Recover and/or recycle metals from waste waters
Best Available Technique (BAT)
It is BAT to recover and/or recycle metals from waste waters.
Brief technical description
This refers to recovery systems within installations, not to external processes.
Metals may be recovered by electrolysis. The system is widely used for precious metal recovery, but can also be used to recover other metals such as nickel and chromium from drag-outs. Suitable electrolysis cells are marketed in different sizes and can operate down to metal contents of less than 100 mg/l.
May be operated in conjunction with other techniques to achieve low emission levels for water, or recycling of rinse-waters, etc.
Achieved environmental benefits
Recovery of metals for re-use.
Reduction of metals in drag-out and their consequent decrease in effluent concentrations.
In the electrolytic separation of metal solutions containing cyanide, the anodically oxidative destruction of the cyanide takes place in parallel to the metal winning.
Power consumption at low current efficiencies.
Precious metals electrolytic recovery requires the electrolytic reactor to be able to reduce the metal concentration down to a very low concentration (1 ppm or less). The current efficiency at this level is very low. In all cases, a simple flat plate cathode would be sufficient in theory, but when high current efficiencies are required (for both precious and transition metals) sophisticated cathode design is needed (rotating tube cell, graphite fibre cathode), or a fluidised bed to overcome cathode surface depletion. In all cases (including anodic oxidation) the anode must be of the ‘insoluble’ type.
Cathodes are usually sheets, foil or particles, generally made of the same metal to be recovered, but also of stainless steel or other metals, which allow either a mechanical parting of the deposit from the cathode blank, or its removal by anodic dissolution. Iron, stainless steel, porous carbon, graphite particles, glass or plastic metallised beads and metallised fabrics are all examples of common materials used. Cathode material selection is largely determined by the nature of the treatment, which follows the metal deposition. In any case, maximising both the cathode surface area and the diffusion process are the most important means to enhance the efficiency of the electrolytic reactor.
Anodic material includes: graphite, lead, lead alloys with antimony, silver or tin, stainless steel, cast iron, ferro-silicon and the valve metals (titanium, tantalum, tungsten, niobium) coated with noble metals (platinum iridium) or with noble metal oxides (iridium, ruthenium oxides).
Anodic material selection is usually a compromise based on:
- over-voltage behaviour for the particular reaction on a given material
- anode corrosion, mechanical properties and the form in which the material is available
Operating conditions vary as a function of the metal to be recovered; for gold the recommended conditions are: pH minimum of 10, cell voltage 8 V, current density 20 A/dm2 temperature >60 °C, and an anode-cathode gap from 8 to 16 cm.
Further advantages of the electrolytic recovery over the ion exchange method are:
- it does not produce any increase in the dissolved salt concentration
- the presence of other metals in similar concentrations does not affect the rate of removal of the desired species
- may also oxidise unwanted species, such as cyanide
Noble metals, because of their electropositive character, are more readily electrodeposited than non-noble ones.
For electrolytic metal recovery, the following streams are particularly suitable:
- rinsing (drag-out) concentrates from electroplating metal
- rinsing (drag-out) concentrates and used process solutions from chemical metal plating excluding solutions-containing phosphate
- sulphuric acid regenerates of cation exchangers from the treatment of rinsing waters: these contain non-ferrous metals.
The purity of the generated metals may permit a direct in-house use as an anode material, otherwise re-use is via the scrap metal trade.
Gold and silver have been recovered electrolytically for well over 50 years.
Electrolytic recovery has wider applicability than precious metals: it can also be used for transition metals.
Fluidised bed cells increase the process efficiency.
Cost-effective for precious metals.
Can be cost-effective for transition metals, for example, where it reduces the waste water treatment costs (capital and running costs).
In-house electrolysis has costs in investments and personnel (both time and skills) as well as a substantial energy expenditure because of the low electricity yield (kg/amp hour). This may be offset for cyanide solutions where the cyanide is destroyed in parallel.
For a- fluidised bed cell: although the technique can be utilised on most metals, economic considerations limit the application to either valuable or easily re-usable metals. Units can recover from 1 kg/week to 150 kg/week of electrolytically pure metal from solution. The solutions can be very dilute, typically containing 100 – 500 parts per million (0.1 – 0.5 gm/l).
Surface treatment of metals plants e.g.
- Silver recovery from waste photographic solutions
- Copper Recovery – Printed Circuit Manufacturer
- electroplating nickel
Electrodialysis technique allows to maintain a sufficiently low concentration of nickel in the wash water while the concentration of the metal in the concentrate solution.
The resulting concentrate may serve to supplement the content of plating baths.
The degree of recovery of this method exceeds 90%.
Energy consumption is 3.1 kWh/kg Ni
Example Plant: Asahi Glass – Japan
- electroplating copper
As a result of electrodialysis desalinated water produced (which can be re-used for washing) and concentrate copper cyanide plating baths directed to.
The copper concentration in the rinsing water is less than 1 g Cu/dm3.
The concentrate has a concentration of 65 g Cu/dm3.
Energy consumption is 1-2 kWh/kg Cu 94% in the recovery of copper from the rinsing water.
Example Plant: fractional installation Technical France.
Annual net profit plating is 1,500 Euro for the recovery of 292 kg Cu.
- electroplating chrome
As a result of electrodialysis water is formed (which can be reused for irrigation) and chromic acid deprived of 60-90% of heavy metals.
Energy consumption is 12-15 kWh/kg CrO3.
After application of reverse osmosis water is obtained with a high purity (95% retention of chromium) and concentrate all the components of sewage strip.
Example Plant: installation of fractional technical in Germany.