The technology involves the disposal of environmentally hazardous phosphogypsum dumps with the extraction of valuable products for the chemical and metallurgical industries, as well as agriculture, along with the simultaneous extraction of rare earth metals (REMs).
As is well known, REMs have become a central topic in today's political agenda - U.S. President Donald Trump highlighted access to these resources as a key national interest in Ukraine. REMs are indispensable, for example, in the production of various electronic device components and metal alloys. Academician Alexander Fersman called them "vitamins of industry." China is the main producer of REMs, accounting for 70% of global production.
Despite their name, rare earth metal deposits are quite widespread. The problem is that all territorially accessible natural deposits contain no more than 2% of these elements. Their extraction is economically viable only as a by-product. A promising approach, in particular, is the production of REMs in combination with phosphoric acid from apatite concentrates.
The most common dihydrate method of sulfuric acid processing of apatite raw materials leaves about 15% of rare earth metals (REMs) in the phosphoric acid solution. The rest ends up in the phosphogypsum dumps. These dumps are classified as a fourth class of environmental hazards. The state charges a substantial fee for such disposal because the volumes of phosphogypsum in the dumps are quite large. To illustrate, for every ton of phosphoric acid produced, 4-5 tons of this by-product are generated, which is currently hardly used anywhere. For instance, only phosphogypsum with a high degree of purification meets construction standards. However, this is an unreasonably expensive process.
Researchers from the Department of General and Physical Chemistry at the Saint Petersburg Mining University of Empress Catherine II, using the laboratory potential of the university's scientific center "Mineral and Technogenic Resource Processing Problems," have proven the effectiveness of utilizing any phosphogypsum dumps using the carbonate conversion method.
«We are talking about treating phosphogypsum with an ammonium carbonate solution. The main products of the conversion are chalk (calcium carbonate) and ammonium sulfate. Chalk is suitable for use in metallurgy, chemical, and construction industries. Ammonium sulfate is a complex mineral fertilizer.
At a conversion temperature of about 100 degrees Celsius, all rare earth metals completely transfer into the carbonate solution. They are extracted from the solution through fractional crystallization or by using ion-exchange resins, yielding individual rare earth elements, which become standalone products» -said the developer of the technology, Professor Tatiana Litvinova from the Department of General and Physical Chemistry.
During laboratory experiments, the scientists at the Mining University obtained new data on the kinetics and thermodynamics of REM extraction and their forms of existence in acidic phosphate-sulfate environments, as well as the behavior of phosphogypsum components during carbonate conversion.
Alternative research in the field of efficient rare earth metal (REM) extraction from phosphogypsum is being conducted at a number of universities and corporate research laboratories in Russia and abroad. Typically, this involves leaching with sulfuric acid. However, this method has two significant drawbacks: it is quite expensive, and the extraction of REMs does not include the utilization of technogenic waste.
The development by the scientists at the Mining University is free of these drawbacks. The authorship rights for this technology are confirmed by patents. Further research will focus on the extraction of individual rare earth elements from the solution. In this regard, active collaboration is underway with colleagues from several Chinese universities.
The economic effect of the development, when implemented at apatite ore processing enterprises, consists of income from the sale of the final products of processing (rare earth metals, chalk, and ammonium sulfate) and the reduction of phosphogypsum disposal areas, which in turn reduces environmental fees. The emphasis on the co-extraction of rare earth metals makes the proposed method independent of the composition of the original natural raw materials. The extraction of rare earth metals does not require specially trained personnel and, if necessary, allows for the separation of lanthanides from uranium-thorium group elements.
The development has already attracted interest from companies such as "FOSAGRO" and "AKRON." It can also be successfully used outside of Russia. For example, in North African countries, where phosphogypsum dumps already pose a serious threat not only due to the pollution of surrounding areas but also in terms of climate change and damage to the resort business.