Platinum recovery

Platinum loss has been reduced by 25-50% when base metal gauzes have been incorporated into the pad as they help to disperse the heat of reaction and reduce 

Since 1920 the platinum recpiired to produce a given amout of nitric acid has fallen to less than 30% of the earlier levels a resnlt of using catalysts with higher activity, greater selectivity, and longer operating fives. It is unlikely that better catalysts will be developed that can replace platinum/rhodium gauze. 

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Recent developments in the ammonia oxidation process have included efforts to reduce catalyst losses in the process. Platinum recovery filters have been installed at various stages in the process. Gold/palladium gauze filter pads have been added on the exit side of the catalyst bed, inside the reactor/converter units. These filters have reportedly ensured a platinum recovery of 80% (Ref. PT4). Another trend has been for the use of additional filters in the downstream units. These filters are of alumino-silicate construction. 

A floating head-type shell-and-tube heat exchanger is recommended for this application because of the need to provide capacity for thermal expansion of the tube bundle. The floating head also enables easy withdrawal of the tube bundle for cleaning purposes. This factor may be very advantageous, not because the streams are subjected to fouling, but because of the possibility that platinum carryover from the reactor will be deposited on the walls of the tubes. Platinum recovery is improved by providing easy access. 

The nitrogen oxide reaction gas stream cannot be directly controlled from the steam superheater. Instead the flowrate, temperature and pressure are predetermined by the reactor feed conditions. No direct control exists on this stream as far as the production of steam is concerned, both inlet and outlet lines possess isolation valves for plant shutdown. These lines would be blanked before any platinum recovery work was attempted on the steam superheater. Inlet and outlet linesalsofeature temperature indicators, consistent with the policy of constant monitoring of this parameter throughout the process. 

The platinum recovery on palladium gauzes depends on the transfer of platinum oxide vapor from the gas stream to the surface of the palladium gauzes. The speed of transfer depends on the design conditions of the nitric acid plant and also on the structure of the palladium gauzes237. 

At temperatures lower than 800°C, the palladium oxide is stable and progressively recovers the palladium wire. This makes it inactive for further platinum recovery. For this reason MTL gauzes are not used in atmospheric nitric acid plants. The speed of diffusion of platinum into the core of the palladium wire decreases over time due to a lower differential of concentration of platinum on the surface and in the core of the wire237. 

A 11.46-g (30.00mmol) quantity of bis(l,2-ethanediamine)platinum(II) chloride (Alfa) is dissolved in a minimal amount of water in a SOO-mL Erlenmeyer flask. [The bis(l,2-ethanediamine)platinum(II) chloride must first have been carefully recrystallized twice to insure its purity. Recrystallization is best accomplished by dissolving the solid in SOmL of water, gravity filtering, and allowing the filtrate to evaporate slowly. Pure [Pt(en)2]Cl2 gives a colorless (not brown or yellow) solution when dissolved in water. Impure [Pt(en)2]Cl2 drastically lowers the yield of this procedure.] A 200-mL volume of concentrated HCl is added, and the solution is heated to 89 °C for 7 h. The solution must not be boiled. The reaction mixture is allowed to cool to room temperature, then it is chilled in an ice-water bath. The solid that forms is collected by vacuum filtration, and the filtrate is set aside for platinum recovery. The solid is then washed with lOOmL of water acidified with five drops of concentrated HCl. The product dissolves in this washing, and some insoluble yellow by-product, which is [Pt(en)Cl2], remains on the paper. This second filtrate is treated with 200 mL of concentrated HCl at room temperature, which precipitates the pure product. Yield 4.65 g (30%). 

Chassary P., Vincent T., Marcano J.S., Macaskie L.E., Guibal E. Palladium and platinum recovery from bicomponent mixtures using chitosan derivatives. Hydrometallurgy 2005 76 131-147. 

Platinum Recovery - During operation the surface of the catalyst is damaged by abrasion and... 

The possible range of platinum losses is given in Figure 8.2. Platinum from the catalyst passes into the gas stream in the form of very fine particles, and its loss can substantially inCTease the production cost. Therefore, several methods of platinum recovery were developed and instaUed in many plants. Two types of recovery systems - catchment gauzes and mechanical filters - are usually offered. ... 

Catalyst inventory required for normal production of the API depends on catalyst consnmption in production and catalyst regeneration cycle time. Time reqnired for cycle of regeneration of catalyst (platinum recovery and new catalyst production) was about 3 months. During that period it was necessary to have enough catalyst in possession for normal production. Since significant improvements in the production of the API were already achieved in reducing catalyst consumption, the scope of the project included only activities related to the reduction of regeneration cycle time. 

Porous supports were used to make commercial platinum sulfuric acid catalysts. These included asbestos, kieselguhr, and silica gel. Rather surprisingly, water-soluble carriers such as magnesium sulfate were also successftrl and made platinum recovery more convenient. A flow sheet of a typical modem susphuric acid plant is shown in Figure 2.3. Table 2.5 describes four typical industrial catalysts. 

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