The LCA was a cradle-to-gate study spanning the extraction of manganese ore and manganese smelting in submerged electric arc furnaces and associated on-site and off-site processes including, but not limited to, grid electricity generation, fuel refining, coal mining, and cokemaking. The end-gate of the LCA is saleable lump manganese alloy, prior to distribution to iron and steelmaking facilities.
Global industry representation
The LCA covers 2010 production based on data collected from 16 mines and smelters. The site selection process aimed to capture a broad range of regional factors, facility sizes, production methods, and technologies. The LCA data and results are representative of global manganese ore and alloy production and cover production in Australia, China, France, India, South Africa, and the USA. The participating sites included in the LCA study represented 26 % of global manganese extraction and 8 % of global manganese alloy production in the study year.
The LCA results are presented using four functional units to satisfy the internal industry and external communication objectives of the study (Table 1). The production of the three alloy systems represented by the LCA were modeled and reported independently to increase the value of the study as a tool for internal industry benchmarking and optimization. Data from each respective alloy system was represented relative to 1 kg of alloy of average compositions defined by the American Society for Testing and Materials (ASTM A99-03 (2009) and ASTM A483/A483M (2010)). An additional, fourth functional unit “average manganese alloy” was defined based on an average of the three alloy systems to provide results representative of the entire industry. The aggregated manganese alloy system contains an equal weighting of each production facility considered in the study, resulting in a 2:2:1 ratio between HC FeMn, SiMn, and Ref. FeMn, respectively.
Life cycle impact assessment categories and environmental indicators
The life cycle impact assessment (LCIA) considered the following potential environmental impact categories: global warming potential (GWP), acidification potential (AP), and photochemical ozone creation potential (POCP). The impact assessment was carried out using the CML 2001 mid-point method developed at the Institute of Environmental Sciences, Leiden University, Netherlands. In addition to the potential environmental impact categories, the LCA results highlighted a series of key process and environmental indicators. Table 2 summarizes the LCIA environmental impact categories and indicators presented in the results.
Manganese alloy supply chain description
The manganese supply chain was divided into modules representing primary (on-site) and associated upstream and downstream processes for each step of the manganese alloy production process (Fig. 1). Manganese ore is mined via conventional surface and underground mining methods. Surface mining involves the removal of overburden and blasting and extraction of ore and waste rock using primarily diesel-powered mobile equipment. Underground mines, in comparison, rely more on electricity and produce less waste rock to selectively extract manganese ore. Ore is hauled by mobile equipment and conveyors from the point of extraction to ore processing and beneficiation plants, where run-of-mine (ROM) ore is crushed and separated from waste rock using crushers, wet screening, dense media separation drums, and cyclones. The waste rock from the processing plant is either stockpiled (dry) or sent to tailings impoundments (wet slurry). The manganese mining life cycle includes upstream diesel refining to power mobile equipment and on-site generators, electricity when connected to the grid, and the manufacture of tires to replace worn tires on mobile equipment.
Manganese ore fines and internally recycled manganese-bearing fines can be agglomerated in sinter plants located at mining and/or smelting facilities. Sintering agglomerates fines and partially reduces manganese oxides in the ore using coal and coke. Sinter production requires upstream electricity, coal mining, and cokemaking. Emissions from sinter plants can be controlled using cyclones, scrubbers, baghouses, and electrostatic precipitators.
Manganese smelting occurs in submerged electric arc furnaces (EAFs), where electrical energy and coal and coke act to chemically reduce the manganese ore and sinter and produce metallic manganese in the form of molten HC FeMn and SiMn alloys. SiMn production involves the addition of a silicon-bearing material and often involves the recycling of slag generated during FeMn production. Manganese smelting is an energy and carbon-intensive process. Particulate emissions are controlled by baghouses and wet scrubbers. Slag waste generated from smelting is generally managed through internal recovery and utilization in by-product applications. Manganese alloy production involves a variety of auxiliary processes around the furnace, including slag processing, metal casting, crushing and screening, and raw material handling. The entire life cycle includes, but is not limited to, the production of upstream electricity, cokemaking, and coal, quartz, and limestone mining.
Refining of ferromanganese is performed in its molten state at oxygen converters following the furnace smelting stage. Refining reduces the carbon content of the alloy and produces MnO-rich dusts suitable for use in pigment manufacturing. The refining life cycle requires upstream electricity, oxygen, and a variety of other industrial gases.
Life cycle modeling
A customized modeling approach was devised to accommodate the multiple objectives and target audiences of the LCA by simultaneously developing LCA models at process, site, and global scales.
Each participating mine and smelter was modeled independently and later assembled with equal weighting to form the complete global system. Each site model was composed of common process modules to allow for the comparison of individual processes across multiple sites for site benchmarking and data validation purposes. Individual site models were compared to industry averages in confidential site-specific reports to help drive optimization within each participating facility.
Primary site data was collected by site visit and data questionnaire and formed the flows into, out of, and within the primary manganese supply chain. Secondary upstream and downstream processes were linked to the primary material flows using Ecoinvent and GaBi LCI datasets and modeled using GaBi Life Cycle Engineering software. Region-specific LCI data was used to reflect differing electricity grid mixes in each participating country.
The life cycle models for each site were added to form the global models representing each manganese alloy system. Each alloy system was then summed to form the global average manganese alloy system, resulting in a 2:2:1 ratio between HC FeMn, SiMn, and Ref. FeMn, respectively. This ratio is reflective of the representation of participating facilities, and in comparison to global industry production, is over-weighted, under-weighted, and balanced for HC FeMn, SiMn, and Ref. FeMn, respectively.
Allocation and co-products
A number of co-products result from manganese alloy production and leave the primary manganese supply chain. Noteworthy co-products include slag produced during smelting and MnO dusts collected from the refinery off-gas stream. Slag is often used as an aggregate material for local construction, diverting a waste stream but receiving no allocation of environmental impacts. However, FeMn slag can be an important manganese-bearing material for SiMn production, and a mass allocation is used to transfer a portion of the environmental burden of FeMn furnaces to slag destined for SiMn production. MnO dusts are typically sold as a valuable pigment, although no allocation is allotted due to the small quantity of MnO produced, which yields proportionally low economic value compared to alloy production.