Having survived more than a century of turbulent economic and political upheavals, this producer is intent upon modernizing its mining, processing and smelting operations
By Sasa Stojadinovic, Miodrag Zikic and Radoje Pantovic
Rudarsko Topionicki Basen Bor (RTB Bor Group), located in Bor, East Serbia, is one of the largest regional producers of copper and precious metals in Eastern Europe, and conducts business as a corporation comprising several companies operating under central management. Operations involving ore extraction and processing from surface and underground mines are located in Bor (Rudnici Bakra Bor–RBB, or Copper Mines Bor; and Majdanpek Rudnici Bakra Majdanpek–RBM, Copper Mines Majdanpek) with smelting operations in Bor.
Archeological mining sites in the Bor and Majdanpek areas are among the oldest in Europe and date back to 4500 B.C. However, the beginning of modern mining in the Bor area was in 1904 when French investors established “The French Society of the Bor Mines, the Concession St. George,” headquartered in Paris. French ownership continued until World War II, when the Germans took over the operations. With the birth of the Socialistic Federative Republic of Yugoslavia (SFRY) just after the war, the Bor copper mines became a state enterprise. Mining operations at Majdanpek started in the late 1950s, the Veliki Krivelj open-pit mine was opened in 1979, and the Cerovo open-pit mine started operations in 1993.
In the 1990s, the collapse of the SFRY and the civil wars in Croatia and Bosnia resulted in the economic isolation of Yugoslavia (later becoming Serbia and Montenegro). Foreign markets were inaccessible due to embargo; consequently fuel and spare parts were unobtainable and production dropped to minimal levels as overburden removal in the open-pits and underground development activity faltered.
After the democratic changes in 2000, the new government had no idea what to do with RTB. After two unsuccessful attempts at privatization, RTB struggled to sustain production with what was left of the equipment—loading shovels were 30 or more years old, haul trucks were almost as old, auxiliary equipment practically did not exist, underground drills were barely operational, processing equipment and smelting technology were obsolete etc. However, a spike in the price of copper in 2007–2008 and changes in the political establishment helped the government to recognize the benefits of owning a copper-producing company; a government decision to invest in RTB enabled the renewal of equipment and boosted to RTB’s recovery.
Most new equipment, primarily for surface mining, was purchased during 2010–2011, and the current situation at RTB Bor can be best described as one of rapid renewal and recovery. Intensive overburden removal activity at all RTB’s open-pits are characteristic of present mining operations. The result is production of almost 14,000 metric tons of cathode copper, mostly from domestic concentrates, in the first six months of 2011. In the same period, RTB produced 5.7 million mt of ore, of which 4.1 million mt were taken from Veliki Krivelj. Processing facilities produced 13,820 mt of copper in concentrate (10,800 mt from Veliki Krivelj and Bor and 3,020 mt from Majdanpek). During the first half of 2011, 12.3 million mt of overburden were removed—an increase of 120% compared with the corresponding period in 2010.
RTB also has ambitious plans for the next several years, primarily involving reconstruction of the smelter; construction of a new sulphuric acid plant; an increase in capacity at Veliki Krivelj from the present 8 million mt/y of ore to 10.6 million mt/y; and reconstruction and throughput increases at the Veliki Krivelj ore processing facilities.
The main challenge for the near future is development of the Borska Reka ore deposit. With reserves of 319 million mt of ore (0.5% Cu, 0.204 g/t Au and 35.89 g/t Mo) and more than 1 billion mt of estimated reserves (within the 0.3% Cu range) Borska Reka is the future of RTB. The depth of the deposit will pose problems, extending more than 1,300 m (the terrain elevation is at K+430 m and the explored area of the deposit is between elevation K+25 m and K-995 m.) Environmental issues are also a challenge, with Bor representing an ecological “black spot” for Serbia, but RTB is determined to resolve the issues and become a modern, environmentally friendly corporation.
Mining and Processing Aspects
RTB’s current underground operations are at the Jama copper mine in Bor and Coka Marin 1 gold mine in Majdanpek. Coka Marin is still under development.
Jama’s nominal annual production capacity is 2 million mt of ore but that level has never been attained; the mine’s production record of 1.9 million mt/y occurred in the 1990s. There have been several orebodies involved in mining at this site, but currently mining is concentrated in orebody T, with occasional small-scale operations in the Brezanik orebody. Orebody T is relatively small with 200,000 mt of ore, but rich in copper (grading 5%-plus). Production from orebody T and production from Jama is 10,000 mt/month. Because the flotation facilities at Bor are set to process low-grade ore, the ore from orebody T is not floated but is directly fed, after grinding and milling, to the smelter to prevent copper loss.
Characteristic of the underground mining operations is the application of a roof support system similar, if not identical, to the New Austrian Tunneling method. Numerous stress, strain and displacement gauges and sensors are placed in the roof and inside drillholes and the roof is under constant geodetic survey. All of this is necessary because the mining method designed for this orebody is an open-stoping technique with ore extraction in horizontal sections. Consequently, the height of open space above the working area constantly increases and will reach 75 m at the end of operations. This makes any intervention in case of roof failure impossible and quality bolting support and constant monitoring are imperative.
Although the reserves were not fully exhausted, mining operations in the Tilva Ros orebody were abandoned after a mine flood and loss of life in 2006. However, considering the significant amounts of ore left unmined, the restart of mining operations is being considered. While in operation, Tilva Ros was mined by a modified sublevel caving method. This method probably contributed to the flood because the water and mud penetrated the mine from old workings and from the floor of the inactive Bor open-pit. Future mining in this orebody will include a different mining method and require sealing of all abandoned works.
The future of the Jama underground mine and of RTB copper production is the Borska Reka orebody. This orebody lies beneath the present mining operations at Jama and its explored part lies between elevations K+25 and K-995 m. A portion of the orebody has been developed but is not yet in production due to lack of investment. Existing equipment, even after having been overhauled, is not sufficient in number. However, the main problem is the mining method. To achieve profitability the orebody must be mined by applying a highly productive method. Block caving or variants of sublevel caving have been considered but the application of any of these requires relocation of a village and graveyard located within the subsidence zone. Because of that, and the depth of the deposit, mining of Borska Reka has been postponed until a feasible surface-preservation scheme can be designed.
Currently, the Veliki Krivelj open-pit carries the major burden of RTB Bor copper production—more than 75%. Consequently, the majority of investment has been related to Veliki Krivelj, with the main spending focus on the mine’s load/haul fleet. The first new haul trucks to arrive on site were two Komatsu HD 1500-7s (140-mt payload) purchased in 2008. During the last two years, 11 more haul trucks were purchased (220-mt Belaz 75306s and 130-mt Belaz 75131s). With these new trucks in operation the haulage fleet now has 23 units (including rebuilt Unit Rig MT 3600s and Euclid R 170s). There are currentlyfive operational loading shovels: three are older 11.4-m3-bucket P&H and Marion units, and two new machines—a 15-m3-bucket Terex O&K RH 120 and a 22-m3-bucket Komatsu PC 4000, with one Komatsu still on order.
The mine’s nominal production capacity is 8 million mt/y of ore, at a stripping ratio of 1.2. However, the pit is being redesigned to achieve 10.6 million mt/y output and overburden removal for the fourth pushback has already begun. The increase in productivity requires reconstruction of the Veliki Krivelj flotation facilities since current capacity there is only 8 million mt/y. To prevent eventual bottlenecks in primary crushing, a system connecting the primary ore crusher and primary waste crusher No. 2 has been installed that allows the use of waste crusher No. 2 as a primary ore crusher when necessary.
The ore from primary crushing is further comminuted and processed in the Veliki Krivelj concentrator and the resulting concentrate is sent to the smelter at Bor. Waste/overburden is disposed at three active dumps. There are two conventional dumps—Todorov potok and Saraka potok—and one High Slope Dump. Overburden from operations on the fourth pushback is disposed at Todorov potok, because of short haulage distances, but all other waste from Veliki Krivelj is taken to the High Slope Dump. The Saraka potok dump is almost full and is used only when the haulage system for the High Slope Dump is out of service.
The Majdanpek open-pit is, in fact, two open-pits with North and South areas separated by a road, river and high voltage power line. The North area deposit is poly-metallic, containing copper and precious metals as well as lead and zinc. As at Veliki Krivelj, Majdanpek also has renewed equipment, most of which is engaged in overburden removal in the North area open-pit. The South area is currently inactive but restart of mining activities there is planned for 2014 with annual production targeted at 6 million mt of ore. Ore is concentrated by flotation at the Majdanpek concentrator, capable of producing 12.35 million mt/y. The resulting product is sent to the Bor smelter.
The Cerovo open-pit is the newest, having started operations in 1993 with a design production capacity of 2.5 million mt/y. Although mining was terminated in 2002, Cerovo’s surface-mineable reserves stand at 14 million mt of ore. The Cerovo complex consists of several adjacent ore deposits, and although studies indicate that ore extraction could be profitable in the long term, the necessary inclusion of all these deposits in an integrated mining plan would require relocation of a railroad line and tunnel—an undertaking that is presently economically unfeasible. Despite the fact that infrastructure already exists (access roads, power and water, grinding and milling facilities, etc.) the decision to reopen Cerovo is controversial, considering that the primary crusher was dismantled and relocated to Veliki Krivelj. Despite these obstacles, overburden removal is presently under way, utilizing a Terex O&K RC 120 excavator and three small Belaz haul trucks. Construction of a pipeline that will be used for hydraulic transport of milled ore to the Veliki Krivelj flotation plant also has begun and the first tons of ore are expected by the end of the year.
Air pollution from the smelter—which uses 1970s smelting technology—is the company’s most serious environmental problem; the presence of SO2 from smelter emissions is constant and SO2 concentrations can reach 12,000 µg/m3, compared with a Serbian regulatory limit of 350 µg/m3. During the past several years, smelter emissions have periodically caused severe damage to crops, orchards and vineyards, with the latest event occurring as recently as July 2010. However, the smelter is being rebuilt to employ modern technology from Outotec. The smelter and new sulphuric acid plant construction are being financed by loans from the World Bank, with the Serbian government as guarantor.
Overall, a century of mining has left its mark on the Bor area landscapes. Almost 1,000 hectares (2,500 acres) have been degraded by surface mining operations and waste or tailings disposal. The Serbian government initiated a regional development project, financed by the World Bank, which includes reclamation and remediation of degraded areas. The Dutch company Witteveen+Bos has been retained to conduct site investigation, design work, development of an environmental impact assessment, preparation of an environmental management plan, and a plan for stability, rehabilitation and remediation of several older tailing dumps and dams in the area.
A major environmental concern is the Krivelj River collector, which reroutes the river beneath a tailings dump at the Veliki Krivelj flotation plant and which was severely damaged, creating a possibility for tailings from the dump to enter into the collector and contaminate downstream waters. It has been repaired but construction of a new collector is part of the regional development project. The Spanish consulting company Eurostudios-IK has been engaged to investigate sites and design the remediation and construction of the collector.
Localized environmental issues include the seismic effects of blasting at the Veliki Krivelj open-pit. The proximity of the nearby village and the likelihood of ground vibration causing damage to village structures has led to development of a seismic monitoring system. The system is currently in the design phase but when completed will combine ground vibration monitoring, crack response monitoring, structural health monitoring and Internet technology into a reliable, accurate and transparent system. It will provide necessary data for changes in blast design that will result in optimal blasting results with minimum effect to surrounding structures.
High Slope Waste Dumps: A Proven Possibility
During the Serbian economic crisis, the Veliki Krivelj operation looked for a solution that would lower the cost of waste haulage by truck. With two active waste dumps almost full, a new location for waste disposal was urgently needed. The solution was found in the cavity of the closed Bor open-pit, located 3 km south of Veliki Krivelj, where:
- The volume of the cavity, nearly 240 million m3, could provide room for disposal of 225 million m3 of waste rock from Veliki Krivelj without need for additional area degradation; and
- Upon completion of dumping operations, the cavity could be filled and reclamation of the area could be accomplished more easily.
Considering the terrain configuration, haul distances, the lack of haul trucks at the time and the costs of haulage, the High Slope Dump solution required a combined haulage system. This system includes truck haulage within the pit outline, a waste crusher followed by a belt conveyor to the dump point, and a boom spreader at the end. The system was designed in 1995, construction began in 1996 and it became operational during 1997. The design included foundations for two cone crushers, but only one crusher was installed initially; the second crusher was finally installed and became operational just this year.
Once crushed, waste material is fed to a 2.8-km-long main belt conveyor and transported to the dumping location. The conveyor has installed power of 4 MW (1 MW in the main drive station with three 1-MW booster stations along the conveyor route). Material from the main conveyor is fed to a dump conveyor, which in turns feeds a boom spreader that dumps the waste material into the cavity of the Bor open-pit, forming a single-slope dump with maximum slope height of 400 m. With both crushers in operation, system capacity is 4,700 mt/h.
Because of the lack of data on disposed material behavior in high slope conditions, the High Slope Dump concept was designed as a phased operation. The first phase was waste disposal from the main platform level down; the second phase was waste disposal from the main dumping platform up to the final dump top platform. The first phase also was subdivided into two stages with the first, experimental stage objective aimed at reaching the maximum slope height as quickly as possible in order to collect necessary data for the following stage and phase. Another reason to include the experimental phase was legal: the Serbian ministry of mining would not give permission for the dumping operations if there was no experimental phase since no similar operation existed at the time. Still another other reason for phased development of the waste dump was the existence, at that time, of certain objects within the Bor pit outline that could not be covered with waste.
The designers cautiously set the “safe” distance of the 300-mt boom spreader at 90 m from the crest of the dump slope. With a boom length of 30 m the spreader formed piles of disposed material on the main platform, and the original design suggested tandem bulldozers be used to push the material over the edge. However, due to a chronic shortage of bulldozers the boom spreader was generally set at a distance of less than 30 m from the slope crest and fed the material directly into the cavity of the pit. Although this setup was not in accordance with the design and could be considered a “gray area” in legal terms, it provided significant data on slope stability issues.
However, with only one crusher installed at the beginning the productivity of the system was restricted by 50%. In addition, shortages of fuel and spare parts caused by economic problems in the 1990s reduced the haulage fleet to only a couple of operational trucks. With the primary need to sustain copper production, overburden removal was limited during that period, resulting in in low utilization of the dumping system over the years (only 0.8 % in 2004 and 12.7% overall utilization from 1997 through mid-2010.)
Early in 2010, RTB engaged the Technical Faculty in Bor to redesign the dump system, including installation of a new 50-m-long spreader boom. The first problem we faced was the lack of data on material characteristics such as cohesion and the angle of internal friction. Because of the nature of the disposed material and variations in fragmentation and type of material, no existing method for determination of these parameters was available. It was assumed the slope, under the then-present conditions of 350 m height and overall inclination of 32°, was stable with a safety factor of 1. Using these assumptions we reverse engineered the stability calculations and calculated the values of needed parameters. These values were fed to another slope stability calculation which determined a new “safe” distance of 35 m with a safety factor of 1.3. The 35-m distance was considered adequate after a landslide of material occurred on September 12, 2010, during which 511,000 m3 or 970,900 mt of material slid downslope but the unstable ground never extended within 30 m of the main platform.
Although material consolidation and settling have resulted in minor slides at the crest and tension cracks in the main platform, 13 years of dumping material down the high slope provide sufficient experience to claim that high slope waste dumps are possible and safe. Apart from the September 2010 event, there were no significant problems at the dump until recently when, after the reconstruction of the spreader in December 2010 and installation and testing of the second crusher, the system became fully operational and capable of achieving full productivity; this consequently resulted in a change of dynamics. The material is disposed of much faster, the boom spreader advances more frequently and the material does not have time to consolidate, resulting in relatively frequent large landslides. Fortunately, the slides have not endangered the spreader.
The principal characteristic of these slides is swelling of the slope, leading us to conclude that unconsolidated material loses its bearing capacity under rapidly increasing load. We have proposed a monitoring system and procedures to monitor slope deformations and tension crack widths. The proposed system would provide data on the behavior of the slope and tension cracks prior to a slide and allow formulation of a slide prediction model.
The proposed system has not yet been installed. However, this outcome does not necessarily mean that high-slope waste dumps should be disregarded as an option. Recent events have provided new insights in the behavior of the material in these extreme conditions and future research and monitoring will only serve to make this type of dump safer.
Sasa Stojadinovic, M.Sc., Miodrag Zikic, Ph.D., and Radoje Pantovic, Ph.D. are members of the Technical Faculty in Bor, University of Belgrade, Bor, Serbia.