Published by The EPRI Center for Materials Production lndustrial and Agriculfural Technotogies and Services CMP-109

Submerged Arc Furnaces

a wide range of metal products by smelting various minerals to metal products. in some processes a valuable slag or vapor product arises as well as the metal. This Techcommentary covers the princi- pal submerged arc processes in use today, but is in no way an exhaustive review of all submerged arc process- es. Table 1 lists some of the products made in submerged arc furnaces.

In the submerged arc fumace electric power heats the raw rnateri- als and provides the energy to reduce the ore to a metallic state. Generally, carbon serves as the reducing agent and fluxes are often added to facili- tate the process. The m m o n practice is to mix the ore materials, the reducing agent, and any fluxes outside of the fumace and then to pe- riodically charge this mixture (often called charge mix) into the furnace. Although the charge mix is added periodically, the reduction reactions and metal production proceed contin- uously. The metal is usually allowed to accumulate until tapping occurs at appropriate intervals; however, con- tinuous tapping is not uncommon.

The term "submerged arc" is used because the electrodes are usu- ally buried deep in the furnace bur- den and the reduction reaction takes place near the tip of the electrodes. At the top of the burden little current flows between the electrodes because of the high resistivity of the unmelted charge. As the burden descends In the furnace the noncar- bon portion of the charge begins to melt. As the carbon heats. its resis- tance decreases providing a conduc- tivity path between the electrodes. This current flow creates the intense heat needed for the high temperature and energy required for the reduction reactions.

Due to the relatively low resistivi-

Submerged arc furnaces produce

ty of most smelting process charge 1) Slagless processes with an arc mixtures, submerged arc furnaces between the electrode and the metal generally operate at lower secondary bath. voltages and higher currents than 2) Slag processes-frequently with Steelmaking furnaces. Depending on a cdte bed formed under the elec- the size of the furnace and the resis- trcde. The coke bed floats on me slag tivity of the charge mix, the sec- layer and the current passes through

Figurn 1 Submerged Arc Furnace (Open) tor Produc%ng Ferrosiihn.

ondary voltage will typically range between 160 to 280 volts in modern furnaces. The current can reach 200,OOO amperes in the largest fut- naces. Since changes in the resis- tance are very slow in submerged arc furnaces and the processes are con- tinuous, electric control is much simpler, and any disturbance to the electrical distribution network is much less severe than is the case with typical electric arc furnace steel production.

The term "submerged arc fur- nace" does not really describe all electric smelting processes because. in some cases, arcing is to be avoid- ed. Although each smelting process is unique, most processes fall into one of three operating modes:

the coke bed and the slag before reaching the molten metal. In mese slag processes arcing IS usually minimized.

where the heat is generated by the current passing through a slag layer. There is no coke bed and no arcing should occur.

three-phase ac power with three elec- trodes arranged in a conventional delta configuration in a round fur- nace, see Figure l . However. there are some six+lectrode systems used to produce pig iron. copper-nickel matte, ferronickel, and for slag cleaning. The six-electrode furnaces are rectan- gular, usually with in-line electrodes, see Figure 2.

3) slsg resistance processes

Most submerged arc furnaces use

TechCornmentary/CMP-l09 1

Self-Baking Electrodes

Mast submerged arc furnaces that produce ferroalloys use a s e l f - baking electrode system. invented nearly eighty years ago by a team led by C.W. Snderberg. This system takes advantage of the electric power used for the process to bake the electrode in place.

A series of cylindrical steel cas- ings welded on top of each other forms the electrode column. Operators periodically add solid green carbon "paste" blocks to the top of the column. The paste blocks melt to form the unbaked electrode shape. Vertical fins, which are attached to the casing, carry the power into the carbon paste so that the paste bakes to a solid carbon electrode in the casing. As the process consumes the baked elec- trode, more electrode is "slipped' into the furnace.

tem reduces electrode cost by at least 8% over the purchase of pre- baked amorphous carbon elec- trodes. It also allows Ute furnace to be larger since prebaked electrodes are limited to 55 inches in diameter, whereas self-baking electrodes can be as large as 72 inches in diameter.

Slagless Processes The Silicon Alloys-Ferrosilicon and Silicon Metal

The most widespread use of submerged arc furnaces in the United States today IS in the produc- tion of silicon alloys. In these slag- less processes an arc is made between the electrode and the metal

The self-baking electrode sys-

Table 1

Arc Furnace Products Commonly Made by Submerged

Slag Resistance

bath within a cavity under the charge mix. see Figure 1.

deoxidizer and alloying element in the iron and steel industry. Most grades of ferrosilicon contain either'; 5056 or 750k silicon by weight, com- monly referred to as 50% FeSi and 75% Fe8. Silicon metal (usually 99% Si punty) is primarily used as an alloying element in aluminum

Ferrosilicon is widely used as a

Figurn 2 Furnace tor Slag Cleaning

complex side reactions that gener- ate Si0 gas and CO gas which bum to Si0 and CO ,as they escape the burden. Successful operations keep the electrodes deeply buried so that the evolving Si0 gas condenses within the mix. The Si0 .fume which is aerosol in nature exits the furnace through a hooding system and collects in baghouse filters. The

. collected fume is very ligM (9 to 12 products and as a feedstock for the production of silicones.

A mixture of silica (usually quartz or quarzite), coke or coal, and wood chips makeup the charge mix. The coal or coke acts as the reduc- ing agent and the wood chips improve charge porosity and resis- tiwty. Iron, usually in the form of light steel scrap, is added for fer- rosilicon production. The raw maie- rials are thoroughly mixed, transferred io overhead mix bins.

and then gravw fed ' through chutes into the furnace. Silicon metal and 75% FeSi are typi- cally made in an open furnace where a stoking machine spreads the charge mix after it drops from the chutes. The stoking machine adds raw matenal as needed

I ~

Calcium CaM to correct the charge comwsition and stokes

~ ~~ ~~

Fenvbumo the burden to prevent mm t*mdj h-t,&ous sintering and to help release any gaseous

Pig lrwr with bitrmiumslq or vadium pressure buildup in the ~ O i U m S u i m burden. The reaction to

fananiokd produce silicon metal

: can be summarized as CopperMdte follows:

C c p p a r - N i i I Matte SiO,+ 2C Si + 2CO However, there are

lbMt3). If the fume is pure enough and handled correctly, it can be resold as an additive to cement, ceramic, and refractory products.

Figure 1 deptcts an open fur- nace used to produce silicon metal. Note that prebaked amorphous car- bon electrodes are used in this case. The steel casing used for a self-bak- ing electrode would contaminate the silicon metal product with iron as the casing is consumed by the process; therefore, setf-baking elec- trodes are not used when producing silicon metal.

Silicon metal furnaces typically range from 10 MW to 25 MW in elec- tric power rating and ferrosilicon furnaces range from 10 to 40 MW in the United States. Table 2 gives typi- cal operating data for sllicon alloys furnaces.

Slag Processes

Manganese Alloys-Ferromanganese and Silko-manganese.

a deoxidizer and to counteract the deleterious effects of sulfur. Manganese is also an alloying ele- ment since it stabilizes austenite. Manganese is also used in the pro- duction of abrasion-resistant steel products, such as "Hadfield type steels.

Steelmaher use manganese as

TechComrnentary!CMP-l09 2

Table 2 Typical operating data for 75% ferrosillcon

and siltcon metal operations

Submerged arc furnaces pro- duce high-carbon or "standard" ferromanganese (typically 79% Mn and 7% C) and a silicon-manganese- iron alloy commonly referred to as silico-manganese. Silico-man- ganese typically contains 68 to 72% Mn. 15 to 23% Si, and 1 to 2% C. H i g h c a m ferromanganese can be refined to medium-carbon or low- carbon ferromanganese in post tapping processes.

Manganese ores, which usually contain iron oxides, are mixed with coal or coke reductant, and usually a small quantity of fluxes to form the charge mix. Silico-manganese is produced by reprocessing high car- bon ferromanganese by-product slag and smelting cheaper high silicon-manganese ores.

Modern ferromanganese fur- naces have a closed roof designed to prevent ambient air from mixing with the process off-gas. The off-gas contains 65 to 70%. CO and can be used as fuel for other plant prccess- es such as raw material drying.

The raw materials are premtxed and then transported to overhead mix bins. From the overhead mix bins the mix drops through chutes which distribute the mix around the electrodes. The CO gas produced in the reaction zone pre-reduces the higher manganese oxide forms to MnO; however, gas reduction of MnO is impossible. The electric current passes through the coke bed and the slag layer and a reaction between the solid carbon reductant and the MnO occurs in the slag. The coke layer minimizes arcing between me electrode and the slag bath.

volatilization, ferromanganese pro- ducers operate at as low a tempera- ture as possible while still keeping the slag fluid. The lower tempera-

In order to reduce manganese

ture limits the amount of MnO reducbon; therefore, the slag may contain up to 45% MnO and can be used as a raw material for the pro- duction of silico-manganese, man- ganese chemicals. or electrotytic manganese.

romanganese mix is lower than it is for silicon alloys; therefore, ferro- manganese furnaces operate at lower voltages and higher currents. Table 3 gives typical operating data for manganese alloys furnaces.

Slag Resistance Furnaces Matte Smelting

The treatment of copper and nickel sulfides to produce copper- nickel matte or copper matte is often done in an electric furnace. In the matte smelting operation, the

The electric resistance of the fer- ,

sulflde mmerals are separately con- centrated and readied for separation Into mane (a molten solutm of copper. nlckel, iron. and sulfur) and slag.

The smelting process is carried out using electric resistance heating of the slag. Arcing is to be avoided. Thus, the term slag resistance fur- nace is applied to th~s type of process. The solid concentrate charge floats on the slag and gradu- ally melts into the slag. The slag is tn constant motion due to the passage of the electric current. In addiion to providing resistance heating, the slag serves as means for discarding some of the iron and gangue. The matte particles sink to the bottom of the furnace to form a liquid metal pool. This type of smelting is usually carried out tn a rectangular furnace wth six electrodes. See Figure 2. Slag Cleaning Furnaces

in conjunction with the flash smelting of copper and nickel con- centrates. In the most widely used flash smelting process a mixture of concentrate and flux is suspended in a stream of preheated air and then fired down a shaft. Droplets of molten matte and slag collect at the bottom of the shaft. Due to the rapidity of the matte formation the slag still contains magnetii (Fe :O,) and an excess of copper and nickel, some in an oxidized form. Depend- ing on the grade of the matte, the copper and nickel contents can be as high as 1.5% which may be

Slag cleaning furnaces are used

Table 3 Typical operating data for ferrornanganese and

silico-manganese operations

TechCommentav/CMP-l09 3

as hlgh as their values in the orlglnal ore.

smelter IS transferted to a slag cleaning furnace where It undergoes a secondary smelting operation. Coke breeze IS added to reduce the Fe,O,to FeO. and allow separation from the copper and nickel. Dunng the settling time in the furnace the matte preclpitates from the slag into the matte bath under the slag. By using this procedure 75 to 80% of the copper and nickel contained in the slag can be recovered in the form of matte. The slag cleaning fur- nace also processes solid revert and converter slags which can contain up to 5 to 6% Cu and Ni. For this fur- nace, the coke addition is about 2% of the liquid slag treated, and the energy consumed is 90 to 135 kWhiton of liquid slag. Additional energy is required if solid revert is used.

New Developments

The molten slag from the flash

To date, direct current (de) smelt- ing furnaces have limited cwnmer-

clal usage: however. there has been some prcrntslng work done In the pilot scale. DC furnaces ate often consideled for use In combinatlon with hollow-electrode systems. Fine material is Injected mrough the electrode so that it can be used without a costly agglomeration step. This promises to increase me metal yleld from the ore and from the reprocessed slags as fine material is generally a waste product today.

Rapid advances are also being made with self-baking electrode sys- tems for silicon metal furnaces. The traditional SBderberg electrode is not suitable for silicon metal pro- dudon since the steel casing would melt into the charge mix and increase the iron content of the sili- con metal. Significant strides have been made with a self-baking elec- trode system where the electrode paste bakes onto a center core of graphfie material. The steel casing is nonconsumable; therefore, iron contamination is avoided.

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Key Words: Submerged Arc, Smelting, Silicon Alloys Applicable SIC Codes: 2819,3313,3331, 3339,3559

81996 Electnc Power Research Institvte. Inc. All rigks reserved. Primed 7/96 TechCommentary/CMP-l09~GlO6672 4