Bilag: BAT checkliste

Brancheorientering for varmforzinkning

BAT-cheklisten gennemgår punkt for punkt de enkelte BAT-løsninger fra BREF-dokumentet proces for proces. I kolonnen "Specification of BAT" står helt præcis angivet, hvad BREF-dokumentet forlanger af den konkrete BAT-løsning. Under COWI's kommentarer er angivet nogle praktiske og vejledende kommentarer bl.a. med udgangspunkt i typiske danske produktionsforhold.

BAT issue	BAT description	BREF reference no.	Specification of BAT	COWI comments
Degreasing
General	Installation of a degreasing step, unless items are totally grease-free which is rarely the case in job galvanizing	C.4.2.1	Customers should reduce oil coating of items	For job galvanizing it is difficult to control oil coating. Sometimes it is sufficient to include degreasing chemicals in the pickling acid.
Optimum operation	Optimum bath operation to enhance efficiency, e.g. by agitation	C.4.2.2	1. Monitor and control of bath chemistry.
			2. Movement of items or liquid.
			3. Use cascade degreasing
Extend lifetime of degreasing bath	Cleaning of degreasing solution to extend lifetime (by skimming, centrifuge, etc.) and recirculation; reuse of oily sludge, e.g. thermally	C.4.2.3, C.4.2.5, C.4.2.6	Removal of oil, grease and sludge can be done in several ways:	Treatment of spent degreasing baths is also an issue for consideration
		D.4.3.1	1. Settling and skimming
		D.4.3.1	2. Separation by centrifuge
		D.3.2.6, D.4.3.4,	3. Separation by ultra- or micro-filtration
Biological degreasing	Biological degreasing with in situ cleaning (grease and oil removal from degreaser solution) by bacteria	C.4.2.4	Grease and oil are degraded continuously by micro-organisms. Sludge is separated and removed. Chemicals, micro-organisms and water are added.	Biological degreasing is an alternative to 2 and 3
Pickling and stripping
General:	Use separate vessels for pickling and stripping	C.4.3.6		To facilitate regeneration of spent acid
Optimum pickling parameters	Close monitoring of bath temperature and concentration parameters and operating within the limits given in Part D/Chapter D.6.1 "Open Pickling Bath Operation".	C.4.3.1, C.4.3.3, D.6.1	Pickling time is depending of acid and iron concentration and temperature. By gradually replacing pickling acid with fresh acid uniform pickling conditions can be maintained	By activated pickling free HCl is 4 - 6% and iron = 120 - 180 g/l.
Extraction from pickling tank	If operation outside the operational range given in D.6.1 is desired, e.g. if heated or higher concentrated HCl-baths are used, installation of an extraction unit and treatment of the extracted air (e.g. by scrubbing) are considered BAT. The associated HCl emission level is 2 - 30 mg/Nm³.	C.4.3.12, C.4.3.13, D.5.1, D.5.2, D.5.3, D.5.4
Use inhibitors	Special attention to the actual pickling effect of the bath and use of pickling inhibitors to avoid over-pickling	C.4.3.2	Can reduce acid attack on the item surface and this way reduce acid consumption and generation of spent pickling acid	Especially important to use where pickling time is long and out of control
Recovery of acid	Recovery of free acid fraction from spent pickle liquor	C.4.3.4.1, D.5.10.5	The following methods are referred:	Alternative to no.5
		D.5.9.2	1. Evaporation recovery of HCl	Evaporation and condensation
		D.5.9.3	2. Recovery of HCl by retardation	Ion-exchange technology
		D.5.9.4	3. Recovery of HCl by diffusion dialysis	Membrane technology
		C.4.3.5.1, D.5.10.1.1	4. Pyrohydrolysis (fluidised bed process)	Mainly external centralised treatment
		C.4.3.5.1, D.5.10.1.2	5. Pyrohydrolysis (spray roasting process)	Mainly external centralised treatment
		D.5.10.2	6. Electrolytic regeneration of HCl
External recovery	External regeneration of pickling liquor			Alternative to internal treatment (no.4). For regeneration of free acid or for preparation of FeCl3 precipitation agent
Zinc removal from mixed pickling liquor	Zn removal from acid	C.4.3.8, C.4.3.7	Extraction of zinc from mixed pickle liquor by an organic extractant (Tributyphosphate). Recovery of pure zinc chloride.	By increasing Fe:Zn ratio by dissolving iron swarft it may be possible to recover the spent acid at an external plant
Use spent pickle liquor for flux	Use spent pickle liquor for flux production	C.4.3.9.1	1. Oxidation, neutralisation af separation of iron. Reuse of liquid	Oxidation of iron by hydrogen peroxide and neutralisation by ammonia. Separation of iron sludge and reuse of liquid as flux.
		C.4.3.9.2	2. Zinc removal by ion-exchange. Regeneration and neutralisation. Reuse of liquid.	After ion-exchange the pickling bath can be adjusted and reused.
Rinsing
General	Good drainage between pre-treatment tanks is advocated. Furthermore, rinsing after degreasing and after pickling to avoid carry-over into subsequent process bath and thus to prolong the lifetime of these baths are essential. BAT is:	C.4.4
Static rinse and cascades	Static rinse or rinsing cascades	C.4.4.1	1. Reuse of static rinse for process bath
		C.4.4.2	2. Water saving technology with reuse potential
Reuse rinse water	Reuse of rinse water to replenish preceding process baths	C.4.4.1, C.4.4.2
Wastewater-free operation	Wastewater-free operation (wastewater may be generated in exceptional cases, in which wastewater treatment is then required)	C.4.4.1, C.4.4.2
Fluxing
Bath control	Control of flux bath parameters and the optimized amount of flux bath used to reduce emission.	C.4.5.1	Monitor and control concentration of flux salt (ZnCl₂ and NH₄Cl) and pollution with iron
Iron removal	Removal of iron to purify and maintain the flux bath properly	C.4.5.2	1. Aeration and precipitation of iron	Simple basic method
		C.4.5.3, D.7.1.1	2. Iron removal by oxidation with H₂O₂	Classical chemical method
		C.4.5.4, D.7.1.2	3. Iron removal by electrolytic oxidation	Electrolytic oxidation of iron
		C.4.5.5, D.7.1.3	4. Removal of iron by ion-exchange	Ion-exchange purification
		C.4.5.6, D.7.2	5. External regeneration of flux bath	External treatment
Hot dipping
General:	The main problem by hot dipping is emission of fume and dust generated from the flux by dipping the item into molten zinc
Emission reduction	Capture of emissions by enclosure of the pot or by lip extraction followed by dust abatement (e.g. by fabric filters or wet scrubbers). The dust level associated with this techniques is < 5 mg/Nm³.			By enclosing the pot a lower suction flow is needed. By lip-extraction a very high air-flow is needed.
		C.4.6.1	1. Enclosed galvanizing pot	Low suction flow saving energy
		C.4.6.2	2. Lip extraction at galvanizing pot	Air purification before discharge
		C.4.6.3	3. Smoke-reduced flux agent	Partly substitution of NH₄Cl by KCl
Reuse of dust	Internal or external reuse of collected dust for flux production. As this dust may occasionally contain dioxins at low concentrations due to upset conditions in the plant (badly degreased items being galvanised), only recovery processes yielding fluxing agents free of dioxins are BAT.	C.4.6.4	Dust will contain zinc chloride and ammonium chloride which can be regenerated by an external company.	Dust contains a lot of metallic zinc and need dissolution by HCl to prepare flux salt. For bigger amounts of dust regeneration can be done internal.
Energy saving
General:	It is good praxis to recover heat from flue gas from the kettle even the flue gas temperature is low (450 oC) and air volume is not very high. The heat can be recovered for hot water or for air drying.	C.4.6.8	Recovery of heat from flue gas by heat exchanger and transferred to air or water.
Saving	Better utilisation of heat and reduction of energy loss	C.4.6.9	Energy saving can be done by making the transfer of heat from the flue gas to the kettle correctly. Loss of heat can be reduced by covering the zinc surface when not use.
Zinc containing wastes
General:	All zinc containing wastes should be separately stored and protected from rain and wind. The wastes should be reused in the non-ferrous industry or recovered in other sectors.
Reduce dross	Hard zinc or dross should be reduced	C.4.6.5	1. Sufficient rinsing after pickling	All methods are based on reduction of iron impurities in the zinc kettle because iron react with zinc and generate dross
			2. Continuous regeneration of flux bath
			3. Use of fluxing agent with low NH₄Cl
			4. Avoid local over-heating of the kettle
Reduce splashes	Splashes should be reduced or recovered	C.4.6.6	1. Sufficient drying after fluxing
			2. Areas around the kettle must be kept clean to obtain recoverable zinc with a minimum of impurities
Reuse zinc ash	Ash contains 60-70% zinc which should be recovered	C.4.6.7	Use special melting pots for collection of ash. The pot is treated in a Zinkof-furnace at 520 °C by rotation and 60-65 % molten zinc is extracted and reused in the kettle. The residue containing ZnO is sold for external recovery