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MINERAL JIGS

MINERAL JIGS

Mineral jigs or gravity concentrators were designed to allow for separation of materials with different densities. This process is accomplished by flowing a stream of liquid-suspended material over a screen and subjecting the screen to a vertical hydraulic pulsation. This pulsation momentarily expands or dilates the screen bed and allows the heavier materials to work toward the bottom. Heavier material, or concentrate, that is finer than the screen openings will gradually work through the beds and the retention screen into the hutch, or lower compartment. That material, the concentrate, is continuously discharged from this compartment or hutch through a spigot. If the concentrate is coarser than the screen, it will work down to the top of the shot bed, and can be withdrawn either continuously or intermittently. The lighter material, or tailing, will be rejected over the end of the jig.

The mineral jig has been widely used in recovering valuable heavy minerals such as gold, platinum, tin, tungsten and lower density minerals, gemstones such as diamond and sapphire from alluvial or placer deposits. Jigs have also been widely used in hard rock mill applications for recovering of coarse heavy minerals liberated in open or closed grinding circuits. Base metals, iron, manganese ores and barite have also been recovered using jigs. Where the difference between the density of the mineral being sought and the gangue is large, as in the case of gold or platinum, the “Placer Jig” achieves very efficient recoveries even of minus 50 mesh particles.

The mineral jig has certain advantages in placer and hard rock mill flow sheets. In gold recovery the jigs produce highly concentrated products which can be easily upgraded by methods such as barrel amalgamation, treating across shaking tables or processing through centrifugal concentrators. In other placer operations the heavy minerals being sought are recovered efficiently and cheaply with similar high ratios of concentration. In iron, manganese and base metal treatment flow sheets, the jigs are operated to produce marketable grades of concentrate; or, as pre-concentration devices, to reject barren gangue prior to the ore entering the fine grinding section of the mill flow sheet.

The construction of the mineral jig results in maximum utilization of floor area and minimum head room requirements, permitting greater capacity per unit of operating floor area than, for example, shaking tables or other devices. The design of the machine contributes to minimum operating and maintenance costs and negligible parts replacement expense.

MECHANISM OF OPERATION OF MINERAL JIG

Jig concentrators are devices used mainly in the mining industry for mineral processing, to separate particles within the ore body, based on their specific gravity (relative density).

The particles would usually be of a similar size, often crushed and screened prior to being fed over the jig bed. There are many variations in design; however the basic principles are constant: The particles are introduced to the jig bed (usually a screen) where they are thrust upward by a pulsing water column or body, resulting in the particles being suspended within the water. As the pulse dissipates, the water level returns to its lower starting position and the particles once again settle on the jig bed. As the particles are exposed to gravitational energy whilst in suspension within the water, those with a higher specific gravity (density) settle faster than those with a lower count, resulting in a concentration of material with higher density at the bottom, on the jig bed. The particles are now concentrated according to density and can be extracted from the jig bed separately. In the mining of most heavy minerals, the denser material would be the desired mineral and the rest would be discarded as floats (or tailings).

There are some minerals, notably coal, that are lighter (lower in density) than the surrounding rock and in such instances the process of extraction would work in reverse, i.e. the coal would settle on top with the rock below (on the jig bed). There are several designs and methods of extraction from the jig bed.

WORKING PRINCIPLE FOR EFFECTIVE JIGGING

The Mineral Jig operates on the basic principle of hindered settling, and in order to effect a concentration it is necessary that there be a differential in the settling rate of the solids treated. As there is an accumulated differential in settling rates in the grinding mill discharge of a conventional closed grinding circuit, this is an ideal feed for the MINERAL JIG. In addition, this is an enriched product due to the settling and trapping of the higher specific gravity mineral particles in the classifier pool, and concentrate removal from the closed circuit is imperative.

In the upper sectional view the rotating water valve (synchronized with the eccentric shaft) has closed and the plunger is moving the diaphragm on its downward stroke which creates an upward pulsation to the JIG bed. During this period lighter gangue particles are carried further than the heavier mineral particles which may even settle against the upward movement in the pulp. At the completion of the upward pulsation the rotating water valve has passed through a 180° arc or half of a circle.

HOW DOES A JIG WORK

It is often hard for people to understand exactly how jigs work. For what it’s worth, I will put forth my understanding of the principles. Simply stated a jig is a device which has a couple of basic advantages over a sluice box. It is continuous and only needs occasional clean up (its biggest drawing card to hard rock mills).

It maintains, if properly balanced, an intermittent fluidized bed capable of retaining finer gold than a sluice box. If not properly balanced it can be more disastrous than a sluice box. A properly functioning jig is dependent upon a number of factors; feed flow density, feed flow rate, pulse rate, hutch water input, hutch concentrate delivery flow and of course the makeup of the gangue and specifically the particle shape and size of the gold.

However, the following are what we are attempting to do in a jig:

·   Create a cross flow on the top that:

a] is fast enough so that only the desired product and as little as possible worthless gangue accompanies it into the jig hutch and

b] is slow enough to allow the desired product to settle into the ragging without being rushed right over the top to tails for, once trapped in the ragging, capture of a particle is reasonably certain as all forward motion should now be defeated.

·   Put enough hutch water in to make sure that:

a] the negative pull of the diaphram or pulsing device does not create a sucking effect on the jig bed and

b] it’s not so much that all settling rate is defeated.

·   Draw off enough water with the hutch product to:

a] flow it to whatever point of delivery and

b] at same time balance this with the incoming hutch water to accomplish the aims  above.

If we analyze the foregoing statements, we must realize that if the incoming hutch water rate is too large, then the upward thrust during the positive stroke will be too severe and will push fine gold particles upwards and not allow them to settle out and if the incoming rate of feed and water is too large, then forward motion will be too fast and again not allow fine gold particles to settle out. Most of us know from experience that the settling rate of fine gold is quite slow so we can assume that the settling rate on the negative pulse of the jig mist overcame the forward movement turbulence of the gangue; for on the positive pulse, no fine sized gold particles will settle out. We can be quite certain that forward motion takes place on both negative and positive strokes albeit at different speeds depending on the portion of the pulse cycle it is in and depending on where the particle is in relation to the feed entrance and tails exit of the jig cell – for the flow rate speeds up the closer you get to the tails exit and in the same manner as a sluice box at same point the speed of the gangue destroys recovery of fine gold. One assumes that forward motion will be much reduced in the very last portion of the negative pulse but that settling rate is ranging from positive to negative in the positive pulse of the jig, and if there is the slightest bit too much hutch take up water the settling rate will go to the negative side sufficiently to stop all fine gold recovery and in the writer’s experience, some quite coarse gold recovery (up to 10 mesh in size).

Interstitial trickling is the redeeming feature of jigs for fine particle recovery. At the end of each settling cycle, coarse particles will bridge together first, and cone to rest. Finer particles will then trickle through the interstices of the larger one, and came to rest much later. Flaky particles are generally not well recovered by jigs. Firstly, their terminal velocity is much lower than that of a spherical particle of equal mass. Secondly, their shape restricts their motion during the trickling phase.

Jigs are often used in gold concentration. Their effectiveness, however, rapidly decreases below 300 µm (50 Mesh). This seriously limits their use as sole recovery unit. An interesting configuration is the placing of two identical sized jig cells, one behind the other. One of those jig cells absolutely has to be out of sync. If the first one is balanced, it will have sufficient hutch water added that the following combination will be in effect. In jig  solids will be correct but liquid content will be far too large, thus the forward speed of the total will be too fast for effective settling. Conversely, if volume and flow are correct for the second jig, then forward motion will be too slow in the first unit and either too much product will go to the hutch or blinding of the ragging and jig bed will take place. In essence, the same can be said of jigs as of sluice boxes. The large and dense particles will almost immediately drop out of flow and become part of the ragging. Like all 1 g systems the finer the gold and larger the aspect ratio the less likely you are to catch it. Jigs are not a fine gold recovery unit. Simply stated, unless you can find a, way to change the laws of the settling rate in relation to the laws of the flow rate you cannot improve the recovery beyond some fairly definite limits. The laws governing settling and laminar flow are quite absolute.

INDUCTION FURNACE

INDUCTION FURNACE

ABSTRACT

A new generation of industrial induction melting furnaces has been developed during the last 25 years. Present practices followed in Induction Furnaces are discussed in this paper. Through a literature review account of various practices presently being followed in steel industries using Induction Furnaces has been carried out with a view to gather principal of working. Apart from this a pilot study has also been carried out in few industries in India. We provide some recommendations for the productivity improvement .Due to non availability of the proper instrumentations the effect of the ill practices can not be precisely judged. If this is properly measured, the percentage of productivity improvement in steel melting Induction Furnace can be calculated. The review is carried out from the literature in the various journals and manuals. Keywords Induction Furnace, molten metal , productivity, Melt rate.

INTRODUCTION.

An Induction Furnace uses induction to heat a metal to its melting point which is based on the theory of Electro Magnetic Induction. Depending on their frequency (50 Hz  250 kHz) these can be divided to three types:  

1. High Frequency

2. Medium Frequency

3. Low Frequency  

Their capacities range from less than 1kg to 100MT, which are used for remelting of iron and steel (steel scrap), copper, aluminium, precious metals and alloys. Even most modern foundries use this type of furnaces and now more iron foundries are replacing Cupolas with Induction Furnace to melt cast iron as the former emit lots of dust and other pollutants. The Steel making via Induction Furnace route has certain advantages and disadvantages: 

 ADVANTAGES OF INDUCTION FURNACE  

1. It has no electrodes and electric arcs which allow productions of steel and alloys low in

   carbon and  occluded gases without any quality problem.

2. Low melting losses and alloying elements.

3. High power efficiency, therefore, cost effective.

4. Precise control of the operating parameters.  

DISADVANTAGES OF INDUCTION FURNACE  

1. Refining in Induction Furnace is not as intensive or effective as in Electric Arc Furnace  (EAF).

2. Life of Refractory lining is low as compared to EAF.

3. Removal of S and P is limited, so selection of charges with less impurity is required.       

CONSTRUCTION AND WORKING

Combustion furnaces and induction furnaces produce heat in two entirely different ways. In a combustion furnace, heat is created by burning a fuel such as coke, oil or natural gas. The burning fuel brings the interior temperature of the furnace above the melting point of the charge material placed inside. This heats the surface of the charge material, causing it to melt. Induction furnaces produce their heat cleanly, without combustion. Alternating electric current from an induction power unit flows into a furnace and through a coil made of hollow copper tubing. This creates an electromagnetic field that passes through the refractory material and couples with conductive metal charge inside the furnace. This induces electric current to flow inside the metal charge itself, producing heat that rapidly causes the metal to melt. Although some furnace surfaces may become hot enough to present a burn hazard, with induction, you heat the charge directly, not the furnace.

 

Fig. 2: Current flowing in one direction in the induction coil induces a current flow in the opposite direction in the metal charge. This current heats the metal and causes it to melt

A. Induction Electrical System Configurations: 

Induction furnaces require two separate electrical systems: one for the cooling system, furnace tilting and instrumentation, and the other for the induction coil power. A line to the plant’s power distribution panel typically furnishes power for the pumps in the induction coil cooling system, the hydraulic furnace tilting mechanism, and instrumentation and control systems.  Electricity for the induction coils is furnished from a threephase, high voltage, high amperage utility line. The complexity of the power supply connected to the induction coils varies with the type of furnace and its use.  A channel furnace that holds and pours liquefied metal can operate efficiently using mains frequency provided by the local utility. By contrast, most coreless furnaces for melting require a medium to high frequency power supply. Raising the frequency of the alternating current flowing through the induction coils increases the amount of power that can be applied to a given size furnace. This, in turn, means faster melting. A 10 ton coreless furnace operating at 60 Hz can melt its capacity in two hours. At 275 Hz, the same furnace can melt the full 10 ton charge in 26 minutes, or four times faster. An added advantage of higher frequency operation is that furnaces can be started using less bulky scrap and can be emptied completely between heats. The transformers, inverters and capacitors needed to “tune” the frequency required for high efficiency induction furnaces can pose a serious electrical hazard. For this reason, furnace power supplies are housed in key locked steel enclosures, equipped with safety interlocks.                 

B.  Safety Implications:    

Typically, the induction coil power supply and the other furnace systems are energized from multiple electric services. This means that foundry workers cannot assume that the power to the furnace coil has stopped because service has been interrupted to the furnace’s cooling system or hydraulic pumps. Review the lock out/tag out section provided in this safety guide  

C. Input And Output Parameters Of  The Induction Furnaces:     

 In order to study the prevailing practices in steel plants using Induction Furnaces, the following parameters have been identified as

1)  Raw Material: Induction Furnaces are using Steel melting scrap, Sponge Iron & Pig Iron/Cast Irons. On an average the ratio of these items is 40% sponge Iron + 10% Cast Irons or Pig Iron. The technology of melting these input materials varies according to the availability of raw materials and location of the plant and inputs of sponge iron consumed is as high as 85 % as charge mix on bigger furnaces.               

2) Power Supply: An A.C. current from the transformer is fed to the rectifier of the furnaces electronic circuit. This converts A.C. to D.C, voltage is smoothed out by a D.C. choke, and then fed to the inverted section of the furnace. Here the D.C is converted to a high frequency A.C. current and this is fed to the coil.

3) Refractory Lining: The material used for lining is crushed quarts. This is a high purity silica material. The linings are of two types, acidic lining and basic lining.                               

 4) Water: The cooling system is a through one way flow system with the tubular copper coils connected to water source through flexible rubber hoses. The inlet is from the top while the outlet is at the bottom. The cooling process is important because the circuit of the furnace appears resistive, and the real power is not only consumed in the charged material but also in the resistance of the coil. This coil loss as well as the loss of heat conducted from the charge through the refractory crucible requires the coil to be cooled with water as the cooling medium to prevent undue temperature rise of the copper coils.              

5) Molten Metal : The molten metal is the desired output of the Induction furnace. The quantity depends upon the capacity of the furnace, and the quality depends upon the raw material and alloy composition.  The tapping temperature depends upon the type of steel, as well as the distance  of end use of the molten metal.                                     

6) Waste Heat: The surface of the molten metal bath is exposed to atmosphere. This results in the major thermal energy  loss  through radiation. The Coils of furnace are water cooled this also results in heat loss.

7) Slag : During the operation of electric induction melting furnaces, non-metallic are produced from the various sources described earlier. Depending on the specific process being used and the type of iron or steel being melted, the composition of the slag will vary.

8) Slag Composition: The composition of furnace and ladle slags is often very complex. The slags that form in electric furnace melting are the results of complex reactions between silica (adhering sand on casting returns or dirt), iron oxide from steel scrap, other oxidation by products from melting, and reactions with refractory linings. The resulting slag will thus consist of a complex liquid phase of oxides of iron, manganese, magnesium and silicon, silicates and sulfides plus a host of other compounds, which may include alumina, calcium oxides and sulfides, rare earth oxides and sulfides and spinels and fosterites. 

TYPES OF INDUCTION FURNACES

A. Coreless Induction Furnaces:

The coreless induction furnace is a refractory lined vessel with electrical current carrying coils surrounding the refractory crucible. A metallic charge consisting of scrap, pig iron and ferroalloys are typically melted in this vessel.

 B. Channel Furnaces : 

In a channel furnace, induction heating takes place in the “channel,” a relatively small and narrow area at the bottom of the main bath. The channel passes through a laminated steel core and around the coil assembly. 

C. Pressure Pour Furnace:         

A pressure pour is, in essence, a channel furnace, as described above, that is carefully sealed so that the metal can be moved out of the furnace by way of pressurizing the chamber above the molten metal bath in the furnace. 

SAFETY IMPLICATIONS:

Accident investigation reports indicate that most foundry accidents happen due to one of the following reasons:           

• The introduction of wet or damp metal into the melt, causing a water/molten metal explosion

• Lack of operator skill during temperature taking, sampling or the addition of alloying compounds, causing metal splash.

•Dropping large pieces of charge material into a molten bath, causing metal splash

• Improper attention to charging, causing a  bridging conditions

• Failure to stand behind safety lines, causing a  tapping situation             

• Coming into contact with electrical conductors, overriding safety interlock switches or coming into contact with incompletely discharged capacitors, causing electric shock or electrocution

• Lack of operator training

 

ELECTRONBEAM FURNACE

An electron beam furnace (EB furnace) is a type of vacuum furnace employing high energy electron beam in vacuum as the means for delivery of heat to the material being melted. It is one of the electronbeam technologies.

Electron beam furnaces are used for production and refining of high purity metals (especially titanium, vanadium, tantalum, niobium, hafnium, etc.) and some exotic alloys.^[1] The EB furnaces use a hot cathode for production of electrons and high voltage for accelerating them towards the target to be melted.

An alternative for an electron beam furnace can be an electric arc furnace in vacuum.

Somewhat similar technologies are electron beam melting and electron beam welding.

ELECTRIC ARC FURNACE

INTRODUCTION

The use of electric  arc  furnaces  (EAF) for  steelmaking has  grown  dramatically in the  last  decade  in  the  United States.  In  1975  electric  furnaces  accounted  for 20% of  the  steel  produced in  the U.S.; by  1985  this  figure  had grown to 34%. Electric  furnaces  range in capacity  from  a  few  tons  to  as  many as 400, and  a  steelmaking  shop  can have  a  single  furnace  or  up  to  three or  four. In  brief,  these  furnaces  melt steel  by  applying an AC  current  to  a steel  scrap  charge  by  means of graphite electrodes.  It  requires  about 500 kwh of electricity  to  produce  a  ton of steel; consequently,  these  furnaces  use  a tremendous  quantity  of  electricity.  Trans former  loads  may reach 120 MVA. The  melting  process  involves  the  use of large  quantities of  energy in a  short time (1-2 hr)  and in some  instances  the process  has  caused  disturbances  in power  grids.  These  disturbances  have usually  been  characterized  as  “flicker”  brief  irregularities in voltage  a  fraction  of  the 60 Hz  cycle  in length,  and “harmonics”  irregularities  that  tend  to occur in a  pattern  repetitive to the 60 Hz  cycle. The  features of  electric  arc  furnaces were  described in a  CMP Tech Commentary  on  Electric  Arc  Furnaces (Vol. 1, No. 3, 1985).  The  purpose of the  present  Technology commentary,  is  to give  utilities  and  steel  mills  a  better understanding  of  electric  furnaces  from an  electrical  viewpoint.

ENERGY NEEDS

Furnaces are  often  classified  by  power requirement  levels.  A  scale  indicating power classification  ranging  from  ultra high power (UHP),  with over 700 kVA per  ton,  down to low power,  with  less than 200 kVA per  ton. It  is  important  to  consider  the  energy balance  for  a  typical  modem EAF because; 70% of the  total  energy  is electrical,  the  remainder  being  chemical energy  arising  from  the  oxidation of elements  such  as  carbon,  iron,  and silicon  and  the  burning  of  natural  gas with  oxy-fuel  burners.  About 53% of the total  energy  leaves  the  furnace in the liquid  steel,  while  the  remainder  is  lost to  slag,  waste  gas,  or  cooling.

STEEL MAKING  CYCLE

To achieve  meltdown  as  quickly  as possible,  electrodes  are  initially  lowered to a  point  above  the  material,  the current  is  initiated,  and  the  electrodes bore  through  the  scrap  to  form  a  pool of  liquid  metal.  The scrap itself protects the furnace  lining  from  the  high intensity  arc.  Subsequently, the arc is lengthened by increasing  the  voltage to  maximum  power.  Most  modem furnaces  are  equipped  with  water cooled panels in the  upper  half of the  sidewall, rather  than  refractories,  which  allows for  longer  arcs  and  higher  energy  input into  the  furnace. In the  final  stage, when  there  is  a  nearly  complete  metal pool,  the  arc is  shortened  to  reduce radiation  heat  losses  and  to  avoid  refractory  damage  and  hot  spots. After  melt  dawn, oxygen  usually  is injected  to  oxidize  the  carbon in the  steel or  the  charged  carbon.  This  process is an  important  source  of  energy;  the  car bon  monoxide  that  evolves  helps  minimize  the  absorption  of  nitrogen  and flushes  hydrogen  out  of  the  metal.  It also foams  the  slag,  which  helps  minimize  heat loss.

DETAILED  ELECTRICAL OPERATION

After  an  electric  furnace is  charged  with scrap  and  the  roof is  in  place,  the operator  lowers  the  electrodes,  each of  which  has  its  own  regulator  and mechanical  drive.  The  electrodes  are connected  to  the  furnace  transformer‘s secondary  delta  winding,  which  may be  rated  from ;bout 600 to 850 volts. No current  flows  when  the  first  electrode  contacts  scrap,  but a line to line path  through  the  scrap  and  an  arc  are established  when  the  second  electrode completes  the  circuit.  The  regulators for  each  of  these two electrodes  then signal  the  drives  to  raise  the  electrodes  until  the  selected  current voltage ratio  for  the  arc is achieved.  Initiation of  the third  arc  depends  on  the  scrap’s location,  which is unpredictable,  hence the  duration  of  the  unbalance is short but  random.  While  the  scrap  is  still un-melted,  the  arc  may  easily be extinguished  by  a  minor  overshoot in  an electrode  regulator  or  by  physical  movement  of  the  scrap. As the  scrap  melts, it  can  often  shift  and  fall  away  from an  electrode  extinguishing  the  arc, or  against  the electrode  possibly breaking  it. Because  of  the physical  movement and  settling of the  scrap,  wide  excursions  can  take  place  on  a  random  basis in  the  secondary  circuit.  The  abrupt initiation  and  interruption  of  current flow  provides  a  source  of  harmonic currents  and  causes  considerable  disturbance  to  high impedance  circuits. (About 75% of  the  total  impedance is  in the secondary  circuit.)  Voltage  and  current waves  deviate  considerably  from  symmetrical  sinusoidal  patterns,  but  they do  not  attain  full  rectangular  shape, according  to  findings in the  CMP  report, “Arc  Furnace  Power  Delivery  Scoping Study.”’  Disturbances  are  worst  during early  meltdown,  and  they  occur  at  varying  frequencies. Many  attempts  have  been  made to establish  the  human  eye’s  reaction  to the  flicker of a  lamp. 

Generation of harmonics  may  result  in further  flicker  problems,  and  equipment  on  the  power  system  may  also be  damaged. If static  capacitors  are  to be  used  to  improve  the  power  factor, an  analysis to ensure  that  resonance does  not  exist  at  any of the  harmonic frequencies  should  be  made.  Harmonics  contribute to wave  distortion  and to the  increase in effective  inductive reactance.  This  increase  is  often  in  the 10 to 15% range  and  has  been  re- ported  as  high  as 25%. Current  into the  furnace is  therefore  less  than  what would  be  expected  from  calculations based  on  sinusoidal  wave  shapes,  and losses in  frequency-sensitive  equip- ment  such  as  transformers  are  higher than  the  sinusoidal  wave  shape  would produce.

IMPORTANCE OF SCRAP

Scrap is available in a wide  variety of  sizes,  densities, and  chemical  compositions,  and a  mixture is usually  used.  If only  the  lightest,  least  dense  material is  charged,  several  buckets  of  scrap must be  placed in the  furnace to make a full  heat.  This is generally  uneconomical  due to oxidation  losses  and  the need to open  the  furnace  for  several separate  charges,  which  results  in loss of both  time  and  heat.  Nor is the  use of large  heavy  scrap  alone  optimum. A large  piece  might  protrude  and  interfere with  roof  closure  or  require  placement by  magnet, a  process  which  takes  time. The  furnace  operator  therefore  tries to blend  several  types  of  available  scrap to suit  his  needs. It is  beneficial to arrange  the  heavier  pieces  near  the bottom  of  the  charge.  After  about 20 minutes  of operation,  depending on available  power  and  other  practices, the  electrodes  will  have  opened  some voids,  and  cave-ins  can  occur.  If  large pieces  of  scrap  are  on  top  of  the pile,  they  can  possibly  slide  into  and break  an  electrode.  It  is  generally  believed  that  light,  uniform  scrap  produces a smoother  meltdown  than  does  large heavy  scrap.  However,  this  is  not al- ways  the  case. If heavy  scrap  is charged,  full  power  can be applied. If all  the  scrap is  light,  on  full  power  the electrodes  may  bore  through,  damaging  the  furnace  bottom  before a sufficient  pool  of  liquid  metal  has  formed. Generally,  the  initial  period  of  melting causes  the  most  electrical  disturbances. As the scrap  temperature  begins to rise, a liquid  pool  forms,  and  disturbances begin to diminish.  This  is  generally  about 10 minutes  or  so  after  power-on  and can  vary  depending  on  power  levels and  shop  practices.

MELT  DOWN

Heating steel  scrap to approximately 3000" F requires  large  quantities of energy  rapidly  applied.  Therefore,  full power  is  called  for  during  meltdown. The  arc  during meltdown  can be long because  the  electrode  and  arc  are  boring a hole  down  through  the  scrap, and  the  roof  and sidewalls  are  not exposed to arc  radiation. If the  arc is extiquished, the  regulator  will  lower  the electrode to re-establish  it.  This  can take  several  seconds  if  the  scrap  has moved  out  from  under  the  electrode.

MAIN  MELTING  PERIOD

After  about 20 minutes,  most  electric furnaces  will  have  begun  converting scrap to liquid  metal.  Hence,  wide swings in  disturbances  will  diminish considerably.  When  sufficient  molten metal  exists (in some  high-powered furnaces  only 8-10 minutes  is  required), the  arc  is  shortened  by  an  adjustment to the  electrode  regulators.  The  cur- rent  will  rise  since  overall  resistance  is reduced,  and  the  power  factor  and  arc power  will  decline.  Arc  length  is  changed so that  the  shorter  arc  will  deliver a higher  portion  of  its  heat to the  metal below  the  electrode  than  will  the  longer arc,  which  radiates  more  heat to furnace  sidewalls.  Many  studies  have been  conducted  which  confirm  the  ad- vantages  of  the long  arc  for  meltdown of  heavy  scrap  and the  short  arc  for operation  after  sufficient  liquid  metal  has been  formed.  The  short  arc is  much more  stable  than  the  long  arc,  and operation  during  the  refining  period follows  sinusoidal  concepts  much  more closely.

 

REFERENCES

                                  

1. Presidential address on the occasion of xx annual general meeting & interactive seminar on 16th November 2005 at New Delhi overview  of iron and steel industry steel markets Asia conference 2005.

2. Ikbal Nathani ( November 15, 2005) importance of recycling and current ferrous scrap scenario. Ambassador, Indian sub-continent bureau of international recycling Brussels.

3. www.steelworld.com

4. Advanced Melting Technologies: (November 2005 Energy Saving Concepts and Opportunities for the Metal Casting Industry BCS, Incorporated 5550 Sterrett Place, Suite 306 Columbia, MD 21044 www.bcs-hq.com.

5. D. Sager, S. Tambe, R. Stadler, R. Pai, Steel Grips 6 (1) (2008) 8–15.

6. T. Handrich, M. Huck, M. Abel, P. Petrov, in: AISTech 2013 Proceedings, 2013, pp. 791–794.

7. H.R. Manouchehri, Looking at shredding plant configuration and its performance for developing shredding product stream (an overview): Report JK 88011 2007-09-03, 2007).

8. J. Madias, Acero Latinoamericano 527 (2011) 48–56 (in Spanish).