The Renison mine is 50% owned by Metals X through the Bluestone Mines Tasmania Joint Venture (BMTJV). It is the only major tin project in production in Australia with a mining rate of close to 1Mtpa, although the concentrator is restricted at 750,000tpa. The extreme humidity, with an annual rainfall of over 3,000 mm, and the highly acidic processing water with pH around 4.5, creates unique challenges for the sorting process, the equipment, and waste management, which BMTJV resolved with TOMRA’s help.
A complex flowsheet
The underground mine operates a primary crushing system before the material is transported to the surface through a shaft. Once there, it enters the pre-concentration plant, where it undergoes a 3-stage crushing, screening and cleaning. The particles are split into two fractions – 10 to 25mm and 25 to 60mm – which are fed into two TOMRA X-Ray Transmission (XRT) sorters. The output consists of two streams: the product, which is transferred to the wet plant, and the waste, which is fed into a TOMRA EM sorter to separate acid-forming sulphides.
In the wet plant, the product goes through primary grinding followed by bulk sulphide flotation. The tailings are processed downstream to concentrate the cassiterite tin mineral through gravity concentration, gravity tails are further treated via desliming and tin flotation. The combined concentrates are fed to a leaching circuit to remove carbonate minerals, after a final wash stage the concentrate is de-watered and dispatched.
BMTJV approached TOMRA to address two key requirements. The first was the need to upgrade their tin feed to the plant: “The Renison Tin operation wanted to achieve economies of scale, putting more tons through the front end of the plant without upgrading the back end downstream – we wanted to do more with less,” explains Ben Wraith, Principal Project Metallurgist at BMTJV. The second requirement was to address the environmental issue of removing acid-forming sulphides from the waste.
Following site visits and extensive discussions with BMTJV’s teams, TOMRA proposed a solution that addressed the tin feed quality with two COM Tertiary XRT 1200 sorters and the waste issue with a COM Tertiary EM 1200 sorter. A team from BMTJV visited the TOMRA Test Center in Sydney, where they observed what the XRT sorter operating at capacity is capable of and experienced first-hand the exceptional accuracy of TOMRA’s XRT sensor. “Our XRT stands out for the high spatial and density resolution and its ability to do contrast sorting, identifying fine high-density tin inclusions in the ore with an accuracy that has no equal on the market,” says Gavin Rech, Technical Manager at TOMRA. “On top of that, it can separate it from the acid-forming sulphides, so that we have the ability of pulling the tin into the first product and sending the rest to the EM sorter.”
TOMRA XRT sorters: the right solution for Renison’s specific conditions
The two COM Tertiary XRT 1200 sorters went into operation in 2018. Initially, BMTJV’s strategy focused on low reject grades, devoting less attention to achieving the mass reject rate and overall process plant throughput. However, the specific conditions at the Renison mine affected the results achieved. The large variance in Run of Mine (ROM) particle size distribution resulted in insufficient stability in the feed to the circuit. In addition, the extremely wet conditions in West Tasmania and consequent high ambient moisture content, combined with the high moisture ore delivered from underground, further affected the sorting process.
In 2019, a new investigation was opened into the ore sorting performance and led to a change of direction: “a paradigm shift away from targeting low reject grades, towards sorting as aggressively as possible, moving from a tin recovery-based operating strategy to a mass reduction one,” explains Ben Wraith. “We are getting 20 to 25% mass reduction, so 75 to 80% of the materials are going into the wet plant, and we are still achieving 97 to 98% tin recovery overall across the crushing circuit. Pre-concentration didn’t materially impact overall recovery, because the tin in the material that is rejected as waste is extremely fine grained and a proportion is associated with sulphides, so it wouldn’t have been fully recovered in the downstream wet plant and would have been lost to tailings. This approach is best for Bluestone’s application: this configuration gives us the ability to process 15 to 20% more tin units without having to upgrade the downstream concentrator.”
TOMRA’s XRT sorters have proved to be a sound investment that is delivering significant returns: “operating the sorting circuit has slightly increased our overall processing cost, but this is more than offset by the large increase in ROM throughput by 15 to 20%, and thus tin production, so the unit cost per ton of tin produced is reduced by almost 10%,” says Ben Wraith. They have shown to perform consistently, giving BMTJV the confidence in forecasting: “we’ve broken multiple production records in the last year in tin units, and this gives us confidence in what we can achieve, because the machine performs over and over again if you treat it right and if you prepare your feed correctly.”
TOMRA XRT success leads to upgrade decision
In view of the results achieved with the two TOMRA XRT sorters, BMTJV decided to upgrade the ore sorting circuit with two new, recently launched XRT models with stainless steel internal parts, and advanced features such as the TOMRA ACT user interface and the TOMRA Insight cloud-based platform: “the decision to buy new machines was easy,” says Ben Wraith. “The stainless steel will assist prolonging the TOMRA sorter’s life by protecting the unit from our high-moisture and corrosive environment. The more ergonomic design will help our maintenance teams, which is particularly important for machinery operating in these harsh conditions.”
Ben Wraith is particularly looking forward to TOMRA Insight: “It will enhance everybody’s understanding and experience of the machines. It will be more of an analytical platform for our metallurgy and maintenance staff, providing ample opportunity to gain valuable information that can be analyzed and optimized over time. The one feature I am personally interested in the particle size monitoring through the machines, which will enhance our overall circuit performance, because we have an integrated circuit with the crushing and screening plant. Feed preparation is key to maintain a consistent performance – knowing how well you’re preparing your feed in a live fashion can only end up with a better result.”
TOMRA will also be able to log into the machine and check the daily reports generated by TOMRA Insight, so that their technical teams will be well prepared ahead of site visits for maintenance or optimization.
A close collaboration to achieve the best results
Throughout the process, BMTJV worked closely with TOMRA. “TOMRA has been working with the site maintenance team to tailor solutions to our operating environment, which has been invaluable,” says Ben Wraith “TOMRA supported me through site visits, which included equipment inspections, site-based training of our personnel, and an openness to continually improving the technology and fine tune it to our site-specific requirements. They assisted the site with troubleshooting, optimization, discussing the nuts and bolts of the issues as they arose and finding a solution that works.”