Eco-threat Minimization in HCl Leaching of PGMs from Spent Automobile Catalysts by Formic Acid Prereduction Ha Bich Trinh, †,‡ Jae-chun Lee, †,‡ Rajiv R. Srivastava, †,§ Sookyung Kim,* ,†,‡ and Sadia Ilyas ∥ † Resources Recycling, Korea University of Science and Technology (UST), 217, Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of Korea ‡ Mineral Resources Research Division, Korea Institute of Geoscience and Mineral Resources (KIGAM), 124, Gwahak-ro, Yuseong-gu, Daejeon 34132, Republic of Korea § Research and Product Development, Tae-Hyung Recycling, Nongnam-ro, Gimcheon-si, Gyeongsangbuk-do 740-872, Republic of Korea ∥ Mineral and Material Chemistry Laboratory, Department of Chemistry, University of Agriculture Faisalabad (UAF), Main Road, Faisalabad, 38040, Pakistan ABSTRACT: Reclamation of spent automobile catalysts via aqueous processing for the efficient recovery of Pt, Pd, and Rh (PGMs) has remained a challenge. In this research, the effect of prereduction by HCOOH on PGMs leaching using mild acid was investigated and compared to the typical leaching procedure that uses concentrated HCl. Prereduced samples with particle sizes of <0.59 mm demonstrated a remarkable enhancement in the leaching efficiency of PGMs at a lower lixiviant concentration. Using higher acid concentration (>4.0 M HCl), the prereduction effect on PGMs extraction was negligible compared to leaching without HCOOH prereduction. A detailed study of the influential parameters revealed the optimal prereduction conditions to be HCOOH concentration, 15 vol %; pulp density, 10%; temperature, 60 °C; and time, 1 h. Postreduction leaching in 2.0 M HCl at 90 °C for 2 h yielded > 80% Pt, > 85% Pd, and >62% Rh in leach liquor. A subsequent study on the addition of oxidant during the leaching step further enhanced the extraction efficacy up to ∼95% PGMs by introducing ≥1.5 M NaClO 3 . The results revealed that a HCOOH prereduction step can significantly minimize the environmental impact and cost of reagents with the maximum yield of PGMs in a less acidic solution. KEYWORDS: Sustainability, Platinum group metals, Recycling, Automobiles catalytic converter, HCOOH prereduction, HCl leaching, X-ray photoelectron spectroscopy ■ INTRODUCTION Mandated by environmental legislation on the gaseous emission of automobiles, the use of a three-way catalytic converter is essential for transforming (i) CO to CO 2 , (ii) NO x to N 2 , and (iii) unburnt HC to CO 2 and H 2 O. 1−3 High exposure to gaseous emissions causes serious health problems; thus, emissions need to be regulated within the limits stated in Table 1. 4−11 Gaseous emissions are controlled by the catalytic reactivity of platinum group metals (PGMs) in which Pt and Pd mainly catalyze transformation steps (i) and (iii) and Rh performs step (ii). 12 The newly imposed “Euro 6” and “Tier 3 vehicle emission and fuel standard program” may further increase PGMs consumption from its current figure of ∼10 million ounces. 13−15 In view of the limited natural resources, reclamation of PGMs from spent automobile catalysts is therefore of vital importance. 16 Notably, the recovery of hundred-folds more concentrated PGMs from spent automo- bile catalysts instead of mining primary ores can save ∼41% of the energy costs from ore mining and reduce water consumption by 391.5 m 3 for each kilogram of PGMs in ore milling, 17 which are major advantages of recycling spent autocatalysts. Currently, reclamation of spent autocatalysts is achieved via high temperature smelting using Cu/Ni/Fe to collect the PGMs alloy. 16,18 The concentrated PGMs in the collector-alloy then undergo chemical dissolution and separation to produce pure metals. Generation of a large quantity of slag and the consumption of a significant amount of energy in the smelting process are the major drawbacks. 16 To overcome these limitations, hydrometallurgical routes with low calorie inputs have been explored by several researchers. However, the chemical inertness of PGMs and the refractory cordierite substrate limit their dissolution in an ordinary acid environ- ment, and hence, aggressive HCl leaching with a high dosage of oxidizing agents (HNO 3 /halogens/NaOCl/NaClO 3 /H 2 O 2 ) is commonly applied. 19−25 The use of concentrated HCl and Received: May 16, 2017 Revised: June 21, 2017 Published: June 24, 2017 Research Article pubs.acs.org/journal/ascecg © XXXX American Chemical Society A DOI: 10.1021/acssuschemeng.7b01538 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX