PROCESS DESIGN
research and development is now on new sensors and sensor accuracy with the aim of improving detection accuracy.
Outotec provides pneumatic sensor-based sorting machines for pre-concentration in the mining and minerals processing industry, via its partnership agreement with Tomra Mining. Different sensor and machine types cover a vast selection of ore and mineral types.“ All machines are built with the same three principles: material presentation, data collection by sensors, and separation. Material is presented to the sensors as a monolayer of liberated particles. Different sensor types analyse every single particle, and the decision of accepting or rejecting is made within 30 milliseconds. Physical separation of particles is achieved by altering the particle trajectory with precise air jets into the product or waste chutes.”
Sorting enables the removal of sub-economic waste material in a very early process stage. When sorting is situated in the crushing circuit, the removed waste does not have to be milled further down and processed in the beneficiating plant. The removed material allows for more throughput of valuables in the downstream process, or major capital investment savings by decreasing plant size. This decreases the use of consumables such as water or electricity per unit of metal produced. A sorting machine can also be placed in the pebble circuit of an AG or SAG mill. Removing waste from the milling circuit can debottleneck many processes.
With the addition of sorting to the processing plant, significant economic opportunity exists in optimising mining with reference to preconcentration. The resource size can be increased by considering a lower cutoff grade and less selective mining techniques that offer a lower operating cost per tonne. The additional waste material mined due to less selective mining is removed by sorting and the feed grade to the plant remains high. In addition, flotation and leaching recoveries are generally increased due to the higher head grade.
Many mining operations accumulate stockpiles of material below the economic cut-off grade threshold. Numerous successful test work and installations have been done on reclaiming waste dumps by upgrading the low-grade material. Applying sorting to such material has low operating costs because the material has already been blasted and mined. Every material is unique. The sortability of any given deposit is dependent on the heterogeneity of the ore. Any successful sorting operation requires differences in some material properties.“ These properties may be colour, density or electro-magnetic. Different mineralisations can also be distinguished based on near infrared spectra or differences in the crystalline structure. In some
cases, the disseminated hints of metal are not visible to any sensor and sorting relies on tracer properties or mineral associations. This is especially true for gold deposits, where gold particles within the rock are almost always below the resolution of the machine(< 1 mm pixels). To verify the correlation of tracer properties and the metal grades test work has to be done for every sorting application prior to implementation.”
Sorting test work starts on the desktop with a sorting amenability evaluation. Experts validate all data available and provided by the customer. Based on expertise and prior test results a first gate decision is made to continue or not.
Next is the first stage of hands on sensor test work on the material. Outotec asks for 10-20 carefully selected samples of each rock type representing the whole ore body for a first inspection test. These rock types may consist of: ore, marginal ore, waste, intermediate waste or SAG pebbles. Testing can be completed at the Tomra Mining test centres in Hamburg, Germany and Sydney, Australia. Responses from different sensors to the material is tested to determine differences between rock types. The aim of this test work stage is to provide sufficient information for initial sensor selection.
After a successful first inspection a performance test is carried out on industrial scale machines. Test sample sizes are typically 1-4 t of material for each sample. The sample should be as representative as possible of the expected feed including a mix of all rock types.
After assaying the results this test provides all necessary information for proof of sortability and expected performance. During the performance test the sensor system is adjusted and calibrated according to the requirements of the orebody. This provides crucial information such as grades, recovery and capacities. More complicated flow sheets including scavenger sorters or cleaner sorters or combining different sensor types can be tested as well.
“ A successful performance test campaign provides all relevant information for process design and implementation of a sorting solution. Test work is completed using industrial sized machines, so therefore no scale-up factor has to be applied and piloting is generally not required. The main focus of process design is the pretreatment of the sorter feed. The key to successful operation is sufficient fines removal and a narrow particle size distribution. To maximise the benefit of sorting, the circuit should be designed to deliver the maximum amount of material in the size range for sorting. The optimal location for a sorting solution is within the secondary crushing stage where a minimum amount of material bypasses sorting through under- or over-size streams.”
Capacities for individual machines varies based on the rock type and can be up to 400 t / h for certain applications. By combining various machines in different particle size ranges plants have been implemented with a maximum capacity of 2,000 t / h total plant feed.“ Outotec’ s expertise in crushing, grinding, materials handling, flotation, leaching and dewatering ensures smooth incorporation of sorting in any given process. Our offering includes everything from sorting equipment packages including auxiliaries to full turnkey solutions.”
Sensor-based ore particle sorting can be used to pre-concentrate ROM ore feed, SAG mill scats and low grade or waste stockpiles. Plant throughput increase, reduced operating costs and mine optimisation should be considered in evaluating sorting as well as increased recovery in flotation and leach circuits. The economic benefit to the operation can be very powerful and also typically reduces sensitivity to metal prices. A recently published example is Minsur’ s San Rafael mine in Peru, showing a pay-back period of only four months after the start-up of the stockpile sorting plant.“ The machines have proven to be very efficient,” said Pedro Condori, Process Operation Manager for Minsur.
Eriez and HydroFloat
The copper sulphide processing flowsheet has remained virtually untouched for decades and follows a logical progression – crush, grind, float, regrind and refloat to produce a final copper concentrate. This well-defined and proven method of copper sulphide processing has served the industry well for over a century and is based on the particle size range for which conventional flotation is most effective. Work by numerous experts has shown that mechanical flotation works well for a limited size range, from approximately 15 to 150 microns. Particles outside this critical size range are typically lost in industrial operations and rejected to tailings due to inherent constraints associated with the physical interactions that occur in the pulp and froth phases of conventional flotation equipment.
Theoretical and experimental studies conducted by researchers working in the Eriez Flotation Division indicate the inherent limitations of conventional flotation can be overcome through the utilisation of a fluidised-bed flotation machine specifically engineered for the selective recovery of feeds containing very coarse particles. The HydroFloat™ separator, was specifically designed in the early 2000s to address the limitations of traditional flotation systems. By using a quiescent, aerated fluidised bed, the turbulence commonly found in a mixed-tank contacting environment is greatly minimised. As a result, delicate bubble-particle aggregates are more likely to report to the concentrate without disruption. Furthermore, the absence of a
72 International Mining | JANUARY 2018