Feasibility Study (FS) of the Re-opening of the Aranzazu Mine, Zacatecas, Mexico

The feasibility study describes a conventional crushing, grinding, and flotation processing route for the Aranzazu Mine, based on existing equipment with selected modifications and allowances for arsenic management.

Report context

This feasibility study for the re-opening of the Aranzazu Mine, located in Zacatecas, Mexico, presents the processing design for treating 2,597 tonnes per day (TPD) of run of mine ore over a 365-day operating year. The report relies on historical operating data from the previous operation, with particular reference to 2014 plant utilization, and incorporates testwork results from a locked cycle test used to develop the metallurgical balance.

Processing route

Crushing

Run of mine ore is scalped across a static slotted grizzly screen with 15-inch openings before being fed to a vibrating screen with 4-inch wide slotted openings ahead of the primary jaw crusher, which is a 36 inch by 42 inch unit with 250 horsepower. Screen undersize and crushed ore are conveyed to a secondary vibrating screen with a slotted 9.5 mm lower deck. Oversize is treated in a secondary cone crusher with 300 horsepower. Secondary crusher product is conveyed to the tertiary vibrating screen with a 9.5 mm slotted lower deck; undersize forms final crushed product, while oversize reports to the tertiary cone crusher with 400 horsepower, the product of which is recycled to the tertiary screen in closed circuit. The crushing circuits have demonstrated capacity to crush 2,597 TPD of run of mine ore to 80% passing 8 mm. Crushed ore is conveyed to a fine ore stockpile providing approximately 12 hours of live capacity.

Primary Grinding

The grinding circuit design feed tonnage is 116.7 tonnes per hour. The circuit comprises three 373 kilowatt (500 horsepower) single stage ball mills as primary mills, plus the existing 186 kilowatt (250 horsepower) regrind mill, now designated as the number 4 primary mill, which has been converted to primary grinding service. The number 4 mill receives ball mill discharge from two of the 298 kilowatt (425 horsepower) primary mills. Tonnage is controlled by adjusting the speed of conveyors under the stockpile feed pipes.

Ball mills discharge to primary cyclone feed pump boxes, with the primary cyclones operating in closed circuit to produce flotation feed at 80% passing 135 microns. Lime and Cytec 5100 are added to each ball mill feed to maintain pH 11.0 in the cyclone overflow. The lime addition rate is controlled using pH measurement in each cyclone feed pump box. Additional Cytec 5100 is added to the number 4 primary mill to provide collector for fresh surfaces created there.

Conditioning and Flotation

Cyclone overflow feed flows by gravity through a static sampler into two conditioning tanks providing approximately 7 minutes of residence time. Cytec 3477 and MIBC frother are added. Conditioned feed overflows into the first of four banks of four 300 cubic foot Denver DR flotation cells. The first bank, and at operator discretion the second bank, produces final concentrate at 22-23% copper grade. Additional Cytec 3477 is added to the feed box of each flotation bank. Banks 3 and 4 produce a scavenger concentrate that reports back to the first flotation bank feed box.

Proposed changes include adding baffles between the second and third cells of the first and second banks to prevent back mixing, with level control valves and level sensors ensuring a pulp level difference between the first two and second two cells. Final concentrate is sampled by cross cut sampler in the discharge line from the concentrate pump box. Tailings from bank 4 constitute final tailings, pumped directly to the tailings area with a static sampler on the final tailings stream.

Process Control Improvements

Process control improvements over the previous operation include automatic feed tonnage control based on pulp levels in the primary cyclone feed pump boxes and circulating load, automatic pulp density control in flotation feed, and addition of an Online Stream Analyser (OSA) providing copper, iron, and arsenic assays of four process streams approximately every ten minutes. The OSA system includes pumps to a multiplexer with sample collection and transmission in the same unit, with the feed, tailings sample, and scavenger concentrate stream flowing by gravity to flotation feed while final concentrate is pumped to the thickener. Additional provisions include pulp density measurement on thickener underflow for pressure filter feed control, a variable speed pump and level sensor for smooth scavenger concentrate flow, and a Programmable Logic Controller upgrade with back-up processor.

Concentrate Dewatering

Concentrate thickening occurs in a 40 foot (12.2 metre) diameter conventional thickener without flocculant addition, although flocculant addition may become necessary with the increased concentrate production in years 3 to 5 of the production plan. Thickener overflow flows by gravity to a concrete lined decant pond, with clarified water pumped back to process. Thickened concentrate at 60-65% solids is pumped to a TEFSA plate and frame pressure filter. The filter currently has 31 plates installed with capacity for up to 42, expected to easily handle increased concentrate production from the higher ore grade.

Arsenic Management

During previous operations, arsenic in ore was generally in the 0.08 to 0.15% range with average recoveries to concentrate of 33%. The deeper, higher grade ores to be processed in the next five years have arsenic in the 0.2-0.3% range with expected arsenic recovery of 80-90%, so that arsenic in concentrate could exceed 3% at times, incurring significant penalties. A copper to arsenic ratio below 7.7 will almost certainly produce concentrate with 3% arsenic or higher. The new mine plan uses net smelter returns to determine mining limits and will consider this critical ratio. Concentrate produced when the ore copper to arsenic ratio is below 7.7 will be stockpiled and blended with higher ratio concentrate, or alternatively blended with purchased concentrate.

Key reported parameters

Technical qualifications

The metallurgical balance is derived from a locked cycle test described in Section 13 of the report, with adjustments made for the final concentrate grade produced and the ore grade mined compared to the composite sample grade. Water requirements include a 10% contingency, with water recovered from the underground mine contributing a small fraction of plant water requirements; use of this water depends on quality, which has historically been good but may deteriorate when mining higher grade, higher pyrite ores. Water in lime slurry and water with reagents are included in the mass balance. The report notes that flocculant addition to the concentrate thickener may become necessary with the much-increased concentrate production rate in years 3 to 5 of the production plan.

Source: Feasibility Study Report, Re-opening of the Aranzazu Mine, Section 17 Recovery Methods, Tables 17-1 and 17-3, Section 18 Infrastructure.