Measurement device for determining oxygen activity in molten metal or slag

A measurement device is provided for determining the oxygen activity in molten metal or slag with a measurement head, which is arranged at one end of a carrier tube and on which an electrochemical measurement cell is arranged. The electrochemical measurement cell has a solid electrolyte tube, which is closed at one end and which contains a reference material and an electrode at its closed end. The electrode projects from the opposite end of the solid electrolyte tube. The outer surface of the solid electrolyte tube has a coating of a mixture of calcium zirconate with a fluoride. The measurement device can be used for calculating the sulfur, silicon and/or carbon content of the molten metal or slag from reaction of the sulfur with the calcium zirconate, which releases oxygen, and measurement of the resulting change in oxygen activity of the melt or slag at the outer surface of the solid electrolyte tube.

BACKGROUND OF THE INVENTION

The invention relates to a measurement device for determining the oxygen activity in molten metal or slag with a measurement head, which is arranged at one end of a carrier tube and on which an electrochemical measurement cell is arranged. The electrochemical measurement cell has a solid electrolyte tube, which is closed on one end and which contains a reference material and an electrode at its closed end, wherein the electrode projects out of the opposite end of the solid electrolyte tube. In addition, the invention relates to a solid electrolyte tube for an electrochemical measurement cell.

Such measurement devices are known, for example, from German Patent DE 31 52 318 C2. The sensor described in this document is used to measure the concentration of oxygen in molten metal. Similar measurement devices are also known from U.S. Pat. No. 3,578,578, German published patent application DE 28 10 134 A1, or German Patent DE 26 00 103 C2.

BRIEF SUMMARY OF THE INVENTION

In addition to oxygen, there is the need to measure other materials contained in molten metals. Therefore, the invention is based on the object of providing a simple measurement device and also a corresponding solid electrolyte tube, with which, in addition to the oxygen content, the concentration of other elements can also be determined.

The object is achieved in that the outer surface of the solid electrolyte tube has a coating of a mixture of calcium zirconate with a fluoride. It has been shown that this coating allows the determination of the concentration, for example, of sulfur, silicon, or carbon in melts. The effect can be explained in that the sulfur, for example, found in the liquid metal reacts with the CaO from the calcium zirconate, producing oxygen as a reaction product, and the change of the oxygen activity at the surface of the solid electrolyte is measured and correlated with the sulfur. The measurement device can be used in molten metal or slag, especially in steel or iron melts, for measuring the concentration of sulfur, silicon, or carbon. A quick measurement is obtained thereby. It is advantageous if the measurement device has, in addition to the electrochemical measurement cell, a temperature sensor, for example a thermocouple, so that the temperature of the molten metal can also be measured. Silicon is correlated with the sulfur content. Carbon can be calculated from the silicon-carbon thermal equilibrium of the molten metal.

With the measurement device according to the invention, a sample analysis in the laboratory can be avoided, so that considerable savings of time in the production process and consequently an improved and quicker influence of production process can be achieved.

Advantageous embodiments of the invention are given in the below and in the dependent claims. It is expedient that the fluoride be at least one from the group of CaF2, NaF, SrF2, BaF2, MgF2. It is advantageous for high measurement sensitivity that the calcium zirconate be stoichiometric. Likewise, it is advantageous that the coating have a thickness of approximately 10–100 μm, especially about 30 μm. Here, a thinner layer is sufficient at higher temperatures of use (for example, before a desulfurization treatment). In this case, the response time is rather short. At lower temperatures of use (for example, after the desulfurization treatment) a thicker coating is required. The response time is then somewhat longer.

The solid electrolyte tube is advantageously stabilized ZrO2. The layer can also only partially cover the outer surface of the solid electrolyte tube, wherein the surface of the tube should be coated at least in the region in which the reference mass is arranged.

DETAILED DESCRIPTION OF THE INVENTION

The measurement device has a carrier tube1, in which the measurement head2is held, wherein the measurement head2connects inside the carrier tube1to a supply line to the measurement and evaluation circuits through a contact piece3. The carrier tube1is shown only at its beginning inFIG. 1.

At the immersion end of the measurement head2, in addition to a thermocouple4, there is a solid electrolyte tube5. Thermocouple4and solid electrolyte tube5are surrounded by a protective cap6and are each protected before and during the immersion of the measurement head into the melt, particularly molten iron or steel.

InFIG. 2the solid electrolyte tube5is shown in section. It is produced from stabilized zirconium dioxide and has in its interior a mixture of chromium and chromium dioxide as a reference material7. The filling material8arranged on top of this mixture is aluminum oxide, for example. In the center of the solid electrolyte tube5there is arranged a molybdenum rod as electrode9. The electrode9projects from the open end of the solid electrolyte tube5. This open end is closed by a cap10, wherein the filling material8is held at its upper end by a gas-permeable cement11. The solid electrolyte tube5is surrounded by a steel cap12, which also protects the tube during the immersion in the molten metal. It then melts and exposes the coating13arranged on the solid electrolyte tube5. The coating is preferably a mixture of calcium zirconate and magnesium fluoride.

In the melt CaO reacts with sulfur with formation of CaS, whereby oxygen is released, whose activity is measured with the aid of the solid electrolyte tube.