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Drum ovens. Rotary Drum Smelting Furnace for Non-Ferrous Metal Waste Recycling Zinc Alloy Smelting Furnaces

2.1. Purpose of induction channel furnaces

Induction channel furnaces are mainly used for melting non-ferrous metals (copper and copper-based alloys - brass, bronze, nickel silver, nickel silver, kunial; zinc; aluminum and their alloys) and cast iron, as well as mixers for the same metals. The use of induction channel furnaces for steel melting is limited due to the insufficient resistance of the lining.

The presence in induction channel furnaces of electrodynamic and thermal motion of the molten metal or alloy ensures the uniformity of the chemical composition and uniformity of the temperature of the molten metal or alloy in the furnace bath.

Induction channel furnaces are recommended for use in cases where high requirements are imposed on the melted metal and the castings obtained from it, in particular, on minimum gas saturation and non-metallic inclusions.

Induction channel mixers are designed for superheating liquid metal, leveling the composition, creating constant temperature conditions for casting, and in some cases for dosing and controlling the speed of casting into molds of casting machines or into molds.

The charge for induction channel furnaces must be prepared in accordance with the specified composition of the metal or alloy being smelted, must be dry and consist mainly of primary pure metal.

The use of channel furnaces is not recommended when using contaminated secondary charge, using chips, especially when smelting aluminum alloys, as well as when smelting all kinds of ligatures and copper-based alloys containing lead and tin, since this dramatically reduces the life of the lining, and the operation of channel stoves becomes difficult.

The following classification of induction channel furnaces and mixers is given.

The ILK furnace - shaft and drum types - is designed for melting copper and copper-based alloys.

Mixer ILKM is designed for exposure, overheating and pouring of copper and copper-based alloys.

The IAK furnace is designed for melting aluminum and its alloys.

The IAKR mixer is designed for overheating, maintaining a stable temperature of liquid aluminum and pouring it directly into casting molds.

The ICK furnace is designed for melting cathode zinc.

The ICHKM mixer - shaft and drum types - is designed for holding, overheating, pouring liquid iron, it can work in conjunction with cupolas or induction crucible furnaces, or arc furnaces (duplex process) 2 .

The distributing mixer ICHKR is designed for overheating, maintaining a stable temperature of liquid iron and pouring it directly into molds, it works in conjunction with foundry machines and foundry conveyors.

Channel furnaces can operate independently with periodic pouring of molten metal or alloy, or as part of melting-distributing units. For example, the ILKA-6 unit consists of an ILK-6 furnace (useful capacity 6 tons, power consumption 1264 kW, voltage 475 V), an overflow chute and an ILKM-6 mixer (useful capacity 6 tons, power consumption 500 kW, voltage 350 V) . This unit is designed for melting and semi-continuous casting of copper and its alloys into round and flat ingots. The ILKA-16M2 unit consists of two ILK-16M2 furnaces (usable capacity 16 t, power consumption 1656 kW, voltage 475 V), a system of overflow heated chutes and an ILKM-16M2 mixer (usable capacity 16 t, power consumption 500 kW, voltage 350 V). ), designed for continuous melting and pouring of high-quality oxygen-free copper onto wire rod.

TO main advantages induction channel furnaces can be classified

1. Minimum waste (oxidation) and evaporation of the metal, since heating occurs from below. The most heated part of the melt, located in the channels, has no air access, and the metal surface in the bath has a relatively low temperature.

2. Low energy consumption for melting, overheating and metal holding. The channel furnace has a high electrical efficiency due to the use of a closed magnetic circuit.

At the same time, the thermal efficiency of the furnace is also high, since the bulk of the melt is in the bath, which has a thick heat-insulating lining.

2 The use of duplex processes for melting in two different melting units is advisable with full use of the advantages of each furnace, such as energy, heat engineering, operational, economic, etc. . For example, when melting in a cupola, the efficiency during melting reaches 60%, and when overheated, only 5%. In an induction furnace during melting, the efficiency is low, no more than 30%, and during overheating it is high - about 60%, therefore, the connection of a cupola with an induction furnace gives a clear advantage in the use of thermal energy. In addition, metal with a more precise chemical composition and a more stable temperature can be obtained in induction furnaces than in cupolas and electric arc furnaces.

3. Homogeneity of the chemical composition of the metal in the bath due to the circulation of the melt, due to electrodynamic and thermal forces. The circulation also speeds up the melting process.

TO major shortcomings channel induction furnaces include:

1. Severe operating conditions of the channel lining - hearth stone. The resistance of this lining decreases with an increase in the temperature of the melt, during the melting of alloys containing chemically active components (for example, bronzes containing tin and lead). Melting in these furnaces is also difficult for low-grade, contaminated charge - due to the overgrowth of channels.

2. The need to constantly (even during long breaks in work) keep a relatively large amount of molten metal in the furnace. Complete draining of the metal leads to a sharp cooling of the lining of the channels and to its cracking. For this reason, it is also impossible to quickly switch from one brand of melted alloy to another. In this case, it is necessary to carry out a number of ballast transition heats. Gradual loading of a new charge changes the composition of the alloy from the original to the desired one.

3. The slag on the surface of the bath has a low temperature. This makes it difficult to carry out the necessary metallurgical operations between the metal and the slag. For the same reason, and also due to the low circulation of the melt near the surface, the melting of chips and light scrap is difficult.

2.2. The principle of operation of the induction channel furnace

The principle of operation of an induction channel furnace is similar to the principle of operation of a power transformer operating in short circuit mode. However, the electrical parameters of a channel electric furnace and a conventional transformer differ markedly. This is due to the difference in their designs. Structurally, the furnace consists (Fig. 2.1) of a lined bath 2, in which almost the entire mass of the melted metal 3 is placed, and an induction unit located under the bath.

The bath communicates with the melting channel 5, also filled with melt. The melt in the channel and adjacent section of the bath forms a closed conductive ring.

The inductor - magnetic circuit system is called a furnace transformer

Rice. 2.1. The device of the induction channel furnace of shaft type

The induction unit combines a furnace transformer and a hearthstone with a channel.

The inductor is the primary winding of the transformer, and the role of the secondary coil is played by the molten metal that fills the channel and is located in the lower part of the bath.

The current flowing in the secondary circuit causes heating of the melt, while almost all the energy is released in the channel, which has a small cross section (90-95% of the electrical energy supplied to the furnace is absorbed in the channel). The metal is heated due to heat and mass transfer between the channel and the bath.

The movement of the metal is due

mainly by electrodynamic forces arising in the channel, and to a lesser extent by convection associated with overheating of the metal in the channel in relation to the bath. Overheating is limited by a certain allowable value, which limits the allowable power in the channel.

The operating principle of the channel furnace requires a permanently closed secondary circuit. Therefore, only partial discharge of the molten metal and additional loading of the corresponding amount of new charge is allowed. All channel furnaces operate with a residual capacity that is usually 20-50% of the furnace's total capacity and ensures that the channel is constantly filled with liquid metal. Freezing of the metal in the channel is not allowed; during the intermelting downtime, the metal in the channel must be maintained in a molten state.

The induction channel furnace has the following differences from power transformers:

1) the secondary winding is aligned with the load and has only one turn N 2 with a relatively low height compared to the height of the primary winding with the number of turns N 1 (Fig. 2.2);

2) the secondary turn - the channel - is located at a relatively large distance from the inductor, since it is separated from it not only by electrical, but also by thermal insulation (air gap and lining). In this regard, the leakage fluxes of the inductor and channel are significantly higher than the leakage fluxes of the primary and secondary windings of a conventional power transformer of the same power, therefore, the leakage reactances of an induction channel furnace are higher than those of a transformer. This, in turn, leads to the fact that the energy performance of the induction duct furnace - this is the electrical efficiency and power factor - is noticeably lower than that of a conventional transformer.

R 2 ′ , X 2 ′

R1,X1

Rice. 2.2. Schematic diagram of the induction channel furnace

The basic equations (equation of currents and equations of electrical state) for an induction channel furnace are similar to those for a transformer operating in short circuit mode (no voltage

U 2 ):

I & 1 = I & 10 + (− I & 2′ ) ;

U & 1 = (− E & 1 ) + R 1I & 1 + jX 1I & 1 ;

E 2 ′ = R 2 ′I & 2 ′ + jX 2 ′I & 2 ′ .

The equivalent circuit and the vector diagram of the induction channel furnace are shown in fig. 2.3.

Rice. 2.3. Equivalent circuit and vector diagram:

U 1 - voltage on the inductor; I 1 - current in the inductor; I 10 - no-load current in the inductor; I 2 ′ - reduced current in the furnace channel; E 1 - EMF of self-induction (induced by the main stream in the inductor winding); E 2 ′ - EMF of mutual induction (induced by the main flow in the furnace channel); - parameters of the inductor; - channel parameters

The intensive movement of the molten metal from the channels to the bath and back is of the utmost importance, since almost all the heat is released in the channels. In the occurrence of metal circulation, convection plays a certain role, associated with overheating of the metal in the channels, but the main factor

rum is the electrodynamic interaction of the current in the channel with the magnetic leakage flux passing between the channel and the inductor (Fig. 2.4).

Rice. 2.4. Scheme of the interaction of the channel current with the magnetic field

The electrodynamic forces F r are directed from the inductor and towards the metal in the channel K at the axial direction of the current density in the channel δ z . Created

with them, the pressure is equal to zero on the inner surface of the channel and is maximum on its outer surface. As a result, the metal is displaced into the bath from the mouth of the channel along its outer wall and sucked into the channel along its inner wall (Fig. 2.5, b). To enhance circulation, the mouths of the channel are given a rounded shape, providing a minimum hydraulic resistance.

nie (Fig. 2.5, a; 2.6).

In those cases where it is necessary to weaken the circulation (for example, when melting aluminum), the mouths are made without expansion, with high hydraulic resistance.

The through unidirectional movement of the metal through the channel and the bath instead of symmetrical circulation makes it possible to enhance heat and mass transfer, reduce the overheating of the metal in the channels and thereby increase the durability of the hearth stone. To ensure such movement of the metal, various technical solutions were proposed: a screw channel with mouths opening into the bath on

different heights, which sharply enhances convection; channels of variable cross section, in which there is not only a radial (compressing), but also an axial component of the forces of the electrodynamic interaction of the current in the channel with its own magnetic field; an additional electromagnet to create an electrodynamic force that moves the metal up the central channel of the dual induction unit.

The use of screw channels and channels of variable cross section on single-channel units did not justify itself. The use of an additional electromagnet is connected with the complication and rise in the cost of the furnace and therefore has found only limited application. The use of channels with mouths of variable cross section on double induction units gave a positive result. In a double unit with a different shape of the central and side mouths, the unidirectional movement of the metal is caused, which is especially intense in the absence of a phase shift between the magnetic fluxes of the inductors. Such units are used in practice and provide a doubling of the life of the lining.

2.3. The design of induction channel furnaces

With a wide variety of types of induction channel furnaces, the main structural units are common to all of them: lining, furnace transformer, casing, ventilation unit, tilt mechanism

(Fig. 2.7, 2.8).

Rice. 2.7. Induction channel furnace for melting copper alloys with three-phase induction unit (shaft type):

1, 2 - lining; 3 - 5 - furnace transformer; 6 - 8 - housing; 9 - cover; 10 - 11 - ventilation unit; 12 - 13 - tilt mechanism

Rice. 2.8. Induction channel furnace (drum type):

1- casing; 2 – turning mechanism; 3 - lining; 4 - induction unit; 5- air cooling of the lining of the channel part; 6 - supply of current and water to the inductors

furnace transformer

The scheme of the furnace transformer, the elements of which are the magnetic circuit, the inductor, the channel, is determined by the design of the furnace.

The main elements of the transformer are the magnetic circuit and in-

The furnace with one induction unit has a single-phase transformer with an armored magnetic circuit. Transformers with core magnetic circuits are also widely used. The voltage to the primary winding (inductor) is supplied from a supply autotransformer with a large number of voltage steps, which allows you to adjust the power of the furnace. The autotransformer is connected to the line voltage of the workshop network, usually without a balancing device, since the power of single-phase furnaces is relatively small.

A furnace with a double induction unit (Fig. 2.9,) is a two-phase load, just like a furnace with two separate single-phase induction units. Inductors in a two-phase system are connected to a three-phase network in an open delta circuit, if this does not cause unacceptable voltage unbalance, or in a Scott circuit, which ensures uniform loading of the three phases. Structurally, the dual unit consists of two rod-type transformers.

A furnace with a three-phase induction unit can have a three-phase transformer or three single-phase transformers. The latter is preferable, despite the large mass of the magnetic circuit, as it provides more convenient assembly and disassembly, which have to be done periodically when changing the lining.

Rice. 2.9. Typical unified detachable induction units:

a - for ILK furnaces (power for melting copper 300 kW, for melting brass - 350 kW, for a double unit, respectively, 600 and 700 kW); b - for IAK furnaces (power 400 kW); c - for ICHKM furnaces (power 500 kW - single-phase unit and 1000 kW - dual unit);

1 - casing; 2 - lining; 3 - channel; 4 - magnetic circuit; 5 - inductor

Three-phase induction units or groups of single-phase units, the number of which is a multiple of three, allow you to evenly load the supply network. Power supply of multiphase furnaces is carried out through regulating autotransformers.

The magnetic circuit of the furnace transformer is made of sheet electrical steel, the yoke is removable due to regular assembly and disassembly.

The cross-sectional shape of the rod with a small transformer power is square or rectangular, and with a large power it is cruciform or stepped.

The inductor is a spiral coil made of copper wire. As a rule, the inductor coil has a circular cross section. However, in furnaces having a rectangular contour of the melting channel, the inductor coil can repeat its shape. The diameter of the inductor obtained from the electrical calculation determines the dimensions of the core located inside it.

Furnace transformer operates in severe temperature conditions. It heats up not only due to electrical losses in copper and steel, like a conventional transformer, but also due to heat losses through the lining of the melting channel. Therefore, forced cooling of the furnace transformer is always used.

The inductor of the channel furnace has forced air or water cooling. With air cooling, the inductor is made of a copper winding wire of rectangular cross section, the average current density is 2.5 - 4 A/mm2. With water cooling, an inductor made of a profiled copper tube, preferably unequal, with a working wall thickness (facing the channel) of 10 - 15 mm; the average current density reaches 20 A/mm2. The inductor, as a rule, is made single-layer, in rare cases - two-layer. The latter is much more structurally complex and has a lower power factor.

The rated voltage on the inductor does not exceed 1000 V and most often corresponds to the standard mains voltage (220, 380 or 500 V). The turn voltage at low power of the induction unit is 7 - 10 V, and at high power it increases to 13 - 20 V. The shape of the inductor turns is usually circular, only for aluminum melting furnaces, the channels of which consist of straight segments, and the core always has a rectangular section, the turns of the inductor are also made rectangular. The inductor is insulated with a keeper, asbestos tape or fiberglass tape. Between the inductor and the core there is an insulating cylinder 5–10 mm thick made of bakelite or fiberglass. The cylinder is fixed on the core with hammered wooden wedges.

When the furnace is not powered by a special adjustable power transformer, taps are made from several extreme turns of the inductor. By applying the supply voltage to various taps, it is possible to change the transformation ratio of the furnace transformer and thereby control the amount of power released in the channel.

Furnace body

Typically, the furnace body consists of a frame, a bath casing and an induction unit casing. The casing of the bath for furnaces of small capacity, and for drum furnaces also of large capacity, can be made sufficiently strong and

rigid, which allows you to abandon the frame. Housing structures and fastenings must be designed to withstand the loads that occur when the furnace is tilted to provide the necessary rigidity in the tilted position.

The framework is made of steel shaped beams. The trunnions of the tilt axis are supported by bearings mounted on supports mounted on the foundation. The casing of the bathtub is made of sheet steel with a thickness of 6 - 15 mm and is supplied with stiffening ribs.

The casing of the induction unit is used to connect the hearth stone and the furnace transformer of the furnace into a single structural element. Two-chamber ovens do not have a separate casing of the induction unit, it is one with the casing of the bath. The casing of the induction unit covers the inductor, therefore, to reduce eddy current losses, it is made of two halves with an insulating gasket between them. The screed is made with bolts equipped with insulating bushings and washers. In the same way, the casing of the induction unit is attached to the casing of the bath.

The casings of induction units can be cast or welded, often with stiffening ribs. It is preferable to use non-magnetic alloys as the material for casings. Two-chamber furnaces have one common casing of the bath and the induction unit.

ventilation unit

In small-capacity furnaces that do not have water cooling, the ventilation unit serves to remove heat from the inductor and the surface of the opening of the hearth stone, which is heated by heat conduction from molten metal in closely spaced channels. The use of a water-cooled inductor does not exempt from the need to ventilate the opening of the hearth stone in order to avoid overheating of its surface. Although modern removable induction units have not only water-cooled inductors, but also water-cooled casings and hearthstone openings (an in-

subcooled caisson), the ventilation unit is an obligatory element of the equipment of the channel furnace.

Fans with drive motors are often mounted on the furnace frame. In this case, the fan is connected to a duct that distributes air through ventilated openings, a short rigid air duct. The mass of the ventilation unit can be significant, which leads to a significant increase in the load on the tilting mechanism of the furnace. Therefore, another layout is used, in which the fans are installed next to the furnace and connected to it with flexible sleeves that allow for tilting. Instead of flexible hoses, an air duct can be used, consisting of two rigid sections, which are joined by means of a rotary joint on the continuation of the axis of inclination, which also makes it possible to overturn the furnace. With this arrangement, the load on the tilt mechanism is reduced, but the design of the air ducts becomes more complicated and the space around the furnace is cluttered.

Furnaces with removable induction units are equipped with individual fans to cool each unit. Failure of the fan can lead to an oven failure. Therefore, the ventilation unit must have a standby fan, ready for immediate activation and separated from the duct by a damper. The exception is furnaces with individual fans on induction units. Individual fans are small in size and weight, and in case of failure they can be replaced very quickly, so there is no need to install backup fans on the furnace.

Furnaces with removable induction units are equipped with individual fans to cool each unit.

tilt mechanism

Channel furnaces of small capacity (up to 150-200 kg) are usually equipped with a manual tilt mechanism, the tilt axis passes near the center of gravity of the furnace.

Large ovens are equipped with hydraulic tilting mechanisms. The axis of inclination is located at the drain sock.

The inclination of drum furnaces is carried out by turning around an axis parallel to the longitudinal axis of the bath. When the furnace is in a vertical position, the tap hole is above the liquid metal level; when the furnace is turned on the rollers, it is under the bath mirror. The position of the taphole relative to the ladle does not change during the process of draining the metal, since the taphole is located in the center of the support disk, on the axis of rotation.

Any type of tilting mechanism must ensure that all metal is drained from the furnace.

2.4. Lining of induction channel furnaces

The lining of a channel furnace is one of the main and critical elements on which many technical and economic indicators, productivity and reliability of its operation depend. The requirements for the furnace bath lining and the induction units (hearthstone) are different. The bath lining must have high durability and a long service life, since the cost of lining materials is high, and the time required to replace and dry it can be several weeks. In addition, the lining of the furnace bath must have good thermal insulation properties so as to improve the thermal efficiency of the furnace.

The materials used for lining the bath must have a constant volume during firing and have a minimum temperature coefficient.

Expansion factor (t.c.r.) when heated to eliminate the possibility of dangerous thermal and mechanical stresses.

The refractory layer of the bath lining must withstand high thermal, chemical and mechanical loads. Refractory materials used for this purpose must have high density, fire resistance, slag resistance, thermal stability and high mechanical strength.

With high-quality performance of lining work with appropriate refractories, the durability of the furnace bath for hot holding of cast iron reaches two years, and for melting copper alloys - up to three years.

The lining of the channel part of the furnace (hearthstone) is operated even under more difficult conditions than the lining of the bath, since it operates under a high hydrostatic pressure of the metal column. The temperature of the metal in the channel is higher than in the furnace bath. The metal movement caused by the magnetic flux leads to rapid mechanical wear of the refractory material in cast iron and copper alloy furnaces. In the channels of aluminum melting furnaces, magnetic fields lead to a layering of aluminum oxides in a certain area and contribute to the overgrowth of the channels.

The thickness of the lining of the channel furnace (hearthstone) should be as small as possible so as not to impair the energy performance of the furnace. Small thickness sometimes leads to excessive weakening of the mechanical strength of the lining and to high temperature differences across the thickness of the lining between the outer and inner walls of the channel, which causes the formation of cracks. The temperature of the inner walls of the channel corresponds to the temperature of the overheated metal, and the outer walls are cooled by a water-cooled cylinder or a jet of cold air.

One of the main causes of lining failure is the penetration of molten metal from the hearthstone channel to the inductor and casing through cracks in the lining. An additional factor in the formation of cracks is the impregnation of the channel walls with metal oxides or slag, which causes additional stresses. For lining the hearth stone, the best refractory materials and the most modern technology are used.

Refractory materials used for lining electric melting furnaces are divided into acidic, basic

and neutral.

TO acidic refractory materials include siliceous stuffed

masses with a high content of silicon oxide (97 - 99% SiO2), dinas, as well as fireclay containing silicon oxide not associated with alumina (Al2 O3< 27 % ).

TO The main materials include refractories, which consist mainly of oxides of magnesium or calcium (magnesite, magnesite-chromite, periclase spinel, periclase and dolomite refractories).

TO neutral refractory materials include those refractories that are characterized by a predominant content of amphoteric oxides of aluminum, zirconium, and chromium oxide (corundum, mullite, chromite, zircon and bacor refractories).

IN In the lining of induction channel furnaces, refractory materials must first of all have a refractoriness exceeding the temperature of the molten metal, since at temperatures approaching the refractoriness temperature, these materials begin to soften and lose structural strength. The quality of refractory materials is also evaluated by their ability to withstand stress at high temperatures.

The refractory lining is most often destroyed as a result of chemical interaction with slags and metal melted in the furnace. The degree of its destruction depends on the chemical composition of the metal acting on the lining, its temperature, as well as on the chemical composition of the lining and its porosity.

When exposed to high temperatures, most of the refractories shrink in volume due to additional sintering and compaction. Some refractory materials (quartzite, dinas, etc.) increase in volume. Excessive change in volume can cause cracking, blistering, and even failure of the lining, so refractory materials must have volume consistency at operating temperatures.

Temperature changes during heating and especially during cooling of furnaces cause cracking of the refractory material due to its insufficient thermal stability, which is one of the most important factors determining the service life of the lining of induction furnaces.

IN In practice, an isolated effect of only one of the listed destructive factors is rarely encountered.

IN At present, there are no refractory materials that combine all the working properties necessary for the stable service of the lining in induction melting furnaces. Each type of refractory material is characterized by its inherent properties, on the basis of which the area of its rational application is determined.

For the correct choice and efficient use of refractory material in specific furnaces, it is necessary to know in detail, on the one hand, all the most important properties of the material, and on the other hand, the service conditions of the lining.

According to the classification, all refractory products are further subdivided according to the following features:

1) according to the degree of refractoriness - refractory (from 1580 to 1770°C), high refractory (1770 to 2000°C) and highest refractoriness (above

2000°C);

2) in shape, size - for normal brick "straight" and "wedge", shaped products are simple, complex, especially complex, large-block and monolithic refractory concretes, which are at the same time non-firing refractories;

3) according to the method of manufacture - for products obtained by plastic molding (pressing), semi-dry pressing, tamping from powdered non-plastic dry and semi-dry masses, casting from slip

ra and melt, vibrating from refractory concretes, sawing from fused blocks and rocks;

4) according to the nature of heat treatment - on non-fired, fired and cast from the melt;

5) by the nature of their porosity (density) - especially dense, sintered with

with a porosity of less than 3%, high-density with a porosity of 3-10%, dense with a porosity of 10-20%, ordinary with a porosity of 20-30%, lightweight, heat-insulating with a porosity of 45-85%.

2.5. Features of channel furnaces for melting various metals

Furnaces for melting copper and its alloys

The pouring temperature of copper is 1230 o C, and so that overheating of the metal does not lead to a significant reduction in the service life of the hearth stone, the specific power

density in the channels should not exceed 50 10 6 W/m 3 .

For brass, the pouring temperature is approximately 1050 o C, and the specific power in the channels does not exceed (50 - 60) 10 6 W/m 3 . With more

specific power, the so-called zinc ripple occurs, which consists in interrupting the current in the channels. Zinc, whose melting point is less than the melting point of brass, boils in the channels when brass is melted. Its vapors in the form of bubbles rise to the mouths of the channels, where, in contact with the colder metal, they condense. The presence of bubbles leads to a narrowing of the channel cross section and, consequently, to an increase in the current density in it and an increase in the forces of electrodynamic compression of the metal in the channel by the magnetic field of its own current. At a specific power higher than indicated, intense boiling of zinc occurs, the working cross section is significantly reduced, the electrodynamic pressure exceeds the hydrostatic pressure of the metal column above the channel, as a result of which the metal is pinched, and the current stops. After a break in the current, the electrodynamic forces disappear, the bubbles rise, after which the passage of the current resumes, the current breaks occur 2–3 times per second, disrupting the normal operation of the furnace.

At a specific power less than indicated, the zinc pulsation begins

It is emitted when the entire bath is heated to a temperature of about 1000 o C and serves as a signal that the brass is ready for pouring.

For smelting copper and its alloys, shaft furnaces are used, and when loading more than 3 tons, drum furnaces and mixers are used. The power factor for copper melting is approximately 0.5; when melting bronzes and brasses - 0.7; when melting copper-nickel alloys - 0.8.

Furnaces for melting aluminum and its alloys

Features of channel furnaces for melting aluminum and its alloys (Fig. 2.10, 2.11) are associated with the easy oxidation of aluminum and other properties

stvami metal and its oxide. Aluminum has a melting point of 658 o C,

pouring about 730 o C . The low density of liquid aluminum makes intensive circulation of the melt undesirable, since non-metallic inclusions, entrained to the depth of the bath, float up very slowly.

Rice. 2.10. General view of the induction channel electric furnace IA-0.5 for melting aluminum and aluminum alloys

(Useful furnace capacity 500 kg, residual capacity 250 kg, furnace power 125 kW):

1 - cover with lifting mechanism; 2 - upper casing; 3 - lower casing; 4 - magnetic circuit; 5 - fan installation; 6 - plunger; 7 - bearings; 8 - plumbing; 9 - inductor; 10 - lining

Molten aluminum in the furnace is covered with a film of solid oxide, which, due to the surface tension of aluminum, is held on its surface, protecting the metal from further oxidation. However, if a continuous film is broken, then its fragments sink and sink to the bottom of the bath, falling into the channels. Aluminum oxide is chemically active, and film fragments are attached to the channel walls due to chemical interaction, reducing their cross section. During operation, the channels "overgrow" and they have to be periodically cleaned.

Rice. 2.11. Interchangeable induction units for aluminum melting

With rectangular channels: a - with access to vertical and horizontal channels;

b - with access to vertical channels

These properties of aluminum and its oxide make it necessary to work with a low specific power in the channels. In this case, the overheating of the metal in the channels decreases, and the temperature on the surface is maintained at a minimum level, which weakens the oxidation, the rate of which increases with increasing temperature.

At a low specific power, the metal circulation decreases, which contributes to the preservation of the oxide film and a decrease in the number of non-metallic inclusions.

It is impossible to ensure the safety of the oxide film, since it is destroyed when the charge is loaded. During the melting period, the cracking of the film occurs mainly due to the circulation of the metal. Therefore, in furnaces for aluminum melting, measures are taken to weaken it, especially in the upper part of the bath: they reduce the specific power in the channels, often use the horizontal arrangement of the channels, and with their vertical arrangement, increase the depth of the bath, the transition from the channel to the bath is performed at a right angle, which increases the hydraulic resistance of the channel mouth. The horizontal arrangement of the channels also has the advantage that it is difficult for film fragments to enter the channels, but it does not completely exclude it, since the fragments can be entrained into the channels by the metal circulation.

The channels of aluminum melting furnaces consist of straight sections, which makes them easier to clean.

The overgrowth of the channel affects the electrical mode when its size becomes approximately equal to the depth of current penetration into the metal, which for molten aluminum at a frequency of 50 Hz is 3.5 cm. Therefore, in order to clean the channels less often, the radial size of the channel is taken to be 6 - 10 cm For a horizontal section, which is especially difficult to clean, the radial size of the channel of this section is taken to be approximately (1.3 - 1.5) d 2. Vertical sections are cleaned approximately once per shift, go-

horizontal - once a day.

Along with the use of furnaces of other constructive types, two-chamber furnaces are used. It can be single-phase with two channels connecting the baths, or three-phase with four channels. In the walls of the baths along the axes of the channels, holes are made for cleaning the channels, closed with clay plugs. Cleaning is done after draining the metal.

The power factor due to the large cross section of the channels is low, it is 0.3 - 0.4.

Zinc smelting furnaces

High purity cathodic zinc is melted in channel furnaces, which does not require refining. Molten zinc, having a high fluidity, comes into contact with the lining materials. Since the process of impregnating the lining with zinc accelerates with an increase in the hydrostatic pressure of the metal, zinc melting furnaces have a shallow rectangular bath and induction units with horizontal channels.

(Fig. 2.12) ..

Rice. 2.12. Induction channel furnace type ITs-40 with a capacity of 40 tons for melting zinc:

1 - melting chamber; 2 - distributing chamber; 3 - induction unit; 4 - loading roller table

The bath is divided into melting and pouring chambers by an internal partition, in the lower part of which there is a window. Pure metal flows through the window into the pouring chamber, impurities and impurities located near the surface remain in the melting chamber. The furnaces are equipped with loading and pouring devices and operate in a continuous mode: cathode zinc is loaded into the melting chamber through an opening in the roof, and the remelted metal is poured into molds. Pouring can be carried out by scooping out the metal with a ladle, releasing it through a valve, or pumping it out. Loading and unloading devices are designed to prevent the ingress of zinc vapor into the workshop, and are equipped with powerful exhaust ventilation.

Furnaces with the use of removable induction units are made swinging, and with fixed ones - stationary. The slope is used to replace the induction unit without draining the metal.

The power factor of zinc furnaces is 0.5 - 0.6 .

Cast Iron Furnaces

Channel furnaces are used to melt iron as mixers in the duplex process with cupola, arc and induction crucible furnaces, allowing the temperature to be increased, alloying and homogenizing the iron prior to casting. The power factor of iron smelting furnaces is 0.6 - 0.8.

Furnaces with a capacity of up to 16 tons are shaft furnaces with one or two removable units, larger capacity furnaces are shaft and drum furnaces, with the number of removable units from one to four.

There are special channel distributing mixers for servicing casting conveyors. The issuance of a dosed portion from such a mixer is carried out either by tilting the furnace, or by displacing the metal by supplying compressed gas to a sealed furnace.

Channel mixers for cast iron have siphon systems for pouring and creaming metal; the filling and outlet channels go into the bath near its bottom, below the melt surface. Due to this, the metal is not contaminated with slag. Filling and draining of metal can occur simultaneously.

2.6. Operation of induction channel furnaces

The charge for channel furnaces is made up of pure raw materials, production waste and ligatures (intermediate alloys). The refractory components of the charge are loaded into the furnace first, then those that make up the bulk of the alloy, and the last ones are fusible. In the process of melting the charge

should be periodically upset to avoid welding of pieces and the formation of a bridge over the molten metal.

When melting aluminum and its alloys, charge materials must be cleaned of non-metallic contaminants, since, due to the low density of aluminum, they are removed from the melt with great difficulty. Since the latent heat of fusion of aluminum is high, when a large amount of charge is loaded into the furnace, the metal can solidify in the channels; therefore, the mixture is loaded in small batches. The voltage on the inductor at the beginning of the melt should be reduced; as the liquid metal accumulates, the voltage is increased, making sure that the bath remains calm and the oxide film on its surface does not crack.

During temporary shutdowns, the channel furnace is switched to idle mode, when only such an amount of metal is left in it, which ensures the filling of the channels and the preservation of a closed metal ring in each of them. This metal residue is maintained in a liquid state. Power in this mode is 10 - 15% of the nominal power of the furnace.

When the furnace is stopped for a long time, all the metal from it must be drained, since during solidification and subsequent cooling, it breaks in the channels due to compression, after which it becomes impossible to start the furnace. To start an empty furnace, molten metal is poured into it, and the bath and hearth stone must be preheated to a temperature close to the temperature of the melt, in order to avoid cracking of the lining and solidification of the metal in the channels. The heating of the lining is a lengthy process, since its speed should not exceed a few degrees per hour.

The transition to a new alloy composition is possible only if the lining is suitable for the new alloy in terms of its temperature characteristics and chemical properties. The old alloy is completely drained from the furnace and a new one is poured into it. If the old alloy did not contain components that are not acceptable for the new alloy, then a suitable metal can be obtained during the first melting. If such components were contained, then it is necessary to carry out several transitional heats, after each of which the content of undesirable components remaining in the channels and on the walls of the bath when the metal is drained decreases.

For normal operation of a channel furnace with removable induction units, it is necessary to have a full set of heated units in reserve, ready for immediate replacement. Replacement is carried out on a hot furnace with a temporary shutdown of the cooling unit being replaced. Therefore, all replacement operations must be carried out quickly so that the duration of the interruption in the supply of cooling water and air does not exceed 10 - 15 minutes, otherwise the electrical insulation will be destroyed.

The condition of the bath lining during operation is monitored visually. The control of channels inaccessible for inspection is carried out by an indirect method, by registering the active and reactive resistances of each inductor, which are determined by the readings of a kilowattmeter and a phase meter. Active resistance in the first approximation is inversely proportional to

is proportional to the cross-sectional area of the channel, and the reactive is proportional to the distance from the channel to the inductor. Therefore, with a uniform expansion (washout) of the channel, the active and reactive resistances decrease, and with a uniform overgrowth of the channel, they increase; when the channel is shifted towards the inductor, the reactance decreases, and when the channel is shifted towards the casing, it increases. According to the measurement data, diagrams and graphs of the change in resistance are built, which make it possible to judge the wear of the channel lining. The condition of the lining of the channel furnace is also judged by the shell temperature, which is regularly measured at many control points. A local increase in the temperature of the casing or an increase in the temperature of the water in any branch of the cooling system indicates the beginning of the destruction of the lining.

The lining of induction duct electric furnaces simultaneously performs the functions of electrical and thermal insulation. However, when damp (cold furnace) or saturated with electrically conductive materials (from a melt or gaseous medium), the electrical resistance of the lining drops sharply. This creates a risk of electric shock.

Due to a malfunction, electrical contact may occur between live parts and other metal parts of the electric furnace; As a result, assembly units, such as a frame, with which personnel come into contact during operation, may become energized.

When operating electric furnaces, devices and electrical equipment included in installations (control panels, transformers, etc.), conventional means are used to protect against electric shock: grounding of metal parts (furnace frames, platforms, etc.), protective insulating means (mittens , handles, stands; scaffolds, etc.), interlocks that prevent the doors from opening until the unit is turned off, etc.

The source of explosion hazard is water-cooled units (molds, inductors, casings and other elements of electric furnaces). In case of malfunctions, their tightness is broken and water enters the working space of the furnace; under the influence of high temperature, water evaporates intensively and an explosion may occur in a hermetically sealed furnace as a result of an increase in pressure; in some cases, water decomposes and when air enters the furnace, an explosive mixture may form. Such accidents occur when the lining is eaten away in induction melting furnaces.

An explosion can be caused by the accumulation of easily flammable substances in the furnace (sodium, magnesium, etc.) formed during the technological process, as well as wet charge. The source of the explosion may be defects in the elements of electric furnaces.

During the operation of the furnace, it is necessary to constantly monitor the uninterrupted supply of cooling water and air and their temperatures at the outlet of the cooling systems. When the water or air pressure decreases, the corresponding relays are activated, the power supply of the faulty induction unit is turned off, and light and sound signals are given. In the event of a decrease in pressure in the water supply line, the furnace is transferred to backup cooling from the fire water supply or emergency tank, which provides

gravity water supply to the furnace cooling systems for 0.5 - 1 hour. The interruption of the uninterrupted supply of cooling water and air leads to an emergency: the inductor winding melts.

Stopping the water supply to the water-cooled jackets of the molds leads to the fact that the metal poured from the transfer case into the mold freezes in the mold, which leads to the failure of the mold and disruption of the process.

If the power supply fails, the metal in the furnace may freeze, which is a serious accident. Therefore, in the power supply systems of channel furnaces, it is desirable to provide for redundancy. The backup power must be sufficient to keep the metal in the furnace in a molten state.

Violation of the furnace lining (not fixed visually or by instruments) leads to the fact that the metal from the bath or channel part of the furnace falls on the furnace transformer, which can lead to failure of the furnace transformer and to an explosive situation.

Explosion safety is ensured by reliable monitoring of the process, signaling violations of the regime, immediate troubleshooting, and staff training.

2.7. The location of the equipment of the foundry installation

The furnace installation includes the actual channel furnace with a tilt mechanism and a number of equipment elements necessary to ensure its normal operation.

Furnaces of relatively small capacity are powered by low voltage busbars of the workshop step-down substation. If there are several furnaces, they are distributed among the phases so that, if possible, the three-phase network is evenly loaded. An autotransformer for voltage regulation can sometimes be provided one for several furnaces, in which case the switching circuit should allow it to be quickly included in the circuit of any furnace. This is possible, for example, when melting brass and zinc in foundries with a constant rhythm of work, when a voltage reduction may be required at the first start-up of a furnace after replacing the induction unit, or during occasional downtime to keep the metal in the furnace heated.

Furnaces with a power of over 1000 kW are usually fed from a 6 (10) kV network through individual power step-down transformers equipped with built-in voltage step switches.

A compensating capacitor bank, as a rule, is part of the furnace installation, however, a furnace with a small capacity and a relatively high power factor (0.8 and higher) may not have it. Ele-

The elements of each furnace installation are the current supply and protection and signaling equipment, measuring and switching equipment.

The location of the equipment of the furnace installation may be different (Fig. 2.13). It is determined mainly by the convenience of transporting the liquid metal, especially if the channel furnace is operated in conjunction with other melting furnaces and pouring means.

Rice. 2.13. Location of the equipment of the channel induction furnace ILK-1.6

The mark at which the furnace is installed is selected from the condition of convenience of loading or pouring and draining metal, as well as installation and change of induction units. As a rule, small-capacity furnaces are installed at the floor level of the workshop, medium and large-capacity tilting furnaces are installed on an elevated working platform, large drum furnaces with service platforms are also at floor level. A description of the types of baths of induction channel furnaces is given in section 3.3.

The capacitor bank is located in the immediate vicinity of the furnace, usually under the work platform or in the basement, in a forced ventilation room, since the 50 Hz capacitors are air-cooled. When the door of the condenser room is opened, the unit is switched off by a safety interlock. An autotransformer and an oil pressure unit for the hydraulic drive of the tilt mechanism are also installed under the working platform.

When the furnace is powered from a separate power transformer, its cell should be located as close as possible to the furnace in order to reduce losses in the current supply.

Near the furnaces, an area for lining, drying and calcining induction units should be equipped.

As an example, Fig. 2.13 shows a melting plant with a channel furnace with a capacity of 1.6 tons for melting copper alloys. Transformer cell 6, which houses a 1000 kVA transformer with high voltage switching equipment and protection, is shown in dashed lines, since it can be located elsewhere. On the working platform 7, a control panel 4 is installed, on the front panel of which there are measuring instruments, signal lamps, buttons for turning on and off heating and controlling the switching of voltage levels. The tilt of the furnace 8 is controlled from the remote control 9, which is installed in a place convenient for monitoring the metal drain. The level of the working platform provides the convenience of bringing the ladle under the drain spout of the furnace. Platform 7, tilting together with the furnace, closes the cutout in the main working platform and allows the furnace to rotate freely around the axis of inclination. Under the working platform there is a power shield 1 with electrical equipment and a hydraulic mechanism for tilting the furnace 2; current lead 3 is also mounted here, connected to the furnace by flexible cables. A condenser battery and an oil pressure unit are also located under the working platform.

3. ELECTRICAL CALCULATION OF INDUCTION DUCT FURNACE

There are two main methods for calculating induction duct furnaces. One of them is based on the theory of absorption of electromagnetic waves in a metal. This method was proposed by A.M. Weinberg and described in the monograph "Induction Channel Furnaces". The second method is based on the theory of a transformer operating in short circuit mode. One of the authors of this method are S.A. Fardman and I.F. Kolobnev. This method has found wide application as an engineering method for calculating channel induction furnaces.

This chapter provides a sequence of engineering electrical calculation with elements of calculation for an induction duct furnace and examples of calculation for individual stages.

The scheme of engineering calculation of an induction channel furnace is given

FORM SELECTION			INITIAL	GRADE
FURNACES. CALCULATION OF USEFUL			CORRECTIONAL	PERFORMANCE
AND DRAINING CONTAINER
THERMAL ENERGY CALCULATION				FURNACE POWER CALCULATION
	TYPE AND CALCULATION			QUANTITY DETERMINATION
TRANSVERSAL				INDUCTION UNITS AND
				NUMBER OF PHASES OF FURNACE
TRANSFORMER
				SELECTING THE TYPE OF ELECTRIC FURNACE
				TRANSFORMER.
	TOKA,		INDUCTOR VOLTAGE SELECTION
GEOMETRIC			INDUCTOR VOLTAGE SELECTION
GEOMETRIC
DIMENSIONS
AND NUMBER OF TURNS				CALCULATION OF GEOMETRIC
AND NDUCTOR.				CALCULATION OF GEOMETRIC
				DIMENSIONS AND CURRENT DUCT
GEOMETRIC				PARTS OF INDUCTION
DIMENSIONS
MAGNETIC CIRCUIT
				CALCULATION OF ELECTRIC
				OVEN PARAMETERS
CALCULATION CORRECTION
				POWER CALCULATION
				CAPACITOR BATTERY,
			REQUIRED FOR INCREASE
	COOLING CALCULATION			cosϕ
	COOLING CALCULATION
		INDUCTOR
THERMAL CALCULATION OF THE FURNACE

As a rule, the following are accepted as initial data for calculation:

Characteristics of the melted metal or alloy:

melting and pouring temperature;

density in the solid and molten state;

the heat content or enthalpy of the alloy at the pouring temperature (the dependence of enthalpy on temperature is shown in Fig. 3.1) or the heat capacity and latent heat of fusion;

resistivity in the solid and molten state (depending on

The dependence of resistivity on temperature is shown in fig. 3.2);

Wed

- furnace characteristics:

the purpose of the furnace;

furnace capacity;

oven performance;

duration of melting and duration of loading and pouring;

- mains characteristics:

mains frequency;

mains voltage or voltage of the secondary winding of the electric furnace transformer supplying the furnace.

3.1. Determination of furnace capacity

The total capacity of the furnace G consists of a useful (drained) capacity G p and a residual capacity (bog capacity) G b

where k b - coefficient taking into account the residual capacity (mass of the swamp). This

the coefficient is taken equal to 0.2 - 0.5; with smaller values - for furnaces with a capacity of more than 1 ton, and large - for furnaces with a capacity of less than 1 ton.

Usable capacity (drainable capacity)


G p =
G p =

where A p - daily productivity of the furnace in tons (t / day); m p - the number of heats per day.

Number of swims per day

	m p =

where τ 1 is the duration of melting and heating of liquid metal in hours, τ 2 is the duration of pouring, loading, cleaning, etc. in hours.

It should be noted that the value of performance is very relative. In the reference literature, performance values are given approximately (Table 3.1).

The duration of melting and heating of liquid metal (τ 1) depends on the physical

ical properties (heat capacity and latent heat of fusion) of melted metals and alloys. The increase in productivity is associated with a decrease

the value of τ 1 , which leads to an increase in the power supplied to the furnace, and affects the design of the furnace, i.e. instead of a single-phase furnace, it will be necessary to develop

to build a three-phase furnace, instead of one induction unit, it will be necessary to use several induction units, etc.

On the other hand, an increase in τ 1 can disrupt the technological pro-

metal or alloy smelting process, for example, alloying additives may evaporate prior to the casting process.

Depending on the type of charge to be loaded, the speed of casting, the size of the section of the cast ingot, etc. the value of τ 2 can also change up to

over a wide range.

Therefore, when carrying out calculations, it is necessary to evaluate the productivity value, taking into account both the technology of melting metals or alloys, and the design features of the furnace being developed.

If the useful capacity of the furnace is given, then the total capacity is determined by the expression

where γ mf is the density of the metal in the liquid state, kg m 3 .

In table. 3.2 shows the values of the density of some metals and alloys.

The cross section of the furnace bath S vp is determined after calculating the furnace channel. The height of the furnace bath h vp is determined by the expression

		V VP

		S ch
		S ch


	Capacity, t

	Useful
	power, kWt
	Manufacturer-
	ness (orientation-
	overnight), t/day
	Number of induction
	units
	Number of phases
	Coefficient
	power without com-
	pensions
	Furnace weight, total
	with metal, t

The choice and use of a particular furnace for melting zinc and zinc alloys depends on the volume and nature of production, the properties and purpose of the alloy, the provision of production with electricity, fuel and other factors. In addition, when choosing a melting unit, it is necessary to proceed from the need to obtain high-quality alloys with minimal losses of zinc and alloying components due to waste, minimum duration and high productivity, minimum consumption of electricity (or fuel) and lining materials per unit of molten charge, reliability, ease of maintenance ovens, etc. Depending on the source of energy and design features, the following main melting furnaces for the preparation of zinc and zinc alloys are distinguished: fuel and electric (crucible and induction).

fuel stoves

Fuel stoves use coal dust, fuel oil, natural and sometimes coke oven gas as fuel. These furnaces include flame reverberatory and crucible furnaces. In foundries, for the remelting of significant amounts of zinc, several modifications of reverberatory furnaces are used: one-, two- and three-chamber. The most widely used one- and two-chamber furnaces. These types of reverberatory furnaces are large and suitable for remelting low-grade zinc containing a large amount of iron and lead impurities. The main parts of reverberatory two-chamber continuous furnaces are the melting chamber and the hoarder (Fig. 52). Burners or nozzles are located in the end wall of the melting chamber.

Under the melting chamber is made inclined, with the rise to the threshold of the loading windows. This makes it possible to easily separate iron and lead-containing phases from zinc melts, which precipitate and settle in the melting chamber. Molten zinc flows from the melting chamber to the storage tank through a special channel. The lining of fuel reverberatory furnaces is made of fireclay bricks.

With a small number of manufactured zinc alloys, stationary and rotary crucible fuel furnaces are used. For melting zinc alloys, graphite, fireclay-graphite, cast iron or steel (more resistant than cast iron) crucibles are used. To increase the resistance of the crucible and prevent the interaction of melts with the material of the crucible, its inner surface is covered with refractory coatings, the compositions of some of them are given below,% (by weight):

1) quartz sand 60, refractory clay 30, liquid glass 10;

2) magnesite chips 59, ground asbestos 12, water glass 10, fireclay powder 18, sodium fluorosilicone 1.0; 3) refractory clay 20, magnesite chips 60, powdered graphite 10, liquid glass 10; 4) powdered graphite 70, talc 20, liquid glass 10; 5) refractory clay 18, powdered graphite 17, liquid glass 5, fireclay powder 60.

The refractory coating is prepared in a mixer by mixing the dry ingredients and then moistening the dry mass with liquid glass. The prepared coating in a pasty state is applied up to 3-10 mm thick on the inner surface of the crucible. Cracks in the coating and other defects are sealed with a coating of the original composition, followed by drying. To obtain a smooth surface layer of the coating, it is coated with special paints.

The composition of paints includes as a filler aqueous solutions of powdered chalk, or zinc oxide (II), talc, alumina, magnesite and others with the addition of binders, such as liquid glass. Some compositions of paints are given below, % (by weight): 1) liquid glass 5, chalk eluted 60, ground asbestos 15, water 20; 2) liquid glass 5, refractory clay 19, water 76; 3) zinc oxide (II) 10, liquid glass 6, refractory clay 4, water 80; 3) zinc oxide (II) 1, liquid glass 4, water 89; 4) liquid glass 4, elutriated chalk 12, water 84.

Paints are applied to the inner surface of the crucible heated to 120-150 ° C, and then dried and even calcined to 350-400 ° C, if binders are included in the composition.

Crucible furnaces have the following positive qualities:

Versatility (you can melt alloys of various composition),

Maneuverability (simplicity of transition from one heat to another),

The minimum contact surface of the metal with furnace gases (small waste and gas saturation of the metal),

Ease of device and maintenance.

However, crucible furnaces also have disadvantages: low productivity, low thermal efficiency. (7-10%) due to heat loss with exhaust gases and high fuel consumption (20-25% fuel oil and 50-60% coke by weight of the smelted metal). Until now, a wide variety of crucible furnaces have been used in foundries, ranging from the simplest coke and oil furnaces to more advanced gas and electric crucible furnaces.

Crucible electric furnaces. Electric resistance crucible furnaces are the most versatile units suitable for melting zinc alloys at. relatively small production scale. The greatest distribution for melting and holding zinc alloys was found by resistance furnaces of the CAT type (Fig. 53) of three types: rotary melting, stationary melting and stationary distributing (Table 38).

The main advantages of crucible electric furnaces over furnaces with oil or gas heating: a significant reduction in waste and the possibility of obtaining liquid metal of better quality. The disadvantage of these furnaces is the relatively slow heating of the mixture, which does not allow high-speed melting in the furnaces. In some plants, when melting electrolytic and printing zinc, reflective electric resistance furnaces are used.

Induction electric furnaces are currently the most advanced melting units for melting zinc and zinc alloys, since they provide high quality alloys, have high thermal and electrical efficiency, are very economical and most convenient to maintain. The advantages of induction furnaces include also low metal losses, their high productivity, which is 2-3 times higher than the productivity of fuel furnaces, and low consumption of crucibles due to the fact that their outer surface is not exposed to hot gases and is not subject to active oxidation.

If we take the cost of melting 1 ton of metal in an induction furnace as 1, then in electric resistance crucible furnaces it will be 2.5, and in fuel oil furnaces 8.

The most widespread for melting zinc alloys are induction crucible furnaces (furnaces without an iron core) of industrial frequency of the IAT and IGT type. Industrial frequency induction crucible furnaces of both types have the same design and differ mainly in the capacity of the crucible and the power of the electrical equipment. The crucibles of the IAT furnaces are made by stuffing and sintering refractory masses; the IGT furnaces are equipped with a steel crucible. Below are the technical data of IAT type furnaces:

Induction channel furnaces (furnaces with an iron core) are used in blank casting shops for smelting primary zinc and alloys based on it. It is expedient to use these furnaces in the presence of a charge consisting mainly of cathode or primary pig zinc, as well as in cases where high requirements are imposed on the melted metal and the castings obtained from it, in particular, on gas saturation and on non-metallic inclusions.

Induction channel furnaces have a higher efficiency compared to induction crucible furnaces. and consequently lower specific power consumption as well as a higher power factor. They are designed for continuous operation. A feature of induction furnaces of this type is the difficulty of transferring them from melting one alloy to another, which is associated with the need to replace the metal in the channel with a new one. For this reason, it is recommended to use channel furnaces for melting zinc or its alloys of constant chemical composition. Below are the main characteristics of induction channel furnaces of the type ITs, ITsK, ILK for melting and holding zinc and alloys based on it:

Structurally, channel furnaces are a lined bath enclosed in a casing and equipped with one or more induction units. Consider a stationary induction furnace with a pump for remelting cathode zinc with a capacity of 20 tons. The furnace has six single-phase transformers connected in two independent parallel three-phase groups and connected to a three-phase network. Both groups can be interconnected and will supply 100% power to the oven (or 50% if only one group is turned on). The furnace has 2 chambers: melting and pouring. The chambers are separated by a wall in which an opening is made near the bottom. Through the opening, pure zinc flows from the melting chamber into the pouring chamber, from where it is pumped out by a pump. All impurities and non-metallic inclusions remain in the melting chamber. The pump is a multi-bladed cast iron propeller driven by an electric motor.

The furnace is loaded from above using a loading device, which is a tilting table, on which zinc is placed by crane to be melted. Then, with a handwheel or an electric motor, the table is rotated on an axis, and the zinc is loaded into the bath. At this point, the pipe is connected to the ventilation exhaust system; as a result of suction, zinc vapor and the furnace atmosphere do not enter the workshop. In case of moisture entering the molten zinc and sudden vaporization, a damper is provided that plays the role of a safety valve. To drain the "swamp" during the repair of the lining, a hole is used, which is closed during operation of the furnace with a refractory plug.

To completely drain the metal from the channels, the furnace is slightly tilted towards the drain hole using a special jack. The induction unit is located so that the mouths of its channel are on both sides of the partition, due to which the metal is heated in the pouring chamber and its mixing in the bath is improved.

Induction channel furnaces, along with crucible electric resistance furnaces, are widely used as holding furnaces for injection molding, blank casting, liquid stamping, etc. In fig. 56 shows a dispensing induction furnace installed directly at the casting machines.

For the production of small zinc castings, stationary induction channel furnaces with a capacity of 200-400 kg are used in various ways. The consumption of electrical energy for melting and overheating of zinc to a temperature of 480 C, including the operation of all auxiliary devices, is 95-120 kW. h/t.

Administration Overall rating of the article: Published: 2012.08.17

Furnaces for melting copper and its alloys

Copper pouring temperature, and so that overheating of the metal does not lead to a significant reduction in the service life of the hearth stone, the specific power in the channels should not exceed .

For brass, the pouring temperature is approximately , and the specific power in the channels does not exceed . With a higher specific power, the so-called zinc ripple occurs, which consists in interrupting the current in the channels. Zinc, whose melting point is less than the melting point of brass, boils in the channels when brass is melted. Its vapors in the form of bubbles rise to the mouths of the channels, where, in contact with the colder metal, they condense. The presence of bubbles leads to a narrowing of the channel cross section and, consequently, to an increase in the current density in it and an increase in the forces of electrodynamic compression of the metal in the channel by the magnetic field of its own current. At a specific power higher than indicated, intense boiling of zinc occurs, the working cross section is significantly reduced, the electrodynamic pressure exceeds the hydrostatic pressure of the metal column above the channel, as a result of which the metal is pinched, and the current stops. After a break in the current, the electrodynamic forces disappear, the bubbles rise, after which the passage of the current resumes, the current breaks occur 2–3 times per second, disrupting the normal operation of the furnace.

At a specific power less than indicated, zinc pulsation begins when the entire bath is heated to a temperature of the order of magnitude and serves as a signal that the brass is ready for casting.

For smelting copper and its alloys, shaft furnaces are used, and when loading more than 3 tons, drum furnaces and mixers are used. The power factor for copper melting is approximately 0,5 ; when melting bronzes and brasses - 0,7 ; when melting copper-nickel alloys - 0,8 .

Furnaces for melting aluminum and its alloys

Features of channel furnaces for melting aluminum and its alloys (Fig. 2.10, 2.11) are associated with the easy oxidation of aluminum and other properties of the metal and its oxide. Aluminum has a melting point, pouring about. The low density of liquid aluminum makes intensive circulation of the melt undesirable, since non-metallic inclusions, entrained to the depth of the bath, float up very slowly.

At a low specific power, the metal circulation decreases, which contributes to the preservation of the oxide film and a decrease in the number of non-metallic inclusions.

The channels of aluminum melting furnaces consist of straight sections, which makes them easier to clean.

The overgrowth of the channel affects the electric mode when its size becomes approximately equal to the depth of current penetration into the metal, which for molten aluminum at a frequency 50 Hz is 3,5 see Therefore, in order to clean the channels less often, take the radial size of the channel 6 – 10 see For a horizontal section, which is especially difficult to clean, take the radial size of the channel of this section approximately (1,3 – 1,5) . Vertical sections are cleaned approximately once a shift, horizontal - once a day.

The power factor due to the large cross section of the channels is low, it is 0,3 – 0,4 .

Zinc smelting furnaces

Furnaces with the use of removable induction units are made swinging, and with fixed ones - stationary. The slope is used to replace the induction unit without draining the metal.

The power factor of zinc furnaces is 0,5 – 0,6 .

Cast Iron Furnaces

Zinc is a heavy fusible metal; Tmelt = 420 °C, p = 7.13 kg/dm3. The low boiling point of zinc (*bp = 907 ° C) limits the allowable temperature of the metal during the melting of all alloys in which it is included. The enthalpy of zinc at 500°C (about 300 kJ/kg) is three times lower than the enthalpy of molten aluminum. The specific electrical resistance of the zinc melt is 0.35-10~6 Ohm.

At low temperatures in air, zinc oxidizes, forming a dense protective film of ZnO3* 3Zn(OH)2. However, in melting furnaces, zinc is oxidized according to the reactions:
2Zn + 02 = 2ZnO, Zn + H20 = ZnO + H2, Zn + C02 = ZnO + CO.

To protect against oxidation, melting can be carried out in a protective or neutral atmosphere, for example, in a nitrogen atmosphere. However, in practice, in most cases, it is sufficient to prevent overheating of the metal above a temperature of 480 °C, at which intense oxidation and gas saturation of zinc begins. At this temperature, zinc and its alloys do not have a noticeable effect on the refractory lining of the furnace and the iron or steel crucible. An increase in temperature leads to the dissolution of the iron in the crucible in the zinc melt.

Furnaces for melting zinc alloys

Given the low melting and boiling point of zinc, zinc alloys are usually smelted in crucible furnaces heated by burning fuel or using electrical resistance and induction. Zinc alloys should not be melted in arc furnaces, since the inevitable local overheating of the metal near the arc burning leads to intense evaporation and oxidation of zinc. Induction channel furnaces are used for melting zinc alloys. At KamAZ, the TsAM10-5 alloy for pressure casting was smelted in three induction channel furnaces with a capacity of 2 tons each with a neutral lining. However, overheating of the metal in the channel leads to instability of the electric mode of melting (the so-called zinc pulsation) and forces to limit the power transmitted to the furnace.

Melting technology

The main part of the charge is usually made up of zinc alloys foundry in ingots, their return and scrap of zinc alloys. As coating fluxes, a mixture of calcium, potassium and sodium chlorides, ammonium chloride or cryolite is used. Primary aluminum in ingots, cathode copper and metallic magnesium are used for charging. All batch components must be free of oils, moisture and other impurities. Melting is carried out without overheating the bath above 480 °C. Based on the results of express analysis, the chemical composition is adjusted.
A steel bell is used to introduce magnesium. Upon receipt of a given chemical composition, the metal is overheated to 440 ... 450 ° C and poured into a ladle heated to the same temperature. In a ladle under an exhaust hood, the melt is refined with tablets of the Degazer complex degasser, which include 87% hexachloroethane, 12.7% NaCl, 0.3% ultramarine. Refining can also be carried out by settling, purging with inert gases and filtration.

The purpose of the drum furnace

The purpose of this rotary kiln is to heat the feed material up to a maximum temperature of 950 °C. The design of the equipment is based on the following process conditions in a rotary kiln.

Raw material Raw material Raw material feed rate Raw material moisture Raw material temperature Specific heat capacity of raw materials Bulk density of raw materials	uranium peroxide (UO 4 . 2H 2 O) 300 kg/h 30 wt. % 16°C 0.76 kJ/kg K 2.85 g/cm³
Product Product material Product feed rate Product Moisture (Wet Mass) Product temperature: on the discharge side of the kiln on the discharge side of the cooler Specific heat capacity of the product Bulk density of product material Particle size	uranium oxide (U3O8) 174.4 kg/h ≈ 0 wt.% 650 - 850 °C 60°C 0.76 kJ/kg K 2.0 g/cm³ 8 – 20 µm
Furnace power consumption	206 kW
Drum speed range normal	1-5 rpm 2.6 rpm

The material is heated in the following heat transfer modes, listed in order of increasing importance:
1. Heat of radiation.
2. Heat from direct contact with the inner surface of the drum.

The required amount of heat is determined taking into account the following requirements:
1. Heat to increase the temperature of solid components.
2. Heat to heat the wet feed material to evaporation temperature.
3. Heat to evaporate wet feed material.
4. Heat to increase the temperature of the air jet.

Description of the drum kiln operation process
Wet cake (UO 4 . 2H 2 O) is placed on the feed conveyor of the kiln. The loading side of the drum is equipped with screw plates and a feed pad that removes material from this side of the drum at high speed. Immediately after leaving the screw plates, the material flows down by gravity along the longitudinal axis of the drum. In the kiln section of the kiln, hydrated uranium peroxide (UO 4 . 2H 2 O) is heated by the electric heating elements of the kiln. The electric oven is divided into three temperature control zones, which provides flexibility in temperature response. In the first two zones, uranium peroxide (UO 4 . 2H 2 O) is gradually heated to a temperature of about 680 °C. In the third zone, the temperature rises to about 880 ° C, and the conversion of uranium peroxide (UO 4 . 2H 2 O) into uranium oxide (U3O8) occurs.

The fully reacted yellow uranium cake (U3O8) is fed into the cooling section of the drum. Heat is removed from the solid components, by high thermal conductivity, through the wall of the kiln drum and removed with cooling water sprayed onto the outer part of the drum. The temperature of the material is reduced to approximately 60 °C, then the material is fed into the discharge pipeline, through which it enters the transport system by gravity. Through the discharge pipeline, a powerful air stream is supplied to the rotary kiln, passing through the drum against the material flow, in order to remove the water vapor formed during the heating stage of the process. Humid air is removed from the loading pipeline by means of ventilation.

Rotary Kiln Components

Rotary Kiln Drum

Welded sections of the drum have seams located alternately at angles of 90° and 180° to each other and obtained by welding with full penetration of the base metal. Tires and gears are mounted on machined surfaces separated from the drum by spacers to allow for differences in radial thermal expansion. The design of the drum takes into account any thermal and mechanical stress and therefore ensures reliable operation. On the loading side of the drum there are pads holding the material, blocking the return flow of material into the pipeline and screw plates for supplying material to the heated sections.
The open sections of the drum on the loading and unloading side are equipped with personnel thermal protection screens.

Bandage
The drum has two shrouds without welds and joints made of forged steel. Each band has a one-piece rectangular section and is hardened for increased durability.

Support wheels
The furnace drum rotates on four support wheels made of forged steel. The support wheels are hardened for longer service life. The wheels are mounted with an interference fit on a high-strength shaft mounted between two bearings that have a service life of at least 60,000 hours. The base of the wheels is equipped with pressure screws for leveling and adjusting the wheels.

Thrust rollers
The unit contains two thrust rollers, consisting of two steel wheels with sealed spherical roller bearings, the service life of which is at least 60,000 hours. Thrust rollers are hardened to increase their service life.

Drive unit

The drum is designed for rotation at a frequency of 1-5 rpm at a power of 1.5 kW from an electric motor with a rotation speed of 1425 rpm, powered by a three-phase AC network with a voltage of 380 V, a frequency of 50 Hz and made in a sealed version with air cooling. The motor shaft is directly connected to the input shaft of the main gearbox through a flexible coupling.

The cycloid main gearbox has an accurate reduction ratio of 71:1 with one reduction stage. The low speed gearbox shaft is designed for the required torque and load limits.

Prevention of kiln drum deformation

In order to prevent deformation of the furnace drum during failures in the power supply system of the electric motor, an additional diesel engine is provided to continue rotating the drum. The diesel engine has an adjustable speed (1500-3000 rpm) and a rated output power of 1.5 - 3.8 kW. The diesel engine is started manually or by a DC electric starter and is directly connected to the electric motor shaft through a coupling.

Drum Kiln">

ring gear
The ring gear is made of carbon steel. Each sprocket has 96 hardened teeth, is mounted on a drum and has connectors for easy removal.

drive gear
Made from carbon steel. Each gear has 14 hardened teeth and is mounted on a low speed gearbox shaft.

Drive chain
A tilted chain is used to ensure the rotation of the kiln drum.

Kiln system

The furnace casing covers the drum and is made of carbon steel. The walls and floor of the casings are made as one complete section. The oven roof consists of three sections, one for each heating zone, and can be removed for oven or drum maintenance.

Characteristics of the chamber / heating elements:

Nozzle water cooler
Nozzle water cooler - reduces the temperature of the furnace product. The body of the cooler is made of carbon steel with internal surfaces coated with epoxy resin (to reduce the effects of corrosion). The housing is equipped with two top-mounted piping having spray nozzles, inlet and outlet rotary labyrinth seals, top steam outlet, bottom drain nozzle, side bypass nozzle, access doors and inspection holes. Water is piped into the spray nozzles and discharged by gravity through the bottom drain flange.

screw feeder

The kiln is equipped with a loading screw conveyor for feeding uranium peroxide cake into the drum, it is a screw located at a zero angle to the horizontal, subjected to finishing.

Kiln thermocouples
Thermocouples are provided for continuous monitoring of the temperature in the furnace zones and the temperature of the unloaded product.

Zero speed switches
The kiln is supplied with two zero speed switches, one of which continuously controls the rotation of the drum, the other - the rotation of the charging helix. The speed switch assemblies are mounted on the ends of the shafts and are of the type of disk pulse generators that create an alternating magnetic field recorded by the measuring device.