- Email: [email protected]
Hot-dip galvanizing and the environment
Hot-dip galvanizing and the environment
13
Zinc as metal is a natural element and therefore its use for protection of steel against corrosion is quite natural, too. Its presence in the environment does not necessarily produce a disturbing effect (Figure 13.1). On the contrary, in a suitable combination with other natural materials as wood, stone, or glass an impressive aesthetic effect may be achieved (Figure 13.2). Hot-dip galvanizing plants should be properly managed with regard to production of waste and emissions to air. In hot-dip galvanizing plants, pickling is generally based on hydrochloric acid solutions, so the description of the environmental impacts in this chapter uses this as its starting point. Modern hot-dip galvanizing plants are designed to meet strict legislative requirements for environmental protection with a large margin of safety. Operations using chemicals with hazardous characteristics are carried out in operating units separated from the working environment and are equipped with efficient pollutant filters.
13.1
Degreasing solutions
In the EU, some waste from degreasing falls under the dangerous waste catalog number 11 01 13. In the case of an alkaline degreasing agent, the whole bath saturated with grease may be required to be disposed of by an authorized contractual partner. For acidic agents, the waste has the character of solid sediment that is collected from the tank bottom when the solution has been pumped out. The volume of this waste is relatively small and it can be disposed of without any further treatment.
13.2
Pickling solutions
Ferrous chloride is generated in the pickling solution (when it is based on hydrochloric acid) during its gradual saturation with iron from steel parts pre-treated for galvanizing. A new pickling bath is started with the free acid content of 120 to 150 g/l and with a ferrous chloride content of about 50 g/l. During the useful life of the pickling solution, the free acid is gradually used up while the bath becomes saturated with iron. After reaching the saturation limit, no further pickling is possible and the bath may be handed over to an authorized entity for disposal as pickling waste. Hot-Dip Galvanizing of Steel Structures. DOI: http://dx.doi.org/10.1016/B978-0-08-100753-2.00013-6 © 2016 Elsevier Ltd. All rights reserved.
200
Hot-Dip Galvanizing of Steel Structures
Figure 13.1 Hot-dip galvanized parts in a natural setting.
Figure 13.2 Hot-dip galvanized zinc coatings in architecture [54].
Saturated solutions of ferrous chloride can be used in wastewater treatment plants for separation of phosphorus. The action of bivalent or trivalent iron salts converts phosphorus to a solid, insoluble phase that precipitates in the sedimentation tank and may be landfilled after separation.
13.3
Stripping
Separated stripping baths are used to remove old zinc coatings or to remove zinc from process hooks/jigs or from rejected galvanized parts. During stripping, the content of iron in the solution must be kept to the minimum as iron reduces the
Hot-dip galvanizing and the environment
201
yield of zinc from the solution for its further processing. As zinc reacts relatively strongly with hydrochloric acid while iron shows a higher resistance to the acid, the content of dissolved iron in the stripping bath can be eliminated by simple supervision of the operator, who must intervene at the moment when all zinc from the surface of the parts has been dissolved and effervescence of the bath has ceased. A saturated solution of zinc chloride is an important raw material from which zinc can be separated successfully for further use in the chemical or pharmaceutical industry. Solutions from a well-managed stripping bath saturated with zinc and with a minimal content of iron are hazardous from the point of view of classification, equally to the pickling bath saturated with ferrous chloride, but it can be sold to an authorized company as raw material for further processing. Zinc chloride is also one of the constituents of flux and so it can be used for the preparation of flux in the produced form without any requirements for treatment.
13.4
Zinc ashes
During immersion of galvanized parts in the zinc bath, ashes with a high content of zinc are produced by oxidation of the zinc at the bath surface and from combustion of the applied flux, which are then present on the bath surface. Before the removal of the galvanized batch the ashes are skimmed to the end of the galvanizing bath with paddles. From there they are collected and stored as the other residues under the EWC catalogue number 11 05 02 so that they can be sold as a valuable raw material for further processing. The content of pure zinc in zinc ashes can reach approximately 70% and zinc can be relatively easily recycled by melting in a rotary furnace.
13.5
Skimmings
In hot-dip galvanizing plants that use the wet process, decontaminated parts are immersed in the zinc bath through a sal-ammoniac layer. In the course of time this will deteriorate. Deteriorated sal-ammoniac is periodically removed from the zinc bath and collected as skimmings with the EWC catalogue number 10 05 11 and is subsequently handed over to an authorized contractual company for disposal or recycling.
13.6
Filter dusts from air pollution control equipment
Particulates emissions from the galvanizing bath operation, mainly from flux combustion, are separated in high-efficiency fabric filters. This fine dust, which contains mostly sal-ammoniac, is a solid air pollutant and its concentration in the released air matter must not exceed the limit prescribed by the legislation. Captured filter dusts are collected as dangerous waste with the catalog number 11 05 03 and can be sent to a flux manufacturer for external recycling.
202
13.7
Hot-Dip Galvanizing of Steel Structures
Dross
During the metallurgical diffusion of the galvanizing reaction that forms the coating, iron is released from the substrate and combines with zinc in the zinc melt in the ratio of 1:13, producing so-called dross (sometimes referred to as “hard zinc”), or iron-zinc alloy phase ζ. Dross has a higher specific weight than pure zinc and therefore it drops to the bottom of the galvanizing bath from where it is periodically collected. This waste has a high zinc content and is categorized under the EWC catalogue number 11 05 01. In the same way as zinc ashes, it is sold as an important raw material for processing in the chemical industry where zinc is recycled from it.
13.8
HCl emissions
The most significant air pollutant produced during the chemical pre-treatment of parts to be hot-dip galvanized is HCl emissions above the pickling baths. Therefore, in modern hot-dip galvanizing plants the chemical pre-treatment workplaces are separated from the working environment by using enclosures that have a high resistance to these chemicals and inside of which these emissions can be captured and separated. There are agents added to pickling baths that can be used to partly eliminate HCl evaporation. The pickling bath should not be operated at elevated temperatures to increase the pickling efficiency. The efficiency of the baths can be adequately controlled by proper management of the baths and timely adjustment of the chemical composition so that they can be operated in the ranges of optimum pickling times.
13.9
Heat recovery
The temperature of flue gas from galvanizing bath heating is used by means of efficient heat exchangers elsewhere in the production process for heating of hot service water, drying of the batch after flux application, or for supplementary heating of the degreasing or flux baths to ensure their required efficiency. Thus, besides reducing the energy demands of the process and the resulting cost savings, the impact of hot-dip galvanizing technology on the environment is also reduced.
13.10
Low water consumption
Proper management of the pre-treatment baths also leads to a considerable reduction or elimination of water consumption as rinsing water can be completely reused to replenish losses in the pickling baths instead of service water.
13
Zinc as metal is a natural element and therefore its use for protection of steel against corrosion is quite natural, too. Its presence in the environment does not necessarily produce a disturbing effect (Figure 13.1). On the contrary, in a suitable combination with other natural materials as wood, stone, or glass an impressive aesthetic effect may be achieved (Figure 13.2). Hot-dip galvanizing plants should be properly managed with regard to production of waste and emissions to air. In hot-dip galvanizing plants, pickling is generally based on hydrochloric acid solutions, so the description of the environmental impacts in this chapter uses this as its starting point. Modern hot-dip galvanizing plants are designed to meet strict legislative requirements for environmental protection with a large margin of safety. Operations using chemicals with hazardous characteristics are carried out in operating units separated from the working environment and are equipped with efficient pollutant filters.
13.1
Degreasing solutions
In the EU, some waste from degreasing falls under the dangerous waste catalog number 11 01 13. In the case of an alkaline degreasing agent, the whole bath saturated with grease may be required to be disposed of by an authorized contractual partner. For acidic agents, the waste has the character of solid sediment that is collected from the tank bottom when the solution has been pumped out. The volume of this waste is relatively small and it can be disposed of without any further treatment.
13.2
Pickling solutions
Ferrous chloride is generated in the pickling solution (when it is based on hydrochloric acid) during its gradual saturation with iron from steel parts pre-treated for galvanizing. A new pickling bath is started with the free acid content of 120 to 150 g/l and with a ferrous chloride content of about 50 g/l. During the useful life of the pickling solution, the free acid is gradually used up while the bath becomes saturated with iron. After reaching the saturation limit, no further pickling is possible and the bath may be handed over to an authorized entity for disposal as pickling waste. Hot-Dip Galvanizing of Steel Structures. DOI: http://dx.doi.org/10.1016/B978-0-08-100753-2.00013-6 © 2016 Elsevier Ltd. All rights reserved.
200
Hot-Dip Galvanizing of Steel Structures
Figure 13.1 Hot-dip galvanized parts in a natural setting.
Figure 13.2 Hot-dip galvanized zinc coatings in architecture [54].
Saturated solutions of ferrous chloride can be used in wastewater treatment plants for separation of phosphorus. The action of bivalent or trivalent iron salts converts phosphorus to a solid, insoluble phase that precipitates in the sedimentation tank and may be landfilled after separation.
13.3
Stripping
Separated stripping baths are used to remove old zinc coatings or to remove zinc from process hooks/jigs or from rejected galvanized parts. During stripping, the content of iron in the solution must be kept to the minimum as iron reduces the
Hot-dip galvanizing and the environment
201
yield of zinc from the solution for its further processing. As zinc reacts relatively strongly with hydrochloric acid while iron shows a higher resistance to the acid, the content of dissolved iron in the stripping bath can be eliminated by simple supervision of the operator, who must intervene at the moment when all zinc from the surface of the parts has been dissolved and effervescence of the bath has ceased. A saturated solution of zinc chloride is an important raw material from which zinc can be separated successfully for further use in the chemical or pharmaceutical industry. Solutions from a well-managed stripping bath saturated with zinc and with a minimal content of iron are hazardous from the point of view of classification, equally to the pickling bath saturated with ferrous chloride, but it can be sold to an authorized company as raw material for further processing. Zinc chloride is also one of the constituents of flux and so it can be used for the preparation of flux in the produced form without any requirements for treatment.
13.4
Zinc ashes
During immersion of galvanized parts in the zinc bath, ashes with a high content of zinc are produced by oxidation of the zinc at the bath surface and from combustion of the applied flux, which are then present on the bath surface. Before the removal of the galvanized batch the ashes are skimmed to the end of the galvanizing bath with paddles. From there they are collected and stored as the other residues under the EWC catalogue number 11 05 02 so that they can be sold as a valuable raw material for further processing. The content of pure zinc in zinc ashes can reach approximately 70% and zinc can be relatively easily recycled by melting in a rotary furnace.
13.5
Skimmings
In hot-dip galvanizing plants that use the wet process, decontaminated parts are immersed in the zinc bath through a sal-ammoniac layer. In the course of time this will deteriorate. Deteriorated sal-ammoniac is periodically removed from the zinc bath and collected as skimmings with the EWC catalogue number 10 05 11 and is subsequently handed over to an authorized contractual company for disposal or recycling.
13.6
Filter dusts from air pollution control equipment
Particulates emissions from the galvanizing bath operation, mainly from flux combustion, are separated in high-efficiency fabric filters. This fine dust, which contains mostly sal-ammoniac, is a solid air pollutant and its concentration in the released air matter must not exceed the limit prescribed by the legislation. Captured filter dusts are collected as dangerous waste with the catalog number 11 05 03 and can be sent to a flux manufacturer for external recycling.
202
13.7
Hot-Dip Galvanizing of Steel Structures
Dross
During the metallurgical diffusion of the galvanizing reaction that forms the coating, iron is released from the substrate and combines with zinc in the zinc melt in the ratio of 1:13, producing so-called dross (sometimes referred to as “hard zinc”), or iron-zinc alloy phase ζ. Dross has a higher specific weight than pure zinc and therefore it drops to the bottom of the galvanizing bath from where it is periodically collected. This waste has a high zinc content and is categorized under the EWC catalogue number 11 05 01. In the same way as zinc ashes, it is sold as an important raw material for processing in the chemical industry where zinc is recycled from it.
13.8
HCl emissions
The most significant air pollutant produced during the chemical pre-treatment of parts to be hot-dip galvanized is HCl emissions above the pickling baths. Therefore, in modern hot-dip galvanizing plants the chemical pre-treatment workplaces are separated from the working environment by using enclosures that have a high resistance to these chemicals and inside of which these emissions can be captured and separated. There are agents added to pickling baths that can be used to partly eliminate HCl evaporation. The pickling bath should not be operated at elevated temperatures to increase the pickling efficiency. The efficiency of the baths can be adequately controlled by proper management of the baths and timely adjustment of the chemical composition so that they can be operated in the ranges of optimum pickling times.
13.9
Heat recovery
The temperature of flue gas from galvanizing bath heating is used by means of efficient heat exchangers elsewhere in the production process for heating of hot service water, drying of the batch after flux application, or for supplementary heating of the degreasing or flux baths to ensure their required efficiency. Thus, besides reducing the energy demands of the process and the resulting cost savings, the impact of hot-dip galvanizing technology on the environment is also reduced.
13.10
Low water consumption
Proper management of the pre-treatment baths also leads to a considerable reduction or elimination of water consumption as rinsing water can be completely reused to replenish losses in the pickling baths instead of service water.