Electrolytic sensors

Abstract: Electrolytic sensors, for detecting and measuring concentrations of minor polyvalent elements in metallurgical melts, have two electrodes separated by an oxidic solid electrolyte matrix, on which is deposited a compound of the element to be measured and an element which is mobile in the matrix. A preferred electrolyte matrix is β-alumina, and preferred deposition compounds are phosphates, sulphates and silicates of the alkali or alkaline earth metals.

Description: This invention relates to electrolytic sensors which are useful for the detection and measurement of concentrations of minor elements, especially non-metallic and semi-metallic elements, in molten metallurgical phases, e.g. metals, alloys, slags, mattes and drosses. There is a need, especially in metal refining and alloying processes, for a sensor which can be used to monitor the concentrations of minor elements in the melts as the process proceeds, so that the process can be controlled. EP-A-0208072 describes an apparatus for electro-chemical determination of silicon dissolved in molten metal including a layer of solid electrolyte for conducting oxygen ions, a reference electrode contacting one surface of the layer of electrolyte, means for determining a potential difference between the reference electrode and molten metal, and a solid auxiliary electrode comprised of SiO₂. In this device the sensor utilizes an electrolyte which conducts oxygen ions. GB-A-1470558 describes an apparatus for detecting an element in a substance by monitoring the EMF generated between the substance and reference material wherein the reference material is a solid electrolyte comprising a beta-alumina containing the element, or a solid compound of the element separated from the substance by the electrolyte, one electrode for contacting the substance and a second electrode separated from the first electrode by a solid electrolyte. The above-mentioned GB-A-1470558 and also GB-A-1602564 describe and claim electrolytic sensors wherein a solid electrolyte, based on a β-alumina matrix, is interposed between the melt being tested and the elements to be detected or measured. However, the ionic species which can be measured by this prior method are limited to ions which are mobile in the solid electrolyte, i.e. monovalent and some divalent and trivalent ions such as alkali metal/alkaline earth metal ions. There is a problem where detection or measurement of multi-valent elements such as P, As, Sb, or S in metallurgical phases is required, as it may be in many refining or alloying processes. The present invention therefore seeks to meet this requirement and provide a sensor which overcomes the disadvantages of the prior art. The present invention in one aspect provides an electrolytic sensor for the detection and measurement of a polyvalent minor element in a molten metallurgical phase, wherein the minor element is selected from phosphorus, arsenic, antimony and sulphur, comprising: two electrodes separated by a solid electrolyte matrix, on which solid electrolyte matrix is deposited a compound of (i) the said element to be detected and measured, (ii) oxygen, and (iii) an element selected from alkali and alkaline earth metal elements whose ions are mobile in the solid electrolyte matrix; and means for measuring the EMF between the said electrical leads. Preferably the solid electrolyte is based on β-alumina or α + β alumina. The element to be detected will be phosphorus, arsenic, antimony or sulphur; while the mobile element will be an alkali or alkaline-earth element ion such as Na, Li, K or Ca. It may also be necessary to deposit with the deposition phase a conducting phase, e.g. a metal, oxide or carbide, which is electrically conducting. Metals in particular may be used as conducting phases and precious metals may be especially useful. The solid electrolyte is suitably used either in the form of a closed-end tube or is sealed into a silica tube. A reference material (which is usually a mixture of ferric oxide and a metal ferrite) is placed in the tube and the deposition and conducting phases are coated on the external surface of the solid electrolyte. Thus, in a typical example, the conducting and deposition phases may be platinum/sodium phosphate which may be vapour-deposited on a solid electrolyte matrix which contains sodium ions. In such an example, the platinum acts as an electrical conductor between the solid electrolyte (β-alumina) and the molten metal. On the surface of the electrolyte the following equilibrium is set up between dissolved Na, P and O in the molten metal or alloy and the solid Na₃PO₄. If the oxygen content of the melt remains constant (as it usually does by virtue of the metal/metal oxide equilibrium) any change in phosphorus content results in a change in the sodium activity in the solid Na₃PO₄ and this is detected by the β-alumina sensor. In this way phosphorus can be detected in melts based on aluminium, tin or copper. A similar approach can be used to detect other polyvalent elements in metals or alloys, e.g. NaAsO₂ or NaSbO₂ may be deposited on the β-alumina matrix if detection of As or Sb respectively is required. For determination of sulphur, it is necessary to use a relatively high-melting salt such as CaSO₄ in contact with Ca/β-alumina matrix. Normally such a salt will be oxidic in character. By "oxidic" as used herein, there is meant a compound in which the elements are chemically combined with oxygen. The sensor according to the invention may be used for on-line detection of minor elements, to monitor the progress of refining or alloying operations carried out on alloy or metal melts. The invention in another aspect provides a method of measuring the concentration of a polyvalent minor element in a molten metallurgical phase, wherein the minor element is selected from phosphorus, arsenic, antimony and sulphur, characterized in that a sensor in accordance with the first aspect of the invention is immersed in the said molten phase and the EMF (electro-motive force) of the resulting cell is measured. Preferably the measured EMF is used as a measure of the concentration of the minor element in the molten phase at constant oxygen concentration. The invention will be further illustrated by the following Examples, in which Example 1 relates to the detection and measurement of phosphorus in molten tin, Example 2 to the detection and measurement of phosphorus in aluminium/silicon alloys, Example 3 to the measurement of oxygen and sulphur in molten copper, and Example 4 to the measurement of oxygen and sulphur in lead. Reference will also be made to the accompanying drawings, wherein figures 1 to 5 are graphs of EMF plotted against concentration of the element being detected (temperature in figures 4 and 5) in the various Examples, as will be described in more detail subsequently. An electrolytic sensor for phosphorus detection was prepared as follows: An isopressed pellet of Na₃PO₄ + Pt was heated by a W-coil to a temperature T of at least 1600°C within an evacuated chamber and the vapour was allowed to deposit on the outer surface of a β-alumina tube. A reference material (ferric oxide + metal ferrite) was enclosed within the tube. Electrical connections were made via iron wires inserted into the reference material and into the melt to be tested, these being connected via a voltmeter. EXAMPLE 1 Measurement of phosphorus in molten tin A phosphorus sensor prepared as described above was immersed in molten tin at 600°C. The EMF obtained with pure (99.999%) tin prior to P additions was 0.62V approximately. This initial attainment of equilibration takes about 20-30 mins. Successive addition of phosphorus in the form of red P led to continuous decrease in the EMF values and an apparent saturation at -0.05V approximately. When oxygen measurements were carried out in conjunction, the EMF's appeared to be independent of P addition, and attained a steady EMF in 60-90 minutes. Experiments at 700°C improved the response time of both the P and O sensors by 10 and 15 mins. respectively. The oxygen saturation was observed at 0.968V, while the P saturation occurred at 0.152V. EXAMPLE 2 Measurement of phosphorus in molten aluminium/silicon alloy Using a phosphorus sensor prepared as described above, tests were carried out on a molten Al/Si alloy containing known amounts of phosphorus. The reaction at the working surface can be represented as: for the electrolytic cell represented as: The relationship between the EMF of this cell and the phosphorus content of the melt can be expressed as: log [ppmP] = A + B [EMF] where A and B are constants. A plot of EMF vs. log [ppmP] is shown in Figure 1. Further phosphorus additions were made to the melt, in the form of P₂0₅. Once the EMF reached the value of -0.260 V (±0.015) further additions of P₂0₅ gave no change in the EMF value. This is consistent with the attainment of P saturation of the melt and the onset of AℓP precipitation, as is predicted by the Aℓ-P phase-diagram. EXAMPLE 3 Measurement of oxygen and sulphur in molten copper (a) Oxygen measurement Molten copper at 1100°C was saturated with oxygen using Cu₂O and oxygen gas. The following cells were set up to measure the oxygen content of the melt: The average EMF readings from these two cells were CELL A 0.541 V (±0.015V) CELL B 0.550 V (±0.02V) (b) Sulphur measurement Sulphur additions were made to the molten copper at 1100°C, using Cu₂S in copper foil. The following cell was set up: The following EMF values were measured for different sulphur concentrations at constant (saturated) oxygen concentration: Wt % S EMF (mV) 0.0012 64 0.0032 12 0.0066 -23 0.0158 -80 0.0407 -149 0.0631 -287 0.1122 -268 0.3020 -272 0.6050 -320 A plot of EMF (mV) vs. log (ppmS) in melt is shown in figure 2. In a second experiment copper at 1100°C containing 400ppm oxygen was measured using cells A + B (see above). The following EMF measurements were obtained: CELL A 0.398 v (±0.025) CELL B 0.411 v (±0.025) With added sulphur at the concentrations specified the following EMF values were obtained at constant oxygen concentration: Wt % S Average EMF (V) ±15 mV) 0.0017 NO STABLE EMF 0.0199 0.464 V. 0.0631 0.420 V. 0.1096 0.349 V. 0.1995 0.322 V. 0.5543 0.242 V. 0.6218 0.208 V. Further sulphur additions had no added effect. A plot of EMF (mV) vs. log (ppmS) in melt is shown in Figure 3. EXAMPLE 4 Measurement of oxygen and sulphur in molten lead The following cells were set up: The following measurements were made: The CELL B EMF being a measure of oxygen concentration and CELL A EMF being a measure of sulphur concentration. T °C T °K EMF CELL A (mV) EMF CELL B (mV) 590 863 39 405 615 888 19 412 655 928 -17 417 701 974 -47 427 749 1022 -86 437 810 1083 -133 450 907 1180 -211 467 CELL A EMF = 713.5 - 0.78 T (K) (mV) CELL B EMF = 236.5 + 0.196 T (K) (mV) The plots of EMF (mV) vs TEMP (°C) for CELLS A and B are shown in Figures 4 and 5.

Claims: An electrolytic sensor for the detection and measurement of a polyvalent minor element in a molten metallurgical phase, wherein the minor element is selected from phosphorus, arsenic, antimony and sulphur, comprising: two electrodes separated by a solid electrolyte matrix, on which solid electrolyte matrix is deposited a compound of (i) the said element to be detected and measured, (ii) oxygen, and (iii) an element selected from alkali and alkaline earth metal elements whose ions are mobile in the solid electrolyte matrix; and means for measuring the EMF between the said electrical leads. A sensor as claimed in claim 1, characterized in that the solid electrolyte is based on β-alumina or α + β-alumina. A sensor as claimed in claim 1 or 2, characterized in that a metallic electrical conductor has been deposited with the said compound, on the solid electrolyte matrix prior to disposing the solid electrolyte matrix in the said molten metal. A sensor as claimed in claim 3, characterized in that the metallic conductor is a precious metal. A method of measuring the concentration of a polyvalent minor element in a molten metallurgical phase, wherein the minor element is selected from phosphorus, arsenic, antimony and sulphur, characterized in that a sensor as claimed in any of claims 1 to 4 is immersed in the said molten phase and the EMF of the resulting cell is measured. A method as claimed in claim 5, characterized in that the measured EMF is used as a measure of the concentration of the minor element in the molten phase at constant oxygen concentration.

Inventors: FRAY DEREK JOHN, KUMAR VASANT RAMACHANDRAN, FRAY, DEREK JOHN, KUMAR, VASANT RAMACHANDRAN
Applicants: MINERAL IND RES ORG, MINERAL INDUSTRY RESEARCH ORGANISATION
Application year: 1989
Main classification: G01N27
IPC codes: G01N27