CLICK IMAGE TO ENLARGE (+)
Those in the plating industry are continually working on the development of a process to recover valuable nickel from spent electroless nickel solutions. Cleaning Service Group (CSG) has come up with a two-stage method to do just that.
The EN Technique
Electroless nickel is a hard, silver-colored coating comprised of nickel alloy with between 4 and 14 percent phosphorous. It is deposited by immersion of parts in a solution of nickel salts and reducing agents at a temperature of 90oC. It is most commonly used in engineering coating applications where excellent wear resistance, hardness and corrosion protection are required, although it is also can be used in a range of other applications, including as a coating in electronics printed circuit board manufacturing.
Electroless nickel plating offers several advantages over conventional electroplating, including that it 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 workpiece made of metal or plastic. The process relies on the presence of a reducing agent such as hydrated sodium hypophosphite, which reacts with the nickel ions to deposit metallic nickel. This plating technique prevents corrosion and wear.
Waste from electroless nickel plating contains as much as 5 g/L of nickel, but due to the complexity of chemical reactions occurring during the process, a host of contaminants also are present, such as complexing agents, solids (suspended and dissolved), organics, and contamination from handling in tanks, tankers, containers, etc. These wastes also include other chemicals present in electroless nickel such as sodium succinate, sodium chloride, ammonium compounds and ethylenediaminetetraacetic acid (EDTA). Other metal contaminants also are 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.
A Two-Stage Recovery 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.
CSG’s treatment and recovery facility in Cadishead, Manchester, U.K., recovers valuable metal, particularly nickel and now copper, from metal-bearing waste solutions. It uses a multi-stage pretreatment process centered around ion exchange technology to concentrate and purify the waste solutions, followed by electrowinning to recover the pure metal. The process is suitable for most nickel or copper solutions greater than 5,000 ppm that contain predominately one metal, although other metals such as chromium, zinc and iron can be tolerated at low concentrations. Nickel and copper filter cakes also can be made suitable for processing by first solubilising in acid.
The parameters of metal concentration, pH, temperature, flow rate and levels of contamination are critical for producing a viable feed for the electrowinning process. Electrolysis technology is available “off the shelf,” but CSG has developed an extensive intermediate process that makes it unique.
The first pretreatment step is through an ion-exchange unit that selectively retains the nickel while enabling other contaminants to pass through. After the metal is removed through the ion exchange, the aqueous effluent undergoes further treatment to make it suitable to be discharged to a sewer.
Pretreatment is followed by 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
CSG’s electrowinning plant contains 15 cylindrical cells, each capable of plating as much as 25 kg of metal. In operation, the concentrated metal solution from the ion exchange is heated and circulated through the electrowinning cells. This has the effect of producing intense turbulence across the electrodes, thereby maintaining a high plating efficiency.
Electrowinning continues for as many as 240 hours, or until the concentration in the metal reduces from approximately 60 g/L to 5 g/L. The depleted feed solution is then returned to the ion exchange 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 1,200-mm long and weighing about 25 kg.
The recovered metal is up to 99.9 percent pure, and the final concentration in the effluent is <4 ppm, which means that 99.9 percent of the metal has been recovered.
Parameters Affecting Plating
Development of the CSG process, which is affected by a number of factors, has been a three-year research and development project. The optimum conditions for plating nickel vary with each batch and the margins are very narrow, as almost no contamination can be tolerated. The correct parameters have been identified as:
· Metals. The presence of other metals prevents nickel from plating, therefore the conditions for plating nickel have to be as near perfect as possible to have a chance of coaxing it from its ionic state to its elemental state.
· pH. Hydrogen liberation during plating affects pH which, therefore, has to continually be adjusted.
· 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.
· 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.
· Temperature. Temperature must be between the narrow margin of 50-55oC and has a profound effect on pH and stress.
· Flow. The system runs at a constant flow rate. Low velocity leads to anodic stress, while high velocity leads to cathodic stress and causes dendrite formation. The acceptable range appears to be 47-53 m3/h.
A considerable amount of trial and error was required for CSG to identify the optimum conditions. The company often achieved optimum conditions on a number of parameters only to discover that a previously unknown variable was now affecting plating performance. A test lab was established alongside the electrowinning plant to enable the operator to mimic conditions on a smaller and more manageable scale.
As described above, recovering nickel is fraught with difficulties, and CSG is continuing research into developing the optimum conditions for plating nickel. In the meantime, its Manchester plant also is being used to plate copper, which has none of the tight variables associated with plating nickel.
Phil Manley is technical manager at CSG’s hazardous waste treatment and recovery complex in Manchester, U.K.