A plant for treating mined waste and low grade material at the world’s largest underground tin mine has repaid its capital cost of US$24 million in just four months, thanks in part to the role of a TOMRA X-ray transmission (XRT)-based ore sorting machine.
The plant went into operation in 2016 and is projected to contribute 6,000 tonnes of refined tin to the San Rafael mine’s 2017 production. The mine, located in the La Venecia district of Peru’s San Rafael municipality, produced about 6% of global tin production in 2015.
PERSERVING ASSET VALUE
The new ore sorting plant was commissioned to treat material from a marginal development waste dumps, and also treats low-grade material from underground. The genesis of this project was an initiative by mine owner MINSUR to preserve asset value in the face of low commodity prices and declining head grades as richer parts of the ore body were depleted.
The project retreats tailings by rejecting low grade material in coarse particle size, initially targeting the ‘Cancha 35’ waste dump, made up of about a million tonnes of marginal grade waste containing 0,6% tin. This compares to an average grade of 2,13% tin mined and processed by San Rafael in 2015.
The aim was to separate sub-economic material before it entered the more cost intensive wet processing, thereby avoiding bottlenecks in the wet section and improving productivity by increasing the feed grade and overall recovery. This was expected to lower the total cash cost per tonne of tin produced, decreasing the cut-off grade and giving a longer life-of-mine.
APPLYING X-RAY TRANSMISSION TECHNOLOGY
There were a number of factors relating to the ore that made this technology option possible. The mine’s ore body is Cassiterite, a tin dioxide with a high specific gravity of 6,8 to 7,1 grams per cubic centimetre, so it is usually beneficiated using gravimetric separation technologies.
Alongside its specific gravity, however, it also has a high atomic density, dominated by the atomic number (Z) 50 which governs the attenuation of transmitting X-rays. This makes it amenable to the application of sensor-based particle sorting for waste rejection, as the tin contained in Cassiterite inherits a high absorption capacity for transmitted X-rays.
Another important factor was that the mineralisation occurred as larger clusters of Cassiterite in a lighter host rock, meaning that the structures of Cassiterite were large enough to be detected by the XRT imaging system. And thirdly, there was a significant degree of liberation of sub-economic waste on the particle level that could be subject to sensor-based particle sorting.
XRT sorting is one type of sensor-based sorting (SBS), an umbrella term for applications where particles are individually detected by a sensor technique and then rejected by an amplified mechanical, hydraulic or pneumatic process. Based on a planar projection of X-ray attenuation of a particle stream, XRT sorting distributes particles either on a chute or fast conveyor so each particle can be scanned and evaluated while passing by.
Hard X-ray radiation is emitted on one side of the material stream, and an image is recorded by a line scan detector on the other side. The particles are fed into the machine from a vibrating feeder, are diverted down a shaped chute, stabilised on the high speed conveyor, and detected by the X-ray system. The particles are analysed and classified in the sorter control and then either retained or rejected depending on the classification of each individual particle.
TESTING OF CAPABILITIES AND PERFORMANCE
In order to test the performance of this technology at San Rafael, MINSUR prepared 1.4 tonnes of composite sample from the Cancha 35 stockpile in the size range 10 to 75 mm by double-stage crushing and screening. This material was sent to TOMRA Sorting’s test facility near Hamburg in Germany.
The performance test work proved that, through all size fractions, a significant amount of material (almost 75% of the feed mass) could be removed by the SBS unit. The average grade is below 0.1% tin and considered sub-economic and financially not feasible to feed to the mine’s main plant. The product grade though is significantly above the required cut-off for mill feed.
The project was approved on the strength of a positive feasibility study, and 14 months later the construction of a 3,000 tonne per day plant was completed. This fast track performance, encouraged by the significant economic potential that it heralded, saw the sensor-based ore sorting equipment being purchased from TOMRA Sorting through its partner Outotec.
REDUCING CAPITAL AND OPERATING COSTS
Several positive impacts from this operation have contributed to reducing capital and operating costs. Most significant amongst these was that the use of sensor-based ore sorting converted uneconomic waste material into economic ore. It means that material below the cut-off for the main plant, set at 0.9% tin in 2014, can now be treated with lower specific operating costs and converted into reserves.
There is now more capacity in the main plant, rising to 3,600 tonnes per day compared to the 2,950 tonnes per day before the implementation of the project. The products from the sensor-based ore sorting process, combined with the concentrate from the -6 mm gravimetric concentrate, inherit a smaller overall size distribution in comparison to unconcentrated run-of-mine ore.
The plant has increased its overall recovery of tin in the main concentrator. Sensor-based ore sorting rejects particles with very fine, mineralised Cassiterite, which is too small to be detected by the XRT-system. The increased grade and size of mineralisation thus increases the performance of the main plant. While plant recovery was about 90,5% before the implementation of the sorting plant, the current recovery is 92,5% – a significant increase.
INCREASING THE MINES RESERVE AND EXTENDING THE LIFE-OF-MINE
The project has increased the mine’s reserves and therefore significantly extended the life-of-mine. Previously parts of San Rafael’s deposit were excluded from mine development because they were below the cut-off grade. Today almost a quarter of the feed to the sensor-based ore sorting plant is contributed by low grade ore from underground.
Another valuable impact of the project has been to eliminate the liability presented by the Chancha 35 stockpile, which has been a source of acid mine drainage. Through the treatment of all this material, it can be properly treated and disposed of, with substantial financial benefits. This has also meant that there is less tailings disposal necessary. By increasing the grade in the feed to the plant, a coarse waste with no acid mine drainage potential can now be disposed of at lower cost. In addition, this coarse waste material can be used in the mine as aggregate for road construction and functional backfill.