This is decided by the shape of article to be galvanized. It should be possible to  immerse and  withdraw  articles without much agitation of  the  bath.  The pans for wire, tube, strip, fasteners, have suitable designs to take care of this factor

Mahajan, R C

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SPELTER PAN - DESIGN, HEATING SYSTEMS AND OPERATIONSR.C. MahajanGeneral Manager (Works), ISWP, Lidranagar,Jamshedpur 831008The design of spelter pan depends on:a. cubic content and shape h. heating areac. thickness of wall d. material of pane. welding electrodes and method of weldingCubic content and shape of panThis is decided by the shape of article to be galvanized. It should be possible to immerse andwithdraw articles without much agitation of the bath. The pans for wire, tube, strip, fasteners,have suitable designs to take care of this factor.The galvanizing bath works like a heat container. The articles being immersed will draw heatto raise itself to galvanizing temperature and the bath should be able to supply this heat withoutmuch fall in temperature. As a thumb rule, the pan should contain 20 times zinc than thequantity of iron goods galvanized per hour, minimum, but in many high temperature galvanizingit may be as high as 40 times. The shape of tank should he such that no pockets should beformed, as such, round cornered tanks are preferred over right angled cornered tanks whichaccumulate dross and are causes of local heating and resulting higher dissolutions and reductionin wall thickness. Fig. 1 shows such design with round corners & welding away from the corners.Heating areaThe galvanizing bath should not be heated from bottom and generally heated from sides. Inorder to have small difference in temperature on outside and inside temperature and to supplyenough heat input, large surface area of the pan is required which means deep tanks. Also theexposed zinc surface of the tank should he as small as possible to avoid formation of ash andheat loss.Wall ThicknessThe real calculations of the wall thickness is very complex. It involves temperature of outsidewall, the hydrostatic pressure of molten zinc which is practically unsupported from sides and isjust lying on the refractory base. At temperature around 500°C the permanent resistance is 4.510Kg /mm` for most of the silicon free iron plates. The tanks mechanical strength can be calculatedby standard structural formulae considering it as freely supported beam. In order to compensatefor dissolution of iron in zinc, additional allowances are made. The wall thickness less than 20nt/m and more than 50 lm/m are not recommended.MaterialMaterial of the tank is selected by consideration of dissolution of iron in zinc at the galvanizingtemperature. The phenomenon of dissolution of iron is complex and depends on various factors:* Temperature of zinc* chemical composition of iron to he galvanized* iron pot and the accessories like sinker, rolls, etc. immersed in zinc* various alloying elements in the bathComplex zinc iron alloy is formed by dissolution of iron in zinc known as dross, which is 94%of zinc and 6% of iron. The iron comes from:* Dissolution of iron from kettle walls* iron salts from pickling* reaction of molten zinc with iron* dissolution of fixtures like sinker rollsSlides 2 to 5 show the various factors affecting the dissolution of iron in zinc. Figures 2,3 and4 show the effect of bath temperature. Please note that the maximum rate of dissolution is at500°C. Figure 5 shows effect of presence of various metals in the bath. Nearly pure iron is usedfor manufacture of galvanizing pan: silicon, carbon and phosphorus accelerate the rate ofdissolution of iron in molten zinc. Highly refined iron with tine grains is used. Grain size lessthan 5 microns is recommended. Figure 6 shows the typical steel chemistry and IS 8917recommendation of steel chemistry.Method of weldingThe tank should he designed to avoid any blind corners and the welding should he confined tostraight portions away from the corners. Figure I shows one such design. The preparation ofweld joints is very important and should be made in consultation with electrode manufacturingcompanies. Figure 7 and 8 show typical electrodes used for welding of galvanizing pans.INI'T'IAL Il1?A'1'INGS AND GOOD OI'EIZA'I'ING PRACTICESThe pan when installed should he charged with lead plates touching the walls of the pan as faras possible to avoid initial local heating. Also, before tiring, the pot is filled with water alongItwith lead and zinc slabs to at least do uniform heating upto 100"C. The drossing and cleaningis very important to avoid hard deposits and also to avoid further formation of dross. The ironcontent of flux tank should be controlled at low level, beneficial metals like Al can he added.Intermittent heating and cooling will shorten the pot life. The tanks should he utilized to itsmaximum capacity. It is not only necessary to keep it hot for 24 hrs, but material should passthrough it continuously.HEATING SYSTEMSSince the inception of galvanizing, various heating media have been used, some with success, andconstant development is being done to achieve the optimum results. Various energy sources usedare :* pulverized coal* producer gas, coke oven gas, natural gas* liquid fuelselectricity, coil heating and induction* immersion burnersFigure 10 to 19 show the different heating arrangements.HIGH TEMPERATURE GALVANIZINGGalvanizing of structural work cast iron fittings and fasteners can only he carried satisfactorilyat temperatures of 550-560°C. As such kettles for this type of galvanizing cannot he made fromsteel plates since it will result in excessive dross. formation and very short kettle life. Thereforekettles for high temperature galvanizing have to ceramic lined and cannot he externally firedbecause of poor conductivity of ceramics. Till recently, only method of heating such ceramicbaths was by top heating. Figure 20 shows one such design. The heat transfer is only by surfaceof zinc, as such you need a large exposed area of zinc bath resulting in poor energy efficiencyand formation of large quantities of ash which is not only wasted but also spoil the surface ofthe galvanizing articles during withdrawing. To overcome this drawback, immersion typerecuperative heaters have been developed as shown in figures 13 and 14. Recently work is beingdone on floating burners as shown in figure 21.CONCLUSIONSProperly designed spelter pans , proper tiring system and correct operating practice will not onlyprolong the life of the pan but will also reduce the formation of dross and ash.I2.

Spelter Pan - Design, Heating Systems and Operations

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Spelter Pan - Design, Heating Systems and Operations

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