Electrolytic Smelting
General Process Overview
In Summary, the Process Sequence is:
Process Phases in Detail
Set out below is a description of the electrolytic smelting process that our zinc smelters use to produce their products.
Electrolytic Smelting
Zinc smelting is the process of recovering and refining zinc metal out of zinc-containing feed material such as zinc-containing concentrates or zinc oxides. Globally, two main zinc-smelting processes are in use:
(i) a pyrometallurgical process run at high temperatures to produce liquid zinc.
(ii) a hydrometallurgical or electrolytic process using aqueous solution in combination with electrolysis to produce a solid zinc deposit.
The vast majority of zinc smelting plants in the western world use the electrolytic process, also called the Roast-Leach-Electrowin (’RLE’) process, since it has various advantages over the pyrometallurgical process (overall more energy-efficient, higher recovery rates, easier to automate hence higher productivity, etc.).
All of Nyrstar’s smelters use the electrolytic smelting process.
General Process Overview
The electrolytic zinc smelting process can be divided into a number of generic sequential process steps, as presented in the general flow sheet set out below.
Source: Brook Hunt
In Summary, the Process Sequence is:
Step 1: Receipt of feed materials (concentrates and secondary feed materials such as zinc oxides) and storage;
Step 2: Roasting: an oxidation stage removing sulphur from the sulphide feed materials, resulting in so-called calcine;
Step 3: Leaching transforms the zinc contained in the calcine into a solution such as zinc sulphate, using diluted sulphuric acid;
Step 4: Purification: removing impurities that could affect the quality of the electrolysis process (such as cadmium, copper, cobalt or nickel) from the leach solution;
Step 5: Electrolysis or electro-winning: zinc metal extraction from the purified solution by means of electrolysis leaving a zinc metal deposit (zinc cathodes);
Step 6: Melting and casting: melting of the zinc cathodes typically using electrical induction furnaces and casting the molten zinc into ingots.
Additional steps can be added to the process transforming the pure zinc (typically 99.995% pure zinc known as Special High Grade (’SHG’)) into various types of alloys or other marketable products.
Process Phases in Detail
The various steps in the process are described in more detail below.
Step 1: Feed Materials Receipt and Storage
Smelters use a mix of zinc-containing concentrates or secondary zinc material such as zinc oxides as feed to their roasting plant. Nyrstar’s Auby and Balen smelters, in particular, are characterised by a relatively high input of secondary materials. Smelters located inland receive their feed by road, rail or canal depending on site-specific logistical factors and the type of feedstock (eg, secondary zinc oxides come in smaller volumes and are typically transported by road). Concentrate deliveries typically happen in large batches (eg, 5,000 to 10,000 tonnes).
Nyrstar’s European smelters are strategically located close to the Antwerp seaport that serves as a global concentrate hub and provides for an extensive multi-modal logistical infrastructure connecting Belgium, the Netherlands and France. Nyrstar’s Australian smelters are located on or near the coast, allowing for direct maritime access, while Clarksville is connected to the major seaport of New Orleans via the Mississippi and Cumberland Rivers.
Most zinc smelters use several sources of concentrates. These different materials are blended to obtain an optimal mix of feedstock for the roasting process.
Step 2: Roasting
Through the roasting process, the zinc sulphides in the concentrates are converted into zinc oxide, known as calcine. A roasting furnace operates at a temperature of approximately 950° C, generating enough energy to make the process autogenous. The roasting process is fully automated, controlled and operated from a central control room. Nyrstar operates some of the world’s largest roasters, which are modelled after those used throughout the industry (e.g., the Lurgi-VM fluidised-bed roaster, which was developed in Balen).
The roasting step results in the production of calcine material (which is transported to the subsequent leaching plant) and sulphur dioxide-rich waste gases. Waste heat boilers remove the calcine contained in these gases as well as recovering the heat in the form of steam that is used in the leaching plant and/or converted into electricity. Finally, the sulphur dioxide is converted into sulphuric acid in a contact process, generating an important smelter by-product.
Step 3: Leaching
The main purpose of the leaching process is to dissolve the zinc oxide contained in the roasted calcine material and to transform it into zinc sulphate prior to the electrolysis stage. For this process, diluted sulphuric acid is used. Approximately 90% of the zinc in roaster calcine is in the form of zinc oxide, with the balance being present as zinc ferrite, an iron-zinc oxide. Zinc oxide can be leached with weak sulphuric acid solutions, but leaching the zinc from ferrites requires more aggressive acid conditions. The contained zinc dissolves while the other metals come out as a solid generically called ’Leach Product’. This leach-residue containing precious metals is sold as a by-product to third parties for further refining. The dissolved iron is removed from the zinc sulphate solution as goethite, jarosite or haematite which is usually stored in ponds.
Step 4: Purification
The leach solution is subsequently sent to a purification installation that removes other non-iron dissolved impurities such as cadmium, copper, cobalt or nickel, which could also affect the electrolysis operation. These impurities are removed through cementation by adding zinc dust to the solution. The resulting by-products are generally sold to third parties for further refining. The purified zinc sulphate solution is sent to the cell house for the electro-winning phase of the smelting process.
Step 5: Electrolysis or Electro-winning
Zinc metal is extracted from the purified solution by means of electrolysis. An electric current is passed through the solution and the zinc is deposited on aluminium cathodes. At regular intervals, these cathodes are removed from the cells and the zinc deposit is stripped from the cathodes. Most of the Nyrstar smelters use a mechanised and automated process of stripping machines, which was initially developed at the Balen plant.
The zinc produced with the electrolysis process is typically of SHG grade containing 99.995% zinc.
The electrolysis phase uses large amounts of electrical energy and is responsible for the high proportion of the energy-cost in the overall smelting process (typically about one third of total plant cash costs) [to be validated based on actual financials]. Hence, cell house productivity (and electrical current and energy efficiency in particular) is a crucial driver in overall plant efficiency. Nyrstar runs some of the industry’s largest and most efficient cell houses.
Step 6: Melting and Casting
Depending on the type of end-products produced, the zinc cathodes coming out of the electro-winning plant can undergo an additional transformation step in a foundry. Zinc cathodes are melted in induction furnaces and cast into marketable products such as ingots. Other metals and alloy components may be added to produce zinc containing alloys used in die-casting or general galvanization applications. Finally, molten zinc may be transported to nearby conversion plants or third parties using specially-designed insulated containers. This is the case with Balen and Budel.
