WO2005082797A1

WO2005082797A1 - Method for removing impurities from molten tin

Info

Publication number: WO2005082797A1
Application number: PCT/GB2005/000702
Authority: WO
Inventors: Paul Arthur Holmes
Original assignee: Pilkington Plc
Priority date: 2004-02-27
Filing date: 2005-02-25
Publication date: 2005-09-09

Abstract

Disclosed are methods whereby sulphur and/or oxygen impurities are removed from molten tin electrolytically; this technique is useful for reducing contamination of the molten tin used in the float bath of a glass manufacturing process, leading to a reduction in faults in the glass surface due to ‘tin speck’ and an increase in glass yield. The contaminated tin may be treated within the float bath, or removed from the float bath, electrolysed and returned to the bath.

Description

METHOD FOR REMOVING IMPURITIES FROM MOLTEN TIN

Technical Field

The present invention relates to the manufacture of glass. In particular it relates to the manufacture of float glass and improving its quality by removing contaminants from the molten metal used in float glass manufacture. The term "float glass" refers to the glass in sheet or ribbon form which is manufactured by the float glass process, where batch materials are melted in a tank to form molten glass which is subsequently presented to a forming means (a bath containing a molten metal, commonly tin, upon which the molten glass floats) where the glass is formed into a solid ribbon. The term "ribbon" refers to the uninterrupted strip of glass having major surfaces which are substantially parallel continually produced in the float glass process. On leaving the bath the ribbon is annealed (i:e. cooled in a controlled manner) and, normally, cut into sheets.

Background Art

The presence of contaminants in the molten tin may cause problems in the manufacture of glass. Two particular contaminants, oxygen and sulphur, are known to be responsible for a problem known as "tin drip". Oxygen may be present in the molten tin due to leakage of atmospheric air into the bath atmosphere caused by incomplete sealing of the bath, or by migrating from the glass ribbon. Sulphur is usually present by migrating from the glass ribbon. If oxygen or sulphur is present in the molten tin, tin sublimes as SnO or SnS at temperatures greater than 850°C and can deposit on the bath ceiling towards the (relatively) cold end of the bath (~ 600°C). The SnO and SnS are reduced by hydrogen present in the bath atmosphere to metallic tin which drips onto the glass ribbon (thus "tin-drip") which causes defects and leads to reduced yield. Attempts have been made to reduce glass loss from tin drip. GB 1025581, US3330635 and US3330637 each disclose a method of removing oxygen and/or sulphur from molten tin by addition to the molten tin of an additive element with which the oxygen and/or sulphur reacts preferentially. Resulting reaction product is removed from the molten metal by take up in a molten layer of a compound maintained in contact with the molten metal. US3494755 discloses a method and apparatus which prevents tin drips from contacting the glass ribbon. The apparatus comprises a collecting surface located towards the cold end of the bath and positioned between the ceiling and the ribbon. Tin drips from the ceiling deposit on the collecting surface and are returned to the bath. GB2235447 discloses a method and apparatus similar to that of US3494755 in which the collecting surface is made from carbon fibre composite material having specific properties. Whilst these prior known methods appear to work well, the provision of additional apparatus above the ribbon is cumbersome and other methods of reducing yield loss from tin drip are desirable.

Disclosure of the Invention

An article published in the Journal of Applied Electrochemistry 31:155-164, 2001 entitled "Cathodic refining in molten salts: Removal of oxygen sulphur and selenium from static and flowing molten copper", discloses a method and apparatus for removing non-metallic impurities from molten copper. We have realised that this method and apparatus may be modified and used in the manufacture of float glass to at least reduce the formation of metallic tin on the bath ceiling, thereby alleviating the problem of tin drip, and reducing yield loss. According to an aspect of the invention there is provided as a method of removing oxygen and/or sulphur contamination from molten tin by treating the molten tin by a process of electrolysis not reliant on the addition of additive elements to the molten tin. According to a further aspect of the invention there is provided a method of treating molten tin used in float glass production to reduce the amount of oxygen and/or sulphur contamination therein, the method comprising the step of subjecting the molten tin to a process of electrolysis not reliant on addition of additive elements to the molten tin. This method removes oxygen and/or sulphur from the molten tin which in turn reduces the volatilisation of SnO and/or SnS which causes tin drip. In each case, the method is normally carried out without the addition of any additive element to the molten tin. Preferably the contaminated tin is removed from the float bath in which it is contained, treated by a process of electrolysis and then returned to the bath. According to a still further aspect of the invention, there is provided a method of manufacturing float glass including the aforementioned method of removing oxygen and/or sulphur contaminants from molten tin used in the float bath. In a preferred method of operating the invention, the molten tin is in electrolytic contact with a cathode and a molten conductive salt connected to an anode.

Brief Description of the Drawings

Embodiments of the invention will now be described with reference to the accompanying drawings in which: - Figure 1 is a schematic cross section through an apparatus used to demonstrate the electrolytic removal of sulphur from molten tin in accordance with the invention. Figure 2 is a schematic plan view of part of the bath used in a float glass manufacturing operation, incorporating apparatus to treat the molten metal according to the present invention. Figure 3 is a schematic side section of an electrolytic cell which includes a recessed channel electrode.

Modes for Carrying Out the Invention

Figure 1 shows a tube reactor 1 constructed of Inconel™ alloy, approximately 650mm in length, 110mm internal diameter with a 5mm wall thickness, with lid 2. An O ring 3 provides a gas tight seal between reactor and lid, with a duct 4 around the neck of the reactor providing for water cooling of the O ring to protect it from overheating. A graphite crucible 5 sits on a ceramic brick 14 at the base of the reactor and contains the layer of molten tin 6 to be purified and a layer of molten salt 7 electrolyte over the molten tin. Graphite electrodes, an anode 8 and a cathode 9 extend through rubber bungs 10 in the Inconel™ lid; the anode 8 terminates in the molten salt layer 7, while cathode 9 is shown insulated by a surrounding alumina sleeve 11 where it passes through the molten salt layer and terminating in the molten tin. An inlet duct 12 and an outlet duct 13 are provided for circulation of inert atmosphere through the reactor. The apparatus was used to treat molten tin containing sulphur and oxygen contaminants removed from the float bath of a float glass manufacturing line. A sample of molten tin was extracted from the tin bath of an operating float glass line into a fused silica tube of about 1cm internal diameter. The tin was allowed to cool to room temperature inside the tube and then removed and immediately cut into aliquots of approximately 1cm in length, discarding the end pieces.

Samples of this tin were then purified to remove sulphur and oxygen using the apparatus shown in Figure 1, with a molten salt electrolyte 7. A wide range of salt electrolytes are available, including calcium, strontium and barium chlorides. The important features are that the electrolytes should be molten at the temperature of electrolysis and should be dried rigorously to exclude water. As shown in Table 1, two electrolytes were used, pure calcium chloride (melting point 770°C) and a eutectic mixture of calcium and strontium chlorides (melting point 600°C).

The apparatus shown in Figure 1 was initially set up so that both carbon electrodes were immersed in the salt electrolyte but not touching the tin sample. The Inconel™ reactor was placed in a vertical furnace and brought to the required temperature. The reactor had been flushed out with dry argon and dry argon was passed through the apparatus via inlet duct 12 and outlet duct 13 during the entire procedure.

In the first experiment, a eutectic mixture of calcium and strontium chlorides used as molten electrolyte was first subjected to a pre-electrolysis procedure to remove any water in the salt. At all times the voltage applied to the electrodes was kept well below that required to decompose the salt (approx. 3.2V). The pre-electrolysis involved applying a voltage of 2N with both graphite electrodes dipping into the molten salt but not touching the molten tin surface. When the current had stabilised, the power was removed and the cathode lowered into contact with the molten tin (as shown in Figure 1), with the alumina sleeve preventing contact between the cathode and the electrolyte. The voltage indicated in Table 1 was then applied between cathode dipping into the molten tin and anode dipping into the molten electrolyte. The initial current flowing declined rapidly to a few hundred milliamps after 1 hour and remained fairly constant thereafter. In order to ensure that the electrolysis had gone to completion, the voltage was applied for four hours before allowing the apparatus to cool to room temperature under the flow of dry argon.

The procedure was repeated with a second sample (sample 2 in Table 1), and then, using a calcium chloride electrolyte in place of the eutectic mixture, with three further samples, samples 3, 4 and 5.

The treated tin samples were then removed and sent, together with a sample of the original untreated tin, for analysis of the sulphur content. The analyses were conducted by the Bundesanstalt fur Material forschung und -priifund in Berlin under the supervision of Dr Heinrich Kipphardt.

For sulphur in tin analysis, a sample of approximately 660mg was washed in methanol and mixed with ~ lg of tungsten flux in an Eltra CS800 instrument that had been calibrated using barium sulphate as a standard and flux only as a blank. The limit of detection (9 σ) was 5.2 mg/kg and the results of duplicate measurements on each tin sample are given in Table 1.

The sulphur in tin data in Table 1 are unequivocal and show that the electrolysis procedure reduced the sulphur concentration from about 30 mg/kg to below the limit of detection of the method (5.2 mg/kg) with many of the individual results being close to zero. The level of oxygen impurity in the original untreated tin were very low, and no conclusive evidence proving the reduction of the oxygen content by electrolysis was obtained. However, it is to be anticipated that the oxygen content would be reduced, in a similar way to the sulphur content, by electrolysis. TABLE 1

Figure 2 shows a bath 20 used in a process of manufacturing float glass. The bath 20 contains molten tin 22 and a ribbon of glass 24 which floats on the molten tin 22. An electrolysis apparatus, generally designated 26, is associated with the bath to remove contaminants from the molten tin contained in the bath. A suitable electrolysis apparatus is disclosed in EP0272803, the disclosure of which is incorporated herein by reference. The apparatus of EP0272803, known as a recessed channel electrode (RCE), allows for electrolysis of a flowing molten metal and thus contaminated molten tin from the bath can be removed therefrom and treated in an RCE and the treated tin can be subsequently returned to the bath. The electrolytic cell shown in Figure 3 is similar to that disclosed in EP0272803 and has a cathode 30 associated with an anode 32 and a diaphragm 34. The cathode 30 contains molten electrolyte 36 which may be a eutectic mixture of BaCl₂ and CaCl₂ whose melting point is about 600°C. Contaminated molten tin is removed from the bath and trickled through the channels of the cathode. Molten tin in the cathode is depicted as reference numeral 38. The electrolytic action of the anode and the cathode oxidises the oxygen and sulphur contained in the molten tin as contaminants at the anode and tin is reduced at the cathode- at the anode: O^ [0] + 2e- S + 2e^" at the cathode: Sn + 2e^~ — > Sn

The atomic oxygen and sulphur generated at the carbon anode can undergo further chemical reaction, for example to carbon dioxide and sulphur dioxide. The result of this process is that the oxygen and sulphur contaminants are removed from the molten tin which is now returned to the bath. It will be appreciated that the contaminated tin to be treated may be removed from any point along the bath from the hot end (~1000°C) to the cold end (~ 600°C). It will also be appreciated that the treated tin may need to be heated before it is returned to the bath to reduce the temperature gradient between the molten tin in the bath and the returning treated tin. Moreover, while the invention has been specifically described with reference to embodiments in which the tin is removed from the float bath for purification, it will be appreciated that the invention is applicable to the treatment of tin within the float bath, e.g. in a pocket at the side of the bath as described in GB1025581, or even within the body of the bath, when, however, it may be necessary to provide means for removing oxygen and or sulphur impurities released by electrolysis from the bath.

Claims

1 A method of removing oxygen and/or sulphur contamination from molten tin by treating the molten tin by a process of electrolysis not reliant on the addition of additive elements to the molten tin.

2 A method of treating molten tin used in float glass production to reduce the amount of oxygen and/or sulphur contamination therein, the method comprising the step of subjecting the molten tin to a process of electrolysis not reliant on the addition of additive elements to the molten tin.

3 A method as claimed in claim 2 wherein the contaminated tin is removed from the bath in which it is contained, treated by a process of electrolysis and then returned to the bath.

4. A method as claimed in any of the preceding claims carried out without the addition of additive elements to the molten tin.

5. A method as claimed in any of the preceding claims wherein the molten tin is in electrolytic contact with a cathode and a molten conductive salt connected as an anode.

6. A method of removing oxygen and/or sulphur contamination from molten tin by treating the molten tin by a process of electrolysis without the addition of additive elements to the molten tin.

7. A method of treating molten tin used in float glass production to reduce the amount of oxygen and/or sulphur contamination therein, the method comprising the step of subjecting the molten tin to a process of electrolysis without the addition of additive elements to the molten tin.

8. A method of manufacturing float glass including the method of any of the preceding claims 1 to 7.