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5.3 What to do during a long idling period
The heat loss from a clear zinc surface is about 54000 kJ/m2h (15 kW/m2). Therefore steps must be taken to decrease these losses.
When production stops, during weekends and public holidays the entire surface of the zinc should be covered by an insulating cover. This cover must lay close to the kettle edge and if sufficiently insulated will reduce the heat losses great ly.
With fossil fuel fired furnaces, with heat• retaining brickwork, a check must be made to ensure that the zinc temperature does not rise too much when the zinc surface is covered. When this is the case the procedure of chapter 5.2.2 must be followed. It has often been said that energy can be saved by lowering the zinc temperature during long stops, this is not correct. If.the zinc surface is adequately covered the necessary heat to maintain the zinc temperature at the same level is far less than• the heat required to heat up the zinc again from 430 deg C to the operating temperature. By lowering the zinc temperature the solubility of iron in zinc decreases too. Fine zeta crystals are formed which do not dissolve when the temperature is raised again and they remain floating in the liquid zinc? causing a roughening of the zinc surface on the articles. Therefore a lowering of the
zinc temperature is always combined with a higher dross production.
When the heating system has to be switched off, and with it the corresponding control and alarm system, the zinc temperature will decrease constantly. At 420 deg C the zinc becomes solid. It is important and necessary to carry out the next heating up procedure carefully and very slowly to avoid temperature differences in the kettle wall of more than 50 deg c. Heating• should be carried out according to chapter 4.4
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5.4 Addition of zinc
Depending upon the zinc consumption the equivalent amount of zinc must be added to the kettle after every shift or after every two shifts. It is advantageous to add small'amounts of zinc. Small additions can be carried out by hand, bigger quantities must be carried out with the help of a crane or other suitable equipment.
If more than one load of zinc blocks must be added at the same time, they should be distributed over the whole length of the kettle.
5.5 Addition of aluminium
Aluminium should be added regularly to the bath as solid aluminium or preferably as a 10% aluminium-zinc alloy. It retards the zinc ash production and gives the zinc surface a shiny appearance. The maximum content in the zinc should not exceed
0,02%. Too little aluminium in the zinc gives the surface of the galvanized articles a yellowish hue.
5.6 Addition of lead
The maximum solubility of lead in zinc is 1.2% at 450 deg C. Further addition of lead to a zinc melt already saturated with it leads to sinking Of the superfluous lead to the bottom of the bath, because of the higher specific gravity of the lead over zinc. When prime western or remelted zinc is used no lead should be added. The portion of lead in this zinc is sometimes higher than the above mentioned solubility. When pure zinc is used lead must be added to retain a minimum lead cOntent of at least 0.6%, otherwise the zeta crystals do not settle properly on the bottom of the bath and remain floating. Lead is difficult to dissolve in liquid zinc. When lead blocks are put into the zinc, they sink directly to the bottom of the bath and melt without dissolving in it. The lead seeps through the dross layer and accumulates in the lead layer present under the dross layer. Lead should therefore be added regularly to the zinc in a fine divided form. However, some lead is dissolved in the zinc during the dressing operation.