Abstract
In order to reduce the use of fossil fuels in pyrometallurgy, different approaches are required. One potential alternative is hydrogen, which can be used in industrial burners instead of fossil fuels such as natural gas. When it reacts with oxygen, it produces water vapour instead of CO or CO₂. Another decarbonisation approach involves using electric energy; however, traditional furnaces may not be compatible with existing systems. Plasma torches can offer a way to deliver electric heat similar to that of gas burners.
Thermal plasma technology exists for quite some time but are gaining renown as a possible solution in decarbonisation. Plasma is known as the 4th state of matter, which is defined as ionized gas with the same amount of positive and negative charges. To ionize a gas, different techniques can be applied. For example, Radiofrequency (RF)-Plasma through induction, transferred and non-transferred electric arc plasma or microwave coupled plasma, which is used in these investigations.
The aim of this study is to investigate the feasibility of producing hydrogen plasma using microwaves and to combine hydrogen combustion with a plasma torch thereby providing heat as the sum of chemical and electrical energy. Combined with controlled addition of air or oxygen into the reaction chamber, neutral, oxidative or reductive atmospheres can be achieved.
The reductive behaviour of hydrogen speciation will play a key role in these investigations. With different levels of ionisation, a different speciation of the hydrogen will take place inside of the plasma. For example, H2, H2+, H, H+ could occur, depending on temperature and other parameters like gas composition, pressure or density.
Equipment used in the test setup is a 10-kW microwave torch with a free mixable input gas mixture between 0 to 100% H2 in N2 to reduce a granulated black copper furnace slag, which contains high amount of cuprous oxide. To cool the plasma leading quartz tube, a controlled air stream is applied on the outside. Additional heating, to ensure a molten slag phase for interaction, is delivered through positioning of the crucible inside a resistance heated furnace. With entering the reaction chamber, both flow streams are combined, leading to a partial combustion of existing hydrogen. This hot gas provides a reductive atmosphere and supplies heat to the slag, which leads to a reduction and the production of a molten metallic copper and secondary slag phase. Focus of the studies is to test melting and reductive behaviour whilst varying parameters like hydrogen content in the gas mixture, radial and axial gas flow and atmospheric conditions resulting from overall gas input including combustion air. Key benchmark is the production of a metallic, copper rich, phase. Furthermore, measurements of the speciation of the plasma contents with regard to the position to the gas ionisation area and the slag phase are made with OES. as well as parameters like power input, gas flows and gas mixtures.