Introduction
Platinum is a precious metal which finds huge applications in the field of catalytic converters, chemical and petroleum refining, in electronics and even in cancer therapy. The global recovery of platinum from spent catalyst is about 35%. Large amounts of catalysts are used in the fluid catalytic cracking, residue fluid catalytic cracking, dehydrogenation, reforming and hydrogenation in petroleum refining, and petrochemical industries. Solid catalytic materials play major role in oil refining industry. Once catalysts complete their life cycle, they are withdrawn from the catalytic refining process and at this stage, catalysts are considered “spent”. Platinum is the main active ingredient that is dispersed on the surface of these catalysts. Even though the platinum loading onto the surface of the catalyst is low, the economic value of the metal makes it essential to recycle it in order to meet future demand and mitigate its supply risk. The aim of this work was to study the recovery of platinum from spent catalyst using a leaching solution consisting of HCl, HNO3 and H2O2. The leaching solution employed promotes a highly efficient dissolution of Pt without or minimal leaching of supports. Chloride media are frequently used for leaching precious metals as chloride forms highly stable complexes with precious metals. Oxidants, such as NaClO3, Cl2, H2O2 and HNO3 are used which converts metallic PGMs (platinum group metals) into ionic state. The ionic state of platinum depends on the concentrations of hydronium and chloride ions, and temperature. The complexes of Pt (II) and Pt (IV) are stable in strong acidic solution (pH < 3). Based on these considerations we optimised a leaching solution which effectively leaches platinum from spent catalyst and offer an advantage over fire assay analysis of precious metals as it is cheap, less time consuming and less hazardous.
Results and discussions
In this study, we took two samples, one reference material and one commercial sample, which were calcined in order to remove the organic and volatile matters. The calcination temperature, H2O2 concentration, solid (sample) to liquid (acid) ratio, reaction time and temperature were varied and their effects were studied for optimal recovery. After several tests the calcination temperature was fixed at 800 ℃, H2O2 concentration at 1%, sample to acid ratio of 0.05 (w/v) and temperature at 95 ℃ for 2 hours. The resulting solution obtained was filtered and platinum concentration was measured in ICP-OES spectrophotometer at 214.4 nm. The results obtained for the quality control sample SRM 2557 was found to be consistent with the certified value with a recovery percentage of more than 99 %. The high value of acid ratio required is due to the refractory nature of platinum which need vigorous oxidising conditions and excess acid to be dissolved.
Table 1: Pt determination in spent automotive catalyst (SRM 2557) and commercial sample (spent petroleum catalyst)
| Sample | Platinum (g/t) | |||
| 600 ℃ | 800 ℃ | 950 ℃ | Declared | |
| SRM 2557 | 1110 | 1127 | 1000 | 1131 |
| Commercial sample | 1210 | 1260 | 1220 | |
Conclusion
Recovery of Pt from spent petroleum catalysts was investigated taking a spent automotive catalyst as the quality control sample. The main parameters affecting the Pt chloride leaching were systematically assessed in order to achieve a satisfactory recovery rate of Pt with minimum reagent consumption, less time consumption and environmental pollution. Pre-treatment by calcination not only removed the organics but also decomposed PtO2 when the temperature was over 600 ℃ and enhanced the leaching efficiency of Pt. However, the leaching efficiency (chemical reactivity) of Pt decreased at 950 ℃. Therefore, the optimum calcination temperature was found to be at 800 ℃. Pt leaching rate was increased steadily with the increasing leaching temperature and reached 99.6 % at 95 ℃ (other conditions being aqua regia S/L of 1:10, 10% H2O2: spent catalysts of 0.5 mL/g). Therefore, in conclusion we can say that an effective leaching method has been optimised for platinum determination in spent catalyst which requires less time, energy and hazardous chemicals compared to fire assay analysis and can be very effective for routine analysis of platinum in such catalysts.
Contributed by Dr. Satirtha Sengupta and Dr.Arijit Goswami under the guidance of Prof. Barun Kumar Gupta