The global energy shortage, which is particularly threatening to South Africa, makes it imperative to apply and operate the most energy-efficient smelter technology, says metallurgical plant and machine construction company SMS Siemag.

Over the years, the group has developed several measures to reduce the overall energy consumption of metal and slag production lines.

“And, with ongoing innovation and adaptation, we are convinced that significant additional energy reduction can be achieved in the future,” says SMS Siemag submerged-arc furnaces and electric smelters GM Rolf Degel.

Submerged-arc furnaces and electric smelters form an essential part of the production line, especially in the ferro- alloy, silicon metal and non- ferrous metal processes. Most of these electric furnaces are highly energy intensive, with the metallurgical process consuming the bulk of electricity.

Energy consumption is mainly determined by the smelting process implemented, and is also influenced by the design of the furnace, notes smelter design and construction company Metix MD Reinier Meyjes. SMS Siemag owns a majority share of Metix.

The production of silicon metal and iron silicon requires the highest specific energy consumption. The electrical energy required to operate a sili- con metal furnace is between 11 000 kWh and 13 500 kWh.

Meanwhile, iron-nickel units consume between 5 500 kWh and 8 000 kWh of energy for every ton of iron nickel produced, depending on the minerals and nickel content of the product.

Energy Reduction Measures

The measures the SMS group has developed to reduce the overall energy consumption of smelting technology include the application of energy recovery systems, such as the solution SMS Siemag installed for Turkey’s sole high-carbon ferrochrome producer, Eti Krom.

“In semiopen-type electric smelters, the combustible gas that is generated during the process is fully combusted in the freeboard below the furnace roof. The furnaces are equipped with adjustable doors, which allow precise control of the offgas temperature. Depending on the process, the temperature is controlled at between 550 °C and 750 °C,” explains Degel.

A boiler produces 30 t/h of superheated steam, which is converted into 5 MW of electrical power by a generator. Eti Krom calculated the amortisation period of the installation at three-and-a-half years.

This system increases the financial feasibility of processes with higher offgas generation capacity, such as silicon metal furnaces. Internal calculation shows that about 20% of the input electric power of a furnace can be recovered.

Meanwhile, a closed-type furnace produces carbon oxide-rich gas, which can be used as an energy source in various up- and downstream processes.

“A few companies have installed electric power generators driven by gas turbines or reciprocating engines. However, this requires intensive cleaning of the furnace gas. The SMS group can offer a ‘dirty gas’-combustion boiler system, which combusts the raw, hot carbon oxide-rich gas directly in a boiler. Up to 15% of a furnace’s electricity requirements can be recovered with this solution,” says Degel.

Electricity Efficiency Improvement

The design and material used for the electrode current lines and the roof design of a furnace also influence its overall energy efficiency.

The SMS group improved the high-current line system by finite element analysis, as well as financial return calculations. By increasing the copper area in the high-current line system, as well as using forged copper contact shoes, the overall electricity consumption of these parts is reduced.

Furnace transformers with low copper losses are specified and placed as close as possible to the electrodes to shorten conductor lengths, while simultaneously reducing the reactance of the complete system.

Advanced Cooling Concepts

Limiting the amount of heat lost by furnaces, using a balanced cooling or refractory system, also reduces energy consumption.

“All furnaces require cooling for parts that are exposed, or potentially exposed, to high temperatures. Heat absorbed by this cooling system is normally not economically recoverable.

“The SMS group believes a balance must be struck by using the cooling system to protect the equipment under harsh conditions, while refractories reduce heat uptake under normal conditions. Refractory linings are specifically designed keeping this principle in mind,” notes Meyjes.

For four 72 MW dc ferrochrome furnaces, currently under construction at chrome producer Kazchrome’s chrome project, in Kazakhstan, a new roof cooling system was developed to lower the overall energy consumption by more than 5%, compared with other dc smelters.

Further, the SMS group offers special cooling arrangements designed especially for large- capacity iron-nickel and platinum furnaces, which involves copper plates being inserted into the sidewall.

“This cooling arrangement is successfully operating in the group’s largest rectangular-shaped furnaces at projects owned by miners Anglo American, in two 114 MVA iron- nickel furnaces, and Vale, in two 120 MVA iron- nickel furnaces, in Brazil, as well as French multi- national mining and metallurgy company Eramet, in New Caledonia, in two 99 MVA iron-nickel furnaces,” Degel adds.

This cooling system extracts less energy when compared to cooling elements such as waffle coolers or copper staves.

“The system is a superior solution in terms of furnace integrity, cooling rate, maintenance and safety, as all water passages are kept outside the furnace, eliminating the risk of explosions.

“In case of cooling water being interrupted, there are also no break-out risks. It uses an easy refractory concept (the use of standard bricks) in comparison with other systems. This is easily installed and cheaper in comparison with other systems and is used in numerous furnaces,” says Degel.

Reduce Process Steps

Reducing the number of process steps by eliminating agglomeration and sintering also promotes energy efficiency.

The need for costly, energy-intensive furnace feed preparation is eliminated by dc smelting technology, which enables direct charging of fine-feed materials to the furnace. This process principle has been extensively applied in ferrochrome production in Kazakhstan and South Africa.

Meanwhile, SMS Siemag says the four 72 MW furnaces supplied to Kazchrome, which should be commissioned in 2012, represent a new benchmark in dc furnace technology.

“The client chose this furnace type as it enables the direct use of ore fines, dust and low-grade reductants and eliminates coke as a reductant. It also offers a higher chrome-recovery rate of more than 92%, eliminates agglomer-ation, sintering and preheating and enables the use of fewer process steps and equipment, which makes operation easier and improves the overall plant availability. “Operating costs are also significantly reduced, which complements Kazchrome’s aim to be the lowest-cost ferrochrome producer worldwide,” adds Degel.

Combining dc furnaces with an efficient energy recovery system lowers energy consumption, taking into account all the carbon energy requirements of conventional ac smelting routes.

The advantages, compared with conventional dc furnaces, enable an energy efficient operation and maximum furnace availability.

"A patented, reliable long-life electrode column enables slipping (electrode lengthening) under full power, while maximum power-on time prolongs furnace life and improves furnace and off-gas control. Further, unique in-house knowledge decreases arc deflection, a high-speed thyristor ignition controller improves and stabilises energy consumption and an innovative cooling or refractory concept reduces heat loss,” explains Meyjes.

Process Optimisation

Through process optimisation, metal yield is increased and recovery rates are improved, while reducing energy consumption.

Submerged-arc furnaces are also applied to deplete metal from slag, mainly in the nonferrous industry. SMS Siemag developed a continuous operating rectangular furnace, which ensures a better recovery rate compared with circular-shaped furnaces.

The group commissioned this furnace at mining company Mopani Copper Mines, in Zambia. A metallic content significantly below 0.6% in the discharged slag is being achieved.

The results of this unit convinced integrated zinc producer Kazzinc to install the same process at its operation, in Ust-Kamenogorsk, Kazakhstan.

The Mopani mine’s furnace has a transformer connection of 11 MVA, while Kazzinc’s unit has a 12 MVA transformer.

Maximised Power-On Time

Heat and electrical losses are reduced when furnaces operate at constant nominal load and when the furnace’s metallurgical conditions, including the raw material mix, are kept constant, says Degel.

"Furnace interruptions have a significant negative impact on energy consumption, owing to the additional energy required to heat up the furnace and to restabilise the process. The SMS design assures the highest availability levels. Many of the units operate for several years with availability close to 100%,” he concludes.