WO2013131697A1

WO2013131697A1 - Probe for continuous measurement of the oxygen saturation in metal melts

Info

Publication number: WO2013131697A1
Application number: PCT/EP2013/052201
Authority: WO
Inventors: Andreas Kasper; Amelie NEUMANN
Original assignee: Saint-Gobain Glass France
Priority date: 2012-03-05
Filing date: 2013-02-05
Publication date: 2013-09-12
Also published as: EP2823293A1

Abstract

1. Probe for continuous measurement of the oxygen saturation in metal melts, comprising: a) a tubular anode (3) with at least one opening (5) for receiving liquid metal from a metal bath (6); b) a reference electrode (4) arranged inside the tubular anode (3), comprising a recording electrode (4.1) which is immersed into an oxygen ion-conducting solid-state electrolyte cylinder (4.2), said solid-state electrolyte cylinder (4.2) being filled with a reference material (4.3, 4.4); c) a thermal probe (1) arranged inside the tubular anode (3) and d) wherein the tubular anode (3) and the recording electrode (4.1) contain silicon carbide.

Description

Probe for continuous measurement of oxygen saturation in molten metals

The invention relates to a probe for the continuous measurement of the oxygen concentration in molten metals, their use and a method of measurement.

Glass is predominantly produced in the endless-continuous float glass process. In this case, a molten glass is passed to a bath of liquid metal, in particular tin, and spreads evenly on the surface of the bath due to their lower density. This allows the production of high quality base glass with particularly smooth surfaces. Due to the high oxygen affinity of the tin, the process must be carried out under inert conditions. For this purpose, the system must be kept under a slight overpressure of forming gas. A complete exclusion of oxygen is hardly possible in production. Thus, for example, when changing a radiator or in case of accidents, such as leaks in the outer tub, air penetrate. The atmospheric oxygen disturbs the production process massively. Oxygen dissolved in the liquid tin oxidizes the tin when the temperature-dependent solubility limit is exceeded to tin (IV) oxide (Sn0 ₂ ), which lies above the bath surface, especially in colder areas of the bath. The Dross causes serious quality problems in the form of glass adhesions. In the event of accidents, tin oxide (SnO) forms at the hot end of the bath at about 1000 ° C., which is extremely volatile at the present temperatures and condenses in colder areas of the system, such as the bath cover. There, tin disubstituted tin (II) oxide to liquid tin and tin (IV) oxide. Dripping tin and falling cassiterite particles cause surface defects in the still soft glass surface. The undesirable effects can be reduced by increasing the hydrogen concentration in the forming gas. The tin oxide is reduced to liquid tin. However, the reduction occurs only at the surface of the particles and therefore only slowly, resulting in very long decay times of the disturbance. An early discovery of the disorder is therefore enormously important. The oxygen concentration of the molten metal must be constantly monitored in the various temperature ranges of the system.

Although the oxygen concentration of metal melts can be determined by classical analytical methods, this requires the removal of a sample. These Sample is reacted with carbon in vacuo and the resulting carbon monoxide is determined.

Furthermore, indirect methods for determining the oxygen concentration are known. These include the extraction of the bath atmosphere and the measurement of the tin count. However, the extraction of the bath atmosphere does not provide the required accuracy. The measurement of the degree of compensation takes place only on the finished product, whereby the measurement results are available only with a time delay.

In modern measuring methods for the in-situ determination of the oxygen content in liquid tin, probes are used which allow a direct monitoring of the local oxygen concentration.

US 3,625,026 A discloses a probe for determining the oxygen concentration in molten metals. The probe comprises a cathode of a doped zirconia cylinder as the oxygen ion conducting solid electrolyte, and an anode of platinum outside the cathode with a rhenium tip protruding into the tin. The different oxygen concentrations inside and outside the solid electrolyte create an electrochemical potential. The oxygen concentration in the liquid tin is to be determined by measuring the voltage generated.

DE 102004022763 B3 discloses a probe whose solid electrolyte cylinder is coated with a mixture of zirconium silicate and a fluoride. As a result, the measurement of other impurities in molten metal is possible.

EP 0562801 B1 discloses a further development of the probe type described in US 3,625,026. Only the probe tip consists of zirconium dioxide, while the other probe body is made of a different heat-resistant material. Furthermore, a thermocouple is inserted in the probe tip.

However, the high fragility and temperature sensitivity of the zirconia probe body is a disadvantage of the described probe. Zirconia has over a high thermal expansion coefficient. For this reason, a so-called thermal shock often occurs when inserting the probe and the probe is completely destroyed. When removing the probe from the molten metal can also occur a thermal shock. Therefore, the probe can not be removed without destruction after installation and used elsewhere.

The lifetime of the probe is very limited, since in the float bath environment rapid aging of the probe material takes place. The crystal structure of the zirconia is stable at the temperatures prevailing in the molten metal. However, along the longitudinal direction of the probe, there is a temperature gradient. The upper part of the probe protrudes from the liquid metal and has a lower temperature. At a temperature of about 400 ° C, the crystal structure of the zirconia converts to a second crystalline form. Along the longitudinal direction of the probe thus occur on two different crystal structures. The change in the crystal structure also causes a change in the volume, whereby high material loads occur at the junction of the two crystal structures. At this point, the probe material breaks frequently. The probe described in EP 0562801 Bl of doped zirconia has an improved temperature resistance, since the zirconia tip completely immersed in the liquid tin. Thus, the described crystal structure change along the longitudinal axis of the probe is prevented.

The hitherto known probes have an anode with rhenium, tungsten, molybdenum and alloys thereof. Rhenium is preferably used as the anode material. The components which come into contact with liquid tin must be insoluble in this. This applies only to the refractory metals of the sixth and seventh subgroups of the periodic table. These are produced in the sintering process due to their high melting points. The sintering aids used are preferably cobalt, nickel and alloys thereof which are readily soluble in the liquid tin. Consequently, the sintering aids are dissolved out of the material over time, which leads to the disintegration of the components.

The much needed in-situ monitoring of the oxygen concentration of tin baths in float glass plants is difficult to accomplish in the prior art. Of Furthermore, due to the short life of the probes, no long-term measurement and no multiple use of the probes is possible.

The object of the invention is to provide a stable and reversible measuring unit, which allows a continuous monitoring of the oxygen concentration in molten metals.

The object of the present invention is achieved according fiction, by a probe for the continuous measurement of the oxygen concentration in metal baths according to the independent claim 1. Preferred embodiments will become apparent from the dependent claims.

An inventive method for measuring the oxygen concentration in metal baths is apparent from the independent claim 13.

An inventive use of the probe described in claim 1 for measuring the oxygen concentration in molten metal is apparent from the independent claim 14.

The probe according to the invention for the continuous measurement of oxygen saturation in molten metals comprises a tubular anode in the interior of which a reference electrode is contained as a cathode. The outer tubular anode thus protects the internal thermocouple and the cathode against mechanical stress. When inserting the probe, tilting can be particularly easy. The reference electrode comprises a deflection electrode immersed in an oxygen ion conducting solid electrolyte cylinder. The solid electrolyte cylinder is filled with a reference material. The tubular anode and the reference electrode lead-off electrode contain silicon carbide, more preferably silicon-doped silicon carbide. Silicon-doped silicon carbide has a low electrical resistance and is therefore particularly suitable. The probe according to the invention is distinguished from the common probes by a high mechanical and chemical stability as well as a resistance to thermal shock. The tubular anode may have both a closed wall surface and one or more slots along the longitudinal axis. The slots generally occupy 5% to 70%, more preferably 10% to 50%, of the circumference of the tubular anode. This includes probe designs with semi-open tubular anodes and channel-shaped anodes. Preference is given to using probes with a closed wall surface, since this protects the internal components against mechanical stress.

The opening of the tube anode serves for the entry of the molten metal into the interior of the anode and is preferably arranged at its lower end. The opening is preferably slot-shaped and preferably has a width of 2 mm to 15 mm, particularly preferably 3 mm to 8 mm. The advantages of a slot-shaped opening over other designs are mainly due to their ease of manufacture. However, other embodiments of the opening are conceivable. It must only be ensured that the liquid tin of the metal bath can enter the probe and flow past the reference electrode. For semi-open tubular anodes or channel-shaped anodes, such an opening is not necessary.

The solid electrolyte cylinder preferably contains zirconia, more preferably yttria-stabilized zirconia. In addition, the solid-state electrolyte cylinder may also contain zirconium dioxide and / or thorium dioxide or zirconium dioxide and / or thorium dioxide doped with calcium oxide, magnesium oxide and / or yttrium oxide. Zirconia or thorium dioxide serves as a diaphragm and allows the migration of oxygen ions between the molten metal and the reference material. The solid electrolyte cylinder is sealed so that the reference material does not evaporate and does not react with the hydrogen of the float bath atmosphere. The solid electrolyte cylinder has an opening through which pressure can escape from its interior.

The reference material contains a metal, preferably Sn, Ni, Cu, Cr and a mixture of this metal and its oxide, preferably Sn / SnO ₂ , Ni / 10, Cu / Cu ₂ O, Cr / CrO. The mixing ratio of metal / metal oxide is between 95/5 weight percent to 65/35 weight percent, preferably between 75/25 weight percent to 85/15 weight percent. The reference electrode thus receives a constant and accurate defined electrochemical potential. For the measurement of the oxygen concentration in tin baths of float glass plants Sn / Sn0 ₂ is preferred as the reference material, since the potential difference is zero at the oxygen saturation critical for tin oxide deposition.

The temperature measurement is preferably carried out by means of a commercially available thermocouple, which is surrounded by a protective cover made of a thermostable material. The protective sheath preferably contains AI2O _3, ZrC> 2, quartz glass, SiC, SiSiC, S1 ₃ N 4, TiB _2, BN and / or mixtures thereof, most preferably AI2O. ₃ The electrochemical potential is temperature dependent. Consequently, the measurement of the temperature of the tin bath, to avoid deviations, takes place spatially close to the location of the potential measurement.

The tubular anode and the discharge electrode of the reference electrode are connected to a contact wire via a conductive metal paste. The conductive metal paste is applied and baked on the electrode portion wrapped with the contact wire. This ensures the permanent electrical contact between the electrode and the contact wire.

The metal paste preferably contains silver, gold, platinum, palladium, copper, nickel, manganese, iron and / or mixtures or alloys thereof, particularly preferably silver.

The invention further comprises a method for measuring the oxygen saturation in molten metals with the probe according to the invention. In a first step, the probe is immersed in the molten metal through the installation opening of the float glass plant. The liquid metal enters the probe through the opening of the tubular anode. In a second step, the probe is attached via a screw in the mounting hole. The anode contacts the float bath casing and is grounded. Subsequently, the generated voltage between anode and reference electrode and the temperature of the liquid metal is measured. The dimensions of the probe holder correspond to those of the usual thermocouples, so that no structural changes to the use of the probe according to the invention are necessary. By measuring the voltage difference and the temperature, the oxygen partial pressure of the molten metal can be calculated by means of Nernst's equation:

With AE: measured voltage difference

R gas constant

T measured temperature

F: Faraday constant

Oxygen partial pressure of the bath

Oxygen partial pressure of the reference

Special embodiments of the measuring method include the measurement of the oxygen concentration in tin baths of float glass plants. The various temperature ranges of the system are around 600 ° C to 1000 ° C. The solubility of the oxygen in the liquid tin is dependent on the bath temperature. At lower temperatures, supersaturation of the molten metal with oxygen occurs more quickly. Especially at the end of the system with about 600 ° C a measurement of the oxygen concentration is therefore important, because here the fastest supersaturation with oxygen occurs. However, the probe according to the invention allows monitoring in the entire temperature range of 600 ° C to 1000 ° C.

The invention further includes the use of the probe according to the invention for measuring the oxygen concentration in molten metals. The probe is preferably used in metal melts in a bath for the production of glass, particularly preferably in tin baths in float glass plants. The measurement of the oxygen concentration in tin baths for glass production is of particular importance, since too high an oxygen content in the molten metal interferes with the production process. When the limit of solubility of the oxygen in the liquid metal is exceeded, the formation of tin dioxide begins. This leads to serious glass defects. The probe according to the invention has a higher thermal shock resistance and lifetime. This makes a permanent in-situ measurement of the oxygen concentration possible. The probe of the invention is assembled so that according to the invention, the internal components are only loosely inserted into the outer tubular anode. As a result, self-destruction is excluded by the different thermal expansion of the various components in normal operation. It must also be ensured in this process step that no tin-containing vapors (Sn, SnO, SnS) get inside the probe, because they would significantly affect the life of the probe by forming undesirable chemical compounds. These unwanted chemical compounds would lead to embrittlement and breakage of the contact wires and the destruction of the solder contacts within a short time. From the point of view of the longest possible duration of use of the exact assembly of the probe is therefore very essential.

In the following the invention will be explained in more detail with reference to a drawing. The drawing does not limit the invention in any way.

Show it:

FIG. 1 is a schematic view of the probe on,

Figure 2 is a schematic external view of the outer contacting of the probe, Figure 3 is a cross-section A-A 'of the probe in Figure 1, Figure 4 is a schematic view of the probe used in the float bath, Figure 5 is a flow chart of the method according to the invention.

Figure 1 shows a schematic view of the inventive probe (15). The probe (15) comprises a tubular anode (3), a reference electrode (4) and a thermocouple (1), the reference electrode (4) and the thermocouple (1) being disposed inside the tubular anode (3). The thermocouple (1) is surrounded by a protective cover (2). The tubular anode (3) has at its lower end an opening (5) through which the liquid tin of the tin bath (6) enters the tubular anode (3). The reference electrode (4) contains a discharge electrode (4.1), which dips into a solid electrolyte cylinder (4.2). The solid electrolyte cylinder (4.2) is filled with a reference material (4.3, 4.4) containing a mixture of tin and tin dioxide (4.3) and tin (4.4). The contacting of the electrodes via two contact wires (7) and the ground (8) via the Floatbad- Casing (11). Since the tubular anode (3) and the discharge electrode (4.1) of the reference electrode (4) are both made of the same material, thermal stresses are also avoided. Otherwise they could falsify the electrochemical potential. Furthermore, the ceramic materials used have a lower density than the liquid metal. The inner components are not firmly connected, but only used loosely and float in the molten metal. As a result, a material break in thermal expansion, as occurs in permanently installed components avoided.

FIG. 2 shows a schematic external view of the probe (15). The contacting of the tubular anode (3) is carried out by burning a conductive metal paste (9) and wrapping this anode portion with the contact wire of the anode (7.1).

FIG. 3 shows a cross-section of the probe (15) in the tin bath (6) along the line A-A '. Through the opening (5) of the tubular anode (3) liquid tin enters the inside of the tubular anode (3). The thermocouple (1) is surrounded by a protective cover (2) and does not directly contact the tin bath (6). The solid electrolyte cylinder (4.2) is sealed and immersed in the tin bath (6). The discharge electrode (4.1) dips into the interior of the solid electrolyte cylinder (4.2) and is not in direct contact with the tin bath (6).

Figure 4 shows a schematic view of the inventive probe (15) used in a float glass plant. The probe (15) is fitted in the mounting hole (13) for thermocouples, wherein the potential measurement via electrical connections (14) takes place while at the same time grounding with the float bath casing (11). The inner components of the probe (15) float freely in the tin bath (6). On the surface of the tin bath (6) is the solidifying glass melt (10). The gas atmosphere within the float bath casing (11) includes forming gas (12). FIG. 5 shows a flow chart of the method according to the invention. In a first step, the probe (15) is introduced through the installation opening (13) of the float glass line into the tin bath (6). Thereafter, the probe (15) is connected to the ground (8) with the float bath casing (1 1). In a final step, the temperature and the voltage generated are measured.

The invention is explained in more detail below with reference to an example for the measurement with the probe according to the invention and a comparative example for measurement with a probe according to the prior art.

In two series of tests, the life and the reusability of the probe according to the invention and a probe according to the prior art were compared when measuring the oxygen concentration in the tin melt of a float glass plant. a) Example 1: Measurement of the oxygen concentration in a tin melt with the probe according to the invention

The probe (15) was inserted at the cold end of the float glass plant at a temperature of 600 ° C in the tin melt. For this purpose, the probe (15) through the mounting hole (13) of the float bath casing (1 1) was introduced into the system. The probe (15) was initially inserted only a short distance into the system so that it did not touch the tin bath (6) yet. In order to avoid a thermal shock, the probe (15) was first left in this position for a short time and then further advanced in sections into the tin melt. This allowed the probe (15) to slowly adjust to the temperature of the float bath atmosphere. The inserted probe (15) was fastened by a screwing device with the tubular anode (3) grounded to the float bath casing (11). The temperature and the voltage generated were measured via the electrical connections (14). b) Comparative Example 2: Measurement of the oxygen concentration in a tin melt with a probe according to the prior art A probe from the model Continox from Heraeus Electro-Nite was used as described for the probe according to the invention in the float glass plant and the oxygen concentration was measured.

Table 1

Table 1 shows the life of the inventive probe (Example 1) and the probe according to the prior art (Comparative Example 2) in comparison. The probe of the prior art is often destroyed by the first insertion of the probe into the system by thermal shock, whereby the short lifetimes of a few minutes come to pass. If the probe known from the prior art could be used successfully in the system, a maximum service life of up to 1.5 years was achieved in individual cases. The fiction, contemporary probe could be successfully used in all cases in the system without a thermal shock occurred. However, the maximum lifetime of the probe according to the invention is not yet known, since it has not come to any failure of the probe. The probe according to the invention has already been heated in total from room temperature to the float bath temperature of 600 ° C. 20 times without damaging the probe. The fiction, contemporary probe can therefore be reused over 20 times, while the probe is destroyed in the prior art when reinserting in 80% of the cases. Due to the multiple usability of the probe according to the invention this is much cheaper. Due to the often short life of the prior art probe, long term monitoring of oxygen concentration with this probe is difficult.

A continuous measurement of the oxygen concentration of the tin melt is enormously important, since otherwise a fault is noticed only very late if production errors already occur in the finished product. The discs produced since the occurrence of the fault must be discarded due to such production errors. This results in immense costs. The probe according to the invention is insensitive to thermal shock and thus facilitates a long-term measurement of the oxygen concentration. Such constant monitoring of the oxygen concentration allows a quick detection of the disturbance. The probe of the invention is suitable for the various temperature ranges of the system from 600 ° C to 1000 ° C. Thus, several probes can be used along the system. As a result, the fault can not only be found very quickly, but also the system section in which the fault is present can be located. Such monitoring has previously been difficult due to the often short life of prior art probes. Early detection, however, is especially important because of the long decay times of the disorder. The disturbance should be detected early so that only the smallest possible amount of oxygen penetrates into the system. If the fault is detected earlier, the resulting leak can be sealed faster. As a result, the total amount of oxygen introduced is lower and the disorder sounds faster. When using the probe according to the invention thus also the production costs can be reduced by shortening the production loss in case of incidents.

LIST OF REFERENCE NUMBERS

1 thermocouple

2 protective sleeve

3 tubular anode

4 reference electrode

4.1 discharge electrode

4.2 solid phase electrolyte cylinder

4.3 Sn / Sn0 ₂

4.4 Sn

5 opening

6 tin bath

7 contact wires

7.1 contact wire anode

7.2 Contact Wire Cathode

8 grounding

9 metal paste

10 glass melt

1 1 Float bath casing

12 forming gas

13 installation opening

14 electrical connections

15 probe

A-A 'cutting line

Claims

claims

A probe for continuously measuring oxygen saturation in molten metals, comprising:

a) a tubular anode (3) having at least one opening (5) for receiving liquid metal from a metal bath (6),

b) a reference electrode (4) arranged inside the tubular anode (3), comprising a discharge electrode (4.1) immersed in an oxygen-ion-conducting solid electrolyte cylinder (4.2), the solid electrolyte cylinder (4.2) being filled with a reference material (4.3, 4.4),

c) a thermocouple (1) and d) disposed within the tubular anode (3), the tubular anode (3) and the diverting electrode (4.1) containing silicon carbide.

2. A probe according to claim 1, wherein the tubular anode (3) along the longitudinal axis has one or more slots, which occupy preferably 5% to 70%, particularly preferably 10% to 50% of the circumference of the tubular anode (3).

3. A probe according to claim 1 or 2, wherein the opening (5) is located at the lower end of the tubular anode (3).

4. Probe according to one of claims 1 to 3, wherein the opening (5) is slot-shaped.

5. Probe according to one of claims 1 to 4, wherein the opening (5) has a width of 2 mm to 15 mm, preferably 3 mm to 8 mm.

6. Probe according to one of claims 1 to 5, wherein the tubular anode (3) and / or the discharge electrode (4.1) contains silicon-doped silicon carbide.

7. A probe according to any one of claims 1 to 6, wherein the solid electrolyte cylinder (4.2) contains zirconia, preferably yttrium-stabilized zirconia.

8. Probe according to one of claims 1 to 7, wherein the reference material (4.3, 4.4) is a metal (4.4), preferably Sn, Ni, Cu, Cr and a metal / metal oxide mixture (4.3), preferably Sn / Sn0 ₂ , Ni / NiO, Cu / Cu ₂ O, Cr / CrO.

9. Probe according to one of claims 1 to 8, wherein the thermal sensor (1) by a protective cover (2) is surrounded.

10. A probe according to claim 9, wherein the protective sheath (2) AI2O ₃ , Zr0 ₂ , quartz glass, SiC, SiSiC, S1 contains ₃ N4, TiB ₂ , BN and / or mixtures thereof.

1 1. A probe according to any one of claims 1 to 10, wherein the tubular anode (3) and / or the Ableitelektrode (4.1) via a conductive metal paste (9) with a contact wire (7.1, 7.2) are connected.

12. A probe according to claim 11, wherein the metal paste (9) contains silver, gold, platinum, palladium, copper, nickel, manganese, iron and / or mixtures or alloys thereof.

A method of measuring oxygen concentration in molten metal with a probe according to any one of claims 1 to 12, wherein the probe (15) is immersed in the liquid metal, the probe (15) is fixed via a screw in the mounting hole (13) and thereby to the ground (8) is connected to the float bath casing (11) and the generated voltage between the tubular anode (3) and reference electrode (4) and the temperature of the liquid metal are measured.

14. Use of a probe according to one of claims 1 to 12 for measuring the oxygen concentration in molten metal, preferably molten metal in a bath for the production of glass, particularly preferably tin baths in float glass plants.