South Crofty PEA Update

The PEA update for the South Crofty project outlines a processing flowsheet that combines pre-concentration with gravity and flotation methods, based on historic operating data and recent testwork.

Report context

This article is based on the South Crofty PEA Update for Cornish Metals Inc., which includes processing route details in Section 17 (Recovery methods). The report presents a preliminary economic assessment-level process plant design for the South Crofty Project.

Processing route

Design basis and testwork programme

Cornish Metals engaged Orway Mineral Consultants (OMC) to carry out a review and process circuit modelling to develop the process plant design for the South Crofty Project. The flowsheet was developed based on historic plant performance from the Wheal Jane plant from 1994-1998, after the 1993 installation of a secondary hydrosizer and MGS which had a dramatic effect on metallurgical recovery rates, and supporting testwork completed in 2023/2024 by WAI’s Metallurgical Laboratory.

The Wheal Jane Concentrator, incorporating key metallurgical improvements made in 1993, was utilised as the basis for the flowsheet development with the incorporation of a pre-concentration plant. This pre-concentration plant consists of XRT ore sorting of the -50 mm+15 mm size fraction, updated to include 15 mm+5 mm using TOMRA’s latest development, with inline pressure jig (IPJ) separation of the 5+0.85 mm fraction, leaving the -0.85 mm fraction only to report downstream without pre-concentration.

Cornish Metals’ metallurgical test programme was primarily aimed at testing the suitability of XRT and IPJ pre-concentration whilst verifying the historic (1994-1998) production records, operating data and flowsheet from the Wheal Jane plant and developing the process design criteria and equipment.

Pre-concentration circuit

The processing flowsheet includes underground primary single stage crushing. ROM material will pass through a static grizzly and 20” x 30” (510 mm x 800 mm) underground jaw crusher in open circuit producing a product of 175 mm (7 in), prior to hoisting. Three 500 m³ live capacity underground bunkers will receive hoisted material for storage.

Feed then passes to a dry double deck screen for secondary crushing and XRT sorting. The +50 mm oversize reports to a vibrating feeder and 90 kW cone crusher in closed circuit. The -50 mm+15 mm stream passes to 20 t hoppers ahead of XRT ore sorting. The -15 mm stream passes to a new wet double deck screen with +5 mm passing to a new XRT Ore Sorter. Accepted products from both stages report to a 430 t rod mill storage bin.

The -5 mm stream from the new wet screen passes to two IPJs (operating as a rougher/rougher configuration). Concentrate from the jigs passes to a dewatering screen, with oversize conveyed to the rod mill storage bin and undersize pumped to fines thickener. The -0.85 mm material from the wet screen is pumped to the Fines Screening Area.

XRT products are screened, with +15 mm reporting to a vibrating feeder and closed circuit 132 kW tertiary cone crusher. The -15 mm+0.85 mm material passes to the 430 t rod mill feed storage bin and -0.85 mm is pumped to the Fines Screening Area.

Grinding and spiral concentration

A 2.7 m diameter rod mill grinds material to nominally P80 size of 850 µm and pumps to a primary spiral circuit. Spiral concentrate is cleaned using shaking tables, with the table tailings returned and combined with the spiral tailings to report to screening ahead of the ball mill circuit.

The -850 +212 µm screen oversize reports to a 2.4 m diameter ball mill which pumps back to the screen in closed circuit, whereas the -212 µm screen undersize is combined with the crusher fines thickener underflow and pumped either to the Cu-Zn Flotation or Primary Gravity sections.

Gravity concentration stages

Primary gravity involves ball mill product and crusher fines (both nominally P80 of 150 µm) combined and pumped via hydrocyclones to two 17 m³ gravity buffer tanks. The classified feed is deslimed and fed at a controlled rate into a primary three-spigot hydrosizer ahead of a combination of shaking tables and MGS units. The total number of primary shaking tables is 20, split as 7, 4, and 9 tables for spigots 1, 2, and 3 respectively. Five C902 MGS units treat the primary fines hydrocyclones underflow.

Primary gravity tailings are re-ground using a ball mill operating in closed circuit with hydrocyclones. Cyclone overflow is deslimed, then separated into size fractions by a secondary five-spigot hydrosizer and fed to shaking tables and MGS. The secondary gravity section uses 26 shaking tables split as 11, 4, 6, 2, and 3 for spigots 1 to 5 respectively. Five secondary MGS C902 units treat the secondary fines hydrocyclones underflow.

Tertiary ultrafine gravity

Slimes from the primary and secondary gravity circuits are processed using Falcon concentrators, with the concentrate cleaned via a single stage MGS. The tertiary gravity section is fed by the primary and secondary fines hydrocyclone overflows with nominal -25 µm cassiterite. These streams are combined and passed over a pair of sieve bends before being pumped into two Falcon C2000 concentrators. Falcon concentrate is pumped to a buffer tank, then to a distribution box where two C902 production MGS units (each a twin-drum machine) provide final cleaning.

Tin dressing and optional circuits

Spiral plus primary and secondary gravity bulk concentrates (combined Sn + S TOT of 55-57%) pass through a small sulphide flotation section (reverse float) to remove sulphide minerals and reduce smelter penalties. This tin dressing concentrate is combined with the tertiary ultrafine MGS concentrate, filtered and containerised for shipment. The detailed design allows space for a WHIMS unit for the option to remove Ferberite if required.

An optional polymetallic Cu-Zn flotation circuit includes a bulk sulphide float with copper and zinc flotation sections producing separate concentrates.

Historical processing data

Historical gravity-only processing of the South Crofty Lower Mine ore grading at 0.84% Sn resulted in average Sn recovery of 73%. From 1988 to 1998, South Crofty ore was processed at the nearby Wheal Jane mill which achieved 88.5% recovery by recovering Sn in fine fractions by froth flotation in addition to gravity recovery of coarse Sn. The Wheal Jane process achieved significantly improved efficiency following improvements (installation of hydrosizer in the secondary gravity circuit and replacing column flotation with MGS) producing final concentrate grades between 55-58% Sn.

Key reported parameters

Technical qualifications

The report notes several specific limitations:

• Pre-concentrator recovery is based on metallurgical testwork and BRUNO modelling mass splits.
• Concentrator recovery is based on the historic Wheal Jane mill recovery/head grade relationship (Section 13.2 of the report).
• The overall plant recovery of 87.13% is calculated as 96.9% x 89.9% = 87.1%.
• The Cu-Zn flotation area is currently under consideration as an option to the mine plan, with the regrind size to be determined but potentially in the range of ≈45 µm depending on test work and historic data.
• The flowsheet has been developed based on historic plant performance from the Wheal Jane plant from 1994-1998 and supporting testwork completed in 2023/2024.
• The overall plant may have either a net positive or negative water balance under certain circumstances.

The processing plant has been designed with a start-up production rate of 500,000 tpa, with the ability for expansion. The qualified person considers the design to have drawn on the best features of the historical South Crofty and Wheal Jane flowsheets whilst introducing new technologies that were not available in 1998 when the mine was last in operation.

Source: South Crofty PEA Update, Cornish Metals Inc., Section 17 Recovery methods.