Zinc Recovery from Wastes

 

A Multilevel Sustainability Analysis of Zinc Recovery from Wastes

Zinc is important transition metal which has wide range of applications in different fields of life. A specific amount of zinc is required to human body for healthy growth and its deficiency cam cause adverse effects to human health. Zinc is an important nutrient for plant growth. Deficiency of zinc in plants can results in discolouration of leafs and decreases in crop yield. Zinc is important element in steel coating (galvanising) due to its corrosion resistant properties. Zinc combines with other elements to form different alloys (like Bronze and Brass) which have various applications. Almost 50% of total zinc is utilised in galvanising, 17% is used for alloying such as die casting and similar percentage is used for making bronze and brass. Other applications include roofing, gutters and construction purposes (6%), as chemicals (6%) and miscellaneous (4%).

The world consumption of zinc is increasing 4.7% per year and its production is increasing 2.7% per year. At current rate of usage, the demand of zinc will reach 2.7 times of today’s demand by 2050. Zinc production mostly relay on primary mining. Zinc production from its ores uses larger fossil fuels and produces an extensive amount of CO₂ which lead to increase in global warming. Also the global production and consumption is increasing and the primary resources of zinc from ore is depleting rapidly. Therefore, the recovery of zinc from secondary source (which is waste) is important in order to meet the increasing demands of zinc. Effective zinc production by secondary sources has several advantages including usage of small amounts of fossil resources, reduced landfilling, waste remediation and mitigation of environmental and health effects.

Zinc consumption is increasing every year. China, Australia and Peru are top three largest producer of zinc by primary mining. Ireland, Turkey and Sweden are largest producer within Europe. Due to higher consumption of zinc, depletion of primary source, increasing environmental and health effects, world has shifted towards production of zinc from secondary sources which is in the form of waste.

One of major secondary source for recovery of zinc is non rechargeable spent batteries. These batteries are used in large amount and available as waste when these are disposed. These batteries contain many hazardous materials like mercury, lead and other carcinogenic metals which are dangerous for health. In these batteries zinc is present as anode and can be recovered.

With the development and advancement in technology and use of electronic devices the amount of E-waste is increasing rapidly which contains metals like Cu, Zn, and Ag etc. These metals can be recovered from e-waste. But the recovery rate of zinc from E-waste is very low as compared to other metals. Therefore recovery of zinc from this source is economically not favourable. Heavy metals like zinc are from industries are washed away with wastewater and cause environmental pollution so it is valuable to recover these metals from wastewater. Zinc is present in different concentrations in wastewater treatment plant, mining plant, copper smelting, metal finishing industry and industrially contaminated groundwater. Zinc can be recovered from these types of wastewater in order to reduce environmental pollution.

Zinc can also be recovered from construction and demolition waste. This is famous method in Netherland where 82% of Zn production is done by using such waste. This waste collected from chemical and electroplating industries and other sectors contains higher concentration of zinc. Zinc and copper recovery from secondary resources like infrastructural materials and alloys is done via a series of pyro-metallurgical reduction processes, hydrometallurgical process microwave irradiation assisted carbothermic methods and electrometallurgical processes etc.

 

Pyrometallurgical process for recovery of zinc involves two steps volatilization and condensation of metals. It is most frequently used method in the world because of higher recovery rate of zinc. But it has some limitations such as high temperature requirement and produces gasses which pollute environment. Hydrometallurgy is modern method used for recovery of zinc. In this process metal recovery is done by precipitation, electrochemical method and leaching method (using acidic or basic solutions). Hydrometallurgy using biological methods (bioleaching) is important technology that recovers zinc from spent batteries with high recovery rate and lower environmental impacts. The recovery of zinc from E-waste can also be done through pyrometallurgical and hydrometallurgical processes.

 

The recovery of zinc from soil and wastewater can be carried out by using physical methods such as chemical precipitation, reverse osmosis, solvent extraction, ion exchange, lime coagulation and electrochemical methods. However, conventional approaches are insufficient in recovery of low concentration of heavy metals from wastewater. Microbial electrosynthesis process serves the purpose of recovery of heavy metals including iron, copper and zinc, present at low concentration in wastewater. To recover zinc from air pollution particles and steel making dust Pyrometallurgical and hydrometallurgical methods are used. Hydrometallurgical methods using acid leaching such as sulfuric acid are more common and favorable due to its less cost.

 

In short zinc production from primary mining resulted in depletion of ores containing zinc and causes environmental pollution. In order to meet increasing demand of zinc and reduce environmental pollution zinc is recovered from secondary resource which also contributes to economy of country. Therefore this is a great contribution towards the green world and to meet increasing demands of zinc