No_4_November_and_December__1952 — Page 60

Fig. 3.

are its corrosive and unpleasant fumes, its uncontrollable pickling speed and the pro- blem of dumping strong factory wastes.

A 10--12 per cent solution of sulphuric acid at a thermostatically controlled tem- perature of 140-150 deg. F. is used which gives a constant pickling rate of 15-20 minutes.

The work is hung up by a corner on specially designed hooks (Fig. 2) and pro- ceeds by means of drop hoists and monorail (Fig. 1)

The acid contains a small percentage of inhibitor to prevent pickling the basis metal, thus ensuring that only scale is removed. No wetting agent is used, for the compressed air agitation would cause frothing.

Tank Details.

The tanks, of special construction (Ac- crington acid resisting tiles and cement) are heated by steam coils made of tellurium lead of special design. The coils are closed and the steam condensate passes into the wash tanks.

The acid used is 95 per cent. C.O.V. (Concentrated oil of vitrol) and is stored in a mild steel boiler shell. At this strength the sulphuric acid has no action on mild steel. A tanker comes in and pumps up 5 or 10 tons each week to the storage tanks. 100 gal. are run in as required (this is con- venient, as 100 gal. C.O.V. is equivalent to 1 per cent increase in strength on the 20,000 gal. pickle tank.) The iron figure is kept constant by using a Kestner crystalliser.

This crystalliser works on the following principle: 500 gal. of pickle liquor are drawn up into a chamber agitated by an electrically driven paddle. The sides of the chamber are lined with lead cooling coils through which mains water passes. The cooling pickle liquor deposits fine crystals of copperas-FeSO4, 7H 2 O- when the temperature drops from 150 dg. F. to 60 deg. F., i.e. two or three hours, depending on the time of the year the load is filtered. The mother liquor goes back to the pickle tank and the crystals are shovelled down a chute to the ground floor. A charge normally makes 5 cwt, and an output of 300 tons of steel per week will produce some 15 tons of crystals.

After pickling the loads go through a hot wash (about 100 deg. F. and heated by the condensate from pickle coils) and then

through a cold wash. All the wash waters from six tanks (dealing with some 300 tons/week) flow into a

water treatment

system. The water flow is about 600 gal. per hr. and has a PH of 2-3 and an iron and zinc content of about 50 parts/100,000. A stream of lime slurry, made in a mixer by adding 1 cwt. lime to 200 gal. of water, is continuously added and compressed air is bubbled in to oxidise the iron. The brown sludge passes to settling tanks. The clear water is pumped back to the wash tanks at a PH of 8-10 and an iron and zinc content of less than 10 parts per million. The sludge is pumped out and dumped.

After washing the work is fluxed (Fig. 4) by dipping in a 30 per cent solution at 120 deg. F. of zinc ammonium chloride incor- porating a wetting agent (No. 20 Flux) and is then dried.

Drying (Fig. 3) is carried out with large quantities of slightly warm clean air so that the dried work never exceeds 180 deg. F. This is most important, for if it did, a reaction would take place between the zinc chloride and basis iron, leading to excessive dross formation. The heat for the oven comes from the combustion products of the bath using a heat exchanger and the can produce fifty air changes per minute.

oven

Heating Galvanizing Pot

The galvanizing pot is heated by pro- ducer gas at the rate of 13,500 c. ft. per hr. (C. V. normal 120-140 B. Th. U. per c. ft.). The spelter baths in question measure 10 ft. long by 5 ft. wide by 9 ft. deep and are constructed from 2 in. thick special quality steel plate. The capacity of each bath is about 85 tons of molten zinc. Each bath is surrounded by a refractory lined setting and this refractory lining is in turn sur- rounded by thick insulation to minimise heat losses, the whole being contained in a heavy mild steel casing. An extension at one end of the setting contains the com- bustion chamber, the recirculating fan- housing and the necessary collecting ducts.

The flow of heating gases around the bath allows the high temperature gases from the fan to pass around the upper half of the bath and return around the lower half before being collected and passed through the combustion chamber, where they are mixed with high temperature products of combustion before being again

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Fig. 4.

recirculated round the bath. As it is necessary to "bleed off" a proportion of the hot gases from the system advantage is taken of this by utilising these waste gases to heat the drying ovens. The system of heating described has been found to be most effective and fulfils all the require- ments found to be essential in heating galvanizing baths, particularly since each bath is under automatic temperature control both with regard to zinc temperature and the maximum permissible temperature in the hot gas flues. The combustion was installed by British Furnaces Ltd.

A CO/CO 2 recorder exists to make sure combustion is complete and the burner has a safety device to prevent it going out. The temperature of the pot is controlled by a Honeywell Brown recorder with two- point control; one in the flues and one in the zinc. The pot is made of 2 in. mild steel, and the temperature range is 450-460 deg. C. with a 454 deg. C. constant figure. The metal used is Prime Western Spelter quality or its English equivalent. Lead is added to maintain a 6 in. to 9 in. layer on the bottom. No addition of tin, antimony, are permitted. The work is dipped by drop hoist with a downward speed say, of 18 ft. per min. which can be modified to suit production. The work remains in the pot long enough to reach the required temperature and is then removed through a clean skimmed surface. The window is then allowed to cool somewhat to about 200-300 deg. C. before it is "etched".

This latter process is necessary because fresh galvanizing will not take normal decorative paints. It is true that the weather will ultimately roughen galvani- zing, but this is a very variable factor in Great Britain from a week or two in humid heavy industrial atmospheres or marine atmospheres to several weeks in drier rural

areas.

To avoid this variation and also the use of copper sulphate type mordants which have been used on site, and which spoil the corrosion resistance of the coating, all windows are pretreated by dipping the galvanized coating whilst still hot in a weak solution of phosphoric acid/phos- phates. This is supplied by Imperial Chemical Industries, Ltd., and the Pyrene Co., Ltd.

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