Procedure of Destructive Chemical Recovery of Precious Metals in Nitric Acid Production

Abstract

Svi selektivni heterogeni katalizatori platinske grupe plemenitih metala koji se upotrebljavaju za oksidaciju plinovitog amonijaka do dušikovih oksida u proizvodnji dušične kiseline troše se tijekom svojeg radnog vijeka. Što je veći tlak oksidacije plinovitog amonijaka, to je veći maseni gubitak plemenitih metala platinske grupe s površine primijenjenog selektivnog heterogenog katalizatora. Ukupni gubici tijekom jedne šarže upotrebe selektivnog heterogenog katalizatora mogu iznositi od 20 do 40 % od ukupno ugrađene mase plemenitih metala. Jedan dio izgubljene mase plemenitih metala može se oporabiti ugradnjom odgovarajućih sustava "hvatača" u obliku mreža smještenih ispod katalizatora ili postavljenjem različitih filtara u procesni tok, gdje dolazi do izdvajanja čvrstih čestica plemenitih metala iz plinovite ili tekuće faze. Iako je učinkovitost njihove oporabe relativno velika, oveća količina plemenitih metala zadržava se i na površini operativne opreme zadužene za proizvodnju i predgrijavanje pare u proizvodnji dušične kiseline. Iz navedene operativne opreme zadržana masa plemenitih metala može se ponovno oporabiti postupcima nedestruktivne i destruktivne kemijske ekstrakcije čvrsto-tekuće. U radu je prikazan postupak destruktivne kemijske oporabe predgrijača i kotla za proizvodnju pare primjenom vodene otopine sumporne kiseline masenog udjela od 20 % te naknadni postupak prerade dobivenog taloga do konačne oporabe plemenitih metala. Metodom destruktivne kemijske oporabe ukupno je ekstrahirano 212,64 kg taloga u kojem je nakon postupka prerade određen kvalitativno-kvantitativan sastav s obzirom na Pt, Pd i Rh čiji su maseni udjeli iznosili w(Pt) = 18,119 %, w(Pd) = 1,749 % i w(Rh) = 0,419 %. Opisanim postupkom uspješno je oporabljeno u procesu proizvodnje dušične kiseline 38 528,2 g Pt, 3719,5 g Pd i 891,1 g Rh minimalne čistoće 99,90 %.The heart of the nitric acid production process is the chemical reactor containing a platinum-based catalyst pack and an associated catchment system, which allows the ammonia oxidation reaction to take place efficiently. Under the severe operating conditions imposed by the high-pressure ammonia oxidation process, the catalyst gauzes experience progressive deterioration, as shown by the restricted surface of the catalyst wires, the loss of catalytic activity and the loss of catalytic materials. The higher the pressure of gaseous ammonia oxidation, the greater the loss of platinum group metals from the surface of the applied selective heterogeneous catalysts. Total losses for one batch over the whole period of using selective heterogeneous catalysts may account in the range from 20 to 40 % of the total installed quantity of precious metals. An important part of the platinum removed from the platinum-rhodium alloy wires can be recovered at the outlet of the reactor by means of palladium catchment gauzes. However, this catchment process, which is based on the great ability of palladium to alloy with platinum, is not 100 % effective and a fraction of the platinum and practically all of the rhodium lost by the catalyst wires, evades the catchment package and is then deposited in other parts of the plant, especially heat exchangers. From the above mentioned operating equipment, the retained mass of precious metals can be recovered by the technical procedure of non-destructive and destructive chemical solid-liquid extraction. Shown is the technical procedure of destructive chemical recovery of preheater and boiler for preheating and production of steam by applying sulfuric acid (w = 20 %) and subsequent procedure of raffination of derived sludge, to the final recovery of precious metals. The technical procedure of destructive chemical recovery of precious metals from preheater and boiler for preheating and production of steam in nitric acid production is shown in Fig. 1. The technical and technological characteristics of the preheater and boiler for preheating and production of steam in nitric acid production at Petrokemija d. d. is shown in Table 1. The overall results of the destructive chemical cleaning of the preheater and boiler by H 2 SO 4 (w = 20 %) is shown in Table 3. By the method of destructive chemical recovery, 212.64 kg of dry sludge were extracted, which following the refining procedure of determined qualitative and quantitative composition of Pt, Pd and Rh amounted to: w(Pt) = 18.118 %, w(Pd) = 1.749 % and w(Rh) = 0.419 %. With the applied technical procedure, the mass of the precious metals successfully recovered in the process of nitric acid production was as follows: 38528.2 g of Pt, 3719.5 g of Pd and 891.1 g of Rh with minimum purity of 99.90 %. The entire quantity of recovered precious metals is used for preparation of new catalytic gauzes, which will serve in the nitric acid production for ammonia oxidation