CS228227B1 - Method for direct heating ionic melt by alternating current passage - Google Patents

Abstract

The present invention provides a method for reducing loss metal electrodes and reduce the amount of secreted metal from an ionic melt such as for example, enamel, in direct heating by passage alternating current. Effect is achieved by having the usual network frequency 50 Hz current is reduced below this value.

Description

The present invention relates to a method of directly heating an ionic melt by passing an alternating current and solves the problem of electrode loss and reduction of metals from the melt during heating.

As the glass melts, more and more electrical heating is used by introducing alternating current directly into the glass. Electrodes made of various metal and ceramic materials serve for this purpose.

At present, molybdenum is used most for its advantageous properties. When electrodes are used to a greater or lesser extent, they always corrode. The most important role in the corrosion of metal electrodes is played by the oxidation of their material by some of the enamel components referred to as corrosion depolarizers. The most important corrosion depolarisers are, in addition to dissolved oxygen from the basic enamel components, lead oxide, sulfuric, arsenic and antimony oxides from clarifying and coloring agents. Corrosion processes are greatly accelerated by the action of alternating current passing through the electrodes. Alternating current can cause corrosion even when spontaneous corrosion does not occur, ie the enamel does not contain substances capable of oxidizing the electrode material.

The result of the corrosion reaction is, on the one hand, the loss of valuable electrode material, which, in combination with intercrystalline corrosion, can also lead to the electrode fracture; Conversely, if the reduced metal collects at the bottom of the furnace, this may lead to secondary corrosion of the vertical electrodes. The corrosion process as a whole is therefore extremely undesirable from the technological point of view and the effort is to minimize both the oxidation of electrodes and the deposition of metals from the glass.

For this purpose, in addition to selecting the appropriate electrode material, cathodic electrode protection, anodic electrode protection or an increase in AC frequency is used. Cathodic protection of the electrodes in which the protected electrode is coupled to the negative pole of the auxiliary direct current source is known from U.S. Pat. . In addition, it requires an auxiliary external DC power source, making the device more complex.

The anodic protection of the electrodes described in US Patent No. 2,545,619 requires the use of an auxiliary negative electrode. The combined electrode protection disclosed in U.S. Patent No. 3,530,221 utilizes two kinds of electrode materials. The first metal material resists the effect of cations and is polarized negatively, and the second ceramic material resists the effect of anions and is polarized positively. The use of this procedure is limited by the properties of the ceramic electrode, which does not allow operation at higher temperatures and the bar-shaped electrodes cannot be used. In addition, the combined method of protection does not prevent the formation of metals on the metal electrode.

Improvement of this combined procedure brings MS. No. 178,528. The essence of the process is to create a protective layer of glass with an increased electrical resistance in the electrode eye. The formed layer prevents both electrode oxidation and electrolytic deposition of metals. The disadvantage of the process is the necessity of introducing the negative pole with the auxiliary electrode into the glass, moreover, the method is designed for melting using lower current densities, usually

below 0.5 A / cm.

In order to reduce the concomitant interference caused by direct heating of ion melts by passing alternating current, DE patent No. 1 019 804 recommends using a higher frequency current, the value of which is generally several kHz. The procedure is energy intensive, a considerable part of the energy is lost in current generators with higher frequency.

The aforementioned drawbacks are limited by the method of the invention, which consists in introducing an alternating current of less than 50 Hz into the ionic melt with metal electrodes. Preferably, an alternating current of 5 to 45 Hz is used. By reducing the frequency to heat the alternating current used, ee reduces both the corrosion of the metal electrodes and the deposition of metals from the melt.

228227!

³

A further advantage of the method according to the invention is that the required alternating current of lower frequency can be obtained with a high current with high efficiency. The attached graphs demonstrate the effect of frequency reduction on the loss of metal electrodes in Figures 1 and 3, and on the amount of melt deposited in Figures 2 and 4.

Fig. 1 shows graphically the amount of corrosion of the metal electrodes, measured by the decrease of their radius Δ na, at the frequency f of alternating current. The results were measured by melting lead glass at 1350 ° C and at a current density of 1 A / cm ^{1 2} .

In FIG. 2, it graphically depicts the amount of lead excreted as a function of the frequency of alternating current. The results were measured under the same working conditions of the previous case.

Fig. 3 * 4 is a graphical representation of the measurement results found when melting lead glass with a current density of 0.5 A / cm 2 at 1350 ° C. Fig. 3 shows the dependence of electrode loss Δρ on frequency f, on Fig. 4 shows the dependence of lead excretion on frequency f.

The essence and effectiveness of the inventions is apparent from the examples, to which the invention is not limited.

He did

A current of 2 A / cm was fed through molybdenum electrodes into the strongly corrosive model glass having a content of 21% PbO, 14% KgO, and 65% SiO ₂ free of fins, melted at 1400 ° C. By reducing the frequency from 55 Hz to 25 Hz, corrosion was reduced by 40% and lead was reduced by 28%.

Example 2

For highly corrosive, process-made glass with a synthetic composition of 58.78% SiO ₂ J 25.27% PbO; 12.06% K ₂ 0; 1.75% Na ₂ 0; 1.5% ZnOJ 0.2% As ₂ O ₃ and 0.435% NiO, melted at 1350 ° C, were fed through the molybdenum electrodes in a single phase alternating current at a density of 1 A / cm ² . Reducing the frequency from 52.5 to 18 Hz reduced the corrosion rate by 3.4x, lead excretion * 3.65x ·

Example 1 and d 3

Under the same conditions as in Example 2, but at a current density of 0.5 A / cm and reducing the frequency from 50 to 10 Hz, the corrosion reduction reached 3.3 times the amount of lead excreted and decreased 3.7 times.

Example 4

Alternating current with a current density of 1.5 A / cm ² was fed through molybdenum electrodes into the molten glass-melted glass melt at 1400 ° C. By reducing the frequency from 52.5 to 14.5 Hz, electrode corrosion decreased by 22%. A frequency of 14.5 Hz represents the minimum corrosion for a given type of glass.

Claims (2)

A method of directly heating an ionic melt by passing an alternating current introduced into the melt by means of electrodes, characterized in that an alternating current of less than 50 Hz is introduced into the metal electrodes. 2. Method according to claim 1, characterized in that an alternating current of 5 Hz to 45 Hz is introduced.