Patent GB-1025581-A (GB)
Title: Improvements in or relating to the manufacture of flat glass
IPC: C03B18/18
Filed: 1963-08-19
Abstract: <PICT:1025581/C1/1> In the manufacture of flat glass by the floatation technique such as described in Specification 893,663 there is maintained in the molten metal a trace of an additive metal with which an impurity in the molten metal preferentially reacts, this product of reaction then being removed from the molten metal by a molten layer of a compound maintained in contact with a zone of the molten metal, which takes up the reaction product.  A fresh surface of the compound is continually presented at the interface between the compound and the molten metal.  This is preferably done by making the molten layer a fluoride compound which when an electric current is applied to it causes the electrolytic release of the impurity which is then removed from the molten layer. Pockets 31 of the molten metal bath are formed in the side walls of the tank. The wall 6 forms a partition extending down into the molten metal to define a channel 34 connecting the pocket 31 to the bath.  The pocket 31 is enclosed and a protective atmosphere is maintained therein through duct 38.  A layer 39 of a fluoride of an additive element is confined in the pocket on the surface of the molten metal.  Any dross formed rises to the surface and into the layer 39.  By passing a direct current through an electrode 40 in the layer 39 and an electrode 42 in the molten metal such that the molten metal becomes the negative electrode, electrolysis is set up which releases the impurity from the additive element in the layer 39 and allows the additive element to be returned to the molten metal bath.  To maintain the layer 39 molten at the cold end of the tank auxiliary electrodes may be inserted in the layer 39 and connected to an A.C. supply. The current passing between the electrodes 40, 41 is controlled at 100 amps and 5 volts.  In an alternative embodiment, Fig. 4 (not shown), the molten layer 67 is confined within a carbon frame 68 having a thin inner lining 69 of a non-porous refractory.  A graphite electrode 70 passes through the pocket roof into the layer 67 and a connector bar 71, Fig. 5 (not shown), is mounted in an exit channel 80 of the pocket. These are connected to a D.C. source.  Auxiliary electrodes 72, 73 are provided to maintain the layer 67 molten.  The molten metal containing dross is caused to enter the pocket through a channel 79. To aid in the removal of dross an upward circulation of the molten metal to the interface is caused by an induction heater 76 having two legs 77, 78, through which the molten metal flows, mounted in the floor of the pocket.  To help the circulation through the pocket a paddle-wheel 81 is mounted in the exit channel 80.  If a non-electrolytic method is used such as with a molten layer of lithium potassium chloride or sodium potassium chloride or calcium barium chloride, the molten layer having taken up the dross may be decanted off, the impurity removed and the compound returned to the molten layer.  The additive element is preferably selected from the group lithium, sodium, potassium, calcium, magnesium, strontium, barium, cerium, manganese, iron, zinc, boron, aluminium, silicon, titanium, zirconium, niobium and tantalum.  Most preferably it is a fluoride compound of one or a mixture of these.