Great Mines: Finland’s Kemi Chrome – by John Chadwick and Hugh Boden (International Mining – October 2015)

http://im-mining.com/

In September 2014 Kemi mine celebrated exactly 50 years since Outokumpu made the decision to begin chrome mining operations there in Kemi, Finland. Today it is one of the most efficient and environmentally friendly mines in the world. The deposit had been found five years earlier. Mining began in 1967, with large-scale mining operations and ferrochrome production beginning in 1968.

The chrome mine and ferrochrome works were the first steps in Outokumpu’s transition from state mining company to one of the world’s foremost stainless steel producers. Today the annual mill capacity is 2.7 Mt/y of ore (up from 1.3 Mt/y in 2010), producing lumpy ore and fine concentrate (all for internal use). The mine employs some 400 people including contractors, and the nearby ferrochrome works and stainless steel mill in Tornio employ some 1,900 (plus contractors).

CEO Mika Seitovirta: “Kemi is an essential part of the integrated production chain in the Tornio site. Chromium is what makes steel stainless, and our own chrome mine guarantees us competitive sourcing of chromium for the future. The Kemi chrome mine is a unique competitive advantage for us globally.” Outokumpu sees the competitive advantages of its ferrochrome operation as:

  • An integrated, world-class operation
  • World-class chrome deposit in the EU
  • Stable, cost efficient electricity supply
  • Excellent cost curve position.

Kemi has sufficient reserves to continue mining for several decades. Current production is some 2.4 Mt/y. There are Proven Reserves of 48 Mt and Probable and Inferred Resources estimated to be some 100 Mt. The grade of the resources is 29.4% Cr2O3 and that of the ore reserves is 25.9% Cr2O3. The deposit remains open at depth with seismic measurements indicating a depth of several kilometers. The deposit is a layered intrusion consisting of several orebodies (more than 10 bodies) dipping at 70°. The average width is 40 m (but varies from less than 1 m to 100 m) and length 3 km. Today five orebodies with a dip of 70° northwest make up the 2.5 km-long ore zone that is now being mined below the Elijärvi and Surmaoja open pits.

Outokumpu Chrome Oy targets minimal environmental effects from its operations. The mine, surrounded by nature, uses no hazardous chemicals in the concentration process: it is simply based on gravity. Environmental effects are continuously monitored, and in underground mining, dust emissions are minimal and metal discharges small.

The main decline portal lies in the footwall of the main pit, about 90 m below the rim. This decline is mainly 8 m-wide x 5.5 m-high, to accommodate passing vehicles. It descends at 1:7 to a depth of 600 m at the base of the hoisting shaft, and connects with several intermediate sublevels. The decline has been asphalted along most of its length to reduce dust and to minimise maintenance costs.

There is a 14,000 m3 workshop at the 350 m level for the mobile equipment fleet and a 23,000 m3 main workshop at the 500 m level. The 350 m level workshop is enclosed by Megadoors, which keep in the heat so that an ambient 18°C can be maintained.

The main pumping station (capacity two x 250 m3/h) on the 350 m level is equipped with slurry pumps with mechanical seals. These pump the unsettled mine water to the surface over a total head of 360 m. Two other dewatering pumping stations are located at the 500 m and 580 m levels.

The crusher station at the 560 m level is equipped with a 1,000 t/h Metso gyratory crusher. This is fed from two sides by vibrating feeders from separate 8 m-diameter main ore passes from the 500 m level, and from one side by a plate feeder, into which ore can be dumped from the 550 m level. A 40 t travelling gantry crane services the entire crusher house. Crushed ore gravitates onto a conveyor in a tunnel below the crusher for transport to the shaft loading pockets 500 m away.

ABB supplied complete mine hoisting systems to the mine. The friction-type production hoist has a pulley diameter of 3.2 m. It has a skip with integrated cage on one side and a counterweight on the other side It is equipped with a 1,600 kW induction motor and ACS600 fully digital frequency
converter with DTC (Direct Torque Control) with active front end that controls the power factor at
unity, i.e. it does not produce any reactive power. Further, the harmonic current generation in the converter is very low.

The Advant controller AC110-based hoist control system provides fully automatic control of both skip hoisting and personnel transportation from three levels in the shaft. The primary mining method is bottom up transverse underground benching with delayed backfill of stopes, where due to the thickness of the orebody three stopes can be developed in a row.

Stopes in the Elijarvi orebody have 4 m pillars secondary as well as some 20 m long and 25 m high. The Surmaoja stopes have no pillars, and are 15-20 m wide for primary and secondary, as well as some 20 m long and 25 m high. The transverse stopes are developed with cable bolt and mesh support to minimise dilution. Cemented fill is placed in the primary stopes, while secondary stopes are backfilled with mine waste rock.

The 5 m x 5 m cross section sublevels are established at 25 m vertical intervals. The development drills are an Atlas Copco Rocket and are 12 m-15 m wide for primary and Boomer E2C C18 and a Boomer E2 C22 that drill the drifts for the primary stopes from the footwall through the ore zone. Any raise drilling is undertaken by a Rhino 100 HM. The cable bolters are a Sandvik DS 520 TC (Cabolt) and
two Atlas Copco Cabletec LCs. The Cabletec LC is based on the Simba M7 longhole production drill rig (widely used at the mine), with a second boom for grouting and cable insertion.

The booms offer a long reach and can drill a line of up to 4.7 m of parallel holes from the same rig setup. Likewise, the booms can reach up to 8 m roof height, allowing the Cabletec LC to install up to 25 m long cable bolts. To date, most holes drilled have been in the 6-11 m range, with cable installed at a rate of more than 40 m/h.

Production blastholes are usually 51 mm diameter downholes drilled in fans 2.3-2.8 m apart by a Sandvik DL 420-7C (Solo) and the Atlas Copco Simba fleet of one M6C, one L7C, one M7C and the newest rig, an ME7C. There is a Simba M4C mounted on a Scania truck that drills slot holes. The LHD fleet comprises one Sandvik LH517 and five Caterpillar R2900Gs, with haulage by Mercedes Benz 4155 K 8 x 4 trucks. The holes are charged with pumped emulsion explosives with 200 mm of stemming. Two Nonel detonators are used in each blasthole.

The Intelligent Mine

Grade control and mineability were key considerations in the original, detailed underground mine planning. In order to provide continuously the data necessary for grade control from underground, as well as the equally important broader array of management information needed to maximise mine efficiency, Outokumpu developed an Intelligent Mine Information Technology program. An Atlas Copco Diamec U6 APC (Automatic Performance Control) is used to obtain the geological data, for grade control and other core drilling. The Diamec was retrofitted with Atlas Copco’s RCS remote control system. This works very well with Kemi’s W-LAN based communication system, and most of the mine’s Atlas Copco units have it fitted.

The WLAN communications network, based on commercially available equipment, covers the whole mine. This network infrastructure not only allows effective underground communication. Also all the Atlas Copco drill rigs equipped with the Rig Remote Access (RRA) are logically integrated into the information systems.

The RRA consists of a communication server onboard the rig and a network adapter and integrates with the mine’s network to allow data transfer and remote monitoring and troubleshooting. It works as a two-way communication system, since data can be sent and received in real time between Atlas Copco and the mine.

For instance, should one of the drill rigs encounter a problem, the warning seen by the operator will also be shown in the mine office, which can then contact Atlas Copco immediately, enabling its people to enter the rig’s electronic system and diagnose the fault. The main benefits of RRA are that the administrative system can be updated automatically with the latest information with no manual handling and the rig operator always has access to the latest production planning.

There is no need to write work reports after each shift, since all log files are automatically saved to the planning department and work orders can be issued during the shift and directed onto the specific drill rig instead of being written before each shift. Fault diagnostics can be conducted remotely, which allows the service technician to diagnose the problem and choose the correct spare parts before travelling to the drill.

The Intelligent Mine Program provides the mine and process plant with an advanced communications system to access and complement the information on Kemi’s master database. This system has been expanded and upgraded more or less continuously. It provides:

  • Fast mine-wide communication and data networks
  • Systems integrated to the networks’ production management system
  • Machines and equipment integrated to the networks
  • Mine infrastructure and personnel training.

Kemi was the first mine to be fully equipped with Finland’s Intelligent Mine™. This was developed by Outokumpu, Tamrock (now Sandvik), Normet and Nordberg-Lokomo (now Metso), the Helsinki University of Technology (now Aalto University), Laboratory of Rock Engineering and Technology Development Centre
of Finland (TEKES).

The basic elements of an Intelligent Mine are:

  • Mine-wide information and data acquisition systems
  • High-speed, two-directional, mine-wide communication and information systems network for real-time monitoring and control
  • Computerised information management, mine planning, control and maintenance systems
  • Autonomous and tele-operated machinery and equipment connected to the mine-wide communication networks
  • Communication and monitoring systems to other mines within a company, machine manufacturers and public networks.

A green concentrator

Crushed ore is delivered to surface at less than 250 mm in size. It then reports to the surface
crushing plant, which is equipped with a Lokomo jaw crusher and two Morgårdshammar gyratory crushers. The plant has Sandvik feeders and screens and Firotec belt conveyors. Its capacity is 385 t/h and utilisation rate 80%.

It has minimal impact on nature and the community due to the oxide ore, the gravity separation process and the closed water circuit. Ore concentration is a two-stage process based on gravity separation – lump ore and fine concentration. The specific gravity of the waste rock is 2.7 t/m3, while that of the ore is 3.45 t/m3. The coarser crushed ore (10–100 mm) reports to the lump concentrating plant operating in conjunction with the crushing plant and the finer ore (grain size <10 mm) is sent to the fine concentration plant where it starts in the grinding circuit.

The lump ore is concentrated in a two-phase heavy-medium (sink-float) circuit in which the medium is a ferrosilicon/water mixture in FLSmidth Wemco separation drums. The planned capacity is 231 t/h at a utilisation of 80%. The circuit for FeSi regeneration has IFE heavy medium recovery magnets. The slurry density in the first phase is 3.2 kg/dm3 and in the second 3.6 kg/dm3. In the first stage waste rock is separated from the ore as a light product and is returned underground for use as backfill. In the second stage, a lump concentrate is separated from the ore as a heavy product and is stored under cover. The remaining (light/intermediate) fraction is crushed to -25 mm and reports with the fine fraction (under 10 mm) to the fine concentrating plant.

There are two Sandvik washing and two dewatering screens, Weir pumps and again all belt conveyors are Firotec. The capacity of the heavy-medium separation plant is 400,000 t/y (35.5 % Cr2O3) upgraded
lumpy ore with grain size of 10-120 mm. The utilisation of the plant is 80%.

The Outotec rod mill is used in primary grinding, in open circuit and Wärtsilä ball mill, in closed circuit, with 800 μm Derrick screens functioning as classifiers, producing a grain size of less than 0.7 mm. A Wärtsilä rod mill is used in open circuit for secondary grinding. Separation performance is strongly dependent on the particle size distribution of the grinding circuit product. Particle size distribution has a significant effect on the concentrate grade and recovery.

The grinding circuit capacity is 300 t/h and that of the spiral plant is 242 t/h. Utilisation of the fine concentrating plant is 94%. The capacity of the fine concentrating plant is 850,000 t/y (45% Cr2O3) metallurgical grade fine concentrate. The final fine concentrate is dried in a drum filter to a moisture content of less than 4%, homogenised and stored in indoor storage facilities. The concentrate is then transported by truck to the ferrochrome plant in Tornio.

Envimine

Outokumpu’s 2014 Sustainability Report noted that “as the ore-bearing minerals are very stable and chemicals are not used in the beneficiation process, mining operations have only a minor effect on local water quality. Metal discharges from mining activities are small, and their effect is only observable as slightly elevated concentrations of nitrogen, solids, calcium and iron in watercourses. The largest emissions into the air result from the transportation of ore and barren rock, from operations in the product loading area and from piles of concentrate.”

Since mining moved wholly underground “dust emissions into the air have become minimal [totalling less than 0.5 t in 2014], but the effect of particulate emissions on air quality is still monitored regularly by studying levels of suspended particulate matter. The results of the monitoring showed that the emissions situation has remained stable and that concentrations of dust in air at and around the site are low.” Dust emissions are kept to a minimum using dust removal systems operating in the mine and concentration plant. Dusting of roads and waste areas is prevented by sprinkling and using dustbinding agents. Concentrates are stored indoors.

Loading areas are asphalt surfaced and are washed regularly during the summer. Concentrate stored outdoors is sprinkled in the summer. Covering and landscaping of the tailings areas as soon as these are full, reduces dust there.

“Piles of barren rock, former open-pit mining activities and the beneficiation and clarification basins all have long-term effects on the landscape. Tailings basins are landscaped when they are full. Barren rock is used in backfilling underground workings.

“As the concentration processes at the mine employed are based on gravimetric separation, only water and small amounts of flocculant are used.

“Of the total amount of water used, 95% was recycled rainwater. Noise generated by blasting operations is almost inaudible, even within the mine area. In April-May the spring flooding in the streams nearby the mine was normal. Almost all the water required for the concentration process is recycled from the 2.6 million m3 settling pond of the waste area. All surface water of the mining area is conducted to the settling pond.

A great deal of analysis and environmental testing has been carried out around the mine. For example, the paper Assessment of the impact of opencast chrome mining on the ambient air concentrations of TSP, Cr, Ni and Pb around a mining complex in Northern Finland, International Journal of Environmental Analytical Chemistry, Volume 82, Issue 5, 2002, describes the results of an air sampling program designed to evaluate the impact of the Kemi open-pit mine.

More recently a Barents region project, Envimine, considered this area’s great potential for mining and examined the challenges associated with mine closure arising from mining-induced impacts on the surrounding water, soil and air, and possible impacts on biota.

The geochemical studies on the active Kemi chromite mine focused on the origin of elevated concentrations of some elements in waters and organic stream sediments at the mine site and in its surroundings. Chemical impacts of tailings and settling ponds on watercourses were estimated based on surface water and organic stream sediment data. The studies provided new data on the composition and geochemical features of the pad of the settling ponds and local peat, and their impacts on water quality in the downstream watercourses.

The project activities embraced:

  • Collecting data of the history and background of the case mines
  • Field studies by GPR, XRF measurements, sampling of waters and soil, analyses
  • Meetings and workshops
  • Reports and recommendations for monitoring groundwater and surface water quality, and for after care plans
  • Publications, presentations at workshops, seminars and conferences.
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