Apparatus for Monitoring the Oxygen Content of High Temperature Fluids

[57] ABSTRACT

A probe assembly including an oxygen ion conductive solid electrolyte electrochemical cell is preheated under controlled conditions to a temperature equivalent to the temperature of a high temperature oxygen containing fluid such as a gas or molten metal prior to insertion of the probe into the fluid for measuring the oxygen concentration of the fluid. The preheating of the solid electrolyte electro-chemical cell coupled with the use of thermal shield elements within the probe assembly prevents physical deterioration of the solid electrolyte electrochemical cell due to the thermal shock produced when contacting the high temperature fluid.

Claim, 4 Drawing Figures

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PATENTED™ 41975

3,864,232

SHEET 1 OF 2

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PATENTEDFEB 41975

3,864.232

SHEET 2 OF 2

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FIG.2.

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FIG.3.

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FIG.4.

3,864,232 and subsequently positioned within a thermal insulating tubular member for insertion directly into the molten metal. The thermal insulating tubular member is a part of a probe assembly capable of being mounted directly 5 on the tundish within which the molten metal is contained. A vertical positioning mechanism associated with the probe assembly functions to lower the solid electrolyte electrochemical cell assembly through the tubular insulating member and into the molten metal 1° for direct contact measurement of the oxygen concentration in the molten metal. Suitable thermal insulating devices within the probe assembly permit subsequent retraction of the solid electrolyte electrochemical cell to a position within the tubular thermal shield member ¹⁵ and the subsequent removal of the probe assembly from the tundissh.

DESCRIPTION OF THE DRAWINGS

The invention will become more readily apparent ²⁰ from the following exemplary description in connection with the accompanying drawings:

FIG. 1 is a sectioned illustration of a probe assembly embodying the invention;

FIG. 2 is a detailed sectioned illustration of the solid electrolyte electrochemical cell assembly of the probe assembly of FIG. 1;

FIG. 3 is a schematic illustration of an alternate configuration of the solid electrolyte electrochemical cell ₃₀ assembly for use in the probe assembly of FIG. 1; and

FIG. 4 is a modification of the configuration of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT 35

Referring to FIG. 1 there is illustrated an oxygen probe assembly 10 inserted through a flange member F in the top of a tundish T containing molten copper C. The oxygen measuring probe assembly is supported on 40 the flange F by tubular housing 12. Extending from the tubular housing 12 into the tundish T and contacting molten copper C is an open ended tubular thermal shield 14 composed of a thermal insulating material capable of withstanding the temperature of the molten 45 copper which is typically in the range of 2,000° to 2,500°F. A clay-graphite composition has proven to be most suitable for fabrication of the tubular shield 14. The tubular solid electrolyte member 20 having a closed end forming an electrochemical cell 22 extends ⁵⁰ through the tubular stainless steel member 30 to connector assembly 38 which is secured to mounting plate 36. The tubular stainless steel member 30 extends from the connector assembly 38 through the stuffing gland 32.

³³ The vertical position of the mounting plate 36 and consequently the vertical position of the combination of the solid electrolyte electrochemical cell 22, which as seen in FIG. 2, is comprised of the solid electrolyte material forming the closed end of the electrolyte ⁶⁰ member 20 and the electrode 23, and the tubular stainless steel member 30 is controlled by knob 40 and gear train 42.

Extending from the tubular stainless steel member 30 is a closed end tubular screen member 44 which sur⁶⁵ rounds the solid electrolyte member 20 and the electrochemical cell 22. The tubular screen member functions to:

APPARATUS FOR MONITORING THE OXYGEN CONTENT OF HIGH TEMPERATURE FLUIDS

BACKGROUND OF THE INVENTION

The measurement of the oxygen content of various fluids used in industrial processes is often a very important parameter. For example the oxygen concentration of molten copper, prior to casting is a key process parameter in continuous melting, casting and rolling operations. A high concentration of oxygen in molten metal results in frequent breaks of the billet and loss of production due to excessive gas evolution in the billet during solidification. A low concentration of oxygen also results in loss of production due to lower strength of the hot billet. Excessive fluctuation of oxygen beyond specification limits adversely affects the quality of the metal product thereby seriously affecting the marketability of the product.

At present, the oxygen content of molten copper is controlled by blowing either a reducing gas or an oxidizing gas stream on the copper as it flows from the holding furnace tothe tundish. If the oxygen content is low, air is blown into the copper. High oxygen is corrected by blowing the hydrogen rich gas stream into the copper. Tundish samples taken periodically are removed and analyzed in a remote laboratory. During the 5 to 10 minute period required to sample and analyze the molten metal an additional 2,500 to 5,000 pounds of copper can be cast at the typical rate of 15 tons/hour. There exists an urgent need for an in situ oxygen measuring technique which will not only provide an accurate oxygen content measurement of high temperature fluids but which will eliminate the time consuming practice of removing samples for analysis in a remote laboratory.

While the conventional solid electrolyte oxygen analyzers offers a rapid direct measurement of oxygen, such analyzers have not been successful in applications requiring continuous measurement of oxygen in high temperature industrial processes. The main disadvantage results from the fact that material compositions of the solid electrolyte at room temperature generally cannot withstand the thermal shock encountered when contacting a high temperature fluid such as a molten metal at 700°C or higher.

SUMMARY OF THE INVENTION

An improved technique and apparatus for monitoring the oxygen content of high temperature environments including gases and/or liquids is disclosed herein.

The traditional fully stabilized solid electrolyte is replaced with a partially stabilized solid electolyte to improve the physical characteristics of the electrolyte to enable it to better withstand elevated temperatures.

The solid electrolyte electrochemical cell is incorporated into a probe assembly and is controllably preheated to a temperature representative of the fluid to be monitored for oxygen content. The preheating of the solid electrolyte electrochemical cell prior to insertion into the fluid eliminates the occurrence of thermal shock when the cell contacts the fluid thus avoiding physical deterioration of the cell.

The specific embodiment of the invention described below with reference tothe accompanying drawings refers to an oxygen ion conductive solid electrolyte electrochemical cell which is controllably preheated to the temperature of the molten metal under consideration

3,864,232

1. dislogdge the thermal plug element 46 prior to the insertion of the electrochemical cell 22 into the molten copper,

2. prevent damaging contact between the fragile solid electrolyte member 20 and the thermal shield 14, 3. provide controlled contact between the electrochemical cell 22 and the molten copper by shielding the cell 22 from sudden splashes of molten metal during the time after the plug 46 is dislodged and before the cell 22 is submerged in the molten copper.

A collar of thermal insulating material 50 is secured to the tubular screen member 44 and moves vertically within the tubular shield member 14 to maintain alignment of the solid electrolyte member 20 within the thermal shield 14. The thermal plug element 46 serves to prevent hot metal and gases from rising within the probe assembly 10 before insertion of the electrochemical cell 22 into the molten metal.

The solid electrolyte electrochemical cell 22 corresponds essentially in method of operation to the solid electrolyte oxygen analyzer described in U.S. Pat. No. 3,400,054, issued Sept. 3, 1968 and assigned tothe assignee of the present invention. An oxygen reference of known oxygen concentration is established within the solid electrolyte member 20 at the closed end by an air supply source 54. When brought in contact with the molten copper C, the solid electrolyte electrochemical cell assembly transmits an EMF signal to signal measuring circuit 56. The magnitude of EMF signal is a function of the difference in oxygen partial pressure between the oxygen reference established by the air supply and the oxygen concentration of the molten copper C. The EMF signal is an indication of the oxygen concentration of the molten copper C.

Prior to positioning the probe assembly 10 on flange F, the probe assembly 10 is positioned on preheat furnace PF such that the thermal shield 14 and solid electrolyte member 20 are inserted within the furnace PF. The electrochemical cell 22 is subjected to a controlled heating process by temperature controller TC to preheat the electrochemical cell 22 to a temperature equal to or exceeding the temperature of the molten copper. The gradual heating of the electrochemical cell 22 by the preheat furnace PF establishes the cell 22 at the temperature of the molten copper without subjecting the cell 22 to a thermal shock which could be detrimental to the physical integrity of the solid electrolyte material. Once the temperature of the electrochemical cell 22 has been established at the temperature of the molten copper C by the preheat furncae PF the oxygen probe assembly 10 is ready to be transferred to the tundish T. During the transfer process and prior to insertion of the oxygen probe assembly 10 into the flange F the tubular thermal shield member 14 and the thermal plug element 46 function to minimize heat loss from the solid electrolyte electrochemical assembly 20 to the enviroment.

After the oxygen probe assembly 10 has been heated to the desired temperature by the preheat furnace PF the assembly is installed on the tundish T with the bottom of the tubular housing 12 position in the flange F. With the assembly 10 in position on the tundish T, the tubular thermal shield member 14 extends inside the tundish T through the gas burner flame to a position below the molten copper level.

When the oxygen concentration measurements of the molten copper are desired, the combination of the solid electrolyte member 20 and the tubular stainless steel member 30 are lowered through the thermal shield member 14 by the adjustment of knob 40. The closed end tubular metallic screen member 44 dislodges the thermal plug element 46, which then assumes a floating position on the surface of the molten copper and is subsequently removed as part of the slag. The tubular metallic screen member 44 contacts the molten copper and functions as an electrical ground through the stainless steel tube 30 to the electrical connector 41 mounted on the mounting plate 36. The molten coppr C contacting the electrochemical cell 22 upon immersion into the molten copper serves to provide an external electrode on the external surface of the solid electrolyte member 22 in opposed relationship with the electrode 23. The molten copper thus completes the ingredients required to form an operational solid electrolyte electrochemical cell as defined in the above reference U.S. Patent and the electrochemical cell 22 immediately functions to produce an EMF signal which is measured by the signal measuring circuit 56 as indication of the oxygen concentration of the molten copper.

The stuffing gland 32, which provides a slidable seal for the vertical movement of the tubular stainless steel member 30, also functions to reduce the natural convection inside the tubular thermal shield member 14 thus minimizing heat losses while also preventing tundish burner flames from rising up through the probe assembly 10.

The requirement for removing the probe assembly 10 for preheating in a remote preheater furnace PF can be eliminated by incorporating a preheating furnace PF' as part of the probe assembly 10 of FIG. 1. The incorporation of a tubular preheating furnace within the tubular thermal shield element 14 coupled with a temperature control circuit can provide direct heating of the electrochemical cell 22 while the probe assembly 10 is positioned in the tundish T.

Referring to FIG. 2 there is illustrated the solid electrolyte electrochemical cell 22 as including the closed end of the oxygen ion conductive solid electrolyte member 20 and the electrode 23 disposed on the inner surface of the closed end to serve as the oxygen reference electrode in response to the air flow introduced into the interior of the solid electrolyte member 20 by the air supply source 54. The essential requirement for the material composition of the solid electrolyte material is that it support substantial oxygen ion conductivity while withstanding relatively high temperatures. While this material requirement can be satisfied by the numberous fully stabilized compositions described in the above-referenced U.S. Patent, such as the calcium stabilized zirconia composition represented as ZrO₂ · 15 mol percent CaO, experimental analysis indicates that partially stabilized electrolyte compositions offer additional advantages. The partial stabilization in contrast to complete stabilization provides for improved physical strength and is therefore desirable for the relatively severe environmental operating conditions to which the solid electrolyte material of a molten metal sensor is subjected. A particularly useful partially stabilized calcia-zirconia electrolyte composition is represented as ZrO₂ ’ 3.5 mol percent CaO.

The conical top of the solid electrolyte member 20 sits inside connector 60, to which the tubular stainless

3,864,232 steel member 30 is affixed. Washer 62 is made of a high temperature insulating material and serves as a cushion for the solid electrolyte member 20. Reference oxygen from the oxygen supply 54 enters the solid electrolyte member 20 through the connector 70 which mates with connector 60. Washer 68 provides a mechanical cushion between the connector 70 and the solid electrolyte member 20.

Electrical continuity between the electrode 23 and the connector 41 mounted on the mounting plate is provided by electrical lead members 72 and 73 which extend through longitudinal passages in a tubular feed through member 75 inserted within the solid electrolyte member 20 to which is attached a metallic mesh electrode 76 which is maintained in intimate contact with electrode 23. The metallic mesh electrode 76, which can be typically fabricated from platinum wire mesh, provides positive electrical contact with the electrode 23 while also isolating the solid electrolyte member 22 from direct mechanical contact by the tubular feed-through member 75.

In addition to being a function of oxygen differential pressure, the EMF signal developed by the electrochemical cell 22 is also responsive to variations in temperature. Therefore in an effort to maintain the temperature of the cell 22 at a third wire 74, known value having material composition different from the electrical leads 72 and 73, is passed through another passage in the feedthrough tube 75 to contact the metallic mesh electrode 76 thus forming a thermocouple combination with the electrical leads 72 and 73. A particular useful thermocouple for monitoring temperatures in the range of molten copper is formed by using platinum for the electrical lead wires 72 and 73 and a platinum 10 percent rhodium material for wire 74. The thermocouple signal produced is transmitted through the electrical connector 41 to a temperature monitoring circuit 82 illustrated in FIG. 1.

An alternate solid electrolyte electrochemical cell assembly is illustrated in FIG. 3. The tubular electrolyte member 20 of FIG. 1 has been replaced by a tubular stainless steel member 80 which supports a dish shape solid electrolyte electrochemical cell 82. The stainless steel tubular member 80 functions as an electrical lead when in contact with the molten metal and functions in combination with the electrical lead 84 which contacts the inner electrode 85 to transmit the EMF signal de5 veloped by the cell 82 to a suitable monitoring circuit.

The configuration of FIG. 3 can be modified as shown in FIG. 4 to monitor high temperature gas environments by adding an external electrode 86 for contacting the gas enviroment and extending the stainless 10 steel tube 80 to contact the electrode 86 and thus functions as an electrical lead wire from the electrode 86.