KR20180039647A - Apparatus and method for vacuum degassing of molten steel - Google Patents

Abstract

The present invention relates to an apparatus for vacuum degassing of liquid steel comprising: at least one vacuum chamber (2) suitable for temporarily accommodating molten steel therein; - a vacuum generating system (10) connected to the at least one vacuum chamber (2) through an intake duct (20). The vacuum generating system 10 comprises at least two compression stages connected together in series, of which the first compression stage 11 operates closer to the at least one vacuum chamber 2, and one or more screw pumps And the second compression stage 12 is operated more distantly with respect to the at least one vacuum chamber 2 to bring the gases to at least atmospheric pressure and comprises one or more liquid ring pumps 120). The one or more screw pumps 110 may be configured such that, if the discharge pressure is at atmospheric pressure, at a compression ratio not exceeding 1:12, and if the discharge pressure is comprised between 50 and 120 mbar absolute, And is sized to operate at a compression ratio. Preferably, the screw pumps are operated at a compression ratio comprised between 1: 3 and 1:10 if the discharge pressure is at atmospheric pressure and between 1:25 and 1: 200 if the discharge pressure is between 50 and 120 mbar absolute , Preferably at a compression ratio between 1:70 and 1:90. The present invention also relates to a method for vacuum degassing of molten steel.

Description

Apparatus and method for vacuum degassing of molten steel

The present invention relates to an apparatus and a method for vacuum degassing liquid steel.

The equipment and method according to the present invention can be used for vacuum degassing with VD (vacuum degassing) technology or VOD (vacuum oxygen de-carbon) technology and can be used for all applications where vacuum treatment of molten steel is required.

Vacuum degassing processes (simply referred to as VD / VOD, in English, "Vacuum Degassing" and "Vacuum Oxigen Decarburisation") are each a combination of steel and stainless steel The steel process is the main purpose of producing steel.

Vacuum treatment achieves extremely low levels of sulfur, hydrogen, and nitrogen, improves micro purity and macro purity of steel, and, in the case of VOD, enables decarbonization of steel.

Generally, VD / VOD systems are designed to continue to operate, and each process lasts between 35 and 120 minutes, depending on the productivity and mode of operation required for the facility.

Although there are a variety of installation solutions to meet specific requirements (available installation space, required productivity), a typical steel degassing installation is comprised of the following components, as shown in the overall diagram in Figure 1:

- A vacuum chamber (A) enclosed against the outside and containing a ladle (L) for containing molten steel therein.

- a system capable of drawing gases until a vacuum generator (B), ie a pressure less than 1 mbar absolute, is achieved within the vacuum chamber.

An intake line C communicating the vacuum chamber A with the vacuum generator B and communicating the vacuum generator B with a stack D discharging the gases generated by the process; ;

Apparatuses aimed at process control (valves and gas pressure and temperature measuring instruments shown below, heat exchangers for cooling the process gas output from the vacuum chamber) installed along the inspiratory line;

A dust separation unit (F), generally consisting of a cyclone (to remove large particles) and a filter (to trap small particles);

- a gas injection system (G) for stirring molten steel and removing impurities in the molten steel, usually argon, in some cases nitrogen;

An injection system (E) for introducing an inert gas into the intake duct to raise the pressure inside the isolated system to manage the foamy slag;

- Vacuum removal carbon (VOD) of steel is also provided with an oxygen injector which is controlled by an auxiliary system and is installed in the lid of the vacuum chamber.

The vacuum chamber A is composed of a lid A1 and a tank A2. There are two types of configurations, depending on what is a fixed part and what is a moving part: these configurations are the opposite of the "wheeled lid" when the tank is fixed and the lid is movable, Wheeled tank ".

Generally speaking, the vacuum generator B may be of two types: a steam ejector / liquid ring (more conventional technical solution in the past) or a mechanical pump (recently more prevalent technology) depending on the operating principle .

As shown in Fig. 1, the apparatuses which manage the process and are installed along the intake line C generally include a valve V1 for returning the vacuum chamber to atmospheric pressure, a main valve V2 for isolating the vacuum chamber from the vacuum generator, , And a valve (V3) for injecting nitrogen to manage the process.

The degassing facility is generally divided into two parts by the main valve V2. Thus, there are two volumes: the tank volume and the retained volume. The tank volume is returned to atmospheric pressure by opening a valve (V1) which, after each vacuum treatment, effectively communicates the vacuum chamber with the external environment. Instead, the reserved volume is generally kept vacuum by virtue of the main valve V2 which keeps it isolated from the outside environment. The maintenance of the vacuum in the reserved volume is achieved by using the reserved volume as a "plenum chamber" that equalizes the pressure between the tank and the reserved volume at the moment of opening the main valve V2, So that the time required to lower the pressure can be shortened. It should be noted that the tank is at atmospheric pressure prior to the opening of the main valve V2.

Generally, a vacuum degassing process involves the following steps:

Placing the ladle containing molten steel inside the vacuum chamber and closing the lid;

- aspirating the gases contained within the installation volume to achieve the required vacuum level (typically < 1 mbar);

Maintaining the operating pressure for a period of time which is considered appropriate to achieve metallurgical objectives (typically 15 to 20 minutes);

Restoring the atmospheric pressure inside the vacuum chamber (opening of the valve V1) and improving the chemical analysis by adding the correct amounts of materials.

The inhaled gas consists mainly of air up to a pressure of approximately 100-150 mbar, metal vapors, hydrogen and nitrogen from steel. The suction capability of the vacuum generating system is automatically adjusted over a range of pressures. The operator is only able to control the bubble formation of the slag present in the ladle with molten steel in order to prevent leakage of the incandescent material from the ladle itself, especially in the case of abnormal chemical reactions only in the vacuum chamber To perform the adjustment.

Control of the entire process is commanded by an automatic cycle for movement of the lid and / or tank and adjustment of the operating state of the system (i. E., The operating point of the vacuum generator to regulate the pressure inside the vacuum chamber).

It is known that a large amount of dust is generated during the entire degassing process.

The material constituting the dust is mainly derived from the evaporation of the metallic elements present in the liquid bath, from the reaction between the steel and the refractory, and to a lesser extent from the iron-alloys and scorifiers, Lt; / RTI &gt; and the filter.

During the VD process, approximately 0.1 to 0.2 kg of dust per ton of treated steel is generated: up to 20 to 40 kg during complete processing (considering lakes of 200 tonnes of molten steel as an example). Analysis of the components of common dusts reveals significant contents of Zn, MgO, CaO, Pb and Mn.

In a VOD process ("vacuum oxygen decarbonization", a vacuum process that injects oxygen to achieve low levels of carbon in molten steel), the amount of dust generated (for 200 tonnes of molten steel) can reach 800-1000 kg have.

It is essential to have an efficient dust collection system to protect the vacuum generator from wear and clogging phenomena and to prevent dust emissions to the atmosphere.

If pressure filtration is required, a cyclone separator (tangential air intake) and a bag filter are installed in series with the intake line. However, filter installations also exist with integrated cyclones.

In general, dust from these processes is very easily ignited when oxygen is present, because of their components. For this reason, bag filters often require efficient cleaning, which is currently the most common technique in these applications, which is typically done automatically by blowing an inert gas (nitrogen) back into the canvas bag after each treatment, This technique is known as "reverse purge jet ".

In addition to environmental requirements related to air emissions, the need to install elements for dust reduction is determined by the degree of dust tolerated by the vacuum system to be installed.

So far, there are two vacuum generation techniques based on completely different operating principles, which are mechanical pump and vapor ejector systems.

Vacuum generation with mechanical pump

In terms commonly used in the steel industry, mechanical pump vacuum generation refers to a vacuum generator with a series of lobe type blowers (roots pump) and screw pumps (screw pump) as shown in Figure 2 do. In this case, the screw pumps are referred to as "pre-vacuum pumps ".

As a general principle, since each of these machines performs the compression of the sucked gas, the compressed "stage" is referred to as representing one or more machines operating in the same pressure range between suction and discharge.

Currently the most prevalent installation solutions consist of a series of serially connected screw pumps and at least two roots pumps, as shown in FIG.

The stages are customarily named in ascending numerical order (stage 1, ....., stage n), starting from the closest to the vacuum chamber (A). The last stage is the stage where the gases are finally discharged to the atmosphere (pre-vacuum stage). Each stage may be composed of several pumps connected in parallel, as shown in Fig.

The criteria for determining the tandem arrangement are as follows: The screw pump can operate at a very high compression ratio (up to 1: 1000), but with a low volumetric flow rate; Instead, roots pumps can handle large volumes of gas, but do not allow for high compression ratios (typically 1: 6).

In typical VD / VOD installations, in operation, the screw pump alone can maintain a pressure of at least 20-50 mbar inside the vacuum chamber and release the gas into the atmosphere. In order to achieve a higher level of low pressure (< 1 mbar), at least two stage roots pumps are required to be installed upstream. The Roots pump is most effective in moving very low lean gases with low pressure of the gas, thanks to its type of construction (dual internal chambers alternately released and blocked by rotating lobes).

In summary, the roots pumps of the initial stage draw process gases at very low pressure (> 1 mbar) in a stable operating state (i.e. ignoring the initial exhaust transient state of the vacuum chamber starting from atmospheric pressure) Delivered to the screw pump in a pressure range operating at high compression efficiency.

A major disadvantage of the use of mechanical pumps in the configuration described above is that they can block and / or damage (seize) the rotating mechanical bodies and contaminate the lubricating oil (in the gear chamber when the gasket is aged) It is necessary to perform filtration of the inhaled gases in order to collect the solid particulate that can be obtained. The Roots pump does not imply its use in a dust environment - it is theoretically possible to treat dust gases without having to deal with seizure operating problems. However, problems of oil contamination may occur in the long term. The biggest problem relates to the fact that the screw pump can be forced to process the dust gases discharged by the Roots pump without filtration, which suffers from the above-mentioned problems of the procedure and results in immediate interruption of the system.

In describing the general operating state of a vacuum generating system with mechanical pumps, refer to four automatic cycles to determine the function of the main devices (valves, filters, pumps) installed:

- Activation cycle of the system: the pumps are started and the volume is evacuated until the volume to the main valve (the "retained volume" reaches a final pressure of typically <5 mbar; in this stage the vacuum chamber remains at atmospheric pressure And the pump is maintained at the minimum rotational speed;

Degassing cycle: the main valve is opened in a controlled manner so as to equalize the pressure between the vacuum chamber and the reserved volume; Slow equalization is designed to avoid excessive stresses to the system from the mechanical point of view (pumps and filter bags) and to prevent immediate and violent oxidation of flammable dust remaining on the surface of the filter bag after prior treatment. The pumps gradually increase the speed to reach the maximum rotational speed. While lowering the pressure, the level of vacuum in the system can be controlled by slowing / bypassing the pump or by injecting nitrogen. Typically, the process pressure (<1 mbar) is reached in 6 to 8 minutes.

- Vacuum stop cycle: When degassing is complete, the main valve is closed and the vacuum chamber returns to atmospheric pressure (after which the lid opening and the addition of material are allowed).

Cleaning cycle: The pumps are isolated and the filter bags are cleaned by a nitrogen blowing system followed by a dust drain cycle after the cleaning cycle, if necessary.

When the cleaning cycle of the bag is completed, the reserved volume is again exhausted (up to a pressure of <5 mbr) to prepare the system for the next degassing cycle. Cleaning of the filter bags is an important aspect for the performance of a mechanical pump system because:

- If dust accumulates in the bags, the pressure loss through the filter is increased and the minimum pressure that can be reached inside the vacuum chamber is limited.

Possible damage to the bags allows large amounts of dust to reach the pumps. Thus, if the cleaning and maintenance of the filters is not properly performed (cleaning cycle by nitrogen, regular inspection of bags ...), operation of the system is at stake.

Ejector  Vacuum generated by an ejector pump

Ejector vacuum generators use superheated steam from a boiler or from other sources as propellent fluid. As a result of the acceleration of the vapor and the architecture of the ejector, the process gas is sucked and compressed.

Each ejector is sized to compress a given amount of gas to achieve a specific ratio between suction pressure and discharge pressure (typically about 1: 5/1: 15). To operate between the pressure required by the process (1 mbar) and atmospheric pressure (1000 mbar), several different ejectors operating in series are required.

In this case, in a tandem arrangement, each ejector is considered a compression "stage ". However, in order to increase the suction capacity of the system at higher pressures (which is generally required during the evacuation step of the vacuum chamber), the stage may consist of several ejectors arranged in parallel.

Figure 4 shows the layout of a typical ejector pumping station wherein S1, S2, S3 and S4 refer to the ejector stages, C1, C2 and C3 refer to the condenser between stages, and P is the collection tank or " Quot; hot pit ". The S3 and S4 stages, in a special case, consist of a pair of A / B ejectors operating in parallel. The operating sequence of the individual stages is usually controlled by the pressure reached by the vacuum chamber (see FIG. 4) as follows: S4-S3-S2-S1.

Heat exchangers are installed in series with the ejectors to condense the vapor contained in the main gas stream to ensure maximum efficiency of the ejector system (treatment of the maximum flow of the process gas).

The steam actually acts only as a propeller for aspirating process gases and condenses as the pressure increases and the temperature decreases.

Thus, the vapors are made to condense inside the "condensers" and the condensers are drained into a tank called a "hot pit".

When there is no filtration system upstream of the ejector groups, the condensate has a high concentration of dust, and therefore requires maintenance work for the treatment of appropriate wastewater treatment facilities and sludge sent into "hot pits" .

Figure 4 shows a diagram of a typical ejector consisting of four compression stages.

This variant of the diagram provides a fourth stage in which a liquid ring pump is constructed in lieu of the ejector, or alternatively a possible fifth stage. This solution is generally desirable in systems that have limited steam availability or are required by installation or process requirements (limited installation space required to operate reliably at pressures above 100 mbar in a VOD system).

The liquid ring pump is a mechanical, centrifugal pump, and compression in the centrifugal pump due to confinement of the gas in the changing (gradually decreasing) volume is caused by centrifugal effect of the rotor, eccentric to the casing (body) of the pump It has a significant influence on the rotation of the generated liquid ring.

Except for the configuration of the pumping system and the dust reduction group connected thereto, the operation of the ejector / liquid ring facility goes through an operation sequence that is quite similar to that described in the mechanical pump.

In an ejector system, it is not necessary to reduce the dust to such an extent as to require the installation of a bag filter for the purpose of protecting the pumping system, since there is no geometric tolerance required by the mechanical system, typically a roots or screw pump.

On the other hand, in some systems, a cyclone with an associated automatic cleaning system or even a bag filter may be installed to minimize maintenance (cleaning of the ejector and hot-pit water treatment).

Finally, when there are no filter elements, a large amount of dust is collected by the injected steam and possibly by the water of the liquid ring pump. Vapor condensed between one ejector stage and another ejector stage helps to capture some of the dust generated during the process. As mentioned above, the condensate is drained into the "hot pit" (indicated by P in Fig. 4). Also, a possible liquid ring in contact with the process gas helps to collect the remaining dust. In a liquid ring / ejector system, the amount of dust contained in the gases discharged to the chimney is very small.

In conclusion, the main difference with regard to the system layout between the ejector system and the mechanical pump system is that it is necessary to maintain the integrity of the machine in the mechanical pump (with all auxiliary elements for cleaning the bag and for discharging the dust) The presence of a filter.

The main limitations of the injector vacuum generation system are complexity and high installation and operating costs.

It is therefore an object of the present invention to combine the technical / operational simplicity of a mechanical pump facility with the possibility of operating without a filter system of an ejector facility, in order to completely or partially eliminate the disadvantages of the prior art mentioned above, and an apparatus and a method for vacuum degassing liquid steel.

A further object of the present invention is to make available a facility for vacuum degassing of molten steel operating more reliably.

A further object of the present invention is to make available equipment for vacuum degassing of molten steel, which is cheaper to operate.

A further object of the present invention is to make available equipment for vacuum degassing of molten steel comparable to conventional systems with at least mechanical pumps in terms of equipment cost.

Technical features of the present invention, in accordance with the above objects, may be best understood from the following description of the claims, the benefit of which is set forth in the following description, taken in conjunction with the accompanying drawings, which illustrate, by way of non- Will be more clearly understood.

1 shows a general diagram of a steel degassing plant;
- Figure 2 shows a general diagram of a conventional vacuum generating system with mechanical pumps of the roots and screw types;
3 shows a general diagram of a conventional vacuum generating system comprising several pumps having loose and screw type mechanical pumps and each stage arranged in parallel; Fig.
- Figure 4 is a diagram of a conventional ejector vacuum generating system;
5 shows a general diagram of a vacuum degassing plant for molten steel according to a preferred embodiment of the present invention;
6 shows a general diagram of a vacuum degassing plant for molten steel according to an alternative embodiment of the present invention;
7 shows a general diagram of a vacuum generating system in a vacuum degassing plant for molten steel according to a preferred embodiment of the present invention;
Figure 8 shows a general diagram of a liquid ring pump.

Referring to the accompanying drawings, reference numeral 1 generally denotes an apparatus for vacuum degassing of molten steel according to the present invention.

The facility 1 according to the present invention can be used for vacuum degassing with VD (Vacuum Degassing) technology or VOD (Vacuum Oxygen Decarburization) technology and can be used for vacuum degassing of liquid steel Can be used for all the applications required.

Here and hereafter, in the description and the claims, the molten steel vacuum degassing apparatus 1 is referred to in use.

According to a general embodiment of the present invention, the equipment for vacuum degassing of the molten steel comprises:

At least one vacuum chamber (2) suitable for temporarily accommodating molten steel therein; And

- a vacuum generating system (10) connected to said at least one vacuum chamber (2) through an intake duct (20).

The vacuum chamber 2 may be of any type suitable for the purpose.

Preferably, the vacuum chamber 2 is configured such that the molten steel enters through the ladle L, but may also be used directly to receive the molten steel.

5 and 6, the vacuum chamber 2 includes a tank 3 and a lid 4, the tank 3 defining the volume of the chamber 2 And the lid 4 is suitable for sealing the tank 3 when the ladle L is housed inside the tank 3. The lid L is adapted to receive the ladle L therein. The vacuum chamber may be of the "wheeled lid" type when the tank is fixed and the lid is movable, or in the opposite case, a "wheeled tank" type.

Advantageously, as shown in Figures 5 and 6, the vacuum chamber can be combined with a cleaning gas, in some cases an injection system of nitrogen, for stirring the molten steel and removing impurities inside the molten steel. In particular, the injection system 30 is designed to supply one or more porous septums disposed at the bottom of the lasers.

In the second case, according to an embodiment not shown in the accompanying drawings, the vacuum chamber 2 may be configured to directly receive the molten steel therein in accordance with the RH process. In this case, the molten steel is temporarily moved from the ladle into the chamber. For this purpose, the vacuum chamber is connected to the ladle through two ducts: a delivery duct and a return duct, and the molten steel from the ladle through the delivery duct is vacuumed The molten steel is returned to the inside of the chamber, and the processed molten steel returns from the vacuum chamber into the ladle through the return duct.

According to the invention, as shown in Figures 5 and 6, the vacuum generating system 10 comprises at least two compression stages connected in series with one another,

- the first compression stage (11) operates closer to said at least one vacuum chamber (2) and consists of one or more screw pumps (110);

The second compression stage 12 operates further away from the at least one vacuum chamber 2 to bring the gas to at least atmospheric pressure and consists of one or more liquid ring pumps 120.

The one or more screw pumps 110 may be operated at a compression ratio not exceeding 1:12 when the discharge pressure is at atmospheric pressure and may be operated at a rate of 1: 200 if the discharge pressure is comprised between 50 and 120 mbar absolute So that it can be operated at a compression ratio not exceeding a predetermined value.

As noted below, the one or more screw pumps 110 are thus sized in a manner that is fundamentally different from conventional screw pumps.

Preferably, the one or more screw pumps 110 can be operated at a compression ratio comprised between 1: 3 and 1:10 when the discharge pressure is at atmospheric pressure, and when the discharge pressure is between 50 and 120 mbar absolute To be operated at a compression ratio of 1:25 to 1: 200, preferably 1:70 to 1:90.

Due to the fact that it operates within the compression ratio range, the one or more screw pumps 110 are provided to conventional screw pumps used as pre-vacuum stages as described above A size is imposed that imposes a much larger internal tolerance (rotor / rotor and rotor / case) than internal tolerance. As such, the screw pumps 110 can operate in direct contact with the dust gases with high dust concentrations, without the side effects of moving mechanical components, and thus used as pre-vacuum stages in conventional mechanical degassing systems Do not lead to the general problems of screw pumps.

This is made possible by the fact that according to the invention the screw pumps are used in stages closer to the vacuum chamber and the selection of operating these pumps within the compression ranges.

In operation, the operation of compressing the gas is completed by one or more liquid ring pumps which form a second compression stage (final) away from the vacuum chamber, which utilizes the fact that the liquid ring pumps are dust insensitive.

Advantageously, the liquid ring pumps also perform an important function of collecting solid particles drawn along the main flow of gas. Thus, the pump service water is used to collect the dust generated by the degassing process and then to collect the dust at one point. Thus, the discharge of dust in the discharge chimney 40 as possible is minimized and the environmental impact is reduced.

Thanks to the invention, the vacuum generating system 10 can directly draw in the dust-containing gas from the at least one vacuum chamber 2 at a high concentration, without the side effects of a typical conventional mechanical pump system.

Conventionally, screw pumps are used to form compression stages farthest from the vacuum chamber (A) in the vacuum generating system of the degassing apparatus, unlike the configuration of the present invention. Although these pumps (pre-vacuum) operate at a compression ratio between 1: 1 and 1:50, preferably between 1: 2 and 1:40, Compression ratio. Conversely, screw pumps 110 according to the present invention are sized to operate at a maximum compression ratio of 1:12 for discharge at atmospheric pressure. Thus, conventional screw pumps are configured to have very tight internal tolerances (rotor / rotor and rotor / case). This in particular makes the screw pumps sensitive to the presence of dust in the gases to be treated.

On the one hand, thanks to the invention, the design of the molten steel degassing plant can be freed from the installation of a filtration device (usually a bag filter) required in the case of a mechanical pump and, on the other hand, Thereby significantly reducing facility costs.

Advantageously, the vacuum generating system 10 is sized to bring the vacuum chamber 2 in a degree of vacuum between 0.2 and 5 mbar, preferably between 0.5 and 1.5 mbar. As a result, the vacuum generating system 10 is sized to produce a total compression ratio between 1: 5,000 and 1: 200.

With regard to the sizing of the vacuum generating system 10 according to the present invention, the possible combinations in terms of the number of screw pumps 110 and liquid ring pumps 120 will depend on the level of performance required by the process, Depending on the design choices made from time to time to minimize the evacuation time of the vacuum chamber and the number of machines installed to obtain a final vacuum degree of approximately <1 mbar.

Advantageously, the vacuum generating system 10 may include one or more intermediate compression stages arranged in series between the first stage 11 and the second stage 12, And one or more screw pumps 110 having characteristics similar to the screw pumps of the one stage 11.

The term "similar characteristics" means that the one or more screw pumps of the intermediate stages are sized to operate within the same compression range as the screw pumps of the first stage, and therefore, Allowing for much higher internal tolerances (rotor / rotor and rotor / case). The size of the screw pumps of the intermediate stages may be the same or different from the size of the screw pumps of the first stage. The choice of size depends on the sizing of the vacuum generating system.

At least one of the compression stages (first, second or middle) may comprise two or more pumps connected in parallel.

According to an embodiment not shown in the accompanying drawings, the vacuum generating system may consist of two or more parallel pumping modules, each of which comprises a first compression stage with at least screw pumps (11) and a second compression stage (12) with liquid ring pumps.

The total number of pumps and modules installed per module is determined at the design stage with the aim of minimizing installation optimization and consumption of auxiliary components (water, nitrogen, electricity).

Advantageously, a modular configuration may be employed for the vacuum generating system 10, i.e. it may be separated into units installed in parallel, or a "hybride " system in which the pumps are grouped in two stages, "Installation can be.

Advantageously, the vacuum generating system 10 can be isolated from the rest of the system by closing the appropriate shut-off valves located upstream of the pumps.

5 and 6, the intake duct 20 includes a by-pass duct 21, which can exclude compression stages formed from the gas streams from the screw pumps 110, . This solution can be employed for both modular and non-modular structures.

In operation, the presence of the bypass duct 21 can be used to exclude screw pumps from the function of some stages of the degassing process, as will be described below again.

Preferably, each of the screw pumps 110 used in the degassing plant 1 according to the present invention comprises two screw rotors which are kinematically synchronized with one another via an electric axis.

For the connection and synchronization of the two screw rotors, these pumps are a conventional "mechanical " type, in which the engine delivers motion to one screw rotor and the other rotor is driven / synchronized by a series of gears in an oil bath Mechanical axis "is not used.

The term "electric axis " refers to software synchronization of a pair of engines by an inverter and a pair of encoders (one for each screw). The software immediately manages the parameters of the two inverters so that the rotors are continuously synchronized. Moreover, the two encoders control the angular deviation of the axes of the screw rotors so that these axes are perfectly parallel to one another.

In operation, any functional anomaly (e.g., internal friction due to the accumulation of dust) results in an increase in torque and current absorption of the motors and, as a result, can cause deviations in the angular speed of the rotors. Advantageously, the software can act at speed in real time until the balance is restored, preventing stress and overheating of the pump.

Compared to solutions with mechanical axes, the construction of this electrical axis does not require oil for lubrication of the gears. It is advantageous that there is no lubricant. In fact, due to the possible pressure differential between the compression chamber (low pressure) and the possible (coexisting) gear chamber (high pressure), the oil may be drawn into the process gas, mixed with the dust, and produce an obstruction.

The liquid ring pumps 120 used in the degassing plant 1 according to the present invention are of a type known per se and therefore their operation is blown by those skilled in the art. Thus, a detailed description of a liquid ring pump is not provided, but is simply referred to a number of concepts useful for introducing some specific elements.

In particular, the liquid ring pumps used in the present invention may have the structure shown in Fig.

8, the liquid ring pump compresses the process gas G 'between an eccentric vane rotor 121 and a ring 122 of water called a service water W . In operation, the dust carried by the process gas (G ') inevitably comes into contact with the constant (W), and the constant (W) acts as a collector. The pump 120 discharges the compressed gas G "together with a minimum amount of dust-containing constant. The mixture of gas and dust-containing constant G" + W is supplied to a separator 123 And the separator 123 separates the gas from the collected dirty water at the bottom of the separator 123 (the gas is now at atmospheric pressure and is sent to the chimney). Advantageously, the constant W is supplemented (124) to offset losses from evaporation.

More specifically, the constant W can be treated as an open circuit and a closed circuit in two ways.

In closed-circuit management, the constant W is recirculated to the saturation limit of the dust, which degrades the performance of the pump. At this point, all the constants (W) are discharged and replaced with clean constants.

In open-circuit management, the constant is continuously discharged from the separator (via opening 125 shown in FIG. 8), and a clean constant line 124 continuously replenishes the service circuit of the liquid ring pump do.

Advantageously, the dust-containing constants are now inert and can be handled in two different ways.

According to a first method, the water containing the dust is collected in a decanting bath with overflow, and the overflow is led to a second bath. Constant containing dust is sent from the second tank to the water treatment plant by a centrifugal pump, where it is treated in a conventional manner.

According to a second method, as schematically shown in Fig. 7, the dust-containing constant leaving the separator can be filtered using methods known in the field.

Advantageously, as shown in Figure 7, in addition to supplementing the constants dispersed in the process gases by the liquid ring pumps, the installation 1 separates the dust contained in the water and pumps it And an auxiliary unit 50 for recirculation.

Continuous cycle operation ensures controlled removal (prevention of internal accumulation) of dust and optimal operation of the liquid ring pump 120 due to cooling and cleaning of the makeup water.

The auxiliary unit 50 may be concentrated or disposed on each liquid ring pump or module but retains the same functions.

As an alternative to the auxiliary unit 50, the installation 1 may comprise at least one continuous replacement device of a constant used by the liquid ring pump, where the constant does not recirculate and does not return.

Advantageously, as shown in FIGS. 5 and 6, at a portion comprised between the vacuum chamber 2 and the vacuum generating system 10, the intake duct 20 is provided with a first control valve 23 And a connecting branch 28 to an atmosphere. The first control valve 23 is opened to return the vacuum chamber 2 to the atmospheric pressure before removing the processed molten steel at the end of the degassing step.

Advantageously, as shown in FIGS. 5 and 6, at a portion comprised between the vacuum chamber 2 and the vacuum generating system 10, the intake duct 20 contains an inert gas (nitrogen or argon) (Not shown), and the connecting branch 29 is equipped with a second control valve 24. The second control valve 24 is connected to the branch 29, The inert gas can be injected by opening the second valve 24, which raises the internal pressure to manage the foamy slag.

According to the preferred embodiment shown in Fig. 5, the degassing installation 1 does not include a filtration device for the gases which must leave the vacuum chamber 2 and pass through the vacuum generating system 1. In Fig. Regardless of the concentration level of dust in the gases, the gases discharged from the vacuum chamber 2 are drawn directly by the vacuum generating system without a gas filtration step for prevention. As mentioned previously, this is possible thanks to the invention.

According to an alternative embodiment shown in Figure 6 the degassing installation 1 may comprise at least one filtering device 25 of gases leaving the vacuum chamber 2 and passing through the vacuum generating system 10 . This filtering device 25 is disposed between the vacuum chamber 2 and the vacuum generating system 10.

In operation, gases leaving the vacuum chamber (2) are filtered to at least partially reduce the amount of dust present in the gases before being drawn by the vacuum generating system. Given that the presence of possible dust does not affect the operation of the vacuum generating system 10, thanks to the present invention, the reduction of dust can be partial and passable. The filtration step for prevention may be provided to optimize the management of the dust in the system and to reduce the load of dust to be managed by the liquid ring pumps.

The filtration device 25 may comprise a system in which a bag filter, a cyclone or a bag filter and a cyclone are integrated.

6, the facility 1 comprises at least one shut-off valve 22, which is connected to the vacuum chamber 2 and the filtration device 25, And is installed in the intake duct (20). This shut-off valve 22 is disposed downstream of the branching point of the intake duct 20 into the connecting branch 28 of the atmosphere. The shutoff valve 22 divides the facility 1 into two parts, thereby identifying two volumes. The first part comprises a vacuum chamber (tank volume); The second part includes a filtration device and a vacuum generating system (reservoir volume).

In operation, the tank volume returns to atmospheric pressure by opening the first control valve 23, which, after each vacuum treatment, causes the vacuum chamber to communicate with the external environment. Instead, the reserved volume is always kept vacuum by virtue of the shut-off valve 22, which keeps it in an air-tight condition. The maintenance of the vacuum of the reserved volume can be accomplished by using the reserved volume as a "plenum chamber" that equalizes the pressure between the reservoir and the reserved volume at the moment of opening the shut- Thereby shortening the time required to lower the pressure in the chamber.

The presence of the shut-off valve 22 is preferable when the facility 1 is equipped with the filtration device 25 (particularly when the filtration device is a bag filter), as shown in Fig. In this case, the reserved volume is very large due to the presence of the filtration device.

Advantageously, when the facility 1 does not have a filtration device 25 (see FIG. 5), the reserved volume is reduced and therefore the advantages associated with maintaining the volume in vacuum are limited, The valve 22 need not be installed.

Advantageously, when the facility 1 is used for vacuum degassing with VOD (vacuum oxygen-decarbonization) technology, the facility 1 comprises a heat exchanger for cooling the process gases (Not shown). In fact, in VOD technology, as a result of the injection of oxygen and, as a consequence, of the decarbonization of steel, the temperature involved is considerably increased. The heat exchanger should be located upstream of the filtration device 22 (if present) and downstream of the shut-off valve within the second part (reservoir volume) of the system.

* * *

The present invention also relates to a method for vacuum degassing of molten steel.

In particular, the process according to the invention can be carried out in a degassing system according to the invention, in particular as described above. Parts common to the facility 1 described above are indicated using the same reference numerals.

According to a general embodiment of the invention, a method for vacuum degassing of molten steel comprises the following operating steps:

a) providing at least one vacuum chamber (2) suitable for temporarily accommodating molten steel therein;

b) disposing molten steel in the vacuum chamber (2);

c) generating a predetermined degree of vacuum inside the vacuum chamber to complete the degassing operation of the molten steel and maintaining the vacuum for a predetermined period of time; ; And

d) withdrawing the degassed molten steel while bringing the vacuum chamber (2) back to atmospheric pressure.

According to the invention, the vacuum evacuation step (c) is carried out by a vacuum generating system (10) comprising at least two compression stages connected together in series, and wherein, among the at least two compression stages:

- the first compression stage (11) operates closer to the at least one vacuum chamber (2) and consists of one or more screw pumps (110);

The second compression stage 12 operates further from the at least one vacuum chamber 2 to bring the gas to at least atmospheric pressure and consists of one or more liquid ring pumps 120.

The one or more screw pumps 110 are sized to operate at a compression ratio not exceeding 1:12 when the discharge pressure is at atmospheric pressure and the discharge pressure is comprised between 50 and 120 mbar absolute To be operated at a compression ratio not exceeding 1: 200.

Preferably, the one or more screw pumps 110 are sized to be operable at a compression ratio comprised between 1: 3 and 1:10 when the discharge pressure is at atmospheric pressure, the discharge pressure is between 50 and 120 mbar absolute, is sized to be operable at a compression ratio between 1:25 and 1: 200, preferably between 1:70 and 1:90.

Preferably, in the evacuation step (c), the vacuum chamber 2 is provided to operate at a degree of vacuum between 0.2 and 5 mbar, preferably between 0.5 and 1.5 mbar.

According to a preferred embodiment of the method, the evacuation step (c) is carried out in the vacuum chamber (2) through the vacuum generating system (10) without the precautionary filtering step of the gases, Lt; RTI ID = 0.0 &gt; of gases. &Lt; / RTI &gt;

According to an alternative embodiment of the method, the evacuation step (c) comprises the step of pre-filtering the gases to reduce the concentration of dust in the gases before they pass through the vacuum generating system And provides suction of gases from the vacuum chamber (2) through the vacuum generating system (10).

Advantageously, said evacuation step (c) comprises:

- an initial evacuation step (c1) in which the vacuum chamber (2) is provided with at most about 300 mbar from atmospheric pressure using only the liquid ring pumps of the vacuum generating system (10); And

- a final evacuation step (c2) using said screw pumps to provide said vacuum pump (2) from a pressure of approximately 300 mbar to a predetermined degree of vacuum.

This mode of operation makes it possible to minimize the amount of dust that the screw pumps must handle, up to the benefit of the operation of these pumps. This mode of operation has the advantage of the presence of a by-pass 21 present in the intake duct and allowing the exclusion of the screw pumps from the passageways of the gases.

Advantageously, during said evacuating step (c), the suction capacity of the vacuum generating system 10 can be varied to reduce any phenomenon in which slag is formed in the molten steel. The suction capacity is varied by slowing or eliminating one or more pumps of the vacuum generation system 10, preferably the liquid ring pumps 120.

Preferably, the change in the suction capacity is performed when the internal pressure of the vacuum chamber 2 is between 300 mbar and 1 mbar, i.e. during the final evacuation step c2.

Preferably, the method according to the invention comprises the step (f) of processing a constant used by said one or more liquid ring pumps. This treatment step (f) is preferably carried out during the evacuation step (c). The process consists of filtering the dust from the constant or continuously changing the constant.

Advantageously, the method comprises a step (e) of mixing molten steel at least during the evacuation step (c), in particular by injecting an inert gas into the molten steel.

The present invention permits many advantages to be achieved and has already been partially described.

The apparatus (1) for vacuum degassing of molten steel according to the present invention combines the possibility of operating the simplicity of a mechanical pump facility without the filter system of the ejector facility.

Thus, thanks to the invention, the equipment installed in the degassing facility can be minimized.

As such, the present invention ensures greater flexibility in the design phase (layout and auxiliary devices) and allows operation of the system with minimal maintenance costs and maintenance of the components most affected by wear in conventional systems. In particular, the present invention reduces periodic inspection and eliminates the need to replace filter bags.

Accordingly, the apparatus (1) for vacuum degassing of molten steel according to the present invention comprises:

- operate more reliably;

- Driving is more expensive.

In view of the cost of the equipment, the equipment 1 for vacuum degassing of molten steel is comparatively less expensive than conventional systems with at least mechanical pumps, which is clearly less expensive than conventional systems with ejectors.

The advantages presented above for the installation 1 according to the invention extend to the degassing process according to the invention.

Thus, the envisioned invention achieves its intended objectives.

Obviously, the practical embodiments of the present invention may assume different forms and configurations than those described while remaining within the protection zone of the present invention.

Moreover, all the details may be replaced by technically equivalent elements and dimensions, and the shapes and materials used may be any as needed.

Claims (20)

Equipment for vacuum degassing of liquid steel:
At least one vacuum chamber (2) suitable for temporarily accommodating molten steel therein;
- a vacuum generating system (10) connected to said at least one vacuum chamber (2) through an intake duct (20)
The vacuum generating system (10) comprises at least two compression stages connected together in series, wherein a first one of the at least two compression stages (11) is connected to the at least one vacuum chamber And is formed of one or more screw pumps 110 and the second compression stage 12 is operated more distant to the at least one vacuum chamber 2 to bring the gases to at least atmospheric pressure Wherein the at least one screw pump (110) is formed of one or more liquid ring pumps (120), wherein the at least one screw pump (110) is sized to operate at a compression ratio not exceeding 1:12 if the discharge pressure is at atmospheric pressure And is sized to operate at a compression ratio not exceeding 1: 200 if the discharge pressure is comprised between 50 and 120 mbar absolute , Vacuum degassing equipment for molten steel. The method according to claim 1,
The one or more screw pumps 110 are sized to operate at a compression ratio comprised between 1: 3 and 1:10 if the discharge pressure is at atmospheric pressure, and if the discharge pressure is between 50 and 120 mbar absolute , A size is imparted to operate at a compression ratio between 1:25 and 1: 200, preferably between 1:70 and 1:90. 3. The method according to claim 1 or 2,
The vacuum generating system (10) is dimensioned to provide a vacuum degree in the vacuum chamber (2) between 0.2 and 5 mbar, preferably between 0.5 and 1.5 mbar. The method according to claim 1, 2, or 3,
The vacuum generating system 10 comprises at least one intermediate compression stage disposed between and connected in series between the first stage 11 and the second stage 12, 1 stage &lt; RTI ID = 0.0 &gt; 11, &lt; / RTI &gt; 5. The method according to one or more of the preceding claims,
Each of the compression stages being formed of two or more pumps connected in parallel. 5. The method according to one or more of the preceding claims,
Wherein the intake duct (20) comprises a by-pass duct (21) capable of removing compression stages formed by the screw pumps (110) from the gas flow. 5. The method according to one or more of the preceding claims,
Wherein each screw pump (110) comprises two screw rotors kinematically synchronized with each other via an electric axis. 5. The method according to one or more of the preceding claims,
At least one filtration device of water used by the one or more liquid ring pumps (120), the filtration device being suitable for removing dust accumulated in the water itself during operation of the pump or the replacement device of the water itself , Vacuum degassing equipment for molten steel. 5. The method according to one or more of the preceding claims,
In the portion included between the vacuum chamber 2 and the vacuum generating system 10 the intake duct 20 is connected to a connection branch 28 ). &Lt; / RTI &gt; 5. The method according to one or more of the preceding claims,
And at least one gas filtration device (25) disposed between the vacuum chamber (2) and the vacuum generating system (10). 11. The method according to claim 9 or 10,
At least one shutoff valve 22 is provided in the intake duct 20 between the vacuum chamber 2 and the filtration device 25 and downstream of the branch point of the connecting branch 28 in the atmosphere Vacuum degassing equipment for molten steel. A method for vacuum degassing of liquid steel comprising:
a) providing at least one vacuum chamber (2) suitable for temporarily accommodating molten steel therein;
b) placing molten steel in the vacuum chamber (2);
c) the vacuum chamber (2) is evacuated through a vacuum generating system (10) which creates a predetermined degree of vacuum in the vacuum chamber to complete the degassing operation of the molten steel and maintains the degree of vacuum for a predetermined period of time Evacuating; And
d) withdrawing the degassed molten steel while bringing the vacuum chamber (2) back to atmospheric pressure,
The vacuum evacuation step (c) is carried out by a vacuum generating system (10) comprising at least two compression stages connected together in series, wherein the first one of the at least two compression stages ) Operates closer to the at least one vacuum chamber (2) and is formed by one or more screw pumps (110), and the second compression stage (12) is connected to the at least one vacuum chamber Operating more distally with respect to the chamber 2 and formed by one or more liquid ring pumps 120 and wherein the one or more screw pumps 110 may be operated at a pressure of 1:12 So as to be capable of operating at a compression ratio not exceeding 1: 200 if the discharge pressure is between 50 and 120 mbar absolute Wherein the groups are assigned, the vacuum degassing method for molten steel. 13. The method of claim 12,
The one or more screw pumps 110 are sized to operate at a compression ratio comprised between 1: 3 and 1:10 if the discharge pressure is at atmospheric pressure, and if the discharge pressure is between 50 and 120 mbar absolute , A size is imparted to operate at a compression ratio between 1:25 and 1: 200, preferably between 1:70 and 1:90. The method according to claim 12 or 13,
In the evacuation step (c), the vacuum chamber (2) is provided to operate at a degree of vacuum between 0.2 and 5 mbar, preferably between 0.5 and 1.5 mbar. 15. The method according to any one of claims 12 to 14,
The evacuation step (c) comprises the steps of providing a direct aspiration of gases from the vacuum chamber (2) through the vacuum generating system, without the need for the pre-filtering step of the gases, irrespective of the level of dust concentration in the gases themselves Vacuum degassing method. 15. The method according to any one of claims 12 to 14,
The evacuation step (c) is performed through the vacuum generating system (10) with the preventive filtering steps of the gases to reduce the dust concentration in the gases themselves before the gases pass through the vacuum generating system Wherein the vacuum chamber (2) is provided with a vacuum. 17. The method according to any one of claims 12 to 16,
The evacuation step (c) comprises:
- an initial evacuation step (c1) in which the vacuum chamber (2) is provided with at most about 300 mbar from atmospheric pressure using only the liquid ring pumps of the vacuum generating system (10); And
- a final evacuation step (c2) using the screw pumps (110) to provide the vacuum pump (2) with a pressure of about 300 mbar to a predetermined degree of vacuum. The method according to any one of claims 12 to 17,
During the evacuation step (c), the suction capacity of the vacuum generating system (10) is changed to reduce the occurrence of slag formation in the molten steel, and the suction capacity is adjusted by one or more pumps of the vacuum generating system The change in the suction capacity is preferably carried out when the internal pressure of the vacuum chamber 2 is between 300 mbar and 1 mbar, Vacuum degassing method. 19. The method according to any one of claims 12-18,
(F) of water used by said one or more liquid ring pumps, said treatment step being preferably carried out during said evacuation step (c), said treatment being carried out by filtration of water from the dust or replacement of the water itself Wherein the molten steel has a surface roughness of at least 20 占 퐉. The method of any one of claims 12 to 19,
(E) of mixing molten steel during at least said evacuation step (c).

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