ES2742861T3 - Plant and method for vacuum degassing of liquid steel - Google Patents

Abstract

Plant for vacuum degassing of liquid steel, comprising: - at least one vacuum chamber (2), suitable for temporarily receiving liquid steel inside, - a vacuum generation system (10), connected to said at least a vacuum chamber (2) by means of a supply duct (20), the vacuum generation system (10) comprises at least two compression phases connected to each other in series, of which a first compression phase (11 ) works closer to said at least one vacuum chamber (2) and is formed by one or more screw pumps (110); characterized in that a second compression phase (12) works further away from said at least one vacuum chamber (2) to bring the gases to at least atmospheric pressure and is made up of one or more liquid ring pumps (120) and because said one or more screw pumps (110) are dimensioned to be able to operate with compression ratios that do not exceed 1:12, if the discharge pressure is atmospheric and with compression ratios that do not exceed 1: 200, if the pressure discharge is between 50 and 120 mbar absolute.

Description

DESCRIPTION

Plant and method for vacuum degassing of liquid steel

Scope

The present invention relates to a plant and a method for vacuum degassing of liquid steel.

The plant and the method according to the invention can be used for vacuum degassing either with the VD technique (vacuum degassing) or with the VOD technique (vacuum oxygen decarburization) and for all applications where a vacuum treatment of liquid steel.

State of the art

The vacuum degassing process (called for simplicity VD / VOD, of the English “vacuum degassing” and “decarburization with vacuum oxygen”), respectively, is a steel process whose main objective is to produce steels that meet the standards High quality and stainless steels, for example, as described in EP 0366293 A2, make it possible to achieve extremely low levels of sulfur, hydrogen and nitrogen, improve the micro and macro purity of steel and, in the case of VOD, decarbonize (reduce carbon content) steel.

In general, VD / VOD systems are designed to operate 24 hours a day and each treatment lasts 35 to 120 minutes, depending on the plant's required productivity and operating practices.

Although there are various solutions for plants designed to meet specific requirements (available installation space, required productivity), normally a steel degassing plant consists of the following components, as shown in the general diagram in Figure 1:

- a vacuum chamber A, hermetic to the outside, in which the bucket L containing the liquid steel is housed; - a vacuum generator B, that is, a system that can aspirate gases until a pressure of less than 1 mbar absolute is achieved inside the vacuum chamber;

- a supply line C that places the vacuum chamber A in communication with the vacuum generator B and the latter with the stacking D through which the gases generated by the process are discharged;

- devices intended to manage the process, installed along the supply line (valves illustrated below and gas pressure and temperature measuring instruments, a heat exchanger to cool the process gas leaving the vacuum chamber );

- a dust separation unit F in general consisting of a cyclone (to remove larger particles) and a filter (to retain smaller particles);

- a gas insufflation system G, normally argon, in some cases even nitrogen, for the agitation of liquid steel and the removal of impurities within it;

- an insufflation system E of inert gas in the supply duct to handle the foamy slag, raising the pressure within the isolated system;

- where vacuum decarburization of steel (VOD) is also provided, an oxygen injector controlled by an auxiliary system is installed in the vacuum chamber lid.

The vacuum chamber A consists of a cover A1 and a tank A2. Depending on which is the fixed part and which is the mobile part, there are two types of construction: a "wheel cover" when the tank is fixed and the cover is mobile, and a "wheel tank" in the opposite case.

In general, depending on the principle of operation, the vacuum generator B can be of two types: with a steam ejector / liquid ring (most popular technical solution in the past) or mechanical pumps (technology that is recently being generalized).

As shown in Figure 1, the devices for managing the process and installed along the supply line C normally comprise a valve V1 to return the vacuum chamber to atmospheric pressure, a main valve V2 to isolate the chamber from Vacuum generator vacuum, a V3 valve to insufflate nitrogen to control the process.

In general, the degassing plant is divided into two parts by the main valve V2. Therefore, there are two volumes: a deposit volume and a retained volume.

The volume of the tank is returned to atmospheric pressure after each vacuum treatment by opening the valve V1 that effectively places the vacuum chamber in communication with the external environment. The volume retained, on the other hand, is generally kept empty thanks to the main valve V2, which keeps it isolated from the external environment. The maintenance of the vacuum in the retained volume allows the time required to reduce the pressure in the system to be shortened by using it as an "impending chamber" that equalizes the pressure between the reservoir and the retained volume at the time of opening the main valve V2. It should be noted that the tank is at atmospheric pressure before opening the main valve V2.

In general, a vacuum degassing process comprises the following steps:

- placing the spoon containing liquid steel inside the vacuum chamber and closing the lid;

- aspiration of the gases contained within the volume of the plant to achieve the required vacuum level (typically <1 mbar);

- permanence at the operating pressure for the time deemed appropriate (typically 15 to 25 minutes) to achieve the metallurgical objectives;

- restoration of atmospheric pressure inside the vacuum chamber (opening of valve VI), and refining of the chemical analysis by adding materials in precise quantities.

The supply gas is mainly composed of air up to a pressure of approximately 100-150 mbar, and after vapors of metal, hydrogen and nitrogen from the steel. The suction capacity of the vacuum generation system is automatically adjusted over the entire pressure range. The operator is requested to make an adjustment only in the case of anomalous chemical reactions inside the vacuum chamber (especially in cases of foaming of the slag, present in the spoon with the molten steel, to avoid leaks of incandescent material itself spoon).

The control of the whole process passes through the movement of the lid and / or the tank and the command of the automatic cycles for the adjustment of the operating conditions of the system (that is, the working points of the vacuum generator for check the pressure inside the vacuum chamber).

It is also known that a large amount of dust is produced throughout the degassing process.

The material that constitutes the powder is mainly derived from the evaporation of the metallic elements present in the liquid bath, (subsequently condensed along the supply line) and the filter, from the reaction between the steel and the refractory and, in lesser extent, of iron alloys and scorifiers.

During a DV process, approximately 0.1-0.2 kg of powder are produced per ton of treated steel: up to 20-40 kg can be produced during a complete treatment (considering, for example, a spoon with a capacity of 200 tons of liquid steel). A typical analysis of the powder composition reveals a significant content of Zn, MgO, CaO, Pb, Mn.

In the VOD process ("vacuum decarburization with oxygen", a vacuum process with oxygen insufflation to achieve low levels of carbon in the liquid steel), the amount of dust generated can reach 800-1000 kg (for 200 tons of liquid steel).

It is essential to have an effective dust collection system to preserve the vacuum generator against wear or obstruction phenomena, as well as to prevent dust emissions into the atmosphere.

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

Typically, the dust from these processes, due to its composition, burns very easily in the presence of oxygen. For this reason, bag filters (which are currently the most common technology for such applications) require frequent and efficient cleaning, which is typically done automatically after each treatment, blowing inert gas (nitrogen) backflow to the canvas bag , a technology known as "reverse pulse jet".

Apart from the environmental requirements related to atmospheric emissions, the need to install or not install elements for dust reduction (bag and cyclone filters) is determined by the degree of dust tolerated by the vacuum system to be installed.

To date, there are two vacuum generation technologies based on operating principles completely different: mechanical pumps and steam ejector systems.

Vacuum generation with mechanical pumps

In the terminology commonly used in the steel industry, vacuum generation with mechanical pumps refers to a vacuum generator that provides serial installation of lobe type blowers (root pumps) and screw pumps (screw pump) , as illustrated in Figure 2. In this case, screw pumps are also called "prevalence pumps".

As a general principle, since each of these machines performs a compression of the aspirated gas, there is talk of a "phase" of compression that refers to one or more machines operating in the same pressure range between the supply and the discharge.

The most widespread plant solutions currently consist of a series of screw pumps and at least two root pumps in series, as shown in Figure 2.

The phases are conventionally called in ascending numerical order (phase 1, ..., phase n) starting from those closest to the vacuum chamber A. The last phase is the one that finally discharges the gases into the atmosphere (phase of prevalent). Each phase can consist of several pumps connected in parallel, as shown in Figure 3.

The criterion that determines the serial arrangement is as follows: screw pumps can operate with very high compression ratios (up to 1: 1000) but with low volumetric flow rates; instead, root pumps can eliminate large volumes of gas, but do not allow high compression ratios (typically about 1: 6).

In typical VD / VOD installations, functionally, the screw pump alone can maintain a pressure of not less than 20-50 mbar inside the vacuum chamber, discharging gases into the atmosphere. To achieve a higher degree of depression (<1 mbar), installation upstream of at least two phases of root pumps is required. The latter, thanks to the type of construction (a double inner chamber released and obstructed alternately by the rotating lobes) are more effective for moving rarefied gases, such as gases at low pressures. In summary, under stable operating conditions (that is, without taking into account the initial evacuation transient of the vacuum chamber starting from atmospheric pressure), early phase root pumps suck process gases at very pressures low (<1 mbar) and send them to the screw pumps in the pressure range in which the latter operate with a higher compression efficiency.

The main disadvantage in the use of mechanical pumps in a configuration like the one described above is related to the need to filter the aspirated gases to retain the solid particles that could block and / or damage the rotating mechanical bodies (arrest) and possibly Contaminate the lubricating oil (contained in the gear chamber in case of deterioration of the seals). Root pumps, although not intended for use in a dusty environment, could theoretically treat dusty gases without encountering stop operation problems. In the long run, however, oil contamination problems would arise. The biggest problem concerns the screw pumps that would be forced, without filtration, to treat the dust gases discharged by the root pumps, incurring the stop problems mentioned above and leading to an immediate system blockage.

When describing the typical operating conditions of a vacuum generation system with mechanical pumps, in general reference is made to 4 automatic cycles that determine the operation of the main installed devices (valves, filter, pumps):

- System activation cycle: the pumps are started and the volume until the main valve ("volume retained") is evacuated reaching a final pressure typically <5 mbar; The vacuum chamber in this phase remains at atmospheric pressure and the pumps are maintained at a minimum rotation speed.

- Degassing cycle: the main valve opens to equalize the pressure in a controlled manner between the vacuum chamber and the “retained” volume; The slow equalization is designed not to overload the system from a mechanical point of view (pumps and filter bags) and avoid the instantaneous and violent oxidation of pyrophoric dust that remains on the surface of the filter bags after previous treatments. The pumps gradually accelerate to the maximum speed of rotation. During the pressure drop, the vacuum level in the system can be controlled by slowing / circumvalating the pumps or blowing nitrogen. Typically, the process pressure (<1 mbar) is reached in 6-8 minutes.

- Vacuum stop cycle: when degassing is completed, the main valve closes and the vacuum chamber is returned to atmospheric pressure (which allows the lid to be opened later and materials added).

- Cleaning cycle: With the pumps isolated, the filter bags are cleaned by means of a nitrogen burst system, and then the cleaning cycle is followed by a dust discharge cycle as necessary. Once the bag cleaning cycle is completed, the retained volume is evacuated again (up to pressures of <5 mbar) preparing the system for the next degassing cycle.

The cleaning of the filter bags is a fundamental aspect for the performance of the mechanical pump system because:

- an excessive accumulation of dust in the bags increases the pressure losses through the filter, limiting the minimum pressure that can be reached inside the vacuum chamber;

- the possible damage of the bags causes large amounts of dust to reach the pumps. In this way, the operation of the system can be compromised if the cleaning and maintenance of the filters are not carried out properly (correct adjustment of the nitrogen wash cycle, periodic inspections of the bags ...). Vacuum generation with ejector pumps

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

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

Also in this case, in the serial arrangement, each ejector is considered as a "phase" of compression. However, one phase may be composed of several ejectors in parallel to increase the suction capacity of the system at higher pressures (typically required during the evacuation phase of the vacuum chamber).

Figure 4 shows the layout of a plant of a typical ejector pumping station where S1, S2, S3 and S4 indicate the ejector phases, C1, C2 and C3, the condensers between phases and P, the collection tank or "hot well ". The phases S3 and S4, in this particular case, consist of pairs of A / B ejectors that operate in parallel. The activation sequence of the individual phases is normally controlled by the pressure reached by the vacuum chamber, and is as follows (with reference to Figure 4): S4-S3-S2-S1.

To ensure maximum efficiency of the ejector system (elimination of the maximum process gas flow), the heat exchangers are installed in series with the ejectors to condense the vapor contained in the main gas flow.

In fact, steam acts only as a propellant to aspirate process gases and condense as the pressure increases and the temperature decreases.

Therefore, the steam is made to condense inside the "condensers" that drain into a tank, called "hot pit".

It is clear that, in the absence of filtration systems upstream of the ejector groups, the condensed water has a higher dust concentration, so it requires wastewater treatment plants and proper maintenance operations for the disposal of channeled sludge to the "hot pit."

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

A variation of this diagram provides that the fourth phase, or alternatively a possible fifth phase, consists of a liquid ring pump instead of an ejector. This solution is generally preferred in systems with limited steam availability or where required by the requirements of the plant or process (limited space for installation, the need to operate stably at pressures greater than 100 mbar for VOD systems) .

The liquid ring pump is a mechanical centrifugal pump in which the compression of the gas, by means of its confinement in a variable volume (gradually reduced), is derived from the rotation of a liquid ring generated by a centrifugal effect of a rotor, eccentric to the casing (body) of said pump.

With the exception of the composition of the pumping system and the dust reduction group connected to it, the operation of an ejector / liquid ring plant passes through operating sequences completely similar to those described for mechanical pumps.

For ejection systems it is not necessary, with the purpose of protecting the pumping system, to reduce dust to the point of requiring the installation of a bag filter, since it lacks the geometric tolerances required by the mechanical system, typical of Root or screw pumps.

On the other hand, in some systems, a cyclone or even a bag filter with a related automatic cleaning system can be installed to minimize maintenance (ejector cleaning and hot well water treatment).

Finally, it should be noted that in the absence of filter elements, a large amount of dust is retained by the injected steam and by the water of the possible liquid ring pumps. Condensed steam between an ejector phase and another helps retain some of the dust generated during the process. The condensed water is drained, as mentioned above, to the "hot well" (indicated as P in Figure 4). In addition, the possible liquid ring in its contact with the process gas helps retain some of the residual dust. It follows that, in a liquid ring / ejector system, the amount of dust contained in the gases discharged in the stack is very low.

In conclusion, the main difference with respect to the system layout between ejector systems and mechanical pumping systems lies in the presence of a bag filter (with all auxiliary elements to clean the bags and discharge the dust), required in the latter case to preserve the integrity of the machines. The main limitation of the vacuum generation systems of injectors lies in their complexity and in the high operating and plant costs.

Presentation of the invention

Accordingly, the purpose of the present invention is to totally or partially eliminate the drawbacks of the prior art mentioned above, by providing a plant and a method for vacuum degassing of liquid steel by combining the simplicity of engineering / operation of a pump plant. mechanical with the possibility of operating without filter systems of an ejector plant.

Another purpose of the present invention is to make available a plant for vacuum degassing of liquid steel that is more functionally reliable.

Another purpose of the present invention is to make available a plant for vacuum degassing of liquid steel that is cheaper in its operation.

Another purpose of the present invention is to make available a plant for vacuum degassing of liquid steel that is at least comparable to conventional systems with mechanical pumps, in terms of plant costs.

Brief description of the drawings

The technical characteristics of the invention, according to the aforementioned purposes, can be clearly seen from the content of the following claims and the advantages thereof will be more clearly understood from the description detailed below, with reference to the drawings. attached, showing one or more embodiments by way of non-limiting examples, in which:

- Figure 1 shows a general diagram of a steel degassing plant;

- Figure 2 shows a general diagram of a conventional vacuum generation system with mechanical pumps of the root and screw type;

- Figure 3 shows a general diagram of a conventional vacuum generation system with mechanical pumps of the root and screw type, with each phase composed of several pumps in parallel;

- Figure 4 is a diagram of a conventional ejector vacuum generation system;

- Figure 5 shows a general diagram of a liquid steel vacuum degassing plant according to a preferred embodiment of the present invention;

- Figure 6 shows a general diagram of a liquid steel vacuum degassing plant according to an alternative embodiment of the present invention;

- Figure 7 shows a general diagram of the vacuum generation system in a liquid steel vacuum degassing plant according to a preferred embodiment of the present invention; Y

- Figure 8 shows a general diagram of a liquid ring pump.

Detailed description

With reference to the accompanying drawings, reference number 1 globally indicates a vacuum degassing plant of liquid steel according to the invention.

The plant 1 according to the invention can be used for vacuum degassing either with the VD technique (Vacuum degassing) or with the v Od technique (Decarburization with vacuum oxygen) and for all applications where treatment is required Vacuum liquid steel.

Hereinafter, in the description and claims, reference will be made to the vacuum degassing plant of liquid steel 1 under conditions of use.

According to a general embodiment of the invention, the vacuum degassing plant for liquid steel comprises:

- at least one vacuum chamber 2, suitable for temporarily receiving liquid steel inside; Y

- a vacuum generation system 10, connected to the at least one vacuum chamber 2 mentioned above by means of a supply conduit 20.

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

Preferably, the vacuum chamber 2 is configured so that the liquid steel is introduced inside by means of a spoon L, but can also be used directly to receive the liquid steel.

In the first case, as shown in Figures 5 and 6, the vacuum chamber 2 comprises a reservoir 3, which defines the volume of the chamber 2 and is suitable for receiving in it the spoon L, and a lid 4 suitable for tightly seal the tank 3 when the spoon L is housed inside. The vacuum chamber can be of the "wheel cover" type when the tank is fixed and the mobile cover or of the "wheel tank" in the opposite case. Advantageously, as shown in Figures 5 and 6, the vacuum chamber can be equipped with an insufflation system 30 of a washing gas, in some cases even nitrogen, for the agitation of liquid steel and the removal of impurities. inside. In particular, this insufflation system 30 is designed to feed one or more porous partitions located at the bottom of the spoon.

In the second case, in accordance with an embodiment not illustrated in the attached figures, the vacuum chamber 2 can be configured to directly house the liquid steel therein according to an RH process. In this case, the liquid steel is temporarily transferred from the bucket into the chamber. For this purpose, the vacuum chamber is connected to a spoon by means of two conduits: a delivery conduit through which the molten steel of the spoon is driven by the pressure difference within the vacuum chamber, and a conduit return, through which the treated molten steel flows back from the vacuum chamber into the bucket. According to the invention, as shown in Figures 5 and 6, the vacuum generation system 10 comprises at least two compression phases connected to each other in series, of which:

- a first compression phase 11 works closer to the at least one vacuum chamber 2 mentioned above and consists of one or more screw pumps 110; Y

- a second compression phase 12 works further away from the vacuum chamber 2 mentioned above to bring the gas at least to atmospheric pressure and consists of one or more liquid ring pumps 120.

The one or more screw pumps 110 mentioned above are sized to be able to operate with compression ratios not exceeding 1:12, if the discharge pressure is atmospheric, and with compression ratios not exceeding 1: 200, if the pressure of download is between 50 and 120 mbar absolute. As will be specified below, the one or more screw pumps 110 mentioned above are therefore radically different in size from conventional screw pumps.

Preferably the one or more screw pumps 110 mentioned above are sized to operate with compression ratios between 1: 3 and 1:10, if the discharge pressure is atmospheric and, if the discharge pressure is between 50 and 120 mbar absolute, with compression ratios between 1:25 and 1: 200, and preferably between 1:70 and 1:90.

Thanks to the fact that they operate at the intervals of compression ratios mentioned above, the one or more screw pumps 110 mentioned above are sized by imposing internal tolerances (rotor / rotor and rotor / case), much higher than those provided in the pump pumps. Conventional screw used as prevalence phases as described above. In this way, the screw pumps 110 mentioned above can operate in direct contact with powder gases with high concentrations of dust without contraindications to the moving mechanical parts and, therefore, without incurring the typical problems of the screw pumps used as Prevalence phases in conventional mechanical degassing systems. This is made possible by the fact that, according to the invention, screw pumps are used in phases closer to the vacuum chamber and by the option of operating said pumps at the compression intervals mentioned above.

Functionally, the work of compressing the gas is completed by the one or more liquid ring pumps mentioned above that define the second (final) compression phase, further away from the vacuum chamber, taking advantage of the fact that the liquid ring pumps They are insensitive to dust.

Advantageously, the liquid ring pumps also perform an important function of retaining the solid particles entrained by the main gas flow. Therefore, the service water of the pump is used to trap the dust generated by the degassing process and then collect it at a single point. In this way, the emission of dust in a possible discharge stack 40 is minimized, which guarantees a low environmental impact.

Thanks to the invention, the vacuum generation system 10 can therefore directly aspirate from the at least one vacuum chamber 2 mentioned above the gases containing dust in high concentrations, without the typical contraindications of a conventional mechanical pump system.

Conventionally, against the provisions of the present invention, screw pumps are instead used in the vacuum generation systems of the degassing plants to define the compression phases furthest from the vacuum chamber A. These pumps (of prevailing), despite operating with compression ratios between 1: 1 and 1:50, and preferably between 1: 2 and 1:40, they are designed to work with compression ratios of up to 1: 1000 with atmospheric pressure discharge . Conversely, screw pumps 110 according to the invention are sized to operate at maximum compression ratios of 1:12 with atmospheric pressure discharge. Therefore, traditional screw pumps must be built with very strict internal tolerances (rotor / rotor and rotor / case). This makes them particularly sensitive to the presence of dust in the treated gases.

Thanks to the present invention, it is therefore possible, on the one hand, to release the design of a liquid steel degassing plant from the installation of a filtration device (usually a bag filter) required in the case of mechanical pumps, and on the other, drastically reduce the costs of the plant that involves a conventional steam ejector system.

Advantageously, the vacuum generation system 10 is sized to bring the vacuum chamber 2 to a vacuum degree between 0.2 and 5 mbar, and preferably between 0.5 and 1.5 mbar. As a result, the vacuum generation system 10 is sized to generate total compression ratios between 1: 5,000 and 1: 200.

With respect to the dimensioning of the vacuum generation system 10 according to the present invention, the possible combinations in terms of number of screw pumps 110 and liquid ring pumps 120 are determined by the design choices made at any time to minimize the number of machines installed to obtain the level of performance required by the process, that is, the evacuation times of the vacuum chamber are limited and the degree of final vacuum is approximately <1 mbar.

Advantageously, the vacuum generation system 10 may comprise one or more intermediate compression phases, located in series between the first phase 11 and the second phase 12 and each composed of one or more screw pumps 110 having similar characteristics to those of the first phase 11.

It is understood that the term "similar characteristics" means that said one or more intermediate phase screw pumps are sized to operate at the same compression intervals as the first phase screw pumps, which makes it possible to adopt internal tolerances ( rotor / rotor and rotor / case) much higher than those provided in conventional screw pumps. The size of the screw pumps of the intermediate phases may be the same as or different from that of the screw pumps of the first phases. The choice of size is determined by the sizing of the vacuum generation system.

One or more of the compression phases mentioned above (first, second or intermediate) may each consist of two or more pumps connected in parallel.

According to embodiments not shown in the attached figures, the vacuum generation system may consist of two or more parallel pumping modules, each of which is composed of at least a first compression phase 11 with screw pumps and a second compression phase 12 with liquid ring pumps.

The total number of pumps installed per module and the number of modules are defined in the design phase in order to optimize the installation and minimize the consumption of auxiliary elements (water, nitrogen, electricity). Advantageously, modular configurations can be adopted for the vacuum generation system 10, that is, separable in units installed in parallel, or "hybrid" installations where the pumps are grouped into two phases without modularity.

Preferably, the vacuum generation system 10 can be isolated from the rest of the system by closing the appropriate isolation valves installed immediately upstream of the pumps.

Preferably, as shown in Figures 5 and 6, the supply conduit 20 comprises a bypass conduit 21 which can exclude from the gas flow the compressor phases formed by the screw pumps 110. This solution can be adopted both in the case of a modular structure as of a non-modular structure. Functionally, as will be summarized below, the presence of the bypass 21 mentioned above can be used to exclude the operation of the screw pumps in some phases of the degassing process.

Preferably, each of the screw pumps 110 used in the degassing plant 1 according to the invention comprises two screw rotors, kinematically synchronized with each other by means of an electric shaft.

For the connection and synchronization of the two screw rotors, these pumps do not use the conventional "mechanical shaft", where one motor transmits the movement to a screw rotor while the other rotor is dragged / synchronized by means of a series of gears in oil bath

The term "electric axis" means the synchronization of the computer program of a pair of motors by means of an inverter (one for each screw) and a pair of encoders. The computer program instantly manages the parameters of the two inverters, so that the rotors are constantly synchronized. In addition, the two encoders control the angular deviation of the shafts of the screw rotors, so that they are perfectly parallel to each other.

Functionally, any functional anomaly (for example, internal friction due to dust accumulation) results in an increase in torque and current absorption of the motors (or one of them) and, consequently, a possible deviation in the angular speed of the rotors. Advantageously, the computer program can act on the speed in real time until the balance is restored, avoiding tensions and overheating of the pump.

Compared to a solution with a mechanical shaft, this electric shaft configuration does not require oil for gear lubrication. The absence of lubricating oil is an advantage. In fact, due to the possible difference in pressure between the compression chamber (lower pressure) and the possible (concurrent) gear chambers (higher pressure), the oil can be aspirated to the process gas, mixing with the powder and generating obstructions. Similarly, in certain phases of operation, dusty gases can flood the gear chambers by contaminating the oil.

The liquid ring pumps 120 used in the degassing plant 1 according to the present invention are of the type known per se and, therefore, their operation is well known to a technician in the sector. Therefore, a detailed description of them is not provided, but simply a reference to a series of useful concepts for introducing some particular elements.

In particular, the liquid ring pumps used in the present invention may have the structure shown in Figure 8.

As shown in Figure 8, a liquid ring pump compresses the process gas G 'between an eccentric vane rotor 121 and a water ring 122, called service water W. Functionally, the dust transported by the process gas G 'necessarily comes into contact with the service water W that acts as a collector. The pump 120 ejects the compressed gas G "together with a minimum amount of pulverulent service water. The mixture G "W of gas and powder water reaches a separator 123 that separates the gas (now at atmospheric pressure and goes to the stack) of the" dirty "water that is collected at the bottom of the separator 123. Advantageously, a Water replenishment 124 W is provided to compensate for evaporation losses.

More specifically, the service water W can be handled in two ways: in an open circuit or in a closed circuit.

With closed circuit management, the water W is recirculated to the dust saturation limit, at which the pump performance falls. At this point, all service water W is discharged and replaced with clean water. With open circuit management, water is continuously discharged from the separator (through the opening 125 illustrated in Figure 8), while a clean water line 124 continuously fills the service circuit of the liquid ring pump.

Advantageously, the resulting water contains dust that is now inert and can be handled in two different ways.

According to a first method, the pulverulent water is collected in a settling bath with an overflow leading to a second bath. From here, the pulverulent water is sent to a water treatment plant, by means of centrifugal pumps, and is treated therein in the conventional manner.

According to a second method, as schematically shown in Figure 7, the pulverulent water leaving the separator can be filtered in situ using known methods.

Advantageously, as shown in Figure 7, the plant 1 comprises an auxiliary unit 50 which, in addition to replenishing the water dispersed by the liquid ring pumps in the process gases, separates the dust contained in the water and recirculates it to the bomb

A continuous cycle operation guarantees both the controlled elimination of dust (avoiding internal accumulation) and the optimal operation of the liquid ring pump 120 thanks to the cooling and cleaning of the filling water.

The auxiliary unit 50 may be centralized or located on board each liquid ring pump or module, while still maintaining the same functions.

Alternatively to the auxiliary unit 50 mentioned above, plant 1 may comprise at least one continuous service water replacement device used by the liquid ring pump, without recirculation, with non-returnable water.

Advantageously, as shown in Figures 5 and 6, in the section between the vacuum chamber 2 and the vacuum generating system 10, the supply duct 20 comprises a branch of connection 28 to the atmosphere equipped with a first control valve 23. This first control valve 23 opens at the end of the degassing process to return the vacuum chamber 2 at atmospheric pressure before extracting the treated liquid steel. Advantageously, as shown in Figures 5 and 6, in the section between the vacuum chamber 2 and the vacuum generating system 10, the supply conduit 20 can comprise a connecting branch 29 to a reservoir (not shown) containing inert gas (nitrogen or argon), equipped with a second control valve 24. The inert gas can be insufflated by opening the second valve 24 to manage the foamy slag, raising the internal pressure.

According to a preferred embodiment illustrated in Figure 5, the degassing plant 1 does not comprise a gas filtration device, which leaves the vacuum chamber 2 and has to pass through the vacuum generating system 10. Regardless of the level of concentration of the dust in said gases, the gases leaving the vacuum chamber 2 are sucked directly by the vacuum generation system without a preventive gas filtration stage. As previously noted, this is possible thanks to the present invention.

According to an alternative embodiment illustrated in FIG. 6, the degassing plant 1 may comprise at least one filtration device 25 of the gases leaving the vacuum chamber 2 and passing through the vacuum generating system 10 Said filtration device 25 is disposed between the vacuum chamber 2 and the vacuum generating system 10.

Functionally, the gases leaving the vacuum chamber 2, before being aspirated by the vacuum generating system, are subjected to filtration to at least partially reduce the dust content present in the gases. Thanks to the present invention, the dust reduction can be partial and smooth, since the possible presence of dust does not affect the operation of the vacuum generation system 10. The preventive filtration stage can be provided to optimize the dust management in the system, reducing the dust load to be managed by means of liquid ring pumps.

The filtration device 25 mentioned above may consist of a bag filter, a cyclone or an integrated bag and cyclone filter system.

In particular, according to the alternative embodiment illustrated in FIG. 6, the plant 1 comprises at least one isolation valve 22 that is installed in the supply duct 20 between the vacuum chamber 2 and the filtration device 25. Said isolation valve 22 is placed downstream of the bifurcation point of the supply conduit 20 in the connection branch 28 mentioned above to the atmosphere. The isolation valve 22 divides the plant 1 into two parts, thus identifying two volumes. A first part comprises the vacuum chamber (tank volume); The second part comprises the filtration device and the vacuum generation system (retained volume).

Functionally, the volume of the tank is returned to atmospheric pressure after each vacuum treatment by opening the first control valve 23 mentioned above that puts the vacuum chamber in communication with the external environment. The volume retained can, however, always be kept empty thanks to the isolation valve 22 which effectively keeps it tight. Maintaining the vacuum of the retained volume allows the time required to reduce the pressure in the system to be shortened by using it as an "impending chamber" that equalizes the pressure between the reservoir and the retained volume at the time of opening the isolation valve 22.

The presence of the isolation valve 22 is preferred in the case where the plant 1 is equipped with a filtration device 25 (in particular if it is a bag filter) as shown in Figure 6. In this case, the volume Retained is very high due to the presence of the filtration device.

Advantageously, in the case where the plant 1 is not equipped with a filtration device 25 (see Figure 5), the isolation valve 22 does not need to be installed, since, in the absence of the filtration device, the volume retained is reduced and, therefore, the advantages associated with maintaining said empty volume are limited.

Advantageously, in the case where the plant 1 is used for vacuum degassing with the VOD technique (vacuum oxygen decarburization), it can comprise a heat exchanger (not shown in the attached figures) to cool the process gases In fact, with the VOD technique, as a result of oxygen injection and the subsequent decarburization of steel, the temperatures involved increase significantly. The heat exchanger must be placed upstream of the possible filtration device 22 and downstream of the possible isolation valve (if present), in the second part of the system (retained volume).

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

In particular, the method according to the invention can be implemented in a degassing system according to the invention, in particular as described above. The parts in common with the plant 1 described above have been indicated using the same alphanumeric references.

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

a) provide at least one vacuum chamber 2 suitable for temporarily receiving liquid steel inside; b) placing liquid steel in said vacuum chamber 2;

c) evacuating the vacuum chamber 2 through a vacuum generation system 10 creating in said chamber a predefined degree of vacuum and maintaining it for a predetermined period of time in order to complete the degassing operation of the liquid steel; Y

d) bring the vacuum chamber 2 back to atmospheric pressure and remove the degassed liquid steel.

According to the invention, the vacuum evacuation step c) is carried out by means of a vacuum generation system 10 comprising at least two compression phases connected to each other in series, of which:

- a first compression phase 11 works closer to the at least one vacuum chamber 2 mentioned above and consists of one or more screw pumps 110; Y

- a second compression phase 12 works further away from the vacuum chamber 2 mentioned above to bring the gas at least to atmospheric pressure and consists of one or more liquid ring pumps 120.

The one or more screw pumps 110 mentioned above are sized to be able to operate with compression ratios not exceeding 1:12, if the discharge pressure is atmospheric, and with compression ratios not exceeding 1: 200, if the pressure of download is between 50 and 120 mbar absolute.

Preferably the one or more screw pumps 110 mentioned above are sized to operate with compression ratios between 1: 3 and 1:10, if the discharge pressure is atmospheric and, if the discharge pressure is between 50 and 120 mbar absolute, with compression ratios between 1:25 and 1: 200, and preferably between 1:70 and 1:90.

Preferably, in the evacuation stage c) mentioned above, the vacuum chamber 2 is operated at a vacuum degree of between 0.2 and 5 mbar, and preferably between 0.5 and 1.5 mbar.

According to a preferred embodiment of the method, the evacuation stage c) provides the direct aspiration of the gases from the vacuum chamber 2 through the vacuum generation system 10 mentioned above without a stage of preventive gas filtration , regardless of the level of dust concentration in the gases themselves.

According to an alternative embodiment of the method, the evacuation stage c) provides the aspiration of the gases from the vacuum chamber 2 through the vacuum generation system 10 mentioned above with a stage of preventive gas filtration, to reduce the concentration of dust in said gases before passing through the vacuum generation system 10.

Preferably, the evacuation stage c) comprises:

- an initial evacuation stage c1) in which the vacuum chamber 2 is brought from atmospheric pressure to approximately 300 mbar using only the liquid ring pumps of the vacuum generating system 10; and - a final evacuation stage c2) in which the vacuum chamber 2 is carried from the pressure of approximately 300 mbar to the predetermined degree of vacuum also using the screw pumps.

This mode of operation makes it possible to minimize the amount of dust that the screw pumps must handle, for the benefit of the operation of said pumps. This mode of operation takes advantage of the presence of the branch 21 which is present in the supply duct and which allows the exclusion of the screw pumps from the gas passageway.

Advantageously, during the evacuation stage c) the suction capacity of the vacuum generating system 10 can be varied to reduce any scum formation phenomenon in the liquid steel. The suction capacity is varied by slowing down or excluding one or more of the pumps of the vacuum generating system 10, preferably the liquid ring pumps 120.

Preferably, the above change in suction capacity is carried out when the internal pressure of the vacuum chamber 2 is between 300 mbar and 1 mbar, that is, during the final evacuation stage c2).

Preferably, the method according to the invention comprises a step f) of treating the service water used by the one or more liquid ring pumps 120 mentioned above. This treatment stage f) is preferably carried out during the evacuation stage c). The treatment consists in filtering water dust or continuously replacing water.

Advantageously, the method comprises a step e) of mixing the molten steel at least during the evacuation stage c), in particular by blowing inert gases into said steel.

The invention allows to achieve numerous advantages, partly already described.

The plant 1 for vacuum degassing of liquid steel according to the invention combines the simplicity of the plant of a mechanical pump plant with the possibility of operating without the filter systems of an ejector plant.

Thanks to the invention, therefore, it is possible to minimize the equipment installed in a degassing plant.

In this way, it guarantees greater flexibility in the design phase (layout and auxiliary), as well as allows the operation of the system while minimizing maintenance costs and possible repairs of the parts most subject to wear in a conventional system . In particular, it reduces periodic inspections and eliminates the need to replace filter bags.

The plant 1 for vacuum degassing of liquid steel according to the invention is therefore:

- functionally more reliable; Y

- cheaper in your operation.

In terms of plant costs, plant 1 for vacuum degassing of liquid steel is at least comparable to conventional systems with mechanical pumps and certainly less expensive than conventional systems with ejectors.

The advantages set forth above for plant 1 according to the invention extend to the degassing method according to the invention.

The invention thus conceived thus achieves the intended objectives.

Obviously, their practical embodiments can assume different shapes and configurations than those described as long as they remain within the sphere of protection of the invention.

In addition, all the details can be replaced by technically equivalent elements and the dimensions, shapes and materials used can be whatever they are necessary.

Claims (20)

1. Plant for vacuum degassing of liquid steel, comprising: - at least one vacuum chamber (2), suitable for temporarily receiving liquid steel inside, - a vacuum generating system (10), connected to said at least one vacuum chamber (2) by means of a supply conduit (20), the vacuum generating system (10) comprises at least two compression phases connected to each other in series, of which a first compression phase (11) works closer to said at least one vacuum chamber (2) and is formed by one or more screw pumps (110); characterized in that a second compression phase (12) works further away from said at least one vacuum chamber (2) to bring the gases to at least atmospheric pressure and is formed by one or more liquid ring pumps (120) and because said one or more screw pumps (110) are sized to be able to operate with compression ratios not exceeding 1:12, if the discharge pressure is atmospheric and with compression ratios not exceeding 1: 200, if the pressure Discharge is between 50 and 120 mbar absolute. 2. Plant according to claim 1, wherein said one or more screw pumps (110) are sized to be able to work with compression ratios between 1: 3 and 1:10, if the discharge pressure is atmospheric and , if the discharge pressure is between 50 and 120 mbar absolute, with compression ratios between 1:25 and 1: 200, and preferably between 1:70 and 1:90. 3. Plant according to claim 1 or 2, wherein the vacuum generation system (10) is sized to bring the vacuum chamber (2) to a vacuum degree between 0.2 and 5 mbar, and preferably between 0.5 and 1.5 mbar. 4. Plant according to claim 1, 2 or 3, wherein the vacuum generation system (10) comprises at least one intermediate compression phase, which is located between the first phase (11) and the second phase ( 12) and is connected to them in series, said intermediate compression phase being formed by one or more screw pumps (110) having characteristics similar to those of the first phase (11). 5. Plant according to one or more of the preceding claims, wherein one or more of said compression phases are each formed by two or more pumps connected in parallel. 6. Plant according to one or more of the preceding claims, wherein the supply conduit (20) comprises a bypass conduit (21) that can exclude from the gas flow the compressor phases formed by screw pumps (110 ). 7. Plant according to one or more of the preceding claims, wherein each screw pump (110) comprises two screw rotors, kinematically synchronized with each other by means of an electric shaft. 8. Plant according to one or more of the preceding claims, comprising at least one water filtration device used by said one or more liquid ring pumps (120) suitable for removing the dust accumulated in the water itself during operation of the pump or a water replacement device itself. 9. Plant according to one or more of the preceding claims, wherein, in the section between the vacuum chamber (2) and the vacuum generating system (10), the supply duct (20) comprises a connecting branch (28) to the atmosphere equipped with a control valve (23). 10. Plant according to one or more of the preceding claims, comprising at least one gas filtration device (25) located between the vacuum chamber (2) and the vacuum generation system (10). 11. Plant according to claims 9 and 10, comprising at least one shut-off valve (22) that is installed in said supply duct (20) between the vacuum chamber (2) and the filtration device (25) , downstream of the branch point of the connecting branch (28) to the atmosphere. 12. Method for vacuum degassing of liquid steel, comprising the following operating steps: a) provide at least one vacuum chamber (2) suitable for temporarily receiving liquid steel inside; b) placing liquid steel in said vacuum chamber (2); c) evacuate the vacuum chamber (2) through a vacuum generation system (10) creating in said chamber a predefined degree of vacuum and maintaining it for a predetermined period of time in order to complete the degassing operation of liquid steel; d) bring the vacuum chamber (2) back to atmospheric pressure and remove the degassed liquid steel; characterized in that the vacuum evacuation stage c) is carried out by means of a vacuum generation system (10) comprising at least two compression phases connected to each other in series, of which a first compression phase (11) works closer of said at least one vacuum chamber (2) and is formed by one or more screw pumps (110), and a second compression phase (12) works further away from said at least one vacuum chamber (2) to bring the gases to at least atmospheric pressure and is formed by one or more liquid ring pumps (120) and because said one or more screw pumps (110) are sized to be able to operate with compression ratios not exceeding 1: 12, if the discharge pressure is atmospheric, and with compression ratios not exceeding 1: 200, if the discharge pressure is between 50 and 120 mbar absolute. 13. A method according to claim 12, wherein said one or more screw pumps (110) are sized to operate with compression ratios between 1: 3 and 1:10, if the discharge pressure is atmospheric and , if the discharge pressure is between 50 and 120 mbar absolute, with compression ratios between 1:25 and 1: 200, and preferably between 1:70 and 1:90. 14. A method according to claim 12 or 13, wherein, in the evacuation stage c), the vacuum chamber (2) is operated at a vacuum degree of between 0.2 and 5 mbar, and preferably between 0.5 and 1.5 mbar. 15. A method according to one or more of claims 12 to 14, wherein said evacuation stage c) provides direct aspiration of the gases from said vacuum chamber (2) through said vacuum generating system without a stage of preventive filtration of gases, regardless of the level of dust concentration in the gases themselves. 16. A method according to one or more of claims 12 to 14, wherein said evacuation stage c) provides the aspiration of the gases from said vacuum chamber (2) through said vacuum generating system with a stage of preventive filtration of gases, to reduce the concentration of dust in the gases themselves before they pass through the vacuum generation system (10). 17. Method according to one or more of claims 12 to 16, wherein the evacuation stage c) comprises: - an initial evacuation stage c1) in which the vacuum chamber (2) is brought from atmospheric pressure to approximately 300 mbar using only the liquid ring pumps (120) of the vacuum generation system (10); Y - a final evacuation stage c2) in which the vacuum chamber (2) is carried from the pressure of approximately 300 mbar to the predefined degree of vacuum also using the screw pumps (110). 18. Method according to one or more of claims 12 to 17, wherein during the evacuation stage c), the suction capacity of the vacuum generating system (10) is varied to reduce the foaming phenomena of the slag in the liquid steel, varying the suction capacity by slowing or excluding one or more of the pumps (110, 120) of the vacuum generation system (10), preferably the liquid ring pumps (120), being carried out preferably said variation of the suction capacity when the internal pressure of the vacuum chamber (2) is between 300 mbar and 1 mbar. 19. Method according to one or more of claims 12 to 18, comprising a stage of treatment f) of the water used by said one or more liquid ring pumps (120), said stage being preferably carried out during the stage of evacuation c), said treatment consisting of a filtration of water dust or a replacement of the water itself. 20. Method according to one or more of claims 12 to 19, which comprises a mixing stage e) of the molten steel at least during the evacuation stage c).