The tailings material in the lysimeters was installed at the same level as the tailings surface in Rautuvaara in spring 2018. Photo: Raija Pietilä, GTK

Biochar offers new opportunities for the remediation of mining environments

Mining operations always cause changes in the environment. The magnitude and
quality of these changes depend, for example, on the type of ore, the processing
technique, the amount of mining waste and the location of the mine site.

Text: Kristina Karvonen

In addition to reducing impacts during active mining operations, Finnish legislation defines that mine closure and remediation should already be addressed when mine planning and permitting processes start.

The correct closure and remediation of mining waste facilities are integral parts of environmental impact management.

The most significant and the most long-term environmental impact of mining operations is associated with the disposal of mining waste, sulphide-bearing tailings and waste rock, in particular, and the resulting metal leaching and acid rock drainage.

Interaction between sulphide minerals, oxygen and water, together with bacterial activity, causes the weathering of rock material and the oxidation of sulphides, resulting the acidification of seepage waters.

In addition to acid mine drainage, dust generated from the topsoil of the waste area can cause environmental loads in surrounding areas due to the lack of cover material or an insufficient cover solution.

The use of glacial till is the most common solution to cover mine waste facilities in Finland.

Its efficiency is affected, above all, by its thickness and water conductivity.

Vegetation also has an impact on the water status of till cover through evaporation and water movements in tree root systems.

High-quality till is not always available, and transportation costs can be high.

– Due to the inorganic composition of till and its poor water-containing capacity, its properties as a sealing layer and also its greening potential in landscaping do not always meet the required criteria when considering optional cover solutions for waste rock piles and tailings impoundments. Modern landscaping calls for solutions that also contribute to greening. There is demand for new cover solutions, and related research is being conducted to an increasing extent, says geologist Raija Pietilä.

Biochar and its diverse uses

Biochar is charcoal produced from biomass via pyrolysis.
It effectively binds water and nutrients, and withstands drought.

As biochar also helps to regulate the volume of carbon dioxide in the atmosphere, it is regarded as one solution in climate change mitigation.

When used as a raw material in soil improvement, its effectiveness is based on its ability to improve water retention, the binding of hazardous substances and the recycling of nutrients. Biochar was not studied actively in Finland before the 2000s, and Nordic research on its use in mining environments is only taking its first steps.

– The use of biochar as a cover material offers new opportunities for the treatment of mining environments, Pietilä says.

The use of low-sulphur tailings improved using biochar in cover solutions can reduce the volume of cover material transported from other locations, as well as the environmental impact. Biochar can also improve the properties of till. This in turn decreases the thickness of the cover layer and the need for any virginal soil. The use of composted material produced from other industrial by-products and municipal waste in the production of biochar contributes to the growth of a sustainable circular economy.

The field study of Rautuvaara tailings consists of six lysimeters which all have different cover solutions. Anna Tornivaara, GTK.

The Biopeitto project, coordinated by the Geological Survey of Finland (GTK), is seeking new solutions for mineral waste cover structures and landscaping in northern climate conditions, considering the needs of the mining industry and environmental aspects.

– The aim is to improve the long-term sustainability, stability and greening of dry cover and to reduce erosion. Another purpose is to improve the water management of the cover and its carbon and nutrient balance, project leader Pietilä says.

The project consists of several parts, such as the planning of test structures and greening solutions, the production of biochar, the testing of materials, the laboratory, greenhouse and in-situ testing of cover structures and landscaping solutions.

Funded by the European Regional Development Fund (ERDF), the three-year Biopeitto project started in summer 2017. In addition to GTK, its partners include the Natural Resources Institute Finland (Luke) and the Oulu Mining School at the University of Oulu. It is also funded by Hannukainen Mining Oy and Agnico Eagle Finland Oy, while Noireco Oy acts as the biochar producer. Raw material for the pyrolysis process has been obtained from Levin vesihuolto Oy, Suomen hyötymurskaus Oy and Stora Enso Veitsiluoto Mill. Rautaruukki Oyj has enabled field research in the Rautuvaara tailings area. Rautaruukki also delivers the till used in the test field from a nearby area.

Of the three work packages of the Biopeitto project, GTK is responsible for project management and biochar testing in cover structures of mine waste, while Luke is in charge of the production of biochar and its use together with paper mill sludge in landscaping

Biochar tested in cover structures of mine waste areas

– The goal of GTK’s work package is to test biochar-based cover solutions that support the circular economy for mine waste facilities by studying the suitability of biochar in cover structures using laboratory and field tests, says research scientist Anna Tornivaara, who leads GTK’s work package.

Test results will be presented during and after the project in various publications and in the Wiki-based open access database focused on mine closure technologies. This database is maintained by GTK. During testing, the ability of biochar to bind hazardous elements and to improve the technical properties of cover material, such as the water retention capacity, will be studied and optimal mixing ratios between different cover materials will be assessed.

Biochar has been selected for the test mixtures on the basis of usability and availability, considering the secondary raw materials of mining processes or other industrial fields. Cooperation with companies is hoped to lead to the innovative development of regional business activities.

The project is making use use of research data on the most potential cover structures, cover materials and mixing ratios.

–We have placed special focus on the practical feasibility of cover structures, such as the availability and cost-efficiency of different materials, Tornivaara says.

To support this research, a survey related to the use of biomaterial in cover solutions has been sent to mining companies, the authorities and other parties operating in the sector in Finland and Sweden. This will help to place focus on any problems and optional solutions associated with the remediation planning, implementation and costs of mine waste management.

– Leaching studies are being conducted as long-term column tests in GTK’s laboratory, Tornivaara says. The test consists of ten columns 40 cm in height and 10 cm in diameter. The columns contain till, tailings from Rautuvaara and pyrolysed bio-based materials at different mixing ratios. Water is pumped into the columns at a low speed. The chemistry of seeped water is analysed and water parameters (pH, EC, redox) are measured. In this way, any differences in mixing ratios and their impact on the quality of drainage can be assessed and optimal mixing ratios for suitable cover solutions can be identified.

– The project is being piloted in the Rautuvaara tailings area in Kolari, where we are studying the functionality of a till cover improved with biochar. On-site studies on long-term behaviour are being conducted by using field lysimeter tests, Tornivaara continues.

Five lysimeters of 30 cm in height and 1 m in diameter filled with tailings have been covered with different materials or material mixtures of different thicknesses. The sixth lysimeter will remain uncovered. The materials used in the cover structure tests have been characterised to identify their chemistry and mineralogy. Characterisation helps to identify the acid generation potential and hazardous substance content of specific materials and to assess their solubility. Electric conductivity, temperature, moisture and oxygen sensors have been installed in the cover solutions. Results are being saved to the data-logger established in the test area. Water samples seeped through the cover layers and lysimeters will be collected in separate containers, and the water samples will be measured and analysed at regular intervals.

The greening of waste areas is being studied in the Rautuvaara tailings area under natural conditions by using different cover solutions, seed mixtures and plants. Photo: Raija Pietilä, GTK

New solutions for landscaping

The landscaping of tailings areas and waste rock piles is challenging. Mine waste facilities are susceptible to erosion resulting from surface runoff, and building a growth layer to protect the pile requires proper planning. Often, the minimum requirement for the growth layer is its water retention, as moisture is poorly retained by rock material. The high saline content of rock material reduces water activity down to a level where only rare plants are able to use it. In addition, the waste material of extractives may contain high concentrations of metals that are toxic to plants and imitate nutrients.

–From the points of view of plants and greening, a sealing layer that contains organic matter and water-retaining material is needed on top of rock material, Pietilä says.

In mine waste areas, biochar added to the cover layer retains water, which plants can then use, improving the success of the growth layer. In addition, water evaporated from plants reduces the seepage of water down into lower layers in the waste pile or tailings impoundments. Another material studied in the project is paper mill sludge, which is a by-product from the forest industry. It has been used, for example, in sealing structures at landfill sites because of its low costs and water infiltration capacity. It also neutralises acidity.

The aim of Luke’s work package is to develop a sealing and landscaping solution based on biochar and paper mill sludge for waste rock piles. The solution needs to be suitable for the demanding conditions of northern regions. The goal is to improve biochar production skills in the region and help northern companies to develop or start business activities in the sector. Using greenhouse tests, the aim is to identify the most promising layers and mixtures of cover materials for landscaping research. Pot tests conducted in greenhouse conditions will serve to identify the impact of till and biochar on the growth of different types of plants. The success of plants and, in particular, the development and physiological state of roots, will be monitored, while their nutrient state and that of the growing material will be studied by means of laboratory analyses.

The landscaping and greening of the cover layer will be tested in a pilot project, for which information on the properties of the materials has been collected using preliminary tests in the greenhouse. The greening pilot has also been established in the Rautuvaara tailings pond. Using the information obtained, the water and nutrient status of the sealing layer will be modelled (3D) and the model will be compared to the success of plants. These models will then be calibrated and validated using column tests.

– The goal of the landscaping package is to build a solution that combines the efficient separation of waste material, greening and effective landscape design, Tornivaara says.

– The starting point of the project is that the combination of biochar and paper mill sludge is a functional cover solution from the environmental point of view when closing mine waste areas, says Pietilä.

The water-retaining layer reduces the contact of seepage water with waste material, and it can be used by plants. Here, paper mill sludge, biochar and the vegetation layer play a key part.

In addition to improving water quality in the catchment areas and reclamation, the aim is to better blend mine waste areas in the surrounding landscape and to improve the carbon balance of these areas. Innovative technologies will produce a carbon-neutral solution that may have significant impacts on the regional economy and the environment.

– The production of biochar and the use of paper mill sludge are also solutions of the circular economy that can reduce the volume of wastes, Pietilä says in conclusion.