The National Green Tribunal (NGT) recently conducted a hearing on the scientific disposal of spent pot lining (SPL) from aluminium industries, which has been classified as hazardous waste under the Hazardous Waste Management Rules, 2016. Currently, most SPL is kept at aluminium smelter sites or in landfills. Leaching from these landfill sites, where dissolved fluorides and cyanides are released from the SPL, can have detrimental effects on the environment.
Understanding Spent Pot Lining
Spent Pot Lining (SPL) is a byproduct of smelting plants that contains high levels of carcinogenic cyanide and fluoride. It requires scientific methods for its utilization or detoxification. Other names for it include spent pot liner and spent cell liner.
The extraction process of aluminium metal from aluminium oxide occurs in electrolytic cells known as pots. The pots are constructed from steel shells with two particular linings: an outer insulating lining (refractory lining) and an inner carbon lining acting as the electrolytic cell’s cathode. During the cell operation, various substances, including aluminium and fluorides, get absorbed into the cell lining. Once the pot lining fails after years of operation, the removed material is labeled as spent pot lining.
Hazardous Nature of SPL
The hazardous properties of SPL stem primarily from its toxic fluoride and cyanide compounds, which can leach into water and are carcinogenic. Furthermore, SPL is corrosive, showcasing a high pH level due to alkali metals and oxides. It also reacts with water, producing dangerous gases that are flammable, toxic, and explosive. Due to its harmful properties, specialized care is required when handling, transporting, and storing SPL.
| Property | Description |
|---|---|
| Toxicity | SPL contains fluoride and cyanide compounds that leach into water and are carcinogenic. |
| Corrosiveness | SPL has a high pH level. This is due to alkali metals and oxides present in it. |
| Reactivity | In contact with water, SPL produces dangerous gases that are flammable, toxic and explosive. |
pH Scale and the Role of Temperature
The pH scale is used to designate how acidic or basic a water-based solution is. Acidic solutions have lower pH levels and basic solutions have higher levels. The pH value regarded as ‘neutral’ depends on temperature. Typically, the pH scale ranges from 0 to 14. At room temperature (25 °C), pure water is considered neither acidic nor basic, sitting neutral on the scale with a pH value of 7. This understanding of pH levels is crucial for handling and managing hazardous wastes like SPL.