In pgm recovery from effluent are there alternatives to using hydrazine?

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Category: Sustainability

Subject: In pgm recovery from effluent are there alternatives to using hydrazine?

Question

Can you please explain the above reduction process using relevant chemical equations? Is there any alternatives as the above reaction does produce hydrogen gas?

Answer

One way of treating effluent streams for the recovery of platinum group metal (pgm) values is to reduce the metals to ‘pgm blacks’ with hydrazine (N₂H₄); a generic chemical equation is shown below:

2M²⁺ + 4OH^– + N₂H₄ → 2M⁰ + N₂ + 4H₂O

With careful control of the conditions quite an effective recovery can be carried out. However, the use of hydrazine is becoming less acceptable in industry. In the presence of pgms, hydrazine can be catalytically decomposed to nitrogen and hydrogen. Hydrazine is also a known carcinogen. The Control of Major Accident Hazards (COMAH) 1999 regulations and the Seveso II directive have taken account of these facts and have proposed that hydrazine concentrations above 5% should not be permissible, and that only quantities up to 500 kg should be stored at any one time.

There are, of course, other chemical alternatives to hydrazine, such as thioform and zinc, but these also carry their own problems and can make effluent disposal costly.

For a safe and more environmentally friendly recovery, a metal scavenger could be used - for instance, Smopex - fibrous precious metal scavengers. These can recover pgms from process or effluent streams. The fibres consist of a polypropylene or viscose backbone (making them suitable for use in both organic and aqueous streams), grafted with specific functional groups that can selectively remove pgms from solution.

Currently there are 10 different types of scavengers available which can be used for metal recovery from a range of effluent or process streams, and many more types of scavengers are in development.

Smopex is capable of recovering pgms down to ppb levels and with metal loadings in the region of 10 wt.% (that is 100 g metal/kg fibre) the quantity of fibre required to achieve excellent recovery is not large. The fibres are thermally stable up to at least 120ºC so they can be used at elevated temperatures, where metal recovery is often easier.

Smopex fibres are non-hazardous and, after washing with water can be returned for refining of the captured metal. Fibres are typically used in a column through which the pgm-containing liquor is fed. Smopex removes the metals to leave a metal-free ‘clean’ liquor.

Answer posted February 2005

Submitted by: Mr Ronald Chetty

Affiliation: Johnson Matthey South Africa

Answered by: Stephanie Phillips

Affiliation: Johnson Matthey