Mining industry : innovations and prospects for the development of materials science III : selected peer-reviewed full text papers from the 9th International Scientific Conference on Innovations and Prospects of Development of Mining Machinery and Electrical Engineering (IPDME 2021) : selected peer-reviewed full text papers from the 9th International Scientific Conference on Innovations and Prospects of Development of Mining Machinery and Electrical Engineering (IPDME 2021), April 22-23, 2021, Saint-Petersburg, Russian Federation