Electrowinning

Electrowinning

Metals can be recovered from solution by electrolysis, a process that is known in the extractive metallurgy industry as electrowinning. When two electrodes are placed in a solution containing metal ions and an electric current is passed between them, the metal can be deposited on the negative electrode. In the recovery of most metals, oxygen is evolved from water at the positive electrode. An electrolyte, and a current density, is generally chosen that gives a dense, compact electrodeposit, and additives included in the electrolyte to further improve product quality (a practice also used in electroplating).
Large-scale electrowinning of copper was developed between 1912 and 1915 at one of the world largest copper mines, that at Chuquicamata in Chile. Electrowinning has become the exclusive choice for recovering copper following leaching of oxide or sulfide ores and solvent extraction. SX/EW produced more than 3.3 million tonnes of copper in 2009, which is 21% of total world copper production.

In a typical electrowinning or electrorefining operation, the electrolytic cells are rectangular tanks and each contains 20-50 negative electrodes and a similar number of positives. An additional positive electrode is required in each cell in order to ensure plating occurs on both sides of each negative. The electrodes are designed to rest on busbars that supply electricity; these are situated outside the top of each tank, one for the negatives and another for the positives. Thus, the electrode pairs in each tank operate in parallel. In the tankhouse, there is a multiple system of practical sections with the banks of cells connected in series and parallel to obtain optimum use of the electrical power derived from the rectifiers, while keeping the voltage to earth at any point at a level that does not pose a risk to personnel.

In copper electrowinning, the positive, oxygen evolving, electrodes are usually made from lead calcium alloys similar to those used for the grids in lead-acid batteries. The traditional negative electrodes for copper electrowinning were thin copper starter sheets made by plating copper onto titanium or stainless steel electrodes. The copper was then stripped from the substrate and inserted into the electrolysis cell and copper deposited from a solution usually containing a concentration of 25 to 60 grams/liter (g/l) copper as copper sulfate and 50 to 180 g/l sulfuric acid, held at 50 to 60oC (122-140oF). Small quantities of a hydrocolloid such as Guar gum are included in the electrolyte, as this helps to form a dense electrodeposit. Typically, a current equivalent to a current density of 300 A/m2 is passed between each pair of positive and negative electrodes; this gives a cell voltage of ~2.0 V and the energy consumption is ~2.0 kWh/kg.

In an electrolysis cell such as in electrowinning or refining, the negative electrode is the cathode and the positive electrode is the anode. Hence electrowon, or electrorefined, copper is known as cathode copper.

Modern developments in copper electrowinning include the use of re-usable stainless steel electrodes to replace copper starter sheets. This technology was established in the Isa Process and later in the Kydd Creek Process. In addition to lowering manpower requirements, it allows the current density to be increased by ~10%. Following electrolysis, the copper is stripped from the stainless steel in special automated machines.

In a typical commercial copper electrowinning plant, there will be thousands of pairs of electrodes in operation together. In modern systems, insertion of negative electrode blanks, removal of loaded electrodes, and transfer to the stripping machines are fully automated.


As pointed out above, zinc electrowinning from zinc sulfate solution is the usual final stage in the recovery of zinc from sulfide flotation concentrates. Electrowinning was introduced in the second decade of the twentieth century to replace distillation. As with copper, zinc is deposited on the negative electrode and oxygen is evolved, derived from water, at the positive electrode. The electrolyte typically contains 55 to 70 grams/liter of zinc as zinc sulfate and 150 to 200 g/l of sulfuric acid and is held at 35 to 38oC (95-100oF). Small, controlled quantities of glue and antimony are added to the electrolyte to help form a smooth deposit. A current density between 400 and 800 A/m2 is employed, and this gives a cell voltage of ~3.5 V and an electrical energy consumption of ~3.3 kWh/kg. The overall reaction converts aqueous zinc sulfate to zinc metal and sulfuric acid. The acid is then recycled to the leaching stage where zinc oxide is dissolved. The positive electrodes in zinc electrowinning are made from lead silver alloys; oxygen evolution is promoted on these alloys and lead dissolution, which would contaminate the zinc product, is avoided. More than 80% of the 2009 world zinc production of 11 million tonnes was produced by electrowinning.

Zinc is, in principle, more difficult to electrowin than copper because zinc lies considerably above hydrogen in the electrochemical series, whereas copper lies below. Thus, the evolution of hydrogen from an acidic zinc sulfate solution is energetically favored over zinc deposition. As it happens, hydrogen evolution on a zinc surface is a very slow process (it occurs at a high overpotential) and hence zinc can be electrodeposited from acid solutions. But impurity metal ions in the solution can plate out as well, if they are below zinc in the electrochemical series. These include antimony, arsenic, cobalt, copper, germanium, and nickel. Not only would these elements contaminate the zinc product if they are present in significant quantities, but also they can promote cathodic hydrogen evolution on the negative electrodes and hence diminish the current efficiency of electrolysis to deposit zinc. This means that the solution needs to be purified before electrolysis. Purification of the solution is achieved by adding zinc dust. Zinc will displace elements below it in the electrochemical series and remove the impurities that effect electrowinning efficiency. These cementation, or metal displacement, reactions also take place by an electrochemical process. The anodic dissolution of zinc gives up electrons to the zinc particles and these are used up in the coupled cathodic reactions, which are the deposition (and removal) of the impurity elements from the solution.

Cadmium metal produced in the purification of zinc electrowinning electrolyte is dissolved in sulfuric acid solution and electrowon in a process similar to that for zinc.

About 60% of the world 2009 nickel production of 1.4 million tonnes was derived from sulfide ores, but an ever-increasing proportion is coming from other ores (such as laterites, in which nickel is present in an oxy-hydroxide or silicate matrix). Indeed, over 70% of the world reserves of nickel are lateritic. Laterites are usually leached to dissolve the nickel. The recovery of nickel from solution, following leaching of either type of nickel ore, can be achieved by hydrogen reduction, as discussed above, or by electrolysis. Outokumpu uses the latter technology in Finland. Nickel is deposited from nickel sulfate solutions (~60 grams/liter nickel) containing sodium sulfate and boric acid to improve the deposit. The latter additive adjusts the pH to ~3.5. The metal is deposited at 65oC (149oF) onto nickel starter sheets and oxygen is evolved at the lead positive electrodes. Nickel is also electrowon in a form suitable for use as soluble positive electrodes in nickel electroplating.

About 70% of the cobalt world 2009 production of 62,000 tonnes was also recovered by electrolysis in a process similar to that for nickel.