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Waste management specialist Cleansing Service Group is continuing with the development of a process to recover valuable nickel from spent electroless nickel solutions.
In this article CSG technical manager Phil Manley details the principles of the process which has been developed over a period of more than two years.
What is nickel?
Nickel is named after the devil and derived from the German word Kupfernickel, a term used originally by German miners and which means ‘Old Nick’s copper’.
Nickel is mined in Russia, Australia, New Caledonia, Cuba, Canada and South Africa, and more than 500,000 tonnes are produced annually. It is estimated reserves will last for at least 150 years. The total amount of nickel dissolved in the sea has been calculated to be around 8 billion tonnes.
Organic matter has a strong ability to absorb nickel although it is a relatively unreactive element. At room temperature it does not combine with oxygen or water, or dissolve in most acids, although it becomes more active at higher temperatures.
What is electroless nickel?
Electroless nickel is a hard, silver coloured coating comprised of nickel alloyed with between 4 – 14% phosphorous. It is deposited by immersion of parts in a solution of nickel salts and reducing agents at a temperature of 90° C.
The uses
It is most commonly used in engineering coating applications where excellent wear resistance, hardness and corrosion protection are required. Included in a very wide range of other applications are oil field valves, rotors, drive shafts, paper handling equipment, fuel rails, optical surfaces for diamond turning, door knobs, kitchen utensils, bathroom fixtures, electrical/mechanical tools and office equipment. It is also used as a coating in electronics printed circuit board manufacturing, usually with an overlay of gold to prevent corrosion.
The plating technique
In contrast to conventional electroplating, electroless nickel plating is produced by a chemical process alone without the need for an electric current. In practice, an auto-catalytic chemical technique is used to deposit a layer of nickel-phosphorous, or nickel-boron alloy, on to a solid work piece. This can be metals or plastic. The process relies on the presence of a reducing agent; for example, hydrated sodium hypophosphite, which reacts with the nickel ions to deposit metallic nickel. This plating technique prevents corrosion and wear.
Electroless nickel plating has several advantages versus electroplating. It is free from flux-density and power supply issues; it provides an even deposit regardless of the shape of the work piece. With the proper pre-plate catalyst it also has the advantage of being able to deposit on non-conductive surfaces.
The composition of electroless nickel solutions varies in chemical makeup, but typically it contains Nickel Sulphate or Nickel Chloride between 1 – 45%, and Sodium Hypophosphite from 1 – 12%.
Due to evolution of hydrogen gas during plating process, pH will decrease so adjustment chemicals such as ammonium hydroxide, potassium hydroxide or sulphuric acid are added. Nickel solutions are also added in order to maintain nickel concentration.
But electroless nickel solutions do have a finite life. During the plating process the nickel is depleted until the electroless nickel solution is spent and the solution must be discarded after being replenished once, or when the nickel concentration <3.5g/ltr.
The waste
Waste electroless nickel contains up to 5g/ltr of nickel, but due to the complexity of chemical reactions occurring during the electroless nickel plating process, a host of contaminants are also present such as complexing agents, solids (suspended and dissolved), organics and contamination from handling in tanks, tankers or containers. These include other chemicals present in electroless nickel such as Sodium Succinate, Sodium Chloride, Ammonium Compounds and EDTA.
Other metal contaminants are also often picked up through the plating process including cadmium, copper, chromium, lead, nitrates, iron and zinc. Where the nickel has been depleted during plating, the concentrations of other chemicals remains relatively constant.
The process
It has been possible for a long time to use electrolysis to recover nickel from solution but, as the electrolyte needs to be relatively pure to achieve any degree of success, recovery of waste nickel solutions on a commercial scale has remained elusive.
The pioneering, purpose-built plant developed at CSG’s award winning treatment and recovery facility at Cadishead, Manchester, recovers valuable metal, particularly nickel, but also now copper, from metal bearing waste solutions using a multi-stage pre-treatment process centred around IEX (Ion Exchange) technology to concentrate and purify the waste solutions, followed by Electrowinning to recover the pure metal.
The alternative physiochemical treatment processes previously resulted in the metals being lost to landfill as the metal hydroxide or sulphide.
The process is suitable for most nickel or copper solutions > 5,000 ppm containing predominately one metal, although other metals such as chromium, zinc and iron can be tolerated at low concentrations. Nickel and copper filter cakes can also be made suitable for processing by first solubilising in acid.
The method of recovery – two separate processes
1. Pre-treatment: The parameters of metal concentration, pH, temperature, flow rate and levels of contamination are critical for producing a viable feed for the electrowinning process.
The first treatment step is through an Ion-Exchange Unit that selectively retains the nickel while allowing other contaminants to pass through. The IEX is a selective process that has a high affinity for metals such as nickel and copper, and lower affinity for zinc, iron and lead. The process does not remove alkaline metals although other metals present at significant levels can also be retained.
Following removal of the metals through the Ion Exchange the aqueous effluent undergoes further treatment to make it suitable to be discharged to a sewer.
Following initial testing and screening of the incoming waste, each suitable batch is analysed for metal concentration before processing through the IEX unit to determine the volume and flow rate to achieve full loading.
When the IEX is saturated, it is rinsed with water to remove residual contamination before the adsorbed metal is extracted using sulphuric acid. At this stage, the nickel concentration is around 16,000 ppm and the concentration of other metals is reduced.
To further remove contaminants such as iron, the nickel sulphate solution is dosed with hydrogen peroxide to bring the iron into suspension before being filtered out. Filtration, chemical dosing and carbon absorption are used to remove other contaminants. The nickel solution is then treated and precipitated as nickel carbonate. When re-dissolved in sulphuric acid the nickel carbonate produces a clean nickel sulphate solution of 60g/l that is suitable for the electrowinning process.
2. Electrowinning: Passing an electric current through a nickel solution or molten salt – the electrolyte – results in the migration of ions to the electrodes, positive ions (cations) to the negative electrode (cathode) and negative ions (anions) to the positive electrode (anode). On reaching their respective electrodes these ions lose their charge and get deposited on the electrode, or discharged as a gas.
The Electrowinning plant used consists of 15 cylindrical cells, each capable of plating up to 25kg of metal.
In operation, the concentrated metal solution from the Ion Exchange is heated and circulated through the electrowinning cells at >50 m³ per hour. This has the effect of producing intense turbulence across the electrodes, thereby maintaining a high plating efficiency.
As the metal is progressively removed from the solution the pH changes, so a dosing system in the feed tank is used to automatically keep the solution at the correct pH by adding a solution of either sulphuric acid or sodium hydroxide. As each atom of nickel plated from the electrolyte releases an atom of hydrogen which is replaced in the solution by 2 atoms of sodium, the sodium is automatically added as the pH drops. An efficiency of 0.02 pH is maintained throughout the time the process is run.
Electrowinning continues for up to 240 hours, or until the concentration in the metal reduces from approximately 60g/ltr to 5g/ltr.
The depleted feed solution is then returned to the IEX to be re-concentrated and the recovered metal is removed from the cells, thereby ensuring no nickel is lost from the process.
Each cell produces a cylinder of pure metal 1200mm long and weighing about 25kg.
The recovered metal is up to 99.9% pure and the final concentration in the effluent is <4 ppm, which means that 99.9% of the metal has been recovered.
Parameters affecting plating
Development of the process has been a three year R&D project as a number of factors affect the process and identifying the correct parameters has been a painstaking process. The optimum conditions for plating nickel vary with each batch and the margins are very narrow as almost no contamination can be tolerated.
1. Metals: The presence of other metals prevents nickel from plating as nickel is more stable in solution as Nickel Sulphate, whereas Zinc or Copper easily revert to their elemental state. The conditions for plating nickel, therefore, have to be as near perfect as possible to have a chance of coaxing it from its ionic state to its elemental state.
During electrolysis, the ions react with the electrode, either receiving or giving up electrons. NiSO4 Ni2+ + SO42– Ni2+ + 2e Ni
The Copper ion has an electrode potential of +0.34V. This is more positive and therefore more easily reduced. Cu2+ + 2e Cu Reduction
Therefore in a competition, copper ions will preferentially pick up the electrons. Ni2+ + Cu2+ + 2e Ni2+ + Cu
When two ions with similar reactivity are in competition then the relative concentration of the two ions becomes an important factor.
Metals which are higher in the electrochemical series effectively displace metals which are lower in the sequence. The more positive the electrode potential results in a greater tendency for reduction to occur.
Copper Cu2+ has a greater tendency to accept electrons than Nickel Ni2+ and therefore copper will form over nickel. Relative concentrations of ions also impacts rates and purity of plating, as ions compete for electrons. In a solution saturated with nickel with low levels of copper, nickel will plate predominantly as it competes for electrons despite have a lower electrode potential, demonstrating why a pure and concentrated solution is required first.
2. pH: Hydrogen liberation during plating affects pH which therefore has to continually be adjusted. The band for pH is very narrow – a range of around 0.05.
The correct pH range needs to be identified for each batch as it is affected by variables in the waste solution.
3. Complexing agents: The presence of complexing agents, such as EDTA and ammonia, keep the nickel in solution thus preventing plating. Another negative effect of complexing agents is that they can destroy the iridium oxide coating on the titanium anodes in the main electrowinning cells.
4. Stress: Virtually all electroplating produces deposits with some degree of internal stress. It is hard to find a process variable that does not influence deposit internal stress. A list of typical variables includes: • Current density • Concentration of every major component of the plating set up • Concentration of additives • Concentration of impurities • Temperature • pH • Plating cell geometry • Composition and condition of anodes • Anode/cathode surface area ratio • Quality of DC power • Nature and condition of substrate • Numerous others
Too much stress makes the cathodes hard to harvest because they become tightly wedged, whereas too little stress causes the cathodes to curve inwards and short circuit the anodes.
5. Temperature: Narrow margin of 50 – 55° C has a profound effect on pH and stress.
6. Flow: The system runs at a constant rate. Low velocity leads to anodic stress, high velocity leads to cathodic stress and causes dendrite formation. The acceptable range appears to be 47 – 53m3ph.
A research and development project – about CSG
CSG believe they are the only UK organisation consistently recovering nickel from spent electroless nickel solutions in the UK. Therefore, a considerable amount of trial and error has been required to identify the optimum conditions.
With such a wide range of variables this has meant it has often felt like to trying to predict six correct lottery numbers. Often we have achieved optimum conditions on a number of parameters, only to discover that a previously unknown variable is now affecting plating performance. Without a dedicated operator diligently working through each stage with careful documentation, the project could easily have been mothballed.
A test lab was established alongside the plant to enable the operator to mimic conditions on a smaller and manageable scale. The original plant consisted of the electrowinning cells with only ion exchange as the pre-treatment. Every other stage has been developed and installed on site – not just chemical treatment, but even the physical harvesting of the nickel was altered and modified to adapt to actual circumstances, as opposed to the predicted performance when the plant was designed.
We have learned to be patient and methodical; it has been an agonising learning curve. There has been a lot of frustration along the way – as each problem is solved a new problem seems to present itself. But the satisfaction of beginning to achieve something that we were told couldn’t be done has made it worthwhile.
As the foregoing illustrates, recovering nickel is fraught with difficulties and CSG is continuing research into developing the optimum conditions for plating nickel.
If anyone in the wider scientific community reading this article has any suggestions as to how some of the difficulties might be overcome, CSG would be pleased to hear from them.
In the meantime, the plant is also being used to plate copper which has none of the tight variables associated with plating nickel.
Whether it’s nickel or copper being plated, the company believes it is vitally important to our increasingly fragile environment that as much of these metals as possible is recycled so that the waste does not finish up in landfill sites, or far worse, is disposed of illegally. ?
Author
Phil Manley is Technical Manager at CSG Lanstar. He has 28 years of experience in chemical waste treatment, starting out as a site chemist, and holding roles of Process Development Manager and Plant Manager. Prior to this he was involved in research at Manchester University.
As Technical Manager he has full responsibility for chemical waste processing at CSG Lanstar and has been instrumental in the development of a number of unique treatment methods for waste materials which have focused on waste recovery and landfill diversion.
While electrolysis technology is available ‘off the shelf’, Phil has been directly involved in the development of an extensive intermediate process that makes CSG’s process unique.
For more information log on to: www.csg.co.uk
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Published: 10th Dec 2011 in AWE International
