Measurement device for determining oxygen activity in metal or slag melts

A measurement device is provided for determining the oxygen activity in molten metal or slag with a measurement head, which is arranged on one end of a carrier tube and on which an electrochemical measurement cell is arranged. The electrochemical measurement cell has a solid electrolyte tubule, which is closed on one side and which contains on its closed end a reference material and an electrode, which projects from the opposite end of the solid electrolyte tubule. The solid electrolyte tubule has on its outer surface a coating made from a mixture of zirconium silicate with a fluoride. The measurement head may also have a thermo-element, and the device may be used to determine the concentration of silicon and/or carbon in the melt, based on the oxygen activity measurement.

BACKGROUND OF THE INVENTION

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

Such measurement devices are known, for example, from German Patent DE 31 52 318C2. The sensor described there is used for measuring 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. In U.S. Pat. No. 4,657,641 a sensor is disclosed, which has a coating made from zirconium silicate and zirconium dioxide.

BRIEF SUMMARY OF THE INVENTION

In addition to measuring oxygen, there is also the need to measure other materials contained in the molten metal. Therefore, the invention is based on the problem of providing a simple measurement device and also a corresponding solid electrolyte tubule, which can be used to determine also the concentration of other elements, in addition to the oxygen content.

The problem is solved in that the solid electrolyte tubule has on its outer surface a coating made from a mixture of zirconium silicate with a fluoride. It has been shown that this method makes possible the determination, for example, of the concentration of silicon or carbon in the molten mass. The effect is explained in that, for example, the silicon contained in the liquid metal reacts with the SiO2from the zirconium silicate. In the equilibrium reaction, oxygen is obtained as a reaction product, and the change in the oxygen activity at the surface of the solid electrolyte is measured and correlated with the silicon content.

The measurement device can be used in molten metal or slag melts, especially in molten steel or iron, for measuring the concentration of silicon or carbon. A rapid measurement process is thereby realized. It is advantageous if the measurement device has a temperature sensor, for example a thermo-element, in addition to the electrochemical measurement cell, so that the temperature of the molten metal can also be measured. Carbon can be calculated from the silicon-carbon temperature equilibrium of the liquid metal.

By the measurement device according to the invention, sample analysis in the laboratory can be avoided, so that considerable time savings in the production process and consequently better and faster actions for influencing the production process can be achieved.

Advantageous embodiments of the invention are given below. It is expedient if the fluoride is at least one from the group CaF2, NaF, SrF2, BaF2, and MgF2. It is advantageous if the layer thickness equals approximately 10–100 μm, especially about 10–50 μm, and advantageously about 30 μm. Here, a thicker layer is sufficient for higher application temperatures (above approximately 1500° C.), for example before a desulfurization treatment. Here, the response time is quite short. At lower application temperatures (up to approximately 1400° C.), for example after the desulfurization treatment, a thinner layer is necessary. The response time is then somewhat greater. Advantageously, about 2–10 wt % fluoride, especially about 3–4 wt % fluoride is used (relative to the weight of the coating). At high fluoride contents, the response time at lower melt temperatures is shorter, because equilibrium can be reached more quickly. The coating can be produced very uniformly by means of plasma spraying or flame spraying.

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

DETAILED DESCRIPTION OF THE INVENTION

The measurement device has a carrier tube1, in which the measurement head2is held, wherein by means of a contact piece3within the carrier tube1the measurement head2contacts a supply line to measurement and evaluation instruments. The carrier tube1is shown inFIG. 1only at the attachment part.

At the immersion end of the measurement head, in addition to a thermo-element4, there is a solid electrolyte tubule5. Thermo-element4and solid electrolyte tubule5are surrounded by a protective cap6and protected before or during the immersion of the measurement head into the melt, especially an iron or steel melt.

InFIG. 2the solid electrolyte tubule5is shown in section. It is produced from stabilized zirconium dioxide and has in its interior as reference material7a mixture of molybdenum and molybdenum dioxide, a mixture of chromium and chromium dioxide, or a mixture of nickel and nickel oxide. The filler material8arranged above the reference material is, for example, aluminum oxide. In the solid electrolyte tubule5, a molybdenum rod is arranged centrally as an electrode9. The electrode9projects from the open end of the solid electrolyte tubule5. This open end is closed by a cap10, wherein the filler material8is held at its upper end by a gas-permeable cement11. The solid electrolyte tubule5is surrounded by a steel cap12, which also protects the tubule during the immersion into the melt. The steel cap12then melts and exposes the coating13arranged on the solid electrolyte tubule5. The coating is preferably a mixture of zirconium silicate and about 3–4 wt % magnesium fluoride. The coating is approximately 30 μm and is applied by plasma-spraying.

In the melt SiO2reacts with the Si of the melt, wherein oxygen is released in an equilibrium reaction, and its activity is measured with the help of the solid electrolyte tubule.