Extracting valuable metals from slag
Text: Timo Hämäläinen
Boliden Harjavalta aims to even better valuable metals recovery from the slags generated in the production of metals. To achieve this the company ordered a pilot-scale project from GTK Mintec.
Boliden Harjavalta’s plants are located on the coast of Southwestern Finland. The company’s main products are copper anodes and nickel matte. The copper anodes will be further refined into copper cathodes at the Pori unit. As by-products, the plants produce metals such as gold, silver, selenium and platinum- palladium concentrate. In addition, the plants produce sulfuric acid and liquid sulfur dioxide.
The raw materials used in the smelters are concentrates from the Boliden group’s own mines and external suppliers. Also recycled metals are used as raw materials.
As part of continuous process development and sustainability, Boliden Harjavalta is researching different options to utilization of the slags and improve the recovery of the valuable metals.
– Our goal is to produce metals with maximum recovery and with minimal amount of waste. It means that we have to extract valuable metals from slag and find different ways to utilize the slags – for example in land construction, concrete industry or manufacturing bricks, says research and development engineer Ville Naakka.
Getting rid of impurities
The biggest challenge for making slags as a product instead of waste is the impurity content of the slags and the solubility of the impurities. The first step for utilising slags as a product is to fill up the environmental limits set by the authorities. They can be achieved by extracting impurities from slag or by processing the slag into a more harmless form.
– We have to discover suitable and economical methods. The profitability of utilisation of slags depends on the recovery of valuable metals, the deposition costs of slags and the price of possible new products, Naakka says.
Slag becomes a cost if it has to be deposited. First you have to find an appropriate deposit site. Then you have to do the groundwork required by environmental safety. In addition, you must transport and pile up the slag.
– The cost savings can be surprisingly big if metal recovery from the slags can be improved and slags can be utilized as a product. And of course it would be sustainable and environmentally friendly, Naakka says.
Boliden Harjavalta has earlier done some lab scale tests with slags to improve valuable metals recovery. At the end of 2016 the company decided to start pilot scale tests to confirm the findings and to see how the continuous processing works.
– We have a laboratory in Harjavalta, but our research equipment is limited. We turned to GTK Mintec, because they have a pilot-scale test plant and laboratory equipment suitable for mineralogical research.
Naakka and his colleagues monitored the test runs at GTK Mintec’s premises in Outokumpu. The tested process included crushing, grinding and separation of the different fractions.
– With GTK personnel we discussed both general issues related to the tests and details. When we saw how the material behaves, we received a lot of knowledgeable comments and suggestion on what we could try out and how we could refine the process. Overall, Naakka considers the test run a success.
Boliden Harjavalta is part of Swedish Boliden group, which has operations in Sweden, Finland, Norway, and Ireland.
Boliden is a metals company with a commitment to sustainable development. Core competence is in the fields of exploration, mining, smelting and metals recycling.
Boliden’s main metals are zinc and copper. Other important metals include nickel, lead, gold and silver.
Concentrate from Boliden’s own and external mines is sold to own and external smelters. Zinc metal is sold to steelworks for rust-proofing (galvanising) and copper metal to wire rod and copper rod manufacturers. Nickel matte is sold as an intermediate raw material to external customers for further refining into pure metals and/or compounds.
Lead is sold to car battery manufacturers. Gold and silver are sold for industrial production and as an investment object.