reducing. its electrodes and refractory work, … · bath refractory is combining one. carbon...

$: REDUCING. ITS ELECTRODES AND REFRACTORY WORK, … · Bath refractory is combining one. Carbon blocks are used for outer part and bath bottom current conducting part, the rest uses$
DESIGN AND CONTROL

DESIGN OF DIRECT CURRENT FURNACE POWER 6.4 MW FOR SILICON REDUCING. ITS ELECTRODES AND REFRACTORY WORK, CONTROL REGIMES

AND SUMMARY OF MANUFACTURING

Nekhamin Sergey

«COMTERM», 18/1, Sokolinoy Gory 5-ya street, Moscow, 105275, Russia, e-mail: [email protected]

ABSTRACT

There are shown scheme and design projects for industrial two-electrode 6. 4 MW electrical furnace created on the basic analysis of power distribution inside the furnace bath. It also reflects data of refractory of current bath bottom electrodes and automatic control system. Additionally are shown results of9.5 years of industrial operations which proves of its effectiveness

KEYWORDS: Ore-termic furnace (OTF), ore-reduced furnace (ORF), electrical arc, silicon, direct current, rectifier, refractory, current bath bottom, electrode, automatical control.

Space power distribution affects main processes in the ore-termic furnace bath, including carbotermic reducing of silicon. On the other side power distribution inside OTF depends on power transaction scheme to the furnace bath.

It is becoming possible using current bath bottom scheme which could not use in case of 50 Hrtz because ofbig induction resistance and unequal phase power.

Type of power distribution in piece-equal field was studied at the special mathematical model [1] by the way of Laplace equation for electrical potential u:

V2 u=O, (1)

taking in account contact at the bounder of materials (k, m) with different electrical currency r:

(2)

Equation (1) under condition (2) allow to get dependence of electrical field potecial from coordinates and calculate tension of electrical field E, current density o , specific volume power Pv, and power P pouring inside the bath by volume V:

The thirteenth International Ferroalloys Congress Efficient technologies in ferroalloy industry

E=-gradU; o = y E;

P= J Pv dv, v

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June 9 - 13, 2013 Almaty, Kazakhstan

DESIGN AND CONTROL

And also electrical resistance Rex of furnace bath as total definition of scheme of furnace connecting.

Analysis of power distribution inside the OTF bath modeling results shows (shown in figure 1) that scheme «electrode-bottom bath»(shown in the top of the figure l,b) has the best figures providing:

•the biggest power concentration under electrodes; •the most effectiveness for strong energy consuming ORF (silicon melting); •bigger equality power distribution on the surfilce «slag-metal; •the highest electrical resistance of bath; •increase of bath resistance during electrodes approaching (in the same time resistance decreases for other schemes) ; •possibility of bath sizes decrease (declining its weight) and increase power concentration relatively, lower specific energy consumption; •create conditions for more cleaning reducers and quality growing of products.

Rex

1,2 ------,.-, ----,J , ~~-

+---- --.-- ----11-_ __ _ _ I

a) for different schemes of connecting, b) and bath electrical resistance depending of the distance between electrodes s,

related to electrode diameter d, c) relative measure.

Figure 1: Distribution of specific volume power

Electrical resistance growing may explained by eliminating currents straight connecting with electrodes (traditionally it is called <<triangle currents») and successfully affects technological process and furnace parameters.


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DESIGN AND CONTROL

Researches showed that you can calculate electrical field into bath materials independently on refractory field because electrical conductivity of bath bottom hundred times bigger than bath materials. There should be boarder condition on the refractory surface U=const.

As a result of bath electrical field and current density in the arc calculation under condition (2) were defined boarder conditions on the conductive part of refractory surface, and then it was allowed to calculate into the conductive part in the bath bottom and also make design calculation.

Force scheme of rectifier affects energetic parameters and price of direct current furnace. Stretch criteriums were created for different rectifier schemes comparison. Main characteristics of rectifiering schemes reduced to base furnace parameters - active power P and useful electrode voltage Uu:

K = S I P -coefficient of growning of calculating transformer power S in comparison to active furnace power P; average and active meaning of calculating tyristors current:

Iavg = n I avg Uu IP; Iact = n I act Uu IP;

calculative opposite voltage at valve:

calculative meaning of valve transformer chains current

lw= lw1 Uu nw IP;

where: n - valve number in the force scheme ; Iavg , Iact - average and active meaning of valve current ; U0p.m - maximum opposite valve voltage ; nw, lw1 - number of valve coils active current meaning for each of them.

Types of rectifier force schemes are shown at figure 2, and comparison criteriums - at table 1. Criteriums meanings were calculated for two of the most popular scheme of power rectifiers: «three-phase bridging» (B) and« two stars with equaling reactor» (S) - at reversive and unreversive types.

I I

J 3

6 8

Figure 2: Power transaction scheme to the furnace bath and types of rectifier force schemes


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DESIGN AND CONTROL

Table 1: Comparative OTF with rectifiers scheme characteristics

Position at Scheme of

Figure 2 furnace Rectifier scheme K lavg lad Uop lw

connectiru! 1

Electrode-B-reversing 1,05 1 2,45 2,1 0,707

2 electrode S-reversing 1,26

0,5 1,22 4,19

0,866 (1,33)* (4,84)**

3 Electrode-bath

B-reversing 1,05 2 4,9 1,05 1,41

4 bottom S-reversing 1,26

1 2,45 2,09

1,73 (1,33)* (2,42)**

5 Electrode-

B-unreversing 1,05 1 1,73 2,1 0,707

6 electrode S- unreversing 1,26

0,5 0,866 4,19

0,866 (1,33)* (4,84)**

7 Electrode-bath

B-unreversing 1,05 2 3,46 1,05 1,41

8 bottom S- unreversing 1,26

1 1,73 2,09

1,73 (1,33)* (2,42)**

• take in attention equating reactor typical power • • corresponds valve back voltage under rectifier idling

Liquid metal stirring is important advantage of direct current furnaces, ant that one provides effective process improving. Usually liquid metal is heated by arc above and this reason does not allow natural convection development, besides arc energy is not absorbed effectively. High specific producing and big furnace power demands excellency of heat absorption mechanism by liquid bath. Using of magnetic hydrodynamic effect allows to achieve required bath stirring without design complication. Equating of liquid metal temperature creates conditions for local metal overheating declining, lowering of metal vapouring and improving metal quality.

Mathematical model of big whirlwinds was used for describing of nonpermanent effects known as LES (Large Eddy Simulation). Using the model were made calculations of liquid metal moving and its structure. Results of modelling were checked at industrial furnaces and allow to create furnace design without undesired effect at refractory [2]. There was created special way of calculation and furnace structure. We can not use previous methods of designing because of special demands for direct current furnaces.

The most effective method is analysis of energy and material streams at the furnace entrance, exit and at different zones. There were developed three directions of material and energetic balances.

First of all, are researching dynamic changing of energy and material streams. Secondly, were separated informative data in the real time. At last for the third time are formed canals of streams transfer and control. Figure 3 reflects streams structure inside the furnace bath in which are separated three

components: WS - working space, CS - control system, E - the rest of furnace equipment. Contact between furnace and outside field depends on outside streams: Ass - electromagnetic; Qss -heatings and Css - raw materials streams ( hard, liquid and gas), exiting streams : Qo - heating and Co - creating material streams; N - forcing conditions. Furnace equipment creates outside streams (Ass, Qss, Css) into entrancing: Ai - electromagnetic, Qi - heating and Ci - entrancing material streams, Gi-mechanical forcings.

Working space inner structure is formed by results of interaction of entrancing Ai, Qi, Ci, Gi and exiting Qo, Co streams which is shown at drawing 3 by zones: 1 - garnisagh, 2 - raw materials, 3 - hot charge, 4 - soft charge, 5 - reducing zone, 6 - metalcarbididing zone, 7 - hollow under


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DESIGN AND CONTROL

electrodes, 8 - silicon liquid, 9 - electroconducting part of bath bottom. Zone condition, power entering them, electrode working end position are inner parameters of process which are not available for straight measures.

CS Control system

[BJ ws

~ ® Ci

~ Gi

~ Work space

C'lrnngings. limitatios (AN: Q N: CN ; G N)

Figure 3: Streams control inside the ore-termical furnace

Energy and material streams could go to the same direction or having two-direction way. As usually elecromagnetical stream has two-way direction. Furnace consumes active power from output system and generates reactive power, distortings and unsymmetries. Process of communications between furnace, outfit source and network reflects inner processes and may look as information coding of technological process condition. Thats why informative part of material and energetical streams carries special role allowing to chose optimal controlling action and increase effectivity of process.

Researching results became the basic foundament for designing of industrial 6.4 MW twoelectrode furnace for metallurgical silicon smelting.

The main parameters of direct current OTF were calculated on the basic design of one-phase 50 Hrtz industrial furnace with two grafit electrodes by diameter 710 mm. which showed the best results of producing. We choosed rectifier voltage interval from 66 V to 123 V and maximum electrode current 53 kA.

The main bath feeding scheme: « electrode- bath bottom », electrode polarity- negative, and conducting bath bottom is positive. Bath bottom was calculated for two electrodes total current 106 kA. There was provided also working regime with electrodes different polarity, allowing to change power distribution during the process without bath bottom feeding.


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DESIGN AND CONTROL

Bath form is rectangular and exiting hole is located on the middle of long rectangular side. Bath refractory is combining one. Carbon blocks are used for outer part and bath bottom current conducting part, the rest uses refractory materials. Bath bottom connects with feeding by water cooling copper line established outside of furnace shell. Steel shell stands on the f:.beams creating outside bath bottom air cooling.

Furnace cover is made by oval water cooling roof with vertical lifting shells. One charging and two picking machines are moving around furnace at the rails.

There was choosed reversing tyristor rectifier scheme « two stars with equalling reactor » using data of table 1. Here is provided 6 parallel canals for electrical energy transfer from transform.er to each electrode having 6 contacting plates. Respectively transformer second chain, equalling reactor, tyristor groups have 6 isolated lanes. There is also provided current distribution control between lanes and created tyristor control giving equal current in the plates for its safety.

Another canal includes electromechanical driving gear with signal of electrode vertical moving electrical regime regulator, equipment of rectifier tyristor group reverse and programme device providing changing of electrode polarity and giving signal for charge changing.

System of automatical control SAU « Silicon » coordinates all control apparatuses and illustrates at figure 4. SAU «Silicon>> has two-level structure of two USA companies «Texas Micro» and <<Analogue Devices» equipment.

Figure 4: The main SAU «Silicon» mnemo scheme

SAU «Silicon>> possesses traditional control canal set for OTF, and also group of canals giving new possibilities for employers, namely:

• electrode voltage control by tyrictor rectifier; • stabilization and flat changing of electrode voltage by impulse-phase tyristor control and

automatical transformer positions changing; • control of electrode direct current and control power distribution inside the furnace bath; • temperature and heating stream checking of furnace shell providing estimating of shell

conditions without breaking germetic state;


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DESIGN AND CONTROL

• adaptive control of furnace regime with changing of control parameters according to process condition.

Experimental researchings and 9.5 year industrial manufacturing of direct current furnace confirmed that the calculations were right. There were achieved raw material saving (near 8 %), quality improving of exiting silicon, and also declinings of admixtures quantities ( proved by industrial users). Stability of technological process improved significantly related to raw materials quality and has allowed not interrupt the production of silicon despite the lack of high-quality raw material.

Heating losses comparison illustrates at figure 5. Heating losses through bath bottom current conducting part for 6.4 MW furnace are equal losses from bath bottom air cooling and do not exceed 20 % of contact plates losses.

Researching of influence of direct current furnace at feeding equipment showed significant its improving in comparison with 50 Hrtz furnace : increasing of power coefficient above 0.9 and liquidation a difference ofphaze power consumption. Forcing changing of feeding voltage quality is suitable to active standarts. In final results, we were able to create high effective direct current 6.4 MW OTF for silicon smelting which was built successfully at OAO <<ZalK>> (Ukraine) [3].

Low current feedng 50kW

Side refractory 200kW

Contact plates 260 kW

/

a) refractory

180 kW

a) direct current, b) 50 Hrtz current

Contact plates 270 kW T

b)

Figure S: Heating losses of current feeding elements and 6.4 MW furnaces bath refractory

Now we can recommend to distribute this experience for creating OTF for different alloys production which can provide product quality improving, lowering electrode consumption at 20-40 %, declining reactive power consumption, increase durability of furnace refractory and shell

REFERENCES

[1]

[2]

[3]

Mironov Ju.M.,etc. Main developments of power distribution inside Ore-reduced furnace// Special issues of electroheating. ChGU. 1997, p.14-24. Nehamin S.M. Magneto-dinamic metal stirring inside ore-reduced direct current furnace in the aiming of technological process intencification. Proceedings conf. "Electroheating-2008". St.P ., p.146-149. Nehamin S.M., etc. Silicon smelting at the dirrect current furnace.// Non-ferrous metals. 2008, N!! 2, p. 60-63.



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reducing. its electrodes and refractory work, … · bath refractory is combining one. carbon...

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