Zinc (zinc)

Zinc (Zn) is a heavy metal and essential trace nutrient that, like copper, is toxic to aquatic life at elevated concentrations. Its effluent discharge limit is 5.0 mg/L for inland surface water, relaxed to 15 mg/L for public sewers and marine coastal discharge, reflecting that it is less acutely toxic than copper but still a regulated pollutant.

Zinc enters effluent from galvanising and metal coating, die-casting, battery manufacture, electroplating, rubber processing (zinc oxide is a vulcanisation activator), and viscose fibre production. For recyclers, zinc is relevant across several sectors: metal recycling (galvanised steel scrap, zinc die-castings, brass which is a copper-zinc alloy), tyre and rubber recycling (rubber contains zinc oxide from vulcanisation, so zinc appears in tyre-derived char, pyro-oil and any wash effluent), and battery recycling (zinc-carbon and zinc-based batteries).

The tyre/rubber connection is particularly characteristic: because zinc oxide is used to activate vulcanisation, all vulcanised rubber carries zinc, and tyre pyrolysis concentrates it in the char. Zinc is therefore a signature element of tyre-derived materials, present in the char (where it can be recovered) and in any aqueous stream from rubber processing.

The practical relevance is that zinc, like copper, can often be managed as a recoverable resource as well as a controlled pollutant. In effluent it is removed by precipitation as hydroxide (with lime) or carbonate, and the recovered zinc may have value. For tyre pyrolysis operators, the zinc concentrated in the char affects the char's composition and potential uses, and any rubber-processing wash water needs zinc removal to meet the 5.0 mg/L limit. For metal recyclers, galvanised and zinc-alloy scrap is a routine zinc source. Managing zinc to the discharge limit is straightforward precipitation chemistry, with the bonus that recovered zinc can offset some treatment cost.