Kemija u Industriji (Jul 2012)
Procedure of Destructive Chemical Recovery of Precious Metals in Nitric Acid Production
Abstract
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.
