New Technology Produces Cleaner Off-gases in Closed Smelting Operations
According to Tenova Pyromet, which is part of Tenova Mining & Minerals, the new method and process to cool and clean the mainly carbon monoxide rich furnace off-gas was developed through a combination and modification of current technologies, successfully addressing the shortcomings of the traditional industry method of wet scrubbing systems. These include reducing the solid content in the cleaned process off-gas to below 5 mg/Nm3 , reducing moisture in the cleaned process gas by reducing or eliminating the use of water, eliminating condensation of tars on colder surfaces for certain process off-gases by operating above the tar dew point of the process gas, and eliminating the wet scrubber liquor water treatment system, which can be problematic when a process off-gas with tars present is to be treated.
The basic principle of the new technology is to clean the process gas as hot as possible using filtration separation technology. Solids are separated and collected from the process gas by passing it through high temperature filtration elements, which are periodically cleaned using an inert gas. The resultant solid gas content of the cleaned gas is below 5 mg/Nm3, as opposed to the 10 to 50 mg/Nm3 of wet scrubbing systems, and is suitable for direct use in any power generation or heating system. If high-volatile reductants are used, the hot gas with tar in vapor form passes through the filtering section for cleaning before entering a suitable tar removal system, prior to being used in a power generation system or heating system. The collected tars from the tar removal systems can be used as a fuel or safely disposed of.
The process, said the company, can thus be applied beneficially as an alternative to conventional wet gas cleaning systems in a number of applications. These include applications where there are medium off-gas volumes at high-gas temperatures, off-gas containing unwanted components such as tars, and where specific clean gas properties are required, such as low moisture content, low solid content and a gas temperature above the dew point for further processing/cleaning.
“This new technology is an important development for ferroalloy producers, as the industry is facing increasing challenges in terms of harder to access and lower grade carbon reductants, compounded by escalating power costs, as well as increasing legislative and social pressure to minimize impact on the environment,” said Chris Oertel, managing director of Tenova Pyromet.
“The increasing cost and diminishing supply of metallurgical grade coke as the main source of carbon for reduction reactions in closed ferroalloy smelting have led producers to use alternative sources of carbon, such as coal with high-volatile matter. When a high percentage of this lower-grade coal is used, tar vapors can be present in the furnace off-gas, which, in wet gas cleaning, will condense when the gas is in contact with colder surfaces or water, creating operating, maintenance and water treatment problems.
“In addition, the rise in electricity prices has made it necessary to consider utilizing all the gases as a source of fuel for producing electrical energy in a cogeneration installation,” he said.
Tenova Pyromet specializes in the design and supply of AC and DC furnaces for the production of ferroalloys, platinum group metals, base metals, slag cleaning and alloy refining, and also designs and supplies plant equipment that is associated with furnaces such as material handling and pre-treatment, alloy conversion and refining, granulation of metal, matte and slag, furnace off-gas fume collection and treatment, and treatment of hazardous dusts and waste.
Earlier this year, Tenova Pyromet reported it had successfully completed cold commissioning of two 33-MVA ferromanganese/silicomanganese furnaces for SARDA Metals and Alloys (SMAL) in India.
Awarded in 2010, the contract from SMAL covered supply of the full electrode columns and automatic furnace controller (AutoFurn), as well as the full basic plant design. This greenfield development near Vishakhapatnam included an 80-MW captive power plant, and the generated capacity will be distributed between the two furnaces and the local power grid. Included in plans for the plant are an extensive greenery project and a rainwater harvesting dam, which will reduce the plant’s overall carbon footprint, making it one of the most environmentally friendly plants in India.
In carrying out the project, Tenova Pyromet said it drew on its operational plant experience as well as on state-ofthe-art software tools to fully assess the characteristics of the client’s raw material in the development of the critical furnace dimensions. Mass and energy balance models enabled the cmpany to study the effects that input changes, such as different raw materials, would have on the process. The process design was complicated by the need to blend the lower manganese-content Indian ores with other higher grade manganese ore from South Africa or Australia, in a two-step process route required to produce 78% high-carbon ferromanganese and silicomanganese with 65% manganese.
The company said its understanding of the different mineralogies of the South African, Australian and Indian ores, enabled an optimized process to be designed, in conjunction with the use of Tenova Pyromet’s submerged arc furnaces and furnace controllers. Optimization included not only the process but, as the mass and energy balance models are directly linked to financial models, also the project NPVs, IRRs, break-even and cash flows, in conjunction with changes in input costs, assumptions, input raw materials and process parameters. These models were used “live” to be able to find the best solution for the client’s specific situation.
Tenova Pyromet said the electrode columns designed and supplied to SMAL offer exceptional operational availability and power efficiency, with features including a robust lower electrode system with protection for key equipment, and a modular arrangement for easy maintenance. The slipping device, a market leader in electrode control, has a clamping shoe configuration that is able to exert adequate pressures on the electrode casings, reducing the risk of casing buckling/damage.
Tenova Pyromet’s AutoFurn is used to automate the furnace, controlling electrode movement and the transformer. By prioritizing the alarming convention, it is possible to operate these large plants with a lean workforce.
The raw material handling systems to feed the furnaces make use of a rotary conveyor—a recent Tenova Pyromet development—which feeds the furnace charging feedbins. By being able to rotate both clockwise and counter clockwise, the rotary conveyor reduces lag time between charging of each bin.
Tenova Pyromet said it has worked with both the private and state sector ferroalloy industry in India since 2004, when it was contracted by Nava Bharat Ferro Alloys Ltd. to design and supply the electrode column and electrode seals for its 24 MVA silicomanganese furnace. Besides significant repeat work from Nava Bharat, it has also since worked for a number of other leading companies, including Steel Authority of India Ltd. (SAIL), Indian Metal & Ferro Alloys, Visa Steel and Visa Bao.