Design is not just what is looks and feels like. Design is how it works.
Between 1982 and 1987, British Steel (Outokumpu today) made extensive investigations into the treatment of steel dust. Their research was far and wide and covered off many various methods of recycling, including alternative to plasma technologies. But, in 1985, they engaged Tetronics International to carry out a series of consultancy programmes on a plasma solution for the recovery of chromium, nickel and molybdenum from fine airborne dusts generated in the production of stainless steels. Until this time, these dusts were disposed of in landfill sites, leading to potential problems of leaching of heavy metals to ground water, including lead, cadmium and hexavalent chrome.
Our consultancy programme, helped British Steel’s decision to go for a plasma arc smelting system. The key findings included:
a) The plasma furnace was able to treat a wide range of waste materials from the melting unit, including from the EAF, AOD unit and various cutting and rolling residues.
b) The energy input to the furnace is independent of the atmosphere and processing conditions.
c) The totally sealed furnace allowed the processing conditions to be controlled very closely for optimum recovery of valuable metals.
d) The high energy density resulted in high processing rates and a small furnace size.
e) The capital and operating costs were lower than competing technologies. Advice and engineering helped British Steel make a series of decisions to install a Tetronics DC plasma arc smelting unit feeding it into their AC EAF unit at the smelting shop in Sheffield, usually referred to as SMACC.
In 1987, Tetronics International were briefed by British Steel to develop a new plasma approach that could ensure British Steel complied with increasingly tough environmental regulations and offered opportunity to recover as much economic value from these difficult wastes a possible.
Celebrating 25 years of success
In 1991, we began working in the SMACC melting shop which produced around 355,000 tons per annum (tpa) of liquid steel using an AC electric arc furnace (EAF) and an argon-oxygen decarburisation (AOD) unit, for continuous casting into slabs, billets or blooms. On July 1st 1991, we pressed the go button on the Tetronics Plasma Arc programme with the objective of collecting around 8,000 tpa of steelmaking fume and dust using the extraction/recovery system from the baghouse filters.
The DC plasma arc smelting unit was positioned in a self-contained facility, housed in its own building and comprising raw materials storage and feeding equipment, a plasma furnace, associated services and utilities (gas, water, plasma power, low voltage power, etc.), metal and slag handling equipment, off-gas abatement systems and plant control room. The plasma furnace consists of a cylindrical steel shell, with water cooling to the roof and upper side walls, air cooled base, a conductive hearth and lined with a bricked refractory lining. The 7,000 Amp, 2 MW plasma arc is provided by a single graphite electrode, acting as the cathode, with the conductive hearth acting as the anode.
A mixture of dust, carbon (usually in the form of coke or anthracite) and burnt lime (CaO) are fed into the furnace, where the carbon acts as a reductant to recover the valuable steel-making metals (iron, chromium, nickel, molybdenum, etc.) as a ferroalloy. Volatile heavy metals including zinc, cadmium, lead etc. are driven into the off-gas system and collected as a dust for resale to zinc smelters, whilst other heavy metals are incorporated into a low-leaching slag product. This safe and inert slag is then used for making road stone.
This dust has been treated in the Tetronics DC plasma arc smelting unit continuously since the plant’s installation in 1991 and continues to provide a valuable service to what is now the Outokumpu Stainless Steel plant.
Over its period of operation, approximately $100 million of value has been recovered and realised by the DC plasma smelting unit from stainless steel dusts, scales and other materials.