Treating the surfaces of plastics
2 APPLIED PROCESSES AND TECHNIQUES
2.9 Continuous coil – large scale steel
2.9.8 Continuous electrolytic zinc or zinc nickel plating activities
This is the electrolytic deposition of a thin layer of pure or alloyed zinc onto the surface of a steel strip substrate.
On continuous electrolytic zinc plating line, the cold rolled, annealed and tempered steel strip is passed continuously through degreasing and pickling pretreatments, then through a series of electrolytic cells containing a zinc electrolyte. It is then passed through one or more post-treatments, either layer conversion such as chromating or phosphating, or through an oiling step.
Plating lines layout may vary in design. As an example, Figure 2.15 shows a typical layout of a continuous electrolytic zinc plating line.
Figure 2.15: Schematic of an electrolytic zinc coating line layout
2.9.8.1 Entry equipment See Section 2.9.1.
2.9.8.2 Degreasing
Degreasing of steel strip surface is by means of both chemical action (alkaline agent) and mechanical action (spray and brushes).
The cleaning process involves the following steps:
• alkaline degreasing via immersion or by spraying. This may be coupled with brushing
• rinsing with water and brushing between intermediate rinsing tanks
• drying.
The degreasing and rinsing sections may be in horizontal or vertical tanks.
Chapter 2
PT/EIPPCB/STM_BREF_FINAL September 2005 91 Alkali degreasing
A typical horizontal spray degreasing section for the treatment of oiled coils consists of three successive stages:
• stage 1: spray nozzles and brushes
• stage 2: spray nozzles
• stage 3: spray nozzles and brushes.
Cascade degreasing has the advantage of achieving a high cleaning rate using only a small amount of degreasing solution. Degreasing is performed in counter-flow direction to the steel coil movement, which means that clean degreasing solution is used in the third (final) stage where it becomes slightly contaminated. This degreasing agent is used for cleaning in the second (more contaminated) stage and subsequently in the first (most contaminated) degreasing stage. The degreasing agent overflows between any two stages are controlled by means of wring rollers.
Section 2.9.3.1 describes typical chemical degreasing systems. In steel coil coating, the degreasing agent is usually an alkaline containing phosphate (a non-siliceous solution) containing mainly sodium hydroxides, orthophosphates and surfactant compounds with a concentration of 5 to 40 g/l, a temperature between 60 and 85 °C and a pH of about 13. The degreasing solution may be used first in the electrolytic degreasing section and be moved to the chemical degreasing stage to replace the spent chemical degreaser.
Environmental considerations
The use of cascade degreasing minimises the amount of raw materials and water used.
When the cleaning solution has reached the maximum oil content it is regenerated by centrifuges.
The solution may be used first in the electrolytic degreasing section.
Fumes generated in degreasing and brushing are normally collected, scrubbed and treated prior to release.
Spent degreasing agent is sent to waste water treatment before release.
The oily waste is treated off-site.
Figure 2.16: Spray degreasing and brushing
Chapter 2
92 September 2005 PT/EIPPCB/STM_BREF_FINAL Rinsing
See Section 2.4. The alkaline degreaser is completely removed by cascade rinsing in three stages. Cascade rinsing has the advantage of achieving a high cleaning rate using only a small amount of rinsing water. Rinsing is performed in a counter-flow direction to the movement of the steel strip: the clean water is used in the third stage where it consequently becomes contaminated. This slightly contaminated water is then used for cleaning in the second stage and subsequently in the previous rinsing stage. The water overflows between two stages are controlled by means of wring rollers. The rinsing equipment involves the following steps:
• stage 1: spray nozzles and brushes
• stage 2: spray nozzles
• stage 3: spray nozzles.
The process solution in the first stage is demineralised water at a temperature of 70 to 85 °C.
Quality control techniques are used to determine the oil content in the water which gives the required standard of cleaning with minimum water usage. During processing, when the oil content in the water has reached this maximum allowed concentration, the water is sent to waste water treatment before discharge.
Environmental considerations
Countercurrent rinsing optimises process efficiency and minimises use of heated demineralised water.
Used water is treated to remove oil prior to discharge.
Water vapour and/or fumes generated in rinsing and brushing are collected and injected into the rinsing tanks.
Figure 2.17: Spray rinsing
2.9.8.3 Drying
The degreased strip is dried by means of a hot air drying device. The hot air temperature lies between 100 and 120 °C.
Environmental considerations Energy efficiency.
2.9.8.4 Entry looper and tension leveller See Section 2.9.1
Chapter 2
PT/EIPPCB/STM_BREF_FINAL September 2005 93 2.9.8.5 Electrolytic degreasing
The final degreasing of steel strip surface is carried out by means of chemical (alkaline agent) and mechanical (sprays and H2 and O2 gases bubbles generated by electrolysis at the strip surface) actions, see Section 2.3.8.
The electrolytic degreasing section involves the following steps carried in cells:
• alkaline degreasing by an electrolytic system
• rinsing with water, possibly coupled with brushing.
The degreasing and rinsing sections may be in horizontal or vertical tanks.
The electrolytic degreasing is carried out by the electrolysis of the alkaline solution releasing H2
gas at the cathode and O2 gas at the anode. The polarity of the electrodes is inverted after each coil or after a certain period in order to avoid polarisation. The solution drag-out to the next stage is controlled by means of wringer rollers.
The degreasing solution is usually the same as that used in the degreasing section. Once the solution has reached a certain oil level it is re-used on the degreasing section (cascade use). The operating temperature of the solution lies between 60 and 85 °C.
Environmental considerations
Re-use of the solution in the more contaminated degreasing section is possible.
Alkali fumes generated in electrolytic degreasing cells are usually collected, scrubbed and treated prior to release to maintain a healthy working environment and to prevent corrosion of equipment and substrates.
Figure 2.18: Electrolytic degreasing
Chapter 2
94 September 2005 PT/EIPPCB/STM_BREF_FINAL Rinsing
Cascade rinsing in two stages completely removes the alkaline solution. Cascade rinsing has the advantage of achieving a high cleaning rate using only a small amount of rinsing water. Rinsing is performed in the counter-flow direction (i.e. the water flows in the opposite direction to the coil). This means that clean water is used in the last stage where it consequently becomes contaminated. This slightly contaminated water is used for cleaning in the second stage and subsequently in the previous rinsing stage. The water drag-over between two stages are controlled by means of wringer rolls. The rinsing equipment involves the following steps:
• stage 1: spray nozzles and brushes
• stage 2: spray nozzles.
Demineralised water having a temperature of 70 to 85 °C is used in the first stage.
Environmental considerations
Use of countercurrent rinsing to minimise use of heated demineralised water.
Water vapour and alkali fumes generated in rinsing and brushing are usually collected and injected in the rinsing tanks.
When the water has reached the maximum allowed oil content commensurate with process quality, it is sent to waste water treatment before release.
2.9.8.6 Pickling
Pickling removes any oxides formed during the various stages of steel processing and prepares a reactive steel surface for the plating section. For further information, see [86, EIPPCB, ].
Coil pickling can be carried out by spraying, or immersion with or without electrolysis.
Typical pickling sections involve the following steps:
• stage 1: pickling
• stage 2: rinsing.
Spray pickling
Two types of pickling solution may be used:
• sulphuric acid at a concentration within a range of 10 to 60 g/l with a temperature range of 25 to 60 °C.
• hydrochloric acid at a concentration within a range of 100 to 150 g/l with a temperature range of 20 to 40 °C.
Both the temperature and the acid concentration are dependent on the available pickling time (which is a function of the contact length and the maximum strip speed).
Environmental considerations
Fumes generated in pickling are usually collected and scrubbed prior to release. In some lines, the contaminated water of the scrubber is treated together with the fumes from the electroplating cells in an evaporator and both the concentrate and the evaporated water are returned to the process.
Spent pickling solution is sent to waste water treatment before release.
Spray rinsing
The same system is used as for degreasing (see Section 2.9.8.5).
Chapter 2
PT/EIPPCB/STM_BREF_FINAL September 2005 95 Figure 2.19: Pickling section
2.9.8.7 Electroplating
In this section, thin deposits of pure or alloyed zinc are electrolytically deposited onto the surface of a steel strip substrate. A typical plating section involves the following steps:
• stage 1: plating through several electrolytic cells
• stage 2: rinsing.
Sections 2.9.4 discuss the process and describe types of electrolytic cell. Current density for various zinc and zinc alloy thicknesses, and the industries they are used in are set out in Table 2.1.
Types of electrolyte bath
Electrolyte baths in continuous electrolytic zinc plating lines are predominantly acid-based.
Alkaline-based electrolytes are no longer in common use. Electrolytes can be sulphate-based or chloride-based. The sulphate-based bath is used with soluble and insoluble anode processes.
The chloride-based bath is only used with soluble anodes as chlorine gas is generated with insoluble anodes. In both baths, ionic additions are made to increase the conductivity of the electrolyte bath (e.g. sodium sulphate, aluminium sulphate, sodium chloride). Buffer additions are made to stabilise the pH (e.g. CH3COONa).
Typical sulphuric base electrolytic bath compositions are:
• zinc 70 - 120 g/l
• free H2SO4 3 - 25 g/l
• Na2SO4 0 - 100 g/l
• pH 1.0 - 3.0 pH units.
Anodes types
These are generally described at the start of Chapter 2. For this process:
• soluble anodes: the zinc anodes are fixed on a supporting rail and, as they are consumed during the process, they are displaced from the entry side rail to the exit side rail
• insoluble anodes: two materials are used for the anode plate: a plate substrate in titanium coated with a thin layer of tantalum oxides or iridium oxides and a plate in lead alloyed with Sn or with Ag and In.
Chapter 2
96 September 2005 PT/EIPPCB/STM_BREF_FINAL Rinsing step
The coil is then rinsed to completely remove the electrolyte from the strip surface. Also, in some cases where the coil is coated only on one side, the rinse may be used to pickle the zinc residues on the non-coated side. Rinsing can be carried out by immersion or by cascade spraying, and in horizontal or vertical tanks depending on the available surface. The same cascade principles described in the degreasing section are applied here, see Section 2.9.3.1.
Demineralised or fresh water is used, with a temperature of between 20 – 50 °C and a pH 0.3 to 3 (among other control parameters).
When the water has reached the maximum allowed contaminant (electrolyte) content determined by quality control, it is sent for waste water treatment before discharge. In some lines, the contaminated water is treated in an evaporator. The evaporated water can be re-used for rinsing and the concentrate re-used in the process electrolyte.
Environmental considerations
Counter-flow cascade rinsing can maximise the rinse efficiency and minimise the use of hot demineralised water.
Re-use of the water outputs distilled from evaporators.
2.9.8.8 Strip polisher
The strip polisher is used to clean the tarnished/oxidised uncoated side of the strip and to remove any stray coating particles which may have been deposited. When producing single sided coated material, some lines omit the polishing technology.
A typical polishing section involves the following stages:
• stage 1: polishing
• stage 2: rinsing
• stage 3: drying.
Polishing
Brushes are used to polish the cold reduced surface on the uncoated side. Demineralised water is used. The metal particulates are filtered from the used water.
Environmental considerations
The used water is sent to waste water treatment before release.
Rinsing
The strip is passed through a hot water rinse to remove any debris, before being dried. Rinsing can be carried out by immersion or by cascade spraying, in horizontal or vertical tanks depending of the available surface. The same principles as described for the degreasing section are applied. See Section 2.9.8.2.
Environmental considerations
The used water is sent to waste water treatment before release.
Drying
The polished strip is dried by means of a hot air drying device. The hot air temperature lies between 100 and 120 °C. The drier device is usually situated at the end of the post-treatment;
the same type of device is used for the phosphate and the chromate sections. See Section 2.6.
Environmental considerations Heat efficiency and losses.
Chapter 2
PT/EIPPCB/STM_BREF_FINAL September 2005 97 2.9.8.9 Phosphating
This process forms a high quality light phosphate coat with the hopeite chemical structure which is widely used in the automotive industry and for household appliances. Phosphating can improve the performance of the zinc-coated strip with regard to drawability, corrosion resistance and subsequent paintability. See Section 2.5.16.
The strip is treated after zinc coating using spray banks in a two-stage process:
• stage 1: activation
• stage 2: phosphating.
Activating
The first stage consists of spray of a titanium refiner, which provides activation sites for the second stage of phosphate coating. The activation solution circulates in a closed loop. The pH lies in a range of 8 to 10, and the temperature is maintained below 40 °C. Spent (used, out of specification) solution it is treated in the waste water treatment section.
Phosphating
Phosphate coatings are described in Section 2.5.16. A phosphate coat of approximately 1 - 1.8 g/m2 is applied. The phosphate system used can either be single or tricationic, depending on customer requirements. The hopeite crystal Zn3(PO4)2.4H2O incorporates approximately 1 % Ni and 5 % Mn. A refiner is added to ensure that the phosphate crystals deposited are small, uniform and tight, which enhances the performance of the coating.
Following phosphating, the strip is rinsed in a spray of dilute chromate solution to form a chromate seal. This seal further enhances the corrosion performance of the phosphate coating.
The strip is then dried.
The phosphate solution circulates in a closed loop at a temperature below 40 °C.
Environmental considerations See Section 2.5.16.
Spent solution is treated in the waste water treatment section.
Effluents may contain traces of nickel and manganese.
2.9.8.10 Full chromating and chromate rinsing
This is described in Section 2.5.17. A typical chromating section involves the following steps:
• stage 1: chromating
• stage 2: rinsing
• stage 3: drying.
Chromating
Full chromating or chromate rinsing of the strip after zinc coating is achieved using spray banks. Chromate rinse or passivation is a spray treatment with solvents containing chromic acids. A coat of 10 – 35 mg/m2 per side is applied. During passivation, Cr(VI) is largely converted to Cr(III). The strip is treated with solutions between 0.5 – 2 % of chromium and at temperatures below 40 °C. The chromate solution circulates in closed loop.
Environmental considerations
General health and environmental issues for chromium passivation are described in Sections 1.4.4.1 and 2.5.17.
Chapter 2
98 September 2005 PT/EIPPCB/STM_BREF_FINAL Spent (used, out of specification) solution may be treated in a typical waste water treatment
plant or disposed of as hazardous waste.
Rinsing
Rinsing can be carried out by cascade spraying in horizontal tanks. The same principles as described for the degreasing section are applied. See Section 2.9.8.2
Environmental considerations
The used water is sent to waste water treatment before release.
Drying
The chromate rinse strip is dried by means of a hot air drying device. The hot air temperature lies between 100 and 120 °C which is necessary for the chemical reaction of the excess Cr(VI) with the solution additives to become Cr(III).
Environmental considerations Heat efficiency and losses.
Anti-finger marking section
This provides a chromate coating for improving protection against white rust corrosion. This coating has the advantage of not showing fingerprints, which is critical for some customers.
A typical chromating section involves the following steps:
• stage 1: coating. The chromate coating is applied via a roller coater system, similar to that used in paint lines
• stage 2: drying. The wet coating is passed through an air oven to dry.
Environmental considerations
The coating is dried in place with no rinsing, so no effluent arises.
Heat efficiency and losses may need to be addressed for the oven.
2.9.8.11 Oiling
A wet film of oil is applied on the surface of the strip, see Section 2.9.5. The oiling coat is between 0.25 – 3 g/m2/side, applied in an electrostatic oiler, with the oil circulating in close loop.
2.9.8.12 Exit looper See Section 2.9.7.2