiency vs. wire diameter graph, such as Fi.gures 1-3. Thcn the appropriate recovery gauze cycle lengths "T" for a variety of mesh sizes and wire dia-meters above this horizontal line are determ1ned using -the formula T - W
1.25~ bL
wherein "W" is the weight of each square rneter of the recovery gauze sheet and "b" is the amount of platinum lost per ton of ammonia processed. In accordance with the mode] 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 thereafter. "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 from first principles or if necessary may be dete:rm.ined empirically. If no hetter data is avai]able from the plant hi.story or the history of a similar plant, "b" may be estimated from Figure 5, presenting loss of 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 leng-th "~ " co:incides with -the planned catalyst cycle length of the plan-t, "T~ Then a gauze giving an acceptable efflciency and pressure drop is chosen near this line. Preferably, to mi.ni~li.ze interest costs, the minimum weight gauze which will both yield an efficiency within the range of this invention and match the planned catalyst cycle lenyth of the plant should be chosen. It is preferred that the gauze sheets used have a weight of less than 2.05 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 length of the nth gauze is determined by using the formula Wn Tn n~1 1.25~n bL [ ~ i)]

where " ~ i" is the recovery efficiency of the ith re-covery gauze sheet and Wn is the weight 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 lost from the recovery gauze for each gram of platinum recovered. In many cases, it will be advantageous to use gauxes of relatively coarse mesh and large diameter wires in the initial layers of the recovery gauze, and to use finer mesh, thinner wires, or both, in the succeeding gauzes, even though the efficiency of the initia:L yauzes may not be as high as could be obtained. ~y appropriately choosing -the mesh and wire diameter for each gauze, it is possible to obtain recovery gauze cycle lenyths 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 rnade yreater than the efficiency of the upstream yauzes.
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-viously, so that it will have an average recovery effi-ciency over the catalyst cycle within the range of this invention; i.eO, 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-fiyuration of each succeediny yauze sheet may then ~e chosen, so tha-t the followiny relationship is approxi-mately satisfied for each gauze sheet:

an(~n)l-m= 2- (G)mln ~ i ¦

where an, dn and ~n are the specific bulk surface area, wire diameter and void fraction, respectively, 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 h gauze sheet; Sc, G, C, m and ~ are as defined previously, while n is the number of the gauze sheet ~12-being desiyned in the pack~ For instance, for the second gauze sheet in the pack, the relationship ~hould be approximately satisfied wi-th n = 2, the third with n - 3, and so on. Greatly improved results can be obtained by insuring that at least one gauze (preferably at leas-t two) in the pack has an average recovery efEiciency exceeding 1 - exp (-3.45/L-7~ and that at least one, but preferably at leas~ two, gauze sh~ets have a recovery gauze cycle length of from about nine- terlths to about eleven-tenths of the p:Lanned catalyst gauze cycle len~th.
Using the method of the present invention 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 configura-tions, it is possible to obtain average efficiencies greater than those given in column 3. Figure 6 is a graph illustrating the average recovery efficie~cies obtainable over the cata-lyst cycle length with the gauzes of the present inven-tion as a function of nitrogen loading, as compared to eEficiencies reported in the prior art.
In practice, recovery gauzes almost always contain a major proportion of palladium or gold and minor additions of other alloying elements which improve mechanical properties. By major proportion of palladium, it is meallt 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 hy 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, horon, and the like. Particularly useful palladium alloys -13~
include palladium/gold, palladium/platinum, palla-dium/nickel, palladium/copper, palladlum/ruthenium, and palladi.um/silver. Alternatively, gauzes containing major proportion of gold and a minor proportion of a platinum group me~al have been suggested, since it has been reported tha-t gold does no-t 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 sarne fashion as the palladium-rich alloys, the mechanical properties of the gold-rich alloys may be improved by adding metals which have a greater a-ffinity for platinum than for oxygen, such as tantalum, niobium, and the like.
Other suitab]e alloying elements include -titanium, zirconium, chromium, nickel, manganese, and the like~
TABLE I
Plant Loading Ef~iciency Tons of Ni-trogen of Gauzes of Efficiency As Ammonia Per the Present of Preferred m2 Per Day _ Invention Gauzes 35-~0 25 27 65-75 17 1~
75~85 16 16 100 + 13 14 r~

For purposes of the present inventlon, the preferred alloys are pal]adium/gold and palladium/nickel alloys, particularly alloys containing at least about 80% palla-dium. 95~ palladium and 5% nickel is a particularly advantageous alloy for the practice of the prescrlt invention, since it is re]atively inexpensive, is easily fabricated and upon exposure to the hot platinum-containing effluent, the wires swell and may double in diameter before they are to be removed. In some cases, the diameter of -the wires in the gauze may more than double, reaching approximately 2~ times thelr ini.tial diameter~ When properly allowed for, this swelling can be particularly advantageous, as the efEiciency 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 ini-tial efficiency of about ll~ could provide an efficiency of about 14% after the wires swell to .012 in., and over about 18% if the wires reach 2~ times their initial diameter, Thus, a gauze which provided an instan-taneous ef~iciency which was initially outside the range of the present invention, can swell to provide an average eEEiciency in the range of the present invention provid-ing a much higher efficiency than would have been pre-dicted based on i-ts initial configuration.
Thus, when nickel/palladium gauzes are used, a gauze may be selected such that its recovery efficiency based on its initial configuration is less than 1 - exp (-3.45/L-7), but upon swelling, these gauzes provide an average recovery efficiency over the catalyst cycle in excess of that glven in Table IA.

_BLE IA

Efficienc~ Loading

3~ 20-25 2~ 35-40 22 ~0-~5 1~ 55-65 12 100-~

In the case of 95% Pd:5% Ni, the average recovery efficlencies over the catalyst cycle ( ~ ~ correlate best when recovery ls predicted based upon the geometric mean of the initial and swelled diameters, but adequate correla-tion 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. If it is desired to account for the effect of swelling in a palladium-gold alloy gauze, the geometric mean wire diameter may be es-timated by multiplying the initial diamete.r by 1.1. Often for 95~
Pd:5~ Ni, the geometric mean diameter can be estimated satisfactorily by multiplying the initial diameter ~y 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 al 50 be used to estimate average efficiencies if geometric mean wire diameters are used and the recovery gauze cycle length is not exceeded.

Specific Embodiments As illustrated in Fiyure 8, the recovery gauzes of the ~present invention may be employed in the form of screens 10 having wires 20 and openings 30. ~s ex~
plained, the combination of -the diameter of wi~es 20 and the mesh or number of wires per lineal inch determininy the mass transfer parameters (MTP) of the screen accord-ing to the formula ~TP = a dw Then, the number of mass transfer units (MTU) represented by a single gauze may be determine~ 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 catalyst 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 strengt~ to withstand the force of the process stream at high reaction tempcra-tures while simultaneous]y enduring the corrosive effects of -the residual ammonia, oxygen and ni-trogen oxide proclucts which are formed during the process.

Design Example I

A recovery gauze is to be desi.gned for a nitric acid plant operating at 900C., 10% NH3 and a loadi.ng of 15 U.S. tons of nitrogen in amrnonia per square meter per day. The plant operates on a cyc:Le length of 130 days, at a pressure of 100 p~s.i.g. To begin, a diagram (Figure 1) is prepared of the single sheet efficiency oE
a recovery gauze as a function of mesh size and wire diameter. Figure 6 is then consulted~ and i-t 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 geome-tric average wire diameter of .0095) in excess of ~0%. From Figure 5, it can be estimated that such a plant can be expected to lose about .~ to .9 grams oi platinum for each ton of nitrogen converted.
Thus, about 12.75 grams of platinum per day are presented to each square meter of gauze which weighs about 916 g/m2. Upon operationl 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 ca-taylst 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 -]8-be lost for each gram 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 Example II

A gauze is to be designed for a plant similar to -that in Design Example I, except that the loading is 57 tons/m~-day, and the cycle leng-th is 60 days. According to Figure 5, a plant of this type can be expectecl to lose between about 1.4 and 1.6 yrams of Pt per -ton oE amrnonia converted. Figure ~ 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. An 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 average recovery efficiency of 67%.

Design Exarnple 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 l.9 grams of platinum for each ton of nitrogen converted, ~hile an efficiency in excess of 12% can be obtained. However, if an 80% Pd:20% Au gauze 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 ca-talyst gauze. Since an 80 mesh by 0.005 in. wire diameter gauze of 80~ Pd-20% ~u has a recovery gauze cycle length of 60 days with a platinum recovery of 948 g/m2, the number of grams of Pt presented to each square meter of the first gauze mus-t be decreased from about 180 grams -to about 105. Thus, 4 coarse gauzes of 50 mesh by .0085 wire diameter should be followed by 4 fine gauzes of 68 mesh by .006 wire diameter to achieve an overall recovery oE 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 length of 150 days. The catalys-t loss rate is known to be 0.144 g. of Pt and Rh per ton of nitrogen.
The production rate of the plant is 330 tons of HNO3 per day, and the effective area of the reactor is 5.8 square meters.
If two s-tandard 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, resul-ting in an average recovery efficiency of the plant cycle length of approximatley 46% per gauze or a total of 71% for both yauzes.
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 days 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.

~ 3 In th~ following Operating R~amples, a gaseous stream of air containing about 10% N~13 by volume was fed to the reactor at a rate of 680 standard cublc feet per hour.
Prior to beginning a run, the feed gas was preheated to a temperature within the range of from about 290-310C.; during the run the gauze exit -temperature was maintained at a rela-tively constant 930C. In Operating Examples 1~6, infra, the run was conduc-ted over an appro~imately 1~6 hour period, and in Operating Examples 7 and 8, the runs were maintained for approxirnately 292 and 483 hours, respec-tively; however, it will be appreciated that, in practice, the reaction period may be varied over a wide range. In Operating Examples 9, 10, 11 and 12, the experiments were conducted in opera-ting nitric acid plants.
'rhis invention will now be illustrated by making reference to examples which specifically describe the gauzes of this invention and their use in recovery processes; however, these examples are illustrative only ~nd 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., short) tons.

Compar~ = ~ ti ~

A recovery gauze pack consisting of two 80% Pd:19.~%
Au:.6~ Ru, 80 mesh by .0039 inch wire diameter gauze sheets was placed between three separa-tor 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 ~.6769 g. The recovery gauzes had a mesh (N~ of 80 and wire diameter (d ) of 0.0031 inches. The sur-Eace area of each recovery screen B5~

(bulk surface area of the wires ~er unit volume of screen) was 263 i.n. 1.
Feed gas consisting of ammoni.a and air was ~orced through the oxi.dation 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 ) was about 95%.
The average recovery ef:Eiciency (~*) for -the P-l/Au recovery gauze pack was determined from assay data by measuring the Pt gain of each recovery gauze and the Pt loss of the ammonia o~idation catalyst ( l.e., the oxidation gauze pack) as follow~:

= Pt Gain Per Recovery Gauze Pt Loss of Oxida- Total Pt Gain For tion Gauze Pack - Preceding Gauzes Following the run, the catalyst weighed 4.3951 g., and subsequent assay data showed a Pt loss of 0.4203 g.
in the oxidation catalys-t. By comparison, the first layer of the recovery gauze pack weighed 0.5965 g., with a P-t 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 efficiency of the second recovery gauze, the weight of Pt gained by the first recovery gauze must be taken into account. The second ]ayer 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 l 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 gau~e sheets -to obtain the average Pt recovery efficiency (~). Table II summari~es the geometry o~ the recovery gauzes employed in Operating Examples 1-6. Based upon these geometries and the flow conditions, the dimensionless mass transfer unit for a slngle screen was calculated from MTU = 2C _ . ad Sc 2/3 Re ~

Predicted average recovery efficiency (~ ) of a single sheet could then be estimated using the formula ~ = l-exp ~-MTU). In a].l runs, the conversion of ni-trogen oxides was in the range of 95-98.9~.
The Pt loss for the catalyst of Operating Examples 1-6, the P-t 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 be-tween recovery efficiency (~) and MTU.

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A recovery gauze pack, designed accordiny to -the principles of thc present invention, consistiny of two recovery gauze sheets (95Pd/5Ni) was placed between three separator screens, as depictecl in Fiyure 9, and this ensemble was placed into a reaction chamber of the type shown in Figure 8 below a 90P-t/5Rh/5Pd oxidation catalyst (15 sheets). The recovery yauzes 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.
Fo]lowing 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 over 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 thcreafter. Assays were conducted on the catalyst and individual gauze sheets in the manner described in the preceding paragraph to determine the average platinum recovery efficiency (~).
The configurations of the recovery gauzes employed in this study are set forth in Table VI:

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On the basis of this study, -tile geometric average mass transfer unit for the recovery gauzes was calculated at 0.260.

Operating Example 8 The procedure of Operating Example 7 was repeated, except that the recovery gauze was opera-ted over a period of 483 hours. The results of this study, inclusive of platinum loss, platinum gain for the recovery gauzes, and their recovery efficiencies are se-t forth in Table V.
These results are also represented in Figure 7.

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-The improvement in Pt recovery efficiency ~ for the 95Pd/5Ni recovery gauzes of Operating EY~amples 7 and 8 is illustrated by Table VI. The benef:i.cial effec-ts attributable to the use of palladium/ni.ckel and the high average mass transfer unlts fo.r the Pd/Ni yauzes of this invention makes -them particularly suitable for platinum/rhodium metal recovery. The data in Table VI
demonstrates the advantages of the Pd/Ni recovery gauzes of this invention and the improvement in platinum recovery efficiency for the 95Pd/5Ni recovery yauzes of Examples 7 ard 8, when compared against an 80Pd/19.4Au/
0.6Ru recovery gauze of Examples 3 and 4 having a similar initial MTU.

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Comparative Operating Example 9 A recovery gauze pack consisting of five 80%Pd:
20% Au, 24 m~sh by 0.008 inch diameter wire gauze sheets were placed between six separator screens in an arrangement similar to that shown in E'iyure 9. This ensemble was placed immediately downstream of a platinum alloy ammonia o~idation catalys-t pack (90Pt/5Rh/5Pd) in a nitric acid plant having a nitrogen loading of 7g tons nitrogen (calculated as ammonia) per square me-ter oE the effective cross-sectional area of -the recovery gauze per day ~ i _ , 78T(N)/m2/d). The plant was operated for 77 days, during which the oxidation catalyst los-t 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 platinum recoveredO Platinum recovery was found to be 42 troy ounccs 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 platinum recovery Eor the five sheets was 24%, a figure which compares favorably with the observed recovery of 22%.
*o illustrate the effectiveness of this system, a recovery gau2e pack was constructed by placing five gauze sheets (manufactured from an alloy of 80Pd/19.4 Au/0.6Ru, having d mesh of 36 and a wire diameter of 0.0071 inches) individually between six separator screens. The recovery gauze pack thus constructed was placed into a reactor with a nitrogen loading of 78~(N)/m2/d. In this operation, the single gauze mass transfer unit (MTU) was calculated at 0.082, and it was predicted that five sheets of recovery gauze would recover about 34% of the pla-tinum lost from the oxidation gauze catalyst.

The recovery gauze pack was installed in the plant immediately downstream of the oxidation qauze pack and the plant was operated for 78 days, ~uring which the oxidation gauze lost 213 troy ounces in weiyht, of which 92~ or 196 troy ounces was estimated to be platinum. At the end of the 7~ day cycle, the recovery c~auze pack was removed and the quan-tity of platinum recovere~ was foun~
to be 35%, based on the recovery yauze pack weight and platinum assay. This figure compares favorably with the predicted recovery of 3~%. 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 manufactured from an alloy composed of 80 weight percent palladium, 19.4 weight percent gold and 0.6 weigh-t percent ruthenium. The recovery gauze was placed immediately downstream of a platinum alloy ammonia oxidation catalyst pack (90Pt/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 estimatcd 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.

Operating Example 11 A platinum recovery gauze pack consisting of 95%
Pd/5% Ni were individually placed between seven separa-tor screensO This pack contained six sheets of pla-tinum recovery gauze, the first three having a mesh of 45 and a wire diame-ter 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 90Pt/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 located 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 tro~ ounces in weight, of which 408 troy ounces (92%) 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 accord.ing to the following equation:

S = (Avera~e Final Wire Diameter~-(Initial Wire Diameter) ~Inltial Wire Diameter) and the results of these determinations are set forth in Table VII.

~ .

TAsLE VII

___ Average Swelling Gauze Sheet Factor (S) ~.

l 1.~5 2 1.30 3 1.08 4 1.0~
0.90 6 0.90 On the basis of the initial wire diameter, mesh size and nitrogen thxoughpu~, 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, t.he total platinum recovery could be predicted to be 83.7~. The recovery, properly based on geometric mean of the final and initial diameters of the wires in the recovery gau7.e, is 76.4%.
An as~ay of the platinum recovery gauze pack showed an actual 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, infra.
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~perating Example 12 A recovery gauze pack consisting of two yauze sheets (80 Pd/19.6 Au/0.4 Ru) were placed be-tween three separator screens, and this ensemble was placed into a first reaction chamher below a 90 Pt/5 Rh/5 Pd oxidation catalyst llO sheets). The Pd/Au gauzes have a mesh of 36 wires per linear inch (N~ and a wire diameter (dw) of 0.0071 inches (N x dw: 0.256~. The oY.idation catalyst weighecl 4.6963 g., and the recovery gauze pack weighed

5.1737 g. prior to the run. The surface area "a" of each recovery screen, tha-t is, the surface area of the wires per unit volume of screen packing, was 117 ina/in3 and 0.555 ftZ 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 preheated to 300C. and ammonia and air were channeled therethrough as a mixed gas stream under a pressure of lO0 p.s.i.g. at a total flow of 680 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 where n and ~' are as defined hereinabove, and ~ is the weight of precious metal recovered by the recovery yauze pack divided by the weight of precious metal in the stream presented to the pack.
Following -the run, the catalyst weighed 4.3973 g., a loss of 0.2989 g. from its starting weight. By comparison, the recovery gauze pack in the first chaTnber weighed 5.2350 g., a gain 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 14.6%. In calculating the weight pick-up efficiency of th~ 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 wîre diameter (dw). In each study, the gauze screen and catalyst employed were weighed prior to use and immediately thereafter, and the the weight changes were converted to weight recovery efficiency ( ~'). Bo-th 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. Bo-th experiments afforded yields of nitrogen oxides in the range of 96-98.9%.

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O ~ ~1 The weight loss for the catalyst of Operating Examples 12-17 and the weight gains for the respective recovery screens and their recovery efficiencies are set forth in Table X:

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This data confirms the hiyh order of recovery (~'~
attributed to the recovery gauzes of this invention.
Fur-thermore, it demonstrates that -the gauzes of Operating Examples 15 & 16, having values of -the 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 (~'1.
Also, these assays confirm that the gauzes recover hoth platinum and rhodium. See in this regard Table XI, where the results of these studies are set forth inclusive of platinurn and rhodium recovery expressed as a ratio.

TABLE XI
_ Type N x dw ~' (%) ~Pt/~h Recovery 3 .195 11.1 16.5 ----2 .248 13.2 10.9 46.3 4 .312 15.9 17.5 39.9 On the hasis of these studies, it was determined that palladium/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 leas-t about 0.3.
It was also found that the recovery gauzes in the first reaction chamber and the recovery gauzes 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 ~3-values o~ N x d . Significantly, -these particular recovery gauæes exhibit an initial N x d parameter of at least 0.3~
The foregoing data shows that the efficierlcy of an 80% palladium and 20~ gold recovery gauze in ammonia oxidation process is significantly improved by cons-truc-ting said gauze to an initial ~ x dw parame-ter of at least about 0.3. A preferred embodiment 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 fmprovement in weight recovery efficiency which can be realized with Pd/Ni recovery gauzes.

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 catalyst weighed 7O107 g., and the recovery gauze pack weighed 5.164 g. 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 Erom 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. I'he recovery gauzes contained 45 wires per linear inch (N) and had a wire diameter (dw) of 0.006 inches (N ~ dw = 0.27). The recovery gauze pack weighed 4.666 g. prior -to the run.
The -two chambers were preheated to 300C. and ammoni,a and air were channeled therethrough as a mixed r j t~
~4-gas stream under a pressure of 100 p.s.i.g. at a total flow oE 6~0 SCFII. Duriny the operation, the first chamber was maintained at a temperature of 930~C. and -the second chamber was maintained at 890C. Arnmonia consti-tuted 10% of the gaseous mix-ture, represen-ting a throughput of 57 tons ni-trogen per square meter per day, that is, 57 t(N2)/m2d.
The weight recovery efficiency ( ~'~ for the Pd/Ni recovery gauzes ln each reactor chamber was determined by measuring the weight gain of each recove~y gauze pack and the weight loss for the ammonia oxida-tion cataJyst.
These measurements were then converted to weight pick-up efficiency (~ ') as per -the equa-tion~ (1 - R) /
wherein n, ~l 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 we ght. 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 ~.826 g., a gain of 0.160 g. On the basis of this da-ta, 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 extent as the Pd/Au alloy recovery gauzes.
The results of these assays are shown in Table ~II where platinurn and rhodium recoveries are expressed as a ratio.

~59..~r;

TABLE XII

Efficiencies Recove.ry Chamber Type N d (in) Nxdw ~ ) of Pt/~h 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 fxom ammonia oxidation catalysts, and that the characteristically improved recovery efficiency associated with a high N x dw produc-t applies equally to the gold and non-gold-containing alloy recovery gauzes of this invention.

Claims (20)

-46-

1. A method for the recovery of platinum and/or rhodium lost from a platinum-containing catalyst during ammonia oxidation processes at temperatures above 850°C;
which comprises bringing said lost platinum and/or rhodium into contact with a foraminate element fabricated from material consisting essentially of palladium and nickel characterized by (a) a novel configuration whereby the initial product of mesh size (N) and wire diameter (dw) for said element is greater than at least about 0.2 and (b) where, for a given nitrogen throughput, the weight recovery efficiency ( ?') is a function of the wire diameter (dw) and mesh size (N) combination, and such that weight recovery efficiency is improved by increasing either the mesh size (N) at a given recovery, wire diameter (dw) or the wire diameter at a given mesh size.

2. The method according to claim 1 wherein the initial product of N and dw is in the range of from about 0.3 to about 0.9.

3. The method according to claim 1 wherein said element is a woven gauze.

4. The method according to claim 1 wherein one or more of said elements are sandwiched between a plurality of separator screens.

5. The method according to claim 1 wherein said element is comprised of at least about 80% palladium by weight and the balance nickel.

6. The method according to claim 1 wherein N is in the range of from about 10-80, dw is in the range of from about 0.003 to 0.090 and their respective values are such that the initial product of N x dw is greater than at least about 0.2.

7. The method according to claim 1 wherein the initial product of N x dw is in the range of from about 0.2 to 0.9.

8. A foraminate element for the recovery of platinum and/or rhodium lost from a platinum-containing catalyst; said element being fabricated from a material consisting essentially of palladium and nickel charac-terized by (a) a novel configuration whereby the initial product of mesh size (N) and wire diameter (dw) for said element is greater than at least about 0.2 and (b) where, for a given nitrogen throughput, the weight recovery efficiency (? ') is a function of the wire diameter (dw) and mesh size (N) combination and weight recovery effi-ciency is improved by increasing the mesh size (N) for a given wire diameter (dw) or increasing wire diameter (dw) for a given mesh size.

9. The element of claim 8 wherein the initial value of Ndw is in the range of from about 0.2 to about 0.9.

10. The element of claim 8 in the form of a woven gauze.

11. The element of claim 8 consisting essentially of from at least about 80% palladium by weight and the balance nickel.

12. The element of claim 8 wherein N is in the range of from about 10-80, dw is in the range of from about 0.003 to 0.090 and their respective values are such that the initial product of N x dw is greater than at least about 0.2.

13. The element of claim 12 wherein the initial product of N x dw is in the range of from about 0.2 to 0.9.

14. In the method for the recovery of platinum and/or rhodium lost from a platinum-containing catalyst during ammonia oxidation processes at temperatures above 850°C. by bringing said lost platinum and/or rhodium into contact with a foraminate element; the improvement which comprises utilizing for recovery purposes a foraminate element fabricated from a material consisting essentially of palladium and nickel, characterized by (a) a novel configuration whereby the initial product of mesh size (N) and wire diameter (dw) for said element is greater than at least about 0.2 and (b) where, for a given nitrogen throughput, the weight recovery efficiency (? ') is a function of the wire diameter (dw) and mesh size (N) combination and weight recovery efficiency is improved by increasing the mesh size (N) at a given wire diameter or increasing the wire diameter (dw) for a given mesh size.

15. The method according to claim 14 wherein the initial value of Ndw is from about 0.2 to about 0.9.

16. The method according to claim 14 wherein said element is a woven gauze.

17. The method according to claim 14 wherein one or more of said elements are sandwiched between a plurality of separator screens.

18. The method according to claim 14 wherein said element is comprised of at least about 80% palladium by weight and the balance nickel.

19. The method according to claim 14 wherein N is in the range of from about 10-80, dw is in the range of from about 0.003 to 0.090 and their respective values are such that the initial product of N x dw is greater than at least about 0.2.

20. The method according to claim 14 wherein the initial product of N x dw is in the range of from about 0.2 to 0.9.