South Crofty feasibility study - metallurgical testing services for Cornish Metals

Challenge

To reopen the South Crofty tin mine, the client needed detailed metallurgical testing to demonstrate the commercial and financial viability of the deposit. The primary challenge was to determine the optimal process flowsheets to achieve the best results. Key areas of optimisation included pre-concentration methods (such as HLS, ore sorting, and jigging), fine gravity recoverable tin (using Multi Gravity Separators or shaking tables), and ultrafine tin recovery (through gravity or tin flotation). The goal was to prove that saleable tin concentrates could be produced while maintaining flowsheet simplicity and minimising expected capital and operational expenditures (CAPEX/OPEX).

Solution

SLR was selected for this project due to our historical knowledge and expertise in gravity processing, particularly in tin processing. Our role involved providing metallurgical expertise and conducting metallurgical testwork to support the feasibility study's objectives.

To address the challenges, we conducted trade-off studies in key areas requiring optimisation to determine the best processing method. During the characterisation phase of the study (Phase One), material from five distinct areas of the deposit was submitted. Head assaying confirmed tin grades within the anticipated ranges. Mineralogical and comminution studies verified the bulk mineral deportment and liberation, as well as the hardness and abrasivity of the material. Preliminary gravity separation studies confirmed tin grade and recovery performance aligned with historical records and expectations.

"The team readily supported the project studies very diligently and undertook the work to a very high standard, offering practical advice, solutions, and recommendations during the testwork." - Cornish Metals

A significant advancement in technology since the mine's previous operation was the implementation of ore sorting as a pre-concentration method. When applied to the South Crofty material, combined with dense media separation, it was determined that a high percentage of ROM ore could be rejected to waste with very low tin losses. These results fed into Phase Two of the study, which aimed to utilise the available sample mass to verify the historical flowsheet metallurgy while exploring newer technologies that could provide Cornish Metals with additional value in terms of revenue and/or CAPEX/OPEX costs. The focus was on making the operation as easy to operate as possible, given the global skills shortage.

Key flowsheet developments from Phase Two testing included the provision for coarse cassiterite recovery using spiral concentrators. Spiral concentration provided a low-cost, easy-to-operate method of recovering coarse cassiterite in a simple rougher spiral stage, followed by cleaning using two shaking tables. A large proportion of the overall cassiterite present in the sample was recovered at this stage.

Additionally, the replacement of shaking tables with MGS separators demonstrated that certain size fractions in both the primary and secondary gravity circuits could potentially reduce floor space and capital costs while increasing stage tin recovery of these size fractions. The replacement of deslime, tin sulphide scavenging, and tin flotation with a Falcon-MGS circuit provided a viable, lower-cost, easy-to-operate alternative to desliming and tin flotation. Open circuit Falcon continuous roughing followed by MGS cleaning conducted on the finest size fraction produced products that were sized, assayed, and mineralogically examined to estimate closed-circuit performance.

Finally, review of WHIMS in the tin dressing circuit involved dry high-intensity magnetic separation (HIMS) testwork followed by SLon testwork on final tin concentrates.

Impact

The success of ore sorting as a method of pre-concentration was notable and is expected to significantly benefit the operation. An acceptable final product was produced in terms of both grade and recovery. The project remains active, and the client is rapidly progressing towards the goal of bringing the mine back into production.

These outcomes align with the client’s goals of demonstrating the commercial and financial viability of the South Crofty tin mine. The implementation of advanced technologies and optimisation methods has provided measurable improvements in processing efficiency and product quality. As a result, the client is well-positioned to achieve their objective of reopening the mine, contributing to the local economy and community development.