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ISSN 0036-0295, Russian Metallurgy (Metally), Vol. 2007, No. 8, pp. 705–710. © Pleiades Publishing, Ltd., 2007.Original Russian Text © V.S. Malinovskii, 2007, published in Elektrometallurgiya, 2007, No. 7, pp. 8–14.

INTRODUCTION

The experience of the commercial use of the univer-sal next-generation dc arc furnaces designed at NTFEKTA and installed in a number of metallurgical andmachine-building enterprises has recently been accu-mulated. In these enterprises, new furnaces wereerected or existing arc steel furnaces (ASFs) were mod-ernized and changed into dc furnaces. As an example,Fig. 1 shows a next-generation 25-t DPPTU-25 dc arcfurnace modernized from a 22-t DSV-20 ac arc furnace.

DPPTU-NP furnaces allow the production of steel,cast iron (including synthetic and high-strength castiron), and alloys based on copper, aluminum, nickel,cobalt, titanium, and other metals with a high effi-ciency, which is reflected in the technical-and-eco-nomic and ecological indices and the metal quality[1–7].

DPPTU-NP furnaces have been assembled by NTFEKTA in many enterprises. NTF EKTA mixers forsteel, cast iron, and alloys based on aluminum, copper,and other metals are also characterized by a high effi-ciency.

In this work, we consider the technical solutions thatallowed us to successfully realize the energetic andengineering possibilities of DPPTU-NP furnaces and topresent the results of DPPTU-NP furnace operation insome enterprises.

DPPTU-NP POWER AND ENGINEERING

Arc dc furnaces designed abroad use a power supplycircuit consisting of a three-phase transformer withswitched voltage steps and one controlled thyristor rec-tifier, which cannot always maintain the required powerconditions for a heat. This circuit favors rapid chargemelting at the beginning of a heat during operation atthe maximum voltage and arc current. The voltageshould then be decreased to exclude overheating of thefurnace lining during long-arc operation. A decrease inthe voltage is accompanied by a decrease in the powerintroduced into the furnace, which can only be compen-sated by the heat of chemical reactions that is generatedby burners of various types, oxygen, and other meth-ods. To protect the lining of modern furnaces, metallur-

gists shield arcs using a foamed slag. The problem ofmixing a melt and the energy transfer to the melt issolved via oxygen (or another gas) blowing of this melt.Otherwise, additional melting of the charge and heatingof the melt are performed at a reduced power, and thefurnace has a low capacity. All methods of forcing aheat using the heat of chemical reactions result in a high(9–12%) charge loss and a high load to the system ofdust and gas removal.

In contrast to this power circuit, a DPPTU-NP (NTFEKTA) power supply includes a transformer with sev-eral three-phase secondary windings, each of which isconnected to a thyristor converter section. The sectionsof a thyristor converter are connected to bottom elec-trodes BE1 and BE2 through reactors

L

1

and

L

2

.DPPTU-NP is equipped with not only a voltage-stepswitch but also a system for switching the sections of athyristor converter [8, 9].

With this power supply, the beginning of melting isprovided by four series connected sections of a thyris-tor converter operating at a reduced current and a highvoltage, with one bottom electrode being turned on[8, 9].

Power and Engineering Possibilitiesof Next-Generation DC Arc Furnaces

V. S. Malinovskii

NTF EKTA

DOI:

10.1134/S0036029507080137

Fig. 1.

DPPTU-25 next-generation dc arc furnace for melt-ing cast iron and steel.

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In this period, the heat is run at a long arc, and theanode spot of the arc is located on metallic chargelumps. The following aims are achieved:

(i) The melt is not overheated, since molten metallicdrops flow down to the hearth.

(ii) The heat is run at a stabilized power without sig-nificant changes in the electric conditions.

At the second heat period, the main charge mass ismelted and the sections of a thyristor converter are sub-jected to parallel-series commutation: every two sec-tions are connected in parallel, and these pairs are con-nected in series (Fig. 2b). The arc current doubles, thevoltage halves, and the power of the first heat period

remains the same. The second bottom electrode isturned on to mix the melt accumulated at the firstperiod. The anode spot of the arc is located on the meltaccumulated at the first period, inside the funnel meltedin the charge. To eliminate local metal overheatingunder the anode spot of the arc, a melt-mixing systemis turned on [8, 9]. This system is organized as follows.For two bottom electrodes shifted with respect to thecenter of the furnace, the current vector inside the melthas horizontal and vertical components. The interactionof the horizontal and vertical current components withthe magnetic field of the current forms a melt flow sothat the metal moves toward the anode spot of the arc ata high velocity and, then, moves deep into the melt. Thehorizontal mixing of the melt is similar. Such mixingimproves the arc energy transfer to the melt anddecreases the furnace lining erosion, since the rate ofmetal mixing near the lining is minimal.

However, the character of metal mixing changeswith time: eddy flows appear in the melt, and themotion of the main metal mass terminates. To preventthis phenomenon and to control the velocity and direc-tion of metal flows, the current controller of the thyris-tor converter periodically decreases the arc current fora short time; as a result, the eddy flows disappear, andregular melt mixing is restored.

Upon melting the main portion of the charge, thethird heat period begins, and all of the sections of thethyristor converter are connected in parallel (Fig. 2c).

As compared to the first and second periods, the arccurrent at the third period increases by a factor of fourand two, respectively. The power supply voltagedecreases in proportion to the increase in the current; inother words, the heat is run at a constant power. Anincrease in the current leads to an increase in the melt-mixing intensity. In contrast to the first and second peri-ods (where the main fraction of arc energy is transmit-ted to the charge), the arc energy (up to 80–90%) at thethird period is directly transmitted to a well-mixedmelt. This scheme allows one to reject a foamed slag,oxygen, and burners used to accelerate melting in ASFsand dc arc furnaces produced abroad (Fig. 3).

As a result of melt mixing, DPPTU-NP provides alarge effective slag–metal interaction area; a high melttemperature and composition homogeneity; rapid dis-solution and high assimilation of alloying elements;rapid desulfuration, deposphoration, decarburization,and degassing of the melt; and rapid removal of nonme-tallic inclusions.

For example, the designed heat scheme favors deephydrogen removal from aluminum alloys [1–4]. Theorganization of the energy transfer from an arc into themelt ensures the minimum energy consumed for melt-ing a charge and melt processing. A new power supplycircuit has been designed for small furnaces; here, atransformer has one high-voltage and at least two low-voltage windings. The winding with a voltage adjusteris intended for the first heat period, and the low-voltage

(a)

T

GE

L

1

L

2

BE2 BE1

1-

A

3

(b)

T

GE

L

1

L

2

BE2 BE1

1-

A

3

(c)

T

GE

L

1

L

2

BE2 BE1

1-

A

3

Fig. 2.

Schematic diagram for the circuits of a DPPTU-NPpower supply at the (a) first, (b) second, and (c) third heatperiods. GE is a graphitized electrode (see text).

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POWER AND ENGINEERING POSSIBILITIES OF NEXT-GENERATION DC ARC FURNACES 707

windings are intended for the second and third heatperiods. The ratios of the arc current to arc voltage atthe first, second, and third periods can be arbitrary.

DPPTU-NP POWER, ENGINEERING,AND ECOLOGY

The costs of the system of dust and gas removal andcleaning in DPPTU-NP are minimal, since it does useoxygen, chemical types of fuel, does not acceleratecharge melting, and does not blow powder carbon tofoam a slag.

The charge loss depends substantially on the gasexchange between the furnace medium and the atmo-sphere, and this gas exchange, in turn, depends on theelectric-arc stability (Fig. 4).

Arc-current oscillations can easily be generated inDPPTU-NP to increase the nitrogen (supplied from air)content in the furnace atmosphere, and the nitrogencontent is independent of a change in the current. At aconstant furnace volume, the pressure inside the fur-nace is a function of the gas temperature. When electricconditions oscillate, the furnace gas temperaturechanges, and the furnace gases are ejected from the fur-nace space or are sucked into it. This regime is charac-teristic of ASFs and dc arc furnaces produced abroad,and it is eliminated in DPPTU-NP due to the specialheat conditions. The elimination of the gas exchangeallowed one to exclude forced-gas evacuation from afurnace, to sharply decrease the degree of metal oxida-tion by atmospheric oxygen, and to prevent the nitrogenand oxygen saturation of a metal. A heat can run in theatmosphere of the gases evolved from a melt; duringsteelmaking, these gases contain a high CO content. Ifnecessary, one can control the furnace atmosphere viathe introduction of the gases required for a heat into thefurnace.

A dc electric arc is a powerful pump for furnacegases. The gas temperature inside a furnace can reach1000

°

C or above. At such a high temperature, dioxins,furans, cyanides, and other harmful compounds cannot

form. At the first heat period, the organic and othermaterials that contaminate a charge evaporate and areheated inside the furnace to a high temperature; at theexit of the furnace, they ignite and are oxidized to formsimple compounds. The small amount of forming gasesand an intense air flow blowing into the furnace gasflow evolved from the furnace provide a high combus-tion rate and rapid cooling of the furnace gases to a tem-perature below 100

°

C. In other words, the best condi-tions for preventing the formation of detrimental chem-ical compounds are provided. In many cases, DPPTU-NP may have no system of gas and dust cleaning. As aresult, one can modernize an ac furnace and transformit into a dc furnace instead of building an expensive sys-tem of gas and dust cleaning for an ASF.

We measured the pollutant-gas content upon mak-ing 110G13L steel in a DPPTU-6AG furnace (which

Bottom electrodes

(a) (b) (c)

Top view

Fig. 3.

Metal mixing during melting in a DPPTU-NP furnace: (a) schematic diagram for vertical mixing and (b, c) metal surfaceupon mixing by normal and distorted eddy flows, respectively.

220200

U

a

, V

5 6

78 9 10

8

I

a

, kA

5 6 7

10

6

10

8 9

N

2

, % tot

6

7

10

18

16

14

12

20

10

5

9

8

454035

τ

, min

Fig. 4.

Nitrogen content in the furnace atmosphere as afunction of arc-current oscillations.

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was modernized from a DSP-6 furnace) and obtainedthe following results:

The furnaces assembled at NTF EKTA are charac-terized by a sharp decrease in the dust–gas emissions.

It is important for Russian conditions that DPPTU-NPfurnaces completely pour a melt, which makes the useof a wet charge safe.

DPPTU-NP OPERATING SAFETYThe mechanical part of DPPTU-NP is based on

the mechanical part of an ASF, which is slightlymodernized. The furnace hearth is lined by conven-tional, basic, or acid refractories designed for ASFs.Part of the body enveloping the furnace hearth doesnot have cooled elements. The furnace walls androof can be cooled, and water-cooled units arealways located above a melt; therefore, their failuredoes cause explosions.

The design of the bottom electrode is of interest.Bottom electrodes provide a current supply from

a power supply to the metal and are located inside thelining of the furnace hearth. Various types of bottomelectrodes have been applied during the long-termuse of dc furnaces, and the best design ensuringsafety, explosion safety, and repair capability is rep-resented by the design of the DPPTU-NP bottomelectrode shown in Fig. 5 [10].

The bottom electrode consists of steel sheets that aremounted in the furnace hearth lining and are welded to

Pollutant emission, g/s MPE, g/s

Dust 0.3301 0.9853

Including Mn 0.0266 0.1486

the base of the bottom electrode, which consists of asteel tube whose inside is covered by copper built-in bythe electroslag method. The electrode base is located inthe lower portion of the lining far from its working sur-face. Cooling channels are placed under the furnacebody, and temperature-sensitive elements are placedinside the base to control the temperature.

This design is explosion-proof, since the coolingchannels are placed outside the furnace and the state ofthe bottom electrode is continuously controlled [11].When a lining is replaced, the worked-out steel sheetsof the bottom electrode are cut off, and new sheets arewelded. Steel sheets can easily be welded to the base,since they are made of homogeneous materials. Wedesigned and patented a number of measures to providelong-term bottom-electrode operation, bottom elec-trode–charge contact, and the complete pouring of amelt.

The level of commercial safety of DPPTU-NP iscomparable to or higher than that of ASF and, ofcourse, is higher than that of induction melting fur-naces, where current-carrying cooled elements and amelt are separated by a relatively thin lining.

COMMERCIAL APPLICATION OF DPPTU-NP

The results of the commercial application ofDPPTU-NP are well known [1–7].

All of the enterprises that introduced dc arc fur-naces decreased their energy consumption by 20%,electrode consumption by a factor of 2.5–3, the dustand gas emissions by an order of magnitude, and themetal loss by five times; increased the furnace capac-ity and the alloy quality; and decreased the alloy andingot costs.

Metallurgists use dc arc furnaces to make steel, castiron, and other alloys.

Metallurgists at NTF EKTA designed a series ofuniversal next-generation dc arc furnaces that can beused to make steels of any grades; gray cast iron; high-strength cast iron; alloys and master alloys based onaluminum and deoxidizers; and copper-based alloys.Furnaces for making lead-based alloys are being devel-oped.

To illustrate the efficiency of the modernization ofASFs into dc furnaces, Table 1 lists the technical andeconomic indices of a DSV-20 ac arc furnace (beforemodernization) and a DPPTU-20 (after modernization)located at OAO Tyazhpressmash. The heat weight is22–30 t, and the DPPTU-NP power increased onlyfrom 8.5 to 10.79 MV A because of the power supplyconditions (i.e., the furnace has a low specific power).The furnace contains a water-cooled roof, and ore-boil-ing melting technology is used.

The dc operation substantially increased the chemi-cal and temperature homogeneity of the melt anddecreased the content of nonmetallic inclusions. As aresult, the degree of supercooling during solidification

12

3

4

5 6

Fig. 5.

DPPTU-NP bottom electrode: (

1

) lining, (

2

) steelsheets, (

3

) steel tube, (

4

) copper rod, (

5

) cooling unit, and(

6

) furnace body.

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increased, which led to favorable conditions forimproving the structure of the metal. On average, thedeviations in the chemical composition decreased by35%. The fractions of heats with >0.035% P in the oldand new furnaces are 18 and 2%, respectively, and thefractions of heats with >0.025% S in them are 33 and15%, respectively.

The main laboratory of OAO Tyazhpressmash stud-ied the macro- and microstructure of shaft workpiecesbefore and after modernization and obtained the fol-lowing results: point inhomogeneity decreased fromnumber 3–4 to number 1, segregation zones and pooraxial soundness are eliminated, nonmetallic inclusionsin the form of aggregations are absent (inclusions areindividual and smaller than number 1.5), and a stablenumber 6–7 microstructure (instead of number 4–5microstructure) was produced.

The discrepancy between the qualities of forgingsand castings and the standard requirements for mechan-ical properties became less pronounced: by 90% for theyield strength, by 60% for the ultimate tensile strength,by 45% for the relative elongation, and by 80% for theimpact toughness.

The ultrasonic check spoilage of forgings decreasedby 15%, and that of export shafts, by 45%.

The annual benefit of the modernization of theASF into DPPTU-NP furnace was about 52 millionrubles, and the savings for some steels per 1 t liquidmetal was 3600 rubles. The modernization paybackperiod was ten months. Thus, the installation of thenext-generation arc furnaces is justified and rapidlycompensated. It should be noted that this calculationdoes not take into account the savings of ecologicalcosts, which is one of the important economic com-ponents.

DPPTU-NP uses new technologies, which provide ahigh metal quality and are considered to be inapplicableto arc furnaces. For example, AK7ch aluminum-basedalloy, which corresponds to the required chemical com-position and exceeds the requirements of State Stan-dard GOST 1583-93 for mechanical properties, is pro-duced. In the as-cast heat-treated state, the yieldstrength of samples cast in a metallic mold is higherthan 216 MPa, the relative elongation is more than 2%,and the Brinell hardness is higher than 94.9. The siliconcontent varies from 6.15 to 7.15%, the magnesium con-tent varies from 0.25 to 0.4%, and the iron content,from 0.1 to 0.3%. The structure of this alloy containsvery fine nonmetallic inclusions. The hydrogen contentis 0.1–0.2 cm

3

/100 g metal, and the porosity of theingots corresponds to number 1 on the porosity scale ofState Standard GOST 1589-93.

The processing of secondary aluminum treated inDPPTU-NP produces high-quality castings. In thiscase, many technological operations are eliminated, thequality of the alloys increases, and the process costsdecrease by five times compared to the process in

induction furnaces and by 15 times compared to gasfurnaces.

NTF EKTA has designed and certified a number ofuniversal dynamic conductivity melting furnaces,whose main parameters are given in Table 2.

The mechanical part of the furnaces is made accord-ing to a classical scheme, which makes it possible toflush slag through a working window and to pour metalthrough a pouring spout as the furnace is tilted.

DPPTU furnaces of various capacities have beenassembled and put into operation to cast lead-basedalloys, ferroalloys, and various types of master alloys.

NTF EKTA has designed 0.5- to 150-t arc dcDMPTU mixers with electromechanical and hydraulicdrives.

The main mixer operation regime is the third heatregime described above, i.e., the use of several bottomelectrodes and the system of electromagnetic melt mix-ing. DMPTU can be used for both melt heating and pro-cessing. For example, for steel and cast iron, it can be

Table 1.

Parameters of the 20-t ac and dc arc furnaceslocated at the Tyazhpressmash works

Parameter DSV-20 DPPTU-20

Dust content in effluent gases, mg/m

3

27.2 9.9

Noise level, dB 98 84

Electric power,kW h/t liquid steel

890 710

Capacity (for liquid metal), t/h 4.54 7.16

Average heat time, h 4.92 3.0

Total metal loss, % 5 3

Consumptionper 1 t liquid steel, kg

electrodes 14.0 2.12

FeSi 13.8 11.2

FeMn 6.3 5.1

lime 48.0 20.7

chamotte 12.1 2.7

Consumptionper 1 t tool liquid steel, kg

FeCr 14.6 12.0

FeV 2.4 1.1

FeMo 2.1 2.1

deoxidizing mixture(lime, FeSi 45, coke)

78.22 46.18

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used for carburizing, decarburization, dephosphoration,desulfuration, alloying, and refining.

CONCLUSIONS

In the course of the long-term commercial use ofDPPTU-NP furnaces in Russia and abroad, we havedetermined their possibilities for making various high-quality alloys from an ordinary cheap charge.

The operation of DPPTU-NP is characterized by ahigh level of ecological and industrial safety.

The DPPTU-NP installation costs are rapidly com-pensated for upon both the replacement or moderniza-tion of ASFs and the replacement of induction meltingfurnaces.

DPPTU-NP furnaces decrease not only the operat-ing costs but also the productive investments upon thecreation of new enterprises or the modernization ofexisting ones. This decrease is achieved owing to adecrease in the costs of charge preparation and dust–gas cleaning systems, out-of-furnace melt treatment,and the use of systems that decrease the effect of fur-nace units on the powering systems and other accompa-nying costs.

REFERENCES

1. A. V. Afanaskin, I. D. Andreev, V. S. Malinovskii, et

al.,“Results of the First Stage in the Implementation of aNext-Generation DC Arc Melting Furnace at OAO Kur-

ganmashzavod,” Liteinoe Proizvod., No. 11, 20–23(2000).

2. A. M. Volodin, A. S. Bogdanovskii, and V. S. Mali-novskii, “Results of the Work of the DPPTU-20 DC Fur-nace at AOOT Tyazhpressmash,” Liteinoe Proizvod.,No. 11, 31–35 (2004).

3. M. K. Zakomarkin, M. M. Lipovetskii, and V. S. Mali-novskii, “25-t DC Arc Steelmaking Furnace at PO Izh-stal’,” Stal’, No. 4, 31–34 (1991).

4. N. S. Ovsov, V. S. Malinovskii, and L. V. Yarnykh, “DCArc Furnace for Melting Cast Iron Chips,” LiteinoeProizvod., No. 4, 23–25 (2003).

5. V. S. Malinovskii, L. V. Brezhnev, S. A. Gaevskii, andA. S. Kryukov, “Experience of the Commercial Use ofDPPT for Melting Aluminum alloys in DPPT,” LiteinoeProizvod., No. 5, 32–34 (2001).

6. V. A. Zyskin, S. I. Pozdnyakov, and V. S. Malinovskii,“Melting of Aluminum alloys in Next-Generation DCArc Furnaces,” Tekhnolog. Legkikh Splavov, No. 1/2,152 (2006).

7. V. S. Malinovskii, V. D. Malinovskii, M. A. Meshkov,and L. V. Yarnykh, “Melting of Aluminum alloys in DCArc Furnaces. Status and Prospects of the New Process,”Metallurg. Mashinostroeniya, No. 4, 2–7 (2004).

8. V. S. Malinovskii, “Method of Metal Melting in a DCArc Furnace,” RF Patent 21 090 773.

9. V. S. Malinovskii, “Method of Electric Melting and theRelated Arc Furnace,” RF Patent 2 104 450.

10. V. S. Malinovskii, “Electric Furnace Bottom Electrode,”RF Patent 2 112 187.

11. V. S. Malinovskii, “DC Arc Furnace,” RF Patent 1 464 639.

Table 2.

Main parameters of universal dc arc furnaces for melting steel, cast iron, and aluminum and copper alloys

Type of furnace

Steel and cast iron Aluminum and copper alloys

nominalcapacity*, t

meltingtime**, min

electric power con-sumed for melting

a solid charge, kW h/t

nominalcapacity*, t

melting time**, min

electric power con-sumed for melting

a solid charge, kW h/t

DPPTU-0.1 0.1 35–40 540 0.1 15–20 470

DPPTU-0.16 0.16 35–40 520 0.16 15–20 450

DPPTU-0.25 0.25 35–40 500 0.25 15–20 430

DPPTU-0.5 0.5 35–40 480 0.5 15–20 410

DPPTU-1.5 1.5 35–40 470 1.5 15–20 390

DPPTU-3 3 35–40 470 3 15–20 380

DPPTU-6 6 35–40 460 6 15–20 380

DPPTU-12 12 35–40 430 12 15–20 370

DPPTU-25 25 35–40 420

DPPTU-50 50 35–40 410

DPPTU-100 100 40–45 400

*Furnaces can be 20% overloaded and 50% underloaded.**Under current for a single bucket charge.