Hot-dip galvanizing, like any other industrial technology, generates waste material. Although efforts have been made in order to minimize the production of waste, the complete elimination of waste material during the process is not possible. The production of waste is one of the main factors affecting the processing costs, environmental damage and an inevitable shortage of natural resources in near future. The depletion of natural resources has motivated researchers to develop new/improved techniques in order to reuse industrial wastes as secondary resources. In addition, the metallic wastes are usually considered hazardous materials that need especially designed disposal/recovery processes and can, otherwise, cause extreme damage to the environment.
In addition to high efficiency, short processing time, and low initial costs, such methods must offer simplicity and involve materials and procedures that are environmentally friendly. While various waste matters of steel production processes are recovered via industrial methods, the recovery of residues produced during the hot-dip galvanizing of steel, such as ash, dross and dust, has not yet been fully developed in an industrial scale mainly due to the complexity of the recovery steps, and therefore, the waste products are preferably disposed of.
During the hot-dip galvanizing of steel, the reaction between iron and zinc results in the formation of dross in the molten bath, which either floats on the bath surface, also known as zinc ash or skimming, or precipitates at the bottom of it, known as zinc dross. Residual zinc obtained from the hot-dip galvanizing process can be classified into metallic (intermetallic compounds of iron-zinc) and non-metallic (mainly zinc oxide) compounds. Zinc ash that is skimmed manually from the surface of the molten zinc bath, contains around 70-96 % zinc in metallic form as well as oxide that accounts for 12-15 % of the zinc consumed. The dross normally contains about 92-94 % zinc. Another source of secondary zinc is from the end-of-life products.
Hot-dip galvanizing is considered a great contributor to sustainability due to 100 % recyclability, long lifetime, minimal environmental impact, and cost efficiency . Various techniques have already been explored for an efficient recovery of hot-dip galvanizing waste. According to earlier studies, around 15 % of the zinc in the ash can be recovered by simply stirring the ash in the zinc bath, while the addition of sawdust has been shown to increase the zinc recovery even more to ~55%. It is proven that the heat treatment of ash at 700 oC in a sloping hearth or reverberatory furnace can recover around 50 % of the metallic zinc from its ash . Improving the zinc processing recovery is a key factor in increasing the hot-dip galvanizing efficiency and reducing the costs.
The recovery processes are based on either hydrometallurgical or pyrometallurgical processes or a combination of both. Hydrometallurgical methods typically deal with metal separation via acid leaching and selective precipitation, electrowinning, or liquid-liquid extraction. Although these recovery processes give satisfactory results, most of the techniques mentioned above are costly and complicated.
The pyrometallurgical processing technique deals with the recovery of zinc through thermal treatment of the zinc residue. The zinc ash needs to be pre-treated via ball milling and screening in order to separate the oxide from the zinc metal. The oxide is consumed to produce zinc salts or in the process of primary zinc beneficiation. The metallic zinc is remelted and treated into secondary zinc.
Galvanic dross containing high amounts of copper, nickel and cobalt can be treated via the combination of hydrometallurgical and pyrometallurgical processes. The dross is first mixed with iron sulphide and heat treated under an oxidizing atmosphere, during which iron oxide and sulphur dioxide are produced. The atmosphere and temperature of the treatment is adjusted so that selective metallic elements can form sulphates. The target zinc sulphate, along with other target metal sulphates, is treated via a hydrometallurgical process, such as leaching, in order to extract the metals. The thermodynamic free energy of zinc sulphate is -0.09 MJ at 800 K. A comparison between the thermodynamic free energy of zinc sulphate with other metal sulphates present in the system can give an estimation of the recovery efficiency of each metal. The extraction efficiency of zinc from the dross via this process is reported to be ~49% .
An alternative method is to reuse the hot-dip galvanizing waste in other industries such as in cement production , and as additions to pigments , asphalt emulsions , and ceramics . Although this solution seems to be a suitable alternative in galvanic waste management, a simple and effective process for metal recovery, which can have a great impact on preserving natural resources and reducing the cost of galvanizing process, is yet to be developed.
Ark Novin is actively working towards the sustainable development of hot-dip galvanizing. We are proud to announce that significant improvements have recently been made in terms of reducing the production of hot-dip galvanizing waste and increasing the efficiency of zinc recovery, which is attributed to several years of research and experience of our team of experts. The results of our findings are soon to be presented as publications and patents.
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