Abstract
The enrichment of platinum group metals (PGMs) from low-grade ores needs a new and efficient technique to separate PGMs from other metals. High-pressure leaching (HPL) is a potential method for PGM extraction because it uses high pressure, temperature and chemical environment to leach out the PGMs from the low-grade ores and then recover the metals from an aqueous solution. The HPL efficiency and quality depend largely on the type of leaching reagents, leaching time, temperature and pressure of leaching, as well as leaching kinetics. This paper briefly introduces the basic principles of HPL, discusses the effects of operating conditions on the quality of the extracted pgms, and summarizes the current research progress in the area.
Introduction
Platinum group metals (PGMs), consisting of platinum, osmium, rhodium, iridium, palladium and ruthenium, are widely used in the production of alloys, catalysts and other aspects, such as fuel cell and catalytic converters. Due to their increased demand, the cost of PGMs has been increasing significantly and the parallel exploration of low-grade PGMs ore has become necessary. However, it is very difficult to extract PGMs from these low-grade PGMs using traditional separation methods. Therefore, it is of utmost importance to develop an efficient, economical and environmentally friendly method for separating PGMs from other metals. High-pressure leaching (HPL) is one of the promising methods for PGM extraction as it can effectively enrich PGMs from low-grade ores.
Basic Principles of High-pressure Leaching
High-pressure leaching (HPL) is a leaching process carried out at elevated temperatures and pressures. It has been used in the chemical and metallurgical industries for decades for ore treatment. In HPL, metals such as PGM can be extracted from low-grade ores using the following steps:
1) Pre-oxidation process: usually an oxidative leaching reagent with a HPL leaching reagent is used to oxidize ore minerals. This pre-oxidation process can reduce the leaching time and improve leaching efficiency.
2) High pressure leaching: in the leaching process, leaching reagents such as sulphuric acid, hydrochloric acid or sodium cyanide are used to leach out pgms from low-grade ores. These leaching reagents can be added into a closed and pressure resistant vessel and heated. The pressure in the vessel is then increased to the desired pressure and temperature to leach out PGM.
3) Separation and purification: the leached solution is then subjected to electrochemical or chemical separation processes to purify the extracted pgms. The electrolytic process can remove impurities present in the solution and precipitate the pgms to form an insoluble electrolytic concentrate.
4) Recovery: finally, the precipitated metals can be further recovered using common techniques such as solvent extraction, ion exchange, and precipitation.
Effect of Operating Conditions on Quality of Extracted PGMs
The efficiency and quality of pgms extracted from low-grade ores using HPL depend mainly on the operating conditions employed in the process. The operating conditions, such as type of leaching reagents, leaching time, temperature and pressure of leaching, leaching kinetics, etc. have a significant impact on the quality of extracted pgms. It is therefore important to understand the effect of different operating conditions on the quality of extracted pgms.
Leaching reagent: different leaching reagents such as hydrochloric acid, sulphuric acid, phosphoric acid and sodium cyanide can be used for HPL processing. The selection of leaching reagents depends on the type of ore being processed and the targeted pgms to be extracted. For example, sodium cyanide is generally used for gold extraction and hydrochloric acid or sulphuric acid are used for PGM extraction.
Leaching time: the leaching time is an important factor to consider when leaching low-grade ores. A longer leaching time can improve the PGM extraction yield, however, it can also lead to undesired oxidation of the PGM and increased acid consumption.
Temperature and pressure: the temperature and pressure of leaching can have a great effect on the leaching efficiency and quality. Generally, the leaching temperature should not be too high or too low and should be selected according to the type of ore and leaching reagents used.
Leaching kinetics: the leaching kinetics mainly depends on the chemical reaction rate of leaching reagents and minerals. The leaching rate is influenced by factors such as particle size and type, leaching reagent, temperature and pressure.
Conclusions
High-pressure leaching (HPL) is a promising technique for the enrichment of Platinum group metals (PGMs) from low-grade ores. The efficiency and quality of the PGM extraction depend largely on the type of leaching reagents, leaching time, temperature and pressure of leaching, and leaching kinetics. In order to achieve efficient and quality PGM extraction, an optimal combination of operating conditions is necessary. Further research is needed to improve the performance of HPL extraction and to develop better understanding of the underlying processes.
